浙北大银山金矿石英闪长岩体的年代学、地球化学特征及其地质意义

Geochronology,Geochemical Characteristics and Its Geological Significance of the Quartz Diorite Pluton from the Dayinshan Gold Deposit in Northern Zhejiang Province

浙北大银山金矿位于下扬子被动陆缘与江南古岛弧的接触带附近,为一矽卡岩相关的金矿床。文章对矿区内与矿化密切相关的石英闪长岩开展了主量与微量元素、锆石U-Pb测年以及Lu-Hf同位素地球化学研究。研究表明,该岩体为一套高钾钙碱性-钙碱性、准铝质向过铝质过渡的岩石系列,富集大离子亲石元素(Ba、Rb、K、Sr等)、亏损高场强元素(Nb、Ta、Ti等),亏损重稀土元素,Eu异常不明显,具有类似俯冲带岛弧岩石的特征。同时,该岩体具有较低的Sr/Y值,高Y和Yb含量,表明其岩浆源区的大陆地壳较薄。锆石U-Pb年龄表明,石英闪长岩的206Pb/238U加权平均年龄为144±1 Ma。锆石Lu-Hf同位素数据表明,εHf(t)值为-5.0^-9.9,二阶段Hf同位素模式年龄在1507~1815 Ma之间,显示该岩体的源区物质以扬子板块下地壳为主,与长江中下游成矿带多数金矿成矿岩体具有相似的物质来源。大银山金矿与长江中下游地区的金矿化时间相近,其成矿可能受晚中生代古太平洋板块俯冲作用影响,是在陆内断裂坳陷带中不同构造环境下的响应。

The Dayinshan deposit,located in the conjunction zone between the lower Yangtze passive margin and the Jiangnan paleo-island arc,is a skarn gold deposit associated with the quartz diorites.In this study,U-Pb ages and Lu-Hf isotope compositions of zircon grains extracted from quartz diorites were measured combining with major and trace elemental geochemistry,with aims to elucidate the timing of the gold mineralization and the origin of the ore-related quartz diorites.LA-ICP-MS zircon U-Pb dating gives the weighted mean ages of 144±1 Ma for quartz diorites.The quartz diorites are high K calcium alkaline to calcium alkaline and aluminum to peraluminous.All the samples are enriched in large ion lithophile elements(Ba,Rb,Sr,K)and depleted in high field strength elements(Nb,Ta,Ti)and heavy rare earth elements,with weakly negative Eu anomalies.The results show that quartz diorites of Dayinshan have a similar characteristics of arc magmatic rocks associated with subduction zone.The quartz diorites also have lower Sr/Y value and higher Y and Yb content,indicating that its magma comes from the thinner continental crust.Zircon Hf isotope model ages vary from 1.51 to 1.82 Ga,with an zirconεHf(t)value of-5.0^-9.9.All of these features are consistent with the gold deposit from the Yangtze River metallogenic belt.Detailed elemental and isotopic data demonstrate that these rocks were derived from partial melting of the lower crust of the Yangtze plate induced by subduction of the paleo-Pacific in late Mesozoic.