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.