Garnet Biotite Schist Research Articles

3D modeling and age of the collision metamorphism of the Khan-Khukhei Block, Northern Mongolia

The reconstruction of the Early Paleozoic collision metamorphism of the Khan-Khukhei Block (Northern Mongolia) based on 3D modeling of thermal state of the crust and isotope dating is presented. The age of garnet-biotite schist from a metamorphic complex of the Khan-Khukhei Block of 517.4±7.4 Ma is determined for the first time. 3D modeling was conducted to compare this age of metamorphism with the available age determinations of (post-)collision granite formation in the range of 513–505 Ma. The model considers radiogenic heating at an increased value of heat release in the rocks of the thickened crust of the Khan-Khukhei block as the cause of the migmatization and formation of granitoid melts. Three-dimensional modeling was carried out for the first time for the thermal-dome type of metamorphism. The results provide a realistic conception of magma generation in typical collision settings in the absence of mantle magmatic heat sources. The time interval between metamorphism and the stage of anatectic magma formation is 5–12 million years.

Far-field response to the closure of the Meso-Tethys Ocean: New geochronological evidence from the Chem Co graben in the westernmost part of Central Tibet

The Chem Co graben is located in the westernmost part of the Qiangtang block, central Tibet. It is adjacent to the Longmu Co Fault to the north and approximately 50 km away from the Karakoram Fault to the west. The formation of the graben resulted in the exposure of basement rocks in the footwalls of the graben bounding normal fault, which hold crucial information on the Mesozoic closure of the Meso-Tethys Ocean. Garnet-biotite schist crops out sporadically in the footwall of the graben-boundary normal fault, and is intruded by leucogranite dikes. Pseudosection modeling indicates peak metamorphic conditions for the schist of 590–670 °C and 4.5–7.5 kbar, similar to the conditions of mid-crustal rocks at the western end of the Qiangtang block. Field investigations and microstructural analysis suggest syn-kinematical left-lateral strike-slip in both the biotite schist and granitoid veins. Zircon U − Pb, monazite U − Th − Pb, and 40Ar/39Ar ages show that intense regional intensive tectonic deformation and contemporaneous magmatism began at ∼120.6 Ma and ended with the peak metamorphism conditions at 105.3 ± 6.0 Ma. These results indicate that the closure of the Meso-Tethys Ocean in the westernmost part of central Tibet occurred over this period (i.e., 121–105 Ma) with final closure during the late Early Cretaceous. The closure of the Meso-Tethys Ocean likely triggered widespread far-field responses, extending from the Altyn Tagh Fault to the Longmu Co Fault, and reaching the Pangong and Hunza regions around the Western Himalayan Syntaxes. Episodic crustal thickening and surface uplift since the closure of the Meso-Tethys Ocean caused the upper crust to be extruded along the westernmost part of central Tibet, leading to the formation of the Chem Co graben.

Metamorphic P–T conditions and ages of garnet-biotite schists in the Dahongshan Group from the southwestern Yangtze Block

Metamorphic P–T conditions and ages of garnet-biotite schists in the Dahongshan Group from the southwestern Yangtze Block

Discrepancies and Research Gaps on the Lithostratigraphy of the Jajarkot Thrust Sheet, Western Nepal Himalaya

The interaction between the Indian and Asian plates has led to the formation of imbricated thrust sheets in the Himalayas. This study focuses on a segment of the Jajarkot Thrust Sheet, characterized by metamorphic rocks, including garnet-biotite schist, biotite schist, quartzite interlayers, and marbles. The upper stratigraphic sequence of this thrust sheet contains fossiliferous rocks from the Paleozoic era. Prior classifications have identified three main units: Chaurjhari Formation, Thabang Formation, and Jaljala Formation. However, the Department of Mines and Geology recognizes two units: Siuri Formation and Surbang Formation. This discrepancy, along with inconsistent terminologies like "Jajarkot nappe" and "Jajarkot klippe", highlights the need for a unified stratigraphic framework. Such clarity is crucial for assessing mineralization potentials, particularly for iron and copper, in western Nepal and understanding the thrust sheet's tectonic history within the broader Himalayan context.

Late Permian medium-pressure metamorphism in the eastern Songnen Massif, eastern Central Asian Orogenic Belt (NE China): Implications for the final closure of the Paleo-Asian Ocean

The nature of late Paleozoic metamorphism in the eastern Central Asian Orogenic Belt (CAOB) is controversial. This paper reports mineral chemistry, petrological, and zircon and titanite U–Pb age data for representative rocks in the Dongfengshan Group from the eastern margin of the Songnen Massif. We use these data to determine the rock associations, P–T conditions, and timing of metamorphism. The P–T conditions were estimated from 54 mineral pairs in representative rocks, including garnet–biotite schist, garnet–two-mica schist, garnet–biotite–plagioclase gneiss, and garnet amphibolite, and indicate the Dongfengshan Group experienced metamorphism at P = 5.5–7.1 kbar and T = 592–630 °C (i.e., a geothermal gradient ranging from 25 to 30 °C/km) under medium-pressure lower amphibolite-facies conditions. The biotite–plagioclase gneiss and garnet amphibolite yielded titanite U–Pb ages of 251 and 260 Ma, respectively, which are interpreted to represent the regional metamorphic age (260–251 Ma). The consistency of these dates and ages of metamorphic rims in zircons from a biotite plagiogneiss (252 Ma) confirm that medium-pressure metamorphism occurred in the late Permian rather than the Neoproterozoic, as assumed previously. Based on metamorphic, magmatic, stratigraphic, and paleontological constraints, as well as the regional tectonic history, we propose that late Permian medium-pressure metamorphism in the Dongfengshan Group occurred during the collision and amalgamation of the Siberian and North China cratons following the final closure of the eastern Paleo-Asian Ocean.

Zircon and monazite dating of pelitic high-pressure granulite in the Eastern Himalayan Syntaxis and geological significance

东喜马拉雅构造结的南迦巴瓦杂岩含有广泛分布的高压麻粒岩，但由于以前获得了许多不同的年龄，对这些麻粒岩的变质与深熔时代、持续时间和成因存在不同认识。本文对泥质高压麻粒岩（蓝晶石榴黑云片岩）中的锆石和独居石进行了系统的内部结构、U-（Th）-Pb定年和微量元素分析，以求揭示这些岩石是否具有相同的演化过程。所研究的6个蓝晶石榴黑云片岩由石榴石、蓝晶石、黑云母、石英、钾长石、斜长石、夕线石、白云母、石墨和副矿物金红石、钛铁矿、锆石和独居石组成，峰期矿物组合是石榴石+蓝晶石+斜长石+钾长石+黑云母+石英+金红石。6个样品中的锆石均由继承碎屑核+变质（深熔）幔+变质（深熔）边组成。其中3个样品中的锆石幔和边较宽，均可进行原位定年，幔部给出了类似的较老年龄范围（39.6~31.6Ma、40.8~32.0Ma和38.1~31.3Ma），而边部给出了类似的较年轻年龄范围（26.8~17.3Ma、28.3~18.6Ma和28.4~18.8Ma）。另外3个样品的锆石幔部较窄，不能进行分析，其边部给出了与前3个样品锆石边部类似的年轻年龄范围（22.0~17.0Ma、20.9~16.9Ma和22.2~16.6Ma）。一个片岩样品中的独居石给出了与其锆石幔部+边部年龄类似的较宽年龄范围（38.1~17.5Ma），而另外3个样品中的独居石获得了与其锆石边部年龄相似的年轻年龄范围（26.0~18.8Ma、22.3~16.9Ma和26.4~19.4Ma）。随着年龄的减小，锆石和独居石的Th/U比值增大，Eu/Eu^*减小，独居石的HREE和Y含量减小。基于这些分析结果，笔者认为所研究的6个片岩记录了相同的、从~41Ma持续到~17Ma的进变质与深熔过程。但是，由于某些样品中的锆石和独居石在早期变质和深熔过程中形成的结晶域（锆石幔部）很窄，无法定年，导致不同的样品获得了不同的年龄范围。结合现有研究成果，笔者推测南迦巴瓦杂岩中的高压麻粒岩经历了相似的长期进变质与深熔过程。;The Namche Barwa complex in the Eastern Himalayan Syntaxis contains widely distributed high-pressure granulites. However, due to many different ages previously obtained, there are different understandings on the metamorphic and anatectic age, duration and genesis of these granulites. In this paper, zircon and monazite in high-pressure pelitic granulite (kyanite garnet biotite schist) samples are systematically analyzed by internal structure, U-(Th)-Pb isotope and trace element, in order to reveal whether these rocks have the same evolution process. The six schists studied are composed of garnet, kyanite, biotite, K-feldspar, plagioclase, quartz, sillimanite, muscovite, graphite and accessory minerals rutile, ilmenite, zircon and monazite. The peak mineral assemblages are garnet+kyanite+plagioclase+K-feldspar+biotite+quartz+rutile. The zircons in the six samples are composed of inherited detrital core+metamorphic (anatectic) mantle+metamorphic (anatectic) rim. The zircon mantle and rim in three samples are sufficiently wide and can be dated in situ. The zircon mantles give a similar older age range of 39.6~31.6Ma, 40.8~32.0Ma and 38.1~31.3Ma, respectively, while the rims give similar younger age ranges of 26.8~17.3Ma, 28.3~18.6Ma and 28.4~18.8Ma, respectively. The zircon mantles of the other three samples are narrow and cannot be analyzed, while the zircon rims give young age ranges of 22.0~17.0Ma, 20.9~16.9Ma and 22.2~16.6Ma, respectively. Monazite in one schist sample gives a wide age range of 38.1~17.5Ma, which is similar to that obtained by zircon mantle+rim from the same sample. Monazite in the other three samples obtains young age ranges of 26.0~18.8Ma, 22.3~16.9Ma and 26.4~19.4Ma, respectively, which are similar to those given by the zircon rims from the same samples. With the decrease of age, Th/U ratio of zircon and monazite increases, Eu/Eu^* of zircon and monazite decreases, HREE and Y of monazite decrease. Based on these analytical results, we suggest that the six schists recorded the same and long-lasting prograde metamorphic and anatectic process from ca.41Ma to ca.17Ma. However, the domains of zircon and monazite in some samples formed during early metamorphism and anatexis are too narrow to date via the in-situ analysis method adopted by this study. Combined with the existing research results, we speculate that the high-pressure granulites in the Namche Barwa complex have experienced similar prolonged prograde metamorphism and anatexis.

Geochemical characteristics of Precambrian Basement rocks from southeastern Nigeria: an approach to their petrogenesis and tectonic setting.

Trace and Rare-Earth element geochemical study of twenty samples of migmatitic banded gneisses, garnet biotite schists, dolerites, granites and rhyolites was carried out in a bid to determine their petrogenetic and tectonic significance in the evolution of the southeastern Basement complex of Nigeria. The data shows that partial melting (crustal anatexis) of migmatitic gneisses and schists played a significant role in the evolution of the granitic intrusions. This is supported by the high incompatible (Rb/Sr = 0.16 to 1.31 and Ba/Sr = 0.75 to 6.21) elements ratio in the granitic intrusions than that of the migmatitic gneisses and schists (Rb/Sr, 0.051 to 0.824; Ba/Sr, 0.7 to 5). High ratios of Ba/Sr and Rb/Sr and lesser values of Ba/Rb ratios in some granitic intrusions than in others suggests increasing fractionation during the anatexis. The role of partial melting is also evident in the smooth REE patterns shown by most of these rocks and the negative Eu anomaly as indicated by the values of Eu/Eu* (0.097 to 0.7). LREE enrichment is evident in the high values of Ce/YbN (12.08-174.5), La/YbN (15.2-228.4) and La/SmN (2.6-7.2) in the granitic intrusions. Tectonic discrimination diagrams of the rocks indicate that the basement rocks were most probably formed in a post-collision orogenic setting while the dolerite and the rhyolite were formed in within-plate anorogenic setting.

Early Eocene high-flux magmatism and concurrent high-temperature metamorphism in the Gangdese belt, southern Tibet

Abstract The Himalayan-Tibetan orogen represents one of the major Cenozoic tectonic features on Earth, and yet considerable debate continues over the timing and sequence of collisional events leading to its formation. In this contribution, we present new field relations, petrology, geochemistry, geochronology, and phase equilibria modeling in the Gangdese belt of southern Tibet in an effort to address Indo-Asian collisional events in the region. These investigations reveal that the dominantly dioritic Nymo intrusive complex was formed at ca. 50–47 Ma. We establish that the Jurassic-aged Bima volcano-sedimentary sequence underwent early Eocene (50–47 Ma) high-temperature (HT) amphibolite-facies metamorphism. Petrology and phase equilibria modeling of garnet-biotite schists in the Bima rocks reveals mineral assemblages of melt + plagioclase + garnet + biotite + magnetite + ilmenite + sillimanite formed under conditions of 5.3–7.5 kbar and 700–800 °C. We contend that the early Eocene Nymo intrusive complex represents part of the ca. 50 Ma high-flux magmatic “flare-up” that triggered the HT amphibolite-facies metamorphism within the overlying plate during Indo-Asian collision. The synchroneity of high-flux magmatism and HT metamorphism in the Gangdese belt roughly coincided with the continuing Indo-Asian collisional process, implying the early Eocene closure of the Neotethys Ocean along the southern margin of the Lhasa terrane.

Metamorphic evolution and U-Pb geochronology of metapelite, northeastern Wutai Complex: Implications for Paleoproterozoic tectonic evolution of the Trans-North China Orogen

The Wutai Complex lies in the middle part of the Trans-North China Orogen, mainly consisting of metamorphosed volcanic-sedimentary rocks (named as “Wutai Group”) and Late Archean TTG gneisses. Garnet biotite schist from the northeastern part of the Wutai Group (lower Shizui Subgroup) mainly consists of garnet porphyroblast plus biotite, staurolite, kyanite, sillimanite/fibrolite, plagioclase, quartz as well as accessory minerals including ilmenite, magnetite, hematite, apatite, monazite and zircon. Petrographic observation and mineral chemistry indicate that these metapelites underwent the sequential prograde (M1), peak (M2) and retrograde (M3) metamorphic stages. Geothermobarometry computation and phase equilibrium modelling show that the representative samples record clockwise P-T paths passing from 3–7 kbar/570–630 °C (M1) through 9–10 kbar/615–660 °C (M2P) to 6–9 kbar/655–680 °C (M2T) and finally to a speculated exhumation stage (M3), indicative of orogenic metamorphism. SRHIMP and LA-ICP-MS U-Pb dating on inherited detrital zircons yielded three age peaks of ~2.7 Ga, ~2.5 Ga and ~2.1 Ga. The youngest 207Pb/206Pb age of 2064 ± 21 Ma of detrital zircon possibly constrains the maximum depositional age of the Shizui Subgroup. SIMS and SHRIMP U-Pb dating of metamorphic monazite and zircon constrain the metamorphic age of the garnet-bearing mica schist to be 1853–1823 Ma. Combined with previous data, the Wutai Complex was demonstrated to have been involved in a prolonged subduction-collision process (1.97–1.82 Ga) between the Western and Eastern Blocks in the Paleoproterozoic.

Tectonic evolution of the Dadongcha Formation, Ji'an group, Qinghe area, Tonghua, NE China: Implications from geochronological and geochemical evidence of metapelites

The Jiao–Liao–Ji Belt (JLJB) is an important Paleoproterozoic orogenic belt that has been a focus of research to understand the Precambrian evolution of the North China Craton (NCC). The widespread Paleoproterozoic Ji’an group, which was exposed in the northern margin of JLJB, is the key to solving controversies in the tectonic setting of this special belt. An intensive study on metamorphic reactions, zircon U–Pb ages, mineral thermobarometry, geochemistry, and quantitative phase equilibria modeling (pressure–temperature pseudosections) of typical metapelites from the Dadongcha Formation of the Ji’an group provides important insight to understand the tectonic setting and processes of the Paleoproterozoic JLJB. The analyzed metapelites are exposed in Toudao and Qinghe towns in NE China, include garnet–biotite schist, garnet–biotite–plagioclase gneiss, garnet–sillimanite–cordierite–biotite gneiss, and hypersthene-spinel–cordierite–biotite gneiss. Garnets contain mineral inclusions such as sillimanite, biotite, feldspar, and quartz and retain the typical cordierite reactive edge structures. The protoliths of metapelites are sets of claystone or miscellaneous sandstone with an average chemical composition that is close to that of the upper continental crust (UCC). Chemical Index of Alteration (CIA) and Index of Compositional Variability (ICV) values suggest a high degree of weathering of their sources and low compositional maturity of clastic sediments. The chondrite-normalized rare earth element (REE) pattern of analyzed samples displays characteristics of medium fractionation in light and heavy REEs, and strong enrichment in light REEs. The primitive mantle-normalized trace element pattern shows enrichment of large ion lithophile elements (LILEs) such as Ba, and the content of high field strength elements (HFSEs) such as Zr and Hf is relatively low; furthermore, Nb, Ta, P, and Ti are distinctly depleted. The emphasis is on the metamorphism of metapelites, which shows a clockwise P–T path. In combination with zircon U–Pb dating, a general evolution model for the metapelites was drawn. The metapelites were formed in an active continental margin environment where the evolved island arcs developed around 1.95–2.02 Ga, then were involved in the collision between the Longgang and Langrim blocks in the eastern part of the NCC, and returned to the surface after undergoing regional metamorphism with widespread anatexis around 1.8–1.95 Ga.

Petrography and geochemistry of Neoproterozoic charnockite–granite association and metasedimentary rocks around Okpella, southwestern Nigeria

The Neoproterozoic charnockite–granite association of Okpella intrudes metasedimentary and migmatite-gneiss complex rocks in the eastern Igarra Schist Belt, southwestern Nigeria. In order to unravel the complex processes involved in the formation and tectonic evolution of the Igarra Schist Belt during the Neoproterozoic, detailed field, petrographic and whole-rock geochemical study of the charnockite–granite association and metasedimentary rocks around Okpella was conducted. Published data on the metasedimentary rocks and Pan-African granitoids in different sectors of the belt were also compiled for detailed interpretations. The charnockites and granites of Okpella show primary magmatic mineralogy and geochemical characteristics. They are silicic (> 63 wt% SiO2), metaluminous to peraluminous, high-K calc-alkaline, ferroan, post-collisional granitoids. The garnet-biotite schist, calc-silicate gneiss and quartzite in the area are low–medium grade metasedimentary rocks. The mineralogy and geochemistry of the charnockites suggest that the charnockitic melt was derived from mafic lower continental crust through partial melting and assimilation-fractionation processes. The granites probably originated from mixed melts derived from lower- to mid-crustal tonalites-trondhjemites-granodiorites and/or subducted metagreywackes and mantle-derived magmas (probably the charnockitic melt and/or its progenitor). The granites and charnockites are coeval and were presumably emplaced during the post-collisional stages of the Pan-African Orogeny. The garnet-biotite schist, calc-silicate gneiss and quartzite represent metamorphosed immature to slightly mature sedimentary rocks, probably greywacke, marl and subarkose, respectively, that were sourced from intermediate–acid rocks which underwent low–moderate chemical weathering with minor contribution from recycled sediment sources and deposited in active continental environments. The Neoproterozoic evolution of the Igarra Schist Belt, therefore, involved deposition and infolding of sediments in active continental margin during the early Pan-African followed by upwelling of basaltic magma from the mantle which underplated and crystallized in the lower continental crust and was subsequently partially melted to generate the charnockitic and granitic melts through mantle-crust interaction during the late Pan-African.

Metamorphic evolution of metasedimentary rocks of the Feira Nova region: Tectonic implications for the Brasiliano Orogeny in eastern Borborema Province, Northeast Brazil

Thermobarometric estimations and X-ray maps were combined to evaluate the metamorphic evolution of metasedimentary rocks (Surubim Complex) in the Feira Nova region, eastern Borborema Province (NE Brazil). Sillimanite-bearing biotite schists, garnet-biotite schists and gneisses were studied. X-ray maps show different growing phases and retrograde zoning patterns for biotite and garnet. High Ca and Mg contents in garnet cores along with high Ti in biotite are interpreted as equilibrium compositions at peak metamorphism. Lower contents of Ca and Mg at garnet rims and of Ti in biotite rims suggest cooling and decompression. Garnet-biotite geothermometer and garnet-Al2SiO5-quartz-plagioclase (GASP) geobarometer constrain high temperature (∼650–760 °C) and medium pressure (∼0.6–0.9 GPa; burial to ∼23–34 km) conditions for the metamorphic peak, which was synchronous to development of a flat-lying foliation and local anatexis. Garnet rims indicate exhumation to ∼19-11 km (∼0.4–0.3 GPa) at ∼590-520 °C, probably during a transpressional regime. The data points to low exhumation and cooling rates. The lack of extensional fabrics in the region right after the Brasiliano Orogeny supports erosional unroofing rather than extensional collapse. Based on kyanite occurrences nearby, we infer a clockwise P-T path related to thickening in a predominant intracontinental setting, followed by exhumation with low erosional and cooling rates.

Neoproterozoic granitoid magmatism and granulite metamorphism in the Chu-Kendyktas terrane (Southern Kazakhstan, Central Asian Orogenic Belt): Zircon dating, Nd isotopy and tectono-magmatic evolution

Within the Chu-Kendyktas Precambrian terrane located in Southern Kazakhstan (in the western part of the Central Asian Orogenic Belt), medium- and high-grade metamorphic formations have been identified and attributed to the Aydaly and Shukyr Complexes. The Aydaly Complex is predominantly composed of orthogneisses with a Neoproterozoic protolith age of 790 Ma and subordinate high-temperature granulites comprising a succession of amphibolites, amphibole-rich granulites, mesocratic granulites, and melanocratic (clinopyroxene-rich) and leucocratic (melt-rich) granulites, which were evidently formed at moderate pressures of 6–9 kbar and high temperatures, increasing from 700 to 900 °C or more. The clinopyroxene-rich granulites of the Aydaly Complex contain detrital zircon grains of mainly Palaeoproterozoic (1790–2058 Ma) and, to a more limited extent, Neoarchean (~2500 Ma) ages, with preserved magmatic zoning and indicating a sedimentary origin of the protolith via mafic greywackes that were subsequently metamorphosed to amphibolites. The age estimates of the leucocratic melt-rich granulites, which form veins or layers within the clinopyroxene-rich melanosome, correspond to the range 770–790 Ma and define the Neoproterozoic stage of magmatism and granulite metamorphism. Hence, the deposition of the sedimentary protolith for the Aydaly Complex granulites and its subsequent emplacement at mid-crustal levels occurred from the middle Palaeoproterozoic (~1800 Ma) to the late Tonian (~770–790 Ma). In turn, the Shukyr Complex consists of garnet-biotite schists, which are characterised by a significantly lower grade of metamorphism compared to the Aydaly Complex, at around T ~600 °C. The garnet-biotite schists are thought to represent moderate-temperature hornfels, located at the contact between the protolith (shales) and the Early Palaeozoic Ulken massif granitoids. The deposition of the Shukyr Complex protolith occurred during the Ediacaran–Cambrian, as a result of the erosion of mainly Mesoproterozoic (1000 Ma), and to a lesser extent Palaeoproterozoic (2460 Ma), rocks. The Sm-Nd whole-rock isotopic compositions suggest that the formation of the protoliths of the Aydaly and Shukyr Complexes was related to the reworking of Palaeoproterozoic continental crust.

Gold-Bearing Pyrrhotite Ores in East Sayan: Composition and Formation Conditions (by the Example of the Ol’ginskoe Ore Occurrence)

Abstract —We present results of study of the chemical composition, formation conditions, and genesis of gold-bearing pyrite–pyrrhotite ores widespread within the ophiolite belts in the southeast of East Sayan. The study was performed at the Ol’ginskoe ore occurrence localized in the Ol’gino gold ore zone. Sulfide mineralization zones are composed of lenticular interbeds and bodies subconfoimable with the bedding of the enclosing schist strata. The ores are carbonaceous siliceous garnet–biotite schists variably enriched in sulfide minerals, mainly pyrrhotite, with impurities of other silicate minerals: tremolite, chlorite, albite, plagioclase, diopside, and epidote. In addition, sphalerite, chalcopyrite, pyrite, and arsenopyrite are present in small amounts in the ores. The established Р–Т conditions of ore metamorphism (T = 430–540 °C, P ~ 5 kbar) correspond to the lower boundary of the epidote–amphibolite facies. Thermobarogeochemical studies of fluid inclusions in vein quartz showed close temperatures (536–340 °C) but lower pressures, 200–800 bars, which indicates that the quartz vein formation was related to a pressure drop. The predominant salts of the fluid inclusions are Fe and Mg chlorides with Na and K impurities. The geologic location, structures, textures, and mineral, geochemical, and isotope compositions of the studied sulfide ores indicate their formation in submarine deep-water environments as a result of the activity of hydrothermal systems, analogues of “black smokers”, later subjected to metamorphism. During metamorphism, primary ores underwent mineral and chemical transformations: pyrrhotitization of pyrite, appearance and coarsening of native gold particles, and redistribution of components (Mn, As, etc). The low contents of gold and some ore-forming elements (Zn, Cu, and Pb) in pyrrhotite ores testify to the removal of these elements with a metamorphogenic fluid. The removed ore-forming components might have been a source of material for later gold–sulfide–quartz deposits of the Urik–Kitoi zone in East Sayan.

Jardim do Seridó Suite: first example of Ediacaran peraluminous magmatism in the Rio Piranhas-Seridó Domain, Borborema Province, Northeast Brazil

Peraluminous granitoids were recently mapped in the Rio Piranhas-Serido Domain, the northeastportion of the Borborema Province. These bodies occur as stock and intrusive dikes in the metapelites of the Serido Formation, constituting the Jardim do Serido Suite. This suite has two facies. The first and most expressive facies consists of a medium-grained and whitish leucocratic monzogranite with muscovite, garnet, and biotite, while the second is represented by fine-grained and greyish leucocratic granodiorites with biotite, cropping out mainly on the edges of the main body. The thermobarometric conditions for the crystallization of the monzogranites were estimated between 2-5 kbar and 690-950 oC, whereas the lithochemical data indicate a syn-collisional peraluminous signature. Neoproterozoic garnet-biotite schist xenoliths of the Serido Formation are sometimes observed partially assimilated in the main body (stock). A U-Pb zircon age of 592 ± 2 Ma obtained in the main body is considered as the crystallization age of the Jardim do Serido Suite, corroborating the proposed existence of peraluminous magmas associated with the Brasiliano Orogenesis in the Rio Piranhas-Serido Domain. The chemical and petrographic data identified this suite as fertile to that it can generate dikes of rareelements-bearing pegmatites.

Basement Characteristic Western Part of Java, Indonesia ; Case Study in Bayah Area, Banten Province

Recent study reveals that in Bayah Complex, 20 km west of Ciletuh Melange Complex, discovered a metamorphic rock that interpreted as the basement of Java. The aim of this research is to know the characteristic of metamorphic in Bayah areas. The result shows that the metamorphic rocks of Bayah Geological Complex are dominated by mica schist group i.e. muscovite schist, muscovite - biotite schist, garnet biotite schist and chlorite schist associated with Pelitic - Psammitic protolith. The amphibolite, epidote amphibolite and actinolite schist found were metamorphosed of mafic rock protolith. All of them have been deformed and altered. Based on mineral assemblage, mica schist group included lower greenschist - epidote amphibolite facies, whereas actinolite schist, epidote amphibolite schist and hornblende schist included greenschist facies, epidote amphibolite facies and amphibolite facies respectively. Based on the data, these metamorphic rocks are associated with orogenic style. The metamorphic rocks exposed to the surface through a complex process since Late Cretaceous. Metamorphic rocks have been deformed, folded and faulted since its formation. Its possible this rock was uplifted to the surface due to intrusion of Cihara Granodiorite.

Triassic rejuvenation of unexposed Archean-Paleoproterozoic deep crust beneath the western Cathaysia block, South China

Jurassic (ca. 150 Ma) Daoxian basalts from the western Cathaysia block (South China) entrained a suite of deep-seated crustal xenoliths, including felsic schist, gneiss and granulite, and mafic two-pyroxene granulite and metagabbro. Zircon U–Pb–Hf isotopic, whole-rock elemental and Sr–Nd–Pb isotopic compositions have been determined for these valuable xenoliths to reveal the poorly-known, unexposed deep crust beneath South China. Detrital zircons from the garnet-biotite schists show several populations of ages at 0.65–0.5 Ga, 1.1–0.75 Ga, 1.6–1.4 Ga, 1.8–1.7 Ga, 2.5–2.4 Ga, ~2.8 Ga, and ~3.5 Ga, representing a multi-sourced, meta-sedimentary origin with deposition time at the early Cambrian. One mafic granulite contains zircons with concordant U–Pb ages of Neoarchean (~2520 Ma), as well as Hf model ages of 2.8–2.6 Ga and positive εHf(t) values (up to 6.3), suggesting an accretion of juvenile crust in Neoarchean, probably as the main framework of the lower crust. Geochemical and geochronological evidence shows the mafic granulite and metagabbro were produced by underplating of magmas derived from the depleted asthenosphere and mixed with EM2-type materials during the Late Triassic (205–196 Ma). This magmatic underplating also resulted in the widespread metamorphism of the mafic lower crust and felsic middle crust (e.g., the felsic granulite and gneiss) at 202–201 Ma. We suggest the existence of a highly evolved Archean-Paleoproterozoic basement beneath the western Cathaysia block, which experienced episodic accretion and reworking and the strong rejuvenation during the Triassic. A three-layered structure of the lower crust could exist beneath the Daoxian area during the Jurassic time: its upper layer is an evolved Archean-Paleoproterozoic basement; the middle hybrid layer represents a mixture of Archean-Paleoproterozoic basement with newly accreted/reworked Proterozoic to Phanerozoic materials; and the deeper layer consists of mafic granulites derived from the intensive magmatic underplating during the Triassic.

Mesoproterozoic suturing of Archean crustal blocks in western peninsular India: Implications for India–Madagascar correlations

The Kumta and Mercara suture zones welding together Archean crustal blocks in western peninsular India offer critical insights into Precambrian continental juxtapositions and the crustal evolution of eastern Gondwana. Here we present the results from an integrated study of the structure, geology, petrology, mineral chemistry, metamorphic P–T conditions, zircon U–Pb ages and Lu–Hf isotopes of metasedimentary rocks from the two sutures. The dominant rocks in the Kumta suture are greenschist- to amphibolite-facies quartz–phengite schist, garnet–biotite schist, chlorite schist, fuchsite schist and marble. The textural relations, mineral chemistry and thermodynamic modelling of garnet–biotite schist from the Kumta suture indicate peak metamorphic P–T conditions of ca. 11kbar at 790°C, with detrital SHRIMP U–Pb zircon ages ranging from 3420 to 2547Ma, εHf (t) values from −9.2 to 5.6, and TDMc model ages from 3747 to 2792Ma. The K–Ar age of phengite from quartz–phengite schist is ca. 1326Ma and that of biotite from garnet–biotite schist is ca. 1385Ma, which are interpreted to broadly constrain the timing of metamorphism related to the suturing event. The Mercara suture contains amphibolite- to granulite-facies mylonitic quartzo-feldspathic gneiss, garnet–kyanite–sillimanite gneiss, garnet–biotite–kyanite–gedrite–cordierite gneiss, garnet–biotite–hornblende gneiss, calc-silicate granulite and metagabbro. The textural relations, mineral chemistry and thermodynamic modelling of garnet–biotite–kyanite–gedrite–cordierite gneiss from the Mercara suture indicate peak metamorphic P–T conditions of ca. 13kbar at 825°C, followed by isothermal decompression and cooling. For pelitic gneisses from the Mercara suture, LA-ICP-MS U–Pb zircon ages vary from 3249 to 3045Ma, εHf (t) values range from −18.9 to 4.2, and TDMc model ages vary from 4094 to 3314Ma. The lower intercept age of detrital zircons in the pelitic gneisses from the Mercara suture ranges from 1464 to 1106Ma, indicating the approximate timing of a major lead-loss event, possibly corresponding to metamorphism, and is broadly coeval with events in the Kumta suture. Synthesis of the above results indicates that the Kumta and Mercara suture zones incorporated sediments from Palaeoarchean to Mesoproterozoic sources and underwent high-pressure metamorphism in the late Mesoproterozoic. The protolith sediments were derived from regions containing juvenile Palaeoarchean crust, together with detritus from the recycling of older continental crust. Integration of the above results with published data suggests that the Mesoproterozoic (1460–1100Ma) Kumta and Mercara suture zones separate the Archean (3400–2500Ma) Karwar–Coorg block and Dharwar Craton in western peninsular India. Based on regional structural and other geological data we interpret the Kumta and Mercara suture zones as extensions of the Betsimisaraka suture of eastern Madagascar into western India.

Formation age and genesis of the Gongchangling Neoarchean banded iron deposit in eastern Liaoning Province: Constraints from geochemistry and SHRIMP zircon U–Pb dating

The Gongchang iron deposit, a typical Algoma-type banded iron formation (BIF) deposit located in the Eastern Block of the North China Craton (NCC), is hosted in Neoarchean metamorphic rocks, including amphibolite, biotite leptynite, garnet biotite schist, plagioclase-hornblende schist, garnet chlorite schist, muscovite quartz schist and actinolite magnetite quartzite. Geochemical data suggest that protoliths of the amphibolites and biotite leptynite should be basalt and dacite, respectively. SHRIMP U–Pb dating of zircons from the plagioclase-hornblende schist shows that the Gongchangling iron deposit was formed at 2554±12Ma. Major elements and REE data of the ores indicate that the Gongchangling BIF precipitated from seawater, and the ultimate iron and silica origins may have been related to nearby volcanic activity. The δ18O values of quartz from the BIFs range between 9.1 and 12.9‰, suggesting that exotic high δ18O fluids were involved during metamorphism. The pyrite have δ34S values of range between −3.5‰ and 14.7‰, suggesting that mineralization were originally from metamorphism. Comparison of the mineralization age of the Gongchangling BIF with those of other local BIFs in North China and other cratons throughout the world indicates that the Gongchangling BIF formed coevally with the global BIF mineralization at ∼2.5Ga. Taken together, these data suggest that the Gongchangling iron deposit was formed in a volcanic submarine environment in which hydrothermal fluids associated with volcanic activity supplied both iron and silica for the banded iron formations.

A Rodinian suture in western India: New insights on India-Madagascar correlations

We report detailed evidence for a new paleo-suture zone (the Kumta suture) on the western margin of southern India. The c. 15-km-wide, westward dipping suture zone contains garnet-biotite, fuchsite-haematite, chlorite-quartz, quartz-phengite schists, biotite augen gneiss, marble and amphibolite. The isochemical phase diagram estimations and the high-Si phengite composition of quartz-phengite schist suggest a near-peak condition of c. 18kbar at c. 550°C, followed by near-isothermal decompression. The detrital SHRIMP U–Pb zircon ages from quartz-phengite schist give four age populations ranging from 3280 to 2993Ma. Phengite from quartz-phengite schist and biotite from garnet-biotite schist have K–Ar metamorphic ages of ca. 1326 and ca. 1385Ma respectively. Electron microprobe-CHIME ages of in situ zircons in quartz-phengite schist (ca. 3750Ma and ca. 1697Ma) are consistent with the above results. The Bondla ultramafic-gabbro complex in the west of the Kumta suture compositionally represents an arc with K–Ar biotite ages from gabbro in the range 1644–1536Ma. On the eastern side of the suture are weakly deformed and unmetamorphosed shallow westward-dipping sedimentary rocks of the Sirsi shelf, which has the following upward stratigraphy: pebbly quartzite/sandstone, turbidite, magnetite iron formation, and limestone; farther east the lower lying quartzite has an unconformable contact with ca. 2571Ma quartzo-feldspathic gneisses of the Dharwar block with a ca. 1733Ma biotite cooling age. To the west of the suture is a c. 60-km-wide Karwar block mainly consisting of tonalite-trondhjemite-granodiorite (TTG) and amphibolite. The TTGs have U–Pb zircon magmatic ages of ca. 3200Ma with a rare inherited core age of ca. 3601Ma. The K–Ar biotite cooling age from the TTGs (1746Ma and 1796Ma) and amphibolite (ca. 1697Ma) represents late-stage uplift. Integration of geological, structural and geochronological data from western India and eastern Madagascar suggest diachronous ocean closure during the amalgamation of Rodinia; in the north at around ca. 1380Ma, and a progression toward the south until ca. 750Ma. Satellite imagery based regional structural lineaments suggests that the Betsimisaraka suture continues into western India as the Kumta suture and possibly farther south toward a suture in the Coorg area, representing in total a c. 1000km long Rodinian suture.