GEON 14 MAGMATISM, DEFORMATION, AND METASOMATISM IN THE SOUTHERN LAKE SUPERIOR REGION: DEFINING THE BARABOO OROGENY

It is now known that the 1476–1470 Ma Wolf River batholith in northeastern Wisconsin was not an isolated igneous phenomenon, but was accompanied by geon 14 regional deformation, metamorphism, and K–metasomatism, features that are most clearly recorded by Baraboo Interval sedimentary rocks in the southern Lake Superior region. These sediments were deposited after the geon 16 Mazatzal orogeny and are among the most chemically mature in the geological record; sandstones and siltstones are devoid of detrital feldspar, and mudstones contain kaolinite (or its metamorphic equivalent, pyrophyllite), rather than detrital muscovite. Such chemical maturity is due to complete removal by weathering of feldspar and mica from mature paleosols beneath Baraboo Interval sediments.

In slate overlying folded quartzite in the Baraboo Range, metasomatic muscovite aligned parallel to axial–plane cleavage yields ⁴⁰Ar/³⁹Ar cooling ages of 1496, 1493, 1483, and 1473 ± 3 Ma. Elsewhere in the Baraboo Range, Ar ages for metasomatic muscovite are 1469 ± 23 Ma in paleosol beneath quartzite, 1480 ± 22 Ma in hydrothermal veins low in the quartzite section, and 1472 ± 3 Ma in a quartz breccia zone high in the section. The mass flux of K₂O introduced during geon 14 metasomatism in the 8–m thick Baraboo paleosol is 0.89 mols cm^-2, whereas the mass flux of K₂O in five other Paleoproterozoic paleosols in the region ranges from 1.75 to 0.30 mols cm^-2. For all six paleosols the mass flux of K₂O is highly correlated with paleosol thickness:

K₂O_flux (mols cm^-2) = 0.16 × thickness (m) – 0.42; R² = 0.95

suggesting that the specific geochemical process responsible for K–metasomatism in the Baraboo paleosol was similar to that during earlier episodes of Precambrian fluid–rock interaction along unconformities.

Because geon 14 deformation, metamorphism, and metasomatism in the region is best documented by Baraboo Interval sedimentary rocks, especially in the Baraboo Range, we propose that this geon 14 tectonomagmatic event be called the “Baraboo orogeny”, rather than the previous, informal term “Wolf River orogeny”. The Baraboo orogeny provides a midcontinental link between the northeastern Pinware orogeny and the southwestern Picuris orogeny, and completes the transcontinental extent of geon 14 orogenesis for 5000 km along the southern margin of Laurentia.