ARCHEAN PROVENANCE OF GRANITE COBBLES IN WOODFORDIAN TILLS IN CENTRAL ILLINOIS

The glaciated landscape of much of Illinois is a product of the most recent glacial episode, which is characterized by a series of end moraines composed predominantly of till and deposited by ice flowing into Illinois from the east and northeast around 20,000 years before present time. To better understand till provenance and sediment recycling in central Illinois, a study was completed using U-Pb dating of zircon grains in phaneritic quartzofeldspathic cobbles in tills from two moraines in McLean County. Samples were collected from six sites, three sites each from both the Bloomington and the Normal moraines, and then crushed with usual techniques. In an attempt to avoid zircon fertility bias, only one zircon was picked from each individual clast. These zircons were then analyzed for U-Pb isotope ratios at the University of Arizona LaserChron Center by laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS), and isotope ratios were used to calculate zircon crystallization ages.

The Bloomington Moraine localities yielded 96 ages, all of which were Archean, with the peak age being 2704 Ma. The Normal Moraine localities yielded 76 ages, all but three of which are Archean, with the peak age being 2714 Ma. The three non-Archean grains ranged in age from 1719-1786 Ma. The similarity of clast provenance age, and the domination of only Archean cobbles is astonishing. The absence of non-Archean ages indicate that these clasts were most likely originally derived from basement rocks in southern Ontario east of the Lake Superior Rift and west of the Grenville front. Discordant zircons have a lower Concordia intercept at about 1100 Ma, which may suggest a proximity to the Grenville Front.