Characteristics and geological significance of olivine xenocrysts in Cenozoic volcanic rocks from western Qinling

Abstract

Abstract Cenozoic volcanic rocks from the Haoti, Dangchang County of the western Qinling Mountains, contain a few clearly-zoned olivines. These olivines are relatively big in grain sizes and usually have cracks or broken features. Their cores have similar composition (Mg# 90.4–91.0) to those for the peridotitic xenoliths entrained in host volcanic rocks and their rims are close to the composition of olivine phenocrysts (Mg#=85.5–51.9). The CaO contents in these zoned olivines are lower than 0.1%. These features demonstrate that the clearly-zoned olivines are xenocrysts and disaggregated from mantle periodotites. The zoned texture was the result of the interaction between the olivine and host magna. Available data show that the volcanic rocks would have been derived from the mantle source metasomatized by subducted hydrathermally-altered oceanic crust. The formation of these Cenozoic volcanic rocks was perhaps related to the rapid uplift of the Tibetan Plateau.