Constraining the metamorphic evolution of a cryptic hot Mesoproterozoic orogen in the Central Indian Tectonic Zone, using P– T pseudosection modelling of mafic intrusions and host reworked granulites

Abstract

In this study, we reconstruct the metamorphic pathways of reworking of a deep crustal granulite terrane in the Central Indian Tectonic Zone, using Archaean/Palaeoproterozoic (?) polycyclic felsic granulites and two groups of intrusive Mesoproterozoic mafic granulites, a coarse-grained noritic gabbro and a fine-grained gabbroic norite. The granulite terrane, locally referred to as the Bhandara–Balaghat granulite domain, is bounded by the South Indian Block and a Grenville-aged, younger tectonic domain of the Central Indian Tectonic Zone. The granulites and the mafic intrusions were multiply deformed and metamorphosed in the early Mesoproterozoic. Using P– T pseudosection modelling of host felsic granulites and the mafic intrusions, two distinct metamorphic events (BM 2 and BM 3) with contrasting P– T paths have been established. The P– T path of BM 2 metamorphism has a clockwise sense, having an important prograde segment of heating of more than 250 °C with pressure fall, followed by cooling. The peak BM 2 metamorphism has been constrained at ∼6 kbar, ∼725 °C. Emplacement of the mafic intrusions was broadly coincident with the low-pressure metamorphism, and they underwent a phase of subsolidus cooling. During subsequent metamorphism, BM 3, the mafic intrusions and the host felsic granulites were re-metamorphosed along a counterclockwise P– T path. This led to tectonic burial of the mid-crust to ∼9.4 kbar, ∼760 °C, which was followed by cooling accompanying pressure decline. Although, the clockwise P– T path is generally interpreted in terms of thermal relaxation of crust following thrusting, the scale of heating (∼250 °C) during initial decompression, documented here, is much larger than modelled for tectonically thickened crust, but consistent with magmatically active extensional zones. Based on this evidence and also the syn-metamorphic mafic intrusions, we present an alternate interpretation in terms of mid-crustal extension for the BM 2 metamorphism. In contrast, the counter clockwise P– T path during BM 3, recording cooling following prograde burial is explained by tectonic thickening of a hot mid-crust. Collating available geochronological data, the two metamorphic events appear to indicate tectonic switching from lithospheric extension to contraction in the early Mesoproterozoic. The findings provide the first quantitative constraints on an early Mesoproterozoic hot orogen at the craton–mobile belt interface in the Central Indian Tectonic Zone.