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 Köli and Seve Nappes in the Trondelag-Jämtland region. Thermobarometry and thermochronology reveal that the oceanic Köli Nappes reached peak conditions of 9–10 kbar and 550–650 °C prior to muscovite closure to Ar beginning at ca. 425 Ma. The continental Seve Nappes attained slightly higher pressures and temperatures (∼11–12 kbar and 700–725 °C) and closed to Ar loss in muscovite by 415 Ma in the east and by 400 Ma in the west. In contrast, the ultrahigh-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.

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