Isotopic systemafics of ultrahigh pressure metamorphic rocks and relative dating methods
超高压变质岩的放射性同位素体系及年代学方法

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.