Zircon geochronology of ultrahigh-pressure eclogites and exhumation of the Western Gneiss Region, southern Norway

Numerous eclogite samples within the Western Gneiss Region (WGR), southern Norway, contain coesite or evidence of former coesite, a high-pressure polymorph of quartz. This transition, at ∼2.7 GPa, defines the lower bound of ultrahigh-pressure (UHP) metamorphism. The occurrence of large volumes of rock preserving UHP mineralogy requires fast rates of subduction to, and exhumation from depths of 100–150 km, challenging existing geodynamic models.; The WGR is a composite metamorphic terrane, comprising a series of stacked thrust sheets that were emplaced ∼420 Ma (106 years ago) during collision of the continents of Baltica and Laurentia. The study area consists mainly of relatively untransposed Baltica craton (“basement”), with belts of transposed (“allochthonous”) rock from thrust sheets derived from external parts of Baltica and from Laurentia. The basement contains UHP eclogites, while the allochthonous rocks within the study area apparently did not experience UHP metamorphism.; This study assesses three poorly understood aspects of the subduction and exhumation of the UHP rocks of the WGR: (1) the size and shape of the UHP terrane(s); (2) the timing of UHP metamorphism; and (3) the different histories of the basement and allochthonous rocks. Newly discovered UHP eclogite locales define three discrete UHP terranes ranging from ∼100 km2 to ∼2500 km2 in size. UHP metamorphism occurred at ∼405 Ma, determined using the U/Pb decay scheme in zircon from four eclogite samples. Petrologic and thermobarometric analysis of five pelitic allochthonous samples yields peak metamorphic conditions of ∼1.0–1.5 GPa and ∼750°C, followed by decompression to ∼0.5 GPa. Comparison of the U/Pb ages with lower temperature 40Ar/39Ar ages indicates rapid (∼15–20°C/m.y.) cooling of the UHP rocks down to ∼350°C, followed by remarkably slow (<5°C/m.y.) cooling.; Taken as a whole, these data yield two important conclusions: (1) juxtaposition of the allochthonous rocks with the UHP rocks occurred at depths of ∼20–45 km, after a phase of rapid exhumation of the UHP rocks; and (2) the UHP rocks presently occupy the cores of three broad antiforms, the result of late (<335 Ma) regional folding.