Charnockite Forming Metamorphism in Palghat, Southern India: A Fluid Inclusion Study of Gneiss, Charnockite, Charnoenderbite and Pegmatite

The Palghat region in southern India presents excellent examples of fluid controlled gneiss to charnockite transition zones. The paper constitutes results of detailed fluid inclusion studies carried out to understand the nature and role of fluids, across transition zones of such transformations and in associated charnoenderbite and pegmatite. Fluid inclusion petrographic studies suggest presence of at least three generations of fluid inclusions in different rock types viz , CO₂, CO₂-H₂O, and CO₂-H₂O-NaCl. The high-Density CO₂ inclusions (1 03 to 0 91 g/cc) are characteristics of charnoenderbites with moderate density CO₂ inclusions(0 92 to 0 70 g/cc) in hornblende gneiss and relatively low CO₂ density( 0 84 to 0 71 g/cc) inclusions in arrested charnockites. Salinities range from 2 to 35 wt% NaCl equivalent. Majority of the inclusions are low-Density CO₂, indicating preferential leakage and re-Entrapment of these inclusions under shallower conditions. In charnoenderbites, a distinct bi-Modal distribution in CO₂ densities is documented with early high-Density CO₂ inclusions, trapped under higher pressure, deep seated conditions and a late, low-Density CO₂ inclusions trapped under relatively shallower conditions. Occurrence of generally low-Density CO₂ inclusions in the arrested charnockites suggest release and re-Entrapment of CO₂ inclusions along structurally controlled weak zones. The higher concentration of low-Density monophase CO₂ inclusions in the arrested charnockites and pegmatite are interpreted as a result of selective removal of H₂O from the mixed CO₂-H₂O inclusions, during ductile and ductile-Brittle shear deformation. The CO₂ inclusions showing large density differences between the early charnockites (charnoenderbite) and late (arrested) charnockites are suggestive of entrapment at different periods and that the CO₂ densities of the charnoenderbite have been re-Equilibrated during the latest tectono-Metamorphic event. The fluids responsible for the formation of arrested charnockites are low P-T type late fluids, formed at a shallower, mid crustal levels. The study further illustrates major role of saline brines in the formation of arrested charnockites.