Metamorphism and geochronology of the Foping gneiss dome: Insights into Early Triassic collision of the Qinling Orogen, Central China

Abstract

The Foping gneiss dome, which outcrops in the narrowest part of the Qinling Orogen, contains high-grade metamorphic rocks that record the collision between the North China Block (NCB) and South China Block (SCB). Herein, petrography, mineral chemistry, phase equilibria modeling and UPb geochronology are used to constrain the collision events responsible for the high-grade metamorphism and exhumation of the Foping gneiss dome. The pelitic and mafic granulites sampled from the dome reveal peak metamorphism at P/T conditions of 6.2–7.4 kbar/ 805–855 °C, and document clockwise P-T-t paths with retrograde metamorphism of amphibolite-facies at 3.4–4.7 kbar/ 605–715 °C. While the pelitic schist from the rim of this dome witnessed peak metamorphism at P/T conditions of 4.9–5.1 kbar/ 590–605 °C and retrograde metamorphism at 3.5–3.7 kbar/ 510–520 °C. UPb dating of zircon and monazite confirm ages of ∼247–240 Ma for peak metamorphism with granulite-facies, ages of ∼208–198 Ma for retrograde metamorphism. The two metamorphic stages may record the transition from crustal thickening to dome exhumation, and the age of peak metamorphism probably marks the timing of the collision between the NCB and SCB along the Qinling Orogen in the Early Triassic. The collision probably terminated in the Late Triassic. The new discovery of the Triassic metamorphic event in the Qinling, combined with previously reported collision event of the Dabie Orogen to the east, supports the idea that the continental collision between the NCB and SCB along the Qinling-Dabie Orogen was synchronous.