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

• The Seve Nappe Complex (SNC) of the Scandinavian Caledonides records a well-documented history of high pressure (HP) and ultra-high pressure (UHP) metamorphism. Eclogites of the SNC occur in two areas in Sweden, namely Jamtland and Norrbotten. The Jamtland eclogites and associated rocks are well-studied and provide evidence for late Ordovician UHP metamorphism, whereas the Norrbotten eclogites, formed during the late Cambrian (Furongian)/Early Ordovician, have not been studied in such detail, especially in terms of the P-T conditions of their formation. Within the studied eclogite, clinopyroxene contains a high-Na core and two rims: inner, medium-Na and outer, low-Na. Garnet consists of a high-Ca euhedral core, low-Ca inner rim and medium-Ca outer rim. A similar pattern occurs within phengite, where high-Si cores are enveloped by medium and low-Si rims. The compositions of the mineral cores, inner rims and outer rims reflect three stages in the metamorphic evolution of the eclogite. Applied Quartz-in-Garnet geobarometry, coupled with Zr-in-rutile geothermometry reveal that garnet nucleation (E0 stage) took place at 1.5-1.6GPa and 620-660 degrees C. The eclogite peak-pressure assemblage developed during the E1 stage, it consists of garnet+omphacite+phengite+rutile+coesite? and yields P-T conditions of 2.8-3.1GPa and 660-780 degrees C as constrained by conventional geothermobarometry and thermodynamic modelling in the NCKFMMnASHT system. Later, lower-pressure stages E2 and E3 record conditions of 2.2-2.8GPa, 680-780 degrees C and 2.1GPa, 735 degrees C, respectively. The prograde metamorphic evolution of the eclogite is inferred from inclusions of epidote, amphibole and clinopyroxene within garnet. The presence of amphibole-quartz-plagioclase symplectites, secondary epidote/zoisite and titanite replacing rutile record the later retrograde changes taking place at <1.5GPa (referred as E4 stage). The obtained P-T conditions indicate that the Norrbotten eclogites underwent a metamorphic evolution characterized by a clockwise P-T path with peak metamorphism reaching up to coesite stability field within a relatively cold subduction regime (7.8 degrees C/km). The obtained results provide the first evidence for UHP metamorphism in the SNC above the Arctic Circle and document cold subduction regime and multistage exhumation of the deeply subducted Baltican margin at early stage of the Caledonian Orogeny.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences (hsv//eng)

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geologi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geology (hsv//eng)

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geokemi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geochemistry (hsv//eng)

Nyckelord

QuiG barometry

Seve Nappe Complex

Subduction-exhumation

thermodynamic modelling

Zr in rutile thermometry

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