鲁西中、新生代镁铁质岩浆作用与地幔化学演化  

Mesozoic-Cenozoic Mafic Magmatism in Western Shandong Province and Its Implication for the Chemical Evolution of the Mantle

作　　者：邱检生[1] 胡建[1] 蒋少涌[1] 王汝成[1] 徐夕生[1] 

机构地区：[1]南京大学成矿作用国家重点实验室,江苏南京210093

出　　处：《地球科学（中国地质大学学报）》2005年第6期646-658,共13页Earth Science-Journal of China University of Geosciences

基　　金：国家自然科学基金项目(No.40272035);国家创新研究群体科学基金项目(No.40221301);教育部新世纪优秀人才支持计划资助项目(NCET-04-0459).

摘　　要：镁铁质火成岩作为分布最为广泛的典型幔源岩石,已成为探索地幔化学性状及示踪岩石圈深部过程的主要研究对象.通过对典型样品元素-同位素组成的系统测定,并结合前人已有资料,综合研究了鲁西中生代和新生代镁铁质岩石的地质与地球化学特征.研究结果表明,中生代镁铁质火成岩总体具有富轻稀土和大离子亲石元素、贫高场强元素、ISr值变化范围大(0.70396～0.71247)、εNd(t)值显著偏低(-9.20～-21.21)的地球化学特征,但该区南部和北部的中生代镁铁质岩石在元素-同位素组成上仍存在一定差别,主要表现在南部较之北部镁铁质岩石具有更高的稀土总量(REE为325.52×10-6～555.75×10-6)和轻、重稀土比值(LREE/HREE=17.75～25.97),以及更高的LILE/HFSE比值(如La/Nb=6.37～13.85,Th/Nb=0.52～1.53).南部镁铁质岩石较之北部镁铁质岩石也更富放射成因锶,ISr值分别为0.70844～0.71247和0.70396～0.70598.元素-同位素综合示踪指示鲁西中生代地幔总体具有因岩石圈大规模拆沉作用形成的EMⅠ型富集地幔特征,但其南部叠加了因深俯冲而进入地幔的扬子陆壳的影响,因而表现出EMⅠ和EMⅡ组分混合的富集地幔特征.新生代玄武岩具有类似于大洋玄武岩的地球化学特征,其源区应为亏损的软流圈地幔,但在部分熔融形成岩浆之前遭受了近期的交代作用.自中生代至新生代,华北克拉通地幔具有由富集向亏损演变的趋势,这一化学性状的演变最可能是中生代以来岩石圈大规模拆沉作用,导致软流圈地幔上涌并对原有岩石圈地幔再改造所致.Mafic igneous rock, a widely distributed and typical mantle-derived rock, has become a major research source for revealing possible mechanisms for the chemical evolution of the mantle. Combined geological and geochemical studies have been carried out on the Mesozoic and Cenozoic mafic igneous rocks in western Shandong Province, determining the elemental and isotopic compositions of typical samples. Generally, the Mesozoic mafic igneous rocks have enriched LREE and LILE concentrations, low HFSE abundance, and display highly variable Isr ratios （0. 703 96--0. 712 47） and distinctly negative εNd （t） values （-9. 20 to -21. 21）. Compared to the north of western Shandong Province, the Mesozoic mafic rocks in the south have higher total REE contents （∑ REE = 325. 52 × 10^-6- 555. 75 × 10^-6）, higher LREE/HREE ratios （17.75--25.97）, and higher LILE/HFSE ratios （e. g. , La/Nb= 6. 37--13. 85, Th/Nb= 0. 52--1.53）. They also have more radiogenic Sr isotopic compositions, with Isr values of 0. 708 44--0. 712 47 and 0. 703 96--0. 705 98, respectively. Integrated elemental and isotopic tracing suggest that the Mesozoic mantle of western Shandong Province is generally characterized by the EMⅠ component, which was probably formed by large-scale lithospheric delamination, whereas the mantle source in the south part has been superimposed by the influence of the deep subducted Yangtze continental materials, and thus displays the mixed features of EM Ⅰ and EM Ⅲ components. The geochemical characteristics of the Cenozoic basalts are similar to those of the oceanic basalts. It is likely that the rocks were derived from a depleted asthenosphere source but underwent metasomatism shortly before partial melting. From the Mesozoic to the Cenozoic, the nature of the mantle beneath the North China craton evolved from an enriched one to a depleted one. This chemical evolution most likely resulted from an asthenosphere upwelling, which was induced by large-scale lithospheric delamination and thus replaced the

关 键 词：镁铁质火成岩 地球化学 源区示踪 地幔化学演化 鲁西 

分 类 号：P588.12[天文地球—岩石学]