内蒙古阿拉善右旗吊井子一带镁铁质—超镁铁质岩石地球化学特征及成因分析

GEOCHEMICAL CHARACTERISTICS OF MAFIC AND ULTRAMAFIC ROCKS IN DIAOJINGZI,ALXA, INNER MONGOLIA

内蒙古自治区西部阿拉善右旗吊井子一带发育有3处镁铁质—超镁铁质岩体,位于华北板块之巴丹吉林坳陷西缘,其主要岩石类型为角闪石岩及橄榄岩。相对比与北大山镁铁质—超镁铁质岩体,本文研究区内角闪石岩分异明显,具有低钾、高钛、高铝、低磷等特征;大离子亲石元素富集,高场强元素亏损,稀土元素分异不明显,La/Sm、La/Ce、La/Yb较低,为亏损地幔来源的(N-MORB)洋中脊玄武岩的特点;橄榄岩富镁贫铁,具有镁质超基性岩的特点,岩浆未发生明显分异,微量元素中Cu、Ni、Co、Cr、V等地幔来源的元素强烈富集,不相容元素富集,稀土元素分异明显,稀土曲线右倾,具有弱Eu异常,La/Sm、La/Ce、La/Yb值较高,岩浆源于富集地幔物质的部分熔融。两种岩石地球化学成分及特征差异较大,不具备同源性特征。

Three small-sized Mafic-ultra mafic rock masses were found developed in Diaojingzi of Alxa Youqi, west of Inner Mongolia Autonomous Region. The main lithology is amphibolite and peridotite. The fractionation of amphibolite is obvious. It has the characteristics of low-potassium, high-titanium, high-aluminum and low-phosphorus. The large ionic lithophile elements are enriched, while the high field-strength elements are depleted. The fractionation of rare earth elements is not obvious, the ratios of La/Sm, La/Ce, La/Yb are lower,which has the characteristics of the mid-ocean ridge basalt（N-MORB）, ie. the depleted mantle source. The olivinite is rich in magnesium but poor in iron, which is the characteristics of magnesia ultrabasic rock magma.And there is no obvious fractionation of the magma. There shows a strong enrichment of mantle source elements,like Cu, Ni, Co, Cr, V. The incompatible elements are enriched, and the fractionation of rare earth elements is distinct. The rare earth elements curve is rightward, with characteristics of a weak Eu anomaly, and a high ratio of La/Sm, La/Ce and La/Yb. This suggests that the magma is derived from partial melting of mantle material.The geochemical components and characteristics of the two types of rocks are very different, so they are not homologous.