New Insights into the Southern Margin of the Archean–Proterozoic Boundary in the North-Central United States Based on U–Pb, Sm–Nd, and Ar–Ar Geochronology01/01/2007
New geophysical analysis of the Precambrian basement in Minnesota–Iowa–Wisconsin indicates that an Archean–Proterozoic boundary (Spirit Lake trend) previously recognized in NW Iowa can be continued eastward into central Wisconsin and farther east as the Spirit Lake tectonic zone (SLtz). To test the age of Paleoproterozoic crust south of this structure, several subsurface samples of Precambrian basement from the north-central United States have been analyzed or re-examined using modern techniques of U–Pb, Sm–Nd, and 40Ar/39Ar 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 (1700–1800 Ma). Bedrock core samples from eastern Nebraska are ca. 1760–1800 Ma, and two samples from SE South Dakota, immediately south of the SLtz, yield ages of 1762 ± 28 Ma (Vermillion) and 1733 ± 2 Ma (Elk Point). Xenoliths from impact breccia in the Manson structure in north-central Iowa yield a similar age of ca. 1705 ± 30 Ma and metagabbro from SE Minnesota yields an age of 1760 ± 9 Ma. Farther to the northeast, zircons from Paleoproterozoic gneiss in the basement of Manitoulin Island, only a few km south of the Superior craton in Ontario, also yield a geon 17 age (1714 ± 10 Ma). Sm–Nd model ages (TDM) for samples immediately south of the SLtz fall in the range 1.9–2.2 Ga, indicating limited involvement of Archean crust. In contrast, Sm–Nd TDM ages for samples north of the SLtz typically range from 2.5 to 3.0 Ga, for both Paleoproterozoic plutons and Archean gneisses. Ion microprobe analyses of zircons from the Elk Point and Manson samples also show the presence of geon 16 overgrowths, indicating a strong regional thermal overprint during geon 16 accretion. This is supported by mid-geon 16 hornblende 40Ar/39Ar ages for samples from SE South Dakota and SE Minnesota. Although no U–Pb ages are available for juvenile basement beneath the ca. 1760 Ma granite–rhyolite suite of southern Wisconsin, south of the SLtz, Sm–Nd model ages are typically ca. 1.9–2.0 Ga, consistent with basement to the rhyolites being geon 17 in age. Collectively, the data require that most, if not all, of the Paleoproterozoic 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. Penokean (geon 18) crustal rocks are limited mainly to northern and central Wisconsin, east-central Minnesota, and northern Michigan. These results also show that medium grade (>500 ◦C) tectonothermal effects of the subsequent geon 16 (≈Mazatzal) orogeny to the south continue into the north-central United States. Both terranes probably also continue eastward into Ontario, Canada and farther east into protolith of the Grenville Province.