Genesis Of Cenozoic Alkaline Basalts In The Southeastern Margin Of North China Craton

Cenozoic volcanism in eastern China is wide-spread, and the most important rock type in this area is the sodic alkaline basalt. It provides an ideal setting in which to study the genesis of intra-continental alkaline basalts, the characteristics of the mantle source, and the potential geodynamic mechanism inducing continental volcanism. Here we present major, trace-element, and Sr-Nd-Hf isotopes data for the Cenozoic alkaline basalts from Shandong and Jiangsu-Anhui area in the southeastern margin of North China Craton, as well as the major, trace-element data for minernals in peridotite xenoliths hosted in Tashan basalts (one of the volcanoes in Jiangsu-Anhui area), to argue the multiple genesis of the sodic alkaline basalt. We also suggest that the geochemical heterogeneity of the mantle is induced by different lithologies in the source, and it provides a new clue to discuss the potential geodynamic mechanism for the Cenozoic continental volcanism in eastern China.The Cenozoic alkaline basalts in Shandong can be divided into two types:the strongly alkaline basalts and the weakly alkaline basalts. Inverse rare earth element (REE) modeling suggests that the strongly and the weakly alkaline basalts represent melting amounts of1.5-3%and3-10%, respectively. In comparison with the weakly alkaline basalts, these strongly alkaline basalts have relatively lower contents of SiO2(39.2-45.1wt.%) and Al2O3(10.3-13.8wt.%), higher contents of alkalis (Na2O+K2O=4.3-8.6wt.%) and CaO (8.0-12.6wt.%), higher Ca/Al ratios (0.7-1.3), and higher concentrations of most incompatible elements. On the whole, primitive-mantle normalized spidergrams reveal that the strongly alkaline basalts 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 basalts. These main characteristics of the strongly alkaline basalts resemble those of carbonatites. Therefore, we suggest that the carbonatitic "fingerprints" of these rocks are inherited from an asthenospheric source that had undergone enrichment with carbonatitic liquids, and our observations indicate that the source rocks of the strongly alkaline basalts in Shandong were mainly carbonated peridotite. Sr-Nd-Hf isotopic compositions for the weakly alkaline basalts form separate binary mixing arrays which converge on the compositions of Dashan, an isolated, nephelinitic volcano with the most depleted isotopic signature. The two enriched endmembers have higher SiO2contents, lower CaO contents, lower Ce/Pb, Th/La, Zr/Hf ratios, and significantly diverging isotopic enrichment trends from this common endmember. Both trends deviate from the normal Nd-Hf mantle array toward higher εHf values. All these features point to a recycled eclogitic source for the weakly alkaline basalts in Shandong. In addition, the isotopic compositions of basalts from the two volcanic chains show two slightly different mixing trends, which indicates that the enriched endmembers of the two chains are different. Such differences reflect an additional, small-scale chemical and isotopic difference between the recycled crustal components in the respective mantle sources. These are explained by different proportions of melts extracted from eclogite during the first (Mesozoic) stage of evolution.Panshishan and Tanshan are two Cenozoic volcanoes in the Jiangsu-Anhui area, which are characterized with abundant mantle xenoliths. Basalts from these two volcanoes have lower Sc contents (10.3-17.8ppm), lower Ca/Al ratios (0.4-0.6) and higher Na/Ti ratios (2.8-5.0) than those for other alkaline basalts in Jiangsu-Anhui area, indicating the influence of clinopyroxene in the genesis of these basalts. Peridotite xenoliths from these two volcanoes are usually composed of core and rim with different lithologies. The core of peridotite xenoliths generally contains more amounts of pyroxene than the rim. The spongy textures are observed in clinopyroxene, which indicates that it has been melted during the ascending process, and the "clinopyroxene fingerprints" are inherited from these melts during melting. In addition, we observed that the orthopyroxenes are included in the olivine, suggesting the reaction between peridotite and basaltic melts. This reaction can consume the orthopyroxenes, produce the olivines, and raise the SiO2contents of basalts. Therefore, the Panshishan and Tashan basalts are produced by the interactions between basaltic magma and peridotite xenoliths.