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.

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.

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.

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.

Conventional U-Pb dating versus SHRIMP of the Santa Barbara Granite Massif, Rondonia, Brazil

USGS Publications Warehouse

Sparrenberger, I.; Bettencourt, Jorge S.; Tosdal, R.M.; Wooden, J.L.

2002-01-01

The Santa Ba??rbara Granite Massif is part of the Younger Granites of Rondo??nia (998 - 974 Ma) and is included in the Rondo??nia Tin Province (SW Amazonian Craton). It comprises three highly fractionated metaluminous to peraluminous within-plate A-type granite units emplaced in older medium-grade metamorphic rocks. Sn-mineralization is closely associated with the late-stage unit. U-Pb monazite conventional dating of the early-stage Serra do Cicero facies and late-stage Serra Azul facies yielded ages of 993 ?? 5 Ma and 989 ?? 13 Ma, respectively. Conventional multigrain U-Pb isotope analyses of zircon demonstrate isotopic disturbance (discordance) and the preservation of inherited older zircons of several different ages and thus yield little about the ages of Sn-granite magmatism. SHRIMP U-Pb ages for the Santa Ba??rbara facies association yielded a 207Pb/206Pb weighted-mean age of 978 ?? 13 Ma. The textural complexity of the zircon crystals of the Santa Ba??rbara facies association, the variable concentrations of U, Th and Pb, as well as the mixed inheritance of zircon populations are major obstacles to using conventional multigrain U-Pb isotopic analyses. Sm-Nd model ages and ??Nd (T) values reveal anomalous isotopic data, attesting to the complex isotopic behaviour within these highly fractionated granites. Thus, SHRIMP U-Pb zircon and conventional U-Pb monazite dating methods are the most appropriate to constrain the crystallization age of the Sn-bearing granite systems in the Rondo??nia Tin Province.

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

Age of the Lava Creek supereruption and magma chamber assembly at Yellowstone based on 40Ar/39Ar and U-Pb dating of sanidine and zircon crystals

NASA Astrophysics Data System (ADS)

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

2015-12-01

The last supereruption from the Yellowstone Plateau formed Yellowstone caldera and ejected the >1000 km3 of rhyolite that composes the Lava Creek Tuff (LCT). Tephra from the eruption blanketed much of the western United States, and is a key Quaternary chronostratigraphic marker, in particular for dating deposition of mid-Pleistocene glacial and pluvial deposits in western North America. We performed 40Ar/39Ar dating of single sanidines to delimit eruption age, and ion microprobe U-Pb and trace-element analyses of crystal faces on single zircons to characterize magmatic evolution and date near-eruption crystallization, as well as analyses of crystal interiors to date the interval of zircon crystallization. Sanidines from the two LCT members A and B yield an 40Ar/39Ar isochron date of 631 ± 4 ka (2σ). Crystal faces on zircons from both members yield a weighted mean 206Pb/238U date of 627 ± 6 ka (2σ) and have trace element concentrations that vary with eruptive stratigraphy. Zircon interiors yield a weighted mean 206Pb/238U date of 660 ± 6 ka, and reveal reverse and/or oscillatory zoning of trace element concentrations, with many crystals containing high-U concentrations and dark cathodoluminescence (CL) cores. These crystals with high-U cores are possibly sourced from 'defrosting' of melt-impregnated margins of the growing subvolcanic reservoir. LCT sanidines mirror the variation of zircon composition within the eruptive stratigraphy, with crystals from upper LCT-A and basal LCT-B having bright-CL rims with high Ba concentrations, suggesting late crystallization after addition of less evolved silicic magma. The occurrence of distal LCT in stratigraphic sequences marking the Marine Isotope Stage 16-15 transition supports the apparent eruption age of ca. 631 ka. These results reveal that Lava Creek zircons record episodic heating, renewed crystallization, and an overall up-temperature evolution for Yellowstone's subvolcanic reservoir in the 103-104 year interval

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.

Age of the Lava Creek supereruption and magma chamber assembly at Yellowstone based on 40Ar/39Ar and U-Pb dating of sanidine and zircon crystals

USGS Publications Warehouse

Matthews, Naomi E.; Vazquez, Jorge A.; Calvert, Andrew T.

2015-01-01

The last supereruption from the Yellowstone Plateau formed Yellowstone caldera and ejected the >1000 km3 of rhyolite that composes the Lava Creek Tuff. Tephra from the Lava Creek eruption is a key Quaternary chronostratigraphic marker, in particular for dating the deposition of mid Pleistocene glacial and pluvial deposits in western North America. To resolve the timing of eruption and crystallization history for the Lava Creek magma, we performed (1) 40Ar/39Ar dating of single sanidine crystals to delimit eruption age and (2) ion microprobe U-Pb and trace-element analyses of the crystal faces and interiors of single zircons to date the interval of zircon crystallization and characterize magmatic evolution. Sanidines from the two informal members composing Lava Creek Tuff yield a preferred 40Ar/39Ar isochron date of 631.3 ± 4.3 ka. Crystal faces on zircons from both members yield a weighted mean 206Pb/238U date of 626.5 ± 5.8 ka, and have trace element concentrations that vary with the eruptive stratigraphy. Zircon interiors yield a mean 206Pb/238U date of 659.8 ± 5.5 ka, and reveal reverse and/or oscillatory zoning of trace element concentrations, with many crystals containing high U concentration cores that likely grew from highly evolved melt. The occurrence of distal Lava Creek tephra in stratigraphic sequences marking the Marine Isotope Stage 16–15 transition supports the apparent eruption age of ∼631 ka. The combined results reveal that Lava Creek zircons record episodic heating, renewed crystallization, and an overall up-temperature evolution for Yellowstone's subvolcanic reservoir in the 103−104 year interval before eruption.

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.

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

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

Zircon Zoning, Trace Elements and U-Pb Dates Reveal Crustal Foundering Beneath the Pamir

NASA Astrophysics Data System (ADS)

Hacker, B. R.; Shaffer, M. E. F.; Ratschbacher, L.; Kylander-Clark, A. R.

2017-12-01

Xenoliths that erupted in the SE Pamir of Tajikistan at 11.2 Ma from 1000-1050°C and 90 km depth illuminate what happens when crust founders into the mantle. The xenoliths are a broad range of crustal rock types and contain abundant xenoliths whose U-Pb isotopic ratios and trace-element contents were examined by laser-ablation split stream inductively coupled plasma mass spectrometry. Cathodoluminescence imaging of the grains shows igneous cores with oscillatory zoning overprinted by substantial recrystallization. The bulk of the U-Pb dates are concordant and range from 160 Ma to 11 Ma. The range of dates suggest that the xenoliths were likely derived from the Jurassic-Cretaceous Andean-style magmatic arc and its Proterozoic-Mesozoic host rocks along the southern margin of Asia. The zircons show distinct changes in Eu anomaly, Lu/Gd ratio, and Ti concentrations that are interpreted to indicate garnet growth and minimal heating at 22-20 Ma, and then 200-300°C of heating, 25 km of burial, and alkali-carbonate melt injection at 14-11 Ma. These changes are interpreted to coincide with: i) heat input due to Indian slab breakoff at 22‒20 Ma; ii) rapid thickening and foundering of the Pamir lithosphere at 14‒11 Ma, prior to and synchronous with collision between deep Indian and Asian lithospheres beneath the Pamir.

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

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

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.

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

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.

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

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

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.

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

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.

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.

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

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

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.

Zircon U-Th and U-Pb Ages From Quaternary Silicic Volcanic and Plutonic Rocks, and Their Bearing on Granitoid Batholiths

NASA Astrophysics Data System (ADS)

Bacon, C. R.

2007-12-01

In the ten years since publication of M. Reid et al.'s seminal paper on zircon ages from rhyolites (EPSL 150:2-39, 1997) >20 papers have appeared on SIMS 238U-230Th and 238U-206Pb geochronology of zircon from silicic volcanic rocks, plutonic xenoliths, and young intrusions. In some cases, as well as for U-Pb studies of Tertiary granitoids, plutonic samples are interpreted in the context of related volcanism. These geochronologic data have advanced conceptual models of silicic magma genesis and pluton construction. Of fundamental importance are discoveries that zircons in volcanic rocks typically pre-date eruption by 10's to 100's of kyr and that multiple zircon populations are common; these crystals are "antecrysts" recycled from intrusive rocks or crystal mush of the system that vented. Resolving such age differences is possible with U-Th at <300 ka but is challenging with U-Pb, where SIMS precision limits resolution of differences on the order of 100 kyr for Pleistocene-Miocene zircons. Cathodoluminescence (CL) imaging of polished crystals guides beam placement but leads to sampling bias that favors high-U regions. Thus, although model-age histograms and relative probability plots identify zircon age populations, they are unlikely to accurately define relative abundances of age groups. Microbeam analysis collects data for the entire volume sampled but only SIMS depth-profiling into crystal faces can spatially resolve fine zones. ID-TIMS analysis of CL-imaged zircon fragments can improve U-Pb precision. SIMS complements geochronology with trace element fingerprints of zircon growth environments and enables Ti-in-zircon thermometry. Literature examples illustrate recent findings: (1) rhyodacite lava at Crater Lake contains zircons derived from late Pleistocene granodiorite represented by blocks ejected in the caldera-forming eruption; (2) zircons in Mount St. Helens dacites grew at sub-eruption temperatures and pre-date eruptions by up to 250 kyr; (3) Miocene

LA-ICP-MS Pb-U Dating of Young Zircons from the Kos-Nisyros Volcanic Centre, SE Aegean Arc (Greece)

NASA Astrophysics Data System (ADS)

Guillong, M.; Von Quadt, A.; Peytcheva, I.; Bachmann, O.

2014-12-01

Zircon Pb-U dating has become a key technique for answering many important questions in geosciences. This paper describes a new LA-ICP-MS approach. We show, using previously dated samples of a large quaternary rhyolitic eruption in the Kos-Nisyros volcanic centre (the 161 ka Kos Plateau Tuff), that the precision of our LA-ICP-MS method is as good as via SHRIMP, while ID-TIMS measurements confirm the accuracy. Gradational age distribution over >140 ka of the Kos zircons and the near-absence of inherited cores indicate near-continuous crystallisation in a growing magma reservoir with little input from wall rocks. Previously undated silicic eruptions from Nisyros volcano (Lower Pumice, Nikia Flow, Upper Pumice), which are stratigraphically constrained to have happened after the Kos Plateau Tuff, are dated to be younger than respectively 124 ± 35 ka, 111 ± 42 ka and 70 ± 24 ka. Samples younger than 1 Ma were corrected for initial thorium disequilibrium using a new formula that also accounts for disequilibrium in 230Th decay. Guillong, M. et al., 2014, JAAS, 29, p. 963-967; doi: 10.1039/c4ja00009a.

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.

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.

LA-ICP-MS and SIMS U-Pb and U-Th zircon geochronological data of Late Pleistocene lava domes of the Ciomadul Volcanic Dome Complex (Eastern Carpathians).

PubMed

Lukács, Réka; Guillong, Marcel; Schmitt, Axel K; Molnár, Kata; Bachmann, Olivier; Harangi, Szabolcs

2018-06-01

This article provides laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and secondary ionization mass spectrometry (SIMS) U-Pb and U-Th zircon dates for crystals separated from Late Pleistocene dacitic lava dome rocks of the Ciomadul Volcanic Dome Complex (Eastern Carpathians, Romania). The analyses were performed on unpolished zircon prism faces (termed rim analyses) and on crystal interiors exposed through mechanical grinding an polishing (interior analyses). 206 Pb/ 238 U ages are corrected for Th-disequilibrium based on published and calculated distribution coefficients for U and Th using average whole-rock and individually analyzed zircon compositions. The data presented in this article were used for the Th-disequilibrium correction of (U-Th)/He zircon geochronology data in the research article entitled "The onset of the volcanism in the Ciomadul Volcanic Dome Complex (Eastern Carpathians): eruption chronology and magma type variation" (Molnár et al., 2018) [1].

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

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.

Resolution, the key to unlocking granite petrogenesis using zircon U-Pb - Lu-Hf studies

NASA Astrophysics Data System (ADS)

Tapster, Simon; Horstwood, Matthew; Roberts, Nick M. W.; Deady, Eimear; Shail, Robin

2017-04-01

Coarse-scale understanding of crustal evolution and source contributions to igneous systems has been drastically enhanced by coupled zircon U-Pb and Lu-Hf data sets. These are now common place and potentially offer advantages over whole-rock analyses by resolving heterogeneous source components in the complex crystal cargos of single hand-samples. However, the application of coupled zircon U-Pb and Lu-Hf studies to address detailed petrogenetic questions faces a crisis of resolution - On the one hand, micro-beam analytical techniques have high spatial resolution, capable of interrogating crystals with complex growth histories. Yet, the >1-2% temporal resolution of these techniques places a fundamental limitation on their utility for developing petrogenetic models. This limitation in data interpretation arises from timescales of crystal recycling or changes in source evolution that are often shorter than the U-Pb analytical precision. Conversely, high-precision CA-ID-TIMS U-Pb analysis of single whole zircons and solution MC-ICP-MS Lu-Hf isotopes of column washes (Hf masses equating to ca. 10-50 ng) have much greater temporal resolution (<0.1%), yet lack the spatial resolution to deal with complex crystal growth. Analyses homogenize any heterogeneity within the zircon and convolute the petrogenetic model. A balance must be struck between spatial and temporal resolution to address petrogenetic issues. Here, we demonstrate that micro-sampling of complex xenocryst-rich zircon crystals (e.g. <40 µm zircon tips) from the granitic post-Variscan Cornubian Batholith (SW England), in tandem with low-common Pb blank CA-ID-TIMS U-Pb chemistry, permits the analysis of zircon volumes that approach those of LA-ICPMS analyses, whilst simultaneously retaining the majority of the temporal resolution associated with the CA-ID-TIMS U-Pb technique. The low volume of zircon within these analyses may only provide <5 ng Hf, and therefore gaining useful precision from Lu-Hf isotopes is

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.

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

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.

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

Dating kimberlite emplacement with zircon and perovskite (U-Th)/He geochronology

NASA Astrophysics Data System (ADS)

Stanley, Jessica; Flowers, Rebecca

2017-04-01

Kimberlites provide rich information about the composition and evolution of cratonic lithosphere. They can entrain xenoliths and xenocrysts from the entire lithospheric column as they transit rapidly to the surface, providing information on the state of the deep lithosphere as well as any sedimentary units covering the craton at the time of eruption. Accurate geochronology of these eruptions is key for interpreting this information and discerning spatiotemporal trends in lithospheric evolution, but kimberlites can sometimes be difficult to date with available methods. Here we explore whether (U-Th)/He dating of zircon and perovskite can serve as reliable techniques for determining kimberlite emplacement ages by dating a suite of sixteen southern African kimberlites by zircon and/or perovskite (U-Th)/He (ZHe, PHe). Most samples with abundant zircon yielded ZHe dates reproducible to ≤15% dispersion that are in good agreement with published eruption ages, though there were several samples that were more scattered. Since the majority of dated zircon were xenocrystic, zircon with reproducible dates were fully reset during eruption or resided at temperatures above the ZHe closure temperature ( 180 °C) prior to entrainment in the kimberlite magma. We attribute scattered ZHe dates to shallowly sourced zircon that underwent incomplete damage annealing and/or partial He loss during the eruptive process. All seven kimberlites dated with PHe yielded dates reproducible to ≤15% dispersion and reasonable results. As perovskite has not previously been used as a (U-Th)/He chronometer, we conducted two preliminary perovskite 4He diffusion experiments to obtain initial estimates of its temperature sensitivity. These experiments suggest a PHe closure temperature of >300 °C. Perovskite in kimberlites is unlikely to be xenocrystic and its relatively high temperature sensitivity suggests that PHe dates will typically record emplacement rather than post-emplacement processes. ZHe

Alteration and chemical U-Th-total Pb dating of heterogeneous high-uranium zircon from a pegmatite from the Aduiskii massif, middle Urals, Russia

NASA Astrophysics Data System (ADS)

Zamyatin, Dmitry A.; Shchapova, Yuliya V.; Votyakov, Sergey L.; Nasdala, Lutz; Lenz, Christoph

2017-09-01

The U-Th-Pb isotope system in the accessory mineral zircon may be disturbed, as for instance by the secondary loss of radiogenic lead. The recognition of such alteration is crucial for the sound interpretation of geochronology results, in particular for chemical dating by means of an electron probe micro-analyser (EPMA). Here we present the example of high-U zircon samples from a granite pegmatite from the Aduiskii Massif, Middle Urals, Russia. The structural and chemical heterogeneity of samples was characterised by EPMA, including joint probability distribution (JPD) analysis of back-scattered electrons (BSE), cathodoluminescence (CL) and U M β images, and by Raman and photoluminescence (PL) spectroscopy. We found a high-U interior region (U up to 11.4 wt%) without any obvious indication of alteration. This domain has stoichiometric composition, and its Raman spectrum is similar to that of amorphous ZrSiO4. In addition, altered lower-U regions are present that are non-stoichiometric and contain non-formula elements such as Ca, Al, Fe, and water up to several wt%. Their Raman spectra yielded a band near 760-810 cm-1 which is not related to any ZrSiO4 vibration; we assign it tentatively to the symmetric stretching of (UO2)2+ groups. This assignment is supported by the observation of a fairly intense PL phenomenon whose spectral position and vibrational-coupling structure strongly indicates a uranyl-related emission. Altered zones were formed by both fluid-driven diffusion reaction and coupled dissolution-reprecipitation processes. The variation of BSE and CL intensities in amorphous high-U zircon is controlled by its chemical composition and the presence of water and uranyl groups. We have determined a weighted mean EPMA age of 246 ± 2 Ma, which agrees reasonably well with previous dating results for the Aduiskii Massif.

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

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.

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.

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.

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.

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

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.

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

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

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.

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.

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

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.

U-Pb Dating of Calcite by LA-ICPMS

NASA Astrophysics Data System (ADS)

Hacker, B. R.; Kylander-Clark, A. R.; Holder, R. M.; Nuriel, P.

2016-12-01

An emerging frontier area in geochronology is U-Pb dating of carbonate minerals by laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The spate of papers over the last few years applying LA-ICPMS to carbonate dating stems from the capability of LA-ICPMS to deal with the variable, and often low, U/Pb ratios of carbonate. LA-ICPMS is an excellent tool for efficiently screening out samples with low U/Pb ratios and provides the ability to measure many spots with different U/Pb ratios and obtain dates free of assumptions about the composition of common Pb. Because this technique is in its infancy, important questions remain. What percentage of carbonate samples have high enough U/Pbc ratios that they can be dated? What percentage of samples yield isochronous datasets? What are the limits on precision and accuracy of carbonate U/Pb dates? What is the best analytical method in the absence of isotopically homogeneous reference materials? Through the generosity of our colleagues we have acquired 8 reference materials ranging in age from 3 to 250 Ma. We have analyzed 125 unknowns from a variety of locations using a 193 nm ns laser with an 80-100 μm spot and a Nu Plasma HR-ES. We measure 207Pb/206Pb using NIST 614 glass and then calculate a 206Pb/238U correction factor based on the measured vs. known ages of the reference materials. Sixty of these samples ( 50%) have high enough U/Pb ratios that they can be dated. There is great heterogeneity among the sample suites: some have no datable samples, whereas one suite of 68 samples yielded 53 datable rocks. Of the samples with high U/Pbc ratios, a majority yielded isochronous U-Pb data, indicating that the U-Pb system closed at a given time and was not subsequently disturbed.

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.

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.

Molybdenite Re-Os, zircon U-Pb dating and Lu-Hf isotopic analysis of the Xiaerchulu Au deposit, Inner Mongolia Province, China

NASA Astrophysics Data System (ADS)

Wang, Jia-xin; Nie, Feng-Jun; Zhang, Xue-ni; Jiang, Si-hong

2016-09-01

The Xiaerchulu Au deposit, located in the Southern Orogenic Belt (SOB) of Western Inner Mongolia (WIM), is hosted in an Early Permian (271-261 Ma) volcanic-plutonic sequence. Mineralization took place in silicified biotite granites or along the contact zone between the Neoproterozoic Baiyinbaolage Group and the biotite granite. In order to constrain the timing of the Xiaerchulu mineralization and discuss the petrogenesis of the hosting granites, molybdenite Re-Os, and zircon U-Pb and, Lu-Hf, and REE, geochemical, and Sr-Nd isotopic studies were completed in this study. We measured Re-Os isotopes of six molybdenite samples from the main ore body, which yielded a weighted average model age of 261.7 ± 1.5 Ma with a MSWD of 0.55, indicating that the time of mineralization was at ca. 262 Ma. High precision U-Pb dating for the studied granites yields Permian 206Pb/238U ages ranging from 271 to 269 Ma. These age data confirm that both the intrusion and related mineralization were initiated in Early Permian period. These granites are strongly peraluminous with A/CNK = 1.11-1.12, high SiO2-K2O contents, as well as containing biotite and muscovite, indicating a petrogenesis of typical S-type granites, the above consideration is also consistent with the result of discrimination diagrams. The Re contents of molybdenite, εNd(t), and zircon εHf(t), as well as the 176Hf/177Hf values of the granites, fall into the ranges from 1.153 to 2.740 μg/g, - 11.1 to - 9.3, - 8.8 to - 0.9, and 0.282358 to 0.282688, respectively. All of this evidence suggests that the metals were derived from a predominantly crustal source, the granites originated from crust in an extensional setting, and the rejuvenation of the continent may have play an important role during the ore-forming processes of the Early Permian epoch.

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

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.

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.

Origin and evolution of multi-stage felsic melts in eastern Gangdese belt: Constraints from U-Pb zircon dating and Hf isotopic composition

NASA Astrophysics Data System (ADS)

Guo, Liang; Zhang, Hong-Fei; Harris, Nigel; Pan, Fa-Bin; Xu, Wang-Chun

2011-11-01

This integrated study of whole rock geochemistry, zircon U-Pb dating and Hf isotope composition for seven felsic rocks from the Nyingchi Complex in eastern Himalayan syntaxis has revealed a complex magmatic history for the eastern Gangdese belt. This involves multiple melt sources and mechanisms that uniquely identify the tectonic evolution of this part of the Himalayan orogen. Our U-Pb zircon dating reveals five stages of magmatic or anatectic events: 165, 81, 61, 50 and 25 Ma. The Jurassic granitic gneiss (165 Ma) exhibits εHf(t) values of + 1.4 to + 3.5. The late Cretaceous granite (81 Ma) shows variable εHf(t) values from - 0.9 to + 6.2, indicating a binary mixing between juvenile and old crustal materials. The Paleocene granodioritic gneiss (61 Ma) has εHf(t) values of + 5.4 to + 8.0, suggesting that it originated from partial melting of a juvenile crustal material. The Eocene anatexis is recorded in the leucosome, which has Hf isotopic composition similar to that of the Jurassic granite, indicating that the leucosome could be derived from partial melting of the Jurassic granite. The late Oligocene biotite granite (25 Ma) shows adakitic geochemical characteristics, with Sr/Y = 49.3-56.6. The presence of a large number of inherited zircons and negative εHf(t) values suggest that it sourced from anatexis of crustal materials. In contrast to the Gangdese batholiths that are mainly derived from juvenile crustal source in central Tibet, the old crustal materials play an important role for the magma generation of the felsic rocks, suggesting the existence of a crustal basement in the eastern Gangdese belt. These correspond to specific magmatic evolution stages during the convergence between India and Asia. The middle Jurassic granitic gneiss resulted from the northward subduction of the Neo-Tethyan oceanic slab. The late Cretaceous magmatism is probably related to the ocean ridge subduction. The Paleocene-Eocene magmatism, metamorphism and anatexis are

Combined apatite fission track and U-Pb dating by LA-ICPMS

NASA Astrophysics Data System (ADS)

Chew, D. M.; Donelick, R. A.

2012-04-01

Apatite is a common accessory mineral in igneous, metamorphic and clastic sedimentary rocks. It is a nearly ubiquitous accessory phase in igneous rocks, is common in metamorphic rocks of pelitic, carbonate, basaltic, and ultramafic composition and is virtually ubiquitous in clastic sedimentary rocks. In contrast to the polycyclic behavior of the stable heavy mineral zircon, apatite is unstable in acidic groundwaters and has limited mechanical stability in sedimentary transport systems. Apatite has many potential applications in provenance studies, particularly as it likely represents first-cycle detritus. Fission track and U-Pb dating are very powerful techniques in apatite provenance studies. They yield complementary information, with the apatite fission-track system yielding low-temperature exhumation ages and the U-Pb system yielding high-temperature cooling ages which constrain the timing of apatite crystallization. This study focuses on integrating apatite fission track and U-Pb dating by the LA-ICPMS method. Our approach is intentionally broad in scope, and is applicable to any quadrupole or rapid-scanning magnetic-sector LA-ICPMS system. Calculating uranium concentrations in fission-track dating by LA-ICPMS increases the speed of analysis and sample throughput compared to the conventional external detector method and avoids the need for neutron irradiation (Hasebe et al., 2004). LA-ICPMS-based uranium measurements in apatite are measured relative to an internal concentration standard (typically 43Ca). Ca in apatite is not always stochiometric as minor cations (Mn2+, Sr2+, Ba2+ and Fe2+) and REE can substitute with Ca2+. These substitutions must be quantified by multi-elemental LA-ICPMS analyses. Such data are also useful for discriminating between different apatite populations in sedimentary or volcaniclastic rocks based on their trace-element chemistry. Low U, Th and radiogenic Pb concentrations, elevated common Pb / radiogenic Pb ratios and U-Pb elemental

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.

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.

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

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

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

U-Pb Dating of Unabraded Detrital Zircon Metamorphic Rims in the Nanaimo Basin, British Columbia

NASA Astrophysics Data System (ADS)

Boivin, M. P.; Guest, B.; Matthews, W.

2016-12-01

Thin metamorphic rims on detrital zircons from the Nanaimo Basin in SW British Columbia offer a unique opportunity to further constrain the source of these zircons, helping to resolve the long standing Baja BC controversy. Here we present an analytical approach for dating thin zircon rims and use it to show that zircons from the Nanaimo Basin are most likely derived from metamorphic rocks in southern California. Conventional in-situ laser ablation sample preparation typically requires mounting and polishing zircon grains to expose their core. However, in order to date these thin metamorphic zircon rims a depth-profiling approach on unabraded grains was employed. Zircon grains from the Upper Cretaceous Geoffrey, Spray, and Gabriola formations of the Nanaimo Group exposed on Denman and Hornby Islands (British Columbia) were sorted into five groups based on morphology. The zircons were then mounted on tape along with several grains of a well-characterised zircon reference material to validate the uncertainty of the method. The zircons were then imaged using a Zygo Zescope optical profilometer in order to correct for grain-to-grain variations in elevation relative to mounting medium and ensure consistent laser focus. Backscatter electron images (BSE) were used to further characterised the grains and optimize the location of laser ablation targets. Zircons were ablated using a Resonetics 193 nm excimer laser and uranium and lead isotopic ratios were measured using an Agilent 7700 quadrupole mass spectrometer. A low frequency laser repetition rate extended the data collection period on relatively thin zircon rims. Our results show that metamorphic zircon growth occurred in two main phases at 100 Ma and 77 Ma suggesting two sources of detrital zircons with differing metamorphic histories were present in the catchment area. The timing of metamorphism of the source area for the Nanaimo basin is inconsistent with derivation from sources in the Rocky Mountains (Lemhi sub

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.

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

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

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

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

Comparative use of TIMS and SHRIMP for U Pb zircon dating of A-type granites and mafic tholeiitic layered complexes and dykes from the Corsican Batholith (France)

NASA Astrophysics Data System (ADS)

Cocherie, A.; Rossi, Ph.; Fanning, C. M.; Guerrot, C.

2005-05-01

The Corsica-Sardinia batholith in the southern realm of the Hercynian belt of Europe shows evidence for gravitational collapse of this part of the mountain belt, causing major felsic and mafic magmatism. The latest intrusions are composed of leucomonzogranite and late metaluminous and alkaline granite, associated with tholeiitic layered complexes and dykes. Three dating methods on zircon (Pb-evaporation, ID-TIMS and SHRIMP) were used to unravel the chronology of these felsic and mafic rocks. Dating of zircons by the conventional U-Pb method, using TIMS after zircon dissolution, achieved an analytical uncertainty of 1 Ma for favourable cases. The TIMS Pb-evaporation technique resulted in ages with an uncertainty range of 4 to 8 Ma. After 15 to 20 analyses with the SHRIMP method, a precision ranging from 2 to 5 Ma was obtained (all at 2 σ). The three methods applied to the same zircon population extracted from four A-type granites, show that the uncertainty ranges within 2-5 Ma according to the sample. This error seems to correspond to the real geochronological uncertainty that can be achieved. The results obtained show that all six tested alkaline granites were emplaced during a very short interval of about 3-5 Ma at about 288 Ma, almost contemporaneous with the leucomonzogranite emplacement (291-287 Ma) that ended the batholith formation. In addition, there is no significant gap with the age of emplacement of the mafic tholeiitic magmatism (around 286 Ma) crosscutting the "A-type" granites. The late alkaline granites definitely do not show up here as precursors of the Tethyan rifting that began at about 170 Ma, i.e. some 100 Ma after their emplacement. It is thus necessary to examine if alternative hypotheses to the anorogenic model of the A-type Younger Granite province better fit the new geochronological data. A model involving depleted continental-crust derived magma should be compatible with the timing and geodynamical constraints as far as isotopic data are

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.

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

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

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.

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.

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.

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

Dating high-grade metamorphism: constraints from zircon and garnet REE compositions

NASA Astrophysics Data System (ADS)

Whitehouse, M. J.; Platt, J. P.

2001-12-01

We present high spatial resolution ion microprobe REE analyses of zircon and garnet from pelitic granulite adjacent to the Ronda peridotite, Betic Cordillera, southern Spain. The zircons exhibit polyphase growth, with thick structureless (in cathodoluminescence) overgrowths over detrital cores. These overgrowths yield a U-Pb age of 21.3 +/- 0.3 Ma [1, unpublished data] which we intepret as dating an episode of zircon growth during the Alpine orogeny. REE analyses of the dated portions of these zircons reveal profound differences between cores and rims. Cores show patterns typical of magmatic zircon (steep upward slopes from La to Lu with marked positive Ce anomaly), while the overgrowths are characterised by flat or even negatively sloping HREE profiles (Gd - Lu). Garnet, which occupies ca. 30 % by volume of the rock, is the most likely phase to host the HREEs in the rock and has been the subject of further ion-microprobe REE, textural and trace element investigations. The garnets are themselves zoned, with dominant central regions that are relatively free of inclusions overgrown by inclusion-rich, more calcic rims. Inclusions of kyanite +rutile in the central regions and sillimanite +ilmenite in the rims suggests that the garnets grew during decompression, and the Ca-enrichment in the rims suggests that their growth coincided with the initiation of partial melting. The presence of rimmed zircons only in the garnet rims and the matrix further suggests that the zircons also grew during this late decompressional history. An REE traverse of the garnet from core to rim reveals marked HREE depletion in the rims relative to the cores which we suggest is consistent with the textural evidence and probably results from early garnet core growth strongly depleting the HREEs available to subsequent growth. This mechanism can also be invoked to explain depletion in the zircon rims and more closely ties their formation to this stage of garnet growth. We therefore interpret the

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.

Detrital U-Pb zircon dating of lower Ordovician syn-arc-continent collision conglomerates in the Irish Caledonides

USGS Publications Warehouse

Clift, P.D.; Carter, A.; Draut, A.E.; Long, H.V.; Chew, D.M.; Schouten, H.A.

2009-01-01

The Early Ordovician Grampian Orogeny in the British Isles represents a classic example of collision between an oceanic island arc and a passive continental margin, starting around 480??Ma. The South Mayo Trough in western Ireland preserves a complete and well-dated sedimentary record of arc collision. We sampled sandstones and conglomerates from the Rosroe, Maumtrasna and Derryveeny Formations in order to assess erosion rates and patterns during and after arc collision. U-Pb dating of zircons reveals a provenance dominated by erosion from the upper levels of the Dalradian Supergroup (Southern Highland and Argyll Groups), with up to 20% influx from the colliding arc into the Rosroe Formation, but only 6% in the Maumtrasna Formation (~ 465??Ma). The dominant source regions lay to the northeast (e.g. in the vicinity of the Ox Mountains, 50??km distant, along strike). The older portions of the North Mayo Dalradian and its depositional basement (the Annagh Gneiss Complex) do not appear to have been important sources, while the Connemara Dalradian only plays a part after 460??Ma, when it supplies the Derryveeny Formation. By this time all erosion from the arc had effectively ceased and exhumation rates had slowed greatly. The Irish Grampian Orogeny parallels the modern Taiwan collision in showing little role for the colliding arc in the production of sediment. Negligible volumes of arc crust are lost because of erosion during accretion to the continental margin. ?? 2008 Elsevier B.V.

U-Pb SHRIMP dating of uraniferous opals

USGS Publications Warehouse

Nemchin, A.A.; Neymark, L.A.; Simons, S.L.

2006-01-01

U-Pb and U-series analyses of four U-rich opal samples using sensitive high-resolution ion microprobe (SHRIMP) demonstrate the potential of this technique for the dating of opals with ages ranging from several tens of thousand years to millions of years. The major advantages of the technique, compared to the conventional thermal ionisation mass spectrometry (TIMS), are the high spatial resolution (???20 ??m), the ability to analyse in situ all isotopes required to determine both U-Pb and U-series ages, and a relatively short analysis time which allows obtaining a growth rate of opal as a result of a single SHRIMP session. There are two major limitations to this method, determined by both current level of development of ion probes and understanding of ion sputtering processes. First, sufficient secondary ion beam intensities can only be obtained for opal samples with U concentrations in excess of ???20 ??g/g. However, this restriction still permits dating of a large variety of opals. Second, U-Pb ratios in all analyses drifted with time and were only weakly correlated with changes in other ratios (such as U/UO). This drift, which is difficult to correct for, remains the main factor currently limiting the precision and accuracy of the U-Pb SHRIMP opal ages. Nevertheless, an assumption of similar behaviour of standard and unknown opals under similar analytical conditions allowed successful determination of ages with precisions of ???10% for the samples investigated in this study. SHRIMP-based U-series and U-Pb ages are consistent with TIMS dating results of the same materials and known geological timeframes. ?? 2005 Elsevier B.V. All rights reserved.

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

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

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

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

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

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

Detrital zircon U-Pb and (U-Th)/He double-dating of Upper Cretaceous-Cenozoic Zagros foreland basin strata in the Kurdistan Region of northern Iraq

NASA Astrophysics Data System (ADS)

Barber, D. E.; Stockli, D. F.; Koshnaw, R. I.; Horton, B. K.; Tamar-Agha, M. Y.; Kendall, J. J.

2014-12-01

The NW Zagros orogen is the result of the multistage collisional history associated with Late Cretaceous-Cenozoic convergence of the Arabian and Eurasian continents and final closure of Neotethys. Siliciclastic strata preserved within a ~400 km segment of the NW Zagros fold-thrust belt and foreland basin in the Iraqi Kurdistan Region (IKR) provide a widespread record of exhumation and sedimentation. As a means of assessing NW Zagros foreland basin evolution and chronostratigraphy, we present coupled detrital zircon (DZ) U-Pb and (U-Th)/He geo-thermochronometric data of Upper Cretaceous to Pliocene siliciclastic strata from the Duhok, Erbil, and Suleimaniyah provinces of IKR. LA-ICP-MS U-Pb age analyses reveal that the foreland basin fill in IKR in general was dominantly derived from Pan-African/Arabian-Nubian, Peri-Gondwandan, Eurasian, and Cretaceous volcanic arc terrenes. However, the provenance of these strata varies systematically along strike and through time, with an overall increase in complexity upsection. DZ age distribution of Paleocene-Eocene strata is dominated by a ~95 Ma grain age population, likely sourced from the Late Cretaceous Hassanbag-Bitlis volcanic arc complex along the northern margin of Arabia. In contrast, DZ U-Pb age distributions of Neogene strata show a major contribution derived from various Eurasian (e.g., Iranian, Tauride, Pontide; ~45, 150, 300 Ma) and Pan-African (~550, 950 Ma) sources. The introduction of Eurasian DZ ages at the Paleogene-Neogene transition likely records the onset of Arabian-Eurasian collision. Along strike to the southeast, the DZ U-Pb spectra of Neogene strata show a decreased percentage of Pan-African, Peri-Gondwandan, Tauride, and Ordovician ages, coupled with a dramatic increase in 40-50 Ma DZ ages that correspond to Urumieh-Dokhtar magmatic rocks in Iran. Combined with paleocurrent data, this suggests that Neogene sediments were transported longitudinally southeastward through an unbroken foreland basin

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

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.

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.

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.

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

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

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

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.

Internal morphology, habit and U-Th-Pb microanalysis of amphibolite-to-granulite facies zircons: geochronology of the Ivrea Zone (Southern Alps)

NASA Astrophysics Data System (ADS)

Vavra, Gerhard; Schmid, Rolf; Gebauer, Dieter

Several types of growth morphologies and alteration mechanisms of zircon crystals in the high-grade metamorphic Ivrea Zone (IZ) are distinguished and attributed to magmatic, metamorphic and fluid-related events. Anatexis of pelitic metasediments in the IZ produced prograde zircon overgrowths on detrital cores in the restites and new crystallization of magmatic zircons in the associated leucosomes. The primary morphology and Th-U chemistry of the zircon overgrowth in the restites show a systematic variation apparently corresponding to the metamorphic grade: prismatic (prism-blocked) low-Th/U types in the upper amphibolite facies, stubby (fir-tree zoned) medium-Th/U types in the transitional facies and isometric (roundly zoned) high-Th/U types in the granulite facies. The primary crystallization ages of prograde zircons in the restites and magmatic zircons in the leucosomes cannot be resolved from each other, indicating that anatexis in large parts of the IZ was a single and short lived event at 299+/-5Ma (95% c. l.). Identical U/Pb ages of magmatic zircons from a metagabbro (293+/-6Ma) and a metaperidotite (300+/-6Ma) from the Mafic Formation confirm the genetic context of magmatic underplating and granulite facies anatexis in the IZ. The U-Pb age of 299+/-5Ma from prograde zircon overgrowths in the metasediments also shows that high-grade metamorphic (anatectic) conditions in the IZ did not start earlier than 20Ma after the Variscan amphibolite facies metamorphism in the adjacent Strona-Ceneri Zone (SCZ). This makes it clear that the SCZ cannot represent the middle to upper crustal continuation of the IZ. Most parts of zircon crystals that have grown during the granulite facies metamorphism became affected by alteration and Pb-loss. Two types of alteration and Pb-loss mechanisms can be distinguished by cathodoluminescence imaging: zoning-controlled alteration (ZCA) and surface-controlled alteration (SCA). The ZCA is attributed to thermal and/or decompression pulses

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.

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

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

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

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

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

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.

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

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.

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

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.

Metamorphic P-T path and zircon U-Pb dating of HP mafic granulites in the Yushugou granulite-peridotite complex, Chinese South Tianshan, NW China

NASA Astrophysics Data System (ADS)

Zhang, Lu; Zhang, Lifei; Xia, Bin; Lü, Zeng

2018-03-01

Co-existing granulite and peridotite may represent relics of the paleo-suture zone and provides an optimal opportunity for better understanding of orogeny between two blocks. In this study, we carried out petrological and U-Pb zircon dating investigation on the HP mafic granulites associated with peridotite complex at Yushugou in Chinese South Tianshan. The studied samples include garnet-bearing high-pressure mafic granulites which can be subdivided into two types: Type I orthopyroxene-free and Type II orthopyroxene-bearing granulites and amphibolite. Type I granulite (Y21-2) has a mineral assemblage of garnet (33 vol.%), clinopyroxene (32 vol.%) and plagioclase (30 vol.%); and Type II granulite (Y18-8) has a mineral assemblage of garnet (22 vol.%), clinopyroxene (10 vol.%), orthopyroxene (14 vol.%), plagioclase (45 vol.%) and quartz. Garnet in both granulites exhibits core-rim structure characterized by increasing grossular and decreasing pyrope from core to rim. Petrographic observations and phase equilibrium modeling using THERMOCALC in the NCFMASHTO system for the mafic granulites (Y21-2 and Y18-8) show three stages of metamorphism: Stage I (granulite facies) was recognized by the large porphyroblastic garnet core, with P-T conditions of 9.8-10.4 Kbar and 860-900 °C (Y21-2) and 9.9-10.6 Kbar and 875-890 °C (Y18-8), respectively; Stage II (HP granulite facies) has peak P-T conditions of 12.1 Kbar at 755 °C (Y21-2) and 13.8 Kbar at 815 °C (Y18-8) using mineral assemblages combining with garnet rim compositions with maximum grossular and minimum pyrope contents; Stage III (amphibolite facies) was characterized by the development of calcic amphibole in granulites with temperature of 446-563 °C. Therefore, an anticlockwise P-T path characterized by simultaneous temperature-decreasing and pressure-increasing was inferred for the Yushugou HP mafic granulite. Studies of zircon morphology and inclusions, combined with zircon U-Pb dating and REE geochemistry

Ion microprobe U-Pb dating and strontium isotope analysis of biogenic apatite

NASA Astrophysics Data System (ADS)

Sano, Y.; Toyoshima, K.; Takahata, N.; Shirai, K.

2012-12-01

Conodonts are micro-fossils chemically composed of apatite which occurred in the body of one animal. They are guide fossils to show formation ages of sedimentary sequences with the highest resolution [1] and good samples to verify the dating method. We developed the ion microprobe U-Pb dating of apatite [2] and applied the method to a Carboniferous conodont [3] by using a SHRIMP II installed at Department of Earth and Planetary Sciences, Hiroshima University. Recently we have developed the NanoSIMS U-Pb dating method and successfully measured the formation ages of monazite [4] and zircon [5] at Atmosphere and Ocean Research Institute, University of Tokyo. In this work we carried out the NanoSIMS U-Pb dating of biogenic apatite such as conodont. Since the spot size of NanoSIMS is smaller than SHRIMP II, it is easier to have multi-spots on the single fragment of biogenic apatite. Based on the isochron method of U-Pb system, we have calculated the formation ages. They are consistent with those in literature. In order to study the chemical evolution of ocean during the past 600 Million years, strontium isotopes (87Sr/86Sr) of fossil marine carbonate such as coral skeletons and foraminifera tests were measured and compiled [6]. However they are not robust when the age is older than 500Ma, partly due to post-depositional histories. Apatite is more stable and more resistant to the alteration than carbonate [7]. Recently we have developed the method of NanoSIMS strontium isotopic analysis of a fish otolith, which composed of aragonite [8]. In this work we carried out the strontium isotopic analysis of biogenic apatite. The advantage of the ion microprobe technique over the TIMS (thermal ionization mass spectrometer) and MC-ICP-MS (multi-collector inductively coupled argon plasma mass spectrometer) method is preservation of the important textural context and to provide an opportunity for other simultaneous analytical work with high spatial resolution. This is the case for

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.

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

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

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

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

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.

Evaluating the utility of detrital thermochronometric studies: detrital laser ablation (U-Th)/He dating and conventional bedrock zircon (U-Th)/He analyses from the eastern Sierra Nevada, California

NASA Astrophysics Data System (ADS)

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

2016-12-01

Recent applications of the newly developed `laser ablation double dating' (LADD) technique, an integrated laser microprobe U/Pb and (U-Th)/He dating method, have showcased the potential utility of LADD for detrital thermochronologic studies. However, detrital thermochronologic techniques rely on confidence that detrital data adequately represent the full range of bedrock cooling ages within a catchment. To test this primary assumption, we compare (U-Th)/He zircon ages from age-elevation transects to LADD (U-Th)/He zircon ages from modern fluvial detritus collected at the range front in the eastern Sierra Nevada, California. Terminated by a normal fault escarpment, the small, steep catchments along the eastern side of the Sierra Nevada batholith are apropos locations for comparing the ability of detrital data to deduce the exhumation history of a source terrain with standard age-elevation transects. Additionally, the exhumation of the Sierra Nevada batholith is also intriguing, as past evaluations of the post-emplacement exhumation history of the range have yielded discrepant results. Thus far, analyses from the southern extent of the eastern Sierra Nevada show narrow ranges of cooling ages consistent with simple, relatively rapid exhumation. Ongoing analyses will expand the dataset such that we can fully compare bedrock and detrital age ranges as well as characterize the exhumation history of the range with a thermochronometer that has not been used to date the batholith.

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

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

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

Zircon age-temperature-compositional spectra in plutonic rocks

DOE PAGES

Samperton, Kyle M.; Bell, Elizabeth A.; Barboni, Mélanie; ...

2017-08-23

We present that geochronology can resolve dispersed zircon dates in plutonic rocks when magma cooling time scales exceed the temporal precision of individual U-Pb analyses; such age heterogeneity may indicate protracted crystallization between the temperatures of zircon saturation (T sat) and rock solidification (T solid). Diffusive growth models predict asymmetric distributions of zircon dates and crystallization temperatures in a cooling magma, with volumetrically abundant old, hot crystallization at T sat decreasing continuously to volumetrically minor young, cold crystallization at T solid. We present integrated geochronological and geochemical data from Bergell Intrusion tonalites (Central Alps, Europe) that document zircon compositional changemore » over hundreds of thousands of years at the hand-sample scale, indicating melt compositional evolution during solidification. Ti-in-zircon thermometry, crystallization simulation using MELTS software, and U-Pb dates produce zircon mass-temperature-time distributions that are in excellent agreement with zircon growth models. In conclusion, these findings provide the first quantitative validation of longstanding expectations from zircon saturation theory by direct geochronological investigation, underscoring zircon’s capacity to quantify supersolidus cooling rates in magmas and resolve dynamic differentiation histories in the plutonic rock record.« less

Zircon age-temperature-compositional spectra in plutonic rocks

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

Samperton, Kyle M.; Bell, Elizabeth A.; Barboni, Mélanie

We present that geochronology can resolve dispersed zircon dates in plutonic rocks when magma cooling time scales exceed the temporal precision of individual U-Pb analyses; such age heterogeneity may indicate protracted crystallization between the temperatures of zircon saturation (T sat) and rock solidification (T solid). Diffusive growth models predict asymmetric distributions of zircon dates and crystallization temperatures in a cooling magma, with volumetrically abundant old, hot crystallization at T sat decreasing continuously to volumetrically minor young, cold crystallization at T solid. We present integrated geochronological and geochemical data from Bergell Intrusion tonalites (Central Alps, Europe) that document zircon compositional changemore » over hundreds of thousands of years at the hand-sample scale, indicating melt compositional evolution during solidification. Ti-in-zircon thermometry, crystallization simulation using MELTS software, and U-Pb dates produce zircon mass-temperature-time distributions that are in excellent agreement with zircon growth models. In conclusion, these findings provide the first quantitative validation of longstanding expectations from zircon saturation theory by direct geochronological investigation, underscoring zircon’s capacity to quantify supersolidus cooling rates in magmas and resolve dynamic differentiation histories in the plutonic rock record.« less

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.

Intra-grain Common Pb Correction and Detrital Apatite U-Pb Dating via LA-ICPMS Depth Profiling

NASA Astrophysics Data System (ADS)

Boyd, P. D.; Galster, F.; Stockli, D. F.

2017-12-01

Apatite is a common accessory phase in igneous and sedimentary rocks. While apatite is widely employed as a low-temperature thermochronometric tool, it has been increasingly utilized to constrain moderate temperature cooling histories by U-Pb dating. Apatite U-Pb is characterized by a thermal sensitivity window of 375-550°C. This unique temperature window recorded by the apatite U-Pb system, and the near-ubiquitous presence of apatite in igneous and clastic sedimentary rocks makes it a powerful tool able to illuminate mid-crustal tectono-thermal processes. However, as apatite incorporates only modest amounts of U and Th (1-10s of ppm) the significant amounts of non-radiogenic "common" Pb incorporated during its formation presents a major hurdle for apatite U-Pb dating. In bedrock samples common Pb in apatite can be corrected for by the measurement of Pb in a cogenetic mineral phase, such as feldspar, that does not incorporate U or from determination of a common Pb composition from multiple analyses in Tera-Wasserburg space. While these methods for common Pb correction in apatite can work for igneous samples, they cannot be applied to detrital apatite in sedimentary rocks with variable common Pb compositions. The obstacle of common Pb in apatite has hindered the application of detrital apatite U-Pb dating in provenance studies, despite the fact that it would be a powerful tool. This study presents a new method for the in situ correction of common Pb in apatite through the utilization of novel LA-ICP-MS depth profiling, which can recover U-Pb ratios at micron-scale spatial resolution during ablation of a grain. Due to the intra-grain U variability in apatite, a mixing line for a single grain can be generated in Tera-Wasserburg Concordia space. As a case study, apatite from a Variscan alpine granite were analyzed using both the single and multi-grain method, with both methods giving identical results. As a second case study the intra-grain method was then performed

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.

Diachronous evolution of volcano-sedimentary basins north of the Congo craton: Insights from U Pb ion microprobe dating of zircons from the Poli, Lom and Yaoundé Groups (Cameroon)

NASA Astrophysics Data System (ADS)

Toteu, Sadrack Félix; Penaye, Joseph; Deloule, Etienne; Van Schmus, William Randall; Tchameni, Rigobert

2006-04-01

Ion microprobe U-Pb dating of zircons from Neoproterozoic volcano-sedimentary sequences in Cameroon north of the Congo craton is presented. For the Poli basin, the depositional age is constrained between 700-665 Ma; detrital sources comprise ca. 920, 830, 780 and 736 Ma magmatic zircons. In the Lom basin, the depositional age is constrained between 613 and 600 Ma, and detrital sources include Archaean to Palaeoproterozoic, late Mesoproterozoic to early Neoproterozoic (1100-950 Ma), and Neoproterozoic (735, 644 and 613 Ma) zircons. The Yaoundé Group is probably younger than 625 Ma, and detrital sources include Palaeoproterozoic and Neoproterozoic zircons. The depositional age of the Mahan metavolcano-sedimentary sequence is post-820 Ma, and detrital sources include late Mesoproterozoic (1070 Ma) and early Neoproterozoic volcanic rocks (824 Ma). The following conclusions can be made from these data. (1) The three basins evolved during the Pan-African event but are significantly different in age and tectonic setting; the Poli is a pre- to syn-collisional basin developed upon, or in the vicinity of young magmatic arcs; the Lom basin is post-collisional and intracontinental and developed on old crust; the tectono-metamorphic evolution of the Yaoundé Group resulted from rapid tectonic burial and subsequent collision between the Congo craton and the Adamawa-Yade block. (2) Late Mesoproterozoic to early Neoproterozoic inheritance reflects the presence of magmatic event(s) of this age in west-central Africa.

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

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.

Quantifying the timescales of Archean UHT metamorphism through U-Pb monazite and zircon petrochronology

NASA Astrophysics Data System (ADS)

Guevara, V.; MacLennan, S. A.; Schoene, B.; Dragovic, B.; Caddick, M. J.; Kylander-Clark, A. R.; Couëslan, C. G.

2016-12-01

Unraveling the timescales of metamorphism is crucial to understanding the mechanisms behind mass/heat transfer through Earth's crust. Though such mechanisms and their durations are becoming well constrained in modern (Phanerozoic) settings, the drivers of metamorphism in the ancient geologic record remain more enigmatic. The development of accessory phase petrochronology has allowed metamorphic evolution to be closely linked to isotopic dates, ultimately improving quantification of metamorphic durations. While in-situ petrochronological methods preserve textural and spatial context, they often lack the temporal resolution required to accurately quantify metamorphic duration in Archean terranes. Here we combine in-situ U-Pb monazite (mnz) and zircon (zrn) laser ablation split-stream (LASS) and high-precision ID-TIMS-TEA petrochronology of distinct grain domains to resolve the timescales of ultrahigh temperature (UHT) metamorphism in the Archean Pikwitonei granulite domain (PGD). The PGD encompasses >1.5x105 km2 of granulite-facies rocks on the NW edge of the Superior Province. Themodynamic modelling of a pelite from the western part of the PGD suggests peak P-T conditions of >8 kbar, 900-940 °C and UHT decompression to 8 kbar followed by cooling. LASS analysis of zrn inclusions in garnet (grt) yields a date of 2701 Ma, with Ti in zrn thermometry yielding T of 800-900 °C. LASS analysis of mnz yields dates of 2720-2680 Ma for low HREE domains with no to shallow negative Eu anomalies, suggestive of growth during plagioclase (plg) breakdown and grt stability. ID-TIMS analysis of a mnz fragment with a strong negative Eu anomaly, suggestive of growth during plg stability, gives a concordant 207Pb/206Pb date of 2666 Ma, consistent with LASS results of 2660-2640 Ma for chemically similar domains. ID-TIMS analyses of zrn rims yield a range of 207Pb/206Pb dates from 2671 to 2656 Ma (±<1 Ma). Ti in zrn yields 800 °C for these rims, indicating they grew at similar T

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

Parent zonation in thermochronometers - resolving complexity revealed by ID-TIMS U-Pb dates and implications for the application of decay-based thermochronometers

NASA Astrophysics Data System (ADS)

Navin Paul, Andre; Spikings, Richard; Chew, David; Daly, J. Stephen; Ulyanov, Alexey

2017-04-01

High temperature (>350℃) U-Pb thermochronometers primarily use accessory minerals such as apatite, titanite and rutile, and assume that daughter isotopes are lost by thermally activated volume diffusion while the parent remains immobile. Studies exploiting such behaviour have been successfully used to reconstruct thermal histories spanning several hundred million years (e.g. Cochrane et al., 2014). However, outliers in date (ID-TIMS) vs diffusion length space are frequently observed, and grains are frequently found to be either too young or too old for expected thermal history solutions using the diffusion data of Cherniak et al. (2010). These deviations of single grain apatite U-Pb dates from expected behaviour could be caused by a combination of i) metamorphic (over-)growth, ii) fluid-aided Pb mobilisation during alteration/recrystallization, iii) parent isotope zonation, iv) metamictisation, and v) changes in diffusion length with time (e.g. fracturing). We present a large data set from the northern Andes of South America, where we compare apatite U-Pb ID-TIMS-(TEA) data with LA-ICP-MS element maps and in-situ apatite U-Pb LA-(MC)-ICP-MS dates. These are combined with U-Pb zircon and 40Ar/39Ar (muscovite) data to attempt to distinguish between thermally activated volume diffusion and secondary overgrowth/recrystallization. We demonstrate that in young (e.g. Phanerozoic) apatites that have not recrystallized or experienced metasomatic overgrowths, U-Pb dates are dominantly controlled by volume diffusion and intra-crystal uranium zonation. This implies that ID-TIMS analyses of apatites with zoned parent isotope distributions will not usually recover accurate thermal history solutions, and an in-situ dating method is required. Recovering the uranium distribution during in-situ analysis provides a means to account for parent zonation, substantially increasing the accuracy of the modelled t-T-paths. We present in-situ data from apatites where scatter in date v

Temporal and Spatial Fluctuations in Ancestral Northern Cascade Arc Magmatism from New LA-ICP-MS U-Pb Zircon Dating

NASA Astrophysics Data System (ADS)

McCallum, I. S.; Mullen, E.; Jean-Louis, P.; Tepper, J. H.

2015-12-01

Mt. Baker and the adjacent Chilliwack batholith (MBC focus) in NW Washington preserve the longest magmatic record in the Cascade Arc, providing an excellent natural laboratory for examining the spatial, temporal and geochemical evolution of Cascade magmatism and links to tectonic processes. We present new U-Pb zircon LA-ICP-MS ages for 14 samples from MBC and neighboring regions of the north Cascades. The new results are up to 8 Myr different from previous K-Ar ages, illustrating the need for new age determinations in the Cascades. A maximum age of 34.74±0.24 Ma (2σ) (Post Creek stock) is consistent with 35-40 Ma ages for arc inception in the southern Cascades. The most voluminous MBC plutons cluster at 32-29 Ma, consistent with an early flare-up that also coincides with intrusion of the Index batholith farther south (2 samples at 33.26±0.19, 33.53±0.15 Ma). This flare-up is absent in the northernmost Cascades where the oldest pluton (Fall Creek stock) is 6.646±0.046 Ma, 4 Myr younger than previously cited. Earliest Cascade magmatism is progressively younger to the north of MBC, possibly tracing the northerly passage of the slab edge. MBC activity was continuous to 22.75±0.17 Ma (Whatcom Arm), marking the initiation of an 11 Myr hiatus. Magmatism resumed at 11.33±0.08 Ma (Indian Creek) and continued to the modern Mt. Baker cone, defining a pattern of southwesterly migration over ~55 km that may be attributable to slab rollback and arc rotation (e.g. Wells & McCaffrey 2013). Uniformity of the rate and direction of migration implies that rollback and rotation began at least 11 Myr ago. Post-hiatus magmas show distinct geochemical and petrologic characteristics including a major Pb isotopic shift. The 2.430±0.016 Ma Lake Ann stock contains 4.2 Ma zircon antecrysts, recording prolonged activity in that area. The 1.165±0.013 Ma Kulshan caldera ignimbrite contains ~200 Ma inherited zircons that may provide the first direct record of Wrangellian basement beneath

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.

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

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

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.

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.

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.

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

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).

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.

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

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

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

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

Single grain U/Pb geochronology of detrital zircons from Midcontinent rift arkoses, NE Kansas: Implications for depositional history

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

Martin, M.W.; Van Schmus, W.R.; Berendsen, P.

1993-03-01

The Midcontinent rift system in the subsurface south of the Lake Superior region has been well imaged by magnetic, gravity and seismic surveys, however only a few wells have penetrated and recovered core from rift-basin fill in this region. Texaco's exploratory Noel Poersch well [number sign]1 in northeastern Kansas, penetrated [approximately] 2,600 m of rift-related volcanic, igneous, and arkosic sedimentary rocks from which a total of 35 m of core was taken from fourteen different horizons in the rift-related section. To determine provenance ages and to constrain better the depositional patterns of clastic sedimentary rocks within the Mid-continent rift basin,more » the authors have undertaken U/Pb geochronology of detrital zircon from arkosic horizons along the depth of recovered core from the Texaco Poersch [number sign]1 well. Preliminary analyses indicate that the stratigraphically lowest arkoses recovered in core have provenance ages that range in age from 1.7--1.8 Ga, 1.4--1.5 Ga and 1.1--1.2 Ga. These data suggest the following conclusions: (1) The arkosic sediments were primarily derived proximally from the adjacent rift margin, which is known to consist of 1.75--1.80 Ga gneissic and granitic basement intruded by 1.35--1.45 Ga granitic plutons in Nebraska and northernmost Kansas plus 1.63--1.68 Ga granitic basement intruded by 1.35--1.45 granitic plutons in most of Kansas; 1.63--1.70 detrital zircons were absent, suggesting that most of the detritus was derived from northerly directions. (2) No Archean or 1.85 to 1.90 Ga Early proterozoic detrital zircons were found, suggesting very little to no transport of detritus along the rift axis from farther north, e.g., from Penokean, Trans-Hudson, or Superior Province regions. (3) One nearly concordant zircon with a Pb-Pb age of 1.18 Ga was found, suggesting that some of the detritus was derived either from older phases of igneous rift fill or from ca. 1.2 Ga intrusions that pre-date rifting.« less

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).

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

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

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.

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.

U-Pb isotopic systematics of zircons from prograde and retrograde transition zones in high-grade orthogneisses, Sri Lanka

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

Baur, N.; Liew, T.C.; Todt, W.

1991-07-01

The authors present U-Pb zircon isotopic data from locally restricted prograde (arrested in situ charnockitization) and retrograde metamorphic transition zones, which are well exposed in Proterozoic orthogneisses tectonically interbanded with granulite facies supracrustal rocks of the Highland Group in Sri Lanka. These granitoid rocks yield apparent ages of 1942 {plus minus} 22 Ma, {approximately} 770 Ma, {approximately} 660 Ma, and {approximately} 560 Ma. All samples show severe Pb-loss some 550-560 Ma ago. The main phase of granulite-formation could not be dated unambiguously but is bracketed between {approximately} 660 Ma and {approximately} 550 Ma. The pervasive Pb-loss event around 550-560 Mamore » reflects the end of this period of high-grade metamorphism and was associated with widespread igneous activity and retrogression. This is constrained by the 550 {plus minus} 3 Ma intrusion age for a post-tectonic granite. They relate this late phase of thermal activity to crustal uplift of the Sri Lankan granulites. This data unambiguously prove the high-grade history of the Sri Lanka gneisses to be a late Precambrian event that may be related to the Pan-African evolution along the eastern part of Africa.« less

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

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

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.

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

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

New zircon U-Pb LA-ICP-MS ages and Hf isotope data from the Central Pontides (Turkey): Geological and geodynamic constraints

NASA Astrophysics Data System (ADS)

Çimen, Okay; Göncüoğlu, M. Cemal; Simonetti, Antonio; Sayit, Kaan

2018-05-01

The Central Pontides in northern Anatolia is located on the accretionary complex formed by the closure of Neotethyan Intra-Pontide Ocean between the southern Eurasian margin (Istanbul-Zonguldak Terrane) and the Cimmerian Sakarya Composite Terrane. Among other components of the oceanic lithosphere, it comprises not yet well-dated felsic igneous rocks formed in arc-basin as well as continent margin settings. In-situ U-Pb age results for zircons from the arc-basin system (öangaldağ Metamorphic Complex) and the continental arc (Devrekani Metadiorite and Granitoid) yield ages of 176 ± 6 Ma, 163 ± 9 Ma and 165 ± 3 Ma, respectively. Corresponding in-situ average (initial) 176Hf/177Hf initial ratios are 0.28261 ± 0.00003, 0.28267 ± 0.00002 and 0.28290 ± 0.00004 for these units and indicative of a subduction-modified mantle source. The new U-Pb ages and Hf isotope data from these oceanic and continental arc units together with regional geological constraints support the presence of a multiple subduction system within the Intra-Pontide Ocean during the Middle Jurassic.

Precambrian-Cambrian provenance of Matinde Formation, Karoo Supergroup, northwestern Mozambique, constrained from detrital zircon U-Pb age and Lu-Hf isotope data

NASA Astrophysics Data System (ADS)

Bicca, Marcos Müller; Jelinek, Andrea Ritter; Philipp, Ruy Paulo; de Carvalho Lana, Cristiano; Alkmim, Ana Ramalho

2018-02-01

The Permian-Triassic time interval was a period of high sedimentation rates in the intracontinental Karoo rift basin of northwestern Mozambique, reflecting high exhumation rates in the surrounding high ground Precambrian-Cambrian basement and juxtaposed nappes. U-Pb LA-MC-ICPMS dating and Lu-Hf isotopic analysis of detrital zircons from the Late Permian-Early Triassic Matinde Formation of the Karoo Supergroup is used as a reliable proxy to map denudation patterns of source regions. Data allow discrimination of U-Pb age populations of ca. 1250-900 Ma, a secondary population between ca. 900-700 and a major contribution of ages around ca. 700-490 Ma. Zircon grains of the Mesoproterozoic age population present Mesoproterozoic (1000-1500 Ma) to Paleoproterozoic (1800-2300 Ma) Hf TDM ages, with positive (0 to +11) and negative εHf values (-3 to -15), respectively. The younger U-Pb age population also presents two different groups of zircon grains according to Lu-Hf isotopes. The first group comprise Paleoproterozoic (1800-2300 Ma) ages, with highly negative εHf values, between -10 and -22, and the second group exhibits Mesoproterozoic ages (1200-1500 Ma), with increased juvenile εHf values (ca. 0 to -5). These Hf isotopes reinforce the presence of unexposed ancient crust in this region. The oldest U-Pb age population resembles the late stages of Grenville Orogeny and the Rodinia Supercontinent geotectonic activity mostly represented by magmatic rocks, which are widely present in the basement of northern Mozambique. The juvenile Hf-isotope signature with an older age component is associated to rocks generated from subduction processes with crust assimilation by continental arcs, which we correlate to rocks of the Nampula Complex, south and east of the Moatize-Minjova Basin. The U-Pb ages between 900 and 700 Ma were correlated to the calc-alkaline magmatism registered in the Guro Suite, related to the breakup phase of Rodinia, and mark the western limit of the Moatize

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

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.

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 geochronology and geochemistry of granites in the Zhuguangshan complex, South China: Implications for uranium mineralization

NASA Astrophysics Data System (ADS)

Zhang, Long; Chen, Zhenyu; Li, Xiaofeng; Li, Shengrong; Santosh, M.; Huang, Guolong

2018-05-01

The Zhuguangshan complex, composed of Caledonian, Indosinian, and Yanshanian granites, and Cretaceous mafic dykes, is one of the most important granite-hosted uranium producers in South China. Here we present LA-ICP-MS zircon U-Pb and hornblende 40Ar/39Ar geochronology and whole-rock and biotite geochemistry for the granites in this complex to evaluate the magmatism and its constraints on uranium mineralization. Samples collected from the Fuxi, Youdong, Longhuashan, Chikeng, Qiling, and Sanjiangkou intrusions yield zircon weighted 206Pb/238U ages of 426.7 ± 5.4 Ma, 226.4 ± 3.5 Ma, 225.0 ± 2.7 Ma, 152.2 ± 3.0 Ma, 153.9 ± 2.1 Ma, and 155.2 ± 2.1 Ma, respectively. A new Ar-Ar dating of the hornblende of the diabase from the Changjiang uranium ore field yields a plateau age of 145.1 ± 1.5 Ma. These results coupled with published geochronological data indicate that six major magmatic events occurred in the study area at 420-435 Ma, 225-240 Ma, 150-165 Ma, 140 Ma, 105 Ma, and 90 Ma. Both U-bearing and barren granites occur in this complex, and they display differences in whole-rock and biotite geochemistry. The barren granites show higher Al2O3, CaO, TFMM, Rb, Zr, Ba, SI, Mg#, (La/Yb)N, and Eu/Eu*, but lower SiO2, ALK, Rb, DI, Rb/Sr, and TiO2/MgO than those of the U-bearing granites. Biotites in the U-bearing granites are close to the Fe-rich siderophyllite-annite end member with Fe/(Fe + Mg) ratios higher than 0.66, whereas those in the barren granites are relatively close to the Mg-rich eastonite-phlogopite end member with Fe/(Fe + Mg) ratios <0.66. The U-bearing granites were mainly derived from the partial melting of pelitic sedimentary source, whereas the psammitic source generated the barren granites. In addition, the barren granites show higher TFMM, Ba, and Eu/Eu* but lower SiO2, Rb/Sr and Al2O3/TiO2 ratios with higher zircon saturation temperatures relative to the U-bearing granites. These results indicate that the geochemical compositions of the U

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.

Neoproterozoic transpression and granite magmatism in the Gavilgarh-Tan Shear Zone, central India: Tectonic significance of U-Pb zircon and U-Th-total Pb monazite ages

NASA Astrophysics Data System (ADS)

Chattopadhyay, Anupam; Chatterjee, Amitava; Das, Kaushik; Sarkar, Arindam

2017-10-01

The Gavilgarh-Tan Shear Zone (GTSZ) is a crustal-scale shear/fault zone that dissects the unclassified basement gneisses separating two major supracrustal belts, viz. the Paleo- to Mesoproterozoic (≥1.5 Ga) Betul Belt and the Neoproterozoic (∼1.0 Ga) Sausar Belt, of the Central Indian Tectonic Zone (CITZ). The GTSZ extends for more than 300 km strike length, partly covered by the Deccan Trap flows. Granitoid rocks ranging from syenogranite to granodiorite in composition, sheared at temperatures corresponding to the amphibolite facies metamorphic condition, define the GTSZ in the Kanhan River Valley. Earlier geological studies have suggested that the GTSZ underwent a sinistral-sense partitioned transpression in response to an oblique collision between two continental fragments, possibly related to crustal thickening and high-pressure granulite metamorphism (the Ramakona-Katangi granulite: RKG) in the northern part of the Sausar Belt. LA-ICP-MS U-Pb dating of zircon and EPMA U-Th-total Pb dating of monazite grains from four different types of syn-tectonic granitoids of the GTSZ carried out in the present study show that granitoids intruded the basement gneisses between 1.2 Ga and 0.95 Ga, given the error limit of the calculated ages. The age of transpression and mylonitization is more definitely bracketed between 1.0 Ga and 0.95 Ga, which correlates well with the published ages of deformation and metamorphism in the Sausar Belt. This age data strongly supports the suggested collisional tectonic model involving the GTSZ and the RKG granulites of the Sausar Belt and underlines a Grenvillian-age tectonic history for the southern part of the Central Indian Tectonic Zone (CITZ), which possibly culminated in the crustal assembly of the Neoproterozoic supercontinent Rodinia.

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

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

Scanning ion imaging - a potent tool in SIMS U -Pb zircon geochronology

NASA Astrophysics Data System (ADS)

Whitehouse, M. J.; Fedo, C.; Kusiak, M.; Nemchin, A.

2012-12-01

The application of high spatial resolution (< 15-20 μm lateral) U-Pb data obtained by sec-ondary ion mass spectrometers (SIMS) coupled with textural information from scanning electron microscope (SEM) based cathodoluminescence (CL) and/or back-scattered elec-tron (BSE) characterisation, has revolutionised geochronology over the past 25 years, re-vealing complexities of crustal evolution from zoned zircons. In addition to ge-ochronology, such studies now commonly form the basis of broader investigations using O- and Hf- isotopes and trace elements obtained from the same growth zone as age, circumventing ambiguities commonly present in bulk-rock isotope studies. The choice of analytical beam diameter is often made to maximise the precision of data obtained from a given area of analysis within an identifiable growth zone. In cases where zircons yield poorly constrained internal structures in SEM, high spatial resolution spot analyses may yield uninterpretable and/or meaningless mixed ages by inadvertent sampling across regions with real age differences. Scanning ion imaging (SII) has the potential to generate accurate and precise geochrono-logical data with a spatial resolution down to ca. 2 μm, much higher than that of a normal spot analysis. SII acquisition utilises a rastered primary beam to image an area of the sample with a spatial resolution dependent on the selected primary beam diameter. On the Cameca ims1270/80 instruments, the primary beam scanning is coupled with the dynamic transfer optical system (DTOS) which deflects the secondary ions back on to the ion optical axis of the instrument regardless of where in the raster illuminated area the ions originated. This feature allows retention of a high field magnification (= high transmission) mode and the ability to operate the mass spectrometer at high mass resolution without any compromise in the quality of the peak shape. Secondary ions may be detected either in a sequential (peak hopping) mono

Uranium-lead dating of hydrothermal zircon and monazite from the Sin Quyen Fe-Cu-REE-Au-(U) deposit, northwestern Vietnam

NASA Astrophysics Data System (ADS)

Li, Xiao-Chun; Zhou, Mei-Fu; Chen, Wei Terry; Zhao, Xin-Fu; Tran, MyDung

2018-03-01

The Sin Quyen deposit in northwestern Vietnam contains economic concentrations of Cu, Au and LREE, and sub-economic concentration of U. In this deposit, massive and banded replacement ores are hosted in Neoproterozoic metapelite. The paragenetic sequence includes sodic alteration (stage I), calcic-potassic alteration and associated Fe-REE-(U) mineralization (stage II), Cu-Au mineralization (stage III), and sulfide-(quartz-carbonate) veins (stage IV). The Sin Quyen deposit experienced an extensive post-ore metamorphic overprint, which makes it difficult to precisely determine the mineralization age. In this study, zircon and monazite U-Pb geochronometers and the Rb-Sr isochron method are used to constrain the timing of mineralization. Zircon grains in the ore are closely intergrown or texturally associated with hydrothermal minerals of stage II (e.g., garnet, allanite, and hedenbergite). They may contain primary fluid inclusions and display irregular zoning in cathodoluminescence (CL) images. Zircon grains are rich in U (688 to 2902 ppm) and poor in Th (0.2 to 2.9 ppm). Their δ18OV-SMOW values range from 11.9 to 14.0‰, higher than those of typical magmatic zircon. These textural and compositional features imply that zircon precipitated from 18O- and U-rich hydrothermal fluids, coeval with the minerals of stage II. Monazite occurs in close association with stage II magnetite and allanite and has low contents of Th (<2700 ppm), indicative of a hydrothermal origin. Hydrothermal zircon and monazite have indistinguishable U-Pb ages of 841 ± 12 and 836 ± 18 Ma, respectively, representing the timing of Fe-REE mineralization. There is no direct isotopic constraint on the timing of the Cu-Au mineralization, but geological observations suggest that the Cu-Au and Fe-REE ores most likely formed within a single evolved hydrothermal process. In the plot of 87Rb/86Sr vs. 87Sr/86Sr, the composition of bulk-ore and biotite separates from ore lie along a reference line for 30 Ma

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

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.

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

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, 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.

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).

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

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

Titanite chronology, thermometry, and speedometry of ultrahigh-temperature (UHT) calc-silicates from south Madagascar: U-Pb dates, Zr temperatures, and lengthscales of trace-element diffusion

NASA Astrophysics Data System (ADS)

Holder, R. M.; Hacker, B. R.

2017-12-01

Calc-silicate rocks are often overlooked as sources of pressure-temperature-time data in granulite-UHT metamorphic terranes due to the strong dependence of calc-silicate mineral assemblages on complex fluid compositions and a lack of thermodynamic data on common high-temperature calc-silicate minerals such as scapolite. In the Ediacaran-Cambrian UHT rocks of southern Madagascar, clinopyroxene-scapolite-feldspar-quartz-zircon-titanite calc-silicate rocks are wide-spread. U-Pb dates of 540-520 Ma from unaltered portions of titanite correspond to cooling of the rocks through upper-amphibolite facies and indicate UHT metamorphism occurred before 540 Ma. Zr concentrations in these domains preserve growth temperatures of 900-950 °C, consistent with peak temperatures calculated by pseudosection modeling of nearby osumilite-bearing gneisses. Younger U-Pb dates (510-490 Ma) correspond to fluid-mediated Pb loss from titanite grains, which occurred below their diffusive Pb-closure temperature, along fractures. The extent of fluid alteration is seen clearly in back-scattered electron images and Zr-, Al-, Fe-, Ce-, and Nb-concentration maps. Laser-ablation depth profiling of idioblastic titanite grains shows preserved Pb diffusion profiles at grain rims, but there is no evidence for Zr diffusion, indicating that it was effectively immobile even at UHT.

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).

Petrogenesis of granodiorite in the Balong region, eastern Kunlun Orogen, China: Constraints from zircon U-Pb geochronology, geochemistry and Sr-Nd-Hf isotopes

NASA Astrophysics Data System (ADS)

Huang, X.; Li, H.; Wang, Y.; Liu, Y.

2017-12-01

Numerous granitoid intrusions that close to the Balong region have great scientific significance to reveal tectonic evolution and geodynamic background of eastern Kunlun Orogen (EKO). Balong granodiorite (BLG) is located at the northern of the EKO. It generally emplaced into the Proterozoic to Lower Palaeozoic rocks and contains abundant mafic microgranular enclaves. LA-ICP-MS zircon U-Pb dating of the BLG gives a 206Pb/238U age of 230.7±1.9 Ma, indicating that it was emplaced in the Late Triassic. The BLG is high-K calc-alkaline series and metaluminous, with SiO2 of 59.86 61.83%, K2O+Na2O of 5.98 6.40%, CaO of 4.95 5.77% and P2O5 of 0.14% 0.18%. The granodioritic rocks are enriched in LILE (Ba, Rb, Sr), but depleted in HFSE (Nb, Ta, P, Ti), with weak negative Eu anomalies (δEu=0.70 0.82). Mineralogy and geochemistry of the rocks show an affinity to I-type granite. The BLG, having (87Sr/86Sr)i ratios of 0.70819 to 0.70832, ɛNd(t) values of -5.27 to -5.75, and zircon ɛHf(t) values ranging from -3.86 to -1.34. The whole-rock Nd isotopic model ages (1432 1471 Ma) and zircon Hf isotopic model ages (1205 1357 Ma) indicate that the BLG is generated by partial melting of lower crust (Precambrian metabasaltic basement rocks) with different degree of involvement of mantle material. Combined with regional geological data, the granodiorite was derived from dehydration melting of mafic lower crustal rocks during the subduction of the Anyemaqen ocean lithosphere at Late Permian-Triassic in a subduction setting. Basaltic magma underplating and crust-mantle mixing are main mechanisms for the origin of large-scale I-type granitoid in Balong.

Late accretion to the Moon recorded in zircon (U-Th)/He thermochronometry

NASA Astrophysics Data System (ADS)

Kelly, Nigel M.; Flowers, Rebecca M.; Metcalf, James R.; Mojzsis, Stephen J.

2018-01-01

We conducted zircon (U-Th)/He (ZHe) analysis of lunar impact-melt breccia 14311 with the aim of leveraging radiation damage accumulated in zircon over extended intervals to detect low-temperature or short-lived impact events that have previously eluded traditional isotopic dating techniques. Our ZHe data record a coherent date vs. effective Uranium concentration (eU) trend characterized by >3500 Ma dates from low (≤75 ppm) eU zircon grains, and ca. 110 Ma dates for high (≥100 ppm) eU grains. A progression between these date populations is apparent for intermediate (75-100 ppm) eU grains. Thermal history modeling constrains permissible temperatures and cooling rates during and following impacts. Modeling shows that the data are most simply explained by impact events at ca. 3950 Ma and ca. 110 Ma, and limits allowable temperatures of heating events between 3950-110 Ma. Modeling of solar cycling thermal effects at the lunar surface precludes this as the explanation for the ca. 110 Ma ZHe dates. We propose a sample history characterized by zircon resetting during the ca. 3950 Ma Imbrium impact event, with subsequent heating during an impact at ca. 110 Ma that ejected the sample to the vicinity of its collection site. Our data show that zircon has the potential to retain 4He over immense timescales (≥3950 Myrs), thus providing a valuable new thermochronometer for probing the impact histories of lunar samples, and martian or asteroidal meteorites.

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.

Status Report on the 40Ar/39Ar and U/Pb Dating of Tuffs in the Dewey Lake Formation of West Texas Towards Constraining the Permo-Triassic Magnetostratigraphic Time Scale

NASA Astrophysics Data System (ADS)

Chang, S.; Renne, P. R.; Mundil, R.

2007-12-01

A detailed magnetic polarity time scale for the Permo-Triassic Boundary interval, critical for correlating events in marine and terrestrial paleoenvironments, is not yet well-established. Recently, late Permian magnetostratigraphic studies have been reported for non-marine sections in Europe and South Africa (Szurlies et al., 2003; Nawrocki, 2004; Ward et al., 2005). However, these sections are devoid of index fossil suitable for correlation with marine successions and also lack age constraints from radioisotopic dating methods. In other words, it is dubious to correlate these magnetostratigraphic data with the GSSP Permo-Triassic boundary and mass extinction. The Dewey Lake red beds formation of West Texas, believed to be the youngest Permian formation in North America, has yielded high-quality paleomagnetic data (Molina-Garza et al., 1989; Steiner, 2001) and contains several silicic tuffs potentially enabling high-resolution calibration of the magnetic polarity time scale in this critical age range. The tuffs have yet to be placed into a regional stratigraphic or magnetostratigraphic framework, and it is unclear exactly how many distinct eruptive units are represented by the 7 distinct samples collected to date from widely separated (>160 km) localities. 40Ar/39Ar (sanidine and biotite) and U/Pb (zircon) studies reveal that all 7 sampled tuffs were probably erupted within several hundred ka of the Permo-Triassic boundary as dated at the Meishan GSSP section (Renne et al., 1995; Mundil et al., 2004) but results thus far are inadequate to convincingly resolve age differences between the various samples. U/Pb dating of some samples is severely challenged by Pb-loss from the zircons despite application of the Mattinson (2005) annealing/chemical abrasion technique. 40Ar/39Ar data have been obtained from as many as four different irradiations in order to reduce neutron fluence related error. We observe the familiar ~1% bias between U/Pb and 40Ar/39Ar ages. Biotite

Zircon U-Pb Ages from an Ultra-High Temperature Metapelite, Rauer Group, East Antarctica: Implications for Overprints by Grenvillian and Pan-African Events

USGS Publications Warehouse

Wang, Yanbin; Tong, Laixi; Liu, Dunyi

2007-01-01

SHRIMP U-Pb dating of zircon from an ultra-high temperature (UHT, ~1000 °C) granulite-facies metapelite from the Rauer Group, Mather Peninsula, east Antarctica, has yielded evidence for two episodes of metamorphic zircon growth, at ~1.00 Ga and ~530 Ma, and two episodes of magmatism in the source region for the protolith sediment, at ~2.53 and ~2.65 Ga, were identified from the zircon cores. Successive zircon growth at ~1.00 Ga and ~530 Ma records a sequence of distinct, widely spaced high-temperature metamorphic and/or anatectic events related to Grenvillian and Pan-African orogenesis. This study presents the first robust geochronological evidence for the timing of UHT metamorphism of the Rauer Group, supporting arguments that the peak UHT metamorphic event occurred at ~1.00 Ga and was overprinted by a separate high-grade event at ~530 Ma. The new age data indicate that the UHT granulites of the Rauer Group experienced a complex, multi-stage tectonothermal history, which cannot simply be explained via a single Pan-African (~500 Ma) high-grade tectonic event. This is critical in understanding the role of the eastern Prydz Bay region during the assembly of the east Gondwana supercontinent, and the newly recognized inherited Archaean ages (~2.53 and ~2.65 Ga) suggest a close tectonic relationship between the Rauer Group and the adjacent Archaean of the Vestfold Hills

Zircons as a Probe of Early Luanr History

NASA Astrophysics Data System (ADS)

Crow, C. A.; McKeegan, K. D.; Gilmour, J. D.; Crowther, S. A.; Talor, D. J.

2013-09-01

Zircons are ideal for investigating the early lunar bombardment because we can measure both U-Pb crystallization ages and fissiongenic Xe degassing ages for the same crystal. We report U-Pb, Pb-Pb and U-Xe ages for three lunar zircons.

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

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

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

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.

Provenance from zircon U-Pb age distributions in crustally contaminated granitoids

NASA Astrophysics Data System (ADS)

Bahlburg, Heinrich; Berndt, Jasper

2016-05-01

The basement of sedimentary basins is often entirely covered by a potentially multi-stage basin fill and therefore removed from direct observation and sampling. Melts intruding through the basin stratigraphy at a subsequent stage in the geological evolution of a region may assimilate significant volumes of country rocks. This component may be preserved in the intrusive body either as xenoliths or it may be reflected only by the age spectrum of incorporated zircons. Here we present the case of an Ordovician calc-alkaline intrusive belt in NW Argentina named the "Faja Eruptiva de la Puna Oriental" (Faja Eruptiva), which in the course of intrusion sampled the unexposed and unknown basement of the Ordovician basin in this region, and parts of the basin stratigraphy. We present new LA-ICP-MS U-Pb ages on zircons from 9 granodiorites and granites of the Faja Eruptiva. The main part of the Faja Eruptiva intruded c. 445 Ma in the Late Ordovician. The zircon ages obtained from the intrusive rocks have a large spread between 2683.5 ± 21.6 and 440.0 ± 4.9 Ma and reflect the underlying crust and may be interpreted in several ways. The inherited zircons may have been derived from the oldest known unit in the region, the thick siliciclastic turbidite successions of the upper Neoproterozoic-lower Cambrian Puncoviscana Formation, which is inferred to represent the basement of the NW Argentina. The basement to the Puncoviscana Formation is not known. Alternatively, the inherited zircons may reflect the geochronological structure of the entire unexposed Early Paleozoic crust underlying this region of which the Puncoviscana Formation was only one component. This crust likely contained rocks pertaining to and detritus derived from earlier orogenic cycles of the southwestern Amazonia craton, including sources of Early Meso- and Paleoproterozoic age. Detritus derived, in turn, from the Faja Eruptiva intrusive belt reflects the origin of the granitoids as well as the inherited

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

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

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

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.

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.

Permian U-Pb (CA-TIMS) zircon ages from Australia and China: Constraining the time scale of environmental and biotic change

NASA Astrophysics Data System (ADS)

Denyszyn, S. W.; Mundil, R.; Metcalfe, I.; He, B.

2010-12-01

In eastern Australia, the interconnected Bowen and Sydney Basins are filled with terrestrial sediments of late Paleozoic to early Mesozoic age. These sedimentary units record significant evolutionary events of eastern Gondwana during the time interval between two major mass extinctions (end Middle Permian and Permian-Triassic), and also provide lithological evidence for the Carboniferous-Permian Late Paleozoic Ice Age of southern Pangea, considered to be divisible into up to seven discrete glaciation events in Australia [e.g., 1]. These glaciations are currently assigned ages that indicate that the last of the glaciations predate the end Middle Permian mass extinction at ca. 260 Ma. However, the estimates for the time and durations are largely based on biostratigraphy and lithostratigraphy that, in the absence of robust and precise radioisotopic ages, are unacceptably fragile for providing an accurate high-resolution framework. Interbedded with the sediments are numerous tuff layers that contain zircon, many of which are associated with extensive coal measures in the Sydney and Bowen Basins. Published SHRIMP U-Pb zircon ages [2, 3] have been shown to be less precise and inaccurate when compared to ages applying the CA-TIMS method to the same horizons. Also within the late Middle Permian, the eruption of the Emeishan flood basalts in SW China has been proposed to have caused the end Middle Permian mass extinction [e.g., 4], though a causal link between these events demands a rigorous test that can only be provided by high-resolution geochronology. We present new U-Pb (CA-TIMS) zircon ages on tuff layers from the Sydney and Bowen Basins, with the purpose of generating a timescale for the Upper Permian of Australia to allow correlation with different parts of the world. Initial results, with permil precision, date a tuff layer within the uppermost Bandanna Fm. to ca. 252 Ma, a tuff within the Moranbah Coal Measures to ca. 256 Ma, and a tuff within the Ingelara Fm. to

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.

Zircons as a Probe of Early Lunar Impact History

NASA Astrophysics Data System (ADS)

Crow, C. A.; McKeegan, K. D.; Gilmour, J. D.; Crowther, S. A.; Taylor, D. J.

2013-08-01

Zircons are ideal for investigating the early lunar bombardment because we can measure both U-Pb crystallization ages and fissiongenic Xe degassing ages for the same crystal. We report U-Pb, Pb-Pb and U-Xe ages for two lunar zircons.

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

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.

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.

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

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.

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.

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.

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.

Two types of gneisses associated with eclogite at Shuanghe in the Dabie terrane: carbon isotope, zircon U-Pb dating and oxygen isotope

NASA Astrophysics Data System (ADS)

Zheng, Yong-Fei; Gong, Bing; Zhao, Zi-Fu; Fu, Bin; Li, Yi-Liang

2003-10-01

There are two types of gneisses, biotite paragneiss and granitic orthogneiss, to be closely associated with UHP eclogite at Shuanghe in the Dabie terrane. Both concentration and isotope composition of bulk carbon in apatite and host gneisses were determined by the EA-MS online technique. Structural carbonate within the apatite was detected by the XRD and FTIR techniques. Significant 13C-depletion was observed in the apatite with δ13C values of -28.6‰ to -22.3‰ and the carbon concentrations of 0.70-4.98 wt.% CO 2 despite a large variation in δ18O from -4.3‰ to +10.6‰ for these gneisses. There is significant heterogeneity in both δ13C and δ18O within the gneisses on the scale of several tens meters, pointing to the presence of secondary processes after the UHP metamorphism. Considerable amounts of carbonate carbon occur in some of the gneisses that were also depleted in 13C primarily, but subjected to overprint of 13C-rich CO 2-bearing fluid after the UHP metamorphism. The 13C-depleted carbon in the gneisses is interpreted to be inherited from their precursors that suffered meteoric-hydrothermal alteration before plate subduction. Both low δ13C values and structural carbonate in the apatite suggest the presence of 13C-poor CO 2 in the UHP metamorphic fluid. The 13C-poor CO 2 is undoubtedly derived from oxidation of organic matter in the subsurface fluid during the prograde UHP metamorphism. Zircons from two samples of the granitic orthogneiss exhibit low δ18O values of -4.1‰ to -1.1‰, demonstrating that its protolith was significantly depleted in 18O prior to magma crystallization. U-Pb discordia datings for the 18O-depleted zircons yield Neoproterozoic ages of 724-768 Ma for the protolith of the granitic orthogneiss, consistent with protolith ages of most eclogites and orthogneisses from the other regions in the Dabie-Sulu orogen. Therefore, the meteoric-hydrothermal alteration is directly dated to occur at mid-Neoproterozoic, and may be correlated

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.

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.

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.

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

Tracing source terranes using U-Pb-Hf isotopic analysis of detrital zircons: provenance of the Orhanlar Unit of the Palaeotethyan Karakaya subduction-accretion complex, NW Turkey

NASA Astrophysics Data System (ADS)

Ustaömer, Timur; Ayda Ustaömer, Petek; Robertson, Alastair; Gerdes, Axel

2016-04-01

Sandstones of the Late Palaeozoic-Early Mesozoic Karakaya Complex are interpreted to have accumulated along an active continental margin related to northward subduction of Palaeotethys. The age of deposition and provenance of the sandstones are currently being determined using radiometric dating of detrital zircons, coupled with dating of potential source terranes. Our previous work shows that the U-Pb-Hf isotopic characteristics of the sandstones of all but one of the main tectonostratigraphic units of the Karakaya Complex are compatible with a provenance that was dominated by Triassic and Permo-Carboniferous magmatic arc-type rocks, together with a minor contribution from Lower to Mid-Devonian igneous rocks (Ustaömer et al. 2015). However, one of the tectono-stratigraphic units, the Orhanlar Unit, which occurs in a structurally high position, differs in sedimentary facies and composition from the other units of the Karakaya Complex. Here, we report new isotopic age data for the sandstones of the Orhanlar Unit and also from an extensive, associated tectonic slice of continental metamorphic rocks (part of the regional Sakarya Terrane). Our main aim is to assess the provenance of the Orhanlar Unit sandstones in relation to the tectonic development of the Karakaya Complex as a whole. The Orhanlar Unit is composed of shales, sandstone turbidites and debris-flow deposits, which include blocks of Devonian radiolarian chert and Carboniferous and Permian neritic limestones. The sandstones are dominated by rock fragments, principally volcanic and plutonic rocks of basic-to-intermediate composition, metamorphic rocks and chert, together with common quartz, feldspar and mica. This modal composition contrasts significantly with the dominantly arkosic composition of the other Karakaya Complex sandstones. The detrital zircons were dated by the U-Pb method, coupled with determination of Lu-Hf isotopic compositions using a laser ablation microprobe attached to a multicollector

In situ detrital zircon (U-Th)/He thermochronology

NASA Astrophysics Data System (ADS)

Tripathy, A.; Monteleone, B. D.; van Soest, M. C.; Hodges, K.; Hourigan, J. K.

2010-12-01

Detrital studies of both sand and rock are relevant to many problems, ranging from the climate and tectonics feedback debate to the long-term record of orogenic evolution. When applying the conventional (U-Th)/He technique to such studies, two important issues arise. Often, only euhedral grains are permissible for analysis in order to make simple geometric corrections for α-recoil. In detrital samples, this is problematic because euhedral grains can be scarce due to mechanical abrasion during transport, and potentially introduce bias in favour of more proximally sourced grains. Second, inherent to detrital studies is the need to date many grains (>100) per sample to ensure a representative sampling of the sediment source region, thus making robust conventional detrital studies both expensive and time-consuming. UV laser microprobes can improve this by permitting careful targeting of the grain interior away from the α-ejection zone, rendering the α-recoil correction unnecessary, thus eliminating bias toward euhedral grains. In the Noble Gas, Geochemistry, and Geochronology Laboratory at ASU, apatite and zircon have been successfully dated using in situ methods. For this study, the conventional and in situ techniques are compared by dating zircons from a modern river sand that drains a small catchment in the Mesozoic-Cenozoic Ladakh Batholith in NW India. This sample has a simple provenance, which allows us to demonstrate the robustness of the in situ method. Moreover, different microbeam techniques will be explored to establish the most efficient approach to obtain accurate and precise U-Th concentrations using synrock, which is our powdered, homogenized, and reconstituted zircon-rock standard. Without this, such in situ U-Th data would be difficult to obtain. 117 zircons were dated using the conventional (U-Th)/He method, revealing dates ranging from 9.70±0.35 to 106.6±3.5 Ma (2σ) with the major mode at 26 Ma. For comparison, 44 grains were dated using the in

U-Pb (zircon) and geochemical constraints on the age, origin, and evolution of Paleozoic arc magmas in the Oyu Tolgoi porphyry Cu-Au district, southern Mongolia

USGS Publications Warehouse

Wainwright, A.J.; Tosdal, R.M.; Wooden, J.L.; Mazdab, F.K.; Friedman, R.M.

2011-01-01

Uranium-Pb (zircon) ages are linked with geochemical data for porphyry intrusions associated with giant porphyry Cu-Au systems at Oyu Tolgoi to place those rocks within the petrochemical framework of Devonian and Carboniferous rocks of southern Mongolia. In this part of the Gurvansayhan terrane within the Central Asian Orogenic Belt, the transition from Devonian tholeiitic marine rocks to unconformably overlying Carboniferous calc-alkaline subaerial to shallow marine volcanic rocks reflects volcanic arc thickening and maturation. Radiogenic Nd and Pb isotopic compositions (??Nd(t) range from +3.1 to +7.5 and 206Pb/204Pb values for feldspars range from 17.97 to 18.72), as well as low high-field strength element (HFSE) contents of most rocks (mafic rocks typically have <1.5% TiO2) are consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc. The Late Devonian and Carboniferous felsic rocks are dominantly medium- to high-K calc-alkaline and characterized by a decrease in Sr/Y ratios through time, with the Carboniferous rocks being more felsic than those of Devonian age. Porphyry Cu-Au related intrusions were emplaced in the Late Devonian during the transition from tholeiitic to calc-alkaline arc magmatism. Uranium-Pb (zircon) geochronology indicates that the Late Devonian pre- to syn-mineral quartz monzodiorite intrusions associated with the porphyry Cu-Au deposits are ~372Ma, whereas granodiorite intrusions that post-date major shortening and are associated with less well-developed porphyry Cu-Au mineralization are ~366Ma. Trace element geochemistry of zircons in the Late Devonian intrusions associated with the porphyry Cu-Au systems contain distinct Th/U and Yb/Gd ratios, as well as Hf and Y concentrations that reflect mixing of magma of distinct compositions. These characteristics are missing in the unmineralized Carboniferous intrusions. High Sr/Y and evidence for magma mixing in syn- to late-mineral intrusions distinguish the Late

U-Pb geochronology of zircon and monazite from Mesoproterozoic granitic gneisses of the northern Blue Ridge, Virginia and Maryland, USA

USGS Publications Warehouse

Aleinikoff, J.N.; Burton, W.C.; Lyttle, P.T.; Nelson, A.E.; Southworth, C.S.

2000-01-01

Mesoproterozoic granitic gneisses comprise most of the basement of the northern Blue Ridge geologic province in Virginia and Maryland. Lithology, structure, and U-Pb geochronology have been used to subdivide the gneisses into three groups. The oldest rocks, Group 1, are layered granitic gneiss (1153 ?? 6 Ma), hornblende monzonite gneiss (1149 ?? 19 Ma), porphyroblastic granite gneiss (1144 ?? 2 Ma), coarse-grained metagranite (about 1140 Ma), and charnockite (>1145 Ma?). These gneisses contain three Proterozoic deformational fabrics. Because of complex U-Pb systematics due to extensive overgrowths on magmatic cores, zircons from hornblende monzonite gneiss were dated using the sensitive high-resolution ion microprobe (SHRIMP), whereas all other ages are based on conventional U-Pb geochronology. Group 2 rocks are leucocratic and biotic varieties of Marshall Metagranite, dated at 1112??3 Ma and 1111 ?? 2 Ma respectively. Group 3 rocks are subdivided into two age groups: (1) garnetiferous metagranite (1077 ?? 4 Ma) and quartz-plagioclase gneiss (1077 ?? 4 Ma); (2) white leucocratic metagranite (1060 ?? 2 Ma), pink leucocratic metagranite (1059 ?? 2), biotite granite gneiss (1055 ?? 4 Ma), and megacrystic metagranite (1055 ?? 2 Ma). Groups 2 and 3 gneisses contain only the two younger Proterozoic deformational fabrics. Ages of monazite, seprated from seven samples, indicate growth during both igneous and metamorphic (thermal) events. However, ages obtained from individual grains may be mixtures of different age components, as suggested by backscatter electron (BSE) imaging of complexly zoned grains. Analyses of unzoned monazite (imaged by BSE and thought to contain only one age component) from porphyroblastic granite gneiss yield ages of 1070, 1060, and 1050 Ma. The range of ages of monazite (not reset to a uniform date) indicates that the Grenville granulite event at about 1035 Ma did not exceed about 750??C. Lack of evidence for 1110 Ma growth of monazite in

Mineral inclusions and SHRIMP U-Pb dating of zircons from the Alamas nephrite and granodiorite: Implications for the genesis of a magnesian skarn deposit

NASA Astrophysics Data System (ADS)

Liu, Yan; Zhang, Rongqing; Zhang, Zhiyu; Shi, Guanghai; Zhang, Qichao; Abuduwayiti, Maituohuti; Liu, Jianhui

2015-01-01

Extending approximately 1300 km and located in the Western Kunlun Mountains, the Hetian nephrite belt is the largest nephrite belt in the world and contains approximately 11 major deposits and more than 20 orebodies including the Alamas deposit. Hetian nephrite deposits can be classified as Mg-skarn deposits with Precambrian dolomitic marble host rock and green, green-white and white nephrite zones are distributed gradually in the zone of a granodiorite pluton. The green nephrite is mainly predominately composed of tremolite with generally minor to trace constituents of diopside, grossularitic garnet, actinolite and other minerals. Also green nephrite has higher content of TFe2O3, than green-white and white nephrites have. We subdivided the zircons from the green nephrites into four types, depending on their internal textures, mineral inclusions, and SHRIMP U-Pb ages. Type I zircons are round instead of idiomorphic in shape and lack obvious zoning. Type II and IV zircons have broad, clear oscillatory zoning and are hypidiomorphic or idiomorphic in shape; they contain inclusions of diopside, tremolite, chlorite and calcite. Most Type III zircons are narrow rims (< 10 μm) surrounding Type II and Type I zircons with highly luminous brightness and no zoning. Both Type I and Type II zircons have individual ages of 411 to 445 Ma and Type IV zircons have younger ages (388 to 406 Ma). Among the concordant ages, 425.7 ± 5.8 Ma and 420.0 ± 9.9 Ma for the QYZr1 and QYZr2 are consistent within error, with the 418.5 ± 2.8 Ma of the Alamas granodiorite formation age and the maximum age of the Alamas nephrite deposit. The partially recrystallization of zircons during skarn formation possibly lead to some younger individual ages (406.5 to 308 Ma). In the Western Kunlun Mountain, both Buya granite and Alamas grandiorite are high Ba-Sr granites and crystallized in Western Kunlun Orogen. The Buya granite formed at about 430 Ma in a post-orogenic tectonic environment. Considering

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.

Provenance of the exotic Northern Sierra terrane (North American Cordillera) based on U-Pb detrital zircon data

NASA Astrophysics Data System (ADS)

Powerman, V.; Girty, G.; Hanson, R. E.; Grove, M.; Miller, E. L.; Hourigan, J. K.

2017-12-01

Ages of detrital zircons from the Northern Sierra terrane (NST) suggest an exotic provenance with respect to NW Laurentia. We have acquired U-Pb LA-ICPMS dz ages from 16 samples collected from the uppermost NST allochthon, the Sierra City mélange, and 1 sample from the lower Culbertson Lake allochthon. Age distributions can be divided into 3 partly intersecting groups: (a) 6 mélange samples and the 1 Culbertson Lake allochthon sample are dominated by >1 Ga grains; (b)5 samples are characterized by the additional presence of Early Paleozoic and Neoproterozoic grains (520-640;680-800;840-1000Ma); (c) 9 samples, 8 feldspathic, 1—qtz-rich, can be also characterized by the presence of 360-520Ma grains. These results strengthen the non Laurentian nature of detrital sources:(1)most of the detrital age distributions possess ages in the 1.49-1.61Ga interval, the "N.American magmatic gap";(2) Ediacaran zircons cannot be linked to any igneous event within West Laurentia. Most samples possess detrital age distributions that include the 1.0-2.0 Ga peak, characteristic of Baltica rather than Laurentia. These data, supplemented by SHRIMP-RG data (353-368Ma) from stitching igneous units suggest the following model: parts of NST were located at the NE margin of Baltica in the early Paleozoic, receiving "Baltica" (1.0-2.0 Ga) and "Timanide"(Late Vendian — Early Cambrian) zircons. This crustal block was later rifted away from Baltica and by mid-Paleozoic was juxtaposed with allochthons of presumably NW Laurentia provenance. The assembled terrane was involved in a subduction zone, resulting in the emplacement of 353-368Ma igneous rocks. The U-Pb detrital zircon age distributions presented here are similar to signatures of strata in along strike exotic terranes of the North American Cordillera (such as the Yreka terrane of the Klamath Mts., the Alexander terrane of S.Alaska and the Arctic Chukotka-Alaska terrane) by having Timanian, Baltica, and Caledonian signatures. Hence, it

Petrologic and zircon U-Pb geochronological characteristics of the pelitic granulites from the Badu Complex of the Cathaysia Block, South China

NASA Astrophysics Data System (ADS)

Zhao, Lei; Zhou, Xiwen; Zhai, Mingguo; Liu, Bo; Cui, Xiahong

2018-06-01

The recognition of the Indosinian Orogeny in the South China block has been controversial and difficult because of strong weathering and thick cover. High temperature (HT) and high pressure (HP) metamorphic rocks related to this orogeny were considered to be absent from this orogenic belt until the recent discovery of eclogite and granulite facies meta-igneous rocks, occurring as lenses within the meta-sedimentary rocks of the Badu Complex. However, metamorphic state of these meta-sedimentary rocks is still not clear. Besides, there have been no geochronological data of HT pelitic granulites previously reported from the Badu Complex. This paper presents petrographic characteristics and zircon geochronological results on the newly discovered kyanite garnet gneiss, pyroxene garnet gneiss and the HT pelitic granulites (sillimanite garnet gneiss). Mineral assemblages are garnet + sillimanite + ternary feldspar + plagioclase + quartz + biotite for the HT pelitic granulite, kyanite + ternary feldspar + garnet + sillimanite + plagioclase + quartz + biotite for the kyanite garnet gneiss, and garnet + biotite + pyroxene + plagioclase + ternary feldspar + quartz for the pyroxene garnet gneiss, respectively. Decompressional coronas around garnet grains can be observed in all these pelitic rocks. Typical granulite facies mineral assemblages and reaction textures suggest that these rocks experienced HP granulite facies metamorphism and overprinted decompression along a clockwise P-T loop. Results from integrated U-Pb dating and REE analysis indicate the growth of metamorphic zircons from depleted heavy REE sources (100-50 chondrite) compared with detrital zircons derived from granitic sources (typically > 1000 chondrite). Metamorphic zircons in HP granulite exhibit no or subdued negative Eu anomalies, which perhaps indicate zircon overgrowth under eclogite facies conditions. The zircon overgrowth ages range from 250 to 235 Ma, suggesting that HP granulite (eclogite) to

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

Zircon U-Pb and molybdenite Re-Os geochronology and geological significance of the Baoshan porphyry Cu polymetallic deposit in Jiangxi province

NASA Astrophysics Data System (ADS)

Jia, Liqiong; Wang, Liang

2017-10-01

Baoshan porphyry Cu polymetallic deposit belongs to Jiujiang-Ruichang Cu-Au ore field, which is a component part of the Middle-Lower Yangtze River Cu-Au metallogenic belt. The U-Pb LA-MC-TCP MS dating of the zircons from Baoshan granodiorite porphyry yields an age of 147.81±0.48Ma (MSWD=1.07). Six molybdenite samples separated from Baoshan deposit are used for Re-Os dating and obtained the weighted average age of 147.42±0.84Ma and an isochron age of 147.7±1.2Ma. These ages suggest that the mineralization in the Baoshan deposit is genetically associated to the granodiorite porphyry, and the process of rock-and ore-forming is continuous. These data indicate that ages of intrusion and ore-body from Baoshan deposit are almost identical to other typical magmatic intrusion and deposits in Jiujiang-Ruichang metallogenic district. Tt is inferred that the Baoshan deposit was formed in the transition from EW-striking Tndosinian tectonic domain to NE-striking Paleo-Pacific tectonic domain.

North Qinling Terrain as a provenance of Kuanping Group: LA-ICP-MS U-Pb Geochronology of detrital zircons

NASA Astrophysics Data System (ADS)

Hu, B.; Li, S.; Zhai, M.; Wu, J.; Jia, X.

2017-12-01

Though some Neoproterozoic S-type granites in the North Qinling Terrain (NQT), China indicate the collision between the NQT and an unknown block, there are still controversial. The LA-ICP-MS U-Pb ages of detrital zircons of meta-sandstones from the Kuanping Group in Luonan area, NQT, provide sedimentology evidence to prove that the NQT and an unknown block from Rodinia supercontinent have been collided during Meso-Neoproterozoic. The U-Pb ages of detrital zircons from the Kuanping Group show that the main age peaks are at 2.58 Ga, 2.46 Ga, 2.0 Ga, 1.78 Ga, 1.6 Ga, 1.45 Ga and 1.27 Ga. The youngest age of 880 Ma indicates that the sedimentary age of the Kuanping Group is less than 880 Ma. The provenances, which provide 1.45 - 0.88 Ga sediments may come from NQT, which magmatic and metamorphic rocks during this period outcropped. Whereas provenances providing 2.6- 1.6 Ga sediments may come from an unknown block. This indicates that the Kuangping Group received both NQT and the unknown block materials. Therefore, the NQT and the unknown block may have collided before 880 Ma. 889 - 848 Ma A-type granites distributing the NQT was considered forming under a post-collisional tectonics. According the youngest detrital zircon ages of 880 Ma, it is inferred that the Kuanping Basin may also form in the same tectonic environments. Neoproterozoic Kuanping basin and 889 - 848 Ma A-type granites may be a result which NQT broken off a block of Rodinia supercontinent. Acknowledgments: This research is supported by National Key Research and Development Plan of China (2016YFC0601002), Special Fund for Basic Scientific Research of Central Colleges, Chang'an University (310827172201, 0009-2014G1271067) and National Nature Science Foundation of China (41402042).

U-Pb Detrital Zircon Ages from Sarawak: Changes in Provenance Reflecting the Tectonic Evolution of Southeast Asia

NASA Astrophysics Data System (ADS)

Breitfeld, H. T.; Galin, T.; Hall, R.

2014-12-01

Sarawak is located on the northern edge of Sundaland in NW Borneo. Five sedimentary basins are distinguished with ages from Triassic to Cenozoic. New light mineral, heavy mineral and U-Pb detrital zircon ages show differences in provenance reflecting the tectonic evolution of the region. The oldest clastic sediments are Triassic of the Sadong-Kuching Basin and were sourced by a Carnian to Norian volcanic arc and erosion of Cathaysian rocks containing zircons of Paleoproterozoic age. Sandstones of the Upper Jurassic to Cretaceous Bau-Pedawan Basin have distinctive zircon populations indicating a major change of tectonic setting, including initiation of subduction below present-day West Sarawak in the Late Jurassic. A wide range of inherited zircon ages indicates various Cathaysian fragments as major source areas and the arrival of the SW Borneo Block following subduction beneath the Schwaner Mountains in the early Late Cretaceous. After collision of the SW Borneo Block and the microcontinental fragments with Sundaland in the early Late Cretaceous, deep marine sedimentation (Pedawan Formation) ceased, and there was uplift forming the regional Pedawan-Kayan unconformity. Two episodes of extension were responsible for basin development on land from the latest Cretaceous onwards, probably in a strike-slip setting. The first episode formed the Kayan Basin in the Latest Cretaceous (Maastrichtian) to Early Paleocene, and the second formed the Ketungau Basin and the Penrissen Sandstone in the Middle to Late Eocene. Zircons indicate nearby volcanic activity throughout the Early Cenozoic in NW Borneo. Inherited zircon ages indicate an alternation between Borneo and Tin Belt source rocks. A large deep marine basin, the Rajang Basin, formed north of the Lupar Line fault. Zircons from sediments of the Rajang Basin indicate they are of similar age and provenance as the contemporaneous terrestrial sediments to the south suggesting a narrow steep continental Sundaland margin at the

Zircon U-Pb chronology, geochemistry and Sr-Nd-Pb isotopic compositions of the Volcanic Rocks in the Elashan area, NW China: petrogenesis and tectonic implications

NASA Astrophysics Data System (ADS)

Zhou, H.; Wei, J.; Shi, W.; Li, P.; Chen, M.; Zhao, X.

2017-12-01

Elashan area is located in the intersection of the East Kunlun Orogenic Belt (EKOB) and the West Qinling Orogenic (WQOB). We present petrology, zircon U-Pb ages, whole-rock geochemistry and Sr-Nd-Pb isotopic compositions from the andesite and felsic volcanic rocks (rhyolite and rhyolitic tuffs) in Elashan group volcanic rock. The LA-ICP-MS zircon U-Pb age data indicate that the volcanic rocks are emplaced at 250 247 Ma. The volcanic rocks have high -K and aluminum - peraluminous characteristics, A/CNK = 1.07 1.82, δ ranges from 1.56 2.95, the main body is calc-alkaline rock. They are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depleted in some high field strength elements (HFSEs, e.g., Nb, Ta, P and Ti), while having a flat heavy REE (HREEs) pattern. The ∑REE values of 178.68 to 298.11 ppm, average 230.50 ppm. The LREE/HREE values of 4.39 to 11.78 ppm, average 6.77 ppm. REE fractionation is obvious, REE distribution curve was right smooth, and have slightly negative Eu anomalies (Eu/Eu*=0.44-0.80, average 0.60), which as similar to the island arc volcanic rocks. The volcanic rocks have initial 87Sr/86Sr ratios of 0.71028-0.71232, ɛNd(t) values of -6.7 to -7.6, with T2DM-Nd ranging from 1561 to 1640 Ma. Pb isotopic composition (206 Pb / 204 Pb)t = 18.055 18.330, (207 Pb / 204 Pb)t = 15.586 15.618, (208 Pb / 204 Pb)t = 37.677 38.332. Geochemical and Sr-Nd-Pb isotopes indicates that Elashan group volcanic magma derived mainly from the lower crust. Elashan group volcanic rocks is the productive East Kunlun block and West Qinling block collision, which makes the thicken crust caused partial melting in the study area. The source rocks is probably from metamorphic sandstone of Bayankala. But with Y-Nb and Rb-(Y+Nb), R1-R2 and Rb/10-Hf-Ta*3 diagrams showing that intermediate-acid rocks mainly formed in volcanic arc-collision environment, probably the collision event is short , therefore rocks retain the original island

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

U-Pb SHRIMP geochronology of zircon in garnet peridotite from the Sulu UHP terrane, China: Implications for mantle metasomatism and subduction-zone UHP metamorphism

USGS Publications Warehouse

Zhang, R.Y.; Yang, J.S.; Wooden, J.L.; Liou, J.G.; Li, T.F.

2005-01-01

We studied the Zhimafang ultrahigh-pressure metamorphic (UHP) peridotite from pre-pilot drill hole PP-1 of Chinese Continental Scientific Drilling project in the Sulu UHP terrane, eastern China. The peridotite occurs as lens within quartofeldspathic gneiss, and has an assemblage of Ol + Opx + Cpx + Phl + Ti-clinohumite (Ti-Chu) + Grt (or chromite) ?? magnesite (Mgs). Zircons were separated from cores at depths of 152 m (C24, garnet lhezolite), 160 m (C27, strongly retrograded phlogopite-rich peridotite) and 225 m (C50, banded peridotite), and were dated by SHRIMP mass spectrometer. Isometric zircons without inherited cores contain inclusions of olivine (Fo91-92), enstatite (En91-92), Ti-clinohumite, diopside, phlogopite and apatite. The enstatite inclusions have low Al2O3 contents of only 0.04-0.13 wt.%, indicating a UHP metamorphic origin. The weighted mean 206Pb/238U zircon age for garnet lherzolite (C24) is 221 ?? 3 Ma, and a discordia lower intercept age for peridotite (C50) is 220 ?? 2 Ma. These ages are within error and represent the time of subduction-zone UHP metamorphism. A younger lower intercept age of 212 ?? 3 Ma for a foliated wehrlite (C27) was probably caused by Pb loss during retrograde metamorphism. The source of zirconium may be partially attributed to melt/fluid metasomatism within the mantle wedge. Geochronological and geochemical data confirm that the mantle-derived Zhimafang garnet peridotites (probably the most representative type of Sulu garnet peridotites) were tectonically inserted into a subducting crustal slab and subjected to in situ Triassic subduction-zone UHP metamorphism. ?? 2005 Elsevier B.V. All rights reserved.

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.

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

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

Comparing pre- and post-chemical abrasion ages for Miocene Peach Springs Tuff zircon from ID-TIMS and SIMS analyses

NASA Astrophysics Data System (ADS)

Lidzbarski, M. I.; Mundil, R.; Miller, J. S.; Vazquez, J. A.

2012-12-01

The Miocene Peach Spring Tuff (PST) is a voluminous (>600 km3), zoned ignimbrite (trachyte to high-SiO2 rhyolite) that is exposed widely in eastern California, western Arizona, and southernmost Nevada, which was erupted from the Silver Creek caldera in the southwestern Black Mountains, AZ. PST serves as a regionally widespread marker unit and its eruption age has been determined to 18.8 to 18.9 Ma by 40Ar/39Ar methods, when corrected for systematic bias and normalized to the U-Pb system (Renne et al., 2010,). We performed ion-microprobe (SIMS) U-Pb dating of zircon from individual pumice clasts from PST to evaluate the growth history of zircon in the PST magma system. Sectioned, polished zircon from conventional epoxy mounts allows dating of internal growth domains (e.g. cores, interiors, and near-rim), whereas mounting unpolished zircon in indium and analyzing unpolished crystal faces provides a means to selectively sample the final increments of crystal growth (Reid and Coath, 2000). Combining U-Pb ages of unpolished zircon rims with near-rim interior analyses on sectioned grains yields a mean age of ca. 18.3 Ma, whereas ages of cores of sectioned crystals yield a mean of ca. 18.9 Ma. Several zircons have rim and/or core ages that are several hundred thousand years older or younger than these means (up to 1 m.y. total spread), although the uncertainties for individual SIMS ages are 2 to 5% (2 sigma uncertainty). Therefore, the distribution of ages is challenging to resolve. A modest number of the older grains are plausibly recycled antecrysts, but we suspect that the youngest zircons may have experienced Pb-loss. Failure to account for the possibility of inheritance and Pb-loss may lead to erroneous interpretations about crystallization in the PST system. In order to evaluate and mitigate the effects of Pb-loss, we employed the chemical abrasion (CA) technique of Mattinson (2005), which effectively eliminates domains in zircon that have suffered Pb-loss, and

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

Detrital zircon and apatite (U-Th)/He geochronology of intercalated baked sediments: A new approach to dating young basalt flows

NASA Astrophysics Data System (ADS)

Cooper, Frances J.; van Soest, Matthijs C.; Hodges, Kip V.

2011-07-01

Simple numerical models suggest that many basaltic lava flows should sufficiently heat the sediments beneath them to reset (U-Th)/He systematics in detrital zircon and apatite. This result suggests a useful way to date such flows when more conventional geochronological approaches are either impractical or yield specious results. We present here a test of this method on sediments interstratified with basalt flows of the Taos Plateau Volcanic Field of New Mexico. Nineteen zircons and apatites from two samples of baked sand collected from the uppermost 2 cm of a fluvial channel beneath a flow of the Upper Member of the Servilleta Basalt yielded an apparent age of 3.487 ± 0.047 Ma (2 SE confidence level), within the range of all published 40Ar/39Ar dates for other flows in the Upper Member (2.81-3.72 Ma) and statistically indistinguishable from the 40Ar/39Ar dates for basal flows of the Upper Member with which the studied flow is broadly correlative (3.61 ± 0.13 Ma). Given the high yield of 4He from U and Th decay, this technique may be especially useful for dating Pleistocene basalt flows. Detailed studies of the variation of (U-Th)/He detrital mineral dates in sedimentary substrates, combined with thermal modeling, may be a valuable tool for physical volcanologists who wish to explore the temporal and spatial evolution of individual flows and lava fields.

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

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.

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

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

Genesis of the Bangbule Pb-Zn-Cu polymetallic deposit in Tibet, western China: Evidence from zircon U-Pb geochronology and S-Pb isotopes

NASA Astrophysics Data System (ADS)

Kan, Tian; Zheng, Youye; Gao, Shunbao

2016-04-01

The Banbule Pb-Zn-Cu skarn deposit is located in the Longger-Gongbujiangda volcanic magma arc in the Gangdese-Nyainqentanglha Plate. It is the only lead-zinc polymetallic deposit discovered in the westernmost Nyainqentanglha metallogenic belt. The measured and indicated resources include 0.9 Mt of Pb+Zn (4.77% Pb and 4.74% Zn, respectively), 6499 t of Cu, and 178 t of Ag (18.75g/t Ag). The orebodies mainly occur as lenses, veins and irregular shapes in the contact zone between the quartz-porphyry and limestone of the Upper Permian Xiala Formation, or in the boundaries between limestone and sandstone. Pb-Zn-Cu mineralization in the Banbule deposit is closely associated with skarns. The ore minerals are dominated by galena, sphalerite, chalcopyrite, bornite, and magnetite, with subordinate pyrite, malachite, and azurite. The gangue minerals are mainly garnet, actinolite, diopside, quartz, and calcite. The ore-related quartz-porphyry displays LA-ICP-MS zircon U-Pb age of 77.31±0.74 Ma. The δ34S values of sulfides define a narrow range of -0.8 to 4.7‰ indicating a magmatic source for the ore-forming materials. Lead isotopic systematics yield 206Pb/204Pb of 18.698 to 18.752, 207Pb/204Pb of 15.696 to 15.760, and 208Pb/204Pb of 39.097 to 39.320. The data points are constrained around the growth curves of upper crust and orogenic belt according to the tectonic discrimination diagrams. The calculated Δβ - Δγ values plot within the magmatic field according to the discrimination diagram of Zhu et al. (1995). The S-Pb isotopic data suggest that Bangbule is a typical skarn deposit, and the Pb-Zn-Cu mineralization is genetically related to the quartz-porphyry in the mining district. The discovery of the Bangbule deposit indicates that there is metallogenic potential in the westernmost Nyainqentanglha belt, which is of great importance for the exploration work in this area.

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.

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

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.

Petrography and U-Pb detrital zircon geochronology of metasedimentary strata dredged from the Chukchi Borderland, Amerasia Basin, Arctic Ocean

NASA Astrophysics Data System (ADS)

Brumley, K.; Miller, E. L.; Mayer, L. A.; Andronikov, A.; Wooden, J. L.; Dumitru, T. A.; Elliott, B.; Gehrels, G. E.; Mukasa, S. B.

2010-12-01

In 2008-2009, twelve dredges were taken aboard the USCGC Healy from outcrops along the Alpha Ridge, Northern Chukchi Borderland, Northwind Ridge and the Chukchi Plateau in the Arctic Ocean as part of the U.S. Extended Continental Shelf Project. To ensure sampling of outcrop, steep bathymetric slopes (>40°) with little mud cover were identified with multibeam sonar and targeted for dredging. The first dredge from Alpha Ridge yielded volcaniclastic sedimentary rocks deposited from a phreatomagmatic eruption in shallow water (<200m). This is inconsistent with tectonic reconstructions suggesting that the Alpha Ridge was created as an oceanic plateau on deep oceanic crust of the Canada Basin. Another dredge, taken from the northern tip of Northwind Ridge, yielded metasedimentary rocks deformed under greenschist facies conditions (chlorite+white mica). These rocks are intruded and/or overlain by mid-Cretaceous alkalic basalts, also taken in this dredge, and dated by 40Ar/39Ar (plagioclase separate) to be 112±1 Ma. The metasedimentary rocks, from this single dredge, range in grain size from mud to coarse sandstone and grit which all contain grains and sub-angular clasts of volcanic, plutonic, metamorphic and fine grained sedimentary rocks as well as monocrystalline quartz, potassium feldspar, and plagioclase. All of these samples display the same bedding to foliation angle and lithology, which further indicates that they were dredged from in situ outcrop and are not random samples of ice rafted debris. Based on grain size variations and graded beds, they are interpreted as Silurian gravity flow deposits fed by proximal syn-orogenic and/or magmatic arc sources. Detrital zircons were separated from four sandstone samples of the Northwind Ridge dredge, and their U-Pb single grain ages determined by LA-MC-ICPMS and SHRIMP, (N= 393). Their detrital zircon populations are dominated by euhedral first-cycle zircon ca. 430 and 980 Ma with lesser older recycled zircons between

Refined depositional history and dating of the Tongaporutuan reference section, north Taranaki, New Zealand: new volcanic ash U-Pb zircon ages, biostratigraphy and sedimentation rates

USGS Publications Warehouse

Maier, K.L.; Crundwell, Martin P.; Coble, Matthew A.; Kingsley-Smith, Peter R.; Graham, Stephan A.

2016-01-01

This study presents new radiometric ages from volcanic ash beds within a c. 1900 m thick, progradational, deep-water clastic slope succession of late Miocene age exposed along the north Taranaki coast of the North Island, New Zealand. The ash beds yield U–Pb zircon ages ranging from 10.63 ± 0.65 Ma to 8.97 ± 0.22 Ma. The new ages are compatible with and provide corroboration of New Zealand Tongaporutuan Stage planktic foraminiferal and bolboformid biostratigraphic events identified in the same section. The close accord between these two age datasets provides a stratigraphically consistent and coherent basis for examining margin evolution. The arrival of a prograding clastic wedge and ensuing upward shoaling is recorded by sedimentation rates c. 2000 m/Ma–1 that are an order of magnitude higher than sedimentation rates on the precursor deep basin floor. This outcrop study provides new constraints for interpreting analogous subsurface deposits in Taranaki Basin and complements the regional late Miocene biostratigraphic dating framework.

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

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

U-Pb zircon constraints on the tectonic evolution of southeastern Tibet, Namche Barwa area

USGS Publications Warehouse

Booth, A.L.; Zeitler, P.K.; Kidd, W.S.F.; Wooden, J.; Liu, Yajing; Idleman, B.; Hren, M.; Chamberlain, C.P.

2004-01-01

The eastern syntaxis of the Himalayas is expressed in the crust as a pronounced southward bend in the orogen. The change in strike of geologic features coincides with the high topography of the Namche Barwa region, the exposure of granulite-grade metamorphic rocks, and a 180-degree bend in the Yalu Tsangpo. We have conducted a geochronologic and geochemical investigation of several suites of granitoids collected from the Namche Barwa massif and subjacent terranes of southeastern Tibet, ranging from cm-scale dikes and sills to larger, outcrop-scale intrusions. U-Pb SHRIMP-RG zircon ages establish at least five magmatic episodes: ???400 to 500 Ma, ???120 Ma, 40 to 70 Ma, 18 to 25 Ma, and 3 to 10 Ma. These episodes broadly correlate to spatial patterns in sample localities, as follows: 400 to 500 Ma ages occur in zircon cores collected from within the massif proper; ???120 Ma granites, related to early Gangdese arc plutonism, are primarily located northeast of Namche Barwa; later (40-70 Ma) Gangdese activity is expressed in granites west of Namche Barwa. 18 to 25 Ma granites occur both along the suture zone west of Gyala Peri, and directly north of Namche Barwa along the area of the Jiali fault zone, and are attributed both to shearing within the Jiali fault zone and to an early Miocene Gangdese Thrust event. Exceptionally young (<10 Ma) zircon ages are clustered near the core of the massif, along the Yalu Tsangpo gorge. Trace-element geochemical data indicates the presence of both fluid-present and fluid absent melts, with a fluid-absent (decompression) melting regime dominating near the core of Namche Barwa.

Origin of northern Gondwana Cambrian sandstone revealed by detrital zircon SHRIMP dating

USGS Publications Warehouse

Avigad, D.; Kolodner, K.; McWilliams, M.; Persing, H.; Weissbrod, T.

2003-01-01

Voluminous Paleozoic sandstone sequences were deposited in northern Africa and Arabia following an extended Neoproterozoic orogenic cycle that culminated in the assembly of Gondwana. We measured sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages of detrital zircons separated from several Cambrian units in the Elat area of southern Israel in order to unravel their provenance. This sandstone forms the base of the widespread siliciclastic section now exposed on the periphery of the Arabian-Nubian shield in northeastern Africa and Arabia. Most of the detrital zircons we analyzed yielded Neoproterozoic concordant ages with a marked concentration at 0.55–0.65 Ga. The most likely provenance of the Neoproterozoic detritus is the Arabian-Nubian shield; 0.55–0.65 Ga was a time of posttectonic igneous activity, rift-related volcanism, and strike-slip faulting there. Of the zircons, 30% yielded pre-Neoproterozoic ages grouped at 0.9–1.1 Ga (Kibaran), 1.65–1.85 Ga, and 2.45–2.7 Ga. The majority of the pre-Neoproterozoic zircons underwent Pb loss, possibly as a consequence of the Pan-African orogeny resetting their provenance. Rocks of the Saharan metacraton and the southern Afif terrane in Saudi Arabia (∼1000 km south of Elat) are plausible sources of these zircons. Kibaran basement rocks are currently exposed more than 3000 km south of Elat (flanking the Mozambique belt), but the shape of the detrital zircons of that age and the presence of feldspar in the host sandstone are not fully consistent with such a long-distance transport. Reworking of Neoproteorozoic glacial detritus may explain the presence of Kibaran detrital zircons in the Cambrian of Elat, but the possibility that the Arabian-Nubian shield contains Kibaran rocks (hitherto not recognized) should also be explored.

New insights into Arctic paleogeography and tectonics from U-Pb detrital zircon geochronology

USGS Publications Warehouse

Miller, E.L.; Toro, J.; Gehrels, G.; Amato, J.M.; Prokopiev, A.; Tuchkova, M.I.; Akinin, V.V.; Dumitru, T.A.; Moore, Thomas E.; Cecile, M.P.

2006-01-01

To test existing models for the formation of the Amerasian Basin, detrital zircon suites from 12 samples of Triassic sandstone from the circum-Arctic region were dated by laser ablation-inductively coupled plasma-mass spectrometry (ICP-MS). The northern Verkhoyansk (NE Russia) has Permo-Carboniferous (265-320 Ma) and Cambro-Silurian (410-505 Ma) zircon populations derived via river systems from the active Baikal Mountain region along the southern Siberian craton. Chukotka, Wrangel Island (Russia), and the Lisburne Hills (western Alaska) also have Permo-Carboniferous (280-330 Ma) and late Precambrian-Silurian (420-580 Ma) zircons in addition to Permo-Triassic (235-265 Ma), Devonian (340-390 Ma), and late Precambrian (1000-1300 Ma) zircons. These ages suggest at least partial derivation from the Taimyr, Siberian Trap, and/ or east Urals regions of Arctic Russia. The northerly derived Ivishak Formation (Sadlerochit Mountains, Alaska) and Pat Bay Formation (Sverdrup Basin, Canada) are dominated by Cambrian-latest Precambrian (500-600 Ma) and 445-490 Ma zircons. Permo-Carboniferous and Permo-Triassic zircons are absent. The Bjorne Formation (Sverdrup Basin), derived from the south, differs from other samples studied with mostly 1130-1240 Ma and older Precambrian zircons in addition to 430-470 Ma zircons. The most popular tectonic model for the origin of the Amerasian Basin involves counterclockwise rotation of the Arctic Alaska-Chukotka microplate away from the Canadian Arctic margin. The detrital zircon data suggest that the Chukotka part of the microplate originated closer to the Taimyr and Verkhoyansk, east of the Polar Urals of Russia, and not from the Canadian Arctic. Copyright 2006 by the American Geophysical Union.

Thermal and exhumation history of the central Tianshan (NW China): Constraints by U-Pb geochronology and Ar-Ar and (U-Th)/He thermochronology

NASA Astrophysics Data System (ADS)

Yin, J.; Chen, W.; Hodges, K. V.; Xiao, W.; Van Soest, M. C.; Cai, K.; Zhang, B.; Mercer, C. M.; Yuan, C.

2015-12-01

Geochronology and thermochronology using multiple mineral-isotopic chronometers reveals the thermo-tectonic history of the central Tianshan (NW China) from emplacement to exhumation. Granites from the central Tianshan, which are associated with the southward subduction of the northern Tianshan Ocean, have been dated at 362-354 Ma using the LA-ICP-MS Zircon U-Pb method. A younger diorite sample (282 ± 1 Ma, Zircon U-Pb method by LA-ICP-MS) from northern Tianshan formed during the final closure of the Northern Tianshan Ocean when the Junggar Block collided with the Yili-Central Tianshan Block. 40Ar/39Ar step-wise heating plateau dates (biotite Ar/Ar: 312-293 Ma; Plagioclase Ar/Ar: 270-229 Ma) from the Central Tianshan show rapid post-magmatic cooling during the Late Carboniferous-Early Permian followed by a more modest rate of cooling from the middle Permian to the middle Jurassic. The northern Tianshan diorite (biotite Ar/Ar: 240 ± 1 Ma) also reveals a middle Jurassic cooling. Apatite (U-Th )/He dates from the central Tianshan samples range from ca. 130 Ma to ca. 116 Ma. The Apatite (U-Th )/He date for the northern Tianshan sample is ca. 27 Ma. Previous studies also reported Apatite (U-Th)/He ages of ca. 44 Ma-11 Ma in the Baluntai area of the southern Central Tianshan[1]. Two episodes of cooling are distinguished by thermal history modelling: (1) Mesozoic cooling occurred as the result of the exhumation and tectonic reactivation of the central Tianshan; and (2) The Tianshan orogenic belt has been rapidly exhumed since the Middle Cenozoic. References [1] Lü, H.H., Chang, Y., Wang, W., Zhou, Z.Y., 2013. Rapid exhumation of the Tianshan Mountains since the early Miocene: Evidence from combined apatite fission track and (U-Th)/He thermochronology. Science China: Earth Sciences, 43(12): 1964-1974 (in Chinese).

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

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

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

U-Pb zircon geochronology of the Paleoproterozoic Tagragra de Tata inlier and its Neoproterozoic cover, western Anti-Atlas, Morocco

USGS Publications Warehouse

Walsh, G.J.; Aleinikoff, J.N.; Benziane, F.; Yazidi, A.; Armstrong, T.R.

2002-01-01

New U-Pb zircon data obtained by sensitive high resolution ion microprobe (SHRIMP) from the Tagragra de Tata inlier in the western Anti-Atlas, Morocco establish Paleoproterozoic ages for the basement schists, granites, and metadolerites, and a Neoproterozoic age for an ignimbrite of the Ouarzazate Series in the cover sequence. The age of interbedded felsic metatuff in the metasedimentary and metavolcanic sequence of the basement schists is 2072 ?? 8 Ma. This date represents: (1) the first reliable age from the metasedimentary and metavolcanic sequence; (2) the oldest reliable age for the basement of the Anti-Atlas; (3) the first date on the timing of deposition of the sediments on the northern edge of the Paleoproterozoic West African craton; (4) a lower age limit on deformation during the Eburnean orogeny; and (5) the first date obtained from the non-granitic Paleoproterozoic basement of Morocco. Ages of 2046 ?? 7 Ma (Targant granite) and 2041 ?? 6 Ma (Oudad granite) support earlier interpretations of a Paleoproterozoic Eburnean igneous event in the Anti-Atlas. The granites post-date the Eburnean D1 deformation event in the Paleoproterozoic schist sequence, and place a ???2046 Ma limit on short-lived Eburnean deformation in the area. Cross-cutting metadolerite is 2040 ?? 6 Ma; this is the first date from a metadolerite in the western Anti-Atlas. All of the dolerites in the area post-date emplacement of the two granites and the new age constrains the onset of late- or post-Eburnean extension. Ignimbrite of the Ouarzazate Series, immediately above the Paleoproterozoic basement is 565 ?? 7 Ma. This Neoproterozoic age agrees with ages of similar volcanic rocks elsewhere from the Ouarzazate Series. The date also agrees with the ages of associated hypabyssal intrusions, and marks the second and final stage of Pan-African orogenic activity in the western Anti-Atlas. ?? 2002 Elsevier Science B.V. All rights reserved.

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

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.

Zircon ion microprobe dating of high-grade rocks in Sri Lanka

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

Kroener, A.; Williams, I.S.; Compston, W.

1987-11-01

The high-grade gneisses of Sri Lanka display spectacular in-situ granulitization phenomena similar to those observed in southern India and of current interest for evolutionary models of the lower continental crust. The absolute ages of these rocks are poorly constrained and so, using the SHRIMP ion microprobe, the authors have analyzed small spots on zircons from upper amphibolite to granulite grade quartzitic and pelitic metasediments. Detrital grains from a metaquartzite of the Highland Group preserve premetamorphic U-Pb ages of between 3.17 and 2.4 Ga and indicate derivation of the sediment from an unidentified Archean source terrain. The Pb-loss patterns of thesemore » zircons and the other samples suggest severe disturbance at ca 1100 Ma ago, which the authors attribute to high-grade regional metamorphism. Two pelitic gneisses contain detrital zircons with ages up to 2.04 Ga and also record an approx. = 1100 Ma event that is also apparent from metamorphic rims around old cores and new zircon growth. A granite intrusive into the Highland Group granulites records an emplacement age of 1000-1100 Ma as well as metamorphic disturbance some 550 Ma ago but also contains older, crustally derived xenocrysts. Zircons from a metaquartzite xenolith within the granitoid Vijayan Complex are not older than approx. 1100 Ma; therefore the Vijayan is neither Archean in age nor acted as basement to the Highland Group, as previously proposed. The authors suggest that the Vijayan Complex formed significantly later than the Highland Group and that the two units were brought into contact through post-1.1 Ga thrusting. Although the granulitization phenomena in India and Sri Lanka are similar, the granulite event in Sri Lanka is not Archean in age but took place in the late Proterozoic.« less

Direct U-Pb dating of Cretaceous and Paleocene dinosaur bones, San Juan Basin, New Mexico: COMMENT

USGS Publications Warehouse

Koenig, Alan E.; Lucas, Spencer G.; Neymark, Leonid A.; Heckert, Andrew B.; Sullivan, Robert M.; Jasinski, Steven E.; Fowler, Denver W.

2012-01-01

Based on U-Pb dating of two dinosaur bones from the San Juan Basin of New Mexico (United States), Fassett et al. (2011) claim to provide the first successful direct dating of fossil bones and to establish the presence of Paleocene dinosaurs. Fassett et al. ignore previously published work that directly questions their stratigraphic interpretations (Lucas et al., 2009), and fail to provide sufficient descriptions of instrumental, geochronological, and statistical treatments of the data to allow evaluation of the potentially complex diagenetic and recrystallization history of bone. These shortcomings lead us to question the validity of the U-Pb dates published by Fassett et al. and their conclusions regarding the existence of Paleocene dinosaurs.

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.

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

(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

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

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

Petrogenesis of the Yaochong granite and Mo deposit, Western Dabie orogen, eastern-central China: Constraints from zircon U-Pb and molybdenite Re-Os ages, whole-rock geochemistry and Sr-Nd-Pb-Hf isotopes

NASA Astrophysics Data System (ADS)

Chen, Wei; Xu, Zhaowen; Qiu, Wenhong; Li, Chao; Yu, Yang; Wang, Hao; Su, Yang

2015-05-01

The Dabie orogen is among the most famous continent-continent collisional orogenic belts in the world, and is characterized by intensive post-collisional extension, magmatism and Mo mineralization. However, the genetic links between the mineralization and the geodynamic evolution of the orogen remain unresolved. In this paper, the Yaochong Mo deposit and its associated granitic stocks were investigated to elucidate this issue. Our new zircon U-Pb ages yielded an Early Cretaceous age (133.3 ± 1.3 Ma) for the Yaochong granite, and our molybdenite Re-Os dating gave a similar age (135 ± 1 Ma) for the Mo deposit. The Yaochong stock is characterized by high silica and alkali but low Mg, Fe and Ca. It is enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs: Rb, K, Th and U), but strongly depleted in heavy REEs, and high field strength elements (HFSEs: Nb, Ta, Ti and Y). The Yaochong granite has initial 87Sr/86Sr ratios of 0.7087-0.7096, and Pb isotopic ratios of (206Pb/204Pb)i = 16.599-16.704, (207Pb/204Pb)i = 15.170-15.618 and (208Pb/204Pb)i = 36.376-38.248. The granite has εNd(t) of -18.0 to -16.3 and εHf(t) values of -26.5 to -20.0. All these data indicate that the Yaochong granite is a high-K calc-alkaline fractionated I-type granite, and may have originated from partial melting of the thickened Yangtze continental crust. The Mo ores also show low radiogenic Pb isotopes similar to the Yaochong stock. Medium Re content in molybdenite (21.8-74.8 ppm) also suggests that the ore-forming materials were derived from the thickened lower crust with possibly minor mixing with the mantle. Similar to the Eastern Dabie orogen, the thickened crust beneath the Western Dabie orogen may also have experienced tectonic collapse, which may have exerted fundamental geodynamic controls on the two-stage Mo mineralization in the region.

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

Upper crustal emplacement and deformation of granitoids inside the Uppermost Unit of the Cretan nappe stack: constraints from U-Pb zircon dating, microfabrics and paleostress analyses

NASA Astrophysics Data System (ADS)

Kneuker, Tilo; Dörr, Wolfgang; Petschick, Rainer; Zulauf, Gernold

2015-03-01

The present study is dealing with the emplacement and deformation of diorite and quartz diorite exposed along new road cuts between Agios Nikolaos and Sitia (Uppermost Unit, eastern Crete). Mingling of both melt types is indicated by enclaves of diorite inside quartz diorite and vice versa. The diorite and quartz diorite intruded into coarse-grained white marble, which is in lateral contact to, but also forms the roof of, the intrusive body. Evidence for contact metamorphism is indicated by increasing grain size of calcite in the marble with decreasing distance from the diorite. U-Pb (TIMS) dating of zircons, separated from quartz diorite, yielded a concordant age at 74.0 ± 0.25 Ma, which is interpreted as emplacement age. As this age is close to published K-Ar cooling ages of hornblende and biotite, the melt should have intruded and cooled down rapidly at upper structural levels, which is not common for granitoids of the Uppermost Unit of Crete. Upper crustal melt emplacement is also documented by stoped blocks and by the lack of any ductile (viscous) deformation. The diorite and quartz diorite, however, are affected by strong post-Oligocene brittle faulting. Paleostress analysis, based on these faults, revealed a change in stress field from N-S and NNW-SSE shortening by thrusting (convergence between African and European plates) to NNE-SSW and NE-SW shortening accommodated by strike-slip (SW-ward extrusion of the Anatolian microplate). Calcite-twin density indicates high differential stress (260 ± 20 MPa) related to these phases of crustal shortening.

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

Connecting the U-Th and U-Pb Chronometers: New Algorithms and Applications

NASA Astrophysics Data System (ADS)

McLean, N. M.; Smith, C. J. M.; Roberts, N. M. W.; Richards, D. A.

2016-12-01

The U-Th and U-Pb geochronometers are important clocks for separate intervals of the geologic timescale. U-Th dates exploit disequilibrium in the 238U intermediate daughter isotopes 234U and 230Th, and are often used to date corals and speleothems that are zero age through 800 ka. The U-Pb system relies on secular equilibrium decay of 238U to 206Pb and 235U to 207Pb over longer timescales, and can be used to date samples from <1 Ma to 4.5 Ga. Disequilibrium plays a role in young U-Pb dates, but only as a nuisance correction. Both chronometers can produce dates with uncertainties <0.1% near the center of their applicable age ranges, but become less precise at their intersection, when the 238U decay chain approaches secular equilibrium and there has been little time for ingrowth of radiogenic Pb. However, if measurements or assumptions about both chronometers can be made, then they can be combined into a single, more informed date. Coupling the datasets can improve their precision and accuracy and help interrogate the assumptions that underpin each. Working with this data is difficult for two reasons. The Bateman equations are long and cumbersome for U decay chains that include 238U, 234U, 230Th, 226Ra, 206Pb and 235U, 231Pa, and 207Pb. Also, Pb measurements often comprise varying amounts of radiogenic Pb from locally heterogeneous U concentrations mixed with varying amounts of common Pb. At present there is no established, flexible computational framework to combine information from measurements and/or assumptions of these parameters, and no way to visualize and interpret the results. We present new algorithms to quickly and accurately solve the system of differential equations defined by both of the uranium decay chains and the linear regression through the U-Pb isochron. The results are illustrated on a new concordia diagram, where the concordia curve is determined by measured and/or assumed U-series disequilibrium and can have unfamiliar topologies. We

Detrital zircon U-Pb geochronology and provenance of the Carboniferous-Permian glaciomarine pebbly slates in the Tibetan Plateau

NASA Astrophysics Data System (ADS)

Wang, Q.; Zhu, D.; Zhao, Z.; Chung, S.; Li, C.; Sui, Q.; Fu, X.; Mo, X.

2011-12-01

Glaciomarine diamictites (including pebbly slate, pebbly siltstone, and pebbly sandstone) in the Tibetan Plateau are widely interpreted to have been associated with the deglaciation of the Indian continent. Guiding by zircon cathodoluminescence images, we determined U-Pb ages for detrital zircons from five typical Carboniferous-Permian pebbly slate samples from the Qiangtang, Lhasa, and Tethyan Himalaya of the Tibetan Plateau. The age distributions of detrital zircons from two samples (180 analyses) from Qiwu and Gangma Tso of the Qiangtang Terrane are similar, with two main age peaks ca. 579 and ca. 816 Ma and one minor age peak ca. 2490 Ma. Two samples (177 analyses) from Jiangrang and Damxung of the Lhasa Terrane define similar age distributions with two main age peaks ca. 539 and ca. 1175 Ma. Ages of detrital zircons from one sample (110 analyses) from Kangmar of the Tethyan Himalaya display main age peaks ca. 535, ca. 949, and ca. 2490 Ma. The ca. 816-Ma detrital zircons from the Qiangtang Terrane were most likely derived from the Lesser Himalaya, and the ca. 950-Ma detrital zircons from the Tethyan Himalaya might have been sourced from the High Himalaya, Eastern Ghats Province of the Indian plate and the Rayner Province of East Antarctica. The distinctive ca. 1175-Ma age population characteristic of zircons in the pebbly slates from the Lhasa Terrane is identical to the detrital zircons from the late Paleozoic sandstones (Zhu et al., 2011a) and the inherited zircons from the Mesozoic peraluminous granites (Zhu et al., 2011b) in this terrane, but significantly absent in the pebbly slates from both the Qiangtang and the Tethyan Himalayan terranes. The ca. 1175-Ma detrital zircons in the Lhasa Terrane were most likely sourced from the Albany-Fraser-Wilkes in southwestern Australia and East Antarctica. These new data obtained in this study reveal a distinct difference of detrital zircon provenance for the coeval Carboniferous-Permian glaciomarine pebbly slates

U-Pb dating and emplacement history of granitoid plutons in the northern Sanandaj-Sirjan Zone, Iran

NASA Astrophysics Data System (ADS)

Mahmoudi, Shahryar; Corfu, Fernando; Masoudi, Fariborz; Mehrabi, Behzad; Mohajjel, Mohammad

2011-05-01

The Sanandaj-Sirjan Zone (SSZ), which runs parallel to the Zagros fold and thrust belt of Iran, underwent a multistage evolution starting with Neotethys initiation, its subsequent subduction below the Iranian continental crust, and eventual closure during convergence of Arabia towards central Iran. Plutonic complexes are well developed in the northern part of the SSZ and we have dated a number of them by ID-TIMS U-Pb on zircon. The new data record the following events: a Mid Jurassic period that formed the Boroujerd Plutonic Complex (169 Ma), the Astaneh Pluton (168 Ma) and the Alvand Pluton (165 Ma); Late Jurassic emplacement of the Gorveh Pluton (157-149 Ma); Mid Cretaceous (109 Ma) formation of a I-type phase in the Hasan Salary Pluton near Saqqez, followed by Early Paleocene (60 Ma) intrusion of A-type granite in the same pluton; and the youngest intrusive event recorded so far in the SSZ with the intrusion of granite in the Gosheh-Tavandasht Complex near Boroujerd at 34.9 Ma. These different events reflect specific stages of subduction-related magmatism prior to the eventual Miocene collision between the two continental blocks.

Tectonic Evolution of the Izmir Ankara Suture Zone in Northwest Turkey Using Zircon U-Pb Geochronology and Zircon Lu-Hf Isotopic Tracers

NASA Astrophysics Data System (ADS)

Campbell, C.; Taylor, M. H.; Licht, A.; Mueller, M.; Ocakglu, F.; Moeller, A.; Metais, G.; Beard, K. C.

2017-12-01

Detrital zircons from a Cretaceous forearc basin and Tertiary foreland basin located along the Sakarya Zone of the Western Pontides were analyzed to better understand the closure history of the Tethyan oceans. The Variscan Orogeny is characterized by abundant 350-300 Ma U-Pb ages and vertical ɛHf arrays, consistent with a mature magmatic arc that emplaced plutons through a southward growing accretionary margin. An ɛHf pull-up is observed from 300-250 Ma interpreted as rifting of the Intra-Pontide Ocean. The Cimmerian Orogeny is characterized by a 250-230 Ma ɛHf pull-down, followed by a 230-200 Ma magmatic gap consistent with underthrusting of the Karakaya Complex. From 200-120 Ma another magmatic lull is observed. The Alpine Orogeny is characterized by an ɛHf pull-down from 120-85 Ma within Cretaceous forearc sediments and a 100 Ma deviant ɛHf vertical array within Tertiary foreland basin sediments. Minor zircon U-Pb age peaks and contrasting inter-basinal ɛHf evolution are interpreted to represent onset of Andean-style subduction along the southern margin of the Sakarya Zone at 120 Ma followed by crustal thickening until 85 Ma. The deviant 100 Ma ɛHf vertical array within foreland basin detritus is interpreted as initiation of intra-oceanic subduction within the Izmir-Ankara Ocean. An 85-75 Ma ɛHf pull-up from forearc basin sediments is interpreted as slab roll-back along the southern margin of the Sakarya Zone, responsible for final rifting of the Western Black Sea. At 80 Ma, a vertical ɛHf array from Tertiary foreland basin deposits is interpreted to represent synchronous melting of the Tavsanli Zone and intra-oceanic slab break-off. A single 66 Myr pre-collisional grain defines a sharp ɛHf pull-down immediately prior to total arc shut-off, interpreted to represent incipient collision between the Sakarya and Tavsanli zones. A 52 Ma syn-collisional tuff yields minimally intermediate ɛHf values followed by a slight 48 Ma ɛHf pull-down, interpreted as a

Timing of mid-crustal ductile extension in the northern Snake Range metamorphic core complex, Nevada: Evidence from U/Pb zircon ages

NASA Astrophysics Data System (ADS)

Lee, J.; Blackburn, T.; Johnston, S. M.

2016-12-01

Metamorphic core complexes (Mccs) within the western U.S. record a history of Cenozoic ductile and brittle extensional deformation, metamorphism, and magmatism, and exhumation within the footwall of high-angle Basin and Range normal faults. Documenting these histories within Mccs have been topics of research for over 40 years, yet there remains disagreement about: 1) whether the detachment fault formed and moved at low angles or initiated at high angles and rotated to a low angle; 2) whether brittle and ductile extensional deformation were linked in space and time; and 3) the temporal relationship of both modes of extension to the development of the detachment fault. The northern Snake Range metamorphic core complex (NSR), Nevada has been central to this debate. To address these issues, we report new U/Pb dates from zircon in deformed and undeformed rhyolite dikes emplaced into ductilely thinned and horizontally stretched lower plate rocks that provide tight bounds on the timing of ductile extension at between 38.2 ± 0.3 Ma and 22.50 ± 0.36 Ma. The maximum age constraint is from the Northern dike swarm (NDS), which was emplaced in the northwest part of the range pre- to syn-tectonic with ductile extension. The minimum age constraint is from the Silver Creek dike swarm (SDS) that was emplaced in the southern part of the range post ductile extensional deformation. Our field observations, petrography, and U/Pb zircon ages on the dikes combined with published data on the geology and kinematics of extension, moderate and low temperature thermochronology on lower plate rocks, and age and faulting histories of Cenozoic sedimentary basins adjacent to the NSR are interpreted as recording an episode of localized upper crustal brittle extension during the Eocene that drove upward ductile extensional flow of hot middle crustal rocks from beneath the NSR detachment soon after, or simultaneous with, emplacement of the NDS. Exhumation of the lower plate continued in a rolling

Evolution Of An Upper Crustal Plutonic-Volcanic Plumbing System:Insights From High Precision U-Pb Zircon Geochronology Of Intracaldera Tuff And Intrusions In Silver Creek Caldera, Arizona, USA

NASA Astrophysics Data System (ADS)

Zhang, T.; Mundil, R.; Miller, C. F.; Miller, J. S.; Paterson, S. R.

2010-12-01

Study of both plutonic and volcanic regimes in one single magmatic system is a powerful approach towards obtaining a more complete view of the long-term evolution of magma systems. The recently discovered Silver Creek caldera is the source of the voluminous Peach Spring Tuff (PST) (Ferguson, 2008) and presents a unique opportunity to study a field laboratory of a linked plutonic-volcanic system. This relict west-facing half caldera is predominantly filled with trachytic intracaldera tuff with the caldera margin intruded by several petrologically distinct hypabyssal intrusions. These include porphyritic granite with granophyric texture, felsic leucogranite, porphyritic monzonite exposed on NE side of the caldera that is zoned from more felsic to more mafic, and quartz-phyric dikes that intrude the caldera fill. We present preliminary single zircon ages from 4 samples that have been analyzed using the CA-TIMS method after thermal annealing and chemical leaching (Mattinson 2005), including 1 sample from intracaldera tuff and 3 samples from caldera-related intrusions. 3-D total U/Pb isochron ages from all four samples fall within a range of 18.32-18.90 Ma with uncertainties between 0.09 and 0.39 Ma, although some of them lack precision and are compromised by elevated common Pb. For example, zircon from the dated porphyritic monzonite yields an age of 18.32±0.42 Ma (MSWD=2.7) where the excess scatter may result from real age dispersion and/or different compositions of the common Pb contribution. The PST had been dated to ~18.5 Ma by 40Ar/39Ar techniques (Nielson et al., 1990). In order to be compared to U/Pb ages the 40Ar/39Ar age must be adjusted for a revised age for the then used flux monitor (MMbh-1) and corrected for the now quantified systematic bias between 40Ar/39Ar and U/Pb ages (Renne et al., 2010), which results in a corrected age of 18.8 Ma. Thus, the ages for our samples match that of the PST within error. Based on current results, the age difference

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

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.

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

SHRIMP U-Pb in zircon geochronology of granitoids from Myanmar: temporal constraints on the tectonic evolution of Southeast Asia

NASA Astrophysics Data System (ADS)

Barley, M. E.; Zaw, Khin

2009-04-01

The Mesozoic to Tertiary tectonic evolution of Southeast Asia is the result of the convergence and collision of fragments of Gondwanaland with Eurasia culminating in the collision of India. A rapidly growing geochronological database is placing tight constraints on the timing and duration of magmatic episodes, metallogenic and tectonic events in the Himalayas, Tibet and eastern Indochina. However, there is little comparable geochronology for Myanmar. This SHRIMP U-Pb in zircon geochronology focuses on granitoids from the Mogok Metamorphic Belt (MMB, a belt of high grade metamorphic rocks at the edge of the Shan-Thai Terrane), the Myeik Archipelago (Shan-Thai Terrane) and the west Myanmar Terrane. Strongly deformed granitic orthogneisses in the MMB near Mandalay contain Jurassic (~170 Ma) zircons that have partly recrystallised during ~43 Ma high-grade metamorphism. A hornblende syenite from Mandalay also contains Jurassic zircons with evidence of Eocene metamorphism rimmed by thin zones of 30.9 ±0.7 Ma magmatic zircon. The relative abundance of Jurassic zircons in these rocks is consistent with suggestions that southern Eurasia had an Andean-type margin at that time. Mid-Cretaceous to earliest Eocene (120 to 50 Ma). I-type granitoids in the MMB, Myeik Archipelago and west Myanmar confirm that prior to the collision of India, an up to 200km wide magmatic belt extended along the Eurasian margin. The primitive I-type Khanza Chaung granodiorite in the Wuntho batholith in the west Myanmar terrane hosts porphyry-style mineralisation and has a magmatic age of 94  1 Ma. Triassic (~240 Ma), Jurassic (~170 Ma) and Early Cretaceous xenocryst zircons in this granitoid correspond with peaks of granitoid magmatism in the Shan-Thai terrane and establish that west Myanmar was part of the margin of Eurasia during the Mesozoic. A suite of highly fractionated metaluminous to peraluminous I-type granitoids with associated Sn-W-Ta mineralisation emplaced in the Myeik Archipelago of

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.

Ca. 890 Ma magmatism in the northwest Yangtze block, South China: SIMS U-Pb dating, in-situ Hf-O isotopes, and tectonic implications

NASA Astrophysics Data System (ADS)

Zhou, Jiu-Long; Li, Xian-Hua; Tang, Guo-Qiang; Gao, Bing-Yu; Bao, Zhi-An; Ling, Xiao-Xiao; Wu, Li-Guang; Lu, Kai; Zhu, Yu-Sheng; Liao, Xin

2018-01-01

Early Neoproterozoic tectonics of the Yangtze block remains poorly understood because very limited igneous records are available from the time interval of ∼1000-870 Ma. In this paper, our new SIMS U-Pb dating results demonstrate that the Liushudian mafic intrusion and Pinghe alkaline complex in the northwest Yangtze block were emplaced at 888 ± 6 Ma and 891 ± 7 Ma, respectively, representing the products of a ∼890 Ma igneous event. Gabbros from the Liushudian intrusion have rather depleted zircon ɛHf(t) (mean = 10.4) and normal mantle-like zircon δ18O (mean = 5.97‰). Their parental magma was thus probably derived from asthenospheric mantle. Geochemically, these mafic rocks have an affinity to continental flood tholeiitic basalts rather than ocean island basalts, as previously thought. In contrast, an ijolite sample from the Pinghe complex has less depleted zircon ɛHf(t) (mean = 5.7) and anomalously high zircon and apatite δ18O (mean = 13.76‰ and 13.80‰, respectively). Such a characteristic δ18O signal, among the highest yet known for igneous zircons, could be either inherited from a magma source in metasomatized lithospheric mantle or acquired by assimilation of high-δ18O supracrustal materials (e.g., limestone, chert) during magma evolution. An intra-plate extensional environment is suggested for the ∼890 Ma igneous event in the northwest Yangtze block, although it is as yet unclear whether this igneous event is related to a mantle plume or not. It could be concluded that magmatism on the western periphery of the Yangtze block was not shut down between ∼1000 and ∼870 Ma, and the ∼890 Ma intra-plate igneous event may mark either the onset of Neoproterozoic continental rifting or the ending of Late Mesoproterozoic to Early Neoproterozoic lithospheric extension.

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

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

Zircon U-Pb age, Hf isotope and geochemistry of Carboniferous intrusions from the Langshan area, Inner Mongolia: Petrogenesis and tectonic implications

NASA Astrophysics Data System (ADS)

Liu, Min; Zhang, Da; Xiong, Guangqiang; Zhao, Hongtao; Di, Yongjun; Wang, Zhong; Zhou, Zhiguang

2016-04-01

Late Paleozoic was a critical period for the tectonic evolution of the northern margin of the Alxa-North China craton, but the evolutionary history is not well constrained. The Carboniferous intrusions in the Langshan area in the western part of the northern margin of the Alxa-North China craton are mainly composed of tonalite, quartz diorite, olivine gabbro and pyroxene peridotite. Zircon LA-ICP-MS U-Pb dating indicates that the Langshan Carboniferous intrusions were emplaced at ca. 338-324 Ma. The quartz diorites are characterized by high amounts of compatible trace elements (Cr, Ni and V) and high Mg# values, which may suggest a significant mantle source. The positive Pb and negative Nb-Ta-Ti anomalies, the variable εHf(t) (-6.9 to 2.0) values and the old Hf model ages (1218-1783 Ma) imply some involvement of ancient continental materials in its petrogenesis. The tonalite has relatively high Sr/Y ratios, low Mg#, Yb and Y contents, features of adakite-like rocks, negative εHf(t) values (-9.8 to -0.1) and older Hf model ages (1344-1953 Ma), which suggest significant involvement of ancient crust materials and mantle-derived basaltic component in its petrogenesis. The high Mg# values, high Cr and Ni contents, and low Zr and Hf contents of the mafic-ultramafic rocks show evidence of a mantle source, and the relatively low zircon εHf(t) values (-5.9 to 3.2) might point to an enriched mantle. The trace element characteristics indicate the influence of subducted sediments and slab-derived fluids. In the tectonic discrimination diagrams, all the rocks plot in subduction-related environment, such as volcanic arc and continental arc. Considering the regional geology, we suggest that the Carboniferous intrusions in the Langshan area were likely emplaced during the late stage of the southward subduction of the Paleo-Asian Ocean plate, which formed a continental arc along the northern margin of the Alxa-North China craton.

Calibrating Late Cretaceous Terrestrial Cyclostratigraphy with High-precision U-Pb Zircon Geochronology: Qingshankou Formation of the Songliao Basin, China

NASA Astrophysics Data System (ADS)

Wang, T.; Ramezani, J.; Wang, C.

2015-12-01

A continuous succession of Late Cretaceous lacustrine strata has been recovered from the SK-I south (SK-Is) and SKI north (SK-In) boreholes in the long-lived Cretaceous Songliao Basin in Northeast China. Establishing a high-resolution chronostratigraphic framework is a prerequisite for integrating the Songliao record with the global marine Cretaceous. We present high-precision U-Pb zircon geochronology by the chemical abrasion isotope dilution thermal-ionization mass spectrometry method from multiple bentonite core samples from the Late Cretaceous Qingshankou Formation in order to assess the astrochronological model for the Songliao Basin cyclostratigraphy. Our results from the SK-Is core present major improvements in precision and accuracy over the previously published geochronology and allow a cycle-level calibration of the cyclostratigraphy. The resulting choronostratigraphy suggest a good first-order agreement between the radioisotope geochronology and the established astrochronological time scale over the corresponding interval. The dated bentonite beds near the 1780 m depth straddle a prominent oil shale layer of the Qingshankou Formation, which records a basin-wide lake anoxic event (LAE1), providing a direct age constraint for the LAE1. The latter appears to coincide in time with the Late Cretaceous (Turonian) global sea level change event Tu4 presently constrained at 91.8 Ma.

Detrital zircon U-Pb reconnaissance of the Franciscan subduction complex in northwestern California

USGS Publications Warehouse

Dimitru, Trevor; Ernst, W. Gary; Hourigan, Jeremy K.; McLaughlin, Robert J.

2015-01-01

In northwestern California, the Franciscan subduction complex has been subdivided into seven major tectonostratigraphic units. We report U-Pb ages of ≈2400 detrital zircon grains from 26 sandstone samples from 5 of these units. Here, we tabulate each unit's interpreted predominant sediment source areas and depositional age range, ordered from the oldest to the youngest unit. (1) Yolla Bolly terrane: nearby Sierra Nevada batholith (SNB); ca. 118 to 98 Ma. Rare fossils had indicated that this unit was mostly 151-137 Ma, but it is mostly much younger. (2) Central Belt: SND; ca. 103 too 53 Ma (but poorly constrained), again mostly younger than previously thought. (3) Yager terrane: distant Idaho batholith (IB); ca. 52 to 50 Ma. Much of the Yager's detritus was shed during major core complex extension and erosion in Idaho that started 53 Ma. An eocene Princeton River-Princeton submarine canyon system transported this detritus to the Great Valley forearc basin and thence to the Franciscan trench. (4) Coastal terrane: mostly IB, ±SNB, ±nearby Cascade arc, ±Nevada Cenozoic ignimbrite belt; 52 to <32 Ma. (5) King Range terrane: dominated by IB and SNB zircons; parts 16-14 Ma based on microfossils. Overall, some Franciscan units are younger than previously thought, making them more compatible with models for the growth of subduction complexes by positive accretion. From ca. 118 to 70 Ma, Franciscan sediments were sourced mainly from the nearby Sierra Nevada region and were isolated from southwestern US and Mexican sources. From 53 to 49 Ma, the Franciscan was sourced from both Idaho and the Sierra Nevada. By 37-32 Ma, input from Idaho had ceased. The influx from Idaho probably reflects major tectonism in Idaho, Oregon, and Washington, plus development of a through-going Princeton River to California, rather than radical changes in the subduction system at the Franciscan trench itself.

U-Pb zircon age data for selected sedimentary, metasedimentary, and igneous rocks from northern and central Alaska

USGS Publications Warehouse

Moore, Thomas E.

2014-01-01

Data from two studies are included in this report. The first study, by Dumoulin and others (2013), reported the detrital zircon U-Pb age analysis of a single sample from the Upper Mississippian Ikalukrok unit of the Kuna Formation (table 1). The second study is that of Moore and others (in press), which focuses on the Upper Jurassic and Lower Cretaceous part of the Brookian sequence in the western Brooks Range (17 samples; table 2). For the latter study, samples were analyzed from the following units (1) the Upper Jurassic unit, Jw, of Curtis and others (1984), (2) the Lower Cretaceous Igrarok Hills unit of Moore and others (2002), (3) the Upper Jurassic and Lower Cretaceous Okpikruak Formation, (4) the Lower Cretaceous lower Brookian shale of Mull (1995), (5) the Lower Cretaceous Mount Kelly Graywacke Tongue of the Fortress Mountain Formation, (6) and the upper Lower Cretaceous Nanushuk Formation as redefined by Mull and others (2003). The results for each study are reported in separate Excel files, with individual samples in each study being shown as separate sheets within the files. The analyses of individual zircons are listed separately on the sheet according to the filtering schemes of the study and by the type of mass spectrometer used.

U-Pb SHRIMP-RG zircon ages and Nd signature of lower Paleozoic rifting-related magmatism in the Variscan basement of the Eastern Pyrenees

USGS Publications Warehouse

Martinez, F.J.; Iriondo, A.; Dietsch, C.; Aleinikoff, J.N.; Peucat, J.J.; Cires, J.; Reche, J.; Capdevila, R.

2011-01-01

The ages of orthogneisses exposed in massifs of the Variscan chain can determine whether they are part of a pre-Neoproterozoic basement, a Neoproterozoic, Panafrican arc, or are, in fact, lower Paleozoic, and their isotopic compositions can be used to probe the nature of their source rocks, adding to the understanding of the types, distribution, and tectonic evolution of peri-Gondwanan crystalline basement. Using SHRIMP U-Pb zircon geochronology and Nd isotopic analysis, pre-Variscan metaigneous rocks from the N??ria massif in the Eastern Pyrenean axial zone and the Guilleries massif, 70km to the south, have been dated and their Nd signatures characterized. All dated orthogneisses from the N??ria massif have the same age within error, ~457Ma, including the Ribes granophyre, interpreted as a subvolcanic unit within Caradocian sediments contemporaneous with granitic magmas intruded into Cambro-Ordovician sediments at deeper levels. Orthogneisses in the Guilleries massif record essentially continuous magmatic activity during the Ordovician, beginning at the Cambro-Ordovician boundary (488??3Ma) and reaching a peak in the volume of magma in the early Late Ordovician (~460Ma). Metavolcanic rocks in the Guilleries massif were extruded at 452??4Ma and appear to have their intrusive equivalent in thin, deformed veins of granitic gneiss (451??7Ma) within metasedimentary rocks. In orthogneisses from both massifs, the cores of some zircons yield Neoproterozoic ages between ~520 and 900Ma. The age of deposition of a pre-Late Ordovician metapelite in the Guilleries massif is bracketed by the weighted average age of the youngest detrital zircon population, 582??11Ma, and the age of cross-cutting granitic veins, 451??7Ma. Older detrital zircons populations in this metapelite include Neoproterozoic (749-610Ma; n=10), Neo- to Mesoproterozoic (1.04-0.86Ga; n=7), Paleoproterozoic (2.02-1.59Ga; n=5), and Neoarchean (2.74-2.58Ga; n=3). Nd isotopic analyses of the N??ria and Guilleries

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

Natural radionuclide mobility and its influence on U-Th-Pb dating of secondary minerals from the unsaturated zone at Yucca Mountain, Nevada

USGS Publications Warehouse

Neymark, L.A.; Amelin, Y.V.

2008-01-01

Extreme U and Pb isotope variations produced by disequilibrium in decay chains of 238U and 232Th are found in calcite, opal/chalcedony, and Mn-oxides occurring as secondary mineral coatings in the unsaturated zone at Yucca Mountain, Nevada. These very slowly growing minerals (mm my-1) contain excess 206Pb and 208Pb formed from excesses of intermediate daughter isotopes and cannot be used as reliable 206Pb/238U geochronometers. The presence of excess intermediate daughter isotopes does not appreciably affect 207Pb/235U ages of U-enriched opal/chalcedony, which are interpreted as mineral formation ages. Opal and calcite from outer (younger) portions of coatings have 230Th/U ages from 94.6 ?? 3.7 to 361.3 ?? 9.8 ka and initial 234U/238U activity ratios (AR) from 4.351 ?? 0.070 to 7.02 ?? 0.12, which indicate 234U enrichment from percolating water. Present-day 234U/238U AR is ???1 in opal/chalcedony from older portions of the coatings. The 207Pb/235U ages of opal/chalcedony samples range from 0.1329 ?? 0.0080 to 9.10 ?? 0.21 Ma, increase with microstratigraphic depth, and define slow long-term average growth rates of about 1.2-2.0 mm my-1, in good agreement with previous results. Measured 234U/238U AR in Mn-oxides, which pre-date the oldest calcite and opal/chalcedony, range from 0.939 ?? 0.006 to 2.091 ?? 0.006 and are >1 in most samples. The range of 87Sr/86Sr ratios (0.71156-0.71280) in Mn-oxides overlaps that in the late calcite. These data indicate that Mn-oxides exchange U and Sr with percolating water and cannot be used as a reliable dating tool. In the U-poor calcite samples, measured 206Pb/207Pb ratios have a wide range, do not correlate with Ba concentration as would be expected if excess Ra was present, and reach a value of about 1400, the highest ever reported for natural Pb. Calcite intergrown with opal contains excesses of both 206Pb and 207Pb derived from Rn diffusion and from direct ??-recoil from U-rich opal. Calcite from coatings devoid of opal

Monazite paragenesis and U-Pb systematics in rocks of the eastern Mojave Desert, California, U.S.A.: implications for thermochronometry

USGS Publications Warehouse

Kingsbury, J.A.; Miller, C.F.; Wooden, J.L.; Harrison, T.M.

1993-01-01

Studies of the paragenesis and U-Pb systematics of monazite in rocks from the eastern Mojave Desert, California, corroborate its potential usefulness as a prograde thermochronometer and in dating granite inheritance. Unmetamorphosed Latham Shale and its equivalents at grades ranging from greenschist to upper amphibolite facies are virtually identical in composition. Monazite is absent in the shale and low-grade schists, but it is abundant in schists at staurolite and higher grades. Lower-grade schists instead include minute Th- and Ce-oxides and unidentified Ce-poor LREE-phosphates that apparently are lower-temperature precursors to monazite. Thus monazite originates when the pelite passes through lower-amphibolite-facies conditions. Monazites from three Upper Cretaceous granites yield ages that are strongly discordant. Upper intercepts of 1.6-1.7 Ga are similar to those defined by U-Pb data for coexisting zircons and coincide with a period of copious magmatism in the Mojave crust. As the host Upper Cretaceous granitic magmas were all above 700??C, effective closure of the restitic monazites to Pb loss must be well in excess of this temperature. U-Pb compositions of monazite from Proterozoic granitoids and schist also indicate high Pb retentivity. Taken together, these studies support the suggestion that monazite can be an effective prograde thermochronometer. At least in pelites, it is not usually retained as a detrital mineral, but rather forms during moderate-temperature metamorphism. Its U-Pb system should not be reset by subsequent higher-grade metamorphism. ?? 1993.

SHRIMP U-Pb zircon dating from eclogite lenses in marble, Dabie-Sulu UHP terrane: restriction on the prograde, UHP and retrograde metamorphic ages

NASA Astrophysics Data System (ADS)

Liu, F.; Gerdes, A.; Xue, H.; Liang, F.

2006-12-01

Eclogite as lenses in impure marbles from Dabie-Sulu UHP terrane, represent parts of deeply subducted meta- sedimentary rocks. To constrain the age of metamorphism during subduction and exhumation, zircons from 2 eclogite samples in Dabie-Sulu impure marbles have been investigated. Beside Inherited (detrital) grains, 3 different metamorphic zircon domains have been identified based on distribution of mineral inclusion, trace elements and cathodoluminescence (CL) imaging: 1. Dark-luminescent rounded cores with quartz eclogite- facies mineral inclusions suggest formation at high-pressure (HP) metamorphic conditions. 2. White- luminescent zircon, either surrounding domain 1 or as rounded to spindly cores with index coesite eclogite- facies mineral inclusions indicates formation at UHP conditions. 3. Grey-luminescent rims around domain 2 with low-pressure mineral inclusions suggest formation during late regional amphibolite-facies retrogression. The three distinct zircon domains were dated by SHRIMP and yielded three discrete and meaningful age groups: 245±4 Ma for prograde HP metamorphism, 235±3 Ma for UHP metamorphism and 215±6 Ma for late amphibolite-facies retrogression from Dabie-Sulu eclogite. This data suggests that subduction and exhumation took place in about 10-11 Myr and 19-20 Myr, respectively. Continental materials was subducted from surface to the deep mantle depth at rates of 10 km/Myr, and subsequently exhumed from the mantle to the base of the crust at rates of 7 km/Myr. Ultrafast exhumation of the Dabie-Sulu UHP terrane from depth of 160 to 30 km was probably driven by buoyancy forces after UHP slab break-off at deep mantle depths.

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.

Proterozoic tectonostratigraphy and paleogeography of central Madagascar derived from detrital zircon U-Pb age populations

USGS Publications Warehouse

Cox, R.; Coleman, D.S.; Chokel, C.B.; DeOreo, S.B.; Wooden, Joseph L.; Collins, A.S.; De Waele, B.; Kroner, A.

2004-01-01

Detrital zircon U‐Pb ages determined by SHRIMP distinguish two clastic sequences among Proterozoic metasedimentary rocks from central Madagascar. The Itremo Group is older: zircon data, stromatolite characteristics, and carbon isotope data all point to a depositional age around 1500–1700 Ma. The Molo Group is younger, deposited between ∼620 Ma (the age of the youngest zircon) and ∼560 Ma (the age of metamorphic overgrowths on detrital cores). Geochronologic provenance analysis of the Itremo Group points to sources in East Africa as well as local sources in central and southern Madagascar but provides no evidence for a detrital contribution from northern and eastern Madagascar nor from southern India. Detrital zircon and sedimentologic similarities between rocks of the Itremo Group and the Zambian Muva Supergroup suggest a lithostratigraphic correlation between the two. The Molo Group has a strong 1000–1100 Ma detrital signature that also indicates an east African provenance and suggests a Neoproterozoic geographic connection with Sri Lanka but shows no indication of input from the Dharwar craton and eastern Madagascar. Central Madagascar was probably juxtaposed with the Tanzanian craton in the Paleo‐ and Mesoproterozoic, whereas northern and eastern Madagascar were connected to India. Internal assembly of Madagascar postdates Neoproterozoic Molo Group sedimentation and is likely to have occurred at about 560 Ma.

Petrography, geochemistry and U-Pb zircon age of the Matongo carbonatite Massif (Burundi): Implication for the Neoproterozoic geodynamic evolution of Central Africa

NASA Astrophysics Data System (ADS)

Midende, Gilbert; Boulvais, Philippe; Tack, Luc; Melcher, Frank; Gerdes, Axel; Dewaele, Stijn; Demaiffe, Daniel; Decrée, Sophie

2014-12-01

the carbonatite can directly be related to the carbonatite evolution. They have been dated at 705.5 ± 4.5 Ma (U-Pb concordant age, LA-ICP-MS). Similar zircon megacrysts of the Lueshe carbonatite (DRCongo) have been dated and give a concordant age at 798.5 ± 4.9 Ma (U-Pb, LA-ICP-MS). Considering that an extensional tectonic regime occured at that time in Central Africa - what remains debated - both ages could relate to different stages of Rodinia breakup, with uprise of mantle-derived magmas along Palaeoproterozoic lithospheric zones of weakness.

Tectonic evolution of the NE section of the Pamir Plateau: New evidence from field observations and zircon U-Pb geochronology

NASA Astrophysics Data System (ADS)

Zhang, Chuan-Lin; Zou, Hai-Bo; Ye, Xian-Tao; Chen, Xiang-Yan

2018-01-01

The Pamir Plateau at the western end of the India-Asia collision zone underwent long-term terrane drifting, accretion and collision between early Paleozoic and Mesozoic. However, the detailed evolution of this plateau, in particular, the timing of the Proto- and Palaeo-Tethys ocean subduction and closure, remains enigmatic. Here we report new field observations and zircon U-Pb ages and Hf isotopic compositions of the representative rocks from the so-called Precambrian basement in the northeastern Pamir, i.e., the Bulunkuole Group. The rock associations of the Bulunkuole Group indicate volcano-sedimentary sequences with arc affinities. Geochronological data demonstrate that the deposition age of the Bulunkuole Group in the NE section of the Pamir was Middle to Late Cambrian (530-508 Ma) rather than Paleoproterozoic. The deposition age became progressively younger from south to north. The amphibolite- to granulite facies metamorphism of the Bulunkuole Group took place at ca. 200-180 Ma. Unlike the scenario in the Southern Kunlun terrane (SKT) in the eastern section of the West Kunlun Orogenic Belt (WKOB), early Paleozoic metamorphism (ca. 440 Ma) was absent in this area. Two phases of magmatic intrusions, composed of granites and minor gabbros with arc geochemical signatures, emplaced at 510-480 Ma and 240-200 Ma. The amphibolite (meta mafic sheet? 519 Ma) and the meta-rhyolite (508 Ma) have zircon εHf(t) values of 1.6 to 5.9 and - 1.5 to 1.4, respectively. The 511 Ma gneissic granite sheet and the 486 Ma gabbro have zircon εHf(t) values of - 0.1 to 2.4 and 1.3 to 3.6, respectively. Zircon εHf(t) of the 245 Ma augen gneissic granite sheet varies from - 2.2 to 2.0 whereas the metamorphic zircons from the amphibolite (193 Ma) and high-pressure mafic granulite sample (187 Ma) have negative εHf(t) values of - 5.3 to - 2 and - 15 to - 12, respectively. In line with rock association and the deposition age of the Bulunkuole Group and the Saitula Group in the eastern

Developing Zircon as a Probe of Planetary Impact History

NASA Astrophysics Data System (ADS)

Wielicki, Matthew

2014-12-01

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.

Sandstone provenance and U-Pb ages of detrital zircons from Permian-Triassic forearc sediments within the Sukhothai Arc, northern Thailand: Record of volcanic-arc evolution in response to Paleo-Tethys subduction

NASA Astrophysics Data System (ADS)

Hara, Hidetoshi; Kunii, Miyuki; Miyake, Yoshihiro; Hisada, Ken-ichiro; Kamata, Yoshihito; Ueno, Katsumi; Kon, Yoshiaki; Kurihara, Toshiyuki; Ueda, Hayato; Assavapatchara, San; Treerotchananon, Anuwat; Charoentitirat, Thasinee; Charusiri, Punya

2017-09-01

Provenance analysis and U-Pb dating of detrital zircons in Permian-Triassic forearc sediments from the Sukhothai Arc in northern Thailand clarify the evolution of a missing arc system associated with Paleo-Tethys subduction. The turbidite-dominant formations within the forearc sediments include the Permian Ngao Group (Kiu Lom, Pha Huat, and Huai Thak formations), the Early to earliest Late Triassic Lampang Group (Phra That and Hong Hoi formations), and the Late Triassic Song Group (Pha Daeng and Wang Chin formations). The sandstones are quartzose in the Pha Huat, Huai Thak, and Wang Chin formations, and lithic wacke in the Kiu Lom, Phra That, Hong Hoi and Pha Daeng formations. The quartzose sandstones contain abundant quartz, felsic volcanic and plutonic fragments, whereas the lithic sandstones contain mainly basaltic to felsic volcanic fragments. The youngest single-grain (YSG) zircon U-Pb age generally approximates the depositional age in the study area, but in the case of the limestone-dominant Pha Huat Formation the YSG age is clearly older. On the other hand, the youngest cluster U-Pb age (YC1σ) represents the peak of igneous activity in the source area. Geological evidence, geochemical signatures, and the YC1σ ages of the sandstones have allowed us to reconstruct the Sukhothai arc evolution. The initial Sukhothai Arc (Late Carboniferous-Early Permian) developed as a continental island arc. Subsequently, there was general magmatic quiescence with minor I-type granitic activity during the Middle to early Late Permian. In the latest Permian to early Late Triassic, the Sukhothai Arc developed in tandem with Early to Middle Triassic I-type granitic activity, Middle to Late Triassic volcanism, evolution of an accretionary complex, and an abundant supply of sediments from the volcanic rocks to the trench through a forearc basin. Subsequently, the Sukhothai Arc became quiescent as the Paleo-Tethys closed after the Late Triassic. In addition, parts of sediments of

New U-Pb zircon geochronology of the Choma-Kalomo Block (Zambia) and the Dete-Kamativi Inlier (Zimbabwe), with implications for the extent of the Zimbabwe Craton.

NASA Astrophysics Data System (ADS)

Glynn, Sarah; Wiedenbeck, Michael; Master, Sharad; Frei, Dirk

2015-04-01

The Choma-Kalomo Block is a north-east trending, Mesoproterozoic terrane located in southern Zambia. It is composed of as yet undated gneissic basement with a high-grade metamorphosed supracrustal metasedimentary sequence, which is intruded by hornblende granites and gneisses of the Choma-Kalomo Batholith, that is dated between ca. 1.37 and 1.18 Ga. Our new zircon U-Pb age data on metasedimentary rocks of the Choma-Kalomo Block identifies samples of different ages, with slightly different provenances. The oldest metasedimentary rock is a muscovite-biotite schist, which has only Palaeoproterozoic detrital zircons, the two age clusters around 2.03-2.02 Ga and 1.8-1.9 Ga, correspond to the ages of granitic intrusion, and metamorphism, in the Magondi Mobile Belt on the western side of the Archaean Zimbabwe Craton. The second sample is a garnetiferous paragneiss, which contains both Palaeoproterozoic (2.04 Ga), and Mesoproterozoic zircons, ca. 1.36 Ga, derived from the granites of the Choma-Kalomo Batholith. The third sample is a biotite-muscovite schist, in which the detrital zircon ages fall into four separate clusters: ca. 3.39 Ga, ca. 2.7-2.6 Ga, ca. 2.1-1.7 Ga (with a peak at ca. 1.18 Ga), and 1.55 - 1.28 Ga. The Archaean zircons in this sample are derived from the Zimbabwe Craton, while the Palaeoproterozoic samples come from the Magondi belt, and the youngest zircons come from both phases of the Choma-Kalomo Batholith. A possible connection between the Choma-Kalomo Block and the Dete-Kamativi Inlier - some 150 km to the south-east in western Zimbabwe - has been proposed on the basis of similarities in the nature of their Sn-Ta-muscovite pegmatite mineralisation. The Dete-Kamativi Inlier, which is part of the Magondi Mobile Belt, is a window into Palaeoproterozoic north-east trending belts of deformed and metamorphosed supracrustal rocks. By dating localities which we suspect form the basement to the surrounding younger sediments, along with selected pegmatites

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

Sedimentary provenance of Trinity Peninsula Group, Antarctic Peninsula: petrography, geochemistry and SHRIMP U-Pb zircon age constraints.

NASA Astrophysics Data System (ADS)

Castillo, P.; Lacassie, J. P.; Hervé, F.; Fanning, C. M.

2009-04-01

The Trinity Peninsula Group (TPG) crops out in northern Graham Land and consists of a mostly non-fossiliferous metasedimentary succession of Permo-Triassic(?) age, which was accreted prior to the initiation of the Gondwana breakup. This succession has been sub-divided, from north to south, into five formations, namely: Hope Bay (HBF), View Point (VPF), Legoupil (LgF), Charlotte Bay (ChBF) and Paradise Harbour (PHF) formations. However, there are still large areas with unknown stratigraphic allocation, age and extension. Twenty TPG samples (12 sandstones and 8 mudstones) were collected from four localities in the Antarctic Peninsula, including Hope Bay; Paradise Harbour, Cape Legoupil and Charlotte Bay. Twelve sandstones were selected for modal analysis and 15 samples (7 sandstones and 8 mudstones) for whole rock chemical analysis. The geochemical data of the TPG samples was compared with the geochemical data of other sedimentary successions of different provenance and tectonic setting, by using unsupervised artificial neural networks. The modal composition of the sandstones is dominated by quartz and, in similar but smaller proportions by feldspar, and according to the discrimination scheme of Dickinson et al. (1983) is consistent with the product of erosion of the plutonic roots of a magmatic arc. The chemical data suggest a relatively evolved source, with a composition similar to a typical granodioritic continental magmatic arc. The deposition of the detritus is most likely to have occurred within an active continental margin. Three sandstone samples from the HBF, LgF and PHF were selected for U-Pb dating of detrital zircons by SHRIMP. For the HBF and PHF samples, the major age component is Permian (270-280 Ma). Only the sample from LgF has two important peaks at ~270 and ~470 Ma. In all cases, the youngest dated zircon is Permian (~257 Ma). These results show that there are strong chemical and chronological similarities between the TPG, the Duque de York Complex

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

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.

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