$$^{40}Ar/^{39}Ar$$ Age Spectra of Minerals from the Fleur de Lys Terrane in Northwest Newfoundland: Their Bearing on Chronology of Metamorphism within the Appalachian Orthotectonic Zone

Abstract

$$^{40}Ar/^{39}Ar$$ age spectra have been determined for hornblende, muscovite, and biotite from the Fleur de Lys metamorphic terrane. Age spectra of all samples are undisturbed, and indicate that total-gas ages date times of post-metamorphic cooling. Within the western division of the Fleur de Lys total-gas ages are variable (hornblende 420-386 m.y., muscovite 411-390 m.y., and biotite 386-368 m.y.), but for each mineral phase show a consistent geographic distribution (older ages recorded in the southwest). At each locality examined, hornblende and muscovite have similar total-gas ages which average ~20 m.y. older than biotite dates. Although distinctly younger dates are recorded in the eastern division of the Fleur de Lys (hornblende 364 m.y., biotite 346-342 m.y.), a similar age discordance is observed. The regional distribution of total-gas ages suggests diachronous southwest-northeast post-metamorphic cooling. This diachronisim may be a result of earlier and, in total, more extensive vertical displacement of the southwestern Fleur de Lys terrane. Relatively rapid post-metamorphic uplift is indicated by the limited discordancy of hornblende and biotite total-gas dates at each locality. When combined with regional stratigraphic relations, the data suggest that in the south and southwest, uppermost structural levels of the Fleur de Lys terrane may have been exhumed by ~460-470 m.y. However, the present erosional level diachronously cooled and was exhumed later. The data do not support previous suggestions of Acadian metamorphism within the Fleur de Lys terrane.