Ak. Engvik et Tb. Andersen, Evolution of Caledonian deformation fabrics under eclogite and amphibolitefacies at Vardalsneset, Western Gneiss Region, Norway, J METAMORPH, 18(3), 2000, pp. 241-257
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.