Paleogeographic Reconstruction of Part of Eastern Gondwanaland:Petrofacies and Detrital Geochronologic Study of Permo-Carboniferousgondwanan Sequences of India and Bangladesh
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Geology and Geography
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This study has been carried out focusing on petrofacies and geochronology of Permo-Carboniferous Gondwanan sequences of Barapukuria, Dighipara, and Khalaspir coal basins in the Bengal Basin and Jharia Basin in the Eastern India in order to better reconstruct regional detrital and tectonic histories of Late Paleozoic Gondwanan basins of South Asia. Sandstone petrographic studies of Permo-Carboniferous Gondwanan sequences reveal differences in sandstone modes between sediments from India and the Bengal Basin. Modal compositions vary among various basins and among sandstones from different stratigraphic levels of the same basin. These compositional differences may be attributed to differences in source rocks. Heavy mineral assemblages in Gondwanan sequences of Bangladesh and India are dominated by a few mineral groups, including garnet, apatite, zircon, rutile, tourmaline, sphene, epidote, siderite and opaques. Microprobe studies of garnets show all analyzed grains are almandine rich and indicate sources from amphibolite facies, granulite facies, and pegmatites. Ca-Fe(tot)-Mg plots suggest that tourmaline grains iii were derived from Ca-poor metapelites, metapsammites, and quartz tourmaline rocks, with one exception. The exceptional sample from Dighipara appears to have derived from Ca-rich metapelites, metapsammites, and calc-silicate rocks. Laser 40Ar/39Ar analyses of detrital muscovites show mostly polymodal distributions of cooling ages indicating multiple source terranes. Age distributions for Indian sediments significantly differ from those of the Bengal Basin samples. Age distributions from Bengal samples strongly fall in a narrow zone of Cambro-Ordovician (470 – 520 Ma). In contrast, Indian samples are more scattered (480 – 1830 Ma) and include more Neoproterozoic cooling ages. However, Indan samples also include significant age distributions of Cambro-Ordovician. Within the Indian basin, the age distributions of the older Talchir Formation significantly differ from those of the younger Barakar sandstones. Stratigraphic changes in the modes of age distributions for muscovites may reflect changes in source terranes through time. Similarities between the ages of the present study to published geochronology of the Kuunga orogen, Shingbhum craton, Meghalayan craton, and/or Eastern Ghats Mobile Belt are interpreted to indicate these regions are the most probable sources of Gondwanan sediments in Bengal Basin and the Jharia Basin of eastern India.