PALEOPROTEROZOIC-CAMBRIAN TECTONIC EVOLUTION OF CENTRAL LAURENTIA: EVIDENCE FROM DETRITAL ZIRCON GEOCHRONOLOGY OF BASAL CAMBRIAN STRATA IN THE DEEP ILLINOIS BASIN, USA

Here we present 960 new detrital zircon ages from three new, deep boreholes from the Illinois Basin, USA. These zircon age spectra reveal new details of the late Paleoproterozoic through early Cambrian tectonic history of the Illinois Basin, including evidence of the timing of the development of the Reelfoot Rift and break-up of Rodinia. The oldest detrital zircon population is ~1650 Ma, older than any known age for crystalline rocks from other deep drill cores in the basin, and conspicuously absent in other Cambrian strata in the region. We interpret this population to have been derived from late Paleoproterozoic crust that exists beneath the Illinois Basin and was exposed during the deposition of Cambrian strata. The principle age peak is ~1375 Ma, characteristic of igneous rocks of the Southern Granite-Rhyolite province (SGRP), but also present in the St. Francois Mountains of the Eastern Granite-Rhyolite province (EGRP). A secondary peak of ~1460 Ma detrital zircon reflects the known ages of crystalline rocks that characterize EGRP determined from the along the periphery of the Illinois Basin. This population of detrital zircon in Cambrian strata may reflect a greater abundance of ~1370 Ma crust beneath the Illinois Basin than presently recognized. These data reveal basal Cambrian sandstones in the Illinois Basin have a detrital zircon provenance that is distinct from the overlying late Cambrian arenites, and that a previously unknown northern arm of the Reelfoot Rift extends into central Illinois, which is more than 300 km further north than currently mapped. 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, probably from Reelfoot Rift igneous rocks, that range from 540-525 Ma.