Repeated Caledonian burial and ultrafast cooling and exhumation of high-pressure granulite facies rocks from the Blaho Nappe on the island of Fjortoft, Western Gneiss Region, Norway

The allochthonous Blaho Nappe in the Nordoyane ultra high pressure (UHP) domain, Western Gneiss Region in Norway, acts as a window to examine geological processes occurring in continent-continent collisional zones, but many aspects regarding its tectonometamorphic evolution remain debated and elusive. In this contribution, an integrated study including major- and trace-element zoning in garnet, phase equilibrium modelling and the simulation of cation diffusion in garnet was conducted on two high-pressure (HP) granulite facies rocks from the Blaho Nappe on the island of Fjortoft. The results shed new light on the complex geodynamic processes that act in continent-continent collisional zones and finally shape collisional orogens. Phengite, biotite, amphibole, zoisite-allanite and low-Zr rutile enclosed in garnet likely attest to a prograde eclogite facies metamorphism for the studied rocks. Pressure-temperature (P-T) conditions of similar to 1.5-1.6 GPa and 615-670 degrees C were retrieved for this stage. An extensive re-equilibration under peak HP granulite facies conditions of similar to 1.5 GPa and 925 +/- 50 degrees C followed. Subsequently, the rocks were cooled and reburied to eclogite facies conditions of similar to 1.8-1.9 GPa and 805-825 degrees C. This was followed by a final stage of decompression and cooling to amphibolite facies conditions of similar to 650-780 degrees C and 0.5-1.0 GPa. Cooling and exhumation rates of >400 degrees C/Ma and >75 km/Ma, respectively, indicating an ultrafast temperature and pressure decrease are estimated for this stage from simulations of cation diffusion in garnet. The anticlockwise P-T path obtained here is relatively complete and compatible with a repeated burial history during the Caledonian orogeny but not with UHP conditions proposed for the Blaho Nappe. Our model proposes that the rocks later forming the Blaho Nappe were buried to lower crustal depths of approximately 55 km equating to a geothermal gradient of similar to 13 degrees C/km during the early Caledonian orogeny. Subsequent heating of these rocks to HP granulite facies conditions was likely driven by slab break-off and hot mantle upwelling. Baltica underthrusting during the Scandian continent-continent collision cooled and transported the Blaho Nappe to greater depths. The obtained cooling and exhumation rates indicate ultrafast exhumation, presumably in an exhumation channel.