Crystalline Basement

Crystalline Basement Rock made up of minerals in a clearly crystalline state. Alternatively, igneous or metamorphic rock, as opposed to a sedimentary rock; eg crystalline limestone is equivalent to marble. The thickness of the geological group-formations overlying the crystalline basement varies from 0.12 km to 3 km in the northern districts of Bangladesh and 3 km to 21 km in the basin part of Bangladesh.

The crystalline basement rocks encountered in the boreholes drilled at the precambrian indian platform part comprise granite, granodiorite, tonalite, diorite, gabbro, dolerite, gneiss, schist, amphibolite. Granitic and gneissic rocks are predominant. Granitic rocks are mainly composed of quartz, K-feldspar, sodic plagioclase, biotite, and hornblende. Chloritisation of hornblende and biotite are conspicuous. Mostly they show hypidiomorphic inequigranular texture. Perthite, myrmekite, graphic intergrowth textures are common in the granitic crystalline basement. Basic crystalline basement is mainly composed of plagioclase, pyroxene and hornblende. Mineralogical composition of the gneisses is quite similar to that of the granitic rocks. The crystalline basement rocks have been subjected to intense faulting. Anomalous contents of copper, iron and other metallic minerals are found in the crystalline basement in the northern districts of Bangladesh.The extensional tectonic force acting on the crystalline basement has given rise to fault bounded graben basins favoring accumulation of coal. The crystalline basement complex is underlain by the coalbearing gondwana sediments.In the northern part of Bangladesh where the basement complex occurs at relativcely shallow depth,mining work is being conducted to extract hard rock for construction purposes. [Eunuse Akon]