The Southern Granulite Terrain is composed of a collage of crustal blocks with intervening shear/suture zones. The terrain has witnessed 550 Ma old granulite events (high temperature metamorphism) which occur well south of the broad Palaghat-Cauvery shear zone. It is evidenced that the Palghat Cauvery shear zone itself contains the older rocks like 3.0 Ga Sittampundi anorthosite and the younger Tiruchengodu granite of 660 Ma old (Bhaskar Rao et al., 1996). SGT is characterized by the presence of few major shear zones and deep crustal faults and is bounded by shear zone/fault immediately bordering the Kodaikanal hill ranges (Janardhan, 1999). Prominent among them are Moyar Bhavani, Palghat-Cauvery and Achankovil shear zones. These shear zones represent major terrain boundaries in south India. The major shear zones, E-W trending Palghat-Cauvery in the north and NW-SE trending Achankovil in the south, divide the SGT into (a) Northern Block (b) Madurai Block and (c) Trivandrum Block. The Northern Block composed of orthopyroxene bearing granulites and hornblende biotite gneiss and syenite and carbonatite intrusive bodies (750 Ma) that occur along Salem-Attur and Mettur shear zones. The triangular shaped land mass bounded by Moyar shear zone in the north and Bhavani shear zone in the south is known as Nilgiri block. Nilgiri block represents the deepest level of exposure (9-10 kb; palaeo-depth of ~ 35 km) of granulite grade lower crust in south India. The crustal section is essentially composed of garnetiferous hypersthene bearing tonalitic granulites with smaller amount of kyanite-bearing gneisses, quartzites and banded magnetite quartzites. Its deeper northern part is encroached by intermediate and basic magmatic rocks presently represented by non-garnetiferous hypersthene bearing tonalite and garnetiferous basic granulites, basaltic to anorthositic granulites with both the pyroxenes and pyroxenites (Raith et al., 1999). The block lying between Palaghat Cauvery shear zone in the north and Achankovil shear zone in the south is known as Madurai block.
