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 isolatedvolcanoes 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.%大陆碱性玄武岩在地球化学特征上与洋岛玄武岩高度相似,被看做是板内玄武岩在大陆上的典型代表.本文以山东新生代碱性玄武岩为例,探讨大陆碱性玄武岩的成因.山东新生代碱性玄武岩按时空分布特征可以分为两类:早期定向分布、相互平行的三个火山群(包括鲁西的潍坊火山群、沂水火山群和胶东的蓬莱火山群)和晚期杂乱分布的孤立小火山.早期火山群碱性较弱,以碱性橄榄玄武岩和碧玄岩为主,微量元素特征和同位素组成变化大；晚期孤立小火山碱性强,以碧玄岩和霞石岩为主,微量元素特征和同位素组成较均一.因此,从岩性组成和时空分布特征看,山东的火山群相当于洋岛/海山的造盾期玄武岩,而孤立小火山接近于洋岛/海山上的复苏期玄武岩.潍坊火山群和沂水火山群在Sr-Nd,Nd-Hf同位素相关图上都存在从亏损到富集的两端元混合排列趋势,但两者的排列趋势有一点区别.其中同位素富集的端元相对于原始地幔具有偏低的Ce/Pb比和偏高的Ba/Th比,指示其为大陆下地壳物质.同时,这种富集端元的Th/La比值明显低于大陆下地壳的平均值,其放射成因Hf相对于放射成因Nd过剩(即Nd-Hf同位素解耦),说明这种富集端元不是岩浆上升过程中混染的下地壳物质,而是经历过早期熔融的再循环大陆下地壳(榴辉岩或者石榴辉石岩).鲁西两个平行火山群在同位素排列上的区别类似于夏威夷玄武岩中的KEA链和LOA链,因此,山东的平行火山群的深部动力学背景可能是地幔柱,再循环大陆下地壳物质可能是这种地幔柱的重要组成物质.晚期的孤立小火山在地球化学特征上与火成碳酸岩非常相似,如在原始地幔标准化图上都具有K,Pb,Zr,Hf,Ti的负异常等特征,因此我们认为其地幔源区为碳酸盐化的橄榄岩.孤立小火山中等亏损的Sr,Nd,Hf同位素特征支持碳酸岩熔体来自年轻的(中生代?)再循环洋壳.
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