6. Implications
In interpreting the ages in the context of re- gional cooling and thermal history, closure tem-
peratures for biotite, muscovite, and hornblende are assumed to be 300, 350, and 500 9 50°C,
respectively McDougall and Harrison, 1999.
6
.
1
. Mica thermochronology In the southern Lake Superior region, the ther-
mal and deformational front identified by Holm et al. 1998b separates basement rocks with pre-
dominantly primary post-Penokean cooling ages to the north from rocks with thermally reset i.e.
1100 – 1650 Ma RbSr and
40
Ar
39
Ar mica ages to the south Fig. 2. The large scatter of the RbSr
biotite dates has long been considered problem- atic Peterman and Sims, 1988 and requires bet-
ter assessment of Mesoproterozoic overprinting effects related to intrusion of the 1470 Ma Wolf
river batholith and to 1100 Ma midcontinent rift activity. The 1170 9 4 Ma biotite date obtained in
this study is from a sample that has clearly been affected by the midcontinent rifting event. This
sample is located approximately 140 km south- west of the Goodman Swell and is surrounded by
rocks yielding mineral ages as old as 2500 Ma Fig. 2. This suggests that localized midcontinent
rift resetting occurred well outside of the area of the Goodman Swell. The biotite date of 1357 9 5
Ma Fig. 3 is interpreted to represent partial resetting due to intrusion of midcontinent rift
dikes.
All of the samples dated in this study occur well away \ 50 km from the exposed western margin
of the Wolf river batholith Fig. 2. This, plus the fact that mica ages south of the deformational
front cluster around 1600 Ma i.e. they do not ‘young’ toward the batholith, suggests that this
area has not been thermally affected by the Wolf river batholith. Using the MacArgon computer
program of Lister and Baldwin 1996, Loofboro and Holm 1998 modeled the effects of various
thermal histories in an attempt to evaluate the possible influence of Wolf river batholith reheat-
ing on mica age data from western Wisconsin. Several short duration 2 – 4 million year thermal
spikes between 200 and 450°C were imposed at 1470 Ma to simulate intrusion of the batholith.
The initial conditions were chosen to reflect cool- ing through 350 – 300°C at 1760 – 1750 Ma and
final conditions reflect exposure by Cambrian time.
Loofboro and Holm 1998 concluded that in- trusion of the Wolf river batholith could not have
caused the cluster of 1600 Ma mica dates in western Wisconsin by partial resetting of 1760 –
1750 mica cooling ages. Because of the difference in closure temperature between biotite and mus-
covite, the modeling revealed that significant dif- ferences in the degree of partial resetting and
hence apparent ages obtained are expected for imposed 1470 Ma thermal pulses between 300 and
450°C. For instance, a short duration 350°C ther- mal pulse imposed at 1470 Ma would partially
reset biotite to a ca. 1600 Ma age, but would only reset muscovite to about 1725 Ma. Similarly, a
short duration 400°C thermal pulse at 1470 Ma, which would partially reset muscovite to ca. 1630
Ma, would also cause complete resetting of biotite to 1470 Ma. Because our cluster of ca. 1600
Ma ages include both muscovite and biotite Fig. 2, the modeling results indicate that intrusion of
the Wolf river batholith was not responsible for generating these ages by partial resetting of mica
argon systematics.
Mica ages between 1576 and 1614 Ma are interpreted to represent complete resetting of mica
argon systematics during the long-established but ‘enigmatic’ 1630 Ma event. Deformation of the
Paleoproterozoic post-Penokean quartzites, which are correlatable with undeformed quartzite bod-
ies, has recently been interpreted as a result of Mazatzal orogenic activity at 1650 Ma during
the assembly of southern Laurentia Holm et al., 1998b. This activity is likely the cause for the
thermal disturbance affecting the mica samples. Mica
40
Ar
39
Ar ages of 1760 – 1750 Ma from bedrock sampled beneath undeformed quartzites
are interpreted as representing the time of initial crustal
stabilization and
cooling after
the Penokean orogeny. These older mica ages were
unaffected by Mazatzal orogenic activity as sug- gested by their location north of the thermalde-
formational front Fig. 2.
6
.
2
. Hornblende thermochronology A hornblende plateau date of 1638 9 5 Ma is
interpreted to represent complete resetting caused by the 1650 Ma activity noted above. It was
reported two similar
40
Ar
39
Ar hornblende dates from northeastern Wisconsin and northern Michi-
gan were interpreted as representing examples of complete, albeit localized, resetting of high-tem-
perature minerals in association with Mazatzal- age deformation, perhaps caused by fluid-related
activity. Such localized hydrothermal, fluid-re- lated resetting has been documented in other Pre-
cambrian rocks such as the Elat area of southern Israel Heimann et al., 1995.
Six hornblende samples in this study yield ap- parent ages which scatter over a 70 million year
interval between 1723 and 1796 Ma. The spectra are admittedly complex and the scatter in the data
are difficult to interpret. In east – central Minne- sota, where Penokean rocks of 5 – 6 kbar paleo-
pressures are exposed, hornblende ArAr ages are uniformly 1760 Ma and indicate crustal stabi-
lization Holm et al., 1998a. In the lower-grade region of Wisconsin, Penokean hornblende ArAr
ages are preserved because overall less unroofing occurred there during the 1760 Ma stabiliza-
tion event rocks with paleopressures of 2 – 4 kbar are predominant; Geiger and Guidotti, 1989.
Given that crustal stabilization in northern Wis- consin involved only isolated plutonism and
lower-temperature cooling, it is unlikely to be responsible for the scatter of post-Penokean horn-
blende ages to as young as 1723 Ma. We suggest instead that the hornblende ages reflect variable
retention of radiogenic argon associated with episodic loss some time after initial closure during
Penokean time 1870 – 1820 Ma. Given the evi- dence for widespread Mazatzal-age resetting
based on the mica dates described above, the scatter in the 1723 – 1796 Ma hornblende dates
can be interpreted to reflect varying degrees of partial
resetting due
to Mazatzal
orogenic activity.
An increasing number of thermochronologic studies of Proterozoic rocks in New Mexico
Thompson et al., 1996; Karlstrom et al., 1997 and Colorado Shaw et al., 1999 document per-
vasive Mesoproterozoic metamorphism followed by a protracted upliftcooling history. The results
of those studies differ considerably from the ther- mochronologic results obtained from western
Wisconsin. Those studies yield numerous horn- blende and mica dates in the 1500 – 1300 Ma
interval from rocks collected both near and far from similar age midcrustal plutons. The absence
of Mesoproterozoic hornblende cooling ages and the cluster of 1600 Ma mica dates in western
Wisconsin suggests that this area has remained below 300°C since the end of the Paleoproterozoic
ca. 1600 Ma. We suggest that a combination of elevated temperatures 350 – 500°C and localized
areas of enhanced fluid activity associated with Mazatzal deformation provide the simplest expla-
nation for preservation of the scattered Penokean, intermediate, and Mazatzal hornblende dates of
the southern Lake Superior region.
Finally, the southernmost hornblende analyzed gives the oldest date, but the discordant spectrum
is complex and ambiguous. It gives a near-plateau date of 2503 9 18 Ma for the highest temperature
fractions that may have age significance. This area, part of the Archean Marshfield terrane
which collided with the magmatic arc rocks to- ward the end of the Penokean orogeny, was possi-
bly beyond that which was significantly affected by the Penokean orogeny to the north. A low-
temperature increment of Penokean age on this sample suggests that this area was only partially
reset during the Penokean orogeny. Surprisingly, the area must also have escaped the effects of
fluid-inducedmoderate-temperature
reheating during the Mazatzal orogeny. This 2503 Ma date
may reflect an upper amphibolite facies metamor- phic event that Cummings 1984 recognized in
the Big falls area near Little falls in Fig. 2 at approximately this time.
6
.
3
. Microtextural studies Petrographic study of dated samples from three
localities Jim fall, Little falls, and Cornell; Fig. 2 focused on microtextural indicators of ductile de-
formation in both quartz and feldspars Perham, 1992; Romano, 1999. Samples from Jim falls and
Little falls exhibit both high- and intermediate-
temperature deformational features. High-temper- ature features \ 450 – 500°C of recrystallized pla-
gioclase and quartz with very few signs of strain were present in one of the Jim falls sections and
two of the Little falls sections. Hornblende from Jim falls yields a Penokean date 1853 Ma whereas
hornblende from Little Falls yields a much younger date 1638 9 5 Ma, interpreted as repre-
senting high-temperature i.e. 500°C total reset- ting
due to
Mazatzal orogenic
activity. A
hornblende sample from Cornell gives an appar- ently partially reset date of 1733 9 6 Ma. The
microstructures in Cornell samples indicate that the temperature of deformation was approximately
450 – 500°C plagioclase ductilely deformed. Mi- crostructures from all areas show at least some
ductile deformation in plagioclase whereas horn- blende dates from these samples yield highly dis-
cordant dates. This suggests that there is not a link between the resetting of hornblende and mi-
crostructural features that imply high-temperature deformation. Instead, the microstructures appear
to dominantly record earlier high-temperature events on which partial argon loss was subse-
quently superimposed.
7. Temperature-time reconstruction
Kategori