Carbonic fluid inclusions in amphibolite-facies pelitic schists from Bodonch area, western Mongolian Altai

Abstract

We report first fluid inclusion data on amphibolite -facies pelitic schists from Bodonch area of western Mongolian Altai in the Central Asian Orogenic Belt. Three categories of fluid inclusions have been observed in quartz: dominant primary and secondary inclusions, and least dominant pseudosecondary inclusions. The melting temperatures of all the categories of inclusions lie in the narrow range of −57.5 °C to −56.6 °C, close to the triple point of pure CO 2 . Homogenization of fluids occurs into liquid phase at temperature between −33.3 °C to +19.4 °C, which convert into densities in the range of 0.78 g/cm 3 to 1.09 g/cm 3. The estimated CO 2 isochores for primary and pseudosecondary high -density inclusions is broadly consistent with the peak metamorphic condition of the studied area (6.3 -7.3 kbar at 655 °C). The results of this study, together with the primary and pseudosecondary nature of the inclusions, indicate CO 2 was the dominant fluid component during the peak amphibolitefacies metamorphism of the study area. The examined quartz grains are texturally associated with biotite, kyanite and staurolite, which are regarded as high -grade minerals formed during prograde to peak metamorphism. Therefore quartz probably formed by high -grade metamorphism and the primary fluid inclusions trapped in the minerals probably preserve fluids at around peak metamorphism.