Geochemistry and Petrogenesis of Cenozoic Andesite-Dacite Associations from the Hoh Xil Region, Tibetan Plateau

Volcanic units with ages in the range 4.27-44.60 Ma in the Zhentouya area south to Ulan Ul Lake, Hoh Xil region, are mainly lava, predominantly dacite in composition, together with lesser amounts of andesite. The high-potassium calc-alkaline dacite series is characterized by enriched LIL elements such as Rb, Ba, Th, U, and K, and depleted HFSE such as Ti, Nb,Ta, and Sr, whereas the andesite series containing high-potassium caic-alkaline and shoshonitic components is enriched in the LIL elements Cs, Rb, Ba, Th, U, and Sr, and depleted in the HFSE Ti, Nb, and Ta. La/Sm-La and Zr/Sm-Zr discrimination diagrams indicate that both the dacite and the andesite series, with their contrasting evolutionary trends, were generated from partial melting of the continental crust rather than by fractional crystallization of a primary basaltic magma derived from the mantle. Nd, Sr, and Pb isotopic compositions of the dacite (radiogenic Sr-87/Sr-86 = 0.709227-0.709578, Pb-208/Pb-204 = 39.1356-39.2622. Pb-207/Pb-204 = 15.6675-1.5.7063, Pb-206/Pb-204 = 18.7635-18.7970 and nonradiogenic Nd-143/Nd-144 0.512287-0.512591, epsilon(Nd) = -0.92 to -6.85) provide evidence that crustal anatexis in the Tibetan Plateau played an important role in petrogenesis of the dacite series. Moreover, the presence of granulite xenoliths in the dacite series places P-T constraints on the magma source at a temperature range of 780-820degreesC and pressure of 0.86 GPa (equal to a depth of 28 km). Combined with P-T conditions inferred from the granulite-facies xenoliths, the geochemistry of the dacite and the isotopic signature of crustal anatexis suggest that partial melting of the middle part of thickened Tibetan crust could have produced the primary dacite magma series. The andesite, to some degree, shows an adakitic compositional signature involving high Sr >1000 ppm, Sr/Y ratios >50, and low Yb contents <2 ppm. Evidently plagioclase broke down in the andesitic magma source region under the P-T conditions of partial melting. Compared with the dacite series, it appears that partial melting of the lower part of the thickened, eclogite-facies Tibetan crust was responsible for generation of the andesitic magma.