SHOCK METAMORPHISM AND SHOCK BAROMETRY AT A COMPLEX IMPACT STRUCTURE - SLATE ISLANDS, CANADA

The Slate Islands archipelago is believed to represent the central upl ifted portion of a complex impact structure. Planar microstructures in quartz and feldspars and shock vitrification of rocks are the most co mmon shock metamorphic features encountered. No diaplectic quartz was identified in the exposed rocks, but minor maskelynite is present. Sha tter cones occur on all islands of the archipelago suggesting minimum pressures of 4 +/- 2 GPa. The relative frequency of low index planar m icrostructures of specific, optically determined crystallographic orie ntations in quartz are correlated with results from shock barometric e xperiments to estimate peak shock pressures experienced by the exposed target rocks. In general, there is a decrease in shock pressure recor ded in the target rocks from about 20-25 GPa in east-central Patterson Island to about 5-10 GPa at the western shore of this island and on M ortimer Island. The shock attenuation gradient is similar to 4.5 GPa/k m across this section of the island group. However, the shock attenuat ion has a roughly concentric plan only over the western part of the ar chipelago. There is no distinct shock center and there are other devia tions from circularity. This is probably the result of: (1) the shock wave not having expanded from a point or spherical source because of t he similar to 1.0 to 1.5 km size of the impactor; (2) differential mov ement of large target rock blocks during the central uplift and crater modification phases of the impact process. The orientation of planar deformation features in quartz appears to be independent of the shock wave direction suggesting that crystal structure exerts the primary co ntrol on microstructure development. Based on the results of XRD analy ses, residual, post-impact temperatures were high enough to cause anne aling of submicroscopic damage in shocked quartz.