超高压变质岩的放射性同位素体系及年代学方法
Isotopic systemafics of ultrahigh pressure metamorphic rocks and relative dating methods
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摘要: 要深刻理解同位素在超高压变质及退变质过程中的地球化学行为对获得超高压变质岩准确并有明确意义的年龄值是非常重要的。对Sm—Nd，Rb—Sr同位素体系，只有变质矿物同位素体系达到平衡才能给出精确有意义的等时线年龄。研究表明，与副变质岩互层的细粒榴辉岩的高压变质矿物之间，或者强退变质岩石的退变质矿物之间，其Nd，Sr同位素可以达到平衡；然而高压变质矿物与退变质矿物之间Nd，Sr同位素不平衡。由于全岩样品总是含有数量不等的退变质矿物，因此石榴石＋全岩Sm—Nd法或多硅白云母＋全岩Rb—Sr法将有可能给出无地质意义的年龄。通常低温榴辉岩的高压变质矿物之间存在Nd同位素不平衡。超高压变质岩多硅白云母所含过剩Ar主要源于榴辉岩原岩中角闪石在变质分解时释放出来的放射成因Ar。因此，不舍榴辉岩的花岗片麻岩多硅白云母基本不舍过剩Ar。对变质锆石成因的准确判断是正确理解锆石U-Pb年龄意义的关键。本文对不同成因锆石的判别标志及年龄意义做了总结，并指出将阴极发光图形，锆石痕量元素组成及矿物包裹体鉴定相结合是进行锆石成因鉴定的有效方法。高压变质或退变质增生锆石组成单一，是理想变质定年对象。然而变质重结晶锆石域常是重结晶锆石和继承晶质锆石的混合区，因而给出混合年龄。只有完全变质重结晶锆石才能给出准确变质时代。Abstract: Comprehensive understanding on isotopic geochemical behaviors in the ultrahigh pressure metamorphism (UHPM) and retrograde metamorphism is very important to obtain meaningful and precise ages for UHPM rocks.For Sm-Nd and Rb-Sr isotopic systematics,mineral isochron is meaningful only under the condition that isotopic equilibrium between metamorphic minerals was achieved.It is documented that the Nd and Sr isotopic equilibrium between high-pressure metamorphic minerals from fine grain eclogite interbeded with parametamorphic rocks or between retrograde metamorphic minerals from intensively retrograde metamorphic rocks can be achieved.However,the Nd and Sr isotopes can not be in equilibrium between UHPM minerals and retrograde metamorphic minerals.Because whole rock samples always contain some retrograde minerals,the tie line of garnet whole-rock in Sm-Nd isochron diagram or the tie line of phengite whole- rock in Rb-Sr isochron diagram can give a geologically meaningless age.Nd isotopic disequilibrium between high pressure metamorphic minerals in cold eclogite is a common situation.The excess Ar in phengite from UHPM rocks were mainly provided by decomposition of amphibole in the protolith of eclogite during eclogite-facies metamorphism. Therefore,phengite from granitic gneiss without eclogite basically contains no excess Ar.Carefully study on zircon origin is a key for understanding of geological meaning for zircon U-Pb age.The differentiation criterions for zircon origin and their geological meanings are summarized in this paper.A combined study of cathode luminescence (CL) image,trace elements and mineral inclusions on zircons from UHPM rocks is an effective way to correctly distinguish different origins of zircons.High pressure and retrograde metamorphic overgrowth zircons are simple in composition,thus are ideal minerals for metamorphic dating.However,the metamorphic recrystallized zircon domain is often a mixture between metamorphic recrystallized zircons and inherited crystalline zircons,thus gives a mixing age.Only complete recrystallized zircon domain can yield a meaningful metamorphic age.
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