Late Proterozoic Charnockites in Orissa, India: A U-Pb and Rb-Sr Isotopic Study

Charnockite formation in the Angul district of Orissa took place between 1088 + 26/- 17 Ma, the U-Pb zircon upper intercept crystallization age of a leptynite neosome, and 957 + 8/ - 4-956 ± 4 Ma, the U-Pb zircon-monazite upper intercept and U-Pb monazite crystallization ages of a granite. Confirmation of the Proterozoic age of the charnockites is given by (1) a U-Pb zircon upper intercept 1159 -H 59/- 30 Ma age and a Rb-Sr whole-rock 1080 ± 65 Ma age for an augen gneiss which pre-dates the leptynite, and (2) U-Pb monazite ages of 973 ± 5, 964 ± 4, and 953 ± 4 Ma for a gray quartzofeldspathic gneiss, the augen gneiss, and the leptynite, respectively: these late Proterozoic dates are interpreted as representing ages recorded during charnockitization. The ca. 950-980 Ma charnockite- and granite-forming events are related to the evolution of mantle-derived, CO₂-bearing basic magma emplaced into the deeper levels of an extensional tectonic-transcurrent fault regime. The ca. 1100-1150 Ma tectonothermal and igneous events represent compressional tectonism in reactivated crystalline basement in the late mid-Proterozoic Eastern Ghats orogenic belt.