苏鲁-大别超高压岩石中锆石SHRIMP U-Pb定年研究——综述和最新进展  

Review and prospect of SHRIMP U-Pb dating on zircons from Sulu-Dabie UHP metamorphic rocks.

作　　者：刘福来[1] 薛怀民[1] 

机构地区：[1]中国地质科学院地质研究所国土资源部大陆动力学重点实验室,北京100037

出　　处：《岩石学报》2007年第11期2737-2756,共20页Acta Petrologica Sinica

基　　金：国家重点基础研究发展规划项目(编号2003CB716502);国家自然科学基金重大项目(编号40399143);中国地质调查局地质调查工作项目(编号12120105660)的联合资助成果。

摘　　要：如何建立苏鲁-大别超高压岩石深俯冲-超高压-快速折返过程连续而完整的P-T-t轨迹及精细的年代谱系,是目前地学界研究的热点。而变质锆石是否记录深俯冲石英榴辉岩相进变质阶段的年代学信息和超高压峰期变质时代的准确归属,是目前苏鲁-大别超高压变质带需要深入研究的核心问题。本文在对前人同位素年代学方面所取得的成果进行系统总结的基础上。采用锆石中矿物包体激光拉曼和电子探针测试、锆石阴极发光图像成因分析以及SHRIMP U-Pb定年等综合研究手段,确定苏鲁-大别地体榴辉岩及其强退变质围岩在深俯冲-构造折返过程中主要经历了四个阶段的变质演化:深俯冲石英榴辉岩相进变质(Ⅰ)、超高压峰期变质(Ⅱ)、构造折返初期石英榴辉岩相退变质(Ⅲ)和构造折返晚期角闪岩相退变质(Ⅳ)。研究发现,扬子板块(中)新元古代巨量的陆壳物质在早三叠纪(246～244Ma)俯冲到华北板块之下约65km的深处。发生了石英榴辉岩相进变质,相应的变质温压条件为T=542～693℃,P=1.7～2.02GPa。这些高压石英榴辉岩相岩石在中-新三叠纪继续向下俯冲,在235～225Ma期间,俯冲的深度至少达到了170km的地幔深处,并发生了峰期柯石英榴辉岩相超高压变质,相应的变质温压条件为T=722～866℃,P>5.5GPa。苏鲁-大别超高压地体自石英榴辉岩相进变质阶段到超高压峰期变质阶段的俯冲速率为7.0km/Myr。这些超高压岩石在219～216Ma期间,发生了第一次构造抬升至75km的深处,并经历了石英榴辉岩相退变质作用的改造,退变质温压条件为T=730～780℃,P=1.7～2.6GPa。这些退变质岩石在212～205Ma期间,又经历了第二次抬升至25km中-下地壳深处,并叠加了角闪岩相退变质作用,该阶段变质温压条件为T=610～710℃,P=0.7～1.2GPa。苏鲁-大别超高压地体两次构造抬升的速率大致相同,为5.6km/Myr。该项成果不仅确定了Eclogite and its country-rocks from the Sulu-Dabie ultrahigh-pressure (UHP) terrane are deeply subducted continental crustal rocks. The related UHP metamorphic P-T-t path has been reconstructed which could be subdivided into four stages, including prograde ( Stage 1) and UHP metamorphism (Stage 11) during deep-subduction, and initial quartz eclogite-facies ( Stage 111) and later amphibolite-facies ( Stage IV) retrograde metamorphism during exhumation. The distribution of mineral inclusions, cathodoluminescence ( CL) imaging combined with SHRIMP U-Pb dating of the metamorphic zircon make it possible to related zircon zones ( domains) to different metamorphic stages. The prograde metamorphic domains preserve quartz eclogite-facies mineral assemblage, formed at 542 - 693 degrees C and 1.7 - 2.02GPa. The UHP metamorphic domains retain index coesite-bearing UHP mineral assemblage and record UHP condition of 722 - 866 degrees C and > 5. 5 GPa. The matrix of eclogite commonly preserves quartz eclogite-facies mineral assemblage and record initial retrograde P-T condition of 730 - 780 degrees C and 1.7 - 2.6GPa. The outmost retrograde rims of zircons, however, contain no-UHP minerals such as quartz, albite, amphibole and calcite, related to the later regional amphibolitefacies retrogression with 610 - 710 degrees C and 0.7 - 1.2GPa. SHRIMP U-Pb dating on these zoned zircons combined with previous geochronologic data identify four discrete and meaningful age groups: the prograde metamorphism of quartz eclogite-facies ( Stage I) occurred at 246 - 244Ma, the UHP peak metamorphic event (Stage 11) was at 235 - 225Ma, the initial quartz eclogite-facies retrogression (Stage 111) was limited at 219 - 216Ma, and the later amphibolite-facies retrograde (Stage IV) overprint was restricted at 212 - 205 Ma. These meaningful age groups suggest that subduction to UHP depths of Proterozoic voluminous continental materials of the Sulu-Dabie UHP terrane took place about 15Ma, and exhumation of the metamorphic rocks from Stage 11 to Stage I

关 键 词：锆石 SHRIMP U-PB定年 巨量陆壳物质 P-T-T轨迹 苏鲁-大别超高压地体 

分 类 号：P597.3[天文地球—地球化学]