Petrography, Geochemistry and Rb-Sr geochronology of the basement granitoids from Umthongkut area, West Khasi Hills district, Meghalaya, India: implications on petrogenesis and uranium mineralization

The basement granitoids from Umthongkut, West Khasi Hills district, Meghalaya is exposed in a very small area (1.5 km x 0.5 km) and unconformably overlain by Mahadek sandstone of upper Cretaceous age and successively followed by rocks of Tertiary age. Megascopically, granitoids are leucocratic, mesocratic to mild melanocratic and differ in texture and mineral abundance without any definite contact relation in field. The leucocratic granitoid is medium-grained, designated as granite (syenogranite and monzogranite) with appreciable content of biotite and garnet and has peraluminous to mild metaluminous character (mol. Al₂O₃/mol. CaO+Na₂O+K₂O = 0.9-1.17). However, mesocratic granitoid is relatively fine-grained, identified as quartz monzonite with abundant biotite, minor hornblende and opaque like magnetite, ilmenite, anatase, pyrite and chalcopyrite and metaluminous to mild peraluminous (mol. A/CNK = 0.76-1.02) in nature. The presence of characteristic minerals and textural replacement features point towards S-type signature for granite and I-type for quartz monzonite. On the other hand, mixed S and I-type characters are observed in geochemical plot of SiO₂ vs. A/CNK. The coexistence of granite and quartz monzonite with former being more evolved, S-type signature and presence of garnet with biotite suggests that the granite appears to be generated from melting of lower crustal source of monzonitic composition. The Rb-Sr dating is also attempted and the errorchron age of 1130±53 with low initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7063±0.001 Ma appears to be the emplacement age for these coexisting phases of granitoid. Considering the Meghalaya plateau being northeastern extension of Chhotanagpur granite-gneiss complex (CGGC), the calculated age for the studied samples corresponds to the 1200-1000 Ma. which is defined as fourth tectono-magmatic thermal event of CGGc. Interestingly, these granitoids are considered as the probable source for uranium in the sandstones of Mahadek Formation of Cretaceous age hosting ore grade uranium mineralization at Umthongkut. (author)