Origin of Lake Superior Region Early Paleozoic Super-Mature Quartz Arenites: Evidence from U-Pb Detrital Zircon Ages

Cambrian-Ordovician super-mature quartz arenites of the Lake Superior region are largely considered a result of recycling on the basis of sedimentologic and mineralogic evidence. The goal of this study is to investigate the recycling model using evidence from detrital zircon ages.

Zircon grains of the Franconia Formation, Jordan Sandstone, basal sandstones of the Oneota Dolomite, and St. Peter Sandstone average 100 - 400 microns in length and typically display compositional zoning. 207Pb/206Pb ages (determined by LA-ICP-MS) of detrital zircons range from 3,200 to 950 Ma. Several zircon age populations are identified: (1) an Archean population (3,000 - 2,600 Ma) is present in all arenites; (2) a less prominent Middle Proterozoic population (1,500 - 1,350 Ma) is present in all arenites except those of the lower St. Peter Sandstone; and (3) a younger Middle Proterozoic population (1,300 - 900 Ma) is present in all arenites except those of the Franconia Formation. Most zircon age groups correlate well with the ages of nearby geologic provinces (e.g., Minnesota River Valley, Superior Province, and anorogenic suites), however those from the younger Middle Proterozoic population were likely derived from more distal sources (e.g., Grenville).

The presence of similar zircon populations in Early Paleozoic arenites suggests that their sediment sources remained relatively constant. However, several observations suggest that although some recycling might have occurred, multiple sources must have been involved: (1) the relative proportions of the zircon age populations vary significantly in each of the arenites, (2) Middle Proterozoic zircon populations are absent, or nearly absent in two of the arenites, and (3) the detrital zircon populations of the Early Paleozoic arenites differ significantly from those of the underlying Late Proterozoic sandstones (e.g., Hinckley and Fond du Lac). These observations suggest variable inputs of sediment from several sources that remained stable throughout Early Paleozoic deposition.