A comparative Ar-40/Ar-39 conventional and laserprobe study of muscovite from the Port Mouton pluton, southwest Nova Scotia

We report Ar-40/Ar-39 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, south western Nova Scotia. Laserprobe data from the cores of thin grain fragments suggest that this pluton cooled rapidly following intrusion at 373 +/- 1 M a, the U-Pb monazite age. The rims of thicker more complete grains record a ges 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 g radients may be the result of prolonged reheating during the Late Carbonife rous Alleghanian Orogeny or, alternatively, the result of rapid cooling at this time to temperatures below the closure value for muscovite rims. Conve ntional age spectra obtained from muscovite separates record neither the ol der intrusion age nor the younger reset-cooling age. Instead, these interme diate ages appear to reflect the averaging of intragrain (core-rim) age var iations in thick grains and thus have no chronological significance. For th ese Port Mouton muscovite minerals, the record of initial cooling appears t o reside only in certain limited regions of a given grain, a record that ca n be recovered by the laserprobe technique applied to carefully prepared su bgrain fragments. A contrast in the early tectono-thermal histories of plut onic rocks in southwestern Nova Scotia relative to those in the northeast m ay be the result of perturbation by a mantle plume.