Paired high-pressure granulite and eclogite in collision orogens and their geodynamic implications.

In some typical collision orogens, high-pressure granulites are spatially and temporally associated with eclogites. Understanding the relationship between eclogites and high-pressure granulites can provide crucial constraints on the orogenic process and mechanism in the continental collision orogens. This contribution presents the field relationships, metamorphic conditions and chronological data from three areas (South Altyn, North Qaidam and North Qinling) to establish a tectonic model for the formation of high-pressure granulites and associated eclogites in continental collision orogens. South Altyn Tagh (SAT) eclogites occur in the Yushikuanggou-Jianggeleshiyi area near Qiemo. These eclogites and associated sillimanite-bearing gneisses, granitic gneisses and small amount of marbles form an eclogite-gneiss unit. Quartz pseudomorphs after coesite have been recognized in the SAT eclogites. Peak P-T conditions have been estimated at 2.8～3.0GPa and 730～850℃. Subsequent retrogression overprint developed under amphibolite-granulite facies conditions. Geochronology interpreted to reflect eclogite-facies metamorphism yields ages between 485 and 500Ma. SAT high-pressure granulites, located at the Bashiwake area, consist of mafic granulites and felsic granulite. The granulites and enclosed ultramafic rocks compose a HP/HT metamorphic unit about 5 km wide, bounded by sinistral strike-slip shear zones against amphibolite-facies gneisses that lack evidence for high-pressure metamorphism. Peak metamorphic assemblages of high-pressure granulites contain ternary feldspar (now mesoperthite) and kyanite. Thermobarometry indicates that they experienced peak P-T conditions of 930～ 1020℃ and 1.8～2. 5GPa, and postdated by a medium- high-pressure granulite facies overprint. Zircon SHRIMP geochronology dated the high-pressure facies metamorphic events at 493Ma to 497Ma. Dulan eclogite unit is located at eastern end of North Qaidam HP/ UHP metamorphic belt. Coesite inclusions have been recognized in both eclogites and paragneisses, and are also supported by P-T estimates of peak conditions of eclogites (2.7～3.25GPa, 670～730℃). Subsequent retrogression overprints were suggested to occur under amphibolite facies conditions. Geochronology interpreted to reflect eclogite-facies metamorphism yields ages between 420Ma and 450Ma. Dulan HP granulite unit, located at the western part of Aercituoshan, mainly consists of mafic granulites and felsic granulite. Mineral assemblages in equilibrium in mafic granulites contain mainly garnet, clinopyroxene, kyanite, plagioclase, zoisite/clinozosite, rutile and quartz, whereas felsic granulites contain garnet, kyanite, K-feldspar, plagioclase and quartz. Thermobarometry of the peak mineral assemblages yields P-T conditions of 1.4～1.85GPa and 800～925℃, postdated by subsequent retrogression under amphibolite facies-greenschist facies conditions. High-pressure granulite facies metamorphism was estimated to have occurred at ca. 450Ma. North Qinling eclogites occur along Guanpo-Shuanghuaishu with near east-west trend in the North Qinling orogens. North Qlinling eclogites contains peak assemblages of Grt + Omp ± Phe + Qtz + Rt, yielding peak metamorphic conditions at 2.25～2.65GPa and 680～770℃. Geochronological data suggest eclogite facies metamorphism took place ca. 500Ma. North Qinling high-pressure granulite is situated in the Songshugou, southern side of eclogite-bearing unit. Mafic granulites and felsic granulites are spatially associated ultramafic rocks. Peak P-T conditions were estimated at 1.45～1.80GPa and 850～925℃. Zircon U-Pb datings indicate peak high-pressure granulite facies event occurred between 485Ma and 507Ma. The three examples above indicate that high-pressure granulite and spatially close eclogite experienced contrasting metamorphic histories but granulite-facies conditions in high-pressure granulite and eclogite facies onditions in eclogites developed at the same time. This possibly implies that high-pressure granulites and eclogites formed in different thermal environments, i. e. eclogites in continental subduction slabs, whereas high-pressure granulites at the base of the overriding continent crust thickened as a result of continental subduction.