| Abstract: | Volcanism is the direct indication of the upper mantle thermodynamic activities, and it is an efficient path to study the lithosphere tectonic evolution. Cenozoic volcanism in North Tibet is of obviously time circles. In the space, from south to north, the chemical composition of volcanics changed from Na|rich alkalic basaltic series formed 44Ma. ago into high|K calcalkali series formed in about 40～31Ma.in Qiangtang area, into minor amount of leucite basalt and phonolite series of 26Ma., further into shoshonite series aged at 19～7Ma. in Cocoxili basin, until Quaternary shoshonite series in Karakunlun and Qilian Mt.|Yumen region(Xiaoguo Chi etc.1999).Isotopes of Sr,Nd,Pb and trace elements in volcanic rocks provide wide range of information for the evolution of lithosphere in North Tibet after 45Ma. Present research shows that Na|rich alkalic basalt series formed 44Ma. ago has the features of a primitive mantle in Sr,Nd,Pb isotopes. Along with the evolutionary tendency to high|K calcalkali series and phonolite series, isotopes of Sr,Nd,Pb evolve towards EM2|rich mantle end. The North Tibet is a compound continental mass combined from several terrains of different geological periods and the lithosphere mantle sees a complicated evolutionary history. Hence, the mixing of different rocks in ancient subduction zones and long history of metasomatism in the upper mantle offered excellent and favorable conditions for the formation of EM2 mantle end and for potassic enrichment. Isotopic evolutionary features indicate that the early magma came from the asthenosphere while later magmas were derived from partial remelting of lithosphere mantle. |
