SEVE NAPPE COMPLEX METAMORPHISM AND U-PB SPHENE GEOCHRONOLOGY IN THE TARFALA VALLEY OF THE KEBNEKAISE MASSIF, NORTHERN SWEDISH CALEDONIDES

The Seve Nappe Complex (SNC) is an amphibolite to eclogite grade thrust complex in the Scandinavian Caledonides. SNC protoliths include mafic igneous and sedimentary rocks initially formed during Rodinia rifting in the late Neoproterozoic. Highgrade metamorphism occurred during an Ordovician subduction event, but the complex was thrust onto the Baltic craton during the Silurian. The Tarfala valley in the Kebnekaise Massif, northern Swedish Caledonides, offers excellent exposure of a poorly studied part of the northern SNC.

Thermodynamic modelling using Thermocalc of a Tarfala valley meta-dolerite indicate peak SNC metamorphic conditions reached approximately 9 kbar and 700^oC based on metamorphic garnet and plagioclase compositions. Adjacent garnet-amphibolite units, with the same protolith as the meta-dolerite, contain lower grade sphene-rich metamorphic assemblages. Garnet inclusion textures and proximity of the studied rocks suggest that the garnet-amphibolites experienced similar peak conditions as the meta-dolerite. However, the garnet-amphibolites preserve lower grade assemblages due to retrograde hydration. The modelling predicts that sphene was not part of the peak metamorphic assemblage but stabilized in the garnet-amphibolites at temperatures below ~650^oC during hydrous retrogression.

U-Pb ID-TIMS sphene dates from the garnet-amphibolites range from ~480-450 Ma, with age at least partial dependent on grain size. The sphene dates are also somewhat dependent on choice of initial Pb isotopic compositions, but the range of ²⁰⁶Pb/²³⁸U dates are approximately the same when either coexisting feldspar or model mantle compositions are used. Overall, the spread in dates is interpreted to indicate diffusive Pb loss during retrogression and constrain peak SNC metamorphism to shortly before ~480 Ma during the Ordovician subduction event.