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Age and Isotopic Composition of Mafic Dikes within the Wyoming Province

Permanent Link: http://ufdc.ufl.edu/UFE0020561/00001

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Title: Age and Isotopic Composition of Mafic Dikes within the Wyoming Province A Window into the Evolution of the Subcontinental Lithosphere
Physical Description: 1 online resource (63 p.)
Language: english
Creator: Richards, Joshua L
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: bbmz, mafic, map, mmp, montana
Geological Sciences -- Dissertations, Academic -- UF
Genre: Geology thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Mafic dikes provide geochemical and isotopic insight into processes affecting mantle evolution. Two specific areas within the Wyoming Province, the Beartooth-Bighorn Magmatic Zone (BBMZ), and the Montana Metasedimentary Province (MMP), and one area within the Great Falls tectonic zone (GFTZ), the Montana Alkali Province (MAP), each contain mafic intrusions, primarily as dikes. One of three dikes sampled from the BBMZ has a U-Pb zircon age of 2.8 Ga. Geochemical and isotopic similarities for the other dikes suggest a Late Archean emplacement for all three dikes. No dikes sampled from the MMP yielded reliable, datable phases, however, geochronology and paleomagnetic data suggest that dikes were either emplaced at 1450 or 780 Ma. The MAP, a petrologic province, formed ~50 Ma ago within the GFTZ, a largely Proterozoic feature. Major and trace element geochemistry for all samples exhibit features, such as a relative depletion in high field strength elements, characteristic of modern convergent margin volcanism. Nd and Pb isotopic data, however, suggest that the geochemical features were derived from the source of the mafic magmas and that these signatures were established well before the time that the magmas formed. In particular, Sm/Nd isotopic data from whole rocks suggest metasomatic enrichment in the mantle in the BBMZ at ~3.4 Ga and ~2.0 Ga for the MMP, whereas Pb/Pb whole rock analysis suggests metasomatism of the mantle in the BBMZ at ~3.2 Ga, ~1.9 Ga for the MMP, and ~1.8 Ga for the MAP. The ages quoted above are based on secondary isotopic ratios and are only loosely constrained (i.e., the 3.2 Ga and the 3.4 Ga ?ages? from the BBMZ samples are not statistically distinct). Similarly, the 1.9 Ga and 1.8 Ga ?ages? for the MMP and MAP samples are also indistinguishable from each other. What is distinct, however, is that samples from the Proterozoic GFTZ (MMP and MAP) suggest that the underlying mantle was modified (metasomatized) at this time and has remained largely undisturbed. The mantle-altering events inferred for all three areas appear to be similar to modern day island arc settings. We propose that the evolution recorded in the isotopic data suggests that metasomatism of the dike sources occurred at different times throughout the area and we conclude that the source of this metasomatism is from subduction related processes similar to modern day island arc settings.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Joshua L Richards.
Thesis: Thesis (M.S.)--University of Florida, 2007.
Local: Adviser: Mueller, Paul A.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2017-08-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0020561:00001

Permanent Link: http://ufdc.ufl.edu/UFE0020561/00001

Material Information

Title: Age and Isotopic Composition of Mafic Dikes within the Wyoming Province A Window into the Evolution of the Subcontinental Lithosphere
Physical Description: 1 online resource (63 p.)
Language: english
Creator: Richards, Joshua L
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: bbmz, mafic, map, mmp, montana
Geological Sciences -- Dissertations, Academic -- UF
Genre: Geology thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Mafic dikes provide geochemical and isotopic insight into processes affecting mantle evolution. Two specific areas within the Wyoming Province, the Beartooth-Bighorn Magmatic Zone (BBMZ), and the Montana Metasedimentary Province (MMP), and one area within the Great Falls tectonic zone (GFTZ), the Montana Alkali Province (MAP), each contain mafic intrusions, primarily as dikes. One of three dikes sampled from the BBMZ has a U-Pb zircon age of 2.8 Ga. Geochemical and isotopic similarities for the other dikes suggest a Late Archean emplacement for all three dikes. No dikes sampled from the MMP yielded reliable, datable phases, however, geochronology and paleomagnetic data suggest that dikes were either emplaced at 1450 or 780 Ma. The MAP, a petrologic province, formed ~50 Ma ago within the GFTZ, a largely Proterozoic feature. Major and trace element geochemistry for all samples exhibit features, such as a relative depletion in high field strength elements, characteristic of modern convergent margin volcanism. Nd and Pb isotopic data, however, suggest that the geochemical features were derived from the source of the mafic magmas and that these signatures were established well before the time that the magmas formed. In particular, Sm/Nd isotopic data from whole rocks suggest metasomatic enrichment in the mantle in the BBMZ at ~3.4 Ga and ~2.0 Ga for the MMP, whereas Pb/Pb whole rock analysis suggests metasomatism of the mantle in the BBMZ at ~3.2 Ga, ~1.9 Ga for the MMP, and ~1.8 Ga for the MAP. The ages quoted above are based on secondary isotopic ratios and are only loosely constrained (i.e., the 3.2 Ga and the 3.4 Ga ?ages? from the BBMZ samples are not statistically distinct). Similarly, the 1.9 Ga and 1.8 Ga ?ages? for the MMP and MAP samples are also indistinguishable from each other. What is distinct, however, is that samples from the Proterozoic GFTZ (MMP and MAP) suggest that the underlying mantle was modified (metasomatized) at this time and has remained largely undisturbed. The mantle-altering events inferred for all three areas appear to be similar to modern day island arc settings. We propose that the evolution recorded in the isotopic data suggests that metasomatism of the dike sources occurred at different times throughout the area and we conclude that the source of this metasomatism is from subduction related processes similar to modern day island arc settings.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Joshua L Richards.
Thesis: Thesis (M.S.)--University of Florida, 2007.
Local: Adviser: Mueller, Paul A.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2017-08-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0020561:00001


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1 AGE AND COMPOSITION OF MAFIC DIKE S WITHIN THE WYOMING PROVINCE: A WINDOW INTO THE EVOLUTION OF THE SUBCONTINENTAL LITHOSPHERE By JOSHUA LEE RICHARDS A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2007

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2 2007 Joshua Richards

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3 To Robert Earl Roach you will forever be in my heart

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4 ACKNOWLEDGMENTS At this time, I would like to acknowledge my friends and family for their constant persistence and belief that I can accomplish anyt hing. I would also like to thank my committee members, Dr. David Foster and Dr. Michael Perfit, especially Dr. Paul Mueller for his patience throughout this process. I woul d like to thank Dr. Jim Vogl, Dr. David Mogk, and Kelly Probst for the necessary help in the field. I would also like to tha nk Dr. George Kamenov and Warren Grice for the assistance in data collection and in terpretation. An extended thanks goes to all the faculty, staff and fellow graduate students who ha ve made my time here some of the best years of my life. Lastly, I would lik e to acknowledge the or ganizations which provided the necessary funding which made this project a reality: Depart ment of Geological Scien ces at the University of Florida, Tobacco Root Geological Society, United States Geological Survey (Grant # 05HQGR0156), and National Sc ience Foundation (EAR 0106592).

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES................................................................................................................. ..........7 LIST OF FIGURES................................................................................................................ .........8 ABSTRACT....................................................................................................................... ..............9 CHAPTER 1 INTRODUCTION..................................................................................................................11 2 PRECAMBRIAN MAFIC DIKES OF TH E BEARTOOTH-BIGHORN MAGMATIC ZONE (bbmz).................................................................................................................... .....15 Geologic Setting and Samples................................................................................................15 Results........................................................................................................................ .............16 Discussion..................................................................................................................... ..........18 3 PRECAMBRIAN MAFIC DIKES OF THE MONTANA METASEDIMENTARY PROVINCE (MMP)...............................................................................................................35 Geologic Setting and Previous Work.....................................................................................35 Age Relations.................................................................................................................. ........36 Tobacco Root Mountains........................................................................................................37 Ruby Range..................................................................................................................... .......38 Highland Mountains............................................................................................................. ..39 Results........................................................................................................................ .............40 Discussion..................................................................................................................... ..........41 4 EOCENE MAFIC DIKES OF THE MONTANA ALKALI PROVINCE (MAP).................46 Geologic Setting and Previous Work.....................................................................................46 Castle Mountains............................................................................................................... .....47 Crazy Mountains................................................................................................................ .....48 Results........................................................................................................................ .............48 Discussion..................................................................................................................... ..........49 5 CONCLUSION..................................................................................................................... ..52 APPENDIX MATERIALS AND METHODS..............................................................................53 Sampling Strategy.............................................................................................................. .....53 Sample Processing.............................................................................................................. ....53

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6 Zircon Separation.............................................................................................................. ......55 LIST OF REFERENCES............................................................................................................. ..57 BIOGRAPHICAL SKETCH.........................................................................................................63

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7 LIST OF TABLES Table page 2-1 General information on mafic dikes sampled....................................................................29 2-2 Major, Trace, and Isotopic concentra tions of mafic dikes within the Wyoming Province and Great Falls tectonic zone (GFTZ)................................................................31 2-3 U-Pb data for BT01 sample from the Southeastern Beartooth Mountains........................34

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8 LIST OF FIGURES Figure page 1-1 Field map of the Wyoming Province.................................................................................13 1-2 This field schematic comprises sa mples collected for this research..................................14 2-1 SiO2 vs. FeO*/MgO discrimination diagram.....................................................................21 2-2 The Ti-Zr-Y discrimination diagram.................................................................................22 2-3 Trace element plots for all three areas...............................................................................23 2-4 REE plots for all three areas..............................................................................................24 2-5 U-Pb concordia diagram for BBMZ sample BT01............................................................25 2-6 Sm/Nd plot for samples within the BBMZ........................................................................26 2-7 Diagram 206Pb/204Pb vs. 207Pb/204Pb for dikes within the BBMZ......................................27 2-8 Age vs. initial Nd...............................................................................................................28 3-1 Sm/Nd plot for samples within the MMP..........................................................................44 3-2 Whole rock 206Pb/204Pb vs. 207Pb/204Pb diagram for samples within the MMP.................45 4-1 Whole rock 206Pb/204Pb vs. 207Pb/204Pb isotopic diagram for MAP samples.....................51

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9 Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science AGE AND COMPOSITION OF MAFIC DIKE S WITHIN THE WYOMING PROVINCE: A WINDOW INTO THE EVOLUTION OF THE SUBCONTINENTAL LITHOSPHERE By Joshua Lee Richards August 2007 Chair: Paul Mueller Major: Geology Mafic dikes provide geochemical and isotopic insight into processes affecting mantle evolution. Two specific areas within the Wyom ing Province, the Bear tooth-Bighorn Magmatic Zone (BBMZ), and the Montana Metasedimentar y Province (MMP), and one area within the Great Falls tectonic zone (GFTZ), the Montan a Alkali Province (MAP), each contain mafic intrusions, primarily as dikes. One of three di kes sampled from the BBMZ has a U-Pb zircon age of 2.8 Ga. Geochemical and isotopic similarities for the other dikes suggest a Late Archean emplacement for all three dikes. No dikes sa mpled from the MMP yielded reliable, datable phases, however, geochronology and paleomagnetic data suggest that dikes were either emplaced at 1450 or 780 Ma. The MAP, a petrologic prov ince, formed ~50 Ma ago within the GFTZ, a largely Proterozoic feature. Major and trace element geochemistry for all sa mples exhibit features, such as a relative depletion in high field strength elements, characteristic of modern convergent margin volcanism. Nd and Pb isotopic data, however, suggest that th e geochemical features were derived from the source of the mafic magmas and that these signatu res were established well before the time that the magmas formed. In particular, Sm/Nd isotop ic data from whole rocks suggest metasomatic enrichment in the mantle in the BBMZ at ~3.4 Ga and ~2.0 Ga for the MMP, whereas Pb/Pb

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10 whole rock analysis suggests metasomatism of the mantle in the BBMZ at ~3.2 Ga, ~1.9 Ga for the MMP, and ~1.8 Ga for the MAP. The ages quoted above are base d on secondary isotopic ratios and are only loosely constrained (i.e., the 3.2 Ga and the 3.4 Ga ages from the BBMZ samples are not statistically di stinct). Similarly, the 1.9 Ga and 1.8 Ga ages for the MMP and MAP samples are also indistingu ishable from each other. What is distinct, however, is that samples from the Proterozoic GFTZ (MMP and MA P) suggest that the underlying mantle was modified (metasomatized) at this time and has remained largely undisturbed. The mantle-altering events inferred for all three areas appear to be similar to modern day island arc settings. We propose that the evolution recorded in the isotopi c data suggests that metasomatism of the dike sources occurred at different times throughout the ar ea and we conclude that the source of this metasomatism is from subduction related processe s similar to modern day island arc settings.

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11 CHAPTER 1 INTRODUCTION Mafic dikes are useful geologi c tools for investigating pro cesses such as continental breakup (Harlan et al., 2003), ancient mantle pl umes (i.e., Heaman et al., 1992; Park et al., 1995), magmatic underplating (Chamberlain et al., 2003), and the evolution of the subcontinental lithosphere in general. Study of mafic dikes within the Wyo ming Province, therefore, can enhance our understanding of the subcontinental lit hosphere/ mantle system below this Archean terrain. Within the Wyoming province, dikes were sampled from two sub-provinces, the Beartooth-Bighorn Magmatic Z one (BBMZ) and the Montana Metasedimentary Province (MMP) (Figure 1-1 and 1-2). Dikes within th e BBMZ and MMP are thought to represent the only magmatic events that occu r between the cratonization of the Wyoming Province at ~2.8 Ga (Mueller and Frost, 2006) and se dimentation during the Paleozoic (Harlan et al., 2003), with the exception of the formation of the Belt Basin (1.45 Ga) in the MMP. Dikes, th erefore, are integral for understanding processes which pr oduced this mafic magmatism. Mafic magmatism within the MAP is consid erably younger than that in the other two field areas. These magmas intruded Phanerozoic rocks exposed in the Gr eat Falls tectonic zone (GFTZ) (Figure 1-1). In the past, the origin of the GFTZ has been argued as either an Archean continental suture zone (i.e., Boerner et al., 1998 ) or as a Proterozoic collisional zone (i.e., ONeill and Lopez, 1985; Mueller et al., 2002; Mue ller et al., 2006; Foster et al., 2006), but the latter is currently the most acc epted hypothesis. Dikes within this area provide information about the subcontinental lithosphere/ ma ntle beneath the MAP and subsequently can help constrain age and origin of the GFTZ.

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12 Dikes within the Wyoming Province, Great Fall s tectonic zone, and in other locals are postulated to result from mantle plumes, as thenosphere upwelling, generation by decompression melting of the subcontinental lithosphere, and/ or subsequent magmatism due to subduction related processes (LeCheminant and Heaman, 1989; Heaman et al., 1992; Harlan et al., 2003). In this paper, we present major element, trace elemen t, and isotopic data for mafic dikes within the Wyoming Province and the Great Fa lls tectonic zone. These data provide new insights into the development of the subcontinenta l lithosphere/ mantle beneath the Northern Wyoming Province and Great Falls tectonic zone.

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13 Figure 1-1. Field map of the Wyoming Provin ce and associated geoc hronological events occurring along its boundaries. The box denotes the specific field area for this project and is enlarged in Fi gure 1B. [Reproduced with the direct permission from Foster, D.A., Mueller, P.A., Mogk, D.W., Wooden, J.L., and Vogl, J.J., 2006, Proterozoic evolution of th e western margin of the Wyoming craton: implications for the tectonic a nd magmatic evolution of the northern Rocky Mountains: Canadian Journal of Earth Sciences (page 1604, Figure 1).

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14 Figure 1-2. This field schematic comprises samples collected for this research. The Beartoot h-Bighorn Magmatic Zone (BBMZ), the Montana Metasediment ary Province (MMP) and the Montana Alkali Province (MAP) are represen ted. Samples are represented by numbers as followed: 1-4 =BTL01, BTL02, BT01 and BT02; 5=CA04; 6-7= CZ01 and CZ02; 8=TR01; 9=RR01; 10-12=HM03, SJM-10, a nd SJM-18; 14-15= KG and 247FM. Geographic locations are noted in Table 2-1.

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15 CHAPTER 2 PRECAMBRIAN MAFIC DIKES OF THE BE ARTOOTH-BIGHORN MAGMATIC ZONE (BBMZ) Geologic Setting and Samples The Beartooth-Bighorn magmatic zone (BBMZ) is located in the nor th central portion of the Wyoming Province and encompasses the Be artooth Mountains and the Bighorn Mountains (Figure 1-1 and 1-2). The BBMZ has experienced multiple periods of both felsic and mafic magmatism between ~3.5 Ga, earliest gneisses, and 0.8 Ga, latest mafic dikes (Mogk et al., 1992; Wooden and Mueller, 1988; Harlan et al., 2003; Mueller et al., 2006), but is composed dominantly of Late Archean igne ous to metaigneous rocks that ar e tonalitic to granodioritic to trondhjemitic (TTG) in composition (e.g., Mogk et al., 1992; Chamberlain et al., 2003). Precambrian mafic dikes intruded the basement of the BBMZ and have been studied by multiple workers (Prinz, 1964; Mueller, 1971; Wooden, 1975; Harlan et al., 2003). These dikes range in composition from Archean orthoamphibolites, metadolerites, and late Precambrian quartz dolerites to Neoproterozoic olivine doler ites (Prinz, 1964; Mu eller and Rogers, 1973; Baadsgaard and Mueller, 1973; Wood en, 1975; Harlan et al., 1997). Samples BTL02, BT01 and BT02 are fine graine d samples from the chilled margins of three separate dikes. These dikes are tholeiitic with ophitic to sub-ophiti c texture. Plagioclase and clinopyroxene are primary minerals, but am phibole is also common and interpreted as a product of deuteric alteration al ong with seritization of plagiocl ase (Winter, 2001). In another dike (BTL), BTL01 is from a zone of intermediate texture and contains An60-An40 plagioclase (labradorite) phenocrysts measuring up to 2 cm in le ngth and 1 cm in width. It is located between the chilled margin (BTL02) and the central zone of the dike. The central zone of the dike contains even larger plagioclase (up to 4cm in length) phenocrysts than BTL01, and, therefore, was not sampled due to concerns for having a ho mogeneous, representative sample. The relative

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16 proportion of plagioclase in the groundmass of BTL 01 is less (~25% labradorit e) than that of the other three samples (~50% plagioclase). Based on chilled margin compositions, all samples are from Group I-II of Mueller and Rogers (1973). These groups were designated based on TiO2 content, i.e., Mueller and Rogers (1973) defined Group I as ha ving the lowest wt % TiO2 (< 2.0) of all dikes in their study. This group made up ~64 % of the dikes in their study and are compositionally similar to the metadolerite samples of Prinz (1964). Group I dike s were estimated to have a coherent minimum age of 2.55 Ga based on K-Ar and Rb-Sr whole-ro ck analyses (Baadsgaard and Mueller, 1973). Results Major element, trace element, and isotopic da ta are listed in Table 2-2. Dikes from the BBMZ plot near the boundary between tholeiitic and calc-alkaline compositions (Figure 2-1). Samples range in SiO2 from 5357 wt % and have low-to-moderate TiO2 contents equivalent to Group I of Mueller and Rogers (1973). This simila rity extends into the mi neral assemblages also, with both having plagioclase and clinopyroxene (augite) as primar y minerals. BTL01, a variation of Prinzs (1964) leopard rock, can be classifi ed as an anorthositic gabbro because of its porphyritic texture and composition. Some shearing fabric is noted in thin sections of all BBMZ samples with the exception of BTL01. This fabric is likely associated with post emplacement movement along the fractures filled by the dikes and associated hydrothermal alteration for at least sample BTL01 (e.g., seritiza tion of the large pl agioclase phenocrysts), in addition to deuteric alteration seen in most dikes of this age (Prinz, 1964). Trace element abundances normalized to primitive mantle for all BBMZ dike samples show depletion of Nb in relation to U and K (Fi gure 2-3a), similar to island arc calc-alkaline basalts (Wilson, 1989; Winter, 2001). BTL01, how ever, has much lower LREE content and yields a pattern that more closely resembles thos e of primitive, arc type basalts. An important

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17 difference in the patterns is that the rare eart h element (REE) abundances in the chilled margin samples BT01, BT02, and BTL02 show pronounced en richment of the light REE (e.g., La = >100X) with respect to chondritic values, while BTL01 (phenocryst-rich sample) shows distinctly lower abundances (Figure 2-4a). Th e HREE contents of all samples, however, are slightly enriched (Lu=~10X) w ith little to no Eu anomaly in any sample (Figure 2-4a). Mueller and Rogers (1973) used the data of Baadsgaard and Mueller (1973) to assign an age of ~2.5 Ga to these Group I (low TiO2) mafic dikes. In an effort to obtain a more constrained age for this group, U-Pb data were obtained from one sample (Table 2-3). Small zircons (<50 microns) were extracted from BT01 and analyzed using the Sensitive High Resolution Ion Micro ProbeReverse Geometry (SHRIMP-RG) at the Stanford-U.S.G.S. Microanalytical Center. Data plot close to concordia and suggest a crystallization age of 2.8 Ga (Figure 2-5). The Quad Creek Metanorite, a sm all pluton equated to Group I dikes by Mueller and Rogers (1973), also yields a U-Pb age of ~2.8 Ga (Mueller unpublished), consistent with BT01 and within error of the 2.55 Ga whole-rock Rb-Sr age of Mueller and Rogers (1973). Overall, these zircon data sugge st the Group I dikes of Muelle r and Rogers (1973) are better assigned an age of ~2.8 Ga. Whole-rock Sm-Nd isotopic data (Table 22) for samples BTL02, BT01 and BT02 yield present day Nd values of -33.1 to -43.2 and calculated initial Nd values of -2.6 to -3.6 using 2.8 Ga as an approximate crysta llization age (Table 2-3). 147 Sm/144 Nd ratios for these samples (BTL02, BT01, and BT02) range from 0.08836 to 0.11132 and produce a range of TDM from 3.03.1 Ga. Regression line for Sm/Nd for the di kes in the BBMZ produce an age of ~3.4 Ga (Figure 2-6) and is w ithin error of the TDM and TCHUR. Sample BTL01 yields a present day value of -7.4, an initial Nd of 2.64, and has a 147Sm/144Nd value of 0.16784 (Table 2-2). Though its

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18 higher Nd value is commensurate with its less evolved trace element chemical composition compared to the other BBMZ samples, it is difficult to calculate reliable TDM and TCHUR for this sample because of the high 147Sm/ 144Nd ratio (i.e., close to the values of model mantle compositions). The relatively high 147 Sm/ 144 Nd does, however, allow for a comparatively more reliable estimate of initial Nd because there is little change re lative to the evolving mantle over time. Figure 13 depicts the initial Nd values from all four samples and the initial Nd for 2.8 Ga felsic rocks within the BBMZ (Wooden and Mueller, 1988) and shows that the dike compositions are strongly overl apping with the initial Nd compositions of the more felsic rocks. Whole-rock Pb isotopic compositions (Figure 27) of all BBMZ samples produce a linear array equivalent to an age of ~3.2 Ga, though they are clearly younger because they intrude 2.8 Ga gneisses and granitoids (W ooden and Mueller, 1988). Discussion Major element data for dike samples B TL01, BTL02, BT01, and BT02 show relatively high SiO2 (54-57 wt. %), have moderately high total iron (10-11.5 wt. %), and are interpreted as calc-alkaline bordering on t holeiitic (Figure 2-1). Trace elemental data are particularly useful in determining the source of (e.g., primitive mantle vs. enriched mantle) and modes of magmatism (e.g., subduction related vs. plumerelated) of mafic rocks. For sample BTL01, a less enriched pattern of LREE compared to BTL02, BT01, and BT02 (20X chondritic values ve rsus 200-600X chondritic values), indicates a more primitive signature for the intermediate zone of the leopard rock sa mple than that of the chill margin samples within the BBMZ (Figure 2-4a ). This is likely due to analysis of a higher percentage of groundmass, which is possibly depleted in plagio clase relative to the large phenocrysts within sample BTL 01. Alternatively, the dike ma y be a composite intrusion.

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19 The relative depletion in H FSE (Nb, Ta, Ti) evident in all samples from the BBMZ (Figure 2-3a) suggests the source material is more similar to the metasomatized mantle (mantle wedges) in modern island arc environments, as opposed to plume type magmatism proposed for dikes such as the Mackenzie or Franklin mafic dike swarms of Northern Canada (e.g. Park, 1981; Armstrong et al., 1982; Fahrig and West, 1986; LeCheminant and Heaman, 1989; Park et al, 1994). It is important to note that these samples do not plot within field D (within plate basalts), indicating a lack of evidence for pl ume related magmatism (Figure 2-2). The time at which this signature was acquire d, however, is more difficult to specify. Isotopic data (Sm/Nd, U/Pb) are useful in understanding the evolution, mixing, and/or contamination of dike magmas (e.g., contaminati on of mafic intrusions when emplaced in more felsic crust) and, therefore, he lp constrain the time at which the HFSE signature was acquired. Sm/Nd data for dikes within the BBMZ form an a rray at ~3.4 Ga (Figure 2-6), which is clearly older than the dominant age of th e country rock in the eastern B eartooth Mountains, but is within error of whole rock lead isot opic data, which produce an arra y suggesting an age of ~3.2 Ga (Figure 2-7). For the Beartooth Mountains, isotopic Nd and Pb values from the mafic dikes are best interpreted to indicate that the mantle sour ce of the dike magmas was modified prior to formation of the dike magmas. The LREE enrich ment, HFSE depletion, and isotopic systematics seen in the dikes suggest this modification was likely to have been a metasomatic event associated with convergence and subduction about 3.1-3.4 Ga ago. Due to the differences in concentration of Pb in the crust and mantle, the Nd value of 3.4 Ga is likely more representative of the metasomatic event which occurred in the source BBMZ magmas. This metasomatic event resulted in resetting of pare nt/daughter ratios in the U/Pb and Sm/Nd isotopic systems and

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20 significant homogenization of isotopic compositions to yield the isochr on-like arrays. This proposed metasomatic event is compatible with evidence for 3.1-3.5 Ga felsic magmatism in the eastern Beartooth Mountains (Mueller et al., 2006 ) indicating that a series of crust forming events in this interval were substantial enough to impact the crus tal and mantle systems coevally.

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21 0 1 2 3 4 4247525762SiO2 wt. %FeO*/MgO wt. % BBMZ MMPCalc-Alkaline Tholeiitic BTL01 Figure 2-1. SiO2 vs. FeO*/MgO discrimination diagra m for island-arc basalts. Field designation line extr apolated from diagram in Winter (2001). FeO* is calculated total iron as FeO. This diagram represents an approximation of values. Samples from the MAP were not plotted due to their high alkalic and silica undersatu rated composition.

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22 Figure 2-2. The Ti-Zr-Y discrimination diag ram for basalts (after Pearce and Cann, 1973). Fields are A= island-arc tholeiites; B= MORB, island-arc tholeiites, and calc-alkali basa lts; C= calc-alkali basalts; D= within-plate basalts. Coordinates for the de signated fields (A, B, C, and D) are listed in both Pearce and Cann (1973) and Rollinson (2003).

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23 1 10 100 1000 BTL01 BTL02 BT01 BT02 Sample/ Primitive Mantle 0.1 1 10 100 1000 SJM-10 SJM-18 TR01 HM03 247FM KG RR01 RbBaThUNbTaKLaCePbPrSrPNdZrHfSmEuTiDyYHoYbLu 1 10 100 1000 CA04 CZ01 CZ02 A B C BBMZ MMP MAP Figure 2-3. Trace element plots for all three areas A) BBMZ. B) MMP. C) MAP. The spider diagrams are all norm alized using primitive mantle values from Sun and McDonough (1989).

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24 1 10 100 1000 BTL01 BTL02 BT02 BT01 Sample/ Chondrite 1 10 100 1000 SJM-10 SJM-18 TR01 HM03 247FM KG RR01 LaCePrNdSmEuGdTbDyHoErTmYbLu 1 10 100 1000 10000 CA04 CZ01 CZ02 A B C BBMZ MMP MAP Figure 2-4. REE plots for all three areas. A) BBMZ. B) MMP. C) MAP. The REE plots are normalized using chondritic values from Sun and McDonough (1989).

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25 2880 2840 2800 2760 2720 2680 2640 2600 25600.44 0.48 0.52 0.56 0.60 111213141516207Pb/235U206Pb/238U data-point error ellipses are 2 Intercepts at 2802.4 +5.5/-4.4 Ma MSWD = 0.99 Figure 2-5. U-Pb concordia diagram for BBM Z sample BT01, a southern Beartooth sample located just east of Beartooth Butte (Prinz, 1964) and correlates with other ~2.8 Ga dikes also located within this area of the BBMZ.

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26 0.5100 0.5104 0.5108 0.5112 0.5116 0.5120 0.5124 0.5128 0.060.080.100.120.140.160.18147Sm/144Nd143Nd/144Nd Age = 3393 Ma Initial 143Nd/144Nd =0.50847.00015 Figure 2-6. Sm/Nd plot for samples within the BBMZ with a calculated array age of ~3.4 Ga using Isoplot 3.2 (Ludwig, 2004).

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27 15.2 15.4 15.6 15.8 16.0 16.2 16.4 15.516.517.518.519.520.5206Pb/204Pb207Pb/204Pb Approximate "Age" = 3.2 Ga Figure 2-7. Diagram 206Pb/204Pb vs. 207Pb/204Pb for dikes within the BBMZ. Using U/Pb geochronology from zircon grains within BT01, emplacement age for the BBMZ dikes is 2.8 Ga.

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28 -20.00 -15.00 -10.00 -05.00 00.00 05.00 10.00 15.0000.511.522.533.544.5Age (Ga)Epsilon Nd BBMZ MAP MMP Dudas et al., 1987 Mueller et al., 2004 MMDS CHUR Depleted Mantle MMDS in MMP Generalized Evolution of Northern Wyoming Province Crust Wooden and Mueller, 1988 Figure 2-8. Age vs. Initial Nd for the BBMZ, MMP, MAP and a Metamorphosed Mafic Dikes and Sills (MMDS) sample from within the MMP. MMDS value was taken from Mu eller et al., 2004 and dated at 2.06 Ga using U/Pb geochronology of zircon. There are two samples plotted for the MMP; TR01 and SJM-18. These samples represent th e only dikes with known ages for the sample s in the MMP for this project. Both samples are ~1.45 Ga in age (refer to MMP section for clarification). Samples added from Duds et al., 1987 ar e alkalic to subalkalic in composition and within the MAP. Wooden and Mueller (1988) samples are felsic 2.8 Ga crustal samples from th e BBMZ and defined by the ligh t gray field overlying the BBMZ mafic dike samples.

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29 Table 2-1. General information on mafic dike s sampled. Coordinates for the latitude a nd longitude are in WGS 84. Age determinat ions are as follows; 1 zircon SHRIMP, U-Pb; 3 Prinz, 1964 and Wooden, 1975, K-Ar; 4 Duds et al., 1987; Harlan et al., 1988; 5 Johnson and Swapp, 1989, Harlan et al, 2005; 6 Wooden, 1975; 7 Mueller, 1971. 8 Harlan et al., 1990, BBMZ refers to the Bighorn-Beartooth Magmatic Zone and the MMP refers to the Montana Metasedimentary Prov ince. Both are represented in figure 1. Textural de scriptions were compiled in this research. All observations are of th e author. Sample Location Coordinates Name Age (Ma) Strike Size Composition General Key Notes Alteration BT01 Beartooth/ BBMZ 44.97932 N 109.5934 W Diabase 28025.51 N 75 W 3 m width >1 km length Plag + pyx + opx Fine to medium grained containing three coexistsing pyroxenes minimal BT02 Beartooth/ BBMZ 45.00760 N 109.5213 W Diabase 2800 N 45 W 10 m width >1 km length Plag + pyx + opx + bi Fine to medium grained with orthopyrexene crystals minimal BTL01 Beartooth/ BBMZ 42.24055 N 109.6630 W Diabase 2800-25003 N 25 W 6 m width <1 km length Plag+ opx+ cpx Porphyritic 60% large (up to 8 inches) euhedral and subeuhedral plagioclase phenocrysts minimal BTL02 Beartooth/ BBMZ 42.24055 N 109.6630 W Diabase 2800-25003 N 25 W 1 m width <1 km length Plag + opx + cpx with metadolerite and Metanorite within the groundmass Nonporpyhritic fine grained, metadolerite groundmass minimal TR01 Tobacco Root/ MMP 45.42093 N 112.1561 W Diabase 1450 5 N 75 W 6 m width ~500 m length Plag + cpx+ mafic groundmass Fi ne grained fresh diabasic intrusion none RR01 Ruby Range/ MMP 45.1743 N 112.4239 W Diabase 14506 N 23 W 4 m width not exposed at surface Kspar + cpx + neph Fine to medium grained amphibolite lens Amphibolite grade HM03 Highland/ MMP 45.59607 N 112.4870 W Diabase N 68 W 9 m width >2 km length Plag + cpx + hornblende + biotite Dark greenish-gray to black, Fine to medium grained diabase. Ophitic to subophitic cpx and plag. Chloritization has been noted as minor hydrothermal alteration chloritization of pyroxene SJM-10 Highland/ MMP 45.67377 N 112.3491 W Diabase N/A N/A Plag + cpx+ biotite + hornblende No textural description available N/A SJM-18 Highland/ MMP 45.69483 N 112.3659 W Diabase 1450 N/A N/A Plag + cpx+ biotite + hornblende No textural description available N/A KG Gravelly/ MMP 44.88678 N 111.7346 W Diabase Not Available NW strike As wide as 30 m Plag+ opx+ cpx Fine to medium grained black to greenish fresh minimal 247FM Gravelly/ MMP N/A Diabase Not Available NW strike As wide as 30 m Plag+ Hbl + Kspar + Qtz Fine to medium grained black to greenish fresh minimal

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30 Table 2-1. Continued Sample Location Coordinates Name Age (Ma) Strike Size Composition General Key Notes Alteration CA04 Castle/ GFTZ 46.41131 N 110.5519 W Diabase 60-50 4 N 64 W 30 m width >3 km length Cpx + Kspar+ phl+ nepheline + sodalite Fine to medium grained, porphyritic with sodalite reported but none noted none CZ01 Crazy/ GFTZ 46.17021 N 110.5214 W Diabase 48 2 4 N 18 W 5 m width 50 m length Cpx + Kspar+ phl + nepheline fi ne to medium grained with primary phase phlogopite and mafic groundmass none CZ02 Crazy/ GFTZ 46.16797 N 110.4925 W Diabase 48 2 4 N 79 W 15 m width ~750 m length Cpx + Kspar+ phl + varying sedimentary xenoliths Fine to medium grained, amphibolite occurs in the groundmass along with phlogopite none

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31 Table 2-2. Major, Trace, and Isotopic concentrations of mafic dikes within the Wyoming Provin ce and Great Falls tectonic zone (GFTZ). Major element analyses was accomplished using x-ray fluorescence (XRF). Fe2O3 and FeO were calculated by assuming Fe O accounts for 85% of Fe in the system. in the TDM field of Sm/Nd indicates difficu lty expressing a valid age due to slope of sample evaluation line with respect to CHUR's evolu tion. for CZ01 and RR01 signifies th at the major element chemistry wa s normalized due to low total. Sub-province BBMZ MMP MAP Sample BTL01 BTL02 BT01 BT02 TR01 RR01* HM03 SJ M-10 SJM-18 KG 247 FM CA04 CZ01* CZ02 SiO2 (%) 53.39 54.01 56.92 54.54 50.02 47.38 51.32 50.99 48.98 49.18 54.36 46.52 44.85 49.48 TiO2 0.49 1.14 0.63 1.06 1.25 0.34 1.32 1.37 1.25 0.65 1.12 0.53 0.45 2.06 Al2O3 14.97 15.82 15.92 16.01 14.21 13.31 13.79 12.89 13.36 23.75 15.3 13.97 16.47 13.23 Fe2O3 1.47 1.70 1.26 1.53 2.02 1.44 2.40 2.54 2.26 1.10 1.66 1.42 1.57 2.40 FeO 8.34 9.65 7.17 8.67 11.44 8.16 13.60 14.40 12.80 6.24 9.40 8.06 8.90 13.60 MnO 0.15 0.14 0.13 0.14 0.2 0.16 0.25 0.25 0.25 0.11 0.15 0.17 0.17 0.23 MgO 7.8 4.43 3.61 4.3 7.41 7.61 5.69 5.18 7.02 3.71 4.98 5.98 4.05 6.65 CaO 8.2 6.8 5.52 6.54 11.24 10.46 9.7 9.45 11.46 12.54 8.26 8.59 7.08 7.65 K2O 1.55 1.73 3.01 2.2 0.21 3.68 0.39 0.4 0.28 0.3 1.15 2.48 2.43 1.01 Na2O 2.52 3.11 3.46 3.35 2.13 4.80 2.18 2.2 2.02 2.14 3.03 6.37 8.68 0.96 P2O5 0.08 0.78 0.81 0.6 0.12 1.48 0.11 0.12 0.09 0.06 0.15 0.92 1.14 0.22 LOI 0.73 0.74 1.05 0.66 0.56 1.18 0.24 0.36 1.08 0.9 0.97 4.04 4.17 3.12 Total 99.68 100.03 99.5 99.6 100.81 100.00 100.99 100.15 100.86 100.68 100.54 99.05 100.00 100.61 Normative Qz 0.96 4.63 5.88 3.31 3.00 3.54 1.00 4.00 6.92 Or 9.15 10.21 17.77 12.99 1.00 21.19 2.00 2.36 2.00 2.00 7.00 14.64 14.05 5.96 Ab 21.30 26.29 29.25 28.32 18.00 2.46 18.00 18.60 17.00 18.00 25.00 11.25 5.80 8.11 An 24.95 24.09 19.01 22.14 28.00 3.82 26.00 24.11 26.00 54.00 25.00 2.21 28.79 Ne 20.15 23.09 34.53 Di 12.44 3.83 2.60 5.34 22.00 30.72 17.00 18.54 25.00 6.00 13.00 28.41 22.79 6.43 Hy 26.86 23.78 18.99 21.18 24.00 26.00 26.06 20.00 16.00 20.00 33.33 Ac 1.00 15.59 Ol 3.00 12.22 4.00 10.27 9.89 Mt 2.14 2.47 1.83 2.22 2.00 2.05 3.00 3.69 3.00 2.00 2.00 2.06 1.24 3.49 Il 0.93 2.17 1.20 2.02 0.63 2.00 2.61 2.00 1.00 2.00 1.01 0.84 3.92 Ap 0.17 1.70 1.77 1.31 3.17 0.26 2.01 2.42 0.48

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32 Table 2-2. Continued Sample BTL01 BTL02 BT01 BT02 TR01 RR01 HM03 SJM-10 SJM-18 KG 247 FM CA04 CZ01 CZ02 Element (ppm) Sc 17 21 18 20 42 25 41 43 43 27 24 15 7 19 V 136 225 168 189 297 227 319 353 348 154 187 195 246 193 Cr 111 73 79 94 193 64 70 36 95 255 115 84 9 163 Co 24 28 23 27 49 34 47 49 57 46 39 29 28 32 Ni 55 54 51 48 81 47 49 40 69 156 69 37 14 54 Cu 59 46 34 1 163 40 114 107 417 51 148 107 248 133 Zn 49 112 93 106 86 83 102 111 117 72 90 93 106 99 Ga 17 20 20 19 16 12 17 19 17 12 18 15 13 14 Rb 7 45 108 105 2 18 9 8 2 76 54 64 66 84 Sr 153 1056 1036 469 125 91 72 59 92 140 290 2400 3691 3396 Y 15 29 30 26 23 24 28 33 26 20 25 24 26 29 Zr 52 297 252 254 78 99 45 41 40 82 130 188 163 345 Nb 2 9 12 10 5 7 2 3 2 3 9 47 47 51 Ba 67 1537 2034 1217 56 143 84 112 30 357 405 2921 3985 4346 La 5 111 127 44 6 9 5 7 3 11 22 120 174 197 Ce 9 215 273 91 18 21 15 19 12 21 44 211 327 350 Pr 1 23 26 11 2 3 2 3 2 3 5 24 40 38 Nd 6 89 103 53 11 14 9 12 9 11 21 95 165 155 Sm 2 13 15 9 3 4 3 4 3 3 5 13 20 21 Eu 0.57 3.26 3.50 2.39 1.03 1.09 1.01 1.18 1.00 0.72 1.27 3.38 4.63 5.43 Gd 1.79 10.73 11.80 6.30 3.98 3.82 4.03 4.32 3.90 2.65 4.40 10.17 14.48 14.30 Tb 0.37 1.19 1.25 0.99 0.65 0.67 0.71 0.82 0.68 0.50 0.71 1.22 1.72 1.77 Dy 2.23 5.31 5.53 4.83 3.88 4.01 4.62 5.34 4.29 3.00 4.16 4.70 5.50 6.09 Ho 0.45 1.00 1.05 0.93 0.84 0.82 1.02 1.16 0.92 0.62 0.86 0.86 0.96 1.03 Er 1.11 2.96 3.27 2.58 2.15 2.13 2.75 3.22 2.41 1.65 2.38 2.67 3.55 3.39 Tm 0.24 0.39 0.39 0.39 0.39 0.35 0.47 0.52 0.42 0.32 0.38 0.35 0.37 0.36 Yb 1.22 2.32 2.45 2.26 2.28 2.01 3.00 3.41 2.53 1.75 2.31 1.95 2.03 2.04 Lu 0.20 0.35 0.37 0.33 0.34 0.30 0.44 0.50 0.38 0.28 0.35 0.29 0.29 0.30 Hf 1.25 6.63 5.64 6.23 2.25 2.73 1.49 1.45 1.40 2.06 3.37 4.58 3.69 7.41 Ta 0.14 0.40 0.52 0.47 0.36 0.47 0.20 0.24 0.25 0.23 0.69 1.63 1.63 1.83 Pb 2.48 17.52 19.48 12.56 1.12 3.30 1.13 0.87 2.17 7.60 10.57 19.48 34.81 29.53 Th 0.17 20.13 11.97 4.72 0.84 0.87 1.11 1.83 0.32 2.30 5.85 14.74 18.53 23.39 U 0.17 1.88 1.50 1.96 0.27 0.25 0.40 0.52 0.13 0.58 1.60 3.36 5.16 5.06

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33 Table 2-2. Continued Sample BTL01 BTL02 BT01 BT02 TR01 RR01 HM03 SJM-10 SJM-18 KG 247 FM CA04 CZ01 CZ02 Whole Rock Isotopes Wr Rb-Sr 87 Rb/86 Sr 0.1028 0.2159 0.2935 0.6001 0.1038 0.5492 0.3938 0.3949 ND 1.7651 0.4736 0.0493 0.0539 0.0300 87Sr/86Sr 0.7064 0.7122 0.7136 0.7248 0.7044 0.7200 0.7138 0.7136 ND 0.7648 0.7209 0.7059 0.7053 0.7057 WR Sm-Nd 147Sm/144Nd 0.1678 0.0889 0.0864 0.1113 0.1690 0.1622 0.1953 0.1728 0.2030 0.1378 0.1242 0.0824 0.0758 0.0859 143Nd/144Nd 0.5122 0.5105 0.5104 0.5109 0.5126 0.5122 0.5126 0.5123 0.5129 0.5114 0.5113 0.5120 0.5125 0.5120 TCHUR (Ga) 2.2 3.1 3.0 3.0 0.5 1.8 2.2 3.2 2.8 0.8 0.2 0.8 TDM 2.7 3.2 3.2 3.2 1.7 2.5 2.8 3.4 3.0 1.2 0.7 1.2 Nd Present -7.78 -42.41 -43.34 -33.12 -1.66 -8.10 -1.62 -6.55 5.13 -24.25 -25.63 -11.59 -3.60 -12.35 Nd Initial 2.64 -3.56 -3.59 -2.36 3.11 -1.92 -1.37 -2.28 4.01 -13.73 -12.67 -10.86 -2.78 -11.64 WR Pb 206/204 17.522 19.937 16.472 18.720 20.116 17.697 21.764 25.407 17.924 16.252 18.639 16.831 17.377 16.901 207/204 15.624 16.293 15.437 15.835 15.741 15.670 16.055 16.387 15.513 15.268 15.740 15.359 15.416 15.360 208/204 37.159 45.984 39.679 46.583 39.849 37.350 41.319 44.925 37.777 36.274 37.665 37.333 37.557 37.271

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34 Table 2-3. U-Pb data for BT01 sample from the Southeastern Beartooth Mountains (Figure 1-2). U-Pb age calculated from twelve spots on a single zircon grain analy zed by SHRIMP in Menlo Park, Ca. Spot Name U (ppm) Th (ppm) Corr 206 /238 % err ppm Rad 206P b 204cor r 206Pb /238U Age 1s err 204cor r 207Pb /206Pb Age 1s err 204corr 208Pb /232Th Age 1s err % Discordant 4cor r 208r /232 % err 238/ 206r % err 207r /206r % err 207r /235 % err BT01-1 158.7 134.8 0.5335 1.1 72.7 2756.1 25.2 2806.8 7.3 2712.9 237.7 1.8 0.1436 8.8 1.8745 1.1 0.1976 0.4 14.54 1.2 BT01-2 252.5 27.2 0.5465 1.1 118.5 2810.5 24.1 2804.0 6. 0 2918.3 48.4 -0.2 0.1553 1.7 1.8299 1.1 0.1973 0.4 14.87 1.1 BT01-3 174.7 210.6 0.5044 1.1 75.7 2624.3 23.9 2790.7 15.8 2775.9 36.5 6.3 0.1472 1.3 1.9902 1.1 0.1957 1.0 13.56 1.5 BT01-4 224.3 25.2 0.5295 1.1 102.1 2739.1 23.8 2813.1 6.3 2691.8 48.2 2.7 0.1425 1.8 1.8888 1.1 0.1984 0.4 14.48 1.1 BT01-5 245.2 71.7 0.5600 1.1 118.0 2866.8 24.6 2797.8 6. 0 2715.3 36.7 -2.4 0.1438 1.4 1.7856 1.1 0.1966 0.4 15.18 1.1 BT01-6 224.1 68.1 0.5447 1.1 104.9 2802.8 24.4 2793.2 6. 4 2870.4 66.2 -0.3 0.1526 2.3 1.8361 1.1 0.1960 0.4 14.72 1.1 BT01-7 475.2 419.6 0.4736 1.0 193.3 2491.7 20.4 2710.5 5.6 2407.0 37.7 8.8 0.1265 1.6 2.1193 1.0 0.1864 0.3 12.13 1.0 BT01-8 253.1 61.3 0.5419 1.1 117.8 2791.0 24.1 2809.7 8.5 2679.5 112.3 0.7 0.1418 4.2 1.8457 1.1 0.1980 0.5 14.79 1.2 BT01-9 232.3 42.7 0.5316 1.1 106.1 2747.4 23.9 2806.2 8.1 2512.6 71.2 2.1 0.1324 2.8 1.8818 1.1 0.1976 0.5 14.48 1.2 BT0110 169.1 160.1 0.5108 1.1 74.2 2656.7 24.4 2793.3 8.2 2756.0 104.5 5.1 0.1461 3.8 1.9608 1.1 0.1960 0.5 13.78 1.2 BT0111 163.3 103.8 0.5503 1.1 77.2 2825.3 26.1 2804.8 7.5 2854.7 38.8 -0.7 0.1517 1.4 1. 8181 1.1 0.1974 0.5 14.97 1.2 BT0112 176.7 93.0 0.5474 1.1 83.1 2814.6 25.6 2800.3 7.1 2880.4 38.6 -0.5 0.1532 1.3 1. 8267 1.1 0.1969 0.4 14.86 1.2 4corr = 204 common Pb correction applied r and rad = radiogenic after common Pb correction

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35 CHAPTER 3 PRECAMBRIAN MAFIC DIKES OF THE MONTANA METASEDIMENTARY PROVINCE (MMP) Geologic Setting and Previous Work The Montana Metasedimentary Province (MMP) is located on the nor thwestern part of the Wyoming Province (Figures 1-1 and 1-2). The MMP is characterized by metasupracrustal sequences comprised of quartzofe ldspathic gneisses with dist inctive marble-iron formationquartzite associations, calcare ous gneisses, pelitic to mafic schists, amphibolites, and metaultramafic rocks (Mogk et al., 1992; Mueller et al., 1993). The protoliths for the high grade metamorphic rocks in the MMP are believed to be from deposition of volcanic and sedimentary rocks along a passive continen tal margin in Archean time (Harlan, 1992; Mogk et al., 1994). Gneisses range from 3.2 to 3.5 Ga and detrital zirc ons from the metasupracrustal sequences yield ages up to 3.9 Ga, comparable to published da tes within quartzites in the BBMZ (e.g. Mogk et al., 1992; Stevenson and Patchett, 1990, Mueller et al., 1982; Mueller et al., 1998). Magmatism in the MMP occurred at multip le times between 3.5-1.8 Ga. The 1.8 Ga magmatism is coeval with a significant metamorphic event that affected the rocks and led to their inclusion within the GFTZ (Wooden et al., 1978; Johnson and Sw app, 1989; Harlan, 1992; Harlan et al., 2005; Mueller et al., 2002). Mafic dikes from four ranges (Tobacco Root Mountains, Ruby Range, Highland Mountains, and Gravelly Range) within the MMP we re analyzed for this project. The Tobacco Root Mountains, Highland Range, and Ruby Range a ll contain country rock lithologies that are typical of the MMP (i.e ., Wooden, 1972; Wooden et al., 1978; Harlan et al, 2005; Johnson and Swapp, 1989). The Gravelly Range is compositionally similar to the other ra nges but lack pelitic rocks found within the MMP (ONeill and Christiansen, 2004) The Gravelly Range has exposed

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36 low-grade metamorphic rocks, ranging from green schist to lower amphibolite facies, whereas, the Tobacco Root, Ruby Range, and Highland M ountains have experienced upper amphibolite facies metamorphism last recorded at ~1.8 Ga (Johnson and Swapp, 1989; Harlan, 1992; Harlan et al., 2005; Mueller et al., 2005). Age Relations Dike swarms of the Tobacco Root and Ruby Range were separated into three groups (A, B, and C) by earlier workers (Wooden et al ., 1978, Johnson and Swapp, 1989, Harlan et al,. 2005) based on major elements (e.g., FeO*, MgO, and CaO), whole rock Rb/Sr (Wooden et al., 1978), Sm/Nd mineral isochrons (Harlan et al., 2005) correlation with k nown regional tectonic events (Johnson and Swapp, 1989; Harlan et al., 2005), and correlating paleomagnetic data (Harlan et al., 2005). Group A dikes are slightly altered, fine-to-me dium grained diabase, with clinopyroxene (augite) and plagioclase as primary minerals. Group A dikes from the Ruby Range gave a whole rock Rb/Sr isochron of 1455 125 Ma with an initial 87Sr/86Sr ratio of 0.7019.0008 (Wooden, 1975; Wooden et al., 1978). Group A and C dikes from the Tobacco Root Mountains gave both whole rock and individual mine ral (plagioclase a nd pyroxene) Rb/Sr and Sm/Nd isochrons of 1448 Ma (Harlan et al., 2005). Because of these similarities and because the paleomagnetic data for Group A and Group C dikes are essentia lly identical, it has been suggested that the Group C dikes were also emplaced at ~1450 Ma (Harlan et al., 2005). For reference, Group B dikes have clinopyrox ene and plagioclase as primary minerals, but also contain biotite as a mi nor primary mineral. Group B dike s have also experienced more alteration. The most commonly alte red mineral is plagioclase that is partially replaced by clays and micas and chloritization of biotite and pyr oxene (Wooden et al., 1978). Group B dikes also have generally lower MgO wt %, strong enrichme nt of FeO*, and whole rock Rb/Sr parallel

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37 isochrons of 1130 to 1160 Ma (Wooden et al., 19 78; Harlan et al., 2005 ). The paleomagnetic pole direction for Group B dikes is not compatible with pole directions established for ca. 1100 and 1450 Ma (Harlan et al., 1997; Harlan et al., 2005). There is correlation, however, of poles from Group B dikes with well da ted Neoproterozoic (ca. 780-723 Ma) igneous rocks from the Cordillera. In addition to these samples, Harlan et al. (2005) reports pale omagnetic data reported by Harlan et al. (1996) and Ha rlan et al. (1997) strongly suggest Group B dikes are Neoproterozoic in age. Samples SJM-18 and TR01 plot within Group A, with respect to fields (i.e., MgO vs CaO, K2O, Ni, Cr) lain out by ear lier workers (Wooden et al., 1978; Johnson and Swapp, 1989; Harlan et al., 2005) and are, therefore, in ferred to have an emplacement age of ~1450 Ma. Published U/Pb data (sample TR42; Harlan et al., 2005) from baddeleyite and zircon grains also indicate 1448 Ma emplacement age for sample TR01. Group A and C dikes are attributed to extension accompanying the formation of the Belt Basin (ca. 1450 Ma) (Johnson and Swapp, 1989; Harlan et al., 2005). The ca. 780 Ma mafic magmatism of Group B dikes is at tributed to the breakup of R odinia along the western boundary of Laurentia (Harlan et al ., 2005; Mueller et al., 2006). Tobacco Root Mountains The Tobacco Root Mountains are a north-tre nding, east-tilted bloc k raised along a high angle normal fault located on the western margin of the range (Burger, 2004). The majority of exposed rocks are high-grade Archean metamo rphic rocks that include quartzofeldspathic gneisses, but there are some Paleozoic and Mesozo ic rocks in the northern part of the range. The central and eastern portion of th e range contains granitic to di oritic intrusions of the Late Cretaceous Tobacco Root batholith (Burger, 2004). The southern part of the range is contains dike swarms that show a uniform strike of WNW and range in composition from unaltered to

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38 metamorphosed mafic and ultramafic rocks (i.e ., Wooden, 1975; Wooden et al., 1978; Harlan et al., 2005). The metamorphosed dikes have been extensiv ely studied by earlier workers (Reid, 1963; Cordua, 1973; Brady et al., 2004) and in summar y; the MMDS (metamorphosed mafic dikes and sills) of the Tobacco Root Mountains are describe d as diabase intrusions were metamorphosed at ~1.8 Ga metamorphic event of the MMP/ GFTZ. Co mpared to samples from this project from the same region, the MMDS has a similar SiO2 contents (48% and 52%), similar TiO2 (~.5 to 2%), general mineralogy (clinopyroxene and pl agioclase as primary minerals) to the unmetamorphosed dikes of the MMP (Brady et al., 2004; Burger et al., 2004). Trace element chemistry is reported for the MMDS in Brady et al. (2004) and is al most identical (i.e., range of LREE enrichment is 10-100 times chondritic va lues; enrichment of HREE is ~10 times chondritic values) to values fo r the MMP dikes (Figure 5b). Ruby Range The Ruby Range is an uplifted Precambrian bl ock whose exposed core is comprised of deformed, high grade, metamorphic rocks (i.e., qu artzofeldspathic gneiss es) (Garihan, 1979), in which three major stratigraphic divisions we re recognized; Cherry Creek Group, the Dillon quartzofeldspathic gneiss, and the pre-Cherry Creek rocks (oldest). Garihan (1979) reports a complex history of recurring intrusions, metamo rphism, deposition, and r eactivation of faults. The dike sampled from the Ruby Range is locat ed in the southern portion of this range (Figure 1b) and its field charac teristics are listed in Table 2-1. Early workers (Wooden, 1975; Garihan, 1979 and references with in) report that metabasites are the primary mafic intrusion in the area. These metabasites are compositionally similar to RR01 and have the same mineral assemblages (Garihan, 1979). Garnet is presen t in both the reported metabasites and RR01, although RR01s garnet is attributed to post em placement metamorphic process. RR01s exposed

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39 at the surface was very limited and therefore difficult to interpret its field re lations as a dike or a possible mafic pod or lens. Due to the high total alkali content (Na2O + K2O=8.5 wt. %) (Table 2-2) and the presence of garnet, it is not considered likely that this sample is a pristine intrusion, but and its bulk composition likely refl ects alteration to a significant degree. Highland Mountains The Highland Mountains are similar to th e Tobacco Root Mountains and Ruby Range, i.e., a block of mainly Archean rocks uplifted during the Laramide Orogeny of Late Cretaceous and early Cenozoic time (Johnson and Swapp, 1989; Harrison et al., 1974). The southern Highland Range consists of Archean quartzofeld spathic gneiss similar to what is present throughout the MMP, although rocks such as marbles and pelitic schists that are present in the Tobacco Root Mountains and Ruby Range have not been reported in the Highland Mountains (Harlan, 1992). Dikes occur in two separate groups disti nguished by strike; an east-west group and a northwest-southeast group. The east-west trending dikes are more numerous than the northwest trending dikes and Johnson and Swapp (1989) exam ined their intersection in an attempt to determine relative age. They noted that no petr ologic or geochemical di fferences were evident and proposed that both groups were from the same emplacement event and source. Unfortunately, the age of the Highland Range di kes is poorly constraine d (Harlan, 1992; Johnson and Swapp, 1989), but is proposed to be Mesopr oterozoic (~1.45 Ga) based on two arguments presented above. Sample SJM-18 is believed to be ~1.45 Ga due to its geochemical similarities to Group A dikes as discussed above. Also, dikes in the Highland Mountains cut basement rocks that have been metamorphosed at 1.8 Ga (i .e., Harlan, 1992; Johnson and Swapp, 1989) implying an age of less than 1.8 Ga. Although moderate al teration of HM03 is reported in Table 2-1, it was not metamorphosed during the 1.8 Ga metamo rphic event (Mueller et al, 2004, Brady et al,

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40 2004), but was more likely affected by locali zed hydrothermal alteration (Johnson and Swapp, 1989). Results Major, trace element, and isotopic ratios are reported in Table 2-2. Samples from the MMP have a range of 46-51 wt. % SiO2 and, except for RR01 and KG, are similar with respect to other oxides. All samples fall near the tholeiitic-cal c alkaline transition line in Figure 2-1. Sample RR01 contains potassium feldspar and minimal plagioclase, wher eas no potassium feldspar was noted in any other MMP samples (Table 2-1). RR01 is the only MMP sample with metamorphic garnet porphyoblasts. Further examination in thin section indicates that this is post emplacement metamorphism due to the orientation of clinopyr oxene around the porphyoblas ts of garnet. These observations, along with unclear fi eld observations make it difficult to interpret the composition or age of RR01 with any certainty. Sample KG from the Gravelly Range is also distinct in having twice as much Al2O3, but only half of th e total iron and TiO2 as the other samples in the MMP. Trace element abundances (Figure 2-3b) for samples within the MMP show variable depletion of high field strength elements (H FSE) when normalized to primitive mantle. The Highland samples (HM03, SJM-10, and SJM-18) are the most depleted in some HFSE, especially in Zr relative to other samples in the MMP (Figure 2-3b). The Gravelly Range samples (247FM and KG) show enrichment of U as well as Pb relative to primitive mantle and other MMP samples. Sample RR01 shows HFSE depletions, but also show large (100x) enrichment of K with respect to primitive mantle. All samples from the MMP have HREE concentrations ~10-20 times and LREE of 10-50 times chondritic values (Figure 2-4b), with the exception of the two samp les from the Gravelly Range (247FM and KG). Both of these samples s how a slightly greater enrichment of LREE (80100 times chondritic values), but similar HREE abunda nces to the rest of the MMP samples. All

PAGE 41

41 samples from the MMP show a pattern similar to calc-alkaline basalt s (Figure 2-4b) (Wilson, 1989) and also similar to samples from the MMP in Burger et al. (2004) Figure 2-2 indicates that samples from the MMP, using an incompatible trace element ternary diagram, show variation across different compositi onal/tectonic settings (i.e., MORB, IAB, and calc-alkaline). The three samples that fall into field A (island arc tholeiites) are all from the Highland Range, whereas the other four (KG, 247FM, RR01, and TR 01) samples scatter between fields B and C. It is important to note that none of the MMP samples plot into field D of Figure 2-2, indicating that they are not likely to be within -plate basalts (i.e., plume related). Although discussed in the same context due to some geochemical and geographic similarities, there are differences in isotopic systematics between the samples within the MMP. The 147Sm/ 144Nd values of the MMP dikes (Table 2-2) are high and pose a difficulty for calculating TDM and TCHUR values. As with TDM and TCHUR, both samples HM03 and SJM-18 exhibit initial Nd values that are unlikely to be accurate. Both show a change of just one epsilon unit from initial to present day values (Table 2-2). Initial Nd values for samples in the MMP range from 5 to -8, whereas present day Nd values yield a range of -1.62 to -25.63. Due to the range and high Sm/Nd ratios, only two initial Nd values (SJM-10 and TR01) were plotted for the MMP (Figure 8) because the age assignments can be made more comfortably. A sample from the MMDS of the Tobacco Root Mountains reported in Mueller et al., (2004) was also plotted on Figure 8 to provide initial Nd values for 2.06 Ga from U/Pb zirc on ages. Whole rock Pb data for samples within the MMP are listed in Table 2-2. 206Pb/204Pb ratios vary between 16.252 and 25.407, 207 Pb/204Pb ratios vary from 15.268 to 16.387. Discussion Samples from the MMP show a wide compositional variation and resemble compositions of basalt and basaltic andesite due to the wt % of SiO2, TiO2, Al2O3, and total iron, with the

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42 exception of RR01, which has distinctly higher total alkali of 8.5 wt. % and lower TiO2 than the other MMP samples. Due to these differences, th e presence of garnet (likely post-emplacement), and a poor understanding of field relations, RR01 is not used in the source evolution interpretations for the MMP. Due to the depletion in HFSE (Nb, Ta, Ti) for all samples from the MMP (Figure 2-3b), the source is interpreted to be similar to the metasomatized mantle created in the mantle wedges in modern island arc environments. REE patterns for the MMP have slight enrichments in LREE, but not to the extent of average calc-alkaline basa lt (Figure 2-4b and 2-4c). This is also shown in Figure 2-1 where dikes from the MMP plot near the transition lin e for tholeiitic/calc-alkaline. Island-arc tholeiitic basalts usua lly have LREE depleted patterns whereas calc-alkaline basalts are typically LREE enriched (Wilson, 1989). Sm/Nd ratios and Nd values indicate the source may have had parent/daughter ratios reset ca. 2.0 Ga (Figure 3-1). Although this age is co nsistent with the pr oposed Paleoproterozoic metamorphic event within the western MMP (Johnson and Swapp, 1989; Mueller et al., 2004; Harlan et al., 2005), difficulties calculating TDM and TCHUR values arise due to high 147Sm/ 144Nd ratios for 5 of the 7 MMP samples (0.1622.2030) (TR01, HM03, SJM-10, SJM-18, and RR01). The two highest values of 147Sm/144Nd are for samples HM03 (0.1953), which produces a TDM of 6.2 Ga, and SJM-18 (0.2030), which produces a TDM of 8.5 Ga. Although th is should cause concern for the integrity of the REE and tr ace element data, HM03 and SJM-18 show close similarities to the Highl and sample (SJM-10 has a TDM and TCHUR of 2.8 Ga and 2.2 Ga, respectively) in major, trace, and isotopic ratio s (Sm/Nd and U-Pb). Consequently, the rather extreme TDM for these samples more likely reflects the difficulty in extrapolating evolution of samples with Sm/Nd near that of CHUR and depleted mantle (DM).

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43 Whole rock lead isotopes for the MMP produc e a ~1.9 Ga age (Figure 3-2). This age is indistinguishable from either the proposed ~ 1.8 Ga metamorphic event within the MMP or the Sm/Nd reference line of ~2.0 Ga. Metasomatism of source (s) for the MMP dikes is likely constrained to this time frame of ~1.9-2.0 Ga.

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44 0.5120 0.5122 0.5124 0.5126 0.5128 0.5130 0.150.160.170.180.190.200.21147Sm/144Nd143Nd/144Nd Reference Lines are ~ 2.0 Ga RR01 SJM-10 HM03 SJM-18 TR01 Figure 3-1. Sm/Nd plot for samples within th e MMP form two closely parallel reference lines that represen t a ~2.0 Ga age, which were calculated using Isoplot 3.2 (Ludwig, 2004). The Gravelly Range samples, KG and 247 FM, were not included in this graph due to the different history of th e Gravelly Range from the rest of the MMP with respects to Sm/Nd isotopic systematics.

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45 15.0 15.4 15.8 16.2 16.6 1416182022242628206Pb/204Pb207Pb/204Pb Approximate "Age" = ~ 1.9 Ga Figure 3-2. Whole rock 206Pb/204Pb vs. 207Pb/204Pb diagram for samples within the MMP. Age for these samples is actually a reference line a nd although there is variation of composition within the dikes of the MMP, the da ta suggest that the source material(s) within the MMP is isotopically similar with respect to Pb.

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46 CHAPTER 4 EOCENE MAFIC DIKES OF THE M ONTANA ALKALI PROVINCE (MAP) Geologic Setting and Previous Work The Crazy Mountains and the Ca stle Mountains are part of the high-potassium igneous province of Montana (referred to as the M ontana Alkali Province (M AP)) (Hearn et al, 1992; Duds et al., 1987) (Figure 1-2). This petr ologic province contains unique Mid-Eocene, feldspathoidal, mafic, alkalic rocks that are char acterized by a strong enrich ment in incompatible elements with respect to primitive mantle and have isotopic compositions (Rb/Sr, Sm/Nd, and Pb/Pb) that reflect an ancient source (Hearn et al ., 1989). In addition to these unique mafic rocks, there are also felsic varieties of the feldspathoidal rocks which are texturally and compositionally different from their mafic counterparts. The MAP magmas, at one time, were thought to be penetrating Archean basement (Duds and Eggler, 1989), however, U-Pb zircon ages from mafic and felsic igneous and metamorphic rocks within the Li ttle Belt Mountains give domina ntly Paleoproterozoic ages (1.86 Ga; Mueller et al., 2002; Vogl et al., 2005) indicating little to no Archean component. These rocks within the Little Belt Mountains were also analyzed for trace elements and Sm/Nd isotopic ratios, which show characteristics of petroge nesis in a convergent zone where juvenile lithosphere was subducted (i.e., Mue ller et al., 2002; Vogl et al., 2005; Foster et al., 2006). This convergent boundary is referred to as the Great Falls tectonic z one (ONeill and Lopez, 1985; Mueller et al., 2002; Vogl et al., 2005; Foster et al., 2006) The GFTZ is postulated to be Paleoproterozoic collisional orogen between the Archean Wyoming and Medicine Hat provinces (O Neill a nd Lopez, 1985; Mueller et al., 2002; Foster et al., 2006). This is a zone contai ning NE trending faults, intrus ions, and depositional patterns which range in age from Proterozoic to Te rtiary (ONeill and Lopez, 1985). Even though

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47 completely covered by Proterozoic and Phaner ozoic sedimentary rocks, the GFTZ provides insight into Proterozoic accreti onary processes along the western margin of Laurentia (Dahl et al., 1999; Mueller et al., 2005). Th e western section (i.e. Tobacco Root and Highland Mountains) of the GFTZ (Figure 1a) include s Archean rocks with metamorphic zircon and monazite ages ~ 1.77 Ga (Harlan, 1996; Mueller et al., 2004; Mueller et al, 2005; Foster et al., 2006), while the northwestern portion of the GFTZ appears to be Paleoproterozoic (see above). With the exception of the uplifted exposed cores in the Little Belts a nd Little Rocky Mountains, the basement is concealed in MAP. It is this c oncealment that causes inherent difficulty in understanding where Archean basement (i.e., Bear tooth Mountains) to the south ends and where Proterozoic (i.e., Little Belt Mountains) basement to the north begins. Between 69 and 27 Ma, the MAP experienced alkalic magmatic activity focused in eight major centers and multiple minor centers (Hearn et al., 1989). The main periods of magmatism took place between 69-60 Ma and 54-50 Ma. In the early Eocene, numerous locations experienced this alkalic igneous activity (i.e., Cr azy, Castle, and Little Belt Mountains; Figure 11 and 1-2) (Hearn, 1989; Duds and Eggler, 1989) Mafic dikes were sampled from the Crazy and Castle Mountains, which are desc ribed in McDonald et al (2005). Castle Mountains The Eocene Castle Granite, Blackhawk Diorite and a porphyritic da cite are exposed in the Castle Mountains, Montana, where they intrude Precambrian Belt series rocks and younger Paleozoic and Mesozoic sedimentary rocks. Samp le CA04 is a malignite dike from the eastern side of the Castle Mountains that radiates from the Comb Creek Laccolith, a nepheline syenite, and intrudes the Bearpaw shale (Duds and Eggler 1989). This laccolith lies ~10km south of this exposure and includes dikes and sills that have compositions of phonolite, trachyte, malignite, and lamprophyre (Duds and Eggler, 1989).

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48 Crazy Mountains The mafic intrusions of the Crazy Mountains were emplaced into Cretaceous and Paleocene sedimentary strata; adjacent basement cored uplifts include the Archean-cored blocks of the Beartooth Mountains to th e south, and the Proterozoic rocks of the Little Belt Mountains to the northwest. Rb/Sr mineral isochrons for the mafic igneous activity in the area produce an ~48 Ma age for the Crazy Mountains (Duds et al., 1987) and is used as the reference age for these samples. The mafic alkalic rocks (including malignite, nepheline syenite, analcite, syenite, theralite, trachyte porphyry, a nd quartz latite) are sodium-rich silica-undersaturated, strongly alkaline, and were emplaced as dikes, sills, phacc oliths, and laccoliths that are generally located in the northern half of the Crazy Mountains. Samples CZ01 and CZ02 are dikes located in the northwestern portion of the Crazy Mountains (Figure 1b) and are di scussed in Duds and Eggler (1989). Sample CZ01 is a malignite with the reported highest 143Nd/144Nd of any sample analyzed in the Crazy Mountains (Duds and Eggler, 1989). Sample CZ02 is a minette with reported xenoliths ranging in composition from amphibolite to clinopyroxenite with some sedimentary xenoliths occurring (Duds and Eggler, 1989), although none were observed. Results The field characteristics and geochemical data for samples from the MAP are presented in Tables 2-1 and 2-2, respectively. The samp les from this area range from 44-49.5% SiO2, but CZ02 has lower values for Na2O, K2O, and P2O5 wt. % (0.96, 1.01, and 0.22, respectively) than CA04 (6.4, 2.48, and 0.92) and CZ01 (8.6, 2.43, and 1.14). CZ02 (2.06 and 13.60) has the highest TiO2 and total iron wt % compared to CA04 (0.53 and 8.06) and CZ01 (0.45 and 8.90) (Table 2). These differences refl ect variations in mineralogy. For example, CZ02 has twice as

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49 much magnetite and contains no nepheline co mpared to CA04 and CZ01. Normatively, CZ02 has 7% olivine (Table 2-2), but olivine is not present in thin section or hand sample. For the normalized trace element diagram (F igure 2-3c) samples from the MAP show a tendency for depletion in the high field strength elements (HFSE) (i.e. P, Ti, Zr, Hf). CZ02 has the largest depletion of P, but ha s the smallest depletion of Ti co mpared to the other two samples (Figure 2-3c). The REE plot (Figure 5c) for the MAP shows a strong enrichment in the LREE (~800x chondritic values) for CZ01 and CZ02 a nd a slight enrichment in the HREE (20x chondritic values). CA04 shows a slightly le ss enriched LREE pattern (~500x chondritic values) in comparison to CZ01 and CZ02. Sm-Nd isotopic data show a variation in initial nd of -3 to -12 and a present day value of -2.75 to -11.6. It is interes ting to note that the low 147Sm/ 144Nd ratio of CZ01 (0.07584) is paired with the high 143Nd/144Nd (0.51246) (Table 2-2). Sample s CA04 and CZ02 have similar 207Pb/204Pb and 206Pb/204Pb ratios, whereas, CZ01 has the highest ratios of the MAP samples (Table 2-2). Discussion Due to the high alkalic compositions of th e MAP dikes, presentation within Figure 2-1 proved to be misleading. The high Na2O wt % for sample CZ01 (8.5) compared to CA04 and CZ02 is attributed to the amount of nephelin e present in CZ01 (~30 %). CA04 does have nepheline present (~15-20 %) but at lesser extent than CZ01. The presence of feldspathoidal rocks within this petrogra phic province is consistent with the observed high Na2O relative to K2O reported by Duds et al., (1987), Duds and Eggler (1989), and H earn et al., (1989). The REE patterns (Figure 2-4c) for the MAP s how strong enrichment in LREE relative to HREE. This pattern is similar to modern alka li basalt (Wilson, 1989; Winter, 2001).Variations in the concentration of Cr and Ni, the presence of phologopite in CZ01 and CZ02, and lack of

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50 garnet within the MAP samples, suggests that th e source(s) of these dikes was likely to be a metasomatized, clinopyroxene rich mantle and le ss likely to be predomin ately peridotitic type source (Hearn et al., 1989). Sm/Nd data for samples in the MAP, when co mbined with samples reported in Hearn et al. (1989), show a diverse range of ra tios for Sm/Nd with less variance in 143Nd/144Nd ratios. Values for CZ01 and CZ02 are within error of pu blished ratios for samples within the Crazy Mountains (Duds et al., 1987; Duds and Eggl er, 1989). Consequently, there is no secondary array for these samples. Whole rock U/Pb data form an array indicativ e of an age of ~1.8 Ga (Figure 4-1). This age represents a time of resetti ng of parent/daughter isotopic ra tios of the source for the MAP magmas, likely from metasomatism of the source ma terial initiated in a convergent environment. This age is also undistinguishab le from the Sm/Nd and U/Pb data from the MMP (Figure 3-1 and 3-2, respectively). The whole rock U/Pb da ta for the MAP allow us to propose that the subduction related metasomatism is likely attribut ed to the Great Falls collision between the Wyoming Province and the Hearne-Medicine Ha t Block at ~1.86 Ga (Mueller et al., 2002; Mueller and Frost, 2006; Foster et al., 2006).

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51 15.34 15.36 15.38 15.40 15.42 16.716.917.117.317.5206Pb/204Pb207Pb/204Pb Approximate "Age"= ~1.8 Ga Figure 4-1. Whole rock 206Pb/204Pb vs. 207Pb/204Pb isotopic diagram for MAP samples. MAP Pb/Pb data form an array with an age of ~ 1.8 Ga.

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52 CHAPTER 5 CONCLUSION The Wyoming Province has a diverse and comp licated geologic history that includes intrusion of both Archean and Proterozoic mafic di kes. All samples for this study are borderline tholeiitic to calc alkaline in composition. Normalized trace element diagrams for samples within all three areas show a range of de pletions in high field strength el ements (i.e. Nb, Ta, Ti) that is characteristic of modern conve rgent margin magmatism. REE pa tterns differ, particularly in terms of LREE enrichment, suggesting distinct sources and/or petrogenesis. These distinct patterns are consistent with modern magmatic arc signatures, suggesting that the source of these dikes was the subcontinental lithosphere and that this lithosphere was modified by distinct episodes of Archean and Proter ozoic subduction and mantle meta somatism. This metasomatism, in turn, reset isotopic parent/daughter ratios fo r the source material of the dikes within the BBMZ, MMP, and MAP. Pb and Nd isotopes from mafic dikes within the BBMZ indicate that this metasomatic event occurred at ~3.1-3.4 Ga, whereas Pb and Nd isotopic data for the MMP and MAP are indistinguishable from one anot her at ~1.9-2.0 Ga and ~1.8 Ga, respectively.

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53 APPENDIX MATERIALS AND METHODS Sampling Strategy Diabase dikes and sills were sampled using lo cations within the citied literature coupled with additional samples located during fieldwork. These dikes and sills were examined to determine physical weathering and metamorphism of the rock units. In addition to these above mentioned samples, samples SJM-10, SJM-18, and KG were collected by The Keck Group and processed along with the samples that I collected. Sample Processing Samples were all processed using standard crushing procedures (i.e jaw crusher, disk mill), and mineral separation using water table t echniques with the heavy mineral separates used. This fraction was then sieved using a number 50 (350 microns) pan. From this sieved fraction, heavy liquids were used to further separate hi gher density minerals from lower densities. The heavy liquids used were Tetramethylbromide (T BE), which has a reported density of 3.06g/cc3, and Methyl Iodide (MeI), which has a reporte d density of 3.26 g/cc3. Zircons were then extracted at the smallest degr ee possible at 1.5 amps using a Fr antz 300M Magnetic Separator. These nonmagnetic fractions were then separated into the best least fr actured zircons using a microscope. All collected samples were also processed in the ball mill for fine rock powders. These powders were used for major, trace element, a nd whole rock isotope determination. For major elemental analysis, Rock powders were sent o ff-premise for x-ray fluorescence and are reported in Table 2. The remaining rock powders were pro cessed at the University of Florida. For trace elements and isotopic data, these powders were disso lved in sealed Teflon vials for several days

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54 at 100 C in HF-HNO3 mixture. Trace element data was acquired using Element ICP-MS and were corrected for standard and machine drift. Radiogenic isotopic analyses were performed at the Department of Geological Sciences at the University of Florida. Sr, Pb, and Nd were separated using standard chromatographic methods. Once separated, whole rock Pb, Sm-Nd, and Rb were analyzed using Nu Plasma Multiple-Collector magnetic separator Inductive ly Coupled Mass Spectrometer (MC-ICP-MS), whereas Sr was analyzed using a Micromass S ector 54 Thermal Ionization Mass Spectrometer (TIMS). Samples and standard solutions analyzed on the (MC-ICP-MS) were aspirated into the plasma source either via a Micromist nebulizer with GE spray chamber (wet plasma) or through DSN-100 desolvating nebulizer (dry plasma ), depending on the Nd concentration. The instrument settings were carefully tuned to maximize the signal intensities on a daily basis. Preamplifier gain calibration was performed be fore each analytical session. Nd isotope measurements were conducted for 60 ratios in static mode simultaneously acquiring 142Nd on low-2, 143Nd on low-1, 144Nd on Axial, 145Nd on high-1, 146Nd on high-2, 147Sm on high-3, 148Nd on high-4, and 150Nd on high-5 Faraday detectors. The measured 144Nd, 148Nd, and 150Nd beams were corrected for isobaric interference from Sm using 147Sm/144Sm = 4.88, 147Sm/148Sm = 1.33, and 147Sm/150Sm = 2.03. All measured ratios were normalized to 146Nd/144Nd = 0.7219 using an exponential law for mass-bias co rrection. The mean value of 143Nd/144Nd for our Ames Nd inhouse standard based on 23 repeat analys es during the samples analyses was 0.512140 (2 = 0.000012). Three repeat analyses of the JNdi-1 a nd LaJolla Nd standards during the same time interval produced mean values of 0.512106 (2 = 0.000013) and 0.511856 (2 = 0.000013), respectively. Three separate di ssolutions of USGS SRM BCR-1 we re prepared and analyzed for

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55 Nd isotopes together with the samples in order to further evaluate the analytical protocol. The mean value of 143Nd/144Nd for the analyses of BCR-1 was 0.512645 (2 = 0.000011), which is indistinguishable from the published TI MS value of 0.51264 (Gladney et al., 1990). Pb isotopic analyses were conducted on a Nu Plasma multi collector ICP-MS using the Tl normalization technique on fresh mixtures to prevent oxidation of thallium to Tl3+ (for more details see Kamenov et al. 2004, and Kamenov et al, 2005). Analyses of NBS 981 conducted in wet plasma mode together with the samp le analyses gave the following results: 206Pb/204Pb=16.937 (+/-0.004 2 ), 207Pb/204Pb=15.490 (+/-0.003 2 ), and 208Pb/204Pb=36.695 (+/0.009 2 ). Due to the low Pb content the ultramafic xenoliths were analyzed in dry plasma mode and the reported data are relative to the following NBS 981 values (n=29): 206Pb/204Pb=16.937 (+/-0.001, 2 ), 207Pb/204Pb=15.491 (+/-0.001, 2 ), and 208Pb/204Pb=36.694 (+/-0.004, 2 ). The TIMS is equipped with seven Faraday co llectors and one Daly collector. Sr samples were loaded on oxidized W singl e filaments and run in dynamic collection mode. Data were acquired at a beam intensity of 1.5V for 88Sr, with corrections for in strumental discrimination made assuming 86Sr/88Sr=0.1194. Errors in measured 87Sr/86Sr are better th an +/0.00002 (2 ) based on long-term reproducibility of NBS 987 (87Sr/86Sr=0.71024). Rb was analyzed by both mass spectrometers to develop an accuracy curve and determine whether a certain machine produces higher precision data. Zircon Separation Zircons were separated with the aid of a mi croscope. Using the lowest magnetic angle at 1.5 amps, the zircons were hand picked on the basi s of shape, size, and degree of inclusions, although due to the scarcity of zi rcons within these samples, any grain thought to be a zircon was extracted. The grains that were smaller than 50 microns were analyzed on the Sensitive High

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56 Resolution Ion Micro Probe (SHRIMP), by Dr. Pa ul Mueller and Dr. Joe Wooden. The grains larger than 50 microns were mounted on epoxy di sks and polished to expose grain centers. The disks also contained the standard Forest Center (FC-1). Forest Ce nter is a gabbro that has been precisely dated at 1099 Ma and is located in Duluth Minnesota (Paces and Miller, 1993). Titanite Separation Titanite was extracted from one sample (B TO2) and analyzed using the ICP-MS as mentioned above. This procedure was administ ered by Sam Coyner and used jointly for his research.

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57 LIST OF REFERENCES Baadsgaard, H. and Mueller, P.A., 1973. K-Ar and Rb-Sr ages of intrusive Precambrian mafic rocks, southern Beartooth Mountains, Montana and Wyoming: Geological Society of America Bulletin, v.84, p. 3634-3644. Burger, H. R., 2004, General geology and tectonic setting of the Tobacco Root Mountains in Precambrian Geology of the Tobacco Root Mountains, Montana: Boulder, Colorado Geological Society of America, Special Paper 377, p. 1-14. Brady, J.B., Mohlman, H.K., Harris, C., Carm ichael, S.K., Jacob, L.J., Chaparro, W.R., 2004, General geology and geochemistry of metamorphosed Proterozoic mafic dikes and sills, Tobacco Root Mountains, Montana in Precambrian Geology of the Tobacco root Mountains, Montana: Boul der, Colorado Geological Society of America, Special Paper 377, p. 89-104. Boerner, D.E., Crave, J.A., Kurtz, R.D., Ross, G.M., and Jones, F.W., 1997, The Great Falls tectonic zone: suture or intracontin ental shear zone?: Ca nadian Journal of Earth Science, v. 35, p. 175-183. Casella, C.J., Levay, J., Eble, E., Hirst, B ., Huffman, K., Lahti, V., Metzger, R., 1982, Precambrian geology of the southwestern Beartooth Mountains, Yellowstone National Park, Montana and Wyoming: Montana Bureau of Mines and Geology, Special Publication 84, p. 1-24. Chamberlain, K.R., Frost, C.D., Frost, B. R., 2003, Early Archean to Mesoproterozoic evolution of the Wyoming Province: Arch ean origins to modern lithospheric architecture: Canadian Journa l of Earth Sciences, v.40, p.1357-1374. Cordua, W.S, 1973, Precambrian geology of th e southern Tobacco Root Mountains, Madison County, Montana [Ph.D. disse rtation]: Bloomington Indiana University, 247 p. Corra da Costa, P.C., Girardi, V.A.V., Teixeira, W., 2006. 40Ar/39Ar and Rb/Sr geochronology of the Gois-Crixs dike sw arm, Central Brazil: constraints on the Neoarchean-Paleoproterozoic tectonic boundary in South America, and Nd-Sr signature in the subcontinental mantle: Inte rnational Geology Review, v.48, p.547-560. DePaolo, D.J., 1980, Sources of continental crust: neodymium isotope evidence from the Sierra Nevada and Peninsular ranges: Science, v. 209, p.684-687. Duds, F.., Carlson, R.W., Eggler, D.H. 1987. Regional Middle Proterozoic enrichment of the subcontinental mantle source of igneous rocks from central Montana, Geology, v.15, p. 22-25.

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58 Duds, F. and Eggler, D.H., 1989, Crazy Mountains, Montana, in Montana High Potassium Province, Crazy Mountains to Jordan, Montana: International Geologic Congress Field Trip 346 Guidebook, p. 7-22. Fahrig, W.F., and West T.D., 1986, Diabase dyke swarms of the Canadian Shield: Geological Survey of Canada Map 1627a. Faure, G. and Mensing, T.M, 2005, Isotopes: principles and applications: Wiley and Sons, Hoboken, New Jersey, 896 p. Foster, D.A., Mueller, P.A., Mogk, D.W., W ooden, J.L., and Vogl, J.J., 2006, Proterozoic evolution of the western margin of the Wyom ing craton: implications for the tectonic and magmatic evolution of the northern Roc ky Mountains: Canadian Journal of Earth Sciences, v. 43, p. 1601-1619. Frost, B.R., Chamberlain, K. R., Swapp, S., Frost, C.D., Hulsebosch, T.P., 2000, Late Archean structural and metamorphic hist ory of the Wind River Range: Evidence for a long-lived active margin on the Archean Wyoming craton: Geological Society of America Bulletin, v. 112, no. 4, p. 564-578. Garihan, J.M., 1979, Geology and structure of the central Ruby Range, Madison County, Montana: Summary: Geological Societ y of America Bulletin, v. 90, p. 323-326. Giletti, B.J., 1966, Isotopic ages from sout hwestern Montana: Journal of Geophysical Research, v. 71, no. 16, p. 4029-2036. Greenough, J.D., and Kyser T.K., 2003, Cont rasting Archean and Proterozoic lithospheric mantle: isotopic evidence from the Shonkin Sag sill (Montana): Contributions to Mineralogy and Petrology, v. 154, p. 169-181. Harlan, S.S., Geissman, J.W., Lageson, D. R., Snee, L.W., 1988. Paleomagnetic and isotopic dating of thrust-belt deformati on along the eastern edge of the Helena salient, northern Crazy Mountains Basi n, Montana: Geological Society of America Bulletin, v.100, p. 492-499. Harlan, S.S., 1992, Paleomagnetism and 40Ar/39Ar geochronology of selected Proterozoic intrusions, s outhwest Montana, southeas tern Wyoming, and central Arizona: Ph.D. dissertation, University of New Mexico, Albuquerque, NM, 170 p. Harlan, S.S., Geissman, J.W., Snee, L. W., Reynolds, R.L., 1996, Late Cretaceous remagnetization of Proterozoic mafic dikes, southern Highland Mountains, southwestern Montana: A paleomagne tic and 40Ar/39Ar study: Geological Society of America Bulletin, v. 108, no. 6, p. 653-668.

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59 Harlan, S.S., Geissman, J.W., Snee, L.W., 1997, Paleomagnetic and 40Ar/39Ar geochronologic data from Late Proteroz oic mafic dikes and sills, Montana and Wyoming: USGS Professional Paper1580, 16 p. Harlan, S. S., Heaman, L., LeCheminant, A. N., Premo, W. R., 2003, Gunbarrel mafic magmatic event: A key 780 Ma time marker for Rodinia plate reconstructions: Geology, v.31, p.1053-1056. Harlan, S. S., Premo, W. R., Unruh, D., Geissman, J. W., 2004, Isotopic dating of Meso-and Neoproterozoic mafic magmatism in the southern Tobacco Root Mountains, Southwestern Montana: Precam brian Research, v. 136, p. 269-281. Hearn, B.C., Dudas, F.O., Eggler, D.H., H yndman, D.W., OBrien, H.E., McCallum, I.S., Irving, A.J., Berg, R.B., 1989, Montana Hi gh Potassium Igneous Province: Field Trip Guidebook T346: 28th International Geolog ical Congress, 79 p. Hoffman, P.F., 1988, United plates of America, the birth of a craton: Early Proterozoic assembly and growth of Laurentia: Annual reviews of Earth and Planetary Sciences, v.16, p.543-603. Irvine, T.N. and Baragar, W.R., 1971, A guide to the chemical classification of the common volcanic rocks: Canadian Jour nal of Earth Sciences, v. 8, p. 523-548. Johnson, E.L., and Swapp, S.M., 1989, The geochemi stry and structural significance of a set of Middle Precambrian diabase dikes from the Highland Range, southwestern Montana, Canadian Journal of Earth Science, v. 26, p. 119-128. LeBas, M.J., Le Maitre, R.W., Streckei sen, A., and Zanettin, B., 1986. A chemical classification of volcanic rocks based on the total alkali silica di agram: Journal of Petrology, v. 27, p. 745-750. LeCheminant, A.N., and Heaman L.M., 1989, Mackenzie igneous events, Canada: Middle Proterozoic hotspot magmatism associated with ocean opening: Earth and Planetary Science Letters, v. 96, p. 38-48. Ludwig, K.R., 2005, Isoplot: Regression analys is for radiogenic isotopes software, Version 3.4. McDonald, C., Lopez, D.A, Berg, R.B., Gi bson, R.I., 2005, Preliminary Geologic Map of the Ringling 30 x 60 Quadrangle, Centra l Montana, Montana Bureau of Mines and Geology, Open File Report 511, Mogk, D.W., Mueller, P.A., and Wooden, J. L., 1992, The nature of Archean terrane boundaries: an example from the nor thern Wyoming Province: Precambrian Research, v. 55, p. 155-168.

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60 Mueller, P.A., 1971, Geochemistry and geochr onology of the mafic rocks of the southern Beartooth Mountains, Montana and Wyoming, Ph.D. dissertation, Rice University, Houston, TX, 58 p. Mueller, P.A., and Rogers, J.J.W., 1973, Secu lar chemical variation in a series of Precambrian mafic rocks, Beartooth Mountains, Montana and Wyoming: Geological Society of Amer ica Bulletin, v. 84, p. 3645-3652. Mueller, P.A., and Wooden, J.L., 1988, Evid ence for Archean subduction and crustal recycling, Wyoming province: Geology, v.16, p. 871-874. Mueller, P.A., Shuster, R.D., Wooden, J.L ., Erslev, E.A., Bowes, D.R., 1993, Age and composition of Archean crystalline rock s from the southern Madison Range, Montana: implications for crustal evolu tion in the Wyoming craton: Geological Society of America Bulletin, v. 105, p. 437-446. Mueller, P. A., Heatherington, A. L., DArc y, K. A., Wooden, J. L., Nutman, A.P., 1996, Contrasts between Sm-Nd whole-rock and U-Pb zircon systematics in the Tobacco Root batholith, Montana: implications for the determination of crustal age provinces: Tectonophysics, v. 265, p. 169-179. Mueller, P.A, Heatherington, A.L., Kell y, D.M., Wooden J.L., and Mogk, D.W., 2002, Paleoproterozoic crust with the Great Falls tectonic zone: implications for the assembly of southern Laurentia: Geol ogical Society of America, v.30, no. 2, p. 127-130. Mueller, P.A., Burger, H.R., Wooden, J.L., Heatherington, A.L., Mogk, D.W., and DArcy, K., 2004, Age and evolution of th e Precambrian crust of the Tobacco Root Mountains, Montana, in Brady, J.B., Burger, H.R., Cheney, J.T., and Harms, T.A., eds., Precambrian geology of the Tobacco Root Mountains, Montana: Boulder, Colorado, Geological Society of America Special Paper 377, p. 181-202. Mueller, P.A, and Frost, C.D., 2006, the Wy oming province: a distinctive Archean craton in Laurentian North America: Canadian Journal of Earth Science, v. 43, p.13911397. ONeill, J. M., and Christiansen, R. L., 2004, Geologic Map of the Hebgen Lake Quadrangle, Beaverhead, Madison, and Ga llatin Counties, Montana, Park and Teton Counties, Wyoming, and Clark a nd Fremont Counties, Idaho: U.S. Geological Survey Scientific Investig ations Map 2816, 1 plate, scale 1:100,000. Park, J.K., 1981, Paleomagnetism of the La te Proterozoic sills in the Tsezotene Formation, Mackenzie Mountains, Northw est Territories, Canada: Canadian Journal of Earth Sciences, v. 18, p. 1572-1580.

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61 Park, J.K., Buchan, K.L., and Harlan, S ., 1994 A proposed giant radiating dyke swarm fragmented by the separation of Laurentia and Australia based on Paleomagnetism of ca. 780 Ma mafic intrus ions in western No rth America: Earth and Planetary Science Letters, v. 132, p. 129-139. Pearce, J.A., and Cann, J.R., 1973, Tectonic sett ing of basic volcanic rocks determined using trace element analyses: Earth and Planetary Science Letters, v. 19, p. 290 300. Plank, T., and Langmuir, C.H., 1988, An evalua tion of the global variation in the major element chemistry of arc basalts: Eart h and Planetary Science Letters, v. 90, p. 349-370. Prinz, M., 1964, Geologic evolution of the Beartooth Mountains, Montana and Wyoming, Part 5: Mafic dike swarms of the s outhern Beartooth Mountains, Geological Society of America Bulletin, v. 75, p. 1217-1248. Reid, R.R., 1963, Metamorphic rocks of the northern Tobacco Root Mountains, Madison County, Montana: Geological Societ y of America Bulletin, v.74, p. 293-305. Rollinson, H., 1993, Using geochemical data: evaluation, presentation, interpretation. Pearson-Prentice Hall, UK, 352 p. Schmitz, M.D., Wirth, K.R., Craddock, J.P., 1995, Major and trace element geochemistry of Early Proterozoic mafic dykes of north ern Minnesota and southwestern Ontario in Physics and Chemistry of Dykes: A. A. Balkema, Rotterdam, Netherlands, 337 p. Snyder, G.L., Hughes, D.J., Hall R. P., Ludw ig, K.R., 1989, Distribution of Precambrian mafic intrusives penetrating some Archean rocks of western North America, U.S Geological Survey Open-File Report, p. 89-125. Stevenson, R.K. and Patchett, P.J., 1990, Imp lications for the evol ution of continental crust from Hf isotopic systematics of Archean detrital zircons. Geochimica Cosmochimica Acta, v. 54, p1683-1698. Sun, S.S. and McDonough, W.F., 1989, Chemical and isotopic systematics of oceanic basalts: implications for man tle composition and processes: in Saunders, A.D., Norry, M.J. (editors), Magmatism in O cean Basins, Geological Society Special Publication 42. p. 313-345. Wooden, J.L., 1975, Geochemistry and Rb-Sr geochronology of Precambrian mafic dikes from the Beartooth, Ruby Range, and Tobacco Root Mountains, Montana. Ph.D. dissertation, University of North Carolina, Chapel Hill, NC. 194 p.

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62 Wooden, J.L., Vitaliano, C.T., Koehler, S.W., and Ragland, P.C., 1978, The late Precambrian mafic dikes of the southe rn Tobacco Root Mountains, Montana: geochemistry, Rb-Sr geochronology and relatio nship to belt tectonics : Canadian Journal of Earth Sciences, v. 15, no.4, p. 467-479. Wooden, J.L., Mueller, P.A., Hunt, D.K., Bo wes, D.R., 1982, Geochemistry and Rb-Sr geochronology of Archean rocks from the in terior pf the southeastern Beartooth Mountains, Montana and Wyoming: M ontana Bureau of Mines and Geology, Special Publication 84, p. 45-55. Wooden J.L., and Mueller, P.A., 1988, Pb, Sr, and Nd isotopic compositions of a Late Archean, igneous rocks, eastern Beartoot h Mountains: implications for crust mantle evolution: Earth and Plan etary Science Letters, v. 87, p.59-72. Zindler, A. and Hart, S. 1986, Chemical geodynamics: Annual Review of Earth and Planetary Sciences, v. 14, p. 493-571.

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63 BIOGRAPHICAL SKETCH Joshua Richards was born in Indianapolis Indiana in 1974. He attended Center Grove High School in Greenwood, Indiana in 1992. After hi gh school, he joined the United States Navy in 1994. After serving his tour of duty, he bega n his undergraduate studies in chemistry at Indiana University-Purdue University at Indi anapolis (IUPUI) in 1997. In 2000, he changed his major to geology and graduated with his Bachel or of Arts in 2004. In August of 2004, he began his graduate studies at the Univ ersity of Florida in the depart ment of geological sciences. In August 2007, he received his Master of Science in geology.