Abstract:Cenozoic volcanic units in Zhentouya area of north Qiangtang are mainly andesite and dacite lavas. The andesite, to some extent, shows an adakitic compositional characteristics, such as high Sr (>1000×10-6), Sr/Yratios >50 and low Yb contents (<2×10-6). Evidently plagioclase broke down in the andesitic magma source region under the p-Tconditions of partial melting. Compared with the dacite series, it seems that partial melting of the lower part of the thickened eclogite-facies Tibetan crust was responsible for the generation of the andesitic magma. The dacite is characterized by enriched LILE such as Rb, Ba, Th, U and K, and depleted HFSE such as Ti, Nb, Ta and Sr. It is noteworthy that the dacite exhibits a relatively low Sr (Sr<660×10－6), and this may imply that the plagioclase was still stable in the dacitic magma source region at the pressure and temperature of partial melting. Two_pyroxene granulite and clinopyroxene granulite xeneloths were discovered in the andesite and dacite volcanic rocks outcropped in north Qiangtang of central Tibetan Plateau. Based on electron microprobe analyses, the authors have concluded that the xenoliths provide constraints for the equilibrium temperatures of the two-pyroxene granulite, i.e., about 783 to 818℃ as determined by two_pyroxene thermometry, and that the formation pressure of the clinopyroxene granulite should be about 0.845 to 0.858 GPa, equal to 27.9～28.3 km in depth It is indicated that these granulite xenoliths represent the samples derived from the middlepart of the thickened Tibetan crust. This discovery is of great importance to the furtherinvestigation of the components and thermal regime of the deep crustof Tibetan Plateau. Geochemistry of dacite and characteristics of granulite xenolith suggest that partial melting of the middle part of thickened Tibetan crust might have produced the primary dacite magma series.
