Structure and Metamorphic Characteristics of Cordierite-Bearing Gneiss of South Kerala

Abstract

Cordierite-bearing gneiss of the Quilon district, Kerala, occurs in a 8-10km wide zone within quartzo-feldspathic gneiss, being bordered on the north by a charnockite massif. These gneisses are deformed and are polymetamorphic in character. The cordierite-forming metamorphism is the second in metamorphic chronology. The reaction producing the bulk of cordierite was: biotite + sillimanite + quartz ⇌ cordierite + garnet + K-fetdspar + H₂O. The K_D (Mg-Fe) values of coexisting garnet-cordierite and garnet-biotite pairs indicate that the PT conditions of metamorphism (∼ 7 kb, ∼ 720°C) were those of granulite facies. Under these conditions cordierite was produced from Fe-Mg enriched melt restite, being trapped at the fringe of the charnockite massif.