NEW DETRITAL ZIRCON AGE DATA FROM BARABOO INTERVAL QUARTZITES, SOUTHERN WISCONSIN

Baraboo Interval Quartzites were deposited between ~1725 and 1630 Ma across the southern Superior Province. Detrital zircon data constrains their depositional age and provenance; yet most published data sets are atypical due to small number of analyses per sample and/or sample preparation (selecting for zircon size), which may bias results. We present 291 individual detrital zircon LA-MC-ICPMS ages from (1) the basal Baraboo quartzite, Sauk County; (2) the ‘Dake’ quartzite, Sauk County; and (3) a previously undocumented quartzite sampled from the subsurface of Fond du Lac County. These provide sufficient analyses to identify small age-populations, thus improving interpretation of provenance and maximum depositional age.

Archean grains encompass 2515 – 3350 Ma with a continuous span between 2515 – 2750 Ma and scattered peaks after 2880 Ma. Paleoproterozoic grains encompass 1725 – 2485 Ma with a continuous span between 1725 – 2000 Ma and peaks at 1780 and 1850-1890 Ma. Each sample has a distinct detrital zircon signature: Basal Baraboo analyses span 1725 – 1925 Ma with a small scattering of Archean grains. The ‘Dake’ analyses span 1740 – 1900 Ma with a significant scattering of grains > 2000 Ma. The Fond du Lac analyses span 1730 – 2000 and 2480 - 2750 Ma, with scatter between 2100 – 2320 Ma.

The Superior Province was a minor source of grains > 3200 Ma. About 12% of total grains were sourced from the Marshfield Terrane (S. WI; ~2515- 2530, 2740- 3000 Ma). About ~10% of total grains fall between 1900- 2400 and likely have a distal source, perhaps magmatism related to ~2100 Ma continental rifting of the Superior and Wyoming cratons represented in the southern Superior Region by the Marathon Large Igneous Province. The Penokean orogen (1875 – 1835 Ma) sourced ~20% of grains. Post-Penokean magmatism (East-central MI & WI; ~1800, 1775, and 1750 Ma) sourced ~55% of grains. About 1% of grains are <1750 Ma and constrain the quartzite’s maximum depositional age. These young grains are likely distally sourced, as no rocks of this age are known in WI. The samples’ varied zircon signatures reflect the depositional complexity of the Baraboo Interval Quartzites, indicating changes in provenance controlled by changes in depositional environment, sediment recycling, unroofing, and/or distinct drainage systems.