THE EFFECTS OF HYPERVELOCITY IMPACT ON CRYSTALLINE ROCKS FROM THE HAUGHTON IMPACT STRUCTURE, DEVON ISLAND, CANADA

The Earth is constantly being struck by objects from space, which can create enormous pressures and temperatures on a limited area over a very short period of time creating conditions unlike those created by any other geological process. The resulting “shock” metamorphism can occur at pressures that reach hundreds of GigaPascals and well over a thousand degrees Kelvin altering the target material on both megascopic and microscopic scales. Much remains to be understood regarding effects at small length scales such as the degree to which vaporization and melting create porosity in target rocks, and the nature of distinctive microfeatures in accessory geochronology minerals such as zircon (ZrSiO4). This thesis explores the effects of shock metamorphism on crystalline, quartzofeldspathic basement material from the ~39 million year old Haughton impact structure on Devon Island, selected because of its high degree of preservation of shock assemblages. This thesis consists of three main lines of research. One main portion of this study involved assigning shock levels to these samples based on petrographic examination of * main mineral phases and conventional shock classification schemes, and a classification system created for this project. A total of 52 crystalline bedrock samples from a breccia unit in the crater, and one reference site outside of the crater, were classified using this system. The shock levels determined in this way range from 0 to 7 indicating shock pressures ranging from 2 to 80 GPa at the time of the impact which agrees with previously published estimates for Haughton samples. The second main line of research involved measuring physical characteristics (e.g. density and porosity) of the shocked samples to discover how these relate to the assigned shock levels. The bulk density, grain density, and porosity of a wide range of samples were determined using a water displacement method, a bead displacement method, and analysis using a He-pycnometer. Results suggest a nonlinear, negative correlation between density and shock level such 2 that densities of crystalline rocks with density ~3 g/cm are reduced by degrees to less than 1.0 g/cm . Also the results show a positive nonlinear correlation between porosity and shock level. The third portion of this study focuses on the microstructural effects and phase changes in zircon grains across the range of shock metamorphic conditions using optical and electron microscopy, followed by Raman spectroscopy. A petrographic survey of 255 zircons revealed microfeatures including fracturing, planar features and granular texture, as well as new microporosity textures in zircon from shock level 7. This survey showed that general trends exist in some microscopic features, including fractures and granular textures, related to increasing shock pressures. Raman spectroscopy data from 22 zircon grains showed evidence of radiation damage, impurities, transitions to reidite, and recrystallized grains. The overall pattern is that of metamict, low crystallinity zircon in basement gneiss outside the impact structure, increased crystallinity and possible preservation of reidite in an intermediate shock level sample and highly crystalline, annealed zircon (no reidite) in samples composed entirely of glass presumably due to high temperature melting. The overall sequence of structural and phase changes compares favorably to results from similar studies of other impact craters such as Ries, Germany. In summary, this thesis expands our knowledge of the detailed shock metamorphic effects of the Haughton impact event on one class of target lithologies with implications for interpreting shock history at craters or impactites with lower degrees of preservation.

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