DETRITAL ZIRCON U-PB GEOCHRONOLOGY OF THE MIDDLE RUN SEQUENCE IN KENTUCKY AND OHIO SUPPORTS THE SPATIOTEMPORAL CONVERGENCE OF LATE MESOPROTEROZOIC GRENVILLE COLLISIONAL AND MIDCONTINENT RIFTING PROCESSES, AND THE PRESENCE OF A LATE GRENVILLE FORELAND BASIN UNDER CENTRAL KY

Geochronological data indicate that Midcontinent rifting (MCR) occurred at 1115-1085 Ma, temporally bracketed by Grenville orogenesis (Shawinigan and Ottawan at ~1150 and 1050, respectively). Basement drill core from Kentucky and Ohio provides additional insight into the spatial distribution and temporal evolution of the Grenville, MCR, and foreland basin formation in the region where collision and rifting are inferred to merge. The Kentucky Geological Survey (KGS) mapped a boundary under central KY that separates Grenville metamorphic rocks to the east from granite/rhyolite; the Middle Run (MR) (a Precambrian redbed sequence interpreted as rift fill or Grenville foreland basin); volcanics; and mafic intrusives to the west. The boundary was interpreted as the Grenville Front bordering a rift basin. Granulite facies orthogneiss (1018 ± 19 Ma) of eastern KY basement represents late Ottawan Grenville collision. 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 MR discovery well features two dominant DZ age modes at ~1110 and ~1180 Ma, and minor age modes at ~1350 (pre-Elzevirian and Southern Granite Rhyolite [SGR]), and at ~1450 (Eastern Granite Rhyolite [EGR]). Analytical precision precludes resolution of a much broader dominant age mode (~1120-1080 Ma) for the Blan well into both Grenville Shawinigan and Midcontinent components with minor modes at ~1450 (EGR) and ~1650 (Mazatzal). The most restricted age mode (1115 Ma) from the redbeds in the Sherrer well is interpreted as syn-rift sediment due to abundant feldspar clasts from early rift felsic volcanism and 500’ thick, overlying basalt flows. Some input from the Elzevirian (~1250 Ma), SGR, EGR, Mazatzal, and late Archean was also recorded. This evidence shows that the Sherrer 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 the Grenville orogen and brackets the time of Midcontinent magmatism (1115-1085 Ma). The lithologic map and DZ ages are consistent with a rift basin capped by basalt (Sherrer) that evolved into a foreland basin or over-filled rift basin (OHMR and Blan) as post-Ottawan exhumation occurred.