Coupling of Uranium and Thorium Series Isotope Systematics for Age Determination of Late Pleistocene Zircons using LA-ICP-MS

NASA Astrophysics Data System (ADS)

Sakata, S.; Hirakawa, S.; Iwano, H.; Danhara, T.; Hirata, T.

2014-12-01

Zircon U-Th-Pb dating method is one of the most important tools for estimating the duration of magmatism by means of coupling of uranium, actinium and thorium decay series. Using U-Pb dating method, its reliability is principally guaranteed by the concordance between 238U-206Pb and 235U-207Pb ages. In case of dating Quaternary zircons, however, the initial disequilibrium effect on 230Th and 231Pa should be considered. On the other hands, 232Th-208Pb dating method can be a simple but powerful approach for investigating the age of crystallization because of negligible influence from initial disequilibrium effect. We have developed a new correction model for accurate U-Pb dating of the young zircon samples by taking into consideration of initial disequilibrium and a U-Pb vs Th-Pb concordia diagram for reliable age calibration was successfully established. Hence, the U-Th-Pb dating method can be applied to various zircons ranging from Hadean (4,600 Ma) to Quaternary (~50 ka) ages, and this suggests that further detailed information concerning the thermal history of the geological sequences can be made by the coupling of U-Th-Pb, fission track and Ar-Ar ages. In this presentation, we will show an example of U-Th-Pb dating for zircon samples from Sambe Volcano (3 to 100 ka), southwest Japan and the present dating technique using LA-ICP-MS.

Apollo 12 breccia 12013: Impact-induced partial Pb loss in zircon and its implications for lunar geochronology

NASA Astrophysics Data System (ADS)

Thiessen, F.; Nemchin, A. A.; Snape, J. F.; Bellucci, J. J.; Whitehouse, M. J.

2018-06-01

Apollo 12 breccia 12013 is composed of two portions, one grey in colour, the other black. The grey portion of the breccia consists mainly of felsite thought to have formed during a single crystallisation event, while the black part is characterized by presence of lithic fragments of noritic rocks and individual plagioclase crystals. In this study, U-Pb analyses of Ca-phosphate and zircon grains were conducted in both portions of the breccia. The zircon grains within the grey portion yielded a large range of ages (4154 ± 7 to 4308 ± 6 Ma, 2σ) and show decreasing U and Th concentrations within the younger grains. Moreover, some grains exhibit recrystallisation features and potentially formation of neoblasts. The latter process requires high temperatures above 1600-1700 °C leading to the decomposition of the primary zircon grain and subsequent formation of new zircon occurring as neoblasts. As a result of the high temperatures, the U-Pb system of the remaining original zircon grains was most likely open for Pb diffusion causing partial resetting and the observed range of 207Pb/206Pb ages. The event that led to the Pb loss in zircon could potentially be dated by the U-Pb system in Ca-phosphates, which have a weighted average 207Pb/206Pb age across both lithologies of 3924 ± 3 Ma (95% conf.). This age is identical within error to the combined average 207Pb/206Pb age of 3926 ± 2 Ma that was previously obtained from Ca-phosphates within Apollo 14 breccias, zircon grains in Apollo 12 impact melt breccias, and the lunar meteorite SaU 169. This age was interpreted to date the Imbrium impact. The zircon grains located within the black portion of the breccia yielded a similar range of ages (4123 ± 13 to 4328 ± 14 Ma, 2σ) to those in the grey portion. Given the brecciated nature of this part of the sample, the interpretation of these ages as representing igneous crystallisation or resetting by impact events remains ambiguous since there is no direct link to their source rocks via textural relationships or crystal chemistry. Similarly, the currently available zircon data set for all lunar samples may be distorted by partial Pb loss, resulting in meaningless and misleading age distribution patterns. Therefore, it is crucial to fully understand and recognize the processes and conditions that may lead to partial resetting of the U-Pb system in zircon in order to better constrain the magmatic and impact history of the Moon.

Textural and U-Pb systematics (CA-TIMS) of stepwise leaching in zircon from granophyres in the Archean Stillwater Complex

NASA Astrophysics Data System (ADS)

Wall, C. J.; Scoates, J. S.; Friedman, R. M.; Meurer, W. P.

2011-12-01

The chemical abrasion-TIMS method or CA-TIMS uses a high-temperature annealing treatment to remove the effects of Pb-loss from radiation damaged parts of the zircon lattice and allows for highly precise and accurate U-Pb dating [1]. Zircon with high U-Th concentrations can be strongly metamict and it is not yet clear how effective the chemical abrasion treatment is when applied to these types of grains. In this study, we evaluate the link between the textural response and U-Pb systematics of zircon during chemical leaching for a granophyric rock from the Archean Stillwater Complex in Montana. The sample was selected based on the high abundance of zircon and the high degree of metamictization of the grains. Untreated and leached zircon grains were analyzed by scanning electron microscopy (SEM) and isotope dilution thermal ionization mass spectrometry (ID-TIMS). In thin section, zircon grains are euhedral in morphology and tend to be associated with amphibole-rich zones. Under the SEM, zircon grains typically have two distinct zones, a Ca-rich amorphous zone in the core and a more intact outer shell. Five acid-leaching steps were carried out on grains with each step increasing in temperature and acid strength until the zircon residue completely dissolved (starting at a 50% strength HF/HNO3 mixture at 100°C for 4 hours and finishing at full strength acid at 170°C for 4 hours). SEM imaging was conducted on grains after each step with a noticeable change in the morphology of the grains. As the leaching progressed, the acid leach created large pathways through the crystal lattice until only grain fragments remained, in some cases even boring large holes into the centre of the zircon grain. The acid preferentially dissolved the more soluble Ca-rich zones leaving behind fragile zircon "shells". U-Pb results of untreated grains are highly discordant (37-80%) and yield a chord with an upper intercept age of 1981 ± 140 Ma, whereas leached grains are slightly less discordant (12-69%) and define a much different chord with an upper intercept age of 2618 ± 4 Ma. Both ages are too young in comparison to the crystallization age of the Stillwater Complex (ca. 2709 Ma) and are considered to represent two discrete Pb-loss events that have reset the U-Pb systematics of these zircon grains. These Pb-loss events may be due to hydrothermal events associated with the emplacement of mafic dikes or the crystallization of regionally extensive monzonite plutons that intruded the Stillwater Complex. Dating different geological events using the U-Pb isotopic compositions of untreated metamict zircon and leached zircon by systematic acid leaching could prove to be a useful application of CA-TIMS. [1] Mattinson (2005) Chemical Geology 220, 47-66.

Coordinated U-Pb geochronology, trace element, Ti-in-zircon thermometry and microstructural analysis of Apollo zircons

NASA Astrophysics Data System (ADS)

Crow, Carolyn A.; McKeegan, Kevin D.; Moser, Desmond E.

2017-04-01

We present the results of a coordinated SIMS U-Pb, trace element, Ti-in-zircon thermometry, and microstructural study of 155 lunar zircons separated from Apollo 14, 15, and 17 breccia and soil samples that help resolve discrepancies between the zircon data, the lunar whole rock history and lunar magma ocean crystallization models. The majority of lunar grains are detrital fragments, some nearly 1 mm in length, of large parent crystals suggesting that they crystallized in highly enriched KREEP magmas. The zircon age distributions for all three landing sites exhibit an abundance of ages at ∼4.33 Ga, however they differ in that only Apollo 14 samples have a population of zircons with ages between 4.1 and 3.9 Ga. These younger grains comprise only 10% of all dated lunar zircons and are usually small and highly shocked making them more susceptible to Pb-loss. These observations suggest that the majority of zircons crystallized before 4.1 Ga and that KREEP magmatism had predominantly ceased by this time. We also observed that trace element analyses are easily affected by contributions from inclusions (typically injected impact melt) within SIMS analyses spots. After filtering for these effects, rare-earth element (REE) abundances of pristine zircon are consistent with one pattern characterized by a negative Eu anomaly and no positive Ce anomaly, implying that the zircons formed in a reducing environment. This inference is consistent with crystallization temperatures based on measured Ti concentrations and new estimates of oxide activities which imply temperatures ranging between 958 ± 57 and 1321 ± 100 °C, suggesting that zircon parent magmas were anhydrous. Together, the lunar zircon ages and trace elements are consistent with a ⩽300 My duration of KREEP magmatism under anhydrous, reducing conditions. We also report two granular texture zircons that contain baddeleyite cores, which both yield 207Pb-206Pb ages of 4.33 Ga. These grains are our best constraints on impact ages within our sample population, and suggest at least one large impact is contemporaneous with the most common time of magmatic zircon formation on the Moon's crust visited by the Apollo missions.

Implications of Bishop Tuff zircon U-Pb ages for rates of zircon growth and magma accumulation

NASA Astrophysics Data System (ADS)

Reid, M. R.; Schmitt, A. K.

2012-12-01

Rates of geologic processes obtained from natural studies rely on accurate geochronologic information. An important benchmark in geochronology as well as a valuable source of insights into the evolution of voluminous explosive eruptions is the >600 km3 Bishop Tuff (BT). A recently determined weighted mean 206Pb/238U date of 767.1±0.9 ka for a BT zircon population [1] is indistinguishable from the recalibrated 40Ar/39Ar sanidine date of 767.4±2.2 ka [2], potentially providing a key intercalibration point between astronomical and radio-isotopic dating approaches. Consequences of these results are linear zircon growth rates of >1×10-14 cm/sec and magma accumulation rates of >200 km3/ka. In contrast, spatially selective SIMS U-Pb dating of BT zircons yielded mean pre-eruption ages of 850 ka [3], a difference that raises questions about the validity of intercalibration between U-Pb and K-Ar dating methods and the history of magma accumulation. We obtained new SIMS analyses of the BT zircons using more spatially and analytically sensitive methods and verifying our accuracy against the TIMS dated Quaternary zircon 61.308A (2.488±0.002 Ma). Analyses were performed on zircon rims and on oriented cross-sections exposed during optical interferometry-calibrated serial sectioning removing the outermost ~31 μm. Sputtering by a 100 nA ion beam versus the normally employed 10-12 nA beam resulted in enhanced radiogenic Pb yields and analytical uncertainties for Quaternary zircon approaching the U-Pb age reproducibility of the primary zircon standard (~1-2 % for AS3). Ages obtained at ~31 μm depth (representing <5% of crystal growth in most cases) average 892±26ka (MSWD=0.29), corroborating previous evidence for residence times of several tens of ka. Rim ages average 781±22 ka (MSWD=0.61), overlapping Ar/Ar determinations of eruption age and corroborating the importance of near-eruption aged zircon growth. Our results confirm the presence of BT zircon domains that predate eruption by tens of ka, domains that could be difficult to detect by single crystal analyses. SIMS analyses can selectively sample more interior portions of grains whereas whole grain TIMS analyses will be volumetrically biased towards the outermost ~20% of the BT zircons. Spots centered on BT zircon cores sample a domain representing only a few volume percent of the crystal; if mantled completely by eruption-aged growth, single crystal zircon ages are predicted to be <5 ka older than eruption, extrapolating from the SIMS dates. When, compared to grain interiors, the greater average near-rim U concentrations are factored into this mass balance, the influence of these older domains is further diminished. A scenario that can at least partially reconcile the disparate zircon dates in the context of evolution of the BT and the dynamics of zircon crystallization is as follows: (1) zircon nucleation and relatively rapid growth an average of ~100 ka before eruption; (2) significant decay in zircon growth rates as temperatures and Zr melt gradients relax; and, finally, (3) mantling of earlier zircon cores by rejuvenation and renewed BT zircon growth during the lead-up to eruption. Comparisons between spot and single grain dating show that most zircon growth likely occurred close to eruption. [1] Crowley et al., 2007.Geology 35, 1123. [2] Rivera et al., 2011. Earth Planet. Sci. Lett. 311, 420. [3] Simon and Reid, 2005. Earth Planet. Sci. Lett. 235, 123.

Evolution of the depleted mantle and growth of the continental crust: improving on the imperfect detrital zircon record

NASA Astrophysics Data System (ADS)

Vervoort, J. D.; Kemp, A. I. S.; Patchett, P. J.

2012-04-01

One of the basic tenets of terrestrial geochemistry is that the continental crust has been extracted from the mantle leaving the latter depleted in incompatible elements. Nd and Hf isotopes have long shown that this process has been an essential feature of the Earth throughout its history. There is wide agreement on the general nature of this process, but the details of the isotopic record—and their implications for the depletion of the mantle and the extraction of continental crust—remain debated. Recently, much attention has been given to detrital zircons in both modern and ancient sediments. An advantage of this approach is the integration of the crystallization history of the zircon from the U-Pb chronometer with its Hf isotopic composition, which can provide important information on whether the zircons have been derived from juvenile or reworked crust. One essential requirement in this approach, however, is to unambiguously determine the crystallization ages of the zircons. We suggest that this represents an important—but generally ignored—source of uncertainty in the Hf isotopic record from detrital zircons. The quality filter most often used to assess the integrity of zircon U-Pb systematics is concordance; if a zircon is concordant, it is assumed that the U-Pb age is accurate. A concordance filter is less effective in old zircons, however, because ancient Pb loss, viewed today, parallels concordia. Without the benefit from the geological context of the host rock to the zircons, it is impossible to unambiguously determine it true crystallization age. Ancient Pb loss in zircons produces an apparent age less than the true magmatic age. The initial Hf isotopic composition of these zircons, as a result, will be calculated at the wrong age and will be anomalously low (by ~2.2 epsilon Hf units per 0.1 Ga). Hf model ages, calculated from these parameters, will be artificially old and spurious. The combination of unradiogenic Hf and Hf model ages > U-Pb ages in the zircon record are often given as prima facie evidence of crustal reworking and recycling during Earth's early history, and underpin models for large volumes of ancient continental crust. For many of these old zircons it may have nothing to do with crustal reworking, but simply reflect unrecognized ancient Pb loss. A more robust picture of the isotopic evolution of the Earth can be gained from an integrated approach of Hf and Nd isotopes in well age-constrained magmatic samples: careful U-Pb zircon geochronology to determine the crystallization age of the rock; Hf isotopic composition of the zircons; and Hf and Nd isotopic measurements of the whole-rocks. We demonstrate this with respect to evolution of the depleted mantle, and discuss the implications for the timing of crust formation. An important part of this approach is the realization that not all rock samples (or zircons!) yield useful, unambiguous results. Inclusion of all Hf isotope data from large zircon databases, unscrutinized for quality and lacking in context, will do more to obscure our understanding of the isotopic evolution of the Earth than to clarify it.

Magma evolution as seen through zircon geochemistry: an example from the Southern Adamello Batholith, N. Italy

NASA Astrophysics Data System (ADS)

Broderick, C.; Schaltegger, U.; Gerdes, A.; Frick, D.; Guenther, D.; Brack, P.

2012-04-01

Zircon is an ubiquitous accessory mineral often used for U-Pb geochronology but is also an important recorder of geochemical information. The trace element and isotopic characteristics of zircon yield potential for tracking changes in an evolving magma through time. With recent advances in U-Pb zircon geochronology, 10-100 ka to Ma timescales are observed for incremental pluton construction (Michel et al., 2008, Schaltegger et al., 2009). In observed 100 ka timescales of zircon crystallization, can zircon record the processes that produce trace element variations in a magma? This study focuses on the Val Fredda Complex (VFC) in the southern tip of the 43 to 33 Ma Adamello batholith, N. Italy. The VFC displays complex relationships among mafic melts that were injected into solidifying felsic magmas. Single zircon crystals were dated using CA-ID-TIMS. With permil uncertainties on 206Pb/238U zircon dates, zircons reveal complexities within single populations. The mafic units crystallized potential autocrystic zircons over a duration of 100 - 150ka, whereas the felsic units record up to 200ka of zircon crystallization. In order to understand these complex zircon populations, we analyzed Hf isotopes and trace elements, on the same volume of zircon used for U-Pb dating, following the TIMS-TEA method (Schoene et al., 2010). This detailed zircon study will allow us to look at how magmas are evolving with time. Hf isotopes of VFC mafic zircons reveal distinct ɛHf values between the three mafic units and their ɛHf values remain consistent through time, whereas the VFC felsic units record more complexity in their ɛHf values. We observe changes such as increasing and slight decreases in ɛHf with time which suggest different processes are occurring to produce the different felsic units. Trace element ratios in zircon reveal differences which allow us to make distinctions between felsic and mafic units (e.g. Th/U, (Lu/Gd)N, REEs). The VFC records 200 ka of zircon crystallization and our data suggests that zircons do in fact reflect changes in isotopic and in trace element signatures on 100 ka timescales. Although we observe changes in our trace elements, the TIMS-TEA method provides an average of trace element concentrations from a zircon volume, dominated by more marginal growth zones. Therefore we will compare our data with in situ methods to determine how our trace element data compares with trace elements across zircon profiles. We acknowledge funding of FNS in the frame of ProDoc Adamello 4-D. Michel et al., 2008, Geol. 36 : 459-462 ; Schaltegger et al., 2009, Earth Planet. Sci.Lett. 286: 208-218; Schoene et al., 2010, Geochim. Cosmochim. Acta 74, 7144-7159.

Time scales of intra-oceanic arc magmatism from combined U-Th and (U-Th)/He zircon geochronology of Dominica, Lesser Antilles

NASA Astrophysics Data System (ADS)

Howe, T. M.; Schmitt, A. K.; Lindsay, J. M.; Shane, P.; Stockli, D. F.

2015-02-01

The island of Dominica, located in the intra-oceanic Lesser Antilles arc, has produced a series of intermediate (mostly andesitic) lava domes and ignimbrites since the early Pleistocene. (U-Th)/He eruption ages from centers across the island range from ˜3 to ˜770 ka, with at least 10 eruptions occurring in the last 80 ka. Three eruptions occurred near the southern tip of Dominica (Plat Pays Volcanic Complex) in the past 15 ka alone. Zircon U-Th ages from individual centers range from near-eruption to secular equilibrium implicating protracted storage and recycling of zircons within the crust. Overlapping zircon crystallization peaks within deposits from geographically separated vents (up to 40 km apart) indicate that magma associated with separate volcanic edifices crystallized zircon contemporaneously. Two lava domes from the southern sector of the island display exclusively young zircon rim ages (<50 ka) with narrow crystallization peaks consistent with the construction of a new magma reservoir. The younging of eruption and crystallization ages implies that the magmatic foci leading to the construction of this reservoir have migrated southward, arc-parallel over time. Overall, our data support geochemical models for the ongoing construction of a silicic intrusive complex, consisting of varying amounts of crystal mush, beneath the island. U-Pb zircon ages <1-2 Ma indicate that accumulation of this complex is entirely Quaternary in age. Together zircon U-Th and U-Pb ages for Dominica suggest that the magmatic processes and time scales operating in intra-oceanic arcs are similar to those documented for continental arcs. This article was corrected on 18 MAR 2015. See the end of the full text for details.

Constraints on the timing of multiple thermal events and re-equilibration recorded by high-U zircon and xenotime: Case study of pegmatite from Piława Górna (Góry Sowie Block, SW Poland)

NASA Astrophysics Data System (ADS)

Budzyń, Bartosz; Sláma, Jiří; Kozub-Budzyń, Gabriela A.; Konečný, Patrik; Holický, Ivan; Rzepa, Grzegorz; Jastrzębski, Mirosław

2018-06-01

The application of zircon and xenotime geochronometers requires knowledge of their potential and limitations related to possible disturbance of the age record. The alteration of the intergrown zircon and xenotime in pegmatite from the Góry Sowie Block (SW Poland) was studied using the electron microprobe analysis, X-ray WDS compositional mapping, micro-Raman analysis, and LA-ICP-MS U-Pb dating of zircon and xenotime, as well as the U-Th-total Pb dating of uraninite. These microanalytical techniques were applied to understand the formation mechanisms of the secondary textures related to post-magmatic processes in the zircon and xenotime intergrowth, and to constrain their timing. Textural and compositional features combined with U-Pb data indicate that the pegmatite-related crystallization of the zircon and xenotime intergrowth occurred ca. 2.09 Ga (2086 ± 35 Ma for zircon and 2093 ± 52 Ma for xenotime), followed by the re-equilibration of zircon and xenotime ca. 370 Ma (373 ± 18 Ma and 368 ± 6 Ma, respectively) during the formation of the younger pegmatite. The zircon and xenotime were most likely derived from Precambrian basement rocks and emplaced in the pegmatite as a restite. The zircon preserved textures related to diffusion-reaction processes that affected its high-U core (up to ca. 9.6 wt% UO2), which underwent further metamictization and amorphization due to self-radiation damage. The zircon rim and xenotime were affected by coupled dissolution-reprecipitation processes that resulted in patchy zoning, age disturbance and sponge-like textures. Xenotime was also partially replaced by fluorapatite or hingganite-(Y) and Y-enriched allanite-(Ce). The termination of the low-temperature alteration was constrained by the U-Th-total Pb age of the uraninite inclusions that crystallized in zircon at 281 ± 2 Ma, which is consistent with the age of 278 ± 15 Ma obtained from the youngest cluster of U-Pb ages in the re-equilibrated high-U zircon domains. This study demonstrates the importance of the careful examination of compositional, microtextural and geochronological data obtained using microanalytical techniques to reconstruct the complex thermal histories recorded by accessory minerals.

Late Proterozoic charnockites in Orissa, India: A U-Pb and Rb-Sr isotopic study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Aftalion, M.; Bowes, D.R.; Dash, B.

1988-11-01

Charnockite formation in the Angul district of Orissa took place between 1088 + 26/ -17 Ma, the U-Pb zircon upper intercept crystallization age of a leptynite neosome, and 957 +8/ -4-956 {plus minus} 4 Ma, the U-Pb zircon-monazite upper intercept and U-Pb monazite crystallization ages of a granite. Confirmation of the Proterozoic age of the charnockites is given by (1) a U-Pb zircon upper intercept 1159 + 59/ -30 Ma age and a Rb-Sr whole-rock 1080 {plus minus} 65 Ma age for an augen gneiss which pre-dates the leptynite, and (2) U-Pb monazite ages of 973 {plus minus} 5,964 {plusmore » minus} 4, and 953 {plus minus} 4 Ma for a gray quartzofeldspathic gneiss, the augen gneiss, and the leptynite, respectively: these late Proterozoic dates are interpreted as representing ages recorded during charnockitization. The ca. 950-980 Ma charnockite- and granite-forming events are related to the evolution of mantle-derived, CO{sub 2}-bearing basic magma emplaced into the deeper levels of an extensional tectonic-transcurrent fault regime. The ca. 1100-1150 Ma tectonothermal and igneous events represent compressional tectonism in reactivated crystalline basement in the late mid-Proterozoic Eastern Ghats orogenic belt.« less

Zircon U-Pb dating of eclogite from the Qiangtang terrane, north-central Tibet: a case of metamorphic zircon with magmatic geochemical features

NASA Astrophysics Data System (ADS)

Zhai, Qing-guo; Jahn, Bor-ming; Li, Xian-hua; Zhang, Ru-yuan; Li, Qiu-li; Yang, Ya-nan; Wang, Jun; Liu, Tong; Hu, Pei-yuan; Tang, Suo-han

2017-06-01

Zircon is probably the most important mineral used in the dating formation of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks. The origin of zircon, i.e., magmatic or metamorphic, is commonly assessed by its external morphology, internal structure, mineral inclusions, Th/U ratios and trace element composition. In this study, we present an unusual case of metamorphic zircon from the Qiangtang eclogite, north-central Tibet. The zircon grains contain numerous eclogite-facies mineral inclusions, including omphacite, phengite, garnet and rutile; hence, they are clearly of metamorphic origin. However, they display features similar to common magmatic zircon, including euhedral crystal habit, high Th/U ratios and enriched heavy rare earth elements pattern. We suggest that these zircon grains formed from a different reservoir from that for garnet where no trace elements was present and trace element equilibrium between zircon and garnet was achieved. U-Pb dating of zircon gave an age of 232-237 Ma for the eclogite, and that of rutile yielded a slightly younger age of ca. 217 Ma. These ages are consistent with the reported Lu-Hf mineral isochron and phengite Ar-Ar ages. The zircon U-Pb and mineral Lu-Hf isochron ages are interpreted as the time of the peak eclogite-facies metamorphism, whereas the rutile U-Pb and phengite Ar-Ar ages represent the time of exhumation to the middle crust. Thus, the distinction between metamorphic and magmatic zircons cannot be made using only Th/U ratios and heavy REE compositions for HP-UHP metamorphic rocks of oceanic derivation.

The multistage crystallization of zircon in calc-alkaline granitoids: U-Pb age constraints on the timing of Variscan tectonic activity in SW Iberia

NASA Astrophysics Data System (ADS)

Pereira, M. F.; Chichorro, M.; Moita, P.; Santos, J. F.; Solá, A. M. R.; Williams, I. S.; Silva, J. B.; Armstrong, R. A.

2015-07-01

CL imaging and U-Th-Pb data for a population of zircons from two of the Évora Massif granitoids (Ossa-Morena Zone, SW Iberia) show that both calc-alkaline granitoids have zircon populations dominated by grains with cores and rims either showing or not showing differences in Th/U ratio, and having ages in the range ca. 350-335 Ma (Early Carboniferous). Multistage crystallization of zircon is revealed in two main growth stages (ca. 344-342 Ma and ca. 336-335 Ma), well represented by morphologically complex zircons with cores and rims with different ages and different Th/U ratios that can be explained by: (1) crystallization from melts with different compositions (felsic peraluminous to felsic-intermediate metaluminous; 0.001 < Th/U ratio < 0.5) and (2) transient temperature fluctuations in a system where anatectic felsic melts periodically underwent injection of more mafic magmas at higher temperatures. The two studied calc-alkaline granitoids do not include inherited zircons (pre-Carboniferous), probably because they were formed at the highest grade of metamorphism ( T > 837 °C; granulite facies) and/or because they were derived from inheritance-poor felsic and mafic rocks from a previous cycle, as suggested by the internal structures of zircon cores. These Variscan magmatic rocks with crystallization ages estimated at ca. 336-335 Ma are spatially and temporally related to high-temperature metamorphism, anatexis, processes of interaction between crustal- and mantle-derived magmas and intra-orogenic extension that acted in SW Iberia during the Early Carboniferous.

Lifetime of an ocean island volcano feeder zone: constraints from U-Pb dating on coexisting zircon and baddeleyite, and 40/39Ar age determinations, Fuerteventura, Canary Islands

USGS Publications Warehouse

Allibon, James; Ovtcharova, Maria; Bussy, Francois; Cosca, Michael; Schaltegger, Urs; Bussien, Denise; Lewin, Eric

2011-01-01

High-precision isotope dilution - thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon and baddeleyite ages from the PX1 vertically layered mafic intrusion Fuerteventura, Canary Islands, indicate initiation of magma crystallization at 22.10 ± 0.07 Ma. The magmatic activity lasted a minimum of 0.52 Ma. 40Ar/39Ar amphibole dating yielded ages from 21.9 ± 0.6 to 21.8 ± 0.3, identical within errors to the U-Pb ages, despite the expected 1% theoretical bias between 40Ar/39Ar and U-Pb dates. This overlap could result from (i) rapid cooling of the intrusion (i.e., less than the 0.3 to 0.6 Ma 40Ar/39Ar age uncertainties) from closure temperatures (Tc) of zircon (699-988 °C) to amphibole (500-600 °C); (ii) lead loss affecting the youngest zircons; or (iii) excess argon shifting the plateau ages towards older values. The combination of the 40Ar/39Ar and U/Pb datasets implies that the maximum amount of time PX1 intrusion took to cool below amphibole Tc is 0.8 Ma, suggesting PX1 lifetime of 520,000 to 800,000 Ma. Age disparities among coexisting baddeleyite and zircon (22.10 ± 0.07/0.08/0.15 Ma and 21.58 ± 0.15/0.16/0.31 Ma) in a gabbro sample from the pluton margin suggest complex genetic relationships between phases. Baddeleyite is found preserved in plagioclase cores and crystallized early from low silica activity magma. Zircon crystallized later in a higher silica activity environment and is found in secondary scapolite and is found close to calcite veins, in secondary scapolite that recrystallised from plagioclase. close to calcite veins. Oxygen isotope δ18O values of altered plagioclase are high (+7.7), indicating interaction with fluids derived from host-rock carbonatites. The coexistence of baddeleyite and zircon is ascribed to interaction of the PX1 gabbro with CO2-rich carbonatite-derived fluids released during contact metamorphism.

Precise U-Pb Zircon Constraints on the Earliest Magmatic History of the Carolina Terrane.

PubMed

Wortman; Samson; Hibbard

2000-05-01

The early magmatic and tectonic history of the Carolina terrane and its possible affinities with other Neoproterozoic circum-Atlantic arc terranes have been poorly understood, in large part because of a lack of reliable geochronological data. Precise U-Pb zircon dates for the Virgilina sequence, the oldest exposed part, constrain the timing of the earliest known stage of magmatism in the terrane and of the Virgilina orogeny. A flow-banded rhyolite sampled from a metavolcanic sequence near Chapel Hill, North Carolina, yielded a U-Pb zircon date of 632.9 +2.6/-1.9 Ma. A granitic unit of the Chapel Hill pluton, which intrudes the metavolcanic sequence, yielded a nearly identical U-Pb zircon date of 633 +2/-1.5 Ma, interpreted as its crystallization age. A felsic gneiss and a dacitic tuff from the Hyco Formation yielded U-Pb zircon dates of 619.9 +4.5/-3 Ma and 615.7 +3.7/-1.9 Ma, respectively. Diorite and granite of the Flat River complex have indistinguishable U-Pb upper-intercept dates of 613.9 +1.6/-1.5 Ma and 613.4 +2.8/-2 Ma. The Osmond biotite-granite gneiss, which intruded the Hyco Formation before the Virgilina orogeny, crystallized at 612.4 +5.2/-1.7 Ma. Granite of the Roxboro pluton, an intrusion that postdated the Virgilina orogeny, yielded a U-Pb upper intercept date of 546.5 +3.0/-2.4 Ma, interpreted as the time of its crystallization. These new dates both provide the first reliable estimates of the age of the Virgilina sequence and document that the earliest known stage of magmatism in the Carolina terrane had begun by 633 +2/-1.5 Ma and continued at least until 612.4 +5.2/-1.7 Ma, an interval of approximately 25 m.yr. Timing of the Virgilina orogeny is bracketed between 612.4 +5.2/-1.7 Ma and 586+/-10 Ma (reported age of the upper Uwharrie Formation). The U-Pb systematics of all units studied in the Virgilina sequence are simple and lack any evidence of an older xenocrystic zircon component, which would indicate the presence of a continental-type basement. This observation, together with the juvenile Nd isotopic character of the Virgilina volcanic arc sequence, suggests that the oldest part of the Carolina terrane was built on oceanic crust away from a continental crustal influence.

Zircon trace element and isotopic (Sr, Nd, Hf, Pb) effects of assimilation-fractional crystallization of pegmatite magma: A case study of the Guangshigou biotite pegmatites from the North Qinling Orogen, central China

NASA Astrophysics Data System (ADS)

Yuan, Feng; Liu, Jia-Jun; Carranza, Emmanuel John M.; Zhang, Shuai; Zhai, De-Gao; Liu, Gang; Wang, Gong-Wen; Zhang, Hong-Yuan; Sha, Ya-Zhou; Yang, Shang-Song

2018-03-01

Evidence for open-system magmatic processes related to wallrock assimilation accompanied by fractional crystallization (AFC) is present in the Guangshigou biotite pegmatites, North Qinling Orogen. The biotite pegmatite-gneiss contacts generally coincide with the greatest enrichment of U and Th. Zircon Usbnd Pb dating constrains the crystallization ages of the biotite pegmatite (rim zone-415 ± 2.6 Ma; internal zone-413.5 ± 2.5 Ma), in line with a pyrite Pbsbnd Pb isochron age (413 ± 22 Ma). Metamict areas in zircon show generally elevated concentrations of trace elements and expulsion of radioactive Pb. Internal zone samples, representing uncontaminated magma, have negative to positive zircon ( 413 Ma) εHf(t) (- 1.53 - + 3.24), low εNd(t) values (- 2.4), and old Hf and Nd model ages (tDM2 = 1.5-1.19 Ga, T2DM = 1.35 Ga, respectively), indicating a dominantly recycled Mesoproterozoic lower crustal material with involvement of some juvenile materials in the source region. The magmatic oxygen fugacity (fO2) and crystallization temperatures ranges from - 24.81 to - 13.34 of log fO2 and 570 °C to 793 °C, respectively. Compared to the internal zone, pegmatite rim samples display a variable and lower εNd(t) values (- 3.9 to - 2.8) and T2DM (1.47-1.37 Ga), but similar Hf isotopic compositions, favouring a three-component isotopic mixing model (recycled Mesoproterozoic lower crust materials, juvenile materials, and host gneiss). Pronounced variations of Ti, Y, U, Th, Hf, and REE concentrations in zircon from grain to grain in individual samples and from area to area within individual grains suggest a fluctuating crystallization environment in hybridized magma from which the rim-hosted zircons crystallized. Variable and high radiogenic Pb ratios of pyrites forming in the hybridized magma were inherited from the matrix. Zircons from both zones exhibit similar Hf isotope patterns, indicating the rim-hosted zircons crystallized during the early stage of hybridization of magma. However, the heterogeneous Nd isotopic composition in individual pegmatites imply an incorporation of wallrock-derived melt with less radiogenic Nd.

Magmatic Longevity Constrained by ID-TIMS U-Pb Dating of Zircon and Titanite

NASA Astrophysics Data System (ADS)

Szymanowski, D.; Wotzlaw, J. F.; Ellis, B. S.; Bachmann, O.; Von Quadt, A.

2016-12-01

Clues about the timescales and thermal conditions associated with the growth and evacuation of large silicic magma reservoirs are frequently drawn from radiometric dating, diffusion modelling, or thermomechanical modelling. A growing amount of petrological and geochronological evidence, supported by thermal modelling, suggests that many silicic magma reservoirs may exist for some 104-106 years in the form of high-crystallinity mushes at relatively low temperatures ( 700-750°C; [1-3]). Geochronological studies addressing this issue typically utilise the U-Pb system in zircon capable of recording extended periods of crystallisation, particularly in evolved calc-alkaline systems that spend most of their lifetime zircon-saturated. In this study, we integrate U-Pb dating of zircon and titanite to investigate the longevity of the magma reservoir that produced the Kneeling Nun Tuff, a 35 Ma, >900 km3 crystal-rich rhyolitic super-eruption from the Mogollon-Datil volcanic field in New Mexico (USA). High-precision ID-TIMS U-Pb dates of single crystals of both zircon and titanite independently record a continuous crystallisation history over >400,000 years. We combine the dating of both accessory phases with textural, major, trace element and isotopic studies of single crystals, placing tight constraints on the thermal conditions of magma accumulation and storage while recording differentiation and rejuvenation processes within the magma reservoir. The results suggest a protracted `cool' upper-crustal storage of magma prior to the Kneeling Nun Tuff eruption followed by a melting event which reduced the magma crystallinity and conditioned it for eruption. [1] Bachmann & Bergantz (2004), J. Petrol. 45, 1565-1582. [2] Gelman et al. (2013), Geology 41, 759-762. [3] Cooper & Kent (2014), Nature 506, 480-483.

U-Pb zircon and CHIME monazite dating of granitoids and high-grade metamorphic rocks from the Eastern and Peninsular Thailand - A new report of Early Paleozoic granite

NASA Astrophysics Data System (ADS)

Kawakami, T.; Nakano, N.; Higashino, F.; Hokada, T.; Osanai, Y.; Yuhara, M.; Charusiri, P.; Kamikubo, H.; Yonemura, K.; Hirata, T.

2014-07-01

In order to understand the age and tectonic framework of Eastern to Peninsular Thailand from the viewpoint of basement (metamorphic and plutonic) geology, the LA-ICP-MS U-Pb zircon dating and the chemical Th-U-total Pb isochron method (CHIME) monazite dating were performed in the Khao Chao, Hub-Kapong to Pran Buri, and Khanom areas in Eastern to Peninsular Thailand. The LA-ICP-MS U-Pb zircon dating of the garnet-hornblende gneiss from the Khao Chao area gave 229 ± 3 Ma representing the crystallization age of the gabbro, and that of the garnet-biotite gneisses gave 193 ± 4 Ma representing the timing of an upper amphibolite facies metamorphism. The CHIME monazite dating of pelitic gneiss from the Khao Chao gneiss gave scattered result of 68 ± 22 Ma, due to low PbO content and rejuvenation of older monazite grains during another metamorphism in the Late Cretaceous to Tertiary time. The U-Pb ages of zircon from the Hua Hin gneissic granite in the Hub-Kapong to Pran Buri area scatter from 250 Ma to 170 Ma on the concordia. Granite crystallization was at 219 ± 2 Ma, followed by the sillimanite-grade regional metamorphism at 185 ± 2 Ma. Monazite in the pelitic gneiss from this area also preserves Early to Middle Jurassic metamorphism and rejuvenation by later contact metamorphism by non-foliated granite or by another fluid infiltration event in the Late Cretaceous to Tertiary time. The Khao Dat Fa granite from the Khanom area of Peninsular Thailand gave a U-Pb zircon age of 477 ± 7 Ma. This is the second oldest granite pluton ever reported from Thailand, and is a clear evidence for the Sibumasu block having a crystalline basement that was formed during the Pan-African Orogeny. The Khao Pret granite gives U-Pb zircon concordia age of 67.5 ± 1.3 Ma, which represents the timing of zircon crystallization from the granitic melt and accompanied sillimanite-grade contact metamorphism against surrounding metapelites and gneisses. Metamorphic rocks in the Doi Inthanon area also share the similar plutono-metamorphic history with the Khanom and the Hub-Kapong to Pran Buri areas. This suggests that these three areas belong to the Sibumasu block, and the Sibumasu block records similar plutono-metamorphic history from Northern to Peninsular Thailand. Relative abundance of oceanic components in the Khao Chao gneiss, their Late Triassic magmatic ages, and the Early Jurassic metamorphic ages prefer the interpretation that the Khao Chao gneiss belongs to the Sukhothai Arc.

Laser Ablation in situ (U-Th-Sm)/He and U-Pb Double-Dating of Apatite and Zircon: Techniques and Applications

NASA Astrophysics Data System (ADS)

McInnes, B.; Danišík, M.; Evans, N.; McDonald, B.; Becker, T.; Vermeesch, P.

2015-12-01

We present a new laser-based technique for rapid, quantitative and automated in situ microanalysis of U, Th, Sm, Pb and He for applications in geochronology, thermochronometry and geochemistry (Evans et al., 2015). This novel capability permits a detailed interrogation of the time-temperature history of rocks containing apatite, zircon and other accessory phases by providing both (U-Th-Sm)/He and U-Pb ages (+trace element analysis) on single crystals. In situ laser microanalysis offers several advantages over conventional bulk crystal methods in terms of safety, cost, productivity and spatial resolution. We developed and integrated a suite of analytical instruments including a 193 nm ArF excimer laser system (RESOlution M-50A-LR), a quadrupole ICP-MS (Agilent 7700s), an Alphachron helium mass spectrometry system and swappable flow-through and ultra-high vacuum analytical chambers. The analytical protocols include the following steps: mounting/polishing in PFA Teflon using methods similar to those adopted for fission track etching; laser He extraction and analysis using a 2 s ablation at 5 Hz and 2-3 J/cm2fluence; He pit volume measurement using atomic force microscopy, and U-Th-Sm-Pb (plus optional trace element) analysis using traditional laser ablation methods. The major analytical challenges for apatite include the low U, Th and He contents relative to zircon and the elevated common Pb content. On the other hand, apatite typically has less extreme and less complex zoning of parent isotopes (primarily U and Th). A freeware application has been developed for determining (U-Th-Sm)/He ages from the raw analytical data and Iolite software was used for U-Pb age and trace element determination. In situ double-dating has successfully replicated conventional U-Pb and (U-Th)/He age variations in xenocrystic zircon from the diamondiferous Ellendale lamproite pipe, Western Australia and increased zircon analytical throughput by a factor of 50 over conventional methods.Reference: Evans NJ, McInnes BIA, McDonald B, Becker T, Vermeesch P, Danisik M, Shelley M, Marillo-Sialer E and Patterson D. An in situ technique for (U-Th-Sm)/He and U-Pb double dating. J Analytical Atomic Spectrometry, 30, 1636 - 1645.

The 230Th correction is the 1st priority for accurate dates of young zircons: U/Th partitioning experiments and measurements

NASA Astrophysics Data System (ADS)

Krawczynski, M.; McLean, N.

2017-12-01

One of the most accurate and useful ways of determining the age of rocks that formed more than about 500,000 years ago is uranium-lead (U-Pb) geochronology. Earth scientists use U-Pb geochronology to put together the geologic history of entire regions and of specific events, like the mass extinction of all non-avian dinosaurs about 66 million years ago or the catastrophic eruptions of supervolcanoes like the one currently centered at Yellowstone. The mineral zircon is often utilized because it is abundant, durable, and readily incorporates uranium into its crystal structure. But it excludes thorium, whose isotope 230Th is part of the naturally occurring isotopic decay chain from 238U to 206Pb. Calculating a date from the relative abundances of 206Pb and 238U therefore requires a correction for the missing 230Th. Existing experimental and observational constraints on the way U and Th behave when zircon crystallizes from a melt are not known precisely enough, and thus currently the uncertainty in dates introduced by they `Th correction' is one of the largest sources of systematic error in determining dates. Here we present preliminary results on our study of actinide partitioning between zircon and melt. Experiments have been conducted to grow zircon from melts doped with U and Th that mimic natural magmas at a range of temperatures, and compositions. Synthetic zircons are separated from their coexisting glass and using high precision and high-spatial-resolution techniques, the abundance and distribution of U and Th in each phase is determined. These preliminary experiments are the beginning of a study that will result in precise determination of the zircon/melt uranium and thorium partition coefficients under a wide variety of naturally occurring conditions. This data will be fit to a multidimensional surface using maximum likelihood regression techniques, so that the ratio of partition coefficients can be calculated for any set of known parameters. The results of this study will reduce the largest source of uncertainty in dating young zircons and improve the accuracy of U-Pb dates, improving our ability to tell time during geologic processes. The attainment of more accurate timing of the geologic timescale is important to geologists of all disciplines, from paleontology to planetary cosmochemistry to geobiology.

U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the south shore of Lake Superior

USGS Publications Warehouse

Zartman, R.E.; Nicholson, S.W.; Cannon, W.F.; Morey, G.B.

1997-01-01

New single-crystal zircon U-Th-Pb ages for plutonic and rhyolitic Keweenawan Supergroup rocks from the south shore of Lake Superior provide geochronological constraints on magmatic evolution associated with the 1.1 Ga Midcontinent rift. Analyses of a granophyric phase of the Mineral Lake intrusion and the Meilen granite, both parts of the Meilen Intrusive Complex, and a laterally extensive rhyolite from the top of the Kallander Creek Volcanics have weighted average 207Pb/206Pb ages of 1102.0 ?? 2.8 Ma (N = 2), 1100.9 ?? 1.4 Ma (N = 5), and 1098.8 ?? 1.9 Ma (N = 4), respectively. Analyses of a pyroclastic rhyolite flow at the top of the Porcupine Volcanics result in variable 207Pb/206Pb ages that range from 1080 to 1137 Ma. This rhyolite exhibits a continuum between morphologically complex and simpler prismatic zircon crystals, the latter yielding concordant analyses having a weighted average 207Pb/206Pb age of 1093.6 ?? 1.8 Ma (N = 2). Four prismatic zircons from an aphyric rhyolite of the Chengwatana Volcanics in the Ashland syncline form a linear array intersecting concordia at 1094.6 ?? 2.1 Ma (MSWD = 1.3). Another presumed Chengwatana rhyolite recovered from drill core intersecting the Hudson-Afton horst in southeast Minnesota yielded only ???20 morphologically indistinguishable zircons. Six analyses give 207Pb/206Pb ages ranging from 1112 to 1136 Ma, including one analysis with a virtually concordant age of 1130 Ma. This age, however, is considerably older than that obtained for the Chengwatana Volcanics in the Ashland syncline or any other precisely dated rock from the Midcontinent rift.

Zircon U-Pb Age Distributions in Cogenetic Crystal-Rich Dacitic and Crystal-Poor Rhyolitic Members of Zoned Ignimbrites in the Southern Rocky Mountains by Chemical Abrasion Inductively-Coupled-Plasma Mass Spectrometry (CA-LA-ICP-MS).

NASA Astrophysics Data System (ADS)

Sliwinski, J.; Zimmerer, M. J.; Guillong, M.; Bachmann, O.; Lipman, P. W.

2015-12-01

The San Juan locus of the Southern Rocky Mountain Volcanic Field (SRMVF) in SW Colorado represents an erosional remnant of a mid-Tertiary (~37-23 Ma) ignimbrite flare up that produced some of the most voluminous ignimbrites on Earth. A key feature of many SRMVF ignimbrites is compositional zonation, with many volcanic units comprising both dacitic and rhyolitic horizons. Geochemical, field and petrographic evidence suggests that dacites and rhyolites are cogenetic. Here, we report U-Pb zircon ages by chemical abrasion inductively-coupled-plasma mass spectrometry (CA-LA-ICPMS) for rhyolitic and dacitic components in four units: the Bonanza, Rat Creek, Carpenter Ridge and Nelson Mountain Tuffs. All units show zircon age spectra that are either within analytical uncertainty of Ar/Ar ages or are appreciably older, indicating prolonged magma residence times (~500 ka) prior to eruption. Anomalously young Pb-loss zones in zircon have been largely removed by chemical abrasion. Older, inherited zircons and zircon cores (60-2000 Ma) are rare in all samples, suggesting limited assimilation of upper crustal Precambrian country rock or complete resorption during recharge events and magma chamber growth.

Assessment of Alternative [U] and [Th] Zircon Standards for SIMS

NASA Astrophysics Data System (ADS)

Monteleone, B. D.; van Soest, M. C.; Hodges, K.; Moore, G. M.; Boyce, J. W.; Hervig, R. L.

2009-12-01

The quality of in situ (U-Th)/He zircon dates is dependent upon the accuracy and precision of spatially distributed [U] and [Th] measurements on often complexly zoned zircon crystals. Natural zircon standards for SIMS traditionally have been used to obtain precise U-Pb ages rather than precise U and Th concentration. [U] and [Th] distributions within even the most homogeneous U-Pb age standards are not sufficient to make good microbeam standards (i.e., yield good precision: 2σ < 5%) for (U-Th)/He dates. In the absence of sufficiently homogeneous natural zircon crystals, we evaluate the use of the NIST 610 glass standard and a synthetic polycrystalline solid “zircon synrock” made by powdering and pressing natural zircon crystals at 2 GPa and 1100°C within a 13 mm piston cylinder for 24 hours. SIMS energy spectra and multiple spot analyses help assess the matrix-dependence of secondary ion emission and [U] and [Th] homogeneity of these materials. Although spot analyses on NIST 610 glass yielded spatially consistent ratios of 238U/30Si and 232Th/30Si (2σ = 2%, n = 14), comparison of energy spectra collected on glass and zircon reveal significant differences in U, UO, Th, and ThO ion intensities over the range of initial kinetic energies commonly used for trace element analyses. Computing [U] and [Th] in zircon using NIST glass yields concentrations that vary by more than 10% for [U] and [Th], depending on the initial kinetic energy and ion mass (elemental, oxide, or sum of elemental and oxide) used for the analysis. The observed effect of chemistry on secondary ion energy spectra suggests that NIST glass cannot be used as a standard for trace [U] and [Th] in zircon without a correction factor (presently unknown). Energy spectra of the zircon synrock are similar to those of natural zircon, suggesting matrix compatibility and therefore potential for accurate standardization. Spot analyses on the zircon powder pellets, however, show that adequate homogeneity of [U] and [Th] (2σ = 37% and 33% for 238U/30Si and 232Th/30Si, respectively, n = 8) has yet to be achieved. Modeling shows that homogenization of [U] and [Th] within these pellets requires preparation of powders with <2 micron sized particles, which has yet to be achieved in sample preparation. Thus, the zircon synrock pellet remains a viable potential [U], [Th] standard, although the preparation of a sufficiently fine grained, homogeneous pellet is a work in progress.

The Mesozoic metamorphic-magmatic events in the Medog area, the Eastern Himalayan Syntaxis: constraints from zircon U-Pb geochronology, trace elements and Hf isotope compositions in granitoids

NASA Astrophysics Data System (ADS)

Dong, Hanwen; Xu, Zhiqin; Li, Yuan; Liu, Zhao; Li, Huaqi

2015-01-01

Based on the regional geological mapping, several granitoid intrusions had been discovered in the Eastern Himalayan Syntaxis (EHS). In order to constrain their petrogenesis and discuss their relations with the regional tectonics, we carried out U-Pb dating, trace elements and Hf isotope geochemistry studies on zircons separated from the granitoid rocks, in the area of the EHS. In this contribution, the granitoid rocks are mainly composed of diorites (X20-1-6) and granitic gneissic rocks (X2-15-1). The U-Pb zircon dating of diorites yields a crystallization age of 193.8 ± 2.0 Ma. These zircon have ɛ Hf( t) values ranging from -6.48 to -0.05, indicating an involvement of ancient crustal materials in the generation of these igneous rocks. The zircons from the Medog granitic gneissic rock commonly show zoning structures. The REE patterns and abundances of the inherited cores are different from those of the oscillatory rims. The LA-ICP-MS U-Pb zircon in situ analyses indicate that: (1) the zircon cores give multi-stage magmatic event ages ranging from 516 to 1,826 Ma, of which six ages are converged on 1,330-911 Ma, it is considered that the migmatitic gneiss has been formed in this time, and (2) while the zircon rims yield 206Pb/238U weighted mean ages of 217.4 ± 3.0 Ma (MSWD = 3.2), which was interpreted to represent the ages of the Triassic anatexis. Their ɛ Hf( t) values range from -18.98 to -8.36 and -14.22 to 8.72, respectively. The timing of the anatexis in the Medog area is coeval with the widespread metamorphism in Lhasa terrane.

Using zircon (U-Th)/He damage-diffusivity patterns to quantify detachment-related basement exhumation in the Mecca Hills, CA

NASA Astrophysics Data System (ADS)

Moser, A. C.; Ault, A. K.; Evans, J. P.; Reiners, P. W.; Stearns, M.; Guenthner, W.

2017-12-01

Exposures of gneiss and Orocopia Schist (OS) in the Mecca Hills, California, adjacent to the southernmost San Andreas Fault system, preserve the exhumation history of Oligocene detachment faulting. We investigate the duration, magnitude, and mechanisms of exhumation of these units at regional and local scales using in situ U-Pb zircon dating (n = 248), (U-Th)/He (He) thermochronometry (n = 39), and He date-effective U (eU) patterns. Zircons with variable preserved visual metamictization were targeted for He analyses to purposefully build a dataset with a range in eU concentration and zircon He closure temperatures, as well as induce a He date-eU correlation. Analyzed zircon crystals range from clear and transparent to purple-brown and translucent in each sample. Zircon cathodoluminescence images reveal oscillatory and sector chemical zoning. Each sample contains a population of largely Proterozoic U-Pb dates implying some grains accumulated radiation damage since 1.9-1.1 Ga. Zircon (U-Th)/He dates from seven samples of OS and gneiss yield a mean date of 24 ± 3.5 Ma (n = 32) and uniform dates over an 90-2950 ppm eU range. One gneiss sample yields a mean date of 65 ± 5.6 Ma (n = 7) over a limited eU spread ( 500-950 ppm). Mean zircon He dates from these two units overlap, but dates are broadly younger in northeastern exposures dominated by OS. Preliminary thermal history simulations integrating zircon U-Pb data, He date-eU patterns, and independent geologic constraints require at least 200 °C of cooling through the zircon He partial retention zone 30-21 Ma and show that the pre-70 Ma thermal history does not affect the predicted date-eU correlation. This shared rapid cooling history documented in the OS and gneiss imply these units were juxtaposed prior to 30 Ma and exhumed as a coherent structural block within the footwall of the Orocopia Mountains Detachment Fault in the Mecca Hills. Spatio-temporal variation in mean zircon He dates may delineate time-transgressive cooling of these units as they exhumed. Zircon textures and overall invariant He dates regardless of eU imply that visual metamictization persists in zircons at temperatures >200 °C and indicates these grains experienced a thermal history characterized by temperatures that preserved visual damage but fully induced He loss in the crystals prior to exhumation.

Detrital zircon ages from southern Norway - implications for the Proterozoic evolution of the southwestern Baltic Shield

NASA Astrophysics Data System (ADS)

Knudsen, T.-L.; Andersen, T.; Whitehouse, M. J.; Vestin, J.

An ion-microprobe (SIMS) U-Pb zircon dating study on four samples of Precambrian metasediments from the high-grade Bamble Sector, southern Norway, gives the first information on the timing of discrete crust-forming events in the SW part of the Baltic Shield. Recent Nd and Pb studies have indicated that the sources of the clastic metasediments in this area have crustal histories extending back to 1.7 to 2.1Ga, although there is no record of rocks older than 1.6Ga in southern Norway. The analysed metasediments are from a sequence of intercalated, centimetre to 10-metre wide units of quartzites, semi-metapelites, metapelites and mafic granulites. The zircons can be grouped in two morphological populations: (1) long prismatic; (2) rounded, often flattened. The BSE images reveal that both populations consist of oscillatory zoned, rounded and corroded cores (detrital grains of magmatic origin), surrounded by homogeneous rims (metamorphic overgrowths). The detrital zircons have 207Pb/206Pb ages between 1367 and 1939Ma, with frequency maxima in the range 1.85 to 1.70Ga and 1.60 to 1.50Ga. There is no correlation between crystal habit and age of the zircon. One resorbed, inner zircon core in a detrital grain is strongly discordant and gives a composite inner core-magmatic outer core 207Pb/206Pb age of 2383 Ma. Two discrete, unzoned zircons have 207Pb/206Pb ages of 1122 and 1133Ma, representing zircon growth during the Sveconorwegian high-grade metamorphism. Also the μm wide overgrowths, embayments in the detrital cores and apparent ``inner cores'' which represent secondary metamorphic zircon growth in deep embayments in detrital grains, are of Sveconorwegian age. The composite-detrital-metamorphic zircon analyses give generally discordant 206Pb/238U versus 207Pb/235U ratios and maximum 207Pb/206Pb ages of 1438Ma. These data demonstrate the existence of a protocrust of 1.7 to 2.0Ga in the southwestern part of the Baltic Shield, implying a break in the overall westward younging trend of the Precambrian crust, inferred from the southeastern part of the Baltic Shield.

Do Jack Hills Detrital Zircons Contain Records of the Early Geodynamo?

NASA Astrophysics Data System (ADS)

Weiss, B. P.; Maloof, A. C.; Tailby, N. D.; Ramezani, J.; Fu, R. R.; Glenn, D. R.; Kehayias, P.; Walsworth, R. L.; Hanus, V.; Trail, D.; Watson, E. B.; Harrison, T. M.; Bowring, S. A.; Kirschvink, J. L.; Swanson-Hysell, N.; Coe, R. S.; Einsle, J. F.; Harrison, R. J.

2015-12-01

It is unknown when Earth's dynamo magnetic field originated. With crystallization ages ranging from 3.0-4.38 Ga, detrital zircon crystals found in the Jack Hills of Western Australia might preserve a record of the missing first billion years of Earth's magnetic field history. Recently, Tarduno et al. (2015) argued that magnetization in Jack Hills zircons provides evidence for a substantial geomagnetic field dating back to their U/Pb formation ages (3.3 and 4.2 Ga). However, the identification of such ancient field records requires establishing that the zircons have avoided remagnetization since their formation. At a minimum, it should be demonstrated that they have not been remagnetized since being deposited at ~3.0 Ga. To establish the timing and intensity of the metamorphic and alteration events experienced by the zircon, we conducted 12 paleomagnetic field tests in combination with U-Pb geochronology on their host rocks (see Weiss et al. 2015, EPSL). Our data show that the Hadean zircon-bearing rocks and surrounding region have been pervasively remagnetized, with the final major overprinting likely from emplacement of the Warakurna large igneous province at 1.1 Ga (see Figure). Even if some Jack Hills zircons do record a pre-depositional magnetization, they still could have been remagnetized sometime during the 1.4 Gy between their crystallization and their deposition. First, the temperatures capable of remagnetizing magnetite inclusions are well below those that could reset a U-Pb date or result in significant discordance. Therefore, thermal events capable of completely remagnetizing Jack Hills zircons could be undetected by the techniques reported by Tarduno at al. (2015). Second, the zircons' magnetization might be dominated by secondary ferromagnetic inclusions or contamination. To address the latter possibility, we are conducting electron microscopy, x-ray tomography, and magnetic field mapping on the zircons. Our initial quantum diamond magnetometry high-resolution (<20 um) imaging has found that the magnetization of many zircons resides in grain exteriors, suggesting a dominantly secondary origin. We conclude that there is currently no robust evidence for pre-depositional (>3.0 Ga) magnetization in the Jack Hills detrital zircons. We are continuing to search for such magnetic records.

Quantifying time in sedimentary successions by radio-isotopic dating of ash beds

NASA Astrophysics Data System (ADS)

Schaltegger, Urs

2014-05-01

Sedimentary rock sequences are an accurate record of geological, chemical and biological processes throughout the history of our planet. If we want to know more about the duration or the rates of some of these processes, we can apply methods of absolute age determination, i.e. of radio-isotopic dating. Data of highest precision and accuracy, and therefore of highest degree of confidence, are obtained by chemical abrasion, isotope-dilution, thermal ionization mass spectrometry (CA-ID-TIMS) 238U-206Pb dating techniques, applied to magmatic zircon from ash beds that are interbedded with the sediments. This techniques allows high-precision estimates of age at the 0.1% uncertainty for single analyses, and down to 0.03% uncertainty for groups of statistically equivalent 206Pb/238U dates. Such high precision is needed, since we would like the precision to be approximately equivalent or better than the (interpolated) duration of ammonoid zones in the Mesozoic (e.g., Ovtcharova et al. 2006), or to match short feedback rates of biological, climatic, or geochemical cycles after giant volcanic eruptions in large igneous provinces (LIP's), e.g., at the Permian/Triassic or the Triassic/Jurassic boundaries. We also wish to establish as precisely as possible temporal coincidence between the sedimentary record and short-lived volcanic events within the LIP's. Precision and accuracy of the U-Pb data has to be traceable and quantifiable in absolute terms, achieved by direct reference to the international kilogram, via an absolute calibration of the standard and isotopic tracer solutions. Only with a perfect control on precision and accuracy of radio-isotopic data, we can confidently determine whether two ages of geological events are really different, and avoid mistaking interlaboratory or interchronometer biases for age difference. The development of unprecedented precision of CA-ID-TIMS 238U-206Pb dates led to the recognition of protracted growth of zircon in a magmatic liquid (see, e.g., Schoene et al. 2012), which then becomes transferred into volcanic ashes as excess dispersion of 238U-206Pb dates (see, e.g., Guex et al. 2012). Zircon is crystallizing in the magmatic liquid shortly before the volcanic eruption; we therefore aim at finding the youngest zircon date or youngest statistically equivalent cluster of 238U-206Pb dates as an approximation of ash deposition (Wotzlaw et al. 2013). Time gaps between last zircon crystallization and eruption ("Δt") may be as large as 100-200 ka, at the limits of analytical precision. Understanding the magmatic crystallization history of zircon is the fundamental background for interpreting ash bed dates in a sedimentary succession. Ash beds of different stratigraphic position and age my be generated within different magmatic systems, showing different crystallization histories. A sufficient number of samples (N) is therefore of paramount importance, not to lose the stratigraphic age control in a given section, and to be able to discard samples with large Δt - but, how large has to be "N"? In order to use the youngest zircon or zircons as an approximation of the age of eruption and ash deposition, we need to be sure that we have quantitatively solved the problem of post-crystallization lead loss - but, how can we be sure?! Ash bed zircons are prone to partial loss of radiogenic lead, because the ashes have been flushed by volcanic gases, as well as brines during sediment compaction. We therefore need to analyze a sufficient number of zircons (n) to be sure not to miss the youngest - but, how large has to be "n"? Analysis of trace elements or oxygen, hafnium isotopic compositions in dated zircon may sometimes help to distinguish zircon that is in equilibrium with the last magmatic liquid, from those that are recycled from earlier crystallization episodes, or to recognize zircon with partial lead loss (Schoene et al. 2010). Respecting these constraints, we may arrive at accurate correlation of periods of global environmental and biotic disturbance (from ash bed analysis in biostratigraphically or cyclostratigraphically well constrained marine sections) with volcanic activity; examples are the Triassic-Jurassic boundary and the Central Atlantic Magmatic Province (Schoene et al. 2010), or the lower Toarcian oceanic anoxic event and the Karoo Province volcanism (Sell et al. in prep.). High-precision temporal correlations may also be obtained by combining high-precision U-Pb dating with biochronology in the Middle Triassic (Ovtcharova et al., in prep.), or by comparing U-Pb dates with astronomical timescales in the Upper Miocene (Wotzlaw et al., in prep.). References Guex, J., Schoene, B., Bartolini, A., Spangenberg, J., Schaltegger, U., O'Dogherty, L., et al. (2012). Geochronological constraints on post-extinction recovery of the ammonoids and carbon cycle perturbations during the Early Jurassic. Palaeogeography, Palaeoclimatology, Palaeoecology, 346-347(C), 1-11. Ovtcharova, M., Bucher, H., Schaltegger, U., Galfetti, T., Brayard, A., & Guex, J. (2006). New Early to Middle Triassic U-Pb ages from South China: Calibration with ammonoid biochronozones and implications for the timing of the Triassic biotic recovery. Earth and Planetary Science Letters, 243(3-4), 463-475. Ovtcharova, M., Goudemand, N., Galfetti, Th., Guodun, K., Hammer, O., Schaltegger, U., Bucher, H. Improving accuracy and precision of radio-isotopic and biochronological approaches in dating geological boundaries: The Early-Middle Triassic boundary case. In preparation. Schoene, B., Schaltegger, U., Brack, P., Latkoczy, C., Stracke, A., & Günther, D. (2012). Rates of magma differentiation and emplacement in a ballooning pluton recorded by U-Pb TIMS-TEA, Adamello batholith, Italy. Earth and Planetary Science Letters, 355-356, 162-173. Schoene, B., Latkoczy, C., Schaltegger, U., & Günther, D. (2010). A new method integrating high-precision U-Pb geochronology with zircon trace element analysis (U-Pb TIMS-TEA). Geochimica Et Cosmochimica Acta, 74(24), 7144-7159. Schoene, B., Guex, J., Bartolini, A., Schaltegger, U., & Blackburn, T. J. (2010). Correlating the end-Triassic mass extinction and flood basalt volcanism at the 100 ka level. Geology, 38(5), 387-390. Sell, B., Ovtcharova, M., Guex, J., Jourdan, F., Schaltegger, U. Evaluating the link between the Karoo LIP and climatic-biologic events of the Toarcian Stage with high-precision U-Pb geochronology. In preparation. Wotzlaw, J. F., Schaltegger, U., Frick, D. A., Dungan, M. A., Gerdes, A., & Günther, D. (2013). Tracking the evolution of large-volume silicic magma reservoirs from assembly to supereruption. Geology, 41(8), 867-870. Wotzlaw, J.F., Hüsing, S.K., Hilgen, F.J.., Schaltegger, U. Testing the gold standard of geochronology against astronomical time: High-precision U-Pb geochronology of orbitally tuned ash beds from the Mediterranean Miocene. In preparation.

Application of U-Pb ID-TIMS dating to the end-Triassic global crisis: testing the limits on precision and accuracy in a multidisciplinary whodunnit (Invited)

NASA Astrophysics Data System (ADS)

Schoene, B.; Schaltegger, U.; Guex, J.; Bartolini, A.

2010-12-01

The ca. 201.4 Ma Triassic-Jurassic boundary is characterized by one of the most devastating mass-extinctions in Earth history, subsequent biologic radiation, rapid carbon cycle disturbances and enormous flood basalt volcanism (Central Atlantic Magmatic Province - CAMP). Considerable uncertainty remains regarding the temporal and causal relationship between these events though this link is important for understanding global environmental change under extreme stresses. We present ID-TIMS U-Pb zircon geochronology on volcanic ash beds from two marine sections that span the Triassic-Jurassic boundary and from the CAMP in North America. To compare the timing of the extinction with the onset of the CAMP, we assess the precision and accuracy of ID-TIMS U-Pb zircon geochronology by exploring random and systematic uncertainties, reproducibility, open-system behavior, and pre-eruptive crystallization of zircon. We find that U-Pb ID-TIMS dates on single zircons can be internally and externally reproducible at 0.05% of the age, consistent with recent experiments coordinated through the EARTHTIME network. Increased precision combined with methods alleviating Pb-loss in zircon reveals that these ash beds contain zircon that crystallized between 10^5 and 10^6 years prior to eruption. Mineral dates older than eruption ages are prone to affect all geochronologic methods and therefore new tools exploring this form of “geologic uncertainty” will lead to better time constraints for ash bed deposition. In an effort to understand zircon dates within the framework of a magmatic system, we analyzed zircon trace elements by solution ICPMS for the same volume of zircon dated by ID-TIMS. In one example we argue that zircon trace element patterns as a function of time result from a mix of xeno-, ante-, and autocrystic zircons in the ash bed, and approximate eruption age with the youngest zircon date. In a contrasting example from a suite of Cretaceous andesites, zircon trace elements record crystallization in a differentiating magma over ~200 ka. This new tool can lead to more robust ages for the deposition of volcanic rocks and therefore aids in calibrating the stratigraphic record. Our new U-Pb geochronology shows that the end-Triassic extinction and a concomitant negative spike in organic carbon isotopes and sea level occurred in far less than 300 ka and that these events coincided within 150 ka to the onset of CAMP volcanism in North America. These data substantially tighten the correlation between extinction and the CAMP, though more high-precision age constraints from sections that record these events, coupled with ongoing bio-, chemo- and magnetostratigraphic efforts, are needed to substantiate and understand causality. The post extinction recovery in ammonite diversity occurred in 2.0±0.2 Ma, coinciding with the duration of the Hettangian stage. Recovery of numerous ash beds from within the Hettangian allow determination of the absolute rates of ammonite diversification at the sub-zone level, constraining the interaction between volcanism, animal evolution and global environment in the wake of catastrophe.

Integrated Laser Ablation U/Pb and (U-Th)/He Dating of Detrital Accessory Minerals from the Naryani River, Central Nepal

NASA Astrophysics Data System (ADS)

Horne, A.; Hodges, K. V.; Van Soest, M. C.

2015-12-01

The newly developed 'laser ablation double dating' (LADD) technique, an integrated laser microprobe U/Pb and (U-Th)/He dating method, could be an exceptionally valuable tool in detrital thermochronology for identifying sedimentary provenance and evaluating the exhumation history of a source region. A recent proof-of-concept study has used LADD to successfully date both zircon and titanite crystals from the well-characterized Fish Canyon tuff, but we also believe that another accessory mineral, rutile, could be amenable to dating via the LADD technique. To continue the development of the method, we present an application of LADD to detrital zircon, titanite, and rutile from a sample collected on the lower Naryani River of central Nepal. Preliminary analyses of the sample have yielded zircon U/Pb dates ranging from 31.4 to 2405 Ma; zircon (U-Th)/He from 1.8 to 15.4 Ma; titanite U/Pb between 18 and 110 Ma; titanite (U-Th)/He between 1 and 16 Ma; rutile U/Pb from 6 to 45 Ma; and rutile (U-Th)/He from 2 to 25 Ma. In addition to the initial data, we can use Ti-in-zircon, Zr-in-titanite, and Zr-in-rutile thermometers to determine the range of possible long-term cooling rates from grains with U/Pb ages younger than collision. Thus far our results from zircon analyses imply a cooling rate of approximately 15°C/Myr; titanite analyses imply between 10 and 67°C/Myr; and rutile between 9 and 267°C/Myr. This spread in potential cooling rates, especially in the order of magnitude differences of cooling rates calculated from the rutile grains, suggests that the hinterland source regions of the Naryani river experienced dramatically different exhumation histories during Himalayan orogenisis. Ongoing analyses will expand the dataset such that we can more adequately characterize the range of possibilities represented in the sample.

U-Pb ID-TIMS zircon ages of TTG gneisses of the Aravalli Craton of India

NASA Astrophysics Data System (ADS)

Chauhan, Hiredya; Saikia, Ashima; Kaulina, Tatiana; Bayanova, Tamara; Ahmad, Talat

2015-04-01

The crystalline basement of the Aravalli Craton is a heterogeneous assemblage dominated by granitic gneisses and granites with sporadic occurrences of amphibolites and dismembered sedimentary enclaves (Upadhyaya et al., 1992). This assemblage is known to have experienced multiple deformation and metamorphic events followed by emplacement of voluminous granites and basaltic dykes. Based on Sm-Nd whole rock data on the basement Mewar orthogneisses of Jhamarkotra region (Gopalan et al., 1990) and Pb/Pb ages of zircon from Gingla Granites which intrudes the basement (Wiedenbeck et al., 1996), it has been inferred that the whole magmatic episode leading to the formation of the basement spanned from 3300 to 2400 Ma and that the Aravalli cratonic block had broadly stabilized by 2500 Ma on which the younger Aravalli and Delhi Supergroup unconformably deposited. However, no comprehensive age data on the basement gneisses from the study area spanning the entire magmatic episode is available. This work attempts to provide a time frame work for evolution of the basement gneisses of the Aravalli Craton. We present here U-Pb zircon ages from the Precambrian basement TTG gneisses of the Aravalli Craton of north western India. Pb and U were measured on multicollector Finnigan-MAT 262 mass spectrometer. The temperatures of measurements were 1300°C for Pb and 1500°C for U. Pb isotope ratios were corrected for mass fractionation with a factor of 0.10% per amu, based on repeat analyses of the standard NBS SRM 982. The U analyses were corrected for mass fractionation with a factor of 0.003% per amu, based on repeat analyses of the NBS U 500 standard. Reproducibility of the U-Pb ratios was determined from the repeated analysis of standard zircon IGFM-87 (Ukraine) and taken as 0.5% for 207Pb/235U and 206Pb/238U ratios, respectively, at 95% confidence level. All calculations were done using the programs PBDAT and ISOPLOT (Ludwig 1991, 2008). Four zircon fractions corresponding to four zircon types from UD-16 sample yield a U-Pb discordant age of 2680±30 Ma. Two zircon fractions from UD-17 sample show discordant 207Pb/206Pb ages of 2506 and 2577 Ma. Zircons in our samples have moderate to high U contents (180-770 ppm) with low Th/U ratios (0.2-0.5) in the sample UD-16, characteristic for magmatic zircons from TTG rocks. Thus the obtained age of 2680±30 Ma is interpreted as an age of magmatic crystallization of tonalites. Gopalan, K. et al., (1990): Precambrian Res., 48, 287-297. Ludwig, K.R. (1991): PBDAT program. US. Geol. Surv. Open-file report 88-542, 38 p. Ludwig, K. R. (2008): Isoplot/Ex, version 3.6, Berkeley Geochronology Center, Special Publication no. 4. Upadhyaya, R. et al., (1992): Current Sci., 62(2): 87-92. Wiedenbeck, M. et al., (1996): Chem Geol. 129: 325-340.

Radiometric ages of the Fire Clay tonstein [Pennsylvanian (Upper Carboniferous), Westphalian, Duckmantian]: A comparison of U-Pb zircon single-crystal ages and 40Ar/39Ar sanidine single-crystal plateau ages

USGS Publications Warehouse

Lyons, P.C.; Krogh, T.E.; Kwok, Y.Y.; Davis, D.W.; Outerbridge, W.F.; Evans, H.T.

2006-01-01

The Fire Clay tonstein [Pennsylvanian (Upper Carboniferous), Westphalian Series, Duckmantian Stage]-a kaolinized, volcanic-ash deposit occurring in Kentucky, West Virginia, Tennessee, and Virginia-is the most widespread bed in the Middle Pennsylvanian of the central Appalachian basin, USA. A concordant single-crystal U-Pb zircon datum for this tonstein gives a 206Pb/238U age of 314.6 ?? 0.9 Ma (2??). This age is in approximate agreement with a mean sanidine plateau age of 311.5 ?? 1.3 Ma (1??, n = 11) for the Fire Clay tonstein. The difference between the two ages may be due to bias between the 40K and 238U decay constants and other factors. The age of the Fire Clay tonstein has important implications for Duckmantian Stage (Westphalian Series) sedimentation rates, correlations with the Westphalian Series of Europe, Middle Pennsylvanian volcanic events, and the late Paleozoic time scale. ?? 2006 Elsevier B.V. All rights reserved.

Empirical constraints on the effects of radiation damage on helium diffusion in zircon

NASA Astrophysics Data System (ADS)

Anderson, Alyssa J.; Hodges, Kip V.; van Soest, Matthijs C.

2017-12-01

In this study, we empirically evaluate the impact of radiation damage on zircon (U-Th)/He closure temperatures for a suite of zircon crystals from the slowly cooled McClure Mountain syenite of south-central Colorado, USA. We present new zircon, titanite, and apatite conventional (U-Th)/He dates, zircon laser ablation (U-Th)/He and U-Pb dates, and zircon Raman spectra for crystals from the syenite. Titanite and apatite (U-Th)/He dates range from 447 to 523 Ma and 88.0 to 138.9 Ma, respectively, and display no clear correlation between (U-Th)/He date and effective uranium concentration. Conventional zircon (U-Th)/He dates range from 230.3 to 474 Ma, while laser ablation zircon (U-Th)/He dates show even greater dispersion, ranging from 5.31 to 520 Ma. Dates from both zircon (U-Th)/He datasets decrease with increasing alpha dose, indicating that most of the dispersion can be attributed to radiation damage. Alpha dose values for the dated zircon crystals range from effectively zero to 2.15 × 1019 α /g, spanning the complete damage spectrum. We use an independently constrained thermal model to empirically assign a closure temperature to each dated zircon grain. If we assume that this thermal model is robust, the zircon radiation damage accumulation and annealing model of Guenthner et al. (2013) does not accurately predict closure temperatures for many of the analyzed zircon crystals. Raman maps of the zircons dated by laser ablation document complex radiation damage zoning, sometimes revealing crystalline zones in grains with alpha dose values suggestive of amorphous material. Such zoning likely resulted in heterogeneous intra-crystalline helium diffusion and may help explain some of the discrepancies between our empirical findings and the Guenthner et al. (2013) model predictions. Because U-Th zoning is a common feature in zircon, radiation damage zoning is likely to be a concern for most ancient, slowly cooled zircon (U-Th)/He datasets. Whenever possible, multiple mineral-isotopic systems should be employed to add additional, independent constraints to a sample's thermal history.

From opening to subduction of an oceanic domain constrained by LA-ICP-MS U-Pb zircon dating (Variscan belt, Southern Armorican Massif, France)

NASA Astrophysics Data System (ADS)

Paquette, J.-L.; Ballèvre, M.; Peucat, J.-J.; Cornen, G.

2017-12-01

In the Variscan belt of Western Europe, the lifetime and evolution of the oceanic domain is poorly constrained by sparse, outdated and unreliable multigrain ID-TIMS U-Pb zircon dating. In this article, we present a complete in situ LA-ICP-MS dataset of about 300 U-Pb zircon analyses obtained on most of the ophiolitic and eclogitic outcrops of Southern Brittany, comprising new dating of previously published zircon populations and newly discovered rock samples. In situ dating and cathodo-luminescence imaging of each zircon grain yields new absolute time-constraints on the evolution of the Galicia-Moldanubian Ocean. The new results confirm that the opening of this oceanic domain is well defined at about 490 Ma. In contrast, the generally-quoted 400-410 Ma-age for the high-pressure event related to the subduction of the oceanic crust is definitely not recorded in the zircons of the eclogites. In light of these new data, we propose that the obduction of oceanic rocks occurred at about 370-380 Ma while the high-pressure event is recorded at 355 Ma in only a few zircon grains of some eclogite samples. Additionally, this large scale dating project demonstrates that the zircons from eclogites do not systematically recrystallise during the high pressure event and consequently their U-Pb systems do not record that metamorphism systematically. These zircons rather preserve the isotopic memory of the magmatic crystallization of their igneous protolith. Another example of an eclogite sample from the French Massif Central illustrates the frequent mistake in the interpretation of the ages of the early hydrothermal alteration of zircons in the oceanic crust versus partial or complete recrystallization during eclogite facies metamorphism.

Tracking the evolution of a giant magmatic system from assembly to supereruption

NASA Astrophysics Data System (ADS)

Wotzlaw, J. F.; Schaltegger, U.; Frick, D. A.; Dungan, M. A.; Gerdes, A.; Günther, D.

2012-12-01

The chemical and physical characteristics of large magma bodies prior to eruption are closely related consequences of the balance between the rate of magma influx and the rate of cooling. The products of caldera-related silicic supereruptions, and less voluminous associated precursor and post-caldera activity preserve information about the thermal and chemical states of the system at the moment of evacuation of the magma chamber. Numerous studies of the Oligocene Fish Canyon Tuff (5000 km3; Colorado, USA) and related eruptive products of the La Garita caldera have addressed the origin and evolution of large-volume crystal-rich magmas. We use the presence of zircon in all eruptive products of the Fish Canyon magmatic system to gain a high-resolution geochronologic control on the thermal evolution of the magmatic system by using the trace element composition of U-Pb dated zircons as a proxy for magma crystallinity. New analytical protocols permit to obtain age, chemical and isotopic information from the exact same volume of single zircons (Schoene et al., 2010, GCA) allowing us to trace magma crystallinity as a function of time. Zircon U-Pb dates record ~400,000 years of crystallization. Variations in trace element composition in U-Pb dated zircons can be attributed to trace element fractionation imposed by co-crystallization of titanite. Due to the relatively high modal abundance of titanite in Fish Canyon magma and the extreme compatibility of the rare earth elements (REE) in this phase, titanite is the major control on many trace element concentrations and ratios (e.g. Yb/Dy) in coexisting zircons via varying degrees of depletion of these elements in the host melt. Modeling of compositional variations as a result of fractional crystallization suggests that the range of zircon compositions can be explained by 50-70% crystallization of a fractionating assemblage containing 0.4-0.8 vol.% titanite. The period of cooling centered around 28.4 Ma and is marked by an apparent crystallinity that is ~30% higher than at the time of eruption, implying that the Fish Canyon magma was perilously close to complete solidification, hence 'plutonic death', prior to late reheating. We estimate the duration of the reheating event from the age difference of the chemically most evolved (highest Yb/Dy) and the youngest dated zircon to be ~170 ka. This duration for reactivation of the near solidus crystal mush is in excellent agreement with estimates derived from numerical modeling of remelting by upward percolation of a hot gas-phase derived from underplated mafic magma ('gas sparging'; Bachmann and Bergantz, 2003, Geology). This investigation, which is the first in which high resolution U-Pb zircon geochronology has been coupled with geochemical modeling of systematic compositional variations in an accessory mineral, ties together many of the threads which have been developed independently during previous studies of the Fish Canyon magmatic system and places related petrogenetic processes into an absolute time frame.

Single-crystal 40Ar/39Ar incremental heating reveals bimodal sanidine ages in the Bishop Tuff

NASA Astrophysics Data System (ADS)

Andersen, N. L.; Jicha, B. R.; Singer, B. S.

2015-12-01

The 650 km3 Bishop Tuff (BT) is among the most studied volcanic deposits because it is an extensive marker bed deposited just after the Matuyama-Brunhes boundary. Reconstructions of the vast BT magma reservoir from which high-silica rhyolite erupted have long influenced thinking about how large silicic magma systems are assembled, crystallized, and mixed. Yet, the longevity of the high silica rhyolitic melt and exact timing of the eruption remain controversial due to recent conflicting 40Ar/39Ar sanidine vs. SIMS and ID-TIMS U-Pb zircon dates. We have undertaken 21 40Ar/39Ar incremental heating ages on 2 mm BT sanidine crystals from pumice in 3 widely separated outcrops of early-erupted fall and flow units. Plateau ages yield a bimodal distribution: a younger group has a mean of 766 ka and an older group gives a range between 772 and 782 ka. The younger population is concordant with the youngest ID-TIMS and SIMS U-Pb zircon ages recently published, as well as the astronomical age of BT in marine sediment. Of 21 crystals, 17 yield older, non-plateau, steps likely affected by excess Ar that would bias traditional 40Ar/39Ar total crystal fusion ages. The small spread in older sanidine ages, together with 25+ kyr of pre-eruptive zircon growth, suggest that the older sanidines are not partially outgassed xenocrysts. A bimodal 40Ar/39Ar age distribution implies that some fraction of rhyolitic melt cooled below the Ar closure temperature at least 10 ky prior to eruption. We propose that rapid "thawing" of a crystalline mush layer released older crystals into rhyolitic melt from which sanidine also nucleated and grew immediately prior to the eruption. High precision 40Ar/39Ar dating can thus provide essential information on thermo-physical processes at the millenial time scale that are critical to interpreting U-Pb zircon age distributions that are complicated by large uncertainties associated with zircon-melt U-Th systematics.

Relict zircon U-Pb age and O isotope evidence for reworking of Neoproterozoic crustal rocks in the origin of Triassic S-type granites in South China

NASA Astrophysics Data System (ADS)

Gao, Peng; Zheng, Yong-Fei; Chen, Yi-Xiang; Zhao, Zi-Fu; Xia, Xiao-Ping

2018-02-01

Granites derived from partial melting of sedimentary rocks are generally characterized by high δ18O values and abundant relict zircons. Such relict zircons are valuable in tracing the source rocks of granites and the history of crustal anatexis. Here we report in-situ U-Pb ages, O isotopes and trace elements in zircons from Triassic granites in the Zhuguangshan and Jiuzhou regions, which are located in the Nanling Range and the Darongshan area, respectively, in South China. Zircon U-Pb dating yields magma crystallization ages of 236 ± 2 Ma for the Zhuguangshan granites and 246 ± 2 Ma to 252 ± 3 Ma for the Jiuzhou granites. The Triassic syn-magmatic zircons are characterized by high δ18O values of 10.1-11.9‰ in Zhuguangshan and 8.5-13.5‰ in Jiuzhou. The relict zircons show a wide range of U-Pb ages from 315 to 2185 Ma in Zhuguangshan and from 304 to 3121 Ma in Jiuzhou. Nevertheless, a dominant age peak of 700-1000 Ma is prominent in both occurrences, demonstrating that their source rocks were dominated by detrital sediments weathered from Neoproterozoic magmatic rocks. Taking previous results for regional granites together, Neoproterozoic relict zircons show δ18O values in a small range from 5 to 8‰ for the Nanling granites but a large range from 5 to 11‰ for the Darongshan granites. In addition, relict zircons of Paleozoic U-Pb age occur in the two granitic plutons. They exhibit consistently high δ18O values similar to the Triassic syn-magmatic zircons in the host granites. These Paleozoic relict zircons are interpreted as the peritectic product during transient melting of the metasedimentary rocks in response to the intracontinental orogenesis in South China. Therefore, the relict zircons of Neoproterozoic age are directly inherited from the source rocks of S-type granites, and those of Paleozoic age record the transient melting of metasedimentary rocks before intensive melting for granitic magmatism in the Triassic.

When the CA-TIMS therapy fails: the over-enthusiastic, the mixed-up, and the stubborn zircon

NASA Astrophysics Data System (ADS)

Corfu, F.

2009-12-01

Mattinson’s CA-TIMS technique has proven to be highly successful in efficiently removing disturbed domains of zircon, thus enabling the determination of precise and accurate ages in a variety of geological situations. The method generally produces equal or better results than mechanical abrasion. There are, however, cases where CA-TIMS fails to achieve perfect concordance, and occasionally actually increases the degree of discordance. Such a behaviour (the over-reaction) is observed for example in U-rich (>1000 ppm) but texturally homogeneous zircon, a type quite common in highly differentiated portions of mafic intrusions, in granitic pegmatites, and in some metamorphic rocks. Because of their internal uniformity, such zircons do not exhibit large contrasts in crystallinity and solubility and after the baking stage of the CA-TIMS procedure they will either totally dissolve in the first HF attack, or produce discordant results. By contrast, mechanical abrasion of this type of zircon can isolate alteration-free zircon domains that yield concordant and reproducible data. This is due to the homogeneity of the crystals, the lack of zoning limiting contrasts in the degrees of metamictization and brittleness and preventing fracturing. The absence of fractures helps to confine alteration of zircon to the marginal domains, and these can be removed very efficiently by mechanical abrasion. This behaviour is exemplified by zircon populations from 252.0 Ma and 182.5 Ma mafic sills in Siberia and the Karoo basin. Another class of zircons that in general does not, or only partially comply with CA-TIMS includes populations from metamorphic and poly-orogenic rocks, typical of many Proterozoic orogens. The metamorphic reprocessing of zircon into low-U domains can freeze-in old Pb-loss patterns that cannot be undone by any technique. New metamorphic zircon growth can also create mixtures of different aged domains that cannot be resolved by CA-TIMS. In favourable circumstances, however, CA-TIMS could provide elegant ways to isolate the metamorphic components. A third category includes zircon populations that fail to achieve consistent ages, or concordant plateaus in multi-step partial dissolutions, even though they may have all the attributes of ideal CA-TIMS objects. The category is exemplified by a 62 Ma magmatic suite with a simple geological history and good quality zircon populations. The zircons have moderate U contents, regular growth zoning and few visible imperfections, yet the data reveal very extensive Pb loss requiring the almost total dissolution of the zircon before reaching the concordant residues. Baking of some of these zircon populations turns the colourless crystals brick-red (but still transparent), the red colour persisting long into the partial dissolution procedure. These features suggest that Pb loss may be related to a submicroscopic network of impurities and primary defects in the crystal structures which form pathways for the diffusion of Pb. Subsequent annealing appears to make the discordant domains impervious to partial dissolution. In conclusion, it is evident that CA-TIMS is a very helpful new technique for U-Pb geochronology, but some limitations must be considered that require case-by-case judgment and good Krogh-type abraders in reserve.

Multiple Hadean crystallization and reworking events preserved in individual Jack Hills zircon grains

NASA Astrophysics Data System (ADS)

Bellucci, Jeremy; Nemchin, Alexander; Whitehouse, Martin; Snape, Joshua

2017-04-01

Five Hadean (>3.9 Ga) aged zircon grains from the Jack Hills metasedimentary belt have been investigated by an improved secondary ion mass spectrometry scanning ion image technique. This technique has the ability to obtain accurate and precise full U-Pb systematics on a scale <5 μm, as well as document the spatial distribution of U, Th and Pb. All five of the grains investigated here have complex cathodoluminescence patterns that correlate to different U, Th, and Pb concentration domains. The age determinations for these different chemical zones indicate multiple reworking events that are preserved in each grain and have affected the primary crystalized zircon on the scale of <10 μm, smaller than traditional ion microprobe spot analyses. These new scanning ion images and age determinations suggest that roughly half, if not all, previous analyses, including those of trace elements and various isotope systems, could have intersected several domains of unfractured zircon, thus making the interpretation of any trace element, Hf, or O isotopic data tenuous. Lastly, all of the grains analyzed here preserve at least two distinguishable 207Pb/206Pb ages. These ages are preserved in core-rim and/or complex internal textural relationships. These secondary events took place during at ca. 4.3, 4.2, 4.1, 4.0, and 3.7 Ga potentially indicating a sequence of magmatic and/or metamorphic events that recycled some volume of early crust during the Hadean and into Paleo- to Mesoarchean several times with an apparent periodicity of ca. 100 Ma.

Exploring the U-Pb systematics of titanite from the Archean Stillwater Complex

NASA Astrophysics Data System (ADS)

Friedman, R. M.; Wall, C. J.; Scoates, J. S.; Weis, D. A.; Meurer, W. P.

2011-12-01

The Stillwater Complex is a large mafic-ultramafic layered intrusion in the Beartooth Mountains of Montana (USA) and host to the world-class J-M Reef platinum group element deposit. The size and geologic/economic importance of this igneous complex make it an important target for high-precision U-Pb dating. As a part of a comprehensive U-Pb study of the Stillwater Complex, we present ID-TIMS U-Pb titanite data, including new single grain results produced using the EARTHTIME ET535 spike, for very low-volume, relatively felsic granophyric and pegmatitic rocks associated with Stillwater layered rocks. Four samples studied include a pegmatitic ksp-qtz core to a gabbroic pegmatoid in the Lower Banded Series (N1), an alaskite (quartz diorite) and an amphibole-rich reaction zone between the alaskite and anorthosite (AN1) in the Middle Banded Series, and an amphibole-bearing granophyre from the Upper Banded Series (GN3). CA-TIMS U-Pb dating of zircon from these samples yielded concordant results only for the pegmatitic rock (weighted 207Pb/206Pb: 2709.65 ± 0.80 Ma, n = 5), which agrees with new zircon ages from Stillwater layered rocks. Results for high-U (up to 1438 ppm) metamict zircon that occurs in the other three rocks were highly discordant and did not yield precise ages. Titanite U-Pb results for the pegmatite are about -1% to +1% discordant with two groupings of 207Pb/206Pb dates: one with a weighted average of 2708.1 ± 2.0 Ma (n = 2), which overlaps in age with zircon from the same sample and the crystallization age of the Stillwater Complex, and a second, younger grouping of 2701.1 ± 1.3 Ma (n = 5). Younger dates record an early Pb-loss event, possibly related to intrusion of cross-cutting quartz monzonites. The alaskite data also shows two groupings of 207Pb/206Pb dates, although more subtle: a weighted average of 2709.3 ± 1.8 Ma (n = 3) and a single result of 2706.5 ± 1.7 Ma. Titanite from the other two samples has undergone significant Pb-loss. Results for four analyses from the reaction zone sample are 1.74-54.3% discordant and lie in a quasi-linear array with intercepts at ca. 2700 Ma and 325 Ma. Data for the granophyre in the Upper Banded Series also have a wide range of discordance (1.5-3.4%, 21.4% and 61.3%). Titanites from these samples appear to have undergone a two stage Pb-loss history: an early, relatively minor event as suggested for the pegmatite, and a subsequent episode, which for some grains resulted in significant discordance. Concordant U-Pb titanite data from two of the Stillwater Complex granophyres confirms nearly synchronous crystallization with that of the layered rocks in the intrusion. For the pegmatite, the ages and overlap of zircon and titanite suggest a direct relationship between the granophyres and the layered rock and relatively rapid cooling of the complex through titanite closure temperature. Titanite from the alaskite yields useful age information, whereas coexisting zircon are highly discordant and not age diagnostic.

Pre-Variscan evolution of the Western Tatra Mountains: new insights from U-Pb zircon dating.

PubMed

Burda, Jolanta; Klötzli, Urs

In situ LA-MC-ICP-MS U-Pb zircon geochronology combined with cathodoluminescence imaging were carried out to determine protolith and metamorphic ages of orthogneisses from the Western Tatra Mountains (Central Western Carpathians). The metamorphic complex is subdivided into two units (the Lower Unit and the Upper Unit). Orthogneisses of the Lower Unit are mostly banded, fine- to medium-grained rocks while in the Upper Unit varieties with augen structures predominate. Orthogneisses show a dynamically recrystallised mineral assemblage of Qz + Pl + Bt ± Grt with accessory zircon and apatite. They are peraluminous (ASI = 1.20-1.27) and interpreted to belong to a high-K calc-alkaline suite of a VAG-type tectonic setting. LA-MC-ICP-MS U-Pb zircon data from samples from both units, from crystals with oscillatory zoning and Th/U > 0.1, yield similar concordia ages of ca. 534 Ma. This is interpreted to reflect the magmatic crystallization age of igneous precursors. These oldest meta-magmatics so far dated in the Western Tatra Mountains could be linked to the fragmentation of the northern margin of Gondwana. In zircons from a gneiss from the Upper Unit, cores with well-developed oscillatory zoning are surrounded by weakly luminescent, low contrast rims (Th/U < 0.1). These yield a concordia age of ca. 387 Ma corresponding to a subsequent, Eo-Variscan, high-grade metamorphic event, connected with the formation of crustal-scale nappe structures and collision-related magmatism.

Tectono-magmatic evolution of the Chihuahua-Sinaloa border region in northern Mexico: Insights from zircon-apatite U-Pb geochronology, zircon Hf isotope composition and geochemistry of granodiorite intrusions

NASA Astrophysics Data System (ADS)

Mahar, Munazzam Ali; Goodell, Philip C.; Feinstein, Michael Nicholas

2016-11-01

We present the whole-rock geochemistry, LA-ICP-MS zircon-apatite U-Pb ages and zircon Hf isotope composition of the granodioritic plutons at the southwestern boundary of Chihuahua with the states of Sinaloa and Sonora. These granodiorites are exposed in the north and south of the Rio El Fuerte in southwest Chihuahua and northern Sinaloa. The magmatism spans over a time period of 37 Ma from 90 to 53 Ma. Zircons are exclusively magmatic with strong oscillatory zoning. No inheritance of any age has been observed. Our new U-Pb dating ( 250 analyses) does not support the involvement of older basement lithologies in the generation of the granitic magmas. The U-Pb apatite ages from granodiorites in southwest Chihuahua vary from 52 to 70 Ma. These apatite ages are 1 to 20 Ma younger than the corresponding zircon U-Pb crystallization ages, suggesting variable cooling rates from very fast to 15 °C/Ma ( 800 °C to 500 °C) and shallow to moderate emplacement depths. In contrast, U-Pb apatite ages from the Sinaloa batholith are restricted from 64 to 61 Ma and are indistinguishable from the zircon U-Pb ages range from 67 to 60 Ma within the error, indicating rapid cooling and very shallow emplacement. However, one sample from El Realito showed a larger difference of 20 Ma in zircon-apatite age pair: zircon 80 ± 0.8 Ma and apatite 60.6 ± 4 Ma, suggesting a slower cooling rate of 15 °C/Ma. The weighted mean initial εHf (t) isotope composition (2σ) of granodiorites varies from + 1.8 to + 5.2. The radiogenic Hf isotope composition coupled with previous Sr-Nd isotope data demonstrates a significant shift from multiple crustal sources in the Sonoran batholithic belt to the predominant contribution of the mantle-derived magmas in the southwest Chihuahua and northern Sinaloa. Based on U-Pb ages, the absence of inheritance, typical high Th/U ratio and radiogenic Hf isotope composition, we suggest that the Late Cretaceous-Paleogene magmatic rocks in this region are not derived from melting of a felsic older crust, neither evolved North American Proterozoic basement nor Jurassic metasedimentary rocks of the Guerrero terrane. Instead, the magma was primarily derived from partial melting of mantle related sources, possibly in the subcontinental mantle wedge above the Farallon plate. Our younger U-Pb zircon-apatite ages are well correlated with the late-stage cogenetic porphyry Cu-Au mineralization at 57 to 59 Ma. If the origin of ore mineralization is related to the magma sources of host batholiths, then a minimal input from crustal melt is suggested in the genesis of Cu-Au porphyry system.

Concordant ages for the Lava Creek Tuff from high-spatial-resolution U-Pb dating of zircon rim faces and single-crystal sanidine 40Ar/39Ar dating

NASA Astrophysics Data System (ADS)

Matthews, N. E.; Vazquez, J. A.; Calvert, A. T.

2013-12-01

The last great explosive supereruption from the Yellowstone Plateau formed present-day Yellowstone caldera and ejected the >1000 km3 of rhyolite that composes the Lava Creek Tuff (LCT). The LCT eruption blanketed much of the western United States in ash, and consequently is a key chronostratigraphic marker bed for delimiting Quaternary uplift rates, the age of middle Pleistocene glacial and pluvial deposits, and tephra correlation in North America. Previous 40Ar/39Ar dating of the two mineralogically distinct LCT members (A & B) yield ages ranging from ca. 600 ka (Gansecki et al., 1998) to ca. 640 ka (Lanphere et al., 2002). To resolve the timing of eruption and crystallization timescale for the LCT magma, we dated both LCT members using a dual-method approach as follows: (1) ion microprobe (SHRIMP-RG) U-Pb dating and trace-element characterization of the final few micrometers of zircon crystallization by analysis of unpolished rims on indium-mounted crystals, and dating of the onset of zircon crystallization by traditional analysis of sectioned crystal interiors, and (2) laser-fusion 40Ar/39Ar dating of single sanidine crystals from bulk LCT ignimbrite and pumice. The unpolished rims of zircon from LCT members A & B yield indistinguishable ages, with a mean age of 621.8 × 2.5 ka (1σ) after correction for initial 230Th disequilibrium as constrained by ion-probe analyses of LCT melt inclusions. Single sanidine crystals from LCT-B yield a mean age of 624.9 × 2.6 ka (FCT=28.17 Ma) that is indistinguishable from the zircon rim ages for both members. These results indicate that LCT members A & B erupted over a geologically brief interval, which is supported by the direct and gradational contact of their equivalent fallout in distal lacustrine deposits and a lack of field evidence for a significant time-break between the LCT A & B in proximal deposits (Christiansen, 2001), but contrasts with older Yellowstone ignimbrite (e.g., Huckleberry Ridge) that may have erupted as multiple pulses over tens of kyr (Ellis et al., 2012). Both LCT members contain zircon whose interiors yield U-Pb ages that are mostly ca. 40 kyr older than their rims, suggesting a short timescale for assembly of the LCT magma chamber relative to some other voluminous bodies of silicic magma (e.g., Fish Canyon). Trace-element concentrations in zircon from Members A and B appear diagnostic, with the former containing higher U and REE concentrations. Our concordant results from U-Pb and 40Ar/39Ar dating indicating a ca. 625 ka age for the LCT eruption are supported by the stratigraphic position of LCT-B tephra at or near the top of Great Basin pluvial lake bed sequences associated with Marine Isotope Stage 16, whose termination is astronomically dated at 621 ka. Christiansen, 2001, USGS Prof. Pap. 729-G; Ellis et al., 2012, Quat Geochron 9: 31-41; Gansecki et al., 1998, Geology 26(4): 343-346; Lanphere et al., 2002, GSA Bull. 114(5): 559-568.

Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

NASA Astrophysics Data System (ADS)

Kunz, Barbara E.; Regis, Daniele; Engi, Martin

2018-03-01

Granulite facies rocks frequently show a large spread in their zircon ages, the interpretation of which raises questions: Has the isotopic system been disturbed? By what process(es) and conditions did the alteration occur? Can the dates be regarded as real ages, reflecting several growth episodes? Furthermore, under some circumstances of (ultra-)high-temperature metamorphism, decoupling of zircon U-Pb dates from their trace element geochemistry has been reported. Understanding these processes is crucial to help interpret such dates in the context of the P-T history. Our study presents evidence for decoupling in zircon from the highest grade metapelites (> 850 °C) taken along a continuous high-temperature metamorphic field gradient in the Ivrea Zone (NW Italy). These rocks represent a well-characterised segment of Permian lower continental crust with a protracted high-temperature history. Cathodoluminescence images reveal that zircons in the mid-amphibolite facies preserve mainly detrital cores with narrow overgrowths. In the upper amphibolite and granulite facies, preserved detrital cores decrease and metamorphic zircon increases in quantity. Across all samples we document a sequence of four rim generations based on textures. U-Pb dates, Th/U ratios and Ti-in-zircon concentrations show an essentially continuous evolution with increasing metamorphic grade, except in the samples from the granulite facies, which display significant scatter in age and chemistry. We associate the observed decoupling of zircon systematics in high-grade non-metamict zircon with disturbance processes related to differences in behaviour of non-formula elements (i.e. Pb, Th, U, Ti) at high-temperature conditions, notably differences in compatibility within the crystal structure.

Timing of metamorphism and exhumation in the Nordøyane ultra-high-pressure domain, Western Gneiss Region, Norway: New constraints from complementary CA-ID-TIMS and LA-MC-ICP-MS geochronology

NASA Astrophysics Data System (ADS)

Butler, J. P.; Jamieson, R. A.; Dunning, G. R.; Pecha, M. E.; Robinson, P.; Steenkamp, H. M.

2018-06-01

We present the results of a combined CA-ID-TIMS and LA-MC-ICP-MS U-Pb geochronology study of zircon and associated rutile and titanite from the Nordøyane ultra-high-pressure (UHP) domain in the Western Gneiss Region (WGR) of Norway. The dated samples include 4 eclogite bodies, 2 host-rock migmatites, and 2 cross-cutting pegmatites and leucosomes, all from the island of Harøya. Zircon from a coesite eclogite yielded an age of ca. 413 Ma, interpreted as the time of UHP metamorphism in this sample. Zircon data from the other eclogite bodies yielded metamorphic ages of ca. 413 Ma, 407 Ma, and 406 Ma; zircon trace-element data associated with 413 Ma and 407 Ma ages are consistent with eclogite-facies crystallization. In all of the eclogites, U-Pb dates from zircon cores, interpreted as the times of protolith crystallization, range from ca. 1680-1586 Ma, consistent with Gothian ages from orthogneisses in Nordøyane and elsewhere in the WGR. A zircon core age of ca. 943 Ma from one sample agrees with Sveconorwegian ages of felsic gneisses and pegmatites in the western part of the area. Migmatites hosting the eclogite bodies yielded zircon core ages of ca. 1657-1591 Ma and rim ages of ca. 395-392 Ma, interpreted as the times of Gothian protolith formation and Scandian partial melt crystallization, respectively. Pegmatite in an eclogite boudin neck yielded a crystallization age of ca. 388 Ma, interpreted as the time of melt crystallization. Rutile and titanite from 3 samples (an eclogite and two migmatites) yielded concordant ID-TIMS ages of 378-376 Ma. The results are similar to existing U-Pb data from other Nordøyane eclogites (415-405 Ma). In combination with previous pressure-temperature data from the coesite eclogite, these ages indicate that peak metamorphic conditions of 3 GPa/760 °C were reached ca. 413 Ma, followed by decompression to 1 GPa/810 °C by ca. 397 Ma and cooling below ca. 600 °C by ca. 375 Ma. The results are compatible with protracted UHP metamorphism followed by relatively slow exhumation. The question of whether partial melting began at UHP conditions is not resolved by this study.

Listwaenite in the Sartohay ophiolitic mélange (Xinjiang, China): A genetic model based on petrology, U-Pb chronology and trace element geochemistry

NASA Astrophysics Data System (ADS)

Qiu, Tian; Zhu, Yongfeng

2018-03-01

Listwaenite lenses in the Sartohay ophiolitic mélange (Xinjiang, China) were formed via reactions between serpentinite and metasomatic fluids. First, serpentinite changed into talc schist via the reaction of serpentine + CO2 → talc + magnesite + H2O. Second, talc schist changed into listwaenite via the reaction of talc + CO2 → magnesite + quartz + H2O. Magnetite was progressively destroyed during transformation from serpentinite to talc schist, and completely consumed in listwaenite. Zircon crystals 30-100 μm long, disseminating in talc schist, undeformed listwaenite and mylonitized listwaenite, coexist with talc, quartz and magnesite, while micron-sized zircon grains (<5 μm in length) occur along the shearing foliation in the weakly deformed listwaenite and mylonitized listwaenite. We postulate that these micron-sized zircon crystals may have grown in-situ from medium-temperature hydrothermal fluids. Concentrations of most trace elements including high field strength elements (HFSE) increase from the undeformed, through the weakly deformed, to the mylonitized listwaenite, showing a positive correlation with the degrees of deformation and proportions of micron-size zircon, apatite, rutile and monazite. The large zircon crystals recovered from talc schist, undeformed listwaenite and mylonitized listwaenite yield similar weighted mean U-Pb ages (302.9 ± 6.8 Ma, 299.7 ± 5.5 Ma and 296.5 ± 3.5 Ma), and are thought to represent the age of formation of the talc schist and listwaenite. These ages are indistinguishable within errors and suggest a rapid transformation from talc schist to listwaenite. Some zircon rims in samples of the undeformed listwaenite and mylonitized listwaenite give much younger apparent U-Pb ages (280-277 Ma), which could be interpreted as a recrystallization age reflecting late-stage shearing in the Sartohay ophiolitic mélange.

Zircon from charnockite gneiss, charnockite, and leucosome of migmatite in the Nimnyr Block of the Aldan Shield

NASA Astrophysics Data System (ADS)

Glebovitsky, V. A.; Sedova, I. S.; Berezhnaya, N. G.; Skublov, S. G.; Samorukova, L. M.

2015-12-01

The microgeochemistry of zircon was studied in three samples: charnockite gneiss (1594), charnockite (1594a), and migmatite leucosome Lc4 (1594c). Prismatic (Zrn I) and oval (Zrn II) zircon morphotypes are distinguished in the first two samples. Most zircon grains consist of two-phase cores and overgrowth rims variable in thickness. The average weighted concordant U-Pb age of Zrn II cores from charnockite gneiss is 2436 ± 10 Ma. The concordant ages of Zrn I and Zrn II cores from charnockite are 2402 ± 16 Ma and 2453 ± 14 Ma, respectively. Some overgrowth rims are 1.9-2.1 Ga in age. In leucosome Lc4, all measured prismatic zircon crystals yielded a discordant age of 1942 ± 11 Ma (the upper intersection of discordia with concordia). These zircons are strongly altered and anomalously enriched in U and Th. Zrn I grains are enriched relative to Zrn II in REE, Li, Ca, Sr, Ba, Hf, Th, and U. Zrn I is considered to be a product of melt crystallization or subsolidus recrystallization in the presence of melt. Zrn II is relict or crystallizing from melt and then partly fused again. Zrn I from charnockite gneiss and especially from charnockite are markedly altered and have a more discordant age than Zrn II. This is probably related to concentration of fluid in the residual melt left after zircon crystallization.

Chemical-abrasion SIMS dating of zircon from the Eocene Caetano caldera, Nevada

NASA Astrophysics Data System (ADS)

Colgan, J.; Watts, K. E.; John, D. A.; Henry, C. D.; Coble, M. A.; Vazquez, J. A.

2012-12-01

The Eocene Caetano caldera in northern Nevada formed during eruption of ~1100 km3 of crystal-rich rhyolite. Miocene extension cut the caldera into a set of fault blocks that expose minor pre-caldera volcanic rocks, two units of intracaldera Caetano Tuff up to 4 km thick, ash-flow tuff feeder dikes and ring-fracture intrusions, caldera collapse breccias, and post-collapse resurgent intrusions. Single-crystal 40Ar/39Ar sanidine dates on all parts of the caldera system overlap, yielding a 34.01 ± 0.05 Ma (n=17, Fish Canyon sanidine = 28.201 Ma) age for the eruption. 40Ar/39Ar dating also documents several preceding episodes of magmatism: 35.69 ± 0.06 Ma (sanidine, n =13) rhyolite dikes in the nearby Cortez gold district, 35.21 ± 0.18 Ma (plagioclase, n=1) andesite lava underlying Caetano Tuff, and a 38.90 ± 0.11 Ma (biotite, n=1), dacite dike in the northeastern caldera wall. Extensive U-Pb SHRIMP dating of zircon from both the Cortez dikes and all phases of the Caetano system suggests continuous magmatism from 40-34 Ma. However, all samples contain at least some—sometimes many—zircons with U-Pb ages younger than the 34.0 Ma argon age. To determine if anomalously young zircon ages are due to Pb-loss, we analyzed representative samples of the upper Caetano Tuff and the Redrock Canyon resurgent pluton with and without chemical abrasion to mitigate Pb-loss. Bulk zircon separates were annealed at 850°C for 48 hours, then chemically abraded with 10:1 HF/HNO3 vapor in a Parr bomb at 225°C for 8 hours, based on protocols outlined by Mattinson (2005). Both treated and untreated zircons from the same sample were mounted in epoxy and polished to their midsections, then imaged on the SEM using BSE and CL. The SHRIMP-RG at Stanford University was used to determine U-Pb ages and trace element concentrations in single spots for ~25 to 30 individual zircons per sample, using a round-robin procedure and two zircon age standards (R33 and 080) to monitor external precision. Analyses revealed distinctly different age populations for the abraded and untreated zircons. The chemically abraded populations yielded unimodal zircon age distributions with mean ages that overlap with the 40Ar/39Ar age. Untreated zircon populations yielded mean ages 0.9-1.5 Ma younger than the 40Ar/39Ar. In the untreated populations, 50-60% of zircon ages are younger than 34.0 Ma at 1σ, versus 15-20% in the chemically abraded populations. Comparison of trace element data from treated and untreated populations indicates that trace element concentrations are apparently unaffected by the chemical abrasion procedure. Further experiments are underway, but we tentatively conclude that chemical abrasion is effective for removing damaged Pb-loss portions of zircons while still enabling high spatial resolution U-Pb dating and trace element analysis. It appears to be a relatively fast and low-cost way to improve the accuracy of SIMS dating of large populations of zircon from Tertiary and older plutonic and volcanic rocks where Pb-loss is frequently an issue.

Zircon-pyrochlore ores of Proterozoic Gremyakha-Vyrmes polyphase massif, Kola Peninsula: source and evolution

NASA Astrophysics Data System (ADS)

Sorokhtina, Natalia; Belyatsky, Boris; Antonov, Anton; Kononkova, Natalia; Lepekhina, Elena; Kogarko, Lia

2017-04-01

The alkaline-ultrabasic Gremyakha-Vyrmes massif occurs within the Central Kola terrane in the northern part of the Fennoscandian Shield and consists of diverse rock complexes: basic-ultrabasic rocks, foidolites, alkaline metasomatic rocks and carbonatites, alkaline granites and granosyenites. Nb-Zr ore deposit is confined to alkaline metasomatic rocks developed over foidolites. The metasomatites are represented by albitites and aegirinites occur as submeridionally orientated zones extending up to 6-8 km and several hundred meters thickness. They are mainly composed of albite and aegirine, but amphibole, annite, microcline, fluorapatite, titanite, ilmenite, pyrochlore group minerals, zircon are present [Sorokhtina et al., 2016]. Carbonatites are developed sporadically and accessory zircon but not the pyrochlore is observed only in contact zones with albitites and aegerinites. In metasomatites, zircon and pyrochlore are main rare metal minerals, which are formed at the latest stages of crystallization. Ca-dominant fluorcalcio- and hydroxycalciopyrochlores are the most abundant, whereas U-dominant pyrochlore, oxyuranobetafite, zero-valent-dominant (Ba, Sr-dominant) pyrochlore, hydro- or kenopyrochlore are rare. The pyrochlore-group minerals form heterogeneous metacrystals containing inclusions of host rock minerals, calcite, ilmenite, zircon, sulfides, and graphite. While pyrochlore is replaced by Si-rich "pyrochlore" (SiO2 is up to 18 wt.%.), cation-deficient hydrated pyrochlore, Fe-Si-Nb, U-Si-Nb, and Al-Si-Nb phases along fracture zones and margins. The early generation zircon is represented by large heterogeneous metacrystals filled with inclusions of various host rock minerals, calcite, ilmenite, thorite, thorianite and sulfides, while the late zircons are empty of inclusions. Zircons are nearly stoichiometric in composition; but intermediate zones are enriched in Pb, Y and Th, and overgrowths are enriched Hf only. According to CL and ion-microprobe analysis zircon has polygenetic nature: some relics inherited from foidolite crystallized at about 800°C, whereas the newly formed - at 600°C [Watson et al., 2006]. The time interval of the magmatic massif formation may be estimated as long as 80-100 Ma only. The basic-ultrabasic rocks and foidolites were intruded consistently at 1982 ± 6 Ma and 1894±12 according to SHRIMP-II U-Pb zircon dating, but the whole-rock Sm-Nd isotope dating has resulted in 1879±99 Ma and reflects the impact of alkaline granite intrusion (1871±9 Ma). The late differentiates from alkaline magma crystallization were the main source of rare metals for zircon-pyrochlore ores of alkaline metasomatites. The metasomatic rocks (aegirinites, albitites) and carbonatites were formed as late as 1910 ± 15 Ma (SHRIMP-II U-Pb zircon, titanite, pyrochlore). While some pyrochlore grains from metasomatites are showed that U-Pb age of ore formation is 1766 ± 24 and 1764 ± 19 respectively. That can be attributed to additional source of rare metals connected with fluids formed during regional metamorphism 1750 m.y. ago [Glebovitskii et al., 2014]. The last probable source of rare-metal material and ore-deposit evolution stage (recrystallization) is established by individual pyrochlore grain Sm-Nd and U-Pb systems and evidences tectono-thermal activity at the Paleozoic plume magmatism, which was followed by structural and chemical mineral changes. The research was done within the framework of the scientific program of Russian Academy of Sciences and state contract K41.2014.014 with Sevzapnedra. References: Watson E. B., Wark D. A., Thomas J. B. Crystallization thermometers for zircon and rutile // Contrib. Mineral. Petrol. 2006. 151, 413-433. Glebovitskii V.A., Bushmin S.A., Belyatsky B.V., Bogomolov E.S., Borozdin A.P., Savva E.V., Lebedeva Y.M. Rb-Sr age of metasomatism and ore formation in the low-temperature shear zones of the Fenno-Karelian craton, Baltic Shield // Petrology. 2014. 22(2). 184-204. Sorokhtina N.V., Kogarko L.N., Shpachenko A.K., Senin V. G. Composition and Conditions of Crystallization of zircon from the rare-metal ores of the Gremyakha-Vyrmes massif, Kola Peninsula // Geochemistry International. 2016. 54 (12). 1035-1048.

40Ar/39Ar dating and zircon chronochemistry for the Izu-Bonin rear arc, IODP site U1437

NASA Astrophysics Data System (ADS)

Schmitt, A. K.; Konrad, K.; Andrews, G. D.; Horie, K.; Brown, S. R.; Koppers, A. A. P.; Busby, C.; Tamura, Y.

2016-12-01

The scientific objective of IODP Expedition 350 drilling at Site U1437 (31°47.390'N, 139°01.580'E) was to reveal the "missing half of the subduction factory": the rear arc of a long-lived intraoceanic subduction zone. Site U1437 lies in a 50 km long and 20 km wide volcano-bounded basin, 90 km west of the Izu arc front, and is the only IODP site drilled in the rear arc. The Izu rear arc is dominated by Miocene basaltic to dacitic seamount chains, which strike at a high angle to the arc front. Radiometric dating targeted a single igneous unit (1390 mbsf), and fine to coarse volcaniclastic units for which we present zircon and 40Ar/39Ar (hornblende, plagioclase, and groundmass) age determinations. All zircons analyzed as grain separates were screened for contamination from drill-mud (Andrews et al., 2016) by analyzing trace elements and, where material was available, O and Hf isotope compositions. Igneous Unit 1 is a rhyolite sheet and yielded concordant in-situ and crystal separate U-Pb zircon ages (13.7±0.3 Ma; MSWD = 1.3; n = 40 spots), whereas the 40Ar/39Ar hornblende plateau age (12.9±0.3; MSWD = 1.1; n = 9 steps) is slightly younger, possibly reflecting pre-eruptive zircon crystallization, or alteration of hornblende. U-Pb zircon and 40Ar/39Ar plateau ages from samples above igneous Unit 1 are concordant with biostratigraphic and paleomagnetic ages (available to 1300 mbsf), but plagioclase and groundmass samples below 1300 m become younger with depth, hinting at post-depositional alteration. A single zircon from 1600 mbsf yielded a U-Pb age of 15.4±1.8 Ma; its trace element composition resembles other igneous zircons from U1437, and is tentatively interpreted as a Middle Miocene age for the lowermost lithostratigraphic unit VII. Oxygen and Hf isotopic values of igneous zircon indicate mantle origins, with some influence of assimilation of hydrothermally altered oceanic crust evident in sub-mantle oxygen isotopic compositions. Lessons from site U1437 are that integrated chronochemistry is essential for achieving accurate age models in oceanic drilling. Reference: Andrews, G. D., Schmitt, A. K., Busby, C. J., Brown, S. R., Blum, P., & Harvey, J. (2016). Age and compositional data of zircon from sepiolite drilling mud to identify contamination of ocean drilling samples. G3. doi: 10.1002/2016GC006397.

Isotope U-Pb age on single zircon and REE distribution in rocks and zircon from paleoproterozoic Kandalaksha-Kolvitsa complex Baltic shield

NASA Astrophysics Data System (ADS)

Steshenko, Ekaterina; Bayanova, Tamara; Drogobuzhskaya, Svetlana; Lyalina, Ludmila; Serov, Pavel; Chashchin, Viktor; Elizarov, Dmitriy

2017-04-01

Kandalaksha-Kolvitsa paleoproterozoic complex located in the N-E part of Baltic shield and consists of three zones. Marginal zone (mesocratic metanorite) lies at the base of the massif. Main zone is composed of leucocratic metagabbro. The upper zone is alteration of mataanorthosite and leucocratic metagabbro. All rocks were subjected to granulate and anorthositic metamorphism. Age of magmatic crystallization of the massif was determined for the first time, using the U-Pb isotope method for single zircon grains. Three fractions of single zircons from anorthosite of the Kandalaksha massif gave precise U-Pb age of 2435.5 ± 4.8 Ma. For the first time REE concentration (WR) was determined using a quadrupole mass spectrometer (Agilent 7500 ce ICP-MS) in the main varieties of rocks of the Kandalaksha-Kolvitsa paleoproterozoic complex. Anorthosite and leucocratic metagabbros (main zone) are characterized by a flat spectrum distribution of HREE, which were normalized by [1]. The REE pattern is characterized by significant positive anomalies of Eu ((Eu / Eu *)n = 3.72-3.91) in anorthosite and leucogabbros and 7.26 - in ortoamfibolitah. General content of individual elements that are common for this type of rocks: Cen = 5.82-8.54, Ybn = 1.54-1.58, which indicates that the process of crystallization of the rock occurred with predominant accumulation of plagioclase. According to geochemical and Nd-Sr isotopic data (ISr=0.702 - 0.706, ɛNd(T) = +1 - (-3)) Kandalaksha Kolvitsa complex, appear to have a general plume source with Paleoproterozoic layered intrusions of the Baltic Shield [2] Distribution of REE (ELAN-9000 ICP-MS) in zircon have a typical magmatic species: a positive Ce, negative Eu anomaly and HREE flat spectrum. Titanium content in zircons were measured for the calculation of their crystallization temperature with 8350C. These data are evidence of magmatic origin of zircon [3]. The scientific researches are supported by RFBR (projects № 15-35-20501, № 16-05-00305, 16-05-00367, 16-05-00427) and theme of state assignment № 0231-2015-0005. References: 1. Boynton W.V. Cosmochemistry of the rare earth elements: meteorite studies // Ed. Henderson P. Rare earth element geochemistry. Amsterdam: Elsevier. 1984. P. 63-114. 2. Watson E. B., Wark D.A., Thomas J.B. Crystallization thermometers for zircon and rutile // Contrib. Miner. Petrol. 2006. V. 151. P. 413-433. 3. Hoskin P.W.O. and Schaltegger U. The Composition of zirconand igneous and metamorphic petrogenesis // Reviews in mineralogy & geochemistry. 2003. V. 53. P. 27-62.

Approaches for the accurate definition of geological time boundaries

NASA Astrophysics Data System (ADS)

Schaltegger, Urs; Baresel, Björn; Ovtcharova, Maria; Goudemand, Nicolas; Bucher, Hugo

2015-04-01

Which strategies lead to the most precise and accurate date of a given geological boundary? Geological units are usually defined by the occurrence of characteristic taxa and hence boundaries between these geological units correspond to dramatic faunal and/or floral turnovers and they are primarily defined using first or last occurrences of index species, or ideally by the separation interval between two consecutive, characteristic associations of fossil taxa. These boundaries need to be defined in a way that enables their worldwide recognition and correlation across different stratigraphic successions, using tools as different as bio-, magneto-, and chemo-stratigraphy, and astrochronology. Sedimentary sequences can be dated in numerical terms by applying high-precision chemical-abrasion, isotope-dilution, thermal-ionization mass spectrometry (CA-ID-TIMS) U-Pb age determination to zircon (ZrSiO4) in intercalated volcanic ashes. But, though volcanic activity is common in geological history, ashes are not necessarily close to the boundary we would like to date precisely and accurately. In addition, U-Pb zircon data sets may be very complex and difficult to interpret in terms of the age of ash deposition. To overcome these difficulties we use a multi-proxy approach we applied to the precise and accurate dating of the Permo-Triassic and Early-Middle Triassic boundaries in South China. a) Dense sampling of ashes across the critical time interval and a sufficiently large number of analysed zircons per ash sample can guarantee the recognition of all system complexities. Geochronological datasets from U-Pb dating of volcanic zircon may indeed combine effects of i) post-crystallization Pb loss from percolation of hydrothermal fluids (even using chemical abrasion), with ii) age dispersion from prolonged residence of earlier crystallized zircon in the magmatic system. As a result, U-Pb dates of individual zircons are both apparently younger and older than the depositional age of the ash, therefore masking the true age of deposition. Trace element ratios such as Th/U, Yb/Gd, as well as Hf isotope analysis of dated zircon can be used to decipher the temporal evolution of the magmatic system before the eruption and deposition of the studied ashes, and resolve the complex system behaviour of the zircons. b) Changes in the source of the magma may happen between the deposition of two stratigraphically consecutive ash beds. They result in the modification of the trace element signature of zircon, but also of apatite (Ca5 (F, Cl, OH) (PO4)3). Trace element characteristics in apatite (e.g. Mg, Mn, Fe, F, Cl, Ce, and Y) are a reliable tool for distinguishing chemically similar groups of apatite crystals to unravel the geochemical fingerprint of one single ash bed. By establishing this fingerprint, ash beds of geographically separated geologic sections can be correlated even if they have not all been dated by U-Pb techniques. c) The ultimate goal of quantitative stratigraphy is to establish an age model that predicts the age of a synchronous time line with an associated 95% confidence interval for any such line within a stratigraphic sequence. We show how a Bayesian, non-parametric interpolation approach can be applied to very complex data sets and leads to a well-defined age solution, possibly identifying changes in sedimentation rate. The age of a geological time boundary bracketed by dated samples in such an age model can be defined with an associated uncertainty.

Geochronology and Geochemistry of Zircons from the IODP Site U1437 in the Rear of the Izu-Bonin Volcanic Arc

NASA Astrophysics Data System (ADS)

Andrews, G. D.; Schmitt, A. K.; Busby, C. J.; Brown, S. R.

2015-12-01

Zircons recovered from International Ocean Discovery Program Expedition 350 Site U1437 (31°47.390'N, 139°01.580'E) in the Izu-Bonin arc were analyzed by SIMS to constrain their age (U/Pb geochronology) and geochemistry (trace elements, δ18O); LA-ICP-MS ɛHf analyses are pending. Seven intervals were dated successfully: six tuffs and lapilli-tuffs between 680.99 and 1722.46 m below sea floor (mbsf) and a single peperitic rhyolitic intrusion at 1388.86 - 1390.07 mbsf. Thirty-two intervals which underwent mineral separation lacked zircon, or yielded zircon much older than age expectations for U1437. Geochronology results from separated zircons confirm and extend the shipboard age model to 1360.77 mbsf where Late Miocene (Tortonian) submarine volcanic rocks (11.3 ±0.7 Ma; n = 17) were sampled. In-situ measurement of zircons associated with magnetite crystals in the rhyolite intrusion yield an age of 13.6 ±1.7 Ma (n = 9). Zircon U contents are low (typically <300 ppm), with trace element ratios characteristic of oceanic lithosphere and near-mantle δ18O values (4-6 ‰). Individual Miocene zircon crystals are difficult to distinguish by age alone from those in the drilling mud (sepiolite) used during Expedition 350; the sepiolite is quarried by IMV Nevada in the Amargosa Valley. Our analysis of thirty-three zircons from the sepiolite finds that they have a broad and varied age distribution (2 - 2033 Ma) with a prominent peak at 12-14 Ma, bimodal δ18O values (peaks at 5-5.5 and 6.5-7.5 ‰), and dominantly continental trace element signatures. Three zircons from U1437 are tentatively identified as sepiolite-derived, but a single Eocene grain (51.7 ±2.4 Ma) recovered from 1722.46 mbsf has an age unlike those in the sepiolite, and potentially is genuinely xenocrystic. The majority of U1437 zircons thus crystallized from evolved melts lacking continental characteristics, although thermal and compositional conditions conducive for zircon crystallization appear to have been rarely attained.

Combined garnet and zircon geochronology of the ultra-high temperature metamorphism: Constraints on the rise of the Orlica-Śnieżnik Dome, NE Bohemian Massif, SW Poland

NASA Astrophysics Data System (ADS)

Walczak, Katarzyna; Anczkiewicz, Robert; Szczepański, Jacek; Rubatto, Daniela; Košler, Jan

2017-11-01

Garnet and zircon geochronology combined with trace element partitioning and petrological studies provide tight constraints on evolution of the UHT-(U)HP terrain of the Orlica-Śnieżnik Dome (OSD) in the NE Bohemian massif. Lu-Hf dating of peritectic garnet from two mesocratic granulites constrained the time of its initial growth at 346.9 ± 1.2 and 348.3 ± 2.0 Ma recording peak 2.5 GPa pressure and 950 °C temperature. In situ, U-Pb SHRIMP dating of zircon from the same granulite gave a younger age of 341.9 ± 3.4 Ma. Ti-in-zircon thermometry indicates crystallization at 810-860 °C pointing to zircon formation on the retrograde path. Lu partitioning between garnet rim and zircon suggest equilibrium growth and thus U-Pb zircon age constrain the terminal phase of garnet crystallization which lasted about 6 Ma. All Sm-Nd garnet ages obtained for mesocratic and mafic granulites are identical and consistently younger than the corresponding Lu-Hf dates. They are interpreted as reflecting cooling of granulites through the Sm-Nd closure temperature at about 337 Ma. The estimated PTt path documents the ca. 10 Ma evolution cycle of the OSD characterized by two distinct periods: (1) 347 - > 342 Ma period corresponds to nearly isothermal decompression resulting from crustal scale folding and vertical extrusion of granulites, and (2) at > 342-337 Ma which corresponds to a fast, nearly isobaric cooling.

Observations and controls on the occurrence of inherited zircon in Concord-type granitoids, New Hampshire

USGS Publications Warehouse

Harrison, T.M.; Aleinikoff, J.N.; Compston, W.

1987-01-01

U-Pb analyses of zircons separated from two Concord-type plutons near Sunapee and Dixville Notch, New Hampshire, reveal differences in the pattern and magnitude of zircon inheritance which are related to differences in melt chemistry. The Sunapee pluton contains only slightly more Zr than required to saturate the melt at the peak temperature of 700 ?? 30??C. Traces of inherited zircon in this separate are inferred to be present as small, largely resorbed grains. In contrast, the Long Mountain pluton, near Dixville Notch, contains about 240% more Zr than required to saturate the melt. Thus, more than half of the Zr existed as stable, inherited zircon crystals during the partial fusion event, consistent with the observation of substantial inheritance in all grain size fractions. Ion probe intra-grain analyses of zircon from the Long Mountain pluton indicate a complex pattern of inheritance with contributions from at least two Proterozoic terrenes and caution against simple interpretations of upper and lower intercepts of chords containing an inherited component. Ion probe analyses of zircons from the Sunapee pluton reveal clear evidence of U loss which results in incorrect apparent conventional U-Pb ages. Ages of crystallization for the Long Mountain and Sunapee pluton are ~350 and 354 ?? 5 Ma, respectively. A Sm/Nd measurement for the Long Mountain pluton yields a depleted mantle model age of 1.5 Ga, consistent with the observed inheritance pattern. In contrast, a Sm/Nd model age for the Sunapee pluton is improbably old due to minor monazite fractionation. ?? 1987.

Unraveling the Switch from Subduction to Exhumation within a Collisional Orogen: Split-stream U-Pb and Trace-element Results from the Western Gneiss Region, Norway (Invited)

NASA Astrophysics Data System (ADS)

Gordon, S. M.; Whitney, D. L.; Teyssier, C. P.; Fossen, H.; Desormeau, J. W.; Jessen, B.

2013-12-01

During continental collision, crustal material may be subducted to great depths and subsequently exhumed. Ultrahigh-pressure (UHP) terranes preserve a record of the subduction of crustal material during suturing of colliding continents and the exhumation of this material during extension and, in some cases, collapse of the orogen. The UHP rocks of the Western Gneiss Region (WGR), Norway, resulted from the collision of Baltica with Laurentia during the final stages of the Caledonian orogeny. The WGR represents one of the two largest UHP terranes on Earth and consists of a UHP eclogite-bearing domain south of the Møre-Trøndelag strike-slip fault and a HP mafic granulite-bearing domain north of the fault. At least some of the HP granulite is overprinted eclogite. To evaluate the metamorphic and structural relationship of mafic rocks and associated migmatite in both regions, we obtained LA-ICP-MS U-Pb dates and trace-element analyses for zircon from a variety of textural types of leucosome associated with mafic layers and lenses. Five leucosomes within highly deformed migmatite in the HP granulite complex on the Roan Peninsula reveal U-Pb lower-intercept ages from ca. 405 to 409 Ma and upper-intercept Proterozoic dates. These zircons have distinct trace-elements patterns: all of the zircons that yield Proterozoic dates have overall much higher REE concentrations, a more significant negative Eu anomaly (-0.3 to -0.7) and steeper HREE patterns (Lu/Dy = 5-12). In comparison, the Caledonian zircons reveal flatter Eu anomalies (-0.3 to 0.2) and less steep HREE patterns (Lu/Dy = 2-7), although the individual patterns do not seem to correlate with age. The Caledonian zircon patterns suggest crystallization at high-pressures and are distinct from the inherited Proterozoic grains. Similar results were obtained from zircon rims extracted from layer-parallel to crosscutting leucosomes from the UHP domain. Trace elements in zircon in these samples record the transition from high-pressure (garnet-present, plagioclase-absent) crystallization to lower-pressure (plagioclase-present) crystallization with garnet-present × plagioclase-absent REE patterns. Moreover, dates from the layer-parallel leucosomes are as old as 410-406 Ma. The new U-Pb dates suggest a similar melt crystallization history that was coeval with previously determined ages of (U)HP metamorphism of WGR eclogite. The More-Trondelag fault acted as a transform fault and accommodated coeval extension that exhumed both the (U)HP and granulite domains. Results are consistent with the presence of partially molten crust in a large part of the WGR at HP or UHP conditions during the latest stages of the Caledonian orogeny. The decreased viscosity and increased buoyancy and strain weakening induced by partial melting may have triggered or at least contributed to the switch from subduction to exhumation in the WGR, marking the end of collisional orogeny.

Geochemistry, thermometry and isotope ratios on the same zircon crystals: the tandem use of quadrupole LA-ICPMS and CA-TIMS

NASA Astrophysics Data System (ADS)

Olin, P. H.; Schmitz, M. D.; Crowley, J. L.

2011-12-01

Current trends in igneous petrology include the extraction of diverse geochemical information from smaller sample targets by ever more efficient and cost effective means. Igneous zircons are repositories of several types of petrogenetic information, such as magmatic crystallization ages obtained using U-Pb geochronology, magmatic temperatures using Ti-in-zircon geothermometry, and magmatic differentiation and/or mixing trends using trace element contents. Here we demonstrate a tandem quadrupole LA-ICPMS and CA-TIMS approach on single zircon crystals and within domains in single crystals, which extracts all of these data from a single laser spot analysis and then guides the acquisition of CA-TIMS ages at precisions relevant to magmatic histories. We present data from zircon-bearing intrusive and extrusive rocks spanning the compositional spectrum, and highlight results from silicic volcanic rocks with different affinities. The utility of our approach is illustrated in zircons from the Temora diorite, a commonly used standard material which we analysed using 25-μm ablation spots placed on dozens of grains which had been previously annealed and chemically abraded prior to mounting in epoxy. Our LA-ICPMS results illustrate a 3- to 5-fold variation in trace element concentrations and trace element ratios over >150 degrees of cooling as estimated from Ti-in-zircon thermometry. Some geochemical parameters (e.g., Nb/Ta variations and Eu anomalies) are consistent with crystal fractionation during progressive crystallization, while others are bimodal (e.g., Hf and U contents), suggesting the mixing of crystal/magma batches prior to final solidification. LA-ICPMS U-Pb spot ages reproduce the accepted CA-TIMS age within 2% precision and accuracy, while our CA-TIMS results on the same grains constrain the development of the observed geochemical variability to within 100 ka. Other zircon standard materials to be presented include Plesovich syenite, FC1 gabbro, and R33 diorite. Analyses of zircons from selected western Snake River Plain silicic volcanic units further demonstrate the capability of our approach. These units are targeted with the overarching goal of better understanding magmatism in the region and to identify geochemical fingerprints to better distinguish among and to correlate between units. Several rhyolites distributed along the northern margin of the plain have LA-ICPMS ages of ca. 11 Ma, within error of each other and their CA-TIMS ages, and reveal differences in temperatures of crystallization and trace element contents and ratios. Individual units have crystallization temperatures that span 100 degrees or more, and show correlations with whole-rock major and trace element contents and ratios. In many cases, zircon geochemical parameters such as REE and Y contents, and Nb/Ta and Th/U ratios allow units proximal to each other to be distinguished from one another while also providing fingerprints to correlate to distal units on the south side of the plain or elsewhere in the province.

Zircon geochronology and ca. 400 Ma exhumation of Norwegian ultrahigh-pressure rocks: An ion microprobe and chemical abrasion study

USGS Publications Warehouse

Root, D.B.; Hacker, B.R.; Mattinson, J.M.; Wooden, J.L.

2004-01-01

Understanding the formation and exhumation of the remarkable ultrahigh-pressure (UHP) rocks of the Western Gneiss Region, Norway, hinges on precise determination of the time of eclogite recrystallization. We conducted detailed thermal ionization mass spectrometry, chemical abrasion analysis and sensitive high-resolution ion-microprobe analysis of zircons from four ultrahigh- and high-pressure (HP) rocks. Ion-microprobe analyses from the Flatraket eclogite yielded a broad range of apparently concordant Caledonian ages, suggesting long-term growth. In contrast, higher precision thermal ionization mass spectrometry analysis of zircon subject to combined thermal annealing and multi-step chemical abrasion yielded moderate Pb loss from the first (lowest temperature) abrasion step, possible minor Pb loss or minor growth at 400 Ma from the second step and a 407-404 Ma cluster of slightly discordant 206Pb/238U ages, most likely free from Pb loss, from the remaining abrasion steps. We interpret the latter to reflect zircon crystallization at ???405-400 Ma with minor discordance from inherited cores. Zircon crystallization occurred at eclogite-facies, possibly post-peak conditions, based on compositions of garnet inclusions in zircon as well as nearly flat HREE profiles and lack of Eu anomalies in zircon fractions subjected to chemical abrasion. These ages are significantly younger than the 425 Ma age often cited for western Norway eclogite recrystallization, implying faster rates of exhumation (>2.5-8.5 km/Myr), and coeval formation of eclogites across the UHP portion of the Western Gneiss Region. ?? 2004 Published by Elsevier B.V.

Evaluating the mush extraction + multiple magma batch model for the Lake City magmatic system (Colorado, USA) using zircon U/Pb TIMS-TEA

NASA Astrophysics Data System (ADS)

Pamukcu, A. S.; Schoene, B.; Deering, C. D.

2016-12-01

Volcanic eruptions that involve a wide range of magma types highlight questions on genetic and geometric relationships between magmas in the crust prior to eruption. The Lake City magmatic system (Colorado, USA) is one such example: exposed in the caldera are ignimbrites from the 23 Ma Sunshine Peak Tuff, which range in composition and crystallinity with time (crystal-poor rhyolite to crystal-rich trachyte), and resurgent intrusions of porphyritic syenite, monzonite, and dacite (Hon 1987). Field relations and bulk rock geochemistry suggest the Lake City magmatic system was complex, with magmas of these various types existing concurrently as multiple magma batches, though not necessarily always in contact (Kennedy et al. 2015). Geochemical modeling further suggests that the crystal-poor rhyolites were liquids extracted from a syenitic mush and that the crystal-rich trachytes are remobilized portions of this cumulate. To address the genetic and geometric links between these magmas in more detail, we utilize TIMS-TEA to assess U/Pb zircon geochronology and trace element geochemistry in concert. For each eruptive unit/magma type, zircons were roughly separated into size groupings (small, medium, large), imaged by cathodoluminescence (CL), and analyzed individually by CA-ID-TIMS. Preliminary results indicate that zircons crystallized over a period of 177±31 ky, which is within the range suggested by Ar/Ar geochronology (80-300 ky, Bove et al. 2001). Consistent with the current model for the Lake City system, zircons from the rhyolites and trachytes overlap in age, while those of the dacites are younger. There is no clear relationship between age and CL zoning pattern or crystal size (e.g., small crystals are not always the youngest). We can further address relationships between the rhyolite, trachyte, and syenite using TEA to assess trace elements of the dated zircons. Rhyolite-MELTS models suggest that zircons crystallized in a rhyolitic melt derived from the trachyte/syenite will have similar LREEs and MREEs but higher HREEs than those that crystallized in the mush. In addition, if the trachytes are remobilized cumulates that are represented by the post-caldera syenites, we expect that trace element contents of zircon from these units will be largely similar.

The origin of zircon and the significance of U-Pb ages in high-grade metamorphic rocks: a case study from the Variscan orogenic root (Vosges Mountains, NE France)

NASA Astrophysics Data System (ADS)

Skrzypek, E.; Štípská, P.; Cocherie, A.

2012-12-01

U-Pb zircon dating is combined with petrology, Zr-in-rutile thermometry and mineral equilibria modelling to discuss zircon petrogenesis and the age of metamorphism in three units of the Variscan Vosges Mountains (NE France). The monotonous gneiss unit shows results at 700-500 Ma, but no Variscan ages. The varied gneiss unit preserves ages between 600 and 460 Ma and a Variscan group at 340-335 Ma. Zircon analyses from the felsic granulite unit define a continuous array of ages between 500 and 340 Ma. In varied gneiss samples, zoned garnet includes kyanite and rutile and is surrounded by matrix sillimanite and cordierite. In a pseudosection, it points to peak conditions of ~16 kbar/850 °C followed by isothermal decompression to 8-10 kbar/820-860 °C. In felsic granulite samples, the assemblage K-feldspar-garnet-kyanite-Zr-rich rutile is replaced by sillimanite and Zr-poor rutile. Modelling these assemblages supports minimum conditions of ~13 kbar/925 °C, and a subsequent P-T decrease to 6.5-8.5 kbar/800-820 °C. The internal structure and chemistry of zircons, and modelling of zircon dissolution/growth along the inferred P-T paths are used to discuss the significance of the U-Pb ages. In the monotonous unit, inherited zircon ages of 700-500 Ma point to sedimentation during the Late Cambrian, while medium-grade metamorphism did not allow the formation of Variscan zircon domains. In both the varied gneiss and felsic granulite units, zircons with a blurred oscillatory-zoned pattern could reflect solid-state recrystallization of older grains during HT metamorphism, whereas zircons with a dark cathodoluminescence pattern are thought to derive from crystallization of an anatectic melt during cooling at middle pressure conditions. The present work proposes that U-Pb zircon ages of ca. 340 Ma probably reflect the end of a widespread HT metamorphic event at middle crustal level.

Baddeleyite in PGE paleoproterozoic layered intrusions on Fennoscandian Shield (Arctic region): significance for timing, duration and continental reconstraction

NASA Astrophysics Data System (ADS)

Bayanova, Tamara; Korchagin, Aleksey; Chachshin, Viktor; Nerovich, Ludmila; Drogobuzhskaya, Svetlana

2017-04-01

Baddeleyite was firstly found and U-Pb dating in PGE layered intrusions of the Fennoscandian Shield in the rock-forming orthopyroxene (Lukkulaisvaara intrusion in Karelia region) and in magmatic zircon from gabbronorite Mt. Generalskaya (Kola region). Real crystals of baddeleyite were separated and U-Pb measured from Fedorovo-Pansky complex in gabbronorites lower part of the Pt-Pd reef intrusion (as first phase 2.50 Ga) and in upper part of Pt-Pd reef in anorthosites (second phase -2.45 Ga) and reflect time interval about 50 Ma of magmatic complex activity. In basite dykes from Cr-Ti-V Imandra lopolith baddeleyite were dating by U-Pb with 2.40 Ga. Therefore total duration time of Kola LIP and magmatic origin of the multimetal deposits are estimated as 100 Ma [1]. New additional isotope Nd-Sr-He data for the WR of the layered PGE intrusions in the Kola-Karelia-Finland big belt more than 500 km reflect EM-1 mantle reservoir. New REE (ELAN- 9000) distributions in the WR and dykes complexes of the Fedorovo-Pansky and Monshegorck Cu-Ni and PGE ore deposits gave OIB, N-MORB and E-MORB primary plume mantle source due to Re-Os data [2]. LA-ICP-MS data of REE investigations in baddeleyite crystals from Monchegorsk ore region yielded 1000 C forming of the grains and high U-Pb closure temperatures compared with zircon. Baddeleyite also primary magmatic minerals in the layered PGE intrusions and dykes complexes from Fennoscandian Shield and U-Pb precise data using artificial 205 Pb spike of the crystals together with time data for different continents gave new important information concerning break up and super continental reconstruction of geological history in paleoproterozoic time [3]. Acknowledgements: Many thanks to G.Wasserburg for 205 Pb artificial spike, J. Ludden, F. Corfu, V. Todt and U. Poller for assistance in the establishing of the U-Pb for single zircon and baddeleyite. All studies are supported by RFBR 16-05-00305. All investigations are devoted to memory of academician RAS F.P. Mitrofanov due to whom baddeleyite was found, separation and studied by U-Pb (ID-TIMS and LA-ICP-MS) methods. References: [1] Bayanova et al. (2014), INTECH, 143-193; [2] Yang Sheng-Hong et. al. (2016), Mineralium Deposita 51, 1055-1073; [3] Ernst R. Large igneous provinces. (2016), London, 500

Chemical abrasion-SIMS (CA-SIMS) U-Pb dating of zircon from the late Eocene Caetano caldera, Nevada

USGS Publications Warehouse

Watts, Kathryn E.; Coble, Matthew A.; Vazquez, Jorge A.; Henry, Christopher D.; Colgan, Joseph P.; John, David A.

2016-01-01

Zircon geochronology is a critical tool for establishing geologic ages and time scales of processes in the Earth's crust. However, for zircons compromised by open system behavior, achieving robust dates can be difficult. Chemical abrasion (CA) is a routine step prior to thermal ionization mass spectrometry (TIMS) dating of zircon to remove radiation-damaged parts of grains that may have experienced open system behavior and loss of radiogenic Pb. While this technique has been shown to improve the accuracy and precision of TIMS dating, its application to high-spatial resolution dating methods, such as secondary ion mass spectrometry (SIMS), is relatively uncommon. In our efforts to U-Pb date zircons from the late Eocene Caetano caldera by SIMS (SHRIMP-RG: sensitive high resolution ion microprobe, reverse geometry), some grains yielded anomalously young U-Pb ages that implicated Pb-loss and motivated us to investigate with a comparative CA and non-CA dating study. We present CA and non-CA 206Pb/238U ages and trace elements determined by SHRIMP-RG for zircons from three Caetano samples (Caetano Tuff, Redrock Canyon porphyry, and a silicic ring-fracture intrusion) and for R33 and TEMORA-2 reference zircons. We find that non-CA Caetano zircons have weighted mean or bimodal U-Pb ages that are 2–4% younger than CA zircons for the same samples. CA Caetano zircons have mean U-Pb ages that are 0.4–0.6 Myr older than the 40Ar/39Ar sanidine eruption age (34.00 ± 0.03 Ma; error-weighted mean, 2σ), whereas non-CA zircons have ages that are 0.7–1.3 Myr younger. U-Pb ages do not correlate with U (~ 100–800 ppm), Th (~ 50–300 ppm) or any other measured zircon trace elements (Y, Hf, REE), and CA and non-CA Caetano zircons define identical trace element ranges. No statistically significant difference in U-Pb age is observed for CA versus non-CA R33 or TEMORA-2 zircons. Optical profiler measurements of ion microprobe pits demonstrate consistent depths of ~ 1.6 μm for CA and non-CA Caetano, R33 and TEMORA-2 zircons, and do not indicate variations in secondary ion sputtering rates due to chemical or structural changes from the CA treatment. Our new data underscore the potential for cryptic Pb-loss to go unrecognized in other geologically young magmatic centers that do not have zircons with high U, statistically discordant isotope ratios, high common Pb, or metamict textures.

Re-evaluation of the origin and evolution of > 4.2 Ga zircons from the Jack Hills metasedimentary rocks

NASA Astrophysics Data System (ADS)

Nemchin, A. A.; Pidgeon, R. T.; Whitehouse, M. J.

2006-04-01

New data are presented on internal structures, U-Pb systematics and oxygen isotope compositions of eight detrital zircons with ages greater than 4.2 Ga, from the Jack Hills metasedimentary belt, Australia. Cathodoluminescence imaging, ion-microprobe U-Pb and oxygen isotope results show evidence for an extensive period of complex zircon growth, secondary reaction and U-Pb isotopic disturbance from 4.36 to 3.90 Ga. In addition many of the zircons have discordant U-Pb systems and excess common Pb indicating a superimposed, relatively recent, reaction between radiation damaged zircon and low temperature fluids. The significance of oxygen isotope compositions for zircons with complex internal structures and U-Pb systems is complicated by uncertainty in the origin of the grains and the unknown effect of later reactions. However, a minority of grains with sharp oscillatory zoning, uniform and concordant U-Pb systems, igneous Th-U ratios and low common Pb contents, are interpreted as undisturbed primary magmatic zircons. The oldest identified, oscillatory zoned, magmatic grain, with an age 4363 ± 20 Ma, is one of a few reported magmatic grains with this age, which is interpreted as the oldest reliable age for Hadean magmatic zircons. Mantle δ18O values are reported for these zircons. Younger oscillatory zoned zircon, including oscillatory zoned cores in complex grains, have δ18O values lower than 6.5‰, which are within the range of ion microprobe analysed δ18O values for zircons in high temperature equilibrium with the normal mantle rocks of 5.3 ± 0.6‰ (2 standard deviations). These values are also within the range of δ18O values found in lunar zircons. The absence of heavy oxygen in the grains that can be interpreted as primary magmatic zircons and the complex history over the period from 4.36 to 3.9 Ga, seen in all other Jack Hills zircons and reflected in the internal structures and U-Pb isotopic systems, questions the model for the early Earth involving long intervals of relatively temperate conditions from 4.4 to 4.0 Ga that were conducive to oceans and possibly life.

Mid-Neoproterozoic intraplate magmatism in the northern margin of the Southern Granulite Terrane, India: Constraints from geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes

NASA Astrophysics Data System (ADS)

Deeju, T. R.; Santosh, M.; Yang, Qiong-Yan; Pradeepkumar, A. P.; Shaji, E.

2016-11-01

The northern margin of the Southern Granulite Terrane in India hosts a number of mafic, felsic and alkaline magmatic suites proximal to major shear/paleo-suture zones and mostly represents magmatism in rift-settings. Here we investigate a suite of gabbros and granite together with intermediate (dioritic) units generated through mixing and mingling of a bimodal magmatic suite. The massive gabbro exposures represent the cumulate fraction of a basic magma whereas the granitoids represent the product of crystallization in felsic magma chambers generated through crustal melting. Diorites and dioritic gabbros mostly occur as enclaves and lenses within host granitoids resembling mafic magmatic enclaves. Geochemistry of the felsic units shows volcanic arc granite and syn-collisional granite affinity. The gabbro samples show mixed E-MORB signature and the magma might have been generated in a rift setting. The trace and REE features of the rocks show variable features of subduction zone enrichment, crustal contamination and within plate enrichment, typical of intraplate magmatism involving the melting of source components derived from both depleted mantle sources and crustal components derived from older subduction events. The zircons in all the rock types show magmatic crystallization features and high Th/U values. Their U-Pb data are concordant with no major Pb loss. The gabbroic suite yields 206Pb/238U weighted mean ages in the range of 715 ± 4-832.5 ± 5 Ma marking a major phase of mid Neoproterozoic magmatism. The diorites crystallized during 206Pb/238U weighted mean age of 724 ± 6-830 ± 2 Ma. Zircons in the granite yield 206Pb/238U weighted mean age of 823 ± 4 Ma. The age data show broadly similar age ranges for the mafic, intermediate and felsic rocks and indicate a major phase of bi-modal magmatism during mid Neoproterozoic. The zircons studied show both positive and negative εHf(t) values for the gabbros (-6.4 to 12.4), and negative values for the diorites (-7.8 to -16.7) and granite (-16.6 to -6.7). Together with the Hf depleted model ages and crustal model ages, we infer that the magma sources involved both juvenile depleted mantle and reworked Mesoproterozoic, Paleoproterozoic and Neoarchean components. The mid Neoproterozoic intraplate magmatism is considered to be a response to mantle upwelling in an aborted rift setting.

Zircon crystallization and recycling in the magma chamber of the rhyolitic Kos Plateau Tuff (Aegean arc)

USGS Publications Warehouse

Bachman, O.; Charlier, B.L.A.; Lowenstern, J. B.

2007-01-01

In contrast to most large-volume silicic magmas in continental arcs, which are thought to evolve as open systems with significant assimilation of preexisting crust, the Kos Plateau Tuff magma formed dominantly by crystal fractionation of mafic parents. Deposits from this ~60 km3 pyroclastic eruption (the largest known in the Aegean arc) lack xenocrystic zircons [secondary ion mass spectrometry (SIMS) U-Pb ages on zircon cores never older than 500 ka] and display Sr-Nd whole-rock isotopic ratios within the range of European mantle in an area with exposed Paleozoic and Tertiary continental crust; this evidence implies a nearly closed-system chemical differentiation. Consequently, the age range provided by zircon SIMS U-Th-Pb dating is a reliable indicator of the duration of assembly and longevity of the silicic magma body above its solidus. The age distribution from 160 ka (age of eruption by sanidine 40Ar/39Ar dating; Smith et al., 1996) to ca. 500 ka combined with textural characteristics (high crystal content, corrosion of most anhydrous phenocrysts, but stability of hydrous phases) suggest (1) a protracted residence in the crust as a crystal mush and (2) rejuvenation (reduced crystallization and even partial resorption of minerals) prior to eruption probably induced by new influx of heat (and volatiles). This extended evolution chemically isolated from the surrounding crust is a likely consequence of the regional geodynamics because the thinned Aegean microplate acts as a refractory container for magmas in the dying Aegean subduction zone (continent-continent subduction).

Oxygen isotopic composition and U-Pb discordance in zircon

USGS Publications Warehouse

Booth, A.L.; Kolodny, Y.; Chamberlain, C.P.; McWilliams, M.; Schmitt, A.K.; Wooden, J.

2005-01-01

We have investigated U-Pb discordance and oxygen isotopic composition of zircon using high-spatial resolution ??18O measurement by ion microprobe. ??18O in both concordant and discordant zircon grains provides an indication of the relationship between fluid interaction and discordance. Our results suggest that three characteristics of zircon are interrelated: (1) U-Pb systematics and concomitant age discordance, (2) ??18O and the water-rock interactions implied therein, and (3) zircon texture, as revealed by cathodoluminescence and BSE imaging. A key observation is that U-Pb-disturbed zircons are often also variably depleted in 18O, but the relationship between discordance and ??18O is not systematic. ??18O values of discordant zircons are generally lighter but irregular in their distribution. Textural differences between zircon grains can be correlated with both U-Pb discordance and ??18O. Discordant grains exhibit either a recrystallized, fractured, or strongly zoned CL texture, and are characteristic of 18O depletion. We interpret this to be a result of metamictization, leading to destruction of the zircon lattice and an increased susceptibility to lead loss. Conversely, grains that are concordant have less-expressed zoning and a smoother CL texture and are enriched in 18O. From this it is apparent that various stages of water-rock interaction, as evidenced by systematic variations in ??18O, leave their imprint on both the texture and U-Pb systematics of zircon. Copyright ?? 2005 Elsevier Ltd.

Geochemical signatures and magmatic stability of terrestrial impact produced zircon

NASA Astrophysics Data System (ADS)

Wielicki, Matthew M.; Harrison, T. Mark; Schmitt, Axel K.

2012-03-01

Understanding the role of impacts on early Earth has major implications to near surface conditions, but the apparent lack of preserved terrestrial craters > 2 Ga does not allow a direct sampling of such events. Ion microprobe U-Pb ages, REE abundances and Ti-in-zircon thermometry for impact produced zircon are reported here. These results from terrestrial impactites, ranging in age from ~ 35 Ma to ~ 2 Ga, are compared with the detrital Hadean zircon population from Western Australia. Such comparisons may provide the only terrestrial constraints on the role of impacts during the Hadean and early Archean, a time predicted to have a high bolide flux. Ti-in-zircon thermometry indicates an average of 773 °C for impact-produced zircon, ~ 100 °C higher than the average for Hadean zircon crystals. The agreement between whole-rock based zircon saturation temperatures for impactites and Ti-in-zircon thermometry (at aTiO2 = 1) implies that Ti-in-zircon thermometry record actual crystallization temperatures for impact melts. Zircon saturation modeling of Archean crustal rock compositions undergoing thermal excursions associated with the Late Heavy Bombardment predicts equally high zircon crystallization temperatures. The lack of such thermal signatures in the Hadean zircon record implies that impacts were not a dominant mechanism of producing the preserved Hadean detrital zircon record.

Ancient Pb and Ti mobilization revealed by Scanning Ion Imaging

NASA Astrophysics Data System (ADS)

Kusiak, Monika A.; Whitehouse, Martin J.; Wilde, Simon A.

2014-05-01

Zircons from strongly layered early Archean ortho- and paragneisses in ultra-high temperature (UHT) metamorphic rocks of the Napier Complex, Enderby Land, East Antarctica are characterized by complex U-Th-Pb systematics [1,2,3]. A large number of zircons from three samples, Gage Ridge, Mount Sones and Dallwitz Nunatak, are reversely discordant (U/Pb ages older than 207Pb/206Pb ages) with the oldest date of 3.9 Ga [4] (for the grain from Gage Ridge orthogneiss). To further investigate this process, we utilized a novel high spatial resolution Scanning Ion Imaging technique on the CAMECA IMS 1280 at the Natural History Museum in Stockholm. Areas of 70 μm x 70 μm were selected for imaging in mono- and multicollection modes using a ~2 μm rastered primary beam to map out the distribution of 48Ti, 89Y, 180Hf, 232Th, 238U, 204Pb, 206Pb and 207Pb. The ion maps reveal variable distribution of certain elements within analysed grains that can be compared to their CL response. Yttrium, together with U and Th, exhibits zonation visible on the CL images, Hf shows expected minimal variation. Unusual patchiness is visible in the map for Ti and Pb distribution. The bright patches with enhanced signal do not correspond to any zones or to crystal imperfections (e.g. cracks). The presence of patchy titanium is likely to affect Ti-in-zircon thermometry, and patchy Pb affecting 207Pb/206Pb ages, usually considered as more robust for Archean zircons. Using the WinImage program, we produced 207Pb/206Pb ratio maps that allow calculation of 207Pb/206Pb ages for spots of any size within the frame of the picture and at any time after data collection. This provides a new and unique method for obtaining age information from zircon. These maps show areas of enhanced brightness where the 207Pb/206Pb ratio is higher and demonstrate that within these small areas (μm scale) the apparent 207Pb/206Pb age is older, in some of these patches even > 4 Ga. These data are a result of ancient Pb mobilization, which is independent of the degree of metamictisation, oxygen isotope and REE content of the zircons [5]. The Antarctic zircons experienced ancient Ti and Pb mobilization and redistribution, most likely caused by polyphase metamorphism at ~2.8 Ga and ~2.5 Ga, the latter documented as reaching temperatures of >1100ºC [5]. References: [1] Williams et al., 1984, Contr. Min. Petrol. 88, 322-327. [2] Black et al., (1986), Contr. Min. Petrol. 94, 427-437. [3] Harley & Kelly, 2007, Earth's Oldest Rocks: Developments in Precambrian Geology 15, 149-186. [4] Kusiak et al., 2013, Geology 41, 291-294. [5] Kusiak et al., 2013, American J. of Sci. 313, 933-967. [5] Hokada et al., 2004, Contr. Min. Petrol. 147, 1-20.

Brittle-ductile deformation effects on zircon crystal-chemistry and U-Pb ages: an example from the Finero Mafic Complex (Ivrea-Verbano Zone, western Alps)

NASA Astrophysics Data System (ADS)

Langone, Antonio; José Alberto, Padrón-Navarta; Zanetti, Alberto; Mazzucchelli, Maurizio; Tiepolo, Massimo; Giovanardi, Tommaso; Bonazzi, Mattia

2016-04-01

A detailed structural, geochemical and geochronological survey was performed on zircon grains from a leucocratic dioritic dyke discordantly intruded within meta-diorites/gabbros forming the External Gabbro unit of the Finero Mafic Complex. This latter is nowadays exposed as part of a near complete crustal section spanning from mantle rocks to upper crustal metasediments (Val Cannobina, Ivrea-Verbano Zone, Italy). The leucocratic dyke consists mainly of plagioclase (An18-24Ab79-82Or0.3-0.7) with subordinate amounts of biotite, spinel, zircon and corundum. Both the leucocratic dyke and the surrounding meta-diorites show evidence of ductile deformation occurred under amphibolite-facies conditions. Zircon grains (up to 2 mm in length) occur mainly as euhedral grains surrounded by fine grained plagioclase-dominated matrix and pressure shadows, typically filled by oxides. Fractures and cracks within zircon are common and can be associated with grain displacement or they can be filled by secondary minerals (oxides and chlorite). Cathodoluminescence (CL) images show that zircon grains have internal features typical of magmatic growth, but with local disturbances. However EBSD maps on two selected zircon grains revealed a profuse mosaic texture resulting in an internal misorientation of ca. 10o. The majority of the domains of the mosaic texture are related to parting and fractures, but some domains show no clear relation with brittle features. Rotation angles related to the mosaic texture are not crystallographically controlled. In addition, one of the analysed zircons shows clear evidence of plastic deformation at one of its corners due to indentation. Plastic deformation results in gradual misorientations of up to 12o, which are crystallographically controlled. Trace elements and U-Pb analyses were carried out by LA-ICP-MS directly on petrographic thin sections and designed to cover the entire exposed surface of selected grains. Such investigations revealed a strong correlation between internal zircon structures, chemistry, U-Pb isotope ratios and mylonitic fabric. U-Pb data return highly discordant and variable ages: in particular, the 206Pb/238U ages range from Carboniferous to Triassic within the same zircon grain. The youngest 206Pb/238U data derive from narrow axial stripes oriented parallel or at low angle with respect to the foliation planes. These stripes are characterized by an overall HREE, Y, U and Th enrichment possibly reflecting deformation of the grain in presence of interstitial fluid phases, likely related to a concomitant magmatic activity. Deformation related structures (cracks and fractures) within zircon grains acted as fast-diffusion pathways allowing fluids to modify the geochemistry and isotopic systems of zircon. Our results suggest that fluid-assisted brittle-ductile deformation can severely modify the trace elements and isotopic composition of zircon with unexpected patterns constrained by stress regime. In similar cases, our observations suggest that, for a more appropriate interpretation of the petrologic evolution and age variability, a direct characterization of the internal structures of zircons still placed in their microtextural site is highly recommended.

Decoding a protracted zircon geochronological record in ultrahigh temperature granulite, and persistence of partial melting in the crust, Rogaland, Norway

NASA Astrophysics Data System (ADS)

Laurent, Antonin T.; Bingen, Bernard; Duchene, Stephanie; Whitehouse, Martin J.; Seydoux-Guillaume, Anne-magali; Bosse, Valerie

2018-04-01

This contribution evaluates the relation between protracted zircon geochronological signal and protracted crustal melting in the course of polyphase high to ultrahigh temperature (UHT; T > 900 °C) granulite facies metamorphism. New U-Pb, oxygen isotope, trace element, ion imaging and cathodoluminescence (CL) imaging data in zircon are reported from five samples from Rogaland, South Norway. The data reveal that the spread of apparent age captured by zircon, between 1040 and 930 Ma, results both from open-system growth and closed-system post-crystallization disturbance. Post-crystallization disturbance is evidenced by inverse age zoning induced by solid-state recrystallization of metamict cores that received an alpha dose above 35 × 1017 α g-1. Zircon neocrystallization is documented by CL-dark domains displaying O isotope open-system behaviour. In UHT samples, O isotopic ratios are homogenous (δ18O = 8.91 ± 0.08‰), pointing to high-temperature diffusion. Scanning ion imaging of these CL-dark domains did not reveal unsupported radiogenic Pb. The continuous geochronological signal retrieved from the CL-dark zircon in UHT samples is similar to that of monazite for the two recognized metamorphic phases (M1: 1040-990 Ma; M2: 940-930 Ma). A specific zircon-forming event is identified in the orthopyroxene and UHT zone with a probability peak at ca. 975 Ma, lasting until ca. 955 Ma. Coupling U-Pb geochronology and Ti-in-zircon thermometry provides firm evidence of protracted melting lasting up to 110 My (1040-930 Ma) in the UHT zone, 85 My (ca. 1040-955 Ma) in the orthopyroxene zone and some 40 My (ca. 1040-1000 Ma) in the regional basement. These results demonstrate the persistence of melt over long timescales in the crust, punctuated by two UHT incursions.

Cooling rates and the depth of detachment faulting at oceanic core complexes: Evidence from zircon Pb/U and (U-Th)/He ages

USGS Publications Warehouse

Grimes, Craig B.; Cheadle, Michael J.; John, Barbara E.; Reiners, P.W.; Wooden, J.L.

2011-01-01

Oceanic detachment faulting represents a distinct mode of seafloor spreading at slow spreading mid-ocean ridges, but many questions persist about the thermal evolution and depth of faulting. We present new Pb/U and (U-Th)/He zircon ages and combine them with magnetic anomaly ages to define the cooling histories of gabbroic crust exposed by oceanic detachment faults at three sites along the Mid-Atlantic Ridge (Ocean Drilling Program (ODP) holes 1270D and 1275D near the 15??20???N Transform, and Atlantis Massif at 30??N). Closure temperatures for the Pb/U (???800??C-850??C) and (U-Th)/He (???210??C) isotopic systems in zircon bracket acquisition of magnetic remanence, collectively providing a temperature-time history during faulting. Results indicate cooling to ???200??C in 0.3-0.5 Myr after zircon crystallization, recording time-averaged cooling rates of ???1000??C- 2000??C/Myr. Assuming the footwalls were denuded along single continuous faults, differences in Pb/U and (U-Th)/He zircon ages together with independently determined slip rates allow the distance between the ???850??C and ???200??C isotherms along the fault plane to be estimated. Calculated distances are 8.4 ?? 4.2 km and 5.0 2.1 km from holes 1275D and 1270D and 8.4 ?? 1.4 km at Atlantis Massif. Estimating an initial subsurface fault dip of 50 and a depth of 1.5 km to the 200??C isotherm leads to the prediction that the ???850??C isotherm lies ???5-7 km below seafloor at the time of faulting. These depth estimates for active fault systems are consistent with depths of microseismicity observed beneath the hypothesized detachment fault at the TAG hydrothermal field and high-temperature fault rocks recovered from many oceanic detachment faults. Copyright 2011 by the American Geophysical Union.

Ti-in-zircon thermometry: applications and limitations

NASA Astrophysics Data System (ADS)

Fu, Bin; Page, F. Zeb; Cavosie, Aaron J.; Fournelle, John; Kita, Noriko T.; Lackey, Jade Star; Wilde, Simon A.; Valley, John W.

2008-08-01

The titanium concentrations of 484 zircons with U-Pb ages of ˜1 Ma to 4.4 Ga were measured by ion microprobe. Samples come from 45 different igneous rocks (365 zircons), as well as zircon megacrysts (84) from kimberlite, Early Archean detrital zircons (32), and zircon reference materials (3). Samples were chosen to represent a large range of igneous rock compositions. Most of the zircons contain less than 20 ppm Ti. Apparent temperatures for zircon crystallization were calculated using the Ti-in-zircon thermometer (Watson et al. 2006, Contrib Mineral Petrol 151:413-433) without making corrections for reduced oxide activities (e.g., TiO2 or SiO2), or variable pressure. Average apparent Ti-in-zircon temperatures range from 500° to 850°C, and are lower than either zircon saturation temperatures (for granitic rocks) or predicted crystallization temperatures of evolved melts (˜15% melt residue for mafic rocks). Temperatures average: 653 ± 124°C (2 standard deviations, 60 zircons) for felsic to intermediate igneous rocks, 758 ± 111°C (261 zircons) for mafic rocks, and 758 ± 98°C (84 zircons) for mantle megacrysts from kimberlite. Individually, the effects of reduced a_{TiO2} or a_{SiO2}, variable pressure, deviations from Henry’s Law, and subsolidus Ti exchange are insufficient to explain the seemingly low temperatures for zircon crystallization in igneous rocks. MELTs calculations show that mafic magmas can evolve to hydrous melts with significantly lower crystallization temperature for the last 10-15% melt residue than that of the main rock. While some magmatic zircons surely form in such late hydrous melts, low apparent temperatures are found in zircons that are included within phenocrysts or glass showing that those zircons are not from evolved residue melts. Intracrystalline variability in Ti concentration, in excess of analytical precision, is observed for nearly all zircons that were analyzed more than once. However, there is no systematic change in Ti content from core to rim, or correlation with zoning, age, U content, Th/U ratio, or concordance in U-Pb age. Thus, it is likely that other variables, in addition to temperature and a_{TiO2}, are important in controlling the Ti content of zircon. The Ti contents of igneous zircons from different rock types worldwide overlap significantly. However, on a more restricted regional scale, apparent Ti-in-zircon temperatures correlate with whole-rock SiO2 and HfO2 for plutonic rocks of the Sierra Nevada batholith, averaging 750°C at 50 wt.% SiO2 and 600°C at 75 wt.%. Among felsic plutons in the Sierra, peraluminous granites average 610 ± 88°C, while metaluminous rocks average 694 ± 94°C. Detrital zircons from the Jack Hills, Western Australia with ages from 4.4 to 4.0 Ga have apparent temperatures of 717 ± 108°C, which are intermediate between values for felsic rocks and those for mafic rocks. Although some mafic zircons have higher Ti content, values for Early Archean detrital zircons from a proposed granitic provenance are similar to zircons from many mafic rocks, including anorthosites from the Adirondack Mts (709 ± 76°C). Furthermore, the Jack Hills zircon apparent Ti-temperatures are significantly higher than measured values for peraluminous granites (610 ± 88°C). Thus the Ti concentration in detrital zircons and apparent Ti-in-zircon temperatures are not sufficient to independently identify parent melt composition.

Timing of global regression and microbial bloom linked with the Permian-Triassic boundary mass extinction: implications for driving mechanisms

NASA Astrophysics Data System (ADS)

Baresel, Bjoern; Bucher, Hugo; Bagherpour, Borhan; Brosse, Morgane; Guodun, Kuang; Schaltegger, Urs

2017-04-01

High-precision U-Pb dating of single-zircon crystals by chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) is applied to volcanic beds that are intercalated in sedimentary sequences across the Permian-Triassic boundary (PTB). By assuming that the zircon crystallization age closely approximate that of the volcanic eruption and subsequent deposition, U-Pb zircon geochronology is the preferred approach for dating abiotic and biotic events, such as the formational PTB and the Permian-Triassic boundary mass extinction (PTBME). We will present new U-Pb zircon dates for a series of volcanic ash beds in shallow-marine Permian-Triassic sections in the Nanpanjiang Basin, South China. These high-resolution U-Pb dates indicate a duration of 90 ± 38 kyr for the Permian sedimentary hiatus and a duration of 13 ± 57 kyr for the overlying Triassic microbial limestone in the shallow water settings of the Nanpanjiang pull apart Basin. The age and duration of the hiatus coincides with the formational PTB and the extinction interval in the Meishan Global Stratotype Section and Point, thus strongly supporting a glacio-eustatic regression, which best explains the genesis of the worldwide hiatus straddling the PTB in shallow water records. In adjacent deep marine troughs, rates of sediment accumulation display a six-fold decrease across the PTB compatible with a dryer and cooler climate during the Griesbachian as indicated by terrestrial plants. Our model of the PTBME hinges on the synchronicity of the hiatus with the onset of the Siberian Traps volcanism. This early eruptive phase likely released sulfur-rich volatiles into the stratosphere, thus simultaneously eliciting a short-lived ice age responsible for the global regression and a brief but intense acidification. Abrupt cooling, shrunk habitats on shelves and acidification may all have synergistically triggered the PTBME. Subsequently, the build-up of volcanic CO2 induced this transient cool climate whose early phase saw the deposition of the microbial limestone.

Assessing the isotopic evolution of S-type granites of the Carlos Chagas Batholith, SE Brazil: Clues from U-Pb, Hf isotopes, Ti geothermometry and trace element composition of zircon

NASA Astrophysics Data System (ADS)

Melo, Marilane G.; Lana, Cristiano; Stevens, Gary; Pedrosa-Soares, Antônio C.; Gerdes, Axel; Alkmin, Leonardo A.; Nalini, Hermínio A.; Alkmim, Fernando F.

2017-07-01

The Carlos Chagas batholith (CCB) is a very large ( 14,000 km2) S-type granitic body formed during the syn-collisional stage of the Araçuaí orogen (southeastern Brazil). Zircons extracted from the CCB record a wide range of U-Pb ages (from 825 to 490 Ma), indicating a complex history of inheritance, magmatic crystallization and partial melting during the evolution of the orogeny. Magmatic zircons (ca. 578-588 Ma) are marked by similar Hf isotope compositions and REE patterns to those of inherited cores (ca. 825-600 Ma), indicating that these aspects of the chemical signature of the magmatic zircons have likely been inherited from the source. The U-Pb ages and initial 176Hf/177Hf ratios from anatectic and metamorphic zircon domains are consistent with a two-stage metamorphic evolution marked by contrasting mechanisms of zircon growth and recrystallization during the orogeny. Ti-in-zircon thermometry is consistent with the findings of previous metamorphic work and indicates that the two metamorphic events in the batholith reached granulite facies conditions (> 800 °C) producing two generations of garnet via fluid-absent partial melting reactions. The oldest metamorphic episode (ca. 570-550 Ma) is recorded by development of thin anatectic overgrowths on older cores and by growth of new anatectic zircon crystals. Both domains have higher initial 176Hf/177Hf values compared to relict cores and display REE patterns typical of zircon that grew contemporaneously with peritectic garnet through biotite-absent fluid partial melting reactions. Hf isotopic and chemical evidences indicate that a second anatectic episode (ca. 535-500 Ma) is only recorded in parts from the CCB. In these rocks, the growth of new anatectic zircon and/or overgrowths is marked by high initial 176Hf/177Hf values and also by formation of second generation of garnet, as indicated by petrographic observations and REE patterns. In addition, some rocks contain zircon crystals formed by solid-state recrystallization of pre-existing zircon, which exhibit similar Hf isotope composition to those of inherited/magmatic core domains. The first anatectic event is interpreted as result of crustal thickening after the intrusion of the batholith. This introduced the batholith to a depth in excess of 30 km and produced widespread anatexis throughout the batholith. The second event was associated with asthenospheric upwelling during extensional thinning and gravitational collapse of the orogen, this produced anatexis in parts from the CCB that had been re-fertilized for anatexis by retrogression along shear zones following the first granulite facies event.

Hf isotope compositions In detrital zircons as a new tool for provenance studies

NASA Astrophysics Data System (ADS)

Jacobsen, Y. J.; Münker, C.; Mezger, K.

2003-04-01

Identifying the provenance of continental sediments is a major issue in palaeo-tectonic studies, providing important information for paleogeographic reconstructions. Isotope studies, e.g. those of whole rock Sm-Nd or detrital zircon U-Pb dating, have widely been used for this purpose. Here we assess the potential of combined Lu-Hf data and U-Pb ages determined on the same single detrital zircons as a new tool for provenance studies. Due to the low Lu/Hf ratios in zircons the Hf isotope composition of a zircon changes insignificantly after its crystallization. Thus each particular grain preserves information on the Hf-siotpe composition of its source and the age of this source. Provided that both the U-Pb and Lu-Hf isotope systems have not been disturbed, this information can be used to constrain the sources of each individual zircon. In order to demonstrate the capability of Hf isotope studies on detrital zircons for provenance studies, we obtained combined U-Pb ages and Lu-Hf isotope data for zircons from the Cambrian Junction Formation in New Zealand. The Junction Formation was deposited on the (present) SE margin of Gondwana near the Australian continent and consists of turbidites, siltstones and conglomerates [1]. Typical continent derived Paleozoic sediments in SE Gondwana generally show characteristic age maxima at 500-600 Ma, 1000-1200 Ma (Grenvillian) and additional older peaks (early Proterozoic to Archean) [2]. We focused on two groups of detrital zircons with Grenvillian and Proterozoic to Late Archean ages. The initial ɛHf values for these zircons range from 0.7 to -15.5 for the Grenvillian and from -5.2 to -14.1 for the Proterozoic/Archean zircons. Corresponding two stage Hf model ages range from ca. 1500 to 2500 Ma for the Grenvillian and from ca. 3200 to 3600 Ma for the Proterozoic/Archean zircons. Furthermore it can be shown that the Grenvillian zircons must have been derived from recycled Grenvillian provinces. Comparison of these Hf model ages with Nd crustal residence ages from the possible sources in Australia, Antarctica and Laurentia reveals the possible sources of the zircons. Based on the paleogeographic setting in Cambrian time the Grenville-age zircons were most likely derived from Drauning Maud Land (Antarctica), thus confirming earlier models by [1] and [3]. The Archean zircons were most likely derived from W-Australia (Yilgarn or Pilbara Kraton) or E-Antarctica (Miller Range). [1] Wombacher and Münker 2000: J. Geol. 108, [2] Ireland et al. 1998: Geology 26, [3] Flöttmann et al. 1998: J. Geol. Soc. 155.

Complex Histories of Two Lunar Zircons as Evidenced by their Internal Structures and U-Pb Ages

NASA Technical Reports Server (NTRS)

Pidgeon, R. T.; Nemchin, A. A.; Meyer, Charles

2006-01-01

The U-Pb dating of lunar zircon by ion-microprobe provides a robust technique for investigating the timing of lunar events [1,2]. However, we have now identified two cases where the U-Pb systems in a single zircon show more than one age. These complex zircons provide new opportunities for extending our knowledge on the timing of events in the early history of the Moon.

Fission track dating of kimberlitic zircons

NASA Astrophysics Data System (ADS)

Haggerty, Stephen E.; Raber, Ellen; Naeser, Charles W.

1983-04-01

The only reliable method for dating kimberlites at present is the lengthy and specialized hydrothermal procedure that extracts 206Pb and 238U from low-uranium zircons. This paper describes a second successful method by fission track dating of large single-crystal zircons, 1.0-1.5 cm in dimension. The use of large crystals overcomes the limitations imposed in conventional fission track analysis which utilizes crushed fragments. Low track densities, optical track dispersion, and the random orientation of polished surfaces in the etch and irradiation cycle are effectively overcome. Fission track ages of zircons from five African kimberlites are reported, from the Kimberley Pool (90.3 ± 6.5 m.y.), Orapa (87.4 ± 5.7 and 92.4 ± 6.1 m.y.), Nzega (51.1 ± 3.8 m.y.), Koffiefontein (90.0 ± 8.2 m.y.), and Val do Queve (133.4 ± 11.5 m.y.). In addition we report the first radiometric ages (707.9 ± 59.6 and 705.5 ± 61.0 m.y.) of crustal zircons from kimberlites in northwest Liberia. The fission track ages agree well with earlier age estimates. Most of the zircons examined in this study are zoned with respect to uranium but linear correlations are established (by regression analysis) between zones of variable uranium content, and within zones of constant uranium content (by analysis of variance). Concordance between the fission track method and the U/Pb technique is established and we concluded that track fading from thermal annealing has not taken place. Kimberlitic zircons dated in this study, therefore, record the time of eruption.

Fission track dating of kimberlitic zircons

USGS Publications Warehouse

Haggerty, S.E.; Raber, E.; Naeser, C.W.

1983-01-01

The only reliable method for dating kimberlites at present is the lengthy and specialized hydrothermal procedure that extracts 206Pb and 238U from low-uranium zircons. This paper describes a second successful method by fission track dating of large single-crystal zircons, 1.0-1.5 cm in dimension. The use of large crystals overcomes the limitations imposed in conventional fission track analysis which utilizes crushed fragments. Low track densities, optical track dispersion, and the random orientation of polished surfaces in the etch and irradiation cycle are effectively overcome. Fission track ages of zircons from five African kimberlites are reported, from the Kimberley Pool (90.3 ?? 6.5 m.y.), Orapa (87.4 ?? 5.7 and 92.4 ?? 6.1 m.y.), Nzega (51.1 ?? 3.8 m.y.), Koffiefontein (90.0 ?? 8.2 m.y.), and Val do Queve (133.4 ?? 11.5 m.y.). In addition we report the first radiometric ages (707.9 ?? 59.6 and 705.5 ?? 61.0 m.y.) of crustal zircons from kimberlites in northwest Liberia. The fission track ages agree well with earlier age estimates. Most of the zircons examined in this study are zoned with respect to uranium but linear correlations are established (by regression analysis) between zones of variable uranium content, and within zones of constant uranium content (by analysis of variance). Concordance between the fission track method and the U/Pb technique is established and we concluded that track fading from thermal annealing has not taken place. Kimberlitic zircons dated in this study, therefore, record the time of eruption. ?? 1983.

Fitful and protracted magma assembly leading to a giant eruption, Youngest Toba Tuff, Indonesia

USGS Publications Warehouse

Reid, Mary R; Vazquez, Jorge A.

2017-01-01

The paroxysmal eruption of the 74 ka Youngest Toba Tuff (YTT) of northern Sumatra produced an extraordinary 2800 km3 of non-welded to densely welded ignimbrite and co-ignimbrite ash-fall. We report insights into the duration of YTT magma assembly obtained from ion microprobe U-Th and U-Pb dates, including continuous age spectra over >50% of final zircon growth, for pumices and a welded tuff spanning the compositional range of the YTT. A relatively large subpopulation of zircon crystals nucleated before the penultimate caldera-related eruption at 501 ka, but most zircons yielded interior dates 100-300 ka thereafter. Zircon nucleation and growth was likely episodic and from diverse conditions over protracted time intervals of >100 to >500 ka. Final zircon growth is evident as thin rim plateaus that are in Th/U chemical equilibrium with hosts, and that give crystallization ages within tens of ka of eruption. The longevity and chemical characteristics of the YTT zircons, as well as evidence for intermittent zircon isolation and remobilization associated with magma recharge, is especially favored at the cool and wet eutectoid conditions that characterize at least half of the YTT, wherein heat fluxes could dissolve major phases but have only a minor effect on larger zircon crystals. Repeated magma recharge may have contributed to the development of compositional zoning in the YTT but, considered together with limited allanite, quartz, and other mineral dating and geospeedometry, regular perturbations to the magma reservoir over >400 ka did not lead to eruption until 74 ka ago.

Small Volume Isotopic Analysis of Zircon Using LA-MC-ICP-MS U-Pb and Lu-Hf and Sub-ng Amounts of Hf in Solution

NASA Astrophysics Data System (ADS)

Bauer, A.; Horstwood, M. S.

2016-12-01

Crust-mantle evolution studies are greatly informed by zircon U-Pb and Lu-Hf isotopic datasets and the ease with which these data can now be acquired has seen their application become commonplace. In order to deconvolute geochemical change and interpret geologic variation in complexly zoned zircons, this information is most ideally obtained on the smallest volume of zircon by successive SIMS U-Pb and LA-MC-ICP-MS Lu-Hf isotopic analyses. However, due to variations in zircon growth zone geometry at depth, the Lu-Hf analysis may not relate to the lower volume U-Pb analysis, potentially causing inaccuracy of the resultant age-corrected Hf isotope signature. Laser ablation split-stream methods are applied to be certain that U-Pb and Lu-Hf data represent the same volume of zircon, however, the sampling volume remains relatively large at 40x30µm1. Coupled ID-TIMS U-Pb and solution MC-ICP-MS Lu-Hf work traditionally utilize whole-zircon dissolution ( 10-50ng Hf), which has the potential to homogenize different zones of geologic significance within an analysis. Conversely, modern ID-TIMS U-Pb methods utilize microsampling of zircon grains, often providing < 5ng Hf, thereby challenging conventional Lu-Hf acquisition protocols to achieve the required precision. In order to obtain usable precision on minimal zircon volumes, we developed laser ablation methods using successive 25um spot U-Pb and Lu-Hf ablation pits with a combined depth of 18um, and low-volume solution introduction methods without Hf-REE separation utilizing Hf amounts as low as 0.4ng, while retaining an uncertainty level of ca. 1 ɛHf for both methods. We investigated methods of Yb interference correction and the potential for matrix effects, with a particular focus on the accurate quantification of 176Lu/177Hf. These improvements reduce the minimum amount of material required for U-Pb and Hf isotopic analysis of zircon by about an order of magnitude. 1Ibanez-Mejia et al (2015). PreRes, 267, 285-310.

Zircon crytallization and recycling in the magma chamber of the rhyolitic Kos Plateau Tuff (Aegean arc)

USGS Publications Warehouse

Bachman, O.; Charlier, B.L.A.; Lowenstern, J. B.

2007-01-01

In contrast to most large-volume silicic magmas in continental arcs, which are thought to evolve as open systems with significant assimilation of preexisting crust, the Kos Plateau Miff magma formed dominantly by crystal fractionation of mafic parents. Deposits from this ??? 60 km3 pyroclastic eruption (the largest known in the Aegean arc) lack xenocrystic zircons [secondary ion mass spectrometry (SIMS) U-Pb ages on zircon cores never older than 500 ka] and display Sr-Nd whole-rock isotopic ratios within the range of European mantle in an area with exposed Paleozoic and Tertiary continental crust; this evidence implies a nearly closed-system chemical differentiation. Consequently, the age range provided by zircon SIMS U-Th-Pb dating is a reliable indicator of the duration of assembly and longevity of the silicic magma body above its solidus. The age distribution from 160 ka (age of eruption by sanidine 40Ar/39Ar dating; Smith et al., 1996) to ca. 500 ka combined with textural characteristics (high crystal content, corrosion of most anhydrous phenocrysts, but stability of hydrous phases) suggest (1) a protracted residence in the crust as a crystal mush and (2) rejuvenation (reduced crystallization and even partial resorption of minerals) prior to eruption probably induced by new influx of heat (and volatiles). This extended evolution chemically isolated from the surrounding crust is a likely consequence of the regional geodynamics because the thinned Aegean microplate acts as a refractory container for magmas in the dying Aegean subduction zone (continent-continent subduction). ?? 2007 Geological Society of America.

The cooling history and the depth of detachment faulting at the Atlantis Massif oceanic core complex

NASA Astrophysics Data System (ADS)

Schoolmeesters, Nicole; Cheadle, Michael J.; John, Barbara E.; Reiners, Peter W.; Gee, Jeffrey; Grimes, Craig B.

2012-10-01

Oceanic core complexes (OCCs) are domal exposures of oceanic crust and mantle interpreted to be denuded to the seafloor by large slip oceanic detachment faults. We combine previously reported U-Pb zircon crystallization ages with (U-Th)/He zircon thermochronometry and multicomponent magnetic remanence data to determine the cooling history of the footwall to the Atlantis Massif OCC (30°N, MAR) and help establish cooling rates, as well as depths of detachment faulting and gabbro emplacement. We present nine new (U-Th)/He zircon ages for samples from IODP Hole U1309D ranging from 40 to 1415 m below seafloor. These data paired with U-Pb zircon ages and magnetic remanence data constrain cooling rates of gabbroic rocks from the upper 800 m of the central dome at Atlantis Massif as 2895 (+1276/-1162) °C Myr-1 (from ˜780°C to ˜250°C); the lower 600 m of the borehole cooled more slowly at mean rates of ˜500 (+125/-102) °C Myr-1(from ˜780°C to present-day temperatures). Rocks from the uppermost part of the hole also reveal a brief period of slow cooling at rates of ˜300°C Myr-1, possibly due to hydrothermal circulation to ˜4 km depth through the detachment fault zone. Assuming a fault slip rate of 20 mm/yr (from U-Pb zircon ages of surface samples) and a rolling hinge model for the sub-surface fault geometry, we predict that the 780°C isotherm lies at ˜7 km below the axial valley floor, likely corresponding both to the depth at which the semi-brittle detachment fault roots and the probable upper limit of significant gabbro emplacement.

Crustal melting and recycling: geochronology and sources of Variscan syn-kinematic anatectic granitoids of the Tormes Dome (Central Iberian Zone). A U-Pb LA-ICP-MS study

NASA Astrophysics Data System (ADS)

López-Moro, F. J.; López-Plaza, M.; Gutiérrez-Alonso, G.; Fernández-Suárez, J.; López-Carmona, A.; Hofmann, M.; Romer, R. L.

2018-04-01

In this study, we report U-Pb Laser Ablation ICP-MS zircon and ID-TIMS monazite ages for peraluminous granitoid plutons (biotite ± muscovite ± cordierite ± sillimanite) in the Tormes Dome, one of the gneiss-cored domes located in the Central Iberian Zone of the Variscan belt of northern Spain. Textural domains in zircon, interpreted to represent the magmatic crystallization of the granitoids (and one monazite fraction in the Ledesma pluton) yielded ages around 320 Ma, in agreement with other geochronological studies in the region. This age is interpreted to date the timing of decompression crustal melting driven by the extensional collapse of the orogenic belt in this domain of the Variscan chain of western Europe. In addition, there are several populations of inherited (xenocrystic) zircon: (1) Carboniferous zircon crystals (ca. 345 Ma) as well as one of the monazite fractions in the coarse-grained facies of the Ledesma pluton that also yielded an age of ca. 343 Ma. (2) Devonian-Silurian zircon xenocrysts with scattered ages between ca. 390 and 432 Ma. (3) Middle Cambrian-Ordovician (ca. 450-511 Ma). (4) Ediacaran-Cryogenian zircon ages (ca. 540-840 Ma). (5) Mesoproterozoic to Archaean zircon (900-2700 Ma). The abundance of Carboniferous-inherited zircon shows that crustal recycling/cannibalization may often happen at a fast pace in orogenic scenarios with only short lapses of quiescence. In our case study, it seems plausible that a "crustal layer" of ca. 340 Ma granitoids/migmatites was recycled, partially or totally, only 15-20 My after its emplacement.

Creation of a continent recorded in zircon zoning

USGS Publications Warehouse

Moser, D.E.; Bowman, J.R.; Wooden, J.; Valley, J.W.; Mazdab, F.; Kita, N.

2008-01-01

We have discovered a robust microcrystalline record of the early genesis of North American lithosphere preserved in the U-Pb age and oxygen isotope zoning of zircons from a lower crustal paragneiss in the Neoarchean Superior province. Detrital igneous zircon cores with ??18O values of 5.1???-7.1??? record creation of primitive to increasingly evolved crust from 2.85 ?? 0.02 Ga to 2.67 ?? 0.02 Ga. Sharp chemical unconformity between cores and higher ??18O (8.4???-10.4???) metamorphic overgrowths as old as 2.66 ?? 0.01 Ga dictates a rapid sequence of arc unroofing, burial of detrital zircons in hydrosphere-altered sediment, and transport to lower crust late in upper plate assembly. The period to 2.58 ?? 0.01 Ga included ???80 m.y. of high-temperature (???700-650 ??C), nearly continuous overgrowth events reflecting stages in maturation of the subjacent mantle root. Huronian continental rifting is recorded by the youngest zircon tip growth at 2512 ?? 8 Ma (??? 600 ??C) signaling magma intraplating and the onset of rigid plate behavior. This >150 m.y. microscopic isotope record in single crystals demonstrates the sluggish volume diffusion of U, Pb, and O in zircon throughout protracted regional metamorphism, and the consequent advances now possible in reconstructing planetary dynamics with zircon zoning. ?? 2008 The Geological Society of America.

Dating High Temperature Mineral Fabrics in Lower Crustal Granulite Facies Rocks

NASA Astrophysics Data System (ADS)

Stowell, H. H.; Schwartz, J. J.; Tulloch, A. J.; Klepeis, K. A.; Odom Parker, K.; Palin, M.; Ramezani, J.

2015-12-01

Granulite facies rocks may record strain that provides a record of compressional and/or extensional crustal events in hot orogenic cores and the roots of magmatic arcs. Although the precise timing of these events is important for constructing tectonic histories, it is often difficult to determine due to uncertain relationships between isotopic signatures, mineral growth, and textural features that record strain. In addition, there may be large uncertainties in isotope data due to intracrystalline diffusion and multiple crystallization events. L-S tectonites in lower crustal rocks from Fiordland, NZ record the early stages of extensional collapse of thickened magmatic arc crust. The precise age of these fabrics is important for constraining the timing of extension that led to opening of the Tasman Sea. High temperature granulite facies L-S fabrics in garnet reaction zones (GRZ) border syn- to post-deformational leucosomes. U-Pb zircon, Lu-Hf garnet, and Sm-Nd garnet ages, and trace elements in these phases indicate the complexity of assigning precise and useful ages. Zircon have soccer ball morphology with patchy and sector zoned CL. Zircon dates for igneous host and adjacent GRZ range over ca. 17 Ma. 236U-208Pb LA-ICP-MS are 108-125 Ma, N=124 (host & GRZ); however, chemical abrasion (CA) shifts GRZ dates ca. 2 Ma older. 236U-208Pb SHRIMP-RG dates cluster in 2 groups: 118.5±0.8 Ma, N=23 and 111.0±0.8 Ma, N=6. CA single crystal TIMS dates also fall into 2 groups: 117.6±0.1 Ma, N=4 and 116.6±0.2 Ma N=4. Garnet isochron ages determined from coarse garnet selvages adjacent to leucosomes range from 112.8±2.2 (147Sm-143Nd, 10 pts.) to 114.8±3.5 (177Lu-176Hf, 6 pts.) Ma. Zircon dates from all methods show ranges (>10 Ma) and 2 distinct populations. Host and GRZ zircon cannot be readily distinguished by age, lack younger rims, but have distinct Th/U trends and Eu/Eu* vs. Hf ratios. Difference in zircon trace element composition indicates either early leucosome emplacement or xenocrystic zircon in leucosomes. We conclude that the small number of oldest zircon grains are inherited, older zircon age populations (CA LA-ICP-MS, SHRIMP-RG and TIMS) are near identical ca. 118 Ma and date intrusion, and that the youngest zircon and indistinguishable garnet ages (113-116 Ma) date syn-deformational granulite facies metamorphism.

LA-SF-ICP-MS zircon U-Pb geochronology of granitic rocks from the central Bundelkhand greenstone complex, Bundelkhand craton, India

NASA Astrophysics Data System (ADS)

Verma, Sanjeet K.; Verma, Surendra P.; Oliveira, Elson P.; Singh, Vinod K.; Moreno, Juan A.

2016-03-01

The central Bundelkhand greenstone complex in Bundelkhand craton, northern India is one of the well exposed Archaean supracrustal amphibolite, banded iron formation (BIF) and felsic volcanic rocks (FV) and associated with grey and pink porphyritic granite, tonalite-trondhjemite-granodiorite (TTG). Here we present high precision zircon U-Pb geochronological data for the pinkish porphyritic granites and TTG. The zircons from the grey-pinkish porphyritic granite show three different concordia ages of 2531 ± 21 Ma, 2516 ± 38 Ma, and 2514 ± 13 Ma, which are interpreted as the best estimate of the magmatic crystallization age for the studied granites. We also report the concordia age of 2669 ± 7.4 Ma for a trondhjemite gneiss sample, which is so far the youngest U-Pb geochronological data for a TTG rock suite in the Bundelkhand craton. This TTG formation at 2669 Ma is also more similar to Precambrian basement TTG gneisses of the Aravalli Craton of north western India and suggests that crust formation in the Bundelkhand Craton occurred in a similar time-frame to that recorded from the Aravalli craton of the North-western India.

Fingerprinting the K/T impact site and determining the time of impact by U-Pb dating of single shocked zircons from distal ejecta

NASA Technical Reports Server (NTRS)

Krogh, T. E.; Kamo, S. L.; Bohor, B. F.

1993-01-01

U-Pb isotopic dating of single 1 - 3 micrograms zircons from K/T distal ejecta from a site in the Raton Basin, Colorado provides a powerful new tool with which to determine both the time of the impact event and the age of the basement at the impact site. Data for the least shocked zircons are slightly displaced from the 544 +/- 5 Ma primary age for a component of the target site, while those for highly shocked and granular grains are strongly displaced towards the time of impact at 65.5 +/- 3.0 Ma. Such shocked and granular zircons have never been reported from any source, including explosive volcanic rocks. Zircon is refractory and has one of the highest thermal blocking temperatures; hence, it can record both shock features and primary and secondary ages without modification by post-crystallization processes. Unlike shocked quartz, which can come from almost anywhere on the Earth's crust, shocked zircons can be shown to come from a specific site because basement ages vary on the scale of meters to kilometers. With U-Pb zircon dating, it is now possible to correlate ejecta layers derived from the same target site, test the single versus multiple impact hypothesis, and identify the target source of impact ejecta. The ages obtained in this study indicate that the Manson impact site, Iowa, which has basement rocks that are mid-Proterozoic in age, cannot be the source of K/T distal ejecta. The K/T distal ejecta probably originated from a single impact site because most grains have the same primary age.

GHR1 - A new Eocene natural reference material for U-Pb and Hf isotopic measurements in zircon

NASA Astrophysics Data System (ADS)

Ibanez-Mejia, M.; Eddy, M. P.

2017-12-01

We present chemical abrasion-isotope dilution-thermal ionization (CA-ID-TIMS) U-Pb zircon geochronology and solution multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) Hf isotopic data from a proposed natural zircon reference material for use during in situ analyses of U-Pb and Hf isotopic ratios. The sample, GHR1, was collected from the rapakivi intrusive phase of the Eocene Golden Horn batholith in Washington, USA. Zircons separated from this sample range up to 250-300 μm in length and have moderate aspect ratios. A weighted mean of 15 Th-corrected 206Pb/238U zircon dates from GHR1 produced at the Massachusetts Institute of Technology is 48.132 ± 0.023 Ma (2σ analytical and tracer uncertainties only, MSWD=1.70) confirming that there is little or no inter-crystal age heterogeneity at the scale of a few 10 kyr. Solution MC-ICP-MS measurements of chemically purified aliquots give a 176Hf/177Hf weighted mean of 0.283050 ± 17 (2σ, n=10), corresponding to a ɛHf0 of ca. +9.3. The 2σ variability of these measurements is comparable to our reproducibility of the JMC-475 Hf isotopic standard 0.282160 ± 14 (n= 13), suggesting that GHR1 zircons are homogenous with respect to 176Hf/177Hf. In situ 206Pb/238U dates from collaborating secondary ion mass spectrometry (SIMS), sensitive high-resolution ion microprobe (SHRIMP), and laser ablation ICP-MS (LA-ICP-MS) laboratories are in excellent agreement with the CA-ID-TIMS date and illustrate the reproducibility and potential value of this reference zircon. The mean values of 176Hf/177Hf measurements from two LA-ICP-MS laboratories are in agreement with the solution MC-ICP-MS value, but show slightly greater dispersion and higher (Lu+Yb)/Hf values. We attribute this discrepancy to apatite inclusions that are high in REE and may lead to greater isobaric interferences on 176Hf. These inclusions and potential isobaric interferences from REE were removed during the chemical abrasion step prior to bulk dissolution and ion-exchange purification in the solution data. Nevertheless, the apparent isotopic homogeneity and reproducibility of 206Pb/238U and 176Hf/177Hf ratios and the potentially unlimited reserves of GHR1 suggest that it is a promising reference material. We plan to distribute GHR1 at the meeting to interested laboratories.

Provenance of Marine Sediment in the Gulf of Alaska, IODP Expedition 341: Links Between Sediment Derivation, Glacial Systems, and Exhumation of the Coastal Mountain Belts

NASA Astrophysics Data System (ADS)

Allen, W. K.; Dunn, C. A.; Enkelmann, E.; Ridgway, K.; Colliver, L.

2015-12-01

Provenance analysis of Neogene sand and diamict beds from marine boreholes drilled by the IODP Expedition 341 provides a marine sedimentary record of the interactions between tectonics, climate and sediment deposition along a glaciated convergent margin. The 341 boreholes represent a cross-margin transect that sampled the continental shelf, slope, and deep sea Surveyor Fan of the Gulf of Alaska. Our dataset currently consists of ~ 650 detrital zircons selected for double dating method utilizing both detrital zircon fission track (FT) and U-Pb analysis from sand and diamict beds, as well as zircon U-Pb geochronology and apatite FT from igneous and gneissic clasts. Detrital zircon U-Pb geochronology of sand records dominant peak ages of 53, 62, 70, and 98 Ma with minor populations of 117, 154, and 170 Ma. Most of these ages can be correlated to primary igneous sources in the Coast Plutonic Complex, the Chugach Metamorphic Complex, the plutonic rocks of Wrangellia, and the Sanak-Baranoff plutonic belt. All samples analyzed to date, covering a 10 Myr range, share nearly identical detrital zircon populations suggesting similar primary sediment sources and reworking of sediment in thrust belts and accretionary prisms along this convergent margin. Plutonic and gneissic clasts collected from the boreholes on the shelf have already been double dated. These clasts have general U-Pb zircon crystallization ages of 52-54 Ma and apatite fission track cooling ages of 10-12 Ma. These results, along with previous published studies, indicate that these clasts were derived from the Chugach Metamorphic Complex and were eroded and transported by the Bagley Ice Field and Bering Glacier. Future results using this approach should allow us to pinpoint which parts of the exhumed onshore ranges and which glacial systems provided sediment to marine environments in the Gulf of Alaska.

Significance of zircon U-Pb ages from the Pescadero felsite, west-central California coast ranges

USGS Publications Warehouse

McLaughlin, Robert J.; Moore, Diane E.; ,; Martens, UWE C.; Clark, J.C.

2011-01-01

Weathered felsite is associated with the late Campanian–Maastrichtian Pigeon Point Formation near Pescadero, California. Poorly exposed, its age and correlation are uncertain. Is it part of the Pigeon Point section west of the San Gregorio–Hosgri fault? Does it rest on Nacimiento block basement? Is it dextrally offset from the Oligocene Cambria Felsite, ∼185 km to the southeast? Why is a calc-alkaline hypabyssal igneous rock intrusive into the outboard accretionary prism? To address these questions, we analyzed 43 oscillatory-zoned zircon crystals from three incipiently recrystallized pumpellyite ± prehnite ± laumontite-bearing Pescadero felsite samples by sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) techniques. Thirty-three zircons gave late Mesozoic U-Pb ages, with single-grain values ranging from 81 to 167 Ma; ten have pre-Mesozoic, chiefly Proterozoic ages. A group of the four youngest Pescadero zircons yielded an apparent maximum igneous age of ca. 86–90 Ma. Reflecting broad age scatter and presence of partly digested sandstone inclusions, we interpret the rest of the zircons (perhaps all) as xenocrysts. Twenty-three zircons were separated and analyzed from two samples of the similar Cambria Felsite, yielding a unimodal 27 Ma U-Pb age. Clearly, the origin of the Upper Oligocene Cambria Felsite is different from that of the Upper Cretaceous Pescadero felsite; these rocks are not correlated, and do not constrain displacement along the San Gregorio–Hosgri fault. Peak ages differ slightly, but relative probability curves for Mesozoic and pre-Mesozoic Pescadero zircons compare well, for example, with abundant U-Pb age data for detrital zircons from Franciscan metaclastic strata ∼100 km to the east in the Diablo Range–San Francisco Bay area, San Joaquin Great Valley Group turbidites, Upper Cretaceous Nacimiento block Franciscan strata, and Upper Cretaceous forearc units of the Transverse Ranges. Based on zircon U-Pb ages, geologic and petrographic relations, the Pescadero felsite and a capping, sheared metaconglomerate underlie the Pigeon Point Formation. We infer that the magma formed by anatexis of Franciscan or Great Valley clastic sedimentary rocks originating from a parental Mesozoic Sierran-Mojave-Salinian calc-alkaline arc. The felsite erupted during Late Cretaceous time, was metamorphosed to pumpellyite-prehnite grade within the subduction zone, and then was rapidly exhumed, weakly zeolitized, and exposed before Pigeon Point forearc deposition. Pescadero volcanism apparently reflects a previously unrecognized ca. 86–90 Ma felsic igneous event in the accretionary margin.

Zircon U-Pb age of the Pescadero felsite: A late Cretaceous igneous event in the forearc, west-central California Coast Ranges

USGS Publications Warehouse

Ernst, W.G.; Martens, U.C.; McLaughlin, R.J.; Clark, J.C.; Moore, Diane E.

2011-01-01

Weathered felsite is associated with the late Campanian-Maastrichtian Pigeon Point Formation near Pescadero, California. Poorly exposed, its age and correlation are uncertain. Is it part of the Pigeon Point section west of the San Gregorio-Hosgri fault? Does it rest on Nacimiento block basement? Is it dextrally offset from the Oligocene Cambria Felsite, ~185 km to the southeast? Why is a calc-alkaline hypabyssal igneous rock intrusive into the outboard accretionary prism? To address these questions, we analyzed 43 oscillatory-zoned zircon crystals from three incipiently recrystallized pumpellyite ?? prehnite ?? laumontite-bearing Pescadero felsite samples by sensitive high-resolution ion microprobe-reverse geometry (SHRIMPRG) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) techniques. Thirty-three zircons gave late Mesozoic U-Pb ages, with single-grain values ranging from 81 to 167 Ma; ten have pre-Mesozoic, chiefl y Proterozoic ages. A group of the four youngest Pescadero zircons yielded an apparent maximum igneous age of ca. 86-90 Ma. Refl ecting broad age scatter and presence of partly digested sandstone inclusions, we interpret the rest of the zircons (perhaps all) as xenocrysts. Twenty-three zircons were separated and analyzed from two samples of the similar Cambria Felsite, yielding a unimodal 27 Ma U-Pb age. Clearly, the origin of the Upper Oligocene Cambria Felsite is different from that of the Upper Cretaceous Pescadero felsite; these rocks are not correlated, and do not constrain displacement along the San Gregorio-Hosgri fault. Peak ages differ slightly, but relative probability curves for Mesozoic and pre-Mesozoic Pescadero zircons compare well, for example, with abundant U-Pb age data for detrital zircons from Franciscan metaclastic strata ~100 km to the east in the Diablo Range- San Francisco Bay area, San Joaquin Great Valley Group turbidites, Upper Cretaceous Nacimiento block Franciscan strata, and Upper Cretaceous forearc units of the Transverse Ranges. Based on zircon U-Pb ages, geologic and petrographic relations, the Pescadero felsite and a capping, sheared metaconglomerate underlie the Pigeon Point Formation. We infer that the magma formed by anatexis of Franciscan or Great Valley clastic sedimentary rocks originating from a parental Mesozoic Sierran-Mojave-Salinian calcalkaline arc. The felsite erupted during Late Cretaceous time, was metamorphosed to pumpellyite-prehnite grade within the subduction zone, and then was rapidly exhumed, weakly zeolitized, and exposed before Pigeon Point forearc deposition. Pescadero vol canism apparently reflects a previously unrecognized ca. 86-90 Ma felsic igneous event in the accretionary margin. ?? 2011 Geological Society of America.

Late magmatic stage of the zoned Caleu pluton (Central Chile): insights from zircon crystallization conditions

NASA Astrophysics Data System (ADS)

Molina, P. G.; Parada, M.; Gutierrez, F. J.; Chang-Qiang, M.; Jianwei, L.; Yuanyuan, L.

2012-12-01

The Caleu pluton consists of three N-S elongated lithological zones: Gabbro-Diorite Zone (GDZ), Tonalite Zone (TZ) and Granodiorite Zone (GZ); western, middle and eastern portions of the pluton, respectively. The zones are thought to be previously differentiated in a common, isotopically depleted (Sr-Nd), subjacent magma reservoir at a 4 kbar equivalent depth. The emplacement should have occurred at the climax of the Cretaceous rifting. We present preliminary results of U238/Pb206 zircon geochronology; zircon saturation, Tsat(Zrn), and crystallization temperatures (Ti-in-Zrn); as well as relative oxidation states at time of crystallization, based on: (i) the sluggish REE and HFSE subsolidus diffusivities in zircon; (ii) the behavior of Ti4+↔Si4+ and Ce4+↔Zr4+ isovalent replacement, in addition to a constrained TiO2 activity in almost all typical crustal rocks; and (iii) relative oxidation states at time of crystallization, respectively. The latter are obtained by interpolation of the partition coefficients of trivalent (REE) and tetravalent (HFSE) curves in Onuma diagrams for each zircon, and then estimating relative Ce(IV)/Ce(III) ratios. Results obtained from 4 samples (a total of 77 zircon grains) collected from the three mentioned lithological zones indicate U/Pb ages of approximately 99.5 ±1.5 Ma, 96.8 ±0.6 Ma, and 94.4 +2.2 -0.8 Ma; and Ti-in Zrn ranges of ca. 720-870°C, ca. 680-820°C and ca. 750-840°C, for the GDZ, TZ and GZ samples, respectively. On the other hand Tsat(Zrn) of ca. 750-780°C in the TZ, and ca. 830-890°C in the GZ, were obtained. As expected saturation temperatures are similar or higher than Ti-in-Zrn obtained in zircon grains of TZ and GZ, respectively. Cathodoluminiscence images in zircon suggest a magmatic origin, due to absence of complex zoning patterns and fairly well conserved morphologies. Exceptionally the GDZ sample zircons show evidence of inheritance, indicating a xenocrystic and/or antecrystic origin. A relative Ce(IV)/Ce(III) decrease with temperature gives rise to separate trends for each zone, suggesting an independent unbuffered development at time of zircon formation. Taking into account variations in zircon ages, morphologies and crystallization ranges in the collected samples, it is suggested that zircon crystallization took place independent of the magma composition. Furthermore, old inherited zircon grains in the GDZ could be antecrysts, or products of a restricted and not major host rock assimilation, owing to the impossibility of basaltic magmas to saturate zircon crystals, and the isotopically depleted signature. This study is financed by CONICYT-FONDAP grant 15090013, Centro de Excelencia en Geotermia de los Andes (CEGA); and the CONICYT PBCT-PDA07, Programa Bicentenario de Ciencia y Tecnología.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kogawa, M.; Watson, E. B.; Ewing, R. C.

Lead-doped zircon crystals, which were synthesized under three different conditions (Watson et al. 1997): dry at 1430 °C at atmospheric pressure without P2O5; wet at 900 °C at 1.5 GPa in the presence of P2O5; and wet at 800 °C at 1.0 GPa without P2O5, have been investigated to understand the mechanisms of Pb incorporation into zircon at the sub-micrometer scale, using various electron microscopy techniques including high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Four different mechanisms in which Pb may be incorporated into zircon have been identified. In the P-free synthesis, Pb-oxidemore » hydrate particles, 50–200 nm in size, are embedded in zircon. Each Pb-particle is associated with a single vesicle, ~50 nm. Lead in the zircon structure is possibly incorporated under the detection limit value of energy-dispersive X-ray analysis (EDX) by means of: (1) Zr4+ = Pb2+ + 2H+ at less than ~0.1 wt%. In the system with P, Pb-phases occur in three different forms: Pb-rich domains concentrated along cleavage planes or grain boundaries without any evident crystal form; numerous Pb-phosphate particles, as large as 100 nm, embedded heterogeneously in the zircon crystal; and homogeneous distribution of Pb in the zircon structure at less than ~1 wt% as determined by EDX. These results suggest that charge balance is maintained by the xenotime-type coupled substitution: (2) Zr4+ + 2Si4+ = Pb2+ + 2P5+ with a possible minor contribution from mechanism 1. The apparent solubility limit of Pb, <1 wt%, is constrained mainly by the xenotime-type coupled substitution mechanism, which is probably due to increasing strain at higher Pb-concentrations. The presence of Pb2+ in natural zircon is consistent with the low-level Pb allowed by substitution mechanism 2, with only a minor contribution from substitution mechanism 1, the latter of which causes distortion in the local structure.« less

Growth and Construction of Oceanic Crust at Atlantis Bank, Southwest Indian Ridge

NASA Astrophysics Data System (ADS)

Schwartz, J. J.; John, B. E.; Cheadle, M. J.; Miranda, E. A.; Grimes, C. B.; Wooden, J. L.; Dick, H. J.

2005-12-01

Magmatic zircon is a common accessory mineral in oceanic crustal rocks including gabbro, oxide gabbro, diabase and felsic veins. Its presence in these rocks provides an exceptional opportunity to document crustal growth processes at slow-spreading mid-ocean ridges. We present nineteen Pb/U zircon SHRIMP-RG ion probe ages of lower crustal rocks collected by manned submersible, ROV, dredging and ODP drilling from a 20 x 30 km2 area of Atlantis Bank, Southwest Indian Ridge, which allow us to constrain the growth and construction of oceanic crust. Weighted average 206Pb/238U ages of these samples range from 10.7 to 13.9 Ma, with errors of 0.1-0.6 m.y. (<1 - 4%). At least 75% of these gabbros accreted within error of the predicted sea-surface magnetic age, whereas up to 25% are between 700,000 and 2.5 m.y. older. In one sample, we identified zircon with inherited cores as much as 1.5 m.y. older than their corresponding rims. There is no observable correlation between age and lithology, and the anomalously old samples are not from any specific part of Atlantis Bank; they appear to be randomly distributed amongst the non-anomalous age samples and come from various structural depths. We consider two models to explain the presence of these anomalously old rocks: i) a stochastic intrusion model whereby magma was intruded at different spatial locations within the rift valley as the plates spread apart, resulting in the entrapment of older lower crust by subsequent intrusions; and/or ii) a model in which some gabbroic bodies originally crystallized at depths of ~5-18 km below the base of the crust in a thick, cold, axial lithosphere and were subsequently uplifted along flow-lines and intruded by shallow-level magmas during the creation of Atlantis Bank. In this model, the difference in time between the Pb/U zircon crystallization age and the magnetic age is a proxy for the depth at which zircon crystallized (assuming a constant mantle upwelling rate during the construction of Atlantis Bank over ~1.3 m.y.). We prefer the latter model, although aspects of both models may apply.

Age constraints on felsic intrusions, metamorphism and gold mineralisation in the Palaeoproterozoic Rio Itapicuru greenstone belt, NE Bahia State, Brazil

USGS Publications Warehouse

Mello, E.F.; Xavier, R.P.; McNaughton, N.J.; Hagemann, S.G.; Fletcher, I.; Snee, L.

2006-01-01

U-Pb sensitive high resolution ion microprobe mass spectrometer (SHRIMP) ages of zircon, monazite and xenotime crystals from felsic intrusive rocks from the Rio Itapicuru greenstone belt show two development stages between 2,152 and 2,130 Ma, and between 2,130 and 2,080 Ma. The older intrusions yielded ages of 2,152??6 Ma in monazite crystals and 2,155??9 Ma in zircon crystals derived from the Trilhado granodiorite, and ages of 2,130??7 Ma and 2,128??8 Ma in zircon crystals derived from the Teofila??ndia tonalite. The emplacement age of the syntectonic Ambro??sio dome as indicated by a 2,080??2-Ma xenotime age for a granite dyke probably marks the end of the felsic magmatism. This age shows good agreement with the Ar-Ar plateau age of 2,080??5 Ma obtained in hornblendes from an amphibolite and with a U-Pb SHRIMP age of 2,076??10 Ma in detrital zircon crystals from a quartzite, interpreted as the age of the peak of the metamorphism. The predominance of inherited zircons in the syntectonic Ambro??sio dome suggests that the basement of the supracrustal rocks was composed of Archaean continental crust with components of 2,937??16, 3,111??13 and 3,162??13 Ma. Ar-Ar plateau ages of 2,050??4 Ma and 2,054??2 Ma on hydrothermal muscovite samples from the Fazenda Brasileiro gold deposit are interpreted as minimum ages for gold mineralisation and close to the true age of gold deposition. The Ar-Ar data indicate that the mineralisation must have occurred less than 30 million years after the peak of the metamorphism, or episodically between 2,080 Ma and 2,050 Ma, during uplift and exhumation of the orogen. ?? Springer-Verlag 2006.

Peninsular terrane basement ages recorded by Paleozoic and Paleoproterozoic zircon in gabbro xenoliths and andesite from Redoubt volcano, Alaska

USGS Publications Warehouse

Bacon, Charles R.; Vazquez, Jorge A.; Wooden, Joseph L.

2012-01-01

Historically Sactive Redoubt volcano is an Aleutian arc basalt-to-dacite cone constructed upon the Jurassic–Early Tertiary Alaska–Aleutian Range batholith. The batholith intrudes the Peninsular tectonostratigraphic terrane, which is considered to have developed on oceanic basement and to have accreted to North America, possibly in Late Jurassic time. Xenoliths in Redoubt magmas have been thought to be modern cumulate gabbros and fragments of the batholith. However, new sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages for zircon from gabbro xenoliths from a late Pleistocene pyroclastic deposit are dominated by much older, ca. 310 Ma Pennsylvanian and ca. 1865 Ma Paleoproterozoic grains. Zircon age distributions and trace-element concentrations indicate that the ca. 310 Ma zircons date gabbroic intrusive rocks, and the ca. 1865 Ma zircons also are likely from igneous rocks in or beneath Peninsular terrane basement. The trace-element data imply that four of five Cretaceous–Paleocene zircons, and Pennsylvanian low-U, low-Th zircons in one sample, grew from metamorphic or hydrothermal fluids. Textural evidence of xenocrysts and a dominant population of ca. 1865 Ma zircon in juvenile crystal-rich andesite from the same pyroclastic deposit show that this basement has been assimilated by Redoubt magma. Equilibration temperatures and oxygen fugacities indicated by Fe-Ti–oxide minerals in the gabbros and crystal-rich andesite suggest sources near the margins of the Redoubt magmatic system, most likely in the magma accumulation and storage region currently outlined by seismicity and magma petrology at ∼4–10 km below sea level. Additionally, a partially melted gabbro from the 1990 eruption contains zircon with U-Pb ages between ca. 620 Ma and ca. 1705 Ma, as well as one zircon with a U-Th disequilibrium model age of 0 ka. The zircon ages demonstrate that Pennsylvanian, and probably Paleoproterozoic, igneous rocks exist in, or possibly beneath, Peninsular terrane basement. Discovery of Pennsylvanian gabbro similar in age to Skolai arc plutons 500 km to the northeast indicates that the Peninsular terrane, along with the Wrangellia and Alexander terranes, has been part of the Wrangellia composite terrane since at least Pennsylvanian time. Moreover, the zircon data suggest that a Paleoproterozoic continental fragment may be present in the mid-to-upper crust in southern Alaska.

U-Pb age constraints for the La Tuna Granite and Montevideo Formation (Paleoproterozoic, Uruguay): Unravelling the structure of the Río de la Plata Craton

NASA Astrophysics Data System (ADS)

Pamoukaghlián, Karina; Gaucher, Claudio; Frei, Robert; Poiré, Daniel G.; Chemale, Farid; Frei, Dirk; Will, Thomas M.

2017-11-01

The Río de la Plata Craton is a continental block that crops out in Uruguay, eastern Argentina, southernmost Brazil and Paraguay. It comprises in Uruguay the Piedra Alta, Tandilia and Nico Pérez terranes, separated by the Colonia and the Sarandí del Yí megashears. The La Tuna Granite, which intrudes the Araminda metasandstones in the Tandilia Terrane, was considered Cambrian in age and the intruded sandstones were assigned to the Neoproterozoic Piedras de Afilar Formation. We show that the granite is Paleoproterozoic in age and that the host metasandstones do not belong to the Piedras de Afilar Formation, but to the Paleoproterozoic Montevideo Formation. U-Pb LA ICP-MS of zircon ages for the La Tuna Granite yielded a concordant crystallization age of 2156 ± 26 Ma. Furthermore a metamorphic event at 2010 ± 9 Ma is revealed by Pb stepwise leaching dating of monazites. U-Pb detrital zircon ages of the host Araminda metasandstone yield an upper intercept discordia age of 2152 ± 29 Ma, which marks the intrusion of the La Tuna pluton, and which is in accordance with the zircon U-Pb LA ICP MS constraints. A concordant U-Pb detrital zircon age of 2465 ± 40 Ma provides a maximum depositional age constraint for the metapsammites. Comparing quartz arenites of the Ediacaran Piedras de Afilar Formation with the Araminda metaquartzites, we conclude that they are very similar regarding petrology but they differ in age and metamorphic overprint. Detrital zircons in quartz arenites of the Piedras de Afilar Formation show youngest ages of 1.0 Ga. On the other hand, detrital zircons recovered from the Araminda metasandstones and the age of the intruding granite allow interpreting a depositional age between 2465 and 2150 Ma. Nd model ages show crustal residence times in average more than 200 myr older for the Tandilia Terrane both in Uruguay and Argentina, with a significant Neoarchean component, which is lacking in the Piedra Alta Terrane. Whereas the Piedra Alta Terrane was formed rapidly from a juvenile source, the Tandilia Terrane shows a more protracted history. Our data support the interpretation of the Tandilia Terrane as a separate tectonostratigraphic unit.

Post-caldera volcanism: In situ measurement of U-Pb age and oxygen isotope ratio in Pleistocene zircons from Yellowstone caldera

USGS Publications Warehouse

Bindeman, I.N.; Valley, J.W.; Wooden, J.L.; Persing, H.M.

2001-01-01

The Yellowstone Plateau volcanic field, the site of some of the largest known silicic volcanic eruptions, is the present location of NE-migrating hotspot volcanic activity. Most volcanic rocks in the Yellowstone caldera (0.6 Ma), which formed in response to the climactic eruption of 1000 km3 of Lava Creek Tuff (LCT), have unusually low oxygen isotope ratios. Ion microprobe analysis of both U-Pb age and ??18O in zircons from these low-??18O lavas reveals evidence of complex inheritance and remelting. A majority of analyzed zircons from low-??18O lavas erupted inside the Yellowstone caldera have cores that range in age from 2.4 to 0.7 Ma, significantly older than their eruption ages (0.5-0.4 Ma). These ages and the high-??18O cores indicate that these lavas are largely derived from nearly total remelting of normal-??18O Huckleberry Ridge Tuff (HRT) and other pre-LCT volcanic rocks. A post-HRT low-??18O lava shows similar inheritance of HRT-age zircons. The recycling of volcanic rocks by shallow remelting can change the water content and eruptive potential of magma. This newly proposed mechanism of intracaldera volcanism is best studied by combining in situ analysis of oxygen and U-Pb isotope ratios of individual crystals. ?? 2001 Elsevier Science B.V. All rights reserved.

The Age of the Moon

NASA Astrophysics Data System (ADS)

Barboni, M.; Boehnke, P.; Keller, C. B.; Kohl, I. E.; McKeegan, K. D.; Schoene, B.; Young, E. D.

2016-12-01

Knowledge of the age of the Moon is important for understanding the early evolution of the solar system, including the timing of the hypothesized Giant Impact (GI). There have been many attempts to determine the Moon's age, but significant disagreement remains with some authors favoring an early formation and others arguing for a relatively young Moon formed at 4.4 Ga. Attempts to date the GI indirectly through its effects on the asteroid belt are problematic as there is no way to uniquely ascertain the cause of the observed disturbances (e.g., GI or meteorite parent body breakup). Determining the timing of the Lunar Magma Ocean (LMO) crystallization provides a more direct constraint on the age of the Moon, but interpreting the chronologic significance of LMO products is complicated by the fact that the only rock samples available are breccias. A better approach is to construct a model age for the fractional crystallization of the LMO since this should provide a global signature. Zircons from the Apollo samples are ancient, robust against later disturbances, and amenable to precise U-Pb geochronology and Hf isotope analyses that can be used to construct Lu-Hf model ages for the silicate differentiation of the Moon. Previous isotopic studies of Apollo zircons yielded artificially young Hf model ages because of the (then unknown) effect of neutron capture on Hf isotopic ratios generated by long exposure to cosmic radiation, and were unable to determine whether or not the U-Pb dates were concordant due to insufficient precision of in situ dating techniques. We have addressed these issues by carrying out CA-ID-TIMS U-Pb geochronology on Apollo 14 zircon fragments, followed by Hf isotope determination by solution MC-ICP-MS on the same volume of zircon. By constructing Hf model ages from zircons that are concordant to the sub-permil level, we show that the minimum age for the end of differentiation of the LMO, and by extension, the formation of the Moon, is 4.52 ± 0.01 Ga.

U-series in zircon and 40Ar/39Ar geochronology reveal the most recent stage of a supervolcanic cycle in the Altiplano-Puna Volcanic Complex, Central Andes

NASA Astrophysics Data System (ADS)

Tierney, C.; de Silva, S. L.; Schmitt, A. K.; Jicha, B.; Singer, B. S.

2010-12-01

The ignimbrite flare up that produced the Altiplano-Puna Volcanic Complex of the Central Andes is characterized by episodic supervolcanism over a ~10 Ma time-span that climaxed about 4Ma. Since peak activity, the temporal and spatial record of volcanism suggests a waning of the system with only one other supervolcanic eruption at 2.6Ma. The most recent phase of volcanism from the APVC comprises a series of late Pleistocene domes that share a general petrochemical resemblance to the ignimbrites. New U-series data on zircons and high precision 40Ar/39Ar age determinations reveal that these effusive eruptions represent a temporally coherent magmatic episode. The five largest domes (Chao, Chillahuita, Chanka, Chascon-Runtu Jarita, and Tocopuri) have a combined volume >40 km3, and are distributed over an elliptical area of over 3000km2 centered at 22°S 68°W. They are crystal rich (>50%) dacites to rhyolites. New 40Ar/39Ar age determinations on biotite for the domes range range from 108±6 to 190±50 ka. However, 40Ar/39Ar ages from sanidine for some of the domes are more precise and span from 87±4 to 97±2 ka. We therefore interpret the eruption age of all these domes to be ~90 - 100 ka. This is consistent with SIMS U-series crystallization ages from the rims of 66 zircon crystals from four of the domes that reveal a fairly continuous spread of ages from ~90 ka to >300 ka with potentially common peaks in zircon ages at 100 ka and ~200 ka. U-Pb dating on the interiors of some of these zircon crystals indicates crystallization ages of up to 1.5 Ma. The common peaks of zircon crystallization between domes suggest that magma that fed these domes shared a larger regional source. Furthermore, the large volume of this potential source and the crystal-rich nature of the lava imply that this source was likely a large body of crystal-mush. The continuous nature of the zircon rim age population indicates that the residence time of this magma body was likely >200kyr. Potential peaks in zircon crystallization ages could result from periodic injections of andesitic magma that reinvigorated crystallization. However, the ubiquity of likely antecrystic zircon interiors suggests that the associated temperature rise was insufficient to cause complete resorbtion of the antecrysts. A shared peak of zircon crystallization just prior to eruptions, as well as co-eruption of andesitic lava connotes recharge as the eruption trigger.

High-Precision U-Pb Geochronology of Ice River Perovskite: A Possible Interlaboratory and Intertechnique EARTHTIME Standard

NASA Astrophysics Data System (ADS)

Burgess, S. D.; Bowring, S. A.; Heaman, L. M.

2012-12-01

Accurate and precise U-Pb geochronology of accessory phases other than zircon are required for dating some LIP basalts or determining the temporal patterns of kimberlite pipes, for example. Advances in precision and accuracy lead directly to an increase in the complexity of questions that can be posed. U-Pb geochronology of perovskite (CaTiO3) has been applied to silica-undersaturated basalts, carbonatites, alkaline igneous rocks, and kimberlites. Most published IDTIMS perovskite dates have 2-sigma precisions at the ~0.2% level for weighted mean 206Pb/238U dates, much less than possible with IDTIMS analyses of zircons, which limits the applicability of perovskite in high-precision applications. Precision on perovskite dates is lower than zircon because of common Pb, which in some cases can be up to 50% of the total Pb and must be corrected for and accurately partitioned between blank and initial. Relatively small changes in the composition of common Pb can result in inaccurate but precise dates. In many cases minerals with significant common Pb are corrected using Stacey and Kramers (1975) two stage Pb evolution model. This can be done without serious consequence to the final date for minerals with high U/Pb ratios. In the more common case where U/Pb ratios are relatively low and the proportion of common Pb is large, applying a model-derived Pb isotopic composition rather than measuring it directly can introduce percent-level inaccuracy to dates calculated with precisely known U/Pb ratios. Direct measurement of the common Pb composition can be done on a U-poor mineral that co-crystallized with perovskite; feldspar and clinopyroxene are commonly used. Clinopyroxene can contain significant in-grown radiogenic Pb and our experiments indicate that it is not eliminated by aggressive step-wise leaching. The U/Pb ratio in clinopyroxene is generally low (20 < mu < 50) but significant. Other workers (e.g. Kamo et al., 2003; Corfu and Dahlgren, 2008), have used two methods to determine the amount of ingrown Pb. First, by measuring the U/Pb ratio in clinopyroxene and assuming a crystallization age the amount of ingrown Pb can be calculated. Second, by assuming that perovskite and clinopyroxene (± other phases) are isochronous, the initial Pb isotopic composition can be calculated using the y-intercept on 206Pb/238U, 207Pb/235U, and 3-D isochron diagrams. To further develop a perovskite mineral standard for use in high-precision dating applications, we have focused on single grains/fragments of perovskite and multi-grain clinopyroxene fractions from a melteigite sample (IR90.3) within the Ice River complex, a zoned alkaline-ultramafic intrusion in southeastern British Columbia. Perovskite from this sample has variable measured 206Pb/204Pb (22-263), making this an ideal sample on which to test the sensitivity of the date on grains with variable amounts of radiogenic Pb to changes in common Pb composition. Using co-existing clinopyroxene for the initial common Pb composition by both direct measurement and by the isochron method allows us to calculate an accurate weighted-mean 206Pb/238U date on perovskite at the < 0.1% level, which overlaps within uncertainty for the two different methods. We recommend the Ice River 90.3 perovskite as a suitable EARTHTIME standard for interlaboratory and intertechnique comparison.

New Zircon U-Pb Age Constrain of the Origin of Devil's River Uplift (SW Texas) and Insights into the Late Proterozoic and Paleozoic Evolution of the Southern Margin of Laurentia

NASA Astrophysics Data System (ADS)

Rodriguez, E.; Dickerson, P. W.; Stockli, D. F.

2017-12-01

The Devils River Uplift (DRU) in SW Texas records the evolution of the southern Laurentian margin from Grenvillian orogenesis and assembly of Rodinia, to its fragmentation by rifting, and to the amalgamation of Pangaea. It was cored by a well (Shell No. 1 Stewart), penetrating Precambrian gneisses and Cambrian metasediments and sandstones. New zircon LA-ICP-MS data from a total of 10 samples elucidate the crystallization and depositional ages, as well as the detrital provenance, of Precambrian and Cambrian rocks from the DRU. Zircons from five Precambrian crystalline basement samples (6000-9693') yield uniform U-Pb crystallization ages of 1230 Ma that are similar to ages for young gneisses of the Valley Spring Domain (Llano uplift) in central Texas, where they mark the cessation of arc magmatism within the Grenville orogenic belt. The 1230 Ma igneous basement is overlain by L.-M. Cambrian metasedimentary rocks ( 4000-6000') with maximum depositional ages of 533-545 Ma. Detrital zircons from Cambrian strata are dominated by a 1070-1080 Ma population, likely derived from basement units exposed in Texas (Llano uplift, Franklin Mts.), with minor contributions from local 1230 Ma Precambrian basement and the 1380-1500 Ma Granite Rhyolite Province. The L.-M. Cambrian interval is dominated (>80%) by Neoproterozoic detrital magmatic zircons with two major distinct age clusters at 570-700 Ma and 780-820 Ma, supporting a two-stage Rodinia rift model and providing strong evidence for major Cryogenian-Eocambrian intraplate magmatism along the southern margin of Rodinia. Moreover, detrital zircon signatures for L.-M. and U. Cambrian strata strongly correlate with those from the Cuyania terrane of W. Argentina - notably the W. Sierras Pampeanas (Sa. Pie de Palo, Sa. de Maz): 1230 Ma from metasandstones (PdP); 1081-1038 Ma from metasiliciclastics (PdP, SdM); Cryogenian-Eocambrian [774 & 570 Ma] plutons (SdM, PdP). In summary, these new zircon U-Pb data from DRU in SW Texas show that it is part of the Grenville orogenic belt, characterized by 1230 Ma magmatism, and that it experienced Cryogenian-Eocambrian intraplate magmatism as well. Significant correlations between DRU and the Cuyania terrane imply that both participated in Rodinia rifting and creation of the southern Laurentian margin.

Precambrrian crustal evolution in the great falls tectonic zone

NASA Astrophysics Data System (ADS)

Gifford, Jennifer N.

The Great Falls Tectonic Zone (GFTZ) is a zone of northeast trending geological structures in central Montana that parallel structures in the underlying basement. U-Pb zircon and Nd isotopic data from the Little Belt Mountains (LBM) suggest that the GFTZ formed at ~1.86 to 1.80 Ga due to ocean subduction followed by collision between the Archean Wyoming Province (WP) and Medicine Hat Block (MHB). This study characterizes the GFTZ basement by geochronological and geochemical analysis of crustal xenoliths collected from Montana Alkali Province volcanics and exposed basement rock in the Little Rocky Mountains (LRM). Xenoliths collected from the Grassrange and Missouri Breaks diatremes and volcanics in the Bearpaw and Highwood Mountains have igneous crystallization ages from ~1.7 Ga to 1.9 Ga and 2.4 Ga to 2.7 Ga, and metamorphic ages from ~1.65 Ga to 1.84 Ga. Zircon Lu-Hf and whole-rock Sm-Nd data indicate that the xenoliths originated from reworked older continental crust mixed with mantle-derived components in all cases. Trace element patterns show fluid mobile element enrichments and fluid immobile element depletions suggestive of a subduction origin. Igneous ages in the LRM range older, from ~2.4 Ga to 3.2 Ga. Geochemical evidence suggests that the LRM meta-igneous units also formed in a subduction setting. Detrital zircon ages span the early Paleoproterozoic to Mesoarchean, with abundant 2.8 Ga ages. Zircon U-Pb igneous crystallization age data from xenoliths and the LRM are consistent with U-Pb zircon igneous crystallization ages from the MHB, suggesting that this segment of the GFTZ shares an affinity with concealed MHB crust. Published detrital zircon ages from the northern Wyoming Province reveal more abundant >3.0 Ga ages than the MHB or GFTZ samples. These geochronologic and geochemical data from the xenoliths and LRM samples allow for a refined model for crustal evolution in the GFTZ. Subduction under the Neoarchean to Paleoproterozoic crust of the MHB formed an igneous arc followed by metamorphism during the MHB-WP collision. Later Paleoproterozoic tectonothermal activity represents post-orogenic collapse after the terminal collision. Tectonic activity in the Cenozoic led to basement uplift and the formation of xenolith bearing volcanic units sampled for this study.

The effect of weathering on U-Th-Pb and oxygen isotope systems of ancient zircons from the Jack Hills, Western Australia

NASA Astrophysics Data System (ADS)

Pidgeon, R. T.; Nemchin, A. A.; Whitehouse, M. J.

2017-01-01

We report the result of a SIMS U-Th-Pb and O-OH study of 44 ancient zircons from the Jack Hills in Western Australia with ages ranging from 4.3 Ga to 3.3 Ga. We have investigated the behaviour of oxygen isotopes and water in the grains by determining δ18O and OH values at a number of locations on the polished surfaces of each grain. We have divided the zircons into five groups on the basis of their U-Th-Pb and OH-oxygen isotopic behaviour. The first group has concordant U-Th-Pb ages, minimal common Pb, δ18O values consistent with zircons derived from mantle source rocks and no detectable OH content. U-Th-Pb systems in zircons from Groups 2, 3 and 4 vary from concordant to extremely discordant where influenced by cracks. Discordia intercepts with concordia at approximately zero Ma age are interpreted as disturbance of the zircon U-Th-Pb systems by weathering solutions during the extensive, deep weathering that has affected the Archean Yilgarn Craton of Western Australia since at least the Permian. Weathering solutions entering cracks have resulted in an influx of Th and U. δ18O values of Group 2 grains fall approximately within the "mantle" range and OH is within background levels or slightly elevated. δ18O values of Group 3 grains are characterised by an initial trend of decreasing δ18O with increasing OH content. With further increase in OH this trend reverses and δ18O becomes heavier with increasing OH. Group 4 grains have a distinct trend of increasing δ18O with increasing OH. These trends are explained in terms of the reaction of percolating water with the metamict zircon structure and appear to be independent of analytical overlap with cracks. Group five zircons are characterised by U-Pb systems that appear to consist of more than one age but show only minor U-Pb discordance. Nevertheless trends in δ18O versus OH in this group of grains resemble trends seen in the other groups. The observed trends of δ18O with OH in the Jack Hills zircons are similar to those reported in a previous study of zircons from an Archean granite from south-western Australia.

A search for thermal excursions from ancient extraterrestrial impacts using Hadean zircon Ti-U-Th-Pb depth profiles.

PubMed

Abbott, Sunshine S; Harrison, T Mark; Schmitt, Axel K; Mojzsis, Stephen J

2012-08-21

Few terrestrial localities preserve more than a trace lithic record prior to ca. 3.8 Ga greatly limiting our understanding of the first 700 Ma of Earth history, a period inferred to have included a spike in the bolide flux to the inner solar system at ca. 3.85-3.95 Ga (the Late Heavy Bombardment, LHB). An accessible record of this era may be found in Hadean detrital zircons from the Jack Hills, Western Australia, in the form of μm-scale epitaxial overgrowths. By comparing crystallization temperatures of pre-3.8 Ga zircon overgrowths to the archive of zircon temperature spectra, it should, in principle, be possible to identify a distinctive impact signature. We have developed Ti-U-Th-Pb ion microprobe depth profiling to obtain age and temperature information within these zircon overgrowths and undertaken a feasibility study of its possible use in identifying impact events. Of eight grains profiled in this fashion, four have overgrowths of LHB-era age. Age vs. temperature profiles reveal a period between ca. 3.85-3.95 Ga (i.e., LHB era) characterized by significantly higher temperatures (approximately 840-875 °C) than do older or younger zircons or zircon domains (approximately 630-750 °C). However, temperatures approaching 900 °C can result in Pb isotopic exchange rendering interpretation of these profiles nonunique. Coupled age-temperature depth profiling shows promise in this role, and the preliminary data we report could represent the first terrestrial evidence for impact-related heating during the LHB.

Zircon U-Pb ages and geochemistry of migmatites and granites in the Foping dome: Evidence for Late Triassic crustal evolution in South Qinling, China

NASA Astrophysics Data System (ADS)

Zhang, He; Li, Shuang-Qing; Fang, Bo-Wen; He, Jian-Feng; Xue, Ying-Yu; Siebel, Wolfgang; Chen, Fukun

2018-01-01

Migmatites provide a record of melt formation and crustal rheology. In this study we present zircon U-Pb ages and geochemical composition of migmatites from the Foping dome and granites from the Wulong pluton. U-Pb results from migmatite zircons indicate two episodes of partial melting. Rim domains from a leucosome in the Longcaoping area yield an age of ca. 209 Ma. Migmatites collected from the Foping dome yield U-Pb zircon ages of 2910 to 190 Ma, suggesting the involvement of meta-sedimentary source components. Rim domains of the zircons with low Th/U ratios (< 0.1) give ages of 225-190 Ma and the youngest age domains (ca. 195 Ma) are characterized by low contents of heavy rare earth elements, which is related to crystallization of garnet. Magmatic rocks from the Wulong pluton can be subdivided into high Sr/Y and low Sr/Y granites. U-Pb zircon ages vary from 219 to 214 Ma for the high Sr/Y granites and from 214 to 192 Ma for the low Sr/Y granites. High Sr/Y granites have higher Na2O and Sr contents than the low Sr/Y granites. They also lack negative Eu anomalies and are depleted in HREE compared to the low Sr/Y granites. Initial 87Sr/86Sr ratios and εNd values of all the samples roughly overlap with those of Neoproterozoic basement rocks exposed in South Qinling. Including previous studies, we propose that the high and low Sr/Y granites formed by melting of thickened and normal crust, respectively. Close temporal-spatial relationship of the high and low Sr/Y granites with the two-stage migmatization events implies variation of crustal thickness and thermal overprints of the orogenic crust in post-collisional collapse. Following the collision of South Qinling and the Yangtze block prior to 219 Ma, partial melting of the deep crust occurred. The melts migrated upwards to form the high Sr/Y granites. This process occurred rapidly and caused collapse of the thickened crust and carried heat upwards, leading to further partial melting within the shallower crust and formation of the low Sr/Y granites.

In situ U-Pb and Lu-Hf isotopic studies of zircons from the Sancheong-Hadong AMCG suite, Yeongnam Massif, Korea: Implications for the petrogenesis of ∼1.86 Ga massif-type anorthosite

NASA Astrophysics Data System (ADS)

Lee, Yuyoung; Cho, Moonsup; Yi, Keewook

2017-05-01

Isotopic and geochemical characteristics of Proterozoic anorthosite-mangerite-charnockite-granite (AMCG) suite have long been used for tracing the mantle-crustal source and magmatic evolution. We analyzed Lu-Hf isotopic compositions of zircon from the Sancheong-Hadong AMCG complex, Yeongnam Massif, Korea, in order to understand tectonomagmatic evolution of the Paleoproterozoic AMCG suite occurring at the southeastern margin of the North China Craton (NCC). The anorthositic rocks in this complex, associated with charnockitic and granitic gneisses, were recrystallized to eradicate magmatic features. In situ SHRIMP (sensitive high-resolution ion microprobe) U-Pb analyses of zircon from a leuconorite and an oxide-bearing gabbroic dyke yielded weighted mean 207Pb/206Pb ages of 1870 ± 2 Ma and 1861 ± 6 Ma, respectively. Charnockitic, granitic, and porphyroblastic gneisses yielded weighted mean 207Pb/206Pb zircon ages of 1861 ± 6 Ma, 1872 ± 6 Ma, and 1873 ± 4 Ma, respectively. These crystallization ages, together with our previous geochronological data for anorthosites (1862 ± 2 Ma), are indicative of episodic AMCG magmatism over an ∼10 Ma interval. Initial εHf(t) values of zircon analyzed from five anorthositic rocks and four felsic gneisses range from +2.1 to -6.1 and -0.3 to -5.4, respectively. Zircon Hf isotopic data in combination with available whole rock Sr-Nd isotopic data suggest that anorthositic parental magma was most likely derived from a mantle source and variably affected by crustal contamination. This crustal component is also reflected in charnockitic-granitic magmas produced primarily by the melting of lower crust. Taken together, the AMCG magmatism at 1.87-1.86 Ga in the Yeongnam Massif is most likely a late orogenic product of Paleoproterozoic NCC amalgamation tectonically linked to assembly of the Columbia supercontinent.

Nanogeochronology of discordant zircon measured by atom probe microscopy of Pb-enriched dislocation loops

PubMed Central

Peterman, Emily M.; Reddy, Steven M.; Saxey, David W.; Snoeyenbos, David R.; Rickard, William D. A.; Fougerouse, Denis; Kylander-Clark, Andrew R. C.

2016-01-01

Isotopic discordance is a common feature in zircon that can lead to an erroneous age determination, and it is attributed to the mobilization and escape of radiogenic Pb during its post-crystallization geological evolution. The degree of isotopic discordance measured at analytical scales of ~10 μm often differs among adjacent analysis locations, indicating heterogeneous distributions of Pb at shorter length scales. We use atom probe microscopy to establish the nature of these sites and the mechanisms by which they form. We show that the nanoscale distribution of Pb in a ~2.1 billion year old discordant zircon that was metamorphosed c. 150 million years ago is defined by two distinct Pb reservoirs. Despite overall Pb loss during peak metamorphic conditions, the atom probe data indicate that a component of radiogenic Pb was trapped in 10-nm dislocation loops that formed during the annealing of radiation damage associated with the metamorphic event. A second Pb component, found outside the dislocation loops, represents homogeneous accumulation of radiogenic Pb in the zircon matrix after metamorphism. The 207Pb/206Pb ratios measured from eight dislocation loops are equivalent within uncertainty and yield an age consistent with the original crystallization age of the zircon, as determined by laser ablation spot analysis. Our results provide a specific mechanism for the trapping and retention of radiogenic Pb during metamorphism and confirm that isotopic discordance in this zircon is characterized by discrete nanoscale reservoirs of Pb that record different isotopic compositions and yield age data consistent with distinct geological events. These data may provide a framework for interpreting discordance in zircon as the heterogeneous distribution of discrete radiogenic Pb populations, each yielding geologically meaningful ages. PMID:27617295

Investigating sources of ignimbrites in the Altiplano-Puna Volcanic Complex using U-Pb dating of zircons

NASA Astrophysics Data System (ADS)

Kern, J. M.; de Silva, S. L.; Schmitt, A. K.

2011-12-01

Large silicic volcanic fields (LSVFs) are thought to represent the surface expression of upper crustal batholith emplacement, with the spatiotemporal distribution of the vents and eruptions representing the development of the system. The Altiplano-Puna Volcanic Complex (APVC) in the Central Andes is a LSVF active from 11-1 Ma that erupted over 13,000 km3 of magma from large, multicyclic caldera centers and smaller ignimbrite shields during 3 distinct pulses of volcanism at 8.4, 5.5, and 4.0 Ma. Links to the magmatic system beneath are being pursued through U-Pb zircon dating of APVC ignimbrites. Initial results comprise 61 238U/206Pb zircon ages of mostly marginal crystal domains from five APVC ignimbrites-the 0.98 ± 0.03 Ma Purico, 3.96 ± 0.08 Ma Atana, 4.0 ± 0.9 Ma Toconao, 4.09 ± 0.02 Ma Puripicar, and 8.33 ± 0.06 Ma Sifon ignimbrites-dated by high-resolution secondary ionization mass spectrometry (SIMS). Each zircon analyzed was less than 350 μm in length and cathodoluminescence images reveal zonations within individual zircons, though significant core-rim age differences are rare. The ~1 Ma Purico ignimbrite displays multiple zircon age populations significantly predating the 40Ar/39Ar eruption age, but younger than ages from the nearby large-volume Atana ignimbrite erupted from La Pacana caldera. Some peaks do, however, coincide with later resurgent activity within La Pacana as expressed by the 2.7 Ma Cerro Bola dome. Zircon ages in the Atana ignimbrite are indistinguishable from its eruption, while those from the 4.0 Ma Toconao ignimbrite-the volatile-rich cap of the Atana magma chamber-contains three populations of xenocrystic zircons from the Proterozoic-Ordivician, ~13 Ma, and ~9 Ma. The ~9 Ma zircons correlate with K-Ar ages from an underlying ignimbrite, whereas the 13 Ma xenocrysts likely have a plutonic source. The Purico ignimbrite thus provides direct evidence of zircon inheritance from previous eruption cycles, while the Toconao records a much more complex history of inheritance and assimilation absent from its consanguineous counterpart, the Atana. The 4.09 Ma Puripicar and 8.33 Ma Sifon ignimbrites display a single zircon age peak up to ~ 0.5 Ma prior to the eruption age. While these data may suggest relatively long magma residence times, the time periods are significantly shorter than the repose period between episodes of ignimbrite eruptions. New U-Pb zircon ages further the understanding of the connections between LSFV ignimbrites and their underlying batholiths.

U-Pb systematics in coexisting zircon, rutile and titanite from granophyres in the Archean Stillwater Complex: metamictization and the fate of radiogenic Pb

NASA Astrophysics Data System (ADS)

Friedman, R. M.; Wall, C. J.; Scoates, J. S.; Meurer, W. P.

2009-12-01

Self-irradiation of zircon causes structural damage (metamictization) that can result in the loss of radiogenic Pb during interaction with aqueous solutions. To evaluate this behavior in metamict zircon, and in other U-bearing accessory phases like titanite and rutile, we are examining the U-Pb systematics of granophyric rocks from the ca. 2.7 Ga Stillwater layered intrusion, Montana. Four samples were studied in detail, including a pegmatitic ksp-qtz core to a gabbroic pegmatoid in the Lower Banded Series (N1), an alaskite and an amphibole-rich reaction zone between the alaskite and anorthosite (AN1) in the Middle Banded Series, and an amphibole-bearing granophyre from the Upper Banded Series (GN3). Except in the pegmatite, zircon is variably metamict with amorphous zones characterized by distinctive Ca-enrichment. Single zircon grains were analyzed by ID-TIMS following annealing and chemical abrasion, and multi-grain (n=4-5) fractions of titanite and rutile were analyzed by conventional ID-TIMS; the UBC 233-235U-205Pb isotopic tracer is calibrated against mixed U-Pb gravimetric reference solutions made available through the EarthTime initiative. The U-Pb systematics are coherent only for the pegmatite yielding both a Concordia age of 2709.60 ± 0.80 Ma (2σ, including tracer calibration, decay-constant errors not included) for low-U zircon (76-237 ppm) and concordant titanite results with 207Pb/206Pb ages from 2701-2710 Ma. The results for high-U zircon (up to 1438 ppm) for the other three samples are strongly discordant (9-43%, 85-89%, 28-71%, respectively) with a wide range of 207Pb/206Pb ages (2583-2647 Ma, 2210-2357 Ma, 2345-2499 Ma). Given the extreme incompatibility of Pb2+ in zircon and the highly metamict state of zircon in these granophyres, we are investigating the extent to which radiogenic lead is selectively removed during the chemical abrasion and annealing process from step-wise leaching experiments and image analysis (CL, SEM). In contrast, titanite and rutile analyses yield highly variable results that are grouped based on degree of discordance: (1) minor to moderate discordance (0-20%) with titanite ages at ca. 2.7 Ga and rutile ages from 2.6-2.7 Ga, (2) strong discordance (34-78%) with variable and young ages defined by fractions with relatively high-U contents (60-774 ppm), and (3) strong reverse discordance (3-21%), especially titanite from the reaction zone, which nonetheless gives 207Pb/206Pb ages of ca. 2705 Ma. We speculate that reverse discordance may be related to elemental redistribution in the granophyres during hydrothermal alteration of the co-existing highly metamict zircon, and other U-bearing phases, and subsequent incorporation of Pb into co-existing titanite and rutile.

Timing and duration of garnet granulite metamorphism in magmatic arc crust, Fiordland, New Zealand

USGS Publications Warehouse

Stowell, H.; Tulloch, A.; Zuluaga, C.; Koenig, A.

2010-01-01

Pembroke Granulite from Fiordland, New Zealand provides a window into the mid- to lower crust of magmatic arcs. Garnet Sm-Nd and zircon U-Pb ages constrain the timing and duration of high-P partial melting that produced trondhjemitic high Sr/Y magma. Trace element zoning in large, euhedral garnet is compatible with little post growth modification and supports the interpretation that garnet Sm-Nd ages of 126.1??2.0 and 122.6??2.0. Ma date crystal growth. Integration of the garnet ages with U-Pb zircon ages elucidates a history of intrusion(?) and a protracted period of high-temperature metamorphism and partial melting. The oldest zircon ages of 163 to 150. Ma reflect inheritance or intrusion and a cluster of zircon ages ca. 134. Ma date orthopyroxene-bearing mineral assemblages that may be magmatic or metamorphic in origin. Zircon and garnet ages from unmelted gneiss and garnet reaction zones record garnet granulite facies metamorphism at 128 to 126. Ma. Peritectic garnet and additional zircon ages from trondhjemite veins and garnet reaction zones indicate that garnet growth and partial melting lasted until ca. 123. Ma. Two single fraction garnet ages and young zircon ages suggest continued high-temperature re-equilibration until ca. 95. Ma. Phase diagram sections constrain orthopyroxene assemblages to <0.6 GPa @ 650??C, peak garnet granulite facies metamorphic conditions to 680-815??C @ 1.1-1.4. GPa, and a P-T path with a P increase of???0.5. GPa. These sections are compatible with water contents???0.28wt.%, local dehydration during garnet granulite metamorphism, and <0.3. GPa P increases during garnet growth. Results demonstrate the utility of integrated U-Pb zircon and Sm-Nd garnet ages, and phase diagram sections for understanding the nature, duration, and conditions of deep crustal metamorphism and melting. Geochronologic and thermobarometric data for garnet granulite indicate that thickening of arc crust, which caused high-pressure metamorphism in northern Fiordland, must have occurred prior to 126. Ma, that loading occurred at a rate of ca. 0.06. GPa/m.y., and that garnet granulite metamorphism lasted 3-7m.y. Locally-derived partial melts formed and crystallized in considerably less than 10 and perhaps as little as 3m.y. ?? 2010 Elsevier B.V.

Permian A-type rhyolites of the Muráň Nappe, Inner Western Carpathians, Slovakia: in-situ zircon U-Pb SIMS ages and tectonic setting

NASA Astrophysics Data System (ADS)

Ondrejka, Martin; Li, Xian-Hua; Vojtko, Rastislav; Putis, Marian; Uher, Pavel; Sobocký, Tomas

2018-04-01

Three representative A-type rhyolitic rock samples from the Muráň Nappe of the inferred Silicic Unit of the Inner Western Carpathians (Slovakia) were dated using the high-precision SIMS U-Pb isotope technique on zircons. The geochronological data presented in this paper is the first in-situ isotopic dating of these volcanic rocks. Oscillatory zoned zircon crystals mostly revealed concordant Permian (Guadalupian) ages: 266.6 ± 2.4 Ma in Tisovec-Rejkovo (TIS-1), 263.3 ± 1.9 Ma in Telgárt-Gregová Hill (TEL-1) and 269.5 ± 1.8 Ma in Veľká Stožka-Dudlavka (SD-2) rhyolites. The results indicate that the formation of A-type rhyolites and their plutonic equivalents are connected to magmatic activity during the Permian extensional tectonics and most likely related to the Pangea supercontinent break-up.

U-Pb zircon, geochemical and Sr-Nd-Hf-O isotopic constraints on age and origin of the ore-bearing intrusions from the Nurkazgan porphyry Cu-Au deposit in Kazakhstan

NASA Astrophysics Data System (ADS)

Shen, Ping; Pan, Hongdi; Seitmuratova, Eleonora; Jakupova, Sholpan

2016-02-01

Nurkazgan, located in northeastern Kazakhstan, is a super-large porphyry Cu-Au deposit with 3.9 Mt metal copper and 229 tonnage gold. We report in situ zircon U-Pb age and Hf-O isotope data, whole rock geochemical and Sr-Nd isotopic data for the ore-bearing intrusions from the Nurkazgan deposit. The ore-bearing intrusions include the granodiorite porphyry, quartz diorite porphyry, quartz diorite, and diorite. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating indicates that the granodiorite porphyry and quartz diorite porphyry emplaced at 440 ± 3 Ma and 437 ± 3 Ma, respectively. All host rocks have low initial 87Sr/86Sr ratios (0.70338-0.70439), high whole-rock εNd(t) values (+5.9 to +6.3) and very high zircon εHf(t) values (+13.4 to +16.5), young whole-rock Nd and zircon Hf model ages, and consistent and slightly high zircon O values (+5.7 to +6.7), indicating that the ore-bearing magmas derived from the mantle without old continental crust involvement and without marked sediment contamination during magma emplacement. The granodiorite porphyry and quartz diorite porphyry are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) and depleted in high-field strength elements (HFSE), Eu, Ba, Nb, Sr, P and Ti. The diorite and quartz diorite have also LILE and LREE enrichment and HFSE, Nb and Ti depletion, but have not negative Eu, Ba, Sr, and P anomalies. These features suggest that the parental magma of the granodiorite porphyry and quartz diorite porphyry originated from melting of a lithospheric mantle and experienced fractional crystallization, whereas the diorite and quartz diorite has a relatively deeper lithospheric mantle source region and has not experienced strong fractional crystallization. Based on these, together with the coeval ophiolites in the area, we propose that a subduction of the Balkhash-Junggar oceanic plate took place during the Early Silurian and the ore-bearing intrusions and associated Nurkazgan porphyry Cu-Au deposit occurred in an intra-oceanic arc setting.

U-Pb geochronology of zircon and polygenetic titanite from the Glastonbury Complex, Connecticut, USA: An integrated SEM, EMPA, TIMS, and SHRIMP study

USGS Publications Warehouse

Aleinikoff, J.N.; Wintsch, R.P.; Fanning, C.M.; Dorais, M.J.

2002-01-01

U-Pb ages for zircon and titanite from a granodioritic gneiss in the Glastonbury Complex, Connecticut, have been determined using both isotope dilution thermal ionization mass spectrometry (TIMS) and the sensitive high resolution ion microprobe (SHRIMP). Zircons occur in three morphologic populations: (1) equant to stubby, multifaceted, colorless, (2) prismatic, dark brown, with numerous cracks, and (3) elongate, prismatic, light tan to colorless. Cathodoluminescence (CL) imaging of the three populations shows simple concentric oscillatory zoning. The zircon TIMS age [weighted average of 207Pb/206Pb ages from Group 3 grains-450.5 ?? 1.6 Ma (MSWD=1.11)] and SHRIMP age [composite of 206Pb/238 U age data from all three groups-448.2 ?? 2.7 Ma (MSWD = 1.3)], are interpreted to suggest a relatively simple crystallization history. Titanite from the granodioritic gneiss occurs as both brown and colorless varieties. Scanning electron microscope backscatter (BSE) images of brown grains show multiple cross-cutting oscillatory zones of variable brightness and dark overgrowths. Colorless grains are unzoned or contain subtle wispy or very faint oscillatory zoning. Electron microprobe analysis (EMPA) clearly distinguishes the two populations. Brown grains contain relatively high concentrations of Fe2O3, Ce2O3 (up to ~ 1.5 wt.%), Nb2O5, and Zr. Cerium concentration is positively correlated with total REE + Y concentration, which together can exceed 3.5 wt.%. Oscillatory zoning in brown titanite is correlated with variations in REE concentrations. In contrast, colorless titanite (both as discrete grains and overgrowths on brown titanite) contains lower concentrations of Y, REE, Fe2O3, and Zr, but somewhat higher Al2O3 and Nb2O5. Uranium concentrations and Th/U discriminate between brown grains (typically 200-400 ppm U; all analyses but one have Th/U between about 0.8 and 2) and colorless grains (10-60 ppm U; Th/U of 0-0.17). In contrast to the zircon U-Pb age results, SHRIMP U-Pb data from titanite indicate multiple growth episodes. In brown grains, oscillatory zoned cores formed at 443 ?? 6 Ma, whereas white (in BSE) cross-cutting zones are 425 ?? 9 Ma. Colorless grains and overgrowths on brown grains yield an age of 265 ?? 8 Ma (using the Total Pb method) or 265 ?? 5 Ma (using the weighted average of the 206Pb/238U ages). However, EMPA chemical data identify zoning that suggests that this colorless titanite may preserve three growth events. Oscillatory zoned portions of brown titanite grains are igneous in origin; white cross-cutting zones probably formed during a previously unrecognized event that caused partial dissolution of earlier titanite and reprecipitation of a slightly younger generation of brown titanite. Colorless titanite replaced and grew over the magmatic titanite during the Permian Alleghanian orogeny. These isotopic data indicate that titanite, like zircon, can contain multiple age components. Coupling SHRIMP microanalysis with EMPA and SEM results on dated zones as presented in this study is an efficient and effective technique to extract additional chronologic ?? 2002 Elsevier Science B.V. All rights reserved.

Exploring the pre-eruptive history of the Central Atlantic Magmatic Province (CAMP) and the link with the end Triassic extinction using high precision U-Pb zircon and baddeleyite geochronology

NASA Astrophysics Data System (ADS)

Davies, Joshua; Marzoli, Andrea; Bertrand, Hervé; Youbi, Nasrrddine; Schaltegger, Urs

2015-04-01

The Central Atlantic Magmatic Province (CAMP) is a massive outpouring of basaltic lava, dykes and sills that was predominantly emplaced into the Triassic-Jurassic basins of North and South America, Europe and Africa. These basins were, at the time, in the center of the paleo-supercontinent Pangea, and the CAMP flood basalts are associated with Pangea's break-up and the opening of the Atlantic Ocean. The global climatic and environmental impact of the basalt eruption has been temporally linked with the end-Triassic mass extinction, although the extinction horizon, defined by a carbon isotope excursion, is stratigraphically below the first basaltic flows in all of the currently identified basins. Therefore, if the extinction is related to the CAMP, it must be related to a process that occurred before the eruption of the first basalt flow, or is co-incident with a currently unidentified older basalt flow. Here we present high precision TIMS zircon U-Pb geochronology on zircons from the North Mountain basalt (NMB) in the Fundy basin, Canada, and also baddeleyite from the Foum Zuid dyke (FZD) in the Anti-Atlas, Morocco. The NMB zircons have been separated from the lowermost accessible basalt flow of the NMB sequence in a coarse-grained section, rather than from a felsic residual melt pod, which is the usual target for zircon geochronology in basalts. The baddeleyites from the FZD were also separated from a coarse-grained section of the dyke. The zircons and baddeleyites from the NMB and FZD samples contain an antecrystic population with ages more than 1 Ma older than the emplacement of the basalts. The U-Pb ages presented here suggest that there was magmatic activity relating to the CAMP before the eruption of the first basalts. There are a number of possible explanations for the old zircons 1) recycling of zircon from earlier phases of magmatism, which then would have to have been re-molten and entrained into the NMB and FZD magmas. 2) Recycling of crystal mush from the same magmatic system indicating that the system stayed at temperatures which enabled the magmas/crystal mushes to stay saturated in zircon and baddeleyite. 3) The older zircons are all xenocrysts or inherited cores from earlier magmatism. The identification of antecrystic zircon and baddeleyite in the basalts has significant implications for the relationship between the CAMP and the end Triassic extinction. Recycling of older Zr-phases, which crystallized earlier in the magmatic system that produced CAMP basalt melts, also bear important information on the chemical and physical dynamics of the magmatic plumbing system of the CAMP flood basalt province. We present geochronological information, CL images, and Hf isotopic information to support our interpretations for the origins of these important grains.

Electron Backscatter Diffraction (EBSD) Analysis and U-Pb Geochronology of the Oldest Lunar Zircon: Constraining Early Lunar Differentiation and Dating Impact-Related Deformation

NASA Technical Reports Server (NTRS)

Timms, Nick; Nemchin, Alexander; Grange, Marion; Reddy, Steve; Pidgeon, Bob; Geisler, Thorsten; Meyer, Chuck

2009-01-01

The evolution of the early moon was dominated by two processes (i) crystallization of the Lunar Magma Ocean (LMO) and differentiation of potassium-rare earth element-phosphorous-rich residual magma reservoir referred to as KREEP, and (ii) an intense meteorite bombardment referred to as lunar cataclysm . The exact timing of these processes is disputed, and resolution relies on collection and interpretation of precise age data. This study examines the microstructure and geochronology of zircon from lunar impact breccias collected during the Apollo 17 mission. A large zircon clast within lunar breccia 72215,195 shows sector zoning in optical microscopy, cathodoluminescence (CL) imaging and Raman mapping, and indicates that it was a relict fragment of a much larger magmatic grain. Sensitive high resolution ion microprobe (SHRIMP) U-Pb analysis of the zircon shows that U and Th concentration correlate with sector zoning, with darkest CL domains corresponding with high-U and Th (approx.150 and approx.100 ppm respectively), and the brightest-CL sectors containing approx.30-50 ppm U and approx.10-20 ppm Th. This indicates that variations in optical CL and Raman properties correspond to differential accumulation of alpha-radiation damage in each sector. Electron backscatter diffraction (EBSD) mapping shows that the quality of electron backscatter patterns (band contrast) varies with sector zoning, with the poorest quality patterns obtained from high-U and Th, dark-CL zones. EBSD mapping also reveals a deformation microstructure that is cryptic in optical, CL and Raman imaging. Two orthogonal sets of straight discrete and gradational low-angle boundaries accommodate approx.12 misorientation across the grain. The deformation bands are parallel to the crystallographic {a}-planes of the zircon, have misorientation axes parallel to the c-axis, and are geometrically consistent with formation by dislocation creep associated with <100>{010} slip. The deformation bands are unlike curved morphology of crystal-plastic microstructures in tectonically deformed terrestrial zircon, and geometrically similar to dislocation microstructures reported in experimentally shocked zircon. We interpret these crystal-plastic deformation microstructures to have resulted from a significant impact, either directly from impact shock, or during ductile flow directly following the impact. The deformation bands appear to continue undeflected through the non-indexed, radiation-damaged areas of the grain, which suggests that the orientation variation predates any significant mechanical weakening from radiation damage in the grain, and therefore occurred early in its history.

U Pb zircon age, geochemical and Sr Nd Pb Hf isotopic constraints on age and origin of alkaline intrusions and associated mafic dikes from Sulu orogenic belt, Eastern China

NASA Astrophysics Data System (ADS)

Liu, Shen; Hu, Ruizhong; Gao, Shan; Feng, Caixia; Qi, Youqiang; Wang, Tao; Feng, Guangying; Coulson, Ian M.

2008-12-01

Post-orogenic alkaline intrusions and associated mafic dikes from the Sulu orogenic belt of eastern China consist of quartz monzonites, A-type granites and associated mafic dikes. We report here U-Pb zircon ages, geochemical data and Sr-Nd-Pb-Hf isotopic data for these rocks. The SHRIMP U-Pb zircon analyses yield consistent ages ranging from 120.3 ± 2.1 Ma to 126.9 ± 1.9 Ma for five samples from the felsic rocks, and two crystallization ages of 119.0 ± 1.7 Ma and 120.2 ± 1.9 Ma for the mafic dikes. The felsic rocks and mafic dikes are characterized by high ( 87Sr/ 86Sr) i ranging from 0.7079 to 0.7089, low ɛNd( t) values from - 15.3 to - 19.2, 206Pb/ 204Pb = 16.54-17.25, 207Pb/ 204Pb = 15.38-15.63, 208Pb/ 204Pb = 37.15-38.45, and relatively uniform ɛHf( t) values of between - 21.6 ± 0.6 and - 23.7 ± 1.0, for the magmatic zircons. The results suggest that they were derived from a common enriched lithospheric mantle source that was metasomatized by foundered lower crustal eclogitic materials before magma generation. Geochemical and isotopic characteristics imply that the primary magma to these rocks originated through partial melting of ancient lithospheric mantle that was variably hybridized by melts derived from foundered lower crustal eclogite. The mafic dikes may have been generated by subsequent fractionation of clinopyroxene, whereas the felsic rocks resulted from fractionation of potassium feldspar, plagioclase and ilmenite or rutile. Both were not affected by crustal contamination. Combined with previous studies, these findings provide new evidence that the intense lithospheric thinning beneath the Sulu belt of eastern China occurred between 119 and 127 Ma, and that this was caused by the removal of the lower lithosphere (mantle and lower crust).

Coordinated U–Pb geochronology, trace element, Ti-in-zircon thermometry and microstructural analysis of Apollo zircons

DOE Office of Scientific and Technical Information (OSTI.GOV)

Crow, Carolyn A.; McKeegan, Kevin D.; Moser, Desmond E.

Here, we present the results of a coordinated SIMS U–Pb, trace element, Ti-in-zircon thermometry, and microstructural study of 155 lunar zircons separated from Apollo 14, 15, and 17 breccia and soil samples that help resolve discrepancies between the zircon data, the lunar whole rock history and lunar magma ocean crystallization models. The majority of lunar grains are detrital fragments, some nearly 1 mm in length, of large parent crystals suggesting that they crystallized in highly enriched KREEP magmas. The zircon age distributions for all three landing sites exhibit an abundance of ages at ~4.33 Ga, however they differ in thatmore » only Apollo 14 samples have a population of zircons with ages between 4.1 and 3.9 Ga. These younger grains comprise only 10% of all dated lunar zircons and are usually small and highly shocked making them more susceptible to Pb-loss. These observations suggest that the majority of zircons crystallized before 4.1 Ga and that KREEP magmatism had predominantly ceased by this time. We also observed that trace element analyses are easily affected by contributions from inclusions (typically injected impact melt) within SIMS analyses spots. After filtering for these effects, rare-earth element (REE) abundances of pristine zircon are consistent with one pattern characterized by a negative Eu anomaly and no positive Ce anomaly, implying that the zircons formed in a reducing environment. This inference is consistent with crystallization temperatures based on measured Ti concentrations and new estimates of oxide activities which imply temperatures ranging between 958 ± 57 and 1321 ± 100 °C, suggesting that zircon parent magmas were anhydrous. Together, the lunar zircon ages and trace elements are consistent with a ≤300 My duration of KREEP magmatism under anhydrous, reducing conditions. We also report two granular texture zircons that contain baddeleyite cores, which both yield 207Pb– 206Pb ages of 4.33 Ga. These grains are our best constraints on impact ages within our sample population, and suggest at least one large impact is contemporaneous with the most common time of magmatic zircon formation on the Moon’s crust visited by the Apollo missions.« less

Coordinated U–Pb geochronology, trace element, Ti-in-zircon thermometry and microstructural analysis of Apollo zircons

DOE PAGES

Crow, Carolyn A.; McKeegan, Kevin D.; Moser, Desmond E.

2016-12-28

Here, we present the results of a coordinated SIMS U–Pb, trace element, Ti-in-zircon thermometry, and microstructural study of 155 lunar zircons separated from Apollo 14, 15, and 17 breccia and soil samples that help resolve discrepancies between the zircon data, the lunar whole rock history and lunar magma ocean crystallization models. The majority of lunar grains are detrital fragments, some nearly 1 mm in length, of large parent crystals suggesting that they crystallized in highly enriched KREEP magmas. The zircon age distributions for all three landing sites exhibit an abundance of ages at ~4.33 Ga, however they differ in thatmore » only Apollo 14 samples have a population of zircons with ages between 4.1 and 3.9 Ga. These younger grains comprise only 10% of all dated lunar zircons and are usually small and highly shocked making them more susceptible to Pb-loss. These observations suggest that the majority of zircons crystallized before 4.1 Ga and that KREEP magmatism had predominantly ceased by this time. We also observed that trace element analyses are easily affected by contributions from inclusions (typically injected impact melt) within SIMS analyses spots. After filtering for these effects, rare-earth element (REE) abundances of pristine zircon are consistent with one pattern characterized by a negative Eu anomaly and no positive Ce anomaly, implying that the zircons formed in a reducing environment. This inference is consistent with crystallization temperatures based on measured Ti concentrations and new estimates of oxide activities which imply temperatures ranging between 958 ± 57 and 1321 ± 100 °C, suggesting that zircon parent magmas were anhydrous. Together, the lunar zircon ages and trace elements are consistent with a ≤300 My duration of KREEP magmatism under anhydrous, reducing conditions. We also report two granular texture zircons that contain baddeleyite cores, which both yield 207Pb– 206Pb ages of 4.33 Ga. These grains are our best constraints on impact ages within our sample population, and suggest at least one large impact is contemporaneous with the most common time of magmatic zircon formation on the Moon’s crust visited by the Apollo missions.« less

Dating sub-20 micron zircons in granulite-facies mafic dikes from SW Montana: a new approach using automated mineralogy and SIMS U-Pb geochronology

NASA Astrophysics Data System (ADS)

Ault, A. K.; Mahan, K. H.; Flowers, R. M.; Chamberlain, K.; Appleby, S. K.; Schmitt, A. K.

2010-12-01

Geochronological data is fundamental to all tectonic studies, but a major limitation for many lithologies is a paucity of sizeable zircons suitable for conventional U-Pb techniques. In particular, mafic dike swarms provide important time markers for tectonometamorphic activity in Precambrian terranes, but commonly yield little or no zircon or baddeleyite sufficient for TIMS or standard ion-probe analysis of crystal separates. We apply a new approach involving in-situ automated mineralogy and high spatial resolution Secondary Ion Mass Spectrometry (SIMS) geochronology to a mafic dike swarm exposed in the Northern Madison Range of SW Montana. The dikes cross-cut early fabrics but are also variably deformed and metamorphosed to P-T conditions as high as 1.2 GPa and 850 C. The swarm emplacement age is inferred to be ca. 2.1 Ga based on similarities to dated dikes in the adjacent Tobacco Root Mountains. Resolving the timing of dike emplacement and high-grade metamorphism in the study area is important for understanding the extent of post-Archean modification to the northwest margin of the Wyoming craton. Identification and textural characterization of zircons were facilitated by in-situ automated mineralogical analysis, in contrast to a standard elemental X-ray mapping approach. Our technique uses an SEM-based platform coupling calibrated BSE data with X-ray data collected by multiple energy dispersive spectrometers to rapidly identify target accessory phases at high spatial resolution. Whole thin section search maps were generated in ~30 minutes at 4 µm pixel resolution. Our dike thin sections commonly contained >300 zircons in a variety of textural settings, with 80% having a short dimension <10 µm. Zircons were dated in-situ by adjusting the field aperture of the CAMECA ims1270 to preferentially collect secondary ions emitted from within the inner few microns of the ~15 µm diameter analysis pit. This allows us to analyze zircon grains with a minimum dimension as small as 8 μm at radiogenic yields typically >95% for 206Pb. SIMS data for 22 zircons from a granulite-facies mafic dike thin section define a chord with upper and lower intercepts of 1753.1 ± 9.5 Ma and 63.2 ± 7.9 Ma, respectively (2 sigma error, MSWD = 1.6). A positive correlation between U concentration and degree of discordance indicates that the more radiation-damaged zircons underwent greater Pb loss. We infer Pb loss to reflect re-heating linked to emplacement of the nearby Tobacco Roots Batholith (ca. 74-71 Ma). Metamorphic zircon growth ca. 1750 Ma indicates that the high-grade metamorphic core of the Big Sky orogeny extends into the Northern Madison Range, farther inboard into the Wyoming craton than previously recognized. Coupling automated mineralogy searching with refined SIMS methods enables acquisition and interpretation of in-situ U-Pb data from zircons of a size that would not be feasible with most other techniques.

The 3.5 Ga granulites of the Bug polymetamorphic complex, Ukraine (U-Pb SHRIMP-II zircon data)

NASA Astrophysics Data System (ADS)

Lobach-Zhuchenko, Svetlana; Kaulina, Tatiana; Baltybaev, Shauket; Yurchenko, Anastasija; Balagansky, Victor; Skublov, Sergei; Sukach, Vitaliji

2014-05-01

The Bug polymetamorphic complex composes the south-west of the Ukrainian Shield. It experienced multistage deformation and metamorphism of 3.6 to 1.95 Ga. The age of protolith of the gneiss enderbite is up to 3.7-3.6 Ga (Claesson et al., 2006: Lobach-Zhuchenko et al., 2010, 2013; Bibikova et al, 2013). The 3.5 old granulitic zircon (sample UR132) was found in a light grey massive medium grained with weak foliation granulite from the Odessa quarry located at the right riverside of the Bug (N 48° 13' 55.2''; E 29° 59' 75''). The rock is mafic in composition (SiO2 = 51.50 wt%, #mg=0.43, (La/Yb)n =7.5), and shows enrichment in Sr (333 ppm), Zr (244 ppm), and Nb (12 ppm) compared to MORB. This contains antiperthitic plagioclase (An30-35), hypersthene (En0.46 Fs0.53 Wo0.01 ), diopside (En0.33 Fs0.20Wo0.47), quartz, ilmenite, magnetite, apatite, and zircon. Three types of zircon are recognized. (1) Large (~300 μm) isometric and oval grains displaying bright colour and sector zoning in CL. Some crystals have fir-tree texture. Isometric shape together with fir-tree zoning is typical for zircons growing under conditions of granulite- facies metamorphism (Hoskin and Schaltegger, 2003). These zircons are low in U and Th, have high Th/U ratios (0.61-1.1), and display decreasing of element contents from the centre to margin (ppm): U - 68 to 44, Th - 58 to 19, total REE - 723 to 406, Y - 1049 to 553, and Li - 1.23 to 0.91 at the constant (Lu/La)n ratios (4200-4600). Zircon's crystallization temperature calculated from the Ti content is 705 °C and is consistent with those calculated on the base of the mineral composition. (2) Small sized (60-100 μm) zircons. Most of these occur within plagioclase and truncate antiperthite lamellae. They show euhedral zoned cores and bright rims in CL. (3) Zircons principally distinguished from those of groups 1 and 2. These are dark in CL, some grains are severely structurally damaged and contain numerous inclusions (Qzt, Pl, Kfsp, rare Opx and Cpx). Zircon U-Pb isotopic analysis was carried out using SHRIMP II ion microprobe technique at the Isotopic Centre of VSEGEI, St.-Petersburg. Six transparent grains of the (1) group form a discordia line with Concordia intercepts at 3499+/-33 Ma (and 2638+/-240 Ma (MSWD=2.3). According to internal textures and chemical composition of zircons their formation is associated with granulite metamorphism. The 207 Pb/ 206 Pb data for 11 grains from (3) group are highly variable in age from 3330+/-5 to 2356+/-7 Ma indicating isotopic disturbance. They do not form an isochrone, thus reliable determination of their age is not yet possible. Thus, the oldest granulitic event at 3499 ± 33 Ma has been identified and justified for rocks of the Bug polymetamorphic granulite complex. Recognition of this oldest granulite metamorphism proved possible due to preserved isotopic and geochemical features of zircon. The work was financially supported by program ONZ - 6.

High-Resolution Zircon U-Pb CA-TIMS Dating of the Carboniferous—Permian Successions, Paraná Basin, Brazil

NASA Astrophysics Data System (ADS)

Griffis, N. P.; Mundil, R.; Montanez, I. P.; Isbell, J.; Fedorchuk, N.; Lopes, R.; Vesely, F.; Iannuzzi, R.

2015-12-01

The late Paleozoic Ice Age (LPIA) is Earth's only record of a CO2-forced climatic transition from an icehouse to greenhouse state in a vegetated world. Despite a refined framework of Gondwanan ice distribution, questions remain about the timing, volume, and synchronicity of high-latitude continental ice and the subsequent deglaciation. These questions ultimately preclude our understanding of linkages between ice volume, sea level, and high- and low-latitude climate. Poor constraints on the timing and synchronicity of glacial and interglacial transitions reflect a lack of high-resolution radioisotopic dates from high-latitude, ice-proximal Carboniferous-Permian successions. The Rio Bonito Fm in Rio Grande do Sul State of southern Brazil hosts the oldest non-glaciogenic Carboniferous- Permian deposits of the Paraná Basin, thus recording the icehouse-to-greenhouse transition. Despite a widespread effort over the last two decades to constrain these deposits in time by means of U-Pb zircon geochronology, published data sets of the Candiota and Faxinal coals of the Rio Bonito Fm host discrepancies that may reflect post- eruptive open system behavior of zircon and analytical artifacts. These discrepancies have hindered the correlation of the Candiota and Faxinal sediments within the larger Gondwanan framework. Here we present the first U-Pb ages on closed system single zircons using CA-TIMS techniques on Permo-Carboniferous ash deposits of the Paraná Basin. Preliminary results indicate two major and distinct coal-forming periods that are separated by ca 10 Ma. Our results and conclusions are not in agreement with multi- crystal U-Pb TIMS and SIMS ages that suggest coeval deposition of the Candiota and Faxinal coals. CA-TIMS analyses applied to zircons from additional ash deposits are aimed at constructing a robust chronostratigraphic framework for the Carboniferous- Permian succession of the Paraná Basin, which will facilitate a better understanding of the timing and ice dynamics of the LPIA.

Zircon U-Pb Ages Chronicle 3 Myr of Episodic Crystallization in the Composite Miocene Tatoosh Pluton, Mount Rainier National Park, Washington Cascades

NASA Astrophysics Data System (ADS)

Bacon, C. R.; Du Bray, E. A.; Wooden, J. L.; Mazdab, F. K.

2007-12-01

Zircon geochronology of upper crustal plutons can constrain longevities of intermediate to silicic magmatic systems. As part of a larger study of the geochemistry and metallogeny of Tertiary Cascades magmatic arc rocks, we used the USGS-Stanford SHRIMP RG to determine 20 to 28 238U-206Pb ages for zircons from each of 6 quartz monzodiorite (qmd), quartz monzonite (qm), or granodiorite (grd) samples representative of the Tatoosh pluton, and one grd from the nearby Carbon River stock. The 7x12 km composite Tatoosh pluton, discontinuously exposed on the south flank of Mount Rainier, consists of at least 4 petrographic/compositional phases, here termed Pyramid Peak, Nisqually, Reflection Lake, and Tatoosh. These collectively intrude gently folded and weakly metamorphosed basaltic andesite flows and volcaniclastic rocks of the Eocene Ohanapecosh Formation, silicic ignimbrites and sedimentary rocks of the Oligocene Stevens Ridge Formation, and basaltic to intermediate volcanic rocks of the Miocene Fifes Peak Formation. Histograms and relative probability plots of U- Pb ages indicate 2 to 4 age populations within each sample. The weighted mean age of each of the youngest populations (all ±2σ) is interpreted as the time of final solidification: Pyramid Peak qmd (58.5% SiO2) 17.4±0.2 Ma, Nisqually grd (in Paradise Valley; 65.4% SiO2) 16.7±0.2 Ma, Nisqually grd (at Christine Falls; 66.4% SiO2) 17.3±0.2 Ma, Reflection Lake qm (along Pinnacle Peak trail; 66.6% SiO2) 17.1±0.2 Ma, Tatoosh grd (in Stevens Canyon; 67.8% SiO2) 18.2±0.2 Ma, Tatoosh grd (south of Louise Lake; 69.3% SiO2) 19.3±0.1 Ma, and Carbon River grd (68.0% SiO2) 17.4±0.3 Ma. The older Nisqually grd age is indistinguishable from a TIMS zircon age of 17.5±0.1 Ma reported by Mattinson (GSA Bulletin 88:1509-1514, 1977) for grd from a nearby locality. None of the 164 SHRIMP-RG U-Pb ages, including cores, is older than 21 Ma. The relatively small, high-level pluton likely was emplaced and solidified in pulses; zircons with ages significantly greater than the final solidification age of a given sample are considered to be antecrysts recycled from earlier crystallization episodes within the larger magmatic system. Although interpretation of the age populations is subjective, we have identified 17.4-Ma antecrysts in the 16.7-Ma Nisqually grd and antecrysts of 1-3 ages averaging 18.1, 18.8, 19.3, and 20.0 Ma in the other samples. Notably, age populations are separated by 0.5-0.7 Myr, intervals similar to lifetimes of large arc volcanoes. The total duration of pluton assembly is ~2.6 Myr, or as much as ~3.6 Myr if the oldest antecrysts are considered. The oldest, most differentiated rocks are on the east side of the pluton (Tatoosh phase) and the youngest, least differentiated on the west (Pyramid Peak phase). Two samples each of the Nisqually and Tatoosh phases yield apparent crystallization age differences of 0.7- and 1.1-Myr, respectively, for petrographically and chemically similar rocks. The Tatoosh U-Pb data appear to chronicle repeated waxing and waning of a long-lived igneous system as recorded by crystal-rich magma that periodically solidified near the system's roof at the close of well-defined crystallization episodes.

Late Permian volcanic dykes in the crystalline basement of the Považský Inovec Mts. (Western Carpathians): U-Th-Pb zircon SHRIMP and monazite chemical dating

NASA Astrophysics Data System (ADS)

Pelech, Ondrej; Vozárová, Anna; Uher, Pavel; Petrík, Igor; Plašienka, Dušan; Šarinová, Katarína; Rodionov, Nikolay

2017-08-01

This paper presents geochronological data for the volcanic dykes located in the northern Považský Inovec Mts. The dykes are up to 5 m thick and tens to hundreds of metres long. They comprise variously inclined and oriented lenses, composed of strongly altered grey-green alkali basalts. Their age was variously interpreted and discussed in the past. Dykes were emplaced into the Tatricum metamorphic rocks, mostly consisting of mica schists and gneisses of the Variscan (early Carboniferous) age. Two different methods, zircon SHRIMP and monazite chemical dating, were applied to determine the age of these dykes. U-Pb SHRIMP dating of magmatic zircons yielded the concordia age of 260.2 ± 1.4 Ma. The Th-U-Pb monazite dating of the same dyke gave the CHIME age of 259 ± 3Ma. Both ages confirm the magmatic crystallization at the boundary of the latest Middle Permian to the Late Permian. Dyke emplacement was coeval with development of the Late Paleozoic sedimentary basin known in the northern Považský Inovec Mts. and could be correlated with other pre-Mesozoic Tethyan regions especially in the Southern Alps.

From Crustal Anatexis to Pluton Emplacement: High-Precision Zircon Geochronology Reveals the Thermal History of the Larderello-Travale Geothermal System (Italy)

NASA Astrophysics Data System (ADS)

Farina, F.; Dini, A.; Ovtcharova, M.; Davies, J.; Bouvier, A. S.; Baumgartner, L. P.; Caricchi, L.; Schaltegger, U.

2017-12-01

Late Miocene to recent post-collisional extension in Tuscany (Italy) led to the emplacement of shallow-level granitic plutons and to the eruption of small rhyolitic bodies. The intrusion of peraluminous two-mica and tourmaline-bearing granites triggered the formation of the steam-dominated Larderello-Travale geothermal system. In this study, zircon crystals from granite samples obtained from drill holes at 3.0-4.5 km depth were investigated by combining in-situ oxygen isotopes analysis and high-precision CA-ID-TIMS U-Pb age determinations to gain insight into the nature of the magmatic heat source fuelling the geothermal field. Magmatic zircon crystals display δ18O values ranging from 8.6 to 13.5‰ and crystals from individual samples exhibit inter- and intra-grain oxygen isotope variability exceeding 3‰. The geochronological data indicates the existence of three magmatic pulses with ages between 3.637 ± 0.008 and 1.671 ± 0.004 Ma. More importantly, zircon crystals from individual samples exhibit an age spread as large as 200-400 ky. This age dispersion, which is more than one order of magnitude greater than the uncertainty on a single date, suggest that most of the zircon did not crystallize at the emplacement level, but within isolated and isotopically distinct magma batches before large-scale homogenization in a magmatic reservoir at depth. The rate of assembly and final volume of this reservoir is estimated using the distribution of precise U-Pb zircon dates following the approach of Caricchi et al. (2014). Thermal modelling indicates that the heat flow at the surface in the geothermal field cannot be sustained by the inferred reservoir or by heat advection from the mantle. Our data suggest the existence of a younger shallow-level intrusion, whose occurrence also accounts for the existence of confined magmatic fluids at the top of the Larderello-Travale intrusion. We conclude that a multi-disciplinary approach, integrating high-precision zircon dating, in-situ oxygen isotopes and thermal modelling can be used to resolve the thermal structure of the crust in active geothermal systems. Ref: Caricchi et al., (2014). Nature, 511, 457-461. This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 701494.

Stability of Zircon and its Isotopic Ratios in High-Temperature Fluids: Long-Term (4 months) Isotope Exchange Experiment at 850 °C and 50 MPa

NASA Astrophysics Data System (ADS)

Bindeman, Ilya N.; Schmitt, Axel K.; Lundstrom, Craig C.; Hervig, Richard L.

2018-05-01

Stability of zircon in hydrothermal fluids and vanishingly slow rates of diffusion identify zircon as a reliable recorder of its formation conditions in recent and ancient rocks. Debate, however, persists on how rapidly oxygen and key trace elements (e.g., Li, B, Pb) diffuse when zircon is exposed to hot aqueous fluids. Here, we report results of a nano- to micrometer-scale investigation of isotopic exchange using natural zircon from Mesa Falls Tuff (Yellowstone) treated with quartz-saturated, isotopically (18O, D, 7Li, and 11B) labeled water with a nominal δ18O value of +450‰ over 4 months at 850°C and 50 MPa. Frontside (crystal rim inwards) δ18O depth profiling of zircon by magnetic sector SIMS shows initially high but decreasing 18O/16O over a 130 nm non-Fickian profile, with a decay length comparable to the signal from surficial Au coating deposited onto zircon. In contrast, backside (crystal interior outwards) depth profiling on a 2-3 µm thick wafer cut and thinned from treated zircon by focused ion beam (FIB) milling lacks any significant increase in 18O/16O during penetration of the original surface layer. Near-surface time-of-flight (TOF-SIMS) frontside profiles of uncoated zircon from 4-month and 1-day-long experiments as well as untreated zircons display similar enrichments of 18O over a distance of 20 nm. All frontside 18O profiles are here interpreted as transient surface signals from nm-thick surface enrichment or contamination unrelated to diffusion. Likewise, frontside depth profiling of H, Li, and B isotopes are similar for long- and short-duration experiments. Additionally, surface U-Pb dating of zircon from the 4-month experiment returned U-Pb ages by depth profiling with 1 µm penetration that were identical to untreated samples. Frontside and backside depth-profiling thus demonstrate that diffusive 18O enrichment in the presence of H2O is much slower than predicted from experiments in Watson and Cherniak (1997). Instead, intracrystalline exchange of oxygen between fluid and zircon in wet experimental conditions with excess silica occurred over length-scales equivalent to those predicted for dry diffusion. Oxygen diffusion coefficients even under wet conditions and elevated temperatures (850 °C) are <1-3×10-23 m2/sec, underscoring a virtual lack of oxygen diffusion and an outstanding survivability of zircons

U-Pb ages and Hf isotope compositions of zircons in plutonic rocks from the central Famatinian arc, Argentina

NASA Astrophysics Data System (ADS)

Otamendi, Juan E.; Ducea, Mihai N.; Cristofolini, Eber A.; Tibaldi, Alina M.; Camilletti, Giuliano C.; Bergantz, George W.

2017-07-01

The Famatinian arc formed around the South Iapetus rim during the Ordovician, when oceanic lithosphere subducted beneath the West Gondwana margin. We present combined in situ U-Th-Pb and Lu-Hf isotope analyses for zircon to gain insights into the origin and evolution of Famatinian magmatism. Zircon crystals sampled from four intermediate and silicic plutonic rocks confirm previous observations showing that voluminous magmatism took place during a relatively short pulse between the Early and Middle Ordovician (472-465 Ma). The entire zircon population for the four plutonic rocks yields coherent εHf negative values and spreads over several ranges of initial εHf(t) units (-0.3 to -8.0). The range of εHf units in detrital zircons of Famatinian metasedimentary rocks reflects a prolonged history of the cratonic sources during the Proterozoic to the earliest Phanerozoic. Typical tonalites and granodiorites that contain zircons with evolved Hf isotopic compositions formed upon incorporating (meta)sedimentary materials into calc-alkaline metaluminous magmas. The evolved Hf isotope ratios of zircons in the subduction related plutonic rocks strongly reflect the Hf isotopic character of the metasedimentary contaminant, even though the linked differentiation and growth of the Famatinian arc crust was driven by ascending and evolving mantle magmas. Geochronology and Hf isotope systematics in plutonic zircons allow us understanding the petrogenesis of igneous series and the provenance of magma sources. However, these data could be inadequate for computing model ages and supporting models of crustal evolution.

New uppermost Cambrian U-Pb date from Avalonian Wales and age of the Cambrian-Ordovician boundary

USGS Publications Warehouse

Davidek, K.; Landing, E.; Bowring, S.A.; Westrop, S.R.; Rushton, A.W.A.; Fortey, R.A.; Adrain, J.M.

1998-01-01

A crystal-rich volcaniclastic sandatone in the lower Peltura scarabaeoides Zone at Ogof-odi near Criccieth, North Wales, yields a U-Pb zircon age of 491 ?? 1 Ma. This late Late Cambrian date indicates a remarkably young age for the Cambrian-Ordovician boundary whose age must be less than 491 Ma. Hence the revised duration of the post-Placentian (trilobite-bearing) Cambrian indicates that local trilobite zonations allow a biostratigraphic resolution comparble to that provided by Ordovician graptolites and Mesozoic ammonites.

U-Pb-Th geochronology of monazite and zircon in albitite metasomatites of the Rožňava-Nadabula ore field (Western Carpathians, Slovakia): implications for the origin of hydrothermal polymetallic siderite veins

NASA Astrophysics Data System (ADS)

Hurai, V.; Paquette, J.-L.; Lexa, O.; Konečný, P.; Dianiška, I.

2015-10-01

Sodic metasomatites (albitites) occur around and within siderite veins in the southern part of the Gemeric tectonic unit of the Western Carpathians. Accessory minerals of the metasomatites represented by monazite, zircon, apatite, rutile, tourmaline and siderite are basically identical with the quartz-tourmaline stage of other siderite and stibnite veins of the tectonic unit. Statistical analysis of chemical Th-U(total)-Pb isochron method (CHIME) of monazite dating yielded Jurassic-Cretaceous ages subdivided into 3-4 modes, spreading over time interval between 78 and 185 Ma. In contrast, LA-ICPMS 206Pb/238U dating carried out on the same monazite grains revealed a narrow crystallization interval, showing ages of Th-poor cores with phengite inclusions identical within the error limit with Th-rich rims with cauliflower-like structure. The determined lower intercept at 139 ± 1 Ma overlapped the Vallanginian-Berriasian boundary, thus corroborating the model of formation of hydrothermal vein structures within an arcuate deformation front built up in the Variscan basement as a response to Early Cretaceous compression, folding and thrusting. In contrast, associated zircons are considerably older than the surrounding Early-Palaeozoic volcano-sedimentary rocks, showing Neoproterozoic ages. The zircon grains in albitite metasomatites are thus interpreted as fragments of Pan-African magmatic detritus incorporated in the vein structures by buoyant hydrothermal fluids.

Is Myanmar jadeitite of Jurassic age? A result from incompletely recrystallized inherited zircon

NASA Astrophysics Data System (ADS)

Yui, Tzen-Fu; Fukoyama, Mayuko; Iizuka, Yoshiyuki; Wu, Chao-Ming; Wu, Tsai-Way; Liou, J. G.; Grove, Marty

2013-02-01

Zircons from two Myanmar jadeitite samples were separated for texture, mineral inclusion, U-Pb dating and trace element composition analyses. Three types of zircons, with respect to U-Pb isotope system, were recognized. Type I zircons are inherited ones, yielding an igneous protolith age of 160 ± 1 Ma; Type II zircons are metasomatic/hydrothermal ones, giving a (minimum) jadeitite formation age of 77 ± 3 Ma; and Type III zircons are incompletely recrystallized ones, with non-coherent and geologically meaningless ages from 153 to 105 Ma. These Myanmar jadeitites would therefore have formed through whole-sale metasomatic replacement processes. Compared with Type I zircons, Type II zircons show typical metasomatic/hydrothermal geochemical signatures, with low Th/U ratio (< 0.1), small Ce anomaly (Ce/Ce* = < 5) and low ΣREE content (40-115 ppm). Type III zircons, however, commonly have the above geochemical signatures straddle in between Type I and Type II zircons. It is shown that the resetting rates of various trace element compositions and U-Pb isotope system of inherited zircons are not coupled "in phase" in response to zircon recrystallization during jadeitite formation. The observed abnormally low Th/U ratio and small Ce anomaly of some Type I zircons, as well as the lack of negative Eu anomaly of all Type I zircons, should be suspected to be of secondary origin. In extreme cases, incompletely recrystallized zircons may show typical metasomatic/hydrothermal geochemical signatures, but leave U-Pb isotope system partially reset or even largely unchanged. Such zircons easily lead to incorrect age interpretation, and hence erroneous geological implication. The Myanmar jadeitites, based on the present study, might have formed during the Late Cretaceous subduction before the beginning of India-Asia continental collision at Paleocene. Previously proposed Late Jurassic ages for Myanmar jadeitites are suggested as results rooted on data retrieved from incompletely recrystallized inherited zircons.

A search for thermal excursions from ancient extraterrestrial impacts using Hadean zircon Ti-U-Th-Pb depth profiles

PubMed Central

Abbott, Sunshine S.; Harrison, T. Mark; Schmitt, Axel K.; Mojzsis, Stephen J.

2012-01-01

Few terrestrial localities preserve more than a trace lithic record prior to ca. 3.8 Ga greatly limiting our understanding of the first 700 Ma of Earth history, a period inferred to have included a spike in the bolide flux to the inner solar system at ca. 3.85–3.95 Ga (the Late Heavy Bombardment, LHB). An accessible record of this era may be found in Hadean detrital zircons from the Jack Hills, Western Australia, in the form of μm-scale epitaxial overgrowths. By comparing crystallization temperatures of pre-3.8 Ga zircon overgrowths to the archive of zircon temperature spectra, it should, in principle, be possible to identify a distinctive impact signature. We have developed Ti-U-Th-Pb ion microprobe depth profiling to obtain age and temperature information within these zircon overgrowths and undertaken a feasibility study of its possible use in identifying impact events. Of eight grains profiled in this fashion, four have overgrowths of LHB-era age. Age vs. temperature profiles reveal a period between ca. 3.85–3.95 Ga (i.e., LHB era) characterized by significantly higher temperatures (approximately 840–875 °C) than do older or younger zircons or zircon domains (approximately 630–750 °C). However, temperatures approaching 900 °C can result in Pb isotopic exchange rendering interpretation of these profiles nonunique. Coupled age-temperature depth profiling shows promise in this role, and the preliminary data we report could represent the first terrestrial evidence for impact-related heating during the LHB. PMID:22869711

Permian single crystal U-Pb zircon age of the Rožňava Formation volcanites (Southern Gemeric Unit, Western Carpathians, Slovakia)

NASA Astrophysics Data System (ADS)

Vozárová, Anna; Šmelko, Miloš; Paderin, Ilya

2009-12-01

Zircon populations from the Rožňava Formation volcanic rock complex have been analysed. Euhedral zircons from the 1st volcanogenic horizon with fine oscillatory growth zoning, typical of magmatic origin, gave the average concordia age of 273.3 ± 2.8 Ma, with Th/U ratios in the range of 0.44-0.73. The Permian ages ranging from 266 to 284 Ma were identified in the wider, zoned or unzoned, central zircon parts, as well as in their fine-zoned oscillatory rims. The average concordia age of 275.3 ± 2.9 was obtained from the euhedral zircon population of the 2nd volcanogenic horizon of the Rožňava Formation. The analyses were performed on zoned magmatic zircons in the age interval from 267 to 287 Ma, with Th/U ratios in the range of 0.39-0.75. In the later zircon population two inherited zircon grains were dated giving the age of 842 ± 12 Ma (Neoproterozoic) and 456 ± 7 Ma (Late Ordovician). The magmatic zircon ages document the Kungurian age of Permian volcanic activity and contemporaneous establishment of the south-Gemeric basin. The time span of volcanic activity corresponds to the collapse of the Western Carpathian Variscan foreland which expanded southward.

Linking rapid magma reservoir assembly and eruption trigger mechanisms at evolved Yellowstone-type supervolcanoes

USGS Publications Warehouse

Wotzlaw, J.F.; Bindeman, I.N.; Watts, Kathryn E.; Schmitt, A.K.; Caricchi, L.; Schaltegger, U.

2014-01-01

The geological record contains evidence of volcanic eruptions that were as much as two orders of magnitude larger than the most voluminous eruption experienced by modern civilizations, the A.D. 1815 Tambora (Indonesia) eruption. Perhaps nowhere on Earth are deposits of such supereruptions more prominent than in the Snake River Plain–Yellowstone Plateau (SRP-YP) volcanic province (northwest United States). While magmatic activity at Yellowstone is still ongoing, the Heise volcanic field in eastern Idaho represents the youngest complete caldera cycle in the SRP-YP, and thus is particularly instructive for current and future volcanic activity at Yellowstone. The Heise caldera cycle culminated 4.5 Ma ago in the eruption of the ∼1800 km3 Kilgore Tuff. Accessory zircons in the Kilgore Tuff display significant intercrystalline and intracrystalline oxygen isotopic heterogeneity, and the vast majority are 18O depleted. This suggests that zircons crystallized from isotopically distinct magma batches that were generated by remelting of subcaldera silicic rocks previously altered by low-δ18O meteoric-hydrothermal fluids. Prior to eruption these magma batches were assembled and homogenized into a single voluminous reservoir. U-Pb geochronology of isotopically diverse zircons using chemical abrasion–isotope dilution–thermal ionization mass spectrometry yielded indistinguishable crystallization ages with a weighted mean 206Pb/238U date of 4.4876 ± 0.0023 Ma (MSWD = 1.5; n = 24). These zircon crystallization ages are also indistinguishable from the sanidine 40Ar/39Ar dates, and thus zircons crystallized close to eruption. This requires that shallow crustal melting, assembly of isolated batches into a supervolcanic magma reservoir, homogenization, and eruption occurred extremely rapidly, within the resolution of our geochronology (103–104 yr). The crystal-scale image of the reservoir configuration, with several isolated magma batches, is very similar to the reservoir configurations inferred from seismic data at active supervolcanoes. The connection of magma batches vertically distributed over several kilometers in the upper crust would cause a substantial increase of buoyancy overpressure, providing an eruption trigger mechanism that is the direct consequence of the reservoir assembly process.

Extremely High Magma Emplacement Rates Recorded in the Golden Horn Batholith, WA

NASA Astrophysics Data System (ADS)

Eddy, M. P.; Bowring, S. A.; Tepper, J. H.; Miller, R. B.

2015-12-01

High SiO2 rhyolites emplaced during 'super-eruptions' demonstrate that large volumes of eruptible magma can exist in the upper crust. However, the timescale over which the magma reservoirs that source these eruptions are built remains controversial. Thermal models suggest that magma emplacement rates need to be > 0.005-0.01 km3/yr in order to accumulate enough eruptible magma to source a 'super-eruption'. Yet, these rates are higher than the time-averaged rates (< 0.001 km3/yr) for nearly all well-studied granitoid plutonic complexes. This disparity contradicts geologic evidence suggesting that the high SiO2 rhyolites emplaced during 'super-eruptions' are extracted from crystal rich magma chambers that should be preserved in the geologic record as granodioritic and granitic plutons. We quantify time-averaged magma emplacement rates for the upper crustal Golden Horn batholith, WA based on new geologic mapping and U-Pb zircon CA-IDTIMS geochronology. The batholith is exposed over 310 km3 and can be separated in the field into five intrusive units. High topography allows the 3D geometry of each phase to be constrained and their volumes range from < 100 km3 to > 400 km3. U-Pb zircon geochronology reveals that four of the five phases were assembled incrementally and distinct zircon populations from samples within these phases suggest that individual magmatic pulses had fully crystallized before the next arrived. However, six nearly identical U-Pb zircon dates from a > 400 km3 rapakivi granite show that this phase was built in ca. 50 kyr and that large portions may have been emplaced nearly simultaneously. The implied emplacement rate for this phase (≥ 0.008 km3/yr) is in agreement with those predicted for assembly of the upper crustal magma chambers that source 'super-eruptions', and it may provide a rare and unprecedented opportunity to study the processes that occur in such chambers.

SHRIMP U-Pb zircon geochronology and thermal modeling of multilayer granitoid intrusions. Implications for the building and thermal evolution of the Central System batholith, Iberian Massif, Spain

NASA Astrophysics Data System (ADS)

Díaz Alvarado, Juan; Fernández, Carlos; Castro, Antonio; Moreno-Ventas, Ignacio

2013-08-01

This work shows the results of a U-Pb SHRIMP zircon geochronological study of the central part of the Gredos massif (Spanish Central System batholith). The studied batholith is composed of several granodiorite and monzogranite tabular bodies, around 1 km thick each, intruded into partially molten pelitic metasediments. Granodiorites and monzogranites, belonging to three distinct intrusive bodies, and samples of anatectic leucogranites have been selected for SHRIMP U-Pb zircon geochronology. Distinct age groups, separated by up to 20 Ma, have been distinguished in each sample. Important age differences have also been determined among the most representative age groups of the three analyzed granitoid bodies: 312.6 ± 2.8 Ma for the Circo de Gredos Bt-granodiorites (floor intrusive layer), 306.9 ± 1.5 Ma for the Barbellido-Plataforma granitoids (top intrusive layer) and 303.5 ± 2.8 Ma for Las Pozas Crd-monzogranites (middle intrusive layer). These age differences are interpreted in terms of sequential emplacement of the three intrusive bodies, contemporary with the Late Paleozoic D3 deformation phase. The anatectic leucogranites are coeval to slightly younger than the adjacent intrusive granodiorites and monzogranites (305.4 ± 1.6 Ma for Refugio del Rey leucogranites and 303 ± 2 Ma for migmatitic hornfelses). It is suggested that these anatectic magmas were generated in response to the thermal effects of granodiorite intrusions. Thermal modeling with COMSOL Multiphysics® reveals that sequential emplacement was able to keep the thermal conditions of the batholith around the temperature of zircon crystallization in granitic melts (around 750 °C) for several million of years, favoring the partial melting of host rocks and the existence of large magma chambers composed of crystal mush prone to be rejuvenated after new intrusions.

What can zircon ages from the Jack Hills detrital zircon suite really tell us about Hadean geodynamics?

NASA Astrophysics Data System (ADS)

Whitehouse, Martin; Nemchin, Alexander

2015-04-01

As the only direct sample of the Hadean Earth, detrital zircon grains from the Jack Hills, Western Australia, have been the subject of intense investigation over the almost three decades since their discovery. A wide variety of geochemical and isotopic analyses of these grains, as well as their mineral inclusions, have been used variously to support two fundamentally different models for Hadean geodynamics: (i) Some form of (not necessarily modern-style) plate recycling generating felsic (continental-type?) crust at the boundaries [1, 2], or conversely (ii) the persistence of a long-lived, stagnant basaltic lid within which magmatism occurred as a result of internal temperature perturbations and/or impacts [3, 4], a model also generally consistent with a wide range of observations from post-Hadean geochemical reservoirs. Despite the considerable time and resources expended, the majority of these studies uncritically accept the individual U-Pb zircon ages, even though their veracity is key to many of the interpretations [5, 6]. We report here the results of an in-depth evaluation of all published (and new) U-Pb ages from the Jack Hills zircon suite in order to define age populations that can be used with a high degree of confidence in geodynamic interpretations. A notable problem in the interpretation of U-Pb data from ancient zircon grains (including those as young as the Neoarchean) is that disturbance of the systematics even several 100 Ma after crystallization causes data to spread along the concordia curve without becoming discernably discordant within the relatively large error bounds associated with U/Pb ages from in situ dating methods (e.g. SIMS). While 207Pb/206Pb ages are typically more precise, individually they provide no means to detect Pb-loss-induced younging. However, if two or preferably more analyses have been made in the same zircon growth zone, a reasonable evaluation of the possibility of Pb-loss can be made. In the available Jack Hills zircon dataset, only 111 grains have been analysed at least twice and of these, only 48 give a consistent internal age, while only 14 have been analysed more than twice and can strictly be considered to yield true ages. Two resulting age peaks at 4.18 - 4.08 Ga and 4.05 - 3.98 Ga potentially represent major magmatic events in the Hadean. In order to explain ages >4.18 Ga, a magmatic event as old as the oldest reliable Jack Hills zircon age of 4.374 Ga is also required. The significance of this limited number of magmatic events for Hadean global geodynamic models will be discussed. References: [1] Harrison, T.M. et al. Geochim Cosmochim Ac 69 (10), A390-A390 (2005), [2] Peck, et al. Geochim Cosmochim Acta 65 (22), 4215-4229 (2001), [3] Kemp, A.I.S. et al. EPSL, 296 (1-2), 45-56 (2010), [4] Kamber, B.S., et al., Contrib Mineral Petr 145 (1), 25-46 (2003), [5] Cavosie, A.J., et al., Precambrian Res 135 (4), 251-279 (2004). [6] Holden P, et al., Int. J. Masspectrometry, 286, 53-63 (2009)

Origin of zircon-bearing mantle eclogites entrained in the V. Grib kimberlite (Arkhangelsk region, NW Russia): Evidence from mineral geochemistry and the U-Pb and Lu-Hf isotope compositions of zircon

NASA Astrophysics Data System (ADS)

Shchukina, Elena V.; Agashev, Alexey M.; Zedgenizov, Dmitry A.

2018-05-01

The concentrations of major and trace elements in minerals, reconstructed whole-rock compositions of zircon-bearing equigranular eclogites from the V. Grib kimberlite pipe located within the Arkhangelsk Diamondiferous Province (North-Western Russia), and results of the U-Pb and Lu-Hf isotope analyses of zircon grains from eclogites and granulite xenoliths are reported. These data suggest that the equigranular eclogites could represent the fragments of mid-ocean-ridge basalt that were metamorphosed during Paleoproterozoic subduction at 1.7-1.9 Ga. The Hf isotope compositions of the eclogitic zircon display uniformity and indicate corresponding Hf-depleted mantle model ages of 2.2-2.3 Ga. The formation of zircon in eclogites could have resulted from interactions with metasomatic/subduction-related fluids just prior to, but associated with, Paleoproterozoic eclogite formation. A link between eclogitic zircon formation and continental lower-crustal rocks can be excluded based on differences in the Hf isotope compositions of eclogitic and granulitic zircon grains. The U-Pb upper intercept age of granulitic zircon of 2716 ± 61 Ma provides a new minimum age constraint for zircon crystallisation and granulite formation. The U-Pb ages obtained from granulitic zircon show two stages of Pb loss at 2.2-2.6 Ga and 1.7-2.0 Ga. The late Paleoproterozoic stage of Pb loss recorded in granulitic zircon is due to the intensive reworking of basement crustal rocks, which was caused by a tectonic process/subduction event associated with equigranular eclogite formation. Our data, along with evidence previously obtained from the V. Grib pipe coarse-granular eclogites, show at least two main subduction events in the lithospheric mantle of the Arkhangelsk region: the Archean (2.8 Ga) and Paleoproterozoic (1.7-1.9 Ga) subductions, which correspond to major magmatic and metamorphic events in the Baltic Shield.

In situ Pb-Pb dating of rutile from slowly cooled granulites by LA-MC-ICP-MS: confirmation of the high closure temperature (>=600°C) for Pb diffusion in rutile

NASA Astrophysics Data System (ADS)

Vry, J.; Baker, J.; Waight, T.

2003-04-01

We have analysed Pb isotopes in natural rutile crystals by laser ablation MC-ICP-MS to assess the potential of rapid Pb-Pb dating of rutile with this method. The rutile samples are from granulite-facies Mg- and Al-rich rocks from the Reynolds Range, Northern Territory, Australia. This metamorphic terrane has a well-constrained high-T cooling history (ca. 3^oC/Myr) defined by previous U-Pb dating of monazite and zircon (peak metamorphism at 1584 Ma), which we have supplemented with additional Rb-Sr dates of phlogopite, biotite and muscovite. The dated rutiles vary in size from 3 to 0.05 mm, have Pb concentrations of ca. 20 ppm, and were analysed with a 266 nm laser coupled to an AXIOM MC-ICP-MS (spot size of 200-50 μm). Individual larger crystals (>= 200 μm) exhibit sufficient Pb isotopic heterogeneity (206Pb/204Pb = 10000-80000) to perform isochron calculations on several short analyses of a single grain (30-60 s). The largest rutiles yielded Pb-Pb isochron ages of 1540-1555 Ma with typical uncertainties of ± 1 to 10 Ma. 207Pb/206Pb ages are typically within 1% of the Pb-Pb isochron ages testifying to the radiogenic nature of Pb in the rutile. A mean age for all the analysed rutiles was 1548.4 ± 9.1 Ma (n = 33). Comparable 207Pb/206Pb ages were also obtained from individual smaller crystals (50 μm) where the 204Pb ion beam could not be measured precisely. The results demonstrate that even small rutile crystals are extremely resistant to isotopic resetting, and that this mineral is a high-T chronometer. Phlogopite and muscovite Rb-Sr ages are <1454 and 1400-1480 Ma, respectively, with some of the phlogopite and biotite micas having been partially reset by later thermal events younger than 400 Ma. All the mica ages are considerably younger (100-70 My) than the rutile ages, which approach U-Pb ages for monazite and zircon overgrowths, even though the mica closure temperatures (350-500^oC) are comparable or slightly higher than earlier geological estimates [1] of the rutile closure temperature. Thus, our results confirm a recent experimental study [2] that suggested the closure temperature for Pb diffusion in rutile (e.g. 100 μm) is much higher (200^oC) than previously thought [1]. [1] Mezger et al., 1989. High precision U-Pb ages of metamorphic rutile: applications to the cooling history of high-grade terranes. EPSL 96, 106-118. [2] Cherniak, 2000. Pb diffusion in rutile. Contrib. Mineral. Petrol., 139, 198-207.

Geochemical and zircon isotopic evidence for extensive high level crustal contamination in Miocene to mid-Pleistocene intra-plate volcanic rocks from the Tengchong field, western Yunnan, China

NASA Astrophysics Data System (ADS)

Li, Linlin; Shi, Yuruo; Williams, Ian S.; Anderson, J. Lawford; Wu, Zhonghai; Wang, Shubing

2017-08-01

SHRIMP zircon Pb/U dating of Cenozoic volcanic rocks in the Tengchong area, western Yunnan Province, China, shows that the dacite and andesitic breccia lavas from Qushi village were intruded at 480 ± 10 ka and 800 ± 40 ka, respectively. Moreover, Pb/U dating of trachyandesite from Tuantian village and olivine basalt from Wuhe village give weighted mean 206Pb/238U ages of 2.82 ± 0.08 Ma and 12.28 ± 0.30 Ma. Corrections for initial 230Th disequilibrium of zircon were used for the former two younger ages. The Tengchong volcanic rocks have a large range of SiO2 (48.6-66.9 wt.%) and mostly belong to a high-K calc-alkaline series. The lavas originated from heterogeneous sources and were modified by subsequent fractional crystallization. The REE and other trace element patterns of the Tengchong volcanic rocks resemble magmas having a large component of continental crust. All have similar degrees of LREE and HREE fractionation and are enriched in LILE, La, Ce and Pb, with depletions in Nb, Ta, Ti, Sr and P relative to primitive mantle. Zircon δ18O values of 6.96 ± 0.17 and 7.01 ± 0.24‰ and highly varied negative εHf(t) values of - 1.5 to - 11.0 and - 10.3 to - 13.7, as well as the presence of inherited zircon grains in the studied samples, indicate that the magmas contain crustal material on a large scale. The Tengchong volcanic rocks have HFSE ratios (e.g., Nb/Ta, La/Nb, Zr/Y) similar to continental flood basalts, indicative of an intra-plate extensional tectonic setting. Widespread distributed faults might have facilitated upwelling of mantle-derived melts and eruptions from shallow crustal magma chambers to form the large volcanic field.

U-Pb systematics of zircon and titanite from the Gardnos impact structure, Norway: Evidence for impact at 546 Ma?

NASA Astrophysics Data System (ADS)

Kalleson, E.; Corfu, F.; Dypvik, H.

2009-05-01

Zircon and titanite were investigated in impactites of the Gardnos structure, a crater formed in Sveconorwegian (ca. 1 Ga) crust, which was then overridden in the Devonian by Caledonian nappes. Observed deformation features in zircons are granular texture, planar microstructures, and likely the incorporation of organic carbon during impact causing black staining of the zircon grains. The grains were studied by scanning electron microscopy (SEM) and cathode luminescence (CL) and dated by U-Pb isotope dilution - thermo-ionization mass spectrometry (ID-TIMS). Zircon grains without impact related features have U-Pb data showing moderate discordance (5-13%) and indicating formation ages mostly in the range of 1600-1000 Ma, except detrital zircon ages as old as >2481 Ma, reflecting the diversity of target rocks in the area. Titanite with concordant ages of 995-999 Ma dates metamorphism during final juxtaposition of the Telemarkia on the Idefjorden terrane to the east. Zircon grains with demonstrated or presumed shock features yield highly discordant (14-40%) U-Pb data, with a majority of them plotting along an array with a lower intercept of about 340 Ma reflecting the influence of the Caledonian orogeny and recent Pb-loss. One zircon grain was totally reset at 379 Ma during late Caledonian metamorphism, which also caused local growth of new titanite. A specific group of zircon grains yields data with relatively high discordance for moderate U contents, and five of these analyses, including that of a grain with proven granular or aggregate texture, fit a discordia line with an upper intercept of 546 ± 5 Ma. These features are interpreted as indicating zircon break-down to an amorphous state during impact, with subsequent recrystallization into microcrystalline aggregates causing extensive to complete Pb loss. We further suggest that their crystallinity prevented Pb loss during the Caledonian orogeny, while the small subgrain size and increasing metamictisation allowed more recent disturbances. We thus interpret the 546 Ma age as the approximate time of impact.

Tectonic Implications of Paleoproterozoic Deo Khe Granitoids in Northwestern Vietnam

NASA Astrophysics Data System (ADS)

Hoang, T. H. A.; Yu, Y.; Pham, T. H.; Choi, S. H.; Tu, V. L.; Son, L. M.

2015-12-01

An integrated study of petrographic description, zircon U/Pb geochronology, and Hf isotopic analysis was carried out on the medium-grained two-mica Deo Khe Granitoids (DKG) in northwestern Vietnam. U/Pb zircon ages were 1855-1873 Ma, interpreted as the time of magma crystallization. On the basis of Hf isotopic compositions, a single-stage Hf model ages were estimated as 3.3-2.8 Ga. Values of Hf isotopes ɛHf (t) range from -23.6 to -17.5, suggesting that the DKG are products of reworked Archean crustal rocks. A similar sequence of tectonic events including the presence of 2.8-2.9 Ga tonalite-trondhjemite-granodiorite (TTG) gneiss, metamorphic development of TTG gneiss at 1.9-2.0 Ga, and 1.85 Ga magmatic activity were recognized both in Yangtze block and northwestern Vietnam. Therefore we propose that basement rocks in northern Vietnam are similar to those found along southern China.

Ti-in-Zircon Thermometer: Preliminary Results

NASA Astrophysics Data System (ADS)

Fu, B.; Cavosie, A. J.; Clechenko, C. C.; Fournelle, J.; Kita, N. T.; Lackey, J.; Page, F.; Wilde, S. A.; Valley, J. W.

2005-12-01

The titanium in zircon thermometer has been applied to 167 zircons from diverse rock types. These rocks include metamorphosed anorthosite and gabbro (1.15 Ga, intrusion age), and unmetamorphosed granitic pegmatite (0.9 Ga) from the Adirondack Highlands; metaluminous and peraluminous granites (114-90 Ma) of the Sierra Nevada Batholith; megacrysts from kimberlite pipes in southern Africa, Brazil, and Siberia; and detrital zircons (4.4-3.9 Ga) of metaconglomerate from Jack Hills, Western Australia. Titanium concentration in zircon was analysed using a CAMECA IMS-1280 ion microprobe (see Page et al., this volume). Spot analyses were correlated to U-Pb SHRIMP pits especially for Adirondack and Jack Hills zircons. The majority of zircons have Ti-content less than 10 ppm. Variability, in excess of analytical precision, within individual zircons is observed in about one-third of crystals. In general, there is no systematic change in Ti from core to rim (identified by cathodoluminescence) of zircons, or with regard to age, U content, Th/U ratio, or U-Pb age concordance for these non-metamict grains. The average temperatures for zircon crystallization in different rock suites using the experimental/empirical calibration of Watson and Harrison (W&H, 2005, Science 308:841), assuming the presence of rutile and quartz, are estimated to be: anorthosite 735±41°C (1SD, n=24; Ti = 10±5 ppm); metagabbro 714±31°C (n=19; Ti = 8±4 ppm); Adirondack pegmatite 500±16°C (n=5; Ti = 0.3±0.1 ppm); metaluminous and peraluminous granites from Sierra Nevada 681±67°C (n=53; Ti = 6±5 ppm) and 613±75°C (n=68; Ti = 3±3 ppm); kimberlite megacrysts 740±64°C (n=169; Ti = 14±13 ppm) (Page et al., this volume); and detrital zircons from Jack Hills metaconglomerate 718±63°C (n=64; Ti = 10±9 ppm). Most of the host rocks contain ilmenite or titanite suggesting that α(TiO2)>0.5, but rutile activity is unknown for megacrysts and detrital zircons. Pegmatite contains no Ti-rich minerals, permissive of low rutile activity. Peraluminous granites have lower whole-rock TiO2 content (0.02-0.21 wt.%) than metaluminous granites (0.54-0.91 wt.%), and both have similar zircon saturation temperatures (710-780°C). The low Ti content and apparent temperature in zircons from anorthosite and metagabbro (magmatic T = ca. 1000-1100°C) and the homogeneity of igneous zircon cores vs. metamorphic rims in metagabbro suggest that Ti content was reset during granulite-facies metamorphism (750°C, 1.0 Ga), in contrast to the current estimate of extremely low diffusivity of Ti in zircon. The Ti contents of >3.9 Ga Jack Hills detrital zircons are consistent with previously published data that were used to constrain magmatic temperature and thus the composition of unknown host rocks (W&H, 2005, Science 308:841). However, these values overlap those from both felsic and mafic rocks, and the Ti data are not sufficient to independently identify parent melt chemistry unless post-crystallization changes can be ruled out. The best evidence of pre-4 Ga evolved crust is provided by previously reported oxygen isotope ratios and felsic mineral inclusions in zircon (see Cavosie et al., 2005, EPSL 235:663).

CL-imaging and ion microprobe dating of single zircons from a high-grade rock from the Central Zone, Limpopo Belt, South Africa: Evidence for a single metamorphic event at ˜2.0 Ga

NASA Astrophysics Data System (ADS)

Mouri, H.; Brandl, G.; Whitehouse, M.; de Waal, S.; Guiraud, M.

2008-02-01

The combination of ion microprobe dating and cathodoluminescence (CL) imaging of zircons from a high-grade rock from the Central Zone of the Limpopo Belt were used to constrain the age of metamorphic events in the area. Zircon grains extracted from an orthopyroxene-gedrite-bearing granulite were prepared for single crystal CL-imaging and ion microprobe dating. The grains display complex zoning when using SEM-based CL-imaging. A common feature in most grains is the presence of a distinct core with a broken oscillatory zoned structure, which clearly appears to be the remnant of an original grain of igneous origin. This core is overgrown by an unzoned thin rim measuring about 10-30 μm in diameter, which is considered as new zircon growth during a single metamorphic event. Selected domains of the zircon grains were analysed for U, Pb and Th isotopic composition using a CAMECA IMS 1270 ion microprobe (Nordsim facility). Most of the grains define a near-concordant cluster with some evidence of Pb loss. The most concordant ages of the cores yielded a weighted mean 207Pb/ 206Pb age of 2689 ± 15 (2 σ) Ma, interpreted as the age of the protolith of an igneous origin. The unzoned overgrowths of the zircon grains yielded a considerably younger weighted mean 207Pb/ 206Pb age of ˜2006.5 ± 8.0 Ma (2 σ), and these data are interpreted to reflect closely the age of the ubiquitous high-grade metamorphic event in the Central Zone. This study shows clearly, based on both the internal structure of the zircons and the data obtained by ion microprobe dating, that only a single metamorphic event is recorded by the studied 2.69 Ga old rocks, and we found no evidence of an earlier metamorphic event at ˜2.5 Ga as postulated earlier by some workers.

Age of UHP metamorphism in the Western Mediterranean: Insight from rutile and minute zircon inclusions in a diamond-bearing garnet megacryst (Edough Massif, NE Algeria)

NASA Astrophysics Data System (ADS)

Bruguier, Olivier; Bosch, Delphine; Caby, Renaud; Vitale-Brovarone, Alberto; Fernandez, Laure; Hammor, Dalila; Laouar, Rabah; Ouabadi, Aziouz; Abdallah, Nachida; Mechati, Mehdi

2017-09-01

Diamond-bearing UHP metamorphic rocks witness for subduction of lithospheric slabs into the mantle and their return to shallow levels. In this study we present U-Pb and trace elements analyses of zircon and rutile inclusions from a diamond-bearing garnet megacryst collected in a mélange unit exposed on the northern margin of Africa (Edough Massif, NE Algeria). Large rutile crystals (up to 300 μm in size) analyzed in situ provide a U-Pb age of 32.4 ± 3.3 Ma interpreted as dating the prograde to peak subduction stage of the mafic protolith. Trace element analyses of minute zircons (≤30 μm) indicate that they formed in equilibrium with the garnet megacryst at a temperature of 740-810 °C, most likely during HP retrograde metamorphism. U-Pb analyses provide a significantly younger age of 20.7 ± 2.3 Ma attributed to exhumation of the UHP units. This study allows bracketing the age of UHP metamorphism in the Western Mediterranean Orogen to the Oligocene/early Miocene, thus unambiguously relating UHP metamorphism to the Alpine history. Exhumation of these UHP units is coeval with the counterclockwise rotation of the Corsica-Sardinia block and most likely resulted from subduction rollback that was driven by slab pull.

Genesis of zircon megacrysts in Cenozoic alkali basalts and the heterogeneity of subcontinental lithospheric mantle, eastern China

NASA Astrophysics Data System (ADS)

Yu, Yao; Xu, Xisheng; Chen, Xiaoming

2010-09-01

Zircon megacrysts are found in alluvial deposits associated with Cenozoic basalts from Changle in Shandong Province, Mingxi in Fujian Province and Penglai in Hainan Province within the coastal area of eastern China. They are colourless, transparent to light brown-maroon, and some of them are up to 16 mm long. U-Pb ages of zircon megacrysts from Changle, Mingxi and Penglai are 19.2 ± 0.7 Ma, 1.2 ± 0.1 Ma and 4.1 ± 0.2 Ma respectively, slightly older than the eruption ages of their corresponding host rocks (16.05-18.87 Ma, 0.9-2.2 Ma, 3 Ma). ɛHf(t) values of zircon megacrysts are 9.02 ± 0.49, 6.83 ± 0.47, 4.46 ± 0.48 for Changle, Mingxi and Penglai, respectively, which indicates their mantle origin. We suggest that the zircon megacrysts originated from metasomatised lithospheric mantle and were later brought up quickly by the host basaltic magma. The euhedral forms, uniform internal structure and chemical homogeneity within a single grain suggest crystallization under stable conditions. Pronounced positive Ce anomalies and negligible Eu anomalies suggest oxidizing conditions and little or no fractional crystallization of plagioclase. The differences in Hf-isotope compositions among the zircon megacrysts from different localities are consistent with the Sr-Nd-Pb isotopic compositions of their respective host basalts. This indicates that the host basalts acquired their isotopic signatures from the lithospheric mantle from which the zircon megacrysts derived. These data document the lateral compositional heterogeneity in the upper mantle beneath eastern China. Like mantle xenoliths, zircon megacrysts also have the potential to fingerprint the composition and evolution of the subcontinental lithospheric mantle.

Coeval Formation of Zircon Megacrysts and Host Magmas in the Eifel Volcanic Field (Germany) Based on High Spatial Resolution Petrochronology

NASA Astrophysics Data System (ADS)

Schmitt, Axel; Klitzke, Malte; Gerdes, Axel; Ludwig, Thomas; Schäfer, Christof

2017-04-01

Zircon megacrysts (approx. 0.5-6 mm in diameter) from the Quaternary West and East Eifel volcanic fields, Germany, occur as euhedral crystals in porous K-spar rich plutonic ejecta clasts, and as partially resorbed xenocrysts in tephrite lava. Their relation to the host volcanic rocks has remained contentious because the dominantly basanitic to phonolitic magma compositions in the Eifel are typically zircon undersaturated. We carried out a detailed microanalytical study of zircon megacrysts from seven locations (Emmelberg and Rockeskyll in the West Eifel; Bellerberg, Laacher See, Mendig, Rieden, and Wehr in the East Eifel). Crystals were embedded in epoxy, sectioned to expose interiors through grinding with abrasives, diamond-polished, and mapped by optical microscopy, backscattered electron, and cathodoluminescence imaging. Subsequently, isotope-specific analysis using secondary ionization mass spectrometry (SIMS) and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) was carried out placing 100 correlated spots on 20 selected crystals. Concordant U-Th disequilibrium and U-Pb ages determined by SIMS are between ca. 430 ka (Rieden) and 170 ka (Mendig) and indicate that the megacryst zircons crystallized almost always briefly before eruption. A significant gap between zircon megacryst crystallization (ca. 230 ka) and eruption (ca. 45 ka) ages was only detected for the Emmelberg location. SIMS trace element abundances (e.g., rare earth elements) vary by orders-of-magnitude and correlate with domain boundaries visible in cathodoluminescence; trace element patterns match those reported for zircon from syenitic origins. Isotopic compositions are homogeneous within individual crystals, but show some heterogeneity between different crystals from the same locality. Average isotopic values (δ18O SMOW = +5.3±0.6 ‰ by SIMS; present-day ɛHf = +1.7±2.5 ‰ by LA-ICP-MS; 1 standard deviation), however, are consistent with source magmas being dominantly mantle-derived. The porous structure and relatively small grain size of the host enclaves suggests that they originated from subvolcanic intrusions. Moreover, the preservation of zircon in hot, zircon undersaturated magmas requires brief residence times. Zircon megacrysts thus appear to have crystallized in highly differentiated magmas or nearly solidified intrusions from which crystals or rock aggregates were incorporated into more primitive magmas en route to surface. This implies that chemical signatures of apparently primitive magmas in basaltic volcanic fields can be modified by interaction with evolved melts that differentiated prior to eruption, mostly within an interval less than the ca. 10-25 ka uncertainty range of the radiometric ages.

The Age of the intra-Danubian Suture (Southern Carpathians, Romania)

NASA Astrophysics Data System (ADS)

Balica, C.; Hann, H. P.; Chen, F.; Balintoni, I. C.; Zaharia, L.

2007-12-01

The Southern Carpathians, as an Alpine chain are formed of two domains, namely the Getic Domain (GD) and the Danubian Domain (DD). The basement of DD is represented by two terranes, named Dragsan and Lainici- Paius sutured through Tisovita-Iuti ophiolitic complex. The two terranes were invaded by large granitic plutons, some of them being dated as Late Proterozoic by U/Pb method. Yet, along the inferred suture there are four granitic bodies whose ages have been only assumed by their geological relations. From North to South the four bodies sampled for LA-ICP-MS zircon U/Pb dating are: Muntele Mic, Sfardinu, Cherbelezu and Ogradena. The previously CL imaged zircon crystals were ablated at the China's University of Geosciences facilities in Wuhan. The zircons from all samples showed quite complex structures, with many inherited cores or affected by lead loss processes. In order to get a mean age for every pluton, we used the weighted average plots by projecting the 206Pb/238U apparent ages. The crystallization age of the Poiana Marului pluton is around 326.7±7 Ma (MSWD 1.6). A set of sixteen apparent ages ranging between 400 to 648 Ma together with other tree points indicating 897, 1353 and 1693 Ma, represent inheritances. There was no observable lead loss process in this data set. The Sfardinu granite crystallized at 310±7.9 Ma (MSWD 4.8). The inheritances found in this sample are at 427 and 723 Ma, but an important lead loss process occurred later, as indicated by ten apparent ages between 240-292 Ma. Cherbelezu granite gave a crystallization mean age of 326.9±4.9 Ma (MSWD 1.9). A single inherited core appears at 502 Ma, yet eight apparent ages ranging between 239-295 Ma signalize again an important lead loss process. Other two ages at 176 and 193 Ma confirm the presence of this process. The Ogradena pluton zircons display two possible crystallization ages for the outer zones of zircon grains, at 356.6±7.8 Ma (twelve apparent ages, MSWD 12) and 314.1±7.8 Ma (nine apparent ages, MSWD 13) respectively. It is difficult to explain this situation, and we suppose the second figure as the probable crystallization age. Inheritances are represented by a number of 37 206Pb/238U apparent ages scattered between 400 and 612 Ma, only one apparent age of 265 Ma possibly proving the presence of lead loss. The geochemical parameters ASI, Fe-number and MALI calculated from the major oxides analyses showed that all four plutons are metaluminous and magnezian. Both Cherbelezu and Ogradena have a calcalkaline character near to alkalicalcic while Muntele Mic and Sfardinu are alkalicalcic. Geochemically, all four plutons are closed to Cordilleran type, main portion or a little inboard of it. Out of these data several conclusions can be drawn. The age of the intra-Danubian suture is late Visean to Bashkirian or late Variscan. Muntele Mic and Ogradena granites exhibit abundant Cadomian inheritances, while Cherbelezu and Sfardinu plutons seem affected by lead loss processes due to an important thermotectonic event happened during 250 to 290 Ma interval. The emplacement of the four plutons probably reflects a subductional process. Finally, the inheritances suggest a Gondwanan source for the anatectic material.

Zircon and monazite response to prograde metamorphism in the Reynolds Range, central Australia

NASA Astrophysics Data System (ADS)

Rubatto, Daniela; Williams, Ian S.; Buick, Ian S.

2001-01-01

We report an extensive field-based study of zircon and monazite in the metamorphic sequence of the Reynolds Range (central Australia), where greenschist- to granulite-facies metamorphism is recorded over a continuous crustal section. Detailed cathodoluminescence and back-scattered electron imaging, supported by SHRIMP U-Pb dating, has revealed the different behaviours of zircon and monazite during metamorphism. Monazite first recorded regional metamorphic ages (1576 ± 5 Ma), at amphibolite-facies grade, at ˜600 °C. Abundant monazite yielding similar ages (1557 ± 2 to 1585 ± 3 Ma) is found at granulite-facies conditions in both partial melt segregations and restites. New zircon growth occurred between 1562 ± 4 and 1587 ± 4 Ma, but, in contrast to monazite, is only recorded in granulite-facies rocks where melt was present (≥700 °C). New zircon appears to form at the expense of pre-existing detrital and inherited cores, which are partly resorbed. The amount of metamorphic growth in both accessory minerals increases with temperature and metamorphic grade. However, new zircon growth is influenced by rock composition and driven by partial melting, factors that appear to have little effect on the formation of metamorphic monazite. The growth of these accessory phases in response to metamorphism extends over the 30 Ma period of melt crystallisation (1557-1587 Ma) in a stable high geothermal regime. Rare earth element patterns of zircon overgrowths in leucosome and restite indicate that, during the protracted metamorphism, melt-restite equilibrium was reached. Even in the extreme conditions of long-lasting high temperature (750-800 °C) metamorphism, Pb inheritance is widely preserved in the detrital zircon cores. A trace of inheritance is found in monazite, indicating that the closure temperature of the U-Pb system in relatively large monazite crystals can exceed 750-800 °C.

Provenance of the lower Miocene of the Gulf of Mexico from detrital zircon double dating

NASA Astrophysics Data System (ADS)

xu, J.

2013-12-01

The lower Miocene interval of the Gulf of Mexico (GOM) has recently gained increasing attention from oil and gas industry due to its hydrocarbon potential below the salt canopy. However, it has been less well studied than both the underlying Oligocene and overlying middle Miocene strata. The lower Miocene worldwide is a transitional period of tectonic, climatic, and oceanographic change. In particular, it is a period of major tectonic reorganization in the western interior of North America (Rocky Mountains), involving a shift from the Oligocene thermal phase, with abundant volcanic activity recorded in the thick Frio/Vicksburg succession of the GOM, to the Miocene Basin-Range extensional phase. Climatic conditions also changed from a relatively arid Oligocene to wetter Miocene, resulting in increased sediment yields from exhumed tectonic structures. Previous provenance studies used proportions of quartz, feldspar and lithic fragments and consideration of likely river courses through known paleogeomorphological elements. Only limited detrital zircon (DZ) U-Pb studies on Paleocene strata have been undertaken and there has been no previous U-Pb and (U-Th)/He double dating in the GOM. In this study we apply the latest analytical approaches, such as DZ U-Pb dating to gain robust source terranes ages and more fully elucidate the complex sediment provenance and dispersal history of GOM. We also employ DZ (U-Th)/He (ZHe) dating, combined with DZ U-Pb, to not only define sedimentary provenance but also the exhumation histories of detrital source regions. Samples of lower Miocene outcrop exposures in Texas and Louisiana have been collected to discriminate the varied tectonic and drainage system changes across the basin in lateral. In addition, samples from the Eocene, Oligocene and middle Miocene have been obtained to reveal vertical shift of source terranes contributions. Our initial age data show detrital zircons of lower Miocene sediments come from a wide range of source terranes including a large populations from the western interior of North America (Rocky Mountains), Grenville, Mid-Continent, and Yavapai-Mazatzal provinces, with smaller populations from the Appalachian-Ouachita, Wyoming or Superior regions. Based on U-Pb dating results, we will carry out (U-Th)/He dating on selected zircons to reveal the detailed exhumation histories of the sediment source regions. Using the dual criteria of DZ crystallization age (U-Pb) and cooling age (U-Th/He) to constrain provenance will enable us to generate rigorous reconstructions of the lower Miocene depositional systems from source terrane to deep-water sink for this key transitional period in geologic history.

Major zircon megacryst suites of the Indo-Pacific lithospheric margin (ZIP) and their petrogenetic and regional implications

NASA Astrophysics Data System (ADS)

Sutherland, Lin; Graham, Ian; Yaxley, Gregory; Armstrong, Richard; Giuliani, Gaston; Hoskin, Paul; Nechaev, Victor; Woodhead, Jon

2016-04-01

Zircon megacrysts (± gem corundum) appear in basalt fields of Indo-Pacific origin over a 12,000 km zone (ZIP) along West Pacific continental margins. Age-dating, trace element, oxygen and hafnium isotope studies on representative zircons (East Australia-Asia) indicate diverse magmatic sources. The U-Pb (249 to 1 Ma) and zircon fission track (ZFT) ages (65 to 1 Ma) suggest thermal annealing during later basalt transport, with < 1 to 203 Ma gaps between the U-Pb and ZFT ages. Magmatic growth zonation and Zr/Hf ratios (0.01-0.02) suggest alkaline magmatic sources, while Ti—in—zircon thermometry suggests that most zircons crystallized within ranges between 550 and 830 °C. Chondrite-normalised multi-element plots show variable enrichment patterns, mostly without marked Eu depletion, indicating little plagioclase fractionation in source melts. Key elements and ratios matched against zircons from magmatic rocks suggest a range of ultramafic to felsic source melts. Zircon O-isotope ratios (δ18O in the range 4 to 11‰) and initial Hf isotope ratios (ɛHf in the range +2 to +14) encompass ranges for both mantle and crustal melts. Calculated Depleted Mantle (TDM 0.03-0.56 Ga) and Crustal Residence (0.20-1.02 Ga) model ages suggest several mantle events, continental break-ups (Rodinia and Gondwana) and convergent margin collisions left imprints in the zircon source melts. East Australian ZIP sites reflect prolonged intraplate magmatism (~85 Ma), often during times of fast-migrating lithosphere. In contrast, East Asian-Russian ZIP sites reflect later basaltic magmatism (<40 Ma), often linked to episodes of back-arc rifting and spreading, slow-migrating lithosphere and slab subduction.

Signatures of mountain building: Detrital zircon U/Pb ages from northeast Tibet

USGS Publications Warehouse

Lease, Richard O.; Burbank, Douglas W.; Gehrels, George E.; Wang, Zhicai; Yuan, Daoyang

2007-01-01

Although detrital zircon has proven to be a powerful tool for determining provenance, past work has focused primarily on delimiting regional source terranes. Here we explore the limits of spatial resolution and stratigraphic sensitivity of detrital zircon in ascertaining provenance, and we demonstrate its ability to detect source changes for terranes separated by only a few tens of kilometers. For such an analysis to succeed for a given mountain, discrete intrarange source terranes must have unique U/Pb zircon age signatures and sediments eroded from the range must have well-defined depositional ages. Here we use ∼1400 single-grain U/Pb zircon ages from northeastern Tibet to identify and analyze an area that satisfies these conditions. This analysis shows that the edges of intermontane basins are stratigraphically sensitive to discrete, punctuated changes in local source terranes. By tracking eroding rock units chronologically through the stratigraphic record, this sensitivity permits the detection of the differential rock uplift and progressive erosion that began ca. 8 Ma in the Laji Shan, a 10-25-km-wide range in northeastern Tibet with a unique U/Pb age signature.

An ion microprobe study of individual zircon phenocrysts from voluminous post-caldera rhyolites of the Yellowstone caldera

NASA Astrophysics Data System (ADS)

Watts, K. E.; Bindeman, I. N.; Schmitt, A. K.

2010-12-01

Following the formation of the Yellowstone caldera from the 640 ka supereruption of the Lava Creek Tuff (LCT), a voluminous episode of post-caldera volcanism filled the caldera with >600 km3 of low-δ18O rhyolite. Such low-δ18O signatures require remelting of 100s of km3 of hydrothermally altered (18O-depleted) rock in the shallow crust. We present a high resolution oxygen isotope and geochronology (U-Th and U-Pb) study of individual zircon crystals from seven of these voluminous post-caldera rhyolites in order to elucidate their genesis. Oxygen isotope and geochronology analyses of zircon were performed with an ion microprobe that enabled us to doubly fingerprint 25-30 µm diameter spots. Host groundmass glasses and coexisting quartz were analyzed in bulk for oxygen isotopes by laser fluorination. We find that zircons from the youngest (200-80 ka) post-caldera rhyolites have oxygen isotopic compositions that are in equilibrium with low-δ18O host groundmass glasses and quartz and are unzoned in oxygen and U-Th age. This finding is in contrast to prior work on older (500-250 ka) post-caldera rhyolites, which exhibit isotopic disequilibria and age zoning, including the presence of clearly inherited zircon cores. Average U-Th crystallization ages and δ18O zircon values for Pitchstone Plateau flow (81±7 ka, 2.8±0.2‰), West Yellowstone flow (118±8 ka, 2.8±0.1‰), Elephant Back flow (175±22 ka, 2.7±0.2‰) and Tuff of Bluff Point (176±20 ka, 2.7±0.1‰) are overlapping or nearly overlapping in age and identical in oxygen isotope composition within uncertainty (2 SE). New U-Pb geochronology and oxygen isotope data for the North Biscuit Basin flow establish that it has an age (188±33 ka) and δ18O signature (2.8±0.2‰) that is distinctive of the youngest post-caldera rhyolites. Conversely, the South Biscuit Basin flow has a heterogeneous zircon population with ages that range from 550-250 ka. In this unit, older and larger (200-400 µm) zircons have more disparity in δ18O signatures (-0.2-3.6‰) while the younger and smaller (<100-200 µm) zircons have δ18O signatures (2.6±0.3‰) that are identical to the youngest post-caldera rhyolites. Our results are consistent with derivation of the youngest post-caldera rhyolites from a common magma reservoir that was assembled from heterogeneous pockets of low-δ18O melt. The magma was homogenized prior to eruption, erasing evidence of batch assembly in all but the oldest South Biscuit Basin unit. An important new finding of this study is that the newly defined post-LCT East Biscuit Basin flow is the oldest (U-Pb age of 761±66 ka) and most primitive (70 wt% SiO2) rhyolite to erupt from within the Yellowstone caldera. Unlike the youngest post-caldera rhyolites, the average δ18O composition of East Biscuit Basin zircons (4.4±0.3‰) is significantly higher and out of equilibrium with the host groundmass glass (δ18O=1.0‰). Its formation requires remelting of hydrothermally altered LCT and pre-LCT subcaldera rocks that retain oxygen isotopic evidence of inherited zircon phenocrysts.

Evidence for prolonged mid-Paleozoic plutonism and ages of crustal sources in east-central Alaska from SHRIMP U-Pb dating of syn-magmatic, inherited, and detrital zircon

USGS Publications Warehouse

Dusel-Bacon, C.; Williams, I.S.

2009-01-01

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb analyses of igneous zircons from the Lake George assemblage in the eastern Yukon-Tanana Upland (Tanacross quadrangle) indicate both Late Devonian (???370 Ma) and Early Mississippian (???350 Ma) magmatic pulses. The zircons occur in four textural variants of granitic orthogneiss from a large area of muscovite-biotite augen gneiss. Granitic orthogneiss from the nearby Fiftymile batholith, which straddles the Alaska-Yukon border, yielded a similar range in zircon U-Pb ages, suggesting that both the Fiftymile batholith and the Tanacross orthogneiss body consist of multiple intrusions. We interpret the overall tectonic setting for the Late Devonian and Early Mississippian magmatism as an extending continental margin (broad back-arc region) inboard of a northeast-dipping (present coordinates) subduction zone. New SHRIMP U-Pb ages of inherited zircon cores in the Tanacross orthogneisses and of detrital zircons from quartzite from the Jarvis belt in the Alaska Range (Mount Hayes quadrangle) include major 2.0-1.7 Ga clusters and lesser 2.7-2.3 Ga clusters, with subordinate 3.2, 1.4, and 1.1 Ga clusters in some orthogneiss samples. For the most part, these inherited and core U-Pb ages match those of basement provinces of the western Canadian Shield and indicate widespread potential sources within western Laurentia for most grain populations; these ages also match the detrital zircon reference for the northern North American miogeocline and support a correlation between the two areas.

Reply to Comment on "Zircon U-Th-Pb dating using LA-ICP-MS: Simultaneous U-Pb and U-Th dating on the 0.1 Ma Toya Tephra, Japan"

NASA Astrophysics Data System (ADS)

Ito, Hisatoshi

2015-04-01

Guillong et al. (2015) mentioned that corrections for abundance sensitivity for 232Th and molecular zirconium sesquioxide ions (Zr2O3+) are critical for reliable determination of 230Th abundances in zircon for LA-ICP-MS analyses. There is no denying that more rigorous treatments are necessary to obtain more reliable ages than those in Ito (2014). However, as shown in Fig. 2 in Guillong et al. (2015), the uncorrected (230Th)/(238U) for reference zircons except for Mud Tank are only 5-20% higher than unity. Since U abundance of Toya Tephra zircons that have U-Pb ages < 1 Ma is in-between that of FCT and Plesovice, the overestimation of 230Th by both abundance sensitivity and molecular interferences is expected to be 5-20% for the Toya Tephra. Moreover Ito (2014) obtained U-Th ages of the Toya Tephra by comparison with Fish Canyon Tuff (FCT) data. Because both the FCT and the Toya Tephra have similar trends of overestimation of 230Th, the effect of overestimation of 230Th to cause overestimation of U-Th age should be cancelled out or negligible. Therefore the pivotal conclusion in Ito (2014) that simultaneous U-Pb and U-Th dating using LA-ICP-MS is possible and useful for Quaternary zircons holds true.

U-Xe Degassing Ages of Terrestrial and Lunar Impact Zircons

NASA Astrophysics Data System (ADS)

Crow, C. A.; Crowther, S. A.; Gilmour, J. D.; Busemann, H.; Moser, D. E.; McKeegan, K. D.

2015-07-01

We present U-Xe degassing ages for individual zircons from Apollo 14 samples and the terrestrial impact structure at Vrederfort, South Africa. Preliminary results suggest the degassing ages are consistent with or younger than their 207Pb-206Pb ages.

U-Pb Ages of Lunar Apatites

NASA Technical Reports Server (NTRS)

Vaughan, J.; Nemchin, A. A.; Pidgeon, R. T.; Meyer, Charles

2006-01-01

Apatite is one of the minerals that is rarely utilized in U-Pb geochronology, compared to some other U-rich accessory phases. Relatively low U concentration, commonly high proportion of common Pb and low closure temperature of U-Pb system of apatite inhibit its application as geochronological tool when other minerals such as zircon are widely available. However, zircon appear to be restricted to certain type of lunar rocks, carrying so called KREEP signature, whereas apatite (and whitlockite) is a common accessory mineral in the lunar samples. Therefore, utilizing apatite for lunar chronology may increase the pool of rocks that are available for U-Pb dating. The low stability of U-Pb systematics of apatite may also result in the resetting of the system during meteoritic bombardment, in which case apatite may provide an additional tool for the study of the impact history of the Moon. In order to investigate these possibilities, we have analysed apatites and zircons from two breccia samples collected during the Apollo 14 mission. Both samples were collected within the Fra Mauro formation, which is interpreted as a material ejected during the impact that formed the Imbrium Basin.

Applications of primary and secondary inclusion assemblages for zircon petrogenesis and alteration

NASA Astrophysics Data System (ADS)

Bell, E. A.

2017-12-01

Igneous zircon often contains abundant mineral inclusions which represent a mixture of primary phases captured during crystallization in magma and secondary phases formed either during late-stage deuteric alteration of a solidifying pluton, during later metamorphism, or during detrital transport and diagenesis in groundwater. Microstructural examination of zircon from both magmatic and metamorphic rocks reveals varying abundances of clearly secondary phases filling cracks and potentially secondary phases in contact with cracks or in disturbed regions of the host zircon. We used EDS and WDS X-ray spectroscopy to examine crack-isolated, crack-intersecting, and crack-filling phases in zircon from Phanerozoic magmatic rocks (USA, Victoria), several Grenville (Blue Ridge, VA) orthogneisses, and detrital zircons in metasediments from Jack Hills, Mt. Narryer (Western Australia) and the Nuvvuagittuq supracrustal belt (northern Quebec). Orthogneiss and detrital zircon appear to retain primary inclusion compositions away from contact with cracks or disturbed regions of zircon (as distinguished by U-Pb). Characteristic trace element patterns associated with chemical alteration of zircon match well with the apparently dominant secondary phases in metasedimentary detrital zircons and magmatic zircon subjected to deuteric alteration. Additionally, high spatial resolution Pb isotopic analyses of secondary phosphates using the CAMECA ims1290 ion microprobe reveal preservation of multiple generations of metamorphic phosphate, in some cases juxtaposed within a single inclusion on the 5-10 micron scale. Zircon can therefore in many cases preserve the compositions of its primary inclusion cargo through later metamorphism. Zircon can also preserve information about individual hydrothermal or metamorphic events during the grain's residence in the crust.

Middle Proterozoic age for the Montpelier Anorthosite, Goochland terrane, eastern Piedmont, Virginia

USGS Publications Warehouse

Aleinikoff, J.N.; Horton, J. Wright; Walter, M.

1996-01-01

Uranium-lead dating of zircons from the Montpelier Anorthosite confirms previous interpretations, based on equivocal evidence, that the Goochland terrane in the eastern Piedmont of Virginia contains Grenvillian basement rocks of Middle Proterozoic age. A very few prismatic, elongate, euhedral zircons, which contain 12-29 ppm uranium, are interpreted to be igneous in origin. The vast majority of zircons are more equant, subangular to anhedral, contain 38-52 ppm uranium, and are interpreted to be metamorphic in origin. One fraction of elongate zircon, and four fragments of a very large zircon (occurring in a nelsonite segregation) yield an upper intercept age of 1045 ?? 10 Ma, interpreted as the time of anorthosite crystallization. Irregularly shaped metamorphic zircons are dated at 1011 ?? 2 Ma (weighted average of the 207Pb/206Pb ages). The U-Pb isotopic systematics of metamorphic titanite were reset during the Alleghanian orogeny at 297 ?? 5 Ma. These data provide a minimum age for gneisses of the Goochland terrane that are intruded by the anorthosite. Middle Proterozoic basement rocks of the Goochland terrane may be correlative with those in the Shenandoah massif of the Blue Ridge tectonic province, as suggested by similarities between the Montpelier Anorthosite and the Roseland anorthosite. Although the areal extent of Middle Proterozoic basement and basement-cover relations in the eastern Piedmont remain unresolved, results of this investigation indicate that the Goochland terrane is an internal massif of Laurentian crust rather than an exotic accreted terrane.

New Insights on the Recrystallization and New Growth of Extensively Radiation-damaged Zircon

NASA Astrophysics Data System (ADS)

Hanchar, J. M.; Schmitz, M. D.; Wirth, R.

2012-12-01

Approximately 10 grams of cm-sized nearly metamict zircon crystals from the Saranac Prospect in the Bancroft District of Ontario were combined by breaking into small pieces and then ground under ethanol to a fine powder with an agate mortar and pestle in order to make enough homogeneously mixed material for multiple powder X-Ray and diffraction scans, high-resolution transmission electron microscopy (HR-TEM) measurements, and chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-TIMS). While these large zircon crystals ground to a powder have a larger surface area and not in the same physical state (i.e., brown and metamict) as what is typically analyzed in single zircon CA-ID-TIMS U-Pb analysis (clear euhedral grains), the physical and chemical changes that occur during the heat treatment used in CA-TIMS are thought to be similar processes. Aliquots of the ground zircon powder were annealed in situ using a Pt furnace in a powder diffractometer during which time simultaneous powder diffraction patterns were acquired starting at 25°C, at elevated temperature (from 500°C to 1400°C) at selected time intervals, and then again at 25°C. The powder X-ray diffraction results indicate that below ~900°C the recrystallization of the zircon powder commences but is incomplete, even after 36 hours, with diffuse low intensity diffraction peaks. At 1150°C the zircon powder shows significant recrystallization. At 1150°C, the recrystallization is essentially complete in less than one hour. Before heating the zircon powder samples consisted of clear, transparent to brown, translucent, complexly zoned fragments. After heating at 900°C the zircon powder retained a smaller percentage of clear or brown complexly zoned fragments, while the majority of material had transformed to oscillatory or irregularly zoned, dominantly white opaque microcrystalline fragments. The clear fragments were hypothesized to be preexisting original crystalline zircon, the brown complexly zoned fragments preexisting metamict zircon, and the white opaque fragment new recrystallized zircon and other oxides. At 1150°C all that remained after heating were dominantly white opaque fragments and extremely rare clear fragments. A variety of fragment types from unannealed, 900°C and 1150°C anneals were chemically abraded in concentrated hydrofluoric acid at 190°C for 12 hours. Upon treatment with chemical abrasion, all unannealed material, nearly all material from the 900°C anneal, and all white opaque microcrystalline material from the 1150°C anneal dissolved; only the rare residual clear, transparent fragments from the 1150°C anneal were robust to chemical abrasion at these conditions. Residual clear fragments yielded concordant U-Pb ID-TIMS dates of 1064 Ma (considering updated U decay constant ratio), confirming the hypothesis that low-U closed system domains are preserved through annealing up to 1150°C and can be extracted via chemical abrasion from even dominantly metamict zircon crystals. By contrast, newly formed crystallites resulting from metamict zircon breakdown during annealing appear to be quite soluble during chemical abrasion. Further experiments are underway to refine minimum threshold chemical abrasion conditions necessary to eliminate open-system domains in the Saranac zircon.

Zircon U-Pb geochronology and Sr-Nd-Pb-Hf isotopic constraints on the timing and origin of Mesozoic granitoids hosting the Mo deposits in northern Xilamulun district, NE China

NASA Astrophysics Data System (ADS)

Shu, Qihai; Lai, Yong; Zhou, Yitao; Xu, Jiajia; Wu, Huaying

2015-12-01

Located in the east section of the Central Asian orogen in northeastern China, the Xilamulun district comprises several newly discovered molybdenum deposits, primarily of porphyry type and Mesozoic ages. This district is divided by the Xilamulun fault into the southern and the northern parts. In this paper, we present new zircon U-Pb dating, trace elements and Hf isotope, and/or whole rock Sr-Nd-Pb isotopic results for the host granitoids from three Mo deposits (Yangchang, Haisugou and Shabutai) in northern Xilamulun. Our aim is to constrain the age and petrogenesis of these intrusions and their implications for Mo mineralization. Zircon U-Pb LA-ICP-MS dating shows that the monzogranites from the Shabutai and Yangchang deposits formed at 138.4 ± 1.5 and 137.4 ± 2.1 Ma, respectively, which is identical to the molybdenite Re-Os ages and coeval well with the other Mo deposits in this region, thereby indicating an Early Cretaceous magmatism and Mo mineralization event. Zircon Ce/Nd ratios from the mineralized intrusions are significantly higher than the barren granites, implying that the mineralization-related magmas are characterized by higher oxygen fugacity. These mineralized intrusions share similar zircon in-situ Hf and whole rock Sr-Nd isotopic compositions, with slightly negative to positive εHf(t) ranging from - 0.8 to + 10.0, restricted εNd(t) values from - 3.7 to + 1.6 but a little variable (87Sr/86Sr)i ratios between 0.7021 and 0.7074, indicative of formation from primary magmas generated from a dominantly juvenile lower crust source derived from depleted mantle, despite diverse consequent processes (e.g., magma mixing, fractional crystallization and crustal contamination) during their evolution. The Pb isotopes (whole rock) also show a narrow range of initial compositions, with (206Pb/204Pb)i = 18.03-18.88, (207Pb/204Pb)i = 15.48-15.58 and (208Pb/204Pb)i = 37.72-38.28, in agreement with Sr-Nd-Hf isotopes reflecting the dominance of a mantle component. An integration of geochronological and isotopic data points to three stages of Mo mineralization events (i.e., Triassic, Late Jurassic, and Early Cretaceous) associated with magmas generated from three different source regions in the Xilamulun district, NE China. The variation in the origin of the magmas from which the porphyry Mo systems were generated suggests that the formation of Mo deposit lies not in the composition of magma sources. Other factors, including high magma oxygen fugacity, may have taken a fundamental role in Mo enrichment and subsequent mineralization.

Petrogenesis and magmatic evolution of ∼130 Ma A-type granites in Southeast China

NASA Astrophysics Data System (ADS)

Sun, Fajun; Xu, Xisheng; Zou, Haibo; Xia, Yan

2015-02-01

A number of Late Mesozoic (∼130 Ma) A-type granitic plutons have been identified in Southeast China. Here we investigate the petrogenesis of one of these granitic plutons in Southeast China, the Sanqingshan-Damaoshan (SD) granites in northeastern Jiangxi Province, using zircon U-Pb geochronology, Hf isotopic analyses, and major and trace element analyses. The SD granites are metaluminous to weakly peraluminous and show typical A-type affinity, which is characterized by high SiO2, Na2O + K2O, rare earth element (REE), high field strength element (HFSE) contents, Ga/Al and Fe# [FeOt/(FeOt + MgO)] values. Zircon grains from the SD granites and some other ∼130 Ma A-type granites commonly contain oscillatory zoning ;cores; surrounded by unzoned to weakly zoned ;rims;. Detailed studies of zircons from the SD granites show that ;rims; are enriched in LREE, Th and U compared with ;cores;. Chondrite-normalized REE patterns of the ;cores; increase steeply from La to Lu and show pronounced Ce and Eu anomalies, while REE patterns of the ;rims; display higher REE abundances with flatter LREE patterns and moderate Ce anomalies. Nevertheless, Lu-Hf isotopic analyses and Ti-in zircon thermometer show similar characteristics between ;rims; and ;cores;, indicating that the ;rims; may crystallize under the effect of internal magmatic hydrothermal fluids. U-rich ;rims; are more susceptible to Pb loss caused by self-irradiation, which may lead to significant younger U-Pb ages. As a result, U-Pb ages of zircon ;cores; (∼130 Ma) represent crystallization ages of the SD granites. εHf(t) values of zircon grains from the SD granites are between -6.4 and -0.4 with Mesoproterozoic model ages (T2DM) ranging from 1.22 to 1.59 Ga, suggesting that the granites may be formed by partial melting of Proterozoic basement. Compared with other adjacent ∼130 Ma A-type granitic plutons in SE China, the SD granites have similar geochemical characteristics and Hf isotopic compositions to those of Xiangshan, Daqiaowu, Yangmeiwan, and Tongshan granites, but different from the Baijuhuajian granite. εHf(t) values of the Baijuhuajian granites are higher than other granites, indicating significant participation of juvenile materials. These ∼130 Ma A-type granites indicate a back-arc extension setting due to the roll-back of paleo-Pacific plate, where the crust and lithospheric mantle became progressively thinned. The upwelling of asthenosphere triggered the partial melting of crustal rocks and generated the Sanqingshan-Damaoshan, Tongshan, Daqiaowu and Yangmeiwan granitic plutons. With ongoing back-arc extension and increased subduction angle during the roll-back of subducted paleo-Pacific slab, the back-arc extension gradually intensified, resulting in significant additions of mantle juvenile materials to the crustal magma and the formation of the Baijuhuajian granite.

Advantages of conducting in-situ U-Pb age dating of multiple U-bearing minerals from a single complex: Case in point - the Oka Carbonatite Complex

NASA Astrophysics Data System (ADS)

Chen, W.; Simonetti, A.

2012-12-01

A detailed radiometric investigation is currently underway focusing on U-bearing accessory minerals apatite, perovskite, and niocalite from the Oka Carbonatite Complex (Canada). One of the main objectives is to obtain a comparative chronology of melt crystallization for the complex. Unlike other commonly adopted U-bearing minerals (e.g., zircon, monazite) for in-situ dating investigations, apatite, perovskite, and niocalite contain relatively high contents of common Pb. Hence, careful assessment of the proportion and composition of the common Pb, and usage of appropriate matrix-matched external standards are imperative. The Madagascar apatite was utilized as the external standard for apatite dating, and the Emerald Lake and Durango apatites were adopted as secondary standards; the latter yield ages of 92.6 ±1.8 and 32.2 ±1.1 Ma, respectively, and these are identical to their accepted ages. Pb/U ages for apatite from Oka were obtained for different rock types, including 8 carbonatites, 4 okaites, 3 ijolites and 3 alnoites, and these define a range of ages between ~105 and ~135 Ma; this result suggests a protracted crystallization history. In total, 266 individual analyses define two peaks at ~115 and ~125Ma. For perovskite dating, the Ice River perovskite standard was utilized as the external standard. The perovskites from one okaite sample yield an age of 112.2 ±1.9 Ma, and is much younger than the previously reported U-Pb perovskite age of 131 ±7 Ma. Hence, the combined U-Pb perovskite ages also suggest a rather prolonged time of melt crystallization. Niocalite is a rare, accessory silicate mineral that occurs within the carbonatites at Oka. The international zircon standard BR266 was selected for use as the external standard and rastering was employed to minimize the Pb-U fractionation. Two niocalite samples give young ages at 110.6 ±1.2 and 115.0 ±1.9 Ma, and are identical to their respective apatite ages (given associated uncertainties) from the same sample. The niocalite for a carbonatite sample Oka153 defines a bimodal age distribution, with weighted average 206Pb/238U ages of 110.1 ±5.0 and 133.2 ±6.1 Ma. Apatite from the same sample also records a similar bimodal age distribution of 111.4 ±2.8 and 126.9 ±1.8 Ma. The combined in situ U-Pb dating results for apatite, pervoskite, niocalite from Oka clearly support a protracted history of magmatic activity (~30 Myr) for this carbonatite complex. Of importance, the U-Pb results from this study clearly indicate the significance of conducting a thorough geochronological investigation rather than defining the age of any one alkaline complex solely on the basis of a single radiometric age determination.

Thermomagmatic evolution of Mesoproterozoic crust in the Blue Ridge of SW Virginia and NW North Carolina: Evidence from U-Pb geochronology and zircon geothermometry

USGS Publications Warehouse

Tollo, Richard P.; Aleinikoff, John N.; Wooden, Joseph L.; Mazdab, Frank K.; Southworth, Scott; Fanning, Mark C.

2010-01-01

New geologic mapping, petrology, and U-Pb geochronology indicate that Mesoproterozoic crust near Mount Rogers consists of felsic to mafic meta-igneous rocks emplaced over 260 m.y. The oldest rocks are compositionally diverse and migmatitic, whereas younger granitoids are porphyritic to porphyroclastic. Cathodoluminescence imaging indicates that zircon from four representative units preserves textural evidence of multiple episodes of growth, including domains of igneous, metamorphic, and inherited origin. Sensitive high-resolution ion microprobe (SHRIMP) trace-element analyses indicate that metamorphic zircon is characterized by lower Th/U, higher Yb/Gd, and lower overall rare earth element (REE) concentrations than igneous zircon. SHRIMP U-Pb isotopic analyses of zircon define three episodes of magmatism: 1327 ± 7 Ma, 1180–1155 Ma, and 1061 ± 5 Ma. Crustal recycling is recorded by inherited igneous cores of 1.33–1.29 Ga age in 1161 ± 7 Ma meta-monzogranite. Overlapping ages of igneous and metamorphic crystallization indicate that plutons of ca. 1170 and 1060 Ma age were emplaced during episodes of regional heating. Local development of hornblende + plagioclase + quartz ± clinopyroxene indicates that prograde metamorphism at 1170–1145 Ma and 1060–1020 Ma reached upper-amphibolite-facies conditions, with temperatures estimated using Ti-in-zircon geothermometry at ~740 ± 40 °C during both episodes. The chemical composition of 1327 ± 7 Ma orthogranofels from migmatite preserves the first evidence of arc-generated rocks in the Blue Ridge, indicating a subduction-related environment that may have been comparable to similar-age systems in inliers of the Northern Appalachians and the Composite Arc belt of Canada. Granitic magmatism at 1180–1155 Ma and ca. 1060 Ma near Mount Rogers was contemporaneous with anorthosite-mangerite-charnockite-granite (AMCG) plutonism in the Northern Appalachian inliers and Canadian Grenville Province. Metamorphism at ca. 1160 and 1060 Ma correlates temporally with the Shawinigan orogeny and Ottawan phase of the Grenvillian orogeny, respectively, suggesting that the Blue Ridge was part of Rodinia dating back to ca. 1180 Ma.

A combined geochronological approach to investigating long lived granite magmatism, the Shap granite, UK

NASA Astrophysics Data System (ADS)

Miles, A. J.; Woodcock, N. H.

2018-04-01

With the advent of more precise dating methods, it has become apparent that zircon dates from granite plutons frequently indicate older emplacement ages than other dating methods. Here we attempt to reconcile a number of dating methods from the c. 5 km2 Caledonian Shap granite, Northern England. The results reveal a more complex and protracted evolution than indicated by application of any single dating method. Zircon U-Pb dates give a weighted mean age of 415.6 ± 1.4 (2σ) Ma. A mafic enclave, dated at 412 ± 2 (2σ) Ma (revised Rb-Sr feldspar age from Davidson et al., 2005), contains resorbed K-feldspar and zircon crystals scavenged from the host crystal mush. These ages are at odds with field relations in the thermal aureole that suggest final emplacement at approximately 404 Ma or later during Acadian deformation. Previously reported Re-Os ages on molybdenites associated with magmatic fluids, have given ages of 405.2 ± 1.8 (2σ) Ma (Selby et al., 2008) and confirm the overlap of at least some magmatic activity with Acadian deformation. A similar emplacement age is supported by Rb-Sr whole-rock-mineral and biotite K-Ar dates when adjusted for revised decay constants (402 ± 3 Ma and 401 ± 7 Ma, respectively, Wadge et al., 1978). The lower closure temperatures of these systems relative to the U-Pb system in zircon means that they are more likely to record the timing of final granite emplacement. These data suggest that most zircons grew before final granite emplacement, by about 10 Ma on average. We suggest that the majority of zircon crystals record pre-emplacement magmatic activity within a deeper part of the system. Mafic enclaves and their scavenged cargo of crystals record the assembly of a mid-crustal batholith where crystals remained at least locally mobile at 412 Ma. Gravity data support the existence of an extensive, 1500 km2 intrusive body, originally at about 15 km depth beneath Shap. This batholith is likely to have remained below the granite solidus for much of its existence due to conductive heat loss, but episodic influxes of silicic magma between c. 412 and 405 Ma are thought to have enabled periods of rejuvenation. These influxes are recorded by complex compositional zoning patterns within K-feldspar megacrysts. The Shap granite itself is likely to represent a rejuvenated crystal slurry, emplaced as a cylindrical cupola above the main magma body during Acadian transpression. This study highlights the importance of integrating different dating techniques and that final emplacement of granites can only be indicated by the youngest zircon ages.

Accessory Mineral Records of Early Earth Crust-Mantle Systematics: an Example From West Greenland

NASA Astrophysics Data System (ADS)

Storey, C. D.; Hawkesworth, C. J.

2008-12-01

Conditions for the formation and the nature of Earth's early crust are enigmatic due to poor preservation. Before c.4 Ga the only archives are detrital minerals eroded from earlier crust, such as the Jack Hills zircons in western Australia, or extinct isotope systematics. Zircons are particularly powerful since they retain precise records of their ages of crystallisation, and the Lu-Hf radiogenic isotope and O stable isotope systematics of the reservoir from which they crystallised. In principle, this allows insight into the nature of the crust, the mantle reservoir from which the melt was extracted and any reworked material incorporated into that melt. We have used in situ methods to measure U-Pb, O and Lu-Hf within single zircon crystals from tonalitic gneisses from West Greenland in the vicinity of the Isua Supracrustal Belt. They have little disturbed ages of c.3.8 Ga, mantle-like O isotope signatures and Lu-Hf isotope signatures that lie on the CHUR evolution line at 3.8 Ga. These samples have previously been subjected to Pb isotope feldspar and 142Nd whole rock analysis and have helped constrain models in which early differentiation of a proto-crust must have occurred. The CHUR-like Lu-Hf signature, along with mantle-like O signature from these zircons suggests juvenile melt production at 3.8 Ga from undifferentiated mantle, yet the other isotope systems preclude this possibility. Alternatively, this is further strong evidence for a heterogeneous mantle in the early Earth. Whilst zircons afford insight into the nature of the early crust and mantle, it is through the Sm-Nd system that the mantle has traditionally been viewed. Titanite often contains several thousand ppm Nd, making it amenable to precise analysis, and is a common accessory phase. It has a reasonably high closure temperature for Pb and O, and it can retain cores with older ages and distinct REE chemistry. It is often the main accessory phase alongside zircon, and it is the main carrier of Nd within the whole rock such that Nd isotope analysis of titanite may be able to see-through later alteration that may have partially reset the whole rock system. We present new in-situ U-Pb, O and Sm-Nd and high-precision U-Pb ID-TIMS and Sm-Nd MC- ICPMS data from individual or fragmented titanite grains. We discuss how these data complement the zircon data and may help to resolve long-standing debates in ancient gneiss terranes, with utility to the nature and formation of crust on the early Earth.

Looking through the Zircon Kaleidoscope: Durations, Rates, and Fluxes in Silicic Magmatic System

NASA Astrophysics Data System (ADS)

Schaltegger, U.; Wotzlaw, J. F.

2014-12-01

The crystallization rate of zircon in a cooling magma depends on the cooling rate through the saturation interval in addition to compositional and kinetic factors. Repeated influx of hot magma over 10-20 ka leads to short-amplitude temperature oscillations, which are recorded by resorption/crystallization cycles of zircon. Plotting the number of dated zircons versus their high-precision U-Pb date results in curves that qualitatively relate to the evolution of magma temperature over time [1], [2]. The trace elemental, O and Hf isotopic composition of zircon gives indications about the degree of magma homogenization and thermal evolution. Zircons from systems with small volumes and magma fluxes record non-systematic chemical and Hf isotopic heterogeneity, suggesting crystallization in non-homogenized ephemeral magma batches. Such systems typically lead to small, mid-upper crustal plutons [3]. Zircons from large-volume crystal-poor rhyolites record initial heterogeneities and rapid amalgamation of smaller magma batches over 10 ka [4], while zircons from monotonous intermediates record magma evolution over several 100 ka with coherent fractionation trends suggesting homogenization and a coherent thermal evolution [2]. In both cases, volumes and flux rates were sufficient to produce large volumes of eruptible magma on very contrasting time scales. Zircon is therefore recording cyclic crystallization-rejuvenation processes during temperature fluctuations in intermediate to upper crustal magma reservoirs but may not relate to the physical pluton emplacement or eruption. We can quantify volumes, rates of magma influx, rates of cooling and crystallization, and the degree of convective homogenization from zircon data, and infer reservoir assembly and eruption trigger mechanisms. These parameters largely control the evolution of long-lived, low-flux silicic magmatic system typical for mid-to-upper crustal plutons, monotonous intermediates are characterized by intermediate durations and fluxes while short-lived, high-flux systems preferentially produce crystal-poor rhyolites. References: [1] Caricchi et al. (2014) Nature 511, 457-461; [2] Wotzlaw et al. (2013) Geology 41, 867-870; [3] Broderick (2013) PhD thesis, Univ. of Geneva; [3] Wotzlaw et al. (2014) Geology, doi:10.1130/G35979.1

Devonian to Carboniferous collision in the Greenland Caledonides: U-Pb zircon and Sm-Nd ages of high-pressure and ultrahigh-pressure metamorphism

NASA Astrophysics Data System (ADS)

Gilotti, Jane A.; Nutman, Allen P.; Brueckner, Hannes K.

2004-10-01

A variety of eclogites from an east-west transect across the North-East Greenland eclogite province have been studied to establish the timing of high pressure (HP) and ultrahigh-pressure (UHP) metamorphism in this northern segment of the Laurentian margin. Garnet + omphacite ± amphibole + whole rock Sm-Nd isochrons from a quartz eclogite, a garnet + omphacite + rutile eclogite and a partially melted zoisite eclogite in the western HP belt are 401±2, 402±9 and 414±18 Ma, respectively. Corresponding sensitive high-resolution ion microprobe (SHRIMP) 206Pb/238U ages of metamorphic zircon in the same samples are 401±7, 414±13, and 393 ±10 Ma. Metamorphic zircon domains were identified using morphology, cathodoluminescence (CL) imaging, U, Th, Th/U and trace element contents. Zircon from the quartz eclogite and the garnet + omphacite + rutile eclogite are typical of eclogite facies zircon with rounded to subhedral shapes, patchy to homogenous CL domains, low U, and very low Th and Th/U. The partially melted eclogite contains euhedral zircons with dark, sector-zoned, higher U, Th and Th/U inherited cores. Three cores give a Paleoproterozoic 207Pb/206Pb age of 1,962±27 Ma, interpreted as the age of the leucogabbroic protolith. CL images of the bright overgrowths show faint oscillatory zoning next to homogenous areas that indicate zircon growth in the presence of a HP melt and later recrystallization. Additional evidence that zircon grew during eclogite facies conditions is the lack of a Eu anomaly in the trace element data for all the samples. These results, combined with additional less precise Sm-Nd ages and our earlier work, point to a Devonian age of HP metamorphism in the western and central portions of the eclogite province. An UHP kyanite eclogite from the eastern part of the transect contains equant metamorphic zircon with homogeneous to patchy zoning in CL and HP inclusions of garnet, omphacite and kyanite. These zircons have slightly higher U, Th and Th/U values than the HP ones, no Eu anomaly, and are thus comparable to UHP zircons in the literature. The 206Pb/238U age of these zircons is 360±5 Ma, much younger than the HP eclogites. The same sample gives a Sm-Nd age of 342±6 Ma. Unlike the HP eclogites, the Sm-Nd age of the UHP rock is ca. 20 Ma younger than the U-Pb zircon age and most likely records slow cooling through the closure temperature, since peak temperatures were in excess of 900°C. Widespread HP metamorphism of both the Laurentian and Baltica continental margins marks the culmination of this continent continent collision in the Devonian. Carboniferous UHP conditions, though localized in the east, suggest a prolonged collisional history rather than a short-lived Scandian orogeny. The traditional Silurian Scandian orogeny should thus be extended through the Devonian.

Geology and U-Pb Zircon ages of the Kavacik Leucogranite in the Bornova Flysch Zone (Western Anatolia, Turkey)

NASA Astrophysics Data System (ADS)

Güngör, Talip; Hasözbek, Altuǧ; Akal, Cüneyt; Mertz-Kraus, Regina; Peştemalci Üregel, Reyhan

2016-04-01

The Bornova Flysch Zone comprises an olistostrome-melange situated NE-SW direction between the Izmir Ankara Suture Zone and the Menderes Massif. The Bornova Flysch Zone is mainly composed of slightly deformed Late Cretaceous to Paleocene sandstone and shale with Mesozoic limestone and oceanic crustal associations. These large-scale blocks in the matrix of the Bornova Flysch Zone are mostly defined as limestone, basalt, serpentinite and radiolarian cherts. In this study, granitic bodies, situated in the Bornova Flysch Zone, named as Kavacik leucogranite is examined for the first time, in terms its geological features and its U-Pb zircon crystallization ages. Kavacik leucogranite displays a typical granitic texture and its composition indicates ranging between granitic to granodioritic in composition with lack of mafic minerals. The geochemical features of the granite indicate the I-type and subalkaline nature of the granitic body. The geochemical signatures of the Kavacik granite points out Volcanic Arc Granitoids as similarly seen in Karaburun granite. U-Pb zircon LA ages were also obtained from the Kavacik granite ranging between 224.5 ± 2.0 Ma and 230.0 ± 2.8 Ma. Early Triassic zircon ages are also previously observed in the Karaburun Peninsula (Karaburun Granite) and the Menderes Massif (Odemis-Kiraz Submassif). The initial geological boundary relation of the Kavacik Leucogranite is not clear in the field and likely displays tectonic boundary features in the matrix of the Bornova Flysch Zone. Overall, the geochemical features of the Kavacik leucogranite and similar leucomagmatic bodies in the Western Anatolia points out the subduction-related tectonic setting is favorable during the Triassic time.

In-Situ U-Pb Dating of Apatite by Hiroshima-SHRIMP: Contributions to Earth and Planetary Science.

PubMed

Terada, Kentaro; Sano, Yuji

2012-01-01

The Sensitive High Resolution Ion MicroProbe (SHRIMP) is the first ion microprobe dedicated to geological isotopic analyses, especially in-situ analyses related to the geochronology of zircon. Such a sophisticated ion probe, which can attain a high sensitivity at a high mass resolution, based on a double focusing high mass-resolution spectrometer, designed by Matsuda (1974), was constructed at the Australian National University. In 1996, such an instrument was installed at Hiroshima University and was the first SHRIMP to be installed in Japan. Since its installation, our focus has been on the in-situ U-Pb dating of the mineral apatite, as well as zircon, which is a more common U-bearing mineral. This provides the possibility for extending the use of in-situ U-Pb dating from determining the age of formation of volcanic, granitic, sedimentary and metamorphic minerals to the direct determination of the diagenetic age of fossils and/or the crystallization age of various meteorites, which can provide new insights into the thermal history on the Earth and/or the Solar System. In this paper, we review the methodology associated with in-situ apatite dating and our contribution to Earth and Planetary Science over the past 16 years.

In situ location and U-Pb dating of small zircon grains in igneous rocks using laser ablation-inductively coupled plasma-quadrupole mass spectrometry

NASA Astrophysics Data System (ADS)

Sack, Patrick J.; Berry, Ron F.; Meffre, Sebastien; Falloon, Trevor J.; Gemmell, J. Bruce; Friedman, Richard M.

2011-05-01

A new U-Pb zircon dating protocol for small (10-50 μm) zircons has been developed using an automated searching method to locate zircon grains in a polished rock mount. The scanning electron microscope-energy-dispersive X ray spectrum-based automated searching method can routinely find in situ zircon grains larger than 5 μm across. A selection of these grains was ablated using a 10 μm laser spot and analyzed in an inductively coupled plasma-quadrupole mass spectrometer (ICP-QMS). The technique has lower precision (˜6% uncertainty at 95% confidence on individual spot analyses) than typical laser ablation ICP-MS (˜2%), secondary ion mass spectrometry (<1%), and isotope dilution-thermal ionization mass spectrometry (˜0.4%) methods. However, it is accurate and has been used successfully on fine-grained lithologies, including mafic rocks from island arcs, ocean basins, and ophiolites, which have traditionally been considered devoid of dateable zircons. This technique is particularly well suited for medium- to fine-grained mafic volcanic rocks where zircon separation is challenging and can also be used to date rocks where only small amounts of sample are available (clasts, xenoliths, dredge rocks). The most significant problem with dating small in situ zircon grains is Pb loss. In our study, many of the small zircons analyzed have high U contents, and the isotopic compositions of these grains are consistent with Pb loss resulting from internal α radiation damage. This problem is not significant in very young rocks and can be minimized in older rocks by avoiding high-U zircon grains.

SHRIMP-RG U-Pb isotopic systematics of zircon from the Angel Lake orthogneiss, East Humboldt Range, Nevada: Is this really archean crust?

USGS Publications Warehouse

Premo, Wayne R.; Castineiras, Pedro; Wooden, Joseph L.

2008-01-01

New SHRIMP-RG (sensitive high-resolution ion microprobe-reverse geometry) data confirm the existence of Archean components within zircon grains of a sample from the orthogneiss of Angel Lake, Nevada, United States, previously interpreted as a nappe of Archean crust. However, the combined evidence strongly suggests that this orthogneiss is a highly deformed, Late Cretaceous monzogranite derived from melting of a sedimentary source dominated by Archean detritus. Zircon grains from the same sample used previously for isotope dilution-thermal ionization mass spectrometry (ID-TIMS) isotopic work were analyzed using the SHRIMP-RG to better define the age and origin of the orthogneiss. Prior to analysis, imaging revealed a morphological variability and intragrain, polyphase nature of the zircon population. The SHRIMP-RG yielded 207Pb/206Pb ages between ca. 2430 and 2580 Ma (a best-fit mean 207Pb/206Pb age of 2531 ± 19 Ma; 95% confidence) from mostly rounded to subrounded zircons and zircon components (cores). In addition, several analyses from rounded to subrounded cores or grains yielded discordant 207Pb/206Pb ages between ca. 1460 and ca. 2170 Ma, consistent with known regional magmatic events. All cores of Proterozoic to latest Archean age were encased within clear, typically low Th/U (206Pb/238U ages between 72 and 91 Ma, consistent with magmatic ages from Lamoille Canyon to the south. An age of ca. 90 Ma is suggested, the younger 206Pb/238U ages resulting from Pb loss. The Cretaceous and Precambrian zircon components also have distinct trace element characteristics, indicating that these age groups are not related to the same igneous source. These results support recent geophysical interpretations and negate the contention that the Archean-Proterozoic boundary extends into the central Great Basin area. They further suggest that the world-class gold deposits along the Carlin Trend are not underlain by Archean cratonal crust, but rather by the Proterozoic Mojave province and Neoproterozoic and Paleozoic metasedimentary sequences dominated by detritus derived from Late Archean sources rather than Proterozoic sources, as is evident farther to the south in the Ruby Mountains.

A Modern Analog to the Depositional Age Problem: Zircon and Apatite Fission Track and U-Pb Age Distributions by LA-ICP-MS

NASA Astrophysics Data System (ADS)

Donelick, H. M.; Donelick, M. B.; Donelick, R. A.

2012-12-01

Sand from three river systems in North Idaho (Snake River near Lewiston, Clearwater River near Lewiston and the Salmon River near White Bird) and two regional ash fall events (Mt. Mazama and Mt. St. Helens) were collected for zircon U-Pb detrital age analysis. Up to 120 grains of zircon per sample were ablated using a Resonetics M-50 193 nm ArF Excimer laser ablation (LA) system and the Pb, Th, and U isotopic signals were quantified using an Agilent 7700x quadrupole inductively coupled plasma-mass spectrometer (ICP-MS). Isotopic signals for major, minor, and trace elements, including all REEs, were also monitored. The youngest zircon U-Pb ages from the river samples were approximately 44 Ma; Cenozoic Idaho Batholith and Precambrian Belt Supergroup ages were well represented. Significant common Pb contamination of the Clearwater River sample (e.g., placer native Cu was observed in the sample) precluded detailed analysis of the zircon U-Pb ages but no interpretable ages <44 Ma were observed. Interestingly, not one of the river samples yielded zircon U-Pb ages near 0 Ma, despite all three catchment areas having received significant ash from Mt. St. Helens in 1980, and Mount Mazama 7,700 years ago, and no doubt other events during the Quaternary. Work currently in progress seeks to address bias against near 0 Ma ages in the catchment areas due to: a) small, local ash fall grain sizes and b) overwhelming number of older grains relative to the ash fall grains. Data from Mt. St. Helens ash from several localities near the mountain (Toutle River and Maple Flats, WA) and several far from the mountain (Spokane, WA; Princeton, ID; Kalispell, MT) and Mt. Mazama ash fall deposits near Lewiston, ID and Spokane, WA will be presented to address these possibilities. Additionally, fission track and U-Pb ages from apatites collected from these river and ash fall samples will also be shown to help constrain the problem.

Elemental and Isotopic Tomography at Single-Atom-Scale in 4.0 and 2.4 Ga Zircons

NASA Astrophysics Data System (ADS)

Valley, J. W.; Reinhard, D. A.; Snoeyenbos, D.; Lawrence, D.; Martin, I.; Kelly, T. F.; Ushikubo, T.; Strickland, A.; Cavosie, A. J.

2012-12-01

Atom probe tomography can determine identity (mass/charge ratio) and 3-D position of individual atoms in minerals such as zircon. These data provide unique information for understanding the thermal history and mechanisms of mineral reaction and exchange, including radiation damage. Nine needle-shaped specimens ~100 nm in diameter (at the apex) were sampled from 2 zircons by FIB and analyzed with a local-electrode atom probe (LEAP), CAMECA LEAP 4000X HR. The LEAP uses pulsed-laser heating to field evaporate the tip of a zircon needle and accelerates the ions into a position-sensitive TOF-MS. With due care for complex isobaric interferences (molecules, multiple ionizations) and background correction, it is possible to individually identify up to 10E8 atoms/needle (36% detection efficiency) by mass/charge (MRP ~ 1000@ m/n=16Da) and position (X-Y-Z coordinates on 0.2 nm scale) (Kelly & Larson 2012). The 3-D distribution of Pb and Y differ at atom-scale in the 2 zircons. Zircon #1 (4007 Ma, Jack Hills, W. Australia, Cavosie 2005, Ushikubo et al. 2008, Bouvier et al. 2011) is homogeneous in Pb and Y. In contrast, incompatible elements, including Pb and Y, are concentrated in equant 5-10 nm dia. domains, spaced ~50 nm apart in zircon #2 (2438 Ma, Albion-Raft R-Grouse Ck core complex, Utah, Strickland et al. 2011). U is homogeneously distributed in both zircons. The analyzed domains suffered 4-8 x 10E15 α-decay events/mg due to U and Th decay and yet both zircons yield >97% concordant U-Pb ages by SIMS, suggesting annealing of radiation damage during the life of the zircons. The 207-Pb/206-Pb ratios for these nm-scale domains, as measured by LEAP, average 0.17 for the 2.4 Ga Zrc2 (3 needles) and 0.43 for the 4.0 Ga Zrc1 (5 needles). These ratios are less precise (±40% 2σ) due to ultra-small sample size, but are in excellent agreement with values measured by SIMS, 0.1684 and 0.4269, respectively. Thus Pb in both zircons is radiogenic. The Pb-Y-rich domains and lack of correlation with U in Zrc2 is best explained by diffusion of Pb and other incompatible elements (Y, REEs) into 5-10 nm domains that were damaged by α-recoil and may have been metamict as the result of single U- or Th-decay chains. Diffusion distances of ~20 nm for these elements in crystalline zircon require temperatures above 700 C for ~10 myrs (Cherniak 2010). This is consistent with the known history of Zrc2, as a xenocrystic 2.4 Ga zircon in a 29 Ma granodiorite phase of a long-lived, extension-related plutonic suite that experienced high-grade regional metamorphism. In contrast, the absence of enriched domains in Zrc1 suggests that this zircon did not experience similar high-grade metamorphism before or after its deposition within the ~3 Ga Jack Hills metaconglomerate.

Petrology, geochemistry and U-Pb geochronology of magmatic rocks from the high-sulfidation epithermal Au-Cu Chelopech deposit, Srednogorie zone, Bulgaria

NASA Astrophysics Data System (ADS)

Chambefort, Isabelle; Moritz, Robert; von Quadt, Albrecht

2007-10-01

The Chelopech deposit is one of the largest European gold deposits and is located 60 km east of Sofia, within the northern part of the Panagyurishte mineral district. It lies within the Banat-Srednegorie metallogenic belt, which extends from Romania through Serbia to Bulgaria. The magmatic rocks define a typical calc-alkaline suite. The magmatic rocks surrounding the Chelopech deposit have been affected by propylitic, quartz-sericite, and advanced argillic alteration, but the igneous textures have been preserved. Alteration processes have resulted in leaching of Na2O, CaO, P2O5, and Sr and enrichment in K2O and Rb. Trace element variation diagrams are typical of subduction-related volcanism, with negative anomalies in high field strength elements (HFSE) and light element, lithophile elements. HFSE and rare earth elements were relatively immobile during the hydrothermal alteration related to ore formation. Based on immobile element classification diagrams, the magmatic rocks are andesitic to dacitic in compositions. Single zircon grains, from three different magmatic rocks spanning the time of the Chelopech magmatism, were dated by high-precision U-Pb geochronology. Zircons of an altered andesitic body, which has been thrust over the deposit, yield a concordant 206Pb/238U age of 92.21 ± 0.21 Ma. This age is interpreted as the crystallization age and the maximum age for magmatism at Chelopech. Zircon analyses of a dacitic dome-like body, which crops out to the north of the Chelopech deposit, give a mean 206Pb/238U age of 91.95 ± 0.28 Ma. Zircons of the andesitic hypabyssal body hosting the high-sulfidation mineralization and overprinted by hydrothermal alteration give a concordant 206Pb/238U age of 91.45 ± 0.15 Ma. This age is interpreted as the intrusion age of the andesite and as the maximum age of the Chelopech epithermal high-sulfidation deposit. 176Hf/177Hf isotope ratios of zircons from the Chelopech magmatic rocks, together with published data on the Chelopech area and the about 92-Ma-old Elatsite porphyry-Cu deposit, suggest two different magma sources in the Chelopech-Elatsite magmatic area. Magmatic rocks associated with the Elatsite porphyry-Cu deposit and the dacitic dome-like body north of Chelopech are characterized by zircons with ɛHfT90 values of ˜5, which suggest an important input of mantle-derived magma. Some zircons display lower ɛHfT90 values, as low as -6, and correlate with increasing 206Pb/238U ages up to about 350 Ma, suggesting assimilation of basement rocks during magmatism. In contrast, zircon grains in andesitic rocks from Chelopech are characterized by homogeneous 176Hf/177Hf isotope ratios with ɛHfT90 values of ˜1 and suggest a homogeneous mixed crust-mantle magma source. We conclude that the Elatsite porphyry-Cu and the Chelopech high-sulfidation epithermal deposits were formed within a very short time span and could be partly contemporaneous. However, they are related to two distinct upper crustal magmatic reservoirs, and they cannot be considered as a genetically paired porphyry-Cu and high-sulfidation epithermal related to a single magmatic-hydrothermal system centered on the same intrusion.

Middle Neoproterozoic (ca. 705-716 Ma) arc to rift transitional magmatism in the northern margin of the Yangtze Block: Constraints from geochemistry, zircon U-Pb geochronology and Hf isotopes

NASA Astrophysics Data System (ADS)

Wang, Ruirui; Xu, Zhiqin; Santosh, M.; Xu, Xianbing; Deng, Qi; Fu, Xuehai

2017-09-01

The South Qinling Belt in Central China is an important window to investigate the Neoproterozoic tectono-magmatic processes along the northern margin of the Yangtze Block. Here we present whole-rock geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes of a suite of Middle Neoproterozoic intrusion from the Wudang Uplift in South Qinling. Zircon LA-ICP-MS U-Pb ages reveal that these rocks were formed at ca. 705-716 Ma. Geochemical features indicate that the felsic magmatic rocks are I-type granitoids, belong to calcic- to calc-alkaline series, and display marked negative Nb, Ta and Ti anomalies. Moreover, the enrichment of light rare earth elements (LREEs) and large ion lithophile elements (LILEs), combined with depletion of heavy rare earth elements (HREEs) support that these rocks have affinity to typical arc magmatic rocks formed in Andean-type active continental margins. The REE patterns are highly to moderately fractionated, with (La/Yb)N = 5.13-8.10 in meta-granites, and 2.32-2.35 in granodiorite. The granitoids have a wide range of zircon εHf(t) values (-29.91 to 14.76) and zircon Hf two-stage model ages (696-3482 Ma). We suggest that the ca. 705-716 Ma granitoids were sourced from different degrees of magma mixing between partial melting of the overlying mantle wedge triggered by hydrous fluids released from subducted materials and crustal melting. The hybrid magmas were emplaced in the shallow crust accompanied by assimilation and fractional crystallization (AFC). Both isotopic and geochemical data suggest that the ca. 705-716 Ma felsic magmatic rocks were formed along a continental arc. These rocks as well as the contemporary A-type granite may mark a transitional tectonic regime from continental arc to rifting, probably related to slab rollback during the oceanic subduction beneath the northern margin of Yangtze Block.

Micrometer-scale U-Pb age domains in eucrite zircons, impact re-setting, and the thermal history of the HED parent body

NASA Astrophysics Data System (ADS)

Hopkins, M. D.; Mojzsis, S. J.; Bottke, W. F.; Abramov, O.

2015-01-01

Meteoritic zircons are rare, but some are documented to occur in asteroidal meteorites, including those of the howardite-eucrite-diogenite (HED) achondrite clan (Rubin, A. [1997]. Meteorit. Planet. Sci. 32, 231-247). The HEDs are widely considered to originate from the Asteroid 4 Vesta. Vesta and the other large main belt asteroids record an early bombardment history. To explore this record, we describe sub-micrometer distributions of trace elements (U, Th) and 235,238U-207,206Pb ages from four zircons (>7-40 μm ∅) separated from bulk samples of the brecciated eucrite Millbillillie. Ultra-high resolution (∼100 nm) ion microprobe depth profiles reveal different zircon age domains correlative to mineral chemistry and to possible impact scenarios. Our new U-Pb zircon geochronology shows that Vesta's crust solidified within a few million years of Solar System formation (4561 ± 13 Ma), in good agreement with previous work (e.g. Carlson, R.W., Lugmair, G.W. [2000]. Timescales of planetesimal formation and differentiation based on extinct and extant radioisotopes. In: Canup, R., Righter, K. (Eds.), Origin of the Earth and Moon. University of Arizona Press, Tucson, pp. 25-44). Younger zircon age domains (ca. 4530 Ma) also record crustal processes, but these are interpreted to be exogenous because they are well after the effective extinction of 26Al (t1/2 = 0.72 Myr). An origin via impact-resetting was evaluated with a suite of analytical impact models. Output shows that if a single impactor was responsible for the ca. 4530 Ma zircon ages, it had to have been ⩾10 km in diameter and at high enough velocity (>5 km s-1) to account for the thermal field required to re-set U-Pb ages. Such an impact would have penetrated at least 10 km into Vesta's crust. Later events at ca. 4200 Ma are documented in HED apatite 235,238U-207,206Pb ages (Zhou, Q. et al. [2011]. Early basaltic volcanism and Late Heavy Bombardment on Vesta: U-Pb ages of small zircons and phosphates in eucrites. Lunar Planet. Sci. 42. Abstract #2575) and 40-39Ar age spectra (Bogard, D.D. [2011]. Chem. Erde 71, 207-226). Yet younger ages, including those coincident with the Late Heavy Bombardment (LHB; ca. 3900 Ma), are absent from Millbillillie zircon. This is attributable to primordial changes to the velocity distributions of impactors in the asteroid belt, and differences in mineral closure temperatures (Tc zircon ≫ apatite).

Permian-Carboniferous arc magmatism in southern Mexico: U-Pb dating, trace element and Hf isotopic evidence on zircons of earliest subduction beneath the western margin of Gondwana

NASA Astrophysics Data System (ADS)

Ortega-Obregón, C.; Solari, L.; Gómez-Tuena, A.; Elías-Herrera, M.; Ortega-Gutiérrez, F.; Macías-Romo, C.

2014-07-01

Undeformed felsic to mafic igneous rocks, dated by U-Pb zircon geochronology between 311 and 255 Ma, intrude different units of the Oaxacan and Acatlán metamorphic complexes in southwestern Mexico. Rare earth element concentrations on zircons from most of these magmatic rocks have a typical igneous character, with fractionated heavy rare earths and negative Eu anomalies. Only inherited Precambrian zircons are depleted in heavy rare earth elements, which suggest contemporaneous crystallization in equilibrium with metamorphic garnet during granulite facies metamorphism. Hf isotopic signatures are, however, different among these magmatic units. For example, zircons from two of these magmatic units (Cuanana pluton and Honduras batholith) have positive ɛHf values (+3.8-+8.5) and depleted mantle model ages (using a mean crustal value of 176Lu/177Hf = 0.015) ( T DMC) ranging between 756 and 1,057 Ma, whereas zircons from the rest of the magmatic units (Etla granite, Zaniza batholith, Carbonera stock and Sosola rhyolite) have negative ɛHf values (-1 to -14) and model ages between 1,330 and 2,160 Ma. This suggests either recycling of different crustal sources or, more likely, different extents of crustal contamination of arc-related mafic magmas in which the Oaxacan Complex acted as the main contaminant. These plutons thus represent the magmatic expression of the initial stages of eastward subduction of the Pacific plate beneath the western margin of Gondwana, and confirm the existence of a Late Carboniferous-Permian magmatic arc that extended from southern North America to Central America.

U-Pb isotopic systematics of shock-loaded and annealed baddeleyite: Implications for crystallization ages of Martian meteorite shergottites

NASA Astrophysics Data System (ADS)

Niihara, Takafumi; Kaiden, Hiroshi; Misawa, Keiji; Sekine, Toshimori; Mikouchi, Takashi

2012-08-01

Shock-recovery and annealing experiments on basalt-baddeleyite mixtures were undertaken to evaluate shock effects on U-Pb isotopic systematics of baddeleyite. Shock pressures up to 57 GPa caused fracturing of constituent phases, mosaicism of olivine, maskelynitization of plagioclase, and melting, but the phase transition from monoclinic baddeleyite structure to high-pressure/temperature polymorphs of ZrO2 was not confirmed. The U-Pb isotopic systems of the shock-loaded baddeleyite did not show a large-scale isotopic disturbance. The samples shock-recovered from 47 GPa were then employed for annealing experiments at 1000 or 1300 °C, indicating that the basalt-baddeleyite mixture was almost totally melted except olivine and baddeleyite. Fine-grained euhedral zircon crystallized from the melt was observed around the relict baddeleyite in the sample annealed at 1300 °C for 1 h. The U-Pb isotopic systems of baddeleyite showed isotopic disturbances: many data points for the samples annealed at 1000 °C plotted above the concordia. Both radiogenic lead loss/uranium gain and radiogenic lead gain/uranium loss were observed in the baddeleyite annealed at 1300 °C. Complete radiogenic lead loss due to shock metamorphism and subsequent annealing was not observed in the shock-loaded/annealed baddeleyites studied here. These results confirm that the U-Pb isotopic systematics of baddeleyite are durable for shock metamorphism. Since shergottites still preserve Fe-Mg and/or Ca zonings in major constituent phases (i.e. pyroxene and olivine), the shock effects observed in Martian baddeleyites seem to be less intense compared to that under the present experimental conditions. An implication is that the U-Pb systems of baddeleyite in shergottites will provide crystallization ages of Martian magmatic rocks.

Paleozoic and Paleoproterozoic Zircon in Igneous Xenoliths Assimilated at Redoubt Volcano, Alaska

NASA Astrophysics Data System (ADS)

Bacon, C. R.; Vazquez, J. A.; Wooden, J. L.

2010-12-01

Historically active Redoubt Volcano is a basalt-to-dacite cone constructed upon the Jurassic-early Tertiary Alaska-Aleutian Range batholith. New SHRIMP-RG U-Pb age and trace-element concentration results for zircons from gabbroic xenoliths and crystal-rich andesitic mush from a late Pleistocene pyroclastic deposit indicate that ~310 Ma and ~1865 Ma igneous rocks underlie Redoubt at depth. Two gabbros have sharply terminated prismatic zircons that yield ages of ~310 Ma. Zircons from a crystal mush sample are overwhelmingly ~1865 Ma and appear rounded due to incomplete dissolution. Binary plots of element concentrations or ratios show clustering of data for ~310-Ma grains and markedly coherent trends for ~1865-Ma grains; e.g., ~310-Ma grains have higher Eu/Eu* than most of the ~1865-Ma grains, the majority of which form a narrow band of decreasing Eu/Eu* with increasing Hf content which suggests that ~1865-Ma zircons come from igneous source rocks. It is very unlikely that detrital zircons from a metasedimentary rock would have this level of homogeneity in age and composition. One gabbro contains abundant ~1865 Ma igneous zircons, ~300-310 Ma fluid-precipitated zircons characterized by very low U and Th concentrations and Th/U ratios, and uncommon ~100 Ma zircons. We propose that (1) ~310 Ma gabbro xenoliths from Redoubt Volcano belong to the same family of plutons dated by Aleinikoff et al. (USGS Circular 1016, 1988) and Gardner et al. (Geology, 1988) located ≥500 km to the northeast in basement rocks of the Wrangellia and Alexander terranes and (2) ~1865 Ma zircons are inherited from igneous rock, potentially from a continental fragment that possibly correlates with the Fort Simpson terrane or Great Bear magmatic zone of the Wopmay Orogen of northwestern Laurentia. Possibly, elements of these Paleoproterozoic terranes intersected the Paleozoic North American continental margin where they may have formed a component of the basement to the Wrangellia-Alexander-Peninsular composite terrane prior to transport to its present location (e.g., Colpron and Nelson, Geological Society, London, Special Publication 318, 2009). Xenocrysts from the ~1865 Ma igneous rocks, and possibly also ~310 Ma gabbros, are contained in relatively low-temperature mush and partially melted gabbro that we interpret to have been derived from the margin of the subvolcanic magma accumulation and storage region defined by seismicity at 4-10 km bsl. The Redoubt crystal mush contains evidence for assimilation of ~1865 Ma igneous rocks that have no equivalent exposed in Alaska. The discovery of Paleoproterozoic grains as the dominant zircon component in crystal mush raises the question of the origin of other crystals in Redoubt magmas.

Zircon U-Pb ages and Hf-O isotopic composition of migmatites from the Zanjan-Takab complex, NW Iran: Constraints on partial melting of metasediments

NASA Astrophysics Data System (ADS)

Moghadam, Hadi Shafaii; Li, Xian-Hua; Stern, Robert J.; Ghorbani, Ghasem; Bakhshizad, Farzaneh

2016-01-01

We study migmatites and other metamorphic rocks in the Zanjan-Takab region of NW Iran and use these results to report the first evidence of Oligocene core complex formation in Iran. Four samples of migmatites associated with paragneisses, including leucosomes and associated para-amphibolite melanosomes were selected for U-Pb dating and Hf-O isotopic analysis. Zircon cores - interpreted as originally detrital zircons - have variable ages that peak at ca. 100-110 Ma, but their sedimentation age - indicated by the youngest 206Pb/238U ages - is ca. 35-40 Ma. New zircons associated with incipient melting occur as overgrowths around zircon cores and/or as newly grown grains. Morphologies and internal structures suggest that rim growth and formation of new zircons were associated with partial melting. All four samples contain zircons with rims that yield 206Pb/238U ages of 28-25 Ma, indicating that partial melting occurred in Late Oligocene time. δ18O values for zircon rims vary between 8.2 and 12.3‰, significantly higher than expected for mantle inputs (δ18O 6‰) and consistent with equilibrium with surface materials. Zircon rims yield εHf(t) between 2.2 and 12.4 and two-stage Hf model ages of 448-562 Ma, indicating that the region is underlain by Cadomian-Caledonian crust. According to the Hf-O isotopic values, the main mechanism forming zircon rims was dissolution of pre-existing detrital zircons with reprecipitation of new zircon shortly thereafter. Oligocene ages indicate that partial melting accompanied core complex formation in the Zanjan-Takab region. Extension, melting, and core complex formation in south-central Iran are Eocene in age, but younger ages of Oligocene-Miocene in NW Iran and Turkey indicate that extension was distributed throughout the region during Cenozoic time.

U-Pb SHRIMP geochronology and trace-element geochemistry of coesite-bearing zircons, North-East Greenland Caledonides

USGS Publications Warehouse

McClelland, W.C.; Power, S.E.; Gilotti, J.A.; Mazdab, F.K.; Wopenka, B.

2006-01-01

Obtaining reliable estimates for the timing of eclogite-facies metamorphism is critical to establishing models for the formation and exhumation of high-pressure and ultrahigh-pressure (UHP) metamorphic terranes in collisional orogens. The presence of pressure-dependent phases, such as coesite, included in metamorphic zircon is generally regarded as evidence that zircon growth occurred at UHP conditions and, ifdated, should provide the necessary timing information. We report U-Pb sensitive high-resolution ion microprobe (SHRIMP) ages and trace-element SHRIMP data from coesite-bearing zircon suites formed during UHP metamorphism in the North- East Greenland Caledonides. Kyanite eclogite and quartzofeldspathic host gneiss samples from an island in J??kelbugt (78??00'N, 18??04'W) contained subspherical zircons with well-defined domains in cathodoluminescence (CL) images. The presence of coesite is confirmed by Raman spectroscopy in six zircons from four samples. Additional components of the eclogite-facies inclusion suite include kyanite, omphacite, garnet, and rutile. The trace-element signatures in core domains reflect modification of igneous protolith zircon. Rim signatures show flat heavy rare earth element (HREE) patterns that are characteristic of eclogite-facies zircon. The kyanite eclogites generally lack a Eu anomaly, whereas a negative Eu anomaly persists in all domains of the host gneiss. The 207Pb- corrected 206Pb/238U ages range from 330 to 390 Ma for the host gneiss and 330-370 Ma for the kyanite eclogite. Weighted mean 206Pb/238U ages for coesite-bearing domains vary from 364 ?? 8 Ma for the host gneiss to 350 ?? 4 Ma for kyanite eclogite. The combined U-Pb and REE data interpreted in conjunction with observed CL domains and inclusion suites suggest that (1) Caledonian metamorphic zircon formed by both new zircon growth and recrystallization, (2) UHP metamorphism occurred near the end of the Caledonian collision, and (3) the 30-50m.y. span of ages records long residence times at eclogite-facies conditions for the UHProcks of North-East Greenland. This spread in observed ages is interpreted to be characteristic of metamorphic rocks that have experienced relatively long (longer than 10 m.y.) residence times at UHP conditions. ?? 2006 Geological Society of America.

TEMORA 1: A new zircon standard for Phanerozoic U-Pb geochronology

USGS Publications Warehouse

Black, L.P.; Kamo, S.L.; Allen, C.M.; Aleinikoff, J.N.; Davis, D.W.; Korsch, R.J.; Foudoulis, C.

2003-01-01

The role of the standard is critical to the derivation of reliable U-Pb zircon ages by micro-beam analysis. For maximum reliability, it is critically important that the utilised standard be homogeneous at all scales of analysis. It is equally important that the standard has been precisely and accurately dated by an independent technique. This study reports the emergence of a new zircon standard that meets those criteria, as demonstrated by Sensitive High Resolution Ion MicroProbe (SHRIMP), isotope dilution thermal ionisation mass-spectrometry (IDTIMS) and excimer laser ablation- inductively coupled plasma-mass-spectrometry (ELA-ICP-MS) documentation. The TEMORA 1 zircon standard derives from the Middledale Gabbroic Diorite, a high-level mafic stock within the Palaeozoic Lachlan Orogen of eastern Australia. Its 206Pb/238U IDTIMS age has been determined to be 416.75??0.24 Ma (95% confidence limits), based on measurement errors alone. Spike-calibration uncertainty limits the accuracy to 416.8??1.1 Ma for U-Pb intercomparisons between different laboratories that do not use a common spike. ?? 2003 Published by Elsevier Science B.V. All rights reserved.

Zircon morphology and U-Pb geochronology of seven metaluminous and peralkaline post-orogenic granite complexes of the Arabian Shield, Kingdom of Saudi Arabia

USGS Publications Warehouse

Aleinikoff, John Nicholas; Stoeser, D.B.

1988-01-01

The U-Pb zircon method was used to determine the ages of seven metaluminous-to-peralkaline post-orogenic granites located throughout the Late Proterozoic Arabian Shield of Saudi Arabia. Zircons from the metaluminous rocks are prismatic, with length-to-width ratios of about 2-4:1 and small pyramidal terminations. In contrast, zircons from three of the four peralkaline complexes either lack well developed prismatic faces (are pseudo-octahedral) or are anhedral. Some of the zircons from the peralkaline granites contain inherited radiogenic lead. This complicates interpretation of the isotopic data and. in many cases, may make the U-Pb method unsuitable for determining the age of a peralkaline granite. Zircons in the metaluminous granites do not contain inheritance and thus, best-fit chords calculated through the data have upper concordia intercepts that indicate the age of intrusion, and lower intercepts that indicate simple episodic lead loss. The results show that these granites were emplaced during multiple intrusive episodes from 670 to 510 Ma (Late Proterozoic to Cambrian).

Comment on "Zircon U-Th-Pb dating using LA-ICP-MS: Simultaneous U-Pb and U-Th dating on 0.1 Ma Toya Tephra, Japan" by Hisatoshi Ito

NASA Astrophysics Data System (ADS)

Guillong, M.; Schmitt, A. K.; Bachmann, O.

2015-04-01

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of eight zircon reference materials and synthetic zircon-hafnon end-members indicate that corrections for abundance sensitivity and molecular zirconium sesquioxide ions (Zr2O3+) are critical for reliable determination of 230Th abundances in zircon. Other polyatomic interferences in the mass range 223-233 amu are insignificant. When corrected for abundance sensitivity and interferences, activity ratios of (230Th)/(238U) for the zircon reference materials we used average 1.001 ± 0.010 (1σ error; mean square of weighted deviates MSWD = 1.45; n = 8). This includes the 91500 and Plešovice zircons, which were deemed unsuitable for calibration of (230Th)/(238U) by Ito (2014). Uranium series zircon ages generated by LA-ICP-MS without mitigating (e.g., by high mass resolution) or correcting for abundance sensitivity and molecular interferences on 230Th such as those presented by Ito (2014) are potentially unreliable.

SHRIMP-RG U-Pb ages of provenance and metamorphism from detrital zircon populations and Pb-Sr-Nd signatures of prebatholithic metasedimentary rocks at Searl Ridge, northern Peninsular Ranges batholith, southern California: Implications for their age, origin, and tectonic setting

USGS Publications Warehouse

Premo, Wayne R.; Morton, Douglas M.

2014-01-01

Twenty-four samples were collected from prebatholithic metasedimentary rocks along Searl Ridge, the north rim of the Diamond Valley Reservoir, Domenigoni Valley, centrally located in the northern Peninsular Ranges of southern California. These rocks exhibit progressive metamorphism from west to east across fundamental structural discontinuities now referred to as a “transition zone.” Documented structural and mineralogical changes occur across this metamorphic gradient. Sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) U-Pb ages were obtained from detrital zircons from metasedimentary rocks through the transition zone. To the west, metapelitic and minor metasandstone units yielded numerous concordant 206Pb/238U ages between 210 and 240 Ma, and concordant 207Pb/206Pb ages at 1075–1125 Ma, 1375–1430 Ma, and 1615–1735 Ma, although distinct differences in provenance were noted between units. A few older 207Pb/206Pb ages obtained were ca. 2250 Ma and ca. 2800 Ma. Rocks of the eastern part of the transition zone include high-grade paragneisses that yielded numerous concordant 206Pb/238U ages between 103 and 123 Ma and between 200 and 255 Ma, and concordant 207Pb/206Pb ages at 1060–1150 Ma, 1375–1435 Ma, and 1595–1710 Ma. Some zircon results from these high-grade gneisses are marked by distinct Pb-loss discordia with lower-intercept ages of ca. 215 Ma and Paleoproterozoic upper-intercept ages. Younger ages between 100 and 105 Ma are mainly obtained from rims of some zircon grains that are characterized by low Th/U values (<0.1) and high U contents (>1000 ppm), indicating the likelihood of metamorphic zircon growth at that time. The similarity of zircon age populations between western and eastern units through the transition zone indicates that this fundamental structure probably dissects sediments of the same basin. This supposition is further supported by initial whole-rock Pb-Sr-Nd isotopic data that show similar average initial 206Pb/204Pb (18.65 to 18.9), 87Sr/86Sr (0.713 to 0.718), and εNd (−7 to −12) values for both the western and eastern units—values that also indicate the presence of significantly older crustal material in their provenance.Magmatic zircons from a diorite dike that crosscuts the foliation, but is itself subsequently metamorphosed, yielded a SHRIMP-RG concordia age of 103.3 ± 0.73 Ma, which is within agreement of an isotope dilution–thermal ionization mass spectrometry (ID-TIMS) U-Pb age of 103.37 ± 0.25 Ma. A postmetamorphic, cross-cutting pegmatite yielded discordant U-Pb zircon age data, but euhedral, glassy monazite from the pegmatite yielded a slightly discordant 207Pb/235U age of 101.85 ± 0.35 Ma and a Th-Pb age of 97.53 ± 0.18 Ma, suggesting that this pegmatite was injected during or just after deformation ceased. The age and initial Pb-Sr-Nd signature for the dioritic dike indicate it was produced during the transition zone plutonism elsewhere in the northern Peninsular Ranges batholith, whereas the pegmatitic dike was derived from crustal anatexis.Collectively, these results indicate that this sequence of metasedimentary rocks was derived from mainly a Late Permian to Early Triassic igneous provenance that probably intruded Proterozoic crust. The sequence was subsequently metamorphosed during deformation of the Cretaceous continental margin at ca. 105 to 97 Ma.

New Hf isotope data from the Jack Hills zircons: constraints on the Hadean crustal evolution

NASA Astrophysics Data System (ADS)

Amelin, Y.; Davis, D.; Lee, D.

2004-05-01

Here we present a follow-up of our study of the "older" population of detrital zircons from the Jack Hills metaconglomerate W-74 [1]. We report Lu-Hf data for zircon grains, which have been previously analyzed with a number of techniques: BSE and CL imaging, detailed U-Pb SHRIMP geochronology, trace element concentrations, and oxygen isotopic compositions. After completion of non-destructive SIMS analyses and imaging, the zircons were extracted from the mounts, dissolved and analyzed for U-Pb and Lu-Hf using isotope dilution. Twenty five grains were air abraded before digestion, and eight grains were digested without abrasion. Four grains were cut, and the fragments were analyzed for U-Pb and Lu-Hf separately. The 207Pb/206Pb ages determined by isotope dilution vary between 3788-4186 Ma; the maximum SHRIMP spot 207Pb/206Pb ages of the same grains are between 3871-4276 Ma. The spot 207Pb/206Pb ages averaged over each grain are close to the whole grain isotope dilution values. The U-Pb discordance depends mainly on whether the grains were abraded: the median discordance of 27 abraded grains and fragments is 2.7 (the range is -0.4 to 20.2), whereas the median discordance of 11 unabraded grains and fragments is 66.5 (the range is 20.5 to 83.5). The epsilon176Hf values, calculated using the whole grain TIMS 207Pb/206Pb ages and the 176Lu decay constant of 1.865*10-11, are between -1.4 and -10.6. Using maximum SHRIMP spot 207Pb/206Pb ages and the same decay constant yields the range of epsilon176Hf of 0.1 to -8.6. If the decay constant of 1.983*10-11 is used instead, then the range of epsilon176Hf becomes 4.7 to -5.0 using the whole grain ages, or 6.3 to -3.0 using the maximum SHRIMP spot ages. Grain fragment analyses show internal variations of initial 176Hf/177Hf in three grains out of five. This observation is consistent with multi-episodic zircon growth rather than with ancient Pb loss. In the presentation we shall discuss the prospect of reliable interpretation of the Hf isotope from the Jack Hills detrital zircons and from other complex ancient zircon populations. [1] Amelin et al. 2001, Proceedings of the Archean-2001 symposium, Perth, WA.

The petrology and chronology of NWA 8009 impact melt breccia: Implication for early thermal and impact histories of Vesta

NASA Astrophysics Data System (ADS)

Liao, Shiyong; Hsu, Weibiao

2017-05-01

Studies of petrology, mineralogy and geochronology of eucrites are keys to reconstruct the thermal and impact history of 4 Vesta, the proposed parent body for HED meteorites. Here we report the petrography, mineralogy and geochemistry of NWA 8009, a newly found eucritic impact-melt breccia, and present SIMS U-Pb ages of zircon and phosphates. NWA 8009 consists of coarse- and fine-grained lithic and mineral clasts set in fine-grained recrystallized matrix. It was derived from a protolith of monomict non-cumulate eucrite. Evidence for intense shock metamorphism observed in NWA 8009 includes mosaicism, deformed exsolution lamellae and partial melting of pyroxene, melting and incipient flow of plagioclase, planar fractures and granular textures of zircon. These shock effects indicate NWA 8009 was subjected to an impact metamorphism with peak pressure of ∼50-60 GPa and post-shock temperature of ∼1160-1200 °C. NWA 8009 is among the most intensely shocked HEDs reported yet. After the impact, the sample was buried near the surface in target rocks and experienced rapid cooling (∼23 °C/h) and annealing, resulting in recrystallization of the matrix and devitrification of plagioclase and silica glasses. U-Pb isotopic system of apatite within plagioclase groundmass of lithic clasts is completely reset and constrains the timing of impact at 4143 ± 61 Ma, providing a new robust impact age on Vesta. Combined with the presence of synchronous impact resetting events, especially those recorded by Lu-Hf, Sm-Nd, and Pb-Pb isotopic systems, we identified a period of high impacts flux at ca. 4.1-4.2 Ga on Vesta. This impact flux occurred coincident with the uptick at ca. 4.1-4.2 Ga in impact age spectra of the moon, probably reflects widespread intense bombardment throughout the inner solar system at ca. 4.1-4.2 Ga. Based on evidence from zircon chemical zoning, petrographic occurrences, as well as the distinctive Zr/Hf ratios, we suggested that zircons in NWA 8009 have had a metamorphic, instead of magmatic origin. They mainly crystallized from melts produced by partial melting of mesostasis area due to reheating event during early global thermal metamorphism, rather than by Zr release from Zr-rich minerals. The U-Pb isotopic system in zircons was not disturbed by subsequent impacts, the weighted-mean 207Pb/206Pb age of 4560 ± 8 Ma represents the timing of zircon growth during thermal metamorphism. Zircons from NWA 8009 and other eucrites may share a common origin during metamorphic growth events, and constraining the global thermal metamorphism on Vesta at ca. 4.55 Ga. The main heat sources responsible for global metamorphism in basaltic crust of Vesta might be heating from the hot interior, especially heat flow related to magmatism, rather than impact.

Zircon Trace Element Contents and Refined U-Pb Crystallization Ages for the Tatoosh Pluton, Mount Rainier National Park, Washington Cascades

NASA Astrophysics Data System (ADS)

Bacon, C. R.; Du Bray, E. A.; John, D. A.; Mazdab, F. K.; Wooden, J. L.

2008-12-01

The 7x12 km Tatoosh pluton south of Mount Rainier consists of 4 petrographic/compositional phases, here termed Nisqually, Reflection, Pyramid, and Stevens, that intrude Tertiary volcanic and sedimentary wall and roof rocks; contacts between the 4 intrusive units are rarely exposed. We used the USGS-Stanford SHRIMP- RG to analyze, in a continuous session, zircons from each of 6 quartz monzodiorite (qmd), quartz monzonite (qm), or granodiorite (grd) samples for 206Pb/238U ages and, concurrently, U, Th, Hf, and REE concentrations. A round-robin procedure yielded statistically robust geochronological results. Ages that we reported previously (FM07) were compromised by instrument instability and by calibration differences between analytical sessions. Between 11 and 31 new analyses of zircons from each sample were evaluated using the TuffZirc and Umix Ages routines of Isoplot 3.41 (Ludwig, 2003). TuffZirc solidification ages for the intrusions are: Nisqually grd (Paradise Valley; 65.4% SiO2) 17.29 +0.37/-0.24 Ma, Nisqually grd (Christine Falls; 66.4%) 17.70 +0.30/-0.16 Ma, Reflection qm (Pinnacle Peak trail; 66.6%) 18.38 +0.45/-0.28 Ma, Pyramid qmd (58.5%) 18.58 +0.20/-0.15 Ma, Stevens grd (Stevens Canyon; 67.8%) 19.15 +0.15/-0.12 Ma, and Stevens grd (south of Louise Lake; 69.3%) 19.20 +0.31/-0.26 Ma (U-Th initial-disequilibrium corrected, ±2σ). Precision of the U-Pb data limits rigorous identification of antecrysts to those with ages ~1 Myr > solidification ages. Antecryst ages that produce subsidiary modes in relative probability diagrams for the two Stevens samples give weighted mean values of 20.18 ±0.26 Ma and 20.07 ±0.18 Ma. Wide ranges in trace element concentrations and ratios indicate that many analyzed zircons grew in highly fractionated residual liquids in high-crystallinity environments. Concentrations of Th and U in Tatoosh zircons vary by two orders of magnitude, cores tend to have higher Th, U, and Th/U than rims, and overgrowths that fill reentrants have high U contents and low Th/U ratios. Chondrite-normalized REE patterns have familiar convex-up shapes with positive Ce and negative Eu anomalies: LaN = 0.03-6 (10 values >6 may reflect inclusions), YbN = 380-33,900, Ce/Ce* = 0.7-505, and Eu/Eu* = 0.06-1.25. Slopes of REE patterns increase subtly in the order Reflection < Pyramid ≤ Nisqually < Stevens. Fractionation of plagioclase + pyroxene and(or) amphibole + Fe-Ti oxide + apatite + zircon should produce relative-LREE- enriched successive liquids. Observed marginally steeper REE patterns (greater positive slopes) for rims, lower REE concentrations, and lower Th/U ratios may reflect co-precipitation of allanite ± thorite. Two parallel arrays in Eu/Eu* versus Hf, in which Eu/Eu* decreases as Hf increases, are consistent with growth of higher-Hf, lower-Eu/Eu* zircon from more evolved melts; separate arrays imply Nisqually and Pyramid + Reflection intrusions. Zircon Eu/Eu* and Hf in the Stevens granodiorite define one high-Eu/Eu* field and another similar to that for Pyramid and Reflection samples that imply more than one parental magma. The zircon ages indicate solidification in three episodes: Stevens ~19.2 Ma, Pyramid + Reflection ~18.5 Ma, and Nisqually ~17.5 Ma. An ~20.1 Ma stage is represented by antecrysts in Stevens samples. The source(s) of the pluton were active for at least ~2.7 Myr and major crystallization episodes were separated by intervals of 0.7-1.0 Myr.

The double-edged sword of high-precision U-Pb geochronology or be careful what you wish for. (Invited)

NASA Astrophysics Data System (ADS)

Bowring, S. A.

2010-12-01

Over the past two decades, U-Pb geochronology by ID-TIMS has been refined to achieve internal (analytical) uncertainties on a single grain analysis of ± ~ 0.1-0.2%, and 0.05% or better on weighted mean dates. This level of precision enables unprecedented evaluation of the rates and durations of geological processes, from magma chamber evolution to mass extinctions and recoveries. The increased precision, however, exposes complexity in magmatic/volcanic systems and highlights the importance of corrections related to disequilibrium partitioning of intermediate daughter products, and raises questions as to how best to interpret the complex spectrum of dates characteristic of many volcanic rocks. In addition, the increased precision requires renewed emphasis on the accuracy of U decay constants, the isotopic composition of U, the calibration of isotopic tracers, and the accurate propagation of uncertainties It is now commonplace in the high precision dating of volcanic ash-beds to analyze 5-20 single grains of zircon in an attempt to resolve the eruption/depositional age. Data sets with dispersion far in excess of analytical uncertainties are interpreted to reflect Pb-loss, inheritance, and protracted crystallization, often supported with zircon chemistry. In most cases, a weighted mean of the youngest reproducible dates is interpreted as the time of eruption/deposition. Crystallization histories of silicic magmatic systems recovered from plutonic rocks may also be protracted, though may not be directly applicable to silicic eruptions; each sample must be evaluated independently. A key to robust interpretations is the integration high-spatial resolution zircon trace element geochemistry with high-precision ID-TIMS analyses. The EARTHTIME initiative has focused on many of these issues, and the larger subject of constructing a timeline for earth history using both U-Pb and Ar-Ar chronometers. Despite continuing improvements in both, comparing dates for the same rock with both chronometers is not straightforward. Compelling issues range from pre-eruptive magma chamber residence, recognizing open system behavior, accurately correcting for disequilibrium amounts of 230Th and 231Pa, precise and accurate dates of fluence monitors for 40Ar/39Ar, and inter-laboratory biases. At present, despite the level of internal precision achievable by each technique, obstacles remain to combining both chronometers.

Refined Proterozoic evolution of the Gawler Craton, South Australia, through U-Pb zircon geochronology

USGS Publications Warehouse

Fanning, C.M.; Flint, R.B.; Parker, A.J.; Ludwig, K. R.; Blissett, A.H.

1988-01-01

Through the application of both conventional U-Pb zircon analyses and small-sample U-Pb isotopic analyses, the nature and timing of tectonic events leading to the formation of the Gawler Craton have been defined more precisely. Constraints on deposition of Early Proterozoic iron formation-bearing sediments have been narrowed down to the period 1960-1847 Ma. Deformed acid volcanics, including the economically important Moonta Porphyry, have zircon ages of ??? 1790 and 1740 Ma. The voluminous acid Gawler Range Volcanics and correlatives to the east were erupted over a short interval at 1592 ?? 2 Ma, and were intruded by anorogenic granites at ??? 1575 Ma. Small-sample zircon analyses proved to be an extremely valuable adjunct to conventional analyses, generally yielding more-concordant data which forced a curved discordia through an upper intercept slightly younger than from a conventional straight-line discordia. ?? 1988.

U-Pb zircon and biostratigraphic data of high-pressure/low-temperature metamorphic rocks of the Talea Ori: tracking the Paleotethys suture in central Crete, Greece

NASA Astrophysics Data System (ADS)

Zulauf, G.; Dörr, W.; Krahl, J.; Lahaye, Y.; Chatzaras, V.; Xypolias, P.

2016-10-01

Inherited deformation microfabrics of detrital quartz grains and U-Pb (Laser ablation (LA)-ICPMS and ID TIMS) ages of detrital zircons separated from the Phyllite-Quartzite Unit s.l. of the Talea Ori, central Crete, suggest strikingly different source rocks. Albite gneiss of the lower Rogdia Beds includes Cambrian and Neoproterozoic rounded zircons with main U-Pb age peaks at 628 and 988 Ma. These and minor Paleoproterozoic and Archean peaks, together with the lack of Variscan-aged and Mesoproterozoic zircons, are similar to the age spectra obtained from the Phyllite-Quartzite Unit s.str. of the Peloponnesus and eastern Crete and from the Taurides. All of these zircons should be derived from the northeastern passive margin of Gondwana (Cimmeria). Metatuffites of the uppermost Rogdia Beds and metasandstone of Bali beach, on the other hand, include euhedral detrital zircons displaying a Variscan U-Pb age spectra at ca. 300 Ma with concordia ages at 291 ± 3, 300 ± 1 Ma (Rogdia) and 286 ± 3, 300 ± 3, 313 ± 2 Ma (Bali). Both types of metasediments and their zircons are similar to those of the pre-Alpine basement and overlying Tyros Beds of eastern Crete, revealing a provenance at the southern active margin of Laurasia. Thus, in central Crete the Paleotethys suture should be situated inside the Rogdia Beds. Magmatic zircons separated from a rhyolite boulder of the lower Achlada Beds yielded a concordant U-Pb zircon age at 242 ± 2 Ma placing a maximum age for the deposition of the (meta)conglomerate from which the boulder was collected. This age is compatible with an Olenekian-early Anisian age of the underlying Vasilikon marble suggested by new findings of the foraminifera Meandrospira aff. pusilla. Both the Achlada Beds and the Vasilikon marble can be attributed to the lower Tyros Beds of eastern Crete. The Alpine deformation led to a pervasive mylonitic foliation, which is affecting most of the studied rocks. This foliation results from D2 top-to-the-north shearing, which post-dates the growth of blue amphiboles (crossite).

Exhumation of the Black Mountains in Death Valley, California, with new thermochronometric data from the Badwater Turtleback

NASA Astrophysics Data System (ADS)

Sizemore, T. M.; Cemen, I.; Wielicki, M. M.; Stockli, D. F.; Heizler, M. T.; Lutz, B. M.

2017-12-01

The Black Mountains, in Death Valley, California, are one of the key areas to better understand Basin and Range extension because they contain Cenozoic igneous and sedimentary rocks overlying mid- to deep-crustal, 1.74 Ga basement gneiss with abundant fault striations, large-scale extensional folds, and tectonite fabrics containing top-to-the-northwest shear-sense indicators. These rocks make up the footwall of three prominent, high-relief "turtleback" fault surfaces in the western flank of the Black Mountains, which are thought to have accommodated a significant amount of strain in the Death Valley area. It is unknown whether the missing Paleozoic and Mesozoic strata in the Black Mountains were removed in association with high-angle faulting, or along a continuous detachment surface with a rolling-hinge style of faulting as the hanging wall moved to the west, now forming the Panamint Range. The turtlebacks play an important role in resolving this question because they are commonly cited as containing conflicting evidence of both hypotheses. To provide insight into this problem, we are building an exhumation model across the Black Mountains using previously published thermochronometric data as well as new transect-based (U-Th)/He and Ar-Ar thermochronology and U-Pb geochronology for the Badwater turtleback. The model will provide a four-dimensional view of the exhumation history of the Black Mountains, to serve as evidence for either of the two previously mentioned hypotheses, or possibly some other style of exhumation. Additionally, we will compare the exhumation history of the Black Mountains to that of the Panamint Range using previously published data and interpretations. Our preliminary zircon U-Pb data suggest a crystallization age for the gneissic rocks on the Badwater turtleback of 1.74 Ga (207Pb/206Pb, 2σ error=31.8 Ma, n=6) with two younger populations at 1.46 Ga (207Pb/206Pb, 2σ error=51.8 Ma, n=3) and 79.6 Ma (206Pb/238U, 2σ error=10.0 Ma, n=2), possibly reflecting post-crystallization alteration. This is in agreement with previously published ages. The zircon 206Pb/238U ages of the Smith Mountain Granite, exposed at the top of the Badwater turtleback, range from 3.06-9.71 Ma (n=11), confirming Cenozoic intrusion of the pluton.

Prospects for Practical Laser Ablation U/Pb and (U-Th)/He Double-Dating (LADD) of Detrital Apatite

NASA Astrophysics Data System (ADS)

Horne, A.; Hodges, K. V.; Van Soest, M. C.

2017-12-01

A laser ablation micro-analytical technique for (U-Th)/He dating has been shown to be an effective approach to the thermochronologic study of detrital zircons (Tripathy-Lang et al., J. Geophys. Res., 2013), while Evans et al. (J. Anal. At. Spectrom., 2015) and Horne et al. (Geochim. Cosmochim. Acta, 2016) demonstrated how the technique could be modified to enable laser ablation U/Pb and (U-Th)/He double-dating (LADD) of detrital zircon and titanite. These successes beg the question of whether or not LADD is viable for another commonly encountered detrital mineral: apatite. Exploratory LADD studies in Arizona State University's Group 18 Laboratories - using Durango fluorapatite, apatite from the Fish Canyon tuff, and detrital apatite from modern fluvial sediments in the eastern Sierra Nevada of California - illustrate that the method is indeed viable for detrital apatite. However, the method may not be appropriate for all detrital samples. For example, many apatite grains encountered in detrital samples from young orogenic settings have low concentrations of U and Th and small crystal sizes. This can lead to imprecise laser ablation (U-Th)/He dates, especially for very young grains potentially obscuring or inhibiting relevant interpretations of the data set.

Magmatic tempo of Earth's youngest exposed plutons as revealed by detrital zircon U-Pb geochronology.

PubMed

Ito, Hisatoshi; Spencer, Christopher J; Danišík, Martin; Hoiland, Carl W

2017-09-29

Plutons are formed by protracted crystallization of magma bodies several kilometers deep within the crust. The temporal frequency (i.e. episodicity or 'tempo') of pluton formation is often poorly constrained as timescales of pluton formation are largely variable and may be difficult to resolve by traditional dating methods. The Hida Mountain Range of central Japan hosts the youngest exposed plutons on Earth and provides a unique opportunity to assess the temporal and spatial characteristics of pluton emplacement at high temporal resolution. Here we apply U-Pb geochronology to zircon from the Quaternary Kurobegawa Granite and Takidani Granodiorite in the Hida Mountain Range, and from modern river sediments whose fluvial catchments include these plutons in order to reconstruct their formation. The U-Pb data demonstrate that the Kurobegawa pluton experienced two magmatic pulses at ~2.3 Ma and ~0.9 Ma; whereas, to the south, the Takidani pluton experienced only one magmatic pulse at ~1.6 Ma. These data imply that each of these magmatic systems were both spatially and temporally distinct. The apparent ~0.7 Myr age gap between each of the three magmatic pulses potentially constrains the recharge duration of a single pluton within a larger arc plutonic complex.

Equilibrium and Disequilibrium of 230Th-238U in Zircon from the Minoan Eruption, Santorini, Aegean Sea, Greece

NASA Astrophysics Data System (ADS)

Schmitt, A. K.; Stockli, D. F.; Song, E. J.; Storm, S.

2016-12-01

The Minoan eruption (ca. 1600 BCE; 40-80 km3 dense rock equivalent) occurred after a ca. 18 ka period of dormancy followed by rapid reinvigoration through arrival of new magma from deep reservoirs colliding with evolved magmas in shallow storage. Although zoned phenocrysts indicate brief timescales ranging between years to decades for final pre-eruptive magma recharge and mixing, it remains unclear how magma accumulation vs. crystallization were balanced in the subvolcanic reservoir during the preceding inter-eruptive cycle. To directly probe magma presence over the repose interval prior to the Minoan eruption and further back in time, we reconnoitered the potential of U-Th zircon geochronology to date the crystallization of individual zircon crystals from pumice from the Minoan eruption. Zircon crystals were extracted from composite pumice samples (several kg each) from basal fall out deposits using gravity and magnetic separation. Etching in cold HF removed adherent glass and revealed the shape of crystals, which were pressed into indium metal to expose unpolished rims to the ion beam of a CAMECA IMS 1270 secondary ionization mass spectrometer. Adherent glass was ubiquitous, indicating that crystals were in contact with melt at the time of eruption. Six of 18 crystals were in 230Th/238U secular equilibrium, two crystals yielded ages of ca. 160 ka, and the remaining rims dated between eruption age and ca. 20 ka. Low Th/U of some secular equilibrium zircon suggests recycling of metamorphic basement zircon, which is also indicated by the presence of rutile in heavy mineral separates. U-Th dates also reveal recycling of zircon from Pleistocene intrusions that likely represent left-over magma from antecedent eruption cycles. We tentatively interpret the dominant zircon population with near-eruption to ca. 20 ka ages to indicate continuous melt presence underneath Santorini during the last repose interval. Distinguishing a hiatus in zircon crystallization between 20 ka and 160 ka from continuous presence of a crystal mush capable of crystallizing zircon throughout the late Pleistocene requires further analyses. A corollary of mixed zircon populations of heterogeneous provenance in Minoan pumice is that the age significance of U-series data for major phases analyzed in bulk is suspect.

Interesting Inclusions From Podiform Chromitites in Luobusa Ophiolite, Tibet

NASA Astrophysics Data System (ADS)

Yamamoto, S.; Komiya, T.; Hirose, K.; Maruyama, S.

2003-12-01

For the past decade, diamonds and unusual mineral asemblages were reported in podiform chromitites of the Luobusa ophiolite, southern Tibet, China (Bai 1993, Bai 2000, Yan 2001). These minerals were found from heavy mineral separation of chromitites. These minerals include (1) native elements, (2) alloys, (3) carbide (SiC, CrC), (4) platinium group elements (PGE) and arsenides, (5) silicates (Ol, Opx, Cpx, Amp, Srp, Chl, Uv, Prp, Alm, Wo, Zrn, Ap, Bt, Spn, Rt, Pl, Kfs, Phl, Sil, Qz and octahedral serpentine (possible pseduomorph after ringwoodite?), (5) oxide (corundum and chromite), (6) carbonates. Despite many questions as to these minerals above still remain open, these mineral inclusions would provide us the important infomation on the formation of the podiform chromitites. In this study, octahedral serpentine was discovered both on a thin section and from the heavy mineral separation. These octahedral inclusions exist within chromites, forming a line. These minerals are approximately 5-15μ m in diameter and have well octahedral morphology. EPMA, laser raman spectrometer and transmission electron microscopy (TEM) were used to determine the structure and chemical composition of this crystal. For the present, there are several interpretations of this octahedral silicate. One possibility is that if the octahedral structuer is euhedral so this octahedral serpentine may be pseudomorph after ringwoodite because of its chemical composition and octahedral crystal shape. Another is that ocahedral minerals are melt inclusions. Linear occurrence of octahedral minerals is similar to that of fuluid inclusions. If the octahedral structuer is negative crystal shape reflecting octahedral crystal of cromian spinel, then octahedral inclusions may be melt inclusions judging from linear occurrence. At the same time, zircons were obtained from the mineral separation from chromitites. U-Pb dating of these zircons by LA-ICP-MS yielded two different ages. One group has relatively younger age 107-534 Ma, which nearly plots on a concordia line. Another group has older age 1460-1822 Ma, which plots off the concordia line. Cathode luminescence images of these zircons indicate that some zircons have clear oscillatory zoning whereas other zircons show apparent homogeneous overgrowth. But any correlation between CL image and the U-Pb age was not identified in particular. Luobusa ophiolite has been recognized as fragment of Tethys oceanic crust formed in Cretaceous at 100-120 Ma (Allegre et al. 1984). The minimum age 107 Ma corresponds to the age of the formation of Luobusa ophiolite and all other age of zircons in chromitites is much older than that of ophiolite. In addition, the inclusions in the zircons were analyzed by EPMA and laser raman spectrometer. Several zircons contain some inclusions, which are quartz, feldsper, mica, apatite, titanite and others. These inclusions are the minerals composed of crustal material, which means that these zircons were crystalized in the low pressuer crustal condition. On the other hand, Yu et al. (2001) reported that zircons from chromitites in Luobusa ophiolite have shorter inter-atomic distances for Zr-O and Si-O bonds. They concluded that Tibetan-zircons were derived from the high-pressure mantle environment. Judging from the line of evidence mentioned avobe, it is highly possible that these zircons captured by chromitites were originated from recycled crustal materials convecting through upper mantle.

Crustal Zircons from the Podiform Chromitites in Luobusa Ophiolite, Tibet

NASA Astrophysics Data System (ADS)

Yamamoto, S.; Komiya, T.; Maruyama, S.

2004-12-01

For the past decade, diamonds and unusual mineral asemblages were reported in podiform chromitites of the Luobusa ophiolite, southern Tibet, China (Bai 1993, Bai 2000, Yan 2001) by heavy mineral separation. These include (1) native elements, (2) alloys, (3) carbide, (4) platinium group elements (PGE) and arsenides, (5) silicates (6) oxide, (7) carbonates, (8) minerals with unusual compositons. Despite many questions as to these minerals above still remain open, these mineral inclusions would provide us the important infomation on the formation of the podiform chromitites. In this study, over 100 zircons were discovered by heavy mineral separation of podiform chromitite in Luobusa ophiolite. The discovery of accessory zircons in chromitites allowed us to date the formation of the chromitite and history of tectonic evolutions. Here we report the U-Pb age and mineral inclusions of zircons and discuss with unusually old age zircons. 20 zircon grains in chromitites from No. 1 site were analyzed. Zircons from the chromitites in Luobusa ophiolite are usually euhedral-subhedral and some are rounded. Cathodoluminescence images of these zircons indicate that some zircons have clear oscillatory zoning, whereas other zircons show apparent homogeneous overgrowth. U-Pb dating of these zircons by LA-ICP-MS yielded two different ages. One group has relatively younger age, 107-534Ma, which plots nearly on a concordia line. Another group has older age 1460-1822Ma, which plots off the concordia line. There is insignificant difference of apparent ages within a single zircon grain. For example, a zircon has 1650 Ma in the core, whereas does 1654 Ma in the rim. We identified several mineral inclusions, quartz, feldspar, mica, apatite, within both yonger and older zircons using laser-Raman spectrometry and EPMA. No high-pressure minerals or mantle minerals were identified. This means that these unusually old zircons were formed in low-pressure crustal emvironment. Where did the zircons in chromitites come from? It has been recognized that this ophiolite was formed at 110-120 Ma based on radiolaria in cherts overlying the pillow lavas (ALLEGRE et al., 1984; ZIABREV et al., 2003). In this study, the minimum age of 107 Ma, which we obtained from zircon in chromitites, is consistent to the age of the ophiolite. But, all other ages of zircons are much older than that of ophiolite. Yang et al. (2001) also reported U-Pb zircon ages of 450-910 Ma and Re-Os iridosmine age of 400 Ma from chromitites in Luobusa ophiolite. Mineral inclusions within zircons are crustal materials, which means that these zircons were crystallized in the low pressure crustal condition. Thus these zircons within chromitites are interpreted as xenocrysts from old crustal materials. Recently, old-age zircons (330 to 1600 Ma) were also reported from the Mid Atlantic Ridge MORBs (PILOT et al., 1998). They suggest one possibility that these old-age zircons may have derived from old continental crustal material, which have assimilated with the MORB magma during ascent. Moreover, Archean zircons were reported from pyroxenite dikes in Jormua ophiolite (PETRI et al., 2003). YU et al. (2001) reported that zircons from chromitites in Luobusa ophiolite have shorter inter-atomic distances of Zr-O and Si-O bonds. As a result, they concluded that Tibetan-zircons were derived from the high-pressure mantle environment. Judging from the line of evidence mentioned above, it is highly possible that these zircons captured by chromitites were originated from recycled crustal materials, convecting through upper mantle.

The Comparison of Detrital Zircon Ages to Point Count Provenance Analysis for the Pottsville Sandstone in the Northern Appalachian Foreland Basin Venango County, Pennsylvania

NASA Astrophysics Data System (ADS)

Loveday, S.; Harris, D. B.; Schiappa, T.; Pecha, M.

2017-12-01

The specific sources of sediments deposited in the Appalachian basin prior to and immediately following the Alleghenian orogeny has long been a topic of debate. Recent advances in U-Pb dating of detrital zircons have greatly helped to determine some of the sources of these sediments. For this study, sandstone samples were collected from the Pottsville Formation in the northern Appalachian Foreland Basin, Venango County, Pennsylvania to provide supplementary data for previous work that sought to describe the provenance of the same sediments by point counts of thin sections of the same units. Results of this previous work established that the provenance for these units was transitional recycled orogenic, including multiple recycled sediments, and that a cratonic contribution was not able to be determined clearly. The previous results suggested that the paleoenvironment was a fluvial dominated delta prograding in the northern direction. However, no geochronologic data was found during this study to confirm this interpretation. We sought to verify these results by U-Pb analysis of detrital zircons. Samples were collected from the areas where the previous research took place. U-Pb ages were found from sample at the highest elevation and lowest elevation. In the first sample, sample 17SL01 (younger sample stratigraphically), the zircons yield U-Pb age range peaks at 442-468 ma and 1037-1081 ma. The probability density plot for this specific sample displays a complete age gap from 500 ma to 811 ma. In the second sample, sample 17SL03 (older rock stratigraphically), the zircons yield U-Pb ages range peaks of 424-616 ma and 975-1057 ma. This sample doesn't show any ages younger than 424 ma and it doesn't display the sample age gap as sample 17SL01 does. The ages of zircons are consistent with thin section point counting provenance results from previous research suggesting zircon transport from the northern direction.

Crustal evolution at mantle depths constrained from Pamir xenoliths

NASA Astrophysics Data System (ADS)

Kooijman, E.; Hacker, B. R.; Smit, M. A.; Kylander-Clark, A. R.; Ratschbacher, L.

2012-12-01

Lower crustal xenoliths erupted in the Pamir at ~11 Ma provide an exclusive opportunity to study the evolution of crust at mantle depths during a continent-continent collision. To investigate, and constrain the timing of, the petrologic processes that occurred during burial to the peak conditions (2.5-2.8 GPa, 1000-1100 °C; [1]), we performed chemical- and isotope analyses of accessory minerals in 10 xenoliths, ranging from eclogites to grt-ky-qtz granulites. In situ laser ablation split-stream ICPMS yielded 1) U-Pb ages, Ti concentrations and REE in zircon, 2) U/Th-Pb ages and REE in monazite, and 3) U-Pb ages and trace elements in rutile. In addition, garnet, and biotite and K-feldspar were dated using Lu-Hf and 40Ar/39Ar geochronology, respectively. Zircon and monazite U-(Th-)Pb ages are 101.9±1.8, 53.7±1.0, 39.1±0.8, 21.7±0.4, 18.2±0.5, 16.9±0.8, 15.1±0.3 (2σ) and 12.5-11.1 Ma; most samples showed several or all of these populations. The 53.7 Ma and older ages are xenocrystic or detrital. For younger ages, zircon and monazite in individual samples recorded different ages-although zircon in one rock and monazite in another can be the same age. The 39.1 Ma zircon and monazite mostly occur as inclusions in minerals of the garnet-bearing assemblage that represents the early, low-P stages of burial. Garnet Lu-Hf ages of 37.8±0.3 Ma support garnet growth at this time. Spinifex-like textures containing 21.7-11.1 Ma zircon and monazite record short-lived partial melting events during burial. Aligned kyanite near these patches indicates associated deformation. Zircons yielding ≤12.5 Ma exhibit increased Eu/Eu* and markedly decreased HREE concentrations, interpreted to record feldspar breakdown and omphacite growth during increasing pressure. Rutile U-Pb cooling ages are 10.8±0.3 Ma in all samples. This agrees with the weighted mean 40Ar/39Ar age of eight biotite, K-feldspar and whole rock separates of 11.00+0.16/-0.09 Ma. Rutile in eclogites provides Zr/Hf and Nb/Ta trends that indicate clinopyroxene fractionation. This is consistent with the occurrence of rutile in omphacite-rich parts of the rocks and supports their HP petrogenesis. In the felsic granulites rutile is associated with the amphibolite-facies garnet-bearing assemblage and its Nb/Ta and Zr/Hf primarily reflect fractionation by rutile. Zirconium-in-rutile temperatures are 800-835 °C for the felsic granulites and 860-895 °C for the eclogites. Titanium-in-zircon temperatures increase from ~735 °C (0.7-1.0 GPa) at 39.1 Ma to ~900 °C (>2.5 GPa) at 11.5 Ma; a further, abrupt increase toward 1000 °C at 11.1 Ma marks melting at the onset of eruption. The analytical uncertainty on the Miocene ages is small compared to the 28-Myr burial record, enabling precise dating of individual reaction and deformation events. These events are at least an order of magnitude shorter than the duration of burial, and evidently occurred in pulses recorded by the (re)crystallization of zircon or monazite. Reference: [1] Hacker et al. (2005) J Petrol 46 (8): 1661-1687.

Progress integrating ID-TIMS U-Pb geochronology with accessory mineral geochemistry: towards better accuracy and higher precision time

NASA Astrophysics Data System (ADS)

Schoene, B.; Samperton, K. M.; Crowley, J. L.; Cottle, J. M.

2012-12-01

It is increasingly common that hand samples of plutonic and volcanic rocks contain zircon with dates that span between zero and >100 ka. This recognition comes from the increased application of U-series geochronology on young volcanic rocks and the increased precision to better than 0.1% on single zircons by the U-Pb ID-TIMS method. It has thus become more difficult to interpret such complicated datasets in terms of ashbed eruption or magma emplacement, which are critical constraints for geochronologic applications ranging from biotic evolution and the stratigraphic record to magmatic and metamorphic processes in orogenic belts. It is important, therefore, to develop methods that aid in interpreting which minerals, if any, date the targeted process. One promising tactic is to better integrate accessory mineral geochemistry with high-precision ID-TIMS U-Pb geochronology. These dual constraints can 1) identify cogenetic populations of minerals, and 2) record magmatic or metamorphic fluid evolution through time. Goal (1) has been widely sought with in situ geochronology and geochemical analysis but is limited by low-precision dates. Recent work has attempted to bridge this gap by retrieving the typically discarded elution from ion exchange chemistry that precedes ID-TIMS U-Pb geochronology and analyzing it by ICP-MS (U-Pb TIMS-TEA). The result integrates geochemistry and high-precision geochronology from the exact same volume of material. The limitation of this method is the relatively coarse spatial resolution compared to in situ techniques, and thus averages potentially complicated trace element profiles through single minerals or mineral fragments. In continued work, we test the effect of this on zircon by beginning with CL imaging to reveal internal zonation and growth histories. This is followed by in situ LA-ICPMS trace element transects of imaged grains to reveal internal geochemical zonation. The same grains are then removed from grain-mount, fragmented, and analyzed by U-Pb TIMS-TEA. In situ trace element transects are used to model predicted TIMS-TEA trace element concentrations to test whether complicated trace element profiles undermine U-Pb TIMS-TEA data. We find good agreement between predicted and measured TIMS-TEA data, and can argue that the measured ID-TIMS U-Pb date corresponds to the time at which the geochemical signature measured by TIMS-TEA was acquired. Thus, in a hypothetical magma that is differentiating through AFC processes on timescales resolvable by geochronology, U-Pb TIMS-TEA should usually be a robust indicator of magma evolution through time. We present data from two ca. 40-30 Ma alpine intrusions from northern Italy: the southern Adamello batholith and the Bergell intrusion. The relatively young age of these intrusions permits uncertainties on individual zircon or zircon fragments as good as 10 ka, while zircon populations from individual hand samples often record zircon growth of >200 ka. Using the methodologies described above, we explore whether these zircons record in situ magmatic differentiation or introduction of antecrystic zircon to magma batches, and integrate these data to gain a better understanding of magma storage, differentiation and emplacement as a function of pressure, temperature, and time. These methods are a promising step towards interpreting complicated geochronologic data in ashbed samples as well through a better understanding of magmatic processes that precede eruption.

Zircon ages delimit the provenance of a sand extrudite from the Botucatu Formation in the Paraná volcanic province, Iraí, Brazil.

PubMed

Pinto, Viter M; Hartmann, Léo A; Santos, João O S; McNaughton, Neal J

2015-09-01

Ion microprobe age determinations of 102 detrital zircon crystals from a sand extrudite, Cretaceous Paraná volcanic province, set limits on the origin of the numerous sand layers present in this major flood basalt province. The zircon U-Pb ages reflect four main orogenic cycles: Mesoproterozoic (1155-962 Ma), latest Proterozoic-early Cambrian (808-500 Ma) and two Palaeozoic (Ordovician- 480 to 450 Ma, and Permian to Lower Triassic- 296 to 250 Ma). Two additional small concentrations are present in the Neoarchean (2.8 to 2.6 Ga) and Paleoproterozoic (2.0 to 1.7 Ga). Zircon age peaks closely match the several pulses of igneous activity in the Precambrian Brazilian Shield and active orogeny in Argentina. A main delimitation of the origin of the sand is the absence of zircon ages from the underlying Cretaceous basalts, thus supporting an injectite origin of the sand as an extrudite that emanated from the paleoerg that constitutes the Botucatu Formation.

U-Pb detrital zircon geochronology from the basement of the Central Qilian Terrane: implications for tectonic evolution of northeastern Tibetan Plateau

NASA Astrophysics Data System (ADS)

Liu, Changfeng; Wu, Chen; Zhou, Zhiguang; Yan, Zhu; Jiang, Tian; Song, Zhijie; Liu, Wencan; Yang, Xin; Zhang, Hongyuan

2018-03-01

The Tuolai Group dominates the Central Qilian Terrane, and there are different opinions on the age and tectonic attribute of the Tuolai Group. Based on large-scale geologic mapping and zircon dating, the Tuolai Group is divided into four parts: metamorphic supracrustal rocks, Neoproterozoic acid intrusive rocks, early-middle Ordovician acid intrusive rocks and middle Ordovician basic intrusive rocks. The metamorphic supracrustal rocks are the redefined Tuolai complex-group and include gneiss and schist assemblage by faulting contact. Zircon U-Pb LA-MC-ICP-MS dating was conducted on these samples of gneiss and migmatite from the gneiss assemblage, quartzite, two-mica schist and slate from the schist assemblage. The five detrital samples possess similar age spectra; have detrital zircon U-Pb main peak ages of 1.7 Ga with youngest U-Pb ages of 1150 Ma. They are intruded by Neoproterozoic acid intrusive rocks. Therefore, the Tuolai Group belonging to late Mesoproterozoic and early Neoproterozoic. With this caveat in mind, we believe that U-Pb detrital zircon dating, together with the geologic constraints obtained from this study and early work in the neighboring regions. We suggest that the formation age of the entire crystalline basement rocks of metasedimentary sequence from the Central Qilian Terrane should be constrained between the Late Mesoproterozoic and the Late Neoproterozoic, but not the previous Paleoproterozoic. The basement of the Central Qilian Terrane contains the typical Grenville ages, which indicates the Centre Qilian Terrane have been experienced the Grenville orogeny event.

Timing and duration of Variscan high-pressure metamorphism in the French Massif Central: A multimethod geochronological study from the Najac Massif

NASA Astrophysics Data System (ADS)

Lotout, Caroline; Pitra, Pavel; Poujol, Marc; Anczkiewicz, Robert; Van Den Driessche, Jean

2018-05-01

Accurate dating of eclogite-facies metamorphism is of paramount importance in order to understand the tectonic evolution of an orogen. An eclogite sample from the Najac Massif (French Massif Central, Variscan belt) displays a zircon-bearing garnet-omphacite-amphibole-rutile-quartz peak assemblage. Pseudosection modeling suggests peak pressure conditions of 15-20 kbar, 560-630 °C. Eclogite-facies garnet displays Lu-enriched cores and Sm-rich rims and yields a Lu-Hf age of 382.8 ± 1.0 Ma and a Sm-Nd age of 376.7 ± 3.3 Ma. The ages are interpreted as marking the beginning of the prograde garnet growth during the initial stages of the eclogite-facies metamorphism, and the high-pressure (and temperature) peak reached by the rock, respectively. Zircon grains display chondrite-normalized REE spectra with variably negative, positive or no Eu anomalies and are characterized by either enriched or flat HREE patterns. However, they yield a well constrained in situ LA-ICP-MS U-Pb age of 385.5 ± 2.3 Ma, despite this REE pattern variability. Zr zonation in garnet, Y content in zircon and the diversity of zircon HREE spectra may suggest that zircon crystallized prior to and during incipient garnet growth on the prograde P-T path, recording the initial stages of the eclogite-facies conditions. Consequently, the zircon age of 385.5 ± 2.3 Ma, comparable within error with the Lu-Hf age obtained on garnet, is interpreted as dating the beginning of the eclogite-facies metamorphism. Accordingly, the duration of the prograde part of the eclogite-facies event is estimated at 6.1 ± 4.3 Myr. Subsequent exhumation is constrained by an apatite U-Pb age at 369 ± 13 Ma.

A cathodoluminescence study on zircons with a complex thermal history traces back Permian crustal events in the Ivrea Zone (South Alpine, Northern Italy)

NASA Astrophysics Data System (ADS)

Peressini, G.; Poller, U.

2003-04-01

In the context of a U-Pb SHRIMP data-set, a cathodoluminescence (CL) study has been performed on zircons from the Ivrea-Verbano Zone; this is a tectonically bounded section of intermediate to deep crust, tilted and obducted in Alpine time, in which a large deep-crustal intrusion of mantle origin, the Mafic Complex (MC), was emplaced in the Early Permian. Zircons from 16 samples from the different units of the MC have been studied using CL. Three samples collected from the Paragneiss Bearing Belt (PBB) yield some partially reset spot-ages, reflecting the field observation that country rock slabs are frequent in that area. On the other side, unlike in zircons from granites, the cores are invisible under CL-imaging, and this makes the U-Pb spot-age results unpredictable, with a continuous range of ages in the same sample, spanning for an interval of over 35 Ma, followed by some much older peaks, up to 600 Ma. A fourth sample, collected 2 km far, but still within the PBB, defines instead a unique, well-defined age at 287±3 Ma, with no older peaks, its zircons showing a CL pattern typical for metamorphic grains. A different case is displayed by a sample collected from the deeper Amphibole Gabbro unit: each single grain records a complex story of magmatic growth with variable diffusivity conditions. All of them show a second major overprint, that lead to both (re)crystallization and resorption, always corresponding to much lower U and Th contents, with no sensible modification of the Th-U ratio. The age of the second event, though, is not distinguishable from that of first crystallization of the grains, and has not been at such a temperature as to obliterate the fine zoning pattern of the primary grain. The CL patterns of each single grain, composed of different domains, allow considerations on the environmental conditions of growth and (re)crystallization. CL is a very powerful tool itself, revealing crystal-chemical processes. The integration of the CL-study with the SHRIMP spot-age determination relates them to the sequence of geologic events. The study, performed on samples coming from all over the Complex, has allowed a better understanding of processes in the Carboniferous-Permian lower crust of the Ivrea Zone: the knowledge of the Complex, its structure and the field relationships has become a key for interpreting the results. Where the CL displays a composite pattern, mechanisms are needed to explain geochemical fingerprints: therefore, trace and REE analyses by ion probe are in progress to integrate the SHRIMP data.

Extinct Plutonium Geochemistry of Ancient Hadean Zircons

NASA Astrophysics Data System (ADS)

Turner, G.; Gilmour, J.; Crowther, S.; Busfield, A.; Mojzsis, S.; Harrison, M.

2005-12-01

The abundance of 244Pu in the early solar system has important implications for r-process nucleosynthesis and models of noble gas transport within the Earth's mantle. Our recent discovery(1) of xenon isotopes from the in-situ decay of 244Pu in ancient Jack Hills zircons promises to provide a new time-sensitive window on the first 500 Ma of Earth history. We have extended this initial work by the use of resonance ioniisation mass spectrometry to analyse xenon released by stepped heating from 17 individual zircons with Pb-Pb ages in the range 3.95 to 4.18 Ga. Our immediate objectives are to determine the causes of variations in the inferred Pu/U ratios and in the longer term to determine the initial Pu/U ratio of the Earth. The Pu/U ratios calculated for individual zircons may be expected to vary as a result of igneous fractionation and also from differential loss of Pu and U fission xenon in the last 4 Ga. We have studied the effects of xenon loss by irradiating the zircons with thermal neutrons to generate xenon from 235U neutron fission in order to determine U/Xe ratios and apparent ages. 131Xe/134Xe and 132Xe/134Xe ratios can be used to calculate the relative contributions from 244Pu and 238U spontaneous fission and 235U neutron fission. The measured Pu/U ratios (back calculated to 4.56 Ga on the basis of the individual Pb-Pb ages) range from zero to 0.012. The highest ratio in our initial study was 0.008 (note that the published ratio has been revised upwards on the basis of improved decay parameters for 238U spontaneous fission). Comparison of Pb-Pb and U-Xe ages indicate varying amounts of xenon loss, over 50% in some cases. While this accounts for some of the variability in the inferred Pu/U, igneous fractionation may also play a part, and we are currently attempting to investigate this by a comparison with REE abundances. Reference: (1) Turner et al. (2004) Science, 306, 89-91.

An Integrated Analytical Approach to Obtaining Reliable U-Pb and Hf Isotopic Data from Complex (>3.9 to 3.3 Ga) Zircon from the Acasta Gneiss Complex

NASA Astrophysics Data System (ADS)

Bauer, A.; Bowring, S. A.; Vervoort, J. D.; Fisher, C. M.

2014-12-01

The Acasta Gneiss Complex (AGC) of northwestern Canada preserves some of Earth's oldest granitic crust (>4.03 Ga) and thereby contains important insight into crust forming processes on the early Earth. In general, rocks of the AGC have undergone a complex history of metamorphism and deformation (Archean and Paleoproterozoic)1,2, and, as a consequence, the zircons retain a complex history including inheritance, magmatic and metamorphic overgrowths, recrystallization, and multi-stage Pb loss. Previously published Hf isotopic data on zircons show within sample variability in excess of analytical uncertainty2,3,4. In order to assess the meaning and significance of this apparent isotopic variability, we are using two different methods to obtain coupled U-Pb and Lu-Hf isotopic data in zircon from a suite of rocks ranging in age from ca. > 3.9 Ga to 3.3 Ga. To obtain these data from the same volume of zircon, our approach involves: 1) split stream LA-ICPMS for U-Pb and Lu-Hf; 2) mechanical isolation of zircon domains for chemical abrasion and ID-TIMS U-Pb analyses and solution ICPMS for Lu-Hf recovered from U-Pb ion exchange chromatography. The deconvolution of complex histories requires this integrated approach and permits us to take advantage of both high spatial resolution and highest precision measurements to ultimately decipher the age and isotopic composition of discrete domains of multi-phase zircon. We demonstrate our approach with both relatively simple and complex grain populations in an attempt to understand within and between grain heterogeneity. The samples with the simplest zircon systematics have increasingly negative ɛHf from oldest to youngest, consistent with involvement of 4.0 Ga or older crust in later generations; also, none of our samples have been derived solely from strongly depleted sources. The presence of intra-zircon variability within samples from the AGC reflects a complex history of magmatic additions requiring melting/assimilation of older rocks consistent with the inherited zircon record (4.2-4.06 Ga)1,5. [1]Bowring and Williams (1999). CoMP, 134(1), 3-16. [2]Iizuka et al (2007). Precambrian Res, 153(3), 179-208. [3]Amelin et al (2000). Geochim Cosmochim Ac, 64(24), 4205-4225. [4] Guitreau, et al (2012). EPSL, 337, 211-223. [5]Iizuka, et al (2006). Geology, 34(4), 245-248.

Developing Zircon as a Probe of Planetary Impact History

NASA Astrophysics Data System (ADS)

Wielicki, Matthew

2014-12-01

The identification of Meteor Crater in Arizona as an extraterrestrial impact by Eugene Shoemaker provided the first evidence of this geologic phenomenon and opened the door to a new field of research that has eventually lead to the identification of over ~150 terrestrial impact structures. Subsequently impacts have been evoked in the formation of the moon, delivery of volatiles and bio-precursors to early Earth, creation of habitats for the earliest life and, in more recent times, major mass extinction events. However, understanding the impact flux to the Earth-Moon system has been complicated by the constant weathering and erosion at Earth's surface and the complex nature of impactite samples such that only a hand full of terrestrial craters have been accurately and precisely dated. Currently 40Ar/39Ar step-heating analysis of impactite samples is commonly used to infer impact ages but can be problematic due to the presence of relic clasts, incomplete 40Ar outgassing or excess 40Ar, and recoil and shock effects. The work presented here attempts to develop zircon geochronology to probe planetary impact histories as an alternative to current methods and provides another tool by which to constrain the bolide flux to the Earth-Moon system. Zircon has become the premier geo-chronometer in earth science and geochemical investigation of Hadean zircon from Western Australia has challenged the long-standing, popular conception that the near-surface Hadean Earth was an uninhabitable and hellish world; Zircons may preserve environmental information regarding their formation and thus provide a rare window into conditions on early Earth. Isotopic and petrologic analyses of these ancient grains have been interpreted to suggest that early Earth was more habitable than previously envisioned, with water oceans, continental crust, and possibly even plate tectonics. The Hadean is also suspected to be a time of major planetary bombardment however identifying impact signatures within the Hadean population remains difficult and this study hopes to develop criteria to recognize impact zircon and possibly provide constraints on the early impactor flux. Five large terrestrial craters, Vredefort and Morokweng, South Africa, Sudbury and Manicouagan, Canada, and Popigai, Russia, are the focus of this study as smaller craters do not have the energy to produce thick melt sheets, which persist over time-scales sufficient for crystallization of zircon, permitting geochemical and geochronological analysis. Geochemical analysis of these impact-produced zircons yields similar chemical signatures to endogenic igneous zircon from crustal melts and highlights the need for well-developed criteria for discriminating impact and endogenic grains for impact geochronology. One such criterion is modeling of impact zircon crystallization temperature spectra for simulated impact events on targets of varying composition. Provided some assumptions the zircon crystallization spectra can be estimated from well established Zr systematics in crustal melts. Results for impacts into an Archean terrestrial surface (used as a proxy for the Hadean as little to no rock record exists >4.0 Ga) yields a crystallization spectra significantly higher than that reported for the Hadean zircon population and appears to rule out impacts as a dominant source for these ancient grains. When no dateable impact melt sheet exists, either due to the lack of energy of the impact itself or from subsequent erosion at Earth's surface, loss of radiogenic lead, Pb*, has been suggested as an alternative method to date the event. Pb*-loss was investigated from target rocks from Vredefort and Morokweng and suggests that Pb* diffusion, even in zircon isolated from shocked and brecciated target rocks, is remarkably slow. This may explain the seeming lack of 'reset' zircon in terrestrial impactites. Little is known about Pb* diffusion pathways associated with shock microstructures introduced during impact cratering and future diffusion studies may provide better constraints on this problem. Although little disturbance was identified in Pb* of target zircon, other low temperature geochronometers, zircon (U-Th)/He dating in this case, have been shown to be completely 'reset' and accurately date impacts. Zircon (U-Th)/He ages isolated from the target rock below ~850 m of well-dated impact melt at Morokweng yield ages consistent with the impact melt sheet and provide an alternative tool to dating events where such melts no longer exists. This geochronometer was also applied to impactites from Popigai, Russia and results in an age that is significantly younger than that reported in the literature and coincident with the Eocene-Oligocene boundary mass extinction event however the lack of any impact signatures at this boundary is puzzling. Constraining the impact flux to the Earth-Moon system not only allows for a better understanding into early Earth evolution and the formation of a habitable planet but also provides constraints on the modern impactor flux, important criteria for estimating the likelihood of future impact events. Zircon geochronology offers an exciting new tool by which to date impact events and has the potential to assist understanding of complex impactite samples from terrestrial craters and future sample return missions.

Hadean Crustal Processes Revealed from Oxygen Isotopes and U-Th-Pb Depth Profiling of Pre-4.0 Ga Detrital Zircons from Western Australia

NASA Technical Reports Server (NTRS)

Trail, D.; Mojzsis, S. J.; Harrison, T. M.

2005-01-01

Because physical and chemical processes of the past are determined from analysis of a preserved geologic record, little is known about terrestrial crustal processes of the first 500 Ma during the so-called Hadean Eon. What is known from direct measurements has been derived almost exclusively from the study of greater than 4.0 Ga detrital zircons from the Jack Hills, Western Australia. The geochemistry of these zircons has direct application to understanding the origin and evolution of the rocks during the Hadean because: (i) U-Th-Pb age determinations by ion microprobe suggests the presence of crust as early as 4.37 Ga, or shortly after lunar formation; (ii) high-resolution U-Th-Pb zircon depth profiles reported here reveal several episodes of zircon growth in the Hadean previously unrecognized; (iii) core regions of pre-4.0 Ga zircons with igneous compositions are enriched in O-18 and contain metaluminous and peraluminous mineral inclusions, both features indicative of S-type grainitod protoliths. Study of these ancient zircons provides a unique window into the first half billion years that permits assessment of the potential of the Hadean Earth to host an emergent biosphere.

High-precision ID-TIMS zircon U-Pb geochronology using new 1013 Ohm resistors

NASA Astrophysics Data System (ADS)

Von Quadt, A.; Buret, Y.; Large, S.; Peytcheva, I.; Trinquier, A.; Wotzlaw, J. F.

2015-12-01

Faraday cups equipped with high gain amplifiers provide a means to measure small ion beams in static mode without the limited linear range of ion counting systems. We tested the application of newly available 1013 Ohm resistors to ID-TIMS zircon U-Pb geochronology using a range of natural and synthetic reference materials. The TritonPlus-RPQ at the Institute of Geochemistry and Petrology, ETH Zurich, is equipped with five new 1013 Ohm resistors and one MasCom secondary electron multiplier, allowing to measure the 202-204-205-206-207-208Pb masses in static mode. U is measured subsequently as U-oxide (265-267-270UO2) during a second step, also in static Faraday mode. The gain calibration of the 1013 Ohm resistors was performed using the procedure of Trinquier (2014), with 144Nd-146Nd being measured using 1011 Ohm resistor and 142-143-145-148-150Nd being measured using 1013 Ohm resitors (Trinquier, 2014; Koornneef et al., 2014). Standard deviations of the noise in all five new 1013 Ohm resistors are lower than 5.0 x 10-6 over a 6 month period, with no shift occurring over this time interval. This new detector set-up was tested by analyzing natural zircon standard materials and synthetic U/Pb solutions (www.earthime.org), ranging in age from ~2 Ma to ~600 Ma. All natural zircon standards were chemically abraded (Mattinson, 2005) and all samples were spiked with the ET2535 tracer solution. U-Pb dates obtained using the static measurement routine are compared to measurements employing dynamic peak jumping routines on the MasCom multiplier. This study illustrates the benefits and current limitations of using high gain amplifiers to measure small ion beams for zircon U-Pb geochronology compared to conventional dynamic ion counting techniques. Mattinson, J.M. (2005) Chemical Geology 220:47-66; Trinquier, A. (2014) Application Note 30281; Koornneef, J. et al (2014) Analytica Chimica Acta 819:49-55.

Development of a petrochronology laboratory in Russia

NASA Astrophysics Data System (ADS)

Yavryan, M.; Powerman, V.; Nourgaliev, D. K.

2017-12-01

Each year more and more U-Pb detrital zircon studies are coming from Russia. However, the latest developments in zircon studies, namely combining U-Pb age with the measurements of REE concentrations and Lu-Hf isotopy on the same grain, have been unavailable in Russia. Kazan Federal U. has undertaken certain efforts in order to develop Russia's first petrochronology laboratory. The following equipment was recently purchased, installed and put into operation: Neptune Plus HR MC ICPMS, with jet-interface and RPQ filter. iCAP-Qc quadruple ICPMS. 2 x NWR213 laser ablation systems; one of them equipped with the TwoVol2 ablation chamber. Helium gas is used to flush the ablation chamber; 5 ml/min. of nitrogen is mixed downstream before plasma in order to increase the signal. The following methods have been set up at KFU on the Neptune Plus ICPMS: (1) U-Pb geochronology on zircons. A set of standards have been dated (R33, 91500, Plesovice, Mudtank, GJ-1, AS-3); (2) U-Pb geochronology on perovskites. We have collected, separated and analyzed Baikal Tazheran 463 Ma perovskite, previously dated on TIMS (e.g., Ireland et al., 1990; Li et al., 2010), using 91500 zircon as a primary standard. We intend to start using Tazheran perovskite as an in-house standard for dating perovskites. (3) Lu-Hf isotopy on a set of standard zircons. All results will be presented during the poster session. Our next steps will include setting the (a) U-Pb geochronology and (b) REE measurements on the quadruple MS; (c) splitting the ablation stream into two and directing them to Neptune for Lu-Hf measurements and to iCAP for either U-Pb, or REE, or both types of measurements; (d) experimenting with pre-ablation in order to diminish common lead, with (e) rastering during ablation to diminish downhole fractionation, with (f) squid signal smoothing device.

Zircon U-Pb age and Hf-O isotopes of felsic rocks from the Atlantis Bank, Southwest Indian Ridge

NASA Astrophysics Data System (ADS)

Liu, C. Z.; Zhang, W. Q.

2017-12-01

Hole U1473A was drilled to 790 meters below seafloor on the Atlantis Bank, an oceanic core complex in the Southwest Indian Ridge, where the upper crust has been removed by detachment faulting. The recovered core consists dominantly of olivine gabbro, with subordinate gabbro, gabbro with varying Fe-Ti oxide concentrations. Felsic veins intermittently occur throughout the whole core section. Zircons separated from twenty-four felsic samples have been conducted for U-Pb dating and O isotope analyses on the Cameca 1280 and Lu-Hf isotopes by laser ablation coupled with a MC-ICPMS. The zircons have highly variable contents of U (12-2078 ppm) and Th (5-801 ppm), yielding Th/U ratios of 0.33-0.81. They are typical oceanic zircons as defined by the trace element discrimination plots of Grimes et al. (2015). The weighted mean 206Pb/238U ages of the analyzed zircons vary from 11.29 to 12.57 Ma. Age differences between felsic veins throughout the whole core are not resolved within analytical uncertainty of the SIMS measurements. All felsic samples have similar zircon Hf isotope compositions, with initial 176Hf/177Hf ratios of 0.283126-0.283197 and ɛHf values of 12.76-15.27. Zircons from all felsic samples but one have mantle-like δ18O values of 5.14-5.50‰. Zircons from one sample show partial resorption or total recrystallization; in comparison, they have lower δ18O values of 4.81±0.21‰. Such characteristics provide clear evidence for hydrothermal alteration after magmatic intrusion.

Episodic intrusion, internal differentiation, and hydrothermal alteration of the miocene tatoosh intrusive suite south of Mount Rainier, Washington

USGS Publications Warehouse

du Bray, E.A.; Bacon, C.R.; John, D.A.; Wooden, J.L.; Mazdab, F.K.

2011-01-01

The Miocene Tatoosh intrusive suite south of Mount Rainier is composed of three broadly granodioritic plutons that are manifestations of ancestral Cascades arc magmatism. Tatoosh intrusive suite plutons have individually diagnostic characteristics, including texture, mineralogy, and geochemistry, and apparently lack internal contacts. New ion-microprobe U-Pb zircon ages indicate crystallization of the Stevens pluton ca. 19.2 Ma, Reflection-Pyramid pluton ca. 18.5 Ma, and Nisqually pluton ca. 17.5 Ma. The Stevens pluton includes rare, statistically distinct ca. 20.1 Ma zircon antecrysts. Wide-ranging zircon rare earth element (REE), Hf, U, and Th concentrations suggest late crystallization from variably evolved residual liquids. Zircon Eu/Eu*-Hf covariation is distinct for each of the Reflection-Pyramid, Nisqually, and Stevens plutons. Although most Tatoosh intrusive suite rocks have been affected by weak hydrothermal alteration, and sparse mineralized veins cut some of these rocks, significant base or precious metal mineralization is absent. At the time of shallow emplacement, each of these magma bodies was largely homogeneous in bulk composition and petrographic features, but, prior to final solidification, each of the Tatoosh intrusive suite plutons developed internal compositional variation. Geochemical and petrographic trends within each pluton are most consistent with differential loss of residual melt, possibly represented by late aplite dikes or erupted as rhyolite, from crystal-rich magma. Crystal-rich magma that formed each pluton evidently accumulated in reservoirs below the present level of exposure and then intruded to a shallow depth. Assembled by episodic intrusion, the Tatoosh intrusive suite may be representative of midsized composite plutonic complexes beneath arc volcanoes. ?? 2011 Geological Society of America.

Timing of metasomatism in a subcontinental mantle: evidence from zircon at Finero (Italy)

NASA Astrophysics Data System (ADS)

Badanina, I. Yu.; Malitch, K. N.

2012-04-01

The Finero phlogopite-peridotite represents a metasomatized residual mantle harzburgite, exposed at the base of the lower-crust section in the Ivrea Zone, Western Alps (Hartmann and Wedepohl 1993). It forms the core of a concentrically zoned sequence of internal layered gabbro, amphibole-rich peridotite and external gabbro. The phlogopite peridotite contains small-size chromitite bodies, with a suite of accessory minerals such as phlogopite, apatite, Ca-Mg carbonates, zirconolite, zircon, thorianite and uraninite, proposed to form during alkaline-carbonatitic metasomatism process within the mantle (Zaccarini et al. 2004). In this study, the combined application of a non-destructive technique to separate zircon from their host rocks (see details at http://www.natires.com) and in-situ analytical technique for compositional and isotopic analysis (SHRIMP-II at Russian Geological Research Institute, St. Petersburg) has provided new more detailed age constraints on the formation of chromitite and related metasomatic events within a mantle tectonite at Finero. Chromitite samples derived from the dump in the prospecting trenches of Rio Creves. In thin sections, zircon occurs as relatively large (up to 200 μm) grains characterized by subhedral to euhedral shapes. Separated grains of zircon form two distinct populations. Dominant zircon population is pale pink and characterized by different shapes (subhedral, subrounded or elongated). In cathodoluminescense, the main set of population is represented by complex grains, which show development of core-rim relationship (most likely recrystallized rim on a preserved core). Subordinate zircon grains are colourless. They are characterized by a smoky cathodoluminescense, with almost no internal pattern. Three main U-Pb age clusters have been recognized. The youngest age cluster, typical for subordinate colourless zircon population and rims in complex grains of dominant pale pink population, show two concordant 206Pb/238U ages (e.g., 208.6 ± 4.0 Ma, MSWD=2.0; P=0.16, n=8 and 194.9 ± 3.4 Ma, MSWD=0.45; P=0.50, n=3, respectively). Other age clusters are characterized by the cores and rims observed in composite grains. They yielded concordant 206Pb/238U ages of 288.3 ± 7.3 Ma (MSWD=3.3, n=6) and 248.6 ± 3.3 Ma (MSWD=0.13, P=0.72, n=8), respectively. Since the pioneering work of Exley et al. (1982), the complex metasomatic history at Finero has received much attention. New U-Pb results are consistent with the age range obtained for mantle rocks, the phlogopite peridotite (293 ± 13 Ma, Voshage et al. 1987) and chromitite (208 ± 2 Ma, Grieco et al. 2001). The former age estimate, based on a Rb-Sr whole-rock isochron for six phlogopite-bearing peridotites and one phlogopite pyroxenite, has been interpreted as time of K metasomatic enrichment of the harzburgite. This event has been coeval with the intrusion of alkaline ultramafic magmas into the deep crust of the Ivrea Zone during the late Carboniferous (287 ± 3 Ma, Garuti et al. 2001). The U-Pb age of 208±2 Ma for zircon at Alpe Polunia, attributed by Grieco et al. (2001) to one of the major metasomatic episodes, is corroborated by a subordinate subset of zircon grains at Rio Creves. The U-Pb zircon ages identified in this study thus show notable differences. Our U-Pb data do not concur with the assumption of a single metasomatic event during chromitite formation. In contrast, we suggest a prolonged formation and multistage evolution of zircon growth, as mirrored by multiple U-Pb ages. U-Pb results for zircons from two chromitite localities (Alpe Polunia and Rio Creves) place tight constraints on their different temporal evolution. We presume that Hf-isotope data of zircon and Os-isotope data of laurite, to be investigated in the future, will shed new light on the sources of materials involved in a subcontinental mantle at Finero. This investigation was supported by Uralian Branch of Russian Academy of Sciences (grant 12-P-5-1020).

U-Pb SHRIMP II age and origin of zircon from lhertzolite of the bug Paleoarchean complex, Ukrainian Shield

NASA Astrophysics Data System (ADS)

Lobach-Zhuchenko, S. B.; Baltybaev, Sh. K.; Glebovitsky, V. A.; Sergeev, S. A.; Lokhov, K. O.; Egorova, Yu. S.; Balagansky, V. V.; Skublov, S. G.; Galankina, O. L.; Stepanyuk, L. M.

2017-12-01

Complex study of the U-Pb and Lu-Hf systems of zircon from a lhertzolite lens of Archean gneiss enderbites of the Bug complex, Ukrainian Shield, showed that ultramafic magma was contaminated by the material of the country gneiss enderbites. The age of the zircons of 2.81 ± 0.05 Ga corresponds to the period of ultramafic magmatism within the Bug complex. Previously, this peak of endogenic activity was considered the stage of manifestation of metamorphism and magmatism of mafic composition.

Hunting for eruption ages in accessory minerals

NASA Astrophysics Data System (ADS)

Vazquez, J. A.

2012-12-01

A primary goal in geochronology is to provide precise and accurate ages for tephras that serve as chronostratigraphic markers for constraining the timing and rates of volcanism, sedimentation, climate change, and catastrophic events in Earth history. Zircon remains the most versatile accessory mineral for dating silicic tephras due to its common preservation in distal pyroclastic deposits, as well as the robustness of its U-Pb and U-series systems even after host materials have been hydrothermally altered or weathered. Countless studies document that zircon may be complexly zoned in age due to inheritance, contamination, recycling of antecrysts, protracted crystallization in long-lived magma reservoirs, or any combination of these. Other accessory minerals such as allanite or chevkinite can retain similar records of protracted crystallization. If the goal is to date the durations of magmatic crystallization, differentiation, and/or magma residence, then these protracted chronologies within and between accessory minerals are a blessing. However, if the goal is to date the timing of eruption with high precision, i.e., absolute ages with millennial-scale uncertainties, then this age zoning is a curse. Observations from ion microprobe 238U-230Th dating of Pleistocene zircon and allanite provide insight into the record of near-eruption crystallization in accessory minerals and serve as a guide for high-precision whole-crystal dating. Although imprecise relative to conventional techniques, ion probe analysis allows high-spatial resolution 238U-230Th dating that can document multi-millennial age distributions at the crystal scale. Analysis of unpolished rims and continuous depth profiling of zircon from small and large volume eruptions (e.g., Coso, Mono Craters, Yellowstone) reveals that the final several micrometers of crystallization often yield ages that are indistinguishable from associated eruption ages from the 40Ar/39Ar or (U-Th)/He methods. Using this approach, we have derived relatively precise (± ~ 5%, 2σ) U-Th isochron ages from the unpolished rims of pumice-derived allanite and zircon from late Pleistocene Wilson Creek Formation tephras in eastern California, whose ages are controversial and have been difficult to resolve via 40Ar/39Ar and radiocarbon dating. Allanite and zircon rims from Ashes 7-19 in the lower portion of Wilson Creek sediments yield stratigraphically consistent ages of ca. 27 ka to ca. 62 ka, with a minority of crystals identifiable as xenocrysts from early Mono Craters rhyolites. Model ages for the interiors of allanite crystals are mostly < 10 k.y. older than their rims. Tephra deposited during the geomagnetic excursion debated to be either the Mono Lake or Laschamp event yields a rim isochron age of ca. 41 ka. This age is indistinguishable from an independent age of 41 ka derived at the latter excursion's type locality in France (Singer et al., 2009) as well as from age-models for deep-sea sediments. The results are in excellent agreement with a previously determined chronology derived from magnetostratigraphy (Zimmerman et al., 2006). Refs: Singer et al., 2009, EPSL 286: 80-88; Zimmerman et al., 2006, EPSL 252: 94-106

Using U-Pb Detrital Zircon Geochronology to Study Ice Streams in the Weddell Sea Embayment, Antarctica

NASA Astrophysics Data System (ADS)

Agrios, L.; Licht, K.; Hemming, S. R.; Williams, T.

2016-12-01

Till from major ice streams of the Weddell Sea Embayment contain detrital zircons with distinct U-Pb age populations that can be used as a provenance tool to better understand ice stream dynamics. The ice streams in this study include the Foundation Ice Stream, and Academy, Slessor, and Recovery glaciers, all of which drain ice from the continent's interior into the Weddell Sea. Characterizing the U-Pb detrital zircon ages in till and rocks will (1) provide the zircon provenance signatures of the material carried by the ice stream - when these signatures are found in LGM and older deposits downstream they can enable interpretation of past ice flow history; and (2) constrain ice-covered upstream bedrock geology that supplies the till carried by ice streams and glaciers. U-Pb ages of detrital zircons were measured in 21 samples of onshore till, erratics, and bedrock of potential source rocks. Grains were analyzed by LA-ICPMS at the University of Arizona (n=300). Relative probability U-Pb age density plots of till in moraines along the Foundation Ice Stream and Academy Glacier show prominent peaks at 500-530 and 615-650 Ma, which overlap with the timing of the Ross and Pan-African orogenies. Zircon ages of 1000-1095 Ma are also present. Local bedrock in the Patuxent Range has the most prominent peak at 510 Ma, suggesting the till is predominantly derived from local Patuxent Formation. However, local bedrock also has fewer grains at 1030 Ma which suggests that this age population is carried in the till as well. Prominent peaks in U-Pb ages from till transported by the Recovery Glacier are 530, 635, 1610 and 1770 Ma. Bedrock of this area contains similar age peaks, with the exception of the 635 Ma peak, suggesting that this ice stream is carrying a signature from an unexposed source of this age completely buried by ice. The Slessor Glacier carries zircons with prominent populations at 1710 and 2260-2420 Ma, which overlap with a high-grade metamorphic event in the Shackleton Range between 1710-1680 Ma. In order to gain the offshore signature of ice streams, these data will be compared to 40Ar/39Ar hornblende and biotite thermochronological data, and U-Pb geochronology data from subglacial till and proximal glaciomarine sediment from existing core sites located at the edge of the Ronne-Filchner Ice Shelf.

Generation of Silicic Melts in the Early Izu-Bonin Arc Recorded by Detrital Zircons in Proximal Arc Volcaniclastic Rocks From the Philippine Sea

NASA Astrophysics Data System (ADS)

Barth, A. P.; Tani, K.; Meffre, S.; Wooden, J. L.; Coble, M. A.; Arculus, R. J.; Ishizuka, O.; Shukle, J. T.

2017-10-01

A 1.2 km thick Paleogene volcaniclastic section at International Ocean Discovery Program Site 351-U1438 preserves the deep-marine, proximal record of Izu-Bonin oceanic arc initiation, and volcano evolution along the Kyushu-Palau Ridge (KPR). Pb/U ages and trace element compositions of zircons recovered from volcaniclastic sandstones preserve a remarkable temporal record of juvenile island arc evolution. Pb/U ages ranging from 43 to 27 Ma are compatible with provenance in one or more active arc edifices of the northern KPR. The abundances of selected trace elements with high concentrations provide insight into the genesis of U1438 detrital zircon host melts, and represent useful indicators of both short and long-term variations in melt compositions in arc settings. The Site U1438 zircons span the compositional range between zircons from mid-ocean ridge gabbros and zircons from relatively enriched continental arcs, as predicted for melts in a primitive oceanic arc setting derived from a highly depleted mantle source. Melt zircon saturation temperatures and Ti-in-zircon thermometry suggest a provenance in relatively cool and silicic melts that evolved toward more Th and U-rich compositions with time. Th, U, and light rare earth element enrichments beginning about 35 Ma are consistent with detrital zircons recording development of regional arc asymmetry and selective trace element-enriched rear arc silicic melts as the juvenile Izu-Bonin arc evolved.

U-Pb Geochronology of Grandite Skarn Garnet: Case Studies From Jurassic Skarns of California

NASA Astrophysics Data System (ADS)

Gevedon, M. L.; Seman, S.; Barnes, J.; Stockli, D. F.; Lackey, J. S.

2016-12-01

We present 3 case studies using a new method for U-Pb dating grossular-andradite (grandite) skarn garnet via LA-ICP-MS (Seman et al., in prep). Grandite is commonly rich in U, with high Fe3+ contents generally correlating with higher U concentrations. Micron-scale non-radiogenic Pb heterogeneities allow for regression of age data using Tera-Wasserberg concordia. Although others have dated accessory skarn minerals, garnet U-Pb ages are powerful because garnet grows early and is nearly ubiquitous in skarns, resists alteration, and provides a formation age independent of that of the causative pluton. The Darwin stock (Argus range, eastern CA) was likely a short-lived, single pulse of magmatism, genetically related to the Darwin skarn. A robust skarn garnet U-Pb age of 176.8 ± 1.3 Ma agrees well with the pluton U-Pb zircon age of 175 Ma (Chen and Moore, 1982). Furthermore, zircon separated from, and in textural equilibrium with, exoskarn garnetite yields a U-Pb age of 176.8 ± 1 Ma. Such agreement between plutonic and skarn zircon ages with a skarn garnet age in a geologically simple field area is the ideal scenario for establishing grandite U-Pb as a viable tool for directly dating skarns. The Black Rock skarn (BRS; eastern CA) is more complex: multiple plutons and ambiguous field relations complicate determination of a causative pluton. A skarn garnet U-Pb age of 172.0 ± 3 Ma confirms a middle Jurassic BRS formation age. Investigation of 4 local plutons yield zircon U-Pb ages of 222 ± 3 Ma, 213 ± 4 Ma, 207 ± 4 Ma and 176.2 ± 2 Ma. Comparison of the skarn garnet U-Pb and pluton ages suggest the BRS is genetically related to the youngest pluton, providing basis for further field and geochemical investigation. The Whitehorse skarn (WS; Mojave Desert, CA) lies in an important region for studying the changing tectono-magmatic regime of the Jurassic North American Cordillera; basin fill suggests a tectonically-controlled oscillating regional shoreline (Busby, 2012). Values of δ18O of WS garnet are strongly negative, —9.8‰ to 1.2‰, and can only be the result of access to meteoric water. A grandite U-Pb age of 162.3 ± 2 Ma requires the region of the WS not be submerged below the sea at this time. Future work coupling grandite U-Pb and δ18O may be vital in establishing the timing of Jurassic Cordilleran tectonic changes in the Mojave Desert.

Mesozoic high-Mg andesites from the Daohugou area, Inner Mongolia: Upper-crustal fractional crystallization of parental melt derived from metasomatized lithospheric mantle wedge

NASA Astrophysics Data System (ADS)

Meng, Fanxue; Gao, Shan; Song, Zhaojun; Niu, Yaoling; Li, Xuping

2018-03-01

Mineral chemistry, major- and trace-element data, zircon U-Pb ages, and Sr-Nd isotopic data are presented for a suite of volcanic rocks from the Daohugou area, Ningcheng City, Inner Mongolia, on the northern margin of the North China Craton. Samples from the suite are of basaltic andesite to rhyolite in composition, with the rocks containing <60 wt% SiO2 have high MgO, Cr, and Ni contents, and classify as high-Mg andesites (HMAs). Zircons from a rhyolite yielded weighted mean 206Pb/238U age of 164 ± 1 Ma, indicating that the Daohugou volcanic suite is coeval with the Tiaojishan Formation of northern Hebei and western Liaoning Province. The HMAs have similar enriched-mantle I (EMI)-type isotopic compositions to each other, with low εNd (t) values, moderate (87Sr/86Sr) i ratios, enrichment in LREEs relative to LILEs, and depletion in HFSEs (e.g., Nb, Ta, Ti), indicating formation through protracted fractional crystallization of a common parental magma. The unusually low CaO contents and CaO/FeO ratios of olivine phenocrysts in the HMAs suggest that the parental melt was subduction-related. The results of Rhyolite-MELTS modelling indicates that HMAs may form through upper-crustal fractional crystallization from arc basalts. Therefore, the Daohugou HMAs were most likely formed through fractional crystallization of a parental melt derived from metasomatized lithospheric mantle at crustal depths. The addition of "water" to the cratonic keel may have played a key role in the destruction of the North China Craton.

Geochemistry, mineralogy, and zircon U-Pb-Hf isotopes in peraluminous A-type granite xenoliths in Pliocene-Pleistocene basalts of northern Pannonian Basin (Slovakia)

NASA Astrophysics Data System (ADS)

Huraiová, Monika; Paquette, Jean-Louis; Konečný, Patrik; Gannoun, Abdel-Mouhcine; Hurai, Vratislav

2017-08-01

Anorogenic granite xenoliths occur in alkali basalts coeval with the Pliocene-Pleistocene continental rifting of the Pannonian Basin. Observed granite varieties include peraluminous, calcic to peralkalic, magnesian to ferroan types. Quartz and feldspars are dominant rock-forming minerals, accompanied by minor early ilmenite and late magnetite-ulvöspinel. Zircon and Nb-U-REE minerals (oxycalciopyrochlore, fergusonite, columbite) are locally abundant accessory phases in calc-alkalic types. Absence of OH-bearing Fe, Mg-silicates and presence of single homogeneous feldspars (plagioclase in calcic types, anorthoclase in calc-alkalic types, ferrian Na-sanidine to anorthoclase in alkalic types) indicate water-deficient, hypersolvus crystallization conditions. Variable volumes of interstitial glass, absence of exsolutions, and lacking deuteric hydrothermal alteration and/or metamorphic/metasomatic overprint are diagnostic of rapid quenching from hypersolidus temperatures. U-Pb zircon ages determined in calcic and calc-alkalic granite xenoliths correspond to a time interval between 5.7 and 5.2 Ma. Positive ɛHf values (14.2 ± 3.9) in zircons from a 5.2-Ma-old calc-alkalic granite xenolith indicate mantle-derived magmas largely unaffected by the assimilation of crustal material. This is in accordance with abundances of diagnostic trace elements (Rb, Y, Nb, Ta), indicating A1-type, OIB-like source magmas. Increased accumulations of Nb-U-REE minerals in these granites indicate higher degree of the magmatic differentiation reflected in Rb-enrichment, contrasting with Ba-enrichment in barren xenoliths. Incipient charnockitization, i.e. orthopyroxene and ilmenite crystallization from interstitial silicate melt, was observed in many granite xenoliths. Thermodynamic modeling using pseudosections showed that the orthopyroxene growth may have been triggered by water exsolution from the melt during ascent of xenoliths in basaltic magma. Euhedral-to-skeletal orthopyroxene growth probably reflects contrasting ascent rates of basaltic magma with xenoliths, intermitted by the stagnation in various crustal levels at a <3 kbar pressure. The Tertiary suite of intra-plate, mantle-derived A1-type granites and syenites is geochemically distinct from pre-Tertiary, post-orogenic A2-type granites of the Carpatho-Pannonian region, which exhibit geochemical features diagnostic of crustal melting along continental margins.

On the longevity of silicic magma based on multi-isotope investigation of zircons and modeling their survivals destinies

NASA Astrophysics Data System (ADS)

Bindeman, I. N.; Wotzlaw, J. F.; Melnik, O. E.

2015-12-01

Large volumes of crystal poor, near-liquidus rhyolites are erupted worldwide as tuffs and lavas in rift and hot spots more common previously on early earth, creating temporally very high magma production rates. In this contribution we combine results of IDTIMS dating of zircons with numerical modeling of zircon crystallization. New investigation of zircons in major Yellowstone tuffs: Huckleberry Ridge (Members A,B,C), Mesa Falls, and Lava Creek (A,B) tuffs was done by a combination of in situ measurements of oxygen isotopes followed by ID-TIMS U-Pb dating, Hf isotopes and trace elemental investigation of single crystals. We discover that nearly all zircons are of eruption age, but display significant isotope (O,Hf) diversity and often show decoupled O and Hf isotope systematics. This record rapid (~103yrs) double or triple remelting and sequestration from diverse Archean crust and hydrothermally altered shallow-crustal rocks from previous eruptive cycles, followed by effective mixing of co-existing magma reservoirs with diverse zircons prior to eruptions. Similar results characterize other studied Snake River Plain rhyolites in pre-Yellowstone Heise complex. These results collectively suggest that zircons crystallize after reheating above saturation rejuvenation in isotopically-diverse areas of the crust in the magma plumbing system. Modeling of zircon and quartz dissolution and crystallization trajectories outline conditions of survival (inheritance) vs complete dissolution on conductive timescales, and when combined with a phase diagram, magma T-t paths can be computed. Zircon rejuvenation requires hot, >770-800°C peak temperatures lasting 10-102yrs. We speculate that near liquidus hot and dry Yellowstone rhyolites are kept alive in a multi-batch state by a series of interconnected pods and sills that can rapidly get thermomechanically assembled into large, shallow and eruptable supervolcanoic magma bodies. We suggest that overpressure and roof dynamics and rheology plays a more important role than magma buyoncy. The runaway batch assembly process creates temporally very high magma production rates, orders of magnitude higher than for arc volcanoes. Such views have implication for the state of the magma chamber under Yellowstone and similar supervolcanoes elsewhere.

Multi-stage evolution of xenotime-(Y) from Písek pegmatites, Czech Republic: an electron probe micro-analysis and Raman spectroscopy study

NASA Astrophysics Data System (ADS)

Švecová, E.; Čopjaková, R.; Losos, Z.; Škoda, R.; Nasdala, L.; Cícha, J.

2016-12-01

The chemical variability, degree of radiation damage, and alteration of xenotime from the Písek granitic pegmatites (Czech Republic) were investigated by micro-chemical analysis and Raman spectroscopy. Dominant large xenotime-(Y) grains enriched in U, Th and Zr crystallized from a melt almost simultaneously with zircon, monazite and tourmaline. Xenotime is well to poorly crystalline depending on its U and Th contents. It shows complex secondary textures cutting magmatic growth zones as a result of its interaction with F,Ca,alkali-rich fluids during the hydrothermal stage of the pegmatite evolution. The magmatic xenotime underwent intense secondary alteration, from rims inwards, resulting in the formation of inclusion-rich well crystalline xenotime domains of near end-member composition. Two types of recrystallization were distinguished in relation to the type of inclusions: i) xenotime with coffinite-thorite, cheralite and monazite inclusions and ii) xenotime with zirconcheralite and zircon inclusions. Additionally, inner poorly crystalline U,Th-rich xenotime domains were locally altered, hydrated, depleted in P, Y, HREE, U, Si and radiogenic Pb, and enriched in fluid-borne cations (mainly Ca, F, Th, Zr, Fe). Interaction of radiation-damaged xenotime with hydrothermal fluids resulted in the disturbance of the U-Th-Pb system. Alteration of radiation-damaged xenotime was followed by intensive recrystallization indicating the presence of fluids >200 °C. Subsequently other types of xenotime formed as a consequence of fluid-driven alteration of magmatic monazite, and Y,REE,Ti,Nb-oxides or crystallized from hydrothermal fluids along cracks in magmatic monazite and xenotime.

Geochronological study of zircons from continental crust rocks in the Frido Unit (southern Apennines)

NASA Astrophysics Data System (ADS)

Laurita, Salvatore; Prosser, Giacomo; Rizzo, Giovanna; Langone, Antonio; Tiepolo, Massimo; Laurita, Alessandro

2015-01-01

Zircon crystals have been separated from gneisses and metagranitoids of the Pollino area (southern Apennines) in order to unravel the origin of these crustal slices within the ophiolite-bearing Frido Unit. The morphology of the zircon has been investigated by SEM, and the internal structure was revealed by cathodoluminescence. Data obtained by U/Pb dating have been used to deduce the age and significance of the different crystallization stages of zircon, connected to the evolutionary stages of the continental crust (Late Paleozoic-Early Mesozoic). Zircons in gneisses are characterized by inherited cores of magmatic origin, bordered by metamorphic rims. Inherited zircons generally show Paleoproterozoic to Ordovician ages, indicating the provenance of the sedimentary protolith from different sources. The exclusive presence of Late Neoproterozoic zircon cores in leucocratic gneisses may suggest a different magmatic source possibly connected to Pan-African events. Late Carboniferous-Early Permian ages are found mainly in zircon rims of metamorphic origin. These are similar to the emplacement ages of protolith of the metagranites in the middle crust portion. Late Carboniferous-Early Permian metamorphism and magmatism testify the extensional collapse of the Hercynian belt, recorded in European, particularly, in the Corsica-Sardinia block and in Calabria. Late Permian-Triassic ages have been detected in zircon rims from gneisses and metagranitoids. These younger ages appear related to deformation and emplacement of albite-quartz veins in both lithologies, and are related to an extensional episode predating the Middle Triassic to Middle Jurassic rifting in the Tethyan domain, followed by Middle to Late Jurassic spreading.

New Robust Reference Materials for In Situ Single Grain Rutile U-Pb Geochronology and Method Refinements for Detrital Rutile Analysis by LA-MC-ICP-MS

NASA Astrophysics Data System (ADS)

Parrish, R. R.; Bracciali, L.; Condon, D. J.; Horstwood, M. S.; Najman, Y.

2012-12-01

While rutile (TiO2) occurs in the heavy mineral suite of detrital sediments and originates mainly in medium- to high-grade metamorphic and some igneous rocks, there are very few applications of U-Pb dating of rutile to provenance studies; this is due to an overreliance on zircon, low U content of rutile limiting measurement quality by in situ methods, a higher proportion of common Pb relative to zircon, and a lack of widely available good quality reference materials. We have addressed these issues and characterized two ~ 1.8 Ga rutile reference materials by SEM, trace elements, U-Pb ID-TIMS, and intra-grain and inter-grain U-Pb LA-MC-ICP-MS analysis using mixed faraday and multiple ion counting detectors with high sensitivity. We have assessed U-Pb discordance and in situ variations in relative common Pb and age and their bearing on the quality of the reference materials for in situ U-Pb dating. The rutiles (Sugluk-4 and PCA-S207) come from granulite facies belts of the Canadian Shield, namely the northern Cape Smith Belt of Quebec and the Snowbird Tectonic Zone (Sasatchewan). The ID-TIMS data are slightly discordant due to variable common Pb and limited Pb loss; the variation in 6 single grains of Sugluk-4, that we use as the primary reference material, is <1% in 206Pb/238U, and <2% for 207Pb/206Pb (95 % conf.); after common Pb correction these variations are <1%. The measured variations are smaller than in existing reference materials (i.e. R10) in current use. LA-ICP-MC-MS data (n ~ 500 for each) have a reproducibility of 206Pb/238U and 207Pb/206Pb of ~2-4% (at the 2S level), which is only modestly worse than long-term data for multiple zircon standards, this being due to the real variation in measured values arising from limited Pb loss, age variation, and common Pb variability [1]. We have applied our refined method to the provenance of rutile from drainages from British Columbia, Bhutan, and the Brahmaputra River of NE India (predominant rutile ages ~ 50, 15, and 2 Ma, respectively; Bracciali et al., this meeting). Our method successfully dates >75% of all rutile grains in a sediment; unsuccessful analyses are due to poor quality rutiles with massive common Pb and/or U contents < ~1ppm. While some analyses are therefore unusable, unlike zircon age zoning is rare to absent in rutile and there is little need to image grains to identify 'inheritance' to arrive at a correct interpretation of measured ages. Rutile has a ~ 500°C closure temperature and thus records mainly the time of cooling; it is therefore a sensitive recorder of metamorphic thermochronological information, and an excellent complement to detrital zircon analysis. There appears to be huge scope of in situ application of U-Pb dating to detrital rutile in provenance studies in the future. [1] Bracciali L., Parrish R.R., Condon D., Horstwood M.S.A., Najman,Y., Two new rutile reference materials for in situ U-Pb LA-MC-ICP-MS dating and applications to sedimentary provenance, submitted to Chem. Geol.

Is the Macquarie Arc (Lachlan Orogen) An Exotic Terrane or Formed on the Gondwanan Margin? Reappraisal by SHRIMP U-Pb Dating of Volcano-Sedimentary Rocks

NASA Astrophysics Data System (ADS)

Buckman, S.; Zhang, Q.; Nutman, A. P.

2017-12-01

A fundamental question concerning the Ordovician Macquarie Arc rocks is did they form within the palaeo-Pacific Ocean and are entirely juvenile, or did they evolve on the periphery of Gondwana? This is a key issue to ongoing debates concerning the growth of the eastern Gondwanan margin throughout the Palaeozoic. This problem is complicated by the arc now occurs as several slices, in post-arc tectonic contact with the eastern Gondwanan Ordovician Adaminaby Group. The dispersal of the arc as tectonic slices means that the temporal correlation of lithologies across the extent of the arc's exposure needs to be verified via U-Pb zircon geochronology. Our zircon U-Pb geochronology reveals that samples with the oldest zircons of Palaeozoic volcanic origin do not contain any Gondwanan-sourced zircons (particularly Cambrian and Neoproteorzoic). These samples, particularly some ascribed to the Weemalla Formation and Mitchell Formation have unimodal zircon populations of 450.5 Ma and 479.8 Ma. On the other hand, some samples with somewhat younger volcanic populations of 415 Ma and 458 Ma contain some Gondwanan-sourced older detrital zircons as well. Some of these latter samples are derived from outcrops that have previously consigned to the Yarrimbah Formation, should be older than, or equivalent to, the samples with the unimodal volcanic zircon populations of 480 Ma. This shows clearly that the consignment of some Macquarie Arc units to particular formations needs to be revised. The geochemical and radiogenic isotopic characteristics of the Macquarie Arc indicate that it is dominated by products of an intra-oceanic island arc that developed contemporaneously but spatially separated from Adaminaby Group passive margin sedimentation along eastern Gondwana. However, because our new U-Pb zircon data reveals that only samples with the youngest volcanic zircons also contain Gondwanan zircons, it shows that before the death of the arc, it was proximal to the Gondwanan margin, leading to a mixed provenance. This model is being tested by zircon Hf isotopic analysis.

U-Th-Pb geochronology of the Massabesic Gneiss and the granite near Milford, South-Central New Hampshire: New evidence for avalonian basement and taconic and alleghenian disturbances in Eastern New England

USGS Publications Warehouse

Aleinikoff, J.N.; Zartman, R.E.; Lyons, J.B.

1979-01-01

U-Th-Pb systematics for zircon and monazite from Massabesic Gneiss (paragneiss and orthogneiss) and the granite near Milford, New Hampshire, were determined. Zircon morphology suggests that the paragneiss may be volcaniclastic (igneous) in origin, and thus the age data probably record the date (minimum of 646 m.y.) at which the rock was extruded. A two-stage lead-loss model is proposed to explain the present array of data points on a concordia diagram. Orthogneiss ages range only narrowly and are clustered around 475 m.y. Data for the granite of Milford, New Hampshire, are scattered, but may be interpreted in terms of inheritance and modern lead loss, yielding a crystallization age of 275 m.y. This is the only known occurrence of Avalonian-type basement in New Hampshire and as such provides evidence for the location of the paleo-Africa-paleo- North America suture. The geochronology also further documents the occurrence of disturbances during the Ordovician and Permian. ?? 1979 Springer-Verlag.

Long-Term (4 mo) Oxygen Isotope Exchange Experiment between Zircon and Hydrothermal Fluid

NASA Astrophysics Data System (ADS)

Bindeman, I. N.; Schmitt, A. K.; Lundstrom, C.; Golledge, S.

2013-12-01

Knowing oxygen diffusivity in zircon has several critical applications: 1) establishing zircon stability and solubility in hot silica-saturated hydrothermal solutions; 2) deriving metamorphic and magmatic heating timescales from intra-crystal oxygen isotopic gradients; 3) assessing the survivability of oxygen isotopic signatures in Hadean zircons. We report results of a microanalytical investigation of an isotope exchange experiment using a cold-seal pressure apparatus at 850°C and 500 MPa over 4 months duration. Natural zircon, quartz and rutile were sealed with a silica-rich solution doped with 18-O, D, 7-Li and 10-B in a gold capsule. The diffusion length-scales were examined by depth profiling using time-of-flight (TOF) and high-sensitivity dynamic secondary ionization mass spectrometry (SIMS). Starting materials had distinct and homogeneous δ18O: zircon from Mesa Falls tuff of Yellowstone (+3.6‰), rutile from Karelia (-29‰), Bishop Tuff Quartz (+8.4‰), and δ18O doped water (+400‰). Starting material zircon showed invariant 18O/16O during depth profiling. After the 4 month experiment, rutile crystal surfaces displayed etching (100's of nm), while zircon exteriors lacked visible change. Quartz was completely dissolved and reprecipitated in a minor residue. Rutile developed ~2 μm long Fickian diffusion profiles largely consistent with the wet diffusion coefficients for rutile previously reported [1]. Surface U-Pb dating of zircon detected no significant Pb loss from the outermost ~300 nm of the crystal face and returned identical core-face ages. We performed δ18O depth profiling of zircon in two directions. First, forward profiles (crystal rim inwards) by dynamic SIMS (no surface treatment besides Au-coating; Cs+ beam of 20 kV impact energy) showed initially high and decreasing 18O/16O over ~130 nm; TOF-SIMS forward profiles using a 2 kV Cs+ sputter beam and 25 kV Bi3+ primary ions on uncoated zircon surfaces (cleaned for 2 min with HF) yielded decreasing 18O/16O over a similar length scale. These profile lengths are largely consistent with wet diffusion coefficient for zircon reported by [2]. In contrast, back-side depth profiling was conducted by dynamic SIMS on a 1 μm thick wafer cut from the zircon by FIB. No significant elevation in 18O/16O was detected when the surface layer was penetrated, consistent with dry diffusion coefficients of [2]. The results suggest that nm-scale SIMS surface analysis of isotope ratios is challenging. We are investigating if they can be critically affected by knock-on effects and/or continuous mixing of a very thin enriched surface layer during depth profiling in our and previous experiments. [1] Moore et al., 1998, Am. Min. 83, 700-711 [2] Watson and Cherniak, 1997, EPSL 148, 537-544

Detrital zircon provenance from three turbidite depocenters of the Middle-Upper Triassic Songpan-Ganzi complex, central China: Record of collisional tectonics, erosional exhumation, and sediment production

USGS Publications Warehouse

Weislogel, A.L.; Graham, S.A.; Chang, E.Z.; Wooden, J.L.; Gehrels, G.E.

2010-01-01

To test the idea that the voluminous upper Middle to Upper Triassic turbidite strata in the Songpan-Ganzi complex of central China archive a detrital record of Dabie ultrahigh-pressure (UHP) terrane unroofing, we report 2080 single detrital U-Pb zircon ages by sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis from 29 eastern Songpan-Ganzi complex sandstone samples. Low (<0.07) Th/U zircons, consistent with crystallization under UHP conditions, are rare in eastern Songpan-Ganzi complex zircon, and U-Pb ages of low Th/U zircons are incompatible with a Dabie terrane source. An unweighted pair group method with arithmetic mean nearest-neighbor analysis of Kolmogorov-Smirnov two-sample test results reveals that the eastern Songpan-Ganzi complex is not a single contiguous turbidite system but is instead composed of three subsidiary depocenters, each associated with distinct sediment sources. The northeastern depocenter contains zircon ages characterized by Paleozoic and bimodally distributed Precambrian zircon populations, which, together with south-to southeast-directed paleocurrent data, indicate derivation from the retro-side of the Qinling-Dabie (Q-D) collisional orogen wedge. In the central depocenter, the dominantly Paleozoic detrital zircon signature and south-to southwest-oriented paleocurrent indicators reflect a profusion of Paleozoic zircon grains. These data are interpreted to reflect an influx of material derived from erosion of Paleozoic supra-UHP rocks of the Dabie terrane in the eastern Qinling-Dabie orogen, which we speculate may have been enhanced by development of a monsoonal climate. This suggests that erosional unroofing played a significant role in the initial phase of UHP exhumation and likely influenced the petrotectonic and structural evolution of the Qinling-Dabie orogen, as evidenced by compressed Triassic isotherms/grads reported in the Huwan shear zone that bounds the Dabie terrane to the north. The central depocenter deposits reflect a later influx of bimodally distributed Precambrian zircon, signifying either a decrease in the influx of Paleozoic zircon grains due to stalled UHP exhumation and/or dilution of the same influx of Paleozoic zircons by spilling of Precambrian zircon from the northeastern depocenter into the central depocenter basin, perhaps due to infilling and bypass of sediment from the northern depocenter or due to initial collapse and constriction of the eastern Songpan-Ganzi complex basin. The southeastern depocenter of the eastern Songpan-Ganzi complex bears significant Paleozoic, Neoproterozoic, and Paleoproterozoic zircon populations derived from the South China block and Yidun arc complex, likely recording nascent uplift of the Longmenshan deformation belt due to impingement of the Yidun arc complex upon the western margin of the South China block. ?? 2010 Geological Society of America.

Contemporaneous alkaline and tholeiitic magmatism in the Ponta Grossa Arch, Paraná-Etendeka Magmatic Province: Constraints from U-Pb zircon/baddeleyite and 40Ar/39Ar phlogopite dating of the José Fernandes Gabbro and mafic dykes

NASA Astrophysics Data System (ADS)

Almeida, Vidyã V.; Janasi, Valdecir A.; Heaman, Larry M.; Shaulis, Barry J.; Hollanda, Maria Helena B. M.; Renne, Paul R.

2018-04-01

We report the first high-precision ID-TIMS U-Pb baddeleyite/zircon and 40Ar/39Ar step-heating phlogopite age data for diabase and lamprophyre dykes and a mafic intrusion (José Fernandes Gabbro) located within the Ponta Grossa Arch, Brazil, in order to constrain the temporal evolution between Early Cretaceous tholeiitic and alkaline magmatism of the Paraná-Etendeka Magmatic Province. U-Pb dates from chemically abraded zircon data yielded the best estimate for the emplacement ages of a high Ti-P-Sr basaltic dyke (133.9 ± 0.2 Ma), a dyke with basaltic andesite composition (133.4 ± 0.2 Ma) and the José Fernandes Gabbro (134.5 ± 0.1 Ma). A 40Ar/39Ar phlogopite step-heating age of 133.7 ± 0.1 Ma from a lamprophyre dyke is identical within error to the U-Pb age of the diabase dykes, indicating that tholeiitic and alkaline magmatism were coeval in the Ponta Grossa Arch. Although nearly all analysed fractions are concordant and show low analytical uncertainties (± 0.3-0.9 Ma for baddeleyite; 0.1-0.4 Ma for zircon; 2σ), Pb loss is observed in all baddeleyite fractions and in some initial zircon fractions not submitted to the most extreme chemical abrasion treatment. The resulting age spread may reflect intense and continued magmatic activity in the Ponta Grossa Arch.

Record of Fluctuating Magmatic Environments, Melt Fractionation, and Mixing of Crystals: Elemental Compositions of Zircon Zones, Spirit Mountain Batholith, Nevada

NASA Astrophysics Data System (ADS)

Miller, C. F.; Claiborne, L. L.; Wooden, J. L.; Mazdab, F. K.; Walker, B. A.

2006-12-01

Spirit Mountain batholith is a large, tilted, subvolcanic intrusion in southernmost Nevada (Walker et al., in press; Lowery Claiborne et al., in press). Field relations and elemental chemistry of rocks ranging from felsic cumulates to leucogranites demonstrate both fractionation and frequent recharging. SHRIMP U-Pb analysis of zircon reveals a 2 m.y. history (17.4-15.3 Ma) for the batholith; almost all of the samples record multiple age populations. Elemental concentrations and zoning patterns document the utility of zircon in tracking magmatic environments and crystal transfer processes and provide important insights into the complex and protracted history of the batholith. The data lend strong support to the Watson et al. (2006) Ti-in-zircon thermometer. At reasonable a(TiO2) between 0.5 and 0.9, all calculated T's are consistent with petrological constraints and granite phase equilibria; using a(TiO2) = 0.7, T ranges from 675-900 C. Over this apparent T interval, which reflects a range in Ti from 3.2-34 ppm, concentrations of Hf (6000-18000 ppm), U (20-5000 ppm), and Th (50-13,000 ppm), and REE patterns all vary dramatically and systematically. Hf, U, Th, and Ce/Ce* are negatively correlated with T; LREE/MREE, MREE/HREE, and Eu/Eu* are positively correlated with T. These variations indicate that zircon preferentially incorporated Zr over Hf (hence falling Zr/Hf); U and Th behaved as strongly incompatible elements in the crystallizing assemblage as a whole; compatibility of REE increased with decreasing atomic number (effect of LREE accessories?); Eu+2 was more compatible (feldspars) and Ce+4 less compatible than equivalent +3 REE. All of these trends are consistent with the observed crystallizing assemblage and with general trends in whole rocks. More striking, however, is intrasample and intragrain variability. Although leucogranite samples have a larger proportion of zircon with compositions indicating low T and growth from fractionated melt, all samples have a very wide, and overlapping, range of zircon compositions. Many grains are very strongly zoned, and some individual grains show almost the full range of calculated T and elemental composition. Rims of zircons from the leucogranites are typically "cold" and evolved, but many have high-T interiors that contrast with host rock. Core-to-rim zoning is commonly not monotonic but rather strongly fluctuating, indicating that during growth zircon experienced episodes of increased T and immersion in less evolved melts. These patterns of compositional variability reveal that zircons survived heating events; were entrained in ascending fractionated melt; and were exchanged among contrasting magma batches. Such a history is consistent with protracted residence in a large, multiply replenished and rejuvenated mushy magma system in which melt fraction fluctuated in space and time.

U-Pb age of the Diana Complex and Adirondack granulite petrogenesis

USGS Publications Warehouse

Basu, A.R.; Premo, W.R.

2001-01-01

U-Pb isotopic analyses of eight single and multi-grain zircon fractions separated from a syenite of the Diana Complex of the Adirondack Mountains do not define a single linear array, but a scatter along a chord that intersects the Concordia curve at 1145 ?? 29 and 285 ?? 204 Ma. For the most concordant analyses, the 207Pb/206Pb ages range between 1115 and 1150 Ma. Detailed petrographic studies revealed that most grains contained at least two phases of zircon growth, either primary magmatic cores enclosed by variable thickness of metamorphic overgrowths or magmatic portions enclosing presumably older xenocrystic zircon cores. The magmatic portions are characterized by typical dipyramidal prismatic zoning and numerous black inclusions that make them quite distinct from adjacent overgrowths or cores when observed in polarizing light microscopy and in back-scattered electron micrographs. Careful handpicking and analysis of the "best" magmatic grains, devoid of visible overgrowth of core material, produced two nearly concordant points that along with two of the multi-grain analyses yielded an upper-intercept age of 1118 ?? 2.8 Ma and a lower-intercept age of 251 ?? 13 Ma. The older age is interpreted as the crystallization age of the syenite and the younger one is consistent with late stage uplift of the Appalachian region. The 1118 Ma age for the Diana Complex, some 35 Ma younger than previously believed, is now approximately synchronous with the main Adirondack anorthosite intrusion, implying a cogenetic relationship among the various meta-igneous rocks of the Adirondacks. The retention of a high-temperature contact metamorphic aureole around Diana convincingly places the timing of Adirondack regional metamorphism as early as 1118 Ma. This result also implies that the sources of anomalous high-temperature during granulite metamorphism are the syn-metamorphic intrusions, such as the Diana Complex.

Detrital zircon U-Pb geochronology, Lu-Hf isotopes and REE geochemistry constrains on the provenance and tectonic setting of Indochina Block in the Paleozoic

NASA Astrophysics Data System (ADS)

Wang, Ce; Liang, Xinquan; Foster, David A.; Fu, Jiangang; Jiang, Ying; Dong, Chaoge; Zhou, Yun; Wen, Shunv; Van Quynh, Phan

2016-05-01

In situ U-Pb geochronology, Lu-Hf isotopes and REE geochemical analyses of detrital zircons from Cambrian-Devonian sandstones in the Truong Son Belt, central Vietnam, are used to provide the information of provenance and tectonic evolution of the Indochina Block. The combined detrital zircon age spectra of all of the samples ranges from 3699 Ma to 443 Ma and shows with dominant age peaks at ca. 445 Ma and 964 Ma, along with a number of age populations at 618-532 Ma, 1160-1076 Ma, 1454 Ma, 1728 Ma and 2516 Ma. The zircon age populations are similar to those from time equivalent sedimentary sequences in continental blocks disintegrated from the East Gondwana during the Phanerozoic. The younger zircon grains with age peaks at ca. 445 Ma were apparently derived from middle Ordovician-Silurian igneous and metamorphic rocks in Indochina. Zircons with ages older than about 600 Ma were derived from other Gondwana terrains or recycled from the Precambrian basement of the Indochina Block. Similarities in the detrital zircon U-Pb ages suggest that Paleozoic strata in the Indochina, Yangtze, Cathaysia and Tethyan Himalayas has similar provenance. This is consistent with other geological constrains indicating that the Indochina Block was located close to Tethyan Himalaya, northern margin of the India, and northwestern Australia in Gondwana.

U-Pb zircon geochronology of plutonism in the northern Peninsular Ranges batholith, southern California: Implications for the Late Cretaceous tectonic evolution of southern California

USGS Publications Warehouse

Premo, Wayne R.; Morton, Douglas M.; Wooden, Joseph L.; Fanning, C. Mark

2014-01-01

Utilizing both sensitive high-resolution ion microprobe (SHRIMP) and conventional isotope dilution–thermal ionization mass spectrometry (ID-TIMS) methods, crystallization and/or emplacement ages have been obtained for a suite of Cretaceous intermediate-composition plutonic samples collected along a roughly E-W–trending traverse through the northern Peninsular Ranges batholith. Previously noted petrologic, mineralogic, and textural differences delineated four major zonations from west to east and raised the need for detailed geochemical and isotopic work. U-Pb zircon geochronology establishes that these zonations are essentially temporally separate. Mean 206Pb/238U ages date the three older zones from west to east at 126–107 Ma, 107–98 Ma, and 98–91 Ma. Despite petrologic differences, a relatively smooth progression of magmatism is seen from west to east. A fourth zone is defined by magmatism at ca. 85 Ma, which represents emplacement of deeper-level plutons east of the Eastern Peninsular Ranges mylonite zone in an allochthonous thrust sheet in the northeastern Peninsular Ranges batholith.The age data presented here differ slightly from those presented in earlier work for similar rocks exposed across the middle and southern portions of the Peninsular Ranges batholith in that our data define a relatively smooth progression of magmatism from west to east, and that the transition from western-type to eastern-type plutonism is interpreted to have occurred at ca. 98 Ma and not at ca. 105 Ma.The progressive involvement of older crustal components in the enrichment of eastern Peninsular Ranges batholith–type magma sources is documented by the occurrence of Proterozoic zircon inheritance within samples of the eastern part of the batholith.

Jurassic cooling ages in Paleozoic to early Mesozoic granitoids of northeastern Patagonia: 40Ar/39Ar, 40K-40Ar mica and U-Pb zircon evidence

NASA Astrophysics Data System (ADS)

Martínez Dopico, Carmen I.; Tohver, Eric; López de Luchi, Mónica G.; Wemmer, Klaus; Rapalini, Augusto E.; Cawood, Peter A.

2017-10-01

U-Pb SHRIMP zircon crystallization ages and Ar-Ar and K-Ar mica cooling ages for basement rocks of the Yaminué and Nahuel Niyeu areas in northeastern Patagonia are presented. Granitoids that cover the time span from Ordovician to Early Triassic constitute the main outcrops of the western sector of the Yaminué block. The southern Yaminué Metaigneous Complex comprises highly deformed Ordovician and Permian granitoids crosscut by undeformed leucogranite dikes (U-Pb SHRIMP zircon age of 254 ± 2 Ma). Mica separates from highly deformed granitoids from the southern sector yielded an Ar-Ar muscovite age of 182 ± 3 Ma and a K-Ar biotite age of 186 ± 2 Ma. Moderately to highly deformed Permian to Early Triassic granitoids made up the northern Yaminué Complex. The Late Permian to Early Triassic (U-Pb SHRIMP zircon age of 252 ± 6 Ma) Cabeza de Vaca Granite of the Yaminué block yielded Jurassic mica K-Ar cooling ages (198 ± 2, 191 ± 1, and 190 ± 2 Ma). At the boundary between the Yaminué and Nahuel Niyeu blocks, K-Ar muscovite ages of 188 ± 3 and 193 ± 5 Ma were calculated for the Flores Granite, whereas the Early Permian Navarrete granodiorite, located in the Nahuel Niyeu block, yielded a K-Ar biotite age of 274 ± 4 Ma. The Jurassic thermal history is not regionally uniform. In the supracrustal exposures of the Nahuel Niyeu block, the Early Permian granitoids of its western sector as well as other Permian plutons and Ordovician leucogranites located further east show no evidence of cooling age reset since mica ages suggest cooling in the wake of crystallization of these intrusive rocks. In contrast, deeper crustal levels are inferred for Permian-Early Triassic granitoids in the Yaminué block since cooling ages for these rocks are of Jurassic age (198-182 Ma). Jurassic resetting is contemporaneous with the massive Lower Jurassic Flores Granite, and the Marifil and Chon Aike volcanic provinces. This intraplate deformational pulse that affected northeastern Patagonia during the Early Jurassic (Sinemurian-Pliensbachian) was responsible for the partial (re)exhumation of the mid-crustal Paleozoic basement along reactivated discrete NE-SW to ENE-WSW lineaments and the resetting of isotopic systems. These new thermochronological data indicate that Early Permian magmatic rocks of the Nahuel Niyeu block were below 300 °C for ca. 20 Ma prior to the onset of the main magmatic episode of the Late Permian to Triassic igneous and metaigneous rocks of the Yaminué block.

Deciphering igneous and metamorphic events in high-grade rocks of the Wilmington complex, Delaware: Morphology, cathodoluminescence and backscattered electron zoning, and SHRIMP U-Pb geochronology of zircon and monazite

USGS Publications Warehouse

Aleinikoff, J.N.; Schenck, W.S.; Plank, M.O.; Srogi, L.A.; Fanning, C.M.; Kamo, S.L.; Bosbyshell, H.

2006-01-01

High-grade rocks of the Wilmington Complex, northern Delaware and adjacent Maryland and Pennsylvania, contain morphologically complex zircons that formed through both igneous and metamorphic processes during the development of an island-arc complex and suturing of the arc to Laurentia. The arc complex has been divided into several members, the protoliths of which include both intrusive and extrusive rocks. Metasedimentary rocks are interlayered with the complex and are believed to be the infrastructure upon which the arc was built. In the Wilmingto n Complex rocks, both igneous and metamorphic zircons occur as elongate and equant forms. Chemical zoning, shown by cathodoluminescence (CL), includes both concentric, oscillatory patterns, indicative of igneous origin, and patchwork and sector patterns, suggestive of metamorphic growth. Metamorphic monazites are chemically homogeneous, or show oscillatory or spotted chemical zoning in backscattered electron images. U-Pb geochronology by sensitive high resolution ion microprobe (SHRIMP) was used to date complexly zoned zircon and monazite. All but one member of the Wilmington Complex crystallized in the Ordovician between ca. 475 and 485 Ma; these rocks were intruded by a suite of gabbro-to-granite plutonic rocks at 434 ?? Ma. Detrital zircons in metavolcanic and metasedimentary units were derived predominantly from 0.9 to 1.4 Ga (Grenvillian) basement, presumably of Laurentian origin. Amphibolite to granulite facies metamorphism of the Wilmington Complex, recorded by ages of metamorphic zircon (428 ?? 4 and 432 ?? 6 Ma) and monazite (429 ?? 2 and 426 ?? 3 Ma), occurred contemporaneously with emplacement of the younger plutonic rocks. On the basis of varying CL zoning patterns and external morphologies, metamorphic zircons formed by different processes (presumably controlled by rock chemistry) at slightly different times and temperatures during prograde metamorphism. In addition, at least three other thermal episodes are recorded by monazite growth at 447 ?? 4, 411 ?? 3, and 398 ?? 3 Ma. ?? 2006 Geological Society of America.

Zircon and monazite petrochronologic record of prolonged amphibolite to granulite facies metamorphism in the Ivrea-Verbano and Strona-Ceneri Zones, NW Italy

NASA Astrophysics Data System (ADS)

Guergouz, Celia; Martin, Laure; Vanderhaeghe, Olivier; Thébaud, Nicolas; Fiorentini, Marco

2018-05-01

In order to improve the understanding of thermal-tectonic evolution of high-grade terranes, we conducted a systematic study of textures, REE content and U-Pb ages of zircon and monazite grains extracted from migmatitic metapelites across the amphibolite to granulite facies metamorphic gradient exposed in the Ivrea-Verbano and Strona-Ceneri Zones (Italy). This study documents the behaviour of these accessory minerals in the presence of melt. The absence of relict monazite grains in the metasediments and the gradual decrease in the size of inherited zircon grains from amphibolite to granulite facies cores indicate partial to total dissolution of accessory minerals during the prograde path and partial melting. The retrograde path is marked by (i) growth of new zircon rims (R1 and R2) around inherited cores in the mesosome, (ii) crystallisation of stubby zircon grains in the leucosome, especially at granulite facies, and (iii) crystallisation of new monazite in the mesosome. Stubby zircon grains have a distinctive fir-tree zoning and a constant Th/U ratio of 0.20. Together, these features reflect growth in the melt; conversely, the new zircon grains with R1 rims have dark prismatic habits and Th/U ratios < 0.1, pointing to growth in solid residues. U-Pb ages obtained on both types are similar, indicating contemporaneous growth of stubby zircon and rims around unresorbed zircon grains, reflecting the heterogeneous distribution of the melt at the grain scale. In the Ivrea-Verbano Zone the interquartile range (IQR) of U-Pb ages on zircon and monazite are interpreted to represent the length of zircon and monazite crystallisation in the presence of melt. Accordingly, they provide an indication on the minimum duration for high-temperature metamorphism and partial melting of the lower crust: 20 Ma and 30 Ma in amphibolite and granulite facies, respectively. In amphibolite facies, zircon crystallisation between 310 and 294 Ma (IQR) is interpreted to reflect metamorphic peak condition and earlier retrograde history; conversely, monazite crystallisation between 297 and 271 Ma (IQR) reflects cooling under 750 °C to a temperature close to the solidus. In granulite facies, zircon crystallisation between 295 and 265 Ma (IQR) is interpreted to reflect high-temperature conditions, which were attained after peak of metamorphism during isothermal decompression and subsequent cooling under 850-950 °C. The observed decrease of U-Pb ages in metamorphic zircon and monazite from amphibolite to granulite facies (i.e. from the middle to the lower crust) is interpreted to record slow cooling and crystallisation of the Variscan orogenic root at the transition from orogenic collapse to opening of the Tethys Ocean.

Mass-spectrometric mining of Hadean zircons by automated SHRIMP multi-collector and single-collector U/Pb zircon age dating: The first 100,000 grains

NASA Astrophysics Data System (ADS)

Holden, Peter; Lanc, Peter; Ireland, Trevor R.; Harrison, T. Mark; Foster, John J.; Bruce, Zane

2009-09-01

The identification and retrieval of a large population of ancient zircons (>4 Ga; Hadean) is of utmost priority if models of the early evolution of Earth are to be rigorously tested. We have developed a rapid and accurate U-Pb zircon age determination protocol utilizing a fully automated multi-collector ion microprobe, the ANU SHRIMP II, to screen and date these zircons. Unattended data acquisition relies on the calibration of a digitized sample map to the Sensitive High Resolution Ion MicroProbe (SHRIMP) sample-stage co-ordinate system. High precision positioning of individual grains can be produced through optical image processing of a specified mount location. The focal position of the mount can be optimized through a correlation between secondary-ion steering and the spot position on the target. For the Hadean zircon project, sample mounts are photographed and sample locations (normally grain centers) are determined off-line. The sample is loaded, reference points calibrated, and the target positions are then visited sequentially. In SHRIMP II multiple-collector mode, zircons are initially screened (ca. 5 s data acquisition) through their 204Pb corrected 207Pb/206Pb ratio; suitable candidates are then analyzed in a longer routine to obtain better measurement statistics, U/Pb, and concentration data. In SHRIMP I and SHRIMP RG, we have incorporated the automated analysis protocol to single-collector measurements. These routines have been used to analyze over 100,000 zircons from the Jack Hills quartzite. Of these, ca. 7%, have an age greater than 3.8 Ga, the oldest grain being 4372 +/- 6 Ma (2[sigma]), and this age is part of a group of analyses around 4350 Ma which we interpret as the age when continental crust first began to coalesce in this region. In multi-collector mode, the analytical time taken for a single mount with 400 zircons is approximately 6 h; whereas in single-collector mode, the analytical time is ca. 17 h. With this productivity, we can produce significant numbers of zircons for statistically limited studies including correlations between age and morphology, mineral-inclusion paragenesis, as well as isotopic studies including Hf and O isotopic compositions, Pu-Xe, and Sm-Nd isotopes.

Separating Multiple Episodes of Partial Melting in Polyorogenic Crust: AN Example from the Haiyangsuo Complex, Northern Sulu Belt, Eastern China

NASA Astrophysics Data System (ADS)

Feng, P.; Wang, L.; Brown, M.; Wang, S.

2017-12-01

Determining the timing, mechanism and source of partial melts in polyorogenic crust is challenging. In the Sulu belt, the tectonic affinity of the Haiyangsuo (HYS) complex is controversial due to its polyphase metamorphic history. Here we use detailed field mapping, petrology, microstructural analysis and zircon geochronology to study thin stromatic leucosomes in host granite gneiss, and crosscutting leucogranite dykes to decipher the melting history. Zircon grains from both granite gneiss and thin leucosomes exhibit core-mantle-rim structures. Zircon cores yield protolith ages of 2.86-2.81 Ga, whereas the mantles and rims yield younger metamorphic/melt crystallization ages of ca. 1.82-1.80 Ga. The mantles are characterized by gray luminescence, flat HREE distribution patterns and relatively low Th/U ratios, indicating crystallization during granulite-facies metamorphism. Whereas rims show bright luminescence, steep HREE distribution patterns and higher Th/U ratios, suggesting they crystallized from melt. The mantles and rims have ɛHf (t) of -18.2 to -11.0. Using 176Lu/177Hf = 0.001, these data project back to the array of ɛHf (t) values for the zircon cores. This demonstrates that the thin leucosomes were derived from the gneiss without any mass input from a mantle source. These features are consistent with an origin of the HYS as part of the eastern margin of the NCC prior to juxtaposition with the Sulu belt. Zircons from the leucogranite dykes also show core-mantle-rim structure. Inherited cores yield concordant 206Pb/238U ages of 776-701 Ma consistent with the dominant age range for protoliths of the UHP metamorphic rocks in the Sulu belt. Zircon mantle and rim domains, which both contain multiphase solid inclusions (Kfs + Pl + Qz and Hem + Pl + Qz in mantles and Kfs + Pl + Qz + Bt in rims), yield melt crystallization ages of 226-217 and 169-156 Ma, respectively. High Sr, low Y and Yb contents, high Sr/Y ratios, and the range of ɛNd (t) values (-18.2- -15.0) and initial 87Sr/86Sr ratios (0.7106 - 0.7146) for the leucogranites are consistent with melting of thickened lower continental crust of the Sulu belt. We interpret the dykes to have been emplaced during post-collisional collapse of the orogenic root of this belt in the Middle-Upper Jurassic.

Petrology of Zircon-Bearing Diogenite Northwest Africa 10666

NASA Technical Reports Server (NTRS)

Tanner, T. B.; Jeffcoat, C. R.; Righter, M.; Berger, E. L.; Lapen, T. J.; Irving, A. J.; Kuehner, S. M.; Fujihara, G.

2017-01-01

The howardite, eucrite, and diogenite (HED) meteorites are a group of achondrites thought to be derived from the asteroid 4 Vesta, though there is active debate as to whether all diogenites are part of the HED suite. Petrologic investigation of the HED meteorite group provides a means of understanding early planetary differentiation processes and early evolution of planets in our solar system. Diogenites are predominantly coarse grained ortho-pyroxenites with some samples containing appreciable amounts of clinopyroxene, olivine, chromite, and plagioclase. Accessory metal, troilite, and apatite are common. Many diogenites are brecciated, however, there are few poorly to unbrecciated samples. Diogenites are important because they may represent the lower crust of 4 Vesta. Although Mg isotope data indicates that the sources of diogenites are ancient, their crystallization ages are difficult to constrain due to their protracted thermal histories. The limited chronologic data for diogenites also limits the ability to test petrogenetic connections with eucrites and even parent body. A reliable and high closure-temperature isotope system, such as U-Pb in zircon, is needed to address the timing of diogenite igneous crystallization. Description of the textures and mineralogy of diogenites are essential to their classification and understanding their formation, in particular, whether all phases are petrogenetically related. Here, we present detailed petrographic data from a rare zircon-bearing feldspathic diogenite, Northwest Africa (NWA) 10666 and provide textural evidence for igneous crystallization of the zircon.

Deformation-related microstructures in magmatic zircon and implications for diffusion

NASA Astrophysics Data System (ADS)

Reddy, Steven Michael; Timms, Nicholas E.; Hamilton, Patrick Joseph; Smyth, Helen R.

2009-02-01

An undeformed glomeroporphyritic andesite from the Sunda Arc of Java, Indonesia, contains zoned plagioclase and amphibole glomerocrysts in a fine-grained groundmass and records a complex history of adcumulate formation and subsequent magmatic disaggregation. A suite of xenocrystic zircon records Proterozoic and Archaean dates whilst a discrete population of zoned, euhedral, igneous zircon yields a SHRIMP U-Pb crystallisation age of 9.3 ± 0.2 Ma. Quantitative microstructural analysis of zircon by electron backscatter diffraction (EBSD) shows no deformation in the inherited xenocrysts, but intragrain orientation variations of up to 30° in 80% of the young zircon population. These variations are typically accommodated by both progressive crystallographic bending and discrete low angle boundaries that overprint compositional growth zoning. Dispersion of crystallographic orientations are dominantly by rotation about an axis parallel to the zircon c-axis [001], which is coincident with the dominant orientation of misorientation axes of adjacent analysis points in EBSD maps. Less common <100> misorientation axes account for minor components of crystallographic dispersion. These observations are consistent with zircon deformation by dislocation creep and the formation of tilt and twist boundaries associated with the operation of <001>{100} and <100>{010} slip systems. The restriction of deformation microstructures to large glomerocrysts and the young magmatic zircon population, and the absence of deformation within the host igneous rock and inherited zircon grains, indicate that zircon deformation took place within a low-melt fraction (<5% melt), mid-lower crustal cumulate prior to fragmentation during magmatic disaggregation and entrainment of xenocrystic zircons during magmatic decompression. Tectonic stresses within the compressional Sunda Arc at the time of magmatism are considered to be the probable driver for low-strain deformation of the cumulate in the late stages of initial crystallisation. These results provide the first evidence of crystal plastic dislocation creep in zircon associated with magmatic crystallisation and indicate that the development of crystal-plastic microstructures in zircon is not restricted to high-strain rocks. Such microstructures have previously been shown to enhance bulk diffusion of trace elements (U, Th and REE) in zircon. The development of deformation microstructures, and therefore multiple diffusion pathways in zircon in the magmatic environment, has significant implications for the interpretation of geochemical data from igneous zircon and the trace element budgets of melts due to the potential enhancement of bulk diffusion and dissolution rates.

Early Mesozoic rift basin architecture and sediment routing system in the Moroccan High Atlas

NASA Astrophysics Data System (ADS)

Perez, N.; Teixell, A.; Gomez, D.

2016-12-01

Late Permian to Triassic extensional systems associated with Pangea breakup governed the structural framework and rift basin architecture that was inherited by Cenozoic High Atlas Mountains in Morocco. U-Pb detrital zircon geochronologic and mapping results from Permo-Triassic deposits now incorporated into the High Atlas Mountains provide new constraints on the geometry and interconnectivity among synextensional depocenters. U-Pb detrital zircon data provide provenance constraints of Permo-Triassic deposits, highlighting temporal changes in sediment sources and revealing the spatial pattern of sediment routing along the rift. We also characterize the U-Pb detrital zircon geochronologic signature of distinctive interfingering fluvial, tidal, and aeolian facies that are preferentially preserved near the controlling normal faults. These results highlight complex local sediment mixing patterns potentially linked to the interplay between fault motion, eustatic, and erosion/transport processes. We compare our U-Pb geochronologic results with existing studies of Gondwanan and Laurentian cratonic blocks to investigate continent scale sediment routing pathways, and with analogous early Mesozoic extensional systems situated in South America (Mitu basin, Peru) and North America (Newark Basin) to assess sediment mixing patterns in rift basins.

U-Pb zircon geochronologycal investigation on the Morro dos Seis Lagos Carbonatite Complex and associated Nb deposit (Amazonas, Brazil)

NASA Astrophysics Data System (ADS)

Rossoni, Marco B.; Bastos Neto, Artur C.; Souza, Valmir S.; Marques, Juliana C.; Dantas, Elton; Botelho, Nilson F.; Giovannini, Arthur L.; Pereira, Vitor P.

2017-12-01

We present results of U-Pb dating (by MC-ICP-MS) of zircons from samples that cover all of the known lithotypes in the Seis Lagos Carbonatite Complex and associated lateritic mineralization (the Morro dos Seis Lagos Nb deposit). The host rock (gneiss) yielded an age of 1828 ± 09 Ma interpreted as the crystallization time of this unit. The altered feldspar vein in the same gneiss yielded an age of 1839 ± 29 Ma. Carbonatite samples provided 3 groups of ages. The first group comprises inherited zircons with ages compatible with the gneissic host rock: 1819 ± 10 Ma (superior intercept), 1826 ± 5 Ma (concordant age), and 1812 ± 27 Ma (superior intercept), all from the Orosirian. The second and the third group of ages are from the same carbonatite sample: the superior intercept age of 1525 ± 21 Ma (MSWD = 0.77) and the superior intercept age of 1328 ± 58 Ma (MSWD = 1.4). The mineralogical study indicates that the ∼1.3 Ga zircons have affinity with carbonatite. It is, however, a tendence rather than a well-defined result. The data allow state that the age of 1328 ± 58 Ma represents the maximum age of the carbonatite. Without the same certainty, we consider that the data suggest that this age may be the carbonatite age, whose emplacement would have been related to the evolution of the K'Mudku belt. The best age obtained in laterite samples (a superior intercept age of 1828 ± 12 Ma) is considered the age of the main source for the inherited zircons related to the gneissic host rock.

The timing of tertiary metamorphism and deformation in the Albion-Raft River-Grouse Creek metamorphic core complex, Utah and Idaho

USGS Publications Warehouse

Strickland, A.; Miller, E.L.; Wooden, J.L.

2011-01-01

The Albion-Raft River-Grouse Creek metamorphic core complex of southern Idaho and northern Utah exposes 2.56-Ga orthogneisses and Neoproterozoic metasedimentary rocks that were intruded by 32-25-Ma granitic plutons. Pluton emplacement was contemporaneous with peak metamorphism, ductile thinning of the country rocks, and top-to-thewest, normal-sense shear along the Middle Mountain shear zone. Monazite and zircon from an attenuated stratigraphic section in the Middle Mountain were dated with U-Pb, using a SHRIMP-RG (reverse geometry) ion microprobe. Zircons from the deformed Archean gneiss preserve a crystallization age of 2532 ?? 33 Ma, while monazites range from 32.6 ?? 0.6 to 27.1 ?? 0.6 Ma. In the schist of the Upper Narrows, detrital zircons lack metamorphic overgrowths, and monazites produced discordant U-Pb ages that range from 52.8 ?? 0.6 to 37.5 ?? 0.3 Ma. From the structurally and stratigraphically highest unit sampled, the schist of Stevens Spring, narrow metamorphic rims on detrital zircons yield ages from 140-110 Ma, and monazite grains contained cores that yield an age of 141 ??2 Ma, whereas rims and some whole grains ranged from 35.5 ?? 0.5 to 30.0 ?? 0.4 Ma. A boudinaged pegmatite exposed in Basin Creek is deformed by the Middle Mountains shear zone and yields a monazite age of 27.6 ?? 0.2 Ma. We interpret these data to indicate two periods of monazite and metamorphic zircon growth: a poorly preserved Early Cretaceous period (???140 Ma) that is strongly overprinted by Oligocene metamorphism (???32-27 Ma) related to regional plutonism and extension. ?? 2011 by The University of Chicago.

Archean crustal evolution of the Narryer Gneiss Terrane, Western Australia, as revealed by the U-Pb age and Hf-isotope compositions of zircon from the granitic gneisses

NASA Astrophysics Data System (ADS)

Sylvester, P.; Souders, K.; Crowley, J. L.; Myers, J.

2011-12-01

The Narryer Gneiss Terrane of the Yilgarn Craton, Western Australia, is an important area for studies of early crustal evolution because of the preservation of (1) detrital zircons of Hadean to Archean age in the Jack Hills and Mt. Narryer metasedimentary belts, and (2) several widespread units of granitic gneisses emplaced between ca. 3.7 and 2.6 Ga. We have analyzed the U-Pb geochronology and Hf-isotope geochemistry of magmatic zircons from 38 samples of the granitic gneisses using laser ablation - (multicollector) - ICPMS. The sample suite is dominated by the Meeberrie gneiss, a banded quartz-microcline-oligoclase-biotite gneiss of monzogranite to granodiorite composition, and the Dugel gneiss, a leucocratic, pegmatite-layered syenogranite gneiss, but gneisses of dioritic to tonalitic composition, as well as less deformed granite sheets, are also represented. Magmatic zircons were identified on the basis of the preservation of oscillatory zoning in BSE and CL images, igneous Th/U ratios (>0.2), and concordant U-Pb isotopic systematics with low common Pb contents. The results indicate many of the gneisses are composed of the products of multiple magmatic events, as has been reported previously for samples of the Meeberrie gneiss (Kinny & Nutman, 1996, Precambrian Res. 78, 165-178). Major ages of magmatism preserved in the gneisses occurred at ca. 3685-3665 Ma, 3620-3565 Ma, 3495-3440 Ma, 3375-3330 Ma, and 3300-3260 Ma. The late granite sheets crystallized at 2710-2645 Ma. Hf-isotope compositions of the zircons trend to less radiogenic values with decreasing age, with ɛHf values of ca. 0 to -5 for 3.7-3.4 Ga gneisses, ca. -1 to -9 for 3.4-3.2 Ga gneisses and ca. -5 to -20 for the late granite sheets. The array of the Hf isotopic compositions with time for the entire sample set are fit well by a regression indicating a source reservoir with a 176Lu/177Hf of 0.022 extracted from the depleted mantle at 3.9 Ga. This suggests that the Narryer gneisses and late granite sheets were derived by repeated partial melting of an Eoarchean mafic to intermediate crustal source that persisted throughout the Archean. The ɛHf evolution trend for the Narryer gneisses plots distinctly above the ɛHf trend of the Hadean mafic crustal reservoir inferred for the source of the Hadean detrital zircons from the Jack Hills. The implication is that the inferred Hadean crustal reservoir was not an important source of the Narryer gneisses, either because it was largely isolated from zones of melting in the crust of the Narryer Gneiss Terrane during the Archean, or because it was simply of limited volume.

Zircon U-Pb and Hf-O isotopes trace the architecture of polymetallic deposits: A case study of the Jurassic ore-forming porphyries in the Qin-Hang metallogenic belt, China

NASA Astrophysics Data System (ADS)

Zhao, Panlao; Yuan, Shunda; Mao, Jingwen; Santosh, M.; Zhang, Dongliang

2017-11-01

The Qin-Hang intra-continental porphyry-skarn Cu polymetallic belt (QHMB) is among the economically important metallogenic belts in South China. The significant differences in the size and metal assemblage of the Jurassic magmatic-hydrothermal ore deposits in this belt remain as an enigma. Here we employ zircon U-Pb and Hf-O isotopes of the Tongshanling and Baoshan Cu-Pb-Zn deposits in the central part of the QHMB to investigate the contrasting metallogenic architecture. Our SIMS zircon U-Pb data indicate that the Tongshanling and Baoshan granodiorite formed at 160 Ma. These rocks show high Mg# values, and negative zircon εHf(t) and high δ18O values suggesting that the magmas of the granodiorite porphyries were mainly generated through the anatexis of older crustal components triggered by the input of mantle-derived magma. The minor content of amphibole phenocrysts, low Sr/Y ratios, negative Eu anomaly, and low zircon Ce4 +/Ce3 + ratios indicate that the porphyries are relatively less oxidized with less water content compared with the ore-bearing porphyries in the Dexing and Yuanzhuding porphyry Cu deposits in the northern and southern part of the QHMB, suggesting that high magmatic water content and oxidation state are important prerequisites for the formation of large size porphyry-skarn copper deposits in the QHMB. The positive correlation between zircon εHf(t) values with the Cu reserves, as well as zircon δ18O values with the Cu/(Cu + Pb + Zn) ratios of the deposits indicate that the magmatic sources exerted a first-order control on the volume and metal assemblage of deposits in the QHMB. The Hf and Nd isotope contour maps indicate that the central part of the QHMB has high potential for Pb-Zn-dominated magmatic-hydrothermal deposits, whereas the northern and southern part of the QHMB are prospective for large Cu deposits. Our results have important implications in formulating regional exploration strategies for Jurassic porphyry-skarn Cu-Pb-Zn deposits in the Qin-Hang belt.

Thermal events documented in Hadean zircons by ion microprobe depth profiles

NASA Astrophysics Data System (ADS)

Trail, Dustin; Mojzsis, Stephen J.; Harrison, T. Mark

2007-08-01

We report the first U-Th-Pb ion microprobe depth profiles of four Hadean zircons from the Jack Hills and Mount Narryer supracrustal belts of the Narryer Gneiss Complex (NGC), Western Australia. This ultra-high spatial resolution technique probes the age and origin of sub-micron features in individual crystals that can record episodes of zircon growth. Near-surface grain dates of 2700 Ma or older are coincident with post-depositional growth/modification. Some ages may coincide with documented pre-deposition metamorphic events for the NGC and igneous emplacement at ca. 3700 Ma. Separate events that do not correlate in time with known geologic episodes prior to the preserved rock record are also present on pre-4000 Ma zircons. We find evidence for a ˜3.9 Ga event, which is coterminous within age uncertainty with one or several large basin-forming impacts (e.g. Nectaris) on the Moon attributed to the late heavy bombardment of the inner solar system.

SHRIMP U-Pb dating, trace elements and the Lu-Hf isotope system of coesite-bearing zircon from amphibolite in the SW Sulu UHP terrane, eastern China

NASA Astrophysics Data System (ADS)

Liu, Fulai; Gerdes, Axel; Zeng, Lingsen; Xue, Huaimin

2008-06-01

In this study, we link mineral inclusion data, trace element analyses, U-Pb age and Hf isotope composition obtained from distinct zircon domains of complex zircon to unravel the origin and multi-stage metamorphic evolution of amphibolites from the Sulu ultrahigh-pressure (UHP) terrane, eastern China. Zircon grains separated from amphibolites from the CCSD-MH drill hole (G12) and Niushan outcrop (G13) were subdivided into two main types based on cathodoluminescence (CL) and Laser Raman spectroscopy: big dusty zircons with inherited cores and UHP metamorphic rims and small clear zircons. Weakly zoned, grey-white luminescent inherited cores preserve mineral inclusions of Cpx + Pl + Ap ± Qtz indicative of a mafic igneous protolith. Dark grey luminescent overgrowth rims contain the coesite eclogite-facies mineral inclusion assemblage Coe + Grt + Omp + Phe + Ap, and formed at T = 732-839 °C and P = 3.0-4.0 GPa. In contrast, white luminescent small clear zircons preserve mineral inclusions formed during retrograde HP quartz eclogite to LP amphibolite-facies metamorphism (T = 612-698 °C and P = 0.70-1.05 GPa). Inherited zircons from both samples yield SHRIMP 206Pb/238U ages of 695-520 Ma with an upper intercept age of 800 ± 31 Ma. The UHP rims yield consistent Triassic ages around 236-225 and 239-225 Ma for G12 and G13 with weighted means of 229 ± 3 and 231 ± 3 Ma, respectively. Small clear zircons from both samples give 206Pb/238U ages around 219-210 Ma with a weighted mean of 214 ± 3 Ma, interpreted as the age of retrograde quartz eclogite-facies metamorphism. Matrix amphibole from both samples indicate Ar-Ar ages of 209 ± 0.7 and 207 ± 0.7 Ma, respectively, probably dating late amphibolite-facies retrogression. The data suggest subduction of Neoproterozoic mafic igneous rocks to UHP conditions in Middle Triassic (∼230 Ma) times and subsequent exhumation to an early HP (∼214 Ma) and a late LP stage (∼208 Ma) over a period of ∼16 and 6 Myr, respectively. Thus, early exhumation from a mantle depth of 120-100 km to about 60 km occurred at an average rate of 0.3 cm/y, while subsequent exhumation to a middle crustal level took place at approximately 0.54 cm/y. These exhumation rates are considerably slower than those obtained for UHP rocks in the Dora Maira and Kokchetav massifs (2-3 cm/y). Based on similar P-T estimates and trace element and Hf isotope compositions, Sulu amphibolites can be identified as retrograde UHP eclogites. The εHf(800) of +8 implies a significant input from the depleted mantle to the Sulu-Dabie terrane during the middle Neoproterozoic. Overgrown rims are characterized by a distinct trace element composition with low Lu/Hf and Th/U and significantly higher 176Hf/177Hf ratios than inherited cores, consistent with formation during/after garnet (re-)crystallization and fractionation of the Lu-Hf system during UHP metamorphism. The combined dataset suggests homogenization of the 176Hf/177Hf ratio within the metamorphic mineral assemblage and during protolith formation. Observed variations are explained by mixing of material from both domains during laser ablation, e.g., due to partial recrystallization of inherited cores.

Age and P-T Conditions of the Gridino eclogite in the Belomorian Province, Russia

NASA Astrophysics Data System (ADS)

Yu, Huanglu; Zhang, Lifei; Guo, Jinghui

2017-04-01

The Russian Belomorian eclogite was once regarded as Archean in age and the oldest eclogite in the world. However, its Archean age is disputed. The Gridino ecogite, the abundant eclogite in Belomorian province, is located in the southwest of the Paleoproterozoic Lapland-Kola collisional orogeny, and occurs as boudins and metamorphosed dykes within the tonalite-trondhjemite-granodiorite (TTG) gneisses. Zircons from these eclogites have magmatic cores and metamorphic rims. Metamorphic rims, which contain typical metamorphic mineral inclusions of omphacite and garnet, and are characterized by low Th/U ratios (< 0.035) and flat HREE patterns, yield a U-Pb age of ca. 1.90 Ga. The δ18O values of 6.23 to 6.80 ‰ of zircon rims are acquired during the eclogite-facies metamorphism. On the contrary, zircon cores display higher Th/U ratios 0.18-0.45, negative Eu anomalies and strong enrichment in HREE and have Neoarchean U-Pb ages of ca. 2.70 Ga. δ18O values of 5.64 to 6.07 ‰ suggest the possibility of crystallization from slightly evolved mantle-derived magmas. A three-stage metamorphic evolution has been recognized in the Gridino eclogite based on phase equilibria modeling: prograde epidote amphibolite facies, peak eclogite facies and retrogressed high-pressure granulite facies. The peak metamorphic P-T conditions (790-815 °C, 21-22 kbar) give an apparent geothermal gradient of 11-12 °C/km for Lapland-Kola collisional orogeny during Paleoproterozoic. The Gridino eclogite is not Archean, but the known oldest Paleoproterozoic eclogite, which may respond to the assembly of Columbia supercontinent.

Generation and Evolution of Quaternary Magmas Beneath Tengchong: Sr-Nd-Pb-Hf Isotope and Zircon U-series Age Constraints

NASA Astrophysics Data System (ADS)

Zou, H.; Ma, M.; Fan, Q.; Xu, B.; Li, S. Q.; Zhao, Y.; King, D. T., Jr.

2017-12-01

The Tengchong volcanic field on the southeastern margin of the Tibetan Plateau represents rare Quaternary volcanic eruptions on the plateau. The Quaternary Tengchong volcanic field formed high-potassium calc-alkaline volcanic rocks that include trachybasalts, basaltic trachyandesites, trachyandesites, and dacites. Herein, we present comprehensive Nd-Sr-Pb-Hf isotopic and elemental data for trachybasalts, basaltic trachyandesites, and trachyandesites from four young Tengchong volcanoes at Maanshan, Dayingshan, Heikongshan, and Laoguipo, in order to understand their magma genesis and evolution. Nd-Sr-Pb-Hf isotopes for the primitive Tengchong magma (trachybasalts with SiO2 <52.5 wt. % and MgO >5.5% wt. %) reflect a heterogeneous enriched mantle source. High Th/U, Th/Ta, and Rb/Nb ratios and Nd-Sr-Pb-Hf isotope characteristics of the primitive magmas suggest that the enriched mantle beneath Tengchong formed as a result of subduction of clay-rich sediments, which probably came from the Indian continental plate. Partial melting of the enriched mantle was generated by deep continental subduction coupled with recent regional extension in the Tengchong area. With regard to the evolved magmas (basaltic trachyandesites and trachyandesites), good correlations between SiO2 content and the ratios 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, and 177Hf/176Hf strongly suggest that the combined assimilation and fractional crystallization (AFC) was an important process during magma evolution to form these basaltic trachyandesites and trachyandesites. Uranium-series zircon dating on these evolved lavas from Tengchong is used to constrain their magma evolution and residence timescales.

Zircon U-Pb age, Lu-Hf isotope, mineral chemistry and geochemistry of Sundamalai peralkaline pluton from the Salem Block, southern India: Implications for Cryogenian adakite-like magmatism in an aborted-rift

NASA Astrophysics Data System (ADS)

Renjith, M. L.; Santosh, M.; Li, Tang; Satyanarayanan, M.; Korakoppa, M. M.; Tsunogae, T.; Subba Rao, D. V.; Kesav Krishna, A.; Nirmal Charan, S.

2016-01-01

The Sundamalai peralkaline pluton is one among the Cryogenian alkaline plutons occurring in the Dharmapuri Rift Zone (DRZ) of the Salem Block in the Southern Granulite Terrane (SGT) of India. Here we present zircon U-Pb age and Lu-Hf isotopic composition, mineral chemistry and geochemistry of the pluton to explore the petrogenesis and geodynamic implications. Systematic modal variation of orthoclase, Na-plagioclase, Ca-amphibole (ferro-edenite and hastingsite) and quartz developed quartz-monzonite and granite litho units in the Sundamalai pluton. Thermometry based on amphibole-plagioclase pair suggests that the pluton was emplaced and solidified at around 4.6 kbar pressure with crystallization of the major phases between 748 and 661 °C. Estimated saturation temperature of zircon (712-698 °C) is also well within this range. However, apatite saturation occurred at higher temperatures between 835 and 870 °C, in contrast with monazite saturation (718-613 °C) that continued up to the late stage of crystallization. Estimated oxygen fugacity values (log fO2: -14 to -17) indicate high oxidation state for the magma that stabilized titanite and magnetite. The magmatic zircons from Sundamalai pluton yielded a weighted mean 206Pb/238U age of 832.6 ± 3.2 Ma. Geochemically, the Sundamalai rocks are high-K to shoshonitic, persodic (Na2O/K2O ratio > 1), silica-saturated (SiO2:65-72 wt.%), and peralkaline in composition (aluminum saturation index, ASI < 1; Alkalinity index, AI < 0). The initial magma was mildly metaluminous which evolved to strongly peralkaline as result of fractional crystallization (plagioclase effect) controlled differentiation between quartz-monzonite and granite. Both rock types have high content of Na2O (5.1-6.3 wt.%), Ba (350-2589 ppm) and Sr (264-1036 ppm); low content of Y (8.7-17 ppm) and Yb (0.96-1.69 ppm); elevated ratios of La/Yb (11-46) and Sr/Y (46-69) and are depleted in Ti, with a positive Sr anomaly suggesting an adakite-like composition and garnet controlled melting of a plagioclase-poor source. The low content of MgO (<1 wt.%), Cr (7-29 ppm), Ni (6-19 ppm), Sc(2-3 ppm), positive Sr anomaly and predominantly negative zircon εHf(t) values (-10.8 to -9.3 with an average of -10.2) and initial 176Hf/177Hf ratios (0.281947-0.282022) confirm a Paleoproterozoic crustal source. Based on the field and geochemical evidences, we propose that a previously metasomatized mafic lower-crustal source enriched in alkalis has undergone CO2-present partial melting as a result of asthenospheric upwelling beneath an aborted rifting along the DRZ generating the magma that crystallized the Sundamalai rocks. Age of this pluton is comparable with that of the other Cryogenian felsic alkaline plutons from Salem Block suggesting extensive rift-related magmatism at this time in the SGT.

U-Pb geochronology documents out-of-sequence emplacement of ultramafic layers in the Bushveld Igneous Complex of South Africa.

PubMed

Mungall, James E; Kamo, Sandra L; McQuade, Stewart

2016-11-14

Layered intrusions represent part of the plumbing systems that deliver vast quantities of magma through the Earth's crust during the formation of large igneous provinces, which disrupt global ecosystems and host most of the Earth's endowment of Pt, Ni and Cr deposits. The Rustenburg Layered Suite of the enormous Bushveld Igneous Complex of South Africa has been presumed to have formed by deposition of crystals at the floor of a subterranean sea of magma several km deep and hundreds of km wide called a magma chamber. Here we show, using U-Pb isotopic dating of zircon and baddeleyite, that individual chromitite layers of the Rustenburg Layered Suite formed within a stack of discrete sheet-like intrusions emplaced and solidified as separate bodies beneath older layers. Our U-Pb ages and modelling necessitate reassessment of the genesis of layered intrusions and their ore deposits, and challenge even the venerable concept of the magma chamber itself.

Pervasive remagnetization of detrital zircon host rocks in the Jack Hills, Western Australia and implications for records of the early geodynamo

NASA Astrophysics Data System (ADS)

Weiss, Benjamin P.; Maloof, Adam C.; Tailby, Nicholas; Ramezani, Jahandar; Fu, Roger R.; Hanus, Veronica; Trail, Dustin; Bruce Watson, E.; Harrison, T. Mark; Bowring, Samuel A.; Kirschvink, Joseph L.; Swanson-Hysell, Nicholas L.; Coe, Robert S.

2015-11-01

It currently is unknown when Earth's dynamo magnetic field originated. Paleomagnetic studies indicate that a field with an intensity similar to that of the present day existed 3.5 billion years ago (Ga). Detrital zircon crystals found in the Jack Hills of Western Australia are some of the very few samples known to substantially predate this time. With crystallization ages ranging from 3.0-4.38 Ga, these zircons might preserve a record of the missing first billion years of Earth's magnetic field history. However, a key unknown is the age and origin of magnetization in the Jack Hills zircons. The identification of >3.9 Ga (i.e., Hadean) field records requires first establishing that the zircons have avoided remagnetization since being deposited in quartz-rich conglomerates at 2.65-3.05 Ga. To address this issue, we have conducted paleomagnetic conglomerate, baked contact, and fold tests in combination with U-Pb geochronology to establish the timing of the metamorphic and alteration events and the peak temperatures experienced by the zircon host rocks. These tests include the first conglomerate test directly on the Hadean-zircon bearing conglomerate at Erawandoo Hill. Although we observed little evidence for remagnetization by recent lightning strikes, we found that the Hadean zircon-bearing rocks and surrounding region have been pervasively remagnetized, with the final major overprinting likely due to thermal and/or aqueous effects from the emplacement of the Warakurna large igneous province at ∼1070 million years ago (Ma). Although localized regions of the Jack Hills might have escaped complete remagnetization, there currently is no robust evidence for pre-depositional (>3.0 Ga) magnetization in the Jack Hills detrital zircons.

Geochemical contrasts between Late Triassic ore-bearing and barren intrusions in the Weibao Cu-Pb-Zn deposit, East Kunlun Mountains, NW China: constraints from accessory minerals (zircon and apatite)

NASA Astrophysics Data System (ADS)

Zhong, Shihua; Feng, Chengyou; Seltmann, Reimar; Li, Daxin; Dai, Zhihui

2017-12-01

The Weibao copper-lead-zinc skarn deposit is located in the northern East Kunlun terrane, NW China. Igneous intrusions in this deposit consist of barren diorite porphyry (U-Pb zircon age of 232.0 ± 2.0 Ma) and ore-bearing quartz diorite and pyroxene diorite (U-Pb zircon ages of 223.3 ± 1.5 and 224.6 ± 2.9 Ma, respectively). Whole-rock major and trace element and accessory mineral (zircon and apatite) composition from these intrusions are studied to examine the different geochemical characteristics of ore-bearing and barren intrusions. Compared to the barren diorite porphyry, the ore-bearing intrusions have higher Ce4+/Ce3+ ratios of zircon and lower Mn contents of apatite, indicating higher oxidation state. Besides, apatite from the ore-bearing intrusions shows higher Cl contents and lower F/Cl ratios. These characteristics collectively suggest the higher productivity of ore-bearing quartz diorite and pyroxene diorite. When compared with ore-bearing intrusions from global porphyry Cu deposits, those from Cu-Pb-Zn skarn deposits display lower Ce4+/Ce3+ and EuN/EuN* ratios of zircon and lower Cl and higher F/Cl ratios of apatite. We conclude that these differences reflect a general geochemical feature, and that zircon and apatite composition is a sensitive tool to infer economic potential of magmas and the resulting mineralization types in intrusion-related exploration targets.

Silicic melt evolution in the early Izu-Bonin arc recorded in detrital zircons: Zircon U-Pb geochronology and trace element geochemistry for Site U1438, Amami Sankaku Basin

NASA Astrophysics Data System (ADS)

Barth, A. P.; Tani, K.; Meffre, S.; Wooden, J. L.; Coble, M. A.

2016-12-01

Understanding the petrologic evolution of oceanic arc magmas through time is important because these arcs reveal the processes of formation and the early evolution of juvenile continental crust. The Izu-Bonin (IB) arc system has been targeted because it is one of several western Pacific intraoceanic arcs initiated at 50 Ma and because of its prominent spatial asymmetry, with widespread development of relatively enriched rear arc lavas. We examined Pb/U and trace element compositions in zircons recovered at IODP Site 351-U1438 and compared them to regional and global zircon suites. These new arc zircon data indicate that detrital zircons will yield new insights into the generation of IB silicic melts and form a set of useful geochemical proxies for interpreting ancient arc detrital zircon provenance. Project IBM drilling target IBM1 was explored by Expedition 351 at Site U1438, located in the proximal back-arc of the northern Kyushu-Palau Ridge (KPR) at 27.3°N. A 1.2 km thick section of Paleogene volcaniclastic rocks, increasingly lithified and hydrothermally altered with depth, constitutes a proximal rear arc sedimentary record of IB arc initiation and early arc evolution. The ages and compositions of U1438 zircons are compatible with provenance in one or more edifices of the northern KPR and are incompatible with drilling contamination. Melt zircon saturation temperatures and Ti-in-zircon thermometry suggest a provenance in relatively cool and silicic KPR melts. The abundances of selected trace elements with high native concentrations provide insight into the petrogenesis of U1438 detrital zircon host melts, and may be useful indicators of both short and long-term variations in melt compositions in arc settings. The U1438 zircons are slightly enriched in U and LREE and are depleted in Nb compared to zircons from mid-ocean ridges and the Parece-Vela Basin, as predicted for melts in a primitive oceanic arc setting with magmas derived from a highly depleted mantle source. Close age and geochemical affinity of U1438 detrital zircons to igneous zircons in Eocene leucotonalite from the partially exhumed intrusive suite at Komahashi-Daini Seamount in the northernmost KPR suggests that these zircons also can yield insight into the link between silicic volcanism and evolving tonalitic intrusions in the Paleogene IB arc.

Testing the reliability of information extracted from ancient zircon

NASA Astrophysics Data System (ADS)

Kielman, Ross; Whitehouse, Martin; Nemchin, Alexander

2015-04-01

Studies combining zircon U-Pb chronology, trace element distribution as well as O and Hf isotope systematics are a powerful way to gain understanding of the processes shaping Earth's evolution, especially in detrital populations where constraints from the original host are missing. Such studies of the Hadean detrital zircon population abundant in sedimentary rocks in Western Australia have involved analysis of an unusually large number of individual grains, but also highlighted potential problems with the approach, only apparent when multiple analyses are obtained from individual grains. A common feature of the Hadean as well as many early Archaean zircon populations is their apparent inhomogeneity, which reduces confidence in conclusions based on studies combining chemistry and isotopic characteristics of zircon. In order to test the reliability of information extracted from early Earth zircon, we report results from one of the first in-depth multi-method study of zircon from a relatively simple early Archean magmatic rock, used as an analogue to ancient detrital zircon. The approach involves making multiple SIMS analyses in individual grains in order to be comparable to the most advanced studies of detrital zircon populations. The investigated sample is a relatively undeformed, non-migmatitic ca. 3.8 Ga tonalite collected a few kms south of the Isua Greenstone Belt, southwest Greenland. Extracted zircon grains can be combined into three different groups based on the behavior of their U-Pb systems: (i) grains that show internally consistent and concordant ages and define an average age of 3805±15 Ma, taken to be the age of the rock, (ii) grains that are distributed close to the concordia line, but with significant variability between multiple analyses, suggesting an ancient Pb loss and (iii) grains that have multiple analyses distributed along a discordia pointing towards a zero intercept, indicating geologically recent Pb-loss. This overall behavior has important implications for the studies of detrital populations, suggesting that even zircon from a supposedly single population can form a complex age distribution pattern. Notably, the age groups are also very similar to those identified at this time interval in the zircon population from Jack Hills in Western Australia. Interestingly 18O in zircon from the Greenland tonalite sample is very consistent at the mantle value and independent of the behavior of the U-Pb system, suggesting general stability of even metamict zircon as related to the O isotope exchange. At least it indicates that if this exchange takes place as it appears to be the case for the Hadean zircon, factors other than just the radiation damage have to play a role in the zircon modification.

Tiny twists in time; exploring angular resolution of in situ EBSD orientation microstructures in solar system zircon

NASA Astrophysics Data System (ADS)

Moser, D. E.

2012-12-01

Kikuchi discovered electron diffraction in samples of calcite in the 1920's, and orientation of lattice planes by Electron Backscatter Diffraction (EBSD) is now routinely measured by automated camera systems at a spatial resolution of tens of nanometers using Field Emission Gun SEM. The current methodology is proving particularly powerful when measuring lattice orientation microstructure in U-Pb geochronology minerals such as zircon and baddeleyite that have experienced high temperature deformation or shock metamorphism. These are among the oldest preserved mineral phases in inner solar system materials, and we have been applying EBSD to rare samples of the Early Earth and grains from extraterrestrial environments such as the Moon and Mars. In these cases the EBSD orientation data are useful for identifying high diffusivity pathways that may have afforded isotopic and trace element disturbance, microstructural proxies for shock metamorphic pressures, as well as resolving glide plane systems in ductile zircon and shear twin mechanisms. Blanket estimates of angular resolution for automated EBSD misorientation measurements are often in the range of 0.5 degrees. In some cases strain giving rise to only a few degrees of lattice misorientation has facilitated 100% Pb-loss. In some cases, however, there is a spatial correlation between trace element or cathodoluminescence zoning in zircon and what appears to be low magnitudes misorientation close to the limits of resolution. Given the proven value of performing EBSD analysis on geochronology minerals, a more thorough exploration of the precision and accuracy of EBSD lattice misorientation measurements is warranted. In this talk the relative weighting of the factors that limit EBSD angular resolution will be investigated, focusing on U-Pb dating minerals such as zircon. These factors include; sample surface preparation, phase symmetry, pseudo-symmetry effects, degree of crystallinity, Kikuchi band contrast and indexing, solid solution effects on unit cell, dimension camera calibration and camera-sample distance, beam conditions and focussing, and general microscope operating conditions (e.g. high vacuum vs. variable pressure). An assessment of potential zircon EBSD reference materials and sample preparation protocols will be presented, along with case studies of zircon orientation microstructures from meteorites and terrestrial craters representative of different strain and thermal environments in the inner solar system.BSD lattice misorientation maps of a) crystal-plastically deformed and partly recrystallized zircon, after Rayner et al. (in prep.), and b) shock-metamorphosed lunar zircon (Darling et al., in prep.).

Evaluating the provenance of Permian-Triassic and Palaeocene-Eocene ash beds by high precision U-Pb and Lu-Hf isotopic analyses of zircons: linking local sedimentary records to global events

NASA Astrophysics Data System (ADS)

Eivind Augland, Lars; Jones, Morgan; Planke, Sverre; Svensen, Henrik; Tegner, Christian

2016-04-01

Zircons are a powerful tool in geochronology and isotope geochemistry, as their affinity for U and Hf in the crystal structure and the low initial Pb and Lu allow for precise and accurate dating by U-Pb ID-TIMS and precise and accurate determination of initial Hf isotopic composition by solution MC-ICP-MS analysis. The U-Pb analyses provide accurate chronostratigraphic controls on the sedimentary successions and absolute age frames for the biotic evolution across geological boundaries. Moreover, the analyses of Lu-Hf by solution MC-ICP-MS after Hf-purification column chemistry provide a powerful and robust fingerprinting tool to test the provenance of individual ash beds. Here we focus on ash beds from Permian-Triassic and Palaeocene successions in Svalbard and from the Palaeocene-Eocene Thermal Maximum (PETM) in Fur, Denmark. Used in combination with whole rock geochemistry from the ash layers and the available geochemical and isotopic data from potential source volcanoes, these data are used to evaluate the provenance of the Permian-Triassic and Palaeocene ashes preserved in Svalbard and PETM ashes in Denmark. If explosive eruptions from volcanic centres such as the Siberian Traps and the North Atlantic Igneous Province (NAIP) can be traced to distal basins as ash layers, they provide robust tests of hypotheses of global synchronicity of environmental changes and biotic crises. In addition, the potential correlation of ash layers with source volcanoes will aid in constraining the extent of explosive volcanism in the respective volcanic centres. The new integrated data sets will also contribute to establish new reference sections for the study of these boundary events when combined with stable isotope data and biostratigraphy.

Data Reduction of Laser Ablation Split-Stream (LASS) Analyses Using Newly Developed Features Within Iolite: With Applications to Lu-Hf + U-Pb in Detrital Zircon and Sm-Nd +U-Pb in Igneous Monazite

NASA Astrophysics Data System (ADS)

Fisher, Christopher M.; Paton, Chad; Pearson, D. Graham; Sarkar, Chiranjeeb; Luo, Yan; Tersmette, Daniel B.; Chacko, Thomas

2017-12-01

A robust platform to view and integrate multiple data sets collected simultaneously is required to realize the utility and potential of the Laser Ablation Split-Stream (LASS) method. This capability, until now, has been unavailable and practitioners have had to laboriously process each data set separately, making it challenging to take full advantage of the benefits of LASS. We describe a new program for handling multiple mass spectrometric data sets collected simultaneously, designed specifically for the LASS technique, by which a laser aerosol is been split into two or more separate "streams" to be measured on separate mass spectrometers. New features within Iolite (https://iolite-software.com) enable the capability of loading, synchronizing, viewing, and reducing two or more data sets acquired simultaneously, as multiple DRSs (data reduction schemes) can be run concurrently. While this version of Iolite accommodates any combination of simultaneously collected mass spectrometer data, we demonstrate the utility using case studies where U-Pb and Lu-Hf isotope composition of zircon, and U-Pb and Sm-Nd isotope composition of monazite were analyzed simultaneously, in crystals showing complex isotopic zonation. These studies demonstrate the importance of being able to view and integrate simultaneously acquired data sets, especially for samples with complicated zoning and decoupled isotope systematics, in order to extract accurate and geologically meaningful isotopic and compositional data. This contribution provides instructions and examples for handling simultaneously collected laser ablation data. An instructional video is also provided. The updated Iolite software will help to fully develop the applications of both LASS and multi-instrument mass spectrometric measurement capabilities.

Trace element chemistry of zircons from oceanic crust: A method for distinguishing detrital zircon provenance

USGS Publications Warehouse

Grimes, Craig B.; John, Barbara E.; Kelemen, P.B.; Mazdab, F.K.; Wooden, J.L.; Cheadle, Michael J.; Hanghoj, K.; Schwartz, J.J.

2007-01-01

We present newly acquired trace element compositions for more than 300 zircon grains in 36 gabbros formed at the slow-spreading Mid-Atlantic and Southwest Indian Ridges. Rare earth element patterns for zircon from modern oceanic crust completely overlap with those for zircon crystallized in continental granitoids. However, plots of U versus Yb and U/Yb versus Hf or Y discriminate zircons crystallized in oceanic crust from continental zircon, and provide a relatively robust method for distinguishing zircons from these environments. Approximately 80% of the modern ocean crust zircons are distinct from the field defined by more than 1700 continental zircons from Archean and Phanerozoic samples. These discrimination diagrams provide a new tool for fingerprinting ocean crust zircons derived from reservoirs like that of modern mid-ocean ridge basalt (MORB) in both modern and ancient detrital zircon populations. Hadean detrital zircons previously reported from the Acasta Gneiss, Canada, and the Narryer Gneiss terrane, Western Australia, plot in the continental granitoid field, supporting hypotheses that at least some Hadean detrital zircons crystallized in continental crust forming magmas and not from a reservoir like modern MORB. ?? 2007 The Geological Society of America.

Neoproterozoic magmatic flare-up along the N. margin of Gondwana: The Taknar complex, NE Iran

NASA Astrophysics Data System (ADS)

Moghadam, Hadi Shafaii; Li, Xian-Hua; Santos, Jose F.; Stern, Robert J.; Griffin, William L.; Ghorbani, Ghasem; Sarebani, Nazila

2017-09-01

Magmatic ;flare-ups; are common in continental arcs. The best-studied examples of such flare-ups are from Cretaceous and younger continental arcs, but a more ancient example is preserved in Late Ediacaran-Cambrian or Cadomian arcs that formed along the northern margin of Gondwana. In this paper, we report new trace-element, isotopic and geochronological data on ∼550 Ma magmatic rocks from the Taknar complex, NE Iran, and use this information to better understand episodes of flare-up, crustal thickening and magmatic periodicity in the Cadomian arcs of Iran and Anatolia. Igneous rocks in the Taknar complex include gabbros, diorites, and granitoids, which grade upward into a sequence of metamorphosed volcano-sedimentary rocks with interlayered rhyolites. Granodioritic dikes crosscut the Taknar gabbros and diorites. Gabbros are the oldest units and have zircon U-Pb ages of ca 556 Ma. Granites are younger and have U-Pb zircon ages of ca 552-547 Ma. Rhyolites are coeval with the granites, with U-Pb zircon ages of ∼551 Ma. Granodioritic dikes show two U-Pb zircon ages; ca 531 and 548 Ma. Geochemically, the Taknar igneous rocks have calc-alkaline signatures typical of continental arcs. Whole-rock Nd and zircon O-Hf isotopic data show that from Taknar igneous rocks were generated via mixing of juvenile magmas with older continental crust components at an active continental margin. Compiled geochronological and geochemical data from Iran and Anatolia allow identification of a Cadomian flare-up along northern Gondwana. The compiled U-Pb results from both magmatic and detrital zircons indicate the flare-up started ∼572 Ma and ended ∼528 Ma. The Cadomian flare-up was linked to strong crustal extension above a S-dipping subduction zone beneath northern Gondwana. The Iran-Anatolian Cadomian arc represents a site of crustal differentiation and stratification and involved older (Archean?) continental lower-middle crust, which has yet to be identified in situ, to form the continental nuclei of Anatolia and Iran. The Cadomian crust of Anatolia and Iran formed a single block ;Cimmeria; that rifted away from northern Gondwana and was accreted to southern Eurasia in late Paleozoic time.

U-Pb Geochronology of Devonian Granites in the Meguma Terrane of Nova Scotia, Canada: Evidence for Hotspot Melting of a Neoproterozoic Source.

PubMed

Keppie; Krogh

1999-09-01

U-Pb isotopic analyses of monazite and zircon from six granitic plutons in the Meguma Terrane yield nearly concordant ages of 373+/-3 Ma, interpreted as the time of intrusion. U-Pb analyses of euhedral zircons with thick rims overgrowing cores, which were abraded to remove all or most of the rim, plot on chords between 370+/-3 and 628+/-33 Ma (Larrys River and Halfway Cove plutons), 372+/-3 and approximately 660 Ma (Shelburne pluton), and 373+/-2 and approximately 732 Ma (Barrington Passage pluton). The upper intercepts are interpreted as the age of magma source, correlatives of which are present in the Avalon Composite Terrane to the north. This basement may be either in depositional or tectonic contact with the overlying Cambro-Ordovician Meguma Group. Other zircons in the granites are generally irregular-euhedral with thin rims, and most U-Pb isotopic analyses fall between two chords from 373-2040 and 373-2300 Ma, with a few lying outside this field. These zircons are probably derived from the country rock (Goldenville Formation), which a previous study has shown contains detrital zircons with concordant U-Pb ages of 3000, 2000, and 600 Ma, and numerous intermediate discordant ages. These new ages, along with published data, document a relatively short (5-10 m.yr.) but voluminous period of magmatism. This age is approximately synchronous with intrusion of mafic rocks and lamprophyre dikes and regional low-pressure metamorphism and was followed by rapid denudation of 5-12 km. These observations may be interpreted in terms of shallowly dipping subduction and overriding of a mantle plume that eventually penetrates through the subducting plate to melt the overriding continental plate. Subsequent northward migration of the plume could explain both the approximately 360 Ma magmatism in the Cobequid Highlands (Avalon Composite Terrane) and the mid-Carboniferous plume-related intrusions around the Magdalen Basin.

Air abrasion experiments in U-Pb dating of zircon

USGS Publications Warehouse

Goldich, S.S.; Fischer, L.B.

1986-01-01

Air abrasion of zircon grains can remove metamict material that has lost radiogenic Pb and zircon overgrowths that were added during younger events and thereby improve the precision of the age measurements and permit closer estimates of the original age. Age discordance that resulted from a single disturbance of the U-Pb isotopic decay systems, as had been demonstrated by T.E. Krogh, can be considerably reduced, and, under favorable conditions, the ages brought into concordancy. Two or more events complicate the U-Pb systematics, but a series of abrasion experiments can be helpful in deciphering the geologic history and in arriving at a useful interpretation of the probable times of origin and disturbances. In east-central Minnesota, U.S.A., Penokean tonalite gneiss is dated at 1869 ?? 5 Ma, and sheared granite gneiss is shown to have been a high-level granite intrusion at 1982 ?? 5 Ma in the McGrath Gneiss precursor. Tonalite gneiss and a mafic granodiorite in the Rainy Lake area, Ontario, Canada, are dated at 2736 ?? 16 and 2682 ?? 4 Ma, respectively. The tonalitic phase of the Morton Gneiss, southwestern Minnesota, is dated at 3662 ?? 42 Ma. ?? 1986.

Ca. 400 Ma Recrystallization of Norwegian Ultrahigh-pressure Eclogites: an ion Microprobe and Chemical Abrasion Study

NASA Astrophysics Data System (ADS)

Root, D. B.; Mattinson, J. M.; Hacker, B. R.; Wooden, J. L.

2002-12-01

Understanding the formation and exhumation of the ultrahigh-pressure (UHP) rocks of western Norway hinges on precise determination of the time of eclogite recrystallization. Our study consists of SHRIMP analysis, in conjunction with CL imagery, of zircon from four UHP and high-pressure (HP) eclogites; and detailed TIMS analysis of zircon from two samples subjected to combined thermal annealing and multi-step chemical abrasion (CA). SHRIMP analyses of the Otnheim and Langenes eclogites yield Caledonian spot ages of ca. 400 Ma from zircon rims. CL imagery and Th/U ratios from the Langenes eclogite indicate formation of rims by recrystallization of inherited zircon. SHRIMP analysis of the UHP Flatraket eclogite yielded a broad range of apparently concordant Caledonian ages. CA analyses of two fractions yielded moderate Pb loss from the first (lowest T) steps; possible minor Pb loss or minor growth at 400 Ma from the second steps; and a 407-404 Ma cluster of slightly discordant 206Pb/238U ages, most likely free from Pb loss, from the remaining steps. We interpret the latter to reflect recrystallization of inherited zircon, with possible new growth, at ca. 400-395 Ma. Alternatively, the high-temperature CA steps could represent growth at 407-404 Ma, with apparent discordance due to intermediate daughter product effects. HP/UHP zircon recrystallization in the Flatraket eclogite is inferred from three lines of evidence: i) zircon occurs as inclusions in garnet, omphacite, breunnerite, dolomite, and quartz, as well as in symplectites after phengite and omphacite; ii) association of zircon with rutile implies zircon formation during HP breakdown of Zr-ilmenite; and iii) chondrite-normalized ICP-MS analyses of the CA steps reveal small Eu anomalies and shallow HREE profiles, indicating zircon recrystallization in the presence of garnet. CA analysis of the Verpeneset eclogite yielded distinctly discordant step ages from two steps comprising <90% of the sample, with 206Pb/238U ages of 408 and 414 Ma. CL imagery indicates incomplete recrystallization of inherited igneous zircon, in keeping with steep HREE profiles determined from chondrite-normalized ICP-MS analyses. Our zircon age of ca. 400-395 Ma for the Flatraket eclogite is significantly younger than the 425 Ma age often cited for western Norway eclogite recrystallization, implying, in conjunction with 390-385 Ma 40Ar/39Ar white mica cooling ages, faster rates of exhumation (ca. 15 km/m.y.), and weakening the link between UHP metamorphism and ophiolite emplacement at 430-425 Ma.

The Wall-Rock Record of Incremental Emplacement in the Little Cottonwood-Alta Magmatic and Hydrothermal System, Wasatch Mountains, Utah, U.S.A.

NASA Astrophysics Data System (ADS)

Stearns, M.; Callis, S.; Beno, C.; Bowman, J. R.; Bartley, J. M.

2017-12-01

Contact aureoles record the cumulative effects on wall rocks of magma emplacement. Like the plutons they surround, contact aureoles have long been regarded to form geologically instantaneously. Protracted incremental emplacement of plutons must be reconciled with the wall-rock record of heat and mass transfer. Fundamental questions include how heat and material move from intrusions into their aureoles and how long that process takes. The Little Cottonwood stock is surrounded by a 2 km-wide contact aureole that contains prograde AFM mineral assemblages in the pelitic layers of the Proterozoic Big Cottonwood Formation. The Alta stock is surrounded by a well characterized 1 km-wide contact aureole containing both prograde AFM and CMS mineral assemblages in Ophir Shale and Mississippian dolostones, respectively. Understanding the petrogenesis of these aureoles requires the timing of magmatism and wall-rock metamorphism to be independently determined. Preliminary petrochronology (U/Th-Pb dates and trace element concentrations collected by LASS-ICP-MS) from the inner aureoles of both intrusions establishes a protracted history of monazite (re)crystallization from 35-25 Ma in the Little Cottonwood aureole and 35 Ma in the Alta aureole. Little Cottonwood aureole monazites are characterized by a positive age correlation with heavy rare earth elements (HREE) and a negative correlation with Eu/Eu*. Alta aureole monazites have a similar range of the HREE concentrations and Eu/Eu* variation. Zircon growth interpreted to record emplacement-level magmatic crystallization of the western Little Cottonwood stock ranges from 33-28 Ma near the contact. Multi-grain U-Pb zircon TIMS dates from the Alta stock range from 35-33 Ma and are interpreted to suggest the full range of emplacement-level magmatism in the Alta stock. Additionally, in situ U-Pb titanite dates from the Alta stock record intermittent high temperature hydrothermal activity in the stock margin from 35-24 Ma. These new data suggest that the Little Cottonwood aureole developed over several million years and overlapped in time with hydrothermal (re)crystallization of titanite within the Alta Stock. Both systems continued to develop after monazite (re)crystallization within Ophir Shale, which was concurrent with emplacement of the Alta Stock.

U-Th-Pb zircon dating of the 13.8-Ma dacite volcanic dome at Cerro Rico de Potosi, Bolivia

USGS Publications Warehouse

Zartman, R.E.; Cunningham, C.G.

1995-01-01

The temporal relationship between the extrusion of the Miocene dacite volcanic dome at Cerro Rico de Potasi, Bolivia, and the associated Ag-Sn mineralization has an important bearing on the heat and metal sources for this world class mineral deposit. The present study uses U-Th-Pb dating of sparse zircon contained in the dacite to demonstrate that, at most, only several hundred thousand years separate dome emplacement from main stage mineralization. -from Authors

Precise U/Pb zircons dates of bentonites in Upper Ordovician and Lower Silurian reference sections in North America and Britain.

NASA Astrophysics Data System (ADS)

Suarez, S. E.; Brookfield, M. E.; Catlos, E. J.; Stockli, D. F.; Batchelor, R. A.

2016-12-01

The end of the Ordovician marks one of the greatest of the Earth's mass extinctions. One hypothesis explains this mass extinction as the result of a short-lived, major glaciation preceded by episodes of increased volcanism brought on by the Taconic orogeny. K-bentonites, weathered volcanic ash, provide evidence for increased volcanism. However, there is a lack of modern precise U-Pb dating of these ashes and some confusion in the biostratigraphy. The aim of this study is to obtain more precise U-Pb zircon ages from biostratigraphically constrained bentonites which will lead to better correlation of the Upper Ordovician and Lower Silurian relative time scales, as well as time the pulses of eruption. Zircon grains were extracted from the samples by heavy mineral separation and U-Pb dated using the Laser Ablation-Inductively Coupled Plasma-Mass Spectrometer at the University of Texas-Austin. We report here 3 precise U-Pb zircon ages from the Trenton Group, Ontario, Canada, and Dob's Linn, Scotland. The youngest age from the top of the Kirkfield Formation in Ontario is 448.0 +/- 18 Ma, which fits with existing late Ordovician stratigraphic ages. At Dob's Linn, Scotland, the site of the Ordovician/Silurian Global Boundary Stratigraphic Section and Point (GSSP), the youngest age for DL7, a bentonite 5 meters below the GSSP is 402.0 +/- 12.0 Ma, and for DL24L, a bentonite 8 meters above the GSSP is 358.2 +/- 7.9 Ma. These are Devonian ages in current timescales - the current age for the GSSP is 443.8 +/- 1.8 Ma, based on an U/Pb dates from a bentonite 1.6 meters above the GSSP at Dob's Linn. We are confident that our techniques rule out contamination and the most likely explanation is that the small zircons we analyzed either suffered Pb loss, or grew overgrowths during low grade hydrothermal metamorphism of the sediments during the intrusion of the Southern Upland Devonian granites during the Caledonian orogeny. These Devonian ages suggest that the 443.8 +/- 1.8 Ma age may also be suspect. The Dob's Linn site is therefore unsuitable for calibrating the biostratigraphic horizons. Work in progress will provide more U-Pb dating of bentonites from around the Ordovician-Silurian boundary in Canada, United States, Britain and Scandinavia with the aim of calibrating the local series and stages in order to help in International correlations.

New Insights into Arctic Tectonics: Uranium-Lead, (Uranium-Thorium)/Helium, and Hafnium Isotopic Data from the Franklinian Basin, Canadian Arctic Islands

NASA Astrophysics Data System (ADS)

Anfinson, Owen Anthony

More than 2300 detrital zircon uranium-lead (U-Pb) ages, 32 176Hf/177Hf (eHf) isotopic values, 37 apatite helium (AHe) ages, and 72 zircon helium (ZHe) ages represent the first in-depth geochronologic and thermochronologic study of Franklinian Basin strata in the Canadian Arctic and provide new insight on >500 M.y. of geologic history along the northern Laurentian margin (modern orientation). Detrital zircon U-Pb age data demonstrate that the Franklinian Basin succession is composed of strata with three distinctly different provenance signatures. Neoproterozoic and Lower Cambrian formations contain detrital zircon populations consistent with derivation from Archean to Paleoproterozoic gneisses and granites of the west Greenland--northeast Canadian Shield. Lower Silurian to Middle Devonian strata are primarily derived from foreland basin strata of the East Greenland Caledonides (Caledonian orogen). Middle Devonian to Upper Devonian strata also contain detrital zircon populations interpreted as being primarily northerly derived from the continental landmass responsible for the Ellesmerian Orogen (often referred to as Crockerland). U-Pb age data from basal turbidites of the Middle to Upper Devonian clastic succession suggest Crockerland contributed sediment to the northern Laurentian margin by early-Middle Devonian time and that prior to the Ellesmerian Orogeny Crockerland had a comparable geologic history to the northern Baltica Craton. Detrital zircon U-Pb ages in Upper Devonian strata suggest Crockerland became the dominant source by the end of Franklinian Basin sedimentation. Mean eHf values from Paleozoic detrital zircon derived from Crockerland suggest the zircons were primarily formed in either an island arc or continental arc built on accreted oceanic crust setting. ZHe cooling ages from Middle and Upper Devonian strata were not buried deeper than 7 km since deposition and suggest Crockerland was partially exhumed during the Caledonian Orogen. AHe cooling ages are partially reset since deposition and experienced varying burial histories depending on stratigraphic and geographic location within the basin. AHe ages from Middle Devonian strata from the western margin of the basin indicate episodes of exhumation associated with clastic influxes of sediment into the Sverdrup Basin during the Late Jurassic-Early Cretaceous and Late Cretaceous.

Two mineralization events in the Baiyinnuoer Zn-Pb deposit in Inner Mongolia, China: Evidence from field observations, S-Pb isotopic compositions and U-Pb zircon ages

NASA Astrophysics Data System (ADS)

Jiang, Si-Hong; Chen, Chun-Liang; Bagas, Leon; Liu, Yuan; Han, Ning; Kang, Huan; Wang, Ze-Hai

2017-08-01

The Xing-Mong Orogenic Belt (XMOB) is located in the eastern part of the Central Asian Orogenic Belt (CAOB) and has experienced multiple tectonic events. The Baiyinnuoer Pb-Zn deposit may be a rare case that documents two periods of mineralization in the tectonically complex XMOB. There are two types of Pb-Zn mineralization in the deposit: (1) skarn-type ore, hosted by the skarn in the contact zone between marble and granodiorite and within the marble and (2) vein-type ore, hosted by crystal tuff and feldspar porphyry. This study revealed that the host rocks, mineral assemblages, mineralization occurrences, S-Pb isotopes, and ages between the two types of ore are notably different. Zircon U-Pb dating indicates that the granodiorite was emplaced in the Early Triassic (244 ± 1 to 242 ± 1 Ma), the crystal tuff was deposited in the Early Cretaceous (140 ± 1 to 136 ± 1 Ma), and the feldspar porphyry was intruded in the Early Cretaceous (138 ± 2 to 136 ± 2 Ma). The first skarn mineralization occurred at ∼240 Ma and the second vein-type Pb-Zn mineralization took place between 136 and 129 Ma. Thus the Triassic orebodies were overprinted by Early Cretaceous mineralization. The sphalerite and galena from the skarn mineralization have higher δ34S values (-4.7 to +0.3‰) than the sphalerite, galena and aresenopyrite from the vein-type mineralization (-7.5 to -4.2‰), indicating different sulfur sources or ore-forming processes for the two types of mineralization. The Pb isotopic compositions of the two types of ore are very similar, suggesting similar lead sources. Geochemistry and Nd-Pb-Hf isotopic systematics of the igneous rocks in the region show that the Triassic granodiorite was generated from hybridization of mafic and felsic magmas due to strong crust-mantle interaction under the collisional setting that resulted following the closure of the Paleo-Asian Ocean and the collision of North China and Siberian cratons at the end of the Permian; while the Cretaceous igneous rocks at Baiyinnuoer originated from the partial melting of a juvenile lower crust with minor input from the crust caused by the underplating of mafic magma in an extensional setting.

(De)coupled zircon metamictization, radiation damage, and He diffusivity

NASA Astrophysics Data System (ADS)

Ault, A. K.; Guenthner, W.; Reiners, P. W.; Moser, A. C.; Miller, G. H.; Refsnider, K. A.

2017-12-01

We develop and apply a new protocol for targeting crystals for the zircon (U-Th)/He (He) thermochronometry to maximize effective U (eU) and corresponding closure temperature variability to develop zircon He date-eU correlations observed in some datasets. Our approach exploits visual proxies for radiation damage accumulation (metamictization) during zircon selection. We show that by purposefully targeting a spectrum of zircon textures from pristine to metamict grains, it is possible to generate broad eU variation in suites of zircon from a single sample and zircon He date-eU-metamictization trends that can be exploited to resolve increasingly complex thermal histories. We present plane light photographs, eU concentration, and zircon He results from 59 individual zircons from nine crystalline rock samples. Six of the nine samples come from exposed Proterozoic granitoids on SE Baffin Island, Canada; Boulder Creek, CO; Sandia Mountains, NM; and Mecca Hills, CA. We report data from three Archean Baffin samples to compare with the Proterozoic Baffin sample date-eU-metamictization trend. In each Proterozoic sample, target zircons display a spectrum of metamictization from pristine, transparent crystals to purple-brown, translucent grains. Progressive loss of transparency and increase in discoloration consistently corresponds to an increase in eU in all samples. Individual zircon eU varies from 89-1885 ppm and, within each sample, the total eU spread is 538 ppm to 1374 ppm. For any given eU value, the Archean zircon appear comparatively more metamict than the Proterozoic Baffin grains and samples collectively define a 1681 ppm range in eU, with more restrictive intrasample eU spreads (199-1120 ppm). Proterozoic samples from Baffin, Sandia, and Front Range yield negative zircon He date-eU correlations with intrasample date ranges of 90-783 Ma. Increasing eU and younger dates correspond with increasing metamictization. In contrast, all three Proterozoic Mecca Hills samples yield uniform 25 Ma zircon He dates over 1800 ppm eU. We apply simple thermal history models that account for the coevolution of zircon radiation damage and He-diffusivity to demonstrate that visible zircon metamictization and He diffusivity can be either coupled or decoupled depending on a sample's thermal history.

Zircon U-Pb ages and Hf isotopic compositions indicate multiple sources for Grenvillian detrital zircon deposited in western Laurentia

NASA Astrophysics Data System (ADS)

Howard, Amanda L.; Farmer, G. Lang; Amato, Jeffrey M.; Fedo, Christopher M.

2015-12-01

Combined U-Pb ages and Hf isotopic data from 1.0 Ga to 1.3 Ga (Grenvillian) detrital zircon in Neoproterozoic and Cambrian siliciclastic sedimentary rocks in southwest North America, and from igneous zircon in potential Mesoproterozoic source rocks, are used to better assess the provenance of detrital zircon potentially transported across Laurentia in major river systems originating in the Grenville orogenic highlands. High-precision hafnium isotopic analyses of individual ∼1.1 Ga detrital zircon from Neoproterozoic siliciclastic sedimentary rocks in Sonora, northern Mexico, reveal that these zircons have low εHf (0) (-22 to -26) and were most likely derived from ∼1.1 Ga granitic rocks embedded in local Mojave Province Paleoproterozoic crust. In contrast, Grenvillian detrital zircons in Cambrian sedimentary rocks in Sonora, the Great Basin, and the Mojave Desert, have generally higher εHf (0) (-15 to -21) as demonstrated both by high precision solution-based, and by lower precision laser ablation, ICPMS data and were likely derived from more distal sources further to the east/southeast in Laurentia. Comparison to new and existing zircon U-Pb geochronology and Hf isotopic data from Grenvillian crystalline rocks from the Appalachian Mountains, central and west Texas, and from Paleoproterozoic terranes throughout southwest North America reveals that zircon in Cambrian sandstones need not entirely represent detritus transported across the continent from Grenville province rocks in the vicinity of the present-day southern Appalachian Mountains. Instead, these zircons could have been derived from more proximal, high εHf (0), ∼1.1 Ga, crystalline rocks such as those exposed today in the Llano Uplift in central Texas and in the Franklin Mountains of west Texas. Regardless of the exact source(s) of the Grenvillian detrital zircon, new and existing whole-rock Nd isotopic data from Neoproterozoic to Cambrian siliciclastic sedimentary rocks in the Mojave Desert demonstrate that the occurrences of higher εHf (0), Grenvillian detrital zircons are decoupled from the sources of the bulk of the sedimentary detritus in which the zircons are entrained. The Cambrian Wood Canyon Formation and the underlying ;off craton; Neoproterozoic Johnnie Formation and Stirling Quartzite all contain higher εHf (0), Grenvillian detrital zircon, in some cases as the dominant detrital zircon population. However, only portions of the Wood Canyon Formation have whole rock Nd isotopic compositions consistent with a bulk sediment source in ∼1.1 Ga sources rocks. Whole rock Nd isotopic compositions of the remaining portions of this unit, and all of the Johnnie Formation and Stirling Quartzite, require bulk sediment sources principally in Paleoproterozoic continental crust. We consider the observed decoupling in the sources of Grenvillian detrital zircon and bulk sediment in the Wood Canyon Formation and underlying siliciclastic sediments as a demonstration that detrital zircon U-Pb and Hf isotopic data alone can provide an incomplete picture of the source of sediments that comprise a given siliciclastic stratigraphic unit.

The relationship between plutonism and volcanism: zircon ages from granitoid clasts in recent pyroclastic deposits from Tarawera volcano

NASA Astrophysics Data System (ADS)

Shane, P. A.; Storm, S.; Schmitt, A. K.; Lindsay, J.

2011-12-01

In Quaternary magmatic systems that have not undergone extensive uplift that would expose their intrusive roots, co-magmatic (and xenolithic) plutonic clasts entrained in eruptive deposits are an important source of information on the temporal relationship between plutonism and volcanism. Granitoid clasts in pyroclastic deposits of the 0.7 ka (Kaharoa) eruption from the Tarawera volcano of the Okataina Volcanic Centre (OVC), New Zealand, provide a rare insight to the plutonic processes beneath one of the most productive Quaternary rhyolite centers on Earth. SIMS U-Th and U-Pb data on 79 granitoid zircon crystals from six clasts reveal a unimodal age spectrum yielding a weighted average model age of 211 ± 4 ka (MSWD = 1.1). This crystallization event coincides with relative quiescence in OVC volcanism. A few outlier antecrysts date back to ~700 ka, a period significantly longer than the known volcanic record at OVC (probably ~330 ka). In contrast, zircon crystallization in co-erupted pumice and lava of the 0.7 ka Kaharoa event, and that of the three preceding rhyolite eruptions, occurred mostly during 0-50 ka. Thus, the granitoid clasts represent part of the system immediately beneath the volcano that survived assimilation and/or destruction in subsequent eruption and caldera collapse episodes. Brittle deformation features, incipient alteration and low-d18O whole-rock compositions (+3%) are consistent with a shallow solid carapace that has interacted with hydrothermal fluids. However, d18O SIMS analyses of zircons (+5.4 ± 0.2 %; n = 11) are consistent with magmatic compositions, and thus meteoric interaction occurred post-emplacement. The Kaharoa granitoids contrast with those ejected in the ~60 ka caldera-forming Rotoiti event, that were partly molten and display zircon age spectra indistinguishable from that in co-erupted pumices, suggesting the latter were derived from contemporaneous crystal mush. The 0.7 ka Kaharoa case shows that, over time, eruptible parts of a magmatic reservoir can become armored by a solidified intrusive carapace that minimizes interaction with other parts of the magmatic system and the surrounding wall-rocks. Thus, plutonic and volcanic evolution can diverge even in close proximity of the same magmatic system.

Petrogenesis and U-Pb zircon chronology of felsic tuffs interbedded with turbidites (Eastern Pontides Orogenic Belt, NE Turkey): Implications for Mesozoic geodynamic evolution of the eastern Mediterranean region and accumulation rates of turbidite sequences

NASA Astrophysics Data System (ADS)

Eyuboglu, Yener

2015-01-01

The Meso-Cenozoic geodynamic evolution of the Eastern Pontides Orogenic Belt, which is one of the key areas of the Alpine-Himalayan system, is still controversial due to lack of systematic geological, geophysical, geochemical and chronological data. The prevailing interpretation is that this belt represents the southern margin of Eurasia during the Mesozoic and its geodynamic evolution is related to northward subduction of oceanic lithosphere. This paper reports the first detailed geological, geochemical and chronological data from felsic tuffs interbedded with late Cretaceous turbidites in the Southern Zone of the Eastern Pontides Orogenic Belt. Individual tuff layers are thin, mostly < 2 m in thickness, implying that these are dominantly air-fall tuffs. Petrographic data indicate that the felsic tuffs, which exhibit various degrees of alteration, can be classified as crystal-rich and crystal-poor tuffs. The crystal-poor tuffs consist mainly of 45-65% devitrified glass shards and 10-20% broken quartz crystals, whereas the crystal-rich tuffs consist of > 50% crystals. The zircon U-Pb data show three statistically distinct ages at 84, 81 and 77 Ma, with uncertainties of about 1 Ma, suggesting that tuff-forming late Cretaceous magmatism started about 84 Ma ago and was episodically active over a minimum of 7 Ma. The age data also indicate that the average accumulation rate of the turbiditic sequence that hosts the felsic tuffs remained constant between 36 and 40 cm/10 ky. Their enrichment in LIL and LRE elements relative to HFS and HRE elements, and also strongly negative Nb, Ta and Ti anomalies, are consistent with those of magmas generated by subduction-related processes. The tuffs have relatively low initial ratios of 143Nd/144Nd (0.512296-0.512484; εNd: - 2.1 and - 7.2) and 87Sr/86Sr (0.704896-0.706159). Their initial Pb isotopic compositions range from 18.604 to 18.646 for 206Pb/204Pb, from 15.644 to 15.654 for 207Pb/206Pb and from 38.712 to 38.763 for 208Pb/204Pb. The distribution of Sr-Nd isotopic compositions in the late Cretaceous igneous rocks from different locations of the Eastern Pontides Orogenic Belt is consistent with two-component mixing between depleted mantle and crust. However, the Pb isotopic data are not compatible with two-component mixing and require at least a third component. Considering all of the new data and also previous data such as southward migration and increasing potassium content of the late Cretaceous arc volcanism, the northward migration of Cenozoic igneous activity, northward drift of the belt since the late Cretaceous and the existence of south-dipping reverse fault systems in the whole region, the Meso-Cenozoic geodynamic evolution of the Eastern Pontides Orogenic Belt can be best explained by southward subduction of Tethys oceanic lithosphere, rather than northward subduction.

Magma hybridization in the Western Tatra Mts. granitoid intrusion (S-Poland, Western Carpathians).

PubMed

Burda, Jolanta; Gawęda, Aleksandra; Klötzli, Urs

In the Variscan Western Tatra granites hybridization phenomena such as mixing and mingling can be observed at the contact of mafic precursors of dioritic composition and more felsic granitic host rocks. The textural evidence of hybridization include: plagioclase-K-feldspar-sphene ocelli, hornblende- and biotite-rimmed quartz ocelli, plagioclase with Ca-rich spike zonation, inversely zoned K-feldspar crystals, mafic clots, poikilitic plagioclase and quartz crystals, mixed apatite morphologies, zoned K-feldspar phenocrysts. The apparent pressure range of the magma hybridization event was calculated at 6.1 kbar to 4.6 kbar, while the temperature, calculated by independent methods, is in the range of 810°C-770°C. U-Pb age data of the hybrid rocks were obtained by in-situ LA-MC-ICP-MS analysis of zircon. The oscillatory zoned zircon crystals yield a concordia age of 368 ± 8 Ma (MSWD = 1.1), interpreted as the age of magma hybridization and timing of formation of the magmatic precursors. It is the oldest Variscan magmatic event in that part of the Tatra Mountains.

The formation age of ores from the Pebble Cu-Au-Mo giant deposit (Alaska, United States)

NASA Astrophysics Data System (ADS)

Kremenetskii, A. A.; Popov, V. S.; Gromalova, N. A.

2012-02-01

Zircons from the porphyry-like quartz-diorite boss of the Pebble Cu-Au-Mo deposit (southwest Alaska) have been examined. By their appearance and internal structure (cathode luminescence and electron probing), the zircons have been subdivided into four genetic groups: (1) xenogenic detrital (mainly rounded); (2) magmatogene (protolith crystal in the center and growth zone at the edge); (3) hydrothermally altered (with new-formed regeneration edges in growth zones); (4) metamict-altered (unconsolidated center of the crystal and sectoring in growth zones). Based on SHRIMP U-Pb dating for the principal heterogeneous elements in every group, the following stages of ore formation have been identified for the Pebble deposit: (a) crystallization of quartz diorite-porphyry bosses (95-92 Ma, the concordant age is 94.7 ± 1.5 Ma); (b) late magmatic metasomatic alterations with copper-molybdenum mineralization (92-85 Ma, the concordant age is 90.15 ± 0.78 Ma); (c) postmagmatic argillization with epithermal gold-sulfide mineralization (82-80 Ma, the concordant age is 82.9 ± 2.7 Ma).

Theoretical modeling of zircon's crystal morphology according to data of atomistic calculations

NASA Astrophysics Data System (ADS)

Gromalova, Natalia; Nikishaeva, Nadezhda; Eremin, Nikolay

2017-04-01

Zircon is an essential mineral that is used in the U-Pb dating. Moreover, zircon is highly resistant to radioactive exposure. It is of great interest in solving both fundamental and applied problems associated with the isolation of high-level radioactive waste. There is significant progress in forecasting of the most energetically favorable crystal structures at the present time. Unfortunately, the theoretical forecast of crystal morphology at high technological level is under-explored nowadays, though the estimation of crystal equilibrium habit is extremely important in studying the physical and chemical properties of new materials. For the first time, the thesis about relation of the equilibrium shape of a crystal with its crystal structure was put forward in the works by O.Brave. According to it, the idealized habit is determined in the simplest case by a correspondence with the reticular densities Rhkl of individual faces. This approach, along with all subsequent corrections, does not take into account the nature of atoms and the specific features of the chemical bond in crystals. The atomistic calculations of crystal surfaces are commonly performed using the energetic characteristics of faces, namely, the surface energy (Esurf), which is a measure of the thermodynamic stability of the crystal face. The stable crystal faces are characterized by small positive values of Esurf. As we know from our previous research (Gromalova et al.,2015) one of the constitutive factors affecting the value of the surface energy in calculations is a choice of potentials model. In this regard, we studied several sets of parameters of atomistic interatomic potentials optimized previously. As the first test model («Zircon 1») were used sets of interatomic potentials of interaction Zr-O, Si-O and O-O in the form of Buckingham potentials. To improve playback properties of zircon additionally used Morse potential for a couple of Zr-Si, as well as the three-particle angular harmonic potential. The other sets of interatomic potentials («Zircon 2, Zircon 3») differed from the first in that parameters was found with the help of quantum-chemical calculations of the structure «ab initio».The surface energies for different faces of zircon were calculated using Metadise code (Watson et al., 1996) at P4-3000 personal computer with Windows XP operating system. The computation time for one simple form was from 30 minutes to 12 hours. Calculations have shown that depending on the chosen model the surface energy of zircons faces several changes. For example, Esurf of face (331) obtained using models of potentials «Zircon 2», «Zircon 3» sufficiently similar (2.82 and 3.01 J/mol2 respectively). Meaning of Esurf of this face, calculated on the basis of set «Zircon 1» significantly lower (1,54 J/mol2). With regard to the face (100), it has low surface energies when selecting all three models, with a minimum value (1,14 J/mol2) in the model «Zircon 1». References: Gromalova N.A., Eremin N.N., Urusov V.S. Atomistic computer modeling of the crystal-morpology of corundum group minerals // Zapiski RMO. V. 144. №4. 2015. p. 84-92. Watson G.W., Kelsey E.T., de Leeuw N.H., Harris D.J, Parker S.C. Atomistic simulation of dislocations, surfaces and interfaces in MgO. Journal of the Chemical Society Faraday Transactions. 1996. V.92 P. 433-438.

Petrogenesis of Cretaceous volcanic-intrusive complex from the giant Yanbei tin deposit, South China: Implication for multiple magma sources, tin mineralization, and geodynamic setting

NASA Astrophysics Data System (ADS)

Li, Qian; Zhao, Kui-Dong; Lai, Pan-Chen; Jiang, Shao-Yong; Chen, Wei

2018-01-01

The giant Yanbei tin ore deposit is the largest porphyry-type tin deposit in South China. The orebodies are hosted by the granite porphyry in the central part of the Yanbei volcanic basin in southern Jiangxi Province. The Yanbei volcanic-intrusive complex mainly consists of dacitic-rhyolitic volcanic rocks, granite, granite porphyry and diabase dikes. In previous papers, the granite porphyry was considered as subvolcanic rocks, which came from the same single magma chamber with the volcanic rocks. In this study, zircon U-Pb ages and Hf isotope data, as well as whole-rock geochemical and Sr-Nd isotopic compositions of different magmatic units in the Yanbei complex are reported. Geochronologic results show that various magmatic units have different formation ages. The dacite yielded a zircon U-Pb age of 143 ± 1 Ma, and the granite porphyry has the emplacement age of 138 ± 1 Ma. Diabase dikes which represented the final stage of magmatism, yielded a zircon U-Pb age of 128 ± 1 Ma. Distinctive whole rock Sr-Nd and zircon Hf isotopic compositions suggest that these magmatic units were derived from different magma sources. The volcanic rocks were mainly derived from the partial melting of Paleoproterozoic metasedimentary rocks without additions of mantle-derived magma. The granite porphyry has an A-type geochemical affinity, and was derived from remelting of Paleo-Mesoproterozoic crustal source with involvement of a subordinate mantle-derived magma. The granite porphyry is also a typical stanniferous granite with high F (4070-6090 ppm) and Sn (7-39 ppm) contents. It underwent strongly crystal fractionation of plagioclase, K-feldspar, and accessory minerals (like apatite, Fe-Ti oxides), which may contribute to the tin mineralization. The diabase was derived by partial melting of enriched lithospheric mantle which had been metasomatised by slab-derived fluids. The change of magmatic sources reflected an increasing extensional tectonic environment, perhaps induced by slab rollback of subducted paleo-Pacific plate.

Rb-Sr, Sm-Nd, and U-Pb geochronology of the rocks within the Khlong Marui shear zone, southern Thailand

NASA Astrophysics Data System (ADS)

Kanjanapayont, Pitsanupong; Klötzli, Urs; Thöni, Martin; Grasemann, Bernhard; Edwards, Michael A.

2012-08-01

In southern Thailand, the Khlong Marui shear zone is dominated by a NNE-SSW striking high topographic lozenge shaped area of ca. 40 km long and 6 km wide between the Khlong Marui Fault and the Bang Kram Fault. The geology within this strike-slip zone consists of strongly deformed layers of mylonitic meta-sedimentary rocks associated with orthogneisses, mylonitic granites, and pegmatitic veins with a steeply dipping foliation. The strike-slip deformation is characterized by dextral ductile deformation under amphibolite facies and low to medium greenschist facies. In situ U-Pb ages of inherited zircon cores from all zircons in the Khlong Marui shear zone indicate that they have the same material from the Archean. Late Triassic to Late Cretaceous ages obtained for zircon outer cores of the mylonitic granite are probably related to a period of magmatic activity that was significantly influenced by the West Burma and Shan-Thai collision and the subduction along the Sunda Trench. The early dextral ductile deformation phase of the Khlong Marui shear zone in the Early Eocene suggested by U-Pb ages of zircon rims, and the later dextral transpressional deformation in the Late Eocene indicated by mica Rb-Sr ages. Rb-Sr, Sm-Nd, and U-Pb dating correlation implies that the major exhumation period of the ductile lens was in the Eocene. This period was tectonically influenced in the SE Asia region by the early India-Asia collision.

Initiation of the Bukadaban Feng Normal Fault and Implications for the Topographic Evolution of Northern Tibet

NASA Astrophysics Data System (ADS)

Niemi, N. A.; Chang, H.; Li, L.; Molnar, P. H.

2017-12-01

The Bukadaban Feng massif in northern Tibet forms the footwall of an east-west trending graben that is kinematically linked to the Kunlun fault. Extension across this graben accommodates left-lateral slip on the Kunlun fault, as evidenced by the 2001 Kunlun earthquake rupture. New geochronologic and thermochronologic data from Bukadaban Feng provide insight into the evolution of this normal fault system. The Bukadaban Feng massif is composed of two plutonic units, an eastern unit of dacitic composition and a western unit of rhyolitic composition. Sixty-five LA-ICP-MS zircon U-Pb age determinations on the rhyolitic unit reveal a range of ages from 873 - 6.3 Ma. CA-TIMS U-Pb zircon geochronology on the nine youngest of these zircons yields an emplacement age of 6.8 Ma. Twenty-seven LA-ICP-MS zircon U-Pb ages on the dacite range from 208 to 7.9 Ma. No coherent population of young zircons was observed, and CA-TIMS analysis was not performed. Zircon (U-Th)/He analysis on the dacite and rhyolite yield ages of 3.9 and 5.0 Ma, respectively, while apatite (U-Th-Sm)/He thermochronology on 5 samples collected from both units along the trace of the normal fault yield ages ranging from 1.4 - 2.6 Ma. The emplacement ages and compositions of plutonic rocks at Bukadaban Feng are consistent with the eruptive timing and geochemistry of silicic volcanic rocks in the graben (Zhang et al., 2012). Silicic magmatism is often associated with the onset of crustal extension, and the combination of plutonism and correlative silicic volcanism provides an indirect constraint on the initiation of this graben at 7 Ma. The distinct zircon (U-Pb) and (U-Th)/He ages indicates that the rocks presently exposed at Bukadaban Feng were emplaced at ambient temperatures in excess of 180°C. The zircon and apatite thermochronologic data require exhumation at rates of 1-2 mm/yr since the late Miocene. A 7 Ma initiation age for the Bukadaban Feng normal fault is consistent with both published estimates of total offset across the Kunlun fault ( 70 km; Kidd and Molnar, 1988) and recent fault slip rates ( 10 mm/yr; van der Woerd et al., 2002). To the extent that the onset of extension and strike-slip faulting can be related to the attainment of high topography, these new data imply that northernmost Tibet may have reached maximum elevations as recently as the late Miocene.

Provenance of sediments from Sumatra, Indonesia - Insights from detrital U-Pb zircon geochronology, heavy mineral analyses and Raman spectroscopy

NASA Astrophysics Data System (ADS)

Liebermann, C.; Hall, R.; Gough, A.

2017-12-01

The island of Sumatra is situated at the southwestern margin of the Indonesian archipelago. Although it is the sixth largest island in the world, the geology of the Sumatra sedimentary basins and their underlying basement is relatively poorly understood in terms of their provenance. This work is a multi-proxy provenance study utilizing U-Pb detrital zircon dating by LA-ICP-MS combined with optical and Raman spectroscopy-based heavy mineral analysis. It will help to unravel the stratigraphy of Sumatra, contribute to paleogeographic reconstruction of western SE Asia, and aid a wider understanding of Sumatran petroleum plays. Thin section analyses, heavy mineral assemblages, and >3500 concordant U-Pb zircon ages, from samples acquired during two fieldwork seasons indicate a mixed provenance for Cenozoic sedimentary formations, including both local igneous sources and mature basement rocks. Characteristic Precambrian zircon age spectra are found in all analysed Cenozoic sedimentary strata. These can be correlated with zircon age populations found in Sumatran basement rocks; Neoproterozoic and Mesoproterozoic age groups are dominant (c. 500-600 Ma, c. 850-1000 Ma, c. 1050-1200 Ma). Paleoproterozoic to Archaean zircons occur as minor populations. The Phanerozoic age spectra of the Cenozoic formations are characterised by distinct Carboniferous, Permo-Triassic, and Jurassic-Cretaceous zircon populations. Permo-Triassic zircons are interpreted to come from granitoids in the Malay peninsula or Sumatra itself. Eocene to Lower Miocene strata are characterised by ultrastable heavy minerals such as zircon, tourmaline, and rutile, which together with garnet, suggest the principal sources were igneous and metamorphic basement rocks. Cenozoic zircons appear only from the Middle Miocene onwards. This change is interpreted to indicate a new contribution from a local volcanic arc, and is supported by the occurrence of unstable heavy minerals such as apatite and clinopyroxene, and the presence of volcanic quartz. The absence of an earlier volcanic contribution is surprising since subduction is widely considered to have been active from the Eocene.

Late Proterozoic-Paleozoic evolution of the Arctic Alaska-Chukotka terrane based on U-Pb igneous and detrital zircon ages: Implications for Neoproterozoic paleogeographic reconstructions

USGS Publications Warehouse

Amato, J.M.; Toro, J.; Miller, E.L.; Gehrels, G.E.; Farmer, G.L.; Gottlieb, E.S.; Till, A.B.

2009-01-01

The Seward Peninsula of northwestern Alaska is part of the Arctic Alaska-Chukotka terrane, a crustal fragment exotic to western Laurentia with an uncertain origin and pre-Mesozoic evolution. U-Pb zircon geochronology on deformed igneous rocks reveals a previously unknown intermediate-felsic volcanic event at 870 Ma, coeval with rift-related magmatism associated with early breakup of eastern Rodinia. Orthogneiss bodies on Seward Peninsula yielded numerous 680 Ma U-Pb ages. The Arctic Alaska-Chukotka terrane has pre-Neoproterozoic basement based on Mesoproterozoic Nd model ages from both 870 Ma and 680 Ma igneous rocks, and detrital zircon ages between 2.0 and 1.0 Ga in overlying cover rocks. Small-volume magmatism occurred in Devonian time, based on U-Pb dating of granitic rocks. U-Pb dating of detrital zircons in 12 samples of metamorphosed Paleozoic siliciclastic cover rocks to this basement indicates that the dominant zircon age populations in the 934 zircons analyzed are found in the range 700-540 Ma, with prominent peaks at 720-660 Ma, 620-590 Ma, 560-510 Ma, 485 Ma, and 440-400 Ma. Devonian- and Pennsylvanian-age peaks are present in the samples with the youngest detrital zircons. These data show that the Seward Peninsula is exotic to western Laurentia because of the abundance of Neoproterozoic detrital zircons, which are rare or absent in Lower Paleozoic Cordilleran continental shelf rocks. Maximum depositional ages inferred from the youngest detrital age peaks include latest Proterozoic-Early Cambrian, Cambrian, Ordovician, Silurian, Devonian, and Pennsylvanian. These maximum depositional ages overlap with conodont ages reported from fossiliferous carbonate rocks on Seward Peninsula. The distinctive features of the Arctic Alaska-Chukotka terrane include Neoproterozoic felsic magmatic rocks intruding 2.0-1.1 Ga crust overlain by Paleozoic carbonate rocks and Paleozoic siliciclastic rocks with Neoproterozoic detrital zircons. The Neoproterozoic ages are similar to those in the peri-Gondwanan Avalonian-Cadomian arc system, the Timanide orogen of Baltica, and other circum-Arctic terranes that were proximal to Arctic Alaska prior to the opening of the Amerasian basin in the Early Cretaceous. Our Neoproterozoic reconstruction places the Arctic Alaska-Chukotka terrane in a position near Baltica, northeast of Laurentia, in an arc system along strike with the Avalonian-Cadomian arc terranes. Previously published faunal data indicate that Seward Peninsula had Siberian and Laurentian links by Early Ordovician time. The geologic links between the Arctic Alaska-Chukotka terrane and eastern Laurentia, Baltica, peri-Gondwanan arc terranes, and Siberia from the Paleoproterozoic to the Paleozoic help to constrain paleogeographic models from the Neoproterozoic history of Rodinia to the Mesozoic opening of the Arctic basin. ?? 2009 Geological Society of America.

Crustal nature and origin of the Russian Altai: Implications for the continental evolution and growth of the Central Asian Orogenic Belt (CAOB)

NASA Astrophysics Data System (ADS)

Cai, Keda; Sun, Min; Buslov, M. M.; Jahn, Bor-ming; Xiao, Wenjiao; Long, Xiaoping; Chen, Huayong; Wan, Bo; Chen, Ming; Rubanova, E. S.; Kulikova, A. V.; Voytishek, E. E.

2016-04-01

The Central Asian Orogenic Belt is a gigantic tectonic collage of numerous accreted terranes. However, its geodynamic evolution has been hotly debated primarily due to incomplete knowledge on the nature of these enigmatic terranes. This work presents new detrital zircon U-Pb and Hf isotopic data to constrain the crustal nature and origin of the Russian Altai, a critical segment of Altai-Mongolian terrane. The youngest zircon 206Pb/238U ages of 470 Ma constrain that the Terekta Formation, previously envisaged as Precambrian basement, was actually deposited after the Middle Ordovician. As for the three more sedimentary sequences above the Terekta Formation, they have youngest zircon 206Pb/238U ages of 425 Ma, 440 Ma and 380 Ma, respectively, indicating their depositions likely in the Late Silurian to Devonian. From all analyses, it is noted that many zircon U-Pb ages cluster at ca. 520 Ma and ca. 800 Ma, and these zircons display oscillatory zoning and have subhedral to euhedral morphology, which, collectively, suggests that adjacent Neoproterozoic to Paleozoic igneous rocks were possibly dominant in the sedimentary provenance. Additionally, a few rounded Archean to Mesoproterozoic zircon grains are characterized by complex texture, which are interpreted as recycling materials probably derived from the Tuva-Mongolian microcontinent. Precambrian rocks have not been identified in the Russian Altai, Chinese Altai and Mongolian Altai so far, therefore, Precambrian basement may not exist in the Altai-Mongolian terrane, but this terrane probably represents a large subduction-accretion complex built on the margin of the Tuva-Mongolian microcontinent in the Early Paleozoic. Multiple episodes of ridge-trench interaction may have caused inputs of mantle-derived magmas to trigger partial melting of the newly accreted crustal materials, which contributed to the accretionary complex. During accretionary orogenesis of the CAOB, formation of such subduction-accretion complex is likely ubiquitous, indicating continental crust growth by both lateral accumulation and vertical basaltic injection.

Detrital and volcanic zircon U-Pb ages from southern Mendoza (Argentina): An insight on the source regions in the northern part of the Neuquén Basin

NASA Astrophysics Data System (ADS)

Naipauer, Maximiliano; Tapia, Felipe; Mescua, José; Farías, Marcelo; Pimentel, Marcio M.; Ramos, Victor A.

2015-12-01

The infill of the Neuquén Basin recorded the Meso-Cenozoic geological and tectonic evolution of the southern Central Andes being an excellent site to investigate how the pattern of detrital zircon ages varies trough time. In this work we analyze the U-Pb (LA-MC-ICP-MS) zircon ages from sedimentary and volcanic rocks related to synrift and retroarc stages of the northern part of the Neuquén Basin. These data define the crystallization age of the synrift volcanism at 223 ± 2 Ma (Cerro Negro Andesite) and the maximum depositional age of the original synrift sediments at ca. 204 Ma (El Freno Formation). Two different pulses of rifting could be recognized according to the absolute ages, the oldest developed during the Norian and the younger during the Rhaetian-Sinemurian. The source regions of the El Freno Formation show that the Choiyoi magmatic province was the main source rock of sediment supply. An important amount of detrital zircons with Triassic ages was identified and interpreted as a source area related to the synrift magmatism. The maximum depositional age calculated for the Tordillo Formation in the Atuel-La Valenciana depocenter is at ca. 149 Ma; as well as in other places of the Neuquén Basin, the U-Pb ages calculated in the Late Jurassic Tordillo Formation do not agree with the absolute age of the Kimmeridgian-Tithonian boundary (ca. 152 Ma). The main source region of sediment in the Tordillo Formation was the Andean magmatic arc. Basement regions were also present with age peaks at the Carboniferous, Neoproterozoic, and Mesoproterozoic; these regions were probably located to the east in the San Rafael Block. The pattern of zircon ages summarized for the Late Jurassic Tordillo and Lagunillas formations were interpreted as a record of the magmatic activity during the Triassic and Jurassic in the southern Central Andes. A waning of the magmatism is inferred to have happened during the Triassic. The evident lack of ages observed around ca. 200 Ma suggests cessation of the synrift magmatism. The later increase in magmatic activity during the Early Jurassic is attributed to the onset of Andean subduction, with maximum peaks at ca. 191 and 179 Ma. The trough at ca. 165 Ma and the later increase in the Late Jurassic could be explained by changes in the relative convergence rate in the Andean subduction regime, or by the shift to a more mafic composition of the magmatism with minor zircon fertility.

Correlated microanalysis of zircon: Trace element, δ 18O, and U-Th-Pb isotopic constraints on the igneous origin of complex >3900 Ma detrital grains

NASA Astrophysics Data System (ADS)

Cavosie, Aaron J.; Valley, John W.; Wilde, Simon A.; E. I. M. F.

2006-11-01

The origins of >3900 Ma detrital zircons from Western Australia are controversial, in part due to their complexity and long geologic histories. Conflicting interpretations for the genesis of these zircons propose magmatic, hydrothermal, or metamorphic origins. To test the hypothesis that these zircons preserve magmatic compositions, trace elements [rare earth elements (REE), Y, P, Th, U] were analyzed by ion microprobe from a suite of >3900 Ma zircons from Jack Hills, Western Australia, and include some of the oldest detrital zircons known (4400-4300 Ma). The same ˜20 μm domains previously characterized for U/Pb age, oxygen isotope composition (δ 18O), and cathodoluminescence (CL) zoning were specifically targeted for analysis. The zircons are classified into two types based on the light-REE (LREE) composition of the domain analyzed. Zircons with Type 1 domains form the largest group (37 of 42), consisting of grains that preserve evolved REE compositions typical of igneous zircon from crustal rocks. Grains with Type 1 domains display a wide range of CL zoning patterns, yield nearly concordant U/Pb ages from 4400 to 3900 Ma, and preserve a narrow range of δ 18O values from 4.7‰ to 7.3‰ that overlap or are slightly elevated relative to mantle oxygen isotope composition. Type 1 domains are interpreted to preserve magmatic compositions. Type 2 domains occur in six zircons that contain spots with enriched light-REE (LREE) compositions, here defined as having chondrite normalized values of La N > 1 and Pr N > 10. A subset of analyses in Type 2 domains appear to result from incorporation of sub-surface mineral inclusions in the analysis volume, as evidenced by positively correlated secondary ion beam intensities for LREE, P, and Y, which are anti-correlated to Si, although not all Type 2 analyses show these features. The LREE enrichment also occurs in areas with discordant U/Pb ages and/or high Th/U ratios, and is apparently associated with past or present radiation damage. The enrichment is not attributed to hydrothermal alteration, however, as oxygen isotope ratios in Type 2 domains overlap with magmatic values of Type 1 domains, and do not appear re-set as might be expected from dissolution or ion-exchange processes operating at variable temperatures. Thus, REE compositions in Type 2 domains where mineral inclusions are not suspected are best interpreted to result from localized enrichment of LREE in areas with past or present radiation damage, and with a very low fluid/rock ratio. Correlated in situ analyses allow magmatic compositions in these complex zircons to be distinguished from the effects of secondary processes. These results are additional evidence for preservation of magmatic compositions in Jack Hills zircons, and demonstrate the benefits of detailed imaging in studies of complicated detrital zircons of unknown origin. The data reported here support previous interpretations that the majority of >3900 Ma zircons from the Jack Hills have an origin in evolved granitic melts, and are evidence for the existence of continental crust very early in Earth's history.

2.9, 2.36, and 1.96 Ga zircons in orthogneiss south of the Red River shear zone in Viet Nam: evidence from SHRIMP U-Pb dating and tectonothermal implications

NASA Astrophysics Data System (ADS)

Nam, Tran Ngoc; Toriumi, Mitsuhiro; Sano, Yuji; Terada, Kentaro; Thang, Ta Trong

2003-05-01

Orthogneissic rocks coexisting with migmatites and containing small amphibolite lenses are exposed in the center of the metamorphic belt which runs parallel to the Day Nui Con Voi-Red River shear zone in northern Viet Nam. The orthogneiss complex has given some radiogenic dates of Early Proterozoic and Late Archean, which are the oldest ages ever registered for the Southeast Asian continent. Zircon grains separated from three samples of the orthogneiss complex have been dated to establish the protolith age and the timing of high-grade tectonothermal events in the complex. Sixty-five SHRIMP U-Th-Pb analyses of these zircons define three age groups of 2.84-2.91, 2.36, and 1.96 Ga. The age groups correspond to three periods of zircon generation. The oldest ˜2.9 Ga cores indicate a minimum age for the protolith of the orthogneiss complex. Two younger generations (including ˜2.36 Ga outer-cores and ˜1.96 Ga rims) probably grew during later high-grade tectono-metamorphic events, which were previously suggested by K-Ar and 40Ar/ 39Ar cooling ages of ˜2.0 Ga for synkinematic hornblendes. An early thermal history of the orthogneiss complex has been constrained, including a primary magma-crystallization stage starting at ˜2.9 Ga, followed by two Early Proterozoic (˜2.36 and ˜1.96 Ga) high-grade tectonothermal events. The ca. 2.9 Ga protolith age of the orthogneiss complex documented in this study provides new convincing evidence for the presence of Archean rocks in Indochina, and clearly indicates that the crustal evolution of northern Viet Nam started as early as Late Archean time.

Constraining Metamorphic Timing and Processes by Dating Garnet, Zircon, Titanite and Monazite in UHP and HP Rocks from Weihai, Sulu UHP Terrane, Eastern China

NASA Astrophysics Data System (ADS)

Wang, D.; Vervoort, J. D.; Fisher, C. M.; Cao, H.

2016-12-01

The Sulu UHP terrane is the extension of the Dabie orogenic belt to the east, offset 500 km to the northeast by the Tanlu fault [1]. The focus of this study, the Weihai area, is located at the northernmost part of the Sulu UHP terrane, and consists mainly of gneisses overprinted by amphibolite-facies assemblages, in addition to minor eclogite, granulite, and some ultramafic rocks [1]. Time constrains are critical to our understanding of the processes of UHP metamorphism, as well as the tectonic evolution of the region. In the last decade, U-Pb dating of metamorphic domains of zircons has been widely applied to determine the history of the UHP metamorphism (240 - 220 Ma) [1]. Recent garnet Lu-Hf dating from the Dabie terrane (240 - 220Ma) suggests the initiation of prograde metamorphism to be prior to ca. 240 Ma [2]. In-situ U-Pb dating of accessary minerals using LA-ICPMS (i.e. monazite, titanite, rutile, etc.), can provide important information to augment and complement the zircon U-Pb metamorphic dates. In this study, we collected samples throughout the Weihai area. Protolith ages of these samples range from Paleoproterozoic to Neoproterozoic ( 1850 - 700 Ma) as indicated by U-Pb dating of zircon cores. Zircon metamorphic rims yield U-Pb ages of 240 - 220 Ma, likely indicating the UHP stage of the Sulu terrane [3]. Four eclogites yield Lu-Hf garnet isochrons with dates between 239 and 224 Ma, consistent with garnet Lu-Hf dates from Dabie UHP terrane [2]. Sm-Nd isochrons indicate systematic younger dates (220 - 210 Ma) interpreted as cooling ages. Titanites extracted from four samples give U-Pb ages ranging from 220 to 200 Ma, in agreement with the titanite dates from the southern Sulu terrane [4]. Monazites from three samples give precise dates between 214 and 211 Ma. Collectively, monazite and titanite U-Pb ages are broadly consistent with the garnet Sm-Nd isochrons, and thus we interpret these as cooling ages. Based on the dates of different systems/minerals presented above, we suggest the prograde metamorphism of Weihai UHP terrane likely took place prior to 240 Ma, and the peak of the UHP stage is likely between 240 and 220 Ma. [1] Zhang et al., Gondwana Res., 16 (2009) 1-26 [2] Cheng et al., J. Metamorphic Geol., 26 (2008), 741-758 [3] Liou et al., J. Asian Earth Sci., 35 (2009), 199-231 [4] Chen and Zheng, GCA, 150(2015), 53-73

Mechanisms and timescales of generating eruptible rhyolitic magmas at Yellowstone caldera from zircon and sanidine geochronology and geochemistry

USGS Publications Warehouse

Stelten, Mark; Cooper, Kari M.; Vazquez, Jorge A.; Calvert, Andrew T.; Glessner, Justin G

2015-01-01

We constrain the physical nature of the magma reservoir and the mechanisms of rhyolite generation at Yellowstone caldera via detailed characterization of zircon and sanidine crystals hosted in three rhyolites erupted during the (ca. 170 – 70 ka) Central Plateau Member eruptive episode – the most recent post-caldera magmatism at Yellowstone. We present 238U-230Th crystallization ages and trace-element compositions of the interiors and surfaces (i.e., unpolished rims) of individual zircon crystals from each rhyolite. We compare these zircon data to 238U- 230Th crystallization ages of bulk sanidine separates coupled with chemical and isotopic data from single sanidine crystals. Zircon age and trace-element data demonstrate that the magma reservoir that sourced the Central Plateau Member rhyolites was long-lived (150 – 250 kyr) and genetically related to the preceding episode of magmatism, which occurred ca. 256 ka. The interiors of most zircons in each rhyolite were inherited from unerupted material related to older stages of Central Plateau Member magmatism or the preceding late Upper Basin Member magmatism (i.e., are antecrysts). Conversely, most zircon surfaces crystallized near the time of eruption from their host liquids (i.e., are autocrystic). The repeated recycling of zircon interiors from older stages of magmatism demonstrates that sequentially erupted Central Plateau Member rhyolites are genetically related. Sanidine separates from each rhyolite yield 238U-230Th crystallization ages at or near the eruption age of their host magmas, coeval with the coexisting zircon surfaces, but are younger than the coexisting zircon interiors. Chemical and isotopic data from single sanidine crystals demonstrate that the sanidines in each rhyolite are in equilibrium with their host melts, which considered along with their near-eruption crystallization ages suggests that nearly all CPM sanidines are autocrystic. The paucity of antecrystic sanidine crystals relative to antecrystic zircons require a model where eruptible rhyolites are generated by extracting melt and zircons from a long-lived mush of immobile crystal-rich magma. In this process the larger sanidine crystals remain trapped in the locked crystal network. The extracted melts (plus antecrystic zircon) amalgamate into a liquid dominated (i.e., eruptible) magma body that is maintained as a physically distinct entity relative to the bulk of the long-lived crystal mush. Zircon surfaces and sanidines in each rhyolite crystallize after melt extraction/amalgamation and their ages constrain the residence time of eruptible magmas at Yellowstone. Residence times of the large volume rhyolites (~40 – 70 km3) are ≤ 1 kyr (conservatively < 6 kyr), which suggests that large volumes of rhyolite can be generated rapidly by extracting melt from a crystal mush. Because the lifespan of the crystal mush that sourced the Central Plateau Member rhyolites is two orders of magnitude longer than the residence time of eruptible magma bodies within the reservoir, it is apparent that the Yellowstone magma reservoir spends most of its time in a largely-crystalline (i.e., uneruptible) state, similar to the present-day magma reservoir, and that eruptible magma bodies are ephemeral features.

Precise K-Ar, 40Ar/39Ar, Rb-Sr and U/Pb mineral ages from the 27.5 Ma fish canyon tuff reference standard

USGS Publications Warehouse

Lanphere, M.A.; Baadsgaard, H.

2001-01-01

The accuracy of ages measured using the 40Ar/39Ar technique is affected by uncertainties in the age of radiation fluence-monitor minerals. At present, there is lack of agreement about the ages of certain minerals used as fluence monitors. The accuracy of the age of a standard may be improved if the age can be measured using different decay schemes. This has been done by measuring ages on minerals from the Oligocene Fish Canyon Tuff (FCT) using the K-Ar, 40Ar/39Ar. Rb-Sr and U/Pb methods. K-Ar and 40Ar/39Ar total fusion ages of sanidine, biotite and hornblende yielded a mean age of 27.57 ?? 0.36 Ma. The weighted mean 40Ar/39Ar plateau age of sanidine and biotite is 27.57 ?? 0.18 Ma. A biotite-feldspar Rb-Sr isochron yielded an age of 27.44 ?? 0.16 Ma. The U-Pb data for zircon are complex because of the presence of Precambrian zircons and inheritance of radiogenic Pb. Zircons with 207Pb/235U < 0.4 yielded a discordia line with a lower concordia intercept of 27.52 ?? 0.09 Ma. Evaluation of the combined data suggests that the best age for FCT is 27.51 Ma. Published by Elsevier Science B.V.

Correlating rates of magmatic arc unroofing and sedimentation using detrital zircon U/Pb and (U-Th)/He thermochronology

NASA Astrophysics Data System (ADS)

Fosdick, J. C.

2017-12-01

Double and triple dating of minerals using multiple geo-thermochronometers has revolutionized efforts to evaluate complex thermotectonic histories of orogens, isolate unique sedimentary sources, and quantify basin burial reheating. A persisting challenge is to distinguish volcanic sources from rapidly exhumed sources, with the simplistic premise that coincident cooling dates among high- to low-temperature thermochronometers are diagnostic of volcanic sources. Coupled zircon U/Pb and (U-Th)/He geo-thermochronometry from the Miocene Bermejo foreland basin in the southern Central Andes reveals a high temporal resolution of unroofing signatures of the Choiyoi Group, a Permian-Triassic silicic volcanic and plutonic complex, and the Pennsylvanian-Permian Colangüil batholith. Both units are important sediment sources within the High Andes for the Cenozoic east-flowing sediment routing systems. Results show fluvial sourcing of Colangüil detrital zircons with progressively greater partial loss of He (<8% to 12-23% fractional loss from 9.5 Ma to 6 Ma), as indicated by upsection younging of zircon He dates for a given U/Pb age cluster. These findings suggest erosion of increasingly deeper levels of the Colangüil arc during late Miocene development of the High Andes. This progression of higher He loss and thus younger He dates during sedimentation for a given U/Pb age cluster is analogous to the magmatic arc unroofing trend revealed by undissected to dissected arc provenance fields in sandstone petrography. Multi-method thermochronometry of detrital minerals may reveal an added level of information regarding rates of cooling, unroofing, and thermal evolution of magmatic systems as preserved in the detrital record.

Tectonic evolution of the Yarlung suture zone, Lopu Range region, southern Tibet

NASA Astrophysics Data System (ADS)

Laskowski, Andrew K.; Kapp, Paul; Ding, Lin; Campbell, Clay; Liu, XiaoHui

2017-01-01

The Lopu Range, located 600 km west of Lhasa, exposes a continental high-pressure metamorphic complex beneath India-Asia (Yarlung) suture zone assemblages. Geologic mapping, 14 detrital U-Pb zircon (n = 1895 ages), 11 igneous U-Pb zircon, and nine zircon (U-Th)/He samples reveal the structure, age, provenance, and time-temperature histories of Lopu Range rocks. A hornblende-plagioclase-epidote paragneiss block in ophiolitic mélange, deposited during Middle Jurassic time, records Late Jurassic or Early Cretaceous subduction initiation followed by Early Cretaceous fore-arc extension. A depositional contact between fore-arc strata (maximum depositional age 97 ± 1 Ma) and ophiolitic mélange indicates that the ophiolites were in a suprasubduction zone position prior to Late Cretaceous time. Five Gangdese arc granitoids that intrude subduction-accretion mélange yield U-Pb ages between 49 and 37 Ma, recording Eocene southward trench migration after collision initiation. The south dipping Great Counter Thrust system cuts older suture zone structures, placing fore-arc strata on the Kailas Formation, and sedimentary-matrix mélange on fore-arc strata during early Miocene time. The north-south, range-bounding Lopukangri and Rujiao faults comprise a horst that cuts the Great Counter Thrust system, recording the early Miocene ( 16 Ma) transition from north-south contraction to orogen-parallel (E-W) extension. Five early Miocene (17-15 Ma) U-Pb ages from leucogranite dikes and plutons record crustal melting during extension onset. Seven zircon (U-Th)/He ages from the horst block record 12-6 Ma tectonic exhumation. Jurassic—Eocene Yarlung suture zone tectonics, characterized by alternating episodes of contraction and extension, can be explained by cycles of slab rollback, breakoff, and shallow underthrusting—suggesting that subduction dynamics controlled deformation.

Petrogenesis of the Baishan granite stock, Eastern Tianshan, NW China: Geodynamic setting and implications for potential mineralization

NASA Astrophysics Data System (ADS)

Cao, MingJian; Qin, KeZhang; Li, GuangMing; Evans, Noreen J.; McInnes, Brent I. A.; Lu, WeiWei; Deng, Gang

2017-11-01

Located in a region rich in Cu-Ni and Mo mineralization, the Baishan granitic stock is barren for reasons that remain enigmatic. Whole rock elemental and Sr-Nd isotope analysis, major element analysis of a number of minerals, and zircon trace element, U-Pb and Hf isotope analysis were undertaken in order to reveal the petrogenesis of the granites. All granites show typical I-type characteristics including metaluminous to slightly peraluminous, calc-alkaline signatures with a strong depletion of Nb, Ta, Ti and P, enrichment of light rare earth elements and large ion lithophile elements (e.g., Cs, Rb, Th, U, K). In addition, a strong depletion in Ti and P, highly fractionated light rare earth element patterns and less fractionated heavy rare earth element patterns, and negative correlations between SiO2 and TiO2, Al2O3, MgO, FeOT, P2O5, Zr and Hf suggest significant fractional crystallization of amphibole, apatite, zircon and Ti-bearing minerals. Whole rock Sr-Nd and zircon Hf isotopic compositions show wide variations with (87Sr/86Sr)i values of 0.70358 to 0.70505, εNd (t) of 3.8 to 7.2, and εHf (t) of 2.4 to 12.2 indicating derivation from partial melting of juvenile lower crust with obvious addition of ancient crust. Zircon U-Pb ages indicate a formation age of 292 Ma, significantly older than the ore-forming granite porphyry and slightly older than the regional mafic-ultramafic, A-type and diabase magmatism of Eastern Tianshan. The granite stocks were likely derived during heating of ascending asthenospheric mantle above a mantle plume in the Early Permian. Mineral chemistry, saturation thermometry, mineral species and whole rock Fe2O3/FeO ratios indicate a crystallization temperature of > 980 to 665 °C, pressure of 1.6 kbar and oxygen fugacity of ≤ NNO for the granite stock. Comparing the geochemistry, magma source and crystallization environment for the Early Permian barren granite and Late Triassic ore-related granite porphyry, the low ratios of Sr/Y and low (La/Yb)N, and reduced oxidation state (≤ NNO) in the granitic stock are signatures of infertility for the Early Permian granite. This study implies high Mo mineralization potential for granitic rocks with high Sr/Y, (La/Yb)N and highly oxidized conditions.

Timing of deformation and rapid subsidence in the northern Altiplano, Peru: Insights from detrital zircon geochronology of the Ayaviri hinterland basin

NASA Astrophysics Data System (ADS)

Horton, B. K.; Perez, N. D.; Saylor, J. E.

2011-12-01

Although age constraints on crustal deformation and sediment accumulation prove critical to testing hypotheses of orogenic plateau construction, a common lack of marine facies, volcanic tuffs, and suitable fossils hinders many attempts at chronological reconstructions. A series of elevated retroarc basins along the axis of the Andean orogenic belt provide opportunities to define the timing of deformation and transformation from foreland to hinterland basin configurations. In this study, we present new U-Pb ages of detrital zircons in the Ayaviri intermontane basin of southern Peru (~4 km elevation) in the northern part of the central Andean (Altiplano) plateau. Nearly all sandstone samples show strong unimodal U-Pb age peaks (generally defined by > 5-50 zircons), suggesting these age peaks represent syndepositional volcanism and can be regarded as accurate estimates of true depositional (stratigraphic) age. Integration of these ages with structural and stratigraphic relationships demonstrate the utility of zircon U-Pb geochronology in defining both (1) the timing of basin partitioning and (2) the pace of sediment accumulation. (1) U-Pb ages for several sandstone samples from growth-strata packages associated with two basin-bounding faults reveal structural partitioning of the Ayaviri basin from late Oligocene to Miocene time. In the north, displacement along the southwest-directed Ayaviri thrust fault commenced in late Oligocene time (~28-24 Ma), inducing initial structural partitioning of an upper Eocene-Oligocene, > 5 km thick succession potentially representing an early Andean retroarc foreland basin. In the south, the Ayaviri basin was further disrupted by initial displacement along the northeast-directed Pasani thrust fault in early to middle Miocene time (~18-15 Ma). (2) Additional U-Pb analyses from the Ayaviri basin fill help delimit the long-term rates of sedimentation, suggesting relatively short-lived (< 5 Myr) pulses of accelerated accumulation. Rapid increases in Miocene sediment accumulation rates may reflect rapid subsidence driven by local thrust loading or piecemeal removal of lower crust/lithosphere, hypotheses to be considered by ongoing paleoelevation studies in the region. This approach highlights the potential for detrital zircon U-Pb geochronology to constrain deformation timing and tempo of sedimentation in proximal, coarse-grained basin-fill successions that are typically not amenable to chronostratigraphic techniques.

Zircon U-Pb dating, Hf analysis from the Horoman perdiotite -age constraint for lithospheric process, and tectonic juxtaposition of collision root zone-

NASA Astrophysics Data System (ADS)

Okamoto, K.; Yi, K.; Wang, K. L.; Chung, S. L.

2017-12-01

Hidaka metamorphic belt, Hokkaido, Japan is known as youngest arc-arc collision in the world. It ncludes the youngest granulite and the Horoman peridotite complex in the highest grade zone. Age of these rocks have been determined by various methods (K-Ar, U-Pb, Rb-Sr). However, the age of Horoman peridotite complex has not been determined yet. Only Yoshikawa et al 1993) reported the cooling age of the complex as 23 Ma according to whole rock Rb-Sr isochron. This study has performed U-Pb dating of zircons from the Horoman peridotite, and from the paragneiss surrounding the peridotite complex in order to determine the intrusive age of the Horoman peridotite complex into the lower crustal conditions. Several zircon grains were separated from the peridotite. All zircons are homogeneous exhibiting different age group; 267-278 Ma, 33-40 Ma and 18-20 Ma. Hf isotope analysis indicates that the 267-278 Ma is juvenile age and other two are recycled. As a result of this measurement, rims of the zircons from the gneisses show that 238U-206Pb ages are 20 Ma and detrital cores are ranging from 580-510 Ma, 60-50 Ma, 46-40 Ma and 27 Ma. The rim ages are from the gneiss suffered amphibolite facies and granulite faices, and there is a consistancy with zircon rim ages (19 Ma) from the granulite (Kemp et al 2007, Usuki et al 2006 and so on). That is, granulite faices metamorphism was coeval to regional metamorphism in the lower crust at 20 Ma. The zircon ages from the peridotite was probably related to local hydration related to precipitation of phlogopite at 20 Ma, I type magma infiltration at 40 Ma and lithosphere formation at 270 Ma. It is considered that the Horoman peridotite complex was part of the lithosphere at 270 Ma, and the joined as subarc mantle prior to I type magma activity at 40 Ma, aud suffered local hydration and regional metamorphism at 20 Ma. Ref. Kemp, A.I.S., et al., 2007, Geology, 35, 807-810; Usuki, T. et al, 2006, Island Arc, 14, 503-516.

A Late Silurian U-Pb zircon age for Linville metadiabase, Grandfather Mountain window, North Carolina

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fetter, A.H.; Goldberg, S.A.

1993-03-01

Linville metadiabase intrudes both Precambrian basement within the Grandfather Mountain window and nonconformably overlying Grandfather Mountain Formation. It occurs as sill-like and dike-like bodies, and is apparently not recognized outside of the window. Major element data classify the composition of the rock as tholeiitic basalt. Zircon was separated from a metadiabase body intruding metasiltstone of the Grandfather Mountain Formation west of Cranberry Knob. Zircon are clear, euhedral, with a l/w ratio of 3:1. Two abraded fractions (75-150 and < 75 [mu]m, both NM-2[degree]) yield concordant ages, which the authors report as 415 [plus minus] 3 Ma (2[sigma]), a weighted meanmore » of the two [sup 207]Pb/[sup 206]Pb ages. This Late Silurian (Ludlow) age is interpreted as the time of crystallization. Previously, similarities in mode of occurrence and major element composition have been used to correlate Linville metadiabase with late Precambrian mafic rocks within and outside of the window, as field relations do not constrain its age. Linville metadiabase thus is an unlikely candidate for feeders to the Montezuma metabasalt, which occurs as a flow immediately above metarhyolite dated as 742 [plus minus] 2 Ma (2[sigma]). Linville metadiabase may be one component of a magmatic pulse spanning 10-20 m.y. associated with the Acadian orogeny. The new zircon age places constraints on the timing of metamorphism and deformation, as Linville metadiabase is foliated, containing metamorphic assemblages from the biotite zone of the greenschist facies. The age and fabric relations are permissive evidence of post-Taconic, Acadian or Alleghanian orogeny.« less

Using the methods of radiospectroscopy (EPR, NMR) to study the nature of the defect structure of solid solutions based on lead zirconate titanate (PZT).

PubMed

Bykov, Igor; Zagorodniy, Yuriy; Yurchenko, Lesya; Korduban, Alexander; Nejezchleb, Karel; Trachevsky, Vladimir; Dimza, Vilnis; Jastrabik, Lubomir; Dejneka, Alexander

2014-08-01

The nature of intrinsic and impurity point defects in lead zirconate titanate (PZT) ceramics has been explored. Using electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), and X-ray photoelectron spectroscopy (XPS) methods, several impurity sites have been identified in the materials, including the Fe(3+)-oxygen vacancy (VO) complex and Pb ions. Both of these centers are incorporated into the PZT lattice. The Fe(3+) –VО paramagnetic complex serves as a sensitive probe of the local crystal field in the ceramic; the symmetry of this defect roughly correlates with PZT phase diagram as the composition is varied from PbTiO3 to PbZrO3. NMR spectra (207)Pb in PbTiO3, PbZrO3, and PZT with iron content from 0 to 0.4 wt% showed that increasing the iron concentration leads to a distortion of the crystal structure and to improvement of the electrophysical parameters of the piezoceramics. This is due to the formation of a phase which has a higher symmetry, but at high concentrations of iron (>0.4 wt%), it leads to sharp degradation of electrophysical parameters.

New geochronological history of the Mbuji-Mayi Supergroup (Proterozoic, DRC) through U-Pb and Sm-Nd dating

NASA Astrophysics Data System (ADS)

François, Camille; Baludikay, Blaise K.; Storme, Jean-Yves; Baudet, Daniel; Paquette, Jean-Louis; Fialin, Michel; Debaille, Vinciane; Javaux, Emmanuelle J.

2016-04-01

The Mbuji-Mayi Supergroup, DRC is located between the Archean-Paleoproterozoic Kasai Craton and the Mesoproterozoic Kibaran Belt. This sedimentary sequence, unaffected by regional metamorphism, preserves a large diversity of well-preserved acritarchs (organic-walled microfossils), evidencing the diversification of complex life (early eukaryotes) for the first time in mid-Proterozoic redox stratified oceans of Central Africa (Baludikay et al., in review). This Supergroup is composed of two distinct lithostratigraphic successions (i) BI Group: a lower siliciclastic sequence (ca. 1175 Myr to ca. 882 Myr or ca. 1050 Myr (Cahen, 1954; Holmes & Cahen, 1955; Delpomdor et al., 2013) unconformably overlying the ca. 2.82-2.56 Gyr granitoid Dibaya Complex to the North (Cahen & Snelling; recent notice on DRC geological map); and (ii) BII Group: a poorly age-constrained upper carbonate sequence with sparse shales . Basaltic lavas (including pillow lavas) overlying the Mbuji-Mayi Supergroup were dated around 950 Myr (Cahen et al., 1974; Cahen et al., 1984). To better constraint the age of this Supergroup in the Meso-Neoproterozoic limit, we combine different geochronological methods, in particular on diagenetic minerals such as monazite (Montel et al., 1996; Rasmussen & Muhling, 2007) and xenotime (McNaughton et al., 1999) but also on detrital zircons. For the BI Group, results of in situ U-Pb dating with LA-ICP-MS on monazite, xenotime and zircon (Laboratoire Magmas et Volcans, Clermont-Ferrand) provide ages between 2.9 and 1.2 Gyr for zircons and between 1.4 and 1.03 Gyr for monazites and xenotimes. New results of in situ U-Th-Pb dating of well-crystallized monazites and xenotimes with Electron MicroProbe (Camparis, UPMC, Paris), highlight that some crystals display zonations with an inherited core older than 1125 Myr and diagenetic rims around 1050-1075 Myr. This suggests that the diagenesis of BI Group is younger than 1175 Myr (Delpomdor et al., 2013) and probably around 1030-1075 Myr, coherent with an age on 2 syngenetic galenas around 1055 Myr for the top of BI Group (Cahen, 1954; Holmes & Cahen, 1955). Sm-Nd datings on basaltic pillow lavas overlying the Mbuji-Mayi Supergroup (previously dated around 950 Myr (Cahen et al., 1974; Cahen et al., 1984) are in progress (Laboratoire G-Time, ULB, Bruxelles) to precisely limit in time the end of deposition of this Supergroup.

Provenance of Modern Soils and Limestone and Chert Bedrock of Middle Tennessee Assessed Using Detrital Zircon U-Pb Geochronology

NASA Astrophysics Data System (ADS)

Ayers, J. C.; Katsiaficas, N. J.; Wang, X.

2014-12-01

Relatively thick soils mantle limestone bedrock throughout much of middle TN. Detrital zircon U-Pb geochronology was used to test two hypotheses: 1) That soil formed by accumulation of insoluble residue during chemical weathering of "dirty" limestone bedrock. 2) That an exotic component, perhaps wind-blown loess, was deposited and weathered to form soil. Samples of soil and underlying bedrock were collected from flat surfaces at the tops of cliffs. At Site 1 the Mississippian cherty limestone of the Fort Payne Formation was collected along with the B1 and B2 horizons of the overlying ultisol. At Site 2 a composite sample of A and B horizons of an alfisol and a sample of the underlying Ordovician limestone of the Hermitage Formation were collected. Zircon was recovered from soil and limestone samples, imaged using cathodoluminescence, and analyzed for trace elements and U-Pb isotopes using a 193 nm laser and quadrupole ICP-MS. Discordant analyses were discarded and 206Pb/238U ages are reported. Trace element concentrations and ratios in zircon seem to not be useful as provenance indicators. However, comparison of U-Pb age spectra showed that soils at both sites predominantly formed by weathering of limestone, with a small exotic component. The Hermitage has significant age peaks at ~1330, 1043, 955 and 439 Ma, and its overlying soil has age peaks at 1410, 1235, 1036 and 442 Ma. The age spectra are significantly different (Kolmogorov-Smirnov probability P = 0.01 < 0.05 significance). The Fort Payne has age peaks at ~1253, 967 and 417 Ma, while the B1 has age peaks at 1440, 1182, 1012 and 450 Ma (K-S P = 0.051) and the B2 at 1240, 941, 362, 81 and 33 Ma (K-S P = 0.073). The young ages in B2 require an exotic component that may account for ~25% of the measured ages. The source of the exotic material has not yet been identified, but its zircon age spectrum does not match previously published age spectra for the regional Pleistocene Peoria loess. Bedrock age peaks overlap with the Grenville, Taconic and Acadian orogenies of eastern North America. This study demonstrates that dating of detrital zircon is a powerful tool for determining the provenance of soil and limestone.

U-Pb zircon geochronology and Zr-in-rutile thermometry of eclogites from the Dulan area, North Qaidam ultra-high pressure (UHP) terrane, western China

NASA Astrophysics Data System (ADS)

Hernández Uribe, D.; Stubbs, K.; Lehman, M. R.; Gilmore, V.; Kylander-Clark, A. R.; Mattinson, C. G.

2016-12-01

The Dulan area, in the North Qaidam terrane, exposes UHP eclogites and gneisses that experienced a 20 Myr UHP event at P-T conditions of 30 kbar and 700 °C. Two eclogites were analyzed using Zr-in-rutile thermometry and zircon U-Pb + trace element analysis to constrain the metamorphic evolution of the area. A kyanite-phengite eclogite presents a mineral assemblage of grt + omp + ph + ky + rt + zo + qz. Rutile analyses show a Zr concentration of 173-250 ppm with a mean of 207 ± 19 ppm. The calculated temperatures yielded 685-716 °C with an average of 700 ± 7°C. Zircon U-Pb analyses gave an upper intercept age of 880 ± 89 Ma. These analyses from cathodoluminiscence (CL)-dark core zircons show a negative Eu anomaly and a steep HREE slope suggesting a magmatic origin for the protolith. Analyses from CL-bright rims gave a weighted mean age of 427 ± 2 Ma. These zircons show an eclogite facies trace elements pattern suggesting that the age represent the HP-UHP event. Titanium concentration in zircons gave a weighted mean of 4.41 ± 0.25 ppm. This Ti concentration yielded a calculated temperature of 674 °C A phengite eclogite shows a mineral assemblage of grt + omp + ph + rt + zo + qz. Rutile in matrix analyses show a Zr concentration of 123-161 ppm with a mean of 139 ± 9 ppm. Calculated temperatures for these rutiles ranges from 659-680 °C with a mean temperature of 668 ± 5 °C. U-Pb analyses from CL-dark zircon cores gave a weighted mean age of 844 ± 7 Ma. These zircons show a negative Eu anomaly and a steep HREE slope suggesting a magmatic origin for the protolith. Analyses from CL-grey rims gave a weighted mean age of 433 ± 4 Ma. These zircons show an eclogite facies trace elements pattern, representing the timing of the HP-UHP event. Titanium concentration in zircons gave a weighted mean of 3.13 ± 0.34 ppm. This concentration yielded calculated temperature 647 °C. The obtained ages are in the same range as the ones obtained for the northern and southern Dulan area suggesting that all the area belongs to the same tectonic unit and experienced the same continental collision metamorphism.

The Mesoarchean Tiejiashan-Gongchangling potassic granite in the Anshan-Benxi area, North China Craton: Origin by recycling of Paleo- to Eoarchean crust from U-Pb-Nd-Hf-O isotopic studies

NASA Astrophysics Data System (ADS)

Dong, Chunyan; Wan, Yusheng; Xie, Hangqiang; Nutman, Allen P.; Xie, Shiwen; Liu, Shoujie; Ma, Mingzhu; Liu, Dunyi

2017-10-01

Mesoarchean and older potassic granites are important indicators of recycling of ancient continental crust early in Earth's history. This study of integrated whole rock and zircon geochemistry and geochronology reports the age and identification of the source materials of the > 200 km2 Mesoarchean Tiejiashan-Gongchangling granite in the Anshan-Benxi area, North China Craton, the largest pre-Neoarchean granite domain in the craton. SHRIMP U-Pb zircon dating on 15 samples indicates the magmatic crystallization of the granites between 2.95 and 3.0 Ga and reveals a superimposed tectonothermal event at 2.91 Ga. The granites are characterized by high SiO2 and K2O, low CaO, FeOt, MgO and TiO2 with peraluminuous features. They show large variations in (La/Yb)n and strong negative Eu and Ba anomalies and Nb, P and Ti depletions. Whole rock Nd and magmatic zircon Hf isotopic compositions show large variations, but with most having εNd(t) and εHf(t) values < 0, with tDM(Nd) and tDM(Hf) values varying from 3.3 to 3.9 Ga and 3.3 to 4.0 Ga, respectively. Magmatic zircons without very strong lead loss (discordance ≤ 20%) have δ18O values of + 3.14 to + 8.39. 3.3-3.7 Ga xenocrystic zircons occur in some samples. The granite formed as a result of recycling of Paleo- to Eoarchean continental material in an intracontinental environment, with little if any contribution from Mesoarchean mantle sources. The sources could be predominantly unaltered ancient gneisses, together with yet to be identified Paleo- to Eoarchean materials affected by early low temperature alteration (weathered rocks or clastic sediment).

The significance of the Medicine Hat Block (southern Alberta, northern Montana) in the assembly of Laurentia: New interpretations from recent single grain zircon geochronological and geochemical data

NASA Astrophysics Data System (ADS)

LaDouceur, B. O.; Gifford, J.; Malone, S.; Davis, B.

2017-12-01

Keywords: Medicine Hat Block, Zircon, U/Pb ages, Hf isotopes, Laurentia The Medicine Hat Block (MHB) is one of the core cratonic elements that amalgamated in the Paleoproterozoic to form Laurentia. However, unlike many of the other cratons, the role of the MHB in the formation of Laurentia is poorly constrained. Virtually all of the MHB is concealed by Proterozoic and younger supracrustal sequences, limiting the data collected from this craton. The primary source of samples from the MHB comes from two sources: 1) xenoliths of variably metamorphoses gneisses, amphibolites, and meta-plutonic rocks collected from Eocene volcanic rock, and 2) similar lithologies recovered from boreholes that penetrate to the MHB basement. Multigrain zircon TIMS analyses yielded U/Pb ages ranging from 1.70 Ga to 3.26 Ga. Recent zircon single-grain LA-ICPMS U-Pb ages revealed a slightly older range of Archean ages, 2.63 Ga to 3.27 Ga, and two samples yielding Paleoproterozoic ages at 1.78 and 1.82 Ga. Whole-rock Sm/Nd data indicated that the samples formed from crustal sources, with model ages ranging between 1.80 Ga to 3.48 Ga. In-situ zircon Hf isotopic results revealed that Archean-aged zircon are generally suprachondritic, with eHf(t) values between 8.3 and -8.7. In contrast, the Paleoproterozoic grains yielded negative eHf(t) values ranging from -6.8 to -21.2, suggestive of a reworked Archean crustal component in their genesis. In particular, the Sweetgrass Hill xenolith suite is characterized solely by Paleoproterozoic ages, with evolved eHf(t) suggesting that any older U-Pb ages were reset by granulite facies metamorphism and zircon recrystallization. The combined U-Pb and Hf isotopic data from these samples helps illuminate the character of the MHB and its relationships to the Wyoming and Hearne cratons, as well as the Great Falls Tectonic Zone (GFTZ). The ages overlap between cratonic elements; however, the abundance of positive eHf(t) values of the 2.8 Ga ages suggests that the MHB is distinct from the Wyoming Craton, and that the GFTZ must indeed be a collisional zone as proposed by others. The Paleoproterozoic ages observed in the granulite xenolith samples supports this distinctness as well, and also supports proposed models of a Paleoproterozoic underplating event observed in other xenoliths and in seismic sections.

Assessing the origin of old apparent ages derived by Pb stepwise leaching of vein-hosted epidote from Mount Isa, northwest Queensland, Australia

NASA Astrophysics Data System (ADS)

Duncan, Robert J.; Maas, Roland

2014-12-01

Epidote metasomatism affected large areas of tholeiitic metabasalts of the ~1,780 Ma Eastern Creek Volcanics in the Western Fold Belt of the Proterozoic Mount Isa inlier. Hydrothermal epidote generally occurs in quartz veins parallel to or boudinaged within the dominant S2 fabrics which formed during the regional metamorphic peak at ~1,570 Ma associated with the Isan orogeny. Previously published stable isotopic and halogen data suggest that the fluids responsible for epidote formation are metamorphic in origin (with an evaporitic component). Application of the Pb stepwise leaching technique to the epidote does not separate radiogenic Pb4+ and common Pb2+, generating little spread in 206Pb/204Pb (between 16.0 and 30.5). The causes for this relatively low range are twofold: There is little radiogenic Pb in the epidotes (the most radiogenic steps account for <1 % of Pb released) and both Pb2+ and uranogenic Pb4+ substitute into the same site in the epidote crystal lattice. Consequently, age regressions using the Pb stepwise leaching data give ages between 150 and 1,500 myrs older than the host rocks and over 450 myrs older than the thermal metamorphic peak. These old ages are attributed to chemical inheritance from the host metabasalts, via radiogenic Pb release by breakdown of phases such as zircon, monazite, titanomagnetite, and ilmenite during metamorphism. This idea is supported by trace element data and chrondrite-normalized rare earth element patterns that are similar to both the metabasalts and epidotes (except for a variable Eu anomaly in the latter). Relatively high fO2 during vein formation (Fe3+ dominates in the epidote crystal lattice) would allow the incorporation of Th4+ and exclusion of U6+ and would explain elevated Th/U ratios (up to 12) in epidote compared with the host metabasalts. Non-incorporation of U would explain the relatively low U/Pb ratios and non-radiogenic character of the epidote. This process may provide a source of metal for the small U deposits around Mount Isa and may also suggest a relationship between U mineralization and regional Cu mobilization during the Isan orogeny. Our work suggests that non-conventional geochronometers should be used only if additional geological information and geochemical data (e.g., mineral chemistry, trace elements) are available to evaluate any resulting age calculations.

Geology, zircon geochronology, and petrogenesis of Sabalan volcano (northwestern Iran)

NASA Astrophysics Data System (ADS)

Ghalamghash, J.; Mousavi, S. Z.; Hassanzadeh, J.; Schmitt, A. K.

2016-11-01

Sabalan Volcano (NW Iran) is an isolated voluminous (4821 m elevation; > 800 km2) composite volcano that is located within the Arabia-Eurasia collision zone. Its edifice was assembled by recurrent eruptions of trachyandesite and dacite magma falling into a relatively restricted compositional range (56-67% SiO2) with high-K calc-alkaline and adakitic trace element (Sr/Y) signatures. Previous K-Ar dating suggested protracted eruptive activity between 5.6 and 1.4 Ma, and a two stage evolution which resulted in the construction of the Paleo- and Neo-Sabalan edifices, respectively. The presence of a topographic moat surrounding Neo-Sabalan and volcanic breccias with locally intense hydrothermal alteration are indicative of intermittent caldera collapse of the central part of Paleo-Sabalan. Volcanic debris-flow and debris-avalanche deposits indicate earlier episodes of volcanic edifice collapse during the Paleo-Sabalan stage. In the Neo-Sabalan stage, three dacitic domes extruded to form the summits of Sabalan (Soltan, Heram, and Kasra). Ignimbrites and minor pumice fall-out deposits are exposed in strongly dissected drainages that in part have breached the caldera depression. Lavas and pyroclastic rocks are varyingly porphyritic with Paleo-Sabalan rocks being trachyandesites carrying abundant phenocrysts (plagioclase + amphibole + pyroxene + biotite). The Neo-Sabalan rocks are slightly more evolved and include dacitic compositions with phenocrysts of plagioclase + amphibole ± alkali-feldspar ± quartz. All Sabalan rock types share a common accessory assemblage (oxides + apatite + zircon). High spatial resolution and sensitivity U-Pb geochronology using Secondary Ionization Mass Spectrometry yielded two clusters of zircon ages which range from 4.5 to 1.3 Ma and 545 to 149 ka, respectively (all ages are averages of multiple determinations per sample). U-Th zircon geochronology for selected Neo-Sabalan rocks agrees with the U-Pb ages, with the youngest zircon rims dating to ca. 110 ka. Because zircon crystallization predates eruption, this age represents the upper limit for the youngest eruptions of Sabalan. Valley-filling ignimbrites yielded variable U-Pb zircon ages which argue against these pyroclastic rocks being generated in a single caldera forming event. These results indicate that eruptions occurred more recently than previously indicated by K-Ar dating. Paleo-Sabalan and Neo-Sabalan volcanic rocks have similar geochemical characteristics, including enrichment of LILE and LREE relative to HFSE and HREE, respectively, and prominent negative Ti, Nb, and Ta anomalies. The trachyandesitic to dacitic rocks of Sabalan also share negative Eu anomalies. This, together with horizontal or slightly increasing Y vs. Rb trends, indicates fractionation of plagioclase-amphibole or plagioclase-clinopyroxene assemblages with negligible crustal assimilation (based on low and invariant Rb/Th). High degrees of mantle partial melting are inferred from high (La/Yb)N (from 28 to 48). Overall, the subduction-affinity of Sabalan volcanic rocks agrees with models of melt generation following a Quaternary slab break-off event coeval with continental collision.

Petrologic evolution of the Caetano magmatic system: What can we learn from a dissected, 34 Ma caldera in the northern Great Basin, western U.S.A.?

NASA Astrophysics Data System (ADS)

Watts, K. E.; Colgan, J. P.; John, D. A.; Henry, C. D.

2012-12-01

Eruption of the >1,100 km3 Caetano Tuff and formation of the Caetano caldera occurred during the mid-Tertiary ignimbrite flare-up in the Great Basin. Post-collapse extension and faulting created a series of tilted fault blocks that expose >4 km thick intracaldera tuff, two generations of resurgent granitic plutons, silicic ring-fracture intrusions, a tuff dike that fed the early eruption, and pre- and post-caldera andesites. We integrate new petrologic data for extrusive and intrusive Caetano units with geologic mapping and geochronology to provide an exceptional view into the inner workings of a large caldera center. The Caetano Tuff is a phenocryst-rich (~30-50%) ignimbrite with a mineralogy of plagioclase + sanidine + quartz + biotite + orthopyroxene + Fe-Ti oxides ± hornblende + accessory zircon and allanite. Plagioclase crystals in the Caetano Tuff and cogenetic intrusive units span a wide compositional range (>30 mol% An) and have diverse petrographic textures ranging from euhedral phenocrysts to anhedral, sieved crystals with melt-rich cores. Plagioclase compositions measured by electron microprobe for whole rock thin sections are consistent with compositional zoning of the intracaldera tuff shown by XRF whole rock analyses, oligoclase (~10-30 mol% An) and andesine (~30-50 mol% An) in the most evolved (75-77% SiO2) and least evolved (72-74% SiO2) tuff units, respectively. However, orthopyroxene compositions are apparently decoupled from the host tuff composition, with the highest Mg#s (~60-70%) occurring in the most evolved tuff samples. In the Caetano Tuff, equilibrium pairs of Fe-Ti oxides yield an average eruption temperature of 745°C, which is consistent with the average Ti-in-zircon temperature of 750±70°C (1 stdev, n=90 spots) obtained from Ti concentrations measured by SHRIMP for single zircons. Application of Al-in-hornblende geobarometry indicates an average equilibration pressure of 4.5±0.1 kbar, corresponding to mid-crustal magma storage depths of ~14-15 km. In light of our new petrologic data, we highlight the following key points: (1) Diverse crystal cargoes, disequilibrium textures, and wide compositional oscillations in single phenocrysts and among discrete mineral populations indicate prolonged and complex episodes of magma assembly and growth. Based on zircon U-Pb SHRIMP ages that range from ~34-37 Ma, assembly and growth may have spanned ~2-3 Ma, or a 34 Ma Caetano magma chamber may have assimilated older igneous rocks in and around the caldera. (2) Mineral chemistry, U-Pb and Ar-Ar geochronology, O isotope geochemistry, and whole rock major and trace element geochemistry indicate a genetic connection between the Caetano Tuff and resurgent granitic plutons, supporting the role of linked volcanic-plutonic components in caldera settings. (3) Generation and eruption of crystal-rich "monotonous" rhyolite calls into question the prevailing paradigms of crystal-poor rhyolites derived from crystal mushes, or crystal-rich "monotonous intermediates" derived from homogeneous dacitic magma reservoirs. The Caetano Tuff may be a representative end member of caldera-forming eruptions that is important for understanding large-volume rhyolite genesis in the shallow-middle crust.

Zircon and baddeleyite from the economic ultramafic-mafic Noril'sk-1 intrusion (Russia): Hf-isotope constraints on source composition

NASA Astrophysics Data System (ADS)

Malitch, K. N.; Belousova, E. A.; Badanina, I. Yu.; Griffin, W. L.

2012-04-01

The ultramafic-mafic Noril'sk-1 intrusion in the northwestern part of the Siberian Craton (Russia) represents one of three known Noril'sk-type, ore-bearing intrusions, which host one of the world's major economic sulphide platinum-group-element (PGE)-Cu-Ni deposits. Zircon and baddeleyite dated previously both by SHRIMP (i.e. 248.0 ± 3.7 Ma, Campbell et al. 1992) and ID-TIMS (251.1 ± 3.6 Ma, Kamo et al. 1996) have been restricted to one lithology (e.g. leucogabbro) of the Noril'sk-1 intrusion. To better constrain the age of igneous event and sources involved in its generation our multi-technique study utilized ten rock samples characteristic of unmineralized and mineralized lithologies. The rocks investigated comprise (from top to bottom) gabbro-diorite (sample N1-1), leucogabbro (N1-3), olivine-free gabbro (N1-2 and N1-4), olivine-bearing gabbro (N1-5), olivine gabbro (N1-6), plagiowehrlite and plagiodunite (N1-7), taxitic-textured rocks comprising melanotroctolite, olivine gabbro with relics of ultramafic rocks (N1-8, N1-9) and contact fine-grained gabbro (N1-10). Sulphide PGE-Cu-Ni ores occur in ultramafic (N1-7) and taxitic-textured rocks (N1-8 and N1-9), which have thickness of about 17 m, whereas the low-sulphide horizon of about 1 m thick occurs in the upper part of intrusion (N1-3). In situ U-Pb analyses of zircon from these rocks, combined with detailed study of crystal morphology and internal structure, identify four zircon populations (Malitch et al. 2012). The U-Pb ages of baddeleyite and the defined zircon populations cover a significant time span, from Late Paleozoic to Early Mesozoic (e.g., 290 ± 2.8; 261.3 ± 1.6; 245.7 ± 1.1; 236.5 ± 1.8 and 226.7 ± 0.9 Ma). The established distribution of U-Pb ages implies that crystallization of baddeleyite and zircon corresponds to several stages of protracted evolution of ultramafic-mafic magmas at deep-seated staging chambers and/or probably characterizes interaction of distinct magmas during formation of the Noril'sk-1 intrusion, which served as the favorable factor for accumulation of ores of unique scales and concentrations, To test this hypothesis, in situ Hf-isotope data were collected on the dated spots within single zircon grains. The analysis used a New Wave LUV213 laser-ablation microprobe attached to a Nu plasma MC-ICP-MS at GEMOC (Griffin et al. 2002). Hf-isotope results grouped on the basis of lithology show notable differences. Zircons from the unmineralized 'layered rock sequence' (e.g., olivine-free gabbro, olivine-bearing gabbro and olivine gabbro) are characterized by the most 'radiogenic' initial 176Hf/177Hf and some of ɛHf values close to those of the Depleted Mantle. Irrespective of zircon population most radiogenic Hf-isotope compositions are typical for olivine-free gabbro (mean ɛHf 7.3 ± 1.1 for sample N1-4), olivine-bearing gabbro (9.2 ± 3.8, sample N1-5) and olivine gabbro (8.3 ± 2.0, sample N1-6). In contrast, zircons from the leucogabbro that encloses the low-sulphide horizon (N1-3), and plagiowehrlite (N1-7) and taxitic-textured rocks (N1-8 and N1-9) with disseminated sulphide ores have less radiogenic Hf-isotope values (e.g., mean ɛHf6.2 ± 1.4, 5.9 ± 2.3, 6.4 ± 1.2 and 4.9±1.4, respectively). The least radiogenic values (ɛHffrom -2.9 to +2.3, mean ɛHf = 0.1 ± 1.9) are recorded in gabbro-diorite from the upper part of intrusion. The baddeleyite from olivine-free gabbro has the narrowest range of ɛHf values (e.g. 6.8-8.4), with a mean of ɛHfof 7.6 ± 0.8, closely matching that of zircon (mean ɛHf= 7.3 ± 1.1). Zircons from the leucogabbro that hosts the low-sulphide horizon (N1-3), and ultramafic and taxitic-textured lithologies with disseminated sulphide ores (N1-7, N1-8 and N1-9) have less radiogenic ɛHf values than those in barren lithologies. The Hf-isotope data for zircons from ore-bearing rocks thus suggest that the Noril'sk magmas represent mixing between a juvenile source equivalent to the Depleted Mantle and a subcontinental lithospheric source probably at least Neoproterozoic in age. We propose that the SCLM component is especially prominent in the mineralized portions of the intrusion. This is consistent with the suggestion of Zhang et al (2008) that ancient cratonic lithospheric mantle may have contributed significantly to the PGE and Ni budget of the "fertile" Siberian Large Igneous Province. Small population of zircons from the gabbro-diorite show the least 'radiogenic' Hf-isotope values, indicating the input of a distinctly older lithospheric, possibly crustal, component, being consistent with a hybrid nature of this lithology. Our approach for deciphering the origin of zircon and baddeleyite from mafic and ultramafic rocks provides a unique set of U-Pb and Hf-isotope constraints on temporal evolution and petrologic history of the Noril'sk-1 intrusion. The study was supported by Uralian Branch of Russian Academy of Sciences (12-U-5-1038). Refereneces: Campbell I.H., Czamanske G.K., Fedorenko V.A., Hill R.I., Stepanov V. (1992) Synchronism of the Siberian traps and the Permian-Triassic boundary. Science 255, 1760-1763. Griffin W.L., Wang X., Jackson S.E., Pearson N.J., O'Reilly S.Y., Xu X., Zhou X. (2002) Zircon chemistry and magma genesis, SE China: in-situ analysis of Hf isotopes, Pingtan and Tonglu igneous complexes. Lithos 61, 237-269. Kamo S.L., Czamanske G.K., Krogh T.E. (1996) A minimum U-Pb age for Siberian flood-basalt volcanism. Geochim. Cosmochim. Acta 60, 3505-3511. Malitch K.N., Badanina I.Yu., Belousova E.A., Tuganova E.V. (2012) Results of U-Pb dating of zircon and baddeleyite from the Noril'sk-1 ultramafic-mafic intrusion (Russia). Russian Geology and Geophysics 53(2), 123-130. Zhang M., O'Reilly S.Y., Wang K-L., Hronsky J., Griffin W.L. (2008) Flood basalts and metallogeny: The lithospheric connection. Earth-Science Reviews 86, 145-174.

The Chachil Limestone (Pliensbachian-earliest Toarcian) Neuquén Basin, Argentina: U-Pb age calibration and its significance on the Early Jurassic evolution of southwestern Gondwana

NASA Astrophysics Data System (ADS)

Leanza, H. A.; Mazzini, A.; Corfu, F.; Llambías, E. J.; Svensen, H.; Planke, S.; Galland, O.

2013-03-01

New radiometric U-Pb ages obtained on zircon crystals from Early Jurassic ash layers found within beds of the Chachil Limestone at its type locality in the Chachil depocentre (southern Neuquén Basin) confirm a Pliensbachian age (186.0 ± 0.4 Ma). Additionally, two ash layers found in limestone beds in Chacay Melehue at the Cordillera del Viento depocentre (central Neuquén Basin) gave Early Pliensbachian (185.7 ± 0.4 Ma) and earliest Toarcian (182.3 ± 0.4 Ma) U-Pb zircon ages. Based on these new datings and regional geological observations, we propose that the limestones cropping out at Chacay Melehue are correlatable with the Chachil Limestone. Recent data by other authors from limestones at Serrucho creek in the upper Puesto Araya Formation (Valenciana depocentre, southern Mendoza) reveal ages of 182.16 ± 0.6 Ma. Based on these new evidences, we consider the Chachil Limestone an important Early Jurassic stratigraphic marker, representing an almost instantaneous widespread flooding episode in western Gondwana. The unit marks the initiation in the Neuquén Basin of the Cuyo Group, followed by widespread black shale deposition. Accordingly, these limestones can be regarded as the natural seal of the Late Triassic -earliest Jurassic Precuyano Cycle, which represents the infill of halfgrabens and/or grabens related to a strong extensional regime. Paleontological evidence supports that during Pliensbachian-earliest Toarcian times these limestones were deposited in western Gondwana in marine warm water environments.

Detrital zircon U-Pb geochronological and sedimentological study of the Simao Basin, Yunnan: Implications for the Early Cenozoic evolution of the Red River

NASA Astrophysics Data System (ADS)

Chen, Yi; Yan, Maodu; Fang, Xiaomin; Song, Chunhui; Zhang, Weilin; Zan, Jinbo; Zhang, Zhiguo; Li, Bingshuai; Yang, Yongpeng; Zhang, Dawen

2017-10-01

The paleo-Red River is suggested to have been a continental-scale drainage system connecting the Tibetan Plateau to the South China Sea. However, the evolution of the paleo-Red River is still under debate. This study presents new results from sedimentological analyses and detrital zircon U-Pb geochronologic data from fluvial sedimentary rocks of Paleocene to Oligocene age of the Simao Basin to constrain the nature of the paleo-drainage system of the Red River. The detrital zircon U-Pb results reveal multiple age groups at 190-240 Ma, 260-280 Ma, 450-540 Ma, 1700-1900 Ma and 2400-2600 Ma for the Paleocene to late Eocene Denghei Formation (Fm.), but only one conspicuous peak at 220-240 Ma for the late Eocene-Oligocene Mengla Fm. Provenance analyses illustrate that the former likely had source areas that included the Hoh-Xil, Songpan-Ganzi, northern Qiangtang, Yidun and western Yangtze Terranes, which are consistent with the catchments of the Upper and Lower Jinshajiang Segments, whereas the latter mainly transported material from a limited number of sources, such as the Lincang granitic intrusions west of the Simao Basin. Integrated with available detrital zircon U-Pb geochronologic and paleogeographic data, our study suggests the existence of a paleo-Red River during the Paleocene to late Eocene that was truncated and lost its northern sources after approximately 35 Ma, due to left-lateral strike-slip faulting of the Ailao Shan-Red River and clockwise rotation of the Lanping-Simao Terrane.

Controls on Cenozoic exhumation of the Tethyan Himalaya from fission-track thermochronology and detrital zircon U-Pb geochronology in the Gyirong basin area, southern Tibet

NASA Astrophysics Data System (ADS)

Shen, Tianyi; Wang, Guocan; Leloup, Philippe Hervé; van der Beek, Peter; Bernet, Matthias; Cao, Kai; Wang, An; Liu, Chao; Zhang, Kexin

2016-07-01

The Gyirong basin, southern Tibet, contains the record of Miocene-Pliocene exhumation, drainage development, and sedimentation along the northern flank of the Himalaya. The tectonic controls on basin formation and their potential link to the South Tibetan Detachment System (STDS) are not well understood. We use detrital zircon (ZFT) and apatite (AFT) fission-track analysis, together with detrital zircon U-Pb dating to decipher the provenance of Gyirong basin sediments and the exhumation history of the source areas. Results are presented for nine detrital samples of Gyirong basin sediments (AFT, ZFT, and U-Pb), two modern river-sediment samples (ZFT and AFT), and six bedrock samples (ZFT) from transect across the Gyirong fault bounding the basin to the east. The combination of detrital zircon U-Pb and fission-track data demonstrates that the Gyirong basin sediments were sourced locally from the Tethyan Sedimentary Sequence. This provenance pattern indicates that deposition was controlled by the Gyirong fault, active since 10 Ma, whose vertical throw was probably < 5000 m, rather than being controlled by normal faults associated with the STDS. The detrital thermochronology data contain two prominent age groups at 37-41 and 15-18 Ma, suggesting rapid exhumation at these times. A 15-18 Ma phase of rapid exhumation has been recorded widely in both southern Tibet and the Himalaya. A possible interpretation for such a major regional exhumation event might be detachment of the subducting Indian plate slab during the middle Miocene, inducing dynamic uplift of the Indian plate overriding its own slab.

LA-ICP-MS zircon U-Pb and muscovite K-Ar ages of basement rocks from the south arm of Sulawesi, Indonesia

NASA Astrophysics Data System (ADS)

Jaya, Asri; Nishikawa, Osamu; Hayasaka, Yasutaka

2017-11-01

The zircon U-Pb and muscovite K-Ar age from the Bantimala, Barru and Biru basement complexes in the South Arm of Sulawesi, Indonesia provide new information regarding the timing of magmatism, metamorphism and sedimentation in this region and have implications for the origin and evolution of the study area. The study area is at the juncture between the southeast margin of Sundaland and Bird's Head-Australia. The age of both the zircon U-Pb of detrital materials in the Bantimala Complex and the muscovite K-Ar of amphibolite in the Biru Complex fall in the Late Early Cretaceous (between 109 and 115 Ma), which is a similar age range to previous data for both the sedimentary and metamorphic rocks. The youngest detrital zircon in the schist samples from the Barru Complex fall into the Triassic in age (between 243 and 247 Ma). These age data indicate that the protolith of all three basement complexes were involved in the subduction system and metamorphosed in the late Early Cretaceous, but there are several differences in their deposition environment under and out of the influence of the late Early Cretaceous magmatism in the Bantimala and Barru Complexes, respectively. Felsic igneous activities are confirmed in the Late Cretaceous and the Eocene by the zircon U-Pb age of igneous rocks intruding or included as detrital fragments in three basement complexes. These dates are similar to those reported from the Meratus Complex of South Kalimantan. The detrital zircon age distributions of the basement rocks in the South Arm of Sulawesi display predominant Mesozoic (Cretaceous and Triassic) and Paleozoic populations with a small population of Proterozoic ages supporting the hypothesis that the West Sulawesi block originated from the region of the circum Bird's Head-Australian, namely the Inner Banda block. The absence of Jurassic zircon age population in the South Arm of Sulawesi suggests the division of the South Arm of Sulawesi from the Inner Banda block in early stage of rifting. Western Sulawesi is composed of several blocks separated from Inner Banda block with different histories, which is supported by the varieties of zircon population distribution in the basement rocks in the Western Sulawesi and also difference of general orientations of structural features between the Bantimala and Barru Complexes.

Timing and sources of late Archean magmatism, Kolar area, south India: Implications for Archean tectonics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Krogstad, E.J.

1988-01-01

The N-S trending 80 km long by 4-8 km wide Kolar Schist Belt in the Achean Dharwar craton of south India is bounded on its east and west by gneiss terranes. The contacts between the schist belt and surrounding gneisses are tectonic, rather than intrusive or unconformable. On the west side of the schist belt, monzodioritic to granitic gneisses have U-Pb zircon ages of 2631 +6.5/{minus}6 Ma, 2610 +10/{minus}10 Ma, and 2551 +3/{minus}3 Ma. The U-Pb sphene ages of these orthogneisses are between 2553 and 2551 Ma. Later granitic intrusions have U-Pb sphene and garnet ages as young as 2400more » Ma. Gneisses occurring as tectonic and magmatic inclusions in the area contain zircons older than 3140 Ma. The dominant gneiss unit on the east side of the schist belt has a U-Pb zircon age of 2532 +3.5/{minus}3Ma; U-Pb sphene ages east of the belt range from 2520 to 2500 Ma. The last major shearing episode, probably represented by Pb-Pb K-feldspar-whole rock ages on both sides of the schist belt, and by an {sup 40}Ar/{sup 39}Ar muscovite plateau age from sheared gneisses, occurred between 2520 and 2420 Ma. Pb, Nd and Sr initial ratios for the western gneisses suggest that their parent magmas were mantle-derived, but were contaminated by continental crust older than 3200 Ma. Nd, Sr and Pb initial ratios for the eastern gneisses show no evidence of older continental crust either having contaminated the magmas, or acting as part of the source materials. The Kolar Schist Belt is interpreted as the site of a latest Archean or earliest Proterozoic (2520 to 2420 Ma) suture zone where newly generated continental crust on the east was tectonically accreted to the margin of an older (3400 to 2550 Ma) continental nucleus to the west.« less

Paleoproterozoic mojaveprovince in northwestern Mexico? Isotopic and U-Pb zircon geochronologic studies of precambrian and Cambrian crystalline and sedimentary rocks, Caborca, Sonora

USGS Publications Warehouse

Lang, Farmer G.; Bowring, S.A.; Matzel, J.; Maldonado, G.E.; Fedo, C.; Wooden, J.

2005-01-01

Whole-rock Nd isotopic data and U-Pb zircon geochronology from Precambrian crystalline rocks in the Caborca area, northern Sonora, reveal that these rocks are most likely a segment of the Paleoproterozoic Mojave province. Supporting this conclusion are the observations that paragneiss from the ??? 1.75 Ga Bamori Complex has a 2.4 Ga Nd model age and contains detrital zircons ranging in age from Paleo- proterozoic (1.75 Ga) to Archean (3.2 Ga). Paragneisses with similar age and isotopic characteristics occur in the Mojave province in southern California. In addition, "A-type" granite exposed at the southern end of Cerro Rajon has ca 2.0 Ga Nd model age and a U-Pb zircon age of 1.71 Ga, which are similar to those of Paleoproterozoic granites in the Mojave province. Unlike the U.S. Mojave province, the Caborcan crust contains ca. 1.1 Ga granite (Aibo Granite), which our new Nd isotopic data suggest is largely the product of anatexis of the local Precambrian basement. Detrital zircons from Neoproterozoic to early Cambrian miogeoclinal arenites at Caborca show dominant populations ca. 1.7 Ga, ca. 1.4 Ga, and ca. 1.1 Ga, with subordinate Early Cambrian and Archean zircons. These zircons were likely derived predominately from North American crust to the east and northeast, and not from the underlying Caborcan basement. The general age and isotopic similarities between Mojave province basement and overlying miogeoclinal sedimentary rocks in Sonora and southern California is necessary, but not sufficient, proof of the hypothesis that Sonoran crust is allochthonous and was transported to its current position during the Mesozoic along the proposed Mojave-Sonora megashear. One viable alternative model is that the Caborcan Precambrian crust is an isolated, autochthonous segment of Mojave province crust that shares a similar, but not identical, Proterozoic geological history with Mojave province crust found in the southwest United States ?? 2005 Geological Society of America.

Origin of the Bashierxi monzogranite, Qiman Tagh, East Kunlun Orogen, NW China: A magmatic response to the evolution of the Proto-Tethys Ocean

NASA Astrophysics Data System (ADS)

Zheng, Zhen; Chen, Yan-Jing; Deng, Xiao-Hua; Yue, Su-Wei; Chen, Hong-Jin; Wang, Qing-Fei

2018-01-01

The Qiman Tagh of the East Kunlun Orogen, NW China, lies within the Tethysides and hosts a large W-Sn belt associated with the Bashierxi monzogranite. To constrain the origin of the granitic magmatism and its relationship with W-Sn mineralization and the tectonic evolution of the East Kunlun Orogen and the Tethys, we present zircon U-Pb ages and Hf isotopic data, and whole-rock compositional and Sr-Nd-Pb isotopic data of the Bashierxi monzogranite. The granite comprises quartz, K-feldspar, plagioclase, and minor muscovite, tourmaline, biotite, and garnet. It contains high concentrations of SiO2, K2O, and Al2O3, and low concentrations of TiO2 and MgO, indicating a peraluminous high-K calc-alkaline affinity. The rocks are enriched in Rb, U, Pb, and light rare earth elements, and relatively depleted in Eu, Ba, Nb, Sr, P, and Ti, and are classified as S-type granites. Twenty zircon grains yield a weighted mean 238U/206Pb age of 432 ± 2.6 Ma (mean square weighted deviation = 1.3), indicating the occurrence of a middle Silurian magmatic event in the region. Magmatic zircons yield εHf(t) values of -6.7 to 0.7 and corresponding two-stage Hf model ages of 1663-1250 Ma, suggesting that the granite was derived from Mesoproterozoic crust, as also indicated by 207Pb/206Pb ages of 1621-1609 Ma obtained from inherited zircon cores. The inherited zircon cores yield εHf(t) values of 8.3-9.6, which indicate the generation of juvenile crust in the late Paleoproterozoic. Samples of the Bashierxi granite yield high initial 87Sr/86Sr ratios and radiogenic Pb concentrations, and negative εNd(t) values. Isotopic data from the Bashierxi granite indicate that it was derived from partial melting of ancient (early Paleozoic to Mesoproterozoic) sediments, possibly representing recycled Proterozoic juvenile crust. Middle Silurian granitic magmatism resulted from continental collision following closure of the Proto-Tethys Ocean. The Qiman Tagh represents a Caledonian orogenic belt containing S-type granites and associated W-Sn deposits.

Isotopic age of the Black Forest Bed, Petrified Forest Member, Chinle Formation, Arizona: An example of dating a continental sandstone

USGS Publications Warehouse

Riggs, N.R.; Ash, S.R.; Barth, A.P.; Gehrels, G.E.; Wooden, J.L.

2003-01-01

Zircons from the Black Forest Bed, Petrified Forest Member, Chinle Formation, in Petrified Forest National Park, yield ages that range from Late Triassic to Late Archean. Grains were analyzed by multigrain TIMS (thermal-ionization mass spectrometry), single-crystal TIMS, and SHRIMP (sensitive, high-resolution ion-microprobe). Multiple-grain analysis yielded a discordia trajectory with a lower intercept of 207 ?? 2 Ma, which because of the nature of multiple-grain sampling of a detrital bed, is not considered conclusive. Analysis of 29 detrital-zircon grains by TIMS yielded U-PB ages of 2706 ?? 6 Ma to 206 ?? 6 Ma. Eleven of these ages lie between 211 and 216 ?? 6.8 Ma. Our statistical analysis of these grains indicates that the mean of the ages, 213 ?? 1.7 Ma, reflects more analytical error than geologic variability in sources of the grains. Grains with ages of ca. 1400 Ma were derived from the widespread plutons of that age exposed throughout the southwestern Cordillera and central United States. Twelve grains analyzed by SHRIMP provide 206Pb*/238U ages from 214 ?? 2 Ma to 200 ?? 4 Ma. We use these data to infer that cores of inherited material were present in many zircons and that single-crystal analysis provides an accurate estimation of the age of the bed. We further propose that, even if some degree of reworking has occurred, the very strong concentration of ages at ca. 213 Ma provides a maximum age for the Black Forest Bed of 213 ?? 1.7 Ma. The actual age of the bed may be closer to 209 Ma. Dating continental successions is very difficult when distinct ash beds are not clearly identified, as is the case in the Chinle Formation. Detrital zircons in the Black Forest Bed, however, are dominated by an acicular morphology with preserved delicate terminations. The shape of these crystals and their inferred environment of deposition in slow-water settings suggest that the crystals were not far removed from their site of deposition in space and likely not far in time. Plinian ash clouds derived from explosive eruptions along the early Mesozoic Cordilleran margin provided the crystals to the Chinle basin, where local conditions insured their preservation. In the case of the Black Forest Bed, the products of one major eruption may dominate the volcanic contribution to the unit. Volcanic detritus in the Chinle Formation was derived from multiple, distinct sources. Coarse pebble- to cobble-size material may have originated in eastern California and/or western Arizona, where Triassic plutons are exposed. Fine-grained detritus, in contrast, was carried in ash clouds that derived from caldera eruptions in east-central California or western Nevada.

U-Pb geochronologic constraints on the origin of a unique monazite- xenotime gneiss, Hudson Highlands, New York

USGS Publications Warehouse

Aleinikoff, J.N.; Grauch, R.I.

1990-01-01

A unique rock composed almost entirely of equal proportions of monazite and xenotime occurs as a small, lenticular body (2 ?? 0.5 ?? 0.3 m) in association with paragneiss, migmatite, and Canada Hill Granite in an outcrop in the Hudson Highlands of southeastern New York. The paragneiss contains detrital zircon (207Pb/206Pb ages of 1150-1460 Ma), monazite, and xenotime (both dated at about 1000 Ma). Zircons from the monazite-xenotime gneiss contain dark, rounded cores and clear rims, a morphology suggestive of derivation from the paragneiss, with subsequent metamorphic overgrowth. We conclude, based on results from xenotime and zircon rims, that the monazite-xenotime gneiss formed at about 985 Ma. Based on zircon morphology and age relations within the outcrop, we prefer a metasomatic origin over other possibilities such as a paleo-placer or anatectic restite. -from Authors

Isotope age of the rare metal pegmatite formation in the Kolmozero-Voron'ya greenstone belt (Kola region of the Fennoscandian shield): U-Pb (TIMS) microlite and tourmaline dating

NASA Astrophysics Data System (ADS)

Kudryashov, Nikolay; Lyalina, Ludmila; Mokrushin, Artem; Zozulya, Dmitry; Groshev, Nikolay; Steshenko, Ekaterina; Kunakkuzin, Evgeniy

2016-04-01

The Kolmozero-Voron'ya greenstone belt is located in the central suture zone, which separates the Murmansk block from the Central-Kola and the Keivy blocks. The belt is represented by volcano-sedimentary rocks of Archaean age of 2.9-2.5 Ga. Rare metal pegmatites (Li, Cs with accessory Nb, Ta, and Be) occur among amphibolite and gabbroid intrusions in the northwestern and southeastern parts of the belt. According to the Rb-Sr data, the age of pegmatites was considered to be 2.7 Ga. Until recently there was no generally accepted point of view on the origin of pegmatites. Now we have isotopic data for a range of rock complexes that could pretend to be parental granites for the rare metal pegmatites. These are granodiorites with the zircon age of 2733±Ma, and microcline and tourmaline granites, which Pb-Pb isochronal age on tourmaline from the tourmaline granite located near the deposit is estimated to be 2520±70 Ma. The pegmatite field of the Vasin Myl'k deposit with the lepidolite--albite--microcline--spodumene--pollucite association is located among amphibolites in the northwestern part of the belt. The deposit is represented by subparallel low-angle zoned veins up to 220 m long and 5 m thick dipping in the southeastern direction at an angle of 10° too 30°. The minerals of the columbite--tonalite group from Vasin Myl'k deposit include microlite, simpsonite, and torolite, and are the oldest among different minerals represented by several generations in pegmatites under consideration. Zircons from the pegmatites are mostly represented by crystals with the structure affected by the action of fluids that put certain restrictions on its use as a geochronometer of the crystallization process. Microlite from the pegmatite taken from the dump of a prospecting drill hole was used for U--Pb (TIMS). The mineral is represented by 0.5--1.0 mm long euhedral octahedral crystals. It is brown in color, and transparent. The microlite crystals were preliminarily cleaned from surface contamination being placed the ultrasonic bath with the 7N HNO3 solution. The discordia constructed for seven measured microlite weights is characterized by upper intercepts with concordia at 2454±8 Ma, which probably reflects the time of rare metal pegmatite crystallization, coeval with the age of tourmaline granites. Taking into consideration the obtained U--Pb age of microlite, it may be assumed that the tourmaline granites dated back to 2520±70 Ma served as the most probable parental rocks for rare metal pegmatites of the Vasin-Myl'k deposit. The work is supported by RFBR No 16-05-00367, No 16-05-00427 and project No 0231-2015-0005

Detrital K-feldspar thermochronology of the Nanaimo Group: Characterization of Basement and Extraregional Basin Contributions

NASA Astrophysics Data System (ADS)

Isava, V.; Grove, M.; Mahoney, J. B.; Kimbrough, D. L.

2016-12-01

The Late Cretaceous-Early Paleogene Nanaimo Group covers the contact between Triassic basement Wrangellia terrane and the Jurassic-Cretaceous Coast Plutonic Complex (CPC) in southern British Columbia. Prior detrital zircon U-Pb and Hf studies indicate a change in sediment source for the Nanaimo basin, from the primitive CPC in Santonian-Early Campanian time to an isotopically evolved continental extraregional source during the late Campanian/Maastrictian. Two notably different areas have been proposed as potential source regions: (1) the Idaho/Boulder batholith and Belt Supergroup, and (2) the Mojave/Salinia segment of structurally disrupted late Cretaceous southern California margin. Single crystal 40Ar/39Ar laser fusion of ca. 100-200 grains apiece from seven detrital K-feldspar samples from Santonian-Maastrichtian strata of the northern Nanaimo Group constrain the history of the sediments' source regions. The two oldest samples, from the K-feldspar poor Comox and Extension Fms., display a monotonic increasing distribution of cooling ages 80-125 Ma that reflects shallow erosion of the CPC. In contrast, Late Campanian strata of the Cedar District and De Courcy Fms. exhibit a more pronounced cluster of cooling ages 80-95 Ma as well as a greater proportion of Jurassic ages that represent progressively deeper erosion of the CPC. Evidence for an extraregional sediment source appears abruptly in the Geoffrey Fm. by 72 Ma, matching the time of local-to-extraregional shift indicated in detrital zircon U-Pb studies. Over 90% of the detrital K-feldspars from these arkosic sandstones yield cooling ages of 70-80 Ma, with sparse older ages associated with the CPC. Samples from the successively younger Spray and Gabriola Fms. also yield >90% K-feldspar ages younger than 80 Ma and exhibit age maxima of 68 Ma and 65 Ma, respectively. These results are distinct from detrital zircon U-Pb and K-feldspar 40Ar/39Ar ages of the southern Sierra Nevada, Mojave/Salina, and northern Peninsular Ranges of southern California. Resemblance with the Idaho/Boulder batholith indicates the latter as a more likely source of extraregional detritus. Measurements are underway to couple detrital K-feldspar 40Ar/39Ar thermochronology with U-Pb isotopes on the same grains to further distinguish between these two possibilities.

Trace-element record in zircons during exhumation from UHP conditions, North-East Greenland Caledonides

USGS Publications Warehouse

McClelland, W.C.; Gilotti, J.A.; Mazdab, F.K.; Wooden, J.L.

2009-01-01

Coesite-bearing zircon formed at ultrahigh-pressure (UHP) conditions share general characteristics of eclogite-facies zircon with trace-element signatures characterized by depleted heavy rare earth elements (HREE), lack of an Eu anomaly, and low Th/ U ratios. Trace-element signatures of zircons from the Caledonian UHP terrane in North-East Greenland were used to examine the possible changes in signature with age during exhumation. Collection and interpretation of age and trace-element analyses of zircon from three samples of quartzofeldspathic gneiss and two leucocratic intrusions were guided by core vs. rim zoning patterns as imaged by cathodoluminesence. Change from igneous to eclogite-facies metamorphic trace-element signature in protolith zircon is characterized by gradual depletion of HREE, whereas newly formed metamorphic rims have flat HREE patterns and REE concentrations that are distinct from the recrystallized inherited cores. The signature associated with eclogite-facies metamorphic zircon is observed in coesite-bearing zircon formed at 358 ?? 4 Ma, metamorphic rims formed at 348 ?? 5 Ma during the initial stages of exhumation, and metamorphic rims formed at 337 ?? 5 Ma. Zircons from a garnet-bearing granite emplaced in the neck of an eclogite boudin and a leucocratic dike that cross-cuts amphibolite-facies structural fabrics have steeply sloping HREE patterns, variably developed negative Eu anomalies, and low Th/U ratios. The granite records initial decompression melting and exhumation at 347 ?? 2 Ma and later zircon rim growth at 329 ?? 5. The leucocratic dike was likely emplaced at amphibolite-facies conditions at 330 ?? 2 Ma, but records additional growth of compositionally similar zircon at 321 ??2 Ma. The difference between the trace-element signature of metamorphic zircon in the gneisses and in part coeval leucocratic intrusions indicates that the zircon signature varies as a function of lithology and context, thus enhancing its ability to aid in the interpretation of U-Pb data and track the exhumation history of UHP terranes. The differences may reflect variation in elemental availability through breakdown reactions in quartzofeldpathic gneiss vs. availability during melt production and/or crystallization. UHP rocks in North-East Greenland began exhumation by 347 ?? 2 Ma, were still at HP eclogite-facies conditions at 337 ?? 5 Ma and were at amphibolite-facies conditions by 330 ?? 2 Ma. ?? 2009 E. Schweizerbart'sche Verlagsbuchhandlung.

Magmatic origin and fluid alteration versus inheritance: Complex history of accessory minerals from I-type granites from northern Victoria Land (Antarctica)

NASA Astrophysics Data System (ADS)

Menneken, M.; John, T.; Läufer, A.; Berndt, J.; Henjes-Kunst, F.; Giese, J.

2016-12-01

When reconstructing the formation and evolution of Earths earliest crust, one is still heavily reliant on information that can be gathered from detrital zircon grains, which have been proven to be a useful tool concerning the various isotopic, chemical, and mineralogical features that can be utilized within. However, detailed investigations have shown that some of these tracers might be prone to alteration and will not necessarily reflect conditions during formation [1, 2]. In this study, zircons and their host rocks from the Granite Harbour Intrusives (GHI) of northern Victoria Land, Antarctica, have been investigated with respect to regional evolution of continental crust as well as the reliability of zircon as a recorder of crustal formation. Here we present U-Pb, δ18O, trace element and inclusion data of zircons from Cambro-Ordovician granitoids, as well as geochemical analyses of accessory apatites. Our initial results show that even in I-type granitoids with a presumably simple formation history, U-Pb-age data and related isotopic, chemical, or mineralogical features have to be evaluated carefully in order to constrain the timing of magmatic events, inheritance or fluid alteration events. They do, for example, incorporate a large portion of recycled material, which is clearly reflected by a strong component of inherited U-Pb-zircon ages, but can also be inferred by comparing main- and trace-element compositions of apatite inclusions in zircon with accessory apatites. Apatite inclusions from all investigated samples for example, are clearly higher in F concentration, than their accessory counterparts. However, not all zircon grains record the youngest event, emphasizing that not only alteration might be a key factor to consider when evaluating zircon characteristics with respect to their host rock, but also, the timing and preservation of predating features. [1] Rasmussen et al. (2011) Metamorphic replacement of mineral inclusions in detrital zircon from Jack Hills, Australia: Implications for the Hadean Earth: Geology 39, 1143-1146. [2] Nemchin et al. (2006) Re-evaluation of the origin and evolution of > 4.2 Ga zircons from the Jack Hills metasedimentary rocks: Earth and Planetary Science Letters, v. 244, p. 218-233.

Cadomian basement and Paleozoic to Triassic siliciclastics of the Taurides (Karacahisar dome, south-central Turkey): Paleogeographic constraints from U-Pb-Hf in zircons

NASA Astrophysics Data System (ADS)

Abbo, Avishai; Avigad, Dov; Gerdes, Axel; Güngör, Talip

2015-06-01

The Tauride block in Turkey is a peri-Gondwana, Cadomian-type terrane that rifted from the Afro-Arabian margin of Gondwana in the Permo-Triassic and re-accreted to Arabia in the Neogene. In the Karacahisar dome in the southern-central Taurides, Neoproterozoic basement metasediments and intrusive rocks are overlain by Cambro-Ordovician, Carboniferous and Triassic sediments. We studied U-Pb-Hf in zircons from major rock units exposed in Karacahisar in order to constrain the Cadomian crustal evolution of the Taurides, to evaluate the provenance of the Neoproterozoic and overlying sediments, to constrain the paleogeography of the Taurides, and to assess their linkage to Gondwana. The Neoproterozoic metasediments are low-grade metamorphic wacke-type turbidites that evolved in a broad back-arc basin peripheral to Afro-Arabia. Their detrital zircon U-Pb signal comprises a preponderance (40-68%) of Neoproterozoic-aged zircons (peak ages defined at 635 and 830 Ma), indicating that the sedimentary pile was built mainly from the erosion of Pan-African terranes from Afro-Arabia. The εHf values of the younger population (635 Ma) are mostly positive, indicating derivation from a juvenile arc, whereas Cryogenian-Tonian detrital zircons spread vertically (- 25 < εHf < 15), indicating a different provenance where mixing of juvenile magmas with Paleoproterozoic to Neoarchean crust was widespread. An unusually high proportion of pre-Neoproterozoic zircons is found in all Cadomian metasediments, including up to 31% Grenvillian-aged (ca. 1.0 Ga) and up to 35% of ca. 2.5 Ga zircons; about a third of the latter possess positive εHf values. Because only minor exposures of 1.0 and 2.5 Ga crustal vestiges are currently known in North Africa and Arabia, we infer that pre-Neoproterozoic terranes were dispersed within the Cadomian realm itself. The youngest detrital zircons in all Cadomian metasediments concentrate at 0.58 Ga, indicating that the proto-Cadomian back-arc basin was formed towards the Late Ediacaran. The termination of Neoproterozoic sedimentation is marked by the intrusion of dyke swarms yielding a U-Pb zircon age of 544 ± 4 Ma, coeval with magmatism in other Cadomian basement units in the Taurides (e.g., Sandıklı and Menderes massif). Shortly afterwards the Tauride basement was overstepped by the Cambro-Ordovician platform. From the Cambrian to the Triassic, the U-Pb-Hf detrital zircon signal of the cover sediments in the Taurides in Karacahisar portrays remarkable resemblance to that of typical North Gondwana Cambro-Ordovician cover sediments, and shows that the Taurides accreted to Afro-Arabia by the onset of the Cambrian. The short time interval between Cadomian basin fill and deposition of the Cambrian platform implies that the Taurides evolved within a short distance from their present position relative to the Afro-Arabian margin of Gondwana. The Gondwana provenance of the Tauride sedimentary cover is maintained through the Upper Triassic, indicating that Neo-Tethys rifting was incipient at that time, allowing Afro-Arabian detritals to reach the Taurides.

How to build a mid-crustal intrusive suite: geologic mapping, U-Pb geo-/thermochronology, and thermal modeling of the Bergell Intrusion, Central Alps

NASA Astrophysics Data System (ADS)

Samperton, K. M.; Schoene, B.; Annen, C.

2015-12-01

Insights into the characteristic rates and processes of crustal magmatic systems can best be made through the integration of observational, analytical and modeling perspectives. We present such an approach in reconstructing the emplacement, differentiation and cooling history of the Bergell Intrusion (N Italy/SE Switzerland), a normally-zoned pluton preserving a ~10 km mid-crustal transect. U-Pb zircon, titanite and allanite geo-/thermochronology of Bergell granitoids provide key empirical constraints for informing numerical simulations of pulse-wise, incremental assembly. Protracted zircon crystallization histories, representing the time between magma zircon saturation and cooling to the solidus, provide a direct petrologic link to forward models of magma emplacement, both of which can be used to derive quantitative magmatic cooling rates for the middle crust. Titanite and allanite dates provide additional constraints on the timing of solidification. Geochronology and modeling are performed in the context of detailed field and structural observations, including those previously interpreted as evidence of upward, pluton-scale melt migration via floor convergence/roof ballooning. Combined Bergell data and modeling demonstrate that pulsed assembly can lead to the formation of substantial melt reservoirs in the middle crust: this finding is largely in contrast to similar models of shallow crustal plutons, highlighting the importance of factors such as ambient country rock temperature in affecting melt residence timescales. This work emphasizes the importance of implementing joint data/modeling studies to intrusive rocks across the full range of spatial scales, emplacement levels and tectonic settings observed on Earth.

Determinations of rare earth element abundance and U-Pb age of zircons using multispot laser ablation-inductively coupled plasma mass spectrometry.

PubMed

Yokoyama, Takaomi D; Suzuki, Toshihiro; Kon, Yoshiaki; Hirata, Takafumi

2011-12-01

We have developed a new calibration technique for multielement determination and U-Pb dating of zircon samples using laser ablation-inductively coupled plasma mass spectrometry (ICPMS) coupled with galvanometric optics. With the galvanometric optics, laser ablation of two or more sample materials could be achieved in very short time intervals (~10 ms). The resulting sample aerosols released from different ablation pits or different solid samples were mixed and homogenized within the sample cell and then transported into the ICP ion source. Multiple spot laser ablation enables spiking of analytes or internal standard elements directly into the solid samples, and therefore the standard addition calibration method can be applied for the determination of trace elements in solid samples. In this study, we have measured the rare earth element (REE) abundances of two zircon samples (Nancy 91500 and Prešovice) based on the standard addition technique, using a direct spiking of analytes through a multispot laser ablation of the glass standard material (NIST SRM612). The resulting REE abundance data show good agreement with previously reported values within analytical uncertainties achieved in this study (10% for most elements). Our experiments demonstrated that nonspectroscopic interferences on 14 REEs could be significantly reduced by the standard addition technique employed here. Another advantage of galvanometric devices is the accumulation of sample aerosol released from multiple spots. In this study we have measured the U-Pb age of a zircon sample (LMR) using an accumulation of sample aerosols released from 10 separate ablation pits of low diameters (~8 μm). The resulting (238)U-(206)Pb age data for the LMR zircons was 369 ± 64 Ma, which is in good agreement with previously reported age data (367.6 ± 1.5 Ma). (1) The data obtained here clearly demonstrate that the multiple spot laser ablation-ICPMS technique can become a powerful approach for elemental and isotopic ratio measurements in solid materials.

In-Situ U–Pb Dating of Apatite by Hiroshima-SHRIMP: Contributions to Earth and Planetary Science

PubMed Central

Terada, Kentaro; Sano, Yuji

2012-01-01

The Sensitive High Resolution Ion MicroProbe (SHRIMP) is the first ion microprobe dedicated to geological isotopic analyses, especially in-situ analyses related to the geochronology of zircon. Such a sophisticated ion probe, which can attain a high sensitivity at a high mass resolution, based on a double focusing high mass-resolution spectrometer, designed by Matsuda (1974), was constructed at the Australian National University. In 1996, such an instrument was installed at Hiroshima University and was the first SHRIMP to be installed in Japan. Since its installation, our focus has been on the in-situ U–Pb dating of the mineral apatite, as well as zircon, which is a more common U-bearing mineral. This provides the possibility for extending the use of in-situ U–Pb dating from determining the age of formation of volcanic, granitic, sedimentary and metamorphic minerals to the direct determination of the diagenetic age of fossils and/or the crystallization age of various meteorites, which can provide new insights into the thermal history on the Earth and/or the Solar System. In this paper, we review the methodology associated with in-situ apatite dating and our contribution to Earth and Planetary Science over the past 16 years. PMID:24349912

Combined garnet and zircon geochronology and trace elements studies - constraints of the UHP-(U)HT evolution of Orlica-Śnieżnik Dome (NE Bohemian Massif).

NASA Astrophysics Data System (ADS)

Walczak, Katarzyna; Anczkiewicz, Robert; Szczepański, Jacek; Rubatto, Daniela

2017-04-01

The Orlica-Śnieżnik Dome (OSD), located on the NE margin of the Bohemian Massif, is predominantly composed of amphibolite-facies orthogneiss that contain bodies of HP and UHP eclogites and granulites. Numerous geochronological studies have been undertaken to constrain the timing of the ultra-high grade metamorphic event. Despite this, the exact timing of UHP-(U)HT conditions remain dubious (e.g. Brueckner et al., 1991; Anczkiewicz et al., 2007; Bröcker et al., 2009 & 2010). We have utilized garnet and zircon geochronology to provide time constraints on the evolution of the UHT-(U)HP rocks of the OSD. We have combined the ages with trace element analyses in garnet and zircon to better understand the significance of the obtained ages in petrological context. Lu-Hf grt-wr dating of peritectic garnet from two felsic granulites constrained the time of its initial growth at 346.9 ± 1.2 and 348.3 ± 2.0 Ma, recording peak conditions of 2.7 GPa and 950°C (e.g. Ferrero et al., 2015). In situ U-Pb SHRIMP dating of zircon from the same granulite gave a younger age of 342.2 ± 3.4 Ma. HREE partitioning between garnet rim and metamorphic zircon indicate their growth in equilibrium, hence, the U-Pb zircon date constrains the terminal phase of garnet crystallization. Similar ages were obtained from two eclogite bodies from Międzygórze and Nowa Wieś localities; Lu-Hf (grt-cpx-wr) dating provided ages of 346.5 ± 2.4 and 348.1 ± 9.1 Ma for samples from Międzygórze and Nowa Wieś, respectively. The same age (within error) of 346.3 ± 5.2 Ma was reported by Bröcker et al. (2010) for zircon from the Międzygórze eclogite. Comparison of REE concentrations in garnet (this study) and in metamorphic zircon (reported in Bröcker et al., 2010) indicate that garnet and zircon crystallized in equilibrium. Furthermore, M-HREE patterns observed in both garnet and zircon strongly suggest their growth at eclogite facies conditions. Sm-Nd garnet ages obtained for both felsic and mafic granulites and eclogites are identical within error and are consistently younger than corresponding Lu-Hf dates. Sm-Nd grt-wr ages of two samples of felsic granulite provide 332.4 ± 5.2 and 337.6 ± 2.3 Ma, while Sm-Nd grt-cpx age of a mafic granulite provides 336.9 ± 6.0 Ma. Sm-Nd grt-cpx(-wr) ages obtained for three eclogite samples range from 336.2 ± 3.5 to 337.7 ± 2.6 Ma. The foregoing ages are interpreted to reflect cooling through the Sm-Nd closure temperature at about 337 Ma. The estimated PTt path documents the evolution of the OSD, characterized by two distinct periods: (1) nearly isothermal decompression resulting from crustal scale folding and vertical extrusion of granulites at 347-342 Ma, and (2) fast, nearly isobaric cooling at 342 - 337 Ma, becoming very rapid towards the end of this period. Anczkiewicz, R. et al. 2007. Lithos, 95, 363-380. Bröcker , M. et al. 2009. Journal of Metamorphic Geology, 27, 385-403. Bröcker, M. et al. 2010. Geological Magazine, 147(3), 339-362. Brueckner, H. K. et al. 1991. Neues Jahrb Mineral Abh, 63, 169-193. Ferrero S. et al. 2015. Geology, 43, 447-450.

Geochronology and tectonic significance of Middle Proterozoic granitic orthogneiss, North Qaidam HP/UHP terrane, Western China

USGS Publications Warehouse

Mattinson, C.G.; Wooden, J.L.; Liou, J.G.; Bird, D.K.; Wu, C.L.

2006-01-01

Amphibolite-facies para- and orthogneisses near Dulan, in the southeast part of the North Qaidam terrane, enclose minor ultra-high pressure (UHP) eclogite and peridotite. Field relations and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. Ion microprobe U-Pb and REE analyses of zircons from two granitic orthogneisses indicate magmatic crystallization at 927 ?? Ma and 921 ?? 7 Ma. Zircon rims in one of these samples yield younger ages (397-618 Ma) compatible with partial zircon recrystallization during in-situ Ordovician-Silurian eclogite-facies metamorphism previously determined from eclogite and paragneiss in this area. The similarity between a 2496 ?? 18 Ma xenocrystic core and 2.4-2.5 Ga zircon cores in the surrounding paragneiss suggests that the granites intruded the sediments or that the granite is a melt of the older basement which supplied detritus to the sediments. The magmatic ages of the granitic orthogneisses are similar to 920-930 Ma ages of (meta)granitoids described further northwest in the North Qaidam terrane and its correlative west of the Altyn Tagh fault, suggesting that these areas formed a coherent block prior to widespread Mid Proterozoic granitic magmatism. ?? Springer-Verlag 2006.

U-Pb detrital zircon dates and provenance data from the Beaufort Group (Karoo Supergroup) reflect sedimentary recycling and air-fall tuff deposition in the Permo-Triassic Karoo foreland basin

NASA Astrophysics Data System (ADS)

Viglietti, Pia A.; Frei, Dirk; Rubidge, Bruce S.; Smith, Roger M. H.

2018-07-01

Detrital zircon U-Pb age dating was used for provenance determination and maximum age of deposition for the Upper Permian (upper Teekloof and Balfour formations) and Lower Triassic (Katberg Formation) lithostratigraphic subdivisions of the Beaufort Group of South Africa's Karoo Basin. Ten samples were analysed using laser ablation - single collector - magnetic sectorfield - inductively coupled plasma - mass spectrometry (LA-SF-ICP-MS). The results reveal a dominant Late Carboniferous-Late Permian population (250 ± 5 Ma - 339 ± 5 Ma), a secondary Cambrian-Neoproterozoic (489 ± 5 Ma to 878 ± 24 Ma) population, a minor Mesoproterozoic (908 ± 24 Ma to 1308 ± 23) population, and minor occurrences of Devonian, Ordovician, Proterozoic and Archean zircon grains. Multiple lines of evidence (e.g. roundness and fragmentary nature of zircons, palaeo-current directions, and previous work), suggest the older zircon populations are related to sedimentary recycling in the Gondwanide Orogeny. The youngest and dominant population contain elongate euhedral grains interpreted to be directly derived from their protolith. Since zircons form in felsic igneous rocks, and no igneous rocks of Late Permian age occur in the Karoo Basin, these findings suggest significant input of volcanic material by ash falls. These results support sedimentological and palaeontological data for a Lopingian (Late Permian) age for the upper Beaufort Group, but contradict previous workers who retrieved Early Triassic dates from zircons in ashes for the Beaufort and Ecca Groups. Pb-loss not revealed by resolvable discordance on the concordia diagram, and metamictization of natural zircons are not factored into the conclusions of earlier workers.

Subduction Initiation Existed Along the Ancient Continent Margins? Evidence of U-Pb ages of zircons from the Bonin Trench, Japan

NASA Astrophysics Data System (ADS)

Li, Y. B.; Pearce, J. A.; Ryan, J. G.; Li, X. H.; Haraguchi, S.; Iizuka, T.; Kon, Y.; Yamamoto, S.; Sawaki, Y.; Ishii, T.; Maruyama, S.

2017-12-01

Although it is not cleanly known when and where the subduction initiation began on the Paleo-Izu-Bonin-Mariana (IBM) Trench, Jurassic and Cretaceous plutonic rocks, such as gabbroic, granitic and metamorphic rocks had been sampled from the Amami Plateau-Daito Ridge-Okidaito Ridge (ADO) in the Philippine Sea Plate. Furthermore, Mesozonic to Paleozonic ages zircons were obtained from volcaniclastic sandstones collected from northern Izu-Bonin forarc (Tani et al., 2012). We present U-Pb ages, Hf-O isotopes and trace element compositions of zircon grains separated from sediment, volcanic rock, dolerite and gabbro, collected from Chichijima Island and Bonin forearc seafloor (KH03-3, KT04-28 cruise of the University of Tokyo, IODP Leg 352). In the zircon age histogram, several age groups were identified. The age peaks are 0-3 Ma and 13 Ma (Hahajima Seamount: soft mud and volcanic tuff); 38 Ma (Oomachi Seamount: sandstone); 45 Ma (Chichijima Island: volcanic rock); 40 Ma, 48 Ma and 52 Ma (Hahajima Seamount: dolerite and gabbro); 45 Ma and 164-165 Ma (IODP Leg 352: volcanic rock), respectively. Zircon U-Pb ages ranging 0-52 Ma correspond well to the multi-stages of magmatism in the IBM. However, 164-165 Ma maybe represent the ages of zircon xenocryst including in forearc volcanic rock , which pre-existing in ancient continent crustal materials (SE China Continent Crust?) as the basement of Paleo-IBM. It seems reasonable to suppose that the subduction initiation of IBM existed along the ancient SE China Continent margins. The initiation of subduction zone is a consequence of lateral compositional buoyancy contrast within the lithosphere, that advocated by Niu et al. (2003, 2016).

Southernmost Andes and South Georgia Island, North Scotia Ridge: Zircon U-Pb and muscovite {40Ar }/{39Ar } age constraints on tectonic evolution of Southwestern Gondwanaland

NASA Astrophysics Data System (ADS)

Mukasa, Samuel B.; Dalziel, Ian W. D.

1996-11-01

Zircon U-Pb and muscovite {40Ar }/{39Ar } isotopic ages have been determined on rocks from the southernmost Andes and South Georgia Island, North Scotia Ridge, to provide absolute time constraints on the kinematic evolution of southwestern Gondwanaland, until now known mainly from stratigraphic relations. The U-Pb systematics of four zircon fractions from one sample show that proto-marginal basin magmatism in the northern Scotia arc, creating the peraluminous Darwin granite suite and submarine rhyolite sequences of the Tobifera Formation, had begun by the Middle Jurassic (164.1 ± 1.7 Ma). Seven zircon fractions from two other Darwin granites are discordant with non-linear patterns, suggesting a complex history of inheritances and Pb loss. Reference lines drawn through these points on concordia diagrams give upper intercept ages of ca. 1500 Ma, interpreted as a minimum age for the inherited zircon component. This component is believed to have been derived from sedimentary rocks in the Gondwanaland margin accretionary wedge that forms the basement of the region, or else directly from the cratonic "back stop" of that wedge. Ophiolitic remnants of the Rocas Verdes marginal basin preserved in the Larsen Harbour complex on South Georgia yield the first clear evidence that Gondwanaland fragmentation had resulted in the formation of oceanic crust in the Weddell Sea region by the Late Jurassic (150 ± 1 Ma). The geographic pattern in the observed age range of 8 to 13 million years in these ophiolitic materials, while not definitive, is in keeping with propagation of the marginal basin floor northwestward from South Georgia Island to the Sarmiento Complex in southern Chile. Rocks of the Beagle granite suite, emplaced post-tectonically within the uplifted marginal basin floor, have complex zircon U-Pb systematics with gross discordances dominated by inheritances in some samples and Pb loss in others. Of eleven samples processed, only two had sufficient amounts of zircon for multiple fractions, and only one yielded colinear points. These points lie close to the lower concordia intercept for which the age is 68.9 ± 1.0 Ma, but their upper intercept is not well known. Inasmuch as this age is similar to the {40Ar }/{39Ar } age of secondary muscovite growing in extensional fractures of pulled-apart feldspar phenocrysts in a Beagle suite granitic pluton (plateau age is 68.1 ± 0.4 Ma), we interpret the two dates as good time constraints for cooling following a period of extensional deformation probably related to the tectonic denudation of the highgrade metamorphic complex of Cordillera Darwin in Tierra del Fuego.

Growth of a Large Composite Magma System: the EJB Pluton, Eastern California.

NASA Astrophysics Data System (ADS)

Matty, D. J.; Vervoort, J.; Dufrane, A.; Hart, G.; Student, J.; Morgan, S.

2008-12-01

The composite EJB pluton crops out in the White-Inyo Mountains of eastern California, and comprises the Eureka Valley monzonite (EVM), the Joshua Flat quartz monzonite (JFQM), the Beer Creek granite (BCG), and an unnamed diorite. While sometimes equivocal, field relationships suggest that the EVM was emplaced first, followed by the JFQM, and finally the BCG; the diorite predates the BCG. Sylvester and others (GSAB, 1978) reported zircon U-Pb ages of 179±2 Ma for the EVM and 174±5 Ma for the JFQM. Coleman and others (GSAB, 2003) determined a U-Pb age of 179±3 Ma (via Pb-loss trajectory) for the BCG. Because of the uncertainty in the ages and ambiguous field relations, the sequence and duration of EJB magmatism remain unclear. To understand more fully the timing of EJB magmatism, we separated zircons from 12 samples collected from each of the main EJB units. These samples were characterized using light microscopy, SEM and CL techniques. U-Pb ages were determined from individual zircons by LA-ICP-MS following the method of Chang and others (G3, 2006). For the ages reported below, the reported uncertainties are based on factors within the analysis, but do not include external factors such as sample/standard bias or other matrix effects. Overall uncertainty in LA-ICPMS U-Pb geochronology is hard to assess, but we estimate that all ages reported below are subject to a minimum 2% uncertainty. We determined a concordant U-Pb age of 180±2 Ma for the EVM, which agrees with the results of Sylvester and others (1978). The unnamed diorite produced a concordant U-Pb age of 177±3 Ma. Concordant U-Pb ages of 172±2, 172±3, 173±2, 174±2, and 175±2 Ma were determined for individual samples of the JFQM and agree with the age reported by Sylvester and others (1978) of 174±5 Ma. Concordant U-Pb ages of 168±4, 168±3, 169±1, 172±2, and 172±2 Ma were determined for individual BCG samples. Within the reported error, there is no difference in age between individual samples of the BCG, but taken as absolute, the ages tantalizingly decrease from NW to SE within the exposed area of the BCG. No such pattern is suggested within the JFQM. Collectively, these new LA-ICP-MS zircon age data support the observed field relationships and suggest that the EJB magma system was periodically active for as long as 10-12 million years. This time scale agrees well with current models of incremental growth of plutons and has important implications for strain accumulation in mid-crustal arc environments.

The Grand St Bernard-Briançonnais Nappe System and the Paleozoic Inheritance of the Western Alps Unraveled by Zircon U-Pb Dating

NASA Astrophysics Data System (ADS)

Bergomi, M. A.; Dal Piaz, G. V.; Malusà, M. G.; Monopoli, B.; Tunesi, A.

2017-12-01

The continental crust involved in the Alpine orogeny was largely shaped by Paleozoic tectono-metamorphic and igneous events during oblique collision between Gondwana and Laurussia. In order to shed light on the pre-Alpine basement puzzle disrupted and reamalgamated during the Tethyan rifting and the Alpine orogeny, we provide sensitive high-resolution ion microprobe U-Pb zircon and geochemical whole rock data from selected basement units of the Grand St Bernard-Briançonnais nappe system in the Western Alps and from the Penninic and Lower Austroalpine units in the Central Alps. Zircon U-Pb ages, ranging from 459.0 ± 2.3 Ma to 279.1 ± 1.1 Ma, provide evidence of a complex evolution along the northern margin of Gondwana including Ordovician transtension, Devonian subduction, and Carboniferous-to-Permian tectonic reorganization. Original zircon U-Pb ages of 371 ± 0.9 Ma and 369.3 ± 1.5 Ma, from calc-alkaline granitoids of the Grand Nomenon and Gneiss del Monte Canale units, provide the first compelling evidence of Late Devonian orogenic magmatism in the Alps. We propose that rocks belonging to these units were originally part of the Moldanubian domain and were displaced toward the SW by Late Carboniferous strike-slip faulting. The resulting assemblage of basement units was disrupted by Permian tectonics and by Mesozoic opening of the Alpine Tethys. Remnants of the Moldanubian domain became either part of the European paleomargin (Grand Nomenon unit) or part of the Adriatic paleomargin (Gneiss del Monte Canale unit), to be finally accreted into the Alpine orogenic wedge during the Cenozoic.

Detrital Record of Phanerozoic Tectonics in Iran: Evidence From U-Pb Zircon Geochronology

NASA Astrophysics Data System (ADS)

Horton, B. K.; Gillis, R. J.; Stockli, D. F.; Hassanzadeh, J.; Axen, G. J.; Grove, M.

2004-12-01

Ion-microprobe U-Pb ages of 91 detrital zircon grains supplement ongoing investigations of the tectonic history of Iran, a critical region bridging the gap between the Alpine and Himalayan orogenic belts. These data improve understanding of the distribution of continental blocks during a complex history of Late Proterozoic (Pan-African) crustal growth, Paleozoic passive-margin sedimentation, early Mesozoic collision with Eurasia, and Cenozoic collision with Arabia. U-Pb analyses of detrital zircon grains from four sandstone samples (two Lower Cambrian, one uppermost Triassic-Lower Jurassic, one Neogene) collected from the Alborz mountains of northern Iran reveal a spectrum of ages ranging from 50 to 2900 Ma. Most analyses yield concordant to moderately discordant ages. The Lower Cambrian Lalun and Barut sandstones yield age distribution peaks at approximately 550-650, 1000, and 2500 Ma, consistent with a Gondwanan source area presently to the south and west in parts of Iran and the Arabian-Nubian shield (Saudi Arabia and northwestern Africa). The uppermost Triassic-Lower Jurassic Shemshak Formation exhibits a broad range of U-Pb ages, including peaks of approximately 200-260, 330, 430, 600, and 1900 Ma, requiring a Eurasian source area presently to the north and east in the Turan plate (Turkmenistan and southwestern Asia). Neogene strata display both the youngest and oldest ages (approximately 50 and 2900 Ma) of any samples, a result of substantial sedimentary recycling of older Phanerozoic cover rocks. Because the youngest zircon ages for three of the four samples are indistinguishable from their stratigraphic (depositional) ages, these data suggest rapid exhumation and help constrain the termination age of Late Proterozoic-Early Cambrian (Pan-African) orogenesis and the timing of the Iran-Eurasia collision.

A Triassic to Cretaceous Sundaland-Pacific subduction margin in West Sarawak, Borneo

NASA Astrophysics Data System (ADS)

Breitfeld, H. Tim; Hall, Robert; Galin, Thomson; Forster, Margaret A.; BouDagher-Fadel, Marcelle K.

2017-01-01

Metamorphic rocks in West Sarawak are poorly exposed and studied. They were previously assumed to be pre-Carboniferous basement but had never been dated. New 40Ar/39Ar ages from white mica in quartz-mica schists reveal metamorphism between c. 216 to 220 Ma. The metamorphic rocks are associated with Triassic acid and basic igneous rocks, which indicate widespread magmatism. New U-Pb dating of zircons from the Jagoi Granodiorite indicates Triassic magmatism at c. 208 Ma and c. 240 Ma. U-Pb dating of zircons from volcaniclastic sediments of the Sadong and Kuching Formations confirms contemporaneous volcanism. The magmatic activity is interpreted to represent a Triassic subduction margin in westernmost West Sarawak with sediments deposited in a forearc basin derived from the magmatic arc at the Sundaland-Pacific margin. West Sarawak and NW Kalimantan are underlain by continental crust that was already part of Sundaland or accreted to Sundaland in the Triassic. One metabasite sample, also previously assumed to be pre-Carboniferous basement, yielded Early Cretaceous 40Ar/39Ar ages. They are interpreted to indicate resumption of subduction which led to deposition of volcaniclastic sediments and widespread magmatism. U-Pb ages from detrital zircons in the Cretaceous Pedawan Formation are similar to those from the Schwaner granites of NW Kalimantan, and the Pedawan Formation is interpreted as part of a Cretaceous forearc basin containing material eroded from a magmatic arc that extended from Vietnam to west Borneo. The youngest U-Pb ages from zircons in a tuff layer from the uppermost part of the Pedawan Formation indicate that volcanic activity continued until c. 86 to 88 Ma when subduction terminated.

Ore genesis and geodynamic setting of the Lianhuashan porphyry tungsten deposit, eastern Guangdong Province, SE China: constraints from muscovite 40Ar-39Ar and zircon U-Pb dating and Hf isotopes

NASA Astrophysics Data System (ADS)

Liu, Peng; Mao, Jingwen; Pirajno, Franco; Jia, Lihui; Zhang, Feng; Li, Yang

2017-12-01

The Lianhuashan deposit has long been regarded as a typical tungsten porphyry deposit, located in the eastern Guangdong Province, in the Southeastern Coastal Metallogenic Belt (SCMB). LA-MC-ICP-MS zircon U-Pb dating of the quartz porphyry yielded a weighted mean 206Pb/238U age of 137.3 ± 2.0 Ma, which is interpreted as the emplacement age of the quartz porphyry. Hydrothermal muscovite yielded a plateau 40Ar/39Ar age of 133.2 ± 0.9 Ma, which is consistent with the zircon U-Pb age, suggesting that the tungsten mineralization is genetically related to the quartz porphyry. Combined with previous studies, we suggest that there is a 145-135 Ma episode linking the granitic magmas with W-Sn ore systems in the SCMB. Zircon ɛHf (t) values of the quartz porphyry are in range of - 3.8 to 0.9, and the two-stage Hf model ages (TDM2) are 1.1-1.4 Ga, which is younger than the basement rocks in the Cathaysia Block (1.8-2.2 Ga), signifying that the quartz porphyry was predominantly derived from melting of Mesoproterozoic crust containing variable amounts of mantle components. In combination with the newly recognized coeval alkaline/bimodal magmatism and A-type granites in eastern Guangdong, we suggest that the 145-135 Ma W-Sn metallogenic event of the SCMB is related to a geodynamic setting of large-scale lithospheric extension and thinning, which can be ascribed to melting of the crust caused by mantle upwelling, triggered by the oblique subduction of the Izanagi plate.

First U-Pb geochronology on detrital zircons from Early-Middle Cambrian strata of the Torgau-Doberlug Syncline (eastern Germany) and palaeogeographic implications

NASA Astrophysics Data System (ADS)

Abubaker, Atnisha; Hofmann, Mandy; Gärtner, Andreas; Linnemann, Ulf; Elicki, Olaf

2017-10-01

LA-ICP-MS U-Pb data from detrital zircons of the Ediacaran to Cambrian siliciclastic sequence of the Torgau-Doberlug Syncline (TDS, Saxo-Thuringia, Germany) are reported for the first time. The majority of 203 analysed zircon grains is Proterozoic with minor amount of Archean and Palaeozoic grains. The U-Pb ages fall into three groups: 2.8-2.4 Ga (3%), Neoarchean to earliest Palaeoproterozoic; 2.3-1.6 Ga (46%), early to late Palaeoproterozoic; 1.0-0.5 Ga (47%), Neoproterozoic to Cambrian. This age distribution is typical for the West African Craton as the source area and for Cadomian orogenic events in northwestern Gondwana. The samples show an age gap between 1.6 and 1.0 Ga, which is characteristic for West African provenance and diagnostic in distinguishing this unit from East Avalonia and Baltica. The dataset shows clusters of Palaeoproterozoic ages at 2.2-1.7 Ga, that is typical for western Gondwana, which was affected by abundant magmatic intrusions (ca. 2.2-1.8 Ga) during the Eburnean orogeny (West African craton). Neoarchean zircon ages (3%) point to recycling of magmatic rocks formed during the Liberian and Leonian orogenies. Ediacaran to earliest Cambrian rocks of the TDS originated in an active margin regime of the Gondwanan shelf. The following early Palaeozoic overstep sequence was deposited within rift settings that reflects instability of the West-Gondwanan shelf and the separation of terranes from Ordovician onward. The results of this study demonstrate distinct northwestern African provenance of the Cambrian siliciclastics of the TDS. Due to Th-U ratios from concordant zircon analysis, igneous origin from felsic melts is concluded as the source of these grains.

Geochronology, geochemistry and Sr-Nd-Pb-Hf isotopes of the Early Jurassic granodiorite from the Sankuanggou intrusion, Heilongjiang Province, Northeastern China: Petrogenesis and geodynamic implications

NASA Astrophysics Data System (ADS)

Deng, Ke; Li, Qiugen; Chen, Yanjing; Zhang, Cheng; Zhu, Xuefeng; Xu, Qiangwei

2018-01-01

Mesozoic granitoid rocks represent a volumetrically component of the Northeastern (NE) China and preserve useful information about the tectonomagmatic history of this region. The Sankuanggou intrusion associated with skarn Fe-Cu deposit in the Duobaoshan ore field within NE China primarily consists of granodiorite with minor alkali-feldspar granite and diorite, which intrudes the Ordovician Duobaoshan Formation in the region. Zircon LA-ICP-MS U-Pb geochronology and whole-rock geochemistry, and Sr-Nd-Pb-Hf isotope analysis were performed on the Sankuanggou intrusion to investigate the petrogenesis and geodynamic implications. Zircon U-Pb dating of magmatic zircons from the granodiorite rock suggests that the intrusion was emplaced in the Early Jurassic (177 ± 1 Ma). Geochemically, it belongs to the metaluminous to slightly peraluminous high-K calc-alkaline I-type granitoids with a narrow range of SiO2 concentration (65.73-67.33 wt.%), high Ba, Sr, LREE and LILE contents and low abundance of Rb, Y, HREE and HFSE. All of these studied samples have homogeneous initial isotope traits with (87Sr/86Sr)i ranging from 0.70415 to 0.70423, εNd(t) of + 3.6 to + 4.0, (206Pb/204Pb)i = 17.933-18.458, (207Pb/204Pb)i = 15.520-15.587 and (208Pb/204Pb)i = 37.523-38.087, and zircon εHf(t) values varying from + 4.8 to + 9.9. These results, combined with the previous data, demonstrate that the Sankuanggou granitoids were formed by partial melting of the pre-existing juvenile crust in an extensional regime related to the post-collisional setting following the closure of the CAOB rather than previously proposed continental arc setting related to Paleo-Pacific or the Mongol-Okhotsk subduction, although their potential influence should not be dismissed.

Integrated in situ U-Pb Age and Hf-O Analyses of Zircon from the Northern Yangtze Block: New Insights into the Neoproterozoic Low-δ18O Magmas in the South China Block

NASA Astrophysics Data System (ADS)

Yang, Y. N.; Wang, X. C.; Li, Q. L.; Li, X. H.

2015-12-01

The oxygen isotopic composition of Neoproterozoic magmas from the northern Yangtze Block holds a key for the origin of large-scale 18O depletion in the HP and UHP metamorphic rocks in the Dabie-Sulu orogenic belt, northern margin of the South China Block. We report here the integrated in situ U-Pb dating and O-Hf isotope analyses of zircon grains from sedimentary and volcanic rocks of the late Neoproterozoic Suixian Group (SG) from the northern Yangtze Block. Detrital zircon grains display age peaks of 0.73-0.74 Ga, 0.79 Ga, and 2.0 Ga. Zircon U-Pb ages together with Hf-O isotopic composition indicate provenance of SG dominantly from proximal Neoproterozoic igneous rock and likely hidden Paleoproterozoic basement along the northern margin of the Yangtze Block. The zircon δ18O values from SG range from 10.5‰ to 1.3‰. Zircon grains with negative δ18O value, typical result of magma-ice interaction, were not identified in this study. The major phase of low-δ18O (< 4‰) magmas initiated at ca. 780 Ma, long before the first glaciation event (< 715 Ma) in the South China Block. Thus caution should be taken when using low-δ18O zircon grains to infer cold climate. Low-δ18O zircon grains have large ranges of ɛHf(t) values, varying from -15.5 to 10.7, concentrating on negative ɛHf(t). This strongly argues against the possibility that the low-δ18O magma was produced by partial melting of high-temperature hydrothermally altered oceanic crust because this model predicted MORB-like Hf isotopes for the resultant low-δ18O magmas. This study emphasizes that high-T water-rock interaction and continental rifting tectonic setting are essential to generate abundant low-δ18O magmas. The important application of our study is to confirm that most of negative-δ18O zircons identified in HP and UHP metamorphic rocks may not have been inherited from their Neoproterozoic protoliths.

Late Paleozoic-Early Mesozoic tectonic evolution of the Paleo-Asian Ocean: geochronological and geochemical evidence from granitoids in the northern margin of Alxa, Western China

NASA Astrophysics Data System (ADS)

Sha, Xin; Wang, Jinrong; Chen, Wanfeng; Liu, Zheng; Zhai, Xinwei; Ma, Jinlong; Wang, Shuhua

2018-03-01

The Paleo-Asian Ocean (Southern Mongolian Ocean) ophiolitic belts and massive granitoids are exposed in the Alxa block, in response to oceanic subduction processes. In this work, we report petrographic, geochemical, and zircon U-Pb age data of some granitoid intrusions from the northern Alxa. Zircon U-Pb dating for the quartz diorite, tonalite, monzogranite, and biotite granite yielded weighted mean 206Pb/238U ages of 302±9.2 Ma, 246.5±4.6 Ma, 235±4.4 Ma, and 229.5±5.6 Ma, respectively. The quartz diorites ( 302 Ma) exhibit geochemical similarities to adakites, likely derived from partial melting of the initially subducted Chaganchulu back-arc oceanic slab. The tonalites ( 246.5 Ma) display geochemical affinities of I-type granites. They were probably derived by fractional crystallization of the modified lithospheric mantle-derived basaltic magmas in a volcanic arc setting. The monzogranites ( 235 Ma) are characterized by low Al2O3, but high Y and Yb with notably negative Eu anomalies. In contrast, the biotite granites ( 229.5 Ma) show high Al2O3 but low Y and Yb with steep HREE patterns and the absence of negative Eu anomalies. Elemental data suggested that the biotite granites were likely derived from a thickened lower crust, but the monzogranites originated from a thin crust. Our data suggested that the initial subduction of the Chaganchulu oceanic slab towards the Alxa block occurred at 302 Ma. This subduction process continued to the Early Triassic ( 246 Ma) and the basin was finally closed before the Middle Triassic ( 235 Ma). Subsequently, the break-off of the subducted slab triggered asthenosphere upwelling (240-230 Ma).

SHRIMP U-Pb detrital zircon dating to check subdivisions in metamorphic complexes: a case of study in the Nevado-Filábride complex (Betic Cordillera, Spain)

NASA Astrophysics Data System (ADS)

Santamaría-López, Ángel; Sanz de Galdeano, Carlos

2018-04-01

U-Pb dating on inherited detrital zircons has been applied to obtain the probable maximum age of deposition of the detrital protolith of the Nevado-Filábride complex (Betic Cordillera, Spain). Five of eight samples correspond to the lower part of the lithologic sequence of this complex, where radiometric dating of metasediments has not been presented till the present. The youngest age populations in the majority of samples are Carboniferous. The estimation of the maximum age of deposition in the lower and upper units is 349.1 ± 1.6 and 334.6 ± 2.9 Ma, respectively. In addition, samples show common age populations at ca. 490-630 and ca. 910-1010 Ma. Observations agree with the Carboniferous to early Permian U-Pb ages previously obtained in orthogneisses levels which are situated in the upper part of the complex. Combination of the minimum age of deposition deducible from the orthogneisses studies and the maximum ages of deposition obtained from the detrital zircons of this work, allow establishing the deposition of de studied lithological succession comprised between ca. 282 and 349 Ma or a shorter period.

Arc-continent collision of the Coastal Range in Taiwan: Geochronological constraints from U-Pb ages of zircons

NASA Astrophysics Data System (ADS)

Geng, Wei; Zhang, Xun-Hua; Huang, Long

2018-04-01

The oblique arc-continent collision between the Luzon arc and the southeastern margin of the Eurasian continent caused the uplift of Taiwan. The Coastal Range in eastern Taiwan is the northern section of the Luzon arc in the collision zone and thus records important information about the arc-continent collision. In this paper, we determine and analyze the U-Pb ages of magmatic zircons from the volcanic arc and clastic zircons from the fore-arc basin in the Coastal Range. For the volcanic arc in the Coastal Range, the eruption ages range from 16.8-5 Ma. Given that the initial subduction of the South China Sea oceanic crust (17 Ma) occurred before the Luzon arc formed, we conclude that the volcanic activity of the Coastal Range began at 16.8 ± 1.3 Ma; it was most active from 14 to 8 Ma and continued until approximately 5 Ma. The U-Pb chronology also indicates that the initial stage of arc-continent collision of the Coastal Range started at approximately 5 Ma, when the northern section of the Luzon arc moved away from the magmatic chamber because of the kinematics of the Philippine Sea Plate.

Capability of U-Pb dating of zircons from Quaternary tephra: Jemez Mountains, NM, and La Sal Mountains, UT, USA

NASA Astrophysics Data System (ADS)

Krautz, Jana; Hofmann, Mandy; Gärtner, Andreas; Linnemann, Ulf; Kleber, Arno

2018-01-01

Two Quaternary tephras derived from the Jemez Mountains, New Mexico - the Guaje and Tsankawi tephras - are difficult to distinguish due to their similar glass-shard chemical composition. Differences in bulk chemical composition are small as well. Here we examine the feasibility to assign an age to a distal tephra layer in the La Sal Mountains, Utah, by U-Pb dating of zircons and to correlate it with one of the two Jemez eruptions. We also dated original Jemez tephras for comparison. Even though the tephras are very young, we obtained reasonable age determinations using the youngest cluster of zircon grains overlapping in age at 2σ. Thereafter, the Guaje tephra is 1.513 ± 0.021 Myr old. The La Sal Mountains tephra is correlated with the Tsankawi tephra. Three samples yielded a common age range of 1.31-1.40 Myr. All ages are in slight disagreement with published age determinations obtained by 40Ar / 39Ar dating. These findings indicate that distal Jemez tephras can be distinguished by U-Pb dating. Furthermore, we encourage giving this method a try for age assignments even of Quaternary volcanic material.

U Pb and Lu Hf isotope record of detrital zircon grains from the Limpopo Belt Evidence for crustal recycling at the Hadean to early-Archean transition

NASA Astrophysics Data System (ADS)

Zeh, Armin; Gerdes, Axel; Klemd, Reiner; Barton, J. M., Jr.

2008-11-01

Detrital zircon grains from Beit Bridge Group quartzite from the Central Zone of the Limpopo Belt near Musina yield mostly ages of 3.35-3.15 Ga, minor 3.15-2.51 Ga components, and numerous older grains grouped at approximately 3.4, 3.5 and 3.6 Ga. Two grains yielded concordant Late Hadean U-Pb ages of 3881 ± 11 Ma and 3909 ± 26 Ma, which are the oldest zircon grains so far found in Africa. The combined U-Pb and Lu-Hf datasets and field relationships provide evidence that the sedimentary protolith of the Beit Bridge Group quartzite was deposited after the emplacement of the Sand River Gneisses (3.35-3.15 Ga), but prior to the Neoarchean magmatic-metamorphic events at 2.65-2.60 Ga. The finding of abundant magmatic zircon detritus with concordant U-Pb ages of 3.35-3.15 Ga, and 176Hf/ 177Hf of 0.28066 ± 0.00004 indicate that the Sand River Gneiss-type rocks were a predominant source. In contrast, detrital zircon grains older than approximately 3.35 Ga were derived from the hinterland of the Limpopo Belt; either from a so far unknown crustal source in southern Africa, possibly from the Zimbabwe Craton and/or a source, which was similar but not necessarily identical to the one that supplied the Hadean zircons to Jack Hills, Western Australia. The Beit Bridge Group zircon population at >3.35 Ga shows a general ɛHf t increase with decreasing age from ɛHf 3.9Ga = -6.3 to ɛHf 3.3-3.1Ga = -0.2, indicating that Hadean crust older than 4.0 Ga ( TDM = 4.45-4.36 Ga) was rejuvenated during magmatic events between >3.9 and 3.1 Ga, due to a successive mixing of crustal rocks with mantle derived magmas. The existence of a depleted mantle reservoir in the Limpopo's hinterland is reflected by the ˜3.6 Ga zircon population, which shows ɛHf 3.6Ga between -4.6 and +3.2. In a global context, our data suggest that a long-lived, mafic Hadean protocrust with some tonalite-trondhjemite-granodiorite constituents was destroyed and partly recycled at the Hadean/Archean transition, perhaps due to the onset of modern-style plate tectonics.

The 3.66 Ga Nuvvuagittuq Tonalite-Trondhjemite-Granodiorite Suite: A Case of Hadean Anatexis

NASA Astrophysics Data System (ADS)

Stevenson, R. K.; Bizzarro, M.

2011-12-01

The ca 3.8-4.3 Ga Nuvvuagittuq supracrustal sequence in northern Quebec, Canada, ranks as one of the most exciting early Earth terrain discoveries of the past fifteen years. The supracrustal sequence consists of mafic amphibolite rocks with rare felsic schists along with ultramafic sills and oxide-rich and quartz-rich iron formation. The supracrustal sequence is semi-oval in form and is folded around a core of foliated tonalite-trondhjemite-granodiorite (TTG) suite. A similar TTG suite forms a sheath around the exterior margin of the Nuvvuagittuq supracrustal sequence. Zircons from a felsic volcanic unit within the sequence yielded a minimum age of 3.8 Ga (U-Pb) for volcanic rocks of the sequence (David et al. 2009). A study of the Nd142 isotope systematics of the sequence found evidence for a Nd142 deficit in the sequence and a Nd142-Sm147 isochron yielded an age of 4.28 Ga (O'Neil et al. 2008). Dating of zircons from the tonalite sheath (U-Pb TIMS; David et al., 2009) and from the inner core (U-Pb SHRIMP; this study) yield similar ages (3.66 Ga). Zircons from this sequence undoubtedly contributed to the Hf isotopic composition of Archean sedimentary sequences of the Canadian Shield (Stevenson and Patchett 1990).The zircons from the TTG suite are very homogeneous and only have rare thin metamorphic rims that yield an age of 2.7 Ga (U-Pb Shrimp; this study). Radiogenic isotope studies (Sm-Nd and Lu-Hf) indicate that the TTG suite is an anatectic product of the melting of the Nuvvuagittuq supracrustal suite. The geochemical compositions of the Nuvvuagittuq TTG suite are compared with the compositions of ca 3.6 TTG suites from other localities around the world. David, J. Godin, L., Stevenson, R., O'Neil, J. and Francis, D. 2009. U-Pb ages (3.8-2.7 Ga) and Nd isotope data from the newly- identified Eoarchean Nuvvuagittuq supracrustal sequence, Superior Craton, Canada. GSA Bulletin 121; 150-163. O'Neil, J., Carlson, R.W., Francis, D., and Stevenson, R.K. 2008. Neodymium-142 Evidence Hadean Mafic Crust. Science 321 (Sept. 26), 1828-1831. Stevenson, R.K. and Patchett, P.J. (1990): Implications for the evolution of continental crust from Hf isotope systematics of Archean detrital zircons. Geochimica et Cosmochimica Acta 54, 1683-1697.

Detrital Zircon U-Pb Analysis of the Liuqu Conglomerate Along the Yarlung-Zangbo Suture Zone, and Implications for the Mode and Timing of Collision Tectonics in Southern Tibet

NASA Astrophysics Data System (ADS)

Xie, Y.; Dilek, Y.

2016-12-01

The Liuqu Conglomerate (LQC) along the Yarlung-Zangbo suture zone (YZSZ) in Southern Tibet is a terrestrial deposit that provides significant spatial and temporal constraints for the timing and nature of collisional events in the tectonic evolution of the Tibetan-Himalayan orogenic belt. The 10-km-wide (N-S) LQC is exposed discontinuously for more than 1000 km in an E-W direction, and is tectonically overlain to the north by the Cretaceous Neotethyan oceanic lithosphere along a S-vergent thrust fault system and to the south by Triassic-Jurassic metamorphosed sedimentary-volcanic rocks of the Tethyan Himalaya along N-vergent reverse-thrust faults. The major facies of the LQC are the matrix-supported to clast-supported conglomerates. The matrix is poorly to moderate sorted red quartz sandstone, mudstone and sub-rounded pebble to cobble-sized clasts. The clast lithology present in central and southern parts includes dark red sandstone, siltstone and mudstone greyish-green shale, grey phyllite and slate with their provenance in the Triassic Tethyan Himalaya to the south. The clastic material making up its stratigraphy in the northern part of the LQC includes quartz sandstone, radiolarian chert, minor dolerite, gabbro and peridotite derived from the Cretaceous ophiolite. Here we report in-situ detrital zircon U-Pb age analysis of sandstone from the LQC near Liuqu area. 163 concordant U-Pb ages obtained from sample 22-LQ-15, 27-LQ-15 and 35-LQ-15 show the youngest age is 307±13 Ma with discordance of -17.02%, and the oldest zircon grain is 3362 ±51 Ma with discordance of 2.63%. Statistically, the age spectrum of these zircons from the three sandstone samples display a prominent peak centred in 935 Ma, a large peak around 516 Ma, and two small clusters around 2429 Ma and 2772 Ma. The zircon U-Pb results provide evidence of age clusters of the sandstone in LQC are consistent with the detrital U-Pb age signature of the sandstone in Tethyan Himalaya. Thus, the sediments in the LQC could be derived from the northern edge of the Indian margin and a late Jurassic-Cretaceous intra-oceanic island arc that lay within Thethys and developed prior to the final collision between India and Eurasia plates.

Matrix effects for elemental fractionation within ICPMS: applications for U-Th-Pb geochronology

NASA Astrophysics Data System (ADS)

Chen, W.

2016-12-01

Recent development in instruments provides significant technical supports for daily, quick, money saving geochemical analyses. Laser ablation ICPMS stands out due to these reasons, especially for the U-Th-Pb isotopic dating. Matrix-matched external standardization is by far the most common approach used in U-Th-Pb dating via LA-ICPMS. However, matrix-effects between standard and sample for in-situ dating have shown to be both significant and insignificant. It remains mysterious whether a well matrix-matched standard is needed for U-Th-Pb dating by LA-ICPMS. This study provides an experimental framework for the understanding of matrix effects induced elemental fractionation for U-Th-Pb associated with ICPMS. A preliminary study on the influence of varied U, Th and Pb amounts on their fractionations has been carried out. Experimental data show that different U, Th and Pb contents result in varied 238U/206Pb and 232Th/208Pb ratios. The fractionations of U/Pb and Th/Pb increase with the increasing contents (1 ppb to 100 ppb) with a strong positive anomaly at 10 ppb. Matrixes representing minerals frequently used in dating have been investigated for the influences on U/Pb and Th/Pb fractionations, which suggest a complicated effect. Little fractionations observed between mineral pairs (e.g., monazite and apatite; zircon and perovskite; rutile and perovskite; xenotime and baddeleyite), whereas large fractionations identified for other minerals (e.g., zircon and baddeleyite; monazite and sphene; rutile and baddeleyite). Single element matrix (i.e., Si, P, Ca, Zr, Ti) has been studied to identify their effects on the fractionations. U/Pb ratio increases with the increasing Si and P contents, whereas it decreases for Zr, Ca and Ti. Th/Pb ratio increases with increasing Si contents, decreases for P and Zr, and increases first then decreases for Ca and Ti. Above all, different matrix and U, Th and Pb amounts show distinct U/Pb and Th/Pb fractionations within ICPMS. The minerals with matrix indicating similar fractionations in ICPMS can be used as suitable reference standards for each other with proper laser ablation settings via LA-ICPMS. Moreover, matched U, Th and Pb contents should be considered when using a matrix-matched standard in laser ablation analysis.

Two-pyroxene syenitoids from the Moldanubian Zone of the Bohemian Massif: peculiar magmas derived from a strongly enriched lithospheric mantle source

NASA Astrophysics Data System (ADS)

Janoušek, Vojtěch; Holub, František; Gerdes, Axel; Verner, Kryštof

2013-04-01

(Ultra-)potassic plutonic rocks constitute a conspicuous association with metamorphic rocks of the high-grade, lower crustal/upper mantle Gföhl Unit (Moldanubian Zone). They can be subdivided into two contrasting suites: (1) coarse Kfs-phyric amphibole-biotite melagranite to quartz syenite (the durbachite series sensu Holub 1997), and (2) essentially even-grained biotite-two-pyroxene quartz syenites to melagranites (Tábor and Jihlava plutons). The latter, "syenitoid suite", characterized by an originally 'dry' mineral assemblage orthopyroxene + clinopyroxene + Mg-biotite, with accessoric zircon, apatite, ilmenite, monazite and/or rutile ± Cr-spinel, is a subject of the current study. Our conventional U-Pb ages for zircon (336.9 ± 0.6 Ma) and rutile (336.8 ± 0.8 Ma) from the Tábor Pluton, together with the age from the Jihlava body (U-Pb zircon: 335.1 ± 0.6 Ma; Kotková et al. 2010), provide a precise time bracket for the emplacement and rapid cooling of the syenitoids below c.600 ° C (closure temperature of U-Pb system in rutile: Cherniak 2000). This is in line with post-tectonic emplacement of hot dry melt into shallow levels of essentially consolidated orogenic crust. Comparably low temperatures obtained by zircon and rutile saturation calculations document probably a delayed onset of crystallization of the accessories in a hot, alkalis and ferromagnesian components-rich magma derived from a mantle source. Indeed, the structural relations inside and around the ultrapotassic plutons suggest that the most important regional HT/LP flat-lying fabric(s) in the Moldanubian Zone are closely related with the emplacement and crystallization of the durbachite suite at 343-338 Ma. They have formed prior to the relatively shallower emplacement of the essentially post-tectonic syenitoids dated at ~337-336 Ma (Žák et al. 2005; Verner et al. 2006, 2008). The two magmatic suites are thus essentially diachronous and not (nearly) contemporaneous (c. 335 Ma) intrusions at contrasting crustal levels as assumed by Kotková et al. (2010). The syenitoid plutons show mutually comparable, crustal-like radiogenic isotope signatures with highly radiogenic Sr (87Sr/86Sr337= 0.7119-0.7125) and unradiogenic Nd (?Nd337 = -6.8 to -7.6). This, together with the rest of the whole-rock geochemical variation, is in line with a generation from a strongly enriched lithospheric mantle source. It was, shortly before, modified by a deep subduction and relamination of the upper crustal material, similar to the felsic HP granulites common in the Moldanubian Zone (Janoušek & Holub 2007; Lexa et al. 2011). The petrology and chemical data indicate that large-scale mixing with crustally-derived acid magmas can be largely or fully discounted and the key role is ascribed to closed-system fractional crystallization with, or without, crystal accumulation of various combinations of biotite, clinopyroxene and/or orthopyroxene with minor amounts of apatite. This stands in a sharp contrast with the history of volumetrically prevalent, slightly older, durbachite suite, in genesis of which the magma mixing of chemically and isotopically contrasting mantle and crustal components was clearly much more significant (Holub 1997). This research was financially supported by the GAR Project P210-11-2358 (to VJ).

Petrogenesis of Late Jurassic granodiorites from Gutian, Fujian Province, South China: Implications for multiple magma sources and origin of porphyry Cu-Mo mineralization

NASA Astrophysics Data System (ADS)

Li, Bin; Jiang, Shao-Yong; Lu, An-Huai; Lai, Jian-Qing; Zhao, Kui-Dong; Yang, Tao

2016-11-01

The Gutian porphyry Cu-Mo deposit is a newly proved porphyry copper deposit in the coastal South China associated with granodioritic porphyries. In this study, zircon U-Pb ages and Hf isotope data, as well as geochemical and Sr-Nd-Pb-Re-Os isotopic compositions, are reported for these intrusions and minerals. Both zircon U-Pb and molybdenite Re-Os dating suggest that the Gutian granodiorite porphyries and related mineralization formed at 160 Ma. The Gutian granodiorites show a low-Mg adakitic geochemical affinity, with relatively high K2O but low Cr and Ni contents. These rocks have initial (87Sr/86Sr)i ratios of 0.7085 to 0.7097, negative εNd(t) values (- 12.5 to - 7.8), (206Pb/204Pb)t ratios of 18.048 to 18.241, (207Pb/204Pb)t ratios of 15.609 to 15.628, and (208Pb/204Pb)t ratios of 38.494 to 38.667. Zircons from the granodiorites have negative εHf(t) values of - 15.7 to - 8.5, which are close to those of Cathaysia crust-derived melts. Geochemical and Sr-Nd-Pb-Hf isotopic compositions suggest that they may be derived from Late Jurassic thickened juvenile lower crust. These lower crustal magma sources may not only contain pre-Proterozoic basement rocks, but also involve Triassic and Middle-Late Jurassic arc magmas within the lower crust, which were likely derived from an enriched mantle source associated with paleo-Pacific Plate subduction from the Middle to Late Jurassic. The Gutian ore-related granodiorites represent a new example for significant contributions of ancient subduction melts and enriched mantle-derived sources for porphyry-type magmatism and Cu-Mo mineralization, which occurred in response to an arc regime during the Middle to Late Jurassic in South China. Supplemental Table S2. Hf isotopic compositions of zircons from the studied rocks from the Gutian porphyry deposit in South China. Supplemental Table S3. Statistics for zircon U-Pb ages and Hf isotope compositions from Gutian granodiorites in South China Supplemental Table S4. Major element (wt.%) and trace element (ppm) concentrations of Gutian intrusions in Fujian Province, South China. Supplemental Table S5. Sr and Nd isotopic compositions of the studied rocks from the Gutian porphyry deposit in Fujian Province, South China. Supplemental Table S6. Pb isotopic compositions of the studied rocks from the Gutian porphyry deposit in South China. Supplemental Table S7. Re-Os isotopic compositions of molybdenite from the Gutian porphyry deposit in South China.

Geology and geochronology of the Spirit Mountain batholith, southern Nevada: Implications for timescales and physical processes of batholith construction

USGS Publications Warehouse

Walker, B.A.; Miller, C.F.; Lowery, Claiborne L.; Wooden, J.L.; Miller, J.S.

2007-01-01

The Spirit Mountain batholith (SMB) is a ??? 250??km2 composite silicic intrusion located within the Colorado River Extensional Corridor in southernmost Nevada. Westward tilting of 40-50?? has exposed a cross-section from the roof through deep levels of the batholith. Piecemeal construction is indicated by zircon geochronology, field relations, and elemental geochemistry. Zircon U/Pb data (SHRIMP) demonstrates a ??? 2??million year (17.4-15.3??Ma) history for the SMB. Individual samples contain zircons with ages that span the lifetime of the batholith, suggesting recycling of extant zircon into new magma pulses. Field relations reveal several distinct intrusive episodes and suggest a common injection geometry of stacked horizontal sheets. The largest unit of the SMB is a gradational section (from roof downward) of high-silica leucogranite through coarse granite into foliated quartz monzonite. Solidification of this unit spans most of the history of the batholith. The 25??km ?? 2??km leucogranite was emplaced incrementally as subhorizontal sheets over most or all of the history of this section, suggesting repeated fractional crystallization and melt segregation events. The quartz monzonite and coarse granite are interpreted to be cumulate residuum of this fractionation. Age data from throughout this gradational unit show multiple zircon populations within individual samples. Subsequent distinct intrusions that cut this large unit, which include minor populations of zircons that record the ages of earlier events in construction of the batholith, preserve a sheeted, sill-on-sill geometry. We envision the SMB to have been a patchwork of melt-rich, melt-poor, and entirely solid zones throughout its active life. Preservation of intrusion geometries and contacts depended on the consistency of the host rock. Zircons recycled into new pulses of magma document remobilization of previously emplaced crystal mush, suggesting the mechanisms by which evidence for initial construction of the batholith became blurred. ?? 2007.

U-Pb and K-Ar geochronology in Paleozoic and Mesozoic intrusive rocks of the Coastal Cordillera, Valparaiso, Chile

USGS Publications Warehouse

Gana, Paulina; Tosdal, Richard M.

1996-01-01

The U-Pb and K-Ar geochronology applied to intrusive rocks from the Coastal Batholith of Central Chile, demonstrates the existence of a basement block of the Mirasol Unit, with a crystallization age of 299??10 Ma, exposed in the northern block of the Melipilla Fault. The age of 214??1 Ma obtained in the 'Dioritas Gne??isicas de Cartagena Unit', indicates that a Late Triassic magmatism took place in this region; it coincides with the end of an extensive crustal melting period, proposed for northern Chile. The ages of the Jurassic plutonic units (Laguna Verde, Sauce, Pen??uelas and Limache) are restricted to the 156-161 Ma interval, showing in certain cases, inherited zircons from an unknown source. The difference between ages obtained using both chronological methods is a few million years, indicating that a short time passed between the crystallization and the cooling of the plutonic bodies, as well as a fast magmatic differentiation process. The Laguna Verde and Sauce Units, experienced a fast uplift, probably as a result of an extensional tectonic process in the magmatic arc, or induced by the magmatic pressure through fracture zones during Middle Jurassic.

Zircon Messengers Reveal the Age and History of Great Basin Crust, Kern Mountains, Nevada

NASA Astrophysics Data System (ADS)

Gottlieb, E. S.; Miller, E. L.; Wooden, J. L.

2011-12-01

Results of SHRIMP-RG analyses of complexly zoned zircons from muscovite-bearing granitic rocks exposed in the Kerns Mountains of East-Central Nevada constrain the timing, duration, and loci of zircon growth within the interior of the U.S. Cordillera during Late Cretaceous through Eocene time. The Kern Mountains are an exhumed block of greenschist to amphibolite facies metamorphosed miogeoclinal rocks that were pervasively intruded by the Late Cretaceous Tungstonia granite pluton and the Eocene Skinner Canyon and Uvada plutons (Best et al., 1974). Euhedral zircons separated from a coarse-grained (2-3 cm) muscovite-bearing phase of the Tungstonia pluton exhibit complex cathodeluminescence (CL) zonation. Sub-angular to sub-rounded cores with highly variable CL are overgrown by oscillatory-zoned zircon which in turn is rimmed by dark CL zircon (U>5000 ppm). A weighted mean Pb/U age of 70.2±0.9 Ma (n=20, MSWD=2.5) obtained from the oscillatory-zoned zircon coincides with the end of Cretaceous peak metamorphism at shallow crustal levels. Pb/U ages from core zones (n=18) predominantly are 0.9-1.4 Ga (n=11; 7 of which <15% discordant) or 2.4-2.7 Ga (n=5; 1 of which <15% discordant), consistent with ages of detrital zircons within the Late Proterozoic McCoy Creek Group exposed in adjacent ranges. A previously undated muscovite-bearing dike in Skinner Canyon yielded a texturally complex population of subhedral zircon grains. CL imaging of these grains reveals fragmental, ghost-like cores surrounded by irregularly shaped overgrowth zones with diffuse boundaries which are rimmed by oscillatory-zoned zircon. Both oscillatory zoned and gradational rim areas (n=32) yielded Late Cretaceous to Eocene ages. Twelve spots define the age of intrusion at 41.7±0.3 Ma (MSWD=1.8), consistent with the local onset of Eocene magmatism. An older period of zircon growth from ~75-45 Ma, coincident with the proposed duration of the Laramide shallow slab, is defined by zircon with flat to shallow HREE patterns and systematically increasing Yb content through time (n=16). Zircon defining a slightly older growth period (85-90 Ma, n=4) is geochemically distinct (>>Yb content, steep HREE slope). Within the cores, Pb/U ages cluster around 1.55-1.68 Ga (n=9) and 2.28-2.48 Ga (n=8) with concordia upper intercepts at 1.68 and 2.45 Ga respectively. The younger age is consistent with widespread magmatism in the SW U.S. (Whitmeyer and Karlstrom, 2007) and the older with intrusion of the nearby Chimney Rock orthogneiss in the East Humboldt Range (W.R. Premo, unpub.). These data imply the Tungstonia was partially derived from a shallow (detrital) source, while the dike assimilated deeper (basement) sources. Trace element geochemistry and CL-inferred textures of the ~75-45 Ma span of zircon growth in the dike reveal a prolonged period of near-zircon solidus conditions in the lower crust, curtailed by assimilation into Eocene intrusions. Coupling age and geochemistry of the xenocrystic areas of complexly zoned zircons provides a powerful tool for understanding the timing and conditions of the crustal evolution processes.

Dating the Gaofan and Hutuo Groups - Targets to investigate the Paleoproterozoic Great Oxidation Event in North China

NASA Astrophysics Data System (ADS)

Peng, Peng; Feng, Lianjun; Sun, Fengbo; Yang, Shuyan; Su, Xiangdong; Zhang, Zhiyue; Wang, Chong

2017-05-01

There are several sedimentary units in North China that are proposed to be associated with the Paleoproterozoic Great Oxidation Event (GOE) and/or subsequent events; however, few of them have been precisely dated. In this study, deposition age of the greenschist facies Gaofan and Hutuo Groups is determined. Zircon grains liberated from a tuff layer (metamorphosed to sericite-quartz schist) in the upper part of the Mohe Formation (the second of the three formations of the Gaofan Group) yield a weighted average 207Pb/206Pb age of 2186 ± 8 Ma (n = 7, MSWD = 1.3), representing time of deposition. This age and the detrital zircon U-Pb ages of the basal feldspar quartzite (meta-siltstone), as well as the initial deposition age of the unconformably overlying Hutuo Group, confine the deposition age of the Gaofan Group to 2350-2150 Ma. This result negates the Gaofan Group as one subgroup of the 2560-2510 Ma Wutai greenstone belt. Zircons from the Banlaoyao mafic sill (meta-diabase) that intruded the Dongye Subgroup of the Hutuo Group yield an upper intercept U-Pb age of 2057 ± 25 Ma (n = 14, MSWD = 1.3), representing time of crystallization. Considering the age of the basalt in the first formation of the Doucun Subgroup and the tuff in the first formation of the Dongye Subgroup, the deposition age of the Doucun and Dongye Subgroups of the Hutuo Group is confined to 2150-2090 Ma and 2090-2060 Ma, respectively. These age brackets, as well as the available carbon and nitrogen isotope data indicate that the Zhangxianbu Formation of the Gaofan Group possibly recorded the GOE; whereas the Mohe-Yaokouqian Formations of the Gaofan Group and the Doucun-Dongye Subgroups of the Hutuo Group recorded the subsequent Lomagundi-Jatuli Event (LJE). However, the Lomagundi-Jatuli carbon excursions are hardly distinguishable from the Gaofan Group and the Doucun Subgroup (Hutuo Group) as both units consist of little inorganic carbon but terrestrial clastic turbidites.

Isotopic constraints on contamination processes in the Tonian Goiás Stratiform Complex

NASA Astrophysics Data System (ADS)

Giovanardi, Tommaso; Mazzucchelli, Maurizio; Lugli, Federico; Girardi, Vicente A. V.; Correia, Ciro T.; Tassinari, Colombo C. G.; Cipriani, Anna

2018-06-01

The Tonian Goiás Stratiform Complex (TGSC, Goiás, central Brazil), is one of the largest mafic-ultramafic layered complexes in the world, emplaced during the geotectonic events that led to the Gondwana accretion. In this study, we present trace elements and in-situ U/Pb-Lu-Hf analyses of zircons and 87Sr/86Sr ratios of plagioclases from anorthosites and gabbros of the TGSC. Although formed by three isolated bodies (Cana Brava, Niquelândia and Barro Alto), and characterized by a Lower and Upper Sequence (LS and US), our new U/Pb zircon data confirm recent geochemical, geochronological, and structural evidences that the TGSC has originated from a single intrusive body in the Neoproterozoic. New Hf and Sr isotope ratios construe a complex contamination history for the TGSC, with different geochemical signatures in the two sequences. The low Hf and high Sr isotope ratios of the Lower Sequence (εHf(t) from -4.2 down to -27.5; 87Sr/86Sr = 0.706605-0.729226), suggest the presence of a crustal component and are consistent with contamination from meta-pelitic and calc-silicate rocks found as xenoliths within the Sequence. The more radiogenic Hf isotope ratios and low Sr isotope composition of the Upper Sequence (εHf(t) from 11.3 down to -8.4; 87Sr/86Sr = 0.702368-0.702452), suggest a contamination from mantle-derived metabasalts in agreement with the occurrences of amphibolite xenoliths in the US stratigraphy. The differential contamination of the two sequences is explained by the intrusion of the TGSC in a stratified crust dominated by metasedimentary rocks in its deeper part and metavolcanics at shallower levels. Moreover, the differential thermal gradient in the two crystallizing sequences might have contributed to the preservation and recrystallization of inherited zircon grains in the US and total dissolution or magmatic overgrowth of the LS zircons via melt/rock reaction processes.

Whole-rock Pb and Sm-Nd isotopic constraints on the growth of southeastern Laurentia during Grenvillian orogenesis

USGS Publications Warehouse

Fisher, C.M.; Loewy, S.L.; Miller, C.F.; Berquist, P.; Van Schmus, W. R.; Hatcher, R.D.; Wooden, J.L.; Fullagar, P.D.

2010-01-01

The conventional view that the basement of the southern and central Appalachians represents juvenile Mesoproterozoic crust, the final stage of growth of Laurentia prior to Grenville collision, has recently been challenged. New whole-rock Pb and Sm-Nd isotopic data are presented from Meso protero zoic basement in the southern and central Appalachians and the Granite-Rhyolite province, as well as one new U-Pb zircon age from the Granite-Rhyolite province. These data, combined with existing data from Mesoproterozoic terranes throughout southeastern Laurentia, further substantiate recent suggestions that the southern and central Appalachian basement is exotic with respect to Laurentia. Sm-Nd isotopic compositions of most rocks from the southern and central Appalachian basement are consistent with progressive growth through reworking of the adjacent Granite-Rhyolite province. However, Pb isotopic data, including new analyses from important regions not sampled in previous studies, do not correspond with Pb isotopic compositions of any adjacent crust. The most distinct ages and isotopic compositions in the southern and central Appalachian basement come from the Roan Mountain area, eastern Tennessee-western North Carolina. The data set indicates U-Pb zircon ages up to 1.8 Ga for igneous rocks, inherited and detrital zircon ages >2.0 Ga, Sm-Nd depleted mantle model (TDM) ages >2.0 Ga, and the most elevated 207Pb/204Pb observed in southeastern Laurentia. The combined U-Pb geochronologic and Sm-Nd and Pb isotopic data preclude derivation of southern and central Appalachian basement from any nearby crustal material and demonstrate that Grenville age crust in southeastern Laurentia is exotic and probably was transferred during collision and assembly of Rodinia. These new data better define the boundary between the exotic southern and central Appalachian basement and adjacent Laurentian Granite-Rhyolite province. ?? 2010 Geological Society of America.

Late Cretaceous porphyry copper mineralization in Sonora, Mexico: Implications for the evolution of the Southwest North America porphyry copper province

NASA Astrophysics Data System (ADS)

Barra, Fernando; Valencia, Victor A.

2014-10-01

Two porphyry Cu-Mo prospects in northern Sonora, Mexico (Fortuna del Cobre and Los Humos) located within the southwestern North American porphyry province have been dated in order to constrain the timing of crystallization and mineralization of these ore deposits. In Fortuna del Cobre, the pre-mineralization granodiorite porphyry yielded an U-Pb zircon age of 76.5 ± 2.3 Ma, whereas two samples from the ore-bearing quartz feldespathic porphyry were dated at 74.6 ± 1.3 and 75.0 ± 1.4 Ma. Four molybdenite samples from Los Humos porphyry Cu prospect yielded a weighted average Re-Os age of 73.5 ± 0.2 Ma, whereas two samples from the ore-bearing quartz monzonite porphyry gave U-Pb zircon ages of 74.4 ± 1.1 and 74.5 ± 1.3 Ma, showing a Late Cretaceous age for the emplacement of this ore deposit. The results indicate that Laramide porphyry Cu mineralization of Late Cretaceous age is not restricted to northern Arizona as previously thought and provide evidence for the definition of NS trending metallogenic belts that are parallel to the paleo-trench. Porphyry copper mineralization follows the inland migration trend of the magmatic arc as a result of the Farallon slab flattening during the Laramide orogeny.

S-type granite generation and emplacement during a regional switch from extensional to contractional deformation (Central Iberian Zone, Iberian autochthonous domain, Variscan Orogeny)

NASA Astrophysics Data System (ADS)

Pereira, M. F.; Díez Fernández, R.; Gama, C.; Hofmann, M.; Gärtner, A.; Linnemann, U.

2018-01-01

Zircon grains extracted from S-type granites of the Mêda-Escalhão-Penedono Massif (Central Iberian Zone, Variscan Orogen) constrain the timing of emplacement and provide information about potential magma sources. Simple and composite zircon grains from three samples of S-type granite were analyzed by LA-ICP-MS. New U-Pb data indicate that granites crystallized in the Bashkirian (318.7 ± 4.8 Ma) overlapping the proposed age range of ca. 321-317 Ma of the nearby S-type granitic rocks of the Carrazeda de Anciães, Lamego and Ucanha-Vilar massifs. The timing of emplacement of such S-type granites seems to coincide with the waning stages of activity of a D2 extensional shear zone (i.e. Pinhel shear zone) developed in metamorphic conditions that reached partial melting and anatexis (ca. 321-317 Ma). Dykes of two-mica granites (resembling diatexite migmatite) are concordant and discordant to the compositional layering and S2 (main) foliation of the high-grade metamorphic rocks of the Pinhel shear zone. Much of the planar fabric in these dykes was formed during magmatic crystallization and subsequent solid-state deformation. Field relationships suggest contemporaneity between the ca. 319-317 Ma old magmatism of the study area and the switch from late D2 extensional deformation to early D3 contractional deformation. Inherited zircon cores are well preserved in these late D2-early D3 S-type granite plutons. U-Pb ages of inherited zircon cores range from ca. 2576 to ca. 421 Ma. The spectra of inherited cores overlap closely the range of detrital and magmatic zircon grains displayed by the Ediacaran to Silurian metasedimentary and metaigneous rocks of the Iberian autochthonous and parautochthonous domains. This is evidence of a genetic relationship between S-type granites and the host metamorphic rocks. There is no substantial evidence for the addition of mantle-derived material in the genesis of these late D2-early D3 S-type granitic rocks. The ɛNd arrays of heterogeneous crustal anatectic melts may be just inherited from the source, probably reflecting mixing of a range of crustal materials with different ages and primary isotopic signatures. The generation of the Bashkirian S-type granites has been dominated by continental crust recycling, rather than the addition of new material from mantle sources.

U-Pb Geochronology of non-marine Upper Triassic strata of the Colorado Plateau (western North America): implications for stratigraphic correlation and paleoenvironmental reconstruction

NASA Astrophysics Data System (ADS)

Rasmussen, C.; Mundil, R.; Irmis, R. B.; Keller, C. B.; Giesler, D.; Gehrels, G. E.

2017-12-01

The Triassic is a critical period in Earth history that witnessed the origin of modern ecosystems and frequent climate fluctuations, as well as major environmental events such as flood basalt volcanism and bolide impacts. The Chinle Formation contains a primary non-marine archive for past ecosystems in North America due to its fossil richness and well-studied sedimentology. Moreover, within these highly fossiliferous strata, a biotic turnover has been reported that has been hypothesized to coincide with one or more of the aforementioned environmental events. Unfortunately, few radioisotopic ages have been published for the Late Triassic, limiting our ability for lithological and paleoenvironmental correlations. In addition, the superposition of the Chinle Formation remains illusive due to frequent lateral facies changes and discontinuous outcrops across the Colorado Plateau. The 520 m long core 1A of the Colorado Plateau Coring Project from Petrified Forest National Park (PFNP) (Arizona) provides, for the first time, a continuous section of these early Mesozoic sedimentary strata. Many of the sand- and siltstones from this continuous succession throughout most of the Upper Triassic Chinle Formation contain euhedral zircons suitable for U-Pb analyses. We analyzed >300 crystals each from 10 samples using LA-ICPMS; these results indicated abundant Late Triassic crystals that appear to be closely associated with the depositional age of the host rock. We then selected the youngest grains from these samples to obtain precise CA-TIMS U-Pb single zircon ages in order to constrain the maximum depositional ages (using quantitative methods) of these formations. We are able to revise the proposed time scale (based on outcrop samples) for Upper Triassic strata at PFNP and evaluate whether the biotic turnover observed within the Sonsela Member of these strata coincides with the Manicouagan bolide impact event. This revised chronostratigraphic framework allows intercalibration with the Newark astrochronostratigraphic polarity time scale and the correlation with other key non-marine and marine Late Triassic sections globally (e.g., Ischigualasto-Villa Uníon Basin, Argentina; Tethyan region, Europe) improving our understanding of paleoenvironmental and evolutionary changes during the Triassic.

Time-scale calibration by U-Pb geochronology: Examples from the Triassic Period

NASA Astrophysics Data System (ADS)

Mundil, R.

2009-05-01

U-Pb zircon geochronology, pioneered by Tom Krogh, is a cornerstone for the calibration of the time scale. Before Krogh's innovations, U-Pb geochronology was essentially limited by laboratory blank Pb (typically hundreds of nanograms) inherent in the then existing zircon dissolution and purification methods. The introduction of high pressure HF dissolution combined with miniature ion exchange columns (1) reduced the blank by orders of magnitude and allowed mass-spectrometric analyses of minute amounts of material (picograms of Pb and U). Krogh also recognized the need for minimizing the effects of Pb loss, and the introduction of the air-abrasion technique was the method of choice for two decades (2), until the development of the combined annealing and chemical abrasion technique resulted in essentially closed system zircons (3). These are the prerequisite for obtaining precise (permil-level) and accurate radio-isotopic ages of individual zircons contained in primary volcanic ash deposits, which are primary targets for the calibration of the time scale if they occur within fossil bearing sediments. A prime example is the calibration of the Triassic time scale which improved significantly using these techniques. The ages for the base and the top of the Triassic are constrained by U-Pb ages to 252.3 (4) and 201.5 Ma (5), respectively. These dates also constrain the ages of major extinction events at the Permian-Triassic and Triassic-Jurassic boundaries, and are statistically indistinguishable from ages obtained for the Siberian Traps and volcanic products from the Central Atlantic Magmatic Province, respectively, suggesting a causal link. Ages for these continental volcanics, however, are mostly from the K-Ar (40Ar/39Ar) system which requires accounting and correcting for a systematic bias of ca 1 % between U-Pb and 40Ar/39Ar isotopic ages (the 40Ar/39Ar ages being younger) (6). Robust U-Pb age constraints also exist for the Induan- Olenekian boundary (251.2 Ma, (7)) and the Early-Middle Triassic (Olenekian-Anisian) boundary (247.2 Ma, (8, 9)), resulting in a surprisingly short duration of the Early Triassic which has implications for the timing of biotic recovery and major changes in ocean chemistry during this time. Furthermore, the Anisian-Ladinian boundary is constrained to 242.0 Ma by new U-Pb and 40Ar/39Ar ages. Radio-isotopic ages for the Late Triassic are scarce and the only reliable and biostratigraphically controlled age is from an upper Carnian tuff dated to 230.9 Ma (10), yielding a duration of more than 35 Ma for the Late Triassic. The resulting time-scale is at odds with the most recent compilation (11) but arguably more accurate because it is entirely based on U-Pb analyses applied to closed-system zircons with uncertainties at the permil level or better. 1. T. E. Krogh, Geochimica et Cosmochimica Acta 37, 485 (1973); 2. T. E. Krogh, Geochimica et Cosmochimica Acta 46, 637 (1982); 3. J. M. Mattinson, Chemical Geology 220, 47 (2005); 4. R. Mundil, K. R. Ludwig, I. Metcalfe, P. R. Renne, Science 305, 1760 (2004); 5. U. Schaltegger, J. Guex, A. Bartolini, B. Schoene, M. Ovtcharova, Earth and Planetary Science Letters 267, 266 (2008); 6. R. Mundil, P. R. Renne, K. K. Min, K. R. Ludwig, in Eos Trans. AGU, Fall Meet. Suppl. (2006), vol. 87(52), pp. V21A-0543; 7. T. Galfetti et al., Earth and Planetary Science Letters 258, 593 (2007). 8. M. Ovtcharova et al., Earth and Planetary Science Letters 243, 463 (2006). 9. J. Ramezani et al., Earth and Planetary Science Letters 256, 244 (2007). 10. S. Furin et al., Geology 34, 1009 (2006); 11. J. G. Ogg, in A Geologic Time Scale 2004 F. M. Gradstein, J. G. Ogg, A. G. Smith, Eds. (University Press, Cambridge, 2004) pp. 271-306.

Geologic history of Martian regolith breccia Northwest Africa 7034: Evidence for hydrothermal activity and lithologic diversity in the Martian crust

USGS Publications Warehouse

McCubbin, Francis M.; Boyce, Jeremy W.; Novak-Szabo, Timea; Santos, Alison; Tartese, Romain; Muttik, Nele; Domokos, Gabor; Vazquez, Jorge A.; Keller, Lindsay P.; Moser, Desmond E.; Jerolmack, Douglas J.; Shearer, Charles K.; Steele, Andrew; Elardo, Stephen M.; Rahman, Zia; Anand, Mahesh; Delhaye, Thomas; Agee, Carl B.

2016-01-01

The timing and mode of deposition for Martian regolith breccia Northwest Africa (NWA) 7034 were determined by combining petrography, shape analysis, and thermochronology. NWA 7034 is composed of igneous, impact, and brecciated clasts within a thermally annealed submicron matrix of pulverized crustal rocks and devitrified impact/volcanic glass. The brecciated clasts are likely lithified portions of Martian regolith with some evidence of past hydrothermal activity. Represented lithologies are primarily ancient crustal materials with crystallization ages as old as 4.4 Ga. One ancient zircon was hosted by an alkali-rich basalt clast, confirming that alkalic volcanism occurred on Mars very early. NWA 7034 is composed of fragmented particles that do not exhibit evidence of having undergone bed load transport by wind or water. The clast size distribution is similar to terrestrial pyroclastic deposits. We infer that the clasts were deposited by atmospheric rainout subsequent to a pyroclastic eruption(s) and/or impact event(s), although the ancient ages of igneous components favor mobilization by impact(s). Despite ancient components, the breccia has undergone a single pervasive thermal event at 500–800°C, evident by groundmass texture and concordance of ~1.5 Ga dates for bulk rock K-Ar, U-Pb in apatite, and U-Pb in metamict zircons. The 1.5 Ga age is likely a thermal event that coincides with rainout/breccia lithification. We infer that the episodic process of regolith lithification dominated sedimentary processes during the Amazonian Epoch. The absence of pre-Amazonian high-temperature metamorphic events recorded in ancient zircons indicates source domains of static southern highland crust punctuated by episodic impact modification.

Rapid exhumation of Cretaceous arc-rocks along the Blue Mountains restraining bend of the Enriquillo-Plantain Garden fault, Jamaica, using thermochronometry from multiple closure systems

NASA Astrophysics Data System (ADS)

Cochran, William J.; Spotila, James A.; Prince, Philip S.; McAleer, Ryan J.

2017-11-01

The effect of rapid erosion on kinematic partitioning along transpressional plate margins is not well understood, particularly in highly erosive climates. The Blue Mountains restraining bend (BMRB) of eastern Jamaica, bound to the south by the left-lateral Enriquillo-Plantain Garden fault (EPGF), offers an opportunity to test the effects of highly erosive climatic conditions on a 30-km-wide restraining bend system. No previous thermochronometric data exists in Jamaica to describe the spatial or temporal pattern of rock uplift and how oblique (> 20°) plate motion is partitioned into vertical strain. To define the exhumation history, we measured apatite (n = 10) and zircon (n = 6) (U-Th)/He ages, 40Ar/39Ar (n = 2; amphibole and K-spar) ages, and U/Pb zircon (n = 2) crystallization ages. Late Cretaceous U/Pb and 40Ar/39Ar ages (74-68 Ma) indicate rapid cooling following shallow emplacement of plutons during north-south subduction along the Great Caribbean Arc. Early to middle Miocene zircon helium ages (19-14 Ma) along a vertical transect suggest exhumation and island emergence at 0.2 mm/yr. Older zircon ages 10-15 km to the north (44-35 Ma) imply less rock uplift. Apatite helium ages are young (6-1 Ma) across the entire orogen, suggesting rapid exhumation of the BMRB since the late Miocene. These constraints are consistent with previous reports of restraining bend formation and early emergence of eastern Jamaica. An age-elevation relationship from a vertical transect implies an exhumation rate of 0.8 mm/yr, while calculated closure depths and thermal modeling suggests exhumation as rapid as 2 mm/yr. The rapid rock uplift rates in Jamaica are comparable to the most intense transpressive zones worldwide, despite the relatively slow (5-7 mm/yr) strike-slip rate. We hypothesize highly erosive conditions in Jamaica enable a higher fraction of plate motion to be accommodated by vertical deformation. Thus, strike-slip restraining bends may evolve differently depending on erosivity and local climate.

Rapid exhumation of Cretaceous arc-rocks along the Blue Mountains restraining bend of the Enriquillo-Plantain Garden fault, Jamaica, using thermochronometry from multiple closure systems

USGS Publications Warehouse

Cochran, William J.; Spotila, James A.; Prince, Philip S.; McAleer, Ryan J.

2017-01-01

The effect of rapid erosion on kinematic partitioning along transpressional plate margins is not well understood, particularly in highly erosive climates. The Blue Mountains restraining bend (BMRB) of eastern Jamaica, bound to the south by the left-lateral Enriquillo-Plantain Garden fault (EPGF), offers an opportunity to test the effects of highly erosive climatic conditions on a 30-km-wide restraining bend system. No previous thermochronometric data exists in Jamaica to describe the spatial or temporal pattern of rock uplift and how oblique (> 20°) plate motion is partitioned into vertical strain. To define the exhumation history, we measured apatite (n = 10) and zircon (n = 6) (U-Th)/He ages, 40Ar/39Ar (n = 2; amphibole and K-spar) ages, and U/Pb zircon (n = 2) crystallization ages. Late Cretaceous U/Pb and 40Ar/39Ar ages (74–68 Ma) indicate rapid cooling following shallow emplacement of plutons during north-south subduction along the Great Caribbean Arc. Early to middle Miocene zircon helium ages (19–14 Ma) along a vertical transect suggest exhumation and island emergence at ~ 0.2 mm/yr. Older zircon ages 10–15 km to the north (44–35 Ma) imply less rock uplift. Apatite helium ages are young (6–1 Ma) across the entire orogen, suggesting rapid exhumation of the BMRB since the late Miocene. These constraints are consistent with previous reports of restraining bend formation and early emergence of eastern Jamaica. An age-elevation relationship from a vertical transect implies an exhumation rate of 0.8 mm/yr, while calculated closure depths and thermal modeling suggests exhumation as rapid as 2 mm/yr. The rapid rock uplift rates in Jamaica are comparable to the most intense transpressive zones worldwide, despite the relatively slow (5–7 mm/yr) strike-slip rate. We hypothesize highly erosive conditions in Jamaica enable a higher fraction of plate motion to be accommodated by vertical deformation. Thus, strike-slip restraining bends may evolve differently depending on erosivity and local climate.

The Amazon-Laurentian connection as viewed from the Middle Proterozoic rocks in the central Andes, western Bolivia and northern Chile

USGS Publications Warehouse

Tosdal, R.M.

1996-01-01

Middle Proterozoic rocks underlying the Andes in western Bolivia, western Argentina, and northern Chile and Early Proterozoic rocks of the Arequipa massif in southern Peru?? from the Arequipa-Antofalla craton. These rocks are discontinuously exposed beneath Mesozoic and Cenozoic rocks, but abundant crystalline clasts in Tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton. Near Berenguela, western Bolivia, the Oligocene and Miocene Mauri Formation contains boulders of granodiorite augen gneiss (1171??20 Ma and 1158??12 Ma; U-Pb zircon), quartzose gneiss and granofels that are inferred to have arkosic protoliths (1100 Ma source region; U-Pb zircon), quartzofeldspathic and mafic orthogneisses that have amphibolite- and granulite-facies metamorphic mineral assemblages (???1080 Ma metamorphism; U-Pb zircon), and undeformed granitic rocks of Phanerozoic(?) age. The Middle Proterozoic crystalline rocks from Berenguela and elsewhere in western Bolivia and from the Middle Proterozoic Bele??n Schist in northern Chile generally have present-day low 206Pb/204Pb ( 15.57), and elevated 208Pb/204Pb (37.2 to 50.7) indicative of high time-averaged Th/U values. The Middle Proterozoic rocks in general have higher presentday 206Pb/204Pb values than those of the Early Proterozoic rocks of the Arequipa massif (206Pb/204Pb between 16.1 and 17.1) but lower than rocks of the southern Arequipa-Antofalla craton (206Pb/204Pb> 18.5), a difference inferred to reflect Grenvillian granulite metamorphism. The Pb isotopic compositions for the various Proterozoic rocks lie on common Pb isotopic growth curves, implying that Pb incorporated in rocks composing the Arequipa-Antofalla craton was extracted from a similar evolving Pb isotopic reservoir. Evidently, the craton has been a coherent terrane since the Middle Proterozoic. Moreover, the Pb isotopic compositions for the Arequipa-Antofalla craton overlap those of the Amazon craton, thereby supporting a link between these cratons and seemingly precluding part of the Arequipa-Antofalla craton from being a detached fragment of another craton such as eastern Laurentia, which has been characterized by a different U/Pb history. Pb isotopic compositions for the Arequipa-Antofalla craton are, furthermore, distinct from those of the Proterozoic basement in the Precordillera terrane, western Argentina, indicating a Pb isotopic and presumably a tectonic boundary between them. The Pb isotopic compositions for the Precordillera basement are similar to those of eastern Laurentia, and support other data indicating that these rocks are a detached fragment of North America. Finally, the distinct Pb isotopic evolution history of the Arequipa-Antofalla craton and eastern Laurentia require minor modification to tectonic models linking eastern North America-Scotland to the oroclinal bend in western South America.

Component geochronology in the polyphase ca. 3920 Ma Acasta Gneiss

USGS Publications Warehouse

Mojzsis, Stephen J.; Cates, Nicole L.; Caro, Guillaume; Trail, Dustin; Abramov, Oleg; Guitreau, Martin; Blichert-Toft, Janne; Hopkins, Michelle D.; Bleeker, Wouter

2014-01-01

The oldest compiled U–Pb zircon ages for the Acasta Gneiss Complex in the Northwest Territories of Canada span about 4050–3850 Ma; yet older ca. 4200 Ma xenocrystic U–Pb zircon ages have also been reported for this terrane. The AGC expresses at least 25 km2 of outcrop exposure, but only a small subset of this has been documented in the detail required to investigate a complex history and resolve disputes over emplacement ages. To better understand this history, we combined new ion microprobe235,238U–207,206Pb zircon geochronology with whole-rock and zircon rare earth element compositions ([REE]zirc), Ti-in-zircon thermometry (Tixln) and 147Sm–143Nd geochronology for an individual subdivided ∼60 cm2 slab of Acasta banded gneiss comprising five separate lithologic components. Results were compared to other variably deformed granitoid-gneisses and plagioclase-hornblende rocks from elsewhere in the AGC. We show that different gneissic components carry distinct [Th/U]zirc vs. Tixln and [REE]zirc signatures correlative with different zircon U–Pb age populations and WR compositions, but not with 147Sm–143Nd isotope systematics. Modeled  [REE] from lattice-strain theory reconciles only the ca. 3920 Ma zircons with the oldest component that also preserves strong positive Eu∗ anomalies. Magmas which gave rise to the somewhat older (inherited) ca. 4020 Ma AGC zircon age population formed at ∼IW (iron–wüstite) to

Geochronologic Constraints on the Location of the Sino-Korean/Yangtze Suture and Evolution of the Northern Dabie Shan

NASA Astrophysics Data System (ADS)

Bryant, D. L.; Ayers, J. C.; Gao, S.; Miller, C. F.; Zhang, H.

2002-05-01

The Northern Dabie Complex (NDC) has been proposed to be either a Paleozoic magmatic arc, an exhumed piece of subducted continental crust, or young crust produced almost entirely by Cretaceous extensional magmatism. Ion microprobe zircon 238U-206Pb ages of separates from NDC gneisses center around 689Ma (+/- 31(95%CL)), consistent with the characteristic zircon dates of the Yangtze Craton [1]. Field observations also show that these gneisses, ranging from granitic to dioritic composition, make up a sizeable area ( ~30%) of the NDC. Zircon separates from the Baimajian granitoid, the largest of the widespread Cretaceous intrusions in the NDC, have yielded ages clustered around 677Ma (+/- 79), and 120Ma (+/- 3.4), the latter of which agrees with ion probe Th-Pb monazite ages. The ~700Ma age indicates that this intrusion may be linked with partial melting of underlying Yangtze crust, while the 120Ma age is the age of its crystallization. Granitic intrusions from Sanzushi and Yerenshai in the Dabie ultrahigh-pressure (UHP) region also show clusters of ages at 714Ma (+/- 55) from zircon cores, as well as rims around 250Ma (+/- 38), which is interpreted as the time of collision of the two continental blocks. These age data support the hypothesis set forth by Zhang et al. [2] using Sm-Nd and Pb isotopic data, that the Yangtze block lies beneath the exhumed UHP belt and outcrops as the NDC, which lies between the UHP belt and the Sino-Korean/Yangtze suture. The Baimajian granitoid, however, also shows a range of older zircon core ages from 1.4-2.0Ga, which may represent the early stages of formation of the Yangtze craton. Zhang et al. [2] suggested craton formation at 1.6-2.4Ga but few such ages have been reported for rocks of the Yangtze or Sino-Korean cratons. 1. Hacker, et al. (2000) Journal of Geophysical Research. Vol. 105. p. 13,339. 2. Zhang, et al. (In press) Chemical Geology.

Geochemical constraints on genesis of Paleoproterozoic A-type granite in the south margin of North China Craton

NASA Astrophysics Data System (ADS)

Xue, Shuo; Xu, Yang; Ling, Ming-Xing; Kang, Qing-Qing; Jiang, Xiao-Yan; Sun, Sai-Jun; Wu, Kai; Zhang, Zhe-Kun; Luo, Ze-Bin; Liu, Yu-Long; Sun, Weidong

2018-04-01

Paleoproterozoic A-type granites are widely outcropped in the North China Craton (NCC), particularly in the Trans-North China Orogen. However, their genesis and tectonic significance remain obscure. Here we report systematic studies on geochronology and geochemical characteristics of A-type granite in Huayangchuan, south margin of the NCC. The samples are enriched in total alkali (K2O + Na2O > 8.97 wt%), and depleted in MgO (0.84-0.93 wt%), CaO (1.28-1.90 wt%) and P2O5 (0.18-0.20 wt%), with high FeOT/MgO (5.69-6.67). They are characterized by high Zr + Y + Nb + Ce values (1293-1392 ppm) and 10,000 × Ga/Al ratios (3.14-3.35), which are typical characteristics of A-type granite. The Huayangchuan A-type granite can be further classified as A1-type subgroup based on particular geochemical features, e.g., low Y/Nb (0.87-1.00) and Yb/Ta (0.88-1.10). High precision zircon U-Pb dating of the A-type granite by secondary ion mass spectrometry (SIMS) yields Paleoproterozoic 207Pb/206Pb ages of 1829.5 ± 2.5 Ma. The low zircon ɛHf(t) values (-6.97 to -10.45), along with zircon Hf model age of 2.7-2.9 Ga, indicate that the Huayangchuan A-type granite was derived from partial melting of the ancient continental crust with contribution of enriched mantle components. The low zircon δ18O composition (4.00 to 6.78‰) indicates that the zircons were crystallized from low δ18O magmas, which derived from the crust metasomatized by low δ18O mantle fluids or melts. The E-W trend A1-type granitic plutons in the NCC are generally outcropped in a rift tectonic regime, which is consistent with the development of the mantle plume in the Xiong'er district. The large volume of basaltic magmas, generated by mantle plume head, underplated the lower continental crust and formed the Huayangchuan A-type granite.

Primary Data on U/Pb-Isotope Ages and Lu/Hf-Isotope Geochemical Systematization of Detrital Zircons from the Lopatinskii Formation (Vendian-Cambrian Transition Levels) and the Tectonic Nature of Teya-Chapa Depression (Northeastern Yenisei Ridge)

NASA Astrophysics Data System (ADS)

Kuznetsov, N. B.; Priyatkina, N. S.; Rud'ko, S. V.; Shatsillo, A. V.; Collins, W. J.; Romanyuk, T. V.

2018-03-01

The main results are presented on U/Pb-isotope dating of 100 detrital zircons and, selectively, on the Lu/Hf-isotope system of 43 grains from sandstones of the Lopatinskii formation (the lower stratigraphic level of the Chingasan group). Ages from 896 ± 51 to 2925 ± 38 Ma were obtained with a pronounced maximum of 1890 Ma in the curve of probability density, along with ɛHf estimates from +8.4 to-15.1, which allow one to throw doubt upon the molasse nature of the Lopatinskii formation.

Mesoproterozoic evolution of the Río de la Plata Craton in Uruguay: at the heart of Rodinia?

NASA Astrophysics Data System (ADS)

Gaucher, Claudio; Frei, Robert; Chemale, Farid; Frei, Dirk; Bossi, Jorge; Martínez, Gabriela; Chiglino, Leticia; Cernuschi, Federico

2011-04-01

Mesoproterozoic volcanosedimentary units and tectonic events occurring in the Río de la Plata Craton (RPC) are reviewed. A belt consisting of volcanosedimentary successions exhibiting greenschist-facies metamorphism is exposed in the eastern RPC (Nico Pérez Terrane) in Uruguay. The Parque UTE Group consists of basic volcanics and gabbros at the base (1,492 ± 4 Ma, U-Pb on zircon), carbonates in its middle part and interbedded carbonates, shales and acid volcanics (1,429 ± 21 Ma, U-Pb on zircon) at the top. The Mina Verdún Group is made up of rhyolites and acid pyroclastics at its base and top, and Conophyton-bearing limestones and massive dolostones in the middle. A U-Pb LA-ICP MS zircon age of 1,433 ± 6 Ma is reported here for lapilli-tuffs at the base of the Mina Verdún Group (Cerro de las Víboras Formation). This age shows that the Mina Verdún Group immediately postdates the Parque UTE Group, a fact supported by carbon isotope chemostratigraphy. Both units were deformed and metamorphosed between 1.25 and 1.20 Ga, as shown by K-Ar and Ar-Ar ages. This tectonic event affected most of the RPC and led to the accretion of the Nico Pérez Terrane to the remainder of the RPC along the Sarandí del Yí megashear. We report a U-Pb LA-ICP MS zircon age (upper intercept) of 3,096 ± 45 Ma for metatonalites of the La China Complex (Nico Pérez Terrane), which yield a lower intercept age of 1,252 Ma. A proto-Andean, Mesoproterozoic belt is envisaged to account for abundant Mesoproterozoic detrital zircon ages occurring in Ediacaran sandstones of the RPC. If the RPC is fringed at both sides by Mesoproterozoic, Grenville-aged belts it is likely that it occupied a rather central position in Rodinia. A possible location between Laurentia and the Kalahari Craton, and to the south of Amazonia, is suggested.

Link between SSZ ophiolite formation, emplacement and arc inception, Northland, New Zealand: U Pb SHRIMP constraints; Cenozoic SW Pacific tectonic implications

NASA Astrophysics Data System (ADS)

Whattam, Scott A.; Malpas, John; Smith, Ian E. M.; Ali, Jason R.

2006-10-01

New U-Pb age-data from zircons separated from a Northland ophiolite gabbro yield a mean 206Pb/ 238U age of 31.6 ± 0.2 Ma, providing support for a recently determined 28.3 ± 0.2 Ma SHRIMP age of an associated plagiogranite and ˜ 29-26 Ma 40Ar/ 39Ar ages ( n = 9) of basalts of the ophiolite. Elsewhere, Miocene arc-related calc-alkaline andesite dikes which intrude the ophiolitic rocks contain zircons which yield mean 206Pb/ 238U ages of 20.1 ± 0.2 and 19.8 ± 0.2 Ma. The ophiolite gabbro and the andesites both contain rare inherited zircons ranging from 122-104 Ma. The Early Cretaceous zircons in the arc andesites are interpreted as xenocrysts from the Mt. Camel basement terrane through which magmas of the Northland Miocene arc lavas erupted. The inherited zircons in the ophiolite gabbros suggest that a small fraction of this basement was introduced into the suboceanic mantle by subduction and mixed with mantle melts during ophiolite formation. We postulate that the tholeiitic suite of the ophiolite represents the crustal segment of SSZ lithosphere (SSZL) generated in the southern South Fiji Basin (SFB) at a northeast-dipping subduction zone that was initiated at about 35 Ma. The subduction zone nucleated along a pre-existing transform boundary separating circa 45-20 Ma oceanic lithosphere to the north and west of the Northland Peninsula from nascent back arc basin lithosphere of the SFB. Construction of the SSZL propagated southward along the transform boundary as the SFB continued to unzip to the southeast. After subduction of a large portion of oceanic lithosphere by about 26 Ma and collision of the SSZL with New Zealand, compression between the Australian Plate and the Pacific Plate was taken up along a new southwest-dipping subduction zone behind the SSZL. Renewed volcanism began in the oceanic forearc at 25 Ma producing boninitic-like, SSZ and within-plate alkalic and calc-alkaline rocks. Rocks of these types temporally overlap ophiolite emplacement and subsequent Miocene continental arc construction.

Zircon U-Pb ages, Hf isotope data, and tectonic implications of Early-Middle Triassic granitoids in the Ailaoshan high-grade metamorphic belt of Southeast Tibet

NASA Astrophysics Data System (ADS)

Wu, Wenbin; Liu, Junlai; Chen, Xiaoyu; Zhang, Lisheng

2017-04-01

The Ailaoshan tectonic belt, where the effects of the Paleo-Tethyan ocean evolution and Indian-Eurasian plate collision are superimposed, is one of the most significant geological discontinuities in western Yunnan province of southeast Tibet. An Ailaoshan micro-block within the belt is bounded by the Ailaoshan suture zone to the west and the Red River Fault to the east, and consists of low- and high-grade metamorphic belts. Late Permian-Middle Triassic granitoids that are widely distributed to the west of the Ailaoshan suture zone and within the Ailaoshan micro-block may yield significant information on the Tethyan tectonic evolution of the Ailaoshan tectonic belt. This study reports new LA-ICP-MS zircon U-Pb geochronology and Hf isotope data of four granitoids from the Ailaoshan high-grade metamorphic belt. Zircon grains from the Yinjie granitoid do not have inherited cores and yield a weighted mean U-Pb age of 247.1 ± 2.0 Ma. The zircon ɛ Hf( t) values range from 7.8 to 12.1, and Hf model ages from 775 to 546 Ma, indicating that the granitoid was derived from juvenile crust. The rims of zircons from the Majie and Yuanjiang granitoids yield weighted mean U-Pb ages of 239.5 ± 1.8 and 237.9 ± 2.6 Ma, respectively, whereas the cores yield ages of 1608-352 Ma. The ɛ Hf( t) values of zircon rims range from -20.4 to -5.3, yielding Hf model ages from 2557 to 1606 Ma and suggesting that the source magma of the Majie and Yuanjiang granitoids was derived from ancient crust. An additional granitoid located near the Majie Village yields a zircon U-Pb age of 241.2 ± 1.0 Ma. Based on our geochronological and geochemical data, combined with geological observations, we propose that the Ailaoshan micro-block was derived from the western margin of the Yangtze block, and is comparable to the Zhongzan and Nam Co micro-blocks. The presence of late Permian mafic rocks with rift-related geochemical characteristics within the Ailaoshan micro-block, together with granitoids derived from partial melting of ancient/juvenile crust, indicates the presence of an Ailaoshan rift. This possible rift may correspond to the Ganzi-Litang Ocean to the northwest and the Jinping-Song Da rift to the southeast. It is suggested that westward subduction of the Jinshajiang-Ailaoshan-Song Ma oceanic lithosphere triggered the separation of the Zhongzan, Ailaoshan, and Nam Co micro-blocks from the western passive continental margin of the Yangtze block through the opening of the Ganzi-Litang-Ailaoshan-Jinping-Song Da ocean/rift. This ocean/rift may represent a subsidiary branch of the Paleo-Tethyan Ocean along the western margin of the Yangtze block.

Archean Arctic continental crust fingerprints revealing by zircons from Alpha Ridge bottom rocks

NASA Astrophysics Data System (ADS)

Sergeev, Sergey; Petrov, Oleg; Morozov, Andrey; Shevchenko, Sergey; Presnyakov, Sergey; Antonov, Anton; Belyatsky, Boris

2015-04-01

Whereas thick Cenozoic sedimentary cover overlapping bedrock of the Arctic Ocean, some tectonic windows were sampled by scientific submarine manipulator, as well as by grabbing, dredging and drilling during «Arctic-2012» Russian High-Arctic expedition (21 thousands samples in total, from 400-km profile along Alpha-Mendeleev Ridges). Among others, on the western slope of Alpha Ridge one 10x10 cm fragment without any tracks of glacial transportation of fine-layered migmatitic-gneiss with prominent quartz veinlets was studied. Its mineral (47.5 vol.% plagioclase + 29.6% quartz + 16.6% biotite + 6.1% orthoclase) and chemical composition (SiO2:68.2, Al2O3:14.9, Fe2O3:4.44, TiO2:0.54, MgO:2.03, CaO:3.13, Na2O:3.23, K2O:2.16%) corresponds to trachydacite vulcanite, deformed and metamorphozed under amphibolite facies. Most zircon grains (>80%) from this sample has an concordant U-Pb age 3450 Ma with Th/U 0.8-1.4 and U content of 100-400 ppm, epsilon Hf from -4 up to 0, and ca 20% - ca 3.3 Ga with Th/U 0.7-1.4 and 90-190 ppm U, epsilon Hf -6.5 to -4.5, while only 2% of the grains show Proterozoic age of ca 1.9 Ga (Th/U: 0.02-0.07, U~500 ppm, epsilon Hf about 0). No younger zircons were revealed at all. We suppose that magmatic zircon crystallized as early as 3450 Ma ago during acid volcanism, the second phase zircon crystallization from partial melt (or by volcanics remelting) under amphibolite facies metamorphism was at 3.3 Ga ago with formation of migmatitie gneisses. Last zircon formation from crustal fluids under low-grade metamorphic conditions was 1.9 Ga ago. There are two principal possibilities for the provenance of this metavolcanic rock. The first one - this is ice-rafted debris deposited by melted glacial iceberg. However, presently there are no temporal and compositional analogues of such rocks in basement geology of peri-oceanic regions, including Archean Itsaq Gneiss Complex, Lewisian Complex and Baltic Shield but these regions are far from the places of nowadays iceberg formation. Moreover, no Grenvillian-age zircons were revealed in studied sample. The nearest areas of Paleo- and Mesoarchean rocks appearance are deeply inland territories like Acasta Gneiss Complex, Uivak Gneisses, the Superior Province, Anabar and Aldan Shields, Northern China craton (Anshan complex), thus, a only possibility to bring some fragments of basement rocks from these areas to oceanic coast is transcontinental river transportation. The second possibility - this gneissic clast has a local provenance and has undergone a submarine weathering, shallow marine avalanche, proximal transportation by alongside ocean flows, tidal waves abrasion, and as so, reflects local bedrock geology, i.e. adjacent Alpha Ridge rock composition. Additional confirmation of this can be seen in a wide distribution of Qu-sandstones with Paleo-Mezoarchean zircons, and finds of similar allochtonous zircons in dolerites along Alpha-Mendeleev Ridge profile. The studied fragment is a unique evidence for the possible existence of Paleoarchean continental crust within the submarine Alpha-Mendeleev Ridge in Arctic Ocean.

Episodic Holocene eruption of the Salton Buttes rhyolites, California, from paleomagnetic, U-Th, and Ar/Ar dating

USGS Publications Warehouse

Wright, Heather M.; Vazquez, Jorge A.; Champion, Duane E.; Calvert, Andrew T.; Mangan, Margaret T.; Stelten, Mark E.; Cooper, Kari M.; Herzig, Charles; Schriener Jr., Alexander

2015-01-01

In the Salton Trough, CA, five rhyolite domes form the Salton Buttes: Mullet Island, Obsidian Butte, Rock Hill, North and South Red Hill, from oldest to youngest. Results presented here include 40Ar/39Ar anorthoclase ages, 238U-230Th zircon crystallization ages, and comparison of remanent paleomagnetic directions with the secular variation curve, which indicate that all domes are Holocene. 238U-230Th zircon crystallization ages are more precise than but within uncertainty of 40Ar/39Ar anorthoclase ages, suggesting that zircon crystallization proceeded until shortly before eruption in all cases except one. Remanent paleomagnetic directions require three eruption periods: (1) Mullet Island, (2) Obsidian Butte, and (3) Rock Hill, North Red Hill, and South Red Hill. Borehole cuttings logs document up to two shallow tephra layers. North and South Red Hills likely erupted within 100 years of each other, with a combined 238U-230Th zircon isochron age of: 2.83 ± 0.60 ka (2 sigma); paleomagnetic evidence suggests this age predates eruption by hundreds of years (1800 cal BP). Rock Hill erupted closely in time to these eruptions. The Obsidian Butte 238U-230Th isochron age (2.86 ± 0.96 ka) is nearly identical to the combined Red Hill age, but its Virtual Geomagnetic Pole position suggests a slightly older age. The age of aphyric Mullet Island dome is the least well constrained: zircon crystals are resorbed and the paleomagnetic direction is most distinct; possible Mullet Island ages include ca. 2300, 5900, 6900, and 7700 cal BP. Our results constrain the duration of Salton Buttes volcanism to between ca. 5900 and 500 years.

U-Pb and Lu-Hf zircon geochronology of the Cañadón Asfalto Basin, Chubut, Argentina: Implications for the magmatic evolution in central Patagonia

NASA Astrophysics Data System (ADS)

Hauser, N.; Cabaleri, N. G.; Gallego, O. F.; Monferran, M. D.; Silva Nieto, D.; Armella, C.; Matteini, M.; Aparicio González, P. A.; Pimentel, M. M.; Volkheimer, W.; Reimold, W. U.

2017-10-01

The Cañadón Asfalto basin, central Chubut, Argentina, comprises a volcano-sedimentary sequence related to the opening of the Atlantic Ocean during Mesozoic times. The Lonco Trapial, Cañadón Asfalto and Cañadón Calcáreo formations are the main units related to the evolution of this basin. The Las Chacritas and Puesto Almada members are distinguished in the Cañadón Asfalto Formation. LA-HR-ICP-MS U-Pb and Lu-Hf data on zircon were obtained on these units. The Lonco Trapial Formation gave a weighted average age of 172.3 ± 1.8 Ma. A pyroclastic level from the Las Chacritas Member gave a weighted average age of 168.2 ± 2.2 Ma. Two U-Pb concordant ages of 160.3 ± 1.7 Ma on a laminated tuffite and 158.3 ± 1.3 Ma on a pyroclastic level were obtained for the Puesto Almada Member. Two maximum depositional ages constrain the sedimentary provenance areas for the basin: 1) A sample from the Sierra de la Manea range, where a controversial unit related either to the Cañadón Asfalto or to the Cañadón Calcáreo formation occurs, gave an age of 176.6 ± 1.0 Ma. Two younger zircon crystals indicate that this unit may be related to the Cañadón Calcáreo Formation. 2) A sandstone with cross-stratification from the Puesto Almada Member gave a maximum depositional age of 173.6 ± 6.4 Ma. In terms of U-Pb and Lu-Hf isotopes, two magmatic events are identified in central Patagonia: the Mamil Choique magmatic event characterized by negative εHf values around -5.0 and representing recycling during Permian times of Mesoproterozoic crust (TDM of ∼1.5 Ga), and the Cañadón Asfalto magmatic event with negative (-8.2) to positive (+4) εHf values and Meso- to Neoproterozoic TDM between 1.5 and 0.8 Ga. The younger event is characterized by three main cycles: C1 related to the Lonco Trapial magmatism, C2 to the Las Chacritas volcanism, and C3 to the Puesto Almada volcanism. These cycles are related with Marifil, Chon Aike and El Quemado formations volcanics events of Patagonia and the Neuquén Basin during the Mesozoic.

Exotic island arc Paleozoic terranes on the eastern margin of Gondwana: Geochemical whole rock and zircon U-Pb-Hf isotope evidence from Barry Station, New South Wales, Australia

NASA Astrophysics Data System (ADS)

Manton, Ryan J.; Buckman, Solomon; Nutman, Allen P.; Bennett, Vickie C.

2017-08-01

Early Paleozoic intra-oceanic terranes crop out along the Peel-Manning Fault System, in the southern New England Orogen, NSW Australia. These are the Cambrian ophiolitic Weraerai terrane and the Siluro-Devonian island arc Gamilaroi terrane. There has been debate whether these terranes formed at the Gondwana margin or if they are intra-oceanic, and were accreted to Gondwana later in the Paleozoic. Major-trace-REE elemental data indicate Weraerai terrane formed in a supra-subduction environment. Rare zircons extracted from Weraerai terrane gabbro-plagiogranite suites at Barry Station yield a U-Pb zircon date of 504.9 ± 3.5 Ma with initial εHf values of + 11.1 indicating a juvenile source. Amphibole-bearing felsic dykes and net-vein complexes are also found within the gabbro with a U-Pb zircon date of 503.2 ± 5.7 Ma and initial εHf values of + 11.6. These are coeval in age with their host rocks and we propose they represent partial melts of the mafic crust during the circulation of seawater. The Gamilaroi trondhjemites of prehnite-pumpellyite-greenschist metamorphic grade terrane yielded very few zircons with an age of 413 ± 8.7 Ma. Zircon initial εHf values range from + 5.0 to + 2.9, indicating an input from an evolved crustal source, unlike the purely oceanic Weraerai terrane. Gamilaroi terrane trondhjemites are enriched in LREE have low K2O and K2O/Na2O ratios and strong negative Nb anomalies consistent with supra-subduction zone environments. Multiple subduction zones may well have existed within the Panthalassa Ocean during the early-mid Paleozoic with the Weraerai-Gamilaroi being accreted onto the Gondwanan margin during the latest Devonian.

CONCH: A Visual Basic program for interactive processing of ion-microprobe analytical data

NASA Astrophysics Data System (ADS)

Nelson, David R.

2006-11-01

A Visual Basic program for flexible, interactive processing of ion-microprobe data acquired for quantitative trace element, 26Al- 26Mg, 53Mn- 53Cr, 60Fe- 60Ni and U-Th-Pb geochronology applications is described. Default but editable run-tables enable software identification of secondary ion species analyzed and for characterization of the standard used. Counts obtained for each species may be displayed in plots against analysis time and edited interactively. Count outliers can be automatically identified via a set of editable count-rejection criteria and displayed for assessment. Standard analyses are distinguished from Unknowns by matching of the analysis label with a string specified in the Set-up dialog, and processed separately. A generalized routine writes background-corrected count rates, ratios and uncertainties, plus weighted means and uncertainties for Standards and Unknowns, to a spreadsheet that may be saved as a text-delimited file. Specialized routines process trace-element concentration, 26Al- 26Mg, 53Mn- 53Cr, 60Fe- 60Ni, and Th-U disequilibrium analysis types, and U-Th-Pb isotopic data obtained for zircon, titanite, perovskite, monazite, xenotime and baddeleyite. Correction to measured Pb-isotopic, Pb/U and Pb/Th ratios for the presence of common Pb may be made using measured 204Pb counts, or the 207Pb or 208Pb counts following subtraction from these of the radiogenic component. Common-Pb corrections may be made automatically, using a (user-specified) common-Pb isotopic composition appropriate for that on the sample surface, or for that incorporated within the mineral at the time of its crystallization, depending on whether the 204Pb count rate determined for the Unknown is substantially higher than the average 204Pb count rate for all session standards. Pb/U inter-element fractionation corrections are determined using an interactive log e-log e plot of common-Pb corrected 206Pb/ 238U ratios against any nominated fractionation-sensitive species pair (commonly 238U 16O +/ 238U +) for session standards. Also displayed with this plot are calculated Pb/U and Pb/Th calibration line regression slopes, y-intercepts, calibration uncertainties, standard 204Pb- and 208Pb-corrected 207Pb/ 206Pb dates and other parameters useful for assessment of the calibration-line data. Calibrated data for Unknowns may be automatically grouped according to calculated date and displayed in color on interactive Wetherill Concordia, Tera-Wasserburg Concordia, Linearized Gaussian ("Probability Paper") and Gaussian-summation probability density diagrams.

Contrasting Cu-Au and Sn-W Granite Metallogeny through the Zircon Geochemical and Isotopic Record

NASA Astrophysics Data System (ADS)

Gardiner, Nicholas; Hawkesworth, Chris; Robb, Laurence; Whitehouse, Martin; Roberts, Nick; Kirkland, Chris

2017-04-01

Magmatic genesis and evolution - mediated by geodynamic setting - exert a primary control on the propensity of granites to be metal fertile. A revolution in our understanding of these petrogenetic processes has been made through a range of mineral-based tools, most notably the common accessory mineral zircon. There is consequently considerable interest in whether the geochemical and isotopic compositions of zircon can be applied to metallogenic problems. The paired magmatic belts of Myanmar have broadly contrasting metallogenic affinities (Sn-W versus Cu-Au), and are interpreted to have formed on the accretionary margin of the subducting Neo-Tethys Ocean. They therefore present the opportunity to geochemically compare and contrast the zircon compositions in two end-member types of granite-hosted mineral deposits generated in collisional settings. We present an integrated zircon isotope (U-Pb, Lu-Hf, O) and trace element dataset that fingerprint: (a) source; (b) redox conditions; and (c) degree of fractionation. These variables all impact on magma fertility, and our key question to address is whether they can be reliably traced and calibrated within the Myanmar zircon record. Granitoid-hosted zircons from the I-type copper arc have juvenile ɛHf (+7 to +12) and mantle-like δ18O (5.3 ‰), whereas zircons from the S-type tin belt have low ɛHf (-7 to -13) and heavier δ18O (6.2-7.7 ‰). Plotting Hf versus U/Yb reaffirms that the tin belt magmas contain greater crustal contributions than the copper arc rocks. Links between whole rock Rb/Sr and zircon Eu/Eu* highlights that the latter can be used to monitor magma fractionation in systems that crystallize plagioclase (low Sr/Y). Ce/Ce* and Eu/Eu* in zircon are thus sensitive to redox and fractionation respectively, and can be used to evaluate the sensitivity of zircons to the metallogenic affinity of their host rocks. Tin contents that exceed the solubility limit are required in order to make a magmatic-hydrothermal deposit, and empirical observations suggest that this threshold may be marked by zircon Eu/Eu* values of ca. < 0.08. The isotope and trace element signatures of both magmatic and detrital zircons can be developed into a useful exploration tool.

Geochronology of Zircon in Eclogite Reveals Imbrication of the Ultrahigh-Pressure Western Gneiss Region of Norway.

NASA Astrophysics Data System (ADS)

Young, D. J.; Kylander-Clark, A. R.; Root, D. B.

2014-12-01

Eclogite provides the only record of kinematic events at the deepest levels of orogens. Integrating the U-Pb geochronology and trace element chemistry of zircon in eclogite reveals the most complete view of the PTt history, yet low concentrations of uranium and zirconium and drier compositions that hinder zircon growth at peak conditions render it a challenging rocktype for this approach. The iconic Western Gneiss Region (WGR) in Norway is one of the largest terranes of deeply subducted continental rocks in the world, and contains many indicators of ultrahigh-pressure metamorphic conditions (P>2.8 GPa) that developed during the Siluro-Devonian Caledonian Orogeny. A metamorphic transition from amphibolite-facies to ultrahigh-pressure eclogite facies broadly coincides with a km-scale shear zone that underlies the majority of the WGR. A critical unknown is the timing of movement on this feature, which emplaced allochthonous units above the Baltica basement, but might also have accommodated late-orogenic exhumation of the WGR from mantle depths. We carried out laser ablation split-stream ICPMS (LASS) and selected multigrain TIMS analyses of zircons from eleven eclogites across the southern WGR, of which eight are located within or above the shear zone. LASS spots on polished grains mostly yield weakly discordant Proterozoic intrusive ages, and often minimal indication of a Caledonian (U)HP metamorphic overprint. Direct ablation into unpolished zircon reveals thin rims of Caledonian age in some cases. Overall, the dataset shows that all samples began zircon growth at approximately the same time (ca. 430-420 Ma). Eclogite from lower levels of the shear zone does not contain any dates younger than ca. 410 Ma, however, while eclogite from higher levels continued growth until ca. 400 Ma. We interpret this to result from thrusting of the WGR above cooler basement after 410 Ma, terminating new zircon crystallization within the shear zone but allowing limited further growth in rocks above.

Tectonic Recycling in the Paleozoic Ouachita Assemblage from U-Pb Detrital Zircon Studies

NASA Astrophysics Data System (ADS)

Gleason, J. D.; Gehrels, G. E.; Finney, S. C.

2001-05-01

The Paleozoic Ouachita deep-marine clastic sedimentary assemblage records a complex provenance over the course of its 200 m.y. history, with evidence for mixed sources and multiple dispersal paths. Combined neodymium and U-Pb detrital zircon work has established that most of the assemblage in Arkansas and Oklahoma is derived from Laurentian sources, meaning that regardless of the multiple pathways by which sediment was delivered to Ouachita seafloor, the material had its ultimate origin on the North American continent. More detailed work is in progress to elucidate specific dispersal paths, in particular for the middle to late Ordovician when a major change in provenance is recorded, and during the Carboniferous when voluminous turbidites entered the basin. We sampled three formations for U-Pb detrital zircon studies: the lower Middle Ordovician Blakely Sandstone, the Upper Ordovician/Lower Silurian Blaylock Sandstone, and the Pennsylvanian Jackfork Group. Individual zircon ages from these units document a major change in provenance between deposition of the Blakely Sandstone and Blaylock Sandstone, which is also reflected in the neodymium isotopic signature. Both units have a large population of Grenvillian-age zircons (1.0-1.2 Ga), and a less abundant population of 1.3-1.4 Ga zircons likely derived from sources in the mid-continent region. The Blakely Sandstone also contains abundant Archean zircons (2.5-2.7 Ga, likely derived from the Superior Province), and one grain apparently derived from the Penokean orogen (1.9 Ga). Zircon morphology (highly rounded, spherical), combined with the pure quartz sandstone lithology of the Blakely Sandstone, indicates very mature sedimentary sources. We conclude that zircons from this source were recycled ultimately from source terranes in the North American craton. This is reinforced by neodymium isotopes (eNd = -15), paleocurrents (from the north) and olistoliths (1.3 Ga granites), the latter indicating that Blakely turbidites were delivered to Ouachita seafloor from the North American shelf. In contrast, the Blaylock Sandstone lacks any grains older than 1.4 Ga. A single grain dated at 467 Ma (Taconian) is consistent with the primary source of the Blaylock turbidites being the southern Appalachian Mountains. This is reinforced by neodymium isotopes (eNd = -8), paleocurrent data (sources to the east-southeast), sandstone petrography (quartzolithic, indicating recycled fold-thrust belt sources), and the zircon morphology we observed (fewer rounded grains, indicating less mature sources). Sandstone from the Carboniferous Jackfork Group yields a wide spectrum of zircon ages (1.0 - 3.5 Ga), suggesting that it was derived in part by tectonic recycling of the pre-Carboniferous seafloor assemblage as the Ouachita remnant ocean basin closed between North America and Gondwana. In addition to Grenvillian-, Penokean- and Archean-age grains, there are also grains with ages of 1.4 and 1.5 Ga, all of which suggest a North American provenance. Dispersal paths for sediment entering the Carboniferous Ouachita basin are still a matter of debate, but the U-Pb zircon data are consistent with well-mixed material from the Appalachian-Ouachita orogen entering the basin from multiple directions. The preponderance of Grenvillian-age zircons in all three units reinforces the notion that sediment eroded from the Grenville orogen had widespread distribution across much of the North American continent.

Contrasting sources of Late Paleozoic rhyolite magma in the Polish Lowlands: evidence from U-Pb ages and Hf and O isotope composition in zircon

NASA Astrophysics Data System (ADS)

Słodczyk, Elżbieta; Pietranik, Anna; Glynn, Sarah; Wiedenbeck, Michael; Breitkreuz, Christoph; Dhuime, Bruno

2018-02-01

The Polish Lowlands, located southwest of the Teisseyre-Tornquist Zone, within Trans-European Suture Zone, were affected by bimodal, but dominantly rhyolitic, magmatism during the Late Paleozoic. Thanks to the inherited zircon they contain, these rhyolitic rocks provide a direct source of information about the pre-Permian rocks underlying the Polish Lowland. This paper presents zircon U-Pb geochronology and Hf and O isotopic results from five drill core samples representing four rhyolites and one granite. Based on the ratio of inherited vs. autocrystic zircon, the rhyolites can be divided into two groups: northern rhyolites, where autocrystic zircon is more abundant and southern rhyolites, where inherited zircon dominates. We suggest that the magma sources and the processes responsible for generating high silica magmas differ between the northern and southern rhyolites. Isotopically distinct sources were available during formation of northern rhyolites, as the Hf and O isotopes in magmatic zircon differ between the two analysed localities of northern rhyolites. A mixing between magmas formed from Baltica-derived mudstone-siltstone sediments and Avalonian basement or mantle can explain the diversity between the zircon compositions from the northern localities Daszewo and Wysoka Kamieńska. Conversely, the southern rhyolites from our two localities contain zircon with similar compositions, and these units can be further correlated with results from the North East German Basin, suggesting uniform source rocks over this larger region. Based on the ages of inherited zircon and the isotopic composition of magmatic ones, we suggest that the dominant source of the southern rhyolites is Variscan foreland sediments mixed with Baltica/Avalonia-derived sediments.

Sequence stratigraphy, geodynamics, and detrital geothermochronology of Cretaceous foreland basin deposits, western interior U.S.A

NASA Astrophysics Data System (ADS)

Painter, Clayton S.

Three studies on Cordilleran foreland basin deposits in the western U.S.A. constitute this dissertation. These studies differ in scale, time and discipline. The first two studies include basin analysis, flexural modeling and detailed stratigraphic analysis of Upper Cretaceous depocenters and strata in the western U.S.A. The third study consists of detrital zircon U-Pb analysis (DZ U-Pb) and thermochronology, both zircon (U-Th)/He and apatite fission track (AFT), of Upper Jurassic to Upper Cretaceous foreland-basin conglomerates and sandstones. Five electronic supplementary files are a part of this dissertation and are available online; these include 3 raw data files (Appendix_A_raw_isopach_data.txt, Appendix_C_DZ_Data.xls, Appendix_C_U-Pb_apatite.xls), 1 oversized stratigraphic cross section (Appendix_B_figure_5.pdf), and 1 figure containing apatite U-Pb concordia plots (Appendix_C_Concordia.pdf). Appendix A is a combination of detailed isopach maps of the Upper Cretaceous Western Interior, flexural modeling and a comparison to dynamic subsidence models as applied to the region. Using these new isopach maps and modeling, I place the previously recognized but poorly constrained shift from flexural to non-flexural subsidence at 81 Ma. Appendix B is a detailed stratigraphic study of the Upper Cretaceous, (Campanian, ~76 Ma) Sego Sandstone Member of the Mesaverde Group in northwestern Colorado, an area where little research has been done on this formation. Appendix C is a geo-thermochronologic study to measure the lag time of Upper Jurassic to Upper Cretaceous conglomerates and sandstones in the Cordilleran foreland basin. The maximum depositional ages using DZ U-Pb match existing biostratigraphic age controls. AFT is an effective thermochronometer for Lower to Upper Cretaceous foreland stratigraphy and indicates that source material was exhumed from >4--5 km depth in the Cordilleran orogenic belt between 118 and 66 Ma, and zircon (U-Th)/He suggests that it was exhumed from <8--9 km depth. Apatite U-Pb analyses indicate that volcanic contamination is a significant issue, without which, one cannot exclude the possibility that the youngest detrital AFT population is contaminated with significant amounts of volcanogenic apatite and does not represent source exhumation. AFT lag times are <5 Myr with relatively steady-state to slightly increasing exhumation rates. Lag time measurements indicate exhumation rates of ~0.9->>1 km/Myr.

The Triassic reworking of the Yunkai massif (South China): EMP monazite and U-Pb zircon geochronologic evidence

NASA Astrophysics Data System (ADS)

Chen, Cheng-Hong; Liu, Yung-Hsin; Lee, Chi-Yu; Sano, Yuji; Zhou, Han-Wen; Xiang, Hua; Takahata, Naoto

2017-01-01

Geohistory of the Yunkai massif in South China Block is important in understanding the geodynamics for the build-up of this block during the Phanerozoic orogenies. To investigate this massif, we conduct EMP monazite and U-Pb zircon geochronological determinations on mineral inclusions and separate for seventeen samples in four groups, representing metamorphic rocks from core domain, the Gaozhou Complex (amphibolite facies, NE-striking) and the Yunkai Group (greenschist facies, NW-striking) of this massif and adjacent undeformed granites. Some EMP monazite ages are consistent with the NanoSIMS results. Monazite inclusions, mostly with long axis parallel to the cleavage of platy and elongated hosts, give distinguishable age results for NW- and NE-trending deformations at 244-236 Ma and 236-233 Ma, respectively. They also yield ages of 233-230 Ma for core domain gneissic granites and 232-229 Ma for undefomed granites. Combining U-Pb zircon ages of the same group, 245 Ma and 230 Ma are suggested to constrain the time of two phases of deformation. Aside from ubiquity of Triassic ages in studied rocks, ages of detrital monazite in the meta-sandstone match the major U-Pb zircon age clusters of the metamorphic rock that are largely concentrated at Neoproterozoic (1.0-0.9 Ga) and Early Paleozoic (444-431 Ma). Based on these geochronological data, Triassic is interpreted as representing the time for recrystallization of these host minerals on the Early Paleozoic protolith, and the also popular Neoproterozoic age is probably inherited. With this context, Yunkai massif is regarded as a strongly reactivated Triassic metamorphic terrain on an Early Paleozoic basement which had incorporated sediments with Neoproterozoic provenances. Triassic tectonic evolution of the Yunkai massif is suggested to have been controlled by converging geodynamics of the South China and Indochina Blocks as well as mafic magma emplacement related to the Emeishan large igneous province (E-LIP).

Provenance analysis of the Late Paleozoic sedimentary rocks in the Xilinhot Terrane, NE China, and their tectonic implications

NASA Astrophysics Data System (ADS)

Han, Jie; Zhou, Jian-Bo; Wilde, Simon A.; Song, Min-Chun

2017-08-01

The Xilinhot Terrane is located in the eastern segment of the Central Asian Orogenic Belt in NE China, and is a key to a hotly debated issue on the Paleozoic tectonic evolution of this giant progenic belt. To constrain the tectonic evolution of the Xilinhot Terrane in the Late Paleozoic, we undertook zircon U-Pb dating and geochemical analyses of the Zhesi and Benbatu formations in the Suolun and Xi Ujimqin areas in the Xilinhot Terrane. Samples of the Benbatu Formation yield detrital zircon U-Pb ages ranging from 2659 Ma to 316 Ma, with four age populations at: 2659-1826 Ma, 1719-963 Ma, 590-402 Ma, and 396-316 Ma, whereas samples from the Zhesi Formation yield detrital zircon U-Pb ages ranging from 1967 Ma to 250 Ma, with four age populations at: 1967-1278 Ma, 971-693 Ma, 561-403 Ma, and 399-250 Ma. The age groups of both the Benbatu and Zhesi formations in the Xilinhot Terrane are similar to those in other parts of the Central Asian Orogenic Belt (CAOB). This evidence indicates that the Xilinhot Terrane is a microcontinent, and not an accretionary complex as previously thought. Furthermore, the youngest zircon grains in the Benbatu and Zhesi formations yield weighted mean 206Pb/238U ages of 322 ± 12 Ma (MSDW = 0.12, n = 4) and 257 ± 2.8 Ma (MSDW = 1.6, n = 8), respectively. Combined with fossil data, our new data suggest that the Benbatu and Zhesi formations in the Xilinhot Terrane were possibly deposited at ∼322 Ma and ∼257 Ma, respectively. Based on the provenance of the Carboniferous-Permian sandstones came from the blocks of NE China, we speculate that the Xilinhot Terrane is the western part of the Songliao block.

Late Pleistocene granodiorite source for recycled zircon and phenocrysts in rhyodacite lava at Crater Lake, Oregon

USGS Publications Warehouse

Bacon, C.R.; Lowenstern, J. B.

2005-01-01

Rhyodacite tephra and three lavas erupted ???27 ka, interpreted to be early leaks from the climactic magma chamber of Mount Mazama, contain ubiquitous resorbed crystals (antecrysts) that were recycled from young granodiorite and related plutonic rocks of the same magmatic system. The shallow composite pluton is represented by blocks ejected in the 7.7-ka climactic eruption that formed Crater Lake caldera. Plagioclase crystals in both rhyodacite and granodiorites commonly have cores with crystallographically oriented Fe-oxide needles exsolved at subsolidus conditions. At least 80% of plagioclase crystals in the rhyodacite are antecrysts derived from plutonic rocks. Other crystals in the rhyodacite, notably zircon, also were recycled. SIMS 238U- 230Th dating indicates that zircons in 4 granodiorite blocks crystallized at various times between ???20 ka and ???300 ka with concentrations of analyses near 50-70, ???110, and ???200 ka that correspond to periods of dacitic volcanism dated by K- Ar. U-Th ages of zircon from a rhyodacite sample yield similar results. No analyzed zircons from the granodiorite or rhyodacite are pre-Quaternary. Zircon minimum ages in blocks from different locations around the caldera reflect ages of nearby volcanic vents and may map the distribution of intrusions within a composite pluton. Survival of zircon in zircon-undersaturated hydrous magma and of Fe-oxide needles in plagioclase suggests that little time elapsed from entrainment of antecrysts to the ???27-ka eruption of the rhyodacite. The ???27-ka rhyodacite is an example of young silicic magma that preserved unstable antecrysts from a known source early during growth of a large high-level magma chamber. In contrast, the voluminous 7.7-ka climactic rhyodacite pumice is virtually lacking in zircon, indicating dissolution of any granodioritic debris in the intervening period. Mineralogical evidence of assimilation may be destroyed in hot, vigorously growing silicic magma bodies such as ultimately produced the climactic eruption of Mount Mazama. ?? 2005 Elsevier B.V. All rights reserved.

Detrital zircon U-Pb Geochronology of the Boleo Formation of Santa RosalÍa Basin, Baja California Sur, México

NASA Astrophysics Data System (ADS)

Henry, M.; Alvarez Ortega, K. G.; Banes, A.; Holm-Denoma, C.; Busby, C.; Niemi, T.

2017-12-01

The Santa Rosalía Basin (SRB) is a rift basin related to the opening of the Gulf of California. The Boleo Formation is the oldest and dominant sedimentary fill of the SRB, with a poorly constrained age. We carried out a U-Pb detrital zircon (DZ) study of the Boleo Formation to constrain its maximum depositional age. The Boleo Formation has a basal limestone-gypsum section, overlain by an up to 250 m thick clastic sequence, with coarsening upward cycles of mudstone, sandstone, and conglomerate. Cu-Zn-Co-Mn stratiform ore deposits ("mantos") cap the conglomerate in each cycle, numbered 0, 1, 2, 3 and 4 (from top to bottom of section1). Sandstone samples were collected for U-Pb detrital zircon geochronology from four stratigraphic levels beneath a manto, including one each below mantos 1, 3 and 4, as well as two localities beneath manto 2. Additionally, one sample was collected above the gypsum. The sandstones are lithic feldspathic wackes derived from erosion of andesitic arc volcanic rocks, which generally lack zircon, so large DZ samples were collected. A field Wilfley table was constructed from local materials as a first step to concentrate heavy minerals, from 88 kg/sample to 16 kg/sample. The field-processed samples were further concentrated in the lab using standard zircon separation methods. Yields were excellent, 1,000 zircons per sample. We analyzed 315 zircons per sample by LA-ICPMS, using the Arizona LaserChron Center. DZ ages from the Boleo Formation range dominantly from Late Miocene through Early Cretaceous, with minor Paleozoic and Precambrian ages. However, the maximum depositional age of the formation is constrained by 40 Ar/39 Ar age of 9.42 +/- 0.29 Ma on underlying volcanic rocks2. Only 5 to 22 zircons per sample are less than 10 Ma, and of those, all stratigraphic levels are dominated mostly by 9 Ma zircons, except for the stratigraphically highest sample. Zircons from this form a coherent group of 3 with a TuffZirc age of 6.04 +/- 0.02 (75% confidence level). Thus the age of the top of the Boleo Formation appears to be well-constrained at 6 Ma, while the remainder of the section remains poorly constrained at 6-9 Ma. Future work will examine the provenance of the zircon in a Gulf of California tectonic framework. 1 Wilson 1995 USGS PP 273 2 Gutierrez et al., 2016 GSA Annual Mtg abstr.

Melting depths associated with Jack Hills zircons crystallization as revealed by in situ trace element measurements

NASA Astrophysics Data System (ADS)

Profeta, L.; Ducea, M. N.; Gehrels, G. E.

2016-12-01

The Jack Hills zircons hosted within the Narryer Gneiss Complex, Yilgarn craton have ages from 4.4 Ga up to Mesoarchean. These zircons crystallized from low temperature granitoid magmas (Harrison, 2009). Here, we use trace element measurements obtained simultaneously with U-Pb ages using LA-ICP-MS on 276 Jack Hills zircons in order to estimate the depth of melting. La/Yb are converted to whole rock equivalent values using newly determined REE -whole rock partition coefficients (Chapman et al., 2016). La/Yb are subsequently transformed into depth estimates using the correlation between whole rock La/Yb and crustal thickness put forward in Profeta et al. (2015) for modern arcs. Our data pertains to 4.2 to 3.2 Ga zircons, which are supplemented with previously published data on 4.4.-4.3 Ga zircons (Peck et al. 2001). Depth estimates are averaged over 100 Ma bins, revealing a remarkably constant trend throughout the investigated period with values around 50 ± 10 km. We interpret that these depths may not be the result of a thick continental crust, as is the case for modern arcs, but rather the existence of different melting conditions during the Hadean and Paleoarchean due to elevated thermal regimes within the mantle. The high La/Yb whole rock ratios (with computed values greater than 10) coupled with elevated mantle temperatures point towards granitoid generation from partial melting of hydrated basalts (e.g. Martin et al., 2014). [1] Harrison, T.M., Annu. Rev. Earth Planet. Sci. 37, 479-505 (2009). [2] Chapman, J. B. et al., Chem. Geol. 439, 59-70 (2016). doi: 10.1016/j.chemgeo.2016.06.014. [3] Profeta, L. et al., Sci. Rep. 5, 17786 (2015). doi: 10.1038/srep17786 [4] Peck, W. et al., Cosmochim. Acta 65, 4215-4229 (2001). doi: 10.1016/S0016-7037(01)00711-6 [5] Martin, H. et al. Lithos 198, 1-13 (2014). doi: 10.1016/j.lithos.2014.02.017

Petrotectonic characteristics, geochemistry, and U-Pb geochronology of Jurassic plutons in the Upper Magdalena Valley-Colombia: Implications on the evolution of magmatic arcs in the NW Andes

NASA Astrophysics Data System (ADS)

Rodríguez, G.; Arango, M. I.; Zapata, G.; Bermúdez, J. G.

2018-01-01

Field, petrographic, and geochemical characterization along with U-Pb zircon geochronology of the Jurassic plutons exposed in the Upper Magdalena Valley (Colombia) allowed recognizing distinct western and eastern suites formed in at least three magmatic pulses. The western plutons crop out between the eastern flank of the Central Cordillera and the Las Minas range, being limited by the Avirama and the Betania-El Agrado faults. The western suite comprises a quartz monzonite - quartz monzodiorite - quartz diorite series and subordinate monzogranites. Chemically, the rocks are high-K calc-alkaline I-type granitoids (some reaching the shoshonitic series) with metaluminous of magnesium affinity. Trace-element tectonic discrimination is consistent with magmatism in a continental arc environment. Most rocks of this suite crystallized between 195 and 186 Ma (Early Jurassic, Pliensbachian), but locally some plutons yielded younger ages between 182 and 179 Ma (Early Jurassic, Toarcian). The eastern suite crops out in the eastern margin of the Upper Magdalena Valley, east of the Betania - El Agrado fault. Plutons of this unit belong to the monzogranite series with rock types ranging between syenogranites and granodiorites. They are high-K calc-alkaline continental granitoids, some metaluminous and some peraluminous, related to I-type granites generated in a volcanic arc. Crystallization of the suite was between 173 and 169 Ma (Middle Jurassic, Aalenian-Bajocian), but locally these rocks contain zircon with earlier inherited ages related to the magmatic pulse of the western suite between 182 and 179 Ma (Early Jurassic, Toarcian). The evolution of the Jurassic plutons in the Upper Magdalena Valley is best explained by onset or increase in subduction erosion of the accretionary prism. This explains the eastward migration of the arc away from the trench. Subduction of prism sediments increased the water flux from the subducting slab, decreasing solidus temperatures, therefore increasing the volume of magma and the amount of crustal melts involved in the magma. This is explains the crystallization of older and more primitive quartz-monzodiorite stocks in the west and the later crystallization of granitic bodies with batholitic dimensions in the east.

Advances in Laser Microprobe (U-Th)/He Geochronology

NASA Astrophysics Data System (ADS)

van Soest, M. C.; Monteleone, B. D.; Boyce, J. W.; Hodges, K. V.

2008-12-01

The development of the laser microprobe (U-Th)/He dating method has the potential to overcome many of the limitations that affect conventional (U-Th)/He geochronology. Conventional single- or multi-crystal (U- Th)/He geochronology requires the use of pristine, inclusion-free, euhedral crystals. Furthermore, the ages that are obtained require corrections for the effects of zoning and alpha ejection based on an ensemble of assumptions before interpretation of their geological relevance is possible. With the utilization of microbeam techniques many of the limitations of conventional (U-Th)/He geochronology can either be eliminated by careful spot selection or accounted for by detailed depth profiling analyses of He, U and Th on the same crystal. Combined He, Th, and U depth profiling on the same crystal potentially even offers the ability to extract thermal histories from the analyzed grains. Boyce et al. (2006) first demonstrated the laser microprobe (U-Th)/He dating technique by successfully dating monazite crystals using UV laser ablation to liberate He and determined U and Th concentrations using a Cameca SX-Ultrachron microprobe. At Arizona State University, further development of the microprobe (U-Th)/He dating technique continues using an ArF Excimer laser connected to a GVI Helix Split Flight Tube noble gas mass spectrometer for He analysis and SIMS techniques for U and Th. The Durango apatite age standard has been successfully dated at 30.7 +/- 1.7 Ma (2SD). Work on dating zircons by laser ablation is currently underway, with initial results from Sri Lanka zircon at 437 +/- 14 Ma (2SD) confirmed by conventional (U-Th)/He analysis and in agreement with the published (U-Th)/He age of 443 +/- 9 Ma (2SD) for zircons from this region in Sri Lanka (Nasdala et al., 2004). The results presented here demonstrate the laser microprobe (U-Th)/He method as a powerful tool that allows application of (U- Th)/He dating to areas of research such as detrital apatite and zircon dating, where conventional (U-Th)/He geochronology has limited applicability. Boyce et al. (2006) GCA 70 (3031-3039), Nasdala et al. (2004) Am. Min. 89 (219-231)

Uranium-lead isotopic ages from the Sierra Nevada Batholith, California

NASA Astrophysics Data System (ADS)

Chen, James H.; Moore, James G.

1982-06-01

This study provides new information on the timing and distribution of Mesozoic magmatic events in the Sierra Nevada batholithic complex chiefly between 36° and 37°N. latitude. U-Pb ages have been determined for 133 zircon and 7 sphene separates from 82 samples of granitoid rocks. Granitoid rocks in this area range in age from 217 to 80 m.y. Triassic intrusions are restricted to the east side of the batholith; Jurassic plutons occur south of the Triassic plutons east of the Sierra Nevada, as isolated masses within the Cretaceous batholith, and in the western foothills of the range; Cretaceous plutons form a continuous belt along the axis of the batholith and occur as isolated masses east of the Sierra Nevada. No granitic intrusions were emplaced for 37 m.y. east of the Sierra Nevada following the end of Jurassic plutonism. However, following emplacement of the eastern Jurassic granitoids, regional extension produced a fracture system at least 350 km long into which the dominantly mafic, calc-alkalic Independence dike swarm was intruded 148 m.y. ago. The dike fractures probably represents a period of regional crustal extension caused by a redistribution of the regional stress pattern accompanying the Nevadan orogeny. Intrusion of Cretaceous granitic plutons began in large volume about 120 m.y. ago in the western Sierra Nevada and migrated steadily eastward for 40 m.y. at a rate of 2.7 mm/y. This slow and constant migration indicates remarkably uniform conditions of subduction with perhaps downward migration of parent magma generation or a slight flattening of the subduction zone. Such steady conditions could be necessary for the production of large batholithic complexes such as the Sierra Nevada. The abrupt termination of plutonism 80 m.y. ago may have resulted from an increased rate of convergence of the American and eastern Pacific plates and dramatic flattening of the subduction zone. U-Pb ages of the Giant Forest-alaskite sequence in Sequoia National Park are all in the range 99±3 m.y., indicating a relatively short period of emplacement and cooling for this nested group of plutons. U-Pb ages of a mafic inclusion and its host granodiorite indicate that both were derived from a common source or that the mafic inclusion was totally equilibrated with the granodioritic magma. Comparison of isotopic ages determined by different methods such as zircon U-Pb, sphene U-Pb, hornblende K-Ar, and biotite K-Ar suggests that zircon U-Pb ages generally approximate the emplacement age of a pluton. However, some plutons probably contain inherited or entrained old zircons, and the zircons of some samples are disturbed by younger thermal and metamorphic events. The ages reported here are consistent with U-Pb age determinations previously made on granitic rocks to the north [Stern et al., 1981], The age distribution of granitic belts determined here is in general agreement with those established by K-Ar dating [Evernden and Kistler, 1970] but does not differentiate the five epochs of plutonism determined in their study.

Uranium-lead isotopic ages from the Sierra Nevada Batholith, California

USGS Publications Warehouse

Chen, J.

1982-01-01

This study provides new information on the timing and distribution of Mesozoic magmatic events in the Sierra Nevada batholithic complex chiefly between 36° and 37°N. latitude. U-Pb ages have been determined for 133 zircon and 7 sphene separates from 82 samples of granitoid rocks. Granitoid rocks in this area range in age from 217 to 80 m.y. Triassic intrusions are restricted to the east side of the batholith; Jurassic plutons occur south of the Triassic plutons east of the Sierra Nevada, as isolated masses within the Cretaceous batholith, and in the western foothills of the range; Cretaceous plutons form a continuous belt along the axis of the batholith and occur as isolated masses east of the Sierra Nevada. No granitic intrusions were emplaced for 37 m.y. east of the Sierra Nevada following the end of Jurassic plutonism. However, following emplacement of the eastern Jurassic granitoids, regional extension produced a fracture system at least 350 km long into which the dominantly mafic, calc-alkalic Independence dike swarm was intruded 148 m.y. ago. The dike fractures probably represents a period of regional crustal extension caused by a redistribution of the regional stress pattern accompanying the Nevadan orogeny. Intrusion of Cretaceous granitic plutons began in large volume about 120 m.y. ago in the western Sierra Nevada and migrated steadily eastward for 40 m.y. at a rate of 2.7 mm/y. This slow and constant migration indicates remarkably uniform conditions of subduction with perhaps downward migration of parent magma generation or a slight flattening of the subduction zone. Such steady conditions could be necessary for the production of large batholithic complexes such as the Sierra Nevada. The abrupt termination of plutonism 80 m.y. ago may have resulted from an increased rate of convergence of the American and eastern Pacific plates and dramatic flattening of the subduction zone. U-Pb ages of the Giant Forest-alaskite sequence in Sequoia National Park are all in the range 99±3 m.y., indicating a relatively short period of emplacement and cooling for this nested group of plutons. U-Pb ages of a mafic inclusion and its host granodiorite indicate that both were derived from a common source or that the mafic inclusion was totally equilibrated with the granodioritic magma. Comparison of isotopic ages determined by different methods such as zircon U-Pb, sphene U-Pb, hornblende K-Ar, and biotite K-Ar suggests that zircon U-Pb ages generally approximate the emplacement age of a pluton. However, some plutons probably contain inherited or entrained old zircons, and the zircons of some samples are disturbed by younger thermal and metamorphic events. The ages reported here are consistent with U-Pb age determinations previously made on granitic rocks to the north [Stern et al., 1981], The age distribution of granitic belts determined here is in general agreement with those established by K-Ar dating [Evernden and Kistler, 1970] but does not differentiate the five epochs of plutonism determined in their study.

Zircon geochronology and Hf-O isotope geochemistry from granites in the Iapetus Suture Zone in Ireland and the Isle of Man

NASA Astrophysics Data System (ADS)

Fritschle, Tobias; Daly, J. Stephen; Whitehouse, Martin J.; McConnell, Brian; Buhre, Stephan

2014-05-01

Late Caledonian syn- to post-orogenic granites located in the Iapetus Suture Zone (ISZ) in Ireland and Britain have been related to A-type subduction and possible slab breakoff [1] following the Laurentia-Avalonian collision. Lack of reliable age data (especially in Ireland) has inhibited petrogenetic investigations of these rocks. Hence, ion microprobe U-Pb and oxygen isotope analyses as well as LA-MC-ICPMS Lu-Hf isotopic measurements on zircons from Irish and Isle of Man granites have been undertaken to provide better constraints on this enigmatic episode of the Caledonian Orogeny. Four stages of Late Caledonian granitic magmatism (c. 435, 417, 410 and 394 Ma) are indicated by U-Pb dating of oscillatory-zoned magmatic zircons. The Crossdoney, Kentstown, Drogheda and Ballynamuddagh granites together with a rhyolite from Glenamaddy have yielded U-Pb concordia ages, interpreted as intrusion-ages, between 419.9 ± 4.3 Ma (Glenamaddy) and 415.8 ± 2.0 Ma (Crossdoney) with a weighted average of 417.5 ± 0.9 Ma (MSWD = 1.3). The Glenamaddy Granite - which intruded the Glenamaddy Rhyolite - yielded an age of 410 ± 2.1 Ma. In addition, the Rockabill Granite yielded a younger age of 393.9 ± 1.9 Ma, whereas the Carnsore Granite yielded an older age of 434.6 ± 1.9 Ma. Inherited zircons (487 to 453 Ma) occur in several of the granites, and are interpreted to have been derived from Ordovician arc magmatic rocks accreted within the ISZ. A younger group of c. 440 Ma inherited zircons occurs in the c. 417 Ma Crossdoney and Ballynamuddagh granites. These grains could be related to continued or renewed Silurian arc magmatism. Hf-O isotopic measurements on the dated zircon grains range between -2 and +7 ɛHfi units and 5.5 to 8.5 o δ18O. These are interpreted to indicate the contribution of juvenile mantle melts - possibly derived from the Ordovician arc - to some of the granites. Significant heterogeneities in zircon oxygen isotopes in at least four of the granites further suggest the involvement of isotopically distinct protoliths. The Dhoon and Foxdale granites in the Isle of Man were previously regarded to be of Late Caledonian age. Surprisingly, zircons from these granites yielded concordant U-Pb ages of 455.6 ± 2.1 Ma (Dhoon) and 455.9 ± 2.1 Ma (Foxdale), respectively. Inherited cores are of Meso- and Palaeoproterozoic age. Hafnium isotopic analyses of the dated magmatic zircon rims from both intrusions are slightly more radiogenic than those from the Late Caledonian granites. Their δ18O values range between 5.5 to 7.5 o. The Isle of Man granites are now suggested to be the plutonic equivalents of a Late Ordovician (Caradocian) volcanic arc generated in the Iapetus Ocean. [1] Atherton & Ghani (2002), Lithos 62, 65-85.

3-D Characterization of Detrital Zircon Grains and its Implications for Fluvial Transport, Mixing, and Preservation Bias

NASA Astrophysics Data System (ADS)

Markwitz, V.; Kirkland, C. L.; Mehnert, A.; Gessner, K.; Shaw, J.

2017-12-01

Detrital zircon studies can suffer from selective loss of provenance information due to U-Pb age discordance, metamictization, metamorphic overprinting and fluviatile transport processes. The relationship between isotopic composition and zircon grain shape, and how grain shape is modified during transport, is largely unknown. We combine X-ray tomography with U-Pb geochronology to quantify how fluvial transport affects 3-D zircon shape, detrital age signature, and grain density along the Murchison River, whose catchment comprises Eoarchean to Early Paleozoic source rocks in Western Australia. We acquired tomographic volumes and isotopic data from 373 detrital zircons to document changes in size, shape and density in transport direction, and explore how grain shape, age spectra and the proportion of discordant material vary along the channel. Results show that shape characteristics are sensitive to transport distance, stream gradient, proximity to source material, and whether the source consists of primary or recycled zircons. With increasing transport distance, grain lengths decrease more than their widths. Furthermore, the loss of metamict grains occurs at a near constant rate, resulting in a linear increase of mean calculated zircon density by ca. 0.03 g/cm3 per 100 km transport distance. 3-D grain shape is therefore strongly linked to detrital age signature, and mean grain density is a function of the absolute transport distance. 3-D shape characteristics provide valuable information on detrital zircon populations, including the interaction between source materials with fluvial transport processes, which significantly affects preservation bias and, by inference, the representativeness of the sampled data.

U-Pb Detrital Zircon Geochronologic Constraints on Depositional Age and Sediment Source Terrains of the Late Paleozoic Tepuel-Genoa Basin

NASA Astrophysics Data System (ADS)

Griffis, N. P.; Montanez, I. P.; Isbell, J.; Gulbranson, E. L.; Wimpenny, J.; Yin, Q. Z.; Cúneo, N. R.; Pagani, M. A.; Taboada, A. C.

2014-12-01

The late Paleozoic Ice Age (LPIA) is the longest-lived icehouse of the Phanerozoic and the only time a metazoan dominated and vegetated world transitioned from an icehouse climate into a greenhouse. Despite several decades of research, the timing, extent of glaciation and the location of ice centers remain unresolved, which prohibits reconstruction of ice volume. The Permo-Carboniferous sediments in the Tepuel-Genoa Basin, Patagonia contains a near complete record of sedimentation from the lower Carboniferous through lower Permian. Outsized clasts, thin pebble-rich diamictites and slumps represent the last of the late Paleozoic glacially influenced deep-water marine sediments in the Mojón de Hierro Fm. and the Paleozoic of Patagonia. U-Pb analysis of detrital zircons separated from slope sediments reveal groupings (20 myr bins, n≥5 zircons) with peak depositional ages of 420, 540 to 660 and 1040 Ma. Zircon age populations recovered from the Mojón de Hierro Fm. compare well with bedrock ages of the Deseado Massif of SE Patagonia, suggesting this may be a potential source of sediments. The maximum depositional age of the sediments is 306.05 ± 3.7 Ma (2σ) as determined by the median age of the two youngest concordant zircons that overlap in error. The youngest zircon from the analysis yields a 238U/206Pb age of 301.3 ± 4.5 Ma (2σ; MSWD = 2.3). Younger zircons from the analysis compare well with the age of granite bedrock exposed along the basin margin to the E-NE suggesting they may reflect a more proximal source. These data, which indicate a maximum age of late Carboniferous for the Mojón de Hierro Fm, provide the first geochemical constraints for the timing of final deposition of glaciomarine sediments in the Tepuel-Genoa Basin, and contributes to the biostratigraphic correlation of the late Paleozoic succession in Patagonia with other key LPIA basins that has thus far been hindered by faunal provincialism.

Geochronology of high-grade metamorphic rocks from the Anjul area, Lut block, eastern Iran

NASA Astrophysics Data System (ADS)

Bröcker, Michael; Fotoohi Rad, Gholamreza; Abbaslu, Fateme; Rodionov, Nikolay

2014-03-01

U-Pb and Rb-Sr geochronology has been used to constrain robust ages for leucosomes and high-grade gneisses from the Anjul area in the eastern part of the Lut block, Iran. The new results do not support the previously suggested Proterozoic age for this occurrence, but instead reveal the importance of Jurassic and Cretaceous magmatic and/or metamorphic processes. Ionprobe U-Pb zircon dating yielded four age groups (>200, ˜168, ˜120 and ˜110 Ma). Textural observations suggest that ages >200 Ma represent inherited zircons. The majority of zircons yielded Jurassic (168 ± 2 and 169 ± 2 Ma) and Cretaceous (120 ± 3, 108 ± 2, 111 ± 3 Ma) intercept ages. Explanations for the two dominant age groups (˜168 and ˜110 Ma) include the following alternatives: (a) the Jurassic ages constrain the protolith age of magmatic precursors that experienced metamorphic overprinting at ˜110 Ma; and (b) both the ˜168 Ma and ˜110 Ma ages indicate the time of metamorphic episodes, e.g. zircon-formation during different anatectic events or migmatization followed by a lower temperature overprint associated with new zircon growth. Multi-point Rb-Sr mineral isochrons of three additional gneisses indicated ages of 102 ± 3 Ma, 102 ± 1 Ma and 97 ± 2 Ma. These ages further document the importance of Cretaceous metamorphism in the Anjul area. The difference compared to the U-Pb ages of zircon overgrowths is interpreted to indicate cooling after a thermal event with or without partial melting. The two major occurrences of metamorphic rocks in the eastern Lut block are exposed in the Deh-Salm and the Anjul region. These occurrences may represent two different segments of a single metamorphic belt that can broadly be related to accretionary and/or collisional processes induced by convergence between the Afro-Arabian and Eurasian plates. Our geochronological study provides a conclusive evidence for Cretaceous metamorphism. We speculate that zircon overgrowths with Cretaceous ages reflect metamorphic processes unrelated to melt formation that overprinted a pre-existing population recording Jurassic anatexis.

Testing the age calibration of the Newark-Hartford APTS by magnetostratigraphic correlation of U-Pb zircon-dated tuffaceous beds in the Late Traissic Chinle Formation in core PFNP-1A from the Petrified Forest National Park (Arizona, USA)

NASA Astrophysics Data System (ADS)

Kent, D. V.; Olsen, P. E.; Mundil, R.; Lepre, C. J.

2017-12-01

The Newark-Hartford APTS extends over 27 Myr according to cycle stratigraphy of the Norian and Rhaetian of the Late Triassic and Hettangian and Sinemurian of the Early Jurassic and an additional 6 Myr by extrapolation into the Carnian; the entire sequence is anchored by U-Pb zircon dating of CAMP activity that provides a calibration date of 201.6 Ma for Chron E23r just below the end-Triassic extinction and the earliest CAMP basalts in the Newark basin (Blackburn+2013 Science; Kent+2017 ESR). The developing APTS has been successfully used for global correlations in marine and non-marine facies but there have been ongoing suggestions that millions of years of Rhaetian time are missing in a cryptic unconformity that supposedly occurs just above E23r in the Newark Supergroup basins. Testing the continuity of the APTS by magnetostratigraphic correlation of U-Pb zircon-dated tuffaceous beds in the Chinle Formation was a prime scientific objective for core PFNP-1A. Paleomagnetic results were obtained using stepwise thermal demagnetization to 680°C from >150 samples of finer-grained red lithologies from the upper 250 m of the cored section of the Chinle (upper Sonsela, Petrified Forest including the Black Forest Bed, and lower Owl Rock Members). Characteristic directions isolated in 2/3 of the samples showed antipodal directions that were shallow with respect to reference directions (flattening factor 0.5), consistent with early acquisition of remanence. Seven polarity magnetozones produce a distinctive pattern correlated to Chrons E17r to E14r of the APTS. The Black Forest Bed at 209.93±0.26 Ma (Ramezani+2011 GSAB), confirmed by our new U-Pb dates from core PFNP-1A, occurs in a reverse polarity magnetozone correlated to E16r (209.95-210.25 Ma), which puts the U-Pb zircon date(s) in excellent agreement with the inferred APTS age. Rather than a 'missing Rhaetian', the apparent regional differences in appearances and disappearances of palynoflora, conchostracans, and other endemic taxa in continental deposits are more likely a reflection of demonstrated continental drift across climate belts and the misinterpretation of ecostratigraphy as chronostratigraphy. A suite of new U-Pb dates in conjunction with paleomagnetic analyses in PFNP-1A is expected to calibrate much of Triassic succession of the Colorado Plateau.

Geochronology, petrogenesis and tectonic settings of pre- and syn-ore granites from the W-Mo deposits (East Kounrad, Zhanet and Akshatau), Central Kazakhstan

NASA Astrophysics Data System (ADS)

Li, GuangMing; Cao, MingJian; Qin, KeZhang; Evans, Noreen J.; Hollings, Pete; Seitmuratova, Eleonora Yusupovha

2016-05-01

There is significant debate regarding the mineralization ages of the East Kounrad, Zhanet and Akshatau W-Mo deposits of Central Kazakhstan, and the petrogenesis and tectono-magmatic evolution of the granites associated with these deposits. To address these issues, we present molybdenite Re-Os dating, zircon U-Pb dating, whole rock geochemistry as well as Sr-Nd-Pb and zircon O-Hf isotopic analyses on the pre-mineralization and ore-forming granites. U-Pb dating of zircons from pre-mineralization granitic rocks yield Late Carboniferous ages of 320-309 Ma, whereas ore-forming granites have Early Permian ages of 298-285 Ma. Molybdenite Re-Os isotopic data indicate a mineralization age of 296 Ma at East Kounrad, 294 Ma at Akshatau and 285 Ma at Zhanet. The pre-ore and ore-forming granites are high-K calc-alkaline, metaluminous to slightly peraluminous I-type granites. The pre-mineralization granites are relatively unfractionated, whereas the ore-forming granites are highly fractionated. The fractionating mineral phases are probably K-feldspar, apatite, Ti-bearing phases and minor plagioclase. The pre-mineralization and ore-forming rocks are characterized by similar Sr-Nd-Pb-Hf-O isotopic compositions ((87Sr/86Sr)i = 0.70308-0.70501, εNd (t) = - 0.5 to + 2.8, 207Pb/204Pb = 15.60-15.82, zircon εHf (t) = + 1.2 to + 15.6 and δ18O = + 4.6 to + 10.3‰), whole rock TDMC (Nd) (840-1120 Ma) and zircon TDMC (Hf) (320-1240 Ma). The isotopic characteristics are consistent with a hybrid magma source caused by 10-30% assimilation of ancient crust by juvenile lower crust. The geochronology and geochemistry of these granites show that the Late Carboniferous pre-mineralization granitic rocks formed during subduction, whereas the Early Permian ore-forming, highly fractionated granite probably underwent significant fractionation with a restite assemblage of K-feldspar, apatite, Ti-bearing phases and minor plagioclase and developed during collision between the Yili and Kazakhstan terranes commenced at the latest Late Carboniferous.

The formation and rejuvenation of continental crust in the central North China Craton: Evidence from zircon U-Pb geochronology and Hf isotope

NASA Astrophysics Data System (ADS)

Li, Qing; Santosh, M.; Li, Sheng-Rong; Guo, Pu

2014-12-01

The Trans-North China Orogen (TNCO) along the central part of the North China Craton (NCC) is considered as a Paleoproterozoic suture along which the Eastern and Western Blocks of the NCC were amalgamated. Here we investigate the Precambrian crustal evolution history in the Fuping segment of the TNCO and the subsequent reactivation associated with extensive craton destruction during Mesozoic. We present zircon LA-ICP-MS U-Pb and Lu-Hf data on TTG (tonalite-trondhjemite-granodiorite) gneiss, felsic orthogneiss, amphibolite and granite from the Paleoproterozoic suite which show magmatic ages in the range of 2450-1900 Ma suggesting a long-lived convergent margin. The εHf(t) values of these zircons range from -11.9 to 12 and their model ages suggest magma derivation from both juvenile components and reworked Archean crust. The Mesozoic magmatic units in the Fuping area includes granite, diorite and mafic microgranular enclaves, the zircons from which define a tight range of 120-130 Ma ages suggesting a prominent Early Cretaceous magmatic event. However, the εHf(t) values of these zircons show wide a range from -30.3 to 0.2, indicating that the magmatic activity involved extensive rejuvenation of the older continental crust.

Pan-Africa/Pan-Brazilian detrital zircons in Lower Palaeozoic schists of SW Norway - enigmatic detrital zircon U-Pb ages

NASA Astrophysics Data System (ADS)

Zimmermann, Udo; Bjørheim, Maren; Clark, Chris

2013-04-01

We present Sensitive High Resolution Ion Microprobe (SHRIMP) U-Pb zircon age data from metasedimentary rocks (schists and quartzites) located in the town of Stavanger (SW Norway). The metasedimentary sequence is composed of schists, medium grained quartz-rich metawackes and quartzites. Quartzites and meta-quartz-wackes exhibit a mylonitic fabric with newly grown fine-grained muscovite defining the fabric. Accessory minerals are zircon, allanite, detrital apatite, monazite, ilmenite, rutile and zircon. The schists are dark and dominated by quartz and feldspar in a fine chloritic and silica-rich matrix and represent the dominant lithology of the region. While quartzites and metawackes show typical geochemical characteristics for strongly reworked rocks, the schists have very low Zr/Sc and Th/Sc ratios below 0.9 and point together with other trace element ratios (La/Sc, Ti/Zr) to the strong influence of less fractionated, mafic, sources in the detritus, possibly arc derived. U-Pb ages of detrital zircon from quartzites range between 740 to 1800 Ma. There is a defined population at 1135 and 1010 Ma tentatively correlated with the Sveconorwegian orogeny. A second population at ~1450 Ma that can be related to a tectono-magmatic event during the Earliest Mesoproterozoic, also recorded in Oslo, southern Sweden and Bornholm, mapped along the proposed southern margin of Baltica. Other detrital zircons record ages between 1586 - 1664 Ma that are not related to the latter event. The oldest U-Pb detrital zircon grain age was 1796 Ma and is potentially associated with the terminal phase of the Svecofennian orogeny. Detrital zircons from the associated schists do show a similar abundance of main age clusters but the oldest found zircons dates to 2013 Ma while the maximum depositional age could be determined by grains of Cambrian to even Ordovician ages with a large 1 sigma error, as such that we rather propose a Cambrian maximum depositional age. It is possible to speculate that the black schists are an equivalent of the Alum shale successions, which is exposed in the Oslo region, southern Sweden and Bornholm (Denmark) and would be then belong to the margin of Baltica. However, detrital zircons with Ediacaran to Lower Palaeozoic ages are exotic to Baltica, and especially unexpected for the proposed passive margin. Magmatic events in SW Baltica of such an age are yet unknown, besides the intrusion of mafic dykes which cannot account for this large number of detrital zircons in the schists. Hence, there are several possibilities to explain this population: 1. The source area was not in Baltica and this sliver of schists is exotic to Baltica and was accreted during the Caledonian orogeny as the rocks show Caledonian deformation and metamorphism. 2. The depositional area had been in Baltica but the source area has drifted away and the schists are younger than Middle Cambrian, possibly Caledonian. 3. The schists are one of the few relicts which reflect magmatic events of Ediacaran and Lower Paleozoic ages (pre-Caledonian) in Baltica, which we have not been aware of so far and for which we have no geodynamic explanation (as the current opinion interprets a passive margin at the western boundary of Baltica) and might indicate unexpectedly young rift magmatism. If possibility (1) is taken into account then the candidates for the origin are somewhat restricted to Gondwana as on the eastern margin of Laurentia massive magmatism of Ediacaran to Lower Paleozoic ages is as well not well constrained.

Late Mesoproterozoic to Early Paleozoic history of metamorphic basement from the southeastern Chiapas Massif Complex, Mexico, and implications for the evolution of NW Gondwana

NASA Astrophysics Data System (ADS)

Weber, Bodo; González-Guzmán, Reneé; Manjarrez-Juárez, Román; Cisneros de León, Alejandro; Martens, Uwe; Solari, Luigi; Hecht, Lutz; Valencia, Victor

2018-02-01

In this paper, U-Pb zircon geochronology, Lu-Hf and Sm-Nd isotope systematics, geochemistry and geothermobarometry of metaigneous basement rocks exposed in the southeastern Chiapas Massif Complex are presented. Geologic mapping of the newly defined "El Triunfo Complex" located at the southeastern edge of the Chiapas Massif reveals (1) partial melting of a metamorphic basement mainly constituted by mafic metaigneous rocks (Candelaria unit), (2) an Ediacaran metasedimentary sequence (Jocote unit), and (3) occurrence of massif-type anorthosite. All these units are intruded by undeformed Ordovician plutonic rocks of the Motozintla suite. Pressure and temperature estimates using Ca-amphiboles, plagioclase and phengite revealed prograde metamorphism that reached peak conditions at 650 °C and 6 kbar, sufficient for partial melting under water saturated conditions. Relict rutile in titanite and clinopyroxene in amphibolite further indicate a previous metamorphic event at higher P-T conditions. U-Pb zircon ages from felsic orthogneiss boudins hosted in deformed amphibolite and migmatite yield crystallization ages of 1.0 Ga, indicating that dry granitic protoliths represent remnants of Rodinia-type basement. Additionally, a mid-Tonian ( 920 Ma) metamorphic overprint is suggested by recrystallized zircon from a banded gneiss. Zircon from folded amphibolite samples yield mainly Ordovician ages ranging from 457 to 444 Ma that are indistinguishable from the age of the undeformed Motozintla plutonic suite. Similar ages between igneous- and metamorphic- zircon suggest a coeval formation during a high-grade metamorphic event, in which textural discrepancies are explained in terms of differing zircon formation mechanisms such as sub-solidus recrystallization and precipitation from anatectic melts. In addition, some amphibolite samples contain inherited zircon yielding Stenian-Tonian ages around 1.0 Ga. Lu-Hf and Sm-Nd isotopes and geochemical data indicate that the protoliths of the amphibolite have E-MORB characteristics and were derived from a depleted mantle source younger than the Rodinia-type basement. Inasmuch as similar amphibolites also occur in the Ediacaran metasedimentary rocks as dykes or lenses, Late Neoproterozoic magmatism in a rift setting is suggested. Hence, the geologic record of the El Triunfo Complex includes evidences for Rodinia assemblage, Tonian circum-Rodinia subduction, and breakup during the Late Neoproterozoic. Metamorphism, and partial melting are interpreted in terms of a convergent margin setting during the Ordovician. The results place the southern Chiapas Massif along with Oaxaquia and similar Northern Andes terranes on the NW margin of Gondwana interpreted as the extension of the Famatinian orogen that evolved during the closure of the Iapetus Ocean.

Early cretaceous lower crustal reworking in NE China: insights from geochronology and geochemistry of felsic igneous rocks from the Great Xing'an range

NASA Astrophysics Data System (ADS)

Li, Yinglei; Liu, Huichuan; Huangfu, Pengpeng; He, Hongyun; Liu, Yongzheng

2018-01-01

This paper presents new zircon LA-ICP-MS U-Pb ages and whole-rock geochemical data for two granitic plutons and rhyolites of the Baiyingaolao Formation in the western Xing'an range (NE China). The two syenogranite granitic plutons yield identical zircon U-Pb age of 142 ± 1 Ma, and the Baiyingaolao rhyolites yield zircon U-Pb age of 138 ± 2 Ma. The granites contain some hornblendes, and show low Zr and Zr + Nb + Ce + Y contents, and low A/CNK (0.98-1.11), Mg# (6-55), and FeOT/MgO values. Rhyolite samples show similar geochemical characteristics with A/CNK of 0.99-1.10 and Mg# of 14-21. In combination with the high K2O contents (4.43-5.61 wt%) and negative correlations between P2O5 and SiO2, both the granites and rhyolites were classified as high-K calc-alkaline I-type granitoids. All samples give high zirconium saturation temperature of 794-964 °C with few initially inherited zircons, and belong to high-temperature I-type granitoids. They were generated by dehydration melting of biotite/muscovite from sub-alkaline meta-basalts in lower crust depth, leaving garnet, amphibole, and plagioclase as the major residual minerals. The syenogranites and rhyolites are likely formed in Mongol-Okhotsk oceanic subduction setting. Incorporating other lower crust-originated felsic rocks in Erguna and Xing'an massifs and Songliao basin, it is argued that lower crustal reworking is pronounced in NE China during Early Cretaceous.

New U-Pb zircon ages and the duration and division of Devonian time

USGS Publications Warehouse

Tucker, R.D.; Bradley, D.C.; Ver Straeten, C.A.; Harris, A.G.; Ebert, J.R.; McCutcheon, S.R.

1998-01-01

Newly determined U-Pb zircon ages of volcanic ashes closely tied to biostratigraphic zones are used to revise the Devonian time-scale. They are: 1) 417.6 ?? 1.0 Ma for an ash within the conodont zone of Icriodus woschmidti/I. w. hesperius Lochkovian); 2) 408.3 ?? 1.9 Ma for an ash of early Emsian age correlated with the conodont zones of Po. dehiscens--Lower Po. inversus; 3) 391.4 ?? 1.8 Ma for an ash within the Po. c. costatus Zone and probably within the upper half of the zone (Eifelian); and 4) 381.1 ?? 1.3 Ma for an ash within the range of the Frasnian conodont Palmatolepis punctata (Pa. punctata Zone to Upper Pa. hassi Zone). U-Pb zircon ages for two rhyolites bracketing a palyniferous bed of the pusillites-lepidophyta spore zone, are dated at 363.8 ?? 2.2 Ma and 363 ?? 2.2 Ma and 363.4 ?? 1.8 Ma, respectively, suggesting an age of ~363 Ma for a level within the late Famennian Pa. g. expansa Zone. These data, together with other published zircon ages, suggest that the base and top of the Devonian lie close to 418 Ma and 362 Ma, respectively, thus lengthening the period of ~20% over current estimates. We suggest that the duration of the Middle Devonian (Eifelian and Givitian) is rather brief, perhaps no longer than 11.5 Myr (394 Ma-382.5 Ma), and that the Emsian and Famennian are the longest stages in the period with estimated durations of ~15.5 Myr and 14.5 Myr, respectively.

Age and origin of the Merrimack terrane, southeastern New England: A detrital zircon U-Pb geochronology study

NASA Astrophysics Data System (ADS)

Sorota, Kristin

Metasedimentary rocks of the Merrimack terrane (MT) originated as a thick cover sequence on Ganderia consisting of sandstones, calcareous sandstones, pelitic rocks and turbidites. In order to investigate the age, provenance and stratigraphic order of these rocks and correlations with adjoining terranes, detrital zircon suites from 7 formations across the MT along a NNE-trending transect from east-central Massachusetts to SE New Hampshire were analyzed by U-Pb LA-ICP-MS methods on 90-140 grains per sample. The youngest detrital zircons in the western units, the Worcester, Oakdale and Paxton Formations, are ca. 438 Ma while those in the Kittery, Eliot and Berwick Formations in the northeast are ca. 426 Ma. The Tower Hill Formation previously interpreted to form the easternmost unit of the MT in MA, has a distinctly different zircon distribution with its youngest zircon population in the Cambrian. All samples except for the Tower Hill Formation have detrital zircon age distributions with significant peaks in the mid-to late Ordovician, similar abundances of early Paleozoic and late Neoproterozoic zircons, significant input from ˜1.0 to ˜1.8 Ga sources and limited Archean grains. The similarities in zircon provenance suggest that all units across the terrane, except for the Tower Hill Formation, belong to a single sequence of rocks, with similar sources and with the units in the NE possibly being somewhat younger than those in east-central Massachusetts. The continuous zircon age distributions observed throughout the Mesoproterozoic and late Paleoproterozoic are consistent with an Amazonian source. All samples, except the Tower Hill Formation, show sedimentary input from both Ganderian and Laurentian sources and suggest that Laurentian input increases as the maximum depositional age decreases.

Age and thermal history of the Geysers plutonic complex (felsite unit), Geysers geothermal field, California: A 40Ar/39Ar and U-Pb study

USGS Publications Warehouse

Dalrymple, G.B.; Grove, M.; Lovera, O.M.; Harrison, T.M.; Hulen, J.B.; Lanphere, M.A.

1999-01-01

Sixty-nine ion microprobe spot analyses of zircons from four granite samples from the plutonic complex that underlies the Geysers geothermal field yield 207Pb/206Pb vs. 238U/206Pb concordia ages ranging from 1.13 ?? 0.04 Ma to 1.25 ?? 0.04 (1??) Ma. The weighted mean of the U/Pb model ages is 1.18 ?? 0.03 Ma. The U-Pb ages coincide closely with 40Ar/39Ar age spectrum plateau and 'terminal' ages from coexisting K-feldspars and with the eruption ages of overlying volcanic rocks. The data indicate that the granite crystallized at 1.18 Ma and had cooled below 350??C by ~0.9-1.0 Ma. Interpretation of the feldspar 40Ar/39Ar age data using multi-diffusion domain theory indicates that post-emplacement rapid cooling was succeeded either by slower cooling from 350??to 300??C between 1.0 and 0.4 Ma or transitory reheating to 300-350??C at about 0.4-0.6 Ma. Subsequent rapid cooling to below 260??C between 0.4 and 0.2 Ma is in agreement with previous proposals that vapor-dominated conditions were initiated within the hydrothermal system at this time. Heat flow calculations constrained with K-feldspar thermal histories and the present elevated regional heat flow anomaly demonstrate that appreciable heat input from sources external to the known Geysers plutonic complex is required to maintain the geothermal system. This requirement is satisfied by either a large, underlying, convecting magma chamber (now solidified) emplaced at 1.2 Ma or episodic intrusion of smaller bodies from 1.2 to 0.6 Ma.

Hydrothermal titanite from the Chengchao iron skarn deposit: temporal constraints on iron mineralization, and its potential as a reference material for titanite U-Pb dating

NASA Astrophysics Data System (ADS)

Hu, Hao; Li, Jian-Wei; McFarlane, Christopher R. M.

2017-09-01

Uranium-lead isotopes and trace elements of titanite from the Chengchao iron skarn deposit (Daye district, Eastern China), located along the contact zones between Triassic marine carbonates and an early Cretaceous intrusive complex consisting of granite and quartz diorite, were analyzed using laser ablation inductively coupled plasma mass spectrometry to provide temporal constraints on iron mineralization and to evaluate its potential as a reference material for titanite U-Pb geochronology. Titanite grains from mineralized endoskarn have simple growth zoning patterns, exhibit intergrowth with magnetite, diopside, K-feldspar, albite and actinolite, and typically contain abundant primary two-phase fluid inclusions. These paragenetic and textural features suggest that these titanite grains are of hydrothermal origin. Hydrothermal titanite is distinct from the magmatic variety from the ore-related granitic intrusion in that it contains unusually high concentrations of U (up to 2995 ppm), low levels of Th (12.5-453 ppm), and virtually no common Pb. The REE concentrations are much lower, as are the Th/U and Lu/Hf ratios. The hydrothermal titanite grains yield reproducible uncorrected U-Pb ages ranging from 129.7 ± 0.7 to 132.1 ± 2.7 Ma (2σ), with a weighted mean of 131.2 ± 0.2 Ma [mean standard weighted deviation (MSWD) = 1.7] that is interpreted as the timing of iron skarn mineralization. This age closely corresponds to the zircon U-Pb age of 130.9 ± 0.7 Ma (MSWD = 0.7) determined for the quartz diorite, and the U-Pb ages for zircon and titanite (130.1 ± 1.0 Ma and 131.3 ± 0.3 Ma) in the granite, confirming a close temporal and likely genetic relationship between granitic magmatism and iron mineralization. Different hydrothermal titanite grains have virtually identical uncorrected U-Pb ratios suggestive of negligible common Pb in the mineral. The homogeneous textures and U-Pb characteristics of Chengchao hydrothermal titanite suggest that the mineral might be a suitable internal reference material for U-Pb dating.

Cogenetic late Pleistocene rhyolite and cumulate diorites from Augustine Volcano revealed by SIMS 238U-230Th dating of zircon, and implications for silicic magma generation by extraction from mush

USGS Publications Warehouse

Coombs, Michelle L.; Vazquez, Jorge A.

2014-01-01

Augustine Volcano, a frequently active andesitic island stratocone, erupted a late Pleistocene rhyolite pumice fall that is temporally linked through zircon geochronology to cumulate dioritic blocks brought to the surface in Augustine's 2006 eruption. Zircon from the rhyolite yield a 238U-230Th age of ∼25 ka for their unpolished rims, and their interiors yield a bimodal age populations at ∼26 ka and a minority at ∼41 ka. Zircon from dioritic blocks, ripped from Augustine's shallow magmatic plumbing system and ejected during the 2006 eruption, have interiors defining a ∼26 ka age population that is indistinguishable from that for the rhyolite; unpolished rims on the dioritic zircon are dominantly younger (≤12 ka) indicating subsequent crystallization. Zircon from rhyolite and diorite overlap in U, Hf, Ti, and REE concentrations although diorites also contain a second population of high-U, high temperature grains. Andesites that brought dioritic blocks to the surface in 2006 contain zircon with young (≤9 ka) rims and a scattering of older ages, but few zircon that crystallized during the 26 ka interval. Both the Pleistocene-age rhyolite and the 2006 dioritic inclusions plot along a whole-rock compositional trend distinct from mid-Holocene–present andesites and dacites, and the diorites, rhyolite, and two early Holocene dacites define linear unmixing trends often oblique to the main andesite array and consistent with melt (rhyolite) extraction from a mush (dacites), leaving behind a cumulate amphibole-bearing residue (diorites). Rare zircon antecrysts up to ∼300 ka from all rock types indicate that a Quaternary center has been present longer than preserved surficial deposits.

U-Pb zircon geochronology and evolution of some Adirondack meta-igneous rocks

NASA Technical Reports Server (NTRS)

Mclelland, J. M.

1988-01-01

An update was presented of the recent U-Pb isotope geochronology and models for evolution of some of the meta-igneous rocks of the Adirondacks, New York. Uranium-lead zircon data from charnockites and mangerites and on baddeleyite from anorthosite suggest that the emplacement of these rocks into a stable crust took place in the range 1160 to 1130 Ma. Granulite facies metamorphism was approximately 1050 Ma as indicated by metamorphic zircon and sphene ages of the anorthosite and by development of magmatitic alaskitic gneiss. The concentric isotherms that are observed in this area are due to later doming. However, an older contact metamorphic aureole associated with anorthosite intrusion is observed where wollastonite develops in metacarbonates. Zenoliths found in the anorthosite indicate a metamorphic event prior to anorthosite emplacement. The most probable mechanism for anorthosite genesis is thought to be ponding of gabbroic magmas at the Moho. The emplacement of the anorogenic anorthosite-mangerite-charnockite suite was apparently bracketed by compressional orogenies.

Temporal Evolution of Volcanic and Plutonic Magmas Related to Porphyry Copper Ores Based on Zircon Geochemistry

NASA Astrophysics Data System (ADS)

Dilles, J. H.; Lee, R. G.; Wooden, J. L.; Koleszar, A. M.

2015-12-01

Porphyry Cu (Mo-Au) and epithermal Au-Ag ores are globally associated with shallow hydrous, strongly oxidized, and sulfur-rich arc intrusions. In many localities, long-lived magmatism includes evolution from early andesitic volcanic (v) and plutonic (p) rocks to later dacitic or rhyolitic compositions dominated by plutons. We compare zircon compositions from three igneous suites with different time spans: Yerington, USA (1 m.y., p>v), El Salvador, Chile (4 m.y., p>v), and Yanacocha, Peru (6 m.y., v>p). At Yerington granite dikes and ores formed in one event, at ES in 2 to 3 events spanning 3 m.y., and at Yanacocha in 6 events spanning 5 m.y. At both ES and Yanacocha, high-Al amphiboles likely crystallized at high temperature in the mid-crust and attest to deep magmas that periodically recharged the shallow chambers. At Yanacocha, these amphiboles contain anhydrite inclusions that require magmas were sulfur-rich and strongly oxidized (~NNO+2). The Ti-in-zircon geothermometer provides estimates of 920º to 620º C for zircon crystallization, and records both core to rim cooling and locally high temperature rim overgrowths. Ore-related silicic porphyries yield near-solidus crystallization temperatures of 750-650°C consistent with low zircon saturation temperatures. The latter zircons have large positive Ce/Ce* and small negative Eu/Eu*≥0.4 anomalies attesting to strongly oxidized conditions (Ballard et al., 2001), which we propose result from crystallization and SO2 loss to the magmatic-hydrothermal ore fluid (Dilles et al., 2015). The Hf, REE, Y, U, and Th contents of zircons are diverse in the magma suites, and Th/U vs Yb/Gd plots suggest a dominant role of crystal fractionation with lesser roles for both crustal contamination and mixing with high temperature deep-sourced mafic magma. Ce/Sm vs Yb/Gd plots suggest that magma REE contents at <900°C are dominated by early crystallization of hornblende and apatite, and late crystallization (~<780°C) of titanite. Magma mixing and crustal contamination are most evident in pre-ore magmas, whereas ore-forming intrusions at low temperatures are dominated by crystal fractionation. Thus, zircon provides evidence for cyclic crystallization and mafic recharge that enrich late silicic melts in incompatible ore components water, sulfur, chlorine and metals.

Newly Described Tephra Provide Middle Pleistocene Age Constraints to Stegodon Fossils in West (Indonesian) Timor

NASA Astrophysics Data System (ADS)

Jensen, B. J. L.; Dufrane, A.; Mark, D.; Zaim, Y.; Rizal, Y.; Aswan, A.; Hascaryo, A.; Ciochon, R.; Gunnell, G.; Larick, R.; Zonnveld, J. P.

2017-12-01

As the Asian proboscidian Stegodon dispersed across Island Southeast Asia during the Pleistocene, multiple forms developed. On Timor, a southerly island east of Wallace's Line, the Ainaro gravels have yielded a highly dwarfed S. timorensis and a larger S. `trigonocephalus.' During a half-century of exploration, the age of the fossil bearing gravels remains in question, with only one age determination of >130 ka derived from six 230Th- 238U dates on a tusk fragment found in the Raebia area (Louys et al. 2016). Here we present radiometric ages for two tephra deposits bracketing Ainaro gravels at Raebia, a S. timorensis fossil locality 8 km northeast of Atambua city. The Raebia ravine exposes 2-10 meters of coarse-grained gravels incised into silt and clay deposits, bracketed by two indurated and largely devitrified tephras. Some intact glass was present to geochemically characterize each unit, which are both high-silica rhyolites. Biotite and zircons for 40Ar/39Ar and laser ablation U-Pb dating were extracted from the upper unit (Raebia Tuff 1; RT1), and zircons from the lower unit (Raebia Tuff 2; RT2). RT1 had zircons with two distinct age populations, but the youngest yield a 230Th deficiency corrected 206Pb/238U age of 665 ± 19 ka, (2s, n = 23, MSWD = 0.81), consistent with the 40Ar/39Ar age 614.9 ± 16.4 ka (2s, full external precision). Preliminary zircon dates on RT2 are more problematic, providing a large range that suggests inheritance by xenoliths and/or locally-sourced detrital zircons. However, a single zircon yielded 230Th deficiency corrected 206Pb/238U age of 708 ± 66 ka (2s, n=17, MSWD = 0.41), which is stratigraphically consistent. These are the first reliable age constraints on a higher elevation Ainaro gravel terrace and fossils they contain. The only other direct ages on the gravels are 230Th- 238U dates on lower terraces interbedded with coral, ranging from 130 ka to Holocene in age (Roosmawati and Harris 2009). These two newly described and dated tephra are likely regionally distributed and may represent important stratigraphic horizons for this portion of Southern Wallacea. They also provide useful data for calculating uplift rates for the region from the middle Pleistocene.

A hidden Late Cretaceous arc and subsequent magmatic events in the Caucasus-Iran-Anatolia (CIA) orogenic belt: Detrital zircon U-Pb and Hf isotopic constraints

NASA Astrophysics Data System (ADS)

Tien, C. Y.; Lin, Y. C.; Chu, M. F.; Chung, S. L.; Bi˙ngöl, A. F.

2017-12-01

The Caucasus-Iran-Anatolia (CIA) orogenic belt formed by "Turkic-type orogeny" consists mainly of subduction-accretion complexes following the collision between Eurasia and Arabia and the closure of Neotethy. This study reports U-Pb and Hf isotopic data of detrital zircon separates from five Eocene to mid-Miocene sandstone samples from Divrigi and Duranlar in the west to the Mus basin in the east, all locating in the northern part of the Bitlis-Zagros suture zone. The U-Pb age data suggest four main magmatic episodes: (1) 100-70 Ma, (2) 60-40 Ma, (3) 30 Ma, and (4) 15 Ma. The Late Cretaceous zircons recovered mainly from the Mus basin are marked by a significant Hf isotopic variation over time, with ɛHf(T) values dropping from +15 to -10. Zircons from the second and third episodes show spatial variations in isotopic compositions, with positive ɛHf(T) values (+10 to +5) in the Mus basin and heterogeneous ɛHf(T) values (+10 to -10) in the west. The fourth and youngest episode of zircons, mainly from Duranlar area, shows uniform ɛHf(T) values around +5. We attribute the Late Cretaceous episode of zircons to the broadly coeval Elazig arc magmatism that, according to our counterpart study, occurred as a short-lived, intra-oceanic arc system by subduction initiation after the formation of Neotethyan ophiolites in the region. Moreover, we argue that this Late Cretaceous arc system may have existed more widely within the southern branch of Neothethys than that suggested by present-day outcrops. The dramatic change in Hf isotopic composition from 100 to 70 Ma, also observed in the rock record by our counterpart study, may be interpreted as a result of subduction to accretion processes. The remaining three episodes of zircons are related to younger stages of magmatism within or around the suture zone that remains poorly studied. Our results indicate that detrital zircon is a useful tool to uncover "hidden" magmatic records in the CIA and other "Turkic-type" orogenic belts where complex interaction of multiple micro-terrains may have taken place during accretionary and collisional orogenesis.

Discovery of Miocene adakitic dacite from the Eastern Pontides Belt (NE Turkey) and a revised geodynamic model for the late Cenozoic evolution of the Eastern Mediterranean region

NASA Astrophysics Data System (ADS)

Eyuboglu, Yener; Santosh, M.; Yi, Keewook; Bektaş, Osman; Kwon, Sanghoon

2012-08-01

The Cenozoic magmatic record within the ca. 500 km long eastern Pontides orogen, located within the Alpine metallogenic belt, is critical to evaluate the tectonic history and geodynamic evolution of the eastern Mediterranean region. In this paper we report for the first time late Miocene adakitic rocks from the southeastern part of the eastern Pontides belt and present results from geochemical and Sr-Nd isotopic studies as well as zircon U-Pb geochronology. The Tavdagi dacite that we investigate in this study is exposed as round or ellipsoidal shaped bodies, sills, and dikes in the southeastern part of the belt. Zircons in the dacite show euhedral crystal morphology with oscillatory zoning and high Th/U values (up to 1.69) typical of magmatic origin. Zircon LA-ICPMS analysis yielded a weighted mean 206Pb/238U age of 7.86 ± 0.15 Ma. SHRIMP analyses of zircons with typical magmatic zoning from another sample yielded a weighted mean 206Pb/238U age of 8.79 ± 0.19 Ma. Both ages are identical and constrain the timing of dacitic magmatism as late Miocene. The Miocene Tavdagi dacite shows adakitic affinity with high SiO2 (68.95-71.41 wt.%), Al2O3 (14.88-16.02 wt.%), Na2O (3.27-4.12 wt.%), Sr (331.4-462.1 ppm), Sr/Y (85-103.7), LaN/YbN (34.3-50.9) and low Y (3.2-5 ppm) values. Their initial 143Nd/144Nd (0.512723-0.512736) and 87Sr/86Sr (0.70484-0.70494) ratios are, respectively, lower and higher than those of normal oceanic crust. The geological, geochemical and isotopic data suggest that the adakitic magmatism was generated by partial melting of the mafic lower crust in the southeastern part of the eastern Pontide belt during the late Miocene. Based on the results presented in this study and a synthesis of the geological and tectonic information on the region, we propose that the entire northern edge of the eastern Pontides-Lesser Caucasus-Elbruz magmatic arc was an active continental margin during the Cenozoic. We identify a migration of the Cenozoic magmatism towards north over time resulting from the roll-back of the southward subducted Tethys oceanic lithosphere. Slab break-off during Pliocene is proposed to have triggered asthenospheric upwelling and partial melting of the subduction-modified mantle wedge which generated the alkaline magmatic rocks exposed in the northern part of the magmatic arc.

Opening the closed box: lattice diffusion in zircon?

NASA Astrophysics Data System (ADS)

Wheeler, J.; MacDonald, J.; Goodenough, K. M.; Crowley, Q.; Harley, S.; Mariani, E.

2015-12-01

In principle, any radiogenic parent or daughter element can diffuse through any crystalline lattice. Given improved analytic techniques and mathematical models, geochronology is beginning to take such diffusion into account in a quantitative fashion. Whilst lattice diffusion compromises simple interpretation of radiometric data, it can, when combined with spatially resolved data, provide more detailed insight into thermal histories. In regions that have experienced particularly high temperatures diffusion may become significant in minerals normally thought to be reliably closed. We have modelled Pb diffusion in zircon, building on earlier work on Ar diffusion in micas - the mathematics being basically the same. We are motivated by some challenging isotope data from zircon in the Lewisian Complex of NW Scotland (a TTG region with a long Archaean and Proterozoic history). For example we have grains with old rims and younger cores. Whilst other explanations are possible, we show how lattice diffusion of Pb is plausible, using experimental diffusion data together with estimates of ultra-high temperatures from the region. We have modified a previous model for Ar diffusion ("Diffarg") to include variations in parent isotope concentration, so we can understand the consequences of U zonation within zircon grains during prolonged thermal histories. This is also relevant to asking why Pb has apparently not diffused in zircon from other UHT regions - or has it?

The mesoproterozoic Beaverhead impact structure and its tectonic setting, Montana-Idaho: 40Ar/39 and U-Pb isotopic constraints

USGS Publications Warehouse

Kellogg, K.S.; Snee, L.W.; Unruh, D.M.

2003-01-01

New 40Ar/39Ar and uranium-lead (U-Pb) zircon data from the Beaverhead impact structure, first identified by extensive shatter coning of Proterozoic quartzite and gneiss from the Beaverhead Mountains near the Montana-Idaho border, indicate that the structure formed at or after 900 Ma. The 40Ar/39Ar age spectra from fine-grained muscovite and biotite from a breccia zone in high-grade gneiss show significant argon loss but yield dates for highest-temperature steps that cluster between 899 and 908 Ma. The dated minerals probably formed by recrystallization of impact glass, so on both geologic and isotopic grounds, the dates probably represent the minimum age of impact. U-Pb data for zircons from the same breccia are strongly discordant and yield an upper intercept apparent age of 2464 ?? 56 Ma and a lower intercept apparent age of 779 ?? 69 Ma. Another brecciated gneiss about 7 km to the northeast that does not contain secondary mica does contain zircons that yield a concordant apparent age of 2455 ?? 9 Ma. Nearby gneiss that neither is brecciated nor contains shatter cones yields an apparent age of 2451 ?? 46 Ma. The 40Ar/39Ar results constrain the age of the shatter-coned quartzite and indicate that it is >900 Ma and possibly correlative with the Gunsight Formation of the Mesoproterozoic Lemhi Group. The upper intercept U-Pb age of ???2450 Ma from all three dated samples also shows that the Paleoproterozoic basement rocks of the area are among the youngest in the mostly Archean Wyoming province of North America. The impact site lies near the margin of the province, along the northeast-trending Great Falls tectonic zone, and the relatively young crustal age may reflect Early Proterozoic marginal accretion.

Improved 206Pb/238U microprobe geochronology by the monitoring of a trace-element-related matrix effect; SHRIMP, ID-TIMS, ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards

USGS Publications Warehouse

Black, L.P.; Kamo, S.L.; Allen, C.M.; Davis, D.W.; Aleinikoff, J.N.; Valley, J.W.; Mundil, R.; Campbell, I.H.; Korsch, R.J.; Williams, I.S.; Foudoulis, C.

2004-01-01

Precise isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) documentation is given for two new Palaeozoic zircon standards (TEMORA 2 and R33). These data, in combination with results for previously documented standards (AS3, SL13, QGNG and TEMORA 1), provide the basis for a detailed investigation of inconsistencies in 206Pb/238U ages measured by microprobe. Although these ages are normally consistent between any two standards, their relative age offsets are often different from those established by ID-TIMS. This is true for both sensitive high-resolution ion-microprobe (SHRIMP) and excimer laser ablation-inductively coupled plasma-mass spectrometry (ELA-ICP-MS) dating, although the age offsets are in the opposite sense for the two techniques. Various factors have been investigated for possible correlations with age bias, in an attempt to resolve why the accuracy of the method is worse than the indicated precision. Crystallographic orientation, position on the grain-mount and oxygen isotopic composition are unrelated to the bias. There are, however, striking correlations between the 206Pb/238U age offsets and P, Sm and, most particularly, Nd abundances in the zircons. Although these are not believed to be the primary cause of this apparent matrix effect, they indicate that ionisation of 206Pb/238U is influenced, at least in part, by a combination of trace elements. Nd is sufficiently representative of the controlling trace elements that it provides a quantitative means of correcting for the microprobe age bias. This approach has the potential to reduce age biases associated with different techniques, different instrumentation and different standards within and between laboratories. Crown Copyright ?? 2004 Published by Elsevier B.V. All rights reserved.

The pre-orogenic detrital zircon record of the Variscan orogeny: Preliminary results

NASA Astrophysics Data System (ADS)

Stephan, Tobias; Kroner, Uwe

2017-04-01

To test plate-tectonic constellations in consideration of the long-term development of sedimentary transport paths, temporally and spatially highly resolved records of provenance analysis are mandatory. The interpretation of existing studies focus on small-scale areas within an orogen thereby neglecting the differing distribution of provenance data in the entire orogenic system. This study reviews a large data set of compiled geochronological data to document the development of pre-orogenic tectonic units on the example of the Variscan orogeny. Constrained by tectonic and geological models, the temporal distribution of U-Pb detrital zircon ages, used as a proxy for sedimentary provenance, shows that some minima and maxima of zircon abundance are nearly synchronous for thousands of kilometres along the orogeny. Age spectra of Precambrian to Lower Palaeozoic samples were constructed on the basis of 38729 U-Pb ages from 685 samples that were compiled from 102 publications. The age compilation combines thermal ionization mass spectrometry (TIMS), laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS), sensitive high-resolution ion microprobe (SHRIMP), and secondary ion mass spectrometry (SIMS) analyses. The data was re-processed using a common age calculation and concordance filter to ensure comparability. The concordance of each zircon grain was calculated from 206Pb/238U and 207Pb/235U ages to guarantee that only concordant grains, i.e., with <10% normal and <5% reverse discordance, were included in the age compilation. In order to ignore a metamorphic overprint and hence a blur of the younger age spectra, the compilation is constrained to age data older than 400 Ma only. If a precise sample age is not documented by the author, the weighted-mean age of the youngest zircon population (n > 3) is used for the maximum age of deposition. In addition to the location of >600 samples, the precise depositional ages result in a spatially and temporally high resolution. To avoid the different levels of analytical precision of the compiled TIMS, LA-ICP-MS, SHRIMP, and SIMS data, detrital zircon ages are plotted as kernel density estimates. Spatial and temporal distribution of the kernel density estimates, as well as further statistical techniques (e.g. multidimensional scaling) are used to discriminate groups of similar age distributions. Preliminary results reveal four major sources for the pre-orogenic sedimentary units (i.e. Saharan Metacraton, West-African craton, Amazonas craton and Fennoscandian shield). The mixing of several source signals in Gondwana derived sediment spectra point to vast deltaic systems along the Gondwanan shelf area.

Micrometer-scale U–Pb age domains in eucrite zircons, impact re-setting, and the thermal history of the HED parent body

USGS Publications Warehouse

Hopkins, M.D.; Mojzsis, S.J.; Bottke, W.F.; Abramov, Oleg

2015-01-01

Meteoritic zircons are rare, but some are documented to occur in asteroidal meteorites, including those of the howardite–eucrite–diogenite (HED) achondrite clan (Rubin, A. [1997]. Meteorit. Planet. Sci. 32, 231–247). The HEDs are widely considered to originate from the Asteroid 4 Vesta. Vesta and the other large main belt asteroids record an early bombardment history. To explore this record, we describe sub-micrometer distributions of trace elements (U, Th) and 235,238U–207,206Pb ages from four zircons (>7–40 μm ∅) separated from bulk samples of the brecciated eucrite Millbillillie. Ultra-high resolution (∼100 nm) ion microprobe depth profiles reveal different zircon age domains correlative to mineral chemistry and to possible impact scenarios. Our new U–Pb zircon geochronology shows that Vesta’s crust solidified within a few million years of Solar System formation (4561 ± 13 Ma), in good agreement with previous work (e.g. Carlson, R.W., Lugmair, G.W. [2000]. Timescales of planetesimal formation and differentiation based on extinct and extant radioisotopes. In: Canup, R., Righter, K. (Eds.), Origin of the Earth and Moon. University of Arizona Press, Tucson, pp. 25–44). Younger zircon age domains (ca. 4530 Ma) also record crustal processes, but these are interpreted to be exogenous because they are well after the effective extinction of 26Al (t1/2 = 0.72 Myr). An origin via impact-resetting was evaluated with a suite of analytical impact models. Output shows that if a single impactor was responsible for the ca. 4530 Ma zircon ages, it had to have been ⩾10 km in diameter and at high enough velocity (>5 km s−1) to account for the thermal field required to re-set U–Pb ages. Such an impact would have penetrated at least 10 km into Vesta’s crust. Later events at ca. 4200 Ma are documented in HED apatite 235,238U–207,206Pb ages (Zhou, Q. et al. [2011]. Early basaltic volcanism and Late Heavy Bombardment on Vesta: U–Pb ages of small zircons and phosphates in eucrites. Lunar Planet. Sci. 42. Abstract #2575) and 40–39Ar age spectra (Bogard, D.D. [2011]. Chem. Erde 71, 207–226). Yet younger ages, including those coincident with the Late Heavy Bombardment (LHB; ca. 3900 Ma), are absent from Millbillillie zircon. This is attributable to primordial changes to the velocity distributions of impactors in the asteroid belt, and differences in mineral closure temperatures (Tc zircon ≫ apatite).

Detrital Zircon U-Pb and Hf-isotope Constrains on Basement Ages, Granitic Magmatism, and Sediment Provenance in the Malay Peninsula

NASA Astrophysics Data System (ADS)

Sevastjanova, Inga; Clements, Benjamin; Hall, Robert; Belousova, Elena; Pearson, Norman; Griffin, William

2010-05-01

The Malay Peninsula forms the western part of central Sundaland in SE Asia. Sundaland comprises Indochina, the Thai-Malay Peninsula, Sumatra, Java, Borneo, and the shallow shelf between these landmasses. It is a composite region of continental crustal fragments that are separated by sutures that represent remnant ocean basins and volcanic arcs. The Malay Peninsula includes two of these fragments - East Malaya and Sibumasu - separated by the Bentong-Raub Suture Zone. The latter is a Palaeo-Tethyan ocean remnant. Granitoids of the Malay Peninsula are the major sources of detrital zircon in Sundaland. East Malaya is intruded by Permian-Triassic Eastern Province granitoids interpreted as products of Palaeozoic subduction of oceanic crust beneath the East Malaya Volcanic Arc. Sibumasu is intruded by Triassic Main Range Province granitoids interpreted as syn- to post-collisional magmatism following suturing to East Malaya. Locally, there are minor Late Cretaceous plutons. Basements of Sibumasu and East Malaya are not exposed and their ages are poorly constrained. The exact timing of the collision between these fragments is also contentious. In order to resolve these uncertainties, 752 U-Pb analyses from 9 samples were carried out on detrital zircons from modern rivers draining the Malay Peninsula and, of these, 243 grains from 6 samples were selected for Hf-isotope analyses. U-Pb zircon ages show that small numbers of Neoarchean-Proterozoic grains are consistently present in all samples, but do not form prominent populations. Permian-Triassic populations are dominant. Only one sample contains a small Jurassic population probably sourced from the area of Thailand and most likely recycled from fluvial-alluvial Mesozoic 'red-beds'. Late Cretaceous populations are locally abundant. Hf-isotope crustal model ages suggest that basement beneath the Malay Peninsula is heterogeneous. Some basement may be Neoarchean but there is no evidence for basement older than 2.8 Ga beneath the Malay Peninsula. Both Sibumasu and East Malaya basements are Paleoproterozoic, but of different ages. 176Hf/177Hfi ratios suggest that Permian-Triassic zircons were sourced from three major magmatic suites: (a) Permian crust-derived granitoids, (b) Early-Middle Triassic granitoids with a mixed mantle- and crust-derived source, and (c) Late Triassic crust-derived granitoids. This suggests three major Permian-Triassic episodes of magmatism in the Malay Peninsula. Two of these episodes (a and b) occurred in the Eastern Province. This suggests a multi-phase evolution of the East Malaya Volcanic Arc. Crust-derived zircon Hf-isotope signatures are unusual for a continental margin arc and may indicate contamination from older crust beneath the East Malaya fragment. A Late Permian-Early Triassic gap in magmatism and subsequent change of zircon source may indicate a micro-collision around 260-270 Ma (e.g. with an island arc or a seamount on the Paleo-Tethys oceanic crust). U-Pb ages and Hf-isotope composition of zircons sourced from the Main Range Province granitoids suggest that Sibumasu-East Malaya collision occurred by Late Triassic, but it is not clear when exactly this collision initiated. Different Hf-isotope signatures of Triassic zircons can be used as indicators of sediment provenance from the Malay Peninsula. Crust-derived signatures are diagnostic of Triassic zircons from the Main Range Province source, whereas mixed crust- and mantle-derived signatures of similar age zircons indicate Eastern Province source.

He, U, and Th Depth Profiling of Apatite and Zircon Using Laser Ablation Noble Gas Mass Spectrometry and SIMS

NASA Astrophysics Data System (ADS)

Monteleone, B. D.; van Soest, M. C.; Hodges, K. V.; Hervig, R.; Boyce, J. W.

2008-12-01

Conventional (U-Th)/He thermochronology utilizes single or multiple grain analyses of U- and Th-bearing minerals such as apatite and zircon and does not allow for assessment of spatial variation in concentration of He, U, or Th within individual crystals. As such, age calculation and interpretation require assumptions regarding 4He loss through alpha ejection, diffusive redistribution of 4He, and U and Th distribution as an initial condition for these processes. Although models have been developed to predict 4He diffusion parameters, correct for the effect of alpha ejection on calculated cooling ages, and account for the effect of U and Th zonation within apatite and zircon, measurements of 4He, U, and Th distribution have not been combined within a single crystal. We apply ArF excimer laser ablation, combined with noble gas mass spectrometry, to obtain depth profiles within apatite and zircon crystals in order to assess variations in 4He concentration with depth. Our initial results from pre-cut, pre-heated slabs of Durango apatite, each subjected to different T-t schedules, suggest a general agreement of 4He profiles with those predicted by theoretical diffusion models (Farley, 2000). Depth profiles through unpolished grains give reproducible alpha ejection profiles in Durango apatite that deviate from alpha ejection profiles predicted for ideal, homogenous crystals. SIMS depth profiling utilizes an O2 primary beam capable of sputtering tens of microns and measuring sub-micron resolution variation in [U], [Th], and [Sm]. Preliminary results suggest that sufficient [U] and [Th] zonation is present in Durango apatite to influence the form of the 4He alpha ejection profile. Future work will assess the influence of measured [U] and [Th] zonation on previously measured 4He depth profiles. Farley, K.A., 2000. Helium diffusion from apatite; general behavior as illustrated by Durango fluorapatite. J. Geophys. Res., B Solid Earth Planets 105 (2), 2903-2914.

Metallogeny of precious and base metal mineralization in the Murchison Greenstone Belt, South Africa: indications from U-Pb and Pb-Pb geochronology

NASA Astrophysics Data System (ADS)

Jaguin, J.; Poujol, M.; Boulvais, P.; Robb, L. J.; Paquette, J. L.

2012-10-01

The 3.09 to 2.97 Ga Murchison Greenstone Belt is an important metallotect in the northern Kaapvaal Craton (South Africa), hosting several precious and base metal deposits. Central to the metallotect is the Antimony Line, striking ENE for over 35 km, which hosts a series of structurally controlled Sb-Au deposits. To the north of the Antimony Line, hosted within felsic volcanic rocks, is the Copper-Zinc Line where a series of small, ca. 2.97 Ga Cu-Zn volcanogenic massive sulfide (VMS)-type deposits occur. New data are provided for the Malati Pump gold mine, located at the eastern end of the Antimony Line. Crystallizations of a granodiorite in the Malati Pump Mine and of the Baderoukwe granodiorite are dated at 2,964 ± 7 and 2,970 ± 7 Ma, respectively (zircon U-Pb), while pyrite associated with gold mineralization yielded a Pb-Pb age of 2,967 ± 48 Ma. Therefore, granodiorite emplacement, sulfide mineral deposition and gold mineralization all happened at ca. 2.97 Ga. It is, thus, suggested that the major styles of orogenic Au-Sb and the Cu-Zn VMS mineralization in the Murchison Greenstone Belt are contemporaneous and that the formation of meso- to epithermal Au-Sb mineralization at fairly shallow levels was accompanied by submarine extrusion of felsic volcanic rocks to form associated Cu-Zn VMS mineralization.

The oxygen isotope composition of baddeleyite and a test of crystal orientation effects during SIMS analysis

NASA Astrophysics Data System (ADS)

Ibanez-Mejia, M.; DesOrmeau, J. W.; Eddy, M. P.; Kitajima, K.; Valley, J. W.

2017-12-01

Baddeleyite, the monoclinic polymorph of ZrO2, is a relatively common accessory phase in undersaturated and alkaline igneous rocks that is rapidly developing into a widely-used tool for studying the age and initial 176Hf/177Hf composition of many terrestrial and planetary rocks that typically lack zircon. The prospect of combining U-Pb-Hf data from baddeleyite with δ18O information, as regularly done with zircon, could prove a very powerful addition to the `analytical toolbox' of the igneous petrologist and for studies of crust and mantle evolution. However, the oxygen isotope systematics of this mineral remain poorly explored, as are the potential analytical hurdles involved in obtaining accurate spatially-resolved δ18O data by SIMS. Here, we report laser fluorination δ18O measurements from two baddeleyite megacrysts from Kovdor (δ18O = 0.24 ± 0.11 ‰) and Phalaborwa (4.58 ± 0.11 ‰), which were subsequently analyzed by SIMS to explore their compositional homogeneity and potential as reference materials for correcting instrumental mass fractionation (IMF). Randomly oriented grain fragments analyzed by SIMS were subsequently mapped using high-resolution EBSD, such that the incidence angle of the Cs+ primary ion beam relative to the baddeleyite crystallographic axes could be determined for each spot. We found that: a) δ18O values for both crystals reproduce fairly well, but several apparent outliers (ca. 10% of all data) were measured with no evident correlation to orientation, cracks or inclusions, suggesting these might not be ideal standards; b) there is a systematic difference in mean measured IMFs of ca. 0.6 ‰ between the two baddeleyite crystals; c) mean 16OH/16O values for Phalaborwa (2.5x10-4) are significantly higher than those of Kovdor (2.9x10-6), suggesting that different degrees of radiation damage affect IMF; and d) there is no statistically significant correlation in our dataset (n= 96 spot analyses) between IMF and crystallographic orientation (in contrast to U-Pb [1]), and two twinned crystal fragments analyzed multiple times in various different orientations show no evidence for excessive δ18O dispersion (e.g., 2SD of a Kovdor crystal analyzed 41 times is ±0.27 ‰). [1] Wingate and Compston, 2000

Zircon U-Pb Geochronology, Hf Isotopic Composition and Geological Implications of the Neoproterozoic Huashan Group in the Jingshan Area, Northern Yangtze Block, China

NASA Astrophysics Data System (ADS)

Yang, Z.; Yang, K.

2015-12-01

In the northern Yangtze Block, a clear angular unconformity between the Mesoproterozoic sequences (e.g. Dagushi Group) and the overlying Neoproterozoic strata (e.g. Huashan Group) marks the the Jinning orogeny. A combined study of Lu-Hf isotopes and U-Pb ages for detrital zircons from Huashan Group can provide information on the crustal evolution of sedimentary provenances and the timing of the Jinning orogeny. Detrital zircons from Huashan Group have two major U-Pb age populations of about 2.0Ga, 2.65Ga, and three subordinate age groups of about 0.82Ga, 2.5Ga, 2.9Ga with minor >3.0Ga ages. The youngest five analyses yield a weighted average age of 816±9Ma, which is consistent with that of interlayered basalt (824±9Ma, Deng et al., 2013) and roughly defines the minimum depositional age of Huashan Group. Detrital zircons of Huashan Group mostly have two stage Hf isotope model ages (TDM2) between 3.0 to 3.3Ga, indicating that the northern Yangtze Block experienced significant continental crustal growth during the Paleo- to Meso-archean. Similar U-Pb ages of detrital zircons have been obtained from Precambrian sedimentary rocks in the northern Yangtze Block from previous studies (Liu et al., 2008; Guo et al., 2014 and references therein). Recently, ca. 2.65Ga A-type granites had been reported from the Kongling and Huji area, which likely record the thermally stable lithosphere (Chen et al., 2013; Zhou et al., 2015). In combination with this study, it documents the widespread 2.6-2.7Ga magmatic rocks in the northern Yangtze Block. Zhao et al. (2013) demonstrated both the ca. 850Ma tonalite and trondhjemite of the Huangling igneous complex were formed in a continental arc setting. This suggests the Miaowan-Huashan oceanic basin proposed by Bader et al. (2013) has not been closed at ca. 850Ma. This evidence, together with the depositional age of the Huashan Group, indicates the Jinning orogeny took place at 850-820 Ma. [1] Bader et al., 2013 Tectonics [2] Deng et al., 2013 International Geology Review [3] Guo et al., 2014 Precambrian Research [4] Liu et al., 2008 American Journal of Science [5] Zhao et al., 2013 American Journal of Science

Provenance and recycling of Arabian desert sand

NASA Astrophysics Data System (ADS)

Garzanti, Eduardo; Vermeesch, Pieter; Andò, Sergio; Vezzoli, Giovanni; Valagussa, Manuel; Allen, Kate; Kadi, Khalid; Al-Juboury, Ali

2013-04-01

This study seeks to determine the ultimate origin of aeolian sand in Arabian deserts by high-resolution petrographic and heavy-mineral techniques combined with zircon U-Pb geochronology. Point-counting is used here as the sole method by which unbiased volume percentages of heavy minerals can be obtained. A comprehensive analysis of river and wadi sands from the Red Sea to the Bitlis-Zagros orogen allowed us to characterize all potential sediment sources, and thus to quantitatively constrain provenance of Arabian dune fields. Two main types of aeolian sand can be distinguished. Quartzose sands with very poor heavy-mineral suites including zircon occupy most of the region comprising the Great Nafud and Rub' al-Khali Sand Seas, and are largely recycled from thick Lower Palaeozoic quartzarenites with very minor first-cycle contributions from Precambrian basement, Mesozoic carbonate rocks, or Neogene basalts. Instead, carbonaticlastic sands with richer lithic and heavy-mineral populations characterize coastal dunes bordering the Arabian Gulf from the Jafurah Sand Sea of Saudi Arabia to the United Arab Emirates. The similarity with detritus carried by the axial Tigris-Euphrates system and by transverse rivers draining carbonate rocks of the Zagros indicates that Arabian coastal dunes largely consist of far-travelled sand, deposited on the exposed floor of the Gulf during Pleistocene lowstands and blown inland by dominant Shamal northerly winds. A dataset of detrital zircon U-Pb ages measured on twelve dune samples and two Lower Palaeozoic sandstones yielded fourteen identical age spectra. The age distributions all show a major Neoproterozoic peak corresponding to the Pan-African magmatic and tectonic events by which the Arabian Shield was assembled, with minor late Palaeoproterozoic and Neoarchean peaks. A similar U-Pb signature characterizes also Jafurah dune sands, suggesting that zircons are dominantly derived from interior Arabia, possibly deflated from the Wadi al-Batin fossil alluvial fan or even from Mesozoic sandstones of the Arabian margin accreted to the Cenozoic Zagros orogen. Due to extensive recycling and the fact that zircon is so resistant to weathering and erosion, the U-Pb age signatures are much less powerful a tracer of sedimentary provenance than framework petrography and heavy minerals. Actualistic provenance studies of dune fields at subcontinental scale shed light on the generation and homogenization of aeolian sand, and allow us to trace complex pathways of multistep sediment transport, thus providing crucial independent information for accurate palaeogeographic and palaeoclimatic reconstructions.

Provenance and recycling of Arabian desert sand

NASA Astrophysics Data System (ADS)

Garzanti, Eduardo; Vermeesch, Pieter; Andò, Sergio; Vezzoli, Giovanni; Valagussa, Manuel; Allen, Kate; Kadi, Khalid A.; Al-Juboury, Ali I. A.

2013-05-01

This study seeks to determine the ultimate origin of aeolian sand in Arabian deserts by high-resolution petrographic and heavy-mineral techniques combined with zircon U-Pb geochronology. Point-counting is used here as the sole method by which unbiased volume percentages of heavy minerals can be obtained. A comprehensive analysis of river and wadi sands from the Red Sea to the Bitlis-Zagros orogen allowed us to characterize all potential sediment sources, and thus to quantitatively constrain provenance of Arabian dune fields. Two main types of aeolian sand can be distinguished. Quartzose sands with very poor heavy-mineral suites including zircon occupy most of the region comprising the Great Nafud and Rub' al-Khali Sand Seas, and are largely recycled from thick Lower Palaeozoic quartzarenites with very minor first-cycle contributions from Precambrian basement, Mesozoic carbonate rocks, or Neogene basalts. Instead, carbonaticlastic sands with richer lithic and heavy-mineral populations characterize coastal dunes bordering the Arabian Gulf from the Jafurah Sand Sea of Saudi Arabia to the United Arab Emirates. The similarity with detritus carried by the axial Tigris-Euphrates system and by transverse rivers draining carbonate rocks of the Zagros indicates that Arabian coastal dunes largely consist of far-travelled sand, deposited on the exposed floor of the Gulf during Pleistocene lowstands and blown inland by dominant Shamal northerly winds. A dataset of detrital zircon U-Pb ages measured on twelve dune samples and two Lower Palaeozoic sandstones yielded fourteen identical age spectra. The age distributions all show a major Neoproterozoic peak corresponding to the Pan-African magmatic and tectonic events by which the Arabian Shield was assembled, with minor late Palaeoproterozoic and Neoarchean peaks. A similar U-Pb signature characterizes also Jafurah dune sands, suggesting that zircons are dominantly derived from interior Arabia, possibly deflated from the Wadi al-Batin fossil alluvial fan or even from Mesozoic sandstones of the Arabian margin accreted to the Cenozoic Zagros orogen. Due to extensive recycling and the fact that zircon is so resistant to weathering and erosion, the U-Pb age signatures are much less powerful a tracer of sedimentary provenance than framework petrography and heavy minerals. Actualistic provenance studies of dune fields at subcontinental scale shed light on the generation and homogenization of aeolian sand, and allow us to trace complex pathways of multistep sediment transport, thus providing crucial independent information for accurate palaeogeographic and palaeoclimatic reconstructions.

Late Cretaceous (ca. 95 Ma) magnesian andesites in the Biluoco area, southern Qiangtang subterrane, central Tibet: Petrogenetic and tectonic implications

NASA Astrophysics Data System (ADS)

He, Haiyang; Li, Yalin; Wang, Chengshan; Zhou, Aorigele; Qian, Xinyu; Zhang, Jiawei; Du, Lintao; Bi, Wenjun

2018-03-01

The tectonic evolutionary history of the Lhasa and Qiangtang collision zones remains hotly debated because of the lack of pivotal magmatic records in the southern Qiangtang subterrane, central Tibet. We present zircon U-Pb dating, whole-rock major and trace-element geochemical analyses, and Sr-Nd isotopic data for the newly discovered Biluoco volcanic rocks from the southern Qiangtang subterrane, central Tibet. Zircon U-Pb dating reveals that the Biluoco volcanic rocks were crystallized at ca. 95 Ma. The samples are characterized by low SiO2 (50.26-54.53 wt%), high Cr (109.7-125.92 ppm) and Ni (57.4-71.58 ppm), and a high Mg# value (39-56), which plot in the magnesian andesites field on the rock classification diagram. They display highly fractionated rare earth element patterns with light rare earth element enrichment ([La/Yb]N = 21.04-25.24), high Sr/Y (63.97-78.79) and no negative Eu anomalies (Eu/Eu* = 0.98-1.04). The Biluoco volcanic rocks are depleted in Nb, Ta and Ti and enriched in Ba, Th, U and Pb. Moreover, the eight samples of Biluoco volcanic rocks display constant (87Sr/86Sr)i ratios (0.70514-0.70527), a positive εNd(t) value (2.16-2.68) and younger Nd model ages (0.56-0.62 Ga). These geochemical signatures indicate that the Biluoco volcanic rocks were most likely derived from partial melting of the mantle wedge peridotite metasomatized by melts of subducted slab and sediment in the subducted slab, invoked by asthenospheric upwelling resulting from the slab break-off of the northward subduction of the Bangong-Nujiang oceanic lithosphere. Identification of ca. 95 Ma Biluoco magnesian andesites suggests they were a delayed response of slab break-off of the northward subduction of the Bangong-Nujiang oceanic lithosphere at ca. 100 Ma.

Mixture modeling of multi-component data sets with application to ion-probe zircon ages

NASA Astrophysics Data System (ADS)

Sambridge, M. S.; Compston, W.

1994-12-01

A method is presented for detecting multiple components in a population of analytical observations for zircon and other ages. The procedure uses an approach known as mixture modeling, in order to estimate the most likely ages, proportions and number of distinct components in a given data set. Particular attention is paid to estimating errors in the estimated ages and proportions. At each stage of the procedure several alternative numerical approaches are suggested, each having their own advantages in terms of efficency and accuracy. The methodology is tested on synthetic data sets simulating two or more mixed populations of zircon ages. In this case true ages and proportions of each population are known and compare well with the results of the new procedure. Two examples are presented of its use with sets of SHRIMP U-238 - Pb-206 zircon ages from Palaeozoic rocks. A published data set for altered zircons from bentonite at Meishucun, South China, previously treated as a single-component population after screening for gross alteration effects, can be resolved into two components by the new procedure and their ages, proportions and standard errors estimated. The older component, at 530 +/- 5 Ma (2 sigma), is our best current estimate for the age of the bentonite. Mixture modeling of a data set for unaltered zircons from a tonalite elsewhere defines the magmatic U-238 - Pb-206 age at high precision (2 sigma +/- 1.5 Ma), but one-quarter of the 41 analyses detect hidden and significantly older cores.

Allanite age-dating: Non-matrix-matched standardization in quadrupole LA-ICP-MS

NASA Astrophysics Data System (ADS)

Burn, M.; Lanari, P.; Pettke, T.; Engi, M.

2014-12-01

Allanite Th-U-Pb age-dating has recently been found to be powerful in unraveling the timing of geological processes such as the metamorphic dynamics in subduction zones and crystallization velocity of magmas. However, inconsistencies among analytical techniques have raised doubts about the accuracy of allanite age data. Spot analysis techniques such as LA-ICP-MS are claimed to be crucially dependent on matrix-matched standards, the quality of which is variable. We present a new approach in LA-ICP-MS data reduction that allows non-matrix-matched standardization via well constrained zircon reference materials as primary standards. Our data were obtained using a GeoLas Pro 193 nm ArF excimer laser ablation system coupled to an ELAN DRC-e quadrupole ICP-MS. We use 32 μm and 24 μm spot sizes; laser operating conditions of 9 Hz repetition rate and 2.5 J/cm2 fluence have proven advantageous. Matrix dependent downhole fractionation evolution is empirically determined by analyzing 208Pb/232Th and 206Pb/238U and applied prior to standardization. The new data reduction technique was tested on three magmatic allanite reference materials (SISSb, CAPb, TARA); within error these show the same downhole fractionation evolution for all allanite types and in different analytical sessions, provided measurement conditions remain the same. Although the downhole evolution of allanite and zircon differs significantly, a link between zircon and allanite matrix is established by assuming CAPb and TARA to be fixed at the corresponding reference ages. Our weighted mean 208Pb/232Th ages are 30.06 ± 0.22 (2σ) for SISSb, 275.4 ± 1.3 (2σ) for CAPb, and 409.9 ± 1.8 (2σ) for TARA. Precision of single spot age data varies between 1.5 and 8 % (2σ), dependent on spot size and common lead concentrations. Quadrupole LA-ICP-MS allanite age-dating has thus similar uncertainties as do other spot analysis techniques. The new data reduction technique is much less dependent on quality and homogeneity of allanite standard reference materials. This method of correcting for matrix-dependent downhole fractionation evolution opens new possibilities in the field of LA-ICP-MS data acquisition, e.g. the use of a NIST standard glass to date all material types given a set of well constrained reference materials.

Trace element geochemistry of zircons from mineralizing and non-mineralizing igneous rocks related to gold ores at Yanacocha, Peru

NASA Astrophysics Data System (ADS)

Koleszar, A. M.; Dilles, J. H.; Kent, A. J.; Wooden, J. L.

2012-12-01

Zircons record important details about the evolution of magmatic systems, are relatively insensitive to alteration, and have been used to investigate the geochemistry, temperature, and oxidation state of volcanic and plutonic system. We examine zircons that span 6-7 m.y. of calc-alkaline volcanic activity in the Yanacocha district of northern Peru, where dacitic intrusions are associated with high-sulfidation gold deposits. The 14.5-8.4 Ma Yanacocha Volcanics include cogenetic lavas and pyroclastic rocks and are underlain by the andesites and dacites of the Calipuy Group, the oldest Cenozoic rocks in the region. We present data for magmatic zircons from the Cerro Fraile dacitic pyroclastics (15.5-15.1 Ma) of the Calipuy Group, and multiple eruptive units within the younger Yanacocha Volcanics: the Atazaico Andesite (14.5-13.3 Ma), the Quilish Dacite (~14-12 Ma), the Azufre Andesite (12.1-11.6 Ma), the San Jose Ignimbrite (11.5-11.2 Ma), and the Coriwachay Dacite (11.1-8.4 Ma). Epithermal high sulfidation (alunite-bearing) gold deposits are associated with the dacite intrusions of the Coriwachay and Quilish Dacites. Zircons from the non-mineralizing rocks typically have lower Hf concentrations and record Ti-in-zircon temperatures that are ~100°C hotter than zircons from the mineralizing intrusions. Temperatures recorded by zircons from the mineralizing intrusions are remarkably similar to those of the underlying Cerro Fraile dacite pyroclastics, but the zircons discussed here generally record SHRIMP-RG 206Pb/238U ages within error of previously published Ar-Ar eruption ages (eliminating antecrystic or xenocrystic origins). These observations suggest that zircons in the mineralizing intrusions form after greater extents of crystallization (and thus record elevated Hf concentrations and lower temperatures) than do zircons in the non-mineralized deposits. Unlike zircons from mineralized units associated with the porphyry Cu(Mo) deposits in Yerington, Nevada, which generally have Eu/Eu* ratios approaching 1 (i.e., Eu-anomalies that decrease in magnitude) with increasing Hf (and thus increasing crystallization), zircons associated with mineralized deposits in the Coriwachay and Quilish intrusions at Yanacocha typically have Eu-anomalies that are highly variable but do not vary systematically during magma evolution. These Eu/Eu* versus Hf trends are inconsistent with observations from the porphyry Cu-Mo(Au) deposit at El Salvador, Chile, where zircons from all porphyry intrusions have Eu-anomalies that become systematically more negative during magma evolution but with highest Eu/Eu* occurring in mineralized intrusions. We explore a variety of scenarios to explain the Eu/Eu* systematics of zircons from Yanacocha, including changes to the magmatic oxidation state as a consequence of anhydrite breakdown and progressive degassing.

Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

USGS Publications Warehouse

Bryant, Bruce; Wooden, J.L.

2008-01-01

In west-central Arizona near the northeast margin of the Basin and Range Province, the Rawhide detachment fault separates Tertiary and older rocks lacking significant effects of Tertiary metamorphism from Precambrian, Paleozoic, and Mesozoic rocks in the Harcuvar metamorphic core complex below. Much of the northern part of the Harcuvar complex in the Buckskin and eastern Harcuvar Mountains is layered granitic gneiss, biotite gneiss, amphibolite, and minor pelitic schist that was probably deformed and metamorphosed in Early Proterozoic time. In the eastern Buckskin Mountains, Early and Middle Proterozoic plutons having U-Pb zircon ages of 1,683?6.4 mega-annum (Ma) and 1,388?2.3 Ma, respectively, intruded the layered gneiss. Small plutons of alkaline gabbro and diorite intruded in Late Jurassic time. A sample of mylonitized diorite from this unit has a U-Pb zircon age of 149?2.8 Ma. In the Early Cretaceous, amphibolite facies regional metamorphism was accompanied by partial melting and formation of migmatite. Zircon from a granitic layer in migmatitic gneiss in the eastern Harcuvar Mountains has a U-Pb age of 110?3.7 Ma. In the Late Cretaceous, sills and plutons of the granite of Tank Pass were emplaced in both the Buckskin and eastern Harcuvar Mountains. In the Buckskin Mountains those intrusions are locally numerous enough to form an injection migmatite. A pluton of this granite crops out over almost half the area of the eastern Harcuvar Mountains. Paleozoic and Mesozoic sedimentary rocks were caught as slices along south-vergent Cretaceous thrusts related to the Maria fold and thrust belt and were metamorphosed beneath a thick sheet of Proterozoic crustal rocks. Inception of volcanism and basin formation in upper-plate rocks indicates that regional extension started at about 26 Ma, in late Oligocene. The Swansea Plutonic Suite, composed of rocks ranging from gabbro to granite, intruded the lower-plate rocks in the Miocene and Oligocene(?). Granite and a gabbro from the suite have a U-Pb zircon age of 21.86?0.60 Ma. Previously published 40Ar/39Ar ages of hornblende suggest that some of the Swansea Suite is Oligocene. The felsic rocks contain numerous inclusions ranging from porphyritic granite to porphyritic granodiorite. A sample from one inclusion has a U-Pb zircon age of 1,409?6.3 Ma. A discordia line for the U-Pb zircon data from the Swansea Plutonic Suite has an upper intercept at 1,408?3.4 Ma. The Swansea Plutonic Suite probably formed by interaction between mantle material and plutonic rocks at least as old as Middle Proterozoic. An irregular layer in the middle crust, which is thickest under and adjacent to the Buckskin Mountains, may be the level where that interaction took place. During extensional deformation these rocks and all the older rocks were displaced southwest from beneath the rocks of the Colorado Plateau transition zone below an area extending 50?80 kilometers northeast of the Buckskin Mountains as far as Bagdad, Arizona, or beyond. At that time the rocks were variably mylonitized, and a northeast-trending lineation formed. Much of the evidence for the complex sequence of structural events preserved in these rocks in the western Harcuvar Mountains has been obliterated in the northern Harcuvar complex by Miocene deformation.

Ignimbrites to batholiths: integrating perspectives from geological, geophysical, and geochronological data

USGS Publications Warehouse

Lipman, Peter W.; Bachmann, Olivier

2015-01-01

Multistage histories of incremental accumulation, fractionation, and solidification during construction of large subvolcanic magma bodies that remained sufficiently liquid to erupt are recorded by Tertiary ignimbrites, source calderas, and granitoid intrusions associated with large gravity lows at the Southern Rocky Mountain volcanic field (SRMVF). Geophysical data combined with geological constraints and comparisons with tilted plutons and magmatic-arc sections elsewhere are consistent with the presence of vertically extensive (>20 km) intermediate to silicic batholiths (with intrusive:extrusive ratios of 10:1 or greater) beneath the major SRMVF volcanic loci (Sawatch, San Juan, Questa-Latir). Isotopic data require involvement of voluminous mantle-derived mafic magmas on a scale equal to or greater than that of the intermediate to silicic volcanic and plutonic rocks. Early waxing-stage intrusions (35–30 Ma) that fed intermediate-composition central volcanoes of the San Juan locus are more widespread than the geophysically defined batholith; these likely heated and processed the crust, preparatory for ignimbrite volcanism (32–27 Ma) and large-scale upper-crustal batholith growth. Age and compositional similarities indicate that SRMVF ignimbrites and granitic intrusions are closely related, but the extent to which the plutons record remnants of former magma reservoirs that lost melt to volcanic eruptions has been controversial. Published Ar/Ar-feldspar and U-Pb-zircon ages for plutons spatially associated with ignimbrite calderas document final crystallization of granitoid intrusions at times indistinguishable from the tuff to ages several million years younger. These ages also show that SRMVF caldera-related intrusions cooled and solidified soon after zircon crystallization, as magma supply waned. Some researchers interpret these results as recording pluton assembly in small increments that crystallized rapidly, leading to temporal disconnects between ignimbrite eruption and intrusion growth. Alternatively, crystallization ages of the granitic rocks are here inferred to record late solidification, after protracted open-system evolution involving voluminous mantle input, lengthy residence (105–106yr) as near-solidus crystal mush, and intermittent separation of liquid to supply volcanic eruptions. The compositions of the least-evolved ignimbrite magmas tend to merge with those of caldera-related plutons, suggesting that the plutons record nonerupted parts of long-lived cogenetic magmatic systems, variably modified prior to final solidification. Precambrian-source zircons are scarce in caldera plutons, in contrast to their abundance in some peripheral waning-stage intrusions of the SRMVF, implying dissolution of inherited crustal zircon during lengthy magma assembly for the ignimbrite eruptions and construction of a subvolcanic batholith. Broad age spans of zircons (to several million years) from individual samples of some ignimbrites and intrusions, commonly averaged and interpreted as “intrusion-emplacement age,” alternatively provide an incomplete record of intermittent crystallization during protracted incremental magma-body assembly, with final solidification only when the system began to wane. Analyses of whole zircons cannot resolve late stages of crystal growth, and early growth in a long-lived magmatic system may be poorly recorded due to periods of zircon dissolution. Overall, construction of a batholith can take longer than recorded by zircon-crystallization ages, while the time interval for separation and shallow assembly of eruptible magma may be much shorter. Magma-supply estimates (from ages and volcano-plutonic volumes) yield focused intrusion-assembly rates sufficient to generate ignimbrite-scale volumes of eruptible magma, based on published thermal models. Mid-Tertiary processes of batholith assembly associated with the SRMVF caused drastic chemical and physical reconstruction of the entire lithosphere, probably accompanied by asthenospheric input.

Chronology of early Archaean granite-greenstone evolution in the Barberton Mountain Land, South Africa, based on precise dating by single zircon evaporation

NASA Technical Reports Server (NTRS)

Kruener, Alfred; Byerly, Gary R.; Lowe, Donald R.

1991-01-01

Precise Pb-207/Pb-206 single zircon evaporating ages are reported for low-grade felsic metavolcanic rocks within the Onverwacht and Fig Tree Groups of the Barberton Greenstone Belt (BGB), South Africa, as well as for granitoid plutons bordering the belt. Dacitic tuffs of the Hooggenoeg Formation in the upper part of the Onverwacht Group are shown to yield ages between 3445 + or - 3 and 3416 + or - 5 Ma and to contain older crustal components represented by a 3504 + or - 4 Ma old zircon xenocryst. Fig Tree dacitic tuffs and agglomerates have euhedral zircons between 3259 + or - 3 Ma in age which are interpreted to reflect the time of crystallization. The comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites are keynoted. The data adduced show that the Onverwacht and Fig Tree felsic units have distinctly different ages and thus do not constitute a single, tectonically repeated unit as proposed by others. It is argued that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones, and that the BGB in the Kaapval craton of southern Africa and greenstones in the Pilbara Block of Western Australia may have been part of a larger crustal unit in early Archaean times.

A 565 Ma old glaciation in the Ediacaran of peri-Gondwanan West Africa

NASA Astrophysics Data System (ADS)

Linnemann, Ulf; Pidal, Agustín Pieren; Hofmann, Mandy; Drost, Kerstin; Quesada, Cecilio; Gerdes, Axel; Marko, Linda; Gärtner, Andreas; Zieger, Johannes; Ulrich, Jens; Krause, Rita; Vickers-Rich, Patricia; Horak, Jana

2018-04-01

In the Cadomian orogen of the NE Bohemian Massif and of SW Iberia, a post-Gaskiers glacial event dated at c. 565 Ma has been detected. Such Ediacaran-aged glaciomarine deposits occur in the Weesenstein and Clanzschwitz groups of the Saxo-Thuringian zone (Bohemia) and in the Lower Alcudian group of the southern Central Iberian zone (Iberia). Both areas are parts of Cadomia situated in the Western and Central European Variscides. Glaciomarine sedimentary rocks are characterized by such features as dropstones, flat iron-shaped pebbles (" Bügeleisen- Geschiebe"), facetted pebbles, dreikanters, and zircon grains affected by ice abrasion. For age and provenance determination, LA-ICP-MS U-Pb ages ( n = 1124) and Hf isotope ( n = 446) analyses were performed. The maximum age of the glaciomarine deposits within a Cadomian back-arc basin based on U-Pb analytics resulted in the youngest detrital zircon populations showing ages of 562-565 Ma and of c. 566-576 Ma old zircon derived from granitoid pebbles within the diamictites. The youngest age recorded was 538-540 Ma based on zircon from the plutons which had intruded the previously deformed Ediacaran metasedimentary rocks. Previously described glaciomarine diamictites of Cadomia (Weesenstein, Clanzschwitz, and Orellana diamictites) are most definitely younger than the c. 579-581 Ma Gaskiers glaciation in Newfoundland (Gaskiers) and in SE New England (Squantum). We propose the term Weesenstein- Orellana glaciation for this new Ediacaran glacial event, named after the most relevant regions of exposure. Palaeogeographically, these glaciomarine diamictites and related sedimentary deposits lie on the periphery of the West African Craton (western peri-Gondwana), and evidence has been provided by detrital zircon U-Pb ages and their Hf isotope composition. Correlation with similar glaciomarine deposits in the Anti-Atlas (Bou Azzer) and Saudi Arabia suggests a continued distribution of post-Gaskiers glacial deposits along the Gondwana margin of Northern Africa. The Weesenstein- Orellana glaciation correlates in part with the Shuram-Wonoka δ13C anomaly.

Syn-collisional I-type Esenköy Pluton (Eastern Anatolia-Turkey): An indication for collision between Arabian and Eurasian plates

NASA Astrophysics Data System (ADS)

Açlan, Mustafa; Altun, Yusuf

2018-06-01

The Esenköy pluton which is situated in the East Anatolian Accretionary Complex (EACC) is represented by I-type, metalumino, calc-alkaline, VAG + syn-COLG, gabbro, diorite, quartz diorite, tonalite and granodiorite type rocks. This paper presents the characteristics of the above granitoids on their major, trace, rare earth elements (REE) and their zircon U-Pb dating. Zircon U-Pb crystallisation ages for gabbro, tonalite and granodiorite are 22.3 ± 0.2 Ma, 21.7 ± 0.2 Ma and 21.8 ± 0.2 Ma respectively. Esenköy granitoids show medium and high-K calc-alkaline character, with six exceptional K-poor sample plot in tholeiitic series field. The Rb/Y-Nb/Y diagram for Esenköy granitoids display subduction zone enrichment trend. The data which obtained from major, trace and REE geochemical characteristics and 206Pb/238U ages indicate that the collision which is take place between Arabian and Eurasian plates along the Bitlis-Zagros suture zone has begun in the Early Miocene (Aquitanian) or before from Early Miocene.

Exploring for early bombardments on Earth from pre-3.85 Fa thermal effects recorded in Hadean zircons - a status report

NASA Astrophysics Data System (ADS)

Mojzsis, S. J.; Abramov, O.; Harrison, T. M.; Kring, D. A.; Levison, H. F.; Trail, D.; Watson, E. B.

2008-12-01

We report on our progress with high-resolution ion microprobe U-Th-Pb depth profiles and Ti+REEs spot analysis which show that subsequent to their crystallization in melts under typical crustal conditions on Earth, some Hadean (pre-3.85 Ga) zircons record common age domains with unusual chemical and isotopic characteristics consistent with a high-temperature (possibly impact) origin. We have found evidence for later overprints caused by intense thermal alteration between 3.94-3.97 Ga in six of eight studied grains but no evidence for older events. These findings alert us to two fundamental things we did not know before about the probiotic potential of the Earth in the earliest solar system: (i) that the bombardment epoch did not result in complete 'Doomsday' scale destruction of the Earth's crust since the Moon-forming event at ca. 4.5 Ga; and (ii) age constraints on both sides of the ther-mally altered 3.94-3.97 Ga zircon domains are very good and so far our data show that no detectable thermal events are recorded by the zircons before ~3.97 Ga up to about 4.3 Ga. This observation is consistent with the output of new classes of dynamical models that successfully re-create the decay of impactor populations in the early solar system as recorded on the Moon and in meteorites.

Accessory mineral U-Th-Pb ages and 40Ar/39Ar eruption chronology, and their bearing on rhyolitic magma evolution in the Pleistocene Coso volcanic field, California

USGS Publications Warehouse

Simon, J.I.; Vazquez, J.A.; Renne, P.R.; Schmitt, A.K.; Bacon, C.R.; Reid, M.R.

2009-01-01

We determined Ar/Ar eruption ages of eight extrusions from the Pleistocene Coso volcanic field, a long-lived series of small volume rhyolitic domes in eastern California. Combined with ion-microprobe dating of crystal ages of zircon and allanite from these lavas and from granophyre geothermal well cuttings, we were able to track the range of magma-production rates over the past 650 ka at Coso. In ??? 230 ka rhyolites we find no evidence of protracted magma residence or recycled zircon (or allanite) from Pleistocene predecessors. A significant subset of zircon in the ???85 ka rhyolites yielded ages between ???100 and 200 Ma, requiring that generation of at least some rhyolites involves material from Mesozoic basement. Similar zircon xenocrysts are found in an ???200 ka granophyre. The new age constraints imply that magma evolution at Coso can occur rapidly as demonstrated by significant changes in rhyolite composition over short time intervals (???10's to 100's ka). In conjunction with radioisotopic age constraints from other young silicic volcanic fields, dating of Coso rhyolites highlights the fact that at least some (and often the more voluminous) rhyolites are produced relatively rapidly, but that many small-volume rhyolites likely represent separation from long-lived mushy magma bodies. ?? The Author(s) 2009.

Geochemical constraints on the link between volcanism and plutonism at the Yunshan caldera complex, SE China

NASA Astrophysics Data System (ADS)

Yan, Lili; He, Zhenyu; Beier, Christoph; Klemd, Reiner

2018-01-01

The Yunshan caldera complex is part of a larger scale, ca. 2000-km-long volcanic-plutonic complex belt in the coastal region of SE China. The volcanic rocks in the caldera complex are characterized by high-silica peraluminous and peralkaline rhyolites associated with an intracaldera porphyritic quartz monzonite pluton. In this study, we present zircon U-Pb, Hf and stable O isotopes along with geochemical data of both volcanic and plutonic rocks to evaluate the potential petrogenetic link between volcanism and plutonism in the Yunshan caldera complex. SHRIMP zircon U-Pb geochronology of both volcanic and plutonic rocks yields almost identical ages ranging from 95.6 to 93.1 Ma. The peraluminous and peralkaline rhyolites show negative anomalies of Sr, P, Ti and Ba and to a lesser extent negative Nb and Ta anomalies, along with positive Rb anomalies and `seagull-like' rare earth element (REE) patterns with negative Eu anomalies and low (La/Yb)N ratios. The intracaldera porphyritic quartz monzonite displays minor negative Rb, Nb, Ta, Sr, P and Ti anomalies and a positive Ba anomaly with REE patterns characterized by relatively high (La/Yb)N ratios and lack significant Eu anomalies. The peraluminous and peralkaline rhyolites and the porphyritic quartz monzonite exhibit consistent ɛ Nd( t) of - 3.7 to - 2.2 and display zircon ɛ Hf( t) values of - 2.1 to 3.7. They further have similar, mantle-like, zircon oxygen isotopic compositions (δ18OVSMOW mainly = 4.63 to 5.76‰). We interpret these observations to be in agreement with a crystal mush model in which the parental magma of the volcanic and plutonic rocks of the Yunshan caldera complex was likely produced by interaction of asthenosphere melts with subduction-influenced enriched mantle wedge. The peralkaline rhyolites are interpreted to represent the most differentiated magma that has subsequently experienced significant fluid-melt interactions, whereas the porphyritic quartz monzonite may be representative of the residual crystal mush. The Yunshan rhyolites typically match the geochemical characteristics of `hot-dry-reduced' rhyolites indicating that, during the late Cretaceous, the tectonic setting of SE China changed from a compressional environment to an extensional environment, i.e., from an arc into a back-arc setting. Our results imply that volcanic and plutonic rocks in caldera systems may provide unique constraints on the evolution of the magmatic system in which both the erupting melt and the residual crystalline material are being preserved.

Origin and geodynamic significance of the early Mesozoic Weiya LP and HT granulites from the Chinese Eastern Tianshan

NASA Astrophysics Data System (ADS)

Mao, Ling-Juan; He, Zhen-Yu; Zhang, Ze-Ming; Klemd, Reiner; Xiang, Hua; Tian, Zuo-Lin; Zong, Ke-Qing

2015-12-01

The Chinese Tianshan in the southwestern part of the Central Asian Orogenic Belt (CAOB) is characterized by a variety of high-grade metamorphic rocks, which provide critical constraints for understanding the geodynamic evolution of the CAOB. In this paper, we present detailed petrological and zircon U-Pb geochronological studies of the Weiya low-pressure and high-temperature (LP-HT) granulites of the Chinese Eastern Tianshan. These granulites were previously considered to be a product of a regional metamorphic orogenic event. Due to different bulk-rock chemistries the Weiya granulites, which occur as lenses within the contact metamorphic aureole of the Weiya granitic ring complex, have a variety of felsic-pelitic and mafic granulites with different textural equilibrium mineral assemblages including garnet-cordierite-sillimanite-bearing granulites, cordierite-sillimanite-bearing granulites, cordierite-orthopyroxene-bearing granulites, and orthopyroxene-clinopyroxene-bearing granulites. Average P-T thermobarometric calculations and conventional geothermobarometry indicates that the Weiya granulites underwent early prograde metamorphism under conditions of 600-650 °C at 3.2-4.2 kbar and peak metamorphism of 750-840 °C at 2.9-6.3 kbar, indicating a rather high geothermal gradient of ca. 60 °C/km. Zircon U-Pb LA-ICP-MS dating revealed metamorphic ages between 244 ± 1 to 237 ± 3 Ma, which are in accordance with the crystallization age of the Weiya granitic ring complex. We suggest that the formation of the Weiya granulites was related to contemporaneous granitic magmatism instead of a regional metamorphic orogenic event. In addition, a Late Devonian metamorphic age of ca. 380 Ma was recorded in zircon mantle domains from two pelitic samples which is consistent with the metamorphic age of the Xingxingxia metamorphic complex in the Chinese Eastern Tianshan. This suggests that the mantle domains of the zircon grains of the Weiya granulites probably formed during the Late Devonian regional metamorphism and were overprinted by the Early Triassic contact metamorphism. Therefore, Early Triassic geodynamic models for the southwestern part of the CAOB, which are based on a previously suggested regional metamorphic orogenic event of the Weiya granulites, need to be viewed with caution.

Extreme isotopologue disequilibrium in molecular SIMS species during SHRIMP geochronology

NASA Astrophysics Data System (ADS)

Magee, Charles W., Jr.; Danišík, Martin; Mernagh, Terry

2017-12-01

The current limitation in the accuracy and precision of inter-element analysis in secondary ion mass spectrometry (SIMS) is the ability to find measurable quantities that allow relative differences in ionization and transmission efficiency of secondary ions to be normalized. In uranium-thorium-lead geochronology, the ability to make these corrections, or "calibrate" the data, results in an accuracy limit of approximately 1 %. This study looks at the ionization of uranium and thorium oxide species, which are traditionally used in U-Pb calibration, to explore the conditions under which isotopologues, or molecular species whose composition differs only in the isotopic composition of one or more atoms in the molecule, remain in or deviate from equilibrium. Isotopologue deficits of up to 0.2 (200 ‰) below ideal mixing are observed in UO2+ species during SIMS gechronological analyses using the SHRIMP IIe SIMS instrument. These are identified by bombarding natural U-bearing minerals with an 18O2- primary beam. The large anomalies are associated with repeat analyses down a single SIMS sputtering crater (Compston et al., 1984), analysis of high-uranium, radiation-damaged zircon, and analysis of baddeleyite. Analysis of zircon under routine conditions yield UO2+ isotopologue anomalies generally within a few percent of equilibrium. The conditions under which the isotopologue anomalies are observed are also conditions in which the UOx-based corrections, or calibration, for relative U vs. Pb ionization efficiencies fail. The existence of these isotopologue anomalies suggest that failure of the various UOx species to equilibrate with each other is the reason that none of them will successfully correct the U  / Pb ratio. No simple isotopologue-based correction is apparent. However, isotopologue disequilibrium appears to be a more sensitive tool for detecting high-U calibration breakdowns than Raman spectroscopy, which showed sharper peaks for ˜ 37 Ma high-uranium zircons than for reference zircons OG1 and Temora. U-Th-Sm / He ages were determined for aliquots of reference zircons OG1 (755±71 Ma) and Temora (323±43 Ma), suggesting that the broader Raman lines for the Temora reference zircons may be due to something other than accumulated radiation damage. Isotopologue abundances for UO+ and ThO+ and their energy spectra are consistent with most or all molecular species being the product of atomic recombination when the primary beam impact energy is greater than 5.7 keV. This, in addition to the large UO2+ instrumentally generated isotopologue disequilibria, suggests that any attempts to use SIMS to detect naturally occurring isotopologue deviations could be tricky.

Can Single Crystal (U-Th)/He Zircon Ages from Nördlinger Ries Suevite be Linked to Impact-Related Shock Effects?

NASA Astrophysics Data System (ADS)

van Soest, M. C.; Cooper, F. J.; Wartho, J.; Hodges, K.; Buchner, E.; Schmieder, M.; Koeberl, C.

2010-12-01

Dating of impact-related material is difficult especially when pristine impact melt is unavailable. In the absence of such melts, most geochronometers in shocked basement or melt-poor impact rocks yield only partially reset or non-reset ages. In such cases, application of the low closure temperature apatite and zircon (U-Th)/He geochronometers can be successful, since impact-related physical and thermal shock should reset the He systematics in both minerals in most materials affected by the impact. For a proof of concept study on the well-studied Ries impact structure, we (U-Th)/He dated apatites (14.08 ± 0.26 Ma 2σ, n = 5) and zircons (14.26 ± 0.31 Ma 2σ, n = 10) from two Aumühle quarry suevite samples and one Polsingen quarry impact melt rock, which was dated at 14.37 ± 0.30 Ma (2σ) using Ar-Ar stepwise heating of recrystallized K-feldspar melt (Buchner et al., 2010). The (U-Th)/He ages agree well with the 14.37 Ma age, but are slightly younger than the suggested age of 14.59 ± 0.20 Ma (2σ - based on recent, post 1995, Ar-Ar data, Buchner et al., 2010) for the impact structure. However, among the 27 zircons dated, 6 were partially reset (>16Ma), and 11 zircons yielded younger dates (<13.5 Ma).The younger dates are problematic for successful (U-Th)/He dating of impact structures of unknown age, as they would be identified incorrectly as the age of the impact event. The cause for these younger dates may be: a) partial He loss due to a post-impact thermal event, which at Ries is unlikely as there is no geological evidence for such an event; b) compromised He retention due to metamictization by progressive radiation damage; or c) compromised He retention due to impact shock-related effects. The latter two causes can produce similar visual effects on zircon and the He loss mechanism is also similar, i.e. changes in the zircon crystal structure on a micro scale. However, the effects of these processes on zircon have been documented extensively by non-destructive analytical methods such as Raman spectroscopy, single crystal XRD, and SEM. These techniques have provided a means to distinguish shock from radiation damage effects, based on the presence of the high pressure zircon polymorph reidite, at shock levels >20 GPa (Wittmann et al., 2006). In order to establish if these effects can be identified in individual zircons before undertaking (U-Th)/He dating, and thus if these analytical methods can be used to pre-select suitable zircons for dating, 30 Ries zircons have been picked, based on euhedral shape and size, from two suevite samples for non-destructive geochemical and crystal structure studies prior to (U-Th)/He dating. Initial results from Raman spectroscopy indicate reidite is present in >80% of the grains, providing clear evidence for shock, while other grains show extensive amorphization likely due to extreme shock or radiation damage. References: Buchner et al. (2010) MAPS 45, 5: 662-674; Wittman et al. (2006) MAPS 41, 3: 433-454.

Igneous and tectonic evolution of the Batchawana Greenstone Belt, Superior Province: a U-Pb zircon and titanite study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Corfu, F.; Grunsky, E.C.

1987-01-01

U-Pb isotopic dating of zircon and titanite from all the major litho-tectonic units of the Batchawana belt, an Archean greenstone belt of the Abitibi Subprovince of the Superior Province in Canada, shows that the belt evolved during a period of about 60 Ma between about 2730 and 2670 Ma ago. Subsequent deformation of the supracrustal sequences produced isoclinal folding and culminated in metamorphism ranging from lower greenschist to amphibolite facies and anatexis related to the intrusion of syn- to late-tectonic plutons, four phases of which have ages of 2678 +4/-2 Ma, 2677 +/- 2 Ma, 2677 +/- 3 Ma, andmore » 2676 +/- 2 Ma. Two post-tectonic granitoid plutons in the center of the belt were intruded 2674 +/- 3 Ma and 2673 +/- 5 Ma ago and were followed by the emplacement of a composite mafic to felsic intrusion; a monzonite and a hornblendite from this intrusion yield identical ages of 2668 +/- 2 Ma. Titanite ages are identical or younger than the ages of coexisting zircons and reflect regional metamorphism and post-tectonic plutonism, but in a few cases they are younger and may record increased fluid activity along faults and the intrusion of mafic dikes. U-Pb zircon systematics, together with age and lithological relationships, suggests that the greenstone belt formed in an oceanic environment from material derived initially mainly from the mantle. Subsequent melting at the base of the thickening volcanic succession produced intermediate to felsic volcanic rocks, tonalites, and later granodioritic to granitic plutons leading to the final consolidation of the granite-greenstone terrain. 47 references.« less

A new insight into Pan-African tectonics in the East-West Gondwana collision zone by U-Pb zircon dating of granites from central Madagascar

NASA Astrophysics Data System (ADS)

Nédélec, A.; Paquette, J.-L.

1998-02-01

The assembly of Gondwana was the result of a major collision orogen, the East African Orogen, between East and West Gondwana during Neoproterozoic times. Madagascar, which represents a fragment of East Gondwana, is located in a key area of this Pan-African orogen. Granites of unambiguous tectonic setting have been dated using the U-Pb zircon method in order to constrain the timing of orogenic events. The central part of Madagascar is characterized by syntectonic alkaline granitic sheets, referred to as ``stratoid'' granites. These are of both mantle and crustal derivation. Their U-Pb zircon ages are well defined between 627 and 633 Ma for both plutonic suites, regardless of either mainly mantle or crustally origin. It is not surprising that the crustally-derived suite contains inherited zircons in the 2.2-2.4 Ga range attesting to the existence of Lower Proterozoic crust in northern central Madagascar. The generation of huge amounts of granitic magma is regarded as the result of post-collision extension under a high heat flow regime. Therefore, an age between 700 and 650 Ma is inferred for the beginning of Gondwana assembly along the collision zone between central Madagascar and Kenya, i.e., in the central part of the East African Orogen. Following this, brittle fracturing of the stratoid granite series permitted the emplacement of the Ambatomiranty granitic dyke swarm at a minimum age of 560 Ma, in possible connection with a nearby shear belt. The strike-slip tectonic regime at ~570-560 Ma is well known in southern Madagascar and in its Gondwana connections. This stage corresponds to intracontinental reworking and the final suturing of Gondwana.

A new insight into Pan-African tectonics in the East-West Gondwana collision zone by U-Pb zircon dating of granites from central Madagascar

NASA Astrophysics Data System (ADS)

Paquette, Jean-Louis; Nédélec, Anne

1998-02-01

The assembly of Gondwana was the result of a major collision orogen, the East African Orogen, between East and West Gondwana during Neoproterozoic times. Madagascar, which represents a fragment of East Gondwana, is located in a key area of this Pan-African orogen. Granites of unambiguous tectonic setting have been dated using the U-Pb zircon method in order to constrain the timing of orogenic events. The central part of Madagascar is characterized by syntectonic alkaline granitic sheets, referred to as "stratoid" granites. These are of both mantle and crustal derivation. Their U-Pb zircon ages are well defined between 627 and 633 Ma for both plutonic suites, regardless of either mainly mantle or crustally origin. It is not surprising that the crustally-derived suite contains inherited zircons in the 2.2-2.4 Ga range attesting to the existence of Lower Proterozoic crust in northern central Madagascar. The generation of huge amounts of granitic magma is regarded as the result of post-collision extension under a high heat flow regime. Therefore, an age between 700 and 650 Ma is inferred for the beginning of Gondwana assembly along the collision zone between central Madagascar and Kenya, i.e., in the central part of the East African Orogen. Following this, brittle fracturing of the stratoid granite series permitted the emplacement of the Ambatomiranty granitic dyke swarm at a minimum age of 560 Ma, in possible connection with a nearby shear belt. The strike-slip tectonic regime at ˜570-560 Ma is well known in southern Madagascar and in its Gondwana connections. This stage corresponds to intracontinental reworking and the final suturing of Gondwana.

Evidence of early Archean crust in northwest Gondwana, from U-Pb and Hf isotope analysis of detrital zircon, in Ediacaran surpacrustal rocks of northern Spain

NASA Astrophysics Data System (ADS)

Naidoo, Thanusha; Zimmermann, Udo; Vervoort, Jeff; Tait, Jenny

2018-03-01

The Mora Formation (Narcea Group) is one of the oldest Precambrian supracrustal successions in northern Spain. Here, we use U-Pb and in situ Hf isotope analysis on detrital zircon to determine its age and provenance. The youngest U-Pb dates constrain the maximum depositional age of the Mora Formation at 565 ± 11 Ma. Results indicate: (1) a dominant Ediacaran zircon population (33%; 565-633 Ma, Cadomian) within a spectrum of Neoproterozoic ages (40%; 636-996 Ma); and (2) smaller Mesoproterozoic (5%; 1004-1240 Ma), Palaeoproterozoic (11%; 1890-2476 Ma) and Archean (11%; 2519-3550 Ma) populations. Results here do not point to one specific cratonic source area; instead, detritus may have been derived from the West African craton and Amazonia, or even the concealed Iberian basement. The lack of 1.3-1.8 Ga grains suggests exclusion of the Sahara Craton as a major source, but this is not certain. This mixed composition favours a complex source history with reworking of detritus across terrane/craton boundaries. Hafnium isotope compositions indicate a range of crustal and juvenile sources, with initial ɛHf values between -15.8 and 11.1, and Hf model ages from 0.8 to 3.7 Ga. For Neoproterozoic zircons (80%), juvenile components (ɛHf(i) +10) may be related to Rodinia fragmentation and the onset of an active margin setting leading to the Cadomian orogeny. Palaeoproterozoic to Paleoarchean grains (20%) all have negative ɛHf values and Meso- to Eoarchean Hf model ages. This indicates an early (Archean) sialic crustal component for northwestern Gondwana.

Tectonic isolation from regional sediment sourcing of the Paradox Basin

NASA Astrophysics Data System (ADS)

Smith, T. M.; Saylor, J.; Sundell, K. E.; Lapen, T. J.

2017-12-01

The Appalachian and Ouachita-Marathon mountain ranges were created by a series of tectonic collisions that occurred through the middle and late Paleozoic along North America's eastern and southern margins, respectively. Previous work employing detrital zircon U-Pb geochronology has demonstrated that fluvial and eolian systems transported Appalachian-derived sediment across the continent to North America's Paleozoic western margin. However, contemporaneous intraplate deformation of the Ancestral Rocky Mountains (ARM) compartmentalized much of the North American western interior and mid-continent. We employ lithofacies characterization, stratigraphic thickness, paleocurrent data, sandstone petrography, and detrital zircon U-Pb geochronology to evaluate source-sink relationships of the Paradox Basin, which is one of the most prominent ARM basins. Evaluation of provenance is conducted through quantitative comparison of detrital zircon U-Pb distributions from basin samples and potential sources via detrital zircon mixture modeling, and is augmented with sandstone petrography. Mixing model results provide a measure of individual source contributions to basin stratigraphy, and are combined with outcrop and subsurface data (e.g., stratigraphic thickness and facies distributions) to create tectonic isolation maps. These maps elucidate drainage networks and the degree to which local versus regional sources influence sediment character within a single basin, or multiple depocenters. Results show that despite the cross-continental ubiquity of Appalachian-derived sediment, fluvial and deltaic systems throughout much of the Paradox Basin do not record their influence. Instead, sediment sourcing from the Uncompahgre Uplift, which has been interpreted to drive tectonic subsidence and formation of the Paradox Basin, completely dominated its sedimentary record. Further, the strong degree of tectonic isolation experienced by the Paradox Basin appears to be an emerging, yet common feature among other intraplate, tectonically active basins.

Gondwanan/peri-Gondwanan origin for the Uchee terrane, Alabama and georgia: Carolina zone or Suwannee terrane(?) and its suture with Grenvillian basement of the Pine Mountain window

USGS Publications Warehouse

Steltenpohl, M.G.; Mueller, P.M.; Heatherington, A.L.; Hanley, T.B.; Wooden, J.L.

2008-01-01

The poorly known, suspect, Uchee terrane occupies a critical tectonic position with regard to how and when peri-Gondwanan (Carolina) and Gondwanan (Suwannee) terranes were sutured to Laurentia. It lies sandwiched between Laurentian(?) continental basement exposed in the Pine Mountain window and adjacent buried Gondwanan crust of the Suwannee terrane. The Uchee terrane has been proposed as both a septum of Piedmont rocks that once was continuous across the erosionally breached Pine Mountain window or part of the Carolina zone. To help resolve this issue, we conducted U-Pb (SHRIMP-RG) (sensitive high-resolution ion microprobe-reverse geometry) zircon studies and whole-rock isotopic analyses of principal metasedimentary and metaplutonic units. U-Pb ages for zircons from the Phenix City Gneiss suggest igneous crystallization at ca. 620 Ma, inheritance ca. 1000 to ca. 1700 Ma, and a ca. 300 Ma (Alleghanian) overprint recorded by zircon rims. Zircons from the metasedimentary/metavolcaniclastic Moffits Mill Schist yield bimodal dates at ca. 620 and 640 Ma. The 620 to 640 Ma dates make these rocks age-equivalent to the oldest parts of the Carolina slate belt (Virgilina and Savannah River) and strongly suggest a Gondwanan (Pan-African and/or Trans-Brasiliano) origin for the Uchee terrane. Alternatively, the Uchee terrane may be correlative with metamorphic basement of the Suwannee terrane. The ca. 300 Ma overgrowths on zircons are compatible with previously reported 295 to 288 Ma 40Ar/39Ar hornblende dates on Uchee terrane rocks, which were interpreted to indicate deep tectonic burial of the Uchee terrane contemporaneous with the Alleghanian orogeny recorded in the foreland. Temperature-time paths for the Uchee terrane are similar to that of the Pine Mountain terrane, indicating a minimum age of ca. 295 Ma for docking. In terms of tectono-metamorphic history of the Uchee terrane, it is important to note that no evidence for intermediate "Appalachian" dates (e.g, Acadian or Taconian) has been reported. This younger history, together with the ages of metaigneous rocks and evidence for pre-Grenville basement, suggests the Uchee terrane is likely of Gondwanan origin and may he related to Carolina zone terranes that accreted during the Alleghanian orogeny. ?? 2008 Geological Society of America.

Cretaceous-Eocene provenance connections between the Palawan Continental Terrane and the northern South China Sea margin

NASA Astrophysics Data System (ADS)

Shao, Lei; Cao, Licheng; Qiao, Peijun; Zhang, Xiangtao; Li, Qianyu; van Hinsbergen, Douwe J. J.

2017-11-01

The plate kinematic history of the South China Sea opening is key to reconstructing how the Mesozoic configuration of Panthalassa and Tethyan subduction systems evolved into today's complex Southeast Asian tectonic collage. The South China Sea is currently flanked by the Palawan Continental Terrane in the south and South China in the north and the two blocks have long been assumed to be conjugate margins. However, the paleogeographic history of the Palawan Continental Terrane remains an issue of uncertainty and controversy, especially regarding the questions of where and when it was separated from South China. Here we employ detrital zircon U-Pb geochronology and heavy mineral analysis on Cretaceous and Eocene strata from the northern South China Sea and Palawan to constrain the Late Mesozoic-Early Cenozoic provenance and paleogeographic evolution of the region testing possible connection between the Palawan Continental Terrane and the northern South China Sea margin. In addition to a revision of the regional stratigraphic framework using the youngest zircon U-Pb ages, these analyses show that while the Upper Cretaceous strata from the Palawan Continental Terrane are characterized by a dominance of zircon with crystallization ages clustering around the Cretaceous, the Eocene strata feature a large range of zircon ages and a new mineral group of rutile, anatase, and monazite. On the one hand, this change of sediment compositions seems to exclude the possibility of a latest Cretaceous drift of the Palawan Continental Terrane in response to the Proto-South China Sea opening as previously inferred. On the other hand, the zircon age signatures of the Cretaceous-Eocene strata from the Palawan Continental Terrane are largely comparable to those of contemporary samples from the northeastern South China Sea region, suggesting a possible conjugate relationship between the Palawan Continental Terrane and the eastern Pearl River Mouth Basin. Thus, the Palawan Continental Terrane is interpreted to have been attached to the South China margin from the Cretaceous until the Oligocene oceanization of the South China Sea. In our preferred paleogeographic scenario, the sediment provenance in the northeastern South China Sea region changed from dominantly nearby Cretaceous continental arcs of the South China margin to more distal southeastern South China in the Eocene.

Chemical evolution of Himalayan leucogranites based on an O, U-Pb and Hf study of zircon

NASA Astrophysics Data System (ADS)

Hopkinson, Thomas N.; Warren, Clare J.; Harris, Nigel B. W.; Hammond, Samantha J.; Parrish, Randall R.

2015-04-01

Crustal melting is a characteristic process at convergent plate margins, where crustal rocks are heated and deformed. Miocene leucogranite sheets and plutons are found intruded into the high-grade metasedimentary core (the Greater Himalayan Sequence, GHS) across the Himalayan orogen. Previously-published Himalayan whole-rock data suggest that these leucogranites formed from a purely meta-sedimentary source, isotopically similar to those into which they now intrude. Bulk rock analyses carry inherent uncertainties, however: they may hide contributions from different contributing sources, and post-crystallization processes such as fluid interaction may significantly alter the original chemistry. In contrast, zircon is more able to retain precise information of the contributing sources of the melt from which it crystallises whilst its resistant nature is impervious to post-magmatic processes. This multi-isotope study of Oligocene-Miocene leucogranite zircons from the Bhutan Himalaya, seeks to differentiate between various geochemical processes that contribute to granite formation. Hf and O isotopes are used to detect discrete changes in melt source while U-Pb isotopes provide the timing of zircon crystallisation. Our data show that zircon rims of Himalayan age yield Hf-O signatures that lie within the previously reported whole-rock GHS field, confirming the absence of a discernible mantle contribution to the leucogranite source. Importantly, we document a decrease in the minimum ɛHf values during Himalayan orogenesis through time, correlating to a change in Hf model age from 1.4 Ga to 2.4 Ga. Nd model ages for the older Lesser Himalayan metasediments (LHS) that underthrust the GHS are significantly older than those for the GHS (2.4-2.9 Ga compared with 1.4-2.2 Ga), and as such even minor contributions of LHS material incorporated into a melt would significantly increase the resulting Hf model age. Hence our leucogranite data suggest either a change of source within the GHS over time, or an increasing contribution from older Lesser Himalayan (LHS) material in the melt. This is the first time that an evolutionary trend in the chemistry of Himalayan crustal melts has been recognized. Thus these new data show that, at least in the Himalaya, accessory phase geochemistry can provide more detailed insight into tectonic processes than bulk rock geochemistry.

Zircon U-Pb and molybdenite Re-Os geochronology and Sr-Nd-Pb-Hf isotopic constraints on the genesis of the Xuejiping porphyry copper deposit in Zhongdian, Northwest Yunnan, China

NASA Astrophysics Data System (ADS)

Leng, Cheng-Biao; Zhang, Xing-Chun; Hu, Rui-Zhong; Wang, Shou-Xu; Zhong, Hong; Wang, Wai-Quan; Bi, Xian-Wu

2012-10-01

The Xuejiping porphyry copper deposit is located in northwestern Yunnan Province, China. Tectonically, it lies in the southern part of the Triassic Yidun island arc. The copper mineralization is mainly hosted in quartz-dioritic and quartz-monzonitic porphyries which intruded into clastic-volcanic rocks of the Late Triassic Tumugou Formation. There are several alteration zones including potassic, strong silicific and phyllic, argillic, and propylitic alteration zones from inner to outer of the mineralized porphyry bodies. The ages of ore-bearing quartz-monzonitic porphyry and its host andesite are obtained by using the zircon SIMS U-Pb dating method, with results of 218.3 ± 1.6 Ma (MSWD = 0.31, N = 15) and 218.5 ± 1.6 Ma (MSWD = 0.91, N = 16), respectively. Meanwhile, the molybdenite Re-Os dating yields a Re-Os isochronal age of 221.4 ± 2.3 Ma (MSWD = 0.54, N = 5) and a weighted mean age of 219.9 ± 0.7 Ma (MSWD = 0.88). They are quite in accordance with the zircon U-Pb ages within errors. Furthermore, all of them are contemporary with the timing of the Garzê-Litang oceanic crust subduction in the Yidun arc. Therefore, the Xuejiping deposit could be formed in a continental margin setting. There are negative ɛNd(t) values ranging from -3.8 to -2.1 and relatively high initial 87Sr/86Sr ratios from 0.7051 to 0.7059 for the Xuejiping porphyries and host andesites. The (206Pb/204Pb)t, (207Pb/204Pb)t and (208Pb/204Pb)t values of the Xuejiping porphyries and host andesites vary from 17.899 to 18.654, from 15.529 to 15.626, and from 37.864 to 38.52, respectively, indicative of high radiogenic Pb isotopic features. In situ Hf isotopic analyses on zircons by using LA-MC-ICP-MS exhibit that there are quite uniform and slightly positive ɛHf(t) values ranging from -0.2 to +3.2 (mostly between 0 and +2), corresponding to relatively young single-stage Hf model ages from 735 Ma to 871 Ma. These isotopic features suggest that the primary magmas of the Xuejiping porphyries and their host andesites were mainly derived from a metasomatized mantle, with contamination of about 5-10% crustal rocks during ascending. Comparing with typical porphyry Cu deposits, the Xuejiping porphyry Cu deposit is distinct by strong silicific and phyllic alteration and major stockwork veining mineralization in the ore-bearing porphyries, but lack of pervasive potassic alteration and disseminated mineralization. This indicates that there could be more prospective Cu resources in the Xuejiping ore district.

Geochemistry, zircon U-Pb dating and Hf isotopies composition of Paleozoic granitoids in Jinchuan, NW China: Constraints on their petrogenesis, source characteristics and tectonic implication

NASA Astrophysics Data System (ADS)

Zeng, Renyu; Lai, Jianqing; Mao, Xiancheng; Li, Bin; Ju, Peijiao; Tao, Shilong

2016-05-01

Granitoids are widely distributed in Jinchuan at the southwestern margin of the North China plate, which is also an important area of mineral deposits. The research subject of this article are two Paleozoic granitoids, a cataclastic syenogranite and a granodiorite porphyry. This study presents whole rock geochemistry and zircon U-Pb-Hf isotope data for the two granitoids to determine their petrogenesis, source characteristics and tectonic significance. The cataclastic syenogranite is characterized by metaluminous composition with high potassium, and LaN/YbN from 39 to 48. The composition with strong negative Eu anomalies and Zircon saturation temperatures (TZr) from 947 to 1072 °C classify this intrusion as an A-type granite. The granodiorite porphyry is metaluminous with high sodium, sub-alkaline, LaN/YbN ratios from 27 to 32. These I-type intrusions have no Eu anomalies and TZr ranges from 818 to 845 °C. Both the cataclastic syenogranite and granodiorite porphyry show enrichment of LREE and LILE and depletion of HREE and HFSE, except Hf and Zr. Using single zircon LA-ICP-MS U-Pb dating, the emplacement age of the cataclastic syenogranite and granodiorite porphyry are determined at 433.4 ± 3.7 Ma and 361.7 ± 4.6 Ma, respectively. Zircons from the cataclastic syenogranits have uniform negative εHf(t) values (-11 ± 0.5 to -9 ± 0.5), implying the involvement of an old Palaeoproterozoic crustal source in magma genesis. The zircons from the granodiorite porphyry have εHf(t) values that range from -8 ± 1.0 to +10 ± 0.6, suggesting heterogeneous source materials involving both juvenile and ancient crust reworked crustal components. Based on the geological significance of granites at the southwestern margin of the North China plate, the closure of the North Qilian Ocean occurred at ∼444 Ma. Geochemical features suggest that the cataclastic syenogranite and granodiorite porphyry formed in an intraplate extensional and compressional setting, respectively. Hence after a period of extensional post-collisional intraplate evolution, strong compressive forces affected the area.

Detrital zircon U-Pb geochronology and whole-rock Nd-isotope constraints on sediment provenance in the Neoproterozoic Sergipano orogen, Brazil: From early passive margins to late foreland basins

NASA Astrophysics Data System (ADS)

Oliveira, E. P.; McNaughton, N. J.; Windley, B. F.; Carvalho, M. J.; Nascimento, R. S.

2015-11-01

SHRIMP U-Pb detrital zircon geochronology and depleted-mantle Nd-model ages of clastic rocks were combined to understand the sediment provenance in the Neoproterozoic Sergipano Belt. The Sergipano is the main orogenic belt between the Borborema province and the São Francisco Craton, eastern South America; it is divisible into several lithostratigraphic domains from North to South: Canindé, Poço Redondo-Marancó, Macururé, Vaza Barris, and Estância. Nd model ages (TDM) and detrital zircon U-Pb SHRIMP geochronology indicate that the protoliths of clastic metasedimentary rocks from the Marancó and Macururé domains were mostly derived from eroded late Mesoproterozoic to early Neoproterozoic rocks (1000-900 Ma), whereas detritus of similar rocks from the Canindé domain came from a younger source (ca. 700 Ma and 1000 Ma). Samples from the Vaza Barris domain show the greatest scatter of both TDM and zircon ages amongst all domains, but with important contributions from Proterozoic sources (690-1050 Ma and ca. 2100 Ma) and less from Archaean sources. The Estância domain samples have zircon population peaks at 570 Ma, 600 Ma, and 920-980 Ma, with a few older grains; one diamictite contains only ca. 2150 Ma zircon grains. Our preliminary results support a model in which sediments of the Marancó and Macururé domains were deposited on a continental margin of the ancient Borborema plate before its collision with the São Francisco Craton; the Canindé domain is likely to be an aborted Neoproterozoic rift assemblage within the southern part of the Borborema plate (Pernambuco-Alagoas massif). The basal units of the Vaza Barris and Estância domains have clast sources from the São Francisco Craton and are best interpreted as passive margin sediments. However, the uppermost units of the Estância and Vaza Barris domains come from foreland basins formed during collision of Borborema plate with the São Francisco Craton.

Paleoproterozoic Keulik-Kenirim Ore-Bearing Gabbro-Peridotite Complex, Kola Region: A New Occurrence of Ferropicritic Magmatism

NASA Astrophysics Data System (ADS)

Smolkin, V. F.; Lokhov, K. I.; Skublov, S. G.; Sergeeva, L. Yu.; Lokhov, D. K.; Sergeev, S. A.

2018-03-01

Comprehensive research of ore-bearing differentiated intrusions of the Keulik-Kenirim structural unit, which represents a fragment of the Paleoproterozoic Pechenga-Varzuga Belt, has been carried out for the first time. The intrusions are subvolcanic by type and lenticular in shape, nearly conformable and steeply dipping. They are made up of peridotite, olivine and plagioclase pyroxenites, and gabbro metamorphosed under amphibolite facies conditions along with host basic volcanics. All intrusive rocks are enriched in TiO2 and FeO. Sulfide Cu-Ni mineralization is represented by disseminated, pocket, and stringer-disseminated types, which are clustered in the peridotitic zone as hanging units and bottom lodes. The Ni content in disseminated ore is estimated at 0.45-0.55 wt % and 1.15-3.32 wt % in ore pockets; the Cu grades are 0.17-0.20 and 0.46-5.65 wt %, respectively. To determine the age of intrusions and metamorphism of intrusive and volcanic rocks, various isotopic systems have been used: Sm-Nd (TIMS) in rock and U-Pb (SIMS SHRIMP) and Lu-Hf (LA-ICP-MS) in zircon. Conclusions on the origin of zircons are based on concentrations of trace elements including REE therein and Hf-Nd correlation in zircons and rocks. The U-Pb system of zircons reflects episodes of igneous rock formation (1982 ± 12 Ma) and their postmagmatic transformation (1938 ± 20 Ma). The last disturbance of the U-Pb isotopic system occurred 700 and 425 Ma. Xenogenic zircons dated from 3.17 to 2.65 Ga have been revealed in the studied samples. These zircons were captured by magma from the Archean basement during its ascent. The intrusions were emplaced synchronously with economic ore formation in the Pechenga ore field (1985 ± 10 Ma). The peak metamorphism of intrusive rocks under amphibolite facies conditions is recorded at 40 Ma later. The differentiated intrusions of the Keulik-Kenirim structural unit are close in their internal structure, mineralogy, and geochemistry, as well as in age and features of related Cu-Ni mineralization to ore-bearing intrusions of the Pechenga ore field, which are derivatives of ferropicritic (ferriferous) magmatism.

In situ U-Th-Pb ages of the Miaoya carbonatite complex in the South Qinling orogenic belt, central China

NASA Astrophysics Data System (ADS)

Ying, Yuancan; Chen, Wei; Lu, Jue; Jiang, Shao-Yong; Yang, Yueheng

2017-10-01

The Miaoya carbonatite complex in the South Qinling orogenic belt hosts one of the largest rare earth element (REE)-Nb deposits in China that is composed of carbonatite and syenite. The emplacement age of the complex and the geochronological relationship between the carbonatite and syenite have long been debated. In this study, in situ U-Th-Pb ages have been obtained for the constituent minerals zircon, monazite and columbite from carbonatite and syenite of the Miaoya complex, together with their chemical and isotopic compositions. In situ trace element compositions for zircon from carbonatite and syenite are highly variable. The zircon displays slightly heavy REE (HREE)-enriched chondrite-normalized patterns with no Eu anomaly and various light REE (LREE) contents. In situ Th-Pb dating for zircon from the Miaoya complex by laser ablation ICP-MS yields ages of 442.6 ± 4.0 Ma (n = 53) for syenite and 426.5 ± 8.0 Ma (n = 23) for carbonatite. Monazite from carbonatite and syenite shows similar chondrite-normalized REE patterns and yields a consistent Th-Pb age of 240 Ma. Based on petrographic and chemical composition, columbite from the carbonatite can be identified into two groups. The columbite dispersed within carbonatite is characterized by slightly LREE-enriched chondrite-normalized REE patterns, whereas columbite associated with apatite is characterized by LREE-depleted trends. Columbite has been further determined to have a weighted mean 206Pb/238U age of 232.8 ± 4.5 Ma (n = 9) using LA-ICP-MS. Detailed geochronological and chemical investigations suggest that there were two major episodes of magmatic/metasomatic activities in the formational history of the Miaoya carbonatite complex. The early alkaline magmatism emplaced in the Silurian was related to the opening of the Mianlue Ocean, whereas the late metasomatism or hydrothermal overprint occurred during the Triassic South Qinling orogeny. The latter serves as the major ore formation period for both REE (e.g., monazite) and Nb (e.g., columbite).

Quaternary bimodal volcanism in the Niğde Volcanic Complex (Cappadocia, central Anatolia, Turkey): age, petrogenesis and geodynamic implications

NASA Astrophysics Data System (ADS)

Aydin, Faruk; Schmitt, Axel K.; Siebel, Wolfgang; Sönmez, Mustafa; Ersoy, Yalçın; Lermi, Abdurrahman; Dirik, Kadir; Duncan, Robert

2014-11-01

The late Neogene to Quaternary Cappadocian Volcanic Province (CVP) in central Anatolia is one of the most impressive volcanic fields of Turkey because of its extent and spectacular erosionally sculptured landscape. The late Neogene evolution of the CVP started with the eruption of extensive andesitic-dacitic lavas and ignimbrites with minor basaltic lavas. This stage was followed by Quaternary bimodal volcanism. Here, we present geochemical, isotopic (Sr-Nd-Pb and δ18O isotopes) and geochronological (U-Pb zircon and Ar-Ar amphibole and whole-rock ages) data for bimodal volcanic rocks of the Niğde Volcanic Complex (NVC) in the western part of the CVP to determine mantle melting dynamics and magmatic processes within the overlying continental crust during the Quaternary. Geochronological data suggest that the bimodal volcanic activity in the study area occurred between ca. 1.1 and ca. 0.2 Ma (Pleistocene) and comprises (1) mafic lavas consisting of basalts, trachybasalts, basaltic andesites and scoria lapilli fallout deposits with mainly basaltic composition, (2) felsic lavas consisting of mostly rhyolites and pumice lapilli fall-out and surge deposits with dacitic to rhyolitic composition. The most mafic sample is basalt from a monogenetic cone, which is characterized by 87Sr/86Sr = 0.7038, 143Nd/144Nd = 0.5128, 206Pb/204Pb = 18.80, 207Pb/204Pb = 15.60 and 208Pb/204Pb = 38.68, suggesting a moderately depleted signature of the mantle source. Felsic volcanic rocks define a narrow range of 143Nd/144Nd isotope ratios (0.5126-0.5128) and are homogeneous in Pb isotope composition (206Pb/204Pb = 18.84-18.87, 207Pb/204Pb = 15.64-15.67 and 208Pb/204Pb = 38.93-38.99). 87Sr/86Sr isotopic compositions of mafic (0.7038-0.7053) and felsic (0.7040-0.7052) samples are similar, reflecting a common mantle source. The felsic rocks have relatively low zircon δ18O values (5.6 ± 0.6 ‰) overlapping mantle values (5.3 ± 0.3 %), consistent with an origin by fractional crystallization from a mafic melt with very minor continental crustal contamination. The geochronological and geochemical data suggest that mafic and felsic volcanic rocks of the NVC are genetically closely related to each other. Mafic rocks show a positive trend between 87Sr/86Sr and Th, suggesting simultaneous assimilation and fractional crystallization, whereas the felsic rocks are characterized by a flat or slightly negative variation. High 87Sr/86Sr gneisses are a potential crustal contaminant of the mafic magmas, but the comparatively low and invariant 87Sr/86Sr in the felsic volcanics suggests that these evolved dominantly by fractional crystallization. Mantle-derived basaltic melts, which experienced low degree of crustal assimilation, are proposed to be the parent melt of the felsic volcanics. Geochronological and geochemical results combined with regional geological and geophysical data suggest that bimodal volcanism of the NVC and the CVP, in general, developed in a post-collisional extensional tectonic regime that is caused by ascending asthenosphere, which played a key role during magma genesis.

Stratigraphy, age, and depositional setting of the Miocene Barstow Formation at Harvard Hill, central Mojave Desert, California

USGS Publications Warehouse

Leslie, Shannon R.; Miller, David M.; Wooden, Joseph L.; Vazquez, Jorge A.

2010-01-01

New detailed geologic mapping and geochronology of the Barstow Formation at Harvard Hill, 30 km east of Barstow, CA, help to constrain Miocene paleogeography and tectonics of the central Mojave Desert. A northern strand of the Quaternary ENE-striking, sinistral Manix fault divides the Barstow Formation at Harvard Hill into two distinct lithologic assemblages. Strata north of the fault consist of: a green rhyolitic tuff, informally named the Shamrock tuff; lacustrine sandstone; partially silicified thin-bedded to massive limestone; and alluvial sandstone to pebble conglomerate. Strata south of the fault consist of: lacustrine siltstone and sandstone; a rhyolitic tuff dated at 19.1 Ma (U-Pb); rock-avalanche breccia deposits; partially silicified well-bedded to massive limestone; and alluvial sandstone and conglomerate. Our U-Pb zircon dating of the Shamrock tuff by SHRIMP-RG yields a peak probability age of 18.7 ± 0.1 Ma. Distinctive outcrop characteristics, mineralogy, remanent magnetization, and zircon geochemistry (Th/U) suggest that the Shamrock tuff represents a lacustrine facies of the regionally extensive Peach Spring Tuff (PST). Here we compare zircon age and geochemical analyses from the Shamrock tuff with those of the PST at Stoddard Wash and provide new insight into the age of zircon crystallization in the PST rhyolite. Results of our field studies show that Miocene strata at Harvard Hill mostly accumulated in a lacustrine environment, although depositional environments varied from a relatively deep lake to a very shallow lake or even onshore setting. Rock-avalanche breccias and alluvial deposits near the base of the exposed section indicate proximity to a steep basin margin and detrital studies suggest a southern source for coarse-grained deposits; therefore, we may infer a southern basin-margin setting at Harvard Hill during the early Miocene. Our geochronology demonstrates that deposition of the Barstow Formation at Harvard Hill extended from before ~19.1 Ma until well after ~18.7 Ma, similar to timing of Barstow Formation lake deposition in the Calico Mountains but at least 3 million years older than comparable lacustrine facies in the Mud Hills type section. These observations are consistent with either of two paleogeographic models: westward transgression of lacustrine environments within a single large basin, or sequential development of geographically distinct eastern and western sub-basins.

Pb-Pb systematics of lunar rocks: differentiation, magmatic and impact history of the Moon

NASA Astrophysics Data System (ADS)

Nemchin, A.; Martin, W.; Norman, M. D.; Snape, J.; Bellucci, J. J.; Grange, M.

2016-12-01

Two independent decay chains in U-Pb system allow the determination of both ages and initial isotope compositions by analyzing only Pb in the samples. A typical Pb analysis represents a mixture of radiogenic Pb produced from the in situ U decay, initial Pb and laboratory contamination. Utilizing the ability of ion probes to analyse 10-30 micrometer-sized spots in the samples while avoiding fractures and other imperfections that commonly host contamination, permits extraction of pure lunar Pb compositions from the three component mixtures. This results in both accurate and precise ages of the rocks and their initial compositions. Lunar Mare and KREEP basalts postdating the major lunar bombardment are likely to represent such three component mixtures and are therefore appropriate for this approach, also giving an opportunity to investigate Pb evolution in their sources. A source evolution model constrained using available data indicates a major differentiation on the Moon at 4376±18 Ma and very radiogenic lunar mantle at this time. This age is likely to reflect the mean time of KREEP formation during the last stage of Magma Ocean differentiation. Rocks older than about 3.9 Ga are more complex than basalts and may include an extra Pb component, if modified by impacts. An example of this is presented by Pb-Pb data obtained for the anorthosite sample 62236, where the age of the rock is determined as 4367±29 Ma from analyses of CPx lamellae inside the large Opx grains: however large plagioclase crystals do not contain Pb in quantities sufficient for ion probe analysis, precluding determination of the initial Pb composition of the sample. Most of Pb is found in the brecciated parts of the anorthosite between the large grains. The composition of this Pb is similar to the initial Pb of 3909±17 Ma Apollo 16 breccia 66095, suggesting that is was injected into the anorthosite during a 3.9 Ga impact. Similar ca 3.9 Ga ages were determined for 1-2 millimeter size feldspathic clasts from several Apollo 14 breccias, where they are likely to date Pb homogenization during the Imbrium impact. Combined with U-Pb data obtained previously using U-bearing minerals such as zircon and phosphates, the new Pb-Pb data sets open an opportunity for a detailed chronological and isotopic investigation of lunar differentiation, magmatic evolution and impact history.

U, Th, Pb and REE abundances and Pb 207/Pb 206 ages of individual minerals in returned lunar material by ion microprobe mass analysis.

NASA Technical Reports Server (NTRS)

Andersen, C. A.; Hinthorne, J. R.

1972-01-01

Results of ion microprobe analyses of Apollo 11, 12 and 14 material, showing that U, Th, Pb and REE are concentrated in accessory minerals such as apatite, whitlockite, zircon, baddeleyite, zirkelite, and tranquillityite. Th/U ratios are found to vary by over a factor of 40 in these minerals. K, Ba, Rb and Sr have been localized in a K rich, U and Th poor glass phase that is commonly associated with the U and Th bearing accessory minerals. Li is observed to be fairly evenly distributed between the various accessory phases. The phosphates have been found to have REE abundance patterns (normalized to the chondrite abundances) that are fairly flat, while the Zr bearing minerals have patterns that rise steeply, by factors of ten or more, from La to Gd. All the accessory minerals have large negative Eu anomalies. Radiometric age dates (Pb 207/Pb 206) of the individual U and Th bearing minerals compare favorably with the Pb 207/Pb 206 age of the bulk rocks.

Annealing effects on cathodoluminescence of zircon

NASA Astrophysics Data System (ADS)

Tsuchiya, Y.; Nishido, H.; Noumi, Y.

2011-12-01

U-Pb zircon dating (e. g., SHRIMP) is an important tool to interpret a history of the minerals at a micrometer-scale, where cathodoluminescence (CL) imaging allows us to recognize internal zones and domains with different chemical compositions and structural disorder at high spatial resolution. The CL of zircon is attributed by various types of emission centers, which are extrinsic ones such as REE impurities and intrinsic ones such as structural defects. Metamictization resulted from radiation damage to the lattice by alpha particles from the decay of U and Th mostly causes an effect on the CL features of zircon as a defect center. However, slightly radiation-damaged zircon, which is almost nondetectable by XRD, has not been characterized using CL method. In this study, annealing effects on CL of zircon has been investigated to clarify a recovery process of the damaged lattice at low radiation dose. A single crystal of zircon from Malawi was selected for CL measurements. It contains HfO2: 2.30 w.t %, U: 241 ppm and Th: 177 ppm. Two plate samples perpendicular to c and a axes were prepared for annealing experiments during 12 hours from room temperature to 1400 degree C. Color CL images were captured using a cold-cathode microscope (Luminoscope: Nuclide ELM-3R). CL spectral measurements were conducted using an SEM (JEOL: JSM-5410) combined with a grating monochromator (Oxford: Mono CL2) to measure CL spectra ranging from 300 to 800 nm in 1 nm steps with a temperature controlled stage. The dispersed CL was collected by a photoncounting method using a photomultiplier tube (Hamamatsu: R2228) and converted to digital data. All CL spectra were corrected for the total instrumental response. Spectral analysis reveals an anisotropy of the CL emission bands related to intrinsic defect center in blue region, radiation-induced defect center from 500 to 700 nm, and trivalent Dy impurity center at 480 and 580 nm, but their relative intensities are almost constant. CL on the surface perpendicular to c-axis, at which direction its intensity is maximum, was characterized in this study. Color CL imaging indicates yellow emission below 300 degree C, white to pale blue between 400 ~ 800 degree C and blue above 900 degree C. The broad band emission assigned to radiation-induced defect center in metamict state decreases in an increase of annealed temperature and disappeared above 700 degree C, whereas any change in XRD patterns was not detected. The blue CL emission caused by intrinsic defect center gradually increases in heating due to a recovery of its crystal field. Furthermore, the emission bands of trace amounts of trivalent Gd at 320 nm and Er at 400nm become to be appeared in response to a recrystallization by annealing. Therefore, CL analysis provides useful information on characteristics of the nature of metamict zircon rather than other conventional methods.

The role of trace element partitioning between garnet, zircon and orthopyroxene on the interpretation of zircon U-Pb ages: an example from high-grade basement in Calabria (Southern Italy)

NASA Astrophysics Data System (ADS)

Fornelli, A.; Langone, A.; Micheletti, F.; Pascazio, A.; Piccarreta, G.

2014-03-01

The recognition of the coeval growth of zircon, orthopyroxene and garnet domains formed during the same metamorphic cycle has been attempted with detailed microanalyses coupled with textural analyses. A coronitic garnet-bearing granulite from the lower crust of Calabria has been considered. U-Pb zircon data and zircon, garnet and orthopyroxene chemistries, at different textural sites, on a thin section of the considered granulite have been used to test possible equilibrium and better constrain the geological significance of the U-Pb ages related to zircon separates from other rocks of the same structural level. The garnet is very rich in REE and is characterised by a decrease in HREE from core to outer core and an increase in the margin. Zircons show core-overgrowth structures showing different chemistries, likely reflecting episodic metamorphic new growth. Zircon grains in matrix, corona around garnet and within the inner rim of garnet, are decidedly poorer in HREE up to Ho than garnet interior. Orthopyroxene in matrix and corona is homogeneously poor in REE. Thus, the outer core of garnet and the analysed zircon grains grew or equilibrated in a REE depleted system due to the former growth of garnet core. Zircon ages ranging from 357 to 333 Ma have been determined in the matrix, whereas ages 327-320 Ma and around 300 Ma have been determined, respectively, on cores and overgrowths of zircons from matrix, corona and inner rim of garnet. The calculated DREEzrn/grt and DREEopx/grt are largely different from the equilibrium values of literature due to strong depletion up to Ho in zircon and orthopyroxene with respect to garnet. On the other hand, the literature data show large variability. In the case study, (1) the D zrn/grt values define positive and linear trends from Gd to Lu as many examples from literature do and the values from Er to Lu approach the experimental results at about 900 °C in the combination zircon dated from 339 to 305 Ma with garnet outer core, and (2) D opx/grt values define positive trends reaching values considered as suggestive of equilibrium from Er to Lu only with respect to the outer core of garnet. The presence of a zircon core dated 320 Ma in the inner rim of garnet suggests that it, as well as those dated at 325-320 Ma in the other textural sites and, probably, those dated at 339-336 Ma showing depletion of HREE, grew after the garnet core, which sequestered a lot of HREE and earlier than the HREE rich margin of garnet. The quite uniform REE contents in orthopyroxene from matrix and corona and the low and uniform contents of HREE in the zircon overgrowths dated at about 300 Ma allow to think that homogenisation occurred during or after the corona formation around this age. The domains dated around 325-320 Ma would approximate the stages of decompression, whereas the metamorphic peak probably occurred earlier than 339 Ma.

Precambrian U-Pb zircon ages in eclogites and garnet pyroxenites from South Brittany (France): an old oceanic crust in the West European Hercynian belt?

NASA Astrophysics Data System (ADS)

Peucat, J. J.; Vidal, Ph.; Godard, G.; Postaire, B.

1982-08-01

U-Pb zircon ages have been determined for two eclogites from the Vendée and for two garnet pyroxenites from the Baie d'Audierne. In an episodic Pb loss model, the two discordia would give upper intercept ages around 1300-1250 Ma and lower intercepts ages of 436-384 Ma. Two interpretations are proposed: (1) The 1250-1300 Ma ages may reflect an unspecified upper mantle event or process; the Paleozoic ages correspond to the tectonic emplacement of an eclogitic mantle fragment into the continental crust. (2) The protolith may have been extracted from the upper mantle 1250-1300 Ma ago and stored in a crustal environment until it was metamorphosed under high-pressure conditions around 400 Ma ago. This latter model is favoured by available geologic and isotopic data. Consequently, we propose that a 1300 Ma old oceanic crust was tectonicly incorporated into a sialic basement during the Proterozoic. This mixture was subsequently subducted during the Paleozoic.

Origin of the ca. 50 Ma Linzizong shoshonitic volcanic rocks in the eastern Gangdese arc, southern Tibet

NASA Astrophysics Data System (ADS)

Liu, An-Lin; Wang, Qing; Zhu, Di-Cheng; Zhao, Zhi-Dan; Liu, Sheng-Ao; Wang, Rui; Dai, Jin-Gen; Zheng, Yuan-Chuan; Zhang, Liang-Liang

2018-04-01

The origin of the Eocene shoshonitic rocks within the upper part of the extensive Linzizong volcanic succession (i.e., the Pana Formation) in the Gangdese arc, southern Tibet remains unclear, inhibiting the detailed investigations on the crust-mantle interaction and mantle dynamics that operate the generation of the coeval magmatic flare-up in the arc. We report mineral composition, zircon U-Pb age and zircon Hf isotope, whole-rock element and Sr-Nd-Hf isotope data for the Pana Formation volcanic rocks from Pangduo, eastern Gangdese arc in southern Tibet. The Pana volcanic rocks from Pangduo include basalts, basaltic andesites, and dacites. SIMS and LA-ICPMS zircon U-Pb dating indicates that the Pangduo dacites were erupted at 50 ± 1 Ma, representing the volcanic equivalent of the coeval Gangdese Batholith that define a magmatic flare-up at 51 ± 1 Ma. The Pangduo volcanic rocks are exclusively shoshonitic, differing from typical subduction-related calc-alkaline volcanic rocks. The basalts have positive whole-rock ƐNd(t) (+1.7) and ƐHf(t) (+3.8) with high Zr abundances (121-169 ppm) and Zr/Y ratios (4.3-5.2), most likely derived from the partial melting of an enriched garnet-bearing lithospheric mantle that was metasomatized by subduction-related components with input from asthenosphere. Compared to the basalts, similar trace elemental patterns and decreased whole-rock ƐNd(t) (-3.5 to -3.3) and ƐHf(t) (-2.5 to -1.6) of the basaltic andesites can be attributed to the input of the ancient basement-derived material of the central Lhasa subterrane into the basaltic magmas. The coherent whole-rock Sr-Nd-Hf isotopic compositions ((87Sr/86Sr)i = 0.7064-0.7069, ƐNd(t) = -6.0 to -5.2, ƐHf(t) = -5.6 to -5.0) and varying zircon ƐHf(t) (-6.0 to +4.1) of the dacites can be interpreted by the partial melting of a hybrid lower crust source (juvenile and ancient lower crust) with incorporation of basement-derived components. Calculations of zircon-Ti temperature and whole-rock zircon saturation temperature of the dacites, and clinopyroxene crystallization temperature of the basalts suggest that the Pangduo volcanic rocks are most likely derived from the high-temperature melting of the lithosphere (including lithospheric mantle and overlying continental crust) as a result of the slab breakoff of the Neo-Tethyan oceanic lithosphere.

Westernmost Tian Shan (Uzbekistan): Magmatism and Exhumation

NASA Astrophysics Data System (ADS)

Abdulhameed, Sanaa; Ratchbacher, Lothar; Gagala, Lukasz; Jonkheere, Raymond

2014-05-01

The westernmost segment of the Tian Shan comprises the Ghissar-Alai Range of Tajikistan and Uzbekistan; its southwestern promontory contains the Baysunta crystalline massif. This orocline extends northwest of the Pamir and southeast of the Turan platform and forms the (north) western margin of the intra-orogenic Afghan-Tajik basin that was inverted during the India-Asia collision, mainly as a result of the gravitational collapse of the Pamir Plateau. The area contains Paleozoic slope and shelf clastics mantling crystalline basement rocks, altogether intruded by massif granitoids; it hides the cryptic Late Paleozoic South Ghissar suture. In Uzbekistan, the crystalline basement rocks of the westernmost Tien Shan are involved in the folding and thrusting of the Jurassic to Neogene sediments of the Afghan-Tajik basin, spectacularly proving thick-skinned deformation and demonstrating basement involvement below the Jurassic evaporate décollement underneath the Afghan-Tajik basin. We sampled the crystalline basement rocks of the Tian Shan of Uzbekistan to constrain the formation of the enigmatic Baysunta block and date the crystallization and high-grade metamorphism of the granitoids and associated metamorphic rocks of the Ghissar range; we employed U-Pb zircon geochronology. To time the deformation and exhumation of the fold-and thrust belt of the westernmost Afghan-Tajik basin, we used apatite fission-track thermochronology. Concordant U-Pb crystallization ages of zircons in the orthogneiss and paragneiss comprise 620 to 300 Ma; the Neoproterozoic ages imply a correlation of the Baysunta block with the Garm crystalline massif of the central Ghissar-Alai range of northeastern Tajikistan. The youngest zircon crystallization ages from granitoids are ~220 Ma, revealing enigmatically young magmatism, post-dating the last known collision event by >50 Ma. Together with 270-240 Ma meta-basaltic dykes and stocks in Tajikistan, they may trace a regional post-orogenic delamination event. The apatite fission-track thermochronology suggests a two-phase exhumation history. Exhumation/cooling within the age range of ~17 and 4 Ma and clustering around 10 Ma date slip along the thick-skinned thrusts. This demonstrates the impact of the India-Asia collision on the edge of the Turan platform, far northwest of the western edge of the collision zone. Ages from the northwestern edge of the study area cover 197 to 69 Ma; they trace a fossil partial annealing zone. They show that the Tian Shan along the margin of the Turan platform was never covered by sedimentary rocks >3 km thick.

Two-types of Early Cretaceous adakitic porphyries from the Luxi terrane, eastern North China Block: Melting of subducted Paleo-Pacific slab and delaminated newly underplated lower crust

NASA Astrophysics Data System (ADS)

Wang, Hao; Xu, Zhaowen; Lu, Xiancai; Fu, Bin; Lu, Jianjun; Yang, Xiaonan; Zhao, Zengxia

2016-01-01

The origin and tectonic setting of Early Cretaceous adakitic rocks from the Luxi terrane in the eastern North China Block (NCB) remain debated. To resolve this issue, we determined whole-rock geochemistry, zircon U-Pb ages, and in situ Hf-O isotopes of the Mengyin and Liujing adakitic porphyries from the Luxi terrane. Zircon U-Pb dating results reveal that both the Mengyin and Liujing plutons were emplaced during the Early Cretaceous, with weighted mean 206Pb/238U ages of 130 ± 1 Ma (2σ) and 131 ± 2 Ma (2σ), respectively. In addition, abundant Neoarchean-Paleoproterozoic inherited zircon cores are identified in the Mengyin adakitic porphyry with 207Pb/206Pb ages ranging from 2.53 to 2.42 Ga. Rocks of both plutons are silicic (SiO2 = 65.4-70.2 wt.%), metaluminous, and alkaline in composition, comprising mainly quartz syenite porphyries. Samples from both plutons are enriched in large ion lithophile elements (LILEs) (e.g., Rb, Sr, and Ba), and light rare earth elements (LREEs), depleted in high field strength elements (HFSEs) (e.g., Nb, Ta, and Ti), and heavy rare earth elements (HREEs), and have either positive or no Eu anomalies. In addition, both adakitic porphyries have high Mg# values (51-64), high Sr and La contents, low Y and Yb contents, and high Sr/Y (Mengyin = 149-264; Liujing = 58-110) and (La/Yb)N (Mengyin = 32.4-45.3; Liujing = 43.8-53.1) ratios, similar to adakitic rocks worldwide. The Mengyin adakitic porphyry has higher whole-rock εNd(t) values (-5.8 to - 4.1), more radiogenic Pb [(206Pb/204Pb)i = 18.35-18.39, (207Pb/204Pb)i = 15.55-15.56, (208Pb/204Pb)i = 38.20-38.23], higher zircon rim εHf(t) values (+ 3.3 to + 8.8) and δ18O values (+ 6.5‰ to + 7.9‰), and lower (87Sr/86Sr)i ratios (0.7049-0.7050) than the Liujing adakitic porphyry [εNd(t) = - 12.4 to - 12.2, (206Pb/204Pb)i = 17.63-17.72, (207Pb/204Pb)i = 15.56-15.58, (208Pb/204Pb)i = 37.76-37.94, εHf(t) = - 14.8 to - 11.2, δ18O = + 5.9‰ to + 7.1‰, (87Sr/86Sr)i = 0.7090-0.7091]. The Mengyin adakitic porphyry was most likely derived from partial melting of subducted oceanic slab with some input of NCB Neoarchean-Paleoproterozoic lower crust components. The Liujing adakitic porphyry was probably derived from partial melting of delaminated newly underplated thick lower crust, which then interacted with above asthenospheric mantle peridotite. Slab rollback together with the ridge subduction of the Paleo-Pacific slab was the most likely geodynamic mechanism for formation of the Early Cretaceous Mengyin and Liujing adakitic porphyries.

Contribution of Columbia and Gondwana Supercontinent assembly- and growth-related magmatism in the evolution of the Meghalaya Plateau and the Mikir Hills, Northeast India: Constraints from U-Pb SHRIMP zircon geochronology and geochemistry

NASA Astrophysics Data System (ADS)

Kumar, Santosh; Rino, Vikoleno; Hayasaka, Yasutaka; Kimura, Kosuke; Raju, Shunmugam; Terada, Kentaro; Pathak, Manjari

2017-04-01

The Meghalaya Plateau and the Mikir Hills constitute a northeastern extension of the Precambrian Indian Shield. They are dominantly composed of Proterozoic basement granite gneisses, granites, migmatites, granulites, the Shillong Group metasedimentary cover sequence, and Mesozoic-Tertiary igneous and sedimentary rocks. Medium to coarse grained, equigranular to porphyritic Cambrian granite plutons intrude the basement granite gneisses and the Shillong Group. U-Pb SHRIMP zircon geochronology and geochemistry of the granite gneisses and granites have been carried out in order to understand the nature and timing of granite magmatism, supercontinent cycles, and crustal growth of the Meghalaya Plateau and Mikir Hills. Zircons from the Rongjeng granite gneiss record the oldest magmatism at 1778 ± 37 Ma. An inherited zircon core has an age of 2566.4 ± 26.9 Ma, indicating the presence of recycled Neoarchaean crust in the basement granite gneisses. Zircons from the Sonsak granite have two ages: 523.4 ± 7.9 Ma and 1620.8 ± 9.2 Ma, which indicate partial assimilation of an older granite gneiss by a younger granite melt. Zircons from the Longavalli granite gneiss of the Mikir Hills has a crystallization age of 1430.4 ± 9.6 Ma and a metamorphic age of 514 ± 18.6 Ma. An inherited core of a zircon from Longavalli granite gneiss has an age of 1617.1 ± 14.5 Ma. Zircons from younger granite plutons have Cambrian mean ages of 528.7 ± 5.5 Ma (Kaziranga), 516 ± 9.0 Ma (South Khasi), 512.5 ± 8.7 Ma (Kyrdem), and 506.7 ± 7.1 Ma and 535 ± 11 Ma (Nongpoh). These plutons are products of the global Pan-African tectonothermal event, and their formation markedly coincides with the later stages of East Gondwana assembly (570-500 Ma, Kuunga orogen). The older inherited zircon cores (2566.4 ± 26.9 Ma, 1758.1 ± 54.3 Ma, 1617.1 ± 14 Ma) imply a significant role for recycled ancient crust in the generation of Cambrian granites. Thus the Meghalaya Plateau and Mikir Hills experienced major felsic magmatic episodes at 1800 Ma, 1600 Ma, 1400 Ma, and 500 Ma with recycling of Neoarchaean crust, and later contributions from Paleo-Mesoproterozoic granite gneiss sources. A 258 ± 20 Ma lower intercept age of the Rongjeng granite gneiss perhaps indicates a Permo-Triassic thermal imprint on the Meghalaya Plateau. The granite gneisses and granites have peraluminous to metaluminous compositions, and syn-orogenic to post-collisional affinities. We conclude that the orogenic history of the Meghalaya Plateau and the Mikir Hills records crustal growth of the Columbia and Gondwana supercontinents as noted in other Pan-African-Indian-Prydz-Brasiliano orogens.

Petrogenesis and tectonic implications of Early Cretaceous volcanic rocks from Lingshan Island in the Sulu Orogenic Belt

NASA Astrophysics Data System (ADS)

Meng, Yuanku; Santosh, M.; Li, Rihui; Xu, Yang; Hou, Fanghui

2018-07-01

The Dabie-Sulu orogenic belt in eastern China marks the boundary between the Yangtze Block and the North China Block. Here we investigate a suite of volcanic rocks from Lingshan Island in the Sulu belt comprising rhyolite, trachyte, trachyandesite and basaltic trachyandesite. We present petrological, geochemical and zircon Usbnd Pb ages and Hfsbnd O isotope data with a view to gain insights on the petrogenesis and tectonic implications. SHRIMP II analyses of zircon grains from the rhyolite yield 206Pb/238U age of 127.6 ± 1.3 Ma and LA-MC-ICP-MS dating show 126.3 ± 1.2 Ma and 127.3 ± 1.1 Ma, together constraining the eruption time as Early Cretaceous. LA-MC-ICP-MS analyses of zircon grains from the andesitic rocks yield 206Pb/238U ages of 129.0 ± 1.6 Ma, 129.8 ± 1.5 Ma and 130.9 ± 1.0 Ma. Geochemically, the rhyolite shows shoshonitic features with low MgO and Cr, but high Na2O + K2O. The zircon grains from these rocks yield negative εHf(t) values and low δ18O values, and these together with the presence of Neoproterozoic inherited zircons suggest that the magma source involved melting of the Yangtze crust. The andesitic rocks, including basaltic trachyandesite, trachyandesite and trachyte, show a wide range of SiO2, Mg# values, and Cr, enriched in LILE and LREE, depleted in HFSE (Nb, Ta and Ti), and have significantly negative zircon εHf(t) values, suggesting derivation from subcontinental lithosphere mantle that was metasomatized by felsic melts. Our results, integrated with those from previous studies suggest heterogeneous magma involving the mixing of mantle and crustal sources within an extensional setting in the Early Cretaceous.

Dielectric properties and phase transition behaviors in (1-x)PbZrO3-xPb(Mg1/2W1/2)O3 ceramics

NASA Astrophysics Data System (ADS)

Vittayakorn, Naratip; Charoonsuk, Piyanut; Kasiansin, Panisara; Wirunchit, Supamas; Boonchom, Banjong

2009-09-01

The solid solution of lead zirconate [PbZrO3 (PZ)] and lead magnesium tungstate [Pb(Mg1/2W1/2)O3 (PMW)] has been synthesized by the wolframite precursor method. The crystal structure, phase transformations, dielectric and thermal properties of (1-x)PZ-xPMW, where x =0.00-0.10, were investigated. The crystal structure of sintered ceramics was analyzed by x-ray diffraction. Phase-pure perovskite was obtained for all compositions. Furthermore, a change from orthorhombic to rhombohedral symmetry was observed as the mole fraction of increased PMW. As a result, it was found that PbZrO3-Pb(Mg1/2W1/2)O3 undergoes successive transitions from the antiferroelectric phase to the ferroelectric phase to the paraelectric state. The coexistence of orthorhombic and rhombohedral phases in this binary system is located near the composition x =0.1.

Syn-convergence extension in the southern Lhasa terrane: Evidence from late Cretaceous adakitic granodiorite and coeval gabbroic-dioritic dykes

NASA Astrophysics Data System (ADS)

Ma, Xuxuan; Xu, Zhiqin; Meert, Joseph G.

2017-10-01

Late Cretaceous (∼100-80 Ma) magmatism in the Gangdese magmatic belt plays a pivotal role in understanding the evolutionary history and tectonic regime of the southern Lhasa terrane. The geodynamic process for the formation of the early Late Cretaceous magmatism has long been an issue of hot debates. Here, petrology, geochronology and geochemistry of early Late Cretaceous granodiorite and coeval gabbroic-dioritic dykes in the Caina region, southern Lhasa, were investigated in an effort to ascertain their petrogenesis, age of intrusion, magma mixing and tectonic setting. Zircon U-Pb dating of granodiorite yields 206Pb/238U ages of 85.8 ± 1.7 and 86.4 ± 1.1 Ma, whilst that of the E-W trending dykes yields ages of 82.7 ± 2.6 and 83.5 ± 3.5 Ma. Within error, the crystallization ages of the dykes and the granodiorite are indistinguishable. Field observations and mineralogical microstructures are suggestive of a magma mixing process during the formation of the dykes and the granodiorite. The granodiorite exhibits geochemical features that are in agreement with those of subduction-related high-SiO2 adakites. The granodiorite and dykes have relatively constant εNd(t) values of +2.2 to +4.9 and initial 87Sr/86Sr ratios (0.7045-0.7047). These similar characteristics are herein interpreted as an evolutionary series from the dykes to granodiorite, consistent with magma mixing process. Ti-in-zircon thermometer and Al-in-hornblende barometer indicate that the granodiorite and the dioritic dyke crystallized at temperatures of ca. 750 and 800 °C, depths of ca. 6-10 and 5-9 km, respectively. Taking into account the synchronous magmatic rocks in the Gangdese Belt and the coeval rifted basin within the Lhasa terrane, the granodiorite and dykes reveal an early Late Cretaceous syn-convergence extensional regime in the southern Lhasa terrane, triggered by slab rollback of the Neotethyan oceanic lithosphere.

Magmatic @d^1^8O in 4400-3900 Ma detrital zircons: A record of the alteration and recycling of crust in the Early Archean [rapid communication

NASA Astrophysics Data System (ADS)

Cavosie, A. J.; Valley, J. W.; Wilde, S. A.

2005-07-01

Ion microprobe analyses of δ 18O in 4400-3900 Ma igneous zircons from the Jack Hills, Western Australia, provide a record of the oxygen isotope composition of magmas in the earliest Archean. We have employed a detailed analysis protocol aimed at correlating spatially related micro-volumes of zircon concordant in U/Pb age with δ 18O and internal zoning. Simultaneous analysis of 18O and 16O with dual Faraday cup detectors, combined with frequent standardization, has yielded data with improved accuracy and precision over prior studies, and resulted in a narrower range of what is interpreted as magmatic δ 18O in > 3900 Ma zircons. Preserved magmatic δ 18O values from individual zircons (Zrc) range from 5.3‰ to 7.3‰ (VSMOW), and increasingly deviate from the mantle range of 5.3 ± 0.3‰ as zircons decrease in age from 4400 to 4200 Ma. Elevated δ 18O (Zrc) values up to 6.5‰ occur as early as 4325 Ma, which suggests that evolved rocks were incorporated into magmas within ˜230 Ma of Earth's accretion. Values of magmatic δ 18O (Zrc) as high as 7.3‰ are recorded in zircons by 4200 Ma, and are common thereafter. The protoliths of the magmas these zircons crystallized in were altered by low temperature interaction with liquid water near Earth's surface. These results provide the strongest evidence yet for the existence of liquid water oceans and supracrustal rocks by approximately 4200 Ma, and possibly as early as 4325 Ma. The range of magmatic δ 18O values in the 4400-3900 Ma zircons is indistinguishable from Archean igneous zircons, suggesting similar magmatic processes occurred over the first two billion years of recorded Earth history. Zircons with sub-solidus alteration histories, identified by the presence of disturbed internal zoning patterns, record δ 18O values both below (4.6‰) and above (10.3‰) the observed range for primary magmatic zircon, and are unreliable indicators of Early Archean magma chemistry.

Timing of the Late Paleozoic Ice Age: A Review of the Status Quo and New U-Pb Zircon Ages From Southern Gondwana

NASA Astrophysics Data System (ADS)

Mundil, R.; Griffis, N. P.; Keller, C. B.; Fedorchuk, N.; Montanez, I. P.; Isbell, J.; Vesely, F.; Iannuzzi, R.

2017-12-01

Throughout the Carboniferous and Permian Late Paleozoic Ice Age (LPIA), glaciations in southern Gondwana exerted a profound influence on global climate and environment, ocean chemistry, and the nature of sedimentary processes. The LPIA is widely regarded as an analogue for Pleistocene glaciations. Our understanding of the latter, as well as the validity of predictions for the future global climate and environment, depends therefore on our ability to reconstruct the LPIA. A robust chronostratigraphic framework built on high precision/high accuracy geochronology is crucial for the reconstruction of events and processes that occurred during the LPIA, particularly in the absence of high-resolution terrestrial biostratigraphic constraints that apply to both near- and far-field proxy records. The occurrence of volcaniclastic layers containing primary volcanic zircon at many levels throughout southern Gondwana makes such a reconstruction feasible, but complications inevitably arise due to the mixing of older age components with primary volcanic crystals, as well as the potential of unrecognized open system behavior to produce spurious younger ages. These pitfalls cause age dispersion that may be difficult to interpret, or is unrecognized if low precision geochronological techniques are used, resulting in inaccurate radioisotopic ages. Our current efforts in the Parana Basin (Southern Brazil) and the Karoo Basin (South Africa/Namibia) concentrate on building a robust and exportable chronostratigraphic framework based on U-Pb zircon CA-TIMS ages with sub-permil level precision combined with Bayesian approaches for resolving the eruption age of dispersed age spectra to facilitate the reconstruction of glaciogenic processes through the Carboniferous-Permian transition, as well as their implications for global sea level, atmospheric pCO2 and ocean chemistry. We will also review currently available geochronological data from contemporaneous Australian successions and their potential for robust correlations and paleo-environmental reconstruction.

3.3 Ga SHRIMP U-Pb zircon age of a felsic metavolcanic rock from the Mundo Novo greenstone belt in the São Francisco craton, Bahia (NE Brazil)

NASA Astrophysics Data System (ADS)

Peucat, J. J.; Mascarenhas, J. F.; Barbosa, J. S. F.; de Souza, S. L.; Marinho, M. M.; Fanning, C. M.; Leite, C. M. M.

2002-07-01

Felsic metavolcanics associated with supracrustal rocks provide U-Pb zircon and Sm-Nd TDM ages of approximately 3.3 Ga, which establish an Archean age of the Mundo Novo greenstone belt. A granodioritic gneiss from the Mairi complex, located on the eastern boundary of the Mundo Novo greenstone belt, exhibits a zircon evaporation minimum age of 3.04 Ga and a Nd model age of 3.2 Ga. These results constrain the occurrence of at least three major geological units in this area: the Archean Mundo Novo greenstone belt, the Archean Mairi gneisses, and the adjoining Paleoproterozoic (<2.1 Ga) Jacobina sedimentary basin. The Jacobina basin follows the same trend as the Archean structure, extending southward to the Contendas-Mirante belt, in which a similar Archean-Paleoproterozoic association appears. We postulate that during the Paleoproterozoic in the eastern margin of the Gavião block, these Archean greenstone belts constituted a zone of weakness along which a late-stage orogenic sedimentary basin developed.

The mineralogical and chronological evidences of subducted continent material in deep mantle: diamond, zircon and rutile separated from the Horoman peridotite of Japan

NASA Astrophysics Data System (ADS)

Li, Y.; Yang, J.; Nida, K.; Yamamoto, S.; Lin, Y.; Li, Q.; Tian, M.; Kon, Y.; Komiya, T.; Maruyama, S.

2017-12-01

The Horoman peridotite complex is an Alpine-type orogenic lherzolite massif of upper-mantle in the Hidaka metamorphic belt, Hokkaido, Japan. The peridotite complex is composed of dunite, harzburgite, spinel lherzolite and plagioclase lherzolite, exhibits a conspicuous layered structure, which is a product of a Cretaceous to early Paleogene arc-trench system formed by westward subduction of an oceanic plate between the paleo-Eurasian and paleo-North American Plates. Various combinations of diamond, corundum, moissanite, zircon, monazite, rutile, and kyanite have been separated from spinel harzburgite (700 kg) and lherzolite (500 kg), respectively. The carbon isotopes analyses of diamond grains by Nano-SIMS yielded significant light carbon isotopes feature as δ13 CPDB values ranging from -29.2 ‰ to -17.2 ‰, with an average of -22.8±0.32 ‰. Zircon grains occur as sub-angular to round in morphological characteristics, similar to zircons of crustal sedimentary rocks. Many zircons contain small inclusions, comprise of quartz, apatite, rutile and muscovite. The U-Pb age of zircon grains analyzed using LA-ICP-MS and SIMS gave a wide age range, from the Jurassic to Archean (ca 159 - 3131 Ma). In the zircon age histogram, four age groups were identified; the age peaks are 2385 Ma, 1890 Ma, 1618 Ma and 1212 Ma, respectively. On the other hand, U-Pb ages of rutile grains analyzed using SIMS gave a peak of 370 Ma in age histogram. The mineralogical and chronological evidences of numerous crustal minerals in peridotite of Horoman suggest that the ancient continent material was subducted in deep mantle and recycled through the upper mantle by multicycle subduction processes.

Using detrital zircons from river sands to constrain major tectono-thermal events of the Cathaysia Block, SE China

NASA Astrophysics Data System (ADS)

Xu, Yonghang; Wang, Christina Yan; Zhao, Taiping

2016-07-01

Detrital zircons from the Minjiang and Zhujiang Rivers in SE China have been analyzed for U/Pb ages and Lu-Hf isotopic compositions to constrain their provenance and the growth history of the continental crust of the Cathaysia Block. Zircon U/Pb ages show five major populations at 90-250 Ma, 400-500 Ma, 0.7-1.2 Ga, 1.6-2.0 Ga and 2.3-2.6 Ga. Proterozoic zircons have Hf isotopic signatures consistent with the remelted ancient crust and the involvement of juvenile crust. However, Phanerozoic zircons have Hf isotopic signatures indicative of reworked or recycled ancient crust. Crustal growth rates based on the two-stage Hf model ages of the detrital zircons indicate that 30% and 90% of present crust in the northeastern Cathaysia Block was formed by 2.5 Ga and 1.6 Ga, respectively, whereas <20% continental crust was formed by 2.5 Ga, and 80% by 1.6 Ga in the southwestern Cathaysia Block. Therefore, Neoarchean and Paleoproterozoic were two major periods of crustal growth of the Cathaysia Block. Our results also reveal that the tectono-thermal events at ∼370 Ma and ∼117 Ma may have occurred to the Wuyishan terrane in the northeastern Cathaysia Block. Jurassic zircon grains have Hf isotopic compositions more variable than Cretaceous grains, which may have been derived from different source rocks. The 140-120 Ma tectono-thermal events were likely related to the change of the subduction direction of the Paleo-Pacific plate from northward to northwestward at 140 Ma. The 112-90 Ma tectono-thermal events may be correlated with the rollback of the subducted paleo-Pacific plate at ∼110 Ma.

Interaction of the Siberian craton and Central Asian Orogenic Belt (CAOB) recorded by detrital zircons from Transbaikalia

NASA Astrophysics Data System (ADS)

Powerman, V.; Shatsillo, A.; Chumakov, N.; Kapitonov, I.; Hourigan, J. K.

2015-12-01

The goal of this study is to pinpoint the beginning of interaction of two gigantic crustal structures: the Siberian Craton and the Central Asian Orogenic Belt (CAOB). We hypothesize that the beginning of convergence should be recorded in the Neoproterozoic passive margin strata of Siberian Craton by the first appearance of extraregional Neoproterozoic zircons. In order to test this hypothesis, we have acquired U-Pb zircon age distributions from twelve Neoproterozoic clastic rocks from the Baikal-Patom margin of Siberia and one sample from the volcaniclastic Padrinsky Group that was deposited atop accreted CAOB crust. Stratigraphically lower strata from the Siberian margin yield Archean - Paleoproterozoic detrital zircon ages, which are similar to, and probably derived from the Siberian Precambrian craton. A few extra-regional Mesoproterozoic grains are also present. The provenance shift happens in the upper portion of the section and is marked by a strong influx of extra-regional Neoproterozoic sediments. The youngest grains of 610 Ma constrain the sedimentation age and confine the timing of interaction between CAOB and Siberia in this region. Neoproterozoic zircons also dominate the overlying sedimentary unit, suggesting the continuance of the convergence. The coeval volcanoclastic unit on the CAOB side has a similar U-Pb detrital age distribution, strengthening the provenance link. Analysis of the local tectonics suggests that the beginning of accretion might have started even before the first appearance of Neoproterozoic zircon: during the development of a regional unconformity, capped by 635 Ma (?) "Snowball Earth" tillites of Dzhemkukan Fm. The absence of Neoproterozoic zircons in Dzhemkukan Fm. is probably explained by a thin-skinned tectonics that did not result in massive orogenesis . Our data are in good correlation with other Neoproterozoic sedimentary basins of southern Siberian Craton, including Cisbaikalia and Bodaibo Synclinorium.

Provenance of north Gondwana Cambrian-Ordovician sandstone: U-Pb SHRIMP dating of detrital zircons from Israel and Jordan

USGS Publications Warehouse

Kolodner, K.; Avigad, D.; McWilliams, M.; Wooden, J.L.; Weissbrod, T.; Feinstein, S.

2006-01-01

A vast sequence of quartz-rich sandstone was deposited over North Africa and Arabia during Early Palaeozoic times, in the aftermath of Neoproterozoic Pan-African orogeny and the amalgamation of Gondwana. This rock sequence forms a relatively thin sheet (1-3 km thick) that was transported over a very gentle slope and deposited over a huge area. The sense of transport indicates unroofing of Gondwana terranes but the exact provenance of the siliciclastic deposit remains unclear. Detrital zircons from Cambrian arkoses that immediately overlie the Neoproterozoic Arabian-Nubian Shield in Israel and Jordan yielded Neoproterozoic U-Pb ages (900-530 Ma), suggesting derivation from a proximal source such as the Arabian-Nubian Shield. A minor fraction of earliest Neoproterozoic and older age zircons was also detected. Upward in the section, the proportion of old zircons increases and reaches a maximum (40%) in the Ordovician strata of Jordan. The major earliest Neoproterozoic and older age groups detected are 0.95-1.1, 1.8-1.9 and 2.65-2.7 Ga, among which the 0.95-1.1 Ga group is ubiquitous and makes up as much as 27% in the Ordovician of Jordan, indicating it is a prominent component of the detrital zircon age spectra of northeast Gondwana. The pattern of zircon ages obtained in the present work reflects progressive blanketing of the northern Arabian-Nubian Shield by Cambrian-Ordovician sediments and an increasing contribution from a more distal source, possibly south of the Arabian-Nubian Shield. The significant changes in the zircon age signal reflect many hundreds of kilometres of southward migration of the provenance. ?? 2006 Cambridge University Press.

Localized rejuvenation of a crystal mush recorded in zircon temporal and compositional variation at the Lassen Volcanic Center, northern California

USGS Publications Warehouse

Klemetti, Erik W.; Clynne, Michael A.

2014-01-01

Zircon ages and trace element compositions from recent silicic eruptions in the Lassen Volcanic Center (LVC) allow for an evaluation of the timing and conditions of rejuvenation (reheating and mobilization of crystals) within the LVC magmatic system. The LVC is the southernmost active Cascade volcano and, prior to the 1980 eruption of Mount St. Helens, was the site of the only eruption in the Cascade arc during the last century. The three most recent silicic eruptions from the LVC were very small to moderate-sized lava flows and domes of dacite (1915 and 27 ka eruptions of Lassen Peak) and rhyodacite (1.1 ka eruption of Chaos Crags). These eruptions produced mixed and mingled lavas that contain a diverse crystal cargo, including zircon. 238U-230Th model ages from interior and surface analyses of zircon reveal ages from ~17 ka to secular equilibrium (>350 ka), with most zircon crystallizing during a period between ~60–200 ka. These data support a model for localized rejuvenation of crystal mush beneath the LVC. This crystal mush evidently is the remnant of magmatism that ended ~190 ka. Most zircon are thought to have been captured from “cold storage” in the crystal mush (670–725°C, Hf >10,000 ppm, Eu/Eu* 0.25–0.4) locally remobilized by intrusion of mafic magma. A smaller population of zircon (>730°C, Hf <10,000 ppm, Eu/Eu* >0.4) grew in, and are captured from, rejuvenation zones. These data suggest the dominant method to produce eruptible melt within the LVC is small-scale, local rejuvenation of the crystal mush accompanied by magma mixing and mingling. Based on zircon stability, the time required to heat, erupt and then cool to background conditions is relatively short, lasting a maximum of 10 s–1000 s years. Rejuvenation events in the LVC are ephemeral and permit eruption within an otherwise waning and cooling magmatic body.

Localized rejuvenation of a crystal mush recorded in zircon temporal and compositional variation at the Lassen Volcanic Center, northern California.

PubMed

Klemetti, Erik W; Clynne, Michael A

2014-01-01

Zircon ages and trace element compositions from recent silicic eruptions in the Lassen Volcanic Center (LVC) allow for an evaluation of the timing and conditions of rejuvenation (reheating and mobilization of crystals) within the LVC magmatic system. The LVC is the southernmost active Cascade volcano and, prior to the 1980 eruption of Mount St. Helens, was the site of the only eruption in the Cascade arc during the last century. The three most recent silicic eruptions from the LVC were very small to moderate-sized lava flows and domes of dacite (1915 and 27 ka eruptions of Lassen Peak) and rhyodacite (1.1 ka eruption of Chaos Crags). These eruptions produced mixed and mingled lavas that contain a diverse crystal cargo, including zircon. 238U-230Th model ages from interior and surface analyses of zircon reveal ages from ∼17 ka to secular equilibrium (>350 ka), with most zircon crystallizing during a period between ∼60-200 ka. These data support a model for localized rejuvenation of crystal mush beneath the LVC. This crystal mush evidently is the remnant of magmatism that ended ∼190 ka. Most zircon are thought to have been captured from "cold storage" in the crystal mush (670-725°C, Hf >10,000 ppm, Eu/Eu* 0.25-0.4) locally remobilized by intrusion of mafic magma. A smaller population of zircon (>730°C, Hf <10,000 ppm, Eu/Eu* >0.4) grew in, and are captured from, rejuvenation zones. These data suggest the dominant method to produce eruptible melt within the LVC is small-scale, local rejuvenation of the crystal mush accompanied by magma mixing and mingling. Based on zircon stability, the time required to heat, erupt and then cool to background conditions is relatively short, lasting a maximum of 10 s-1000 s years. Rejuvenation events in the LVC are ephemeral and permit eruption within an otherwise waning and cooling magmatic body.

3. 96 Ga zircons from an Archean quartzite, Beartooth Mountains, Montana

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mueller, P.A.; Wooden, J.L.; Nutman, A.P.

1992-04-01

U-Pb isotopic systematics of detrital zircons incorporated in a middle Archean quartzite from the Beartooth Mountains, Montana, were investigated with the SHRIMP ion microprobe. These new data reveal an extended and previously unrecognized record of crustal evolution for the northern Wyoming province. Seventy-eight analyses of 67 grains yielded a range of {sup 207}Pb/{sup 206}Pb ages from 2.69 to 3.96 Ga. Concordant analyses from 43 separate grains defined a maximum age for the deposition of the quartzite of 3.30 Ga; other provenance ages extend to 3.96 Ga. Ages of < 3.30 Ga are generally discordant, and appear to reflect late Archeanmore » disturbance of the U-Pb system, including metamorphism at {approximately}2.8 Ga. The predominance of ages at {approximately}3.3 Ga is interpreted to represent the last major episode of crust formation prior to deposition of the quartzite. The concordant analyses of > 3.30 Ga indicate that older crustal components with ages up to 3.96 Ga, or detritus from them, were also in the provenance of this quartzite. This older age is equivalent to that of the oldest known rock from the Acasta gneisses of the Slave province and is exceeded only by the > 4.0 Ga age of detrital zircons of the Yilgarn block of Western Australia. These data support an increased probability for the survival of sialic crust created before the cessation of the late bombardment at 3.8 to 3.9 Ga.« less

Source, evolution and emplacement of Permian Tarim Basalts: Evidence from U-Pb dating, Sr-Nd-Pb-Hf isotope systematics and whole rock geochemistry of basalts from the Keping area, Xinjiang Uygur Autonomous region, northwest China

NASA Astrophysics Data System (ADS)

Zhang, Dayu; Zhou, Taofa; Yuan, Feng; Jowitt, Simon M.; Fan, Yu; Liu, Shuai

2012-04-01

Permian basalts distribute at least 250,000 km2, and underlie the southwest Tarim Basin in Xinjiang Uygur Autonomous region, northwest China. This vast accumulation of basalt is the main part of the Tarim Large Igneous Province (LIP). The basaltic units in the Lower Permian Kupukuziman and Kaipaizileike Formations in the Keping area, Tarim Basin; were the best exposure of the Permian basalt sequence in the basin. LA-ICP-MS U-Pb dating of zircon from the basal basaltic unit in the section gives an age of 291.9 ± 2.2 Ma (MSWD = 0.30, n = 17); this age, combined with previously published geochronological data, indicates that the basalts in the Tarim Basin were emplaced between 292 Ma and 272 Ma, with about 90% of the basalts being emplaced between 292 and 287 Ma. Basalts from the Keping area have high FeOT (10.8-18.6 wt.%), low Mg#s (0.26-0.60), and exhibit primitive mantle normalized patterns with positive Pb, P and Ti but negative Zr, Y and Ta anomalies. The basalts from both formations have similar 206Pb/204Pb (18.192-18.934), 207Pb/204Pb (15.555-15.598) and 208Pb/204Pb (38.643-38.793) ratios. The basalts also have high ɛSr(t) (45.7-62.1), low ɛNd(t) (-3.6 to -2.2) and low zircon ɛHf(t) (-4.84 to -0.65) values. These characteristics are typical of alkali basalts and suggest that the basalts within the Tarim Basin were derived from an OIB-type mantle source and interacted with enriched mantle (EMI-type) before emplacement. Rare earth element systematics indicate that the parental melts for the basalts were high-degree partial melts derived from garnet lherzolite mantle at the base of the lithosphere. Prior to emplacement, the Tarim Permian Basalts (TPB) underwent fractional crystallization and assimilated crustal material; the basalts were finally emplaced during crustal extension in an intra-plate setting. The wide distribution, deep source and high degree partial melting of the TPB was consistent with a mantle plume origin. The TPB and other coeval igneous rocks in the Tarim Basin constitute a Permian LIP formed by a mantle plume in a similar fashion to the plume-related Emeishan LIP in southwest China.

Zircon U-Pb geochronology links the end-Triassic extinction with the Central Atlantic Magmatic Province.

PubMed

Blackburn, Terrence J; Olsen, Paul E; Bowring, Samuel A; McLean, Noah M; Kent, Dennis V; Puffer, John; McHone, Greg; Rasbury, E Troy; Et-Touhami, Mohammed

2013-05-24

The end-Triassic extinction is characterized by major losses in both terrestrial and marine diversity, setting the stage for dinosaurs to dominate Earth for the next 136 million years. Despite the approximate coincidence between this extinction and flood basalt volcanism, existing geochronologic dates have insufficient resolution to confirm eruptive rates required to induce major climate perturbations. Here, we present new zircon uranium-lead (U-Pb) geochronologic constraints on the age and duration of flood basalt volcanism within the Central Atlantic Magmatic Province. This chronology demonstrates synchroneity between the earliest volcanism and extinction, tests and corroborates the existing astrochronologic time scale, and shows that the release of magma and associated atmospheric flux occurred in four pulses over about 600,000 years, indicating expansive volcanism even as the biologic recovery was under way.

Petrography and zircon U-Pb isotopic study of the Bayanwulashan Complex: Constrains on the Paleoproterozoic evolution of the Alxa Block, westernmost North China Craton

NASA Astrophysics Data System (ADS)

Wu, Sujuan; Hu, Jianmin; Ren, Minghua; Gong, Wangbin; Liu, Yang; Yan, Jiyuan

2014-11-01

The Bayanwulashan Metamorphic Complex (BMC) exposes along the eastern margin of the Alxa Block, the westernmost part of the North China Craton (NCC). BMC is principally composed of metamorphic rocks with amphibole plagiogneiss, biotite plagioclase gneiss and granitic gneiss. Our research has been focused on the petrography and zircon U-Pb geochronology of the BMC to better understand the evolution of the Alxa Block and its relationship with the NCC. Evidences from field geology, petrography, and mineral chemistry indicate that two distinct metamorphic assemblages, the amphibolite and greenschist facies, had overprinted the preexisting granitic gneiss and suggest that the BMC experienced retrograde metamorphic episodes. The LA-ICP-MS zircon U-Pb ages reveal that the primary magmatic activities of BMC were at ca. 2.30-2.24 Ga and the two metamorphic events were at ca. 1.95-1.91 Ga and ca. 1.88-1.85 Ga respectively. These ages indicate that BMC initially intruded during Paleoproterozoic, not as previously suggested at Archean period. The Early Paleoproterozoic metamorphic records and the magmatic thermochronological data in BMC exhibit different evolution paths between the Alxa Block and the NCC. The Alxa Block was most likely an independent Early Paleoproterozoic terrain. Following different amalgamation processes, The Alxa Block combined with Western Block at ca. 1.95 Ga and then united with NCC at ca. 1.85 Ga.

Nano is the next big thing: Revealing geochemical processes with atom probe microscopy

NASA Astrophysics Data System (ADS)

Reddy, Steven; Saxey, David; Rickard, William; Fougerouse, Denis; Peterman, Emily; van Riessen, Arie; Johnson, Tim

2017-04-01

Characterizing compositional variations in minerals at the nanometre scale has the potential to yield fundamental insights into a range of geological processes associated with nucleation and mineral growth and the subsequent modification of mineral compositions by processes such as diffusion, deformation and recrystallization. However, there are few techniques that allow the quantitative measurement of low abundance trace elements and isotopes signatures at the nanometre scale. Atom probe microscopy is one such technique that has been widely used in the study of metals and, in the last decade, semiconductors. However, the development and application of atom probe microscopy to minerals is in its infancy and only a handful of published studies exist in the literature. Here, we provide an introduction to atom probe microscopy and its potential use in geological studies using two examples from both undeformed and deformed zircon (ZrSiO4). In the first example, we use atom probe microscopy to show that discordant data from the core of an undeformed 2.1 Ga zircon, metamorphosed at granulite facies conditions 150 Myr ago, contains distinct Pb reservoirs that represent both the crystallisation and metamorphic 207Pb/206Pb ages. Crystallisation ages are preserved within ˜10 nm diameter dislocation loops that formed during annealing of radiation-damaged zircon during the prograde path of the metamorphic event. The results highlight the potential for resolving the chronology of multiple, distinct Pb reservoirs within isotopically complex zircon and provide an explanation for varying amounts of discordance within individual zircon grains. In the second example, we illustrate complex trace element distributions associated with near-instantaneous deformation of a shocked zircon during the ˜1.17 Ga Stac Fada bolide impact. Substitutional and interstitial ions show correlated segregation, indicating coupling between different mobility mechanisms associated with the rapid formation and migration of oxygen vacancies and dislocations into low energy configurations. The results of these two studies show how quantification of elemental and isotopic variations at the nanoscale may reveal fundamental new insights into geochemical processes that underpin the interpretation of geochemical data collected at the microscale. Furthermore, these new data highlight the important role of crystal defects, even in undeformed zircon, in the chemical modification of zircon, and allow the interplay amongst radiation damage, recrystallization and deformation to be assessed.

238U-230Th dating of chevkinite in high-silica rhyolites from La Primavera and Yellowstone calderas

USGS Publications Warehouse

Vazquez, Jorge A.; Velasco, Noel O.; Schmitt, Axel K.; Bleick, Heather A.; Stelten, Mark E.

2014-01-01

Application of 238U-230Th disequilibrium dating of accessory minerals with contrasting stabilities and compositions can provide a unique perspective on magmatic evolution by placing the thermochemical evolution of magma within the framework of absolute time. Chevkinite, a Th-rich accessory mineral that occurs in peralkaline and metaluminous rhyolites, may be particularly useful as a chronometer of crystallization and differentiation because its composition may reflect the chemical changes of its host melt. Ion microprobe 128U-230Th dating of single chevkinite microphenocrysts from pre- and post-caldera La Primavera, Mexico, rhyolites yields model crystallization ages that are within 10's of k.y. of their corresponding K-Ar ages of ca. 125 ka to 85 ka, while chevkinite microphenocrysts from a post-caldera Yellowstone, USA, rhyolite yield a range of ages from ca. 110 ka to 250 ka, which is indistinguishable from the age distribution of coexisting zircon. Internal chevkinite-zircon isochrons from La Primavera yield Pleistocene ages with ~5% precision due to the nearly two order difference in Th/U between both minerals. Coupling chevkinite 238U-230Th ages and compositional analyses reveals a secular trend of Th/U and rare earth elements recorded in Yellowstone rhyolite, likely reflecting progressive compositional evolution of host magma. The relatively short timescale between chevkinite-zircon crystallization and eruption suggests that crystal-poor rhyolites at La Primavera were erupted shortly after differentiation and/or reheating. These results indicate that 238U-230Th dating of chevkinite via ion microprobe analysis may be used to date crystallization and chemical evolution of silicic magmas.

Early Paleozoic tectonic reconstruction of Iran: Tales from detrital zircon geochronology

NASA Astrophysics Data System (ADS)

Moghadam, Hadi Shafaii; Li, Xian-Hua; Griffin, William L.; Stern, Robert J.; Thomsen, Tonny B.; Meinhold, Guido; Aharipour, Reza; O'Reilly, Suzanne Y.

2017-01-01

In this study we use detrital zircons to probe the Early Paleozoic history of NE Iran and evaluate the link between sediment sources and Gondwanan pre-Cadomian, Cadomian and younger events. U-Pb zircon ages and Hf isotopic compositions are reported for detrital zircons from Ordovician and Early Devonian sedimentary rocks from NE Iran. These clastic rocks are dominated by zircons with major age populations at 2.5 Ga, 0.8-0.6 Ga, 0.5 Ga and 0.5-0.4 Ga as well as a minor broad peak at 1.0 Ga. The source of 2.5 Ga detrital zircons is enigmatic; they may have been supplied from the Saharan Metacraton (or West African Craton) to the southwest or Afghanistan-Tarim to the east. The detrital zircons with age populations at 0.8-0.6 Ga probably originated from Cryogenian-Ediacaran juvenile igneous rocks of the Arabian-Nubian Shield; this inference is supported by their juvenile Hf isotopes, although some negative εHf (t) values suggest that other sources (such as the West African Craton) were also involved. The age peak at ca 0.5 Ga correlates with Cadomian magmatism reported from Iranian basement and elsewhere in north Gondwana. The variable εHf (t) values of Cadomian detrital zircons, resembling the εHf (t) values of zircons in magmatic Cadomian rocks from Iran and Taurides (Turkey), suggest an Andean-type margin and the involvement of reworked older crust in the generation of the magmatic rocks. The youngest age population at 0.5-0.4 Ga is interpreted to represent Gondwana rifting and the opening of Paleotethys, which probably started in Late Cambrian-Ordovician time. A combination of U-Pb dating and Hf-isotope data from Iran, Turkey and North Gondwana confirms that Iran and Turkey were parts of Gondwana at least until late Paleozoic time.

Peculiar Feldspar And Quartz Inclusions Within Zircons From Anorthosites, North Eastern Desert, Egypt

NASA Astrophysics Data System (ADS)

Eliwa, H. A.; Dawoud, M. I.; Khalaf, I. M.; Negendank, J. F.; Itaya, T.

2004-12-01

Zircons from three anorthosite outcrops along Wadi Dib area, north Eastern Desert of Egypt contain abundant and conspicuous inclusions of quartz, feldspar, amphibole and apatite. These anorthosites, as (50-100m thick) layers, represent the top of mafic-ultramafic intrusions exhibiting rhythmic layering visible by reputation of melanocratic and leucocratic layers. Field and microscopic studies exhibit that these anorthosites were affected by the action of residual magmatic solutions associated with the late stage crystallization of the younger granites, which modified their mineralogical composition. They are composed totally of plagioclase with subordinate amount of clinoenstatite, augite, amphibole, biotite, K-feldspar, and quartz. Accessories are magnetite, ilmenite, apatite and zircon. The abundance and the mode of occurrence of K-feldspar, quartz, and biotite with apatite and zircon among the megacrysts suggest their formation is ascribed to the interaction with the residual solutions. The microprobe data exhibit difference between feldspar and amphiboles contained herein zircons and those as anorthosite mineral constituents. The genetic relationship between zircons and their inclusions suggests later growth of zircons than inclusions and most probably at the final stage of rock modification. Zircons are magmatic and found in the interstitial feldspar and quartz among plagioclase megacrysts in aggregates or as individual grains. The microscopic and SEM images investigation exhibit that most zircons are subhedral to euhedral equant and prismatic crystals. Most zircons have same range of crystal morphologies and internal growth structures with predominance of prism /{100/} and pyramid /{101/} and occasionally prism /{110/} and pyramid /{111/}. No evidences for poly-faceted grains, inherited cores or later overgrowths were detected. CL images distinguished zircons with visible core-rim structures and others with regular and continuous growth zones contained herein various inclusions. The dark CL cores in the core-rim structured zircons are higher in U, Y and sometimes Hf relative to the CL bright rims. Microprobe data and x-ray chemical mapping of various zoned zircons suggest that U and Y with sometimes Hf have a negative correlation to the CL brightness, while Th doesn't exhibit any significant correlation.

Geochronology and thermobarometry of the granitoid rocks within the Vaasa granite-migmatite complex, western Finland

NASA Astrophysics Data System (ADS)

Kurhila, Matti; Kotilainen, Anna; Tiljander, Mia; Hölttä, Pentti; Korja, Annakaisa

2015-04-01

The Vaasa granite-migmatite dome in west-central Finland has been formed in the Svecofennian orogeny, after the main collisional stage at ~1.9 Ga. The structure consists of a granite-migmatite core surrounded by metasedimentary rocks with outward decreasing metamorphic grade. The core comprises anatectic garnet-bearing granites, diatexites, pyroxene granites, and minor intrusive granodiorites. Geochemically, all of the rocks are peraluminous and magnesian. The Vaasa granites have close to average upper crustal compositions, and they show signs of titanite and plagioclase fractionation. The heavy REEs vary strongly according to garnet retention. Zircon U-Pb ages for these rock types indicate crystallization at 1875 Ma for the diatexites and garnet-bearing granites and at 1870 Ma for the pyroxene granites. Melt-forming temperatures are estimated by zircon and monazite saturation temperatures, and by Al/Ti ratios. No clear difference in the melting temperatures of the various rock types could be detected. However, while the monazite and zircon saturation temperatures point to temperatures around 800 ° C, the Al-Ti thermometer gives consistently about 100 ° C degrees higher results. Given the anatectic and felsic nature of the rocks, the lower temperature estimates seem more probable. Crystallization temperatures and pressures were calculated with the help of mineral chemical analyses. Garnet-biotite-plagioclase-quartz thermobarometry, and Al-in-hornblende barometry indicate pressures of 5.5-6 kbars for the diatexites, the pyroxene granites and an intrusive granodiorite. Significantly lower pressures of 2-4 kbars are recorded for the garnet-bearing granites. The garnet-biotite thermometer implies crystallization temperatures between 650 - 700 ° C for the pyroxene granites and the diatexites, and upto 600 ° C for the garnet-bearing granites. These results are markedly lower than those indicated by the whole-rock saturation temperatures of the same rocks. This may suggest that the melting has been non-saturated or that the post-crystallization leaching has affected the mineral compositions.

Zircon U-Pb ages and Sr-Nd isotope ratios for the Sirstan granitoid body, NE Iraq: Evidence of magmatic activity in the Middle Cretaceous Period

NASA Astrophysics Data System (ADS)

Abdulzahra, Imad Kadhim; Hadi, Ayten; Azizi, Hossein; Asahara, Yoshihiro; Yamamoto, Koshi

2017-03-01

The Sirstan granitoid (SG), comprising diorite and granodiorite, is located in the Shalair Valley area, in the northeastern part of Iraq within the Sanandaj-Sirjan Zone (SSZ) of the Zagros Orogenic Belt. The U-Pb zircon dating of the SG rocks has revealed a concordia age of 110 Ma, which is interpreted as the age of crystallization of this granitoid body during the Middle Cretaceous. The whole-rock Rb-Sr isochron data shows an age of 52.4 ± 9.4 Ma (MSWD = 1.7), which implies the reactivation of the granitoid body in the Early Eocene due to the collision between the Arabian and Iranian plates. These rocks show metaluminous affinity with low values of Nb, Ta and Ti compared to chondrite, suggesting the generation of these rocks over the subduction zone in an active continental margin regime. The SG rocks are hornblende-bearing I-type granitoids with microgranular mafic enclaves. The positive values of ɛNd (t = 110 Ma) (+0.1 to +2.7) and the low (87Sr/86Sr)i ratios (0.7044 to 0.7057) indicate that the magma source of the SG granitoids is a depleted subcontinental mantle. The chemical and isotope compositions show that the SG body originated from the metasomatic mantle without a major role for continental contamination. Our findings show that the granitoid bodies distributed in the SSZ were derived from the continuous Neo-Tethys subduction beneath the SSZ in Mesozoic times and that the SSZ was an active margin in the Middle Cretaceous.

Isotopic complexities and the age of the Delfonte volcanic rocks, eastern Mescal Range, southeastern California: Stratigraphic and tectonic implications

USGS Publications Warehouse

Fleck, R.J.; Mattinson, J.M.; Busby, C.J.; Carr, M.D.; Davis, G.A.; Burchfiel, B.C.

1994-01-01

Combined U-Pb zircon, Rb-Sr, 40Ar/39Ar laser-fusion, and conventional K-Ar geochronology establish a late Early Cretaceous age for the Delfonte volcanic rocks. U-Pb zircon analyses define a lower intercept age of 100.5 ± 2 Ma that is interpreted as the crystallization age of the Delfonte sequence. Argon studies document both xenocrystic contamination and postemplacement Ar loss. Rb-Sr results from mafic lavas at the base of the sequence demonstrate compositionally correlated variations in initial 87Sr/86Sr ratios (Sri) from 0.706 for basalts to 0.716 for andesitic compositions. This covariation indicates substantial mixing of subcontinental lithosphere with Proterozoic upper crust. Correlations between Rb/Sr and Sri may result not only in pseudoisochrons approaching the age of the crustal component, but also in reasonable but incorrect apparent ages approaching the true age.Ages obtained in this study require that at least some of the thrust faulting in the Mescal Range-Clark Mountain portion of the foreland fold-and-thrust belt occurred later than ca. 100 Ma and was broadly contemporaneous with emplacement of the Keystone thrust plate in the Spring Mountains to the northeast. Comparison of the age and Rb-Sr systematics of ash-flow tuff boulders in the synorogenic Lavinia Wash sequence near Goodsprings, Nevada, with those of the Delfonte volcanic rocks supports a Delfonte source for the boulders. The 99 Ma age of the Lavinia Wash sequence is nearly identical to the Delfonte age, requiring rapid erosion, transport, and deposition following Delfonte volcanism.

High-pressure granulites in the Fuping Complex of the central North China Craton: Metamorphic P-T-t evolution and tectonic implications

NASA Astrophysics Data System (ADS)

Qian, Jiahui; Yin, Changqing; Zhang, Jian; Ma, Li; Wang, Luojuan

2018-04-01

Mafic granulites in the Fuping Complex occur as lenses or boudins within high-grade TTG (Trondhjemite-Tonalite-Granodiorite) gneisses. Petrographic observations reveal four generations of mineral assemblage in the granulites: an inclusion assemblage of hornblende + plagioclase + ilmenite + quartz within garnet core; an inferred peak assemblage composed of garnet ± hornblende + plagioclase + clinopyroxene + rutile/ilmenite + quartz; a decompression assemblage characterized by symplectites of clinopyroxene ± orthopyroxene + plagioclase, coronae of plagioclase ± clinopyroxene ± hornblende around embayed garnet porphyroblasts or a two-pyroxene association; and a late amphibolite-facies retrogressive assemblage. Two representative samples were used for pseudosection modeling in NCFMASHTO model system to determine their metamorphic evolution. The results show that these granulites experienced a high-pressure stage of metamorphism with peak P-T conditions of 12-13 kbar and 760-800 °C (Pmax) and a post-peak history under P-T conditions of ∼9.0 kbar and 805-835 °C (Tmax), indicating a nearly isothermal decompression process (ITD) with a slight heating. Metamorphic evolution from the Pmax to the Tmax is predicted to be dominated by garnet breakdown through continuous metamorphic reactions of garnet + quartz ± diopside = hornblende + plagioclase + liquid and garnet + quartz + hornblende = plagioclase + diopside + liquid + orthopyroxene. Further metamorphic evolution after the Tmax is dominated by cooling, suggesting that high-pressure (HP) granulites may also exist in the Fuping Complex. Metamorphic zircons in the Fuping HP mafic granulites have left inclined REE patterns, Ti contents of 1.68-6.88 ppm and crystallization temperatures of 602-712 °C. SIMS zircon U-Pb dating on these zircons yields 207Pb/206Pb ages of 1891 ± 14 Ma and 1849 ± 6 Ma, interpreted to represent the cooling stage of metamorphism. The P-T-t evolution of the Fuping HP mafic granulites records well the protracted Paleoproterozoic orogenic event occurred in the central North China Craton.

Unravelling the pre-Variscan evolution of the Habach terrane (Tauern Window, Austria) by U-Pb SHRIMP zircon data

NASA Astrophysics Data System (ADS)

Eichhorn, Roland; Loth, Georg; Kennedy, Allen

2001-08-01

The U-Pb SHRIMP age determinations of zircons from the Habach terrane (Tauern Window, Austria) reveal a complex evolution of this basement unit, which is exposed in the Penninic domain of the Alpine orogen. The oldest components are found in zircons of a metamorphosed granitoid clast, of a migmatitic leucosome, and of a meta-rhyolitic (Variscan) tuff which bear cores of Archean age. The U-Pb ages of discordant zircon cores of the same rocks range between 540 and 520 Ma. It is assumed that the latter zircons were originally also of Archean origin and suffered severe lead loss, whilst being incorporated into Early-Cambrian volcanic arc magmas. The provenance region of the Archean (2.64-2.06 Ga) zircons is assumed to be a terrane of Gondwana affinity: i.e., the West African craton (Hoggar Shield, Reguibat Shield). The Caledonian metamorphism left a pervasive structural imprint in amphibolite facies on rocks of the Habach terrane; it is postdated by discordant zircons of a migmatitic leucosome at <440 Ma (presumably ca. 420 Ma). Alpine and Variscan upper greenschist- to amphibolite-facies conditions caused partial lead loss in zircons of a muscovite gneiss ('white schist') only, where extensive fluid flow and brittle deformation due to its position near a nappe-sole thrust enhanced the grains' susceptibility to isotopic disturbance. The Habach terrane - an active continental margin with ensialic back-arc development - showed subduction-induced magmatic activity approx. between 550 and 507 Ma. Back-arc diorites and arc basalts were intruded by ultramafic sills and subsequently by small patches of mantle-dominated unaltered and (in the vicinity of a major tungsten deposit) altered granitoids. Fore-arc (shales) and back-arc (greywackes, cherts) basin sediments as well as arc and back-arc magmatites were not only nappe-stacked by the Caledonian compressional regime closing the presumably narrow oceanic back-arc basin and squeezing mafic to ultramafic cumulates out of high-level magma chambers (496-482 Ma). It also induced uplift and erosion of deeply rooted crystalline complexes and triggered the development of a successor basin filled with predominantly clastic greywacke-arkosic sediments. The study demonstrates that the basement rocks exposed in the Habach terrane might be the 'missing link' between similar units of the more westerly positioned External domain (i.e., Aar, Aiguilles Rouges, Mont Blanc) and the Austroalpine domain to the east (Oetztal, Silvretta).

Origin and age of zircon-bearing chromitite layers from the Finero phlogopite peridotite (Ivrea-Verbano Zone, Western Alps) and geodynamic consequences

NASA Astrophysics Data System (ADS)

Zanetti, Alberto; Giovanardi, Tommaso; Langone, Antonio; Tiepolo, Massimo; Wu, Fu-Yuan; Dallai, Luigi; Mazzucchelli, Maurizio

2016-10-01

An investigation has been performed on three chromitite layers segregated in dunite bodies of the Phlogopite Peridotite mantle unit in the Finero complex (FPP, Ivrea-Verbano Zone, Southern Alps) aimed at providing new constraints to their origin and evolution. Field relationships, the sub-chondritic Hf isotopic composition of the zircons (εHf(188) as low as - 5.4), the heavy O isotopic composition of zircons and pyroxenes (δ18O up to 6.9‰), the strict similarity of the trace element composition between the clinopyroxenes and amphiboles from the chromitites and those from the phlogopite harzburgites and pyroxenites forming the typical FPP association, as well as the REE composition of zircons, which approaches equilibrium with the associate clinopyroxene, suggest that the studied chromitites were segregated from melts, highly contaminated from continental crust, during the pervasive cycle of metasomatism recorded by the FPP. An LA-ICP-HRMS survey of chromitite zircon grains has provided Early Jurassic U-Pb ages mostly between 199 ± 3 Ma and 178 ± 2 Ma, with a pronounced peak at 187 Ma. Relevant exceptions are inherited domains of two grains giving Triassic ages of 242 ± 7 Ma and 229 ± 7 Ma, and a third homogeneous zircon giving 208 ± 3 Ma. Our geochronological data and those reported in the literature show that the FPP chromitites have zircon populations with different internal CL textures, but the same sub-chondritic Hf isotopic composition, which define an overall U-Pb age span from 290 Ma to 180. The segregation of the chromitite layers and the main pervasive metasomatism likely occurred in the Early Permian (in a post-collisional, transtensional setting) or before (possibly, in a subduction-related setting). The rejuvenation of the zircon ages was accompanied by a progressive disappearance of the internal zoning, interpreted as the result of a prolonged residence at mantle depths with progressive re-equilibration of the U-Pb system due to thermal perturbations. The age peak at 187 Ma is argued to constrain the timing of FPP exhumation at shallower, crustal levels. This process was characterised by an important reheating event, possibly due to lithospheric hyperextension. The evolution of the FPP appears completely different than that of mantle bodies of the central IVZ (i.e., the Val Sesia-type bodies), which were emplaced within the continental crust, as part of accretionary prisms, at or before the end of the Variscan orogeny.

Geochronological and geochemical constraints on the petrogenesis of Middle Paleozoic (Kwangsian) massive granites in the eastern South China Block

NASA Astrophysics Data System (ADS)

Zhang, Feifei; Wang, Yuejun; Zhang, Aimei; Fan, Weiming; Zhang, Yuzhi; Zi, Jianwei

2012-10-01

To achieve a better understanding of the Kwangsian orogenic event of the eastern South China Block, this paper documents a set of new zircon U-Pb geochronological and Hf isotopic data and whole-rock elemental and Sr-Nd isotopic analytical results for the representative massive granite intrusions across the Jiangshan-Shaoxing fault. The studied samples are classified into two groups, representing the rocks from the Cathaysia Block to the east of the Jiangshan-Shaoxing Fault (Group 1) and those from the eastern Yangtze Block between the Anhua-Luocheng and Jiangshan-Shaoxing faults (Group 2). The Group 1 samples gave the zircon U-Pb ages of 405-454 Ma and ɛHf(t) values of - 3.6 to - 15.2 with Hf model ages of 1.6-2.4 Ga. Group 2 yielded the zircon U-Pb ages of 400-432 Ma and ɛHf(t) values of - 0.2 to - 12.7 with Hf model ages of 1.3-2.2 Ga. Geochemically, the Group 1 samples (A/CNK = 1.02-1.43) have relatively lower Al2O3, MgO, CaO, P2O5 and ɛNd(t) but higher K2O + Na2O than those of Group 2 (A/CNK = 0.93-1.44). Both groups show similar chondrite-normalized patterns of rare-earth elements with Eu/Eu* values of 0.15-0.92 and strongly negative Ba, Sr, Nb, P and Ti anomalies in primitive mantle-normalized spider diagrams. Their ɛNd(t) values range from - 11.1 to - 8.0 for Group 1, and - 8.9 to - 5.0 for Group 2, generally similar to those of Precambrian paragneiss and contemporaneous gneissoid granites in the eastern South China Block. Our geochronological results indicate that the Kwangsian massive granites in the eastern South China Block were crystallized between 400 Ma and 454 Ma, synchronous to the Kwangsian gneissoid granites along the Wugong and Wuyi-Baiyun-Yunkai domains in the eastern South China Block. The synthesis of these whole-rock geochemical and in-situ zircon Hf isotopic data suggests that both the Group 1 and 2 granites across the Jiangshan-Shaoxing Fault were predominantly derived from a crustal source with some proportional metapelitic and metaigneous components from the Precambrian basement. The input of juvenile mantle-derived magma is insignificant. Taking into account other available evidences, it is proposed that the Jiangshan-Shaoxing Fault might be a pre-existing boundary between the Yangtze and Cathaysia Blocks, which was reactivated during the middle Paleozoic (Kwangsian) orogenic event. The massive granitic magmatism was probably resulted from the doubly crustal thickening and subsequent isostatic readjustment in an intracontinental tectonic regime.