The Plutonic Concept

Most of the Archaean rocks in India have been commonly referred to as having originated at plutonic or hypometamorphic depths. Data exist to prove that the Archaean crusts were decidedly thin and hotter. Thus, metamorphism in the remote Archaean time took place under lower load pressure around 4-7 kb and under higher temperature around 650°C-750°C. As the crust thickened metamorphism in depth took place under higher pressures and lower temperatures.

The Khondalite tract along the East Coast of India has mineralogical, petrological and structural singularity to prove that it was metamorphosed under the former set of P-T conditions.

This khondalite belt shows a trend of 'facies' variation which is strongly contrasted with the variation in Hietanen's Barrovian or other types. A new 'facies series' and metamorphic' type', viz, 'Khondalite type' is hence proposed to be added to Hietanen's eight 'types' of metamorphism.