U-Pb geochronology of Devonian granites in the Meguma Terrane of Nova Scotia, Canada: Evidence for hotspot melting of a Neoproterozoic source

U-Pb isotopic analyses of monazite and zircon from six granitic plutons in the Meguma Terrane yield nearly concordant ages of 373 +/- 3 Ma, interprete d as the time of intrusion. U-Pb analyses of euhedral zircons with thick ri ms overgrowing cores, which were abraded to remove all or most of the rim, plot on chords between 370 +/- 3 and 628 +/- 33 Ma (Larrys River and Halfwa y Cove plutons), 372 +/- 3 and similar to 660 Ma (Shelburne pluton), and 37 3 +/- 2 and similar to 732 Ma (Barrington Passage pluton). The upper interc epts are interpreted as the age of magma source, correlatives of which are present in the Avalon Composite Terrane to the north. This basement may be either in depositional or tectonic contact with the overlying Cambro-Ordovi cian Meguma Group. Other zircons in the granites are generally irregular-eu hedral with thin rims, and most U-Pb isotopic analyses fall between two cho rds from 373-2040 and 373-2300 Ma, with a few lying outside this field. The se zircons are probably derived from the: country rock (Goldenville Formati on), which a previous study has shown contains detrital zircons with concor dant U-Pb ages of 3000, 2000, and 600 Ma, and numerous intermediate discord ant ages. These new ages, along with published data, document a relatively short (5-10 m.yr.) but voluminous period of magmatism. This age is approxim ately synchronous with intrusion of mafic rocks and lamprophyre dikes and r egional low-pressure metamorphism and was followed by rapid denudation of 5 -12 km. These observations may be interpreted in terms of shallowly dipping subduction and overriding of a mantle plume that eventually penetrates thr ough the subducting plate to melt the overriding continental plate. Subsequ ent north ward migration of the plume could explain both the similar to 360 Ma magmatism in the Cobequid Highlands (Avalon Composite Terrane) and the mid-Carboniferous plume-related intrusions around the Magdalen Basin.