The trachyandesite from Dongka Formation in Yushu county is characterized by high Na2 O, MgO, A12 03 , and low K2 O, TiO2, P2 05, with alumina saturation index A/CNK of 0.98 1.08. These features suggest that the trachyandesite are characteristics of Metaluminous-weak peraluminous, simultaneously, this rock belongs to sodic andesite attribute to low K20, and NaEO/KEO = 12.60 - 15.97. The SiO2-K20 diagram and Rittmann index（2.89 ～ 3.25 ） show the trachyandesite belong to the calc-alkaline series. The chondrite-normalized REE patterns are rightward inclined, with Eu weak negative anomaly （ δEu = 0.90 ～ 0.96 ）. The primitive mantle normalized trace element spidergrams are characterized by enriched Rb, Th and U, depleted Nb,Ta, Ba. The ratios of CE/Pb, Nb/U do not rise, as the MgO content increase, La/Nb,Zr/Nb,Ba/Nb show a good stability,which indicates that the ascent of magma process did not suffer form crustal contamination,may be a reflection of its source region. Studies have shown that the magma experienced fractional significant crystallization of olivine and clinopyroxene during the magmatic process,and the magma source region might be converted by the subducting slab melt. It can be deduced from formation age and geochemical characteristics that the trachyandesite is a geological record of Jinsha ocean basin westward subduction and oblique collision, also provide the basis for Jomda-Weixi volcanic arc evolution and mineralization.
