陕西勉略宁地区铜厂湾金矿床辉绿岩年代学、地球化学特征及其构造意义

Geochronology and geochemistry of diabase veins from Tongchangwan gold deposit in Mianluening area of Shaanxi Province and its tectonic significance

勉(县)-略(阳)-宁(强)地区在大地构造位置上处在秦岭造山带与扬子板块西北缘的接合部,是中国大陆造山系重要的组成部分,也是重要的矿化集中区,区内岩浆活动强烈,对岩浆岩的形成过程进行研究有利于促进对岩浆-成矿活动规律的系统认识。文章通过对勉略宁地区铜厂湾金矿区内辉绿岩脉进行岩石学、锆石U-Pb年代学和地球化学研究,讨论了辉绿岩的成因、物质来源、形成构造背景及其对区域找矿的指示意义。结果表明,辉绿岩脉的成岩年龄为(199.6±1.8)Ma,为早侏罗世。全岩地球化学特征显示,w(SiO_(2))为47.13%~51.56%,w(MgO)为3.83%~6.41%(Mg^(#)=37~48),w(FeO^(T))为9.36%~12.74%,w(TiO_(2))为1.37%~1.86%,呈亚碱性,属拉斑玄武岩系列。岩石稀土元素总量(ΣREE)为60.18×10^(-6)~189.06×10^(-6),轻、重稀土元素分馏显著((La/Yb)N=2.66~9.12),富集轻稀土元素(LREE),具有右倾型稀土元素配分模式,具Eu的弱负异常(δEu=0.6~0.97),与原始地幔相比具有明显的Nb、Ta、Sr负异常和Ba、K正异常。尽管铜厂湾矿区辉绿岩地球化学组成具有弧岩浆岩的特征,但根据区域构造演化历史,笔者认为辉绿岩形成于早侏罗世南秦岭后碰撞阶段,造山带整体处于减压增温的特殊构造体制,地幔物质上涌,底侵作用使得下地壳物质发生部分熔融从而形成了混合型岩浆。

Located at the juncture of the Qinling orogenic belt and the northwestern margin of the Yangtze plate,the Mianluening triangular zone is an important part of China’s orogenic system and mineralization concentration area.This area is characterized by intense magmatic activities,so studies of the formation process of magmatic rocks are helpful to the systematic understanding of magmatic metallogenic activities.In this study,the authors carried out petrologic and geochemical investigation as well as LA-ICP-MS zircon U-Pb dating for diabase in the Tongchangwan gold deposit of the Mianluening triangular area.Accordingly,petrogenesis,material source and tectonic setting of diabase and its significance for regional prospecting were discussed.The results show that the diabase was formed at(199.6±1.8)Ma of early Jurassic period.The diabase has SiO_(2) content of 47.13%~51.56%,MgO content of 3.83%~6.41%(Mg^(#)=37~48),FeO^(T) content of 9.36%~12.74%and TiO_(2) content of 1.37%~1.86%,showing subalkaline and tholeiitic features.The diabase shows high total REE content(60.18×10^(-6)~189.06×10^(-6))and(La/Yb)N ratios(2.66~9.12)with right-inclined REE patterns,weak negative Eu anomaly(δEu=0.6~0.97),depletion of Nb,Ta,Sr and enrichment of Ba and K.Although the geochemical composition of the diabase in the Tongchangwan deposit has the characteristics of arc magmatic rock,the authors,according to the regional tectonic evolution history,believe that the diabase was formed in the post collisional stage of South Qinling in the Early Jurassic period.During the post collisional stage,the orogenic belt was dominated by decompression and heating,accompanied by the upwelling of mantle materials and the partial melting of lower crustal materials,which caused the formation of mixed magma.