Isotopic dating in metamorphic belts

Abstract

Isotopic dating in metamorphic rocks is concerned with the timing of processes whose operation is controlled by continually changing physical conditions. Of these, temperature has a pervasive influence and the use of physical models to elucidate the ways in which temperature is likely to vary is briefly reviewed. The central concept of closure temperature is critically discussed and quantitative estimates for biotite and hornblende are evaluated, in order to illustrate the factors which control closure temperatures and limit their application in practice. The possibility of dating crystallization using minerals which crystallize below their respective closure temperatures is examined, taking examples involving K-Ar, Rb-Sr, Sm-Nd and U-Pb analyses. The indirect dating of metamorphism via structurally dated plutons is briefly considered, and the problems of interpreting both discordant U-Pb zircon ages and Rb-Sr whole rock isochrons in metamorphosed granites are discussed. The value of near-concordant zircon ages is emphasized. The extent of isotopic equilibration during metamorphism, specifically of Sr-isotopes in metasediments, is shown not to be a simple function of metamorphic grade. The flow of fluids produced by metamorphic reactions appears to be crucial and there is a potential for dating metamorphism using whole rock samples where such flow can he demonstrated independently. The concluding discussion briefly examines existing data for the Dalradian and indicates the potential for further progress using both well-established and novel approaches.