Eclogite and garnet pyroxenite from Stor Jougdan, Seve Nappe Complex, Sweden: implications for UHP metamorphism of allochthons in the Scandinavian Caledonides

Ultrahigh-pressure metamorphism (UHPM) has recently been discovered in far-travelled allochthons of the Scandinavian Caledonides, including finding of diamond in the Seve Nappe Complex. This UHPM of Late Ordovician age is older and less recognized than that in the Western Gneiss Region of southwestern Norway, which was related to terminal collision between Baltica and Laurentia. Here we report new evidence of UHPM in the Lower Seve Nappe, recorded by eclogite and garnet pyroxenite from the area of Stor Jougdan in northern Jamtland, central Sweden. Peak-metamorphic assemblage of eclogite, garnet+omphacite+phengite+rutile+coesite? yields P-T conditions of 2.8-4.0GPa and 750-900 degrees C, constrained by conventional geothermobarometry and thermodynamic modelling in the NCKFMTASH system. The prograde metamorphic evolution of the eclogite is inferred from inclusions of zoisite and amphibole in garnet, which are stable at lower pressure, whereas the retrograde evolution is recorded by formation of diopsidic clinopyroxene+plagioclase symplectites after omphacite, growth of amphibole replacing these symplectites, and of titanite around rutile. In garnet pyroxenite the peak-metamorphic assemblage consists of garnet+orthopyroxene+clinopyroxene+olivine. P-T conditions of 2.3-3.8GPa and 810-960 degrees C have been derived based on the conventional geothermobarometry and thermodynamic modelling in the CFMASH and CFMAS systems. Retrograde evolution has been recognized from replacement of pyroxene and garnet by amphibole. The results show that eclogite was metamorphosed during deep subduction of continental crust, most probably derived from the continental margin of Baltica, whereas the origin and tectonic setting of the garnet pyroxenite is ambiguous. The studied pyroxenite/peridotite of Baltican subcontinental affinity could have been metamorphosed as a part of the subducting plate and exhumed due to the downward extraction of a forearc lithospheric block.