新疆乌什塔拉地区晚泥盆世火山岩地球化学特征及构造意义

Geochemical Characteristics and Tectonic Significance of the Late Devonian Volcanics in Wushentara Area,Xinjiang

乌什塔拉地区晚泥盆世火山岩产于破城子组中下段,位于南天山造山带中部,主要由一套酸性火山岩和中性火山岩-沉积岩组成;岩性主要为英安斑岩、流纹斑岩及少量安山岩,呈北西向展布。对岩石地球化学研究,表明破城子组火山岩属钙碱性准铝-过铝质系列;富集轻稀土元素,相对亏损重稀土元素,具有弱的Eu（平均为0.68）负异常;大离子亲石元素（K、Rb、Th等）明显富集,高场强元素相对亏损,具有明显的Ba、Nb、Ta、P、Ti负异常,且Th/Ta（9.04～14.37）值较高,Ta/Yb（0.27～0.69）值较低,暗示其具有陆缘岛弧火山岩的特征。87 Sr/86 Sr为0.713 14～0.725 07,比值较高,反映岩浆演化过程中有地壳物质的混染。结合南天山造山带的构造演化特征,认为晚泥盆世破城子组火山岩形成于活动大陆边缘环境。

Located in the middle of the South Tianshan orogenic belt,Late Devonian volcanics of WuShitala area occurred in the middle and lower section of Pochengzi Formation and is mainly composed of a set of acidic volcanics and neutral volcanics-sedimentary rocks,including dacite porphyry,rhyolite porphyry and a small amount of andesite.The volcanic rocks are distributed in a NW trend.The geochemical characteristics show that the Pochengzi Formation volcanics belong to the Calc-alkaline quasi-aluminium to peraluminous series,enriched in LREE and relatively depleted in HREE,and with weak Eu（an average of 0.68）negative anomaly;There is an obvious enrichment in large ion lithophile elements（such as K,Rb,Th）,a relative deficit in high field strength elements,as well as obvious negative Ba,Nb,Ta,P,Ti anomalies.Furthermore,the ratio of Th/Ta is higher（9.04-14.37）but the value of Ta/Yb is lower（0.27-0.69）,signifying that it has the characteristics of continental marginal island arc volcanic rocks.Ratio of 87Sr/86 Sr from 0.71314to 0.72507is relatively higher,reflecting crustal material contamination in magma evolution.According to the tectonic evolution characteristics of theSouth Tianshan orogenic belt,the characteristics of volcanics in Pochengzi Formation suggest that they were probably formed in an active continental margin environment.