Refining Paleoproterozoic Sedimentary Sequence Boundaries

Home , Animikie Group, Cuyuna Range, Mesabi Range, Penokean orogeny, Rove Formation

Refining Paleoproterozoic Sedimentary Sequence Boundaries in East-Central Minnesota, Carlton County: Implications for Source, Age, Correlations, and Tectonic Histories

A thesis submitted to the Kent State University Graduate College in partial fulfillment of the requirements for the degree of Master of Science

Scott W. Scheiner

December, 2012

Thesis written by

Scott W. Scheiner

B.S. Kent State University, 2009

M.S. Kent State University, 2012

Approved by

______, Advisor Dr. Daniel K. Holm

______, Chair, Department of Geology Dr. Daniel K. Holm

______, Dean, College of Arts and Sciences Dr. Raymond A. Craig

TABLE OF CONTENTS

LIST OF FIGURES...... v

LIST OF TABLES...... vi

ACKNOWLEDGEMENTS...... vii

SUMMARY...... 1

Chapter I. Introduction...... 3 Animikie Basin in Minnesota...... 5 Lake Superior Banded Iron Formations...... 9 Tectonic Model of the Penokean Orogeny...... 11 Southern Margin of Animikie Basin...... 14

II. New Developments...... 17 Yavapai Magmatism...... 17 Yavapai Metamorphic Signature...... 18 Yavapai Structures...... 20 Yavapai Sedimentation...... 21 Field Description of Bedrock Geology: Southern Margin of Animikie Basin...... 21 New Hypothesis...... 22

III. Field Sampling...... 25 Lithologic Variation Across Holst’s Line...... 26 Preparation of Thin Sections...... 27 Thin Section Analysis...... 27 Geochemistry...... 30 Geochemical Analysis Techniques...... 30 Geochemical Results...... 31 Isotope Analysis...... 35 Preparation of Isotope Samples...... 38 Isotope Results...... 38

iii

Discussion...... 41

IV. A New Interpretation...... 45 A New Tectonic Model...... 49 Implication for Inventory of Structures...... 51

REFERENCES...... 53

LIST OF FIGURES

Figure 1. Paleoproterozoic supracrustal sequences in the Lake Superior region...... 4 2. Stratigraphy of major iron ranges in Minnesota and Ontario...... 6 3. Simplified geologic map of east-central Minnesota...... 8 4. Aeromagnetic map of east-central Minnesota...... 10 5. Schematic cross-section of proposed Penokean orogeny tectonic evolution...... 13 6. Large fold structure at Thomson Dam within Carlton County, Minnesota...... 16 7. Bedrock geology map of Carlton County, east-central Minnesota...... 23 8. Schematic synopsis of tectono-sedimentary models along the southern margin of the Animikie Basin...... 24 9. Photomicrographs of SS10-2a, SS10-4, SS10-5, and SS10-6...... 29 10. La/Sc vs. Ti.Zr ratio plot...... 34 11. Sc/Cr vs. La/Y ratio plot...... 34 12. Th vs. Zr plot...... 36 13. Sc vs. V plot...... 36 14. CaO vs K2O+Na2O plot...... 37 15. εNd(Age) vs. Age plot...... 43 16. Th/Sc vs εNd plot...... 44 17. Composite stratigraphic column of the Animikie Basin...... 46 18. Proposed tectono-sedimentary formation of the Animikie Basin...... 48 19. New tectonic interpretation...... 50

LIST OF TABLES

Tables 1. Major and trace element geochemistry of samples from Carlton County, Minnesota...... 32 2. Nd isotopic composition of Carlton County, Minnesota samples...... 40

Acknowledgements

I owe many thanks to Daniel Holm for his guidance and dedication through all stages of this project. I greatly appreciate Tathagata Dasgupta’s guidance on elemental and isotopic analysis, and for connecting me with Syracuse University where my samples were analyzed. I would like to thank Terry Boerboom, David Hacker, and Donald

Palmer for their help in the review process. I appreciate the help and support I received from everyone in the KSU Geology Department.

I cannot thank my family and friends enough for the love, support, and encouragement they provided during this process. I would especially like to thank my loving wife, Aubrey, for her everlasting patience and support. Also, to my parents, Ed and Lisa, for their unwavering support in everything I have undertaken.

Funding for this research was provided by: Kent State University Department of

Geology (School of Hard Rocks Mineral Resources Award), and The Institute of Lake

Superior Geology Student Research Grant.

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Summary

The Animikie Basin in north-eastern Minnesota is usually interpreted as a

Penokean (1875-1835 Ma) foredeep deposit consisting mainly of 2 to 5 km thick turbidite sequences underlain by banded iron formations and quartzite conglomerates, all deposited in a foreland basin north of the accreted Penokean arc terrain. Much of the

Animikie basin sequence is only very weakly metamorphosed and only mildly deformed.

However, along its southern margin in Carlton County, east-central Minnesota, the sedimentary sequence is strongly deformed into what are interpreted to be refolded nappes. The twice deformed sequence occurs south of a similar sedimentary sequence that has undergone a single deformation. The contact between these sequences, although nowhere exposed, was interpreted by Holst (1985) as a Penokean thrust fault and all of the deformation assumed to be Penokean in age. This contact is commonly referred to at

Holst’s Line.

A wealth of new data collected over the last ~15 years document Yavapai-age magmatism, metamorphism, sedimentation and deformation overprinting the Penokean orogen. Metamorphic ages that that post-date the Penokean orogeny and detrital zircon ages indicate that some of the Paleoproterozoic sedimentary rocks may be younger than previously thought. Based on this new evidence, I propose Holst’s Line to represent an unconformity separating sedimentary foredeep packages of different ages and test this hypothesis using geochemical and Sm/Nd isotopic data.

Geochemical analyses were obtained on 10 samples divided into two groups based upon location north or south of Holst’s Line. Relative proportions of major elements varied little within and across these groups. Trace element ratio plots of La/Sc vs Ti/Zr, Sc/Cr vs La/Y, Th vs Zr and Sc vs V were created with the hope of distinguishing between different tectonic settings for the source of the sediments. The results show a concentrated grouping of data from the southern samples and a much larger spread from northern samples, suggesting a more variable source for the northern sedimentary sequence.

Neither Sm nor Nd are significantly fractionated compared to each other during surficial processes, and therefore, Nd isotopes are frequently used in sedimentary rocks as an indicator of provenance. Four samples were analyzed, two from north of Holst’s Line and two to the south. Using previously published detrital zircon ages stratigraphic relationships, ages of 1770 Ma and 1850 Ma were assigned to the northern and southern rocks respectively. ƐNd (T) was then calculated for each sample with the results indicating the northern rocks were derived from an older Archean crustal source whereas the southern rocks were derived from a younger, possibly arc source.

The differences documented here argue against Holst’s Line being a tectonic contact. Instead, Holst’s Line is reinterpreted as an unconformity separating Penokean foredeep and Yavapai foredeep Paleoproterozoic deposits. This reinterpretation has important ramifications for interpreting structures throughout the upper Great Lakes region. Lastly a new tectonic sequence model for the formation and deformation of the

Animikie Basin is proposed.

Chapter 1

Introduction

Paleoproterozoic sedimentary rock sequences on the southern continental margin of the Archean Superior Province are world-renowned for their abundant banded iron formations (BIFs) ( Morey & Southwick, 1993) and well-preserved algal fossils (Han &

Runnegar, 1992). They are preserved over a 700 x 300 km wide swath in Minnesota, northern Wisconsin, upper Michigan and into Canada (Fig. 1). Recent discoveries include the identification of the 1850 Ma Sudbury Impact layer above the iron-formations (Pufahl et al. 2007) and the 2.4-2.0 Ga Great Oxidation Event (Holland, 2006), one of the most significant changes in seawater chemistry in Earth history (Canfield, 1998; Poulton et al.,

2004). These supracrustal rocks lie athwart the Archean (north)-Proterozoic (south) boundary and formed during both 2400-2200 Ma rifting and subsequent foreland basin deposition during 1890-1830 Ma Penokean arc accretion. Because of their sedimentologic and tectonic importance, there has been much research on the correlation of the Paleoproterozoic sedimentary stratigraphic units within the Upper Great Lakes region specifically, and around the margins of the entire Superior craton in general

(Bekker & Rainbird, 2012).

The major Paleoproterozoic sedimentary basins in eastern Minnesota are separated from those in Wisconsin and the Upper Peninsula of Michigan by the 1100 Ma

Figure 1. Paleoproterozoic supracrustal sequences (shaded areas) in the Lake Superior region (modified after Ojakangas et al., 2001). 5

Mid-Continent Rift (Fig. 1; Van Schmus & Hinze, 1985). Limited exposure, together with varying degrees of polyphase deformation and metamorphism has made it difficult to correlate rock units based primarily on lithostratigraphic similarities. In the past decade several studies have produced well constrained ages allowing for the general consensus that many of these sequences are syndepositional, including the majority of the

BIFs. During this same time, however, refinements in the ages of igneous, metamorphic, and hydrothermal events reveal multiple late Paleoproterozoic tectonic events which likely included pulses of sedimentation previously attributed solely to Penokean orogenesis at 1890-1830 Ma, but some of which are now known to post-date this time

(Schneider et al., 2004; Holm et al., 2005; Medaris et al., 2007; Vallini et al., 2007).

Animikie Basin in Minnesota

The Animikie Basin in north-eastern Minnesota (Fig. 1) consists mainly of 2 to 5 km thick (Chandler & Malek, 1991) turbidite sequences known as the

Rove/Virginia/Thomson Formations (Fig. 2) underlain by banded iron formations and quartzite conglomerates, all deposited in a foreland basin north of the accreted Penokean arc terrain. The Gunflint Iron Formation in the lower Animikie Group contains interbedded ash beds dated at 1878 Ma (Fralick et al., 2002) Additionally, Addison et al.

(2005) obtained U/Pb zircon ages of 1832 and 1836 Ma from ash layers near the base of the Rove Formation. These ages are consistent with the lower Animikie Group sediments forming in a broad shallow sea during Penokean accretion.

Figure 2. Stratigraphy of the major iron ranges in Minnesota and Ontario (modified after Schulz and Cannon, 2007; Cannon et al. 2010). Thick black line represents the 1850 Ma Sudbury Ejecta layer.

On its northwest margin the Animikie Basin sedimentary rocks lie nonconformably on Archean basement, and are largely undeformed, dipping only gently to the southeast. On its southern margin in east-central Minnesota the Animikie Basin sedimentary rocks are in angular unconformity with older supracrustal rocks of the North

Range and Mille Lacs Groups, and are themselves strongly deformed (Fig. 3).

The recent discovery of the widespread and rapidly deposited Sudbury Impact ejecta layer (Addison et al. 2005) in the Gunflint (Ontario) and Mesabi (northern

Minnesota) Iron Ranges adds important age information on lower Animikie Basin deposition and provides an accurate means of correlating units throughout the region.

Dated at 1850+1 Ma (Krogh et al., 1984; Davis, 2008; Cannon et al., 2010), the Sudbury ejecta layer separates two completely different sedimentary sequences (Fig. 2).

Described at 10 localities throughout the Lake Superior region, the ejecta layer lies immediately above the BIFs, and is in turn overlain by black shale, followed by thick graywacke turbidite sequences. The ejecta layer thins with increasing distance from the impact site near Sudbury, Ontario, consistent with the fact that the majority of impact horizons recognized are from the base of the Baraga Group (Animikie Basin equivalents) in northern Michigan (closer to the impact crater in Sudbury, Ontario). The Sudbury

Impact layer is locally metamorphosed up to the greenschist facies in the Lake Superior region. However, it is readily distinguished by the well preserved ejecta clasts, and by the presence of shocked quartz and shatter cones.

Central Minnesota

ECMB: ECMB: East

(modified after Boerboom, 200

central Minnesota showing the major structural and depositional

map map of east

Simplified Simplified geologic

3. 3.

Figure elements of the Paleoproterozoic sequences Batholit

Lake Superior Banded Iron Formations

Paleoproterozoic Banded Iron Formations are abundant throughout the Lake

Superior region and have been extensively studied due to their great economic importance (Morey & Southwick, 1995). The identification of the 1850 Ma Sudbury

Ejecta layer immediately above the BIFs provides evidence that the majority of BIFs

(with one exception) throughout the Lake Superior region are correlatable and likely deposited simultaneously in a broad shallow basin.

Economically rich Iron Ranges occur along two sides of the Animikie Basin; the undeformed Mesabi (northwest) and the strongly deformed Cuyuna (southwest). The

BIFs within these ranges, long considered equivalent, are now known to be separated by the Animikie Basin basal unconformity (Chandler, 1993). The magnetic signatures from the BIFs that make up the Cuyuna district, which is comprised of two separate iron formation packages, the Cuyuna North range and Cuyuna South range, separated by a thrust fault, appear to extend under the Animikie Basin toward the northeast. The aeromagnetic image of east-central Minnesota (Fig. 4) denotes magnetic highs as lighter gray tones. The strong BIF aeromagnetic signature clearly delineated as lighter gray tones in the Cuyuna district continues to the northeast although less vividly due to burial of the BIFs under the Animikie Basin. These BIFs make up the base of the North Range

Group, a twice deformed sequence of rocks consisting of phosphorus and manganese enriched carbonaceous argillite and siltite (Southwick & Morey, 1990). The age of the

North Range Group metasedimentary rocks is uncertain but are considered

Figure 4. Aeromagnetic map of east-central Minnesota (modified from Chandler et al., 2007). Highly deformed North and South Ranges can be seen continuing under the Animikie basin in the northeast corner. Carlton County boxed in white for orientation purposes.

older than ca. 1860 Ma, which is the time of the initial low-temperature metamorphism of the region (Vallini et al., 2007).

The North Range Group is overlain unconformably by Animikie Group rocks on the southwest margin of the Animikie Basin. Above the unconformity is a less well known iron formation (the Emily) located at the same relative stratigraphic level as the

Biwabik Iron Formation of the Mesabi Range. The rocks of the Emily district, overall, have been pervasively oxidized and leached (Morey & Southwick, 1995) to varying degrees. The Emily contains several different lithotopes which Morey et al. (1991) interpret to represent the sedimentary environments of a platform transitioning towards a deep basin. The lithotopes representing deeper water environments are dominated by carbonates and silicates, with some lens shaped iron-formations intermingled. Black shales and some turbiditic siltstone are also typical within the deeper basin sediments of the Emily (Morey & Southwick, 1995). Importantly, the iron rich lenses of the Emily only extend laterally for a few dozen meters. Manganese oxide deposits can be found in localized areas within the Emily, and are most likely epigenetic

(Morey & Southwick, 1995).

Tectonic Model of the Penokean Orogeny

The 1890-1835 Ma Penokean orogeny was the result of arc/microcontinent collision during rapid southward growth of Laurentia between 1900 and 1600 Ma. Schulz and Cannon (2007) presented a synthesis of the tectonic evolution of the Penokean orogen (Fig. 5). Beginning at ~1890 Ma ocean closure resulted in formation of the

Pembine-Wausau magmatic arc terrane (Fig. 5a), a tholeittic and calc-alkalic island arc complex, derived from melting of subducting oceanic lithosphere (Chandler et al., 2007).

By ~1875 Ma (Fig. 5b), “soft” accretion of the magmatic arc resulted in downwarping of the continental margin and initiation of a shallow foreland basin, into which the lower

Animikie Group sediments were deposited. Simultaneously, north-directed subduction on the southern side of the Pembine-Wausau, brought the Archean Marshfield Terrane closer to the Superior Craton. By ~1850 Ma (Fig. 5c), the Marshfield Terrane accreted with enough force to uplift the Pembine-Wausau Terrane onto the Superior Craton, creating a foreland bulge. By ~1840 Ma (Fig. 5d), compression of the Pembine-Wausau

Terrane resulted in thrust sheets of sedimentary and volcanic rock being emplaced onto the Superior Craton. Thick turbidite sequences were deposited into the upper Animikie foreland basin. The end of the Penokean orogeny was marked by ~1830 Ma (Fig. 5e) metamorphism of the more deeply buried sediments in the southern foreland basin, and by intrusion of stitching granites.

Historically, Paleoproterozoic deformation and metamorphism in the Upper Great

Lakes has been solely attributed to Penokean orogenesis. However new evidence, described in more detail in the next chapter, suggests a strong overprint related to

Yavapai orogenesis at roughly 1800-1750 Ma (Holm et al., 2005). Therefore, the end of the Penokean Orogeny (as depicted in Fig. 5e), must be viewed primarily as the initial framework or architecture which has been tectonically modified by subsequent

Figure 5. Schematic cross-sections illustrating proposed tectonic evolution of the Penokean Orogeny (modified after Schulz and Cannon, 2007).

orogenesis. Importantly, the Paleoproterozoic sedimentary sequences in southern

Minnesota and northwesternmost Wisconsin are overlain nonconformably by little deformed ~1700 Ma Baraboo Interval quartzites (Dott, 1983) whereas these same quartzites are strongly folded by 1650 Ma tectonism related to the Mazatzal Orogeny (ca.

1600-1700 Ma) throughout most of Wisconsin (Holm et al., 1998). All post-Penokean overprinting west of the Midcontinent Rift (largely MN) are related to Yavapai age tectonism, making this area most amenable for deciphering the geologic history of the region. The Animikie Basin sedimentary sequence just west of the Midcontinent Rift in east-central

Minnesota is the focus of this study.

Southern Margin of the Animikie Basin

Much of the Animikie basin sequence is only very weakly metamorphosed and only mildly deformed. However, along its southern margin in east-central Minnesota within Carlton County, metamorphic grade increases to lower greenschist facies (chlorite and biotite zones) with metamorphism progressively increasing in the underlying Mille

Lacs Group below the Animikie unconformity (southward) to staurolite zone of the amphibolite facies. Thermobarometry from the Mille Lacs Group indicates peak burial of depths 21 – 25 km and temperatures reaching close to 600 oC at ca. 1765 Ma (Holm &

Selverstone, 1990; Holm et al., 1998, 2007). The Mille Lacs Group show textural and geochronologic evidence of two metamorphic events, first at 1830 Ma and again at 1765

Ma (Beck, 1988; Schneider et al., 2004; Holm et al, 2007).

Structural and strain analysis studies reveal that the Animikie rocks along the southern margin of the Animikie Basin are also strongly deformed (Fig. 3; Holm et al.,

1988). Holst (1985) recognized two distinct structural zones: a northern zone characterized by a single well-developed cleavage and upright folds (Fig. 6), and a southern zone containing two cleavage planes and refolded folds. Holst hypothesized that the line separating twice deformed rocks (south) from once deformed rocks (north) represented a tectonic/deformational front (see also Sun et al., 1995). Holst (1985) and

Sun et al. (1995) both considered the entire deformed sedimentary sequence to have been deposited prior to both deformations, which they inferred to be Penokean in age.

The goal of this study is to obtain a more accurate interpretation of the events which have affected the Animikie Basin rocks in east-central Minnesota. Non fossiliferous Paleoproterozoic sedimentary rocks are difficult to date and their intense deformation and metamorphism within a poorly-exposed area has made the study of these rocks challenging. Nonetheless, a wealth of new information about the Animikie Basin rocks obtained from new techniques reveal a strong Yavapai age imprint (Holm et al.,

1996, 1998, 2005). The possibility of Geon 17 age overprinting requires re-examination of the previous depositional and deformational models which assume only Penokean orogenesis throughout the region.

Figure 6. Large fold structure (dashed lines) at Thomson Dam within Carlton County, Minnesota.

Chapter 2

New Developments

New data on the ages of rock units and tectonic events within the Lake Superior region reveal a clear Yavapai-age overprint of much of the Penokean orogen. Modern geochronologic techniques have produced metamorphic ages that post-date the Penokean orogeny. Dating of detrital zircons indicates that some of the Paleoproterozoic sedimentary rocks may be younger than previously thought. Geophysical data, including large-scale aeromagnetic surveys reveal Yavapai age terrane boundaries that cross-cut

Penokean age sutures. In this chapter, I summarize new data collected over the last 15 years which documents Yavapai-age magmatism, metamorphism, sedimentation and deformation. Based on these new findings, I propose a new tectonic sequence model for the formation and deformation of the Animikie Basin in eastern Minnesota.

Yavapai Magmatism

The Paleoproterozoic orogenic architecture preserved in east-central Minnesota

(Fig. 3) consists of an external (northern) foreland basin, a medial fold-thrust belt, and an internal (southern) medium-grade metamorphic and plutonic terrane, all historically interpreted to have formed entirely during the Penokean Orogeny (Southwick et al.,

1988). More recent field, geophysical, and drill core studies indicate that the plutonic terrane is comprised of predominantly late- to post-tectonic plutons that form a physically continuous area of bedrock over 7000 km2 (Jirsa et al., 1995; Boerboom et al., 1995;

1999). U-Pb zircon ages obtained by Holm et al. (2005) from 20 separate plutonic bodies, revealed that all post-tectonic units in east-central Minnesota were emplaced during a series of long-lived magmatic pulses between ca. 1800 and 1772 Ma, well after Penokean orogenesis. They interpreted the East-Central Minnesota Batholith (ECMB) as having intruded the internal zone of the Penokean orogen at mid-crustal depths of roughly 13-17 km (Holm et al., 2005).

The Yavapai age ECMB intrudes Penokean structures and metamorphic rocks and is little deformed aside from minor brittle shear zones. Exhumation of the Penokean internal zone rocks and the batholith occurred shortly after intrusion ended as all plutons cooled simultaneously and rapidly at 1760-1740 Ma (Holm & Lux, 1996; Holm et al.,

1998). Uplift would have resulted in rapid rates of erosion and high amounts of sedimentation as the volume of eroded material, the entire upper crust that overlie the

ECMB when it intruded, would have been quite large, on the order of ~100,000 km3 or more.

Yavapai Metamorphic Signature

Application of modern geochronologic techniques has provided considerable new information on the Proterozoic thermal/metamorphic history of the Lake Superior region.

In the 1990’s, application of the Ar/Ar incremental heating technique on hornblende and mica led to the discovery of widespread Yavapai-age rapid cooling of Paleoproterozoic metasedimentary rocks originally metamorphosed during the Penokean Orogeny (Holm et al., 1998). Holm et al. (1998) proposed a dominant amphibolite facies metamorphic signature occurred at ca. 1760 Ma, well after Penokean orogenesis. Since then, new

monazite geochronology has revealed a pervasive geon 17 imprint throughout the internal and medial zones surrounding the East-Central Minnesota Batholith (Holm et al., 2007).

Monazites record metamorphic histories because of their high closure temperatures

(>850oC) and ability to preserve chemical and age zonation associated with multiple thermal events (Cherniak et al., 2004 & Townsend et al., 2000).

Radiometric dating of metamorphic minerals from amphibolite facies Mille Lacs

Group rocks below the Animikie basin unconformity indicate that they were metamorphosed both during the Penokean and Yavapai orogenies (Holm and Lux, 1996;

Holm et al., 1998, 2007; Schneider et al., 2004). Low grade metamorphism of the younger Animikie Basin rocks is undocumented, but could be either Penokean or

Yavapai in age. Vallini et al. (2007) used xenotime geochronology to date low-grade metamorphism (sub-greenschist and greenschist facies) of metasedimentary rocks from the northern medial (fold-thrust belt) and external (foredeep) regions. Xenotime can precipitate out during diagenesis, commonly forming on detrital zircons (Rasmussen,

1996; Vallini, 2006; Vallini et al. 2005), and can also form as a result of hydrothermal fluids (Vallini et al., 2007), making it ideal for obtaining ages from low-grade metamorphic rocks. Samples of the Mille Lacs and North Range Groups in Minnesota yielded both Penokean and Yavapai greenschist facies ages, whereas samples from the overlying Animikie Group collected in the Mesabi Iron Range yielded only Yavapai ages. Vallini et al. (2007) interpreted these ages as resulting from an extensive regional widespread fluid migration within the Penokean foreland basin. Based on these results, it seems likely that the greenschist facies metamorphism in the Carlton County study area is

Yavapai in age.

Yavapai Structures

The locus of Yavapai age amphibolite facies metamorphism described above coincides with an approximately east-west corridor of Archean cored gneiss domes extending from east-central Minnesota eastward across northern Michigan (Fig. 1; Holm and Lux, 1996; Schneider et al., 2004; Tinkham and Marshak, 2004). Previously attributed to Penokean orogenesis based on their structural trend, the gneiss domes are now known to have formed at ca. 1760 Ma during exhumation of the overthickened

Penokean internal zone.

The gneiss dome-plutonic terrane in east-central Minnesota is separated from the

Penokean fold-thrust belt by the southeast dipping Malmo structural discontinuity

(MSD). West of the McGrath gneiss dome (Fig. 4), the MSD is clearly marked by the sharp truncation of linear aeromagnetic anomalies associated with strongly folded iron- formations in the Cuyuna Range. Here, the MSD juxtaposes post-Penokean plutons and medium-grade metamorphic rocks (Little Falls Fm) to the south against older

(Penokean?) lower-grade metasedimentary rocks to the north, indicating that the MSD is a Yavapai age, possibly reactivated Penokean structure that uplifted exhumed the plutonic terrane. The exhumation of the southern plutonic-metamorphic terrane almost certainly would have resulted in the shedding of sediment into the Animikie Basin area during geon 17.

Yavapai Sedimentation

Historically, the supermature ‘Baraboo Interval’ (ca. 1700 Ma) quartzite bodies scattered throughout Minnesota and Wisconsin have been considered the oldest post-

Penokean sedimentary units in the southern Lake Superior region (Dott, 1983). However,

Medaris et al. (2007) documented a separate Statherian (1800-1600 Ma) unit in central

Wisconsin consisting of an immature meta-arkose. The meta-arkose is crosscut by a

~1762 Ma granitic dike and contains abundant Penokean age detrital monazite grains.

In Ontario, Heaman and Easton (2005) obtained detrital zircons within the Rove

Formation (within the upper units of the northern Animikie Basin) that postdate the

Penokean Orogeny. Ten zircon ages were dated between 1796-1777 Ma with the youngest 1777 Ma zircon being concordant. This zircon age puts a maximum age on sedimentation in the region, indicating that the sediments must be Yavapai in age and likely eroded from a younger southerly source, rather than from the north where Archean age rocks are dominant. Finally, Wirth et al. (2008) obtained detrital zircon ages of 2.05-

1.80 Ga from turbidite sediments of the Thomson Formation at the southern edge of the Animikie basin.

Field Description of Bedrock Geology: Southern Margin of Animikie Basin

Recent bedrock mapping within Carlton County, Minnesota (Fig. 7; Boerboom,

2009), along the southern margin of the Animikie basin revealed discontinuous sulfidic horizons characterized by large cubic pyrite porphyroblasts, in addition to chert horizons and minor mafic hypabyssal intrusions. This layer conjecturally overlies a basal conglomerate (recognized in drill core) and is well delineated geophysically by

discontinuous linear aeromagnetic anomalies (Fig. 4). The distended pyritic rich layer appears similar to portions of the iron rich layers near Emily and may be at roughly the same stratigraphic level as the Emily District rocks to the west (Fig. 3). The sulfidic horizons become less common to the north and are absent north of Holst’s Line (Fig. 7).

New Hypothesis

Since the classic structural and strain analysis studies by Holst (1984, 1985, 1987) in Carlton County, Holst’s Line has been generally accepted to be a thrust fault at the north edge of a nappe terrain (Fig. 8, left). Both once-deformed (D2) rocks to the north and twice-deformed (D1 & D2) to the south were interpreted by Holst to consist of the same Penokean age foredeep deposits which were deformed, then juxtaposed against one another by thrust faulting near the end of the Penokean orogeny. For this interpretation, all structures are Penokean in age.

I hypothesize Holst’s Line to represent an unconformity separating sedimentary foredeep packages of different ages (Fig. 8, right). In this scenario, foredeep deposits north of Holst’s Line were deposited during the Yavapai Orogeny and depositionally overlie Penokean age foredeep deposits to the south. In this interpretation, early nappe structures (D1) are Penokean in age whereas overprinting upright east-west trending folds

(D2) are Yavapai in age. Testing between these two hypotheses is critical for understanding the Paleoproterozoic tectono-depositional history of the region and for assessing the inventory of structural features formed during the two orogenies. In this study I use geochemical data and Sm/Nd isotopic analyses in an effort to test between these two hypotheses.

Figure 7. Bedrock geology of the study area within Carlton County, east-central Minnesota (modified after Boerboom, 2009). SS – represents sample locations for this study.

deposits (after holst, holst, deposits(after

sedimentary sedimentary models along the southern margin of the Animikie

A: Late Penokean thrust faults cuts Penokean thrust faults Late A: Animikie Penokean foredeep

central Minnesota. central

. . Simplified schematic synopsis of tectono

Figure Basin,east B: 1984). Yavapai age from (south) Yavapai Penokean unconformity separates deposits foredeep Animikie foredeep deposits(north).

Chapter 3

Field Sampling

Field samples were collected from multiple outcrops within Carlton County during the summer of 2010. Appropriate sample locations were chosen following consultation with Terry Boerboom of the Minnesota Geological Survey, who recently completed a bedrock geology map of Carlton County (Boerboom, 2009). A north to south transect of the county was decided upon in such a way to allow me to sample each important stratigraphical unit on both sides of Holst’s line within the study area. Using

Terry’s field maps I managed to collect 20 samples from the area. The least weathered samples with the largest grain sizes were chosen; however, poor outcrop exposure led to limited stratigraphical selection. As a result, most of the samples are very fine-grained greywacke, with some being extremely fine-grained. For example, sample SS10-2a collected from Thomson Dam (north of Holst’s Line), is a very fine-grained, well- indurated, dark grey meta-greywacke, mostly massive with few thin intermittent bedding planes <3mm thick. Sample SS10-11 (collected south of Holst’s line) is a very fine- grained, relatively friable, dark grey meta-greywacke that is thinly bedded and crenulated; some voids where pyrite has weathered out are evident. Bedding planes within the sample range in thickness from <1 to 10mm. After collection, each sample was bagged, labeled, and a GPS location was recorded for each site.

Lithologic Variation Across Holst’s Line

Holst’s line doesn’t just represent a transition from once deformed rocks in the north to twice deformed rocks in the south, it also marks a lithological change (Holst,

1984). The rocks to the north consist of mainly metagreywacke, metasiltstone, and slate, whereas the southern metagreywackes contain sulfide-rich graphitic units, metavolcanics, and some carbonaceous layers (Morey & Ojakangas, 1970; Connolly, 1981; Hyrkas,

1982). There is no evidence of any metavolcanics or sulfide-rich graphitic units within the northern rocks. Carbonates are rare and occur mainly as small concretions, not continuous layers (Holst, 1985). Holst used this abrupt transition in lithology to support his interpretation that the contact represents a thrust fault. However, a similar lithologic change occurs along the northwestern edge of the Animikie Basin where little deformation has occurred.

Along the northern extent of the Animikie Basin, thin carbonaceous layers intermingled with black shales have been identified at the base of the Virginia formation which lies immediately above the Biwabik Iron formation (Jirsa et al., 2011). The Emily

District, located along the south-western edge of the Animikie Basin, is correlated with the Biwabik Iron Formation (in the Mesabi Iron Range), and contains both iron-sulfide rich, carbonaceous layers, and black shales (Morey & Southwick, 1995). Also, to the north-east of the Animikie Basin, the Gunflint Formation (Fig. 1) is located at the same stratigraphic level as the lower Thomson Formation (Virginia Formation equivalent), and contains chert-carbonates, volcanics, and shale (Fralick et al., 2002). The twice deformed lower Thomson Formation can be correlated with other units at the same stratigraphic

level from around the Animikie Basin, all of which consist of similar strata and are overlain by thick turbidite sequences. This is consistent with my hypothesis that Holst’s line could represent an unconformity, instead of a thrust fault.

Preparation of Thin Sections

Thin sections were made from each sample. The samples were cut into

(approximately 22mm x 45mm x 15mm) chips using a rock saw in the Geology

Department of Kent State University. Each rock chip was inspected for any signs of fractures or weathering in order to ensure the quality of the thin sections. This proved to be difficult for some of the samples since they were weakly metamorphosed greywacke, and more susceptible to weathering. Any of the samples that displayed evidence of a fabric, were cut perpendicular to the fabric so mineralogic domains could be observed in thin section view. The samples were then sent to Quality Thin Sections to be processed.

Thin Section Analysis

Thin section analysis proved to be more difficult than anticipated due to the nature of the samples. Some of the rocks are extremely fine-grained, even when examined using the 10x objective lens individual minerals were almost indistinguishable.

However, some samples contain grains large enough to identify mineralogically. Also, the presence or absence of fabric can be seen in most of the slides. Evidence of one versus two deformational events can be seen in most of these samples.

Samples 1-4, exhibit a single weak slaty fabric in hand sample. Photomicrograph of SS10-2a (Fig. 9a) from the Thomson Dam area represents a typical lithology from

north of Holst’s Line and does not show significant signs of deformation, however the tabular shaped grains are slightly aligned. The grains are not specifically oriented or deformed, although some plagioclase grains have been sericitized.

Samples 5, 6, 8, 10, and 11, (all south of Holst’s Line) show signs of two deformations. Photomicrographs of SS10-5 (Fig. 9c) and SS10-6 (Fig. 9d) clearly show a crenulation fabric, indicative of two deformations. The S-shaped quartz grain in SS10-6 is a good indicator of the shear direction that created the crenulations. Most large grains are elongated and oriented in a similar direction. Many samples taken from south of

Holst’s line show well developed crenulation cleavage, indicating two deformations.

It is thought that if the sediments in the different deformational zones are from different sources, then the southern (within the 2D zone) sediments might be mineralogically different from the northern (1D) sediments. However, upon examination of the thin sections, there doesn’t appear to be a dramatic mineralogical change. In the photomicrograph of SS10-2a, the quartz, feldspar, and plagioclase grains are easily seen.

Some of the feldspars have been sericitized, which is to be expected since this sample lies within the chlorite zone (Fig. 7). SS10-4 (Fig. 9b) looks very similar to SS10-2a except there is more potassium feldspar and less plagioclase. With more sericite in this sample, it is possible that much of the plagioclase has been replaced. In both SS10-5 and SS10-6, quartz can be seen easily, however the majority of the grains are so small that identification is difficult.

Figure 9. Thin sections from various samples which are typical of sediments found both north (a,b) and south (c,d) of Holst’s Line. (a) Sample SS10-2a. (b) Sample SS10-4. (c) Sample SS10-5. (d) Sample SS10-6.

Geochemistry

The geochemical composition of sedimentary rocks is influenced by a series of different factors; composition of source rocks, intensity of weathering, tectonic setting, diagenesis, and alteration (Holail & Moghazi, 1998). As stated previously, Holst’s line separates once deformed rocks in the north from twice deformed rocks in the south.

Based on previous research, we propose that this line may represent a folded unconformity separating sedimentary packages of different ages. To test this hypothesis, ten samples from Carlton County in East-Central Minnesota were selected for geochemical analysis at varying distances both north and south of Holst’s Line.

Geochemical Analysis Techniques

Ten samples were cut into 15-20 mg chips and sent to GeoAnalytical Laboratory at Washington State University, Pullman for XRF analysis. All weathered crusts were removed and each chip was carefully selected to ensure homogeneity. WSU processed the chips according to the procedures listed on the WSU GeoAnalytical Laboratory web page (http://www.sees.wsu.edu/Geolab/note/xrf.html). The samples were ground into a very fine powder, mixed with di-lithium tetraborate flux, and fused at 1000oC in a muffle furnace. The bead was then reground, refused and polished on diamond labs to provide a smooth flat surface (Johnson et al., 1999). The analysis was performed using their

ThermoALR XRF spectrometer which measured 29 major and trace element concentrations within the samples. The trace element values shown are in parts per million (ppm). The major element values represent a weight percent, and were normalized by the lab on a volatile free basis.

Geochemical Results

Elemental analysis results are shown in Table 1. Samples are divided into two categories based on their location relative to Holst’s Line. It is important to note that when plotting the data, sample SS10-12 was not included due to its distal location from the proposed unconformity. Also, southern sample SS10-7 yielded anomalous results compared to the rest of the southern samples. To show the reproducibility, a duplicate was made from SS10-2a and is labeled as SS10-2aR in Table 1. The variation in results between SS10-2a and SS10-2aR is very small. FeO* had the largest variation with regards to major elements and was less than 0.25%. While chromium had the largest variation for the trace elements and the difference was only 5 ppm. SS10-2aR was plotted with the rest of the data to show the accuracy of the data. The average values for the major elements between the northern and southern samples show little variation, most being under 1.5% with SiO2 having the largest at 3%. Trace element data reveals a similar comparative result, with most varying by less than 30 ppm, the largest difference being Barium (164 ppm).

La, Y, Th, Zr, Ti, and Sc have been shown to have low mobility during the sedimentary process (Holland, 1978), allowing their concentrations to better indicate the source material. The La/Sc -Ti/Zr, Sc/Cr-La/Y, Th-Zr, Sc-V, plots are modeled after

Bhatia and Crook (1986) who used similar plots to distinguish between different tectonic settings. When the data are plotted using La/Sc-Ti/Zr ratios (Fig. 10), used to identify oceanic arc signatures , both the northern and southern points plot in a linear pattern. The northern samples have a larger variation in values, Ti/Zr ranging from 60 to 25.63 and

Above Holst's Line Below Holst's Line SS10-1 SS10-2a SS10-2aR SS10-3a SS10-4 SS10-5 SS10-6 SS10-7 SS10-9 SS10-11 SS10-12 SiO2 61.43 69.74 69.67 65.04 73.81 65.27 58.85 73.01 65.15 60.88 71.33 TiO2 0.885 0.746 0.744 0.783 0.492 0.824 0.973 0.599 0.930 1.043 0.600 Al2O3 19.23 13.64 13.55 16.71 12.43 17.30 20.19 13.14 17.11 19.07 13.54 FeO* 8.91 6.13 6.31 7.88 4.16 8.29 10.24 5.01 7.51 9.83 5.81 MnO 0.057 0.079 0.078 0.064 0.050 0.035 0.062 0.065 0.050 0.084 0.056 MgO 3.62 2.25 2.25 3.24 1.27 2.86 3.90 1.79 2.78 3.41 1.97 CaO 0.32 2.33 2.33 0.63 2.58 0.18 0.32 1.08 0.65 0.29 1.43 Na2O 2.26 3.80 3.80 3.61 3.92 2.52 1.47 4.13 3.04 2.20 4.35 K2O 3.11 1.13 1.12 1.91 1.19 2.57 3.75 1.05 2.61 3.02 0.78 P2O5 0.177 0.159 0.157 0.139 0.101 0.144 0.238 0.126 0.169 0.170 0.127 Ni 83 51 53 73 36 86 103 36 68 88 35 Cr 165 231 236 155 121 148 186 123 152 179 134 Sc 27 15 14 21 12 23 30 13 23 27 13 V 156 102 99 130 83 138 178 82 144 171 96 Ba 594 255 259 454 284 480 843 273 623 586 187 Rb 125 42 43 73 44 97 142 41 102 116 35 Sr 110 256 258 165 224 69 96 227 153 135 278 Zr 137 291 290 113 140 144 157 174 162 161 194 Y 24 20 20 18 15 25 27 19 26 26 24 Nb 9.9 8.9 9.5 9.7 6.7 9.3 10.4 7.6 12.4 11.3 8.6 Ga 23 14 14 18 13 21 26 14 20 25 15 Cu 29 40 40 19 10 33 65 16 45 49 19 Zn 113 91 92 91 54 107 131 88 120 149 68 Pb 7 25 25 12 11 7 11 11 11 5 14 La 27 38 38 10 24 27 30 24 30 27 31 Ce 64 76 75 35 43 60 70 52 63 60 67 Th 8 11 10 6 7 6 9 6 9 8 7 Nd 31 33 33 14 21 31 37 23 30 28 30 U 4 4 5 4 2 3 3 2 2 3 4 Major elements are normalized on a volitile-free basis, with total Fe expressed as FeO. Values are displayed as wt%. Trace elements are un-normalized, values are displayed as ppm. "R" denotes a duplicate made from the same rock powder.

Table 1. Major and trace element geochemistry of samples from Carlton County, Minnesota.

La/Sc ranging from 2.634 to 0.505. For the southern samples Ti/Zr ranged between

64.719 and 34.404, with La/Sc ranging from 1.776 to 0.99. According to Bhatia and

Crook (1986), oceanic arc signatures generally have La/Sc ratios of <1 and Ti/Zr ratios of

>40. With exception of anomalous sample SS10-7, the southern samples are very close to being within this range. In contrast only two of the five northern samples plot within the island arc area.

Sc/Cr – La/Y ratios are plotted in Figure 11. The northern samples plot in a roughly linear pattern with La/Y values between 1.92 and 0.59 and Sc/Cr values ranging from

0.155 to 0.063. The southern samples mostly plot in a tight cluster, with SS10-7 again being anomalous. La/Y values range from 1.28 to 1.06 and Sc/Cr values are between

0.161 and 0.109. According to Bhatia and Crook (1986), oceanic island arcs have lower

Sc/Cr – La/Y ratios when compared to a continental source; however, this differentiation is not clearly seen in the data.

Bhatia and Crook (1986) also attribute oceanic island arcs with having low Zr and

Th, and high ferromagnesium V and Sc trace elements. For the northern samples Th ranges from 11 ppm to 6 ppm and Zr from 291 ppm to 113 ppm. Southern Th values range from 174 ppm to 144 ppm. On average, the southern samples have higher Zr values; however, with the exception of sample SS10-2a, both northern and southern rocks plot within the same area (Fig 12). The Sc-V plot (Fig 13) does seem to indicate a difference between the north and south, as long as SS10-7 is excluded. Northern Sc values range from 27 ppm to 12 ppm with V ranging from 156 ppm to 83 ppm. Southern

Sc values range from 130 ppm to 13 ppm and V ranging from 178 ppm to 82 ppm.

80.00

70.00

60.00

50.00 SS10-2a and 40.00 SS10-2aR Ti/Zr North 30.00 South

20.00 SS10-7 10.00

0.00 0.00 0.50 1.00 1.50 2.00 2.50 3.00 La/Sc

Figure 10. La/Sc vs Ti/Zr ratio plot (after Bhatia and Crook, 1986). Boxed region represents typical island arc ratios. Areas not within the box are typically continental in origin.

2.00 1.80 1.60 SS10-2a 1.40 and SS10-2aR

1.20

1.00 La/Y SS10-7 North 0.80 South 0.60 0.40 0.20 0.00 0.00 0.05 0.10 0.15 0.20 0.25 Sc/Cr

Figure 11. Sc/Cr vs. La/Y ratio plot (after Bhatia and Crook, 1986). Typical island arc ratios should be lower than those of continental origin.

Overall, the samples plot in a linear pattern. However, the southern samples have higher overall V-Sc values, which according to Bhatia and Crook, may indicate an island arc source.

Major elements CaO, Na2O, and K2O were compared using a CaO vs K2O+Na2O plot (Fig 14). Northern K2O+Na2O values range from 5.52 wt.% to 4.93 wt.% and CaO from 2.58 wt.% to 0.32 wt. %. Southern samples have K2O+Na2O values ranging from

5.66 wt. % to 5.09 wt. % with CaO between 1.08 wt. % to 0.18 wt.%. Knowing that the island arc complex was tholeiitic and calc-alkalic enriched, a higher ratio would be expected in the southern samples. However, the results do not show this relationship.

The major oxide and trace element geochemistry data do not show a clear difference between the two groups of samples; however, in most of the plots the southern samples show less geochemical trace-element variation than those from the north. The smaller range in data from the rocks south of Holst’s Line indicates that they may be different from those to the north. Also, it is important to note that values for sample SS10-7 were consistently different from that of the other southern samples.

Isotope Analysis

From erosion to diagenesis, rocks are broken apart, mixed, and sometimes chemically modified. Neither Sm nor Nd are significantly fractionated compared to each other during these surficial processes, and therefore, reflect the original isotopic composition of the source material (Chakrabarti et al., 2007). For this reason, Nd isotopes are frequently used as an indicator of provenance and are particularly useful for

350 SS10-2a 300 and SS10-2aR

250

200

North

Zr (ppm) Zr 150 South 100

0 0 2 4 6 8 10 12 Th (ppm)

Figure 12. Th vs. Zr plot (after Bhatia and Crook, 1986). Island arcs typically have lower Th and Zr than continental sources.

200 180 160 SS10-2a 140 and SS10-2aR

120 100 North V (ppm) V 80 South 60 SS10-7 40 20 0 0 5 10 15 20 25 30 35 Sc (ppm)

Figure 13. Sc vs. V plot (after Bhatia and Crook, 1986). Island arcs typically are enriched in these ferromagnesian elements.

5.70

5.60

5.50

5.40

5.30

5.20 North South K2O+Na2) (wt%) K2O+Na2) 5.10

5.00 SS10-2a and SS10-2aR 4.90

4.80 0.00 0.50 1.00 1.50 2.00 2.50 3.00 CaO (wt%)

Figure 14. CaO vs. K2O+Na2O plot. A tholeittic and calc-alkalic enriched island arc complex should lead to a higher ratio of these major elements.

fine grained rocks (Miller and O’Nions, 1984; Nelson and DePaolo, 1988; Basu et al.,

1990; Gleason et al., 1994; Simien et al., 1999).

Preparation of Isotope Samples

Of the twenty samples collected, four were chosen for isotopic analysis. These four were chosen specifically because of their proximity to the contact between once deformed and twice deformed metasedimentary rocks. Samples were cut down to approximately 30g and the pieces were inspected to be certain that all weathering rind was removed. These samples were then sent to Syracuse University to be analyzed for

Nd isotopic data. Lab preparation and analytical techniques follow that described in

Samson et al. (1995). Samples were powdered using a steel jaw crusher and alumina shatterbox. Approximately 300 mg of each powdered sample was then dissolved in teflon bombs using a specific procedure to produce a solution that was then spiked with a

149Sm-150Nd isotopic tracer solution. This mixture was then dried and re-dissolved in a series of HCl solutions, which allows for the separation of Nd and Sm from the mix.

Isotopic ratios were measured using a fully automatic VG-Sector 54 mass spectrometer

(Samson et al., 1995).

Isotope Results

The results from the isotope analyses are shown in Table 2. The epsilon parameter, ƐNd (T), indicates the difference in initial 143Nd/144Nd in a sample from the

CHUR value at the time of original crystallization (Depaolo & Wasserburg, 1976). To calculate this, the formation age of the rock samples was needed. Samples SS10-2a and

SS10-4 were taken from north of Holst’s Line while SS10-6 and SS10-8 were taken from the south. An age of 1770 Ma was chosen for the northern samples based on detrital zircon ages (Heaman & Easton, 2005 and Wirth et al., 2006). Earlier I proposed that the rocks directly to the south of Holst’s Line in Carlton County are compositionally similar to the Emily iron formation. Morey and Southwick (1995), stated that the Emily district is stratigraphically equivalent to both the Biwabik and Mesabi iron ranges to the north.

Addison et al. (2005) discovered a layer near the top of both the Mesabi and Biwabik iron formations which is correlative to the Sudbury ejecta layer (~1850 Ma) both stratigraphically and compositionally. Hence, the southern samples were assigned a depositional age of 1850 Ma.

Values in the column labeled ƐNd (T), in Table 2, represent the initial 143Nd/144Nd value at the time of formation of the rock sample. Melting lowers the Sm/Nd ratio from the standard CHUR (chondritic uniform reservoir) line, due to the differences in the respective distribution coefficients of Sm and Nd, a model that assumes that Sm/Nd ratio of the earth evolved in a uniform reservoir which has the same ratio as chondritic meteorites (Depaolo & Wasserburg, 1976). During a partial melting event within the mantle, due to the chemical fractionation of Sm and Nd, Sm is relatively partitioned into the residue while Nd goes into the melt. This partitioning creates the differences in the

Sm/Nd ratio between the residue, and the partial melt. Over time, due to the differences in the Sm/Nd ratio, the Nd isotopic ratio (143Nd/144Nd) evolve in different paths; the residue becoming more than and the partial melt fraction becoming less than CHUR.

ƐNd is calculated by comparing the 143Nd/144Nd ratios of a sample against the

d e Sample Field Sm Nd ƐNd ƐNd TCHUR TDM No. No. (ppm) (ppm) 147Sm/144Nd 143Nda/144Nd (0)b (T)c Age (Ga) (Ga) 1 SS10-2a 4.88 28.1 0.1051 0.51157 ± 3 -21.1 -0.4 1770 1.8 2.08 2 SS10-4 3.93 20.8 0.1141 0.511482 ± 19 -22.5 -3.9 1770 2.13 2.39 3 SS10-6 4.79 26 0.1114 0.512075 ± 8 -19.8 9.2 1850 1.81 2.11 4 SS10-8 5.39 28.8 0.1132 0.511687 ± 3 -18.6 1.2 1850 1.73 2.05 Hemming 1f Sand 4.53 24.4 0.1139 0.511679 -18.7 0 1770 Hemming 2f Shale 5.02 25.5 0.1191 0.511768 -17 0.6 1770 a Measured ratio, corrected for spike and normalized to 146Nd/144Nd=0.7219. Uncertainties are + 2σ and refer to least significant digit. b 143 144 eNd (0) indicates present-day epsilon value; present-day Bulk Earth values: Nd/ Nd=0.512638; 147Sm/144Nd=0.1966. c eNd (T) calculated for age of rock. d Bulk Earth (CHondrite Uniform Reservoir) age. e Depleted mantle model age following model of DePaolo (1981). f Data taken from Hemming et al. (1995)

Table 2. Nd isotopic composition of Carlton County, Minnesota samples.

143Nd/144Nd ratio of CHUR. A positive value indicates a more juvenile or ‘depleted’ source rock, whereas a negative number represents an older more ‘enriched’ one

(DePaolo & Wasserburg, 1976). Young Arc type rocks are the result of melting within the depleted mantle and sediments derived from this source and will have a positive ƐNd value when compared to CHUR, whereas sediments derived from an older Archean crustal source will display a negative value as they will have a lower Sm/Nd ratio than

CHUR. Based on this understanding, my data (Fig. 15) indicates an older enriched source for the northern sediments which make up samples SS10-2a and SS10-4 (ƐNd

(T)1.85Ga = -0.4 and -3.9 respectively) and a younger depleted source for the southern samples SS10-6 and SS10-8 (ƐNd (T)1.77Ga = 9.2 and 1.2 respectively). Also, data taken from Hemming et al. (1995) has been plotted in Figure 15. These samples were taken from the Thomson Dam area and when given a 1770 Ma age, their values fall very close to the Thomson Dam sample (SS10-2a).

Discussion

Recent studies of the Samarium-Neodymium isotope model have shown its usefulness and reliability for age and source rock identification (Hemming et al., 1995).

Hemming et al. (1995) and Barovich et al. (1989) used Nd isotope data from Animikie

Basin rocks in Minnesota to infer that the lower portion of the Animikie sequence rocks from the Mesabi Range are composed of sediments derived from older Archean crust, likely derived from the Superior Province to the north. In contrast, similar isotope data from the upper portions of the Animikie sequence from the Mesabi Range indicate a ca.

1.8 Ga source age, likely the younger 1850 Ma magmatic arc to the south.

The isotope data indicates a clear difference between my northern and southern samples within the study area as well, although reversed from the Hemming et al. and

Barovich et al. data (Fig. 15). In the stratigraphically lower twice-deformed (lower

Thomson) unit I see an isotopic signature which indicates a more juvenile source when compared to the upper unit (upper Thomson). It is possible that the southern rocks (lower

Thomson) within the study area which indicate a younger, more depleted source are the result of Penokean age sedimentation derived from the magmatic arc terrain to the south which accreted between 1875-1850 Ma. As discussed earlier, the emplacement of gneiss domes between 1750-1760 Ma caused rapid unroofing and increased sedimentation

(Holm & Lux, 1996). After the arc had eroded away, the gneiss domes were exposed and subjected to erosion. These gneiss domes are composed of altered Archean crust and could have been the source of sediment for the samples taken from the north of Holst’s

Line (upper Thomson) which indicate an older more evolved source. The proximity of both the magmatic arc terrane and Archean gneiss domes, as well as their respective ages make them possible sources for the basin rocks in question.

Finally, McLennan et al. (1993) used an ƐNd vs Th/Sc plot (Fig. 16) to differentiate provenance between turbidite sequences. When my data is applied to their model, I see two different source types. The southern sample (SS10-6) falls within their

Arc zone whereas the northern samples indicate a more upper crustal type source. The difference in sediment source which is indicated in both Figures 15 and 16, supports my hypothesis that Holst’s Line is an unconformity.

εNd (Age) vs Depositional Age (Ma)

15 14 13 12 11 10 9 SS10-6 8 7

5 Thomson Dam data 4 (Hemming et al., 1995) 3

2 Nd (Age) Nd

ε 1 SS10-8 0 -1 SS10-2a CHUR -2 -3 -4 SS10-4 -5 -6 1760 1860 1960 2060 2160 2260 Depositional Age (Ma)

Figure 15. ƐNd(Age) vs. Age plot. Values below the CHUR line indicate an Archean Crustal source is likely for the sediments. Values above indicate a younger more depleted source.

Figure 16. Th/Sc vs. ƐNd plot (after McLennan et al., 1993). Used to compare provenance for different turbidite sequences.

Chapter 4

A New Interpretation

The provenance of sedimentary rocks within the Animikie Basin has helped to constrain the tectonic evolution of the Paleoproterozoic rifted margin of the Superior

Province. Geochemical and Nd/Pb isotopic evidence led Hemming et al. (1995) to conclude that the Thomson Formation turbidite sequences were derived solely from

Paleoproterozoic sources. However, additional data reported in chapter 3, suggest a more variable source. According to new data, the greywackes to the south of Holst’s Line have a Paleoproterozoic signature, whereas those directly to the north of the line indicate an

Archean or possibly mixed source. When the data which Hemming et al. (1995) collected for the Thomson Dam location are recalculated using 1770 Ma as the source age, it indicates an Archean rather than Paleoproterozoic source.

Figure 17 is a composite stratigraphic column of the Animikie Basin where the lower portion represents the preserved stratigraphy at the northern margin of the basin and the upper portion represents the preserved stratigraphy along the southern deformed margin. The Pokegama Quartzite, which has a late Archean provenance (Hemming et al.,

1995), underlies the iron formations along the northwestern edge of the Animikie Basin.

The Virginia Formation greywackes overlie the iron formations in the north, and according to Nd data obtained from a single drill core were derived from a

Figure 17. Composite stratigraphic column of the Animikie Basin.

Paleoproterozoic arc source. The Emily District rocks along the western margin of the

Animikie Basin are at the same stratigraphic level as the iron formations in the northwestern portion of the basin. The lower portion (twice deformed) of the Thomson

Formation is correlated with the Virginia Formation along the northern edge of the basin.

New data from this study indicate the lower portion of the Thomson Formation, like the

Virginia Formation, has a Paleoproterozoic provenance. However, Nd data from the upper Thomson Formation (once deformed) indicate the provenance for the greywackes was more likely older Archean crustal rocks.

The proposed tectono-sedimentary formation and resulting geometry of the

Animikie Basin is shown as Figure 18. At the base of the Animikie Basin the Penokean age unconformity is shown as a nonconformity in the north separating Penokean age sedimentary rocks from the Archean basement below. As the basin progresses southward, the basal Penokean unconformity becomes an angular unconformity separating pre-Penokean sedimentary and volcanic rocks to the south from Penokean foredeep rocks to the north. Only the southern portion of the Animikie Basin which was closest to the orogenic zone experienced deformation during the Penokean orogeny.

During the Yavapai orogeny, the southern margin of the basin experienced uplift and erosion, followed by Yavapai foredeep deposition resulting in the formation of a disconformity in the north and an angular unconformity in the south. Significant uplift in the southern portion of the basin due to the emplacement of the ECMB and gneiss domes led to copious amounts of sediment being shed into the basin, consistent with the bulk of the turbidite sequences within the Animikie basin being Yavapai in age. Near the end of

sedimentary formationtheAnimikie Basin of sedimentary

tectono Figure 18. Proposed FigureProposed 18.

the Yavapai orogeny, deformation led to the folding of the Yavapai foredeep deposits and refolding of Penokean and pre-Penokean rocks in the south. Deformation waned to the north away from the orogenic zone.

A New Tectonic Model

Growing evidence for Yavapai tectonism, and the new data from this study requires a new tectonic model which incorporates both the Penokean orogeny and the

Yavapai orogeny (Fig. 19). Figure 19a depicts the formation of a foredeep basin

(Animikie Basin) initially forming as the result of a southerly dipping subduction zone associated with the accretion of the Wisconsin Magmatic Terrain (WMT) against the southern Archean continental margin. Major sedimentation into the basin from the north commences prior to ~1850 Ma. Thrust faulting of the basin sediments initiates at roughly

1840 Ma due to an orogenic impact from the south (Fig. 19b). This is the first deformation the older basin turbidite sequences (Lower Thomson Formation) experience.

In Figure 19c the East Central Minnesota Batholith intrudes the Penokean overthickened root between 1800-1772 Ma, prior to significant erosion and possibly related to Yavapai subduction further south. Rapid unroofing of the orogenic root together with gneiss dome (MGD) formation resulted from accretion and deformation of the warm overthickened crust. As a result of uplifting of the southern portion, renewed sedimentation into the basin begins around 1770-1750 Ma (Fig. 19d). The youngest ages of the detrital zircons found within this sequence matches the age of the ECMB (Holm et al., 2005), making the batholith a likely source for the sediment. The Nd data from this

Figure 19. New tectonic interpretation. Wisconsin Magmatic Terrane (WMT), East- Central Minnesota Batholith (ECMB), McGrath Gneiss Dome (MGD)

study indicates that this sediment also contains an Archean signature which can be attributed to the erosion of the now exhumed MGD. Between 1750-1700 Ma (Fig. 19e) continued shortening from the south caused the southern portion of the basin sedimentary package to be deformed again, although the deformation from this event wanes to the north (Vallini et al., 2007). The older sequence that has experienced both deformations is exposed in the southern portion of the basin, and is represented in Figure 19 by the D1 and D2 zone. The younger sediment that has only been deformed once is present in the northern portion of the basin (D1 zone).

Implication for Inventory of Structures

In tectonic models proposed as recently as 2007, the Thomson Formation and equivalents are considered Penokean foredeep sediments (Schulz and Cannon, 2007).

The majority of evidence revealing a significant Yavapai overprint comes from studies which focused on higher grade zones, such as the gneiss dome corridor within the orogenic core at the Penokean suture (Holm et al., 2007; Schneider et al., 2004). Yavapai igneous and metamorphic zircons and monazites are plentiful in these zones. Recently, xenotime geochronology (Vallini et al., 2007) and detrital zircon (Heaman and Easton,

2005) studies have focused on the poorly exposed, low grade rocks in the northern portion of the Animikie Basin and have identified a Yavapai signature. However, attributing structures to different north-directed accretionary orogenic events is not trivial.

The reinterpretation of Holst’s Line as an unconformity separating Penokean foredeep and Yavapai foredeep Paleoproterozoic deposits has important ramifications for interpreting structures throughout the upper Great Lakes region. Structures like those found at Thomson Dam in Carlton County, a popular fieldtrip stop, are well known and widely publicized as classic Penokean age folds (Fig. 6). The growing body of evidence suggests that the late open upright folds and the sedimentary rocks that are folded may both be manifestations of Yavapai orogenesis and not classic features of Penokean orogenesis.

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