Neoproterozoic–Early Devonian magmatism in the Antigonish Highlands, Avalon terrane, Nova Scotia: Tracking the evolution of the mantle and crustal sources during the evolution of the Rheic Ocean

Abstract

The Antigonish Highlands lies in the Avalon terrane (Avalonia) in the Canadian Appalachians and records four episodes of continental rift-related bimodal magmatism ranging in age from Late Neoproterozoic to Middle Devonian. The oldest episode (ca. 615 Ma Clydesdale Formation) was emplaced in a local rift setting in an Andean-style arc when Avalonia resided along the northern Gondwanan margin. Early Cambrian magmatism (Arbuckle Brook Formation) occurred in a local transtensional setting after arc magmatism had given way to a San Andreas-type transform environment. Middle Ordovician (Dunn Point and Bears Brook formations) magmatism occurred in a local rift in an ensialic island arc setting, analogous to the modern Taupo Volcanic Zone in northern New Zealand, after Avalonia had separated from Gondwana. Middle Devonian (McArras Brook Formation and correlatives) occurred after Avalonia had collided with Laurentia. In each episode, mafic rocks are characterized by high FeO t, FeO t/MgO, TiO 2, Zr/Y and Ti/Y typical of differentiated within-plate mafic magmas. Multi-element spider-diagrams indicate that all four episodes were generated in an enriched sub-continental lithospheric mantle. LIL, HFS and REE patterns indicate that the Clydesdale, Dunn Point/Bears Brook and McArras Brook formations were sourced in the shallow (spinel lherzolite) mantle, and that the Arbuckle Brook Formation was generated in the deeper (garnet lherzolite) mantle. In each episode, coeval felsic rocks were generated by crustal anatexis. Taken together, the Sm–Nd isotopic data for the mafic rocks form an envelope that defines the evolution of the mantle source beneath the Antigonish Highlands that was enriched between 0.8 and 1.1 Ga and has an average Sm/Nd ratio of ca. 0.24 (a value that is typical of an enriched mantle source). Sm–Nd isotopic data for the felsic rocks are characterized by a recurrence of T DM ages between 0.95 and 1.0 Ga, interpreted to reflect repeated melting of the lower crust that was itself derived from a depleted-mantle source between 0.95 and 1.1 Ga. The remarkable geochemical and isotopic similarity of the Neoproterozoic, to Middle Devonian mafic and felsic magmas indicate that the crust and sub-continental lithospheric mantle beneath the Antigonish Highlands were coupled during the rift and drift of Avalonia from the Gondwanan margin during the formation of the Rheic Ocean in the Ordovician, its accretion to Baltica and to Laurentia in the Silurian, and post-accretionary strike-slip movement of Avalon along the Laurentian margin. Although the Sm–Nd isotopic data suggest that Avalonian basement and lithospheric mantle formed over the same time interval (0.8 to 1.1 Ga), the divergence of their respective envelopes with time reflects their differing ε Nd values and Sm/Nd ratios. We suggest that Avalonian crustal basement and its enriched mantle were both formed between 0.8 and 1.1 Ga in a Panthalassa-type ocean that surrounded the supercontinent Rodinia and were accreted to the northern Gondwanan margin at about 650 Ma, prior to the oldest magmatic event in the Antigonish Highlands.