Madhuparna Roy

Geochronological (Rb-Sr and Sm-Nd) studies on intrusive gabbros and dolerite dykes from parts of Northern and Central Indian cratons: Implications for the age of onset of sedimentation in Bijawar and Chattisgarh basins and uranium mineralisation

The Dargawan gabbros intrusive into the Moli Subgroup of Bijawar Group, yielded Rb-Sr whole rock isochron age of 1967 ± 140 Ma. Based on the oldest age from overlying Lower Vindhyan (1.6Ga) and the underlying youngest basement ages (2.2 Ga), the time range of Bijawar sedimentation may be assigned as 2.1–1.6 Ga (Paleoproterozoic). Sm-Nd Model ages (TDM), obtained, for Dargawan gabbros, is c. 2876–3145 Ma. High initial 87Sr/ 86Sr ratio of 0.70451 (higher than the contemporary mantle) and negative ɛNdi (at 1.9 Ga) value of −1.5 to − 4.5, indicate assimilation of Archaean lower crustal component by the enriched mantle source magma at the time of gabbroic intrusion. The dolerite, from Damdama area, which is intrusive into the basement and overlying sediments of Chandrapur Group in the central Indian craton, yielded Rb-Sr internal isochron age of 1641 ± 120 Ma. The high initial 87Sr/86Sr ratio of 0.7098 and ɛNdi value of −3.5 to −3.7 (at 1.6 Ga) is due to contamination of the mantle source magma with the overlying sediments. These dolerites have younger Sm-Nd Model ages (TDM) than Dargawan gabbros as c. 2462–2675 Ma, which is similar to the age of the Sambalpur granite, from which probably sediments to this part of Chattisgarh basin are derived. Hence mixing of sediments with the Damdama dyke during its emplacement, gives rise to high initial 87Sr/86Sr and low initial 143Nd/144 ratios for these dykes. The c. 1600 Ma age indicates minimum age of onset of the sedimentation in the Chandrapur Group of Chattisgarh basin. Both the above mafic intrusions might have taken place in an intracratonic rift related (anorogenic) tectonic setting. This study is the first reliable age report on the onset of sedimentation in the Chandrapur Group. The total minimum time span of Chandrapur and Raipur Group may be 1.6 Ga to 1.0 Ga (Mesoproterozoic). The unconformably underlying Shingora Group of rocks of Chhattisgarh Supergroup thus indicates Paleoproterozoic age (older than 1.6 Ga). Most part of the recently classified Chattisgarh Supergroup and Bijawar-Vindhyan sequence are of Mesoproterozoic-Paleoproterozoic age and not of Neoproterozoic-Mesoproterozoic age as considered earlier. Petrographic study of basic dykes from Damdama area (eastern margin of Chattisgarh Supergroup) indicated presence of primary uranium mineral brannerite associated with goethite. This is the evidence of mafic intrusive providing geotherm and helping in scavenging the uranium from the surrounding and later alterations causing remobilisation and reconcentration of pre-existing uranium in host rocks as well as in mafic dyke itself otherwise mafic rocks are poor source of uranium and can not have primary uranium minerals initially. It can be concluded that mafic dykes have role in uranium mineralisation although indirectly.

Petrogenetic Model of A-Type Granitoids of the Kullampatti Area, Salem District, Tamil Nadu, India

Abstract The late Proterozoic granitoids at Kullampatti are of A-type and comprise three phases: (1) barren phase I (granite-adamellite), (2) fertile phase II (trondjhemite) and (3) rarely-mineralized phase III (pegmatitic granite). They were emplaced within the Archean cratonic granulite terrane of southern India, close to the Moyar-Bhavani-Attur shear zone, and form a consanguinous association with the Suriyamalai or Sankari granite-adamellite batholithic dome. On the basis of their field, tectonic, petromineragraphic and geochemical aspects, a petrogenetic model involving a two-stage melting of an undepleted middle to lower crustal granulitic protolith is envisaged. In this, the first-stage melting led to the formation of an LCT-rich hydrous melt that was removed from the site of its generation, followed by second-stage partial melting of the depleted granulite during post-orogenic or anorogenic environment, resulting in the HCD- and HFS-rich, NYF- and A-type melt that on fractional crystallization, together with hydrothermal activity, led to the formation of these granitoids.

Petrological evidences of impact-induced shock metamorphism in the basement granitoids and rhyolitic melt breccia of Mohar area, Shivpuri district, Madhya Pradesh

The circular structure at Mohar (Dhala structure) in the western part of Bundelkhand Gneissic Complex, is marked by a prominent outlier of Kaimur sediments surrounded by low lying concentric sequence of sediments of Dhala Formation and basement granite breccia. This has been interpreted as a volcanic eruption related cauldron structure and meteoritic impact crater structure by various authors, on the basis of absence or presence of shock indicators in the clasts of a rhyolite-like rock that crops out scantily in the north western part of the structure. During the course of extensive sub-surface uranium exploration in this structure, the geoscientists of Atomic Minerals Directorate for Exploration and Research observed unequivocal and rampant evidences of shock metamorphic features for the first time in drill core samples of basement granitoids which constitute the bed rock for the rhyolite-like melt breccia, which overlies it. Published data of shock metamorphic features from this area are largely confined to the surface samples of the rhyolite-like melt rock, exposed in sparse outcrops. The shock metamorphic features recorded in the sub-surface granitoid bed rock samples during the present study, comprise planar deformation features (PDF) in quartz, feldspar, apatite and zircon, toasted, diaplectic, ladder-textured feldspars, selectively shock-melted feldspars and melt-veined quartz. The shock metamorphic features recorded in surface and sub-surface samples of the melt rock include ballen quartz, PDF in quartz clasts, toasted and diaplectic feldspar clasts shocked basic rock fragments with isotropised feldspars. Both the shocked bedrock granitoid and the melt rock bear uncharacteristic geochemical signatures with elevated K2O, MgO and depleted CaO. The study also observes that the melt breccia overlying the granitoid bedrock also occurs as pocket-like patches at various depths within the granitoids. Thus, the present findings have helped in understanding the attributes of the basement granitoid and associated melt breccia, thereby linking the genesis of the latter by selective melting of the former, due to the process of impact. It reinforces the already propounded theory of impact as the likely cause for the development of the structure in the basement Bundelkhand granitoid that was later filled by sediments standing out presently as a mesa.

A Petrological and Geochemical Account of Subsurface Noritic Intrusion in the Western Part of Bundelkhand Massif, Shivpuri District, M.P.

Subsurface exploration for uranium in the northwestern part of Bundelkhand massif, near Khor area, Shivpuri dist., M.P., resulted in intercepting a substantial thickness of mafic rock within Bundelkhand granitoid. Intercepts of this mafic rock at various levels in the boreholes, indicate that the rock mainly occurs as dykelike intrusion and fracture-fills within Bundelkhand granite. It is essentially composed of hypersthene and plagioclase, with or without olivine, leading to the characterisation as hypersthene microdolerite, noritic dolerite and norite (±olivine), depending on the grainsize and variation from intergranular to ophitic texture. Chemically, the rock is characterised by av. 49.09% SiO2, 2.46%TiO2, 2.33 Fe2O3, 9.45% FeO, 5.75% MgO, 8.37% CaO and 0.96% K2O. The normative composition ranges from 3.53% quartz, 46.86% plagioclase, 12.58% diopside, 19.24% hypersthene. The olivine normative samples show av. 5.65% olivine. Geochemical plots indicate an intra-plate affinity along with oceanic signature, while presence of mineralogical and normative olivine, together with the REE pattern, point towards a lower crustal or mantle source. The mineralogical and normative presence of either quartz or olivine in these mafic rocks implies that it has an intermediate character between the tholeiitic dolerite dykes and the komatiite-type ultramafics reported from Bundelkhand craton. The complex geochemical signature of the rocks also reveals that both intra-continental as well as a mixture of oceanic- to upper mantle signatures are evident. The present study is a first time report of the occurrence of this hitherto unknown noritic body at depth within the Bundelkhand granite, which has no visible surface expression. The findings may strengthen the existing concept of a continuum between Rajasthan craton in the west and Bundelkhand craton in the east, as a single protocontinent.

Hydrothermal fracture controlled vein type uranium mineralization in the Paleoproterozoic Bijawar Group of rocks, Sonrai basin, Lalitpur district, U.P. – Fresh findings from subsurface borehole data

Sonrai basin, located along the southwestern margin of Bundelkhand Granite-Gneiss Complex (BGC) is known for its base metal and phosphorite mineralization. Uranium mineralization of appreciable grade and thickness was reported in Pisnari block in the northern part of this basin during sub-surface exploration activity in 1976-97. This was in the form of uranium-bitumin association within fractures in the carbonate-terrigenous sediments of Gorakalan shale, Rohini carbonate and Bandai sandstone of Sonrai Formation of Bijawar Group. Similar mineralization was also noted within the lower chloritic shale member of Solda Formation of the same Group. During subsequent drilling activity at a later phase (2005-09), uranium rich zones within Rohini carbonate and chloritic shale members were corroborated by the interception of mineralized bands in two boreholes drilled near Pisnari. Petrographic study of radioactive core samples reveal that uranium mineralization is closely associated with globular carbonaceous matter and sulphides along the fractures. Uranium is either adsorbed in carbonaceous matter or occurs as ultrafine inclusions of coffinite within carbonaceous matter. An additional phase of secondary uranium mineral (uranophane) is also noted in one of the boreholes. The uranium minerals, in association with sulphides, fill up moderately dipping fractures (approximately 15o towards North) which are oblique to the core axis of the inclined borehole and thus, the mineralization is characterized as hydrothermal fracture-controlled vein type mineralisation. C-HN- S analysis of carbonaceous matter occurring in close association with the uranium minerals reveal heterogeneity in composition with respect to carbon and sulfur. The present paper aims at discussing the geological, petrological and radiometric aspects of this mineralization, so as to enhance the understanding of the same.

Structural and luminescent characterisation of uraniferous fluorapatite and haematite associated with phosphatic rocks of the Bijawar group in Sagar District, Madhya Pradesh (India)

The structural and spectroscopic characteristics of phosphatic ferruginous shale samples from the Bijawar Group rocks from Sagar District of Madhya Pradesh (India) have been probed for identification of uranium species. Fluorapatite (

Remnants of supracrustal rocks around Mohar brecciated granite complex, Shivpuri district, Madhya Pradesh: A petrological and geochemical appraisal

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Uranium Mineralisation in the South-Western Part of Cuddapah Basin: A Petromineralogical and Geochemical Study

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