Strontium isotope distribution in a regionally metamorphosed granite from the Zentralgneis,south-east Tauernfenster,Austria

The effect of Alpine (40 my) low amphibolite facies metamorphism on strontium isotopes in the Variscan (250–300 my) granitic complex of the Zentralgneis was investigated by analysing samples of a leucogranite which forms small intrusions into a tonalite. The leucogranite has Rb/Sr ratios up to 30.Two muscovites and three biotites underwent complete reequilibration during the metamorphism. The ages of 12–20 my are consistent with published K-Ar ages on micas from the same area, and reflect post-metamorphic cooling.Six whole-rock samples define an isochron with an age of 244±7 my (=1.47×10^–11yr^–1). Three further samples, collected within 30 cm of contacts with the tonalite, plus 1 cm sawn slabs of leucogranite and tonalite from either side of a contact fit a line with 10% greater slope; this line is interpreted as a mixing line, resulting from contamination of the leucogranite, near its contacts, with strontium from the enclosing tonalite. The time of contamination is uncertain but petrological evidence suggests it is the result of the metamorphism.Neither leucogranite whole rocks nor minerals from the tonalite, at contacts and in a xenolith, show any evidence for preferential mobilisation of radiogenic strontium in the course of metamorphism.The work was carried out in Heidelberg during successive tenure of the Michael Foster Memorial Scholarship from the University of Oxford and a Wates Travelling Fellowship of the Royal Society of London. Field work and sample collection was supported in part by the Geologische Bundesanstalt, Wien. For support from each of these sources I am very grateful. It is also a pleasure to acknowledge with thanks the support and facilities afforded me at the Laboratorium für Geochronologie by Dr. Hans-J. Lippolt.