The timing of stabilisation and the exhumation rate for ultra-high pressure rocks in the Western Gneiss Region of Norway

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Journal of Metamorphic Geology
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New petrographic evidence and a review of the latest radiometric age data are taken to indicate that formation of the ultra-high pressure (UHP) eclogites within the Western Gneiss Region of Norway probably occurred within the 400-410 Ma time frame. Thus, this event took place significantly later than the previous, widely accepted age of c. 425 Ma for the timing of the high pressure metamorphism in this part of the Scandinavian Caledonides. Garnet growth under UHP (coesite-stable) conditions is recognised as a discrete, younger event following on from earlier garnet formed under firstly amphibolite facies then quartz-stable, eclogite facies conditions. Currently, the best constrained and most precise age, specifically for UHP mineral growth, is the 402 +/- 2 Ma U-Pb age for metamorphic zircon (some of which retain coesite inclusions) from the Hareidland eclogite. Exhumation must have followed shortly thereafter and, based on synoptic pressure-temperature and depth-time curves, must have been very fast. Our data and those of others indicate an initial fast exhumation to about 35 km depth by about 395 Ma at a mean rate of about 10 mm a(-1). This rapid exhumation rate may have been driven by the appreciable residual buoyancy of the deeply subducted continental crustal slab due to incomplete eclogitization of the dominant Proterozoic orthogneisses during the short-lived UHP event. Subsequent exhumation to 8-10 km depth by about 375 Ma occurred at a much slower mean rate of about 1.3 mm a(-1) with the late-stage extensional collapse of the Caledonian orogen playing an increasingly important role, especially in the final unroofing of the Western Gneiss Region with some remarkably preserved UHP rocks.


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