Petrology And Fluid Inclusions Of Garnet-clinopyroxene Rocks From Paramati In The Palghat-cauvery Shear Zone System, Southern India

摘要

We report petrological and fluid inclusion data on garnet-clinopyroxene rocks from Paramati in the Palghat-Cauvery Shear Zone System, southern India. The rocks are composed of garnet, clinopyroxene, and plagioclase with or without orthopyroxene and retrograde pargasite. The peak P-T conditions of 9-12 kbar and 830-880 ℃ were obtained from early garnet-clinopyroxene-plagioclase-quartz assemblage. Available petrological data from this region suggest that the high-pressure event was followed by heating to the peak ultrahigh-temperature condition and decompression as supported by the occurrence of orthopyroxene + plagioclase symplectite between garnet + clinopyroxene and plagioclase lamella in clinopyroxene. High Ca-Eskola molecule in the integrated clinopyroxene (～ 8.6%) is also comparable with its high-pressure origin. Primary and secondary fluid inclusions in garnet are dominantly CO_2-rich. They were homogenized into the liquid phase in the temperature range of-11.1 to +14.2 ℃, corresponding to low-CO_2 densities of 0.82 to 0.99 g/cm~3. The fluid densities, when computed into isochores, indicate lower pressures (～ 6-7 kbar at 900 ℃) than the P- T conditions estimated by geothermobarometry. The low-density values and the primary nature of the inclusions trapped in garnet, as well as wide variation in homogenization temperatures, indicate density modification of prograde metamorphic fluid during exhumation. The results of this study support prograde high-pressure metamorphism and subsequent decompression along a clockwise P- T path for the evolution of the rocks in this Gondwana suture zone probably related to subduction of crustal materials to the mantle depth ( > 40 km) and following continent-continent collision during the amalgamation of the Gondwana Supercontinent.