Noble metal abundances of late Archean (2.7 Ga) accretion-related shoshonitic lamprophyres, Superior Province, Canada

Abstract

Shoshonitic magmatism occurred in transpressive geodynamic settings within the Superior Province during the diachronous accretion of allochthonous subprovinces over 2720 to 2680 Ma, which culminated in stabilization of this Archean craton. The dikes are interpreted to have formed from low degrees of partial melting of depleted mantle metasomatically enriched in LILE and LREE during a prior subduction-accretion event. Selenium contents of the dikes range from 50–130 ppb, and S from 150–1000 ppm, with S/Se ratios of 1150–18,000, signifying open-system behaviour of S during autometasomatic and late hydrothermal alteration. Based on the Se contents, and assuming magmatic S/Se ratios of ∼ 5,000, primary S contents of the alkaline magmas were undersaturated at ∼ 650 ppm. MgO and Ni are correlated, and Ir (<0.02–0.24 ppb) and Rh (0.02–0.32 ppb) also decrease with lower MgO. Platinum (0.15–5.8 ppb) and Pd (0.48–4.8 ppb) abundances are systematically greater than averages for MORB, but less than contents of 11 ppb and 15 ppb, respectively, for boninites. Primitive mantle normalized patterns are smooth, except at Ru and Au; Pt/Ir ratios tend to increase in more evolved dikes because of decreased abundances of Ir. Platinum, paladium, and gold do not correlate with La/Yb in least altered dikes, interpreted to imply that mantle metasomatism responsible for enrichment of the source did not contribute Pt, Pd, or Au. During evolution of the shoshonitic magmas, olivine ± pyroxene controlled Ir concentrations and probably Rh. Platinum and palladium correlate with Ti, implying that these PGE were controlled by Timagnetite fractionation.