Denise Romano a, *, Daniel K. Holm a, K.A. Foland b. Abstract

Transcription

1 Precambrian Research 104 (2000) 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 b a Department of Geology, Kent State Uni ersity, 22 McGil rey Hall, P.O. Box 5190, Kent, OH 44242, USA b Department of Geological Sciences, Ohio State Uni ersity, Columbus, OH 43210, USA Received 6 October 1999; accepted 28 April 2000 Abstract Twenty-one hornblende and mica 40 Ar/ Ar 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 ( Ma) Penokean orogenic belt in the southern Lake Superior region Ma mica plateau ages from bedrock beneath undeformed Ma quartzites are interpreted as the time of lower temperature ( C) cooling and crustal stabilization after the Penokean orogeny. Six mica ages from bedrock underlying deformed Paleoproterozoic quartzites cluster around 1600 Ma ( Ma). The complete absence of Ma mica ages beneath regions of deformed quartzites suggests widespread heating to temperatures above C during Mazatzal-related deformation and metamorphism. The 1600 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 40 Ar/ Ar dates obtained in this study address the higher-temperature overprinting effects in the southern Lake Superior region. One latest Archean age of 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 ( Ma) and probably reflect variable retention of radiogenic argon. They are interpreted to indicate variable degrees of partial intermediate-temperature ( C) resetting of hornblende argon systematics at Ma. Collectively, these data suggest that the effects of the Mazatzal orogeny in the southern Lake Superior region involved C metamorphism and penetrative deformation Elsevier Science B.V. All rights reserved. Keywords: Ar/Ar thermochronology; Paleoproterozoic; Post-Penokean overprinting; Crustal stabilization * Corresponding author. Fax: address: d.romanol@juno.com (D. Romano) /00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S (00)

2 26 D. Romano et al. / Precambrian Research 104 (2000) Introduction The Ma Penokean orogenic rocks of the North American midcontinent are part of a vast belt of juvenile crust accreted onto the southern 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 preserved within the Penokean province. In particular, 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 Ma, therefore, provides insight, ultimately, into the cratonization of continental crust. The purpose of this research has been to constrain 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-orogenic 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 Minnesota). Many earlier studies have attributed post- Penokean metamorphism to an enigmatic, low-grade but widespread event at ca 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 ( C) disturbance at that time and assigns its cause to orogenic activity along the southern Fig. 1. Proposed distribution of Paleoproterozoic crustal provinces in the United States prior to formation of the Mesoproterozoic igneous and tectonic provinces (modified after Holm et al., 1998b). Cross-hatchure pattern depicts region of inner accretionary belt and Penokean belt deformed during main ca Ma tectonism of outer tectonic belt (after Van Schmus et al., 1993; Holm et al., 1998b). Ba, Barron quartzite; Bb, Baraboo quartzite; S, Sioux quartzite; Mv, Mojave province; Mz, Mazatzal province; Y, Yavapai province; CB, Cheyenne belt. Inset shows map of Paleoproterozoic belts along the reconstructed southern margin of Laurentia Baltica (after Kerr et al., 1997).

3 D. Romano et al. / Precambrian Research 104 (2000) Fig. 2. Summary map of localities dated in this study. margin of Laurentia (Holm et al., 1998b), compatible 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 continents that formed the bulk of Laurentia at 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 continent from southern California to Labrador, consist of an Ma inner accretionary belt and a Ma outer tectonic belt. These provinces are depicted in Fig. 1, which also shows the region of known pre Ma rocks metamorphosed and deformed during the formation of the outer tectonic belt (ca Ma; Van Schmus et al., 1993; Holm et al., 1998b). The Ma Penokean orogeny in Wisconsin 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 margin assemblage (overlying an Archean basement) separated from a southern assemblage of oceanic arcs (the Wisconsin magmatic terranes, WMT) by the ca 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 Ma Eau Pleine shear zone (EPSZ, Fig. 2). Where observed in outcrops, the volcanic rocks are of upper greenschist 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 period of widespread magmatism at 1760 Ma that included rhyolitic volcanism in central and southern Wisconsin and granitic plutonism

4 28 D. Romano et al. / Precambrian Research 104 (2000) 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 km. They were subsequently unroofed and depositionally overlain by Ma cratonic quartzites (Dott, 1983; Holm et al., 1998b) some of which are deformed and metamorphosed to lower greenschist facies (320 0 C; Medaris et al., 1998). 3. Previous thermochronology Thermochronology in the southern Lake Superior 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 ( Ma) in northeast Wisconsin which they named the Goodman Swell (Fig. 2). They interpreted these ages as recording flexural uplift associated with lithospheric 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 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 Paleoproterozoic quartzites to the south from relatively undeformed Paleoproterozoic quartzites to the north. They proposed that in regions where Ma quartzites are absent (or unexposed), cooling ages might serve as a proxy for identifying regions of significant thermal and deformational overprinting of the Penokean orogenic 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 techniques on the coarsest grains that were not composite (usually m). Final separation was done by hand picking followed by washing. The 40 Ar/ Ar measurements on populations of separated grains were performed in the Radiogenic Isotopes Laboratory at Ohio State University using general procedures that have been described previously (Foland et al., 1993 and references therein). Aliquots of about 6 10 mg for mica and mg for hornblende were irradiated in the Ford Nuclear Reactor of the Phoenix Memorial Laboratory at the University of Michigan 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 mass spectrometer, typically in about or 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 uncertainty of 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 emphasize 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 applies 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

5 D. Romano et al. / Precambrian Research 104 (2000) sample localities and corresponding dates are plotted on Fig. 2; thin-section descriptions are available in Romano (1999). Incremental-heating 40 Ar/ Ar age results are illustrated in normal age-spectra diagrams, Fig. 3 for micas (sample numbers 11 21) and Fig. 4 for hornblende (numbers 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 K/Ca and K/Cl ratios that indicate mineral heterogeneities (Fig. 4). In particular, the hornblende 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 correlated with K/Ca and K/Cl ratios, which are much higher than those for hornblende and are observed for lower temperature increments. These results are consistent with the hornblende discor- Fig Ar/ 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 1 sigma uncertainties.

6 30 D. Romano et al. / Precambrian Research 104 (2000) Fig Ar/ Ar 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 discordance, 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 interpreting and accounting 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 separates yield plateaus, which are defined as contiguous gas fractions constituting a majority of the Ar where the variations of ages of individual increments may be attributed to analytical uncertainties. 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.

7 D. Romano et al. / Precambrian Research 104 (2000) Mica ages Five biotite separates yield virtually identical ages between 1576 and 1605 Ma. Plateau ages of , , and 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 Ar released, of 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 reported in Sims et al., 1989) yields a near-plateau date of Ma. This date is nearly concordant with the crystallization age of the granite, suggesting it cooled rapidly after intruding. Two biotite separates ( c11 and c12) from Penokean syntectonic granites yielded anomalously young ages of Ma (plateau) and 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 and Ma, respectively. A third muscovite ( c19) gives a total-gas date of 1518 Ma but a highly discordant spectrum, the saddle-shape of this spectrum (Fig. 3) may indicate excess argon as is frequently the case, and we therefore attribute no geological significance to the 1518 Ma total-gas age of this sample Hornblende ages Seven of the ten hornblende separates give plateau or near-plateau dates (Fig. 4). Surprisingly, most of the hornblende separates analyzed yield ages 1800 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 Ma. Six other hornblende 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 increments, 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 nearplateau at 1745 Ma. Hornblende from a garnet amphibolite ( c2) collected in Cornell yields a plateau date of Ma. Hornblende separate (c5) from amphibolitic gneiss from Neillsville yielded a near-plateau date of Ma; low temperature fractions are as low as 1320 Ma while a plateau is reached for the higher-temperature fractions, which make up 36% of the Ar released, 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 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 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 Ma. Hornblende from amphibolite 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 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.

8 32 D. Romano et al. / Precambrian Research 104 (2000) Implications In interpreting the ages in the context of regional cooling and thermal history, closure temperatures for biotite, muscovite, and hornblende are assumed to be 300, 350, and C, respectively (McDougall and Harrison, 1999) Mica thermochronology In the southern Lake Superior region, the thermal and deformational front identified by Holm et al. (1998b) separates basement rocks with predominantly primary post-penokean cooling ages to the north from rocks with thermally reset (i.e Ma) Rb/Sr and 40 Ar/ Ar mica ages to the south (Fig. 2). The large scatter of the Rb/Sr biotite dates has long been considered problematic (Peterman and Sims, 1988) and requires better assessment of Mesoproterozoic overprinting effects related to intrusion of the 1470 Ma Wolf river batholith and to 1100 Ma midcontinent rift activity. The 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 southwest 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 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 reheating 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 cooling through C at Ma and final conditions reflect exposure by Cambrian time. Loofboro and Holm (1998) concluded that intrusion of the Wolf river batholith could not have caused the cluster of 1600 Ma mica dates in western Wisconsin by partial resetting of mica cooling ages. Because of the difference in closure temperature between biotite and muscovite, the modeling revealed that significant differences 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 thermal pulse imposed at 1470 Ma would partially reset biotite to a ca 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 Ma, would also cause complete resetting of biotite to 1470 Ma. Because our cluster of ca 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 bodies, 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/ Ar ages of 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 suggested by their location north of the thermal/deformational front (Fig. 2).

9 D. Romano et al. / Precambrian Research 104 (2000) Hornblende thermochronology A hornblende plateau date of Mais interpreted to represent complete resetting caused by the 1650 Ma activity noted above. It was reported two similar 40 Ar/ Ar hornblende dates from northeastern Wisconsin and northern Michigan were interpreted as representing examples of complete, albeit localized, resetting of high-temperature minerals in association with Mazatzalage deformation, perhaps caused by fluid-related activity. Such localized hydrothermal, fluid-related resetting has been documented in other Precambrian rocks such as the Elat area of southern Israel (Heimann et al., 1995). Six hornblende samples in this study yield apparent 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 Minnesota, where Penokean rocks of 5 6 kbar paleopressures are exposed, hornblende Ar/Ar ages are uniformly 1760 Ma and indicate crustal stabilization (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 stabilization event (rocks with paleopressures of 2 4 kbar are predominant; Geiger and Guidotti, 1989). Given that crustal stabilization in northern Wisconsin involved only isolated plutonism and lower-temperature cooling, it is unlikely to be responsible for the scatter of post-penokean hornblende 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 ( Ma). Given the evidence for widespread Mazatzal-age resetting based on the mica dates described above, the scatter in the 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 pervasive Mesoproterozoic metamorphism followed by a protracted uplift/cooling history. The results of those studies differ considerably from the thermochronologic results obtained from western Wisconsin. Those studies yield numerous hornblende and mica dates in the 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 Ma). We suggest that a combination of elevated temperatures ( C) and localized areas of enhanced fluid activity associated with Mazatzal deformation provide the simplest explanation 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 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 toward the end of the Penokean orogeny, was possibly beyond that which was significantly affected by the Penokean orogeny to the north. A lowtemperature 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 metamorphic event that Cummings (1984) recognized in the Big falls area (near Little falls in Fig. 2) at approximately this time Microtextural studies Petrographic study of dated samples from three localities (Jim fall, Little falls, and Cornell; Fig. 2) focused on microtextural indicators of ductile deformation in both quartz and feldspars (Perham, 1992; Romano, 1999). Samples from Jim falls and Little falls exhibit both high- and intermediate-

10 34 D. Romano et al. / Precambrian Research 104 (2000) temperature deformational features. High-temperature features ( C) of recrystallized plagioclase 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 ( Ma), interpreted as representing high-temperature (i.e. 500 C) total resetting due to Mazatzal orogenic activity. A hornblende sample from Cornell gives an apparently partially reset date of Ma. The microstructures in Cornell samples indicate that the temperature of deformation was approximately C (plagioclase ductilely deformed). Microstructures from all areas show at least some ductile deformation in plagioclase whereas hornblende dates from these samples yield highly discordant dates. This suggests that there is not a link between the resetting of hornblende and microstructural features that imply high-temperature deformation. Instead, the microstructures appear to dominantly record earlier high-temperature events on which partial argon loss was subsequently superimposed. 7. Temperature-time reconstruction New mica and hornblende 40 Ar/ Ar thermochronologic data from the southern Lake Superior region provide important information about the timing, extent, and nature of metamorphic overprinting of the Penokean orogenic belt. Previous 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 petrologic investigation of the deformed Baraboo quartzite (Fig. 1) which indicates metamorphic temperatures of C (Medaris et al., 1998). It is suggested that the deformation and metamorphism of the Baraboo quartzite occurred during widespread Mazatzal-related overprinting of the Penokean orogenic belt, and that the cluster of ca Ma mica 40 Ar/ 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).

11 D. Romano et al. / Precambrian Research 104 (2000) 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 partially. One hornblende date of 1638 Ma from Little falls suggests that enough heat was produced locally, probably by the infiltration of hot fluids, to completely reset the mineral. The remainder of the path shows thermal restabilization through mica closure temperatures after the Mazatzal orogeny. 8. Post-Penokean tectonic and crustal evolution Fig. 6. Proposed post-penokean tectonic evolution of central and northern Wisconsin. See text for detailed description. Patterning follows that of Fig. 2. A time-temperature reconstruction (Fig. 5) illustrates the thermal history of the area based on the new data. The cooling path after the Penokean orogeny north of the thermal/deformational front is shown through two micas (represented by filled circles) at 1755 Ma. The 1755 Ma primary cooling ages suggest significant amounts of exhumation immediately after 1760 Ma magmatism at 9 11 km depths and probably represents collapse and stabilization of overthickened crust (Holm and Lux, 1996; Holm et al., 1998a). Stabilization probably involved a return to relatively normal thickness crust and hence was followed shortly by deposition of mature quartzites (Fig. 5). 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 million year was interrupted by magmatism (emplaced at km depths in northern Wisconsin and extruded atop the crust in southern 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 thermal 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 (Cannon 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 proposed nearly 20 years ago by Dott (1983) that the

12 36 D. Romano et al. / Precambrian Research 104 (2000) 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 thermally equilibrated to the point of cratonization. The 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 lithologic and/or age-related strength differences which may have existed across the suture, as also suggested 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 profuse 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 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 constructive reviews, which significantly improved the manuscript. This work benefited much from various discussions with David Schneider, Craig Mancuso, and Jason Rampe. Appendix A. 40Ar/ Ar data tables Summary of 40 Ar/ Ar results. Number Sample name %K t p 2 96-DR-4 hornblende DR-7 hornblende JFS-H hornblende DR-22-H hornblende DR-22-H hornblende DR-8 hornblende DR-12 hornblende DR-15 hornblende DR-17 hornblende WIS-GD-H hornblende WO-2 hornblende WI-RG biotite Biotite JFS-B biotite t tg

13 D. Romano et al. / Precambrian Research 104 (2000) DR-22 B biotite WIS-GD-B biotite GWD-1 biotite W-249 biotite W-55 biotite Flambeau muscovite DR-8 muscovite DR-7 muscovite %K is the approximate K concentration of sample in wt.%, derived from Ar yields. t tg is The total-gas age derived from the summation of all fractions of the incremental-heating analysis. t p is 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. 40 Ar/ Ar analytical results. T a 40Arb / 38 Ar b / 37 Ar b / 36 Ar b / Ar F c Ar d 40 Ar* e K/Ca f K/Cl g Apparent Ar Ar Ar ( 100) (%) (%) ( 100) Age (Ma) h 96-DR-4 hornblende c57a2m (J= wt. = g %K=0.37) Sum Ar age spectrum, C fractions (91% of Ar) DR-7 hornblende c57a5 (J= wt. = g %K=0.55)

14 38 D. Romano et al. / Precambrian Research 104 (2000) Sum Ar age spectrum, C fractions (85% of Ar) JFS-H hornblende c55b12 (J= wt. = g %K=1.1) Sum DR-22-H hornblende c57a18 (J= wt. = g %K=0.54) Sum DR-22-H hornblende c60b28m (J= wt. = g %K=0.50)

15 D. Romano et al. / Precambrian Research 104 (2000) Sum Ar age spectrum, C fractions (36% of Ar) DR-8 hornblende c57a8m (J= wt. = g %K=0.40) Sum Ar age spectrum, C fractions (63% of Ar)

16 40 D. Romano et al. / Precambrian Research 104 (2000) DR-12 hornblende c57a12 (J= wt. = g %K=0.83) Sum Ar age spectrum, C fractions (36% of Ar) DR-15 hornblende c57a14 (J= wt. = g %K=0.84) Sum Ar age spectrum, C fractions (76% of Ar) WO-2 hornblende c55b11 (J= wt. = g %K=0.42) Sum Ar age spectrum, C fractions (80% of Ar) DR-17 hornblende c57a15 (J= wt. = g %K=0.14)