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

Abstract

The petrographic and geochemical variations in the middle to late Archean volcanic rocks of the Michipicoten (Wawa) greenstone belt (MGB), Ontario, record bimodal basalt-rhyolite magmatic activity at approximately 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 Ta 4–35). Cycles 2 and 3 are more evolved than cycle 1, but all three cycles 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 asthenospheric mantle with assimilation of older continental crust. The felsic volcanic 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 lithophile elements (LILE) relative to rhyolites from other Superior Province greenstone belts. We suggest that the rhyolites record the geochemical traits of their sources, and these could be interpreted to be moderate to high degree melts of tonalitic upper crust rather than fractionated magmas or partial melts derived from the underlying mafic volcanics. 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 compositionally intermediate volcanic rocks and the evidence for crustal contributions to MGB cyclical magmatism raises the possibility that these are Archean analogues of the Phanerozoic flood basalts which straddle the margins of continental shield areas, such as the Java-Ontong sequence, and are not relics of either Archean oceanic crust or subduction-related volcanics.