INHERITED FABRIC IN A SYMPLECTITE: A KEY TO DEFORMATION HISTORY?

We studied the deformation structures in a eclogite gneiss from the Western Gneiss Region in Norway which was subducted as part of Baltica lithosphere beneath Laurentia during the Scandian orogeny. Petrologic data indicate that deformation took place at ~4 GPa and 800 °C and produced a foliation and lineation in garnet and omphacite. Whereas the garnet fabric remained rather unchanged during exhumation, omphacite was transformed into a symplectite consisting of diopside, hornblende and plagioclase. Although the original grain structure of the omphacite is still recognizable, the actual grains themselves were completely replaced by the lamellar symplectite. Measurements of the crystallographic preferred orientation (CPO) with electron backscattering diffraction (EBSD) show that all three phases of the symplectite have a systematic orientation relationship with the macroscopic fabric as well as the (presumed) orientation of the host omphacite. The orientation relationship between the two chain silicates is very sharp with the crystallographic axes a, b and c being parallel. Their bulk texture shows a maximum of <001> parallel to the lineation and girdles of {010} and {110} perpendicular to the lineation with maxima subparallel to the foliation. This corresponds to an L type texture of the original omphacite and indicates constrictional strain with an additional component of simple shear. For feldspar the orientation relationship to the host orientation is less clear and has a considerably larger spread with {100} being close to {010} of diopside/hornblende and with the <010> and <001> directions aligned in the foliation plane. Thus the deformation texture of the original omphacite is inherited and preserved by the symplectite phases that formed during retrograde amphibolite facies metamorphism, yielding information about the strain along the PT path at highest metamorphic conditions.