Synmetamorphic High-Density Carbonic Fluids in the Lower Crust: Evidence from the Nilgiri Granulites, Southern India

Abstract

Listen

Translate article

High-density CO2 inclusions occur abundantly in granulite fades rocks (age of metamorphism ∼2–5b.y.) of the Nilgiri massif, southern India. The chronology of carbonic inclusions in the widespread enderbitic granulites studied in relation to the development of micro-textures and mineral assemblages indicates that randomly oriented, negative-crystal-shaped CO2 inclusions (4–20 μim) in garnet and quartz grains (qtz I) armoured by garnet entrap syn-peak-metamorphic pore fluids. The more abundant trail-bound CO2 inclusions in the deformed, polygonized, and partially recrystallized matrix quartz grains (qtz II and III) and plagioclase grains were formed in connection with a stage of compressional deformation and subsequent annealing related to the development of the late-Proterozoic Bhavani shear zone. These inclusions resulted from local re-equilibration of the former peak-granulitic carbonic inclusions and re-entrapment of released fluids. The presence of pure CO2 in all the inclusions is confirmed by microthermometric data and laser-excited Raman microspectrome-try. Temperatures of homogenization (liquid phase) are in the range of ⊟50 to +20°C, and the corresponding CO2 densities are between 1.154 and 0–807 g/cm3. Mineralogical thermobarometry on the enderbitic granulites documents a continuous gradient of near-peak metamorphic conditions from 750°C, 9–10 kb in the northern part to 73O°C, 7 kb in the southwestern part of the Nilgiri massif. Uniform P, Testimates (600–650 °C, 6–7 kb) for late coronitic garnet + quartz assemblages in enderbites and metadolerites indicate that differential uplift of the massif to mid-crustal levels was accomplished before late compressional deformation. In conformity, carbonic inclusions in quartz II and III are characterized by uniformly high density (1.154–1.08 g/cm3). In contrast, early carbonic inclusions in garnet and quartz I preserve the density contrast reflecting the regional P, T gradient during near-peak metamorphic fluid entrapment. The fluid inclusion sys-tematics indicate ‘near-isochoric’ uplift of the northern high-P domain, but near-isobaric cooling of the southwestern low-P domain. The carbonic fluids are thought to have been derived either from internal sources during dehydration-melting processes or from freezing synmetamorphic intrusives into the lower crust.