LINKING METAMORPHISM AND DEFORMATION WITH IN-SITU MONAZITE GEOCHRONOLOGY: INTERPRETATION OF MULTISTAGE TECTONIC HISTORIES

Absolute timing is the key variable in interpreting the history of terranes that experienced multiple tectonic events. Timing in polymetamorphic terranes requires accurate dates that are directly linked to metamorphic reactions and deformational events. The Athabasca granulite terrane (AGT), Saskatchewan, is one of Earth’s largest samples of lower continental crust. The terrane contains three km-scale steeply-dipping shear zones separated by regions of sub-horizontal fabric (flow) and localized steep fabric domains. Zircon data record plutonism and metamorphism at 2.6 Ga, 2.55 Ga, 1.9 Ga, and ca. 1.85 Ga, but conclusions about the tectonic history and deep crustal processes require linking each date with a specific event. Integration of high-resolution compositional X-ray mapping of major and accessory phases with full thin-section compositional maps is a key tool for interpreting monazite domains in the context of deformation and metamorphic reactions. Integrated petrologic, microstructural and in-situ monazite analysis in the AGT allows differentiation of two HT (>750 C) granulite-facies events at 2.55 Ga and 1.9 Ga and characterization of associated deformation events even when directly overlapping. Sub-horizontal flow occurred mainly during the older event (2.55 Ga) and was locally associated with pressures as high as 1.5 GPa. The Neoarchean record is interpreted to reflect collisional orogenesis and associated topographically-driven lower crustal flow. The 1.9 Ga event involved HT-metamorphism and the progressive development and reactivation of the regional shear zones in response to far-field plate collisions during assembly of Laurentia. Dating of garnet breakdown reactions related to thrusting over hydrous middle crustal rocks provide key age constraints on exhumation of the AGT at 1.85 Ga. The AGT provides a rare view of deep crustal processes during accretion, reactivation, and exhumation, and a superb example of the essential role of fabric- and reaction-specific monazite dating for parsing the components of multi-phase tectonic histories.