An exotic Mesoarchean microcontinent: The Coorg Block, southern India

Sandwiched between the Dharwar Craton in the north and the Neoarchean-Proterozoic crustal blocks to the south, the Coorg Block in southern India is composed dominantly of a suite of arc magmatic rocks including charnockites, TTG (tonalite-trondhjemite-granodiorite)-related granitoid suite and felsic volcanic tuffs together with minor accreted oceanic remnants along the periphery of the block. Coeval mafic and felsic magmatism with magma mixing and mingling in an arc setting is well represented in the block. Here we present the petrology, geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes of all the major lithologies from this block. Computation of metamorphic P-T conditions from mineral chemical data shows consistent granulite-facies P-T conditions of 820-870 degrees C and up to 6 kbar. Our geochemical data from major, trace and REE on representative samples of the dominant rock types from the Coorg Block corroborate an arc-related signature, with magma generation in a convergent margin setting. The zircon data yield weighted mean (207)pb/Pb-206 ages of 3153.4 +/- 9 to 3184.0 +/- 55 Ma for syenogranites, 31703 +/- 6.8 Ma for biotite granite, 3275 +/- 5.1 Ma for trondhjemite, 3133 +/- 12 to 3163.8 +/- 6.9 Ma for chamockites, 3156 +/- 10 to 3158.3 +/- 82 for mafic enclaves, 3161 +/- 16 Ma for diorite and 3173 16 Ma for felsic volcanic tuff. An upper intercept age of 3363 +/- 59 Ma and a lower intercept age of 2896 +/- 130 Ma on zircons from a charnockite, as well as an evaluation of the Th/U values of the zircon domains against respective Pb-207/Pb-206 ages suggest that the Mesoarchean magma emplacement which probably ranged from >33 to 3.1 Ga was immediately followed by metamorphism at ca. 3.0 to 2.9 Ga. The ages of magmatic zircons from the chamockites and their mafic granulite enclaves, as well as those from the volcanic tuff and biotite granite, are all remarkably consistent and concordant marking ca. 3.1 Ga as the peak of subduction-related crust building in this block, within the tectonic milieu of an active convergent margin. The majority of zircons from the Coorg rocks show Hf isotope features typical of crystallization from magmas derived from juvenile sources. Their Hf crustal model ages suggest that the crust building might have also involved partial recycling of basement rocks as old as ca. 3.8 Ga. The crustal blocks in the Southern Granulite Terrane in India preserve strong imprints of major tectonothermal events at 25 Ga, 2.0 Ga, 0.8 Ga and 0.55 Ga associated with various subduction-accretion-collision or rifting events. However, the Coorg Block is exceptional with our data suggesting that none of the above events affected this block. Importantly, there is also no record in the Coorg Block for the 2.5 Ga pervasive regional metamorphism that affected all the other blocks in this region. The geochronological data raise the intriguing possibility that this block is an exotic entity within the dominantly Neoarchean collage in the northern domain of the Southern Granulite Terrane of India. The Mesoarchean arc-related rocks in the Coorg Block suggest that the magma factories and their tectonic architecture in the Early Earth were not markedly different from those associated with the modern-style plate tectonics. (C) 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.