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

High-density CO² inclusions occur abundantly in granulite fadesrocks (age of metamorphism 2–5b.y.) of the Nilgiri massif,southern India. The chronology of carbonic inclusions in thewidespread enderbitic granulites studied in relation to thedevelopment of micro-textures and mineral assemblages indicatesthat randomly oriented, negative-crystal-shaped CO² inclusions(4–20 µim) in garnet and quartz grains (qtz I) armouredby garnet entrap syn-peak-metamorphic pore fluids. The moreabundant trail-bound CO₂ inclusions in the deformed, polygonized,and partially recrystallized matrix quartz grains (qtz II andIII) and plagioclase grains were formed in connection with astage of compressional deformation and subsequent annealingrelated to the development of the late-Proterozoic Bhavani shearzone. These inclusions resulted from local re-equilibrationof the former peak-granulitic carbonic inclusions and re-entrapmentof released fluids. The presence of pure CO₂ in all the inclusionsis confirmed by microthermometric data and laser-excited Ramanmicrospectrome-try. Temperatures of homogenization (liquid phase)are in the range of 50 to +20C, and the corresponding CO₂ densitiesare between 1.154 and 0–807 g/cm3. Mineralogical thermobarometry on the enderbitic granulites documentsa continuous gradient of near-peak metamorphic conditions from750C, 9–10 kb in the northern part to 73OC, 7 kb inthe 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 indicatethat differential uplift of the massif to mid-crustal levelswas accomplished before late compressional deformation. In conformity,carbonic inclusions in quartz II and III are characterized byuniformly high density (1.154–1.08 g/cm3). In contrast,early carbonic inclusions in garnet and quartz I preserve thedensity contrast reflecting the regional P, T gradient duringnear-peak metamorphic fluid entrapment. The fluid inclusionsys-tematics indicate ‘near-isochoric’ uplift ofthe northern high-P domain, but near-isobaric cooling of thesouthwestern low-P domain. The carbonic fluids are thought tohave been derived either from internal sources during dehydration-meltingprocesses or from freezing synmetamorphic intrusives into thelower crust.