Geochemistry and tectonic significance of volcanic rocks from Qushenla Formation in Gerze area of northern Tibetan Plateau

By using petrological,isotope chronological,and geochemical methods,the authors studied the volcanic rocks in the studied area,mainly including dacites and trachytes. The results show that they formed during the late Early Cretaceous. Geochemically,the volcanic rocks are relatively enriched in large-ion lithophile elements( Rb,K,and Th) and depleted in high field strength elements( Nb,Ta,and Ti),and rich in light rare earth elements,and depleted in heavy rare earth elements. The fact indicates that the main body of the volcanic rocks in the Qushenla Formation was derived from the partial melting of lower crust. The lithological assemblages are characterized by continental high-K calc-alkaline and shoshonitic series,suggesting that the southward-subducting oceanic slab in southern Bangong Lake had break off and that the Bangong Lake-Nujiang Ocean had closed before 107 Ma. The main dynamic mechanisms for the genesis of this set of intermediate-acidic volcanic rocks were upwelling of the asthenosphere and partial melting of the lower crust caused by slab break-off.