Retrogressive Metamorphism of Mafic Granulites of Manantoddy, North Kerala

The mineral reactions: garnet + quartz = orthopyroxene + plagioclase; garnet + clinopyroxene = orthopyroxene + pargasite + anorthite and [clinopyroxene = hornblende + quartz are seen as symplectitic intergrowths in the mafic granulites of north Kerala, suggesting a retrogressive evolutionary history. The estimate of physical conditions of metamorphism based on the equilibrium mineral chemistry of gar + cpx (+ plag + q + z ), suggest 800°C temperature and 11 kb pressure for the peak period of metamorphism. Uplift of the terrain, following metamorphism caused partial breakdown of garnet in the presence of clinopyroxene and quartz in producing retrogressive textures and mineral assemblages. P-T of 6 kb and 583°C is indicated for tbe development of syrnplectite of garnet and secondary orthopyroxene. The pressures document slow uplift from inferred depth of at least 30 krn during Archaean to a level of 20 km for the development of symplectites. The mineral chemistry and P-T suggest that the retrogressive metamorphism did not transcend but remained 'within' the transitional granulite facies conditions.