CONTRASTING TYPES OF LATE ARCHEAN PLUTONIC ROCKS IN NORTHWESTERN ONTARIO - IMPLICATIONS FOR CRUSTAL EVOLUTION IN THE SUPERIOR PROVINCE

The Late Archean Aulneau and Lount Lake batholiths have contrasting geochemical characteristics that are interpreted to be a consequence of derivation from different sources. The Aulneau batholith, which was intruded into low-grade metavolcanic rocks of the Wabigoon subprovince, is characterized by high Na2O and low K2O, Rb and Y, together with enriched light rare earth elements (LREE), near-chondritic heavy rare earth elements (HREE), negligible Eu anomalies, and mantle-type initial Sr and Nd isotopic compositions. These geochemical characteristics are consistent with derivation from the partial melting of metamorphosed tholeiitic basalt. In contrast, the Lount Lake batholith, which was intruded into older (2.83-3.17 Ga) tonalitic plutonic rocks of the Winnipeg River subprovince, has relatively high K2O, Rb, Y and REE, low Na2O, and prominent Eu anomalies. The initial Sr and Nd isotopic compositions of the Lount Lake batholith are consistent with a crustal source at least 200-400 Ma older than the crystallization age of the batholith. These geochemical characteristics indicate that the Lount Lake batholith originated from the partial melting of deeper-level equivalents of the tonalitic metaplutonic rocks (2.83-3.17 Ga) which they intrude. These results suggest that reworked 3.0-Ga crust may be extensive in the Winnipeg River subprovince, and that the geochemical and isotopic signature of Late Archean plutonic rocks may be a useful reconnaissance tool in the identification of areas of extensive older crust. The results can be interpreted within a uniformitarian model. Diachronous, ensimatic magmatic arcs represented by the Winnipeg River and western Wabigoon subprovinces developed independently. Collision of these arcs at 2707 +/- 8 Ma marks the cessation of subduction and related magmatic activity and the initiation of high-grade metamorphism and melting in tectonically thickened portions of the crust.