The Friningen Garnet Peridotite (central Scandinavian Caledonides): a good example of the characteristic microfabric and PTt path of a cold mantle wedge garnet peridotite
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Orogenic peridotites from the Seve Nappe Complex (SNC), central Scandinavian Caledonides, were traditionally interpreted as fragments of sub-oceanic, Iapetus-related, lithosphere that became incorporated into the Scandian nappe pile during the Caledonian orogeny. The Friningen Garnet Peridotite (FGP) consists dominantly of dunite, garnet-bearing harzburgite and garnet lherzolite with minor internal eclogite- and/or garnet pyroxenite layers/dikes. The FGP shows evidence of an Archean to mid Proterozoic origin for the M1 mineral assemblage, including sulphides, as well as an early Caledonian age (~ 454 Ma) for the second mineral assemblage (M2) which is also characterised by a garnet-olivine HP mineral assemblage. Pre-Caledonian HP-HT metamorphic conditions of 1200°C and P ~ 2.0 GPa are determined for the early M1a assemblage (ol+opx+cpx+grt). These physical conditions are followed by an inferred early Caledonian exhumation event down to 1050-1100°C and 1.5 GPa (M1b), followed by isobaric cooling down to 800°C (M1c). The latter is based on the break down of high-Cr M1a grt (uvt=6%) into opx+cpx+spl+parg symplectites (M1b) and the exsolution of garnet from Al-rich opx and cpx. The early-Caledonian UHPM evidence (M2; T=800°C and P=3.0 GPa), discovered within an eclogitic dyke, is displayed in the FGP by low-Cr M2 grt (uvt=3%) replacing the M1b lower P symplectite assemblage and growth of M2 garnets around M1b spinels. Pl+di symplectites after omphacite in the internal eclogitic dyke and breakdown of kyanite to sapphirine and pl are indicative of isothermal decompression (M3a) down to 800ºC and 1.0 GPa. This M3a phase has not been recognised in the mineral assemblage of the FGP, however, M2 mineral compositions of most grt2, opx2 and cpx2 indicate a metamorphic overprint at T=600°C and P=1.0 GPa (M3b). An exception to this rule is formed by the Cr-content of M1a and M2 garnets. The finding of the stable mineral assemblage dolomite+grt+ky+zoi±ph in the eclogitic dyke and of Sr-bearing carbonates in multiphase solid inclusions within M1a opx, cpx and ol indicates early (i.e. before crustal intrusion) metasomatism by slab-derived COH-bearing fluids. Three olivine microstructures developed in response to different deformation events occurring at different PT conditions: Olivine M1 porphyroclasts were formed in the mantle at HT and low differential stress (1-3 MPa). The main mechanism involved was dislocation creep, producing a mixture of a- and b-type fabrics. These two fabrics are probably the result of two deformation phases, different in temperature and water content. Olivine M2 foam microstructure were formed during subduction, after crustal “intrusion” of the FGP. The main mechanism involved was dislocation creep, producing a-type fabrics. Olivine M3 foam microstructures were formed after strain localization along anastomosed shear zones at low T (650-700°C) and high differential stress (70-100 MPa). The main mechanism involved was grain size sensitive creep. However, dislocation creep was also involved producing a weak e-type olivine CPO. The FGP can be interpreted as a mantle wedge garnet peridotite that originates from a cold and thick lithospheric mantle wedge, underneath either Laurentia or a micro-continent positioned between Baltica and Laurentia, that became incorporated into the subducting continental crust (SNC) during “early Caledonian” subduction (M2) down to UHPM conditions. In addition the M1a assemblage, including primary assemblages of the eclogitic dyke, are interpreted to be the result of Proterozoic (~1.5 Ga) refertilization of an Archean dunite body. Absolute timing of the M1b assemblage is uncertain but interpreted to be related to Iapetus formation. The multiphase solid-and fluid inclusions, present only in M1a-b mineral assemblages, are interpreted as evidence for an early Caledonian COH-bearing subduction zone fluid infiltration event. This newly documented PTt path of the FGP, except for the M2 subduction age, resembles that of cold mantle wedge garnet peridotites from the Western Gneiss Complex. As such the PTt path of the FGP gives important information about the role of the SNC during early Caledonian and Scandian collisions between Baltica and Laurentia.