Permian to Cretaceous polymetamorphic evolution of the Stewart River region, Yukon-Tanana terrane, Yukon, Canada:: P-T evolution linked with in situ SHRIMP monazite geochronology

Textural relationships and the trace element chemistry of accessory minerals and garnet can provide the linkage between in situ SHRIMP ages and quantitative pressure-temperature data that is required to decipher complex polymetamorphic and polydeformational histories. Application of these methods to lower amphibolite facies rocks of the Stewart River area, Yukon (Canada) yields robust new constraints on the tectonic evolution of central Yukon Tanana Terrane (YTT).A TIMS U/Pb titanite age of 365-350 Ma is interpreted to date low-P metamorphism (M1) and D1 deformation associated with arc plutonism above an east-dipping subduction zone. Monazite inclusions in garnet porphyroblasts record a transition from low to high pressure (similar to 9 kbar and 600 degrees C) at c. 239 Ma. These data help to establish a c. 260-240 Ma tectonometamorphic event (M2-D2) reflecting intra-arc thickening during west-dipping subduction of Slide Mountain Ocean. Another transition from low- to high-P (M3-D3; 7.8 kbar and 595 degrees C), dated by c. 195-187 Ma monazite, is interpreted to reflect the change from regional contact metamorphism during arc plutonism to internal duplication of YTT during initial collision of YTT with the North American craton. The Mt Burnham (north-eastern) region records a different history because of its proximity to later plutons and its late exhumation via extensional faulting. Monazite growth at 146 Ma dates similar to 9 kbar metamorphism (M4), interpreted to reflect a previously unrecognized period of plutonism associated with auriferous quartz veins in the Klondike region. Monazite growth at 114-107 Ma reflects low-P (< 4.6 kbar) contact metamorphism (M5) accompanying regional plutonism and extension.