MINERALOGICAL CHARACTERISTICS AND PETROGENESIS OF DEEP-DERIVED XENOLITHS IN CENOZOIC SYENITE-PORPHYRY IN LIUHE, WESTERN YUNNAN PROVINCE

Abstract

Several types of garnet-bearing deep-derived xenoliths have been discovered in syenite-porphyry in Liuhe Village, Heqing County, Yunnan Province. In this paper, we reported in detail the petrographic characteristics and retrogressive metamorphic phenomena of the xenoliths. The result of electronic probe microanalysis (EPMA) of main rock-forming minerals is also presented. Based on the mineral assemblages of the xenoliths and the chemical compositions of the main rock-forming minerals, the xenoliths are divided into five categories. The pressure and temperature of rock forming and metamorphism are estimated with clinopyroxene geothermobarometer, amphibole geobarometer and garnet-clinnopyroxene Fe-Mg geothermometer. It is concluded that garnet diopsidites originated from the upper mantle (87～95 km in depth) and were captured by syenitic magma as they moved upwards into the lower crust rapidly; garnet-diopside amphibolite and granulite were formed in the lower crust (45～55 km in depth); and garnet -plagioclase amphibolite came from the middle-upper crust (about 30 km in depth). With an exception of garnet diopsidite, the other four types of xenoliths all underwent retrogressive metamorphism. According to their textures, chemical compositions and estimated pressure and temperature, the p-T trajectory of the deep-derived xenoliths is constructed. It is inferred that the retrogressive metamorphism occurred in two stages, and fluid rich in Na played an important role in it. The significance of the change of pressure and depth during the retrogressive metamorphism is also discussed in this paper.