蒙古中部Tariat新生代玄武岩的地幔源区特征:橄榄石成分及Sr-Nd-Pb同位素证据

Characteristics of the mantle source for Cenozoic basalts in Tariat, Central Mongolia:Olivine composition and Sr-Nd-Pb isotopes

蒙古中部新生代玄武岩呈弥散状广泛分布,然而有关该地区新生代玄武岩的研究十分有限。本文分析了蒙古中部Tariat地区新生代玄武岩中橄榄石斑晶的成分,并结合新的全岩主量元素、微量元素、Sr-Nd-Pb同位素数据以及前人报道的数据,探讨蒙古中部新生代玄武岩的地幔源区特征。Tariat新生代玄武岩以碱性玄武岩为主,微量元素蛛网图上具有明显Ba、K、Sr正异常,Th、U、Ti负异常,无明显Nb、Ta负异常,显示出与富集地幔(EM1)型洋岛玄武岩(OIB)相似的特征。Tariat玄武岩的Sr-Nd-Pb同位素组成介于亏损地幔(DMM)与EM1组分之间。橄榄石成分表明Tariat玄武岩源区同时存在辉石岩和橄榄岩组分。Tariat玄武岩的Sr/Sr与CaO/AlO值、Nb/U与Ce/Pb值呈良好的负相关关系,Nd/Nd与CaO/AlO值、Ba/Th以及Pb/Pb与Ba/La值呈良好的正相关关系,说明Tariat玄武岩的成分变化主要受控于DMM与EM1端元组分的混合作用,其中DMM端元组分来源于含金云母的岩石圈地幔,而EM1端元组分来源于再循环的洋壳辉长岩与远洋沉积物混合,玄武岩源区母岩中的辉石岩组分则是由再循环的洋壳物质熔体交代地幔橄榄岩形成的二阶段辉石岩。

Cenozoic basalts are widespread throughout Mongolia;however, research on these basalts remains limited. This study analyzed the composition of olivine phenocrysts in the Tariat Cenozoic basalts of central Mongolia, and combines the new major element, trace element, Sr-Nd-Pb isotope whole-rock data, and the data reported by the predecessors to discuss the characteristics of the mantle source of Cenozoic basalts in central Mongolia. The Tariat Cenozoic basalts are dominated by alkaline basalts, and the samples showed similar trace element patterns to EM1-type oceanic island basalt(OIB) in the primitive mantle-normalized trace element spidergrams with obvious positive anomalies of Ba, K, and Sr;negative anomalies of Th, U, and Ti;and no obvious Nb or Ta negative anomaly. The Sr-Nd-Pb isotopic data showed that the isotopic composition of the Tariat basalts is between the depleted mantle(DMM) and enriched mantle(EM1) components. The composition of olivines in the Tariat basalts indicated that pyroxenite and peridotite within the parent rock in the mantle source.TheSr/Sr of the basalt had a good negative correlation with CaO/AlO, Nb/U, and Ce/Pb;Nd/Nd had a good positive correlation with CaO/AlOand Ba/Th;andPb/Pb had a good positive correlation with Ba/La.These correlations indicate that the compositional change of the Tariat basalts were predominantly controlled by the mixing of DMM-and EM1-like components. The components of the DMM originated from the lithospheric mantle containing phlogopite, and the EM1-like components were derived from the mixing of recycled oceanic crust gabbro and pelagic sediments and the pyroxenite components of the source lithology were derived from two-stage pyroxenite which formed by the reaction of mantle peridotite with the melting of recycled ocean crust components.