Mineralogical and Petrological Studies of Gneisses and Charnockites of Dharmapuri District, Tamil Nadu

Detailed mineralogical studies of amphibolite to granulite facies transition zone rocks and granulite facies rocks in Dharmapuri district, Tamil Nadu, were carried out in order to know the gneiss-charnockite relations, nature of protolith, and metamorphic conditions.

The studies show that the rocks are of magmatic origin. and show a gradual decrease in modal composition of hornblende and biotite and gradual increase of hypersthene and iron oxide in a north to south traverse. Breakdown of hydrous minerals to anhydrous minerals is evident with the release of iron oxide suggesting progressive transformation of gneisses to charnockite, Biotite occurs in two generations, an earlier one is of massive type and breaks down to anhydrous minerals, while the latter one occurs as tabular flakes and likely to have formed during the waning stages of amphibolite to granulite facies metamorphism, and forms a zone of concentration just above the stage or CO₂ migration. The mineralogy of pyroxene, plagioclase, hornblende, biotite and iron oxides also suggests that the pressure-temperature conditions in the transition zone are lower than those of the granulite zone.