EVIDENCE OF CONTINENTAL COLLISION IN THE ATHABASCA MYLONITE TRIANGLE. INSIGHT FROM EBSD, LA-ICMPS MAPPING AND LU-HF GARNET GEOCHRONOLOGY

The c.1.9 Snowbird Tectonic Zone (STZ) is a major tectonic lineament of the Canadian shield that separates the Rae and Hearne cratons. The nature of this structure has been debated for the past 30 years in which it has been alternately interpreted as either a major Paleoproterozoic orogenic belt or as a zone of far-field reactivation of pre-existing structures associated with the c. 2.55 Macquoid orogeny. The STZ is characterized by a scattered record of high-pressure metamorphism; characterizing its age and tectonic context is a key factor for understanding the timing of the continental collision between Rae and Hearne and the nature of the STZ itself. In this study we combined Electron Backscattered Diffraction (EBSD), Laser Ablation Inductively coupled plasma mass spectrometry (LA-ICPMS) trace-element mapping, and Lu-Hf geochronology to characterize microstructures, internal zoning, and the timing of growth of eclogitic garnet within the Athabasca Mylonite Triangle, one of the southernmost exposures of the STZ. This area is characterized by a major low angle shear zone which separates granulitic rocks recording high pressure metamorphism at the hanging wall from rocks unaffected by high pressure in the footwall. Using the structural, petrological, and geochemical characterization as a basis to contextualize the results from garnet Lu-Hf geochronology, we were able to explain the observed diachroneity in garnet growth. Garnet growth in the hanging wall occurred at c. 1914 Ma, and it is associated with the high-pressure metamorphic peak, whereas In the footwall, garnet growth occurred at c. 1904 Ma. These results indicate that the high-pressure metamorphism in this area of the STZ occurred during the Snowbird event and that there was active thrusting in the 1920-1904 Ma time frame. This record of c. 1.9 Ga high-pressure metamorphism along the STZ supports the interpretation that this structure represents a collisional orogen which recorded a continental collision between Rae and Hearne cratons during the late Paleoproterozoic.