Osumilite, an approximately 970 Ma old high-temperature index mineral of the granulite-facies metamorphism in Rogaland, SW Norway

Abstract

In SW Norway mineral assemblages in metapelites indicate three stages of Precambrian high-grade metamorphism, M-1, M-2 and M-3. An osumilite-in isograd is recognized in the higher-grade part of the M-2 high-temperature granulite-facies aureole surrounding the Bjerkreim-Sokndal lopolith. Osumilite separated from a metapelite yields K-Ar and Rb-Sr cooling ages of 987 ± 30 Ma and 952 ± 25 Ma, respectively, indicating that it may be a suitable mineral for isotopic dating with a closure temperature to Rb-Sr and K-Ar similar to or slightly higher than that of hornblende to K-Ar.