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M Santosh - One of the best experts on this subject based on the ideXlab platform.
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high ba sr adakitic Charnockite suite from the nagercoil block southern india vestiges of paleoproterozoic arc and implications for columbia to gondwana
Geoscience frontiers, 2020Co-Authors: Pin Gao, M Santosh, Sanghoon Kwon, Chengxue Yang, Mu RamkumarAbstract:Abstract The Nagercoil block is the southernmost crustal segment of the Southern Granulite Terrane (SGT) in India and is mainly composed of charnockitic rocks and felsic gneisses (Charnockite suite). In this study, we present petrologic, geochemical, zircon U–Pb, REE, and Hf isotopic studies on the Charnockites and leucogneiss from the Nagercoil block. Based on field investigations and petrologic studies, the Charnockites can be divided into garnet-bearing and garnet-absent anhydrous granulite facies rocks with orthopyroxene. The Charnockites and leucogneiss show transition from adakites to non-adakitic magmatic rocks, with enrichment in LREEs (light rare earth elements) and LILEs (large ion lithophile elements), and depletion in HREEs (heavy rare earth elements) and HFSEs (high field strength elements). Some of the Charnockites and the leucogneiss show typical HSA (high silica adakite) characters, (high SiO2, Al2O3, Ba–Sr, La/Yb, and Sr/Y). The HSA is considered to have formed from the interaction of slab derived melts and peridotitic mantle wedge. The high Ba–Sr features were possibly inherited from subducted oceanic crust melting under the high thermal gradient condition during Precambrian. The magmas were underplated and subjected to fractional crystallization. Zircon grains from the Charnockite and leucogneiss show zoned magmatic cores surrounded by structureless metamorphic rims. Magmatic zircon grains from the Charnockites show ages ranging from 1983 ± 8.8 Ma to 2046 ± 14 Ma, and the metamorphic domains show an age range of 502 ± 14 Ma to 547 ± 8.7 Ma. Zircon from the leucogneiss yielded magmatic and metamorphic ages of 1860 ± 20 Ma and 575.6 ± 8.8 Ma. Both Charnockites and leucogneiss show two prominent age peaks at 1987 Ma and 568 Ma. The REE data of the zircon grains show LREE depletion and HREE enrichment, with the metamorphic grains showing more depletion in HREE. Zircon Hf isotopic data of the magmatic cores of zircon grains from the Charnockite yielded eHf(t) values from −1.17 to 0.46 with TDM and TC DM and age peaks at 2392 and 2638 Ma, suggesting Neoarchean to Paleoproterozoic juvenile sources. We suggest that the high Ba–Sr adakitic Charnockite suite from the Nagercoil block formed in a Paleoproterozoic magmatic arc setting during the assembly of the Columbia supercontinent, and underwent high-grade metamorphism associated with the amalgamation of the Gondwana supercontinent during the late Neoproterozoic–Cambrian. Our study provides new insights into the vestiges of Columbia fragments within the Gondwana assembly with two distinct cycles of crustal evolution.
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petrogenesis of the late paleoproterozoic luyashan igneous Charnockite enderbite suite north china craton and its comparison with metamorphic counterparts
Lithos, 2020Co-Authors: Vinod O. Samuel, M Santosh, Sanghoon Kwon, Yirang JangAbstract:Abstract Orthopyroxene-bearing granitoids of both magmatic and metamorphic origin occur in various terranes of the world, although their petrogenetic distinction remains equivocal. Here we investigate an example of unmetamorphosed magmatic Charnockites (enderbite-Charnockite) of the Late Paleoproterozoic Luyashan pluton in the North China Craton, where both charnockitic (granitic) and enderbitic (granodioritic) granitoids occurs in the pluton. In a thin section scale, charnockitic domains (~1–2 cm) are present within the outcrop scale enderbitic granitoid, that are composed of orthopyroxene, biotite, plagioclase, K-feldspar, quartz, ilmenite, magnetite, and pyrrhotite. Whereas the enderbite domain shows amphibole, clinopyroxene and apatite additionally. Mineral textures and orthopyroxene inclusions in amphibole and biotite preserved early stage orthopyroxene from both domains, where they carry pure endmember ilmenite and magnetite as the major oxide phases with similar compositions. Log (aH2O) - Temperature pseudosection modeling at log fO2 = −15 and pressure 800 MPa shows that orthopyroxene might be crystallized at ~900 °C. Subsequently, the mixed system achieved a higher water activity due to CO2 escape during cooling. During cooling, both the enderbitic and charnockitic domain are stable at a water content of >1 wt% or water activity (aH2O) of 0.4 or above. Our results suggest that mixing of immiscible granodioritic and granitic magma caused the formation of the enderbitic and charnockitic domains at similar water activity. Orthopyroxene inclusions in amphibole and biotite, lack of dehydration and oxidation textures, and absence of pyrite and apatite with monazite precipitates in the Luyashan igneous Charnockites suggest that these rocks are significantly different from their metamorphic counterparts. Our study suggests that orthopyroxene crystallization in igneous Charnockites is mainly dictated by the temperature and pressure at which magma starts crystallizing, rather than water activity. However, in the case of metamorphic Charnockites, in the absence of partial melting, metasomatic orthopyroxene formation depends on low water activity of fluids and their dissolution capacity.
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ancient crustal recycling in modern island arcs a tale of the world s youngest Charnockite from sw japan
Lithos, 2020Co-Authors: Jingyi Wang, M Santosh, Chengxue Yang, M Nakagawa, Yunpeng DongAbstract:Abstract Modern accretionary orogens are considered as zones of extensive juvenile crustal growth on our planet with the Central Asian Orogenic Belt (CAOB) and the western Pacific domain as typical examples. Here we report magmatic Charnockites (orthopyroxene-bearing anhydrous monzogranite) from the Shikoku Island in SW Japan intruding the Shimanto accretionary belt. Petrological and geochemical features of the intrusion are identical to Archean, Paleoproterozoic and Cretaceous Charnockites from various parts of the world and show typical arc magmatic affinity and formation in a convergent margin setting. Zircon U Pb data indicate that the pluton was emplaced at 13.75 Ma, revealing one of the youngest magmatic Charnockite occurrences in the world. Zircon Lu Hf data yield epsilon Hf(t) values in the range of −3.2 to 2.8 suggesting that the magma source involved a substantial amount of reworked material, in addition to juvenile input. The Hf modal age (TDM) shows a major peak at 701 Ma, suggesting reworking of Neoproterozoic basement rocks beneath the young island arc. The data confirm the model of the birth of Proto-Japan along the periphery of the Cathaysia block in South China. The modal age peak corresponds to the timing of magmatism associated with Rodinia rifting. Our study provides evidence for the involvement of both ancient recycled components and juvenile material associated with crust building in a young island arc within an ongoing subduction system.
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petrogenesis of incipient Charnockite in the ikalamavony sub domain south central madagascar new insights from phase equilibrium modeling
Lithos, 2017Co-Authors: Takahiro Endo, M Santosh, Toshiaki Tsunogae, E Shaji, Roger A RambelosonAbstract:Abstract Incipient Charnockites representing granulite formation on a mesoscopic scale occur in the Ambodin Ifandana area of Ikalamavony sub-domain in south-central Madagascar. Here we report new petrological data from these rocks, and discuss the process of granulite formation on the basis of petrography, mineral equilibrium modeling, and fluid inclusion studies. The incipient Charnockites occur as brownish patches, lenses, and layers characterized by an assemblage of biotite + orthopyroxene + K-feldspar + plagioclase + quartz + magnetite + ilmenite within host orthopyroxene-free biotite gneiss with an assemblage of biotite + K-feldspar + plagioclase + quartz + magnetite + ilmenite. Lenses and layers of calc-silicate rock (clinopyroxene + garnet + plagioclase + quartz + titanite + calcite) are typically associated with the Charnockite. Coarse-grained Charnockite occurs along the contact between the layered Charnockite and calc-silicate rock. The application of mineral equilibrium modeling on the mineral assemblages in Charnockite and biotite gneiss employing the NCKFMASHTO system as well as fluid inclusion study on coarse-grained Charnockite defines a P–T range of 8.5–10.5 kbar and 880–900 °C, which is nearly consistent with the inferred P–T condition of the Ikalamavony sub-domain (8.0–10.5 kbar and 820–880 °C). The result of T versus H 2 O activity ( a (H 2 O)) modeling demonstrates that orthopyroxene-bearing assemblage in Charnockite is stable under relatively low a (H 2 O) condition of 0.42–0.43, which is consistent with the popular models of incipient-Charnockite formation related to the lowering of water activity and stabilization of orthopyroxene through dehydration of biotite. The occurrence of calc-silicate rocks adjacent to the Charnockite suggests that the CO 2 -bearing fluid that caused dehydration and incipient-Charnockite formation might have been derived through decarbonation of calc-silicate rocks during the initial stage of decompression slightly after the peak metamorphism. The calc-silicate rocks might have also behaved as a cap rock that trapped CO 2 infiltrated from an external source. ‘CO 2 -rich fluid ponds’ formed beneath calc-silicate layers could have enhanced dehydration of biotite to orthopyroxene, and produced layers of coarse-grained Charnockite adjacent to calc-silicate layers.
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Charnockite magmatism during a transitional phase implications for late paleoproterozoic ridge subduction in the north china craton
Precambrian Research, 2015Co-Authors: M Santosh, Qiongyan YangAbstract:Abstract The Trans-North China Orogen (TNCO) is a major Paleoproterozoic collisional belt along which the Western and Eastern Blocks of the North China Craton were amalgamated. The Luliang Complex occurs along the western margin of this belt and incorporates several arc magmatic suites, supracrustal sequences, granitoids and Charnockites. Here we investigate the Luyashan Charnockite suite, as well as the surrounding granitoids, from this complex. We present results from petrology, fluid inclusions, mineral chemistry, whole rock geochemistry and zircon U–Pb and Lu–Hf isotopic studies. The Charnockites are dominantly anhydrous with the typical presence of orthopyroxene. Microthermometric and laser Raman spectroscopic data on fluid inclusions in the Charnockite characterize the trapped fluids as dominantly CO2 with traces of N2 and CH4. Geochemically, the Charnockite suite shows compositional variation from monzogabbro through gabbroic diorite and diorite to monzodiorite. They are dominantly calc-alkaline and ferroan type and display distinct enrichment in large ion lithophile elements with depletion in HREE. They also show prominent negative anomalies of Nb, Ta, Sr and Ti, and positive anomalies of Rb, Pb, Zr and Hf indicating crustal components in the magma source. The majority of the granitoid suite shows 207Pb/206Pb mean ages in the range of 2172–2032 Ma, correlating with similar ages reported from various groups in the Luliang Complex, the formation of which is correlated with subduction along an active convergent margin. However, the Charnockites which intrude the granitoids and their dioritic enclaves are clearly younger and show tightly constrained emplacement ages in the range of 1853–1872 Ma. The recrystallized zircons in the older granitoids also show similar ages, suggesting a prominent thermal event at this time. These ages are close to the timing of final collision between the Western and Eastern Blocks of the North China Craton and associated high-grade metamorphism at ca. 1.85 Ga. The zircon Lu–Hf isotope data reveal dominantly negative ɛHf(t) values (−8.7 to −0.1) and their TDMC (2477–3042 Ma) suggest that the magma sources mainly involved Meso- to Neoarchean reworked crustal components with limited input of Paleoproterozoic juvenile components. Our study reveals that Charnockite magmatism and regional metamorphism in the western margin of the TNCO broadly overlap in space and time. The geochemical features, mineral assemblages and fluid inclusion characteristics of the Charnockites suggest that the rocks were derived from high temperature (900–1000 °C), CO2-enriched and anhydrous (less than 3%H2O) magmas. The magma evolved into more hydrous conditions during cooling marked by the crystallization of amphiboles at 800–850 °C. The geochemical features of the Charnockite suite also suggest that these rocks were formed during a transitional phase from syn-subduction to collision. Such a scenario is compatible with a tectonic setting involving the subduction of an oceanic ridge when flushing of heat and CO2-rich fluids from asthenosphere into the overriding plate might have occurred through the slab window, generating hot and anhydrous magmas through melting of the basement rocks. The slab window mechanism would also account for broadly coeval high-temperature and high pressure granulites reported from the TNCO. We therefore speculate that the formation of the Luyashan Charnockite suite might have been associated with ridge subduction at the terminal stage of the convergence.
H. M. Rajesh - One of the best experts on this subject based on the ideXlab platform.
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late paleoproterozoic Charnockite suite within post collisional setting from the north china craton petrology geochemistry zircon u pb geochronology and lu hf isotopes
Lithos, 2014Co-Authors: Qiongyan Yang, M Santosh, H. M. Rajesh, Toshiaki TsunogaeAbstract:Abstract Charnockites (pyroxene-bearing granitoids) of magmatic origin in diverse tectonic settings and ranging in age from Mesoarchean to Cretaceous constitute important components of the continental crust. Here we report Charnockites displaying both magnesian and ferroan compositions associated with gabbros from an AMCG (anorthosite–mangerite–Charnockite–granite) suite in the North China Craton. The orthopyroxene in the magnesian Charnockite is characterized by moderate X Mg of 0.63–0.65 (Wo 1–2 En 62–63 Fs 35–36 ), and low Al 2 O 3 content of 0.59–0.71 wt.%. The magnesian Charnockites show medium- to high-K contents, and high Mg# (~ 47–69) similar to that of gabbros, whereas the Mg# of the ferroan Charnockites is low (~ 6–28). The ferroan Charnockites are alkali-calcic to alkalic, and weakly peralkaline to metaluminous, whereas the magnesian Charnockites are calcic to calc-alkalic, and metaluminous. Although magnesian Charnockites are in general considered to have formed in subduction setting, the medium- to high-K contents, high Mg# values with a wide range, and the highly negative eHf values of the zircons in these rocks (− 8. 4 to − 13.6), suggest inheritance of the arc signature from the melting of ancient arc-related crustal material. The ferroan Charnockites show tholeiitic affinity and define a common differentiation trend with the gabbroic anorthosites and likely represent fractionated end-members with or without crustal interaction in a post-collisional rift setting. We present U–Pb age data from zircon grains on seven samples including two ferroan Charnockites, three magnesian Charnockites, one gabbroic enclave in magnesian Charnockite and one gabbroic anorthosite which show emplacement ages of 1748.8 ± 6.4 Ma, 1747.1 ± 9.5 Ma, 1756.4 ± 7.3 Ma, 1756.7 ± 9.2 Ma, 1731 ± 17 Ma, 1731.6 ± 8.2 Ma and 1746.5 ± 7.3 Ma respectively. The negative eHf values (− 1.2 to − 13.6) of zircon grains from these rocks and the older crustal model ages ranging from Mesoarchean to Paleoproterozoic suggest that the magma sources of these rocks involved the melting of ancient crustal components. The age data suggest that the magmatic suite was emplaced within a relatively short time interval between1.73 and 1.76 Ga, during late Paleoproterozoic, placing the rocks suite in a post-collisional scenario, following the amalgamation between the Eastern and Western Blocks of the North China Craton along the Trans-North China Orogen at ca. 1.85–1.80 Ga.
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A geochemical perspective on Charnockite magmatism in Peninsular India
Geoscience Frontiers, 2012Co-Authors: H. M. RajeshAbstract:Abstract Large Charnockite massifs occur in the high-grade Southern Granulite Terrain (SGT) and Eastern Ghats Belt (EGB) crustal provinces of Peninsular India. Available geochronological data indicate that the magmatism is episodic, associated with distinct orogenic cycles in the different crustal domains. The geochemical data also indicate a change in composition from trondhjemitic at ∼3.0–2.9 Ga to dominantly tonalitic at ∼2.6–2.5 Ga to tonalitic-granodiorite-granitic at ∼2.0–1.9 Ga to dominantly tonalitic at 1.7–1.6 Ga to quartz monzonitic or tonalitic at ∼1.0–0.9 Ga to granodiorite-granitic at ∼0.8–0.7 Ga. The trondhjemitic and tonalitic end members are metaluminous, magnesian and calcic to calc-alkalic, characteristic of magnesian group Charnockites. The granodioritic to granitic end members are metaluminous to slightly peraluminous, ferroan and calc-alkalic to alkali-calcic, characteristic of ferroan group Charnockites. The quartz monzonitic end members are metaluminous to peraluminous, magnesian to ferroan and calcic to calc-alkalic, neither characteristic of the magnesian group nor of the ferroan group of Charnockites. Based on the occurrence and difference in composition of the Charnockite massifs, it is suggested that the Charnockite magmatism registers the crustal growth of the Indian plate on its southern (SGT) and eastern (EGB) sides, along active continental margins by accretion of arcs.
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the nagercoil Charnockite a magnesian calcic to calc alkalic granitoid dehydrated during a granulite facies metamorphic event
Journal of Petrology, 2011Co-Authors: H. M. Rajesh, M Santosh, Shinichi YoshikuraAbstract:The Paleoproterozoic Nagercoil Charnockite massif in southern India consists of garnet-absent and garnet-bearing varieties, with garnet occurring only in those areas where the Charnockite incorporates supracrustal lithologies. There is a late Neoproterozoic^Cambrian granulite-facies metamorphic overprint on the Charnockite massif. This study examines whether orthopyroxene in the Charnockites crystallized during the original magmatic stage or formed during the later high-grade metamorphic event. The textural association of orthopyroxene and relic garnet grains, and reaction intergrowths of garnet and/or biotite with orthopyroxene, similar to those reported in dehydration zones within the gneisses surrounding the Charnockite massif, show that orthopyroxene in the Charnockite massif formed during the late high-grade metamorphic event from dehydration reactions and is not part of the original igneous assemblage. Metasomatic textures such as thin K-feldspar rims, together with the presence of CO2-rich fluid inclusions in these rocks, suggest that CO2 metasomatism, widely prevalent in the gneisses surrounding the massif, probably resulted in the low water activity necessary to form orthopyroxene during the metamorphic overprint. An oxygen isotope traverse along a metapelite enclave^Charnockite contact within the massif shows a distinct variation in d 18 O values, with the Charnockite away from the contact having the lowest d 18 O value (þ8·9o) similar to that of typical metaluminous granitic rocks. The Charnockite at the contact shows an intermediate d 18 O value (þ10·3o) and the surrounding metapelite has the highest d 18 O value (þ12o); the intermediate d 18 O value indicates partial assimilation of metapelite with a high d 18 O value into the Charnockite with a low d 18 O value. Both the garnet-absent and garnet-bearing Charnockites have similar compositional ranges to tonalite^granodiorite^granite (TTG) suites. Irrespective of whether garnet is present or not, the Sr contents of the Charnockites show a bimodal distribution: a high-Sr group, which is magnesian, calcic to calc-alkalic, metaluminous, showing TTG affinities, and a low-Sr group, which is dominantly ferroan, calcic to calc-alkalic, metaluminous, showing similarities to the orthopyroxene-bearing dehydration zones. Petrogenetic characterization suggests that the protoliths of the Nagercoil Charnockites were high-temperature melts of hydrous basalt.
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The petrogenetic characterization of intermediate and silicic Charnockites in high-grade terrains: a case study from southern India
Contributions to Mineralogy and Petrology, 2007Co-Authors: H. M. RajeshAbstract:Large Charnockite massifs occur in some of the Precambrian high-grade terrains like the southern Indian granulite terrain. The Cardamom Hill Charnockite massif from the Madurai Block, southern India, consists of an intermediate type and silicic type, with the intermediate type showing similarities to high-Ba−Sr granitoids with low K_2O/Na_2O ratios and the silicic type showing similarities to high-Ba–Sr granitoids with high K_2O/Na_2O ratios. Within the constraints imposed by near basaltic composition of the most mafic samples and their relatively high concentrations of both compatible and incompatible elements, comparison with recent experimental studies on various source compositions, and trace- and rare-earth-element modeling, the distinctive features of the intermediate Charnockites can be best explained in terms of assimilation–fractional crystallization (AFC) models involving interaction between a mantle-derived basaltic magma and lower crustal materials. Silicic Charnockites on the other hand are high temperature melts of moderately hydrous basaltic magmas. A two-stage model which involves an initial partial melting of hydrous basaltic magma and later fractionation explains the geochemical features of the silicic Charnockites, with the fractionation stage most probably an open system AFC. It is suggested that for massifs showing spatial association of intermediate and silicic Charnockites, a model taking into account their compositional difference in terms of the effect of variations in the conditions (e.g., temperature, water fugacity) that prevailed, can account for plausible petrogenetic scenarios.
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Petrogenesis of two granites from the Nilgiri and Madurai blocks, southwestern India: Implications for Charnockite-calc-alkaline granite and Charnockite-alkali (A-type) granite link in high-grade terrains
Precambrian Research, 2007Co-Authors: H. M. RajeshAbstract:Abstract Igneous Charnockites share some of the characteristics of calc-alkaline and alkali (A-type) granites, and together constitute important rock types in many high-grade terrains. The Neoproterozoic Kalpatta and Munnar biotite-hornblende granites intrude the upper-amphibolite to granulite-facies terrain of southwestern India. Geochemical characteristics of the Kalpatta granite are similar to high-K calc-alkaline magnesian granitoids, whereas those of the Munnar granite are similar to alkali ferroan granitoids. Within the constraints imposed by the high temperature, K-rich nature of the magmas, comparison with experimental studies on various granitoid source compositions, and trace- and rare-earth-element modeling, the distinctive features of both granites reflect a source rock of charnockitic nature. In this context, the northern Kerala (NKM) and Cardamom hill (CM) Charnockite massifs, occurring near the granites, were considered as probable source rock compositions. Both the NKM and CM Charnockites consists of an intermediate (low SiO2) type and silicic (high SiO2) type, with the intermediate type showing similarities to high-Ba–Sr granitoids with low K2O/Na2O ratios and the silicic type showing similarities to high-Ba–Sr granitoids with high K2O/Na2O ratios. The proposed petrogenetic model involves partial melting of the intermediate NKM Charnockite forming the Kalpatta calc-alkaline granite. In contrast, the Munnar alkali (A-type) granite was the product of melting, followed by fractional crystallization of the intermediate CM Charnockite. It is suggested that granitoids that formed by melting of intermediate type Charnockites show high-K calc-alkaline magnesian geochemical characteristics, whereas those formed by intermediate type Charnockite melting-fractional crystallization show alkali ferroan geochemical characteristics.
S. Bhattacharya - One of the best experts on this subject based on the ideXlab platform.
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Charnockite Massifs: Key to Tectonic Evolution of the Eastern Ghats Belt, India, and Its Columbia Connection
Advances in Natural Science, 2014Co-Authors: S. Bhattacharya, Miguel Angelo Stipp Basei, Rajib KarAbstract:The Eastern Ghats Granulite Belt, India, with two major lithological associations: Charnockites and meta sedimentary granulites, is characterized by polyphase deformation and complex, possibly multiple granulite events. Barring the cratonic margins in the north and west, two distinct crustal domains have been identified: the Eastern Ghats Province (EGP) and Ongole domain, separated by the Godavari graben. These domains also have distinct geochronological record of granulite event: in the EGP the first granulite event has been recorded as between 1.2 and 0.9 Ga; while in the Ongole domain the granulite event is recorded as 1.6 - 1.7 Ga. However, Charnockite-massifs in both the domains, interpreted as product of deep crustal anatexis under granulite facies conditions, could provide a link in tectonic evolution of the EGB as a whole. LA-ICP-MS analysis of zircon spot ages of two Charnockite massifs reveals vestiges of the1.6 Ga Charnockite magmatism in the EGP as identical to that in the Ongole domain. Another Charnockite massif in the EGP records concordant zircon spot age of 940 Ma, but single spot age of 990 Ma could indicate a prolonged UHT event. Thus magmatic Charnockites of intracrustal melting origin could represent two granulite events, at ca. 1.6 and 1.0 Ga in the Eastern Ghats Belt. Also, accretionary orogenic processes of the Supercontinent Columbia might have encompassed the Eastern Ghats Belt with Australia, Antarctica and Laurentia.
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Kabbaldurga Charnockites revisited: Incipient growth or anatectic melt?
Natural Science, 2013Co-Authors: S. Bhattacharya, A. K. ChaudharyAbstract:A popular hypothesis of in situ transformation of amphibolite facies gneisses to patchy Charnockites by CO2 influx from mantle was proposed primarily from the Kabbaldurga quarries in South Karnataka and subsequently reported from several south Indian localities. However, presence of abundant mafic granulite enclaves in Kabbaldurga and its neighborhood and its implications in relation to patchy Charnockite genesis were not discussed. In these quarries patchy Charnockites occur in various modes and associations. Some of these patches do occupy structural weak zones, such as shear bands and fold noses in the migmatitic gneisses, but many of the patchy Charnockite bodies occur as branching veins transecting the gneissic foliation and hence do not account for fluid pathways. Most importantly, charnockitic leucosomes at margins of mafic granulite enclaves and charnockitic veins within some mafic granulite enclaves indicate a close genetic link between them via dehydration partial melting. This is further corroborated by trace element distribution between them. Dehydration partial melting in mafic rocks in a migmatite terrain such as Kabbaldurga, can explain all the different modes of the patchy Charnockites as various stages of segregation and mobility relative to deformation. Abundant mafic granulite enclaves and field features suggesting a relatively late origin of the patchy Charnockites, are compelling evidence against the notion of a transition zone. Mantle derivation age of the mafic source rocks (protoliths of mafic granulites) at Kabbaldurga at 3.08 ± 0.08 Ga with small positive ? values is virtually identical to the source of the massive Charnockite of Karnataka craton at 3.08 Ga. This could imply a widespread mafic magmatism in South India around 3.0 Ga.
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New Insights into the Origin of Kabbaldurga Charnockites, Karnataka, South India
Gondwana Research, 2000Co-Authors: S. Bhattacharya, S. K. SenAbstract:Abstract A detailed study of the structure and petrology of the rocks bordering the Kabbaldurga-type Charnockites provides important constraints on the origin of these Charnockites. The structural elements register three phases of deformation and show a uniform pattern in the larger area, a pattern consistent with the regional structure of the Precambrian of Southern Karnataka. In the Kabbaldurga area, however, some of the earlier structures are poorly preserved. Yet there are vestiges of early folds described by banded/layered Charnockites as in the neighbouring Kodamballi area, and a consistent development of dilatant structures which can be related to the kinematics of deformation in the larger terrain. At Kabbaldurga the pegmatitic Charnockites occur as veins of diverse orientation; but they rarely follow the shear - generated structures. The metamorphic reactions invoked by previous workers to explain in situ transformation of gneiss to Charnockite were based on chemical similarity of some close pairs. But the petrographic and chemical variations in the pegmatitic Charnockites and the Peninsular gneisses at Kabbaldurga quarry are compelling features which cannot be explained by the hypothesis of in situ transformation. We have argued, on the basis of rock and mineral chemistry, that derivation of the pegmatitic Charnockites by dehydration melting in metabasites offers a better explanation. Pressure-temperature values (at least 850° to 900° C, 7 kbar) obtained by us for the granulites of this area, viewed against the results of experimental dehydration melting in basic rocks with hornblende and/or biotite, provide strong support for this model. In the field leucosomes within the basic granulites of Kabbaldurga are not uncommon. The compositions of the pegmatitic Charnockites (tonalitic and granitic) match those of the melts produced in experiments. Further, the pattern of variation in the composition of hornblende and plagioclase in the basic granulites of the Kabbaldurga area is compatible with extraction of melts. This alternative model for the origin of the Kabbaldurga Charnockites is petrologically feasible and does not require either in situ transformation or structurally controlled growth, which, incidentally, are not ubiquitous at Kabbaldurga
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The structural setting of the Chilka Lake granulite-migmatite-anorthosite suite with emphasis on the time relation of Charnockites
Precambrian Research, 1994Co-Authors: S. Bhattacharya, S. K. Sen, A. AcharyyaAbstract:Abstract The granulites and anorthosites occurring around Chilka Lake in Orissa display polyphase deformation structures which have been utilized to constrain the deformation history and the time relation between the rock units. The granulites are represented by metapelitic khondalites and migmatitic Charnockite-leptynite complexes. Structural mapping complemented by examination of aerial photographs reveals imprints of three folding episodes. The dominant NE-SW tectonic trend was produced by the first folding F 1 with S 1 preserved in khondalites and Charnockites. The second episode of folding was tight, coaxial with F 1 , resulting in hook-shaped folds described on the map scale by the khondalites. In the leptynitic orthopyroxene-free garnetiferous granite gneiss component of the migmatite complex, S 1 is absent, and the major structural grain is due to the gneissose banding S 2 . The third folding affected the metapelites and the migmatite complex differently. In the metapelites the F 3 folds, which are asymmetric, show a uniformly gentle plunge towards the NE/NNE, and the dome-basin patterns ascribed to F 3 by Sarkar et al. (1981) are not discernible. In the migmatite complex these folds, though nearly upright, are disharmonic, probably a consequence of a different rheology owing to the presence of melts. The anorthosites register the imprint of F 3 only and are thus distinctly younger. The retention of S 1 in the Charnockite-enderbite layers and lenses within composite Charnockite-leptynite exposures, transposition of S 1 to the leptynitic foliation S 2 in several locales, and the different orientation of S 1 in Charnockite patches in the different limbs of F 3 folds are strong indications of the Charnockites being older than the leptynites. The Charnockite “measles” occurring near the hinge regions of the F 3 structures cannot be construed as sites of in-situ arrested charnockitization, instead they are discontinuous relicts of Charnockite layers which have been deformed, torn and displaced. We argue that separation of structures of different generations is necessary for a proper understanding of the field features taken to indicate arrested formation of Charnockites in South India and elsewhere.
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Structural evidence supporting a remnant origin of patchy Charnockites in the Chilka Lake area, India
Geological Magazine, 1993Co-Authors: S. Bhattacharya, S. K. Sen, A. AcharyyaAbstract:AbstractDark patches of charnockitic rocks characterized by orthopyroxene occur within garnetiferous granite gneisses (leptynites) in a granulite-migmatite suite around the Chilka Lake, Orissa, within the Eastern Ghats belt in the Indian Precambrian. Analysis of structures of different scales observed in this terrain establishes the presence of three phases of deformation. S1 is pervasive in the metapelitic granulites (mainlykhondalite), while in the migmatite complex composed of leptynites, Charnockites and quartzofeldspathic veins, S1 is present exclusively within the Charnockite lenses and bands, and shows different stages of obliteration in the associated leptynites. Thus, the Charnockite patches must be earlier than the surrounding migmatitic rocks. The Charnockite patches and the surrounding leptynitic gneisses are chemically quite different and the two rock types are not related by any prograde or retrograde transformation. The shapes and disposition of Charnockite patches in the mixed exposures are found to be largely controlled by the third phase of folding and locally associated shearing. The kinematics of this late deformation are not favourable for fluid ingress from deeper levels.
Gareth R. Davies - One of the best experts on this subject based on the ideXlab platform.
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Charnockites and UHT metamorphism in the Bakhuis Granulite Belt, western Suriname : Evidence for two separate UHT events
Precambrian Research, 2015Co-Authors: Martijn Klaver, Emond W.f. De Roever, Josefine A. M. Nanne, Paul R.d. Mason, Gareth R. DaviesAbstract:Abstract The Bakhuis Granulite Belt in western Suriname is an ultrahigh-temperature (UHT) metamorphic terrain in the centre of the Paleoproterozoic (Transamazonian) Guiana Shield. Next to the UHT granulites, the belt contains a 30 by 30 km body of orthopyroxene-bearing granitoids: the Kabalebo Charnockites. This setting offers an excellent opportunity to investigate the source and origin of Charnockite magmatism and the common association of Charnockites with (ultra)high-temperature metamorphic terrains. We present a detailed geochemical dataset and LA-ICPMS zircon U/Pb ages with the aim to investigate the geochemical and geochronological relationship between Charnockite magmatism and UHT metamorphism in the Bakhuis Granulite Belt. The Kabalebo Charnockites have a characteristic trace element signature with elevated K 2 O, P 2 O 5 , Zr, REE and Ba coupled with mobile element depletion, which is a consequence of high-temperature melting of anhydrous but fertile granulitic crust. Field and geochemical evidence suggests that the intermediate granulites in the Bakhuis Granulite Belt are the source of the Kabalebo Charnockites. The new U/Pb zircon ages indicate that Charnockite magmatism (1993–1984 Ma) postdates UHT metamorphism (2.07–2.05 Ga) by at least 60 Myr. We argue that it is not possible to maintain a thermal anomaly >200 °C in excess of a normal geothermal gradient for such a prolonged period and hence conclude that the Bakhuis Granulite Belt has experienced two distinct periods in which temperatures >950 °C were reached in the lower crust. The presence of comagmatic metadolerite enclaves in the Charnockites establishes that mafic magmatism occurred contemporaneously with, and was the likely heat source for, Charnockite magmatism at 1.99–1.98 Ga. In contrast, the 2.07–2.05 Ga UHT metamorphic event is not associated with felsic or mafic magmatism in the Bakhuis Granulite Belt or nearby Guiana Shield and postdates the suturing of the juvenile North Guiana TTG-greenstone belt with the West African Shield by at least 10 Myr. We postulate that the UHT metamorphism at 2.07–2.05 Ga is the result of mantle upwelling in a slab tear in the subducted West African slab that formed as the result of crustal scale shearing and boudinage. Prior to the final stabilisation of the Amazonian–West African Shield at 1.90 Ga, northward subduction at 1.99–1.98 Ga caused the emplacement of voluminous hot, mafic magma, resulting in partial melting of the Bakhuis granulite suite to form the Kabalebo Charnockites. Charnockite magmatism was roughly contemporaneous with the emplacement of a large belt of shallow granites and felsic volcanic rocks in the SW Guiana Shield. Despite their similar age, the inherited zircon populations suggest that the Charnockites are derived from a distinct, juvenile source while the felsic volcanic rocks include an Archaean protolith.
Toshiaki Tsunogae - One of the best experts on this subject based on the ideXlab platform.
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petrogenesis of incipient Charnockite in the ikalamavony sub domain south central madagascar new insights from phase equilibrium modeling
Lithos, 2017Co-Authors: Takahiro Endo, M Santosh, Toshiaki Tsunogae, E Shaji, Roger A RambelosonAbstract:Abstract Incipient Charnockites representing granulite formation on a mesoscopic scale occur in the Ambodin Ifandana area of Ikalamavony sub-domain in south-central Madagascar. Here we report new petrological data from these rocks, and discuss the process of granulite formation on the basis of petrography, mineral equilibrium modeling, and fluid inclusion studies. The incipient Charnockites occur as brownish patches, lenses, and layers characterized by an assemblage of biotite + orthopyroxene + K-feldspar + plagioclase + quartz + magnetite + ilmenite within host orthopyroxene-free biotite gneiss with an assemblage of biotite + K-feldspar + plagioclase + quartz + magnetite + ilmenite. Lenses and layers of calc-silicate rock (clinopyroxene + garnet + plagioclase + quartz + titanite + calcite) are typically associated with the Charnockite. Coarse-grained Charnockite occurs along the contact between the layered Charnockite and calc-silicate rock. The application of mineral equilibrium modeling on the mineral assemblages in Charnockite and biotite gneiss employing the NCKFMASHTO system as well as fluid inclusion study on coarse-grained Charnockite defines a P–T range of 8.5–10.5 kbar and 880–900 °C, which is nearly consistent with the inferred P–T condition of the Ikalamavony sub-domain (8.0–10.5 kbar and 820–880 °C). The result of T versus H 2 O activity ( a (H 2 O)) modeling demonstrates that orthopyroxene-bearing assemblage in Charnockite is stable under relatively low a (H 2 O) condition of 0.42–0.43, which is consistent with the popular models of incipient-Charnockite formation related to the lowering of water activity and stabilization of orthopyroxene through dehydration of biotite. The occurrence of calc-silicate rocks adjacent to the Charnockite suggests that the CO 2 -bearing fluid that caused dehydration and incipient-Charnockite formation might have been derived through decarbonation of calc-silicate rocks during the initial stage of decompression slightly after the peak metamorphism. The calc-silicate rocks might have also behaved as a cap rock that trapped CO 2 infiltrated from an external source. ‘CO 2 -rich fluid ponds’ formed beneath calc-silicate layers could have enhanced dehydration of biotite to orthopyroxene, and produced layers of coarse-grained Charnockite adjacent to calc-silicate layers.
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late paleoproterozoic Charnockite suite within post collisional setting from the north china craton petrology geochemistry zircon u pb geochronology and lu hf isotopes
Lithos, 2014Co-Authors: Qiongyan Yang, M Santosh, H. M. Rajesh, Toshiaki TsunogaeAbstract:Abstract Charnockites (pyroxene-bearing granitoids) of magmatic origin in diverse tectonic settings and ranging in age from Mesoarchean to Cretaceous constitute important components of the continental crust. Here we report Charnockites displaying both magnesian and ferroan compositions associated with gabbros from an AMCG (anorthosite–mangerite–Charnockite–granite) suite in the North China Craton. The orthopyroxene in the magnesian Charnockite is characterized by moderate X Mg of 0.63–0.65 (Wo 1–2 En 62–63 Fs 35–36 ), and low Al 2 O 3 content of 0.59–0.71 wt.%. The magnesian Charnockites show medium- to high-K contents, and high Mg# (~ 47–69) similar to that of gabbros, whereas the Mg# of the ferroan Charnockites is low (~ 6–28). The ferroan Charnockites are alkali-calcic to alkalic, and weakly peralkaline to metaluminous, whereas the magnesian Charnockites are calcic to calc-alkalic, and metaluminous. Although magnesian Charnockites are in general considered to have formed in subduction setting, the medium- to high-K contents, high Mg# values with a wide range, and the highly negative eHf values of the zircons in these rocks (− 8. 4 to − 13.6), suggest inheritance of the arc signature from the melting of ancient arc-related crustal material. The ferroan Charnockites show tholeiitic affinity and define a common differentiation trend with the gabbroic anorthosites and likely represent fractionated end-members with or without crustal interaction in a post-collisional rift setting. We present U–Pb age data from zircon grains on seven samples including two ferroan Charnockites, three magnesian Charnockites, one gabbroic enclave in magnesian Charnockite and one gabbroic anorthosite which show emplacement ages of 1748.8 ± 6.4 Ma, 1747.1 ± 9.5 Ma, 1756.4 ± 7.3 Ma, 1756.7 ± 9.2 Ma, 1731 ± 17 Ma, 1731.6 ± 8.2 Ma and 1746.5 ± 7.3 Ma respectively. The negative eHf values (− 1.2 to − 13.6) of zircon grains from these rocks and the older crustal model ages ranging from Mesoarchean to Paleoproterozoic suggest that the magma sources of these rocks involved the melting of ancient crustal components. The age data suggest that the magmatic suite was emplaced within a relatively short time interval between1.73 and 1.76 Ga, during late Paleoproterozoic, placing the rocks suite in a post-collisional scenario, following the amalgamation between the Eastern and Western Blocks of the North China Craton along the Trans-North China Orogen at ca. 1.85–1.80 Ga.
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First report of Paleoproterozoic incipient Charnockite from the North China Craton: Implications for ultrahigh-temperature metasomatism
Precambrian Research, 2014Co-Authors: Qiongyan Yang, M Santosh, Toshiaki TsunogaeAbstract:Abstract The assembly of crustal blocks within the North China Craton (NCC) at the terminal stage of craton building during Paleoproterozoic witnessed major subduction-collision events. Here we illustrate a case where hot and anhydrous magmas emplaced in the lower crust during the early Paleoproterozoic subduction realm in the NCC led to the formation of orthopyroxene-bearing dehydrated zones of incipient Charnockites within the adjacent TTG (tonalite-trondhjemite-granodiorite) suite. The coarse greenish anhydrous zones of incipient Charnockite show an assemblage of antiperthitic plagioclase + orthopyroxene + K-feldspar + quartz with minor clinopyroxene, ilmenite + magnetite and secondary biotite. The peak P – T conditions of the incipient Charnockite are estimated as 890–970 °C and 3.5–6.5 kbar based on mineral chemical analysis and pseudosection modeling in the system NCKFMASHTO, suggesting ultra-high temperature metasomatism, and the ‘hottest incipient Charnockites’ so far reported. Zircons from the massive Charnockite show oscillatory-zoned magmatic cores with high Th/U values rimmed by unzoned and luminescent thin metamorphic overgrowths. A concordant group of magmatic zircons yields weighted mean 207 Pb/ 206 Pb age of 2401 ± 29 Ma, and a lower intercept age of ca. 1834 ± 81 Ma. A similar concordant group of zircons from the TTG yields a weighted mean 207 Pb/ 206 Pb age of 2446 ± 18 Ma and a weighted mean lower intercept age of 1834 ± 47 Ma. The ca. 2.45 Ga age from the TTG and 2.40 age from the Charnockite are taken to represent the magma emplacement ages, and the identical 1.83 Ga from zircons in both rocks denoting the timing of late Paleoproterozoic metamorphism. Lu–Hf analysis of magmatic cores of the zircons from the Charnockite and the TTG show clear positive ɛ Hf ( t ) values (1.2–5.9), suggesting juvenile sources. The formation of incipient Charnockite zones close to the contact with the Charnockite pluton is consistent with the quantitative numerical models that predict CO 2 migration in anhydrous silicate melt and metasomatic fluid from a rapidly cooling pluton. The ultra-high temperature metasomatism and solid-state reaction generating the incipient Charnockites provide important insights on CO 2 advection from lower crustal magmas during Paleoproterozoic orogeny in the North China Craton.
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phase equilibrium modeling of incipient Charnockite formation in nckfmashto and mnnckfmashto systems a case study from rajapalaiyam madurai block southern india
Geoscience frontiers, 2012Co-Authors: Takahiro Endo, M Santosh, Toshiaki Tsunogae, E ShajiAbstract:Abstract Incipient Charnockites represent granulite formation on a mesoscopic scale and have received considerable attention in understanding fluid processes in the deep crust. Here we report new petrological data from an incipient Charnockite locality at Rajapalaiyam in the Madurai Block, southern India, and discuss the petrogenesis based on mineral phase equilibrium modeling and pseudosection analysis. Rajapalaiyam is a key locality in southern India from where diagnostic mineral assemblages for ultrahigh-temperature (UHT) metamorphism have been reported. Proximal to the UHT rocks are patches and lenses of Charnockite (Kfs + Qtz + Pl + Bt + Opx + Grt + Ilm) occurring within Opx-free Grt-Bt gneiss (Kfs + Pl + Qtz + Bt + Grt + Ilm + Mt) which we report in this study. The application of mineral equilibrium modeling on the charnockitic assemblage in NCKFMASHTO system yields a p - T range of ∼820 °C and ∼9 kbar. Modeling of the Charnockite assemblage in the MnNCKFMASHTO system indicates a slight shift of the equilibrium condition toward lower p and T (∼760 °C and ∼7.5 kbar), which is consistent with the results obtained from geothermobarometry (710–760 °C, 6.7–7.5 kbar), but significantly lower than the peak temperatures (>1000 °C) recorded from the UHT rocks in this locality, suggesting that charnockitization is a post-peak event. The modeling of T versus molar H 2 O content in the rock (M(H 2 O)) demonstrates that the Opx-bearing assemblage in Charnockite and Opx-free assemblage in Grt-Bt gneiss are both stable at M(H 2 O) = 0.3 mol%–0.6 mol%, and there is no significant difference in water activity between the two domains. Our finding is in contrast to the previous petrogenetic model of incipient Charnockite formation which envisages lowering of water activity and stabilization of orthopyroxene through breakdown of biotite by dehydration caused by the infiltration of CO 2 -rich fluid. T - X Fe 3+ (=Fe 2 O 3 /(FeO + Fe 2 O 3 ) in mole) pseudosections suggest that the oxidation condition of the rocks played a major role on the stability of orthopyroxene; Opx is stable at X Fe 3+ X Fe 3+ >0.12. Such low oxygen fugacity conditions of X Fe 3+ 2 O + CH 4 ) during the retrograde stage.