Abstract

The Duxbury massif, Eastmain District, Quebec, is a complex tonalitic intrusion that is internally divisible into a homogeneous border zone of granodiorite (Rb–Sr total-rock isochron age = 2500 ± 85 Ma; initial 87Sr/86Sr = 0.7023 ± 0.0003) and a heterogeneous core zone of tonalite (Rb–Sr total-rock errorchron age = 3060 ± 180 Ma; initial 87Sr/86Sr = 0.7014 ± 0.0003). The geologic, petrographic, and isotopic data are consistent with a model of remobilization of 3.1 Ga sialic basement during the Kenoran orogeny at 2.5 Ga. Evidence suggests that the source material was of tonalitic composition, that it was never more deeply buried than 15–20 km, and that it was remobilized in a plastic to brittle subsolidus state. Slab-sample studies of the tonalite indicate the 3.1 Ga age is a metamorphic one, and that the source material had a pre-history at that time. Sr-isotope constraints imply source region formation as juvenile material from the mantle at between 3.5 and 3.6 Ga. The Duxbury massif, hence, records a history of crustal development spanning at least 600 Ma and possibly more than 1 Ga. These results may be applicable on a larger scale to many of the regional tonalite massives found in the Superior Province of Canada and elsewhere.

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