TECTONIC EVOLUTION OF THE NORTHERN INTERIOR CONTERMINOUS U.S.: A CHEYENNE BELT-SPIRIT LAKE TECTONIC ZONE CONNECTION

Recent geophysical evidence suggests that the Paleoproterozoic Cheyenne Belt exposed in Wyoming extends eastward into the north-central United States as a major geophysical lineament known as the Spirit Lake tectonic zone (SLtz). To test this proposal, several subsurface samples of Precambrian basement from the north-central United States have been analyzed or reexamined using modern techniques of U-Pb and Ar-Ar geochronology. The results fill in a major data gap for the region and show that all U-Pb crystallization ages for samples south of the SLtz are geon 17 (1800-1700 Ma) or younger. Bedrock drill core samples from eastern Nebraska are 1800-1780 Ma, and two samples from SE South Dakota yield ages of 1762 ± 28 Ma and 1733 ± 2 Ma. Further east, xenoliths from impact breccia in the Manson structure, north-central Iowa yield a highly discordant upper intercept age of 1705 ± 30 Ma, and metagabbro from SE Minnesota yields an age of 1760 ± 9 Ma, within error of the ion microprobe analyses age of 1732 ± 20 Ma. Ion microprobe analyses of zircon from Elk Point and Manson samples also show the presence of geon 16 overgrowths with Th/U values of <0.2, indicating a strong regional thermal overprint during Mazatzal accretion. This is supported by a 1650 ± 2 Ma Ar-Ar hornblende cooling age for Elk Point. Further east along strike of the SLtz in Ontario, a drill core from beneath Manitoulin Island yields a zircon upper intercept age of 1714 ± 10 Ma. Although no U-Pb ages are available for juvenile basement beneath the ca. 1750 Ma granite-rhyolite suite of southern Wisconsin, south of the SLtz, Sm-Nd model ages are typically ca. 1.9 to 2.0 Ga, consistent with basement to the rhyolites being geon 17 in age. Collectively, the data require that most, if not all, of the Proterozoic crust immediately south of the SLtz formed during geon 17 and probably represents eastward continuation, from Colorado through Nebraska, of the Yavapai crustal province in the SW United States. These results also show that medium grade (~500 °C) tectonothermal effects of the subsequent Mazatzal orogeny to the south continue into the north-central United States. Both terranes likely also continue eastward beyond northern Ontario, but are obscured by the younger Grenville orogen.