Geochemistry of sandstones from the Miocene Surma Group, Bengal Basin, Bangladesh: Implications for Provenance, tectonic setting and weathering

抄録

Twenty six sandstone samples from six wells penetrating the Miocene Surma Group in the Bengal Basin, Bangladesh, were analyzed by lithium metaborate/tetraborate fusion Inductively Coupled Plasma (ICP) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and by petrographic microscope. The framework grains of the Surma Group sandstones are rich in quartz followed by lithic grains, feldspar and mica (predominantly white mica). The sandstones are dominantly quartzolithic and quartzose one in composition with abundant low-grade metamorphic, sedimentary lithics, low feldspars and little volcanic detritus, indicating that the sands were derived from a quartzose recycled orogen province. The Surma Group sandstones have moderate to high SiO₂ contents (64-85%; on average 77%), TiO₂ concentrations averaging 0.5%, Al₂O₃ contents of about 8.4%, and Fe₂O₃ (total Fe as Fe₂O₃) + MgO content of around 4.5%. Compared to the average sandstone value, the Surma Group sandstones are depleted in CaO (2.36%) and enriched in Al₂O₃, Fe₂O₃ and Na₂O. Geochemically, the sandstones are classified mainly as litharenites. The Chemical Index of Alteration (CIA) values for the Miocene Surma Group sandstones vary from 53 to 65 with an average of 59, indicating low to moderate weathering of the source areas. The average CIA value (59) is a little above than that (50) of the upper continental crust. The shales from the Miocene Surma Group show higher CIA (∼70-78) values, indicating significant weathering in the source. The geochemical characteristics suggest an active continental margin to passive margin setting for the Surma Group sandstones, and preserve the signatures of a recycled provenance. The Eu/Eu* (∼0.69), (La/Lu)cn (∼10.07), La/Sc (∼3.98), Th/Sc (∼1.44), La/Co (∼3.84), Th/Co (∼1.40), and Cr/Th (∼9.48) ratios as well as chondrite-normalized REE patterns with flat HREE, LREE enrichment, and negative Eu anomalies indicate derivation of the Surma Group sandstones from felsic rock sources.