K–Ar ages of amphiboles from the Bay of Islands ophiolite and the Little Port Complex, western Newfoundland, and their geological implications

Abstract

Over 20 K–Ar ages and one 40Ar/39Ar incremental heating analysis of amphiboles from the Bay of Islands ophiolite complex and the Little Port Complex, western Newfoundland, are reported. Amphiboles were obtained from granitic rocks and amphibolites which outcrop in the northern part of the Little Port Complex, and from the sheeted dykes and gabbros and from the basal metamorphic aureole of the ophiolite. The results indicate that obduction of the ophiolite took place approximately 470 m.y. ago and that movement, associated with the emplacement of the Humber Arm allochthon, ceased after 455 m.y. BP. It is postulated that initial argon in amphiboles taken from the basal metamorphic aureole of the ophiolite has resulted in some anomalously high ages.Since the stratigraphic and paleontological relationships in the Bay of Islands are are well known, the dates for obduction and final movement represent additional geochronological reference points in the lower to middle Ordovician time scale.