BRECCIA FORMATION AT A COMPLEX IMPACT CRATER - SLATE ISLANDS, LAKE-SUPERIOR, ONTARIO, CANADA

The Slate Islands impact structure is the eroded remnant of a similar to 30-32 km-diameter complex impact structure located in northern Lake Superior, Ontario, Canada. Target rocks are Archean supracrustal and igneous rocks and Proterozoic metavolcanics, metasediments, and diabas e. A wide variety of breccias occurs on the islands, many of which con tain fragments exhibiting shock metamorphic features. Aphanitic, narro w and inclusion-poor pseudotachylite veins, commonly with more or less parallel boundaries and apophyses branching off them, represent the e arliest breccias formed during the compression stage of the impact pro cess. Coarse-grained, polymictic elastic matrix breccias form small to very large, inclusion-rich dikes and irregularly shaped bodies that m ay contain altered glass fragments. These breccias have sharp contacts with their host rocks and include a wide range of fragment types some of which were transported over minimum distances of similar to 2 km a way from the center of the structure. They cut across pseudotachylite veins and contain inclusions of them. Field and petrographic evidence indicate that these polymictic breccias formed predominantly during th e excavation and central uplift stages of the impact process. Monomict ic breccias, characterized by angular fragments and transitional conta cts with their host rocks, occur in parautochthonous target rocks, mai nly on the outlying islands of the Slate Islands archipelago. A few co ntain fragmented and disrupted, coarse-grained, polymictic elastic mat rix breccia dikes. This is an indication that at least some of these m onomictic breccias formed late in the impact process and that they are probably related to a late crater modification stage. A small number of relatively large occurrences of glass-poor, suevitic breccias occur at the flanks of the central uplift and along the inner flank of the outer ring of the Slate Islands complex crater. A coarse, glass-free, allogenic breccia, containing shatter-coned fragments derived from Pro terozoic target rocks (upper target strata), observed at two locations may be analogous to the 'Bunte Breccia' of the Ries crater in Germany . At one of these locations, this breccia lies close to a crater suevi te deposit. At the other, it overlies parautochthonous, monomictic bre ccia. The Slate Islands impact breccias are superbly exposed, much bet ter than breccias in most other terrestrial impact structures. Observa tions, including those indicative of multiple and sequential processes , provide insight on how impact breccias form and how they relate to t he various phases of the impact process. Eventually they will lead to an improved understanding of planetary impact processes.