Age and geochemical characteristics of Quaternary volcanic rocks in the northwestern margin of Tibetan Plateau and their significance.

A whole-rock 40Ar-39Ar age of 0.81±0.21Ma was obtained for the North Pulu potassic volcanic rocks in the northwestern margin of Tibetan Plateau. The rocks belong to shoshonite series and are mainly composed of shoshonite and trachyandesite. Geochemically, they have coherent content of SiO2 53.08%~55.12%, total alkali 7.36%~8.04% and K2O/Na2O ratios 1.17~1.2. They are significantly enriched in LREE and LILE (K, Rb, Sr, Ba, Th etc.) and remarkably depleted in HREE and HFSE (Nb, Ta, Ti etc.). Elemental geochemistry, combined with high (87Sr/86Sr)i ratios (0.7088~0.7089) and low εNd ratios (-6.05~-5.54), indicates that the potassic volcanic rocks were most likely derived from EMⅡ type enriched continental lithospheric mantle that had been modified by ancient oceanic subduction. Nevertheless, the ratios of Ti/Y, Zr/Y, La/Rb, Zr/Rb, K/La and Pb/La of the potassic volcanic rocks are comparable with those of OIB (ocean island basalt), suggesting that the magma source might have undergone the process of metasomatism of asthenosphere fluid or the deep fluid overlapped the ancient subduction mantle wedge. The North Pulu potassic volcanic rocks might have been the product that responded to the delamination of the mantle lithosphere beneath the northwestern margin of the Tibetan Plateau during the Late Cenozoic continent-continent collision orogeny.