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

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 Orsdalen 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 Orsdalen 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. 800degreesC, 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 Orsdalen 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. (C) 2003 Elsevier Science B.V. All rights reserved.