RECOGNITION OF WOLF RIVER-AGE METAMORPHISM IN THE BARABOO RANGE BY MEANS OF 40AR/39AR THERMOCHRONOLOGY

It has been argued convincingly that the Baraboo Quartzite was folded at ~1630 Ma (Holm et al., 1998, Geology, v. 26, p. 907-910), and the present investigation, using the laser-heating ⁴⁰Ar/³⁹Ar technique, was undertaken to test for such an age in metamorphic assemblages from the Baraboo Range. Surprisingly, there is little evidence for an age of 1630 Ma; among five analyzed samples all yield plateau ages significantly younger than 1630 Ma, and three are comparable to that of Wolf River magmatism (~1470 Ma).

Muscovite, although absent from the supermature quartzite, is abundant at the base of the quartzite section in hydrothermally altered paleosol and in muscovite-pyrophyllite-diaspore veins. Age spectra for muscovite in samples of metapaleosol and vein are discordant; apparent ages rise from 900 Ma to well-defined plateaux of 1457±6 Ma and 1444±10 Ma, respectively. Nonconformably beneath the quartzite are ~1750 Ma rhyolite, granite, and diorite, largely recrystallized to greenschist facies assemblages, probably at 1630 Ma. Two samples of composite amphibole from diorite yield plateau ages of ~1580±15 and 1420±15 Ma, whereas microcline from granite is 889±8 Ma.

Although the closest outcrop of Wolf River-type granite lies 30 miles east of Baraboo, the Wolf River magmatic event was continental in extent, providing a viable thermal source for promoting large-scale fluid flow in the crust. Such fluid flow was probably responsible for hydrothermal alteration along the nonconformity at the base of the Baraboo Quartzite and for partial to complete isotopic resetting of minerals in the underlying basement. Cooling of these minerals from the closure temperature of muscovite to below that of microcline may either have been very slow, i.e. 0.4 °C/m.y., which is improbable for such shallow rocks, or interrupted by a cryptic thermal event.