Triassic collision of the Qinling Orogen: Evidence from granulite metamorphism of the Foping gneiss dome

Triassic collisional assemblage between the North China Block (NCB) and South China Block (SCB) along the Qinling Orogen still remains controversy in timing and process. As an indicator, collision-type granulite well records textural evidences of orogenic crustal alteration induced by continental collision. Located at the narrowest part of the Qinling Orogen, the Foping gneiss dome exposed pelitic and mafic granulites, migmatite and granitoids, which are critical to understand the Triassic tectonic evolution history of the Qinling Orogen. Representative pelitic granulite, mafic granulite, and pelitic schist samples from the Foping dome were selected to obtain the P-T-t paths to reveal the collision timing and process, through an integrated study of petrography, mineral chemistry, estimation of P/T conditions and U-Pb geochronological dating. Our results indicate that these granulites experienced granulite facies peak metamorphism at 6.2-7.4 kbar/ 805-855 °C and followed by retrograde metamorphism at 3.4-4.7 kbar/ 605-715 °C with characteristic of clockwise P-T paths. The pelitic schist underwent peak and retrograde stages at P/T conditions of 4.9-5.1 kbar/ 590-605 °C and 3.5-3.7 kbar/ 510-520 °C, respectively, and consisting clockwise P-T paths. Zircon U-Pb dating gives an inherited age of ca.580-573 Ma, which indicates the protoliths of the Foping granulites probably derived from Neoproterozoic. In addition, it also generates a peak metamorphic age of ca.247-240 Ma, revealing the collision onset of the Qinling Orogen at Early-Middle Triassic. Integrated results of zircon and monazite U-Pb analyzing yield retrograde age at ca.208-198 Ma, constraining the collision termination at Late Triassic. Together with the previous studies in the Qinling-Dabie Orogen, we propose that the collision between the SCB and the NCB occurred in the Qinling Orogen, and it is coeval with the collision time of late Permian-Middle Triassic in the Dabie Orogen, and ends at ca.208-198 Ma. Then the > 70o main clockwise rotation occurred in Jurassic-Cretaceous probably.