DETRITAL HISTORY OF PERMO-CARBONIFEROUS GONDWANAN SEQUENCES OF EASTERN SOUTH ASIA
Sandstones from Bangladesh are angular to sub-angular and more texturally immature than coeval sandstones from India and Nepal. Sandstone modal analysis shows that Gondwanan sediments from Bangladesh area (~Qt66F20L14) plot in the ‘recycled orogenic’ to ‘transitional continental’ provenance fields of Dickinson (1985), whereas Indian Gondwanan samples (~Qt84F3L13) plot in the ‘craton interior’ to ‘recycled orogenic’ fields. Nepal samples have a wider range in composition, from “dissected arc” (Qt32F36L32) to “craton interior” (Qt100F0L0) provenance fields. Heavy minerals are volumetrically rare but garnets are abundant among the non-opaque varieties. Garnet chemistry revealed source terranes mostly in amphibolite and granulite facies rocks. Interestingly, chrome-spinels were conspicuously absent to rare in these regional sandstones.
Single crystal laser 40Ar/39Ar ages of detrital muscovites from the Bengal Gondwana sequences yielded cooling ages from Neoproterozoic to late Ordovician, with dominant age clustering around late Cambrian to early Ordovician. In contrast, Indian samples are polymodal and are more scattered ranging from Paleo-Proterozoic to early Ordovician which include more Neoproterozoic cooling ages. The age distributions of older sections of the Gondwana significantly differ from upper sections in India. Nepal Gondwanas provide laser 40Ar/39Ar muscovite ages ranging from Meso-Proterozoic to Carboniferous.
The ages and diversity of the composition in Gondwanan units of eastern South Asia suggest deposition in various tectonic settings and derivation of detritus from multiple sources, including magmatism in the adjacent Indian craton, Pinjarra, the Rayner Complex of Antarctica, and the Indo-Australian proto-Himalayan orogen.