CHEMISTRY VARIATIONS AND VOLUME LOSS IN MESO-SCALE DUCTILE SHEAR ZONES: EXAMPLE FROM THE NORTHWEST ADIRONDACKS, NEW YORK

Meso-scale ductile shear zones are often reported as possessing flattening strains. Flattening strains require extrusion of material from within the shear zone unless material can be removed from within the shear zone during deformation (volume loss). It is typically thought that volume loss is produced by the removal of material or elements out of the system by syn-kinematic fluids. If volume loss occurs, then distinct bulk chemical differences between the lesser deformed wall rock and shear zone should exist if the mobility of all species is not uniform. However, shear zone strain and chemical changes are complex and conclusions for a data set are non-unique.

To investigate the roles of volume loss and extrusion in developing strain within shear zones, a meso-scale ductile shear zone within the Diana Syenite of the Carthage-Colton Mylonite Zone (Northwest Adirondacks, New York) has been studied with regards to its three-dimensional strain and chemistry. Strain within this particular shear zone is flattening and by itself suggests significant extrusion, particularly in the direction of shear. However, differences in bulk chemistry across the zone do not suggest significant volume change. The bulk chemistry data is corroborated by wavelength dispersive spectrometer (WDS) x-ray maps of key elements across the shear zone, wall rock, and transition zones, which do not show obvious changes in chemistry. Therefore, either there is significant extrusion of material from within the shear zone, which potentially produce space compatibility problems, or the strain analysis is inaccurate. In this case, strain data could be invalid due to the poor definition of strain markers, effects of superimposed strain, or non-passive strain markers.

Currently, a working hypothesis is that strain within this shear zone is plane strain – produced only by simple shear without extrusion or volume loss. In this scenario, chemical variations across the shear zone are explained by alteration by syn-deformational fluids that do not affect the volume, and therefore, the strain of the shear zone. Other strain analysis methods are being explored and might produce more accurate stain data.