山东半岛高压麻粒岩中锆石的U-Pb定年及其地质意义

在山东半岛早前寒武纪变质基底中,高压麻粗岩常常呈透镜体或不规则脉状体广泛分布于TTG片麻岩之中.锆石中矿物包体激光拉曼测试、阴极发光图像分析及原位U-Pb定年结果表明,山东丰岛高压麻粒岩中锆石成因十分复杂,可划分为3种类型:第一类锆石显示明显或弱的岩浆结晶环带,部分锆石保存磷灰石(Ap)等矿物包体,U-Pb定年结果显示...

In-situ U-Pb dating of zircons from high-pressure granulites in Shandong Peninsula,Eastern China and its geological significance

High-pressure(HP)granulites as enclaves and irregular vein-like blocks are widely distributed in TTG gneisses of the Early Precambrian metamorphic basement of Shandong Peninsula,in the southeast of the North China Craton.Zircon grains separated from HP granulites are subdivided into three main types based on cathodoluminescence(CL),Laser Raman spectroscopy and in-situ U-Pb dating;they are inherited(magmatic)zircon,HP metamorphic zircon and retrogressive zircon.The obviously or weakly zoned,dark luminescent inherited(magmatic)zircon with mineral inclusions of apatite(Ap) records two group 207Pb/206Pb ages of 2890-2915 Ma and 2510-2763 Ma,indicating two magmatic events occurring at Mesoarchean and Neoarchean epoches.In contrast,the homogeneous,HP metamorphic zircon with index HP mineral inclusions of garnet(Grt)+clinopyroxene(Cpx)+plagioclase(Pl)+quartz(Qtz)+rutile(Rt)+apatite(Ap) records younger 207Pb/206Pb ages ranging from 1850 Ma to 1900 Ma,which represent peak HP granulite-facies metamorphic times.The retrogressive zircon with mineral inclusions of orthopyroxene(Opx)+plagioclase(Pl)+quartz(Qtz)+apatite(Ap)±amphibole(Amp),however,records the youngest 207Pb/206Pb ages of 1820-1840 Ma,which represents late medium-and low-granulites-facies retrograde metamorphic times.This result indicates that Mesoarchean and Neoarchean igneous protoliths of mafic HP granulites,together with their TTG country rocks and meta-sedimentary rocks are subducted to coeval Paleoproterozoic HP metamorphism(1850-1900 Ma),followed by late medium-and low-granulites-facies retrogression(1820-1840 Ma).This new result is of very important significance for deeply discussing the genetic mechanism and geodynamic model of the HP granulites distributed in the southeast part of the North China Craton.