MONAZITE PETROGENESIS IN THE GRAND FORKS COMPLEX, B.C.: COMBINING THERMODYNAMIC MODELING AND GEOCHRONOLOGY TO EXAMINE CORE COMPLEX EXHUMATION

The Grand Forks metamorphic core complex in southeastern British Columbia is a tectonic window into middle amphibolite to granulite facies metapelites and amphibolites exhumed during the Tertiary orogenic collapse of the Canadian Cordillera. Previous research has identified a high-temperature, ~2.5kbar exhumation event in the Grand Forks complex that precedes greenschist facies extension on the bounding Kettle River detachment. New in-situ LA-ICP-MS monazite geochronology from this study addresses the timing of this high-temperature exhumation event. Monazite cores and intermediate zones highlight a protracted history of prograde metamorphism extending from the late Jurassic to Paleocene (60-58Ma), with thin rim growth zones yielding ages clustered at 52-50Ma. We propose that this rim growth represents monazite growth during exhumation and decompression melting in the early Eocene. Monazite inclusions in decompression reaction textures, namely Crd+Spl+Ilm coronae around sillimanite and Crd+Qtz coronae around garnet, have these 52-50Ma rim ages, with no younger ages recorded from the rock matrix. Chemical mapping of these rims has identified significant Y-enrichment relative to 60-58Ma cores and intermediate zones, which are markedly Y-depleted. Thermodynamic modeling of monazite growth in migmatitic metapelites was conducted in the MnNCKFMASHPCeY (MnO-Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-P₂O₅-Ce₂O₃-Y₂O₃) system using the program Theriak Domino. A prograde Barrovian P-T path at upper amphibolite facies conditions predicts simultaneous garnet and low-Y monazite growth, followed by melt production, garnet breakdown and high-Y monazite growth with subsequent isothermal decompression. Petrographic analyses of migmatitic leucosomes have identified melt crystallization microtextures in the form of monominerallic biotite melanosomes separating mesosomes and leucosomes, as well as reaction textures of allanite replacing monazite. The incorporation of 52-50Ma monazite in these reaction textures suggests that rocks of the Grand Forks complex were still partially molten at that time, but the lack of younger monazite growth suggests that final melt crystallization occurred shortly thereafter.