Thermochronological constraints on the Eocene exhumation of the Grand Forks complex, British Columbia, based on 40Ar/39Ar and apatite fission track geochronology

Abstract : The Grand Forks complex (GFC) is a metamorphic core complex within the composite Shuswap complex in the southern Omineca belt of the Canadian Cordillera. It is juxtaposed against the surrounding low-grade rocks of the pericratonic Quesnel terrane by outward-dipping Eocene normal faults. The GFC attained peak metamorphic conditions of 750-800 °C and 5.5-6.0 kbar (1 kbar = 100 MPa) in the late Paleocene to early Eocene, followed by 2.5 kbar of near-isothermal decompression at upper-amphibolite to granulite facies conditions ( 725-750 °C) in the early Eocene. Subsequent low-temperature greenschist-facies exhumation ( 0.7- 1.5 kbar) was accommodated by the brittle-ductile Kettle River normal fault (KRF) on the east flank of the complex and the Granby fault (GF) on the west flank. This study presents 16 new 40Ar/39Ar hornblende and biotite dates from the GFC and low-grade rocks in the hanging walls to the KRF and GF. Cooling of the GFC through the closure temperature of hornblende ( 530 °C) is constrained to the interval between 54 and 51.4 ± 0.5 Ma, whereas cooling through the closure temperature of biotite ( 280 °C) occurred at 51.4 ± 0.2 Ma. In the hanging wall of the KRF, cooling through the closure temperature of hornblende and biotite occurred nearly coevally at 51.7 ± 0.6 Ma and 51.0 ± 1.0 Ma, respectively. Five apatite fission track dates (closure temperature 110 °C) from the GFC and adjacent hanging walls are indistinguishable within error, yielding an average age of 34.6 ± 2.0 Ma. The lack of difference in biotite and apatite ages between the GFC and the low-grade hanging wall rocks against which it is juxtaposed suggests no significant movement on the KRF and GF after ca. 51 Ma. Results from this study and a previous study on U-Pb dating of the GFC document rapid cooling of the GFC in excess of 200 °C/Ma in a 4 Ma interval between 55 and 51 Ma (Eocene). This rapid phase of exhumation of the GFC was followed by 15 Ma of slow cooling ( 10 °C/Ma) of the joined GFC and hanging wall between 280 °C (biotite closure) and 110 °C (apatite closure).