Paleozoic Tectonothermal Evolution in the West Qinling Orogen, Central China: Petrological and Chronological Evidence from Garnet Amphibolites

The Qinling Complex is located in the core of the northern Qinling Orogen and plays a key role in understanding the tectonic evolution of the Qinling Orogen, but its metamorphic evolution remains controversial. The combined investigation of petrographic observation, zircon U-Pb dating, and phase equilibria modeling for garnet amphibolites from the Tianshui area in the West Qinling Orogen is reported in this study. The results show that the garnet amphibolites record a clockwise P-T path characterized by a pre-T-Max decompression heating stage, a temperature peak at P-T conditions of 0.84-0.99 GPa and 869-886 degrees C, followed by a decompression cooling stage. Zircon U-Pb dating yields four age populations of similar to 479 +/- 4 Ma, similar to 451 +/- 8 Ma, similar to 411 +/- 4 Ma, and similar to 377 +/- 6 Ma. The 479-450 Ma reflects the timing of the pre-T-Max high-medium pressure upper amphibolite-facies metamorphism. The metamorphism at peak temperature condition occurred at c.411 Ma and was followed by decompression cooling to c.377 Ma. The Ordovician high-medium pressure metamorphism is related to the continental collision, which is slightly later than the HP-UHP eclogite-facies metamorphism in the East Qinling Orogen. The HT granulite-facies metamorphism at peak temperature condition took place at reduced pressures, suggesting thinning of the collision-thickened orogenic crust. Therefore, the northern West Qinling Orogen experienced a tectonothermal evolution from initial crust thickening to thinning during the Paleozoic collisional orogeny.