新疆西天山尼新塔格铁矿区石炭纪火山岩地球化学特征及其成因研究

摘要

In the Awulale metallogenic belt in western Tianshan Mountains in Xinjiang,Carboniferous volcanic rocks that host several large-medium sized iron deposits occur extensively. Thisstudyfocuses on the petrology, major and trace elements and isotope geochemicalcharacteristics of the Carboniferous volcanic rocks exposed in the district of Nixintage iron deposit, so as todiscuss its tectonic settings and petrogenesis. We propose that: (1) These volcanic rocksareNa-rich andbasicallybelong to the calc-alkali basalt series. (2) Trace element patterns indicate that these volcanic rocksareenriched in large ion lithophile elements and depleted in high field elements. Furthermore, significant depletioninNb, Ta and relatively weak depletion in Ti and high ratios of Th/Yb and Ta/Yballsuggest that these volcanic rocksareformed in an active continental margin tectonic setting. (3) Petrographicchracteristics andsimilar geochemistrical chracterisitcs indicate that these volcanic rocks are products formed by a cognate magma in different evolution stages.(4) DepletioninNb and Ta is a distinct characteristic ofepicontinental arc basalts formed by partial melting of hybrid mantle wedge whileεNd(t) range from +0.75 to +2.62 suggeststhatthe original magma comes from depleted mantle.Therefore, the magma source for these volcanic rocks in Nixintage district should be a metasomatic depleted mantle wedge. (5)The high content of Zr and high ratios ofZr/Y and La/Nb as well as the decentralized values of (87Sr/86Sr)i allsuggest that the magma has experienced a relatively strong contamination of crustal materials.%新疆西天山阿吾拉勒成矿带内分布有大量的石炭纪火山岩,由于分布广泛并赋存众多大-中型铁矿床,该套火山岩无论是对于西天山地区构造演化,还是矿产分布及成矿规律研究都有重要意义。为了探索其岩石成因及形成环境并进一步约束矿床成因,本文对研究程度相对较低的尼新塔格铁矿区内石炭纪火山岩进行了系统的岩石学、主元素和微量地球化学及同位素地球化学研究。研究显示,本区火山岩具有明显的富钠特征,多属亚碱性玄武岩系列中的钙碱性系列,岩相学及地球化学特征显示该地区火山岩为同源岩浆演化的产物。岩石富集大离子亲石元素和亏损高场强元素,具有相对较高Th/Yb和Ta/Yb比值,显示该类火山岩应形成于活动大陆边缘环境。明显的Nb、Ta亏损显示岩石具有湿地幔楔部分熔融的陆缘弧玄武岩的特征,而其εNd(t)值为+0.75~+2.62,为亏损型,这表明本区火山岩的岩浆源区应为俯冲板片之上受流体交代的亏损地幔楔。较高的Zr含量、Zr/Y比值、La/Nb比值以及较分散的(87Sr/86Sr)i值表明岩浆在演化过程中受到了较强的地壳物质混染作用。