扬子地块西、北缘中元古代地层的划分与对比

摘要

The Mesoproterozoic strata,distributed in the western and northern margin of Yangtze block,have experienced strong deformation and lower to upper greenschist-facies metamorphism.Based on previous study and formation ages,Mesoproterozoic can be divided into two stages:the Early Mesoproterozoic (1.8～1.4Ga) and Late Mesoproterozoic (1.4 ～ 1.0Ga).The Early Mesoproterozoic strata (1.8 ～ 1.4Ga)contains Dahongshan Group,Dongchuan Group,Hekou Group and Tong'an Formation (1～4 Member),while the Late Mesoproterozoic (1.4～1.0Ga) is composed mainly of Kunyang Group,Huili Group in the southwestern part of the block,and Shennongjia Group and Dagushi Group in the northern part of the block in the Late Mesoproterozoic.New zircon U-Pb geochorology data indicate that the tuff of the Tong' an Formation and the quartz-keratophyrite of the Hekou Group formed at 1744± 14Ma and 1659-±-23Ma,respectively,and the mafic dyke intruded into the Huili Group at 1026± 7Ma.According to the sedimentary succession,formation environment and geochronological data,the Dahongshan Group,Dongchuan Group,Hekou Group and Tong' an Formation formed at almost same time and have similar compositions.They all are related to IOCG (Iron-Oxide-Copper-Gold) or SSC (Sediment-hosted Stratiform Copper) formed in an extensional environment during 1.75～1.45Ga.The Huodiya Group in the northern margin of the Yangtze Block may belong to the Early Mesoproterozoic.Late Mesoproterozoic strata are also present in the western and northern margin of the Yangtze Block,including Kunyang and Huili groups in the southwestern margin of Yangtze Block were formed in 1.2 ～ 1.0 Ga;Shennongjia and Dagushi groups in the northern margin of the block formed during 1.4 ～ 1.0Ga,with the top of them probably extending to the Early Neoproterozoic.A large amount of stromatolite was also found to occur in the Mesoproterozoic strata,indicating that they formed in a warm,humid shallow sea environment.Besides the Mesoproterozoie strata mentioned above,the Julin Group in the Yuanmou area of Yunnan province,and the Dengxiangying Group and fifth Member of the Tong'an Formation in western Sichuan province all belong to the Late Mesoproterozoic.%广泛分布于扬子地块西缘和北缘的中元古代地层经历了强烈的变形和绿片岩相的变质改造.根据形成时代,该区的中元古代可以识别出两个阶段,中元古代早期(1.8～1.4Ga)和中元古代晚期(1.4～1.0Ga).中元古代早期的地层包括大红山群、东川群、河口群和通安组(1～4段),中元古代晚期的地层主要由分布在扬子地块西南缘的昆阳群、会理群和分布于扬子地块北缘的神农架群和打鼓石群组成.新的锆石原位定年结果表明,通安组的凝灰岩形成于1744±14Ma左右,河口群角斑岩形成于1659±23Ma左右,侵入会理群天宝山组的辉长辉绿岩形成于1026±7Ma左右.根据岩石组合、形成环境以及年代学资料,中元古代早期的大红山群、东川群、河口群和通安组(1～4段)形成时代相近,地层组成基本相同,它们都含有与岩浆热液有关的铁氧化物铜金(IOCG)矿床或层状铜矿床(SSC),都在1.75～1.45Ga期间形成于大陆裂解环境.扬子地块北缘的火地垭群也可能属于中元古代早期地层.中元古代晚期地层在扬子地块西缘北缘均有分布,其中的昆阳群和会理群大体形成于1.2～1.0Ga,神农架群和打鼓石群形成于1.4～1.0Ga,它们的顶界可能延伸到新元古代早期.在中元古代晚期的地层中含有大量叠层石,表明它们形成于温暖潮湿的浅海环境.除上述的中元古代晚期地层之外,云南元谋地区的苴林群、川西的登相营群、通安组五段等也属于中元古代晚期的地层.