内蒙古宁城早中生代地幔捕虏体及熔/流体交代作用

内蒙古宁城地区发现的尖晶石二辉橄榄岩捕虏体，其寄主岩为早中生代堆晶成因的辉石岩。尖晶石二辉橄榄岩在矿物组成和Sr、Nd、Pb同位素特征等方面区别于寄主辉石岩，具有地幔岩特征。相对原始地幔岩,尖晶石二辉橄榄岩具有Fe高，Mg、Al低，富集K、Na、Ca、LREE和Rb、Sr、Ba、Th等不相容元素的特征，据此可以认为捕虏体来源于交代的富集地幔。对熔/流体交代反应形成的结构及其交代矿物金云母、韭闪石、白云石、方解石的进一步研究，揭示富K、Al、Ca、LREE和不相容元素的硅酸质和碳酸质熔/流体的交代作用致使地幔岩向不断饱满和富集的趋势演化，导致地幔岩Mg#值、Fo值的降低，Al和其他不相容元素的增高。单斜辉石环带原位微量元素测定也证实交代作用的存在。剪切结构的发育可能与软流圈底辟体上涌引发的塑性流变有关。联系华北古老岩石圈地幔多次的地质事件，笔者认为，早中生代地幔的特征与华北克拉通长期以来自身的深部演化有关。

The Early Mesozoic mantle xenoliths and their melt/fluid metasomatism,Ningcheng Area, Inner Mongolia

The spinel-lherzolite xenoliths were discovered within a host rock of Early Mesozoic cumulate pyroxenite in Ningcheng Area of Inner Mongolia.The spinel-lherzolite is different from the host pyroxenite in mineral composition and Sr-Nd-Pb isotopic characteristics,showing features of pyrolite.Relative to the primary mantle,the spinel-lherzolite is characterized by high Fe,low Mg and Al,and rich K,Na,Ca,LREEs,and such incompatible elements as Rb,Sr,Ba,Th,etc.,and thus the xenoliths are considered as coming from a metasomatic enriched mantle.Further investigation of the textures formed by melt/fluid metasomatic reaction and the metasomatic minerals-phlogopite,pargasite,dolomite,and calcite-reveals that it is the melt/fluid metasomatism of the silicates and carbonates rich in K,Al,Ca,LREEs and incompatible elements that makes the pyrolite evolve towards more and more fertile and enriched due to decreasing Mg# and Fo and increasing Al and other incompatible elements.Measurements of REEs of the Cpx in-situ also demonstrate the existence of metasomatism.The developed shear textures are likely related to plastic rheomorphism caused by the upwelling diapirs of the asthenosphere.In connection to many geologic events happened to the ancient lithospheric mantle of North China,the authors believe that the characteristics of the concerned Early Mesozoic mantle are related to the evolution of the North China Craton itself at depth during a long period of time.