GEOCHEMICAL AND ND/PB ISOTOPIC EVIDENCE FOR THE PROVENANCE OF THE EARLY PROTEROZOIC VIRGINIA FORMATION, MINNESOTA - IMPLICATIONS FOR THE TECTONIC SETTING OF THE ANIMIKIE BASIN

The Early Proterozoic Animikie Basin is located at a major crustal boundary between Archean crust of the Superior craton and penecontemporaneous Early Proterozoic crust of the Wisconsin magmatic terranes. The provenance of sedimentary rocks within the Animikie Basin constrains the tectonic evolution of the Early Proterozoic margin of the Superior craton. Published geological, petrographic, and Nd-Pb isotopic data indicate that the Pokegama Quartzite (lower passive margin unit of the Animikie Group) has a Late Archean provenance. In contrast, the Virginia Formation (upper deep water turbidite-shale unit of the Animikie Group) was derived from dominantly Early Proterozoic sources, with isotopic compositions similar to igneous rocks of the Wisconsin magmatic terranes. Shales of the Virginia Formation have Nd depleted mantle model ages 2.14 to 2.35 Ga, and mu(1) of 7.81 to 8.09. Ashes within the Virginia are more evolved geochemically than shales and have younger model ages of 1.86 and 1.99 Ga, and similar mu(1). Shales of the Virginia Formation have high abundances and light rare earth element-enriched chondrite-normalized patterns, large and variable Eu anomalies, high CIA, high La/Th and low Th/Sc and Th/U. The provenance of the Virginia Formation can best be described as young differentiated are. These data, combined with other data presented here and from published research and combined with published structural and stratigraphic constraints, are used to propose a model for the tectonic evolution of the Superior continental margin, beginning as a passive margin within a marginal back are basin, and ending as a telescoped back are basin that is inferred to have closed as a result of some change in relative plate convergence.