Tectonothermal imprints in a suite of mafic dykes from the Chotanagpur Granite Gneissic complex (CGGC), Jharkhand, India: Evidence for late Tonian reworking of an early Tonian continental crust

Tectonothermal evolution of a set of mafic dykes in the felsic orthogneiss basement of the Chotanagpur Granite Gneissic Complex (CGGC) is presented. The gneissic basement rocks preserve high pressure granulite fades metamorphism (M-2, similar to 800 degrees C and similar to 9 kbar) and develop pervasive planar fabric (S-2) during a continent-continent collision event at 1000-950 Ma. Mafic dykes cut the S-2 fabric of its basement it intrudes. Together with the S2. the mafic dykes have been deformed by three sets of folds viz. DkF(1), DkF(2) and DkF(3). A conspicuous N-S trending foliation defined by metamorphic amphibole is developed parallel to the axial plane of DkF(1-2). The mafic dykes preserve relict igneous textures (e.g. intergranular texture) defined by clinopyroxene, orthopyroxene and plagioclase. Numerically computed phase diagram (pseudosection) with a representative bulk composition (with <10 vol% metamorphic minerals namely amphibole+ plagioclase (rim) + quartz) constrain the depth of emplacement of the mafic dykes (15-18 km). The geological features are consistent with rifting of the early Tonian basement of the CGGC and the then mantle beneath it. This lithospheric extension resulted in outpouring of basaltic magma that punctured the early Tonian basement rocks in the form of mafic dykes. Subsequent metamorphism that affected the mafic dykes and its basement culminated at-700 degrees C and similar to 7.5 kbar (M-3 metamorphism). The estimated physical conditions during the M-3 metamorphic event suggest burial of the previously rifted basement possibly in a continent-continent collisional tectonic setting. The Th-U-total Pb ages of monazite grains from the host felsic orthogneiss date the M-2 (ca. 964 Ma) and M-3 (ca. 917 Ma) metamorphism. Based on commonality of geological and geochronological history among the CGGC, the Eastern Ghats Mobile Belt and Rayner complex (and its equivalent parts) in east Antarctica corroborates the view that Indo-Antarctic landmass were united and shared the similar geological histories during the entire Tonian period. During the Tonian period the Indo-Antarctic landmass witnessed two major phases of orogenesis (1000-950 Ma and < 920 Ma) that are punctuated by a phase of lithospheric extension manifested by the emplacement of mafic dykes. (C) 2018 Elsevier B.V. All rights reserved.