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
Thermochronology in the southern Lake Supe- rior region has largely relied upon RbSr ages.
Biotite RbSr ages from Wisconsin and northern Michigan range from 1100 to 1750 Ma. In
their compilation of over 90 RbSr biotite dates, Peterman and Sims 1988 recognized a locus of
anomalously young dates 1100 – 1200 Ma in northeast Wisconsin which they named the Good-
man Swell Fig. 2. They interpreted these ages as recording flexural uplift associated with litho-
spheric loading by abundant mafic volcanic rocks along the midcontinent rift axis to the north.
Rb – Sr biotite ages, which increase erratically in all directions away from the Goodman Swell to as
old as 1700 – 1750 Ma in northwestern Wisconsin and northern Michigan, show considerable scatter
overall. The pattern is also somewhat complicated by the 1470 Ma Wolf river batholith.
Holm et al. 1998b proposed that the existing biotite dates of the southern Lake Superior region
could be roughly divided into two domains, a northern domain characterized by ages older than
1700 Ma and a southern domain consisting of ages younger than 1630 Ma. They further
noted that in northwest Wisconsin, the boundary between these domains separates deformed Pale-
oproterozoic quartzites to the south from rela- tively undeformed Paleoproterozoic quartzites to
the north. They proposed that in regions where 1750 – 1630 Ma quartzites are absent or unex-
posed, cooling ages might serve as a proxy for identifying regions of significant thermal and de-
formational overprinting of the Penokean oro- genic belt.
4. Methodology
Fine- and
medium-grained amphibolites,
gneisses, and tonalites were sampled from west and northwest Wisconsin. Mica and hornblende
were separated using standard magnetic tech- niques on the coarsest grains that were not com-
posite usually 60 – 80 mm. Final separation was done by hand picking followed by washing.
The
40
Ar
39
Ar measurements on populations of separated grains were performed in the Radio-
genic Isotopes Laboratory at Ohio State Univer- sity using general procedures that have been
described previously Foland et al., 1993 and ref- erences therein. Aliquots of about 6 – 10 mg for
mica and 80 – 100 mg for hornblende were irradi- ated in the Ford Nuclear Reactor of the Phoenix
Memorial Laboratory at the University of Michi- gan for 100 h. Subsequently, the irradiated
aliquots were heated incrementally by resistance heating in high-vacuum, low-blank furnaces to
successively higher temperatures, with a dwell time of about 40 min at each temperature. These
incremental-heating fractions were analyzed by static gas mass analysis with a nuclide 6-60-SGA
or a MAP 215-50 mass spectrometer, typically in about 12 – 15 or 25 – 30 steps, respectively. The
results are summarized in the Appendix A which provides full detail plus information e.g. K, Ca,
and Cl contents, monitor used and all the ages for the total-gas or integrated and the plateau if
observed fractions. An overall systematic uncer- tainty of 9 1 is assigned to J values to reflect
uncertainty in the absolute age of the monitor. Typically, this uncertainty is not included when
age uncertainties are quoted, in order to empha- size the level of apparent age dispersion among
plateau fractions in terms of internal concordance and to compare plateaus among samples using a
common monitor; however, this uncertainty ap- plies when comparison to other ages is made.
5. Results
