Abstract

The 579.8 ± 2.2 Ma (40Ar-39Ar, muscovite) Georgeville Pluton in mainland Nova Scotia is an epizonal body consisting of alkali feldspar granite and related pegmatite. The pluton intrudes the ca. 619-608 Ma arc-related rocks of the Georgeville Group, which comprises part of West Avalonia, the largest terrane in the Canadian Appalachians. The granite is characterized by above-average SiO2, Th, Nb, Y, and Zr; very low CaO, TiO2, MgO, FeO, and MnO; and most notably by positively sloped rare earth element (REE) profiles generated by extreme light REE depletion. Tectonic discrimination diagrams suggest a within-plate environment, with many, but not all, geochemical and mineralogical features resembling A-type granites. Numerous local and regional geological constraints indicate that the pluton was intruded in a trancurrent setting following the cessation of Neoproterozoic arc-related magmatism along the West Avalonian portion of the Gondwanan continental margin. Geochemical data are consistent with derivation by partial melting of depleted crust or upper mantle followed by extreme fractionation, including REE-rich accessory phases.

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