INSIGHTS INTO THE INTERACTION OF GRENVILLE OROGENESIS AND MIDCONTINENT RIFTING FROM NEW DETRITAL ZIRCON U-PB GEOCHRONOLOGY OF THE MIDDLE RUN SEQUENCE IN SUBSURFACE KENTUCKY AND OHIO

Potential interaction between Mesoproterozoic Grenville orogenesis and rifting in the buried eastern midcontinent, and the absence of a Grenville foreland basin, have puzzled geologists for decades. Midcontinent rift (MCR) magmatism occurred in the narrow time (1115-1085 Ma) bracketed by the two main Grenville orogenic phases (Shawinigan: 1180-1120; Ottawan: 1080-1020 Ma). Basement cores from KY and OH provide insight into the spatiotemporal evolution of the Grenville orogen, MCR, and Grenville foreland basin where collision and rifting intersect. Previous studies mapped a lithologic boundary under central KY that separates Grenville metamorphic rocks to the east from granite/rhyolite, Neoproterozoic Middle Run Fm. (MR) (a redbed sequence interpreted as rift fill and/or Grenville foreland basin), and mafic and felsic volcanics, to the west. The boundary was interpreted as the Grenville Front adjacent to an intracratonic rift basin. Detrital zircons (DZ) were analyzed from the MR discovery well in OH and from potential MR equivalent redbeds in western KY (Blan well) and central KY (Sherrer well). The Sherrer MR is capped by 500’ of basalt; the other occurrences are not. The MR discovery well features dominant DZ age modes at ~1110 and ~1170 Ma, minor age modes at ~1350 (pre-Elzevirian and Southern Granite Rhyolite [SGR]), and at ~1450 (Eastern Granite Rhyolite [EGR]). Analytical precision precludes resolution of the much broader dominant age mode (~1120-1080 Ma) for the Blan well into both Shawinigan and MCR components with minor modes at ~1450 (EGR) and ~1650 (Mazatzal). The most restricted dominant age mode (1115 ± 5 Ma) from the Sherrer well is interpreted as sediment related to early syn-rift felsic volcanism. Grenville Elzevirian (~1250 Ma), SGR, EGR, Mazatzal, and late Archean input was probably derived from rift flanks. This evidence shows that the Sherrer redbed sequence is not a MR equivalent but must be an early syn-rift basin fill. Absent from all samples is the Ottawan-Shawinigan “Grenville age doublet” that defines Grenville provenance. The basement lithologic map and DZ ages are consistent with a rift basin capped by basalt that evolved into a proximal foreland basin or over-filled rift basin as post-Ottawan exhumation commenced. DZ double-dating may distinguish MCR volcanic vs. Grenville detritus.