Emplacement and Deformation of Mesozoic Gabbros of the High Atlas (Morocco): Paleomagnetism and Magnetic Fabrics

NASA Astrophysics Data System (ADS)

Calvín, P.; Ruiz-Martínez, V. C.; Villalaín, J. J.; Casas-Sainz, A. M.; Moussaid, B.

2017-12-01

A paleomagnetic and magnetic fabric study is performed in Upper Jurassic gabbros of the central High Atlas (Morocco). These gabbros were emplaced in the core of preexisting structures developed during the extensional stage and linked to basement faults. These structures were reactivated as anticlines during the Cenozoic compressional inversion. Gabbros from 19 out of the 33 sampled sites show a stable characteristic magnetization, carried by magnetite, which has been interpreted as a primary component. This component shows an important dispersion due to postemplacement tectonic movements. The absence of paleoposition markers in these igneous rocks precludes direct restorations. A novel approach analyzing the orientation of the primary magnetization is used here to restore the magmatic bodies and to understand the deformational history recorded by these rocks. Paleomagnetic vectors are distributed along small circles with horizontal axes, indicating horizontal axis rotations of the gabbro bodies. These rotations are higher when the ratio between shales and gabbros in the core of the anticlines increases. Due to the uncertainties inherent to this work (the igneous bodies recording strong rotations), interpretations must be qualitative. The magnetic fabric is carried by ferromagnetic (s.s.) minerals mimicking the magmatic fabric. Anisotropy of magnetic susceptibility (AMS) axes, using the rotation routine inferred from paleomagnetic results, result in more tightly clustered magnetic lineations, which also become horizontal and are considered in terms of magma flow trend during its emplacement: NW-SE (parallel to the general extensional direction) in the western sector and NE-SW (parallel to the main faults) in the easternmost structures.

Compositional and mineralogical zoning by inward crystallization of mafic magma: evidence from the Guwoon hornblende gabbro-diorite Complex, Hwacheon, Korea.

NASA Astrophysics Data System (ADS)

Park, Y.-R.; Kim, G.-Y.

2009-04-01

The small body, ca. 1.3 by 1.6km, of a hot-air ballon shape hornblende gabbro - diorite Complex, in Gowoonri, Hwacheon, Korea consists of marginal diorite and central hornblende gabbro. The volumetrically dominant hornblende gabbro in the core of the Complex shows a zoned distribution with three layers distinguished by different dominant mafic mineral phases. From the margin toward the core of the hornblende gabbro body, the domintant mafic minerals change from amphibole phenocryst of nearly rounded shape in cross section with pyroxene pseudomorph through prismatic shape of amphibole to polycrystalline biotite aggregates. Systematic variations in geochemical characteristics among three distinct zones of hornblende gabbro body are also observed. From the outer zone toward the core, major oxides such as MnO, MgO, and CaO show a decreasing tendency, whereas total FeO/(total FeO + MgO) value shows an increasing tendency. Concentrations of trace elements also show systematic variations. Where incompatible elements such as Ba and Th increase, compatible elements like Cr and Sc decrease from the margin toward the core. The zonal distribution divided by change in dominant mafic mineral phase from pyroxene through amphibole to biotite, and systematic compositional changes in both major and trace elements from the outer zone toward the core of the hornblende gabbro body suggest that an inward crystallization mechanism played a major role in the formation of the hornblende gabbro in Guwoonri, Hwacheon, Korea.

Diffusion-driven magnesium and iron isotope fractionation at a gabbro-granite boundary

NASA Astrophysics Data System (ADS)

Wu, Hongjie; He, Yongsheng; Teng, Fang-Zhen; Ke, Shan; Hou, Zhenhui; Li, Shuguang

2018-02-01

Significant magnesium and iron isotope fractionations were observed in an adjacent gabbro and granite profile from the Dabie Orogen, China. Chilled margin and granitic veins at the gabbro side and gabbro xenoliths in the granite indicate the two intrusions were emplaced simultaneously. The δ26Mg decreases from -0.28 ± 0.04‰ to -0.63 ± 0.08‰ and δ56Fe increases from -0.07 ± 0.03‰ to +0.25 ± 0.03‰ along a ∼16 cm traverse from the contact to the granite. Concentrations of major elements such as Al, Na, Ti and most trace elements also systematically change with distance to the contact. All the observations suggest that weathering, magma mixing, fluid exsolution, fractional crystallization and thermal diffusion are not the major processes responsible for the observed elemental and isotopic variations. Rather, the negatively correlated Mg and Fe isotopic compositions as well as co-variations of Mg and Fe isotopes with Mg# reflect Mg-Fe inter-diffusion driven isotope fractionation, with Mg diffusing from the chilled gabbro into the granitic melt and Fe oppositely. The diffusion modeling yields a characteristic diffusive transport distance of ∼6 cm. Consequently, the diffusion duration, during which the granite may have maintained a molten state, can be constrained to ∼2 My. The cooling rate of the granite is calculated to be 52-107 °C/My. Our study suggests diffusion profiles can be a powerful geospeedometry. The observed isotope fractionations also indicate that Mg-Fe inter-diffusion can produce large stable isotope fractionations at least on a decimeter scale, with implications for Mg and Fe isotope study of mantle xenoliths, mafic dikes, and inter-bedded lavas.

Fluid infiltration of the Tudor Gabbro during regional metamorphism

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

Dunn, S.R.; Valley, J.W.

1985-01-01

The Tudor Gabbro (TG), an ovate body (4 x 9 km) 40 km SE of Bancroft. Ontario, was metamorphosed to upper greenschist facies along with surrounding sediments and volcanics. Allen (1976) delineated concentric isograds around the gabbro, including +sphene, +tremolite (to 1.5 km), +tremolite + clinozoisite, +diopside (approx. 120 m), and +garnet (approx. 80 m). Metamorphic conditions are inferred to be 490+/-50/sup 0/C, 5 kb with no thermal gradient. Allen suggested that H/sub 2/O infiltration of the marble and calc-schist accounts for the isograds. The gabbro mineralogy of titanaugite, andesine to labradorite, and minor hornblende is extensively recrystallized to albitemore » and/or oligoclase + actinolite + epidote + ilmenite + calcite (up to 4 wt%) +/- biotite +/- chlorite +/- sphene +/- scapolite. Isotopic analyses of calcite from 39 TG samples show delta/sup 18/O = 9.4 to 16.6 and delta/sup 13/C = -1.9 to 3.4. Bulk silicate delta/sup 18/O of TG range from 7.1 to 10.2. Calcites in metasediment have delta/sup 18/O = 18.1 to 25.3 and delta/sup 13/C = 1.3 to 5.6. Two whole rock silicate analyses of a skarn developed locally at the contact show intermediate delta/sup 18/O of 16.2 and 17.3. The stability of Czo component in epidote requires H/sub 2/O-rich fluids. The delta/sup 13/C of TG calcites average +0.7 nearly identical to the average of 178 carbonates from Grenville marbles (+1.0), showing that metasediment-derived CO/sub 2/ pervasively infiltrated the TG. The infiltration of H/sub 2/O into both the TG and the metasediment suggests that H/sub 2/O-rich fluids migrated upward along the contact.« less

Mineral chemistry of isotropic gabbros from the Manamedu Ophiolite Complex, Cauvery Suture Zone, southern India: Evidence for neoproterozoic suprasubduction zone tectonics

NASA Astrophysics Data System (ADS)

Yellappa, T.; Tsunogae, T.; Chetty, T. R. K.; Santosh, M.

2016-11-01

The dismembered units of the Neoproterozoic Manamedu Ophiolite Complex (MOC) in the Cauvery Suture Zone, southern India comprises a well preserved ophiolitic sequence of ultramafic cumulates of altered dunites, pyroxenites, mafic cumulates of gabbros, gabbro-norites and anorthosites in association with plagiogranites, isotropic gabbros, metadolerites, metabasalts/amphibolites and thin layers of ferruginous chert bands. The isotropic gabbros occur as intrusions in association with gabbroic anorthosites, plagiogranite and metabasalts/amphibolites. The gabbros are medium to fine grained with euhedral to subhedral orthopyroxenes, clinopyroxenes and subhedral plagioclase, together with rare amphiboles. Mineral chemistry of isotropic gabbros reveal that the clinopyroxenes are diopsidic to augitic in composition within the compositional ranges of En(42-59), Fs(5-12), Wo(31-50). They are Ca-rich and Na poor (Na2O < 0.77 wt%) characterized by high-Mg (Mg# 79-86) and low-Ti (TiO2 < 0.35 wt%) contents. The tectonic discrimination plots of clinopyroxene data indicate island arc signature of the source magma. Our study further confirms the suprasubduction zone origin of the Manamedu ophiolitic suite, associated with the subduction-collision history of the Neoproterozoic Mozambique ocean during the assembly of Gondwana supercontinent.

Sokhatiny differentiated gabbro-monzodiorite intrusion from the example of sin-batholitic gabbroids of Yano-Kolymskaya system

NASA Astrophysics Data System (ADS)

Izokh, A. E.; Goryachev, N. A.; Al'shevskii, A. V.; Akinin, V. V.

2012-05-01

The Sokhatnyi intrusion is an example of a later Mesozoic deferential massif referred to the gabbro-monzodiorite type in Northeastern Russia; according to geological data, it precedes high-alumina granitoids of the Yano-Kolymskaya folded system. It is shown that formation of the layered series of the massif is caused by the crystallized and gravitational fractioning of the high-alumina olivine basalt with increased potassium alkalinity in shallow water media. The boundary facies of the massif are represented by manzogabbroidnorites and monzodiorites. The U-Pb age determinations of zircon (SHRIMP-II) from the taxitite striped biothite gabbro in the lower boundary facies showed 148 ± 1 million years. Thus, taking into account the geological relations and geochronological data on the Sokhatinyi gabbro-monzodiorite, a differential intrusion was formed within the same age period as granite batholites of the Main belt.

Partial eclogitization of the Ambolten gabbro-norite, north-east Greenland Caledonides

USGS Publications Warehouse

Gilotti, J.A.; Elvevold, S.

1998-01-01

Partially eclogitized igneous bodies composed of gabbro, leucogabbro, anorthosite and cross-cutting diabase dikes are well represented in the North-East Greenland Eclogite Province. A 200 x 100 meter intrusive body on Ambolten Island (78?? 20' N, 19?? 15' W) records a prograde transition from gabbro-norite to eclogite facies coronitic metagabro-norite surrounded by hydrated margins of undeformed to strongly foliated amphibolite. Igneous plagioclase + olivine + enstatite + augite + oxides convert to eclogite facies assemblages consisting of garnet, omphacite, diopside, enstatite, kyanite, zoisite, rutile and pargasitic amphibole through several coronitic reactions. Relict cumulus plagioclase laths are replaced by an outer corona of garnet, an inner corona of omphacite and an internal region of sodic plagioclase, garnet, kyanite, omphacite and zoisite. Olivine and intercumulus pyroxene are partly replaced by metamorphic pyroxenes and amphibole. The corona structures, zoning patterns, diversity of mineral compositions in a single thin section, and preservation of metastable asemblages are characteristic of diffusion-controlled metamorphism. The most extreme disequilibrium is found in static amphibolites, where igneous pyroxenes, plagioclase domains with eclogite facies, assemblages, and matrix amphibole coexist. Complete eclogitization was not attained at Ambolten due to a lack of fluids needed to drive diffusion during prograde and retrograde metamorphism. The P-T conditions of the high-pressure metamorphism are estimated at ??? 750??C and > 18 kbar. Well-equilibrated, foliated amphibolites from the margin of the gabbro-norite supports our contention that the entire North-East Greenland Eclogite Province experienced Caledonian high-pressure metamorphism, even though no eclogite facies assemblages have been found in the quartzofeldspathic host gneisses to date.

Shock wave properties of anorthosite and gabbro. [to model hypervelocity impact cratering on planetary surfaces

NASA Technical Reports Server (NTRS)

Boslough, M. B.; Ahrens, T. J.

1985-01-01

Huyoniot data on San Gabriel anorthosite and San Marcos gabbro to 11 GPA are presented. Release paths in the stress-density plane and sound velocities are reported as determined from partial velocity data. Electrical interference effects precluded the determination of accurate release paths for the gabbro. Because of the loss of shear strength in the shocked state, the plastic behavior exhibited by anorthosite indicates that calculations of energy partitioning due to impact onto planetary surfaces based on elastic-plastic models may underestimate the amount of internal energy deposited in the impacted surface material.

Mineral chemistry as a tool for understanding the petrogenesis of Cryogenian (arc-related)-Ediacaran (post-collisional) gabbros in the western Arabian Shield of Saudi Arabia

NASA Astrophysics Data System (ADS)

Surour, Adel A.; Ahmed, Ahmed H.; Harbi, Hesham M.

2017-07-01

Metagabbros and gabbros in the Ablah-Shuwas belt (western Saudi Arabia) represent part of significant mafic magmatism in the Neoproterozoic Arabian Shield. The metagabbros are Cryogenian, occasionally stratified and bear calcic amphiboles (hornblende, magnesio-hornblende and actinolite) typical of calc-alkaline complexes. These amphiboles suggest low pressure ( 1-3 kbar), high f_{{{{O}}2 }} and crystallization temperature up to 727 °C, whereas it is 247-275 °C in the case of retrograde chlorite. Rutile and titanite in metagabbros are Fe-rich and replace Mn-bearing ilmenite precursors at high f_{{{{O}}2 }}. On the other hand, younger gabbros are fresh, layered and comprised of olivine gabbro and olivine-hornblende gabbro with an uppermost layer of anorthositic gabbro. The fresh gabbros are biotite-bearing. They are characterized by secondary magnetite-orthopyroxene symplectitic intergrowth at the outer peripheries of olivine. The symplectite forms by deuteric alteration from residual pore fluids moving along olivine grain boundaries in the sub-solidus state. In fresh gabbros, ortho- and clinopyroxenes indicate crystallization at 1300-900 and 800-600 °C, respectively. Geochemically, the Cryogenian metagabbros ( 850-780 Ma) are tholeiitic to calc-alkaline in composition and interpreted as arc-related. Younger, fresh gabbros are calc-alkaline and post-collisional ( 620-590 Ma, i.e., Ediacaran), forming during the late stages of arc amalgamation in the southern Arabian Shield. The calc-alkaline metagabbros are related to a lithospheric mantle source previously modified by subduction. Younger, fresh gabbros were probably produced by partial melting of an enriched mantle source (e.g., garnet lherzolite).

The Atlantis Bank gabbro-suite was not a "normal" magma-chamber that produced basalts

NASA Astrophysics Data System (ADS)

Kvassnes, A. J.; Dick, H. J. B.; Grove, T. L.

2003-04-01

The differentiation of the basalts sampled at Atlantis II Fracture Zone, South-West Indian Ridge, is not the result of simple fractionation of gabbroic mineral-assemblages like those recovered from the adjacent Atlantis Bank and ODP Hole 735B. Large mineral data sets for the gabbros (Dick, et al 2002) are now available for analysis and comparison to spatially associated basalts. We have used Melts and pMelts (Ghiorso and Sack, 1995) to estimate the fractional crystallization trend gabbros from a primitive mantle melt or of the AII F.Z. MORB. Thermodynamic models (Grove et al (1992), Putirka (1999)) were also used to model the glasses hypothetical mafic and felsic mineral equilibrium-compositions. Our results show that while the basalts suggest 30-50% crystallization, the gabbros indicate 35-90% crystallization of a primary melt. It is therefore unlikely that the gabbros sampled from Atlantis Bank are the fossil magma-chambers that expelled melts that formed the spatially associated basalts. The models also show that the most primitive gabbros have elevated clinopyroxene Mg#s (Mg/(Mg+Fe)) relative to the coexisting plagioclase An%. This was unexpected, as the clinopyroxene frequently occurs as oikocrysts surrounding the plagioclase and encloses rounded olivine chadacrysts, indicating that the clinopyroxene precipitated late. Elthon (1992) noted the same problem for Cayman Trough gabbros; suggesting that this was the result of intermediate pressure fractionation. In our models, pressure does have some effect up to 5kbar, but is not enough to explain the discrepancy. We propose a model where melts are modified in a porous network or mush. Plagioclase-olivine networks form by accumulation of buoyant glomerocrysts and then work as filters as new melts pass through. Dissolution of the minerals would make the new melt appear to be more primitive with regards to increased Mg#s, as the dissolution happens fast without complete internal re-equilibration with the gabbro

The Atlantis Bank Gabbro Massif, SW Indian Ridge: the Largest Know Exposure of the Lower Crust in the Oceans

NASA Astrophysics Data System (ADS)

Dick, H. J.; Kvassnes, A. J.; Kinoshita, H.; MacLeod, C. J.; Robinson, P. T.

2017-12-01

Until the discovery of oceanic core complexes little was known and much inferred about the lower ocean crust at slow-spreading ridges. Their study shows the ocean crust isn't simply a uniform layer-cake of pillow lavas, sheeted dikes and gabbros, but is highly variable in thickness, composition and architecture, and even absent over large regions. The 660 km2 Atlantis Bank Gabbro Massif in the rift-mountains of the SW Indian Ridge flanking the Atlantis II Transform is the magmatic end member for ocean core complexes, and best approximates `average' slow-spread crust. Thus it has been a focus for drilling since its discovery in 1986, leading to the current attempt to drill to Moho there (Project SloMo). There are 3 ODP and IODP drill holes on its crest: 1508-m deep Hole 735B, 158-m deep Hole 1105A, and 809.4-m deep Hole U1473. These provide a 200 Kyr view of lower crustal accretion at a slow-spread ocean ridge. Here we extend this view to 2.7 Myr. Mapping and sampling shows the gabbro massif extends nearly the length of a single 2nd order magmatic ridge segment. With numerous inliers of the dike-gabbro transition at numerous locations, and a crust-mantle boundary, traced for 30-km along the transform wall, it would appear to represent a full section of the lower crust. As Moho is at 5.5 ± 1 km mbsf near Hole 735B, and 4.5 km beneath the transform, it is likely a serpentinization front. The crust-mantle boundary was crossed by dives at 4 locations. In each case gabbros at the base of the crust crystallized from melt that had previously fractionated 50% or more from a likely parent. Thus the gabbro massif must be laterally zoned, and the parental mantle melts had to have been emplaced at the center of the paleo-ridge segment, before intruding laterally to the distal end of the complex. Gabbros on a lithospheric flow line down the center of the massif closely resemble those from the drill holes. This shows that while lateral variations in crustal composition and

Petrology of Ortsog-Uul peridotite-gabbro massif in Western Mongolia

NASA Astrophysics Data System (ADS)

Shapovalova, M.; Tolstykh, N.; Shelepaev, R.; Cherdantseva, M.

2017-12-01

The Ortsog-Uul mafic-ultramafic massif of Western Mongolia is located in a tectonic block with overturned bedding. The massif hosts two intrusions: a rhythmically-layered peridotite-gabbro association (Intrusion 1) and massive Bt-bearing amphibole-olivine gabbro (Intrusion 2). Intrusions 1 and 2 have different petrology features. Early Intrusion 1 (278±2.5Ma) is characterized by lower concentrations of alkalis, titanium and phosphorus than late Intrusion 2 (272±2Ma). The chondrite-normalized REE and primitive mantle-normalized rare elements patterns of Ortsog-Uul intrusions have similar curves of elements distribution. However, Intrusion 2 is characterized higher contents of REE and rare elements. High concentrations of incompatible elements are indicative of strong fractionation process. It has been suggested that Intrusions 1 and 2 derived from compositionally different parental melts. Model calculations (COMAGMAT-3.57) show that parental melts of two intrusions were close to high-Mg picrobasaltic magmas. The concentration of MgO in melt is 16.21 (Intrusion 1) and 16.17 (Intrusion 2). Isotopic data of Ortsog-Uul magmatic rocks exhibit different values of εNd (positive and negative) for Intrusion 1 and 2, respectively.

The gabbro-eclogite phase transition and the elevation of mountain belts on Venus

NASA Astrophysics Data System (ADS)

Namiki, Noriyuki; Solomon, Sean C.

1992-12-01

Among the four mountain belts surrounding Lakshmi Planum, Maxwell Montes is the highest and stands up to 11 km above the mean planetary radius and 7 km above Lakshmi Planum. The bulk composition and radioactive heat production of the crust on Venus, where measured, are similar to those of terrestrial tholeiitic basalt. Because the thickness of the low-density crust may be limited by the gabbro-garnet granulite-eclogite phase transitions, the 7-11 km maximum elevation of Maxwell Montes is difficult to understand except in the unlikely situation that the crust contains a large volume of magma. A possible explanation is that the base of the crust is not in phase equilibrium. It has been suggested that under completely dry conditions, the gabbro-eclogite phase transition takes place by solid-state diffusion and may require a geologically significant time to run to completion. Solid-state diffusion is a strongly temperature-dependent process. In this paper we solve the thermal evolution of the mountain belt to attempt to constrain the depth of the gabbro-eclogite transition and thus to assess this hypothesis quantitatively. The one-dimensional heat equation is solved numerically by a finite difference approximation. The deformation of the horizontally shortening crustal and mantle portions of the thermal boundary layer is assumed to occur by pure shear, and therefore the vertical velocity is given by the product of the horizontal strain rate and depth.

The gabbro-eclogite phase transition and the elevation of mountain belts on Venus

NASA Technical Reports Server (NTRS)

Namiki, Noriyuki; Solomon, Sean C.

1992-01-01

Among the four mountain belts surrounding Lakshmi Planum, Maxwell Montes is the highest and stands up to 11 km above the mean planetary radius and 7 km above Lakshmi Planum. The bulk composition and radioactive heat production of the crust on Venus, where measured, are similar to those of terrestrial tholeiitic basalt. Because the thickness of the low-density crust may be limited by the gabbro-garnet granulite-eclogite phase transitions, the 7-11 km maximum elevation of Maxwell Montes is difficult to understand except in the unlikely situation that the crust contains a large volume of magma. A possible explanation is that the base of the crust is not in phase equilibrium. It has been suggested that under completely dry conditions, the gabbro-eclogite phase transition takes place by solid-state diffusion and may require a geologically significant time to run to completion. Solid-state diffusion is a strongly temperature-dependent process. In this paper we solve the thermal evolution of the mountain belt to attempt to constrain the depth of the gabbro-eclogite transition and thus to assess this hypothesis quantitatively. The one-dimensional heat equation is solved numerically by a finite difference approximation. The deformation of the horizontally shortening crustal and mantle portions of the thermal boundary layer is assumed to occur by pure shear, and therefore the vertical velocity is given by the product of the horizontal strain rate and depth.

Discovery of Latest Cretaceous OIB-type alkaline gabbros in the Eastern Pontides Orogenic Belt, NE Turkey: Evidence for tectonic emplacement of seamounts

NASA Astrophysics Data System (ADS)

Eyuboglu, Yener; Dudas, Francis O.; Chatterjee, Nilanjan; Liu, Ze; Yılmaz-Değerli, Sedanur

2018-06-01

The Meso-Cenozoic geodynamic evolution of the Eastern Pontides Orogenic Belt, a mountain chain extending parallel to the southeastern margin of the Black Sea, has been controversial for the last forty years. Here we present data for a newly discovered alkaline gabbro body and its surrounding basaltic rocks in the northern part of the Eastern Pontides Orogenic Belt. We also provide a comprehensive assessment of the Late Mesozoic-Cenozoic geodynamic evolution of the Eastern Mediterranean region. The gabbroic body is bounded by reverse faults along its northern and southern borders and is surrounded by vesicular, pillow-fragment breccias and pillow basalts. Mineral compositions suggest that crystallization of the gabbros began at about 1170 °C, and the lowest preserved crystallization T is near 1000 °C. Estimated pressure at the beginning of crystallization is 5.7-7.4 kb. The 40Ar/39Ar dating of kaersutite and plagioclase and Usbnd Pb dating of titanite indicated that the Hayrat gabbro crystallized at 67 Ma (Late Maastrichtian). Whole rock major-trace-rare earth element and Sr-Nd-Pb isotope data indicate that the gabbros and basalts have different origins. The gabbros are alkaline and exhibit the geochemical features of OIB, whereas the basalts are tholeiitic and reveal depletions of HFSE that are similar to those of arc rocks. The gabbros are strongly fractionated, and derive from an enriched, lithospheric mantle source, with partial melting occurring in a garnet-stable environment. The basalts are less fractionated, and probably derive from a shallower source in which spinel peridotite was the predominant lithology. Considering all new and old geological, geochemical, geochronological and geophysical data from the Black Sea Basin and the Eastern Pontides-Lesser Caucasus-Alborz Orogenic Belt, we suggest that the alkaline Hayrat gabbro formed in an oceanic intraplate setting, and was accreted to the forearc region of the Eastern Pontides Orogenic Belt during

Mid Carboniferous lamprophyres, Cobequid Fault Zone, eastern Canada, linked to sodic granites, voluminous gabbro, and albitization

NASA Astrophysics Data System (ADS)

Pe-Piper, Georgia; Piper, David J. W.; Papoutsa, Angeliki

2018-01-01

Major intra-continental shear zones developed during the later stages of continental collision in a back-arc setting are sites of prolonged magmatism. Mantle metasomatism results from both melting of subducted sediments and oceanic crust. In the Cobequid Fault Zone of the northern Appalachians, back-arc A-type granites and gabbros dated ca. 360 Ma are locally intruded by lamprophyric dykes dated ca. 335 Ma. All the lamprophyres are kersantites with biotite and albite, lesser ilmenite, titanite and fluorapatite, and minor magmatic calcite, allanite, pyrite, magnetite, quartz and K-feldspar in some samples. The lamprophyres show enrichment in Rb, Ba, K, Th and REE and classify as calc-alkaline lamprophyre on the basis of biotite and whole rock chemistry. Pb isotopes lie on a mixing line between normal mantle-derived gabbro and OIB magma. Nd isotopes range from 1.3-3.5 εNdt, a little lower than in local gabbro. Most lamprophyres have δ18O = 3.8-4.4‰. Country rock is cut by pyrite-(Mg)-chlorite veins with euhedral allanite crystals that resemble the lamprophyres mineralogically, with the Mg-chlorite representing chloritized glass. Early Carboniferous unenriched mafic dykes and minor volcanic rocks are widespread along the major active strike-slip fault zones. The lamprophyres are geographically restricted to within 10 km of a small granitoid pluton with some sodic amphibole and widespread albitization. This was displaced by early Carboniferous strike-slip faulting from its original position close to the large Wentworth Pluton, the site of mantle-derived sodic amphibole granite, a major late gabbro pluton, and a volcanic carapace several kilometres thick, previously demonstrated to be the site of mantle upwelling and metasomatism. The age of the lamprophyres implies that enriched source material in upper lithospheric mantle or lower crust was displaced 50 km by crustal scale strike-slip faulting after enrichment by the mantle upwelling before lamprophyre emplacement

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Gravity and magnetic expression of the San Leandro gabbro with implications for the geometry and evolution of the Hayward Fault zone, northern California

USGS Publications Warehouse

Ponce, D.A.; Hildenbrand, T.G.; Jachens, R.C.

2003-01-01

The Hayward Fault, one of the most hazardous faults in northern California, trends north-northwest and extends for about 90 km along the eastern San Francisco Bay region. At numerous locations along its length, distinct and elongate gravity and magnetic anomalies correlate with mapped mafic and ultramafic rocks. The most prominent of these anomalies reflects the 16-km-long San Leandro gabbroic block. Inversion of magnetic and gravity data constrained with physical property measurements is used to define the subsurface extent of the San Leandro gabbro body and to speculate on its origin and relationship to the Hayward Fault Zone. Modeling indicates that the San Leandro gabbro body is about 3 km wide, dips about 75??-80?? northeast, and extends to a depth of at least 6 km. One of the most striking results of the modeling, which was performed independently of seismicity data, is that accurately relocated seismicity is concentrated along the western edge or stratigraphically lower bounding surface of the San Leandro gabbro. The western boundary of the San Leandro gabbro block is the base of an incomplete ophiolite sequence and represented at one time, a low-angle roof thrust related to the tectonic wedging of the Franciscan Complex. After repeated episodes of extension and attenuation, the roof thrust of this tectonic wedge was rotated to near vertical, and in places, the strike-slip Hayward Fault probably reactivated or preferentially followed this pre-existing feature. Because earthquakes concentrate near the edge of the San Leandro gabbro but tend to avoid its interior, we qualitatively explore mechanical models to explain how this massive igneous block may influence the distribution of stress. The microseismicity cluster along the western flank of the San Leandro gabbro leads us to suggest that this stressed volume may be the site of future moderate to large earthquakes. Improved understanding of the three-dimensional geometry and physical properties along the

High-velocity frictional properties of gabbro

NASA Astrophysics Data System (ADS)

Tsutsumi, Akito; Shimamoto, Toshihiko

High-velocity friction experiments have been performed on a pair of hollow-cylindrical specimens of gabbro initially at room temperature, at slip rates from 7.5 mm/s to 1.8 m/s, with total circumferential displacements of 125 to 174 m, and at normal stresses to 5 MPa, using a rotary-shear high-speed friction testing machine. Steady-state friction increases slightly with increasing slip rate at slip rates to about 100 mm/s (velocity strengthening) and it decreases markedly with increasing slip rate at higher velocities (velocity weakening). Steady-state friction in the velocity weakening regime is lower for the non-melting case than the frictional melting case, due perhaps to severe thermal fracturing. A very large peak friction is always recognized upon the initiation of visible frictional melting, presumably owing to the welding of fault surfaces upon the solidification of melt patches. Frictional properties thus change dramatically with increasing displacement at high velocities, and such a non-linear effect must be incorporated into the analysis of earthquake initiation processes.

Initial report of the physical property measurement, ChikyuOman core description Phase I: sheeted dike and gabbro boundary from ICDP Holes GT1A, GT2A and GT3A

NASA Astrophysics Data System (ADS)

Abe, N.; Okazaki, K.; Hatakeyama, K.; Ildefonse, B.; Leong, J. A. M.; Tateishi, Y.; Teagle, D. A. H.; Takazawa, E.; Kelemen, P. B.; Michibayashi, K.; Coggon, J. A.; Harris, M.; de Obeso, J. C.

2017-12-01

We report results on the physical property measurements of the core samples from ICDP Holes GT1A, GT2A and GT3A drilled at Samail Ophiolite, Sultanate of Oman. Cores from Holes GT1A and GT2A in the lower crust section are mainly composed of gabbros (gabbro and olivine gabbro), and small amounts of ultramafic rocks (wehrlite and dunite), while cores from Hole GT3A at the boundary between sheeted dikes and gabbro are mainly composed of basalt and diabase, followed by gabbros (gabbro, olivine gabbro and oxide gabbro), and less common felsic dikes, trondhjemite and tonalite, intrude the mafic rocks. Measurements of physical properties were undertaken to characterize recovered core material. Onboard the Drilling Vessel Chikyu, whole-round measurements included X-ray CT image, natural gamma radiation, and magnetic susceptibility for Leg 1, and additional P-wave velocity, gamma ray attenuation density, and electrical resistivity during Leg 2. Split-core point magnetic susceptibility and color spectroscopy were measured for all core sections. P-wave velocity, bulk/grain density and porosity of more than 500 discrete cube samples, and thermal conductivity on more than 240 pieces from the working half of the split core sections were also measured. Physical Properties of gabbroic rocks from Holes GT1A and GT2A are similar to typical oceanic gabbros from ODP and IODP expeditions at Atlantis Bank, Southwestern Indian Ridge (ODP Legs 118, 176 and 179; IODP Exp 360) and at Hess Deep, Eastern Pacific (ODP Leg 147 and IODP Exp. 345). Average P-wave velocity, bulk density, grain density, porosity and thermal conductivity are 6.7 km/s, 2.92 g/cm^3, 2.93 g/cm^3, 0.98% and 2.46 W/m/K, respectively. P-wave velocity of samples from all three holes is inversely correlated with porosity. No clear correlation between the original lithology and physical properties is observed. GT3A cores show a wider range (e.g., Vp from 2.2 to 7.1 km/s) of values for the measured physical properties

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

USGS Publications Warehouse

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

2012-01-01

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

Brine/Rock Interaction in Deep Oceanic Layered Gabbros: Petrological Evidence from Cl-Rich Amphibole, High-Temperature Hydrothermal Veins, and Experiments

NASA Astrophysics Data System (ADS)

Currin Sala, A. M.; Koepke, J.; Almeev, R. R.; Teagle, D. A. H.; Zihlmann, B.; Wolff, P. E.

2017-12-01

Evidence of high temperature brine/rock interaction is found in hydrothermal veins and dykelets that cross-cut layered olivine gabbros in the deep palaeocrust of the Sumail Ophiolite, Sultanate of Oman. Here we present petrological and geochemical data from these samples, and an experimental attempt to simulate brine/gabbro interaction using externally heated cold seal pressure vessels. The studied natural veins and dykelets contain pargasite, hornblende, actinolite, and Cl-rich pargasite with up to 5 wt% Cl, showing a range of formation conditions from magmatic to metamorphic (hydrothermal) and thus a complex history of brine/rock interaction. In addition, the isotopic study of the radiogenic 87/86Sr and stable 18O in different amphibole types provide an estimate for the extent of seawater influence as alteration agent in the veins of the studied samples. Experiments performed at 750 °C and 200 MPa with different starting materials (chlorine-free amphibole, olivine gabbro powder) and 20 wt% NaCl aqueous brine, illustrate the process by which gabbro-hosted amphibole-rich veins evolve at subsolidus temperatures in the presence of a seawater-derived fluid. Our results demonstrate a decrease in olivine, plagioclase and magnetite content in favour of hastingsite, pargasite and magnesiohornblende, a decrease of IVAl and Ti in the starting amphibole, and an increase in Cl in amphibole, up to 0.2 Cl wt%. Our experiments show the change of magmatic pargasite towards more magnesium and silica-rich end members with results comparable to mildly chlorine-rich pargasites and hornblendes found in the natural samples studied. However, the experimental setup also presents limitations in the attainment of very high-chlorine amphibole (up to 5 wt%). Our analytical and experimental results provide further evidence for the existence of a hydrothermal cooling system in the deep oceanic crust.

Modelling the seismic properties of fast-spreading ridge crustal Low-Velocity Zones: insights from Oman gabbro textures

NASA Astrophysics Data System (ADS)

Lamoureux, Gwenaëlle; Ildefonse, Benoı̂t; Mainprice, David

1999-11-01

Although considerable progress has been made in the study of fast-spreading, mid-ocean ridge magma chambers over the past fifteen years, the fraction of melt present in the chamber remains poorly constrained and controversial. We present new constraints obtained by modelling the seismic properties of partially molten gabbros at the ridge axis. P-wave velocities at low frequencies are calculated in the foliation/lineation reference frame using a differential effective medium technique. The model takes into account the lattice preferred orientation of the crystalline phase and the average shape of the melt phase. The structural parameters are obtained from the Oman ophiolite. The structural reference frame is given by the general trend of the gabbro foliation and the melt fraction and shape are estimated using the textures of nine upper gabbro samples. The estimated melt fraction and shape depend on the assumptions regarding which part of the observed textures represent the melt in the gabbroic mush of the magma chamber. However, we can put limits on the reasonable values for the melt fraction and shape. Our results are consistent with a melt fraction of the order of 10 to 20% in the Low-Velocity Zone (i.e. the magma chamber), which is anisotropically distributed with the melt pockets preferentially aligned parallel to the foliation and approximated by oblate ellipsoids with approximate dimensions of 4 : 4 : 1. These results are also consistent with the seismic structure of the East Pacific rise at 9°30'. The anisotropic melt distribution can, at least partially, explain the vertical velocity gradient described in the LVZ.

Role of Fluids in Lunar vs. Terrestrial Gabbros During Late-Stage and Post-Magmatic Crystallization, a Case Study

NASA Astrophysics Data System (ADS)

Fagan, T. J.; Fujimoto, A.; Kosaka, D.

2018-04-01

Incompatible elements, including H2O, are concentrated in late-stage magmatic pockets in gabbros from the Earth and Moon. Feldspar near the pockets is albitized by water (Earth case) or has discontinuous, unexplained changes in composition (Moon).

Plagioclase zonation styles in hornblende gabbro inclusions from Little Glass Mountain, Medicine Lake volcano, California: Implications for fractionation mechanisms and the formation of composition gaps

USGS Publications Warehouse

Brophy, J.G.; Dorais, M.J.; Donnelly-Nolan, J.; Singer, B.S.

1997-01-01

The rhyolite of Little Glass Mountain (73-74% SiO2) is a single eruptive unit that contains inclusions of quenched andesite liquid (54-61% SiO2) and partially crystalline cumulate hornblende gabbro (53-55% SiO2). Based on previous studies, the quenched andesite inclusions and host rhyolite lava are related to one another through fractional crystallization and represent an example of a fractionation-generated composition gap. The hornblende gabbros represent the cumulate residue associated with the rhyolite-producing and composition gap-forming fractionation event. This study combines textural (Nomarski Differential Interference Contrast, NDIC, imaging), major element (An content) and trace element (Mg, Fe, Sr, K, Ti, Ba) data on the style of zonation of plagioclase crystals from representative andesite and gabbro inclusions, to assess the physical environment in which the fractionation event and composition gap formation took place. The andesite inclusions (54-61% SiO2) are sparsely phyric with phenocrysts of plagioclase, augite and Fe-oxide??olivine, +/-orthopyroxene, +/-hornblende set within a glassy to crystalline matrix. The gabbro cumulates (53-55% SiO2) consist of an interconnected framework of plagioclase, augite, olivine, orthopyroxene, hornblende and Fe-oxide along with highly vesicular interstitial glass (70-74% SiO2). The gabbros record a two-stage crystallization history of plagioclase + olivine + augite (Stage I) followed by plagioclase+orthopyroxene + hornblende + Fe-oxide (Stage II). Texturally, the plagioclase crystals in the andesite inclusions are characterized by complex, fine-scale oscillatory zonation and abundant dissolution surfaces. Compositionally (An content) the crystals are essentially unzoned from core-to-rim. These features indicate growth within a dynamic (convecting?), reservoir of andesite magma. In contrast, the plagioclase crystals in the gabbros are texturally smooth and featureless with strong normal zonation from An74 at the

Geochronology and geochemistry of the Huilvshan gabbro in west Junggar (NW China): Implications for magma process and tectonic regime

NASA Astrophysics Data System (ADS)

Zhang, Huichao; Zhu, Yongfeng

2018-06-01

Gabbro plutons, consisting of clinopyroxene and plagioclase with trace amounts of magnetite, titanite, and apatite, intruded into Early Carboniferous volcanic-sedimentary strata in the Huilvshan gold mining region (west Junggar, China). Samples collected from two gabbro bodies are tholeiitic in composition with low concentrations of Na2O + K2O, showing weak depletions of light rare earth elements with insignificant Eu, Nb, and Ti anomalies. Zircon U-Pb analyses yield a weighted average U-Pb age of 296.1 ± 2.7 Ma (MSWD = 0.98), which could represent the time corresponding to mafic magma emplacement in the Huilvshan region. Geochemical calculations suggest that this mafic magma was derived from a depleted mantle source in a post-collisional tectonic setting corresponding to 4% partial melting of spinel lherzolite.

Hyperextension of continental to oceanic-like lithosphere: The record of late gabbros in the shallow subcontinental lithospheric mantle of the westernmost Mediterranean

NASA Astrophysics Data System (ADS)

Hidas, Károly; Varas-Reus, Maria Isabel; Garrido, Carlos J.; Marchesi, Claudio; Acosta-Vigil, Antonio; Padrón-Navarta, José Alberto; Targuisti, Kamal; Konc, Zoltán

2015-05-01

We report gabbroic dikes in the plagioclase tectonite domains of the Ojén and Ronda massifs (Betic Cordillera, southern Spain), which record crystallization at low-pressure syn-, or slightly postkinematic to the late ductile history of the Betic Peridotite in the westernmost Mediterranean. We present mineral major and trace element compositional data of discordant gabbroic dikes in the Ojén massif and gabbroic patches in the Ronda massif, complemented by the whole rock and electron backscattered diffraction (EBSD) data of the Ojén occurrence. In the Ojén massif, gabbro occurs as 1-3 centimeter wide discordant dikes that crosscut the plagioclase tectonite foliation at high angle. These dikes are composed of cm-scale igneous plagioclase and clinopyroxene crystals that show shape preferred orientations subparallel to the lineation of the host peridotite and oblique to the trend of the dike. Intrusion of Ojén gabbro dikes is coherent with the stress field that formed the high temperature, ductile plagioclase tectonite foliation and then attests for a mantle igneous event prior to the intracrustal emplacement of the massif. In the Ronda massif, gabbroic rocks crystallized in subcentimeter wide anastomozing veins, or as interstitial patches in the host dunite. They are mostly composed of plagioclase and clinopyroxene. Plagioclase composition is bytownitic in the Ojén, and andesinic in the Ronda massif. Clinopyroxene in both places shows identical, light Rare-Earth Element (LREE) depleted trace element patterns. The calculated trace element composition of melts in exchange equilibrium with the studied igneous clinopyroxenes reflects LREE-enriched character coupled with negative Eu anomaly, and indicates that gabbro-forming melts in Ronda and Ojén share a common melt source with an island arc tholeiitic affinity. Geothermobarometric data and liquidus mineralogy indicate that gabbro crystallization occurred at shallow depths (0.2-0.5 GPa) in a 7-16 km thick

Petrology and chemistry of the Green Acres gabbro complex near Winchester, Riverside County, California

USGS Publications Warehouse

Berger, Byron R.; Morton, Douglas M.; Miller, Fred K.

2014-01-01

The Cretaceous Green Acres layered igneous complex, northeast of Winchester, California, is composed of a suite of olivine- and hornblende-bearing gabbros in the Peninsular Ranges batholith within the Perris tectonic block. A consistent mineral assemblage is observed throughout the complex, but there is considerable textural and modal heterogeneity. Both preclude a consistent set of principles based on appearance and mineralogy on which to delineate map units. Distinct changes in the chemistry of olivine, pyroxene, and hornblende, however, serve to define discrete mappable units, and the complex has been divided into five geochemical map units on this basis.Limited whole-rock data show the Green Acres complex is chemically comparable to other Peninsular Ranges batholith gabbroic rocks, and rare earth element (REE) concentrations and patterns are typical of magmas generated in convergent margin settings. For the complex as a whole, olivine is Fo80–35, plagioclase is An100–64, clinopyroxene is Wo49–41En48–38Fs18–6 and Wo36–26En65–42Fs30–8, and orthopyroxene is Wo5–0En78–42Fs50–21, where Fo is forsterite, An is anorthite, Wo is wollastonite, En is enstatite, and Fs is ferrosilite. The Mg/(Mg + ΣFe) atomic ratio in hornblende ranges from 0.84 to 0.50.Magmatic lineations and modal and textural layering are prevalent throughout the complex. Mineral chemistry does not change in any systematic way within and between layers in any map unit. Although the strike of layering varies, in any map unit at any given location it is the same in all units irrespective of intrusive order. Thin dikes, typically late-stage hornblende gabbro, commonly intrude parallel to layering. The strikes of magmatic lineations and modal layers are consistent with the populations of strikes of fabrics in the metamorphic basement as well as tectonic features in surrounding, postgabbro granitic rocks. These relations imply that the regional state of stress at the time of gabbro

Rb‐Sr resonance ionization geochronology of the Duluth Gabbro: A proof of concept for in situ dating on the Moon

PubMed Central

Levine, Jonathan; Whitaker, Tom J.

2015-01-01

Rationale We report new 87Rb‐87Sr isochron data for the Duluth Gabbro, obtained with a laser ablation resonance ionization mass spectrometer that is a prototype spaceflight instrument. The gabbro has a Rb abundance and a range of Rb/Sr ratios that are similar to those of KREEP‐rich basalts found on the nearside of the Moon. Dating of previously un‐sampled young lunar basalts, which generally have a KREEP‐rich composition, is critical for understanding the bombardment history of the Moon since 3.5 Ga, which in turn informs the chronology of the solar system. Measurements of lunar analogs like the Duluth Gabbro are a proof of concept for in situ dating of rocks on the Moon to constrain lunar history. Methods Using the laser ablation resonance ionization mass spectrometer we ablated hundreds of locations on a sample, and at each one measured the relative abundances of the isotopes of Rb and Sr. A delay between the resonant photoionization processes separates the elements in time, eliminating the potential interference between 87Rb and 87Sr. This enables the determination of 87Rb‐87Sr isochron ages without sophisticated sample preparation that would be impractical in a spaceflight context. Results We successfully dated the Duluth Gabbro to 800 ± 300 Ma using traditional isochron methods like those used in our earlier analysis of the Martian meteorite Zagami. However, we were able to improve this to 1100 ± 200 Ma, an accuracy of <1σ, using a novel normalization approach. Both these results agree with the age determined by Faure et al. in 1969, but our novel normalization improves our precision. Conclusions Demonstrating that this technique can be used for measurements at this level of difficulty makes ~32% of the lunar nearside amenable to in situ dating, which can complement or supplement a sample return program. Given these results and the scientific value of dating young lunar basalts, we have recently proposed a spaceflight mission called the Moon Age

A double Fe-Ti oxide and Fe-sulphide liquid immiscibility in the Itsindro Gabbro Complex, Madagascar

NASA Astrophysics Data System (ADS)

Augé, Thierry; Bailly, Laurent; Roig, Jean-Yves

2017-11-01

The petrology and mineralogy of the Itsindro complex in south-central Madagascar has been investigated through samples obtained from the 320.7 m-deep Lanjanina borehole. The section consists of a 254 m-thick pyroxenite unit with interbedded gabbro layers that overlies a gabbro unit and is itself overlain by a 19 m-thick granite unit. Most of the structures are sub-horizontal. A weak magmatic layering is locally observed but at the scale of the core, the intrusion does not appear to be a layered complex. Pyroxenite and gabbro show a systematic disseminated mineralization consisting of Fe-Ti-P oxides and Fe-(Cu-Ni) sulphides that takes the form of ilmenite-titanomagnetite ± apatite and pyrrhotite ± chalcopyrite ± pentlandite. In the upper zone, from 90 to 72 m, sub-massive centimetre-to decimetre-sized layers of oxides and sulphides comprise a total of 16 m of sub-massive sulphide (the main mineralized zone). In this mineralized zone the oxide/sulphide ratio is close to 1/1. The sulphide is strongly dominated by pyrrhotite, which may locally contain inclusions of molybdenite crystals with the Re sulphide rheniite (ReS2). Oxides are generally euhedral, included in or attached to the Fe-sulphide, and also locally form sub-massive centimetre-sized bands. Apatite as a cumulus phase is ubiquitous. Locally it may account for 30% of the ore-rich samples and some samples consist of apatite-Fe-Ti oxides-Fe-Cu-Ni sulphides with virtually no silicate. Apatite is the main REE carrier but the total REE content remains low (<90 ppm). Mineral compositions and whole rock geochemistry indicate that the rocks are highly differentiated, and in spite of a relatively limited thickness, the differentiation process is observed. Two zones can be distinguished: from the bottom to 162.8 m we see a decrease in the Mg number of olivine and pyroxene, and a drop in TiO2 and Al2O3 for the latter. A reverse trend is then observed within the pyroxenite unit from the 162.8 m level upwards. The

Origin and age of the Eisenkappel gabbro to granite suite (Carinthia, SE Austrian Alps)

PubMed Central

Miller, C.; Thöni, M.; Goessler, W.; Tessadri, R.

2011-01-01

The northern part of the Karawanken plutonic belt is a gabbro–granite complex located just north of the Periadriatic lineament near the Slovenian–Austrian border. Petrographic and geochemical studies of the Eisenkappel intrusive complex indicate that this multiphase plutonic suite developed by a combination of crystal accumulation, fractional crystallization and assimilation processes, magma mixing and mingling. The mafic rocks are alkaline and have within-plate geochemical characteristics, indicating anorogenic magmatism in an extensional setting and derivation from an enriched mantle source. The mafic melts triggered partial melting of the crust and the formation of granite. The granitic rocks are alkalic, metaluminous and have the high Fe/Fe + Mg characteristics of within-plate plutons. Temperature and pressure conditions, derived from amphibole-plagioclase and different amphibole thermobarometers, suggest that the analysed Eisenkappel gabbros crystallized at around 1000 ± 20 °C and 380–470 MPa, whereas the granitic rock crystallized at T ≤ 800 ± 20 °C and ≤ 350 MPa. Mineral-whole rock Sm–Nd analyses of two cumulate gabbros yielded 249 ± 8.4 Ma and 250 ± 26 Ma (εNd: + 3.6), garnet-whole rock Sm–Nd analyses of two silicic samples yielded well-constrained ages of 238.4 ± 1.9 Ma and 242.1 ± 2.1 Ma (εNd: − 2.6). PMID:26525511

Early paleozoic gabbro-amphibolites in the structure of the Bureya Terrane (eastern part of the Central Asian Fold Belt): First geochronological data and tectonic position

NASA Astrophysics Data System (ADS)

Smirnov, Yu. V.; Sorokin, A. A.; Kudryashov, N. M.

2012-07-01

Resulting from U-Pb geochronological study, it has been found that the gabbro-amphibolites composing the Bureya (Turan) Terrane in the eastern part of the Central Asian Fold Belt are Early Paleozoic (Early Ordovician; 455 ± 1.5 Ma) in age rather than Late Proterozoic as was believed earlier. The gabbro-amphibolites and associated metabasalts are close to tholeiites of the intraoceanic island arcs in terms of the geochemical properties. It is suggested that the tectonic block composed of these rocks was initially a seafloor fragment that divided the Bureya and Argun terranes in the Early Paleozoic and was later tectonically incorporated into the modern structure of the Bureya Terrane as a result of Late Paleozoic and Mesozoic events.

Time-resolved interaction of seawater with gabbro: An experimental study of rare-earth element behavior up to 475 °C, 100 MPa

NASA Astrophysics Data System (ADS)

Beermann, Oliver; Garbe-Schönberg, Dieter; Bach, Wolfgang; Holzheid, Astrid

2017-01-01

High metal and rare-earth element (REE) concentrations with unusual ('atypical') normalized REE patterns are documented in fluids from active hydrothermal vent fields on the Mid-Atlantic Ridge, 5°S and the East Scotia Ridge. Those fluids show relative enrichment of middle heavy REEs and almost no Eu anomalies in chondrite-normalized patterns. To understand the processes that produce such atypical REE patterns we ran a series of experiments, in which natural bottom seawater or aqueous solutions (NaCl, NaCl-MgCl2, or NaCl-CaCl2) were reacted with gabbro and gabbro mineral assemblages from 300 to 475 °C and 40 and 100 MPa. These P-T conditions are representative for water-rock interactions in hydrothermal root and discharge zones. Fluid flux variability and kinetics were addressed in the experiments by varying the water-to-rock mass ratio (w/r) from 0.5-10 and using different run durations from 3-720 h. Only seawater and synthetic MgCl2-bearing fluid mobilized significant amounts of REEs, Si, Ca, Fe, and Mn from gabbro, from clinopyroxene, and from plagioclase. At 425 °C and 40 MPa, fluids were initially acidic with pH (25 °C) of ∼2 increasing to values between ∼4 and 7 upon progressing reactions. Rare earth element and Fe contents peaked within 3-6 h after interaction with gabbroic mineral grains (125-500 μm) at w/r of 5 (REEs) and 2-5 (Fe) but decreased with continuing reaction without strong REE fractionation. Most of the REEs that were leached from primary minerals and dissolved in the fluids early became redeposited into solid reaction products after 720 h. Contents of dissolved SiO2 were pressure-dependent, being about twofold higher at 100 MPa than at 40 MPa (425 °C) and were below quartz saturation with gabbro and clinopyroxene as solid starting material and close to quartz saturation with plagioclase reactant. However, Si in fluids from the rock-dominated experiments at 100 MPa with gabbro (w/r 0.5-1) dropped to very low contents. A concomitant

The Sr, Nd, and Hf isotopic geochemistry of rocks of the gabbro-diorite-tonalite association from the Eastern Segment of the Middle Urals as an indicator of the age of the continental crust in this area

NASA Astrophysics Data System (ADS)

Smirnov, V. N.; Ivanov, K. S.; Ronkin, Yu. L.; Koroteev, V. A.; Serov, P. A.; Gerdes, A.

2017-05-01

According to isotopic analysis of rocks of the Reft gabbro-diorite-tonalite complex (Middle Urals), gabbro and related diorite and dikes and vein-shaped bodies of plagiogranitoids, crosscutting gabbro, are similar to the depleted mantle substance in ɛNd( T) = 8.6-9.7 and ɛHf( T) = 15.9-17.9. Their model Hf ages are correlated with the time of crystallization. Here, the tonalites and quartz diorites constituting most of the Reft massif are characterized by lower values: ɛNd( T) = 3.7-6.0, ɛHf( T) = 11.1-12.7, and T DM values significantly exceeding the age datings. This is evidence that Neoproterozoic crustal rocks were a source of parental magma for these rocks. The primary 87Sr/86Sr ratio in rocks of both groups is highly variable (0.70348-0.70495). The data obtained allow us to reach the conclusion that the Reft gabbro-diorite-tonalite complex was formed as a result of nearly synchronous processes occurring in the crust and the mantle within a limited area.

The Easternmost Southwest Indian Ridge: A Laboratory to Study MORB and Oceanic Gabbro Petrogenesis in a Very Low Melt Supply Context

NASA Astrophysics Data System (ADS)

Paquet, M.; Cannat, M.; Hamelin, C.; Brunelli, D.

2014-12-01

Our study area is located at the ultra-slow Southwest Indian Ridge, east of the Melville Fracture Zone, between 61 and 67°E. The melt distribution in this area is very heterogeneous, with corridors of ultramafic seafloor where plate separation is accommodated by large offset normal faults [Sauter, Cannat et al., 2013]. These ultramafic corridors also expose rare gabbros and basalts. We use the major and trace elements composition of these magmatic rocks to document the petrogenesis of MORB in this exceptionnally low melt supply portion of the MOR system. Basalts from the easternmost SWIR represent a global MORB end-member for major element compositions [Meyzen et al., 2003], with higher Na2O and Al2O3 wt%, and lower CaO and FeO wt% at a given MgO. Within this group, basalts from the ultramafic corridors have particularly high Na2O, low CaO and FeO wt%. Best fitting calculated liquid lines of descent are obtained for crystallization pressures of ~8 kbar. Gabbroic rocks recovered in the ultramafic corridors include gabbros, oxide-gabbros and variably impregnated peridotites. This presentation focuses on these impregnated samples, where cpx have high Mg#, yet are in equilibrium with the nearby basalts in terms of their trace element compositions. Plagioclase An contents vary over a broad range, and there is evidence for opx resorption. These characteristics result from melt-mantle interactions in the axial lithosphere, which may explain several peculiar major element characteristics of the basalts. Similar interactions probably occur beneath ridges at intermediate to slow and ultraslow spreading rates. We propose that they are particularly significant in our study area due to its exceptionnally low integrated melt-rock ratio.

Crystallization Temperatures of Lower Crustal Gabbros from the Oman Ophiolite and the Persistence of the 'Mush Zone' at Intermediate/Fast Spreading Ridges

NASA Astrophysics Data System (ADS)

VanTongeren, J. A.

2017-12-01

Oceanic crust is formed when mantle-derived magmas are emplaced at the ridge axis, a zone of intense rifting and extension. Magmas begin to cool and crystallize on-axis, forming what is termed the "Mush Zone", a region of partially molten rocks. Several attempts have been made to understand the nature of the Mush Zone at fast spreading mid-ocean ridges, specifically how much partial melt exists and how far off-axis the Mush Zone extends. Geophysical estimates of P-wave velocity perturbations at the East Pacific Rise show a region of low velocity approximately 1.5-2.5 km off-axis, which can be interpreted to be the result of higher temperature [e.g. Dunn et al., 2000, JGR] or the existence of partial melt. New petrological and geochemical data and methods allow for the calculation of the lateral extent of the Mush Zone in the lower oceanic crust on exposed sections collected from the Oman ophiolite, a paleo-fast/intermediate spreading center. I will present new data quantifying the crystallization temperatures of gabbros from the Wadi Khafifah section of lower oceanic gabbros from the Oman ophiolite. Crystallization temperatures are calculated with the newly developed plagioclase-pyroxene REE thermometer of Sun and Liang [2017, Contrib. Min. Pet.]. There does not appear to be any systematic change in the crystallization temperature of lower crustal gabbros with depth in the crust. In order to quantify the duration of crystallization and the lateral extent of the Mush Zone of the lower crust, crystallization temperatures are paired with estimates of the solidus temperature and cooling rate determined from the same sample, previously constrained by the Ca diffusion in olivine geothermometer/ geospeedometer [e.g. VanTongeren et al., 2008 EPSL]. There is no systematic variation in the closure temperature of Ca in olivine, or the cooling rate to the 800°C isotherm. These results show that gabbros throughout the lower crust of the Oman ophiolite remain in a partially

Fe-Ti-oxide textures and microstructures in shear zones from oceanic gabbros at Atlantis Bank, Southwest Indian Ridge

NASA Astrophysics Data System (ADS)

Till, Jessica; Morales, Luiz F. G.; Rybacki, Erik

2016-04-01

Ocean drilling expeditions at several oceanic core complexes formed at slow- and ultra-slow-spreading ridges have recovered cores containing numerous zones of oxide-rich gabbros containing ilmenite and magnetite. In these cores, high modal concentrations of Fe-Ti-oxides are systematically associated with high-temperature plastic deformation features in silicates. We present observations of Fe-Ti-oxide mineral structures and textural characteristics from a series of oxide-rich shear zones from Atlantis Bank (ODP Site 735B) on the Southwest Indian Ridge aimed at determining how oxide mineral abundances relate to strain localization. Fe-Ti-oxide minerals in undeformed oxide gabbros and in highly deformed samples from natural shear zones generally have morphologies characteristic of crystallized melt, including highly cuspate grains and low dihedral angles. Anisotropy of magnetic susceptibility in oxide-rich shear zones is very strong, with fabrics mainly characterized by strong magnetic foliations parallel to the macroscopic foliation. Crystallographic preferred orientations (CPO) in magnetite are generally weak, with occasionally well-defined textures. Ilmenite typically displays well-developed CPOs, however, the melt-like ilmenite grain shapes indicate that at least part of the crystallographic texture results from oriented ilmenite growth during post-deformation crystallization. The oxides are hypothesized to have initially been present as isolated pockets of trapped melt (intercumulus liquid) in a load-bearing silicate framework. Progressive plastic deformation of silicate phases at high-temperature mainly produced two features: (i) elongated melt pockets, which crystallized to form strings of opaque minerals and (ii), interconnected networks of melt regions. The latter lead to intense strain localization of the rock, which appears as oxide-rich mylonites in the samples. In some samples, abundant low-angle grain boundaries in both magnetite and ilmenite suggest

Partial melting of lower oceanic crust gabbro: Constraints from poikilitic clinopyroxene primocrysts

NASA Astrophysics Data System (ADS)

Leuthold, Julien; Lissenberg, C. Johan; O'Driscoll, Brian; Karakas, Ozge; Falloon, Trevor; Klimentyeva, Dina N.; Ulmer, Peter

2018-03-01

Successive magma batches underplate, ascend, stall and erupt along spreading ridges, building the oceanic crust. It is therefore important to understand the processes and conditions under which magma differentiates at mid ocean ridges. Although fractional crystallization is considered to be the dominant mechanism for magma differentiation, open-system igneous complexes also experience Melting-Assimilation-Storage-Hybridization (MASH, Hildreth and Moorbath, 1988) processes. Here, we examine crystal-scale records of partial melting in lower crustal gabbroic cumulates from the slow-spreading Atlantic oceanic ridge (Kane Megamullion; collected with Jason ROV) and the fast-spreading East Pacific Rise (Hess Deep; IODP expedition 345). Clinopyroxene oikocrysts in these gabbros preserve marked intra-crystal geochemical variations that point to crystallization-dissolution episodes of the gabbro eutectic assemblage. Kane Megamullion and Hess Deep clinopyroxene core1 primocrysts and their plagioclase inclusions indicate crystallization from high temperature basalt (>1160 and >1200°C, respectively), close to clinopyroxene saturation temperature (<50% and <25% crystallization). Step-like compatible Cr (and co-varying Al) and incompatible Ti, Zr, Y and rare earth elements (REE) decrease from anhedral core1 to overgrown core2, while Mg# and Sr/Sr* ratios increase. We show that partial resorption textures and geochemical zoning result from partial melting of REE-poor lower oceanic crust gabbroic cumulate (protolith) following intrusion by hot primitive mantle-derived melt, and subsequent overgrowth crystallization (refertilization) from a hybrid melt. In addition, towards the outer rims of crystals, Ti, Zr, Y and the REE strongly increase and Al, Cr, Mg#, Eu/Eu* and Sr/Sr* decrease, suggesting crystallization either from late-stage percolating relatively differentiated melt or from in situ trapped melt. Intrusion of primitive hot reactive melt and percolation of interstitial

Zoned Cr, Fe-spinel from the La Perouse layered gabbro, Fairweather Range, Alaska

USGS Publications Warehouse

Czamanske, G.K.; Himmelberg, G.R.; Goff, F.E.

1976-01-01

Zoned spinel of unusual composition and morphology has been found in massive pyrrhotite-chalcopyrite-pent-landite ore from the La Perouse layered gabbro intrusion in the Fairweather Range, southeastern Alaska. The spinel grains show continuous zoning from cores with up to 53 wt.% Cr2O3 to rims with less than 11 wt.% Cr2O3. Their composition is exceptional because they contain less than 0.32 wt.% MgO and less than 0.10 wt.% Al2O3 and TiO2. Also notable are the concentrations of MnO and V2O3, which reach 4.73 and 4.50 wt.%, respectively, in the cores. The spinel is thought to have crystallized at low oxygen fugacity and at temperatures above 900??C, directly from a sulfide melt that separated by immiscibility from the gabbroic parental magma. ?? 1976.

Dependencies of pore pressure on elastic wave velocities and Vp/Vs ratio for thermally cracked gabbro

NASA Astrophysics Data System (ADS)

Nishimura, K.; Uehara, S. I.; Mizoguchi, K.

2015-12-01

Marine seismic refraction have found that there are high Vp/Vs ratio regions in oceanic crusts at subducting oceanic plates (e.g, Cascadia subduction zone (2.0-2.8) (Audet et al., 2009)). Previous studies based on laboratory measurements indicated that Vp/Vs ratio is high when porosity and/or pore pressure is high (Christensen, 1984; Peacock et al., 2011). Although several studies have investigated the relationships between fracture distributions and Vp, Vs (e.g., Wang et al., 2012; Blake et al., 2013), the relationships for rocks (e.g., gabbro and basalt) composing oceanic crust are still unclear. This study reports the results of laboratory measurements of Vp, Vs (transmission method) at controlled confining and pore pressure and estimation of Vp/Vs ratio for thermally cracked gabbro which mimic highly fractured rocks in the high Vp/Vs ratio zone, in order to declare the dependence of fracture distributions on Vp/Vs. For the measurements, we prepared three type specimens; a non-heated intact specimen, specimens heated up to 500 °C and 700 °C for 24 hours. Porosities of intact, 500 °C and 700 °C specimens measured under the atmospheric pressure are 0.5, 3.4 and 3.5%, respectively. Measurements were conducted at a constant confining pressure of 50 MPa, with decreasing pore pressure from 49 to 0.1 MPa and then increasing to 49 MPa. While Vp/Vs for the intact specimen is almost constant at elevated pore pressure, the Vp/Vs values for the thermally cracked ones were 2.0~2.2 when pore pressure was larger than 30 MPa. In future, we will reveal the relationship between the measured elastic wave velocities and the characteristics of the microfracture distribution. This work was supported by JSPS Grant-in-Aid for Scientific Research (Grant Number 26400492).

Sm-Nd and Rb-Sr Ages for Northwest Africa 2977, A Young Lunar Gabbro from the PKT

NASA Technical Reports Server (NTRS)

Nyquist, L. E.; Shih, C.-Y.; Reese, Y. D.; Irving, A. J.

2009-01-01

Northwest Africa (NWA) 2977 is an olivine gabbro cumulate equivalent to one of the lithologies in lunar mare breccia NWA 773 [1,2,3]. The Ar-39-Ar-40 age is 2.77+/-0.04 Ga based on the last approx.57% of the gas release [4], similar to results for NWA 773 [5]. A Sm-Nd age (T) of 2.865+/-0.031 Ga and Epsilon(sub Nd) = -7.84+/-0.22 for the NWA 773 gabbro reported by [6] has been revised to T = 2.993+/-=0.032 Ga, Epsilon(sub Nd) -4.5+/-0.3 [7]. Sm-147-Nd-143 isochron for NWA 2977: Whole rock, pyroxene, olivine, plagioclase, whole rock leachate (approx.phosphate) and the combined leachates from the mineral separates yield a well defined Sm-Nd isochron for an age T = 3.10+/-0.05 Ga and Epsilon(sub Nd-CHUR) = -3.74+/-0.26 [8], or Epsilon(sub Nd-HEDR) = -4.61+/-0.26 [9]. Rb-87-Sr-87 isochron: NWA 2977 contains only a modest amount of Rb and/or Sr contamination. The Sr-isotopic composition of the contaminant closely resembles that of seawater. The whole rock residue after leaching combined with leach residues for plagioclase and pyroxene define an isochron age of 3.29+/-0.11 Ga for initial Sr-87/Sr-86 = 0.70287+/-18. The olivine residue, with lower Sr abundance of approx 1.5 ppm, is only slightly displaced from the isochron. The relatively small uncertainties of the Rb-Sr isochron parameters and near-concordancy with the Sm-Nd age indicate that both the Rb-Sr and the Sm-Nd ages are reliable.

Structural Characterization of the Foliated-Layered Gabbro Transition in Wadi Tayin of the Samail Ophiolite, Oman; Oman Drilling Project Holes GT1A and GT2A

NASA Astrophysics Data System (ADS)

Deans, J. R.; Crispini, L.; Cheadle, M. J.; Harris, M.; Kelemen, P. B.; Teagle, D. A. H.; Matter, J. M.; Takazawa, E.; Coggon, J. A.

2017-12-01

Oman Drilling Project Holes GT1A and GT2A were drilled into the Wadi Tayin massif, Samail ophiolite and both recovered ca. 400 m of continuous core through a section of the layered gabbros and the foliated-layered gabbro transition. Hole GT1A is cut by a discrete fault system including localized thin ultracataclastic fault zones. Hole GT2A is cut by a wider zone of brittle deformation and incipient brecciation. Here we report the structural history of the gabbros reflecting formation at the ridge to later obduction. Magmatic and high temperature history- 1) Both cores exhibit a pervasive, commonly well-defined magmatic foliation delineated by plagioclase, olivine and in places clinopyroxene. Minor magmatic deformation is present. 2) The dip of the magmatic foliation varies cyclically, gradually changing dip by 30o from gentle to moderate over a 50 m wavelength. 3) Layering is present throughout both cores, is defined by changes in mode and grain size ranging in thickness from 2 cm to 3 m and is commonly sub-parallel to the foliation. 4) There are no high temperature crystal-plastic shear zones in the core. Key observations include: no simple, systematic shallowing of dip with depth across the foliated-layered gabbro transition and layering is continuous across this transition. Cyclic variation of magmatic foliation dip most likely reflects the process of plate separation at the ridge axis. Near-axis faulting- i) On or near-axis structures consist of epidote-amphibole bearing hydraulic breccias and some zones of intense cataclasis with intensely deformed epidote and seams of clay and chlorite accompanied by syntectonic alteration of the wall rock. Early veins are filled with amphibole, chlorite, epidote, and anhydrite. ii) The deformation ranges from brittle-ductile, causing local deflection of the magmatic foliation, to brittle offset of the foliation and core and mantle structures in anhydrite veins. iii) The prevalent sense of shear is normal and slickenfibers

Uniformly mantle-like δ18O in zircons from oceanic plagiogranites and gabbros

NASA Astrophysics Data System (ADS)

Grimes, Craig B.; Ushikubo, Takayuki; John, Barbara E.; Valley, John W.

2011-01-01

Lower ocean crust is primarily gabbroic, although 1-2% felsic igneous rocks that are referred to collectively as plagiogranites occur locally. Recent experimental evidence suggests that plagiogranite magmas can form by hydrous partial melting of gabbro triggered by seawater-derived fluids, and thus they may indicate early, high-temperature hydrothermal fluid circulation. To explore seawater-rock interaction prior to and during the genesis of plagiogranite and other late-stage magmas, oxygen-isotope ratios preserved in igneous zircon have been measured by ion microprobe. A total of 197 zircons from 43 plagiogranite, evolved gabbro, and hydrothermally altered fault rock samples have been analyzed. Samples originate primarily from drill core acquired during Ocean Drilling Program and Integrated Ocean Drilling Program operations near the Mid-Atlantic and Southwest Indian Ridges. With the exception of rare, distinctively luminescent rims, all zircons from ocean crust record remarkably uniform δ18O with an average value of 5.2 ± 0.5‰ (2SD). The average δ18O(Zrc) would be in magmatic equilibrium with unaltered MORB [δ18O(WR) ~ 5.6-5.7‰], and is consistent with the previously determined value for equilibrium with the mantle. The narrow range of measured δ18O values is predicted for zircon crystallization from variable parent melt compositions and temperatures in a closed system, and provides no indication of any interactions between altered rocks or seawater and the evolved parent melts. If plagiogranite forms by hydrous partial melting, the uniform mantle-like δ18O(Zrc) requires melting and zircon crystallization prior to significant amounts of water-rock interactions that alter the protolith δ18O. Zircons from ocean crust have been proposed as a tectonic analog for >3.9 Ga detrital zircons from the earliest (Hadean) Earth by multiple workers. However, zircons from ocean crust are readily distinguished geochemically from zircons formed in continental crustal

Stick-slip behavior of Indian gabbro as studied using a NIED large-scale biaxial friction apparatus

NASA Astrophysics Data System (ADS)

Togo, Tetsuhiro; Shimamoto, Toshihiko; Yamashita, Futoshi; Fukuyama, Eiichi; Mizoguchi, Kazuo; Urata, Yumi

2015-04-01

This paper reports stick-slip behaviors of Indian gabbro as studied using a new large-scale biaxial friction apparatus, built in the National Research Institute for Earth Science and Disaster Prevention (NIED), Tsukuba, Japan. The apparatus consists of the existing shaking table as the shear-loading device up to 3,600 kN, the main frame for holding two large rectangular prismatic specimens with a sliding area of 0.75 m2 and for applying normal stresses σ n up to 1.33 MPa, and a reaction force unit holding the stationary specimen to the ground. The shaking table can produce loading rates v up to 1.0 m/s, accelerations up to 9.4 m/s2, and displacements d up to 0.44 m, using four servocontrolled actuators. We report results from eight preliminary experiments conducted with room humidity on the same gabbro specimens at v = 0.1-100 mm/s and σ n = 0.66-1.33 MPa, and with d of about 0.39 m. The peak and steady-state friction coefficients were about 0.8 and 0.6, respectively, consistent with the Byerlee friction. The axial force drop or shear stress drop during an abrupt slip is linearly proportional to the amount of displacement, and the slope of this relationship determines the stiffness of the apparatus as 1.15 × 108 N/m or 153 MPa/m for the specimens we used. This low stiffness makes fault motion very unstable and the overshooting of shear stress to a negative value was recognized in some violent stick-slip events. An abrupt slip occurred in a constant rise time of 16-18 ms despite wide variation of the stress drop, and an average velocity during an abrupt slip is linearly proportional to the stress drop. The use of a large-scale shaking table has a great potential in increasing the slip rate and total displacement in biaxial friction experiments with large specimens.

Mineralogy, Petrology, Chemistry, and Ar-39 - Ar-40 and Ages of the Caddo County IAB Iron: Evidence for Early Partial Melt Segregation of a Gabbro Area Rich in Plagioclase-Diopside

NASA Technical Reports Server (NTRS)

Takeda, Hiroshi; Bogard, Donald D.; Mittlefehldt, David W.; Garrison, Daniel H.

2000-01-01

We found coarse-grained gabbroic material rich in plagioclase and diopside in the Caddo County IAB iron meteorite. The polished thin sections studied were made from areas rich in Al and Ca detected by a micro-focus X-ray fluorescence (XRF) mapping technique. The gabbro is not a clast within a breccia, but rather this area is located mainly at silicate-metal boundaries only a few cm away from an area with fine-grained, ultramafic silicate similar to winonaites. Medium-grained orthopyroxene and olivine are found in transitional areas showing no disturbance of their crystalline textures. A vein-like region, starting at the area rich in fine-grained mafic silicate, extends towards the gabbroic area with a gradual increase in abundance of plagioclase and diopside. This texture and our accumulated knowledge of the formation mechanism of IAB/winonaltes meteorites, suggest that the gabbroic materials were formed by inhomogeneous segregation of partial melts of chondritic source materials. Compositional data on two mineralogically distinct samples of the gabbro-rich portion of the inclusion were obtained by INAA. Compared to an average of LAB silicate inclusions or winonaites, the Caddo County gabbro is enriched in the incompatible lithophile elements Na, Ca, Sc, REE and Hf, which is consistent with a melt origin for the gabbro. The cosmogenic space exposure age of Caddo County (511 Ma) is significantly younger than exposure ages of some other IAB meteorites, An 39Ar-40Ar age determination of the gabbroic material indicates a series of upward steps in age from 4.516 Ga to 4.523 Ga, with a few high temperature ages up to 4.54 Ga. The older age could approximate the primary recrystallization age of silicates. The stepped Ar age spectrum may indicate differences in Ar closure temperatures during slow cooling of -2-20'C/Myr in the parent body. Alternatively, the younger Ar-Ar ages may date a shock event which occurred while Caddo County was hot and which also created textures

Percussive tool use by Taï Western chimpanzees and Fazenda Boa Vista bearded capuchin monkeys: a comparison

PubMed Central

Visalberghi, Elisabetta; Sirianni, Giulia; Fragaszy, Dorothy; Boesch, Christophe

2015-01-01

Percussive tool use holds special interest for scientists concerned with human origins. We summarize the findings from two field sites, Taï and Fazenda Boa Vista, where percussive tool use by chimpanzees and bearded capuchins, respectively, has been extensively investigated. We describe the ecological settings in which nut-cracking occurs and focus on four aspects of nut-cracking that have important cognitive implications, namely selection of tools, tool transport, tool modification and modulation of actions to reach the goal of cracking the nut. We comment on similarities and differences in behaviour and consider whether the observed differences reflect ecological, morphological, social and/or cognitive factors. Both species are sensitive to physical properties of tools, adjust their selection of hammers conditionally to the resistance of the nuts and to transport distance, and modulate the energy of their strikes under some conditions. However, chimpanzees transport hammers more frequently and for longer distances, take into account a higher number of combinations of variables and occasionally intentionally modify tools. A parsimonious interpretation of our findings is that morphological, ecological and social factors account for the observed differences. Confirmation of plausible cognitive differences in nut-cracking requires data not yet available. PMID:26483529

Metamorphism Near the Dike-Gabbro Transition in the Ocean Crust Based on Preliminary Results from Oman Drilling Project Hole GT3A

NASA Astrophysics Data System (ADS)

Manning, C. E.; Nozaka, T.; Harris, M.; Michibayashi, K.; de Obeso, J. C.; D'Andres, J.; Lefay, R.; Leong, J. A. M.; Zeko, D.; Kelemen, P. B.; Teagle, D. A. H.

2017-12-01

Oman Drilling Project Hole GT3A intersected 400 m of altered basaltic dikes, gabbros, and diorites. The 100% recovery affords an unprecedented opportunity to study metamorphism and hydrothermal alteration near the dike-gabbro transition in the ocean crust. Hydrothermal alteration is ubiquitous; all rocks are at least moderately altered, and mean alteration intensity is 54%. The earliest alteration in all rock types is background replacement of igneous minerals, some of which occurred at clinopyroxene amphibolite facies, as indicated by brown-green hornblende, calcic plagioclase, and secondary cpx. In addition, background alteration includes greenschist, subgreenschist, and zeolite facies minerals. More extensive alteration is locally observed in halos around veins, patches, and zones related to deformation. Dense networks of hydrothermal veins record a complex history of fluid-rock alteration. During core description, 10,727 individual veins and 371 vein networks were logged in the 400 m of Hole GT3A. The veins displayed a range of textures and connectivities. The total density of veins in Hole GT3A is 26.8 veins m-1. Vein density shows no correlation with depth, but may be higher near dike margins and faults. Vein minerals include amphibole, epidote, quartz, chlorite, prehnite, zeolite (chiefly laumontite) and calcite in a range of combinations. Analysis of crosscutting relations leads to classification of 4 main vein types. In order of generally oldest to youngest these are: amphibole, quartz-epidote-chlorite (QEC), zeolite-prehnite (ZP), and calcite. QEC and ZP vein types may contain any combination of minerals except quartz alone; veins filled only by quartz may occur at any relative time. Macroscopic amphibole veins are rare and show no variation with depth. QEC vein densities appear to be higher (>9.3 veins m-1) in the upper 300 m of GT3A, where dikes predominate. In contrast, there are 5.5 veins m-1 at 300-400 m, where gabbros and diorites are abundant. ZP

Electric Currents Generated by Gabbro during Dynamic Uniaxial Loading

NASA Astrophysics Data System (ADS)

Jones, H. H.; Lau, B. W.; Takeuchi, A. T.; Freund, F. T.

2006-12-01

Igneous rocks, when subjected to deviatory stress, turn into a battery. Dynamically stressed rocks can generate large currents. We report on gabbro (Shanxi, China). We use steel pistons to load ~10 cm3 in the center of 30 x 30 x 0.9 cm3 tiles, from 0 to 60 MPa, 1/3 failure strength. Instantly upon loading, a current begins to flow, increasing rapidly to 200-300 pA. One part of the current is carried by electrons, which flow from the stressed rock into the steel pistons and through the external circuit to the edges of the tile. The other part is carried by holes, which flow inside the rock, from the stressed to the unstressed rock and the edges of the tile. At the edges the two charge carriers meet, thereby closing the circuit. Changing the stress rates from 0.0002 to 100 MPa/sec causes the steady currents to increase from ~30,000 A/km3 to ~50,000 A/km3 and an initial spike to develop reaching 300,000 A/km3 at the highest stress rate. Both, electrons and holes, are associated with oxygen anions that changed their valence from 2- to 1- (peroxy). An O- among O2- represents a defect electron in the O2- sublattice, known as positive hole or p-hole for short. In unstressed rocks the O- exist in an electrically inactive form as O- pairs, chemically equivalent to peroxy links, O3X-OO-XO3 with X = Si4+, Al3+ etc. Stresses cause the peroxy links to break, allowing electrons from neighboring O2- to jump in and p-holes to jump out. The p-holes can spread through unstressed rocks using energy levels in the valence band. Sustained large battery currents can flow, for instance in the aftermath of an impact, when the holes can close the circuit by linking up with the electrons. If the circuit is not closed, no battery currents flow.

Timing of the final closure of the Paleo-Asian Ocean in the Alxa Terrane: Constraints from geochronology and geochemistry of Late Carboniferous to Permian gabbros and diorites

NASA Astrophysics Data System (ADS)

Liu, Qian; Zhao, Guochun

2017-04-01

The Alxa Terrane is a crucial place situated between the North China Craton to the east and the Tarim Craton to the west. The Late Paleozoic magmatic record in the Alxa Terrane places important constraints on the timing of the final closure of the middle segment of the Paleo-Asian Ocean (PAO). In this study, new LA-ICPMS zircon U-Pb dating results reveal ca. 300-268 Ma gabbros and diorites in the Bayan Nuru area in the eastern part of the Alxa Terrane. The 300 Ma gabbros show plagioclase accumulations with anorthite compositions (An92-95), arc-like geochemical affinities with relative enrichment in large ionic lithophile elements and depletion in high field strength elements (e.g., Ti, Nb and Ta), as well as negative Hf(t) (-6.01 to -1.75) and Nd(t) (-9.5 to -7.1) values and high initial 87Sr/86Sr ratios (0.707157-0.707220). These features indicate a magma source of an enriched lithospheric mantle metasomatized by high fluid activities. In comparison, the 280-268 Ma gabbros and diorites also have arc-like geochemical affinities but show increasingly evolved isotope compositions, implying more sediment inputs. Compiled zircon ɛHf(t) and whole-rock ɛNd(t) values of the magmatic rocks in the Alxa Terrane decrease from the Late Carboniferous to the Early Permian, and increase from the Middle Permian to the Triassic. The considerably large spread in ɛHf(t) and ɛNd(t) values at ca. 280-265 Ma likely reflects a tectonic switch from a subduction setting to a post-collisional setting, corresponding to the timing of the final closure of the PAO in the Alxa Terrane. Thus, the PAO progressively closed from west to east along the northern margin of the Tarim Craton, the Alxa Terrane, and then the northern margin of the North China Craton during Late Carboniferous to Middle Triassic time. This work was financially supported by a NSFC Project (41190075) entitled "Final Closure of the Paleo-Asian ocean and Reconstruction of East Asian Blocks in Pangea", the fifth

Slow Slip Predictions Based on Gabbro Dehydration and Friction Data Compared to GPS Measurements in Northern Cascadia

NASA Astrophysics Data System (ADS)

Rice, J. R.; Liu, Y.

2008-12-01

For episodic slow slip transients in subduction zones, a large uncertainty in comparing surface deformations predicted by rate and state friction modeling [Liu and Rice, JGR, 2007] to GPS measurements lies in our limited knowledge of the frictional properties and fluid pore pressure along the fault. In this study, we apply petrological data [Peacock et al., USGS, 2002; Hacker et al., JGR 2003; Wada et al., JGR, 2008] and recently reported friction data [He et al., Tectonophys, 2006, 2007] for gabbro, as a reasonable representation of the seafloor, to a Cascadia-like 2D model in order to produce simulations which show spontaneous aseismic transients. We compare the resulting inter-transient and transient surface deformations to GPS observations along the northern Cascadia margin. An inferred region along dip of elevated fluid pressure is constrained by seismological observations where available, and by thermal and petrological models for the Cascadia and SW Japan subduction zones. For the assumed a and a-b profiles, we search the model parameter space, by varying the level of effective normal stress σ, characteristic slip distance L in the source areas of transients, and the fault width under that low σ, to identify simulation cases which produce transient aseismic slip and recurrence interval similar to the observed 20-30 mm and 14 months, respectively, in northern Cascadia. Using a simple planar fault geometry and extrapolating the 2D fault slip to a 3D distribution, we find that the gabbro gouge friction data allows a much better fit to GPS observations than is possible with the granite data [Blanpied et al., JGR, 1995, 1998] which, for lack of a suitable alternative, has been used as the basis for most previous subduction earthquake modeling, including ours. Nevertheless, the values of L required to reasonably fit the geodetic data during a transient event are somewhat larger than 100 microns, rather than in the range of 10 to a few 10s of microns as might be

Tonga Trench gabbros and peridotites: A suit of temporal and spatial forearc materials

NASA Astrophysics Data System (ADS)

Michibayashi, K.; Shinkai, Y.; Tani, K.; Uehara, S.; Harigane, Y.; Ishii, T.; Bloomer, S. H.

2012-12-01

The Tonga trench is one of the deepest oceanic regions in the world (10,866 m). Various types of rocks have been dredged and drilled at several localities on the landward slopes of the trench during Boomerang Leg8 in 1996. In particular, very pristine peridotites outcrop at the most deep landward trench slope. We show that the trench can be divided into two regions: southern region and northern region. The peridotites in the southern region have high-Cr# (0.46-0.83) which were typical of forearc peridotites, whereas the peridotites in the northern region have evidences of the reaction with magma during partial melting. Olivine fabrics are characterized by E-type and D-type. Although E-type and D-type are no clear relationship of mineral composition, grain size and equilibrium temperature, the only difference between E-type and D-type were fabric intensities: D-type has higher fabric intensity than that for E-type. Geochronological study revealed that the gabbros in the southern region have the oldest ages of ~52 Ma that are as old as the oldest rocks in the Izu-Bonin-Mariana arc recording the subduction infancy (e.g., Ishizuka et al., 2011 EPSL). We argue that the southern region preserves the oldest mantle fabrics that took place during the subduction infancy, where pristine and serpentinized peridotites have been deformed in the region where high strain field occurred due to the dragged flow. Eventually, they expose in a very neat condition (i.e. active tectonic erosion and fast ascent rate) resulting from an unique tectonic setting including fast subducting plate (24 cm/yr), fast spreading plate (15 cm/yr) and slab rollback.

[Diversity and frequency of scientific research design and statistical methods in the "Arquivos Brasileiros de Oftalmologia": a systematic review of the "Arquivos Brasileiros de Oftalmologia"--1993-2002].

PubMed

Crosta, Fernando; Nishiwaki-Dantas, Maria Cristina; Silvino, Wilmar; Dantas, Paulo Elias Correa

2005-01-01

To verify the frequency of study design, applied statistical analysis and approval by institutional review offices (Ethics Committee) of articles published in the "Arquivos Brasileiros de Oftalmologia" during a 10-year interval, with later comparative and critical analysis by some of the main international journals in the field of Ophthalmology. Systematic review without metanalysis was performed. Scientific papers published in the "Arquivos Brasileiros de Oftalmologia" between January 1993 and December 2002 were reviewed by two independent reviewers and classified according to the applied study design, statistical analysis and approval by the institutional review offices. To categorize those variables, a descriptive statistical analysis was used. After applying inclusion and exclusion criteria, 584 articles for evaluation of statistical analysis and, 725 articles for evaluation of study design were reviewed. Contingency table (23.10%) was the most frequently applied statistical method, followed by non-parametric tests (18.19%), Student's t test (12.65%), central tendency measures (10.60%) and analysis of variance (9.81%). Of 584 reviewed articles, 291 (49.82%) presented no statistical analysis. Observational case series (26.48%) was the most frequently used type of study design, followed by interventional case series (18.48%), observational case description (13.37%), non-random clinical study (8.96%) and experimental study (8.55%). We found a higher frequency of observational clinical studies, lack of statistical analysis in almost half of the published papers. Increase in studies with approval by institutional review Ethics Committee was noted since it became mandatory in 1996.

Adakite-gabbro-anorthosite magmatism at the final (576-546 Ma) development stage of the Neoproterozoic active margin in the south-west of the Siberian craton

NASA Astrophysics Data System (ADS)

Vernikovskaya, A. E.; Vernikovsky, V. A.; Matushkin, N. Yu.; Kadilnikov, P. I.; Romanova, I. V.; Larionov, A. N.

2017-12-01

In the late Neoproterozoic a prolonged active continental margin mode dominated the southwestern margin of the Siberian craton. Based on results of geological, petrological-geochemical, U-Th-Pb and Sm-Nd, Rb-Sr isotope investigations, for the first time we established that on the final evolution stage of this margin 576-546 Ma, intrusions of adakites and gabbro-anorthosites of the Zimoveyniy massif were emplaced in the South Yenisei Ridge. These new data indicate genetic relationships of the studied adakites and host NEB-metabasites. The formation of adakites could have been due to a crustal or a mantle-crustal source in a setting of transform sliding of lithospheric plates after the subduction stopped.

Electric Currents in Granite and Gabbro Generated by Impacts Up To 1 km/sec

NASA Astrophysics Data System (ADS)

Hollerman, W. A.; Lau, B. L.; Moore, R. J.; Malespin, C. A.; Bergeron, N. P.; Freund, F. T.; Wasilewski, P. J.

2006-12-01

For many years, radio noise, strange lights coming out of the ground, and other unusual phenomena have been detected prior to major earthquakes. Only recently have these signals been systematically monitored and their correlations to earthquakes have been more firmly established. A glow in the sky sometimes heralds a big quake. In January 1995, white, blue, or orange lights extending some 200 m into the air and spreading 1 to 8 km across the ground were reported by at least 23 eyewitnesses in and around Kobe, Japan. Hours later, a 6.9-magnitude earthquake killed more than 4,500 people. Such signals imply the movement of electric currents through rock and soil and their discharge into the air. During summer 2006 a research project started using the single-stage light gas gun at the NASA Goddard Space Flight Center in Maryland. The gun fires 63 mm diameter aluminum sabots of a few grams to 1.2 kilograms. A catcher was designed to stop the sabot while allowing a smaller projectile to impact a desired target at velocities up to 1 km/s. This presentation documents first results of the production of electric currents during impacts on granite and gabbro instrumented with capacitive sensors, contact electrodes, magnetic pick-up coils and photo diodes for light detection. This research is critical towards the development of techniques that could be used to monitor quakes on the Earth and estimate secondary effects of meteorite impacts on the Moon and Mars during the next phase of human space exploration.

Component mobility at 900 °C and 18 kbar from experimentally grown coronas in a natural gabbro

NASA Astrophysics Data System (ADS)

Keller, Lukas M.; Wunder, Bernd; Rhede, Dieter; Wirth, Richard

2008-09-01

Several approximately 100-μm-wide reaction zones were grown under experimental conditions of 900 °C and 18 kbar along former olivine-plagioclase contacts in a natural gabbro. The reaction zone comprises two distinct domains: (i) an irregularly bounded zone with idiomorphic grains of zoisite and minor corundum and kyanite immersed in a melt developed at the plagioclase side and (ii) a well-defined reaction band comprising a succession of mineral layers forming a corona structure around olivine. Between the olivine and the plagioclase reactant phases we observe the following layer sequence: olivine|pyroxene|garnet|partially molten domain|plagioclase. Within the pyroxene layer two micro-structurally distinct layers comprising enstatite and clinopyroxene can be discerned. Chemical potential gradients persisted for the CaO, Al 2O 3, SiO 2, MgO and FeO components, which drove diffusion of Ca, Al and Si bearing species from the garnet-matrix interface to the pyroxene-olivine interface and diffusion of Mg- and Fe-bearing species in the opposite direction. The systematic mineralogical organization and chemical zoning across the corona suggest that the olivine corona was formed by a "diffusion-controlled" reaction. We estimate a set of diffusion coefficients and conclude that LAlAl < LCaCa < ( LSiSi, LFeFe) < LMgMg during reaction rim growth.

Geochronology and geochemistry of late Carboniferous-middle Permian I- and A-type granites and gabbro-diorites in the eastern Jiamusi Massif, NE China: Implications for petrogenesis and tectonic setting

NASA Astrophysics Data System (ADS)

Bi, Jun-Hui; Ge, Wen-Chun; Yang, Hao; Wang, Zhi-Hui; Xu, Wen-Liang; Yang, Jin-Hui; Xing, De-He; Chen, Hui-Jun

2016-12-01

Late Carboniferous-middle Permian magmatism in the Jiamusi Massif of northeast China, in the eastern segment of the Central Asian Orogenic Belt (CAOB), provides critical evidence regarding the tectonic history and geodynamic processes in the region. The gabbro-diorites of the Longtouqiao pluton and two groups of coeval granite in the study area comprise a bimodal magmatic suite. Precise LA-ICP-MS U-Pb zircon ages indicate that the granitoids and gabbro-diorites were emplaced in the late Carboniferous-middle Permian (302-267 Ma). Group I granites have high SiO2 (70.75-77.04 wt.%) and K2O (3.65-5.89 wt.%) contents, are enriched in LILEs (e.g., Rb, Th, and U) relative to HFSEs and LREEs, and have negative Nb, Ta, P, and Ti anomalies, which collectively indicate affinities with subduction-related magmas. Group II granites are weakly peraluminous (A/CNK = 1.03-1.07) and are characterized by enrichment in alkalis (Na2O + K2O = 8.22-8.90 wt.%), low MgO (0.04-0.09 wt.%) and P2O5 (0.01-0.04 wt.%) contents, high Zr and Nb contents, high 10,000 × Ga/Al ratios, and they are geochemically similar to aluminous A-type granites. All the magmatic zircons in these granitoids have great variations of εHf(t) (+ 7.89 to - 5.60) and two-stage Hf model ages (TDM2) of 0.8-1.7 Ga, which suggest that the precursor magmas originated from a heterogeneous source that involved juvenile components derived from a depleted mantle source during magma generation. The aluminous A-type granite magmas were probably derived by high-temperature partial melting of a felsic crustal source, whereas the other granite magmas probably resulted from partial melting of a mafic lower crust. The gabbro-diorites of the Longtouqiao pluton are depleted in Nb, Ta, P, and Ti, and show flat distributions of most LILEs and HFSEs, except for large positive anomalies in Ba, K, and Pb. These features reflect a limited degree of crustal contamination associated with the subduction-related magmatic processes. These data

Parental Sources of High-Alumina Alkaline Melts: Nd, Sr, Pb, and O Isotopic Evidence from the Devonian Kiya-Shaltyr Gabbro-Urtite Intrusion, South Siberia

NASA Astrophysics Data System (ADS)

Vrublevskii, V. V.; Gertner, I. F.; Chugaev, A. V.

2018-04-01

The isotope geochemistry (ɛNd( t) 4.8-5.4, 206Pb/204Pb in 18.05-18.36, 207Pb/204Pbin 15.53-15.57, 208Pb/204Pb in 37.59-37.83, 87Sr/86Sr( t) 0.7048-0.7057, δ18OSMOW 8-10.5‰) and trace element composition of the Kiya-Shaltyr gabbro-urtite pluton allow us to suggest a heterogeneous source and complex geodynamic settings of the Devonian alkali magmatism in the Kuznetsk Alatau. It is assumed that its evolution took place under conditions of partial mingling of matter of the depleted (PREMA) and enriched (EM) mantle with crustal contamination of the evolving melt. Such an interaction could have been a result of superposition of a mantle plume and an active margin (OIB and IAB components). In fold belts this led to the formation of hybrid high-alumina foidoite magmas.

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 of primitive ocean island basalts by crustal gabbro assimilation and multiple recharge of plume-derived melts

NASA Astrophysics Data System (ADS)

Borisova, Anastassia Y.; Bohrson, Wendy A.; Grégoire, Michel

2017-07-01

Chemical Geodynamics relies on a paradigm that the isotopic composition of ocean island basalt (OIB) represents equilibrium with its primary mantle sources. However, the discovery of huge isotopic heterogeneity within olivine-hosted melt inclusions in primitive basalts from Kerguelen, Iceland, Hawaii and South Pacific Polynesia islands implies open-system behavior of OIBs, where during magma residence and transport, basaltic melts are contaminated by surrounding lithosphere. To constrain the processes of crustal assimilation by OIBs, we employed the Magma Chamber Simulator (MCS), an energy-constrained thermodynamic model of recharge, assimilation and fractional crystallization. For a case study of the 21-19 Ma basaltic series, the most primitive series ever found among the Kerguelen OIBs, we performed sixty-seven simulations in the pressure range from 0.2 to 1.0 GPa using compositions of olivine-hosted melt inclusions as parental magmas, and metagabbro xenoliths from the Kerguelen Archipelago as wallrock. MCS modeling requires that the assimilant is anatectic crustal melts (P2O5 ≤ 0.4 wt.% contents) derived from the Kerguelen oceanic metagabbro wallrock. To best fit the phenocryst assemblage observed in the investigated basaltic series, recharge of relatively large masses of hydrous primitive basaltic melts (H2O = 2-3 wt%; MgO = 7-10 wt.%) into a middle crustal chamber at 0.2 to 0.3 GPa is required. Our results thus highlight the important impact that crustal gabbro assimilation and mantle recharge can have on the geochemistry of mantle-derived olivine-phyric OIBs. The importance of crustal assimilation affecting primitive plume-derived basaltic melts underscores that isotopic and chemical equilibrium between ocean island basalts and associated deep plume mantle source(s) may be the exception rather than the rule.

The Pikes Peak batholith, Colorado front range, and a model for the origin of the gabbro-anorthosite-syenite-potassic granite suite

USGS Publications Warehouse

Barker, F.; Wones, D.R.; Sharp, W.N.; Desborough, G.A.

1975-01-01

, Mass., and syenite of Kungnat, Greenland, among others - allowing for different levels of erosion. A suite that includes gabbro or basalt, anorthosite, quartz syenite, fayalite granite, riebeckite granite, and biotite and/or hornblende granites is of worldwide occurrence. A model is proposed in which mantle-derived, convecting alkali olivine basaltic magma first reacts with K2O-poor lower crust of granulite facies to produce magma of quartz syenitic composition. The syenitic liquid in turn reacts with granodioritic to granitic intermediate crust of amphibolite facies to produce the predominant fayalite-free biotite and biotite-hornblende granites of the batholith. This reaction of magma and roof involves both partial melting and the reconstitution and precipitation of refractory phases, as Bowen proposed. Intermediate liquids include MgO-depleted and Na2O-enriched gabbro, which precipitated anorthosite, and alkali diorite. The heat source is the basaltic magma; the heat required for partial melting of the roof is supplied largely by heats of crystallization of phases that settle out of the liquid - mostly olivine, clinopyroxene and plagioclase. ?? 1975.

Rodingite in Layered Gabbro of the Leka Ophiolite Complex, North-Central Caledonides of Norway

NASA Astrophysics Data System (ADS)

Prestvik, T.; Austrheim, H.

2006-12-01

Both the ultramafic (mantle) and the layered ultramafic to gabbroic (crustal) sequences of the Cambrian (497 Ma) Leka ophiolite are characterized by extensive serpentinization. Rodingite, containing grossular garnet, clinopyroxene, clinozoisite, prehnite, chlorite and preiswerkite, which has been found in the lowermost plagioclase-rich layers of the gabbro sequence seems to represent an unusual (new?) mode of rodingite occurrence compared to the more common rodingitized basaltic dikes described from many ultramafic complexes worldwide. The 5 to 15 cm wide rodingitized plagioclase layers, that alternate with less altered layers of wehrlite, clinopyroxenite, and websterite, are located c. 10 m away from a 10 m wide layer of serpentinized dunite. The whole sequence is cut by numerous fractures oriented almost perpendicular to the layering, and rodingite occurs where the fractures transect the plagioclase layers. In the adjacent lithologies, the fractures can be followed as thin veins filled with grossular, clinopyroxene, amphibole, epidote, and chlorite. These fractures were most likely channelways for the rodingite-forming fluids. Gresen analysis, assuming constant volume, shows that the rodingite formed from the plagioclase-rich layers by addition of c. 22 g of CaO, 6 g of FeO and SiO2 and removal of 10 g of Al2O3 and all (2 g) of Na2O per 100 g of protolith. Microtextures show chlorite and serpentine pseudomorphs after primary clinopyroxene, demonstrating that the alteration took place at constant volume. This reaction is the most likely Ca source for the rodingitization, possibly in addition to the serpentinization of olivine in the dunite layers. Furthermore, Ca-enriched and Al2O3- depleted clinopyroxene of the rodingite - compared to the primary clinopyroxene of the layered sequence - attest to the mobil nature of these elements. While both the protolith and the rodingite are almost K2O-free, one of the plagioclase-rich layers has K2O in the 1.1 to 1.4% range

Paleoproterozoic Keulik-Kenirim Ore-Bearing Gabbro-Peridotite Complex, Kola Region: A New Occurrence of Ferropicritic Magmatism

NASA Astrophysics Data System (ADS)

Smolkin, V. F.; Lokhov, K. I.; Skublov, S. G.; Sergeeva, L. Yu.; Lokhov, D. K.; Sergeev, S. A.

2018-03-01

Comprehensive research of ore-bearing differentiated intrusions of the Keulik-Kenirim structural unit, which represents a fragment of the Paleoproterozoic Pechenga-Varzuga Belt, has been carried out for the first time. The intrusions are subvolcanic by type and lenticular in shape, nearly conformable and steeply dipping. They are made up of peridotite, olivine and plagioclase pyroxenites, and gabbro metamorphosed under amphibolite facies conditions along with host basic volcanics. All intrusive rocks are enriched in TiO2 and FeO. Sulfide Cu-Ni mineralization is represented by disseminated, pocket, and stringer-disseminated types, which are clustered in the peridotitic zone as hanging units and bottom lodes. The Ni content in disseminated ore is estimated at 0.45-0.55 wt % and 1.15-3.32 wt % in ore pockets; the Cu grades are 0.17-0.20 and 0.46-5.65 wt %, respectively. To determine the age of intrusions and metamorphism of intrusive and volcanic rocks, various isotopic systems have been used: Sm-Nd (TIMS) in rock and U-Pb (SIMS SHRIMP) and Lu-Hf (LA-ICP-MS) in zircon. Conclusions on the origin of zircons are based on concentrations of trace elements including REE therein and Hf-Nd correlation in zircons and rocks. The U-Pb system of zircons reflects episodes of igneous rock formation (1982 ± 12 Ma) and their postmagmatic transformation (1938 ± 20 Ma). The last disturbance of the U-Pb isotopic system occurred 700 and 425 Ma. Xenogenic zircons dated from 3.17 to 2.65 Ga have been revealed in the studied samples. These zircons were captured by magma from the Archean basement during its ascent. The intrusions were emplaced synchronously with economic ore formation in the Pechenga ore field (1985 ± 10 Ma). The peak metamorphism of intrusive rocks under amphibolite facies conditions is recorded at 40 Ma later. The differentiated intrusions of the Keulik-Kenirim structural unit are close in their internal structure, mineralogy, and geochemistry, as well as in age and

Pétrologie d'un complexe gabbroïque différencié dans le sillon du Boussouma (Burkina-Faso-Craton Ouest Africain)

NASA Astrophysics Data System (ADS)

Wenmega, U.; Tempier, P.

Boussouma birimian gabbroic complex is a genetic association of early ultramafic cumulates (retromorphic cortlandite and clinopyroxenite) overlain by tholeiitic rocks formed by massive or layered metagabbros and basic metavolcanites and pillow lavas. Intrusive later in birimian pyroclastic and schistosed volcanodetrictic rocks, this non deformed gabbroic complex, shows multiphased metamorphism surimposition, varying from amphibolite facies of low temperature to epizonal facies. The cristallogenetic sequence composed in cortlandite cumulates by spinel-olivine-orthopyroxene-clinopyroxene-brown hornblende ± plagioclase, joined to temperature (1300° C to 1100° C at 10 kb) furnished by geothermometer couple (orthopyroxene-augite), give a mantellic character to magma and indicate some high thermodynamic condition for cristallization of early cortlandite cumulate. The differenciation is accomplished by satured water and silica magma, in a magmatic chamber toward a slow sedimentation of early ferromagnesia cumulus (olivine-orthopyroxene) in cortlandite. The consequence of this fractionation, is the enrichment of clinopyroxene, brown hornblende, plagioclase, particularly in clinopyroxenites and gabbros facies. Comparatively to classic ophiolitic associations this complex is distinguishable from them by his intrusif character, later in birimian rocks series, by the absence of mantellic sequence (tectonites of harzburgite, lherzolite), by the absence of structural charriage characters in this complex, one criterion generally associated with subduction history of oceanic crust. About geotectonic context of this complex, his cristallogenetic, thermodynamic, geochemical characteristics could evoke one section of fringing basin crust, corresponding to ocean-continent transition zones, or one section of rift valley crust with immature evolution.

Timing, petrogenesis and tectonic setting of the Late Paleozoic gabbro-granodiorite-granite intrusions in the Shalazhashan of northern Alxa: Constraints on the southernmost boundary of the Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Shi, Xingjun; Wang, Tao; Zhang, Lei; Castro, Antonio; Xiao, XuChang; Tong, Ying; Zhang, Jianjun; Guo, Lei; Yang, Qidi

2014-11-01

The Late Paleozoic tectonic setting and location of the southernmost boundary of the Central Asian Orogenic Belt (CAOB) with respect to the Alxa Block or Alxa-North China Craton (ANCC) are debated. This paper presents new geochronological, petrological, geochemical and zircon Hf isotopic data of the Late Paleozoic intrusions from the Shalazhashan in northern Alxa and discusses the tectonic setting and boundary between the CAOB and ANCC. Using zircon U-Pb dating, intrusions can be broadly grouped as Late Carboniferous granodiorites (~ 301 Ma), Middle Permian gabbros (~ 264 Ma) and granites (~ 266 Ma) and Late Permian granodiorites, monzogranites and quartz monzodiorites (254-250 Ma). The Late Carboniferous granodiorites are slightly peraluminous and calcic. The remarkably high zircon Hf isotopes (εHf(t) = + 6-+ 10) and characteristics of high silica adakites suggest that these granodiorites were mainly derived from "hot" basaltic slab-melts of the subducted oceanic crust. The Middle Permian gabbros exhibited typical cumulate textures and were derived from the partial melting of depleted mantle. The Middle Permian granites are slightly peraluminous with high-K calc-alkaline and low εHf(t) values from - 0.9 to + 2.9. These granites were most likely derived from juvenile materials mixed with old crustal materials. The Late Permian granodiorites, monzogranites and quartz monzodiorites are characterized as metaluminous to slightly peraluminous, with variable Peacock alkali-lime index values from calc-alkalic to alkali-calcic. These rocks were mainly derived from juvenile crustal materials, as evidenced by their high εHf(t) values (+ 3.3 to + 8.9). The juvenile sources of the above intrusions in the Shalazhashan are similar to those of the granitoids from the CAOB but distinct from the granitoids within the Alxa Block. These findings suggest that the Shalazhashan Zone belongs to the CAOB rather than the Alxa Block and that its boundary with the Alxa block can be

Lower crustal section of the Oman Ophiolite drilled in Hole GT1A, ICDP Oman Drilling Project

NASA Astrophysics Data System (ADS)

Umino, S.; Kelemen, P. B.; Matter, J. M.; Coggon, J. A.; Takazawa, E.; Michibayashi, K.; Teagle, D. A. H.

2017-12-01

Hole GT1A (22° 53.535'N, 58° 30.904'E) was drilled by the Oman Drilling Project (OmDP) into GT1A of the Samail ophiolite, Oman. OmDP is an international collaboration supported by the International Continental Scientific Drilling Program, the Deep Carbon Observatory, NSF, IODP, JAMSTEC, and the European, Japanese, German and Swiss Science Foundations, with in-kind support in Oman from the Ministry of Regional Municipalities and Water Resources, Public Authority of Mining, Sultan Qaboos University, and the German University of Technology. Hole GT1A was diamond cored in 22 Jan to 08 Feb 2017 to a total depth of 403.05 m. The outer surfaces of the cores were imaged and described on site before being curated, boxed and shipped to the IODP drill ship Chikyu, where they underwent comprehensive visual and instrumental analysis. Hole GT1A drilled the lower crustal section in the southern Oman Ophiolite and recovered 401.52 m of total cores (99.6% recovery). The main lithology is dominated by olivine gabbro (65.9%), followed in abundance by olivine-bearing gabbro (21.5%) and olivine melagabbro (3.9%). Minor rock types are orthopyroxene-bearing olivine gabbro (2.4%), oxide-bearing olivine gabbro (1.5%), gabbro (1.1%), anorthositic gabbro (1%), troctolitic gabbro (0.8%); orthopyroxene-bearing gabbro (0.5%), gabbronorite (0.3%); and dunite (0.3%). These rocks are divided into Lithologic Unit I to VII at 26.62 m, 88.16 m, 104.72 m, 154.04 m, 215.22 m, 306.94 m in Chikyu Curated Depth in descending order; Unit I and II consist of medium-grained olivine gabbro with lower olivine abundance in Unit II. Unit III is medium-grained olivine melagabbros, marked by an increase in olivine. Unit IV is relatively homogenous medium-grained olivine gabbros with granular textures. Unit V is identified by the appearance of fine-grained gabbros, but the major rocktypes are medium grained olivine gabbros. Unit VI is medium-grained olivine gabbro, marked by appearance of orthopyroxene. Unit VII

Platinum potential of mafic-ultramafic massifs in the western part of the Dambuka ore district (Upper Amur Region, Russia)

NASA Astrophysics Data System (ADS)

Melnikov, A. V.; Stepanov, V. A.; Moiseenko, V. G.

2016-02-01

New data on the Pt potential of mafic-ultramafic massifs of the Khani-Maya, Uldegit, and Dzhalta complexes in the western part of the Dambuka ore district are discussed. The Khani-Maya Complex is represented by metamorphosed gabbro, gabbronorites, gabbro anorthosites, subordinate pyroxenites, hornblendites, and peridotites. The Uldegit Complex is composed of pyroxenites, hornblendites, gabbro, gabbronorites, norites, troctolites, peridotites, dunites, actinolite-tremolites, serpentinites, anthophyllites, and tremolite-plagioclase rocks. The Dzhalta Complex is formed of peridotites, gabbro, eclogitized gabbro, hornblendites, cortlandites, and pyroxenites. All these complexes differ from each other by the concentrations of Ni, Cu, Co, Au, and platinoids depending on the composition of the constituting rocks and the presence of sulfide minerals.

Amphibole-bearing multiphase solid inclusions in olivine, the Murotomisaki Gabbro, Southwest Japan: An evidence of hydrous flux melting

NASA Astrophysics Data System (ADS)

Hoshide, T.; Obata, M.

2009-12-01

The Murotomisaki Gabbro is a sill-like layered intrusion of up to 220m thickness exposed near Cape Muroto, Southwest Japan. Despite the small size of the intrusion, it contains well-developed centimeter- to meter-scale layered structures of modal variation of olivine, plagioclase and augite. Hoshide et al (2006a, b) identified the ’crystal accumulation zone’ (40m from the bottom) that was formed by gravitational settling of olivine crystals and the ’crystal growth zone’ (40-100m from the bottom), in which olivine crystals grew significantly. The fine-scale compositional layering is best developed in the ’crystal growth zone’. Amphibole-bearing multiphase solid inclusions (called ‘the amphibole-clot inclusions’) are common in olivine crystals from both the crystal accumulation- and the crystal growth zones. The amphibole clot inclusions show spherical or convex-polygonal shapes and are composed of pargasitic amphibole, biotite and orthopyroxene, with minor amounts of augite, apatite and opaque minerals. Plagioclase rarely occurs in the amphibole clot inclusions. Bulk chemical compositions of the inclusions, obtained from mineral microprobe analyses and modal composition, are characteristically high in MgO content (16-23 wt %) and they roughly lie between presumable fractionated melt compositions and olivine compositions. From observations above, it is likely that amphibole clot inclusions are of melt origin, which had formed from some hydrous melts probably entrapped in growing olivine crystals. However, it may be difficult to explain both the very magnesian nature of the inclusion and absence of plagioclase in the inclusion by the crystallization of the normal hydrous basaltic melt. The difficulty may be resolved if we suppose, for a trapped melt composition, a more magnesian (i.e., picritic) composition. The highly magnesian nature of the amphibole clot inclusions may suggest that significant amount of olivine component had been added to

Lateral Variability of the Lower Ocean Crust at Atlantis Bank, SW Indian Ridge, Results of IODP Expedition 360

NASA Astrophysics Data System (ADS)

Dick, H. J.; MacLeod, C. J.; Blum, P.; Scientific Party, E.

2016-12-01

IODP Hole U1473A drilled 809.4 m into a 700-m depth wave-cut platform at Atlantis Bank on the SW Indian Ridge. It is an oceanic core complex where massive gabbro was emplaced into the footwall of a single detachment fault for ≥2.7 Myr, with total slip ≥39 km. It was then uplifted to its present position flanking the 6,100 m deep 199-km Atlantis II Transform. The gabbros are back-tilted 20°S, while a sub-horizontal 15 km long mantle peridotite-gabbro contact lies along the transform wall at 4200 m depth 11.5 km west of Hole U1473A. Hole U1473A is 1.4 km north of 158-m deep Hole 1105A and 2.2 km NNE of 1508-m deep Hole 735B. Thus we examine the lateral continuity of the lower ocean crust at ultraslow rates ( 15-16 mm/yr.), and compare it to 1400-m Hole U1309D in the Atlantis Massif MAR core complex (24 mm/yr.) flanking the 63-km Atlantis Transform. The three Atlantis Bank holes are very similar, consisting of a complex series of oxide-rich gabbros and olivine gabbros. Several dikes crosscutting the gabbro sections show that they passed through the dike-gabbro transition after crystallizing and cooling deeper in the crust. They all show extensive high-temperature crystal-plastic deformation predating dike intrusion. A small amount of troctolite was recovered only in Hole 735B. By contrast, gabbro, rather than olivine gabbro was the dominant lithology in Hole U1309D, with intercalations of troctolite and mantle peridotite, and subordinate oxide gabbro. Oxide gabbro is often associated with crystal-plastic deformation. While these are concentrated in the upper 1/3 of Hole 735B, they are more uniformly distributed in Hole U1309D. While one section cannot be traced directly to the other at Atlantis Bank, it appears that they can be correlated based on chemical and structural similarities, with the 1105A and 1473A sections lying some hundreds of meters deeper structurally than Hole 735B, consistent with erosion on the platform. All these sections represent sequential

Sr isotopic tracer study of the Samail ophiolite, Oman.

USGS Publications Warehouse

Lanphere, M.A.; Coleman, R.G.; Hopson, C.A.

1981-01-01

Rb and Sr concentrations and Sr-isotopic compositions were measured in 41 whole-rock samples and 12 mineral separates from units of the Samail ophiolite, including peridotite, gabbro, plagiogranite, diabase dykes, and gabbro and websterite dykes within the metamorphic peridotite. Ten samples of cumulate gabbro from the Wadir Kadir section and nine samples from the Wadi Khafifah section have 87Sr/86Sr ratios of 0.70314 + or - 0.00030 and 0.70306 + or - 0.00034, respectively. The dispersion in Sr- isotopic composition may reflect real heterogeneities in the magma source region. The average Sr-isotopic composition of cumulate gabbro falls in the range of isotopic compositions of modern MORB. The 87Sr/86Sr ratios of noncumulate gabbro, plagiogranite, and diabase dykes range 0.7034-0.7047, 0.7038-0.7046 and 0.7037- 0.7061, respectively. These higher 87Sr/86Sr ratios are due to alteration of initial magmatic compositions by hydrothermal exchange with sea-water. Mineral separates from dykes that cut harzburgite tectonite have Sr-isotopic compositions which agree with that of cumulate gabbro. These data indicate that the cumulate gabbro and the different dykes were derived from partial melting of source regions that had similar long-term histories and chemical compositions.-T.R.

Geological structure and ore mineralization of the South Sopchinsky and Gabbro-10 massifs and the Moroshkovoe Lake target, Monchegorsk area, Kola Peninsula, Russia

NASA Astrophysics Data System (ADS)

Pripachkin, Pavel V.; Rundkvist, Tatyana V.; Miroshnikova, Yana A.; Chernyavsky, Alexey V.; Borisenko, Elena S.

2016-12-01

The South Sopchinsky massif (SSM), Gabbro-10 (G-10) massif, and Moroshkovoe Lake (ML) target Monchegorsk area, Kola Peninsula, are located at the junction of the Monchepluton and Monchetundra layered intrusions. The intrusions were studied in detail as they are targets for platinum-group element (PGE) mineralization. The rocks in these targets comprise medium- to coarse-grained mesocratic to leucocratic gabbronorites, medium-grained mesocratic to melanocratic norites and pyroxenites, and various veins mainly comprising norite, plagioclase-amphibole-magnetite rocks, and quartz-magnetite rocks. The veins contain Ni-Cu-PGE mineralization associated with magnetite and chromite. In all targets, the contacts between gabbronorite and norite-pyroxenite are undulating, and the presence of magmatic (intrusive) breccias suggests that these rocks formed through mingling of two distinct magmatic pulses. In places, the gabbronorites clearly crosscut the modal layering of the norites and pyroxenites. Trace element data indicate that the gabbronorites have similar compositions to rocks of the upper part of the Monchetundra intrusion, whereas the norites and pyroxenites resemble rocks from the lower to intermediate stratigraphic levels of the Monchepluton, such as in the Nude-Poaz and Sopcha massifs. Sulfide mineralization in the studied targets principally consists of secondary bornite, millerite, and chalcopyrite. In contrast, the primary sulfide assemblage within the layered sequence of the adjacent Monchepluton is characterized by pentlandite, chalcopyrite, and pyrrhotite. Therefore, the mineralization in the studied targets is interpreted to be of a contact style. We argue that the studied area represents the contact zone between gabbronorites of the Monchetundra intrusion and norites and pyroxenites of the Monchepluton. In addition, the rocks were overprinted by postmagmatic veining and remobilization of contact style sulfide and PGE mineralization.

Geochemistry and tectonic implications of the Early Carboniferous Keketuobie intrusion in the West Junggar foldbelt, NW China

NASA Astrophysics Data System (ADS)

Deng, Yu-Feng; Yuan, Feng; Zhou, Taofa; Hollings, Pete; Zhang, Dayu

2018-06-01

The Keketuobie intrusion is situated in the northern part of the West Junggar foldbelt at the southern margin of the Central Asian Orogeic Belt. The intrusion consists of medium- to coarse-grained gabbro, fine-grained gabbro and diorite. Igneous zircons from the medium- to coarse-grained gabbro yielded a LA-ICP-MS U-Pb age of 320.8 ± 5.7 Ma, indicating that the intrusion was emplaced in the Early Carboniferous. The intrusive contact between the medium- to coarse-grained gabbro and the fine-grained gabbro indicates they formed from distinct magma pulses. Magnetite crystals from the fine-grained gabbro have lower V2O3 but higher TiO2 and Al2O3 contents than those of the medium- to coarse-grained gabbro, suggesting that the fine-grained gabbro crystallized in a relatively higher fO2 and temperature magma than the medium- to coarse-grained gabbro. The Keketuobie intrusive rocks are characterized by enriched large ion lithophile elements and depleted high field strength elements relative to N-MORB with restricted (87Sr/86Sr)t ratios (0.70370-0.70400) and εNd(t) values (+5.85 to +6.97). The petrography and geochemistry are comparable to those of subduction-related volcanic rocks. The trace elements and isotopic compositions of the mafic intrusive rocks suggest that the primary magmas were derived from mixing of metasomatized lithospheric mantle and depleted asthenospheric melts, perhaps triggered by slab break-off. The Keketuobie intrusion is younger than adjacent ophiolite sequences, island arc volcanic rocks and porphyry deposits, but predates the post-collisional A-type granites and bimodal volcanic rocks in the district, suggesting that the Keketuobie intrusion likely formed in a syn-collisional setting.

Protótipo do primeiro interferômetro brasileiro - BDA

NASA Astrophysics Data System (ADS)

Cecatto, J. R.; Fernandes, F. C. R.; Neri, J. A. C. F.; Bethi, N.; Felipini, N. S.; Madsen, F. R. H.; Andrade, M. C.; Soares, A. C.; Alonso, E. M. B., Sawant, H. S.

2004-04-01

A interferometria é uma poderosa ferramenta usada para investigar estruturas espaciais de fontes astrofísicas fornecendo uma riqueza de detalhes inatingível pelas técnicas convencionais de imageamento. Em particular, a interferometria com ondas de rádio abre o horizonte de conhecimento do Universo nesta ampla banda do espectro eletromagnético, que vai de cerca de 20 kHz até centenas de GHz já próximo ao infravermelho, e que está acessível a partir de instrumentos instalados em solo. Neste trabalho, apresentamos o interferômetro designado por Arranjo Decimétrico Brasileiro (BDA). Trata-se do primeiro interferômetro a ser desenvolvido no Brasil e América Latina que já está em operação na fase de protótipo. Apresentamos o desenvolvimento realizado até o momento, o sítio de instalação do instrumento, o protótipo e os principais resultados dos testes de sua operação, as perspectivas futuras e a ciência a ser desenvolvida com o instrumento nas fases II e III. Neste trabalho é dada ênfase ao desenvolvimento, testes de operação e principais resultados do protótipo. É discutida brevemente a ciência que pode ser feita com o instrumento. Tanto os detalhes técnicos quanto os principais parâmetros estimados para o instrumento nas próximas fases de desenvolvimento e o desempenho do protótipo serão publicados em breve.

Origin of Fe-Ti Oxide Mineralization in the Middle Paleoproterozoic Elet'ozero Syenite-Gabbro Intrusive Complex (Northern Karelia, Russia)

NASA Astrophysics Data System (ADS)

Sharkov, E. V.; Chistyakov, A. V.; Shchiptsov, V. V.; Bogina, M. M.; Frolov, P. V.

2018-03-01

Magmatic oxide mineralization widely developed in syenite-gabbro intrusive complexes is an important Fe and Ti resource. However, its origin is hotly debatable. Some researchers believe that the oxide ores were formed through precipitation of dense Ti-magnetite in an initial ferrogabbroic magma (Bai et al., 2012), whereas others consider them as a product of immiscible splitting of Fe-rich liquid during crystallization of Fe-Ti basaltic magma (Zhou et al., 2013). We consider this problem with a study of the Middle Paleoproterozoic (2086 ± 30 Ma) Elet'ozero Ti-bearing layered intrusive complex in northern Karelia (Baltic Shield). The first ore-bearing phase of the complex is mainly made up of diverse ferrogabbros, with subordinate clinopyroxenites and peridotites. Fe-Ti oxides (magnetite, Ti-magnetite, and ilmenite) usually account for 10-15 vol %, reaching 30-70% in ore varieties. The second intrusive phase is formed by alkaline and nepheline syenites. Petrographical, mineralogical, and geochemical data indicate that the first phase of the intrusion was derived from a moderately alkaline Fe-Ti basaltic melt, while the parental melt of the second phase was close in composition to alkaline trachyte. The orebodies comprise disseminated and massive ores. The disseminated Fe-Ti oxide ores make up lenses and layers conformable to general layering. Massive ores occur in subordinate amounts as layers and lenses, as well as cross-cutting veins. Elevated Nb and Ta contents in Fe-Ti oxides makes it possible to consider them complex ores. It is shown that the Fe-Ti oxide mineralization is related to the formation of a residual (Fe,Ti)-rich liquid, which lasted for the entire solidification history of the first intrusive phase. The liquid originated through multiple enrichment of Fe and Ti in the crystallization zone of the intrusion owing to the following processes: (1) precipitation of silicate minerals in the crystallization zone with a corresponding increase in the Fe and

Mechanical Properties of Shock-Damaged Rocks

NASA Technical Reports Server (NTRS)

He, Hongliang; Ahrens, T. J.

1994-01-01

Stress-strain tests were performed both on shock-damaged gabbro and limestone. The effective Young's modulus decreases with increasing initial damage parameter value, and an apparent work-softening process occurs prior to failure. To further characterize shock-induced microcracks, the longitudinal elastic wave velocity behavior of shock-damaged gabbro in the direction of compression up to failure was measured using an acoustic transmission technique under uniaxial loading. A dramatic increase in velocity was observed for the static compressive stress range of 0-50 MPa. Above that stress range, the velocity behavior of lightly damaged (D(sub 0) less than 0.1) gabbro is almost equal to unshocked gabbro. The failure strength of heavily-damaged (D(sub 0) greater than 0.1) gabbro is approx. 100-150 MPa, much lower than that of lightly damaged and unshocked gabbros (approx. 230-260 MPa). Following Nur's theory, the crack shape distribution was analyzed. The shock-induced cracks in gabbro appear to be largely thin penny-shaped cracks with c/a values below 5 x 10(exp -4). Moreover, the applicability of Ashby and Sammis's theory relating failure strength and damage parameter of shock-damaged rocks was examined and was found to yield a good estimate of the relation of shock-induced deficit in elastic modulus with the deficit in compressive strength.

Seawater Circulation and Thermal Sink at OCEAN Ridges - FIELD Evidence in Oman Ophiolite

NASA Astrophysics Data System (ADS)

Nicolas, A. A.; Boudier, F. I.; Cathles, L. M.; Buck, W. R.; Celerier, B. P.

2014-12-01

Exceptionally, the lowermost gabbros in the Oman ophiolite are black and totally fresh, except for minute traces of impregnation by seawater fluids at very high temperature (~1000°C). These black gabbros sharply contrast with normal, whitish gabbros altered down to Low-T~500-350°C. These hydrous alterations are ascribed to an unconventional model of seawater circulation and cooling of the permanent magma chambers of fast spreading ocean ridges. In this model, gabbros issued from the magma chamber cross a ~100 m thick thermal boundary layer (TBL) before reaching a narrow, Low-T high permeability channel where the heated return seawater is flowing towards black smokers and the local gabbros are altered. Uprising mantle diapirs in Oman diverge at ~5 km on each side of the palaeo-ridge axis and feed an overlying magma chamber that closes at this distance from axis. Preservation of black gabbros along the Moho implies that the loop of seawater alteration locally does not reach Moho beyond this ~5km distance (otherwise black gabbros would be altered in whitish gabbros). This defines an internal "thermal sink" within ~5 km to the ridge axis. There, the sink is efficiently cooled by the active hydrothermal convection that is ridge transverse. This has been documented near the Galapagos ridge by marine geophysical data, within the same distance. Beyond this critical distance, the cooling system becomes dominantly conductive and ridge-parallel. The TBL and attached return flow channels must be rising into the overcooled, accreted crust. Beyond the thermal sink, the 500°C isotherm rebounds into the crust. It is only after ~ 1My of crustal drift that this isotherm penetrates into the uppermost mantle in a sustained fashion, developing serpentinites at the expense of peridotites.

Petrology of gabbroic xenoliths in 1960 Kilauea basalt: crystalline remnants of prior (1955) magmatism

USGS Publications Warehouse

Fodor, R.V.; Moore, R.B.

1994-01-01

The 1960 Kapoho lavas of Kilauea's east rift zone contain 1-10 cm xenoliths of olivine gabbro, olivine gabbro-norite, and gabbro norite. Textures are poikilitic (ol+sp+cpx in pl) and intergranular (cpx+pl??ol??opx). Poikilitic xenoliths, which we interpret as cumulates, have the most primitive mineral compositions, Fo82.5, cpx Mg# 86.5, and An80.5. Many granular xenoliths (ol and noritic gabbro) contain abundant vesicular glass that gives them intersertal, hyaloophitic, and overall 'open' textures to suggest that they represent 'mush' and 'crust' of a magma crystallization environment. Their phase compositions are more evolved (Fo80-70, cpx Mg# 82-75, and An73-63) than those of the poikilitic xenoliths. Associated glass is basaltic, but evolved (MgO 5 wt%; TiO2 3.7-5.8 wt%). The gabbroic xenolith mineral compositions fit existing fractional crystallization models that relate the origins of various Kilauea lavas to one another. FeO/MgO crystal-liquid partitioning is consistent with the poikilitic ol-gabbro assemblage forming as a crystallization product from Kilauea summit magma with ???8 wt% MgO that was parental to evolved lavas on the east rift zone. For example, least squares calculations link summit magmas to early 1955 rift-zone lavas (???5 wt% MgO) through ???28-34% crystallization of the ol+sp+cpx+pl that comprise the poikilitic ol-gabbros. The other ol-gabbro assemblages and the olivine gabbro-norite assemblages crystallized from evolved liquids, such as represented by the early 1955 and late 1955 lavas (???6.5 wt% MgO) of the east rift zone. The eruption of 1960 Kapoho magmas, then, scoured the rift-zone reservoir system to entrain portions of cumulate and solidification zones that had coated reservoir margins during crystallization of prior east rift-zone magmas. ?? 1994 Springer-Verlag.

Successive reactive liquid flow episodes in a layered intrusion (Unit 9, Rum Eastern Layered Intrusion, Scotland)

NASA Astrophysics Data System (ADS)

Leuthold, Julien; Blundy, Jon; Holness, Marian

2014-05-01

We will present a detailed microstructural and geochemical study of reactive liquid flow in Unit 9 of the Rum Eastern Layered Intrusion. In the study region, Unit 9 comprises an underlying lens-like body of peridotite overlain by a sequence of troctolite and gabbro (termed allivalite), with some local and minor anorthosite. The troctolite is separated from the overlying gabbro by a distinct, sub-horizontal, undulose horizon (the major wavy horizon). Higher in the stratigraphy is another, similar, horizon (the minor wavy horizon) that separates relatively clinopyroxene-poor gabbro from an overlying gabbro. To the north of the peridotite lens, both troctolite and gabbro grade into poikilitic gabbro. Clinopyroxene habit in the allivalite varies from thin rims around olivine in troctolite, to equigranular crystals in gabbro, to oikocrysts in the poikilitic gabbro. The poikilitic gabbros contain multiple generations of clinopyroxene, with Cr-rich (~1.1 wt.% Cr2O3), anhedral cores with moderate REE concentrations (core1) overgrown by an anhedral REE-depleted second generation with moderate Cr (~0.7 wt.% Cr2O3) (core2). These composite cores are rimmed by Cr-poor (~0.2 wt.% Cr2O3) and REE-poor to moderate clinopyroxene. We interpret these microstructures as a consequence of two separate episodes of partial melting triggered by the intrusion of hot olivine-phyric picrite to form the discontinuous lenses that comprise the Unit 9 peridotite. Loss of clinopyroxene-saturated partial melt from the lower part of the allivalite immediately following the early stages of sill intrusion resulted in the formation of clinopyroxene-poor gabbro. The spatial extent of clinopyroxene loss is marked by the minor wavy horizon. A further partial melting event stripped out almost all clinopyroxene from the lowest allivalite, to form a troctolite, with the major wavy horizon marking the extent of melting during this second episode. The poikilitic gabbro formed from clinopyroxene-saturated melt

A Crystallization-Temperature Profile Through Paleo-Oceanic Crust (Wadi Gideah Transect, Oman Ophiolite): Application of the REE-in-Plagioclase-Clinopyroxene Partitioning Thermometer

NASA Astrophysics Data System (ADS)

Mueller, S.; Hasenclever, J.; Garbe-Schönberg, D.; Koepke, J.; Hoernle, K.

2017-12-01

The accretion mechanisms forming oceanic crust at fast spreading ridges are still under controversial discussion. Thermal, petrological, and geochemical observations predict different end-member models, i.e., the gabbro glacier and the sheeted sill model. They all bear implications for heat transport, temperature distribution, mode of crystallization and hydrothermal heat removal over crustal depth. In a typical MOR setting, temperature is the key factor driving partitioning of incompatible elements during crystallization. LA-ICP-MS data for co-genetic plagioclase and clinopyroxene in gabbros along a transect through the plutonic section of paleo-oceanic crust (Wadi Gideah Transect, Oman ophiolite) reveal that REE partitioning coefficients are relatively constant in the layered gabbro section but increase for the overlying foliated gabbros, with an enhanced offset towards HREEs. Along with a systematic enrichment of REE's with crustal height, these trends are consistent with a system dominated by in-situ crystallization for the lower gabbros and a change in crystallization mode for the upper gabbros. Sun and Liang (2017) used experimental REE partitioning data for calibrating a new REE-in-plagioclase-clinopyroxene thermometer that we used here for establishing the first crystallization-temperature depth profile through oceanic crust that facilitates a direct comparison with thermal models of crustal accretion. Our results indicate crystallization temperatures of about 1220±8°C for the layered gabbros and lower temperatures of 1175±8°C for the foliated gabbros and a thermal minimum above the layered-to-foliated gabbro transition. Our findings are consistent with a hybrid accretion model for the oceanic crust. The thermal minimum is assumed to represent a zone where the descending crystal mushes originating from the axial melt lens meet with mushes that have crystallized in situ. These results can be used to verify and test thermal models (e.g., Maclennan et al

Igneous stratigraphy and rock-types from a deep transect of the gabbroic lower crust of the Atlantis Bank core complex (SW Indian Ridge): preliminary results from IODP Expedition 360

NASA Astrophysics Data System (ADS)

Sanfilippo, A.; France, L.; Ghosh, B.; Liu, C. Z.; Morishita, T.; Natland, J. H.; Dick, H. J.; MacLeod, C. J.; Expedition 360 Scientists, I.

2016-12-01

International Ocean Discovery Program (IODP) Expedition 360 represents the first leg of a multi-phase drilling programme ('SloMo' project) aimed at investigating the nature of the lower crust and Moho at slow spreading ridges. As an initial phase of the SloMo project, IODP Exp. 360 intended to recover a representative transect of the lower oceanic crust formed at Atlantis Bank, an oceanic core complex on the SW Indian Ridge. During this expedition, 89 cores of gabbroic rocks were recovered at Hole U1473A, drilled to 789.7 m below seafloor. This hole was subsequently deepened to 809.4 mbsf during transit Expedition 362T, which recovered additional 7 cores. The gabbroic section recovered at Hole U1473A consists of several types of gabbro, diabase, and felsic veins. The main lithology is dominated by olivine gabbro (76.5% in abundance), followed by gabbro containing 1-2% oxide (9.5%), gabbro with >2% oxide (7.4%), gabbro sensu stricto (5.1%), felsic veins (1.5%) and diabase (<0.5%). The different lithologies appear randomly distributed throughout the section, although oxide abundance seems to decrease slightly downhole, except for the lowermost intervals where oxide gabbros are more abundant. Based on changes in rock types, grain size, texture, and the occurrence of felsic material, we identified eight lithologic units, which locally define separate geochemical trends. Each unit is characterized by meter-scale heterogeneity which classically characterizes gabbros formed at slow spreading ridges. Reaction textures in olivine gabbros, crosscutting relationships between oxide gabbros and host rocks, the presence of intrusive to sutured contacts, igneous layering and the widespread occurrence of felsic veins and segregations indicate that the evolution of this section was controlled by complicated interactions of magmatic processes, e.g., fractional crystallization, melt-rock reaction, late-stage melt migration, which were active in a crystal mush formed by multiple

Frictional processes of bimaterial interfaces at seismic slip rates.

NASA Astrophysics Data System (ADS)

Passelegue, F. X.; Fabbri, O.; Leclère, H.; Spagnuolo, E.; Di Toro, G.

2017-12-01

Large subduction earthquakes ruptures propagate from crustal rock toward the sea floor along frictional interfaces of different lythologies. Up to now, frictional processes of rocks were mainly investigated along single material experimental faults. Here, we present the results of high velocity friction experiments coupled with high frequency acoustic monitoring system on biomaterial interfaces including gabbro, pyroxenite and serpentinized peridotite (>95%), following a recent field investigation highlighting bimaterial contacts in the Corsica ophiolitic nappe. We first studied the frictional processes of single materials which result in a mechanical behaviour comparable to previous studies. Both gabbro and pyroxenite exhibit two weakening stages. The first one corresponds to flash heating and the second stage occurs concomitantly with complete melting of the interface. In the case of serpentinite, only one weakening stage is observed, after a weakening slip distance of only few centimeters. We then conducted bimaterial experiments. The two couples tested were gabbro/pyroxenite and gabbro/serpentinite, as observed along natural fault zones (Corsica, France). In the case of gabbro/serpentinite, we observe that frictional processes are controlled by serpentinite. Mechanical curves replicate the behaviour of single serpentinite friction experiments. We observe that few melting occurs, and that the product of experiments consists in fine grained cataclasite, as observed in the field. The case of gabbro/pyroxenite is more complicated. The first weakening is controlled by the lithology of the sample installed on the static part of the rotary apparatus. However, the second weakening is controlled by the gabbro and mechanical curves are identical than those obtained in the case of single gabbro experiments. Supported by microstructural analysis and acoustic activity, our results suggest that frictional processes of bimaterial interfaces are controlled by the material

Geochemistry and mineralogy of Pd in the magnetitite layer within the upper gabbro of the Mesoarchean Nuasahi Massif (Orissa, India)

NASA Astrophysics Data System (ADS)

Prichard, Hazel M.; Mondal, Sisir K.; Mukherjee, Ria; Fisher, Peter C.; Giles, Nicolas

2018-04-01

layers of chromitite. PGE are concentrated in these chromitites and in the base metal sulfide-bearing breccias in the overlying gabbro. The Pd in the magnetitites described here indicates the presence of a third level where PGE are concentrated and a magma that crystallized to produce PGE concentrations at three stratigraphic levels in the massif. This indicates that similar thin sill-like intrusions, hosting unusually thick chromitites, may also have PGE concentrations at a number of stratigraphic levels.

Reaction of seawater with fresh mid-ocean ridge gabbro creates ';atypical' REE pattern and high REE fluid fluxes: Experiments at 425 and 475 °C, 400 and 1000 bar

NASA Astrophysics Data System (ADS)

Beermann, O.; Garbe-Schönberg, D.; Holzheid, A. D.

2013-12-01

High-temperature MOR hydrothermalism significantly affects ocean chemistry. The Sisters Peak (SP) hydrothermal field at 5°S on the slow-spreading Mid-Atlantic Ridge (MAR) emanates fluids >400°C [1] that have high concentrations of H2, transition metals, and rare earth elements (REE) exhibiting ';atypical' REE pattern characterized by depletions of LREE and HREE relative to MREE and no Eu anomaly [2]. This is in contrast to the ';typical' LREE enrichment and strong positive Eu anomaly known from many MOR vent fluids observed world-wide [e.g., 3]. Besides temperature, the seawater-to-rock ratio (w/r ratio) has significant control on the fluid chemistry [e.g., 4, 5]. To understand how vent fluid REE-signatures are generated during water-rock interaction processes we reacted unaltered gabbro with natural bottom seawater at 425 °C and 400 bar and at 425 and 475 °C at 1000 bar at variable w/r (mass) ratios ranging from 0.5-10 by using cold seal pressure vessels (CSPV). The run durations varied from 3-72 h. Reacted fluids were analysed for major and trace elements by ICP-OES and ICP-MS. In our experiments, ';atypical' REE fluid pattern similar to those of SP fluids were obtained at high w/r ratio (5 and 10) that might be characteristic for focused fluid-flow along e.g., detachment faults at slow-spreading MOR [6]. In contrast, more ';typical'-like REE pattern with elevated LREE and slightly positive Eu anomalies have been reproduced at low w/r ratio (0.5-1). Results of numerical simulations imply that strong positive Eu anomalies of fluids and altered gabbro from high temperature MOR hydrothermal systems can be created by intense rock leaching processes at high w/r ratio (5-10). This suggests that hydrothermal circulation through the ocean crust creates ';typical' REE fluid pattern with strong positive Eu anomalies if seawater reacts with gabbroic host rock that has been already leached in REE at high fluid fluxes. Simulations of the temporal chemical evolution of

A deep hydrothermal fault zone in the lower oceanic crust, Samail ophiolite Oman

NASA Astrophysics Data System (ADS)

Zihlmann, B.; Mueller, S.; Koepke, J.; Teagle, D. A. H.

2017-12-01

Hydrothermal circulation is a key process for the exchange of chemical elements between the oceans and the solid Earth and for the extraction of heat from newly accreted crust at mid-ocean ridges. However, due to a dearth of samples from intact oceanic crust, or continuous samples from ophiolites, there remain major short comings in our understanding of hydrothermal circulation in the oceanic crust, especially in the deeper parts. In particular, it is unknown whether fluid recharge and discharge occurs pervasively or if it is mainly channeled within discrete zones such as faults. Here, we present a description of a hydrothermal fault zone that crops out in Wadi Gideah in the layered gabbro section of the Samail ophiolite of Oman. Field observations reveal a one meter thick chlorite-epidote normal fault with disseminated pyrite and chalcopyrite and heavily altered gabbro clasts at its core. In both, the hanging and the footwall the gabbro is altered and abundantly veined with amphibole, epidote, prehnite and zeolite. Whole rock mass balance calculations show enrichments in Fe, Mn, Sc, V, Co, Cu, Rb, Zr, Nb, Th and U and depletions of Si, Ca, Na, Cr, Zn, Sr, Ba and Pb concentrations in the fault rock compared to fresh layered gabbros. Gabbro clasts within the fault zone as well as altered rock from the hanging wall show enrichments in Na, Sc, V, Co, Rb, Zr, Nb and depletion of Cr, Ni, Cu, Zn, Sr and Pb. Strontium isotope whole rock data of the fault rock yield 87Sr/86Sr ratios of 0.7046, which is considerably more radiogenic than fresh layered gabbro from this locality (87Sr/86Sr = 0.7030 - 0.7034), and similar to black smoker hydrothermal signatures based on epidote, measured elsewhere in the ophiolite. Altered gabbro clasts within the fault zone show similar values with 87Sr/86Sr ratios of 0.7045 - 0.7050, whereas hanging wall and foot wall display values only slightly more radiogenic than fresh layered gabbro.The secondary mineral assemblages and strontium isotope

Two types of gabbroic xenoliths from rhyolite dominated Niijima volcano, northern part of Izu-Bonin arc: petrological and geochemical constraints

NASA Astrophysics Data System (ADS)

Arakawa, Yoji; Endo, Daisuke; Ikehata, Kei; Oshika, Junya; Shinmura, Taro; Mori, Yasushi

2017-03-01

We examined the petrography, petrology, and geochemistry of two types of gabbroic xenoliths (A- and B-type xenoliths) in olivine basalt and biotite rhyolite units among the dominantly rhyolitic rocks in Niijima volcano, northern Izu-Bonin volcanic arc, central Japan. A-type gabbroic xenoliths consisting of plagioclase, clinopyroxene, and orthopyroxene with an adcumulate texture were found in both olivine basalt and biotite rhyolite units, and B-type gabbroic xenoliths consisting of plagioclase and amphibole with an orthocumulate texture were found only in biotite rhyolite units. Geothermal- and barometricmodelling based on mineral chemistry indicated that the A-type gabbro formed at higher temperatures (899-955°C) and pressures (3.6-5.9 kbar) than the B-type gabbro (687-824°C and 0.8-3.6 kbar). These findings and whole-rock chemistry suggest different parental magmas for the two types of gabbro. The A-type gabbro was likely formed from basaltic magma, whereas the B-type gabbro was likely formed from an intermediate (andesitic) magma. The gabbroic xenoliths in erupted products at Niijima volcano indicate the presence of mafic to intermediate cumulate bodies of different origins at relatively shallower levels beneath the dominantly rhyolitic volcano.

A Reference Section through the Lower Fast-spreading Oceanic Crust in the Wadi Gideah (Sumail ophiolite, Sultanate Oman): Drill Sites GT1A and GT2A within the ICDP Oman Drilling Project

NASA Astrophysics Data System (ADS)

Mueller, S.; Koepke, J.; Garbe-Schoenberg, C. D.; Müller, T.; Mock, D.; Strauss, H.; Schuth, S.; Ildefonse, B.

2017-12-01

In the absence of a complete profile through fast-spreading oceanic crust in modern oceans, we established a reference profile through the whole paleocrust of the Sumail Ophiolite (Oman), which is regarded as the best analogue for fast-spreading oceanic crust on land. For establishing a coherent data set, we sampled the Wadi Gideah in the Wadi-Tayin massif from the mantle section up to the pillow basalts and performed different analytical and structural investigations on the same suite of samples (pool sample concept). The whole sample set contains about 400 samples focusing on both primary magmatic rocks and hydrothermal fault zones to characterize initial formation processes and cooling of the crust. The Wadi Gideah hosts the sites GT1A (lower crust) and GT2A (foliated / layered gabbro transition) where 400 m long cores have been drilled in the frame of the ICDP Oman Drilling Project (OmanDP). Thus, the Wadi Gideah crustal transect is well-suited for providing a reference frame for these two drill cores. Major and trace element data on minerals and rocks reveal in-situ crystallization in the deep crust, thus strongly supporting a hybrid accretion model that is characterized by sheeted sill intrusion in the lower part of the plutonic crust and gabbro glacier features in the upper section. This hybrid model is also supported by results on crystallographic preferred orientations (CPO) of the minerals within the gabbros, which call for distinct formation mechanisms in the upper and lower gabbro sections. A requirement for our hybrid model is significant hydrothermal cooling in the lower crust for the consumption of the latent heat of crystallization. This was facilitated by channelled hydrothermal flow zones, preserved today in faulted zones of extensively altered gabbro cutting both layered and foliated gabbros. These gabbros show higher Sr87/Sr86 ratios if compared to the background gabbro, the presence of late stage minerals (amphibole, oxides, orthopyroxene

Different slip systems controlling crystallographic preferred orientation and intracrystalline deformation of amphibole in mylonites from the Neyriz mantle diapir, Iran

NASA Astrophysics Data System (ADS)

Elyaszadeh, Ramin; Prior, David J.; Sarkarinejad, Khalil; Mansouri, Hadiseh

2018-02-01

A deformed layered gabbro and a mylonitic gabbro sample from the marginal shear zone of the Neyriz mantle diapir in Iran were analyzed using electron backscatter diffraction (EBSD). Both samples have the common amphibole crystallographic preferred orientation (CPO) in which (100) lies perpendicular to foliation and <001> parallel to lineation. Amphibole grains in the layered gabbro sample have little internal deformation, whereas in the mylonitic gabbro sample the amphibole grains are strongly distorted and contain low angle grain boundaries. There is a subtle change in CPO as a function of grain size in the mylonitic gabbro. Coarse grains (porphyroclasts) have a (100) <001> CPO oriented with the main foliation reference frame whilst fine grains have a (100) <001> CPO oriented with the C‧ shear bands. Detailed analysis of porphyroclast distortions and subgrain boundary trace analysis suggests that hard slip systems, most particularly (110) <1-10> control intracrystalline deformation. Schmid factor analysis suggest that these slip systems are not involved in foliation formation but are linked kinematically to C‧ shear bands. It is unlikely that the slip systems that control intracrystalline deformation are important in CPO formation. We interpret that subgrain rotation recrystallization lead to grain size reduction and the elongate recrystallized grains were rotated towards the C‧ shear bands by grain boundary sliding. This rigid body rotation, possibly in combination with easy slip on (100) <001> are considered the main cause of CPO formation. Amphibole zonation patterns in the layered gabbro sample suggest that oriented growth of amphibole may have contributed to CPO.

Dating the growth of oceanic crust at a slow-spreading ridge

USGS Publications Warehouse

Schwartz, J.J.; John, Barbara E.; Cheadle, Michael J.; Miranda, E.A.; Grimes, Craig B.; Wooden, J.L.; Dick, H.J.B.

2005-01-01

Nineteen uranium-lead zircon ages of lower crustal gabbros from Atlantis Bank, Southwest Indian Ridge, constrain the growth and construction of oceanic crust at this slow-spreading midocean ridge. Approximately 75% of the gabbros accreted within error of the predicted seafloor magnetic age, whereas ???25% are significantly older. These anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or, more likely, crystallization of older gabbros at depths of ???5 to 18 kilometers below the base of crust in the cold, axial lithosphere, which were uplifted and intruded by shallow-level magmas during the creation of Atlantis Bank.

Petrology of gabbroic rocks from the Mid-Cayman rise spreading center

NASA Technical Reports Server (NTRS)

Elthon, Don

1987-01-01

Mineral analyses of oxide and silicate phases from a suite (collected with the DSRV Alvin in January 1976 and July 1977) of 48 gabbroic rocks collected from the vicinity of the Mid-Cayman Rise spreading center are reported. Mineral compositions of these anorthosites, leuco-troctolite, leuco-olivine gabbros, olivine gabbros, leuco-gabbros, and gabbros indicate the cumulate rocks have been produced by the crystal fractionation of basaltic liquids. Certain features of these rocks are inconsistent with the occurrence of this fractionation at low pressures (1 atm to 2 kbar). Although the experimental data are not available to conclusively demonstrate that the effects seen were produced at moderate pressures (5-10 kbar), the effects are similar to those predicted thermodynamically and to those observed in limited experimental studies within this pressure range. It is therefore suggested that the most likely scenario for the production of these gabbroic rocks is the moderate-pressure crystallization of basaltic magmas within deep-seated magma chambers underneath this slow-spreading center.

Geochronology and geochemistry of the Niujuanzi ophiolitic mélange, Gansu Province, NW China: implications for tectonic evolution of the Beishan Orogenic Collage

NASA Astrophysics Data System (ADS)

Wang, Shengdong; Zhang, Kexin; Song, Bowen; Li, Shucai; Li, Ming; Zhou, Jie

2018-01-01

The Niujuanzi ophiolitic mélange (NOM), located in the Beishan Orogenic Collage, marks the termination between the Huaniushan arc and Mingshui-Hanshan Massifs. The NOM is mainly composed of gabbros, diabases, plagiogranites, basalts, and greywacke. Two gabbros have ages of 433.8 ± 3.1 and 354.0 ± 3.3 Ma, two plagiogranites have ages of 429.8 ± 2 and 448.7 ± 2.0 Ma, and a diabase has an age of 433.4 ± 3.2 Ma. The gabbros and diabases are calc-alkaline and tholeiitic, with high Al2O3, CaO, and TiO2 contents and low FeOT contents. The gabbros have high Mg# values (49-82), while the diabases have relatively low Mg# values (46-61). The plagiogranites are calc-alkaline and metaluminous, with high SiO2 and Na2O contents and low Al2O3 and K2O contents. The gabbros and diabases are enriched in large iron lithophile elements and slightly depleted in high field strength elements relative to N-MORB and their trace element characteristics are similar to E-MORB. With respect to rare earth element (REE), they have slightly enriched LREEs relative to HREEs. The majority of the plagiogranite trace elements approximate those of the volcanic arc granite. The plagiogranites have obviously enriched LREEs relative to HREEs, with a slightly to strongly negative Eu anomaly, which is similar to ORG but distinct from volcanic arc and within plate granite. The NOM was formed from the Ordovician to the Carboniferous, representing the expansion period of the Niujuanzi Ocean. The gabbros, diabases, and plagiogranites were formed in a mid-ocean ridge environment. The gabbros and diabases were generated by different degrees of partial melting of the mantle, and the plagiogranites derived from both the crystallization differentiation of basaltic magma and the partial melting of amphibolites in the crust.

Towards Solving the Conundrum of Fast-Spread Ocean Crust Formation: Insights from Textural Analysis of Gabbroic Rocks from Pito Deep and Hess Deep, East Pacific Rise

NASA Astrophysics Data System (ADS)

Brown, T. C.; Cheadle, M. J.; John, B. E.; Coogan, L. A.; Gee, J. S.; Karson, J. A.; Meyer, R.; Ceuleneer, G.; Swapp, S.

2014-12-01

Few examples of in situ fast-spread lower ocean crust exist for sampling. Here we present detailed textural analyses of two sample sets that formed at the East Pacific Rise, collected from tectonic windows at Pito (PD) and Hess (HD) deeps. PD samples (collected by ROV) span the upper ~900 m of lower crust. HD samples (collected by seafloor drilling during IODP Exp. 345) come from >1500 m below the sheeted dike gabbro transition (mbsd). PD gabbroic rock textures are consistent with a gabbro glacier flow model generating the uppermost plutonic crust. Shallow samples (41-72 mbsd) likely formed at the distal edge of the magma lens, analogous to similar rocks from Oman. These gabbros are relatively evolved (cpx Mg#75-77, An53-61 and 1-4% Fe-Ti oxides), and have elongate plagioclase grains (aspect ratios up to 1:2:10) exhibiting a strong shape preferred orientation (SPO) with <40% of grains showing dislocation creep textures. Deeper samples (177-876 mbsd) likely began crystallizing in the magma lens then subsided and 'flowed' through the underlying mush zone. These gabbros are more primitive below 386 mbsd (Fo83-88, cpx Mg# 85-89 and An70-82), and plagioclase grains have more equilibrated morphologies (aspect ratios < 1:2:6) that define ~vertical SPOs which increase in strength with depth. Plagioclase exhibits magmatic crystal-lattice preferred orientations (CPOs) which are also vertical. Significantly, the proportion of grains showing dislocation creep textures increases with depth, and plagioclase grain size distributions show a smaller range of sizes at depth; observations that perhaps reflect the effect of increasing strain with depth. IODP Hole U1415I at HD recovered gabbros and troctolitic gabbros from the mid lower crust that show distinctive cm-dm scale modal layering. Strong plagioclase SPOs parallel layering and magmatic CPOs vary dramatically in strength over just 4.5 m of core. Plagioclase grains are relatively equant (aspect ratios < 1:2:4), wrap around

Geology of the Zambales ophiolite, Luzon, Philippines

USGS Publications Warehouse

Rossman, D.L.; Castanada, G.C.; Bacuta, G.C.

1989-01-01

The Zambales ophiolite of western Luzon, Philippines, exposes a typical succession of basalt flows, diabasic dikes, gabbro and tectonized harzburgite. The age established by limiting strata is late Eocene. Lack of evidence of thrust faulting and the general domal disposition of the lithologie units indicate that the ophiolitic rocks are exposed by uplift. Highly complex internal layered structures within the complex are related to processes developed during formation of the ophiolite and the Zambales ophiolite may be one of the least disturbed (by emplacement) ophiolitic masses known. The exposed mass trends north and the upper surface plunges at low angles (a few degrees) to the north and south. The chemistry and composition of the rocks in the northwest part of the Zambales area (Acoje block) is distinct from that in the southeastern segment (Coto block). The Acoje block, according to Evans (1983) and Hawkins and Evans (1983), resembles (on a chemical basis) arc-tholeiite series rocks from intra-island arcs and the rocks in the Coto block are typical back-arc basin rock series. The present writer believes that the ophiolite composes a single genetic unit and that the changes in composition are the result of changes that took place during the initial formation. The gabbro probably formed below a spreading center in an elongate, in cross section, V-shaped, magma chamber. The gabbro is estimated by the writer to be less than 2 km thick and may be less than 1 km in places. Numerous erosional windows through the gabbro in the northern and eastern side of the Zambales area show that the gabbro remaining in those areas is likely to be only a few hundred meters thick. Harzburgite is exposed to a depth of about 800 m in the Bagsit River area and this may be the deepest part of the ophiolite accessible for study on which there is any control on depth. A transitional zone, about 200 m thick lying between the gabbro and harzburgite, is composed of serpentinized dunite

Spatiotemporal Distribution and Population Structure of Monokalliapseudes schubarti (Tanaidacea: Kalliapseudidae) in an Estuary in Southern Brazil

PubMed Central

Freitas-Júnior, Felipe; Christoffersen, Martin Lindsey; de Araújo, Joafrâncio Pereira; Branco, Joaquim Olinto

2013-01-01

Monokalliapseudes schubarti is an endemic tanaidacean microcrustacean from southeastern Brazil to Uruguay inhabiting low energy estuaries. Saco da Fazenda is located in the estuary of the Itajaí-Açú River, state of Santa Catarina, Brazil. It is exposed to strong anthropic impact and receives intensive flows of domestic wastewater, solid residues, and drainage activities. Specimens of M. schubarti were collected monthly, in the intertidal and subtidal regions of Saco da Fazenda, in four stations defined as a function of the physiography of the environment during the period of July 2003 to June 2004. Fecundity values were high, with continuous reproductive activity during the whole period of study. The greatest population densities were observed in the intertidal region, where they are nevertheless intensely consumed by birds, swimming crabs, and fish. This species represents a fundamental link in the food chain of Saco da Fazenda, transferring energy from the detritus level to higher trophic levels. Habitat disturbance and high organic matter may represent factors controlling the distribution of populations of M. schubarti. For this reason, the species may be used to monitor anthropic effects in estuarine areas. PMID:24298211

The fossil hydrothermal rootzone from the Northern Apennine ophiolites (Italy)

NASA Astrophysics Data System (ADS)

Tribuzio, R.; Zanetti, A.; Dallai, L.

2003-04-01

The Northern Apennine ophiolites are lithosphere remnants of the Late Jurassic -- Early Cretaceous Ligurian Tethys, which is considered to have developed in conjunction with the opening of the Central Atlantic Ocean. In the Bonassola area, a km-scale gabbroic body permits the study of the magmatic-hydrothermal transition. The body mostly consists of coarse-grained gabbros of cumulus origin that exhibit granulite-facies recrystallization along ductile shear zones, which most likely occurred in the absence of seawater-derived fluids. These shear zones are crosscut at high angle by parallel swarms of hornblende (± plagioclase) veins. The development of these veins is correlated with coronal hornblende growth at the expenses of igneous clinopyroxene in the host gabbro. Scattered, elongated bodies of hornblende-bearing albitites also crop out. In particular, two different generations of albitite bodies have been recognized. The albitite (1) bodies show irregular contacts against the host gabbro, which are characterized by hornblende-rich reaction zones. These albitites are inferred to have developed when the gabbro was not completely solidified. The albitite (2) bodies has sharp contacts, post-date the granulite-facies foliation in the host gabbros, and show the same elongation direction of hornblende veins. The albitite (2) bodies are therefore related to the same brittle deformation event that gave rise to the hornblende veins. Major, trace, halogen and oxygen isotope analyses of hornblende from both veins and albitite bodies have been carried out. The geochemical signature of hornblende from albitite (1) bodies and related contact reaction zones is similar to that of accessory titanian pargasite of igneous origin in the host gabbro, thus indicating that these albitites were derived by extreme differentiation of basaltic liquid. Two different chemical fingerprints have been recognized for the vein hornblendes. The first type indicates a formation by local reaction

Oman Drilling Project GT3 site survey: dynamics at the roof of an oceanic magma chamber

NASA Astrophysics Data System (ADS)

France, L.; Nicollet, C.; Debret, B.; Lombard, M.; Berthod, C.; Ildefonse, B.; Koepke, J.

2017-12-01

Oman Drilling Project (OmanDP) aims at bringing new constraints on oceanic crust accretion and evolution by drilling Holes in the whole ophiolite section (mantle and crust). Among those, operations at GT3 in the Sumail massif drilled 400 m to sample the dike - gabbro transition that corresponds to the top (gabbros) and roof (dikes) of the axial magma chamber, an interface where hydrothermal and magmatic system interacts. Previous studies based on oceanic crust formed at present day fast-spreading ridges and preserved in ophiolites have highlighted that this interface is a dynamic horizon where the axial melt lens that top the main magma chamber can intrude, reheat, and partially assimilate previously hydrothermally altered roof rocks. Here we present the preliminary results obtained in GT3 area that have allowed the community to choose the drilling site. We provide a geological and structural map of the area, together with new petrographic and chemical constraints on the dynamics of the dike - gabbro transition. Our new results allow us to quantify the dynamic processes, and to propose that 1/ the intrusive contact of the varitextured gabbro within the dikes highlights the intrusion of the melt lens top in the dike rooting zone, 2/ both dikes and previously crystallized gabbros are reheated, and recrystallized by underlying melt lens dynamics (up to 1050°C, largely above the hydrous solidus temperature of altered dikes and gabbros), 3/ the reheating range can be > 200°C, 4/ the melt lens depth variations for a given ridge position is > 200m, 5/ the reheating stage and associated recrystallization within the dikes occurred under hydrous conditions, 6/ the reheating stage is recorded at the root zone of the sheeted dike complex by one of the highest stable conductive thermal gradient ever recorded on Earth ( 3°C/m), 7/ local chemical variations in recrystallized dikes and gabbros are highlighted and used to quantify crystallization and anatectic processes, and the

Slab break-off and the formation of Permian mafic-ultramafic intrusions in southern margin of Central Asian Orogenic Belt, Xinjiang, NW China

NASA Astrophysics Data System (ADS)

Song, Xie-Yan; Xie, Wei; Deng, Yu-Feng; Crawford, Anthony J.; Zheng, Wen-Qin; Zhou, Guo-Fu; Deng, Gang; Cheng, Song-Lin; Li, Jun

2011-11-01

The Baishiquan and Pobei Early Permian mafic-ultramafic intrusions were emplaced into Proterozoic metamorphic rocks in the Central Tianshan and the Beishan Fold Belt, northern Xinjiang, NW China. The Baishiquan intrusion comprises mainly gabbro, and mela-gabbro sills occurring within and along the margins of the gabbro body. In the Pobei intrusion, two distinct gabbroic packages, a lower gabbro and the main gabbro, are intruded and overlain by small cumulate wehrlite bodies. Both intrusions are characterized by enrichments of large ion lithophile elements and Th and U relative to the high field strength elements, and show strong negative Nb and Ta anomalies and positive K and Pb anomalies, leading to higher Th/Yb and Nb/Yb than in mid-ocean ridge basalt and ocean island basalt. These features are comparable with subduction-related mafic rocks and post-collisional magmas. Geological and geochemical considerations indicate that the parental magmas of the two intrusions were derived from decompression melting of ascending asthenosphere and reacted with overlying subduction-modified lithospheric mantle. We believe that these parental magmas were generated by post-collisional extension along the Chinese Tianshan, perhaps triggered by slab break-off or delamination of thickened lithosphere. Relatively lower (143Nd/144Nd)i and higher (87Sr/86Sr)i than other Permian mafic-ultramafic intrusions in the eastern Chinese Tianshan indicate that the parental magmas of these two intrusions experienced significant contamination by old crustal rocks.

Gabbroic and Peridotitic Enclaves from the 2008 Kasatochi Eruption, Aleutian Islands, Alaska

NASA Astrophysics Data System (ADS)

Kentner, A.; Nadin, E. S.; Izbekov, P. E.; Nye, C. J.; Neill, O. K.

2012-12-01

Kasatochi volcano of the Andreanof Islands in the western Aleutian Arc violently erupted over a two day period from August 7-8, 2008. The eruption involved multiple explosive events generating pyroclastic flows, which included abundant mafic and ultramafic enclaves that have since weathered out and accumulated in talus along the coast. These and other mafic enclaves sampled by modern island arc lavas provide insight into subduction magmatism because they emerge from a section of the subduction system that is less likely than shallower zones to be modified by magmatic processes such as mixing, assimilation, or fractionation. We present new whole rock, clinopyroxene, amphibole, plagioclase, and melt compositions from Kasatochi enclaves of the 2008 eruption. The highly crystalline (~40 vol. % phenocryst content), medium-K basaltic andesite host rock contains ~52-55 wt. % SiO2 and 0.6-0.9 wt. % K2O, and is composed of plagioclase, ortho- and clinopyroxene, amphibole, and Ti-magnetite in a microlite-rich groundmass. Upon eruption, this magma sampled two distinct enclave populations: gabbro and peridotite. The gabbro has abundant amphibole (mostly magnesio-hastingsite) and plagioclase with minor clinopyroxene, olivine, and magnetite, while the peridotite is composed of olivine with minor amounts of clinopyroxene and orthopyroxene. There is little textural variation amongst the peridotitic samples collected, but the gabbroic samples vary from layered to massive and cover a range in grain size from fine-grained to pegmatitic. The layered gabbros display centimeter-scale bands of alternating plagioclase- and amphibole-rich layers, with a strong preferential alignment of the amphibole grains. The coarser-grained samples are very friable, with ~10% pore space; disaggregation of these upon host-magma ascent likely formed the amphibole and plagioclase xenocrysts in the andesitic host. Based on the textural and compositional differences, we divide the enclaves into four groups

Petrology and chemistry of Jebel Tanumah complex, Khamis Mushayt, Southern Arabian shield, Saudi Arabia

NASA Astrophysics Data System (ADS)

Nassief, M. O.; Ali, H. M.; Zakir, F. A.

The mafic intrusive complex at Jebel Tanumah is located 15 km north-west of Khamis Mushayt in the southern Arabian Shield and includes olivine-bearing gabbro as well as amphibole-diopside-hornblende gabbro cumulates. These rocks have been generally metamorphosed to upper greeenschist-lower amphibolite facies. Fourteen white rock silicate analyses indicate that the majority of the rocks are calc-alkaline to tholeiitic in composition. The two major structural units in the Khamis Mushayt region identified by Coleman consist of the basement complex of Asir Mountains and the younger metamorphic rocks. Syntectonic granitic rocks intruded the antiforms characterizing the younger rocks whereas the lower parts of the synforms are intruded by post-tectonic intrusions of layered gabbros such as the one studied at Jebel Tanumah.

Geological and Geochemical Controls on Subsurface Microbial Life in the Samail Ophiolite, Oman.

PubMed

Rempfert, Kaitlin R; Miller, Hannah M; Bompard, Nicolas; Nothaft, Daniel; Matter, Juerg M; Kelemen, Peter; Fierer, Noah; Templeton, Alexis S

2017-01-01

Microbial abundance and diversity in deep subsurface environments is dependent upon the availability of energy and carbon. However, supplies of oxidants and reductants capable of sustaining life within mafic and ultramafic continental aquifers undergoing low-temperature water-rock reaction are relatively unknown. We conducted an extensive analysis of the geochemistry and microbial communities recovered from fluids sampled from boreholes hosted in peridotite and gabbro in the Tayin block of the Samail Ophiolite in the Sultanate of Oman. The geochemical compositions of subsurface fluids in the ophiolite are highly variable, reflecting differences in host rock composition and the extent of fluid-rock interaction. Principal component analysis of fluid geochemistry and geologic context indicate the presence of at least four fluid types in the Samail Ophiolite ("gabbro," "alkaline peridotite," "hyperalkaline peridotite," and "gabbro/peridotite contact") that vary strongly in pH and the concentrations of H 2 , CH 4 , Ca 2+ , Mg 2+ , [Formula: see text], [Formula: see text], trace metals, and DIC. Geochemistry of fluids is strongly correlated with microbial community composition; similar microbial assemblages group according to fluid type. Hyperalkaline fluids exhibit low diversity and are dominated by taxa related to the Deinococcus-Thermus genus Meiothermus , candidate phyla OP1, and the family Thermodesulfovibrionaceae. Gabbro- and alkaline peridotite- aquifers harbor more diverse communities and contain abundant microbial taxa affiliated with Nitrospira , Nitrosospharaceae, OP3, Parvarcheota, and OP1 order Acetothermales. Wells that sit at the contact between gabbro and peridotite host microbial communities distinct from all other fluid types, with an enrichment in betaproteobacterial taxa. Together the taxonomic information and geochemical data suggest that several metabolisms may be operative in subsurface fluids, including methanogenesis, acetogenesis, and

Geological and Geochemical Controls on Subsurface Microbial Life in the Samail Ophiolite, Oman

PubMed Central

Rempfert, Kaitlin R.; Miller, Hannah M.; Bompard, Nicolas; Nothaft, Daniel; Matter, Juerg M.; Kelemen, Peter; Fierer, Noah; Templeton, Alexis S.

2017-01-01

Microbial abundance and diversity in deep subsurface environments is dependent upon the availability of energy and carbon. However, supplies of oxidants and reductants capable of sustaining life within mafic and ultramafic continental aquifers undergoing low-temperature water-rock reaction are relatively unknown. We conducted an extensive analysis of the geochemistry and microbial communities recovered from fluids sampled from boreholes hosted in peridotite and gabbro in the Tayin block of the Samail Ophiolite in the Sultanate of Oman. The geochemical compositions of subsurface fluids in the ophiolite are highly variable, reflecting differences in host rock composition and the extent of fluid-rock interaction. Principal component analysis of fluid geochemistry and geologic context indicate the presence of at least four fluid types in the Samail Ophiolite (“gabbro,” “alkaline peridotite,” “hyperalkaline peridotite,” and “gabbro/peridotite contact”) that vary strongly in pH and the concentrations of H2, CH4, Ca2+, Mg2+, NO3-, SO42-, trace metals, and DIC. Geochemistry of fluids is strongly correlated with microbial community composition; similar microbial assemblages group according to fluid type. Hyperalkaline fluids exhibit low diversity and are dominated by taxa related to the Deinococcus-Thermus genus Meiothermus, candidate phyla OP1, and the family Thermodesulfovibrionaceae. Gabbro- and alkaline peridotite- aquifers harbor more diverse communities and contain abundant microbial taxa affiliated with Nitrospira, Nitrosospharaceae, OP3, Parvarcheota, and OP1 order Acetothermales. Wells that sit at the contact between gabbro and peridotite host microbial communities distinct from all other fluid types, with an enrichment in betaproteobacterial taxa. Together the taxonomic information and geochemical data suggest that several metabolisms may be operative in subsurface fluids, including methanogenesis, acetogenesis, and fermentation, as well as the

Fast Spreading Mid Ocean Ridge Magma Chamber Processes: New Constraints from Hess Deep

NASA Astrophysics Data System (ADS)

MacLeod, C. J.; Lissenberg, J. C.; Howard, K. A.; Ildefonse, B.; Morris, A.; JC21 Scientific Party

2011-12-01

Hess Deep, on the northern edge of the Galapagos Microplate, is a rift valley located at the tip of the Cocos Nazca spreading centre. It is actively propagating westwards into young lithosphere formed at the East Pacific Rise (EPR). Previous studies have shown that the centre of Hess Deep, in the vicinity of a horst block termed the intra-rift ridge (IRR), is characterised by outcrops of gabbro and (minor) peridotite that form the most extensive and complete exposure yet known of lower crust and shallow mantle from a fast spreading mid-ocean ridge. In the absence of a total crustal penetration borehole, the tectonic window of Hess Deep provides our best opportunity to study fast-spreading magma chamber processes and lower crustal accretion by direct observation. Using the Isis ROV we collected high-resolution bathymetry and video data from an 11 sq km area of seafloor, from the nadir of Hess Deep (5400 mbsl) up to the IRR, and sampled outcrops from the region in detail. Of 145 samples in total 94 were gabbro (s.l.). Accounting as much as possible for the complex tectonic disruption of the region we have reassembled these gabbros into a stratigraphic section through an EPR lower crust that we estimate to have been originally about 4350 m thick. The upper half of this plutonic section, which includes a dyke to gabbro transition at the top, is more or less intact on the IRR; however the lower half has been tectonically thinned by active gravity driven faulting and is incomplete. Within this lower section we nevertheless believe we have representative samples from the entire interval. At its base, in addition to primitive olivine gabbro we also recovered dunite, troctolite and residual mantle harzburgite. We here present a synthesis of the petrography and whole rock and mineral compositions of the gabbros from the reconstructed lower crustal section, coupled with a quantitative (electron backscatter diffraction and magnetic) study of their petrofabrics. From this, in

Geochemistry and geochronology of the Mesozoic Lanong ophiolitic mélange, northern Tibet: Implications for petrogenesis and tectonic evolution

NASA Astrophysics Data System (ADS)

Zhong, Yun; Liu, Wei-Liang; Xia, Bin; Liu, Jing-Nan; Guan, Yao; Yin, Zhen-Xing; Huang, Qiang-Tai

2017-11-01

The Lanong ophiolitic mélange is a typical ophiolitic mélange in the middle section of the Bangong-Nujiang suture zone in northern Tibet. It mainly consists of ultramafic and mafic rocks, and its tectonic setting and formation age remain poorly constrained. In this paper, new geochemical and LA-ICP-MS (laser ablation-inductively coupled plasma mass spectrometer) zircon U-Pb age data obtained from gabbro, gabbro-dolerite, dolerite and basalt of the Lanong ophiolitic mélange are provided. The pillow basalts exhibit N-MORB (normal mid-ocean ridge basalt)-like geochemical features with a zircon U-Pb age of 147.6 ± 2.3 Ma. They were generated by 20-30% partial melting of a depleted mantle source composed of spinel lherzolite. The gabbro, massive basalt and gabbro-dolerite samples are characterised by more depleted and "V"-shaped REE (rare earth element) patterns, and they exhibit variable degrees of boninite-like geochemical characteristics, with a zircon U-Pb age of 149.1 ± 1.2 Ma (gabbro-dolerite). They were derived from the remelting of a significantly refractory mantle source following one or more episodes of previous basaltic melt extraction. Geochemical data of these mafic rocks indicate that they were developed in a continental fore-arc setting, and magmas were derived from depleted mantle sources modified by subducted slab-derived fluids and melts with minor crustal contamination. On the other hand, the dolerites show distinct OIB (oceanic island basalt)-like geochemical features, with a zircon U-Pb age of 244.1 ± 3.0 Ma. They were formed in a rift setting on a continental shelf-slope and originated from a low degree of partial melting of a depleted asthenospheric magma source mixed with some ancient sub-continental lithospheric mantle materials. The signatures presented here, combined with the results of previous studies, suggest that the Lanong ophiolitic mélange probably developed in a convergent plate margin under the southward subduction of the

Pito Deep reveals spatial/temporal variability of accretionary processes in the lower oceanic crust at fast-spread MOR

NASA Astrophysics Data System (ADS)

John, B. E.; Cheadle, M. J.; Gee, J. S.; Coogan, L. A.; Gillis, K. M.

2017-12-01

During January and February 2017, the 42-day RV Atlantis PMaG cruise mapped and sampled in-situ fast spread lower crust for 35 km along a flow line at Pito Deep Rift (northeastern Easter microplate). There, ridge-perpendicular escarpments bound Pito Deep and expose up to 3 km sections of crust parallel to the paleo-spreading direction, providing a unique opportunity to test models for the architecture of fast spread lower ocean crust (the plutonic section). Shipboard operations included a >57,000 km2 multi-beam survey; ten Sentry dives over 70 km2 (nominal m-scale resolution) to facilitate acquisition of detailed magnetic and bathymetric data, and optimize Jason II dive siting for rock sampling and geologic mapping; nine Jason II dives in 4 areas, recovering >400 samples of gabbroic lower crust, of which 80% are approximately oriented. Combined Sentry mapping and Jason II sampling and imaging of one area, provides the most detailed documentation of in situ gabbroic crust (>3 km2 of seafloor, over 1000+m vertical section) ever completed. Significantly, the area exposes distinct lateral variation in rock type: in the west 100m of Fe-Ti oxide rich gabbroic rocks overly gabbro and olivine gabbro; however, to the east, exposures of primitive, layered troctolitic rocks extend to within 100m below the dike-gabbro transition. Equivalent troctolitic rocks are found 13 km to the southeast parallel to a flow line, implying shallow primitive rocks are a characteristic feature of EPR lower crust at this location. The high-level position of troctolitic rocks is best explained by construction in a shallow, near steady-state melt lens at a ridge segment center, with some form of gabbro glacier flow active during formation of at least the uppermost lower ocean crust (Perk et al., 2007). Lateral variation in rock type (adjacent oxide gabbro, gabbro, olivine-rich gabbro and troctolite) over short distances taken with complexity in magmatic fabric orientation (mineral and grain size

Cognate xenoliths in Mt. Etna lavas: witnesses of the high-velocity body beneath the volcano

NASA Astrophysics Data System (ADS)

Corsaro, Rosa Anna; Rotolo, Silvio Giuseppe; Cocina, Ornella; Tumbarello, Gianvito

2014-01-01

Various xenoliths have been found in lavas of the 1763 ("La Montagnola"), 2001, and 2002-03 eruptions at Mt. Etna whose petrographic evidence and mineral chemistry exclude a mantle origin and clearly point to a cognate nature. Consequently, cognate xenoliths might represent a proxy to infer the nature of the high-velocity body (HVB) imaged beneath the volcano by seismic tomography. Petrography allows us to group the cognate xenoliths as follows: i) gabbros with amphibole and amphibole-bearing mela-gabbros, ii) olivine-bearing leuco-gabbros, iii) leuco-gabbros with amphibole, and iv) Plg-rich leuco gabbros. Geobarometry estimates the crystallization pressure of the cognate xenoliths between 1.9 and 4.1 kbar. The bulk density of the cognate xenoliths varies from 2.6 to 3.0 g/cm3. P wave velocities (V P ), calculated in relation to xenolith density, range from 4.9 to 6.1 km/s. The integration of mineralogical, compositional, geobarometric data, and density-dependent V P with recent literature data on 3D V P seismic tomography enabled us to formulate the first hypothesis about the nature of the HVB which, in the depth range of 3-13 km b.s.l., is likely made of intrusive gabbroic rocks. These are believed to have formed at the "solidification front", a marginal zone that encompasses a deep region (>5 km b.s.l.) of Mt. Etna's plumbing system, within which magma crystallization takes place. The intrusive rocks were afterwards fragmented and transported as cognate xenoliths by the volatile-rich and fast-ascending magmas of the 1763 "La Montagnola", 2001 and 2002-03 eruptions.

Intermediate-Depth Subduction Earthquakes Recorded by Pseudotachylyte in Dry Eclogite-Facies Oceanic Lithosphere from the Alps

NASA Astrophysics Data System (ADS)

Scambelluri, M.; Pennacchioni, G.; Gilio, M.; Bestmann, M.

2016-12-01

While geophysical studies and laboratory experiments provide much information on subduction earthquakes, field studies identifying the rock types for earthquake development and the deep seismogenic environments are still scarce. To date, fluid overpressure and volume decrease during hydrous mineral breakdown the widely favoured trigger of subduction earthquakes in serpentinized lithospheric mantle and hydrated low-velocity layers atop slabs. Here we document up to 40 cm-thick pseudotachylyte (PST) in Alpine oceanic gabbro and peridotite (2-2.5 GPa-550-620°C), the analogue of a modern cold subducting oceanic lithosphere. These rocks mostly remained unaltered dry systems; only very minor domains (<1%) record partial hydration and static eclogitic metamorphism. Meta-peridotite shows high-pressure olivine + antigorite (garnet + zoisite + chlorite after mantle plagioclase); meta-gabbro develops omphacite + zoisite + talc + chloritoid + garnet. Abundant syn-eclogitic pseudotachylyte cut the dry gabbro-peridotite and the eclogitized domains. In meta-peridotite, PST shows olivine, orthopyroxene, spinel microliths and clasts of high-pressure olivine + antigorite and garnet + zoisite + chlorite aggregates. In metagabbro, microfaults in damage zones near PST cut brecciated igneous pyroxene cemented by omphacite. In unaltered gabbro, glassy PST contains micron-scale garnet replacing plagioclase microliths during, or soon after, PST cooling. In the host rock, garnet coronas between igneous olivine and plagioclase only occur near PST and between closely spaced PST veins. Absence of garnet away from PST indicates that garnet growth was triggered by mineral seeds and by heat released by PST. The above evidence shows that pseudotachylyte formed at eclogite-facies conditions. In such setting, strong, dry, metastable gabbro-peridotite concentrate stress to generate large intermediate depth subduction earthquakes without much involvement of free fluid.

Trace element partitioning in rock forming minerals of co-genetic, subduction-related alkaline and tholeiitic mafic rocks in the Ural Mountains, Russia

NASA Astrophysics Data System (ADS)

Krause, J.; Brügmann, G. E.; Pushkarev, E. V.

2009-04-01

The partitioning of trace elements between rock forming minerals in igneous rocks is largely controlled by physical and chemical parameters e.g. temperature, pressure and chemical composition of the minerals and the coexisting melt. In the present study partition coefficients for REE between hornblende, orthopyroxene, feldspars, apatite and clinopyroxene in a suite of co-genetic alkaline and tholeiitic mafic rocks from the Ural Mountains (Russia) were calculated. The results give insights to the influence of the chemical composition of the parental melt on the partitioning behaviour of the REE. Nepheline-bearing, alkaline melanogabbros (tilaites) are assumed to represent the most fractionated products of the melt that formed the ultramafic cumulates in zoned mafic-ultramafic complexes in the Ural Mountains. Co-genetic with the latter is a suite of olivine gabbros, gabbronorites and hornblende gabbros formed from a tholeiitic parental melt. Negative anomalies for the HFSE along with low Nb and Ta contents and a positive Sr anomaly indicate a subduction related origin of all parental melts. The nepheline gabbros consist predominantly of coarse-grained clinopyroxene phenocrysts in a matrix of fine grained clinopyroxene, olivine, plagioclase, K-feldspar and nepheline with accessory apatite. The tholeiitic gabbros have equigranular to porphyric textures with phenocrysts of olivine, pyroxene and hornblende in a plagioclase rich matrix with olivine hornblende, pyroxene and accessory apatite. Element concentrations of adjacent matrix grains and rims of phenochrysts were measured with LA-ICPMS. The distribution of REE between hornblende and clinopyroxene in the tholeiitic rocks is similar for most of the elements (DHbl•Cpx(La-Tm) = 2.7-2.8, decreasing to 2.6 and 2.4 for Yb and Lu, respectively). These values are about two times higher than published data (e.g. Ionov et al. 1997). Partition coefficients for orthopyroxene/clinopyroxene systematically decrease from the HREE

Pseudotachylitic breccia in mafic and felsic rocks

NASA Astrophysics Data System (ADS)

Kovaleva, Elizaveta; Huber, Matthew S.

2017-04-01

Impact-produced pseudotachylitic breccia (PTB) is abundant in the core of the Vredefort impact structure and was found in many pre-impact lithologies (e.g., Reimold and Colliston, 1994; Gibson et al., 1997). The mechanisms involved in the process of forming this rock remain highly debated, and various authors have discussed many possible models. We investigate PTB from two different rock types: meta-granite and meta-gabbro and test how lithology controls the development of PTB. We also report on clast transport between different lithologies. In the core of the Vredefort impact structure, meta-granite and meta-gabbro are observed in contact with each other, with an extensive set of PTB veins cutting through both lithologies. Microstructural analyses of the PTB veins in thin sections reveals differences between PTBs in meta-granite and meta-gabbro. In granitic samples, PTB often develops along contacts of material with different physical properties, such as a contact with a migmatite or pegmatite vein. Nucleation sites of PTB have features consistent with ductile deformation and shearing, such as sigmoudal-shaped clasts and dragged edges of the veins. Preferential melting of mafic and hydrous minerals takes place (e.g., Reimold and Colliston, 1994; Gibson et al., 2002). Refractory phases remain in the melt as clasts and form reaction rims. In contrast, PTB in meta-gabbro develop in zones with brittle deformation, and do not exploit existing physical contacts. Cataclastic zones develop along the faults and progressively produce ultracataclasites and melt. Thus, PTB veins in meta-gabbro contain fewer clasts. Clasts usually represent multi-phase fragments of host rock and not specific phases. Such fragments often originate from the material trapped between two parallel or horse-tail faults. The lithological control on the development of PTB does not imply that PTB develops independently in different lithologies. We have observed granitic clasts within PTB veins in meta-gabbro

Petrogenetic and geodynamic origin of the Neoarchean Doré Lake Complex, Abitibi subprovince, Superior Province, Canada

NASA Astrophysics Data System (ADS)

Polat, Ali; Frei, Robert; Longstaffe, Fred J.; Woods, Ryan

2018-04-01

The Neoarchean (ca. 2728 Ma) anorthosite-bearing Doré Lake Complex in the northeastern Abitibi subprovince, Quebec, was emplaced into an association of intra-oceanic tholeiitic basalts and gabbros known as the Obatogamau Formation. The Obatogamau Formation constitutes the lower part of the Roy Group, which is composed of two cycles of tholeiitic-to-calc-alkaline volcanic and volcaniclastic rocks, siliciclastic and chemical sedimentary rocks, and layered mafic-to-ultramafic sills. In this study, we report major and trace element results, and Nd, Sr, Pb and O isotope data for anorthosites, leucogabbros, gabbros and mafic dykes from the Doré Lake Complex and spatially associated basalts and gabbros of the Obatogamau Formation to assess their petrogenetic origin and geodynamic setting. Field and petrographic observations indicate that the Doré Lake Complex and associated volcanic rocks underwent extensive metamorphic alteration under greenschist facies conditions, resulting in widespread epidotization (20-40%) and chloritization (10-40%) of many rock types. Plagioclase recrystallized mainly to anorthite and albite endmembers, erasing intermediate compositions. Metamorphic alteration also led to the mobilization of many elements (e.g., LILE and transition metals) and to significant disturbance of the Rb-Sr and U-Pb isotope systems, resulting in 1935 ± 150 and 3326 ± 270 Ma errorchron ages, respectively. The Sm-Nd isotope system was less disturbed, yielding an errorchron age of 2624 ± 160 Ma. On many binary major and trace element diagrams, the least altered anorthosites and leucogabbros, and the gabbros and mafic dykes of the Doré Lake Complex plot in separate fields, signifying the presence of two distinct magma types in the complex. The gabbros and mafic dykes in the Doré Lake Complex share the geochemical characteristics of tholeiitic basalts and gabbros in the Obatogamau Formation, suggesting a possible genetic link between the two rock associations. Initial

Geological and geochemical controls on subsurface microbial life in the Samail Ophiolite, Oman

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

Rempfert, Kaitlin R.; Miller, Hannah M.; Bompard, Nicolas

Microbial abundance and diversity in deep subsurface environments is dependent upon the availability of energy and carbon. However, supplies of oxidants and reductants capable of sustaining life within mafic and ultramafic continental aquifers undergoing low-temperature water-rock reaction are relatively unknown. We conducted an extensive analysis of the geochemistry and microbial communities recovered from fluids sampled from boreholes hosted in peridotite and gabbro in the Tayin block of the Samail Ophiolite in the Sultanate of Oman. The geochemical compositions of subsurface fluids in the ophiolite are highly variable, reflecting differences in host rock composition and the extent of fluid-rock interaction. Principal component analysis of fluid geochemistry and geologic context indicate the presence of at least four fluid types in the Samail Ophiolite (“gabbro,” “alkaline peridotite,” “hyperalkaline peridotite,” and “gabbro/peridotite contact”) that vary strongly in pH and the concentrations of H 2, CH 4, Ca 2+, Mg 2+, NO 3 more » $-$, SO$$2-\\atop{4}$$, trace metals, and DIC. Geochemistry of fluids is strongly correlated with microbial community composition; similar microbial assemblages group according to fluid type. Hyperalkaline fluids exhibit low diversity and are dominated by taxa related to the Deinococcus-Thermus genus Meiothermus, candidate phyla OP1, and the family Thermodesulfovibrionaceae. Gabbro- and alkaline peridotite- aquifers harbor more diverse communities and contain abundant microbial taxa affiliated with Nitrospira, Nitrosospharaceae, OP3, Parvarcheota, and OP1 order Acetothermales. Wells that sit at the contact between gabbro and peridotite host microbial communities distinct from all other fluid types, with an enrichment in betaproteobacterial taxa. Together the taxonomic information and geochemical data suggest that several metabolisms may be operative in subsurface fluids, including methanogenesis, acetogenesis, and

N-MORB crust beneath Fuerteventura in the easternmost part of the Canary Islands: evidence from gabbroic xenoliths

NASA Astrophysics Data System (ADS)

Neumann, Else-Ragnhild; Vannucci, Riccardo; Tiepolo, Massimo

2005-09-01

Gabbro xenoliths reported in this paper were collected in northern Fuerteventura, the Canary Island located closest to the coast of Africa. The xenoliths are very fresh and consist of Ti-Al-poor clinopyroxene + plagioclase (An87-67) + olivine (Fo72-86) ± orthopyroxene. Clinopyroxene and orthopyroxene are constantly and markedly depleted in light rare earth elements (LREE) relative to heavy REE (HREE), as expected for cumulus minerals formed from highly refractory N-MORB-type melts. In contrast, whole-rock Primordial Mantle-normalized trace element patterns range from mildly S-shaped (mildly depleted in Pr-Sm relative to both the strongly incompatible elements Rb-La and the HREE) to enriched. Estimates show that the trace element compositions of the rocks and their minerals are compatible with formation as N-MORB gabbro cumulates, which have been infiltrated at various extents (≤1% to >5%) by enriched alkali basaltic melts. The enriched material is mainly concentrated along grain boundaries and cracks through mineral grains, suggesting that the infiltration is relatively recent, and is thus associated with the Canary Islands magmatism. Our data contradict the hypothesis that a mantle plume was present in this area during the opening of the Atlantic Ocean. No evidence of continental material that might reflect attenuated continental crust in the area has been found. Gabbro xenoliths with REE and trace element compositions similar to those exhibited by the Fuerteventura gabbros are also found among gabbro xenoliths from the islands of La Palma (western Canary Islands) and Lanzarote. The compositions of the most depleted samples from these islands are closely similar, implying that there was no significant change in chemistry during the early stages of formation of the Atlantic oceanic crust in this area. Strongly depleted gabbros similar to those collected in Fuerteventura have also been retrieved in the MARK area along the central Mid-Atlantic Ridge. The presence

An evolved axial melt lens in the Northern Ibra Valley, Southern Oman Ophiolite

NASA Astrophysics Data System (ADS)

Loocke, M. P.; Lissenberg, C. J.; MacLeod, C. J.

2014-12-01

The axial melt lens (AML) is a common feature lying at the base of the upper crust at fast-spreading mid-ocean ridges. It is thought to play a major role in the evolution of MORB and, potentially, accretion of the plutonic lower crust. In order to better understand the petrological processes that operate in AMLs we have examined the nature and variability of the horizon equivalent to the AML preserved in the Oman ophiolite. We present the results of a detailed investigation of a section east of Fahrah in the Ibra Valley. Here, a suite of 'varitextured' gabbros separates the sheeted dykes above from foliated gabbros below. It comprises 3 distinct units: an ophitic gabbro with pegmatitic patches (patchy gabbro; 70 m thick), overlain by a spotty gabbro (50 m), capped by a quartz-diorite (120 m). The sheeted dykes are observed to root in the quartz-diorite. Contacts between the plutonic units are gradational and subhorizontal. All of the units are isotropic. A total of 110 samples were collected for detailed petrographic and chemical analysis. With the exception of a small number of the diorites, all of the samples have a 'cumulate' component. Primary igneous amphibole is ubiquitous, present even as a minor phase in the foliated gabbros beneath, and indicating extensive differentiation and/or the presence of water in the primary liquid. France et al. (2014, Lithos) report patches of granoblastic material from this horizon in the Fahrah area, and suggest they represent the restites of partially melted pieces of the sheeted dykes. We did not, however, find any such granoblastic material, nor can the quartz-diorites represent partial melt; instead, preliminary geochemical modeling suggests that all of the units can be related by simple progressive fractional crystallization of an Oman axial ('V1' or 'Geotimes') melt. Along with the field relationships, as well as the basaltic andesite to dacite composition of the overlying sheeted dykes, this suggests that the AML was the

Geological and geochemical controls on subsurface microbial life in the Samail Ophiolite, Oman

DOE PAGES

Rempfert, Kaitlin R.; Miller, Hannah M.; Bompard, Nicolas; ...

2017-02-07

Microbial abundance and diversity in deep subsurface environments is dependent upon the availability of energy and carbon. However, supplies of oxidants and reductants capable of sustaining life within mafic and ultramafic continental aquifers undergoing low-temperature water-rock reaction are relatively unknown. We conducted an extensive analysis of the geochemistry and microbial communities recovered from fluids sampled from boreholes hosted in peridotite and gabbro in the Tayin block of the Samail Ophiolite in the Sultanate of Oman. The geochemical compositions of subsurface fluids in the ophiolite are highly variable, reflecting differences in host rock composition and the extent of fluid-rock interaction. Principal component analysis of fluid geochemistry and geologic context indicate the presence of at least four fluid types in the Samail Ophiolite (“gabbro,” “alkaline peridotite,” “hyperalkaline peridotite,” and “gabbro/peridotite contact”) that vary strongly in pH and the concentrations of H 2, CH 4, Ca 2+, Mg 2+, NO 3 more » $-$, SO$$2-\\atop{4}$$, trace metals, and DIC. Geochemistry of fluids is strongly correlated with microbial community composition; similar microbial assemblages group according to fluid type. Hyperalkaline fluids exhibit low diversity and are dominated by taxa related to the Deinococcus-Thermus genus Meiothermus, candidate phyla OP1, and the family Thermodesulfovibrionaceae. Gabbro- and alkaline peridotite- aquifers harbor more diverse communities and contain abundant microbial taxa affiliated with Nitrospira, Nitrosospharaceae, OP3, Parvarcheota, and OP1 order Acetothermales. Wells that sit at the contact between gabbro and peridotite host microbial communities distinct from all other fluid types, with an enrichment in betaproteobacterial taxa. Together the taxonomic information and geochemical data suggest that several metabolisms may be operative in subsurface fluids, including methanogenesis, acetogenesis, and

Paleomagnetic evidence bearing on Tertiary tectonics of the Tihamat Asir coastal plain, southwestern Saudi Arabia

USGS Publications Warehouse

Kellogg, K.S.; Blank, H.R.

1982-01-01

Paleomagnetic directions determined for an upper Oligocene to lower Miocene dike swarm and from two lower Miocene layered gabbros in the Tihamat Asir coastal plain of southwestern Saudi Arabia are used to test several hypotheses concerning the tectonics of rifting along the eastern margin of the Red Sea. The dikes and gabbros were emplaced during the initial phases of Red Sea rifting and may mark the transition between continental and oceanic crust. Although these rocks have been hydrothermally altered to varying degrees, reliable remanent directions after alternating-field demagnetization were obtained for 23 dikes and for gabbros at Jabal at Tirf and Wadi Liyyah. Twelve of the dikes are reversely magnetized. After the directions of the reversely magnetized dikes are inverted 180?, the mean direction calculated for the normal dikes is approximately 24? more downward than that calculated for the reversed dikes. This result is similar to that found for the As Sarat volcanic field, 100 km to the north, and may be due to a displaced dipole source for the field. The unrotated mean remanent direction for the dikes (inverting reversed dike directions 180?) is D (declination) = 353.2? and I (inclination) = 6.8? with a95 (radius of the cone of 95 percent confidence) = 8.9? whereas directions from the Jabal at Tirf and Wadi Liyyah gabbros lie at D = 176.2?, I = -1.6? (a95 = 7 1 ?) and D = 17.1?, I = 16.3? (a 95 = 8.7?), respectively. Comparing these results with the results from the As Sarat volcanic field, all the paleomagnetic evidence supports a model for approximately 20 ? of westward tilting of the Wadi Damad and Wadi Jizan areas after the emplacement of the Jabal at Tiff gabbro. The Wadi Liyyah area may have been tilted even more toward the Red Sea. The paleomagnetic directions from three widely separated localities in the Jabal at Tirf gabbro are not significantly different, a fact which indicates that the body cooled in approximately its present bowl shape. Evidence

Evolution of Friction, Wear, and Seismic Radiation Along Experimental Bi-material Faults

NASA Astrophysics Data System (ADS)

Carpenter, B. M.; Zu, X.; Shadoan, T.; Self, A.; Reches, Z.

2017-12-01

Faults are commonly composed by rocks of different lithologies and mechanical properties that are positioned against one another by fault slip; such faults are referred to as bimaterial-faults (BF). We investigate the mechanical behavior, wear production, and seismic radiation of BF via laboratory experiments on a rotary shear apparatus. In the experiments, two rock blocks of dissimilar or similar lithology are sheared against each other. We used contrasting rock pairs of a stiff, igneous block (diorite, granite, or gabbro) against a more compliant, sedimentary block (sandstone, limestone, or dolomite). The cylindrical blocks have a ring-shaped contact, and are loaded under conditions of constant normal stress and shear velocity. Fault behavior was monitored with stress, velocity and dilation sensors. Acoustic activity is monitored with four 3D accelerometers mounted at 2 cm distance from the experimental fault. These sensors can measure accelerations up to 500 g, and their full waveform output is recorded at 1MHz for periods up to 14 sec. Our preliminary results indicate that the bi-material nature of the fault has a strong affect on slip initiation, wear evolution, and acoustic emission activity. In terms of wear, we observe enhanced wear in experiments with a sandstone block sheared against a gabbro or limestone block. Experiments with a limestone or sandstone block produced distinct slickenline striations. Further, significant differences appeared in the number and amplitude of acoustic events depending on the bi-material setting and slip-distance. A gabbro-gabbro fault showed a decrease in both amplitude and number of acoustic events with increasing slip. Conversely, a gabbro-limestone fault showed a decrease in the number of events, but an increase in average event amplitude. Ongoing work focuses on advanced characterization of mechanical, dynamic weakening, and acoustic, frequency content, parameters.

Structure and Geochemistry of the Continental-Oceanic Crust Boundary of the Red Sea and the Rifted Margin of Western Arabia

NASA Astrophysics Data System (ADS)

Dilek, Y.; Furnes, H.; Schoenberg, R.

2009-12-01

The continental-oceanic crust boundary and an incipient oceanic crust of the Red Sea opening are exposed within the Arabian plate along a narrow zone of the Tihama Asir coastal plain in SW Saudi Arabia. Dike swarms, layered gabbros, granophyres and basalts of the 22 Ma Tihama Asir (TA) continental margin ophiolite represent products of magmatic differentiation formed during the initial stages of rifting between the African and Arabian plates. Nearly 4-km-wide zone of NW-trending sheeted dikes are the first products of mafic magmatism associated with incipient oceanic crust formation following the initial continental breakup. Gabbro intrusions are composed of cpx-ol-gabbro, cpx-gabbro, and norite/troctolite, and are crosscut by fine-grained basaltic dikes. Granophyre bodies intrude the sheeted dike swarms and are locally intrusive into the gabbros. Regional Bouger gravity anomalies suggest that the Miocene mafic crust represented by the TA complex extends westward beneath the coastal plain sedimentary rocks and the main trough of the Red Sea. The TA complex marks an incipient Red Sea oceanic crust that was accreted to the NE side of the newly formed continental rift in the earliest stages of seafloor spreading. Its basaltic to trachyandesitic lavas and dikes straddle the subalkaline-mildly alkaline boundary. Incompatible trace element relationships (e.g. Zr-Ti, Zr-P) indicate two distinct populations. The REE concentrations show an overall enrichment compared to N-MORB; light REEs are enriched over the heavy ones ((La/Yb)n > 1), pointing to an E-MORB influence. Nd-isotope data show ɛNd values ranging from +4 to +8, supporting an E-MORB melt source. The relatively large variations in ɛNd values also suggest various degrees of involvement of continental crust during ascent and emplacement, or by mixing of another mantle source.

Estudo de cavidade reentrante supercondutora a ser utilizada nos transdutores paramétricos do detector brasileiro de ondas gravitacionais Mario Schenberg

NASA Astrophysics Data System (ADS)

Ribeiro, K. L.; Furtado, S. R.; Aguiar, O. D.; Frajuca, C.

2003-08-01

Cavidades reentrantes de nióbio vêm sendo utilizadas pelo grupo Australiano nos transdutores eletromecânicos paramétricos do detector de ondas gravitacionais Niobè. Esses transdutores paramétricos são ativados por um sinal AC na faixa de microondas (banda X), que é modulado pelo sinal mecânico da vibração da antena esférica, com a variação de um parâmetro, que, no caso, é a capacitância da cavidade. Nós estudamos esse tipo de cavidade, com o objetivo de transformá-la de reentrante aberta para reentrante fechada, de forma a utilizá-la nos transdutores paramétricos do detector brasileiro Mario Schenberg. O desempenho do transdutor depende de alguns parâmetros da cavidade, como a sua Figura de Mérito (Q elétrico) e o seu acoplamento elétrico com o circuito externo. Neste trabalho mostramos a medida do Q elétrico como função do acoplamento, do acabamento superficial interno e do vazamento de microondas da cavidade, e mostramos o desempenho esperado para o detector Mario Schenberg usando uma cavidade supercondutora reentrante fechada de nióbio.

Basaltic volcanism on the eucrite parent body - Petrology and chemistry of the polymict eucrite ALHA80102

NASA Technical Reports Server (NTRS)

Treiman, A. H.; Drake, M. J.

1985-01-01

The polymict eucrite meteorite ALHA80102 is an unequilibrated breccia of basaltic and gabbroic clasts in a fragmental matrix. Clasts include basalts of many textural types, cumulate gabbro, black 'glass', and ferroan troctolite (plagioclase, silica, Fe-rich olivine, ilmenite, mesostasis). Ferroan troctolite has not been previously reported from eucrites or howardites; it is interpreted as the end-product of fractional crystallization of eucritic magmas. Bulk and trace element compositions (by electron microprobe and INAA) of clasts and matrix from ALHA80102 are similar to those of other eucrites; the meteorite contains clasts similar to Juvinas and to Stannern. A clast of cumulate eucrite gabbro is enriched in the light rare earths (La/Lu = 2XCI). This clast is interpreted as an unrepresentative sample of metamorphically equilibrated gabbro; LREE-enriched magmas need not be invoked. ALHA80102 is similar to other polymict eucrites from the Allan Hills and may be paired with ALHA76005, ALHA77302, and ALHA78040.

Feruvite from the Sullivan Pb-Zn-Ag deposit, British Columbia

USGS Publications Warehouse

Jiang, S.-Y.; Palmer, M.R.; McDonald, A.M.; Slack, J.F.; Leitch, C.H.B.

1996-01-01

Feruvite, an uncommon Ca- and Fe2+-rich tourmaline species, has been discovered in the footwall of the Sullivan Pb-Zn-Ag deposit (British Columbia) near gabbro sills and dikes. Its chemical composition varies according to occurrence: feruvite from the shallow footwall has lower Ca, higher Al, and higher X-site vacancies than that from the deep footwall. The major chemical substitution involved in the feruvite is the exchange vector CaMgO???-1Al-1(OH)-1. The most important factor controlling feruvite formation at Sullivan is likely the reaction of Fe-rich hydrothermal fluids with Ca-rich minerals in gabbro and host rocks. This reaction led to the breakdown of Ca-rich minerals (plagioclase and hornblende), with release of Ca to solution and its incorporation into feruvite. This process probably postdated the main stages of formation of fine-grained, intermediate schorl-dravite in the tourmalinite pipe in the footwall, and is attributed to postore intrusion of gabbro and associated albite-chlorite-pyrite alteration.

An Intensive Survey of Archaeological Resources in the Proposed Long Branch Reservoir. Volume 2B

DTIC Science & Technology

1977-01-01

GcGb - Gneissic Gabbro Hematite TGn - Talc Gneiss c - chipped ShGb - Schistic Gabbro a - scratched ShD - Schistic Dolerite f - flake FH/SS - Flint...Hill Sandstone g - ground Mss - Micaceous Sandstone fss - Ferruginous Sandstone A - Argillite c - chert Qtz - Quartz FGQtt - Fine-grained Quartzite Qtt...ARTIFACTS - LONG BRANCI RESERVOIR 41 0 Points Contracti~ng-@ taed, square-based points l a 23MC55 4-4 50 25 9 9.6g b 23MCSS 2-1 74 39 9 22.8g c

Intermediate-depth earthquake generation: what hydrous minerals can tell us

NASA Astrophysics Data System (ADS)

Deseta, N.; Ashwal, L.; Andersen, T. B.

2012-04-01

Subduction zone seismicity has commonly been causally related to the dehydration of minerals within the subducting slab(Hacker et al. 2004, Jung et al. (2004), Dobson et al. 2002, Rondenay et al. 2008). Other models for release of intermediate- and deep earthquakes include spontaneous reaction(s) affecting large rock-bodies along overstepped phase boundaries ( e.g. Green and Houston, 1995) and various shear heating-localization models (e.g. Kelemen and Hirth 2007, John et al. 2009). These concepts are principally reliant on seismic and thermo-petrological modeling; both of which are indirect methods of analysis. Recent discoveries of pseudotachylytes (PST) formed under high pressure conditions (Ivrea-Verbano Zone, Italy, Western Gneiss Region, Norway and Corsica) provide the first tangible opportunity to evaluate these models (Austrheim and Andersen, 2004, Lund and Austrheim, 2003, Obata and Karato, 1995, Jin et al., 1998). This case study focuses on observations based on ultramafic and mafic PST within the Ligurian Ophiolite of the high pressure-low temperature metamorphic (HP-LT) 'Shistes Lustres' complex in Cima di Gratera, Corsica (Andersen et al. 2008). These PST have been preserved in pristine lenses of peridotite and gabbro surrounded by schistose serpentinites. The PST range in thickness from 1mm to 25 cm (Andersen and Austrheim, 2006). Petrography and geochemistry on PST from the peridotite and gabbro samples indicates that total/near-total fusion of the local host rock mineral assemblage occurred; bringing up the temperature of shear zone from 350° C to 1400 - 1700° C; depending on the host rock (Andersen and Austrheim, 2006). The composition of the PST is highly variable, even at the thin section scale and this has been attributed to the coarse-grained nature of the host rock, its small scale inhomogeneity and poor mixing of the fusion melt. Almost all the bulk analyses of the PST are hydrous; the peridotitic PST is always hydrous (H2O content from 3

Cambrian ophiolite complexes in the Beishan area, China, southern margin of the Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Shi, Yuruo; Zhang, Wei; Kröner, Alfred; Li, Linlin; Jian, Ping

2018-03-01

We present zircon ages and geochemical data for Cambrian ophiolite complexes exposed in the Beishan area at the southern margin of the Central Asian Orogenic Belt (CAOB). The complexes consist of the Xichangjing-Xiaohuangshan and Hongliuhe-Yushishan ophiolites, which both exhibit complete ophiolite stratigraphy: chert, basalt, sheeted dikes, gabbro, mafic and ultramafic cumulates and serpentinized mantle peridotites. Zircon grains of gabbro samples yielded 206Pb/238U ages of 516 ± 8, 521 ± 4, 528 ± 3 and 535 ± 6 Ma that reflect the timing of gabbro emplacement. The geochemical data of the basaltic rocks show enrichment in large-ion lithophile elements and depletion in the high field strength elements relative to normal mid-oceanic ridge basalt (NMORB) in response to aqueous fluids or melts expelled from the subducting slab. The gabbro samples have higher whole-rock initial 87Sr/86Sr ratios and lower positive εNd(t) values than NMORB. These geochemical signatures resulted from processes or conditions that are unique to subduction zones, and the ophiolites are therefore likely to have formed within a supra-subduction zone (SSZ) environment. We suggest that the Cambrian ophiolite complexes in the Beishan area formed within a SSZ setting, reflecting an early Paleozoic subduction of components of the Paleo-Central Asian Ocean and recording an early Paleozoic southward subduction event in the southern CAOB along the northern margin of the Tarim and North China Cratons.

Substratum type affects recruitment and development of marine assemblages over artificial substrata: A case study in the Alboran Sea

NASA Astrophysics Data System (ADS)

Sempere-Valverde, Juan; Ostalé-Valriberas, Enrique; Farfán, Gonzalo M.; Espinosa, Free

2018-05-01

There are more than 27,000 harbours along coasts worldwide and construction is expected to increase. The development and application of new ecological engineering ideas, for both old and new structures, is now imperative in order to reduce perturbation on marine coastal biota and to avoid the spread of non-native species. In this study, the early benthic assessment of subtidal and intertidal communities is tracked on five artificial substrata differing in origin, roughness, and chemical composition: Oyster Sandstone, Limestone, Gabbro, Slate and Concrete. Within substrata, Sandstone was the roughest on a 1-2 mm scale. Also, Sandstone and Limestone had predominantly calcareous composition while silicon was abundant in Concrete, Gabbro and Slate. In the intertidal zone, results showed that primary productivity and diatom abundance markedly increased with substratum roughness. In the subtidal zone, species richness and diversity over experimental substrata were lower than in adjacent rocky reefs. Nonetheless, during the first year of colonization communities varied within the experimental substrata. Coverage was higher on Sandstone than Concrete and Gabbro, and species richness was higher on Sandstone than Limestone. The differences are related to intrinsic substratum-type characteristics and showed Limestone, Gabbro and Concrete hold relatively poor ecological benefits in the first phases of ecological succession. The results may help to promote future research in this field and to test different substrata combinations and heterogeneities, for more environmentally sustainable surfaces in design of coastal structures.

Petrogenesis of the Majiari ophiolite (western Tibet, China): Implications for intra-oceanic subduction in the Bangong-Nujiang Tethys

NASA Astrophysics Data System (ADS)

Huang, Qiang-tai; Liu, Wei-liang; Xia, Bin; Cai, Zhou-rong; Chen, Wei-yan; Li, Jian-feng; Yin, Zheng-xin

2017-09-01

The Majiari ophiolite lies in the western Bangong-Nujiang Suture Zone, which separates the Qiangtang and Lhasa blocks in central Tibet. The ophiolite consists of peridotite, gabbro/diabase and basalt. Zircon U-Pb dating yielded an age of 170.5 ± 1.7 Ma for the gabbro, whereas 40Ar/39Ar dating of plagioclase from the same gabbro yielded ages of 108.4 ± 2.6 Ma (plateau age) and 112 ± 2 Ma (isochron age), indicating that the ophiolite was formed during the Middle Jurassic and was probably emplaced during the Early Cretaceous. Zircons from the gabbro have εHf(t) values ranging from +6.9 to +10.6 and f(Lu/Hf) values ranging from -0.92 to -0.98. Mafic lavas plot in the tholeiitic basalt field but are depleted in Nb, Ta and Ti and enriched in Rb, Ba and Th in the N-MORB-normalized trace element spider diagram. These lavas have whole-rock εNd(t) values of +5.9 to +6.6, suggesting that they were derived from a depleted mantle source, which was probably modified by subducted materials. The Majiari ophiolite probably formed in a typical back-arc basin above a supra-subduction zone (SSZ) mantle wedge. Intra-oceanic subduction occurred during the Middle Jurassic and collision of the Lhasa and South Qiangtang terranes likely occurred in the Early Cretaceous. Thus, closure of the Bangong-Nujiang Tethys Ocean likely occurred before the Early Cretaceous.

Zircon and baddeleyite from the economic ultramafic-mafic Noril'sk-1 intrusion (Russia): Hf-isotope constraints on source composition

NASA Astrophysics Data System (ADS)

Malitch, K. N.; Belousova, E. A.; Badanina, I. Yu.; Griffin, W. L.

2012-04-01

The ultramafic-mafic Noril'sk-1 intrusion in the northwestern part of the Siberian Craton (Russia) represents one of three known Noril'sk-type, ore-bearing intrusions, which host one of the world's major economic sulphide platinum-group-element (PGE)-Cu-Ni deposits. Zircon and baddeleyite dated previously both by SHRIMP (i.e. 248.0 ± 3.7 Ma, Campbell et al. 1992) and ID-TIMS (251.1 ± 3.6 Ma, Kamo et al. 1996) have been restricted to one lithology (e.g. leucogabbro) of the Noril'sk-1 intrusion. To better constrain the age of igneous event and sources involved in its generation our multi-technique study utilized ten rock samples characteristic of unmineralized and mineralized lithologies. The rocks investigated comprise (from top to bottom) gabbro-diorite (sample N1-1), leucogabbro (N1-3), olivine-free gabbro (N1-2 and N1-4), olivine-bearing gabbro (N1-5), olivine gabbro (N1-6), plagiowehrlite and plagiodunite (N1-7), taxitic-textured rocks comprising melanotroctolite, olivine gabbro with relics of ultramafic rocks (N1-8, N1-9) and contact fine-grained gabbro (N1-10). Sulphide PGE-Cu-Ni ores occur in ultramafic (N1-7) and taxitic-textured rocks (N1-8 and N1-9), which have thickness of about 17 m, whereas the low-sulphide horizon of about 1 m thick occurs in the upper part of intrusion (N1-3). In situ U-Pb analyses of zircon from these rocks, combined with detailed study of crystal morphology and internal structure, identify four zircon populations (Malitch et al. 2012). The U-Pb ages of baddeleyite and the defined zircon populations cover a significant time span, from Late Paleozoic to Early Mesozoic (e.g., 290 ± 2.8; 261.3 ± 1.6; 245.7 ± 1.1; 236.5 ± 1.8 and 226.7 ± 0.9 Ma). The established distribution of U-Pb ages implies that crystallization of baddeleyite and zircon corresponds to several stages of protracted evolution of ultramafic-mafic magmas at deep-seated staging chambers and/or probably characterizes interaction of distinct magmas during formation

Reconnaissance geology of the Thaniyah Quadrangle, sheet 20/42 C, Kingdom of Saudi Arabia

USGS Publications Warehouse

Greene, Robert C.

1983-01-01

The Thaniyah quadrangle, sheet 20/42 C, is located in the transition zone between the Hijaz Mountains and the Najd Plateau of southwestern Saudi Arabia between lat 20?00' and 20?30' N., long 42?00' to 42?30' E. The quadrangle is underlain by Precambrian metavolcanic, metasedimentary, plutonic, and dike rocks. Metavolcanic rocks consist of metamorphosed basalt and andesite with minor dacite and rhyolite and underlie three discontinuous northwest-trending belts. Metasedimentary rocks are confined to small areas underlain by quartzite, metasandstone, marble, and calc-silicate rock. Plutonic rocks include an extensive unit of tonalite and quartz diorite and a smaller unit of diorite and quartz diorite, which occupy much of the central part of the quadrangle. A small body of diorite and gabbro and a two-part zone of tonalite gneiss are also present. All of these plutonic rocks are assigned to the An Nimas batholith. Younger plutonic rocks include extensive graphic granite and rhyolite in the northeastern part of the quadrangle and several smaller bodies of granitic rocks and of gabbro. The metavolcanic rocks commonly have strong foliation with northwest strike and steep to vertical dip. Diorite and quartz diorite are sheared and brecciated and apparently syntectonic. Tonalite and quartz diorite are both foliate and nonfoliate and were intruded in episodes both preceding and following shearing. The granitic rocks and gabbro are post-tectonic. Trends of faults and dikes are mostly related to the Najd faulting episode. Radiometric ages, mostly from adjacent quadrangles, suggest that the An Nimas batholith is 835 to 800 Ma, gabbro and granite, except the graphic granite and rhyolite unit, are about 640 to 615 Ma, and the graphic granite and rhyolite 575 to 565 Ma old. Metavolcanic rocks similar to those hosting copper and gold mineralization in the Wadi Shuwas mining district adjacent to the southwestern part of the quadrangle are abundant. An ancient copper mine was

Hugoniot equation of state of rock materials under shock compression

PubMed Central

Braithwaite, C. H.; Zhao, J.

2017-01-01

Two sets of shock compression tests (i.e. conventional and reverse impact) were conducted to determine the shock response of two rock materials using a plate impact facility. Embedded manganin stress gauges were used for the measurements of longitudinal stress and shock velocity. Photon Doppler velocimetry was used to capture the free surface velocity of the target. Experimental data were obtained on a fine-grained marble and a coarse-grained gabbro over a shock pressure range of approximately 1.5–12 GPa. Gabbro exhibited a linear Hugoniot equation of state (EOS) in the pressure–particle velocity (P–up) plane, while for marble a nonlinear response was observed. The EOS relations between shock velocity (US) and particle velocity (up) are linearly fitted as US = 2.62 + 3.319up and US = 5.4 85 + 1.038up for marble and gabbro, respectively. This article is part of the themed issue ‘Experimental testing and modelling of brittle materials at high strain rates’. PMID:27956506

Major chemical characteristics of Mesozoic Coast Range ophiolite in California

USGS Publications Warehouse

Bailey, E.H.; Blake, Jr., M.C.

1974-01-01

Sixty-four major element analyses of rocks representative of the Coast Range ophiolite in California were compared with analyses of other onland ophiolite sequences and those of rocks from oceanic ridges. The rocks can be classed in five groups harzburgite-dunite, clinopyroxenite-wehrlite, gabbro, basalt-spilite, and keratophyre-quartz keratophyre which on various diagrams occupy nonoverlapping fields. The harzburgite-dunite from onland ophiolite and ocean ridges are comparable and very low in alkalies. Possible differentiation trends defined on AFM diagrams by other rocks from onland ophiolites and ocean ridges suggest two lines of descent: (1) A trend much like the calc-alkalic trend, though shifted somewhat toward higher iron, and (2) an iron-enrichment trend defined chiefly by the more iron-rich gabbros and amphibolite. MgO-variation diagrams for rocks from the Coast Range ophiolite further distinguish the iron-rich gabbros and amphibolite from the other rock groups and indicate that the iron enrichment, unlike that of the Skaergaard trend, is related to the formation of amphibole. Ophiolite sequences that include the most silicic rock types, such as quartz keratophyre, also exhibit the most pronounced dual lines of descent, suggesting that the silicic rocks and the amphibole-rich gabbros are somehow related. Although the major element chemistry of the Coast Range ophiolite is clearly like that of rocks dredged from oceanic ridges, it is not sufficiently diagnostic to discriminate among the choices of a spreading ridge, an interarc basin, or perhaps even the root zone of an island arc as the site of ophiolite formation.

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

USGS Publications Warehouse

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

2006-01-01

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

Evidence from the lamarck granodiorite for rapid late cretaceous crust formation in California

USGS Publications Warehouse

Coleman, D.S.; Frost, T.P.; Glazner, A.F.

1992-01-01

Strontium and neodymium isotopic data for rocks from the voluminous 90-million-year-old Lamarck intrusive suite in the Sierra Nevada batholith, California, show little variation across a compositional range from gabbro to granite. Data for three different gabbro intrusions within the suite are identical within analytical error and are consistent with derivation from an enriched mantle source. Recognition of local involvement of enriched mantle during generation of the Sierran batholith modifies estimates of crustal growth rates in the United States. These data indicate that parts of the Sierra Nevada batholith may consist almost entirely of juvenile crust added during Cretaceous magmatism.

Aquisição fonológica do português brasileiro por crianças ouvintes bilíngues bimodais e surdas usuárias de implante coclear

PubMed Central

Cruz, Carina Rebello; Finger, Ingrid

2014-01-01

Resumo O presente estudo investiga a aquisição fonológica do Português Brasileiro (PB) por 24 crianças ouvintes bilíngues bimodais, com acesso irrestrito à Língua Brasileira de Sinais (Libras), e por 6 crianças surdas que utilizam implante coclear (IC), com acesso restrito ou irrestrito à Libras. Para a avaliação do sistema fonológico das crianças em PB, foi utilizada a Parte A, Prova de Nomeação, do ABFW – Teste de Linguagem Infantil (ANDRADE et al. 2004). Os resultados revelaram que as crianças ouvintes bilíngues bimodais e a criança surda usuária de IC com acesso irrestrito à Libras apresentaram processo de aquisição fonológica esperada (normal) para a sua faixa etária. Considera-se que a aquisição precoce e o acesso irrestrito à Libras podem ter sido determinantes para o desempenho dessas crianças no teste oral utilizado. PMID:25506105

Rapid hydrothermal cooling above the axial melt lens at fast-spreading mid-ocean ridge: Quantification through intra-plagioclase diffusion revealed by IODP Hole 1256D

NASA Astrophysics Data System (ADS)

Zhang, C.; Koepke, J.; Kirchner, C.; Götze, N.; Behrens, H.

2014-12-01

At fast-spreading mid-ocean ridges the axial melt lenses sandwiched between the lower oceanic crust and the sheeted dike sequences are assumed to be the major magma source of oceanic crust accretion. According to the widely discussed "gabbro glacier" model, the formation of the lower oceanic crust requires efficient cooling of the axial melt lens, resulting in partly crystallization and leading to crystal-melt mush which may subside down to form the lower crust. These processes are believed to be controlled dominantly by periodical magma supply and hydrothermal circulation above melt lens. Here we quantify the cooling rate above melt lens using chemical zoning of plagioclase from hornfelsic recrystallized sheeted dikes overlying the uppermost gabbros, which are part of the dike-gabbro transition zone drilled in Hole 1256D in the Eastern equatorial Pacific by the Integrated Ocean Drilling Program, where for the first time the dike-gabbro transition zone of an intact oceanic crust was penetrated and sampled. The measured zoning patterns are supposed to be a combined result of diffusion during both on-ridge and off-ridge cooling. We estimate the on-ridge cooling rate using a forward modelling approach based on CaAl-NaSi interdiffusion in plagioclase. The results show that the recrystallized sheeted dikes have been cooled from the peak thermal overprint at 1000-1050 °C to 600 °C within about 5-30 years as a result of hydrothermal circulation above a melt lens during a period of magma starvation, corresponding to a cooling rate of 30±15 °C/yr. Heat balance calculation also approves that in order to balance the heat output of a melt lens at a fast-spreading mid-ocean ridge similar to the case of IODP Hole 1256D, the cooling rate above the melt lens is required to be around 30 °C/yr. The estimated rapid hydrothermal cooling rate coincides with the observed annual to decal episodes of melt lens fluctuation and lava eruption, which favors the "gabbro glacier" model and

Behavior of MORB magmas at uppermost mantle beneath a fast-spreading axis: an example from Wadi Fizh of the northern Oman ophiolite

NASA Astrophysics Data System (ADS)

Akizawa, Norikatsu; Arai, Shoji; Tamura, Akihiro

2012-10-01

Relationships of lithologies in uppermost mantle section of Oman ophiolite are highly complicated, harzburgites especially being closely associated with dunites, wehrlites, and gabbros. The petrology and geochemistry of the uppermost mantle section provide constrains on how MORB (mid-ocean ridge basalt) magmas migrate from the mantle to crust. We conducted detailed sampling at the uppermost mantle section of the northern Oman ophiolite (along Wadi Fizh), and it provides us with centimeter-scale lithological and mineral chemical heterogeneity. In particular, we found peculiar plagioclase-free harzburgites that have not been recorded from the current ocean floor, which contain high-Mg# [Mg/(Mg + Fe2+) atomic ratio] clinopyroxenes that are almost in equilibrium (saturated) with MORB in terms of REE concentrations. They are from the uppermost mantle section underlying the wehrlite-dunite layer (=Moho transition zone; MTZ) just beneath the layered gabbro. MORBs cannot be in equilibrium with harzburgites; however, we call the peculiar harzburgites as "MORB-saturated harzburgite" for simplicity in this paper. The MORB-saturated harzburgites exhibit slightly enriched mineralogy (e.g., spinels with higher Ti and ferric iron, and clinopyroxenes with higher Ti and Na) and contain slightly but clearly more abundant modal clinopyroxene (up to 3.5 vol.%) than ordinary Oman depleted harzburgites (less than 1 vol.% clinopyroxene), which are similar to abyssal harzburgites. Gabbro-clinopyroxenite bands, which were melt lenses beneath the ridge axis, are dominant around the MTZ. Detailed sampling around the gabbro-clinopyroxenite bands revealed that the MORB-saturated harzburgites appear around the bands. The interaction between a melt that was MORB-like and an ordinary harzburgite induced incongruent melting of orthopyroxenes in harzburgites, and the melt chromatographically intruded into the wall harzburgite and was modified to coexist with olivine and two pyroxenes at low melt

The Relation Between Plate Spreading Rate, Crustal Thickness and Axial Relief at Mid-Ocean Ridges

NASA Astrophysics Data System (ADS)

Liu, Z.; Buck, W. R.

2017-12-01

Variations in axial valley relief and in faulting at plate spreading centers are clearly related to magma supply and axial lithospheric structure. Previous models that consider the interaction of magmatic dikes with lithospheric stretching do not successfully reproduce both of these trends. We present the first model that reproduces these trends by making simple assumptions about the partitioning of magma between dikes, gabbros and extrusives. A key concept is that dikes open not only in the brittle axial lithosphere but also into the underlying ductile crust, where they cool to form gabbro. The amount of gabbro so intruded depends on magma pressure that is related to axial relief. The deeper the valley the less magma goes into gabbros and the more magma is available for dikes to accommodate plate separation. We define the fraction of plate separation rate accommodated by dikes as M. If M<1 then part of the plate separation occurs as fault offset which deepens the axial valley. This axial deepening decreases the amount of magma go into gabbros and this increases M. If the valley reaches the depth where M =1 then the faulting ceases and the valley stays at that depth. However, even if M<1, the valley depth cannot increase without limit. Through a distributed pattern of tectonic faults, the valley depth reaches a maximum possible depth that depends on the thickness of the axial lithosphere. If M < 1, where the axial depth reaches this tectonic limit, then moderate to large offset faults can develop. If M = 1 before the depth reaches the tectonic limit, normal faults only develop in response to oscillations in magma supply and fault offset is proportional to the amount of extruded lava. We have derived analytic expressions relates axial lithospheric thickness (HL) and crustal thickness (Hc) to axial valley depth. We then used a 2D model numerical model with a fixed axial lithospheric structure to show that the analytic model predictions are reasonable. Finally, we

Tethyan Anhydrite Preserved in the Lower Ocean Crust of the Samail Ophiolite? Evidence from Oman Drilling Project Holes GT1A and 2A

NASA Astrophysics Data System (ADS)

Teagle, D. A. H.; Harris, M.; Crispini, L.; Deans, J. R.; Cooper, M. J.; Kelemen, P. B.; Alt, J.; Banerjee, N.; Shanks, W. C., III

2017-12-01

Anhydrite is important in mid-ocean ridge hydrothermal systems because of the high concentrations of calcium and sulfate in modern seawater and anhydrite's retrograde solubility. Because anhydrite hosts many powerful tracers of fluid-rock interactions (87Sr/86Sr, δ18O, δ34S, trace elements, fluid inclusions) it is useful for tracing the chemical evolution of hydrothermal recharge fluids and estimating time-integrated fluid fluxes. Anhydrite can form from heated seawater (>100°C), through water-rock reaction, or by mixing of seawater and hydrothermal fluids. Although abundant in active hydrothermal mounds, and predicted to form from downwelling, warming fluids during convection, anhydrite is rare in drill core from seafloor lavas, sheeted dikes and upper gabbros, with only minor amounts in ODP Holes 504B and 1256D. Because anhydrite can dissolve during weathering, its occurrence in ophiolites is unexpected. Instead, gypsum is present in Macquarie Island lavas and Miocene gypsum fills cavities within the Cretaceous Troodos ore deposits. Thus, the occurrence of numerous anhydrite veins in cores from the gabbroic lower crust of the Samail ophiolite in Oman was unanticipated. To our knowledge, anhydrite in Oman gabbros has not been previously reported. Oman Drilling Project Holes GT1A and GT2A were drilled into the Wadi Gideah section of the Wadi Tayin massif. Both recovered 400 m of continuous core from sections of layered gabbros (GT1) and the foliated-layered gabbro transition (GT2). Anhydrite is present throughout both holes, some in vein networks but more commonly as isolated 1-110 mm veins (>60 mm ave). Anhydrite is mostly the sole vein filling but can occur with greenschist minerals such as epidote, quartz, chlorite and prehnite. Anhydrite commonly exhibits prismatic and bladed textures but can also be capriciously microcrystalline. Though definitive cross cutting relationships are elusive, anhydrite veins cut across some greenschist veins. Anhydrite is

Cryogenian (˜830 Ma) mafic magmatism and metamorphism in the northern Madurai Block, southern India: A magmatic link between Sri Lanka and Madagascar?

NASA Astrophysics Data System (ADS)

Teale, William; Collins, Alan S.; Foden, John; Payne, Justin L.; Plavsa, Diana; Chetty, T. R. K.; Santosh, M.; Fanning, Mark

2011-08-01

The northern Madurai Block, southern India, lies directly south of, and partly deformed by, the Palghat-Cauvery Shear Zone System (PCSS) - a potential suture of the Neoproterozoic Mozambique Ocean. The Kadavur gabbro-anorthosite complex lies south of the PCSS, in the northern Madurai Block, and crystallized at 829 ± 14 Ma (LA-ICPMS zircon data) in a supra-subduction zone setting. The complex contains zircon ɛHf(t) values of -12.5 to -8.6 that represent Palaeoproterozoic T(DM) model ages (2.3-2.5 Ga). These broadly agree with a whole rock neodymium T(DM) model age of 2287 Ma. Oxygen isotope δ 18O ratios range from 5.82‰ and 6.74‰. The parental magma for the gabbro-anorthosites are interpreted to be derived from a juvenile Neoproterozoic mantle contaminated by Mesoarchaean igneous infra-crustal sources. The gabbro-anorthosites intrude quartzites with dominantly Palaeoproterozoic detrital zircons that contain Neoarchaean and Mesoarchaean hafnium model ages. These quartzite zircons contain metamorphic rims that yield an age of 843 ± 23 Ma demonstrating the autochthonous nature of the gabbro-anorthosite complex. Later felsic magmatism is recorded by the 766 ± 8 Ma crystallisation age of the protolith of a felsic gneiss. Cryogenian magmatism in the Madurai Block is interpreted to form part of an extensive arc magmatic province within the southern East African Orogen that can be traced from central Madagascar, through southern India to the Wanni Complex of Sri Lanka. This province is interpreted to have formed above a south/west dipping subduction system as the Mozambique Ocean was subducted under the Neoproterozoic continent Azania.

Early Cretaceous gabbroic complex from Yinan, Shandong Province: petrogenesis and mantle domains beneath the North China Craton

NASA Astrophysics Data System (ADS)

Xu, Yi-Gang; Ma, Jin-Long; Huang, Xiao-Long; Iizuka, Yoshiyuki; Chung, Sun-Lin; Wang, Yan-Bin; Wu, Xiang-Yang

2004-12-01

Sensitive high resolution ion microprobe (SHRIMP) zircon U Pb ages, geochemical and Sr-Nd-Pb isotopic data are reported for the gabbroic complex from Yinan (Shandong Province) with the aims of characterizing the nature of the Mesozoic mantle beneath the North China Craton. The Yinan gabbros contain alkali feldspar and biotite, and are characterized by moderate Mg#, high SiO2, low FeO and TiO2 contents and a strong enrichment of light rare earth elements [(La/Yb)n=11 50], but no Eu anomaly. They have low Nb/La (0.07 0.29), radiogenic 87Sr/86Sr (0.710) and unradiogenic ɛNd(t) (-15 to -13). These “crustal fingerprints” cannot be attributed to crustal contamination, given the lack of correlation between isotopic ratios and differentiation indices and the unreasonably high proportion of crustal contaminant (>20%) required in modeling. Instead, compositional similarities to contemporaneous basalts from nearby regions imply that the Yinan gabbros were not significantly affected by crystal cumulation. Isotopic data available for the Mesozoic mafic magmas reveal two distinct mantle domains beneath Shandong. While the EM1-like domain (with low 87Sr/86Sr) is confined to western Shandong, the mantle beneath eastern Shandong is dominated by EM2-type (with high 87Sr/86Sr) affinities. This aerial distinction suggests that the EM2-like signature of the Yinan gabbros may have been inherited from westerly-subducted Yangtze crust during the Triassic North China-South China collision. Emplacement of the Yinan gabbros (127 Ma) is likely affiliated with the widespread and protracted extension during the late Mesozoic in this region.

Formation of thick stratiform Fe-Ti oxide layers in layered intrusion and frequent replenishment of fractionated mafic magma: Evidence from the Panzhihua intrusion, SW China

NASA Astrophysics Data System (ADS)

Song, Xie-Yan; Qi, Hua-Wen; Hu, Rui-Zhong; Chen, Lie-Meng; Yu, Song-Yue; Zhang, Jia-Fei

2013-03-01

Panzhihua intrusion is one of the largest layered intrusions that hosts huge stratiform Fe-Ti oxide layers in the central part of the Emeishan large igneous province, SW China. Up to 60 m thick stratiform massive Fe-Ti oxide layers containing 85 modal% of magnetite and ilmenite and overlying magnetite gabbro compose cyclic units of the Lower Zone of the intrusion. The cyclic units of the Middle Zone consist of magnetite gabbro and overlying gabbro. In these cyclic units, contents of Fe2O3(t), TiO2 and Cr and Fe3+/Ti4+ ratio of the rocks decrease upward, Cr content of magnetite and forsterite percentage of olivine decrease as well. The Upper Zone consists of apatite gabbro characterized by enrichment of incompatible elements (e.g., 12-18 ppm La, 20-28 ppm Y) and increasing of Fe3+/Ti4+ ratio (from 1.3 to 2.3) upward. These features indicate that the Panzhihua intrusion was repeatedly recharged by more primitive magma and evolved magmas had been extracted. Calculations using MELTS indicate that extensive fractionation of olivine and clinopyroxene in deep level resulted in increasing Fe and Ti contents in the magma. When these Fe-Ti-enriched magmas were emplaced along the base of the Panzhihua intrusion, Fe-Ti oxides became an early crystallization phase, leading to a residual magma of lower density. We propose that the unusually thick stratiform Fe-Ti oxide layers resulted from coupling of gravity settling and sorting of the crystallized Fe-Ti oxides from Fe-Ti-enriched magmas and frequent magma replenishment along the floor of the magma chamber.

Possible petrogenetic associations among igneous components in North Massif soils: Evidence in 2-4 mm soil particles from 76503

NASA Technical Reports Server (NTRS)

Jolliff, Bradley L.; Bishop, Kaylynn M.; Haskin, Larry A.

1992-01-01

Studies of Apollo 17 highland igneous rocks and clasts in breccias from the North and South Massifs have described magnesian troctolite, norite, anorthositic gabbro, dunite, spinel cataclasites, and granulitic lithologies that may have noritic anothosite or anorthositic norite/gabbro as igneous precursors, and have speculated on possible petrogenetic relationships among these rock types. Mineral compositions and relative proportions of plagioclase and plagioclase-olivine particles in samples 76503 indicate that the precursor lithology of those particles were troctolitic anorthosite, not troctolite. Mineral and chemical compositions of more pyroxene-rich, magnesian breccias and granulites in 76503 indicate that their precursor lithology was anorthositic norite/gabbro. The combination of mineral compositions and whole-rock trace-element compositional trends supports a genetic relationship among these two groups as would result from differentiation of a single pluton. Although highland igneous lithologies in Apollo 17 materials have been described previously, the proportions of different igneous lithologies present in the massifs, their frequency of association, and how they are related are not well known. We consider the proportions of, and associations among, the igneous lithologies found in a North Massif soil, which may represent those of the North Massif or a major part of it.

Cl-rich hydrous mafic mineral assemblages in the Highiș massif, Apuseni Mountains, Romania

NASA Astrophysics Data System (ADS)

Bonin, Bernard; Tatu, Mihai

2016-08-01

The Guadalupian (Mid-Permian) Highiș massif (Apuseni Mountains, Romania) displays a bimodal igneous suite of mafic (gabbro, diorite) and A-type felsic (alkali feldspar granite, albite granite, and hybrid granodiorite) rocks. Amphibole is widespread throughout the suite, and yields markedly high chlorine contents. Three groups are identified: Cl-rich potassic hastingsite (2.60-3.40 wt% Cl) within A-type felsic rocks and diorite, mildly Cl-rich pargasite to hornblende (0.80-1.90 wt% Cl) within gabbro, and low F-Cl hornblende within gabbro and hybrid granodiorite. Coexisting biotite is either Cl-rich within diorite, or F-Cl-poor to F-rich within A-type felsic rocks. Chlorine and fluorine are distributed in both mafic phases, according to the F-Fe and Cl-Mg avoidance rules. The low-Ti contents suggest subsolidus compositions. Cl-rich amphibole within diorite and A-type felsic rocks yields a restricted temperature range - from 575 °C down to 400 °C, whereas mildly Cl-rich amphibole within gabbro displays the highest range - from 675 to 360 °C. Temperatures recorded by Cl-rich biotite within diorite range from 590 to 410 °C. Biotite within A-type felsic rocks yields higher temperatures than amphibole: the highest values- from 640 to 540 °C - are recorded in low-F-Cl varieties, whereas the lowest values- from 535 to 500 °C - are displayed by F-rich varieties. All data point to halogen-rich hydrothermal fluids at upper greenschist facies conditions percolating through fractures and shear zones and pervasively permeating the whole Highiș massif, with F precipitating as interstitial fluorite and Cl incorporating into amphibole, during one, or possibly several, hydrothermal episodes that would have occurred during a ~ 150 My-long period of time extending from the Guadalupian (Mid-Permian) to the Albian (Mid-Cretaceous).

The petrogenesis of late Neoproterozoic mafic dyke-like intrusion in south Sinai, Egypt

NASA Astrophysics Data System (ADS)

Azer, M. K.; Abu El-Ela, F. F.; Ren, M.

2012-08-01

New field, petrographical and geochemical studies are presented here for the late Neoproterozoic Rimm intrusion (˜15 km long) exposed in the southern Sinai Peninsula, Egypt in the northernmost Arabian-Nubian Shield (ANS). Field relations indicate that the Rimm intrusion is younger than the surrounding metamorphic rocks and calc-alkaline syn-tectonic granodiorite and it was not affected by regional metamorphism. The anorogenic peralkaline granite of Gebel Serbal crosscuts the Rimm intrusion. The Rimm intrusion is made up of several consanguineous rock types with gradational contacts. It is composed chiefly of pyroxene-hornblende gabbro, hornblende gabbro and minor quartz diorite. The chemical composition of the mafic minerals indicated that the studied rocks derived from calc-alkaline mafic magma. Geochemically, the studied rocks are characterized by enrichment in LILE relative to HFSE and LREE relative to HREE [(Ce/Yb)N = 4.50-6.36]. Quartz diorite display slightly concave HREE pattern and slightly negative Eu-anomaly [(Eu/Eu*)n = 0.91] which may be the result of fractionation of amphibole and plagioclase from the source melt, respectively. The Rimm intrusion evolved from mafic mantle magma into different type rocks by fractional crystallization with minor crustal contamination. The initial magma corresponds to pyroxene-hornblende gabbro and the crystallization of hornblende was caused by slight H2O increase in magma after crystallization of near-liquidus clinopyroxene and Ca-rich plagioclase. Amphiboles geobarometer indicate that the gabbroic rocks of the Rimm intrusion crystallized at pressures between 4.8 and 6.4 Kb, while quartz diorite crystallized at 1.3-2.1 Kb. Crystallization temperatures range between 800 and 926 °C for the gabbros and between 667 and 784 °C for the quartz diorite. The Rimm intrusion represents a post-orogenic phase formed during the crustal thinning and extension of the Arabian-Nubian Shield.

Geochemical and Sm-Nd isotope-geochemical patterns of metavolcanic rocks, diabase, and metagabbroids on the northeastern flank of the South Mongolian-Khingan orogenic belt

NASA Astrophysics Data System (ADS)

Smirnov, Yu. V.; Sorokin, A. A.

2017-05-01

The first results of geochemical and Sm-Nd isotope-geochemical studies of metavolcanic rocks, metagabbroids, and diabase of the Nora-Sukhotino terrane, the least studied part of the South Mongolian-Khingan orogenic belt in the system of the Central Asian orogenic belt are reported. It is established that the basic rocks composing this terrane include varieties comparable with E-MORB, tholeiitic, and calc-alkaline basalt of island arc, calc-alkaline gabbro-diabase, and gabbroids of island arcs. Most likely, these formations should be correlated with metabasalt and associated Late Ordovician gabbro-amphibolite of the Sukdulkin "block" of the South Mongolian-Khingan orogenic belt, which are similar to tholeiite of intraplate island arcs by their geochemical characteristics.

Rock types of South Pole-Aitken basin and extent of basaltic volcanism

USGS Publications Warehouse

Pieters, C.M.; Head, J. W.; Gaddis, L.; Jolliff, B.; Duke, M.

2001-01-01

The enormous pre-Nectarian South Pole-Aitken (SPA) basin represents a geophysically and compositionally unique region on the Moon. We present and analyze the mineralogical diversity across this basin and discuss the implications for basin evolution. Rock types are derived from Clementine multispectral data based on diagnostic characteristics of ferrous absorptions in fresh materials. Individual areas are characterized as noritic (dominated by low-Ca pyroxene), gabbroic/basaltic (dominated by high-Ca pyroxene), feldspathic (<3-6% FeO), and olivine-gabbro (dominated by high-Ca pyroxene and olivine). The anorthositic crust has effectively been removed from the interior of the basin. The style of volcanism within the basin extends over several 100 Myr and includes mare basalt and pyroclastic deposits. Several areas of ancient (pre-Orientale) volcanism, or cryptomaria, have also been identified. The nonmare mafic lithology that occurs across the basin is shown to be noritic in composition and is pervasive laterally and vertically. We interpret this to represent impact melt/breccia deposits derived from the lower crust. A few localized areas are identified within the basin that contain more diverse lithologies (gabbro, olivine-gabbro), some of which may represent material from the deepest part of the lower crust and perhaps uppermost mantle involved in the SPA event. Copyright 2001 by the American Geophysical Union.

Tectonic Evolution of the Careón Ophiolite (Northwest Spain): A Remnant of Oceanic Lithosphere in the Variscan Belt.

PubMed

Díaz García F; Arenas; Martínez Catalán JR; González del Tánago J; Dunning

1999-09-01

Analysis of the Careón Unit in the Ordenes Complex (northwest Iberian Massif) has supplied relevant data concerning the existence of a Paleozoic oceanic lithosphere, probably related to the Rheic realm, and the early subduction-related events that were obscured along much of the Variscan belt by subsequent collision tectonics. The ophiolite consists of serpentinized harzburgite and dunite in the lower section and a crustal section made up of coarse-grained and pegmatitic gabbros. An Early Devonian zircon age (395+/-2 Ma, U-Pb) was obtained in a leucocratic gabbro. The whole section was intruded by numerous diabasic gabbro dikes. Convergence processes took place shortly afterward, giving rise to a mantle-rooted synthetic thrust system, with some coeval igneous activity. Garnet amphibolite, developed in metamorphic soles, was found discontinuously attached to the thrust fault. The soles graded downward to epidote-amphibolite facies metabasite and were partially retrogressed to greenschist facies conditions. Thermobarometric estimations carried out at a metamorphic sole (T approximately 650 degrees C; P approximately 11.5 kbar) suggested that imbrications developed in a subduction setting, and regional geology places this subduction in the context of an early Variscan accretionary wedge. Subduction and imbrication of oceanic lithosphere was followed by underthrusting of the Gondwana continental margin.

Spinel and orthopyroxene exsolved from clinopyroxene in the Haladala pluton in the middle Tianshan (Xinjiang, China)

NASA Astrophysics Data System (ADS)

Zhu, Yongfeng; Chen, Jing; Xue, Yunxin; Feng, Wanyi; Jiang, Jiuyang

2017-12-01

The Haladala pluton, consisting of troctolite, olivine gabbro and gabbro with zircon SHRIMP U-Pb age of 309 ± 2 Ma (MSWD = 0.72), intruded the Devonian-Carboniferous arc segments in the middle Tianshan. Amphibole, coexisting with magnetite, amphibole, and phlogopite, crystallized in a magma chamber at depth of 20 km (6.9-7.4 kbar, 934-943 °C) based on various thermobaramoters. Two kinds of exsolution textures (spinel rods in clinopyroxene, orthopyroxene lamellae in clinopyroxene) occur in troctolite and olivine gabbro. We describe oriented spinel rods and orthopyroxene lamellae exsolved from the host clinopyroxene based on optical and high-resolution transmission electron microscope (HRTEM) observations. The spinel rods (100) are parallel to their host clinopyroxene (010). Orthopyroxene lamellae (010) are coherent and strictly parallel to their host clinopyroxene (010). Exsolution of spinel rods from the host clinopyroxene is controlled by the reaction of (Ca0.5M2+ 0.5)Fe3+[AlSiO6]in clinopyroxene → (Ca0.86-0.17M2+ 0.14-0.17)(M2 + 1.00-0.96Al0-0.04)[Al0.17-0.10Si1.83-1.90O6] + Fe3O4 + O2.

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

A long in situ section of the lower ocean crust: results of ODP Leg 176 drilling at the Southwest Indian Ridge

NASA Astrophysics Data System (ADS)

Dick, Henry J. B.; Natland, James H.; Alt, Jeffrey C.; Bach, Wolfgang; Bideau, Daniel; Gee, Jeffrey S.; Haggas, Sarah; Hertogen, Jan G. H.; Hirth, Greg; Holm, Paul Martin; Ildefonse, Benoit; Iturrino, Gerardo J.; John, Barbara E.; Kelley, Deborah S.; Kikawa, Eiichi; Kingdon, Andrew; LeRoux, Petrus J.; Maeda, Jinichiro; Meyer, Peter S.; Miller, D. Jay; Naslund, H. Richard; Niu, Yao-Ling; Robinson, Paul T.; Snow, Jonathan; Stephen, Ralph A.; Trimby, Patrick W.; Worm, Horst-Ulrich; Yoshinobu, Aaron

2000-06-01

Ocean Drilling Program Leg 176 deepened Hole 735B in gabbroic lower ocean crust by 1 km to 1.5 km. The section has the physical properties of seismic layer 3, and a total magnetization sufficient by itself to account for the overlying lineated sea-surface magnetic anomaly. The rocks from Hole 735B are principally olivine gabbro, with evidence for two principal and many secondary intrusive events. There are innumerable late small ferrogabbro intrusions, often associated with shear zones that cross-cut the olivine gabbros. The ferrogabbros dramatically increase upward in the section. Whereas there are many small patches of ferrogabbro representing late iron- and titanium-rich melt trapped intragranularly in olivine gabbro, most late melt was redistributed prior to complete solidification by compaction and deformation. This, rather than in situ upward differentiation of a large magma body, produced the principal igneous stratigraphy. The computed bulk composition of the hole is too evolved to mass balance mid-ocean ridge basalt back to a primary magma, and there must be a significant mass of missing primitive cumulates. These could lie either below the hole or out of the section. Possibly the gabbros were emplaced by along-axis intrusion of moderately differentiated melts into the near-transform environment. Alteration occurred in three stages. High-temperature granulite- to amphibolite-facies alteration is most important, coinciding with brittle-ductile deformation beneath the ridge. Minor greenschist-facies alteration occurred under largely static conditions, likely during block uplift at the ridge transform intersection. Late post-uplift low-temperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high- and low-temperature alteration are their respective associations with ductile and cataclastic deformation, and an overall decrease downhole with hydrothermal alteration generally ≤5% in the

Anatomy of a frozen axial melt lens from a fast-spreading paleo-ridge (Wadi Gideah, Oman ophiolite)

NASA Astrophysics Data System (ADS)

Müller, T.; Koepke, J.; Garbe-Schönberg, C.-D.; Dietrich, M.; Bauer, U.; Wolff, P. E.

2017-02-01

At fast-spreading mid-ocean ridges, axial melt lenses (AMLs) sandwiched between the sheeted dyke section and the uppermost gabbros are assumed to be the major magma source of crust formation. Here, we present our results from a field study based on a single outcrop of a frozen AML in the Samail ophiolite in the Sultanate of Oman which presents a whole suite of different lithologies and complex cutting relationships: varitextured gabbro with relics of primitive poikilitic clinopyroxene is intruded by massive quartz diorites and tonalites bearing relics of assimilated sheeted dykes, which in turn are cut by trondhjemite dykes. The whole is cut by basaltic dykes with chilled margins. The geochemical evolutionary trend of the varitextured gabbros, including some of the quartz diorites and tonalites, can be best modelled by fractional crystallisation of an experimental MORB parental melt composition containing 0.4 to 0.8 wt.% H2O. Patchy varitextured gabbros containing domains of primitive poikilitic clinopyroxene and evolved granular networks represent the record of in situ crystallisation. Some quartz diorites, often with xenoliths of sheeted dykes and exceptionally high Al2O3 contents, show a bulk trace element pattern more in accord with melts generated by experimental partial melting of dyke material. Highly evolved, crosscutting trondhjemite dykes show characteristic trace element patterns implying a formation by partial melting of sheeted dykes under lower water activity which is indicated by relatively low Al2O3 contents. The late basaltic dykes with chilled margins crosscutting all other lithologies show a relatively depleted geochemical character with pronounced negative Nb-Ta anomalies implying a genetic relationship to the second phase of magmatic Oman paleo-ridge activity (V2). The field relationships in combination with the petrological/geochemical trends reveal multiple sequences of MORB-type magma cooling (resulting in fractional crystallisation) and re

Battling through the thermal boundary layer: Deep sampling in ODP Hole 1256D during IODP Expedition 335

NASA Astrophysics Data System (ADS)

Ildefonse, B.; Teagle, D. A.; Blum, P.; IODP Expedition 335 Scientists

2011-12-01

IODP Expedition 335 "Superfast Spreading Rate Crust 4" returned to ODP Hole 1256D with the intent of deepening this reference penetration of intact ocean crust several hundred meters into cumulate gabbros. This was the fourth cruise of the superfast campaign to understand the formation of oceanic crust accreted at fast spreading ridges, by exploiting the inverse relationship between spreading rate and the depth to low velocity zones seismically imaged at active mid-ocean zones, thought to be magma chambers. Site 1256 is located on 15-million-year-old crust formed at the East Pacific Rise during an episode of superfast ocean spreading (>200 mm/yr full rate). Three earlier cruises to Hole 1256D have drilled through the sediments, lavas and dikes and 100 m into a complex dike-gabbro transition zone. The specific objectives of IODP Expedition 335 were to: (1) test models of magmatic accretion at fast spreading ocean ridges; (2) quantify the vigor of hydrothermal cooling of the lower crust; (3) establish the geological meaning of the seismic Layer 2-3 boundary at Site 1256; and (4) estimate the contribution of lower crustal gabbros to marine magnetic anomalies. It was anticipated that even a shortened IODP Expedition could deepen Hole 1256D a significant distance (300 m) into cumulate gabbros. Operations on IODP Expedition 335 proved challenging from the outset with almost three weeks spent re-opening and securing unstable sections of the Hole. When coring commenced, the destruction of a hard-formation C9 rotary coring bit at the bottom of the hole required further remedial operations to remove junk and huge volumes of accumulated drill cuttings. Hole-cleaning operations using junk baskets returned large samples of a contact-metamorphic aureole between the sheeted dikes and a major heat source below. These large (up to 3.5 kg) irregular samples preserve magmatic, hydrothermal and structural relationships hitherto unseen because of the narrow diameter of drill core and

Successive mixing and mingling of magmas in a plutonic complex of Northeast Brazil

NASA Astrophysics Data System (ADS)

Neves, S. P.; Vauchez, A.

1995-02-01

Field and petrographic evidence together with major element geochemistry suggest that mixing and mingling of magmas of contrasting compositions were important petrogenetic processes in the Fazenda Nova/Serra da Japeganga plutonic complex of Northeast Brazil. The complex was emplaced at pressures of 300-500 MPa in amphibolite facies metamorphic rocks of Neoproterozoic age and consists of three main rock types: (1) coarse-grained granite; (2) porphyritic granite and (3) diorite to quartz-monzodiorite. The latter two make up the Fazenda Nova batholith which is located on the northwestern side of the sinistral, NE-trending, Fazenda Nova strike-slip shear zone. NE-plunging stretching lineations in the shear zone suggest that this batholith represents an uplifted, and therefore deeper, portion of the complex. The structure of the complex reflects the stratigraphy in a magma chamber, with the porphyritic granite above the diorite and below the coarse-grained granite. The porphyritic granite has a uniform composition, intermediate in mafic mineral content, quartz, and majorelements between the coarse-grained granite and the diorite. It is free of disequilibrium mineral assemblages, and locally displays gradational contacts with the overlain coarse-grained granite. Most elements display linear correlation with SiO 2 in Harker diagrams. These features are interpreted as resulting from mixing of almost crystal-free felsic and intermediate magmas. Fluid dynamic calculations using the coarse-grained granite and the silica-poorest diorite as end-members in the mixing process show that mechanical mixing was possible, and thermal modelling suggests that the formation of an homogeneous hybrid may have been achieved in less than 50,000 yr. The diorites contain corroded K-feldspar megacrysts, and range in composition from low to relatively high silica contents, partly overlapping with the porphyritic granite. This suggests that a new mixing event occurred during the crystallisation

Compositions of Normal and Anomalous Eucrite-Type Mafic Achondrites

NASA Technical Reports Server (NTRS)

Mittlefehldt, D. W.; Peng, Z. X.; Mertzman, S. A.

2016-01-01

The most common asteroidal igneous meteorites are eucrite-type mafic achondrites - basalts and gabbros composed of ferroan pigeonite, ferroan augite, calcic plagioclase, silica, ilmenite, troilite, Ca-phosphate, chromite and Fe-metal. These rocks are thought to have formed on a single asteroid along with howardites and diogenites. However, high precision O-isotopic analyses have shown that some mafic achondrites have small, well-resolved, non-mass-dependent differences that have been interpreted as indicating derivation from different asteroids. Some of these O-anomalous mafic achondrites also have anomalous petrologic characteristics, strengthening the case that they hail from distinct parent asteroids. We present the results of bulk compositional studies of a suite of normal and anomalous eucrite-type basalts and cumulate gabbros.

Amazon Forest Structure from IKONOS Satellite Data and the Automated Characterization of Forest Canopy Properties

Treesearch

Michael Palace; Michael Keller; Gregory P. Asner; Stephen Hagen; Bobby Braswell

2008-01-01

We developed an automated tree crown analysis algorithm using 1-m panchromatic IKONOS satellite images to examine forest canopy structure in the Brazilian Amazon. The algorithm was calibrated on the landscape level with tree geometry and forest stand data at the Fazenda Cauaxi (3.75◦ S, 48.37◦ W) in the eastern Amazon, and then compared with forest...

Oceanic units in the core of the External Rif (Morocco): Intramargin hiatus or South-Tethyan remnants?

NASA Astrophysics Data System (ADS)

Benzaggagh, Mohamed; Mokhtari, Abdelkader; Rossi, Philippe; Michard, André; El Maz, Abdelkhader; Chalouan, Ahmed; Saddiqi, Omar; Rjimati, Ech-Cherki

2014-07-01

The aim of this paper is to describe the mafic rocks that crop out in the central-western Mesorif Zone (External Rif Belt), and discuss their geodynamic signification. Basalt flows, olistoliths and breccias occur in Oxfordian-Berriasian deposits of Mesorif units ascribed to the distal part of the African paleomargin. The climax of volcanic activity is observed at the northern border of a Kimmeridgian carbonate platform progressively dismembered during the Tithonian-Berriasian. In spite of the alteration of the basalts, their petrological and geochemical characters point to E-MORB affinities. The studied gabbro massifs (Bou Adel, Kef el Rhar west and north) occur as restricted slivers or klippes within the Senhadja nappe or mélange of the internal Mesorif, which overlies the basalt-bearing units and other, more external Mesorif units. The compositions range from troctolitic olivine gabbro to ferrogabbro with frequent ortho- to heteradcumulate textures; they display typical tholeiitic affinity. The gabbro massifs are crosscut by trondjhemite dykes and overlain by metabasalts, fault-scarp breccias, ophicalcites, marbles and radiolarites. Composition featuring initial near liquid composition, display multi elements patterns close to those of E-MORB, with a weak Eu negative anomaly and evidence of slight crustal contamination. These gabbro massifs were regarded as Jurassic-Cretaceous intrusions, locally dated (K-Ar) at 166 ± 3 Ma. Conversely, we assume they represent discrete samples of a Jurassic-Cretaceous oceanic basement (ophiolites), emplaced tectonically in the Senhadja nappe (mélange) of the central Mesorif. The correlation of both these types of mafic rock associations (paleomargin basalts and ophiolite klippes) with the serpentinites of the eastern Mesorif (Beni Malek) and Oran mountains (Algeria) is then briefly discussed. We conclude that the previous hypothesis of an intramargin “Mesorif suture zone” must be reconsidered, being challenged by that of a

Amphibole and felsic veins from the gabbroic oceanic core complex of Atlantis Bank (Southwest Indian Ridge, IODP Hole U1473A): when the fluids meets the melts

NASA Astrophysics Data System (ADS)

Sanfilippo, A.; Tribuzio, R.; Antonicelli, M.; Zanetti, A.

2017-12-01

We present a petrological/geochemical investigation of brown amphibole and felsic veins drilled during IODP 360 expedition at Atlantis Bank, a gabbroic oceanic core complex from Southwest Indian Ridge. The main purpose of this study is to unravel the role of seawater and magmatic components in the origin of these veins. Brown amphibole veins were collected at 90-170 mbsf. These veins typically include minor modal amounts of plagioclase and are associated with alteration halos made up of brown amphibole and whitish milky plagioclase in host gabbros. Two sets of late magmatic felsic veins, which mostly consist of plagioclase and minor brown amphibole, were selected. Amphibole-plagioclase geothermometry (Holland and Blundy, 1994) documents that crystallization of brown amphibole and felsic veins occurred in the 850-700 °C interval. In the brown amphibole veins, amphibole and plagioclase have relatively low concentrations of incompatible trace elements and significant Cl (0.2-0.3 wt%). The development of these veins at near surface levels is therefore attributed to seawater-derived fluids migrating downward through cracks developing in the exhuming gabbro. To explain the high temperature estimates for the development of these shallow veins, however, the seawater-derived fluids must have interacted not only with the gabbros, but also with a high temperature magmatic component. This petrogenetic hypothesis is consistent with oxygen and hydrogen isotopic compositions of amphiboles from shallow veins in adjacent Hole 735B gabbros (Alt and Bach, 2006). Trace element compositions of amphibole and plagioclase from the felsic veins show formation by silicate melts rich in incompatible elements. In addition, Cl concentrations in amphibole from the felsic veins are low, thereby indicating that the melts feeding these veins had low or no seawater component. We cautiously propose that: (i) the felsic veins were generated by SiO2-rich melts residual after crystallization of Fe

Petrogenesis of the Dalongkai ultramafic-mafic intrusion and its tectonic implication for the Paleotethyan evolution along the Ailaoshan tectonic zone (SW China)

NASA Astrophysics Data System (ADS)

Liu, Huichuan; Wang, Yuejun; Zi, Jian-Wei

2017-06-01

Layered ultramafic-mafic intrusions are usually formed in an arc/back-arc or intra-plate tectonic environment, or genetically related to a mantle plume. In this paper, we report on an ultramafic-mafic intrusion, the Dalongkai intrusion in the Ailaoshan tectonic zone (SW China), whose occurrence is closely associated with arc/back-arc magmatic rocks. The Dalongkai intrusion is composed of plagioclase-lherzolite, hornblende-peridotite, lherzolite and wehrlite at the bottom, cumulate plagioclase-pyroxenite at the middle part, changing to fine-grained gabbro towards the upper part of the intrusion, forming layering structure. Zircons from the plagioclase-pyroxenites and gabbros yielded U-Pb ages of 272.1 ± 1.7 Ma and 266.4 ± 5.8 Ma, respectively. The plagioclase-pyroxenites show cumulate textures, and are characterized by high MgO (25.0-28.0 wt.%; mg# = 80.6-82.3), Cr (1606-2089 ppm) and Ni (893-1203 ppm) contents, interpreted as early cumulate phases. By contrast, the gabbros have relatively lower mg# values (56.3-62.7), and Cr (157-218 ppm) and Ni (73-114 ppm) concentrations, and may represent frozen liquids. The plagioclase-pyroxenites and gabbros share similar chondrite-normalized REE patterns and primitive mantle-normalized trace element profiles which are analogous to those of typical back-arc basin basalts. The εNd(t) values for both rock types range from +2.20 to +4.22. These geochemical and isotopic signatures suggest that the Dalongkai ultramafic-mafic rocks originated from a MORB-like mantle source metasomatized by subduction-related, sediment-derived fluids. Our data, together with other geological evidence, indicate that the emplacement of the Dalongkai ultramafic-mafic intrusion most likely occurred in a back-arc extensional setting associated with subduction of the Ailaoshan Paleotethyan branch ocean during the Middle Permian, thus ruling out the previously speculated linkage to the Emeishan mantle plume, or to an intra-continental rift.

Paleogeography of the Amazon craton at 1.2 Ga: early Grenvillian collision with the Llano segment of Laurentia

NASA Astrophysics Data System (ADS)

Tohver, Eric; van der Pluijm, B. A.; Van der Voo, R.; Rizzotto, G.; Scandolara, J. E.

2002-05-01

A paleomagnetic, geochronologic and petrographic study was undertaken on the flat-lying gabbros and basalts of the Nova Floresta Formation of Rondônia state, western Brazil in order to constrain the Mesoproterozoic paleogeography of the Amazon craton. Measurement of the anisotropy of magnetic susceptibility on the gabbroic samples reveals a flat-lying foliation with a radiating pattern of lineations, supporting the field evidence that the gabbros are part of a large, undeformed sill. Petrographic observations of oxides in the gabbros reveals two populations of magnetite grains produced during the original cooling of the sill: large, oxyexsolved titanomagnetite grains and fine-grained magnetite in igneous reaction rims. New 40Ar/39Ar age dating of biotite and plagioclase yield ages of ∼1.2 Ga, which represent the rapid cooling following emplacement of the mafic magma. Whole rock dating of basalt samples yields total gas ages of 1062±3 Ma, similar to the ∼1.0 Ga K/Ar ages reported by previous workers. However, the strong compositional dependence of the age spectrum renders this younger whole rock age unreliable except as a minimum constraint. A single magnetic component is found in the basalts, indistinguishable from the characteristic remanence found in the gabbros that is oriented WNW and steeply upward. This magnetization is considered to be primary and was acquired during the cooling of the sill and associated lavas. A paleomagnetic pole calculated from the Nova Floresta Formation (n=16 sites, Plat.=24.6°N, Plong.=164.6°E, A95=5.5°, Q=5), the first reported pole for the Amazon craton for the 1200-600 Ma Rodinia time period, constrains the paleogeographic position of Amazonia at ∼1.2 Ga. Juxtaposition of the western Amazon craton with the Llano segment of the Laurentia's Grenville margin causes the NF pole to lie on the 1.2 Ga portion of the combined APWP for Laurentia and Greenland, which indicates that a collision with the Amazon craton could have

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

NASA Astrophysics Data System (ADS)

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

2010-12-01

Historically active Redoubt Volcano is a basalt-to-dacite cone constructed upon the Jurassic-early Tertiary Alaska-Aleutian Range batholith. New SHRIMP-RG U-Pb age and trace-element concentration results for zircons from gabbroic xenoliths and crystal-rich andesitic mush from a late Pleistocene pyroclastic deposit indicate that ~310 Ma and ~1865 Ma igneous rocks underlie Redoubt at depth. Two gabbros have sharply terminated prismatic zircons that yield ages of ~310 Ma. Zircons from a crystal mush sample are overwhelmingly ~1865 Ma and appear rounded due to incomplete dissolution. Binary plots of element concentrations or ratios show clustering of data for ~310-Ma grains and markedly coherent trends for ~1865-Ma grains; e.g., ~310-Ma grains have higher Eu/Eu* than most of the ~1865-Ma grains, the majority of which form a narrow band of decreasing Eu/Eu* with increasing Hf content which suggests that ~1865-Ma zircons come from igneous source rocks. It is very unlikely that detrital zircons from a metasedimentary rock would have this level of homogeneity in age and composition. One gabbro contains abundant ~1865 Ma igneous zircons, ~300-310 Ma fluid-precipitated zircons characterized by very low U and Th concentrations and Th/U ratios, and uncommon ~100 Ma zircons. We propose that (1) ~310 Ma gabbro xenoliths from Redoubt Volcano belong to the same family of plutons dated by Aleinikoff et al. (USGS Circular 1016, 1988) and Gardner et al. (Geology, 1988) located ≥500 km to the northeast in basement rocks of the Wrangellia and Alexander terranes and (2) ~1865 Ma zircons are inherited from igneous rock, potentially from a continental fragment that possibly correlates with the Fort Simpson terrane or Great Bear magmatic zone of the Wopmay Orogen of northwestern Laurentia. Possibly, elements of these Paleoproterozoic terranes intersected the Paleozoic North American continental margin where they may have formed a component of the basement to the Wrangellia

Understanding the Magmatic Construction of the Dufek Complex, Antarctica

NASA Astrophysics Data System (ADS)

Cheadle, M. J.; Meurer, W. P.; Grimes, C. B.; Gee, J. S.; McCullough, B. C.

2007-12-01

The Jurassic (~180Ma) Dufex Complex in the Pensacola Mountains of Antarctica is arguably one of the largest layered mafic intrusions in the world, with a minimum areal extent of 6600km2. It is mostly buried beneath the Antarctic Icesheet, but is exposed in two parallel mountain ranges; the 45km long Dufek Massif and the 85km long Forrestal Range, which have exposed stratigraphic thicknesses of ~1.8 km and ~1.7 km respectively (Ford, 1976). The two sections appear to be petrologically related, showing a continuous differentiation trend; although some geophysical studies suggest they may represent separate intrusive events (Ferris et al., 1998). The Dufek Massif section consists of the ~230m thick Walker Anothosite unit overlain by the 1550m thick Augenbaugh Gabbro unit. The bottom of the intrusion is not exposed, although geophysical data suggest the presence of an ultramafic basal unit. In the Antarctic summer of 2006/07, we collected and logged 630 oriented rock cores from the lowermost 600m of the section producing a revised and more detailed stratigraphy for this part of the intrusion. In particular, we re-located the boundary between the Walker Anorthosite upwards, so that the Lower Anorthosite of the Augenbaugh Gabbro unit becomes the top of the Walker Anorthosite. We also collected and logged an additional 210 cores from a 100m section higher in the Augenbaugh Gabbro unit. Magnetic susceptibility variation with height was used to correlate between stratigraphic sections. The Walker Anorthosite consists of ortho- and clinopyroxene-bearing spotted anorthosites, interbedded on the meter scale with norites and layered gabbronorites. Modal plagioclase exceeds 65%. Slumped horizons a few meters thick are common, demonstrating a lack of stability of the accumulating mush. The lower part of the Augenbaugh Gabbro unit consists of massive and weakly banded gabbronorites with both cumulus pyroxene and plagioclase, and modal plagioclase ranging from 55- 65%. Rare, thin

The Generation of Oceanic Lithosphere in an Embryonic Oceanic Crust : the Example of the Chenaillet Ophiolite in the Western Alps

NASA Astrophysics Data System (ADS)

Masini, E.; Manatschal, G.; Muntener, O.

2007-12-01

The Chenaillet Ophiolite exposed in the Franco-Italian Alps represents a well-preserved ocean-floor sequence that was only weakly affected by later Alpine convergence. Based on the similarity between rock types and structures reported from ultraslow spreading ridges and those observed in the Chenaillet Ophiolite, it may represent a field analogue for slow to ultraslow spreading ridges such as the Gakkel Ridge or the Southwest Indian Ridge. Mapping of the Chenaillet Ophiolite enabled to identify an oceanic detachment fault that extends over a surface of about 16 km2 capping exhumed mantle and gabbros onto which clastic sediments have been deposited. The footwall of the detachment is formed by mafic and ultramafic rocks. The mantle rocks are strongly serpentinized lherzolites and subordinate harzburgites and dunites. Microstructures reminiscent of impregnation, and cpx major and trace element chemistry indicate that spinel peridotite is (locally) replaced by plagioclase-bearing assemblages. Pyroxene thermometry on primary minerals indicates high temperatures of equilibration ( max 1200°C) for the mantle rocks. Gabbros range from troctolite and olivine-gabbros to Fe-Ti gabbros and show clear evidence of syn-magmatic deformation, partially obliterated by retrograde amphibolite and low-grade metamorphic conditions. In sections perpendicular to the detachment within the footwall, syn-tectonic gabbros and serpentinized peridotites grade over some tens of meters into cataclasites that are capped by fault gouges. Petro-structural investigations of the fault rocks reveal a syn-tectonic retrograde metamorphic evolution. Clasts of dolerite within the fault zone suggest that detachment faulting was accompanied by magmatic activity. Hydrothermal alteration is indicated by strong mineralogical and chemical modifications. Gabbro and serpentinized peridotite, together with serpentinite cataclasites occur as clasts in tectono-sedimentary breccias overlying directly the detachment

Chemical variations in the Triple Group of the Skaergaard intrusion: insights for the mineralization and crystallization process

NASA Astrophysics Data System (ADS)

Nielsen, T. F.; Bernstein, S.

2009-12-01

The 54 Ma. old Skaergaard intrusion ( East Greenland) is a type example for fractionation of basaltic melt along the Fenner Trend. The Triple Group is the upper most 100 m of the Middle Zone and consists of FeTi-oxide rich layered gabbro with three distinct leugabbro layers 2-5 m thick ( L-layers; L1-L3, 2-5m thick) and a less marked layer (L0) c.20 m below L1. These are the most marked of many such layers. Apart from the pronounced layering the lower part of the Triple Group also hosts a world class Au-PGE mineralization. The mineralization is perfectly concordant with the L-layers, and the Triple Group invites investigation of the relationship between host and mineralization. The mineralization includes 5 main levels defined by palladium concentration. The chemical variation across the mineralization is covered by ca. 250 bulk major and trace element compositions, each representing 25cm of stratigraphy giving a continuum of ca. 60m. Proportions of normative plagioclase (plag) and pyroxene (px, including cpx and opx) are complementary, except in mineralized gabbro which is rich in FeTi-oxides. Cumulus ilmenite (ilm) is strongly enriched in layers (7m apart). They occur in both plag- and px-rich gabbro, whereas magnetite (mt) shows no simple correlation with ilm and is mainly a poikilitic intercumulus phase. The L-layers are composed of an upper part rich in plag and px and poor in FeTi-oxides, and a lower part rich in plag and FeTi-oxides and poor in px. The marked breaks in the mineralogy in the L-layers separate one layered succession from the next. The layered successions consist of a lower oxide-poor px-plag adcumulate, followed by complex mesocratic orthocumulate with poikilitic interstitial FeTi-oxide, and an upper part of increasingly simple plag-rich adcumulate with decreasing content of interstitial mt. The Au-PGE mineralized levels are found in the complex FeTi-rich gabbros at and in the base of the leucogabbro layers. The stratigraphic variation in

Intrusive rocks of the Wadi Hamad Area, North Eastern Desert, Egypt: Change of magma composition with maturity of Neoproterozoic continental island arc and the role of collisional plutonism in the differentiation of arc crust

NASA Astrophysics Data System (ADS)

Basta, Fawzy F.; Maurice, Ayman E.; Bakhit, Bottros R.; Azer, Mokhles K.; El-Sobky, Atef F.

2017-09-01

The igneous rocks of the Wadi Hamad area are exposed in the northernmost segment of the Arabian-Nubian Shield (ANS). These rocks represent part of crustal section of Neoproterozoic continental island arc which is intruded by late to post-collisional alkali feldspar granites. The subduction-related intrusives comprise earlier gabbro-diorites and later granodiorites-granites. Subduction setting of these intrusives is indicated by medium- to high-K calc-alkaline affinity, Ta-Nb troughs on the spider diagrams and pyroxene and biotite compositions similar to those crystallized from arc magmas. The collisional alkali feldspar granites have high-K highly fractionated calc-alkaline nature and their spider diagrams almost devoid of Ta-Nb troughs. The earlier subduction gabbro-diorites have lower alkalis, LREE, Nb, Zr and Hf values compared with the later subduction granodiorites-granites, which display more LILE-enriched spider diagrams with shallower Ta-Nb troughs, reflecting variation of magma composition with arc evolution. The later subduction granitoids were generated by lower degree of partial melting of mantle wedge and contain higher arc crustal component compared with the earlier subduction gabbro-diorites. The highly silicic alkali feldspar granites represent extensively evolved melts derived from partial melting of intermediate arc crustal sources during the collisional stage. Re-melting of arc crustal sources during the collisional stage results in geochemical differentiation of the continental arc crust and the silicic collisional plutonism drives the composition of its upper part towards that of mature continental crust.

Mesosiderite clasts with the most extreme positive europium anomalies among solar system rocks

NASA Technical Reports Server (NTRS)

Mittlefehldt, David W.; Rubin, Alan E.; Davis, Andrew M.

1992-01-01

Pigeonite-plagioclase gabbros that occur as clasts in mesosiderites (brecciated stony-iron meteorites) show extreme fractionations of the rare-earth elements (REEs) with larger positive europium anomalies than any previously known for igneous rocks from the earth, moon, or meteorite parent bodies and greater depletions of light REEs relative to heavy REEs than known for comparable cumulate gabbros. The REE pattern for merrillite in one of these clasts is depleted in light REEs and has a large positive europium anomaly as a result of metamorphic equilibration with the silicates. The extreme REE ratios exhibited by the mesosiderite clasts demonstrate that multistage igneous processes must have occurred on some asteroids in the early solar system. Melting of the crust by large-scale impacts or electrical induction from an early T-Tauri-phase sun may be responsible for these processes.

What is an Oceanic Core Complex?

NASA Astrophysics Data System (ADS)

Schroeder, T.; Cheadle, M. J.

2007-12-01

The Mid-Atlantic Ridge (MAR) 75km north and south of the 15-20 Fracture Zone (FZ) has produced upper oceanic lithosphere composed dominantly of mantle peridotite with gabbro intrusions. In the absence of diapirism, mantle peridotite can only be exposed on the seafloor by extensional faulting, thus the sea floor geology and bathymetry provide widespread evidence for extensive low-angle faulting. However, only 3% of the seafloor in this region has the domal morphology characteristic of features that have been termed oceanic core complexes; suggesting that other processes, in addition to low-angle faulting, are responsible for the generation of domal core complexes. Most low-angle faults near the 15-20 FZ form gently dipping (10-15°), 10-15km-wide dip slopes on the flanks of 2000m relief bathymetric ridges that are up to 15-40km long (parallel to the MAR). Core recovered from ODP Leg 209 drill holes in these ridges is dominantly peridotite with small (<50m thick) gabbro intrusions. The peridotite is cut by a very high density of brittle faults dipping at both steep and gentle angles. Several holes also contain long-lived shear zones/faults in their upper reaches in which strain was localized at granulite facies, indicated by mylonitic olivine and cpx, and remained active during cooling to sub-greenschist grade, indicated by cross-cutting of progressively lower-grade syn-deformation mineral assemblages. These observations suggest that seafloor spreading is largely accommodated here by slip on low-angle faults, and that these faults are correctly termed detachment faults. Holes drilled into a domal oceanic core complex north of the 15-20 FZ during Leg 209 (ODP Site 1275) recovered dominantly gabbro and not mantle peridotite. This hole is cut by significantly fewer brittle and ductile faults than the peridotite drilled at the non-core-complex detachment fault sites. The detachment fault in the upper reaches (50m) of Site 1275 was localized at temperatures near feldspar

Formation of the lower ocean crust and the crystallization of gabbroic cumulates at a very slowly spreading ridge

NASA Astrophysics Data System (ADS)

Natland, James H.; Dick, Henry J. B.

2001-10-01

Ocean Drilling Program Hole 735B was extended to 1508 m below the sea floor during Leg 176, atop a shallow bank near Atlantis II Fracture Zone on the very slowly spreading Southwest Indian Ridge. All the drilling was in gabbro, and recovery averaged nearly 87%. The drill penetrated a series of stacked plutons consisting mostly of olivine gabbro, but some with troctolite. Each pluton is some 200-500 m thick, each has its own internally coherent stratigraphy, and each apparently represents an individual event of significant magma inflation and addition to the crust. The entire column was extensively deformed along inclined zones of distributed shear before it was completely frozen, this marking the onset of unroofing of the rocks and their ascent to high rift mountains. The deformation mobilized late-stage melts into flow patterns which led to concentration of ilmenite and magnetite in hundreds of seams of oxide gabbro along or near zones of strong crystal-plastic deformation, the highly differentiated melts overall being concentrated by buoyancy forces toward the top of the section, especially in one zone nearly 70 m thick. However, upward flow was ultimately blocked or deflected by zones of impermeable rock resulting either from downward freezing or grain-size reduction during shear. A melt lens probably did not form at the base of sheeted dikes, as it does at the East Pacific Rise. Despite this, the rocks are cumulates, and most are adcumulates, with very low residual melt porosities. Cumulate theory based on stratiform, layered intrusions does not entirely apply to these rocks. Instead, all gabbros, including the oxide gabbros, crystallized in a dense crystal mush in patterns dominated by fractures, channelized flow, and intergranular porous flow. Most gabbros are not layered; weak modal layering of uncertain origin is present in <2% of the rocks. Porosity reduction leading to formation of adcumulates at all stages was extremely efficient. This occurred in the

Geology of the Alaska-Juneau lode system, Alaska

USGS Publications Warehouse

Twenhofel, William Stephens

1952-01-01

The Alaska-Juneau lode system for many years was one of the worlds leading gold-producing areas. Total production from the years 1893 to 1946 has amounted to about 94 million dollars, with principal values in contained gold but with some silver and lead values. The principal mine is the Alaska-Juneau mine, from which the lode system takes its name. The lode system is a part of a larger gold-bearing belt, generally referred to as the Juneau gold belt, along the western border of the Coast Range batholith. The rocks of the Alaska-Juneau lode system consist of a monoclinal sequence of steeply northeasterly dipping volcanic, state, and schist rocks, all of which have been metamorphosed by dynamic and thermal processes attendant with the intrusion of the Coast Range batholith. The rocks form a series of belts that trend northwest parallel to the Coast Range. In addition to the Coast Range batholith lying a mile to the east of the lode system, there are numerous smaller intrusives, all of which are sill-like in form and are thus conformable to the regional structure. The bedded rocks are Mesozoic in age; the Coast Range batholith is Upper Jurassic and Lower Cretaceous in age. Some of the smaller intrusives pre-date the batholith, others post-date it. All of the rocks are cut by steeply dipping faults. The Alaska-Juneau lode system is confined exclusively to the footwall portion of the Perseverance slate band. The slate band is composed of black slate and black phyllite with lesser amounts of thin-bedded quartzite. Intrusive into the slate band are many sill-like bodies of rocks generally referred to as meta-gabbro. The gold deposits of the lode system are found both within the slate rocks and the meta-gabbro rocks, and particularly in those places where meta-gabbro bodies interfinger with slate. Thus the ore bodies are found in and near the terminations of meta-gabbro bodies. The ore bodies are quartz stringer-lodes composed of a great number of quartz veins from 6

The origin of Karaj dam basement sill marginal reversal by Soret fractionation

NASA Astrophysics Data System (ADS)

Maghdour-Mashhour, Reza; Esmaeily, Dariush

2010-05-01

The Karaj dam basement sill (KDBS), located North West of Tehran, northern Iran, is one of the several E-W-trending plutons in the Albourz Mountains. The KDBS consists of a layered series between upper and lower chilled margins. The rocks of the chilled margins are gabbroic in composition and porphyritic, with euhedral to subhedral plagioclase and clinopyroxene megacrysts up to 5 mm long. The rocks become coarse-grained toward the center of the sill and show a gradual transition from porphyritic to equigranular texture. Field and petrographic observations reveal a reverse trend in marginal units crystallization from the eutectic point to the main magma composition; i.e., the olivine-bearing gabbro (porphyritic chilled margin), which has a eutectic composition, crystallized prior to the marginal gabbros, which have a cotectic or near-cotectic composition, as plagioclase laths in the gabbroic unit are embedded in large crystals of clinopyroxene and this phenomenon is believed to result from the cotectic crystallization of plagioclase and clinopyroxene. Four major mechanisms are proposed and discussed in order to find the exact mechanism responsible for marginal reversal formation as following: 1) Crystal settling is a gravity-dependent mechanism and phenocrysts must have settled to form a layer at the bottom of the sill, showing sharp upper boundary which is not observable in KDBS. Besides, the reverse fractionation of inwardly-dipping sequence of mentioned sill occurs in layers with primary dips up to 55°. Consequently capability of marginal reversals to develop along steeply inclined chamber margins, by this mechanism is implausible. 2) Multiple injections of successive magma pulses fails to explain the origin of marginal reversal since the transition along the entire length of marginal reversal is gradual also there is no compositional break or chilled contact between two mentioned units of KDBS margin (Olivine-gabbro and marginal gabbro). 3) The idea of

Seismic wave velocity of rocks in the Oman ophiolite: constraints for petrological structure of oceanic crust

NASA Astrophysics Data System (ADS)

Saito, S.; Ishikawa, M.; Shibata, S.; Akizuki, R.; Arima, M.; Tatsumi, Y.; Arai, S.

2010-12-01

Evaluation of rock velocities and comparison with velocity profiles defined by seismic refraction experiments are a crucial approach for understanding the petrological structure of the crust. In this study, we calculated the seismic wave velocities of various types of rocks from the Oman ophiolite in order to constrain a petrological structure of the oceanic crust. Christensen & Smewing (1981, JGR) have reported experimental elastic velocities of rocks from the Oman ophiolite under oceanic crust-mantle conditions (6-430 MPa). However, in their relatively low-pressure experiments, internal pore-spaces might affect the velocity and resulted in lower values than the intrinsic velocity of sample. In this study we calculated the velocities of samples based on their modal proportions and chemical compositions of mineral constituents. Our calculated velocities represent the ‘pore-space-free’ intrinsic velocities of the sample. We calculated seismic velocities of rocks from the Oman ophiolite including pillow lavas, dolerites, plagiogranites, gabbros and peridotites at high-pressure-temperature conditions with an Excel macro (Hacker & Avers 2004, G-cubed). The minerals used for calculations for pillow lavas, dolerites and plagiogranites were Qtz, Pl, Prh, Pmp, Chl, Ep, Act, Hbl, Cpx and Mag. Pl, Hbl, Cpx, Opx and Ol were used for the calculations for gabbros and peridotites. Assuming thermal gradient of 20° C/km and pressure gradient of 25 MPa/km, the velocities were calculated in the ranges from the atmospheric pressure (0° C) to 200 MPa (160° C). The calculation yielded P-wave velocities (Vp) of 6.5-6.7 km/s for the pillow lavas, 6.6-6.8 km/s for the dolerites, 6.1-6.3 km/s for the plagiogranites, 6.9-7.5 km/s for the gabbros and 8.1-8.2 km/s for the peridotites. On the other hand, experimental results reported by Christensen & Smewing (1981, JGR) were 4.5-5.9 km/s for the pillow lavas, 5.5-6.3 km/s for the dolerites, 6.1-6.3 km/s for the plagiogranites, 6

Defense Technical Information Center Cataloging Guidelines

DTIC Science & Technology

1994-02-01

24 Field 10 - Personal Authors ............................................................ 27 Field 11 - Date of...Stanford, Calif. 24 . If the subordinate element of a heading begins with a personal name, that name is written without rearrangement. APPEARS AS...country is not repeated if ’ncluded in the heading. APPEARS AS: ENTER AS: Centro Brasileiro de Pesquisas Centro Brasileiro de Fisicas, Rio de Janeiro

Geological setting, emplacement mechanism and igneous evolution of the Atchiza mafic-ultramafic layered suite in north-west Mozambique

NASA Astrophysics Data System (ADS)

Ibraimo, Daniel Luis; Larsen, Rune B.

2015-11-01

The Atchiza mafic and ultramafic-layered suite (hereafter, "Atchiza Suite) crops out in an area 330 km2 west of the Mozambican Tete province. In an early account of the geology of this intrusion, it was considered the continuation of the Great Dyke of Zimbabwe, an idea that was aborted after detailed studies. Nevertheless, the Ni concentrations in the Atchiza outcrop rocks are considerable. Our investigation used field evidence, hand specimens and petrography descriptions, mineral chemistry studies using electron microprobe analysis and tectonic analysis to arrive at a plausible mineralogical composition and understanding of the tectonic setting for the igneous evolution. The mineral composition from the Atchiza Suite indicates that these are cumulates. The magmatic segregation from the petrographic and mineral composition reasoning indicates that dunite-lherzolitic peridotite-olivine gabbro-gabbronorite-gabbro-pegmatitic gabbro is the rock formation sequence. Olivine and chromite were the first phases formed, followed by pyroxene and plagioclase. In addition, it is shown that these minerals are near-liquidus crystallization products of basaltic magma with olivine Fo: 87.06 in dunite, mean values of clinopyroxene are (Wo: 36.4, En: 48.0, Fs: 15.2), orthopyroxene (Wo: 2.95, En: 73.0, Fs: 24.2) and plagioclase An: 71.3, respectively. Opaque minerals comprise Fe-Ti oxides and (Fe, Cr) spinel up to 4.8 vol.%, but chromitite layers are not present. Most of the opaque minerals are interstitial to pyroxene. Sulphides are common in gabbros, with pyrrhotite, pentlandite, chalcopyrite, pyrite and covellite together comprising 0.4-2.0 vol.%. The whole rock Rare Earth Element (REE) concentrations are mainly a result of differentiation, but slight crustal contamination/assimilation contributed to the REE contents. In addition, they also show Eu enrichment, suggesting that plagioclase fractionation was important in the rock. The Atchiza Suite preserves a deep-seated plumbing

Character of High Temperature Mylonitic Shear Zones Associated with Oceanic Detachment Faults at the Ultra-Slow Mid-Cayman Rise

NASA Astrophysics Data System (ADS)

Marr, C.; John, B. E.; Cheadle, M. J.; German, C. R.

2014-12-01

Two well-preserved core complexes at the Mid-Cayman Rise (MCR), Mt Dent and Mt Hudson, provide an opportunity to examine the deformation history and rheology of detachment faults at an ultra-slow spreading ridge. Samples from the CAYTROUGH (1976-77) project and the Nautilus NA034 cruise (2013) were selected for detailed petrographic and microstructural study. Surface samples from Mt. Dent (near the center of the MCR) provide insight into lateral variation in footwall rock type and deformation history across a core complex in both the across and down dip directions. In contrast, sampling of Mt. Hudson (SE corner of the MCR) focuses on a high-angle, crosscutting normal fault scarp, which provides a cross section of the detachment fault system. Sampling across Mt Dent reveals that the footwall is composed of heterogeneously-distributed gabbro (47%) and peridotite (20%) with basaltic cover (33%) dominating the top of the core complex. Sampling of Mt Hudson is restricted to the normal fault scarp cutting the core complex and suggests the interior is dominated by gabbro (85% gabbro, 11% peridotite, 4% basalt). At Mt. Dent, peridotite is exposed within ~4km of the breakaway indicating that the Mt. Dent detachment does not cut Penrose-style oceanic crust. The sample set provides evidence of a full down-temperature sequence of detachment related-fault rocks, from possible granulite and clear amphibolite mylonitizatization to prehnite-pumpellyite brittle deformation. Both detachments show low-temperature brittle deformation overprinting higher temperature plastic fabrics. Fe-Ti oxide gabbro mylonites dominate the sample set, and plastic deformation of plagioclase is recorded in samples collected as near as ~4km from the inferred breakaway along the southern flank of Mt. Dent, suggesting the brittle-plastic transition was initially at ~3km depth. Recovered samples suggest strain associated with both detachment systems is localized into discrete mylonitic shear zones (~1-10cm

Petrogenesis and origin of the Upper Jurassic-Lower Cretaceous magmatism in Central High Atlas (Morocco): Major, trace element and isotopic (Sr-Nd) constraints

NASA Astrophysics Data System (ADS)

Essaifi, Abderrahim; Zayane, Rachid

2018-01-01

During an uplift phase, which lasted ca. 40 Ma, from the Late Jurassic (165 Ma) to the Early Cretaceous (125 Ma), transitional to moderately alkaline magmatic series were emplaced in the Central High Atlas. The corresponding magmatic products include basaltic lava flows erupted within wide synclines and intrusive complexes composed of layered mafic intrusions and monzonitic to syenitic dykes emplaced along narrow anticlinal ridges. The igneous rock sequence within the intrusive complexes is composed of troctolites, olivine-gabbros, oxide-gabbros, monzonites and syenites. The chemical compositions of the various intrusive rocks can be accounted for by crystal accumulation, fractional crystallization and post-magmatic remobilization. The evolution from the troctolites to the syenites was mainly controlled by a fractional crystallization process marked by early fractionation of olivine, plagioclase and clinopyroxene, followed by separation of biotite, amphibole, apatite, and Ti-magnetite. Hydrothermal activity associated with emplacement of the intrusions within the Jurassic limestones modified the elemental and the Sr isotopic composition of the hydrothermally altered rocks In particular the monzonitic to syenitic dykes underwent an alkali metasomatism marked by depletion in K and Rb and enrichment in Na and Sr. As a result, their Sr isotopic composition was shifted towards higher initial Sr isotopic ratios (0.7067-0.7075) with respect to the associated gabbros (0.7036-0.7046). On the contrary, the Nd isotopic compositions were preserved from isotope exchange with the limestones and vary in a similar range to those of the gabbros (+1.6 < εNdi < +4.1). The isotopic and the trace element ratios of the uncontaminated samples were used to constrain the source characteristics of this magmatism. The Sr-Nd isotopic data and the incompatible element ratios (e.g. La/Nb, Zr/Nb, Th/U, Ce/Pb) are consistent with generation from an enriched upper mantle similar to an ocean

Results from the Apollo passive seismic experiment

NASA Technical Reports Server (NTRS)

Lathum, G.; Nakamura, Y.; Dorman, J.; Duennebier, F.; Ewing, M.; Lammlein, D.

1974-01-01

Recent results from the Apollo seismic network suggest that primitive differentiation occurred in the outer shell of the moon to a depth of approximately 300 km; and the central region of the moon is presently molten to a radius of between 200 and 300 km. If early melting to a depth of 300 to 400 km was a consequence of accretional energy, very short accretion times are required. The best model for the zone of original differentiation appears to be a crust 40 to 80 km thick, ranging in composition from anorthositic gabbro to gabbro; overlying an ultramafic cumulate (olivine-pyroxene) about 250 km thick. The best candidate for the molten core appears to be iron or iron sulphide. A new class of seismic signals has recently been identified that may correspond to shallow moonquakes. These are rare, but much more energetic than the more numerous, deep moonquakes.

Results from the Apollo passive seismic experiment

NASA Technical Reports Server (NTRS)

Latham, G.; Nakamura, Y.; Dorman, J.; Duennebier, F.; Ewing, M.; Lammlein, D.

1977-01-01

Recent results from the Apollo Seismic Network suggest that primitive differentiation occurred in the outer shell of the moon to a depth of approximately 300 km and the central region of the moon is presently molten to a radius of between 200 and 300 km. If early melting to a depth of 300 to 400 km was a consequence of accretional energy, very short accretion times are required. It was shown that the best model for the zone of original differentiation is a crust 40 to 80 km thick, ranging in composition from anorthositic gabbro to gabbro, and overlying an ultramafic cumulate about 250 km thick. The best candidate for the molten core appears to be iron or iron sulphide. A new class of seismic signals recently were identified that may correspond to shallow moonquakes. These are rare, but much more energetic than the more numerous, deep moonquakes.

Mineralogy and geochemistry of Eocene Helete formation , Adiyaman, Turkey

NASA Astrophysics Data System (ADS)

Choi, J.; Lee, I.; Yildirim, E.

2013-12-01

Helete formation is located at Adiyaman, Turkey which is in the Alpine-Himalayan orogeny belt. Helete formation is represented by andesitic, basaltic and gabbroic rocks cut by localized felsic intrusions and overlain by open-marine Nummulitic carbonate sediments. Electron microprobe analyses were conducted for 15 rocks samples of Helete formation. These rock samples are named as basalt, andesite, gabbro, diorite, dacite, and granite. Basalt and andesite samples are composed of clinopyroxene(augite), plagioclase(Ab98-96), carbonate, and hyaline. Gabbro samples have wide range of plagioclase composition from anorthite to albite(Ab92-16), and other minerals like clinopyroxene(augite) and amphibole(hornblende and actinolite). Diabase samples consist of epidote group minerals and sphene with plagioclase(Ab80), pyroxene and hornblende. Dacite samples are composed of dolomite and quartz. Granite samples are composed of quartz, chlorite, and plagioclase which range from albite to oligoclase in composition (Ab98-89).

Dependence of frictional strength on compositional variations of Hayward fault rock gouges

USGS Publications Warehouse

Morrow, Carolyn A.; Moore, Diane E.; Lockner, David A.

2010-01-01

The northern termination of the locked portion of the Hayward Fault near Berkeley, California, is found to coincide with the transition from strong Franciscan metagraywacke to melange on the western side of the fault. Both of these units are juxtaposed with various serpentinite, gabbro and graywacke units to the east, suggesting that the gouges formed within the Hayward Fault zone may vary widely due to the mixing of adjacent rock units and that the mechanical behavior of the fault would be best modeled by determining the frictional properties of mixtures of the principal rock types. To this end, room temperature, water-saturated, triaxial shearing tests were conducted on binary and ternary mixtures of fine-grained gouges prepared from serpentinite and gabbro from the Coast Range Ophiolite, a Great Valley Sequence graywacke, and three different Franciscan Complex metasedimentary rocks. Friction coefficients ranged from 0.36 for the serpentinite to 0.84 for the gabbro, with four of the rock types having coefficients of friction ranging from 0.67-0.84. The friction coefficients of the mixtures can be predicted reliably by a simple weighted average of the end-member dry-weight percentages and strengths for all samples except those containing serpentinite. For the serpentinite mixtures, a linear trend between end-member values slightly overestimates the coefficients of friction in the midcomposition ranges. The range in strength for these rock admixtures suggests that both theoretical and numerical modeling of the fault should attempt to account for variations in rock and gouge properties.

Monzonitoid magmatism of the copper-porphyritic Lazurnoe deposit (South Primor'e): U-Pb and K-Ar geochronology and peculiarities of ore-bearing magma genesis by the data of isotopic-geochemical studies

NASA Astrophysics Data System (ADS)

Sakhno, V. G.; Kovalenko, S. V.; Alenicheva, A. A.

2011-05-01

Magmatic rocks from the copper-porphyritic Lazurnoe deposit (Central Primor'e) have been studied. It has been found that rocks from the Lazurnyi massif are referred to gabbro-monzodiorites, monzodiorites, and monzo-granodiorites formed during two magmatic phases of different ages. The earlier phase is represented by gabbro-monzodiorites and diorites of the North Stock, and the later one, by gabbro-monzodiorites and monzo-grano-diorites of the South Stock. On the basis of isotopic dating by the U-Pb (SHRIMP) method for zircon and by the K-Ar method for hornblendes and biotites, the age of magmatic rocks is determined at 110 ± 4 for the earlier phase and at 103.5 ± 1.5 for the later one. Examination of the isotopic composition for Nd, Sr, Pb, Hf, δ18O, and REE spectra has shown that melts of the first phase are contaminated with crustal rocks and they are typical for a high degree of secondary alterations. Potassiumfeldspar, biotite, propylitic alterations, and sulfidization are manifested in these rocks. The rocks of the later stage of magmatism are characteristic for a primitive composition of isotopes and the absence of secondary alterations. They carry the features of adakite specifics that allows us to consider them derivatives of mantle generation under high fluid pressure. The intrusion of fluid-saturated melts of the second phase into the magmatic source of the first phase caused both an alteration pattern of rocks and copper-porphyritic mineralization. Isotopes of sulfur and oxygen allow us to consider the ore component to be of magmatic origin.

World Epidemiology Review, Number 81

DTIC Science & Technology

1977-04-14

the past several years? 76 The cause might be explained as follows: the number of animals being artificially inseminated by means of whole semen...process consists of allowing the patient suspected of carry- ing Chagas Disease to be bitten by artificially bred "barreiros" ["kissing bug...enlarging service to the people. Next month, health centers will be opened in Iraja and satellite units in Fazenda Coqueiro (Bangu) and in Cidade Alta

Origin and modal petrography of Luna 24 soils

NASA Technical Reports Server (NTRS)

Basu, A.; Mckay, D. S.; Fruland, R. M.

1978-01-01

Petrographic modal analyses of polished grain mounts of fractions in the 20 to 250 micron size range from Luna 24 soil samples are presented and used to infer the nature and relative contributions of source rocks. It is found that more than 90% of the identifiable rock fragments are mare basalts, with about 11% of the soil consisting of the crystalline form. Soil breccias, which make up nearly 10% of the soil, are found to be immature. Electron probe analysis of glass particles reveals principle clusters conforming to anorthosite, anorthositic gabbro and mare basalts. More than half of the soil is composed of monomineralic particles, with pyroxene as the most abundant mineral. It is concluded that 85% of the regolith is derived from local mare basalts and gabbros and about 10% is derived from early cumulates of local mare basalt magma. Highland sources are considered to contribute not more than 3% of the regolith.

Ephemeral magma chambers in the Trinity peridotite, northern California

NASA Astrophysics Data System (ADS)

Cannat, Mathilde; Lécuyer, Christophe

1991-02-01

The Trinity Massif comprises the major lithologies of an ophiolite, as defined at the 1972 Penrose conference. Previous studies have shown, however, that it differs from the Semail (Oman) or Table Mountain (Newfoundland) ophiolitic massifs, particularly because its crustal section is thin, and because its mantle section has vertical plastic flow planes. These features have led to an interpretation of the Trinity Massif as a fragment of slow-spreading oceanic lithosphere (Le Sueur et al., 1984; Boudier and Nicolas, 1985). In this paper, we show that the Trinity gabbros occur in discontinuous, kilometre-sized pockets, intrusive into the mantle peridotites. The internal stratigraphy and the petrological characteristics of these gabbros suggest that they formed in short-lived magma chambers. These ephemeral magma chambers developed after the end of the plastic deformation in the surrounding mantle, when it had cooled down to lithospheric temperatures. We discuss the possibility that these small and ephemeral magma chambers formed at a slow-spreading oceanic ridge.

Igneous Cooling Rate constraints on the Accretion of the lower Oceanic Crust in Mid-ocean Ridges: Insights from a new Thermo-mechanical Model

NASA Astrophysics Data System (ADS)

Garrido, C. J.; Machetel, P.

2005-12-01

We report the results of a new thermo-mechanical model of crustal flow beneath fast spreading mid-ocean ridges to investigate both the effect of deep, near off-axis hydrothermal convection on the thermal structure of the magma chamber and the role of variable number of melt intrusions on the accretion of the oceanic crust. In our model the melt is injected at the center of the axial magma chamber with a 'needle' with adjustable porosity at different depths allowing the simulation of different arrangements of melt injection and supply within the magma chamber. Conversely to previous models, the shape of the magma chamber -defined as the isotherm where 95% solidification of the melt occurs- is not imposed but computed from the steady state reached by the thermal field considering the heat diffusion and advection and the latent heat of crystallization. The motion equation is solved for a temperature and phase dependent viscosity. The thermal diffusivity is also dependent on temperature and depth, with a higher diffusivity in the upper plutonic crust to account for more efficient hydrothermal cooling at these crustal levels. In agreement with previous non-dynamic thermal models, our results show that near, deep off-axis hydrothermal circulation strongly affects the shape of the axial magma by tightening isotherms in the upper half of the plutonic oceanic crust where hydrothermal cooling is more efficient. Different accretion modes have however little effect on the shape of the magma chamber, but result in variable arrangements of flow lines ranging from tent-shape in a single-lens accretion scenario to sub-horizontal in "sheeted-sill" intrusion models. For different intrusion models, we computed the average Igneous Cooling Rates (ICR) of gabbros by dividing the crystallization temperature interval of gabbros by the integrated time, from the initial intrusion to the point where it crossed the 950 °C isotherm where total solidification of gabbro occurs, along individual

Zircon oxygen isotopes reveal Ivrea-Verbano Zone source characteristics of the Sesia Valley Caldera

NASA Astrophysics Data System (ADS)

Economos, R. C.; Quick, J. E.; Sinigoi, S.; de Silva, S. L.

2013-12-01

The Sesia Valley, in the Italian Alpine foothills, contains >14 km diameter caldera adjacent to and structurally shallower than the famous Ivrea-Verbano Zone deep crustal section. The caldera and its associated eruptive sequence presents opportunity to explore volcanic magmatism in light of exposed and well characterized source candidates, namely lower crustal gabbros and the mid-crustal metasedimentary Kinzigite formation. Original geochemical characteristics of volcanic units have been obscured by the effects of subsequent hydrothermal alteration. The resistance of the mineral zircon to fluid alteration makes it a prime candidate for the preservation and exploration of these geochemical signals, such as O isotopes. Lower crustal gabbros in the Ivrea-Verbano Zone have broadly monotonic whole-rock δ18O values between +8 and +9‰VSMOW (Sinigoi et al., 1994). Kinzigites preserve a much higher and more heterogeneous δ18O values, typically ranging from +10‰ up to +15‰ (Baker, 1990). Zircons from the caldera-forming rhyolitic eruption units and a pre-caldera rhyodacitic unit were analyzed by ion microprobe at UCLA for in-situ oxygen isotope ratios. External reproducibility of within-mount standard R33 grains range from 0.27 to 0.36‰. Rhyolites from the caldera-forming eruption yield a range of δ18O(zircon) values from 6.3‰ to 8.3‰. This range displays rough correlation with CL activity - CL active grains have lower δ18O(zircon) values while CL dark grains have higher δ18O(zircon) values. This variation may correlate with U contents, which are notoriously low in zircons from Ivrea-Verbano Zone gabbros. We argue that the range in O isotope values suggests zircons are a good fit for magmas influenced by gabbro and Kinzigite sources. However, these zircons do not appear to be inherited directly from either the gabbro or Kinzigite sources as their O isotope signatures are typically intermediate between the two. The pre-caldera rhyodacite sample displays a

Experimental Impacts into Chondritic Targets. Part 1; Disruption of an L6 Chondrite by Multiple Impacts

NASA Technical Reports Server (NTRS)

Cintala, Mark J.; Horz, Friedrich

2007-01-01

A fragment of an L6 chondrite (ALH 85017,13) with an initial mass (M(sub 0)) of 464.1 g was the target in a series of experimental impacts in which the largest remaining fragment (M(sub R)) after each shot was impacted by a 3.18-mm ceramic sphere at a nominal speed of 2 km/s. This continued until the mass of the largest remaining piece was less than half the mass of the target presented to that shot (M(sub S)). Two chunks of Bushveldt gabbro with similar initial masses were also impacted under the same conditions until M(sub R) was less than half M(sub 0). The two gabbro targets required a total of 1.51x10(exp 7) and 1.75x10(exp 7) erg/g to attain 0.27 and 0.33 M(sub R)/M(sub 0), respectively; the chondrite, however, was considerably tougher, reaching 0.40 and 0.21 M(sub R)/M(sub 0) only after receiving 2.37x10(exp 7) and 3.10x10(exp 7) erg g-1, respectively. The combined ejecta and spallation products from the gabbro impacts were coarser than those from the chondrite and in sufficient quantities that the new surface areas exceeded those from the meteorite until the fifth shot in the chondrite series, which was the number of impacts required to disrupt each gabbro target (i.e., MR/M0 = 0.5). Unlike the behavior shown in previous regolith-evolution series, neither gabbro target produced an enhancement in the size fraction reflecting the mean size of the crystals composing the rock (about 3 mm), an effect possibly related to the width of the shock pulse. The original chondrite was so fine-grained and fractured, and the variance in its grain-size distribution so large, that effects related to grain-size were relegated to the <63- m fraction. Impacts into ALH 85017 produced abundant, fine-grained debris, but otherwise the slopes of its size distributions were comparable to those from other experiments involving natural and fabricated terrestrial targets. The characteristic slopes of the chondrite's size distributions, however, were notably more constant over the entire

Perspectives on basaltic magma crystallization and differentiation: Lava-lake blocks erupted at Mauna Loa volcano summit, Hawaii

USGS Publications Warehouse

McCarter, Renee L.; Fodor, R.V.; Trusdell, Frank A.

2006-01-01

Explosive eruptions at Mauna Loa summit ejected coarse-grained blocks (free of lava coatings) from Moku'aweoweo caldera. Most are gabbronorites and gabbros that have 0–26 vol.% olivine and 1–29 vol.% oikocrystic orthopyroxene. Some blocks are ferrogabbros and diorites with micrographic matrices, and diorite veins (≤2 cm) cross-cut some gabbronorites and gabbros. One block is an open-textured dunite.The MgO of the gabbronorites and gabbros ranges ∼ 7–21 wt.%. Those with MgO >10 wt.% have some incompatible-element abundances (Zr, Y, REE; positive Eu anomalies) lower than those in Mauna Loa lavas of comparable MgO; gabbros (MgO <10 wt.%) generally overlap lava compositions. Olivines range Fo83–58, clinopyroxenes have Mg#s ∼83–62, and orthopyroxene Mg#s are 84–63 — all evolved beyond the mineral-Mg#s of Mauna Loa lavas. Plagioclase is An75–50. Ferrogabbro and diorite blocks have ∼ 3–5 wt.% MgO (TiO2 3.2–5.4%; K2O 0.8–1.3%; La 16–27 ppm), and a diorite vein is the most evolved (SiO2 59%, K2O 1.5%, La 38 ppm). They have clinopyroxene Mg#s 67–46, and plagioclase An57–40. The open-textured dunite has olivine ∼ Fo83.5. Seven isotope ratios are 87Sr/86Sr 0.70394–0.70374 and 143Nd/144Nd 0.51293–0.51286, and identify the suite as belonging to the Mauna Loa system.Gabbronorites and gabbros originated in solidification zones of Moku'aweoweo lava lakes where they acquired orthocumulate textures and incompatible-element depletions. These features suggest deeper and slower cooling lakes than the lava lake paradigm, Kilauea Iki, which is basalt and picrite. Clinopyroxene geobarometry suggests crystallization at <1 kbar P. Highly evolved mineral Mg#s, <75, are largely explained by cumulus phases exposed to evolving intercumulus liquids causing compositional ‘shifts.’ Ferrogabbro and diorite represent segregation veins from differentiated intercumulus liquids filter pressed into rigid zones of cooling lakes. Clinopyroxene

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.

Insights into magmatic processes and hydrothermal alteration of in situ superfast spreading ocean crust at ODP/IODP site 1256 from a cluster analysis of rock magnetic properties

NASA Astrophysics Data System (ADS)

Dekkers, Mark J.; Heslop, David; Herrero-Bervera, Emilio; Acton, Gary; Krasa, David

2014-08-01

We analyze magnetic properties from Ocean Drilling Program (ODP)/Integrated ODP (IODP) Hole 1256D (6°44.1' N, 91°56.1' W) on the Cocos Plate in ˜15.2 Ma oceanic crust generated by superfast seafloor spreading, the only drill hole that has sampled all three oceanic crust layers in a tectonically undisturbed setting. Fuzzy c-means cluster analysis and nonlinear mapping are utilized to study down-hole trends in the ratio of the saturation remanent magnetization and the saturation magnetization, the coercive force, the ratio of the remanent coercive force and coercive force, the low-field magnetic susceptibility, and the Curie temperature, to evaluate the effects of magmatic and hydrothermal processes on magnetic properties. A statistically robust five cluster solution separates the data predominantly into three clusters that express increasing hydrothermal alteration of the lavas, which differ from two distinct clusters mainly representing the dikes and gabbros. Extensive alteration can obliterate magnetic property differences between lavas, dikes, and gabbros. The imprint of thermochemical alteration on the iron-titanium oxides is only partially related to the porosity of the rocks. Thus, the analysis complements interpretation based on electrofacies analysis. All clusters display rock magnetic characteristics compatible with an ability to retain a stable natural remanent magnetization suggesting that the entire sampled sequence of ocean crust can contribute to marine magnetic anomalies. Paleointensity determination is difficult because of the propensity of oxyexsolution during laboratory heating and/or the presence of intergrowths. The upper part of the extrusive sequence, the granoblastic dikes, and moderately altered gabbros may contain a comparatively uncontaminated thermoremanent magnetization.

Characterizing the nature of melt-rock reaction in peridotites from the Santa Elena Ophiolite, NW Costa Rica

NASA Astrophysics Data System (ADS)

Carr, D.; Loocke, M. P.; Snow, J. E.; Gazel, E.

2017-12-01

The Santa Elena Ophiolite (SEO), located on the northwestern coast of Costa Rica, consists primarily of preserved oceanic mantle and crustal rocks thrust above an accretionary complex. The SEO is predominantly characterized by mantle peridotites (i.e., primarily spinel lherzolite with minor amounts of harzburgite and dunite) cut and intruded by minor pegmatitic gabbros, layered gabbros, plagiogranites, and doleritic and basaltic dykes. Previous studies have concluded that the complex formed in a suprasubduction zone (SSZ) setting based on the geochemical nature of the layered gabbros and plagiogranites (i.e., depleted LREE and HFSE and enriched LILE and Pb), as well, as the peridotites (i.e., low-TiO2, Zr, and V, and high MgO, Cr, and Ni)(Denyer and Gazel, 2009). Eighteen ultramafic samples collected during the winter 2010/2011 field season (SECR11) exhibit abundant evidence for melt-rock reaction (e.g., disseminated plagioclase and plagioclase-spinel, clinopyroxene-spinel, and plagioclase-clinopyroxene symplectites) and provide a unique opportunity to characterize the textural and chemical nature of melt-rock reaction in the SEO. We present the results of a petrologic investigation (i.e., petrography and electron probe microanalysis) of 28 thin sections (19 spinel lherzolites, of which 14 are plagioclase-bearing, 4 pyroxenite veins, and 5 harzburgites) derived from the SECR11 sample set. The results of this investigation have the potential to better our understanding of the nature of melt generation and migration and melt-rock interaction in the SEO mantle section and shed further light on the complex petrogenetic history of the SEO. Denyer, P., Gazel, E., 2009, Journal of South American Earth Sciences, 28:429-442.

Gravity driven and in situ fractional crystallization processes in the Centre Hill complex, Abitibi Subprovince, Canada: Evidence from bilaterally-paired cyclic units

NASA Astrophysics Data System (ADS)

Thériault, R. D.; Fowler, A. D.

1996-12-01

The formation of layers in mafic intrusions has been explained by various processes, making it the subject of much controversy. The concept that layering originates from gravitational settling of crystals has been superseded in recent years by models involving in situ fractional crystallization. Here we present evidence from the Centre Hill complex that both processes may be operative simultaneously within the same intrusion. The Centre Hill complex is part of the Munro Lake sill, an Archean layered mafic intrusion emplaced in volcanic rocks of the Abitibi Subprovince. The Centre Hill complex comprises the following lithostratigraphic units: six lower cyclic units of peridotite and clinopyroxenite; a middle unit of leucogabbro; six upper cyclic units of branching-textured gabbro (BTG) and clotted-textured gabbro (CTG), the uppermost of these units being overlain by a marginal zone of fine-grained gabbro. The cyclic units of peridotite/clinopyroxenite and BTG/CTG are interpreted to have formed concurrently through fractional crystallization, associated with periodic replenishment of magma to the chamber. The units of peridotite and clinopyroxenite formed by gravitational accumulation of crystals that grew under the roof. The cyclic units of BTG and CTG formed along the upper margin of the sill by two different mechanisms: (1) layers of BTG crystallized in situ along an inward-growing roof and (2) layers of CTG formed by accumulation of buoyant plagioclase crystals. The layers of BTG are characterized by branching pseudomorphs after fayalite up to 50 cm in length that extend away from the upper margin. The original branching crystals are interpreted to have grown from stagnant intercumulus melt in a high thermal gradient resulting from the injection of new magma to the chamber.

Trace Gases - A Warning Signs of Impending Major Seismic Activity

NASA Astrophysics Data System (ADS)

Baijnath, J.; Freund, F.; Li, J.

2013-12-01

Seismological models can predict future earthquakes only with wide uncertainty windows, typically on the order of decades to centuries. To improve short-term earthquake forecasts, it is essential to understand the non-seismic processes that take place in Earth's crust during the build-up of tectonic stresses. Days prior to the January 2001 M 7.6 Gujurat earthquake in India, there was a significant increase in the regional CO concentration, reaching 240 ppbv over a 100 squared kilometers, as derived from data of the MOPITT sensor onboard the NASA Terra satellite. A possible explanation for these observations is that when stresses in Earth's crust are building, positive hole charge carriers are activated, which are highly mobile and spread from deep below the earth to the surface. Positive holes act as highly oxidizing oxygen radicals, oxidizing water to hydrogen peroxide. It is hypothesized that, as positive hole charge carriers arrive from below and traverse the soil, they are expected to oxidize soil organics, converting aliphatics to ketones, formaldehyde, CO and CO2. This is tested by using a closed chamber with a slab of gabbro rock. Ultrasound generated by a pair of 50 W, 40 kHz piezoelectric transducers, applied to one end of the gabbro slab was used to activate the positive holes. This created a high concentration of positive holes at the end of the rock that the electrical conductivity through the rock increased more than 1000-fold, while the increase in conductivity through the other end of the gabbro slab was on the order of 100-fold. On the other end of the slab, rock dust and various soils were placed. A stainless steel mesh was also placed over the soil and dust to allow a current to flow through the granular material. When the far end of the slab was subjected to the ultrasound, currents as large as 250 nA were recorded flowing through the length of the gabbro slab and through the dust/soil pile. Dry dust/soil and dust samples impregnated with

Jurassic metabasic rocks in the Kızılırmak accretionary complex (Kargı region, Central Pontides, Northern Turkey)

NASA Astrophysics Data System (ADS)

Çelik, Ömer Faruk; Chiaradia, Massimo; Marzoli, Andrea; Özkan, Mutlu; Billor, Zeki; Topuz, Gültekin

2016-03-01

The Kızılırmak accretionary complex near Kargı is tectonically bounded by the Jurassic and Early Cretaceous metamorphic massives of the Central Pontides. It consists mainly of serpentinite, serpentinized peridotite, gabbro, basalt, metabasite and deep-marine sedimentary rocks. The metabasites in the Kızılırmak accretionary complex are tectonically located within a serpentinite, radiolarian chert, spilitized basalt, gabbro association and commonly display a steep contact with serpentinites. Amphiboles from metabasites yielded robust 40Ar/39Ar plateau ages ranging between 159.4 ± 0.4 Ma and 163.5 ± 0.8 Ma. These are interpreted as cooling ages of the metabasites. The metabasites have 87Sr/86Sr(i) between 0.7035 and 0.7044 and 206Pb/204Pb(i) ranging between 18.18 and 18.92. The gabbros have higher 87Sr/86Sr(i) between 0.7044 and 0.7060 and 206Pb/204Pb(i) ranging between 17.98 and 18.43. Three basalt samples display 87Sr/86Sr(i) between 0.7040 and 0.7059. Their 206Pb/204Pb(i) are unrealistically low (15.42 and 15.62), suggesting, most likely, Pb loss which results in over-corrected values for decay through time. Pb-Sr-Nd isotopic compositions for all samples consistently plot between the fields of MORB or the Depleted MORB Mantle reservoirs and enriched mantle reservoirs (EMII rather than EMI). All the samples (except one dolerite dike) have negative ɛNdDM(t = 160 Ma) values, suggesting derivation from a reservoir more enriched than the depleted mantle. The protoliths of metabasites correspond to diverse sources (N-MORB, E-MORB, OIB and IAT) based on whole rock major and trace element composition. An IAT-like protolith for the metabasites indicates that the İzmir-Ankara-Erzincan ocean domain was subducting and the tectonic regime was compressional during Late Jurassic and before. The protoliths of these rocks were metamorphosed during the subduction/accretion processes, as observed in the metamorphic rocks located along the Balkan, Northern Turkey and

Infiltration metasomatism within the Rum layered intrusion (Scotland)

NASA Astrophysics Data System (ADS)

Leuthold, J.; Blundy, J.; Holness, M.

2012-04-01

The well studied Rum Isle layered intrusion was built up by emplacement of a series of 15 macro-rhythmic units. Magma intruded 60.53±0.08 Ma (Hamilton et al., 1998), at a pressure <0.5 kbar (Holness, 1999). Units are composed of feldspathic peridotite at the base, overlain by troctolite and gabbro. Layering is induced by fractional crystallization from picritic magma (Upton et al., 2002; Holness et al., 2007). Rum magma liquid line of descent is complicated by reactive melt percolation within crystal mushes, either originating from continuous compaction of the cumulate pile or from a newly injected reactive picritic magma. The studied Unit 9 sequence is well known for the injection of a picritic sill within the layered intrusion. Clinopyroxene textures are very diverse above the Unit 9 intrusive picritic sill. (1) Late-stage interstitial crystals occur in the basal intrusive peridotite, corresponding to the picritic sill. The Cr2O3 content varies from 1.2 to 0.6 wt% along core to rim profiles. (2) Within the overlying troctolite, poikilitic crystals (up to 2 cm in diameter) enclose randomly oriented plagioclase and olivine, wrapped by a foliated troctolitic assemblage. Within the first 3 meters, the Cr2O3 composition of the few clinopyroxene crystals is very similar to the underlying peridotite one, but it then drops to lower concentration (0.5 to 0 wt%). The plagioclase-clinopyroxene-plagioclase median dihedral angle also decreases from ~90° to ~80° (Sides, 2008). The Cr2O3 content increases progressively towards the troctolite-gabbro wavy horizon. Gabbroic enclaves occur within the troctolite. (3) Cumulus clinopyroxene grains occur in the summit gabbro. Their Cr2O3 content regularly decreases from 1.2-0.8 to 0.7-0.3 across the gabbro pile. Cr-spinel crystals are rare. Median plagioclase-clinopyroxene-plagioclase dihedral angles evolve from ~85° to ~95°. We propose that troctolites, that contain gabbroic enclaves, result from clinopyroxene primocryst

Feldspar basalts in lunar soil and the nature of the lunar continents

NASA Technical Reports Server (NTRS)

Reid, A. M.; Ridley, W. I.; Harmon, R. S.; Warner, J.; Brett, R.; Jakes, P.; Brown, R. W.

1974-01-01

It is found that 25% on the Apollo-14 glasses have the same composition as the glasses in two samples taken from the Luna-16 column. The compositions are equivalent to feldspar basalt and anorthosite gabbro, and are similar to the feldspar basalts identified from Surveyor-7 analysis for lunar continents.

Plagiogranites from Markov Deep, Mid-Atlantic Ridge (MAR): physical conditions and alternative modes of origin

NASA Astrophysics Data System (ADS)

Aranovich, Leonid

2010-05-01

Very fresh samples of plagiogranites (PG) hosted by gabbro and peridotite, were collected from the slopes of slow spreading MAR within the Markov Deep area. The PG form pockets, lenses and veins ranging in size from a few mm to first few cm, and are structurally very similar to the migmatites found in ophiolite complexes. The PG veinlets in peridotite contain no quartz (Qtz) and are separated from the host by clearly seen reaction zones. Their bulk composition (56-58 wt.% SiO2) plots at the extreme SiO2-poor end of the PG compositional range from literature, what could be related to the consumption of SiO2 due to reaction with the host. The PG hosted by gabbro are characterized by the presence of Qtz, and, correspondingly, much higher bulk SiO2 (70-76 wt.%). Some PG-containing gabbro samples show textures indicative of the incipient felsic melt formation via partial melting of the host. In both gabbroic and peridotite samples certain textural and mineral composition changes point to interaction with the PG melt. Pressure (P)-temperature (T) estimates for the melt-forming conditions based on the microprobe analyses of coexisting minerals and multi-mineral thermobarometry approach (TWQ; Berman, 1990) along with the Berman and Aranovich (1996) thermodynamic data set correspond to 2-2.5 kbar and 800-830оС. The consistent (in the sense of TWQ) results could be obtained only taking into account a decreased silica activity in the rocks, which was estimated (relative to the beta-Qtz standard state) at a(SiO2)=0.7 for gabbro and at a(SiO2)=0.5 for peridotite. Under these P-T, generation of felsic melt is only possible in the presence of a water-rich fluid phase. Water activity values (aН2О) were evaluated with two independent methods: (1) TWQ calculations (at a constant P=2.2 kbar and a(SiO2)=0.5) employing compositions of orthopyroxene, clinopyroxene and pargasitic amphibole coexisting in the reaction zones between the PG veinlets and peridotite; (2) model granite melt

Can the composition and structure of the lower ocean crust and upper mantle be known without deep ocean drilling?

NASA Astrophysics Data System (ADS)

Dick, H.; Natland, J.

2003-04-01

No. With few exceptions, lower ocean crust sampled by dredge or submersible in tectonic windows such as Atlantis Bank in the Indian Ocean or the MARK area on the Mid-Atlantic Ridge are not representative of the ocean crust. They represent tectonic mixing of rocks from the mantle and crust on large faults that also localize late magmatic intrusion. Where this can be sorted out, the in-situ crustal sections may generally represent a sub-horizontal cross-section through the lower crust and mantle and not a vertical one. The gabbroic rocks exposed represent largely high-level intrusions, highly hybridized by late melt flow along deep faults, or highly evolved gabbro at the distal ends of larger intrusions emplaced into the mantle near transforms. Oceanic gabbros have average compositions that lie outside the range of primary MORB compositions, and rarely are equivalent to spatially associated MORB either as a parent to, or as a residue of their crystallization. Oceanic gabbros sampled from these complexes generally are very coarse-grained, and are unlike those seen in nearly all ophiolites and layered intrusions. In addition, there are few exposures of gabbro and lower ocean crust and mantle in Pacific tectonic windows, though there the possibility of more representative sections is greater due to their exposure in propagating rifts. Limited samples of the mantle from near the midpoints of ocean ridge segments at slow-spreading rifts are from anomalous crustal environments such as ultra-slow spreading ridges or failed rifts. These include abundant dunites, as opposed to samples from fracture zones, which contain only about 1% dunite. While this indicates focused mantle flow towards the midpoint of a ridge, it also shows that fracture zone peridotites are not fully representative of the oceanic upper mantle. Major classes of rocks common in ophiolites, such as fine to medium grained layered primitive olivine gabbros, troctolites, wherlites and dunites, sheeted dikes

Eclogites and their geodynamic interpretation: a history

NASA Astrophysics Data System (ADS)

Godard, Gaston

2001-09-01

Haüy coined the term eclogite, meaning "chosen rock", in 1822, but de Saussure had already observed rocks of this type in the Alps four decades earlier. Throughout the 19th century, the origin of eclogite remained an enigma, in spite of great progress in our knowledge of this rock. The first chemical analyses, carried out around 1870, showed that its bulk composition was the same as gabbro. Therefore, eclogite was thought to be either an igneous rock of gabbroic composition or a metamorphosed gabbro. This second hypothesis became preferred when progressive transitions were observed between gabbros and eclogites. In 1903, simply by comparing the molar volumes of gabbroic and eclogite parageneses, Becke inferred that eclogite was the high-pressure equivalent of gabbro. In 1920, eclogite was involved in the conception of the metamorphic facies by Eskola. However, a few researchers denied the existence of an eclogite facies, and claimed that high stress instead of high lithostatic pressure could generate eclogites. In the 1960s, consideration of the water pressure parameter also favoured the belief that eclogite was simply the anhydrous equivalent of amphibolite. Finally, eclogite was definitely considered as a high-pressure metamorphic rock following the development of experimental petrology and the application of thermodynamics. In recent years, the discovery of ultrahigh-pressure coesite-bearing rocks in the crust has drastically changed geologists' ideas concerning the limits of eclogite-facies crustal metamorphism. Eclogites have been involved in several geodynamic theories. Around 1900, kimberlite studies favoured the idea that eclogite might be abundant in the interior of the Earth. In 1912, Fermor predicted the existence of a dense eclogite-bearing zone in the mantle. This "eclogite layer" hypothesis was still envisaged as late as 1970. The alternative "peridotite" hypothesis became preferred when experimental investigations demonstrated that the gabbro

In-situ arc crustal section formed at the initial stage of oceanic island arc -Diving survey in the Izu-Bonin forearc-

NASA Astrophysics Data System (ADS)

Ishizuka, O.; Yuasa, M.; Tani, K.; Umino, S.; Reagan, M. K.; Kanayama, K.; Harigane, Y.; Miyajima, Y.

2009-12-01

The Bonin Ridge is an unusually prominent forearc massif in the Izu-Bonin arc that exposes early arc volcanic rocks on Bonin Islands. Submarine parts of the ridge, which could complement the record of volcanism preserved on the islands, had not been extensively investigated. In 2007, dredge sampling in the Izu-Bonin forearc brought us ample evidence of exposure of arc crustal section formed at initial stage of this arc along the landward slope of Izu-Ogasawara trench. Based on this discovery, we conducted Shinkai 6500 submersible survey in May, 2009. This expedition enabled us to obtain general understanding of the crustal section that formed when this oceanic arc began. We investigated 3 areas of the Bonin Ridge. Near 28o25’N, 4 dives were used to look at the lower to upper crustal section. The deepest dive observed both gabbro and basalt/dolerite, and appears to have passed over the boundary between the two. Lower slope is composed of fractured gabbro, whereas pillow lava was observed in the uppermost part of this dive track. Two dives surveyed up-slope of the previous dive found outcrop of numerous doleritic basalt dykes and fractured basaltic lava cut by dykes between water depth of 6000 and 5500m. The shallowest dive recovered volcanic breccia and conglomerate with boninitic and basaltic clasts. Combined with results from other dives and dredging, the members of forearc crustal section are from bottom to top: 1) gabbroic rocks, 2) a sheeted dyke complex, 3) basaltic lava flows, 4) volcanic breccia and conglomerate with boninitic and basaltic clasts, 5) boninite and tholeiitic andesite lava flows and dykes (on the Bonin Islands). In addition to this crustal section, dredge sampling and ROV Kaiko dives recovered mantle peridotite below the gabbro. These observations indicate that almost all of the forearc crust down to Moho has been preserved. Preliminary data indicate that basaltic rocks made of sheeted dykes and lava flows and lower gabbros are generally

Styles of Deformation on Either Side of a Ridge-Transform Intersection, Troodos Ophiolite, Cyprus

NASA Astrophysics Data System (ADS)

Titus, S.; Wagner, C.; Alexander, S. O.; Scott, C. P.; Davis, J. R.

2015-12-01

The Troodos ophiolite in Cyprus includes two orthogonal structures - the NS-striking Solea graben and the EW-striking Arakapas fault - that form a ridge-transform intersection. Sheeted dikes and gabbros are preserved on both the inside and outside corners providing a view of mid-crustal deformation in the system. We examine and model these patterns of deformation using existing map and paleomagnetic data combined with new rock magnetic data. The inside corner of the system has been well studied. The most notable feature is the changing orientation of sheeted dikes, which shift from NW- to NE- to E-striking with increasing proximity to the Arakapas fault. Paleomagnetic data from many studies, including our own, show declination anomalies that vary with distance from the ridge and the transform. The three principal axes from anisotropy of magnetic susceptibility (AMS) ellipsoids in the gabbros seem to be correlated with local sheeted dike orientations. The outside corner of the system has been less well studied. Sheeted dike orientations change more subtly; many are NS-striking and dip towards the Solea Graben, but near the inferred ridge-transform intersection, they are NNE-striking. Our new paleomagnetic data from 26 sites record declination and inclination anomalies that vary spatially within the outside corner. AMS data from the gabbros and sheeted dikes again seem loosely linked to sheeted dike orientations. To summarize, the structural and rock magnetic results on either side of the Solea Graben are distinct, confirming the idea that these rocks formed on different sides of a ridge-transform system. The paleomagnetic data yield insights about the styles of deformation following crystallization. The AMS data may yield insights about magmatic plumbing systems when combined systematically with paleomagnetic results. Our results from the outside corner show that patterns of deformation can be complex even on the non-plate boundary side of a ridge-transform system.

Ultramafic inclusions and host alkali olivine basalts of the southern coastal plain of the Red Sea, Kingdom of Saudi Arabia

USGS Publications Warehouse

Ghent, Edward D.; Coleman, Robert Griffin; Hadley, Donald G.

1979-01-01

A variety of mafic and ultramafic inclusions occur within the pyroclastic components of the Al Birk basalt, erupted on the southern Red Sea coastal plain of Saudi Arabia from Pleistocene time to the present. Depleted harzburgites are the only inclusions contained within the basalts that were erupted through Miocene oceanic crust (15 km thick) in the vicinity of Jizan, whereas to the north in the vicinity of Al Birk, alkali basalts that were erupted through a thicker Precambrian crust (48 km thick) contain mixtures of harzburgites, cumulate gabbro, and websterite inclusions accompanied by large (> 2 cm) megacrysts of glassy alumina-rich clinopyroxene, plagioclase, and spinel. Microprobe analyses of individual minerals from the harzburgites, websterites, and cumulate gabbros reveal variations in composition that can be related to a complex mantle history during the evolution of the alkali basalts. Clinopyroxene and plagioclase megacrysts may represent early phases that crystallized from the alkali olivine basalt magma at depths less than 35 km. Layered websterites and gabbros with cumulate plagioclase and clinopyroxene may represent continuing crystallization of the alkali olivine basalt magma in the lower crust when basaltic magma was not rapidly ascending. It is significant that the megacrysts and cumulate inclusions apparently form only where the magmas have traversed the Precambrian crust, whereas the harzburgite-bearing basalts that penetrated a much thinner Miocene oceanic crust reveal no evidence of mantle fractionation. These alkali olivine basalts and their contained inclusions are related in time to present-day rifting in the Red Sea axial trough. The onshore, deep-seated, undersaturated magmas are separated from the shallow Red Sea rift subalkaline basalts by only 170 km. The contemporaneity of alkaline olivine and subalkaline basalts requires that they must relate directly to the separation of the Arabian plate from the African plate.

Trace element composition of rutile and Zr-in-rutile thermometry in meta-ophiolitic rocks from the Kazdağ Massif, NW Turkey

NASA Astrophysics Data System (ADS)

Şengün, Fırat; Zack, Thomas

2016-08-01

In northwest Turkey, ophiolitic meta-gabbros are exposed on the Kazdağ Massif located in the southern part of the Biga Peninsula. Trace element composition of rutile and Zr-in-rutile temperatures were determined for meta-gabbros from the Kazdağ Massif. The Zr content of all rutiles range from 176 to 428 ppm and rutile grains usually have a homogeneous Zr distribution. The rutile grains from studied samples in the Kazdağ Massif are dominated by subchondritic Nb/Ta (11-19) and Zr/Hf ratios (20-33). Nb/Ta and Zr/Hf show positive correlation, which is probably produced by silicate fractionation. The Nb/Ta and Zr/Hf ratios increase with a decrease in Ta and Hf contents. The core of rutile grains are generally characterized by low Nb/Ta ratios of 17-18 whereas the rims exhibit relatively high Nb/Ta ratios of 19-23. Trace element analyses in rutile suggest that these rutile grains were grown from metamorphic fluids. The P-T conditions of meta-gabbros were estimated by both Fe-Mg exchange and Zr-in-rutile thermometers, as well as by the Grt-Hb-Plg-Q geothermobarometer. The temperature range of 639 to 662 °C calculated at 9 kbar using the Zr-in-rutile thermometer is comparable with temperature estimates of the Fe-Mg exchange thermometer, which records amphibolite-facies metamorphism of intermediate P-T conditions. The P-T conditions of meta-ophiolitic rocks suggest that they occur as a different separate higher-pressure tectonic slice in the Kazdağ metamorphic sequence. Amphibolite-facies metamorphism resulted from northward subduction of the İzmir-Ankara branch of the Neo-Tethyan Ocean under the Sakarya Zone. Metamorphism was followed by internal imbrication of the Kazdağ metamorphic sequence resulting from southerly directed compression during the collision.

Petrogenesis and geodynamics of plagiogranites from Central Turkey (Ekecikdağ/Aksaray): new geochemical and isotopic data for generation in an arc basin system within the northern branch of Neotethys

NASA Astrophysics Data System (ADS)

Köksal, Serhat; Toksoy-Köksal, Fatma; Göncüoglu, M. Cemal

2017-06-01

In the Late Cretaceous, throughout the closure of the Neotethys Ocean, ophiolitic rocks from the İzmir-Ankara-Erzincan ocean branch were overthrusted the northern margin of the Tauride-Anatolide Platform. The ophiolitic rocks in the Ekecikdağ (Aksaray/Central Turkey) region typify the oceanic crust of the İzmir-Ankara-Erzincan branch of Neotethys. The gabbros in the area are cut by copious plagiogranite dykes, and both rock units are intruded by mafic dykes. The plagiogranites are leucocratic, fine- to medium-grained calc-alkaline rocks characterized mainly by plagioclase and quartz, with minor amounts of biotite, hornblende and clinopyroxene, and accessory phases of zircon, titanite, apatite and opaque minerals. They are tonalite and trondhjemite in composition with high SiO2 (69.9-75.9 wt%) and exceptionally low K2O (<0.5 wt%) contents. The plagiogranites in common with gabbros and mafic dykes show high large-ion lithophile elements/high-field strength element ratios with depletion in Nb, Ti and light rare-earth elements with respect to N-MORB. The plagiogranites together with gabbros and mafic dykes show low initial 87Sr/86Sr ratios (0.70419-0.70647), high ƐNd( T) (6.0-7.5) values with 206Pb/204Pb (18.199-18.581), 207Pb/204Pb (15.571-15.639) and 208Pb/204Pb (38.292-38.605) ratios indicating a depleted mantle source modified with a subduction component. They show similar isotopic characteristics to the other supra-subduction zone (SSZ) ophiolites in the Eastern Mediterranean to East Anatolian-Lesser Caucasus and Iran regions. It is suggested that the Ekecikdağ plagiogranite was generated in a short time interval from a depleted mantle source in a SSZ/fore-arc basin setting, and its nature was further modified by a subduction component during intra-oceanic subduction.

New Paleomagnetic Data from the Wadi Abyad Crustal Section and their Implications for the Rotation History of the Oman Ophiolite

NASA Astrophysics Data System (ADS)

Meyer, M.; Morris, A.; Anderson, M.; MacLeod, C. J.

2014-12-01

The Oman ophiolite is an important natural laboratory for understanding the construction of oceanic crust at fast spreading axes and its subsequent tectonic evolution. Previous paleomagnetic research in lavas of the northern ophiolitic blocks (Perrin et al., 2000, Mar. Geophys. Res.) has demonstrated substantial clockwise intraoceanic tectonic rotations. Paleomagnetic data from lower crustal sequences in the southern blocks, however, have been more equivocal due to complications arising from remagnetization, and have been used to infer that clockwise rotations seen in the north are internal to the ophiolite rather than regionally significant (Weiler, 2000, Mar. Geophys. Res.). Here we demonstrate the importance and advantages of sampling crustal transects in the ophiolite in order to understand the nature and variability in magnetization directions. By systematically sampling the lower crustal sequence exposed in Wadi Abyad (Rustaq block) we resolve for the first time in a single section a pattern of remagnetized lowermost gabbros and retention of earlier magnetizations by uppermost gabbros and the overlying dyke-rooting zone. Results are supported by a positive fold test that shows that remagnetization of lower gabbros occurred prior to the Campanian structural disruption of the Moho. NW-directed remagnetized remanences in the lower units are consistent with those used by Weiler (2000, Mar. Geophys. Res.) to infer lack of significant rotation of the southern blocks and to argue, therefore, that rotation of the northern blocks was internal to the ophiolite. In contrast, E/ENE-directed remanences in the uppermost levels of Wadi Abyad imply a large, clockwise rotation of the Rustaq block, of a sense and magnitude consistent with intraoceanic rotations inferred from extrusive sections in the northern blocks. We conclude that without the control provided by systematic crustal sampling, the potential for different remanence directions being acquired at different times

New paleomagnetic data from the Wadi Abyad crustal section and their implications for the rotation history of the Oman ophiolite

NASA Astrophysics Data System (ADS)

Meyer, Matthew; Morris, Antony; Anderson, Mark; MacLeod, Chris

2015-04-01

The Oman ophiolite is an important natural laboratory for understanding the construction of oceanic crust at fast spreading axes and its subsequent tectonic evolution. Previous paleomagnetic research in lavas of the northern ophiolitic blocks (Perrin et al., 2000) has demonstrated substantial clockwise intraoceanic tectonic rotations. Paleomagnetic data from lower crustal sequences in the southern blocks, however, have been more equivocal due to complications arising from remagnetization, and have been used to infer that clockwise rotations seen in the north are internal to the ophiolite rather than regionally significant (Weiler, 2000). Here we demonstrate the importance and advantages of sampling crustal transects in the ophiolite in order to understand the nature and variability in magnetization directions. By systematically sampling the lower crustal sequence exposed in Wadi Abyad (Rustaq block) we resolve for the first time in a single section a pattern of remagnetized lowermost gabbros and retention of earlier magnetizations by uppermost gabbros and the overlying dyke-rooting zone. Results are supported by a positive fold test that shows that remagnetization of lower gabbros occurred prior to the Campanian structural disruption of the Moho. NW-directed remagnetized remanences in the lower units are consistent with those used by Weiler (2000) to infer lack of significant rotation of the southern blocks and to argue, therefore, that rotation of the northern blocks was internal to the ophiolite. In contrast, E/ENE-directed remanences in the uppermost levels of Wadi Abyad imply large, clockwise rotation of the Rustaq block, of a sense and magnitude consistent with intraoceanic rotations inferred from extrusive sections in the northern blocks. We conclude that without the control provided by systematic crustal sampling, the potential for different remanence directions being acquired at different times may lead to erroneous tectonic interpretation.

Evaluation of Different Mineral Filler Aggregates for Asphalt Mixtures

NASA Astrophysics Data System (ADS)

Wasilewska, Marta; Małaszkiewicz, Dorota; Ignatiuk, Natalia

2017-10-01

Mineral filler aggregates play an important role in asphalt mixtures because they fill voids in paving mix and improve the cohesion of asphalt binder. Limestone powder containing over 90% of CaCO3 is the most frequently used type of filler. Waste material from the production of coarse aggregate can be successfully used as a mineral filler aggregate for hot asphalt concrete mixtures as the limestone powder replacement. This paper presents the experimental results of selected properties of filler aggregates which were obtained from rocks with different mineral composition and origin. Five types of rocks were used as a source of the mineral filler aggregate: granite, gabbro, trachybasalt, quartz sandstone and rocks from postglacial deposits. Limestone filler was used in this study as the reference material. The following tests were performed: grading (air jet sieving), quality of fines according to methylene blue test, water content by drying in a ventilated oven, particle density using pyknometer method, Delta ring and ball test, Bitumen Number, fineness determined as Blaine specific surface area. Mineral filler aggregates showed significant differences when they were mixed with bitumen and stiffening effect in Delta ring and ball test was evaluated. The highest values were achieved when gabbro and granite fillers were used. Additionally, Scanning Electron Microscopy (SEM) analysis of grain shape and size was carried out. Significant differences in grain size and shape were observed. The highest non-homogeneity in size was determined for quartz sandstone, gabbro and granite filler. Their Blaine specific surface area was lower than 2800 cm2/g, while for limestone and postglacial fillers with regular and round grains it exceeded 3000 cm2/g. All examined mineral filler aggregates met requirements of Polish National Specification WT-1: 2014 and could be used in asphalt mixtures.

Experimental study of eclogitization and melting of basic rocks at P = 4 GPa and T = 1200-1400°C

NASA Astrophysics Data System (ADS)

Gorbachev, N. S.; Shapovalov, Yu. B.; Kostyuk, A. V.

2017-06-01

Experimental study of gabbro-norite eclogitization and melting at P = 4 GPa has made it possible to reveal the effective influence of fluid and temperature on the phase relationships. The melt composition varies from andesite-dacite in "dry conditions" to phonolite and carbonate in the presence of a fluid. The Grt-containing melting curve is replaced by the Cpx-containing liquidus as the temperature changes or a fluid is added. Hence, the possible presence of "garnetitite" and "clinopyroxenite" in the upper mantle was proved experimentally. The ultimate pressure of the spinel facies at the depth of the eclogite upper mantle is controlled by the stability of Cht ≤ 4 GPa. The revealed similarity of the spectra of REE-adakite, tonalite-trondhjemite-granodiorite (TTG), and melts formed under the partial melting of eclogitized gabbro-norite does not contradict the existing ideas of the eclogite source of the TTG rocks. Wide variations in the interphase microelement distribution factors D (Grt, Cpx)/L are indicative of effective fractionation of the microelements in the course of eclogite melting and differentiation.

Syn-collisional I-type Esenköy Pluton (Eastern Anatolia-Turkey): An indication for collision between Arabian and Eurasian plates

NASA Astrophysics Data System (ADS)

Açlan, Mustafa; Altun, Yusuf

2018-06-01

The Esenköy pluton which is situated in the East Anatolian Accretionary Complex (EACC) is represented by I-type, metalumino, calc-alkaline, VAG + syn-COLG, gabbro, diorite, quartz diorite, tonalite and granodiorite type rocks. This paper presents the characteristics of the above granitoids on their major, trace, rare earth elements (REE) and their zircon U-Pb dating. Zircon U-Pb crystallisation ages for gabbro, tonalite and granodiorite are 22.3 ± 0.2 Ma, 21.7 ± 0.2 Ma and 21.8 ± 0.2 Ma respectively. Esenköy granitoids show medium and high-K calc-alkaline character, with six exceptional K-poor sample plot in tholeiitic series field. The Rb/Y-Nb/Y diagram for Esenköy granitoids display subduction zone enrichment trend. The data which obtained from major, trace and REE geochemical characteristics and 206Pb/238U ages indicate that the collision which is take place between Arabian and Eurasian plates along the Bitlis-Zagros suture zone has begun in the Early Miocene (Aquitanian) or before from Early Miocene.

Attache Extraordinaire: Vernon A. Walters and Brazil

DTIC Science & Technology

2009-03-01

contudo, o primeiro encontro com um brasileiro. De acordo com uma entrevista que Walters concedeu, em 1966, ao Jornal O Globo, o primeiro brasileiro...21 de fevereiro de 1964, às 18 h, Lincoln Gordon enviou um telegrama ao Departamento de Estado. A mensagem dava conta de um encontro que o...disposição de Goulart em assumir riscos extremos, por meio do incentivo a violência esporádica no interior, encontros de grandes multidões e greves com o

The abundances of components of the lunar soils by a least-squares mixing model and the formation age of KREEP.

NASA Technical Reports Server (NTRS)

Schonfeld, E.; Meyer, C., Jr.

1972-01-01

A least-square mixing model incorporating mare basalts, KREEP basalts, anorthosites, anorthositic gabbros, ultramafics, granites, and meteorites was used to estimate the abundances of rock components in lunar soil from the Apollo 11, 12, 15, Luna 16, and Surveyor 5 and 6 landing sites. The predominance of iron-rich mare basalt at the sites is indicated.

Isotopic and trace element compositions of upper mantle and lower crustal xenoliths, Cima volcanic field, California: Implications for evolution of the subcontinental lithospheric mantle

USGS Publications Warehouse

Mukasa, S.B.; Wilshire, H.G.

1997-01-01

Ultramafic and mafic xenoliths from the Cima volcanic field, southern California, provide evidence of episodic modification of the upper mantle and underplating of the crust beneath a portion of the southern Basin and Range province. The upper mantle xenoliths include spinel peridotite and anhydrous and hydrous pyroxenite, some cut by igneous-textured pyroxenite-gabbro veins and dikes and some by veins of amphibole ?? plagioclase. Igneous-textured pyroxenites and gabbros like the dike rocks also occur abundantly as isolated xenoliths inferred to represent underplated crust. Mineral and whole rock trace element compositions among and within the different groups of xenoliths are highly variable, reflecting multiple processes that include magma-mantle wall rock reactions, episodic intrusion and it filtration of basaltic melts of varied sources into the mantle wall rock, and fractionation. Nd, Sr, and Pb isotopic compositions mostly of clinopyroxene and plagioclase mineral separates show distinct differences between mantle xenoliths (??Nd = -5.7 to +3.4; 87Sr/86Sr = 0.7051 - 0.7073; 206Pb/204Pb = 19.045 - 19.195) and the igneous-textured xenoliths (??Nd = +7.7 to +11.7; 87Sr/86Sr = 0.7027 - 0.7036 with one carbonate-affected outlier at 0.7054; and 206Pb/204Pb = 18.751 - 19.068), so that they cannot be related. The igneous-textured pyroxenites and gabbros are similar in their isotopic compositions to the host basaltic rocks, which have ??Nd of+5.1 to +9.3; 87Sr/86Sr of 0.7028 - 0.7050, and 206Pb/204Pb of 18.685 - 21.050. The igneous-textured pyroxenites and gabbros are therefore inferred to be related to the host rocks as earlier cogenetic intrusions in the mantle and in the lower crust. Two samples of peridotite, one modally metasomatized by amphibole and the other by plagioclase, have isotopic compositions intermediate between the igneous-textured xenoliths and the mantle rock, suggesting mixing, but also derivation of the metasomatizing magmas from two separate and

Genesis of the Permian Kemozibayi sulfide-bearing mafic-ultramafic intrusion in Altay, NW China: Evidence from zircon geochronology, Hf and O isotopes and mineral chemistry

NASA Astrophysics Data System (ADS)

Tang, Dongmei; Qin, Kezhang; Xue, Shengchao; Mao, Yajing; Evans, Noreen J.; Niu, Yanjie; Chen, Junlu

2017-11-01

The recently discovered Kemozibayi mafic-ultramafic intrusion and its associated magmatic Cu-Ni sulfide deposits are located at the southern margin of the Chinese Altai Mountain, Central Asian Orogenic Belt in north Xinjiang, NW China. The intrusion is composed of olivine websterite, norite, gabbro and diorite. Disseminated and net-textured Ni-Cu sulfide ores are hosted in the center of the gabbro. In this work, new zircon U-Pb ages, Hf-O isotopic and sulfide S isotopic data, and whole rock and mineral chemical analyses are combined in order to elucidate the characteristics of the mantle source, nature of subduction processes, degree of crustal contamination, geodynamic setting of bimodal magmatism in the region, and the metallogenic potential of economic Cu-Ni sulfide deposit at depth. SIMS zircon U-Pb dating of the gabbro yields Permian ages (278.3 ± 1.9 Ma), coeval with the Kalatongke Cu-Ni deposit and with Cu-Ni deposits in the Eastern Tianshan and Beishan areas. Several lines of evidence (positive εHf(t) from + 7.1 to + 13.3, Al2O3, TiO2 and SiO2 contents in clinopyroxene from olivine websterite, high whole rock TiO2 contents) suggest that the primary magma of the Kemozibayi intrusion was a calc-alkaline basaltic magma derived from depleted mantle, and that the degree of partial melting in the magma source was high. The evolution of the Kemozibayi mafic-ultramafic complex was strongly controlled by fractional crystallization and the crystallization sequence was olivine websterite, norite, and then gabbro. This is evidenced by whole rock Fe2O3 contents that are positively correlated with MgO and negatively correlated with Al2O3, CaO and Na2O, similar LREE enrichment and negative Nb, Ta, Hf anomalies in chondrite and primitive mantle-normalized patterns, and a decrease in total REE and trace elements contents and magnetite content from gabbro through to norite and olivine websterite. Varied and low εHf(t) (+ 7.1 to + 13.3) and high δ18O values (+ 6.4‰ to

Igneous Crystallization Beginning at 20 km Beneath the Mid-Atlantic Ridge, 14 to 16 N

NASA Astrophysics Data System (ADS)

Kelemen, P.

2003-12-01

ODP Leg 209 drilled 19 holes at 8 sites along the Mid-Atlantic Ridge from 14° 43 to 15° 44 N. All sites were previously surveyed by submersible, and were chosen to be < 200 m from peridotite or dunite exposed on the seafloor; outcrops of gabbroic rock were also near some sites. One primary goal of Leg 209 was to constrain melt migration and igneous petrogenesis in this region where residual peridotites are exposed on both sides of the Ridge axis. At Sites 1269 and 1273, we penetrated 112 m of basaltic rubble; recovery was poor (3.7 m) and holes unstable, so drilling was terminated. Lavas form nearly horizontal surfaces overlying cliffs exposing peridotite and gabbro. At 6 other sites, we drilled a mixture of residual peridotite and gabbroic rocks intrusive into peridotite. We penetrated 1075 meters at these 6 sites, and recovered 354 m of core. Drilling at Sites 1268, 1270, 1271 and 1272 recovered 25% gabbroic rocks and 75% residual mantle peridotite. Core from Site 1274 is mainly residual peridotite, with a few m-scale gabbroic intrusions. Core from Site 1275 is mainly gabbroic, but contains 24% poikilitic lherzolite interpreted as residual peridotite "impregnated" by plagioclase and pyroxene crystallized from melt migrating along olivine grain boundaries; these impregnated peridotites were later intruded by evolved gabbros. Impregnated peridotites are also common at Site 1271, and present at Sites 1268 and 1270. The overall proportion of gabbroic rocks versus residual peridotites from these 6 sites is similar to previous dredging and submersible sampling in the area. The proportion of gabbro is larger than in"amagmatic" regions on the ultra-slow spreading SWIR and Gakkel Ridges. Impregnated peridotites from Site 1275 have "equilibrated" textures and contain olivine, 2 pyroxenes, plag and Cr-rich spinel. Their whole rock Mg#, Cr# and Ni are high, extending to residual peridotite values. 87 MORB glasses from 14 to 16° N with Mg# from 60 to 73 [from PetDB] could

Reconstructing the Jurassic Talkeetna Intra-oceanic Arc of Alaska Using Thermobarometry

NASA Astrophysics Data System (ADS)

Hacker, B. R.; Mehl, L.; Kelemen, P. B.; Rioux, M.; Greene, A.

2005-12-01

The Talkeetna arc is one of two intra-oceanic arcs where the entire section from the upper mantle tectonite through the sediments capping the volcanic carapace is well exposed. The objective of this study is to reconstruct the vertical section of the Talkeetna arc by determining the (re) crystallization pressures at various structural levels. This information is crucial if the Talkeetna arc is to be exploited as an archetypal cross section for purposes as diverse as understanding the evolution of the Earth's crust, assessing rates and mechanisms of arc growth, and understanding the tectonic history arcs in general. The base of the arc crust exposed at Bernard and Scarp Mtns includes rare gabbro(norites) with metamorphic garnets-mineral assemblages excellent for thermobarometry. Broad core-to-rim garnet zoning toward lower Mg#, pyroxenes with near-rim, steep increases in Mg# and Al2O3, and unzoned plagioclase document cooling following core crystallization at ~900- 1025 °C and 0.9-1.0 GPa. Hornblende gabbros with magmatic garnet exposed in the Klanelneechena klippe indicate significantly lower P-T of ~700-835°C, 0.69- 0.77 GPa. Hornblende gabbro (norites) that comprise the bulk of the arc were studied in the Tazlina, Barnette, Scarp, and Pippin Ridge sections. Differences in mineral composition indicate qualitatively that the Tazlina, Barnette, and Scarp sections crystallized at successively greater depths. Temperature was calculated using hbl- plg [Holland and Blundy, 1994] and cpx-opx [QUILF; Andersen et al., 1993] net- transfer equilibria, and P was constrained using high δV/δS equilibria among plg, amph, opx, and cpx. Poorly known amphibole and pyroxene Tschermak-component activity models yield large uncertainties for P, but relative P differences can be anchored to the better-determined garnet gabbro P's, revealing that the rocks from the Barnette Creek section crystallized at ~0.40-0.55 GPa and the Tazlina and Pippin sections formed at ~0.25-0.35 GPa. Al

Young cumulate complex beneath Veniaminof caldera, Aleutian arc, dated by zircon in erupted plutonic blocks

USGS Publications Warehouse

Bacon, C.R.; Sison, T.W.; Mazdab, F.K.

2007-01-01

Mount Veniaminof volcano, Alaska Peninsula, provides an opportunity to relate Quaternary volcanic rocks to a coeval intrusive complex. Veniaminof erupted tholeiitic basalt through dacite in the past ???260 k.y. Gabbro, diorite, and miarolitic granodiorite blocks, ejected 3700 14C yr B.P. in the most recent caldera-forming eruption, are fragments of a shallow intrusive complex of cumulate mush and segregated vapor-saturated residual melts. Sensitive high-resolution ion microprobe (SHRIMP) analyses define 238U-230Th isochron ages of 17.6 ?? 2.7 ka, 5+11/-10 ka, and 10.2 ?? 4.0 ka (2??) for zircon in two granodiorites and a diorite, respectively. Sparse zircons from two gabbros give 238-230Th model ages of 36 ?? 8 ka and 26 ?? 7 ka. Zircons from granodiorite and diorite crystallized in the presence of late magmatic aqueous fluid. Although historic eruptions have been weakly explosive Strombolian fountaining and small lava effusions, the young ages of plutonic blocks, as well as late Holocene dacite pumice, are evidence that the intrusive complex remains active and that evolved magmas can segregate at shallow levels to fuel explosive eruptions. ?? 2007 The Geological Society of America.

Structural anatomy of a dismembered ophiolite suite from Gondwana: The Manamedu complex, Cauvery suture zone, southern India

NASA Astrophysics Data System (ADS)

Chetty, T. R. K.; Yellappa, T.; Nagesh, P.; Mohanty, D. P.; Venkatasivappa, V.; Santosh, M.; Tsunogae, T.

2011-08-01

Detailed geological and structural mapping of the Manamedu ophiolite complex (MOC), from the south-eastern part of the Cauvery suture zone (CSZ) within the Gondwana collisional suture in southern India reveals the anatomy of a dismembered ophiolite succession comprising pyroxenite actinolite-hornblendite, hornblendite, gabbro-norite, gabbro, anorthosite, amphibolite, plagiogranite, mafic dykes, and associated pelagic sediments such as chert-magnetite bands and carbonate horizons. The magmatic foliation trajectory map shows inward dipping foliations and a variety of fold structures. Structural cross-sections of the MOC reveal gentle inward dips with repetition and omission of different lithologies often marked by curvilinear hinge lines. The succession displays imbricate thrust sheets and slices of dismembered ophiolite suites distributed along several localities within the CSZ. The MOC can be interpreted as a deformed large duplex structure associated with south-verging back thrust system, consistent with crustal-scale 'flower structure'. The nature and distribution of ophiolitic rocks in the CSZ suggest supra-subduction zone setting associated with the lithospheric subduction of the Neoproterozoic Mozambique Ocean, followed by collision and obduction during the final stage of amalgamation of the Gondwana supercontinent in the end Precambrian.

Petrographic and petrological study of lunar rock materials

NASA Technical Reports Server (NTRS)

Winzer, S. R.

1977-01-01

Impact melts and breccias from the Apollo 15 and 16 landing sites were examined optically and by electron microscope/microprobe. Major and trace element abundances were determined for selected samples. Apollo 16 breccias contained impact melts, metamorphic and primary igneous rocks. Metamorphic rocks may be the equivalents of the impact melts. Apollo 15 breccias studied were fragment-laden melts derived from gabbro and more basalt target rocks.

a Possible Ancient Core Complex in the Northern Cache Creek Terrane, British Columbia

NASA Astrophysics Data System (ADS)

Zagorevski, A.

2013-12-01

The Cache Creek terrane (CCT) in Canadian Cordillera comprises a belt of Mississippian to Jurassic oceanic rocks that include Tethyan carbonates and alkaline basalts that are demonstrably exotic to Laurentia. The exotic Tethyan faunas in the CCT, combined with its inboard position with respect to Stikinia and Yukon-Tanana terranes has led to a variety of tectonic hypotheses including oroclinal enclosure of CCT by Stikinia, Yukon-Tanana and Quesnellia during the Jurassic. Detailed studies have demonstrated that the northern CCT is in fact a composite terrane that includes ophiolitic rocks of both ocean island and island arc origins. The western margin of the CCT is characterized by imbricated harzburgite, island arc tholeiite, sedimentary rocks and locally significant felsic volcanic rocks of the Kutcho arc. Gabbro is volumetrically minor and sheeted dyke complexes are either very rare or not developed. The felsic arc volcanic rocks and the pyroxenite bodies that cut the harzburgite have been previously isotopically dated as Middle Triassic (ca. 245 Ma) suggesting that melt percolation through the mantle was coeval with Kutcho arc magmatism and coincided with a magmatic gap in Stikinia. In general the contact between the mantle and supracrustal rocks is faulted making it difficult to determine the original relationships between the mantle and island arc tholeiites. Locally, the contact appears to be intact and is characterized by mantle tectonites with pyroxenite veins overlain by cumulate plagioclase-orthopyroxene gabbro and fine grained diabase. Elsewhere, volcanic and sedimentary rocks sit in fault contact structurally above the mantle. The absence of voluminous gabbro and sheeted dyke complexes, presence of coeval magmas in the crust and mantle, and low angle extensional faulting in some areas suggests that the western part of the CCT may preserve an ocean core complex similar to the Godzilla Megamullion in the Parece-Vela Basin. Such a hypothesis suggests that

Hf and Nd Isotopic and REE Investigations of Magnetite in a Proterozoic IOCG system: Fingerprinting Sources and Timing of Mineralisation

NASA Astrophysics Data System (ADS)

Schaefer, B. F.

2016-12-01

The Stuart Shelf on the margin of the Gawler Craton, South Australia, contains numerous economic and sub-econmic IOCG mineralised systems, including the giant Olympic Dam Cu-Au-U deposit. Hematite and magnetite have played a critical in the genesis of all of these deposits, and increasingly it appears that magnetite has been in equilibrium with either the final mineralised assemblage or was critical in transporting metals during the ore forming event. 14 magnetites and one hematite from three separate styles of iron oxide mineralisation associated with the Prominent Hill Cu-Au deposit were selected for detailed analysis. The REE and isotopic separations were all conducted by low blank wet chemistry and isotopes determined by TIMS (Nd) and MC-ICPMS (Hf). Magnetites associated with skarn style mineralsiation proximal to the ore body are unformly depleted in REE, whereas hematite within the ore and magmatic magmatites and whole rock gabbros from the nearby 1590Ma White Hill Gabbro intrusion are all relatively LREE enriched and display a comparable range in REE. Significantly however, magnetite separates almost invariably display more evolved Hf isotopic signatures than the host lithologies adjacent the economic mineralisation (dacites and metasediments at Prominent Hill mine) implying that the magnetites were sourcing their REE inventory dominantly from the local crust rather than a mantle derived source. In contrast, the magmatic magnetites from the White Hill Complex display Nd and Hf isotopes which are slightly more primitive, recording a greater relative mantle component, however still requiring a significant crustal input. Significantly, the hematite which contains the Au mineralisation preserves ɛNd (1590) = -4.04 and ɛHf (1590) = -6.05 essentially identical to the magmatic magnetites and their host gabbros in the White Hill complex and the basalts and dacites of the host Gawler Range Volcanics (ɛNd (1590) = -7.10 - -3.72 and ɛHf (1590) = -7.69 - -1

Airborne gamma-ray and magnetic anomaly signatures of serpentinite in relation to soil geochemistry, northern California

USGS Publications Warehouse

McCafferty, A.E.; Van Gosen, B. S.

2009-01-01

Serpentinized ultramafic rocks and associated soils in northern California are characterized by high concentrations of Cr and Ni, low levels of radioelements (K, Th, and U) and high amounts of ferrimagnetic minerals (primarily magnetite). Geophysical attributes over ultramafic rocks, which include airborne gamma-ray and magnetic anomaly data, are quantified and provide indirect measurements on the relative abundance of radioelements and magnetic minerals, respectively. Attributes are defined through a statistical modeling approach and the results are portrayed as probabilities in chart and map form. Two predictive models are presented, including one derived from the aeromagnetic anomaly data and one from a combination of the airborne K, Th and U gamma-ray data. Both models distinguish preferential values within the aerogeophysical data that coincide with mapped and potentially unmapped ultramafic rocks. The magnetic predictive model shows positive probabilities associated with magnetic anomaly highs and, to a lesser degree, anomaly lows, which accurately locate many known ultramafic outcrops, but more interestingly, locate potentially unmapped ultramafic rocks, possible extensions of ultramafic bodies that dip into the shallow subsurface, as well as prospective buried ultramafic rocks. The airborne radiometric model shows positive probabilities in association with anomalously low gamma radiation measurements over ultramafic rock, which is similar to that produced by gabbro, metavolcanic rock, and water bodies. All of these features share the characteristic of being depleted in K, Th and U. Gabbro is the only rock type in the study area that shares similar magnetic properties with the ultramafic rock. The aerogeophysical model results are compared to the distribution of ultramafic outcrops and to Cr, Ni, K, Th and U concentrations and magnetic susceptibility measurements from soil samples. Analysis of the soil data indicates high positive correlation between

Magmatic and tectonic evolution of the Ladakh Block from field studies

NASA Astrophysics Data System (ADS)

Raz, U.; Honegger, K.

1989-04-01

The Ladakh Block is in an intermediate position between the Indian plate in the south and the Karakorum-Tibetan plate in the north. To the west it is separated from the Kohistan Arc by the Nanga Parbat Syntaxis, to the east it is cut off from the Lhasa Block by the Gartok-Nubra Fault. Present data, together with previously published results, show, that the Ladakh Block consists of an island arc in the south and a calc-alkaline batholith in the north with remnants of a continental crust. Migmatitic gneisses and metasedimentary sequences, such as quartzites and metapelites, interbedded with basaltic volcanics and overlain by thick platform carbonates were found as evidence of a continental crust. Remnants of megafossils ( Megalodon and Lithiotis) within the high-grade metamorphic marbles indicate a probable age of Late Triassic to Early Jurassic. These sediments were intruded by a faintly layered hornblende-gabbro, which preceded the calc-alkaline magmatic episode. Gabbro and gabbronorites are found as roof pendants and large inclusions within diorites and granodiorites. The major part of the batholith consists of granodiorite and biotite-granite plutons, ranging from Late Cretaceous to Tertiary. Associated with the intrusives are volcanic rocks with trachyandesite to alkalibasalt and basalt-andesite to rhyolite compositions. Garnet-bearing leucogranites succeeded the emplacement of the major plutons. The magmatic stage ended, finally, by intense fracturing and injections of NE-SW striking andesitic dykes. The southernmost unit of the Ladakh Block is formed by oceanic crust with serpentinized peridotite and hornblende-gabbro and is covered by volcanics of an island-arc type (Dras volcanics). These units are intruded by gabbronorite, as well as Middle and Upper Cretaceous granodiorite and coarse-grained biotite-granite. In a plate tectonic view the Ladakh Block represents a transitional sector between the pure island arc of Kohistan in the west and the Andean type

A Proof of Concept for In-Situ Lunar Dating

NASA Astrophysics Data System (ADS)

Anderson, F. S.; Whitaker, T.; Levine, J.; Draper, D. S.; Harris, W.; Olansen, J.; Devolites, J.

2015-12-01

We have obtained improved 87Rb-87Sr isochrons for the Duluth Gabbro, an analog for lunar KREEP rocks, using a prototype spaceflight laser ablation resonance ionization mass spectrometer (LARIMS). The near-side of the Moon comprises previously un-sampled, KREEP rich, young-lunar basalts critical for calibrating the <3.5 Ga history of the Moon, and hence the solar system, since 3.5 Ga. Measurement of the Duluth Gabbro is a proof of concept of lunar in-situ dating to constrain lunar history. Using a novel normalization approach, and by correcting for matrix-dependent isotope effects, we have been able to obtain a date of 1100 ± 200 Ma (Figure 1), compared to the previously established thermal ionization mass spectrometry measurement of 1096 ± 14 Ma. The precision of LARIMS is sufficient to constrain the current 1 Ga uncertainty of the lunar flux curve, allowing us to reassess the timing of peak lunar volcanism, and constrain lunar thermal evolution. Furthermore, an updated lunar flux curve has implications throughout the solar system. For example, Mars could have undergone a longer epoch of voluminous, shield-forming volcanism and associated mantle evolution, as well as a longer era of abundant volatiles and hence potential habitability. These alternative chronologies could even affect our understanding of the evolution of life on Earth: under the classic chronology, life is thought to have originated after the dwindling of bombardment, but under the alternative chronology, it might have appeared during heavy bombardment. In order to resolve the science questions regarding the history of the Moon, and in light of the Duluth Gabbro results, we recently proposed a Discovery mission called MARE: The Moon Age and Regolith Explorer. MARE would accomplish these goals by landing on a young, nearside lunar basalt flow southwest of Aristarchus that has a crater density corresponding to a highly uncertain absolute age, collecting >10 rock samples, and assessing their

Reconstructing Rodinia: the view from Amazonia

NASA Astrophysics Data System (ADS)

Tohver, E.; van der Pluijm, B.; Van der Voo, R.; Scandolara, J.; Rizzotto, G.

2001-05-01

Many Rodinia reconstructions propose that the North American Grenville orogeny at c.1.1 Ga was due to collision between Laurentia and the western Amazon craton, the position of which is presently unrestricted by paleomagnetic data. New paleomagnetic data was collected from the flat-lying basalts and gabbros of the Nova Floresta Formation (K-Ar whole rock, 982 +/-10 Ma, 1038 +/-14 Ma) of Rondonia, western Brazil to constrain the paleogeography of the proposed Laurentia-Amazonia link. Measurement of the anisotropy of magnetic susceptibility (AMS) on the gabbroic samples reveals a flat-lying foliation with a radiating pattern of lineations, demonstrating that the gabbros are part of a large, undeformed sill. Thermal and alternating field (AF) demagnetization of the basalt samples reveals a single component that is oriented WNW and steeply upward. For the gabbro samples, AF demagnetization is more successful than thermal demagnetization at resolving individual components. A characteristic remanence isolated in fields commonly above 40 mT is identical to the single component recorded in the basalts, suggesting that this magnetization was acquired at the same time. A paleomagnetic N-pole calculated from the Nova Floresta Formation (n = 16 sites, Plat. = 26.1N, PLon. = 163.4E, A95 = 5.9) can be matched to the Laurentia APWP for the 1150-750 Ma interval and permits geographic proximity of the two cratons during Grenvillian times. However, the orientation of the Aguapee-Sunsas belt based on this pole suggests a N-S oriented belt, in contrast with the E-W orientation of the Grenville belt on the Laurentian margin. It is proposed that the Amazon craton was rotated 90 degrees counterclockwise from the orientation required by the Laurentia-Amazonia connection. This N-S orientation of the Sunsas-Aguapei belt suggests that the western margin of the Amazon craton was juxtaposed with the Namaqua belt of the western Kalahari craton. This configuration is supported by a common

Untangling Magmatic Processes and Hydrothermal Alteration of in situ Superfast Spreading Ocean Crust at ODP/IODP Site 1256 with Fuzzy c-means Cluster Analysis of Rock Magnetic Properties

NASA Astrophysics Data System (ADS)

Dekkers, M. J.; Heslop, D.; Herrero-Bervera, E.; Acton, G.; Krasa, D.

2014-12-01

Ocean Drilling Program (ODP)/Integrated ODP (IODP) Hole 1256D (6.44.1' N, 91.56.1' W) on the Cocos Plate occurs in 15.2 Ma oceanic crust generated by superfast seafloor spreading. Presently, it is the only drill hole that has sampled all three oceanic crust layers in a tectonically undisturbed setting. Here we interpret down-hole trends in several rock-magnetic parameters with fuzzy c-means cluster analysis, a multivariate statistical technique. The parameters include the magnetization ratio, the coercivity ratio, the coercive force, the low-field susceptibility, and the Curie temperature. By their combined, multivariate, analysis the effects of magmatic and hydrothermal processes can be evaluated. The optimal number of clusters - a key point in the analysis because there is no a priori information on this - was determined through a combination of approaches: by calculation of several cluster validity indices, by testing for coherent cluster distributions on non-linear-map plots, and importantly by testing for stability of the cluster solution from all possible starting points. Here, we consider a solution robust if the cluster allocation is independent of the starting configuration. The five-cluster solution appeared to be robust. Three clusters are distinguished in the extrusive segment of the Hole that express increasing hydrothermal alteration of the lavas. The sheeted dike and gabbro portions are characterized by two clusters, both with higher coercivities than in lava samples. Extensive alteration, however, can obliterate magnetic property differences between lavas, dikes, and gabbros. The imprint of thermochemical alteration on the iron-titanium oxides is only partially related to the porosity of the rocks. All clusters display rock magnetic characteristics in line with a stable NRM. This implies that the entire sampled sequence of ocean crust can contribute to marine magnetic anomalies. Determination of the absolute paleointensity with thermal techniques is

Late Cretaceous tectonothermal evolution of the southern Lhasa terrane, South Tibet: Consequence of a Mesozoic Andean-type orogeny

NASA Astrophysics Data System (ADS)

Dong, Xin; Zhang, Ze-ming; Klemd, Reiner; He, Zhen-yu; Tian, Zuo-lin

2018-04-01

The Lhasa terrane of the southern Tibetan Plateau participated in a Mesozoic Andean-type orogeny caused by the northward subduction of the Neo-Tethyan oceanic lithosphere. However, metamorphic rocks, which can unravel details of the geodynamic evolution, are rare and only exposed in the south-eastern part of the Lhasa terrane. Therefore, we conducted a detailed petrological, geochemical and U-Pb zircon geochronological study of the late Cretaceous metamorphic rocks and associated gabbros from the Nyemo inlier of the southern Lhasa terrane. The Nyemo metamorphic rocks including gneisses, schists, marbles and calc-silicate rocks, experienced peak amphibolite-facies contact metamorphism under P-T conditions of 3.5-4.0 kbar and 642-657 °C with a very high geothermal gradient of 45-50 °C/km, revealing a distinct deflection from the steady-state geotherm during low-pressure metamorphism. Inherited magmatic zircon cores from the metamorphic rocks yielded protolith ages of 197-194 Ma, while overgrowth zircon rims yielded metamorphic ages of ca. 86 Ma. Whole-rock chemistry and zircon Hf isotopes suggest that the protoliths of the gneisses and schists are andesites and tuffs of the early Jurassic Sangri Group, which were derived from a depleted mantle source of a continental arc affinity. The coeval intimately-associated gabbro (ca. 86 Ma) crystallized under P-T conditions of 3.5-5.3 kbar and 914-970 °C, supplying the heat flux high enough to cause the contact metamorphism of the Sangri Group rock types. We propose that the intrusion of the gabbro and a simultaneous pressure increase of up to 4.0 kbar, which is related to crustal thickening due to crustal overthrusting and the intrusion of mafic material, resulted in the late Cretaceous metamorphism of the early Jurassic Sangri Group during an Andean-type orogeny. Furthermore the Nyemo metamorphic rocks, which have previously been considered to represent slivers of the Precambrian metamorphic basement of the Lhasa terrane

Monitoring of reforested areas using LANDSAT data. [Ribas do Rio Pardo, Mato Grosso do Sul, Brazil

NASA Technical Reports Server (NTRS)

Dejesusparada, N. (Principal Investigator); Filho, P. H.; Shimabukuro, Y. E.

1981-01-01

Imagery obtained with channels 5 and 7 was visually interpreted in an effort to determine the spatial, spectral, and temporal characteristics of a 105,000 hectare area of Fazenda Mutum which was reforested with various species of pine and eucalyptus. It was possible to map a reforested area as small as 6 hectare in its initial implantation using contrast with the surrounding targets. Five classes were mapped: nondeforested areas, partially deforested areas, deforested areas, partially reforested areas, and fully reforested areas. In 1979, 12,000 hectare were deforested, 4,330.83 hectare were partially reforested, and 42,744.71 hectare were reforested.

Permian magmatic sequences of the Bilihe gold deposit in central Inner Mongolia, China: Petrogenesis and tectonic significance

NASA Astrophysics Data System (ADS)

Liu, Chunhua; Nie, Fengjun

2015-08-01

The Bilihe gold deposit is located in the eastern section of the Ondor Sum-Yanji Suture at the southern margin of the Xing'an-Mongolian Orogenic Belt (XMOB) and the northern margin of the North China Craton (NCC), central Inner Mongolia. The magmatic rocks in the ore district are generally high-K calc-alkaline, enriched in LREE, Zr, and Hf, and depleted in HREE, Nb, Ta, and P. The magmatic evolution sequences are norite gabbro → granodiorite porphyry → granite or norite gabbro → andesite → dacite porphyry → granodiorite, which show a trend of decreasing TiO2, FeO, MgO, CaO, and P2O5 with increasing SiO2. In the Bilihe ore district, hydrothermal processes were coeval with granitic magmatism for a period of ~ 17 Myr (272-255 Ma). The ages of the granite, granodiorite porphyry, granodiorite, and dacite porphyry are 271.5-264.1 Ma, 269.8-255.8 Ma, 268.3 Ma, and 268.6-259.4 Ma, respectively. The magmatic rocks contain magmatic, hydrothermal, and magmatic-hydrothermal zircons. The magmatic zircons have δCe > 4, La < 3 ppm, and SmN/LaN > 2.5; the hydrothermal zircons have δCe < 4, La > 3 ppm, and SmN/LaN < 2.5. The Nb/Ta and Zr/Hf ratios of granodiorite are 12.7-14.99 and 40.2-46.56, respectively. The Zr/Hf ratios successively increase in the sequence of granite (27.4-29.02) → granodiorite porphyry (29.19-32.18) → dacite porphyry (33.54-38.5) → norite gabbro (36.75-38.37), and their Nb/Ta ratios are 9.09-12.38. Zircons in granodiorite yield ε Hf (t) values of - 0.29 to - 56 (n = 13) and 2.07-7.62 (n = 5), and they give a Hf two-stage model age (tDM2) of 807-4765 Ma. The ε Hf (t) values of the zircons in granite, granodiorite porphyry, and dacite porphyry are - 0.46 to 8.03, 3.17 to 10.32, and - 0.78 to 6.58, respectively, and their Hf tDM2 ages are 787-1324 Ma, 638-1091 Ma, and 868-1343 Ma, respectively. Dehydration partial melting of subducted oceanic crust resulted in the formation of dacite porphyry; partial melting of depleted mantle resulted in

Portrait of a giant deep-seated magmatic conduit system: The Seiland Igneous Province

NASA Astrophysics Data System (ADS)

Larsen, Rune B.; Grant, Thomas; Sørensen, Bjørn E.; Tegner, Christian; McEnroe, Suzanne; Pastore, Zeudia; Fichler, Christine; Nikolaisen, Even; Grannes, Kim R.; Church, Nathan; ter Maat, Geertje W.; Michels, Alexander

2018-01-01

The Seiland Igneous Province (SIP), Northern Norway, contains > 5000 km2 of mafic and ultramafic intrusions with minor alkaline, carbonatite and felsic rocks that were intruded into the lower continental crust at a depth of 25 to as much as 35 km. The SIP can be geochemically and temporally correlated to numerous dyke swarms throughout Scandinavia at 560-610 Ma, and is linked to magmatic provinces in W-Greenland and NE-America that are collectively known as the Central Iapetus Magmatic Province (CIMP). Revised mapping show that the SIP exposes 85-90% layered tholeiitic- alkaline- and syeno-gabbros, 8-10% peridotitic complexes, 2-5% carbonatite, syenite and diorite that formed within a narrow (< 10 Ma) time frame in the Ediacaran (560-570 Ma). Large peridotite complexes were emplaced into the still hot and unconsolidated gabbro (no dating available) and are regarded as the main-conduit systems. Gravimetric data implies an average thickness of igneous rocks of 4-5 km and also features six deep lithospheric roots of ultramafic rocks extending min 9 km into the crust. Together, the root structures represent the main volcanic conduits conveying thousands of km3 of mafic-ultramafic melts from the asthenosphere to the lithosphere. The ultramafic complexes were predominantly emplaced into the layered gabbros at four major igneous centres, respectively, Nordre Brumandsfjord, Melkvann, Kvalfjord and Reinfjord. All complexes are situated in a right-way-up position and are steep sided forming large plugs. A marginal hybrid zone forms at the contact with country-rock and transitions gradually from olivine-mela-gabbro over pyroxenites that grades in to an olivine-clinopyroxenite zone, which is followed by a wehrlite zone and, finally, the centre of the complexes comprises pure dunite. From pyroxenite to dunite, olivine changes from Fo72 to Fo85 and clinopyroxene from Di80 to Di92 i.e. the complexes observe a reverse fractional crystallisation sequence with time. Parental melt

Construction of the Devonian bimodal Gouldsboro pluton via multiple intrusion, coastal Maine, USA

NASA Astrophysics Data System (ADS)

Koteas, C.

2006-12-01

The Gouldsboro pluton (377 ± 19 ma) (Metzger et al., 1982) of the eastern coastal Maine magmatic province preserves abundant evidence of mechanical interaction between mafic and felsic magmas, as well as the stoping and doming of country rock. Coastal Maine is a unique location where subequal volumes of felsic and mafic material have interacted and are preserved in both plutonic and volcanic complexes. The effects of generation and mobilization of large volumes of magma in the shallow crust are well-preserved. The Gouldsboro pluton provides an excellent perspective from the highest to lowest sections (southwest to northeast) of a hybrid magmatic system. The Gouldsboro magmatic complex is hosted by undated greenstone to the north that is overlain by the 477 ± 18 ma (Metzger, 1977) meta-volcanic/meta-sedimentary Bar Harbor formation. Blocks of both units are discernable as 10 cm to 2 m-diameter blocks within the pluton. The Gouldsboro granite is dominantly a fine to medium grained feldspar- rich leucocratic hornblende granite, although biotite is sometimes present. Miarolitic cavities are common in the highest exposure of the system, especially in the south where the finest grained, most leucocratic granite occurs. Other units include, from lowest to highest in the system, hybrid diorite-gabbro, hybrid granite-granodiorite, granite hosting 30 cm to 3 m-diameter globular basaltic pillows, and a previously unrecognized pyroclastic unit, very similar to the lowermost member of the Cranberry Isle series, a volcanic breccia (Seaman et al., 1999). The pyroclastic unit is intruded by medium grained granite hosting angular fragments of volcanic, mafic plutonic, and country rock. While the contact between gabbro near the base of the pluton and granite in the middle of the pluton is commonly gradational, there are complex intrusive breccias in some areas where fine to coarse, angular diorite to gabbro xenoliths are hosted by granodiorite and granite. Enclave-rich zones

Early Jurassic mafic dykes from the Xiazhuang ore district (South China): Implications for tectonic evolution and uranium metallogenesis

NASA Astrophysics Data System (ADS)

Wang, Lian-Xun; Ma, Chang-Qian; Lai, Zhong-Xin; Marks, Michael A. W.; Zhang, Chao; Zhong, Yu-Fang

2015-12-01

A comprehensive study on zircon U-Pb age dating, whole-rock geochemistry and Sr-Nd isotope data has been conducted on the mafic rocks of the Xiazhuang uranium ore district and adjacent regions in South China. Based on field work and petrographic features, three rock types (the Kuzhukeng gabbro, the WNW-trending dolerite dykes and the NNE-trending lamprophyre dykes) are distinguished. Early Jurassic SHRIMP and LA-ICPMS ages of zircon for the Kuzhukeng gabbro (198 ± 1 Ma) and WNW-trending dolerite dykes (193 ± 4 Ma) have been obtained, which are 50 Ma older than previously thought (being Cretaceous). These geochronologic data provide new evidence for the rarely identified Early Jurassic magmatisms in South China. Whole-rock geochemical data for the Kuzhukeng gabbro and WNW-trending dolerite dykes are similar, both of which being higher in FeO and TiO2 but lower in SiO2 and K2O than the NNE-trending lamprophyre dykes. Trace element characteristics and Sr-Nd isotope data indicate arc-like signatures similar to the Cretaceous southeast coast basalts of China for the lamprophyre dykes, but an OIB-like geochemical affinity for the high-TiO2 mafic rocks similar to the Permo/Triassic Emeishan flood basalts and the Middle Jurassic Ningyuan alkaline basalts. We propose that the lamprophyre dykes formed in an arc volcanic system driven by the subduction of the paleo-Pacific plate. In contrast, the Kuzhukeng gabbro and associated dolerite dykes record the post-orogenic (Indosinian) extension event in the Tethyan tectonic regime. This further implies that the Indosinian extension may have lasted until the Early Jurassic, and therefore, the subduction of the paleo-Pacific plate in south China was probably later than this period. Most U deposits of the Xiazhuang area are located at the intersection between the WNW-trending dolerite dykes and the NNE-trending faults within the Triassic granites of eastern Guidong complex, South China. Previous metallogenesis studies assumed that

A review of cardiopulmonary research in Brazilian medical journals: clinical, surgical and epidemiological data.

PubMed

Serrano, Carlos; Rocha e Silva, Mauricio

2010-04-01

Research in the field of cardiopulmonary disease in Brazil has been very active in recent decades. The combination of PUBMED, SCieLO, open access and online searching has provided a significant increase in the visibility of Brazilian journals. This newly acquired international visibility has in turn resulted in the appearance of more original research reports in the Brazilian scientific press. This review is intended to highlight part of this work for the benefit of the readers of "Clinics." We searched through PUBMED for noteworthy articles published in Brazilian medical journals included in the Journal of Citation Reports of the Institute of Scientific Information to better expose them to our readership. The following journals were examined: "Arquivos Brasileiros de Cardiologia," "Arquivos Brasileiros de Endocrinologia e Metabologia," "Brazilian Journal of Medical and Biological Reviews," "Jornal Brasileiro de Pneumologia," "Jornal de Pediatria," "Revista Brasileira de Cirurgia Cardiovascular," "Revista da Associação Médica Brasileira," Revista da Escola de Enfermagem U.S.P." and "São Paulo Medical Journal." These journals publish original investigations in the field of cardiopulmonary disease. The search produced 71 references, which are briefly examined.

The gabbros and associated hornblende rocks occurring in the neighborhood of Baltimore, Maryland

USGS Publications Warehouse

Williams, George Huntington

1886-01-01

Considerable attention has been devoted during late years to the metamorphism of igneous rocks, and it can now be regarded as placed beyond reasonable doubt that such rocks may be changed to more or less schistose masses, which often closely resemble crystallized sediments. This possibility has heretofore been largely ignored, owing doubtless to the extensive obliteration of those characteristics which are generally regarded as most typical of eruptive rocks. Schistose or banded structure, however, can now hardly be considered as necessarily an indication of sedimentary origin. The minerals which are most characteristic of the so-called crystalline schists have been repeatedly shown to be derived from the alteration of igneous as well as of aqueous formations. These minerals only represent the final and most stable combination of certain elements under certain conditions, and are quite independent of the earlier combinations in which these elements may have existed. A lava bed and a clay bank, if the two may be supposed to have originally had the same chemical composition, might, under the influence of the same metamorphic agencies, ultimately give rise to the same rock in spite of original differences in structure or mineralogical composition. Stratification may be obliterated by metamorphism, while foliation, or even a banded structure, may, by the same means, be secondarily induced. Neither structure nor mineral composition can be taken as an infallible guide in determining the origin or the age of rocks.The present paper is intended as a contribution to our knowledge of a particular phase of metamorphism in eruptive rocks, i. e., that one which is dependent on the secondary development of hornblende by the paramorphism or pseudomorphism of pyroxene. This is a change the frequency of which renders it of fundamental geological importance. It has already received considerable attention from many eminent geologists, but no locality heretofore studied seems to have afforded opportunities for tracing out every stage in the process of alteration superior to those offered by the area of massive rocks west and northwest of the city of Baltimore. Here, covering a district of over fifty square miles, the unchanged pyroxene rock and its resultant hornblendic equivalent occur in the most intimate relations. Exposures of both rocks in situ are numerous, and the opportunity of following out the gradual transition of one into the other is proportionately great.

An oxygen isotope profile in a section of Cretaceous oceanic crust, Samail Ophiolite, Oman: Evidence for δ18O buffering of the oceans by deep (>5 km) seawater-hydrothermal circulation at mid-ocean ridges

NASA Astrophysics Data System (ADS)

Gregory, Robert T.; Taylor, Hugh P., Jr.

1981-04-01

Isotopic analyses of 75 samples from the Samail ophiolite indicate that pervasive subsolidus hydrothermal exchange with seawater occurred throughout the upper 75% of this 8-km-thick oceanic crustal section; locally, the H2O even penetrated down into the tectonized peridotite. Pillow lavas (δ18O = 10.7 to 12.7) and sheeted dikes (4.9 to 11.3) are typically enriched in 18O, and the gabbros (3.7 to 5.9) are depleted in 18O. In the latter rocks, water/rock ≤ 0.3, and δ18Ocpx ≈ 2.9 + 0.44 δ18Ofeld, indicating pronounced isotopic disequilibrium. The mineral δ18O values approximately follow an exchange (mixing) trajectory which requires that plagioclase must exchange with H2O about 3 to 5 times faster than clinopyroxene. The minimum δ18Ofeld value (3.6) occurs about 2.5 km below the diabase-gabbro contact. Although the gabbro plagioclase appears to be generally petrographically unaltered, its oxygen has been thoroughly exchanged; the absence of hydrous alteration minerals, except for minor talc and/or amphibole, suggests that this exchange occurred at T > 400°-500°C. Plagioclase δ18O values increase up section from their minimum values, becoming coincident with primary magmatic values near the gabbro-sheeted diabase contact and reaching 11.8 in the diabase dikes. These 18O enrichments in greenschist facies diabases are in part due to exchange with strongly 18O-shifted fluids, in addition to retrograde exchange at much lower temperatures. The δ18O data and the geometry of the mid-ocean ridge (MOR) magma chamber require that two decoupled hydrothermal systems must be present during much of the early spreading history of the oceanic crust (approximately the first 106 years); one system is centered over the ridge axis and probably involves several convective cells that circulate downward to the roof of the magma chamber, while the other system operates underneath the wings of the chamber, in the layered gabbros. Upward discharge of 18O-shifted water into the

Sr-Nd-Hf isotopes of the intrusive rocks in the Cretaceous Xigaze ophiolite, southern Tibet: Constraints on its formation setting

NASA Astrophysics Data System (ADS)

Zhang, Liang-Liang; Liu, Chuan-Zhou; Wu, Fu-Yuan; Zhang, Chang; Ji, Wei-Qiang; Wang, Jian-Gang

2016-08-01

The Cretaceous Xigaze ophiolite is best exposed at the central part of the Yarlung-Zangbo Suture Zone, Tibet Plateau. It consists of a thick section of mantle peridotites, but a relatively thin mafic sequence. This study presents geochronological and geochemical data for intrusive dykes (both mafic and felsic) and basalts to revisit the formation setting of the Xigaze ophiolite. The rodingites are characterized by high CaO and low Na2O contents relative to mafic dykes and show big variations in trace element compositions. Both gabbros and diabases have similar geochemical compositions, with MgO contents of 6.42-11.48 wt% and Mg# of 0.56-0.71. They display REE patterns similar to N-MORB and are variably enriched in large ion lithophile elements. Basalts have fractionated compositions and display LREE-depleted patterns very similar to N-MORB. They do not show obvious enrichment in LILE and depletion in high-field-strength elements, but a negative Nb anomaly is present. The studied plagiogranites have compositions of trondhjemite to tonalite, with high Na2O and low K2O contents. They have low TiO2 contents less than 1 wt%, consistent with melts formed by anatexis of gabbros rather than by differentiation of basalts. Zircons from seven samples, including three rodingites, three plagiogranites, and one gabbro, have been dated and yielded U-Pb ages of 124.6 130.5 Ma, indicating the Xigaze ophiolite was formed during the Early Cretaceous. They have mantle-like δ18O values of + 4.92 + 5.26‰ and very positive εHf(t) values of + 16 + 13.3. Ages of the rodingites and less altered gabbros indicate that serpentinization was occurred at 125 Ma. Occurrence of both gabbroic and diabase dykes within the serpentinites suggests that the mantle lithosphere of the Xigaze ophiolite was rapidly exhumed. Both mafic and felsic dykes have slightly more radiogenic 87Sr/86Sr ratios relative to MORB, but depleted Hf-Nd isotpe compositions. They have a limited range of εNd(t) values of + 7

References from Brazilian medical journals in national publications.

PubMed

Teixeira, Renan Kleber Costa; Botelho, Nara Macedo; Petroianu, Andy

2013-01-01

To assess whether there is a preference for international journal citation to the detriment of national ones in ten Brazilian medical journals, in two different periods. All references in the articles published in Arquivos Brasileiros de Oftalmologia, Revista Brasileira de Cirurgia Cardiovascular, Revista da Associação Médica Brasileira, São Paulo Medical Journal, Arquivos Brasileiros de Endocrinologia e Metabologia, Clinics, Jornal Brasileiro de Pneumologia, Revista da Sociedade Brasileira de Medicina Tropical, Revista Brasileira de Psiquiatria e Acta Ortopédica Brasileira in the years 2011 and 2007 were analyzed, assessing the number of articles published in national and international journals. A total of 36,125 references from 1,462 articles published in the 10 aforementioned journals were analyzed. Of the total number, 4.242 (11.74%) were from Brazilian journals. There was no significant difference between the two analyzed periods. A total of 453 (30,98%) of the articles studied non-cited brazilian papers,and 81 (5.54%) articles had more Brazilian than international references. Of total references analyzed, 11.74% were related to articles published in Brazilian journals. This number, when compared to the percentage of Brazilian articles published in the medical area, demonstrates a good number of citations of national articles. Copyright © 2012 Elsevier Editora Ltda. All rights reserved.

Link between SSZ ophiolite formation, emplacement and arc inception, Northland, New Zealand: U Pb SHRIMP constraints; Cenozoic SW Pacific tectonic implications

NASA Astrophysics Data System (ADS)

Whattam, Scott A.; Malpas, John; Smith, Ian E. M.; Ali, Jason R.

2006-10-01

New U-Pb age-data from zircons separated from a Northland ophiolite gabbro yield a mean 206Pb/ 238U age of 31.6 ± 0.2 Ma, providing support for a recently determined 28.3 ± 0.2 Ma SHRIMP age of an associated plagiogranite and ˜ 29-26 Ma 40Ar/ 39Ar ages ( n = 9) of basalts of the ophiolite. Elsewhere, Miocene arc-related calc-alkaline andesite dikes which intrude the ophiolitic rocks contain zircons which yield mean 206Pb/ 238U ages of 20.1 ± 0.2 and 19.8 ± 0.2 Ma. The ophiolite gabbro and the andesites both contain rare inherited zircons ranging from 122-104 Ma. The Early Cretaceous zircons in the arc andesites are interpreted as xenocrysts from the Mt. Camel basement terrane through which magmas of the Northland Miocene arc lavas erupted. The inherited zircons in the ophiolite gabbros suggest that a small fraction of this basement was introduced into the suboceanic mantle by subduction and mixed with mantle melts during ophiolite formation. We postulate that the tholeiitic suite of the ophiolite represents the crustal segment of SSZ lithosphere (SSZL) generated in the southern South Fiji Basin (SFB) at a northeast-dipping subduction zone that was initiated at about 35 Ma. The subduction zone nucleated along a pre-existing transform boundary separating circa 45-20 Ma oceanic lithosphere to the north and west of the Northland Peninsula from nascent back arc basin lithosphere of the SFB. Construction of the SSZL propagated southward along the transform boundary as the SFB continued to unzip to the southeast. After subduction of a large portion of oceanic lithosphere by about 26 Ma and collision of the SSZL with New Zealand, compression between the Australian Plate and the Pacific Plate was taken up along a new southwest-dipping subduction zone behind the SSZL. Renewed volcanism began in the oceanic forearc at 25 Ma producing boninitic-like, SSZ and within-plate alkalic and calc-alkaline rocks. Rocks of these types temporally overlap ophiolite emplacement and

High-temperature metamorphism and its relation with magmatism (Svecofennian Belt, the Ladoga region, Russia)

NASA Astrophysics Data System (ADS)

Baltybaev, Shauket

2010-05-01

The Ladoga region, situated in the south-eastern part of the Fennoscandian shield, is subdivided into the Archean (ARD) and the Proterozoic (PRD) domains. The boundary between them is a wide shear-zone. The ARD consists mostly of AR-PR middle-low temperature gneisses and the PRD consists of turbidites, pelites, volcanics metamorphosed under HT-conditions (granulite facies). Metamorphism within the PRD is culminated at T= 800-900C and P=5-6 kbar. The peak of metamorphism of granulite facies is dated at 1881 Ma by Pb-Pb stepwise leaching method of rock-forming minerals of the granulites. Pb-Pb results are within error limits coeval with the U-Pb ages of metamorphic monazites. The same (1881Ma) age has gabbro-enderbites. Next stage of metamorphism lasts from 1881 to 1860 Ma under conditions of amphibolite facies. It was restricted with U-Pb, Pb-Pb, Sm-Nd data based on the closure temperature of zircon, monazite, garnet, sillimanite from gneisses, leucosomes of migmatites and synmetamorphic diorites and tonalites. The lowermost point of the trend shows P-T: ~3-4 kbar, 600C. By the time 1860 Ma K-rich granites were emplaced and the uppermost limit for granulite metamorphism comes from the ages of the aplitic/pegmatitic veins (1860-1850 Ma), which cut the K-rich granites. Thermal and tectonic settings can be described based on spatial and temporal changes during magma emplacement. The granulites of the PRD were produced by the emplacement of the extensive basic intrusion (gabbro-enderbites) into the lower-middle crust. A prolonged thermal flux over all area was supported by new generated dioritic and tonalitic melts, which were intruded into the middle crust. The final stage of tectono-metamorphic evolution was marked by emplacement of the K-rich granites. Numerical simulation of the process of magma emplacement (sequences: gabbro-enderbites, diorites and tonalites) and related heat production shows good correlation between intrusive activity and metamorphism of the

Evidence for large-scale submarine mass wasting associated with growth of oceanic core complexes, 16°N, Mid-Atlantic Ridge.

NASA Astrophysics Data System (ADS)

Urann, B.; Cheadle, M. J.; John, B. E.; Dick, H. J.

2016-12-01

Slow spreading ridges display distinct geomorphologic features, often interpreted as long-lived detachment faults, where mafic and ultramafic rocks are exposed at the seafloor. Many bathymetric features in these regions are viewed as the result of tectonic processes (long lived detachment faults), however other features are clearly the result of mass wasting. Here we report zircon U-Pb dates from four gabbro and Fe-Ti oxide gabbro dredge samples recovered from the North Segment on the western flank of the mid-Atlantic ridge (MAR) at 16°N. Initial SIMS U-Pb zircon dating using the Stanford-USGS SHRIMP-RG ion-microprobe yield 230 Th-corrected zircon 206Pb/238U dates within error of one another. The two westernmost samples are separated by 14km along strike, and come from dredges on the footwall of a high-angle normal fault. They have dates of 1.112 +/-0.083 Ma and 1.181 +/- 0.074Ma, and both lie 12-13km west of the present day axial volcanic ridge, These samples therefore yield a spreading rate of 12km/Ma, as expected for this part of the MAR. The two eastern samples lie up to 4.5 km east of the western samples and yield dates of 1.14_/-0.55Ma and 1,221+/-0.027Ma, indistinguishable from those of the samples to the west. Given the predicted spreading rate of 12 km/Ma, these samples should be 0.375Ma younger than those to the west, and should yield dates of 0.74Ma. To account for the similarity in age, we suggest that mass wasting and large landslides from the high angle fault scarps displaced as much as 40km3 of material into the axial valley, dispersing gabbro of similar age over a wide area. This interpretation is consistent with the available multi-beam bathymetry that can be explained in terms of large landslides flowing from the bounding fault scarps into the axial valley towards the present day axial volcanic ridge. If correct, this interpretation has significant implications for evaluating potential tsunami hazards at mid-ocean ridges.

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

NASA Astrophysics Data System (ADS)

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

2005-12-01

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

A Dual-Porosity, In Situ Crystallisation Model For Fast-Spreading Mid-Ocean Ridge Magma Chambers Based Upon Direct Observation From Hess Deep

NASA Astrophysics Data System (ADS)

MacLeod, C. J.; Lissenberg, C. J.

2014-12-01

We propose a revised magma chamber model for fast-spreading mid-ocean ridges based upon a synthesis of new data from a complete section of lower crust from the East Pacific Rise, reconstructed from samples collected from the Hess Deep rift valley during cruise JC21. Our investigation includes detailed sampling across critical transitions in the upper part of the plutonic section, including the inferred axial melt lens (AML) within the dyke-gabbro transition. We find that an overall petrological progression, from troctolite and primitive gabbro at the base up into evolved (oxide) gabbro and gabbronorite at the top of the lower crustal section, is mirrored by a progressive upward chemical fractionation as recorded in bulk rock and mineral compositions. Crystallographic preferred orientations measured using EBSD show that the downward increase in deformation of mush required in crystal subsidence models is not observed. Together these observations are consistent only with a model in which crystallisation of upward migrating evolving melts occurs in situ in the lower crust. Over-enrichment in incompatible trace element concentrations and ratios above that possible by fractional crystallisation is ubiquitous. This implies redistribution of incompatible trace elements in the lower crust by low porosity, near-pervasive reactive porous flow of interstitial melt moving continuously upward through the mush pile. Mass balance calculations reveal a significant proportion of this trace element enriched melt is trapped at mid-crustal levels. Mineral compositions in the upper third to half of the plutonic section are too evolved to represent the crystal residues of MORB. Erupted MORB therefore must be fed from melts sourced in the deeper part of the crystal mush pile, and which must ascend rapidly without significant modification in the upper plutonics or AML. From physical models of mush processes we posit that primitive melts are transported through transient, high porosity

Magmatic geochemistry and relict textures in blueschist-eclogite facies rocks on the island of Syros, Greece

NASA Astrophysics Data System (ADS)

Schumacher, J. C.; Brady, J. B.; Prinkey, D. R.; Walton, A. J.; Able, L. M.; Sinitsin, A. G.; Cheney, J. T.

2004-05-01

The island of Syros is part of the Attic-Cycladic blueschist belt and high-P mineral assemblages indicating peak metamorphic conditions of at least 15-16 kbar and 500 C are common. Two main marble units, which locally contain Mississippian fossils, are partly dolomitic, contain abundant calcite pseudomorphs after aragonite (Dixon, 1969), and are intercalated with the glaucophane (Glau)-schists, retrograde greenschists, and minor quartzites and Mn-cherts. Discrete, fault-bounded packages of blueschist/eclogite-facies mafic rocks with minor serpentinite are also present. The mineral compositions and assemblages in marbles and associated rocks tightly constrain the metamorphic P, T and the fluid compositions and suggest X(H2O) in the range 0.97-0.99. In general, the mafic rocks have a variety of textures and modes, but most are either fine-grained, blueschists with a well-developed fabric (S approx.=L) or coarse-grained (>1 cm), massive omphacite- or Glau-rich rocks. Based on textures, mineralogy and field relations, previous workers (Dixon and Ridley, 1987) have interpreted the mafic rocks as meta-basalt and metagabbros. Evidence of pillow structures, as well as metamorphosed alteration zones which are interpreted as evidence of ocean-floor metamorphism (?) have survived locally. We obtained whole-rock XRF and INAA analyses for fine- and coarse-grained mafic and felsic rocks and some mica-rich samples. Low chemical index of alteration (CIA) for most samples suggest very minor weathering. On a TAS diagram, mafic rocks span the basalt - basaltic andesite - trachy-basalt - basaltic trachy-andesite fields. REE patterns generally fall between 10-100 times chondrite and show flat to moderately LREE-enriched patterns. Coarse-grained rocks have positive Eu anomalies, consistent with their interpretation by other investigators as fractionally crystallized gabbros. Felsic rocks (now epidote-mica-schists) that are associated with the metamorphosed gabbros have negative Eu

Application of Formation MicroScanner Log in Structural Analysis of an Ultra-slow Spreading Environment, ODP Hole 1105A

NASA Astrophysics Data System (ADS)

Zarian, P.; Casey, J. F.; Miller, J.

2002-12-01

One of the unconventional research efforts that have been directed to reveal the structure of the lower oceanic crust is the application of Formation MicroScanner log in an ultra-slow spreading environment. Hole 1105A was cored during ODP Leg 179 to a depth of 158m on the Atlantis Platform in the Southwest Indian Ridge with a relative high recovery of 82.8% of gabbroic rocks. Open-hole logs, including FMS borehole images were acquired after the coring operation. Detailed modal, grain size and microstructural analyses of 147 thin sections reveal the relationships between deformation intensity, modal composition and rheology. The microstructural analyses showed that the majority of the oxide-rich gabbros exhibit high deformation intensity textures, which are present only in a few of the oxide-free gabbros. The oxide-rich gabbros represent ductile deformation zones and control the location of major deformation zones within these rocks. Oxide-rich layers can be clearly identified on electrical images and enabled the identification of different deformation features.The calibration of borehole images with core data provides us with more detailed information about the geometry of these oxide-rich layers within the magma chamber such as the dip and true pseudo-stratigraphic thickness of these layers. Also within the intervals of no core recovery, electrical images provide valuable information of the borehole wall. Brittle deformation features such as natural mineralized fractures can be documented throughout the borehole. Statistical results show that oxide-rich layers are relatively steep with an average dip of about 50 degrees and have a dominant E-W strike which is parallel to the ridge. These layers may represent the insitu crystallization of cummulate layers on a magma chamber wall elongated parallel to the ridge, or they may represent shear zones associated with near surface high angle normal faults that cut the main low angle detachment surface which caused the

X-ray Fluorescence Core Scanning of Oman Drilling Project Holes BT1B and GT3A Cores on D/V CHIKYU

NASA Astrophysics Data System (ADS)

Johnson, K. T. M.; Kelemen, P. B.; Michibayashi, K.; Greenberger, R. N.; Koepke, J.; Beinlich, A.; Morishita, T.; Jesus, A. P. M.; Lefay, R.

2017-12-01

The JEOL JSX-3600CA1 energy dispersive X-ray fluorescence core logger (XRF-CL) on the D/V Chikyu provides quantitative element concentrations of scanned cores. Scans of selected intervals are made on an x-y grid with point spacing of 5 mm. Element concentrations for Si, Al, Ti, Ca, Mg, Mn, Fe, Na, K, Cr, Ni, S and Zn are collected for each point on the grid. Accuracy of element concentrations provided by the instrument software is improved by applying empirical correction algorithms. Element concentrations were collected for 9,289 points from twenty-seven core intervals in Hole BT1B (basal thrust) and for 6,389 points from forty core intervals in Hole GT3A (sheeted dike-gabbro transition) of the Oman Drilling Project on the D/V Chikyu XRF-CL during Leg 2 of the Oman Drilling Project in August-September, 2017. The geochemical data are used for evaluating downhole compositional details associated with lithological changes, unit contacts and mineralogical variations and are particularly informative when plotted as concentration contour maps or downhole concentration diagrams. On Leg 2 additional core scans were made with X-ray Computed Tomography (X-ray CT) and infrared images from the visible-shortwave infrared imaging spectroscopy (IR) systems on board. XRF-CL, X-ray CT and IR imaging plots used together provide detailed information on rock compositions, textures and mineralogy that assist naked eye visual observations. Examples of some uses of XRF-CL geochemical maps and downhole data are shown. XRF-CL and IR scans of listvenite clearly show zones of magnesite, dolomite and the Cr-rich mica, fuchsite that are subdued in visual observation, and these scans can be used to calculate variations in proportions of these minerals in Hole BT1B cores. In Hole GT3A XRF-CL data can be used to distinguish compositional changes in different generations of sheeted dikes and gabbros and when combined with visual observations of intrusive relationships the detailed geochemical

Paleomagnetism of the Oman Ophiolite: New Results from Oman Drilling Project Cores

NASA Astrophysics Data System (ADS)

Horst, A. J.; Till, J. L.; Koornneef, L.; Usui, Y.; Kim, H.; Morris, A.

2017-12-01

The Oman Drilling Project drilled holes at four sites in a transect through the southern massifs of the Samail ophiolite, and recovered 1500 m of igneous and metamorphic rocks. We focus on three sites from the oceanic crustal section including lower layered gabbros (GT1A), the mid-crustal layered to foliated gabbro transition (GT2A), and the shallower transition from sheeted dikes to varitextured gabbros (GT3A). Detailed core descriptions, analyses, and paleomagnetic measurements, were made on D/V Chikyu from July to September 2017 to utilize the core laboratory facilities similar to IODP expeditions. Shipboard measurements included anisotropy of magnetic susceptibility (AMS) and alternating field and thermal demagnetization of 597 discrete samples. Sample demagnetization behavior is varied from each of the cores, with some revealing multiple components of magnetization, and others yielding nearly univectorial data. The interpretation of results from the lower crustal cores is complicated by the pervasive presence of secondary magnetite. In almost all samples, a stable component was resolved (interpreted as a characteristic remanent magnetization) after removal of a lower-coercivity or lower unblocking-temperature component. The inclinations of the stable components in the core reference frame are very consistent in Hole GT1A. However, a transition from negative to positive inclinations in GT2A suggests some structural complexity, possibly as a result of intense late faulting activity. Both abrupt and gradual transitions between multiple zones of negative and positive inclinations occur in Hole GT3A. Interpretation and direct comparison of remanence between drill sites is difficult as recovered core pieces currently remain azimuthally unoriented, and GT2A was drilled at a plunge of 60°, whereas GT1A and GT3A were both drilled vertically. Work is ongoing to use borehole imagery to reorient the core pieces and paleomagnetic data into a geographic in situ reference

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

NASA Astrophysics Data System (ADS)

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

2017-09-01

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

Effects of chemical alteration on fracture mechanical properties in hydrothermal systems

NASA Astrophysics Data System (ADS)

Callahan, O. A.; Eichhubl, P.; Olson, J. E.

2015-12-01

Fault and fracture networks often control the distribution of fluids and heat in hydrothermal and epithermal systems, and in related geothermal and mineral resources. Additional chemical influences on conduit evolution are well documented, with dissolution and precipitation of mineral species potentially changing the permeability of fault-facture networks. Less well understood are the impacts of chemical alteration on the mechanical properties governing fracture growth and fracture network geometry. We use double-torsion (DT) load relaxation tests under ambient air conditions to measure the mode-I fracture toughness (KIC) and subcritical fracture growth index (SCI) of variably altered rock samples obtained from outcrop in Dixie Valley, NV. Samples from southern Dixie Valley include 1) weakly altered granite, characterized by minor sericite in plagioclase, albitization and vacuolization of feldspars, and incomplete replacement of biotite with chlorite, and 2) granite from an area of locally intense propylitic alteration with chlorite-calcite-hematite-epidote assemblages. We also evaluated samples of completely silicified gabbro obtained from the Dixie Comstock epithermal gold deposit. In the weakly altered granite KIC and SCI are 1.3 ±0.2 MPam1/2 (n=8) and 59 ±25 (n=29), respectively. In the propylitic assemblage KIC is reduced to 0.6 ±0.1 MPam1/2 (n=11), and the SCI increased to 75 ±36 (n = 33). In both cases, the altered materials have lower fracture toughness and higher SCI than is reported for common geomechanical standards such as Westerly Granite (KIC ~1.7 MPam1/2; SCI ~48). Preliminary analysis of the silicified gabbro shows a significant increase in fracture toughness, 3.6 ±0.4 MPam1/2 (n=2), and SCI, 102 ±45 (n=19), compared to published values for gabbro (2.9 MPam1/2 and SCI = 32). These results suggest that mineralogical and textural changes associated with different alteration assemblages may result in spatially variable rates of fracture

Morphology and composition of gold in a lateritic profile, Fazenda Pison “Garimpo”, Amazon, Brazil

NASA Astrophysics Data System (ADS)

Larizzatti, J. H.; Oliveira, S. M. B.; Butt, C. R. M.

2008-05-01

This study describes the morphological evolution of gold grains in a lateritic weathering profile in an equatorial rainforest climate. Primary sources of gold are quartz veins associated with shallow granophyric intrusion. Gold grains were found in fresh ore, saprolite, transition zones, ferruginous duricrust, red latosol, and yellow latosol. Irregularly shaped grains predominate, with smaller proportions of dendritic and prismatic forms. Gold grains are weathered in the uppermost 10 m of the regolith. Mean gold grain size is maximum in the duricrust (>125 μm) and decreases progressively upward into the yellow latosol (<90 μm). Voids and corrosion pits appear on grain surfaces, and progressive rounding is observed from the bottom of the profile to the top. Gold grains can be classified as either homogeneous or zoned with respect to their chemical composition. Homogeneous grains contain 2-15% Ag (mean 8.3%). Zoned grains have more variable Ag contents; grain cores have means of approximately 10% or 23% Ag, with Ag-poor zones of approximately 3.7% Ag along internal discontinuities and/or outer rims. Formation of Ag-poor rims is due to preferential depletion of silver. Processes responsible for duricrust formation may preserve some grains as large aggregates, but subsequent transformation into latosol further modifies them.

Identification and Description of Geophysical Techniques.

DTIC Science & Technology

1980-11-01

electrodes are used for voltage measurements with the potentiometer. Current elec- trodes may be stainless-steel or copper rods, buried copper screens, drill...steel in a borehole, or buried metal culverts. Potential elec- trodes may be stainless steel rods or porous containers filled with copper sulfate...Granite 28-2,700 Diorite 47 Gabbro 68-2,370 Porphyry 47 Diabase 78-1,050 Basalt 680 Olivine-Diabase 2,000 1. Peridotite 12,500 I (1)Adapted from C. A

Production of hybrid granitic magma at the advancing front of basaltic underplating: Inferences from the Sesia Magmatic System (south-western Alps, Italy)

NASA Astrophysics Data System (ADS)

Sinigoi, Silvano; Quick, James E.; Demarchi, Gabriella; Klötzli, Urs S.

2016-05-01

The Permian Sesia Magmatic System of the southwestern Alps displays the plumbing system beneath a Permian caldera, including a deep crustal gabbroic complex, upper crustal granite plutons and a bimodal volcanic field dominated by rhyolitic tuff filling the caldera. Isotopic compositions of the deep crustal gabbro overlap those of coeval andesitic basalts, whereas granites define a distinct, more radiogenic cluster (Sri ≈ 0.708 and 0.710, respectively). AFC computations starting from the best mafic candidate for a starting melt show that Nd and Sr isotopic compositions and trace elements of andesitic basalts may be modeled by reactive bulk assimilation of ≈ 30% of partially depleted crust and ≈ 15%-30% gabbro fractionation. Trace elements of the deep crustal gabbro cumulates require a further ≈ 60% fractionation of the andesitic basalt and loss of ≈ 40% of silica-rich residual melt. The composition of the granite plutons is consistent with a mixture of relatively constant proportions of residual melt delivered from the gabbro and anatectic melt. Chemical and field evidence leads to a conceptual model which links the production of the two granitic components to the evolution of the Mafic Complex. During the growth of the Mafic Complex, progressive incorporation of packages of crustal rocks resulted in a roughly steady state rate of assimilation. Anatectic granite originates in the hot zone of melting crust located above the advancing mafic intrusion. Upward segregation of anatectic melts facilitates the assimilation of the partially depleted restite by stoping. At each cycle of mafic intrusion and incorporation, residual and anatectic melts are produced in roughly constant proportions, because the amount of anatectic melt produced at the roof is a function of volume and latent heat of crystallization of the underplated mafic melt which in turn produces proportional amounts of hybrid gabbro cumulates and residual melt. Such a process can explain the

Palaeopermeability structure within fault-damage zones: A snap-shot from microfracture analyses in a strike-slip system

NASA Astrophysics Data System (ADS)

Gomila, Rodrigo; Arancibia, Gloria; Mitchell, Thomas M.; Cembrano, Jose M.; Faulkner, Daniel R.

2016-02-01

Understanding fault zone permeability and its spatial distribution allows the assessment of fluid-migration leading to precipitation of hydrothermal minerals. This work is aimed at unraveling the conditions and distribution of fluid transport properties in fault zones based on hydrothermally filled microfractures, which reflect the ''frozen-in'' instantaneous advective hydrothermal activity and record palaeopermeability conditions of the fault-fracture system. We studied the Jorgillo Fault, an exposed 20 km long, left-lateral strike-slip fault, which juxtaposes Jurassic gabbro against metadiorite belonging to the Atacama Fault System in northern Chile. Tracings of microfracture networks of 19 oriented thin sections from a 400 m long transect across the main fault trace was carried out to estimate the hydraulic properties of the low-strain fault damagezone, adjacent to the high-strain fault core, by assuming penny-shaped microfractures of constant radius and aperture within an anisotropic fracture system. Palaeopermeability values of 9.1*10-11 to 3.2*10-13 m2 in the gabbro and of 5.0*10-10 to 1.2*10-13 m2 in the metadiorite were determined, both decreasing perpendicularly away from the fault core. Fracture porosity values range from 40.00% to 0.28%. The Jorgillo Fault has acted as a left-lateral dilational fault-bend, generating large-scale dilation sites north of the JF during co-seismic activity.

Gravimetric and magnetic fabric study of the Sintra Igneous complex: laccolith-plug emplacement in the Western Iberian passive margin

NASA Astrophysics Data System (ADS)

Terrinha, Pedro; Pueyo, Emilio L.; Aranguren, Aitor; Kullberg, José Carlos; Kullberg, Maria Carla; Casas-Sainz, Antonio; Azevedo, Maria do Rosário

2017-12-01

The geometry and emplacement of the 96 km2, Late Cretaceous Sintra Igneous complex (SIC, ca. 80 Ma) into the West Iberian passive margin is presented, based on structural data, gravimetric modeling, and magnetic fabrics. A granite laccolith ( 76 km2, < 1 km thick, according to gravimetric modeling) surrounds a suite of gabbro-diorite-syenite plugs ( 20 km2, 4 km deep) and is encircled by cone sheets and radial dykes. Anisotropy of Magnetic Susceptibility was interpreted from 54 sites showing fabrics of para- and ferro-magnetic origin. Most fabrics can be interpreted to have a magmatic origin, according to the scarcity of solid-state deformation in most part of the massif. Magnetic foliations are shallowly dipping in the granite laccolith and contain a sub-horizontal ENE-WSW lineation. The gabbro-syenite body displays concentric magnetic foliations having variable dips and steeply-plunging lineations. The SIC can be interpreted to be intruded along an NNW-SSE, 200 km-long fault, perpendicular to the magnetic lineation within the laccolith, and was preceded by the intrusion of basic sills and plugs. The SIC intruded the Mesozoic series of the Lusitanian Basin during the post-rift, passive margin stage, and its geometry was only slightly modified during the Paleogene inversion that resulted in thrusting of the northern border of the intrusion over the country rocks.

Experimental and geochemical evidence for derivation of the El Capitan Granite, California, by partial melting of hydrous gabbroic lower crust

USGS Publications Warehouse

Ratajeski, K.; Sisson, T.W.; Glazner, A.F.

2005-01-01

Partial melting of mafic intrusions recently emplaced into the lower crust can produce voluminous silicic magmas with isotopic ratios similar to their mafic sources. Low-temperature (825 and 850??C) partial melts synthesized at 700 MPa in biotite-hornblende gabbros from the central Sierra Nevada batholith (Sisson et al. in Contrib Mineral Petrol 148:635-661, 2005) have major-element and modeled trace-element (REE, Rb, Ba, Sr, Th, U) compositions matching those of the Cretaceous El Capitan Granite, a prominent granite and silicic granodiorite pluton in the central part of the Sierra Nevada batholith (Yosemite, CA, USA) locally mingled with coeval, isotopically similar quartz diorite through gabbro intrusions (Ratajeski et al. in Geol Soc Am Bull 113:1486-1502, 2001). These results are evidence that the El Capitan Granite, and perhaps similar intrusions in the Sierra Nevada batholith with lithospheric-mantle-like isotopic values, were extracted from LILE-enriched, hydrous (hornblende-bearing) gabbroic rocks in the Sierran lower crust. Granitic partial melts derived by this process may also be silicic end members for mixing events leading to large-volume intermediate composition Sierran plutons such as the Cretaceous Lamarck Granodiorite. Voluminous gabbroic residues of partial melting may be lost to the mantle by their conversion to garnet-pyroxene assemblages during batholithic magmatic crustal thickening. ?? Springer-Verlag 2005.

K-Ar chronology and geochemistry of the Miocene magmatism of Collo-Bougaroun and Edough-Cap de Fer areas (NE Algeria). Temporal constraints on geodynamic evolution of the Eastern Algerian margin between 6° and 8°E

NASA Astrophysics Data System (ADS)

Abbassene, F.; Bellon, H.; Chazot, G.; Ouabadi, A.

2013-12-01

The ''Petite Kabylie'' corresponds to the eastern Algerian coastal magmatic chain outcropping from Jijel to the west, up to the plain of Annaba to the east. In this area, the Collo-Bougaroun volcano-plutonic complex, of ca. 300 km2, comprises (1) granular rocks, mainly cordierite bearing peraluminous granites, (2) gabbros that occur at the northern and southern parts of Cap Bougaroun pluton where they are associated with ultramafic rocks and form the layered complex of Yadene?; (3) microgranular rocks, mainly microgranites, that outcrop at the eastern part of the Bougaroun pluton, in Collo basin and El Milia, microdiorites in Bouserdoum and some doleritic or microgabbroic metric veins at Cap Bougaroun and (4) of rhyolitic lava in Kef Cheraïa. The Bougaroun complex form a huge elliptical batholite along a major axis of 20km oriented ENE- WSW that intrudes serpentinized peridotites and kinzigites of the Bougaroun basement to the east. This granitic pluton gives time constraints as it induces deformation and contact metamorphism of the Oligo-Miocene Kabyle sediments of Collo-Oued Zhour basin in the south. These sediments reach the Upper Burdigalien which suggests that the lower limit of emplacement of this granite is coeval at least with this age. The majority of these magmatic rocks show subalkaline affinity with strong enrichment (0.13 to 4.13 %) in K2O during fractionation to calc-alkaline and high-K calc-alkaline affinity for the most differentiated rocks. The felsic rocks (granites, microgranites and rhyolites) are marked by a significant crustal contamination (ξNd = -10, I Sr = 0.720, δ18O = +12 ‰ [1], [2]) during their petrogenesis. However, the presence of basic rocks (gabbros and dolerites) that are depleted in K2O (0.13 to 0.44%) provides information on mantle composition and origin of magmas. The geochemical data on these rocks are discussed in the very particular geodynamic context of the northern Algerian margin.Twenty-four 40K-40Ar analyses were

Major element chemistry of glasses in Apollo 14 soil 14156.

NASA Technical Reports Server (NTRS)

Reid, A. M.; Ridley, W. I.; Harmon, R. S.; Jakes, P.

1973-01-01

Glasses in a soil sample (14156) from the middle layer of the trench at the Fra Mauro landing site show a wide range of compositions clustered around certain preferred compositions. Ninety per cent of the glasses are of two major types - Fra Mauro basalt (63%) with high K and 17 wt % Al2O3 and Highland basalt or anorthositic gabbro (27%) with low K and 25 to 26 wt % Al2O3. The glass population is almost identical with that of the comprehensive soil 14259.

76 FR 4129 - Notice on Outer Continental Shelf Oil and Gas Lease Sales

Federal Register 2010, 2011, 2012, 2013, 2014

2011-01-24

... Eni Petroleum Co. Inc. Eni Petroleum US LLC. Eni Oil US LLC. Eni Marketing Inc. Eni BB Petroleum Inc. Eni US Operating Co. Inc. Eni BB Pipeline LLC. Group VII Petrobras America Inc. Petroleo Brasileiro S...

New data for paleoprotherozoic PGE-bearing anorthosite of Kandalaksha massif (Baltic shield): U-Pb and Sm-Nd ages

NASA Astrophysics Data System (ADS)

Steshenko, Ekaterina; Bayanova, Tamara; Serov, Pavel

2015-04-01

The aims of this researches were to study the isotope U-Pb age of zircon and rutile and Sm-Nd (rock forming and sulphide minerals) on Kandalaksha anorthosite massif due to study of polimetamorphic history. In marginal zone firstly have been obtained the presence of sulphide mineralization with PGE (Chashchin, Petrov , 2013). Kandalaksha massif is located in the N-E part of Baltic shield and consists of three parts. 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 polymetamorphism. Two fractions of single grains from anorthosite of the massif gave precise U-Pb age, which is equal to 2450± 3 Ma. Leucocratic gabbro-norite were dated by U-Pb method, with age up to 2230 ± 10 Ma. This age reflects the time of granulite metamorphism according to data of (Mitrofanov, Nirovich, 2003). Two fractions of rutile have been analyzed by U-Pb method and reflect age of 1700 ± 10 Ma. It is known that the closure temperature of U-Pb system rutile is 400-450 ° C (Mezger et.al., 1989), thus cooling processes of massif rocks to these temperatures was about 1.7 Ga. These data reflect one of the stages of metamorphic alteration of the massif. Three stages of metamorphism are distinguished by Sm-Nd method. Isotope Sm-Nd dating on Cpx-WR line gives the age of 2311 Ma which suggested of high pressure granulite metamorphism. Moreover Cpx-Pl line reflect the age 1908 Ma of low pressure granulite metamorphism. Also two-points (Grt-Rt) Sm-Nd isochrone yield the age 1687 Ma of the last metamorphic alterations in Kandalaksha anorthosite massif. Model Sm-Nd age of the leucocratic gabbro-norite is 2796 Ma with positive ɛNd (+0.32). It means that the source of gabbro-norite was mantle reservoir. All investigations are devoted to memory of academician PAS F. MItrofanov which was a leader of scientific school for

Age and geochemistry of the intrusive rocks from the Shaquanzi-Hongyuan Pb-Zn mineral district: Implications for the Late Carboniferous tectonic setting and Pb-Zn mineralization in the Eastern Tianshan, NW China

NASA Astrophysics Data System (ADS)

Lu, Wan-Jian; Chen, Hua-Yong; Zhang, Li; Han, Jin-Sheng; Xiao, Bing; Li, Deng-Feng; Zhang, Wei-Feng; Wang, Cheng-Ming; Zhao, Lian-Dang; Jiang, Hong-Jun

2017-12-01

The Central Tianshan Terrane (CTT) in the Eastern Tianshan (Xinjiang, NW China) is an important Pb-Zn metallogenic belt and played a pivotal role in crustal evolution and collisional tectonics of the southern Central Asian Orogenic Belt. The Shaquanzi gabbro and Hongyuan granodiorite are located in the northern margin of the CTT and associated with Pb-Zn mineralization. Zircon U-Pb dating yielded weighted mean ages of 307.2 ± 1.5 Ma and 301.2 ± 1.5 Ma for the Shaquanzi gabbro and the Hongyuan granodiorite, respectively. These rocks are medium-K calc-alkaline series and enriched in large ion lithophile elements (LILEs; e.g., K, Rb, Ba) and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, Ti), displaying typical arc affinities. The Shaquanzi gabbro shows low Nb/Ta (11.0-14.2), a high Mg# range (56-59), positive zircon εHf(t) (+ 3.30 - + 7.26) and whole rock εNd(t) (+ 0.70 - + 1.38) values, and low ISr ratios (0.704858-0.705137), which indicate that the protolith was probably derived from the sub-continental lithospheric mantle that had been metasomatized by subduction-related fluids. The Hongyuan granodiorite contains hornblende but lack of Al-rich minerals and has low ISr ratios (0.704769-0.706211 < 0.707), suggesting an I-type origin. Moreover, the Hongyuan granodiorite has positive εHf(t) (+ 1.12 - + 5.57) and εNd(t) (+ 0.38 - + 1.86) values, with high Mg# (52), variable Nb/Ta ratios (12.6-12.9), low contents of Ni, Cr and Co and Pb isotopes (206Pb/204Pb = 17.461-18.299, 207Pb/204Pb = 15.541-15.581, 208Pb/204Pb = 37.456-38.129), suggesting the Hongyuan granodiorite was generated by partial melting of juvenile crust sources mixed with some mantle-derived materials. Combined published works with our new geochronological, geological, geochemical and isotopic data, we propose that the CTT may have evolved from a continental arc to a syn-collisional setting during the period of ca. 307-301 Ma. The continuing southward subduction of the Junggar

Allochthonous 2.78 Ga oceanic plateau slivers in a 2.72 Ga continental arc sequence: Vizien greenstone belt, northeastern Superior Province, Canada

NASA Astrophysics Data System (ADS)

Skulski, T.; Percival, J. A.

1996-04-01

Embedded within the vast granitoid terrane of the Minto block of northeastern Superior Province are Late Archean greenstone belts of the Goudalie domain that preserve a long-lived record of continent-ocean interaction. The Vizien greenstone belt is one such belt and it contains four fault-bounded structural panels. The 2786 Ma mafic-ultramafic sequence is an allochthonous package of pillowed basaltic andesite, komatiite and volcaniclastic rocks cut by peridotite and gabbro sills. The mafic rocks are LREE-depleted tholeiites which have primitive mantle (PRIM)-normalized abundances of Th < Nb < La, and ɛNd values of +1.5 to + 3.2 reflecting extraction from a depleted mantle source. The 2724 Ma lac Lintelle continental calc-alkaline volcanic sequence consists of massive basalt, plagioclase-porphyritic andesite, dacite, rhyolite, capped by quartz-rich sandstones/conglomerates with 2.97 Ga Nd model ages. Lac Lintelle volcanic rocks are LREE enriched, with low TiO 2 (< 1%) and Zr (< 200 ppm), PRIM-normalized enrichment in Th > La > Nb, and a range of ɛNd values from -0.1 to +1.7. The ~ 2722 Ma lac Serindac bimodal, subaerial tholeiitic volcanic sequence contains andesite (locally with tonalite xenoliths), basalt, gabbro sills, lenses of quartz-rich sedimentary rocks and a thick, upper rhyolite sequence. The lac Serindac tholeiites are LREE-enriched, have PRIM-normalized Th > La > Nb, high Zr (to 300 ppm) and Ti contents, and low ɛNd values from +0.8 in basalt to -1.4 in rhyolite. The < 2718 Ma basement-cover sequence comprises 2.94 Ga tonalitic gneiss unconformably overlain by clastic sediments and a thin upper sequence of 2700 Ma gabbro, siliceous high-Mg basalt (SHMB) and andesite. The SHMB are characterised by LREE depletion and ɛNd values of +2.6, whereas the andesite is LREE-enriched and has ɛNd values of -0.3. The 2786 Ma mafic-ultramafic sequence is interpreted as a sliver of plume-related oceanic plateau crust. The 2724 lac Lintelle sequence represents a

Timing of mafic magmatism in the Tapajós Province (Brazil) and implications for the evolution of the Amazon Craton: evidence from baddeleyite and zircon U Pb SHRIMP geochronology

NASA Astrophysics Data System (ADS)

Santos, João Orestes Schneider; Hartmann, Léo Afraneo; McNaughton, Neal Jesse; Fletcher, Ian Robert

2002-09-01

The precise timing and possible sources of the mafic rocks in the Amazon craton are critical for reconstruction of the Atlantica supercontinent and correlation of mafic magmatism worldwide. New SHRIMP U-Pb baddeleyite and zircon ages and the reinterpretation of 207 existing dates indicate one orogenic (Ingarana) and four postorogenic (Crepori, Cachoeira Seca, Piranhas, and Periquito) basaltic events in the Tapajós Province, south central Amazon craton. Orogenic gabbro dikes that host gold mineralization are 1893 Ma and interpreted as associated with the Ingarana gabbro intrusions of the bimodal calk-alkalic Parauari intrusive suite. The age of 1893 Ma can be used as a guide to discriminate older and mineralized orogenic dikes from younger and nonmineralized Crepori- and Cachoeira Seca-related mafic dikes. The baddeleyite U-Pb age of the postorogenic Crepori dolerite (gabbro-dolerite sills and dikes) is 1780±9 Ma, ˜150 my older than the ages provided by K-Ar. This value correlates well with the Avanavero tholeiitic intrusions in the Roraima group, in the northern part of the craton in Guyana, Venezuela, and Roraima in Brazil. Early Statherian tholeiitic magmatism was widespread not only in the Amazon craton, but also in the La Plata craton of southern South America, where it is known as the giant Piedra Alta swarm of Uruguay and the post-Trans-Amazonian dikes of Tandil in Argentina. The Cachoeira Seca troctolite represents laccoliths, Feixes, and São Domingos, whose baddeleyite U-Pb age is 1186±12 Ma, 120-150 my older than the known K-Ar ages. This age is comparable to other Stenian gabbroic rocks with alkalic affinity in the craton, such as the Seringa Formation in NE Amazonas and the basaltic flows of the Nova Floresta formation in Rondônia. Dolerite from the giant Piranhas dike swarm in the western Tapajós Province has a Middle Cambrian age (507±4 Ma, baddeleyite) and inherited zircons in the 2238-1229 Ma range. The Piranhas dikes fill extensional NNE and

Building monument materials during the 3rd-4rd millennium (Portugal)

NASA Astrophysics Data System (ADS)

Moita, Patricia; Pedro, Jorge; Boaventura, Rui; Mataloto, Rui; Maximo, Jaime; Almeida, Luís; Nogueira, Pedro

2014-05-01

Dolmens are the most conspicuous remains of the populations of the 4th and first half of 3rd millennia BCE. These tombs are impressive not only for their monumentality, but also because of the socioeconomic investment they represent for those Neolithic communities, namely from the Central-South of Portugal, who built them. Although dolmens have been studied for their funerary content and typologies, an interdisciplinary approach toward the geological characterization and sourcing of stones used in these constructions has not received enough attention from researchers. With MEGAGEO project a multidisciplinary group of geologist and archaeologists intends to assess the relationship between the distribution of dolmens in Central-South Portugal, their source materials, and the geological landscape. GIS will map the information gathered and will be used to analyse these relationships. The selection of the areas, with distinctive geologies (limestone vs granite), will allow to verify if human patterns of behaviour regarding the selection of megaliths are similar or different regionally. Geologically the first target area (Freixo, Alentejo) is dominated by a small intrusion of gabbro mingled/mixed within a granodioritic intrusion both related with variscan orogeny. Granodiorite exhibit several enclaves of igneous and metamorphic nature attesting the interaction between both igneous rocks as well with enclosing gneisses. Despite Alentejo region have a reduced number of outcrops the granodiorite provides rounded to tabular metric blocks. The gabbro is very coarse grained, sometimes with a cumulate texture, and their fracturing and weathering provide very fresh tabular blocks. The five studied dolmens (Quinta do Freixo #1 to #5) are implanted in a large granodioritic intrusion, around the gabbroic rocks, within an area of approximately 9km2. The medium grained granodiorite is ubiquity in all the dolmens slabs and occasionally it can be observed features of mixing and

Source characteristics and tectonic setting of mafic-ultramafic intrusions in North Xinjiang, NW China: Insights from the petrology and geochemistry of the Lubei mafic-ultramafic intrusion

NASA Astrophysics Data System (ADS)

Chen, Bao-Yun; Yu, Jin-Jie; Liu, Shuai-Jie

2018-05-01

The newly discovered Lubei sulfide-bearing mafic-ultramafic intrusion forms the western extension of the Huangshan-Jin'erquan mafic-ultramafic intrusion belt in East Tianshan, NW China. The Lubei intrusion comprises hornblende peridotite, lherzolite, and harzburgite in its southern portion, gabbro in its middle portion, and hornblende gabbro in its northern portion. Intrusive relationships indicate that three magma pulses were involved in the formation of the intrusion, and that they were likely evolved from a common primitive magma. Estimated compositions of the Lubei primitive magma are similar to those of island arc calc-alkaline basalt except for the low Na2O and CaO contents of the Lubei primitive magma. This paper reports on the mineral compositions, whole-rock major and trace element contents, and Rb-Sr and Sm-Nd isotopic compositions of the Lubei intrusion, and a zircon LA-MC-ICP-MS U-Pb age for hornblende gabbro. The Lubei intrusion is characterized by enrichment in large-ion lithophile elements, depletion in high-field-strength elements, and marked negative Nb and Ta anomalies, with enrichment in chondrite-normalized light rare earth elements. It exhibits low (87Sr/86Sr)i ratios of 0.70333-0.70636 and low (143Nd/144Nd)i ratios of 0.51214-0.51260, with positive εNd values of +4.01 to +6.33. LA-ICP-MS U-Pb zircon ages yielded a weighted-mean age of 287.9 ± 1.6 Ma for the Lubei intrusion. Contemporaneous mafic-ultramafic intrusions in different tectonic domains in North Xinjiang show similar geological and geochemical signatures to the Lubei intrusion, suggesting a source region of metasomatized mantle previously modified by hydrous fluids from the slab subducted beneath the North Xinjiang region in the early Permian. Metasomatism of the mantle was dominated by hydrous fluids and was related to subduction of the Paleo-Asian oceanic lithosphere during the Paleozoic. Sr-Nd-Pb isotopic compositions suggest that the mantle source was a mixture of depleted mid

Magmatic Diversity of the Wehrlitic Intrusions in the Oceanic Lower Crust of the Northern Oman Ophiolite

NASA Astrophysics Data System (ADS)

Kaneko, R.; Adachi, Y.; Miyashita, S.

2014-12-01

The Oman ophiolite extends along the east coast of Oman, and is the world's largest and best-preserved slice of obducted oceanic lithosphere. The magmatic history of this ophiolite is complex and is generally regarded as having occurred in three stages (MOR magmatism, subduction magmatism and intraplate magmatism). Wehrlitic intrusions constitute an important element of oceanic lower crust of the ophiolite, and numerous intrusions cut gabbro units in the northern Salahi block of this ophiolite. In this study area, we identified two different types of wehrlitic intrusions. One type of the intrusions mainly consists of dunite, plagioclase (Pl) wehrlite and mela-olivine (Ol) gabbro, in which the crystallization sequence is Ol followed by the contemporaneous crystallization of Pl and clinopyroxene (Cpx). This type is called "ordinary" wehrlitic intrusions and has similar mineral compositions to host gabbros (Adachi and Miyashita 2003; Kaneko et al. 2014). Another type of the intrusions is a single intrusion that crops out in an area 250 m × 150 m along Wadi Salahi. This intrusion consists of Pl-free "true" wehrlite, in which the crystallization sequence is Ol and then Cpx. The forsterite contents (Fo%) of Ol from the "ordinary" wehrlitic intrusions and "true" wehrlitic intrusions have ranges of 90.8-87.0 (NiO = 0.36-0.13 wt%) and 84.7 (NiO = 0.31 wt%), respectively. Cr numbers (Cr#) of Cr-spinel from the "true" wehrlitic intrusions show higher Cr# value of 0.85 than those of the "ordinary" wehrlitic intrusions (0.48-0.64). But the former is characterized by very high Fe3+ values (YFe3+ = 0.49-0.68). Kaneko et al. (2014) showed that the "ordinary" ubiquitous type has similar features to MOR magmatism and the depleted type in the Fizh block (Adachi and Miyashita 2003) links to subduction magmatism. These types are distinguished by their mineral chemistries (TiO2 and Na2O contents of Cpx). The TiO2 and Na2O contents of Cpx from the "true" wehrlitic intrusions have 0

Olivine-Rich Troctolite from the Southwest Indian Ridge: Constrains on Melt-Rock Reaction beneath Mid-Ocean Ridges

NASA Astrophysics Data System (ADS)

Yang, A. Y.

2017-12-01

The origin of olivine-rich troctolite, which have been recovered in the plutonic sample suites from the fast-spreading East Pacific Rise, and slow-spreading Mid-Atlantic Ridge and Central Indian Ridge, has been highly debated. They can form either by fractionation of primitive mid-ocean ridge basalt (MORB) melts or by melt migrating through pre-existing (possibly mantle) olivine matrix, thus recording valuable information of magmatic process at mantle-crust transition. This study presents in situ major and trace element study on the olivine-rich troctolite first reported from the ultraslow-spreading Southwest Indian Ridge (SWIR), together with a series of samples from peridotite, gabbro to basalt from an amagmatic segment at 53°E during Dayangyihao Cruises, to investigate the magmatic processes occurred at mantle-crust transition and origin of olivine-rich troctolite. The olivine in the troctolite shows cumulate textures with interstitial clinopyroxene, plagioclase and minor spinel, which is cross-cut by an olivine gabbro vein. Olivine in the troctolite show only mildly decreasing NiO contents (from 0.31-0.25 wt.%) with decreasing Fo (from 86 to 81), and even olivine in the gabbro have NiO up to 0.24 wt.% with Fo of 75. The Fo vs. Ni correlation of olivine is shifted to uniquely higher NiO at a certain Fo compared to the trends defined by either olivine from other lower oceanic crust (LOC) cumulates worldwide or olivine compositions from classic fractionation model for primitive MORB magma. Interstitial clinopyroxene in the troctolite have high Mg# (88-90), Cr2O3 (up to 1.51 wt.%) and TiO2 contents (up to 1.01 wt.%) with Eu/Eu* from 0.4-0.6. Such high-Ti-Cr-Mg# clinopyroxene is rare among global LOC cumulates, and cannot result from simple fractionation of MORB magma. The mineral compositions in the olivine-rich troctolite could be modeled by reaction between a primitive olivine matrix and a highly evolved melt (with 2.4 wt.% TiO2), and such a melt composition is

Dynamics of the Axial Melt Lens/Dike transition at fast spreading ridges: assimilation and hydrous partial melting

NASA Astrophysics Data System (ADS)

France, L.; Ildefonse, B.; Koepke, J.

2009-04-01

Recent detailed field studies performed in the Oman ophiolite on the gabbro/sheeted dike transition, compared to corresponding rocks from the EPR drilled by IODP (Site 1256), constrain a general model for the dynamics of the axial melt lens (AML) present at fast spreading ridges (France et al., 2008). This model implies that the AML/dike transition is a dynamic interface migrating up- and downward, and that the isotropic gabbro horizon on top of the igneous section represents its fossilization. It is also proposed that upward migrations are associated to reheating of the base of the sheeted dike complex and to assimilation processes. Plagiogranitic lithologies are observed close to the truncated base of the dikes and are interpreted to represent frozen melts generated by partial melting of previously hydrothermalized sheeted dikes. Relicts of previously hydrothermalized lithologies are also observed in the fossil melt lens, and are associated to lithologies that have crystallized under high water activities, with clinopyroxene crystallizing before plagioclase, and An-rich plagioclase. To better understand our field data, we performed hydrous partial melting experiments at shallow pressures (0.1 GPa) under slightly oxidizing conditions (NNO oxygen buffer) and water saturated conditions on hydrothermalized sheeted dike sample from the Oman ophiolite. These experiments have been performed between 850°C and 1030°C; two additional experiments in the subsolidus regime were also conducted (750°C and 800°C). Clinopyroxenes formed during incongruent melting at low temperature (<910°C) have compositions that match those from the corresponding natural rocks (reheated base of the sheeted dike and relicts of assimilated lithologies). In particular, the characteristic low TiO2 and Al2O3 contents are reproduced. The experimental melts produced at low temperatures correspond to compositions of typical natural plagiogranites. In natural settings, these silicic liquids would be

Magnetic mapping of (carbonated) oceanic crust-mantle boundary: New insights from Linnajavri, northern Norway

NASA Astrophysics Data System (ADS)

Tominaga, M.; Beinlich, A.; Tivey, M.; Andrade Lima, E.; Weiss, B. P.

2012-12-01

The contribution of lower oceanic crust and upper mantle to marine magnetic anomalies has long been recognized, but the detailed magnetic character of this non-volcanic source layer remains to be fully defined. Here, we report preliminary results of a magnetic survey and source characterization of a "carbonated" oceanic Moho (petrological "Mohorovicic discontinuity") sequence observed at the Linnajavri Serpentinite Complex (LSC), northern Norway. The LSC is located at 67° 36'N and 16° 24'E within the upper Allochthon of the Norwegian Caledonides and represents a dismembered ophiolite. Particularly in the southern ("Ridoalggicohkka") area of the LSC, gabbro, serpentinite and its talc-carbonate (soapstone) and quartz-carbonate (listvenite) altered equivalents are extraordinarily well-exposed [1]. An intact oceanic Moho is exposed here, despite its complex tectonic setting. The small degree of arctic rock weathering (≤ 2 mm weathering surface) allowed for detailed regional-scale surface magnetic mapping across alteration fronts (serpentinite-soapstone; soapstone-listvenite) and lithological contacts (soapstone-gabbro). Magnetic mapping was conducted using a handheld 3-axis magnetometer, surface-towed resistivity meter and Teka surface magnetic susceptometer with sample spacing of 1 m. Geophysical field mapping was combined with petrological observations and scanning SQUID microscopy (SM) mapping conducted on thin sections from rock samples that were drilled along the survey lines. Regional scale magnetic mapping indicates that the total magnetic field across both the "carbonated" Moho and the soapstone-serpentinite interfaces show higher frequency changes in their magnetic anomaly character and amplitudes than the surface-towed resistivity data. SQUID microscopy mapping of both natural remanence magnetization (NRM) and anhysteretic remanence magnetization (ARM) on gabbro, serpentinite, soapstone, and listvenite samples, with a sensor-sample separation of ˜190 �

Mineralogy and Chemistry of Continental-like Calc-alkaline Plutons on Adak Island in the Oceanic Aleutian arc: Emplacement and Implications for the Eocene History of the Arc

NASA Astrophysics Data System (ADS)

Kay, S. M.; Citron, G. P.; Kay, R. W.; Jicha, B. R.

2016-12-01

The mineralogy and chemistry of the 15 km wide latest Eocene/Oligocene (34.6-30.9 Ma) Hidden Bay and Miocene (14.2-13.7 Ma) Kagalaska calc-alkaline plutons on Adak and Kagalaska Islands in the central Aleutian arc provide insight into the arc's Tertiary evolution. The plutons intrude the moderately light REE-enriched tholeiitic basaltic to mafic andesites of the Eocene Finger Bay Formation. The Hidden Bay pluton largely consists of mid to high-K amphibole-bearing cumulate diorite (53-55% SiO2) and granodiorite (57-64% & 61-64% SiO2) with lesser amounts of gabbro (50-52%), leucogranodiorite (67-69% SiO2) and aplite (76-77% SiO2). REE patterns indicate important fractionation of amphibole and plagioclase with pyroxene and olivine present in mafic units and orthopyroxene, biotite, quartz and K-feldspar in silicic units. Quartz, K-feldspar and biotite occur in interstices in most units. Plagioclase cores are mostly from AN40-60 with K-feldspar at OR95-OR98. Fractionation of homogeneous gabbros with high-Al basalt compositions (51% SiO2) best explains the chemistry and mineralogy of the Hidden Bay pluton. The presence of pargasitic amphibole in medium to course grained diorite cumulates indicates fractionation at 12-14 km at 950-1000°C with 5.5% H2O and a NNO oxygen fugacity. Two pyroxene, Mg hornblende and Ti-Zr zircon thermometers for granodiorite and late crystallized areas record temperatures of 850-750°C at 3.5- 4.5 % H2O and a NNO+2 oxygen fugacity. The Kagalaska pluton differs in being more calc-alkaline (alkali-rich), more bimodal in being dominated by amphibole-bearing gabbro and granodiorite/ leucogranodiorite (63-68% SiO2) and in requiring more amphibole fractionation. Both plutons have compositions approaching continental crust and characteristics that are similar to plutons intruded into continental crust. Differences with the Finger Bay Volcanic are best explained by thickening of the crust to near modern thicknesses ( 35-38 km) by the time of pluton

Hydrothermal Alteration of the Lower Oceanic Crust: Insight from OmanDP Holes GT1A and GT2A.

NASA Astrophysics Data System (ADS)

Harris, M.; Zihlmann, B.; Mock, D.; Akitou, T.; Teagle, D. A. H.; Kondo, K.; Deans, J. R.; Crispini, L.; Takazawa, E.; Coggon, J. A.; Kelemen, P. B.

2017-12-01

Hydrothermal circulation is a fundamental Earth process that is responsible for the cooling of newly formed ocean crust at mid ocean ridges and imparts a chemical signature on both the crust and the oceans. Despite decades of study, the critical samples necessary to resolve the role of hydrothermal circulation during the formation of the lower ocean crust have remained poorly sampled in the ocean basins. The Oman Drilling Project successfully cored 3 boreholes into the lower crust of the Semail ophiolite (Holes GT1A layered gabbros, GT2A foliated gabbros and GT3A dike/gabbro transition). These boreholes have exceptionally high recovery ( 100%) compared to rotary coring in the oceans and provide an unrivalled opportunity to quantitatively characterise the hydrothermal system in the lower oceanic crust. Hydrothermal alteration in Holes GT1A and GT2A is ubiquitous and manifests as secondary minerals replacing primary igneous phases and secondary minerals precipitated in hydrothermal veins and hydrothermal fault zones. Hole GT1A is characterised by total alteration intensities between 10 -100%, with a mean alteration intensity of 60%, and shows no overall trend downhole. However, there are discrete depth intervals (on the scale of 30 -100 m) where the total alteration intensity increases with depth. Alteration assemblages are dominated by chlorite + albite + amphibole, with variable abundances of epidote, clinozoisite and quartz. Hole GT1A intersected several hydrothermal fault zones, these range from 2-3 cm up to >1m in size and are associated with more complex secondary mineral assemblages. Hydrothermal veins are abundant throughout Hole GT1A, with a mean density of 37 vein/m. Hole GT2A is characterised by total alteration intensities between 6-100%, with a mean alteration intensity of 45%, and is highly variable downhole. Alteration halos and patches are slightly more abundant than in Hole GT1A. The secondary mineral assemblage is similar to Hole GT1A, but Hole GT2A

Hydration of the Atlantis Massif: Halogen, Noble Gas and In-Situ δ18O Constraints

NASA Astrophysics Data System (ADS)

Williams, M. J.; Kendrick, M. A.; Rubatto, D.

2017-12-01

A combination of halogen (Cl, Br, I), noble gases (He, Ne, Ar, Kr and Xe) and in situ oxygen isotope analysis have been utilized to investigate the fluid-mobile element record of hydration and alteration processes at the Atlantis Massif (30°N on the Mid-Atlantic Ridge). The sample suite investigated includes serpentinite, talc-amphibole ± chlorite schist and hydrated gabbro recovered by seafloor drilling undertaken at sites on a transect across the Atlantis Massif during IODP Expedition 357. Serpentine mesh and veins analysed in-situ by SHRIMP SI exhibit δ18O from 6‰ down to ≈0‰, suggesting serpentinization temperatures of 150 to >280°C and water/rock ratios >5. Differences of 1.5-2.5‰ are observed between adjacent generations of serpentine, but the δ18O range is similar at each investigated drilling site. Halogen and noble gas abundances in serpentinites, talc-amphibole schist and hydrated gabbro have been measured by noble gas mass spectrometry of both irradiated and non-irradiated samples. Serpentinites contain low abundances of halogens and noble gases (e.g. 70-430 ppm Cl, 4.7-12.2 x 10-14 mol/g 36Ar) relative to other seafloor serpentinites. The samples have systematically different Br/Cl and I/Cl ratios related to their mineralogy. Serpentinites retain mantle-like Br/Cl with a wide variation in I/Cl that stretches toward seawater values. Talc-amphibole schists exhibit depletion of Br and I relative to Cl with increasing Cl abundances, suggesting tremolite exerts strong control on halogen abundance ratios. Serpentinites show no evidence of interaction with halogen-rich sedimentary pore fluids. Iodine abundances are variable across serpentinites, and are decoupled from Br and Cl; iodine enrichment (up to 530 ppb) is observed within relatively oxidised and clay-bearing samples. Serpentinized harzburgites exhibit distinct depletion of Kr and Xe relative to atmospheric 36Ar in seawater. Oxygen isotope compositions and low abundances of both halogens

Timescale of Petrogenetic Processes Recorded in the Mount Perkins Magma System, Northern Colorado River Extension Corridor, Arizona

NASA Technical Reports Server (NTRS)

Danielson, Lisa R.; Metcalf, Rodney V.; Miller, Calvin F.; Rhodes Gregory T.; Wooden, J. L.

2013-01-01

The Miocene Mt. Perkins Pluton is a small composite intrusive body emplaced in the shallow crust as four separate phases during the earliest stages of crustal extension. Phase 1 (oldest) consists of isotropic hornblende gabbro and a layered cumulate sequence. Phase 2 consists of quartz monzonite to quartz monzodiorite hosting mafic microgranitoid enclaves. Phase 3 is composed of quartz monzonite and is subdivided into mafic enclave-rich zones and enclave-free zones. Phase 4 consists of aphanitic dikes of mafic, intermediate and felsic compositions hosting mafic enclaves. Phases 2-4 enclaves record significant isotopic disequilibrium with surrounding granitoid host rocks, but collectively enclaves and host rocks form a cogenetic suite exhibiting systematic variations in Nd-Sr-Pb isotopes that correlate with major and trace elements. Phases 2-4 record multiple episodes of magma mingling among cogenetic hybrid magmas that formed via magma mixing and fractional crystallization at a deeper crustal. The mafic end-member was alkali basalt similar to nearby 6-4 Ma basalt with enriched OIB-like trace elements and Nd-Sr-Pb isotopes. The felsic end-member was a subalkaline crustal-derived magma. Phase 1 isotropic gabbro exhibits elemental and isotopic compositional variations at relatively constant SiO2, suggesting generation of isotropic gabbro by an open-system process involving two mafic end-members. One end-member is similar in composition to the OIB-like mafic end-member for phases 2-4; the second is similar to nearby 11-8 Ma tholeiite basalt exhibiting low epsilon (sub Nd), and depleted incompatible trace elements. Phase 1 cumulates record in situ fractional crystallization of an OIB-like mafic magma with isotopic evidence of crustal contamination by partial melts generated in adjacent Proterozoic gneiss. The Mt Perkins pluton records a complex history in a lithospheric scale magma system involving two distinct mantle-derived mafic magmas and felsic magma sourced in the

Petrographic and Geochemical Characterization of the Cambumbia STOCK in Andean Central Cordillera, Colombia

NASA Astrophysics Data System (ADS)

Rojas Lequerica, S.; Jaramillo Mejía, J.; Concha Perdomo, A.

2012-12-01

The Cambumbia Stock is located on the western flank of the Central Cordillera of the northern Andes. The goals of this study were to petrographic and geochemically characterize the Cambumbia igneous body and to establish its petrogenetic history. 41 samples were collected, 28 for petrographic analysis and 14 for elementary chemical determination by ICP-MS. Petrographically the samples were classified as hornblende and pyroxene-gabbros varying to diorites, gabbronorites and tonalites, the rock texture varies from medium to coarse granular grain, with local microporfiritic texture. It was concluded from the major elements analysis that the samples correspond to the sub-alkaline series with low K content, mainly in the calc-alkaline series, within the gabbros and diorites fields. By using the SiO2 vs TiO2 (Jaramillo, 1980), Th/Yb vs Ta/Yb (Pearce, 1984) (Fig. 1) and Zr/117-Th-Nb/16 (Wood, 1979) diagrams it was determined that these rocks were generated in two geotectonic environments: one type MOR (extension) and other island arc (subduction, compression). Petrographic and geochemical comparisons between the rocks of Cambumbia Stock and Diorite and Gabbro El Pueblito (Giraldo, 2009) (located about 25 km to the north-west) may postulate a possible genetic link between them. Recently, a U/Pb age was obtained by the Universidad de Caldas in zircon in 2009 (not published data), yielded an age of 233.41 ± 3.4 Ma (Middle Triassic). This age is consistent with the global event of the extension and fragmentation of Pangea supercontinent. In addition, the mantle nature of the source and the petrogenetic evolution of the magmatic system were established. References GIRALDO, M.I., (2009): Esquema geodinámica de la parte noroccidental de la cordillera Central de Colombia. (Thesis). p.56-68. Universidad Nacional de Colombia, Medellín. JARAMILLO, J.M. (1980): Petrology and geochemistry of the Nevado del Ruiz Volcano northern Andes, Colombia (Thesis). 167 p. University of Houston

Compositional Evidence for Launch Pairing of the YQ and Elephant Moraine Lunar Meteorites

NASA Technical Reports Server (NTRS)

Korotev, R. L.; Jollitt, B. L.; Zeigler, R. A.; Haskin, L. A.

2003-01-01

Arai and Warren provide convincing evidence that QUE (Queen Alexandra Range) 94281 derives from the same regolith as Y (Yamato) 793274 and, therefore, that the two meteorites were likely ejected from the Moon by the same impact. Recently discovered Y981031 is paired with Y793274. The "YQ" meteorites (Y793274/Y981031 and QUE 94281 are unique among lunar meteorites in being regolith breccias composed of subequal amounts of mare volcanic material (a VLT [very-low-Ti] basalt or gabbro) and feldspathic highland material. EET (Elephant Moraine) 87521 and its pair EET 96008 are fragmental breccias composed mainly of VLT basalt or gabbro. Warren, Arai, and colleagues note that the volcanic components of the YQ and EET meteorites are texturally similar more similar to each other than either is to mare basalts of the Apollo collection. Warren and colleagues address the issue of possible launch pairing of YQ and EET, but note compositional differences between EET and the volcanic component of YQ, as inferred from extrapolations of regressions to high FeO concentration. We show here that: (1) EET 87/96 consists of fragments of a differentiated magma body, (2) subsamples of EET represent a mixing trend between Fe-rich and Mg-rich differentiates, and (3) the inferred volcanic component of YQ is consistent with a point on the EET mixing line. Thus, there is no compositional impediment to the hypothesis that YQ is launch paired with EET.

Reconnaissance geology of the Jabal Hashahish Quadrangle, sheet 17/41 B, Kingdom of Saudi Arabia

USGS Publications Warehouse

Hadley, D.G.

1982-01-01

The Jabal Hashahish quadrangle (sheet 17/41 B) lies between lat 17?30' and 18?00' N. and long 41?30' and 42?00' E. and encompasses an area of 2,950 km2, of which only about 600 km2 is land; the remainder is covered by the Red Sea. The geologic formations exposed in the quadrangle include Precambrian layered and intrusive rocks, Tertiary gabbro dikes, Quaternary basaltic lavas and pyroclastic rocks, and Quaternary surficial deposits. The Precambrian rocks include layered sedimentary and volcanic rocks that have been assigned to the Baish, Bahah, and Ablah groups. These rocks have been folded, metamorphosed, and invaded by intrusions. They are cut by Miocene gabbro dikes that were intruded during the initial stages of the opening of the Red Sea rift. The Quaternary rocks also include basalt that was extruded during a continuation of that opening, after the uplift that formed the escarpment that parallels the eastern shore of the Red Sea, but before the Holocene erosional cycle. Coastal, pediment, and alluvial, and eolian deposits of various kinds are also of Quaternary age. The economic potential of the quadrangle lies essentially in the agricultural value of its flood-plain deposits, though these are not so widely used as those in Wadi Hali and Wadi Yiba, which are located in the Manjamah quadrangle. The coral reefs possibly could provide raw materials for use in a cement industry, if any such industry were ever required in this area.

Zircon U-Pb, O, and Hf isotopic constraints on Mesozoic magmatism in the Cyclades, Aegean Sea, Greece

NASA Astrophysics Data System (ADS)

Fu, Bin; Bröcker, Michael; Ireland, Trevor; Holden, Peter; Kinsley, Leslie P. J.

2015-01-01

Compared to the well-documented Cenozoic magmatic and metamorphic rocks of the Cyclades, Aegean Sea, Greece, the geodynamic context of older meta-igneous rocks occurring in the marble-schist sequences and mélanges of the Cycladic Blueschist Unit is as yet not fully understood. Here, we report O-Hf isotopic compositions of zircons ranging in age from ca. 320 Ma to ca. 80 Ma from metamorphic rocks exposed on the islands of Andros, Ios, Sifnos, and Syros with special emphasis on Triassic source rocks. Ion microprobe (SHRIMP II) single spot oxygen isotope analysis of pre-Cretaceous zircons from various felsic gneisses and meta-gabbros representing both the marble-schist sequences and the mélanges of the study area yielded a large range in δ18O values, varying from 2.7 ‰ to 10.1 ‰ VSMOW, with one outlier at -0.4 %. Initial ɛHf values (-12.5 to +15.7) suggest diverse sources for melts formed between Late Carboniferous to Late Cretaceous time that record derivation from mantle and reworked older continental crust. In particular, variable δ18O and ɛHf( t) values for Triassic igneous zircons suggest that magmatism of this age is more likely rift- than subduction-related. The significant crustal component in 160 Ma meta-gabbros from Andros implies that some Jurassic gabbroic rocks of the Hellenides are not part of SSZ-type (supra-subduction zone) ophiolites that are common elsewhere along the margin of the Pelagonian zone.

Petrogenesis of the ∼500 Ma Fushui mafic intrusion and Early Paleozoic tectonic evolution of the Northern Qinling Belt, Central China

NASA Astrophysics Data System (ADS)

Shi, Yu; Pei, Xiaoli; Castillo, Paterno R.; Liu, Xijun; Ding, Haihong; Guo, Zhichao

2017-06-01

The Fushui mafic intrusion in the Qinling orogenic belt (QOB) is composed of meta-gabbro, meta-gabbro-diorite, diorite, and syenite. Most of these rocks are metamorphosed under the upper greenschist facies to lower amphibolite facies metamorphism. Zircon separates from eight samples have LA-ICP-MS U-Pb ages of 497-501 Ma which are taken to be the emplacement age of magmas that formed the Fushui intrusion. Most of the zircon grains exhibit negative εHf values, correspond to TDM2 model ages of late Paleoproterozoic-early Mesoproterozoic or Neoproterozoic and suggest that the mafic rocks were most probably derived from mafic melts produced by partial melting of a previously metasomatized lithospheric mantle. The intrusion is not extensively contaminated by crustal materials and most chemical compositions of rocks are not modified during the greenschist to amphibolite-facies metamorhism. Rocks from the intrusion have primitive mantle-normalized trace element patterns with significant enrichment in light-REE and large ion lithophile elements (LILE) and depletion in high field-strength elements (HFSE). On the basis of the trace element contents, the Fushui intrusion was derived from parental magmas generated by <10% partial melting of both phlogopite-lherzolite and garnet-lherzolite mantle sources. These sources are best interpreted to be in a subduction-related arc environment and have been modified by fluids released from a subducting slab. The formation of the Fushui intrusion was related to the subduction of the Paleotethyan Shangdan oceanic lithosphere at ∼500 Ma.

Ages and origin of felsic rocks from the Eastern Erenhot ophiolitic complex, southeastern Central Asian Orogenic Belt, Inner Mongolia China

NASA Astrophysics Data System (ADS)

Yang, Jinfu; Zhang, Zhicheng; Chen, Yan; Yu, Haifei; Qian, Xiaoyan

2017-08-01

The Central Asian Orogenic Belt (CAOB) is known for its massive Phanerozoic generation of juvenile crust. The tectonic evolution of the CAOB during the late Paleozoic era is still debated. The Eastern Erenhot ophiolite complex (EOC) has been recognized as one of the numerous late Paleozoic ophiolitic blocks in the southeastern part of the CAOB. Zircon U-Pb dating on rhyolite and plagiogranite from the EOC yielded a tight range of ages from 360 to 348 Ma, indicating that the complex formed in the early Carboniferous. The primitive mantle-normalized spider diagram of rhyolites (εNd(t) values of +6.8 and +7) and basalts almost overlaps. Such rhyolites may have been derived from partial melting of juvenile basaltic rocks during the initial opening of the Erenhot-Hegenshan oceanic basin. All of the plagiogranites exhibit similar trace element behaviours of High Field-Strength Elements, such as U, Zr and Hf, and Large Ion Lithophile Elements, such as Ba and Rb, to these of gabbros. These plagiogranites were considered products of episodes of partial melting of hydrous gabbros during ocean floor spreading. We conclude that the northern subduction of the Paleo-Asian Ocean stopped before 360 Ma and the southeastern CAOB experienced extension during the late Paleozoic era. The Erenhot-Hegenshan Ocean, which is comparable to the present Red Sea, originated from syn-collisional crustal thickening, subsequent lithosphere extension, and upwelling of the asthenosphere during orogenic quiescence with an age of 20 Ma.

Oceanic crustal velocities from laboratory and logging measurements of Integrated Ocean Drilling Program Hole 1256D

NASA Astrophysics Data System (ADS)

Gilbert, Lisa A.; Salisbury, Matthew H.

2011-09-01

Drilling and logging of Integrated Ocean Drilling Program (IODP) Hole 1256D have provided a unique opportunity for systematically studying a fundamental problem in marine geophysics: What influences the seismic structure of oceanic crust, porosity or composition? Compressional wave velocities (Vp) logged in open hole or from regional refraction measurements integrate both the host rock and cracks in the crust. To determine the influence of cracks on Vp at several scales, we first need an accurate ground truth in the form of laboratory Vp on crack-free, or nearly crack-free samples. We measured Vp on 46 water-saturated samples at in situ pressures to determine the baseline velocities of the host rock. These new results match or exceed Vp logs throughout most of the hole, especially in the lower dikes and gabbros, where porosities are low. In contrast, samples measured at sea under ambient laboratory conditions, had consistently lower Vp than the Vp logs, even after correction to in situ pressures. Crack-free Vp calculated from simple models of logging and laboratory porosity data for different lithologies and facies suggest that crustal velocities in the lavas and upper dikes are controlled by porosity. In particular, the models demonstrate significant large-scale porosity in the lavas, especially in the sections identified as fractured flows and breccias. However, crustal velocities in the lower dikes and gabbros are increasingly controlled by petrology as the layer 2-3 boundary is approached.

Smoking: what has been addressed in Brazilian journals.

PubMed

Prado, Gustavo Faibischew; Lombardi, Elisa Maria Siqueira; Morais, Anna Miethke; Martins, Stella Regina; Santos, Ubiratan de Paula

2012-12-01

The topic of tobacco smoking, in its several aspects, has been receiving increasing attention among researchers over the past few years, which has been reflected in more data and more solid scientific literature on the subject in national journals. This article aims to review the studies that focused on smoking published between January 2010 and June 2012, in Arquivos Brasileiros de Cardiologia (Brazilian Archives of Cardiology), Brazilian Journal of Medical and Biological Research, Clinics (Sao Paulo), Jornal Brasileiro de Pneumologia (Brazilian Journal of Pulmonology), Revista da Associação Médica Brasileira (Journal of the Brazilian Medical Association) and Revista Brasileira de Cirurgia Cardiovascular (Brazilian Journal of Cardiovascular Surgery). During the aforementioned period 58 articles were published, 52 of which were original ones, addressing several aspects of smoking, such as effects on health, epidemiology, cessation and experimental studies.

Le complexe annulaire d'âge Oligocène de l'Achkal (hoggar Central, Sud Algérie) : témoin de la transition au Cénozoïque entre les magmatismes tholéitique et alcalin. Évidences par les isotopes du Sr, Nd et Pb

NASA Astrophysics Data System (ADS)

Maza, Mustapha; Briqueu, Louis; Dautria, Jean-Marie; Bosch, Delphine

1998-08-01

The Achkal Oligocene ring complex cross-cuts the Upper Eocene tholeiitic traps located on the top of the Hoggar swell. The plutonic rocks range from tholeiitic gabbros to alkali essexites, monzonites and syenites, whereas the volcanites are restricted to late peralkaline rhyolites. The affinity change linked to the large isotopic heterogeneities (from EM1 to HIMU) suggests that the parental magmas are issued from two distinct mantle sources, first lithospheric then deeper. The Achkal has recorded the magmatic evolution of the Hoggar hot spot, between Eocene and Miocene.

Stratigraphy of the late Proterozoic Murdama Group, Saudi Arabia

USGS Publications Warehouse

Greene, Robert C.

1993-01-01

The Murdama group probably was deposited in a back-arc basin on a continental platform bounded on the west by an active volcanic arc above an east-dipping subduction zone. The position of the subduction zone, which was active during most of the deposition in the Afif belt, is marked by a belt of gabbro and ultramafic rocks herein named the jabal Burqah belt. The subduction zone later stepped out to the southwest to the Nabitah belt, and Murdama strata were deposited in the Jabal Hadhah, Mistahjed, and smaller basins.

Poly-phase Deformation Recorded in the Core of the Coast Plutonic Complex, Western British Columbia

NASA Astrophysics Data System (ADS)

Hamblock, J. M.; Andronicos, C. L.; Hurtado, J. M.

2006-05-01

The Coast Plutonic Complex of western British Columbia constitutes the largest batholith within the North American Cordillera. The field area for this study is Mt. Gamsby, an unexplored region above the Kitlope River, east of the Coast Shear Zone and at the southern end of the Central Gneiss Complex. The dominant lithologies on Mt. Gamsby include amphibolite and metasedimentary gneiss, gabbro-diorite, and orthogneiss. The amphibolite gneiss contains alternating amphibolite and felsic layers, with chlorite and epidote pervasive in some regions and garnet rare. This unit is commonly migmatized and contains various folds, boudins, and shear zones. The metasedimentary gneiss contains quartz, k-spar, graphite, chlorite, and perhaps cordierite, but appears to lack muscovite and aluminosilicates. The gabbro-diorite is salt and pepper in color and contains ca. 50% pyroxene and plagioclase. The orthogneiss is light in color and plagioclase-rich, with a texture varying from coarse-grained and undeformed to mylonitic. In some regions, this unit contains abundant mafic enclaves. At least four deformational events (D1-4) are observed. The second generation of folding, F2, is dominant in the area and resulted in the production of a large synform during sinistral shearing. The S1 foliation is observed only in the amphibolite gneiss and is orthogonal to S2, creating mushroom- type fold interference patterns. S2 foliations strike NW-SE and dip steeply to the SW, suggesting SW-NE directed shortening. L2 lineations developed on S2 plunge shallowly to the NW and SE, implying strike-slip motion. Although both dextral and sinistral motions are indicated by shear band data, sinistral motion is dominant. The average right and left lateral shear band orientation is nearly identical to S2, suggesting that right and left lateral shearing were synchronous. Foliations within the orthogneiss are parallel to the axes of S2 folds and boudins in the amphibolite gneiss, suggesting that emplacement

Timing of tectonic evolution of the East Kunlun Orogen, Northern Tibet Plateau

NASA Astrophysics Data System (ADS)

Dong, Yunpeng

2017-04-01

The East Kunlun Orogen, located at the northern Tibet Plateau, represents the western segment of the Central China Orogenic Belt which was formed by amalgamation of the North China blocks and South China blocks. It is a key to understanding the formation of Eastern Asian continent as well as the evolution of the Pangea supercontinent. Based on detailed geological mapping, geochemical and geochronological investigations, the orogen is divided into three main tectonic belts, from north to south, including the Northern Qimantagh, Central Kunlun and Southern Kunlun Belts by the Qimantagh suture, Central Kunlun suture and South Kunlun fault. The Qimantagh suture is marked by the Early Paleozoic ophiolites outcropped in the Yangziquan, Wutumeiren, and Tatuo areas, which consist mainly of peridotites, gabbros, diabases and basalts. Besides, the ophiolite in the Wutumeiren is characterized by occurring anorthosite while the ophiolite in the Tatuo occurring chert. The basalts and diabases from both Yaziquan and Tatuo areas display depletion of Nb, Ta, P and Ti, and enrichment of LILE, suggesting a subduction related tectonic setting. LA-ICP-MS zircon U-Pb age of 421 Ma for the diabase represents the formation age of the Yaziquan ophiolite, while the U-Pb ages of 490 Ma and 505 Ma for gabbro and anorthosite, respectively, constrain the formation age of the Tatuo ophiolite. The basaltic rocks in the Wutumeiren area display flat distribution of HFSEs (such as Nb, Ta, K, La, Ce, Pr, Nd, Zr, Sm, Eu, Ti, Dy, Y, Yb and Lu) and slightly enrichment in LREEs, while the peridotites showing depletion in MREEs. The LA-ICP-MS zircon U-Pb age of 431 Ma for the gabbro represents the formation age of the Wutumeiren ophiolite. Together with regional geology, we suggest herewith a back-arc basin tectonic setting during ca. 505-421 Ma at least for the Qimantagh suture. The Central Kunlun suture is represented by the ophiolite in the Wutuo area, which is characterized by depletion of Nb, Ta, P

Mechanoelectrically Activated Synthesis of Dense, Bulk Nanostructured, Complex Crystalline and Glassy Hard Materials

DTIC Science & Technology

2005-04-01

Facultad de Quimica , Universidad Autonoma de Queretaro, Queretaro, Mexico, May 7, 2002. (Invited Seminar) "* Recent Advances in the Synthesis and...A. Munir, 14o Congresso Brasileiro de Engenharia e Ciencias dos Materiais (14th Brazilian Congress on Materials Science and Engineering), Sao Pedro

Shock recovery analogs and the origin of mesosiderites. [Abstract only

NASA Technical Reports Server (NTRS)

Rowan, L. R.

1994-01-01

The origin of mesosiderites, which consist of approximately equal-weight proportions of Fe-Ni metal and silicates (gabbros, basalts, orthopyroxenites, dunites), remains an interesting and complex problem in meteoritics. There is general agreement that multiple impact events were probably involved in the formation of these brecciated stony-iron meteorites, but given the heterogeneity of mesosiderites, additional processes have been invoked to explain the unique and intricate textural and compositional makeup of mesosiderites. We conducted a series of shock recovery experiments to test the impact event(s) scenario. The results indicated significant similarities between the shocked analogs and many mesosiderites. We have compared our analogs with a suite of thin sections of Barea mesosiderite. I have conducted a series of flash heating experiments in which equal-weight proportions of gabbro and stainless steel (SS304) powders were compressed into small charges and heated under reducing conditions for short times. These experiments were used to bracket localized, peak postshock temperatures in our analog shots and to compare the mixing relations between the silicate and metal. The shock recovery experiments used porous metal-silicate powder starting mixtures, therefore our experiments are most analogous to an impact scenario where the target is an asteroidal regolith surface composed of a loose mixture of Fe-Ni metal and heterogeneous silicates. Analog experiments may really describe a secondary impact process similar to the late-stage, localized impact melting event. This leaves one of the crucial questions about mesosiderite genesis unanswered, namely what is the source of the Fe-Ni metal that is so intimately distributed in these meteorites?

Oxygen self-diffusion in diopside with application to cooling rate determinations

NASA Astrophysics Data System (ADS)

Farver, John R.

1989-04-01

The kinetics of oxygen self-diffusion in a natural diopside have been measured over the temperature range 700-1250°C. Experiments were run under hydrothermal conditions using 18O-enriched water. Profiles of 18O/( 16O+ 18O) versus depth into the crystal were obtained using an ion microprobe. At 1000 bars (100 MPa) confining pressure, the Arrhenius relation for diffusion parallel to the c crystallographic direction yields a pre-exponential factor ( D0) = 1.5 × 10 -6 cm 2/s and an activation energy ( Q) = 54 ± 5 kcal/g-atom O (226 kJ/g-atom O) over the temperature range of the experiments. Diffusion coefficients parallel to the c crystallographic direction are ≈ 100 times greater than perpendicular to c. The oxygen self-diffusion coefficient obtained for diopside is ≈ 1000 times less than that for diffusion in feldspars, and ≈ 100 times less than that for quartz at 800°C, transport parallel to the c axis. Closure temperatures calculated for oxygen diffusional exchange in natural diopside are significantly higher than for quartz or feldspars. Measurable oxygen isotope exchange in diopside by diffusion would require geological settings with very high temperatures maintained for very long durations. The oxygen diffusional exchange kinetics in diopside presented in this paper find important applications in studies of meteoric hydrothermal circulation systems and the time-temperature history of high-grade regionally metamorphosed terrains. Examples considered include the Outer Unlayered Gabbro, Cuillins Gabbro Complex, Isle of Skye, Scotland, and the granulite-grade Turpentine Hill Metamorphics near Einasleigh, Queensland, Australia.

Refining Rodinia: New Paleomagnetic Results From Amazonia and Paleogeographic Implications For The Grenville Orogeny.

NASA Astrophysics Data System (ADS)

Tohver, E.; van der Pluijm, B. A.; van der Voo, R.

The Grenville province of eastern Laurentia is commonly considered to be the product of continental collision between ancestral North America and an as yet unidentified continent. New paleogeographic information for the Amazon craton in early Grenvil- lian times was determined from a new paleomagnetic pole based on the hypabyssal gabbros and flat-lying basalts of the Nova Floresta Fm. found in the western Brazil- ian state of Rondonia. Measurement of the anisotropy of magnetic susceptibility of the gabbros reveals a flat-lying fabric, suggesting an undeformed, igneous body. A paleomagnetic pole (n = 16 sites, Plat = 24.6N, Plon. = 164.6E, A95 = 5.5, Q = 5) is calculated from a steep, characteristic remanence (ChRM) that is inferred to be primary. This ChRM is isolated at applied field >30 mT and is probably carried by magnetite present in large, oxyexsolved titanomagnetites or igneous reaction rims. Emplacement of the body and acquisition of magnetization is dated by 40Ar/39Ar analysis of igneous biotite and plagioclase, both phases yielding ages of ca.1.2 Ga. Comparison of the position of Amazonia with that of ancestral North America deter- mined from the Laurentian APWP from 1.3 - 1.15 Ga suggests that Amazonia may have collided with the southernmost portion of Laurentia at ca.1.2 Ga. The timing of this collision is in agreement with geochronological constraints on the timing of de- formation in the Llano segment of Laurentia as well as observed deformation of the western Amazon craton.

Elemental and Sr-Nd isotopic geochemistry of the Uradzhongqi magmatic complex in western Inner Mongolia, China: A record of early Permian post-collisional magmatism

NASA Astrophysics Data System (ADS)

Qiao, Xueyuan; Li, Wenbo; Zhong, Richen; Hu, Chuansheng; Zhu, Feng; Li, Zhihua

2017-08-01

The magmatic complex in Uradzhongqi, Inner Mongolia, is located in the western segment of the northern margin of the North China Craton (NCC). The dominant components in the complex include syenogranite, monzogranite, granodiorite, diorite and gabbro. Mafic microgranular enclaves (MMEs) are common in syenogranite and granodiorite. Zircon U-Pb dating shows that the ages of these rocks range from 283 to 270 Ma, suggesting an early Permian emplacement. The syenogranite and monzogranite are peraluminous I-type granites, exhibiting conspicuous negative Eu anomaly, enrichment in large-ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in high field strength elements (HFSE). The granodiorites, diorites and MMEs are metaluminous in composition, show high Al2O3, MgO and Fe2O3T contents and weak negative Eu anomaly, as well as LREE and LILE enrichment and HFSE depletion. The gabbros show weak positive Eu anomaly and slight REE differentiation. The Sr-Nd isotope compositions show that the source of mafic magma was depleted mantle (DM) with possible involvement of enriched mantle II (EM II), whereas the felsic magma was derived from the Archean lower crust. Petrographic observation and analytical results of mineralogy, geochronology, geochemistry and Sr-Nd isotopes indicate that the main petrogenesis of these magmatic rocks is the mixing of underplating mafic magma and felsic magma. Tectonically, the complex pluton was formed within a post-collisional regime, and the underplating in this area provides another piece of evidence for the vertical growth of the western segment of the northern margin of the NCC.

Exploration of gold occurrences in alteration zones at Dungash district, Southeastern Desert of Egypt using ASTER data and geochemical analyses

NASA Astrophysics Data System (ADS)

Salem, S. M.; El Sharkawi, M.; El-Alfy, Z.; Soliman, N. M.; Ahmed, S. E.

2016-05-01

The present study aims at exploration of new gold occurrences in the alteration zones at Dungash district. Processed ASTER images band ratios 7/6 × 4/6 and (7 + 9/8), field geology and mineralogical and geochemical data help characterize three types of alterations in three areas 1 to 3 that may be targeted for Au exploration. Area1 confined to the metavolcanics located in the SE of Dungash gold mine and revealed silicified and sericitized type alterations, composed of quartz, epidote, chlorite, biotite and opaque minerals mainly pyrite and chalcopyrite. Area2 occurs in the gabbro-diorite rocks at Abu Meraiwa area NE of Dungash gold mine, which are rich in kaolinite, illite, sericite, pyrite, arsenopyrite and chalcopyrite that record kaolinitized alteration. Area3 is hosted in carbonaceous listwaenized serpentinite thus indicating the role of listwaenitization type alteration in ore genesis. It is composed of calcite, chromite, pyrite, arsenopyrite, chalcopyrite and Ni-bearing sulphides. Au contents in area 1 range between 0.12 and 14.91 ppm, and between 6.1 and 16.3 ppm in area 2, while gold values in area 3 vary from <0.01 to 0.03 ppm. Dungash district is comprised of Pan-African assemblages of ophiolitic ultramafics thrusted over the island arc metavolcanics of dacitic- andesite composition. Gabbro-diorite rocks are intruded in the ultramafics and the acidic metavolcanics as well as diorite-quartz diorite suite intruded in the intermediate metavolcanics. Several acidic dykes, granitic dykes and quartz veins cut through the different rocks types.

Structural development of the Red Hill portion of the Feather River ultramafic complex, Pulmas County, California

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

Weisenberg, C.W.

1979-01-01

The Feather River Ultramafic Complex is a partially serpentinized body of metamorphosed alpine peridotite and gabbro that lies along the northern part of the Melones fault zone, a NNW trending belt in the Northern Sierra Nevada. The complex was studied in the area of Red Hill, near the canyon of the North Fork, Feather River. The complex is separated from the Calaveras Terrain and Arlington Formation country rocks by steep faults; the Melones Fault on the east and the Rich Bar Fault on the west. Units recognized within the complex include Rich Bar metamorphic rocks, peridotite, metaperidotite, tremolite-olivine schist, hornblendemore » schist, and layered metagabbro. The Rich Bar metamorphic rocks are tectonic slices of amphibolite grade hornblende schist, mica schist, and quartzite found along the Rich Bar Fault. The complex shows evidence of 4 major events. E-1 (Pennsylvania-Permian) was formation of the peridotite-gabbro complex. E-2 (Permo-Triassic) consisted of pervasive shearing parallel to the Rich Bar Fault associated with initial emplacement within the Sierra Nevada. E-3 is believed to be compression and metamorphism (serpentinization) associated with the Nevadan orogeny. E-4 was associated with intrusion of nearby plutons. The regional association of the complex with late paleozoic arc volcanics of the Taylorsville area suggest formation near or under an island arc. Metamorphism during emplacement indicates association with the arc at that time. Left-lateral shear during emplacement along the Rich Bar Fault indicates NW directed thrusting when the layering in metagabbro is rotated to horizontal.« less

STRAWBERRY MOUNTAIN WILDERNESS, OREGON.

USGS Publications Warehouse

Thayer, T.P.; Stotelmeyer, Ronald B.

1984-01-01

The Strawberry Mountain Wilderness extends 18 mi along the crest of the Strawberry Range and comprises about 53 sq mi in the Malheur National Forest, Grant County, Oregon. Systematic geologic mapping, geochemical sampling and detailed sampling of prospect workings was done. A demonstrated copper resource in small quartz veins averaging at most 0. 33 percent copper with traces of silver occurs in shear zones in gabbro. Two small areas with substantiated potential for chrome occur near the northern edge of the wilderness. There is little promise for the occurrence of additional mineral or energy resources in the Strawberry Mountain Wilderness.

Petrologic and Oxygen-Isotopic Investigations of Eucritic and Anomalous Mafic Achondrites

NASA Technical Reports Server (NTRS)

Mittlefehldt, D. W.; Greenwood, R. C.; Peng, Z. X.; Ross, D. K.; Berger, E. L.; Barrett, T. J.

2016-01-01

The most common asteroidal igneous meteorites are eucrite-type basalts and gabbros rocks composed of ferroan pigeonite and augite, calcic plagioclase, silica, ilmenite, troilite, Ca-phosphate, chromite and Fe-metal. These rocks are thought to have formed on a single asteroid along with howardites and diogenites (HEDs). However, Northwest Africa (NWA) 011 is mineralogically identical to eucrites, but has an O-isotopic composition distinct from them and was derived from a different asteroid. Modern analyses with higher precision have shown that some eucrites have smaller O-isotopic differences that are nevertheless well-resolved from the group mean.

PERMEABILITY CHANGES IN CRYSTALLINE ROCKS DUE TO TEMPERATURE: EFFECTS OF MINERAL ASSEMBLAGE.

USGS Publications Warehouse

Morrow, C.A.; Moore, Diane E.; Byerlee, J.D.; ,

1985-01-01

The change in permeability with time of granite, quartzite, anorthosite and gabbro was measured while these rocks were subjected to a temperature gradient. Permeability reductions of up to two orders of magnitude were observed, with the greatest reactions occurring in the quartzite. These changes are thought to be caused by dissolution of minerals at high temperatures, and redeposition of the dissolved material at lower temperatures. Quartz appears to be an important mineral in this self-sealing process. If very low permeability is desired around a nuclear waste repository in crystalline rocks, then a quartz-rich rock may be the most appropriate host.

Discovery of a Devonian mafic magmatism on the western border of the Murzuq basin (Saharan metacraton): Paleomagnetic dating and geodynamical implications

NASA Astrophysics Data System (ADS)

Derder, M. E. M.; Maouche, S.; Liégeois, J. P.; Henry, B.; Amenna, M.; Ouabadi, A.; Bellon, H.; Bruguier, O.; Bayou, B.; Bestandji, R.; Nouar, O.; Bouabdallah, H.; Ayache, M.; Beddiaf, M.

2016-03-01

Intraplate deformation is most often linked to major stress applied on plate margins. When such intraplate events are accompanied by magmatism, the use of several dating methods integrated within a multidisciplinary approach can bring constraints on the age, nature and source mobilized for generating the magma and in turn on the nature of the intraplate deformation. This study focuses on the large gabbro Arrikine sill (35 km in extension) emplaced within the Silurian sediments of the western margin of the Murzuq cratonic basin in southeastern Algeria. Its emplacement is dated during the early Devonian (415-400 Ma) through the determination of a reliable paleomagnetic pole by comparison with the Gondwana Apparent Polar Wander Path (APWP). This age can be correlated with deep phreatic eruptions before Pragian time thought to be at the origin of sand injections and associated circular structures in Algeria and Libya. For the sill, the K-Ar age of 325.6 ± 7.7 Ma is related to a K-rich aplitic phase that has K-enriched by more than 20% the Devonian gabbro. Laser-ICP-MS U-Pb method dates only inherited zircons mostly at c. 2030 Ma with additional ages at c. 2700 Ma and younger ones in the 766-598 Ma age range. The Arrikine sill is a high-Ti alkaline gabbro having the geochemical composition of a hawaiite akin to several intraplate continental and oceanic provinces, including the contemporaneous Aïr ring complexes province in Niger, but also to the Mauna Loa volcano in Hawaii. This peculiar composition akin to that of the contemporaneous Aïr province is in agreement with a lower Devonian age for the Arrikine sill. The lower Devonian Arrikine sill emplacement is related to a "Caledonian" transtensive reactivation of the western metacratonic boundary of the Murzuq craton. This event also generated in the Saharan platform the so-called "Caledonian unconformity" of regional extension, the Aïr ring complexes and magmatic rocks that produced sand injections. It could be

Stable isotopic constraints on fluid-rock interaction and Cu-PGE-S redistribution in the Sonju Lake intrusion, Minnesota

USGS Publications Warehouse

Park, Y.-R.; Ripley, E.M.; Miller, J.D.; Li, C.; Mariga, J.; Shafer, P.

2004-01-01

The Sonju Lake intrusion, part of the 1.1 Ga Midcontinent rift-related Beaver Bay Complex, is a 1,200-m-thick, strongly differentiated, layered sequence of mafic cumulates located in northeastern Minnesota. Basal melatroctolite and dunite layers are overlain by troctolite, gabbro, Fe-Ti oxide-rich gabbro, apatite diorite, and monzodiorite. Stratigraphic intervals rich in Pt + Pd, Cu, and S occur over ???500 m in the Fe-Ti oxide-rich gabbro and apatite diorite units. Peak concentrations show offsets that are similar to those found in other tholeiitic layered intrusions. Concentrations of Pd in excess of 100 ppb are confined to the lowermost 25 m of the interval. Copper shows a sharp increase to 630 ppm above the Pd-rich interval. Sulfur contents are low (<375 ppm) in the Cu-rich interval, but they increase to values as high as 3,150 ppm above in the apatite diorite. Disseminated sulfides in the intrusion have ??34S values that range from -2.2 to 3 per mil Vienna-Canyon Diablo Troilite (V-CDT) and suggest that contamination by country rock sulfur was not an important process in the formation of the metal-rich interval. ??18O values of plagioclase from the intrusion range from 5.6 to 12.0 per mil (V-SMOW) and indicate that a relatively low-18O fluid (??18O ???3-5 ???) interacted with the rocks of the intrusion at temperatures less than ???275??C. Clinopyroxene and Fe-Ti oxides (ilmenite with minor amounts of titanomagnetite) show much more restricted ranges in ??18O values (4.6-5.7 and 5.5-6.7 per mil, respectively) and attest to the kinetic control of the oxygen isotope exchange process. The externally derived fluid that interacted with rocks now enriched in platinum group elements (PGE) + Cu- and Fe-sulfide minerals locally liberated sulfur and replaced chalcopyrite and pyrite with goethite. In the Cu-rich zone, goethite that replaces chalcopyrite may contain up to 8.5 weight percent Cu. It is evident that hydrothermal alteration resulted in a decoupling of copper

A new interpretation of the structure of the Sept Iles Intrusive suite, Canada

NASA Astrophysics Data System (ADS)

Higgins, Michael D.

2005-08-01

The layered mafic intrusion at Sept Iles, Canada, is one of the largest intrusions in the world. A new interpretation of its structure is proposed, based on a review of its geology and a comparison with the Skaergaard intrusion, Greenland. Several different magmatic components are recognized; hence the name Sept Iles Intrusive suite (SIIS) is proposed. Emplacement of the suite may have been preceded by eruption of flood basalts. The first magmas of the suite rose in the crust to accumulate beneath the density filter afforded by the basalts. The largest component is the Sept Iles Mafic intrusion (SIMI). The Lower series of the SIMI is dominated by leucotroctolites and leucogabbros. Above it lie the Layered series, which is largely comprised of gabbro and troctolite. Both these units are unchanged from earlier interpretations. The anorthosites (s.l.), gabbros and monzogabbros, formerly called the Transitional series, are now considered to be the Upper Border series, developed by floatation of plagioclase. Common autoliths in the Layered series are parts of the hydrothermally altered Upper Border series from towards the interior of the intrusion, which have foundered and settled through the magma. The contamination of the magma that accompanied this event oxidised iron in the magma and led to the precipitation of magnetite around the periphery of the intrusion. The subsequent depletion of Fe 3+ and/or increase in SiO 2, CaO and P 2O 5 may have induced apatite saturation and accumulation to form two layers rich in apatite, near the base and at top of the Layered series. Granitic magma was developed by fractional crystallisation and was emplaced along the roof of the chamber, where it acquired large quantities of xenoliths. These were probably derived from the flood basalts, their evolved members and fragments of mafic dykes chilled by the granitic magma. Accumulations of monzonite pillows in this unit testify to another magmatic event and a floor to the granitic magma

Isotopic Equilibrium in Mature Oceanic Lithosphere: Insights From Sm-Nd Isotopes on the Corsica (France) Ophiolites

NASA Astrophysics Data System (ADS)

Rampone, E.; Hofmann, A. W.; Raczek, I.; Romairone, A.

2003-12-01

In mature oceanic lithosphere, formed at mid-ocean ridges, residual mantle peridotites and associated magmatic crust are, in principle, linked by a cogenetic relationship, because the times of asthenospheric mantle melting and magmatic crust production are assumed to be roughly coheval. This implies that oceanic peridotites and associated magmatic rocks should have similar isotopic compositions. Few isotope studies have been devoted to test this assumption. At mid-ocean ridges, similar Nd isotopic compositions in basalts and abyssal peridotites have been found by Snow et al. (1994), thus indicating that oceanic peridotites are indeed residues of MORB melting. By contrast, Salters and Dick (2002) have documented Nd isotope differences between abyssal peridotites and associated basalts, with peridotites showing higher 143Nd/144Nd values, and they concluded that an enriched pyroxenitic source component is required to explain the low end of the 143Nd/144Nd variation of the basalts. Here we present Sm/Nd isotope data on ophiolitic mantle peridotites and intruded gabbroic rocks from Mt.Maggiore (Corsica, France), interpreted as lithosphere remnants of the Jurassic Ligurian Tethys ocean. The peridotites are residual after low-degree (<10%) fractional melting. In places, spinel peridotites grade to plagioclase-rich impregnated peridotites. Clinopyroxene separates from both spinel- and plagioclase- peridotites display high 147Sm/144Nd (0.49-0.59) and 143Nd/144Nd (0.513367-0.513551) ratios, consistent with their depleted signature. The associated gabbros have Nd isotopic compositions typical of MORB (143Nd/144Nd = 0.51312-0.51314). Sm/Nd data on plag, whole rock and cpx from an olivine gabbro define an internal isochron with an age of 162 +/- 10 Ma, and an initial epsilon Nd value (9.0) indicating a MORB-type source. In the Sm-Nd isochron diagram, the peridotite data also conform to the above linear array, their initial (160 Ma) epsilon Nd values varying in the range 7

High precision ages from the Torres del Paine Intrusion, Chile

NASA Astrophysics Data System (ADS)

Michel, J.; Baumgartner, L.; Cosca, M.; Ovtcharova, M.; Putlitz, B.; Schaltegger, U.

2006-12-01

The upper crustal bimodal Torres del Paine Intrusion, southern Chile, consists of the lower Paine-Mafic- Complex and the upper Paine-Granite. Geochronologically this bimodal complex is not well studied except for a few existing data from Halpern (1973) and Sanchez (2006). The aim of this study is to supplement the existing data and to constrain the age relations between the major magmatic pulses by applying high precision U-Pb dating on accessory zircons and 40Ar/39Ar-laser-step-heating-ages on biotites from the Torres del Paine Intrusion. The magmatic rocks from mafic complex are fine to medium-grained and vary in composition from quartz- monzonites to granodiorites and gabbros. Coarse-grained olivine gabbros have intruded these rocks in the west. The granitic body is represented by a peraluminous, biotite-orthoclase-granite and a more evolved leucocratic granite in the outer parts towards the host-rock. Field observations suggest a feeder-zone for the granite in the west and that the granite postdates the mafic complex. Two granite samples of the outermost margins in the Northeast and South were analyzed. The zircons were dated by precise isotope-dilution U-Pb techniques of chemically abraded single grains. The data are concordant within the analytical error and define weighted mean 206/238U ages of 12.59 ± 0.03 Ma and 12.58 ± 0.01 Ma for the two samples respectively. A 40Ar/39Ar-age for the second sample yield a date of 12.37 ± 0.11 Ma. Three 40Ar/39Ar -ages of biotites were obtained for rocks belonging to the mafic complex. A hbl-bio- granodiorite from the central part, approximately 150 m below the subhorizontal contact with the granite, gives an age of 12.81 ± 0.11 Ma. A hbl-bio-granodiorite and an olivine-gabbro west of the feeder-zone date at 12.42 ± 0.14 Ma and 12.49 ± 0.11 Ma, respectively. The obtained older age of 12.81 Ma for the granodiorite in the central part is consistent with structural relationships of brittle fracturing of the mafic

Plagioclase-Hosted Magnetite Inclusions From the Bushveld Complex

NASA Astrophysics Data System (ADS)

Feinberg, J. M.; Scott, G. R.; Renne, P. R.; Wenk, H.

2004-12-01

Gabbros from the Main Zone of the 2.064 Ga Bushveld Complex have long been known to possess unusually stable magnetizations due to the presence of high coercivity, exsolved magnetite inclusions in plagioclase and clinopyroxene. The paleomagnetic pole for these rocks has been used to anchor apparent polar wander paths for the Kaapval craton during the Early-Mid Proterozoic. To better understand the rock magnetic properties of silicate-hosted magnetite inclusions, oriented paleomagnetic samples of gabbro were collected from quarries near Belfast and Rustenberg, South Africa, sampling the eastern and western limbs of the Complex, respectively. Plagioclase composition at both sites ranges from An55 (rims) to An65 (cores) based on optical and electron microprobe data. Four kinds of inclusions are present within the plagioclase: elongate magnetite needles, nanometer-scale magnetite particles (responsible for the "cloudy" appearance of some crystals), translucent brown hematite/ilmenite platelets, and colorless euhedral inclusions of pyroxene and/or feldspar. Magnetite inclusions are most abundant at the cores of the plagioclase crystals. Orientations of the needles and the platelets are crystallographically controlled by the silicate host. Although the elongation direction of the magnetite inclusions can occur in any of five possible orientations, only two or three of these directions dominates each plagioclase crystal. Alternating field demagnetization of bulk samples (NRM = 1.5 x 101 A m-1) shows univectorial remanence with average median destructive fields (MDF) of 115 mT (Belfast) and 90 mT (Rustenberg). AF demagnetization of single plagioclase crystals (NRM = 100 A m-1) also shows single component remanence with average MDFs >150 mT. The NRM coercivity spectra of single plagioclase crystals are indistinguishable from that of the bulk samples. When normalized to their abundance in bulk samples the magnetite-bearing plagioclase fully accounts for the NRM of Bushveld

Chalcophile element geochemistry of the Boggy Plain zoned pluton, southeastern Australia: a S-saturated barren compositionally diverse magmatic system

NASA Astrophysics Data System (ADS)

Park, Jung-Woo; Campbell, Ian H.; Ickert, Ryan B.; Allen, Charlotte M.

2013-02-01

The behavior of the platinum group elements (PGE) and Re in felsic magmas is poorly understood due to scarcity of data. We report the concentrations of Ni, Cu, Re, and PGE in the compositionally diverse Boggy Plain zoned pluton (BPZP), which shows a variation of rock type from gabbro through granodiorite and granite to aplite with a SiO2 range from 52 to 74 wt %. In addition, major silicate and oxide minerals were analyzed for Ni, Cu, and Re, and a systematic sulfide study was carried out to investigate the role of silicate, oxide, and sulfide minerals on chalcophile element geochemistry of the BPZP. Mass balance calculation shows that the whole rock Cu budget hosted by silicate and oxide minerals is <13 wt % and that Cu is dominantly located in sulfide phases, whereas most of the whole rock Ni budget (>70 wt %) is held in major silicate and oxide minerals. Rhenium is dominantly hosted by magnetite and ilmenite. Ovoid-shaped sulfide blebs occur at the boundary between pyroxene phenocrysts and neighboring interstitial phases or within interstitial minerals in the gabbro and the granodiorite. The blebs are composed of pyrrhotite, pyrite, chalcopyrite, and S-bearing Fe-oxide, which contain total trace metals (Co, Ni, Cu, Ag, Pb) up to ~16 wt %. The mineral assemblage, occurrence, shape, and composition of the sulfide blebs are a typical of magmatic sulfide. PGE concentrations in the BPZP vary by more than two orders of magnitude from gabbro (2.7-7.8 ppb Pd, 0.025-0.116 ppb Ir) to aplite (0.05 ppb Pd, 0.001 ppb Ir). Nickel, Cu, Re, and PGE concentrations are positively correlated with MgO in all the rock types although there is a clear discontinuity between the granodiorite and the granite in the trends for Ni, Rh, and Ir when plotted against MgO. Cu/Pd values gradually increase from 6,100 to 52,600 as the MgO content decreases. The sulfide petrology and chalcophile element geochemistry of the BPZP show that sulfide saturation occurred in the late gabbroic stage of

Unusual Rocks of the Yap Ridge - Metamorphosed Basal Cumulates of an Arc ?

NASA Astrophysics Data System (ADS)

Hawkins, J. W.; Castillo, P. R.; Batiza, R.

2002-12-01

The 8 to 9 km deep Yap trench, and adjacent Yap Ridge, extend from the southwest end of the Mariana Trench near 11o N, to near 7o 15' N where the trench swings west to intersect the Palau Trench. Unlike other western Pacific subduction systems, the Yap Ridge rises directly from the trench, it has no forearc, neither a remnant nor active volcanic arc, and no inclined seismic zone. The few seismic events recorded are mainly < 70 km depth. Yap Ridge crest depths range from 2.5 km to emergent; there are no emergent volcanoes. Rocks from the islands Yap and Map, are mainly strongly schistose, amphibole-rich, mafic and ultramafic rocks. Metamorphic lineations, and meter-sized mullions having lenticular cross-sections, define inclined (15o southerly dip) tectonic transport. Yap and Map schists are in greenschist facies (actinolite - chlorite - Na-plagioclase, rare titanite and epidote). Talc - tremolite schists, serpentinite, and chlorite-pyroxenite are less common. Small areas of altered andesite are present; quartz diorite and hornblende-rich gabbro occur as clasts in breccias, bomb craters yielded fragments of basalt and diabase. Scattered blankets of laterite several meters thick, and jungle, obscure many details. Deeper crustal rocks exposed on inner wall of Yap Trench, (5 - 2.5 km depths) include amphibolite (Al-hornblende-andesine-titanite) interlayered with calcite- diopside - grossularite marble, and calc-silicate gneisses. Rocks dredged from Yap Ridge include metabasite similar toYap schists, island arc tholeiite series basalt, basaltic andesite, and 2-PX gabbro. These have late Miocene ages (Beccaluva et al., AGU Mon. 23, 1980). Assuming isochemical behavior for immobile elements, protolith for mafic and ultramafic schists had high Mg# (52-83), CaO/Al2O3 0.7-6, Cr 288-1490, Ni 64-609, Zr 13-145, Y 3-28 (ppm).These data suggest picrite, high-Mg basalt, boninite, or OL-PX rich ultramafic cumulates as parents. REE data, e.g. negative slope and (La/Sm)N 0

Petrogenesis of the Alaskan-type mafic-ultramafic complex in the Makkah quadrangle, western Arabian Shield, Saudi Arabia

NASA Astrophysics Data System (ADS)

Habtoor, Abdelmonem; Ahmed, Ahmed Hassan; Harbi, Hesham

2016-10-01

The Makkah quadrangle is a part of the Jeddah terrane in the Precambrian basement, Western Arabian Shield of Saudi Arabia. Gabal Taftafan mafic-ultramafic complex lies within the central part of the Makkah quadrangle. The Taftafan mafic-ultramafic complex is a well-differentiated rock association which comprises of dunite core, hornblende- and plagioclase-bearing peridotites, troctolite, clinopyroxenite and marginal gabbro, in a distinctive zonal structure. The bulk-rock geochemistry of the Taftafan mafic-ultramafic rocks is characterized by a tholeiitic/sub-alkaline affinity with high Mg in the ultramafic core (0.84) and is systematically decreased towards the marginal gabbro (0.60). The patterns of trace elements show enrichment in the fluid-mobile elements (Sr, Ba) and a pronounced negative Nb anomaly which reflect a hydrous parental magma generated in a subduction tectonic setting. The mafic-ultramafic rocks of the Taftafan complex have low total rare earth elements (REE) displaying sub-parallel patterns leading to the assumption that these rocks are comagmatic and are formed by fractional crystallization from a common magma type. The platinum-group elements (PGE) content of all rock types in the Taftafan complex is very low, with ∑ PPGE > ∑ IPGE; displaying slightly positive slopes of the PGE distribution patterns. The chemistry of ferromagnesian minerals is characterized by a high forsterite (Fo) olivine with wide range (Fo91-67), from ultramafic core to the marginal gabbro, Ca-rich diopsidic clinopyroxene, and calcic hornblende. Orthopyroxene is almost absent from all rock types, or very rare when present. Hornblende and Ca-plagioclase possess the longest crystallization history since they are present in almost all rock types of the complex. Spinels in the dunite and hornblende-bearing peridotite core show homogeneous composition with intermediate Cr# (0.53-0.67). Plagioclase-bearing peridotite and troctolite have two exsolved types of spinel; Al

Transnational Education in the Late Nineteenth Century: Brazil, France and Portugal Connected by a School Museum

ERIC Educational Resources Information Center

Vidal, Diana Gonçalves

2017-01-01

This article focuses on the circulation of a single artefact, the "Museu Escolar Brasileiro" (Brazilian School Museum) and its use in education through the pedagogical method of object lessons. Concentrating on the activities of particular individuals and enterprises (Menezes Vieira, Oliveira Lopes and Maison Deyrolle), within three…

Combined 40Ar/39Ar and Fission-Track study of the Freetown Layered Igneous Complex, Freetown, Sierra Leone, West Africa: Implications for the Initial Break-up of Pangea to form the Central Atlantic Ocean and Insight into the Post-rift Evolution of the Sie

NASA Astrophysics Data System (ADS)

Barrie, Ibrahim; Wijbrans, Jan; Andriessen, Paul; Beunk, Frank; Strasser-King, Victor; Fode, Daniel

2010-05-01

Sierra Leone lies within the south-western part of the West African Craton and comprises two major Archaean structural divisions: a low-grade granite-greenstone terrane characterised by N-S striking structures and a NW-SE striking highly metamorphosed belt of strained rocks that form the coastal margin of the craton. Intruded into the belt is the Freetown Layered Igneous Complex (FLIC), a tholeiitic magamtic body emplaced prior to or during the break-up of Pangea to form the Central Atlantic Ocean and, forming today the high ground of the coastal outline of Sierra Leone which is one of the most distinctive features on the West African coast. The break-up of Pangaea to form the Central Atlantic and its passive margins began in the Early Jurassic. Geo-tectonically, the break-up was particularly characterised by the formation of the Central Atlantic Magmatic Province (CAMP), covering once-contiguous parts of North America, Europe, Africa and South America. The FLIC forming part of the heart of CAMP is the largest single layered igneous intrusive yet known on either side of the Central Atlantic, measuring on surface, 65 x 14 x 7 km. Geophysical investigations indicate that the intrusion extends offshore to a depth of about 20 km. Geologically the Complex is a rhythmically layered elongated ultramafic-mafic lopolith divisible into 4 major zones each comprising repeated sequences of troctolitic, gabbroic and anorthositic rocks. An idealised unit of layering is from base upwards: dunite, troctolite, olivine-gabbro, leuco-gabbro, gabbro-norite and anorthosite cumulates. 40Ar-39Ar age spectra and 40Ar/36Ar versus 39Ar/36Ar isochron plots obtained by stepwise-heating experiments on plagioclases, biotites and amphiboles from troctolites, olivine-gabbros, gabbro-norites and anorthosites of the four zones yield plateau and isochron ages that seem to depict the cooling history of the Complex after emplacement. The biotites and some of the plagioclases and amphiboles give very

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Evolution of the Mazatzal province and the timing of the Mazatzal orogeny: Insights from U-Pb geochronology and geochemistry of igneous and metasedimentary rocks in southern New Mexico

USGS Publications Warehouse

Amato, J.M.; Boullion, A.O.; Serna, A.M.; Sanders, A.E.; Farmer, G.L.; Gehrels, G.E.; Wooden, J.L.

2008-01-01

New U-Pb zircon ages, geochemistry, and Nd isotopic data are presented from three localities in the Paleoproterozoic Mazatzal province of southern New Mexico, United States. These data help in understanding the source regions and tectonic setting of magmatism from 1680 to 1620 Ma, the timing of the Mazatzal orogeny, the nature of postorogenic maginatism, Proterozoic plate tectonics, and provide a link between Mazatzal subblocks in Arizona and northern New Mexico. The data indicate a period from 1680 to 1650 Ma in which juvenile felsic granitoids were formed, and a later event between 1646 and 1633 Ma, when these rocks were deformed together with sedimentary rocks. No evidence of pre-1680 Ma rocks or inherited zircons was observed. The igneous rocks have ENd(t) from -1.2 to +4.3 with most between +2 and +4, suggesting a mantle source or derivation from similar-aged crust. Nd isotope and trace element concentrations are consistent with models for typical are magmatism. Detrital zircon ages from metasedimentary rocks indicate that sedimentation occurred until at least 1646 Ma. Both local and Yavapai province sources contributed to the detritus. All of the samples older than ca. 1650 Ma are deformed, whereas undeformed porphyroblasts were found in the contact aureole of a previously dated 1633 Ma gabbro. Regionally, the Mlazatzal orogeny occurred mainly between 1654 and 1643 Ma, during final accretion of a series of island arcs and intervening basins that may have amalgamated offshore. Rhyolite magmatism in the southern Mazatzal province was coeval with gabbro intrusions at 1633 Ma and this bimodal magmatism may have been related to extensional processes following arc accretion. ?? 2007 Geological Society of America.

Mineralogy and geochemistry of the Tartai massif, East Siberian metallogenic province

NASA Astrophysics Data System (ADS)

Podlipsky, M. Yu.; Mekhonoshin, A. S.; Tolstykh, N. D.; Vishnevskiy, A. V.; Polyakov, G. V.

2015-05-01

The Tartai ultramafic-mafic massif is located in the central part of the East Siberian metallogenic (PGE-Cu-Ni) province (728-712 Ma), which constitutes part of the southern margin of the Siberian craton. This dunite-peridotite-pyroxenite-gabbro massif is the host to low-sulfide PGE-Cu-Ni mineralization. The massif was formed by fractional crystallization of picritic magmas and is composed of wehrlite, dunite, plagiowehrlite, and olivine melanogabbro. The composition of olivine varies from Fo89.9 in dunite to Fo83 in melanocratic olivine gabbro; clinopyroxene is esentially augite. Chrome-spinels crystallized at a low degree of oxidation and have a high iron content. Disseminated sulfide mineralization (pentlandite and heazlewoodite) with high PGE concentrations was identified in wehrlites. Pentlandite is enriched in Fe and Co and depleted in S. These features and the association pentlandite with heazlewoodite suggest that the sulfide mineralization was formed over a wide temperature range (600-400°C) at low sulfur activity (log fS2 from -16 to -9). PGM are represented by Ir-bearing sperrylite, Pd-Cu-Sb panning compounds of variable compositions, Pt-Fe-Cu and Pt-Cu alloys. The evolutionary trend of the ore system was from essentially Ni compositions at the early magmatic stage during formation of disseminated mineralization toward Cu-rich composition at the post-magmatic stage. The PGM assemblage from heavy concentrate haloes differs from bedrock-hosted mineralization in its wider variety of mineral species and the presence of refractory platinoids. Sperrylite from heavy concentrate haloes of the Tartai massif serves as a reliable prospecting guide for bedrock-hosted sulfide Cu-Ni deposits.

The monzonorite-anorthosite connection: The petrogenesis of terrestrial KREEP

NASA Technical Reports Server (NTRS)

Longhi, J.; Auwera, J. Vander

1993-01-01

There is a suite of rocks typically associated with Proterozoic massif anorthosites that bear some interesting similarities to lunar KREEP. In many cases these rocks are plutonic and have traditionally been referred to as the jotunite-mangerite-+/-charnockite-+/-syenite suite. However, in the Rogaland district of southwestern Norway, where they are referred to as 'monzonorites', these rocks are also present as fine-grained dikes and as the chill margin of a layered intrusion, and thus approximate magmatic liquid compositions are readily obtained by chemical analysis. Monzonorites are typically enriched in incompatible lithophile elements such as K (alkali feldspar is present), the rare earths (REE), and P. They have intermediate to low Mg', low-Ca pyroxene, and more evolved types have low Ti/Sm ratios. Much debate has developed over attempts to explain the link between monzonorites and massif anorthosites. One feature seems clear: monzonorites and associated anorthosites have different initial isotopic ratios, so a simple relation is not possible. However, there is apparently a continuum in major elements between the monzonorites and gabbros believed to represent magmas parental to the anorthosites. This continuum suggests a link via high pressure fractionation coupled with assimilation. Although more complicated, this scenario is similar to that evoked for the early Moon: following the formation of ferroan anorthosites, continued fractional crystallization of the residual liquids at the base of the crust led to the formation of KREEP. An attempt is made here to establish a link between monzonorites and high-Al gabbros which are nearly always found as ancillary intrusions associated with anorthosites, and which may record processes in lower crustal magma chambers.

Metabolic Activity and Biosignatures of Microbes in the Lower Ocean Crust of Atlantis Bank, IODP Expedition 360

NASA Astrophysics Data System (ADS)

Wee, S. Y.; Edgcomb, V. P.; Burgaud, G.; Klein, F.; Schubotz, F.; Yvon-Lewis, S. A.; Sylvan, J. B.

2017-12-01

International Ocean Discovery Program (IODP) Expedition 360 represents the first leg of a multi-phase drilling program, SloMo, aimed at investigating the nature of the lower crust and Moho at slow spreading ridges. The goal of Expedition 360 was to recover a representative transect of the lower oceanic crust formed at Atlantis Bank, an oceanic core complex on the SW Indian Ridge. Recovered cores were primarily gabbro and olivine gabbro, which may potentially host serpentinization reactions and associated microbial life. Our goal was to sample this subseafloor environment and determine quantity, diversity and metabolic capabilities of any resident microbial life. Hole U1473A was drilled during Expedition 360 down to 790 m below seafloor and samples for detection of microbial communities and microbial biosignatures were collected throughout. We present here quantification of microbial biomass via fluorescence microscopy, preliminary analysis of nutrient addition experiments, data from sequencing of microbial 16S rRNA genes, analysis of microbial lipids, and data from Raman spectra of subsurface isolates. We initiated and sampled 12 nutrient addition experiments from 71-745 mbsf by adding sampled rocks to artificial seawater with no additions, added ammonium, added ammonium plus phosphate, and added organic acids. In nearly all of the experiment bottles, methane was detected when samples were collected at six months and again after one year of incubation. Phosphate in the incubations was drawn down, indicating active microbial metabolism, and archaeal lipids from in situ samples indicate the presence of methanogens, corroborating the likelihood of methanogens as the source of detected methane in the nutrient addition incubations. Altogether, the interdisciplinary approach used here provides a peek into life in the subseafloor upper ocean crust.

Toward an understanding of disequilibrium dihedral angles in mafic rocks

USGS Publications Warehouse

Holness, Marian B.; Humphreys, Madeleine C.S.; Sides, Rachel; Helz, Rosalind T.; Tegner, Christian

2012-01-01

The median dihedral angle at clinopyroxene-plagioclase-plagioclase junctions in mafic rocks, Θcpp, is generally lower than equilibrium (109˚ {plus minus} 2˚). Observation of a wide range of mafic bodies demonstrates that previous work on systematic variations of Θcpp is incorrect in several important respects. Firstly, the spatial distribution of plagioclase compositional zoning demonstrates that the final geometry of three-grain junctions, and hence Θcpp, is formed during solidification (the igneous process): sub-solidus textural modification in most dolerites and gabbros, previously thought to be the dominant control on Θcpp, is insignificant. Θcpp is governed by mass transport constraints, the inhibiting effects of small pore size on crystallization, and variation in relative growth rates of pyroxene and plagioclase. During rapid cooling, pyroxene preferentially fills wider pores while the narrower pores remain melt-filled, resulting in an initial value of Θcpp of 78˚, rather than 60˚ which would be expected if all melt-filled pores were filled with pyroxene. Lower cooling rates create a higher initial Θcpp due to changes in relative growth rates of the two minerals at the nascent three-grain junction. Low Θcpp (associated with cuspate clinopyroxene grains at triple junctions) can also be diagnostic of infiltration of previously melt-free rocks by late-stage evolved liquids (the metasomatic process). Modification of Θcpp by sub-solidus textural equilibration (the metamorphic process) is only important for fine-grained mafic rocks such as chilled margins and intra-plutonic chill zones. In coarse-grained gabbros from shallow crustal intrusions the metamorphic process occurs only in the centres of oikocrysts, associated with rounding of chadacrysts.

Gases and water isotopes in a geochemical section across the Larderello, Italy, geothermal field

USGS Publications Warehouse

Truesdell, A.H.; Nehring, N.L.

1978-01-01

Steam samples from six wells (Colombaia, Pineta, Larderello 57, Larderello 155, Gabbro 6, and Gabbro 1) in a south to north section across the Larderello geothermal field have been analyzed for inorganic and hydrocarbon gases and for oxygen-18 and deuterium of steam. The wells generally decrease in depth and increase in age toward the south. The steam samples are generally characterized by (1) Total gas contents increasing south to north from 0.003 to 0.05 mole fraction; (2) Constant CO2 (95??2 percent); near constant H2S (1.6??0.8), N2 (1.2??0.8), H2 (2??1), CH4 (1.2??1), and no O2 in the dry gas; (3) Presence of numerous, straight chain and branched C2 to C6 hydrocarbons plus benzene in amounts independent of CH4 contents with highest concentrations in the deeper wells; (4) Oxygen-18 contents of steam increasing south to north from -5.0??? to -0.4??? with little change in deuterium (-42??2???). These observations are interpreted as showing: (1) Decreasing gas contents with amount of production because the proportion of steam boiled from liquid water increases with production; (2) Synthesis of CH4 from H2 and CO2 with CO2 and H2 produced by thermal metamorphism and rock-water reactions; (3) Extraction of C2 to C6 hydrocarbons from rock organic matter; (4) Either oxygen isotope exchange followed by distillation of steam from the north toward the south (2 plates at ???220??C) or mixture of deeper more-exchange waters from the north with shallow, less-exchanged recharging waters from the south. ?? 1978 Birkha??user Verlag.

Cretaceous and Paleogene granitoid suites of the Sikhote-Alin area (Far East Russia): Geochemistry and tectonic implications

NASA Astrophysics Data System (ADS)

Grebennikov, Andrei V.; Khanchuk, Alexander I.; Gonevchuk, Valeriy G.; Kovalenko, Sergey V.

2016-09-01

The Mesozoic and Cenozoic geological history of NE Asia comprises alternating episodes of subduction or transform strike-slip movement of the oceanic plate along the continental margin of Eurasia. This sequence resulted in the regular generation of granitoid suites that are characterized by different ages, compositions, and tectonic settings. The Hauterivian-Aptian orogenic stage of the Sikhote-Alin, associated with the strike-slip displacement of the early Paleozoic continental blocks, the successive deformation of the Jurassic and Early Cretaceous terranes, and the injection of the earliest S-type granitoids. During late Albian, the area underwent syn-strike-slip compression caused by collision with the Aptian island arc and resulted in the injection of voluminous magmas of calc-alkaline magnesian (S- and I-type) and alkali-calcic ferroan (A-type) granitoids into syn-faulting compressional and extensional basins, respectively. Northwestward to westward movement of the Izanagi Plate resulted in the initiation of frontal subduction of the Paleo-Pacific Plate during the Cenomanian-Maastrichtian. In turn, this resulted in the generation of plateau-forming ignimbrites and their intrusive analogs formed from metaluminous I-type felsic magmas. Paleocene-Eocene magmatism in the Sikhote-Alin area commenced after the termination of subduction in a rifting regime related to strike-slip movement of the oceanic plate relative to the continent. The break-off of the subducted plate and the injection of oceanic asthenospheric material into the subcontinental lithosphere resulted in the eruption of lamproites and fayalite rhyolites, and coeval intrusions of gabbro and alkali feldspar granites (A-type). The A-type granitic-rocks and coeval gabbro-monzonites are considered to be reliable indicators of the transform continental margin geodynamic settings.

Structure of Hole 1256D: The role of mechanical deformation in superfast-spread crust

NASA Astrophysics Data System (ADS)

Tartarotti, P.; Hayman, N. W.; Anma, R.; Crispini, L.; Veloso Espinosa, E. A.; Galli, L.

2006-12-01

One view of seafloor spreading is that mechanical deformation is not significant at high spreading rates. With recovery of up to 37%, and the vertical axis known for many pieces, shipboard visual core descriptions from Hole 1256D provide an opportunity to evaluate the significance of deformational structures in EPR-, superfast- (~220 mm-yr) spread crust. From top to bottom, the structural characteristics of crustal units are: (1) A relatively flat-lying, ~100-m thick "lava pond" that is largely free of deformational structures; (2) ~184 m of shallowly dipping lava flows remarkable for hyaloclastites and a cooling-related fracture system; (3) ~466 m of massive and sheet flows with flow-related fractures, hydrothermal veins, and (fault-related) cataclastic domains; (3) A ~61 m thick transition zone that contains a well-developed (fault-related) cataclastic domain; (4) A ~346 m thick sheeted dike complex, with abundant hydrothermal veins, local breccias, and magmatic flow features. Recovered chilled dike margins have a mean dip of 70° and range from 41-88°; (5) A ~100 m thick plutonic suite contains gabbroic rocks that intrude the sheeted dikes. Gabbros contain some local brittle structures and minor (largely static) recrystallized domains, but are more noteworthy for their magmatic features: dike/gabbro contacts and flow foliations are modestly dipping (e.g., ~45°) with leucocratic melt patches concentrated toward the top of the section. Brittle structures were subordinate to magmatic processes in accommodating large extensional strain. Brittle deformation was important, however, in accommodating magmatism and hydrothermal fluid flow, thereby affecting the variation of crustal physical properties and the distribution of oceanic alteration.

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).

Apollo 16 exploration of Descartes - A geologic summary.

NASA Technical Reports Server (NTRS)

1973-01-01

The Cayley Plains at the Apollo 16 landing site consist of crudely stratified breccias to a depth of at least 200 meters, overlain by a regolith 10 to 15 meters thick. Samples, photographs, and observations by the astronauts indicate that most of the rocks are impact breccias derived from an anorthosite-gabbro complex. The least brecciated members of the suite include coarse-grained anorthosite and finer-grained, more mafic rocks, some with igneous and some with metamorphic textures. Much of the transverse area is covered by ejecta from North Ray and South Ray craters, but the abundance of rock fragments increases to the south toward the younger South Ray crater.

Crystal fractionation in the SNC meteorites: Implications for sample selection

NASA Technical Reports Server (NTRS)

Treiman, Allan H.

1988-01-01

Almost all rock types in the SNC meteorites are cumulates, products of magma differentiation by crystal fractionation (addition or removal of crystals). If the SNC meteorites are from the surface of Mars or near subsurface, then most of the igneous units on Mars are differentiated. Basaltic units probably experienced minor to moderate differientation, but ultrabasic units probably experienced extreme differentiation. Products of this differentiation may include Fe-rich gabbro, pyroxenite, periodotite (and thus serpentine), and possibly massive sulfides. The SNC meteorites include ten lithologies (three in EETA79001), eight of which are crystal cumulates. The other lithologies, EETA79001 A and B are subophitic basalts.

Zirconology of lherzolites in the Nurali Massif

NASA Astrophysics Data System (ADS)

Krasnobaev, A. A.; Rusin, A. I.; Anfilogov, V. N.; Valizer, P. M.; Busharina, S. V.; Medvedeva, E. V.

2017-06-01

The age trend (SHRIMP U/Pb) of the evolution of zircon is obtained for the first time in lherzolites of the Nurali Massif. Zircons are subdivided into groups by the crystallomorphological and geochemical features. These specific features in zircon development are confirmed by the age dates. Precambrian dates (no younger than 1190 Ma) correspond to mantle sources of the lherzolite block. The Early Silurian (445-448 and 439-440 Ma) wass the time of lherzolite magmatism of 10-15 Ma in duration. The Middle Devonian (382.9 ± 8.7 Ma) corresponded to postmagmatic processes related to the effect of gabbro-diorite intrusions crowning in the Nurali Massif.

Melting and differentiation in Venus with a cold start: A mechanism of the thin crust formation

NASA Technical Reports Server (NTRS)

Solomatov, Viatcheslav S.; Stevenson, David J.

1992-01-01

Recent works argue that the venusian crust is thin: less than 10-30 km. However, any convective model of Venus unavoidably predicts melting and a fast growth of the basaltic crust, up to its maximum thickness of about 70 km limited, by the gabbro-eclogite phase transition. The crust is highly buoyant due to both its composition and temperature and it is problematic to find a mechanism providing its effective recycling and thinning in the absence of plate tectonics. There are different ways to solve this contradiction. This study suggests that a thin crust can be produced during the entire evolution of Venus if Venus avoided giant impacts.

A geochemical evaluation of the Ash Sha'ib mineral prospect, Asir quadrangle, Kingdom of Saudi Arabia

USGS Publications Warehouse

Allcott, Glenn H.

1970-01-01

The mineralized zone at the remotely located Ash Sha'ib ancient mine contains only a small tonnage of moderately low grade sulfide- bearing rock. Based on present data the gross value of the deposit, with a value of $25.00 or more per ton, is $20,000,000. A belt of metasedimentary rocks, intruded by gabbro to the south and granite to the north, was the host for fissure vein-replacement type mineralization. Most of the mineralization is in a siliceous dolomite transected by fissures. The main sulfide mineral is sphalerite, but minor amounts of chalcopyrlte and argentlferous galena contribute to the value of the mineralized sections.

Platinum-group element geochemistry of zoned ultramafic intrusive suites, Klamath Mountains, California and Oregon.

USGS Publications Warehouse

Gray, F.; Page, N.J.; Carlson, C.A.; Wilson, S.A.; Carlson, R.R.

1986-01-01

Analyses for platinum-group elements of the varied rock suites of three Alaskan-type ultramafic to mafic multi-intrusive bodies are reported. Ir and Ru are less than analytical sensitivities of 100 and 20 ppb; Rh is less than or near 1 ppb. Average Pd assays vary among the rocks within intrusive complexes and between the three complexes (6.3, 13.7, 36.4 ppb); average Pt assays vary little among the same samples (27.9, 60.9, 34.0 ppb). Statistically adjusted Pt/(Pt + Pd) ratios increase in each suite from gabbro through clinopyroxenite to olivine-rich rocks, possibly owing to Pd fractionation.-G.J.N.

Mesoproterozoic juvenile mafic-ultramafic magmatism in the SW Amazonian Craton (Rio Negro-Juruena province): SHRIMP U-Pb geochronology and Nd-Sr constraints of the Figueira Branca Suite

NASA Astrophysics Data System (ADS)

Teixeira, Wilson; Geraldes, Mauro C.; D'Agrella-Filho, Manoel S.; Santos, João O. S.; Sant'Ana Barros, Márcia A.; Ruiz, Amarildo S.; Corrêa da Costa, Paulo C.

2011-12-01

The Figueira Branca Suite (FBS) comprises a layered mafic-ultramafic complex which together with mafic-felsic plugs makes up a string of NW-trending intrusive bodies that are emplaced into the Jauru domain (Rio Negro-Juruena province; 1.80-1.60 Ga). This domain comprises Orosirian calc-alkaline rocks and coeval metamorphic volcanic-sedimentary associations, intruded by voluminous granitoid plutons resulted from outboard Cachoeirinha (1587-1522 Ma) and Santa Helena (1485-1420 Ma) accretionary orogens that eventually created the Rondonian-San Ignacio province along the SW margin of the proto-Amazonian Craton. SHRIMP U-Pb age in zircon for one cumulatic gabbro from the FBS yielded a concordia intercept age of 1425.5 ± 8.0 Ma (MSWD = 1.11). Another gabbroic plug which crops out to the East gives a similar within error concordia intercept zircon age of 1415.9 ± 6.9 Ma (MSWD = 0.25), whereas a nearby monzogranite yields a concordia intercept zircon age of 1428.9 ± 2.8 Ma (MSWD = 1.30). All these results are crystallization ages and constrain an important intraplate magmatic event within the Orosirian continental crust at the time of outboard Santa Helena orogen. On the other hand, igneous titanite from another gabbro located to the West of the FBS yielded a weighted mean 207Pb/ 206Pb crystallization age of 1541 ± 23 Ma (MSWD = 0.74). Therefore this rock is not genetically associated with the FBS, as previously suggested by the field information. Additional Nd-Sr isotopic analyses of the FBS mafic-ultramafic rocks and coeval gabbro showed comparable ɛNd(1.42Ga) values (+3.0 to +4.7) and variable ɛSr(1.42Ga) ones (-39.1 to -8.1). These data plot in the depleted field quadrant of the Nd-Sr diagram, indicating a significant influence of the MORB end-member reservoir in the magma genesis. This interpretation is similarly supported by comparison of the Nd evolutionary path of the FBS with those that characterize the isotopic evolution of the Jauru crust and the

Chromite deposits of the north-central Zambales Range, Luzon, Philippines

USGS Publications Warehouse

Rossman, D.L.

1970-01-01

Peridotite and gabbro form an intrusive complex which is exposed over an area about 35 km wide and 150 km long in the center of the Zambales Range of western Luzon. The Zambales Complex is remarkable for its total known resources, mined and still remaining, of about 15 million metric tons of chromite ore. Twenty percent of Free World production was obtained from this area between 1950 and the end of 1964; in 1960 production reached a high of 606,103 metric tons of refractory-grade ore, mostly from the Coto mine near Masinloc, and 128,426 metric tons of metallurgical ore from the Acoje mine. The United States imports 80 to 90 percent of its refractory-grade chromite from the Philippines, and its basic refractory technology has been designed upon the chemical and physical characteristics of Coto high-alumina chromite ore. Continuation of this pattern will depend upon discovery of additional ore reserves to replace those depleted by mining. The Zambales Ultramafic Complex is of the alpine type in which lenticular or podiform deposits of chromite lie in peridotite or dunite, mostly near Contacts with gabbroic rocks. Layered structures, foliation, and lineation commonly are well developed and transect boundaries between major rock units, including chromite deposits, at any angle. Accordingly, these structures cannot be used as guides in exploration and mining as they are used in stratiform complexes such as the Bushveld, where chromite layers extend for many miles. Probably 90 percent of the known deposits in the Zambales Complex are located in two belts in its northern part. One zone containing high-aluminua refractory-grade deposits extends northeast from the Coto mine and Chromite Reservation No. I along a peridotite contact with olivine gabbro, and another of high-chromium metallurgical grade chromite extends south through the Zambales and Acoje properties, and swings westward around the south side of Mount Lanai along a peridotite contact with norite. The textures

In-situ fractional crystallization as a major mechanism in differentiation of Karaj sill, northern Iran

NASA Astrophysics Data System (ADS)

Maghdour-Mashhour, Reza; Esmaeily, Dariush

2010-05-01

During the upper Eocene-Oligocene, the Karaj Dam basement sill (KDBS), located within the E-W-trending Alborz range in northern Iran, intruded the middle and upper tuff units of the Karaj Formation. The KDBS consists of a layered series between upper and lower chilled margins, with local fine-grained monzonitic dykes. The rocks of the chilled margins are gabbroic in composition and show porphyritic texture. The rocks show a continuous transition from porphyritic to equigranular texture toward the center of the sill. Most of the KDBS is medium to coarse grained, showing magmatic layering defined by modal variations in pyroxene and plagioclase, the main constituent minerals. The layered series, dominated by gabbro, diorite, and monzodiorite, formed almost upward from the lower to the upper chilled margin, with gradational transitions between rock types. The KDBS shows an S-shaped profile of MgO concentration from the base to the top of the sill. In situ crystallization is indicated by an approximately constant modal abundance of pyroxene and plagioclase, a narrow range of major element contents in the main lithologies (e.g., 49-54 wt% SiO2, 17-19 wt% Al2O3, and 2.7-5.2 wt% MgO), and variable trace element concentrations (e.g., 500-1150 ppm Ba, 45-130 ppm Rb, 150-720ppm Sr, and 50-160 ppm Zr). Disequilibrium crystallization, caused by Soret fractionation in the marginal series, is indicated by an unexpected overall enrichment trend in MgO concentration and Mg# of pyroxenes from the chilled margin (olivine-bearing gabbro) to marginal gabbro. Formation of chilled margins form an effective insulating layer between contacts and overlaying hot, turbulent magma causing magma cool considerably slowly and have sufficient time to produce different differentiated sequence by in-situ nucleation and growth from gabbro to diorite and monzodiorite. Following the formation of chilled margins, the parent magma composition was established on the Plg-Cpx cotectic line of the Ol

Kontay intrusion (Polar Siberia)- an example of strong magmatic differentiation in the single magmatic body

NASA Astrophysics Data System (ADS)

Zaitsev, Victor

2016-04-01

Kontay intrusion is located in the north-west part of Maimecha-Kotuy province, (part of Siberian Large Igneous Province) in the northern part of the Siberian Platform. Intrusion is fully overlaid and practically not studied before. Geophysical data evident that intrusion has form of laccolite with diameter ˜7.5 km and thickness ˜ 2.5 km [1]. Initially lower part of intrusion was described as "gabbro-anortozite and anortositic gabbro" and higher part - as "leucocratic granophyric anortosites" [2]. Complex mineralo-petrological-geochemical study allow to divide intrusion section on three zones: Lower zone (below 1100 m) - layered sequence of leucocratic and melanocratic of biotite- and ortopyroxene-bearing gabbro, with minor alkaline feldspar, Middle zone (1100-700m) - biotite- and K-feldspar bearing gabbro and monzonotes, interstitial space often contain micrographical structures. Biotite corroded clinopyroxene and form interstitial crystals and Upper zone (700-214 m) - petrographically monotone porthyric quartz-monzonites and granosyenites with rare phenocrysts of plagioclase, magnetite, clinopyroxene and biotite. The main secondary minerals are albite, amphibole (edenite-feroedenite), secondary biotite, chlorite, and F,Al-bearing titanite. Rocks of the all parts fall in the same trends on the binary petrochemical diagrams, they belong to the High-K calc-alkaline seria. They are clearly more alkaline then intrusions of Norilsk district and less alkaline then alkaline-ultramafic intrusions of Maymecha-Kotuy province. Volcanic rocks of Norilsk and Maymecha-Kotuy area was subdivided for four geochemical types, mainly by the Ti-content, Th/U, Gd/Yb and Sm/Yb ratio: Low-Ti-1 (predominated), Low-Ti-2 (well -developed in Norilsk region), Moderate-Ti (developed only in the lower part of volcanic formation in Norilsk region), High-Ti (rare in Norilsk region, but widely spread in Mailecha-Kotuy area) [3]. Kontay intrusion rocks belong to Moderate-Ti group. The closest

Obstacles to Innovation in Brazil: The Lack of Qualified Individuals to Implement Innovation and Establish University-Firm Interactions

ERIC Educational Resources Information Center

Rapini, Marcia Siqueira; Chiarini, Tulio; Bittencourt, Pablo Felipe

2017-01-01

Through an investigation of data available from the Brazilian Innovation Survey (Pesquisa de Inovação) of the Brazilian Institute of Geography and Statistics (Instituto Brasileiro de Geografia e Estatística) and from a national survey on university-firm interactions (the BR Survey), the authors show that Brazilian industrial firms lack qualified…

MOBRAL--Seminario Interamericano de Educacion de Adultos (MOBRAL--Interamerican Seminar on Adult Education). Final Report.

ERIC Educational Resources Information Center

Ministerio da Educacao e Cultura, Rio de Janeiro (Brazil). Movimento Brasileiro de Alfabetizacao.

The report contains the substance of the MOBRAL (Movimento Brasileiro de Alfabetizacao)--Interamerican Seminar on Adult Education held in Rio de Janeiro from April 9th to 18th, 1973, for invited representatives from 21 Latin America and Caribbean countries. The object was to make a contribution to the collective task of identifying, defining, and…

Reconnaissance geology of the Jabal Dalfa Quadrangle, sheet 21/43 C, Kingdom of Saudi Arabia

USGS Publications Warehouse

Greene, Robert C.

1983-01-01

The Jabal Dalfa quadrangle (sheet 21/43 C) is part of the Najd province in west-central Saudi Arabia. The quadrangle is mostly a plain, tilted gently northeastward, but local inselbergs and two areas of dissected uplands rise as much as 200 m above the plain. Wadi Bishah and Wadi Ranyah terminate in the quadrangle. The quadrangle is underlain by Precambrian metavolcanic, metasedimentary, and plutonic rocks. The gneiss outcrops in the northeast and east-central parts of the quadrangle are apparently the oldest rocks. After they were emplaced, a wide variety of metavolcanic and metasedimentary rocks were deposited at Jabal Dalfa and Umm Shat, and in the northeast part of the quadrangle as the Arfan formation. Subsequently, granite gneiss was emplaced in the west part of the quadrangle and intruded by gabbro. Metabasalt and meta-andesite were extruded in a wide north-trending belt through the middle of the quadrangle and at Jabal Silli. Intrusion of small bodies of granitic rocks and Najd faulting conclude the Precambrian history of the area. Surficial deposits include sand and gravel covering the plains, alluvial fans, and voluminous dune sands. In the southeast part of the quadrangle, the layered rocks strike north and dip steeply. They are oriented parallel to the Nabitah fault zone. In the northeast and east-central parts of the quadrangle, layered rocks and gneiss are sheared into slices by the southernmost faults of the major Najd fault zone. Bedding and foliation in these slices strike northwest, parallel to the faults. Gneiss in the west part of the quadrangle also strikes northwest, and dips steeply to vertically; layered rocks underlying Jabal Silli strike northeast. Layered metamorphic rocks in the Jabal Dalfa quadrangle are mostly in the greenschist facies. Projection of data from other quadrangles suggests that the oldest gneiss is about 780 Ma old and the Arfan formation, Umm Shat, and Jabal Dalfa layered rocks are about 775 to 745 Ma old. The gneiss of

Structure, alteration, and geochemistry of the Charlotte quartz vein stockwork, Mt Charlotte gold mine, Kalgoorlie, Australia: time constraints, down-plunge zonation, and fluid source

NASA Astrophysics Data System (ADS)

Mueller, Andreas G.

2015-02-01

The Kalgoorlie district in the Archean Yilgarn Craton, Western Australia, comprises two world-class gold deposits: Mt Charlotte (144 t Au produced to 2013) in the northwest and the Golden Mile (1,670 t Au) in the southeast. Both occur in a folded greenschist-facies gabbro sill adjacent to the Golden Mile Fault (D2) in propylitic alteration associated with porphyry dikes. At Mt Charlotte, a shear array of fault-fill veins within the Golden Mile Fault indicates sinistral strike-slip during Golden Mile-type pyrite-telluride mineralization. The pipe-shaped Charlotte quartz vein stockwork, mined in bulk more than 1 km down plunge, is separated in time by barren D3 thrusts from Golden Mile mineralization and alteration, and occurs between two dextral strike-slip faults (D4). Movement on these faults generated an organized network of extension and shear fractures opened during the subsequent infiltration of high-pressure H2S-rich fluid at 2,655 ± 13 Ma (U-Pb xenotime). Gold was deposited during wall rock sulphidation in overlapping vein selvages zoned from deep albite-pyrrhotite (3 g/t Au) to upper muscovite-pyrite assemblages (5 g/t Au bulk grade). Chlorite and fluid inclusion thermometry indicate that this kilometre-scale zonation is due to fluid cooling from 410-440 °C at the base to 350-360 °C at the top of the orebody, while the greenstone terrane remained at 250 °C ambient temperature and at 300 MPa lithostatic pressure. The opened fractures filled with barren quartz and scheelite during the retrograde stage (300 °C) of the hydrothermal event. During fracture sealing, fluid flux was periodically restricted at the lower D3 thrust. Cycles of high and low up-flow, represented by juvenile H2O-CO2 and evolved H2O-CO2-CH4 fluid, respectively, are recorded by the REE and Sr isotope compositions of scheelite oscillatory zones. The temperature gradient measured in the vein stockwork points to a hot (>600 °C) fluid source 2-4 km below the mine workings, and several

A Geochemical Comparison of the Northern Peninsular Ranges Batholith in Southern California and the Coastal Batholith in Southern Peru

NASA Astrophysics Data System (ADS)

Clausen, B. L.; Martínez Ardila, A. M.; Morton, D. M.

2010-12-01

An extensive geochemical data set from the northern Peninsular Ranges Batholith (PRB) in southern California is compared and contrasted with the Arequipa segment of the Peruvian Coastal Batholith, including new granitoid samples recently collected near Ica (14°S, 76°W). The data include major and trace elements and Sr isotope ratios. This is part of an on-going study of subduction-related magmatism to refine a petrogenetic model of crust formation at plate boundaries, with a particular interest in the role of magma mixing. Research in the northern PRB suggests that continental crust is formed in several cycles: (1) mantle melting to give mafic volcanics and gabbroic intrusives, (2) basalt/gabbro melting to give felsic granitoids uncontaminated by continental crust and having low initial 87Sr/86Sr (Sri) values less than 0.704, and (3) crustal melting to give high Sri values greater than 0.704. Geochemical evidence was used to determine the extent of mixing between mafic and felsic magma that produced rocks of intermediate SiO2 composition. These differentiation cycles formed a west to east chronologic sequence and yielded granitoids of gabbro, tonalite, and granodiorite composition. Using principal component analysis on the northern PRB granitoids, the four factors affecting geochemical composition were categorized as differentiation, crustal contamination, depth of magma source, and conditions that yield a range from calcic to more alkaline granitoids. A similar major and trace element analysis is being done for a classic result of subduction in the Peruvian Coastal Batholith. The Peruvian samples recently collected include granitoids of the upper Cretaceous Coastal Batholith, as well as the associated volcanics of Cretaceous and Jurassic age. The Coastal Batholith samples include a range of granitoids from the early gabbros and from the four batholithic super-units (from west to east: Linga, Pampahuasi, Tiabaya, and Incahuasi) containing a combination of diorite

New Ages for Gorgona Island, Colombia: Implications for Previous Petrogenetic and Tectonic Models

NASA Astrophysics Data System (ADS)

Serrano Duran, L.; Lopez Martinez, M.; Ferrari, L.

2007-05-01

The Gorgona Island, located 50 km to the west of the Colombian Pacific coast, is the only known site with Phanerozoic komatiites in the world besides a key element in several reconstruction of the interaction between the Caribbean and the South America Plate. The Gorgona komatiites are part of an igneous complex that also includes picritic basalts and breccias, gabbros and peridotites (dunites and wherlites), and is covered by deformed mid-Eocene and younger underformed marine sediments. Datings of the igneous rocks were only performed on basalts and include an 86 Ma K-Ar age, an 88.9 ± 1.2 Ma weighted mean of four Ar-Ar ages and an 89.2 ± 5.2 Ma Re-Os isochron age from basalts. Gorgona rocks are affected by reverse faulting with a general eastward vergence. The island is the only subaerially exposed part of a NE elongated sliver accreted in a dextral transpressional regime to the South America continental margin between the Late Eocene and the Early Miocene. Petrologic studies found large spread in radiogenic isotopes and incompatible trace element ratios in Gorgona ultramafic rocks, which have been interpreted as requiring at least two different sources of: 1) a depleted mantle responsible for the generation of the komatiites and most basalts, and 2) an enriched mantle responsible for some rarer enriched basalts and picrites. Despite the large compositional and isotopic heterogeneity the most common interpretation is that the Gorgona ultramafic rocks are the product of a single mantle plume, although it has recently proposed that this would be a separate plume from that generating the bulk of the Caribbean plateau at ~90 Ma. Our new study focused on the geochronology of the Gorgona igneous suite as we consider that this tectonically and petrologically complex island is unlike to have such a narrow age range. We attempted to date eight samples of komatiites, basalts and gabbros by Ar-Ar laser step heating. For four of these samples we successfully obtain

Preliminary observations and logs of BARB 1 and BARB 2: komatiites from the Tjakastad site

NASA Astrophysics Data System (ADS)

Coetzee, Grace; Arndt, Nicholas; Wilson, Allan

2013-04-01

The BARB 1 and BARB 2 cores intersect a suite of komatiite flows and komatiitic basalts as well as fragmental rocks of the Komati Formation of the Onverwacht Group, Barberton Greenstone Belt. The cores give important and previously unattainable information on the structures, textures and contact relationships between individual komatiite flows and different lithological units within the flows. BARB 1 was drilled at -48° on a 5° azimuth to a depth of 419.9 m. This core contains a unique volcanic tumulus succession in the stratigraphically lower 100 m and the rest of the core consists of about 59 flows of spinifex-textured komatiite (1-3 m thick), massive komatiite (0.5-10 m thick), komatiitic basalt (1-9 m thick) and a single basalt layer (10 m thick), intruded by gabbro (0.5-2 m thick) and a single dolerite dyke (18 m thick). BARB 2, approximately 50 m from BARB 1 and parallel to it, was drilled at -45°on an 8° azimuth to a depth of 431.5 m. This core contains approximately 39 flows of komatiite (0.5-10 m thick) and komatiitic basalt (2-23 m thick) which contain possible selvages of pillows. Basalt flows are more numerous (0.3-4 m thick) in BARB 2 whilst gabbro (0.6-7 m thick) is less prevalent. The dolerite dyke observed in BARB 1 does not occur in BARB 2. As the Barberton strata young towards the east, the cores intersected the stratigraphy in a reverse sequence. The cores were drilled such that there exists a 141 m overlap in stratigraphy between them. The section 141 m from the base of BARB 1 should theoretically correlate with the top 141 m of BARB 2. However, this overlap is not evident in the core or in the core logs. A single gabbro layer appears to be lithologically correlatable between both holes. There is no apparent correlation between the pattern of the komatiite flows leading to an initial conclusion that the komatiite flows were not laterally extensive or changed laterally in form over short distances. In both cores the proportion of komatiitic

An evolving magmatic-hydrothermal system in the formation of the Mesozoic Meishan magnetite-apatite deposit in the Ningwu volcanic basin, eastern China

NASA Astrophysics Data System (ADS)

Liu, Wen-Hao; Jiang, Man-Rong; Zhang, Xiao-Jun; Xia, Yan; Algeo, Thomas J.; Li, Huan

2018-06-01

The Meishan iron deposit contains 338 Mt of iron-ore reserves at 39% Fe and represents the largest magnetite-apatite deposit in the Ningwu Basin of eastern China. Controversy has long existed about whether this deposit had a hydrothermal or iron-oxide melt origin. Iron mineralization is genetically related to plutons that are composed of gabbro-diorite, which were emplaced at 130 ± 1 Ma. These rocks have SiO2 contents of 51.72-54.60 wt%, Na2O contents of 3.47-4.04 wt%, K2O contents of 2.02-2.69 wt%, and K2O/Na2O ratios of 0.51-0.73. These rocks are enriched in LILEs and LREEs and depleted in Nb, Ta, and Ti, which indicates that the magma originated through partial melting of an enriched lithospheric mantle source in a subduction environment. A pattern of decreasing initial Sr isotopic ratios and increasing εNd(t) values with time in Early Cretaceous magmatic rocks of the Ningwu Basin may indicate incorporation of increasing proportions of asthenospheric mantle material into the source magma, which is consistent with the processes of lithospheric thinning and asthenospheric upwelling in eastern China related to Mesozoic subduction of the Paleo-Pacific Plate. Two stages of magnetite are found in the gabbro-diorite: (1) early-crystallized magnetite as euhedral-subhedral crystals in larger clinopyroxene crystals, and (2) later-crystallized magnetite and accompanying ilmenite grains in the voids between plagioclase and clinopyroxene crystals. The formation of magnetite before clinopyroxene, combined with the results of Fe-Ti oxide geothermometry and analysis of magnetite V content, indicates that the oxygen fugacity of the source magma was greater than ΔFMQ +2.2 at an early stage (>640 °C) but decreased to ΔFMQ -2.66 as abundant magnetite crystallized at a later stage (∼489 °C). The early crystallization of magnetite at a high oxygen fugacity does not support a Fenner evolution trend for the primitive magma and diminishes the likelihood of liquid immiscibility

Intrusion of basaltic magma into a crystallizing granitic magma chamber: The Cordillera del Paine pluton in southern Chile

NASA Astrophysics Data System (ADS)

Michael, Peter J.

1991-10-01

The Cordillera del Paine pluton in the southernmost Andes of Chile represents a deeply dissected magma chamber where mafic magma intruded into crystallizing granitic magma. Throughout much of the 10x15 km pluton, there is a sharp and continuous boundary at a remarkably constant elevation of 1,100 m that separates granitic rocks (Cordillera del Paine or CP granite: 69 77% SiO2) which make up the upper levels of the pluton from mafic and comingled rocks (Paine Mafic Complex or PMC: 45 60% SiO2) which dominate the lower exposures of the pluton. Chilled, crenulate, disrupted contacts of mafic rock against granite demonstrate that partly crystallized granite was intruded by mafic magma which solidified prior to complete crystallization of the granitic magma. The boundary at 1,100 m was a large and stable density contrast between the denser, hotter mafic magma and cooler granitic magma. The granitic magma was more solidified near the margins of the chamber when mafic intrusion occurred, and the PMC is less disrupted by granites there. Near the pluton margins, the PMC grades upward irregularly from cumulate gabbros to monzodiorites. Mafic magma differentiated largely by fractional crystallization as indicated by the presence of cumulate rocks and by the low levels of compatible elements in most PMC rocks. The compositional gap between the PMC and CP granite indicates that mixing (blending) of granitic magma into the mafic magma was less important, although it is apparent from mineral assemblages in mafic rocks. Granitic magma may have incorporated small amounts of mafic liquid that had evolved to >60% SiO2 by crystallization. Mixing was inhibited by the extent of crystallization of the granite, and by the thermal contrast and the stable density contrast between the magmas. PMC gabbros display disequilibrium mineral assemblages including early formed zoned olivine (with orthopyroxene coronas), clinopyroxene, calcic plagioclase and paragasite and later-formed amphibole

(DEPSCOR FY 09) Obfuscation and Deobfuscation of Intent of Computer Programs

DTIC Science & Technology

2012-12-21

increased as the malware distribution mechanism has moved to the web through infected sites. In this use a site is hacked so as to distribute malware to...Simpósio Brasileiro em Segurança da Informaçao e de Sistemas Computacionais (2009). (Chen et al., 2012) Jundong Chen, Matthias R. Brust, Vir V. Phoha

The Electroplastic Effect in Metals.

DTIC Science & Technology

1984-05-11

the pinch effect. The differ- oulses (%&0A/mm2 for ^-50iis duration) on the flow ence Acp-Anp (Fig. 4r) should then consist prima - * stress of a number...Silveira, R. A. F. 0. Fortes and W. Rvzhkov, Dokl. Akad. Nauk. SSSR 243 330-333 A. Mannheimer, 5th Congr. Brasileiro Eng. (1978). Ciencia Materias (Dec. 1982

Geochronology and geochemistry of basaltic rocks from the Sartuohai ophiolitic mélange, NW China: Implications for a Devonian mantle plume within the Junggar Ocean

NASA Astrophysics Data System (ADS)

Yang, Gaoxue; Li, Yongjun; Santosh, M.; Yang, Baokai; Yan, Jing; Zhang, Bing; Tong, Lili

2012-10-01

The West Junggar domain in NW China is a distinct tectonic unit of the Central Asian Orogenic Belt (CAOB). It is composed of Paleozoic ophiolitic mélanges, arcs and accretionary complexes. The Sartuohai ophiolitic mélange in the eastern West Junggar forms the northeastern part of the Darbut ophiolitic mélange, which contains serpentinized harzburgite, pyroxenite, dunite, cumulate, pillow lava, abyssal radiolarian chert and podiform chromite, overlain by the Early Carboniferous volcano-sedimentary rocks. In this paper we report new geochronological and geochemical data from basaltic and gabbroic blocks embedded within the Sartuohai ophiolitic mélange, to assess the possible presence of a Devonian mantle plume in the West Junggar, and evaluate the petrogenesis and implications for understanding of the Paleozoic continental accretion of CAOB. Zircon U-Pb analyses from the alkali basalt and gabbro by laser ablation inductively coupled plasma mass spectrometry yielded weighted mean ages of 375 ± 2 Ma and 368 ± 11 Ma. Geochemically, the Sartuohai ophiolitic mélange includes at least two distinct magmatic units: (1) a Late Devonian fragmented ophiolite, which were produced by ca. 2-10% spinel lherzolite partial melting in arc-related setting, and (2) contemporary alkali lavas, which were derived from 5% to 10% garnet + minor spinel lherzolite partial melting in an oceanic plateau or a seamount. Based on detailed zircon U-Pb dating and geochemical data for basalts and gabbros from the Sartuohai ophiolitic mélange, in combination with previous work, indicate a complex evolution by subduction-accretion processes from the Devonian to the Carboniferous. Furthermore, the alkali basalts from the Sartuohai ophiolitic mélange might be correlated to a Devonian mantle plume-related magmatism within the Junggar Ocean. If the plume model as proposed here is correct, it would suggest that mantle plume activity significantly contributed to the crustal growth in the CAOB.

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.

Modeling mechanical and thermo-mechanical erosion by flowing lava at Raglan, Cape Smith Belt, New Québec, Canada

NASA Astrophysics Data System (ADS)

Cataldo, V.; Williams, D. A.; Lesher, C. M.

2015-12-01

The 1.5-D Williams et al. model of thermal erosion by turbulent lava was recently applied to the Athabasca Valles lava channel on Mars, in an attempt to establish the importance of thermal erosion in excavating this ~80-100 m deep outflow channel. The modeled erosion depths (0.4-7.5 m) are far less than the depth of the channel which, combined with the short duration of the eruption, suggests that mechanical erosion may have had a greater role. Several studies suggest that mechanical erosion by lava is more important in channel-tube formation than previously thought, under certain circumstances. How would we be able to distinguish between mechanical and thermal erosion? By investigating model results when substrate properties change, as we move from a consolidated, mechanically strong substrate to a partially consolidated or unconsolidated, mechanically weaker substrate. The Proterozoic Raglan komatiitic basalt lava channel of the Cape Smith Belt, New Québec, Canada is a complex erosional environment involving invasive erosion of both sediment and gabbro substrates - which makes it a critical test case. The lava eroded an upper layer of soft sediment, with erosion at the tops, bottoms, and sides of the conduit, through underlying gabbro, and then burrowed laterally into underlying sediment, a scenario requiring a two-dimensional modeling approach. Using the available field data, we will simulate two-dimensional thermomechanical and mechanical erosion interfaces on all sides of a turbulent lava flow by creating a finite-element mesh. The mesh will be defined by the geometry of the lava flow at those lava conduits for which data on lava and substrate composition, lava thickness, slope of the ground, conduit area and volume, and lava flow length are available. Ultimately, this model will be applied to lunar sinuous rilles and martian lava channels for which the use of a two-dimensional approach is needed.

Model for the Evolution of an Oceanic Core Complex and its Hydrothermal Vent on the Ultraslow-Spreading Mid Cayman Spreading Center

NASA Astrophysics Data System (ADS)

Harding, J.; Van Avendonk, H. J.; Hayman, N. W.; Grevemeyer, I.; Peirce, C.

2016-12-01

The Mid Cayman Spreading Center (MCSC) is an ultraslow-spreading center (15 mm yr-1 full rate) along the Caribbean-North American plate boundary. Despite the paradigm that ultraslow-spreading centers are amagmatic and cold, two hydrothermal vent fields have recently been discovered along the MCSC. The Beebe Vent Field is a black smoker in the northern axial deep, and the Von Damm Vent Field (VDVF) is a moderate-temperature, talc precipitating vent found atop an oceanic core complex (OCC). This OCC, "Mt. Dent", is a large (3 km high) massif that formed beneath a detachment fault, which exhumed lower crustal and upper mantle material. The CaySeis Experiment was conducted in April, 2015 in order to collect wide-angle refraction data of the MCSC crust and upper mantle. We modeled the across-axis crustal structure of Mt. Dent as well as the surrounding lithosphere using 2.5D P-wave tomography. Using this tomographic model, along with geochemistry, we propose a model for the formation and evolution of the OCC Mt. Dent and the VDVF. A detachment fault formed in a magma-poor environment due to a pulse of magmatism, producing a large gabbro body that was then exhumed and rotated into the OCC footwall. Once magmatism waned and the gabbroic body cooled, the OCC was faulted and fractured due to plate flexure and increased tectonic extensional stress in the naturally cold and thick lithosphere. These faults provide a permeable and deep network of hydrothermal pathways that mine deep lithospheric heat and expose gabbro and fresh mantle peridotite. This model is consistent with the basalt geochemistry, hydrothermal fluid geochemistry, and the distribution of brittle vs. ductile structures along the detachment shear zone. The VDVF is therefore a product of a pulse of magmatism in an overall melt-poor environment, conditions that may be found at other ultraslow-spreading ridges.

IODP Expedition 335: Deep Sampling in ODP Hole 1256D

NASA Astrophysics Data System (ADS)

Teagle, D. A. H.; Ildefonse, B.; Blum, P.; IODP Expedition 335 Scientists, the

2012-04-01

Observations of the gabbroic layers of untectonized ocean crust are essential to test theoretical models of the accretion of new crust at mid-ocean ridges. Integrated Ocean Drilling Program (IODP) Expedition 335 ("Superfast Spreading Rate Crust 4") returned to Ocean Drilling Program (ODP) Hole 1256D with the intention of deepening this reference penetration of intact ocean crust a significant distance (~350 m) into cumulate gabbros. Three earlier cruises to Hole 1256D (ODP 206, IODP 309/312) have drilled through the sediments, lavas, and dikes and 100 m into a complex dike-gabbro transition zone. Operations on IODP Expedition 335 proved challenging throughout, with almost three weeks spent re-opening and securing unstable sections of the hole. When coring commenced, the comprehensive destruction of the coring bit required further remedial operations to remove junk and huge volumes of accumulated drill cuttings. Hole-cleaning operations using junk baskets were successful, and they recovered large irregular samples that document a hitherto unseen sequence of evolving geological conditions and the intimate coupling between temporally and spatially intercalated intrusive, hydrothermal, contact-metamorphic, partial melting, and retrogressive processes. Hole 1256D is now clean of junk, and it has been thoroughly cleared of the drill cuttings that hampered operations during this and previous expeditions. At the end of Expedition 335, we briefly resumed coring before undertaking cementing operations to secure problematic intervals. To ensure the greatest scientific return from the huge efforts to stabilize this primary ocean lithosphere reference site, it would be prudent to resume the deepening of Hole 1256D in the nearest possible future while it is open to full depth. doi:10.2204/iodp.sd.13.04.2011

Reconnaissance geology of the Manjamah Quadrangle, sheet 18/41 A, Kingdom of Saudi Arabia

USGS Publications Warehouse

Hadley, D.G.

1982-01-01

The Manjamah quadrangle (sheet 18/41 A) lies between lat 18?30' and 19?00' N. and long 41?00' and 41?30' E. and encompasses an area of 2,932 km2, of which about half is land and the remainder covered by the Red Sea. The geologic formations exposed in the quadrangle comprise Precambrian layered and intrusive rocks,. Tertiary layered rocks and gabbro dikes, and. Quaternary basaltic lavas, pyroclastic rocks, and surficial deposits. The Precambrian rocks are layered metasedimentary and metavolcanic rocks that have been assigned to the Baish and Bahah groups. These rocks are cut by Precambrian biotite quartz monzonite and by Miocene gabbro dikes that were intruded during the initial stages of the opening of the Red Sea rift. Tuffaceous siltstone of the Baid formation was also deposited during the Miocene, followed in the Pliocene by the polymict conglomerate of the Bathan formation. The Quaternary rocks include basalt that was extruded during a continuation of the opening of the Red Sea rift, after uplift of the escarpment parallel with the Red Sea but before the Holocene erosional cycle. The greater part of the land area of the quadrangle is covered by Quaternary coastal, pediment, and alluvial deposits of various kinds associated with the deltaic mouths of Wadi Hall and Wadi Yiba and their tributaries and with the development of fringing reefs and islands. The area also contains extensive Quaternary eolian deposits. The economic potential of the quadrangle lies essentially in the agricultural value .of its flood-plain deposits, which are frequently refreshed during flooding with the products of weathering and erosion of the Precambrian rocks in the valleys of Wadi Hal i and Wadi Yiba; coral reefs could possibly provide raw material for use in a cement industry, if any such industry were ever required in this area.

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

NASA Astrophysics Data System (ADS)

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

2012-10-01

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

Accreted fragments of the Late Cretaceous Caribbean Colombian Plateau in Ecuador

NASA Astrophysics Data System (ADS)

Mamberti, Marc; Lapierre, Henriette; Bosch, Delphine; Jaillard, Etienne; Ethien, Raynald; Hernandez, Jean; Polvé, Mireille

2003-02-01

The eastern part of the Western Cordillera of Ecuador includes fragments of an Early Cretaceous (≈123 Ma) oceanic plateau accreted around 85-80 Ma (San Juan-unit). West of this unit and in fault contact with it, another oceanic plateau sequence (Guaranda unit) is marked by the occurrence of picrites, ankaramites, basalts, dolerites and shallow level gabbros. A comparable unit is also exposed in northwestern coastal Ecuador (Pedernales unit). Picrites have LREE-depleted patterns, high ɛNd i and very low Pb isotopic ratios, suggesting that they were derived from an extremely depleted source. In contrast, the ankaramites and Mg-rich basalts are LREE-enriched and have radiogenic Pb isotopic compositions similar to the Galápagos HIMU component; their ɛNd i are slightly lower than those of the picrites. Basalts, dolerites and gabbros differ from the picrites and ankaramites by flat rare earth element (REE) patterns and lower ɛNd; their Pb isotopic compositions are intermediate between those of the picrites and ankaramites. The ankaramites, Mg-rich basalts, and picrites differ from the lavas from the San Juan-Multitud Unit by higher Pb ratios and lower ɛNd i. The Ecuadorian and Gorgona 88-86 Ma picrites are geochemically similar. The Ecuadorian ankaramites and Mg-rich basalts share with the 92-86 Ma Mg-rich basalts of the Caribbean-Colombian Oceanic Plateau (CCOP) similar trace element and Nd and Pb isotopic chemistry. This suggests that the Pedernales and Guaranda units belong to the Late Cretaceous CCOP. The geochemical diversity of the Guaranda and Pedernales rocks illustrates the heterogeneity of the CCOP plume source and suggests a multi-stage model for the emplacement of these rocks. Stratigraphic and geological relations strongly suggest that the Guaranda unit was accreted in the late Maastrichtian (≈68-65 Ma).

Vertical movements following intracontinental magmatism: An example from southern Israel

NASA Astrophysics Data System (ADS)

Gvirtzman, Zohar; Garfunkel, Zvi

1997-02-01

We present a quantitative thermal model for vertical movements following continental magmatism, focusing on how the associated elevation changes depend on the depth of intrusion. When an intrusion is emplaced within the lithosphere, its buoyancy causes a quick initial movement which is followed by long-term movements caused by thermal relaxation. Intrusions emplaced within the gabbro stability field produce initial uplifting which is about 12% of their thickness. Subsequent thermal relaxation reduces the uplift to a residual value of 9-10% of the intrusion thickness. In contrast, intrusions emplaced within the eclogite stability field produce a small subsidence from the very beginning which is slowly increased by thermal relaxation and may reach a residual value of some 4% of the intrusion thickness. In both cases the rates of the thermal subsidence depend on the depth of intrusion: it is relatively fast when the intrusions are shallow but considerably slower when the intrusions are deep. The model enables us to infer volumes and depths of intrusions from amplitudes and rates of vertical movements. As an example we apply the model to analyze the geodynamic evolution of the central Negev, southern Israel, during the Early Cretaceous. Two distinct magmatic pulses that were recognized there represent the two basic situations envisaged by the model, i.e., shallow magma emplacement in the gabbro field associated with uplifting, and deep intrusion in the eclogite field associated with subsidence. In a wider context we think that this model may help in understanding intracratonic basins in nonextensional settings. In particular, deep and thick eclogite intrusions can explain subsidence of regions which were not extended nor uplifted and in regions where crustal magmatism and heating were not observed.

The Northeast Kingdom batholith, Vermont: magmatic evolution and geochemical constraints on the origin of Acadian granitic rocks

USGS Publications Warehouse

Ayuso, R.A.; Arth, Joseph G.

1992-01-01

Five Devonian plutons (West Charleston, Echo Pond, Nulhegan, Derby, and Willoughby) that constitute the Northeast Kingdom batholith in Vermont show wide ranges in elemental abundances and ratios consistent with major crustal contributions during their evolution. The batholith consists of metaluminous quartz gabbro, diorite and quartz monzodiorite, peraluminous granodiorite and granite, and strongly peraluminous leucogranite. Contents of major elements vary systematically with increasingSiO40) and have small negative Eu anomalies. The strongly peraluminous Willoughby leucogranite has unique trace-element abundances and ratios relative to the rest of the batholith, including low contents of Hf, Zr, Sr, and Ba, low values of K/Rb (80-164), Th/Ta (<9), Rb/Cs (7-40), K/Cs (0.1-0.5), Ce/Pb (0.5-4), high values of Rb/Sr (1-18) low to moderate REE contents and light-REE enriched patterns (with small negative Eu anomalies). Flat REE patterns (with large negative Eu anomalies) are found in a small, hydrothermally-altered area characterized by high abundances of Sn (up to 26 ppm), Rb (up to 670 ppm), Li (up to 310 ppm), Ta (up to 13.1 ppm), and U (up to 10 ppm). There is no single mixing trend, fractional crystallization assemblage, or assimilationscheme that accounts for all trace elementvariations from quartz gabbro to granite in the Northeast Kingdom batholith. The plutons originated by mixing mantle-derived components and crustal melts generated at different levels in the heterogeneous lithosphere in a continental collisional environment. Hybrid rocks in the batholith evolved by fractional crystallization and assimilation of country rocks (<50% by mass), and some of the leucogranitic rocks were subsequently disturbed by a mild hydrothermal event that resulted in the deposition of small amounts of sulfide minerals. ?? 1992 Springer-Verlag.

Sulfur geochemistry and microbial sulfate reduction during low-temperature alteration of uplifted lower oceanic crust: Insights from ODP Hole 735B

USGS Publications Warehouse

Alford, Susan E.; Alt, Jeffrey C.; Shanks, Wayne C.

2011-01-01

Sulfide petrography plus whole rock contents and isotope ratios of sulfur were measured in a 1.5 km section of oceanic gabbros in order to understand the geochemistry of sulfur cycling during low-temperature seawater alteration of the lower oceanic crust, and to test whether microbial effects may be present. Most samples have low SO4/ΣS values (≤ 0.15), have retained igneous globules of pyrrhotite ± chalcopyrite ± pentlandite, and host secondary aggregates of pyrrhotite and pyrite laths in smectite ± iron-oxyhydroxide ± magnetite ± calcite pseudomorphs of olivine and clinopyroxene. Compared to fresh gabbro containing 100–1800 ppm sulfur our data indicate an overall addition of sulfide to the lower crust. Selection of samples altered only at temperatures ≤ 110 °C constrains microbial sulfate reduction as the only viable mechanism for the observed sulfide addition, which may have been enabled by the production of H2 from oxidation of associated olivine and pyroxene. The wide range in δ34Ssulfide values (− 1.5 to + 16.3‰) and variable additions of sulfide are explained by variable εsulfate-sulfide under open system pathways, with a possible progression into closed system pathways. Some samples underwent oxidation related to seawater penetration along permeable fault horizons and have lost sulfur, have high SO4/ΣS (≥ 0.46) and variable δ34Ssulfide (0.7 to 16.9‰). Negative δ34Ssulfate–δ34Ssulfide values for the majority of samples indicate kinetic isotope fractionation during oxidation of sulfide minerals. Depth trends in sulfide–sulfur contents and sulfide mineral assemblages indicate a late-stage downward penetration of seawater into the lower 1 km of Hole 735B. Our results show that under appropriate temperature conditions, a subsurface biosphere can persist in the lower oceanic crust and alter its geochemistry.

Fractional crystallization, impregnation and sulphide saturation recorded in Mesozoic arc-related cumulates at King Mountain, Cache Creek Ophiolite, Northern British Columbia.

NASA Astrophysics Data System (ADS)

Bedard, J. H. J.; Zagorevski, A.; Corriveau, A. S.

2016-12-01

The Cache creek terrane extends from southern B.C. to the Yukon. It accreted to North America at 175Ma and is composed of Paleozoic seamounts, Mesozoic oceanic arcs and mantle rocks. Mantle harzburgite massifs represent intra-oceanic core-complexes. Mantle rocks are cut by gabbroic dykes and overlain by chert, lava, dismembered hypabyssal complexes and rare cumulates. At King Mountain, gabbronorites are in tectonic contact with subjacent peridotite. Other crustal relics exposed nearby include sheeted hypabyssal intrusions and volcanics that range from depleted arc tholeiites to boninites. The King Mountain cumulates are rhythmically layered, foliated gabbronorites with 5% oxides and minor interstitial hornblende that yields temperatures of 652-759oC. Cumulates may show evidence of compaction-related flattening and intra-cumulate shear (boudins, fold noses). A 300m thick continuous section records two fractional crystallization cycles, whole rock mg# varying from 60 to 35 in the 1st cycle and from 52 to 30 in the 2nd. Cumulates formed during passage of evolved multiply-saturated magmas derived from a deeper chamber towards the surface. Inverse trace element models show that the gabbronorite cumulates are compositionally akin to boninites. The lowest-mg# rocks in the differentiation cycles are rusty 10cm-1m interbeds with abundant magnetite+ ilmenite ( 10-15%), high sulphide contents ( 5-10%, pyrrhotite and chalcopyrite) and high V contents (<1200ppm). These are interpreted to record episodic co-accumulation of Fe-Ti-oxides, with the decrease in melt FeO-content triggering sulphide immiscibility. Hornblendite and hornblende tonalite veins are locally transposed into the layered cumulates, forming flaser gabbros with 5-50% cm-scale lensoid hornblendite that impregnates and replaces the foliated gabbro-norite; greatly increasing REE contents. Amphibole oikocrysts show evidence of internal deformation and record temperatures of 753-804 oC.

Accreted seamounts in North Tianshan, NW China: Implications for the evolution of the Central Asian Orogenic Belt

NASA Astrophysics Data System (ADS)

Yang, Gaoxue; Li, Yongjun; Kerr, Andrew C.; Tong, Lili

2018-03-01

The Carboniferous Bayingou ophiolitic mélange is exposed in the North Tianshan accretionary complex in the southwestern part of the Central Asian Orogenic Belt (CAOB). The mélange is mainly composed of serpentinised ultramafic rocks (including harzburgite, lherzolite, pyroxenite, dunite and peridotite), pillowed and massive basalts, layered gabbros, radiolarian cherts, pelagic limestones, breccias and tuffs, and displays block-in-matrix structures. The blocks of ultramafic rocks, gabbros, basalts, cherts, and limestones are set in a matrix of serpentinised ultramafic rocks, massive basalts and tuffs. The basaltic rocks in the mélange show significant geochemical heterogeneity, and two compositional groups, one ocean island basalt-like, and the other mid-ocean ridge-like, can be distinguished on the basis of their isotopic compositions and immobile trace element contents (such as light rare earth element enrichment in the former, but depletion in the latter). The more-enriched basaltic rocks are interpreted as remnants/fragments of seamounts, derived from a deep mantle reservoir with low degrees (2-3%) of garnet lherzolite mantle melting. The depleted basalts most likely formed by melting of a shallower spinel lherzolite mantle source with ∼15% partial melting. It is probable that both groups owe their origin to melting of a mixture between plume and depleted MORB mantle. The results from this study, when integrated with previous work, indicate that the Junggar Ocean crust (comprising a significant number of seamounts) was likely to have been subducted southward beneath the Yili-Central Tianshan block in the Late Devonian-Early Carboniferous. The seamounts were scraped-off and accreted along with the oceanic crust in an accretionary wedge to form the Bayingou ophiolitic mélange. We present a model for the tectonomagmatic evolution of this portion of the CAOB involving prolonged intra-oceanic subduction with seamount accretion.

Education and Outreach from the JOIDES Resolution during IODP Expedition 360 : linking onboard research and classroom activities during and after the Expedition.

NASA Astrophysics Data System (ADS)

Burgio, M.; Zhang, J.; Kavanagh, L.; Martinez, A. O.; Expedition 360 Scientists, I.

2016-12-01

The International Ocean Discovery Program (IODP) expeditions provide an excellent opportunity for onboard Education Officers (EO) to communicate and disseminate exciting shipboard research and discoveries to students around the world. During expedition 360, the EOs carried out 140 live webcasts, using different strategies to create an effective link between both students and scientists. Below are examples of strategies we used: -Primary school: The Beauty of Gabbro! and Life in the rocks! During the webcasts, students could virtually tour the ship, interview scientists, and see and discuss samples of the cored gabbro and minerals in thin sections. Artistic contextualization by J. Zhang, facilitated these activities. Moreover, highlighting the search for microbes in the Earth's crust , was particularly successful in engaging the students. -Middle and High school: Fun and relationships in science. Students were able to email expert scientists in the scientific discipline they chose to research and interview them during a live webcast. Some students created a song about the expedition. "on the boat - cup song - IODP project" https://www.youtube.com/watch?v=qex-w9aSV7c-University: Travels, research and the everyday life of professors onboard. We used webcasts to connect with universities in France, Japan and Italy, to create vibrant interactions between students and scientists that enabled students to get closer to their professors and understand better the life of onboard researchers. In collaboration with the science party we developed new strategies to keep in touch with students after completion of the cruise. We generated teaching kits consisting of pedaqgoical sets of pictures, exercises using onboard data, a continuously updated map "tracking geologists", and live webcasts to be organized from laboratories to schools. We already have had enthusiastic feedback from teachers that took part in our webcasts and the challenge is to continue to foster the

From ocean depths to mountain tops: uplift of the Troodos Massif (Cyprus) constrained by (U-Th)/He thermochronology and geomorphic analysis

NASA Astrophysics Data System (ADS)

Morag, N.; Haviv, I.; Katzir, Y.

2013-12-01

The Troodos Massif of Cyprus, rising to nearly 2000 meters above sea level, encompasses one of the world's classic ophiolites. Following its formation at a seafloor spreading center in Late Cretaceous times, this slice of the NeoTethyan oceanic lithosphere was uplifted and eventually exposed on mountain tops during the Neogene. The final uplift and exhumation of the Troodos was previously assigned to Pleistocene age by observations in the circum-Troodos sedimentary strata. However, quantitative thermochronological and geomorphological data from the Massif itself were not available. Here we use apatite (U-Th)/He low-temperature thermochronology complemented by zircon (U-Th)/He and apatite fission track data, and combined with geomorphic analysis to constrain the exhumation and uplift history of the Troodos ophiolite. Apatite (U-Th)/He ages vary with depth from ~ 22 Ma at the top of the Gabbro sequence to ~ 6 Ma at the bottom of the sequence. The deepest sample from a Gabbro pegmatitic dyke intruding the ultramafic sequence yielded an age of ~ 3 Ma. Thermal modeling of apatite (U-Th)/He and fission track data delineates Plio - Pleistocene initiation of rapid uplift and exhumation of the Troodos ophiolite. The estimated cumulative exhumation since its initiation is 2-3 km. No evidence was found for significant uplift of the central Troodos area prior to that time. The geomorphic analysis delineates a bull's-eye zone at the center of the Troodos Massif, where local relief and channel steepness index are highest. The boundaries of this zone roughly correspond with the Mt. Olympus mantle outcrop and suggest recent, differential uplift of this zone relative to its surroundings. The most likely mechanism, which could drive such a focused bull's-eye uplift pattern is hydration of ultramafic rocks (serpentinization) leading to a decrease in rock density and subsequent diapiric uplift of the serpentinized lithospheric mantle.

Lunar basalt meteorite EET 87521: Petrology of the clast population

NASA Technical Reports Server (NTRS)

Semenova, A. S.; Nazarov, M. A.; Kononkova, N. N.

1993-01-01

The Elephant Moraine meteorite EET 87521 was classified as a lunar mare basalt breccia which is composed mainly of VLT basalt clasts. Here we report on our petrological study of lithic clasts and monomineralic fragments in the thin sections EET 87521,54 and EET 87521,47,1, which were prepared from the meteorite. The results of the study show that EET 87521 consists mainly of Al-rich ferrobasalt clasts and olivine pyroxenite clasts. The bulk composition of the meteorite can be well modelled by the mixing of these lithic components which appear to be differentiates of the Luna 25 basalt melt. KREEP and Mg-rich gabbro components are minor constituents of EET 87521.

Apollo 16 Geochemical X-ray Fluorescence Experiment: Preliminary Report.

PubMed

Adler, I; Trombka, J; Gerard, J; Lowman, P; Schmadebeck, R; Blodget, H; Eller, E; Yin, L; Lamothe, R; Osswald, G; Gorenstein, P; Bjorkholm, P; Gursky, H; Harris, B

1972-07-21

The lunar surface was mapped with respect to magnesium, aluminum, and silicon as aluminum/ silicon and magnesium/ silicon intensity ratios along the projected ground tracks swept out by the orbiting Apollo 16 spacecraft. The results confirm the observations made during the Apollo 15 flight and provide new data for a number of features not covered before. The data are consistent with the idea that the moon has a widespread differentiated crust (the highlands). The aluminum/ silicon and magnesium/ silicon concentration ratios correspond to those for anorthositic gabbros through gabbroic anorthosites or feldspathic basalts. The x-ray results suggest the occurrence of this premare crust or material similar to it at the Descartes landing site.

The fracture strength and frictional strength of Weber Sandstone

USGS Publications Warehouse

Byerlee, J.D.

1975-01-01

The fracture strength and frictional strength of Weber Sandstone have been measured as a function of confining pressure and pore pressure. Both the fracture strength and the frictional strength obey the law of effective stress, that is, the strength is determined not by the confining pressure alone but by the difference between the confining pressure and the pore pressure. The fracture strength of the rock varies by as much as 20 per cent depending on the cement between the grains, but the frictional strength is independent of lithology. Over the range 0 2 kb, ??=0??5 + 0??6??n. This relationship also holds for other rocks such as gabbro, dunite, serpentinite, granite and limestone. ?? 1975.

Apollo 16 geochemical X-ray fluorescene experiment: Preliminary report

NASA Technical Reports Server (NTRS)

Adler, I.; Trombka, J.; Gerard, J.; Lowman, P.; Schmadebeck, R.; Blodgett, H.; Eller, E.; Yin, L.; Lamothe, R.; Osswald, G.

1972-01-01

The lunar surface was mapped with respect to Mg, Al and Si as Al/Si and Mg/Si ratios along the projected ground tracks swept out by the orbiting Apollo 16 spacecraft. The results confirm the observations made during the Apollo 15 flight and provide data for a number of features not covered before. The data are consistent with the idea that the moon has a widespread differentiated crust (the highlands). The Al/Si and Mg/Si chemical ratios correspond to those for anorthositic gabbro through gabbroic anorthosites or feldspathic basalts. The X-ray results suggest the occurrence of this premare crust, or material similar to it, at the Descartes landing site.

Apollo 16 geochemical X-ray fluorescence experiment: Preliminary report

NASA Technical Reports Server (NTRS)

Adler, I.; Trombka, J.; Gerard, J.; Lowman, P.; Schmadebeck, R.; Blodgett, H.; Eller, E.; Yin, L.; Lamothe, R.; Osswald, G.

1972-01-01

The lunar surface was mapped with respect to Mg, Al, and Si, as Al/Si and Mg/Si ratios along the projected ground tracks swept out by the orbiting Apollo 16 spacecraft. The results confirm the observations made during the Apollo 15 flight and provide data for a number of features not covered before. The data are consistent with the idea that the moon has a widespread differentiated crust (the highlands). The Al/Si and Mg/Si chemical ratios correspond to that for anorthositic gabbro through gabbroic anorthosites or feldspathic basalts. The X-ray results suggest the occurrence of this premare crust or material similar to it as the Descartes landing site.

Apollo 16 geochemical X-ray fluorescence experiment - Preliminary report.

NASA Technical Reports Server (NTRS)

Adler, I.; Trombka, J.; Gerard, J.; Lowman, P.; Schmadebeck, R.; Blodget, H.; Eller, E.; Yin, L.; Lamothe, R.; Osswald, G.

1972-01-01

Mapping of the lunar surface with respect to magnesium, aluminum, and silicon as aluminum/silicon and magnesium/silicon intensity ratios along the projected ground tracks swept out by the orbiting Apollo 16 spacecraft. The results confirm the observations made during the Apollo 15 flight and provide new data for a number of features not covered before. The data are consistent with the idea that the moon has a widespread differentiated crust (the highlands). The aluminum/silicon and magnesium/silicon concentration ratios correspond to those for anorthositic gabbros through gabbroic anorthosites or feldspathic basalts. The X-ray results suggest the occurrence of this premare crust or material similar to it at the Descartes landing site.

Petrologic and Chemical Characterization of a Suite of Antarctic Diogenites

NASA Technical Reports Server (NTRS)

Mittlefehldt, D. W.; Mertzman, S. A.; Peng, Z. X.; Mertzman, K. R.

2013-01-01

The origin of diogenites, ultramafic cumulates related to eucrites, is an unresolved problem [1]. Most diogenites are orthopyroxenites, a few are harzburgites [2], and some are transitional to cumulate eucrites [1, 3]. Cumulate eucrites are gabbros formed by crystal fractionation from basaltic eucrites [4]. The consensus view is that basaltic eucrites are residual melts from global-magma-ocean crystallization on their parent asteroid [4] which is plausibly Vesta [5]. However, the petrologic and compositional characteristics of diogenites seem to preclude a magma ocean origin [1, 4]. We are doing a petrologic and chemical study of new or unusual diogenites with the ultimate goals of constraining their genesis, and the geologic evolution of Vesta.

First Evidence of Middle Triassic Basic Magmatism in the Southwestern Part of the Dzhugdzhur-Stanovoi Superterrane (Ilikan Terrane)

NASA Astrophysics Data System (ADS)

Buchko, I. V.; Sorokin, A. A.; Rodionov, A. A.; Kudryashov, N. M.

2018-04-01

U-Pb ID-TIMS zircon analyses of the Dzhigda gabbro-gabbrodiorite Massif (Ilikan block in the southwestern part of the Dzhugdzhur-Stanovoi superterrane) have been carried out. The results demonstrate that the formation of the massif at 244 ± 5 Ma corresponds to one of the stages of formation of the Selenga-Vitim volcano-plutonic belt. The latter stretches along the southeastern margin of the North Asian Craton along its border with the Mongol-Okhotsk fold belt. This indicates that the Selenga-Vitim volcano-plutonic belt along with granitoids and volcanics comprises Permian-Triassic massifs and that this belt is superimposed onto structures of not only the Selenga-Stanovoi terrane but also the Dzhugdzhur-Stanovoi terrane.

Trace geochemistry of lunar material

NASA Technical Reports Server (NTRS)

Morrison, G. H.

1974-01-01

The lunar samples from the Apollo 16 and 17 flights which were analyzed include soil, igneous rock, anorthositic gabbro, orange soil, subfloor basalt, and norite breccia. Up to 57 elements including majors, minors, rare earths and other trace elements were determined in the lunar samples. The analytical techniques used were spark source mass spectrometry and neutron activation analysis. The latter was done either instrumentally or with group radiochemical separations. The differences in abundances of the elements in lunar soils at the various sites are discussed. With regard to the major elements only Si is about the same at all the sites. A detailed analysis which was performed on a sample of the Allende meteorite is summarized.

Comparison of orbital chemistry with crustal thickness and lunar sample chemistry

NASA Technical Reports Server (NTRS)

Schonfeld, E.

1977-01-01

A correlation between orbital chemistry (FeO, Al2O3, Mg/Al, MgO/FeO, Th) and the lunar crustal thickness is examined. The correlation suggests either lack of complete homogenization by lateral or vertical mixing, or lateral variation in the differentiation process. In addition, links between orbital chemistry and lunar sample chemistry are investigated. In regions with crustal thickness between 100 and 110 km, gabbroic anorthosites are very abundant, while in regions with crustal thickness of about 80 km anorthositic gabbros are frequent. Special attention is given to the distribution of low-potassium Fra Mauro basalt, found in high concentrations in regions with 50 to 60 km crustal thickness.

Lithologic mapping of mafic intrusions in East Greenland using Landsat Thematic Mapper data

NASA Technical Reports Server (NTRS)

Naslund, H. Richard; Birnie, R. W.; Parr, J. T.

1989-01-01

The East Greenland Tertiary Igneous Province contains a variety of intrusive and extrusive rock types. The Skaergaard complex is the most well known of the intrusive centers. Landsat thematic mapping (TM) was used in conjunction with field spectrometer data to map these mafic intrusions. These intrusions are of interest as possible precious metal ore deposits. They are spectrally distinct from the surrounding Precambrian gneisses. However, subpixel contamination by snow, oxide surface coatings, lichen cover and severe topography limit the discrimination of lithologic units within the gabbro. Imagery of the Skaergaard and surrounding vicinity, and image processing and enhancement techniques are presented. Student theses and other publications resulting from this work are also listed.

Metabolic and infectious pathologies in Brazilian medical literature: a review.

PubMed

Rocha-e-Silva, Mauricio

2010-06-01

This review of original reports on metabolic and infectious diseases that were recently published in Brazilian journals is designed to inform the readership of CLINICS about their content. I conducted a search in PubMed for original research articles (clinical or basic research) recently published (2008-2009) by Brazilian medical and biological periodicals. Papers on metabolic pathologies were retrieved by searching for appropriate keywords such as metabolic syndrome and obesity. Papers on infectious disease were obtained by entering 15 different keywords for the most commonly occurring pathologies. Review articles, editorials, letters to the editor, and case reports were manually excluded. Selected titles were then categorized into appropriate sub-categories. This search produced a total of 123 articles, which filtered down to 72 articles after eliminating editorials, review articles, letters to the Editor and case reports. Reviewed periodicals were Arquivos Brasileiros de Cardiologia, Arquivos Brasileiros de Endocrinologia e Metabologia, Brazilian Journal of Biological and Medical Research, Brazilian Journal of Infectious Diseases, Jornal de Pediatria, Jornal de Pneumologia, Revista da Associação Médica Brasileira, Revista da Escola de Enfermagem da Universidade de São Paulo, and São Paulo Medical Journal. The articles were then briefly summarized.

Blowflies (Diptera, Calliphoridae) Associated with Pig Carcasses in a Caatinga Area, Northeastern Brazil.

PubMed

Alves, A C F; Santos, W E; Farias, R C A P; Creão-Duarte, A J

2014-04-01

Studies that focused on Calliphoridae associated with pig carcasses are abundant in southern and southeastern Brazil; however, there are few in northeast. Here, we present an inventory of the blowfly species associated with the stages of decomposition of pig carcasses in a caatinga area during dry and rainy seasons. The study took place at the Private Reserve for the Environmental Inheritance "Fazenda Almas," state of Paraíba, Brazil. Using a modified version of the Shannon trap, 32,909 adult specimens belonging to eight species were captured. During the dry season, Cochliomyia macellaria (Fabricius) (52.2%) and Chrysomya albiceps (Wiedemann) (39.9%) were the most abundant species. In the rainy season, when the majority of individuals were captured (93.7%), Chloroprocta idioidea (Robineau-Desvoidy) (71.1%) was the most abundant. Five decomposition stages were recognized, being the active decay the most attractive to colonization by blowflies, except for Lucilia eximia (Wiedemann), which was more abundant in the bloated stage.

Pollen analysis of honey and pollen collected by Apis mellifera linnaeus, 1758 (Hymenoptera, Apidae), in a mixed environment of Eucalyptus plantation and native cerrado in Southeastern Brazil.

PubMed

Simeão, C M G; Silveira, F A; Sampaio, I B M; Bastos, E M A F

2015-11-01

Eucalyptus plantations are frequently used for the establishment of bee yards. This study was carried on at Fazenda Brejão, northwestern region of the State of Minas Gerais, Brazil. This farm is covered both with native Cerrado vegetation (Brazilian savanna) and eucalyptus plantations. This paper reports on the botanic origin of pollen pellets and honey collected from honeybee (Apis mellifera) hives along a thirteen-month period (January 2004 to January 2005). The most frequent pollen types found in the pollen pellets during the rainy season were Trema micrantha (Ulmaceae), Copaifera langsdorffii (Fabaceae), an unidentified Poaceae, unidentified Asteraceae-2, Cecropia sp. 1 (Cecropiaceae) and Eucalyptus spp. (Myrtaceae); during the dry season the most frequent pollen types were Acosmium dasycarpum (Fabaceae), Cecropia sp. 1 (Cecropiaceae) and Eucalyptus spp. (Myrtaceae). Pollen grains of Baccharis sp. (Asteraceae), Cecropia sp. 1 (Cecropiaceae), Copaifera langsdorffii (Fabaceae), Mimosa nuda (Fabaceae), Eucalyptus spp. (Myrtaceae) and Trema micrantha (Ulmaceae) were present in the honey samples throughout the study period.

Photoproduction of charm particles at Fermilab

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

Cumalat, John P.

A brief description of the Fermilab Photoproduction Experiment E831 or FOCUS is presented. The experiment concentrates on the reconstruction of charm particles. The FOCUS collaboration has participants from several Central American and Latin American institutions; CINVESTAV and Universidad Autonoma de Puebla from Mexico, University of Puerto Rico from the United States, and Centro Brasileiro de Pesquisas Fisicas in Rio de Janeiro from Brasil.

New geological data of New Siberian Archipelago

NASA Astrophysics Data System (ADS)

Sobolev, Nikolay; Petrov, Evgeniy

2014-05-01

-bearing unconformably overlies folded basement. The latter encompasses two sedimentary units: the Middle Devonian shallow-marine carbonate and Late-Devonian-Permian olistostrome - flysch deposited in transitional environment from carbonate platform to passive margin. Dating of detrital zircons suggests the Siberian Platform and Taimyr-Severnaya Zemlya areas as the most possible provenance. The magmatic activity on Belkovsky Island resulted in formation of Early Triassic gabbro-dolerite similar to the Siberian Platform traps. Proximity of Belkovsky Island to the north of Verkhoyansk foldbelt allows continuation of the latter into the Laptev Sea shelf. The geology of Bolshoy Lyakhovsky Island is discrepant from the rest of the NSA. In the south of Bolshoy Lyakhovsky Island the ophiolite crops complex out: it is composed of tectonic melange of serpentinized peridotite, bandedf gabbro, pillow-basalt, and pelagic sediments (black shales and cherts). All the rocks underwent epidot - amphibolite, glaucophane and greenschist facies metamorphism. The ophiolite is intruded by various in composition igneous massifs - from gabbro-diorite to leuco-granite, which occurred at 110-120 Ma. The Bolshoy Lyakhovsky Island structure is thought to be a westerly continuation of the South Anui suture of Chukchi.

Igneous and Ore-Forming Processes at the Roots of Giant - Ultra-Mafic Pluming System: the Seiland Igneous Comples, Norway

NASA Astrophysics Data System (ADS)

Larsen, R. B.; Iljina, M.; Schanke, M.

2012-12-01

SIP covers an area of 5500 km2 in N. Norway. 50 % of the volume comprises mafic layered or homogenous plg+px+Fe-Ti±ol gabbros. 25 % of the area comprises ultramafic intrusions, mostly peridotite and subsidiary pyroxenite and hornblendite. 25 % comprises calc-alkaline and alkaline plutons, respectively. Ultramafic plutons intersect gabbros and calc-alkaline plutons. Recent zircon U/Pb geochronology imply that SIP formed at 560-570 Ma, with mafic- and ultramafic rocks being emplaced in <4 Ma (Roberts et al., Geol. Mag, 2007). Geothermobarometry of contact metamorphic mineral assemblages, implies minimum depth of 20-30 kilometres. Accordingly, the Seiland province arguably provides a unique cross section through the deep-seated parts of a huge magmatic plumbing system. Sulphide Cu-Ni-(PGE) deposits are intimately associated with the ultramafic rock suite. One deposit from Stjernøy comprises sulphide dissiminations at the floor of a peridotitic pluton, another deposit occur at the floor of the Reinfjord ultramafic layered complex in the far West of SIP and the third deposit comprises vertical sulphide dykes in the interior of a hornblendite on the Øksfjord peninsula. Currently, only the Reinfjord deposit is studied in detail. The Reinfjord intrusions is layered and develops from olivine clinopyroxenites in the Lower Zone to wherlite in the Middle Zone to wehrlites and dunite in the Upper Zone. Earlier studies suggest parental melts with pyroxenitic compositions whereas the dunites and wherlites formed by fractional crystallization (Bennet et al., Bull. NGU, 405, 1-41). During our fieldwork we observed spectacular examples of cumulus structures, not previously reported, and including modally layered and modally graded dunite/wherlite, cross-bedding, slumping and mush-diapirs. Finally we saw an example of magma-replenishment where an olivine pyroxenitic magma was emplaced in to and mixed with the contemporary olivine/wherlite mushes!. The country rock gabbros were

Geology of the ultrabasic to basic Uitkomst complex, eastern Transvaal, South Africa: an overview

NASA Astrophysics Data System (ADS)

Gauert, C. D. K.; De Waal, S. A.; Wallmach, T.

1995-11-01

The Uitkomst complex in eastern Transvaal, South Africa, is a mineralized, layered ultrabasic to basic intrusion of Bushveld complex age (2.05-2.06 Ga) that intruded into the sedimentary rocks of the Lower Transvaal Supergroup. The complex is situated 20 km north of Badplaas. It is elongated in a northwesterly direction and is exposed over a total distance of 9 km. The intrusion is interpreted to have an anvil-shaped cross-section with a true thickness of approximately 800 m and is enveloped by metamorphosed and, in places, brecciated country rocks. Post-Bushveld diabase intrusions caused considerable vertical dilation of teh complex. The complex consists of six lithological units (from bottom to top): Basal Gabbro, Lower Harzburgite, Chromitiferous Harzburgite, Main Harzburgite, Pyroxenite and Gabbronorite. The Basal Gabbro Unit, developed at the base of the intrusion and showing a narrow chilled margin of 0.2 to 1.5 m against the floor rocks, has an average thickness of 6 m and grades upwards into the sulphide-rich and xenolith-bearing sequence of the Lower Harzburgite Unit. The latter unit averages 50 m in thickness and is gradationally overlain by the chromite-rich harzburgite of the Chromitiferous Harzburgite Unit (average thickness 60 m). Following on from the Chromitiferous Harzburgite Unit is the 330 m thick Main Harzburgite Unit. The Pyroxenite and Gabbronorite Units (total combined thickness of 310 m) form the uppermost formations of the intrusion. The three lower lithological units, Basal Gabbro to Chromitiferous Harzburgite, are highly altered by late magmatic, hydrothermal processes causing widespread serpentinization, steatitization, saussuritization and uralitization. Field relations, petrography and mineral and whole rock chemistry suggest the following sequence of events, The original emplacement of magma took place from northwest to southeast. The intrusion was bounded between two major fracture zones that gave rise to an elongated body, which

The reason for a Daly gap in magmatic series of large igneous provinces: geological and petrological evidences

NASA Astrophysics Data System (ADS)

Sharkov, Evgenii; Bogina, Maria; Chistyakov, Alexeii

2017-04-01

One of the most important problems of magmatic petrology over the past century is a «Daly Gap» [Daly, 1914]. It describes the lack of intermediate compositions (i.e., andesite, trachyandesite) in volcanic provinces like ocean islands, LIPs, & arcs, giving rise to "bimodal" basalt-rhyolite, basalt-trachyte or basanite-phonolite suites (Menzies, 2016). At the same time, the origin of the bimodal distribution still remains unclear. Among models proposed to explain the origin of the bimodal series are liquid immiscibility (Charlier et al 2011), physico-chemical specifics of melts (Mungal, Martin,1995), high water content in a primary melt (Melekhova et al., 2012), influence of latent heat production (Nelson et al., 2011), appearance of differentiated transitional chambers with hawaiites below and trachytes on top (Ferla et al., 2006), etc. In this case, the bimodal series are characterized by similar geochemical and isotopic-geochemical features of mafic and sialic members. At the same time, some bimodal series are produced by melting of sialic crust over basaltic chambers (Philpottas and Ague, 2009). This results in the essentially different isotopic characteristics of mafic and sialic members, as exemplified by the bimodal rapakivi granites-anorthosite complexes (Ramo, 1991; Sharkov, 2010). In addition, the bimodal basalt-trachyte series are widely spread in oceanic islands where sialic crust is absent. Thus, it is generally accepted that two contrasting melts were formed in magma chambers beneath volcanoes. Such chambers survived as intrusions and are available for geological study and deciphering their role in the formation of the bimodal magmatic series. We discuss this problem by the example of alkali Fe-Ti basalts and trachytes usually developed in LIPs. Transitional magmatic chambers of such series are represented by bimodal syenite-gabbro intrusions, in particular, by the Elet'ozero intrusion (2086±30 Ma) in Northern Karelia (Russia). The intrusion intruded

Magma chamber history related to the shield building stage of Piton des Neiges volcano, La Réunion Island

NASA Astrophysics Data System (ADS)

Berthod, Carole; Michon, Laurent; Famin, Vincent; Bascou, Jérôme; Bachelery, Patrick

2016-04-01

Piton des Neiges volcano (La Réunion hotspot) experienced a long-lasting shield building stage before entering its degenerative stage 0.4 my ago. The deep edifice incision due to the intense tropical erosion allowed the description for several decades of a layered gabbroic complex interpreted as a piece of magma chamber, which has been tectonically displaced (Chevallier & Vatin-Perignon, 1982; Upton & Wadsworth, 1972). Here, we combine field investigations, petrographic, mineralogical, geochemical and anisotropy of magnetic susceptibility (AMS) studies to constrain the spatial distribution of the plutonic complex, to identify the physical and chemical processes and to integrate this complex in the evolution of Piton des Neiges (PdN). Field investigations allowed us to discover three additional massifs of gabbro and peridotite along the Mât River. The four massifs are overlaid by a pile of basic sills and a breccia interpreted as a debris avalanche deposit. Albeit spatially disconnected, the massifs show a relatively constant dip of the magnetic foliation toward the current summit of the volcano (i.e. toward the SSE). The two massifs cropping in the upper Mât River are exclusively composed of massive dunite and wherlite units with a cumulate texture and no visible dynamic structures. The two massifs located in the lower Mât River are made of olivine-gabbro, ferrogabbro and gabbro showing numerous flow structures and synmagmatic faults that indicates instabilities which trend NNW-SSE. Minerals (olivine, clinopyroxene and oxide) present primitive compositions in the two upper massifs and slightly differentiated ones in the lower massif. Given the consistency of our dataset, we propose that the four massifs correspond to outcrops of a unique chemically stratified magma chamber, whose center would have been located about 4 km North of the current summit of PdN. The existence of an initial PdN, North of the current edifice, is supported by morphological

Mineralization related to Alvand pluton in the Hamadan, western Iran

NASA Astrophysics Data System (ADS)

Salehi, M. H.; Doosti, E. A.; Ahadnejad, V.

2009-04-01

The Alvand (Hamadan) plutonic batholith is one of the largest plutonic bodies in the Sanandaj-Sirjan metamorphic belt. This complex is consist of mafic part (gabbro-diorite-tonalite), intermediate (granite-granodiorite porphyroids), and hololeucocratic granitoids. Previous studies have shown that S-type granite-granodiorites are mostly peraluminous and calc-alkaline; the gabbro-diorite-tonalite suite is mostly metaluminous and tholeiitic to calc-alkaline (Sepahi, 2008). High initial 87Sr-86Sr ratios (0.7081 to 0.7115), low epsilon Nd values (-1.0 to -3.3), and peraluminous character reflects a different origin for the granites, possibly crustal sources (Ghalamghash et al, 2007). Aplite-pegmatite dikes are intruded in granitoide rocks, metamorphic rocks and the contact of Alvand granite with metamorphic rocks. The contact of Alvand granite with metamorphic rocks is sharp. By using heavy mineral studies on the alluvium of Alvand complex, it is recognized 28 minerals amongst Scheelite, Cassiterite, Ilmenite, Zircon and Garnet. Different geostatistical studies such as variant, bivariant and multivariant studies have been done on rough data of heavy minerals. They showed normal concentration of gold in studied rocks and low enrichment of tin and tungsten. The index of the ore elements average, frequeney distribution criteria of elements, the ratio of elements index and multielements show that Alvand granite is barren. Mineralography studies did not recognized any tin and tungsten minerals. The grains of gold was recognized in some of the microscopic thin sections. Calcopyrotite is the most important ore mineral that is accompanied with oxides and iron carbonates. The contacts of aplite-pegmatite dikes with granitoide rocks mostly are not prolific. For recognizing Scheelite, some samples of rocks studied by ultraviolet and few Scheelite is recognized in the samples. Some alteration zone observed in this area but they are not accompany with main mineralization. Although

One- and three-dimensional impact-induced tensional failure in rock

NASA Technical Reports Server (NTRS)

Ahrens, Thomas J.; Rubin, Allan M.

1991-01-01

Planar impact (10 to 25 m/sec) experiments were employed to induce dynamic tensile failure in Bedford limestone for stress durations of 0.5 and 1.3 microsec. Ultrasonic P- and S-wave velocities of recovered targets demonstrate deficits caused by microcracks at tensile stresses greater than 35 and 60 MPa in 1.3 and 0.5 microsec experiments. Three-dimensional impact experiments on 20 cm-sized blocks of Bedford limestone and San Marcos gabbro demonstrate compressional wave velocity deficits up to 30 percent in the vicinity of the crater. The damage microcrack decreases as r exp -1.56 from the crater, indicating a dependence on the magnitude and duration of the tensile hoop stresses associated with the impact-induced shock wave.

Chemistry of Diogenites and Evolution of their Parent Asteroid

NASA Technical Reports Server (NTRS)

Mittlefehldt, D.W.; Beck, A.W.; McSween, H.Y.; Lee, C-T A.

2009-01-01

Diogenites are orthopyroxenite meteorites [1]. Most are breccias, but remnant textures indicate they were originally coarse-grained rocks, with grain sizes of order of cm. Their petrography, and major and trace element chemistry support an origin as crustal cumulates from a differentiated asteroid. Diogenites are genetically related to the basaltic and cumulate-gabbro eucrites, and the polymict breccias known as howardites, collectively, the HED suite. Spectroscopic observations, orbit data and dynamical arguments strongly support the hypothesis that asteroid 4 Vesta is the parent object for HED meteorites [2]. Here we discuss our new trace element data for a suite of diogenites and integrate these into the body of literature data. We use the combined data set to discuss the petrologic evolution of diogenites and 4 Vesta.

Ocean plateau-seamount origin of basaltic rocks, Angayucham terrane, central Alaska

USGS Publications Warehouse

Barker, F.; Jones, D.L.; Budahn, J.R.; Coney, P.J.

1988-01-01

The Angayucham terrane of north-central Alaska (immediately S of the Brooks Range) is a large (ca. 500 km E-W), allochthonous complex of Devonian to Lower Jurassic pillow basalt, diabase sills, gabbro plutons, and chert. The mafic rocks are transitional normal-to-enriched, mid-ocean-ridge (MORB) type tholeiites (TiO2 1.2-3.4%, Nb 7-23 ppm, Ta 0.24-1.08 ppm, Zr 69-214 ppm, and light REE's slightly depleted to moderately enriched). Geologic and geochemical constraints indicate that Angayucham terrane is the upper "skin' (ca. 3-4 km thick) of a long-lived (ca. 170-200 ma) oceanic plateau whose basaltic-gabbroic rocks are like those of seamounts of the East Pacific Rise. -Authors

Competition and GDP Growth: The Mexican Solution

DTIC Science & Technology

2011-10-28

was to strengthen the functions of the CRE after the reorganization of Mexico’s oil company, Petroleos Mexicanos (Pemex). However, bureaucrats and...MX&metaid=609. 19 Moody’s Investor Service: Credit Opinion Petroleos Mexicanos, last modified December 23, 2009, http://www.ri.pemex.com/files...available for use, the company announced in May 2011 its largest discovery of oil in over two decades.27 Brazil’s Petroleo Brasileiro, or Petrobas

Metabolic and infectious pathologies in Brazilian medical literature: a review

PubMed Central

Rocha-e-Silva, Mauricio

2010-01-01

OBJECTIVE: This review of original reports on metabolic and infectious diseases that were recently published in Brazilian journals is designed to inform the readership of CLINICS about their content. METHODS: I conducted a search in PubMed for original research articles (clinical or basic research) recently published (2008–2009) by Brazilian medical and biological periodicals. Papers on metabolic pathologies were retrieved by searching for appropriate keywords such as metabolic syndrome and obesity. Papers on infectious disease were obtained by entering 15 different keywords for the most commonly occurring pathologies. Review articles, editorials, letters to the editor, and case reports were manually excluded. Selected titles were then categorized into appropriate sub-categories. RESULTS: This search produced a total of 123 articles, which filtered down to 72 articles after eliminating editorials, review articles, letters to the Editor and case reports. Reviewed periodicals were Arquivos Brasileiros de Cardiologia, Arquivos Brasileiros de Endocrinologia e Metabologia, Brazilian Journal of Biological and Medical Research, Brazilian Journal of Infectious Diseases, Jornal de Pediatria, Jornal de Pneumologia, Revista da Associação Médica Brasileira, Revista da Escola de Enfermagem da Universidade de São Paulo, and São Paulo Medical Journal. The articles were then briefly summarized. PMID:20835560

Typology of mafic-ultramafic complexes in Hoggar, Algeria: Implications for PGE, chromite and base-metal sulphide mineralisation

NASA Astrophysics Data System (ADS)

Augé, Thierry; Joubert, Marc; Bailly, Laurent

2012-02-01

With the aims to bring new information about the typology and mineral potential of mafic-ultramafic complexes of the Hoggar, detailed petrological and chemical characterisation were performed on serpentinite bands and layered intrusions. The serpentinite bands locally contain pods, layers and disseminations of chromite showing all the characteristics (mode of occurrence, composition, nature and composition of silicate inclusions, etc.) of an "ophiolite" chromite. Some chromite concentrations in the serpentinite bands also contain inclusions of platinum-group minerals (described for the first time in the Hoggar) such as ruarsite (RuAsS), an Os, Ru, Ir alloy, and complex Os, Ir, Ru sulfarsenides and arsenides. The serpentinite probably corresponds to remnants of oceanic lithosphere—more specifically from the upper part of the mantle sequence, generally where chromitite pods are most abundant, and the basal part of the cumulate series with stratiform chromite concentrations—and marks suture zones; the rest of the oceanic crust has not been preserved. Considering the typology of the serpentinites bands, their potential for precious- and base-metals is suspected to be low. Of the two layered mafic-ultramafic intrusions that were studied, the In Tedeini intrusion has a wehrlite core intruded by olivine gabbronorite and surrounded by an olivine gabbro aureole; three orthocumulate units, containing disseminated magmatic base-metal sulphides and with a plagioclase composition varying around An 58.1 and An 63.3, that could have been derived from a single magma. The East Laouni intrusion has a basal unit of olivine gabbronorite with specific silicate oxide intergrowths, and an upper unit of more differentiated gabbro, both units containing disseminated magmatic Ni-Cu sulphides indicative of early sulphide immiscibility; the mineral composition of these two cumulate units indicates that they also could have been derived from a single magmatic episode. The characteristic of

Cr diffusion in MgAl2O4 synthetic spinels: preliminary results

NASA Astrophysics Data System (ADS)

Freda, C.; Celata, B.; Andreozzi, G.; Perinelli, C.; Misiti, V.

2012-04-01

Chromian spinel is an accessory phase common in crustal and mantle rocks, including peridotites, gabbros and basalts. Spinel, it has been identified as one of the most effective, sensible, and versatile petrogenetic indicator in mafic and ultramafic rock systems due to the strict interdependence between its physico-chemical properties (chemical composition, cation configuration etc.) and genetic conditions (temperature, pressure, and chemical characteristics of the system). In particular, studies on intra- and inter-crystalline Mg-Fe2+, Cr-Al exchange demonstrated the close relationship between spinel composition and both degree of partial melting and equilibrium temperature of spinel-peridotites. Moreover, studies focused on the chemical zoning of Mg-Fe2+ and/or Cr-Al components in spinel have been used, combined with a diffusion model, to provide quantitative information on peridotites and gabbros pressure-temperature paths and on deformation mechanisms. Although these potentials, most of the experimental studies have been performed on spinels hosting a limited content of divalent iron (sensu stricto, MgAl2O4), whereas the scarce studies on Cr-Al inter-diffusion coefficient have been performed at 3-7 GPa as pressure boundary condition. In order to contribute to the understanding of processes occurring in the lithospheric mantle, we have initiated an experimental research project aiming at determining the Cr-Al inter-diffusion in spinel at 2 GPa pressure and temperature ranging from 1100 to 1250 °C. The experiments were performed in a end-loaded piston cylinder by using a 19 mm assembly and graphite-Pt double capsules. As starting materials we used synthetic Mg-Al spinel (200-300 μm in size) and Cr2O3 powder. Microanalyses of experimental charge were performed on polished carbon-coated mounts by electronic microprobe. Line elemental analyses were made perpendicular to the contact surface between Cr2O3 powder and spinel, at interval of 2 μm. By processing these

Paleocene Turbidite Deposition in the Central American Seaway (NW Costa Rica): Geochemical Analysis and Provenance of Detrital Spinel and Clinopyroxene

NASA Astrophysics Data System (ADS)

Giblin, A. C.

2015-12-01

The Central American Land Bridge is the crucial connection between North and South America, and the Miocene closure of the Panama seaway led to a change in global oceanic circulation patterns. Modern Costa Rica is part of the island arc that formed over the western Caribbean subduction zone, and the Santa Elena peninsula is on the northwest coast of Costa Rica next to the Sandino forearc basin. This study focuses on the origin and provenance of the Paleocene deep-water Rivas and Descartes turbidites that crop out on the northern part of the Santa Elena peninsula in northwestern Costa Rica. Understanding the sedimentary fill of the Sandino Basin that contributed to the closing of the seaway may lead to a better understanding of the Late Cretaceous-Paleogene arcs. Provenance studies of the Santa Elena Peninsula turbidite sandstone bodies constrain the history of the paleogeography and tectonics of the region. Petrographic analyses of rock thin sections constrain source areas; geochemical analysis of individual detrital heavy minerals from rock samples give indications of sediment sources and tectonic setting during deposition. This study is a provenance analysis based on (i) semi-quantitative energy-dispersive spectrometry analysis of heavy minerals, (ii) quantitative wavelength-dispersive spectrometry for major elements of detrital clinopyroxene and spinel grains, (iii) trace element analysis through laser ablation of single detrital clinopyroxene grains, and (iv) comparative analysis of the different potential source rocks to clearly identify the most likely sediment sources. The detrital spinel and clinopyroxene are possibly sourced from: mantle ophiolites, mid-ocean ridge gabbros, or volcanic arc tholeiitic basalts or calc-alkaline andesites. Spinel and clinopyroxne geochemistry suggests a possible peridotitic source, linked to mantle rocks that are now covered by Tertiary volcanics or have completely eroded. The character of the crustal minerals indicates

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.

Wide-Angle Refraction Tomographic Inversion of Mid Cayman Spreading Center and its Oceanic Core Complex, CaySEIS Experiment

NASA Astrophysics Data System (ADS)

Harding, J.; Van Avendonk, H. J.; Hayman, N. W.; Grevemeyer, I.; Peirce, C.; Dannowski, A.; Papenberg, C. A.

2015-12-01

The CaySEIS experiment, conducted in April 2015, is a multi-national collaborative seismic study of the Mid Cayman Spreading Center (MCSC), an ultra-slow spreading center [15 mm/yr fr] in the Caribbean Sea. Ultra-slow spreading centers are thought to have very thin crust and a paucity of magmatism due to cooler mantle conditions. However, the suggestion that gabbro-cored oceanic core complexes (OCCs), volcanic deposits, and multiple layers of hydrothermal vents are widespread in the MCSC and other ultra-slow spreading centers has led to questions about the relationship between seafloor spreading rates and magmatism. To investigate this further, we conducted the CaySEIS experiment, with five wide-angle seismic refraction lines parallel and perpendicular to the neovolcanic zone. This analysis is based on two east-west oriented 100-km-long seismic refraction lines, which were each occupied by 18 ocean bottom seismometers. Line 2 lies across the central MCSC and an OCC called Mt. Dent. Line 3 crosses the northern end of the MCSC near the Oriente Transform Zone. With the wide-angle OBS data we can image the seismic velocity structure of Mt. Dent and distinguish between two models of OCCs - either Mt. Dent is composed of mostly gabbro with peridotite lenses identified by a low velocity gradient, or it is composed of mostly peridotite with gabbroic bodies identified by a constant velocity gradient. The crustal structure of both lines gives more insight into the asymmetry of the MCSC and the style of seafloor spreading to the east vs. the west. The 2-D velocity models reveal Mt. Dent has thick crust of 8 km with a low velocity gradient, supporting the magmatic gabbroic origin of OCCs. The surrounding crust to the west of the MCSC is highly variable, with areas of very thin crust. The crust to the east of the MCSC has an approximately constant thickness of 4 km. The development of OCCs may contribute to the crustal heterogeneity of ultra-slow spreading centers.

Subcontinental rift initiation and ocean-continent transitional setting of the Dinarides and Vardar zone: Evidence from the Krivaja-Konjuh Massif, Bosnia and Herzegovina

NASA Astrophysics Data System (ADS)

Faul, Ulrich H.; Garapić, Gordana; Lugović, Boško

2014-08-01

The Dinaride and Vardar zone ophiolite belts extend from the south-eastern margins of the Alps to the Albanian and Greek ophiolites. Detailed sampling of the Krivaja-Konjuh massif, one of the largest massifs in the Dinaride belt, reveals fertile compositions and an extensive record of deformation at spinel peridotite facies conditions. High Na2O clinopyroxene and spinel-orthopyroxene symplectites after garnet indicate a relatively high pressure, subcontinental origin of the southern and western part of Krivaja, similar to orogenic massifs such as Lherz, Ronda and the Eastern Central Alpine peridotites. Clinopyroxene and spinel compositions from Konjuh show similarities with fertile abyssal peridotite. In the central parts of the massif the spinel lherzolites contain locally abundant patches of plagioclase, indicating impregnation by melt. The migrating melt was orthopyroxene undersaturated, locally converting the peridotites to massive olivine-rich troctolites. Massive gabbros and more evolved gabbro veins cross-cutting peridotites indicate continued melt production at depth. Overall we infer that the massif represents the onset of rifting and early stages of formation of a new ocean basin. In the south of Krivaja very localized chromitite occurrences indicate that much more depleted melts with supra-subduction affinity traversed the massif that have no genetic relationship with the peridotites. This indicates that volcanics with supra-subduction affinity at the margins of the Krivaja-Konjuh massif record separate processes during closure of the ocean basin. Comparison with published compositional data from other Balkan massifs shows that the range of compositions within the Krivaja-Konjuh massif is similar to the compositional range of the western massifs of the Dinarides. The compositions of the Balkan massifs show a west to east gradient, ranging from subcontinental on the western side of the Dinarides to depleted mid-ocean ridge/arc compositions in the Vardar

The Caribbean-Colombian cretaceous igneous province: The internal anatomy of an oceanic plateau

NASA Astrophysics Data System (ADS)

Kerr, Andrew C.; Tarney, John; Marriner, Giselle F.; Nivia, Alvaro; Saunders, Andrew D.

The Late Cretaceous Caribbean—Colombian igneous province is one of the world's best-exposed examples of a plume-derived oceanic plateau. The buoyancy of the plateau (resulting from residual heat and thick crust) kept it from being totally subducted as it moved eastward with the Farallon Plate from its site of generation in the eastern Pacific and encountered a destructive plate margin. In effect, the plateau makes up much of the Caribbean Plate; it is well exposed around its margins, but more so in accreted terranes in western Colombia (including the well-known Gorgona komatiites and Bolívar mafic/ultramafic cumulates). Compositionally, the lavas of the plateau form three groups: (a) basalts, picrites, and komatiites with light-rare-earth-element (LREE)-depleted chondrite-normalised patterns; (b) basalts with LREE-enriched patterns; and (c) basalts with essentially flat REE patterns (the most dominant type) similar to many of the basalts from the Ontong Java Plateau. These three types demonstrate the heterogeneous nature of the mantle plume source region. The picrites and the komatiites seem to lie nearer the base of the plateau than the more homogeneous basalts; thus, the more MgO-rich melts may have been erupted before large magma chambers had a chance to develop. A reconstructed crustal cross section through the plateau consists of dunitic and pyroxenitic cumulates near the base which are overlain by layered olivine-rich gabbros and more isotropic gabbros. The lowermost eruptive sequence comprises compositionally heterogeneous picrites/komatiites overlain by more homogeneous pillow basalts. Spectacular hornblende-plagioclase veins cut the Bolívar assemblage and these may represent local partial melts of the plateau's base as it was thrusted onto the continent. Subduction-related batholiths and extrusive rocks found around the margin of the province are of two distinct ages; one suite represents pre-plateau collision-related volcanism whereas the other suite

Tectonic implications of a paleomagnetic direction obtained from a Miocene dike swarm in central Honshu, Japan

NASA Astrophysics Data System (ADS)

Hoshi, H.; Sugisaki, Y.

2017-12-01

Central Honshu of Japan is an ideal field for the study of crustal deformation related to arc-arc collision. In this study we obtained rock magnetic and paleomagnetic results from early Miocene igneous rocks in central Honshu in order to examine rotational deformation caused by the collision of the Izu-Bonin-Mariana (IBM) arc with central Honshu. In Takane of the Hida region, gabbro intrusions and older sedimentary rocks are intruded by numerous andesitic dikes that comprise a parallel dike swarm. The dikes formed under two different normal-faulting paleostress conditions, which were suggested using a method of clustering dike orientations. Cross-cutting relationships indicate that the two paleostress conditions existed during the same period. More than 240 oriented cores were taken at 38 sites in two localities for magnetic study. The andesites and gabbros generally have magnetite, and some andesites also contain pyrrhotite. The magnetite records easterly deflected remanent magnetization directions of dual polarities that pass the reversals test. Positive baked contact tests at two sites demonstrate that the easterly deflected direction is a thermoremanent magnetization acquired at the time of intrusion. The overall in situ (i.e., in geographic coordinates) mean direction for andesitic dikes is judged to be highly reliable, although there are two possible scenarios for explaining the easterly deflection: (1) clockwise rotation and (2) tilting to the northwest. We prefer the former scenario and conclude that 45° clockwise rotation occurred in Takane with respect to the North China Block of the Asian continent. This rotation must represent the clockwise rotation of entire Southwest Japan during the opening period of the Japan Sea. Very little difference is observed between the amount of the easterly deflection in Takane and those in the Tokai and Hokuriku regions, indicating no significant relative rotation. Thus, the crust beneath Takane has not suffered rotation

Geochronology of plutonic rocks and their tectonic terranes in Glacier Bay National Park and Preserve, southeast Alaska: Chapter E in Studies by the U.S. Geological Survey in Alaska, 2008-2009

USGS Publications Warehouse

Brew, David A.; Tellier, Kathleen E.; Lanphere, Marvin A.; Nielsen, Diane C.; Smith, James G.; Sonnevil, Ronald A.

2014-01-01

We have identified six major belts and two nonbelt occurrences of plutonic rocks in Glacier Bay National Park and Preserve and characterized them on the basis of geologic mapping, igneous petrology, geochemistry, and isotopic dating. The six plutonic belts and two other occurrences are, from oldest to youngest: (1) Jurassic (201.6–145.5 Ma) diorite and gabbro of the Lituya belt; (2) Late Jurassic (161.0–145.5 Ma) leucotonalite in Johns Hopkins Inlet; (3) Early Cretaceous (145.5–99.6 Ma) granodiorite and tonalite of the Muir-Chichagof belt; (4) Paleocene tonalite in Johns Hopkins Inlet (65.5–55.8 Ma); (5) Eocene granodiorite of the Sanak-Baranof belt; (6) Eocene and Oligocene (55.8–23.0 Ma) granodiorite, quartz diorite, and granite of the Muir-Fairweather felsic-intermediate belt; (7) Eocene and Oligocene (55.8–23.0 Ma) layered gabbros of the Crillon-La Perouse mafic belt; and (8) Oligocene (33.9–23.0 Ma) quartz monzonite and quartz syenite of the Tkope belt. The rocks are further classified into 17 different combination age-compositional units; some younger belts are superimposed on older ones. Almost all these plutonic rocks are related to Cretaceous and Tertiary subduction events. The six major plutonic belts intrude the three southeast Alaska geographic subregions in Glacier Bay National Park and Preserve, from west to east: (1) the Coastal Islands, (2) the Tarr Inlet Suture Zone (which contains the Border Ranges Fault Zone), and (3) the Central Alexander Archipelago. Each subregion includes rocks assigned to one or more tectonic terranes. The various plutonic belts intrude different terranes in different subregions. In general, the Early Cretaceous plutons intrude rocks of the Alexander and Wrangellia terranes in the Central Alexander Archipelago subregion, and the Paleogene plutons intrude rocks of the Chugach, Alexander, and Wrangellia terranes in the Coastal Islands, Tarr Inlet Suture Zone, and Central Alexander Archipelago subregions.

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

Macente, A.; Fusseis, F.; Menegon, L.

Reaction and deformation microfabrics provide key information to understand the thermodynamic and kinetic controls of tectono-metamorphic processes, however they are usually analysed in two dimensions, omitting important information regarding the third spatial dimension. We applied synchrotron-based X-ray microtomography to document the evolution of a pristine olivine gabbro into a deformed omphacite-garnet eclogite in four dimensions, where the 4th dimension is represented by the degree of strain. In the investigated samples, which cover a strain gradient into a shear zone from the Western Gneiss Region (Norway), we focused on the spatial transformation of garnet coronas into elongated clusters of garnets withmore » increasing strain. Our microtomographic data allowed quantification of garnet volume, shape and spatial arrangement evolution with increasing strain. We combined microtomographic observations with light microscope- and backscatter electron images as well as electron microprobe- (EMPA) and electron backscatter diffraction (EBSD) analysis to correlate mineral composition and orientation data with the X-ray absorption signal of the same mineral grains. With increasing deformation, the garnet volume almost triples. In the low strain domain, garnets form a well interconnected large garnet aggregate that develops throughout the entire Page 1 of 52 sample. We also observed that garnet coronas in the gabbros never completely encapsulate olivine grains. In the most highly deformed eclogites, the oblate shapes of garnet clusters reflect a deformational origin of the microfabrics. We interpret the aligned garnet aggregates to direct synkinematic fluid flow and consequently influence the transport of dissolved chemical components. EBSD analyses reveal that garnet show a near-random crystal preferred orientation that testifies no evidence for crystal plasticity. There is, however evidence for minor fracturing, neo-nucleation and overgrowth. Microprobe chemical

Petrogenesis and tectonic implications of Triassic mafic complexes with MORB/OIB affinities from the western Garzê-Litang ophiolitic mélange, central Tibetan Plateau

NASA Astrophysics Data System (ADS)

Liu, Bin; Ma, Chang-Qian; Guo, Yu-Heng; Xiong, Fu-Hao; Guo, Pan; Zhang, Xin

2016-09-01

Although numerous Paleo-Tethyan ophiolites with mid-oceanic ridge basalts (MORB) and/or oceanic-island basalt (OIB) affinities have been reported in the central Tibetan Plateau (CTP), the origin and tectonic nature of these ophiolites are not well understood. The petrogenesis, mantle sources and geodynamic setting of the mafic rocks from these ophiolites are unclear, which is the main reason for this uncertainty. In this paper, we present new geochronological, mineralogical and Sr-Nd isotopic data for the Chayong and Xiewu mafic complexes in the western Garzê-Litang suture zone (GLS), a typical Paleo-Tethyan suture crossing the CTP. Zircon LA-ICP-MS U-Pb ages of 234 ± 3 Ma and 236 ± 2 Ma can be interpreted as formation times of the Chayong and Xiewu mafic complexes, respectively. The basalts and gabbros of the Chayong complex exhibit enriched MORB (E-MORB) compositional affinities except for a weak depletion of Nb, Ta and Ti relative to the primitive mantle, whereas the basalts and gabbros of the Xiewu complex display distinct E-MORB and OIB affinities. The geochemical features suggest a probable fractionation of olivine ± clinopyroxene ± plagioclase as well as insignificant crustal contamination. The geochemical and Sr-Nd isotopic data reveal that the Chayong mafic rocks may have been derived from depleted MORB-type mantle metasomatized by crustal components and Xiewu mafic rocks from enriched lithospheric mantle metasomatized by OIB-like components. The ratios of Zn/Fet, La/Yb and Sm/Yb indicate that these mafic melts were produced by the partial melting of garnet + minor spinel-bearing peridotite or spinel ± minor garnet-bearing peridotite. We propose that back-arc basin spreading associated with OIB/seamount recycling had occurred in the western GLS at least since the Middle Triassic times, and the decompression melting of the depleted MORB-type asthenosphere mantle and partial melting of sub-continental lithosphere were metasomatized by plume

Stress-Activated Electronic Charge Carriers in Igneous Rocks

NASA Astrophysics Data System (ADS)

Pan, C. T.; Jones, H. H.; Freund, F. T.

2006-12-01

Igneous rocks, when subjected to deviatory stress, turn into a battery. We report on gabbro (Shanxi, China) and anorthosite (Larvik, Norway). We use tiles, 30 x 30 x 0.9 cm3, and a pair of steel pistons, 4.4 cm diameter to subject a small off-center volume of ~10 cm3 to 10 MPa, about 5% failure strength. Instantly upon loading, two types of electronic charge carriers are activated in the stressed rock volume and a current begins to flow. One current leg is carried by holes, which flow from the stressed through the unstressed rock to the edges of the tile. The other current leg is carried by electrons, which flow from the stressed rock into the steel pistons and through the external wire to the edge, where they meet the holes. We have measured the impedance of the gabbro and anorthosite over the frequency range from <1 Hz to 10 MHz. We measured the impedence across the 10 cm3 volume between the two pistons and a similar volume outside the pistons in the path of the holes flowing to the edges of the tile: (1) before loading, (2) during loading. We obtain thus information about both types of charge carriers, electrons and holes. Both are associated with oxygen anions that changed their valence from 2- to 1- (peroxy). An O- among O2- represents a defect electron in the O2- sublattice, known as positive hole or p-hole for short. In unstressed rocks the O- exist in an electrically inactive form as O- pairs, chemically equivalent to peroxy links, O3X-OO-XO3 with X = Si4+, Al3+ etc. Stresses cause the peroxy links to break, allowing electrons from neighboring O2- to jump in and p-holes to jump out. The p-holes can spread through unstressed rocks using energy levels at the upper edge of the valence band.

Hayward Fault rocks: porosity, density, and strength measurements

USGS Publications Warehouse

Morrow, C.A.; Lockner, D.A.

2001-01-01

Porosity, density and strength measurements were conducted on rock samples collected from the Hayward Fault region in Northern California as part of the Hayward Fault Working Group’s efforts to create a working model of the Hayward Fault. The rocks included in this study were both fine and coarse grained gabbros, altered keratophyre, basalt, sandstone, and serpentinite from various rock formations adjacent to the Hayward Fault. Densities ranged from a low of 2.25 gm/cc (altered keratophyre) to 3.05 gm/cc (fine gabbro), with an average of 2.6 gm/cc, typical of many other rocks. Porosities were generally around 1% or less, with the exception of the sandstone (7.6%) and altered keratophyre (13.5%). Failure and frictional sliding tests were conducted on intact rock cylinders at room temperature under effective pressure conditions of up to 192 MPa, simulating depths of burial to 12 km. Axial shortening of the samples progressed at a rate of 0.1 µm/sec (fine samples) or 0.2 µm/sec (porous samples) for 6 mm of displacement. Velocity stepping tests were then conducted for an additional 2 mm of displacement, for a total of 8 mm. Both peak strength (usually failure strength) and frictional strength, determined at 8 mm of displacement, increased systematically with effective pressure. Coefficients of friction, based on the observed fracture angles, ranged from 0.6 to 0.85, consistent with Byerlee’s Law. Possible secondary influences on the strength of the Hayward rock samples may be surface weathering, or a larger number of pre-existing fractures due to the proximity to the Hayward Fault. All samples showed velocity strengthening, so that the average a-b values were all strongly positive. There was no systematic relation between a-b values and effective pressure. Velocity strengthening behavior is associated with stable sliding (creep), as observed in the shallow portions of the Hayward Fault.

Electron-microprobe study of chromitites associated with alpine ultramafic complexes and some genetic implications

USGS Publications Warehouse

Bird, M.L.

1978-01-01

Electron-microprobe and petrographic studies of alpine chromite deposits from around the world demonstrate that they are bimodal with respect to the chromic oxide content of their chromite. The two modes occur at 54 ? 4 and 37 ? 3 weight per cent chromic oxide corresponding to chromite designated as high-chromium and high-aluminum chromite respectively. The high-chromium chromite occurs exclusively with highly magnesian olivine (Fo92-97) and some interstitial diopside. The high-aluminum chromite is associated with more ferrous olivine (Fo88-92), diopside, enstatite, and feldspar. The plot of the mole ratios Cr/(Cr+Al+Fe3+) vs. Mg/(Mg+Fe2+) usually presented for alpine chromite is shown to have a high-chromium, high-iron to low-chromium, low-iron trend contrary to that shown by stratiform chromite. This trend is characteristic of alpine type chromite and is termed the alpine trend. However, a trend similar to that for startiform chromite is discernable on the graph for the high-chromium chromite data. This latter trend is well-developed at Red Mountain, Seldovia, Alaska. Analysis of the iron-magnesium distribution coefficient, Kd=(Fe/Mg)ol/(Fe/Mg)ch, between olivine and chromite shows that Kd for the high-chromium chromite from all ultramafic complexes has essentially the same constant value of .05 while the distribution coefficient for the high-aluminum chromite varies with composition of the chromite. These distribution coefficients are also characteristic of alpine-type chromites. The constant value for Kd for the high-chromium chromite and associated high-magnesium olivine in all alpine complexes suggests that they all crystallized under similar physico-chemical conditions. The two types of massive chromite and their associations of silicate minerals suggest the possibility of two populations with different origins. Recrystallization textures associated with the high-aluminum chromite together with field relationships between the gabbro and the chromite pods

Redistribution of iron and titanium in subduction zones: insights from high-pressure serpentinites

NASA Astrophysics Data System (ADS)

Crossley, Rosalind; Evans, Katy; Reddy, Steven; Lester, Gregory

2017-04-01

The redox state, quantity and composition of subduction zone fluids influence the transport and precipitation of elements including those which are redox-sensitive, of economic importance such as Cu, Au and Ag, and those considered to be immobile, which include Fe3+. However, subduction zone fluids remain poorly understood. The redox state of Fe in high-pressure ultramafic rocks, which host a significant proportion of Fe3+, can be used to provide an insight into Fe cycling and constrain the composition of subduction zone fluids. In this work, a combination of the study of oxide and silicate mineral textures, interpretation of mineral parageneses, mineral composition data, and the whole rock geochemistry of high-pressure retrogressed ultramafic rocks from the Zermatt-Saas Zone constrains the distribution of iron and titanium, and oxidation state of iron, to provide constraints on fluids at depth in subduction zones. Oxide minerals host the bulk of the iron, particularly Fe3+. The increase in mode of magnetite during initial retrogression is most consistent with oxidation of existing iron within the samples during the infiltration of an oxidising fluid since it is difficult to reconcile addition of Fe3+ with the known limited solubility of this species. These fluids may be sourced from hybrid samples and/or serpentinites at greater depths. However, high Ti contents are not typical of serpentinites and additionally cannot be accounted for by simple mixing of a depleted mantle protolith with the nearby Allalin gabbro. Titanium-rich samples are suggested to result from fluid-facilitated hybridisation of gabbro and serpentinite protoliths prior to peak metamorphism, and provides the tantalising possibility that Ti, an element generally perceived as immobile, has been added to the rock during this process. If Ti addition has occurred, then the introduction of Fe3+, also generally considered to be immobile, cannot be disregarded. Aluminosilicate complexing could provide a

Geology of the Ishmas gold district, Kingdom of Saudi Arabia

USGS Publications Warehouse

Doebrich, Jeff L.; White, Willis M.

1991-01-01

The Ishmas gold district was mapped at 1:25,000 scale to place auriferous mineralization into geologic perspective, to assist in creating an ore-deposit model, and to aid in devising a strategy for subsequent exploration elsewhere in the Jabal Ishmas-Wadi Tathlith gold belt. The precratonic evolution of the district began with the deposition of a tholeiitic mafic volcanic and volcaniclastic sequence that was intruded by diabase and gabbro. Basaltic to rhyodacitic volcanism following a calc-alkaline evolutionary trend ensued. Subsequent deposition of a thick wacke and sandstone unit represented the final phase in the volcano sedimentary accumulation. The emplacement of a large lopolithic layered-gabbro complex marked the end of the precratonic evolutionary cycle. The district coincides with the boundary of two allochthonous terranes. The collisional Nabitah orogeny represents the suturing of the two terranes. The effects of this event are manifested by numerous north-trending, steeply dipping faults, shear zones, and mylonite belts, as well as diapiric serpentinite. During the waning stages of the orogeny, auriferous quartz pods were precipitated in dilatant structures within the north-trending shear zones by deep-seated circulating fluids. The emplacement of a tonalite stock was closely followed by the formation of N. 20°-35° W. -trending faults. These faults influenced the emplacement of dacite porphyry stocks and associated auriferous quartz veins. The auriferous veins are massive, tabular open-fracture fillings that are spatially, temporally, and genetically related to the dacite porphyry. The emplacement of a quartz monzodiorite stock was responsible for additional auriferous quartz vein mineralization that is almost exclusively hosted by the intrusion. A nearly random orientation of the veins indicates that no regional structure influenced their formation. The formation of a series of N. 60°-80° W -trending faults represents the final episode in the

Ferrobasalt-rhyolite immiscibility in tholeiitic volcanic and plutonic series (Invited)

NASA Astrophysics Data System (ADS)

Charlier, B.; Namur, O.; Kamenetsky, V. S.; Grove, T. L.

2013-12-01

One atmosphere experiments show that silicate liquid immiscibility develops between Fe-rich and Si-rich melts below 1000-1020°C in compositionally diverse lavas that represent classical tholeiitic trends, such as Mull, Iceland, Snake River Plain and Sept Iles. Extreme iron enrichment along the evolution trend is not necessary; immiscibility also develops during iron depletion and silica enrichment after Fe-Ti oxide saturation. Natural liquid lines of descent for major tholeiitic series also approach or intersect the experimentally-defined compositional space of immiscibility. The importance of ferrobasalt-rhyolite unmixing in both volcanic and plutonic environments is supported by worldwide occurrence of immiscible globules in the mesostasis of erupted basalts, and by unmixed melt inclusions in cumulus phases of major layered intrusions such as Sept Iles, Skaergaard and Sudbury. A clear case of liquid immiscibility is also recorded in intrusive tholeiitic gabbros from the Siberian Large Igneous Province and is evidenced by textures and compositions of millimeter-sized silicate melt pools trapped in native iron. An important implication of immiscibility in natural ferrobasaltic provinces is the development of a compositional gap characterized by the absence of intermediate compositions, a major feature observed in many tholeiitic provinces and referred to as the Daly gap. The compositions of experimental silica-rich immiscible melts coincide with those of natural rhyolites with high FeOtot and low Al2O3, which suggests a potential role for large-scale immiscibility in the petrogenesis of late-stage ferroan silicic melts. No evidence for the paired ferrobasaltic melt is observed in volcanic provinces, probably because of its uneruptable characteristics. Instead, Fe-Ti×P-rich gabbros crystallized at depth and are the cumulate products of immiscible Fe-rich melts in plutonic settings, a feature clearly evidenced in the Sept Iles intrusion. The production of

Violence in Brazil and Its Connection with Transnational Crime

DTIC Science & Technology

2003-04-07

factors on violent behavior is research accomplished by the Núcleo de Estudos da Violência (Center of Studies of Violence) at the Universidade de São...53.98 Rio de Janeiro- RJ 52.68 Source: Almanaque Abril 2002 9 - Aggravation of social differences: one of the largest and most sensitive issues in the...for the worst distribution of income, after Swaziland, Nicaragua and South Africa. According to data published by Instituto Brasileiro de Geografia e

Exposure of a late cretaceous layered mafic-felsic magma system in the central Sierra Nevada batholith, California

USGS Publications Warehouse

Coleman, D.S.; Glazner, A.F.; Miller, J.S.; Bradford, K.J.; Frost, T.P.; Joye, J.L.; Bachl, C.A.

1995-01-01

New U-Pb zircon ages for the Lamarck Granodiorite, associated synplutonic gabbro and diorite plutons, and two large mafic intrusive complexes that underlie them in the Sierra Nevada batholith are 92??1 Ma. These ages establish the Late Cretaceous as a period of extensive mafic-felsic magmatism in the central part of the batholith, and confirm the significance of mafic magmatism in the evolution of the voluminous silicic plutions in the Sierran arc. The lack of significant zircon inheritance in any of the units analyzed supports isotopic evidence that the Lamarck and other Late Cretaceous Sierran plutons were derived predominantly from young crust. Recognition of an extensive mafic-felsic magma system in the Sierra Nevada batholith emphasizes the importance of basaltic liquids in the evolution of continental crust in arc settings. ?? 1995 Springer-Verlag.

Ultra sound absorption measurements in rock samples at low temperatures

NASA Technical Reports Server (NTRS)

Herminghaus, C.; Berckhemer, H.

1974-01-01

A new technique, comparable with the reverberation method in room acoustics, is described. It allows Q-measurements at rock samples of arbitrary shape in the frequency range of 50 to 600 kHz in vacuum (.1 mtorr) and at low temperatures (+20 to -180 C). The method was developed in particular to investigate rock samples under lunar conditions. Ultrasound absorption has been measured at volcanics, breccia, gabbros, feldspar and quartz of different grain size and texture yielding the following results: evacuation raises Q mainly through lowering the humidity in the rock. In a dry compact rock, the effect of evacuation is small. With decreasing temperature, Q generally increases. Between +20 and -30 C, Q does not change much. With further decrease of temperature in many cases distinct anomalies appear, where Q becomes frequency dependent.

Global petrologic variations on the moon: a ternary-diagram approach.

USGS Publications Warehouse

Davis, P.A.; Spudis, P.D.

1987-01-01

A ternary-diagram approach for determination of global petrologic variations on the lunar surface is presented that incorporates valuable improvements in our previous method of using geochemical variation diagrams. Our results are as follows: 1) the highlands contain large areas of relatively pure ferroan anorthosite; 2) the average composition of the upper lunar crust is represented by an 'anorthositic gabbro' composition; 3) KREEP/Mg-suite rocks are a minor fraction of the upper lunar crust; 4) within the farside highlands, areas of KREEP/Mg-suite rocks coincide mostly with areas of crustal thinning; 5) portions of the E limb and farside highlands have considerable amounts of a mafic, chondritic Th/Ti component (like mare basalt) whose occurrences coincide with mapped concentrations of light plains that display dark-halo craters.- from Authors

Compositional stratigraphy of crustal material from near-infrared spectra

NASA Technical Reports Server (NTRS)

Pieters, Carle M.

1987-01-01

An Earth-based telescopic program to acquire near-infrared spectra of freshly exposed lunar material now contains data for 17 large impact craters with central peaks. Noritic, gabbroic, anorthositic and troctolitic rock types can be distinguished for areas within these large craters from characteristic absorptions in individual spectra of their walls and central peaks. Norites dominate the upper lunar crust while the deeper crustal zones also contain significant amounts of gabbros and anorthosites. Data for material associated with large craters indicate that not only is the lunar crust highly heterogeneous across the nearside, but that the compositional stratigraphy of the lunar crust is nonuniform. Crustal complexity should be expected for other planetary bodies, which should be studied using high spatial and spectral resolution data in and around large impact craters.

Geochemical recognition of a captured back-arc basin metabasaltic complex, southwestern Oregon

USGS Publications Warehouse

Donato, M.M.

1991-01-01

An extensive fault-bounded amphibolite terrane of Late Jurassic (145 ?? 2 Ma) metamorphic age occurring in the northeastern Klamath Mountains of southern Oregon has been recognized as the remnants of an ancient back-arc basin. In spite of thorough metamorphic recrystallization under amphibolite-facies conditions, the amphibolite locally displays relict igneous textures which suggest that the protoliths included basaltic dikes or sills, shallow diabase intrusions, and gabbros. The geochemical data, together with the present-day geologic context, indicate that the tectonic setting of eruption/intrusion was probably within a back-arc basin that existed inboard (east) of a pre-Nevadan volcanic arc. The basalt (now amphibolite) and the overlying sediments (now the May Creek Schist) were metamorphosed and deformed during accretion to North America during the Late Jurassic Nevadan orogeny. -from Author

Multiple magmatism in an evolving suprasubduction zone mantle wedge: The case of the composite mafic-ultramafic complex of Gaositai, North China Craton

NASA Astrophysics Data System (ADS)

Yang, Fan; Santosh, M.; Tsunogae, T.; Tang, Li; Teng, Xueming

2017-07-01

The suprasubduction zone mantle wedge of active convergent margins is impregnated by melts and fluids leading to the formation of a variety of magmatic and metasomatic rock suites. Here we investigate a composite mafic-ultramafic intrusion at Gaositai, in the northern margin of the North China Craton (NCC). The hornblende gabbro-serpentinite-dunite-pyroxenite-gabbro-diorite suite surrounded by hornblendites of this complex has long been considered to represent an "Alaskan-type" zoned pluton. We present petrologic, mineral chemical, geochemical and zircon U-Pb and Lu-Hf data from the various rock types from Gaositai including hornblende gabbro, serpentinite, dunite, pyroxenite, diorite and the basement hornblendite which reveal the case of multiple melt generation and melt-peridotite interaction. Our new mineral chemical data from the mafic-ultramafic suite exclude an "Alaskan-type" affinity, and the bulk geochemical features are consistent with subduction-related magmatism with enrichment of LILE (K, Rb, and Ba) and LREE (La and Ce), and depletion of HFSE (Nb, Ta, Zr, and Hf) and HREE. Zircon U-Pb geochronology reveals that the hornblendites surrounding the Gaositai complex are nearly 2 billion years older than the intrusive complex and yield early Paleoproterozoic emplacement ages (2433-2460 Ma), followed by late Paleoproterozoic metamorphism (1897 Ma). The serpentinites trace the history of a long-lived and replenished ancient sub-continental lithospheric mantle with the oldest zircon population dated as 2479 Ma and 1896 Ma, closely corresponding with the ages obtained from the basement rock, followed by Neoproterozoic and Phanerozoic zircon growth. The oldest member in the Gaositai composite intrusion is the dunite that yields emplacement age of 755 Ma, followed by pyroxenite formed through the interaction of slab melt and wedge mantle peridotite at 401 Ma. All the rock suites also carry multiple population of younger zircons ranging in age from Paleozoic to

Groundwater quality in an abandoned metal extraction site: the case study of Campello Monti (NW Italy)

NASA Astrophysics Data System (ADS)

Mehta, Neha; Lasagna, Manuela; Antonella Dino, Giovanna; De Luca, Domenico Antonio

2017-04-01

Extractive activities present threat to natural water systems and their effects are observed even after the cessation of activities. The harmful effects of extractive activities such as deterioration of water sources by low quality waters or by allowing leaching of metals into groundwater makes it necessary to carry out careful, scientific and comprehensive studies on this subject. Consequently, the same problem statement was chosen as part of a PhD research Project. The PhD research is part of REMEDIATE project (A Marie Sklodowska-Curie Action Initial Training Network for Improved decision making in contaminated land site investigation and risk assessment, Grant Agreement No. 643087). The current work thus points out on the contamination of groundwater sources due to past mining activities in the area. Contaminated groundwater may act as possible contamination source to surface water also. The impacts on water systems connected to mining activities depend on the ore type, metal being extracted, exploitation method, ore processing, pollution control efforts, geochemical and hydrogeochemical conditions of water and surroundings. To evaluate the effects posed by past metal extracting activities the study was carried out at an abandoned site used for extracting nickel in Campello Monti (Valstrona municipality, Piedmont region, Italy). Campello Monti is located in basement of Southern Italian Alps in the Ivrea Verbano Zone. The area is composed of mafic rocks intruded by mantle periodite. The mafic formation consists of peridotites, pyroxenites, gabbros, anorthosites, gabbro-norite, gabbro-diorite and diorite. Mines were used for nickel exploitation from 9th Century and continued until 1940s. The long history of nickel extraction has left the waste contaminated with Ni and Co in the mountains alongwith tunnels used for carrying out metal extracting activities. The area around the site is used for housing, shows the presence of domestic animals and has Strona creek

Diversity of insect galls associated with coastal shrub vegetation in Rio de Janeiro, Brazil.

PubMed

Carvalho-Fernandes, Sheila P; Ascendino, Sharlene; Maia, Valéria C; Couri, Márcia S

2016-09-01

Surveys in the coastal sandy plains (restingas) of Rio de Janeiro have shown a great richness of galls. We investigated the galling insects in two preserved restingas areas of Rio de Janeiro state: Parque Estadual da Costa do Sol and Reserva Particular do Patrimônio Natural Fazenda Caruara. The collections were done each two months, from June 2011 to May 2012. We investigated 38 points during 45 minutes each per collection. The galls were taken to the laboratory for rearing the insects. A total number of 151 insect galls were found in 82 plant species distributed into 34 botanic families. Most of the galls occurred on leaves and the plant families with the highest richness of galls were Myrtaceae and Fabaceae. All the six insect orders with galling species were found in this survey, where Cecidomyiidae (Diptera) was the main galler group. Hymenoptera and Thysanoptera were found as parasitoids and inquilines in 29 galls. The richness of galls in the surveyed areas reveals the importance of restinga for the composition and diversity of gall-inducing insect fauna.

The use of polliniferous resources by Melipona capixaba, an endangered stingless bee species.

PubMed

Serra, Bruna Danielle Vieira; da Luz, Cynthia Fernandes Pinto; Campos, Lucio Antonio de Oliveira

2012-01-01

Abstract Pollen types present in samples from corbiculae of Melipona capixaba (Moure and Camargo) (Hymenoptera: Apidae: Meliponina) worker bees were analyzed, as well as pollen samples from food pots inside the hives in three sites located at the bees' original habitat. The aim was to find out the sources used as a trophic resource by this species. The dominant pollen grains in the spectrum of the samples belonged to the families Myrtaceae and Melastomataceae. Eucalyptus was the most frequent pollen type in the corbiculae in Conceição do Castelo municipality; Eucalyptus, Myrcia, and Melastomatacea/Combretaceae in the Fazenda do Estado district; and Eucalyptus and Myrcia in the São Paulo de Aracê district, both in the Domingos Martins municipality. Eucalyptus and Melastomataceae/Combretaceae were the predominant pollen types in the food pots. Eucalyptus was the most prevalent type all year round or most of the year. The most common pollen types in the months that Eucalyptus was not present or dominant in the samples were of remaining native forest species, "ruderal" (field) plants, fruit-bearing plants, and introduced ornamental plants.

Gender differences in the field of sexology: new contexts and old definitions.

PubMed

Rohden, Fabiola; Russo, Jane

2011-08-01

To analyze concepts of gender and sexuality present in the field of sex therapeutic interventions. An ethnographic observation was conducted, and it was completed with the analysis of printed material originated from the X Congresso Brasileiro de Sexualidade Humana (10th Brazilian Congress on Human Sexuality), promoted by the Sociedade Brasileira de Estudos em Sexualidade Humana (Brazilian Society of Human Sexuality Studies), and the VIII Congresso Brasileiro sobre Inadequações Sexuais (8th Brazilian Congress on Sexual Inadequacies), promoted by the Associação Brasileira para o Estudo das Inadequações Sexuais (Brazilian Association of Sexual Inadequacy Studies), both held in 2005. The analysis emphasized the interaction between the quantitative perspective in the processing of the variables (participants' gender and profession and lecture topics) and the qualitative perspective in the analysis and interpretation of the more general set of data. The topics of sessions and focus of presentations suggest that the field is divided by the contrast between two medical specialties: gynecology and urology, the former is aimed at female and couple dysfunctions, while the latter is aimed at male dysfunctions. Male sexuality is approached from the predominantly biomedical perspective, centered on the physiology of erection and drug prescription, whereas female sexuality is considered to be conditioned by relationship problems, when psychological intervention is more adequate.

A Astronomia no Ensino Superior Brasileiro entre 1808 e 1889

NASA Astrophysics Data System (ADS)

Bretones, Paulo S.; Videira, Antonio A. P.

2003-05-01

Este artigo apresenta os principais eventos ocorridos na história do ensino de astronomia nos cursos superiores que existiram no Brasil desde a chegada da Família Real portuguesa em 1808 até o final do período monárquico. Para compor esse esboço histórico, utilizamos, principalmente, livros didáticos, regulamentos, decretos e leis responsáveis pelas organizações dos conteúdos oferecidos e das carreiras dos responsáveis pela disciplina. Na análise do material empregado, investigamos a presença de concepções filosóficas e científicas, que podem ter norteado os conteúdos disciplinares. Não realizamos nenhuma comparação com o ensino de astronomia em outros países. Concluímos mostrando que o estudo de astronomia, durante o período monárquico, foi mais direcionado para a formação d engenheiros do que astrônomos. Gostaríamos de observar que o presente artigo não tem a pretensão de abordar o assunto de maneira completa e detalhada.

Petrology of Aztec Wash pluton, Eldorado Mountains, southern Nevada

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

Falkner, C.M.; Miller, C.F.; Wooden, J.L.

1993-04-01

Aztec Wash pluton, a 50 km[sup 2] intrusive complex in the northern Eldorado Mountains, was emplaced ca. 16 Ma (Faulds et al., 1990) during extension within the Colorado River Corridor. The pluton displays extreme compositional variability, ranging from olivine gabbro (ca. 50 wt% SiO[sub 2]) to highly evolved aplite (76% SiO[sub 2]). Most of the intrusion is medium grained, homogeneous granite (ca. 72% SiO[sub 2]), but 1/3 is highly heterogeneous and dominated by mafic to intermediate rocks; a 6 [times] 3km, N-S mafic zone almost bisects the pluton. Well-displayed magma mingling and late mafic and felsic dikes verify the coexistencemore » of mafic and felsic melts. Hornblende barometry indicates that the entire exposed portion of Aztec Wash pluton was emplaced at very shallow depth (« less

A survey of the selenochemistry of major, minor and trace elements.

NASA Technical Reports Server (NTRS)

Schmitt, R. A.; Laul, J. C.

1973-01-01

Average data for igneous and/or metaigneous rocks and soils from seven lunar sites are presented. There are compositional similarities between Apollo 11 and Luna 16 eastern maria, Ap 12 and 15 western maria, and between Ap 16 and L 20 highlands. Subtle differences do exist between the paired mare sites and the two highland sites and striking differences between the eastern and western maria. Chondritic normalized REE (rare earth element) patterns for igneous rocks and soils from all sites range from 7-350 generally with negative Eu anomalies. Anorthositic gabbroes to anorthosites, presumably highland material, exhibit a positive Eu anomaly. The REE patterns or Sr isotopic ratios suggest two lava flows each for the L 16 and Ap 14 sites, at least four lava flows for the Ap 11 and 12 site and about six for the Ap 15 site.

The Apollo 17 'melt sheet' - Chemistry, age and Rb/Sr systematics

NASA Technical Reports Server (NTRS)

Winzer, S. R.; Nava, D. F.; Schuhmann, S.; Philpotts, J. A.; Schuhmann, P. J.; Lum, R. K. L.; Lindstrom, M. M.; Lindstrom, D. J.

1977-01-01

Major, minor, and trace-element compositions, age data, and Rb/Sr systematics of Apollo 17 boulders have been compiled, and additional analyses performed on a norite breccia clast (77215) included in the Apollo 17, Station 7 boulder. The Apollo 17 boulders are found to be identical or nearly so in major, minor, and trace-element composition, suggesting that they all originated as an impact melt analogous to melt sheets found in larger terrestrial craters. The matrix dates (Ar-40/Ar-39) and Rb/Sr systematics available suggest that this impact melt formed by a single impact about 4 billion years ago. This impact excavated, shocked, brecciated, and melted norites, norite cumulates, and possibly anorthositic gabbros and dunites about 4.4 billion years old. The impact was likely a major one, possibly the Serenitatis basin-forming event.

Magmatic history of Red Sea rifting: perspective from the central Saudi Arabian coastal plain.

USGS Publications Warehouse

Pallister, J.S.

1987-01-01

An early stage of magmatism related to Red Sea rifting is recorded by a Tertiary dyke complex and comagmatic volcanic rocks exposed on the central Saudi Arabian coastal plain. Field relations and new K/Ar dates indicate episodic magmatism from approx 30 m.y. to the present day and rift-related magmatism as early as 50 m.y. Localized volcanism and sheeted dyke injection ceased at approx 20 m.y. and were replaced by the intrusion of thick gabbro dykes, marking the onset of sea-floor spreading in the central Red Sea. Differences in the depths and dynamics of mantle-melt extraction and transport may account for the transition from mixed alkaline-subalkaline bimodal magmatism of the pre-20 m.y. rift basin to exclusively subalkaline (tholeiitic) magmatism of the Red Sea spreading axis and the alkali basalt volcanism inland.-L.C.H.

River Valley pluton, Ontario - A late-Archean/early-Proterozoic anorthositic intrusion in the Grenville Province

NASA Technical Reports Server (NTRS)

Ashwal, Lewis D.; Wooden, Joseph L.

1989-01-01

This paper presents Nd, Sr, and Pb isotopic data indicating a late-Archean/early-Proterozoic age for the River Valley anorthositic pluton of the southwestern Grenville Province of Sudbury, Ontario. Pb-Pb isotopic data on 10 whole-rock samples ranging in composition from anorthosite to gabbro yield an age of 2560 + or - 155 Ma. The River Valley pluton is thus the oldest anorthositic intrusive yet recognized within the Grenville Province. The Sm-Nd isotopic system records an age of 2377 + or - 68 Ma. High Pb-208/Pb-204 of deformed samples relative to igneous-textured rocks implies Th introduction and/or U loss during metamorphism in the River Valley area. Rb-Sr data from igneous-textured and deformed samples and from mineral separates give an age of 2185 + or - 105 Ma, indicating substantial disturbance of the Rb-Sr isotopic system.

Inmetro - Instituto Nacional de Metrologia, Qualidade e Tecnologia

Science.gov Websites

Consumidor Laboratórios Legislação Comunicação Social Programa de Monitoramento BPL Produtos analisados de selos Indique! Sugestão para o Programa de Análise de Produtos Licitações Material de Portal do Governo Brasileiro mdic Carta de Serviços Inmetro nos Estados Acre Alagoas AmapÃ

Carboniferous continental arc in the Hegenshan accretionary belt: Constrains from plutonic complex in central Inner Mongolia

NASA Astrophysics Data System (ADS)

Wei, Ruihua; Gao, Yongfeng; Xu, Shengchuan; Santosh, M.; Xin, Houtian; Zhang, Zhenmin; Li, Weilong; Liu, Yafang

2018-05-01

The architecture and tectonic evolution of the Hegenshan accretionary belt in the Central Asian Orogenic Belt (CAOB) remains debated. Here we present an integrated study of zircon U-Pb isotopic ages, whole rock major-trace elements, and Sr-Nd-Pb isotopic data from the Hegenshan volcanic-plutonic belt in central Inner Mongolia. Field observations and zircon U-Pb ages allow us to divide the intrusive complex into an early phase at 329-306 Ma and a late phase at 304 to 299 Ma. The intrusive bodies belong to two magma series: calc-alkaline rocks with I-type affinity and A-type granites. The early intrusions are composed of granodiorite, monzogranite and porphyritic granite, and the late calc-alkaline intrusions include gabbro though diorite to granodiorite. The calc-alkaline intrusive rocks exhibit a well-defined compositional trend from gabbro to granite, reflecting continuous fractional crystallization. These rocks show obvious enrichment in LILEs and LREEs and relative depletion of HFSEs, typical of subduction-related magma. They also exhibit isotopic characteristics of mantle-derived magmas such as low initial 87Sr/86Sr (0.7029-0.7053), positive ɛNd(t) values (0.06-4.76) and low radiogenic Pb isotopic compositions ((206Pb/204Pb)I = 17.907-19.198, (207Pb/204Pb)I = 15.474-15.555, (208Pb/204Pb)I = 37.408-38.893). The marked consistency in geochemical and isotopic compositions between the intrusive rocks and the coeval Baoligaomiao volcanic rocks define a Carboniferous continental arc. Together with available regional data, we infer that this east-west trending continental arc was generated by northward subduction of the Hegenshan ocean during Carboniferous. The late alkali-feldspar granites and the high-Si rhyolites of the Baoligaomiao volcanic succession show similar geochemical compositions with high SiO2 and variable total alkali contents, and low TiO2, MgO and CaO. These rocks are characterized by unusually low Sr and Ba, and high abundances of Zr, Th, Nb, HREEs

Geologic Map of the Weaverville 15' Quadrangle, Trinity County, California

USGS Publications Warehouse

Irwin, William P.

2009-01-01

The Weaverville 15' quadrangle spans parts of five generally north-northwest-trending accreted terranes. From east to west, these are the Eastern Klamath, Central Metamorphic, North Fork, Eastern Hayfork, and Western Hayfork terranes. The Eastern Klamath terrane was thrust westward over the Central Metamorphic terrane during early Paleozoic (Devonian?) time and, in Early Cretaceous time (approx. 136 Ma), was intruded along its length by the massive Shasta Bally batholith. Remnants of overlap assemblages of the Early Cretaceous (Hauterivian) Great Valley sequence and the Tertiary Weaverville Formation cover nearly 10 percent of the quadrangle. The base of the Eastern Klamath terrane in the Weaverville quadrangle is a peridotite-gabbro complex that probably is correlative to the Trinity ophiolite (Ordovician), which is widely exposed farther north beyond the quadrangle. In the northeast part of the Weaverville quadrangle, the peridotite-gabbro complex is overlain by the Devonian Copley Greenstone and the Mississippian Bragdon Formation. Where these formations were intruded by the Shasta Bally batholith, they formed an aureole of gneissic and other metamorphic rocks around the batholith. Westward thrusting of the Eastern Klamath terrane over an adjacent body of mafic volcanic and overlying quartzose sedimentary rocks during Devonian time formed the Salmon Hornblende Schist and the Abrams Mica Schist of the Central Metamorphic terrane. Substantial beds of limestone in the quartzose sedimentary unit, generally found near the underlying volcanic rock, are too metamorphosed for fossils to have survived. Rb-Sr analysis of the Abrams Mica Schist indicates a metamorphic age of approx. 380 Ma. West of Weavervillle, the Oregon Mountain outlier of the Eastern Klamath terrane consists mainly of Bragdon Formation(?) and is largely separated from the underlying Central Metamorphic terrane by serpentinized peridotite that may be a remnant of the Trinity ophiolite. The North Fork

Geology of the Northern Part of the Harcuvar Complex, West-Central Arizona

USGS Publications Warehouse

Bryant, Bruce; Wooden, J.L.

2008-01-01

In west-central Arizona near the northeast margin of the Basin and Range Province, the Rawhide detachment fault separates Tertiary and older rocks lacking significant effects of Tertiary metamorphism from Precambrian, Paleozoic, and Mesozoic rocks in the Harcuvar metamorphic core complex below. Much of the northern part of the Harcuvar complex in the Buckskin and eastern Harcuvar Mountains is layered granitic gneiss, biotite gneiss, amphibolite, and minor pelitic schist that was probably deformed and metamorphosed in Early Proterozoic time. In the eastern Buckskin Mountains, Early and Middle Proterozoic plutons having U-Pb zircon ages of 1,683?6.4 mega-annum (Ma) and 1,388?2.3 Ma, respectively, intruded the layered gneiss. Small plutons of alkaline gabbro and diorite intruded in Late Jurassic time. A sample of mylonitized diorite from this unit has a U-Pb zircon age of 149?2.8 Ma. In the Early Cretaceous, amphibolite facies regional metamorphism was accompanied by partial melting and formation of migmatite. Zircon from a granitic layer in migmatitic gneiss in the eastern Harcuvar Mountains has a U-Pb age of 110?3.7 Ma. In the Late Cretaceous, sills and plutons of the granite of Tank Pass were emplaced in both the Buckskin and eastern Harcuvar Mountains. In the Buckskin Mountains those intrusions are locally numerous enough to form an injection migmatite. A pluton of this granite crops out over almost half the area of the eastern Harcuvar Mountains. Paleozoic and Mesozoic sedimentary rocks were caught as slices along south-vergent Cretaceous thrusts related to the Maria fold and thrust belt and were metamorphosed beneath a thick sheet of Proterozoic crustal rocks. Inception of volcanism and basin formation in upper-plate rocks indicates that regional extension started at about 26 Ma, in late Oligocene. The Swansea Plutonic Suite, composed of rocks ranging from gabbro to granite, intruded the lower-plate rocks in the Miocene and Oligocene(?). Granite and a gabbro

Accuracy of a pediatric early warning score in the recognition of clinical deterioration.

PubMed

Miranda, Juliana de Oliveira Freitas; Camargo, Climene Laura de; Nascimento, Carlito Lopes; Portela, Daniel Sales; Monaghan, Alan

2017-07-10

to evaluate the accuracy of the version of the Brighton Pediatric Early Warning Score translated and adapted for the Brazilian context, in the recognition of clinical deterioration. a diagnostic test study to measure the accuracy of the Brighton Pediatric Early Warning Score for the Brazilian context, in relation to a reference standard. The sample consisted of 271 children, aged 0 to 10 years, blindly evaluated by a nurse and a physician, specialists in pediatrics, with interval of 5 to 10 minutes between the evaluations, for the application of the Brighton Pediatric Early Warning Score for the Brazilian context and of the reference standard. The data were processed and analyzed using the Statistical Package for the Social Sciences and VassarStats.net programs. The performance of the Brighton Pediatric Early Warning Score for the Brazilian context was evaluated through the indicators of sensitivity, specificity, predictive values, area under the ROC curve, likelihood ratios and post-test probability. the Brighton Pediatric Early Warning Score for the Brazilian context showed sensitivity of 73.9%, specificity of 95.5%, positive predictive value of 73.3%, negative predictive value of 94.7%, area under Receiver Operating Characteristic Curve of 91.9% and the positive post-test probability was 80%. the Brighton Pediatric Early Warning Score for the Brazilian context, presented good performance, considered valid for the recognition of clinical deterioration warning signs of the children studied. avaliar a acurácia da versão traduzida e adaptada do Brighton Paediatric Early Warning Score para o contexto brasileiro, no reconhecimento da deterioração clínica. estudo de teste diagnóstico para medir a acurácia do Brighton Paediatric Early Warning Score, para o contexto brasileiro, em relação a um padrão de referência. A amostra foi composta por 271 crianças de 0 a 10 anos, avaliadas de forma cega por uma enfermeira e um médico, especialistas em pediatria, com

Disequilibrium dihedral angles in layered intrusions: the microstructural record of fractionation

NASA Astrophysics Data System (ADS)

Holness, Marian; Namur, Olivier; Cawthorn, Grant

2013-04-01

The dihedral angle formed at junctions between two plagioclase grains and a grain of augite is only rarely in textural equilibrium in gabbros from km-scale crustal layered intrusions. The median of a population of these disequilibrium angles, Θcpp, varies systematically within individual layered intrusions, remaining constant over large stretches of stratigraphy with significant increases or decreases associated with the addition or reduction respectively of the number of phases on the liquidus of the bulk magma. The step-wise changes in Θcpp are present in Upper Zone of the Bushveld Complex, the Megacyclic Unit I of the Sept Iles Intrusion, and the Layered Series of the Skaergaard Intrusion. The plagioclase-bearing cumulates of Rum have a bimodal distribution of Θcpp, dependent on whether the cumulus assemblage includes clinopyroxene. The presence of the step-wise changes is independent of the order of arrival of cumulus phases and of the composition of either the cumulus phases or the interstitial liquid inferred to be present in the crystal mush. Step-wise changes in the rate of change in enthalpy with temperature (ΔH) of the cooling and crystallizing magma correspond to the observed variation of Θcpp, with increases of both ΔH and Θcpp associated with the addition of another liquidus phase, and decreases of both associated with the removal of a liquidus phase. The replacement of one phase by another (e.g. olivine ⇔ orthpyroxene) has little effect on ΔH and no discernible effect on Θcpp. An increase of ΔH is manifest by an increase in the fraction of the total enthalpy budget that is the latent heat of crystallization (the fractional latent heat). It also results in an increase in the amount crystallized in each incremental temperature drop (the crystal productivity). An increased fractional latent heat and crystal productivity result in an increased rate of plagioclase growth compared to that of augite during the final stages of solidification

Michael Tuomey's 1848 geological survey of South Carolina

USGS Publications Warehouse

Nystrom, P.G.

1999-01-01

One hundred and fifty years ago, Michael Tuomey completed his 'Report on the Geology of South Carolina,' the result of four years of arduous labor. The report is the first detailed and comprehensive geological description of the entire state, and it includes a geological map that shows the distribution of Coastal Plain and Piedmont-Blue Ridge units. In the sesquicentennial of Tuomey's survey, it is fitting that we recognize his important early contribution to the geology of South Carolina and the southeast. Tuomey's report is a 293-page volume with a 48-page appendix and an index. Although he gave a complete depiction of Coastal Plain geology and delineated Cretaceous, Lower Eocene, Eocene, Miocene, Post-Pliocene, and alluvial units on his map, the emphasis herein is on his mapping of the Piedmont and Blue Ridge. The metamorphic units he delineated are clay slate, mica slate, talcose slate, hornblende slate, gneiss, and lime rock. Gneiss is the most extensive unit on the map. His map shows many elements of the geologic framework we recognize today. The distribution of his clay slate unit corresponds closely with the Carolina slate and Bel Air belts as we know them now. The gneiss between the two clay slate areas matches the Kiokee belt. Areas of mica slate approximate the northern part of the Kings Mountain belt and the Chauga belt. He also recognized that his talcose slate unit was associated with gold deposits. Granitic and basaltic intrusive rocks are also delineated on the map. It shows the Newberry, Columbia, and Liberty Hill granites we recognize today. Basaltic intrusives outlined include the Bush River of western Newberry County, Dutchmans Creek, Big Wateree Creek, and Ogden gabbros. He described the regional extent of diabase dikes as occuring from Virginia to Alabama, noted their preferred direction and diagrammed their near-vertical orientation. He also referred to the distinctive soil and topography that develops on the large gabbros. Michael Tuomey

Classification of igneous rocks analyzed by ChemCam at Gale crater, Mars

DOE PAGES

Cousin, Agnes; Sautter, Violaine; Payré, Valérie; ...

2017-02-09

Several recent studies have revealed that Mars is not a simple basalt-covered planet, but has a more complex geological history. In Gale crater on Mars, the Curiosity rover discovered 59 igneous rocks. This article focuses on their textures (acquired from the cameras such as MAHLI and MastCam) and their geochemical compositions that have been obtained using the ChemCam instrument. Light-toned crystals have been observed in most of the rocks. They correspond to feldspars ranging from andesines/oligoclases to anorthoclases and sanidines in the leucocratic vesiculated rocks. Darker crystals observed in all igneous rocks (except the leucocratic vesiculated ones) were analyzed bymore » LIBS and mainly identified as Fe-rich pigeonites and Fe-augites. Iron oxides have been observed in all groups whereas F-bearing minerals have been detected only in few of them. From their textural analysis and their whole-rock compositions, all these 59 igneous rocks have been classified in five different groups; from primitive rocks i.e. dark aphanitic basalts/basanites, trachybasalts, tephrites and fine/coarse-grained gabbros/norites to more evolved materials i.e. porphyritic trachyandesites, leucocratic trachytes and quartz-diorites. The basalts and gabbros are found all along the traverse of the rover, whereas the felsic rocks are located before the Kimberley formation, i.e. close to the Peace Vallis alluvial fan deposits. This suggests that these alkali rocks have been transported by fluvial activity and could come from the Northern rim of the crater, and may correspond to deeper strata buried under basaltic regolith (Sautter et al., 2015). Some of the basaltic igneous rocks are surprisingly enriched in iron, presenting low Mg# similar to the nakhlite parental melt that cannot be produced by direct melting of the Dreibus and Wanke (1986) martian primitive mantle. The basaltic rocks at Gale are thus different from Gusev basalts. They could originate from different mantle reservoirs

Magma traps and driving pressure: consequences for pluton shape and emplacement in an extensional regime

NASA Astrophysics Data System (ADS)

Hogan, John P.; Price, Jonathan D.; Gilbert, M. Charles

1998-09-01

The level of emplacement and final form of felsic and mafic igneous rocks of the Wichita Mountains Igneous Province, southwestern Oklahoma, U.S.A. are discussed in light of magma driving pressure, lithostatic load, and crustal magma traps. Deposition of voluminous A-type rhyolites upon an eroded gabbroic substrate formed a subhorizontal strength anisotropy that acted as a crustal magma trap for subsequent rising felsic and mafic magma. Intruded along this crustal magma trap are the A-type sheet granites (length/thickness 100:1) of the Wichita Granite Group, of which the Mount Scott Granite sheet is typical, and smaller plutons of biotite bearing Roosevelt Gabbro. In marked contrast to the subhorizontal granite sheets, the gabbro plutons form more equant stocks with flat roofs and steep side walls. Late Diabase dikes cross-cut all other units, but accompanying basaltic flows are extremely rare in the volcanic pile. Based on magmastatic calculations, we draw the following conclusions concerning the level of emplacement and the shape of these intrusions. (1) Magma can rise to a depth at which the magma driving pressure becomes negligible. Magma that maintains a positive driving pressure at the surface has the potential to erupt. (2) Magma ascent may be arrested at a deeper level in the crust by a subhorizontal strength anisotropy (i.e. crustal magma trap) if the magma driving pressure is greater than or equal to the lithostatic load at the depth of the subhorizontal strength anisotropy. (3) Subhorizontal sheet-intrusions form along crustal magma traps when the magma driving pressure greatly exceeds the lithostatic load. Under such conditions, the magma driving pressure is sufficent to lift the overburden to create the necessary space for the intrusion. (4) Thicker steep-sided stocks or batholiths, with flat roofs, form at crustal magma traps when the magma driving pressure approximates that of the lithostatic load. Under these conditions, the necessary space for the

Geological development and Phanerozoic crustal accretion in the western segment of the southern Tien Shan (Kyrgyzstan, Uzbekistan and Tajikistan)

NASA Astrophysics Data System (ADS)

Brookfield, M. E.

2000-12-01

The Tien Shan form a high intracontinental mountain belt, lying north of the main India-Asia collision mountains, and consist of re-activated Paleozoic orogens. The western segment of the southern Tien Shan lies northwest of the Pamir and west of the Talas-Fergana fault. The stratigraphy, lithology, igneous and metamorphic petrology and geochemistry of this segment indicate that it was formed by the assembly of Lower Paleozoic arcs which developed into microcontinents with Upper Paleozoic mature shelf and slope clastic and carbonate sediments. Precambrian continental crust is confined to two small blocks along its southern margin. The bulk of the southern Tien Shan consists of ?Vendian to Silurian oceanic and slope clastic rocks, resting on oceanic lithosphere, and overlain by thick passive margin Devonian to mid-Carboniferous mature shelf clastics and carbonates. These are unconformably overlain by syn- and post-orogenic immature clastic sediments derived from mountains on the north formed by closure of a Carboniferus southern Tajik and a northern Vendian to Carboniferous Turkestan ocean with the southern Tien Shan microcontinent sandwiched between. Associated with these collisions are late Carboniferous to Permian intrusives, which form three south to north (though overlapping) suites; a southern calc-alkaline granodiorite-granite suite, an intermediate gabbro-monzodiorite-granite suite, and a northern alkaline monzodiorite-granite-alaskite suite. The gabbro-monzodiorite-granite suite forms the earliest subduction-related magmatism of the southern Tien Shan: rare earth element patterns are consistent with derivation from a primitive or slightly enriched mantle. The other suites show more crustal contamination. Rb and Sr vary with depth and degree of partial melting and are consistent with progressive involvement of crustal material in partial melts during collision. The gradual change in composition within each complex, lasting in some cases from 295 to 250 Ma

Crustal thinning and exhumation along a fossil magma-poor distal margin preserved in Corsica: A hot rift to drift transition?

NASA Astrophysics Data System (ADS)

Beltrando, Marco; Zibra, Ivan; Montanini, Alessandra; Tribuzio, Riccardo

2013-05-01

Rift-related thinning of continental basement along distal margins is likely achieved through the combined activity of ductile shear zones and brittle faults. While extensional detachments responsible for the latest stages of exhumation are being increasingly recognized, rift-related shear zones have never been sampled in ODP sites and have only rarely been identified in fossil distal margins preserved in orogenic belts. Here we report evidence of the Jurassic multi-stage crustal thinning preserved in the Santa Lucia nappe (Alpine Corsica), where amphibolite facies shearing persisted into the rift to drift transition. In this nappe, Lower Permian meta-gabbros to meta-gabbro-norites of the Mafic Complex are separated from Lower Permian granitoids of the Diorite-Granite Complex by a 100-250 m wide shear zone. Fine-grained syn-kinematic andesine + Mg-hornblende assemblages in meta-tonalites of the Diorite-Granite Complex indicate shearing at T = 710 ± 40 °C at P < 0.5 GPa, followed by deformation at greenschist facies conditions. 40Ar/39Ar step-heating analyses on amphiboles reveal that shearing at amphibolite facies conditions possibly began at the Triassic-Jurassic boundary and persisted until t < 188 Ma, with the Mafic Complex cooling rapidly at the footwall of the Diorite-Granite Complex at ca. 165.4 ± 1.7 Ma. Final exhumation to the basin floor was accommodated by low-angle detachment faulting, responsible for the 1-10 m thick damage zone locally capping the Mafic Complex. The top basement surface is onlapped at a low angle by undeformed Mesozoic sandstone, locally containing clasts of footwall rocks. Existing constraints from the neighboring Corsica ophiolites suggest an age of ca. 165-160 Ma for these final stages of exhumation of the Santa Lucia basement. These results imply that middle to lower crustal rocks can be cooled and exhumed rapidly in the last stages of rifting, when significant crustal thinning is accommodated in less than 5 Myr through the

a Comparison of Anisotropy of Magnetic Susceptibility and Anisotropy of Anhysteretic Remanence Data from the Early Jurassic Basal Karoo Igneous Series, South Africa

NASA Astrophysics Data System (ADS)

Geissman, J. W.; Ferre, E. C.; Mason, S. N.; Maes, S. M.; Lehman, A.; Marsh, J.

2011-12-01

Although anisotropy of magnetic susceptibility (AMS) data from plutonic rocks has often been used as a proxy for magma flow, many studies indicate that the relationship between the principal axes of the anisotropy of magnetic susceptibility (AMS) and independently inferred flow patterns is not simple. Data from dikes show that complications may arise from the contribution of single domain (SD) magnetite grains, imbricated symmetric fabrics along dike margins and post-solidification thermal stresses. Limited studies have focused on subhorizontal tabular intrusions and their magnetic fabrics. The Karoo Large Igneous Province in South Africa hosts a well-exposed, laterally extensive set of undeformed, stacked gabbroic sills emplaced parallel to bedding in the Karoo Basin. The sills range in thickness from about 1 m up to 1000 m and have a relatively constant petrologic composition of gabbros and gabbro-norites. Theses sills are distributed throughout the entire Karoo Basin and were emplaced at several stratigraphic heights / depths in the Karoo stratigraphic column. Oriented core samples were collected from 30 different sills and yielded 1598 specimens for AMS and AARM fabric data as well as paleomagnetic measurements. The low-field magnetic susceptibility Km ranges widely from about 100 to 20,000 x 10-6 [SI Vol] and the degree of anisotropy P' ranges from 1.01 to 1.10, with a broad correlation between Km and P'. Thermomagnetic experiments reveal that the main magnetic carrier is titanomagnetite with variable ulvöspinel content; hysteresis data show a pseudo-single domain grain dominance. The AMS and AARM fabric data, collected for all sites, yield normal fabric relations and are consistent with the nearly horizontal attitude of the sills in most of the sills, with subvertical K3 AMS and AARM axes. Generally, AARM parameters are greater than AMS values, although the dispersion of AARM data is more variable among sites. In five of 30 sills, K3 and K1 AMS axes are

Rock- and Paleomagnetic Properties and Modeling of a Deep Crustal Volcanic System, the Reinfjord Ultramafic Complex, Seiland Igneous Province, Northern Norway

NASA Astrophysics Data System (ADS)

ter Maat, G. W.; Pastore, Z.; Michels, A.; Church, N. S.; McEnroe, S. A.; Larsen, R. B.

2017-12-01

The Reinfjord Ultramafic Complex is part of the 5000 km2 Seiland Igneous Province (SIP) in Northern Norway. The SIP is argued to be the deep-seated conduit system of a Large Igneous Province and was emplaced at 25-35 km depth in less than 10 Ma (570-560 Ma). The Reinfjord Ultramafic Complex was emplaced during three major successive events at 22-28km depth at pressures of 6-8kb, with associated temperatures 1450-1500°C (Roberts, 2006). The rocks are divided into three formations: the central series (CS) consisting of mainly dunites, upper layered series (ULS) consisting of dunites and wehrlites, a lower layered series (LLS) containing most pyroxene-rich rocks and a marginal zone (MZ) which formed where the ultramafic melts intruded the gabbro-norite and metasedimentary gneisses. Deep exposures such as the Reinfjord Ultramafic Complex are rare, therefore this study gives a unique insight in the rock magnetic properties of a deep ultramafic system. Localised serpentinised zones provide an opportunity to observe the effect of this alteration process on the magnetic properties of deep-seated rocks. Here, we present the results from the rock magnetic properties, a paleomagnetic study and combined potential-fields modeling. The study of the rock magnetic properties provides insight in primary processes associated with the intrusion, and later serpentinization. The paleomagnetic data yields two distinct directions. One direction corresponds to a Laurentia pole at ≈ 532 Ma while the other, though younger, is not yet fully understood. Rock magnetic properties were measured on > 700 specimens and used to constrain the modelling of gravity, high-resolution helicopter, and ground magnetic data. The intrusion is modelled as a cylindrically shaped complex with a dunite core surrounded by wehrlite and gabbro. The ultramafic part of the complex dips to the NE and its maximum vertical extent is modelled to 1400m. Furthermore, modelling allows estimation of relative volumes of

The Paradox of the Axial Melt Lens: Petrology and Geochemistry of the Upper Plutonics at Hess Deep

NASA Astrophysics Data System (ADS)

Lissenberg, C. J.; Loocke, M. P.; MacLeod, C. J.

2014-12-01

The axial melt lens (AML) is a steady-state magma-rich body located at the dyke-gabbro transition at intermediate- and fast-spreading ridges. It is widely believed to be the reservoir from which mid-ocean ridge basalt (MORB) is erupted. The paradox of the axial melt lens is that the plutonic rocks that occur at this level are far too evolved to be in equilibrium with MORB, which is basaltic by definition; hence, the plutonic and volcanic records do not match. We explore this paradox by study of the first comprehensive sample suite of the uppermost plutonics of a fast-spreading ridge, taken by remotely-operated vehicle from the Hess Deep rift during cruise JC21. 23 samples (8 dolerites, 14 gabbronorites, and 1 gabbro) were collected from a section containing the transition from the uppermost gabbroic section into sheeted dykes. We present the results of a detailed petrographic and microanalytical investigation of these samples. They are dominated by evolved, varitextured (both in hand sample and thin section) oxide gabbronorites; olivine occurs in only one sample. A preponderance of the samples have positive Eu/Eu* and Sr/Sr*, indicating a cumulate origin. However, the minerals have evolved compositions, and are in equilibrium with melts significantly more evolved than East Pacific Rise MORB. Furthermore, the trace element contents of clinopyroxene differ significantly from clinopyroxene in equilibrium with MORB, being more enriched in incompatible elements. To account for both the evidence of derivation of MORB from the AML and the evolved nature of its rock record, we posit that the AML must be fed by melts on two different timescales: continual low-volume feeding by evolved interstitial melt from the cumulus pile below is augmented episodically by delivery of high volumes of more primitive melt. The latter episodes may trigger eruptions; hence the primitive melts are held in the magma chamber for only short periods, and erupt on the seafloor before significant

Sulfide enrichment at an oceanic crust-mantle transition zone: Kane Megamullion (23°N, MAR)

NASA Astrophysics Data System (ADS)

Ciazela, Jakub; Koepke, Juergen; Dick, Henry J. B.; Botcharnikov, Roman; Muszynski, Andrzej; Lazarov, Marina; Schuth, Stephan; Pieterek, Bartosz; Kuhn, Thomas

2018-06-01

The Kane Megamullion oceanic core complex located along the Mid-Atlantic Ridge (23°30‧N, 45°20‧W) exposes lower crust and upper mantle directly on the ocean floor. We studied chalcophile elements and sulfides in the ultramafic and mafic rocks of the crust-mantle transition and the mantle underneath. We determined mineralogical and elemental composition and the Cu isotope composition of the respective sulfides along with the mineralogical and elemental composition of the respective serpentines. The rocks of the crust-mantle transition zone (i.e., plagioclase harzburgite, peridotite-gabbro contacts, and dunite) overlaid by troctolites are by one order of magnitude enriched in several chalcophile elements with respect to the spinel harzburgites of the mantle beneath. Whereas the range of Cu concentrations in spinel harzburgites is 7-69 ppm, the Cu concentrations are highly elevated in plagioclase harzburgites with a range of 90-209 ppm. The zones of the peridotite-gabbro contacts are even more enriched, exhibiting up to 305 ppm Cu and highly elevated concentrations of As, Zn, Ga, Sb and Tl. High Cu concentrations show pronounced correlation with bulk S concentrations at the crust-mantle transition zone implying an enrichment process in this horizon of the oceanic lithosphere. We interpret this enrichment as related to melt-mantle reaction, which is extensive in crust-mantle transition zones. In spite of the ubiquitous serpentinization of primary rocks, we found magmatic chalcopyrites [CuFeS2] as inclusions in plagioclase as well as associated with pentlandite [(Fe,Ni)9S8] and pyrrhotite [Fe1-xS] in polysulfide grains. These chalcopyrites show a primary magmatic δ65Cu signature ranging from -0.04 to +0.29 ‰. Other chalcopyrites have been dissolved during serpentinization. Due to the low temperature (<300 °C) of circulating fluids chalcophile metals from primary sulfides have not been mobilized and transported away but have been trapped in smaller secondary

Mafic and felsic igneous rocks at Gale crater

NASA Astrophysics Data System (ADS)

Sautter, Violaine; Cousin, Agnès; Mangold, Nicolas; Toplis, Michael; Fabre, Cécile; Forni, Olivier; Payré, Valérie; Gasnault, Olivier; Ollila, Anne; Rapin, William; Fisk, Martin; Meslin, Pierre-Yves; Wiens, Roger; Maurice, Sylvestre; Lasue, Jérémie; Newsom, Horton; Lanza, Nina

2015-04-01

The Curiosity rover landed at Gale, an early Hesperian age crater formed within Noachian terrains on Mars. The rover encountered a great variety of igneous rocks to the west of the Yellow Knife Bay sedimentary unit (from sol 13 to 800) which are float rocks or clasts in conglomerates. Textural and compositional analyses using MastCam and ChemCam Remote micro Imager (RMI) and Laser Induced Breakdown Spectroscopy (LIBS) with a ˜300-500 µm laser spot lead to the recognition of 53 massive (non layered) igneous targets, both intrusive and effusive, ranging from mafic rocks where feldspars form less than 50% of the rock to felsic samples where feldspar is the dominant mineral. From morphology, color, grain size, patina and chemistry, at least 5 different groups of rocks have been identified: (1) a basaltic class with shiny aspect, conchoidal frature, no visible grains (less than 0.2mm) in a dark matrix with a few mm sized light-toned crystals (21 targets) (2) a porphyritic trachyandesite class with light-toned, bladed and polygonal crystals 1-20 mm in length set in a dark gray mesostasis (11 targets); (3) light toned trachytes with no visible grains sometimes vesiculated or forming flat targets (6 targets); (4) microgabbro-norite (grain size < 1mm) and gabbro-norite (grain size >1 mm) showing dark and light toned crystals in similar proportion ( 8 targets); (5) light-toned diorite/granodiorite showing coarse granular (>4 mm) texture either pristine or blocky, strongly weathered rocks (9 rock targets). Overall, these rocks comprise 2 distinct geochemical series: (i) an alkali-suite: basanite, gabbro trachy-andesite and trachyte) including porphyritic and aphyric members; (ii) quartz-normative intrusives close to granodioritic composition. The former looks like felsic clasts recently described in two SNC meteorites (NWA 7034 and 7533), the first Noachian breccia sampling the martian regolith. It is geochemically consistent with differentiation of liquids produced by low

La Isla de Gorgona, Colombia: A petrological enigma?

NASA Astrophysics Data System (ADS)

Kerr, Andrew C.

2005-09-01

A wide range of intrusive (wehrlite, dunite, gabbro and olivine gabbro) and extrusive (komatiites picrites and basalts) igneous rocks are found on the small pacific island of Gorgona. The island is best known for its ˜90 Ma spinifex-textured komatiites: the only true Phanerozoic komatiites yet discovered. Early work led to suggestions that the rocks of the island formed at a mid-ocean ridge, however more recent research supports an origin as part of a hot mantle plume-derived oceanic plateau. One of the main lines of evidence for this origin stems from the inferred high mantle source temperatures required to form the high-MgO (> 15 wt.%) komatiites and picrites. Another remarkable feature of the island, considering its small size (8 × 2.5 km), is the degree of chemical and radiogenic isotopic heterogeneity shown by the rocks. This heterogeneity requires a mantle source region with at least three isotopically distinctive source regions (two depleted and one enriched). Although these mantle source regions appear to be derived in significant part from recycled oceanic crust and lithosphere, enrichments in 187Os, 186Os and 3He in Gorgona lavas and intrusive rocks, suggest some degree of transfer of material from the outer core to the plume source region at D″. Modelling reveals that the komatiites probably formed by dynamic melting at an average pressure of 3-4 GPa leaving residual harzburgite. Trace element depletion in Gorgona ultramafic rocks appears to be the result of earlier, deeper melting which produced high-MgO trace element-enriched magmas. The discovery of a trace-element enriched picrite on the island has confirmed this model. Gorgona accreted onto the palaeocontinental margin of northwestern South America in the Eocene and palaeomagnetic work reveals that it was formed at ˜26 °S. It has been proposed that Gorgona is a part of the Caribbean-Colombian Oceanic Plateau (CCOP), however, the CCOP accreted in the Late Cretaceous and was derived from a more

Magmatic microgranular enclaves of the northeast of Mato Grosso, Brazil, SE Amazonian Craton: Insights into the magmatism of the Uatumã Supergroup on the basis of field and petrological data

NASA Astrophysics Data System (ADS)

Tarelow Neto, João; Pierosan, Ronaldo; Barros, Márcia Aparecida de Sant'Ana; Chemale, Farid, Jr.; Santos, Fernanda Silva

2017-10-01

Felsic and mafic microgranular enclaves occurs hosted in Paleoproterozoic felsic volcanic and plutonic rocks of the Iriri Group and Rio Dourado Intrusive Suite that crop out in the northeast of the Brazilian state of Mato Grosso as part of the Uatumã Supergroup in the SE Amazonian Craton. The volcanic rocks consist of dacites, rhyolites and ignimbrites of the Iriri Group among which dacites yielded a crystallization age of 1895 ± 7 Ma. The granitoids are composed of syenogranites, and monzogranites along with subordinate granodiorites. The granite pluton of syenogranite composition belongs to the Rio Dourado Intrusive Suite and was previously dated at 1876 ± 39 Ma. Mafic enclaves show petrographic features that suggest magma mingling processes and may be classified as olivine-clinopyroxene gabbro, clinopyroxene-hornblende gabbro, biotite-clinopyroxene diorite, biotite-hornblende diorite, and biotite-quartz diorite. Geochemical patterns of dioritic enclaves fit with those of the mafic rocks of the Santa Inês Intrusive Suite and suggest that they are comagmatic. The felsic enclaves consist of quartz monzonites, monzogranites and quartz-rich granitoids, with geochemical patterns and ages compatible with those of the Iriri Group. U-Pb dating of a granodioritic enclave hosted by a syenogranite of the Rio Dourado Intrusive Suite yielded three distinct ages: a Paleoproterozoic magmatic age around 1.87 Ga; a Paleoproterozoic inherited age of 2001 ± 7 Ma; and Neoarchean inherited ages ranging from 2512 to 2767 Ma. The felsic enclaves may have been derived from crustal melts, and contain restitic zircon grains, or chilled margins with zircon removed from the wall rocks. The recognition of diverse geochemical patterns of felsic and mafic enclaves suggests that the study area can represent the top of a shallow complex magma chamber that experienced a number of different replenishment events from diverse sources. The mafic enclaves and their coeval enclosing volcanic rocks

The mantle source of island arc magmatism during early subduction: Evidence from Hf isotopes in rutile from the Jijal Complex (Kohistan arc, Pakistan)

NASA Astrophysics Data System (ADS)

Ewing, Tanya A.; Müntener, Othmar

2018-05-01

The Cretaceous-Paleogene Kohistan arc complex, northern Pakistan, is renowned as one of the most complete sections through a preserved paleo-island arc. The Jijal Complex represents a fragment of the plutonic roots of the Kohistan arc, formed during its early intraoceanic history. We present the first Hf isotope determinations for the Jijal Complex, made on rutile from garnet gabbros. These lithologies are zircon-free, but contain rutile that formed as an early phase. Recent developments in analytical capabilities coupled with a careful analytical and data reduction protocol allow the accurate determination of Hf isotope composition for rutile with <30 ppm Hf for the first time. Rutile from the analysed samples contains 5-35 ppm Hf, with sample averages of 13-17 ppm. Rutile from five samples from the Jijal Complex mafic section, sampling 2 km of former crustal thickness, gave indistinguishable Hf isotope compositions with εHf(i) ranging from 11.4 ± 3.2 to 20.1 ± 5.7. These values are within error of or only slightly more enriched than modern depleted mantle. The analysed samples record variable degrees of interaction with late-stage melt segregations, which produced symplectitic overprints on the main mineral assemblage as well as pegmatitic segregations of hydrous minerals. The indistinguishable εHf(i) across this range of lithologies demonstrates the robust preservation of the Hf isotope composition of rutile. The Hf isotope data, combined with previously published Nd isotope data for the Jijal Complex garnet gabbros, favour derivation from an inherently enriched, Indian Ocean type mantle. This implies a smaller contribution from subducted sediments than if the source was a normal (Pacific-type) depleted mantle. The Jijal Complex thus had only a limited recycled continental crustal component in its source, and represents a largely juvenile addition of new continental crust during the early phases of intraoceanic magmatism. The ability to determine the Hf

Magnetization of lower oceanic crust and upper mantle

NASA Astrophysics Data System (ADS)

Kikawa, E.

2004-05-01

The location of the magnetized rocks of the oceanic crust that are responsible for sea-floor spreading magnetic anomalies has been a long-standing problem in geophysics. The recognition of these anomalies was a key stone in the development of the theory of plate tectonics. Our present concept of oceanic crustal magnetization is much more complex than the original, uniformly magnetized model of Vine-Matthews-Morley Hypothesis. Magnetic inversion studies indicated that the upper oceanic extrusive layer (Layer 2A of 0.5km thick) was the only magnetic layer and that it was not necessary to postulate any contribution from deeper parts of oceanic crust. Direct measurements of the magnetic properties of the rocks recovered from the sea floor, however, have shown that the magnetization of Layer 2A, together with the observations that this layer could record geomagnetic field reversals within a vertical section, is insufficient to give the required size of observed magnetic anomalies and that some contribution from lower intrusive rocks is necessary. Magnetization of oceanic intrusive rocks were observed to be reasonably high enough to contribute to sea-floor spreading magnetic anomalies, but were considered somewhat equivocal until late 1980Os, in part because studies had been conducted on unoriented dredged and ophiolite samples and on intermittent DSDP/ODP cores. Since ODP Leg 118 that cored and recovered continuous 500m of oceanic intrusive layer at Site 735B, Southwest Indian Ridge with an extremely high recovery of 87 percent, there have been several ODP Legs (legs 147, 153, 176, 179 and 209) that were devoted to drilling gabbroic rocks and peridotites. In terms of the magnetization intensities, all of the results obtained from these ODP Legs were supportive of the model that a significant contribution must come from gabbros and peridotites and the source of the lineated magnetic anomalies must reside in most of the oceanic crust as well as crust-mantle boundary

Classification of igneous rocks analyzed by ChemCam at Gale crater, Mars

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

Cousin, Agnes; Sautter, Violaine; Payré, Valérie

Several recent studies have revealed that Mars is not a simple basalt-covered planet, but has a more complex geological history. In Gale crater on Mars, the Curiosity rover discovered 59 igneous rocks. This article focuses on their textures (acquired from the cameras such as MAHLI and MastCam) and their geochemical compositions that have been obtained using the ChemCam instrument. Light-toned crystals have been observed in most of the rocks. They correspond to feldspars ranging from andesines/oligoclases to anorthoclases and sanidines in the leucocratic vesiculated rocks. Darker crystals observed in all igneous rocks (except the leucocratic vesiculated ones) were analyzed bymore » LIBS and mainly identified as Fe-rich pigeonites and Fe-augites. Iron oxides have been observed in all groups whereas F-bearing minerals have been detected only in few of them. From their textural analysis and their whole-rock compositions, all these 59 igneous rocks have been classified in five different groups; from primitive rocks i.e. dark aphanitic basalts/basanites, trachybasalts, tephrites and fine/coarse-grained gabbros/norites to more evolved materials i.e. porphyritic trachyandesites, leucocratic trachytes and quartz-diorites. The basalts and gabbros are found all along the traverse of the rover, whereas the felsic rocks are located before the Kimberley formation, i.e. close to the Peace Vallis alluvial fan deposits. This suggests that these alkali rocks have been transported by fluvial activity and could come from the Northern rim of the crater, and may correspond to deeper strata buried under basaltic regolith (Sautter et al., 2015). Some of the basaltic igneous rocks are surprisingly enriched in iron, presenting low Mg# similar to the nakhlite parental melt that cannot be produced by direct melting of the Dreibus and Wanke (1986) martian primitive mantle. The basaltic rocks at Gale are thus different from Gusev basalts. They could originate from different mantle reservoirs

Multi-stage origin of the Coast Range ophiolite, California: Implications for the life cycle of supra-subduction zone ophiolites

USGS Publications Warehouse

Shervais, J.W.; Kimbrough, D.L.; Renne, P.; Hanan, B.B.; Murchey, B.; Snow, C.A.; Zoglman, Schuman M.M.; Beaman, J.

2004-01-01

The Coast Range ophiolite of California is one of the most extensive ophiolite terranes in North America, extending over 700 km from the northernmost Sacramento Valley to the southern Transverse Ranges in central California. This ophiolite, and other ophiolite remnants with similar mid-Jurassic ages, represent a major but short-lived episode of oceanic crust formation that affected much of western North America. The history of this ophiolite is important for models of the tectonic evolution of western North America during the Mesozoic, and a range of conflicting interpretations have arisen. Current petrologic, geochemical, stratigraphic, and radiometric age data all favor the interpretation that the Coast Range ophiolite formed to a large extent by rapid extension in the forearc region of a nascent subduction zone. Closer inspection of these data, however, along with detailed studies of field relationships at several locales, show that formation of the ophiolite was more complex, and requires several stages of formation. Our work shows that exposures of the Coast Range ophiolite preserve evidence for four stages of magmatic development. The first three stages represent formation of the ophiolite above a nascent subduction zone. Rocks associated with the first stage include ophiolite layered gabbros, a sheeted complex, and volcanic rocks vith arc tholeiitic or (roore rarely) low-K calc-alkaline affinities. The second stage is characterized by intrusive wehrlite-clinopyroxenite complexes, intrusive gabbros, Cr-rich diorites, and volcanic rocks with high-Ca boninitic or tholeiitic ankaramite affinities. The third stage includes diorite and quartz diorite plutons, felsic dike and sill complexes, and calc-alkaline volcanic rocks. The first three stages of ophiolite formation were terminated by the intrusion of mid-ocean ridge basalt dikes, and the eruption of mid-ocean ridge basalt or ocean-island basalt volcanic suites. We interpret this final magmatic event (MORB

Local earthquake tomography with the inclusion of full topography and its application to Kīlauea volcano, Hawai'i

NASA Astrophysics Data System (ADS)

Li, Peng; Lin, Guoqing

2016-04-01

We develop a new three-dimensional local earthquake tomography algorithm with the inclusion of full topography (LETFT). We present both synthetic and real data tests based on the P- and S-wave arrival time data for Kīlauea volcano in Hawai'i. A total of 33,768 events with 515,711 P-picks and 272,217 S-picks recorded by 35 stations at the Hawaiian Volcano Observatory are used in these tests. The comparison between the new and traditional methods based on the synthetic test shows that our new algorithm significantly improves the accuracy of the velocity model, especially at shallow depths. In the real data application, the P- and S-wave velocity models of Kīlauea show several intriguing features. We observe discontinuous high Vp (> 7.0 km/s) and Vs (> 3.9 km/s) zones at 5-14 km depth below Kīlauea caldera, its East Rift Zone (ERZ) and the Southwest Rift Zone, which may represent consolidated intrusive gabbro-ultramafic cumulates. At Kīlauea caldera, Vp and Vs decrease from ~ 3.9 km/s and ~ 2.6 km/s from the surface to ~ 3.7 km/s and ~ 2.3 km/s at 2 km depth. We resolve a high Vp zone (> 7.0 km/s) at 5-14 km depth and high Vs zone (> 3.9 km/s) at 5-11 km depth. This high Vp and Vs zone extends to the north of the ERZ at 5-10 km depth and to the upper ERZ at 8-12 km depth. In the Hilina Fault System, there is a high Vp layer (~ 7.0 km/s) at 4-6 km depth and a low Vp body of ~ 5.7 km/s at 6-11 km depth. The high Vp layer could be associated with the intrusive ultramafic gabbro sills. The velocity contrast on the north and south sides of the Koa'e Fault System indicates that the intrusive activities mainly occur to the north of the fault. Our new LETFT method performs well in both the synthetic and real data tests and we expect that it will reveal more robust velocity structures in areas with larger topographic variations.

Ionospheric TEC Weather Map Over South America

NASA Astrophysics Data System (ADS)

Takahashi, H.; Wrasse, C. M.; Denardini, C. M.; Pádua, M. B.; de Paula, E. R.; Costa, S. M. A.; Otsuka, Y.; Shiokawa, K.; Monico, J. F. Galera; Ivo, A.; Sant'Anna, N.

2016-11-01

Ionospheric weather maps using the total electron content (TEC) monitored by ground-based Global Navigation Satellite Systems (GNSS) receivers over South American continent, TECMAP, have been operationally produced by Instituto Nacional de Pesquisas Espaciais's Space Weather Study and Monitoring Program (Estudo e Monitoramento Brasileiro de Clima Especial) since 2013. In order to cover the whole continent, four GNSS receiver networks, (Rede Brasileiro de Monitoramento Contínuo) RBMC/Brazilian Institute for Geography and Statistics, Low-latitude Ionospheric Sensor Network, International GNSS Service, and Red Argentina de Monitoreo Satelital Continuo, in total 140 sites, have been used. TECMAPs with a time resolution of 10 min are produced in 12 h time delay. Spatial resolution of the map is rather low, varying between 50 and 500 km depending on the density of the observation points. Large day-to-day variabilities of the equatorial ionization anomaly have been observed. Spatial gradient of TEC from the anomaly trough (total electron content unit, 1 TECU = 1016 el m-2 (TECU) <10) to the crest region (TECU > 80) causes a large ionospheric range delay in the GNSS positioning system. Ionospheric plasma bubbles, their seeding and development, could be monitored. This plasma density (spatial and temporal) variability causes not only the GNSS-based positioning error but also radio wave scintillations. Monitoring of these phenomena by TEC mapping becomes an important issue for space weather concern for high-technology positioning system and telecommunication.

Grainsize evolution and differential comminution in an experimental regolith

NASA Technical Reports Server (NTRS)

Horz, F.; Cintala, M.; See, T.

1984-01-01

The comminution of planetary surfaces by exposure to continuous meteorite bombardment was simulated by impacting the same fragmental gabbro target 200 times. The role of comminution and in situ gardening of planetary regoliths was addressed. Mean grain size continuously decreased with increasing shot number. Initially it decreased linearly with accumulated energy, but at some stage comminution efficiency started to decrease gradually. Point counting techniques, aided by the electron microprobe for mineral identification, were performed on a number of comminution products. Bulk chemical analyses of specific grain size fractions were also carried out. The finest sizes ( 10 microns) display generally the strongest enrichment/depletion factors. Similar, if not exactly identical, trends are reported from lunar soils. It is, therefore, not necessarily correct to explain the chemical characteristics of various grain sizes via different admixtures of materials from distant source terrains. Differential comminution of local source rocks may be the dominating factor.

Participation in the Apollo passive seismic experiment

NASA Technical Reports Server (NTRS)

Press, F.; Toksoez, M. N.; Dainty, A.

1972-01-01

Computer programs which were written to read digital tapes containing lunar seismic data were studied. Interpreting very early parts of the lunar seismogram as seismic body-wave phases enabled the determination of the structure of the outer part of the moon in the Fra Mauro region. The crust in the Fra Mauro region is 60 to 65 km-thick, overlaying a high velocity mantle. The crust is further divided into an upper part, 25 km thick, apparently made of material similar to the surficial basalts, and a lower part of seemingly different composition, possibly an anorthositic gabbro. The generation of the exceedingly long reverberating wave-train observed in lunar seismogram was also studied. This is believed to be due to an intense scattering layer with very high quality coefficient overlying a more homogeneous elastic medium. Titles and abstracts of related published papers are included.

The regolith at the Apollo 15 site and its stratigraphic implications

USGS Publications Warehouse

Carr, M.H.; Meyer, C.E.

1974-01-01

Regolith samples from the Apollo 15 landing site are described in terms of two major fractions, a homogeneous glass fraction and a non-homogeneous glass fraction. The proportions of different components in the homogeneous glass fraction were determined directly by chemical analyses of individual particles. They are mainly green glass, a mare-like glass, and different types of Fra Mauro and Highland type glasses. The proportions of various components in the remainder of each of the soils were determined indirectly by finding the mix of components that best fits their bulk compositions. The mixing model suggests that the Apennine Front consists mainly of rocks of low-K Fra Mauro basalt composition. These may overlie rocks with the composition of anorthositic gabbro. Green glass, which occurs widely throughout the site is believed to be derived from a green glass layer which darkens upland surfaces and lies beneath the local mare surface. ?? 1974.

The Use of Polliniferous Resources by Melipona capixaba, an Endangered Stingless Bee Species

PubMed Central

Serra, Bruna Danielle Vieira; da Luz, Cynthia Fernandes Pinto; Campos, Lucio Antonio de Oliveira

2012-01-01

Pollen types present in samples from corbiculae of Melipona capixaba (Moure and Camargo) (Hymenoptera: Apidae: Meliponina) worker bees were analyzed, as well as pollen samples from food pots inside the hives in three sites located at the bees’ original habitat. The aim was to find out the sources used as a trophic resource by this species. The dominant pollen grains in the spectrum of the samples belonged to the families Myrtaceae and Melastomataceae. Eucalyptus was the most frequent pollen type in the corbiculae in Conceição do Castelo municipality; Eucalyptus, Myrcia, and Melastomatacea/Combretaceae in the Fazenda do Estado district; and Eucalyptus and Myrcia in the São Paulo de Aracê district, both in the Domingos Martins municipality. Eucalyptus and Melastomataceae/Combretaceae were the predominant pollen types in the food pots. Eucalyptus was the most prevalent type all year round or most of the year. The most common pollen types in the months that Eucalyptus was not present or dominant in the samples were of remaining native forest species, “ruderal” (field) plants, fruit-bearing plants, and introduced ornamental plants. PMID:23464528

Oxalate secretion by ectomycorrhizal Paxillus involutus is mineral-specific and controls calcium weathering from minerals

NASA Astrophysics Data System (ADS)

Schmalenberger, A.; Duran, A. L.; Bray, A. W.; Bridge, J.; Bonneville, S.; Benning, L. G.; Romero-Gonzalez, M. E.; Leake, J. R.; Banwart, S. A.

2015-07-01

Trees and their associated rhizosphere organisms play a major role in mineral weathering driving calcium fluxes from the continents to the oceans that ultimately control long-term atmospheric CO2 and climate through the geochemical carbon cycle. Photosynthate allocation to tree roots and their mycorrhizal fungi is hypothesized to fuel the active secretion of protons and organic chelators that enhance calcium dissolution at fungal-mineral interfaces. This was tested using 14CO2 supplied to shoots of Pinus sylvestris ectomycorrhizal with the widespread fungus Paxillus involutus in monoxenic microcosms, revealing preferential allocation by the fungus of plant photoassimilate to weather grains of limestone and silicates each with a combined calcium and magnesium content of over 10 wt.%. Hyphae had acidic surfaces and linear accumulation of weathered calcium with secreted oxalate, increasing significantly in sequence: quartz, granite < basalt, olivine, limestone < gabbro. These findings confirmed the role of mineral-specific oxalate exudation in ectomycorrhizal weathering to dissolve calcium bearing minerals, thus contributing to the geochemical carbon cycle.

Investigation of Effective Material Properties of Stony Meteorites

NASA Technical Reports Server (NTRS)

Agrawal, Parul; Carlozzi, Alex; Bryson, Kathryn

2016-01-01

To assess the threat posed by an asteroid entering Earth's atmosphere, one must predict if, when, and how it fragments during entry. A comprehensive understanding of the Asteroid material properties is needed to achieve this objective. At present, the meteorite material found on Earth are the only objects from an entering asteroid that can be used as representative material and be tested inside a laboratory setting. Therefore, unit cell models are developed to determine the effective material properties of stony meteorites and in turn deduce the properties of asteroids. The unit cell is representative volume that accounts for diverse minerals, porosity, and matrix composition inside a meteorite. The various classes under investigation includes H-class, L-class, and LL-class chondrites. The effective mechanical properties such as Young's Modulus and Poisson's Ratio of the unit cell are calculated by performing several hundreds of Monte-Carlo simulations. Terrestrial analogs such as Basalt and Gabbro are being used to validate the unit cell methodology.

Ordovician volcanic and plutonic complexes of the Sakmara allochthon in the southern Urals

NASA Astrophysics Data System (ADS)

Ryazantsev, A. V.; Tolmacheva, T. Yu.

2016-11-01

The Ordovician terrigenous, volcanic-sedimentary and volcanic sequences that formed in rifts of the active continental margin and igneous complexes of intraoceanic suprasubduction settings structurally related to ophiolites are closely spaced in allochthons of the Sakmara Zone in the southern Urals. The stratigraphic relationships of the Ordovician sequences have been established. Their age and facies features have been specified on the basis of biostratigraphic and geochronological data. The gabbro-tonalite-trondhjemite complex and the basalt-andesite-rhyolite sequence with massive sulfide mineralization make up a volcanic-plutonic association. These rock complexes vary in age from Late Ordovician to Early Silurian in certain structural units of the Sakmara Allochthon and to the east in the southern Urals. The proposed geodynamic model for the Ordovician in Paleozoides of the southern Urals reconstructs the active continental margin, whose complexes formed under extension settings, and the intraoceanic suprasubduction structures. The intraoceanic complexes display the evolution of a volcanic arc, back-, or interarc trough.

The main features of the Uralian Paleozoic magmatism and the epioceanic nature of the orogen

NASA Astrophysics Data System (ADS)

Fershtater, G. B.

2013-02-01

The 2000 km Uralian Paleozoic orogen is situated on the western flank of the Uralo-Mongolian folded belt. It is characterized by an abundant variety of magmatic rocks and related ore deposits. Uralian Paleozoic magmatism is entirely subduction-related. It is proposed that the Uralian orogen represents a cold mobile belt in which the mantle temperature was 200 to 500 °C cooler than in the adjacent areas; a situation which is similar to the modern West Pacific Triangle Zone including Indonesia, the Philippine Islands, and southern Asia. During the course of the geological evolution of the Uralian orogen, the nature of the magmatism has changed from basic rocks of indisputable mantle origin (460-390 Ma) to mantle-crust gabbro-granitic complexes (370-315 Ma) followed by pure crustal granite magmatism (290-250 Ma). This order in rock type and age reflects the evolution of Paleozoic magmatic complexes from the beginning of subduction to the final stages of the orogen development.

Fossil intermediate-depth earthquakes in subducting slabs linked to differential stress release

NASA Astrophysics Data System (ADS)

Scambelluri, Marco; Pennacchioni, Giorgio; Gilio, Mattia; Bestmann, Michel; Plümper, Oliver; Nestola, Fabrizio

2017-12-01

The cause of intermediate-depth (50-300 km) seismicity in subduction zones is uncertain. It is typically attributed either to rock embrittlement associated with fluid pressurization, or to thermal runaway instabilities. Here we document glassy pseudotachylyte fault rocks—the products of frictional melting during coseismic faulting—in the Lanzo Massif ophiolite in the Italian Western Alps. These pseudotachylytes formed at subduction-zone depths of 60-70 km in poorly hydrated to dry oceanic gabbro and mantle peridotite. This rock suite is a fossil analogue to an oceanic lithospheric mantle that undergoes present-day subduction. The pseudotachylytes locally preserve high-pressure minerals that indicate an intermediate-depth seismic environment. These pseudotachylytes are important because they are hosted in a near-anhydrous lithosphere free of coeval ductile deformation, which excludes an origin by dehydration embrittlement or thermal runaway processes. Instead, our observations indicate that seismicity in cold subducting slabs can be explained by the release of differential stresses accumulated in strong dry metastable rocks.

Interplay between magmatic accretion, spreading asymmetry and detachment faulting at a segment end: Crustal structure south of the Ascension Fracture Zone

NASA Astrophysics Data System (ADS)

Bialas, Jörg; Dannowski, Anke; Reston, Timothy J.

2015-12-01

A wide-angle seismic section across the Mid-Atlantic Ridge just south of the Ascension transform system reveals laterally varying crustal thickness, and to the east a strongly distorted Moho that appears to result from slip along a large-offset normal fault, termed an oceanic detachment fault. Gravity modelling supports the inferred crustal structure. We investigate the interplay between magmatism, detachment faulting and the changing asymmetry of crustal accretion, and consider several possible scenarios. The one that appears most likely is remarkably simple: an episode of detachment faulting which accommodates all plate divergence and results in the westward migration of the ridge axis, is interspersed with dominantly magmatic and moderately asymmetric (most on the western side) spreading which moves the spreading axis back towards the east. Following the runaway weakening of a normal fault and its development into an oceanic detachment fault, magma both intrudes the footwall to the fault, producing a layer of gabbro (subsequently partially exhumed).

Survey of existing underground openings for in-situ experimental facilities

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

Wollenberg, H.; Graf, A.; Strisower, B.

1981-07-01

In an earlier project, a literature search identified 60 underground openings in crystalline rock capable of providing access for an in-situ experimental facility to develop geochemical and hydrological techniques for evaluating sites for radioactive waste isolation. As part of the current project, discussions with state geologists, owners, and operators narrowed the original group to 14. Three additional sites in volcanic rock and one site in granite were also identified. Site visits and application of technical criteria, including the geologic and hydrologic settings and depth, extent of the rock unit, condition, and accessibility of underground workings, determined four primary candidate sites:more » the Helms Pumped Storage Project in grandiodorite of the Sierra Nevada, California; the Tungsten Queen Mine in Precambrian granodiorite of the North Carolina Piedmont; the Mount Hope Mine in Precambrian granite and gneiss of northern New Jersey; and the Minnamax Project in the Duluth gabbro complex of northern Minnesota.« less

Widespread abiotic methane in chromitites.

PubMed

Etiope, G; Ifandi, E; Nazzari, M; Procesi, M; Tsikouras, B; Ventura, G; Steele, A; Tardini, R; Szatmari, P

2018-06-07

Recurring discoveries of abiotic methane in gas seeps and springs in ophiolites and peridotite massifs worldwide raised the question of where, in which rocks, methane was generated. Answers will impact the theories on life origin related to serpentinization of ultramafic rocks, and the origin of methane on rocky planets. Here we document, through molecular and isotopic analyses of gas liberated by rock crushing, that among the several mafic and ultramafic rocks composing classic ophiolites in Greece, i.e., serpentinite, peridotite, chromitite, gabbro, rodingite and basalt, only chromitites, characterized by high concentrations of chromium and ruthenium, host considerable amounts of 13 C-enriched methane, hydrogen and heavier hydrocarbons with inverse isotopic trend, which is typical of abiotic gas origin. Raman analyses are consistent with methane being occluded in widespread microfractures and porous serpentine- or chlorite-filled veins. Chromium and ruthenium may be key metal catalysts for methane production via Sabatier reaction. Chromitites may represent source rocks of abiotic methane on Earth and, potentially, on Mars.

Chemistry of the Apollo 11 highland component

NASA Technical Reports Server (NTRS)

Laul, J. C.; Papike, J. J.; Simon, S. B.; Shearer, C. K.

1983-01-01

Thirty-eight Apollo 11 lunar highland fragments from coarse fines 10085 have been subjected to petrologic and chemical study. Six major chemical groups are identified: (a) high-K KREEP; (b) anorthosite with a 10X chondrite positive Eu anomaly and anorthosite with 30X positive Eu anomaly; (c) ANT; (d) LKFM; (e) anorthositic gabbro with no Eu anomaly, with a positive Eu anomaly, and with a negative Eu anomaly; and (f) dominant Highland component, 2X-10X chondrite with a positive 10X-14X Eu anomaly. Newly recognized groups are presented based on the REE patterns: (a) ANT group with 5X La and a 22X positive Eu anomaly; (b) 10X flat with 14X positive Eu anomaly; and (c) 2-3X flat with a 10X positive Eu anomaly. The highland suite is very low in K and REE, and is overall quite similar to the Apollo 16 suite.

Volatilization and mixing in glasses of some Apollo 14 regolith breccias

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

Vaniman, D.T.; Heiken, G.H.

1989-01-01

Three unique samples can be distinguished by analysis of all glass types, including devitrified glasses, in a suite of 26 Apollo 14 regolith breccias. These unique samples include the well-studied sample 14315, which has an abundance of anorthositic gabbro glasses and devitrified glasses; 14004,77, which has no glasses other than those that match the local soil; and 14076,5, which contains no glasses similar to the local soil or to LKFM. Sample 14076,5 is clearly exotic, for it contains devitrified glasses of anorthositic composition and of a silica-volatilized (HASP) trend that stems from anorthosite; these silica-volatilized glasses contain the new mineralmore » yoshiokite. HASP glasses in this exotic sample and HASP glass spheres that stem from the Apollo 14 soil composition differ greatly from the HASP glasses at Apollo 16. The various HASP glasses can be just as useful as non-volatilized glasses in searching or major crustal or regolith lithologies. 18 refs., 2 figs., 2 tabs.« less

Apollo 16 site geology and impact melts - Implications for the geologic history of the lunar highlands

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

Spudis, P.D.

1984-11-15

The geology of the Apollo 16 site is reconsidered on the basis of data from photogeology, geochemical remote sensing, and lunar samples. The site possesses an upper surface of anorthositic gabbro and related rocks. Mafic components were deposited as basin ejecta. The events involved in its geological evolution were the Nectaris impact and the Imbrium impact. The role of large, local craters in the history of the region was to serve as topographic depressions to accumulate basin ejecta. The most abundant melt composition at Apollo 16 is an aluminous variety of LKFM basalt supplied by the Nectaris impact as ejectedmore » basin impact melt. The mafic LKFM melt may have been supplied by the Imbrium impact. More aluminous melt groups are probably derived from local, small craters. The remainder of the deposits in the region are composed of anorthositic clastic debris derived from the Nectaris basin, the local crustal substrate, and Imbrium and other basins.« less

Feasible metabolisms in high pH springs of the Philippines

PubMed Central

Cardace, Dawn; Meyer-Dombard, D'Arcy R.; Woycheese, Kristin M.; Arcilla, Carlo A.

2015-01-01

A field campaign targeting high pH, H2-, and CH4-emitting serpentinite-associated springs in the Zambales and Palawan Ophiolites of the Philippines was conducted in 2012-2013, and enabled description of several springs sourced in altered pillow basalts, gabbros, and peridotites. We combine field observations of pH, temperature, conductivity, dissolved oxygen, and oxidation-reduction potential with analyses of major ions, dissolved inorganic carbon, dissolved organic carbon, and dissolved gas phases in order to model the activities of selected phases important to microbial metabolism, and to rank feasible metabolic reactions based on energy yield. We document changing geochemical inventories in these springs between sampling years, and examine how the environment supports or prevents the function of certain microbial metabolisms. In all, this geochemistry-based assessment of feasible metabolisms indicates methane cycling, hydrogen oxidation, some iron and sulfur metabolisms, and ammonia oxidation are feasible reactions in this continental site of serpentinization. PMID:25713561

Petrogenesis of the Pd-rich intrusion at Salt Chuck, Prince of Wales island: an early Paleozoic Alaskan-type ultramafic body

USGS Publications Warehouse

Loney, R.A.; Himmelberg, G.R.

1992-01-01

The early Paleozoic Salt Chuck intrusion has petrographic and chemical characteristics that are similar to those of Cretaceous Alaskan-type ultramafic-mafic bodies. The intrusion is markedly discordant to the structure of the early Paleozoic Descon Formation, in which it has produced a rather indistinct contact aureole a few meters wide. Mineral assemblages, sequence of crystallization, and mineral chemistry suggest that the intrusion crystallized under low pressures (~2 kbar) with oxidation conditions near those of the NNO buffer, from a hydrous, silica-saturated, orthopyroxene-normative parental magma. The Salt Chuck deposit was probably formed by a two-stage process: 1) a stage of magmatic crystallization in which the sulfides and PGE accumulated in a disseminated manner in cumulus deposits, possibly largely in the gabbro, and 2) a later magmatic-hydrothermal stage during which the sulfides and PGE were remobilized and concentrated in veins and fracture-fillings. In this model, the source of the sulfides and PGE was the magma that produced the Salt Chuck intrusion. -from Authors

Feasible metabolisms in high pH springs of the Philippines.

PubMed

Cardace, Dawn; Meyer-Dombard, D'Arcy R; Woycheese, Kristin M; Arcilla, Carlo A

2015-01-01

A field campaign targeting high pH, H2-, and CH4-emitting serpentinite-associated springs in the Zambales and Palawan Ophiolites of the Philippines was conducted in 2012-2013, and enabled description of several springs sourced in altered pillow basalts, gabbros, and peridotites. We combine field observations of pH, temperature, conductivity, dissolved oxygen, and oxidation-reduction potential with analyses of major ions, dissolved inorganic carbon, dissolved organic carbon, and dissolved gas phases in order to model the activities of selected phases important to microbial metabolism, and to rank feasible metabolic reactions based on energy yield. We document changing geochemical inventories in these springs between sampling years, and examine how the environment supports or prevents the function of certain microbial metabolisms. In all, this geochemistry-based assessment of feasible metabolisms indicates methane cycling, hydrogen oxidation, some iron and sulfur metabolisms, and ammonia oxidation are feasible reactions in this continental site of serpentinization.

Pb-isotopic Features of Primitive Rocks from Hess Deep: Distinguishing between EPR and Cocos-Nazca Mantle Source(s)

NASA Astrophysics Data System (ADS)

Jean, M. M.; Falloon, T.; Gillis, K. M.

2014-12-01

We have acquired high-precision Pb-isotopic signatures of primitive lithologies (basalts/gabbros) recovered from IODP Expedition 345.The Hess Deep Rift, located in the vicinity of the Galapagos triple junction (Cocos, Nazca, and Pacific), is viewed as one the best-studied tectonic windows into fast-spreading crust because a relatively young (<1.5 Ma) cross section of oceanic crust. This allows for (1) characterization of the mantle source(s) at Hess Deep, (2) insight into the extent of isotopic homogeneity or heterogeneity in the area, and (3) constrain the relative contributions from the intruding Cocos-Nazca spreading center. The observed Pb-isotopic variation at Hess Deep covers almost the entire range of EPR MORB (10°N to -5°S). Hess Deep samples range from 208Pb (37.3-38.25), 207Pb (15.47-15.58), 206Pb (17.69-18.91). These compositions suggest that this part of Hess Deep mantle is no more isotopically homogeneous than EPR mantle. Two distinct arrays are also observed: 208Pb-enriched (r2=0.985; n=30) and 208Pb-depleted (r2=0.988; n=6). The 208Pb/204Pb isotopes indicates that the Pb-source for some of the samples at Hess Deep had very low Th/U ratios, whereas other areas around the Galapagos microplate seem to have more "normal" ratios. These trends are less apparent when viewed with 207Pb-isotopes. Instead, the majority of basalts and gabbros follow the NHRL, however, at the depleted-end of this array a negative excursion to more enriched compositions is observed. This negative but linear trend could signify an alteration trend or mixing with an EMI-type mantle source, yet this mixing is not observed with 208Pb. This trend is also observed at Pito Deep, which has similar origins to Hess Deep (Barker et al., 2008; Pollack et al., 2009). The Galapagos region has been considered a testing ground for mixing of HIMU, Enriched Mantle, and Depleted Mantle reservoirs (e.g., Schilling et al., 2002). According to our data, however, an EPR-component must also be

Normal and anomalous AMS fabrics in gabbroic sills: examples from the Karoo Large Igneous Province

NASA Astrophysics Data System (ADS)

Lehman, A.; Ferre, E. C.; Maes, S. M.; Geissman, J. W.; Marsh, M. C.; Mare, L. P.; Marsh, J.

2010-12-01

The magnetic fabric of plutonic rocks has often been used as a proxy for magma flow. Yet, a number of studies suggest that the relationship between the principal axes of the anisotropy of magnetic susceptibility (AMS) and the flow referencial axes are not simple. Several case studies on mafic dikes have shown that complications may arise from the contribution of single domain (SD) magnetite grains, imbricated symmetric fabrics along dike margins and post-solidification thermal stresses. Fewer investigations have been dedicated to subhorizontal tabular intrusions despite the fact that they could also yield valuable clues regarding the various processes that might cause anomalous AMS fabrics. The Karoo Large Igneous Province in South Africa hosts a remarkably impressive set of undeformed, stacked gabbroic sills that were intruded parallel to bedding in the Karoo Basin. The sills range in thickness from about 1 m up to 1000 m and have a relatively constant petrological composition of gabbros and gabbro-norites. Theses sills are distributed throughout the whole Karoo Basin and were emplaced at various stratigraphic heights / depths in the Karoo stratigraphic column. Oriented core samples were collected from 30 different sills and yielded 1598 specimens for AMS, AARM and paleomagnetic measurements. The low-field magnetic susceptibility Km ranges widely from about 100 to 20,000 x 10-6 [SI], while the degree of anisotropy P' ranges from 1.01 to 1.10. A broad correlation between Km and P' is observed. Thermomagnetic experiments reveal that the main magnetic carrier is titanomagnetite with variable ulvöspinel content. This is confirmed by measurement of hysteresis properties that also indicate that titanomagnetite in general has a pseudo-single domain grain size. The directional data is consistent with the nearly horizontal attitude of the sill in 23 out of 30 sills, with subvertical K3 axes. In 5 out of 30 sills, K3 axes are subhorizontal, characterized by scattered

Potassium Isotopes as a New Tracer of Seafloor Hydrothermal Alteration: The Bay of Islands Ophiolite

NASA Astrophysics Data System (ADS)

Parendo, C. A.; Jacobsen, S. B.; Wang, K.

2016-12-01

Hydrothermal circulation at and around oceanic spreading ridges results in elemental exchange between seawater and oceanic crust, with profound implications for both the ionic composition of seawater and the elemental composition of various solid-Earth reservoirs over geological time. Potassium is among the elements known to be mobile during hydrothermal alteration. Here we investigate the isotopic character of this K exchange by obtaining high-precision 41K/39K data for 6 samples from the Bay of Islands Ophiolite, Newfoundland, Canada—a piece of ca. 485 Ma oceanic crust that was affected by seafloor hydrothermal alteration prior to being obducted. Our 41K/39K analyses are generated using an Isoprobe-P MC-ICPMS equipped with a hexapole collision and reaction cell, which essentially eliminates interferences from the K-isotope mass spectrum. The analyses have an external reproducibility of about 0.07‰ (2SD). We find that the 41K/39K ratios of the ophiolite rocks span a range of approximately 0.70‰ and covary with previously determined 87Sr/86Sr ratios. The stratigraphically deepest, least-altered sample (an olivine gabbro) has a 41K/39K ratio within error of that typically observed in common igneous rocks. The stratigraphically higher, more-altered samples (which include hornblende gabbro, plagiogranite, diabase, and basalt) have 41K/39K ratios that are markedly heavier. This variability in 41K/39K ratios is interpreted to reflect variable addition of seawater K to the rocks. A simple open-system water-rock interaction calculation shows that the covariation between 41K/39K and 87Sr/86Sr can be plausibly explained as the result of hydrothermal alteration. The simplest scenario assumes that the 41K/39K ratio of seawater at the time of interest was similar to its present-day value, in which case the calculation suggests that isotopically heavy seawater K is added to oceanic crust with little fractionation—i.e., an effective fractionation factor near 0.0‰. The

A Little Island With A Big Secret: Isla Rábida, Galápagos

NASA Astrophysics Data System (ADS)

Bercovici, H.; Geist, D.; Harpp, K. S.; Almeida, M.; Mahr, J.; Pimentel, R.; Cleary, Z.

2016-12-01

The Galápagos Archipelago is a hotspot island chain 1000 km west of Ecuador, where the vast majority of the lavas are basaltic. Four volcanoes in the archipelago, Rábida, Santiago, Pinzón, and Alcedo, erupt rhyolites and trachytes. Isla Rábida, a small island 50 km east of the mantle plume center, is the focus of this project. It is 5 km2 in area, and lavas range from 0.9 to 1.1 Ma. About 25% of the rocks in our suite are intermediate to felsic, extending from Mg#=2 to 57. Major and trace element data indicate the evolved rocks formed by advanced crystallization of basaltic magma. One of the unique aspects of Rábida is the cumulate xenolith suite ranging from olivine gabbro to ferroan granite. The basalts have 6 to 58 modal% plagioclase phenocrysts, which we interpret as mixtures of melt and accumulated plagioclase mush at the margins of the shallow reservoir. Thus, Rábida erupts material that has undergone different extents of crystallization and crystal sorting from pure melts, to melt-mush hybrids, to solidified cumulates. This hypothesis is evaluated by comparing plagioclase compositions from the xenoliths and the lavas. Plagioclases in two of the lavas, one with Mg#=57 and the other with Mg#=36, have similar compositions and zonation patterns to each other. There is on average less than 4% change in anorthite content from the core of the plagioclases in the basalts to the rim, with the compositions overall varying between An22 and An37. Both melts likely picked up the crystals from the same plagioclase mush before eruption. In comparison to plagioclases in an olivine-gabbro xenolith from Rábida, those in the lavas are less zoned, suggesting that the lavas' plagioclases experienced a different growth environment. Plagioclases in the xenolith are normally zoned, with cores averaging An37 and rims averaging An32. The xenolith's plagioclases also have more diverse compositions than those in the lavas. The normal zoning in the xenolith's plagioclase is

A holistic model for the role of the axial melt lens at fast-spreading mid-ocean ridges

NASA Astrophysics Data System (ADS)

MacLeod, C. J.; Loocke, M. P.; Lissenberg, J. C. J.

2016-12-01

Axial melt lenses (AML) are melt or crystal mush1 bodies located at the dyke-gabbro transition beneath intermediate- and fast-spreading mid-ocean ridges (MORs)2,3. Although it is generally thought that AMLs play a major role in the storage and differentiation of mid-ocean ridge basalts (MORB)1, the melt compositions within the AML and its role in the accretion of the lower crust are heavily debated4-6. Here we present the first comprehensive study of the AML horizon at a fast-spreading MOR (Hess Deep, equatorial Pacific Ocean). We show that plagioclase and pyroxene within the AML are much too evolved to be in equilibrium with MORB, with mean An (54.85) and Mg# (65.01) consistent with derivation from basaltic andesite to andesite melts (Mg# 43-26). We propose that, in between decadal eruptions, the AML is predominantly crystal mush and is fed by small volumes of evolved interstitial melts. Short-lived, focused injection of primitive melt leads to mixing of primitive melts with the extant highly fractionated melt, and triggers eruptions. This model reconciles the paradoxical compositional mismatch between the volcanic and plutonic records with the geophysical characteristics of the AML, the short residence times of Pacific MORB phenocrysts, and the incompatible trace element over-enrichments in MORB. 1Marjanović, M. et al., 2015. Distribution of melt along the East Pacific Rise from 9°30' to 10°N from an amplitude variation with angle of incidence (AVA) technique. Geophys. J. Int. 203. 2Detrick, R. S. et al., 1987. Multi-channel seismic imaging of a crustal magma chamber along the EPR. Nature 326. 3Sinton, J. M. & Detrick, R. S., 1992. Mid-ocean ridge magma chambers. J. Geophys. Res. 97. 4Coogan, L. A., Thompson, G. & MacLeod, C. J., 2002. A textural and geochemical investigation of high level gabbros from the Oman ophiolite: implications for the role of the axial magma chamber at fast-spreading ridges. Lithos 63. 5Pan, Y. & Batiza, R., 2002. Mid-ocean ridge magma

Zircon U-Pb age, Hf isotopic compositions and geochemistry of the Silurian Fengdingshan I-type granite Pluton and Taoyuan mafic-felsic Complex at the southeastern margin of the Yangtze Block

NASA Astrophysics Data System (ADS)

Zhong, Yufang; Ma, Changqian; Zhang, Chao; Wang, Shiming; She, Zhenbing; Liu, Lei; Xu, Haijin

2013-09-01

This work presents an integrated study of zircon U-Pb ages and Hf isotope along with whole-rock geochemistry on Silurian Fengdingshan I-type granites and Taoyuan mafic-felsic intrusive Complex located at the southeastern margin of the Yangtze Block, filling in a gap in understanding of Paleozoic I-type granites and mafic-intermediate igneous rocks in the eastern South China Craton (SCC). The Fengdingshan granite and Taoyuan hornblende gabbro are dated at 436 ± 5 Ma and 409 ± 2 Ma, respectively. The Fengdingshan granites display characteristics of calc-alkaline I-type granite with high initial 87Sr/86Sr ratios of 0.7093-0.7127, low ɛNd(t) values ranging from -5.6 to -5.4 and corresponding Nd model ages (T2DM) of 1.6 Ga. Their zircon grains have ɛHf(t) values ranging from -2.7 to 2.6 and model ages of 951-1164 Ma. The Taoyuan mafic rocks exhibit typical arc-like geochemistry, with enrichment in Rb, Th, U and Pb and depletion in Nb, Ta. They have initial 87Sr/86Sr ratios of 0.7053-0.7058, ɛNd(t) values of 0.2-1.6 and corresponding T2DM of 1.0-1.1 Ga. Their zircon grains have ɛHf(t) values ranging from 3.2 to 6.1 and model ages of 774-911 Ma. Diorite and granodiorite from the Taoyuan Complex have initial 87Sr/86Sr ratios of 0.7065-0.7117, ɛNd(t) values from -5.7 to -1.9 and Nd model ages of 1.3-1.6 Ga. The petrographic and geochemical characteristics indicate that the Fengdingshan granites probably formed by reworking of Neoproterozoic basalts with very little of juvenile mantle-derived magma. The Taoyuan Complex formed by magma mixing and mingling, in which the mafic member originated from a metasomatized lithospheric mantle. Both the Fengdingshan and Taoyuan Plutons formed in a post-orogenic collapse stage in an intracontinental tectonic regime. Besides the Paleozoic Fengdingshan granites and Taoyuan hornblende gabbro, other Neoproterozoic and Indosinian igneous rocks located along the southeastern and western margin of the Yangtze Block also exhibit decoupled

Velocity Structure of the Subducted Yakutat Terrane, Alaska: Insights from Guided Waves

NASA Astrophysics Data System (ADS)

Coulson, S.; Garth, T.; Rietbrock, A.

2017-12-01

Subduction zone guided wave arrivals from intermediate depth earthquakes provide insight into the fine scale velocity structure of the subducting oceanic crust as it dehydrates. These observations can be used to determine the average velocity and thickness of the crustal low velocity layer (LVL) at depth, allowing inferences to be drawn about composition and degree of hydration. We constrain guided wave dispersion by comparing waveforms recorded in the subduction forearc with simulated waveforms, produced using a 2D finite difference waveform propagation model. The structure of the Aleutian arc is complex due to the accretion of the Yakutat Terrane (YT) to the east, which is partially coupled with the subducting Pacific plate. An unusually thick LVL associated with the YT has been inferred down to 140 km depth by receiver function studies and travel time tomography. Focussing on a profile running NNW-SSE close to Anchorage, we constrain slab geometry using global and local catalogues, as well as the curvature inferred from receiver functions (Kim et al., 2014). P-wave arrivals from 41 earthquakes (2012-2015) show significant guided wave dispersion on at least one station; high frequency (>1-3 Hz) energy is delayed by up to 2-3 seconds. Choosing the clearest dispersion observations, we systematically vary both LVL width and P-wave velocity, to find the lowest misfit between the observed and synthetic waveforms. Multiple modelled events show the thickness of the LVL associated with subducted YT to be 6-10 km, significantly thinner than inferred by receiver function studies. Most events are accounted for by an LVL velocity contrast of 12.5-15% with overriding mantle material, however, observations of the deepest event in the northern corner of the YT require a velocity contrast of 6%. Lower velocities in the shallower slab (70-120 km) cannot be accounted for by reacted or unreacted MORB or gabbro compositions. We postulate the presence of interbedded sediments within

Chronology and volcanology of the 1949 multi-vent rift-zone eruption on La Palma (Canary Islands)

NASA Astrophysics Data System (ADS)

Klügel, A.; Schmincke, H.-U.; White, J. D. L.; Hoernle, K. A.

1999-12-01

The compositionally zoned San Juan eruption on La Palma emanated from three eruptive centers located along a north-south-trending rift zone in the south of the island. Seismic precursors began weakly in 1936 and became strong in March 1949, with their foci progressing from the north of the rift zone towards its south. This suggests that magma ascended beneath the old Taburiente shield volcano and moved southward along the rift. The eruption began on June 24, 1949, with phreatomagmatic activity at Duraznero crater on the ridgetop (ca. 1880 m above sea level), where five vents erupted tephritic lava along a 400-m-long fissure. On June 8, the Duraznero vents shut down abruptly, and the activity shifted to an off-rift fissure at Llano del Banco, located at ca. 550 m lower elevation and 3 km to the northwest. This eruptive center issued initially tephritic aa and later basanitic pahoehoe lava at high rates, producing a lava flow that entered the sea. Two days after basanite began to erupt at Llano del Banco, Hoyo Negro crater (ca. 1880 m asl), located 700 m north of Duraznero along the rift, opened on July 12 and produced ash and bombs of basanitic to phonotephritic composition in violent phreatomagmatic explosions ( White and Schmincke, 1999). Llano del Banco and Hoyo Negro were simultaneously active for 11 days and showed a co-variance of their eruption rates indicating a shallow hydraulic connection. On July 30, after 3 days of quiescence at all vents, Duraznero and Hoyo Negro became active again during a final eruptive phase. Duraznero issued basanitic lava at high rates for 12 h and produced a lava flow that descended towards the east coast. The lava contains ca. 1 vol.% crustal and mantle xenoliths consisting of 40% tholeiitic gabbros from the oceanic crust, 35% alkaline gabbros, and 20% ultramafic cumulates. The occurrence of xenoliths almost exclusively in the final lava is consistent with their origin by wall-rock collapse at depth near the end of the eruption

The evolution of young silicic lavas at Medicine Lake Volcano, California: Implications for the origin of compositional gaps in calc-alkaline series lavas

USGS Publications Warehouse

Grove, T.L.; Donnelly-Nolan, J. M.

1986-01-01

At Medicine Lake Volcano, California, the compositional gap between andesite (57-62 wt.% SiO2) and rhyolite (73-74 wt.% SiO2) has been generated by fractional crystallization. Assimilation of silicic crust has also occurred along with fractionation. Two varieties of inclusions found in Holocene rhyolite flows, hornblende gabbros and aphyric andesites, provide information on the crystallization path followed by lavas parental to the rhyolite. The hornblende gabbros are magmatic cumulate residues and their mineral assemblages are preserved evidence of the phases that crystallized from an andesitic precursor lava to generate the rhyolite lavas. The andesitic inclusions represent samples of a parental andesite and record the early part of the differentiation history. Olivine, plagioclase and augite crystallization begins the differentiation history, followed by the disappearance of olivine and augite through reaction with the liquid to form orthopyroxene and amphibole. Further crystallization of the assemblage plagioclase, amphibole, orthopyroxene, magnetite, and apatite from a high-SiO2 andesite leads to rhyolite. This final crystallization process occurs on a cotectic that is nearly horizontal in temperature-composition space. Since a large amount of crystallization occurs over a limited temperature interval, a compositional gap develops between rhyolite and high SiO2 andesite. Liquidus surfaces with shallow slopes in temperature-composition space are characteristic of several late-stage crystallization assemblages in the andesite to rhyolite compositional range. Experimentally produced plagioclase+ amphibole+orthopyroxene+magnetite and plagioclase+ augite+low-Ca pyroxene+magnetite cotectics have liquidus slopes that are nearly flat. At other calc-alkaline volcanic centers crystallization processes involving large compositional changes over small temperature intervals may also be important in the development of bimodal volcanism (i.e. the existence of a composition

Petrography, geochemistry, and U-Pb geochronology of pegmatites and aplites associated with the Alvand intrusive complex in the Hamedan region, Sanandaj-Sirjan zone, Zagros orogen (Iran)

NASA Astrophysics Data System (ADS)

Sepahi, Ali Asghar; Salami, Sedigheh; Lentz, David; McFarlane, Christopher; Maanijou, Mohammad

2018-04-01

The Alvand intrusive complex in the Hamedan area in Iran is in the Sanandaj-Sirjan zone of the Zagros orogen. It consists of a wide range of plutonic rocks, mainly gabbro, diorite, granodiorite, granite, and leucogranites that were intruded by aplitic and pegmatitic dykes. At least three successive magmatic episodes generated an older gabbro-diorite-tonalite assemblage, followed by a voluminous granodiorite-granite association, which was then followed by minor leucocratic granitoids. Aplitic and pegmatitic dykes and bodies have truncated both plutonic rocks of the Alvand intrusive complex and its metamorphic aureole. Chemically they belong to peraluminous LCT (Li-, Cs-, and Ta-bearing) family of pegmatites. Mineralogically, they resemble Muscovite (MS) and Muscovite Rare Element (MSREL) classes of pegmatites. High amounts of some elements, such as Sn (up to 10,000 ppm), Rb (up to 936 ppm), Ba (up to 706 ppm), and LREE (up to 404 ppm) indicate the highly fractionated nature of some of these aplites and pegmatites. U-Pb dating of monazite, zircon, and allanite by LA-ICPMS indicate the following ages: monazite-bearing aplites of Heydareh-e-Poshteshahr and Barfejin areas, southwest of Hamedan, give an age range of 162-172 Ma; zircon in Heydareh-e-Poshteshar gives an average age of 165 Ma and for allanite-bearing pegmatites of Artiman area, north of Tuyserkan, an age of 154.1 ± 3.7 Ma was determined. These overlap with previously reported ages (ca. 167-153 Ma) for the plutonic rocks of the Alvand complex. Therefore, these data reveal that the Jurassic was a period of magmatism in the Hamedan region and adjacent areas in the Sanandaj-Sirjan zone, which was situated at the southern edge of the central Iranian micro-plate (southern Eurasian plate) at this time. Our results also suggest that advective heating in a continental arc setting has caused melting of fertile supracrustal lithologies, such as meta-pelites. These partial melts were then emplaced at much higher

Geophysical Character and Geochemical Evolution of the Mesoproterozoic Figueira Branca Intrusive Suite, SW Amazon Craton (Brazil)

NASA Astrophysics Data System (ADS)

Louro, Vinicius; Cawood, Peter; Mantovani, Marta

2016-04-01

The Jauru Terrain hosts the Figueira Branca Intrusive Suite (FBS) in the SW of the Amazon Craton (Brazil). The FBS is a series of 1425 Ma layered mafic intrusions, previously interpreted as anorogenic. The FBS area is located in foreland to the Santa Helena orogen, formed by the subduction of the Rio Alegre Terrain under the Jauru Terrain. Potential field methods (magnetic and gravity), gamma-ray spectrometry, geochemical and isotope data were used to characterize and to model the extent of FBS magmatism, the distribution of faults and shear zones in the area, to evaluate affinities of the magmatic activity, and the relation between the FBS and the Santa Helena orogen. The geophysical methods identified three anomalies corresponding with FBS outcrops. A fourth anomaly with significantly higher amplitude was observed to the north of the three anomalies. From south to north, the anomalies were named Indiavaí, Azteca, Figueira Branca and Jauru. These anomalies were modeled and indicated a northwest-southeast trend, parallel to regional shear zones. The gamma-ray data enabled the collection of 50 samples from the FBS rocks, the Alto Jauru group that hosts the FBS, from nearby intrusive suites, and the Rio Alegre Terrain. The 30 freshest samples were analyzed by X-ray fluorescence for oxides and some trace elements, 20 by ICP-MS for Rare-Earth Elements and 10 for Nd-Sr isotope analyses. The FBS samples were gabbros and gabbro-norites with Nb/Yb-Th/Yb and TiO2/Yb-Nb/Yb ratios indicating varying degrees of crustal interaction. The TiO2/Yb-Nb/Yb data suggested a subduction related component and the ɛNd-ɛSr indicated a juvenile source. Samples from coeval adjacent intermediate magma suites displayed similar characteristics, which suggest derivation from a bimodal source probably related with the subduction of the Rio Alegre Terrain. We interpreted the tectonic setting of the FBS as a result of a roll-back of the subducted slab, which resulted in rejuvenation of the

Variable Seawater-Peridotite Interactions - First Insights From ODP Leg 209, MAR 15° N

NASA Astrophysics Data System (ADS)

Bach, W.; Garrido, C. J.; Harvey, J.; Paulick, H.; Rosner, M.; Odp Leg 193 Shipboard Science Party

2003-12-01

Serpentinization of peridotites at slow-spreading mid-ocean ridges has important consequences for the rheology of the oceanic lithosphere, geochemical budgets of the oceans, and microbial processes within, at, and above the seafloor. ODP Leg 209 recovered peridotites that show a remarkable variability of hydrothermal alteration reactions and intensities, including talc-tremolite alteration of pyroxenes associated with incipient serpentinization of olivine, complete alteration of peridotites to serpentine and magnetite followed by the destruction of magnetite and replacement of serpentine by talc, variable degrees of serpentine-brucite alteration, and replacement of brucite by iowaite. These rocks provide a unique opportunity to calibrate our observations against recent results from experimental/theoretical geochemical studies and further our understanding of serpentinization and its role in tectonic accretion and microbial colonization of oceanic lithosphere at slow and ultraslow spreading ridges. We propose that at temperatures above 250° C, pyroxenes react to form serpentine, talc, and tremolite, releasing Ca, Si, H2, and acidity to the reacting fluids that may cause rodingitization in adjacent gabbro bodies. Overall however, rodingites are rare, which may reflect the depleted nature of the mantle protoliths. In the absence of pyroxenes (in dunites) - or at temperatures below 250° C, where pyroxenes react very slowly - the fluids do not become enriched in Ca and Si and serpentine, magnetite, and brucite will form. Many serpentinites lack brucite, tremolite, and talc, because changes in fluid pH and silica activity of the interacting fluids, following the exhaustion of either olivine or pyroxene, caused reaction of these phases to serpentine. Extensive talc alteration of serpentinites and gabbros is usually observed at the intrusive contacts, but large-scale silica metasomatism (or Mg-loss) must be invoked to explain the overall abundance of talc at Site 1268

Hydrothermal Upflow, Serpentinization and Talc Alteration Associated with a High Angle Normal Fault Cutting an Oceanic Detachment, Northern Apennines, Italy

NASA Astrophysics Data System (ADS)

Alt, J.; Crispini, L.; Gaggero, L.; Shanks, W. C., III; Gulbransen, C.; Lavagnino, G.

2017-12-01

Normal faults cutting oceanic core complexes are observed at the seafloor and through geophysics, and may act as flow pathways for hydrothermal fluids, but we know little about such faults in the subsurface. We present bulk rock geochemistry and stable isotope data for a fault that acted as a hydrothermal upflow zone in a seafloor ultramafic-hosted hydrothermal system in the northern Apennines, Italy. Peridotites were exposed on the seafloor by detachment faulting, intruded by MORB gabbros, and are overlain by MORB lavas and pelagic sediments. North of the village of Reppia are fault shear zones in serpentinite, oriented at a high angle to the detachment surface and extending 300 m below the paleo-seafloor. The paleo-seafloor strikes roughly east-west, dipping 30˚ to the north. At depth the fault zone occurs as an anticlinal form plunging 40˚ to the west. A second fault strikes approximately north-south, with a near vertical dip. The fault rock outcrops as reddish weathered talc + sulfide in 0.1-2 m wide anastomosing bands, with numerous splays. Talc replaces serpentinite in the fault rocks, and the talc rocks are enriched in Si, metals (Fe, Cu, Pb), Light Rare Earth Elements (LREE), have variable Eu anomalies, and have low Mg, Cr and Ni contents. In some cases gabbro dikes are associated with talc-alteration and may have enhanced fluid flow. Sulfide from a fault rock has d34S=5.7‰. The mineralogy and chemistry of the fault rocks indicate that the fault acted as the upflow pathway for high-T black-smoker type fluids. Traverses away from the fault (up to 1 km) and with depth below the seafloor (up to 500 m) reveal variable influences of hydrothermal fluids, but there are no consistent trends with distance. Background serpentinites 500 m beneath the paleoseafloor have LREE depleted trends. Other serpentinites exhibit correlations of LREE with HFSE as the result of melt percolation, but there is significant scatter, and hydrothermal effects include LREE enrichment

Field geology, geochronology and geochemistry of mafic-ultramafic rocks from Alxa, China: Implications for Late Permian accretionary tectonics in the southern Altaids

NASA Astrophysics Data System (ADS)

Feng, Jianyun; Xiao, Wenjiao; Windley, Brian; Han, Chunming; Wan, Bo; Zhang, Ji'en; Ao, Songjian; Zhang, Zhiyong; Lin, Lina

2013-12-01

The time of termination of orogenesis for the southern Altaids has been controversial. Systematic investigations of field geology, geochronology and geochemistry on newly discriminated mafic-ultramafic rocks from northern Alxa in the southern Altaids were conducted to address the termination problem. The mafic-ultramafic rocks are located in the Bijiertai, Honggueryulin, and Qinggele areas, stretching from west to east for about 100 km. All rocks occur high-grade gneisses as tectonic lenses that are composed of peridotite, pyroxenite, gabbro, and serpentinite, most of which have undergone pronounced alteration, i.e., serpentinization and chloritization. Geochemically, the rocks are characterized by uniform compositional trends, i.e., with low SiO2-contents (42.51-52.21 wt.%) and alkalinity (Na2O + K2O) (0.01-5.45 wt.%, mostly less than 0.8 wt.%), and enrichments in MgO (7.37-43.36 wt.%), with Mg# = 52.75-91.87. As the rocks have been strongly altered and have a wide range of loss-on-ignition (LOI: 0.44-14.07 wt.%) values, they may have been subjected to considerable alteration by either seawater or metamorphic fluids. The REE and trace element patterns show a relatively fractionated trend with LILE enrichment and HFSE depletion, similar to that of T-MORB between N-MORB and E-MORB, indicating that the parental melt resulted from the partial melting of oceanic lithospheric mantle overprinted by fluid alteration of island-arc origin. The ultramafic rocks are relics derived from the magma after a large degree of partial melting of oceanic lithospheric mantle with superposed island arc processes under the influence of mid-ocean-ridge magmatism. LA-ICP MS U-Pb zircon ages of gabbros from three spots are 274 ± 3 Ma (MSWD = 0.35), 306 ± 3 Ma (MSWD = 0.49), 262 ± 5 Ma (MSWD = 1.2), respectively, representing the formation ages of the mafic-ultramafic rocks. Therefore, considering other previously published data, we suggest that the mafic-ultramafic rocks were products of

Incipient boninitic arc crust built on denudated mantle: the Khantaishir ophiolite (western Mongolia)

NASA Astrophysics Data System (ADS)

Gianola, Omar; Schmidt, Max W.; Jagoutz, Oliver; Sambuu, Oyungerel

2017-12-01

The 570 Ma old Khantaishir ophiolite is built by up to 4 km harzburgitic mantle with abundant pyroxenites and dunites followed by 2 km of hornblende-gabbros and gabbronorites and by a 2.5 km thick volcanic unit composed of a dyke + sill complex capped by pillow lavas and some volcanoclastics. The volcanics are mainly basaltic andesites and andesites (or boninites) with an average of 58.2 ± 1.0 wt% SiO2, X Mg = 0.61 ± 0.03 ( X Mg = molar MgO/(MgO + FeOtot), TiO2 = 0.4 ± 0.1 wt% and CaO = 7.5 ± 0.6 wt% (errors as 2 σ). Normalized trace element patterns show positive anomalies for Pb and Sr, a negative Nb-anomaly, large ion lithophile elements (LILE) concentrations between N- and E-MORB and distinctly depleted HREE. These characteristics indicate that the Khantaishir volcanics were derived from a refractory mantle source modified by a moderate slab-component, similar to boninites erupted along the Izu-Bonin-Mariana subduction system and to the Troodos and Betts Cove ophiolites. Most strikingly and despite almost complete outcrops over 260 km2, there is no remnant of any pre-existing MORB crust, suggesting that the magmatic suite of this ophiolite formed on completely denudated mantle, most likely upon subduction initiation. The architecture of this 4-5 km thick early arc crust resembles oceanic crust formed at mid ocean ridges, but lacks a sheeted dyke complex; volcanic edifices are not observed. Nevertheless, low melting pressures combined with moderate H2O-contents resulted in high-Si primitive melts, in abundant hornblende-gabbros and in a fast enrichment in bulk SiO2. Fractional crystallization modeling starting from the observed primitive melts (56.6 wt% SiO2) suggests that 25 wt% pyroxene + plagioclase fractionation is sufficient to form the average Khantaishir volcanic crust. Most of the fractionation happened in the mantle, the observed pyroxenite lenses and layers in and at the top of the harzburgites account for the required cumulate volumes. Finally

Leaf-cutting ants: an unexpected microenvironment holding human opportunistic black fungi.

PubMed

Duarte, A P M; Attili-Angelis, D; Baron, N C; Forti, L C; Pagnocca, F C

2014-09-01

Fungus-growing ants of the genus Atta are known for their leaf-cutting habit, a lifestyle they have maintained since their 50-million-year-old co-evolution with a mutualistic fungus, cultivated as food. Recent studies have highlighted that, in addition to the mutualistic fungus, nests of ants harbor a great diversity of microbial communities. Such microorganisms include the dematiaceous fungi, which are characterized by their melanized cell walls. In order to contribute to the knowledge of fungal ecology, as well as opportunistic strains that may be dispersed by these social insects, we isolated and identified fungi carried by gynes of Atta capiguara and Atta laevigata, collected from colonies located in Fazenda Santana, Botucatu (São Paulo, Brazil). The isolation was carried out using the oil flotation technique, which is suitable for the growth of black fungi. Inoculated plates were incubated at 25 and 35 °C until black cultures were visible (20-45 days). Isolates were identified based on microscopic and molecular characteristics. Some isolated genera were: Cladophialophora, Cladosporium, Exophiala, Ochroconis, Phaeococcomyces, Phialophora and Penidiella. Hyaline species were also found. The results obtained from this work showed that leaf-cutting gynes may contribute to the dispersal of opportunistic dematiaceous fungi. It is suggested that more attention should be paid to this still unexplored subject.

The effect of the water-to-rock ratio on REE distribution in hydrothermal fluids: An experimental study

NASA Astrophysics Data System (ADS)

Beermann, Oliver; Garbe-Schönberg, Dieter; Holzheid, Astrid

2013-04-01

High-temperature submarine MOR hydrothermalism creates high elemental fluxes into, and out of, oceanic lithosphere significantly affecting ocean chemistry. The Turtle Pits hydrothermal system discovered at 5° S on the slow-spreading Mid-Atlantic Ridge (MAR) in water depths of ~3000 m (~300 bar) emanates 'ultrahot' fluids > 400 ° C [1] with high concentrations of dissolved gases (e.g., H2), transition metals, and rare earth elements (REE). The normalised REE patterns of these 'ultrahot' fluids are uncommon as they exhibit depletions of LREE and no Eu-anomaly ('special' REE-signature in [2]), which is in contrast to the "typical" LREE enrichment and pronounced positive Eu-anomaly known from many MOR vent fluids observed world-wide [e.g., 3]. Although hydrothermal fluid REE-signatures may play a key role in understanding processes during water-rock interaction, only few experimental data have been published on REE distribution in seawater-like fluids reacted with rocks from the ocean crust [e.g., 4, 5]. Besides temperature, the seawater-to-rock ratio (w/r ratio) strongly affects water-rock reaction processes and, thus, has significant control on the fluid chemistry [e.g., 6, 7]. To understand how vent fluid REE-signatures are generated during water-rock interaction processes we designed a series of experiments reacting different fluid types with mineral assemblages from fresh, unaltered gabbro at 425 ° C and 400 bar using cold seal pressure vessels (CSPV). Mixtures of 125-500 μm-sized hand-picked plagioclase and clinopyroxene grains separated from unaltered gabbro reacted in gold capsules with 3.2 wt.% NaCl(aq) fluid (similar to seawater salinity), or with natural seawater. The w/r (mass) ratio ranged from 1 to 100 and the run durations were varied from 3 to 30 d in the NaCl(aq) experiments, and was 3 d in the seawater experiments. The reacted fluids were extracted after quenching and analysed by ICP-OES and ICP-MS. Only in the seawater experiments, the gabbro

Intrusive rocks of the Holden and Lucerne quadrangles, Washington; the relation of depth zones, composition, textures, and emplacement of plutons

USGS Publications Warehouse

Cater, Fred W.

1982-01-01

serpentine. These occur either as included irregular masses in later intrusives or as tectonically emplaced lenses in metamorphic rocks. Also of uncertain age but probably much younger, perhaps as young as Eocene, are larger masses of hornblendite and hornblende periodotite that grade into hornblende gabbro. These are exposed on the surface and in the underground workings of the Holden mine. Oldest of the granitoid intrusives are the narrow, nearly concordant Dumbell Mountain plutons, having a radiometric age of about 220 m.y. They consist of gneissic hornblende-quartz diorite and quartz diorite gneiss. Most contacts consist of lit-par-lit zones, but some are gradational or more rarely sharp. The plutons are typically catazonal. Closely resembling the Dumbell Mountain plutons in outcrop appearance, but differing considerably in composition, are the Bearcat Ridge plutons. These consist of gneissic quartz diorite and granodiorite. The Bearcat Ridge plutons are not in contact with older dated plutons, but because their textural and structural characteristics so closely resemble those of the Dumbell Mountain plutons, they are considered to be the same age. Their composition, however, is suggestive of a much younger age. Cutting the Dumbell Mountain plutons is the Leroy Creek pluton, consisting of gneissic biotite-quartz diorite and trondjhemite. The gneissic foliation in the Leroy Creek is characterized by a strong and pervasive swirling. Cutting both the Dumbell Mountain and Leroy Creek plutons are the almost dikelike Seven-fingered Jack plutons. These range in composition from gabbro to quartz diorite; associated with them are contact complexes of highly varied rocks characterized by gabbro and coarse-grained hornblendite. Most of the rocks are gneissic, but some are massive and structureless. Radiometric ages by various methods range from 100 to 193 m.y. Dikes, sills, small stocks, and irregular clots of leucocratic quartz diorite and granodiorite are abundant in t

Proterozoic Eastern Sayan ophiolites (Central Asian Orogenic Belt) record subduction initiation in vicinity of continental block

NASA Astrophysics Data System (ADS)

Belyaev, Vasilii; Gornova, Marina; Medvedev, Alexander; Dril, Sergey; Karimov, Anas

2017-04-01

Geochemical study of cumulate and volcanic rocks from ˜ 1020 Ma Eastern Sayan ophiolites1 (Siberia, Russia) is used to provide a correlation between two ophiolitic belts and link them to subduction initiation setting. Studied areas include Ospin and Ilchir massifs to the East and Dunzhugur to the West of Early Precambrian Gargan block. Ophiolitic cumulates represent peridotite-pyroxenite-gabbro-norite suite with crystallization orders of Cr-Sp - Ol - Cpx - Opx - Plag, and Cr-Sp - Ol - Opx - Amph - (Cpx) - Plag. Clinopyroxene is augite-diopside with Mg# 85-95, low Al2O3 (1-2.5%) and TiO2 (0.05-0.2%). Amph is Mg-hornblende to edenite (Mg# 84-86, 5-8% Al2O3, 0.3-0.6% TiO2). Cr-Sp has Cr# 65-83 and 0.05-0.3% TiO2 in cumulates with high Opx proportion, while in Cpx-dominating pyroxenites chemistry of Cr-Sp is variable (Cr# 40-75, 0.05-0.5% TiO2). Due to alteration, Ol and Opx chemistry is available only for some samples (Ol: Mg# 88, 0.2-0.3% NiO; Opx: Mg# 89, 1.6% Al2O3). Whole-rock MgO ranges 9 to 38%. Amph-free pyroxenites and gabbro-norites show flat to slightly depleted REE pattern with negative HFSE anomalies. Amph-pyroxenites have fractionated trace-element pattern with LREE enrichment, Nb-Ti minima at slightly higher HFSE abundances. In-situ LA-ICP-MS analysis of Cpx in Amph-free pyroxenites and gabbro-norites revealed moderately depleted to flat REE and Nb-Zr-Hf-Ti depletion, with low trace element abundances (La/SmPM = 0.14-0.9, Zr 0.6-2.3, Nd = 0.2-1.1, Yb = 0.2-0.7 ppm). Melts calculated to be in equilibrium with Cpx using distribution coefficients2 are REE-flat to slightly LREE-enriched (La/SmPM = 1-4) at low HREE abundances (0.5-1.5 ppm Yb). Overall, crystallization orders, mineral and whole-rock chemistry suggest origin of ophiolitic cumulates from low-Ca boninites or primitive andesites (higher Opx or Amph proportion) to high-Ca boninites or primitive island arc tholeiites (Cpx-dominating, Amph-free associations with subordinate Opx). Ophiolitic

A Detailed Geochemical Study of Island Arc Crust: The Talkeetna Arc Section, South-central Alaska

NASA Astrophysics Data System (ADS)

Greene, A. R.; Debari, S. M.; Kelemen, P. B.; Clift, P. D.; Blusztajn, J.

2002-12-01

The Talkeetna arc section in south-central Alaska is recognized as the exposed upper mantle and crust of an accreted, Late Triassic to Middle Jurassic island arc. Detailed geochemical studies of layered gabbronorite from the middle and lower crust of this arc and a diverse suite of volcanic and plutonic rocks from the middle and upper crust provide crucial data for understanding arc magma evolution. We also present new data on parental magma compositions for the arc. The deepest level of the arc section consists of residual mantle and ultramafic cumulates adjacent to garnet gabbro and basal gabbronorite interlayered with pyroxenite. The middle crust is primarily layered gabbronorite, ranging from anorthosite to pyroxenite in composition, and is the most widespread plutonic lithology. The upper mid crust is a heterogenous assemblage of dioritic to tonalitic rocks mixed with gabbro and intruded by abundant mafic dikes and chilled pillows. The upper crust of the arc is comprised of volcanic rocks of the Talkeetna Formation ranging from basalt to rhyolite. Most of these volcanic rocks have evolved compositions (<5% MgO, Mg# <60) and overlap the composition of intermediate to felsic plutonic rocks (<3.5% MgO, Mg# <45). However, several chilled mafic rocks and one basalt have primitive characteristics (>8% MgO, Mg# >60). Ion microprobe analyses of clinopyroxene in mid-crustal layered gabbronorites have parallel REE patterns with positive-sloping LREE segments (La/Sm(N)=0.05-0.17; mean 0.11) and flat HREE segments (5-25xchondrite; mean 10xchondrite). Liquids in REE equilibrium with the clinopyroxene in these gabbronorite cumulates were calculated in order to constrain parental magmas. These calculated liquids(La/Sm(N)=0.77-1.83; mean 1.26) all fall within the range of dike and volcanic rock(La/Sm(N)=0.78-2.12; mean 1.23) compositions. However, three lavas out of the 44 we have analyzed show strong HREE depletion, which is not observed in any of the liquid compositions

Bedrock geologic map of the Nashua South quadrangle, Hillsborough County, New Hampshire, and Middlesex County, Massachusetts

USGS Publications Warehouse

Walsh, Gregory J.; Jahns, Richard H.; Aleinikoff, John N.

2013-01-01

The bedrock geology of the 7.5-minute Nashua South quadrangle consists primarily of deformed Silurian metasedimentary rocks of the Berwick Formation. The metasedimentary rocks are intruded by a Late Silurian to Early Devonian diorite-gabbro suite, Devonian rocks of the Ayer Granodiorite, Devonian granitic rocks of the New Hampshire Plutonic Suite including pegmatite and the Chelmsford Granite, and Jurassic diabase dikes. The bedrock geology was mapped to study the tectonic history of the area and to provide a framework for ongoing hydrogeologic characterization of the fractured bedrock of Massachusetts and New Hampshire. This report presents mapping by G.J. Walsh and R.H. Jahns and zircon U-Pb geochronology by J.N. Aleinikoff. The complete report consists of a map, text pamphlet, and GIS database. The map and text pamphlet are only available as downloadable files (see frame at right). The GIS database is available for download in ESRITM shapefile and Google EarthTM formats, and includes contacts of bedrock geologic units, faults, outcrops, structural geologic information, photographs, and a three-dimensional model.

The Pikwitonei granulite domain: A lower crustal level along the Churchill-Superior boundary in central Manitoba

NASA Technical Reports Server (NTRS)

Weber, W.

1983-01-01

The greenschist to amphibolite facies tonalite-greenstone terrain of the Gods Lake subprovince grades - in a northwesterly direction - into the granulite facies Pikwitonei domain at the western margins of the Superior Province. The transition is the result of prograde metamorphism and takes place over 50 - 100 km without any structural or lithological breaks. Locally the orthopyroxene isograd is oblique to the structural grain and transects greenstone belts, e.g., the Cross Lake belt. The greenstone belts in the granulite facies and adjacent lower grade domain consist mainly of mafic and (minor) ultramafic metavolcanics, and clastic and chemical metasedimentary rocks. Typical for the greenstone belts crossed by the orthopyroxene isograd are anorthositic gabbros and anorthosites, and plagiophyric mafic flows. The Pikwitonei granulite domain has been interpreted as to represent a lower crustal level which was uplifted to the present level of erosion. On the basis of gravimetric data this uplift has been modelled as an obduction onto the Churchill Province during the Hudsonian orogeny, similar to the Ivrea Zone. The fault between the Churchill and Superior Province is described.

Oxalate secretion by ectomycorrhizal Paxillus involutus is mineral-specific and controls calcium weathering from minerals

PubMed Central

Schmalenberger, A.; Duran, A. L.; Bray, A. W.; Bridge, J.; Bonneville, S.; Benning, L. G.; Romero-Gonzalez, M. E.; Leake, J. R.; Banwart, S. A.

2015-01-01

Trees and their associated rhizosphere organisms play a major role in mineral weathering driving calcium fluxes from the continents to the oceans that ultimately control long-term atmospheric CO2 and climate through the geochemical carbon cycle. Photosynthate allocation to tree roots and their mycorrhizal fungi is hypothesized to fuel the active secretion of protons and organic chelators that enhance calcium dissolution at fungal-mineral interfaces. This was tested using 14CO2 supplied to shoots of Pinus sylvestris ectomycorrhizal with the widespread fungus Paxillus involutus in monoxenic microcosms, revealing preferential allocation by the fungus of plant photoassimilate to weather grains of limestone and silicates each with a combined calcium and magnesium content of over 10 wt.%. Hyphae had acidic surfaces and linear accumulation of weathered calcium with secreted oxalate, increasing significantly in sequence: quartz, granite < basalt, olivine, limestone < gabbro. These findings confirmed the role of mineral-specific oxalate exudation in ectomycorrhizal weathering to dissolve calcium bearing minerals, thus contributing to the geochemical carbon cycle. PMID:26197714

Relationships among basaltic lunar meteorites

NASA Technical Reports Server (NTRS)

Lindstrom, Marilyn M.

1991-01-01

During the past two years four meteorites of dominantly mare basalt composition were identified in the Japanese and US Antarctic collections. Basalts represent a much higher proportion of the lunar meteorites than is expected from photogeologic mapping of mare and highland regions. Also, the basaltic lunar meteorites are all described as VLT mare basalt, which is a relatively uncommon type among returned lunar samples. The significance of the basaltic meteorites to the understanding of the lunar crust depends on the evaluation of possible relationships among the individual meteorites. None of the specimens are paired meteorites. They differ from each other in petrography and composition. It is important to determine whether they might be paired ejecta which were ejected from the same mare region by the same impact. The question of paired ejecta must be addressed using a combination of exposure histories and petrographic/compositional characteristics. It is possible that the basaltic lunar meteorites are paired ejecta from the same region of the Moon. However, the relationships among them are more complicated than the basaltic breccias being simply brecciated mare gabbros.

Quintinite-1 M from the Mariinsky Deposit, Ural Emerald Mines, Central Urals, Russia

NASA Astrophysics Data System (ADS)

Zhitova, E. S.; Popov, M. P.; Krivovichev, S. V.; Zaitsev, A. N.; Vlasenko, N. S.

2017-12-01

The paper describes the first finding of quintinite [Mg4Al2(OH)12][(CO3)(H2O)3] at the Mariinsky deposit in the Central Urals, Russia. The mineral occurs as white tabular crystals in cavities within altered gabbro in association with prehnite, calcite, and a chlorite-group mineral. Quintinite is the probable result of late hydrothermal alteration of primary mafic and ultramafic rocks hosting emerald-bearing glimmerite. According to electron microprobe data, the Mg: Al ratio is 2: 1. IR spectroscopy has revealed hydroxyl and carbonate groups and H2O molecules in the mineral. According to single crystal XRD data, quintinite is monoclinic, space group C2/ m, a =5.233(1), b = 9.051(2), c = 7.711(2) Å, β = 103.09(3)°, V = 355.7(2) Å3. Based on structure refinement, the polytype of quintinite should be denoted as 1M. This is the third approved occurrence of quintinite-1M in the world after the Kovdor complex and Bazhenovsky chrysotile-asbestos deposit.

Sr and Nd isotope composition of the metamorphic, sedimentary and ultramafic xenoliths of Lanzarote (Canary Islands): Implications for magma sources

NASA Astrophysics Data System (ADS)

Aparicio, Alfredo; Tassinari, Colombo C. G.; García, Roberto; Araña, Vicente

2010-01-01

The lavas produced by the Timanfaya eruption of 1730-1736 (Lanzarote, Canary Islands) contain a great many sedimentary and metamorphic (metasedimentary), and mafic and ultramafic plutonic xenoliths. Among the metamorphosed carbonate rocks (calc-silicate rocks [CSRs]) are monomineral rocks with forsterite or wollastonite, as well as rocks containing olivine ± orthopyroxene ± clinopyroxene ± plagioclase; their mineralogical compositions are identical to those of the mafic (gabbros) and ultramafic (dunite, wherlite and lherzolite) xenoliths. The 87Sr/ 86Sr (around 0.703) and 143Nd/ 144Nd (around 0.512) isotope ratios of the ultramafic and metasedimentary xenoliths are similar, while the 147Sm/ 144Nd ratios show crustal values (0.13-0.16) in the ultramafic xenoliths and mantle values (0.18-0.25) in some CSRs. The apparent isotopic anomaly of the metamorphic xenoliths can be explained in terms of the heat source (basaltic intrusion) inducing strong isotopic exchange ( 87Sr/ 86Sr and 143Nd/ 144Nd) between metasedimentary and basaltic rocks. Petrofabric analysis also showed a possible relationship between the ultramafic and metamorphic xenoliths.

Relic magma chamber structures preserved within the Mesozoic North Atlantic crust?

USGS Publications Warehouse

McCarthy, J.; Mutter, J.C.; Morton, J.L.; Sleep, Norman H.; Thompson, G.A.

1988-01-01

The North Atlantic Transect seismic reflection data, collected southwest of Bermuda, have been reinterpreted following post-stack migration and reveal two major intracrustal reflections. The shallower of these two events, located ~1 s below the igneous basement, is a subhorizontal, undulating surface that in some places is continuous for as much as 10 km. This upper crustal reflection corresponds to the intermittently sharp contact between the sheeted dikes and the underlying isotropic gabbro. A second set of lower crustal reflections, dipping ~20??-40?? eastward, is also prominent on the migrated profile and terminates downdip against the subhorizontal reflection Moho. Their presence may be ascribed to mafic-ultramafic cumulate layers frozen into the oceanic crust at the time of formation at the paleo-spreading center. The gradual thinning in the crust approaching the fracture zones is shown to be more complex than was originally inferred. An intepretation advocating crustal thickening in this narrow zone is proposed as an alternative to the crustal-thinning model of Mutter and others. -from Authors

Ankaramite: A New Type of High-Magnesium and High-Calcium Primitive Melt in the Magnitogorsk Island-Arc Zone (Southern Urals)

NASA Astrophysics Data System (ADS)

Pushkarev, E. V.; Ryazancev, A. V.; Gottman, I. A.; Degtyarev, K. E.; Kamenetsky, V. S.

2018-04-01

This work describes the geological position, mineral and chemical composition of high-Mg effusive ankaramites occurring as dykes and lava flows. They were found in the mélange zone of the western margin of the Magnitogorsk island arc zone in the Southern Urals. Data on the liquidus association of phenocrysts and on the composition of the matrix of effusives are given. According to the data obtained, the conclusion was drawn that the ankaramites studied can be attributed to the primary island arc melts, which were not subject to essential differentiation. This type of effusives has not been distinguished previously among island arc volcanogenic formations of the Urals. It is shown that ankaramites can be considered to be primary melts parental for dunite-clinopyroxenites-gabbro complexes of Ural-Alaskan type. The occurrence of ankaramites in the Paleozoic island arc formations of the Urals indicates the wehrlite composition of the mantle as the reason for the extremely wide development of wehrlites and clinopyroxenites in different mafic-ultramafic complexes of the Urals.

Breccia 66055 and related clastic materials from the Descartes region, Apollo 16

NASA Technical Reports Server (NTRS)

Fruchter, J. S.; Kridelbaugh, S. J.; Robyn, M. A.; Goles, G. G.

1974-01-01

Trace and major element contents obtained by instrumental neutron activation are reported for a number of Apollo 16 soil samples and miscellaneous breccia fragments. In addition, data obtained by instrumental neutron activation and electron microprobe techniques along with petrographic descriptions are presented for selected subsamples of breccia 66055. The compositions of our soil samples can be modeled by mixtures of various amounts of anorthosite, anorthositic gabbro and low-K Fra Mauro basalt components. These mixtures are typical of those found in a number of petrographic surveys of the fines. Breccia 66055 is a complex regolith breccia which consists of at least four distinct types of microbreccias. No systematic relation with respect to stratigraphic age among the various microbreccia types was observed. Compositionally and texturally, the clasts which compose breccia 66055 are similar to a number of previously reported rock types from the Apollo 16 area. The entire breccia appears to have undergone a complex history of thermal metamorphism. We conclude from the study of these samples that the Cayley Formation is probably homogeneous in its gross compositional and petrographic aspects.

Paleoproterozoic high-pressure metamorphism in the northern North China Craton and implications for the Nuna supercontinent

PubMed Central

Wan, Bo; Windley, Brian F.; Xiao, Wenjiao; Feng, Jianyun; Zhang, Ji'en

2015-01-01

The connection between the North China Craton (NCC) and contiguous cratons is important for the configuration of the Nuna supercontinent. Here we document a new Paleoproterozoic high-pressure (HP) complex dominated by garnet websterite on the northern margin of the NCC. The peak metamorphism of the garnet websterite was after ∼1.90 Ga when it was subducted to eclogite facies at ∼2.4 GPa, then exhumed back to granulite facies at ∼0.9 GPa before ∼1.82 Ga. The rock associations with their structural relationships and geochemical affinities are comparable to those of supra-subduction zone ophiolites, and supported by subduction-related signatures of gabbros and basalts. We propose that a ∼1.90 Ga oceanic fragment was subducted and exhumed into an accretionary complex along the northern margin of the NCC. Presence of the coeval Sharyzhalgai complex with comparable HP garnet websterites in the southern Siberian active margin favours juxtaposition against the NCC in the Paleoproterozoic. PMID:26388458

The mafic-ultramafic complex of Aniyapuram, Cauvery Suture Zone, southern India: Petrological and geochemical constraints for Neoarchean suprasubduction zone tectonics

NASA Astrophysics Data System (ADS)

Yellappa, T.; Venkatasivappa, V.; Koizumi, T.; Chetty, T. R. K.; Santosh, M.; Tsunogae, T.

2014-12-01

Several Precambrian mafic-ultramafic complexes occur along the Cauvery Suture Zone (CSZ) in Southern Granulite Terrain, India. Their origin, magmatic evolution and relationship with the associated high-grade rocks have not been resolved. The Aniyapuram Mafic-Ultramafic Complex (AMUC), the focus of the present study in southern part of the CSZ, is dominantly composed of peridotites, pyroxenites, gabbros, metagabbros/mafic granulites, hornblendites, amphibolites, plagiogranites, felsic granulites and ferruginous cherts. The rock types in the AMUC are structurally emplaced within hornblende gneiss (TTG) basement rocks and are highly deformed. The geochemical signature of the amphibolites indicates tholeiitic affinity for the protolith with magma generation in island arc-setting. N-MORB normalized pattern of the amphibolites show depletion in HFS-elements (P, Zr, Sm, Ti, and Y) and enrichment of LIL-elements (Rb, Ba, Th, Sr) with negative Nb anomalies suggesting involvement of subduction component in the depleted mantle source and formation in a supra-subduction zone tectonic setting. Our new results when correlated with the available age data suggest that the lithological association of AMUC represent the remnants of the Neoarchean oceanic lithosphere.

The Apollo 15 X-ray fluorescence experiment

NASA Technical Reports Server (NTRS)

Adler, I.; Trombka, J.; Gerard, J.; Schmadebeck, R.; Lowman, P.; Blodgett, H.; Yin, L.; Eller, E.; Lamothe, R.; Gorenstein, P.

1972-01-01

The CSM spectrometric data on the lunar surface with respect to its chemical composition are presented for Al, Mg, and Si as Al/Si and Mg/Si ratios for the various features overflow by the spacecraft. The lunar surface measurements involved observations of the intensity and characteristic energy distribution of the secondary or fluorescent X-rays produced by the interaction of solar X-rays with the lunar surface. The results showed that the highlands and maria are chemically different, with the highlands having considerably more Al and less Mg than the maria. The mare-highland contact is quite sharp and puts a limit on the amount of horizontal transport of material. The X-ray data suggest that the dominant rock type of the lunar highlands is a plagioclase-rich pyroxene bearing rock, probably anorthositic gabbro or feldspathic basalt. Thus the moon appears to have a widespread differentiated crust (the highlands) systematically richer in Al and lower in Mg than the maria. This crust is pre-mare and may represent the first major internal differentiation of the moon.

Structure, Geochemistry, and Kinematics at the Slow-Spreading 16°30'N Region of the Mid-Atlantic Ridge

NASA Astrophysics Data System (ADS)

Urann, B.; Dick, H. J.; Smith, D. K.

2017-12-01

The 16°30'N region of the Mid-Atlantic Ridge is characterized by active detachment faulting and weakly magmatic crustal accretion, where abundant mantle peridotite is exposed on the seafloor along with pillow basalts, gabbro, and diabase. We present a comprehensive tectonic, petrologic, and geochemical analysis of this slow-spreading ridge section. We utilize the high density dredging from R/V Knorr in 2013 to structurally map the seafloor, in an attempt to understand crustal architecture and the interplay between ephemeral magmatism and crustal accretion. Geochemically, this region exposes some of the most refractory mantle yet found in the oceans, with olivine Mg# of 91 to 92, spatially consistent yet elevated chromian spinel Cr# of 45 to 60, and clinopyroxene ultra-depleted in rare earth elements. Such refractory mantle is at odds with anhydrous melting at a mid-ocean ridge setting at typical mantle potential temperatures, and is more characteristic of a hydrous melting regime. Our data suggests that portions of the Mid-Atlantic Ridge are underlain by highly refractory buoyant relict mantle, likely recycled from a previous hydrous melting event.

Geology and origin of the late Proterozoic Darb Zubaydah ophiolite, Kingdom of Saudi Arabia

USGS Publications Warehouse

Quick, J.E.

1990-01-01

The Darb Zubaydah ophiolite, north-central Arabian Shield, preserves a largely intact section consisting of ultramafic rocks, gabbro, diabase, granodiorite, and interbedded volcanic and sedimentary rocks. Formation of these rocks within or near an island arc is indicated by the absence of pelagic sediments and the abundance of pillow basalt, turbiditic sediments, lahar deposits, and basaltic to rhyolitic tuff. The oldest extrusive rocks formed in a young, relatively unevolved island arc or in a back-arc basin sufficiently close to an arc to receive calc-alkaline lava flows and coarse-grained, arc-derived detritus. Overlying turbidites and lahar deposits of the Kaffan sandstone point to the initiation of a rifting event. High-Ti basalts, which erupted above the Kaffan sandstone, and related diabase are interpreted to be magmatic products of incipient intra-arc rifting. Renewed arc volcanism produced calc-alkaline volcanic rocks that interfingered with the high-Ti basalt and later dominated the section as the volcanic apron of the arc prograded basinward. Extrusion of voluminous calc-alkaline tuff may have been contemporaneous with intrusion of granodiorite and gravity-driven landsliding. -from Author

Early Paleozoic development of the Maine-Quebec boundary Mountains region

USGS Publications Warehouse

Gerbi, C.C.; Johnson, S.E.; Aleinikoff, J.N.; Bedard, J.H.; Dunning, G.R.; Fanning, C.M.

2006-01-01

Pre-Silurian bedrock units played key roles in the early Paleozoic history of the Maine-Quebec Appalachians. These units represent peri-Laurentian material whose collision with the craton deformed the Neoproteozoic passive margin and initiated the Appalachian mountain-building cycle. We present new field, petrological, geochronological, and geochemical data to support the following interpretations related to these units. (1) The Boil Mountain Complex and Jim Pond Formation do not represent part of a coherent ophiolite. (2) Gabbro and tonalite of the Boil Mountain Complex intruded the Chain Lakes massif at ca. 477 Ma. (3) The Skinner pluton, an arc-related granodiorite, intruded the Chain Lakes massif at ca. 472 Ma. (4) The Attean pluton, with a reconfirmed age of ca. 443 Ma, is unrelated to Early Ordovician orogenesis. (5) The most likely timing for the juxtaposition of the Jim Pond Formation and the Boil Mountain Complex was during regional Devonian deformation. These interpretations suggest that the Boundary Mountains were once part of a series of arcs extending at least from central New England through Newfoundland. ?? 2006 NRC Canada.

Emsian synorogenic paleogeography of the Maine Applachians

USGS Publications Warehouse

Bradley, D.; Tucker, R.

2002-01-01

The Acadian deformation front in the northern Appalachians of Maine and New Hampshire can now be closely located during the early Emsian (Early Devonian; 408-406 Ma). Tight correlations between paleontologically and isotopically dated rocks are possible only because of a new 408-Ma time scale tie point for the early Emsian. The deformation front lay between a belt of Lower Devonian flysch and molasse that were deposited in an Acadian foreland basin and had not yet been folded and a belt of early Emsian plutons that intruded folded Lower Devonian rocks. This plutonic belt includes the newly dated Ore Mountain gabbro (U/Pb; 406 Ma), which hosts magmatic-sulfide mineralization. Along the deformation front, a 407-Ma pluton that locally truncates Acadian folds (Katahdin) was the feeder to volcanic rocks (Traveler Rhyolite; 406-407 Ma) that are part of the foreland-basin succession involved in these same folds. The Emsian igneous rocks thus define a syncollisional magmatic province that straddled the deformation front. These findings bear on three alternative subduction geometries for the Acadian collision.