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

摘要

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 ℃ to -56.6 ℃; close to the triple point of pure CO_2. Homogenization of fluids occurs into liquid phase at temperature between -33.3 ℃ to +19.4 ℃; which convert into densities in the range of 0.78 g/cm_3to 1.09 g/cm3. 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 ℃). The results of this study; together with the primary and pseudosecondary nature of the inclusions; indicate C_O2 was the dominant fluid component during the peak amphibolite-facies metamorphism of the study area. The examined quartz grains are texturally associated with biotite; kya-nite 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.