Geochemistry of Miocene trachytes in Bugasi, Lhasa block, Tibetan Plateau: Mixing products between mantle- and crust-derived melts?

Abstract

It is generally believed that Cenozoic potassic and ultrapotassic volcanic rocks of the Tibetan Plateau were generated by partial melting of an enriched mantle region or lower crustal materials. The Miocene Bugasi volcanic rocks (BVR) in the western part of the Lhasa block are composed mainly of trachyandesites and trachytes, both of which belong to the shoshonite series. The trachytes show somewhat transitional compositions between the mantle-derived trachyandesites of the BVR and the crust-derived potassic rocks of Konglongxiang, most evident in their Sr, Ba, and Ni concentrations, Nb/Ta, Rb/Sr, Th/Nb, Zr/Nb, and Ba/Rb trace element ratios, and Sr and Nd isotopic compositions. These features, coupled with the relatively high Cr and Ni concentrations and Mg#, suggest that the trachytes are the product of mixing between mantle-derived and lower crust-derived melts.