Evidences of felsic volcanism and hydrothermal activities from clays associated with the Palaeoproterozoic Porcellanite Formation of the Vindhyan Supergroup,Central India |
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| Institution: | 1. Mantle Petrology Laboratory, Department of Geology, Banaras Hindu University, Varanasi 221005, India;2. Karpinsky Geological Institute (VSEGEI), Russian Academy of Sciences, St. Petersburg 199034, Russia;3. Institute Instrumentation Centre, Indian Institute of Technology, Roorkee 247667, India;4. Mineral Resources, Technical University of Clausthal, Adolph-Römer-Straße 2A, 38678 Clausthal-Zellerfeld, Germany;5. Kuwait Oil Company, Ahmadi, Kuwait |
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| Abstract: | Chemico-mineralogical attributes of authigenic clays associated with the altered volcanic tuffs that occur in the Palaeoproterozoic Porcellanite Formation contain evidences of hydrothermal alteration and diagenetic processes in a marine environment. Previous sedimentological and geochemical studies on Porcellanite Formation were restricted to the Chopan area, but, the details related to provenance, nature and source of volcanism archived in these clays have not been ascertained. In order to understand these aspects, present study on these authigenic clays were carried out. Clay minerals represent dominance of illite with subordinate amount of montmorillonite. Moreover, low abundance of kaolinite is also noticed. The illite fibers and plates associated with the kaolinite indicate illitization. The kaolinite to illite transformation is favoured by incorporation of K+ ions, derived from the K-feldspar dissolution and its overgrowth. Major oxide contents of these clays and their ratios when plotted over diagrams marked with standard illite, kaolinite, smectite and chlorite compositional fields show clustering within or close to the illite field. Thermodynamic components calculated for these clays when plotted over AR23+AlSi3O10(OH)2 ? R23+Si4O10(OH)2 ? AR2+R3+Si4O10(OH)2 ternary diagram, data plots lie within the illite, mixed layer I/S and smectite fields. Binary major oxide data plots between bulk rock and authigenic clay compositions showed felsic affinity. Montmorillonite and illite predominated in the eastern and western marginal areas of the Vindhyan Basin, respectively. However, former resulted from the hydrothermal alteration of volcanic glass associated with the ferruginous breccia and altered tuffs and remnants of the volcanic vents, whereas, later is associated with the tuffaceous beds. Owing to the adsorption, Ba, Rb and Sr is enriched in clays comparing to the bulk rock composition. Low (< 15 ppm) Sc values suggested major contribution from the felsic component. Also, low Rb/Sr and Th/U values revealed moderate insitu weathering. The dominance of K-feldspar alteration and insitu weathering is also evident from clustering of clay data plots in the A-CN-K ternary diagram. Pronounced negative Eu anomaly together with higher LREE/HREE values associated with these clay minerals implied proximity to source and their possible derivation from the silicified felsic tuffs available in the provenance. |
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| Keywords: | Porcellanite Vindhyan Supergroup Weathering Palaeoenvironment Clay minerals Hydrothermal alteration |
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