Formation of eclogite, and reaction during exhumation to mid-crustal levels, Snowbird tectonic zone, western Canadian Shield

摘要

A re-evaluation of the P-T history of eclogite within the East Athabasca granulite terrane of the Snowbird tectonic zone, northern Saskatchewan, Canada was undertaken. Using calculated pseudosections in combination with new garnet-clinopyroxene and zircon and rutile trace element thermometry, peak metamorphic conditions are constrained to similar to 16 kbar and 750 degrees C, followed by near-isothermal decompression to similar to 10 kbar. Associated with the eclogite are two types of occurrences of sapphirine-bearing rocks preserving a rich variety of reaction textures that allow examination of the retrograde history below 10 kbar. The first occurs as a 1-2 m zone adjacent to the eclogite body with a peak assemblage of garnet-kyanite-quartz interpreted to have formed during the eclogite facies metamorphism. Rims of orthopyroxene and plagioclase developed around garnet, and sapphirine-plagioclase and spinel-plagioclase symplectites developed around kyanite. The second variety of sapphirine-bearing rocks occurs in kyanite veins within the eclogite. The veins involve orthopyroxene, garnet and plagioclase layers spatially organized around a central kyanite layer that are interpreted to have formed following the eclogite facies metamorphism. The layering has itself been modified, with, in particular, kyanite being replaced by sapphirine-plagioclase, spinel-plagioclase and corundum-plagioclase symplectites, as well as the kyanite being replaced by sillimanite. Petrological modelling in the CFMAS system examining chemical potential gradients between kyanite and surrounding quartz indicates that these vein textures probably formed during further essentially isothermal decompression, ultimately reaching similar to 7 kbar and 750 degrees C. These results indicate that the final reaction in these rocks occurred at mid-crustal levels at upper amphibolite facies conditions. Previous geochronological and thermochronological constraints bracket the time interval of decompression to < 5-10 Myr, indicating that similar to 25 km of exhumation took place during this interval. This corresponds to minimum unroofing rates of similar to 2-5 mm year(-1) following eclogite facies metamorphism, after which the rocks resided at mid-crustal levels for 80-100 Myr.