Geochemistry of Cenozoic basalts and mantle xenoliths in Northeast China

Abstract

Volcanic activity in Northeast China and adjacent regions is widespread, with eruption ages ranging from Late Cretaceous (80 Ma) to about 300 years ago. Volcanic rock types are principally basanite, alkali olivine basalt, and tholeiite, with minor evolved trachyte and rhyolite. Ultramafic xenoliths, mainly spinel lherzolite and harzburgite, are common in alkali olivine basalt and basanite. We present new major and trace element data from 11 volcanic fields, and helium isotope data from ultramafic xenoliths of three of those fields. 3He/ 4He ratio ranges from 5 to 7 times the atmospheric ratio, indicating that previously reported high 3He/ 4He ratios are likely due to the contamination by cosmogenic 3He. There is currently no evidence for a high- 3He/ 4He mantle plume component beneath NE China. Sr–Nd–Pb isotopic data from the literature are reinterpreted to result from mixing between a FOZO end-member and a LoMu end-member. The LoMu end-member appears to reside within continental lithosphere. Major and trace elements do not indicate significant contributions from a subducted slab. Differences in depth and the extent of partial melting in the asthenosphere, both locally within individual fields and regionally between the different volcanic fields, explain most of the correlations between major and trace elements observed in this study, with deeper melting usually associated with a smaller degree of melting. Highly potassic basalts are associated with the LoMu end-member and are interpreted to be the products of lithosphere melting.