BARABOO INTERVAL QUARTZITES: THE DOTT LEGACY AND NEW REVELATIONS

Proterozoic quartzites are a prominent and significant feature in the Precambrian framework of North America. The principal characteristics of these quartzites were documented by Bob Dott and Ian Dalziel in 1970 and 1972, and in 1983 Dott defined the Baraboo Interval as including quartzites in the southern Lake Superior region that were deposited between 1.75 Ga, the age of the youngest underlying basement, and 1.45 Ga, the age of intrusive granites. The quartzites are supermature in composition, being devoid of detrital feldspar, and associated pelites are among the most chemically mature in the geologic record, having a Chemical Index of Alteration of 97.6 to 99.6. Such chemical maturity originated from intense weathering of sub-quartzite basement, from which feldspar was completely removed under the influence of a warm, humid climate in a region of low topographic relief. Quartzite detritus was derived from the proximal Yavapai terrane and more distal, northerly Penokean and Algoman terranes, and relative probability plots for detrital zircons in the quartzites accordingly display strong geon 17, 18, and 25–27 signals. However, quartzite from the Michels Quarry in the Waterloo Quartzite yields detrital zircons with a strong geon 16 (Mazatzal) signal, in addition to older populations. The youngest of five samples gives a Maximum Age of Deposition of 1643 ± 11 Ma (n = 42) defined by the youngest statistically homogeneous population (MSWD ≤ 1.0). Until now, folding of the quartzites was thought to have occurred at ~1.63 Ga during the Mazatzal Orogeny, based on ⁴⁰Ar/³⁹Ar cooling ages and Rb-Sr isochrons for the basement. However, the occurrence of geon 16 detrital zircons demonstrates that Baraboo Interval quartzites were deposited after the Mazatzal Orogeny and thus could not have been folded at that time. Muscovite parallel to the axial plane of folding in the Baraboo Range yields a ⁴⁰Ar/³⁹Ar cooling age of 1.46 to 1.48 Ga, and muscovite decorating crenulation cleavage in Waterloo metapelite yields an age of 1.45 Ga. Such ages are consistent with Baraboo Interval quartzites having been folded during the 1.45 Ga transcontinental (Picuris-Wolf River-Pinwarian) tectonomagmatic event.