Timing Constraints and Significance of Paleoproterozoic
TIMING CONSTRAINTS AND SIGNIFICANCE OF PALEOPROTEROZOIC
METAMORPHISM WITHIN THE PENOKEAN OROGEN, NORTHERN
WISCONSIN AND MICHIGAN (USA)
A thesis presented to
the faculty of
the College of Arts and Sciences of Ohio University
In partial fulfillment
of the requirements for the degree
Master of Science
Shellie R. Rose
June 2004 This thesis entitled
TIMING CONSTRAINTS AND SIGNIFICANCE OF PALEOPROTEROZOIC
METAMORPHISM WITHIN THE PENOKEAN OROGEN, NORTHERN
WISCONSIN AND MICHIGAN (USA)
BY
SHELLIE R. ROSE
Has been approved for
the Department of Geological Sciences
and the College of Arts and Sciences by
David Schneider
Assistant Professor of Geological Sciences
Leslie A. Flemming
Dean, College of Arts and Sciences Rose, Shellie R. M.S. June 2004. Geological Sciences
Timing constraints and significance of Paleoproterozoic metamorphism within the
Penokean orogen, northern Wisconsin and Michigan (USA). (84 p.)
Director of Thesis: David Schneider
The Paleoproterozoic Penokean orogen of the Lake Superior region, records a dynamic and protracted tectonic history of terrane accretion and orogenic collapse.
Metamorphic features, located within a narrow corridor of deformed supracrustal rocks, include gneiss domes, fault-bounded structural panels, and concentric metamorphic isograds (nodes). This investigation has led to constraining the timing of metamorphism by utilizing U-Th-Pb monazite geochronometric and Ar-Ar muscovite thermochronometric techniques from samples collected across the orogen. Metamorphic and cooling ages reveal peak Penokean (M1) metamorphism at ~1830 Ma, a weaker 1800
Ma (M2) thermal pulse, tectonic collapse, M3 metamorphism, and rapid unroofing at
1760 Ma related to Yavapai convergence. Tectonic extrusion of a mid-crustal block, and basement diapirism via a density inversion are suggested as the mechanism. South of the deformation corridor, Penokean and Yavapai events are overprinted by 1630 Ma
Mazatzal and 1470 Ma Wolf River Batholith deformational and thermal events.
Approved: David Schneider
Assistant Professor of Geological Sciences ACKNOWLEDGMENTS
I would like to express my sincerest thanks to my advisor, David Schneider for all of his academic, financial, and personal support. Although he may not realize it, Dave has taught me many lessons, both scholastically and personally. His perseverance to constantly challenge my abilities has created a more observant, open-minded, tougher individual than the meek spirit I was two years ago. I will carry the newfound knowledge he has given me for the rest of my life; as a result I have become a better person and scientist.
A special thanks goes to Daniel Holm for being such a supportive and caring committee member. His astounding knowledge has provided countless insight into this project and sparked a whole new interest in the fascinating dynamics of Precambrian geology. Daniel’s encouragement for me to partake in this project, invaluable input, in addition to Dave’s guidance, have successfully brought me to the end of this journey. I am forever grateful.
Last but not least, I would like to send my regards to Damian Nance who also contributed his time and efforts by serving on my committee, in addition to the National
Science Foundation for financially supporting this project. Thanks to Mom, Dad, Nen,
Rob, Ethan, and Joshua for their much needed support and humor at all times. Mare, thanks for your help in the field and labs, and for your friendship. Let’s party! 5 TABLE OF CONTENTS
Page
Abstract…………………………………………………………………………………3
Acknowledgments………………………………………………………………………4
List of Figures…………………………………………………………………………..6
List of Tables..…………………………………………………………………………. 7
Introduction……………………………………………………………………………..8
Geologic Setting………………………………………………………………………...11 Penokean Orogeny……………………………………………………………...11 Post Penokean Orogenic Events……………………………………………….. 16
Methodology…………………………………………………………………………....20 Monazite Geochronometry…………………………………………………….. 20 Muscovite Thermochronometry………………………………………………...27 Sampling Strategy………………………………………………………………30
Results…………………………………………………………………………………..32 Park Falls Panel…………………………………………………………………32 Republic Node…………………………………………………………………. 33 Peavy Node…………………………………………………………………….. 45 Niagara Fault Zone…………………………………………………………….. 49
Discussion………………………………………………………………………………52 Geochronometry and Thermochronometry……………………………………..52 Implications for the Development of the Gneiss Dome Corridor………………58
Conclusions……………………………………………………………………………..64
References Cited……………………………………………………………………….. 66
Appendix A: Electron Microprobe Chemical Compositions of Monazite…………….. 78
Appendix B: Sample Descriptions and Locations……………………………………... 83 6 LIST OF FIGURES
Page
1. Geologic Map of Paleoproterozoic Laurentia……………………………………….9
2. Reconstructed Tectonic Map of the Lake Superior Region…………………………12
3. Modified geologic and thermochronologic map of Wisconsin and Michigan……... 14
4. Metamorphic map pattern of the Penokean orogen………………………………… 16
5. SEM/BSE images of monazite in thin section………………………………………23
6. X-ray elemental maps of sample MIST……………………………………………..25
7. Ar-Ar laser probe cooling ages of muscovite grains……………………………….. 29
8. Ion microprobe age results of sample 96-17, Park Falls terrane………………….…34
9. Ion microprobe age results of sample MIST, Republic node………………………. 39
10. EMPA age results of sample MIST, Republic node……………………………….41
11. Ion microprobe age results of sample CREP, Republic node……………………...43
12. Monazite elemental maps of sample CREP, Republic node…………………….…44
13. EMPA age results of sample CREP, Republic node…………………………….…46
14. Ion microprobe age results of sample PVD, Peavy node…………………………..47
15. EMPA age results of sample HRR, Peavy node…………………………………...48
16. X-ray elemental monazite maps of sample PG-03, Niagara fault zone……………50
17. EMPA age results of sample PG-03, Niagara fault zone…………………………..51
18. Temperature vs. Time graph of Penokean orogen…………………………………53
19. Suggested model for gneiss dome corridor formation……………………………..62 7 LIST OF TABLES
Table Page
1. Ion microprobe Pb-Pb monazite isotopic data, metamorphic ages of Penokean orogen………………………………………………………………………….. 35
2. Ar-Ar laser probe muscovite data, cooling ages of Penokean orogen………………37
3. Summary of EMPA Th-U-total Pb monazite ages…………………………………. 42
4. Summary of 1900-1400 Ma geologic events in the southern Lake Superior Region, U.S.A………………………………………………………………………………. 64 8 INTRODUCTION
Metamorphism is a complex process of pressure and temperature dynamics that can be heterogeneous in any given time and place during orogenesis (Zeitler, 1989).
Presently, the determination of peak metamorphic conditions of a rock suite is difficult, especially when multiple thermal episodes are involved. For this reason, it is important to understand how particular metamorphic events are related to one another in space and time, and how the final product is represented in the observed metamorphic map pattern.
Many Precambrian orogenic belts notably lack intact preservation of primary tectonic events due to subsequent thermal and deformational episodes that have overprinted the original signatures. By dating metamorphic and associated accessory minerals, it is possible to identify metamorphic and cooling episodes that occur across an ancient orogen with respect to crustal thickening and unroofing cycles. Moreover, well- constrained timing of metamorphism in fossil orogenic belts is important in understanding the fundamentals of tectonics, as this information can provide time and depth dimensions unobtainable in contemporary orogens (Murphy and Keppie, 2003).
There are a variety of reasons as to why temperature and pressure change within the crust, but for the Lake Superior region of northern Wisconsin and Michigan, I propose as the cause a long-lived convergent margin and related multiple metamorphic events.
The conventional explanation for the observed metamorphism in the Lake
Superior region attributes it to the result of a single accretionary event that occurred as the Archean Superior craton collided with an island arc (Sims et al., 1989). This region, however, lies within a belt of successive Proterozoic accretionary zones that extend from 9
erior Sup L. THO Superior GFtz GLtz
Wyoming P ? nY
ATION RM FO DE Ga 5 ST 1.6 U sY CR Mv a M 0 NORTH 0 6 1 - E R P Mz F T O EN 300 km XT N E ER ST EA ?
Figure 1. Simplified geologic map of crustal provinces which constitute Laurentia throughout the Paleoproterozoic. Five major tectonic events are exhibited here, including (shaded) the Trans-Hudson, Penokean, and Mojave orogenies, (2.0-1.8 Ga), Yavapai orogeny (1760-1700 Ma), and the Mazatzal orogeny (1660-1600 Ma). Note the parallelism of the accreted terranes, suggesting long-lived convergence along the southern margin (after Karlstrom et al., 2001). Black box = study area. Provinces: P = Penokean, THO = Trans-Hudson Orogeny, GFtz = Great Falls tectonic zone GLtz = Great Lakes tectonic zone, Mv = Mojave, nY = northern Yavapai, sY = southern Yavapai, Mz = Mazatzal. Wyoming and Superior terranes are Archean provinces.
Mexico to Scandinavia (Karlstrom et al., 2001; Figure 1). The three major
Paleoproterozoic accretionary episodes preserved in the North American mid-continent include the Penokean-Mojavian (1875-1835 Ma), Yavapai (~1760-1700 Ma), and
Mazatzal (~1660-1600 Ma) orogenies, and have been suggested to be part of a series of accretionary events during the growth of Laurentia (Karlstrom et al., 2001).
The purpose of this investigation is to document timing constraints on the complex metamorphic map pattern within the Lake Superior region, a pattern recently identified by Schneider et al. (2004) to reside in a corridor containing east-west elongated, Archean-cored gneiss domes. Recent models have been suggested to explain 10 the process of gneiss dome formation in metamorphic belts worldwide (Vanderhaeghe et al., 1999; Zeitler et al., 2001; Rey et al., 2001; Teyssier and Whitney, 2002), including extensive localized erosion and lateral flow and/or vertical flow of the middle to lower crust. Contemporary views on the development of gneiss domes within the Lake
Superior region (Holm and Lux, 1996; Schneider et al., 1996; Marshak et al., 1997) argue that the gneiss domes exposed within the Penokean orogen are not simply compressional in nature (i.e., Attoh and Klasner, 1989; Gregg, 1993; Sims, 1996) but are related to a protracted and complex thermal history (Schneider et al., 2004).
Previous geochronometric studies within northern Wisconsin and Michigan have focused on documenting cooling episodes within the basement and cover rocks, utilizing
40Ar/39Ar methods. Recent technological advancements have allowed for increased
spatial, mass, and time resolution of elemental and isotopic concentrations of the
accessory mineral of interest in order to determine the timing of metamorphism and
cooling. New in-situ U-Th-Pb geochronometric and laser probe Ar-Ar
thermochronometric data collected in this investigation, combined with previous age
dates, allow for constraining the timing of peak metamorphism and subsequent cooling
throughout ~500 m.y. of repeated collisions along the southern margin of Laurentia.
Results reported here will also place further rates on gneiss dome development, via
vertical extrusion of the mid-crust, proposed in Schneider et al. (2004) with respect to this
long-lived convergence. 11 GEOLOGIC SETTING
Penokean Orogeny
The Archean Superior Province of North America is an amalgamation of two micro-continents consisting of variable rock types, metamorphic grade, and crustal histories as old as 3600 Ma (Southwick, 1993). The northern portion of the Superior
Province includes the Wabigoon, Quetico, and Wawa subprovinces, made up of greenschist- to lower amphibolite-facies plutonic, volcanoplutonic, and metasedimentary units, and collectively known as a granite-greenstone terrane. A quartzofeldspathic upper amphibolite- to granulite-facies gneiss known as the Minnesota River Valley subprovince
(Perkins and Chippera, 1985) represents the southern portion of the Superior Province, and is separated from the northern Superior Province by the east-west trending Great
Lakes tectonic zone (GLtz; Figure 2).
The GLTZ is a late Archean paleosuture (Sims, 1991) between the two Superior micro-continents and can be traced from the Grenville front in Ontario southwest to central South Dakota, where it is truncated by the ~1.8 Ga Trans-Hudson orogen (Figure
1). Geochronometric studies on conglomerates and granitic plutons within the GLTZ bracket suture activity between 2696 Ma and 2686 Ma (Davis et al., 1989). Davis et al.
(1989) also suggested that related deformation caused by the collision of the two Archean micro-continents was diachronous and/or younger than these dates. Following deformation, metamorphism, and “cratonization”, the southern gneiss terrane began to rift from the Superior Province just south of the GLTZ at roughly 2.0 Ga (Roscoe and 12
2.7 Ga Archean Paleoproterozoic granite-greenstones metasediments GLtz
FF F MGD NFZ
GLtz ? Wisconsin Magmatic Terrane B M C E North EPSZ >3.0 Ga Archean 100 km gneisses
Figure 2. Reconstructed tectonic map of the Lake Superior region, sans 1.1 Ga Midcontinental Rift (after Schneider et al., 2004). Shaded region illustrates the gneiss dome corridor from which samples were collected; solid black pattern indicates location of gneiss domes. Progressively lighter gray pattern represent granites dated ca. 1800, 1775, and 1750 Ma respectively, and young to the southeast. The four structural panels are outlined just north of the Niagara fault zone and nodes are located to the northeast of the panels. GLtz = Great Lakes tectonic zone, NFZ = Niagara fault zone, EPSZ = Eau Pleine shear zone, MGD = McGrath Gneiss Dome, ECMB = East Central Minnesota Batholith, FFF = Flambeau Flowage Fault.
Card, 1993). The presence of ~2100 Ma mafic dikes that crosscut much of the northern region of Michigan and Wisconsin give evidence for the rifting event prior to
Paleoproterozoic Penokean accretion (Sims et al., 1980). Remnants of the Minnesota
River Valley subprovince, south of the GLTZ, became the basement upon which the passive margin/foredeep sediments were deposited.
Paleoproterozoic sediments produced from rifting, along with a passive margin- to-foredeep sequence, lay atop Archean basement within northern Wisconsin and
Michigan, forming the Marquette Range Supergroup (MRS). This assemblage of sedimentary rocks is primarily comprised of a lower rifted Archean cratonic unit exposed 13 in the north, and an upper Proterozoic volcanic unit to the south (Barovich et al., 1989).
Crystallization ages of an 1875 Ma interbedded rhyolite, constraining sediment deposition and extensional volcansim, are similar to ages of arc volcanism directly to the south and are related to oblique subduction (Schneider et al., 2002).
The Penokean orogeny is the first island arc/microcontinental collision that occurred following initiation of a subduction zone along the southeastern margin of the
Archean Superior Province (Van Schmus, 1976; 1980). Formation of an arc terrane known as the Wisconsin Magmatic Terrane (WMT) began at ~ 1870 Ma on the southern overriding plate. Subduction continued for nearly 40 m.y. (Hoffman, 1989; Schneider et al., 2002), leading to oblique collision, crustal thickening, and accretion of the WMT with the former passive margin sequence of the Archean Superior Province (Holm et al., 1988;
Riller et al., 1999; Schneider et al., 2002). Following deformation, thickening, and accretion, the subduction zone apparently migrated (jumped) south, juxtaposing the arc terrane between the northern Archean Superior Province and a southern Archean microcontinent, the Marshfield terrane (Figures 2 and 3).
The Niagara fault zone (NFZ), a steeply south-dipping east-west striking fault, is interpreted to be the 1860 Ma northern paleosuture between the continental margin and arc terrane (Larue, 1983). This fault contains a dismembered Paleoproterozoic ophiolite suite described by Schultz (1987) that is well exposed in the east. Immediately north of and adjacent to the NFZ lie four fault-bounded metamorphic panels consisting of the Park
Falls, Watersmeet, Beechwood, and Iron River terranes. To the northeast lie the 14
Lake Superior
Keewanawan Rift System 1678 1716 Michigan Wisconsin 1760 1765 1755 1738 Continental Margin Assemblage al Front ormation ult 0Def 1653 Fa 16961696 163 Flambeau Flowage Barron 1673 Quartzite 1569
1080 1759 1067 1265 1576 1358 1135 1128
1366 1759 1605 Wisconsin Magmatic 1456 Terrane McCaslin Quartzite 1435
1460 Flambeau Quartzite 1372 1581 Wolf 1579 River E a Batholith u 1475 Pl ei 1428 1427 ne 1461 1618 Sh ear 1440 Zone 1392 1403 Green Bay 1170 1518 Marshfield 1404 1357 1582 Terrane 1409
1600 1415
Hamilton Paleozoic Cover Mounds Quartzite
PALEOZOIC COVER QUARTZITES WISCONSIN MAGMATIC TERRANE
KEWEENAWAN RIFT MARSHFIELD TERRANE CONTINENTAL MARGIN SYSTEM ASSEMBLAGE
WOLF RIVER BATHOLITH THRUST FAULT
Figure 3. Modified geologic and thermochronologic map of Wisconsin and northern Michigan (modified from Holm et al., 1999). Ages are previous results of Ar-Ar muscovite (open circles) and biotite (closed circles) analyses (see text for references). Dashed line indicates the extent of the 1630 Ma Mazatzal deformational and thermal front (Holm et al., 1998b). Shaded regions indicate metamorphic grade (dark = medium- to high-grade; light = low-grade). 15 Republic and Peavy metamorphic nodes, Archean-cored gneiss domes surrounded by metamorphosed continental sequences of the Marquette Range Supergroup. The western portion of the NFZ is not well defined, as the intrusion of the East Central Minnesota
Batholith has concealed the true extent into Minnesota (Figure 2).
The final product of the Penokean orogeny is a narrow zone approximately 100 km wide, defined by Archean basement and MRS sedimentary rocks that are variably metamorphosed and deformed. Preserved deformational and metamorphic features are located within a gneiss dome corridor that is sub-parallel to the Penokean suture within northern Wisconsin and extends west to the McGrath gneiss dome in eastern Minnesota
(Figure 2). It should be noted that this gneiss dome corridor is bounded to the north and south by the GLTZ and NFZ, respectively, and also represents the boundaries of the field area in this investigation.
Regional metamorphic grade within northern Wisconsin and Michigan increases southward from greenschist-facies in the north to amphibolite-facies (staurolite zone) just north of the NFZ. South of the NFZ, metamorphic grade within the WMT exhibits a low- grade greenschist facies except in the vicinity of plutons (Geiger and Guidotti, 1989).
The Peavy and Republic nodes exhibit concentric isograds culminating at andalusite-
(Republic) and sillimanite-facies (Peavy) metamorphism in the cores. Collectively, these metamorphic features constitute the metamorphic map pattern as illustrated in Figure 4.
The first reliable ages of peak (M1) metamorphism (~1830 Ma) within the
Penokean orogen were recently provided by Schneider et al. (2004). Monazite-bearing rocks collected across the orogen yielded ages of ~1830 Ma within the Peavy node, 16
Metamorphic nodes and structural panels of eastern Penokean orogen MI & WI Republic
KEAN FOLD & THRUST BELT PENO MIST
terrane CREP
Michigamme terrane Watersmeet terrane terrane Iron River 96-17 Beechwood PVD 94-MI-1 Park Falls Niagara Fault Zone HRR DH-PF-99 Peavy terrane WISCONSIN MAGMATIC TERRANES 97-CM-10 PG-03
North
40 Miles
Figure 4. Metamorphic map pattern of the Penokean orogen of northern Wisconsin and Michigan, showing locations of samples collected for this study. The Michigamme subterrane is regional greenschist facies. The structural panels range from kyanite zone (Watersmeet), sillimanite zone (Park Falls), to lower- amphibolite facies (Beechwood and Iron River), while the Republic and Peavy nodes culminate in andalusite and sillimanite zones, respectively. South of the Niagara fault zone, greenchist is dominant. (See text for references.)
Watersmeet, and Park Falls panels. These ages likely reflect the timing of peak metamorphism related to collision of the WMT with the Superior Province. The 1830
Ma thermal pulse also coincides with emplacement of undeformed granites that pierce the
NFZ and Eau Pleine shear zone, the southern suture between the WMT and Archean microcontinent (Schneider et al., 2002; Sims, 1989).
Post Penokean Orogenic Events
Following the Penokean collisional events, isostatic adjustments, and cooling, it appears that the orogen experienced three episodes of magmatism (ca. 1800, 1775, and 17 1750 Ma; Holm et al., 2004). Holm et al. (2004) noted that these plutons exhibit a younging age-trend to the southeast (Figure 2) and perpendicular to the accreted terranes, suggesting that slab rollback of the northwestern subducting Yavapai plate occurred.
Following 1830 Ma metamorphism, a weaker ~1800 Ma thermal event (M2), as eluded by igneous and metamorphic dates, is laterally extensive across the orogen and little evidence exists to support significant differential unroofing until that time.
The Yavapai orogeny (1760-1700 Ma) represents the second subparallel orogenic event to occur along the long-lived convergent margin of Laurentia (Karlstrom et al.,
2001) and is coincident with a third episode (M3) of metamorphism in the orogen.
Evidence for continued collision and accretion is preserved within the Penokean orogen in the form of thermally overprinted or partially reset mineral ages. Previous work
(O’Boyle, 2003; Schneider et al., 2004) report geochronometric results for the western
Park Falls and Watersmeet terranes at 1760 Ma. Thermochronologic 40Ar /39Ar results
(Holm and Lux, 1996; Schneider et al., 1996; Holm et al., 1998; Mancuso, 2000; Romano
et al., 2000; and Holm et al., 2001) indicate that deep crustal rocks within the Penokean
orogen experienced significant unroofing between 1770 to 1730 Ma, possibly during
gneiss dome corridor formation. This, in combination with the new igneous ages,
indicates that the 1775 and 1750 Ma magmatic pulses occurred immediately prior or
synchronous to exhumation, suggesting that the M3 thermal event is possibly a result of
weakened, over-thickened crust promoting orogenic collapse in the area (Vanderhaeghe
et al., 1999). These combined metamorphic and cooling ages suggest continued slab 18 rollback to the southeast, facilitating granitic magma genesis and a subsequent metamorphic event (Yavapai collision) to affect the Penokean orogen.
Shortly after Yavapai collision, the Penokean crust had been exhumed and depositionally overlain by the ~1700 Ma Baraboo Interval quartzites (Holm et al., 1998b;
Medaris et al., 2002). Following deposition, the third and final significant orogenic event of Paleoproterozoic amalgamation, the Mazatzal orogeny (1660-1600 Ma), occurred. At ca. 1650 Ma, Mazatzal collision regionally metamorphosed the Baraboo quartzites south of the Niagara fault zone (Holm et al., 1998b; Medaris et al., 2002; and Schneider et al.,
2004). Subparallel with the NFZ, the 1630 Ma thermal/deformational front, a sharp boundary between northern 1760-1730 Ma cooling ages and southern 1630 Ma thermally reset ages (Holm et al., 1998b), represents the northern-most extent of Mazatzal deformation (Figure 3). Most notably, the Baraboo Interval quartzites, including the
Baraboo, Hamilton Mounds, McClasin, and Waterloo quartzites, are highly folded and metamorphosed south of the front but remain horizontal to the north (Holm et al., 1998b;
Medaris et al., 2003).
Similarly, the 1470 Ma intrusion of the Wolf River Batholith (WRB) into the WMT in central Wisconsin metamorphosed these quartzites locally around the plutons (Medaris et al., 2002; Figure 3). The extent of metamorphism caused by circulating Wolf River
Batholith fluids are reflected in reset Ar-Ar biotite and muscovite ages (~1460 to 1360
Ma; Holm et al., 1999; Mancuso, 2000). Results of those studies indicate that the effects of metamorphism from the WRB may extend north to the NFZ, south to the Marshfield terrane, and west to the central portion of the WMT. However, overprinting caused by 19 feeder dikes from the Keeweenawan Midcontinental Rift (1100 Ma) in addition to
Paleozoic cover rocks to the east mask the true extent of the batholith. Ultimately, the intrusion of the WRB records one of the last Paleoproterozoic thermal events to overprint the Penokean orogen of Wisconsin and represents one of the final steps of cratonization of North America. Previous Ar-Ar work has effectively illustrated how consecutive tectonic events along a ~500 m.y. long-lived convergent margin have been preserved in a southeasterly younging pattern (Karlstrom et al., 2001). This study attempts to further constrain the timing of each accretionary and thermal event, supplementing existing cooling age data and establishing a reliable tectonic history for the Lake Superior region. 20 METHODOLOGY
In an attempt to constrain metamorphism within the Penokean orogen, monazite
U-Th-Pb geochronometry and Ar-Ar muscovite thermochronometry were used to confine the timing of regional metamorphism and establish subsequent cooling rates, respectively. The ages obtained in this study, in compilation with previously published results, are applied to their respective metamorphic features, thus establishing a detailed tectonic history. Overlapping techniques are used to supplement each other and provide greater detail of the isotopic systematics when necessary.
Monazite Geochronometry
Currently, the study of lithospheric processes involving burial and uplift in conjunction with surficial processes controlling exhumation, are necessary in order to understand the geodynamic nature of tectonic histories (Koons, 1990; Beaumont et al.,
1992). The processes active during orogenesis are often ambiguous unless pressure- temperature-time evolutions can be determined. Also, timing of peak metamorphism is difficult to determine from cooling ages obtained through conventional Ar-Ar and U-Pb analysis, because of considerable mineral inheritance and elemental loss during subsequent metamorphic episodes. However, technological advances in isotopic and elemental dating of metamorphic minerals has made the correlation between metamorphism and tectonic processes fairly straightforward. Monazite, a U-, Th-, and
Pb-rich accessory mineral, is commonly used in geochronometric analyses to constrain the timing of metamorphic events in medium- to high-grade metapelitic gneisses and schists (Spear and Parrish, 1996; Foster et al., 2000; Harrison et al., 2002). Advances in 21 high spatial resolution micro-analytical techniques for isotopic and elemental analyses have provided an avenue for examining sub-mineral (chemical) domains within single grains, allowing for multiple episodes of monazite growth to be dated (e.g., Williams and
Jercinovic, 2002).
Monazite, a Rare Earth Element-bearing phosphate mineral, is widely used in U-
Th-Pb dating because it commonly occurs as an accessory mineral in many rock types, making it widely available for geochronometric analysis. Notably, monazite also contains large amounts of Th and U with small amounts of common Pb (204Pb). One
advantage of utilizing monazite in geochronometry is its high closure temperature of
~900°C (Cherniak et al., 2004), below which diffusion of Pb occurs at an extremely slow rate (Parrish, 1990; Cherniak et al., 2000; 2004). Consequently, the ability of daughter
Pb to diffuse out of a monazite crystal, thereby resetting it, is unlikely, unless a significant long-term reheating event (>>800°C), or deformationally enhanced dissolution/reprecipitation occurs. Monazite also has the unique capability for crystal growth between ~800°-300°C on the prograde path following allanite breakdown, and
during retrograde dissolution/reprecipitation within the garnet zone (Catlos et al., 2002).
Therefore, the final products of monazite after metamorphism may be crystal domains
that are temporally and spatially distinct.
Monazite grains are often found within amphibolite-facies and higher-grade
metamorphic rocks along planes of foliation within the matrix that increase susceptibility
to overprinting during subsequent tectonic events. In some cases, however, monazite
occurs as inclusions within porphyroblasts such as garnet and staurolite that may armor 22 the inclusions from radiogenic Pb-loss and dissolution (Catlos et al., 2002). Monazite grains located within porphyroblasts can identify whether the timing of the host mineral growth was synchronous or prior to monazite growth (Catlos et al., 2002), a concept relinquished in conventional dating methods. These petrographic relationships may influence the ages obtained through geochronometry and are considered in the interpretation of metamorphic and tectonic processes. In this study, monazite grains were analyzed in situ via high-spatial resolution geochronometric ion and electron microprobe techniques so that petrographic relationships between monazite and the rock sample could be preserved. Monazite occurrence in this study are described as 1) inclusions within porphyroblasts, 2) located along planes of foliation, and 3) randomly dispersed throughout matrix material.
Ion Microprobe Techniques
Sample preparation and analytical procedures for the ion microprobe technique were carried out according to those outlined in Catlos et al. (2002). Four samples of
Archean and Proterozoic basement and cover units were collected from the Park Falls panel, and Peavy and Republic nodes. Monazite grains were identified in thin section using SEM backscatter electron (BSE) detectors, and then imaged to characterize the crystal texture and domain relationship of each grain (Figure 5). Portions of the thin sections containing the target monazite grains were then cut and fabricated into epoxy mounts that were ultimately Au coated prior to analysis.
Ion microprobe spot analyses of 207Pb/206Pb and 232Th/208Pb isotopes were performed using the Cameca ims1270, located at the University of California, Los 23 A
Qtz St Foliation plane
Mnz Bi
150 µm
B C
20 µm 0.5 mm
D
25 µm
Figure 5. SEM/BSE images of monazite in thin section: A) petrographic relationships of monazite with respect to other minerals and metamorphic fabric (MIST). B) preferred analytical quality of monazite (CREP) with little to no inclusions of high-Th bearing apatite and thorite. C) an example of monazite located within the matrix and not in foliation planes (CREP). D) poor quality, skeletal monazite (CREP) containing large inclusions of apatite and thorite (bright white spots).
Angeles. Variations of Th-Pb concentrations were monitored via a calibration curve, carried out on monazite mineral standard, 554 of known age 45 ± 1 Ma (Harrison et al.,
1999). Precision of the ion microprobe is determined by the reproducibility of the calibration curve, which is commonly ± 1% to 2% (Harrison et al., 1995). Therefore, standards were run intermittently to monitor any changes within the calibration curve 24 during analyses. Analyses were conducted using a primary O- ion beam of ~4 nA, with a
sputter diameter of approximately 13 x 18 µm. Such high spatial resolution allows for
dating monazite grains as small as ~15 µm in diameter while preserving the textural
relationships (Harrison et al., 1995). In situ analyses were preferred due to high-spatial
resolution capabilities and to readily identify metamorphic fabric and petrographic
relationships of monazite with respect to the mineralogy of the samples. In addition, in
situ work can be carried out expeditiously as opposed to standard sample crushing and
mineral separation. Up to four spots were analyzed on each grain to identify any age
variations within each monazite.
Age data, were statistically analyzed through Isoplot to calculate weighted
averages and means (error bars are at the 1 sigma level) and to construct cumulative
probability curves (at the 95% confidence level). The distribution of ages and errors
shown in the weighted average diagrams are used to identify metamorphic events
represented by a series of similar ages within the sample. Cumulative probability peaks
illustrate the timing of metamorphic events that coincide with similar ages given in the
weighted average diagrams. Additional age vs. isotope variation diagrams were also
utilized to identify analytical issues and support statistical results.
Electron Microprobe Analyzer Techniques
Sample preparation and analytical procedures for the electron microprobe
analyzer (EMPA) technique were conducted according to the methods discussed in
Williams and Jercinovic (2002). Similar to the ion microprobe technique, thin sections
were imaged via SEM/BSE methods to locate monazite grains, and to identify the context 25 of monazite occurrence and metamorphic fabric within the sample (Figure 5). The
EMPA produces a focused beam <5 µm in diameter, a far greater spatial resolution than the ion microprobe. With this method, monazite grains of 10 µm or less are capable of producing reliable ages. Up to 84 spots on each sample were collected, specifically targeting Th-rich rims, and lower-Th cores.
Elemental analyses were conducted at the University of Massachusetts-Amherst, using the Cameca SX50 electron microprobe that is equipped with four spectrometers.
The PAP method discussed in Pouchou and Pichoir (1984; 1985) was administered for matrix corrections. Images were collected using a high sample current (>200 nA) and small step sizes (~0.5 mm), while rastering the electron beam with a fixed stage. X-ray maps of Y, Th, U, and Pb were obtained from individual sample monazite grains to indicate chemical domains (Figure 6). Analyses were carried out in two steps: major
Y 10 µm Th
Ca U
Figure 6. X-ray elemental maps of sample MIST displaying relatively homogeneous compositions of Yttrium (Y), Thorium (Th), Calcium (Ca), and Uranium (U). Yellow indicates high concentrations and orange displays lower concentrations. 26 elements of the samples were determined first, and concentrations were then hand entered for trace-element analysis using the Cameca trace-element procedure. Note that a representative single major-element analysis was used for multiple age determinations for individual samples. Background intensities were acquired using high-resolution wavelength scans around the peaks of interest, allowing selection of wavelength regions to use in curve fitting, using appropriate polynomial or exponential models, and calculation of the net intensity to be subtracted from the peak. Quantitative trace-element analysis was done using a beam current of 200 nA at 15kV accelerating voltage with a counting time of 600 seconds for three samples and 900 seconds for the fourth. Once concentrations of U, Th, and Pb were obtained, the age equation of Montel et al. (1996) was solved by iteration. Lead concentration was corrected for Y interference on the Pb
M-alpha line by estimation using the empirical approach of Amlie and Griffen (1975), in
this case based on analysis of Pb-free Y3 Al5 O12 garnet (YAG), to determine the
magnitude of the interference. This estimate was then modified based on the major
compositional differences between YAG and monazite to estimate the shift in relative
line intensities due to matrix properties. Similarly, U concentrations were corrected
based on empirical overlap of Th M-gamma on U M-beta, as suggested by Scherrer et al.
(2000). Ages and errors presented may only reflect analytical precision (analyses are
grouped by identified domains) and were calculated using the equation for standard error
of the mean based on the number of analyses for each domain. Weighted-mean ages
were calculated using the standard error from individual analyses of similar age domains
and are presented graphically as cumulative probability curves. 27 Muscovite Thermochronometry
In addition to monazite geochronometry, 40Ar/39Ar thermochronometric analysis
was conducted on muscovite grains collected from the Park Falls terrane, Peavy node,
and Niagara fault zone. Thermochronometric analysis is ideal for revealing diffusion
profiles of Ar from minerals (Zeitler, 1989), and establishing cooling age gradients within
a single grain. With a medium closure temperature of 350°C, as opposed to the much
higher monazite closure temperature, muscovite is capable of recording multiple
heating/cooling events that may not have been preserved by other minerals.
Conventional Ar-Ar ages are obtained through incremental heating techniques; the lack
of spatial detail displayed by the step-heating method may lead to unreliable age spectra,
which are essential in identifying thermal resetting events (Hodges et al., 1994).
Traditionally, plateau ages were used to represent the loss of Ar gas out of a crystal
during step heating. It was believed that Ar would diffuse out of the crystal during
incremental heating, revealing the crystal’s diffusion profile. More recently, studies
(Hodges and Bowring, 1994) have unveiled discrepancies with the conventional method
due to 40Ar intracrystalline homogenization during heating, thus concealing 40Ar gradients
and important geologic details throughout the sample. Therefore, analyses in this
investigation were carried out by the Ar-Ar laser microprobe method, capable of
obtaining cooling age data and possibly cooling rate information on a single muscovite
grain.
Age mapping by laser microprobe Ar-Ar techniques is utilized to reveal a more
detailed thermal history of a study region (Hodges et al., 1994); thermal histories are 28 revealed by apparent age gradients within a grain. For instance, a slow cooling event would be represented on a muscovite grain by a concentric pattern in which the older ages occur in the center of the grain, younging outwards. Multiple cooling events may appear as a disruption of the age gradients, or overlapping age domains within the crystal.
Ar-Ar analysis allows for the construction of “chrontour” maps on muscovite grains
(Figure 7), to assess the age gradients and thermal resetting events, and identifies changes in the cooling history.
Ar-Ar Laser Probe Technique
Muscovite samples collected from the Penokean orogen were prepared and analyzed using the Ar-Ar laser probe technique outlined in Hodges and Bowring (1995).
Irradiation was conducted at the McMaster Nuclear Reactor in Hamilton Ontario, Canada in the C5 position and irradiated for 35 hours at a total power of 70 MW. The Taylor
Creek rhyolite, with a calculated age of 28.34 Ma (Renne et al., 1998) was used to monitor the conversion efficiency of 39K to 39Ar. Measurements from the monitors were
used to determine the values of irradiation parameter, J, that fell between 0.01682 and
0.01684 with uncertainties less than 0.5%. Synthetic salts of K2 SO4 and CaF2 were used
for the corrections of interfering nuclear reactions.
40Ar/39Ar UV-laser ablation on the muscovite grains was conducted using the
Lambda Physik Compex 102 excimer laser, with a 193 nm wavelength, a minimum spot
size capability of ~8 µm and an ablation depth of <1µm per burst. The duration of laser
heating was 120 seconds at 10 Amps. Gettering time to clean the gas lasted 29 94-MI-1
1723
1725 1673
1725 Ma 1732 1759 1781 1798 1595 1791 1725 1786 1730
1800 Ma 1800 Ma 1750 Ma DH-PF-99 1775 Ma 1763 I II 1700 Ma 1788 500 µm 1753 1725 Ma 1720 1678
1675 Ma
1750 Ma 1670 1688 1721 1665
1655 1650 Ma 1626
1771 1600 Ma 97-CM-10 1635
1690 1619 1685 X 1200 Ma 1163
1649 1400 Ma 1300 Ma III IV 1397 1648 500 µm 1402 1323 1341 1332 1393 1393 1403
1368
1300 Ma 1379
1200 Ma 1349
1257
1269 500 µm
Figure 7. Ar-Ar laser probe cooling ages of muscovite grains from the Peavy node (sample 94-MI-1), Park Falls terrane (sample DH-PF-99), and Niagara fault zone (97-CM-10) displayed as “chrontour” maps (see Table 3 for isotopic details). Roman numerals indicate quadrant numbers formed by the perpendicular spot traverses across the grain (DH-PF-99). 30 approximately 10-15 minutes with Al-Zr and Fe-Zr-V getters, and then the gas was transferred to a MAP 215-50 mass spectrometer, utilizing the electron multiplier to amplify the signal. Blanks were run prior to, intermittently, and immediately following analyses. During analysis, the laser diameter ranged from 106 µm up to 182 µm and three to four sets of 900 bursts were utilized at 15 to 20 Hz frequency. Data reduction was conducted using the program ArArCalc (Koppers, 2002); propagated measurement uncertainties, represented by precision limits, are reported throughout this paper at the 2 level, and 1 level for reported cooling ages. The reported results of these analyses are presented in data tables and as “chrontour” maps, in which contour lines represent similar ages within the grain.
Sampling Strategy
For this investigation, Archean gneisses and Proterozoic metapelitic schists were collected across the length of the gneiss dome corridor, specifically targeting 1) metamorphic nodes, 2) the two higher-grade fault-bounded metamorphic panels, 3) the
Niagara fault zone, and 4) the adjacent Wisconsin Magmatic Terrane (Figure 4). Nine samples were collected across the corridor, including fine- and coarse-grained pelitic schists in addition to biotite-rich gneisses. Both basement and cover units were sampled to determine geographic and structural variability of metamorphism across the orogen, and to augment existing data summarized in Schneider et al. (2004). Sampling was conducted along road cuts, shorelines, and riverbanks within the Republic and Peavy nodes, Park Falls terrane, and NFZ, as these features indicate evidence of significant exhumation displayed as Archean basement cored gneiss domes exposed at the surface. 31 Additionally, sampling strategy was based on mapping of Cannon and Klasner (1976),
Sims (1992), and Cannon and Ottke (2000). The Beechwood and Iron River terranes were not sampled due to lack of outcrop and the low-grade nature of the panels.
Similarly, the Watersmeet panel was not sampled, as Schneider et al. (2004) presents metamorphic age results obtained through similar techniques. Collectively, the sample locations selected provide a thorough coverage of the gneiss dome corridor and of the major metamorphic features found across the Penokean orogen in Wisconsin and
Michigan.
Metamorphic ages were determined by in situ U-Th-Pb monazite geochronometry via electron microprobe analyzer (EMPA) and ion microprobe techniques; muscovite cooling ages were obtained through Ar-Ar laser probe techniques. Geochronometry constrains the timing of the regional metamorphic events recorded in the Lake Superior region and relates them to the overall tectonic history of Laurentia, while thermochronometry assists in establishing the cooling history of the events. 32 RESULTS
The geochronometric results of this study are discussed below in a geographic manner with respect to the panels, nodes, and NFZ. Results of the Th-Pb ion microprobe analyses yielded scattered and erroneous dates, thus age results are reported as 207Pb/206Pb
ages. Consequently, the Pb-Pb vs. Th-Pb ratios failed to show a 1:1 trend on most
samples, as expected if the isotope systematics are well behaved. Also, Th/U ratios were
compared to Th-Pb ages to understand the extrinsic and intrinsic controls of the Pb decay
systematics; unfortunately, it is not clear from the results why the Th-Pb ages are
ambiguous. Where further detail was needed to constrain the age of younger monazite
crystal domains, the EMPA technique was utilized due to its smaller beam size and
higher spatial resolution. These results are reported as U-Th-total Pb ages, collectively
placed within specific age domains. Cooling rates established here are presented as
“chrontour” maps in hopes of identifying a concentric cooling pattern across the
muscovite grain that exhibits an older core radially younging toward the rims.
Park Falls Panel
Sample 96-17 was collected near the eastern shore of Blockhouse Lake in the
Park Falls metamorphic terrane, approximately eight kilometers east of Park Falls,
Wisconsin in Price County. This sample is a fine- to medium-grained biotite schist
containing potassium feldspar, muscovite, and quartz within the matrix. Monazite
located in the sample predominantly occurs along biotite/quartz boundaries in the
foliation planes, with one grain located in the matrix. Overall, monazite grains analyzed 33 from this sample are homogeneous with little to no cracks, and range in size from 40 to
100 µm in length.
Ion microprobe analysis was performed on sample 96-17. In total, 15 spots on 7 grains yielded a mean 207Pb/206Pb age of 1799 ± 3 Ma (MSWD = 1.6; Figure 8). The percentages of radiogenic 208Pb and 206Pb are relatively high (>98%; Table 1). No apparent 1:1 trend exists between the 208Pb/232Th and 207Pb/206Pb ages, and the Th/U ratios are relatively constant (between 8 and 16; Figure 8).
An additional sample collected from the Park Falls terrane was analyzed by Ar-Ar thermochronometry to constrain the cooling history of the terrane. Sample DH-PF-99 is a ~2.5 mm muscovite grain from a two-mica granite located just south of the town of
Park Falls, along the Flambeau River in Price County, Wisconsin. In total 16 laser spots on two main perpendicular traverses (Figure 7) yielded a 150 m.y. cooling age gradient, ranging from 1753 ± 10 Ma to 1619 ± 32 Ma (Table 2). Three spot ages are considered anomalous due to an excessive or insufficient percentage of 40Ar, recorded at 100% and
64%, respectively. Therefore, three additional spots were analyzed to augment these ages and are located within the quadrants formed by the traverses. Ages on this muscovite grain exhibit a non-concentric pattern with older ages in quadrant II younging toward quadrant IV (Figure 7). The lack of complete concentricity may be related to breakage during sample collection or lab preparation.
Republic Node
A fine-grained metapelitic biotite schist, sample MIST, containing large garnet and staurolite porphyroblasts with a predominantly quartz matrix was sampled from the 34 1900 A 96-17 Blockhouse Lake, WI B Mean (n:15) = 1799 ± 3 Ma Paleoproterozoic MSWD = 1.6 Metapelitic Schist 1850 1799 Ma
Pb Age (Ma) 1800 206
Pb/ 1750 207 Cumulative Probability
1700 1650 1700 1750 1800 1850 1900 1950
207 206 Pb/ Pb Age (Ma)
2100 C 35 D 96-17 30 96-17 J J 2000 J J 25 JJJ JJ 20 J Age (Ma) 1900 JJ
Th 15 Th/U Ratio J J J J J
232 JJ J JJJ J 10 JJ
Pb/ J 1800 J J 208 J 5
1700 0 1700 1800 1900 2000 2100 1200 1400 1600 1800 2000 2200
207 206 208 232 Pb/ Pb Age (Ma) Pb/ Th Age (Ma) Figure 8. Ion microprobe Pb-Pb age results of monazite from sample 96-17, Park Falls structural panel. A) weighted average based on 11 spots analyzed. B) cumulative probability curve representative of the mean average. C) scatter plot of Th-Pb vs Pb-Pb ages obtained from the sample lacking a 1:1 ratio. D) scatter plot of Th-Pb ages vs Th/U ratio exhibiting no correlation between the two variables.
Republic node in Marquette County, Michigan. The well defined staurolites range in size from <1 mm up to 4 mm in length, while the garnets occur as euhedral grains of approximately 1 mm in length. Monazite grains within the sample range from approximately 5 µm to 50 µm in length. Although no monazite grains were identified as garnet inclusions, SEM imaging revealed monazite inclusions within the staurolite porphyroblasts. Three of the monazite grains analyzed were located along the boundaries Table 1. Ion Microprobe monazite isotopic data, metamorphic ages of Penokean orogen Apparent Ages (Ma) Spot Th/ Th 208Pb/ ±1 207Pb/ ±1 % Radiogenic % Radiogenic 208Pb/ ±1 207Pb/ ±1 U ppm 232Th 206Pb 208Pb 206Pb 232Th 206Pb 96-17 Blockhouse Lake, WI - Proterozoic biotite schist
1.1 8.26 88149.1 0.1022 0.0011 0.1090 0.0008 99.2 99.3 1968 20 1781 14 1.2 7.98 86222.0 0.1048 0.0012 0.1100 0.0005 99.5 99.6 2015 23 1797 8 5.1 16.20 80077.4 0.1003 0.0009 0.1100 0.0002 99.9 99.9 1932 17 1804 3 5.2 16.00 80837.3 0.0992 0.0009 0.1100 0.0002 99.9 99.9 1912 17 1801 3 6.1 10.80 68724.3 0.1054 0.0011 0.1100 0.0002 99.8 99.8 2025 19 1803 4 6.2 10.80 20437.8 0.1037 0.0010 0.1100 0.0002 99.9 99.9 1995 18 1804 4 7.1 14.40 18830.1 0.0953 0.0010 0.1100 0.0005 99.4 99.1 1840 18 1806 8 7.2 14.50 79053.3 0.0910 0.0009 0.1100 0.0005 99.5 99.2 1761 16 1795 8 8.1 11.70 77897.1 0.0945 0.0013 0.1100 0.0004 99.7 99.7 1825 24 1805 6 8.2 11.50 79592.9 0.0934 0.0010 0.1090 0.0003 99.8 99.7 1804 18 1789 4 8.3 11.90 75639.7 0.0962 0.0014 0.1100 0.0007 99.1 98.9 1857 25 1796 11 11.1 13.00 75595.7 0.1025 0.0011 0.1090 0.0004 99.8 99.7 1973 19 1789 7 11.2 13.00 74527.5 0.1028 0.0009 0.1090 0.0003 99.8 99.7 1979 16 1786 6 12.1 13.30 77511.7 0.1048 0.0010 0.1100 0.0003 99.9 99.9 2014 18 1797 5 12.2 13.20 75474.5 0.1008 0.0009 0.1100 0.0002 99.9 99.9 1942 17 1797 3 CREP Republic, MI - Archean biotite gneiss
2.1 116.60 87637.1 0.0758 0.0023 0.1360 0.0100 95.7 70.4 1477 44 2175 128 2.2 98.09 84729.8 0.0750 0.0022 0.1340 0.0065 96.4 73.2 1461 41 2147 85 3.1 100.40 79550.0 0.0101 0.0092 0.1810 0.0107 85.9 73.5 202 184 2666 98 3.2 98.27 59135.7 0.0086 0.0059 0.1830 0.0103 85.5 73.1 172 119 2683 93 4.1 117.70 79956.3 0.0739 0.0027 0.1210 0.0040 99.6 93.0 1441 51 1977 59 4.2 169.20 81555.9 0.0717 0.0020 0.1280 0.0037 99.8 95.5 1399 37 2069 52 4.3 49.91 472907.5 0.0758 0.0025 0.1260 0.0021 99.9 99.2 1477 47 2048 30 4.4 49.86 443961.7 0.0834 0.0056 0.1370 0.0018 99.7 97.5 1620 104 2183 23 13.1 26.33 125533.1 0.1086 0.0035 0.1680 0.0008 99.2 97.8 2084 64 2535 8 13.2 25.52 125241.1 0.1124 0.0032 0.1700 0.0006 99.5 98.6 2153 58 2558 6 13.3 23.74 124415.9 0.1316 0.0046 0.1730 0.0004 99.9 99.8 2499 82 2583 4 13.4 23.76 123374.9 0.1307 0.0038 0.1730 0.0003 99.9 99.8 2483 68 2588 3 13.5 31.36 114221.4 0.1308 0.0073 0.1660 0.0009 98.7 96.1 2485 130 2519 9 13.6 30.50 113535.9 0.1323 0.0066 0.1690 0.0014 98.6 96.0 2511 117 2544 14 PVD Peavy Dam, MI - Paleoproterozoic biotite schist
1.1 8.52 39761.1 0.0809 0.0018 0.1459 0.0021 95.2 95.9 1572 33 1820 59 1.2 8.17 39446.2 0.0805 0.0017 0.1390 0.0005 96.2 96.9 1566 31 1844 12 3.1 9.17 25524.0 0.0830 0.0015 0.1136 0.0003 99.7 99.7 1612 28 1816 5 3.2 9.18 25722.9 0.0822 0.0015 0.1138 0.0003 99.7 99.7 1596 28 1824 6 4.1 8.73 50070.1 0.0989 0.0017 0.1135 0.0004 99.8 99.8 1905 31 1826 7 4.2 8.35 52771.7 0.0953 0.0011 0.1138 0.0004 99.7 99.7 1840 20 1821 9 5.1 9.01 38087.1 0.0888 0.0018 0.1395 0.0022 95.8 96.2 1719 34 1754 57 5.2 8.46 40192.0 0.0916 0.0010 0.1452 0.0010 94.9 95.3 1771 19 1730 25 6.1 10.05 39230.7 0.0944 0.0089 0.1834 0.0014 89.1 91.2 1823 164 1797 41 6.2 9.42 39777.6 0.0792 0.0026 0.1779 0.0013 90.4 92.0 1541 48 1809 38 7.1 6.77 47318.8 0.1063 0.0019 0.1175 0.0003 99.0 99.3 2042 35 1827 6 7.2 6.94 37009.8 0.1079 0.0028 0.1212 0.0012 98.5 98.9 2071 50 1835 29 11.1 7.06 45678.0 0.1061 0.0015 0.1142 0.0002 99.7 99.8 2038 27 1839 4 11.2 7.07 46539.8 0.1037 0.0015 0.1140 0.0002 99.7 99.8 1995 28 1831 5 Uncertainties reported at 1 (absolute) and are calculated by numerical propagation of all known sources of error, and data corrected according to procedures outlined in Catlos e (2002). "8.1" refers to grain 8, spot 1; ".2", etc., refers to a subsequent spot analysis on the same grain. Table 1. (continued) Ion Microprobe monazite isotopic data, metamorphic ages of Penokean orogen Apparent Ages (Ma) Spot Th/ Th 208Pb/ ±1 207Pb/ ±1 % Radiogenic % Radiogenic 208Pb/ ±1 207Pb/ ±1 U ppm 232Th 206Pb 208Pb 206Pb 232Th 206Pb MIST Republic, MI - Proterozoic garnet + staurolite schist
5.1 9.61 8700.0 0.0915 0.0012 0.1456 0.0015 95.0 95.2 1770 22 1714 45 5.2 9.64 9620.0 0.0867 0.0009 0.1289 0.0012 97.2 97.3 1680 17 1738 27 5.3 8.53 8556.7 0.0678 0.0014 0.1184 0.0003 98.6 98.7 1325 27 1757 8 6.1 12.83 10213.3 0.0786 0.0017 0.1156 0.0006 99.2 99.0 1530 33 1752 13 8.1 6.62 17333.3 0.0935 0.0010 0.1153 0.0006 98.5 99.0 1806 18 1753 14 8.2 6.76 14323.3 0.0908 0.0022 0.1130 0.0004 99.0 99.4 1756 40 1758 10 8.3 6.77 12218.3 0.0740 0.0014 0.1090 0.0004 99.8 99.8 1443 26 1760 7 9.1 9.12 13148.3 0.0846 0.0010 0.1331 0.0012 96.9 97.1 1642 19 1781 33 9.2 8.86 8538.3 0.0835 0.0008 0.1297 0.0008 97.3 97.6 1620 15 1787 26 9.3 8.33 7875.0 0.0665 0.0015 0.1245 0.0008 97.5 97.8 1301 29 1728 20 18.1 9.69 7635.0 0.0883 0.0009 0.1252 0.0018 97.7 97.8 1710 16 1742 47 18.2 9.46 7695.0 0.0863 0.0008 0.1169 0.0005 98.7 98.7 1672 14 1733 14 18.3 10.66 6375.0 0.0709 0.0016 0.1123 0.0003 99.5 99.4 1384 30 1754 7 20.1 8.92 8836.7 0.0887 0.0009 0.1218 0.0015 98.1 98.2 1718 16 1748 38 20.2 9.04 10111.7 0.0886 0.0010 0.1157 0.0009 99.0 99.1 1715 19 1769 19 20.3 8.35 4083.3 0.0769 0.0015 0.1138 0.0006 99.0 99.1 1496 28 1741 12 23.1 11.24 4291.7 0.0872 0.0009 0.1201 0.0010 98.3 98.1 1690 17 1690 28 23.2 11.06 6601.7 0.0833 0.0008 0.1165 0.0005 99.0 98.8 1618 15 1745 14 25.1 8.49 10265.0 0.0880 0.0012 0.1110 0.0009 99.6 99.7 1704 21 1769 17 25.2 8.35 11020.0 0.0900 0.0010 0.1088 0.0005 99.8 99.9 1741 19 1760 10 25.3 7.62 21233.3 0.0863 0.0012 0.1115 0.0003 99.6 99.7 1672 23 1777 6 26.1 8.26 12145.0 0.0912 0.0014 0.1252 0.0010 97.4 97.9 1763 25 1752 30 26.2 8.10 31500.0 0.0890 0.0023 0.1212 0.0009 98.0 98.4 1723 42 1760 17 26.3 9.25 7836.7 0.0756 0.0016 0.1470 0.0008 95.4 95.5 1473 30 1786 25 30.1 10.02 24450.0 0.0902 0.0013 0.1801 0.0031 90.6 91.3 1746 24 1744 97 30.2 9.94 28966.7 0.0858 0.0010 0.1613 0.0007 93.3 93.5 1663 18 1750 29 38.3 11.79 21116.7 0.0698 0.0014 0.1182 0.0003 99.0 98.7 1363 26 1759 11 39.3 8.81 6791.7 0.0868 0.0008 0.1221 0.0015 98.2 98.3 1682 15 1766 39 39.2 8.43 22250.0 0.0835 0.0007 0.1139 0.0006 99.2 99.3 1621 13 1761 12 39.1 8.11 25783.3 0.0770 0.0015 0.1114 0.0003 99.5 99.6 1499 28 1759 7 Uncertainties reported at 1 (absolute) and are calculated by numerical propagation of all known sources of error, and data corrected according to procedures outlined in Catlos et a (2002). "8.1" refers to grain 8, spot 1; ".2", etc., refers to a subsequent spot analysis on the same grain. Table 2. Ar-Ar laser probe muscovite data, cooling ages of the Penokean orogen Age ± 1 40Ar(r) 39Ar(k) Sample and 36Ar(a) 37Ar(ca) 38Ar(cl) 39A(k) 40Ar(r) K/Ca ± 1 Spot Number mV mV mV mV mV (Ma) (%) (%) DH-PF-99 Blockhouse Lake, WI - Park Falls terrane J-value 0.01684 DH-PF-99. 1 0.00051 0.00000 0.00000 0.03417 3.03097 1648.70 ± 16.69 95.29 7.17 0.000 ± 0.000 DH-PF-99. 2 0.00026 0.00000 0.00042 0.03668 3.25618 1649.46 ± 8.84 97.69 7.70 0.000 ± 0.000 DH-PF-99. 3 0.00039 0.00000 0.00041 0.02347 2.02681 1619.58 ± 32.54 94.57 4.93 0.000 ± 0.000 DH-PF-99. 4 0.00042 0.00000 0.00000 0.03274 2.86778 1635.07 ± 22.28 95.87 6.87 0.000 ± 0.000 DH-PF-99. 5 0.00044 0.00000 0.00059 0.04154 3.74288 1665.65 ± 10.58 96.62 8.72 0.000 ± 0.000 DH-PF-99. 6 0.00029 0.00000 0.00055 0.03652 3.30522 1670.49 ± 12.18 97.44 7.67 0.000 ± 0.000 DH-PF-99. 7 0.00000 0.00062 0.00024 0.04140 3.92049 1720.19 ± 8.37 100.00 8.69 32.700 ± 50.076 DH-PF-99. 8 0.00118 0.00000 0.00044 0.04052 3.95545 1753.93 ± 10.28 91.92 8.51 0.000 ± 0.000 DH-PF-99. 9 0.00000 0.00002 0.00018 0.01210 1.11490 1690.20 ± 18.07 100.00 2.54 317.884 ± 16942.092 DH-PF-99. 10 0.00468 0.00000 0.00071 0.02481 2.45993 1771.51 ± 40.11 64.00 5.21 0.000 ± 0.000 DH-PF-99. 11 0.00025 0.00000 0.00043 0.03265 2.83851 1626.79 ± 35.62 97.47 6.86 0.000 ± 0.000 DH-PF-99. 12 0.00023 0.00000 0.00035 0.03800 3.39348 1655.96 ± 25.34 98.08 7.98 0.000 ± 0.000 DH-PF-99. 13 0.00022 0.00000 0.00007 0.02197 2.02070 1688.01 ± 26.41 96.94 4.61 0.000 ± 0.000 DH-PF-99. 14 0.00018 0.00000 0.00002 0.01617 1.53310 1721.43 ± 38.53 96.60 3.39 0.000 ± 0.000 DH-PF-99. 15 0.00016 0.01196 0.00050 0.01603 1.47139 1685.78 ± 16.77 96.91 3.37 0.657 ± 0.037 DH-PF-99. 16 0.00022 0.00000 0.00000 0.02759 2.51602 1678.53 ± 21.40 97.46 5.79 0.000 ± 0.000
94-MI-1 Foster City, MI - Peavy node J-value 0.01682 94-MI-1. 1 0.00035 0.00000 0.00000 0.01005 1.01291 1788.59 ± 12.57 90.81 3.27 0.000 ± 0.000 94-MI-1. 2 0.00045 0.00032 0.00000 0.02545 2.50761 1763.17 ± 12.31 94.94 8.27 39.583 ± 232.317 94-MI-1. 3 0.00034 0.00000 0.00000 0.02318 2.21845 1730.62 ± 15.45 95.62 7.53 0.000 ± 0.000 94-MI-1. 4 0.00021 0.00000 0.00000 0.02085 1.99904 1732.41 ± 17.72 97.03 6.78 0.000 ± 0.000 94-MI-1. 5 0.00019 0.00125 0.00000 0.02132 2.03216 1725.85 ± 19.11 97.25 6.93 8.340 ± 11.438 94-MI-1. 6 0.00013 0.00000 0.00000 0.01959 1.86277 1723.28 ± 22.83 98.05 6.37 0.000 ± 0.000 94-MI-1. 7 0.00000 0.00003 0.00000 0.03151 3.18369 1791.64 ± 13.02 100.00 10.24 474.564 ± 12653.269 94-MI-1. 8 0.00006 0.00149 0.00000 0.02962 3.01119 1798.54 ± 18.93 99.46 9.63 9.712 ± 13.925 94-MI-1. 9 0.00030 0.00000 0.00000 0.02317 2.33133 1786.97 ± 20.84 96.30 7.53 0.000 ± 0.000 94-MI-1. 10 0.00029 0.00000 0.00000 0.02083 1.98525 1725.77 ± 30.97 95.86 6.77 0.000 ± 0.000 94-MI-1. 11 0.00087 0.01162 0.00059 0.02007 1.69553 1595.12 ± 11.68 86.88 6.52 0.847 ± 0.102 94-MI-1. 12 0.00009 0.00000 0.00000 0.01891 1.85671 1759.45 ± 27.69 98.55 6.14 0.000 ± 0.000 94-MI-1. 13 0.00008 0.00000 0.00000 0.02018 2.02143 1781.90 ± 22.45 98.91 6.56 0.000 ± 0.000 94-MI-1. 14 0.00028 0.00000 0.00000 0.02297 2.08775 1673.60 ± 34.77 96.19 7.47 0.000 ± 0.000 Table 2. (continued) Ar-Ar laser probe muscovite data, cooling ages of the Penokean orogen Sample and 36Ar(a) Age ± 1 40Ar(r) 39Ar(k) 37Ar(ca) 38Ar(cl) 39Ar(k) 40Ar(r) K/Ca ± 1 Spot Number mV mV mV mV mV (Ma) (%) (%) 97-CM-10 Pine River Flowage - Niagara Fault Zone, WI J-value 0.01684 97-CM-10. 1 0.00134 0.00991 0.00085 0.00754 0.00623 24.95 ± 395.02 1.55 1.84 0.373 ± 0.104 97-CM-10. 2 0.00137 0.01072 0.00047 0.02324 1.25004 1163.37 ± 85.48 75.54 5.67 1.063 ± 0.285 97-CM-10. 3 0.00133 0.01292 0.00120 0.03485 2.42044 1397.21 ± 54.58 86.07 8.51 1.322 ± 0.510 97-CM-10. 4 0.00205 0.01041 0.00090 0.03658 2.55330 1402.14 ± 108.05 80.79 8.93 1.722 ± 0.524 97-CM-10. 5 0.00242 0.00739 0.00077 0.03158 2.06964 1341.51 ± 53.33 74.34 7.71 2.094 ± 0.397 97-CM-10. 6 0.00306 0.00964 0.00076 0.03342 2.33641 1403.61 ± 46.18 72.11 8.16 1.699 ± 0.228 97-CM-10. 7 0.00177 0.01046 0.00081 0.03414 2.30163 1368.65 ± 51.73 81.44 8.33 1.599 ± 0.220 97-CM-10. 8 0.00118 0.01019 0.00115 0.02976 2.02863 1379.36 ± 68.03 85.28 7.26 1.431 ± 0.300 97-CM-10. 9 0.00109 0.00863 0.00039 0.02433 1.60844 1349.94 ± 67.12 83.27 5.94 1.382 ± 0.448 97-CM-10. 10 0.00101 0.00766 0.00072 0.02514 1.50426 1257.18 ± 77.37 83.50 6.14 1.607 ± 0.535 97-CM-10. 11 0.00086 0.00840 0.00079 0.02579 1.56436 1269.86 ± 104.21 85.99 6.29 1.504 ± 0.587 97-CM-10. 12 0.00102 0.00812 0.00070 0.02940 2.03458 1393.85 ± 42.22 87.09 7.17 1.774 ± 0.591 97-CM-10. 13 0.00199 0.00833 0.00077 0.02277 1.57578 1393.86 ± 97.59 72.83 5.56 1.340 ± 0.248 97-CM-10. 14 0.00112 0.01049 0.00084 0.02855 1.85225 1332.05 ± 62.54 84.88 6.97 1.334 ± 0.336 97-CM-10. 15 0.00101 0.00997 0.00059 0.02270 1.45899 1323.32 ± 109.36 82.98 5.54 1.116 ± 0.370 39 of biotite and quartz grains within the planes of foliation (Figure 5A). The remaining grains were located randomly within the matrix of the sample, also along biotite-quartz boundaries. In total, 30 spots on 12 grains from MIST were analyzed in situ via the ion microprobe and yielded a mean Pb-Pb age of 1758 ± 5 Ma (MSWD = 1.05; Figure 9).
1900 A MIST B Mean (n:30) = 1758 ± 5 Ma Republic, MI MSWD = 1.05 1758 Ma 1850 Paleoproterozoic staurolite schist
1800 Pb Age (Ma) 206
Pb/ 1750 207 Cumulative Probability
1700 1650 1700 1750 1800 1850 1900 1950
207 206 Pb/ Pb Age (Ma) 35 D 1800 C MIST 30 MIST
1700 25
20 1600 Age (Ma)
232 15 1500 Th/U Ratio /Th 10 208
Pb 1400 5
1300 0 1300 1400 1500 1600 1700 1800 1200 1400 1600 1800 2000 2200
207 206 208 232 Pb/ Pb Age (Ma) Pb/ Th Age (Ma) Figure 9. Ion microprobe Pb-Pb age results of monazite from sample MIST, Republic metamorphic node. A) weighted average based on 23 spots analyzed. B) cumulative probability curve representative of the mean average. C) scatter plot of Th-Pb vs Pb-Pb ages obtained from the sample exhibiting a partial 1:1 ratio of the older Th-Pb ages. D) scatter plot of Th-Pb ages vs Th/U ratio exhibiting fairly constant values regardless of age. 40 Due to highly variable Th-Pb dates exhibiting large errors, only the Pb-Pb ages are
reported (Table 1). Large errors of the reported ages appear to be related to slightly
lower (< 90%) radiogenic 208Pb and 206Pb.
Analysis of MIST produces a partial 1:1 ratio of 208Pb/232Th ages with the
207Pb/206Pb ages (Figure 9). The older Th-Pb ages indicate a 1:1 trend with respect to the
Pb/Pb ages. However, the remaining Th-Pb analyses fail to follow that trend and produce younger ages. The variation among isotopic age results are also apparent in the Th/U ratio vs. Th-Pb age diagrams. The older age set of results produced a large cluster of Th-
Pb ages similar to those exhibited in the Pb-Pb analyses, while the younger age set produces a linear and variable trend of Th-Pb ages. Nonetheless, the ratio of Th/U remains fairly constant regardless of the calculated ages.
To determine the presence of age domains smaller than the resolution of the ion
microprobe sputter diameter, a subsequent analysis of sample MIST was conducted
utilizing the EMPA technique. There were 13 monazite grains identified in thin section,
which appear to be relatively homogeneous with respect to elemental concentrations, and
two grains have a skeletal appearance. A total of 23 spots on 6 grains yielded a mean age
of 1768 ± 13 Ma (MSWD = 9.2; Figure 10). Three analytical spots were not considered
in the determination of the mean age due to anomalously low concentrations of U, Th,
and Pb. Five spots on the weighted mean diagram (Figure 10) indicate a potential age
domain at 1830 Ma (Table 3), but this M1 age domain is not supported on the cumulative
probability curve, displaying a prominent ~1770 Ma peak. Notably, the results of the
EMPA technique are the same as those obtained via the ion microprobe method. 41 1840 A B 1820 1768 Ma
1800
1780
1760
1740
U-Th-total Pb* Ages (Ma) U-Th-total Pb* 1720 MIST Cumulative Probability Mean (n: 23) = 1768 ± 13 Ma
MSWD = 9.2 1700 1600 1640 1680 1720 1760 1800 1840 1880 U-Th-total Pb Ages (Ma)
Figure 10. EMPA U-Th-total Pb age results of monazite from sample MIST, Republic metamorphic node. A) weighted average based on 23 spots analyzed. B) cumulative probability curve representative of the mean average, also similar to the ion microprobe results.
An Archean basement gneiss, sample CREP, was collected within the sillimanite
zone of the Republic metamorphic node, south of the Republic synform in Marquette
County, Michigan. The sample is a pink to gray coarse-grained foliated gneiss
containing quartz, microcline, Na-plagioclase, and biotite. Monazite grains generally
occur along quartzofeldspathic boundaries, and range from 60 to 150 µm in length. A
majority of the monazite identified in thin section were skeletal and heterogeneous,
containing inclusions of apatite and thorite (Figure 5B and D). Therefore, reliable
208Pb/232Th ages were unobtainable due to significant concentrations of thorium within the inclusions, which affected the analyses. A total of 14 spots on 4 separate grains were performed on this sample, and a combined monazite age was determined to be 2570 ± 35
Ma (MSWD = 72), however, this probably represents a composite date of two separate 42 Table 3. Summary of EMP Th-U-total Pb monazite ages* Sample domain a domain b domain c domain d domain e mean REPUBLIC NODE CREP bi gneiss 2582 ± 10 1809 ± 9 1770 ± 12 1716 ± 16 n: 9 n: 5 n: 8 n: 8 MIST bi-gt-st schist 1768 ± 13 n: 23 PEAVY NODE HRR bi-amph gneiss 1774 ± 9 1765 ± 28 n: 9 n: 17 NIAGARA FAULT ZONE PG-03 bi schist 1628 ± 16 1496 ± 16 n: 9 n: 9 *expressed in Ma as mean ages ± 2x standard deviation domains: 2576 ± 18 Ma (MSWD = 16) and 2116 ± 83 Ma (MSWD = 4.0; Figure 11).
The percentages of radiogenic 208Pb and 206Pb were as low as 70% (Table 1), while Th/U ratios ranged from 23 to 169, suggesting that this sample was relatively chemically heterogeneous and had a high concentration of 204Pb such that these dates should actually be slightly younger. With such large errors and minimal radiogenic Pb signals, the
EMPA method was also applied to sample CREP to elucidate smaller and/or younger age domains and remedy inconsistencies with this sample.
Three out of four grains analyzed by the ion microprobe method were also analyzed by the in situ EMPA technique, including one additional monazite grain approximately 60 µm in length. Elemental concentration maps indicate the heterogeneity of these grains with respect to Y, Th, Ca, and U, and areas with extreme concentrations of these elements were avoided during analyses (Figure 12). The objective was to specifically target younger age domains within a single grain. In total, 78 spots on these 43
2800 A CREP Republic, MI B Mean (n:8) = 2576 ± 18 Ma Archean basement MSWD = 16 gneiss 2576 Ma 2600
2400 Pb Age (Ma)
206 2200 Pb/ Cumulative Probability
207 2000 CREP Mean (n:6) = 2116 ± 83 Ma MSWD = 4.0 2116 Ma 1800 1800 2000 2200 2400 2600 2800
207 206 Pb/ Pb Age (Ma)
2800 200 C D JJJ 180 2400 J 160 CREP CREP JJ 2000 140 120 1600 J JJ JJJ JJ 100 JJ J ThAge (Ma) 1200 80 232 Th/U Ratio 60
Pb/ 800 JJ
208 40 400 JJ 20 JJ JJ JJ 0 0 0 400 800 1200 1600 2000 2400 2800 0 500 1000 1500 2000 2500 3000
207 206 208 232 Pb/ Pb Age (Ma) Pb/ Th Age (Ma)
Figure 11. Ion microprobe Pb-Pb age results of monazite from sample CREP, Republic metamorphic node. A) weighted average of two Archean age domains based on 5 spot analyses. B) cumulative probability curve exhibiting strong evidence for metamorphism at ~2.6 Ga, and a hint of a ~2.1 Ga thermal event. C) Th-Pb vs Pb-Pb age scatter plots showing variability of the Th-Pb ages probably due to thorite and apatite inclusions. D) Th/U ratio vs Th-Pb age scatter plot indicating no correlation between the two variables. 44 Y Th
50µm 50µm
Ca U
50µm 50µm
Figure 12. Elemental maps of sample CREP from the Republic metamorphic node showing chemical heterogeneity of a monazite. Areas with extreme concentrations (red areas) of each element were avoided, as these represent areas of thorite and apatite inclusions in addition to areas hydrothermally altered. 45 four grains yielded a total-Pb mean age of 1980 ± 98 Ma (MSWD = 1881). Ages of 6 spots are not reported here, as concentrations of U, Th, and Pb, were anomalously low.
Within the grains, four possible age domains were identified (Figure 13), yielding ages of
2582 ± 10 Ma (MSWD = 1.8), 1809 ± 9 Ma (MSWD = 1.10), 1770 ± 12 Ma (MSWD =
2.1), and 1716 ± 16 Ma (MSWD = 3.7), and are displayed in Table 3. The large number of spot analyses used in determining each of these mean ages, supports the presence of two significant age domains at roughly 2600 Ma and 1770 Ma displayed on the cumulative probability curve (Figure 13).
Peavy Node
Within the Peavy metamorphic node in the most southeastern portion of the study area, one sample was collected for geochronometric analyses. The sample was located within the kyanite zone of the node near the Peavy Dam about 24 km west of Foster City,
Michigan in Dickinson County. Sample PVD is a medium to fine-grained micaceous schist on which ion microprobe analysis was conducted. Monazite grains found within the sample ranged from approximately 10 µm to 60 µm in length and were primarily located along the boundaries of biotite and quartz grains within the matrix. The SEM images indicated that no monazite were present along foliation planes, and none were found to exist as inclusions within porphyroblasts. Although some of the monazite grains exhibit a skeletal appearance, the majority display a more pristine appearance compared to most other samples in this study. Collectively, 14 spots on 7 grains were analyzed in situ via the ion microprobe and yielded a mean Pb-Pb age of 1828 ± 7 Ma (MSWD = 2.5;
Figure 14). Only Pb-Pb ages are reported here because large errors are caused by low 46 A 1770 Ma 2582 Ma Cumulative Probability
1000 1400 1800 2200 2600 3000 U-Th-total Pb Ages (Ma)
2620 1845 B C 1835 2600 1825
2580 1815
1805 2560
U-Th-total Pb Ages (Ma) U-Th-total Pb U-Th-total Pb Ages (Ma) U-Th-total Pb CREP 1795 CREP Mean (n: 6) = 2582 ± 10 Ma Mean (n: 5) = 1809 ± 9 Ma
MSWD = 1.8 MSWD = 1.10 2540 1785
1770 1805 DE
1795 1750
1785 1730 1775
1765 1710
1755
1690 U-Th-total Pb Ages (Ma) U-Th-total Pb 1745 CREP Ages (Ma) U-Th-total Pb CREP Mean (n: 8) = 1770 ± 12 Ma Mean (n: 7) = 1716 ± 16 Ma
MSWD = 2.1 MSWD = 3.7 1735 1670
Figure 13. EMPA U-Th-total Pb age results of monazite in sample CREP. A) Cumulative probability curve suggesting metamorphic events occurred at ~2.6 Ga and ~1770 Ma. B) Mean average diagram of 6 spot analyses within age domain A (table 2). C) weighted average diagram of 5 spot analyses that reveal an ~1800 Ma age (B) domain. D) weighted average diagram of which 8 spot analyses suggest a thermal event at ~1760 Ma (domain C). E) weighted average diagram indicates a potential age (D) domain at ~1700 Ma. 47 1900 A Peavy Dam, MI B Paleoproterozoic Metapelitic gneiss 1828 Ma 1850
1800 Pb Age (Ma) 206
Pb/
1750 Cumulative Probability 207 PVD Mean (n:14) = 1828 ± 7 Ma MSWD = 2.5 1700 1650 1700 1750 1800 1850 1900 1950
207 206 Pb/ Pb Age (Ma)
2100 35 C D PVD PVD 2000 30 25 1900 20 Age (Ma) 1800
Th 15 Th/U Ratio 232 1700 10 Pb/ 208 1600 5
1500 0 1500 1600 1700 1800 1900 2000 2100 1200 1400 1600 1800 2000 2200
207 206 208 232 Pb/ Pb Age (Ma) Pb/ Th Age (Ma)
Figure 14. Ion microprobe Pb-Pb age results of monazite from sample PVD, Peavy metamorphic node. A) weighted average based on 9 spots analyzed. B) cumulative probability curve representative of the mean average. C) scatter plot of Th-Pb vs Pb-Pb ages obtained from the sample lacking a 1:1 ratio. D) scatter plot of Th-Pb ages vs Th/U ratio exhibiting no correlation between the two variables. percentages of radiogenic Pb (Table 1). As seen in MIST, scatter plots exhibit a fairly constant value of the Th/U ratio independent of the Th/Pb ages, but lack the clustering present in some of the MIST analyses.
Sample HRR, a fine- to medium-grained gneiss containing quartz, biotite, amphibole, and muscovite, was collected near Peavy Pond in Copper County, Michigan.
Seven monazite grains were identified in thin section ranging in size from 20 to 30 µm in 48 diameter. In total, 19 EMPA spots on 5 grains were analyzed, and 17 spots produced a combined mean total-Pb age of 1765 ± 28 Ma (MSWD = 30; Figure 15). Two spots were not included in determining the mean due to low concentrations of U, Th, and Pb. One domain was identified with an age of 1774 ± 9 Ma (MSWD = 1.4; Table 3), based on 9 analytical spots that produced a “plateau” on the weighted average diagram (Figure 15).
The data also displays a strong concentration of ages at roughly 1775 Ma, as seen in the cumulative probability histogram (Figure 15); the indication of a smaller peak on the cumulative probability curve is the result of only a few spot ages, and is not considered a true age domain. Three of five grains were homogeneous in composition, while two grains indicated potential low-Th cores. However, contrary to existing thought on elemental concentrations within accessory minerals, EMPA results reported here do not exhibit older inherited ages in the corresponding low-Th domains.
1900 A HRR B 1860 Mean (n: 9) = 1774 ± 9 Ma MSWD = 1.4 1774 Ma 1820
1780
1740
1700
U-Th-total Pb* Ages (Ma) U-Th-total Pb* HRR
1660 Cumulative Probability Mean (n: 17) = 1765 ± 28 Ma
MSWD = 30 1620 1550 1650 1750 1850 1950 U-Th-total Pb Ages (Ma)
Figure 15. EMPA U-Th-total Pb age results of monazite from sample HRR, Peavy metamorphic node. A) weighted average mean diagram revealing a total sample age of ~1765 Ma and an age domain of ~1775 Ma. B) cumulative probability curve showing a dominant metamorphic event at ~1775 Ma. Smaller peaks slightly hint at possible age domains, but are only based on a maximum of 2 spot analyses. 49 Muscovite grain 94-MI-1, from an Archean biotite-muscovite schist, was collected near Foster City, in Dickinson County, Michigan. Thermochronometric analyses via the Ar-Ar laser probe were conducted to establish cooling ages within the
Peavy node and to identify any subsequent weak thermal events. This sample yields an age gradient of approximately 100 m.y. from core to rim: the oldest spot ages recorded on the grain are 1798 ± 19 Ma, while the youngest age is 1723 ± 23 Ma (Figure 7). Two anomalously young spot ages of 1673 ± 35 Ma and 1595 ± 12 Ma are also present in the core and rim of the muscovite, respectively. The average percentage of 40Ar for sample
94-MI-1 is roughly 96% (>95% = reliable; Table 2). Overall, ‘chrontours’ of the muscovite grain exhibit a concentric pattern with older ages occurring along the rim and younging towards the core.
Niagara Fault Zone
Sample PG-03 was collected along the Menominee River at Piers Gorge approximately 2 km southeast of the town of Niagara, in Marinette County, Wisconsin.
It is a fine-grained quartz sericite schist, containing numerous muscovite and biotite foliation planes from which 13 monazite grains were identified through BSE imaging.
Monazite grains ranging in size from 15 to 35 µm in diameter appear to be fairly homogeneous in elemental concentrations, but do indicate higher Th-regions (Figure 16).
Another sample, PRF, was also collected from a nearby location, along the NFZ. Only two monazite grains were located in thin section, thus the results of both samples (PG-03 and PRF) were combined into a representative NFZ sample to be analyzed via EMPA techniques. There were a total of 52 spots on 8 grains performed on this sample, and 38 50 of the total 52 spots yielded an unreliable mean total-Pb age of 1581 ± 51 Ma (MSWD =
245). Nevertheless, 18 spot analyses reveal two potential age domains of 1628 ± 16
(MSWD = 0.91) and 1496 ± 16 Ma (MSWD = 0.47; Table 3; Figure 17). Although high-
Th rims were identified in elemental maps, the spot ages determined from these areas are not significantly younger.
Y 10 µm Th
Ca U
Figure 16. X-ray elemental maps of sample PG-03 from the Niagara fault zone displaying low-Th cores and high-Th rims. Yellow indicates high-Th concentrations and orange displays lower-Th concentrations. Multiple spot analyses were conducted on both regions.
A muscovite book from the Pine River Pegmatite, sample 97-CM-10, was collected along the Niagara Fault Zone in Florence County, Wisconsin, south of the
Peavy metamorphic node. Laser probe Ar-Ar thermochronometry of 14 spots revealed an approximate 200 m.y. age gradient across the grain with spots ranging from 1403 ± 46
Ma to 1257 ± 77 Ma with one anomalous outlier at 1163 ± 85 Ma (Figure 7). Two perpendicular traverses along the muscovite grain yield a concentric pattern of older core 51 ages and younger rims. The percentage of 40Ar is in the range of 72-87%, and one Ar-Ar laser spot yielded an unreliable anomalously low 40Ar value of <2% (Table 2).
1900 A PG-03 B Mean (n:9) = 1628 ± 16 Ma 1496 Ma MSWD = 0.91 1628 Ma 1700
1500
1300
1100 Cumulative Probability PG-03 U-Th-total Pb Ages (Ma) U-Th-total Pb Mean (n:9) = 1496 ± 16 Ma MSWD = 0.47 900 1000 1200 1400 1600 1800 2000 U-Th-total Pb Ages (Ma)
Figure 17. EMPA U-Th-total Pb age results of monazite from samples PG-03 and PRF, Niagara fault zone. A) weighted mean diagram revealing two age domains of ~1630 Ma and ~1500 Ma. B) cumulative probability curve showing a dominant metamorphic event at the same time. Smaller peaks slightly hint at possible age domains most likely due to older cores, but are only based on a few spot analyses. 52 DISCUSSION
Geochronology and Thermochronology
Republic Node
Metamorphic ages from across the Penokean orogen provide important timing constraints on the thermal history in the region. The ages obtained through in situ geochronometric techniques in this investigation in addition to previous data, collectively reveal Paleoproterozoic thermal events occurring at ca. 1830 Ma, 1800 Ma, 1760 Ma,
1660 Ma, and 1500 Ma across the central Penokean orogen (Figure 18), all of which can be related to known tectonic events. Because complimentary techniques were used to date monazite, it is important to demonstrate that the analytical methods yielded similar ages, and confirm the robustness of the newer EMPA method. A direct analytical comparison can be made on sample CREP, dated via both ion microprobe and EMPA techniques. The ca. 2600 Ma monazite date from Republic is correlative with the ~2686
Ma age of granitic plutons located within the southern Superior Province (Davis et al.,
1989). Sample CREP also yielded a 2100 Ma metamorphic event. It should be noted that this basement unit also contains numerous crosscutting diabase dikes that may be related to ca. 2.0 Ga rifting of the province prior to Penokean collision (Roscoe and Card,
1993). Alternatively, this age may be associated with metamorphism directly related to the ~2.2 Ga Blezzardian orogeny (Stockwell, 1982). Samples collected from the
Republic area also exhibit high concentrations of Ca within monazite grains, in addition to sericitic alteration of plagioclase (Schneider, 1995), thus providing evidence of hydrothermal fluid 53 A WRBWRB M Y P
900 = Panel thermal history
800 = Republic thermal history
700 C) = Peavy thermal history º
600 = Magmatic pulse
500
400 TEMPERATURE ( TEMPERATURE 300
200
100 Baraboo Quartzites
1200 1300 1400 1500 1600 1700 1800 1900 AGE (Ma)
B WWRBRB M
900 = NFZ history 800
700 C) º
600
500
400 TEMPERATURE ( TEMPERATURE 300
200
100
1200 1300 1400 1500 1600 1700 1800 1900 AGE (Ma)
Figure 18. A) A composite thermal history of cover and basement rocks within the Paleoproterozoic Penokean orogen, including the Republic and Peavy nodes, and the fault-bounded structural panels. B) Metamorphic history of cover and basement rocks along the Paleoproterozoic Niagara fault zone. Thermal events are indicated by peaks in temperature, and exhumation/extensional events are represented by rapid decreases in temperature. Exposure of the orogen at the surface is indicated by deposition of the Baraboo Interval quartzite (brown box). Elongate ellipse indicates cooling age gradients determined by Ar-Ar laser probe. Dashed ellipses indicate monazite results at lower temperatures (metamorphic intensity). P = Penokean, Y = Yavapai, M = Mazatzal, WRB = Wolf River Batholith. 54 introduction into the crust at this time. Ultimately, the 2.6 and 2.1 Ga ion microprobe and
EMPA ages from Republic probably represent protolith inheritance and subsequent thermal activity, respectively.
Sample CREP also yields evidence for monazite growth at 1800, 1760, and 1720
Ma in the Republic metamorphic node, while analysis of sample MIST, a
Paleoproterozoic schist, revealed only a single ~1760 Ma event. Thermobarometric results (Haase, 1979) obtained for the Republic area yield pressures of 2-3 kbar, indicating minimal burial during the collision, possibly explaining the lack of peak
Penokean (M1) metamorphic ages. The Humboldt Granite in northern Republic, recently dated by Holm et al. (2004), yielded a U-Pb age of 1805 Ma, suggesting that magmatism accompanied the thermal pulse during the M2 event. The 1800 Ma event is documented across the orogen into Minnesota, probably as a result of initial Yavapai slab rollback.
Notably, both samples collected from Republic exhibit an M3 1760 Ma age.
Additionally, Rose et al. (2003) reports evidence for Yavapai metamorphism at 1740 Ma within the central portion of the WMT. Although these are the first robust 1760 Ma metamorphic ages collected from the central Penokean orogen, previous workers
(Schneider et al., 1996; Mancuso, 2000) have reported Ar-Ar hornblende, muscovite, and biotite cooling ages from 1720 to 1680 Ma, indicative of fast cooling (5-10°C/m.y.), and lend support to a significant thermal event prior to this time (Figure 18A). The 1760 Ma dates reported here confirm the presence of the M3 metamorphic event that has been long recognized in the western Penokean orogen and related to continued rollback (Holm et al., 1998a). 55 Peavy Node
The Peavy metamorphic node to the south of Republic, records the M1 1830 Ma and M2 1800 Ma events, as revealed in both ion microprobe and EMPA datasets.
Schneider et al. (2004) also reported 1830 and 1800 Ma monazite dates from the metamorphic node, indicating crustal thickening associated with Penokean collision was most likely responsible for node formation. Thermochronometric ages from muscovite reported here suggest that the Peavy node underwent a period of cooling between 1800 and 1720 Ma, consistent with previously reported 1800 and 1785 Ma Ar-Ar cooling ages of hornblende in the area (Mancuso, 2000).
Prior to this study, the Peavy region had yielded an Ar-Ar hornblende cooling age of 1765 Ma indicating a thermal M3 Yavapai event (Mancuso, 2000). This investigation has led to the confirmation of the M3 thermal event also recorded in the Republic node, but not clearly evident in metamorphic dates collected in the western panels.
Furthermore, previous studies conducted by Holm et al. (1999) and Mancuso (2000) on the Peavy node report Ar-Ar cooling ages at 1660 Ma, 1640 Ma, and 1600 Ma. These cooling ages suggest that thermal effects of the subsequent Mazatzal accretion were strong enough (> 500°C) to partially overprint the Ar-Ar systematics of hornblende within the Peavy node. In addition to Mazatzal signatures, Mancuso (2000) obtained biotite Ar-Ar cooling ages at 1379 and 1249 Ma (Figure 18B). These ages are purportedly associated with hydrothermal reheating and cooling subsequent to intrusion of the 1470 Ma Wolf River Batholith (WRB), and are possibly suggestive for the extent of the thermal aureole and/or the pluton itself. 56 Park Falls Panel
Ion microprobe results of sample 96-17, located within the Park Falls panel yield evidence for the M2 1800 Ma thermal event. Previous work conducted by O’Boyle
(2003) and Schneider et al. (2004) also report ion microprobe and EMPA metamorphic dates on the Park Falls and Watersmeet panels to include 1830 and 1800 Ma events.
Previous Ar-Ar thermochronologic studies (Schneider et al., 1996) indicate that the
Watersmeet panel underwent periods of cooling and possibly exhumation shortly after the M1 and M2 thermal events. However, the 1820 Ma hornblende cooling age, located in the northernmost portion of the Watersmeet terrane, is the only preserved record of cooling following crustal thickening. It is apparent that the 1800 Ma tectonothermal event, associated with slab rollback, was strong enough to reset the Ar-Ar system in hornblende elsewhere (> 500°C). Similar to the 1830 Ma peak metamorphic event, the
1800 Ma event included synchronous granitic magmatism as the overthickened Penokean crust weakened and became unsupported by the subducting slab (Holm et al., 2004). The
Ar-Ar muscovite cooling age of sample DH-PF-99, obtained in this investigation, suggests that the panel subsequently cooled through 350°C at 1753 and was probably partially reset thereafter by Mazatzal metamorphism at 1620 Ma (Figure 18A).
Niagara Fault Zone
Along the southern portion of the Niagara fault zone, two metamorphic events are identified at ca. 1630 Ma and 1500 Ma (Table 3) based on EMPA monazite analyses.
The ~1630 Ma metamorphic age reported here is supported by previous Ar-Ar hornblende cooling ages of greenschist-facies rocks (Mancuso, 2000) collected in the 57 vicinity of sample PG-03, suggesting that the area cooled through 500°C at 1620 Ma.
Furthermore, the results of this investigation coincide with the boundary of the 1630 Ma thermal/deformational front to the north. The overall lack of 1760 Ma cooling ages south of the NFZ indicate that Mazatzal thermal effects were strong enough (>500°C) to completely overprint previous Yavapai events.
Thermochronometric ages obtained from nearby sample 97-CM-10 indicate the region south of the NFZ experienced a secondary period of cooling from ~1400 Ma to
~1250 Ma, well after the intrusion of the Wolf River Batholith at ~1470 Ma (Figure
18B). Previously reported Ar-Ar thermochronometric hornblende, muscovite, and biotite ages (Mancuso, 2000) collected from samples throughout the WMT suggest that (hydro) thermal effects extend much further than the exposed batholith. Biotite and muscovite cooling ages (Mancuso, 2000) along the southern exposure of the pluton suggest two possibilities: 1) the batholith dips steeply to the south or 2) the extent of hydrothermal activity was minimal. However, similar cooling ages (ca. 1400 Ma) north of the batholith, not overprinted by the ca. 1100 Ma Keeweenawan rift system, indicate that pluton-derived fluids were highly mobilized near the NFZ shear zone as far north as the
Peavy node. Reactivation and shearing of the NFZ throughout multiple accretionary events may have facilitated efficient pathways for fluid flow during magmatism, thus producing a larger ~1460 Ma age thermal aureole to the north of the WRB (Figure 3).
Based on the lack of older Penokean and Yavapai ages within the Ar systematics, it is apparent that the minerals were completely reset at the time of the WRB intrusion and endured a period of slow cooling thereafter. 58 In summation, geochronometric and thermochronometric analyses applied to several metamorphic features across the Penokean orogen (fault-bounded panels, nodes, and the NFZ) have revealed the specific timing of thermal and cooling events. It is clear from ion microprobe Pb-Pb and EMPA total Pb data sets presented here that peak
Penokean M1 metamorphism occurred at ~1830 Ma during a period of crustal thickening.
Notably, a weaker (M2) thermal event at 1800 Ma was recorded across the orogen by utilizing the Ar-Ar laser probe, ion microprobe, and EMPA techniques which most likely represents initial Yavapai slab rollback. Significant metamorphism and thermal pulses between 1770 and 1760 Ma were recorded in all three datasets of samples collected within the gneiss dome corridor. Implications for gneiss dome corridor formation are discussed below. Subsequent metamorphic events at ~1630 Ma are reflected in Ar-Ar laser probe and EMPA results of Niagara fault zone samples, suggesting a prolonged evolution which completely overprinted Penokean and Yavapai signatures during
Mazatzal accretion. Moreover, the NFZ samples also record EMPA total Pb and Ar-Ar cooling ages that reflect thermal effects associated with the intrusion of the Wolf River
Batholith to the south. Although geochronometric and thermochronometric results are very complex, these data can facilitate the identification of distinct metamorphic pulses in relation to known magmatic and tectonic events along the long-lived convergent margin of Laurentia.
Implications for the development of the gneiss dome corridor
The metamorphic map pattern and timing constraints reported in this study can be directly related to models that describe the tectonic and thermal development of the 59 Paleoproterozoic gneiss dome corridor of the Penokean orogen in northern Wisconsin and Michigan. The tectonically emplaced amphibolite-facies gneiss dome corridor, flanked by greenschist-facies rocks to the north and south, records a polymetamorphic history associated with long-lived convergence on the southern margin of Laurentia.
During Penokean burial and deformation of continental margin sediments, the kyanite-zone Watersmeet structural panel reached pressures of 7 to 8 kbars (Black, 1977).
The eastern sillimanite-zone Peavy node attained higher temperatures, but exhibits pressures of 4 to 5 kbars (Attoh and Klasner, 1989). Geochronometric results obtained in this and previous studies (O’Boyle, 2003; Schneider et al., 2004) indicate an 1830 Ma
(M1) metamorphic event associated with this collision-related burial (Figure 18A).
Accompanying this peak Penokean metamorphism was granitic magmatism, displayed as undeformed ~1835 Ma plutons (Sims et al., 1989; Schneider et al., 2002) indicating that significant Penokean deformation had ceased by that time. Notably, however, the
Republic node records paleopressures of 2-3 kbar but does not yield geochronometric evidence for significant Penokean activity at 1830 Ma.
Recently, Holm et al. (2004) has determined that the Penokean crust endured three subsequent pulses of magmatism at ca. 1800, 1775, and 1750 Ma that apparently migrated in a southeastern direction. Plutonic (and volcanic) activity following Penokean accretion is believed to have initiated due to the rollback of the northward subducting
Yavapai slab, facilitating the production of continental arc magmas between approximately 1800 and 1750 Ma. Metamorphic dates obtained in this study and previous studies (O’Boyle, 2003; Schneider et al. 2004) reveal an orogen-wide thermal 60 event (M2) at ca. 1800 Ma, synchronous with initial post-Penokean magmatism.
Samples analyzed in this study and Schneider et al. (2004) record widespread M2 metamorphism across the orogen, suggesting that minor unroofing occurred between the peak Penokean metamorphism (M1) at 1830 Ma and the following 1775 Ma magmatic pulse (Holm et al., 1998; Figure 18A).
Continued rollback and profuse melting at ca. 1775 Ma initiated the intrusion and emplacement of the East Central Minnesota Batholith (ECMB), exposed in the western portion of the Watersmeet and Park Falls structural panels (Figure 2). Shortly following
ECMB emplacement, the abundance of ca. 1760 Ma geochronometric and thermochronometric data suggest that post Penokean crustal collapse occurred during an episode of widespread thinning (metamorphism) and subsequent rapid exhumation
(cooling). Magmatic events have been shown to significantly weaken overthickened crust and facilitate orogenic collapse (Vanderhaeghe et al., 1999). With burial of rocks in the gneiss dome corridor at 10-15 km depths, and temperatures exceeding 500°C, the
Archean basement was probably considerably weakened. Teyssier and Whitney (2002) propose that hot orogens can collapse due to diapirism of warm, low-density basement rocks underlying high-density cover rocks. The Lake Superior region, specifically the
Marquette Range Supergroup, consists of significant amounts of iron formation sediments (Morey, 1993); higher-density cover rocks atop the warm, buoyant basement created a density inversion, allowing the basement to rise to shallower crustal levels
(Marshak, 1999). Rocks investigated for this study originated from some of the deepest
(4-7 kbar) levels exposed in the Penokean orogen (Schneider et al., 2004), signifying that 61 thinning of over-thickened Penokean crust was concomitant with exhumation, producing the gneiss domes throughout the corridor.
Collectively, monazite ages obtained in this study and Schneider et al. (2004) reveal that the 1830, 1800, and 1760 Ma metamorphic events were widespread across the panels and nodes of the Penokean orogen. Previously determined cooling ages obtained by Ar-Ar thermochronology (Schneider et al., 1996; Mancuso, 2000) suggest that the
Peavy node, Watersmeet and Park Falls structural panels underwent rapid cooling through mid-crustal temperatures between 1800 and 1700 Ma, with a documented 1760
Ma thermal pulse across the orogen. It appears as though the (M2) 1800 Ma thermal pulse was the first in a series of thermal events which led to (M3) 1760 Ma collapse, and extension-related rapid cooling thereafter (Figure 18A). It should be noted, however, that formation of the Republic node is strictly a post-Penokean crustal thinning event (~1760
Ma), as depth of burial during Penokean accretion was not sufficient to record an 1830
Ma signature observed in the structural panels and the Peavy metamorphic node. Based on slightly differing metamorphic ages obtained within the Republic node (1760 Ma) and the Peavy node and panels (1775 Ma), it is evident that the Penokean orogen underwent a period of 15 m.y. collapse beginning at ca. 1775 Ma. Collapse and exhumation of the rocks which make up the gneiss dome corridor, would have been unroofed at this time.
Additioinally, they would be flanked to the north and south by lower-grade greenschist rocks through a vertical tectonic extrusion mechanism.
A model for tectonic extrusion of the mid-crustal rocks is presented in Schneider et al. (2004). This model suggests that the amphibolite-facies rocks within the gneiss 62 dome corridor were juxtaposed between the lower-grade rocks by bounding coeval normal- and reverse sense shear zones (Figure 19). These faults include reactivated
Penokean structures such as the Niagara fault zone, in addition to younger ~1765 Ma
A NS
WMT EPSZ NFZ warm, deformed Penokean crust
YAVAPAI SUBDUCTION
B
GNEISS DOME{ CORRIDOR NS
GLtz WMT
YAVAPAI SUBDUCTION
Figure 19. Suggested model for the development of the gneiss dome corridor, based on a schematic N-S cross-section through the Penokean orogen, ca. A) 1830-1800 Ma and B) 1760 Ma. This model (after Schneider et al., 2004) illustrates the remobilization of the gneiss domes associated with rollback during Yavapai subduction. Black circle indicates the Archean-cored gneiss domes within the kyanite-bearing Watersmeet terrane. Gray circle represents the sillimanite-bearing Park Falls terrane which lies immediately to the south. 63 shear zones. Unfortunately, bedrock exposure is so sparse in this region that there are very few places where these faults outcrop: the Niagara fault zone is exposed in only two places south of the Iron River terrane and there are no exposures of the faults bounding the Watersmeet and Park Falls terranes. Therefore, kinematic indicators cannot be used to test the model of coeval normal- and reverse-sense motion or reactivation. However, the east-central Minnesota Malmo structural discontinuity, the western equivalent of the
Flambeau Flowage fault of Wisconsin and piercing point of the pre-Keeweenawan Rift reconstruction, places post-Penokean plutons next to older metamorphic rocks that lie to the north. Furthermore, the gneiss dome corridor is depositionally overlain by ~1700 Ma red quartzites, indicating that the mid-crustal rocks were exposed at the surface by this time.
Previous investigations of gneiss dome formation in the Lake Superior region
(Holm and Lux, 1996; Schneider et al., 1996) have advocated a model of fault doming at the contacts of basement and cover. However, geochronometric and thermochronometric results of basement and cover units in this and other recent studies, indicates that the
Penokean orogen underwent a period of differential collapse due to overthickened crust, thereby facilitating crustal stabilization following Penokean accretion and Yavapai subduction. 64 CONCLUSIONS
In summary, the orogenic rocks collected and dated from northern Michigan and
Wisconsin substantiate five fundamental ideas concerning the metamorphic evolution of the Penokean orogeny (Table 4): 1) Metamorphism at 1830 Ma was induced by accretion
TABLE 4. SUMMARY OF 1900-1400 Ma GEOLOGIC EVENTS IN THE CENTRAL PENOKEAN OROGEN, U.S.A. Geologic Event Age (Ma) Geographic Location Reference
Penokean accretion 1875 Ma Orogen wide Van Schmus (1976) Schneider and others (2002, 2004)
Peak metamorphism and 1835-1830 Ma Watersmeet terrane Schneider and others (2002, 2004) magmatism Park Falls terrane This study Peavy node
Initial Yavapai rollback, 1800 Ma Watersmeet terrane Holm and others (2004) magmatism and metamorphism Park Falls terrane Schneider and others (2004) Peavy node This study Republic node (?)
Yavapai magmatism 1775-1750 Ma Watersmeet terrane Van Schmus (1980) and metamorphism Park Falls terrane Holm and others (1998a, 2004) Gneiss dome 1760 Ma Peavy node Schneider and others (1996, 2004) corridor formation Republic node Holm and Lux (1996) Cooling and exhumation 1750-1700 Ma Wisconsin magmatic terrane Rose and others (2003) This study
Baraboo-interval quartzites ca. 1700 Ma Orogen wide Holm and others (1998b) deposition Medaris and others (2003)
Mazatzal deformation and ca. 1650-1630 Ma Wisconsin magmatic terrane Holm and others (1998b) metamorphism Niagara Fault Zone Romano and others (2000) This study
Wolf River Batholith related ca. 1500-1470 Ma Wisconsin magmatic terrane Van Schmus and others (1975) metamorphism Niagara fault zone Medaris and others (2003) Van Wyck and others (1994) Geiger and Guidotti (1989) This study of the WMT and peak Penokean crustal thickening. 2) A thermal pulse with concomitant magmatism at ~1800 ma resulted in M2 metamorphism. 3) Metamorphism at 1760 Ma resulted in crustal collapse and rapid exhumation within the gneiss dome corridor. 4)
Diapirism and tectonic extrusion provided efficient mechanisms responsible for the formation of the gneiss dome corridor throughout northern Wisconsin and Michigan. 5)
The gneiss dome corridor endured slow cooling following the exhumational event at 65 ~1760 Ma. Although ancient orogens are known for their poorly understood systematics, it is clear that the Paleoproterozoic Penokean orogen was no less dynamic or complicated than younger orogenic events as seen in results presented in this study. 66 REFERENCES CITED
Amlie, R., and Griffen, W.L., 1975, Microprobe analysis of REE minerals using
empirical correction factors: American Mineralogist, 60, 599-606.
Attoh, K., and Klasner, J., 1989, Tectonic implications of the metamorphism and the
gravity field of the Penokean orogen of northern Michigan: Journal of Geology,
108, 353-361.
Barovich, K.M., Patchett, P.J., Peterman, Z.E., and Sims, P.K., 1989, Nd isotopes and the
origin of 1.9-1.7 Ga Penokean continental crust of the Lake Superior region:
Geological Society of America Bulletin, 101, 333-338.
Beaumont, C., Fullsack, P., and Hamilton, J., 1992, Erosional controls of active
compressional orogens: In McClay et al. (eds) Thrust Tectonics, London,
Chapman and Hall, 1-18.
Black, F.M., 1977, The geology of the Turtle-Flambeau area: Iron and Ashland
Counties, Wisconsin: University of Wisconsin – Madison M.S. thesis, 150 p.
Cannon, W.F., and Klasner, J.S., 1976, Geologic and Geophysical map of the Witch Lake
Quad., Michigan: U.S. Geological Survey Miscellaneous Investigations Series
Map I-987, scale 1:62,500.
Cannon, W.F., LaBerge, G., Klasner, J., and Schultz, K., 1998, Reinterpretation of the
Penokean continental margin in part of northern Wisconsin and Michigan:
Institute on Lake Superior Geology Abstracts, 44, 52-53. 67 Cannon, W.F., and Ottke, D., 2000, Preliminary digital geologic map of the Penokean
(Early Proterozoic) continental margin in northern Michigan and Wisconsin:
U.S. Geological Survey Open-file report 99-547 (CD-ROM).
Catlos, E.J., Gilley, L.D., and Harrison, M.T., 2002, Interpretation of monazite ages
obtained via in situ analysis: Chemical Geology, 188, 193-215.
Cherniak, D.J., Watson, E.B., Grove, M., and Harrison, T.M., 2004, Pb diffusion in
monazite: a combined RBS/SIMS study: Geochimica et Cosmochimica Acta, 68,
829-840.
Davis, D.W., Poulsen, K.H., and Kamb, S.L., 1989, New insights into Archean crustal
development from geochronology in the Rainy Lake area, Superior Province,
Canada: Journal of Geology, 97, 54-64.
Foster, G., Kinny, P., Vance, D., Prince, C., and Harris, N., 2000, The significance of
monazite U-Th-Pb age data in metamorphic assemblages; a combined study of
monazite and garnet chronometry: Earth and Planetary Science Letters, 181, 327-
340.
Geiger, C., and Guidotti, C., 1989, Precambrian metamorphism in the southern Lake
Superior region and its bearing on crustal evolution: Geoscience Wisconsin, 13,
1-13.
Gregg, W.J., 1993, Structural geology of parautochthounous and allochthonous terranes
of the Penokean orogeny in Upper Michigan- comparisons with northern
Appalachian tectonics: U.S. Geological Survey Bulletin 1904-Q, 1-28. 68 Haase, C.S., 1979, Metamorphic petrology of the Negaunee Iron formation, Marquette
district, northern Michigan: Ph.D. dissertation, Indiana University, Bloomington,
Ind.
Harrison, T.M., McKeegan, K.D., and Le Fort, P., 1995, Detection of inherited monazite
in the Manaslu leucogranite by 208Pb/232Th ion microprobe dating: crystallization
age and tectonic implications: Earth and Planetary Science Letters, 133, 271-282.
Harrison, T.M., Grove, M., McKeegan, K.D., Coath, C.D., Lovera, O.M., and Le Fort, P.,
1999, Origin and emplacement of the Manaslu intrusive complex, Central
Himalaya: Journal of Petrology 40, 3-19.
Harrison, T.M., Catlos, E., and Montel, J.-M., 2002, U-Th-Pb dating of phosphate
minerals: In Kohn et al. (eds), Reviews in mineralogy and geochemistry –
phosphates, 48, 523-558.
Hodges, K.V., Willis, E.H., and Samuel, A.B., 1994, 40Ar/39Ar age gradients in micas
from a high-temperature-low-pressure metamorphic terrain: Evidence for very
slow cooling and implications for the interpretation of age spectra: Geology, v.
22, 55-58.
Hodges, K.V., and Bowring, S.A., 1995, 40Ar/39Ar thermochronology of isotopically
zoned micas: Insights from the southwestern USA Proterozoic orogen:
Geochimica et Cosmochimica Acta, 59, 3205-3220. 69 Hoffman, P., 1989, Precambrian geology and tectonic history of North America: In Bally
and Palmer (eds), The geology of North America – an overview: Geological
Society of America Decade of North America Geology, A, 478-480.
Holm, D.K., and Lux, D., 1996, Core complex model proposed for gneiss dome
development during collapse of the Paleoproterozoic Penokean orogen,
Minnesota: Geology, 24, 343-346.
Holm, D.K., and Lux, D., 1998, Depth of emplacement and tilting of the Middle
Proterozoic (1470 Ma) Wolf River batholith, Wisconsin: 40Ar/39Ar
thermochronologic constraints: Canadian Journal of Earth Science, 35, 1143-
1151.
Holm, D., Darrah, K., and Lux, D., 1998a, Evidence for widespread ~1760 Ma
metamorphism and rapid crustal stabilization of the Early Proterozoic Penokean
orogen, Minnesota: American Journal of Science, 298, 60-81.
Holm, D., Schneider D., and Coath, C., 1998b, Age and deformation of Early Proterozoic
quartzites in the southern Lake Superior region: Implications for extent of
foreland deformation during final assemble of Laurentia: Geology, 26, 907-910.
Holm, D.K., Romano, D., Mancuso, C., and Foland, K., 1999, Comparison of mica
40Ar/39Ar and Rb/Sr thermochronologic results from northern Wisconsin and
northern Michigan: Institute on Lake Superior Geology, 45, 23-24. 70 Holm, D.K., Van Schmus, W.R., and MacNeill, L.C., 2001, Age of the Humboldt
granite, northern Michigan: Implications for the origin of the Republic
metamorphic node: Institute on Lake Superior Geology, 47, 31-32.
Holm, D.K., Van Schmus, W.R., MacNeill, L.C., Boerboom, T.J., Schweitzer, D., and
Schneider, D., 2004, Late Paleoproterozoic (1900-1600 Ma) tectonic history of
the northern mid-continent, USA: implications for crustal stabilization.
Geological Society of America Bulletin, in press.
Karlstrom, K., Ahall, K-I, Harlan, S.S., Williams, M.L., McLelland, J., and Geissman,
J.W., 2001, Long-lived (1.8-1.0 Ga) convergent orogen in southern Laurentia, its
extensions to Australia and Baltica, and implications for refining Rodinia:
Precambrain Research, 111, 5-30.
Koons, P., 1990, The two-sided orogen: Collision and erosion from the sandbox to the
Southern Alps, NZ: Geology, 18, 679-682.
Koppers, A.P., 2002, ArArCalc; software for 40Ar/39Ar age calculations: Computers and
Geosciences, 28 (5), 605-619.
Larue, D.K., 1983, Early Proterozoic tectonics of the Lake Superior region;
tectonostratigraphic terranes near the purported collision zone: In Medaris, L.G.,
Jr. (ed) Early Proterozoic geology of the Great Lakes region: Geological Society
of America Memoir 160, p. 33-47. 71 Mancuso, C., 2000, Thermal history of the Dunbar dome and Peavy node areas of
northeastern Wisconsin and the Upper Peninsula of Michigan: M.S. Thesis, Kent
State University, 85 p.
Marshak, S., Tinkham, D., Alkmim, F., Brueckner, H., and Bornhorst, T., 1997, Dome
and keel provinces formed during the Paleoproterozoic orogenic collapse – core
complexes, diapirs, or neither?: Examples from the Quadrillatero Ferrifero and the
Penokean orogen: Geology, 25, 415-418.
Marshak, S., 1999, Deformation style way back when: thoughts on the contrasts between
Archean/Paleoproterozoic and contemporary orogens: Journal of Structural
Geology, 21, 1175-1182.
Medaris, G., Singer, B.S., Dott, R.H., Naymark, A., Johnson, C.M., and Schott, R.C.,
2002, Late Paleoproterozoic climate, tectonics and metamorphism in the southern
Lake Superior region and Proto-North America: Evidence from Baraboo interval
quartzites: Journal of Geology, 111, 243-257.
Montel, J.M., Foret, S., Veschmabrem, M., Nicollet, C., and Provost, A., 1996, Electron
microprobe dating of monazite: Chemical Geology, 131, 37-53.
Morey, G.B., 1993, Early Proterozoic epicratonic rocks, The Lake Superior region and
Trans-Hudson orogen: In Reed et al. (eds) Precambrian – Conterminous
United States: Geological Society of America, The geology of North America,
C2, 47-56.
Murphy, B.J., and Keppie, J.D., 2003, Collisional orogenesis in the geological record and
modern analogues: Tectonophysics, 365, 1-5. 72 O’Boyle, C., 2003, Age pattern and nature of metamorphism of subterranes along the
subterranes along the deformed Penokean continental margin, northwest
Wisconsin: M.S. Thesis, Kent State University, 101 p.
Parrish, R.R., 1990, U-Pb dating of monazite and its application to geological problems:
Canadian Journal of Earth Sciences, 27, 1431-1450.
Perkins, D. III, and Chippera, S.J., 1985, Garnet-orthopyroxene-plagioclase-quartz
barometry – Refinement and application to the English River subprovince and the
Minnesota River Valley: Contributions to Mineralogy and Petrology, 89, 69-80.
Pouchou, J., and Pichoir, F., 1984, A new model for quantitative X-Ray microanalysis.
Part I: application to the analysis of homogeneous samples: La Recherche
Aerospatiale, 3, 13-38.
Pouchou, J., and Pichoir, F., 1985, “PAP” phi-rho-Z procedure for improved quantitative
Microanalysis: In Armstrong, J. (ed), Microbeam Analysis, San Francisco Press
Inc, San Francisco, 104-106.
Renne, P.R., DePaolo, D.J., Deino, A.L., Karner, D.B., Owens, T.L., and Swisher, C.C.,
1998, Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar
dating: Chemical Geology, 145, 1-2.
Rey, P., Vanderhaeghe, O., and Teyssier, C., 2001, Gravitational collapse of the
continental crust: Definition, regimes and modes: Tectonophysics, 342, 435-449. 73 Riller, U., Schwerdtner, W.M., Halls, H.C., and Card, K.D., 1999, Transpressive
tectonism in the eastern Penokean orogen, Canada: Consequences for Proterozoic
crustal kinematics and continental fragmentation: Precambrian Research, 93, 51-
70.
Romano, D., Holm, D., and Foland, K., 2000, Determining the extent and nature of
Mazatzal-related overprinting of the Penokean orogenic belt in the southern Lake
Superior region, north-central USA: Precambrian Research, 104, 25-46.
Roscoe, S.M., and Card, K.D., 1993, The reappearance of the Huronian in Wyoming:
rifting and drifting of ancient continents: Canadian Journal of Earth Sciences, 30,
2475-2480.
Rose, S., Schneider, D.A., Loofboro, J., and Holm, D.K., 2003, Results and implications
of monazite geochronology from the central Penokean orogen, WI & MI:
Geological Society of America, Abstracts, 35, 505.
Scherrer, N.C., Engi, M., Gnos, E., Jakob, V., and Leichi, A., 2000, Monazite analysis;
from sample preparation to microprobe age dating and REE quantification:
Schweiz. Mineral. Petrog. MITT. 80, 93-105.
Schneider, D., 1995, Comparison of early Proterozoic postcollisional tectonothermal
histories of the Watersmeet and Republic districts, upper peninsula of Michigan:
Implications for the origin of gneiss domes and metamorphic nodes: M.S. Thesis,
Kent State University, 75 p. 74 Schneider, D., Holm, D., and Lux, D., 1996, On the origin of Early Proterozoic gneiss
domes and metamorphic nodes, northern Michigan: Canadian Journal of Earth
Sciences, 33, 1053-1063.
Schneider, D.A., Bickford, M.E., Cannon, W.F., Schultz, K.J., and Hamilton, M.A.,
2002, Age of volcanic rocks and syndepositional iron formations, Marquette
Range Supergroup: implications for the tectonic setting of Paleoproterozoic iron
formations of the Lake Superior region: Canadian Journal of Earth Sciences, 39,
999-1012.
Schneider, D.A., Holm, D.K., O’Boyle, C., Hamilton, M., and Jercinovic, M., 2004,
Paleoproterozoic development of a gneiss dome corridor in the southern Lake
Superior region, USA: In Whitney et al. (eds) Gneiss domes and orogeny,
Geological Society of America Special Paper, 380, in press.
Schultz, K.J., 1987, An Early Proterozoic ophiolite in the Penokean orogen: Geological
Association of Canada Abstracts with Programs, 12, 87.
Sims, P., Card, K.D., Morey, G.B., and Peterman, Z.E., 1980, The Great Lakes tectonic
zone – A major crustal structure in central North America: Geological Society of
America Bulletin, 690-698.
Sims, P., 1991, Great Lakes tectonic zone in Marquette area, Michigan – Implications for
Archean tectonics in north-central United States: U.S. Geological Survey Bulletin
1904-E, 17. 75 Sims, P., compiler, 1992, Geologic map of Precambrian rocks, southern Lake Superior
region, Wisconsin and northern Michigan: U.S. Geological Survey Miscellaneous
Investigations Series Map I-2185, scale 1:500,000.
Sims, P., 1996, Minnesota River Valley subprovince (Archean Gneiss Terrane): In Sims
and Carter (eds), Archean and Proterozoic Geology of the Lake Superior Region,
U.S.A., 1993, U.S. Geological Survey Professional Paper 1556, 14-29.
Sims, P., Van Schmus, R., Schultz, K., and Peterman, Z., 1989, Tectono-stratigraphic
evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean
orogen: Canadian Journal of Earth Sciences, 26, 2145-2158.
Southwick, D.L., 1993, Archean Superior Province. In Sims, and Carter,
(eds), Archean and Proterozoic Geology of the Lake Superior Region, U.S.A.,
1993, U.S. Geological Survey Professional Paper 1556, 6-13.
Spear, F., and Parrish, R., 1996, Petrology and cooling of Valhalla Complex, B.C.,
Canada: Journal of Petrology, 37, 733-795.
Stockwell, C.H., 1982, Proposals for time classification and correlation of Precambrian
rocks and events in Canada and adjacent areas of the Canadian Shield: part 1—a
time classification of Precambrian rocks and events: Geological Survey of
Canada, Paper 80-19, 135.
Teyssier, C., and Whitney, D.L., 2002, Gneiss domes and orogeny: Geology, 30, 1139-
1142. 76 Vanderhaeghe, O., Teyssier, C., and Wysoczanski, R., 1999, Structural and
geochronological constraints on the role of partial melting during the formation of
the Shuswap metamorphic core complex at the latitude of the Thor-Odin dome,
British Columbia: Canadian Journal of Earth Sciences, 36, 917-943.
Van Schmus, R., 1976, Early and Middle Proterozoic history of the Great Lakes area,
North America: Royal Society of London Philosophical Transactions, 280, 605-
628.
Van Schmus, R., 1980, Chronology of igneous rocks associated with the Penokean
orogeny in WI: In Morey and Hanson (eds) Selected studies of Archean gneisses
and lower Proterozoic rocks, southern Canadian Shield: Geological Society of
America Special Paper, 182, 159-168.
Van Schmus, W.R., Bickford, M., and Condie, K., 1993, Early Proterozoic crustal
evolution: In Reed et al. (eds), Precambrian: Conterminous United States:
Geological Society of America, The geology of North America, C2, 270-281.
Van Schmus, W.R., MacNeill, L.C., Holm, D.K., Boerboom, T.J., and Jirsa, M.A., 2000,
The 1787-1772 Ma East-central Minnesota Batholith: Precursor to crustal
stabilization in the Lake Superior region: Institute on Lake Superior Geology, 46,
65-66.
Van Schmus, W.R., MacNeill, L.C., Holm, D.K., and Boerboom, T.J., 2001, New U-Pb
ages from Minnesota, Michigan, and Wisconsin: Implications for late
Paleoproterozoic crustal stabilization: Institute on Lake Superior Geology, 47, 99-
100. 77 Van Wyck, N., Medaris, G., and Johnson, C., 1994. The wolf river A-type magmatic
event in Wisconsin: U-Pb and Sm-Nd constraints on timing and petrogenesis:
Institute on Lake Superior Geology, 40, 81-82.
Williams, M.L., and Jercinovic, M.J., 2002, Microprobe monazite geochronology: putting
absolute time into microstructural analysis: Journal of Structural Geology, 24,
1013-1028.
Zeitler, P.K., 1989, The geochronology of metamorphic processes: Geological Society
Special Publication, 43, 131-147.
Zeitler, P.K., et al., 2001, Crustal reworking at Nanga Parbat, Pakistan: Metamorphic
consequences of thermal-mechanical coupling facilitated by erosion: Tectonics, 5,
712-728. 78
Appendix A Electron microprobe chemical compositions of monazite domains and calculated spot ages Sample Y Th Pb U Age (Ma) Republic metamorphic node CREP 2.1 11724 63126 5385 864 1752 2.2 10542 40550 3539 740 1764 2.3 13081 57424 5237 1300 1813 2.4 14871 75482 6693 1894 1752 2.5 14109 74467 6549 1821 1742 2.6 10568 45225 3984 912 1769 2.7 10903 51743 4312 969 1686 4.1 3919 92123 6240 69 1468 4.2 8053 57125 4634 391 1709 4.3 9481 61830 5310 465 1801 4.4 12562 72211 7628 1218 2121 4.5 9169 61472 5519 1188 1806 4.6 12125 81075 10716 2715 2469 4.7 6517 39650 3404 376 1788 4.8 5612 34308 2784 18 1748 4.9 8421 52737 4470 482 1768 4.10 11911 85275 8330 1032 2002 4.11 11385 90709 9312 1748 2048 4.12 11248 77145 7106 1357 1860 4.13 5301 35885 1857 377 1096 4.14 5972 42771 3053 503 1488 4.15 7048 58727 4368 621 1554 4.16 9549 73867 5578 763 1578 4.17 12344 80745 6788 1554 1697 4.18 11532 75991 5212 1496 1396 4.19 6040 58002 4283 295 1571 4.20 6402 51068 4107 -526 1801 4.21 13881 100433 11048 2746 2128 4.22 8440 57194 4405 807 1588 4.23 8276 57502 4771 826 1703 4.24 6654 50923 4870 757 1945 4.25 5851 48875 4749 541 2000 4.26 7731 60067 5304 668 1826 4.27 6206 46738 3674 300 1661 4.28 5960 41513 3489 258 1772 4.29 7995 59045 3588 626 1279 79
Appendix A continued Electron microprobe chemical compositions of monazite domains and calculated spot ages Sample Y Th Pb U Age (Ma) CREP 4.30 7286 60400 3887 464 1365 4.31 6625 65219 4217 553 1368 4.32 10098 74290 4966 863 1398 4.33 11132 73838 4745 2392 1261 4.34 9849 73112 6015 725 1714 4.35 12176 75676 6699 1331 1792 4.36 9136 71797 5829 629 1699 12.1 8338 84514 7005 1037 1713 12.2 9802 86698 8520 1511 1978 12.3 8704 78238 7204 930 1895 12.4 5212 84999 5812 690 1445 12.5 12228 103692 6451 1141 1307 12.6 16757 92417 12170 2935 2474 12.7 16937 91598 12897 3061 2615 12.8 14558 89755 11206 2665 2373 12.9 8000 45117 4100 263 1911 12.10 15792 81515 8506 1680 2071 12.11 4603 45401 4177 94 1959 12.12 9885 76708 7408 1554 1928 12.13 16922 93269 11443 2990 2318 12.14 9803 89205 6273 929 1473 12.15 9009 72261 5240 535 1532 12.16 3827 56389 5324 345 1980 12.19 3797 51200 5210 353 2119 13.1 27715 107477 16975 5931 2707 13.2 28962 113014 16786 5843 2591 13.3 27311 108258 15035 4414 2523 13.4 25959 113681 16899 4203 2714 13.5 25077 111498 17411 4531 2802 13.6 25423 106416 15217 4208 2600 13.7 27598 100848 14807 4510 2620 13.8 26071 95195 13591 4134 2565 13.9 12708 98854 10141 2725 1993 13.10 24652 104914 15101 4110 2618 13.11 25365 110990 15805 4439 2587 13.12 12835 113434 16302 5954 2511 13.13 13828 114420 17605 7163 2590 80
Appendix A continued Electron microprobe chemical compositions of monazite domains and calculated spot ages Sample Y Th Pb U Age (Ma) CREP 13.14 13976 114264 17520 7285 2573 13.15 14005 114270 17357 7196 2558 13.16 14527 119633 17805 7285 2527 13.17 13846 118950 18964 7527 2666 13.18 14075 115157 17849 7494 2588 13.19 14115 115421 17813 7435 2583 MIST 1.1 10554 19941 2478 3047 1723 1.2 10538 24498 2939 3273 1739 1.3 10589 21851 2584 2683 1759 1.4 10422 19931 2429 2550 1787 1.5 10934 24759 2823 3118 1690 2.1 9682 19502 2345 2474 1770 2.2 11642 21691 2786 3555 1734 2.3 11950 23644 2971 3728 1722 2.4 8016 16477 1978 2188 1744 2.5 10553 16709 2308 3227 1746 2.6 11262 25827 2861 2727 1726 2.7 10705 21863 2545 2691 1733 3.1 14281 31889 4760 7095 1772 3.2 10862 26748 3444 4023 1787 3.3 10505 17789 2558 3435 1810 3.4 10017 13408 1937 2843 1754 3.5 9951 14566 2039 2828 1764 4.1 11880 19476 2992 4408 1805 4.2 10915 11511 1854 2973 1782 4.3 10835 11393 1872 2967 1807 4.4 10749 10944 1866 3026 1818 4.5 10758 18233 2584 3429 1804 5.1 11171 16053 2642 4102 1824 5.2 7600 17776 2598 3671 1791 5.3 13114 20757 3126 4456 1811 5.4 10807 15552 2296 3281 1792 81
Appendix A continued Electron microprobe chemical compositions of monazite domains and calculated spot ages Sample Y Th Pb U Age (Ma) Peavy metamorphic node HRR 1.1 3560 29997 2788 1061 1774 1.2 4360 33234 3315 1947 1770 1.3 8016 28593 3650 4169 1791 1.4 12938 27112 3053 2792 1763 2.1 12323 39118 5550 6536 1882 2.2 12101 33281 3880 3272 1840 3.1 13276 24984 3103 3437 1779 3.2 11337 29903 3932 4408 1833 4.1 9804 17144 2099 2809 1661 4.2 9078 24981 2726 2487 1728 4.3 9231 20777 2365 2430 1722 4.4 9559 25557 2729 2606 1684 4.5 10557 29255 3221 3197 1701 4.6 9813 22890 2732 2854 1766 5.1 10367 17831 2594 3646 1790 5.2 11274 17754 2544 3683 1756 5.3 11905 18414 2741 4032 1779 5.4 12203 15813 2772 4349 1867 Sample Y Th Pb U Age (Ma) Niagara fault zone PRF 1.1 3080 19536 1578 155 1696 1.2 3329 18756 1510 76 1713 1.3 4340 41111 3298 377 1678 1.4 3081 24141 1868 -24 1679 2.1 2843 12656 856 158 1410 2.2 2375 9576 688 -52 1588 2.3 2531 5396 310 114 1174 Sample Y Th Pb U Age (Ma) PG-03 1.1 3363 13321 1049 189 1622 1.2 5264 3823 325 95 1687 1.3 4451 4088 277 205 1262 1.4 3410 6576 472 105 1477 2.1 3177 10796 978 308 1769 2.2 3351 15389 1368 365 1764 82
Appendix A continued Electron microprobe chemical compositions of monazite domains and calculated spot ages Sample Y Th Pb U Age (Ma) PG-03 2.3 3953 2955 257 131 1625 2.4 4007 3487 285 174 1508 3.1 4723 1389 117 169 1295 3.2 4665 5017 477 521 1507 3.3 7778 2684 234 203 1494 5.1 2876 19729 1488 422 1521 1.1b 5319 7937 575 86 1515 1.2b 6578 1328 95 -13 1614 1.3b 5482 3432 224 -34 1473 1.5b 5606 4392 359 -43 1828 1.6b 4010 3320 228 -32 1546 2.1b 3714 5662 494 67 1803 2.3b 6965 3522 252 -34 1608 2.4b 5552 2213 172 -22 1745 2.5b 3557 2215 122 -22 1251 3.2b 5221 3059 227 74 1487 4.1 3990 14193 1188 280 1688 4.2 4019 6108 412 -20 1484 4.4 3531 3004 208 -29 1558 5.1b 4712 3017 262 147 1603 6.1b 5895 2614 193 -21 1649 6.2b 6622 4401 349 -37 1767 6.3b 6710 5371 440 32 1732 6.4b 5204 3000 239 58 1615 6.6b 6850 3041 222 -30 1641 83 Appendix B Descriptions and locations of rocks collected within the Penokean orogen
MIST- Paleoproterozoic staurolite schist Description: A fine grained biotite schist containing garnet and large porphyroblasts of staurolite. Euhedral staurolite ranges from < 1 mm up to 4 mm in length. Garnet occur as euhedral grains ~ 1mm in length. Foliations are predominantly biotite, within a biotite and quartz matrix. Location: UTM 16 412714E 5151335N (NAD 27) ~16 km north-north-northwest of Republic, Michigan in Marquette County.
PVD- Paleoproterozoic schist Description: A medium to fine grained micaceous schist consisting of a biotite and quartz-rich matrix. Location: UTM 16 406511E 5093247N (NAD 27) ~32 km northwest of Iron Mountain, MI in Iron County.
PG-03- Paleoproterozoic schist Description: Medium gray, fine-grained quartz serecite schist consisting of numerous muscovite and biotite foliation planes and potassium feldspar augens. Augens are poikilitic with muscovite inclusions and are <1mm in diameter. Location: UTM 16 426631E 5067402N (NAD 27) Piers Gorge on the Menominee River ~2.4 km southeast of Niagara, WI in Marinette County.
PRF- Paleoproterozoic garnet schist Description: Dark gray fine-grained foliated biotite schist with altered calcite and pinhead garnets. Biotite grains are aligned perpendicular to the foliation planes. Location: UTM 16 402638E 5075680N (NAD 27) 12 km west of Iron Mountain, MI in Florence County, WI. Pine River Flowage
CREP- Archean orthogneiss Description: Pink to gray coarse-grained foliated gneiss containing anhedral quartz, microcline, Na-plagioclase, and biotite; ± chlorite. Na-plagioclase crystals show evidence of deuteric alteration with serecite. Location: UTM 16 423710E 5136192N (NAD 27) ~ 3.5 km south of Republic, Michigan on state route 95 in Marquette County.
96-17- Paleoproterozoic orthogneiss Description: Pinkish gray fine- to medium-grained micaceous gneiss. Foliation is defined by biotite layers; matrix is dominantly quartz and potassium feldspar ± muscovite. Outcrop also consists of pegmatites Location: UTM 15 568556E 5181479N (NAD 27) on the southeast side of Blockhouse Lake along Hoffman Rd off of state route 182 west of Park Falls, in Price County, Wisconsin. 84 Appendix B (continued)
HRR- Archean orthogneiss Description: A medium gray fine- to medium-grained amphibole-bearing gneiss. Quartz is dominantly equigranular matrix material intermingled with opaque minerals, muscovite, and biotite. Amphiboles are poikilitic (with opaques) and show deuteric alteration to serecite. Foliation of biotite and amphibole planes are weak but visible. Location: UTM 16 402029E 5093598N (NAD 27) Horse Race Rapids, Florence County, WI
97-CM-10- Paleoproterozoic pegmatitic muscovite Description: Muscovite books collected from the Pine River Pegmatite. Location: UTM 16 395011E 5078065N (NAD 27) 45 m west of Pine River and 91 m south of Hwy 101, between the river and the first knob in Florence County, Wisconsin.
DH-PF-99- Paleoproterozoic granitic muscovite Description: A coarse muscovite grain collected from a 2-mica granite of the East Central Minnesota Batholith. Location: UTM 15 698026E 5087395N (NAD 27) South of Park Falls, Wisconsin and down stream of the dam along the Flambeau River in Price County Wisconsin.
94-MI-1- Archean schist muscovite Description: A coarse-grained muscovite collected from an Archean muscovite- biotite quartzofeldspathic schist. Location: UTM 16 446140E 5090200N (NAD 27) The north side of Hwy 69. The sample locality is 2.4 km east of Foster City in Dickinson County, Michigan.