Evolution of Caledonian deformation fabrics under eclogite and amphibolitefacies at Vardalsneset, Western Gneiss Region, Norway

The Vardalsneset eclogite situated in the Western Gneiss Region, SW Norway, is a well preserved tectonite giving information about the deformation reg imes active in the lower crust during crustal thickening and subsequent exh umation. The eclogite constitutes layers and lenses variably retrograded to amphibolite and is composed of garnet and omphacite with varying amounts o f barroisite, actinolite, clinozoisite, kyanite, quartz, paragonite, phengi te and rutile. The rocks record a five-stage evolution connected to Caledon ian burial and subsequent exhumation. (1) A prograde evolution through amph ibolite facies (T =490 +/- 63 degrees C) is inferred from garnet cores with amphibole inclusions and bell-shaped Mn profile. (2) Formation of L > S-te ctonite eclogite (T =680 +/- 20 degrees C, P=16 +/- 2 kbar) related to the subduction of continental crust during the Caledonian orogeny. Lack of asym metrical fabrics and orientation of eclogite facies extensional veins indic ate that the deformation regime during formation of the L > S fabric was co axial. (3) Formation of sub-horizontal eclogite facies foliation in which t he finite stretching direction had changed by approximately 90 degrees. Dis ruption of eclogite lenses and layers between symmetric shear zones charact erizes the dominantly coaxial deformation regime of stage 3. Locally occurr ing mylonitic eclogites (T =690 +/- 20 degrees C, P=15 +/- 1.5 kbar) with t op-W kinematics may indicate, however, that non-coaxial deformation was als o active at eclogite facies conditions. (4) Development of a widespread reg ional amphibolite facies foliation (T =564 +/- 44 degrees C, P < 10.3-8.1 k bar), quartz veins and development of conjugate shear zones indicate that c oaxial vertical shortening and sub-horizontal stretching were active during exhumation from eclogite to amphibolite facies conditions. (5) Amphibolite facies mylonites mainly formed under non-coaxial top-W movement are relate d to large-scale movement on the extensional detachments active during the late-orogenic extension of the Caledonides. The structural and metamorphic evolution of the Vardalsneset eclogite and related areas support the exhuma tion model, including an extensional detachment in the upper crust and over all coaxial deformation in the lower crust.