The Neoproterozoic subduction complex in southern India: SIMS zircon U-Pb ages and implications for Gondwana assembly

The Palghat-Cauvery Suture Zone (PCSZ) in southern India defines the trace of the collisional suture developed during the assembly of the Gondwana supercontinent through the closure of the Mozambique Ocean in the Late Neoproterozoic-Cambrian. Here we report Secondary Ion Mass Spectrometry (SIMS) U-Pb ages from zircons in plagiogranites and gabbros from a suprasubduction zone ophiolitic complex at Manamedu, located along the southern periphery of the PCSZ. The morphology and internal structures, together with the high Th/U values of the zircons from the plagiogranite suggest a magmatic crystallization history. The dominant population of zircons in the two plagiogranite samples analyzed in this study yield Pb-206/U-238 ages of 737 +/- 23 Ma and 782 +/- 24 Ma corresponding to the timing of emplacement of these rocks. The plagiogranite from Manamedu also contains two other zircon populations: the first group shows a discordant population with Pb-207/Pb-206 ages between 2278 and 2527 Ma with an upper intercept age of 2418 +/- 65 Ma. Similar Neoarchean-early Paleoproterozoic ages have been widely reported from the surrounding rocks within the PCSZ and also from the Salem Block to the north. We interpret these older zircons as xenocrysts entrained in the plagiogranite during magma ascent and consolidation. The third group of zircons in the plagiogranite yield Pb-206/U-238 age of 513 +/- 4.6 Ma, comparing well with the Cambrian ages reported in several recent studies from magmatic and metamorphic rocks in the PCSZ and the crustal blocks to the south, correlating with the tectono-thermal events associated with the collision and post-collisional extension associated with the assembly of the Gondwana supercontinent. Zircons in the two gabbro samples from the Manamedu complex analyzed in this study yield well-defined clusters on the concordia and show weighted mean Pb-206/U-238 ages of 744 +/- 11 Ma and 786 +/- 7.1 Ma. The internal structure of the zircons from these gabbro samples and their high Th/U values also suggest a magmatic crystallization history. The zircon ages that we obtained from the Manamedu forearc complex also compare with the recently reported zircon age of 825 +/- 17 from the gabbro-anorthosite complex and the 766 +/- 8 Ma age from oscillatory-zoned euhedral crystals of magmatic zircons in felsic volcanic suite of the Kadavur Dome, to the south of Manamedu. The ages also compare with the 819 +/- 26 Ma Pb-206/U-238 age reported from zircons in arc-related rapakivi granite from an adjacent locality within the PCSZ. All these data suggest a prominent mid Cryogenian subduction system along the southern periphery of the PCSZ prior to the destruction of the Mozambique Ocean lithosphere and the final amalgamation of the Gondwana supercontinent. (C) 2011 Elsevier B.V. All rights reserved.