Tectonic implication on the lithosphere evolution of the Tibet Plateau
青藏高原岩石圈演化的记录：藏北超钾质及钠质火山岩的岩石学与地球化学特征

The Yulinshan is a medium lava sheet, composited of three volcano stages The earlier two period vocanic rocks are majorly nepheline nosean columbretite and nepheline leucitite, and the later are majorly sodalite ponzite and nephelin aegirine trachyte Ages of the volcanic rocks range from 18 to 30Ma The ultrapotassic volcanic magma is highly SiO 2 unsaturated, highly alkaline, highly enriched in LILE, LREE, radiogenic Sr, Pb and unradiogenic Nd( 86 Sr/ 87 Sr= 0 708766 ~0 709162, 206 Pb/ 204 Pb=18 882888~19 104751, 207 Pb/ 204 Pb=15 642037~15 828458, 208 Pb/ 204 Pb=39 138921~39 686835 and 143 Nd/ 144 Nd=0 512116~0 512342)characteristics which suggests an enriched EMII mantle source in which the crust materials are largely involved The Lagala pass sodic alkiline dorgalite is a monadnock of denudation with age of 60Ma The petrology, REE, LILE and Sr Nd Pb isotopes characteristics proposed a lightly enriched mantle source which had experienced ocean crust subduction The results of this study suggest that the formation of the enriched EMII mantle had possibly resulted from the subduction of the India plate crust