U-PB DETRITAL GEOCHRONOLOGY AND HF ISOTOPIC ANALYSES FROM MESOPROTEROZOIC KEWEENAWAN SUPERGROUP STRATA OF THE MIDCONTINENT RIFT, NORTHERN MICHIGAN

The Midcontinent Rift is a continent-scale (~2000 km long), Meso–Neoproterozoic tectonic feature that has been geophysically imaged throughout much of the subsurface of the U.S. Midcontinent. Volcanism and sedimentation associated with rifting occurred over a >200 million years (1110–900 Ma) and some of the best (and only) outcrops occurs along the shorelines of Lake Superior in northern Michigan. Presented here are U-Pb ages and Hf isotopic analyses from detrital zircons collected from the Copper Harbor Conglomerate, Freda Sandstone, and Jacobsville Sandstone of the Keweenawan Supergroup.

Zircon grains from basal rift-fill strata of the Copper Harbor Conglomerate reveal a majority of ages between 950–1250 Ma (primary peaks at 1115 and 1235 Ma) and 1300-1500 Ma (primary peaks at 1330 and 1470 Ma) with isolated occurrences between 1600–1900 Ma (secondary peaks at 1675, 1785, and 1890 Ma). Overlying strata of the Freda Sandstone contain ages between 1800-1900 Ma (primary peak at 1895 Ma) and 2600–2800 Ma (secondary peaks at 2665 and 2750 Ma) with isolated occurrences between 1050–1250 Ma (secondary peaks at 1100 and 1200 Ma). Syn- to post-rift strata of the Jacobsville Sandstone contains ages between 1050–1250 Ma (primary peaks at 1080, 1115, and 1230 Ma) and 1300–1500 Ma (primary peak at 1470 Ma) with isolated occurrences between 1650–1900 Ma.

A majority of detrital zircons (>85%) exhibit depleted epsilon Hf values (+2 to +14) and correspond in age with magmatic source areas of the Yavapai-Mazatzal, Granite-Rhyolite, and Grenville provinces. Less than 15% of zircons show enriched Hf values (-2 to -25) and overlap in age with the Midcontinent Rift (1109–1087 Ma). Detrital zircons that overlap in age with the Trans Hudson/Penokean and Superior provinces show a range of enriched and depleted Hf values.

Provenance trends from the Keweenawan Supergroup are interpreted to reflect primary detrital contributions from juvenile outboard magmatic source areas of the Trans Hudson/Penokean, Yavapai-Mazatzal, Granite-Rhyolite, and Grenville provinces and enriched source areas of the Superior province and Midcontinent Rift. Rift-age zircons with enriched Hf values were likely derived locally from magmatic systems that developed beneath the Superior province rather than juvenile crust of the Yavapai-Mazatzal province.