Geochemical Characteristics And Tectonic Significance Of The Cenozoic Potassic Volcanic Rocks In The Zameiren Area,northern Tibetan Plateau

The Tibetan Plateau is a product of the convergence between Indian and Eurasian plate,which provides a natural laboratory for the study of global orogenic processes.There are different series of Cenozoic volcanic rocks,which widely spread in the Qiangtang area,northern Tibetan Plateau.The relationship between the transformation of different rock series and the evolution of lithosphere tectonics is still unclear,because of the lack of analysis about their temporal and spatial distribution and geochemical characteristics.In this paper,petrology,mineralogy,chronology and geochemistry of the Cenozoic Potassic volcanic rocks in the Zhameiren area were carried out.Combined with the research results of Cenozoic volcanic rocks in Qiangtang area,its lithospheric tectonic evolution were to be discussed.This study provides new evidence for the formation and evolution of the Tibetan Plateau.The Cenozoic volcanic rocks in the Zhameiren area are mainly composed of Trachyandesite and tephriphonolite,which were formed during Oligocene(26-33Ma).Zhameiren volcanic rocks are rich in K2 O and relatively low in Si O2 and MgO.They are highly enriched in LREE with steep REE patterns and without obviously negative Eu anomalies.The primary mantle-normalized variation diagrams are characterized by relatively negative anomalies at Ta,Nb,P and Ti,and strongly positive anomalies in LILE(Rb,Th,Ba,Sr).The volcanic rocks exhibit higher 87Sr/86 Sr,206Pb/204 Pb,207Pb/204 Pb,208Pb/204 Pb,and lower 143 Nd /144 Nd ratio.The characteristic of Sr,Nd and Pb isotopes indicates that the magmatic source of volcanic rocks is related to EM II-rich mantle,which is controlled by the separation and crystallization of mantle-derived magma.The volcanic rocks have the dual geochemical and tectonic environmental properties of interpolate alkaline volcanic rocks and active continentalmargin and island arc volcanic rocks.It is estimated that the depth of formation of volcanic rocks is about 48 km and the crystallization temperature is 1146-1297? by Clinopyroxene.Based on the characteristics of rock series,age change,spatial distribution and geochemistry of the Cenozoic volcanic rocks in the Qiangtang area of northern Tibet,it is pointed out that a series of Cenozoic volcanic rocks are caused by the subduction plate-off and the upwelling and disturbance of asthenosphere caused by subduction and collision of the Indian plate to the Eurasian continent.