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
New mica and hornblende
40
Ar
39
Ar ther- mochronologic data from the southern Lake Supe-
rior region provide important information about the timing, extent, and nature of metamorphic
overprinting of the Penokean orogenic belt. Previ- ous RbSr biotite studies in the area have suggested
that it experienced a widespread, low-temperature thermal disturbance at 1630 Ma. The results of
this study indicate that the thermal overprinting pulse may have been high enough to cause partial
to locally complete resetting of hornblende argon systematics.
Lower closure-temperature
micas from the southern deformed portion of the area
covered by this study were completely reset during this event. This is consistent with a recent petro-
logic investigation of the deformed Baraboo quartzite Fig. 1 which indicates metamorphic
temperatures of 320 – 390°C Medaris et al., 1998. It is suggested that the deformation and metamor-
phism of the Baraboo quartzite occurred during widespread Mazatzal-related overprinting of the
Penokean orogenic belt, and that the cluster of ca. 1600 Ma mica
40
Ar
39
Ar ages date the cooling phase of this Mazatzal regional metamorphism.
Fig. 5. Proposed time – temperature curve for the area south of the Mazatzal thermal front. Open circles represent mica ages south of the front. Filled circles represent mica ages north of the front. Squares represent hornblende ages all located south of the front.
Fig. 6. Proposed post-Penokean tectonic evolution of central and northern Wisconsin. See text for detailed description.
Patterning follows that of Fig. 2.
Hornblende between 1720 and 1800 Ma indicate that the Mazatzal orogeny produced
enough heat in most areas south of the thermal deformational front to reset these minerals par-
tially. One hornblende date of 1638 Ma from Little falls suggests that enough heat was pro-
duced locally, probably by the infiltration of hot fluids, to completely reset the mineral. The re-
mainder of the path shows thermal restabilization through mica closure temperatures after the
Mazatzal orogeny.
8. Post-Penokean tectonic and crustal evolution
