Journal
GEOLOGICAL SOCIETY OF AMERICA BULLETIN
Volume 117, Issue 1-2, Pages 117-134Publisher
GEOLOGICAL SOC AMER, INC
DOI: 10.1130/B25549.1
Keywords
ultrahigh-pressure; Barrovian; continental collision; argon geochronology; Scandinavian Caledonides
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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.
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