Abstract

Lower Paleozoic supermature quartz arenites in Wisconsin and Michigan were derived from several Proterozoic and Archean terranes. Single-grain, detrital zircon populations from the Galesville and St. Peter Sandstones in Wisconsin yield very similar, closely concordant age distributions (Cambrian Galesville Sandstone: 1.1 Ga [n = 4], 1.4 Ga [n = 2], 1.8 Ga [n = 1], and 2.7 Ga [n = 2]; Ordovician St. Peter Sandstone: 1.1 Ga [n = 2], 1.8 Ga [n = 1], and 2.7 Ga [n = 6]). In contrast, most of the nine detrital zircons that were analyzed from the St. Peter Sandstone in the Michigan basin have high U contents (450–2500 ppm) and are strongly discordant (60%–90%). Six zircons from the Michigan basin that have ca. 1.0 Ga 207Pb*/206Pb* ages define a regression line that has an upper intercept of about 1100 Ma and a lower intercept of 15 Ma; one zircon has a 2.7 Ga 207Pb*/206Pb* age. The zircon data indicate that although a number of different terranes contributed detrital material to the Paleozoic quartz arenites in Wisconsin, 1.1 and 2.7 Ga terranes were the dominant sources, and not the local basement, which primarily consists of the ≥ 2.7 Ga Marshfield terrane, the 1.8 Ga Penokean orogen, and the 1.4 Ga Wolf River batholith. A terrane that has a 1.1 Ga age is probably the main source for the St. Peter Sandstone in the Michigan basin.

Quartz separates were also analyzed for Pb-Pb and Sm-Nd isotope variations, and the data do not indicate significant source differences between the heavy mineral fraction (zircons) and the quartz framework grains. Pb-Pb isochrons and Sm-Nd isotope data for quartz separates reflect mixing of the age groups and approximate relative proportions that are identified from the single zircon results.

All isotope data on quartz separates and U-Pb zircon data indicate that the detrital constituents (zircons and quartz framework grains) that compose the lower Paleozoic quartz arenites in Wisconsin were primarily derived from the 2.7 Ga granite-greenstone terrane of the southern Superior Province and a 1.1 Ga terrane. The latter terrane is either the silicic volcanic rocks associated with the Midcontinent rift system, or, more likely, the voluminous granitic rocks that are associated with the Grenville Province on the eastern margin of North America. The Middle Proterozoic Grenville Province was the most important ultimate source of quartz and zircons to the St. Peter Sandstone in the Michigan basin; lesser amounts of material were contributed from the Archean Superior Province.

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