Abstract

We report 40Ar/39Ar age spectrum and laserprobe data for primary magmatic or fluido-magmatic muscovite minerals from the Port Mouton pluton, one of several weakly peraluminous peripheral plutons in the Meguma terrane, southwestern Nova Scotia. Laserprobe data from the cores of thin grain fragments suggest that this pluton cooled rapidly following intrusion at 373 ± 1 Ma, the U–Pb monazite age. The rims of thicker more complete grains record ages of 315–325 Ma, even in cases where there have been no apparent changes in grain rim chemistry and where deformation is minimal. The observed age gradients may be the result of prolonged reheating during the Late Carboniferous Alleghanian Orogeny or, alternatively, the result of rapid cooling at this time to temperatures below the closure value for muscovite rims. Conventional age spectra obtained from muscovite separates record neither the older intrusion age nor the younger reset–cooling age. Instead, these intermediate ages appear to reflect the averaging of intragrain (core–rim) age variations in thick grains and thus have no chronological significance. For these Port Mouton muscovite minerals, the record of initial cooling appears to reside only in certain limited regions of a given grain, a record that can be recovered by the laserprobe technique applied to carefully prepared subgrain fragments. A contrast in the early tectono-thermal histories of plutonic rocks in southwestern Nova Scotia relative to those in the northeast may be the result of perturbation by a mantle plume.

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