Petrological and Geochemical Characteristics of the Carboniferous Volcanic Rocks, South of the Xinyuan County, Xinjiang
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The Xinyuan volcanic rocks in western Tianshan mountains mainly consist of basalts, basaltic trachyandesites, trachyandesites, andesites, rhyolites and pyroclastic materials. The basalts constitute calc-alkaline association, whilst the andesitic rocks and rhyolites belong to high-K calc-alkaline series. The REE patterns indicate that LREES are slightly enriched and HREES are comparatively depleted in the studied volcanic rocks. In the primitive mantle-normalized trace element diagrams, basalts show LILE, U, Th and Pb enriched patterns with HFSE troughs. This study displays that the volcanic magma probably came from the partial melting of the mantle wedge, which had undergone metasomatism induced by the subduction fluid. According to calculation, 3 to 6 percent partial melting of garnet-lherzolite could produce the basalt magma. The andesitic magma could be produced by the fractional crystallization of basalt magma, the fraction of which is 15 to 28 percent.