SOURCE ROCK AGES AND PATTERNS OF SEDIMENTATION IN THE LAKE SUPERIOR REGION: RESULTS OF PRELIMINARY U-PB DETRITAL ZIRCON STUDIES

Although most commonly applied to Phanerozoic orogenic belts, U-Pb age analysis of detrital zircons has great potential for illuminating the sedimentary evolution of cratonic regions. Here we report preliminary results of U-Pb dates of detrital zircons from Paleoproterozoic, Mesoproterozoic, Neoproterozoic, and early Paleozoic sedimentary rocks from Minnesota and Wisconsin. U-Pb analyses (n = 120 grains per sample) were conducted using laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at Washington State University.

Pre-Animikie (>1.85 Ga) sediments (Denham Fm.) contain zircons with two age populations (3.5–3.4 and 2.8–2.5 Ga). Basal sandstones of the Animikie (Pokegama) and Marquette (Palms) Supergroups (~1.85 Ga) contain mostly Neoarchean zircons (2.9-2.6 Ga). In contrast, overlying basin sediments (Rove, Thomson and Tyler Formations), deposited in a migrating foredeep north of the Penokean orogen, consist mostly of zircons with ages from 2.05-1.80 Ga; few Paleoproterozoic or Archean grains are present. Sediments deposited during the early stages of Midcontinent Rifting (Nopeming and Puckwunge Sandstones) have three zircon populations (2.8–2.5, 2.1-1.8, and 1.2-1.1 Ga), whereas those from interflow sediments of the North Shore Volcanic Group are dominated by zircon ages of 1.15-1.0 Ga. Post-rift (<1.09 Ga) sediments, including the Jacobsville, Fond du Lac, and Hinckley Sandstones (Mesoproterozoic to Neoproterozoic?); Franconia Fm. (Late Cambrian); and St. Peter Sandstone (Middle Ordovician) contain zircons that are mostly 1.5-1.2 Ga, or less commonly 2.0 - 1.5 Ga. Archean populations (>3.2 Ga; 2.8-2.5 Ga) are poorly represented in the Mesoproterozoic to Neoproterozoic(?) sediments, but become increasingly more abundant in the Late Cambrian sandstones.

Many of the observed zircon ages can be correlated with known source rock ages in the Lake Superior region. Some zircon populations (e.g., 2.5–2.1 Ga, 1.6–1.5 Ga, and 1.4–1.1 Ga), however, have few obvious local sources and must have been derived from more distal sources or from regional sources with unrecognized multicyclic components. In particular, most of the post-Midcontinent Rift sediments that we studied have abundant ages between 1.5 and 1.1 Ga that might have been derived from Grenville Province sources.