Abstract

The Portneuf–Mauricie Domain in the Grenville Province consists of the Montauban group rocks (1.45 Ga), intruded by the La Bostonnais complex plutons (1.40–1.37 Ga). This assemblage was formed in a magmatic arc setting. The sequence was intruded by mafic–ultramafic tholeiitic plutons, some of which host Ni–Cu ± PGE (platinum group element) prospects. U–Pb zircon ages determined from these plutons indicate that the mineralized intrusions were emplaced between 1.40 and 1.39 Ga and that they are coeval with the La Bostonnais complex plutons. The Ni–Cu ± PGE-bearing intrusions have mature island-arc trace element signatures, with strong chemical evidence for differentiation (Mg# and Cr content; MgO and TiO2 contents) and crustal contamination (enrichments in K2O, Rb, Ba, Th, and light rare-earth elements; Th/Yb and Ta/Yb ratios). However, one intrusion displays a back-arc trace element signature associated with evidence for weak crust assimilation. The evolution of the Portneuf–Mauricie Domain is interpreted as follows: (1) 1.45 Ga — Northwesterly directed Andean-type subduction beneath the Laurentian craton margin. Furthermore, northwest-dipping intraoceanic subduction offshore from the continent formed the Montauban island arc. (2) 1.45 to 1.40 Ga — Andean-type subduction led to the formation of a back-arc basin behind the Montauban arc. (3) 1.40 Ga — Emplacement of the La Bostonnais complex plutons, some hosting Ni–Cu ± PGE prospects, into the Montauban arc. (4) 1.39 Ga — Subduction beneath Laurentia led to arc–continent collision and to closure of the back-arc basin. Intrusion of the Ni–Cu ± PGE-bearing plutons ceased. (5) 1.37 Ga — Intrusion of all La Bostonnais complex plutons ceased.

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