Genesis of Cenozoic low-Ca alkaline basalts in the Nanjing basaltic field, eastern China: The case for mantle xenolith-magma interaction

Although peridotite xenoliths are common in alkaline basalts, it is still unclear whether the chemical compositions of their host rocks have been affected by these mantle fragments and, if so, what processes are involved in this alteration of the host basalts. Here, we document a kind of xenolith-rich alkaline basalts from the Nanjing basaltic field, eastern China. These basalts contain lower concentrations of CaO (4.1-7.8 wt %) and Sc (3.3-17.8 ppm) and have lower Ca/Al (0.3-0.6) and higher Na/Ti ratios (2.8-11.2) than other Cenozoic basalts in this area. These xenolith-rich basalts show good correlations between elemental ratios (e.g., Lu/Hf and Ca/Al) and ε_Hf values, which are indicative of mixing of two distinct components during the genesis of the magmas that formed these basalts: a high-ε_Hf end-member (with low Lu/Hf and Ca/Al ratios) and the primitive melt-related low-ε_Hf end-member. In addition, peridotite xenoliths hosted in these basalts have distinct core-mantle textures, with the margins having higher modal olivine abundances (70%) than the xenolith cores (52%). Within the xenolith margins, some orthopyroxenes are enclosed in the olivines, and all clinopyroxenes are sponge textured. These sponge-textured clinopyroxenes have higher CaO and Sc concentrations, higher Ca/Al ratios, and lower Na/Ti ratios than clinopyroxenes within the cores of the xenoliths, suggesting that the xenoliths underwent low-pressure melting within the host magma. This indicates that xenolith-rich magmas within the study area were contaminated during ascent by melts derived from mantle xenoliths within the magmas, transforming the magmas into the low-Ca alkaline basalts.