Geochemistry of ultramafic xenoliths in Cenozoic alkali basalts from Jiangsu province, eastern China and their geological implication

Abstract

Twelve ultramafic xenoliths in Cenozoic alkali basalts from Jiangsu province, eastern China have been analyzed for major, trace, Sr–Nd isotopic composition and mineral chemical compositions and the origin of these ultramafic xenoliths is discussed based on the geochemical constraints. Based on classification norms, the ultramafic xenoliths in the present study belong to type I xenolith. The Fo-values of the ultramafic xenoliths range from 90.18 to 92.18. The ultramafic xenoliths have higher MgO content, but lower Al2O3, TiO2, CaO and Na2O contents than those of primitive mantle, indicating that they represent residues formed by different degrees of partial melting from the upper mantle. The enrichment of Rb, Ba, U, Nb, K, La, Ce, Sr, P and Zr of ultramafic xenoliths found in Jiangsu province, eastern China may be related to the CO2–H2O-fluids metasomatism. On the basis of Sr–Nd isotopic ratios, we suggest that the lithospheric mantle beneath the study area mostly belongs to depleted-type mantle but with slightly enriched signatures, indicating the heterogeneous characteristics in the mantle source and the influence of different degrees of fluids-metasomatism on the mantle composition. The equilibrium P–T conditions of the spinel lherzolite xenoliths are: T = 913 ~ 1045°C, P = 13 ~ 22 kb corresponding to depths of 45–83 km. The P–T conditions suggest that the geothermal gradient of the upper mantle beneath the study area is approximately similar to oceanic geotherm which may be caused by asthenosphere upwelling. We suggested that lithospheric mantle thinning accompanied by asthenosphere upwelling has occurred and a newly accreted and cooled asthenospheric mantle may exist beneath the study area.