PROVENANCE RECONSTRUCTION OF PART OF EASTERN GONDWANALAND: PERMO-CARBONIFEROUS GONDWANAN SEQUENCES OF NEPAL HIMALAYA AND THE BENGAL BASIN

The Indian subcontinent, along with Australia and Antarctica, constituted Eastern Gondwananland. Permo-Carboniferous siliciclastic Gondwanan sequences are exposed in two flanks of the Himalayas in Nepal and are drilled in intra-cratonic basins in northwest Bengal Basin, Bangladesh. These ~200 to 1000 m-thick sequences consist primarily of massive and trough cross-bedded sandstones and laminated mudstones, with localized conglomerate, diamictites, and coal layers.

Sandstone compositional analyses suggest that these sequences are mostly poorly sorted arkosic sandstones, belonging to several provenance tectonic fields (transitional continental, basement uplift, recycled orogenic, and dissected arc). Heavy minerals are volumetrically rare but garnets are abundant among the non-opaques. Garnet chemistry revealed source terranes mostly in amphibolite and granulite facies rocks. Interestingly, chrome-spinels were conspicuously absent in eastern Nepal and Bengal basin sandstones.

Single crystal laser ⁴⁰Ar/³⁹Ar ages of detrital muscovites from the Bengal Gondwana sequences yielded cooling ages from Neoproterozoic to Late Ordovician suggesting most possible derivation from the adjacent Indian craton, Pinjarra, and proto-Himalayan orogens formed due to the collision between India and Australia. However, laser ⁴⁰Ar/³⁹Ar muscovite ages and sediment geochemistry of sequences mapped as Permo-Carboniferous age in the eastern Nepal Himalayas (~250 km from Bengal Gondwana locations) indicate a very different provenance and pose questions on their existing stratigraphic assignment to the Late Paleozoic.