CAUSES OF SHEAR LOCALIZATION AND SHEAR ZONE GEOMETRY IN PRECAMBRIAN GRANITIC ROCKS

The causes of deformation localization within ductile shear zones and the manner in which discrete ductile shear zones interact with each other to maintain strain compatibility are not yet clearly understood. The probable causes of shear localization are studied in deformed granitic rocks exposed around Mountain, Oconto county, northeastern WI. These rocks were subjected to multiple deformational events post-dating the major Penokean orogenic event in this region. In these rocks, zones of little or no apparent deformation contain clusters of mafic minerals such as amphibole and biotite. These less deformed zones are surrounded by anastomosing ductile shear zones. The shear zones are typically associated with higher concentrations of the mafic minerals and smaller grain size. Here we explore the role of pre existing mafic minerals in shear localization and in developing shear zone geometry within these rocks. Preliminary results show that within the less deformed regions, there are aggregates of bioitite and amphibole with random shape preferred orientation. The concentrated deformation bands are dark colored and have fine grain size. They are comprised mostly of distinctly smaller grains of quartz and feldspar, and the amphibole and biotite clusters do not necessarily form a preferred alignment as one would expect if they were either rigid or passive particles during deformation. One possible explanation for this may be due to recrystallization of these grains during or post deformation. Further work may demonstrate whether the chemistry of the shear zones is significantly distinct from the surrounding rocks. The shear zone localization may be geometrically and/or lithologically controlled.