云开地区变质基底锆石U-Pb年代学及对华夏地块Grenvillian事件的指示  

Zircon U-Pb geochronology of metamorphic basement in Yunkai area and its implications on the Grenvillian event in the Cathaysia Block

作　　者：王磊[1,2] 龙文国[1,2] 徐德明[1,2] 徐旺春[3] 周岱 金鑫镖 黄皓 张鲲 

机构地区：[1]武汉地质矿产研究所,湖北武汉430205 [2]中国地质调查局花岗岩成岩成矿地质研究中心,湖北武汉430205 [3]中国地质大学(武汉)地球科学学院,湖北武汉430074

出　　处：《地学前缘》2015年第2期25-40,共16页Earth Science Frontiers

基　　金：中国地质调查局地质矿产调查评价专项(1212011085416;12120113063000)

摘　　要：云开地区前寒武纪变质基底主要是以片麻岩类为代表的高州杂岩和以浅变沉积岩类为代表的云开群,均不同程度受到了后期加里东期和印支期构造-岩浆事件的改造。对高州杂岩中1个变粒岩（YK1016）和1个黑云斜长片麻岩（YK1014-3）、云开群中1个黑云斜长片麻岩（YK1017-1）样品进行了锆石LA-ICP-MS U-Pb定年。样品YK1016中获得的不一致曲线上下交点年龄分别为（1 035±43）Ma和（437±30）Ma（n=16,MSWD=1.3）,上交点年龄与10个谐和点207Pb/206Pb加权平均年龄（970±30）Ma（MSWD=0.22）一致,解释为其形成年龄。样品YK1014-3获得的206Pb/238 U表观年龄范围为949~2 395 Ma,主要集中在1 000~1 200 Ma,9个最年轻且谐和的分析点207Pb/206Pb加权平均年龄为（970±32）Ma（MSWD=0.53）,对其最大沉积年龄进行了约束。样品YK1017-1获得的206Pb/238 U表观年龄范围为650~2 383 Ma,也主要集中在1 000~1 200 Ma,最年轻且谐和的碎屑锆石年龄将其时代约束在650 Ma。结合云开地区变质基底中已有的高精度年龄数据,认为高州杂岩形成时代为中元古代晚期—新元古代早期,云开群的形成时代为新元古代晚期。另外,研究结果及云开地区已有的数据表明,该区除出露Grenville期岩石外,新元古代变沉积岩中还存在大量Grenville期（900~1 350 Ma）碎屑锆石,部分晶形较好具岩浆起源,这一现象从云开地区一直延伸到华夏东部;但在华夏地块由南往北,Grenville期锆石呈减少趋势,新元古代锆石（820~860 Ma）逐渐增多。这些暗示华夏地块南缘存在一个由南东向北西推进的Grenvillian造山带,和（或）在华夏地块内部存在略晚于Grenville期的构造-岩浆事件。Precambrian metamorphic basement of the Yunkai area mainly consists of the gneissic rocks and meta-sedimentary rocks, as the representative of the Gaozhou complex and the Yunkai Group, which were overprinted by the later Caledonian and/or Indosinian tectono-magmatic events. In this study, LA-ICP-MS U-Pb dating of zircon was determined for one leptynite （Sample YK1016） and one biotite-plagioclase gneiss （Sample YK1014-3） from the Gaozhou complex, and one biotite-plagioclase gneiss （Sample YK1017-1） from the Yunkai Group. The seventeen analyses of sample YK1016 were plotted on a discordant line, which gave an upper intercept age of 1035±43 Ma and a lower intercept at 437±30 Ma （MSWD= 1.3）. The 207Pb/206pb weighted mean age （970± 30 Ma） of the concordant grains is indistinguishable from the upper intercept age and is interpreted as the protolith emplacement age. The younger weighted mean （449± 10 Ma） is identical to the lower intercept age, and is considered to be the metamorphic/anatexis age. Twenty-eight analyses of sample YK1014-3 gave variable 206Pb/238U apparent ages of 949 - 2395 Ma with relative cluster of 1000-1200 Ma. The youngest and concordant grains yielded 207Pb/206Pb weighted mean of 970±32 Ma （n = 9, MSWD= 0. 53）, which constrained the lower limit of deposition time. Thirty-three analyses of sample YK1017-1 gave variable 206Pb/238U apparent ages of 650-2383 Ma with relative cluster of 1000-1200 Ma. The youngest and concordant zircon age constrained the earliest formation age at 650 Ma. Combined with the previous study, we suggest that the Yunkai Group might be formed in the later Neopmterozoic and the Gaozhou complex was formed during the later Mesopmterozoic to the early Neoproterozoic. Furthermore, this study combined with the published data showed that the Grenvillian （900-1350 Ma） detrital zircon were frequently discovered in the Neopmterozoic meta-sedimentary rocks of the Yunkai area, some with well euhedral crystal and magmatic origin, extending to the easter

关 键 词：变质基底 LA-ICP-MS 锆石U-PB 格林威尔事件 云开地区 华夏地块 

分 类 号：P588.3[天文地球—岩石学]