Incipient charnockites from southern India: The role of brines

Southern India and Sri-Lanka are the places where "incipient charnockites", i.e. the local transformation of amphibolite-facies gneisses into orthopyroxene-bearing, igneous looking charnockites, have been discovered in the early sixties. The fact that some incipient charnockites occur along a network of brittle fractures, together with CO₂ remnants preserved in mineral inclusions, had called for the role of fluids during charnockite alteration. The present work presents new observations on fluid inclusions and microtextures of incipient charnockites from type localities in southern India. In addition to CO₂-rich fluid inclusions in quartz and feldspar, all of the occurrences have disrupted remnants of concentrated aqueous alkali chloride solutions. CO₂ inclusions are more abundant in paragneiss (Kerala) than in orthogneiss (Karnataka/Tamil Nadu). The finding of disrupted brine inclusions in the Kabbal charnockite is a key link between closely associated massive charnockites and Closepet Granite, both of which also share the brine remnants. All of the occurrences studied here have feldspar or feldspar-quartz microvein networks along grain boundaries of recrystallized quartz, feldspar and orthopyroxene. These metasomatic veins again indicate the action of alkali-exchanging fluids (i.e., saline solutions). Feldspar microveins, which have been found in most "massive" charnockites, along with the CO₂-rich fluid inclusions, suggest a commonality of incipient charnockite and massive charnockite, both types differing in intensity of interaction with metasomatizing pore fluids.