40Ar/39Ar age spectra of hornblendes from the metamorphic aureole of the allochthonous Bay of Islands ophiolite complex yield similar dates, which overlap at 460 ± 5 m.y. Recent models indicate that the aureole developed during obduction of the ophiolite suite, and it is suggested that the 460 m.y. age dates this tectonic event. Well-documented paleontological control brackets ophiolite obduction to the late Arenigian, and the 460 m.y. date is an important reference point in the calibration of an absolute Ordovician time-scale.