Revealing the Basement in Barapukuria: A Geochemical Study of a Gondwana Coal Basin Basement from Northwest Bangladesh

The Precambrian basement in the northwest Bengal basin is concealed under mainly Tertiary and in places overlain by Gondwana sediments in the half graben basins. The present work reports the study of the petrographic and geochemical investigations of the basement of Barapukuria, one of the major Gondwana coal basins, for the first time. The basement at the Barapukuria, encountered at a depth of ∼200 to 656 m in several drill holes, comprises of tonalite, granodiorite, and leucogranite, which are primarily composed of the variable amounts of plagioclase, quartz, biotite, and amphibole with accessory leucoxene, calcite, epidote, muscovite, and allanite. The lower part of the felsic sequence in the drill hole GDH 43 was dissected by a hornblendite dike. The tonalite and granodiorites have likely chemical composition with silica, which varies within a moderate range from ∼54 to 63 wt% and ∼64 to 68 wt%, respectively. Both tonalite and granodiorites have high iron content ranging from ∼9 to 16 wt% and ∼6 to 12 wt% respectively. Leucogranite, on the other hand, is siliceous (70.32 to 72.10 wt%) and relatively iron rich (4.91 to 6.89 wt%). Collectively, the felsic rocks mostly show a calc-alkaline, I-type character and a moderate degree [(La/Yb) CN = 17] of LREEs enrichment. The Barapukuria basement rocks share similar petrographic and geochemical signatures to the nearby (∼14km east) ∼1.72 Ga felsic basement in Maddhapara. The scarcity of data on the Gondwana coal basin basement from the northeastern Indian shield, however, precludes a comparison between these basements in northeast India and Barapukuria in northwest Bangladesh.