REFINING THE AGE OF BASEMENT ROCKS OF NORTHWEST IOWA: IMPLICATIONS FOR PRECAMBRIAN TECTONICS OF THE LAURENTIAN MIDCONTINENT AND PALEOZOIC SEDIMENT SOURCING

Precambrian basement rocks in northwest Iowa occur at the confluence of the Archean Superior Province and two Paleoproterozoic tectonic sutures (the 1.89-1.83 Ga Trans-Hudson and 1.8-1.7 Ga Yavapai). Here we present three new U-Pb (LA-ICPMS) ages from drill cores of basement rocks along the Transcontinental Arch in northwest Iowa. The cores are available at the Iowa Geological Survey Oakdale Rock Library. The Le Mars granitic gneiss was sampled at a depth of 1078 ft from the Camp Quest D-21 well (W25498; z=38). The weighted mean and concordia ages were both ~1838 Ma, which is the first Trans-Hudson age determined in Iowa, suggesting a relict tectonic sliver of TH south of the Spirit Lake tectonic zone (SLTZ), which is interpreted as the suture between Yavapai rocks to the south and Archean Superior Province rocks to the north. Nine inherited zircons are mostly Archean in age and interpreted as xenocrysts and indicates that Archean crust occurs at depth south of the SLTZ. The Harris granite (Osceola granite) was sampled at a depth of 919 ft from the Harris D-13 well (W25108; z=41). The weighted mean and Concordia ages are ~1780 Ma, suggesting that Yavapai rocks intrude older Trans-Hudson or Archean rocks north of the SLTZ. A previously report plutonic age of 1804 Ma supports multiple pulses of Yavapai magmatism between 1800 Ma and 1780 Ma. The Hawarden granite also was sampled at a depth of 660 ft from the Hawarden D-7 well (W27270; z=35). The zircon age spectrum reveals three age clusters at ~2895, ~2683, and ~1800 Ma. The older, inherited age clusters represent the Minnesota River Valley terrane and the greater Superior Province, respectively. The ~1800 Ma plutonic age is like the nearby Harris granite and the distant 1805 Ma Humboldt granite (northern Michigan) representing the initiation of north-directed Yavapai subduction and granitic melt production into Archean and previously accreted Trans-Hudson and Penokean rocks north of SLTZ. These new data provide important constraints for interpreting detrital zircon age spectra in overlying Cambrian and Ordovician strata.