40Ar/39Ar ages for minerals from the amphibolite dynamothermal aureole, Mont Albert, Gaspe, Quebec

Abstract

New 40Ar/39Ar ages are presented for two hornblendes and one biotite from the dynamothermal aureole that is situated at the base of the Mont Albeit ophiolite. Analysis by the incremental release technique reveals a small component of excess 40Ar in one hornblende, though both exhibit plateau-type spectra. The biotite was analyzed by the total fusion technique. All apparent ages are concordant and average 456 ± 3 Ma.Dynamothermal aureoles form during the obduction process, which juxtaposes ophiolites and amphibolite protoliths during a compressional tectonic event at a plate margin: in this instance, thrusting of the Mont Albert ophiolite over rocks of the Shickshock Group. The concordance of biotite and hornblende ages indicates rapid postmetamorphic cooling and firmly dates obduction at 456 ± 3 Ma ago, an age younger than any of the other northern Appalachian ophiolites. The high-pressure metamorphism of the amphibolite is inconsistent with the present high structural level of the composite Mont Albert – Shickshock allochthon. This implies that obduction and assembly of the allochthon took place in a different environment prior to emplacement in its present high-level position. The age presented here is therefore a maximum limit for the late thrusting event. A north–south diachronism of Humber Zone ophiolite obduction is supported by the new data.