Crustal response to continental collisions between the Tibet, Indian, South China and North China Blocks: Geochronological constraints from the Songpan-Garzê Orogenic Belt, western China

We report SHRIMP U-Th-Pb monazite, conventional U-Pb titanite, Sm-Nd garnet and Rb-Sr muscovite and biotite ages for metamorphic rocks from the Danba Domal Metamorphic Terrane in the eastern Songpan-Garzê Orogenic Belt (eastern Tibet Plateau). These ages are used to determine the timing of polyphase metamorphic events and the subsequent cooling history. The oldest U-Th-Pb monazite and Sm-Nd garnet ages constrain an early Barrovian metamorphism (M1) in the interval c. 204-190 Ma, coincident with extensive Indosinian granitic magmatism throughout the Songpan- Garzê Orogenic Belt. A second, higher-grade sillimanite-grade metamorphic event (M2), recorded only in the northern part of the Danba terrane, was dated at c. 168-158 Ma by a combination of U-Th-Pb monazite and titanite and Sm-Nd garnet ages. It is suggested that M1 was a thermal event that affected the entire orogenic belt while M2 may represent a local thermal perturbation. Rb-Sr muscovite ages range from c. 138-100 Ma, whereas Rb-Sr biotite ages cluster at c. 34-24 Ma. These ages document regional cooling at rates of c. 2-3°C Myr^-1 following the M1 peak for most of the terrane. However, those parts of the terrane affected by the higher-temperature M2 event (e.g. the migmatite zone) experienced initially more rapid (c. 8°C Myr^-1) cooling after peak M2 before joining the regional slow cooling path defined by the rest of the terrane at c. 138 Ma. Regional slow cooling between c. 138 and c. 30 Ma is thought to be the result of post-tectonic isostatic uplift after extensive crustal thickening caused by collision of the South and North China Blocks. The clustering of biotite Rb-Sr ages marks the onset of rapid uplift across the entire terrane commencing at c. 30-20 Ma. This cooling history is shared with many other regions of the Tibet Plateau, suggesting that uplift of the Tibet Plateau (including the Songpan-Garzê Orogenic Belt) occurred predominantly in the last c. 30 Myr as a response to the continuing northwards collision of India with Eurasia.