Application of hornblende thermobarometry and fluid inclusion studies to the Palaeoproterozoic (1.7 Ga) basement rocks from Maddhapara, NW Bangladesh, provide information on the pressure and temperature (P-T) conditions of crystallization, the emplacement depth and composition of magmatic fluid. The basement rocks are predominantly diorite or quartz diorite with a mineral assemblage of plagioclase, hornblende, biotite, quartz, K-feldspar, titanite, and secondary epidote and chlorite. The calculated P-T conditions of the dioritic rocks are 680-725 °C and 4.9-6.4 kbar, which probably correspond to crystallization conditions. Fluid inclusion studies suggest that low- to medium-salinity (0-6.4 wt.% NaCleq) H2O-rich fluids are trapped during the crystallization of quartz and plagioclase. The isochore range calculated for primary aqueous inclusions is consistent with the P-T condition obtained by geothermobarometry. The basement rocks likely crystallized at a depth of ∼17-22 km, with a minimum average exhumation rate of ∼12-15 m/Ma during Palaeoproterozoic to Lopingian time. Such slow exhumation indicates low relief continental shield surface during this period.
All Science Journal Classification (ASJC) codes
- Earth-Surface Processes