THE PALEOPROTEROZOIC EVOLUTION OF THE BARABOO RANGE, WISCONSIN: 300 MILLION YEARS OF SUCCESSIVE MAGMATISM, CRATONIC STABILIZATION, SEDIMENTATION, DEFORMATION AND K-METASOMATISM

The Baraboo Range is exceptional in preserving evidence for four major Paleoproterozoic events, including 1750 Ma Yavapai magmatism, geon 17 cratonic stabilization and sedimentation, 1630 Ma Mazatzal deformation and metamorphism, and geon 14 hydrothermal metasomatism. The geologic history of the Baraboo Range begins at 1750 Ma with emplacement of diorite, granite, and rhyolite of the Montello Batholith, which is the local magmatic expression of the Yavapai Orogeny in the Lake Superior region. Fluvial pebbly quartz arenite at the base of the Baraboo Quartzite is nonconformable on the underlying igneous basement and contains geon 17, and older, detrital zircons. Located beneath the nonconformity in the igneous basement is an eight meter‒thick mature paleosol, from which plagioclase and K‒feldspar were completely removed, accounting for the absence of feldspar and extreme chemical maturity (CIA = 99) of the overlying quartz arenites, siltstones, and mudrocks. The Baraboo Quartzite is correlative with other supermature quartzites across North America that signify the attainment of widespread cratonic stability at ~1700 Ma.

Although folding and metamorphism of the Baraboo Quartzite has not been dated directly, it is thought to result from the Mazatzal Orogeny. This is based on ⁴⁰Ar/³⁹Ar cooling ages of ~1630 Ma for mica and amphibole in geon 17 and older, surrounding regional basement. The metamorphic mineral assemblage, Qtz + Prl, restricts the temperature of recrystallization (and folding) to between 325 ºC and 375 ºC.

Muscovite is abundant in Seeley Slate overlying the quartzite, but rare in the supermature Baraboo Quartzite, where it occurs only in metapaleosol beneath the quartzite and associated with brittle deformation in anastamosing veins at the base of the quartzite (Ms + Prl + Dsp; T = 325‒375 ºC) and in quartzite breccia high in the quartzite (Ms + Qtz + Kln; T < 300 ºC). The mean ⁴⁰Ar/³⁹Ar age for seven samples of muscovite from all four locations is 1468 ± 11 Ma, which establishes the age of K‒metasomatism responsible for muscovite crystallization. This metasomatism is the local product of a continental‒scale flood of K‒metasomatism related to geon 14 emplacement of granitic rocks across North America, including the Wolf River Batholith of east‒central Wisconsin.