High-temperature Metamorphic Event Research Articles

Graphitization in Paleoproterozoic granulites from the Taihua Complex, North China Craton: Constraints from laser Raman spectroscopy, carbon isotopes and geochronology

The presence of graphite as a refractory form of solid carbon in high grade meta-sedimentary rocks serves as a valuable tool for identifying carbon sources and assessing the movement of carbon through the Earth's interior. In this study, we investigate the graphite formation processes from different lithologies (marbles, metapelites) from the Paleoproterozoic granulites in the southern margin of North China Craton. By utilizing a range of analytical techniques, including petrographic, mineral-chemical, laser Raman spectroscopic, carbon stable isotopic, and geochronological studies, we characterize the textural and isotopic variety of graphite and the timing of metamorphism associated with graphitization. The results suggest a progressive growth and graphitization of graphite during medium to high temperature metamorphic facies. The δ13C values of crystalline graphite in metapelites and marble range in −24.00‰ to −20.10‰, and − 22.50‰ to −21.60‰, respectively, both close to organic carbon averages. The calcite from the graphite-bearing marble shows δ13C values in the range of −2.90‰ to −2.80‰, which corresponds to marine carbonates. In contrast, graphite-bearing quartzo-feldspathic gneiss and diopside/augite-bearing marble show heavier isotopic values of −14.4‰ to −16.5‰, suggesting graphite precipitation from fluids of both organic and magmatic origin. The spectral and isotopic signatures of graphite in these rocks indicate a dominant biogenic signature with mixture of abiogenic carbon sources. Zircon UPb geochronology marks the timing of high-grade metamorphism at ca. 1.93 to 1.92 Ga, associated with graphite formation during subduction and accretion process associated with the final assembly of the North China Craton. We propose that crystalline graphite was formed during the continental collision-triggered medium to high temperature metamorphic event, consistent with a global graphite metallogenic event in the Paleoproterozoic.

Lu–Hf, Sm–Nd, and U–Pb isotopic coupling and decoupling in apatite

Apatite is a useful geochronological tool due to its common occurrence and incorporation of a variety of radioactive parent isotopes during crystallisation. However, an understanding of the geological conditions recorded by each of these isotopic systems in apatite is necessary for their effective application to geological problems. In this study, U–Pb, Sm–Nd, and Lu–Hf dates for apatite grains from samples of ∼3.0 Ga TTG gneisses in the Akia terrane of the North Atlantic Craton (south-western Greenland) were obtained in order to better understand the conditions under which isotopic re-equilibration in apatite is achieved. This isotopic data records at least two thermotectonic events subsequent to the initial magmatic crystallisation of these rocks. Whereas the Lu–Hf and Sm–Nd isotopic systems in these apatites record a regional high temperature metamorphic event at ∼2.7 Ga, the U–Pb system records a milder thermal event at ∼1.7 Ga. This decoupling of the U–Pb from the Sm–Nd and Lu–Hf systems in apatite reflects a difference in closure temperature. The regional thermal effects of the ∼1.7 Ga Nagssugtoqidian orogeny in adjacent parts of the North Atlantic Craton resulted in partial to complete reequilibration of the U–Pb systems in apatite across the Akia terrane but were insufficient to cause open system behaviour of Sm–Nd or Lu–Hf. The indistinguishable apatite Sm–Nd and Lu–Hf ages produced in each analysed sample suggest that these systems are similarly susceptible to isotopic re-equilibration. The older ages produced by the Sm–Nd and Lu–Hf systems relative to U–Pb in apatite demonstrate that the former are more robust and thus more likely to faithfully record high temperature events than the U–Pb system.

~25 Ma Ruby Mineralization in the Mogok Stone Tract, Myanmar: New Evidence from SIMS U–Pb Dating of Coexisting Titanite

Ruby (red corundum) is one of the most prominent colored gemstones in the world. The highest-quality ruby (“pigeon blood” ruby) comes from marbles of the Mogok Stone Tract in central Myanmar. Although Mogok ruby has been exploited since the 6th century AD, the formation time of this gemstone is ambiguous and controversial. In this paper, we describe a mineralogical, geochemical, and geochronological study of ruby and titanite in ruby-bearing marbles obtained from an outcrop in the Mogok Stone Tract, central Myanmar. Petrographic observations have shown that titanite generally occurs in the marble matrix or occurs as inclusions in ruby. These two types of titanite exhibit identical chemical compositions. In situ secondary ion mass spectrometer (SIMS) U–Pb dating of the separated titanite from two representative samples of ruby-bearing marbles yielded lower intercept ages of 25.15 ± 0.24 Ma (MSWD = 0.26) and 25.06 ± 0.22 Ma (MSWD = 0.15), respectively. Because the closure temperature of the U–Pb system in titanite is close to the temperature of ruby growth, the obtained U–Pb ages (~25 Ma) are suggested to represent the timing of the studied ruby formation in Mogok. The acquired ages are in agreement with the timing of post-collisional extension in the Himalaya related to the migration of the eastern Himalayan syntaxis. Combining our dating results with previous geochronological data from the Mogok Stone Tract, we suggest that the formation of the studied ruby is most likely related to the high-temperature metamorphic event in the marbles during the India–Asia collision. Our study not only confirms that texturally constrained titanite could be a precise geochronometer to date the mineralization of different types of ruby, but also provides important geochronological information linking gemstone formation to the India–Asia collision.

Grenvillean evolution of the Beishan Orogen, NW China: Implications for development of an active Rodinian margin

Abstract New geochemical and geochronological data are used to characterize the geodynamic setting of metasediments, felsic orthogneisses, and eclogite and amphibolite lenses forming the Beishan complex, NW China, at the southern part of the Central Asian Orogenic Belt. The metasediments correspond compositionally to immature greywackes receiving detritus from a heterogeneous source involving a magmatic arc and a Precambrian continental crust. Metagranitoids, represented by felsic orthogneisses, show both composition of greywacke-derived granitic melt with incompatible trace element patterns similar to the host metasediments. The eclogite lenses are characterized by high Nb contents (5.34–27.3 ppm), high (Nb/La)N (>1), and low Zr/Nb ratios (<4.5), which together with variable and negative whole-rock εNd(t) (–4.3 to –10.3) and zircon εHf(t) (–5.0 to + 2.3) values indicate an origin of enriched mantle source as commonly manifested by back-arc basalts at stretched continental margins. Combined with monazite rare earth element analysis, the in situ monazite U-Pb dating of metagraywacke (880.7 ± 7.9) suggests garnet growth during a high-temperature (HT) metamorphic event. Together with U-Pb dating of zircon metamorphic rims in amphibolite (910.9 ± 3.0 Ma), this indicates that the whole crustal edifice underwent a Grenvillian-age metamorphic event. The protolith ages of the eclogite (889.3 ± 4.8 Ma) and orthogneiss (867.5 ± 1.9 Ma) suggest that basalt underplating and sediment melting were nearly coeval with this HT metamorphism. Altogether, the new data allow placing the Beishan Orogen into a Grenvillean geodynamic scenario where: (1) The late Mesoproterozoic to early Neoproterozoic was marked by deposition of the greywacke sequence coeval with formation of an early arc. (2) Subsequently, an asthenospheric upwelling generated basaltic magma underneath the thinned subcontinental mantle lithosphere that was responsible for HT metamorphism, melting of the back-arc basin greywackes and intrusion of granitic magmas. These events correspond to a Peri-Rodinian supra-subduction system that differs substantially from the Neoproterozoic ophiolite sequences described in the Mongolian part of the Central Asian Orogenic Belt, thus indicating important lateral variability of supra-subduction processes along the Rodinian margin.

Water budget and partial melting in an Archean crustal column: example from the Dharwar Craton, India

AbstractThe fluid budget of a composite crustal column is a critical parameter that influences many lithospheric processes. The amount of water introduced into the middle and lower crust can be quantified using phase equilibrium modelling. The Dharwar Craton, India, displays a now-exposed continuous crustal section from near-surface conditions to c. 30 km depth. This section records the different steps of a c. 15 myr-long high-temperature metamorphic event (60°C kbar−1) responsible for the formation of syn- to post-tectonic anatectic intrusions. The global water budget is assessed using thermodynamic modelling on bulk-rock compositions of an average early Proterozoic supracrustal unit and c. 3.0 Ga felsic basement, the Peninsular gneisses. Results show the fast burial of a water-saturated supracrustal package (1.6 wt%) will release c. 50% of its mineral-bound water, triggering water-fluxed partial melting of the basement. Modelled anatectic magma compositions match the observed granitoid chemistries, and distinction can be made between water-fluxed melting and water-absent melting in the origin of syn- to post-tectonic anatectic granites. Findings from this study show the importance of crustal pile heterogeneity in controlling the nature of partial melting reactions, the composition of the magmas and the rheology of the crust.

Dumortierite- and corundum-bearing quartz–feldspar–mica rocks of the Belomorian eclogite province: An example of melting of phengite + quartz

In the Salma eclogite of the Belomorian eclogite province, a dumortierite–phengite–corundum–bearing quartz–feldspar rock has been studied: its primary HP mineral paragenesis included garnet, phengite, and quartz. The phengite–quartz rocks were formed during dehydration and/or melting of boroncontaining rocks when they were dipped in the Meso- Neoarchaean subduction zone to a depth of not less than 70 km. As a result of the subsequent superimposed high-temperature metamorphic events under PT conditions of high-pressure granulite facies, the phengite in quartz underwent incongruent dehydration melting with formation of complex polymineral pseudomorphs, consisting of feldspars, biotite, newly formed muscovite, kyanite, corundum, and dumortierite. New estimates of the metamorphic temperature (850–900°C according to the melting reactions of phengite and the dumortierite field of stability; about 1000°C by the reintegrated composition of feldspar–mesoperthite) that affected the HP parageneses of Salma eclogitized rocks are at least 50–100°C (or even more) higher than them estimated earlier.

Integrating geologic and satellite radar data for mapping dome-and-basin patterns in the In Ouzzal Terrane, Western Hoggar, Algeria

The In Ouzzal Terrane (IOT) located in the north-western part of the Tuareg Shield forms an elongated N–S trending block, more than 400km long and 80km wide. It involves an Archaean crust remobilized during a very high-temperature metamorphic event related to the Palaeoproterozoic orogeny. The IOT largely crops out in the rocky and sandy desert of Western Hoggar. It corresponds mainly to a flat area with some reliefs composed of Late Panafrican granites, dyke networks or Cambrian volcanic rocks. These flat areas are generally covered by thin sand veneers. They are favorable for discriminating bedrock geological units using imaging radar, backscattering measurements, and field checking, because the stony desert is particularly sensitive to the radar parameters such as wavelength or polarization. The main radar data used are those obtained with the ALOS-PALSAR sensor (L-band), in ScanSAR mode (large swath) and Fine Beam modes. The PALSAR sensor has been also compared to ENVISAT-ASAR and to optical imagery.Detailed mapping of some key areas indicates extensive Archaean dome-and-basin patterns. In certain parts, the supracrustal synforms and orthogneiss domes exhibit linear or circular features corresponding to shear zones or rolling structures, respectively. The geological mapping of these dome-and-basin structures, and more generally of the Archaean and Proterozoic lithological units, is more accurate with the SAR imagery, particularly when using the L-band, than with the optical imagery. A quantitative approach is carried out in order to estimate the backscatter properties of the main rock types. Due to the large variety of configurations, radar satellite imagery such as ALOS PALSAR represents a key tool for geological mapping in arid region at different scales from the largest (e.g., 1:500,000) to the smallest (e.g., 1:50,000).

Zircon ages and Nd–Hf isotopes in UHT granulites of the Ider Complex: A cratonic terrane within the Central Asian Orogenic Belt in NW Mongolia

The Ider Complex of the Tarbagatai Block in northwestern Mongolia is part of a Precambrian microcontinental terrane in the Central Asian Orogen Belt and has experienced a polymetamorphic tectono-metamorphic evolution. We have investigated an enderbitic gneiss, derived from a quartz diorite and a charnockite, derived from a leucogranite, and zircon SHRIMP data reveal late Archaean protolith ages of 2520–2546Ma for these rocks. Metamorphic overgrowth on these zircons as well as newly-formed metamorphic zircons document a high-temperature metamorphic event (T=930–950°C) at about 1855–1860Ma. Nd whole-rock isotopic systematics show these and other gneisses of the Ider Complex straddling the CHUR-line in a Nd isotope evolution diagram, suggesting both crustal reworking and input of some juvenile material, with Nd model ages ranging between ca. 2.5 and 3.1Ga. Hf-in-zircon isotopic data provide a similar pattern and also yielded Archaean Hf crustal model ages. The metamorphic zircons seem to have inherited their Hf isotopic composition from the igneous grains, suggesting a complex process of dissolution, transportation, and re-precipitation involving a fluid phase during high-grade metamorphism. The zircon age patterns do not make it possible to unambiguously assign the Tarbagatai Block to any of the cratons bordering the Central Asian Orogenic Belt, since age peaks at ca. 2520–2550 and ca. 1860Ma are common in the Siberian, North China and Tarim cratons.

A MATLAB-derived software (geothermMOD1.2) for one-dimensional thermal modeling, and its application to the Corsica-Sardinia batholith

Determination of the thermal state of the crust is of fundamental importance to understand the feedbacks between tectonics, rheology and metamorphism. The most important parameters controlling the variation of temperature with depth in the crust are the heat flow across the Moho, the thermal conductivity and the vertical distribution of heat-producing elements 238,235U, 232Th and 40K. The Corsica-Sardinia batholith formed during a regional high-temperature metamorphic event by extensive partial melting of the Variscan crust. Petrologic observations indicate that most granites and migmatites formed between 0.6–0.2GPa, requiring the geotherms to flatten substantially relative to the stage of crustal thickening. The adjustment of geotherms may be interpreted in terms of enrichment in heat-producing elements or, alternatively, in terms of increased mantle contribution. The validity of these two end-member hypothesis has been tested by performing numerical experiments with a software package appositely developed in Matlab. The software allows to set up crustal model composed of up to five different layers, and to plot the calculated geotherm along with the stability field of the haplogranite system and other thermo-barometric constraints. The steady-state thermal structure of the Sardinian Variscan crust has been computed at four different time steps (320, 310, 300, and 280Ma), which cover the batholith evolution. The geometry and composition of each of these crustal models has been constrained based on geophysical observations, geochronology and petrology. In a first experiment, the heat conduction equation has been solved for all of the four time-steps, assuming constant (15mW/m2) heat flow across the Moho in order to simulate a ‘fully crustal’ evolution of the thermal structure. The alternative has been simulated in a second experiment, by rising the basal heat flow up to 40mW/m2 during the last two time steps.

The 3.98–3.63 Ga zircons as indicators of major processes operating in the ancient continental crust of the east Antarctic shield (Enderby Land)

This study presents new results on zircons from the enderbite-charnockite rocks of Enderby Land, East Antarctica. U-Pb age of 3981 ± 8 Ma (SIMS SHRIMP II), which was first obtained for a protolith of massive enderbites from Aker Peaks, eastern Napier Mts, suggests that the existence of sialic crust in the study area at 4 Ga. Although there was only one magmatic zircon (of 150 grains analyzed) in the study area known with the oldest age, its significance cannot be overestimated, since it may indirectly evidence the existence of an Early Archean crustal block with a minimum age of 4 Ga, which extends for over 300 km across Enderby Land from its western to eastern part. Based on the U-Pb systematics, REE and trace element distributions in zircons from charnockite and enderbite gneisses, high-aluminous gneisses, and basic granulites, we first revealed that an early high-temperature metamorphic event accompanied by the emplacement of granodiorite intrusions occurred in the vicinity of Aker Peaks at 3620–3630 Ma. Although the 2850–3050 metamorphic overprints are clearly observed in some other areas of Enderby Land and are widely considered to be of critical importance on a regional scale, their metamorphic signatures are apparently absent from the U-Pb systematics of the studied zircon, thus suggesting the presence of similar old zircons in the study area. At the same time, all samples in this study record a 2480–2550 Ma granulite-amphibolite facies overprint represented as new zircon growths or recrystallization of earlier phases.

The Timing of Yellowknife Gold Mineralization: A Temporal Relationship with Crustal Anatexis?

In order to thoroughly evaluate the genesis of orogenic gold mineralization in the context of plate tectonics, absolute age data are required to link mineralization with lithological evolution. We show that pyrite, associated with orogenic gold mineralization from the Con deposit in the ca. 2700 Ma Yellowknife greenstone belt, has an Re-Os isochron age of 2591 ± 37 Ma. This is identical to the previously proposed timing of ca. 2592 Ma for S-type pluton-related lode-gold within ca. 2660 Ma graywacke-mudstone turbidites east of the greenstone belt. The Re-Os pyrite data indicate that orogenic gold mineralization was deposited ca. 60 to 100 m.y. after the formation of the host rocks in the Yellowknife district and, when combined with the regional geologic framework, indicate that the gold mineralization formed in a distinct geodynamic setting from that during host-rock formation. Initial Os values in the pyrite (Os i = 0.78 ± 0.17) indicate a significant crustal component of Os to the ore system, compatible with metals being derived from the older host rocks. Regionally preserved granulites, migmatites, and charnockites derived, in part, from the older supracrustal sequences, formed at 2595 to 2585 Ma, corresponding to the timing of gold mineralization. S-type plutons with ca. 2592 Ma crystallization ages in the upper crust reflect this high-temperature metamorphic event; these plutons have turbidite-hosted gold deposits associated with their metamorphic aureoles. Collectively, the isotopic and regional geologic data indicate that gold mineralization was related to regional-scale crustal anatexis, which likely resulted from crustal accretion or collision outside what is now preserved of the craton.

The 390 Ma high-T metamorphic event in the Chinese Altai: A consequence of ridge-subduction?

High-grade rocks occur in the Chinese Altai, but the timing of metamorphism is poorly constrained, which hinders our understanding of the thermo-tectonic history of the region. Representative high-grade samples from the sillimanite zone extending from Hanas to Fuyun were selected for zircon U-Pb dating and temperature estimation. LA-ICP-MS analyses of zircon overgrowth rims and recrystallized domains give consistent ages of ∼390 Ma, which is interpreted to record a regional metamorphic event. Temperature (T) estimations using the amphibole-plagioclase-quartz (Amp-Pl-Qtz) and garnet-biotite (GB) geothermometers give relatively high temperatures ranging from 650 to 700 °C. The zircon metamorphic rims yield temperature estimates of ∼720 °C by using the Ti-in-zircon thermometer. These data suggest that a high-temperature metamorphic event took place in the Chinese Altai in the Middle Devonian, and may imply a tectonic environment involving an unusually elevated heat flux from a deep-seated source. Our data support possible ridge subduction around 390 Ma that caused upwelling of the hot asthenosphere and triggered the high-T metamorphism. This model can also account for coeval volcanic activity with a range of geochemical characteristics, diverse mafic intrusions and extensive hydrothermal mineralization in the Altai orogen.

Age constraints of high-temperature metamorphic events in crystalline complexes of the Irkut block, the Sharyzhalgai ledge of the Siberian platform basement: Results of the U-Pb single zircon dating

Geochronological data obtained in this work and previously known results of U-Pb geochronology suggest that principal metamorphic events, which took place in eastern part of the Irkut block (the Sharyzhalgai marginal ledge of the Siberian platform basement), correspond in age to (1) about 2.8 Ga, (2) 2649 ± 6 to 2562 ± 20 Ma, and (3) 1865 ± 4 to 1855 ± 5 Ma. Structural and metamorphic reworking of the earliest event originated under conditions of the granulite facies, whereas conditions of granulite and amphibolite facies were characteristic of the second and third events. Metasomatites after carbonate rocks originated in eastern part of the Sharyzhalgai ledge during the Early Proterozoic metamorphic event that lasted approximately 20 m.y. Being combined with age data, which are known at present for the reference syn-and post-collision granitoids in the Siberian platform basement and flanking foldbelts, new geochronological results show that accretion of basement blocks to the Siberian craton progressed from the east to the west between 1900 and 1840 Ma. To a first approximation, this geochronological interval characterizes time span of the Paleoproterozoic ocean closure and ultimate time, when the craton and supercontinent Columbia became amalgamated.

A zircon study from the Rhodope metamorphic complex, N-Greece: Time record of a multistage evolution

Zircons from an eclogite and a diamond-bearing metapelite near the Kimi village (north-eastern Rhodope Metamorphic Complex, Greece) have been investigated by Micro Raman Spectroscopy, SEM, SHRIMP and LA-ICPMS to define their inclusion mineralogy, ages and trace element contents. In addition, the host rocks metamorphic evolution was reconstructed and linked to the zircon growth domains. The eclogite contains relicts of a high pressure stage (ca. 700 °C and > 17.5 kbar) characterised by matrix omphacite with Jd 40–35. This assemblage was overprinted by a lower pressure, higher temperature metamorphic event (ca. 820 °C and 15.5–17.5 kbar), as indicated by the presence of clinopyroxene (Jd 35–20) and plagioclase. Biotite and pargasitic amphibole represent a later stage, probably related to an influx of fluids. Zircons separated from the eclogite contain magmatic relicts indicating Permian crystallization of a quartz-bearing gabbroic protolith. Inclusions diagnostic of the high temperature, post-eclogitic overprint are found in metamorphic zircon domain Z2 which ages spread over a long period (160 – 95 Ma). Based on zircon textures, zoning and chemistry, we suggest that the high-temperature peak occurred at or before ca. 160 Ma and the zircons were disturbed by a later event possibly at around 115 Ma. Small metamorphic zircon overgrowths with a different composition yield an age of 79 ± 3 Ma, which is related to a distinct amphibolite-facies metamorphic event. The metapelitic host rock consists of a mesosome with garnet, mica and kyanite, and a quartz- and plagioclase-bearing leucosome, which formed at granulite-facies conditions. Based on previously reported micro-diamond inclusions in garnet, the mesosome is assumed to have experienced UHP conditions. Nevertheless, (U)HP mineral inclusions were not found in the zircons separated from the diamond-bearing metapelite. Inclusions of melt, kyanite and high-Ti biotite in a first metamorphic zircon domain suggest that zircon formation occurred during pervasive granulite-facies metamorphism. An age of 171 ± 1 Ma measured on this zircon domain constrains the high-temperature metamorphic event. A second, inclusion-free metamorphic domain yielded an age of 160 ± 1 Ma that is related to decompression and melt crystallization. The similar age data obtained from the samples indicate that both rock types recorded a high-T metamorphic overprint at granulite-facies conditions at ca. 170 – 160 Ma. This age implies that any high pressure or even ultra-high pressure metamorphism in the Kimi Complex occurred before that time. Our findings define new constraints for the geodynamic evolution for the Alpine orogenic cycle within the northernmost Greek part of the Rhodope Metamorphic Complex. It is proposed that the rocks of the Kimi Complex belong to a suture zone squeezed between two continental blocks and result from a Paleo-ocean basin, which should be located further north of the Jurassic Vardar Ocean.

Geochronology of the Karadere basement (NW Turkey) and implications for the geological evolution of the Istanbul zone

Earlier geological work in the Istanbul zone, western Pontide tectonic belt, has revealed the presence of extensive basement outcrops exposed underneath Palaeozoic and Mesozoic to Tertiary cover sequences. The basement of suspected Neoproterozoic age plays an important role in understanding the crustal accretion process in NW Turkey. We report the first results of a detailed Pb–Pb and U–Pb zircon study complemented by Nd–Sr whole rock and mineral data from basement rocks exposed in the Karadere valley, Safranbolu area. Five samples were selected for this study, comprising three metagranitoids and two metasediments. Zircon geochronology indicates that the metagranitoids were formed during Late Proterozoic pan-African magmatic events between 590 and 560 Ma. The rocks are of tonalitic and granitic composition and have low Nb/Y ratios and Ti contents, consistent with those of arc rocks. A continental arc setting is supported by their Sr and Nd isotope data that indicate a contribution of a mantle source as well as crustal assimilation during magma genesis. The metasediments can clearly be distinguished from the metagranitoids by their higher 87Sr/86Sr ratios and lower eNd-values at 580 Ma, which supports the suggestion that the arc was underlain by mature continental crust. Zircons from the metasediments yield a range of Pb–Pb ages between 1,860 and 710 Ma. Thirty per cent of them fall between 890 and 710 Ma, possibly suggesting a derivation from Gondwana (Afro-Arabian) regions. A Sm–Nd garnet–whole rock analysis obtained on a metagranite gives an age of 559±8 Ma, which either reflects pre-metamorphic magmatic growth of garnet in a felsic melt or a syntectonic high-temperature metamorphic event. Uplift and cooling of the basement is further constrained by Rb–Sr biotite ages of 548–545 Ma. These lower Cambrian mineral ages demonstrate that the Istanbul zone was not thermally reactivated during the Hercynian, Cimmerian or Alpine orogeny, in contrast to its neighbouring tectonic zones, confirming its role as a suspect terrane in the modern western Pontide tectonic belt.

Sub‐ophiolite metamorphic rocks from NW Anatolia, Turkey

The metamorphic rocks from near Kütahya in north‐west Anatolia record different stages in the history of closure of the Neo‐Tethyan İzmir–Ankara–Erzincan ocean. Sub‐ophiolite metamorphic rocks within the Tavşanlı zone are a tectonically composite sequence of quartz–mica schists, amphibole schists, amphibolites and garnet amphibolites. They show increasing metamorphic grade towards the base of the ophiolite. A first metamorphic event, typical of sub‐ophiolite metamorphic sole rocks, was characterized by high‐grade assemblages, and followed by retrograde metamorphism. A second event was marked by a medium‐to high‐pressure overprint of the first‐stage metamorphic assemblages with assemblages indicating a transition between the blueschist and greenschist facies. The chemistry of the sub‐ophiolite metamorphic rocks indicates an ocean island basalt origin, and Ar–Ar dating indicates a high temperature metamorphic event at 93±2 Ma. Counter‐clockwise P–T–t paths recorded by the sub‐ophiolite metamorphic rocks are interpreted to result from intra‐oceanic thrusting during the closure of the İzmir– Ankara–Erzincan ocean, initiating subduction, which formed the high‐temperature assemblages. Further subduction then produced the widespread blueschists of the Tavşanlı zone during the Late Cretaceous. Later cold thrusting obducted the ophiolite (with the metamorphic sole welded to its base), ophiolitic melanges and blueschists onto the Anatolide passive margin in the latest Cretaceous. All these events pre‐date the final Anatolide–Pontide continent–continent collision.

Zircon ages and Nd isotopic and chemical compositions of orthogneisses from the Black Forest, Germany: evidence for a Cambrian magmatic arc

Single zircon U–Pb dating combined with 207Pb/206Pb ages obtained by the evaporation method constrains the emplacement of tonalitic, trondhjemitic, and granodioritic orthogneisses of the Moldanubian zone in the Black Forest between 500 and 510 Ma. Two detrital zircon populations of 1.9 and 1.6 Ga indicate Early-Middle Proterozoic material in the former setting of the basement. The initial eNd values range from –0.1 to –3.4 and mean crustal residence ages of 1.0–1.4 Ga are consistent with involvement of Early-Middle Proterozoic crust, and a subordinate juvenile component probably originating from subduction-related melting of the mantle. The orthogneisses have fractionated REE patterns and slightly higher K2O/Na2O ratios than typical low-K tonalite–trondhjemite–granite suites. The chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust. The emplacement of the orthogneisses was superceded by a high-temperature metamorphic event at ∼480 Ma which we interpret as a result of lithospheric thinning in a marginal basin behind a Cambrian magmatic arc.

2.5 b.y. of punctuated Earth history as recorded in a single rock

Deciphering the absolute chronology of ancient crustal rocks is often problematical because of isotopic reequilibration and/or mixing during multiple geologic events. Ion probe U-Pb age data from texturally distinct zircon crystal domains in a single sample of high-grade gneiss from the Nimrod Group in Antarctica reflect successive resorption and crystallization under disequilibrium conditions. Our data indicate that Archean crust of the East Antarctic shield, initially formed from juvenile magmas ca. 3000 Ma, extends to the central Transantarctic Mountains. Successive zircon growth ca. 2955, 1720, and 530 Ma records a sequence of distinct, widely spaced high-temperature metamorphic and/or anatectic events related to Archean, Paleoproterozoic, and Eocambrian orogenesis.

Changes in the [formula omitted] ratios of fluids as evidence for different metamorphic episodes in high grade gneisses from the Konovalov Mountains area (Rayner Complex, East Antarctica)

Rocks of the Rayner complex in East Antarctica have suffered a complex multiphase metamorphic evolution. Radiogenic age constraints taken from the literature suggest at least two metamorphic events, the `main Rayner event' accompanied by charnockite emplacement 963 Myr ago and an amphibolite facies overprint 500–550 Myr ago. Some authors have suggested that a second high temperature metamorphic event was connected with the widespread intrusion of granitic magmas at about 770 Myr. Petrological and stable isotope thermometry is consistent with three metamorphic events, (i) a first granulite facies metamorphism, as preserved in the cationic equilibria of minerals in enderbitic gneisses yielding 810°C and 6.9 kbar; (ii) a regionally pervasive overprint, also at high temperatures (730°C and 5.9 kbar), previously regarded as the `main Rayner' event and; (iii) an amphibolite facies overprint followed by a greenschist facies retrogression, which is especially pronounced in late shear zones. We show that the two older events can be distinguished by the isotopic composition of the metamorphic fluids. During the first granulite facies event, the rocks preserved their inherited isotopic signatures, whereas the second granulite facies overprint was accompanied by pervasive flux of fluids of magmatic (possibly mantle) origin. During the third, amphibolite grade event, the fluids trapped during the second event were remobilised and led to partial reequilibration.

Crustal xenoliths from Tertiary volcanics of the Northern Hessian Depression

A suite of crustal xenoliths from Tertiary basaltic tuffs of the Northern Hessian Depression (NHD) volcanic field comprises abundant meta-igneous pyroxene granulites of mafic, noritic to anorthositic, IAT and tonalitic composition. Less abundant are granitic, tonalitic and leucogranitic gneisses and metasedimentary xenoliths. A total of 49 samples were analyzed for modal compositions, for major and trace elements (including Li, Rb, Sr, Ba, Cs, V, Sc, Cr, Co, Ni, Y, Zr, Nb, Ta, Hf, Th and REE) and oxygen isotopes. Two-pyroxene thermometry yields temperatures between 700 and 900° C for mafic and noritic granulites. Feldspar thermometry indicates temperatures of 660°–710° C for tonalitic granulites and 470°–520° C for granitic and tonalitic gneisses. One highly depleted sillimanite-rich metasediment contains cordierite and garnet which have equilibrated at temperatures of 780° C. The general lack of garnet in the mafic and noritic granulites and the presence of sillimanite in felsic xenoliths indicates that metamorphic pressures have not exceeded 10 kb. Major and trace element data and oxygen isotope compositions of the mafic granulites are compatible with an origin from spilitized enriched-type MORB rocks (enrichment in δ18O to 11‰ and in Li to 34 ppm at average SiO2 contents of 44.1 wt%). These low-T spilites were transformed into amphibolites and then pyroxene granulites during subsequent high temperature metamorphic events. Low Si, Al, K, and Rb concentrations along with An contents in plagioclase ranging from near 50 to 98 mole percent suggest that amphibolite facies protoliths have generated tonalitic melts during partial melting at temperatures above 700° C. The mafic granulite xenoliths are interpreted as restites whereas the tonalitic samples probably represent the extracted partial melts derived by 20 to 30 percent degree of melting. Metasedimentary xenoliths strongly depleted in granitic component could represent restites from which granitic S-type partial melts have been extracted. Tonalitic and leucogranitic gneisses including one trondhjemite xenolith have many chemical characteristics (e.g. REE distribution) in common with tonalite-trondhjemite-granodiorite suites of the North Atlantic region but cannot be accounted for a more specific origin. Estimated elastic properties of the main types of NHD xenoliths yield P-wave velocities of 6.0–6.4 km-1 for granitic, tonalitic and trondhjemite gneisses including tonalitic granulites and 6.5–7.0 for the more mafic xenoliths. When compared with two seismic depths-Vp profiles these data are in accordance with a model where the mafic, andesitic, noritic and tonalitic granulites comprise abundant rock types at depths between 29 km (Moho) and 20 km which mainly consists of old oceanic crust including subduction related volcanic products. The more felsic xenoliths probably represent material from depths between 12 and 20 km.