北祁连甘禅口基性辉绿岩脉 LA-ICP-MS锆石 U-Pb 定年及其地质意义

摘要

Located in Qilian orogenic belt,Ganchankou basite diabase emplaces in Tuolai terrain of Paleoproterozoic Era,and its lithology is dark grey diabase.In this paper,LA-ICP-MS zircon U-Pb dating method is adopted to date the diabase,and 293.0 ±1.8Ma is obtained which indi-cates that a stretch event happened in early Permian Epoch.This paper finds that the diabase be-longs to tholeiite series,with the ratio of chemical A/CNK changing from 0.60 to 0.76,the value of A/NK varying from 2.40 to 4.35,the content of Al2 O3 ranging from 1 1.60% to 18.36%,and the content of TiO2 fluctuating between 0.66% and 2.85%.Meanwhile,trace elements such as K,Rb,Ba and Th have strong enrichment,but Hf and Zr are apparently depleted while Y and Yb are characterized by slight depletion.The study also shows that light rare earth elements after weak fractionation are enriched while the fractionation of heavy rare earth elements is not obvious in the diabase.At the same time there is a weak negative Eu anomaly.According to the regional tectonic background and the petrological and geochemical characteristics,it can be concluded that Ganchankou diabase dyke is formed by the post-tectonic crust thickening underplating of Qilian o-rogenic belt and the magma intermingling effect.%甘禅口基性辉绿岩脉产于北祁连弧盆系中,岩性为深灰色辉绿岩。脉体主要侵位于古元古代托赖岩群中。利用 LA-ICP-MS 锆石 U-Pb 定年方法对该基性辉绿岩脉测年,获得(293.0±1.8)Ma,表明测区在早二叠世有一次伸展事件发生。岩石化学 A/CNK 值为0.60~0.76,A/NK 为2.40~4.35,Al2 O3含量为11.60%~18.36%,TiO2含量为0.66%~2.85%,属拉斑玄武岩系列；微量元素 K、Rb、Ba、Th 富集强烈,Hf、Zr 明显亏损,而 Y、Yb 具有略亏损的特点。脉体具有明显富集的微弱分馏的轻稀土元素,重稀土元素分馏不明显,同时具 Eu 弱负异常特征。根据区域构造背景及岩石学、地球化学特征,可以认为甘禅口基性辉绿岩脉是祁连山造山带进入造山后陆壳加厚底侵-岩浆混合作用形成的。