Zircon fractions from the variably deformed and metamorphosed Swift Current granite and host volcanic rocks of the Love Cove Group record individually discordant U–Pb ages with well-defined upper concordia intercept ages of 580 ± 20 and 590 ± 30 Ma, respectively. These are interpreted to be crystallization dates and indicate a late Proterozoic cogmagmatic relationship. Primary hornblende from the pluton record disturbed 40Ar/39Ar age spectra that suggest postcrystallization argon loss, probably during Acadian (Devonian) regional metamorphism. 40Ar/39Ar plateau ages of 560–566 Ma are well defined for the hornblende and are interpreted to date times of postmagmatic cooling. The similarity between zircon and hornblende dates suggests relatively rapid postmagmatic cooling. A six-point, Rb–Sr whole-rock isochron age of 548 ± 11 Ma is defined for the pluton. The slight discordancy of this date in comparison with the zircon and hornblende ages may reflect a minor disturbance of whole-rock isotopic systems during Acadian regional metamorphism.
The Ackley granite was intruded into variably deformed and metamorphosed rocks within the Gander and Avalon Zones of the Newfoundland Appalachians. Biotite and hornblende from the pluton record internally concordant 40Ar/39Ar age spectra that define total-gas dates of 352–356 Ma. These are similar to a previously reported 355 ± 5 Ma Rb–Sr whole-rock isochron age, and indicate relatively rapid post-magmatic cooling, a feature consistent with the post-kinematic character of the pluton.In the western Avalon Zone, recrystallized, sericitic metavolcanic rocks from the Late Precambrian Love Cove and Marys-town Groups display variably discordant, whole-rock 40Ar/39Ar age spectra. However, the samples yield well defined plateau ages of 353–400 Ma (11 samples), which are interpreted to closely date the regional metamorphism and associated deformation in the western Avalon Zone. This chronology is similar to that of the easternmost Gander Zone, indicating that deformation and metamorphism in the western Avalon Zone occurred simultaneously with tectonic juxtaposition of the Gander and Avalon terranes during the Acadian Orogeny. Geochronological evidence of previously described Precambrian (Avalonian) tectonothermal activity has not been documented in this study.
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