Chotanagpur Granite Gneiss Complex Research Articles

Petrogenesis of magmatic charnockite-biotite granite suite from parts of the Chotanagpur Granite Gneissic Complex (CGGC), eastern Indian shield: Implication for the break down of the Columbia Supercontinent

A rare porphyritic charnockite that is girdled by and mineralogically grades to biotite granite occurs as a part of the Chotanagpur Granite Gneissic Complex (CGGC) in and around Massanjore, Jharkhand, India. Preservation of certain textural features, including (a) euhedral to subhedral grains of orthopyroxene, (b) low dihedral angle subtended by orthopyroxene and plagioclase grains, and (c) relict intergranular and porphyritic textures, are consistent with the view that orthopyroxene in the studied rocks has a magmatic origin. Preserved magmatic features and other petrological attributes of these rocks do not support any significant mass change beyond a few tens of microns during the overprinting high-grade metamorphism. The geochemical variation of the felsic rock suite (porphyritic charnockite and biotite granite) indicates that they are cogenetic and are derived from a ferroan A-type granitoid magma by crystal fractionation. The observed geochemical trend of the studied felsic rock suites has been simulated by phase equilibria modelling in the open system using the system components Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2. The observed mineralogical and geochemical attributes and the results of the modelling study are consistent with a petrogenetic process in which high magma temperature (> 900 °C), low pressure, and low water activity in the parental melt favoured the separation of an orthopyroxene bearing cumulate assemblage (orthopyroxene + plagioclase + quartz + K-feldspar + ilmenite + magnetite) in the initial part of the magmatic differentiation. Removal of this anhydrous cumulitic assemblage raised the bulk H2O content in the residual melt. Orthopyroxene became unstable with respect to biotite in the evolved melt that eventually crystallised minerals that formed the biotite granite. An increase in magma fO2 also restricts the orthopyroxene stability in felsic magma. Taken together all the petrological and geochemical attributes, we demonstrate that fractionation of an orthopyroxene-bearing crystal cumulate from the melt is essential to form the charnockites, and that the biotite granite forms from the evolved melt after the fractionation. The charnockite-biotite granite association of the studied area was formed in an extensional tectonic setting, presumably during the breakdown of the Columbia Supercontinent.

Origin of calcite by magma mixing in mingled rocks of the Ghansura Rhyolite Dome, Bathani volcano-sedimentary sequence, eastern India

Abstract The Ghansura Rhyolite Dome (GRD) is an integral part of the Bathani volcano-sedimentary sequence (BVSs), which in turn is a vital component of the Chotanagpur Granite Gneiss Complex (CGGC), eastern India. The rhyolite dome represents a shallow-level felsic magma chamber that underwent intrusion by basaltic magma during its evolution. The rocks present in the felsic dome such as basalts, rhyolites, and intermediate hybrid rocks preserve evidence of magma mixing and mingling. The objective of this contribution is to take into account, for the first time, that magma mixing processes can play an important role in the generation of calcite in magmatic rocks. The present study focuses on the mingled rocks exposed within the rhyolite dome. Previous reports have documented the existence of two distinct zones (mafic and felsic) interfacing each other in the mingled rocks of the rhyolite dome. The mafic zones dominantly consist of mineral phases such as actinolite, hornblende, biotite, plagioclase, ilmenite, calcite, and titanite. The felsic zones comprise quartz, K-feldspar, plagioclase, and biotite. When mafic magma intruded into the felsic magma chamber, interaction between the mafic and felsic magmas caused diffusion of H, Al, and K ions from the felsic to the mafic endmember. Such diffusive activities resulted in the breakdown of augite, already crystallized in the mafic magma, to form newer minerals like actinolite, hornblende, biotite, calcite, and ilmenite. The presence of ilmenite in the mingled rocks indicates the prevalence of reducing conditions during magma interaction and evolution. Breakdown of hornblende to biotite in such a reduced environmental condition led to the formation of calcite in the mingled rocks. From the results presented in this work, we are proposing that magma mixing can be a viable mechanism to form calcite in igneous rocks by magmatic processes.

Reaction-diffusion modelling of petrological mixing mechanisms in the evolution of continental crusts

Petrological assimilation is a key process in the evolution of high-grade metamorphic terrains in the continental crusts. This study examines the mechanisms of such macroscopic assimilation between felsic (F) and mafic (M) constituents as two petrologically interacting continuum phases, as observed in the Chotanagpur Granite Gneissic Complex (CGGC), India, which underwent amphibolite to granulite facies metamorphism (∼775 to 900°C and 7 to 11 kb) between from the Paleoproterozoic to the late Mesoproterozoic. From field investigation we could recognized four interface patterns: planar, wavy, fingering and incoherent, which are generated at the interface between the F and M units. We have adopted the Turing type reaction-diffusion (RD) approach, which is a well established theoretical model to interpret any complex auto-regulatory pattern in natural and physical sciences, to understand the physics of the self-organizing interface geometries observed across the CGGC. The RD model findings suggest that these patterns are constrained by a combination of: diffusion coefficients (DF, DM) of F and M, a linear or non-linear reaction term (R) that describes phase interactions and a pinning field (W) that introduces microscale heterogeneity. For linear interactions, F − M undergo homogeneous mixing and show planar/wavy interfaces, when DF = DM and W = 0. The mixing turns heterogeneous as DF ≠ DM and W > 0, resulting in phase boundary migration with a fingering pattern. Non-linear reaction coupling enhances heterogeneous mixing and produces incoherent phase boundaries where F-phases host relics of M-phases, following a power-law size distribution. Striking similarities of interface patterns and fractal dimensions estimated from model and CGGC validate the proposed mechanism of macroscopic petrological assimilation. We argue that RD model provides a new insight into the genesis of hybrid rocks in metamorphic terrains.

Magnetotelluric images of the medium enthalpy Bakreswar geothermal province within a granitic gneissic complex, Eastern Indian Peninsula

AbstractThe Bakreswar geothermal province represents a medium enthalpy geothermal system with its Bakreswar and Tantloie hot springs. It lies within the Chotanagpur Granite Gneissic Complex in the eastern part of the Indian Peninsula. The province has a high heat flow and a high geothermal gradient of 90°C/km. Magnetotelluric data from 95 sites in a frequency range of 10 kHz–10 Hz were acquired over the Bakreswar geothermal province to obtain an electrical conductivity model and map the geothermal reservoir with its fluid pathways and related geological structures. Subsurface conductivity models obtained from three‐dimensional inversions of the Magnetotelluric data exhibit several prominent anomalies, which are supplemented by gravity results. The conductivity model maps three features which act as a conduit (a) a northwest–southeast trending feature, (b) an east–west trending feature to the south of the northwest–southeast trending feature (which lies 1 km north of the Oil and Natural Gas Corporation fault marked by previous studies) and (c) shallow conducting features close to Bakreswar hot spring. The northwest–southeast trending feature coincides with the boundary of the high‐density intrusive block. This northwest–southeast trending feature provides the pathway for the meteoric water to reach a maximum depth of 2.7 km, where it gets heated by interacting with deep‐seated structures and then it rises towards the surface. The radiogenic process occurring within the granites of Chotanagpur Granite Gneissic Complex provides the heat responsible for heating the meteoric water. The northwest–southeast and east–west trending features are responsible for the transport of meteoric water to deeper depths and then towards the shallow regions of the Earth. The near surface features close to the Bakreswar hot spring are responsible for carrying the water further towards the hot spring. The resistivity of these structures plotted as a function of salinity and temperatures for saline crustal fluids suggests the involvement of meteoric water. Further, applying Archie's law to this resistivity suggests that the conduit path has a porosity greater than 10%. This study successfully maps the anomalous structures which might foster the migration of geothermal fluid in Bakreswar geothermal province.

Petrology, geochemistry and U–Pb zircon geochronology of Fe–Ti oxide ore‐bearing mafic sill from Saltora‐Mejia area of Chotanagpur Granite Gneissic Complex, eastern India: Implication for late tectonic emplacement of mafic rock of E‐MORB nature in an orogenic belt

An east–west‐trending medium‐grained mafic sill containing co‐genetic Fe–Ti oxide ore lenses is found disposed within granite gneisses around Saltora‐Mejia area in the eastern part of the Chotanagpur Granite Gneissic Complex (CGGC) of eastern India. CGGC is considered as a Proterozoic mobile belt as it witnessed multiple phases of deformation and high‐ grade metamorphism during 1.8–0.8 Ga. Occurrence of such Fe–Ti oxide ore‐bearing mafic sill is unique in the entire CGGC which is a vast Proterozoic orogenic belt and has witnessed many phases of voluminous mafic and felsic magmatisms. The mafic rock is of gabbronorite composition which contains plagioclase, clinopyroxene, orthopyroxene as major constituent primary minerals and amphibole as late magmatic mineral. The rock shows sub‐ophitic, intergranular, mosaic and poikilitic texture (defined by larger pargasitic grain). The gabbronorite shows iron enriched tholeiitic character, low Mg#, low abundances of Ni and Cr, slight enrichment in LILE, LREE and slight depletion in HFSE like Nb and Ti. The computed melt in equilibrium with the studied gabbronorite shows transitional orogenic to anorogenic, within‐plate and E‐MORB‐like geochemical character. In this study, the U–Pb zircon crystallization age (~960 Ma) of the Saltora‐Mejia gabbronorite is reported for the first time which coincides with the late tectonic stage of the most pervasive orogenic activity in the CGGC around 1.2–0.9 Ga. Transitional orogenic to anorogenic geochemical character, late tectonic evolution and other field and laboratory evidences together suggest evolution of the Saltora‐Mejia gabbronorite sill in a late tectonic extensional environment which might have been facilitated by delamination of a subducted plate and upwelling of asthenospheric mantle during the waning stage of a major orogeny in the CGGC.

Mechanisms of Shear Band Formation in Heterogeneous Materials Under Compression: The Role of Pre‐Existing Mechanical Flaws

AbstractShear bands critically govern the shear failure processes and associated geophysical phenomena, for example, faulting, in Earth's crust. Earlier studies on homogeneous materials recognized temperature and strain rate as principal factors controlling the band growth. However, how inherent mechanical heterogeneities can influence their growth mechanisms and complex internal structures needs further investigations. The present article revisits this problem by combining field observations and laboratory experiments, supported by numerical simulations considering geological strain‐rates. Field studies in the Chotanagpur Granite Gneissic Complex of eastern India reveal varied types of macro‐scale shear zone localization. We conducted plane‐strain compression experiments on rock‐analogue models to investigate their origin. The experiments show that mechanical heterogeneities develop wide bands, localized preferentially in their neighborhood, unlike uniformly distributed, conjugate sets of closely spaced narrow bands in a homogeneous system. The wide bands eventually attain a composite structure, containing a core of densely packed band‐parallel sharp secondary bands, flanked by linear zones of closely spaced, narrow bands. We also demonstrate the effects of global strain‐rate on the band evolution in heterogeneous systems. Decreasing strain‐rates replace the composite bands by well‐defined homogeneous shear bands, containing a core of uniform shear, bordered by weakly sheared narrow zones, grading into almost undeformed walls. The experimental results are complemented with numerical models based on visco‐elasto‐plastic rheology, which establish appropriate scaling of the rock analogue to mid‐crustal deformations. This integrated approach finally leads us to conclude that inherent heterogeneities can result in large variations of shear band structures in geological terrains.

Petrogenesis of high-Mg andesites from the Proterozoic Shillong Basin, Northeast India: evidence for continuation of the Central Indian Tectonic Zone to the Assam-Meghalaya Gneissic Complex and its implications for the Columbia supercontinent reconstruction

ABSTRACT This study provides comprehensive geochemical and petrological data for the newly discovered high-magnesium andesites (HMAs) from the Shillong Group of rocks in the Proterozoic Shillong Basin of the Assam-Meghalaya Gneissic Complex (AMGC). It is the first report of HMAs from the easternmost part of the Indian shield region. The AMGC holds immense geodynamic significance because this region has recorded magmatism related to Columbia, Rodinia, and Pangea supercontinental events. The present work discusses geochemical signatures of the Shillong HMAs characterized by calc-alkaline traits with high-K contents and significant concentrations of TiO₂ (0.54–0.76 wt %) higher than boninites (TiO2 < 0.5 wt %), and Mg# (53–56). They exhibit high LILE/HFSE and LREE/HREE ratios, which are distinctive features of magmas generated in subduction zones. Tectonic discrimination diagrams clearly suggest that the Shillong HMAs were generated in a back-arc basin regime. We argue that the Shillong HMAs were derived by 5% to 10% partial melting of a depleted lherzolite mantle source within the spinel – garnet transition zone. We also argue that the Shillong HMAs may have formed as a result of the interaction between subducted sediment-derived melts and mantle. The Shillong HMAs have undergone low- to medium-grade greenschist facies metamorphism characterized by significant mineral alteration, however, they still preserve bulk original alteration-resistant trace element compositions. From a regional perspective, the synchronous nature of origin and geotectonic setting, associated geochronological ages of formation, and regional structural trends suggest that the Shillong Basin is the easternmost continuation of the Mahakoshal Mobile Belt and specifically, the Bathani volcano-sedimentary sequence of the Central Indian Tectonic Zone (CITZ); and together they form the northern fragments of the same suture zone consisting of the CITZ, the Chotanagpur Granite Gneiss Complex, and the AMGC, which demarcates the northern and southern Indian blocks of the Greater Indian Landmass.

Geochemical constraints on the petrogenesis of mafic rocks (metadolerites) from the Proterozoic Shillong Basin, Northeast India: implications for growth of the Greater Indian Landmass

AbstractThe Paleo-Mesoproterozoic Shillong Basin of the Assam-Meghalaya Gneissic Complex is exposed in parts of Northeast India. The studied metadolerites are from the volcano-sedimentary sequence of Shillong Basin from the Borjuri area in the Mikir Massif. This episode of mafic magmatism can be correlated with the Columbia supercontinent formation and bears significance to its reconstruction. The present work discusses the field, petrography and geochemical characteristics of the metadolerites, which occur in close association with the quartzites of the Shillong Group of rocks (metasedimentary rocks of the Shillong Basin). Our data show distinctive characteristics of subduction-related magmatism exhibiting high LREE/HREE, large ion lithophile element/high field strength element ratios and pronounced negative Nb anomaly. Elemental ratios such as Zr/Ba (0.21–0.46), La/Nb (1.23–2.32) and Ba/Nb (30.08–56.90) point to a fluid-enriched lithospheric mantle source in a subduction regime. Metadolerites plot in the field of ‘back-arc basin basalts’ in tectonic discrimination diagrams reinforcing a subduction zone tectonic setting. The mafic rocks correspond to a 6–10 % partial melting of a mantle source incorporating spinel+garnet lherzolite. The metamorphic P-T of the metadolerites estimated from plagioclase-hornblende geothermobarometer (7–8 kbar, 664 °C) is indicative of amphibolite facies metamorphism in a medium P-T zone. Based on the comparative analysis of field observation, petrography, geochemistry and geological ages given by previous workers, we infer that the Shillong Basin represents a back-arc rift region and is the eastern continuation of the Bathani volcano-sedimentary sequence of the Chotanagpur Granite Gneiss Complex marking continuation of the Central Indian Tectonic Zone to the Mikir Massif.

Formation of aluminous clinopyroxene-ilmenite-spinel symplectic assemblage in a regionally metamorphosed calc-silicate granulite from the Chotanagpur Granite Gneiss Complex, East Indian shield

Formation of aluminous clinopyroxene-ilmenite-spinel symplectic assemblage in a regionally metamorphosed calc-silicate granulite from the Chotanagpur Granite Gneiss Complex, East Indian shield

Estimation of magma overpressure from partially exposed dykes - A new approach

We present a new method to estimate full length and maximum thickness of dykes from their partial exposures having at least one tip. The method is particularly applicable to dykes which originate as dominantly opening mode fractures. The idealized canonical model for Mode I fracture shows it to open up in the shape of an ellipse. We solve the equation of this ellipse using every possible combination of two points lying on the dyke margin measured from the observed tip to acquire the results. The method has been validated with observations made on felsic dykes found within intrusive porphyritic granitoids of Chotanagpur Granite Gneiss Complex (CGGC), Purulia, India which are completely exposed (showing tips at both ends) or have an exposed centre with at least one tip. The method has also been tested against published data collected from incomplete dyke outcrops found in the caldera walls of Miyake-jima volcano, Japan. Acquired results are found to be in good agreement with the ones estimated through a previously published method. The present method has been successfully employed to calculate the aspect ratios (maximum thickness/total length) of partially exposed mafic dykes emplaced within younger granite of Chitradurga Schist Belt, Western Dharwar Craton, India. We comment on the magma overpressure ranges and depth of origin of these dykes and on the fracture toughness of the host granite.

Origin of clinopyroxene-ilmenite symplectites in mafic granulites from eastern parts of the Chotanagpur granite gneissic complex, East Indian shield

Abstract In this manuscript, we report a rare occurrence of rod-like intergrowths of clinopyroxene-ilmenite that variably replace titanite in a suite of high-pressure mafic granulites from the Chotanagpur Granite Gneissic Complex, Eastern Indian Shield. Garnet proximal to the clinopyroxene-ilmenite intergrowth is invariably replaced with symplectic clinopyroxene-plagioclase or a rind of plagioclase. Textural modeling of the reaction textures and mineral compositions suggests that the garnet-titanite became unstable and the ilmenite-clinopyroxene preferentially develop after titanite. The presence and texture of halogen rich apatites within the ilmenite-clinopyroxene symplectite are consistent with a fluid mediated process. Thermodynamic modeling in the NCFMAST (+H2O) system, demonstrates that the clinopyroxene-ilmenite symplectite was formed along a steeply decompressive retrograde P-T path (from ~13 to ~7 kbar, at ~800 °C), in the presence of partial melts. The study demonstrates that relative to Fe, Mg, and Ca, Ti was less mobile and that the mobility of Ti was restricted within the confines of titanite being replaced by the clinopyroxene-ilmenite symplectite. This study implies that besides the ligand activity, the fluid/rock ratio exerts a strong control on the length scale of Ti transport in natural rocks.

Effect of source heterogeneity, melt extraction and crystal separation on the composition of a suite of ferroan (A-type) granites from parts of the Chotanagpur Granite Gneissic Complex (CGGC), India

Ferroan granitoids constitute a considerable proportion of the upper continental crust formed during the Proterozoic eon and their origin is usually linked to high-temperature processes (fractionation or anatexis) occurring at shallow crustal depths. However, the relative role of mantle melts vs. crustal rocks as the source of these granitoids has been largely debated. In the Chotanagpur Granite Gneissic Complex (CGGC), located in the East Indian shield, a suite of Mesoproterozoic (ca. 1450–1350 Ma) ferroan granites has intruded the Paleoproterozoic (ca. 1700–1630 Ma) arc granites and currently occurs either as migmatitic gneisses or as biotite gneisses in different parts of the terrane . Previous studies postulated that these Mesoproterozoic extensional granites were sourced from older crustal rocks with limited mantle input. In the current study, the geochemistry of the biotite gneisses is combined with that of the migmatitic gneisses and coeval ferroan granites from northern and western parts of the CGGC to comprehend the wide spatial and compositional variation observed. With the help of phase equilibrium modelling, the current study quantifies the role of the heterogeneity of the crustal source and path-dependent melting in shaping the composition of the ferroan granites. The study demonstrates that the composition of the modelled melt derived from the older (late Paleoproterozoic) granites under low-pressure high-temperature conditions (750-1000 °C; 4–8 kbar) closely resembles the compositions observed in the younger (Mesoproterozoic) ferroan granites. Compositional variation of the melt is further accentuated by high-temperature progressive melting and extraction of melt in small batches during anatexis . Phase equilibrium studies further delineate the significant role of crystal fractionation in shaping the chemical variations observed in the studied granites. • Mesoproterozoic ferroan granites of CGGC formed via anatexis of older arc granites. • Source heterogeneity produces variable composition of the granites. • Melt extraction in small batches produces chemical variation in ferroan granites. • Post-emplacement crystal separation accentuates the compositional variations.

Nature and origin of anorthosite enclaves within Proterozoic granite of Chotanagpur Granite Gneiss Complex of Eastern India

The Central Indian Tectonic Zone (CITZ) is a Proterozoic suture along which the Northern and Southern Indian Blocks are inferred to have amalgamated, forming the Greater Indian Landmass. The Chotanagpur Granite Gneiss Complex (CGGC) represents the eastern extension of the CITZ and exposes several granite plutons. Enclaves of diverse origin are present as minor constituents within these granitic bodies. This study reports new major and trace element data for anorthosite enclaves hosted within granites in the Pattharkatti and Rajgir area from the northern margin of CGGC to get modern insights into the petrogenesis of anorthosites. Anorthosite enclaves show sharp contact with the host granite. They contain cumulus plagioclase (An87–94), intercumulus amphibole (magnesiohornblende and ferrotschermakite), and biotite (Mg-biotite and phlogopite) along with minor iron oxides. Amphibole crystallization pressure and temperature are constrained between 0.5 and 6.4 kbar and 653–780°C for the anorthosites. The studied anorthosites display a very gradual and steady increase in whole-rock rare earth element (REE) contents from Lu to La [(La/Yb)N = 1.22–13.08]. They also show a sharp decline in Fe2O3(t) and MgO, whereas Al2O3 increases with increasing silica contents from 45.69 to 51.16 wt%. In the chondrite normalized REE diagram, plagioclase exhibits LREE enriched patterns with strong positive Eu anomaly. The composition of parental liquid for anorthosite from the study area was estimated by adopting the equilibrium distribution method. Parental melt curves from Sm to La are near parallel and constrained broadly between trapped melt fractions (TMF) = ∼5%–15%. Anorthosites of the study area may have formed from the plagioclase-saturated basaltic melt.

GIS and geophysics: an integrative and inter-evaluating approach towards groundwater assessment

Efforts have been made to explore groundwater assessment possibilities employing geospatial and geophysical techniques as well as statistically authenticate the connection between these two techniques. The study was carried out at Chotanagpur Granite-Gneiss Complex (CGGC), Ranchi, Jharkhand, India. The analytical hierarchy process (AHP) was used for Multi-Criteria Decision Making (MCDM) for various thematic factors such as land use land cover, drainage density, lineament density, lithology, rainfall, soil texture, and slope based on their interaction with atmospheric water and contribution towards groundwater replenishment. These remotely-sensed raw datasets were enriched and overlaid in the Geographic Information System (GIS) atmosphere using the weighted overlay analysis tool, which yielded the groundwater potential index (GWPI) map. Based on the estimated GWPI, the watershed area has been demarcated into five distinct groundwater potential classes in terms of poor, low, moderate, good, and very good. The inverted resistivity tomography models revealed exploitable groundwater zone lies at five sites at depth ranges between 60–70, 70–100, and 140–170 m underlie in the saturated weathered/fractured rocks where characteristic resistivities range between 80 and 800 Ω.m. In the target area, i.e. the IIAB campus, five different zones have been demarcated for groundwater exploitation in terms of low, moderate, and good considering groundwater exploitable depth and their connectivity to the near-surface recharge source. Geophysical findings were also corroborated with the existing borehole datasets for their authenticity and helped to generate conceptual geological models. The accuracy assessment combining geospatial and geophysical results validated their inter-relationship as per the generated error matrix with the overall accuracy of 87.5%.

Tectono‐stratigraphic evolution of Shillong Plateau, North East India through the Permian‐Eocene window

The Shillong Plateau in the north‐eastern Indian Plate signifies a cut‐off patch of the Indian Peninsular Shield, bordered by fault and thrust sheets, with thick alluvium cover along its periphery. The basement rock in the plateau accomplished about 2 km relief difference with the adjoining Sylhet Trough of Bangladesh in the south. The plateau witnessed Permo‐Carboniferous Gondwana sedimentation at its western margin, with Early Cretaceous basaltic traps and continuous fluvio‐marine sedimentation along its southern edge since the late Cretaceous. Although it is largely believed that strike‐slip movement along the Dauki Fault shifted the plateau about 250 km eastward, however, the driving mechanism for Shillong Plateau detachment and conversion of rift‐intercratonic basin to platformal configuration has drawn more attraction. This article focuses on the tectono‐stratigraphic evolution of Shillong Plateau and its adjoining regions on the basis of previous works, to try to understand the distinct tectonic and environmental factors. Early Permian continental rifting to the south‐west of the plateau and south‐east and south‐west of the Malda High presumably separated the Shillong Plateau from Chotanagpur Granite Gneiss Complex (CGGC). Extrusion of Rajmahal Traps west of the Malda High and Bengal‐Sylhet‐Mikir volcanism along an Early Cretaceous geofracture probably accelerated the east‐northeast migration of Shillong Plateau with respect to the CGGC. Since the Late Cretaceous, sedimentation at the southern fringe of Shillong Plateau commenced with an intracratonic basin which gradually transformed into a passive margin configuration during the Late Palaeocene‐Eocene Epoch during growth of the Indian Ocean. Differential subsidence in southern Shillong Plateau caused clastic sedimentation without considerable transportation. At the beginning, the Late Cretaceous intracratonic basin had received volcanoclastic, fluvio‐lacustrine sediments in a series of graben/half‐grabens associated with extensional faulting and volcanism, as represented by the Lower Mahadek and Jadukata formations in the plateau. The upper Mahadek and later Palaeogene sequences along the southern margin of the plateau record marine sedimentation, initiated with an estuarine‐marshy environment during the Late Cretaceous to Palaeocene, followed by a platformal marine succession since the Mid‐Eocene. The Calcutta‐Mymensingh Eocene Hinge Zone that separates a stable shelf from the deep basinal part might be traced through the south‐eastern fringe of Shillong Plateau and presumably continues further north‐east between the Naga and Disang thrusts in the eastern part of Dhansiri Valley.

Petrogenesis of a nepheline syenite from parts of the Chotanagpur Granite Gneissic Complex: implications for Neoproterozoic crustal extension in the East Indian Shield

AbstractThe North Purulia Shear Zone that dissects the granulite basement of the Chotanagpur Granite Gneissic Complex of the East Indian Shield exposes a deformed and metamorphosed nepheline syenite. The studied ‘foid-monzosyenite’ shows high abundances of large ion lithophile elements and high field strength elements with low abundances of compatible elements. Trace-element signatures show negative U, Th, Zr, Ti and Pb and positive Sr, Ba and Eu anomalies with respect to the primitive mantle. The chondrite-normalized diagram shows strongly fractionated rare earth element patterns ((La/Lu)N ∼23–87). Geochemical fingerprints suggest that the basanitic protolith was formed by low-degree partial melting of garnet peridotite in the sub-continental lithospheric mantle. The enriched large ion lithophile, high field strength element and light rare earth element concentrations (relative to primitive mantle) can be explained by a mixed mantle source with components from a previously deformed alkaline rock/carbonatite. Geochemical data do not support any significant crustal contamination and suggest variable fractionation of clinopyroxene, ilmenite, titanite and apatite from the parental melt. Petrological data are consistent with the view that the nepheline syenite magma was emplaced in a rift setting with a minimum temperature of 800–900°C, low fO2 conditions (below the fayalite–magnetite–quartz buffer) at a mid-crustal depth between 950 and 900 Ma. The continental rift zone, however, did not lead to the formation of an open ocean basin. Subsequently, the studied rock and its basement was deformed and metamorphosed in a continent–continent collisional setting at ∼900 Ma. Combining information from the other Indian occurrences with this study, it is demonstrated that the deformed alkaline rocks and carbonatite are potentially valuable for tracing the birth and demise of the palaeo-supercontinents.

Crustal architecture of North Singhbhum Mobile Belt, Eastern Indian Shield: Constraints from two‐dimensional and three‐dimensional modelling of Bouguer gravity and aeromagnetic data

The North Singhbhum Mobile Belt (NSMB) that lies in the Eastern Indian Shield between the Chotanagpur Granite‐Gneiss Complex (CGGC) and the Singhbhum Craton (SC) provides a unique opportunity to understand the Proterozoic crustal evolution of the Indian shield. In the present study, an integrated interpretation of petrophysical, Bouguer gravity, and aeromagnetic data was carried out to examine the crustal architecture and structural framework of the region. Three‐dimensional (3‐D) inversion of the aeromagnetic anomalies reveals that the magnetic basement depths range from 0.2 to 9.0 km in the region. The crustal models obtained from two‐dimensional (2‐D) and 3‐D forward modelling of potential field data reveal the Moho at 37–38 km depth beneath the NSMB and at a relatively deeper depth of 39–40 km and 40–44 km beneath SC and CGGC, respectively. Further, the distinct gravity high and the coinciding bipolar magnetic anomalies associated with the Dalma Formation can be attributed to the mafic/ultramafic intrusions. Gravity models across the Singhbhum Shear Zone (SSZ) and South Purulia Shear Zone (SPSZ) exhibit the crustal density inhomogeneity and Moho offset, typical of ancient suture zones of Precambrian terrains. In comparison, the absence of such crustal‐scale signatures across the North Purulia Shear Zone represents its intracratonic nature. Further, the Bouguer gravity and aeromagnetic signatures across the SPSZ, together with the crustal architecture and metamorphic ages, suggest that SPSZ marks the eastward continuity of the Gavilgarh‐Tan Shear Zone along which the amalgamation of northern and southern Indian cratonic blocks possibly took place at ca. 1.0–0.9 Ga.

Episodic Proterozoic magmatism in Northwest Bangladesh: Implications for Columbia/Nuna and Rodinia reconstructions

A diorite from drillhole GDH 53 at Voktipur in the Dinajpur Block in NW Bangladesh records an age of 1686 ± 4 Ma, whereas a tonalite from drillhole GDH 54 at Barapaharpur has an age of 1641 ± 7 Ma. Conversely, charnockite and monzogranite gneiss from drillhole GDH 31 at Gaibandha record considerably younger ages of 1144 ± 10 and 1133 ± 16 Ma, respectively. The diorite-tonalite-granodiorite suite is metaluminous, calc-alkaline, magnesian, with weak to moderate negative Eu anomalies and I-type characteristics. This suite is interpreted to have been derived from the melting of heterogeneous mafic crust, followed by a moderate degree of fractionation in an arc setting, identical to the source for similar rocks reported elsewhere in the basement of Bangladesh. The charnockite-monzogranite gneissic rocks from Gaibandha are ferroan, display weak negative Eu anomalies, belong to the high K calc-alkaline series, and plot as A-type granitoids (A2 subtype). It is considered that asthenospheric upwelling in an extensional/post-collisional setting provided the heat required to generate these rocks at the base of thinned crust. The geochemical data, lithostratigraphy and age of 1.73–1.64 Ga for the diorite-tonalite-granodiorite suite in NW Bangladesh do not match rocks of the Chotanagpur Granite Gneiss Complex and Central Indian Tectonic Zone to the west in India, negating their association with the collision that united the North and South Indian blocks. However, the Shillong-Meghalaya Gneissic Complex in NE India, with ~1.78–1.62 Ga granite gneiss and 1.28–1.08 Ga charnockite, provides a closer analogue. Events at these two times have also been recorded within the east Gondwana fragments in the Bunger and Obruchev Hills of East Antarctica. The similarities with both the Shillong-Meghalaya Gneissic Complex, lying to the east in India, and with East Antarctica, suggest that Proterozoic magmatic events in the Dinajpur Block of NW Bangladesh relate to events in both the Columbia/Nuna and Rodinia supercontinents and support a connection with these supercontinents throughout the Proterozoic.

PETROGENETIC MODELING OF THE GRANITIC ROCKS IN AND AROUND LODHMA, DIST RANCHI, JHARKHAND, INDIA

The Lodhma area, in the Ranchi district, constitutes the central segment of Chotanagpur Granite Gneissic Complex ( CGGC) and falls in the northeastern segment of Survey of India toposheet number 73E/4. The CGGC is mainly composed of variants of granitic rocks, amphibolites and metasedimentaries. The granitic rocks find good exposure in the area. They comprise predominantly of granite gneisses, crystalline massive granites and migmatites showing noticeable variations in mineralogy, texture, structure and associations. The regional strike of the lithounits of the area show almost E-W trend. However ambient structural controls have brought about local changes in the strike direction. The area has witnessed at least three phases of deformations which is reflected by three generations of folds, which are generally isoclinals. Petrogenetic characterization of granitic rocks and its variants in the area has been attempted using various geochemical tools involving major/ trace element studies and Niggli values. S –type parentage of these Granites have been established on the basis of distribution behavior of various chemical components present in it, thereby indicating role of pelitic schists in the generation of anatectic melt, which ultimately crystallized into granites. S-type lineage of these granites has also been further cemented with the help of several well established binary and ternary variation diagrams

Mafic rocks with back-arc E-MORB affinity from the Chotanagpur Granite Gneiss Complex of India: relicts of a Proterozoic Ophiolite suite

AbstractThe Proterozoic Chotanagpur Granite Gneiss Complex (CGGC) at the northern boundary of the Central Indian Tectonic Zone (CITZ) of the eastern Indian shield preserves relics of fossilized oceanic back-arc crust. We describe the field, petrographical and geochemical characteristics of the mafic rocks comprising pillow basalts and dolerites from the Bathani area of the northern fringe of the CGGC, eastern India. The basalts consist of plagioclase feldspar, hornblende, opaque minerals (Fe–Ti oxide) and chlorite, and the dolerite consists of plagioclase, hornblende and opaque minerals. Our data indicate that the Bathani mafic rocks have tholeiitic to transitional composition and are overprinted by greenschist facies metamorphic conditions; however, REE and fluid immobile elements preserve their primary geochemical signatures. The (La/Sm)N ratios (1.38–2.15) and chondrite-normalized REE patterns point to an enriched mid-ocean ridge basalt (E-MORB) mantle source. Geochemical characteristics indicate a mixed signature of MORB and arc tholeiite with enrichment of Ba, Th, Eu and Sr, similar to that of back-arc supra-subduction zone ophiolites. These mafic rocks are the product of MORB-like magma derived from a depleted mantle corresponding to &lt; 2% partial melting of spinel lherzolite, enriched by subduction-induced slab metasomatism and melting. The Bathani mafic rocks are representative of the upper part of a supra-subduction zone columnar ophiolite section, which was emplaced onto the present-day northern margin of the CGGC during suturing of the northern and southern Indian block at c. 1.9 Ga during the Nuna amalgamation.