Multiple Tectonothermal Events in the Granulite Blocks of Southern India Revealed from EPMA Dating: Implications on the History of Supercontinents

Abstract

We report age data on zircon, monazite, uraninite and huttonite from a suite of 29 samples covering four major granulite blocks in southern India using an electron microprobe technique. The rocks analysed in this study cover all of the major lithounits in these terrains and include garnet-bearing and garnet-free charnockites, garnet-biotite gneisses, khondalites, calc-silicate rocks, and a suite of orthogneisses (biotite gneiss, biotite-hornblende gneiss). Two pink metagranites representing the magmatic phase were also analysed. Zircons from the Madras Block yield well-defined isochrons at 2.5-2.6 Ga. Core to rim analyses of single zircon grains show age zoning with 2.6-2.9 Ga igneous cores mantled by 2.4-2.5 Ga rims. Detrital zircons show age up to ca. 3.2 Ga. Monazites in this block have cores and rims with 2.5-2.3 Ga. A suite of 19 samples from the Madurai Block brings out the multiple tectonothermal events in this terrain. Zircons from an orthogneiss yield well-defined isochrons at 1.7±0.1 Ga, 0.82±0.05 Ga and 0.58±0.04 Ga from core, inner rim and rim portions, respectively. Zircon grains in other rocks preserve either core or secondary growth ages at 0.8-1.0 Ga. Zircons in a pink metagranite from this block show sharply defined isochrons of 0.68±0.03 Ga for the core and 0.57±0.01 Ga for the secondary portion. A late Pan-African overprint is observed throughout this block with zircon rims, monazite, uraninite and huttonite yielding age values in the range of 0.45-0.60 Ga. Zircons from both the Trivandrum and Nagercoil blocks show a major tectonothermal event at 0.55 Ga with faint indications of previous tectonothermal events during 0.8-1.0 and 1.7-2.0 Ga. Monazite data from both the Trivandrum and Nagercoil blocks are essentially similar to those from the Madurai Block except for presence of relic monazite in the former. Our study confirms the notion that the Palghat-Cauvery Shear Zone marks the major terrain boundary between an Archean craton in the north and Proterozoic terrains in the south. It also strengthens the view of Paleoproterozoic accretion and Pan-African reworking of blocks south of this shear zone. The ages of production, accretion, and reworking in the terrains of southern India yield important information for the histories of Columbia (∼1.8-1.5 Ga), Rodinia (∼1.1-1.0 Ga), and Gondwana (∼0.6-0.5 Ga) supercontinents. The southern Indian terrains formed part of a worldwide network of orogenic belts that is centered around 1.8 Ga and outlines the configuration of Columbia. Accretion of terrains in the Madurai block to Archean rocks north of the Palghat-Cauvery Shear Zone at this time is consistent with the ages suggested to be the oldest metamorphic event in the Eastern Ghats orogen of eastern India and the Rayner belt of coastal East Antarctica. Our data confirm earlier evidence that all of southernmost India underwent resetting of isotopic systems during the final accretion of Gondwana at the time of the Pan-African orogeny (∼0.5 Ga). The possibility that the Trivandrum and Nagercoil terrains accreted to the Madurai block after ∼0.7 Ga suggests that this Pan-African zone may also have involved accretion and closure of ocean basins. If that happened, then this zone may be the long-sought suture between East and West Gondwana.