陆内强碱性火山岩的成因:以山东无棣大山霞石岩为例  

Genesis of intra-continental strongly alkaline volcanic rocks: A case study of Dashan nephelinites in Wudi, Shandong Province, North China.

作　　者：罗丹[1] 陈立辉[1] 曾罡[1] 

机构地区：[1]南京大学内生金属成矿研究国家重点实验室,南京大学地球科学与工程学院,南京210093

出　　处：《岩石学报》2009年第2期311-319,共9页Acta Petrologica Sinica

基　　金：“973计划”(2006CB403508);国家自然科学基金(40772035)资助

摘　　要：本文以山东无棣的大山更新世霞石岩为例,通过全岩的元素地球化学和Sr-Nd-Hf同位素组成特征,探讨陆内强碱性幔源火山岩的成因及其源区特征。山东地区的新生代玄武岩主要分为早晚两期,早期以橄榄玄武岩为主,晚期以强碱性岩为主(包括霞石岩、碧玄岩等,常见地幔橄榄岩捕掳体)。大山霞石岩为晚期火山作用的代表,具有富碱和贫硅铝等强碱性火山岩的典型特征。在微量元素组成上,大山霞石岩以强烈富集轻稀土元素和大离子亲石元素,在原始地幔标准化图上以明显亏损Zr、Hf、Ti为特征(Hf/Hf~*=0.66～0.68,Ti/Ti~*=0.68～0.70)。其Sr-Nd-Hf同位素比值均一(^(87)Sr/^(86)Sr=0.70333～0.70339,ε_(Nd)=5.95～6.34,ε_(Hf)=8.26～8.43),与早期玄武岩组成一个两端元混合的排列,其中大山霞石岩位于亏损端元一侧。在同位素组成相同的情况下,大山霞石岩相对山东新生代早期玄武岩具有明显偏低的Hf/Sm、Hf/Hf~*、Ti/Ti~*比值,说明源区残留矿物组合可以造成高场强元素Zr、Hf、Ti与稀土元素之间的明显分馏,暗示源区存在非橄榄岩组分。大山霞石岩相对橄榄岩来源熔体偏低的CaO含量和偏高的Zr/Hf比(47.9～48.4),说明其源区残留过剩的单斜辉石,源岩中的非橄榄岩成分为辉石岩或角闪石岩。中等亏损的同位素组成特征说明这些非橄榄岩组分可能与该区显生宙的地壳重循环过程有关。Here we report geochemical and Sr-Nd-Hf isotopic compositions of the Dashan nephelinites to discuss genesis and lithology of source for intra-continental strongly alkaline volcanic rocks. Two stages of the Cenozoic magmatism call be distinguished in Shandong Province, including early olivine basalts and late strongly alkaline rocks, e. g. nephelinites, basanites. Mantle xenoliths are common in the late alkaline basalts. The Dashan nephelinites are interpreted as the representative of the late magmatism in this area. They have typical signatures of continental alkaline basalts with low SiO2 and high alkali contents. In the primitive mantle-normalized spidergram, the Dashan nephelinites are characterized by enrichment in LREEs and LILEs, and depletion in Zr, Hf, Ti (Hf/Hf* = 0.66 similar to 0.68, Ti/Ti* = 0.68 similar to 0.70). They show a narrow range of isotopic compositions (Sr-87/Sr-86 = 0.70333 similar to 0.70339, epsilon(Nd) = 5.95 similar to 6.34, epsilon(Hf) = 8.26 similar to 8.43). Together with other Cenozoic basalts in Shandong Province, The Dashan nephelinites compose a two-endmember mixing array in the plots of Sr-87/Sr-86 versus epsilon(Nd) and epsilon(Nd) versus epsilon(Hf). The Dashan nephelinites are located at the side near the depleted endmember. For a given epsilon(Nd) or epsilon(Hf) value, the Dashan nephelinites have Obviously lower Hf/Sm, Hf/Hf*, Ti/Ti* ratios than those of the early basalts, Which Suggests a clear non-peridotitic source because their residual minerals can induce the fractionation between HFSEs (Zr, Hf, and Ti) and REEs. Low CaO contents for given MgO conents and superchondritic Zr/Hf ratios (47.9 similar to 48.4) indicate that there are excess clinopyroxene in the residue and the Source rocks of the Dashan nephelinites are pyroxenite or hornblendite. The moderately depleted isotopic composition of the Dashan nephelinites suggests that these non-peridotitic components might be associated with crustal recycling during the Phanerozoic in the North China Craton.

关 键 词：霞石岩 地壳重循环 辉石岩 华北克拉通 

分 类 号：P588.145[天文地球—岩石学]