华北晚中生代和新生代玄武岩中单斜辉石巨晶来源及其对寄主岩岩浆过程的制约:以莒南和鹤壁为例

本文对华北克拉通晚中生代和新生代碱性玄武质岩石中的单斜辉石巨晶进行了主、微量元素和Sr-Nd同位素的综合研究,发现晚中生代和新生代单斜辉石巨晶存在明显的主、微量元素和同位素组成上的差异。新生代单斜辉石巨晶有Al-普通辉石和次透辉石两类;而中生代单斜辉石巨晶只有Al-普通辉石。新生代单斜辉石SiO_2含量高、REE配分型式为上凸型、LILE和放射性元素含量高,并具有比寄主碱性玄武岩更亏损的Sr和Nd同位素组成;而中生代单斜辉石SiO_2含量低、REE配分型式为LREE富集型、LILE和部分HFSE以及放射性元素含量低,并具有比寄主碱性玄武岩稍富集的Sr和Nd同位素组成;巨晶的结构、矿物成分和地球化学特征,以及Mg-Fe在熔体与单斜辉石间的分配状况皆说明,新生代碱性玄武岩中单斜辉石巨晶是碱性玄武岩浆在高压下结晶的,因此二者是同源的;而中生代单斜辉石巨晶是被寄主岩浆偶然捕获的捕虏晶,是不同源的。华北新生代单斜辉石巨晶存在于碱性玄武岩和拉斑玄武岩中,它们具有比寄主碱性玄武岩更亏损的Sr和Nd同位素组成,说明即使是碱性玄武岩也不能完全代表软流圈来源的原始岩浆,其在上升过程中或多或少存在同位素组成富集的物质的混入。同时,拉斑玄武岩不是碱性玄武质岩浆直接结晶分异的产物,亦不是完全由部分熔融程度的不同造成的。拉斑玄武岩中存在岩石圈地幔物质的贡献或是岩浆房内碱性玄武质岩浆受地壳混染作用的结果。

Origin of clinopyroxene megacrysts in the late Mesozoic and Cenozoic basalts from the North China Craton and their constraints on the magma process of host rocks: a case of the Junan and Hebi

Major- and trace-elements as well as Sr-Nd isotopes of clinopyroxene megacrysts in the late Mesozoic and Cenozoic alkaline basaltic rocks from the North China Craton have been analyzed in this paper.There are two types of clinopyroxene megacrysts (Al-augite and salites) in the Cenozoic alkaline basalts and one type (Al-augite) in the late Mesozoic basalts.The Cenozoic Al-augite megacrysts is different from the late Mesozoic ones in many aspects of major- and trace-elements and Sr-Nd isotopes.The Cenozoic Al-augite megacrysts are characterized by high SiO_2,convex upward Chondrite-normalized REE pattern,and higher LILE and radioactive element contents.Their Sr and Nd isotopic compositions are more depleted than those of the host rocks.The late Mesozoic Al-augite megacrysts are characterized by low SiO_2 and LREE enriched patterns with lower LILE,some HFSE and radioactive element contents.In addition,the Sr and Nd isotopic compositions of the late Mesozoic megacrysts are less depleted than those of the host rocks.The characteristics of texture,mineral compositions and geochemistry as well as the Fe-Mg partitioning between the crystal and the melt indicates that the Al-augites in the Cenozoic basalts represent high-pressure crystallization products of alkaline basaltic melts. Thus,both of clinopyroxene megacrysts and host basalts could be derived from a same primitive magma.However,the Al-augites in the late Mesozoic basaltic rocks represent accidentally-lncluded xenocrysts of basaltic components which had crystallized in the depth from a previously melting episode.The more depleted Sr-Nd isotopic compositions of Cenozoic megacrysts compared with those of host alkaline basalts and tholeiites demonstrate that even the alkali basahs could not completely represent primitive magma initiating in asthenosphere.That is to say,the Cenozoic alkaline basalts were more or less modified by some enriched Sr-Nd isotopic components during their eruption.Meanwhile,the tholeiites were not the products formed only by fractional crystallization of alkaline basaltic magma or different degrees of partial melting.It may result from the contribution of lithospheric mantle materials or crust contamination in magma chamber to alkali basaltic magmas.