内蒙古集宁新生代玄武岩的地幔源区特征:元素及Sr-Nd-Pb同位素地球化学证据  

Characteristics of mantle source for Jining Cenozoic basalts from southern Inner Mongolia:evidence from element and Sr-Nd-Pb isotopic geochemistry.Acta Petrologica

作　　者：张文慧[1] 韩宝福[2] 杜蔚[1] 刘志强[1] 

机构地区：[1]北京大学造山带与地壳演化教育部重点实验室 [2]北京大学地球与空间科学学院

出　　处：《岩石学报》2005年第6期1569-1582,共14页Acta Petrologica Sinica

基　　金：教育部"跨世纪人才培养计划"资助成果

摘　　要：内蒙古集宁玄武岩区位于华北克拉通北缘,东邻汉诺坝玄武岩区。由亚碱性的拉斑玄武岩及碱性的橄榄玄武岩、碧玄岩和碱玄岩组成。玄武岩的 SiO_2与 TiO_2含量分别为44.10～52.27wt.%和1.57～2.95wt.%,Mg~#(43-63)及 Ni 含量(27～210ppm)变化范围较大。集宁玄武岩的微量元素原始地幔标准化曲线及 REE 球粒陨石标准化曲线与 OIB 相似,Sr、Nd同位素比值显示了较汉诺坝玄武岩富集的特征。研究表明,虽然集宁玄武岩浆经历了一定程度的橄榄石、单斜辉石的分离结晶作用,但微量元素及 Sr、Nd、Pb 同位素特征排除了幔源岩浆在喷发到地表过程中受到地壳物质显著混染的可能性。^(143)Nd/^(144)Nd、^(87)Sr/^(86)Sr vs.^(206)Pb/^(204)Pb 的线性相关性表明,集宁玄武岩至少来自两个地幔端元组分:EMI 和 PREMA,且端元组分与汉诺坝玄武岩相似。尖晶石二辉橄榄岩的低程度部分熔融(2～5%)熔体与石榴石二辉橄榄岩更低程度部分熔融(<2%)熔体的混合,可以解释集宁玄武岩稀土元素的变化特征。推测 EMI 位于岩石圈地幔之内且源区深度<70km,PREMA 则来自软流圈地幔。集宁碱性玄武岩与亚碱性玄武岩之间的地球化学差异,可能只是来自岩石圈地幔的 EMI 型熔体和来自软流圈地幔的 PREMA 型熔体在形成玄武岩浆时参与混合的比例不同,暗示该地区的岩石圈地幔与软流囤地幔之间经历了强烈的相互作用。集宁玄武岩与汉诺坝玄武岩相似的 Sr、Nd、Pb 同位素比值及相关性,说明它们具有相似的地幔源区,汉诺坝玄武岩的地球化学差异同样可以用 EMI 与 PREMA 组分对岩浆贡献程度的不同来解释。Jining Cenozoic basaltic field, located on the northern edge of the North China craton and to the west of Hannuoba basaltic field, is mainly made up of subalkaline tholeiitic basalt, alkaline olivine basalt, basanite, and tephrite. Their SiO=and TiO2 contents vary from 44.10-52.27 wt. % and 1.57 - 2.95 wt. %, respectively, with variable Mg-number （43-63 ） and Ni contents （27 -210ppm）. The Primitive-mantle normalized incompatible element diagrams and Chondrite-normalized REE patterns are similar to those of OIB. Sr, Nd isotopic ratios are more enriched than Hannuoba basalts. The basalts were mantle-derived magmas with the fractional crystallization of olivine and clinopyroxene, but their trace-element and Sr, Nd, Pb isotopic geochemistry preclude the possibility that the magmas were contaminated significantly by crustal materials. The correlations of ^143Nd/^144 Nd,^87Sr/^86 Sr vs. ^206Pb/^204Pb indicates that Jining basalts were generated by the mixing of at least two mantle components： EMI and PREMA, similar to Hannuoba basalts. The mixing of low degree melts of spinel lherzolite （2 -5% ） and garnet lherzolite （ 〈 2% ） can explain the REE features in Jining basalts. It can be speculated that EMI mantle component came from the lithospheric mantle of a depth 〈 70kin, and PREMA mantle component from asthenospheric mantle. It is possible that the differences between the subalkaline and alkaline basalts may be produced by the mixing of the melts from lithospheric EMI mantle and asthenospheric PREMA mantle in different proportion, implying that strong interactions have occurred between lithosphere and asthenosphere beneath Jining area. The similar isotopic ratios and correlations suggest a similar mantle source for Jining and Hannuoba basalts. Therefore, the formation of Hannuoba basalts can be also interpreted by the mixing of the melts from EMI mantle and PREMA mantle in different proportion.

关 键 词：新生代玄武岩 集宁 地幔源区 部分熔融 同位素 

分 类 号：P588.145[天文地球—岩石学] P597.3[天文地球—地质学]