MONAZITE AND XENOTIME LASER ABLATION SPLIT-STREAM PETROCHRONOLOGY SHEDS LIGHT ON THE COMPLEX METAMORPHIC EVOLUTION OF THE YUKON-TANANA TERRANE, NORTHERN CANADIAN CORDILLERA

The Canadian Cordillera comprises a collage of Paleozoic to Cenozoic terranes that experienced a complex magmatic, tectonic and metamorphic history prior to, during and following accretion to Laurentia. The peri-Larurentian Yukon-Tanana terrane exposes voluminous eclogite, Barrovian and low-pressure contact metamorphism facies series rocks that preserve an episodic magmatic and metamorphic history spanning from Early Carboniferous to Middle Cretaceous. In the Florence Range of northwestern British Columbia, the Yukon-Tanana terrane comprises abundant metapelite containing garnet, kyanite, andalusite, sillimanite, cordierite, K-feldspar, biotite and muscovite. The presence of all three aluminosilicate polymorphs in disequilibrium in several samples suggests a protracted or polymetamorphic evolution in this area. We identified multiple populations of monazite and xenotime based on their textural relationships and qualitative chemical composition. Monazite commonly forms inclusions in high- and low-P minerals (garnet, kyanite, andalusite and cordierite) and crystallizes along with sillimanite near resorbed Grt, whereas xenotime crystallizes preferentially near resorbed garnet and forms rare inclusions in low-P minerals (andalusite and cordierite). In-situ laser-ablation-split-stream (LASS) petrochronology will be used to constrain the timing of growth and breakdown of index metamorphic minerals. The trace element content of monazite and xenotime will be acquired simultaneously and used to link accessory mineral growth to metamorphic reactions releasing or sequestrating Y and REE. Combined with phase equilibria modelling, in-situ petrochronology on monazite and xenotime will provide critical timing constraints to elucidate the complex metamorphic evolution of the Yukon-Tanana terrane exposed in northwestern British Columbia.