Geochronology of the 1.55 Ga Bengal anorthosite and Grenvillian metamorphism in the Chotanagpur gneissic complex, eastern India

Abstract

The thermal-tectonic history of the eastern part of the Chotanagpur gneissic complex (CGC) in the eastern Indian shield was investigated through U–Pb dating of zircon in anorthosite, U–Th–Pb chemical dating of monazite in metapelite granulite, and metamorphic textural analysis. Gabbroic anorthosite in the Bengal anorthosite massif was emplaced at 1550 ± 12 Ma (2 σ error, including error from decay constants), 50–150 Myr after an earlier metamorphic event probably related to the late stage of continental collision between the northern and the southern Indian blocks. Subsequent metamorphism in the CGC is recorded by 947 ± 27 Ma zircon growth in the anorthosite, and 950 ± 20 Ma and 995 ± 24 Ma monazite growths in the matrix and within garnet of metapelite granulite located north of the anorthosite. Peak pressures and temperatures of 7–10.5 kb and 775–825 °C in the metapelite granulite and 8.5–11 kb and 850–900 °C in the anorthosite indicate granulite facies conditions during the 995–947 Ma metamorphic event. Hence, as in the Eastern Ghats belt, Grenvillian granulite facies metamorphism played an important role in the thermal-tectonic evolution of the CGC.