新疆西天山雾岭铁矿闪长岩地球化学及锆石U-Pb年代学

Geochemistry and zircon U-Pb geochronology of the diorite associated with the Wuling iron deposit in Western Tianshan Mountains, Xinjiang

雾岭闪长岩呈单式岩株状侵入于石炭系大哈拉军山组火山碎屑岩中,在闪长岩内部和接触带发现了铁矿体。闪长岩锆石U-Pb年龄为（307.7±0.8）Ma,于晚石炭世侵位。岩石具有正的εNd（t）值（3.1-4.6）和相对低的初始^87Sr/^86Sr比值（0.7043-0.7046）,Nd模式年龄为695-818 Ma,在^87Sr/^86Sr（t）-εNd（t）图解上位于地幔趋势线上,表明地幔是闪长岩的源区,但Th含量（1.61-3.24μg/g）和Th/Ta比值（6.1-7.9）较高,指示受到了陆壳混染作用的影响。因此认为,该岩浆可能是由玄武质岩浆与地壳物质相互作用而发生分离结晶作用后形成。通过综合分析认为,雾岭闪长岩虽然保留有岛弧岩浆作用的痕迹,但其实是板内环境的产物,是在西天山增生造山结束、伊犁微陆块与塔里木板块发生块体间旋转时侵位的,其成因可能与伊犁微陆块内造山带根部拆沉作用或局部形成的挤压-伸展的构造转变带有关。

The Wuling diorite stock was intruded in volcaniclastic rocks of the Carboniferous Dahalajunshan Formation. Iron ore deposits were discovered in the contact zone or the interior of the stock. Zircon LA-ICP-MS U-Pb dating of the diorite yielded a weighted mean ^206pb/^238U age of （307.7±0.8）Ma. Positive εNd（t） values vary from3.1 to 4.6, initial ^87Sr/^86Sr ratios are relatively low （0.7043-0.7046）, and Nd model ages vary between 695Ma and 818Ma and fall on the mantle trend line in the ^87Sr/^86Sr（t） versus εNd（t） diagram. These data indicate a mantle source for the Wuling diorite. However, high Th values （1.61-3.24μg/g） and Th/Ta ratios （6.1-7.9） imply that the magma experienced a certain degree of contamination by continental crust. Therefore, the Wuling diorite magma might have been formed by the fractional crystallization of basaltic magma affected by crustal material. Although some traces of arc magmatism can be detected, the Wuling dirote was actually formed by intraplate magmatism. This diorite was emplaced when block rotation occurred between the Yili and Tarim plates after the termination of the West Tianshan accretionary orogeny. It is inferred that the generation of the magma was probably related to the detachment of an orogenic root zone or a local squeezing- stretching structural transition zone within the Yili plate.