Characteristics and Zonation of the Cenozoic Volcanic Rocks on the Qinghai-Tibet Plateau

The Cenozoic volcanic rocks on the Qinghai-Tibet plateau are recognized as four zones in space and time, i.e., the North Tertiary Zone （NTZ）, the North Quaternary Zone （NQZ）, the South Zone （SZ）, and the East Zone （EZ）, based on this and previous studies. Petrological and geochemical results show that in the four zones intermediate-acid volcanic rocks are prominent, and basic-intermediate and acid are subordinate. The rocks are characterized by high contents of K2O + Na2O （about 8 wt%）, and K2O/Na2O > 1 or close to 1, and mostly belonged to shoshonite-trachyte association and some to ultra-potassic rocks, showing affinity of orogenic genesis. Sr, Nd and Ph isotopic studies suggest that the majority of volcanic rocks were originated from the lower crush and minor, basic rocks from enriched mantle. The volcanic activity started from Paleocene, much earlier than the continent-continent collision and intercontinental subduction at the Himalayan front that started most likely from Miocene. The volcanic activities were mainly constrained by deep-seated faults which cut through the lower crust and into the mantle. That the volcanic rocks are distributed in the interior of the plateau as well as at its margin also suggests that plate subduction and collision is not the merely factor for the genesis of these rocks.