New U-Pb dating technique by a scecondary ion mass spectrometry

二次离子质谱法（西姆斯）单颗粒锆石U-Pb年代学是目前公认的前寒武纪/古生代岩石最可靠的定年方法。与传统的U-Pb方法相比，西姆斯的优点是它可以在单晶内的小区域进行原位分析。本研究旨在利用商用西姆斯仪器（CAMECA IMS-3F离子探针）获得前寒武纪变质岩U-Pb测年新技术。3 F获得的高质量分辨率（M/DM=约6000）足以测定锆石晶体的Pb/KU比值，而不会产生任何显着的质量干扰。该分析技术应用于南部非洲林波波带北方边缘带的前寒武纪变质岩。NMZ群自形锆石U-Pb年龄为核部2.8Ga左右，边缘2Ga ~ 1.7Ga。尽管林波波带被认为在大约2.65至2.70 Ga的“林波波造山运动”期间遭受了麻粒岩相变质作用，但来自北方边缘带的1.7至2 Ga锆石过度生长意味着更年轻的变质事件。我们的数据表明，北方边缘带至少经历了两个阶段的变质作用：2.7和1.7 - 2Ga。

Single zircon U-Pb geochronology by secondary ion mass spectrometry (SIMS) is now considered to be the most reliable dating technique for Precambrain/Paleozoic rocks. The advantage of SIMS compared to the conventional U-Pb method is its in situ analysis of small areas within a single crystal. This research project aimed to obtain new U-Pb dating technique for Precambrian metamorphic rocks by analyzing with a commercial SIMS instrument (CAMECA IMS-3F ion microprobe). High mass resolution (M/DM=ca.6000) obtained by 3F was sufficient to datermine Pb/KU rations of zircon crystals without any significant mass interferences.The analytical technique was applied to Precambrian metamorphic rocks from the Northern Marginal Zone of the Limpopo Belt in southern Africa. U-Pb ages of euhedral zircons from the NMZ cluster at around 2.8 Ga for core and 2 to 1.7 Ga for rim. Although the Limpopo Belt is considered to have suffered granulite facies metamorphism during "Limpopo Orogeny" at about 2.65 to 2.70 Ga, 1.7 to 2 Ga zircon overgrowth from the Northern Marginal Zone implies younger metamorphic events. Our data suggests that the Northern Marginal Zone suffered at least two stages of metamorphism ; 2.7 and 1.7 to 2Ga.