CO 2-rich fluid inclusions in staurolite and associated minerals in a high-pressure ultrahigh-temperature granulite from the Gondwana suture in southern India

Abstract

We report the first detailed fluid inclusion study on Mg–Al rocks from the Gondwana suture zone along the Palghat–Cauvery Shear Zone System in southern India. These rocks underwent high-pressure ( P > 12 kbar) and ultrahigh-temperature ( T ∼ 990 °C) metamorphism during the late Neoproterozoic associated with the collisional assembly of the Gondwana supercontinent. Fluid inclusions of texturally primary and pseudosecondary categories occur in plagioclase, staurolite, and kyanite in these rocks. Inclusions of secondary nature are found in garnet and apatite. All the trapped fluids show melting temperatures in the range of − 58.6 to − 56.6 °C, suggesting a dominantly pure CO 2 composition. The secondary inclusions show high homogenization temperatures ( T h) of + 12.9 to + 30.0 °C, indicating low density of 0.59–0.83 g/cm 3. Although most of the primary and pseudosecondary inclusions also contain moderate to low-density fluids (0.67–0.95 g/cm 3; T h = − 3.5 to + 27.4 °C), rare high-density inclusions (1.00–1.06 g/cm 3; T h = −26.1 to − 14.1 °C) were discovered in matrix plagioclase and porphyroblastic staurolite. The estimated CO 2 isochores for the high-density primary and pseudosecondary inclusions intersect the clockwise P–T trajectory of the rocks at around 8.3–9.0 kbar at 1000 °C, which broadly coincide with the peak metamorphic conditions of the area. Since these fluid inclusions show textural evidence for entrapment during the growth of the host high-grade minerals, we interpret the high-density CO 2 fluids to be traces of a CO 2-rich synmetamorphic fluid that was present during the peak ultrahigh-temperature metamorphism. Despite the preservation of high-density CO 2 inclusions in staurolite and plagioclase, the primary and pseudosecondary inclusions in most of the other minerals show wide density variation which suggests the fluids in these inclusions underwent significant density reversal (density decrease) during decompression of the rocks from high-pressure through the peak ultrahigh-temperature stage along a clockwise P–T trajectory. The finding of pure CO 2 inclusions in these Mg–Al granulites conforms with similar observations from many other granulite-facies terrains with the carbonic fluids probably derived from mantle reservoirs and transferred through deep-seated shear zones that characterize this Gondwana suture.