Neopaleozoic mantle plume in southern Qiangtang，Tibet Plateau?—geochemical evidences from basic and ultrabasic rocks

There are widespread basic and ultrabasic igneous rocks distributing in south Qiangtang. Basic dykes and basalt are distributed near the east-west direction along southern Longmu Co-Shuanghu suture zone. Geotectonically, the dykes and basalt lay on the northern margin of Gondwanaland. They had a range of 800km from the border in the west to Shuanghu area, intensively formed between Late Carboniferous and Early Permian, which reflects the process of magmatic quick emplacement and expulsion. Basic dykes and basalt are mostly of kalium alkaline series, few subalkaline series, with high content in titanium and iron and low value in Mg#, which indicate that they are residual granitic melts from different magmatic crystallization of primary magma. Rare earth elements(REE), trace elements and forming environment of basic dykes and basalt share similar characteristics with plate basalts and typical basalt formed by mantle column. Besides, picrite mostly formed with basic dykes and basalt, and mainly include clinopyroxene peridotites and lherzolites, poor in volatile components. Characteristics of major elements, rare earth elements(REE) and trace elements reflect that picrite is probably the product of primary magma crystallization of mantle plume, not the capture of magma. Moreover, their sources most likely be garnet lherzolite. It can be seen that basic dykes and basalt in southern Qiangtang are the product of decompression melting in the head part of mantle plume, and picrite is the result for melting of passageway in tail part.