Basement cores from two wells drilled west and east of the Grenville front consist of feldspathic litharenite and granitic orthogneiss, respectively. Detrital zircon U-Pb ages for the litharenite define a broad dominant U-Pb age mode at ca. 1115 Ma. The dominant mode matches that for the type locality of the Middle Run Formation in the Ohio subsurface and is interpreted to consist of detrital zircons sourced from East Continent Rift volcanic sources (ca. 1100 Ma) and Grenville Shawinigan granites/gneisses (1120–1180 Ma). The youngest detrital zircon ages (ca. 1020 Ma) require a maximum depositional age that is at least 70 My younger than the time of Midcontinent and East Continent rifting and magmatism. We propose that the litharenite is correlative with the Middle Run Formation in Ohio and was deposited in an evolving late Grenville rift/foreland basin adjacent to the exhuming Grenville orogen. Zircon U-Pb secondary-ion mass spectrometry ages from orthogneiss define a discordant array with intercepts of ca. 1500 and 1000 Ma. The oldest concordant dates (ca. 1450 Ma, from oscillatory-zoned cores) are interpreted as the crystallization age of the igneous protolith of the orthogneiss. Metamorphic zircon rims define a weighted mean U-Pb age of 1018 ± 19 Ma (2σ) Ma, interpreted to represent the time of high-grade metamorphism during the late Ottawan phase of the Grenville orogeny. This age pattern matches that of exposed basement in the Central Gneiss Belt of the Grenville Province (Ontario) and similar basement orthogneisses in Ohio and Kentucky that are interpreted to be of Eastern Granite-Rhyolite Province affinity. All age data are consistent with a provenance model of an actively exhuming Grenville orogen at ca. 1000 Ma producing sediment that is mixing with recycled East Continent Rift sediments.

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Published in The Journal of Geology, v. 126, no. 1.

© 2017 by The University of Chicago. All rights reserved.

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The upgrade of the CAMECA SX50 at the University of Kentucky (UK) was supported by National Science Foundation (NSF) Division of Earth Sciences Instrumentation and Facilities grant 0824714. The SEM at UK was acquired with NSF Instrumentation and Facilities grant 1551342 and with financial support of the Kentucky Geological Survey.

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