MICROSTRUCTURES IN FELDSPARS FROM A MAJOR CRUSTAL THRUST ZONE - THE GRENVILLE FRONT, ONTARIO, CANADA

Samples collected across the NW Grenville Front in Ontario, Canada, have been used to document the change in feldspar microstructures produced during shearing under metamorphic conditions that range from upper amphibolite to lower greenschist grade. The differential displacement across this 5 km thick section of the Grenville Front tectonic zone is at least 10 km. At upper to mid-amphibolite conditions both plagioclase and potassium feldspars behave plastically. Grain-size reduction occurs through grain boundary migration and/or subgrain rotation and produces a core-and-mantle structure. At middle to lower amphibolite grade, grain-size reduction is accomplished through grain boundary migration. At lower amphibolite to upper greenschist grade, grain-size reduction occurs by the nucleation and growth of new grains with a different composition. The abundance of intracrystalline deformation at this grade is inversely proportional to the amount of fine-grained matrix. In ultramylonite with a high matrix to porphyroclast ratio few intracrystalline strain features occur other than ubiquitous undulatory extinction. Myrmekite is well developed on the high stress boundaries in K-feldspar. At upper to mid-greenshist conditions, feldspars deform mainly by slip on synthetic shear fractures. Deformation twins, undulatory extinction, deformation bands and kink bands occur in plagioclase, and perthite flames and myrmekite occur principally on high stress boundaries in K-feldspar. Grain-size reduction in feldspar is accomplished through cataclasis. At middle to lower greenschist facies, deformation in feldspars is dominated by antithetic fractures. The transition from the dominance of synthetic to antithetic fractures is gradational. Below lower greenschist grade, rocks deform by pervasive cataclasis of all minerals.