Metamorphism during the Archean–Paleoproterozoic Transition Associated with Microblock Amalgamation in the Dharwar Craton, India

Numerous tectonic scenarios have been proposed for terrane growth and accretion within the Archean Dharwar Craton, southern Peninsular India. Previously accepted interpretations involving a two-terrane model-comprising a Western Dharwar Craton (WDC) and Eastern Dharwar Craton (EDC) block-have invoked west-dipping subduction and ocean closure, leading to arc magmatism and accretionary orogeny in the WDC, followed by metamorphic overprinting and collisional orogeny in the EDC. However, recent field investigations have revealed the existence of a previously unrecognized 'central' block (Central Dharwar Craton; CDC) within the craton, which requires revision of this model and reinterpretation of metamorphic and magmatic age data. Five samples of high-pressure, upper amphibolite- and granulite-facies meta-igneous and metasedimentary rocks from the southern portion of the Chitradurga Suture Zone, which divides the WDC and CDC, record minimum peak metamorphic conditions of ~820-875°C at ~10 kbar, indicating equilibration at the base of thickened continental crust. U-Pb zircon and Pb-Pb monazite geochronology indicates crystallization of parent mafic magmas at c. 2·61-2·51 Ga and subsequent regional metamorphism of these intrusions to garnet-amphibolite and garnet-granulite facies at c. 2·48-2·44 Ga, bracketing the timing of microblock accretion to the Archean-Proterozoic boundary. Light rare earth element enrichment within these zircon grains indicates magma generation in a suprasubduction-zone environment. In addition, detrital magmatic zircon grains with ages of c. 3·10-3·03Ga and c. 2·97- 2·86 Ga imply contamination of these magmas with Mesoarchean material sourced from the Western Dharwar Craton continental nucleus. Comparison of these metamorphic and magmatic age data with those recorded in the EDC shows that westward-directed subduction is implausible, and that all three terranes (the WDC, CDC, and EDC) must have accreted synchronously, driven by two separate eastward-dipping ocean-continent convergent plate margins. These data further support a recent abundance of observations from the geological record supporting the hypothesis that subduction-driven plate tectonics had initiated on Earth before c. 2·5 Ga, as opposed to a Neoproterozoic onset (c. 0·8-0·9 Ga) reported by numerous studies