Monazite chronology, metamorphism–anatexis and tectonic relevance of the mid-Neoproterozoic Eastern Indian Tectonic Zone

Abstract

A highly tectonized zone (Eastern Indian Tectonic Zone, EITZ) of N- to NNE-trending non-cylindrical asymmetric folds, associated sinistral shears, and shear-related granitoids, traceable for >150 km along its strike, has been identified in eastern India. The EITZ near-orthogonally truncates the older metamorphic fabrics in the eastern fringes of the Chotanagpur Gneissic Complex (CGC), the North Singhbhum Mobile Belt (NSMB), and the Singhbhum Craton, three incongruently evolved crustal domains last metamorphosed at 0.95 Ga, 1.3 Ga and 2.1 Ga, respectively, before tectonism along the EITZ. Neoproterozoic monazite overgrowth at 872–838 Ma on older monazite in EITZ-hosted CGC gneiss and NSMB mica schist, and new monazite crystallization at 876–784 Ma in syn-EITZ granitoids in CGC constrain the timing of tectonism in the EITZ. Shearing occurred at greenschist to lower amphibolite facies conditions in NSMB and Singhbhum Craton. In CGC, prograde metamorphism and anatexis associated with the EITZ occurred in the presence of 5.9 mole% H 2O at peak P– T of 11.3 kb and 800 °C, followed by an isothermal pressure increase to 12.7 kb and retrogression to 9 kb and 730 °C. These P– T conditions are different from the Mesoproterozoic and Grenvillian P– T conditions deduced for the CGC in earlier studies. The Neoproterozoic deformation, metamorphism, and anatexis in the EITZ are coeval with the 0.9–0.7 Ga deformation and anatexis in the Chilka Lake area of the Eastern Ghats Belt, ∼280 km south-southeast along strike of the EITZ, and felsic magmatism/metamorphism in the Rayner Complex of East Antarctica. The regionally extensive 0.87–0.78 Ga sinistral shearing and anatexis in the EITZ is probably related to a distinct period of southward movement of India after its ∼1 Ga collision with East Antarctica during Rodinia assembly. Therefore, rifting related to Rodinia break-up between India and East Antarctica may not have started until 0.78 Ga.