Geochemistry, Petrogenesis and Tectonic Environment of Circum-Superior Belt Basalts, Canada

Abstract

Summary The Circum-Superior Belt of Proterozoic basalts and sedimentary rocks surrounds, and in some places, unconformably overlies the Archaean Superior Craton in northern Quebec, Canada. Four main groups of volcanic rocks are recognized. The oldest unit, the Eskimo Formation on the Belcher Islands, consists of subaerial massive continental tholeiites with 4–7% MgO, relatively high SiO 2 and K 2 O, enriched LREE and other incompatible elements, negative Nb and Ta anomalies, and ɛ Nd (T = 1.8 Ga) of −7 to −8. The overlying Flaherty Formation contains largely subaqueous, massive or pillowed, mildly alkaline (transitional) basalts with relatively low SiO 2 , high alkalis and TiO 2 , enriched incompatible trace elements (including Nb and Ta), and ɛ Nd of + 3. Stratigraphically higher units (on the Ottawa Islands and the Chukotat Formation, Cape Smith Belt) contain subaqueous, massive or pillowed flows, and layered gabbro-peridotite flows with compositions ranging from MgO-rich ol-phyric basalt and evolved cpx-plag-phyric basalt to ol-cpx cumulate. Maximum MgO in noncumulate samples reaches 17 wt%, and trace element characteristics are MORB-like: LREE and other incompatible elements are depleted, and ɛ Nd is + 4 to + 5. The highest units, exposed on the Ottawa Islands, consist of komatiitic basalts and include unusual spinifex-textured layered flows. Although maximum noncumulate MgO contents are about 16 wt%, olivine xenocrysts in cumulate layers have the composition Fo 92 , and provide evidence of a komatiite (MgO = ∼ 22%) precursor magma. Komatiitic basalts are less depleted than the underlying Mg-basalts, with fiat to enriched LREE, negative Nb and Ta anomalies, and ɛ Nd = 0. The chemical features of Eskimo Formation basalts and spinifex basalts can be explained by crustal contamination of parental magmas derived from depleted mantle with ɛ Nd = + 5: the continental tholeiites by 20–30% contamination of parental basalt and the spinifex basalts by about 7% contamination of parental komatiite. The Mg-basalt series results from partial melting of depleted mantle followed by low pressure fractional crystallization, while Flaherty Formation basalts come from a more enriched mantle source. The stratigraphic sequence—(1) early contaminated continental basalt, (2) transitional basalt, (3) MORB-like basalt, (4) crustally-contaminated komatiite—is considered to have arisen during continental rifting, spreading, and then closure of an ocean basin surrounding the Superior Craton.