COUPLED HF AND U-PB ISOTOPE ANALYSIS OF DETRITAL ZIRCONS FROM THE PALEOPROTEROZOIC HURONIAN SUPERGROUP

We report the U-Pb age and precise hafnium isotope compositions for ~270 detrital zircon grains from the Paleoproterozoic Huronian Supergroup to assess patterns of sedimentary provenance during development of the Huronian basin. The Huronian is an up to 12 km-thick succession of mainly siliciclastic rocks deposited along the rifted (?) southern margin of the Archean Superior Province. Paleocurrents in fluvial sandstones throughout the succession suggest provenance mainly from the west and northwest. U-Pb SHRIMP analysis of detrital zircons from six sandstone samples from the western part of the basin indicates sources with prominent modes ca. 2.72 and 2.68 Ga. Two of the samples contain a significant component of older (3.6-2.9 Ga) material and there are 15 grains ranging in age from 2.60-2.45 Ga. Detrital zircon U-Pb dating supported by paleocurrent data suggested contribution from local sources in the ca. 2.72-2.67 Ga Wawa-Abitibi greenstone belt of the underlying Archean Superior Province, which extends for more than 1000 km north and west of the Huronian basin. Previous Hf isotope studies of these rocks indicate that they were derived from juvenile sources with epsilon Hf values ranging from 4.5 to 1.5. Epsilon Hf values of Neoarchean detrital zircons from two Huronian samples indicate a significant proportion (40-60%) in the juvenile Abitibi field. In contrast, detrital zircons of similar age from the other samples are moderately enriched, with epsilon Hf values ranging from 1 to –6. This indicates that zircons crystallized from magmas that were derived from pre-2.8 Ga crust, of which there is little presently exposed in the western Superior Province. The only potential source is represented by gneisses of the Minnesota River Valley terrane, or possibly from the Winnipeg River subprovince of the northwestern Superior. A sample from the Mississagi Formation contains detrital zircons of such antiquity (3.6-3.2 Ga) but the epsilon Hf values from these grains (0 to -4) are only moderately enriched. Four Mesoarchean (3.1-2.9 Ga) and seven latest Neoarchean (2.55-2.45 Ga) detrital zircons have extremely enriched epsilon Hf values, (-10 to -15) and (-5 to -13) respectively, suggesting the involvement of Mesoarchean crust, even in zircons that formed from magmas produced during the 2.5-2.4 Ga Matachawan igneous events.