Geology of the western margin of the Grand Forks complex, southern British Columbia: high-grade Cretaceous metamorphism followed by early Tertiary extension on the Granby fault

Abstract

The Grand Forks complex, in the southern Omineca belt of British Columbia, is a fault-bounded tectonic window exposing Proterozoic sediments and associated mafic rocks metamorphosed to upper amphibolite to granulite facies. Its western margin is marked by the Granby fault, an Eocene west-dipping, low-angle, normal fault characterized by brittle deformation. The metasediments of the Grand Forks complex consist of migmatitic paragneiss containing a peak metamorphic assemblage of garnet + cordierite + sillimanite + K-feldspar ± biotite + quartz. Pressure–temperature conditions for this assemblage are 800 ± 35 °C and 5.8 ± 0.6 kbar (1 kbar = 100 MPa). Resorption of garnet to cordierite ± spinel suggests nearly isothermal decompression of about 2 kbar from peak conditions, interpreted to have occurred prior to normal displacement on the Granby fault. Laser ablation U–Pb dating of monazite from the metasediments suggests a dominant episode of Late Cretaceous metamorphism at 84 ± 3 Ma, with evidence for earlier episodes of Cretaceous metamorphism at 119 ± 3 and 104 ± 3 Ma. Early Tertiary recrystallization at 51 ± 2 Ma is coeval with the emplacement of the nearby Coryell plutonic suite. In the hanging wall of the Granby fault, allochthonous sedimentary and volcanic rocks of Quesnel terrane contain mineral assemblages indicative of the upper greenschist to lower amphibolite facies. Pressure–temperature conditions are estimated at 425 ± 40 °C and 2.3 ± 0.7 kbar. The throw (vertical displacement) on the Eocene Granby fault is estimated to be on the order of 5 km. While significant, the fault cannot account for the entire amount of tectonic uplift of the core complex.