Chronology of Cenozoic volcanic rocks in the source region of the Yangtze River

Abstract：Volcanic rocks in the source region of the Yangtze River contain all the information of the formation and evolution of Cenozoic volcanic rocks in northern Tibet, and its chronological research has important significance for the understanding of the temporal-spatial evolution of Cenozoic volcanic activities in northern Tibet. Based on K-Ar isotope chronological research，Cenozoic volcanism in the source region of the Yangtze River is divided into three phases. In the first phase (40.1-35.1 Ma) volcanism was weak, forming fine-grained latite and porphyritic trachyte of limited distribution. In the second phase (26.5-17.6 Ma), volcanic eruption was strongest,forming extensive thick-bedded, black,vesicular latite. In the the third phase (5.5-4.61 Ma), volcanic mounds of spherulitic-porphyritic trachyte were produced by subvolcanism.The three phases of volcanic eruption in the source region of the Yangtze River correspond with the phases of volcanic activities in northern Tibet. From early to late, the Cenozoic volcanic rocks in the source region of the Yangtze River evolved from high-potassium calc-alkaline series to shoshonite series and from latite to trachyte.