Friday, August 22, 2014

Evidence of a Continental Collision/Subduction at the NeoArchean/PaleoProterozoic Boundary

Neoarchean to Paleoproterozoic high-pressure mafic granulite from the Jiaodong terrain, North China Craton: Petrology, zircon age determination and geological implications


Liu et al


The North China Craton is an ideal place for studying the transition of the Earth's thermal structure and tectonics at the Archean-Proterozoic boundary due to its good preservation of the ~ 2.5 Ga tectono-thermal events. We report the discovery of a high-pressure mafic granulite from the Jiaodong Terrain in the North China Craton. The mafic granulite occurs as garnet-clinopyroxene-orthopyroxene-hornblende gneiss enclaves within a late-Archean trondhjemite-tonalite-granodiorite (TTG) gneiss. Typical high-pressure mineral assemblage of garnet - clinopyroxene - plagioclase - quartz ± rutile has been identified. Plagioclase + clinopyroxene ± orthpyroxene ± hornblende symplectite surrounding garnet ("white eye") is also observed. Using the conventional geothermobarometry and the pseudosection modeling, a clockwise metamorphic P-T path with the peak conditions at ~ 17 kbar and ~ 880 °C was determined. Zircon U-Pb analyses (SHRIMP) on the overgrowth rim of zircon grains of two samples from the same outcrop yielded a metamorphic age of 2473 ± 6 Ma (MSWD = 0.8). The analyses on magmatic core gave a probable magmatic age of 2527 ± 12 Ma (MSWD = 1.9). The high-pressure granulite facies metamorphism corresponds to a collisional event between the ~ 2.5 Ga crust and ~ 2.9 Ga crust at the dawn of Paleoproterozoic in the North China Craton. It also represents a new but rare case of a subduction-collision tectonics at the Archean-Proterozoic transition and provides insight into the change of the Earth's thermal structure.

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