Zircon Geochemistry and Geochronology of the Seven Devils

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ZIRCON GEOCHEMISTRY AND GEOCHRONOLOGY OF THE SEVEN DEVILS

MOUNTAINS, WESTERN IDAHO: TESTING PROPOSED TIES TO THE WRANGELLIA

TERRANE

A THESIS

SUBMITTED TO THE GRADUATE SCHOOL

IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE

MASTER OF SCIENCE

HEATHER CASARES

DR. KIRSTEN NICHOLSON- ADVISOR

DEPARTMENT OF GEOLOGICAL SCIENCES

BALL STATE UNIVERSITY

MUNCIE INDIANA

MAY 2019

Table of Contents Chapter 1 Overview……………………………………………………………………………….1 Introduction………………………………………………………………………………..1 Previous Work…………………………………………………………………………….3 Objectives and Significance……………………………………………………………….4 Chapter 2 Background…………………………………………………………………………….6 Tectonics of Western North America……………………………………………………..6 Geologic Setting………………………………………………………………………..….9 The Blue Mountains Province…………………………………………………………….9 Wallowa terrane Seven Devils Group…………………………………………………..10 Baker terrane………………………………………………………………………….…13 Izee terrane………………………………………………………………………….…..13 Olds Ferry terrane……………………………………………………………………….13 Wrangellia…………………………………………………………………………….….14 Chapter 3 Analytical Methods…………………………………………………………..……….17 Sample Collection………………………………………….…………………………….17 Thin Section Preparation…………………………………………………………………19 Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) Analysis…………………………………………………………………………………..19 X-ray Fluorescence spectrometry (XRF) Analysis…………………………………..…..21 Chapter 4 Manuscript………………………………………………………………………….....24 Acknowledgements………………………………………………………………………………49 References………………………………………………………………………………………..50 Appendix……………………………………………………………………………………...….59

List of Tables Table A-1: U-Pb dating and of zircons from this study using LA-ICPMS…………………..…68 Table A-2: In Situ Lu-Hf isotopic data of zircons from this study…………………………..…..72

Table A-3: Average εHf(t) and errors for the Seven Devils samples……………...…………….73 Table A-4: Loss on Ignition (LOI)…………………………………………………………….....73 Table A-5: Whole Rock Geochemistry for the Seven Devils terrane determined by XRF……..74

List of Figures Figure 1-1: Location of the Seven Devils Mountains in Western Idaho………………………….1 Figure 2-1: Arc-arc collision model of the Blue Mountains Province……………………………8 Figure 2-2: The Blue Mountains Province and locations of terranes……………………………10 Figure 2-3: Close up view of the Wallowa terrane and rocks…………………………………..11 Figure 2-4: The location of the Wrangellia terrane……………………………………………..16 Figure 3-1: Sample location map…………………………………………………………...……18 Figure A-1: Sample SD001………………………………………………………………………60

Figure A-2: Sample SD002………………………………………………………………………61

Figure A-3: Sample SD003………………………………………………………………………62

Figure A-4: Sample SD004………………………………………………………………………63

Figure A-5: Sample SD005………………………………………………………………………64

Figure A-6: Sample SD006………………………………………………………………………65

Figure A-7: Sample SD007………………………………………………………………………66

Figure A-8: Sample SD008………………………………………………………...…………….67

Figure A-9: Sample SD009………………………………………………………………………68

Figure A-10: Average U-Pb ages for sample SD004…………………………………………….70

Figure A-11: Average U-Pb ages for sample SD006…………………………………………….70

Figure A-12: Average U-Pb ages for sample SD007…………………………………………….71

Figure A-13: Average U-Pb ages for sample SD008………………………………………….…71

Figure A-14: Average U-Pb ages for sample SD009…………………………………………….72

Figure A-15: Volcanic Rock Diagram Using Immobile Trace Elements………………………..76

Figure A-16: Discriminate within-plate granites, volcanic arc granites & ocean ridge granites using Nb vs. Y…………………………………………………………………………...………76

iii

Figure A-17: Discriminate within-plate granites, volcanic arc granites & ocean ridge granites using Rb vs. Nb+Y……………………………………………………………………………….77

Chapter One

1.1 Introduction

The western part of North America preserves a rich tectonic history. The western margin of the North American Craton has many terrains and microcontinents that have accreted to it.

The boundary between terranes is known as the suture zone. These terranes and microcontinents began forming during and after the breakup of Pangaea in the early Triassic (Murphy et al.,

2009). The forces created by such massive movement allowed for changes in the plate boundaries which caused the Farallon plate to begin subducting off of North America (Sigloch and Mihalynuk, 2013). The process of subduction ultimately created much of the volcanic arc terranes and microcontinents that now form the North American Cordillera.

Figure (1-1) Location map of the Seven Devils Mountains field study area from (Farag, 2018).

The Seven Devils Mountains (Figure 1-1) are located along a 70 km stretch on the eastern side of the Snake River in Western Idaho within an area known as Hells Canyon

Wilderness by the town of Riggings, Idaho (Sarewitz, 1983). The Seven Devils Mountains consist of mostly intermediate to mafic igneous rocks with rare felsic rocks that have all been variably metamorphosed. These rocks are considered to be the easternmost exposure of Permian to Triassic volcanic rocks in the North American Cordillera (Vallier, 1977).

Modern analytical tools and scientific understanding of trace element chemistry has improved significantly since the last study of the Seven Devils Mountains, conducted in the

1970s to the 1980s (Jones et al., 1977; Sarewitz, 1982). Conducting modern whole rock geochemical analysis including major and trace element geochemistry, and zircon U-Pb geochronology, will bring new insight on the formation of the Seven Devils, how volcanic arcs evolve over time, and importantly, how western North America has evolved. The North

American Cordillera provides an excellent location to study terrane translation in an accretionary orogeny. Previous studies and speculations have provided the Baja B.C. (Baja-British Columbia) hypothesis of terrane transport (Champion et al., 1984; Umhoefer, 1987; Wyld and Wright,

2001; Wyld et al., 2006). Northward terrane transport in the Cordillera, such as the Wrangellia terrane, is supported by fault displacements and paleomagnetic data suggesting displacements from 1100 to 3000 km in Washington and the British Columbia (Champion et al., 1984; Wynne et al., 1995; Wyld et al., 2006).

Trace element geochemistry, improved U-Pb geochronology, and Hf isotope analysis will allow a better understanding of the tectonic history of the terrane and test models for northward displacement of the arc, by allowing for robust comparisons with data from the Wrangellia terrane and the surrounding Blue Mountains terranes. If the Seven Devils terrane is indeed part

2 of the Wrangellia terrane, then collected data will provide additional evidence for significant long-range transport along the Cordilleran margin. If not, then the Seven Devils terrane may represent an independent tectonic element of the Cordillera that may not require orogenic displacement.

1.2 Previous Work

The main terranes (Wallowa, Baker, Izee, and Olds Ferry) of the Blue Mountains

Province formed as island arc systems along the continental margin of western Laurentia. The

Wallowa terrane has been known by many names such as the “volcanic arc terrane”, “Wallowa

Mountains–Seven Devils Mountains volcanic arc terrane” (Brooks and Vallier, 1978), and the

“Seven Devils terrane” (Vallier, 1977). The Wallowa terrane is the most outboard from the continental margin and is an island arc terrane that formed in an intra- oceanic setting in the Blue

Mountains Province (LaMaskin et al., 2008; 2011). The Seven Devils terrane, which has been interpreted to be part of the Wallowa terrane, and even the Wrangellia terrane, is a volcanic complex in what is now western Idaho. Vallier (1977) described the Seven Devils and identified four separate formations: The Permian Windy Ridge formation, Permian Hunsaker Creek formation, Triassic Wild Sheep Creek formation, and the Triassic Doyle Creek formation. The majority of the volcanic rocks in these formations were called spilite, keratophyre, and quartz keratophyre. They are now reclassified as metabasalt, meta-andesite, metadacite, and metarhyolite based on their greenschist metamorphic facies (Vallier et al. 2016). Previous work comparing the Seven Devils to Wrangellia includes paleomagnetic studies, comparative studies using fossils, and geochemistry.

Several authors including Hillhouse et al. (1982) and Harbert et al. (1995) worked to provide evidence for the migration of the Wallowa and Wrangellia terranes using

3 paleomagnetism. The core samples collected for these studies include volcanoclastic sedimentary, sedimentary, and volcanic flow rocks. Paleomagnetic data provide inclination and declination information that yields a paleolatitude, as well as paleopole locations comparable to other data sets. The two studies from Hillhouse in 1982 and again in 1984 on the Wallowa and

Wrangellia terranes respectively, provide a paleolatitude ranging from 10 to 18 degrees from the

Late Triassic in either the northern or southern hemisphere. The studies also show that there had to have been some kind of rotation in the Seven Devils terrane in order to bring it to where it is present day (Hillhouse, 1984).

Comparative fossil studies and the use of fossils were the main source for age dating the surrounding rocks. The Seven Devils terrane contains fossils in the Wild Sheep Creek formation.

These fossils are concentrations of the bivalves Daonella degeeri and D. frami (Jones, 1977).

These fossils resemble others in beds beneath the Nikolai Greenstone of the Wrangellia terrane in southern Alaska. The only other congeneric bivalves that resemble these two species are seen in only two other localities in the Arctic and the western Pacific (Jones, 1977).

Very few geochronologic and geochemical analysis have been conducted on the

Wrangellia and Wallowa terranes. The majority of studies have used fossils to obtain an estimated age for the Seven Devils terrane. U/Pb age dating is rare, with a few studies on detrital minerals in sedimentary rocks. Only one geochemical study by Kurz (2016) was done on intrusive rocks within the Wallowa region, but not specifically on the Seven Devils terrane.

Trace elements that are recorded include large-ion lithophile elements, high-field-strength elements, and rare earth elements (LaMaskin et al., 2008). The collected data revealed that all of the Blue Mountains Province are consistent with subduction-zone, arc origin based on the trace element patterns and the rare earth element patterns vary from terrane to terrane. The rare earth

4 element patterns in the Wallowa terrane are indicative of first cycle volcanism in a forearc basin with juvenile compositions (LaMaskin et al., 2008). These were necessary for the interpretation and reconstruction of the geologic history of North America.

1.3 Objectives and Significance

The main objective of this study is to use a combination of whole rock geochemistry and zircon U-Pb geochronology to constrain the ages of the four formations within the Seven Devils

Group, and to determine the tectonic setting and relationship to other exotic terranes along western North America. These relationships can provide insight and help test how the Seven

Devils terrane fits into the proposed Baja BC hypothesis of northward terrane transport in the

Cordillera, using its proposed links to the Wrangellia terrane. Whole-rock geochemical analyses from the 1970s and 1980s led authors to propose that the Seven Devils terrane was part of the

Wrangellia terrane, a volcanic arc terrane located further north in the Cordillera. Despite their physical similarities, the distance between the terranes and significant geochemical differences make correlation uncertain. Geochemistry and geochronology generated by this study will be used to test models for northward displacement of the arc by allowing for robust comparisons with extant data from Wrangellia. If the Seven Devils is indeed part of Wrangellia, then collected data will provide additional evidence for significant long range transport along the Cordilleran margin. If not, then the Seven Devils may represent an independent tectonic element of the

Cordillera that may not require long-distance orogenic displacement.

Chapter 2 Background

2.1 Tectonics of Western North America

Collisional plate boundaries along western North America have been active from the Late

Permian to present day. The main contributor to the deformation of western North America is the closure of ocean basins and the subduction of the Farallon plate beneath Laurentia. This active margin is preserved as the Cordillera; a group of accreted terranes and microcontinents that formed on oceanic crust.

The Cordilleran margin, and more specifically the northern region of this margin, recorded very complicated tectonics that included the accretion and transport of exotic terranes.

Remnants of subduction zones and strike slip faulting are observed in many regions. The study of displacement along faults and the reconstruction of paleo-landforms help provide evidence for an alternative model to the Baja B.C. hypothesis. The two original opposing models for the terrane transport include: the accreted terranes originated just west of their current position without any northward movement, and the second idea is the Baja B.C. hypothesis showing that the terranes originated from just west of current day Mexico (Champion et al., 1984; Wynne et al., 1995; Wyld et al., 2006). Movement began with Triassic to Late Jurassic terrane dispersal and rotation. The extent of this movement is poorly constrained. Strike slip displacement in the

Cretaceous can be reconstructed using geochemical data, mapping the fault system, and modeling. Various terranes have been displaced between 450 and 900 km to the north of their original positions based on paleomagnetic data (Wyld et al., 2006). Far more detailed tectonic models have been made for the Wallowa-Seven Devils terrane. The boundary between the

Wallowa-Seven Devils terrane and continental North America is marked by the western Idaho shear zone. This is a zone dividing rocks with initial 87Sr/86Sr ratios < 0.704 in accreted terranes

6 to the west, and > 0.706 in the continental lithologies to the east (Strayer, 1989). The western

Idaho shear zone is thought to have mainly strike-slip movement defined by faulting, geochemistry, and a sharp contrast in 87Sr/86Sr ratios on both sides of the shear zone. This is not the only boundary between exotic terranes and continental North America. There are other shear zones, including the Coast shear zone which separates the Wrangellia terrane and other northern exotic terranes from North America (McClelland et al., 2000).

Tectonic models have been made to speculate what the Wallowa terrane might have looked like at different points in geologic time (Figure 2-1). It is still unknown where the exact location of the Wallowa-Seven Devils terrane originated, but there are some speculations from paleomagnetic and geochemical studies (LaMaskin et al., 2011). These speculations, backed by geochemical studies, suggest that the terrane originated somewhere on the oceanic plate as an island arc (LaMaskin et al. 2008). There were two main subduction zones, and a possible strike- slip fault that brought the Wallowa terrane to the continental margin. Both continental North

America and the Wallowa terrane are on separate plates divided by a small subducting oceanic plate during the Mid-Triassic. Late Triassic plate motions brought the Wallowa terrane closer to the continent as subduction formed an accretionary margin with the sedimentary Baker terrane.

The terrane appears to rotate in the Middle to early Late Jurassic, based on models with a large fold and thrust belt on the eastern side of the terrane (LaMaskin et al., 2011).

Figure (2-1) The arc- arc collisional model for the Blue Mountains Provence (from fig. 12 Kurz et al., 2016). It combines paleomagnetic data and age data to determine the location of the terrane at specific times. Geochemistry and general stratigraphy provides a rock based record for the model. WSD- Wallowa Seven Devils; OF- Olds Ferry.

2.2 Geological Setting

The western part of North America is a mélange of accreted terranes. The more geologically complex terranes that formed between the Permian and the mid-Cretaceous include the Wrangellia terrane, and four terranes that reside within the Blue Mountains Province. These four terranes are called Baker, Izee, Olds Ferry, and Wallowa. The Wrangellia terrane stretches from southern Alaska down to northern Washington with the possibility of stretching further into western Idaho. The Blue Mountains Province is located mostly in Oregon with some of the terranes sitting on both sides of the Snake River.

2.3 The Blue Mountains Province

The Blue Mountains Province is the name of a mélange of pre-Tertiary igneous and sedimentary rocks that are located in eastern Oregon, southeast Washington, and western Idaho

(Figure 2-2). The province is divided into multiple terranes defined by their separate geological histories. Previous studies by Howard Brooks (1979) and Tracy Vallier (1977) divided these terranes based on major faults and unconformities. The major faults and unconformities trend to the east and northeast converging in the north towards the western Idaho shear zone (Tumpane,

2010). The terranes are characterized by their petrological and geochemical differences from the

North American Craton. It has been interpreted that during the late Jurassic the island arc terranes of the Blue Mountains combined before their accretion to the continental margin in the late Jurassic to early Cretaceous (Sarewitz, 1982). Blue Mountains Province contains two distinct rock types: those that come from island arcs which include the Baker, Olds Ferry, and Wallowa terranes, and those that are post-accretion to North America like the Izee terrane (Brooks, 1979).

Studies have strengthened the interpretation that the Baker, Izee, and Olds Ferry terranes formed

9 in close proximity to the continental margin, while the Wallowa formed as an intra-oceanic island arc further out (LaMaskin et al., 2008; 2011).

2.3.1 Wallowa terrane Seven Devils Group

Figure (2.2) The Blue Mountains Province showing the Wallowa, Baker, Izee, and Olds Ferry Terranes. IB, Idaho Batholith; SDM, Seven Devils Mountains; WISZ, Western Idaho Shear Zone (from fig. 1 LaMaskin et al., 2008).

The Wallowa terrane (figure 2-3) is located in northeastern Oregon and western Idaho in the North American Cordillera. The Wallowa terrane is often called “Wallowa Mountains–Seven

Devils Mountains volcanic arc terrane” or the “Seven Devils terrane” (Vallier, 1977). This region is complex and its tectonic origins are continuously debated. The Wallowa terrane ranges in age from Mid-Permian to Early Cretaceous, and contains a range of metamorphic, volcanic, and sedimentary assemblages (Kurz et al., 2016). These rocks are variably metamorphosed to mostly upper greenschist facies (Vallier et al., 2016). The lowest unit is called the Seven Devils

Group followed by late Triassic limestone and dolomite. On top of this are fine-grained calcareous marine clastic rocks. There is a single potential unconformity in the Wallowa terrane, along with many fossils within the sedimentary units. The potential unconformity is a disconformity between the Seven Devils group and the Late Triassic limestone and dolomite

(Vallier, 1977).

Figure (2-3) Close up view of the Wallowa terrane and associated rock types (from fig. 2 Vallier et al., 2016).

The Seven Devils Group is an assemblage of Permian to Triassic volcanic and sedimentary rocks within the Seven Devils Mountains. These mountains are situated on the

Idaho side of the Snake River. There are four formations including: Permian Windy Ridge

Formation, Permian Hunsaker Creek Formation; Triassic Wild Sheep Creek Formation; and

Triassic Doyle Creek Formation.

The Permian Windy Ridge Formation is the oldest unit in the group. This formation is dominated by pyroclastic and epiclastic rocks with epiclastic rocks being rare. The rocks tend to be of pale green color. Rhyolitic pyroclastic breccia and tuff are the dominant types of rocks in this formation. The major mineral components of these rocks are rhyolite rock fragments, quartz

(both primary crystals and secondary chalcedony), plagioclase feldspar (albite), opaque minerals

(including pyrite), chlorite, epidote, and sphene (Vallier, 2016). The age of this formation has been inferred to be 268–248 Ma, but fossils and radioisotopic ages have not yet been obtained.

The Permian Hunsaker Creek formation overlays the Windy Ridge formation by a fault contact. Rhyolitic and andesitic flows and volcaniclastic rocks dominate this formation. Basalt and basaltic andesite are rare in this formation, but can be abundant in certain locations. This formation also contains rhyolitic and basaltic diabase and gabbro that are hypabyssal intrusive rocks (Vallier, 2016). Rhyolites within this formation are generally rich in silica and sodium and have little to no potassium and calcium (Vallier, 1995). This is different compared to the traditional rhyolite that typically has more potassium and calcium.

The Triassic Wild Sheep Creek Formation is above the Hunsaker Creek Formation. This formation commonly contains massive lava flows and intrusive hypabyssal rocks such as basalt and rhyolite. Pyroclastic rocks are a minor contributor to this formation and are abundant is only a few areas. Typical colors of the rocks are grey to green with even rarer colors of red to brown.

Epiclastic rocks in the Wild Sheep Creek Formation include conglomerate, breccia, sandstone, and mudstone. This formation is metamorphosed to greenschist and zeolite facies (Vallier et al.,

2016).

The youngest formation in the Seven Devils is the Doyle Creek Formation. It is only slightly younger than the Wild Sheep Creek Formation with an overlap in ages. It consists of

12 lava flows, most of which are basalt, basaltic andesite, and dacite, and volcaniclastic rocks

(Vallier et al., 2016). The color of the rocks in this formation are mostly red to maroon with some pale green in color. The metamorphic grade is upper greenschist facies.

2.3.2 Baker terrane

The Baker terrane is one of the four terranes in the Blue Mountains Province. The Baker terrane is the most complex out of all the Blue Mountains terranes. It is made up of accretionary- complex and intra-arc to forearc-basin deposits (Gray and Oldow, 2005). The terrane is divided into two different subterranes: the Bourne and Greenhorn subterranes (Ferns and Brooks, 1995).

The Bourne subterrane is composed of a mélange of chert-argillite and the rocks are mostly radiolarian chert, cherty argillite, argillite, basalt, and limestone olistoliths (Vallier, 2016). The

Greenhorn subterrane is a mixture of igneous and sedimentary rocks that are in a serpentine mixture (Vallier, 2016).

2.3.3 Izee terrane

The Izee terrane is made up of accretionary-complex and intra-arc to forearc-basin deposits. These deposits are made up of clastic sedimentary rocks (LaMaskin et al., 2011). This makes it similar to and causes it to be confused with being a part of the Baker terrane or the Olds

Ferry terrane. It has been interpreted by many authors that the Izee terrane sits depositionally on top of the Baker terrane (LaMaskin et al., 2011) and is separated from the Olds Ferry terrane by a thrust fault that was originally a depositional contact (Tumpane, 2010).

2.3.4 Olds Ferry terrane

The Olds Ferry terrane is younger than the Wallowa terrane. The late Middle Triassic to

Late Triassic terrane consists of plutons. Speculations suggest that the arc formed partially on the

Wallowa arc making it an extension of it or that it was its own tectonic element fringing the

North American Craton (Vallier, 1995). The Olds Ferry terrane is the only other arc that formed in an oceanic setting (Gray and Oldow, 2005). The Olds Ferry terrane contains the Huntington

Formation; an assemblage of carbonate and clastic sedimentary rocks along with metavolcanic rocks and some plutonic rocks (Brooks and Vallier, 1978). The formation of the Olds Ferry volcanic submarine arc is interpreted to be late Triassic based upon fossils in the fossiliferous limestone (Vallier, 1995). All of the Olds Ferry rocks have been regionally metamorphosed to zeolite and greenschist facies.

2.4 Wrangellia terrane

The Wrangellia terrane (Figure 2-4) is split between four areas: the Wrangell Mountains in Alaska, Chichagof Island, Queen Charlotte Islands, and Vancouver Island. The Wallowa terrane in northeastern Oregon is often included as this terrane’s southeastern most extension

(Jones et al., 1977). Nearly 2000 km separate the main four sections of the Wrangellia terrane

(Jones et al., 1977). These four sections are combined into one terrane because of similar lithology, fossil assemblages, structure, and unconformities.

The stratigraphy of Wrangellia consists of sedimentary, volcanic, and low-grade metamorphic rocks from the Lower Permian to early Jurassic. The oldest units contain Lower

Permian argillite and limestone (Jones et al., 1977). The unit above this is a 100 m Mid-Triassic sequence of gray- black thinly bedded chert, siltstone, and fissile shale (Jones et al., 1977). In the

Middle to Late Triassic period there was volcanic activity constrained by K-Ar and U-Pb geochemical age dating on Jurassic Plutons and fossils from sedimentary rocks (Samson et al.,

1990). Some low-grade metamorphism to greenstone is visible. This large 3,500 m Mid to Late

Triassic unit makes up the majority of the stratigraphic sequence. The unit consists of greenstone, basaltic flows, pillow lavas, sills, and tuff (Jones et al., 1977). Most of the greenstone

14 is concealed, but the basalts and pillow lavas are exposed throughout all the areas. Above this are mostly Triassic sedimentary rocks, consisting of limestone, calcareous shale, and chert (Jones et al., 1977). One of the four regions located on Chichagof Island has marble, suggesting an event of metamorphism within that region. There is an intrusion of granodiorite dated to the Jurassic from 185 to 190 Ma (Samson et al., 1990). These units follow a similar pattern to the Seven

Devils terrane.

Two unconformities and different invertebrate fossils are observed within the Wrangellia terrane. A disconformity lies between the lowest unit which is Lower Permian and the above

Mid-Triassic unit. There is a nonconformity between the thick greenstone and volcanic sequence with the sedimentary units above it (Jones et al., 1977). There are a few fossils of bivalves in the lowest unit. The upper sedimentary units contain stromatolites, gastropods, bivalves, brachiopods, corals, spongiomorphs, cephalopods, and echinoderms (Jones et al., 1977).

Figure 2-4: The Location of the Wrangellia terrane and proposed southern portion as the Seven Devils Mountains (from fig. 1 Jones et al., 1977).

Chapter 3 Analytical Methods

3.1 Sample Collection

One set of nine samples was collected during a field excursion in July of 2018. The excursion took roughly nine days traveling from Muncie, Indiana, to the Seven Devils Mountains near Riggins, Idaho. Sample locations were remote and far from the main Seven Devils

Campground and it was necessary to backpack and temporarily camp along the trail (Figure 3-1).

Samples were collected all along the loop trail which includes trail 124 and 101. The spread of samples was located on both the east and west sides of the Seven Devils Mountains. Hand samples mostly consisted of intermediate igneous rocks with one being more mafic. See figures

A-1 to A-9 for sample descriptions.

Figure (3-1) Sample location map. Shows locations for SD001-009.

3.2 Thin Section Preparation

The thin sections were prepared using standard methods at Ball State University. The rock samples were cut into 1”x2’ billets using a rock saw. Then, the side of each sample was polished and glued to a slide and cleaned to remove fractured areas. Polishing was done by using the metal wheel with 120, 220, 400, and 600 alumina grits. Finally, to ensure a good surface it is polished again on glass with 600 and 1000 alumina grits for 2 to 3 minutes. The billets on the glass slide were then cut down to an appropriate size using the saw arm of a Hillquist 1010 thin section machine. Then the billets were ground down using the thin section machine to about 10-

20 μm greater than final desired thickness (Lange 2014). The slides were then polished on a glass plate using 600 and 1000 alumina grits until plagioclase was identifiable in a microscope. For sample descriptions see appendix figures A-1 to A-9.

3.3 Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) Analysis

U-Pb geochronology and Lu-Hf isotope geochemistry on the collected samples was conducted at the LaserChron lab located at the University of Arizona in Tucson.

The zircons are extracted from the sample and prepared for analysis using standard methods (Kröner et al., 2000) including the following steps: Each igneous rock sample is broken to a small enough size to fit in the jaw crusher using a sledge hammer. Then, they are put through a two-step process to crush the sample to a fine < 500 µm size. The first step is to run the sample through a BICO Chipmunk jaw crusher in order to reduce the rock into gravel sized particles. The next step is the BICO disk mill which uses two spinning steel plates to pulverize the material into a sand-sized fraction. This sediment is run across a Gemini GT-60 shaker table that uses water, along with a constant shaking motion to separate the denser minerals from lighter ones such as quartz and feldspar. The denser material is caught in three buckets, one in

19 the center and two on the sides. These fractions are rinsed in acetone and dried, with the center bucket containing an enriched heavy mineral fraction used in the final processes. This dense fraction is then run across a Franz Isodynamic magnetic separator, with a 20º forward and a 10º side tilt, and a current range from 0.2 to 0.7 Å. This process removes high magnetic susceptibility material from the sample before further processing. Methylene iodide (MeI) is a heavy liquid used to separate the densest minerals from the lighter ones. Dense minerals, including zircon, will sink to the bottom of the container with the liquid, where they can be extracted by opening a stopcock in the base of the separatory funnel for filtering and cleaning.

They are rinsed in acetone and dried to make sure the MeI is no longer on the sample. This material is then run through the magnetic separator with a current of 1.0 to 1.8 A, further purifying the sample. The resulting separate is then picked for zircon grains that show the least defects.

The samples were sent to University of Arizona’s LaserChron lab to be mounted for analysis. The high-quality grains were mounted with standards Sri Lanka, FC-1, and R33. The mounts were sanded down to a depth of ~20 microns, polished, imaged using high resolution cathodoluminescence (CL) and backscattered electron (BSE) images. These images are important because they help distinguish zircons from apatite and allow for a closer up image for spot picking to avoid any inclusions or cracks in the zircon.

U-Pb analysis was conducted by LA-ICPMS at the University of Arizona’s LaserChron

Center using standard methods (Gehrels et al., 2006, 2008; Gehrels and Pecha, 2014). The analyses involve using a Photon Machines Analyte G2 Excimer laser with a spot diameter of 40 microns to get rid of any added Pb and again at 20 microns to ablate the zircon. The material is then carried in helium to the plasma source of an Element2 HR ICP-MS, which sequences

20 rapidly through U, Th, and Pb isotopes. The intensities of these signals are measured with a

SEM. The ratios of U and Pb are then calculated. The calculated ages are shown on U-Pb concordia diagrams and weighted mean diagrams using the routines in Isoplot (Ludwig, 2008).

Hf isotope analyses was conducted with a Photon Machines Analyte G2 excimer laser in situ of U-Pb spots according to standard methods (Cecil et al., 2011; Mueller et al., 2008). The instrument settings were established for Hf analysis by analysis of 10 ppb solutions of JMC475 and a Spex Hf solution, and then by analysis of 10 ppb solutions containing Spex Hf, Yb, and

Lu. Several different standards: Sri Lanka, FC-1, and R33 are analyzed for precision and accuracy before the unknowns. The CL images are used to ensure that the ablation pits do not overlap multiple domains or inclusions. The samples are analyzed with a laser beam diameter of

40 microns. The Lu and Hf isotopes are calculated and an epsilon plot is created to show where the samples sit compared to the depleted mantle and the Chondritic Uniform Reservoir (CHUR) line. The 176Hf/177Hf at time of crystallization is calculated after the analysis using measurements of present-day 176Hf/177Hf and 176Lu/177Hf, using the decay constant of 176Lu (λ = 1.867e-11) from Scherer et al. (2001) and Söderlund et al. (2004).

3.4 X-ray Fluorescence spectrometry (XRF) Analysis

XRF analysis is conducted in house at Ball State University. Pellets are made for the

XRF by a simple process. The rock is cut down by a rock saw into small cubes and polished using 200 grit followed by 600 grit to remove any residue and contamination from the saw. The polished cubes are put through the tungsten carbide ring mill C+CR machine for 35-60 seconds and crushed into a fine powder. Around 9 to 10 grams of each sample was weighed and put into a weighed ceramic crucible. This is then poured into a plastic ceramic mixing cup. Polyvinyl alcohol (PVA) was made and used to make the mixture more saturated and homogenous. The

PVA solution had been mixed using 5 g of dried 85-90% hydrolyzed grains and 95 g (95 mL)

18Ω deionized water. The mixture was heated and stirred until the PVA grains had dissolved.

Nineteen drops of PVA were added into the 8 grams of each sample’s powder, and then the mixture was mixed until it became homogeneous. A pellet press apparatus was used to make the pellets. Powder was poured inside the piston press and pressed at 2-10 tons/in2 for 5-10 sec for three minutes. Pellets were carefully extracted and set to dry in a Fischer Scientific Isotemp

Oven at 50˚C.

Samples were analyzed using methods from Eric Lange 2019. The Samples were analyzed using a Thermo Noran (now Thermo Fisher) QuanX energy dispersive X-ray fluorescence (ED-XRF) analyzer with WinTrace spectrum processing software. There are 4 different conditions used when analyzing samples in order to optimize the fluorescence yield for the elements of interest. Excitation energies for each condition were 6, 12, 16, and 30 keV which used no filter, an Aluminum, a thin Palladium, and a thick Palladium filter for each energy respectively. Each sample was run for 300 seconds of live time, which approximates to about

2700 seconds of total analysis time for nine samples. Abundances were determined using counts per second within a narrow keV range specific to each element. This was divided by the different currents used. The elements were calibrated linearly using a series of 13 standards supplied by the USGS: AGV-2, BCR-2, BHVO-2, BIR-1a, COQ-1, DNC-1a, DTS-2b, GSP-2, QLO-1, SBC-

1, SDC-1, SGR-1b, and W-2a.

The amount of volatiles is calculated by a loss on ignition (LOI) procedure appropriate for intermediate, mafic, and ultramafic rocks with moderate to high amounts of clay alteration.

The powders made out of each rock were used in this process. Around 8 to 10 grams of each sample was weighed and placed into a weighed ceramic crucible that weighed approximately 2.4

22 grams. The total weight for both the powder and crucibles were recorded before placing them overnight in a Fischer Scientific Isotemp Oven at 65˚C. The following day, all samples were taken out, and the weight was measured for each sample. After that, all samples were put back in for 110 ˚C and measured once again the next day. The final measurement was taken on the samples after they were put into the thermo Scientific Thermolyne F48015-60 muffler furnace at

1000 ˚C. The new weight percent lost was reported as LOI.

Chapter 4 Manuscript

Zircon U-Pb Geochronology and Hf Isotope Geochemistry of the Seven Devils Mountains, Western Idaho: Testing proposed ties to Surrounding Terranes

Abstract

The North American Cordillera provides an excellent location to study terrane translations in an accretionary orogeny. Some previous studies have proposed the Baja -British

Columbia hypothesis of northward terrane transport in the Cordillera. This has been supported by fault displacements and paleomagnetic data showing displacements from 1100 to 3000 km in

Washington and British Columbia. The Seven Devils Mountains are located in the Wallowa-

Whitman National Forest in western Idaho. This geologically complex and variably metamorphosed terrane preserves a Permo-Triassic volcanic arc accreted against the Cordilleran margin. Whole-rock geochemical analyses from the 1970s and 1980s led authors to propose that the Seven Devils terrane was part of the Wrangellia terrane, a volcanic arc terrane located further north in the Cordillera. Despite their physical similarities, the distance between the terranes and significant geochemical differences make correlation uncertain. Zircon U-Pb and Lu-Hf was used to test proposed ties to Wrangellia terrane and the neighboring Blue Mountains Province terranes. LA-ICPMS zircon U-Pb dating on 6 intermediate intrusive samples yield ages ranging from 120.1 ± 1.0 Ma to 137± 2.0 Ma with the average age of 127 ± 7.8 Ma. All the samples have similar Lu-Hf isotopic compositions, with a range in εHf(t) of 8.6 to 13.8 with the average εHf(t) value of 10.8 ± 1.2. Isotopic data and ages show that the intrusive rocks were forming close to the time of accretion. These new data, combined with previous age and isotope data from the surrounding terranes suggest that the Seven Devils terrane is the most similar to the terranes in the Blue Mountains Province and not a match to the Wrangellia terrane.

Introduction

The western part of North America preserves a rich tectonic history. The western margin of the North American Craton has many terrains and microcontinents that have accreted to it after forming during and after the breakup of Pangaea in the early Triassic (Murphy et al., 2009).

The forces created by such massive movement allowed for changes in the plate boundaries which caused the Farallon plate to begin subducting off of North America (Sigloch and Mihalynuk,

2013). These forces ultimately created much of the volcanic arc terranes and microcontinents that now form the North American Cordillera. While Late Paleozoic to Mesozoic volcanism and accretion is agreed upon by many authors (Burchfiel et al., 1992; Dickinson, 2004; Gray, 2013) there are some controversies revolving around the origin of a terrane in the Blue Mountains

Province (BMP) known as the Seven Devils terrane (Jones et al., 1977; Sarawitz, 1983;

Hillhouse and Gromme, 1984; Kalk, 2008).

The origin of the Seven Devils terrane has been questioned and disputed by many studies.

The Seven Devils terrane has been correlated with the greater the Wrangellia terrane (Hillhouse and Gromme, 1984; Jones et al., 1977; Samson et al., 1990) and the Wallowa terrane (Gray and

Oldow, 2005; LaMaskin et al., 2015; Schwartz et al., 2011); however, it could also have been a separate tectonic element. Correlation with the Wrangellia terrane provides its own set of problems of terrane transport due to the distance between the terranes. Previous studies and speculations have provided the Baja-British Columbia (Baja-B.C.) hypothesis of terrane transport (Champion et al., 1984; Umhoefer, 1987; Wyld and Wright, 2001; Wyld et al., 2006).

Northward terrane transport in the Cordillera has already been supported by fault displacements and paleomagnetic data showing displacements from 1100 to 3000 km in Washington and

British Columbia (Champion et al., 1984; Wynne et al., 1995; Wyld et al., 2006).

The Seven Devils terrane is a unique tectonic element along the western Idaho shear zone. If it is indeed part of the Wrangellia terrane it can provide further evidence for long ranged terrane transport along the shear zone. If not, the terrane may belong to surrounding terranes or might be its own separate tectonic element. New data will help show how the isotope geochemistry of the rocks reacts to being right on the edge of the sharp shear zone boundary of

Laurentia, provide a more robust dataset for the Seven Devils terrane, and contribute to the tectonic history of North America.

In this contribution, we report zircon U–Pb ages and Lu-Hf isotopic compositions of the intrusive rocks from the Seven Devils terrane. Based on the results, we discuss the origin of the

Seven Devils terrane and the timing of amalgamation with the North American Craton.

Geologic setting

Blue Mountains Province

Fig. 1 An overall view of the Blue Mountains Province and associated terranes taken from fig. 1 (LaMaskin et al., 2008). The IB, Idaho Batholith; SDM, Seven Devils Mountains; SRB, Salmon River Basin; WISZ, Western Idaho Shear Zone are shown along with the terrane boundaries in dashed lines.

The Blue Mountains Province is the name of a mélange of pre-Tertiary igneous and sedimentary rocks that are located in eastern Oregon, southeast Washington, and western Idaho

(Fig. 1). The province is divided into multiple terranes defined by their separate geological histories. Previous studies by Brooks (1979) and Vallier (1977) divided these terranes based on major faults and unconformities. The major faults and unconformities trend to the east and northeast converging in the north towards the western Idaho shear zone (Tumpane, 2010). The terranes are characterized by their petrological and geochemical differences from the North

American Craton. The four distinct terranes in the Blue Mountains Province are called Baker,

Izee, Olds Ferry, and Wallowa terranes. It has been interpreted that during the late Jurassic the island arc terranes of the Blue Mountains combined before their accretion to the continental margin in the late Jurassic to early Cretaceous (Sarewitz, 1982). The Blue Mountains Province contains two distinct rock types: those that come from island arcs which include the Baker, Olds

Ferry, and Wallowa terranes, and those that are post-accretion to North America like the Izee terrane (Brooks, 1979). Studies have strengthened the interpretation that the Baker, Izee, and

Olds Ferry terranes formed in close proximity to the continental margin, while the Wallowa formed as an intra-oceanic island arc further out (LaMaskin et al., 2008; 2011).

The Wallowa terrane (Fig. 2) is located in northeastern Oregon and western Idaho in the

North American Cordillera. The Wallowa terrane is often called “Wallowa Mountains–Seven

Devils Mountains volcanic arc terrane” or the “Seven Devils terrane” (Vallier, 1977). This region is complex, and its tectonic origins are continuously debated. For example, it has been often linked to the Wrangellia terrane (Jones et al., 1977). The Wallowa terrane ranges in age from Mid-Permian to Early Cretaceous, and contains a range of metamorphic, volcanic, and sedimentary assemblages (Kurz et al., 2016). These rocks are variably metamorphosed to mostly upper greenschist facies (Vallier et al., 2016). The lowest unit is called the Seven Devils Group

Fig. 2 Wallowa terrane located in the Blue Mountains Province showing a close up view of the rock types in the area from fig. 2 (Vallier et al., 2016). The Seven Devils Mountains are located in the middle of the map near the arc continent boundary.

28 followed by late Triassic limestone and dolomite. On top of this are fine-grained calcareous marine clastic rocks. There is a single potential unconformity in the Wallowa terrane, along with many fossils within the sedimentary units. The potential unconformity is a disconformity between the Seven Devils group and the Late Triassic limestone and dolomite (Vallier, 1977).

Wrangellia terrane

The Wrangellia terrane (Fig. 3) is split between four areas: the Wrangell Mountains in

Alaska, Chichagof Island, Queen Charlotte Islands, and Vancouver Island. The Wallowa terrane in northeastern Oregon is often included as this terrane’s southeastern most extension (Jones et al., 1977). Nearly 2000 km separate the main four sections of Wrangellia terrane (Jones et al.,

1977). These four sections are combined into one terrane because of similar lithology, fossil assemblages, structure, and unconformities.

Fig. 3 Map of the Wrangellia Terrane updated from fig. 1 (Hillhouse and Gromme, 1984). WM- Wrangell Mountains, CI-Chichagof Island, QCI- Queen Charlotte Islands, VI-Vancouver Island

The stratigraphy of the Wrangellia terrane consists of sedimentary, volcanic, and low- grade metamorphic rocks from the Lower Permian to early Jurassic. The oldest units contain

Lower Permian argillite and limestone (Jones et al. 1977). The unit above this is a 100 m Mid-

Triassic sequence of gray- black thinly bedded chert, siltstone, and fissile shale (Jones et al.,

1977). In the Middle to Late Triassic period there was volcanic activity and some low-grade metamorphism indicated by geochemical age dating. This large 3,500 m Mid to Late Triassic unit makes up the majority of the stratigraphic sequence. The unit consists of greenstone, basaltic flows, pillow lavas, sills, and tuff (Jones et al., 1977). Most of the greenstone is concealed, but the basalts and pillow lavas are exposed throughout all the areas. Above this are mostly Triassic sedimentary rocks, consisting of limestone, calcareous shale, and chert (Jones et al., 1977).

Two unconformities and different invertebrate fossils are observed within the Wrangellia terrane. A disconformity lies between the lowest unit which is Lower Permian and the above

Mid-Triassic unit. There is a nonconformity between the thick greenstone and volcanic sequence with the sedimentary units above it (Jones et al., 1977). Few bivalves are observed in the lowest unit, while the upper sedimentary units contain stromatolites, gastropods, bivalves, brachiopods, corals, spongiomorphs, cephalopods, and echinoderms (Jones et al., 1977).

Geology of the Seven Devils terrane

The Seven Devils terrane is an assemblage of Permian to Triassic volcanic and sedimentary rocks within the Seven Devils Mountains. These mountains are situated on the

Idaho side of the Snake River. There are four formations within the Seven Devils terrane:

Permian Windy Ridge Formation, Permian Hunsaker Creek Formation; Triassic Wild Sheep

Creek Formation; and Triassic Doyle Creek Formation.

The Permian Windy Ridge Formation is the oldest unit in the group. This formation is dominated by pyroclastic and epiclastic rocks with epiclastic rocks being rare. The rocks tend to be pale green in color. Rhyolitic pyroclastic breccia and tuff are the dominant types of rocks in this formation. The major mineral components of these rocks are rhyolite rock fragments, quartz

(both primary crystals and secondary chalcedony), plagioclase feldspar (albite), opaque minerals

(including pyrite), chlorite, epidote, and sphene (Vallier et al., 2016). The age of this formation has been inferred to be 268–248 Ma, but fossils and isotopic ages have not yet been obtained.

The Permian Hunsaker Creek formation overlays the Windy Ridge formation by a fault contact. Rhyolitic and andesitic flows and volcaniclastic rocks dominate this formation. Basalt and basaltic andesite are rare in this formation but can be abundant in certain locations. This formation also contains rhyolitic and basaltic diabase and gabbro that are hypabyssal intrusive rocks (Vallier et al., 2016). Rhyolites within this formation are generally rich in silica and sodium and have little to no potassium and calcium (Vallier, 1995). This is different compared to the traditional rhyolite that typically has more potassium and calcium.

The Triassic Wild Sheep Creek Formation is above the Hunsaker Creek Formation. This formation commonly contains massive lava flows and intrusive hypabyssal rocks such as basalt and rhyolite. Pyroclastic rocks are a minor contributor to this formation and are abundant in only a few areas. Typical colors of the rocks are grey to green with even rarer colors of red to brown.

Epiclastic rocks in the Wild Sheep Creek Formation include conglomerate, breccia, sandstone, and mudstone. This formation is metamorphosed to greenschist and zeolite facies (Vallier et al.,

2016).

The youngest formation in the Seven Devils is the Doyle Creek Formation. It is only slightly younger than the Wild Sheep Creek Formation with an overlap in ages. It consists of

31 lava flows, most of which are basalt, basaltic andesite, and dacite, and volcaniclastic rocks

(Vallier et al., 2016). The color of the rocks in this formation are mostly red to maroon with some pale green. The metamorphic grade is upper greenschist facies.

Fig. 4 Location of collected samples within the Seven Devils Mountains.

The samples for this study were taken from the Wild Sheep Creek Formation in the

Seven Devils Mountains. These nine samples include porphyritic basalt, diorite, granodiorite, and syenitic diorite based off XRF analysis in table A-5 and figure A-15 intrusive rock diagram

32 for immobile trace elements. The majority of these samples except for the basalt were taken from intrusions within the formation that are seen throughout the Blue Mountains Province terranes.

These are all slightly metamorphosed to greenschist facies. Sample locations for this study are shown in Fig. 4.

Analytical Methods

Zircon U-Pb dating and Hf analysis

Representative samples from the Seven Devils terrane were collected and selected for zircon separation and U-Pb analyses. Out of nine samples collected five yielded zircons. Zircon analysis for intrusive igneous rocks (SD004, SD006, SD007, SD008, SD009) were performed by

Laser Ablation ICPMS (LA-ICPMS) at the Arizona LaserChron Center, University of Arizona,

United States, according to standard methods (Gehrels et al., 2006, 2008; Gehrels and Pecha,

2014). The analyses involve ablation of zircon with a Photon Machines Analyte G2 Excimer laser using a spot diameter of 20 microns for U-Pb. All U-Pb data were using Sri Lanka, FC-1, and R33 as standards. The data was calculated by in-house Python decoding routine and an Excel spreadsheet at the University of Arizona. Hf isotope measurements were made via ablation on top of the preexisting U-Pb pits using a spot size of 40 microns. Cathodoluminescence images were used to ensure that the spots did not overlap multiple age domains or inclusions.

Results

Zircon U–Pb geochronology

New U-Pb dates of zircons from intermediate intrusive rocks were obtained using LA-

ICP-MS at the University of Arizona LaserChron lab. Five samples were dated and reported as

207Pb corrected 206Pb/238U ages. The analytical results of samples from diorite to granodiorite are

33 presented in Table A-1 with Concordia diagrams in Fig. 5 and zircon CL images are shown in

Fig. 6.

Fig. 55 ConcordiaConcordia diagramsdiagrams forfor eacheach sample withsample the with age included.the Concordia age and the mean age.

Zircon grains from sample SD004 display little to no oscillatory zoning with size ranging from 100 µm to 350 µm in length to 50 µm to 100 µm in width. The Th/U ratios of the analyzed fifteen zircon grains range from 0.24 to 0.57. These numbers suggest a magmatic origin for the zircons. This is the stratigraphically highest sample dated in this study. All fifteen of the grains have overlapping errors and they give a weighted mean 206Pb/238U age of 137± 2.0 Ma.

Zircons from SD006 display little to no oscillatory zoning with zircon size ranging from

100 µm to 300 µm in length to 50 µm to 150 µm in width. The Th/U ratios of the analyzed fifteen zircons range from 0.27 to 0.66. The Th/U ratios suggest a magmatic origin. One zircon was an outlier with an age of 114.7 ± 1.3 Ma and was not included in the overall age calculation.

This sample has a weighted mean 206Pb/238U age of 123.0 ± 1.7 Ma.

Zircons from sample SD007 display very clear oscillatory zoning on most of the grains and the zircon size ranges from 100 µm to 300 µm in length and 50 µm to 150 µm in width. The

Th/U ratios of the fifteen analyzed zircons range from 0.19 to 0.51. The Th/U ratios are suggestive of a magmatic origin for the zircons. Only 2 zircon ages were manually rejected for being much older than the average. These two grains were 127.5 ± 1.8 Ma and 126.9 ± 1.5 Ma.

The weighted mean for the sample 206Pb/238U age is 120.1 ± 1.1 Ma.

Zircons from SD008 displays some clear oscillatory zoning and the zircon size ranges from 50 µm to 300 µm in length and 50 µm to 150 µm in width. The Th/U ratios of the fifteen analyzed zircons range from 0.13 to 0.22. The Th/U ratios are suggestive of a magmatic origin.

SD008 is one of the stratigraphically lowest samples. Only one zircon was rejected because it is the oldest age and has the highest correction error. The weighted mean 206Pb/238U age is 120.0 ±

1.0 Ma.

Zircons from sample SD009 display little to no oscillatory zoning and the range in size is from 50 µm to 450 µm in length and 50 µm to 100 µm in width. The Th/U ratios of the fifteen analyzed zircons range from 0.21 to 0.40. The ratios are suggestive of a magmatic origin. Sample

SD009 is the easternmost and lowest stratigraphic sample. Only one zircon age was rejected for being too young at 116.5 ± 1.1 Ma. The weighted mean 206Pb/238U age is 135.7 ± 1.9 Ma.

Fig. 6 Cathodoluminescence imaging of zircons that were age dated and had isotopic Lu-Hf analysis. The zircon spots are marked with black circles.

In Situ Zircon Lu-Hf Isotopes

The Hf isotope results of the zircons from the five Seven Devils samples are listed in

Table A-2 and graphed on an epsilon plot in Fig. 7. The five samples; SD004, SD006, SD007,

SD008, and SD009 that all had ages ranging from 136.7 to 120Ma have εHf(t) values from 8.6 to

13.8. SD004 has an average εHf(t) value of 11.7. SD006 has an average εHf(t) value of 11. SD007 has an average εHf(t) value of 10.8. SD008 has an average εHf(t) value of 10.4. SD009 has an average εHf(t) value of 10.1. These εHf(t) values are very tight showing errors of 0.5 to 0.9 with no distinct differences in the samples. The TDM model age for the samples is 470 Ma to about

240 Ma. There appears to be no distinct pattern or differences between any of the samples.

Fig. 7 εHf(t) values displayed on an epsilon plot. DM- Depleted Mantle, CHUR- Chondritic Uniform Reservoir. All the values plot on the positive side of the CHUR line.

Discussion

Zircon Age Relationships

Previous geochronologic investigations on the Seven Devils terrane are sparse and most of the ages are derived from the Wallowa terrane. The ages of the basement rocks are Permian to

Triassic (Vallier and Brooks, 1986). The Seven Devils terrane is often thought to be part of either the Wrangellia terrane, Wallowa terrane, or as a separate terrane that does not belong to either of

37 them. The idea that the Seven Devils terrane could be part of one of these greater terranes comes from the fact that they are stratigraphically similar with similar fossils.

The Seven Devils rocks in this study have an overall age range from 136.7 Ma to 120.0

Ma. This age range coincides with the ages of plutons throughout the Blue Mountains Province.

All of the terranes in the Blue Mountains province were intruded by post-kinematic Late Jurassic to Early Cretaceous calc-alkaline plutons (Gray and Oldow, 2005). The ages from these plutons provide an upper age constraint for the Late Jurassic tectonic amalgamation of terranes outboard of the continental margin (Vallier and Brooks, 1986; Walker, 1986; Avé Lallemant, 1995; Snee et al., 1995). The U-Pb age ranges from the plutons found throughout the Blue Mountains province are collected from zircons dating 145 Ma to 120 Ma (Walker, 1989). These plutons constrain the age of deformation because they crosscut deformed rocks. A quartz diorite pluton outcrops on the western side of the Seven Devils mountains. It has been K/Ar-dated to early

Cretaceous 115 Ma from biotite and hornblende (Vallier, 1995).

Several deformational events have been documented in the Blue Mountains Province.

The oldest documented dynamothermal event occurred during the amalgamation of the terranes between 144 and 142 Ma (Gray and Oldow, 2005). Another important event occurred between

129 and 100 Ma in the belt of rocks along the terrane boundary with the North American Craton

(Davidson, 1990). This event was recorded with Ar40/Ar39 dates of plutons and metamorphism in the northern part of the Wallowa terrane.

The Seven Devils terrane is also linked to the Wrangellia terrane which spans from

Alaska to northern Washington with the possibility of including the Seven Devils terrane in western Idaho. The Wrangellia terrane has different Rb-Sr and U-Pb ages of granodiorite plutons from the different terrane segments. The ages of the granodiorite plutons from Vancouver Island,

38 the closest segment to the Seven Devils terrane, are between 185 and 190 Ma (Armstrong, 1988;

Sampson et al., 1990). The slightly younger Queen Charlotte Islands have plutons around 172

Ma age and the Burnaby Island suite ranges from 158 to 168 Ma (Anderson and Reichenbach,

1990). The plutons are all related directly to Jurassic volcanic activity in proximity of the north

American craton. Although the ages of the plutons appear to be getting younger further north in the Wrangellia terrane (Sampson et al., 1990), they do not match up with the ages of the plutons in the Seven Devils terrane or in the Blue Mountain Province, which are significantly younger than the ages in the Wrangellia terrane.

Tectonic Implications

The Seven Devils terrane sits along the western boundary of the Salmon River suture zone. This zone trends approximately N to NNE through much of western Idaho. The Salmon

River suture zone is characterized by a sharp isotopic transition known as the 0.706 87Sr/86Sr line. This line represents the change from Precambrian North American basement in the east to younger exotic terranes in the west (Gasching et al., 2011).

This line can be defined by other isotopic variations and by the general age of the rocks.

There is a significant difference between the Lu-Hf values from the Seven Devils terrane on the western side of the suture zone and the Idaho Batholith on the eastern side of the suture zone.

The Idaho Batholith yields εHf values of -5 to -30 which is below the CHUR line indicating a continentally derived terrane (Gasching et al., 2011). The εHf values of the Seven Devils terrane range from 8.6 to 15.6. These values plot above the CHUR line and close to the depleted mantle line. This means that the analyzed samples are derived from juvenile arc crust.

The surrounding terranes in the Blue Mountains Province and the Wrangellia terrane have similar isotopic compositions that tell the same story of a juvenile arc crust. The studies

39 done on these terranes use whole rock Nd isotopic ratios or zircon Hf ratios. Late Jurassic 162 -

145 Ma Plutons from the Baker terrane in the Blue Mountains Province give εHf(t) values from

7.8 to 12.3 (Schwartz, 2011). εNd values were collected from the Olds Ferry terrane in the Blue

Mountains Province. The εNd values range from 3.06 to 6.92 for 121.72 - 173.91 Ma rocks

(Tumpane, 2010). The Wrangellia terrane provides εNd values of 2.9 to 5.6 for igneous and plutonic rocks from the late Jurassic to the early Permian (Samson et al., 1990). While the range of isotopic values from the Wrangellia terrane show a juvenile arc setting, the ages from the granodioritic plutons are much older than those of the Seven Devils terrane. The age pattern of the Wrangellia terrane starting with the oldest to the south, excluding the Seven Devils terrane, and the youngest in the north would not work with the ages of the Seven Devils terrane or the

Baja B.C. hypothesis of terrane transport if these two terranes belonged together.

The Seven Devils terrane has a similar tectonic setup as the Coast Mountains Batholith in western Canada. Both the Seven Devils terrane and the Coast Mountains Batholith are considered a part of a group of terranes or a group of terranes respectively. They both sit along shear zones and the terranes are divided by faulting. The εHf(t) values from the Seven Devils samples come out to be very similar to the values of the plutons in the Stikine terrane which are also juvenile arc derived in the Coast Mountains Batholith as seen in Fig. 8. The average values for the Seven Devils terrane plutons sit between 10 and 11 which are similar to the 10 to 13 values for Hf in the plutons in the Stikine terrane (Cecil et al., 2011). To have such a juvenile crustal signature, the crust under the Seven Devils terrane had to have been of a juvenile oceanic arc origin. It also had not only partial crustal remelting, but also an incorporation of mantle- derived basalts from the subducting slab. The sharp shear zone boundary, as opposed to

40 overthrusting the terrane on the continent, allowed the Seven Devils terrane to keep its sharp contrasting isotopic values as opposed to a gradual shift to a continental signature.

Figure 8: Schematic cross section of the Coast Mountains batholiths (CMB) (from fig. 9 Cecil et al., 2011). This shows the sharp contrast of the shear zone along with εHf(t) values of plutons in each terrane.

The Seven Devils terrane shows a juvenile crustal signature in the intermediate intrusive rocks. The ages collected range from 136.7 to 120.0 Ma. Ages from previously dated plutons from the Blue Mountains Province suggest timing of thrusting in the Salmon River suture zone began somewhere around 135 to 130 Ma (Schwartz et al., 2011; Snee et al., 1995). The isotopic compositions suggest that the plutons were emplaced before the complete collision and incorporation of continental crust in the melt.

Conclusions

The LA-ICPMS zircon U-Pb isotopic dating of the Seven Devils intrusive rocks yield a crystallization age range of 136.7 to 120.0 Ma. The Lu-Hf isotopic data yields εHf(t) values from

8.6 to 15.6. This shows that the samples are from a young arc directly associated with the depleted mantle values. These results suggest two separate things. The Seven Devils terrane can not belong to the Wrangellia terrane due to the age discrepancies between them. The Seven

Devils terrane is similar to the terranes in the Blue Mountains Province based on similarly aged plutons and isotopic data. Information on the isotopic composition of the terranes in the Blue

Mountains Province is still sparse and the addition of new data from each terrane would be beneficial to make a direct comparison of all the terranes.

Acknowledgements: This work was funded by Submit funds from Dr. Kirsten Nicholson and the Department of Geological Sciences at Ball State University in Muncie, Indiana.

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resolution of U-Pb ages by laser ablation–multicollector–inductively coupled plasma–

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Acknowledgements

I would like to thank Kirsten Nicholson, my thesis advisor, for all her help and guidance.

I would also like to thank Shawn Malone and Klaus Neuman for all their help and advice along the way. Special thanks to Mike Kutis and Eric Lange for teaching me how to use all the machines that were crucial to the completion of this thesis.

I would like to thank all my peers for their help and support during the research process and writing process of my thesis. I would like to thank Isaac Burton for helping in my field excursion to collect rock samples. I would like to thank Josh Klier for all his help in processing samples to collect zircons for analysis. I would also like to thank Adel Farag for help teaching me the process of making thin sections.

I thank University of Arizona LaserChron lab for all their help and patience in showing us how to use the LA-ICPMS and taking cathodoluminescence pictures of our zircons and analyzing our samples for ages and Lu-Hf.

I would like to finally thank my parents, Ron and Teresa, and sister, Andie for all their love and support throughout this process.

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Hillhouse, J.W., Gromme, C.S., 1984, Northward displacement and accretion of Wrangellia; new

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Jones, D.L., Silberling, N.J., and Hillhouse, J.W., 1977, Wrangellia—A displaced terrane in

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Kalk, Michael Liam., 2008 "Revisiting the Seven Devils-Wrangellia connection: the

paleogeography of triassic rocks in western Idaho." WWU Masters Thesis Collection.

348 pp.

Košler, J., and Sylvester, P.J., 2003, Present Trends and the Future of Zircon in Geochronology:

Laser Ablation ICPMS: Reviews in Mineralogy and Geochemistry, v. 53, p. 243-275.

Kurz, Gene A., 2010, Geochemical, Isotopic, and U-PB Geochronologic Investigations of