Complex zoning texture of phenocryst plagioclase in andesitic rocks of Tancheng area, Shandong Province: Record of magma mixing

A large amount of coarse grain diamonds and diamond placer deposits have been found in the Cenozoic strata in Tancheng area of Shandong Province, without primary diamond deposits so far. The petrography characteristics of outcrop of andesitic volcanic rocks in this area was investigated to explore the source of diamonds. The results show: ① Most of the phenocryst plagioclases have different complex zoning textures, and the core of phenocryst plagioclase is composed of microcrystalline plagioclases and fine olivines, with normal zoning texture crystallized by plagioclase in the outer rings. ② The olivines are evenly distributed in the rock, with content of more than 20%. It also has three generations characteristics, which is basically consistent with the three generations characteristics of olivine in kimberlite. ③ The mineral combinations of “olivine+phlogopite+garnet” and “plagioclase+pyroxene+hornblende” occurred in the rocks. These evidences indicate that the andesitic volcanic rocks in the area are the product of the mixing of andesite magma and kimberlite magma, which was recorded by the complex zoning texture of phenocryst plagioclase. The research in this paper is of enlightenment for the primary diamond ore research in Tancheng area.