LA-ICP-MS U-PB ZIRCON DATING OF THE BUCKHORN MOUNTAIN INTRUSIONS. BASEMENT FINGERPRINTS AND IMPLICATIONS FOR THE EVOLUTION OF THE ACCRETED QUESNEL TERRANE

In situ U-Pb geochronology was performed on zircons from the intrusive rocks and associated dikes of Buckhorn Mountain, Washington in order to better understand the complex geodynamic history of the region. Distinct concordant crystallization ages were obtained, circa 49 Ma for non-deformed granitic facies, 150 Ma for rhyolite dikes, and 165-170 Ma for the main, reduced, metaluminous, slightly porphyritic, and deformed Buckhorn Mountain granodiorite and associated dikes. This study also found inherited zircons with ages around 1.4 Ga, on both Eocene and Jurassic granodiorites and 2.4 Ga in the rhyolites, reflecting the heterogeneity of the basement below the accreted Quesnel terrane. The analyses were conducted at Washington State University using a ThermoFinnigan Element2 single collector, high resolution magnetic sector ICP-MS, and a New Wave UP 213 Nd-YAG (213 nm) laser ablation system. The analytical parameters included a repetition rate of 10 Hz, 40 and 30 microns spot sizes, and a total analysis time of 30 seconds per spot. Two zircon samples with ages of 564 Ma (Peixe; G. Gehrels, personal communication) and 55.5 Ma (94-35; Klepeis et al., 1998) were used as external standards to correct for elemental fractionation, and mass bias. Accuracy of the analyses based on age determinations of 94-35 normalized using Peixe is generally 2-3% or better. The crystallization ages represent distinct magmatic events occurring during the two main tectonic periods in the region. The Middle Jurassic ages are associated with the accretion of the Quesnel terrane where intrusives and country rocks are affected by extensive sub-horizontal ductile shear zones coeval with the emplacement of extensive Fe and Au-skarn hydrothermal systems. The younger age represents the magmatism during the Eocene extension that is well represented by the Challis Volcanics in the vicinity. The late Jurassic age (150 Ma) was obtained for the quartz-porphyry rhyolite dikes, the same samples where we found the 2.4 Ga ages, consistent with anatexis of a deeper basement source as a response to crustal thickening in the late stages of the accretion. The early Proterozoic age might represent part of the Thorsby domain (1.91-2.38 Ga) of the North America basement speculated elsewhere to extend in this direction beneath the Canadian Cordillera.

Klepeis, K.A., Crawford, M.L., and Gehrels, G., 1998, J. Structural Geol., 20: 883-904.