Continental collisions and the creation of ultrahigh-pressure terranes: Petrology and thermochronology of nappes in the central Scandinavian Caledonides

The formation of the vast Devonian ultrahigh-pressure terrane in the 'Western Gneiss Region of Norway was investigated by determining the relationship between these ultrahigh-pressure rocks and the structurally overlying oceanic and continental Koli and Seve Nappes in the Trondelag-Jamtland region. Thermobarometry and thermochronology reveal that the oceanic Koli Nappes reached peak conditions of 9-10 kbar and 550-650 degreesC prior to muscovite closure to Ar beginning at ca. 425 Ma. The continental Seve Nappes attained slightly higher pressures and temperatures (similar to11-12 kbar and 700-725 degreesC) and closed to Ar loss in muscovite by 415 Ma in the east and by 400 Ma in the west. In contrast, the ultra high-pressure rocks were still deep in the mantle at eclogite-facies pressures at 410-400 Ma. These data, in combination with structural, petrological, and thermochronological data from elsewhere in the orogen, show that the ultrahigh-pressure metamorphism occurred in the late stages of continental collision, after the earlier stages of ophiolite emplacement and passive-margin subduction.