Question

Gregional geology of southern granulitic terrain

Answer 1

This chapter provides a summary of geological and tectonic aspects of Southern Granulite Terrane (SGT), which hosts varied lithologies of high grade rocks like charnockites, khondalites and other granulite facies metamorphic rocks with granitic intrusions of different ages. The SGT extending from 80 to 110 N latitude of India (Tamil Nadu, Karnataka, and Kerala states) is one of few terranes in the world that has preserved Archaean and Proterozoic crust with extensive high-grade granulite facies rocks (Fig 1). The transitional boundary between the high-grade and low-grade terranes is demarcated by well known ‘‘Fermor line’’ representing a fundamental crustal discontinuity along which there must have been considerable vertical displacement (Fermor, 1936). This line separates the exposed charnockitic region from non-charnockitic region. The SGT is believed to be of lower crustal origin through a complex evolutionary history with multiple deformations, anatexis, intrusions and polyphase metamorphic events. More than 80% of the terrane is covered by varied lithologies of Archaean and Proterozoic age groups namely, Sathyamangalam Group (>3200 Ma), layered mafic and ultramafic complexes, Bhavani Group (~3000 Ma), Kolar Group (~2900 Ma), Khondalite Group, Charnockite Group (~2600 Ma) and Migmatitic complex (2200-2250Ma) etc. The Proterozoic rocks include younger granulites/charnockies, granites, alkali syenites, corbonatites, mafic and ultramafic intrusives mostly occurring in and around the Cauvery suture/ shear zone (CSZ) (GSI, 2006). 11 Fig 2.1: Geological frame work of Southern India (after Santosh and Sajeev, 2006) 2.1.1. Sathyamangalam Group The Sathyamangalam group of rocks is considered to be equivalents of ‘Sargurs’ of Dharwar craton exposed in the central and northwestern part of the Indian Peninsula. The 12 group consists of quartzite ±fuchsite ± kyanite ± sillimanite and banded iron formation, sillimanite schist ± garnet, kyanite- schist, corundum bearing mica schist and talc-tremolite schist; calc granulite, crystalline limestone/marble, ortho-and para amphibolite (Gopalakrishnan, et al., 1975). 2.1.2. Layered mafic and ultramafic complexes In the proximity of Sathyamangalam group, ultrabasic rock sequence of dunite, peridotite, websterite, garnetiferous gabbro, gabbroic anorthosite occur near Mettupalayam (NW part) and other areas. They generally occur as enclaves within the migmatitic gneisses as a part of the dismembered sequence. Large volume of garnetiferous gabbro and hornblendic anorthosite with chromitite layers as well as small lenses of eclogitic rocks are the characteristic features of this suite (Gopalakrishnan, 1994b). The well known Sittampundi anorthosite yielded 3000-2900Ma by Sm-Nd systematics (Bhaskara Rao et al., 1996). Sittampundi layered anorthosites define two populations : older yield a Concordia age of 2541± 13 Ma, younger belonging to a high-grade metamorphic event at 2461± 15 Ma by zircon LA-ICPMS U–Pband Hf isotope data (Ram Mohan et al. 2013). Similar complexes also occur as small bodies, lenses and bands, in the northeastern sector of the SGT around