Origin of Late Mesozoic Alkaline Basalts and Nature of Lithospheric Mantle in Liaoyuan Area, Jilin Province

This paper presents whole-rock K-Ar dating results and geochemical data of the Late Mesozoic basalts in the Liaoyuan area, Jilin Province, and geochemical data of minerals from the entrained mantle xenoliths, with the aim of constraining the origin of the basalts and the nature of the Late Mesozoic lithosphere mantle. The whole-rock K-Ar dating results indicate that Liaoyuan basalts formed in the Late Cretaceous (91.7±1.7 Ma, 82.5±1.9 Ma, 99.3±1.6 Ma, 82.2±2.4 Ma). They have SiO₂ = 45.1%-45.9%, Na₂O+K₂O= 4.50%-6.72%, and Mg^# [100×Mg/(Mg+Fe²⁺)]=60-63, suggesting that they belong to the alkaline series. Liaoyuan basalts are enriched in light rare earth elements (LREEs), and depleted in heavy rare earth elements (HREEs) [(La/Yb)_N=12.0-14.0], and have no Eu anomalies. Their trace element patterns are similar to that of OIB. The entrained peridotite xenoliths consist mainly of spinel lherzolites in which olivines have relatively low Mg^# (87.1-91.2). The major and trace element data of the minerals from the xenoliths indicate that the Late Cretaceous lithosphere mantle beneath the Liaoyuan area be composed mainly of the juvenile one. The incompatible elements of clinopyroxenes from the peridotite xenoliths vary from the enrichment to the depletion, suggesting that the lithosphere mantle has experienced partial melting and fluid/melt metasomatism, and that the metasomatizing agents are mainly silicate melt and minor carbonate melt. The Late Cretaceous basalts in the Liaoyuan area were derived from partial melting of asthenospheric mantle. Combined with the studies on the regional geology, we conclude that the basalts formed under the back-arc extensional setting related to the subduction of the Paleo-Pacific plate.