High-resolution dating of granite petrogenesis and deformation in a lower crustal shear zone: Athabasca granulite terrane, western Canadian Shield

The Athabasca granulite terrane represents > 20,000 km2 of continental lower crust exposed in the hanging wall of the ca. 1.85 Ga thrust–sense Legs Lake shear zone in the western Canadian Shield. The terrane and the thrust zone are cut by the Grease River shear zone. The structure hosts a syn-tectonic granite dike with a high-precision ID-TIMS zircon 207Pb/206Pb crystallization age of 1922.6 ± 0.4 Ma (MSWD = 3.1, 2σ). Detailed textural analysis and in situ electron probe microanalysis (EPMA) of monazite in the dike indicate three discrete episodes of strain accumulation, each corresponding to syn-kinematic growth of monazite. The results provide evidence for a previously unrecognized > 100 m.y. episodic record of sub-horizontal NW–SE shortening and dextral shear strain along the Grease River shear zone. High-Ca domains in ca. 1.92–1.90 Ga monazite are linked to protracted high-T strain (ca. 650–700 °C) at lower crustal depths (ca. 1.0 GPa) and correlated with indentation of the Slave craton along the Great Slave Lake shear zone. Syn-kinematic overgrowths of low-Ca monazite + apatite are interpreted as products of deformation-enhanced dissolution and re-precipitation of monazite at 1846 ± 9 Ma (MSWD = 1.1, 2σ). The third generation of syn-kinematic monazite yield ca. 1.8 Ga ages. The dataset constrains, for the first time, coeval movement along the strike–slip Grease River shear zone and the thrust–sense Legs Lake shear zone during ca. 1.85 Ga dextral transpression and uplift of continental lower crust to middle crustal levels (ca. 0.5 GPa). Circa 1.8 Ga syn-kinematic monazite domains in dynamically recrystallized feldspars support brittle–ductile reactivation along the Grease River shear zone at shallower crustal levels, coincident with intra-continental strike–slip strain during culmination of the Trans-Hudson orogeny.