Garnetiferous Metabasites from the Sausar Mobile Belt: Petrology, P-T Path and Implications for the Tectonothermal Evolution of the Central Indian Tectonic Zone

Abstract

A suite of garnetiferous amphibolites and mafic granulites occur as small boudins within layered felsic migmatite gneiss in the northern part of the Sausar Mobile Belt (SMB), the latter constituting the southern component of the Proterozoic Central Indian Tectonic Zone (CITZ). Although the two types of metabasites are in various stages of retrogression, textural, compositional and phase equilibria studies attest to four distinct metamorphic episodes. The early prograde stage (Mo) is represented by an inclusion assemblage of hornblende1 + ilmenite1 + plagioclase1 ± quartz and growth zoning preserved in garnet. The peak assemblage (M1) consists of porphyroblastic garnet + clinopyroxene ± quartz ± rutile ± hornblende in mafic granulites and garnet + quartz + hornblende in amphibolites and stabilized at pressure–temperature conditions of 9–10 kbar and 750–800°C and 8 kbar and 675°C, respectively. This was followed by near-isothermal decompression (M2), and post-decompression cooling (M3) events. In mafic granulites, the former resulted in the development of early clinopyroxene2A–hornblende2A–plagioclase2A symplectites at 8 kbar and 775°C (M2A stage), synchronous with D2 and later anhydrous clinopyroxene2B–plagioclase2B–ilmenite2B symplectites and coronal assemblages at 7 kbar, 750°C (M2B stage) and post-dating D2. In amphibolites, ilmenite + plagioclase + quartz ± hornblende symplectites appeared during M2 at 6·4 kbar and 700°C. During M3, coronal garnet + clinopyroxene + quartz ± hornblende-bearing symplectites in metabasic dykes and hornblende3–plagioclase3 symplectites embaying garnet in mafic granulites were formed. P–T estimates show near-isobaric cooling from 7 kbar and 750°C to 6 kbar and 650°C during M3. It is argued that the decompression in the mafic granulites is not continuous, being punctuated by a distinct heating (prograde?) event. The latter is also coincident with a period of extension, marked by mafic dyke emplacement. The combined P–T path of evolution has a clockwise sense and provides evidence for a major phase of early continental subduction in parts of the CITZ. This was followed by a later continent–continent collision event during which granulites of the first phase became tectonically interleaved with younger lithological units. This tectonothermal event, of possibly Grenvillian age, marks the final amalgamation of the North and the South Indian Blocks along the CITZ to produce the Indian subcontinent.