DECOMPRESSIONAL REACTION TEXTURES IN GARNET-SAPPHIRINE GRANULITES FROM THE SNOWBIRD TECTONIC ZONE, NORTHERN SASKATCHEWAN, CANADA

The southern domain of the East Athabasca mylonite triangle in northern Saskatchewan contains high-pressure, high-temperature granulite and eclogite facies rocks that were metamorphosed at peak P-T conditions of 1.5-2.0 GPa, 900-1000°C. A suite of garnet-sapphirine granulites within this domain record spectacular retrograde reaction textures that give evidence for a near-isothermal decompression path. The main eclogitic garnet clinopyroxenite layer is 10-15 m wide and consists of garnet (Prp50-55) with abundant kyanite inclusions. Garnet is surrounded by kelphytic rims of Opx + Pl. Cpx (~Jd20) contains abundant exsolved Na-rich plagioclase and is surrounded by a thin rim of Opx. Within the eclogitic layer, veins of sillimanite contain inclusions of corundum, monazite, and allanite. These veins are rimmed by a spinel-plagioclase symplectite, which is rimmed by a sapphirine-plagioclase symplectite. A rim of corundum crystals occurs between the vein and matrix plagioclase Adjacent to these veins are zones with large, 1-3 mm Opx porphyroblasts that have reacted to form an Opx-Cpx-Pl symplectite. A 1-2 m zone of garnet-sapphirine granulite adjacent to the eclogitic contains the peak metamorphic assemblage Grt+Ky+Qtz. A succession of mineral reactions is evident from composite corona and symplectite textures around garnet and kyanite. Garnet is rimmed by a corona of plagioclase, which is rimmed by a thin corona of Opx. Quartz is separated from plagioclase by a thin corona of Opx. Kyanite is rimmed by a symplectite of sapphirine-spinel-plagioclase. In addition, large spinel-plagioclase symplectites in the matrix may be pseudomorphs after kyanite. The timing of the high-PT metamorphism is constrained by U-Pb dating of sector-zoned zircon from the eclogitic layer that contains inclusions of Ky, Crn, and Grt and yields a date of 1904 Ma. Exhumation of these rocks occurred ca. 1895-1800 Ma, constrained by U-Pb dates on titanite, apatite, and rutile. P-T modeling and dating of monazite associated with these decompression textures is currently being carried out in order to document the timing and character of the early stages of the decompression history. These constraints are relevant for juxtaposition of blocks with the Snowbird Tectonic Zone as well as for the stabilization of the western Canadian Shield.