Impact-related dykes of the Haughton structure, Devon Island, Nunavut
University of New Brunswick
The Haughton crater, which comprises a sedimentary target dominated by carbonates, contains at least two types of impact-related dykes: breccia dykes and a carbonate dyke. The textures and composition of the Haughton dykes have been investigated via optical and analytical scanning electron microscopy and electron microprobe analysis. The breccia dykes contain no textures indicative of a melt, but are characterized by fracturing, a reduction in porosity and, to a lesser extent, grain size via comminution. It is concluded that the breccia dykes, although similar in appearance to the impact melt breccias that occupy the central part of the structure, are cataclastic and are not injected melt material or frictionally generated pseudotachylites. The breccia dykes were primarily formed during the modification stage of crater formation as result of gravitational sliding. The carbonate dyke, however, was a melt as evidenced by flow structures, interlocking crystals, and (sub-) poikilitc textures. The dyke cooled quickly, or never reached high enough temperatures to cause melting of entrained dolomite clasts, as indicated by an absence of assimilation of the clasts and lack of zonation. Since there is no presence of MgO-enriched glass, which would be indicative of a dolomitic sou rce, the dyke probably originated from a limestone lithology; the most likely source being the limestone underlying the dolomite in which the dyke is emplaced. It is proposed that the melt formed by frictional melting during the excavation or modification stage of the impact process. Collapse of the crater walls then forced the melt upwards into the dolomitic horizon into which it was emplaced.