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