• A K Bhatt

      Articles written in Journal of Earth System Science

    • Mineral chemistry of tourmaline from Mashak Pahar, South Purulia Shear Zone (SPSZ), eastern Indian Shield

      Santanu Acharjee Jyotisankar Ray Payel Dey Debapriya Bhattacharyya Mousumi Banerjee Basab Chattopadhyay Shyamal Sengupta A K Bhatt D Chowdhury A K Dwivedi Sanjoy Mahato Arka Ranjan Jana P B Maithani P V Ramesh Babu

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      The area of investigation at and around Mashak Pahar, Bankura district, West Bengal, India comprises a number of rock types namely: granite gneiss, migmatized quartz tourmaline gneiss, quartz pebbleconglomerate, ferruginous quartzite, quartz tourmaline veins (as veins) and graphite schists. Interestingly, the study area lies in the region extending South Purulia Shear Zone (∼Tamar–Porapahar Shear Zone) which marks the boundary between two contrasting tectonic blocks of eastern India, namely, the Chhotanagpur Gneissic Terrane (CGC) to the north and Singhbhum Group of rocks to the south. The rocks of the study area are poly-phasedly deformed by three phases of folding, namely, F1, F2 and F3. All the tourmalines are classified to be of ‘Alkali Group’. Chemistry of tourmalines from migmatized quartz tourmaline gneiss and those from quartz tourmaline veins are in conformity with their relation to (earthquake induced) shear system evolution in this terrain. In general, the compositional evolutionof tourmaline during prograde metamorphism (∼400°–730°C) has been supported by both petrographic and chemical evidences. Assessment of mineral–chemical data of constituent tourmaline grains clearly suggests compositional variations across zonal boundaries within tourmaline that was controlled by changing metamorphic milieu in this terrane. Field and petrographic evidences clearly indicate activation of earlier and later shears in this region accompanied by infiltration of boron and formation of zoned tourmaline crystals.

    • Crystallochemical studies on davidite from Bichun, Jaipur District, Rajasthan, India

      Yamuna Singh Anubhooti Saxena A K Bhatt R Viswanathan T S Shaji L K Nanda

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      Crystallochemical data on metamict davidite from albitites and albitised rocks from the Bichun area (Jaipur district, Rajasthan, India) of Banded Gneissic Complex (BGC) are provided. Davidite occurs as euhedral, subhedral to anhedral crystals in the form of disseminated grains and also as fracture filled veins. The crystals of davidite are up to 8 cm in length and 6 cm in width. The powder X-ray diffraction (XRD) pattern of the heat-treated davidite (at 900◦C) reveals well-defined reflections of crystallographic planes. The calculated unit-cell parameters of the heat treated davidite are: a0 = b0 = 10.3556˚A and c0 = 20.9067˚A, with unit-cell volume (V) = 1941.6385˚A³; and α = β = 90◦ and γ = 120◦, which are in agreement with the values of davidite standard. Geochemical data reveals that the investigated davidite contains 51.5–52.6% TiO₂, 14.8–15.1% Fe₂O₃, 9.8–10.2% FeO, 6.97–7.12% U₃O₈, 6.72–6.92% RE₂O₃, 3.85–3.61% K₂O, 0.9–1.4% Al₂O₃, and 0.8–1.2% SiO₂. The calculated structural formulae of the two davidite crystals are: D-1: K0.0044/0.004Ba0.0044/0.005Ca0.20/0.20Na0.012/0.012Mn0.053/0.053Mg0.14/0.14Pb0.0076/0.008Fe2.675/2.675Fe1.59/1.59Y0.1175/0.118P0.053/0.053Nb0.008/0.008Sn0.001/0.001Zr0.033/0.033U0.468/0.468Th0.009/0.009REE0.6829/0.683)6.05/6.05(Ti12.15/12.15 Fe1.9022/1.903Si0.372/0.372 Al0.517/0.517 Cr0.018/0.018Co0.009/0.009Ni0.027/0.027)15/15O36/36(OH0.319/0.319[]1.681/1.681)2/2 and D-2: (K0.004/0.004Ba0.005/0.005Ca0.20/0.20Na0.012/0.012Mn0.05/0.05Mg0.094/0.094Pb0.007/0.007Fe2.58/2.58Fe1.71/1.71Y0.112/0.112P0.106/0.106Nb0.006/0.006Sn0.001/0.001Zr0.03/0.03U0.48/0.48Th0.009/0.009REE0.665/0.665)6.088/6.088(Ti12.48/12.48Fe1.87/1.87Si0.249/0.249Al0.334/0.334Cr0.019/0.019Co0.008/0.008Ni0.04/0.04)15/15O36/36(OH0.098/0.098[]1.90/1.90)2/2.The calculated structural formulae are not fully stoichiometric, which could be due to metamict nature of davidite. The characteristic feature of distribution pattern of REE in davidite is unusually high concentration of LREE and HREE and substantially low content of MREE. It may be due to the occupation of REEs in two distinct crystallographic sites in davidite structure, i.e., M(1) and M(O) sites. Chondrite-normalised plot of davidite reveals a pronounced negative Eu-anomaly (Eu/Eu∗ = 0.30−0.39), which suggests extremely fractionated nature of the metasomatising fluids from which davidite had crystallized. Metamict davidite-bearing U ores not only from Rajasthan, but also from other parts of India are likely to yield very high U leachability, thereby making them attractive sources of U, which otherwise are ignored by mineral engineers as uneconomic U ores.

    • An approach of understanding acid volcanics and tuffaceous volcaniclastics from field studies: A case from Tadpatri Formation, Proterozoic Cuddapah basin, Andhra Pradesh, India

      Sukanta Goswami P K Upadhyay Sangeeta Bhagat Syed Zakaulla A K Bhatt V Natarajan Sukanta Dey

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      The lower stratigraphic part of the Cuddapah basin is marked by mafic and felsic volcanism. Tadpatri Formation consists of a greater variety of rock types due to bimodal volcanism in the upper part. Presence of bimodal volcanism is an indication of continental rift setting. Various genetic processes involved in the formation of such volcanic sequence result in original textures which are classified into volcaniclastic and coherent categories. Detailed and systematic field worksin Tadpatri–Tonduru transect of SW Cuddapah basin have provided information on the physical processes producing this diversity of rock types. Felsic volcanism is manifested here with features as finger print of past rhyolite-dacite eruptions. Acid volcanics, tuffs and associated shale of Tadpatri Formation are studied and mapped in the field. With supporting subordinate studies on geochemistry,mineralogy and petrogenesis of the volcanics to validate field features accurately, it is understood that volcanism was associated with rifting and shallow marine environmental condition. Four facies (i.e., surge, flow, fall and resedimented volcaniclastic) are demarcated to describe stratigraphic units and volcanic history of the mapped area. The present contribution focuses on the fundamentalcharacterization and categorization of field-based features diagnostic of silica-rich volcanic activities in the Tadpatri Formation.

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