Geochemistry of Proterozoic Radioactive Arkoses in Khammam District,Andhra Pradesh,India: Evidence for K-rich Evolved Granitoid Upper Crust in the Late Archaean |
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| Institution: | 2. Freie Universität Berlin, Institute of Geological Sciences, Malteserstrasse 74-100, 12249 Berlin, Germany;3. University of Genova, Dipartimento di Scienze della Terra, Ambiente e Vita, Corso Europa 26, 16132 Genova, Italy;1. Université Grenoble Alpes, CNRS, ISTerre, F-38041 Grenoble, France;2. Université de Lyon, Ecole Normale Supérieure de Lyon, CNRS, Université Claude Bernard Lyon 1, F-69342 Lyon, France;3. University Pierre et Marie Curie Paris VI, CNRS, ISTEP, F-75252 Paris cedex 05, France;1. Ifremer, Geochemistry and Metallogeny Laboratory, Plouzané, France;2. HydrISE, Institut Polytechnique LaSalle Beauvais, 60026 Beauvais cedex, France;3. Institut Universitaire Européen de la Mer, UMS 3113, Plouzané, France;4. University of Southampton, National Oceanography Centre, European Way, Southampton S014 3ZH, UK;1. Instituto de Ciencias de la Tierra “Jaume Almera” – CSIC, Barcelona, Spain;2. CNR – Istituto per la Dinamica dei Processi Ambientali – sezione di Milano, Via Mangiagalli 34, 20133 Milano, Italy;3. Departamento de Geologia, Instituto Dom Luiz, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, C6, 1749-016 Lisboa, Portugal |
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| Abstract: | Geochemical studies on radioactive arkoses (43–153 ppm U and 387–862 ppm Th) of the Proterozoic Pakhal Supergroup from Bangaruchilka, Khammam district, Andhra Pradesh, India, indicate that their gross major and trace element chemistry reflect their mineral composition. Chemically, arkoses are rich in silica (83% to 88% SiO2) and potassium (3% to 5% K2O), with consistently high Al2O3/Na2O (36 to 50) and K2O/Na2O (18 to 25) ratios, which indicate that they are chemically mature sediments. The arkoses also show higher concentrations of Ti, V, Cr, Ni, Cu, Y, Zr, Nb, La and Pb.The values (60% to 68%) of chemical alteration index (CIA) of studied arkoses are moderate, and indicate that the source rocks have undergone lesser degree of chemical weathering. Tectonic setting discriminate plots of Fe2O3 (total)/MgO vs. TiO2 and Al2O3/SiO2, and K2O/Na2O vs. SiO2 and SiO2/Al2O3 indicate that the Bangaruchilka arkoses represent the sediments that were deposited in passive continental margin (PM), which is further supported by association of platformal type of sediments (quartzites and phyllites) with them. Unlike middle Archaean sedimentary rocks, the studied arkoses are depleted in Na2O, MgO and CaO, and distinctly enriched in SiO2 and K2O. These geochemical features match with post-Archaean clastic sediments, which argues for the involvement of late Archaean granitic crust in supply of detritus of studied arkoses. Enrichment of potassium alongwith abundant microcline and elevated concentrations of Y, Zr, Nb, U, Th, La, etc. in arkoses indicate K-rich evolved granitoid upper crust in the provenance during the late Archaean-early Proterozoic period.Dominance of mechanical weathering over chemical weathering favoured arkose formation, and also transfer of radio-elements with clastic sediments into the Pakhal basin. After sedimentation, uranium seems to have been remobilised from the rocks of Archaean-Pakhal system, consequent to post-sedimentation structural disruptions, and concentrated along suitable structural loci. Therefore, it is likely to encounter significant concentration of uranium in close proximity of unconformable Archaean and Pakhal contacts and tectonic zones, thereby making Pakhal basin and its environs a suitable terrain to search for concealed uranium mineralisation. |
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