西天山特克斯县东南一带大哈拉军山组中性火山岩地球化学及成因探讨

DISCUSSION ON GEOCHEMISTRY AND FORMATION OF INTERMEDIATE VOLCANICS OF DAHALAJUNSHAN GROUP IN SOUTHEAST TEKESI COUNTY AREA, WEST TIANSHAN MOUNTAINS

西天山特克斯县东南一带的大哈拉军山组中性火山岩具有钙碱性火山岩的地球化学特征,在TAS上多数属于玄武安山岩、安山岩,少数为粗安岩。岩石总体反映高硅低钛、富钠低钾特点,w(SiO2)≥54.18%,w(TiO2)≥0.89%,全碱w(K2O+Na2O)≥4.19%,Na2O/K2O≥1.40%。其地球化学的显著特点是:富集大离子亲石元素(Rb,Th,K等)和轻稀土元素(LREE),亏损高场强元素(Nb,Ti等)和重稀土元素(HREE),Eu负异常明显(δEu=0.39～0.78),与岛弧钙碱性火山岩相似。火山岩具有较高的La/Nb值、Ba/Nb值(1.74～2.77,24.03～59.43)和低La/Ba值(0.038～0.092)。Th/Ta,Nb/Ta和Zr/Hf值分别为2.06～17.38,3.35～13.41和37.94～45.02。元素地球化学特征反映岩浆源区具有富集型地幔岩浆源区的特征。其成因可能是受到地壳物质混染和来自消减残留板片析出流体或熔体交代的双重改造。

The intermediate volcanic rocks of Dahalajunshan Group in southeast Tekesi County area, west Tianshan, are calc-alkaline volcanic rocks, most of them are basaltic andesite, andesite, with minor trachyandesite. These rocks are high in silicon and sodium,low in titanium and potas- sium,w（SiOz） ≥54.18%, w（TiO2） ≥0.89%, alkali w （KzO＋Na20）≥4. 19%, Na20/K20≥ 1.40%. Other significant features are enrichment of large ion lithophile elements （Rb, Th, K, etc. ） and LREE, depleted high field strength elements（Nb, Ti, etc. ） and HREE, obvious Eu nega- tive anomaly（3Eu=0. 39-0. 78）, similar to the island arc calc-alkaline volcanic rocks. Volcanic rocks are high in La/Nb and Ba/Nb values（1.74-2.77,24.03-59.43）, low in La/Ba values （0. 038-0. 092）. Th/Ta,Nb/Ta and Zr/Hf values are 2.06-17.38,3.35-13.41 and 37.94-445.02 respectively. Geochemical characteristics show that the magmatic source is of enriched man- tle magma source,which was resulted from by crust contamination and metasomatic process by fluid or melted materials from subduction residual plate.