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

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,Al2O3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。

LA-ICP-MS Zircon U-Pb Dating of Ganchankou Basite Diabase in Northern Qilian and Its Geological Significance

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 indicates that a stretch event happened in early Permian Epoch.This paper finds that the diabase belongs to tholeiite series,with the ratio of chemical A/CNK changing from 0.60to 0.76,the value of A/NK varying from 2.40to 4.35,the content of Al2O3ranging from 11.60%to 18.36%,and the content of TiO2fluctuating 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 regionaltectonic 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 orogenic belt and the magma intermingling effect.