Provenance of the Neogene Surma Group from the Chittagong Tripura Fold Belt, southeast Bengal Basin, Bangladesh: Constraints from whole-rock geochemistry and detrital zircon U-Pb ages

摘要

Miocene Surma Group from the Chittagong Tripura Fold Belt (CTFB), southeast Bengal Basin has been analyzed to evaluate their provenance, tectonic settings and paleoweathering conditions. The sandstones show moderate to high contents of SiO2 (65-80%; 75% on average), and Al2O3 (9.94% on average), with Fe2O3 (total Fe as Fe2O3) + MgO contents of 5.1%, TiO2 (0.57% on average). Compared to the upper continental crust (UCC), the sandstones are depleted in CaO (1.49%) and enriched in Al2O3, Fe2O3 and Na2O . The Neogene shales of the Surma Group are in fair concurrence when compared to the NASC (North American Shale Composite), UCC (the upper continental crust) with the exception of the low content of CaO but when compared with the PAAS (Post Archaean Australian Shale), the Neogene shales are a little more depleted in Al2O3 content. Sandstones and shales have Eu/Eu* similar to 0.61 and -0.65, (La/Lu)N similar to 9.06 and -8.70, La/Sc- similar to 3.90 and 2.86, Th/Sc W similar to 1.19 and similar to 1.41, La/Co similar to 3.69 and similar to 2.42, Th/Co similar to 1.08 and similar to 1.20 and Cr/Th similar to 7.90 and 5.88 ratios as well as Chondrite-normalized REE patterns with flat HREE, LREE enrichment, and negative Eu anomalies indicate the derivation from predominantly felsic sources subjected to low to moderate chemical weathering [Chemical index of alteration (CIA) values of sandstones- 31.11-74.46, ay. 60.08); shales- 43.96-73.07, ay. 61.80]. Integrated geochemical and zircon U-Pb studies reveal that main sediment input might have been from the Himalaya with mixing influence from the east of the Indo-Burman Ranges in an active margin setting at the convergence of the Indian and Burmese plates.