The Cenozoic volcanic rocks from the Chibuzhang Lake area in the Qiangtang area

Three stages have been recognized for the volcanic activity in the Chibuzhang lake area in northern Xizang on the basis of the K-Ar isotopic age determinations:(1) Stage Ⅰ (40.1-35.1 Ma),during which the volcanic eruption was relatively weak,and only small-scale of fine-grained latite and porphyritic trachyte were formed;(2) Stage Ⅱ (26.5-17.6 Ma),during which the volcanic eruption reached its peak,resulting in the formation of widespread thick-bedded balck vesiculate latite,and Stage Ⅲ (5.5-4.61 Ma),during which the crypto-volcanism permitted the pelletoidal and porphyritic trachyte domes. The Cenozoic volcanic rock series consist of high-potassium calc-alkaline series and shonshonite series. The former ones may be traced back to the Eocene and originated from the crust-derived magmas,whereas the latter ones are of Miocene and Pliocene age,and resulted from the mantle-derived magmas. The rock series record the evolution from the high-potassium calc-alkaline series to shonshonite series,while the lithologies show the evolution from latite to trachyte.