Petrological and geochronological constraints on the metamorphic evolution of high-pressure granulites and eclogites of the Snowbird tectonic zone, Canada

Abstract

This thesis examines the petrology and geochronology of high-pressure granulites and eclogites within the Snowbird tectonic zone of the western Canadian Shield. The focus of this study is the East Athabasca mylonite triangle (EAmt), a well-exposed terrane of granulite facies mylonitic rocks along the trace of the Snowbird tectonic zone in northern Saskatchewan. This study focuses on the 400 km² Southern Domain of the EAmt, which contains a spectacular suite of high- pressure granulites and eclogites that have been metamorphosed at conditions exceeding 1.5 GPa and 1000⁰C. Each chapter of this thesis focuses on a different lithology within the Southern Domain - mafic granulite, eclogite, sapphirine granulite, and felsic granulite. The approach that is taken in understanding each of these rock types is an integrated study of the petrological and geochronological constraints that yield important information about the metamorphic evolution of these unique rocks. The main discovery through these integrated studies is that the Snowbird tectonic zone records a significant Paleoproterozoic high-pressure metamorphism that was previously unrecognized. Petrological and geochronological studies of each of these rock types constrains the pressure-temperature-time path of these rocks from their initial formation to their ultimate exhumation. The granulites and eclogites of the Southern Domain are derived from Archean igneous and sedimentary protoliths.

(cont.) The dominant rock type of the Southern Domain, the felsic gneiss, is interpreted to be derived from a pelitic protolith that underwent an early metamorphism at 2.62- 2.60 Ga. Protoliths of the mafic lithologies intruded at mid-crustal levels by 2.55-2.52 Ga. The eclogite protolith was derived from a plagioclase-bearing cumulate source at pressures <1.0 GPa. The eclogite and mafic granulite, and, to a lesser extent, the felsic gneiss record high-pressure metamorphism at 1.9 Ga. Near-isothermal decompression P-T paths in the mafic granulite and eclogite record rapid exhumation to medium-pressure granulite facies conditions of [approx.]1.0 GPa, 800⁰C. Sapphirine-bearing veins within the eclogite record further decompression, cooling, and re-equilibration in the middle crust at [approx.]0.6 GPa, 600-700⁰C. Final exhumation of these rocks occurred by more protracted erosional processes from 1.9 to 1.8 Ga.