Isotope geochronology of metamorphic rocks:Rb-Sr and Sm-Nd systematics

Timing of prograde and retrograde metamorphisms undergone by metamorphic rocks to construct their p-T-t path is an important way to observe crustal movement process.Whole rock isochron and mineral isochron are two basic methods of Rb-Sr and Sm-Nd isotopic dating of metamorphic rocks.Isotopic homogeneous length during metamorphism is a major factor affecting whole rock isochron dating.In general,the homogeneous length of Rb-Sr isotope system is much larger than that of Sm-Nd isotope system during metamorphism so that whole rock Rb-Sr isochron gives a meaningful metamorphic age but Sm-Nd whole rock data do not.For lower grade metamorphism in which metamorphic fluid is more abundant than higher grade metamorphism,however,the Nd isotopic homogeneous scale could be large so that some Sm-Nd whole rock isochrons can yield a meaningful metamorphic ages similar to Rb-Sr isochron age.Whether isotopic equilibrium is achieved between minerals is a key for mineral isochron dating.It is demonstrated that the retrograde metamorphism of ultrahigh pressure metamorphic(UHPM)rocks occurred in an open chemical system,whereas the Nd and Sr isotopic systematics of UHPM minerals may have remained closed.Nd and Sr isotopic disequilibrium between UHPM minerals and retrograde metamorphic minerals has been observed;therefore,the tie line of high-P mineral(such as garnet or phengite)and retrograde metamorphic minerals or whole rock gives an old Sm-Nd age and a young Rb-Sr age,respectively,with no geologic significance.Good linearity of isochron defined by more than 3 well separated minerals and a consistence between ages obtained using different isotopic dating method as well as O isotope equilibrium between minerals have been applied to diagnose Nd isotopic equilibrium between minerals which defined an isochron.Garnet is an important mineral for Sm-Nd and Lu-Hf isotopic dating of eclogite or garnet pyroxenite because of its high Sm/Nd and Lu/Hf ratios.However,since the P-T stability field of garnet is large so that garnet can grow during both prograde and retrograde metamorphisms for high grade metamorphic rocks and has a complex zoning structure.Therefore,how to sampling and analyze the different domains of the garnet with complex structure and diagnose their origins becomes a challenge to Sm/Nd and Lu/Hf mineral isochron dating of eclogite or garnet pyroxenite,which needs a further study.