Tongka garnet pyroxenite in eastern Tibet and Songduo blue schist in Lhasa block

Abstract：The Proterozoic Kaqiong Group is distributed in northern Basu County of east Tibet, with an area of 80 km2. It is located in the Bandonghu-Neijiang juncture zone. The group consists lithologically of biotite-plagioclase gneiss, black mica-plagioclase granulite, granite gneiss, marble and amphibolite. The biotite-plagioclase gneiss is composed of alamandine, sillimanite and kyanite. The U-Pb zircon age is 1334 Ma, and the U-Pb zircon age of granite-gneiss is （507±10） Ma(SHRIMP). The granite-gneiss might be a intrusive granite which underwent the process of deformation-metamorphism in later periods. The Kaqiong Group is a micro-continent separated from the northern margin of Gondwana continent. Garnet pyroxenite and garnet amphibolite inclusions were found for the first time in the biotite-plagioclase geniss. The fresh garnet amphibolites are composed of hornblende, diopside, garnet and a small amount of quartz and rutile, without feldspar. As for chemical constituents of the inclusions, SiO2 is 48.65%～51.18%，k2O<1%. The REE pattern is a flat pattern, with no obvious En anomaly. Trace elements Rb, Ba, Th are rich, and Sr is low, showing characteristics of MORB. The end member molecules for garnets in eclogite are pyrope 51.44%, almondite 26.73%, and grossular 17.29%. The end member molecules for diopside are jadeite 6.40%. Grt-Cpx is commonly overprinted by Hb-Pl and can be observed under microscope. Inclusions should be the products of eclogite degradation.