Early Permian Ultrahigh Tempera- Ture Metamorphism Of The Altai Oro- Gen In Northern Xinjiang,China

Ultra-high-temperature (UHT) granulite-facies rocks offer important constraints on crustal evolution processes and tectonic history of orogens. In this study, one of the rare occurrences of the Phanerozoic UHT granulites-early Permian UHT granulite from the Altai orogen (north-ern Xinjiang) in the western segment of the Central Asian Orogenic Belt (CAOB) was detailed investigated in respect of petrography, chemical composition, phase equilibrium modeling, fluid inclusion and zircon geochronology.The diagnostic mineral in these rocks includes high alumina-orthopyroxene (Al2O3 up to 9.76 wt.%, and y(opx)=AlVI in orthopyroxene up to 0.21) coexisting with sillimanite and quartz, and low-Zn spinel (ZnO= 1.85-2.50 wt.%) overgrowth with quartz. Cordierite corona separated sillimanite from orthopyroxene. The high alumina orthopyroxene was replaced by symplectites of low-alumina orthopyroxene (5.80 wt.% Al2O3) and cordierite. These textural observations indicate a significant decompression following the peak UHT metamorphism.The peak metamorphic temperature (P)-pressure(T) conditions of 920—1010℃ and> 9 kbar were estimated from the conventional geothermobarometry. Phase equilibrium modeling using pseudosections and the y(opx) isopleths suggested an anti-clockwise P-T path for the exhuma-tion of the Altai orogenic belt. The pre-peak assemblage of spinel+quartz in garnet is stable at high- to ultra-high- temperature and low-pressure conditions (P< 5.8 kbar at T= ca.900℃). The peak P-T values recorded by high alumina-orthopyroxene is>940℃ and 7.8 to 10 kbar. Subsequent near-isothermal decompression (ITD) occurred at 890 to 940℃ a-5 to 6 kbar. The final-stage cooling was recorded at 750 and 800℃ and 4 to 5 kbar accompanied by a decrease in the y(opx) values (0.11-0.12).Newly discovered primary and pseudosecondary fluid inclusions in garnet together with pri-mary fluid inclusions in quartz were studied in terms of petrography and microthermometry as well as laser Raman spectroscopy analyses in detail. Melting temperatures of inclusions obtained indicated that the trapped fluid phase is dominantly carbonic. Raman probe confirmed a near pure CO2 composition with only minor dilutant of N2. The homogenized temperatures of the fluid inclusions both from quartz in matrix and enclosed in garnet porphyblasts range from 10.1 ℃ to 29.0℃ with corresponding density of 0.631-0.865 g/cm3. The range of CO2 isochores computed from density measurements in fluid inclusions passed below the retrograde stage of the P-Ttrajectory.206P6/238U age of 277±2 Ma was obtained from overgrowth rims of the zircon grains through LA-ICP-MS in-situ U-Pb dating, probably represents the timing of decompression and cooling stage based on the common occurrences of zircon grains in cordierites. Combined with the pre-vious studies, the formation of the UHT granulite in the Altai orogenic belt with a anti-clockwise P-T path should be related to an extensional setting caused by the subduction and slab detachment processes during the convergence of the Junggar plate and the Siberian plate. The low-density car-bonic fluid inclusions occurring in these rocks are probably a result of density reversal due to the modification of the inclusion cavity volume during rapid decompression.