Molybdenite Re–Os dating of biotite dehydration melting in the Rogaland high-temperature granulites, S Norway

Abstract

The ability of the Re–Os system in molybdenite to record and preserve the age of granulite-facies metamorphism in polymetamorphic belts is tested using the Ørsdalen W–Mo district, Rogaland, S Norway. A low-pressure high-temperature granulite-facies domain, displaying osumilite and pigeonite isograds, is exposed around the 931±2 Ma Rogaland anorthosite complex. Available U–Pb monazite and zircon data show that a 0.93 Ga contact metamorphism overprints a 1.03–0.97 Ga regional Sveconorwegian metamorphism in the gneiss basement. Molybdenite and scheelite in the Ørsdalen district occur in orthopyroxene-bearing leucocratic veins parallel to the regional foliation. The veins are interpreted as migmatic leucosomes formed by fluid-absent incongruent melting of the biotite-rich host rock above ca. 800°C, producing a granulite-facies orthopyroxene±garnet residual assemblage. Molybdenite is interpreted as a product of the melting reaction, crystallized from trace amounts of Mo released from biotite in the host rock during partial melting. Four Re–Os analyses of molybdenite from three samples representing two mines yield an isochron age of 973±4 Ma. The isochroneity of the data indicates that the precipitation of molybdenite and the partial melting event are recorded on the district scale. The Re–Os system in molybdenite was not affected by subsequent 0.93 Ga contact metamorphism, corresponding to formation of garnet+quartz coronitic textures around molybdenite and other minerals in the deposits. The results indicate that granulite-facies conditions prevailed at 973±4 Ma, near the end of a protracted event of regional metamorphism (1.03–0.97 Ga). Biotite dehydration melting recorded in Ørsdalen took place at a pressure of ca. 5.5 kbar (orthopyroxene–garnet–plagioclase–quartz thermobarometry), possibly in association with regional decompression. The study shows that granulite-facies metamorphism (0.97 Ga) took place before intrusion of massif-type anorthosites in Rogaland (0.93 Ga). The study suggests that the high-temperature osumilite-bearing assemblage may be related to the 0.97 Ga event, and that massif-type anorthosites may not be a cause for the high-temperature thermal anomaly in the crust but a late expression of it.