Precise U-Pb dating of Meguma basement xenoliths: new evidence for Avalonian underthrusting

U-Pb zircon and monazite dates for granulite-facies basement xenoliths from the Popes Harbour dyke on the eastern shore of Nova Scotia provide strong evidence that the Meguma terrane overlies Avalonian basement. Slightly disc ordant (1.6%), "facetted" zircons from a mafic granulite indicate a minimum crystallization age of similar to 629 Ma, with near-concordant (0.7%) roun ded zircons suggesting a maximum age for the last metamorphic event affecti ng the zircons at similar to 575 Ma. Two near-concordant (-0.9 to 0.4%) mon azite fractions from a pelitic granulite indicate a major metamorphic distu rbance at 378 +/- 1 Ma, similar to 10 Ma prior to dyke entrainment and coin cident with Meguma regional metamorphism. Projections from 378 Ma through f our highly discordant (15-42%) metapelite zircon fractions give provenance ages between 880 and 1050 Ma and two others project to maximum ages of simi lar to 1530 Ma. Unlike Meguma sediments which lack Grenvillian-age (similar to 1 Ga) detrital zircons and are dominated by similar to 2000 Ma detrital zircons, these dates indicate a dominantly Grenvillian-age provenance for the pelitic xenoliths. The "Avalonian" igneous, metamorphic, and provenance ages from the xenoliths suggest the Meguma rests on Avalonian basement. Be cause Avalonian sediments need a Grenvillian provenance and Meguma sediment s lack such a source but require a greater than or equal to 2.0 Ga componen t missing in the xenoliths, it seems unlikely the Meguma was deposited on A valonian crust. Thus the dating places on firmer footing the suggestion fro m earlier structural, seismic, and geochemical work that the Meguma structu rally overlies Avalonian terrane. Thrusting occurred between the time of ea rliest Meguma deformation (similar to 400 Ma) and intrusion of the xenolith -bearing dyke (similar to 370 Ma).