U-Pb zircon geochronology of the Butedale pluton, British Columbia

The Butedale pluton has been mapped and interpreted as a northwest-trending, elongate, roughly axisymmetric batholith, approximately 30 km wide and 100 km long, forming part of the Coast Plutonic Complex in British Columbia, Canada (Roddick 1970). It is located between the ca. 91 Ma Ecstall Pluton to the northwest (Butler et al. 2002) and the ca. 70-55 Ma Quottoon pluton to the east (Crawford et al. 1999), but the age of the Butedale pluton itself is not well-established. Its relationship to surrounding geologic units is important as the Ecstall pluton records both shallow and steep paleomagnetic inclinations, permitting both large-scale and minimal northward translation since its emplacement in the Cretaceous. Therefore, it has been an important part of the Baja-BC controversy (Butler et al. 2002; Hollister et al. 2004). Similar to the Ecstall pluton, the Butedale pluton is separated from the Quottoon pluton by the Coast Shear Zone, which has been proposed as capable of accommodating Baja-BC-like translations (Andronicos et al. 1999). A well-defined age for the Butedale pluton will allow comparison of the Ecstall and Butedale plutons, which could have implications for the Baja-BC hypothesis. Single-crystal U-Pb CA-TIMS analyses were carried out on four samples roughly equally spaced across a 30 km east-west transect of the Butedale pluton. They reveal the occurrence of at least two distinct magmatic bodies, the older in the west at ca. 95 Ma, and the younger in the east at ca. 85 Ma. Data from the younger body in particular indicate a complex history and show the occurrence of antecrysts that are 7 Ma older than the time of emplacement. This is consistent with the complex magmatic history found in other batholiths along the continental margin of western North America (e.g., Memeti et al. 2009), including nearby plutons in the Northern Cascades (Matzel et al. 2006), and reflects the growth of the Butedale pluton as a series of pulses as opposed to a single emplacement event. The older western part of the Butedale pluton may represent a continuation of the >80 km long Ecstall pluton, the only existing age information for which are from previous U-Pb dating of multi-grain fractions of non-chemically-abraded zircons (Butler et al. 2002). This inherent averaging of age information from any combination of xenocrysts, antecrysts, grains formed at the time of emplacement, and grains that have suffered Pb loss renders the accuracy of the accepted age of 91 Ma across the Ecstall pluton suspect, and raises the possibility that (a) the emplacement of the Ecstall pluton was more protracted than previously thought and (b) the Butedale and Ecstall plutons could be related. Andronicos et al., Jour. Struct. Geol. 1999; Butler et al., JGR, 2002; Crawford et al., Tectonophysics, 1999; Hollister et el., EPSL, 2004; Mattinson, Chem. Geol., 2005; Matzel et al., GSA Bull., 2006; Memeti et al., GSA Bull., 2009; Roddick, GSC Paper 70-41, 1970.