Determining the extent and nature of Mazatzal-related
overprinting of the Penokean orogenic belt in the southern
Lake Superior region, north – central USA
Denise Romano
a,*, Daniel K. Holm
a, K.A. Foland
baDepartment of Geology,Kent State Uni
6ersity,22 McGil6rey Hall,P.O. Box5190,Kent,OH 44242, USA
bDepartment of Geological Sciences,Ohio State Uni
6ersity,Columbus,OH 43210, USA Received 6 October 1999; accepted 28 April 2000
Abstract
Twenty-one hornblende and mica40Ar/39Ar dates from central and northwest Wisconsin, USA, provide important
information on the timing, spatial extent, and intensity of Mazatzal-age metamorphism and deformation which overprinted the Paleoproterozoic (1870 – 1820 Ma) Penokean orogenic belt in the southern Lake Superior region. 1760 – 1750 Ma mica plateau ages from bedrock beneath undeformed 1750 – 1630 Ma quartzites are interpreted as the time of lower temperature (300 – 350°C) cooling and crustal stabilization after the Penokean orogeny. Six mica ages from bedrock underlying deformed Paleoproterozoic quartzites cluster around 1600 Ma (1576 – 1614 Ma). The complete absence of 1760 – 1750 Ma mica ages beneath regions of deformed quartzites suggests widespread heating to temperatures above 300 – 350°C during Mazatzal-related deformation and metamorphism. The1600 Ma mica ages are interpreted to date the cooling phase of this metamorphism. Two anomalously young biotite dates are interpreted to indicate partial resetting associated with Mesoproterozoic rifting at 1100 Ma. Ten hornblende 40Ar/39Ar dates
obtained in this study address the higher-temperature overprinting effects in the southern Lake Superior region. One latest Archean age of 2503918 Ma and two ages of 1853 and 1830 Ma are interpreted as remnant evidence of Archean and Penokean age amphibolite metamorphic events, respectively. The majority of hornblende ages are younger than the Penokean orogeny, scattering between 1796 and 1638 Ma. Microtextural analysis indicates that similar microstructures exist in samples yielding highly discordant hornblende ages. This suggests that shearing and recrystallization did not play an important role in the retention or loss of argon. The 1638 Ma hornblende age is concordant with the Mazatzal orogeny to the south and is interpreted as representing complete thermal or fluid-related resetting associated with that event. Six other post-Penokean ages scatter over a 70 million year interval (1796 – 1723 Ma) and probably reflect variable retention of radiogenic argon. They are interpreted to indicate variable degrees of partial intermediate-temperature (350 – 500°C) resetting of hornblende argon systematics at 1650 – 1630 Ma. Collectively, these data suggest that the effects of the Mazatzal orogeny in the southern Lake Superior region involved 350 – 500°C metamorphism and penetrative deformation. © 2000 Elsevier Science B.V. All rights reserved.
Keywords:Ar/Ar thermochronology; Paleoproterozoic; Post-Penokean overprinting; Crustal stabilization
www.elsevier.com/locate/precamres
* Corresponding author. Fax: +1-216-6727949. E-mail address:d.romanol@juno.com (D. Romano).
1. Introduction
The 1870 – 1820 Ma Penokean orogenic rocks of the North American midcontinent are part of a vast belt of juvenile crust accreted onto the south-ern margin of Laurentia – Baltica during the late Paleoproterozoic (inset, Fig. 1). This belt formed the source region for subsequent generation of
transcontinental Mesoproterozoic crustal-melt
granites, some of the oldest of which are pre-served within the Penokean province. In particu-lar, the voluminous 1470 Ma Wolf river batholith and its associated plutons are little deformed and much of it is apparently minimally eroded (Allen and Hinze, 1992; Holm and Lux, 1998). The tectonic history of the central Penokean province from ca. 1800 – 1400 Ma, therefore, provides in-sight, ultimately, into the cratonization of conti-nental crust.
The purpose of this research has been to con-strain better the timing, nature, and extent of
post-Penokean metamorphism and deformation in the southern Lake Superior region. Northwest Wisconsin is recognized here as a critical region for study of the post-orogenic history of this Paleoproterozoic accretionary orogen. This is the only part of the orogenic belt where undeformed Paleoproterozoic cratonic quartzites are preserved proximal to correlatable, but deformed, post-oro-genic quartzite. This region also contains sparse bedrock thermochronologic age data, although considerable data exist to the east (in northeast Wisconsin) and to the west (in central Minne-sota). Many earlier studies have attributed
post-Penokean metamorphism to an ‘enigmatic,’
low-grade but widespread event at ca. 1630 Ma (Van Schmus and Woolsey, 1975; Van Schmus et al., 1975; Sims and Peterman, 1980; Peterman et al., 1985; Peterman and Sims, 1988). This study provides evidence for an intermediate temperature (350 – 500°C) disturbance at that time and assigns its cause to orogenic activity along the southern
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-arc/continent 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
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 Rb/Sr ages.
Biotite Rb/Sr ages from Wisconsin and northern
Michigan range from 1100 to 1750 Ma. In
their compilation of over 90 Rb/Sr 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.
The40Ar/39Ar 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 91% 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
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 total40Ar 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 K/Ca and K/Cl 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 K/Ca and K/Cl 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. 3.40Ar/39Ar 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;ttg is the total gas age, and tpis the plateau age where
Fig. 4.40Ar/39Ar spectra for hornblende separates. Notation as explained in Fig. 3.
dance due mainly to K-bearing impurities, sheet
silicates and/or 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, K/Ca and K/Cl, in interpret-ing and accountinterpret-ing for the age variations in the hornblende spectra. Plateaus are constructed
when the K/Ca ratios have reduced to a relatively
low, constant, and appropriate level. Not all
5.1. Mica ages
Five biotite separates yield virtually identical ages between 1576 and 1605 Ma. Plateau ages of
157694, 157995, 158194 and 158294 Ma
were obtained from samples c13 – 16 (Fig. 3).
Sample c17 gives a similar total-gas age; a
near-plateau date, constituting 89% of the 39Ar
released, of 160597 Ma may be defined by
omitting the first two (low-age) fractions.
One biotite (c18) from the undeformed
1765 Ma Radisson granite (U/Pb zircon age
re-ported in Sims et al., 1989) yields a near-plateau
date of 175396 Ma. This date is nearly
concor-dant with the crystallization age of the granite, suggesting it cooled rapidly after intruding. Two
biotite separates (c11 and c12) from
Penokean syntectonic granites yielded
anoma-lously young ages of 117094 Ma (plateau) and
135795 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 (c20) and the Flambeau mine
(c21) give ages of 161495 and 175995 Ma,
respectively. A third muscovite (c19) 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 B1800 Ma, significantly younger than
two of the samples, which give typical Penokean ages.
A hornblende separate (c1) from a sample of
amphibolite collected at Little falls yields a
plateau date of 163895 Ma. Six other
horn-blende separates give dates between 1723 and
1796 Ma. The youngest of these (c3), 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
(c4) from a sample of mafic tonalite yields a
discordant spectrum but with a narrow near-plateau at 1745 Ma.
Hornblende from a garnet amphibolite (c2)
collected in Cornell yields a plateau date of
173396 Ma. Hornblende separate (c5) from
amphibolitic gneiss from Neillsville yielded a
near-plateau date of 177799 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 K/Ca reaches a minimum.
Sample c6 from amphibolite from along the
north fork of the Eau Claire river yields a broad
plateau at 178297 Ma. This amphibolite is cut
by Penokean dikes (1850 Ma, Van Wyck,
1995) but an exact age is not known. Lastly, a
hornblende separate (c7) from an amphibolite
from Neillsville quarry gave a near-plateau date
of 1796920 Ma.
Two hornblende separates yield typical
Penokean dates. The younger of the two (c8) is
from an Archean mafic gneiss collected at the north end of Lake Arbutus. The results yield a
plateau at 183097 Ma. Hornblende from
am-phibolite collected in Jim falls (c9) 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 (c10). It yields
a highly complex spectrum with initially high ages followed by decreasing and then increasing ages culminating in a narrow near-plateau at
2503918 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 40Ar
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 500950°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) Rb/Sr and40Ar
/39Ar mica ages to the south (Fig. 2). The large scatter of the Rb/Sr biotite dates has long been considered problem-atic (Peterman and Sims, 1988) and requires bet-ter assessment of Mesoprobet-terozoic overprinting effects related to intrusion of the 1470 Ma Wolf river batholith and to 1100 Ma midcontinent rift
activity. The 117094 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 135795
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 thermal/
6.2. Hornblende thermochronology
A hornblende plateau date of 163895 Ma is
interpreted to represent complete resetting caused
by the 1650 Ma activity noted above. It was
reported two similar 40Ar/39Ar 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 Ar/Ar ages are
uniformly 1760 Ma and indicate crustal
stabi-lization (Holm et al., 1998a). In the lower-grade
region of Wisconsin, Penokean hornblende Ar/Ar
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 uplift/cooling 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 2503918 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-induced/moderate-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
intermediate-temperature deformational features. High-temper-ature feHigh-temper-atures (\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 (163895 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 173396 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 40Ar
/39Ar 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 Rb/Sr 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. 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
A model for the post-accretionary evolution of the Penokean orogenic belt in Wisconsin and northern Michigan begins with Paleoproterozoic accretion that resulted in crustal thickening and metamorphism between 1870 and 1820 Ma (Fig. 6A). Following the Penokean orogeny, a period of tectonic quiescence and amagmatism lasting ca. 50 – 60 million year was interrupted by magma-tism (emplaced at 10 – 11 km depths in northern Wisconsin and extruded atop the crust in
south-ern Wisconsin) and crustal exhumation/cooling
(Fig. 6B). Stabilization of the orogen resulted in crust of normal thickness and was followed by deposition of mature quartzites between 1750 and 1650 Ma (Fig. 6C). Crustal stabilization was short-lived however, as continued accretion and growth of Laurentia to the south (the Mazatzal orogeny) caused significant shortening and ther-mal resetting of a large portion of the Penokean orogen to the north (Fig. 6D). The geometry we depict for collision of the Mazatzal province is in concert with that recently proposed for Mazatzal collision in the southwest USA (Selverstone et al., 1999) and may account for southward vergence of folds in the Baraboo quartzite (Dalziel and Dott, 1970). Aeromagnetic data from Wisconsin (Can-non et al., 1999) which reveal large wavelength folds in the subsurface suggest that Mazatzal shortening of the Penokean crust was probably not simply thin-skinned in nature (Fig. 6D).
Our results corroborate the hypothesis pro-posed nearly 20 years ago by Dott (1983) that the A time-temperature reconstruction (Fig. 5)
il-lustrates the thermal history of the area based on the new data. The cooling path after the
Penokean orogeny north of the thermal/
deforma-tional front is shown through two micas
(repre-sented by filled circles) at 1755 Ma. The
1755 Ma primary cooling ages suggest
signifi-cant amounts of exhumation immediately after
1760 Ma magmatism at 9 – 11 km depths and
severity of quartzite deformation in Wisconsin
was related to collision from the south at 1630
Ma. Such widespread and severe deformation of much of the Penokean orogenic belt indicates that it remained weak for close to 200 million years after the Penokean orogeny and had not ther-mally equilibrated to the point of cratonization.
The 1630 – 1650 Ma thermal/deformational front
(Fig. 2) approximately coincides with the
Penokean suture zone (NFZ), cross-cutting it a low angle. While perhaps only coincidental, this may indicate that the northernmost extent of Mazatzal deformation was controlled by litho-logic and/or age-related strength differences which may have existed across the suture, as also sug-gested for the Wyoming region (Karlstrom and Humphreys, 1998).
As mentioned in the introduction, Wisconsin harbors some of the oldest igneous rocks of the entire transcontinental Mesoproterozoic suite. Mesoproterozoic plutonism occurs solely within the region, which was strongly deformed after Penokean accretion (i.e. south of the Mazatzal
thermal/deformational front). It is possible that
Mazatzal-related deformation contributed to later melt production (which started as early as 1565 Ma; Van Wyck et al., 1994) by causing reheating and crustal thickening which ultimately delayed
cooling of the juvenile arc terrane. The eventual production of the voluminous 1470 Ma Wolf river magma and its emplacement into the upper crust (Fig. 6E) was apparently the final step in the cratonization of this portion of the Penokean orogen (cf. tectonic evolution of the western Penokean orogen proposed by Holm, 1999). The juvenile crust may have been able to evolve into strong, deformation-resistant crust only after pro-fuse partitioning (by melt migration) of weak silicic components into the upper crust (Holm, 1998).
Acknowledgements
This work was supported by National Science Foundation grant EAR-9526944. We thank Paul Myers and Randy Maass for assistance in the field. We thank Fritz Hubacher for indispensable laboratory support and assistance and Jeff Linder and Frank Huffman for helping with equipment maintenance and programming. D. Schneider, P. Reynolds and K. Karlstrom provided very con-structive reviews, which significantly improved the manuscript. This work benefited much from vari-ous discussions with David Schneider, Craig Mancuso, and Jason Rampe.
Appendix A. 40Ar/39
Ar data tables
Summary of40Ar/39Ar results.
ttg
2 96-DR-4 hornblende 0.37
0.55 1627
1 96-DR-7 hornblende 163895
1853
1.1 –
96-JFS-H hornblende 9
0.54 1744
4 97-DR-22-H hornblende –
1770 ‘175495’
4 97-DR-22-H hornblende 0.50
0.40 1710
7 97-DR-8 hornblende ‘1796920’
0.83 1598 ‘177799’
97-DR-12 hornblende 5
0.84 1774
8 97-DR-15 hornblende ‘183097’
0.14 2478
10 97-DR-17 hornblende ‘2503918’
1723
0.61 –
97-WIS-GD-H hornblende 3
1738 178297
6 WO-2 hornblende 0.42
‘175396’
12 95-11 Biotite ‘135795’
6.9 1558
97-DR-22–B biotite 6.6 1576 158294 16
158194
1503 3.7
15 97-WIS-GD-B biotite
11 GWD-1 biotite 7.2 1144 117094
17 W-249 biotite 6.6 1583 ‘160597’
1567
6.9 157694
W-55 biotite 13
Flambeau muscovite 6.3 1754 175995
21
96-DR-8 muscovite 7.2 1615 161495
20
–
97-DR-7 muscovite 7.3 1518
19
%K is the approximate K concentration of sample in wt.%, derived from39Ar yields.
ttgis The total-gas age derived from the summation of all fractions of the incremental-heating analysis.
tpis The plateau age derived from the incremental-heating age spectrum. For those in quotation, the
dispersion among the included fractions exceeds variations from analytical uncertainties.
40Ar
/39Ar analytical results.
37Arb
500 31.89 0.2278 0.9453 1.683 26.99 2.32
657912
59.10 40.1 94.6
651 49.13 0.0788 1.304 6.834 29.03 0.70
5469104
8.38 35.4
3.50 0.99
701 661.1 1.039 1.476215.9 23.34
0.59 25.00 32.6 38.4
751 108.1 0.1983 1.603 27.47 27.00 618920
0.73 77.80 19.8 61.7
800 38.60 0.1011 2.639 2.958 30.08 67798
10.2 39.5 89499
0.64 77.80
851 54.30 0.1508 5.103 4.199 42.38
0.91 84.00 6.93 15.3
880 81.04 0.3590 7.506 4.587 68.35 128297
1.18 93.70 6.44 16.2
910 97.93 0.3357 8.073 2.278 92.27 157796
36.0 5.96
99.30
11.62 172794
930 106.2 0.1566 8.731 0.4731 106.0
107.5 15.53 99.60 5.78 44.1 174393
950 107.4 0.1299 8.999 0.3734
14.34 99.50 5.72 50.0
970 106.3 0.1161 9.095 0.4125 106.3 173094
6.09 48.3 172195
1.79 98.30
990 106.7 0.1205 8.535 0.8210 105.4
172294
98.70 5.97 40.0
1010 106.2 0.1425 8.713 0.6660 105.5 2.06
174093
42.3 5.85
99.40 4.67
1030 107.2 0.1351 8.883 0.4372 107.2
7.75 99.00 5.50 48.3
1050 105.5 0.1201 9.442 0.5758 105.1 171895
6.20 99.10 5.47 52.2
1070 106.4 0.1121 9.507 0.5604 106.1 172895
5.88 50.4 174794
3.12 99.40
1090 107.9 0.1154 8.837 0.4177 107.9
1.07 97.80 5.66 42.1
1110 109.8 0.1367 9.182 1.023 108.0 174896
0.90 97.80 5.78 40.6
1130 109.3 0.1413 8.991 1.048 107.5 174396
42.3 5.82
96.30
0.59 174798
1150 111.4 0.1373 8.930 1.594 107.9
107.8 0.66 96.20 5.64 39.2 174698
1170 111.4 0.1471 9.219 1.644
0.93 97.80 5.71 47.6
1190 109.9 0.1225 9.098 1.047 108.1 174996
50.4 5.72
97.40
0.92 173596
1210 109.0 0.1168 9.081 1.168 106.8
107.9 1.19 97.70 5.70 54.3 174796
1240 109.8 0.1091 9.120 1.056
174994
60.7 5.71
98.90 2.62
1270 108.6 0.0983 9.102 0.6080 108.1
8.52 99.40 5.43 46.2
1300 106.3 0.1248 9.566 0.4442 106.3 173194
6.07 99.10 5.73 56.6
1400 105.8 0.1044 9.066 0.5523 105.4 172195
5.86 54.6 1754920
1.42 35.90
1618 300.6 0.2297 8.867 65.41 108.5
100.00 90.71 6.03 41.7
Sum 111.7 0.1433 8.628 3.905 101.4 1678
173396
39Ar age spectrum, 930–1400°C fractions (91% of 39Ar)
96-DR-7 hornblende c57A5 (J=0.01531 wt.=0.0629g %K=0.55)
31.1 13.2 154093
86.92 0.88
600 101.2 0.4142 1.680 4.513 88.04
1.30 64.21 36.0 32.9
700 147.2 0.2031 1.451 17.854 94.59 161597
96.34 44.5 58.1 149892
800 87.62 0.1028 1.175 1.106 84.48 1.33
79.01 1.06 95.61 19.4 42.1
2.40 97.69 9.28 23.7 154193
950 89.87 0.2319 5.633 0.8302 88.12
98.43 8.02 22.2 157892
975 92.37 0.2464 6.517 0.6373 91.32 3.41
5.85 99.14 7.43 19.9
1000 94.30 0.2731 7.030 0.4336 93.93 160893
19.7 163192
17.19 99.66 7.22
1025 95.86 0.2757 7.242 0.2758 96.00
96.67 20.31 99.69 7.31 19.8
1050 96.51 0.2742 7.150 0.2634 163992
11.54 99.72 7.56 20.2
1100 96.66 0.2687 6.912 0.2478 96.83 164192
19.7 7.04
99.58
7.53 163792
1150 96.48 0.2760 7.423 0.3060 96.55
12.78 99.65 6.98 19.5
1200 96.61 0.2782 7.485 0.2854 96.75 164092
163593
19.3 7.05
98.93 5.17
1250 96.94 0.2819 7.410 0.5218 96.38
97.03 2.28 98.81 7.06 19.7 164393
1300 97.72 0.2760 7.404 0.5614
98.70 7.10 20.0 164093
1.84
1350 97.54 0.2726 7.358 0.5985 96.74
96.87 1.60 97.63 7.08 19.5
1400 98.74 0.2803 7.382 0.9603 164194
164293
98.54 7.15 19.9
1500 97.89 0.2732 7.314 0.6497 96.93 3.53
95.58 100.00 20.2 1627
Sum 96.58 0.2695 6.921 0.6448 98.51 7.55
39
Ar age spectrum, 1125–1400°C fractions (85% of 39
Ar) 163895
96-JFS-H hornblende c55B12 (J=0.00754 wt.=0.1302 g %K=1.1)
5.66 97.22 110.4 39.5
750 328.9 0.1489 0.4735 3.098 319.9 221494
4.55 98.48 36.5 46.2
900 157.6 0.1254 1.432 0.8397 155.3 139893
23.9
99.06 154693
3.31 26.7
925 181.4 0.2075 2.185 0.6251 179.9
264.3 11.00 99.73 15.0 19.3
970 264.4 0.2814 3.476 0.3193 197793
16.75 99.76 13.9 21.8
990 266.7 0.2500 3.753 0.3004 266.7 198893
26.4 14.1
99.78
19.46 184095
1010 235.2 0.2086 3.697 0.2622 235.3
11.50 99.75 14.6 29.7
1030 223.0 0.1872 3.569 0.2717 223.0 177997
175893
33.2 15.4
99.69 4.71
1050 219.1 0.1687 3.397 0.3076 219.0
222.0 5.82 99.74 14.5 28.1 177493
1115 222.0 0.1969 3.594 0.2730
181193
27.8 14.1
99.78 9.09
1180 229.3 0.1986 3.715 0.2583 229.3
226.8 3.79 99.58 13.9 28.4
1250 227.2 0.1953 3.773 0.4061 179893
2.37 99.45 14.4 28.4
1350 225.0 0.1955 3.624 0.4977 224.3 178593
33.7 16.0
98.90
1.56 162093
1525 194.8 0.1672 3.260 0.7962 193.1
129.0 0.43 95.37 20.8 45.5
1575 135.0 0.1295 2.519 2.168 122593
1853
99.47 15.8 26.5
Sum 238.6 0.2086 3.302 0.5047 237.9 100.00
97-DR-22-H hornblende c57A18 (J=0.01540 wt.=0.0742 g %K=0.54)
223199
2.04 88.55 43.8 31.2
700 179.2 0.1910 1.194 6.970 158.8
90.25 4.49 95.75 92.8
800 94.23 0.0595 0.5635 1.362 113 157193
2.78 94.51 28.9 76.4
900 83.24 0.0822 1.807 1.582 78.76 143395
13.1
98.60 154793
3.80 42.2
975 89.17 0.1354 4.002 0.512 88.16
6.53 99.19 8.66 25.8
1020 105.5 0.2140 6.034 0.430 105.1 173693
178093
22.6 7.29
99.53 19.06
1040 109.3 0.2422 7.165 0.344 109.3
107.2 15.21 99.50 7.28 22.8 175893
1080 107.2 0.2396 7.178 0.351
99.31 7.61 23.7 172693
6.92
1080 104.3 0.2310 6.871 0.403 104.1
100.2 2.81 99.51 8.32 25.6
1110 100.3 0.2146 6.281 0.311 168492
8.10 99.40 7.03 22.7
1180 105.6 0.2412 7.430 0.387 105.5 174193
22.3 7.01
99.54
20.95 176593
1275 107.8 0.2454 7.461 0.345 107.9
107.2 5.61 99.30 7.00 22.1
1400 107.4 0.2471 7.472 0.429 175893
174293
98.37 7.13 21.5
1550 106.9 0.2549 7.335 0.764 105.7 1.70
105.8 100.00 98.78 24.8
Sum 106.6 0.2217 6.464 0.591 8.09 1744
97-DR-22-H hornblende c60B28M (J=0.015665wt.=0.00351 g %K=0.50)
3344911
87.70 0.179 9.51
600 391.0 0.5894 2.918 16.37 343.5 0.40
0.26 89.70 0.293 35.0
701 159.2 0.1704 1.786 5.590 143.0 2121914
23.2 0.142
36.50
0.33 1913930
751 329.0 0.3683 3.670 70.78 120.4
83.70 0.338 38.5 2013914
801 156.3 0.1626 1.546 8.650 131.0 0.35
1592911
61.1 0.386
93.00 0.55
851 97.19 0.1006 1.355 2.330 90.48
78.27 0.43 92.60 0.233 74.8
901 84.41 0.0847 2.237 2.170 1444913
81.97 0.63 94.40 0.125 41.4
95.70 0.108 32.7 156697
976 91.87 0.1729 4.831 1.460 88.21 0.59
1635927
31.9 0.084
99.60 1.04
1001 94.18 0.1748 6.220 0.305 94.14
101.7 1.81 95.90 0.081 23.4 1719914
1020 105.6 0.2360 6.421 1.640
179099
21.2 0.074
99.20 3.37
1040 108.6 0.2579 7.023 0.465 108.3
110.0 9.78 99.50 0.070 20.6
1060 110.0 0.2643 7.387 0.385 180795
34.10 99.60 0.072 20.6
1080 106.4 0.2641 7.231 0.355 106.5 177093
21.2 0.071
99.60
10.25 173395
1100 102.9 0.2567 7.279 0.326 103.0
3.37 99.90 0.073 21.6
1120 103.9 0.2521 7.150 0.237 104.3 174795
174997
21.7 0.070
99.10 6.31
1140 104.1 0.2510 7.438 0.257 104.4
104.9 4.83 98.70 0.070 21.8 1754911
1160 104.9 0.2498 7.400 0.347
99.30 0.071 20.4 1750913
3.02
1180 105.2 0.2666 7.332 0.568 104.6
105.1 2.59 99.00 0.070 21.3
1200 105.6 0.2566 7.442 0.563 1755910
2.04 99.30 0.072 23.3
1221 105.1 0.2352 7.215 0.581 104.5 1749915
20.4 0.070
99.20
1.66 1756919
1240 105.7 0.2671 7.467 0.578 105.1
105.3 1.51 96.90 0.073 20.8 1757915
1260 105.8 0.2617 7.141 0.531
1.24 99.00 0.072 21.7
1280 105.3 0.2514 7.274 0.561 104.7 175298
0.070 19.2 175996
1.67 99.40
1300 105.6 0.2828 7.400 0.421 105.4
99.60 0.074 20.7 175395
1350 104.9 0.2636 7.004 0.350 104.8 3.25
3.95 99.30 0.074 20.9
1400 106.3 0.2607 7.052 0.436 106.1 176694
22.0 0.072
95.10
0.67 175799
1500 110.1 0.2508 7.257 2.030 105.2
98.83 0.073 21.3 1770
Sum 107.7 0.2571 7.087 0.782 106.4 100
39Ar age spectrum, 1120–1500°C fractions (36% of 39Ar) 175495
97-DR-8 hornblende c57A8M (J=0.015357 wt.=0.0031 g %K=0.40)
89.90 47.1 25.8 212796
500 163.0 0.2232 1.110 5.602 146.6 2.05
2.29 30.90 47.7 19.3
681 220.0 0.3785 1.096 51.49 67.94 1289916
42.5 110
45.10
2.66 1230910
741 141.1 0.1830 0.477326.24 63.62
132595
141 60.2
78.50 6.583
801 90.01 0.0603 0.8681 70.64 2.75
93.30 32.9 200 124394
851 69.17 0.0401 1.586 1.609 64.57 2.83
133093
119 15.8
95.20 3.73
891 74.46 0.0573 3.307 1.293 71.01
81.03 3.98 96.60 9.37 71.5 145893
900 83.56 0.0861 5.558 1.082
157294
55.9 7.83
98.00 2.63
910 91.95 0.1059 6.648 0.7801 90.49
95.57 2.33 98.50 6.95 50.2
920 96.61 0.1162 7.491 0.6853 163094
2.50 98.50 5.97 40.2
930 101.0 0.1420 8.704 0.7181 100.0 167994
38.5 5.21
99.10
3.45 173794
940 105.7 0.1474 9.978 0.5653 105.4
5.48 99.30 4.89 35.4
955 107.9 0.1590 10.62 0.5145 107.9 176393
176995
35.9 4.97
99.40 10.86
980 108.4 0.1570 10.46 0.4674 108.5
109.3 20.15 99.60 4.99 37.1 177893
1005 109.0 0.1523 10.41 0.4006
97.20 5.01 39.8 166097
1.47
1035 100.4 0.1441 10.37 1.192 98.29
97.00 4.75 35.9 173894
1050 108.0 0.1586 10.92 1.357 105.5 1.94
2.87 98.70 4.67 34.1
1065 109.7 0.1652 11.11 0.7674 109.0 177493
34.0 4.60
99.00
3.48 178594
1080 110.3 0.1654 11.28 0.6578 110.0
3.61 99.30 4.67 35.4
1130 111.1 0.1593 11.13 0.5475 111.0 179594
180493
35.2 4.64
99.10 5.79
1180 112.1 0.1602 11.19 0.6075 111.9
117.2 1.59 98.10 4.42 33.8 185895
1225 118.6 0.1670 11.74 1.057
187496
34.9 4.34
97.50 1.31
1245 121.0 0.1627 11.95 1.308 118.9
118.6 1.26 98.40 4.64 37.3
1260 119.7 0.1523 11.19 0.9037 187296
1.35 98.40 4.66 34.5
1275 122.0 0.1638 11.14 0.9123 121.0 189596
34.8 4.52
98.90
2.05 188395
1290 120.2 0.1621 11.47 0.7048 119.8
98.30 4.91 34.9 186395
1330 118.9 0.1621 10.58 0.9256 117.7 1.70
187694
35.7 4.85
98.70 2.69
1400 119.9 0.1586 10.70 0.8023 119.1
116.5 1.20 94.30 4.84 37.5
1619 122.7 0.1548 10.72 2.610 185196
102.9 100.00 93.49 5.91 39.3
Sum 110.0 0.1489 8.792 2.828 1710
97-DR-12 hornblende c57A12 (J=0.015423 wt.=0.0566 g %K=0.83)
72.79 4.39 87.56 146
700 83.11 0.0959 0.3570 3.500 66.4 135895
135893
5.51 96.83 209
800 75.13 0.0548 0.2496 0.8050 72.76 123
0.4857 98.03 133 128 133793
900 72.62 0.0524 0.3936 71.21 5.67
69.92 6.64 98.39 51.3 106 132093
975 71.01 0.0607 1.019 0.4049
9.39 98.86 18.4 60.8
1020 77.08 0.0974 2.834 0.3596 76.34 140492
40.3 11.8
98.97
9.57 150793
1040 85.30 0.1412 4.421 0.3963 84.68
92.21 9.11 99.23 9.24 31.5 159692
1060 92.58 0.1769 5.657 0.3730
13.94 99.38 8.03 27.0
1090 100.1 0.2048 6.506 0.3603 99.98 168392
26.5 8.21
99.49
19.51 177292
1170 108.4 0.2084 6.366 0.3345 108.3
98.86 6.98 24.0 177193
1250 108.9 0.2294 7.487 0.5966 108.2 6.39
4.62 98.61 6.93 22.9
1350 110.6 0.2400 7.537 0.6956 109.6 178693
22.3 6.71
98.40
3.69 180193
1450 112.3 0.2458 7.786 0.7895 111.0
108.5 1.56 95.77 7.17 22.1
1550 112.8 0.2495 7.292 1.785 177594
98.44 11.1 35.0 1598
Sum 93.56 0.1609 4.701 0.6009 92.4 100.00
39Ar age spectrum, 1170–1550°C fractions (36% of 39Ar) 177799
97-DR-15 hornblende c57A14 (J=0.01543 wt.=0.0587g %K=0.84)
3.807 92.0 12.2 154895
700 99.25 0.4451 0.5678 88.06 3.83 88.69
0.6996 70.32 2.68 97.16 113
800 72.36 0.1553 0.4617 36.5 132692
2.99 97.78 38.5 45.0
900 68.25 0.1279 1.358 0.5376 66.80 127892
98.76
4.78 10.8 16.4 151693
975 86.15 0.3290 4.833 0.4707 85.35
9.84 99.41 11.5 10.1
1020 108.6 0.5255 4.548 0.3206 108.3 177299
182297
9.71 11.7
99.63 14.67
1040 113.2 0.5477 4.469 0.2432 113.2
113.4 13.69 99.73 11.7 9.77 182596
1060 113.4 0.5445 4.465 0.2063
182592
9.81 11.7
99.77 11.77
1080 113.3 0.5420 4.468 0.1908 113.4
99.70 11.6 9.82 182092
1100 112.9 0.5417 4.525 0.2162 112.9 4.97
7.57 99.56 11.2 9.61
1150 114.9 0.5534 4.683 0.2766 114.7 183892
9.54 11.4
99.65
12.49 184192
1200 115.1 0.5572 4.581 0.2395 115.0
114.6 5.09 99.45 11.2 9.56
1250 114.8 0.5560 4.654 0.3204 183793
99.59 11.4 9.57 184592
1350 115.5 0.5553 4.601 0.2653 115.4 5.62
108.4 100.00 99.14 12.5 10.5
Sum 109.1 0.5091 4.193 0.4139 1774
39Ar age spectrum, 1040–1350°C fractions (76% of 39Ar) 183097
WO-2 hornblende c55B11 (J=0.007534wt.=0.1411 g %K=0.42)
293.9 1.45 86.99 20.0 10.8
700 337.3 0.5226 2.620 14.91 210797
2.63 97.11 29.6 61.6
850 182.6 0.0991 1.766 1.821 177.5 153296
41.3 14.1
97.16
0.89 154196
900 183.8 0.1407 3.715 1.851 179.0
1.34 98.34 8.78 33.3
925 184.6 0.1697 5.950 1.171 182.3 156095
161596
25.3 5.26
97.84 1.68
950 195.0 0.2201 9.941 1.656 192.1
200.9 1.34 98.82 4.51 22.4 166396
970 201.7 0.2455 11.59 1.077
99.31 4.21 17.6 172896
6.37
1010 212.9 0.3070 12.41 0.7896 213.2
225.6 27.95 99.70 4.58 16.9
1030 224.5 0.3199 11.41 0.5014 179296
5.48 99.65 5.37 17.8
1050 220.4 0.3035 9.745 0.4932 221.1 176996
18.1 5.49
99.37
3.80 178696
1070 224.3 0.3002 9.527 0.7057 224.3
2.79 99.09 5.66 19.6
1100 225.1 0.2776 9.232 0.9128 224.5 178696
176596
16.9 4.57
99.18 2.35
1125 220.5 0.3208 11.43 0.8799 220.4
223.1 8.44 99.55 4.61 16.6 177996
1150 222.4 0.3258 11.34 0.6038
177796
16.4 4.63
99.59 12.63
1250 221.8 0.3287 11.28 0.5771 222.6
222.7 16.76 99.75 5.71 19.2 177796
1400 221.9 0.2824 9.159 0.4018
97.89 16.9 52.7 84694
1550 80.97 0.1112 3.087 0.6454 79.43 4.08
215.1 100.00 99.15 5.24 18.47
Sum 215.5 0.2939 9.970 0.8535 1738
178297
39Ar age spectrum, 1030–1400°C fractions (80% of 39Ar)
97-DR-17 hornblende c57A15 (J=0.015400 wt.=0.0053 g %K=0.14)
933.4 1.67 94.10 6.50 3.96 4930921
500 987.0 1.361 8.001 19.99
43.90 6.64 7.86 3313948
700 775.6 0.9493 7.841147.4 342.5 1.89
314.0 1.44 80.80 6.91 12.5
94.60 3.97 7.12 3148910
800 321.6 0.7509 13.06 6.250 306.8 0.92
206798
13.4 3.56
89.30 1.37
850 154.4 0.4091 14.54 5.960 139.3
116.9 1.85 89.80 2.73 10.2 185797
900 128.5 0.5277 18.93 4.942
192694
5.72 2.19
96.20 4.47
942 126.9 0.9150 23.49 2.290 123.9
135.9 12.17 98.10 2.44 4.46
960 136.6 1.171 21.13 1.440 203795
11.23 98.50 2.25 6.12
981 167.0 0.8555 22.94 1.460 166.9 229695
7.16 2.12
98.00
5.18 226294
1001 163.4 0.7339 24.29 1.780 162.6
5.01 97.30 2.24 6.17
1015 162.5 0.8504 23.02 2.090 160.5 224695
232794
6.09 2.13
97.90 5.82
1031 171.9 0.8596 24.15 1.853 171.0
177.0 5.04 97.60 2.02 6.07 237393
1046 178.4 0.8627 25.45 2.150
97.20 2.01 6.10 237194
3.77
1061 178.7 0.8589 25.57 2.360 176.7
243795
97.00 2.08 6.18
1076 188.4 0.8493 24.73 2.580 185.7 3.37
2.77 96.40 2.06 6.11
1090 185.5 0.8587 24.97 2.940 181.7 240896
6.32 2.04
93.80
1.56 246697
1106 199.2 0.8341 25.26 4.880 189.9
1.59 94.20 2.08 6.55
1120 205.5 0.8055 24.76 4.680 196.8 251596
254696
6.57 2.04
93.90 1.51
1141 210.9 0.8039 25.28 5.020 201.4
195.6 2.30 95.50 2.04 6.51 250796
1160 201.4 0.8091 25.25 3.730
248595
6.79 2.05
96.50 3.39
1180 196.2 0.7750 25.09 2.990 192.5
197.5 3.86 96.80 2.07 6.76
1201 200.7 0.7777 24.90 2.810 252094
3.54 95.90 2.05 6.79
1241 206.1 0.7759 25.09 3.520 200.9 254394
6.62 2.07
95.60
6.40 247994
1301 197.3 0.7952 24.82 3.600 191.7
96.50 2.00 6.63 249995
1501 198.1 0.7931 25.69 3.010 194.5 4.93
247395
6.63 2.09
88.90 2.94
1650 211.2 0.8035 24.66 8.610 190.8
190.1 100.00 93.40 2.25 6.14
Sum 203.6 0.8622 22.88 6.105 2478
2503918
39Ar age spectrum, 1106–1500°C fractions (29% of 39Ar)
97-WIS-GD-H hornblende c57A31 (J=0.01508 wt.=0.0620 g %K=0.61)
0.83 44.2 2582912
88.43 2.36
800 238.8 0.1466 0.6293 9.359 211.2
104.0 1.82 90.37 0.40 18.9 170296
900 115.0 0.2938 1.313 3.780
4.63 98.30 0.11 12.1
975 108.9 0.4425 4.614 0.7415 107.4 173793
12.6 0.09
99.18
10.52 169693
1020 104.0 0.4235 5.697 0.4266 103.5
105.8 13.72 99.28 0.08 12.6 172093
1040 106.1 0.4251 6.262 0.4119
29.78 99.54 0.08 12.4
1070 104.4 0.4298 6.446 0.3199 104.4 170693
0.09 13.0 166093
9.21 99.28
1100 100.4 0.4121 6.142 0.3935 100.1
163593
99.25 0.09 13.0
1170 98.15 0.4116 6.085 0.3979 97.80 7.43
9.01 99.22 0.08 12.5
1250 104.6 0.4294 6.344 0.4307 104.3 170493
12.5 0.08
99.10
8.36 169493
1350 103.8 0.4280 6.315 0.4681 103.3
167793
98.10 0.08 12.7
1450 103.2 0.4234 6.277 0.8149 101.7 2.34
166694
13.3 0.09
95.62 0.82
1550 104.9 0.4069 6.084 1.7011 100.7
106.1 100.00 98.48 0.09 12.9
Sum 107.3 0.4164 5.945 0.6973 1723
94-WI-RG biotite c57A7 (J=0.015346 wt.=0.0165 g %K=6.4)
42.9 635926
50.57
0.22 1.30
500 54.39 0.1500 40.09 9.103 27.51
84.27 3.71 128 85397
575 46.74 0.0561 14.09 2.484 39.39 0.29
0.69 95.93 8.66 198
650 70.62 0.0390 6.033 0.9676 67.74 128694
192 11.5
98.62
1.70 170293
750 103.6 0.0388 4.563 0.4765 102.2
3.86 98.49 15.5 229
825 107.2 0.0346 3.364 0.5410 105.6 173893
174792
258 18.2
99.67 9.26
900 106.8 0.0312 2.868 0.1125 106.5
106.7 12.65 99.79 19.0 245 175092
975 106.9 0.0322 2.754 0.0689
175592
260 18.3
99.75 9.10
1025 107.5 0.0310 2.851 0.0852 107.2
107.5 5.91 99.65 15.8 227
1050 107.8 0.0340 3.302 0.1224 175892
3.97 99.63 13.5 228
1090 107.8 0.0339 3.866 0.1279 107.4 175792
236 10.2
99.60
3.74 176192
1150 108.2 0.0332 5.117 0.1388 107.8
99.78 12.1 256 176392
1370 108.2 0.0313 4.314 0.0732 108.0 15.40
174792
244 10.9
99.77 20.44
1500 106.7 0.0323 4.808 0.0750 106.4
106.7 12.78 99.74 5.04 254
1550 107.0 0.0315 10.37 0.0896 175092
106.2 100.00 99.58 11.0 242
Sum 106.7 .0326 4.762 .1450 1745
39
Ar age spectrum, 975–1500°C fractions (84% of 39
95-11 Biotite c53I5 (J=0.007675 wt.=0.0171g %K=6.9)
95.03 4.42 97.21 12.5 109
550 97.76 6.042 4.167 0.9137 98894
8.14 99.40 17.3 441
600 143.5 2.335 3.029 0.2791 142.7 133493
21.0 382 135293
10.29 99.54
630 146.1 2.506 2.490 0.2179 145.4
99.77 19.5 403 136292
670 147.2 2.416 2.680 0.1040 146.9 11.23
9.64 99.81 17.4 421
700 147.7 2.355 2.997 0.0840 147.5 136692
549 16.3
99.74
8.51 136792
750 148.1 2.074 3.214 0.1202 147.7
8.65 99.72 97.9 336
825 146.6 2.677 0.5336 0.1289 146.2 135792
99.74 51.3 302 135892
900 146.7 2.849 1.018 0.1205 146.4 13.16
21.07 99.75 20.4 359
975 145.1 2.577 2.567 0.1133 144.8 134892
4.99 99.06
2.04 239 135395
1050 146.9 3.372 10.48 0.4593 145.5
99.57 3.25 198 138895
1200 151.6 3.775 16.09 0.2122 151.0 2.38
1369910
96.73 0.88 70.1
1500 152.9 8.868 59.27 1.695 147.9 0.48
143.7 100.00 325 1341
Sum 144.3 2.740 3.154 0.1890 99.59 16.6
39
Ar age spectrum, 630–1050°C fractions (85% of 39
Ar) 135795
96-JFS-B biotite c53I10 (J=0.00770 wt.=0.0093 g %K=6.9)
80.54 2.58 23.8 64496
500 69.22 23.86 20.22 4.554 55.76 0.44
8.94 98.27 9.48 120
600 149.1 5.609 5.512 0.8615 146.5 136293
261 61.1
98.97 156893
640 181.8 3.216 0.8560 0.6254 179.9 11.01
182.8 9.51 99.69 89.4 306
670 183.4 2.839 0.5848 0.1807 158593
9.35 99.69 312 321
700 183.6 2.758 0.1676 0.1817 183.0 158693
158493
345 64.3
99.74
740 183.2 2.640 0.8126 0.1487 182.7 8.62
181.8 5.97 99.55 35.9 358
780 182.7 2.607 1.456 0.2660 157993
157393
251 13.7
99.59 7.79
850 181.5 3.226 3.811 0.2402 180.8
180.7 9.40 99.53 9.25 249 157293
900 181.5 3.2511 5.650 0.2780
157693
281 17.4
99.62 9.66
950 181.9 3.000 3.011 0.2212 181.2
181.2 10.08 99.62 6.62 261
1000 181.9 3.140 7.895 0.2220 157593
8.03 99.53 10.7 880
1050 185.7 1.747 4.871 0.2843 184.9 159793
131 16.4
99.28
0.95 161795
1100 189.8 5.172 3.179 0.4509 188.4
171.8 0.24 94.84 0.43 47.1
1300 181.0 12.79 121.41 3.179 1520911
99.40 14.8 250 1558
Sum 179.2 3.254 3.530 0.3519 178.1 100.00
157995
39Ar age spectrum, 640–1000°C fractions (70% of 39Ar)
97-DR-22-B biotite c57A20 (J=0.01537 wt.=0.015 g %K=6.6)
550 65.31 13.04 21.05 7.498 43.15
56.39 0.95 95.02 6.31 203
650 59.35 3.826 8.278 0.9947 112693
2.99 98.11 10.8 265
750 86.06 3.148 4.860 0.5425 84.44 150193
291 12.1
98.13
5.72 157193
825 92.05 2.981 4.331 0.5738 90.34
12.07 99.42 16.6 344
900 91.09 2.629 3.149 0.1724 90.56 157392
157992
334 17.4
99.67 10.75
950 91.36 2.658 3.004 0.0945 91.06
91.53 7.83 99.68 15.2 295 158592
1000 91.82 2.864 3.443 0.0918
99.63 12.9 286 158692
5.87
1050 91.99 2.925 4.040 0.1080 91.65
91.72 4.90 99.66 9.55 291
1125 92.03 2.887 5.471 0.0993 158792
5.31 99.58 9.27 287
1200 91.54 2.922 5.639 0.1224 91.17 158092
285 10.4
99.55
5.96 158092
1250 91.55 2.934 5.023 0.1317 91.14
9.62 99.69 11.0 337
1350 91.22 2.644 4.747 0.0872 90.95 157892
158692
324 6.53
99.70 11.56
1450 91.91 2.712 8.007 0.0878 91.64
92.63 10.56 99.66 2.68 269 159792
1550 92.93 3.042 19.473 0.1036
161792
237 2.00
99.58 5.50
1600 94.76 3.304 26.104 0.1328 94.38
90.75 100.00 99.36 6.96 293 1576
Sum 91.33 2.897 7.509 0.1903
39Ar age spectrum, 900–1450°C fractions (74% of 39Ar) 158294
97-WIS-GD-B biotite c57A33 (J=0.015020wt.=0.0163 g %K=3.7)
18.02 1.44 55.75 2.03 39.9
550 32.32 15.07 25.69 4.840 432915
86.63 6.97 68.7 36595
4.57
650 17.28 8.820 7.500 0.7764 14.97
54.74 3.06 95.68 11.9 83.3 108294
750 57.21 7.502 4.401 0.8291
97.30 16.7 83.0 148693
825 87.51 7.513 3.121 0.7926 85.15 3.81
89.21 4.98 98.19 18.3 93.9
90.69 5.04 99.45 15.8 98.2
950 91.19 6.425 3.312 0.1642 155092
4.88 99.60 36.8 99.9
1000 91.75 6.324 1.420 0.1166 91.39 155892
24.8 97.2 156292
5.01 99.32
1075 92.32 6.486 2.105 0.2055 91.69
99.43 9.94 97.5 156292
1150 92.28 6.463 5.258 0.1728 91.76 4.79
4.02 99.58 8.61 97.1
1225 92.33 6.476 6.068 0.1238 91.95 156592
94.5 6.94
99.47
4.95 158192
1300 93.89 6.629 7.529 0.1611 93.41
158292
99.44 7.16 91.0
1400 93.98 6.849 7.297 0.1722 93.46 6.83
157992
96.7 6.77
99.71 16.16
1500 93.43 6.491 7.718 0.0844 93.17
93.47 30.46 99.81 10.7 105.
1600 93.64 6.082 4.905 0.0523 158292
1503 86.55 100.00 99.05
Sum 87.38 6.697 5.559 0.2751 9.40 93.8
39
Ar age spectrum, 1300–1600°C fractions (58% of 39
Ar) 158194
550 70.45 27.06 2.064 1.438 66.18
99.07 79.0 58.2 113093
600 113.35 10.14 0.662 0.3466 112.3 11.55
116293
302 41.5
98.53 11.01
640 118.41 2.936 1.260 0.5789 116.7
118.2 11.51 99.57 89.7 335 117393
690 118.67 2.686 0.583 0.1616
116893
244 33.5
99.58 9.25
750 118.05 3.266 1.560 0.1573 117.6
99.52 39.2 275 116493
850 117.51 3.030 1.333 0.1821 116.9 9.71
14.78 99.57 32.3 243
925 118.38 3.277 1.617 0.1633 117.9 117192
254 37.5
99.83
19.30 116892
1000 117.74 3.168 1.393 0.0572 117.5
99.56 8.33 249 117693
1250 119.16 3.225 6.274 0.1673 118.6 7.74
114.3 100.00 131 1144
Sum 115.19 5.138 1.656 0.2768 99.26 31.6
117094
39Ar age spectrum, 690–1250°C fractions (72% of 39Ar)
W-249 biotite c53I23 (J=0.00777 wt.=0.0191 g %K=6.6)
8.81 98.06 24.2 86.0
550 150.8 7.352 2.159 0.9806 147.9 138093
98.15
1.41 29.8 111 141395
600 155.9 6.000 1.756 0.9671 153.0
99.61 31.4 1985 160493
640 185.3 1.407 1.666 0.2341 184.5 10.30
12.57 99.85 32.2 5276
690 186.9 1.215 1.623 0.0838 186.6 161693
1796 29.2
99.79
12.77 161293
775 186.2 1.414 1.788 0.1243 185.8
184.1 9.33 99.78 21.8 1747
875 184.5 1.423 2.400 0.1284 160295
16.76 99.78 66.2 159292
950 182.8 1.786 0.7893 0.1281 182.4 789
0.0880 99.84 48.1 959 159992
1000 184.0 1.661 1.0870 183.7 15.56
186.3 12.15 99.81 6.28 5018
1250 186.6 1.226 8.322 0.1126 161493
97.01 4.38 74.4 156796
1500 183.6 8.469 11.94 1.847 178.1 0.34
180.9 100.00 99.63 1583
Sum 181.6 2.079 2.396 0.2176 21.8 556
160597
39Ar age spectrum, 640–1250°C fractions (89% of 39Ar)
W-55 blotite c53121 (J=0.00776 wt.=0.027 g %K=6.9)
550 108.8 5.211 5.701 1.658 103.9
0.3984 178.9 6.57 99.33 124
600 180.1 1.741 0.4228 922 157092
157492
0.2548 99.56 129 558
630 180.4 2.084 0.4047 179.6 8.05
0.0621 180.2 11.30 99.88 164
670 180.5 1.552 0.3182 1185 157892
157792
9.82 99.77 72.3
700 180.4 0.972 0.7232 0.1307 180.0 \10 000
157692
99.83
0.0916 130 648
750 180.3 1.923 0.4023 180.0 12.66
180.0 10.75 99.79 16.6 830
825 180.1 1.751 3.156 0.1170 157592
157692
310 14.9
99.79 6.37
875 180.3 2.808 3.509 0.1179 180.0
180.2 9.36 99.80 17.6 543 157892
925 180.6 2.084 2.969 0.1138
157792
616 73.9
99.82 0.0986
975 180.3 1.966 0.7077 180.0 13.61
99.76 3.98 459 158492
1200 181.7 2.266 13.12 0.1394 181.3 9.36
0.1708 178.4 100.00 1567
Sum 178.9 1.950 2.5055 99.70 20.9 639
39Ar age spectrum, 630–975°C fractions (82% of 39Ar) 157694
Flambeau musco6ite c57A27 (J=0.015211wt.=0.0169g %K=6.3)
550 70.91 25.42 23.93 6.230 52.51
88.58 0.86 95.46 12.7 546
700 92.79 2.300 4.114 1.419 153993
96.63 0.85 96.90 11.0 57.5
105.2 1.29 93.38 12.0 1665
850 112.7 1.864 4.353 2.518 172495
2.03 97.95 14.2 \10 000
900 110.6 1.218 3.672 0.7591 108.3 175693
15.8 \10 000 176293
3.26 98.76
950 110.2 0.9344 3.317 0.4545 108.8
108.8 6.27 99.52 14.7 \10 000
975 109.3 0.9773 3.560 0.1685 176192
8.63 99.68 14.8 \10 000
990 108.7 1.074 3.521 0.1114 108.3 175692
\10 000
14.8 99.73
8.98 175792
1010 108.6 1.035 3.544 0.0911 108.4
108.3 8.70 99.67 14.5 \10 000 175792
1030 108.7 1.081 3.600 0.1124
8.76 99.71 13.1 \10 000
1060 108.7 1.095 3.990 0.0984 108.4 175792
\10 000
12.3 99.72
9.19 175992
1100 108.8 1.036 4.258 0.0964 108.5
175892
99.73 12.0 \10 000
1175 108.7 1.128 4.372 0.0901 108.5 12.90
176192
\10 000
11.3 99.70 10.62
1250 109.1 1.143 4.641 0.1024 108.7
99.69 9.03 3243 176192
1325 109.1 1.259 5.790 0.1081 108.8 6.94
99.65 7.22 2659 176092
1500 109.0 1.297 7.244 0.1223 108.6 10.39
108.0 100.00 99.43 11.6 3016
Sum 108.7 1.288 4.501 0.2018 1754
39Ar age spectrum, 900–1500°C fractions (97% of 39Ar) 175995
96-DR-8 musco6ite c53I7 (J=0.007682 wt.=0.0185 g %K=7.2)
0.86 64.87 9.11 9.12 1628925
550 294.1 64.8888 5.736534.9567 190.78
16.4982 189.47 1.88 79.52
630 238.25294.1538 1.6494 31.69 \10 000 1621913
162195
\10 000
92.31 38.32
5.3282 3.04
700 205.28671.6929 1.3637 189.51
162193
740 193.88800.6518 1.2100 189.50
0.5355 188.86 13.59 99.15
780 190.47520.7277 1.5483 33.76 \10 000 161793
22.43 39.55 \10 000 161392
0.1872 99.69
810 188.61010.2200 1.3215 188.03
161292
\10 000
37.22 99.78
0.1286
860 188.30260.6582 1.4042 187.89 26.98
188.56 10.40 99.79 161693
900 188.95720.1741 1.4251 0.1248 36.67 \10 000
161693
188.68 7.21 99.74
950 189.17540.1817 1.5754 0.1561 33.18 \10 000
40.07
0.2095 99.66 \10 000 162093
1025 189.97900.6488 1.3044 189.33 4.32
\10 000
0.2016 189.93 4.04 99.67 33.25
1300 190.55430.3921 1.5718 162393
1615
1.0413 188.51 100.00 98.38
Sum 191.62031.1222 1.4526 35.98 \10 000
39Ar age spectrum, 780–950°C fractions (81% of 39Ar) 161495
161595
3 All fractions (100% of 39Ar)
97-DR-7 musco6ite c57A25 (J=0.015257wt.=0.0154 g %K=7.3)
550 220.7 14.156 6.718 43.211 92.99
86.65 0.36 63.02 6.05 349 1520930
700 137.5 5.809 8.634 17.20
67.43 18.4 816 1490925
1.28
775 124.8 4.302 2.836 13.75 84.16
1.77 82.39 16.1 \10 000
825 107.0 2.237 3.244 6.369 88.13 1537911
1.72 86.92 16.1 \10 000
875 104.4 1.909 3.243 4.611 90.73 156898
\10 000
21.4 96.32
6.64 160593
925 97.56 1.330 2.446 1.206 93.98
88.67 14.90 99.05 21.4 \10 000 154492
975 89.52 1.134 2.441 0.2796
17.54 99.62 21.8 \10 000
1000 84.70 1.129 2.399 0.1024 84.37 149392
21.8 \10 000 148492
11.62 99.62
1025 83.99 1.135 2.393 0.1009 83.67
148592
99.64 21.7 \10 000
1050 84.01 1.075 2.408 0.0937 83.71 9.50
6.60 99.56 20.9 \10 000
1100 84.94 0.9309 2.498 0.1180 84.57 149592
9942 17.2
99.45
6.90 150892
1175 86.14 1.158 3.041 0.1521 85.67
153292
99.44 20.6 \10 000
1250 88.20 1.106 2.533 0.1600 87.71 7.22
152992
\10 000
21.4 99.51 13.83
1500 87.83 1.132 2.448 0.1386 87.39
86.46 100.00 97.65 20.6 \10 000
Sum 88.54 1.234 2.539 0.6959 1518
Notes to data table.
aTemperature in °C measured with a thermocouple on the outside of the Ta crucible.
bThe isotope ratios given are not corrected for Ca, K and Cl derived Ar isotopic interference but37Ar
is corrected for decay using a half-life of 35.1 days. The ratios are corrected for line blanks of atmospheric Ar composition.
cF is the ratio of radiogenic 40Ar to K-derived 39Ar. It is corrected for atmospheric argon and other
