Geochemical feature and its tectonic significance of Gemucuo Oligocene potassic volcanic rocks in the Qiangtang area ,Tibet,China

There is a suite of potassic volcanic rocks(~30Ma)occurred in Gemucuo,Qiangtang area mainly composed of olivine tephrite,olivine trachy-basalt,aegirite augite trachyte and leucite phonolite.SiO2 content of this suit ranges from 46% to 60%,Al2O3 from 3% to 21%,Mg# from 0.14 to 0.63,K2O/Na2O ratio is above 1.The volcanic rocks are enriched in LREE and depleted in com-patible element(Cr、Ni)and HFSE(Nb、Ta、Ti).The eHf(t)of whole rock mass ranges from-1.28 to-6.20.It is indicated that the pri-mary magama source originated from minor melting of an ancient subduction mantle wedge which was metasomatised of astheno-sphere fluid,and consanguineous igneous magma went through AFC(Assimilation Fractional Crystallization).The potassic to ultra-potassic volcanic rocks in Gemucuo,Yulinshan and Huochetoushan together formed the Oligocene potassic to ultrapotassic volcanic rock belt in Qiangtang area.Combined with the study of the magmatic evolution of Cenozoic volcanic rocks in southern Qiangtang,the genesis of the magma might be related to the upwelling of asthenosphere and thinning of lithosphere,which were induced by the sepreation of Qiangtang intracontinental subduction plate 35Ma ago.