DETRITAL HISTORY OF PERMO-CARBONIFEROUS GONDWANAN SEQUENCES OF EASTERN SOUTH ASIA

The South Asian countries, including Bangladesh, India, and Nepal, along with Australia and Antarctica, constituted the Eastern Gondwanaland. Permo-Carboniferous siliciclastic Gondwanan sequences are exposed in Nepal and India, and are found in subsurface in northwest Bangladesh. These sequences consist primarily of bedded to cross-bedded sandstones and laminated mudstones, with localized conglomerate, diamictites, and coal layers.

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.