Chotanagpur Granite Gneiss Complex Research Articles

Petrogenesis of the gabbronorite sill hosted Fe-Ti oxide ore bodies from the eastern part of Chotanagpur granite Gneissic Complex, India

Magmatic Fe-Ti oxide ore bodies of the eastern Chotanagpur Granite Gneiss Complex (CGGC), India occur as massive conformable lenses within a gabbronorite sill. The occurrence, lithological association, mineralogy, and geochemistry of the Fe-Ti oxide ore bodies are significantly different from the typical Fe-Ti oxide deposits associated with anorthosite complexes and mafic- ultramafic complexes. Textural evidence and mineral chemistry suggest mostly simultaneous fractional crystallization of the comprising minerals e.g. titanomagnetite, ilmenite and magnesio-hercynite. The temperature and fO2 conditions of re-equilibration are calculated using the compositions of ilmenite and pre-exsolved titanomagnetite pairs. Extrapolation of the data along ulvospinel isopleths reveals the probable original magmatic conditions e.g. temperature ~ 900 °C and fO2 ~ ΔFMQ + 2. The ore forming melt calculated by ‘equilibrium distribution method’ shows no crustal contamination and has a trace elemental pattern similar to typical enriched basaltic melts e.g. ferropicrites, alkali basalts, OIB and EMORB. High Ti and Zr contents of the melt suggest a possible within plate tectonic setting and enrichment in HFSEs and LREEs suggest that the parent melt could be generated from an enriched mantle source by low degree partial melting of garnet-peridotite layer of the upper mantle. The ‘quantitative fractional crystallization modelling’ indicates that early crystallization of olivine, pyroxenes, plagioclase and ilmenite from an enriched basaltic melt similar to EMORB (most fit melt) led to a condition of high oxygen fugacity and iron enrichment in the residual melt. The residual Fe-Ti rich melt moved to low pressure zones within the gabbronorite sill during late stage of D2 deformation, and later crystallized in the form of present ore bodies.

Evaluation of springs and waterfalls as Geomorphosites and proposition of strategies to develop Geotourism at Ajodhya hill, Puruliya district, Eastern India

Ajodhya hill is an extended part of Chota Nagpur Granite Gneissic Complex located in Puruliya district, western part of West Bengal, Eastern India. This uplifted plateau is primarily composed of Archean granite and gneissic rock with the sharp break at the plateau fringe. Ground water comes out as spring through the fractures of the granitic rock base situated beneath the permeable red soil. Waterfalls are prominent along the steep slope of the plateau fringe. Thirteen springs and four waterfalls have been identified as important Geomorphosites to analyse their Geotourism potential of Ajodhya hill in response to its geologic and geomorphic background. SWOT analysis has been used for estimating the Geotourism potentialities of Ajodhya hill. To analyse the potentiality, weight and score has been given under each factors and weighted score has been calculated. Few strategies have been suggested for enriching the Geotourism potential of the region with the help of TOWS matrix. Prioritization of the strategies has been ascertained by QSPM (quantitative strategies planning matrix) method through which the Ranking of Total Attractiveness Score (RTAS) has been estimated.

Significance of viscous folding in the migmatites of Chotanagpur Granite Gneiss Complex, eastern India

ABSTRACTTo understand the physico-chemical processes associated with migmatisation is an interesting petrological problem. New developments in microfluidics and chaotic mixing experiments have helped us to better perceive these processes from the migmatic rocks of the Proterozoic Chotanagpur Granite Gneiss Complex (CGGC), eastern India. The migmatic rocks of CGGC have preserved folded leucocratic veins in amphibolites representing viscous folding. The viscous folding phenomenon occurred due to the interaction between leucosome and melanosome. Based on textural features and mineral chemical data interpretations, we infer that when granitic and pegmatitic magmas intruded the gneissic rocks and amphibolites of our study area, diffusion of heat and volatiles from the hotter felsic magmas to the colder country rocks initiated partial melting in the amphibolites, forming melanosomes. After their formation, the highly viscous felsic magmas veined into the melanosomes, by progressively melting them and then interacting, leading to chaotic mixing dynamics. The development of chaotic mixing allowed the leucosome to venture into the melanosome as veins by stretching and folding dynamics. As the leucocratic veins or leucosome traversed through the partially molten rock or melanosome due to advection, the veins underwent viscous folding owing to the exertion of compressional stress brought about by the viscosity difference between the two mediums. The occurrence of viscous folding exponentially increased the contact area between the leucosome and the melanosome, eventually leading to enhanced diffusion and augmented mixing between the two mediums. Evidence of mixing through elemental diffusion is well documented by the compositions of amphibole and biotite occurring in the leucosome and melanosome. These minerals show substitution of magnesium and ferrous ion that show linear variation between the endmember compositions.

Genesis of coronae and implications of an early Neoproterozoic thermal event: a case study from SE Chotanagpur Granite Gneissic Complex, India

AbstractPartial equilibrium textures such as corona provide information on changing pressure–temperature (P-T) conditions experienced by a rock during its geological evolution. Coronae layers may form in single or multiple stages; understanding the genesis of each layer is necessary to correctly extract information regarding the physicochemical conditions experienced by the rock. Mafic rocks from SE Chotanagpur Granite Gneissic Complex, India, show the presence of multi-layered coronae at olivine–plagioclase contact with the mineral sequence: olivine | orthopyroxene | amphibole + spinel | plagioclase. Textural studies indicate that the coronae formed during metamorphism in a single stage due to a reaction between olivine and plagioclase. Reaction modelling shows that the corona formation occurred in an open system and experienced a minor volume loss. Pseudosection modelling and thermobarometry suggest that theP-Tconditions related to corona formation are 860 ± 50°C and 7 ± 0.5 kbar. A μMgO-μCaOdiagram shows that the layers in coronae formed in response to chemical potential gradients between the reactant minerals. A combination of field observations and theP-Tconditions of coronae formation suggest a fluid-driven metamorphism. Correlation with extant geological information indicates that the corona-forming event is possibly related to the accretion of India and Antarctica during the assembly of Rodinia.

Mafic-felsic magma interactions in the Bathani volcanic-plutonic complex of Chotanagpur Granite Gneiss Complex, eastern India: implications for assembly of the Greater Indian Landmass during the Proterozoic

Mafic-felsic magma interactions in the Bathani volcanic-plutonic complex of Chotanagpur Granite Gneiss Complex, eastern India: implications for assembly of the Greater Indian Landmass during the Proterozoic

Bulk rock and zircon geochemistry of granitoids from the Chotanagpur Granite Gneissic Complex (CGGC): implications for the late Paleoproterozoic continental arc magmatism in the East Indian Shield

The north-eastern part of the Chotanagpur Granite Gneissic Complex (CGGC) exposes a vast expanse of felsic orthogneisses (~ 1450 Ma magmatic crystallization age) containing a variety of metamorphosed supracrustal rocks and augen gneiss. However, origin and timing of emplacement of the protolith of the augen gneiss are not well constrained. Owing to lack of detailed study and paucity of exposures, the antiquity of the basements of the CGGC also remains poorly understood. To address this issue, we present bulk rock geochemical data and the U–Pb and Lu–Hf isotope composition of zircon from augen gneisses from two adjoining areas within the CGGC. Our obtained data suggest that the augen gneiss have been derived from hybrid source involving older crustal rocks and metasomatised mantle. The bulk rock and zircon geochemistry also suggest that the protolith of the augen gneiss was emplaced in an early Paleoproterozoic basement in a continental arc setting. U–Pb ages of the magmatic zircon grains constrain the age of intrusion at ca. 1710 Ma and ca. 1635 Ma. The timing of this late Paleoproterozoic felsic magmatism overlaps with the timing of the regional high-to-ultrahigh-temperature granulite facies metamorphism in the CGGC.

Neoproterozoic granitic activity in syn‐collisional setting: Insight from petrology, geochemistry, and zircon–monazite geochronology of S‐type granites of the Chotanagpur Granite Gneissic Complex, eastern India

The evidence of a long‐lived Grenvillian orogeny around 1.0 Ga is preserved in the eastern part of India in the form of large‐scale syn‐collisional granitic activity and high‐grade metamorphism in the Chotanagpur Granite Gneissic Complex (CGGC). Geochemically, granitoid rocks of CGGC were classified as ferroan, per‐aluminous, and calc‐alkalic to alkali‐calcic S‐type granites which were generated from the anatexis of meta‐sedimentary protolith (khondalite) in a volcanic‐arc to syn‐collisional setting. Relatively high crystallization temperatures (800–850°C for garnetiferous granite gneiss, 770–830°C for megacrystic granite gneiss, and 639–725°C for pink granite) of these rocks were calculated by geothermometric calculations. The REE modelling testified that 40–45% partial melting of the khondalite in H2O undersaturated condition at around 5‐kbar pressure might have given rise to the parent magma of the garnetiferous granite gneiss (GGG). LA‐ICPMS zircon (U–Pb) dating yielded the crystallization ages of the megacrystic granite gneiss (MGG) and pink granite (PG) as ~1.0 Ga, and EPMA monazite (U–Th–total Pb) dating suggested that the GGG was metamorphosed during 1.0 Ga. The 1.0–0.90 Ga age of high‐grade metamorphism and anatexis in the CGGC may be correlated with that in the Rayner Complex–Eastern Ghats Belt in the Indo‐Antarctic sector, which is considered as the result of Indo‐Antarctic collision during the assembly of Rodinia.

Petrology and geochronology of a suite of pelitic granulites from parts of the Chotanagpur Granite Gneiss Complex, eastern India: Evidence for Stenian‐Tonian reworking of a late Paleoproterozoic crust

The north‐eastern part of the Chotanagpur Granite Gneiss Complex (CGGC) in the East Indian shield contains enclaves of migmatitic pelitic granulites (PG) within felsic orthogneiss (FOG). Field observations, petrology and geochronology (LA–MC–ICP–MS U–Pb dating of zircon and EPMA Th–U–total‐Pb dating of monazite) of the PG suggest two distinct metamorphic events. The earliest event M1, which is characterized by high‐temperature (>850°C) granulite facies metamorphism, occurred in the timespan of ~1680–1580 Ma. Extensive dehydration melting of biotite + sillimanite + quartz‐rich protoliths led to stabilization of the restitic assemblage (garnet + alkali‐feldspar + quartz + sillimanite + ferrian‐ilmenite) together with large volumes of felsic melts (leucosomes). Collisional tectonics followed by delamination and asthenospheric upwelling could have triggered the M1 event. Subsequently, at ~1470–1400 Ma, the igneous protolith of the host FOG intruded and hydrated the PG. Thereafter, a second metamorphic event, M2, accompanied by compressional structures, affected both the rock types. A clockwise P–T path that culminated at ≥10 kbar ~760–850°C and is followed by a steeply decompressive retrograde path characterizes this event. The P–T path and the inferred geothermal gradient (<27°C/km) are compatible with a continent–continent collisional setting. Geochronological findings suggest a protracted orogeny for the M2 event with its major pulse during ~970–950 Ma. When combined with the published information, this study supports the view that a large (if not the entire) portion of the Indian shield and the granulite terranes of east Antarctica share similar tectonothermal events that led to the formation of two supercontinents, Columbia and Rodinia.

The Genesis of Emulsion Texture Owing To Magma Mixing in the Ghansura Felsic Dome of the Chotanagpur Granite Gneiss Complex of Eastern India

The Genesis of Emulsion Texture Owing To Magma Mixing in the Ghansura Felsic Dome of the Chotanagpur Granite Gneiss Complex of Eastern India

Origin of vesuvianite-garnet veins in calc-silicate rocks from part of the Chotanagpur Granite Gneiss Complex, East Indian Shield: The quantitative P-T-XCO2 topology in parts of the system CaO-MgO-Al2O3-SiO2-H2O-CO2 (+Fe2O3, F)

Origin of vesuvianite-garnet veins in calc-silicate rocks from part of the Chotanagpur Granite Gneiss Complex, East Indian Shield: The quantitative P-T-XCO2 topology in parts of the system CaO-MgO-Al2O3-SiO2-H2O-CO2 (+Fe2O3, F)

High pressure metamorphism of mafic granulites from the Chotanagpur Granite Gneiss Complex, India: Evidence for collisional tectonics during assembly of Rodinia

In the north-eastern part of the Chotanagpur Granite Gneiss Complex (CGGC), India, mafic granulite occurs as enclaves within the ˜1450 Ma old felsic rocks. The mafic granulite and the host felsic rock develop a pervasive planar fabric defined by minerals that are characteristics of high pressure (HP) granulite facies metamorphism. The earliest discernable mineral assemblage in the mafic granulites comprises amphibole + clinopyroxene + plagioclase + quartz + ilmenite, preserved as inclusions within garnet porphyroblasts as a record of early M0 stage. Subsequently, the assemblage garnet + clinopyroxene + plagioclase + rutile + quartz developed during the peak metamorphic stage (Mi) defining the pervasive fabric. Quantitative geothermobarometry and intersection of isopleths in a phase diagram (pseudosection) that was numerically computed for a representative bulk composition of the mafic granulites, yield 12 ± 1 kbar and 800 ± 50 °C for the Mi stage. Subsequent changes in physical conditions in response to tectonic evolution of the complex developed two distinct sets of mineral assemblages (Mii and Miii). Firstly, garnet ± clinopyroxene + quartz broke down to symplectic aggregates of orthopyroxene + plagioclase under static conditions (Mii stage). Afterwards, the anhydrous minerals, which developed during the M0-ii stages, were variably replaced by amphibole-bearing assemblages (Miii stage). Geothermobarometric estimates and the stability of different assemblages that developed during M0-Miii stages, define a clockwise (CW) P–T path with an inferred geothermal gradient of ˜20 °C/km during the culmination of HP Mi metamorphism. The reconstructed CW P-T path and the inferred geothermal gradient are consistent with a continent-continent collisional tectonic setting. Integrating all the existing information, it is considered that a large part, if not the whole, of the CGGC behaved as a coherent lower plate during the globally extensive convergent margin orogenesis at the time of Rodinia supercontinant formation.

Do the deformed alkaline rocks always serve as a marker of continental suture zone? A case study from parts of the Chotanagpur Granite Gneissic complex, India

In Santuri- Kankarkiari area of Purulia, West Bengal, India, a nepheline syenite body cuts across the regional Grenvillian (∼1000–950 Ma) fabric (S2) of the host Chotanagpur Granite Gneissic Complex (CGGC). Subsequently, the nepheline syenite of the studied area and its basement are metamorphosed and develop a pervasive regional fabrics (S3). Isoclinal folding of S2 fabric of the host gneissic rocks resulted in axial planar S3 fabric. Field relations, ‘frozen-in’ textural feature, petrology and geochemistry of the studied rock and the published geological and geochronological information from the adjoining areas are consistent with (a) derivation of its magmatic protolith from the then sub-continental lithospheric mantle and emplacement of the melt in a rift basin (b) the rifting of the Grenvillian basement seems to have occurred during (∼950–900 Ma) and (c) burial, deformation and metamorphism (peaked at 9–10 kbar, 690–770 °C) of the studied alkaline rocks and its Grenvillian basement seem to have occurred in a continent-continent collisional tectonic setting during ∼900–780Ma. The commonly perceived notion that the deformed alkaline rocks (DARs) are invariably linked to continental suture along which a former ocean basin floor was consumed is not supported in this study. Integrating all the information, we propose that the nepheline syenite of the studied area and also from parts of the Granulite Terrane of South India represent metamorphosed and deformed aulacogen rock ensemble that should not be associated with opening and closure of an oceanic crust. Combining the information of this study and the published works, it is demonstrated that the deformed alkaline rocks (DARs) can also form in a coherent crustal block without any connection with a continental suture zone. A plausible tectonic model for the evolution of the CGGC has also been presented.

Role of viscous folding in magma mixing

Magma mixing is a complex phenomenon and occurs through the interplay of a number of processes. Recent developments in microfluidics and chaotic mixing experiments have enabled us to better understand the complex processes associated with magma mixing from the hybrid rocks of the Ghansura Rhyolite Dome (GRD) of Proterozoic Chotanagpur Granite Gneiss Complex (CGGC), Eastern India. The hybrid rocks of GRD have preserved amphibole-rich microzones (ARM) and microfolded mineral veins resulting from the mixing of felsic (rhyolitic) and mafic (basaltic) melts. Based on textural features and mineral chemical data interpretations, we infer that viscous folding can be an important mechanism to obtain chemical equilibrium in a magma mixing scenario. Magma mixing is a chaotic process brought about by the interplay of advection (i.e. stretching and folding) and diffusion. In our scenario, stretching and folding of the mineral veins or filaments were achieved through viscous folding, followed by chemical diffusion between the contrasting magmas to enhance magma mixing. Our findings reveal that when mafic magma carrying phenocrysts of augite came in contact with the felsic magma, there was diffusion of heat from the hotter mafic phase to the colder felsic phase followed by diffusion of elemental components. Diffusion of elements like H+, Al3+ and other cations from the felsic phase to the mafic phase converted clinopyroxene phenocrysts in the mafic system into aggregates of small amphibole crystals. The formation of abundant amphibole microcrysts greatly increased the viscosity of the mafic system, allowing the amphibole-rich magma to venture into the surrounding felsic melt as veins and undergo viscous folding to facilitate mixing.

Tectonothermal imprints in a suite of mafic dykes from the Chotanagpur Granite Gneissic complex (CGGC), Jharkhand, India: Evidence for late Tonian reworking of an early Tonian continental crust

Tectonothermal evolution of a set of mafic dykes in the felsic orthogneiss basement of the Chotanagpur Granite Gneissic Complex (CGGC) is presented. The gneissic basement rocks preserve high pressure granulite facies metamorphism (M2, ~800 °C and ~9 kbar) and develop pervasive planar fabric (S2) during a continent-continent collision event at 1000–950 Ma. Mafic dykes cut the S2 fabric of its basement it intrudes. Together with the S2, the mafic dykes have been deformed by three sets of folds viz. DkF1, DkF2 and DkF3. A conspicuous NS trending foliation defined by metamorphic amphibole is developed parallel to the axial plane of DkF1–2. The mafic dykes preserve relict igneous textures (e.g. intergranular texture) defined by clinopyroxene, orthopyroxene and plagioclase. Numerically computed phase diagram (pseudosection) with a representative bulk composition (with <10 vol% metamorphic minerals namely amphibole+ plagioclase rim + quartz) constrain the depth of emplacement of the mafic dykes (15–18 km). The geological features are consistent with rifting of the early Tonian basement of the CGGC and the then mantle beneath it. This lithospheric extension resulted in outpouring of basaltic magma that punctured the early Tonian basement rocks in the form of mafic dykes. Subsequent metamorphism that affected the mafic dykes and its basement culminated at ~700 °C and ~7.5 kbar (M3 metamorphism).The estimated physical conditions during the M3 metamorphic event suggest burial of the previously rifted basement possibly in a continent-continent collisional tectonic setting. The Th-U-total Pb ages of monazite grains from the host felsic orthogneiss date the M2 (ca. 964 Ma) and M3 (ca. 917 Ma) metamorphism. Based on commonality of geological and geochronological history among the CGGC, the Eastern Ghats Mobile Belt and Rayner complex (and its equivalent parts) in east Antarctica corroborates the view that Indo-Antarctic landmass were united and shared the similar geological histories during the entire Tonian period. During the Tonian period the Indo-Antarctic landmass witnessed two major phases of orogenesis (1000–950 Ma and < 920 Ma) that are punctuated by a phase of lithospheric extension manifested by the emplacement of mafic dykes.

Titanite-centered ocellar texture: A petrological tool to unveil the mechanism facilitating magma mixing

The titanite-centered ocellar texture is a rarely observed texture from magma mixing and mingling zones. This work provides a synthesis of petrographical observations and mineral-chemical data to understand the genesis of this texture. The rock with titanite ocelli is from the mafic-felsic interaction zone of the Nimchak Granite Pluton (NGP) of Chotanagpur Granite Gneiss Complex (CGGC) of Eastern India. Interpretations reveal that when mafic (basaltic) magma intrudes a crystallizing felsic (granite) magma chamber the first interaction that takes place between the two phases is diffusion of heat from the relatively hotter mafic magma to the colder felsic magma followed by diffusion of elements. When thermal equilibrium is reached between the two magmas the rheological contrasts between the two phases gets drastically reduced, allowing the felsic melt to back-vein the mafic magma. The influx of felsic melt results in breakdown of minerals like amphibole and biotite in the mafic system resulting in liquids of titanite composition. These liquids get incorporated into the incoming felsic melt as an immiscible phase and flow with it. The felsic melt and the immiscible titanite-liquid occupy the void spaces left behind by escaping gases/volatiles in the mafic system ultimately forming rounded titanite ocelli.

Field evidence, mineral chemical and geochemical constraints on mafic-felsic magma interactions in a vertically zoned magma chamber from the Chotanagpur Granite Gneiss Complex of Eastern India

The Nimchak granite pluton (NGP) of Chotanagpur Granite Gneiss Complex (CGGC), Eastern India, provides ample evidence of magma interaction in a plutonic regime for the first time in this part of the Indian shield. A number of outcrop level magmatic structures reported from many mafic-felsic mixing and mingling zones worldwide, such as synplutonic dykes, mafic magmatic enclaves and hybrid rocks extensively occur in our study domain. From field observations it appears that the Nimchak pluton was a vertically zoned magma chamber that was intruded by a number of mafic dykes during the whole crystallization history of the magma chamber leading to magma mixing and mingling scenario. The lower part of the pluton is occupied by coarse-grained granodiorite (64.84–66.61 wt.% SiO2), while the upper part is occupied by fine-grained granite (69.80–70.57 wt.% SiO2). Field relationships along with textural and geochemical signatures of the pluton suggest that it is a well-exposed felsic magma chamber that was zoned due to fractional crystallization. The intruding mafic magma interacted differently with the upper and lower granitoids. The lower granodiorite is characterized by mafic feeder dykes and larger mafic magmatic enclaves, whereas the enclaves occurring in the upper granite are comparatively smaller and the feeder dykes could not be traced here, except two late-stage mafic dykes. The mafic enclaves occurring in the upper granite show higher degrees of hybridization with respect to those occurring in the lower granite. Furthermore, enclaves are widely distributed in the upper granite, whereas enclaves in the lower granite occur adjacent to the main feeder dykes.Geochemical signatures confirm that the intermediate rocks occurring in the Nimchak pluton are mixing products formed due to the mixing of mafic and felsic magmas. A number of important physical properties of magmas like temperature, viscosity, glass transition temperature and fragility have been used in magma mixing models to evaluate the process of magma mixing. A geodynamic model of pluton construction and evolution is presented that shows episodic replenishments of mafic magma into the crystallizing felsic magma chamber from below. Data are consistent with a model whereby mafic magma ponded at the crust-mantle boundary and melted the overlying crust to form felsic (granitic) magma. The mafic magma episodically rose, injected and interacted with an overlying felsic magma chamber that was undergoing fractional crystallization forming hybrid intermediate rocks. The intrusion of mafic magma continued after complete solidification of the magma chamber as indicated by the presence of two late-stage mafic dykes.

Geochemistry, U-Pb geochronology and Lu-Hf isotope systematics of a suite of ferroan (A-type) granitoids from the CGGC: Evidence for Mesoproterozoic crustal extension in the east Indian shield

Ferroan (A-type) granitoid batholiths typically intrude during the periods of crustal-scale extension either at the terminal phase of an orogenic cycle or in an unrelated ‘anorogenic’ event. Elevated magmatic temperatures coupled with the anhydrous nature of the melts warrants the involvement of the lower crust along with the mantle as material and/or heat source. Granitoid batholiths from the north-eastern part of the Chotanagpur Granite Gneissic Complex (CGGC) show extreme iron enrichment with high alkali and HFSE contents associated with elevated magmatic temperatures (∼900 °C), typical of ferroan (A-type) granites. Whole-rock trace element compositions indicate that the granitoids were dominantly produced by low-pressure anhydrous melting of middle- to lower-crust. U-Pb zircon geochronology constrains the intrusion age of the granitoids to ca. 1450 Ma. Lu-Hf isotopic compositions of the dated zircon indicate a limited contribution of contemporaneous juvenile mantle material; instead point to a dominant involvement of Palaeoproterozoic crustal source. Based on these findings, we suggest that the ferroan (A-type) granitoids occurring in the north-eastern CGGC are the results of melting of Palaeoproterozoic crust, triggered by mantle upwelling and increased heat transfer resulting from crustal extension associated to the fragmentation of the Indian landmass during the breakdown of the Columbia supercontinent.

Evaluation of magma mixing in the subvolcanic rocks of Ghansura Felsic Dome of Chotanagpur Granite Gneiss Complex, eastern India

The subvolcanic rocks exposed in the Ghansura Felsic Dome (GFD) of the Bathani volcano-sedimentary sequence at the northern fringe of the Rajgir fold belt in the Proterozoic Chotanagpur Granite Gneiss Complex preserves evidence of magma mixing and mingling in mafic (dolerite), felsic (microgranite) and intermediate (hybrid) rocks. Structures like crenulated margins of mafic enclaves, felsic microgranular enclaves and ocelli with reaction surfaces in mafic rocks, hybrid zones at mafic-felsic contacts, back-veining and mafic flows in the granitic host imply magma mingling phenomena. Textural features like quartz and titanite ocelli, acicular apatite, rapakivi and anti-rapakivi feldspar intergrowths, oscillatory zoned plagioclase, plagioclase with resorbed core and intact rim, resorbed crystals, mafic clots and mineral transporting veins are interpreted as evidence of magma mixing. Three distinct hybridized rocks have formed due to varied interactions of the intruding mafic magma with the felsic host, which include porphyritic diorite, mingled rocks and intermediate rocks containing felsic ocelli. Geochemical signatures confirm that the hybrid rocks present in the study area are mixing products formed due to the interaction of mafic and felsic magmas. Physical parameters like temperature, viscosity, glass transition temperature and fragility calculated for different rock types have been used to model the relative contributions of mafic and felsic end-member magmas in forming the porphyritic diorite. From textural and geochemical investigations it appears that the GFD was a partly solidified magma chamber when mafic magma intruded it leading to the formation of a variety of hybrid rock types.

Geochemical and U–Pb zircon age characterization of granites of the Bathani Volcano Sedimentary sequence, Chotanagpur Granite Gneiss Complex, eastern India: vestiges of the Nuna supercontinent in the Central Indian Tectonic Zone

Abstract The Central Indian Tectonic Zone (CITZ) marks the suture zone where the North and South Indian cratonic blocks amalgamated to form the Greater Indian Landmass (GIL). It consists of three broad domains from west to east: the central CITZ occupying the central region of mainland India juxtaposed between two mobile belts, namely the Sausar Mobile Belt (SMB) in the south and the Mahakoshal Mobile Belt (MMB) in the north; the Chotanagpur Granite Gneiss Complex (CGGC) lying east of the main CITZ; and the easternmost Shillong Plateau Gneissic Complex (SPGC). The studied granites are from the Bathani Volcano Sedimentary sequence (BVSs) from the northern margin of the CGGC. These are high-K, calc-alkaline, I-type granites related to arc magmatism and are interpreted to have formed by partial melting of an igneous source at upper-crustal depths. The granitic magma underwent extensive fractional crystallization of plagioclase, biotite, K-feldspar and ilmenite during emplacement. The U–Pb (ID-TIMS) zircon emplacement age is c. 1.7–1.6 Ga for these granites. This episode of magmatism can be correlated to the global event of the Nuna supercontinent assembly also reported from the MMB of the central CITZ. We infer that the BVSs is the eastern continuation of the MMB of the central CITZ.

Petrology and U–Pb geochronology of zircon in a suite of charnockitic gneisses from parts of the Chotanagpur Granite Gneiss Complex (CGGC): evidence for the reworking of a Mesoproterozoic basement during the formation of the Rodinia supercontinent

Abstract Understanding the evolution of the Chotanagpur Granite Gneiss Complex (CGGC) of the East Indian Shield is crucial to decipher the role of the Indian Shield in the formation of the Rodinia supercontinent. The area around Deoghar–Dumka exposes a suite of granulite-facies orthogneisses (variably retrogressed to amphibole–biotite gneiss) that enclose remnants of Palaeoproterozoic metasedimentary and meta-igneous rocks. Results from mineral chemistry, laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS) U–Pb dating of zircon and limited bulk-rock compositions of the studied rocks suggest that the magmatic protoliths of the felsic orthogneisses had A-type chemistry, and that these were emplaced at approximately 1450 Ma presumably in a continental rift setting. Intense deformation and metamorphism of the felsic rock culminated at approximately 9 kbar and 850°C along an apparent geothermal gradient of 26°C km −1 . These peak metamorphic conditions were successively followed by initially a steeply decompressive and then a weakly decompressive retrograde pressure–temperature path. The shape of the retrograde pressure–temperature path and the estimated geothermal gradient at the metamorphic peak are interpreted to be the products of continent–continent collision; U–Pb dates of metamorphic zircon overgrowths suggest an age of approximately 943 Ma for the collisional event. This study demonstrates that ‘Grenville-age’ orogenesis thoroughly reworked the approximately 1450 myr-old basement of the CGGC during the formation of the Rodinia supercontinent.