The Grand Forks complex (GFC) is a fault-bounded metamorphic core complex in the southern Omineca Belt of British Columbia, Canada. It experienced prograde metamorphism ranging from upper-amphibolite to granulite facies conditions during the Mesozoic to early Tertiary compressional stage of the Cordilleran orogeny. Peak metamorphism was followed by multi-stage exhumation in the Early Eocene. This study provides U-Pb monazite and zircon constraints on the timing of metamorphic episodes in the GFC and subsequent high-T, amphibolite fades decompression in the Early Eocene. Monazite LA-ICP-MS ages from metapelitic gneisses record episodic metamorphism from the Late Jurassic to Paleocene, with peak metamorphism occurring between similar to 59 and 50 Ma. Peak metamorphism was followed by rapid, near-isothermal decompression of the GFC between similar to 52 and 50 Ma, and leucosome crystallization at similar to 50 Ma. Thermodynamic modeling of metapelites in the system MnNCKFMASHPYCe predicts that monazite was not stable at peak metamorphic conditions, consistent with the dominant population of similar to 59 Ma ages representing growth along the prograde path, most likely at subsolidus conditions. Growth of widespread high-Y monazite rims (similar to 50 Ma) is predicted along suprasolidus decompression and cooling paths. Zircon SHRIMP ages from igneous bodies in the GFC and hanging wall of the bounding Kettle River fault (KRF) suggest ductile deformation related to high-T decompression of the GFC was ongoing at 51 Ma but had ceased by 50 Ma, truncated by post-kinematic granitoids. This high-T deformation predates subsequent greenschist facies extension on the overlying KRF. A pre-KRF, hanging wall ductile shear zone is constrained to similar to 59-51 Ma. It deforms 59 Ma Ladybird suite leucogranites and may be related to high-T exhumation of the core complex. Rapid, >100 degrees C/Ma cooling rates are required to accommodate high-T (amphibolite fades) exhumation of the GFC at 52-50 Ma followed by low-T (greenschist fades) exhumation at similar to 49 Ma. (C) 2012 Elsevier B.V. All rights reserved.
099ix<br/>Times Cited:10<br/>Cited References Count:114
