Granite Gneiss Complex Research Articles

Unravelling Source Area Composition for the Paleoproterozoic Basal Conglomerate (Vindhyan Supergroup) around Dalla, Sonbhadra District, Central India

Abstract The present investigation reveals that the Basal Conglomerate is ~ 2-3 m thick forming basal part of the Vindhyan Supergroup in the Sonbhadra district, central India. The Basal Conglomerate consists of fragments of different rock types supported by the sandy matrix. It contains pebble-sized clasts of quartzite, jasper, phyllite and slate dominated by the quartzite clasts.The matrix is composed of sandy material that is rich in quartz, chert and iron oxides. The occurrence of quartzite clasts suggests its derivation from the Chhotanagpur Granite Gneiss Complex (CGGC) and the phyllite, slate and jasper suggest their derivation from the Mahakoshal Group. The presence of polycrystalline and monocrystalline quartz suggests their derivation from Chhotanagpur Granite Gneiss Complex and the presence of chert suggests its derivation from the Mahakoshal Group. The iron oxides were produced as a result of leaching of iron during chemical weathering. The X-ray diffractometry of the matrix suggest ubiquitous presence of quarz, muscovite and kaolinite. The kaolinite has formed due to alteration of feldspars under warm and humid conditions. This is also supported by the presence of ferruginous matter in the matrix.

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.

Phase equilibria modelling and geochemistry of high‐grade gneiss from the Chhotanagpur Granite Gneiss Complex, eastern India: Implications for tectono‐metamorphic evolution

We investigated high-grade gneiss from the Daltonganj region of the Chhotanagpur Granite Gneiss Complex (CGGC) in eastern India. EPMA monazite dating from the Daltonganj region revealed three age domains at ca. 1434, ca. 978, and ca. 850 Ma, which have been interpreted as the protolithic age and two metamorphic stages of high-grade gneisses, respectively. The Grt1 + Chl + Amp1 + Bt assemblages are thought to represent the first metamorphism as a pre-peak stage of high-grade gneiss, which could have occurred after ∼1434 Ma. The peak metamorphic stage is defined by relict porphyroblasts of orthopyroxene and garnet2 formed during ∼978 Ma in a burial process followed by an isothermal decompression or post-peak stage represented by a Crd + Grt3 + Amp3 assemblage at ∼850 Ma. The P-T conditions estimated for the pre-peak, peak, and post-peak stages using the NCKFMASH model system are 6.6-6.8 kbar/635-645°C, 8.65-9.42 kbar/772-788°C, and 5.71-6.18 kbar/745-762°C, respectively. The geochronology and metamorphic conditions define a clockwise P-T-t path for the high-grade gneiss, indicating that rocks experienced high-pressure conditions during burial metamorphism at subduction-related tectonic activity, followed by an isothermal decompression condition after the gneissic unit was exhumed. The geochemical composition of the high-grade gneiss reveals that the protolith is calc-alkaline basaltic magma formed in an island arc setting and that the restitic features of the gneisses developed after granitic melt segregation.

Occurrences of Rare Earth Element (REE) Bearing Minerals in Migmatitic Gneiss and Granitoids of Chhotanagpur Granite Gneissic Complex, Bihar, India

Abstract This paper presents the geochemical characteristics, petrogenetic processes and source characteristics of Rare Earth Elements (REE) bearing migmatitic gneiss and granitoids near Tangeshwar village, Banka district, Bihar, in order to demarcate the potential zone of REE mineralization. These migmatitic gneiss and granitoids belongs to the Chhotanagpur granite gneissic complex (CGGC). The study of migmatitic gneiss and granitoids of CGGC suggest that they have high content of SiO2, low contents of Fe2O3 and MgO and SiO2 vs. Na2O and K2O shows positive correlation which suggest, they are dominated by sodic and alkali rich minerals and were derived from a felsic (granitic) source. Also LREE-LILE enrichment, HFSE depletion and calc-alkaline nature indicates that the magmatism is related to the subduction zone. Petrographic and EPMA studies of these migmatitic gneiss and granitoids confirms the presence of REE bearing mineral phases such as allanite, xenotime, monazite, and basnasite. They are generally present at the rim of biotite, as an inclusion in biotite and also along the grain boundaries and within the cracks of major silicate minerals. This suggest that the REE formation takes place at the deuteric stage of magma crystallisation followed by the formation of granite. Chemical analysis of bed rock samples for rare earth elements using ICPMS shows that the highest concentration is in migmatitic gneiss and granitoids, which varies from 47.85- 925.67 ppm and 47.85- 626.48 ppm respectively.

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.

On the origin of shear-band network patterns in ductile shear zones

Ductile yielding of rocks and similar solids localize shear zones, which often show complex internal structures due to the networking of their secondary shear bands. Combining observations from naturally deformed rocks and numerical modelling, this study addresses the following crucial question: What dictates the internal shear bands to network during the evolution of an initially homogeneous ductile shear zone? Natural shear zones, observed in the Chotonagpur Granite Gneiss Complex of the Precambrian craton of Eastern India, show characteristic patterns of their internal shear band structures, classified broadly into three categories: type I (network of antithetic low-angle Riedel (R) and synthetic P-bands), type II (network of shear-parallel C and P/R bands) and type III (distributed shear domains containing isolated undeformed masses). Considering strain-softening rheology, our two-dimensional viscoplastic models reproduce these three types, allowing us to predict the condition of shear band growth with a specific network pattern as a function of the following parameters: normalized shear zone thickness, bulk shear rate and bulk viscosity. This study suggests that complex anastomosing shear-band structures can evolve under simple shear kinematics in the absence of any pure shear component.

Reappraisal of 75 Years of Exploration for Atomic Minerals in India and the Way Forward

Abstract In India, Atomic Minerals Directorate for Exploration and Research (AMD), the oldest unit under the Department of Atomic Energy (DAE) is mandated with the exploration and augmentation of atomic mineral(s) resources of the country to support the Nuclear Power Programme (NPP) of India. AMD, till date have established four (04) generations of uranium metallogeny in India, distributed in 46 deposits and several other occurrences, major share of which are contributed by the globally unique, stratabound carbonate hosted, syn-diagenetic Tummalapalle Group of deposit (~59%), metamorphite type deposits along the Singhbhum Shear Zone (~21%), unconformity related deposits in northern part of Cuddapah Basin (~5%) and sandstone type deposits in Cretaceous Mahadek Basin (~6%). Besides these, metasomatite type deposits along the North Delhi Fold Belt (4%) and granite related-structurally controlled, high grade deposits in Bhima Basin (2%) along with some small hydrothermal/metamorphite/migmatite type deposits in Aravalli, Kotri-Dongargarh, Higher Himalaya belts and Chhotanagpur Granite Gneiss Complex contribute to the national inventory which stands at 3,76,000 tonne uranium oxide. AMD have stockpiled substantial quantities of Nb-Ta, Be and Li minerals needed for the NPP of India from prospective pegmatite belts in the country. AMD have established immense potential for REE mineralization in carbonatite complexes of Ambadungar in Gujarat and per-alkaline granite-rhyolite of Siwana Ring Complex, Barmer district, Rajasthan. Exploration along the coastal and riverine placers of the country has established 130 heavy mineral placer deposits with substantial thorium and REE resources. The progressive advancements by AMD through adaptation of integrated multidisciplinary exploration techniques coupled with major thrust in airborne/ground geophysics and expansion of state-of-the-art analytical facilities have facilitated systematic planning for future augmentation of atomic mineral(s) resources. Exploration inputs are envisaged to be intensified in the brownfield target areas for resource augmentation while R&D and phase-wise exploration inputs will be focused for developing the identified greenfield areas following a planned roadmap to ensure selfsufficiency in atomic mineral resources for sustained growth of the NPP to reach the target of net zero carbon emissions by 2070.

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.

Petrogenesis and geotectonic setting of Berguda granitoids of Neoproterozoic Burji-Finchawa granite-gneiss complex, southern Ethiopia: implication for the cessation of subduction control magmatism in East African Orogeny

Petrogenesis and geotectonic setting of Berguda granitoids of Neoproterozoic Burji-Finchawa granite-gneiss complex, southern Ethiopia: implication for the cessation of subduction control magmatism in East African Orogeny

Petrogenetic and geochemical behavior of the Neoproterozoic low-grade metamorphic rocks from Ropi Megada area in Bule Hora Belt, Southern Ethiopia

The granite gneiss complexes and mafic-ultramafic-sedimentary ophiolitic belts are mostly exposed in southern Ethiopia, but the Bule Hora ophiolitic belt is not well studied yet. For the first time, this paper reveals the petrogenesis and geochemical nature of the rocks from the Ropi Megda area in the Bule Hora ophiolitic belt, southern Ethiopia, through ore rock geochemical and field studies. Major oxides and trace elements were analyzed using ICP-MS and ICP-AES to understand the geochemical trends, petrogenesis and tectonic settings of the Ropi Megada. The SiO2 and TiO2 content in the hornblende-biotite-quartz-plagioclase schist range from 35 to 50 wt % and 0.07–6.38 wt %, respectively. Among the major oxides, Fe2O3, MgO, CaO show negative trends with silica content. Similarly, some trace elements like V and Sr show negative trends with SiO2. The rocks are LREE-enriched and HREE-depleted [(La/Yb)N = 0.64–12.77], with a positive Eu anomaly [(Eu/Eu*)N = 0.56–2.32]. Hornblende-biotite-quartz-plagioclase schist has higher gold content (0.001–0.489 ppm) than the biotite-quartz-feldspathic gneiss (0.001–0.003 ppm). Based on the geochemical and field data, the brittle-ductile deformations and hydrothermal fluid reactions with the ophiolitic rocks might have been the primary agents for the gold leaching and transportation from the source to the deposit in the sheared and fractured zone within the host rocks. Hornblende-biotite-quartz-plagioclase schist and quartz-hornblende-biotite schist geochemical data studies from the Ropi Megada area, supposedly associated with the upper mantle composition, show tholeiitic, calc-alkaline, metaluminous, and boninite affinity that formed in the island arc and mid-oceanic ridge basalt tectonic settings.

Soil-Landform Relationship to Understand Quantitative Pedogenesis under Varying Physiographic Catena of Chhotanagpur Plateau Regions of India

Pedogenic indices were considered to understand maturity of soils in different geomorphic units in quantitative terms. High redness rating and low sand/silt and silt/clay ratio at higher elevations of Rajmahal trap, Hazaribagh plateau and Ranchi plateau indicates rapid pedogenic processes and formation of Paleo-Alfisols (Rhodustalfs and Paleustalfs). High CEC/Clay in lower elevations of Rajmahal trap and Hazaribagh plateau indicates formation of more 2: 1 type of interstratified minerals. Predominant occurrence of amorphous iron (Fe) in soils of Rajmahal basalts was indicated by high ammonium oxalate extractable Fe. Higher proportions of dithionite extractable Fe of clay in gullied lands, uplands of Hazaribagh plateau and hillocks of Ranchi plateau indicate the prevalence of crystalline Fe in Chhotanagpur granite gneissic complex. More crystalline nature of soil clays at higher elevation of Ranchi and Hazaribagh plateaus are noted as indicator of laterization processes. Soils on pediments, uplands and plateaus comprise well matured profiles with formation of paleosols of relict types, whereas that of gullied lands and hillock were least weathered. Undulating plains, old alluvial plains and low-lying plains consists of soils under intermediate stages of weathering with intermediate stages of soil forming processes. These soils (P3, P4, P11 and P12) appear to support agricultural land use comparatively better than other soils despite topographic undulation and soil physicochemical constraints in the region. Presence of considerable amount of interstratified type of 2:1 minerals at lower elevations may attribute to restore better organic carbon sequestering capacity supporting long-term agricultural practices in the region. The present endeavour deals with study of quantitative pedogenesis as a tool to understand soil-landform relationship in varying physiographic catena of Chhotanagpur plateau regions of India.

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.

Metamorphic evolution of the pelitic and mafic granulites from Daltonganj, Chhotanagpur Granite Gneiss Complex, India: Constraints from zircon U–Pb age and phase equilibria modelling

The Daltonganj region is located on the northwestern extension of the Chhotanagpur Granite Gneiss Complex in the eastern Indian Peninsula, which is characterized by pelitic and mafic assemblages of granulite facies rock. The pelitic granulites contain garnet, cordierite, biotite, plagioclase, K‐feldspar, sillimanite, and quartz. Petrographical interpretations divulge prograde and retrograde metamorphic events within mafic granulites, which consist of clinopyroxene, orthopyroxene, amphibole, plagioclase, biotite, and quartz. Field observation, petrography, phase equilibrium modelling, and U–Pb zircon geochronology of the pelitic granulites reveal two stages of metamorphic events along the clockwise P–T path. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICP‐MS) zircon U–Pb age dating of pelitic granulites shows the detrital zircon ages from ~1,734 Ma to 1,677 Ma, and the possible metamorphic domains show the weighted mean age of 1,638 ± 22 Ma, which represents the timing of metamorphism. Subsequently, the magmatic emplacement of mafic granulites was recorded at 1,629 ± 6 Ma age by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICP‐MS) zircon U–Pb dating which coincide with the timing of metamorphism of the pelitic granulite. Phase equilibrium modelling in the NCKFMASHTO system divulges the pre‐peak metamorphic stage at ~3.2 kbar and ~620°C and the first stage which is characterized by a peak metamorphic condition that ranges from 7.40 to 9.10 kbar and from 815 to 835°C during ~1,638 Ma with the mineral assemblage of garnet + sillimanite + biotite + plagioclase + K‐feldspar + melt + quartz + ilmenite. Sequentially, the retrograde metamorphism is observed by a nearly isothermal decompression stage at ~4.0 kbar/~790°C after 1,638 Ma, due to the appearance of garnet + cordierite + biotite + plagioclase + K‐feldspar + melt + quartz + ilmenite + magnetite. The formation of cordierite is due to the decompression and dehydration melting phases. The Nb, Sr, and Ti negative anomalies suggest their generation from crustal sources. The geochemical analyses constrain that the sedimentation of pelitic sediments was recorded along the convergent margin and encountered by a subduction‐related tectonic setting. The geochemical interpretation provides significant evidence that the protoliths of pelitic granulites were derived from the Singhbhum Mobile Belt, Mahakoshal Supracrustal Belt, and Bundelkhand Craton, while their metamorphism was processed by the continent–continent collision and followed by exhumation.

Geomorphic assessment of longitudinal profile of the Kanchi river basin, India: Recognition of dynamic equilibrium condition

ABSTRACT The Kanchi, an eastward flowing river, traverses through the Ranchi plateau of the Proterozoic Chhotanagpur granite gneissic complex to meet its trunk stream, the Subarnarekha River. The present study analyzed longitudinal profiles and associated geomorphic indices of the Kanchi River and its tributaries to identify spatial variation in profile form and fluvial response to prevailing controls. This work was carried out using a digital dataset and maps, which were processed in a GIS environment to obtain longitudinal profiles, and different data required in best-fit modelling and geomorphic investigation of longitudinal profiles. The analyses indicate that the rivers are in dynamic equilibrium condition. Spatial distribution of geomorphic indices values reveals that longitudinal profile forms vary from one segment to another along river channels, and these irregularities in the form of prominent and minor knickpoints are highly concentrated in middle river basin areas. The absence of lithological variation and active tectonics, and close association of anomalously steep river segments with structural lineaments and landforms demonstrates structural control on river channel development. PCA denotes strong similarities between geomorphic indices. This understanding of longitudinal variation in river nature may help to formulate strategies for the sustainable development and management of river related resources.

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

Nature and timing of anatectic event of the Hida Belt (Japan): Constraints from titanite geochemistry and U-Pb age of clinopyroxene-bearing leucogranite

The Hida Belt, central Japan, is a continental fragment, which was once a part of the crustal basement of the East Asian continental margin. It consists mainly of Permo-Triassic granite-gneiss complexes with both syn-to-late-metamorphic migmatite or granite bodies. Clinopyroxene-bearing leucogranite, locally called as ‘Inishi’-type migmatite, occurs characteristically in the migmatite zone associated with amphibolite and marble. The leucogranite is characterized by the presence of coarse-grained diopside–hedenbergite series clinopyroxene and titanite in plagioclase-dominated matrix. Clinopyroxene contains abundant calcite inclusions. Euhedral titanite with high Th/U ratios (~2.8–7.8) and REE contents (~4514–14,069 μg/g) contains polycrystalline ‘granitic’ inclusions. Those mineralogical features indicate the involvement of carbonate during anatexis. Considering a nominal pressure of 0.4–0.7 GPa of the Hida gneiss, Zr-in-titanite thermometry yields a temperature of 730–810 °C and 770–850 °C at aTiO2 = 0.5 and 1, respectively. The titanites show highly variable U/Pb (238U/206Pb = 15.0–24.0) and Pb (207Pb/206Pb = 0.172–0.419) isotope ratios, and the scattered trend follows a discordia line with a lower intercept at 225.4 ± 1.9 Ma. This titanite age would represent the timing of regional anatexis that have occurred in a later stage of the regional metamorphism of the Hida Belt. A high apparent thermal gradient at middle crustal levels might have been caused by regional extension.

Magnetic characteristics of a part of Closepet Granite, Eastern Dharwar Craton

The exposed granite gneissic complex to the west of Cuddapah basin in Eastern Dharwar Craton (EDC) is studied to understand the magnetic character of various plutons in this region with more emphasis on a part of Closepet Granite (CG). The NW–SE trending plutons in this region show gradual decrease in magnetic intensities from southwest to northeast; CG in the southwest corner of the study area have high mafic content, and there onwards show a depletion in mantle enrichment towards NE. Parts of the CG namely, the main mass and northern intrusions separated by a gap zone are differentiated based on magnetic character deduced by processing the aeromagnetic data. Reduced to the pole (RTP) and its upward continued maps revealed NNW trending eastward convex anomaly highs inferring the magnetic nature of CG due to Porphyritic Monzogranite (PMG). Gap zone is identified to be structurally significant, bounded by deep ENE faults and inferred to be associated with PMG at relatively deeper levels. 2.5D modelling of magnetic anomaly profile substantiated our qualitative analysis of magnetic anomalies over CG, inferring magnetic and non-magnetic units of CG. Known shear zones running north–south are identified with a few inferred shears revealed from magnetic data interpretation within CG.

Exsolution intergrowth of cpx-opx and pseudosection modelling of two-pyroxene mafic granulite from Daltonganj of Chhotanagpur Granite Gneiss Complex, Eastern India

In this study, we reconstruct for the first time the evolution of a two-pyroxene mafic granulite from the Daltonganj of Chhotanagpur Granite Gneiss Complex (CGGC). Transmission electron microscopy (TEM) revealed that some of the clinopyroxene have exsolution texture, where orthopyroxene occurs as thin lamellae within the porphyroblast of clinopyroxene. To tightly constrain the P-T conditions at which exsolution lamellae developed after the metamorphic peak, we have applied both conventional and multi-equilibrium thermobarometry, as well as forward thermodynamic modelling. Results from multi-equilibrium thermobarometry, using the software THERMOCALC, suggest peak conditions of the mafic granulite at average pressure-temperature (PTav) conditions of 6.7 ± 1.19 kbar/814 ± 60 °C. In contrast, exsolution bearing opx-cpx minerals crystallised at relatively lower temperature (772 ± 14 °C), determined by the conventional geothermometers. The peak to retrograde evolution of these mafic granulites is constrained through phase equilibrium modelling in the NCKFMASHTO (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O) model system using the software Perple_X. Phase equilibria results of peak conditions (i.e. 6.0–6.78 kbar and 775–808 °C) are consistent with respect to those obtained through multi-equilibrium and conventional thermobarometry, while the retrograde path is defined down to ~4.5 kbar and ~540 °C. Our results have twofold implications: (i) they show how the integrating of different geothermobarometric methods is the best proxy to tightly constrain the evolution of high-grade metamorphic rocks, and (ii) they pavement to new constraints on the Paleoproterozoic to Neoproterozoic evolution of the CGGC.