山东荣成马草夼橄榄岩矿物地球化学研究
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973项目(003CB716500);中国地质调查局项目(2001CCB00900);中国地质科学院重点实验室基金资助项目(JB0703)


Geochemistry of minerals from Macaokuang peridotitein Rongcheng area, Shandong Province
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    摘要:

    马草夼橄榄岩位于苏鲁超高压变质带山东荣成腾家集附近,其原岩为残余地幔方辉橄榄岩,经历了超高压变质作用和后期麻粒岩角闪岩相变质作用。对马草夼橄榄岩及侵入于其中的辉石岩脉和角闪岩脉的主要矿物橄榄石、斜方辉石和角闪石做了单矿物微量元素和稀土元素地球化学分析,结果表明Co和Ni在橄榄石中含量最高且富集,在斜方辉石和角闪石中含量低且亏损。由于橄榄石在全岩中含量高达90%以上,因而全岩Co、Ni富集。虽然角闪石在全岩中体积含量仅占3%~8%,但斜方辉石和橄榄石的微量和稀土元素含量远远低于角闪石,故全岩的微量(Co、Ni除外)和稀土元素化学特征受控于角闪石。斜方辉石、橄榄石与角闪石的微量和稀土元素分布特征基本相同,辉石岩脉和角闪岩脉中的角闪石比地幔橄榄岩中的角闪石更富集微量和稀土元素。微量和稀土元素地球化学特征显示马草夼橄榄岩至少受到了含Co、Ni的流体,富含LREE、Ba、Sr、Ta的硅酸盐流体和少量低Th、U的碳酸盐地幔流体的多次交代,流体交代发生于超高压变质后期浅部地幔驻留期间。马草夼橄榄岩所处的氧逸度(相对于FMQ)为+1.0~+2.0,落入正常大陆地幔的范围,在折返过程中未受到壳源流体的影响,保持了地幔氧同位素特征。

    Abstract:

    Located in Tengjiaji of Rongcheng area, Shandong Province, the Macaokuang peridotite lies in the northeastern part of the Sulu ultrahigh pressure metamorphic belt. The protolith of the peridotite is the residual mantle harzburgite, which underwent ultrahigh pressure and then granuliteamphibolite facies metamorphism. Trace and rare earth element abundances of olivine, orthopyroxene and amphibole in the Macaokuang mantle peridotite and the orthopyroxenite veins and amphibolite veins within the peridotite were analyzed, and the result shows that Co and Ni are high and mainly concentrated in olivine, but their contents are low in orthopyroxene and amphibole, suggesting that the bulk rock is enriched in Co and Ni due to the high proportion of olivine in the rock. Although it makes up only 3%~8% of the whole rock, the amphibole controls the contents and distribution patterns of trace and rare earth elements of the bulk rock with the exception of Co and Ni. Orthopyroxene and olivine have much lower trace and rare earth element abundances than amphibole, and the element distribution patterns of orthopyroxene and olivine are quite similar to the pattern of amphibole. Amphibole in orthopyroxenite veins and amphibolite veins is more enriched in trace and rare earth elements than that in peridotite. Geochemistry of the elements shows that peridotite must have experienced repeated metasomatic actions of at least such components as Co and Nirich fluids, LREE, Ba, Sr, Tarich and Th, Uextremely low silicate fluids and carbonic fluids. Fluid metasomatism occurred after UHP metamorphism at a shallow mantle depth. Olivine, orthopyroxene and amphibole in the peridotite together with the intruded veins are characterized by normal mantle oxygen isotopic ratios and show equilibrium each other. Meanwhile, the oxygen fugacity of the peridotite is +1.0~+2.0, falling in the normal continental mantle range, implying that the rock has not experienced metasomatic action of crustal fluids. 

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任玉峰,高 翔,杨经绥,胡明月,贾泽荣,2009,山东荣成马草夼橄榄岩矿物地球化学研究[J].岩石矿物学杂志,28(3):215~224.

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