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Geological Sciences
Spring 1-1-2011
Geology and Geochemistry of Hadean Zircon
Bearing Supracrustals from Quad Creek, Eastern
Beartooth Mountains (Montana, USA)
Analisa Charlotte Maier
University of Colorado at Boulder, [email protected]
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Maier, Analisa Charlotte, "Geology and Geochemistry of Hadean Zircon Bearing Supracrustals from Quad Creek, Eastern Beartooth
Mountains (Montana, USA)" (2011). Geological Sciences Graduate Theses & Dissertations. Paper 34.
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GEOLOGY AND GEOCHEMISTRY OF HADEAN ZIRCON-BEARING SUPRACRUSTALS
FROM QUAD CREEK, EASTERN BEARTOOTH MOUNTAINS, MONTANA, USA
by
ANALISA C. MAIER
B.A University of Colorado, 2010
A thesis submitted to the Facility of the Graduate School of the University of Colorado in partial
fulfillment of the degree requirements for the degree of Master of Science
Department of Geological Sciences
2011
This thesis entitled:
Geology and Geochemistry of Hadean Zircon Bearing Supracrustals from Quad Creek, Eastern
Beartooth Mountains (Montana, USA)
written by Analisa Charlotte Maier
has been approved for the Department of Geological Sciences
Stephen J. Mojzsis, Thesis Advisor
Rebecca Flowers, Committee Member
Kevin Mahan, Committee Member
Date
The final copy of this thesis has been examined by the signatories, and we
Find that both the content and the form meet acceptable presentation standards
Of scholarly work in the above mentioned discipline.
Maier, Analisa (M.S. Geology)
Geology and Geochemistry of Hadean Zircon Bearing Supracrustals from Quad Creek, Eastern
Beartooth Mountains
Thesis directed by Stephen J. Mojzsis
Quad Creek Paleoarchean quartzites (~3300 Ma) in southern Montana contain >3900 Ma
detrital zircons. Although an accessible resource for investigating Hadean (>3850 Ma) crust
distinct from the well-known Narryer Gneisses in Australia and Acasta Gneisses in Canada, the
outcrop-scale geological and geochemical context of these rocks is not well documented. New
mapping reveals a varied suite of isoclinally folded, sheared and variably deformed chromiterich banded and massive quartzites, garnet-bearing siliceous paragneisses, amphibolite, quartzbiotite schists and a suite of inter-bedded quartz+magnetite rocks (banded iron-formation, BIF).
Conventional ion microprobe U-Pb zircon ages of different populations from these units show
neoform growth or outgrowth rims on inherited detrital igneous cores at ~3350 Ma, ~2900 and
later (<2500 Ma) that match those for regional metamorphic events evidenced elsewhere in the
Wyoming Craton. Weighted mean
207
Pb/206Pb ages for the youngest concordant igneous zircon
cores indicate that the maximum age of deposition for the supracrustals is ~3300 Ma. These data
support the notion that the oldest crust tapped by the Quad Creek quartzites is comparable in age
to the ca. 3960 Ma Acasta Gneiss Complex. This similarity is suggestive of a linkage between
the geology of the Wyoming Craton and the Western Slave Province. An ancient supercraton
called Sclavia could provide the mechanism for this linkage. Sediment source(s) for some other
Paleoarchean supracrustals were also contaminated by Hadean crust suggests that pre-3900 Ma
zircons are present in similarly-aged basins elsewhere in North America and perhaps also
remnants of Sclavia. The Slave and Wyoming craton may be the modern components of a ‘Slave
clan’ which contains the remnants of Sclavia.
iii
Acknowledgements
I owe special thanks to many people, without whom this thesis would have never been
accomplished. First and foremost, I would like to thank my advisor Stephen Mojzsis for his
endless help and support in both my undergraduate and graduate careers, without him none of
this would have been possible. I am grateful to Elizabeth Frank, and Nicole Cates for field
assistance.
Nicole Cates also deserves special recognition for teaching me laboratory and
computer techniques as well as answering hundreds of miscellaneous questions throughout the
last few years. Dustin Trail has been instrumental in sending me his original data from his work
in the Beartooth Mountains, discussions, and his comments on earlier versions this study. I
would like to thank Rebecca Flowers and Kevin Mahan for agreeing to be part of this committee
and their comments on this work.
I also thank Michelle Hopkins and Elizabeth Frank, my
fellow graduate students and group members for their advice and knowledge of navigating
through graduate school. Chris Bermel warrants special thanks for his endless support during
these last few years. Furthermore, I am indebted to the University of Colorado, the Colorado
Scientific Society and NASA Exobiology Program for support of this work. Additional funding
came from the J.W Fulbright Foundation and the Alfred P. Sloan Foundation. Finally,
discussions with Paul Mueller at the University of Florida have been very helpful.
iv
CONTENTS
CHAPTER 1: INTRODUCTION…………………………………………………………………1
1.1 The Hadean Eon…………………………………………………………………….....1
1.2 Ancient Zircon Localities………………………………………………………….....2
CHAPTER 2: GEOLOGY AND SAMPLE PREPARATION………………………..……….....7
2.1 Regional geology of the Wyoming Province including the Beartooth Mountains……7
2.2 Methods………………………………………………………………………………11
2.2.1 Field Mapping…………………………………………………...…………12
2.3 Sample preparation…………………………………………………………………..14
2.3.1 Ion microprobe U-Pb geochronology……………………………………...14
2.3.2 Whole rock geochemistry………………………………………………….19
CHAPTER 3: WHOLE ROCK GEOCHEMISTRY…………………………………………….21
3.1 Sample descriptions and results……………………………………………………...21
3.3 Discussion…………………………………………………………………...……….24
CHAPTER 4: ZIRCON GEOCHRONOLOGY……………………………………………..…..30
4.1 Ion microprobe zircon geochronology……………………………………………....30
4.2 Results…………………………………………………………………………….….33
4.2.1 Depth profile of the oldest zircon found at Quad Creek by Trail, 2006…..33
4.3 Discussion…………………………………………………………………………....34
CHAPTER 5: BEARTOOTH ZIRCONS......................................................................................35
5.1 Zircon morphology…………………………………………………………………..35
v
5.2 Zircon chemistry……………………………………………………………………..40
CHAPTER 6: EVIDENCE OF A SUPERCRATON; SCLAVIA?..............................................42
6.1 Detritus from an ancient continent………………………………………………….42
6.1.2 Hadean detrital zircons in other Paleoarchean sedimentary basins in North
America…………………………………………………………………………..42
6.2 Craton and supercratons……………………………………………………………..46
6.2.1Slave clan: modern compoents of Sclavia……………………………........47
6.3 Discussion…………………………………………………………………………....48
CHAPTER 7: SUMMARY……………………………………………………………………...50
REFERENCES CITED…………………………………………………………………………..52
vi
List of Tables
1. Conventional ion microprobe U-Pb analyses of zircons from Quad Creek, Montana……….15
2. Representative bulk-rock compositions from Quad Creek, southern Montana………………20
vii
List of Figures
1. a. Generalized sketch map of Archean and Proterozoic rocks of North America……………..5
1. b. Discovery sample site of Mueller et al. (1992)………………………………………………5
1.c. Location map of the Quad Creek site…………………………………………...…………...5
2. Figure 2. Tera Wasserburg Plot of the Beartooth zircons < 5% disrocdant, showing a large
concentration of zircons at the 3300-3200 Ma population……………………………………….9
3. Comparative relative probability plots of Beartooth Mountain zircon 207Pb/206Pb ages from
this study, Mueller et al. (1992), and Trail (2006)…………………………………………….…10
4. Geologic map (1:250 scale) of the Quad Creek study area…………………………………..13
5. (a,c) Chondrite-normalized REE plots……………………………………………………….22
5. (b,d) Primitive mantle-normalized multi-element plots of the different lithologies in the Quad
Creek study area…………………………………………………...……………...…………..….22
6. Anorthosite-Albite-Orthoclase (An-Al-Or) diagram………………………………………….25
7. Field photographs of described lithologies…………………………………………………...26
8. Alkalis-Fe oxides (AFM) diagram and K2O vs. silicia diagram of BT0602……………...….27
9. Na2O + CaO – Al2O3 – K2O diagram………………………………………………………...29
10a. Tera-Wasserberg plot with a relative probability plot of the Beartooth zircons…………..31
10b. Plot of relative probability and [Th/U]zircon vs. 207Pb/206Pb zircons <30%………………...31
11. Plot of relative probability and [Th/U]zircon vs. 207Pb/206Pb zircons <5%……………….…..32
viii
12. Back scattered electon micrographs of the Beartooth zircons…………………...………….36
13. Diagram of [Th/U]zircon vs. 207Pb/206Pb age from this work, Trail et al. (2007), Mueller et al.
(1992), the average oldest, most concordant zircon from the Assean Lake Complex (Böhm et al.
2003), and the oldest most concordant zircon from the Central Slave Cover Group (Sircombe et
al. 2001)….....................................................................................................................................45
ix
CHAPTER 1
INTRODUCTION
1.1 The Hadean Eon
The Hadean (pre-3.85 Ga) geologic record on Earth is limited. Information regarding the
formation and evolution of the earliest crust has relied on geodynamical model studies (Reymer
and Schubert, 1985), general comparisons with lunar data, and most importantly, pre-3.9 Ga
zircons from Western Australia (Froude et al., 1983, Compston and Pidegeon, 1986) and
Northern Canada (Bowring and Williams, 1999, Iizuka et al., 2006). Since zircon is fairly
resistant to chemical alteration, physical breakdown during weathering, and is somewhat
refractory, non-metamict crystals can serve as strong sources of radiogenic Pb* and Th/U ratios.
Therefore, the geochemistry of Hadean zircons is an exceptional resource to explore the
establishment of crust and the amount of early crustal types.
There are two main views on early
crustal types; one is that ancient Earth was characterized by appearance of significant volumes of
ultramafic/basaltic crust (Galer and Goldstein, 1991, Kamber et al., 2005) and the absence of
considerable continental crust before about 4.0 Ga (Taylor and Mclennan, 1995). The other
view is that early Earth had considerable continental growth and succeeding recycling
(Armstrong, 1981, 1991, Harrison et al., 2005a). The significant role of recycled, granitic crust
in the Hadean eon (≥3850 Ma; definition of Bleeker, 2004) has been demonstrated with the
discovery and characterization of hundreds of thousands of zircons (Holden et al., 2009)
including a population of greater than 4000 Ma grains, found in quartzites from the Narryer
Gneiss Complex of the Yilgarn Craton at the Mt. Narryer (Froude et al., 1983) and Jack Hills
(Compston and Pidgeon, 1986) localities in Western Australia. The mineral chemistry of the
ancient Australian zircons is explicable with granite-granitoid melt compositions (e.g. Maas and
1
McCulloch, 1991; Peck et al., 2001; Blichert-Toft and Albarède, 2008; Harrison, 2009; Hopkins
et al., 2008, 2010; Trail et al., 2007). Elsewhere in Western Australia, Wyche et al. (2004)
identified detrital zircons from the Southern Cross Granite-Greenstone terrane in the central part
of the Yilgarn craton as old as 4350 Ma. Due to the geographic proximity of the Southern Cross
with the Narryer Gneisses, it is likely that they shared the same crustal source for old zircons.
Because these studies are exclusively of detrital zircon grains orphaned from their parent rocks,
only broad inferences may be made about probable source melt compositions that gave rise to
them. Detrital zircons of similar >3.9 Ga have also been found in the Beartooth Mountains of
southern Montana and in northern Canada with related issues identifying melt source
compositions.
1.2 Ancient zircon localities
As opposed to detrital zircon studies, direct analyses of Hadean rocks are severely limited to the
few locations where actual ~4 billion-year-old crustal components are documented with
certainty. These are the Acasta Gneiss Complex (AGC) in the Northwest Territories of Canada
(King, 1985; Bowring et al., 1989; Stern and Bleeker, 1998; Bowring and Williams, 1999) and
certain components of the Mt. Sones gneisses within the Napier Complex of Antarctica (Harley
and Black, 1997). Although both the Acasta and Napier rocks are older than ~3900 Ma (cf.
Moorbath, 2005), the geology of their respective outcrops is known only in very broad terms,
and further complicated by protracted histories of high grade metamorphism and deformation.
Rare >4000 Ma xenocrystic zircons have also been reported occluded in younger gneisses. One
such 4183 Ma zircon was found in drill core from near to the Narryer Gneiss Complex (Nelson
et al., 2000). A concordant ca. 4200 Ma xenocrystic zircon was discovered by Iizuka et al. (2006,
2
2007) in ~ 3960 Ma tonalitic gneiss from the Acasta area. A 3850 Ma orthogneiss sample from
the southern Færinghavn terrane of the Itsaq Gneiss Complex in West Greenland (Nutman et al.,
1997), yielded a single concordant 4100 Ma zircon encased in biotite (Mojzsis and Harrison,
2002). It is noteworthy that although the Itsaq Gneiss Complex (IGC) is regarded as the largest,
most comprehensively studied Eoarchean-Paleoarchean terrane on Earth (e.g. Nutman et al.,
1996), no detrital zircons older than about 3890 Ma have so far been found there (Nutman et al.,
2004).
Aside from the ancient zircon work, other geochemical evidence has accumulated that previously
extensive >3850 Ma Hadean mafic and felsic crust continued to influence the character of
supracrustal belts well into the Eoarchean. This viewpoint is based on the enhanced
142
Nd/144Nd
values compared to the terrestrial standard of (basaltic) amphibolites – some of which preserve
pillow structures – of parts of the ~3770 Ma Isua Supracrustal Belt in West Greenland (Boyet et
al., 2003; Caro et al., 2006). Mafic rocks from the IGC tapped re-worked/recycled even older
pre-4100 Ma mafic crust when they first formed, which is required to explain the presence of
radiogenic
142
Nd from the decay of
constraints. Pre-4000 Ma
147
146
Sm (half-life = 103 Myr) in light of U-Pb zircon age
Sm/143Nd source model ages have also been reported for basalts of
the ca. 3450-3490 Ma Warrawoona Group in Western Australia (Tessalina et al., 2010). Another
line of evidence comes from some CaO-poor (cummingtonite-rich) amphibolitic components of
the Nuvvuagittuq Supracrustal Belt in northern Québec that record lower 142Nd/144Nd ratios than
the terrestrial standard. This result requires that the source melts for the Nuvvuagittuq
amphibolites were isolated from
146
Sm before it became extinct, and the data were used by
O’Neil et al. (2008) to propose a 4280 Ma age for these rocks. An alternative interpretation,
based on U-Pb detrital zircon geochronology (Cates and Mojzsis, 2007, 2009) has argued that the
3
Nuvvuagittuq belt formed at about 3780 Ma. If the latter interpretation is correct, depleted 142Nd
values in these amphibolites were inherited from a recycled ancient precursor in ways
comparable to the Isua rocks cited above.
In sum, any new source of information on the Hadean Earth holds the potential to greatly
enhance our understanding of the mechanisms that shaped the young planet. These would
include: (i) the role of catastrophic impacts in the destruction of primordial crust; (ii) plausibility
of a biosphere at that time; (iii) extent of continental-type crust; and (iv) initiation of plate
tectonics. To advance what we know of the geology of those places already identified to contain
Hadean detritus, fine-scale mapping to direct sample collection for geochronology and
geochemistry of specific outcrops is warranted. With such knowledge, we can begin to build
hypothesis tests for the origin of the oldest detrital rocks and minerals.
One such example is from a fuchsite- (Cr-muscovite) bearing quartzite which hosts rare (≤2%
yield) pre-3900 Ma detrital zircons identified from samples collected at a road cut in the
northwestern part of the Wyoming Craton at Quad Creek in southern Montana (Mueller et al.,
1992, 1998; Figure 1). The Quad Creek rocks provide an accessible collection of Hadean detrital
zircons, but relatively little is known about their specific field relations (e.g. Chamberlain et al.,
2003; Chamberlain and Mueller, 2007). Some low-resolution sketch maps of the Quad Creek
area have been published (e.g. Mogk and Henry, 1988), but the associations of ancient zirconbearing quartzites with other lithotypes and what those lithologies are, as well as the various
modes of zircon occurrence, range of ages, and compositions need definition. To this end, we
performed 1:250 scale mapping of the associated supracrustal rocks nearby the ancient zircon
discovery site (Mueller et al., 1992, 1996) and used the output to propose a model for the origin
4
5
Figure 1. a. Generalized sketch map of Archean and Proterozoic rocks of North America. b. Discovery sample
site of Mueller et al. (1992; sample “HRQ”). FOV = 200 m. c. Location map of the Quad Creek site in the
Hellroaring Plateau, southern Montana.
of the oldest zircon populations in these and other Paleoarchean metasediments in North
America.
6
CHAPTER 2
GEOLOGY AND SAMPLE PREPARATION
2.1 Regional geology of the Wyoming Province including the Beartooth Mountains
Well known occurrences of Archean paragneisses in the Wyoming Craton are found as
metamorphosed volcano-sedimentary supracrustal enclaves throughout the northern and eastern
Beartooth Mountains of southern Montana and northern Wyoming (Chamberlain and Mueller,
2007). At Quad Creek (Figure 1), a diverse suite of tectonically interwoven lithologies comprise
mafic granitoid gneisses, quartzites and other paragneisses, iron-formations, amphibolites, and
some small outcrops of biotite schists of uncertain affinity. This kind of supracrustal assemblage
has been described throughout the northwestern Wyoming Craton, especially for the Montana
Metasedimentary province (MMP; Mogk et al., 1992) in the North Snowy block (Mogk et al.,
1988), about 75 km northwest of the Quad Creek outcrops. Quartzites from the area have been
known for some time to host zircons as old as 3960 Ma (Mueller et al., 1992). In addition, a
supracrustal sample from the Tobacco Root Mountains ~175 km northeast of Quad Creek also
yielded a discordant 3930 Ma zircon (Mueller et al., 1998), which testifies to the presence of a
small background of Hadean detritus in the wider region. Aside from the rare Hadean zircon
record documented for various supracrustal enclaves, periods of Eoarchean and Paleoarchean
crustal growth have also been inferred based on the compositions of gneisses from throughout
the Wyoming Craton by ~4000 Ma Nd model ages and highly radiogenic Pb isotopic
compositions (Wooden and Mueller, 1988). This includes rocks of the Sacawee block and MMP
(Grace et al., 2006) as well as ~3300-3700 Ma ages for the Granite Mountain region of central
Wyoming (Fisher and Stacey, 1986; Langstaff, 1995).
7
The overall dominance of ~3300 Ma ages – which are also the ages of the youngest, most
concordant zircons of Th/U composition consonant with equilibrium exchange from magma – in
the compiled data set of Quad Creek zircons are also likely representative of the maximum age
of deposition of the supracrustals (see below). This is further supported by Figure 2, showing a
strong concentration of < 5% discordant zircon ages in the 3300-3200 Ma age population. These
rocks later experienced magmatic injections probably associated with the final assembly of the
Wyoming Craton (Mueller and Frost 2006). Magmatism occurred in three separate phases in the
southern margin of the terrane at 2710-2670, 2650-2620, and 2550-2500 Ma. Proterozoic
extension at 2100-2000 Ma and 1500-1400 Ma is also recognized, which saw the emplacement
of mafic dike swarms associated with crustal thinning (Chamberlain et al. 2003). The Beartooth
rocks were brought to granulite facies metamorphic conditions (T = 750-800°C, P = 5-6 kbar)
from 3250-3100 Ma (Henry et al., 1982) based mostly on Rb-Sr and Pb-Pb systematics and other
indicators (Wooden et al., 1988a,b; Mogk et al., 1992). Granulite metamorphism was followed
by retrograde amphibolite facies and several later episodes of granitoid intrusions (Mogk and
Henry, 1988). Archean basement visible throughout the contemporary Wyoming province in the
cores of mountain ranges was uplifted during the Cretaceous Laramide Orogeny and sculpted by
glaciers in the Pleistocene so that outcrop exposure is good.
Various results from prior geochronological work on detrital (igneous) and younger postdepositional (metamorphic) zircons from the northern part of the Wyoming Province have been
used to constrain a broader age range of 2700-3300 Ma for the various quartzites (Mueller et al.,
1988). In a study of 355 detrital zircons from the Quad Creek (Figure 3) locality by Mueller et al.
8
Figure 2. Tera Wasserburg Plot of the Beartooth zircons < 5% disrocdant,
showing a large concentration of zircons at the 3300-3200 population.
9
Figure 3. Comparative relative probability plots of Beartooth Mountain zircon 207Pb/206Pb ages
from this study (light grey), Mueller et al. (1992; black), and Trail (2006; dark grey).
10
(1992, 1998), about 20% of the grains were found to be between 3400-4000 Ma and many of
those were within 10% of concordia. While the oldest zircons found in the Beartooth quartzites
are upward of 3960 Ma, demonstrably detrital zircons of igneous derivation here and from the
neighboring Ruby and Tobacco Root uplifts are no younger than about 3250 Ma (Mueller and
Frost, 2006). The mostly Paleoarchean igneous (as opposed to younger neoform metamorphic)
zircon ages reflect the dominant influence to these sediments from some relatively younger crust
shed into the MMP basins. Metamorphic zircons grew either in situ, or as metamorphic
overgrowths/rims on older grain cores and have ages between 2700 and 3200 Ma (Mueller et al.,
1998); granulite facies metamorphism in the eastern Beartooth Mountains occurred at ca. 3200
Ma (Mogk et al., 1992) which matches well ages for metamorphic zircons and outgrowths in
older detrital grains. Samarium-Neodymium model ages and whole-rock and mineral (feldspar)
Pb isotope models further indicate that ~3900 Ma crust was present in the northern Wyoming
province when the Beartooth quartzites (and probably the whole of the MMT) were deposited
(Mueller et al. 2010); Hafnium model ages of ca. 3850 Ma also support this possibility
(Stevenson and Patchett, 2000).
2.2. Methods
The present study reports on a small (185  210 m) area of outcrop with good exposure (~5075%) at the discovery location of pre-3900 Ma zircons in a road cut described in Mueller et al.
(1992). New conventional zircon U-Pb ion microprobe ages (n=118) were combined with a large
scale survey (204Pb-corrected) of detrital zircon age determinations (n=1156). Further details of
an ion microprobe U-Th-Pb depth-profiled ca. 4000 Ma grain in Trail et al. (2007) are also
reported herein. The purpose of the large survey study was to comprehensively constrain the
frequency of zircons of specific ages in these Paleoarchean sediments. When available, we
11
compare measured Th/U for individual zircon analyses with U-Pb zircon age against expected
values for crystals grown in an igneous environment as a function of (207Pb/235U age vs.
207
Pb/206Pb age) concordance (e.g. Mojzsis and Harrison, 2002). Acquisition of whole-rock
major-, minor- and trace element geochemistry is also described below. Collectively these data
were used to assess protolith(s) based on mineralogy and composition, and maximum ages of
deposition of igneous zircons to suggest geochemical links with other ancient rock localities
mentioned above.
2.2.1 Field Mapping
The Quad Creek exposure was mapped with a 25  25 m E-W oriented grid (Figure 4) annotated
to show sample locations with photo-documentation of specific outcrops discussed in the text.
Initial mapping was done by Stephen Mojzsis and Dustin Trail in 2006 and that draft map was
refined and corrected in 2010 with more samples collected in the same locations for
geochemistry. The focus was on quartzites which contain Hadean detrital zircons. Overall, the
exposure is dominated by strongly deformed isoclinally folded supracrustal rocks comprising
massive quartzites (Aqm) and banded paragneisses (Aqb) with locally highly-variable fuchsite
contents (labeled f on the map where locally enriched in Cr-mica). Paragneisses in the field
range from massive to banded and patchily garnetiferous (gt), and are cut by at least one
generation of mafic dikes (Md). Subordinate units include a finely banded but strongly deformed
and sheared quartz magnetite rock (banded iron-formation s.l.) restricted to the easternmost
mapped area that is in contact with a garnet-bearing amphibolite (Amg). No carbonates were
noted at this particular site (cf. Mogk et al., 1992). The sheared east-west dipping isoclinal
folding within the supracrustal sequence does not appear to be a structure shared by the
12
Figure 4. Geologic map (1:250 scale) of the Quad Creek study area is within grey-green
quartzites along a road cut of Montana Rte. 212 at the southern boundary of the mapped
area.
13
underlying massive intrusive granitoid (trondhjemitic) gneisses (Gb). To the best of our
knowledge, the original sampling locality of old detrital zircons reported by Mueller et al. (1992)
2.3 Sample Preparation
2.3.1 Ion Microprobe U-Pb Zircon Geochronology
Ion microprobe U-Pb zircon geochronological analyses of zircons from five supracrustal
lithologies (three quartzites Aqm, and three paragneisses Agb) are reported in Table 1. Rock
samples were prepared for geochemistry and zircon separations using standard techniques (e.g.
Cates and Mojzsis, 2006) and a brief summary is provided here: Approximately 5 kg of Aqm
samples BT0606, -08 and -11 and Agb samples BT0603, -04 and -10 from the mapped area were
chosen for zircon extractions. These large samples were reduced by crushing and sieving to >350
µm. Geochemistry for sample BT1 is not reported here but mineralogically it is identical to Aqm
sample BT0606. Powders for zircon extractions went through two stages of clean heavy liquid
separations, rinsing in reagent-grade acetone and ultrapure water, followed by hand-magnet and
Frantz magnetic separations of the densest mineral residues. The least magnetic grain fractions
were handpicked under a binocular microscope for individual zircons with a wide spectrum of
morphologies and aspect ratios, placed on double-sided adhesive tape and cast in a 2.52 cm
diameter mould with Buehler© epoxide resin alongside standard zircon AS3 (Paces and Miller,
1993; Black et al. 2003). The sample disk was then beveled and polished to brilliance in stages to
0.25 µm alumina until grain centers were exposed. Transmitted and reflected optical micrograph
sample maps were created followed by imaging with back-scattered electrons of polished centers
on a JEOL JXA-8600 electron microprobe at the University of Colorado under standard
operating conditions. To reduce common Pb contamination prior to ion microprobe analysis, all .
14
Table 1. Geochronology
15
Table 1. Geochronology
16
Table 1. Geochronology
17
TABLE 1 GEOCHRONOLOGY
18
grain mounts were pre-cleaned in 1N HCL solution to remove common-Pb contamination,
washed in ultrapure water, air dried in a 50 ºC oven, and coated with ~100 Å of Au to facilitate
conductivity. Conventional zircon U-Pb ion microprobe geochronology on the sample zircons
was performed using the UCLA Cameca ims1270 high-resolution SIMS (e.g. Cates and Mojzsis,
2006), a brief synopsis is provided here: a ~6-nA O2-primary beam was focused to a 25 -µm spot
and operated at a mass resolving power of ΔM ~6000 to exclude molecular interferences. To
increase the Pb+ ion yields, oxygen flooding to a pressure of 2.7x10-5 torr was used (Schumacher
et al., 1994). Ages for unknown zircons were determined by comparison with a working curve
defined by multiple measurements of standard AS3 that yields concordant
207
206
Pb/238U and
Pb/235U ages of 1099.1+ 0.5 Ma. Data were reduced and the output used to construct Tera-
Wasserburg diagrams (207Pb/206Pb vs.
238
U/206Pb) with the Isoplot software package (Ludwig,
2001). All conventional ion microprobe ages are reported at the 1σ level based on asymmetric
weighted regressions except when noted.
2.3.2 Whole-rock geochemistry
Duplicate whole-rock sample splits were prepared by subdividing ~2 kg unweathered rock pieces
and powdering in pre-cleaned ceramic mills. Analysis of major-, minor- and trace elements
(Table 2) was performed by XRF and ICP-MS at Activation Laboratories (ACTLABS; Ontario,
Canada). Comparison of recommended and analyzed standards from ACTLABS show small
deviations (<1.5%) for oxides, except MnO (6.3%), Na2O (6.5%) and P2O5 (23.1%) and for trace
elements <20% relative, except Ni (21.4%), Pr had a 47.6% relative error. Granitoid rock
assignments are from Barker (1979) based on normative modes of albite (Ab), orthoclase (Or)
and anorthite (An); (trond = trondhjemite).
19
Table 2. Representative bulk-rock compositions from Quad Creek, southern Montana
20
CHAPTER 3
WHOLE ROCK GEOEMISTRY
3.1. Geochemical results and sample descriptions
Quartzites (Aqm) BT0606, BT0608, BT0611
The dominant rock type exposed in the mapped area is a quartzite with local dark green banding.
Bulk compositions of the Aqm rocks are strongly enriched in SiO2 (93.2-95 wt. %) befitting their
protolith (probably as “dirty” quartz sands) so that the narrow variability in SiO2 content is due
to different relative abundances of heavy minerals. Most are massive, grey to greenish in color.
However certain sections locally preserve distinctive “sedimentary” bands rich in chromite,
sulfide, Cr-mica, zircon and other minerals. The quartzites have Al2O3 contents that vary by a
factor of ~2 (2.86-4.57 wt. %). Chromium abundances for different samples show an order-ofmagnitude range (40-360 ppm) but overall lower Zr concentrations (37-81 ppm) compared to
granitoids. Primitive mantle normalized multi-element plots (Figure 5) show negative Nb, Sr,
and Ti anomalies consistent with prevailing felsic sources to the sediment with a small mixture
of mafic component. Chondrite normalized REE patterns also display enriched LREE and
depleted HREE, with a negative Eu anomaly observed only in sample BT0608.
Paragneiss (Agb) BT0610
A suite of banded quartz±garnet paragneisses appear in the field as white to grey rocks with
bands of darker more mafic minerals rich in garnet. In outcrop, they appear to show evidence of
partial melting and recrystallization, and tend to weather to a beige-orange color. Sample
BT0610 has a similar Cr-enrichment (230) ppm to some quartzites, but has lower SiO2 contents
21
22
Figure 5. (a,c) Chondrite-normalized REE plots, and (b,d) Primitive mantle-normalized multi-element
plots of the different lithologies in the Quad Creek study area.
(75.09%) and is relatively enriched in alumina (13.66%). Zirconium content is greatly elevated
in the paragneiss with respect to the typical Aqm lithologies (348 ppm) which may denote its
origin from weathered granite. In a multi-element plot, (Figure 5) the Agb rocks show negative
Sr and Ti anomalies along with a concave-upward trend from Dy-to Yb. Overall REE contents
show enriched LREE compared to HREE with a positive Eu anomaly.
Quartz-magnetite rock (BIF) BT1006
A small and narrow outcrop of silicate-facies BIF is sandwiched between sections of an
amphibolite which defines a tight fold axis pinching out northwards. On the map, the
amphibolite limbs are bounded by quartzites on one side and trondhjemite on the other (Figure
3). The amphibolite was not sampled for geochemistry. The BIF unit is very iron rich (51 wt. %
Fe2O3) with 44% SiO2 and minor Al2O3 (~3%). It also contains 50 ppm Cr and 26 ppm Zr
consistent with a small detrital component. Trends in primitive mantle-normalized multi-element
REE plots reflect this in negative Sr and Ti anomalies and show that the BIF has some
similarities to the quarztites and paragneiss for which it is presumably co-genetic in the
supracrustal sequence. In a chondrite normalized REE plot, (Figure 5) LREE are enriched over
HREE and show a small negative Eu anomaly. With respect to Y/Ho ratio, the Quad Creek
quartz-magnetite rock is low (28.7) in comparison to most Archean BIFs (Bolhar et al., 2004), as
well as relatively low in molar Ni/Fe (Konhauser et al., 2009) and Cr contents (Konhauser et al.,
2011) compared to averages for other BIFs of this age.
Trondhjemitic Granitoid Body (Gb) BT1004 and quartz-biotite schist (Aqbc) BT1002
In several places within the supracrustal succession, pockets of a granitoid penetrate through the
outcrop, and the whole locality is underlain by a large trondhjemitic body (BT1004) as defined
23
by normative An-Al-Or compositions (Barker, 1979; Figure 6). We also identified a quartzbiotite schist (conglomerate? BT1002) near the base of the Rte. 212 road cut west of sample site
HRQ (Figure 7d). As this rock was not found to crop out in the mapped area it is not marked on
the map. Both the quartz-biotite schist and the trondhjemite show similar SiO2 and Al2O3 values
to the paragneiss with 77.3% SiO2, and 14.3% Al2O3 for the conglomerate and 72.6% SiO2 and
15.4% Al2O3 for the trondhjemite, respectively. The quartz-biotite schist contains 70 ppm Cr and
49 ppm Zr, whereas the trondhjemite is Cr-poor (<D.L.), and relatively low in Zr (167 ppm)
compared to granites but is entirely consistent with trondhjemite.
Mafic Dike (Md) BT0602
Sample BT0602 is a black to dark grey unit bordered by paragneisses. It contains 47.58% SiO 2,
14.46% Al2O3, 12.54% Fe2O3, and 10.93% CaO with 310 ppm Cr and 45 ppm Zr. To determine
if the mafic dike is part of the calc-alkaline or tholeiite series, we plotted its composition on a
AFM diagram but this was inconclusive as lies between the determining lines of Kuno (1968)
and Irvine and Baragar (1971) (Figure 8). However, on a K2O vs. silica diagram, using the
subdivisions of Rickwood (1989) the mafic dike appears more akin to calc-alkaline with medium
to low K (Figure 8).
3.3 Discussion
Geochemistry and protolith assignment
Sample BT0608 was mapped as a paragneiss, however, the geochemistry shows its protolith is
quartzitic; it should be noted then that the quarztite may extend down further in this location or
the paragneiss and quarztite are intermixed here. In a multi-element diagram (Figure 5), felsic
24
Figure 6. Granitic assignment using the Anorthorsite-Albite-Orthoclase (An-Al-Or)
composition. The black division lines from Barker (1979).
25
26
Figure 7. Field photographs of the key lithologies described in the text. a. Fuchsitic quartzite
(Aqm). b. Banded paragneisses (Agb). c. Mafic dike (Md). d. possible conglomerate. Note
scales.
Figure 8 a. Alkalis-Fe oxides-Mg oxides (AFM) diagram showing the boundary between the
calc-alkaline field and the tholeiitic field from Kuno (1968; solid black line) and Irvine and
Baragar (1971; dashed black line). Mafic dike BT0602 (black square) plots ambiguously in
between the boundaries. b. Black lines showing the subdivision of subalkalic rocks from
Rickwood (1989) using a K2O vs. silica diagram. BT0602 (black square) plots low in the
medium K, calc-alkaline series region.
27
lithologies (detrital and igneous) show enriched large ion lithophiles, and negative Nb anomalies
with depletions in Sr and Ti for all lithologies except the mafic dike. The quartzites follow
expected weathering trends (Nesbitt and Young, 1989) for a mixed granitoid + mafic source
(Figure 9). The quartzites show general variability in Cr and low have Zr values compared to the
single paragneiss sample, which is elevated in both Cr and Zr (Table 2). The quartzites are
shown in Figure 9 to be more weathered than the paragneiss as it is depleted in K2O and enriched
Al2O3 compared to the paragneiss. This could provide an explanation for why the quartzite is
depleted in Zr compared to the paragneiss. Sediments also have a natural variability in elements
and it could be that this particular area of quartzites have stochastically depleted abundances of
Cr and Zr. Alternatively, the paragneiss could be a product of a mix of rock types. To explore
this further, more analysis of other paragneiss samples would be required.
28
Figure 9. Na2O + CaO – Al2O3 – K2O diagram showing weathering trends for the samples in
this work. Quartzites (BT0606, -08, 11) are shown as gray asterisks, the paragneiss (BT0610) is
shown as an open triangle, the conglomerate an open circle, the BIF an open square, the
trondhjemite (BT1004) a filled gray square, and the mafic dike (BT0602) a filled black diamond.
The arrow shows the average weathering trend predicted for upper continental granite from
Nesbitt and Young (1984).
29
CHAPTER 4
ZIRCON GEOCHRONOLOGY
4.1 Conventional U-Pb zircon ion microprobe geochronology
This chapter reports data for zircons from both the quartzites and paragneisses of the Quad Creek
outcrops. Zircons were extracted from three fuchsitic quartzites (BT0606, -08 and -11) and three
(banded) paragneisses (BT0603, BT0604, and BT0610) from the mapped area. Values for
(Th/U)zircon vs. age (Figure 10) for data < 30% discordant appear similarly scattered for different
age bins and vary by several orders of magnitude in Th/U with distinct age groupings at (i)
~2800 Ma with a Th/U composition of .011 (ii) ~3300-3100 Ma (Th/U=0.14-1.5); (iii) ~34003500 Ma (Th/U=0.32-.57) and another older grouping at ≥3700 Ma with Th/U ratios of 0.350.45. Filtering data to the least disturbed ages (<5% discordant; Figure 11) yields distinct age
groupings at ~3300-3100 Ma (Th/U=0.14-1.5), ~3400-3500 Ma (Th/U=0.32-.57) and another
older grouping at ≥3700 Ma with Th/U ratios of 0.35-0.45. The younger ages are not <5%
discordant attributed to significant periods of metamorphism and granitod intrusions after 3200
Ma. Eoarchean to Hadean zircons were found in several of the banded paragneiss samples
(3650-3902 Ma), but the majority of ages reside in the ca. 3300 Ma population. Younger zircons
show a large scatter in Th/U. Figure 11 shows that there is a pronounced age grouping at ca.
3250 Ma and ca. 3450 Ma with Th/U ratios consonant with igneous derivation (0.4-0.5) for the
peak around 3450 Ma, and highly variable ratios (0.4-1.2). A small subset of 3200 Ma and
younger zircons is also evident with highly variable Th/U as well as internal textures (Corfu et al.
2003; Hoskin and Schaltegger, 2003) attributable to in situ growth during granulite facies
metamorphism at 3200 Ma (Figure 11). The igneous-derived Th/U ratios reported here place a
30
Figure 10 a. Tera Wasserburg Plot of the Beartooth zircons. b. Relative probability and Th/U vs.
age plot of zircons that are <30% discordant. The Th/U values of the quartzites are marked as
hollow squares and the paragneisses as filled squares. The different shades of gray for a
correspond to different age populations as indicated in the legend.
31
Figure 11. Plot of relative probability and [Th/U]zircon vs. 207Pb/206Pb zircon age filtered for
analyses that are <5% discordant.
32
putative maximum age for the quartzites and paragneisses at 3300 Ma, in agreement with
previous work (Chamberlain and Mueller, 2007). Out of the 118 zircons analyzed in this work by
conventional U-Pb zircon geochronology, 70 were less than 30% discordant. One of the four
oldest zircons in our analysis pool (3866 Ma) is from the paragneiss (Agb sample BT0610), and
the three others (3902, 3756, and 3690 Ma) are from two fuchsitic quartzites (Aqm sample,
BT0608 and BT0611). Collectively our ages from conventional ion microprobe geochronology
are somewhat younger than the oldest zircon ages reported by Mueller et al. (1992) and in
subsequent publications (Figure 3).
4.2 Results
4.2.1 U-Th-Pb depth profile of the oldest zircon at Quad Creek
Approximately 10 kg of sample BT-1 (HRQ of Mueller et al., 1992) yielded thousands of zircon
grains of diverse morphologies (Trail, 2006). Large scale geochronological surveys of two
polished zircon grain mounts prepared from this sample yielded two zircons with ages ≥3960 Ma
later verified by depth profile analysis (Trail, 2006) on one grain (BEAR-2_5-6) to be 3997±5
Ma (2; mswd = 1.9) with a weighted mean (Th/U)Zirc = 0.503±0.002 Zircon sample BEAR-2_56 is the oldest zircon thus far documented from Quad Creek.
In the data set of Mueller et al. (1992), two pre-3900 Ma HRQ zircons were within 1% of
concordia: their “grain 6” was 3964 Ma (Th/U=0.403) and “grain 45” had an average age of
3936 Ma and Th/U = 0.513. That these oldest grains are large (~120-160 µm), concordant
zircons with a limited range of igneous (Th/U)zircon can potentially provide a clue as to their
source rocks. Such (Th/U) values for zircon are typical of, but not limited to, those derived from
granitoid melts (e.g. Mojzsis and Harrison, 2002). Based on partitioning rules (Blundy and
33
Wood, 2003) these values may be used to approximate the whole rock Th/U that would have
been sourced these zircons to be in the range 1.3-2.7, assuming a partition of ~2 for Th/U in
zircon vs. melt (Mahood and Hildreth, 1983).
Unfortunately, the value is not particularly
diagnostic of any single rock type.
4.3 Discussion
Zircon geochronology for the Quad Creek rocks shows evidence of three different events, the
precise geological order and context of which are problematic due to the deformational overprint
and the limited information from beyond the mapped area.
Based on what is known so far, the first event in its history was the establishment in the
Paleoarchean of a sedimentary depocenter typified by psammitic quartzites rich in detrital placer
minerals
(chromite, minor sulfides),
emplacement of the protolith to the amphibolite (as
extrusive basalt?) and deposition of the banded iron-formation sometime around 3300 Ma. The
stratigraphic context and order of these events remains unclear. Next appears to have been a
period of deformation coinciding with metamorphism at the granulite facies at ~3200 Ma.
Afterwards, crustal thinning saw the intrusion of mafic dikes. Subsequently, an intrusive
trondjhemite/granodiorite was responsible for local partial melting of the sediments. Finally, a
quartz and biotite leucogranitoid was emplaced in and around the supracrustal rocks. Pervasive
hydrothermal alteration affected much of the area. Uplift was followed by denudation and
erosion of younger rocks to expose the oldest core of the Beartooth Mountains.
34
CHAPTER 5
BEARTOOTH ZIRCONS
5.1 Zircon morphology
Uncertainties between zircons of (primary) igneous or (secondary) metamorphic and
hydrothermal origin befuddle age analysis, especially in ancient complexes. However, different
petrogenetic origins of different zircon populations in the same rock can sometimes be
reconciled on crystal chemical grounds (Th/U, REE partitioning, Ti-thermometry; e.g. Cates and
Mojzsis, 2009) coupled with morphological analysis of grain habit and internal texture (Corfu et
al., 2003). Igneous zircons tend to have relatively high aspect ratios and are euhedral; detrital
zircons of whatever petrogenetic origin tend to be rounded from sedimentary transport, and
metamorphic zircons can have a host of textures depending to a large degree on the host rock
composition and degree of metamorphism. Zircons in this study which were collected from
sedimentary protoliths tend to have subhedral habit with variable aspect ratios; some grew in situ
during metamorphism, others have overgrowths on rounded cores, and still others show complex
internal morphologies indicative of resorption/recrystallization (Figure 12). Back-scattered
electron images of the oldest and most concordant zircons in this study (>3.6 Ga) show that they
are subhedral and display clear overgrowths consistent with neoform zircon growth, remelting/resorption and/or recrystallization (Hay et al., 2010). For example, sample grain #1 of
BT0611 (a.k.a. BT0611_1), as well as BT0608_6, and BT0610_5, are cracked with rounded tips,
while one of the oldest zircons in our sample suite: BT0608_4 (3902 Ma) shows little to no
fracturing with quite angular (sub-rounded) tips. One unusual grain (BT0608_4) is a penetration
twin.
35
Figure 12. Back scattered electron micrographs of the Beartooth zircons showing <30%
discordant (not labeled) and >30% discordant (labeled) zircons morphologies
36
Figure 12. Backscattered electron micrographs of the Beartooth zircons.
37
Figure 12. Back scatter electron micrographs of the Beartooth zircons.
38
Figure 12. Back scatter electron micrographs of the Beartooth Mountain zircons.
39
Gross morphological analysis in BSE and optical microscopy of some of the youngest
demonstrably igneous age populations (from 3400-3500 Ma) shows that they are sub-rounded
inclusion-rich grains with obvious rim overgrowths; zircons of this age population are generally
within a few percent of concordia. The youngest igneous zircons from within the ~3300 Ma
population tend to be euhedral with metamorphic rims, and are generally concordant. All zircons
equal to or younger than ca. 3200 Ma are stubby crystals with some fracturing, and are rich in
inclusions. A significant proportion of grains from this grouping are >30% discordant and show
evidence for radiation damage in BSE images. The youngest metamorphic populations (27002900 Ma) show a wide variety of habit from highly rounded to broken with fracturing and
overgrowths with highly variable concordance. Many of the zircons appear igneous, however,
while images are informative, interpretations can be subjective and the suites of textures inferred
as magmatic or igneous can be ambiguous (Corfu et al., 2003)
Many of the younger grains appear to contain igneous morphology including oscillatory zoning.
However, many studies have shown that zircon formed during high-temperature metamorphism
can exhibit a wide range of morphologies and internal zoning features. Pre-existing zircon can
be partially or completely transformed or changed in situ in response to interactions with fluids
and melts. Harley et al., 2007 detail several metamorphic zircons displaying planar and sector
zoning common in high temperature rocks and similar zoning patters found in zircon crystallized
from partial melt. The new growth can be extensive forming on the older zircon including the
core (Harley et al., 2007). For this reason, the zircons in this study have been largely classified
as igneous or metamorphic based on their concordance and/or known metamorphic events or
widespread intrusions.
5.2 Zircon chemistry
40
Chemically distinguishing the magmatic zircons from metamorphic zircons from sediments is
problematic. Detrital zircon REEs show an extremely limited abundance range with no
systematic difference in pattern shape and are not generally useful as indicators of provenance
(Hoskin and Ireland, 2000). However, there have been some improvements through the use
partition coefficients vs. ionic radius using a lattice-strain model, the results of which are said to
be readily interpretable with magmatic exchange (Trail, 2007). This technique should be
considered for further study on these samples. The Th/U values of the oldest zircons from the
paragneiss and quartzites are comparable (Figure 11) and suggestive, though not conclusive of
the same Hadean sediment source. Known metamorphic events occurred from 3200 Ma and
younger, which may provide a reason for the greater discordance and therefore absence on the
younger zircon populations in Figure 11. Likewise, the overall highly variable < 30% discordant
zircons with inverse Th/U ratios (Figure 10) remains problematic unless separated out in terms of
U-Pb age populations. The scatter in Th/U of the 3200 Ma age grouping is interpreted as
representative of the first widespread metamorphic event that triggered zircon growth at the
granulite facies (Mueller et al., 1998), a feature that has been widely documented for other
ancient supracrustal rocks which have experienced similar protracted and high grade
metamorphic histories (e.g. Manning et al., 2006).
More conclusive provenance information
may be obtained by study of the occurrence and chemistry of inclusions hosted by the detrital
zircons and is a priority for future studies.
41
CHAPTER 6
EVIDENCE OF A SUPERCRATON; SCLAVIA?
6.1 Detritus from an ancient Continent
Soon after their discovery, it was realized that the oldest ages for the Beartooth zircons were akin
to those of the oldest tonalitic and granodioritic gneisses in the ca. 3960 Ma Acasta Gneiss
Complex (AGC) ~2250 km to the north at the westernmost margin of the Slave craton,
Northwest Territories, Canada (Bowring and Williams, 1999). The AGC comprises an exposure
of strongly foliated and transposed amphibolitic to granitic gneisses that may be larger than 180
km2 or the known extent of ancient basement rock in the area (Iizuka et al., 2007 and references
therein). The AGC probably represents the last unequilibrated remnant of a much larger block
(Bleeker, 2003) that in principal could have been the source of widepread contamination by
ancient zircons. Presently, the main exposure of the AGC is dominated by layered mafic gneisses
and felsic gneisses with blocks of mafic- intermediate gneisses (Iizuka et al., 2007). The oldest
components of the AGC were emplaced 4030 – 3940 Ma, but record evidence for possibly older
rocks from a 4200 Ma inherited zircon (Iizuka et al., 2006). The initial AGC magmatic
emplacement was followed by two Eoarchean igneous intrusions at 3740-3720 Ma and 36603590 Ma. The last of these events was accompanied by crustal anatexis and recrystallization of
older units (Bowring and Housh, 1995; Iizuka et al., 2007). Subsequent intrusions and
modifications of the AGC continued into the Proterozoic, culminating with regional
metamorphism at 1900-1800 Ma related to the Wopmay Orogen (King, 1985; Sano et al., 1999).
Many of these ages find complements in the zircon age spectra for Quad Creek.
6.1.2 Hadean detrital zircons in other Paleoarchean sedimentary basins in North America?
42
Approximately 1500 km to the northeast of the Beartooth Mountain locality other ~3900 Ma
ages for detrital zircons have also been reported in the Assean Lake Complex (ALC). The ALC
is a ~120 km2 west-northwest trending slice of largely Neo- to Mesoarchean rocks bounded to
the north by the Paleoproterozoic Trans-Hudson Orogen and the Neoarchean Northwest Superior
Province to the south (Böhm et al., 2000, 2003, 2007). Within the ALC there are at least two ca.
3200 Ma supracrustal packages of quartz-biotite schists (“meta-greywacke” s.l.) that yield
Eoarchean and Hadean detrital zircons. These supracrustals were intruded by ~3100 Ma
tonalites, which in turn were modified by later granitoids. Neodymium model ages of the oldest
tonalites and various schists are all >3500 Ma, and some are as old as 4200 Ma, strongly
indicative of reworked Hadean/Eoarchean crust. The Assean Lake rocks underwent multiple
metamorphic episodes and migmatization.
A solitary detrital 3940 Ma zircon was reported from a drill core in the Thelon Basin in Nunavut
(Palmer et al., 2004), however, it is important to note the Thelon Basin is not a Paleoarchean
sedimentary basin but younger (~1.7 Ga). Also, a fuchsitic quartzite of the Rae Province in
northern Canada to the east of the AGC likewise contains old detrital zircons with age
distrubution peaks at 3860, 3760, and 3720 Ma (Hartlaub et al., 2006).
The fact that the dominant population of pre-3800 Ma ages for detrital zircons from ca. 3300 Ma
Narryer Gneisses as tabulated in Holden et al. (2009) resembles the published ages of the oldest
(igneous) components of the Acasta Gneiss Complex (ca. 4020-4050 Ma; e.g. Stern and Bleeker,
1998) is interesting, but lacks compelling linkage with any other rock outside of the terranes
captured within the Yilgarn Craton. The long tail in the distribution of older ages reported in
Holden et al. (2009) from the Jack Hills to 4380 Ma is so far unmatched by any other detrital
zircon locality yet discovered (Nutman et al., 2001). Such a substantial difference in the (albeit
43
small number) age distributions between the Australian and North American Paleoarchean basin
detrital zircon record maybe used to build the case that another different and very old continental
core with diverse primordial crustal components (accreted terrane?) existed in the vicinity of
what is now the Yilgarn block in Western Australia about 3300 million years ago. The oldest
components of this particular source seem not to have affected Laurentian Paleoarchean
sedimentary basins.
Other clues about the ultimate origin(s) of the Quad Creek zircons may be found in the
composition and population statistics of zircons from the ~2800 Ma Central Slave Cover Group
(CSCG) in Canada (Sircombe et al., 2001). Quartzites of CSCG uncomformably overlay 30004000 Ma tonalitic to dioritic gneisses and foliated granites of the Central Slave Basement
Complex (Bleeker et al., 1999; Pietranik et al., 2008). The oldest zircon noted from the CSCG
comes from fuchsitic quartzite at Dwyer Lake and was dated at 3918±5 Ma (spot 65.1, 101%
concordant, Th/U = 0.6 reported in Sircombe et al., 2001) (Figure 13). The remainder of the
Dwyer Lake zircons range from highly discordant (66%) minimum ages of 2439 Ma to within
6% of concordia at 3885 Ma (Th/U=0.560) and 3901 Ma (Th/U=0.486). Like the Quad Creek
zircons in this study, the oldest CSCG ages from Dwyer Lake are evocative of an ancient “Slave
Continent” source (Bleeker 2003; Reddy and Evans, 2009).
Comparisons with these studies and those of the Beartooth zircons including this, Trail, 2006 and
Mueller et al., 1992, 1998 can be summed up in Figure 13.
44
Figure 13. Diagram of [Th/U]zircon vs. 207Pb/206Pb age from this work, Trail et al. (2007),
Mueller et al. (1992), the average oldest, most concordant zircon from the Assean Lake Complex
(Böhm et al. 2003), and the oldest most concordant zircon from the Central Slave Cover Group
(Sircombe et al. 2001). Similar ages and Th/U ratios are evident for the oldest Quad Creek
zircons in this study to those reported from other ancient detrital zircon localities.
45
6.2 Cratons and Supercratons
Archean cratons are complex fragments of pre-2500 Ma crust with commonly genetically linked
mantle. Supercratons, as defined by Bleeker (2003), are large ancestral landmasses of Archean
age with a stabilized core that upon break up spawned several independently drifting cratons.
Bleeker (2003) found a potential 35 cratons giving three possibilities of how these cratons may
have originated and if they can be matched to one another. The first mechanism to explain their
origin is that all 35 cratons came from a single supercontinent called Kenorland (Williams et al.,
1991, Hoffman, 1992, 1997). The evidence for this comes from the fact that most cratons have
rifted and margins and must have come from a greater older landmass. The second explanation
of origin is that a limited set of somewhat independent late Archean supercratons formed that
were similar in nature but with substantial differences between most of the preserved cratons.
Finally, the last mechanism is that there were many more cratons than the 35 preserved today,
but they were destroyed by erosion or crustal recycling. Bleeker (2003) considered that the most
likely scenario is the second, where there are a few cratons, some of which share certain
likenesses and can be related. The greatest evidence for a limited number of cratons lies in the
knowledge that Archean Earth would have had twice enhanced heat production with a greater
fraction of primordial heat budget being retained, and was most likely characterized by more
dynamic mantle convection and smaller, faster, plates (Pollack, 1997). This would favor several
independent supercratons, and smaller continental accumulations (Bleeker, 2003). Using this
evidence, it can be supposed that several of the 35 known cratons formed from the breakup of the
same supercraton. Of these cratons, numerous can be matched to three distinct ‘clans’ of
cratons, a Slave-like clan, a Superior-like clan, and a Kaapvaal clan, each of these likely traces
its ancestry to a different supercraton (Bleeker 2003). The main difference between these craton
46
groups are their relative cratonization ages. The Kaapvaal craton was cratonized around 3000
Ma (Moser et al., 2001), the majority of the Superior by 2680 and 2640 Ma (Card, 1990), and the
Slave craton ca. 2600 to 2580 Ma (Davis et al., 2003). Other differences between these craton
clans are the chronology and stratigraphy of Paleoproterozoic cover sequences and the
emplacement of mafic dike swarms at different periods. The Superior craton is intruded by the
Matachewan dike swarm at 2473 and 2446 Ma (Heaman, 1997) and draped with glaciogenic
deposits ca. 2200 and 2400 Ma in the Huronian Supergroup (Young et al., 2001). The Slave on
the other hand, does not contain mafic dikes around 2470-2440 Ma or 2200-2400 Ma glaciation
events. The Kaapvaal craton is overlain by extensive Paleoproterozic cover sequences of basins
(Transvaal and Hamersley) along with banded iron formation deposits and carbonate deposits
(Cheney, 1996). These differences between the craton clans advocate an origin from different
supercratons. Bleeker (2003) names three main supercratons that would have resulted in the
Kaapvaal, Superior and Slave clans after their breakup, the Vaalbara (Kaapvaal), the Sclavia
(Slave), and the Superia (Superior). For the purposes of this thesis the focus will be on the Slave
clan and Sclavia supercraton
6.2.1 The Slave clan: Modern component of a Sclavia supercraton
The Slave craton is characterized by an extensive Hadean to Mesoarchean basement complex
including the oldest known whole rocks in the Acasta Gneiss Complex (Bowring and Williams
1999) discussed in section 6.1. Apart from minor regional differences, the overall stratigraphy
and granitoid geochronology of the Slave is reasonably similar across the craton and can be
characterized by a pre-2860 Ma basement covered by a thin sequence of banded iron formation
and fuchsitic quartzites (Bleeker et al., 1999a,b), largely similar to those observed in the eastern
47
Beartooth Mountains. There is also pervasive 2730-2700 Ma basaltic volcanism across the
basement complex with minor komatiitic rocks and some intercalations of rhyolite tuff (dated
2722-2700 Ma), and volcaniclastic rocks that have been reworked (Isachsen and Bowring, 1997).
Furthermore, the Slave contains widespread tonalite-trondhjemite-granodiorite-type subvolcanic
plumes and turbidite sedimentation (2670-2650 Ma) across the craton (Bleeker and Villeneuve,
1995; Isachsen and Bowring, 1994). There are abundant tonalite-granodiorite plutons that are
2630-2610 Ma (Davis and Bleeker, 1999), and 2605-2580 Ma evolved granites (two-mica
granites). The Beartooth Mountains and overall Wyoming Province show a similar plume
history with late Archean magmatism composed of high-Na tonalities and trondjemites to high-K
granites from 2950-2550 Ma (Chamberlain et al., 2003). Similar to the Wyoming Province,
there is little evidence of dike swarms in the Slave at ca. 2470-2410 Ma. The oldest known
Paleoproterozoic mafic dikes in the craton (Malley swarm) have been dated at 2230 Ma
(LeCheminant and van Breemen, 1994), with several other swarms dated between 2200-2000
Ma, suggesting that the breakup of the Sclavia supercraton was occurring during this time. The
broad similarities between the Slave and Wyoming necessitate an explanation, being the modern
components of an ancient supercraton provides a fitting rationalization if it is assumed that the
modern Slave and Wyoming craton were once part of the same Slave clan.
6.3 Discussion
There exists that possibility that the Wyoming craton contains sediments from the Slave craton
and that the Slave and Wyoming cratons are the modern components of what was once the Slave
clan and supercraton Sclavia. They both contain ancient detrital zircons of similar >3.9 Ga age
and rock types including fuchsitic quartzites, thin banded iron formation and trondhjemtic
48
bodies. Other locations like the Assean Lake and Thelon Basin also contain ancient detritus and
may be linked to the Slave, Wyoming, and therefore Sclavia as well. High quality paleomagnetic
data sets on similar lithologic units across several cratons are needed to test for further potential
correlation.
49
CHAPTER 7
SUMMARY
Fine-scale geology and geochemistry of the eastern Beartooth Mountains provide a useful
snapshot in miniature of the long and complex history of the Wyoming Craton. The Quad Creek
rocks show evidence of three different events with the establishment of a sedimentary depo
center and BIF deposit in the Paleoarchean, then a period of deformation coinciding with
metamorphism at granulite facies at 3200 Ma, and later hydrothermal alteration, uplift, and
subsequent denudation and erosion of the younger sediments allowing the oldest core of the
Beartooth Mountains to be exposed. Detrital/xenocrystic zircons with Eoarchean to Hadean ages
confirm that older crustal components of the Wyoming Craton existed in the vicinity of these
quartzites and are comparable to the oldest components of the North American Craton. The
geochemistry of the quartzites and paragneiss shows a general variability in Cr and Zr in the
quartzites and elevated Cr and Zr in the paragneiss suggesting a mixed mafic and granitic source.
This work bolsters the case for a genetic link between the Beartooth quartzites and the geology
of the Western Slave Province as previously suggested by Mueller et al. (1992). The Thelon
Basin in Nunavut (Palmer et al., 2004), Assean Lake Complex (Bohm et al., 2000) and Central
Slave Cover Group (Sircombe et al., 2001) may have also been linked. Comparison of the most
concordant grains (<5% discordant) of this study’s Th/U ratios to others from the Beartooth
Mountains, Assean Lake Complex and Central Slave Cover Group and shows highly variable
Th/U ratios around 3200 Ma, but similar ages and Th/U ratios for the oldest and most concordant
grains from Quad Creek with the other localities mentioned in section 6.1.1 (Figure 13).
50
The new data from this study further lead to a better relation of the ages with the regional
geology of this part of the Wyoming Craton and provide additional information towards
correlating ancient terranes (Reddy and Evans, 2009). Paleoarchean sedimentary basins occupy
much of the supracrustal inventory of southwestern Montana (e.g. Tobacco Root Mountains).
The oldest sediment source(s) of the Beartooth quartzites may share a common origin with the
supracrustals of the Assean Lake Complex (Manitoba), the Thelon Basin (Nunavut) and other
quartzites on the Slave Craton also known to contain pre-3900 Ma zircons. If this hypothesis is
correct, it would mean that some Paleoarchean basins of northern North America, like those
captured as supracrustal enclaves in the Wyoming craton, were at one time in the vicinity of a
larger “Slave continent,” Sclavia that shed Hadean and Eoarchean zircons into them. If the ca.
3960 Ma Acasta Gneisses are the core remnants of Sclavia ( Bleeker, 2003), it would be the
logical source of this detritus.
Future investigation should include the study of mineral
inclusions in the zircons, high quality paleomagnetic data of the mafic dikes in the Wyoming
craton, and the search for other Paleoarchean sedimentary basins in North America.
51
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