Geochemistry, geochronology and petrogenesis of East Kunlun high Nb-Ta rhyolites

The East Kunlun high Nb-Ta rhyolite is located at the eastern end of the Kunlun orogenic belt. Zircon U-Pb analysis gives a magmatic age of about 213 Ma, corresponding to the subduction/collision stage of the Kunlun Orogeny. Compared to the felsic volcanic rocks of similar ages in the region and rhyolites associated with subduction and collision zones worldwide, the rhyolite has high SiO2 and K2O, and low CaO and Al2O3. For trace elements, the rhyolite is enriched in Nb and Ta (with only a very weak negative Nb-Ta anomaly), but strongly depleted in Sr and Eu. Bulk rock samples are characterized by high initial Sr isotopic ratios and low initial Nd isotopic ratios with epsilon(Nd)(t) approximate to -3. Taken together, all these data are best interpreted as resulting from mixing of Nb-Ta rich mantle-derived melt with a crustal component. We propose that the East Kunlun high Nb-Ta rhyolites may be generated as follows; mantle-derived alkali basaltic melts (with inherited high Nb-Ta and epsilon(Nd)(t) >0) intruded the crust and developed a magma chamber, where the melt evolved through fractional crystallization with significant plagioclase separation ( strongly depleted Sr and Eu) while assimilating the crustal material characterized by epsilon(Nd)(t) <0 and relatively low Nb and Ta before eruption.