Geochemistry and petrogenesis of Eocene volcanic-intrusive rocks in Liaohe basin, northeastern China
辽河盆地沙三期火山-侵入岩地球化学与岩石成因

Liaohe basin lies in northeastern China. Volcanites of Eocene Shahejie Formation in this basin are bimodal in composition with mafic rocks being one end member and weakly alkaline intermediate rocks being the other. Alkaline basalts are enriched in high field strength elements (e. g. Nb, Th, Zr, Hf, etc.), light rare earth elements (LREE) and large ion lithophile elements (e. g. Ba, Sr) except Rb and K, indicating that these basalts were formed under an intraplate environment. Incompatible trace element patterns of the basalts are virtually comparable to those of typical ocean island basalts (OIB). Apart from strong Sr and Eu depletion due to feldspar fractionation, trachytic rocks have the same characteristics in trace element behaviours as alkaline basalts. Despite of a gap between alkaline basalts and trachytes, major and trace element distributions indicate both the basalts and trachiytes are produced by the same mantle-derived magma. Lines of evidence in mineralogy, petrology and geochemistry imply that basalts are produced by low- degree partial melting of an asthenospheric source with subsequent fractional crystallization of olivine and clinopyroxene, whereas trachytes were derived from the basaltic magma by fractionation of olivines, clinopyroxenes, plagioclases, magnetites at shallow crust, and crustal contamination is not significant in the evolution of the magmas.