Carbonated mantle sources for Cenozoic intra-plate alkaline basalts in Shandong, North China

The genesis of intra-plate alkaline basalts remains controversial, and three sources have been proposed: silica-deficient eclogite-pyroxenite, hornblendite, and carbonated peridotite. Here, we assess these models by analyzing Cenozoic intra-continental alkaline basalts from Shandong province, North China. The Cenozoic basalts of Shandong province consist of an early sequence of weakly alkaline rocks (alkali olivine basalts) and a late sequence of strongly alkaline rocks (basanites and nephelinites). In comparison with the weakly alkaline rocks, the strongly alkaline rocks have lower concentrations of SiO₂ (39.2-45.1wt.%) and Al₂O₃ (10.3-13.8wt.%), higher alkalis (Na₂O+K₂O=4.3-8.6wt.%) and CaO (8.0-12.6wt.%), higher concentrations of most incompatible elements, and higher values of Ca/Al (0.7-1.3), La/Yb (39.4-65.7), and Sm/Yb (6.1-9.9). On the whole, primitive-mantle normalized spidergrams reveal that the strongly alkaline rocks have stronger negative K, Zr, Hf, and Ti anomalies (Hf/Hf*=0.59-0.77, Ti/Ti*=0.46-0.71) than do the weakly alkaline rocks. All these rocks have superchondritic Zr/Hf ratios (>44). Inverse rare earth element (REE) modeling suggests that the strongly and weakly alkaline rocks represent melting amounts of <3% and 3-10%, respectively. Neither silica-deficient eclogite-pyroxenite melts nor hornblendite melts can satisfy all the features mentioned above. Here we prefer a carbonated mantle source because the main characteristics of the strongly alkaline rocks resemble those of carbonatites (e.g., enrichment of most incompatible elements, high Ca/Al ratios, superchondritic Zr/Hf ratios, and negative K, Zr, Hf, and Ti anomalies). Since dry peridotite has a higher solidus temperature than does carbonated peridotite in the mantle, an increased degree of melting under higher temperatures may result in the dilution of "carbonatitic fingerprints." Although contributions from silica-deficient eclogite-pyroxenite or hornblendite cannot be ruled out, our observations suggest that carbonated peridotite is the main source for the Cenozoic strongly alkaline basalts of Shandong.