Fig. 2. Summary map of localities dated in this study.
margin of Laurentia Holm et al., 1998b, com- patible with models for a long-lived orogen along
southern Laurentia Karlstrom et al., 1999.
2. Geologic setting
The Penokean and Trans-Hudson orogenies represent the rapid aggregation of Archean conti-
nents that formed the bulk of Laurentia at 1900 – 1800
Ma Hoffman,
1989. Continued
growth of Laurentia occurred by accretion of juvenile crust along its southern margin, forming
the Transcontinental Proterozoic provinces. These provinces, which span the North American conti-
nent from southern California to Labrador, con- sist of an 1800 – 1700 Ma inner accretionary belt
and a 1700 – 1600 Ma outer tectonic belt. These provinces are depicted in Fig. 1, which also shows
the region of known pre- 1700 Ma rocks meta- morphosed and deformed during the formation of
the outer tectonic belt ca. 1650 Ma; Van Schmus et al., 1993; Holm et al., 1998b.
The 1870 – 1820 Ma Penokean orogeny in Wis- consin represents an island-arccontinent collision,
which deformed and metamorphosed Archean and Paleoproterozoic rocks of the Lake Superior
region. In this region Fig. 2, the orogenic belt consists of a northern deformed continental mar-
gin assemblage overlying an Archean basement separated from a southern assemblage of oceanic
arcs the Wisconsin magmatic terranes, WMT by the ca. 1860 Ma Niagara fault zone NFZ. In
central Wisconsin, the arc rocks are separated from Archean basement of the Marshfield terrane
by the steeply north-dipping ca. 1840 Ma Eau Pleine shear zone EPSZ, Fig. 2. Where observed
in outcrops, the volcanic rocks are of upper green- schist – amphibolite grade Sims and Peterman,
1980 and are crosscut by large intrusions which are associated with the main phase of the
Penokean orogeny Van Schmus et al., 1975; Van Schmus, 1976.
The Penokean orogeny was followed by a pe- riod of widespread magmatism at 1760 Ma
that included rhyolitic volcanism in central and southern
Wisconsin and
granitic plutonism
throughout northern Wisconsin and east – central Minnesota Van Schmus, 1980. Based on mica
compositions, Anderson et al. 1980 concluded that the 1760 Ma granites in the northern
WMT Fig. 2 were emplaced at depths of 10 – 11 km. They were subsequently unroofed and
depositionally overlain by 1750 – 1630 Ma cra- tonic quartzites Dott, 1983; Holm et al., 1998b
some of which are deformed and metamorphosed to lower greenschist facies 320 – 390°C; Medaris
et al., 1998.
3. Previous thermochronology
