Former melt inclusions in garnet from UHP gneisses of the Seve Nappe Complex, Scandinavian Caledonides

Ultrahigh pressure (UHP) metamorphism has been recently reported in gneisses from the Seve Nappe Complex (SNC) of the Scandinavian Caledonides. This study is focused on crystallized former melt inclusions - so-called nanogranitoids - and the information they preserve about anatectic processes occurring during ultradeep subduction. Nanogranitoids (NG) are small (< 20 µm) fully crystallized droplets of anatectic melt entrapped in peritectic phases. Paragneisses from the Åreskutan Mountain representing the SNC are part of the outermost Baltica margin, which has been subducted, metamorphosed, and melted during the closure of the Iapetus Ocean causing the collision between Baltica and Laurentia in Early Devonian. The mineral assemblage of the paragneiss is K-feldspar, garnet, quartz, plagioclase, biotite, white mica, kyanite, sillimanite and rutile. Clusters of inclusions are composed of small (< 5 µm) multiphase inclusions (MPI). MPI represents former fluid inclusion and consists of carbonates, micas, pyrophyllite, kaolinite ± quartz, graphite, rutile, and a residual fluid made of CO2, CH4 and N2. Within the clusters MPI occurs with larger (5 – 20 µm) and rarer NG inclusions, composed of quartz, biotite, K-feldspar, plagioclase and rarely carbonates. Seven re-homogenization experiments at different P-T conditions were conducted in a multi-anvil apparatus to remelt nanogranitoids and determine the composition of the anatectic melt. The best homogenization was acquired at 850, 900˚C and 4, 4.5 GPa. The entrapped melt is mostly rhyolitic in composition, with a high alkali content (~10 wt% N2O+K2O). The H2O and CO2 content of the remelted nanogranitoids analyzed by NanoSIMS are respectively ~ 4.6 wt% (3.3 – 6.6 wt%) and ~ 0.88 wt% (0.12 – 2.11 wt%). The chemical composition of reconstructed anatectic melt from paragneisses of SNC is consistent with experimental studies of UHP melts. Twp partial melting events in Åreskutan Mountain, SNC have been recognized, one at eclogite-facies conditions (482 Ma) and the second at granulite-facies conditions (441 Ma).