西昆仑造山带东段中新元古代洋内弧火山岩地球化学特征

Geochemistry of Meso-and Neoproterozoic intra-oceanic arc volcanic rocks in the eastern segment of the western Kunlun orogenic belt

分布于新疆西昆仑东段北缘(策勒—于田—民丰一线以南)的中新元古代火山岩主要为一套浅变质的(玄武)安山岩,夹有极少量的流纹岩。安山岩的SiO2在52.36%～58.30%之间,平均为54.55%,TiO2均低于0.3%,平均为0.22%。Na2O+K2O为1.70%～3.75%,Na2O均高于K2O,Na2O/K2O在2.44～4.61之间,M/F为0.62～0.78,MgO/(MgO+TFe)在0.36～0.45之间。岩石硅碱成分显示以钙碱性为主。稀土元素研究表明,安山岩的稀土总量低,在15.52×10-6～17.92×10-6之间,接近大洋拉斑玄武岩。安山岩的(La/Yb)N为0.69～1.33,(Ce/Yb)N为0.75～1.17,轻重稀土分异不明显。除1028-I3号样的δEu为0.6之外,其余样品的δEu在0.91～1.13之间,基本没有Eu异常。在稀土元素的球粒陨石配分曲线上,所有安山岩形成一个群体,稀土配分模式接近大洋拉斑玄武岩。岩石的微量元素安山岩的Th/Yb为0.04～0.10,Th/Ta=5.92～11.22,Zr/Hf=20.46～29.40,Th/Y之比为0.04～0.08。微量元素的N—MORB配分模式显示岩石富集Sr、Ba、Rb、K、Th等大离子亲石元素,Cr亏损,与拉斑质的火山弧玄武岩相似。综合分析火山岩的岩石学、岩石地球化学特征,结合区域构造背景,认为该套火山岩产于洋内弧环境。

Study shows that the Meso-and Neopro terozoic volcanic rocks distributed south of the line of Cele-Mingfeng-Yutian on the northern margin of the eastern segment of the West Kunlun Mountains ,Xinjiang,consist dominantly of a suite of low-grade(basaltic)andesite with very small amount of rhyolite.Andesite contains 52.36%-58.30%(mean 54.55%)SiO 2 ,<0.3%(mean 0.22%)TiO 2 and1.70%-3.75%Na 2 O+K 2 O,with Na 2 O>K 2 O and Na 2 O /K 2 O=2.44-4.61,M /F=0.62-0.78and MgO /(MgO+TFe)=0.36-0.45.The alkali-silica composition shows that the rock is mainly calc-alkaline.REE study indicates that theΣREE content of andesite is low,exhibiting a narrow range between15.52and17.92ppm,close to that of oceanic tholeiite.The(La/Yb ) N of andesite is0.69-1.33and the(Ce /Yb ) N is0.75-1.17,the fraction-ation between HREE and HREE being not pronounced.Except for theδEu of sample1028-13that is0.6,theδEu values of other samples are in the range of 0.91-1.17,basically no Eu anomaly being pre-sent.On the chondrite-normalized REE distribution patterns all the andesite forms a colony.The REE distribution patterns are close to that of oceanic tholeiite.Study of trace elements in andesite indicates the following:Th /Yb=0.04-0.10,Th /Ta=5.92-11.22,Zr /Hf=20.46-29.40and Th /Y=0.04-0.08.The N-MORB distribution patterns of trace elements show that the rock is enriched in such large-ion lithophile elements as Sr,Ba ,Rb ,K and Th and depleted in Cr ,as is similar to tholeiitic volcanic arc basalt.An integrated petrologic and geochemical analysis of the volcanic rocks,combined with the tectonic setting,suggests that this suite of volcanic rock occurred in an intra-oceanic arc environment.