STRAIN AND POSSIBLE VOLUME LOSS IN A HIGH-GRADE DUCTILE SHEAR ZONE

Small-scale ductile shear zones in high-grade metagabbros from the Cal edonides of northern Sweden provide a good opportunity for examining t he development of fabric and strain in rocks undergoing progressive no n-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 shea r and volume change plus a homogeneous deformation. Both size of the f eldspar aggregates and aspect ratio of the aggregates plotted against orientation indicate significant volume decrease (in excess of 50%, an d perhaps as much as 80%) towards the centers of the zones. The data a re broadly consistent with a deformation path involving constant shear strain across the zone followed by variable volume change or by a pat h of progressive loss of volume with increasing shear strain followed by a homogeneous strain. They are not consistent with a single path in volving rates of shear strain and volume loss that are in constant pro portions. There is little difference in chemical composition between t he shear zone and the surrounding rock. An analysis of the concentrati ons of elements in the shear zone and wall rock suggests that volume c hanges were slight (< +/-15%). The discrepancy between the values of v olume change estimated from strain analysis and geochemical analysis m ay be resolved either by geochemical equilibration being attained afte r deformation or by a departure from the geometric constraints of the monoclinic 'band' model for shear zones. Modification of marker partic le size and shape during dynamic recrystallization may also partly acc ount for the apparent volume loss.