SANDSTONE PETROGRAPHY AND HEAVY-MINERAL CONSTRAINTS ON PROVENANCE OF PERMO-CARBONIFEROUS GONDWANAN SEQUENCES FROM INDIA AND BANGLADESH

Permo-Carboniferous Gondwanan sequences are well-distributed in the Indian subcontinent, South Africa, Madagascar, Australia, Antarctica and South America, bearing proof that the southern continents were once united in the form of Gondwanaland. In the Indian subcontinent, these sequences were initially deposited in one master basin that later was segmented during Gondwanan extensional tectonics. Sandstone samples were analyzed for their petrography and heavy mineral studies to constrain regional detrital and paleotectonic history of these siliciclastic sequences.

The percentage of heavy minerals is higher in Indian samples (ranging from 0.0436 to 57.9538 % with an average of ~8.207 %) compared to the samples from northwestern Bangladesh (ranging from 0.0640 to 4.9419 % with an average of ~1.305 %). Gondwanan samples of northwestern Bengal Basin contain low diversity suites of heavy minerals compared to coeval sandstone samples from Jharia Basin, India. The characteristic heavy minerals are garnet, zircon, tourmaline, rutile, biotite, chlorite, chloritoid, staurolite, epidote, sphene and pyroxenes including the opaques. Garnet is the dominant heavy mineral species in almost all of the samples after opaque grains. Sandstone modal analysis shows that sandstones from Bangladesh are angular to sub-angular and show greater immaturity compared to the coeval sandstones from India. Several tectonic provenance fields were identified for these mostly poorly sorted arkosic and quartz arenitic sequences. 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.

Sandstone composition and texural studies suggest that the Gondwanan sequences in these graben basins of Indian subcontinent received detritus from different source terranes. The heavy mineral suites suggest derivation of these Permo-Carboniferous sediments from pegmatite, acid and basic igneous as well as metamorphic rocks including the shield areas of India and Anatractica, and the orogenic systems existed in these continents during the late Paleozoic.