U-Pb zircon geochronology of eclogites from the Scandian Orogen, northern Western Gneiss Region, Norway: 14-20 million years between eclogite crystallization and return to amphibolite-facies conditions

Reconstructing tectonic histories involving continental collision, subduction, and exhumation at plate-tectonic rates of similar to 1 cm/year, requires precise U-Pb zircon geochronology. The Western Gneiss Region has exceptional exposures of high-pressure (HP) and ultra-high-pressure (UHP) rocks. The strategy adopted here involved sampling eclogite and associated late unstrained pegmatites to acquire the time of eclogite crystallization and subsequent exhumation, respectively. The oldest eclogite sampled is 415 +/- 1 Ma from layered, probably UHP eclogite at Tevik, Averoya, also with a garnet-hornblende assemblage at 410 +/- 1 Ma. The Flem Gabbro eclogite margin, with implied UHP conditions, is 410 +/- 2 Ma. Hornblende eclogite at Seth, Lepsoya, never at UHP, is 412 +/- 2 Ma. These compare to Devonian ages of 401 +/- 1 Ma for overgrowths on Proterozoic baddeleyite in Selnes Gabbro, 402 +/- 2 Ma for coesite eclogite at Hareidlandet, 405-400 Ma for coesite eclogite at Flatraket, and 405 +/- 2 Ma for near-UHP eclogite at Hjelmelandsdalen. The 415 Ma eclogite at Tevik compares to granitic pegmatite in the same outcrop at 395.2 +/- 1.3 Ma and to pegmatite in eclogite at Aspoya at 395.3 +/- 2 Ma. The 410 Ma age at Flem compares to nearby pegmatite in eclogite at 396 +/- 4 Ma. Collectively, these results imply 14-20 million years between deep eclogite crystallization at similar to 130 km and return to amphibolite-facies conditions at similar to 30 km, with crystallization of locally derived granitoid melts. Nearby garnet-pyroxenite records older ages (similar to 430) and greater depths (similar to 200 km), but on similar exhumation paths at similar to 0.4-0.7 cm/year.