关键词: 大陆碱性玄武岩；地壳再循环；碳酸盐化地幔；中国东部

Abstract:

         Geochemically, continental alkaline basalts are similar to those oceanic island basalts, and are viewed as the representative of intraplate basalts in continents. Here we provide a case study of the Cenozoic basalts in Shandong Province,eastern China, to discuss the genesis of continental basalts. Spatially, the Cenozoic basalts can be divided into two kinds, the early three parallel volcanic belts and the late small and isolated volcanoes. The three belts including the Weifang belt and the Yishui belt in western Shandong, and the Penglai belt in eastern Shandong are less alkaline and are mainly composed of alkali olivine basalts and basanites, which show broad ranges of incompatible element ratios and isotopic values. The small and isolated volcanoes are more alkaline and are mainly composed of basanites and nephenilites, which are more homogenous in incompatible element ratios and isotopic values. Therefore, compared with oceanic island basalts, the parallel volcanic belts in Shandong can be regarded as shield-building stage basalts, while the small and isolated volcanoes correspond to the posterosional or rejuvenescent basalts. On the isotopic plots of Sr vs. Nd and Nd vs. Hf, both the Weifang belt and the Yishui belt show a two-endmember mixing array. However, there is minor difference of the enriched endmembers of these two arrays. The enriched endmembers are characterized by smaller Ce/Pb and elevated Ba/Th ratios, suggesting a signature of lower continental crust. However, such materials have much lower Th/La ratios and have excess radiogenic Hf contents relative to radiogenic Nd. Such geochemical signatures cannot be produced by assimilation of lower continental crust but by recycled lower continental crust (eclogite or garnet pyroxenite), which had undergone early melting in the mantle. The different arrays in isotopic plots of the two belts are alike those of the KEA chain and the LOA chain of Hawaii islands. Therefore, we propose that the parallel volcanic belts of Shandong be attributed to the plume activity and the recycled lower continental crust be important components of the plume. Geochemically,basanites and nepheninites of the late small and isolated volcanoes share many signatures with the carbonatite, e.g. the depletion of K, Pb, Zr, Hf, Ti in the primitive mantle-normalized grams. Therefore, we conclude that their mantle sources have been carbonated recently. The moderately depleted Sr, Nd, Hf isotopic compositions of these alkaline rocks indicate that the carbonated liquids might be derived from recycled young (Mesozoic?) oceanic crust.

Key words: continental alkaline basalts, crustal recycling , carbonated mantle , eastern China