Petrology, geochemistry and geochronology of late Triassic volcanics, Kunlun orogenic belt, western China: Implications for tectonic setting and petrogenesis

The Qimantage volcanic suite (QVS) of late Triassic age, dominated by dacitic to rhyolitic lavas and pyroclastic rocks with minor basaltic andesites, outcrops mainly in the middle segment of Qimantage Mt. region, Kunlun orogenic belt, which extends along the northern margin of Tibetan plateau, western China. Petrological and geochemical studies indicate that the volcanic rocks are of typical calc-alkaline affinity with silica contents ranging from ∼53 to ∼80 wt%, exhibiting a medium- to high-K characteristics. Major element variations define relatively smooth compositional trends, while large ion lithophile (LIL) trace elements correlate positively with increasing SiO2. Chondrite-normalized incompatible trace element spidergram shows conspicuous sparks at Rb and Th, but pronounced Nb and Ti depletion. The chondrite-normalized REE patterns of QVS are generally paralleled to each other, exhibiting highly fractionated LREE/HREE spectra with small to pronounced negative Eu anomalies. Two whole-rock Rb-Sr isochrons yield ages of ca. 209 and ca. 225 Ma, respectively, indicating the volcanic suite was emplaced during the late Triassic. The initial 87Sr/86Sr ratios (0.708-0.709) and initial eNd(t) values (-2.05∼-2.35) imply that the magmas were derived from a slightly enriched mantle source with considerable crustal contamination. Considering its petrological, geochemical and isotopic characteristics in a regional geological background, it is proposed that QVS was generated upon an active continental margin during the late Triassic north-directed subduction event. Both the volcanics and the closely-related coeval granitoid plutons collectively make up the late Triassic arc magmatic belt on the paleogeographically southern margin of the Tarim continent.

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