| Abstract: | Cenozoic volcanic rocks, forming a series of volcanic rocks platform or highland on different sub|volcanic rocks belt, had distributed widely in northern Qinghai—Tibet plateau, and characterized by potassinm|rich lavas which main petrologic types are latidacite and latite. The petrogenesis of volcanic rocks in northern Tibetan plateau had been well|studied, and thought they play an important role to uplift of Tibetan plateau. Based on the study of lithochemistry and isotopic age determination of the rocks in Qiangtang area, which is the most southern and earliest volcanic rocks belt of Cenozoic volcanic rocks, I contribute another view here to the early uplift of Tibetan plateau. Cenozoic volcanic rocks in Qiangtang region, thickness of the rocks are from 200 to 1000m in different area, belong to shoshonite and high|K calc|alkaline series, highly enrich alkaline specially enrich potassinm, large|ion lithophytes elements (Rb,Sr,Ba,and Th), LREE and deplete high|field strength elements (Ta,Er,Hf,and Ti). Chondrite|normalized REE pattern of volcanic rocks show an absence of significant Eu anomaly, and indicate that the volcanic source was plagioclase|free and probably resulted from melting in lower crust or upper mantle. Cenozoic volcanic rocks in Qiangtang region are similar petrologic and geochemical characteristics to that have been reported in more northern Hoh Xil and Kunlun area in Tibetan plateau. The age scope of volcanic rocks in Qiangtang area were from 44.1～32.6Ma, among which the age mainly were 44.1～42.5Ma, which belong to mid|Eocene epoch to primary Oligocene epoch, and could be classified into four out pouring periods, according to the potassium|argon age dating. The study on lithochemistry and age of the rocks in Qiangtang area suggest that Intracontinental subduction southward Qiangtang tectonic belt plays an important role to Cenozoic volcanism, and imply that the convective removal of the thicked lithosphere, which formed at the collision mountain|building processes, are responsible for the formation of Cenozoic volcanic rocks, the earliest uplift of Tibetan plateau. and regional extension in Qiangtang area. |
