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

Abstract

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 J€ amtland, central Sweden. Peak-metamorphic assemblage of eclogite, garnet + omphacite + phengite + rutile + coesite? yields P-T conditions of 2.8-4.0 GPa and 750-900°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 2.3-3.8 GPa and 810-960°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.

UHP metamorphism recorded by phengite eclogite from the Caledonides of northern Sweden: P–T path and tectonic implications

“…Albrecht, ; Kullerud, Stephens, & Zachrisson, ; Nicholson, ; Santallier, ; Stephens & van Roermund, ) it was only recently discovered that the SNC experienced UHP metamorphism. The latter was documented in eclogites and ultramafic rocks of northern Jämtland (Brueckner, van Roermund, & Pearson, ; Gilio, Clos, & van Romermund, ; Janák, van Roermund, Majka, & Gee, ; Klonowska et al., ; Majka, Janák, et al., ) and in metasedimentary rocks of west‐central Jämtland (Klonowska, Majka, Janák, Gee, & Ladenberger, ; Klonowska et al., ; Majka, Rosén, et al., ). UHP metamorphism was constrained in west‐central Jämtland to >445 Ma via Th–U–Pb monazite geochronology (Klonowska et al., ; Majka et al., ).…”

“…Outcome of the estimated P–T (1) provides insight for regional‐scale reconstructions of the subduction regime along the orogen, (2) helps to answer the question if the Baltica passive margin, presently represented by SNC thrust sheets, underwent metamorphism under similar metamorphic conditions (e.g. Klonowska, Janák, Majka, Froitzheim, & Kośmińska, ), (3) constrain the thermal regime of the subduction zone at early stages of the Iapetus Ocean closure, and (4) unravels the issue of multistage exhumation recorded by the Vaimok eclogites.…”

Microdiamond on Åreskutan confirms regional UHP metamorphism in the Seve Nappe Complex of the Scandinavian Caledonides

“… P–T paths of the UHP rocks in the Seve Nappe Complex. Friningen eclogite (Janák et al ., ), Stor Jougdan eclogite and garnet pyroxenite (Klonowska et al ., ), and Åreskutan diamond‐bearing gneiss (this study).…”

“…The evidence for Ordovician UHPM in the SNC found recently in kyanite eclogite within the Friningen garnet peridotite (Janák et al ., ; Gilio et al ., ), in phengite eclogite and garnet pyroxenite from Stor Jougdan (Klonowska et al ., ), and in kyanite‐bearing paragneisses, Majka et al . (; this study) suggests a widespread distribution of these extreme conditions within this allochthon.…”

“…Only HP eclogites have been found in Norrbotten (Stephens & van Roermund, ; Andréasson et al ., ; Kullerud et al ., ; Albrecht, ) and Västerbotten. UHP eclogites, garnet pyroxenites and peridotites have been identified in northern Jämtland (Brueckner et al ., ; Janák et al ., ; Majka et al ., ; Gilio et al ., ; Klonowska et al ., ). Recently, in central Jämtland near Storlien (Fig.…”