U–Pb geochronological constraints on the timing of episodic regional metamorphism and rapid high-T exhumation of the Grand Forks complex, British Columbia

Abstract

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 facies 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 ~59 and 50Ma. Peak metamorphism was followed by rapid, near-isothermal decompression of the GFC between ~52 and 50Ma, and leucosome crystallization at ~50Ma. Thermodynamic modeling of metapelites in the system MnNCKFMASHPYCe predicts that monazite was not stable at peak metamorphic conditions, consistent with the dominant population of ~59Ma ages representing growth along the prograde path, most likely at subsolidus conditions. Growth of widespread high-Y monazite rims (~50Ma) 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 51Ma but had ceased by 50Ma, 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 ~59–51Ma. It deforms 59Ma Ladybird suite leucogranites and may be related to high-T exhumation of the core complex. Rapid, >100°C/Ma cooling rates are required to accommodate high-T (amphibolite facies) exhumation of the GFC at 52–50Ma followed by low-T (greenschist facies) exhumation at ~49Ma.