Detrital zircon U-Pb geochronology of the Precambrian Middle Run Formation (Eastern North America Basement): Implications for Grenvillian foreland basin evolution and Midcontinent Rifting

Abstract The Grenvillian orogeny and formation of the Midcontinent Rift system (MRS) are the primary tectonic events that modified Mesoproterozoic basement of eastern Laurentia prior to Neoproterozoic breakup of Rodinia. Trends of the Grenville Front Tectonic Zone (GFTZ), the cratonward limit of Grenvillian crustal reworking, and the East Continent Rift, an extension of the MRS, appear to converge in the subsurface of central Kentucky and southwestern Ohio. The Middle Run Formation (MRF) is a terrestrial clastic sequence that accumulated in the region of convergence and was interpreted as either a foreland basin or rift-fill deposit. Detrital zircon U-Pb geochronology for MRF feldspathic litharenites from three locations with varying proximity to the GFTZ provides the following constraints on the origin of the MRF and potential GFTZ-MRS interaction. (1) Youngest detrital zircon ages of 1.00 to 1.04 Ga limit the maximum depositional age of the Middle Run Formation to ~1.0 Ga, requiring the MRF to be a latest Grenvillian foreland deposit and not a syn-rift deposit. (2) The abundance of Midcontinent Rift System-age detrital zircon grains attests to the presence of rift-related felsic volcanism in the East Continent Rift and/or recycling of rift-fill sediments from the East Continent Rift into the MRF basin. (3) Detrital zircon age distributions reflect an actively unroofing Grenville orogen as represented by crustal components corresponding to Shawinigan (Geon 11), Elzevirian (Geon 12), and pre-Elzevirian (Geon 13) phases of the Grenville Orogenic Cycle, with significant Eastern Granite-Rhyolite Province basement (Geon 14) input. (4) The presence of only a minor Ottawan (Geon 10) detrital age component indicates that deeper Ottawan crust in the Grenville orogen had not been significantly exhumed. (5) Age modes in four of five samples correlated with the East Continent Rift magmatic interval (1117–1110 Ma) are only slightly older than the early phase of felsic Midcontinent Rift System magmatism (1109–1106 Ma) indicating that magmatism in the East Continent Rift was contemporaneous with the early phase of magmatism in the MRS. The MRF is confirmed to constitute the most proximal region of a Grenvillian clastic wedge that is only preserved or exposed in more distal regions of Laurentia.