Mesoarchean convergent margin processes and crustal evolution: Petrologic, geochemical and zircon U-Pb and Lu-Hf data from the Mercara Suture Zone, southern India

The Mercara Shear Zone is sandwiched between the Western Dharwar Craton and the Coorg Block in the Southern Granulite Terrain of India, and is marked by steep gravity gradients interpreted to suggest the presence of underplated high-density material in the lower crust. Here we present geological, petrological and geochemical data, together with zircon U-Pb ages and Lu-Hf isotopes from a suite of metaigneous (TTG-related gneisses, charnockite, metagabbro, mafic granulite) and metasedimentary (quartz mica schist, khondalite, garnet biotite gneiss, kyanite-sillimanite bearing metapelite) rocks from this zone. Geochemical data on the magmatic suite suggests formation through subduction-related arc magmatism, whereas the metasediments represent volcano-sedimentary trench sequences. Phase equilibrium modeling of mafic granulites from the Mercara Shear Zone suggests P-T range of 10-12 kbar at 700 degrees C to 900 degrees C. The zircon data yield weighted mean zo7Pb/zosPb ages of 3229 +/- 80 Ma for metagabbro, 3168 +/- 25 Ma for the charnockite, and 3181 +/- 20 Ma for the mafic granulite. Ages ranging from 3248 +/- 28 Ma to 3506 +/- 26 Ma were obtained from zircons in the kyanite/sillimanite bearing metap elite, 3335 44 Ma from khondalite, 3135 +/- 14 Ma from garnet biotite gneiss, 3145 +/- 17 Ma to 3292 +/- 57 Ma from quartz mica schist and 3153 +/- 15 Ma to 3252 +/- 36 from TTG gneiss. The tightly defined ages of 3.1 to 3.2 Ga from igneous zircons in the magmatic suite suggest prominent Mesoarchean convergent margin magmatism. The timing of high grade metamorphism as constrained from metamorphic overgrowths in zircons is ca. 3.0 Ga which might mark the collisional event between the Western Dharwar Craton and the Coorg Block. Hf isotope features suggest magma derivation mostly from juvenile sources and the Lu-Hf model ages indicate that the crust building might have also involved partial recycling of basement rocks as old as ca. 3.8 Ga. Our study defines the Mercara Shear Zone as a terrane boundary, and possible Mesoarchean suture along which the Coorg Block was accreted to the Western Dharwar Craton. The accretion of these continental fragments might have coincided with the birth of the oldest supercontinent Ut. (C) 2016 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.