甘肃白银矿田变酸性火山岩锆石LA-ICP-MS测年———白银式块状硫化物矿床形成时代新证据  

A LA-ICP-MS U-Pb chronological study of zircons from meta-acidic volcanics in Baiyin orefield,Gansu Province: New evidence for metallogenic age of Baiyin type massive sulfide deposits

作　　者：何世平[1] 王洪亮[2] 陈隽璐[1] 徐学义[2] 张宏飞[1] 任光明[2] 余吉远[2] 

机构地区：[1]中国地质大学,湖北武汉430074 [2]中国地质调查局西安地质矿产研究所

出　　处：《矿床地质》2006年第4期401-411,共11页Mineral Deposits

基　　金：中国地质调查局国土资源大调查研究项目(西北地区重要成矿带基础地质综合研究;编号1212010510416)的资助

摘　　要：结合阴极发光,对白银矿田与成矿密切相关的变酸性火山岩中的锆石进行了LA_ICP_MS(激光剥蚀等离子体质谱)U_Pb同位素测年。获得变石英角斑岩内单颗粒锆石的微区U_Pb年龄分别为(467.3±2.9)Ma、(414.2±2.7)Ma,糜棱岩化石英角斑岩内单颗粒锆石的微区U_Pb年龄分别为(467.1±2.2)Ma、(435.9±3.6)Ma和(412.6±2.1)Ma。认为白银矿田变酸性火山岩的形成时代为中奥陶世,由韧性剪切作用引起的糜棱岩化的时代为早志留世晚期,主变质期时代为早泥盆世早期。提出白银铜多金属矿床形成时代为中奥陶世。这一新认识,对进一步深入研究北祁连造山带的构造演化过程以及白银式块状硫化物矿床的形成环境、找矿方向具有重要意义。The Baiyin orefield is located in the eastern part of North Qilian Mountains, and its copper-polymetallic ore bodies are controlled strictly by meta-acid volcanic rocks. However, the long controversy on the problem of metallogenic epoch has hindered the further ore prospecting work. In this paper, cathodoluminescence image （CL） and LA-ICP-MS U-Pb isotope dating techniques were used to determine the metallogenic age. It is found that zircons from meta-acid volcanic rocks show features of magmatic zircon, characterized by rhythmic crystal- lization and main Th/U ratio of 0.4-0.8. Two main populations of zircons from meta-quartz keratophyre were obtained, giving average ^206pb/^238U ages of （467.3 ± 2.9）Ma and （414.2 ± 2.7） Ma, respectively. Meanwhile, three main populations of zircons from mylonitized quartz keratophyre were obtained, giving average ^206pb/^238U ages of （467.1 ± 2.2） Ma, （435.9 ± 3.6） Ma and （412.6 ± 2.1） Ma respectively. It is considered that the age of （467.3 ± 2.9） - （467.1 ± 2.2） Ma is the formation time of meta-acid volcanic rocks, and hence the metallo-genic epoch should be Middle Ordovician. （435.9 ± 3.6） Ma and （414.2 ± 2.7） - （412.6 ± 2.1） Ma are probably the ages of two geological events, i.e., mylonization related to ductile shearing and collision orogeny. The findings are of great significance in further research on the tectonic evolution of the North Qilian orogenic belt, the formation environment of the Baiyin type massive sulfide deposits, and the ore-prospecting directions.

关 键 词：地球化学 变酸性火山岩 锆石U-PB年代学 LA-ICP-MS 白银矿田 甘肃 

分 类 号：P597.3[天文地球—地球化学]