Geochemistry and Petrogenesis of the Basement Rocks from Barapaharpur, Rangpur: Implications for Crustal Evolution of the Shallow Basement in the North-West Bangladesh.

English Abstract:

Bangladesh is one of the world’s largest deltaic plains, almost entirely covered by Tertiary and Quaternary sediments without any surface outcrops of igneous and metamorphic rocks. The crystalline basement drillcore samples from Barapaharpur, Rangpur area in the northwest Bangladesh have been investigated through core logging, petrographic and geochemical study to constrain their origin and to propose a crustal evolution and geodynamic model for the area. The Barapaharpur basement rocks were encountered at a depth of ~1200ft and comprises mostly of diorite, which is dissected by minor granodiorite gneiss and hornblendite dykes. The felsic rocks of Barapaharpur basement are metaluminous with low ASI index, calc-alkaline and have high K₂O and Rb contents. They show progressive depletion in P₂O₅ with increasing silica reflecting a typical internal fractionation trend of I-type granitoid. The Barapaharpur granitoid is characterized by strong enrichment in LREE and distinct negative Nb and Ti and zero to weakly negative Eu anomalies. The source melt of Barapaharpur granitoid has been suggested to have generated in an ocean-continent subduction zone. High La/Yb_CN ratios (16.17-17.67) and positive correlation between Sr/Y and La/Yb ratios of the felsic samples imply a deep source melting (upper mantle) at high pressure (10-12Kbar). Dykes of hornblendite suggests a mantle origin, at deeper level than the parental melt of the diorite. This mafic melt occupied the fissure or weak zones within the I-type granitoid forming the late dykes. Comparison between the studied Barapaharpur granitoid and Maddhapara granitoid (55km NW to Barapaharpur) reveals that both of them share remarkably similar geochemical and petrographic signatures, pointing toward a similar source of parental melt for both granitoids. From the geodynamic study based on gravity-magnetic mapping, the location of the granitoids fall within the regional micro-continental block (Dinajpur Block) suggested by a previous work and these two granitoid basements are considered to have represented an unreported part of Palaeoproterozoic supercontinent Columbia. However, U-Pb precise geochronology and high precision geophysical study of the Barapaharpur granitoid are required to establish the precise age, and limit of this new microcontinental block.