Geochronology and geochemistry of the Mesozoic volcanic rocks in Western Liaoning: Implications for lithospheric thinning of the North China Craton

Abstract

Determining the age and petrogenesis of the voluminous Mesozoic magmatic rocks from the North China Craton (NCC) provides critical data for deducing the process and timing of lithospheric thinning. Four Mesozoic magmatic events in the northeast of the craton (Western Liaoning) are delineated by Ar–Ar and U–Pb zircon dating, i.e. the Xinglonggou Formation (177 Ma), the Lanqi Formation (166–153 Ma), the Yixian Formation (126–120 Ma), and the Zhanglaogongtun Formation (∼ 106 Ma), respectively. The Xinglonggou lavas are high-Mg# adakites with arc-like Sr–Nd–Pb isotopic compositions, suggesting that they originated from the subducted Palaeoasian oceanic crust. The typical “continental” geochemical signatures of the Lanqi basalts and basaltic andesites as well as their low ɛ Nd(t), moderate 87Sr/ 86Sr i, and extremely unradiogenic Pb isotopes indicate significant involvement of lower crust materials in their magma. These features, coupled with the low Mg, Ni, and Cr contents may suggest significant olivine fractionation and a magma underplating event, which caused the partial melting of the low-middle crust to produce the voluminous low-Mg andesites and acidic volcanic rocks overlying the Lanqi basalts. The Yixian high-Mg adakitic rocks with the lower-crustal Sr–Nd–Pb isotopic compositions suggest foundering of the mafic lower crust into the underlying convecting mantle. The Yixian basalts show similar geochemical characteristics to the Lanqi basalts except the relatively higher Mg, Ni and Cr contents, which could be derived from a newly enriched lithosphere mantle hybridized by partial melts from the foundered lower continental crust. The Zhanglaogongtun lavas are alkaline basalts with MORB-like Sr–Nd–Pb isotopic compositions, suggesting derivation from a depleted mantle. Based on the new data, a multi-stage lithospheric thinning model is proposed.