BIMODAL CYCLICAL ARCHEAN BASALTS AND RHYOLITES FROM THE MICHIPICOTEN (WAWA) GREENSTONE-BELT, ONTARIO - GEOCHEMICAL EVIDENCE FOR MAGMA CONTRIBUTIONS FROM THE ASTHENOSPHERIC MANTLE AND ANCIENT CONTINENTAL LITHOSPHERE NEAR THE SOUTHERN MARGIN OF THE SUPERIOR-PROVINCE

The petrographic and geochemical variations in the middle to late Arch ean volcanic rocks of the Michipicoten (Wawa) greenstone belt (MGB), O ntario, record bimodal basalt-rhyolite magmatic activity at approximat ely 2.9, 2.75 and 2.7 Ga along what has previously been interpreted to be a southward-dipping convergent plate margin which is underlain by continental crust. Cycle 1 komatiitic basalts have elevated Mg-number (0.51-0.83) compared to cycle 2 and 3 tholeiites (Mg-number 0.44-0.56) , and dominantly flat REE patterns with negative Ta-Nb anomalies (La/T a 4-35). Cycles 2 and 3 are more evolved than cycle 1, but all three c ycles have broadly similar ratios of the incompatible trace elements. Compared to modern oceanic basalts, the basaltic members of cycles 1-3 have a moderate Ta-Nb anomaly (La/Ta 4-46) and low [La/Yb](N)(1.18-1. 56). Low La/Ta is a feature of oceanic lavas, whereas high La/Ta is a feature of magmas which contain contributions from a crustal reservoir . The mafic volcanic rocks are believed to be derived from asthenosphe ric mantle with assimilation of older continental crust. The felsic vo lcanic rocks have elevated Th/Nb (0.7-1.6), and [Ce/Yb](N)=10.2-16.4, and are enriched in light rare earth elements (LREE) and large ion lit hophile elements (LILE) relative to rhyolites from other Superior Prov ince greenstone belts. We suggest that the rhyolites record the geoche mical traits of their sources, and these could be interpreted to be mo derate to high degree melts of tonalitic upper crust rather than fract ionated magmas or partial melts derived from the underlying mafic volc anics. This contrasts with models for many other rhyolites in Superior Province greenstone belts, such as the Southern Volcanic Zone of the Abitibi greenstone belt, where variations require a more mafic crustal source to generate the fractionated low La/Sm, Gd/Yb, and high Zr/Th rhyolites believed to be locally associated with volcanogenic massive sulphide deposits. The absence of significant volumes of compositional ly intermediate volcanic rocks and the evidence for crustal contributi ons to MGB cyclical magmatism raises the possibility that these are Ar chean analogues of the Phanerozoic hood basalts which straddle the mar gins of continental shield areas, such as the Java-Ontong sequence, an d are not relies of either Archean oceanic crust or subduction-related volcanics.