HIGHLIGHTING THE TRANSIENCE AND RAPID RATE OF ACCRETIONARY TECTONISM USING IN-SITU U-PB MONAZITE GEOCHRONOLOGY AND GARNET THERMOBAROMETRY: AN EXAMPLE FROM THE YUKON TANANA TERRANE IN SOUTHWEST YUKON

Through field mapping, microstructural analysis, thermobarometry and in-situ U-Pb monazite SHRIMP geochronology, we have constrained the timing and rate of tectonic burial and exhumation during convergent-margin orogenesis within the western part of the Yukon Tanana terrane in the Northern Canadian Cordillera. Our data reveal that Neoproterozoic to late-Paleozoic rocks in the Aishihik region of Southwest Yukon (Snowcap and Finlayson assemblages) were tectonically buried to mid-crustal conditions over at least 10 m.y. in the Early Jurassic, prior to rapid exhumation to the near surface within <5 m.y. immediately thereafter. Garnet isopleth thermobarometry, using Theriak-Domino with a MATLAB garnet fractionation routine, reveals garnet growth began at conditions around 5 kbar and 530°C and continued to 6 – 7 kbar and 575°C. A low-Y monazite population that is partially included in garnet porphyroblasts constrains the timing of garnet growth to around 199.8 ± 1.3 to 186.1 ± 1.6 Ma. Thermodynamic modelling using garnet-fractionated bulk-rock compositions suggest peak metamorphic conditions were 7 – 8 kbar and 650 °C. This is constrained by the absence of partial melting, as well as the presence of kyanite and staurolite grains that each contain both rutile and ilmenite inclusions. A bloom of high-Y matrix monazite growth around 182.1 ± 1.4 Ma is attributed to decompression and garnet breakdown, which also coincides with the intrusions of the Early Jurassic Aishihik batholith. Rapid exhumation at c. 182 Ma is implied by both the intrusion of the batholith at around 4 kbar into metamorphic rocks that were previously at 7-8 kbar, as well as evidence for erosion shortly thereafter of the Aishihik batholith, whose detritus is found within Early Jurassic strata of the Whitehorse trough to the east. These data highlight the transient and rapid style of accretionary tectonism. We also emphasize the importance of Early Jurassic crustal thickening within the Yukon Tanana terrane at a time in which large parts of the terrane are thought to have been exhumed.