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
Mica and amphibole from a total of 21 samples were analyzed including, ten hornblende; eight
biotite; and three muscovite separates. Numbered
sample localities and corresponding dates are plotted on Fig. 2; thin-section descriptions are
available in Romano 1999. Incremental-heating
40
Ar
39
Ar age results are illustrated in normal age-spectra diagrams, Fig. 3 for micas sample
numbers 11 – 21 and Fig. 4 for hornblende num- bers 1 – 10 where age scales are expanded to show
the details. Isotope correlation analyses do not provide any additional information because the
percentages of total
40
Ar that is radiogenic is quite high, generally \ 99.
Both micas and hornblende separates generally give variably discordant spectra; the hornblende
discordance is more severe compared with the micas where it is generally not pronounced. The
age discordance in the spectra is highly correlated with KCa and KCl ratios that indicate mineral
heterogeneities Fig. 4. In particular, the horn- blende spectra are compromised by unavoidable,
higher-K mineral phases present as inclusions, intergrowths, and alterations. The low apparent
ages in the spectra of most hornblendes are corre- lated with KCa and KCl ratios, which are much
higher than those for hornblende and are ob- served for lower temperature increments. These
results are consistent with the hornblende discor-
Fig. 3.
40
Ar
39
Ar spectra for mica separates. Sample numbers in the upper left corner of the age panels are keyed to locations in Fig. 2. The plateau, if observed, is shown by the double arrowed line; t
tg
is the total gas age, and t
p
is the plateau age where quotation marks indicate only a ‘near-plateau’. Width of the apparent-age patterns are 9 1 sigma uncertainties.
Fig. 4.
40
Ar
39
Ar spectra for hornblende separates. Notation as explained in Fig. 3.
dance due mainly to K-bearing impurities, sheet silicates andor feldspar. The mineral phases are
expected to have younger ages that reflect their lower closure temperatures. In sum, the discor-
dance, particularly for hornblende, is interpreted to reflect such mineralogic heterogeneities, not
argon gradients within crystal domains.
In large part due to the effects of mineralogic heterogeneities it is important to consider the
chemical signatures, KCa and KCl, in interpret- ing and accounting for the age variations in the
hornblende spectra. Plateaus are constructed when the KCa ratios have reduced to a relatively
low, constant, and appropriate level. Not all sepa- rates yield ‘plateaus’, which are defined as con-
tiguous gas fractions constituting a majority of the Ar where the variations of ages of individual
increments may be attributed to analytical uncer- tainties. In some cases, strict plateaus are not
found because, the proportion of Ar is not a majority of the total Ar released i.e. the plateau
is narrow; or, the variations in apparent age are somewhat more than expected from measurement
uncertainties. For these cases, the apparent ages are interpreted to be significant and are termed
‘near plateau’ dates. Where the dispersion among fractions exceeds analytical uncertainty by a large
degree, no plateau is defined.
5
.
1
. Mica ages Five biotite separates yield virtually identical
ages between 1576 and 1605 Ma. Plateau ages of 1576 9 4, 1579 9 5, 1581 9 4 and 1582 9 4 Ma
were obtained from samples c 13 – 16 Fig. 3. Sample c 17 gives a similar total-gas age; a
near-plateau date, constituting 89 of the
39
Ar released, of 1605 9 7 Ma may be defined by
omitting the first two low-age fractions. One biotite c 18 from the undeformed
1765 Ma Radisson granite UPb zircon age re- ported in Sims et al., 1989 yields a near-plateau
date of 1753 9 6 Ma. This date is nearly concor- dant with the crystallization age of the granite,
suggesting it cooled rapidly after intruding. Two biotite
separates c 11
and c
12 from
Penokean syntectonic granites yielded anoma- lously young ages of 1170 9 4 Ma plateau and
1357 9 5 Ma near-plateau. Two of the three muscovite separates give
plateaus with release patterns showing only very minor discordance. Muscovite from a pegmatite
at Little falls c 20 and the Flambeau mine c 21 give ages of 1614 9 5 and 1759 9 5 Ma,
respectively. A third muscovite c 19 gives a total-gas date of 1518 Ma but a highly discor-
dant spectrum, the saddle-shape of this spectrum Fig. 3 may indicate excess argon as is fre-
quently the case, and we therefore attribute no geological significance to the 1518 Ma total-gas
age of this sample.
5
.
2
. Hornblende ages Seven of the ten hornblende separates give
plateau or near-plateau dates Fig. 4. Surpris- ingly, most of the hornblende separates analyzed
yield ages B 1800 Ma, significantly younger than two of the samples, which give typical Penokean
ages.
A hornblende separate c 1 from a sample of amphibolite collected at Little falls yields a
plateau date of 1638 9 5 Ma. Six other horn- blende separates give dates between 1723 and
1796 Ma. The youngest of these c 3, from a sample of mafic gneiss, gives a total-gas date of
1723 Ma; the spectrum is discordant but six in- crements, representing the majority of the gas
73, average at 1700 Ma. Hornblende c 4 from a sample of mafic tonalite yields a
discordant spectrum but with a narrow near- plateau at 1745 Ma.
Hornblende from a garnet amphibolite c 2 collected in Cornell yields a plateau date of
1733 9 6 Ma. Hornblende separate c 5 from amphibolitic gneiss from Neillsville yielded a
near-plateau date of 1777 9 9 Ma; low tempera- ture fractions are as low as 1320 Ma while a
plateau is reached for the higher-temperature fractions, which make up 36 of the
39
Ar re- leased, when KCa reaches a minimum.
Sample c 6 from amphibolite from along the north fork of the Eau Claire river yields a broad
plateau at 1782 9 7 Ma. This amphibolite is cut by Penokean dikes 1850 Ma, Van Wyck,
1995 but an exact age is not known. Lastly, a hornblende separate c 7 from an amphibolite
from Neillsville quarry gave a near-plateau date of 1796 9 20 Ma.
Two hornblende
separates yield
typical Penokean dates. The younger of the two c 8 is
from an Archean mafic gneiss collected at the north end of Lake Arbutus. The results yield a
plateau at 1830 9 7 Ma. Hornblende from am- phibolite collected in Jim falls c 9 yields a
total-gas date of 1853 Ma but the spectrum is highly discordant.
The southernmost hornblende is from an Archean fine-grained amphibolite sampled from
the south end of Lake Arbutus c 10. It yields a highly complex spectrum with initially high
ages followed by decreasing and then increasing ages culminating in a narrow near-plateau at
2503 9 18 Ma. The significance of this age and the interpretation of the spectrum in not fully
clear. The overall shape of the age spectrum and its youngest early increment age of 1870 Ma may
indicate partial resetting during the Penokean orogeny of late Archean hornblende. Excess
40
Ar that is indicated by very high ages of the early
fractions is also a possibility, in which case the 1870 Ma minimum could be interpreted as a
maximum age.
6. Implications
