Strain and possible volume loss in a high-grade ductile shear zone

Abstract

Small-scale ductile shear zones in high-grade metagabbros from the Caledonides of northern Sweden provide a good opportunity for examining the development of fabric and strain in rocks undergoing progressive non-coaxial deformation. The rocks contain strain markers in the form of aggregates of plagioclase feldspar. A selected shear zone is analyzed assuming monoclinic symmetry, which requires for the zone as a whole variable simple shear, variable volume change, or variable simple shear and volume change plus a homogeneous deformation. Both size of the feldspar aggregates and aspect ratio of the aggregates plotted against orientation indicate significant volume decrease (in excess of 50%, and perhaps as much as 80%) towards the centers of the zones. The data are broadly consistent with a deformation path involving constant shear strain across the zone followed by variable volume change or by a path of progressive loss of volume with increasing shear strain followed by a homogeneous strain. They are not consistent with a single path involving rates of shear strain and volume loss that are in constant proportions. There is little difference in chemical composition between the shear zone and the surrounding rock. An analysis of the concentrations of elements in the shear zone and wall rock suggests that volume changes were slight (< ± 15%). The discrepancy between the values of volume change estimated from strain analysis and geochemical analysis may be resolved either by geochemical equilibration being attained after deformation or by a departure from the geometric constraints of the monoclinic ‘band’ model for shear zones. Modification of marker particle size and shape during dynamic recrystallization may also partly account for the apparent volume loss.