PROVENANCE ANALYSIS OF GRANITE COBBLES IN THE TISKILWA TILL USING U-PB GEOCHRONOLOGY

The Tiskilwa Formation is the basal diamict unit of the Wedron Group in northeastern and central Illinois. It is a pink-grey medium textured diamicton that is as much as 90 m in thickness. It was deposited during the Marengo and Shelby phases of the Wisconsin glacial episode (Michigan Subepisode) between 26,000 and 18,500 radiocarbon years ago. U-Pb LA-ICPMS isotopic ages were determined for zircons extracted from quartzofeldspathic cobbles in the Tiskilwa Till to determine their provenance. The cobbles were crushed and processed separately, and single crystals from each cobble were handpicked and analyzed. This method was used rather than obtaining a composite set of grains from the sand matrix to avoid zircon fertility bias. Cobbles (n = 750) were collected at localities from each of the four sub-lobes of the terminal Wisconsin moraines. Two sites were chosen from the Decatur, Peoria, and Princeton sub-lobes with one sample taken from the Harvard sub-lobe. A total of 301 grains analyzed yielded reliable, concordant ages. The age spectra ranged from 1085-3026 Ma, with all but five grains being Archean in age. The zircon age spectrum for each sampling locality is astonishingly similar, with each sample having a peak age of about 2700 Ma and are identical in terms of Kolmogorov-Smirnov statistical analysis. The dominance of Archean grains indicate a Superior Province provenance for the Tiskilwa Formation, with the Hudson Bay Terrane (3.36 – 2.69 Ga) being the most likely source are for the cobbles. These data also indicate that Proterozoic rocks that occur in the southern Superior Provence were entrained by the south-flowing Laurentide ice sheet.