北祁连西段小柳沟矿区花岗质岩石锆石U-Pb年代学、地球化学及成因研究

Zircon U-Pb geochronology,geochemistry and petrogenesis of the granites from the Xiaoliugou deposit in the western of the North Qilian

甘肃小柳沟钨钼(铜)矿是一个与花岗岩类有关的斑岩-矽卡岩-石英脉型矿床,小柳沟矿区内花岗质岩石类型主要为二长花岗岩和花岗闪长岩。本文对该岩体进行年代学、地球化学研究以约束其形成时代和岩石成因。LA-ICP-MS锆石UPb测年分别获得二长花岗岩与花岗闪长岩谐和年龄为454.0±2.0Ma和417.7±1.7Ma,属于加里东期岩浆活动的产物,并经历了海西期和燕山期岩浆热事件的改造。地球化学数据显示,小柳沟花岗质岩石具有高硅、富碱的特征,属于过铝质高钾钙碱性系列,富集Rb、Th、U、K、Pb,亏损Ba、Sr、Ti、P,具有明显Eu负异常,尤其是二长花岗岩显示强烈的铕负异常(δEu为0.08～0.19),属于高分异I型花岗岩。锆石Hf同位素分析结果显示,二长花岗岩εHf(t)为-4.45～4.04,t DM2=1176～1714Ma,花岗闪长岩εHf(t)为-4.18～4.43,t DM2=1124～1670Ma,二者可能是由壳幔混合作用形成,其壳源源区很可能来源于古元古代-中元古代古老地壳岩石,花岗闪长岩的源区相对较深,北大河岩群与朱龙关群可能是小柳沟钨矿的初始矿源层。综合研究表明,小柳沟钨钼矿床与二长花岗岩在时间、空间及成因上有密切关系。结合区域构造演化,认为小柳沟二长花岗岩应形成于俯冲背景下的活动大陆边缘环境,花岗闪长岩形成于碰撞造山环境。

The Xiaoliugou W-Mo deposit in Gansu is a porphyry-skarn-quartz type deposit related to granitoids. Xiaoliugou granitoids are comprised of monzogranite and granodiorite. In this paper,zircon U-Pb dating and geochemistry of the Xiaoliugou granite are studied to constrain its geochronology and petrogenesis. Zircon LA-ICP-MS dating yields concordant ages of 454. 0 ± 2. 0 and 417. 7 ± 1. 7Ma respectively indicating that the two plutons were formed at Caledonian,later affected by Hercynian and Yanshanian magmaticthermal event. Geochemical data shows that,Xiaoliugou granite is silica enriched in composition with high content of alkali. It＇s a peraluminous granite and belongs to the high-K calc-alkaline series,enriched in Rb,Th,U,K,Pb,depleted in Ba,Sr,Ti,P with strong negative abnormal of Eu,especially,the monzogranite has intensely negative Eu abnormality（ δEu = 0. 08 ～ 0. 19） and should be ascribed to highly fractionated I-type granite. The Hf isotope shows that the ε Hf（ t） value of the monzogranite range from- 4. 45 to 4. 04,with t DM2 ages between 1176Ma and 1714Ma,while the ε Hf（ t） value of the granodiorite range from-4. 18 to 4. 43,with t DM2 ages between 1124Ma and 1670Ma,indicating that the Xiaoliugou granitoids were most likely generated by mixing of a depleted mantlederived and an induced crustal-melted felsic magma in the deep crust. We suggested that the source region of granite were likely from the Paleoproterozoic-Mesoproterozoic ancient crustal rocks and the granodiorite is relatively deeper than monzogranite,the ore-forming metals such as tungsten in the deposit may have likely derived from the Beidahe Group and the Zhulongguan Group. Comprehensive studies indicate that Xiaoliugou W-Mo deposit has a closely temporal relationship with the monzogranite. Combine with the tectonic evolution of the North Qilian,we speculate the monzogranite was generated in a continental arc tectonic setting triggered by slab subduction,the granodiorite probably have been generated in syn-collison setting.