Coupling Relationship Between The Exhumation History Of The Representative Magmatic Rocks And Deep Geological Process In Lhasa-Qiangtang Collision Zone

Tibet as the "Natural laboratory of geodynamics",its uplift process and mechanism has been focused on by the International geoloyists all the time.The exist uplift models are mostly assume that plateau uplift was caused by the collision of India-Asia,while its pre-Cenozoic uplift history and deep process have insufficient attention.The lack of understanding of deep process also limits the interpretation to the plateau uplift process and mechanism.We collect early Cretaceous magmatic rocks around Xainza and Daguo,Lhasa-Qiangtang collision zone,using petrology,geochronology and geochemistry to restrict their deep process.Meanwhile,we select representative magmatic rocks(e.g.Yanhu,Coqen,Yare,Baingoin,Nyainrong and Tanggula rocks which have zircon U-Pb age older than 110 Ma),using apatite(U-Th)/He to investigate the exhumation history.The result of petrology,geochronology and geochemistry indicate that Xainza basalt,andesite,dacites and Daguo rhyolites coev al erupted at ca.113 Ma.The Xainza basalts are calc-alkaline and display geochemical signatures comparable to those of within-plate basalts rather than typical arc-type basalts.They are most likely derived from partial melting of an enriched mantle wedge that was metasomatized by subduction-related components,accompanied with significant contributions from subslab asthenospheric mantle.The Xainza andesites and dacites are high-K calc-alkaline and metaluminous to slightly peraluminous.The andesites are generated from partial melting of an ancient lower crust,whereas the dacites are interpreted as the consequences of varying degrees of interaction between the mature Lhasa basement(Al-rich)and mantle-derived melts.The Daguo rhyolites belong to high-K cale-alkaline-shoshonits and are mostly strongly peraluminous,may derived from partial melting of a juvenile crust and experienced significant fractional crystallization.In combination with the other existing data,we suggest that the generation of the ca.113 Ma magmatic rocks around Xainza is most likely triggered by the slab breakoff of the southward subducting Bangong-Nujiang Tethyan Ocean lithosphere.The geological records of the Lhasa-Qiangtang collision zone,particularly the presence of mafic rocks showing within-plate basalt geochemistry,A2-type silicic rocks,and coeval bimodal volcanic rocks in a continent-continent collision zone suggest a petrological association diagnostic of slab breakoff setting.The apatite(U-Th)/He ages obtained in this paper are early Cretaceous to Miocene(108 to 8.02 Ma)and mostly in late Cretaceous(85 to 70 Ma)and Eocene to Oligocene(58 to 34 Ma),represent three periods of exhumation events.This implies that Lhasa-Qiangtang collision zone were experienced early uplift in Cretaceous and have different uplift history from west to east.We consider that there has tight coupling between uplift and deep process:early Cretaceous He ages are the response to the slab break-off of the southward subducting of Bangong-Nujiang lithosphere.The exhumation event in late Cretaceous relate to the lithosphere delamination,while exhumation in Eocene to Oligocene may be the remote response to the collision between Lhasa and India.