Constraints from phase equilibrium modeling and in -situ U-Pb dating of monazite.

The Beidao area of Tianshui region, located in the northern margin of the West Qinling orogenic belt, lies at the junction between the Qinling orogenic belt and the Qilian orogenic belt. Recently, we have identified a suite of granulite-facies metamorphic rocks ( garnet-sillimanite-biotite gneiss) from the Qinling Complex in this area. The peak mineral assemblage of garnet + ilmenite + rutile + sillimanite + K -feldspar + quartz was modeled to be stable in P-T condition of similar to 1.02GPa and 820 degrees C. It records a clockwise PT path, characterized by a prograde evolution with an increase in pressure and temperature, and a retrograde evolution with a largely isothermal decompression process. The in -situ monazite U-Pb dating yielded three age populations of similar to 421Ma, similar to 388 Ma and 368 362Ma, indicating that multi -stage tectonothermal events took place in the northern part of the West Qinling orogenic belt. Taking into account the occurrence of monazite, we speculate that 1) the age of 421Ma represents the timing of the granulite-facies metamorphism, which is consistent with the reported granulite-facies metamorphic peak age in the Huamiao area of the Tianshui region; 2) the age of 388 Ma probably represents the retrograde age of largely isothermal decompression near the solidus; and 3) the age population of 368 362Ma may document the tectonothermal events related to the Xinyang-Yuanlong ductile shear deformation. The integrated study of mineralogy, clockwise P-T path, and in -situ monazite dating, in combination with regional geological data, suggest that the Beidao granulite-facies rocks may be formed due to the continent -continent collisional orogeny in the Paleozoic.