PROVENANCE CONSTRAINTS FROM THE KEWEENAWAN SUPERGROUP: IMPLICATIONS FOR EXHUMATION AND SEDIMENT DISPERSAL DURING THE MIDDLE- AND LATE-STAGE DEVELOPMENT OF THE MIDCONTINENT RIFT SYSTEM

The Midcontinent Rift system is a continent-scale (~2000 km long), Meso–Neoproterozoic, tectonic feature that has been geophysically imaged throughout much of the subsurface of Michigan, southern Ontario, Wisconsin, Minnesota, Iowa, Nebraska, and Kansas. Sedimentation and volcanism associated with rifting occurred over a ~200-million-year period between 1100–900 Ma and some of the best (and only) outcrop of the Keweenawan Supergroup occurs along the shorelines of Lake Superior in northern Michigan (e.g. Keweenaw Peninsula and surrounding areas). Presented here are modal composition trends as well as the first detrital zircons ages from the Copper Harbor Conglomerate and Jacobsville Sandstone of the Keweenawan Supergroup.

U-Pb detrital zircon ages from older rift-fill strata of the Copper Harbor Conglomerate reveal one primary age peak between 1100–1200 Ma and isolated zircon ages at 1400, 1800, 2100, and 2700 Ma. Modal composition trends from the Copper Harbor Conglomerate reflect pervasive occurrences of feldspar and lithic volcanic detritus. U-Pb ages from overlying rift-flank strata of the Jacobsville Sandstone include primary peak ages between 1100–1600 Ma with isolated occurrences at 1700, 2000, 2700, and 2900 Ma. Although lithic volcanic detritus is present in the Jacobsville, modal composition trends reveal a relative abundance of mono- and polycrystalline quartz with subsequent occurrences of feldspar and lithic fragments.

Preliminary provenance data from the Copper Harbor Conglomerate reflect a largely homogenous detrital age signature that overlaps largely with the Grenville/Keweenaw magmatic provinces. Detrital zircon ages from strata of the Jacobsville Sandstone reveal a more heterogeneous detrital signature and overlap with magmatic provinces of the Grenville, Granite/Rhyolite, Yavapai-Mazatzal/Central Plains/Labrador, and Penokean/Trans Hudson orogens. Provenance trends allow for the preliminary interpretation that early- to middle-stage, rift-fill sedimentation was characterized by isolated detrital contributions from localized sources of the Grenville/Keweenaw. Exhumation and sediment dispersal during the later stages of rifting may have been much more widespread and characterized by detrital contributions from a number of magmatic provinces.