Identification of the adakitic rock association in Chibzhang Co area, northern Tibet, and its significance

Major and trace elements and Sr-Nd isotope data of adakitic high-K calc-alkaline volcanic rocks in Chibzhang Co area are presented, which yield ages of 35～40 Ma. All of the rocks exhibit characteristics of adakites formed by partial melting of an eclogitic lower crust: SiO-2=58%～69%,high K-2O, Al-2O-3 and Sr contents,low Y, Yb contents and Mg~# values, strong depletion of Nb and Zr, weak depletion of Ti and P,LREE enrichment with no remarkable Eu negative anomaly,I-(Sr)=0.707?23～0.707?77 and I-(Nd)=0.512?320～0.512?352. The adakitic geochemical features and crustal anatectic isotopic characteristics suggest a primitive magmatic generation process responsible for the partial melting of an eclogitic lower crust of northern Tibetan Plateau. It is also implied that the crust was thickened to over 40 km and an eclogitic lower crust was formed in northern Tibet area during Eocene. The adakitic volcanic rocks of 40 Ma indicate that the continental crust of Qiangtang area in Tibetan Plateau has been thickened to over 40 km,and the plateau probably started its uplifting 40 Ma ago.