VOLUME STRAIN, STRAIN TYPE AND FLOW PATH IN A NARROW SHEAR ZONE

This study explores the state of finite strain and changes in the mean kinematic vorticity number, grain size, whole-rock chemistry and mine ralogy across an upper amphibolite-facies shear zone in a metadiorite, northern Malawi, east-central Africa. P-T conditions during shear-zon e formation and deformation were approximately 700-750 degrees C and 5 -7 kbar and are slightly less than P-T conditions for the regional pea k of metamorphism. The major rock-forming minerals, plagioclase, hornb lende, biotite, and quartz, were deformed by crystal-plastic processes accompanied by, except for hornblende, dynamic recrystallization. The modal abundance of all four major rock-forming minerals shows no syst ematic change from the country rock into and across the shear zone, in dicating that shear-zone development was not associated with retrograd e mineral reactions. The grain size of the major rock-forming minerals decreases within the shear zone. Plagioclase and hornblende, which oc cur as porphyroblasts outside the shear zone, exhibit a bimodal grain- size distribution within the shear zone. Quartz has a unimodal grain-s ize distribution in the shear zone. Major and trace element chemistry does not change systematically across the shear zone, implying no volu me change in the mylonite. Matrix strain data for plagioclase and horn blende by the Fry method and fabric strain as deduced from R-f/phi ana lysis of plagioclase and quartz grains demonstrate a slightly constric tional strain type (K approximate to 1.5) across the shear zone. The q uantitative finite-strain data for the different residual minerals as obtained by unlike methods show no systematic variation, but recrystal lized plagioclase grains record higher strain than the residual grains . The mean kinematic vorticity number changes from approximately 0.3 o utside to approximately 0.8 within the shear zone, indicating that the bulk deformation path deviated from progressive simple shear. The est imates for finite strain and the degree of noncoaxiality account for a pproximately 50% of thinning normal to the shear zone.