Roots of an Archean volcanic arc complex: the Lac des Iles area in Ontario, Canada

Abstract

The Lac des Iles area is located in the Wabigoon Subprovince, an Archean granite-greenstone terrain in the Superior Province in Ontario, Canada. The area consists of several unmetamorphosed plutons of biotite- and hornblende-tonalite and mafic and ultramafic intrusions which are emplaced into an older tonalitic gneiss. The mafic/ultramafic bodies are concentric in shape and consist of periodotite, pyroxenite, gabbro and hornblende gabbro cumulates and diorite. The largest intrusion, the Lac des Iles complex, consists of several mafic and ultramafic centers. An internal SmNd isochron from the southern ultramafic complex gives an intrusion age of 2736 ± 28 Ma (MSWD = 1.5; ϵ Nd(t) = 1.2). This age is also assumed to be representative of the time of emplacement of the other mafic and ultramafic bodies as well as the unmetamorphosed tonalite plutons. The tonalitic gneiss represents older continental crust and had an ϵ Nd value of −1 at this time. The ϵ Nd(t) values of the felsic and mafic/ultramafic suites are positive, ranging from 0 to +2. This range can be explained if primary magmas derived from a depleted mantle reservoir assimilated continental crust, such as the tonalite gneiss, during their ascent. This implies that the tonalite magmas in the Lac des Iles area represent mixtures of the tonalite country rock and mantle magmas which formed by partial melting of depleted mantle or of mafic rocks underplated beneath the continental lithosphere. The morphology and the rock assemblages of the mafic and ultramafic intrusions in the Lac des Iles area resemble Alaskan-type intrusions which are common components of Phanerozoic magmatic arcs associated with subduction zones. In addition, the crystallization sequence in the mafic/ultramafic intrusions suggests that they are derived from a parental magma with high-Al basalt affinities. The mafic/ultramafic rocks are depleted in high field strength elements but enriched in large ion lithophile elements, and the tonalites belong to calc-alkaline rock series displaying fractionation trends towards trondhjemitic compositions. Thus, the Archean Lac des Iles area and the root zones of Phanerozoic active continental margins share many geological, geochemical and petrological features which would suggest that the Lac des Iles area formed in an Andean-type continental margin.