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Geothermometry of the ultrahigh-temperature Saxon granulites revisited. Part II: Thermal peak conditions and cooling rates inferred from oxygen-isotope fractionations

¹ Mineralogisch-Petrologisches Institut, Universität Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany
² Institut für Geowissenschaften, Universität Potsdam, Karl-Liebknecht-Str. 24, 14476 Golm, Germany

^* Corresponding author, e-mail: stephan.hoernes{at}uni-bonn.de

Anhydrous mineral assemblages in felsic and mafic rocks from the Saxon Granulite Massif, part of the central European Variscan Belt, preserve O-isotope equilibrium fractionations recording temperatures of 1000–1020 °C. Although ultrahigh-temperature (UHT) metamorphism has been reported from Precambrian shields and Phanerozoic fold belts, this is the first time that an independent confirmation of UHT conditions based on both O-isotopic and chemical equilibria in the very same set of samples is presented. The evidence comes from feldspathic prismatine granulite and garnet-two-pyroxene granulite, where decompression-related minerals and the whole rocks are in isotopic equilibrium. The rocks thus behaved as closed systems with respect to oxygen at least during early stages of exhumation.

Based on isotopic fractionations between the whole rock and minerals that cease to exchange oxygen at high temperatures, the closed-system behaviour enabled us to estimate near-peak temperatures, even if the predominant minerals were reset during cooling. This has been demonstrated in several samples mainly for whole-rock/garnet fractionations, which indicate temperatures similar to or slightly higher than the multiphase equilibria. Average cooling rates calculated from O-isotopic disequilibrium assemblages using the program COOL (Jenkin et al., 1991) are in the range of 50–80 °C/Ma and correspond to an average exhumation rate of 10 mm/a. The preservation of the UHT isotopic and chemical equilibria is due partly to rapid cooling, partly to anhydrous conditions during peak metamorphism and early cooling. Partial chemical and isotopic homogenisation of peak-metamorphic mineral compositions under anhydrous conditions suggests that the chemical and isotopic exchange reactions were essentially controlled by volume diffusion.

Key-words: UHT metamorphism, high-pressure granulite, oxygen-isotope thermometry, whole-rock/mineral fractionation, cooling rate, Saxon granulite massif.