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Journal of the Geological Society

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Concurrent HP metamorphism on both margins of Iapetus: Ordovician ages for eclogites and garnet pyroxenites from the Seve Nappe Complex, Swedish Caledonides

^{1 1}School of Earth and Environmental Sciences, Queens College and The Graduate Center, CUNY, Flushing, NY 11367, USA
^{2 2}Present address: Lamont–Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA (e-mail: hannes@ldeo.columbia.edu)
^{3 3}Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, Utrecht, 3508 TA, Netherlands

Eclogites and garnet pyroxenites from the Seve Nappe Complex in the Jämtland area of the Scandinavian Caledonides give Sm–Nd mineral ages (garnet–clinopyroxene–whole rock ± orthopyroxene ± amphibole) that are identical within error and give a weighted average age of 457.9 ± 4.5 Ma (95% confidence). The age is 50 Ma younger than ages determined from eclogites from the apparently similar Norrbotten terrane, roughly 200 km to the north. Both terranes are correlated with the western margin of Baltica, suggesting that at least two eclogite-facies metamorphic events affected this margin prior to the final closure of Iapetus during the 430–400 Ma Scandian Orogeny. The c. 458 Ma ages are nearly identical to ages determined from the eclogite-bearing Tromsø Nappe of the uppermost allochthon. The uppermost allochthon is generally considered part of the Laurentian margin, which, if true, requires that it evolved independently of Baltica until the Scandian Orogeny, when Laurentia and Baltica collided. Thus high-pressure eclogite-facies metamorphism and the introduction of mantle peridotite bodies into the crust appear to have occurred concurrently on opposite sides of Iapetus. We suggest that the HP collision recorded in the Jämtland area be called the ‘Jämtlandian Orogeny’ if further studies confirm its Early Ordovician age.