Petrology of gedrite-bearing rocks in mid-crustal ductile shear zones from the Eastern Ghats Belt, India

Abstract

A suite of metapelitic, basic and quartzofeldspathic rocks intruded by enderbitic gneiss from the southernmost tip of the Eastern Ghats Belt, India, and metamorphosed at c. 750-800 °C, 6 kbar, were subjected to repeated ductile shear deformation, hydration, cooling and accompanying alkali metasomatism along narrow shear zones. Gedrite-bearing assemblages developed in the shear zones traversing metapelitic rocks. Interpretation of the reaction textures in an appropriate P-T grid in the system FMASH, an isothermal-isobaric μ_H2O-μ_Na2O grid in the system NFMASH, and geothermobarometric data suggest a complex evolutionary history for the gedrite-bearing parageneses. Initially, gedrite-bearing assemblages were produced due to increase in μ_Na2O at nearly constant but high μ_H2O accompanying cooling. Gedrite was partially destabilized to orthopyroxene+albite due to progressively increasing μ_Na2O. During further cooling and at increased μ_H2O a second generation of gedrite appeared in the rocks.