Metamorphism of the Krishnagiri Granite Gneisses: The Transition zone between the Dharwar Craton and the Southern Granulite Terrane

Migmatites provide essential insights into the processes of partial melting and the rheological behavior of the middle and lower continental crust. This study investigates the extensively migmatized granite gneisses in the northern part of Tamil Nadu, within the Southern Granulite Terrane (SGT). The term transition zone refers to the metamorphic grade shift from lower to higher grades, observed south of the Dharwar Craton (DC) towards the SGT. The SGT is divided into several blocks, separated by crustal-scale shear zones, with this research focusing on the Krishnagiri area of the Shevaroy Block (SB), where the transition from greenschist to granulite facies is evident. Earlier studies proposed the Fermor Line, a hypothetical boundary marking the change from lower-grade to higher-grade metamorphic facies.

The entire Krishnagiri area has dome-shaped hills. The field evidence shows partial melting, including lensoidal leucosome patches and veins, migmatitic quartzo-feldspathic gneisses, and migmatites appearing as metatexites and diatexites. These rocks exhibit schollen, schlieren, and nebulitic structures, with greater migmatization observed near the Mettur shear zone, forming the western boundary of SB. Mafic enclaves in these rocks are deformed and stretched along the shear zone. Deformation features such as shear zones, mylonites, and shear sense indicators are also prominent. The stretching lineation within the granite gneisses trends at 015°.

Field and microscopic observations allow the Krishnagiri granite gneisses to be categorized into four distinct zones based on mineralogical assemblages. Zones 1 and 2 contain metamorphic minerals like epidote, amphibole, and biotite. Zone 3 lacks epidote but includes amphibole and biotite, with more pronounced foliation. Zone 4 features pyroxenes and garnet, with pyroxenes altered to chlorite and rimmed by amphiboles. Mafic enclaves are abundant in Zone 1 and occur within leucosomes, while Zone 4 contains charnockite as pods or lenses within the granite gneiss. K-feldspar veins, appearing from Zone 2 onward, cross-cut the gneisses and include mafic minerals such as amphiboles and pyroxenes.

All these observations indicate an increasing metamorphic grade from north to south across the Krishnagiri region and thus revealing progressively deeper crustal levels towards the south.