Paragenetic relations in gedrite-cordierite-staurolite-biotite sillimanite-kyanite gneisses at Ajitpura, Rajasthan, India

Abstract

Aluminous parageneses containing gedrite, cordierite, garnet, staurolite, biotite, sillimanite, kyanite, quartz or spinel plus corundum are found as dark colored lenses in the polymetamorphic, multideformed Archean complex at Ajitpura in northwest peninsular India. Staurolite, like kyanite, is a relict phase of earlier metamorphism and is excluded as a paragenetic mineral in view of its incompatibility with quartz and gedrite and its lower X_Mg values than for garnet of the assemblage. Its stability here is attributed to zinc content of up to 3 wt%. The X_Mg in other ferromagnesian minerals decreases in the order: cordierite, biotite, gedrite, garnet, as found elsewhere in high grade rocks. The textural criteria and systematic partitioning of Fe and Mg in the ferromagnesian phases, excluding staurolite, indicate attainment of equilibrium during the second metamorphism. From tie line configurations in the phase diagrams, X_Mg ratios in the constituent minerals, and other petrographic criteria, it is suggested that gedrite-cordierite-garnet-sillimanite-biotite assemblage has been produced by the reactions: Biotite+Sillimanite+Quartz=Cordierite+Garnet+K-feldspar+Vapor (1) and Biotite+Sillimanite+Quartz=Cordierite +Gedrite+K-feldspar+Vapor (2) which occurred during partial melting of the rocks at fixed P and T conditions. By isothermal P-X(Fe-Mg) sections it has been demonstrated that release of FeO, SiO₂ and other components modified the composition of the reactant biotite presumably by the substitution FeSiP_H2O=P_total) about 6 kilobars.