内蒙古集宁橄榄岩包体微区微量元素与Sr同位素特征及其岩石圈地幔演化的指示意义

In-Situ Trace Elements and Sr Isotopes in Peridotite Xenoliths from Jining:Implications for Lithospheric Mantle Evolution

为完整了解华北克拉通的破坏程度和机制,加深对其西部陆块岩石圈地幔的研究十分重要,而位于华北克拉通西部集宁新生代碱性玄武岩中的地幔橄榄岩包体,为研究人员认识该地区的岩石圈地幔的性质和演化起到指示作用.运用LA-ICPMS和LA-MC-ICP-MS对集宁地区橄榄岩矿物进行原位微区测试,获得其主量、微量元素和Sr同位素成分的数据.根据矿物组成,可以将集宁地区的橄榄岩分为两类:第一类为贫单斜辉石橄榄岩(单斜辉石体积分数小于8%),它们经历了高程度的部分熔融,可能是古老难熔岩石圈地幔的残留;第二类为二辉橄榄岩(单斜辉石体积分数大于13%),其熔融程度低,代表了新生饱满的岩石圈地幔.第一类橄榄岩中单斜辉石REE含量整体偏低且轻微富集LREE,第二类橄榄岩中单斜辉石具有LREE富集和轻微亏损两种配分模式,大部分样品的核边有一定的强不相容元素及Sr同位素组成变化.这些微量元素和同位素特征都表明集宁橄榄岩包体经历过交代作用.(La/Yb)_N和Ti/Eu比值特征表明它们经历过多阶段的交代作用,交代介质有硅酸盐、碳酸盐熔/流体,这些交代介质可能为来源于古亚洲洋板块俯冲时释放的熔/流体.

Further research on the lithospheric mantle beneath western North China craton is significant in order to understand the degree and evolution of its thinning and transformation thoroughly.Mantle peridotite xenoliths in the Cenozoic alkaline basalts from Jining of western North China craton provide important constraints on the nature and evolution of the lithospheric mantle beneath this area.The samples were analyzed by using LA-ICP-MS and LA-MC-ICP-MS to obtain major element,trace element and Sr isotopic compositions.According to the mineral composition,the peridotites were classified into two groups：（1）Clinopyroxene（Cpx）-poor peridotites（Cpx〈8%）,which might have experienced high-degree partial melting,suggest relict of old and refractory lithospheric mantle under this area.（2）lherzolites（Cpx〉13%）.They might have experienced lowdegree partial melting,representing the fertilized lithospheric mantle.Cpx grains in Type 1peridotite have relatively low REE contents and show slightly LREE-enriched patterns,while Cpx grains in Type 2peridotite show both LREE-enriched andslightly LREE-depleted REE patterns.Most Cpx grains show core-rim variations in strong incompatible elements and Sr isotopic compositions.These features indicate that the peridotites could have experienced metamorphism.Variations of（La/Yb）_N and Ti/Eu ratios imply multistage metasomatism,which could have been caused by silicate and carbonatite melts/fluids associated with the Paleo-Asian oceanic plate subduction.