BASEMENT CRUST AGE AND RODINIAN RIFTING IN THE ILLINOIS BASIN: DETRITAL ZIRCON GEOCHRONOLOGY OF BASAL CAMBRIAN STRATA

We present the first evidence of early Cambrian magmatism associated with the development of the Reelfoot Rift and the rifting of Rodinia beneath the Illinois Basin. Over 650 new zircon ages from samples of Cambrian sandstone from three deep boreholes reveal a complex tectonic history during the Neoproterozoic through early Cambrian leading to the incipient formation of this intracontinental basin. The oldest zircon peak age is ~1660 Ma, older than any known age for crystalline rocks from other deep drill cores in the basin and confirms that primary Mazatzal crust exists beneath the Illinois Basin. The principal age peak is 1375 Ma, which marks the culminating stage of anorogenic magmatism associated with the Eastern Granite-Rhyolite province. A secondary peak of 1460 Ma reflects the known ages of crystalline rocks present in the Illinois Basin. The opening of the Reelfoot Rift, which was part of the broader rifting of Rodinia during the Neoproterozoic and early Cambrian, is marked by zircons that range from 530-540 Ma. These data reveal basal Cambrian sandstones in the Illinois Basin have a detrital zircon provenance that is distinct from the overlying ubiquitous late Cambrian arenites present throughout the upper Midwest. These data also suggest that a northern arm of the Reelfoot Rift extended several 100 km into central Illinois.