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 bou ndary between Archean crust of the Superior craton and penecontemporan eous Early Proterozoic crust of the Wisconsin magmatic terranes. The p rovenance of sedimentary rocks within the Animikie Basin constrains th e tectonic evolution of the Early Proterozoic margin of the Superior c raton. Published geological, petrographic, and Nd-Pb isotopic data ind icate that the Pokegama Quartzite (lower passive margin unit of the An imikie Group) has a Late Archean provenance. In contrast, the Virginia Formation (upper deep water turbidite-shale unit of the Animikie Grou p) was derived from dominantly Early Proterozoic sources, with isotopi c compositions similar to igneous rocks of the Wisconsin magmatic terr anes. Shales of the Virginia Formation have Nd depleted mantle model a ges 2.14 to 2.35 Ga, and mu(1) of 7.81 to 8.09. Ashes within the Virgi nia 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 Fo rmation have high abundances and light rare earth element-enriched cho ndrite-normalized patterns, large and variable Eu anomalies, high CIA, high La/Th and low Th/Sc and Th/U. The provenance of the Virginia For mation can best be described as young differentiated are. These data, combined with other data presented here and from published research an d combined with published structural and stratigraphic constraints, ar e used to propose a model for the tectonic evolution of the Superior c ontinental margin, beginning as a passive margin within a marginal bac k are basin, and ending as a telescoped back are basin that is inferre d to have closed as a result of some change in relative plate converge nce.