Geochemistry of Cenozoic volcanic rocks in Tengchong, SW China: relationship with the uplift of the Tibetan Plateau.

There are wide spread Cenozoic volcanic rocks in Tengchong ( CVRT), Yunnan province, SW China. These rocks comprise three rock types: basalt, andesite (dominant type) and dacite. Most samples are sub-alkaline, and among the sub-alkaline rocks, most are high- K calc-alkaline. These rocks have a SiO2 range of 49.1 wt.% to 66.9 wt.%. TiO2 contents are not high and have a variation of 0.7 wt.%-1.6 wt.%. Trace element concentrations and element ratios (such as Nb/ U, Ce/ Pb, Nb/ La, etc.) of these rocks have a large variation. 87Sr/ 86Sr values fall in the range of 0.7057-0.7093 and 143Nd/ 144Nd values change from 0.5120 to 0.5125. 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb ratios are in the range of 17.936-19.039, 15.614-15.810, and 38.894-39.735, respectively. These geochemical characteristics of CVRT make them resemble island-arc volcanic rocks. We suggest that the magmas were generated in the lithospheric mantle that had already been metasomatized by previous subduction processes. By the study of the uplift history of the Tibetan Plateau, we found that the beginning of the geotectonic processes to the eruption of CVRT was coeval with one uplift event. Therefore, we propose that the uplift of the Tibetan Plateau caused collapse of the collisional orogeny in Tengchong, which further triggered the generation and eruption of the CVRT magmas. [ABSTRACT FROM AUTHOR]