Carbonic Metasomatism and Charnockitic Alteration

Abstract

Field relations in an orthogneiss quarry within the Palghat Gap region illustrate the development of charnockite alteration halos surrounding alkaline pegmatite dykes. Altered orthopyroxenes in the charnockitic zones are artifacts from concordant melt pockets within the gneisses, where orthopyroxene has developed prior to the charnockitie alteration during regional metamorphic, in a process equivalent to dehydration melting. The charnockitic halos represent metasomatic alteration zones developed through the infiltration of CO₂-H₂O fluids expelled from the dykes at shallow crustal levels. Such examples of greasy green charnockitie alterations could be a widespread phenomenon in different crustal blocks of varying metamorphic grades, and are unrelated to regional prograde granulite processes.