摘要
在详细地质勘查和岩石学研究的基础上,选取了矿区北部磨石山矿带元古宙沉积变质型富锐钛矿矿层和南部羊蹄子山矿带西段与热液改造型富矿体接触的花岗岩体中的锆石,分别进行了较系统的SHRIMP锆石U-Pb定年研究。结果显示,富矿层的火山-沉积变质岩系中锆石存在2540～1158Ma(207Pb/206Pb年龄)的年龄谱范围,具有1751Ma、1859Ma和2525Ma3个主要年龄峰,其中以1751Ma左右的岩浆事件最为强烈,可能代表了海底火山热液喷气或喷流作用形成的锆石年龄或与之有关的成矿年龄;1158Ma的数据点为锆石增生事件年龄,可能记录了含矿岩系遭受低角闪岩相或绿片岩相的变质事件或与之有关的变质成矿作用年龄,由此确定火山-沉积岩的沉积作用应发生于1751Ma,时代为中元古代,是华北地台北缘结晶基底最早的沉积盖层,而非前人认为的古元古界上部二道凹群;其他几组较老年龄的碎屑锆石是沉积物源区的或由强烈的海底火山喷发作用从深部结晶基底带来的,它们主要表现为岩浆事件。花岗岩体的侵位(或结晶)年龄为118Ma,属燕山晚期产物,代表了羊蹄子山矿带西段局部遭受后期热液改造的成矿期年龄。
Based on detailed geological reconnaissance and petrological research, the authors selected zircons trom a Mesoproterozoic metamorphosed sedimentary type anatase-rich ore bed in the Moshishan ore belt within the northern part of the ore district and a granite body at the contact zone between the granite and a hydrothermally modified type ore shoot in the western segment of the Yangtizishan ore belt within the southern part of the ore district for relatively systematic SHRIMP U-Pb zircon dating. Some conclusions have thus been reached： Zircons in the metamorphosed volcanic-sedimentary sequence of the anatase-rich ore bed have a relatively wide age spectrum of 2 540--1 158 Ma （^207Pb-^206pb age） with three main age peaks, i.e., 1 751, 1 859 and 2 525 Ma. The ^207Pb-^206Pb weighted mean age of 16 analyzed spots for the 1 688-1 784 age group is 1 751 Ma, which might represent the age of zircons formed by submarine volcanic-hydrothermal exhalation or related mineralization age. A 1 158 Ma data point signifies the age of the zircon overgrowth event, which might have recorded a low amphibolite facies or greenschist facies metamorphic event that the mineralized rock sequence underwent or its related metamorphic mineralization age. It may thus be ascertained that the sedimentation of sedimentary rocks in the study area took place between 1 751 and 1 158 Ma, indicating a Mesoproterozoic age. These rocks are therefore the earliest sedimentary cover of the crystalline basement on the northern margin of the North China platform, rather than the upper Paleoproterozoic Erdao＇ao Group as thought previously. Other groups of detrital zircons with older ages were derived from sediment source regions or brought up from the deep-seated crystalline basement by strong submarine volcanic eruption, as is manifested chiefly by magmatic events. The granite body has an emplacement （or crystallization） age of 118 Ma, indicating a late Yanshanian product and representing the age of the metallogenic epoch when the western segment of the Yangtizishan ore belt locally underwent the latestage hydrothermal reformation.
出处
《矿床地质》
CAS
CSCD
北大核心
2008年第4期449-458,共10页
Mineral Deposits
基金
国家自然科学基金(40773038)
内蒙古自治区地勘项目(05-1-TK01)
中国地质科学院矿产资源所资金(K0714-3)的联合资助
