Characteristics of melt-rock reaction in Shuangliao peridotite xenoliths and their implications to mantle metasomatism

A harzburgite xenolith sample from Paleogene basalts in Shuangliao area of Jilin Province records the information of upper mantle metasomatism. Primary orthopyroxenes are surrounded by secondary clinopyroxenes and olivines, forming reaction texture or existing as residues in secondary clinopyroxenes. These phenomena appear around spinel only. Electronic probe analyses reveal that secondary clinopyroxenes are characterized by high Mg#, Cr# and CaO/Al2O3 , secondary olivines by high Mg#, CaO and Cr2O3 content, and the metasomatized rim of spinels by high Cr# and CaO. As metasomatism had no influence on Mg# in peridotite, and the melt-rock reaction proceeded in order of Opx+Sp+melt (Ⅰ)→Cpx+Ol+melt (Ⅱ), the authors consider that the lithospheric mantle in Shuangliao area was metasomatized by carbonate melt or silicate melt rich in carbonate. This conclusion is based on experiments and literature. The metasomatism resulted in decreasing content of orthopyroxene, increasing contents of clinopyroxene and olivine in peridotite, and the transformation from harzburgites to wehrlites. This metasomatism may occur in the melt-rock reaction zone located in the asthenosphere-lithosphere boundary, suggesting that the asthenosphere-lithosphere interaction resulting from lithosphere thinning and asthenosphere upwelling remained active in Shuangliao area in Early Tertiary.