| 鄂尔多斯地块北部中、新生代玄武岩地球化学特征及其地质意义 |
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| 引用本文： | 邹和平,张珂,刘玉亮,李刚.鄂尔多斯地块北部中、新生代玄武岩地球化学特征及其地质意义[J].大地构造与成矿学,2010,34(1). |
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| 作者姓名： | 邹和平 张珂 刘玉亮 李刚 |
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| 作者单位： | 中山大学,地球科学系,广东,广州,510275;中山大学,地球科学系,广东,广州,510275;中山大学,地球科学系,广东,广州,510275;中山大学,地球科学系,广东,广州,510275 |
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| 基金项目： | 国家自然科学基金项目 |
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| 摘 要： | 通过对分布于鄂尔多斯地块北部杭锦旗与和林格尔玄武岩地球化学特征的研究,探讨了该区中、新生代岩浆源区的组成、变化及其所反映的地质意义。杭锦旗与和林格尔分别有早白垩世(Ar-Ar年龄为126.2Ma)和晚中新世(Ar-Ar年龄为6.4Ma)玄武岩分布。杭锦旗早白垩世玄武岩的SiO2(46.93%～47.93%)较低而碱含量较高(K2O+Na2O为5.75%～6.21%),组合指数σ为6.7～8.0,K2O(Na2O-2),属于碱性系列的钾质粗面玄武岩;其Mg#值为32.4～32.8,Ni、Cr含量低于20μg/g,反映玄武岩浆形成后经历过橄榄石和单斜辉石分离结晶等后期演化作用;其Nb、Ta相对于La亏损,初始87Sr/86Sr为0.7062,εNd(t)为-12.25,反映岩浆源区主要为富集EMI型地幔。和林格尔晚中新世玄武岩为橄榄拉斑玄武岩,其组合指数σ为2.1～2.8,Mg#值变化于46.7～52.4,Ni含量变化于111.1～125.8μg/g,Cr为123～178μg/g,Mg#值和Ni、Cr含量的变化反映岩浆原生程度随时间变新有所增大;其Nb、Ta相对富集,具有相似于OIB的微量元素特征,但仍表现相对富集的Sr-Nd同位素性质,初始87Sr/86Sr比值变化于0.7051～0.7053,εNd(t)为-1.91～-0.74,推测为软流圈与EMI型岩石圈地幔相互作用的产物。杭锦旗与和林格尔玄武岩的207Pb/204Pb比值都较低,△207/204为3.6～6.5,△208/20460,均具EMI型Dupal异常。玄武岩地球化学特征进一步表明,华北克拉通西部的鄂尔多斯地块所处的岩石圈自早白垩世以来发生过较小规模的伸展减薄作用,但其伸展减薄和被改造的程度远弱于华北东部。
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| 关 键 词： | 玄武岩 地球化学 岩石圈 伸展减薄作用 鄂尔多斯地块 |
Geochemical Characteristics and their Geological Implications of Meso-Cenozoic Basalts in the Northern Ordos Block, North China |
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| ZOU Heping,ZHANG Ke,LIU Yuliang,LI Gang.Geochemical Characteristics and their Geological Implications of Meso-Cenozoic Basalts in the Northern Ordos Block, North China[J].Geotectonica et Metallogenia,2010,34(1). |
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| Authors: | ZOU Heping ZHANG Ke LIU Yuliang LI Gang |
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| Abstract: | Based on the geochemical and isotopic compositions of the Meso-Cenozoic basalts from the northern Ordos Block in North China, the implication of the possible mantle processes of the North China craton breakage beginning from the Mesozoic is discussed. The early Cretaceous basalts with Ar-Ar age of 126.2 Ma are distributed at Hangjin Banner in the northern Ordos and the late Miocene basalts with Ar-Ar age of 6.4 Ma are located at Helinger County in the northeastern Ordos. The early Cretaceous basalts from Hangjin Banner belong to alkalic potassic trachybasalts with lower SiO_2 (46.93%～47.93%), higher alkali contents ( K_2O+Na_2O=5.75%～6.21% and K_2O >(Na_2O-2)),and the serial index (σ)of 6.7～8.0. Their Mg~# indices are from 32.4 to 32.8 and contents of Ni-Cr are lower than 20μg/g, indicating that there had occurred crystallization differentiation of minerals such as olivine and monoclinic pyroxene in basaltic magmas. The early Cretaceous basalts show Nb-Ta depletions relative to La, and their initial ~(87)Sr/~(86)Sr ratios are 0.7062 and ε_(Nd)(t)-12.25, indicating that the magma was mainly derived from the enriched EMI-type mantle. On the other hand, the late Miocene basalts from Helinger belong to the olivine tholeiite associations with Mg~# indices being 46.7～52.4 and the serial index (σ) 2.1～2.8.Their contents of Ni and Cr are 111.1～125.8μg/g and 123～178μg/g, respectively. Variation of the Mg~# indices and contents of Ni-Cr show a higher degree of original magma with the passage of time. The late Miocene basalts also show Nb-Ta enrichment that is similar to OIB in incompatible trace elements and relatively enriched characteristics of Sr-Nd isotope compositions with the initial ~(87)Sr/~(86)Sr ratios ranging from 0.7051 to 0.7053 and ε_(Nd)(t)from -1.91 to -0.74, implying that the late Miocene magmas may have been the products as interaction of the asthenosphere and the EMI-type continental lithospheric mantle. Furthermore, both the basalts from Hangjin Banner and Helinger County are of low ~(207)Pb/~(204)Pb ratios with △207/204 being 6～6.5 and △208/204>60, showing characteristics of the EMI-type Dupal isotope anomaly. Therefore, it is further indicated by the geochemical and Sr-Nd-Pb isotopic data of the basalts that a small-scale extension and thinning of the lithosphere beneath the Ordos block had taken place since the early Cretaceous. The extent of thinning and transformation of the lithosphere beneath the western North China, however, was far weaker than that of the eastern North China. |
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| Keywords: | basalt geochemistry lithosphere extension and thinning the Ordos block |
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