Ultrahigh-density carbonic fluids in ultrahigh-temperature metagabbros from the Palghat-Cauvery Suture Zone, Southern India: Phase equilibria modelling and fluid inclusion study

Abstract

Fluid inclusions trapped in high-grade metamorphic minerals offer direct evidence for the nature and role of fluids involved in lower crustal processes. Here, we report extremely high-density (~1.155 g/cm 3 ) carbonic fluid inclusions preserved within garnet grains in metagabbros from the central segment of the Palghat-Cauvery Suture Zone (PCSZ) in southern India. The metagabbros are composed of coarse-grained garnet, clinopyroxene, orthopyroxene, plagioclase, quartz, and retrograde calcic amphibole. Garnet occurs either as porphyroblastic grains formed during prograde to peak stage, or as fine-grained aggregates around plagioclase possibly formed by post-peak decompressional (near-isobaric) cooling. The peak and retrograde P–T conditions have been estimated as ~940 °C/~11.5 kbar and ~ 840–850 °C/~9.3–10.2 kbar, respectively, based on phase equilibria modelling using pseudosection computation. Fluid inclusions occur as primary and secondary phases within the porphyroblastic garnet and matrix plagioclase. The melting temperatures of these fluid inclusions are in the range of −57.4 to −56.6 °C, indicating nearly pure CO 2 composition. The lowest homogenization temperature from the primary inclusions in garnet (−50.1 °C) translates to extremely high density of 1.155 g/cm 3 . The calculated isochores for the inclusions (~11.3 kbar at 950 °C) are broadly consistent with the peak P–T conditions, suggesting that the fluids are trapped near the peak metamorphic stage under a dry condition. The preservation of ultrahigh-density carbonic inclusions is probably due to isochore-parallel decompressional cooling after the peak metamorphism. The secondary inclusions show slightly lower densities of 1.014–1.053 g/cm 3 (in garnet) and 0.955–0.976 g/cm 3 (in plagioclase), the isochores of which suggest a lower-pressure range of 4.8–5.8 kbar at 800 °C. The high-density pure CO 2 fluids reported in this study as well as in previous studies from the PCSZ, and the isochoric evolution provide robust evidence for the presence of supercritical carbonic fluids in the lower continental crust. • Extremely high-density (~1.155 g/cm 3 ) carbonic fluid inclusions in metagabbros. • Peak and retrograde conditions of ~940 °C/~11.5 kbar and ~ 840–850 °C/~9.3–10.2 kbar. • Garnet aggregates around plagioclase formed by post-peak near-isobaric cooling. • Isochore-parallel decompressional cooling after the peak metamorphism. • Ultrahigh-density fluids are preserved due to isochoric exhumation path.