Gneiss and migmatite of Archean age in the Precambrian basement of central Wisconsin

Many of the southernmost exposures of the Precambrian shield in central Wisconsin consist of gneiss, migmatite, and (or) amphibolite with associated younger intrusive tonalite and granite. Whole-rock Rb-Sr analyses and zircon U-Pb analyses show that the migmatite and gneiss formed about 2.8 b.y. ago. Ages on minerals and younger plutonic rocks indicate that the older gneiss and migmatite were intruded and locally metamorphosed during major events 1.5 to 1.9 b.y. ago.

Rb-Sr data for the migmatite and gneiss indicate elevated initial ⁸⁷Sr/⁸⁶Sr ratios, suggesting that the migmatite and gneiss formed from crustal rocks of intermediate to granitic composition (which were themselves formed more than 3.0 b.y. ago). If this is true, then the gneissic and migmatitic rocks in central Wisconsin may represent an eastward extension of the ancient (≥3.5 b.y. old) Minnesota River Valley terrane.