The Geochemical Characteristics and Zircon U-Pb Dating of the Miocene Continental Volcanic Rocks from Chabaoma Formation in the Mt. Jurhen Ul Area of North Qiangtang,Qinghai-Tibetan Plateau

In the Chabaoma Formation of Mt. Jurhen Ul area,the main rock types of continental volcanic rockare latite and andesite,which were formed in Neogene-Miocene epoch. The geochemical characteristics indicate that the contents of SiO₂(main element)range from 42.45% to 62.60%, and most of them are 58.08%-62.60%, which is relatively higher than the ones of FeO, MgO, K₂O, TiO₂ and P₂O₅, suggesting these rocks belong to the ones of mantle-derived magmatic rock series. The values of σ(Litman index) are 2.87-3.41, with an average of 3.19, showing these rocks have the characteristics of calc-alkaline rock series. The total alkali(Na₂O+K₂O) contents of these volcanic rocks are high, the contents of Na₂O-2 are less than the ones of K₂O, and the ratios of K₂O/Na₂O are 1.12-1.43, suggesting theses rocks belong to shoshoniteseries.For the trace elements, the strong incompatible elements(such as Rb, Ba, Th, U and K) showstrong enrichment, but the weakly incompatible elements(Ta, Nb, La, Ce, P, Zr, Hf and Sm) show weak enrichment, while the transition elements(Ti, Y and Yb) are weakly less, and the compatible element(Lu) is strong loss.The strong enrichment of incompatible elements reveal that these volcanic rocks were soured from the enriched mantle(EM II). The distribution model of REE is right abligue in the orientation, with significant REE fractionation and weak negative Eu anomalies, suggesting that the crustal material had mixed into the magma during the upwelling process, which lead to the occurring of assimilation and hybridization. The analysis show that, these rocks formed in an intraplate setting. The ICP-MS zircon U-Pb dating reveals that the crystallization ages of these volcanic rocks are 20±1Ma, belonging to the Miocene Epoch of Neogene Period.