NOBLE-METAL ABUNDANCES OF LATE ARCHEAN (2.7-GA) ACCRETION-RELATED SHOSHONITIC LAMPROPHYRES, SUPERIOR PROVINCE, CANADA

Shoshonitic magmatism occurred in transpressive geodynamic settings wi thin the Superior Province during the diachronous accretion of allocht honous subprovinces over 2720 to 2680 Ma, which culminated in stabiliz ation of this Archean craton. The dikes are interpreted to have formed from low degrees of partial melting of depleted mantle metasomaticall y 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 beha viour of S during autometasomatic and late hydrothermal alteration. Ba sed on the Se contents, and assuming magmatic S/Se ratios of approxima tely 5,000, primary S contents of the alkaline magmas were undersatura ted at approximately 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. Platin um (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 1 5 ppb, respectively, for boninites. Primitive mantle normalized patter ns are smooth, except at Ru and Au; Pt/Ir ratios tend to increase in m ore evolved dikes because of decreased abundances of Ir. Platinum, pal adium, and gold do not correlate with La/Yb in least altered dikes, in terpreted to imply that mantle metasomatism responsible for enrichment of the source did not contribute Pt, Pd, or Au. During evolution of t he shoshonitic magmas, olivine +/- pyroxene controlled Ir concentratio ns and probably Rh. Platinum and palladium correlate with Ti, implying that these PGE were controlled by Ti-magnetite fractionation.