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Continental mafic magmatism of different ages in the same terrane: Constraints on the evolution of an enriched mantle source

¹ Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada
² Department of Geology, St. Mary's University, Halifax, Nova Scotia B3H 3C3, Canada

The Antigonish Highlands, Nova Scotia, are within the Avalon terrane of the northern Appalachians and contain four distinct episodes of rift-related magmatism: Neoproterozoic, Cambrian, Middle Ordovician, and Late Devonian. All four magmatic suites are composed of basalts and subordinate crustally derived felsic rocks. The mafic rocks of these suites, which do not appear to be significantly contaminated by continental crust, display similar geochemical and Sm-Nd isotopic characteristics consistent with an enriched mantle source that was metasomatically enriched prior to the oldest rifting event, probably between 0.8 and 1.1 Ga. These data also imply that the Avalonian crust and its subcontinental lithospheric mantle remained coupled during four magmatic events. Because the earliest phases of magmatism occurred when Avalonia was located along the Gondwanan margin, and the latest phases after Avalonia accreted to Laurentia, the coupling of crust and mantle in the Antigonish Highlands suggests that the migration of Avalonia did not completely detach its lithospheric mantle from its crustal cover. This study demonstrates the importance of comparing magmatism of different ages in the same terrane.

Key Words: enriched mantle • Sm-Nd isotopes • continental basalts • Avalon terrane

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