Geochemistry and Genesis of Craton-derived Sediments from Active Continental Margins: Insights from the Mizoram Foreland Basin, NE India

- Significance of elevated Meghalaya Plateau in the supply-chain of sediments to the Mizoram Basin.
- The sediment flux in the Brahmaputra drainage may have become relatively felsic from the Miocene to the present.
- Geochemical discrimination of cratonic provenance signal from that of the juvenile sources.
- New geochemical discrimination plots to identify provenance to the sediments.

This study investigates relative controls of source compositions and sedimentary processes (weathering + mineral sorting) on the bulk-rock geochemistry of clastic rocks deposited along an active continental margin setting by considering the Miocene Mizoram basin of NE India. It is a classic foreland accretionary sedimentary basin related to Himalaya orogeny and is ideally located to understand tectonic evolution of the Eastern Himalayas, Indo-Burmese arc, and Indian cratonic margin [the Meghalaya Plateau (also known as Shillong Plateau)]. These three tectonic domains represent potential suppliers of clastic sediments to the basin. The Mizoram basin comprises sandstone, siltstone and shale deposited under shallow marine conditions. Depletions in Ca and Sr indicate plagioclase-dominated weathering of the sources. The Mizoram strata are first-cycle sediments; however, quartz dilution has resulted in the decrease of both major (except SiO2) and trace elements, carbonate dilution has increased Ca and Sr, and heavy minerals have influenced Zr, HREE, Hf, U and Y abundances in a few of the studied Mizoram sediments. Newly formulated Rb/V-Zr/Zn-Sc/Nb and Rb/V-Zr/Zn-Eu/Eu* ternary plots distinguish sediments derived from mafic versus felsic sources, and allow the estimation of mafic:felsic proportions in the sedimentary provenances. We thus suggest an average tonalite-granodiorite source terrain for the Mizoram sediments. The geochemical evidences, including TiO2 (> 0.7 wt%), Cr/Cr* (> 1), V/V* (> 1), Th/U (10.23), Nb/U (9.01) and Th/Rb (0.124) values, indicate that the Mizoram sediments were derived chiefly from old cratonic crust of the Meghalaya Plateau. The source for the Miocene Mizoram sediments is slightly mafic than that supplying the modern Brahmaputra and Ganga sediments (dominated by the juvenile Himalayas). The Mizoram clastics provide indirect evidence for rise of the Meghalaya Plateau and a possible change in the Brahmaputra drainage as controlled by the uplift. We suggest that the Mizoram sediments are archives of the cratonic provenance in a foreland basin dominated by orogeny-derived detritus.

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