Age-integrated tectonic evolution across the orogen-craton boundary: Age zonation and shallow- to deep crustal participation during Late Cambrian cratonisation of Eastern Ghats Belts, India

Abstract

The Eastern Ghats Belt (EGB) is an orogenic belt which is known to evolve through Proterozoic time before cratonisation with Proto-India and holds an important key to the reconstruction of erstwhile supercontinents. To track the comprehensive evolution of the EGB with respect to Proto-India prior to and during its cratonisation, we carried out a detailed petrological and high-resolution geochronological investigation (zircon U-Pb SHRIMP and monazite U-Th-total Pb EPMA) on systematically sampled rocks from a transect across the western boundary of the EGB. Our results reveal that along a narrow zone near the thrust-bound margin between the EGB and Proto-India, the entire crustal segment, i.e. from deep-crustal granulites to mid-crustal cratonic amphibolites to shallow-crustal quartz breccia, was involved actively in the thrust-related tectonics during cratonisation of the EGB at ~550–500Ma. This youngest imprint of the ~550–500Ma event erased all the earlier age histories from the zircon and monazite grains of the marginal granulites. On the other hand, the older age-imprints are better preserved in the interior of the orogeny (~30km east of thrust-boundary between the EGB and the BC and further eastward) with little or no age-imprints of the final tectonic pulse. Combining all the new and earlier geochronological data with P-T calculations and textural characterisation, we propose a comprehensive and geochronologically well-constrained tectonic evolution of the EGB, which encompasses its earliest deep-crustal metamorphic to latest shallow-crustal sedimentation histories. The outcome of this study also reveals, for the first time, that the entire crustal section participated actively during the cratonisation. The new geochronological data coupled with field evidence and textural evidence from the shallow-crustal rock finally suggest that the thrusting vis-à-vis cratonisation was continued, at least, up to Late Cambrian, thus post-dating the Pan-African orogeny.