ROOTS OF AN ARCHEAN VOLCANIC ARC COMPLEX - THE LAC-DES-ILES AREA IN ONTARIO, CANADA

The Lac des Iles area is located in the Wabigoon Subprovince, an Arche an granite-greenstone terrain in the Superior Province in Ontario, Can ada. 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 a re concentric in shape and consist of peridotite, pyroxenite, gabbro a nd hornblende gabbro cumulates and diorite. The largest intrusion, the Lac des Iles complex, consists of several mafic and ultramafic center s. An internal Sm-Nd isochron from the southern ultramafic complex giv es an intrusion age of 2736 +/- 28 Ma (MSWD = 1.5; epsilon(Nd)(t) = 1. 2). This age is also assumed to be representative of the time of empla cement of the other mafic and ultramafic bodies as well as the unmetam orphosed tonalite plutons. The tonalitic gneiss represents older conti nental crust and had an epsilon(Nd) value of -1 at this time. The epsi lon(Nd)(t) values of the felsic and mafic/ultramafic suites are positi ve, ranging from 0 to +2. This range can be explained if primary magma s derived from a depleted mantle reservoir assimilated continental cru st, such as the tonalite gneiss, during their ascent. This implies tha t the tonalite magmas in the Lac des Iles area represent mixtures of t he tonalite country rock and mantle magmas which formed by partial mel ting of depleted mantle or of mafic rocks underplated beneath the cont inental lithosphere. The morphology and the rock assemblages of the ma fic and ultramafic intrusions in the Lac des lies area resemble Alaska n-type intrusions which are common components of Phanerozoic magmatic arcs associated with subduction zones. In addition, the crystallizatio n sequence in the mafic/ultramafic intrusions suggests that they are d erived from a parental magma with high-Al basalt affinities. The mafic /ultramafic rocks are depleted in high field strength elements but enr iched in large ion lithophile elements, and the tonalites belong to ca lc-alkaline rock series displaying fractionation trends towards trondh jemitic 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 L ac des Iles area formed in an Andean-type continental margin.