40Ar/39Ar DATING OF GEON 14 K-METASOMATISM AND HYDROTHERMAL ALTERATION IN THE SOUTHERN LAKE SUPERIOR REGION

The 1.47-1.48 Ga Wolf River Batholith, which is part of a transcontinental A-type granitic igneous event, has long been recognized as a major geologic feature in the Precambrian basement of Wisconsin (Van Schmus et al., 1975, GSA Bull., v. 96, 907-914). In recent years, hydrothermal alteration related to geon 14 magmatism has been identified in Statherian Baraboo Interval sedimentary rocks over a wide area in the southern Lake Superior region.

Baraboo Interval sedimentary rocks consist predominantly of supermature siliciclastics (CIA = 96.8 - 98.6), which contain various proportions of quartz, kaolin or pyrophyllite, and hematite. Muscovite occurs only locally, where it has crystallized due to the introduction of K by hydrothermal fluids.

In the Baraboo Range, ⁴⁰Ar/³⁹Ar plateau ages for muscovite are 1456 ± 11 Ma (metasaprolite), 1467 ± 11 Ma (ms-prl-dsp veins in quartzite), 1459 ± 3 Ma (quartzite breccia), and 1460 ± 3, 1470 ± 3, 1481 ± 3, and 1484 ± 3 Ma (Seeley Slate). Muscovite in the Seeley Slate, which is oriented parallel to axial plane cleavage, constrains the timing of either the folding in the Baraboo syncline or the growth of muscovite along a pre-existing cleavage.

Muscovite in Waterloo Quartzite yields 1452 ± 7 Ma, and in Seven Sisters argillite, 1470 ± 11 Ma. Two samples of Sioux pipestone (ms-prl-dsp) yield 1280 ± 13 Ma and 1370 ± 10 Ma, with these younger ages being attributed to Ar loss from extremely fine-grained muscovite. Farther afield, diagenetic illite in Statherian strata in the Athabasca basin formed at ~1480 Ma.

It appears that geon 14 hydrothermal fluid flow occurred along permeable channels, such as unconformities, on a scale of hundreds of kilometers in the southern Lake Superior region, and beyond. Although this observation was at first surprising, it is less so when considered in the context of the transcontinental emplacement of geon 14 A-type granitic plutons. Such a continental-scale igneous event likely produced significant topographic relief along the belt of pluton emplacement and regional heating, which promoted extensive geon 14 fluid migration in North America.