Garnet-Biotite Thermometry, Garnet-Aluminum Silicate-Silica

UNLV Retrospective Theses & Dissertations

1-1-2003

Garnet-biotite thermometry, garnet-aluminum silicate-silica- plagioclase barometry, and Emp monazite geochronology of the early Proterozoic Ivanpah Mountains, eastern Mojave Desert, Se California

David Allen Shields University of Nevada, Las Vegas

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Repository Citation Shields, David Allen, "Garnet-biotite thermometry, garnet-aluminum silicate-silica-plagioclase barometry, and Emp monazite geochronology of the early Proterozoic Ivanpah Mountains, eastern Mojave Desert, Se California" (2003). UNLV Retrospective Theses & Dissertations. 1507. http://dx.doi.org/10.25669/nrt1-1gf3

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. GARNET-BIOTITE THERMOMETRY, GARNET-AljSiOs-SILICA-PLAGIQCLASE

BAROMETRY, AND EMP MONAZITE GEOCHRONOLOGY OF THE

EARLY PROTEROZOIC IVANPAH MOUNTAINS,

EASTERN MOJAVE DESERT,

SE CALIFORNIA

David A. Shields

Bachelor of Science George Mason University 1999

A thesis submitted in partial fulfillment of the requirements for the

Master of Science Degree Department of Geoscience College of Sciences

Graduate College University of Nevada, Las Vegas May 2003

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Thesis Approval IJNIV The Graduate College University of Nevada, Las Vegas

February IRrh 20 m

The Thesis prepared by

David A. Shields

Entitled

G arnet-Biotite Thermometry. Garnet-A1 oSiO s-Siliea-PIaginr1 a.gp Barnmei-ry,

and EMP Monazite Geochronology of the Early Proterny.m'e Tvanpah Mnnnt-ains,

Eastern Mojave Desert, SE California ______

is approved in partial fulfillment of the requirements for the degree of

Master of Science ______

u a C C ^ xamimtion Committe^hair

L of the Graduate College Examination Qmmittee Member

ExaminationCj

nmuate QmegeTaxulty Representative

1017-53 ii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT

Garnet-Biotite Thermometry, Gamet-AhSiOs-Silica-Plagioclase Barometry, and EMP Monazite Geochronology on the Early Proterozoic Ivanpah Mountains, Eastern Mojave Desert

David Allen Shields

Dr. Rodney Metcalf, Examination Committee Chair Professor of Geology/Geoscience Department Chair University of Nevada, Las Vegas

Metamorphic fabric measurements indicate only one deformational fabric (NNE) is

present in the Ivanpah Mountains, indicating that the area experienced one orogenic event

during the Early Proterozoic. Peak P-T conditions, using gamet-ALSiOg-silica-

plagioclase barometry and gamet-biotite thermometry, were ~4.0kb and >700°C,

respectively. Peak metamorphic-T’s were recorded by the largest garnet studied.

Electron Microprobe Analysis on monazite, using U-Th-Pb radioactive decay,

suggest metamorphism occurred between 1752 ± 8 6 Ma and 1689 ± 34 Ma, similar

to the age of the Yavapai terrane (-1.70-1.69 Ma). These ages are constrained from six

monazite grains included in three large garnets (2250-5000um). Matrix monazite from

granitic rocks indicate magmatism may have occurred between 1559 ±31 Ma and 1416 ±

28 Ma. Large garnets effectively shield monazite from Pb-loss when temperature is

exceeded above what is considered it’s closure-T for radiogenic-Pb (~700°C), thus

pétrographie location is also a function of closure-T of radiogenic-Pb in monazite.

Ill

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS

ABSTRACT...... iii

LIST OF FIGURES...... vii

ACKNOWLEDGEMENTS...... xi

CHAPTER 1 INTRODUCTION AND GEOLOGIC BACKGROUND OF THE SW UNITED STATES...... 1

CHAPTER 2 PREVIOUS RELATED WORKS IN THE IVANPAH MOUNTAINS AND THE MOJAVE CRUSTAL PROVINCE...... 5 Hewett, 1956 ...... 5 Wooden and Miller, 1990 ...... 8 Miller and Wooden, 1992 ...... 8 Barth, 2000 ...... 8 Deubendorfer et al., 2001...... 9 Young et al., 1989 ...... 11

CHAPTER 3 GEOLOGY OF THE EASTERN MOJAVE DESERT AND LITHOLOGIES OF THE IVANPAH MOUNTAIN FIELD AREA ...... 13 Rocks of the Eastern Mojave Desert...... 14 Metamorphic Fabric in the Eastern Ivanpah Mountains ...... 15 Lithologie Field Descriptions of the Eastern Ivanpah Mountains ...... 17

CHAPTER 4 PETROGRAPHY AND SAMPLE SELECTION...... 25 Methods ...... 25 Pétrographie Descriptions ...... 27 Pelitic Gneiss-IV63...... 27 Pelitic Gneiss-IV41...... 29 Augen Gneiss-IVl 1 and IV50 ...... 30 Leucocratic Sill-IV6 6 A ...... 32 Meta-Aplite SilI-IV76A ...... 33 Amphibolite Gneiss-IV5 ...... 35

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 5 ELECTRON MICROPROBE DATA ...... 38 Plagioclase, Biotite, and Garnet Chemical Characteristics ...... 39 Pelitic Gneiss-IV63...... 40 Plagioclase ...... 40 B iotite...... 41 G arnet...... 41 Pelitic gneiss-IV41 and IV41D ...... 42 Plagioclase ...... 42 B iotite...... 44 G arnet...... 45

CHAPTER 6 METAMORPHIC P-T CONDITIONS OF THE IVANPAH MOUNTAINS...... 48 P-T Grid and Thompson Projections ...... 53 Quantitative Thermobarometry...... 53 Garnet-Biotite Thermometer ...... 57 The GASP Barometer ...... 59 Analytical Techniques ...... 60 Selection of Analytical Data for P-T Determinations...... 63 Quantitative P-T Results...... 64 Metapelitic Rocks...... 64 Metagranitic Rocks...... 65

CHAPTER 7 ELECTRON MICROPROBE DATING OF MONAZITE...... 6 8 Background and Theory ...... 6 8 Locating Monazite ...... 71 Analytical Technique ...... 72 Calculated Ages and Errors ...... 74 Weighted Mean Age and Weighted Error ...... 75 MSWD...... 77 Samples and Monazite Selection ...... 78 Presentation of Data...... 79 Monazite Dating Results...... 81 Pelitic Gneiss-rV63...... 82 Pelitic Gneiss-IV41...... *:...... 87 Pelitic Gneiss + Leucosome-IV41D...... 90 Meta-Aplite SÜ1-IV76A ...... 93 Leucocratic Sill-IV6 6 A ...... 93 Augen Gneiss-IV50...... 94

CHAPTER 8 DISCUSSION AND CONCLUSIONS...... 97 Suggestions for Future workers...... 103

APPENDICES...... 105 Appendix A. FIGURES ...... 105 Appendix B. GARNET CHEMICAL ANALYSES ...... 175

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Appendix C. CORDIERITE CHEMICAL ANALYSES ...... 190 Appendix D. PLAGIOCLASE CHEMICAL ANALYSES ...... 191 Appendix E. MONAZITE CHEMICAL ANALYSES ...... 196

BIBLIOGRAPHY...... 208

VITA ...... 214

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES

Figure I Map of Early Proterozoic orogenic provinces of the SW US and generalized geologic map of the Ivanpah Mountains ...... 105 Figure 2 Pb and Nd isotopic map of the SW US ...... 106 Figure 3 Map showing locations of Proterozoic rocks of the Eastern Mojave Desert and surrounding area ...... 107 Figure 4 Differences between the Proterozoic rocks of the Eastern Mojave Desert and the Yavapai orogenic terranes ...... 108 Figure 5 Geologic map of the eastern Ivanaph field area ...... 109 Figure 6 Stereogram of metamorphic fabric measurements ...... 1 1 0 Figure 7 Photographs of NNE foliation planes of pelitic gneiss ...... I l l Figure 8 Photograph ofleucosome in foliation of pelitic gneiss ...... 112 Figure 9 Photograph of leucosome pod in foliation of pelitic gneiss ...... 113 Figure 10 Photographs of leucocratic viens in pelitic gneiss ...... 114 Figure 11 Photographs of small leucosome folds in pelitic gneiss ...... 115 Figure 12 Photographs of various leucosome structures in pelitic gneiss 116 Figure 13 Photograph of large garnet in pelitic gneiss ...... 117 Figure 14 Photographs of spatial relationship between pelitic gneiss, Amphibolite, and meta-aplite ...... 118 Figure 15 Photograph of amphibolite gneiss between pelitic gneiss foliation 119 Figure 16 Photograph of leucocratic dike intruding and cross-cutting foliation in augen gneiss ...... 1 2 0 Figure 17 Photographs of small folds in augen gneiss ...... 121 Figure 18 Photograph of garnet filled meta-aplite sill in pelitic gneiss ...... 122 Figure 19 Pétrographie image of biotite and sillimanite foliation wrapping around garnet porphyroblast ...... 123 Figure 20 Pétrographie image of garnet elongate to NNE foliation ...... 124 Figure 21 Pétrographie image of quartz grains elongate to foliation ...... 124 Figure 22a Pétrographie image of garnet (IV63gtl) enclosing quartz, plagioclase, and biotite ...... 125 Figure 22b Pétrographie image of garnet (IV63gtl) enclosing oval shaped quartz and plagioclase ...... 125 Figure 23 Pétrographie image of a cluster of monazite grains enclosed within the core of garnet IV63gtl ...... 126 Figure 24 Pétrographie image of cordierite being replaced by sillimanite 126 Figure 25 Pétrographie image of well-developed sillimanite foliation ...... 127 Figure 26 Pétrographie image of sillimanite lineation within garnet ...... 127 Figure 27 Pétrographie image of garnet pressure shadow ...... 128 Figure 28 Pétrographie image of cordierite twinning ...... 128 Figure 29 Pétrographie image of well-developed biotite foliation

V ll

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. wrapping around K-feldspar ...... 129 Figure 30 pétrographie image of elongate collection of felsic minerals between biotite foliation...... 129 Figure 31a Pétrographie image of a large zoned monazite grain ...... 130 Figure 3 lb Pétrographie image of a large zoned monazite grain ...... 130 Figure 32a Pétrographie image of prismatic sillimanite and biotite foliation...... 131 Figure 32b Pétrographie image of NNE trending sillimanite lineation wrapping around plagioclase augen ...... 131 Figure 33 Pétrographie image of fibrolite sillimanite enclosed within plagioclase ...... 132 Figure 34 Pétrographie image of opx enclosing cpx, plagioclase, and hornblende ...... 133 Figure 35 Diagram showing the relationship between the radius of garnet, cooling rates, and the maximum temperature that can be recorded by garnet-biotite thermometry ...... 134 Figure 36 Diagram showing the chemical analyses of two core to rim traverses across the two garnets IV63gtl and IV41gt2 ...... 135 Figure 37 Pétrographie image showing decreasing Mg/Fe ratios from core to rim to adjacent biotite in Iv63gtl ...... 136 Figure 38 Pétrographie image showing point traverses from core to rim towards adjacent biotite and quartz in IV41gtl ...... 136 Figure 39 KFMASH P-T grid and corresponding Thompson projections ...... 137 Table 1 Thermometry and barometry analytical points along with determined peak metamorphic P’s and T’s and retrograde T’s ...... 138 Figure 40 Diagram showing the radius of garnet vs. temperature recorded using gamet-biotite thermometry in metapelitic rocks...... 142 Figure 41 Diagram showing the radius of garnet vs. temperature recorded using gamet-biotite thermometry in metagranitic rocks...... 143 Figure 42 Diagram showing peak P ’s vs. T’s and retrograde T’s recorded using gamet-biotite thermometry and GASP barometry...... 144 Figure 43 Diagram showing the new Th/Pb vs. U/Pb isochrons ...... 145 Figure 44 Frequency Distribution Histogram for the 225 monazite EMP ages ...... 146 Table 2 Monazite ages within MSWD statistical requirements ...... 147 Table 3 Monazite ages not within MSWD statistical requirements ...... 148 Figure 45 Th/Pb vs. U/Pb isochron diagram for the gamet enclosed Monazite IV63mztlO...... 149 Figure 46a Weighted mean age plot for the gamet enclosed monazite IV63mzt3...... 150 Figure 46b Th/Pb vs. U/Pb isochron diagram for the gamet enclosed

viu

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. monazite Iv63mzt3...... 150 Figure 47 Weighted mean age plot for the gamet enclosed monazite rV63mzt5...... 151 Figure 48 Th-compositional map and corresponding spot ages of the gamet enclosed monazite IV63mztl ...... 152 Figure 49 Th-compositional map and corresponding spot ages of the gamet enclosed monazite lV63mzt2 ...... 152 Figure 50 Th/Pb vs. U/Pb isochron diagram for the gamet enclosed monazite lV63mzt2...... 153 Figure 51 Weighted mean age plot for the four gamet enclosed monazite grains lV63mztl, lV63mzt2, lV63mzt3, and lV63mzt5 ...... 154 Figure 52 Th/Pb vs. U/Pb isochron diagram for the gamet enclosed monazites IV63mztl, IV63mzt2, lV63mzt3, and lV63mzt5 ...... 155 Figure 53a Th-compositional map and corresponding spot ages of the matrix monazite lV63mzt6...... 156 Figure 53b Th/Pb vs. U/Pb isochron diagram for the matrix monazite lV63mzt6...... 156 Figure 54 Th-compositional map and corresponding spot ages of the matrix monazite lV63mzt8 ...... 157 Figure 55 Th-compositional map and corresponding spot ages of the matrix monazite lV63mzt9 ...... 158 Figure 56 Th/Pb vs. U/Pb isochron diagram for the matrix monazite lV63mzt9 ...... 159 Figure 57 Th-compositional map and corresponding spot ages of the gamet enclosed monazite lV41mztl ...... 160 Figure 58 Th-compositional map and corresponding spot ages of the gamet enclosed monazite lV41mzt2 ...... 161 Figure 59 Th/Pb vs. U/Pb isochron diagram for the gamet enclosed monazite 1V41 mzt2...... 162 Figure 60a Th-compositional map and corresponding spot ages of the matrix monazite 1V41 mzt3...... 163 Figure 60b Th/Pb vs. U/Pb isochron diagram for the matrix monazite lV41mzt3...... 163 Figure 61a Th-compositional map and corresponding spot ages of the matrix monazite IV41 mzt4...... 164 Figure 61b Th/Pb vs. U/Pb isochron diagram for the matrix monazite IV41mzt4...... 164 Figure 62 Th-compositional map and corresponding spot ages of the matrix monazite IV41 mztS...... 165 Figure 63 Th-compositional map and corresponding spot ages of the matrix monazite IV41Dmztl...... 166 Figure 64 Th-compositional map and corresponding spot ages of the gamet enclosed monazite IV41Dmzt2 ...... 167 Figure 65 Th/Pb vs. U/Pb isochron diagram for the gamet enclosed monazite IV4 lDmzt2...... 168 Figure 66 Th-compositional map and corresponding spot ages of the matrix monazite IV76Amztl...... 169

Reproduced with permission ot the copyright owner. Further reproduction prohibited without permission. Figure 67 Th-compositional map and corresponding spot ages of the matrix monazite IV 6 6 A m ztl...... 170 Figure 6 8 Th/Pb vs. U/Pb isochron diagram for the matrix monazite IV76Amztl...... 171 Figure 69 Th-compositional map and corresponding spot ages of the matrix monazite IVSOmztl...... 172 Figure 70 Th/Pb vs. U/Pb isochron diagram for the matrix monazite IV50m ztl...... 173 Figure 71 Weighted mean age plot for the matrix monazite IVSOmztl...... 174

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS

During the past 216 years many people have given their time, knowledge, and

financial support in my pursuit of this Masters Degree. There is no way I could thank

everyone sufficiently in just a few written paragraphs, but I will try. First, thanks to

my Thesis Advisor Dr. Rod Metcalf for giving me this project to work on. It’s was

exactly the type of research I wanted to do. His time and support was greatly appreciated.

Thanks Rod! Special thanks to Dr. Clay Crow for his expertise on the Electron

Microprobe. Without his help on the microprobe this research could not have moved

forward, or been finished. Thanks a million Clay! Thanks to committee member Dr.

Terry Spell for his expertise in statistics and geochronology. Thanks to Leigh Justet for

her effort in formatting the U-Th-Pb Excel age program.

I can not express how thankful I am to The Geological Society of America and

the Geoscience Department here at UNLV for their financial support. It was vital for

the successful completion of this project.

And how could I ever forget to thank my loved ones back in Virginia. I missed

you all a million times over the last couple of years. I know all of you never stopped

believing that I could do this. I thank all of you for your words of support. I also cannot

forget to thank my Aunt in West Virginia for her support. She never let me forget that

there was a reward at the end of the rainbow. Special thanks to my Father for his support

and for sparking my interest in the planets and astronomy, which eventually led to my

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. interest in geology. Thank you Dad!

And Mom, how could I ever thank you enough for your words of strength and

encouragement. You were always there and never stopped believing that I

would see this to the end. THIS THESIS IS DEDICATED TO YOU! ! !

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 1

INTRODUCTION AND GEOLOGIC BACKGROUND OF THE SW UNITED STATES

During the Early Proterozoic, located in what is now the SW United States, several

juvenile crustal terranes were accreted to the supercontinent of Laurentia in a series of

orogenic events (Hoffman, 1988; Bowring and Karlstrom, 1990). Included in these

orogenic events were the well documented Mazatzal Orogeny which occurred at ~ 1 .6 6 -

1.60 Ga producing in the Mazatzal orogenic terrane, and the heavily studied Yavapai

Orogeny at ~1.70-1.69 Ga producing in the Yavapai orogenic terrane (Karlstrom and

Bowring, 1988). Additionally, there is the hypothesized Ivanpah Orogeny at >1.70 Ga,

which is believed resulted in the Mojave orogenic terrane (Wooden and Miller,

1990)(Figure 1). This progression of several continental collisions during the

Paleoproterozoic, adding to the assembly of Laurentia, is seen in the rock record of the

SW United States in a somewhat linear orogenic belt that extends from eastern and

central New Mexico, through Arizona, and into southern Nevada and SE California

(Figure 1). Divisions into separate orogenic terranes was first based on the isotopic

signatures of Nd (Bennett and Depaolo, 1987) and a study by Wooden et al (1988) that

found that the differences in (Pb) isotope ratios in the region could be used to define

boundaries between the different orogenic crustal provinces and roughly coincided with

the proposed Nd isotopic boundaries (Figure 2).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. An abundance of subsequent geochronologic work has been carried out on the timing

and style of accretion of the both the Mazatzal orogenic terrane and as the Yavapai

orogenic terrane, to place them in a correct geochronologic context and to reconstruct

models of Paleoproterozoic geologic history in the region (Karlstrom and Bowring, 1988;

Lerch et al, 1991; Wooden and Dewitt, 1991; Labrenz and Karlstrom, 1991; Daniel et al,

1995). In fact, a fairly complete reconstruction of Proterozoic crustal evolution in most of

the SW United States presently exists (Wooden and Miller, 1990). The exception to this

is the Eastern Mojave Desert, which is part of the proposed Mojave orogenic terrane (also

called the Ivanpah orogenic terrane by some workers). In fact very little work has been

done, and sparse literature exists on Proterozoic crustal evolution in the Eastern Mojave

Desert. Before the mentioned isotopic boundaries were proposed the Eastern Mojave

Desert was thought to be a part of the Yavapai terrane, and thus it was believed by

geologists that the Eastern Mojave Desert was added to the current North American

craton during the Yavapai orogeny. The few geochronologic studies that do exist on the

Mojave crustal province by Miller and Wooden (1992) Barth et al. (2000) Dubendorfer et

al. (2001) (discussed further in Chapter 2) have challenged this assertion and have

concluded that the two regions contrast sharply in orogenic style and timing of accretion.

The also argue that the two regions are the result of two distinct continental collisionary

episodes during the Early Proterozoic that can be spatially separated in space and time.

Thus, in recent years the debate among researchers has evolved to refocusing research in

the area to that of geochronologic studies to determine the actual timing of the addition of

the two regions rather than to continue the old debate concerning whether the two

orogenic terranes represent separate geologic events. That has now been established in

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the minds of some researchers, but much of the Eastern Mojave Desert remains unstudied

and more research is needed to determine decisively if the two regions are deserving of

their separate orogenic status. In addition even with recent research (discussed below) the

timing of the accretion of the Eastern Mojave Desert is still not conclusively established.

This thesis study takes place in the eastern Ivanpah Mountains, Eastern Mojave

Desert, SE California (Figure 3). The main purpose of this study will be to determine the

timing of metamorphism, deformation, and tectonic style of accretion in the area of the

Ivanpah Mountains and add to a limited data base of geologic work that currently exist in

the Eastern Mojave Desert. The thesis project may help geologists finally determine

whether the Mojave crustal terrane represents a separate and distinct orogenic event than

that of the Yavapai terrane and the debate concerning the origin and crustal evolution of

the SW US may be closer to being resolved.

Additionally, this thesis project will represent the first detailed pétrographie study of

the Early Proterozoic metamorphic rocks of the Ivanpah Mountains and will be the first

extensive geothermobarometric study involving the calculation of metamorphic P-T

conditions of these same rocks in the Ivanpah Mountains. Furthermore, the research will

involve using a fairly new technique that uses the Electron Microprobe (EMP) to date the

age of cystallization of the light rare earth element phosphate monazite through the

radioactive decay of U and Th to Pb (discussed further in Chapter 6 ). This technique of

dating monazite with the EMP to reconstruct geochronologic events and using monazite

as a geochronometer in metamorphic terranes is now being widely accepted by geologists

as a valid technique in reconstructing geologic events (Montel et al., 1996). This is the

first time the technique has been developed and attempted at UNLV. This study may also

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. be used as a template for any future attempt at EMP dating of monazite at the university.

No extensive detailed geochronologic data currently exists using EMP dating of

monazite on the Proterozoic rocks of the Eastern Mojave Desert, although Wooden and

Miller (1989) did use the EMP to date one monazite crystal in their study of the Ivanpah

Mountains in 1990. Most geochronologic studies that have been done in the Eastern

Mojave Desert as well as the Mojave crustal province as a whole, has focused on U-Pb

zircon ages of the abundant pre-, syn-, and post-orogenic granitic rocks that can be found

in the region. Supracrustal metamorphosed sedimentary rocks in the region have also

been dated using U-Pb zircon ages.

Additionally, pelitic rocks are of interest in this project because specific mineral

assembleges in the pelites can be used as geothermometers (i.e. gamet-biotite; Hodges

and Spear, 1982) and geobarometers [i.e. GASP (gamet-ALSiOs-quartz-plagioclase)

Hodges and Spear, 1982] to interpret approximate ranges of metamorphic P-T conditions

in the Ivanpah Mountains. Furthermore, high-grade granulite facies pelitic rocks have

been found to contain an abundance of monazite, which occurs as the major phosphate

phase in the Proterozoic rocks of the Eastem Ivanpah Mountains. Pressure, temperature,

and age data from this research will help accomplish the goal of this study, which is a

better understanding of the Early Proterozoic geologic history of the Eastem Mojave

Desert.

Methods for the determination of metamorphic P’s and T’s, as well as a more

detailed discussion of the EMP monazite dating technique, will be fully discussed in

Chapters 5 and 6 of this thesis.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 2

PREVIOUS RELATED WORK IN THE IVANPAH MOUNTAINS AND THE

MOJAVE CRUSTAL PROVINCE

Hewett, 1956

The first extensive published geologic work in the Ivanpah Mountains was by

Hewett (1956). The main purpose of his work was to report on the mineral resources, as

well as to map the Ivanpah quadrangle at a 1:125,000 scale. Hewett mapped all

Proterozoic rocks in this quadrangle as Pre-Cambrian gneiss and granite. The map is still

widely used today and is one of the only detailed geologic maps ever produced in this

particular area. No geochronology, or P-T work, was done by Hewett in this early

reconnaissance of the area, although Hewett did provide the first brief descriptions of the

Pre-Cambrian rocks and also provided a brief interpretation of the relative age

relationships of the different lithologie units of several different mountain ranges in the

Eastem Mojave Desert.

Wooden and Miller, 1990

In 1990, Wooden and Miller put together the first detailed geochronologic study in

the Ivanpah Valley including the Ivanpah, New York, and McCullough Mountain ranges

(Figure 3). They chose this particular area because of the unaltered state of the Early

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Proterozoic rocks that are located there. They stated in their 1990 paper that the Ivanaph

Valley was a good locality for Proterozoic studies because this particular area contained

Proterozoic rocks that were less effected by Phanerozoic deformational and extensional

overprints than in other parts of the Eastem Mojave Desert. Their research expanded on

other work that had been completed in the Eastem Mojave Desert at other localities by

Howard et al. (1982, 1988), Miller et al., (1986), Nielson et al. (1987), Davis et al.

(1982), and John (1982), and put together an isotopic and geochronologic synthesis of

these other works. The goal was to reconstract the Proterozoic crastal evolution in the

Eastem Mojave Desert, and to review the evidence that had been gathered up to that point

that suggested that significant isotopic, as well as, an absolute age difference existed

between the Proterozoic rocks of the Mojave cmstal region and the Proterozoic rocks of

the adjacent Yavapai cmstal region. Figure 4 is a list of the differences between the

Mojave and Yavapai orogenic provinces that Wooden and Miller suggested were

evidence that the two terranes are the result of two separate orogenic episodes.

Differences listed in Figure 4 include age of oldest supracmstals; 1.9-2.3 Ga in the

Mojave versus Yavapai supracmstals with ages of < 1.8 Ga (Pb-Pb zircon ages). (2)

Granitic rocks in the Mojave are pre-, syn-, and post-orogenic while no post-orogenic

(1.69-1.67 Ga) granitoid have been recognized in most of the Yavapai, (3) Nd model ages

of the Proterozoic Mojave are between 2.0 and 2.2 Ga, with crystallization ages between

1.7-1.8 Ga. The Proterozoic rocks in the Yavapai exhibit Nd model and crystallization

that are very similar and are between 1.60-1.85 Ga. (4) Initial Pb from feldspar analyses

of the Early Proterozoic rocks of the Mojave cmstal province indicate average isotopic

ratio values at 1.7 Ga o f ^ b / ^ b = 16.1, ^“’Pb/^'^b = 15.38, and ^ P b /^ b = 35.65.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. In the Yavapai crastal province initial Pb ratios at 1.7 Ga, (from ore galenas) of Early

Proterozoic rocks have average compositions of ^°^Pb/^*^Pb = 15.72, ^"^Pb/^^Pb = 15.27,

and ^°^Pb/^^Pb = 35.34. (5) Extensive Proterozoic migmatitic sequences in the Mojave

province are widespread, but migmatites are rarely found in the Yavapai, indicating a

higher-grade of metamorphism took place in the Mojave. Granulite facies metamorphism

is characteristic of the Proterozoic Mojave while upper-greenschist to amphibolite facies

metamorphism is dominant in most of Arizona and New Mexico (Williams, 1991). ( 6 )

Structural and lithologie trends in most of the Yavapai strike ENE-WSW, while in the

Eastem Mojave Desert and the Ivanpah Mountains the foliation of metamorphic fabric

generally strikes NNE-SSW (Wooden and Miller, 1990; this study, 2002).

It is important to mention, although the Mojave and Yavapai crastal provinces exhibit

differences, a boundary zone exist that was defined by Wooden and Dewitt (1991) based

on the Pb isotopic signatures mentioned above in Figure 4. The boundary zone is ~ 75 km

wide and is characterized by rocks that have Pb isotopic signatures that range from that of

the Mojave to that of the Yavapai (Deubendorfer, 2001). Thus, an abrupt sharp margin

between the two regions where geologic characteristics of one province stops and the

other begins is not sudden, but gradational in nature (Bennett and Depaolo, 1987).

Ages determined from the previous works are mostly from zircon fractions in pre-,

syn-, and post-orogenic granitic rocks found in the region, with some ages from zircons

from supracmstals. The thesis I am presenting here focuses not only on the granitic and

related intrusive rocks but also on the pelitic metasedimentary gneisses (paragneisses)

found in the eastem Ivanaph Mountains. The pelitic rocks have been chosen for an

additional focus because little geochronologic work has been done on these rocks in the

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Eastem Mojave Desert. I will be providing new data by which a better understanding of

the timing of metamorphism, as well as, the style and timing of deformation may be

conclusively established in the Eastem Mojave Desert.

Miller and Wooden, 1992

The only other major known published research on the Ivanpah Mountains was done

by Wooden and Miller (1992). They published a Proterozoic field guide of the Ivanpah,

New York, and Providence Mountains. In this USGS Open-File Report, Miller and

Wooden produced a 1:24,000 geologic and lithologie map of the Ivanpah Mountains

(Figure 5). This particular location within the Ivanpah Mountains is the chosen field area

in this thesis project. It was originally intended in this thesis research to map this area in

more detail but the units where found to be highly variable in lithology on a very small

scale, thus the Miller and Wooden map was used as a guideline for the location of the

varying types of lithologie units, sample collection, and the measurement of metamorphic

fabrics. Lithologies found in the Ivanaph Mountains are shown on the map and are

described in detail in Chapter 3.

Barth, 2000

Barth et al. (2000), reported geochronologic studies (ages also on U-Pb zircon

fractions) from the Baldwin Lake area (westemmost portion of the Mojave cmstal

terrane) of the central San Bemardino Mountains, southem Califomia. The maximum age

of Baldwin Lake orthogneisses (gneisses with igneous protoliths) were determined to be

-1783 +/- 12 Ma, while minimum ages reported were 1675 +/- 19 Ma (ages from U-Pb

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in this westemmost portion of the Mojave cmstal province are greater than the maximum

ages reported for metaigneous found in the eastem portion of the Mojave which have

been determined to be 1760 Ma +/- 20 Ma by Wooden and Miller (1990). Again, this

study by Barth, 2000, is similar to that of the Wooden and Miller (1990), which mainly

reported zircon U-Pb ages from augen and granitic gneiss and other felsic plutonic rocks.

Barth’s model for the accretion of early Proterozoic terranes to the supercontinent of

Laurentian illustrates a composite Mojave-Yavapai terrane that became sutured to the

craton at ~ 1.65-1.60 Ga. He hypothesized that this was the Mazatzal orogeny that is

recorded in the rocks of most of New Mexico and parts of eastem Arizona. The model

shows Ivanpah and Yavapai terranes as separate ancient island-arcs off the coast of the

North American craton and at -1.7 Ga these two separate arc terranes collided in the

Proterozoic ocean becoming a composite arc terrane before accreting to what was then

the supercontinent of Laurentia at -1.65-1.60 Ga during the Mazatzal orogeny. The older

supracmstals (-1.9-2.3 Ga) that can be found in the Mojave can be explained in Barth’s

model as the result of the separate Mojave arc terrane at -1.8-1.7 Ga located near the

margin of Laurentia and receiving Late Archean and Early Proterozoic continental

sediments.

Deubendorfer et al., 2001

The most recent attempt to reconstmct Early Proteroizoic cmstal evolution in the SW

United States was by Dubendorfer (2001). His work took place in the Cerbat Mountains

in northwestem Arizona (Figure 3). These particular mountains are the eastemmost

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Proterozoic range that contains solely Mojave Pb isotopic signatures but exhibits a

similar structural history as the Yavapai province (Duebendorfer et al., 2001). They lie on

the very western edge of the boundary zone. Two metamorphic fabrics were discovered

in the metamorphic Proterozic rocks of the Cerbat Mountains. Si showed a metamorphic

fabric comparable to the pervasive fabric in the Yavapai terrane (strike = NW; dip =

moderately northeast). S 2 was orientated similar to the dominant fabric located in other

Proterozoic ranges in the Mojave (strike = NNE; dip = sub vertical). 82 truncates and

transposes Si (Duebendorfer et al., 2001). Thus the development of S 2 occurred after the

formation of Si.

The study dated orthogneisses (metagranites and metadacites), and metacarbonates

that were patially associated with Di and D 2 (pre-, syn-, and post orogenic) producing Si

and S 2, respectively. Using U-Pb zircon, sphene, and apatite ages and associating the ages

with deformational events Di and D 2, they were able to produce an alternative model to

that of Barth et al.’s (2000) model for the progression of the growth and crustal evolution

of the Proterozoic in the SW United States. Briefly, their research bracketed Di between

1737 +/- 4.3 and 1721 +/- 2.4 Ma by U-Pb zircon ages from granitic rocks that were

associated with late-syn Di and post-Di/Pre-D 2, respectively. D 2 was bracketed between

the post-Di/pre-D 2 age of 1721 +/- 2.4 Ma and a zircon of a granitic dike that was post-

D2, dated at 1682.5 +/- 2.3 Ma. From these ages and their association with Di and D 2, the

model that was produced interpreted the Di event occurring a t-1740-1720 Ma (called

the Medicine Bow orogeny by Duebendorfer et al., 2001) to represent a collisionary

event between the Mojave and Yavapai terranes, before their accretion to the craton

producing a composite Mojave-Yavapai crustal terrane. Eventually this composite terrane

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produced S 2 (the principal fabric that transposes Si) during it’s eventual collision with the

SW margin of Laurentia between 1710-1680 Ma (Yavapai orogeny)(Duebendorfer et al.,

2001).

It’s important to point out that both the model by Barth (2000), and Deubendorfer

(2001), interpret the Mojave and Yavapai to have collided first, becoming a composite

terrane, before their co-addition to what was then Laurentia. Although Wooden and

Miller (1990), did not offer an illustrated model for the Proterozoic continental growth of

the SW US, they did not present any evidence, nor conclude that the Mojave and Yavapai

ever existed as a composite terrane. Their conclusions seemed to indicate to this writer

that both terranes added to the growth of Laurentia by independent accretionary episodes

that occurred at a time interval separated by -10-20 Ma. The Ivanpah orogeny at -1.71-

1.70 Ga and the Yavapai orogeny at <1.70 Ga.

Young et al., 1989

The only review of related works that will be done here concerning petrography,

barometry, and thermometry is that of Young et al. (1989) which took place in the

McCullough Range of the Ivanpah Valley area (Figure 3). The McCullough range is

located about 20 miles east of the Ivanaph Mountains across the Ivanaph Valley.

Thermobarometry, as well as pétrographie research, in the Eastern Mojave Desert is even

more limited than geochronologic data that currently exist. The 1989 study by Young et

al., presented the first in depth report on petrography, barometry, and thermometry of any

Proterozoic mountain ranges in the Ivanpah Valley. Using the four cation exchange

equilibria reactions gamet-biotite, gamet-sillimanite-quartz-plagioclase (GASP), gamet-

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cordierite, and cordierite-spinel it was determined that the Proterozoic metamorphic rocks

in the McCullough Mountains experienced low-P and high-T metamorphic conditions

during deformation. Pressures and temperatures that were determined using these stated

barometers and thermometers restricted the conditions of metamorphism to 3-4 kb and

590-750 °C.

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GEOLOGY OF THE EASTERN MOJAVE DESERT AND LITHOLOGIES OF THE

IVANPAH MOUNTAINS FIELD AREA

It is not the intention or one of the goals of this thesis research project to repeat the

works of Hewett (1956), or Wooden and Miller (1990), or Miller and Wooden (1992),

which included field descriptions of the lithologie units that are located in the Ivanpah

field area and of Proterozoic rocks of other mountain ranges in the Ivanaph Valley.

However, I will try and to expand on, and/or improve upon both the work of Hewett, and

of Wooden and Miller, with the intention of helping the reader understand the various

structures within units, the relative chronologic relationship among the different units,

and most importantly, the relationship of each particular unit to the deformation and to

that of the dominant metamorphic fabric that is pronounced in the field area.

Also it is the opinion of the writer of this thesis that some names given to lithologie

units by Miller and Wooden (1992), are confusing because of the exclusion of the meta

prefix from some the names of the metamorphosed igneous rocks in the field area. Thus,

all metamorphic rocks with igneous protoliths will be changed to include this prefix at the

beginning of the name. For example, a granitic rock with metamorphic characteristics

will be referred to as metagranite, just as metamorphosed gabbro will be called

metagabbro. The changes in the names will also be reflected on the lithologie map of the

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field area (Figure 5. Furthermore, the rock name mafic tonalité, given by Miller and

Wooden (1992), will not be used in this study.

Rocks of the Eastern Mojave Desert

Rocks in the eastern Mojave Desert and the surrounding region are primarily

Proterozoic to Cenozoic in age (Hewett, 1956). The Early Proterozoic rocks exposed in

the Eastern Mojave Desert have been part of North America for at least 2 billion years

(Foster et al., 1990). Proterozoic basement rocks generally range from ~ 1.7 Ga to 1.4 Ga.

and are widely distributed throughout the Eastern Mojave Desert (Glazner et al.,

1994)(Figure 3). Two unique features of eastern Mojave Proterozoic rocks versus

adjacent area include migmatitic gneisses and undeformed to highly deformed plutonic

granite bodies of batholithic dimensions (Wooden and Miller, 1990). Pre-, syn-, and post-

orogenic plutonic igneous granitoids have now been documented in the Eastern Mojave

Desert with ages ranging from 1.76 to 1.66 Ga and contain zircons in their wallrock up to

700 m.y. older than the oldest plutonic rocks (Miller and Wooden, 1992). A

geochronologic sequence of events before 1.76 Ga is rather vague in the Eastern Mojave

Desert, but Miller and Wooden (1992) described this time period as one of deposition of

sedimentary rocks and volcanism. Between 1.76 Ga and 1.73 Ga rocks of the Mojave

province were intruded by mafic, metaluminous, and K-rich magmas (Miller and

Wooden, 1992). A major orogenic event, followed these intrusions at around 1.71 to

1.695 Ga (Ivanpah orogeny?; Yavapai orogeny?) and caused thorough migmatization of

older rocks as well as the synmetamorphic plutonic bodies (Miller and Wooden, 1992).

At ~ 1.66 Ga granitic plutonic bodies intruded the area on a regional extent (Miller and

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Wooden, 1992). These younger granitoids display an evolution of K-rich, peraluminous

magmas to metaluminous calc-alkaline magmas (Miller and Wooden, 1992). Proterozoic

magmatism younger than 1.4 Ga is not well documented, but anorogenic (1.4 Ga)

igneous plutons do occur in abundance in the Mojave Desert Region (Miller and

Wooden, 1992). Minor exposures of ~ 1.1 Ga diabase sills are also found in the eastern

Mojave Desert (Glazner et al., 1994). Paleozoic ( > 570 Ma) to lower Permian

miogeoclinal to cratonic rocks, along with continental slope and rise deposits, are also

present in the eastern Mojave Desert, and overlie the igneous and metamorphic basement

(Burchfiel and Davis, 1975; Martin and Walker, 1992; Stewart and Poole, 1975; Glazner,

1994). The Cenozoic is represented by basaltic and rhyolitic lava flows along with

continental sedimentary rocks and alluvium (Hewett, 1956).

Metamorphic Fabric in the Eastern Ivanpah Mountains

The study area lies in the eastern Ivanpah Mountains just northeast of the Clark

Mountain Fault (Figure 1). Briefly before proceeding, I would like to emphasize that it

was not the intent of this project to include extensive structural techniques to reconstruct

the Proterozoic geologic evolution in the Eastern Mojave Desert. A more intense and

larger scale structural analysis could provide valuable insite into the Proterozoic crustal

evolution of the region. Nevertheless, the only structural analysis offered in this project is

the interpretation of metamorphic fabric orientation through the production of a

stereographic projection, resulting from the ~ 200 fabric measurements taken on the

rocks of the eastern Ivanapah’s during the months of May and June, 2001.

Strike and dip was measured on all major rock types (major Ethologies listed below).

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The majority of fabric measurements were carried out on pelitic gneiss, but occasionally

even migmatites and intruding and/or cross-cutting metaigneous rocks were measured for

orientation of fabric. The results of fabric measurements are shown in the stereogram

(Figure 6). The interpretation from the stereogram is that one metamorphic fabric is

dominant in the Ivanpah field area with a preferential orientation of NNE. Poles to the

foliation planes are located on the stereogram as points. Each point represents a foliation

plane, which is orthogonal to the point. A clustering of points in the NW quadrant of the

stereogram near the center indicates one metamorphic fabric exist in these rocks with a

preferred orientation of N10°E strike and a subvertical dip of 69° to the NW. Figure 8

shows two examples of pelitic gneiss and one example of augen gneiss in outcrop

showing this dominant characteristic NE striking NW dipping metamorphic fabric that is

present in the Ivanpah Mountains. This was the only metamorphic fabric macroscopically

examined or determined to be present or recorded by the rocks in the field area,

evidenced by the results of the stereographic projection. This slightly NE strike with a

subvertical dip to the NW is consistent with the determined orientation of Early

Proterozoic metamorphic fabric at other localities within the Eastern Mojave Desert

(Hewett, 1956; Wooden and Miller, 1990, Young et al, 1989, Barth, 2000; Deubendorfer,

2001). At some other locations within the Eastern Mojave Desert and the transition zone

(one example is the Cerbat Mountains), two metamorphic fabrics are present

(Deubendorfer, 2001), which seemingly record a more complex geologic history than the

Proterozoic metamorphic rocks of the Ivanapah Field area.

Other alternatives to this interpretation are, (1) granulite facies metamorphism

that is characteristic of the Mojave completely obliterated evidence of the first fabric or

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(2), simply the rocks in the eastern Ivanpah’s were not geographically situated to record

both collisionary events. Special attention was given in the pétrographie portion of this

research to locate and/or find evidence for any additional metamorphic fabrics that may

have been preserved by the rocks in the field area.

All rock types in the Ivanaph Mountain field area, which I am going to provide

lithologie, as well as, field descriptions for below, carry this macroscopic characteristic of

one defining deformational fabric with a NNE strike and NW dip. Lineation in

amphibolites and metagabbros also coincided with this orientation of fabric as they were

found to carry a trend of mineral lineation to the NNE (angles similar to strike of foliation

planes).

Lithologie Field Descriptions of the Eastern Ivanpah Mountains

Emphasis for lithologie field descriptions will be placed specifically on several rock

units, specifically on several rock units including pelitic gneiss, migmatite (including

veins, pods, layers), augen gneiss, metaigneous cross cutting and intruding white felsic

dikes (called metaaplite by this study), and crosscutting and intruding metamafic dikes,

including metagabbro, amphibolite, and amphibolite gneiss. Emphasis has been placed on

these five lithologie units because of their overall abundance in the field area, the relative

relationships among the units, and the strong association these units have with the one

dominant metamorphic fabric present. These five rock types are also the most abundant

rocks in the field area. Other felsic metaigneous rocks, which include a variety of

metagranitoids and metadiorites, will be briefly described. Complete modal analysis of

mineralogy of the rock types will be given in Chapter 4 (Pétrographie Descriptions).

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The metasedimentary rock units within the Ivanaph Mountain field area are the most

abundant Ethologies present. These gneissic rocks are composed of pelitic to greywacke

compositions (Miller and Wooden, 1992) and display the extremely well defined foliation

planes present in the field area. An abundance of pelitic gneiss with variable

compositions and Ethologies can be found in the Ivanpah Mountains, including pelitic

gneiss (biotite-sillimanite), senE-pelitic (biotite-rich) gneiss, and biotite-gamet gneiss

(Miller and Wooden, 1992). The foliation of these metasedimentary gneisses are so well

displayed in outcrop they form what appears to be a false stratigraphy (Figure 7) from a

distance. Upon closer examination several metamorphic characteristics reveal the true

nature of these rocks. The first distinctive feature within the variety of pelitic gneisses

units are the abundance of interlayered and intermingled leucosome material. The

leucosome appears in most places to have been injected between foliation planes of these

metasedimentary units (Figure 8) perhaps as a partial melt due to granulite facies

metamorphic conditions (Young et al, 1989). Leucosome lenticular shaped pods are

common in the schistose portion of the pelitic gneiss in which it occurs (Figure 9). In

other locations the leucosome, as well as, small leucocratic veins and dikes (mm’s to

cm’s in diameter), appear to both crosscut foliation and intrude the planes of weakness

between foliation planes (Figure 10). In other locations small leucosome folds occur

within the pelitic gneiss (Figure 11). These leucosome folds probably crosscut foliation in

the pelitic gneiss in the latter stages of deformation event since they both crosscut

metamorphic foliation and are folded, but at other locations the leucosome veins lack any

folding. This widespread relationships between the pelitic gneisses and

leucosome/leucocratic dikes seem to indicate that the melting and intrusion of felsic

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material into the pelitic gneisses occurred late syn-orogenic to post-orogenic. Young et al

(1989) also agreed that considering the weakly developed foliation within the leucosome

material parallel with the NNE striking, NW dipping Si foliation of the paragneisses,

these granitic compositional dikes were apparently emplaced during the latter stages of

the Si deformational event. Because of the abundant association between the pelitic

gneisses and leucosome. Wooden and Miller (1990) called these rocks migmatitic pelitic

gneisses. These “migmatitic pelitic gneisses” are primarily grey to blue-grey in color with

an overwhelming quantity of leucosome pods, hooks, and layers (Figure 12) that are

filled with an abundance of garnet. The garnet in some locations forms clusters in the

shape of paw-prints (Wooden and Miller, 1990). Garnet is also a common mineral in

the greyish-blue schistose portion of this gneissic rock and occurs as porphyroblasts.

Garnets usually range in size from just a few mm’s to 7-8 cm’s in diameter (Figure 13)

and at isolated localities may make up as much as 30-40% the composition of the pelitic

rocks. Small leucocratic folds also are common within the pelitic gneisses and display

axial planes parallel the foliation planes in the migmatites.

The other dominant texture present in the pelitic gneisses is the strong foliation

defined by biotite mica and the abundance of sillimanite needles, which define a lineation

also wrapping around garnet porphyroblast. Coarse (l-5mm) sillimanite can be observed

in hand sample and can be seen defining a lineation that parallels the dominant NE

striking foliation direction in some pelitic gneiss outcrops. Rarely white whisps of

fibrolite sillimanite can be recognized in the pelitic gneisses in hand sample. An

abundance of sillimanite in these rocks attest to their pelitic origin (Miller and Wooden,

1992).

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One very interesting aspect of the migmatitic pelitic gneisses is the occurrence in

many locations throughout the field area strongly associated with the NE striking

metamafic dikes (metagabbro or amphibolite), and other NE striking metaleucocratic

dikes. These three Ethologies appear at several locations in a group of three displaying

again what falsely appears as flat lying layers all having the same orientation of strike to

the NNE and dip to the NW (Figure 14). The chronologic interpretation between the three

units is that the metaigneous rocks metaaplite and amphibolite gneiss, intruded a very

weak foliation plane that was created during metamorphism of the protolith that lead to

the crystallization of the pelitic gneiss. Notice in Figure 14a that not only has the

metaigneous rocks, metagabbro and metaaplite, intruded along weak foliation in the

migmatitic pelitic gneiss, but the metaaplite itself is also intruded by a smaller leucocratic

dike along strike of foliation. This smaller leucocratic dike also appears to have selected

an intrusive route through the direction of foliation in the rock that it is intruding, thus

these metaigneous rocks are late-syn to post-orogenic. More evidence to back this

interpretation was located at several locations where small leucocratic dikes cross-cut

foliation in pelitic gneisses at 90° for a short distance but abruptly change direction of

intrusion 90°, following a weak foliation plane in the pelitic gneiss (Figure 10). Young et

al (1989) found a similar relationship between the pelitic gneisses and leucocratic

material in the near-bye McCullough Range and also agreed that considering the weakly

developed foliation parallel with the NNE striking, NW dipping Si foliation of the

paragneisses these granitic plutons were apparently emplaced during the latter stages of

the Si deformational event.

The large metaaplite dikes and other metaleucocratic dikes may represent large scale

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melts due to in situ anatexsis of the pelitic rocks that were subsequently injected through

weak foliation, thus genetically related to the leucosome melts but on a larger scale.

Within these white leucocratic dikes common occurring minerals are quartz, K-spar, Na-

plagioclase, biotite, and garnet. Euhedral garnet’s (mm’s to cm’s) are especially

spectacular scattered throughout the pure crystalline white rock body of the aplitic

composition leucocratic dikes. All the above mentioned minerals can be identified in

hand sample.

As shown in Figures 14a and 14b, metamafic dikes also occur in close spatial

relationship with the metaleucocratic dikes and the migmatitic pelitic gneiss. The

metamafics include coarsely crystalline metagabbros with hornblende, pyroxene (cpx and

opx), and reddish-brown Mg-rich biotite (phlogopite). Phenocryst often occur as

plagioclase that preserve original igneous textures as polygonized groups of medium-

grained minerals (Miller and Wooden, 1992). No garnet is present in these rocks. Most

samples collected from the metagabbros showed a preferred hornblende lineation to the

NNE, but occasionally no preferred lineation could be observed in either hand sample or

outcrop. At one location a metagabbro cut foliation in an augen gneiss at -70° and

displayed no metamorphic fabric, thus interpreted to be strictly post-orogenic.

The amphibolites occurred both with (Figure 15) and without gneissic layering. Light

colored -1-2 mm layers, when present, were composed mostly of quartzite with other

felsic material. Occasionally, the quartzite layers are several centimeters wide with

hornblende needles aligned to the lineation that they define. The amphibolites, like the

metagabbros, are composed mainly of hornblende and pyroxene (cpx and opx). While

garnet is also absent in these rock types. Most amphibolites can be chemically separated

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from the metagabbros on the basis of SiOa and MgO content (Miller and Wooden, 1992).

The amphibolites contain ~ 44-50% Si02 and 10-20 % MgO, while metagabbros contain

~ 51% Si02 and <10 % MgO (Miller and Wooden, 1992).

Augen gneiss is also abundant in the eastern Ivanpah field area. Extreme weathering

of these units made it difficult to collect fresh samples, as well as, measuring

metamorphic fabric orientation. The abundance of augen gneiss is located in the small

stream valleys that surround the mountain range, thus are usually located topographical

lower than the other Ethologies in the area. Perhaps these rocks represent intrusive

igneous rocks that were metamorphosed and then subsequently sheared, but could also

represent quartzofeldspathic sediments that were metamorphosed (Wooden and Miler,

1990). Feldspar augen can be recognized in many locations and occurs as 1-3 cm

lenticular eyes set in a matrix of felsic mineralogy with an abundance of biotite defining a

well-developed foliation. The augen gneiss is also cut by small leucocratic dikes and

viens similar to that seen in the pelitic gneisses. More evidence to back the earlier

interpretation that the abundant leucocratic dikes in the mountain range are late-syn to

post-orogenic can be found in the augen gneiss at several locations where small

leucocratic dikes cross-cut foliation in augen gneiss at 90° for ~ 1-2 meters but then also

abruptly change direction of intrusion (similar to that seen in the pelitic gneisses) at 90°,

thus also choosing and following a weak foliation plane in the augen gneiss (Figure 16).

Additionally, two larger folds were located within the augen gneiss units (Figure 17)

within two separate dry flash flood stream beds. Wave height and wavelength of the folds

are both -2-3 meters, respectively. These small folds within the field area represent the

largest folded structures ever reported in the eastern Ivanpah Mountains. Miller and

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Wooden (1992) speculated that any large- scale folds that may have once been present

here may have been destroyed by shearing.

Finally, the last lithologie field description that will be presented here is of the

remainder of felsic metaigneous rocks that are prominent in the eastern Ivanpah field

area, which include a variety of metagranitoids and metadiorites of varying composition.

Pink metagranitic rocks commonly occurr in the field area. These metagranites, including

metapegmatite, also intrude the pelitic gneiss and augen gneiss sequences along and

between the Si foliation planes. The contacts between the metagranites and metadioritic

rocks, and the pelitic gneiss, are largely gradational in nature (at most locations), and not

as sharply defined as the contacts that occur between both the majority of metamafic

rocks and metaaplites, with the pelitic gneiss. These rocks units usually occur in a wide

range of sizes from several cm’s wide to several meters wide. Gneissic layering is a

common characteristic of the granitic rocks. Biotite is abundant which defines a foliation

parallel to the Si foliation in the pelitic gneisses. Garnets in these rocks are very small (1-

3mm) compared to the large garnets observed in the other lithologie units, especially the

metapelites. The garnets also define the direction of foliation in these rocks. Sometimes

they form linear string-like trails parallel to the biotite foliation planes found in the rocks.

Occasionally, coarse sillimanite 2-3mm long can be observed in outcrop hand samples.

Sillimanite was so abundant at one location the rocks displayed a cloudy white whispy

appearance. The metagranitic pegmatites contain alkali feldspar crystals as large as 2-3

cm long and are less common than the phaneritic metagranitoids. Chemical

characteristics of these related granitoids are subalkalic with enrichment of total alkalies

at 7-8 wt %, while potassium is more abundant than sodium (Wooden and Miller, 1990).

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Miller and Wooden (1992) describe a variety of metigneous rocks of dioritic

composition which include quartz-diorite, biotite-rich granodiorite, “Mafic tonalité”, and

trondjhemite. These rocks are have been metamorphosed and occur as 1-5 meter layers

scattered throughout the field area. Gneissic layering in this compositionally

heterogeneous group of rocks constantly appears as banding parallel to Si. Biotite and

garnet are also prominent minerals in this group of rocks. Fabric in these rocks is defined

by prominent biotite mica foliation planes. These rocks contain significant amounts of

hornblende and some pyroxene can be identified in outcrop.

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PETROGRAPHY AND SAMPLE SELECTION

Methods

During the coarse of three weeks of fieldwork in the eastern Ivanpah field area over

200 hand samples were collected with the intention of all major Ethologies present to be

represented in the sample set. Special effort was given to collect the freshest crystalline

samples possible from each field station and to take orientated samples if outcrop

conditions permitted. Collecting crystalline orientated samples was successfully carried

out on all Ethologies, although metamafic rocks and augen gneiss presented the most

problem in accomplishing this task because of the high degree of chemical weathering to

these specific rock groups. Out of the ~200 hand samples collected, 65 specimens were

chosen for billet cutting and subsequent thin-section production for pétrographie analysis.

The 65 billets selected for thin section analysis were chosen on the basis of three criteria,

(1) degree of crystallinity, (2) whether they were orientated to metamorphic fabric, and

(3) all major rock types must be represented. Approximately 30 thin sections were of

pelitic and migmatitic pelitic gneissic compositions. Out of the total 65 thin sections that

were petrographically analyzed, 12 were selected for probe section analysis involving the

collection of P-T-time data. The process for selection of these 12 probe sections were

based on the same criteria mentioned above, with one additional condition, which was the

section had to contain the minerals necessary for the P-T-time calculation. Specifically,

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the minerals gamet-sillimanite-quartz-plagioclase for pressure, gamet-biotite for

temperature, and monazite for time calculations (P-T-time data discussed in Chapter’s 5

and 6). It was of these 12 probe sections that the pétrographie descriptions given below

were chosen for all Ethologies except the amphibolite (IV-5). No amphibolites, nor

metagabbros, were selected for electron microprobe (BMP) work because of their lack of

appropriate minerals for barometry (no ALSiOj phase), thermometry (no garnet) and

geochronology (no monazite). The amphibolite was chosen for pétrographie descriptions

because of the extreme abundance of metamafic rocks found in the field area.

Additionally, the mineralogy of metamafic rocks may help confirm or constrain P-T

conditions during metamorphism.

Specifically, seven pelitic gneisses were chosen for probe work including FV63, FV41,

rV29, rV87, IV64B1, FV49, and IV47. Out of these seven a detailed pétrographie analysis

will be given for IV63 and IV41. Two augen gneisses IV ll and IV50 were selected for

BMP work and a detailed pétrographie description will be provided for IV 11. FV66A is a

small intruding metaleucocratic dike and it too was selected for both BMP work and for a

pétrographie a description. IV76A is a larger white crystalline garnet filled metaaplite

dike, injected through foliation and outcropping with pelitic gneiss (Figure 18). A

detailed pétrographie description will be given for this rock. IV41D is composed of -80%

injecting leucosome with -20% of the schistose pelitic rock that it intrudes into, on the

same thin section. IV41D was chosen for BMP work only. It should be noted that the

-20% pelitic portion of IV41D is of the same migmatitic pelitic gneiss as IV41, thus data

collected, and pétrographie characteristics of the schistose pelitic portion of IV41 may

also apply to the schistose pelitic portion of FV41D. The leucosome material that

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constitutes 80% of IV41D has similar pétrographie characteristics as IV66A. The exact

sample location can be found on the lithologie Map (Figure 5).

In the forthcoming pétrographie descriptions special attention to the presence of

garnet in each sample will be given because of the importance of garnet in this study for

the determination of the conditions of metamorphism (i.e. P and T). Garnets are also very

important in this research to help resolve the timing of metamorphism (U-Th-Pb dating of

monazite) because the garnets may shield monazite crystals that are included in their

cores from younger metamorphic events, possibly causing Pb-loss and alteration of the

U-Th-Pb system in the monazites.

Pétrographie Descriptions

Pelitic Gneiss - FV63

IV63 is highly pelitic migmatitic gneiss. Approximate mineral modal percentages are

as follows: 40% quartz, 20% biotite, 15% garnet, 10% plagioclase, 10% K-spar

(microcline), 5% sillimanite, 2% cordierite, and <1% accessory minerals. Accessories

include an overwhelming abundance of monazite (several hundred 5um-75um crystals),

zircon, apatite, hercynite, ilmenite, and other unidentified oxides.

The metamorphic fabric preserved in this sample strikes N10°E and dips 72°NW. An

abundance of biotite and sillimanite define a very continuous foliation planes that wrap

around garnet porphyroblast (Figure 19). Many garnets in this rock are somewhat

elongate with the long dimension parallel to foliation direction (Figure 20). Additionally,

other minerals such as quartz and plagioclase, also occur elongate to strike direction of

the fabric (Figure 21). No observable evidence within the pelitic matrix of this rock

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suggest that more than one metamorphic fabric is preserved in this rock. Note that the

garnet porphyroblast do not contain mineral inclusions that record the NE striking fabric

that is prominent in the matrix of the pelitic gneiss.

Garnet porphyroblast crystal facies are highly anhedral with irregular cystal edges.

The porphyroblast enclose an abundance of rounded and oval shaped quartz grains

(Figure 22a). Plagioclase and K-spar also appear as circular grains (positively identified

with EDS on the BMP) within the rounded quartz, while biotite is occurs as small

rectangular, light tan, blocky crystals (Figure 22b). It is the interpretation of this study

that these rounded quartz grains, which are abundant especially in the larger garnets, are

the product of partial consumption of matrix minerals due to the growth of the garnet

porphyroblast. Most of these circular quartz patches within the garnets are probably

connected to the matrix in the 3""° dimension and thus, this could be a matrix texture. The

2-D thin section gives the illusion that most of these circular minerals are inclusions

within the garnet, but they may actually extend to the matrix perpendicular to the plane of

the thin section.

Additionally, a cluster of -12-15 monazite crystals appear close to the center

of a 1cm in diameter anhedral garnet (Figure 23). Throughout the garnet, monazite is a

common inclusion both in the core and near the rims. Over 100 monazites appear

included in this large garnet alone. Several of these monazites were used for U-Th-Pb

dating of the mineral and the results will be discussed in Chapter 6.

One additional texture observed in this sample was the abundance of cordierite with

the presence of included prismatic and needle-like sillimanite (Figure 24). The sillimanite

usually is located substantially within foliation planes associated with biotite but it also

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occurs routinely within the cordierite.

Pelitic Gneiss - IV41

IV41, the second pelitic gneiss that will be discussed here, contains mineral

proportions in the approximate amounts of 25% quartz, 25% biotite, 20% sillimanite,

15% garnet, 10%, K-spar (microcline), 5% plagioclase, 1-2% cordierite, and <1%

accessories (same accessories as IV63). This rock is slightly more pelitic than IV63

evidenced by the 35% total modal abundance of the Al-rich minerals biotite and

sillimanite. This high abundance of aluminous minerals alternating with leocosome felsic

rich layers may represent a restite due to extreme melting at high temperatures (Yardley

et al, 1990). The continuous, well developed fabric in this rock is defined by these two

Al-bearing minerals, although elongate veins of felsic leucosome layers define the

gneissic texture of this migmatitic gneiss (Figure 25). The fabric is preferentially striking

~ N16° and dipping 75NW. Similar to IV63, no evidence of a second fabric is apparent in

this rock. Figure 26 shows a sillimanite lineation extending through the body of a

garnet porphyroblast, which is a common feature found in IV41. This sillimanite

lineation within the garnets parallels the 2-3mm thick compilations of orientated biotite

and sillimanite in the matrix that extend around the garnets, thus the garnets must be post-

kinematic with respect to the deformation that produced the matrix fabric. The garnet

porphyroblasts have well developed pressure shadows that are filled mostly with quartz

with minor amounts of plagioclase and K-spar (Figure 27). The garnet porphyroblast are

mostly subhedral, -1/2 cm in diameter, and enclose smaller (5-lOum) rounded

plagioclase and (5-lOum) rounded K-feldspar, and very small (20-30um x 10-15um)

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blocky clean biotite of a lighter brownish tan color than the biotites found in the matrix

(similar to IV63). Another characteristic of these garnets, similar to that of IV63, is the

elongated shape similar to several garnet porphyroblast seen in that rock. These garnets

have a longer dimension parallel to the defining foliation in the rock. Smaller (300-

500um) euhedral garnets do occur in the matrix but are not situated in between foliation

planes as the larger garnet porphyroblast are.

Cordierite in this section contains abundant included sillimanite, as with the previous

pelitic gneiss, but also has been pseudomorphed by a large quantity of sericite and pinnite

(fine-grained micas; which is diagnostic of cordierite) during an apparent retrograde

metamorphic reaction (Yardley et al, 1990). Cordierite was identified petrographically by

the yellowish pinnite alteration rimming the crystals seen in PPL, a wealth of sillimanite

needle inclusions, twinning characteristics similar to that of plagioclase (Figure 28), and

the lack of undulatory extinction that is a characteristic of quartz.

Augen Gneiss - IV ll and FV50

The augen gneisses IV ll and IV50 lack significant amounts of aluminous silicates

compared to the pelitic gneisses. Sillimanite and cordierite are not present in either of

these samples, while biotite modal abundance is -15%, thus the chemical composition

does not substantiate a pelitic origin for this Ethology. Approximately 50% of the biotite

present in this sample has been chloritized due to weathering. Additional minerals in

rVl 1 occur in the following approximate percentages: 40% quartz, 25% plagioclase, 10%

garnet, 5% K-spar (microcline), and 1% accessories such as monazite, zircon, apatite, and

ilmenite. Other oxides may be present in this sample but the identification of these

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minerals have not been determined. The abundance of secondary chlorite along with the

presence of a large quantity of oxidation minerals indicate and confirm the high degree of

chemical weathering that the augen gneisses in the Ivanpah field area have been

subjected to. In the field, the rocks sometimes crumble to the touch, but IV11 was much

less altered compared to other augen gneiss samples that were collected, thus the reason it

was chosen for a pétrographie description.

The metamorphic fabric in IV ll strikes N25E. No dip measurement was measured in

this augen gneiss because of the high degree of weathering that prevented the accuracy of

the measurement on this particular sample. Other augen gneisses, however, were

measured to strike slightly to the NE and dip from 55 to 85° to the NW. It might be noted

that several augen gneisses were determined to strike slightly NW -5-10° and dip steeply

to the SW -80-85°. This was not interpreted as the presence of a second metamorphic

fabric. Biotite foliation planes in IV ll are discontinuous across the long dimension of the

thin section. Large l-2cm augen structures in this rock are usually composed of

plagioclase and minor amounts of K-spar. These augen structures are routinely wrapped

by the defining biotite cleavage planes and finely crystalline felsic minerals that occur in

long strands (Figure 29). The finely crystalline minerals are smaller than the larger matrix

minerals and may possibly represent recrystallization during shearing.

Garnet in IV11 is mainly anhedral, unlike the pelitic gneisses where garnet appears

anhedral, and subhedral due to reaction textures. Flattened elongate garnet is also present

sandwiched between foliation planes in IV ll, similar to garnet in IV63 (Figure 20).

Garnet has also been identified in this sample to have a similar occurrence of enclosed

small rounded quartz, and included circular plagioclase within the quartz. This is a

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prominent characteristic seen in the metapelites, but is less prominent in IV11, as well as

other augen gneisses that were petrographically analyzed.

Additionally, the accessory mineral zircon is more abundant than monazite in

rv i 1 compared to that of the pelitic gneisses. Monazite is present in this sample,

but the main phosphate present is the calcium phosphate apatite, which occurs as 300-

500um subhedral to euhedral crystals.

Finally, the textures usually associated with plutonic igneous rocks are abundant in

IV11, as well as, other augen gneiss samples. Specifically, these textures mainly include

the occurrence of myrmekite and granophyric texture. Considering the appearance of

these typical igneous textures, along with the mainly granitic mineralogy within the

augen gneisses, it may be concluded that these rocks originated from an igneous granite

protolith, although metamorphosed quartzofeldspathic sedimentary rocks is also a

possibility.

Leucocratic Sill - IV66A

The thin section IV66A is of a small leucocratic dike associated with a larger

meta-aplite igneous intrusion, seen earlier in Figure 14a. The ~lm wide meta-aplite dike

is intruded by a smaller 10cm wide felsic dike. IV66A is from this smaller leucovein that

is penetrating down strike of the metaaplite. This felsic rock is composed of ~ 50%

quartz, 20% plagioclase, 10% K-spar (microcline), 10% biotite, 10% garnet, and <1%

accessories zircon, monazite, and various oxides. Biotite is highly chloritized, while

secondary muscovite is also present. No sillimanite or cordierite was identified in this

rock.

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The metamorphic fabric measurement was limited to that of a strike measurement

only (N25E) because this small leucocratic dike did not extend far enough above the

surface of the surrounding larger felsic dike for an accurate dip measurement to be

determined. In the upper % of the thin section biotite foliation is discontinuous and weak,

but in the lower 14 of the section a continuous, well- defined biotite fabric is present.

Another texture associated with the defined metamorphic fabric was the presence of

elongate lenticular collections of leucocratic material with quartz and plagioclase

composing these eye shaped structures (Figure 30). Biotite, as well as these thin

leucocratic augen shaped structures, wrap around very small (l/4cm diameter or less)

garnets.

The garnets mainly exhibit anhedral to subhedral crystal shapes. Zircon and monazite

in rV66A are larger than these same minerals that occur in the metapelites Both zircon

and monazite are as large as 200um, while zircon is more abundant than the monazites,

unlike that of the metapelites where monazite occurs in a higher abundance than zircon.

Furthermore, both zircon and monazite was identified as being extensively zoned.

Figures 31a and 31b, illustrate two large monazite grains, one from IV66A and one from

IV41D, that exhibit zoning and that be identified under cross polars in thin section.

Additionally, myrmekite and granophyric textures were identified in IV66A, similar to

that seen in the augen gneisses, IV11 and IV50.

Meta-Aplite Sill - IV76A

FV76A is a leucocratic dike (called meta-aplite by this study) that is closely

associated with a pelitic gneiss unit in the field (Figure 18). The importance of this

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particular leucocratic sample, as opposed to other samples with felsic and leucocratic

mineralogies, was the presence of sillimanite. Other similar samples including IV66A did

not contain any AlaSiOs phase, thus rendering GASP barometry invalid. This sample was

odd in the sense that sillimanite was more abundant than biotite at an approximate ratio

of 3 toi ratio (15% modal sillimanite; 5% modal biotite). Even the pelitic rocks usually

contained more biotite than sillimanite. Furthermore, K-spar (microcline) was the

dominant felsic phase with a modal percent of -35. Other minerals that were present in

this rock sample occurred in the following estimated modal percentages: 30% quartz,

10% plagioclase, 5% garnet, 1-2% accessories. Accessory minerals occur as apatite,

zircon, monazite, and opaque oxides. As with the other felsic rocks described, zircon is

vastly more abundant than monazite. In fact, monazite was sparse in this sample, but it

was identified and ages of crystallization of this rock will be reported in Chapter 6.

Both IV76A and the pelitic gneiss associated with it (located at field station 76) carry

a foliation that strikes N22E and dips 62NW. Again, as with many other locations

throughout the field area, these white felsic plutonic rocks are interpreted to have

penetrated through a weak cleavage plane in the pelitic gneisses that it is spatially

associated with, but exhibit the development of a weak metamorphic fabric defined by

biotite mica (thus late-syn to post-orogenic). Mostly sillimanite, and smaller amounts of

biotite and opaque oxide phases (ilmenite and hercynite), define the metamorphic fabric

in this rock. Although this is not an augen gneiss, sillimanite lineation commonly can be

observed wrapping around larger augen shaped structures that consist of interlocking

quartz, K-spar, and plagioclase (Figures 32a and 32b). Furthermore, larger elongate

quartz and K-spar are common in this rock and exhibit a long dimension parallel to the

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lineation direction defined by the matrix sillimanite.

Garnets in IV76A are typically anhedral and elongate with the long dimension ~3-4

nun across. The garnets commonly contain a large quantity of sillimanite needles that are

not orientated to, or parallel to the sillimanite that defines the matrix fabric direction.

Fibrolite sillimanite is shown included in plagioclase in Figure 33. It appears that the

plagioclase has been partially replaced by a retrograde reaction that produced sillimanite.

One additional feature that commonly occurs in this sample is the intergrowth of

quartz and alkali feldspar known as micrographie texture. IV76A was one of the very few

samples that exhibited this pétrographie characteristic.

Finally, a brief description of the amphibolite gneiss IV-5 will be given below,

although as stated earlier, the metamafic rocks in the field area are not suitable for the

chosen techniques of this study for the determination of metamorphic pressure’s,

temperature’s, nor age calculations, because of the absence of particular minerals

(already mentioned). Nevertheless, mineral assemblages within metamorphic mafic rocks

may give qualitative P-T information. Additionally these rocks may be important

concerning the geologic evolution in the region, thus a brief pétrographie description will

be given here (Wooden and Miler, 1990).

Amphibolite Gneiss - IV5

rV5 can be described as a thinly banded moderately crystalline amphibolite

gneiss. Lighter felsic bands, usually just a l-2mm’s in width, alternate with darker mafic

mineral layers approximately 3-5mm’s in width, although the width of the felsic layers

vary from location to location within the field area. The most commonly occurring

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minerals in this lithology are hornblende (cummingtonite) at -40% modal abundance and

Ca-plagioclase at -35% modal abundance. The presence of two pyroxenes (opx and cpx)

has been identified in this study. Clinopyroxene is present at -15%, while orthopyroxene

is present in an approximate modal amount of 5%. Clinopyroxene + orthopyroxene +

cummingtonite in the amphibolites correspond to metamorphic conditions of the

homblende-granulite subfacies (Young et al, 1989; Abscher and McSween, 1985.

Quartz is present at about 5-10% modal abundance. Biotite has been identified but is very

rare (<1%). No garnet, sillimanite, cordierite, nor K-spar have been identified in this

metamafic rock. The occurrence of cummingtonite instead of garnet in metamafic rocks

and amphibolite is also consistent with homblende-granulite subfacies but at lower

pressure (Young et al, 1989; Miyashiro, 1972. It is important to note that none of the

metamorphic mafic rocks (including metagabbro), or rocks of intermediate compostions

(varying metadiorites) in this study that were petrographically analyzed contained the

minerals garnet, sillimanite, or cordierite.

Additionally, monazite, a key mineral in this study, was extensively searched for in

backscatter mode on the BMP in IV5, and several other metamafic rock probe sections,

and was not identified in any of the metamafic rocks in this research. The major

phosphatic phase in all metamafic rocks of the field area was determined to be apatite.

Zircon was also identified petrographically, in backscatter mode on the BMP.

The amphibolite gneiss IV5 is very similar to the amphibolite gneiss seen earlier in

Figure 14b. The metamorphic fabric (gneissic layering) that is a characteristic of this

mafic gneiss strikes N20B and dips 74NW. It is defined mostly by subhedral hornblende

with additional elongate anhedral opx crystals, and some anhedral to subhedral cpx

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(although cpx throughout the rock is more randomly distributed). Additionally, the opx

has highly irregular crystal faces while enclosing an abundance of anhedral mineral

phases, again possibly due to reactions taking place producing opx and consuming matrix

minerals (Figure 34). Furthermore, plagioclase crystals in IV5 exhibit twinning defined

by very broad lamellae. This indicates a high proportion of anorthite to albite component

in the plagioclase (Yardley et al, 1990).

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ELECTRON MICROPROBE DATA - SILICATE MINERALS

The goal of chemical data collection focused on respective silicate minerals for

geothermometry and geobarometry calculations. But before actual analytical points could

be selected for P and T calculation, a number of important factors had to be considered to

accomplish the goals of thermometry and barometry. Goals specifically being the

calculation of peak metamorphic P-T conditions, as well as, retrograde-T conditions.

Prograde metamorphic conditions are unlikely to be calculated since granulite facies

metamorphism tends to homogenize existing mineral assemblages in the rocks at peak

thermal conditions due to extreme volume diffusion throughout mineral lattice structures.

It will not be possible to determine retrograde P’s in this research because the anorthite

(Ca) component in plagioclase shows very little variation from sample to sample or

throughout individual samples depending on pétrographie location (i.e. included in garnet

or in the matrix), thus no retrograde P’s can be paired with retrograde T’s for the location

of a retrograde P-T point.

A variety of garnet, biotite, and plagioclase grains were analyzed from numerous

samples to confirm that presence or absence of chemical trends that would demonstrate

the existence zoning profiles. This is of primary importance for garnet because although

growth zoning is seldom recorded in granulite minerals, it has been shown by Spear

(1993) and Indares and Martignole (1985) that late Fe-Mg re-equilibration between

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garnet and adjacent biotites takes place during retrograde metamorphism, thus a chemical

zoning profile among Fe and Mg in the garnets may be present adjacent to biotite grains.

Unlike the garnet, the adjacent biotite grains are homogenized on the rim of garnets

during retrogression because redistribution of Fe-Mg occurs throughout the biotite grain.

The pétrographie location of plagioclase and biotite [i.e. included in garnet-enclosed in

quartz (c), in contact with the rim of garnet (r), or in the matrix (m)] was also varied in

the initial analyses so a complete representation of any varying mineral compositions, if

they exist, could be confirmed.

Additionally, the largest garnets in each sample was targeted for chemical analyses

because it has been shown by Spear, 1993, that the size of the garnet (radius), along with

cooling rates, can restrict the maximum temperature recorded by thermometry (Figure

35). No research that this writer is aware of has stated the size of the garnets when T’s are

reported from gamet-biotite thermometry. It will be shown by this study, that this may be

an extremely important consideration for the extraction of accurate peak metamorphic

temperatures particularly in granulite facies rocks.

Plagioclase, Biotite, and Garnet Chemistry

No chemical heterogeneity was recognized among plagioclase and biotite populations

in any sample, thus all plagioclase and biotite analyzed in this study are interpreted to be

homogenous. Garnets showed little chemical heterogeneity from sample to sample and no

zoning from the cores to the rims away from biotite grains. From core to rim towards

adjacent biotites (in direct contact with the garnets), trends especially in Fe and Mg were

recognized. Specifically, Fe increased while Mg decreased from core to rim towards

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adjacent biotite. Figure 36 shows the results of oxide total wt.% of FeO, MgO, CaO, and

MnO from core to rim traverses across garnets analyzed in samples IV63, and IV41D. Ca

and Mn generally showed no variation from core to rim away or towards biotite grains,

although Mn slightly increased from core to rim in a few garnets. Indares and Martignole

(1985) found similar trends in Mn component from garnets of granulite facies

metasedimentary rocks of the Grenville Province in Maniwaki Valley, Canada.

Specific examples of plagioclase, biotite, and garnet elemental trends from samples

IV63 and IV41D (including IV41) are given below, but generally speaking all samples

showed similar trends. The reader should refer to Appendices A, B, and D, which display

the final BMP chemical results for garnet, biotite, and plagioclase, repectively.

Pelitic Gneiss - IV63

Plagioclase

In IV63 seven plagioclase grains were analyzed. Four grains were located enclosed in

one large 1cm in diameter (5000pm radius) subhedral garnet (garnet in Figures 22 and

23), two were located in the matrix close to this large garnet (within l/2cm), and one

crystal was analyzed that was adjacent to the garnet. Results show that only a very small

variation in anorthite component to albite component exist out of 42 points analyzed on

the seven grains of plagioclase analyzed from IV63. Thus, a compositional difference

based on the pétrographie location of plagioclase does not exist. Plagioclase grains

included in quartz enclosed in garnet, showed only a slight difference in An and Ab

component, from that of matrix plagioclase, thus only one plagioclase population was

determined to exist (This has important implications for calculating pressures from the

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GASP method used in this study; see Chapter 6). Anorthite component in plagioclase

ranged from Anzg-Anss.

Biotite

Seven biotite grains were also analyzed in IV63. Two that appeared to be enclosed in

the large garnet (IV63gtl) included in rounded irregularly shaped quartz, two lying

adjacent to the large garnet directly in contact with the garnet rim, and two in the matrix

(1 close and 1 far). Point traverses across several of the biotite crystals were performed to

look for heterogeneity (specifically Mg/Fe differences) but none were determined,

although a substantial variation in Mg/Fe was found to be present among individual

biotite grains depending on the location of the biotite. Two biotite grains (IV63bt4 and

rV63bt5), contained within quartz enclosed in garnet, yielded average Mg/Fe ratios of

1.32 and 1.23, respectively. IV63bt3, also within quartz enclosed in garnet yielded an

average Mg/Fe ratio of 1.88. IV63bt3 and IV63bt6 (matrix), yield Mg/Fe average values

of 1.01 and 1.06, respectively. IV63btl and IV63bt7, in contact with the garnet rim, yield

Mg/Fe ratios of 1.37 and 1.50. Individual Mg/Fe values for each analytical point are

given in Appendix B. Average values will be used in thermometry. Statistically, this is

probably the best approachfor minerals with very little variation in composition.

Garnet

IV63gtl was the only garnet analyzed in this sample. A traverse from the garnet core

away from biotites in contact with the rim was not carried out on this sample. But

considering though that this procedure was carried out on a total of 13 garnets in this

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research (see Appendix A), and all 13 showed the same trend of homogeneity away from

biotites in contact with garnet rims (Mg/Fe decrease towards biotites lying in contact with

garnet rims), there was no need to continuously repeat this procedure on every sample.

rV63gtl was found to be heterogeneous towards biotite in contact with the rim. Figure 37

shows the positions of a BMP point traverse across the large garnet towards a biotite

grain lying in direct contact with the garnet rim (analytical points in Appendix A are

IV 63gtl-l to 1-7 and IV63gtl-8 to 1-12). The Mg/Fe ratio from 1-1 to 1-7, and 1-8 to

1-12, decrease from 0.318 to 0.238 and 0.348 to 0.167 from core to rim, respectively.

Grossular component from core to rim across traverse 1-1 to 1-7 generally decreased

from points 1-1 to 1-6 (0.090 to 0.084) but then increased again from 1-6 to 1-7 (0.084 to

0.094). No trend in Ca composition from traverse 1-8 to 1-12 can be recognized from the

data, thus no zoning in grossular component in garnet is interpreted to be present.

Pelitic Gneiss - IV41 and IV41D

Plagioclase

Including both IV41 and IV41D five plagioclase crystals were analyzed for total

composition on the BMP (two from IV41 and three from IV41D). The two plagioclase

analyses from IV41 were from matrix crystals. One was near the rim of a moderately

sized -3000 x -2000pm garnet porphyroblast (IV41gtl; long dimension of garnet

elongate to foliation direction). The other plagioclase was located and analyzed in the

matrix far from any garnets that could be seen in two-dimensions. In IV41D three

plagioclase grains were analyzed, one circular grain enclosed in quartz and included in a

large -2250pm radius subhedral garnet (IV41Dgt2), one near IV41Dgt2 (matrix close).

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and one far from IV41Dgt2 (matrix far).

No significant compositional difference exists between the rim close plagioclase and

the rim far plagioclase in sample IV41. The rim close grain (IV41pll-l displays a range

of Anorthite component of Anyo-Anyi, and an average albite component of ~Ab 34. The

matrix far grain (IV41pl2-l) has a range of anorthite component of Angz-An?;. No trends

in Ca or Na (i.e from core to rim) were determined to exist in either grain (no Ca or Na

zoning), thus the crystals were determined to be homogenous.

A slight variation in plagioclase composition was found to be present in IV41D

depending on the pétrographie location of the plagioclase analyzed. IV41Dpll enclosed

in rV41Dgt2 (small 10pm circular grain enclosed in quartz) contains an anorthite

component of approximately Anez-Angg (IV41Dpll-2 to 1-5). It was determined that that

the matrix-close plagioclase in IV41D had a similar Ca and Na compositional range as

the enclosed plagioclase (Angy-An?; and Abaz-Abss). The matrix far plagioclase

IV41Dpl6, on the other hand, was found to have a substantially different range in

anorthite component than any of the other four plagioclases in either IV41 or IV41D.

Anorthite in the matrix far plagioclase ranged from Anz^-Ans?. Holdaway, 2001,

recommended that geologic error could be reduced in the calculations if matrix garnet

close plagioclase was used for the determination of peak metamorphic pressure

calculations. He also recommended that garnet close biotites be used for calculation of

peak temperatures. As was the case in IV41, no compositional zoning trends were found

to be present in plagioclase grains in FV41D.

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Biotite

Six biotite grains were analyzed in IV41 and IV41D, two in IV41 and four in IV41D.

Point traverses were performed on several of these grains that were found both in the

matrix and enclosed within garnet. Ratios of Mg/Fe showed no substantial zoning

patterns, or general increases or decreases in Mg/Fe, within individual biotite grains, thus

it was concluded that all biotite crystals, whether situated in the matrix or included in

garnet were individually homogenous. On biotites that were situated in contact with

garnet, traverses were run from the point on the biotite nearest to the garnet rim away

from the garnet rim. Traverses on matrix biotites were generally performed along the

long dimension of the biotite that was parallel to the principal direction of foliation in

each section. The average Mg/Fe ratio of the rim biotite IV41btl, in IV41, was found to

be 1.24, while the average Mg/Fe in the matrix close biotite IV41bt2 has been determined

to be 1.22. In IV41D, both rim biotites TV41Dbtl and IV41Dbt2 have similar Mg/Fe

ratios as the two biotites analyzed in IV41. Mg/Fe average ratios in these two grains are

1.15 and 1.28, respectively. The two biotite grains analyzed in IV41D, which were

situated in the leucosome portion of IV41D (1 matrix far from FV41Dgt2, 1 matrix close

to rV41Dgt2) showed a substantial departure from Mg/Fe values from that of the other

four biotites just discussed. IV41Dbt3, which has an average Mg/Fe ratio of 0.91, is a

matrix close biotite enclosed in quartz. IV41Dbt4 is a matrix far biotite with an average

Mg/Fe ratio of 0.90. It would appear from these findings there is a substantial deviation

in Mg/Fe composition depending upon the pétrographie location of the biotite. Biotite

grains in the matrix have a substantially lower Mg/Fe composition to that compared to

the three rim biotites and the one biotite enclosed in IV41Dgt2, with the exception being

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IV41bt2, in IV41. It is possible that IV41bt2, which is just a few hundred microns from

rV41Dgt2 was also involved in Mg and Fe re-equilibration along with the three rim

biotites from IV41 and IV41D during retogression, while the two matrix biotites in

rV41D were not close enough for Mg-Fe exchange and re-equilibration with IV41Dgt2,

thus the lower Mg/Fe.

Gamet

Two garnets were analyzed from IV41 and IV41D. The gamet porphyroblast from

IV41gtl is situated within well-defined biotite and sillimanite bearing foliation. Two

traverses from the core of IV41gtl to two adjacent biotite grains in contact with the rim

of IV41gtl were performed (IV41gtl-l to 1-5 and IV41gtl-9 to 1-12). One traverse, from

the gamet core away from adjacent biotite, towards a rim quartz grain, was carried out

(FV41gtl-l then IV41gtl-6 to l-8)(Figure 38). Mg, Fe, Ca, and Mn components from

core to rim towards the adjacent quartz were found to be fairly constant with no

substantial increases or decreases from core to rim. Mg/Fe from core to rim away from

biotite showed very little change (0.284 at the core to 0.276 at the rim). In contrast, when

point analyses are carried out from core to rim towards adjacent biotite (Figure 38), an

increase in Fe and decreases in Mg systematically occurs. Mg/Fe ratios from IV41gtl-l

to 1-5 decrease from 0.284 to 0.175, moving from core to rim. Mg/Fe also systematically

decreases from IV41gtl-l to 1-12 (sequence of points in this traverse were IV41gtl-l, 1-

9 tol-11, then 1-12, near a second biotite in contact with the gamet rim. Mg/Fe decreases

from 0.284 in the gamet core, to 0.231 on the gamet rim adjacent to biotite. This also

shows that a re-equilibration among Mg and Fe between gamet and biotite has occurred.

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thus a zoning profile among Mg and Fe exist in the gamet from core to rim towards

adjacent biotite, but no zoning profile is present away from adjacent biotite.

In addition, one large gamet from IV41Dgtl was analyzed on the BMP for

compositional heterogeneity. This gamet lies on the contact between the pelitic schistose

portion and the leucosome portion of the sample. Similar traverses as with IV41 were

performed on IV41D. Two traverses from gamet core to gamet rim near adjacent biotites

and one traverse from gamet core to adjacent quartz. Very similar findings to that of the

traverses in IV41 were determined. No compositional trends or zoning of Fe, Mg, Ca, or

Mn were determined to exist within the gamet towards the adjacent quartz, but definite

increases in Fe and decreases in Mg occurs as point traverses are carried out from gamet

core to gamet rim towards adjacent biotite grains. Mg/Fe decreases from 0.290 in the

gamet core to 0.184 on the gamet rim near adjacent biotite (IV41Dgt2-l to gt2-8). Mg/Fe

decreases from 0.280 in the mid-gamet core, to 0.174 on the gamet rim near another

adjoining biotite on the rim (IV41Dgt2-13 to 2-16). No significant change in Mg/Fe

occurs from core to rim near a bordering quartz (.289 to .276)(IV41Dgt2-9 to 2-12).

Again, as shown with gamets from IV63 and IV41, IV41Dgt2 displays an Mg-Fe

exchange between adjacent gamet and biotite. This is a typical trend usually seen in

granulite grade rocks, among gamet and biotite, where homogenization among the

minerals takes place at peak T’s, then an Mg-Fe exchange takes place during

retrogression and cooling. This trend was found in all gamets, in all samples, analyzed in

this study.

In review, the following chemical characteristics and trends of gamet, biotite, and

plagioclase are applicable to all samples of this research. Indvidual gamets were found to

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be homogenous, except on the rims were biotite was in direct contact with the rims.

Specifically Fe increased and Mg decreased from core to rim towards adjacent biotite

(i.e. lower Mg/Fe). Mn increased from core to rim within several gamets, but this was not

a consistent trend among all gamets analyzed. Grossular component showed no change

from core to rim.

Individual biotite was homogenous independent of pétrographie location. However,

the composition of biotite was found to be a function of the distance from gamet (i.e.

lower Mg/Fe among matrix biotite). This is also similar to the findings of Indares and

Martignole, 1985. They found that the composition in biotite was a function of the

distance from gamet, thus isolated matrix biotites were enriched in Fe (lower Mg/Fe)

compared to biotite located at or near gamet rims.

Plagioclase An and Ab components varied only slightly depending on pétrographie

location (i.e. included in quartz enclosed in gamet or matrix grains). Among individual

plagioclase grains no chemical zoning was determined to be present based on point

traverses ran on several plagioclase grains in each sample.

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METAMORPHIC P-T CONDITIONS OF THE IVANPAH MOUNTAINS

Metamorphic P-T conditions in the Ivanpah Mountians were evaluated primarily from

pelitic and semi-pelitic Ethologies. A semi-quantitative estimate for metamorphic P-T

conditions was made based on metamorphic mineral assemblages by application of the

AFM Thompson projections and the petrogenetic grid of Spear and Cheney (1989).

Mineral assemblages most likely record conditions at the thermal peak of metamorphism.

Electron Microprobe data was used to calculate metamorphic P-T conditions using the

published quantitative gamet-biotite geothermometer (Ferry and Spear, 1978; Indares and

Martignole, 1985; Holdaway, 2000; Holdaway et al., 2001) and the GASP barometer

(Koziol and Newton, 1988; Holdaway, 2001).

P-T Grid and Thompson Projections

The KFMASH P-T grid presented here (Figure 35) is based P-T locations for

reactions in the KFASH and KMASH systems. The grid is taken from Spear (1993) after

Spear and Cheney (1989). The advantage of this grid is that it takes into account and

provides an “intemally consistent framework” for the effects of the Mg-Fe solid solution

behavior. This is important because quantitative temperature, and to a lesser degree

quantitative pressure determinations, are a function of the Mg-Fe solid solution exchange

reaction in gamet and biotite. On the KFMASH grid stability fields for the three AlzSiOs

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polymorphs kyanite, andalusite, and sillimanite are shown.

First approximation of Ivanpah metamorphic P-T conditions, can be constrained

using the KFMASH grid by the following mineralogical characteristics: (1) The presence

of sillimanite as the only occurring AlzSiOj polymorph, which limits P-T conditions to

the sillimanite field only (solid light blue area that outlines a triangular region on the

grid). (2) The complete absence of primary muscovite and the abundance of K-spar,

which is likely due to the KFASH univariant reaction: muscovite + quartz sillimanite

+ K-feldspar 4- HzO. The reaction is shown on the KFMASH P-T grid as a solid red line.

This first presence of K-spar + sillimanite and absence of muscovite + quartz restricts

metamorphic conditions at or above the second sillimanite isograd (also known as the

muscovite-out isograd). This reaction further constrains peak metamorphic conditions to

the sillimanite field but only where muscovite is absent (i.e. to the right of the solid red

reaction line on the P-T grid). (3) The presence of biotite + sillimanite + gamet +

cordierite. The presence of these four minerals in some of the samples is likely due to the

univariant reaction biotite + sillimanite = gamet + cordierite and is shown on the

KFMASH grid as a solid green line. This further constrains P-T conditions at or near this

univariant reaction line in the sillimanite field. Perhaps the occurrence of these four

minerals in several of the samples indicates that this univariant reaction was initiated but

did not go to completion (conversation Metcalf, 2002). Additionally, cordierite may be

present as a relict phase in some samples, evidenced by the textural relationships with

sillimanite (sillimanite included in cordierite; see Figure 24). Perhaps cordierite grew at

the expense of sillimanite on the prograde path, during the reaction biotite + sillimanite 4-

quartz —> gamet 4- cordierite 4- HzO, and on the retrograde path the reverse reaction

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gamet + cordierite + H 2O biotite + sillimanite + quartz failed to go to completion due

to the lack of H2O (shown by the abundance of sillimanite included in cordierite in Figure

24)(conversation Metcalf, 2002).

It should be noted that out of 30 pelitic gneiss thin sections that were petrographically

analyzed only 7 were determined to have cordierite. Further substantiating the hypothesis

that the crystallization of cordierite and the associated reaction may have begun but did

not go to completion, thus that peak metamorphic conditions attained during orogenisis

and deformation may have been on or near the univariant reaction curve biotite +

sillimanite = gamet + cordierite.

On the KFMASH P-T grid (Figure 35) two AFM Thompson projections are also

displayed. The two projections are shown on the grid in their respective approximate P-T

locations that they likely represent. The AFM diagrams assume the presence of quartz, K-

feldspar, and H 2O. Both AFM diagrams are projected from K-feldspar. Plotting positions

on the AFM diagrams (A) and (B) for the associated minerals (data used from IV63 and

IV29) were determined by taking BMP mineral chemical data from Appendices A, B, and

C, and using the following plotting formulas from Spear, 1993:

A = AI2O3’ = AI2O3-K2O = I/2AIO3/2-I/2 KO 1/2

F = FeO' = FeO

M = MgO' = MgO

The three mineral assemblages (KFMASH system) of the pelitic gneisses in the field

area are assemblage (A) gamet + biotite + sillimanite, (B) gamet 4- biotite 4- sillimanite 4-

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cordierite, and (C) gamet + biotite. Of the 30 pelitic thin sections analyzed in this study,

22 contained assemblage (A), 7 contained assemblage (B), and only 1 contained

assemblage (C). No chemical data exist for assemblage (C), thus no AFM diagram will

be presented for this particular mineral assemblage. It can be seen on AFM diagram (A)

(chemical data from IV63) tie-lines exist between gamet, biotite, and sillimanite

enclosing a triangular region indicating the presence of these three minerals in that

particular bulk rock composition. A likely scenario in the metamorphic paragenesis was a

prograde path of metamorphism from assemblage (A) to assemblage (B)(i.e. left to right

on the P-T grid). The tie-line that exist from sillimanite to biotite on AFM diagram (A)

was broken during the reaction biotite + sillimanite gamet + cordierite as prograde

metamorphism proceeded. As the reaction proceeded new tie-lines where created during

the crystallization of cordierite and additional gamet, while partial consumption of

already existing biotite and sillimanite occurred. The new mineral assemblage created as

metamorphism reached this univariant reaction is shown in AFM diagram (B)(chemical

data from IV29) with biotite, sillimanite, gamet, and cordierite all co-existing in the same

rock.

Mineral assemblage (C) is somewhat problematic in the sense that it contains no

sillimanite or cordierite. Perhaps the bulk rock chemical composition of assemblage (C)

lacked substantial A1 and/or Mg for the crystallization of sillimanite and/or cordierite to

occur (conversation Metcalf, 2002).

An additionally approximation of P-T constraints can be based on the presence of

abundant migmatites in the field area, as well as, the mineralogy of the metamafic rocks

reported earlier. For anatexis (partial melting) to occur in rocks of greywacke and pelitic

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compositions temperatures during metamorphism need to exceed ~700°C (Wooden and

Miller, 1990). Furthermore, P-T conditions suggested by the KFMASH grid are

consistent with melting in pelites and felsic compositional rocks (Winkler, 1967). The

anatexis minimum and maximum partial melting curves for wet granitic compositional

rocks is shown on the P-T grid (Figure 35) as two orange dashed lines. Any P-T

conditions to the right of the maximum partial melting curve normally suggest complete

melting of granitic rocks, although compositional differences and the addition of fluids

such as H2O and CO 2 can change the position of this partial melting curve (Winkler,

1967). As shown in the petgrography portion of this thesis, leucocratic leucosomes and

other spatially associated leucocratic dikes and veins are dominated by the presence of

quartz, K-spar, and plagioclase. Possibly suggesting that these partial melts originated

from in situ pelitic sediments that were originally metamorphosed. The presence of

abundant migmatites in the eastern Ivanpah Mountains demonstrates that the minumum

temperature for partial melting during prograde metamorphism must have been achieved,

thus peak metamorphic temperatures must have reached near or in excess of 700°C.

Young et al. (1989) suggest that in the Proterozoic McCullough Range (Figure 3) melting

may have been initiated at or near the second sillimanite isograd at approximately 650°C

and pressures between 3-5 kb.

In summary, this first approximation of the location in P-T space of metamorphism

restricts peak conditions in the sillimanite stability field at or near the range of P-T

conditions restricted by the bitoite + sillimanite —» gamet + cordierite univariant reaction.

Thus, first approximation P-T conditions must be within the range of 625-800°C and 2-5

kb, respectively. In the next two sections of this chapter a more quantitative look at P-T

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conditions during metamorphism were determined using the methods of gamet-biotite

thermometry and GASP barometry.

Quantitative Thermobarometry

The goal of quantitative thermobarometry is to determine equilibrium metamorphic P

and T conditions using the P and T dependence of the equilibrium constant (Keq). The

thermodynamically derived equation that relates Keq as a function of P and T is:

AG = m - T A S + RTlnKeq

Where G = Gibbs Free Energy, H = enthalpy, S - entropy, and R = the gas constant. The

equation can be rewritten in a more general form by employing the P and T derivatives of

free energy as:

AG = -AS (T,P)dT + AV(TJ*)dP + (RTlnKeq)dT + (RTlnKeq)dP

This equation shows that both AS and AV are functions of P and T. In general though, a

good geothermometer has a large AVreaction (more sensitive to pressure change) and a

good geobarometer has a large ASreaction (more sensitive to temeperature change)(Spear,

1993). Additionally, both of these types of reactions are experimentally calibrated and

usually require non-ideal mixing models for solid-solution minerals (i.e. Ca and Mn

substituting for Fe and Mg in the gamet-biotite Fe-Mg solid solution).

Furthermore, some geothermobarometers are based on exchange reactions (gamet-

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biotite geothermometer) and some are based on net transfer reactions (GASP

geobarometer), but most importantly, both exchange reactions and net transfer reactions

relate Keq as a function of P and T (Spear, 1993).

Gamet-Biotite Thermometer

The gamet-biotite Fe-Mg exchange reaction is widely used geothermometer in

metamorphic rocks (Cox, 1992; Spear, 1993). During metamorphism a partitioning of

iron (Fe) and magnesium (Mg) takes place between gamet and biotite if they are in

contact or in close proximity (Ferry and Spear, 1978). The reaction can be modeled in

terms of exchange components in the KFMASH system as follows:

FciAhSUOn + KMg3AlSi30io(OH)2 = Mg3Al2Si30n + KFe3AlSi30w(0H)2

almandine + phlogopite = pyrope + annite

The ratio of Mg/Fe in gamet to the ratio of Mg/Fe in biotite can be determined from

coexisting gamets and biotites by major element analysis on the electron microprobe.

This ratio is known as the Kd (distribution coefficient; K d = K gq^) and is primarily a

function of the temperature at which the minerals formed (Spear, 1993).

K d = (M g/Fe/“'^^'/(Mg/Fef°‘'‘"

The Kd can then be placed in an experimentally derived equation based on the

thermodynamic properties of the minerals gamet and biotite. The equation can then be

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solved for temperature (T), thus a quantitative geothermometer (Spear, 1993). Ferry and

Spear (1978) were the pioneers of gamet-biotite thermometry and originally determined

that the equation that relates Kd as a function of temperature was:

InKo = -2109/T(K) + 0.782

This equation assumes ideal ionic solution behavior between Fe and Mg in gamet and

biotite, however, the incorporation of minor elements commonly results from a departure

from ideality in both gamet and biotite solid solutions (Indares and Martignole, 1985).

Numerous workers have expanded on the work of Ferry and Spear (1978) to determine

the effects of possible non-ideal behavior of Fe and Mg in gamet and biotite (Perchuk

and Lavent'eva, 1983; Indares and Martignole, 1985; Berman and Aranovich, 1988,

Williams and Grambling, 1990; Cox, 1992; Patino-Douce and Rice, 1993; Ganguly et al,

1996; Mukhopadhyay et al, 1997; Holdaway et al, 1997; Holdaway, 2000) to produce a

more accurate model that relates Kd as a function of temperature that accounts for the

non-ideal behavior of Mg and Fe in gamet and biotite. Possible elemental substitutions

that can cause non-ideal behavior in gamet include Ca (grossular component), Mn

(spessartine component), and Fe^^ (andradite component) (Williams and Grambling,

1990). Non-ideal solid solution behavior in biotite results from substitutions of Fe^^, Ti,

and Al'^ (Indares and Martignole, 1985; Holdaway et al, 1997). Many models have been

produced after Ferry and Spear (1978) by the various studies just mentioned, based on

new thermodynamic restrictions for non-ideal mixing of the elements Ca, Mn, Fe"^^, Ti,

and Al'^. It is beyond the scope of this work to review all these models or go into detail

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of the new thermodynamic restrictions that have been determined by the various workers,

but I will briefly descibe the model used in this research project to calculate temperatures

at this time. The model used for the determination of temperatures of metamorphism is

that suggested by Holdaway (2000). This model is referred to in his paper as model 5AV.

A software program for model 5AV may be obtained on PC disk by writing to Michael

Holdaway at [email protected]. This model combines thermodynamic data from

three previous gamet Margules models of Berman and Aranovich (1988), Ganguly et al.

(1996), and Mukhopadhyay et al. (1997) to arrive at model 5AV’s new thermodynamic

values. The model is (in Joules), AGex = 40198 - 7.80T; WpeMg in biotite = 22998 - 17.40T;

AWAiinbiotite = 245559 - 280.3IT. Where G = Gibbs Free Energy, T = temperature, and

W = Margule parameter. A Margule parameter is a thermodynamically derived constraint

that accounts for non-ideal behavior of solid-solution in minerals (i.e. Fe and Mg in

gamet and biotite). Following Holdaway et al. (1997) and Holdaway (2001) Fe^^ in

gamet was set at 3% and Fe^^ in biotite was set at 11.6%. Fe^^ in both gamet and biotite

has to be estimated because the Electron Microprobe (used in this research) can not

measure the amount of Fe^^ and Fe^^ separately. This is an important consideration in

gamet-biotite geothermometry because the Fe measured by the Electron Microprobe is

total Fe (Fe"^^ + Fe^^), and the gamet-biotite thermometer is based on the exchange of

Fe^^ only, between gamet and biotite. The Al'^ in biotite was assumed to be 10% by

model 5AV, but accounts for trivial effects on temperature calculations for gamet-biotite

thermometry (Holdaway, 2001). Holdaway (2001) recommends model 5AV over the use

of any of the three single models just mentioned. The equation for calculating

temperature based on model 5AV is as follows:

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T(K) = (40198 + 0.295P + G + B)/ (7.802 - BRlnKo)

Where R = 8.31441 and P is in bars, and G and B are given in terms of Margules

parameters (W).

G — 3R Tin (WMggame/^Fe in gamet)

B — 3R71n(Wpe in Siotite/^VMg in biotite)

The GASP Barometer

The net transfer reaction (involves the production and consumption of phases; In

K=0):

3CaAÏ2Si208 — Ca3Al2SisOi2 + l A h S i O s + S i 0 2 (anorthite) = (grossular) + (Al-silicate) + (quartz)

forms the basis of an important geobarometer for metapelitic rocks (Koziol and Newton,

1988). The mineral assemblage plagioclase + gamet + sillimanite (or andalusite; or

kyanite) + quartz is common in medium to high-grade metamorphic rocks over broad

ranges in P-T space (See Koziol and Newton, 1988, for the approximate location in P-T

space of the equilibrium curve of the GASP end-member continuous net transfer

reaction). The theory is based on the fact that the ratio of Ca in gamet to Ca in

plagioclase is a function of the temperature and pressure the rock formed at (Ghent,

1976). Similarly as with gamet-biotite geothermometry where a Kd value is calculated

from the ratio of Mg/Fe in gamet to the Mg/Fe in biotite, a Keq value can be calculated

that relates the concentration of Ca in both gamet and plagioclase. The equation that

relates Keq as a function of the ratio of grossular component in gamet to anorthite

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component in plagioclase is:

Keq — (Ct3game/(Caplagioclase)

The components quartz and AlgSiO; are ignored because they are nearly pure end-

member phases, thus activities of both minerals are set at 1.0 and are cancelled from the

equilibrium equation, although it is extreme importance to realize that the IQq does not

apply to rocks that do not contain Al 2SiOs and quartz (Spear, 1993). The Keq value,

calculated from measured compositions of Ca in gamet and Ca in plagioclase along with

a temperature from gamet-biotite geothermometry, can then be substituted into an

equation that is based on the thermodynamic properties of the GASP net transfer reaction,

thus a Pressure (P) can be calculated (Koziol and Newton, 1988). The pressure equation

is:

0 = -48,357 + 150.66T(K) + (P-l)(-6.608 + RTlnKeq)

As with gamet-biotite thermometry where an ideal ionic solution model is assumed,

this equation also assumes ideal ionic substitution, thus corrections to the activity-

composition relations are required for the GASP geobarometer to calculate the correct

pressures. For this study model GASP.AVF obtained from Michael Holdaway will be

used to calculate pressures of equilibrium of the GASP net transfer reaction. This model

has also been obtained on PC disk from Michael Holdaway. The model takes into

account both the non-ideal behavior of Ca in gamet, as well as, Ca in plagioclase using

thermodynamic constraints determined from Fuhrman and Lindsey (1988) of the temary-

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feldspar solid solution.

Analytical Techniques

Major element chemical analyses for both gamet-biotite thermometry, as well as,

GASP barometry was determined using Wavelength Dispersive Spectrometry (WDS) on

the JEOL 8900 Superprobe at the University of Nevada Las Vegas. Operating conditions

were set at 15kV accelerating voltage, with a specimen current of 2 x lO'^A (20 nA’s),

and beam diameter of 1 pm for gamet and biotite analyses. Beam diameter for

plagioclase analyses was set at 1pm. A ZAP correction factor was applied to all chemical

analysis of gamet, biotite, and plagioclase. A complete list of standard deviations of

elemental analyses will not be given here, but the analytical uncertainty on the electron

microprobe can not be better than ±2% for various elements (Cox, 1992). Relative

uncertainties of ±2% in MgO and FeO analyses result in an error of ±30°C when

propagated through the gamet-biotite thermometer equation at 5kb pressure (Ghent et al,

1979). Mineral standards for elemental oxide analyses for gamet and biotite were as

follows: enstatite-SiOz, ilmenite-FeO, microcline-KzO, jadite-NazO, jadite-AlzOs,

ilmenite-TiOz, ilmenite-MnO. Gamet and biotite standards were ran as unknowns

approximately every 10 analytical points. Plagioclase standards were ran approximately

every 6 analytical points. Any total wt.% for gamet and plagioclase, of an individual

analytical point, that did not fall between 98.5 and 101.5% was systematically rejected.

For biotite, a total wt.% of ~95% was acceptable, but was not always achieved because of

the altered nature of the mineral in a few of the samples.

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Selection of Analytical Data for P-T Determinations

This research has two goals concerning geothermometry and geobarometry,

specifically, (1) determine absolute peak metamoiphic P’s and T’s of orogenesis, and (2)

determine retrograde T’s during cooling of the metamorphic event. How can this be

acconplished? In other words, what pairs of minerals, or specific locations within the

minerals need to be used to determine peak P ’s and T’s and retrograde and T ’s?

For the determination of both peak and retrograde T’s the methods of Indares and

Martignole, (1985), and Cox, (1992) were used. Both these studies used the Mg-Fe

compositions of garnet-core and matrix-far biotite pairs for the calculation of peak T’s. It

was shown in the last section that garnets were likely homogenized by volume diffusion

during the granulite facies metamorphism, thus garnet cores likely reflect equilibrium

Mg/Fe composition at the peak T that was reached. Furthermore, approximate T’s

determined from the P-T grid (Figure 35) are in agreement with the approximate T’s

needed (~700°C) for the onset of volume diffusion in garnet (Cox, 1992; Cygan and

Lasaga, 1985). Biotite matrix-far grains are also assumed to have been homogenized

during high-grade metamorphism, thus these biotite Mg/Fe composition also reflect

equilibrium conditions at peak temperatures (Cox, 1992). This research has also

confirmed the findings of Cox (1992) in which biotite chemistry in granulite facies rocks

were found to be a function of the distance from garnet (i.e. Fe higher and Mg/Fe lower

for matrix far biotite), therefore for this thesis project 1 define the peak distribution

coefficient as K dp, where Agf reflects equilibrium at or near peak (P) temperatures, thus:

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It was documented in Chapter 5 that garnet Fe significantly increases, and garnet Mg

decreases, from core to rim, near adjacent biotite, with the lowest Mg/Fe adjacent to

biotite. A readjustment of Mg-Fe between garnet and biotite occurs during retrogressive

cooling, with the Mg/Fe composition of the garnet rim nearest the biotite, along with the

Mg/Fe composition of the adjoining biotite, reflecting the approximate temperature at

which diffusion between the two minerals ceased (Indares and Martignole, 1985). Based

upon these determinations a second distribution coefficient can be defined here as K dc,

which reflects equilibrium Mg/Fe conditions during retrogression and cooling (C). K dc

can be specifically defined as;

K d c =

Given the definition of K dc, mineral pairings for retrograde temperature will be gamet-

rim and biotite-close (average composition). All biotite close grains that were used for

temperature calculations in this research are in direct contact with a garnet rim.

Although many papers have been written concerning the extraction of pressure

conditions of metamorphism using the GASP barometer (Hodges and Spear, 1982;

Holdaway, 2001), very little literature has been published concerning the pétrographie

selection of plagioclase for GASP barometry. Considering that the anorthite component

in plagioclase and garnet varied only slightly depending on the sample, pétrographie

location, or within individual grains, it is unlikely that a retrograde P can be determined

from the methods used in this research.

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Holdaway (2001) recommended using matrix plagioclase, close to the analyzed

garnet, to calculate peak metamorphic conditions. It has been shown in this study though,

that for at least in these granulite facies rocks, matrix close plagioclase falls within the

same compositional range (Ab-An components) as matrix far plagioclase and plagioclase

in contact with garnet rims (rim plagioclase). Thus, selection of plagioclase based on

pétrographie location in the matrix would make little difference for GASP pressure

calculations. For this study, matrix plagioclase independent of pétrographie location will

be used to calculate peak pressures of metamorphism for each individual sample.

It may be worth noting that the only plagioclase grains that were analyzed that show

any slight deviation in Ca and Na conpositions compared to that of matrix plagioclase

and rim plagioclase are the small circular (5-15pm) grains included in circular quartz that

are enclosed in garnet (Figure 22b). Typically, these plagioclases have a slightly lower

anorthite component and a slightly higher albite component than any other

petrographically located plagioclase grains, but the deviation from matrix plagioclase

composition was determined to be insignificant. Given this determination, peak P’s will

be calculated using matrix plagioclase composition. Peak K eq conditions can then be

defined as KEqp'.

^ _ ^^^arnet-core^Q^lagioclase-matrix^3

Where, P = peak metamorphic conditions.

I should reiterate that there is a high degree of confidence that the matrix plagioclase

do represent equilibrium at or near peak-P during metamorphism because it can be shown

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that plagioclase grains are in textural ^equilibrium with matrix biotite, the rims of garnets,

as well as, other matrix minerals.

Quantitative P-T Results

Table 1 shows the quantitative P-T results using gamet-biotite thermometry and

GASP barometry on the 7 metapelitic rocks (IV63, IV41, IV29, IV87, IV64B1, IV47,

IV49) and the 5 metagranitic rocks (IV41D, IV76A, IV66A, IVl 1, IV50) analyzed on the

BMP. The pétrographie locations are given along with the analytical points used for each

mineral pairing to calculate K dp, K dc, and Kgqp. Note that when (c) or (r) are given as the

pétrographie locations for garnet this corresponds to one single point near the center of

the garnet or on the rim of the garnet, respectively. When (r) is given for the pétrographie

location for biotite or plagioclase, then the analyses took place on a biotite or plagioclase

in direct contact with a garnet rim. The letter (m) refers to minerals that were analyzed in

the rocks matrix. When the average composition of a mineral was used to calculate

equilibrium conditions the letter (a) is given after c, r, or m. Peak metamorphic T’s and

P’s were determined using gamet-c, biotite-ma, and plagioclase-ma compositional

pairings. On some smaller garnets peak T’s and P’s were determined using gamet-ma,

biotite-ma, and plagioclase-ma (or ra) compositional pairings. For the calculation of

retrograde T’s gamet-r and biotite-ra pairings were used. On most samples more than one

mineral pairing (indicated by the bold (1), (2), (3), etc.. .on Table 1) was used to

determine peak P’s and T’s, as well as, retrograde T’s. The radius of the garnet used in

each mineral pairing is also given in Table 1 to try and determine if a relationship exist

between the size of the garnet analyzed and the peak-T that it records. For IVl 1 and IV50

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no P’s could be determined because^f the lack of any A^SiOs phase. Additionally, no

retrograde T’s were determined for FV49 because no rim biotites were analyzed from that

sample. The reader should refer to Table 1 for all data presented below concerning P and

T results.

Metapelitic Rocks

Among the pelitic gneisses IV63 and IV41D (garnet from FV41D from the pelitic

portion of the sample) recorded the highest metamorphic peak T’s at ~714°C and ~720°C,

respectively. Peak metamorphic P’s recorded in these two samples were 3.6 and 4.4 kb,

respectively. Peak T’s and P’s determined from compositional pairings from IV29 and

IV49 were similar but slightly less than that determined from IV63 and IV41D, at 690°C

at 3.9 kb, and ~670°C at 4.0 kb, respectively. The remaining metapelitic rocks FV87,

IV64B1, and FV47, all showed peak metamorphic conditions at or below ~664°C and 3.4

kb.

The lowest retrograde T’s, among the metapelitic rocks, were again recorded by

gamet-biotite pairings from FV63. In fact, the ~501°C retrograde temperature

determined for FV63 was the lowest temperatures calculated in this research. Mineral

compositional pairings in IV87 recorded gamet-biotite retrograde equilibrium T’s as low

as ~524°C.

Additionally, a relationship may exist between the size of the gamet and the peak

metamorphic conditions recorded with thermometry and barometry (see Spear, 1993; p.

619-620 and Figure 36). Figure 40 shows the gamet radius plotted against the T recorded

by thermometry for peak metamorphic mineral pairing’s among the pelitic gneisses. The

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two largest garnets analyzed, IV63gtl (~5000um radius) and IV41Dgt2 (~3000um

radius), record the highest T’s among the pelitic gneisses. Other large garnets IV29gt2

(-2000 X lOOOum), IV29gt3 (-2000 x lOOOum) and IV41gtl (-2500 x lOOOum) also

gave gamet-biotite equilibrium peak T’s at or above 670°C. The smaller gamet IV29gt5

(500 X 400um) only recorded equilibrium peak metamorphic T’s as high as -609°C.

Spear (1993), cautioned that the extraction of peak metamorphic T’s using gamet-biotite

thermometry was dependent on the radius of the gamet, as well as, the cooling rate of the

rock. It is shown on Figure 40 that the highest peak-T equilibrium conditions were

recorded by the largest gamets analyzed in this study and a relationship exist between the

radius of the gamet and the peak-T recorded.

In rV29 five gamets of differing sizes, paired with matrix biotite, were analyzed and

Peak-T recorded. The largest gamet FV29gt3 records peak-T equilibrium conditions at

-689®C, while the smallest gamet IV29gt5 records equilibrium peak-T conditions at only

609.1°C. The three gamets with radii between that of IV29gt3 and FV29gt5, give peak-T

conditions between those determined by these two gamets.

Metagranitic Rocks

Peak metamorphic temperatures recorded by the rocks of granitic composition were

determined to be slightly higher than peak-T’s determined from the pelitic gneisses.

Generally, the peak-T’s were determined to be >710°C. The highest peak metamorphic

conditions recorded by the rocks of granitic composition was by the metaaplite IV76A.

The peak-T was determined to be 747°C, while the peak-P, from IV76A, was very similar

to peak-P’s determined on the metapelitic rocks, at or near 4.0kb. The augen gneiss IV50

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recorded similar peak-T’s as IV76A, at~735°C. Augen gneiss FVll recorded gamet-

biotite peak-T equilibrium conditions at ~719°C, while peak-T on the metaleucocratic

dike IV66A, was determined to be ~712°C. FV41D, which contains both leucosome and

pelitic schistose portions on the sanple thin section, gave retrograde T’s at ~557°C. No

GASP barometric calculations could be carried out on the two augen gneisses, IV ll or

IV50, because the two samples lack an aluminosilicate phase.

Retrograde T’s for the metagranitic rocks were also generally determined to be higher

than the retrograde T’s determined on the metapelitic rocks. The lowest retrograde

gamet-biotite equilibrium temperature recorded by the rocks of granitic composition was

IV66A, at ~560°C. IV76A, recorded cooling T’s as low as ~566°C. The lowest cooling

T’s recorded by the two augen gneisses IV ll and FV50, were -576 °C and -623°C,

respectively.

The relationship between the peak-T recorded and the radius of the gamet used in

thermometry is not as obvious in the metagranitic rocks. Figure 41 shows the peak-T

recorded versus gamet radius of the metagranitic rocks. No definite trend, of increasing

peak-T recorded with increasing size of the gamet, can be confirmed from the limited

amount of data points, although the highest temperature of peak equilibrium conditions

was again determined fi'om the largest gamet, IV76Agtl, (- 4000um radius). This would

seem to suggest that the radius of the gamet used in thermometry may again be a limiting

condition when peak-T’s are attempted to be extracted using the Mg-Fe gamet-biotite

exchange equilibrium reaction.

All peak-T’s with corresponding peak-P’s from Table 1 are plotted on Figure 42 to

try and determine an approximate P-T point of the rocks of the eastem Ivanpah

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Mountains. Peak conditions are shown on Figure 42 with the blue diamonds. Since the

largest gamets in this study recorded equilibrium peak-T conditions near or above 700°C,

and generally the smaller gamets recorded T’s <700°C, then it can be concluded that peak

metamorphic T’s and P’s during the orogenic event in question reached ~750°C and ~ 4.0

kb. Lower peak-T’s recorded from smaller gamets may record cooling profiles, therefore

the peak-T’s recorded from these smaller gamets do not record tme metamorphic peak-

T ’s.

Additionally, it has be shown by this research that retrograde Mg-Fe exchange

reactions began at ~575°C and ceased at -501 °C. This lowest retrograde cooling-T of

-501°C corresponds nicely to the temperature at which Mg-Fe exchange reactions and

volume diffusion cease in gamet, which is -500°C (Spear, 1993).

One of the most impressive findings from geothermometry and geobarometry

calculations is the correspondence of peak-T’s and Peak-P’s with the biotite + sillimanite

= gamet + cordierite univariant reaction (blue line on Figure 42). The peak P-T point of

-747°C and 4.0kb, from IV76A, plots almost directly on this reaction curve. The

presence of biotite + sillimanite + gamet + cordierite assemblages in some of the sarrqjles

provides further evidence that suggest that the peak P-T conditions recorded by the rocks

of the eastem Ivanpah Mountains should be at or near this univariant reaction curve.

Finally, best estimates of Peak P-T conditions during metamorphism have been

constrained between -714-747°C and -3.4 - 4.4 kb from both the pelitic and granitic

rocks. Retrograde cooling-T’s are recorded down to -501 °C.

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ELECTRON MICROPROBE DATING OF MONAZITE

Background and Theory

Monazite is a light rare-earth element (LREE) phosphate and is a common accessory

mineral in granitic igneous rocks and metamorphic rocks of varying compositions

(Overstreet, 1967; Kingsbury et al., 1992). The mineral incorporates U and Th into it’s

lattice structure during crystallization but does not readily inherit common Ph. (Williams

et al., 1999). Both the U and Th subsequently decay to radiogenic lead (Pb*)(Jaffey et al,

1971). The electron microprobe monazite dating method is based on measuring

concentrations of U, Th, and Pb*, at a single point and calculating the age (t) by solving

the following equation:

Pb* = Th/232[exp(À^^^ t) - 1]208 + [U(0.9928)/238.04]

x[exp(X^^U) -1J206 + [U(0.0072)/235]

x[exp(X^^^t)-l]207

where U, Th, and Pb are in ppm and , X^^, are the radioactive decay constants

of Th, and respectively (Montel et al., 1996). The equation must be solved

for (r) by iteration. Iteration is an algorithmic technique where an equation can be solved

by repeatedly working on successive parts of the problem. In this case the three separate

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time (t) variables in the age equation^

During crystallization monazite typically incorporates high concentrations of both U

(up to 5 wt%) and Th (3-15 wt%) causing radiogenic of Pb* to accumulate very quickly

(Montel et al., 1996). In less than one hundred million years the Pb* reaches a level

where precise measurements can be determined with the BMP (Montel et al., 1996). The

U-Th-Pb concentrations from a single BMP analysis allow for the calculation of a

meaningful age only if the assumption of no initial incorporation Pb* is valid, and no

other modification of the system has occurred, such as Pb* loss (Roa et al., 1998).

Monazite usually displays concordant behavior in the ^^^U/^°^Pb vs ^^*U/^*^^Pb system

(Albarede et al., 1985; Parrish, 1990; Rao et al., 1998) indicating the U-Pb system in

monazite is usually unaffected by subsequent geologic events (i.e. the system remained

closed)(Rao et al, 1998; Crowley and Ghent, 1999; Smith and Geletti, 1997).

The main advantages of using the electron microprobe versus other conventional

methods to determine U-Th-Pb ages are: (1) the BMP is non-destructive to samples. (2)

The BMP has a spatial resolution of 1-3 pm allowing individual chemical domains of

differing ages within single monazites to be dated. This is a major advantage over the U-

Pb TIMS method which may yield mixed ages from adjacent zones rendering the final

age determination meaningless. (3) Analyses can be run relatively quickly compared to

other methods. For example, one homogenous monazite can be analyzed with the BMP

and an several spot ages determined within 1 hour. (4) The BMP will give precise and

accurate analysis because the interactions between atoms and electrons are well

understood. (5) The instrument is relatively easy to use. Finally, (6) the major advantage

of BMP analysis is that it allows for in situ analyses (Cocherie et al., 1998). The in situ

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nature of the method also allows for individual monazite grains to be correlated to

structural features within a thin section, so that the fabric development in a metamorphic

rock can be dated by correlating individual monazites with fabric orientation.

Also, zones within single monazite grains can be given specific ages (i.e. all points

within in a zone have the same age within analytical error) and can be correlated to

specific geologic events such as metamorphism and recrystallization (Cocherie et al.,

1998; Shaw et al., 2001).

The use of monazite as a geochronometer has increased in recent years due to a better

understanding of the behavior of U-Th-Pb in monazite (Zhu et al., 1997). Before more

recent studies, many geologists believed that monazite retained *Pb only at temperatures

below ~530°C and therefore could not record high-temperature histories and time of

crystallization in metamorphic rocks, nor reflect “inheritance” in igneous rocks (Wagner

et al., 1977; Kingsbury et al., 1992). More recent work has suggested that the U-Th-Pb

system in monazite is not completely reset when exposed to high metamorphic and

igneous temperatures for a significant period of geologic time and the effective closure

temperature of Pb* in monazite is ~700°C (Copeland et al., 1988; Kingsbury et al.,

1992). This suggests that the initial time of crystallization of many magmatic and high-

grade metamorphic events (i.e. eastem Ivanpah Mountains) could be recorded by

monazite (Kingsbury et al., 1992).

During orogenesis in the eastem Ivanpah Mountains many of these granitic and high-

grade metamorphic rocks were emplaced during accretion and intense deformation that

resulted in high-grade metamorphism (Miller and Wooden, 1992). Thus, it should be

possible to place age constraints on the metamorphism and deformation in the eastem

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Ivanpah Mountains using BMP dating of monazite.

Locating Monazite

Candidates for monazite dating (individual monazite crystals) were initially

examined petrographically. Differentiating between monazite and zircon proved to be

difficult because both minerals range in size from only 20-50pm. An efficient way to

locate monazite on a microscopic scale is to map each probe section in BSB mode or map

each section using Ce X-ray images (Williams et al., 1999) using the BMP. This method

can be very time consuming and expensive, since one BSB map on the BMP can take up

to 1 hour for a single monazite grain. The recommendation here is to search for monazite

in BSB mode (40-1OOX) without using the mapping technique. Monazite will almost

always be the brightest mineral in BSB mode since it is composed largely of elements of

very high atomic number. Zircon can be distinguished from monazite by slowly reducing

the brightness of image. Zircon will eventually disappear from the field of view, and all

that will remain will be small (usually 5-100 pm) bright specks on the image. These small

bright specks are almost always monazite grains. One can then record the x-y BMP stage

coordinates of individual monazite grains and relocate the grains later for subsequent

dating.

Another way to differentiate between monazite and zircon is by crystal form. Zircon is

usually euhedral and elongate, displaying a dipyramidal form. Monazite is usually

subhedral displaying a roundish shape (Scherrer et al., 2000). Although this study found

this to be true in many cases, it is not a full-proof method for differentiating between the

two minerals because monazite, at least in the rocks of the eastern Ivanpah Mountains,

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was extremely variable in size and form, including anhedral masses, subhedral rounded

grains, and even euhedral elongate dipyramidal crystals similar to the crystal form of

zircon. Monazite in the granitic samples were usually larger (-50-200 pm), displayed a

more euhedral crystalline structure, and contained well-defined Th-zones as opposed to

monazite within pelitic samples. Monazites enclosed in gamet in pelitic samples were

usually small (-5-25 pm), rounded, and lacked well-defined Th-zones. Monazite in the

matrix of the pelitic samples were slightly larger (-25-75 pm), usually displayed a

subhedral rectangular shape, and contained moderately defined Th-zones. No relationship

was determined to exist between the size of the monazite and its orientation to the

dominant NNE fabric present in all lithologies. Although most monazite grains were

elongate to fabric orientation in all samples (independent of pétrographie location).

In this study, monazite clusters (5-20 grains) were located often within cores of larger

(>2500 pm radius) gamet porphyroblasts. Monazite was also found to be abundant in

smaller (1000-2500 pm) radius gamets (2-5 grains usually), although monazite was

sparse in the extremely small gamets (<1000 pm).

Analytical Technique

U, Th, Pb, and Y, were analyzed on a JEOL 8900 Superprobe at the University of

Nevada, Las Vegas. PET crystals were used to simultaneously determine the amounts of

U, Th, Pb, and Y, using the spectral lines MP for U, M a for Th, M a for Pb, and L a for Y.

Yitrium was analyzed because of it’s well known spectral interferences with PbMa

(Cocherie et al., 1998). An Excel spreadsheet used for age determination also corrects for

the Y interferences. Operating conditions for quantitative chemical analyses of U-Th-Pb-

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Y were 15 kV (accelerating voltage), 200 nA (beam current), and 1pm beam diameter.

Standard count times were 30s (peak + background). Sanple counting times

(peak + background) were 200s + 100s for U, 300s + 150s for Pb, 150s + 75s for Th, and

60s + 30s for Y, respectively. Stated in other words, it takes -6.5 minutes to determine

the amounts of U, Th, Pb, and Y from one spot analysis. The amounts of U, Th, Pb, and

Y are then converted to ppm and placed into the age equation using the Excel macro

program written by Michael Jercinovic (UMass, Amherst)(modified by New Mexico

Tech). This study is using this program to calculate all U-Th-Pb ages for single point

analysis.

Standard measurements utilized pure minerals to reduce uncertainty for measuring U

(uranium glass), Th (thorite; ThO:), Pb (PbO), and Y (yittrium phosphate; YPO4). A ZAP

correction procedure was carried out on each single spot analysis. For each measurement

the statistical uncertainties are given at (la).

Before the above procedures could be carried out, Th and Ce X-ray maps for selected

monazites were made to identify any chemical zoning. This is an important step for the

determination of the final age of a monazite (or zones within monazite) for the following

reasons. (1) If a monazite displays zoning patterns then the monazite may be chemically

heterogeneous, and may display several domains of differing ages. (2) A fixed

composition (short analysis) is initially determined for each homogenous zone in a

monazite for the elements P, Si, La, Ce, Pr, Nd, Sm, Ca, and Gd. This initial composition

is then used as a fixed composition when the long quantitative analysis (conditions

described above) is used for the final determination of the amounts of U, Th, Pb, and Y.

Mapping for the determination of possible zones is probably the most critical step in

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BMP dating of monazite (M.L. Williams, personal communication, 2002).

Operating conditions for the short preliminary analysis were 15kV, 20nA, and 1pm

beam diameter. Counting times for the determination of the amount of P, Si, La, Ce, Pr,

Nd, Sm, Ca, and Gd were 30s (peak) and 15s (background). The spectral lines used for

the determination of these elements are as follows, Za-La, Aa-P, Aa-Si, Za-Ce, Z^-Pr,

Zy5-Nd, LJ3-Sm, Aa-Ca, and LP for Gd. This method of using a short preliminary program

to determine the average amounts of these elements makes the final total wt.% for each

spot analysis more variable than if longer peak and background counting times were used

for all these mentioned chemical constituents of monazite. Therefore, acceptable final

oxide wt.% for each individual spot analysis was set between 97%-105%. Any total oxide

wt.% on any single spot analysis that did not fall within this range was rejected.

Calculated Ages and Errors

Errors on single ages were calculated by using the macro run Excel Monazite

Dating Spreadsheet Program obtained from New Mexico Tech. The program calculates

individual ages from the measured amounts of U, Th, and Pb, (95% confidence) from the

age equation (above) and associated errors by propagating the BMP counting

uncertainties (standard deviations) of U, Th, and Pb into the age equation. This statistical

approach was developed by Ancey et al. (1978), and is based on the fact that photon

counting in X-ray analysis is based on Poisson’s Law (Montel et al., 1996).

The procedure routinely used for determining and reporting ages and errors for BMP

dating of monazite is to simply report the calculated weighted mean age and the standard

weighted mean error (95% confidence interval) for a population of spot analyses within a

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single homogenous monazite crystal, or for separate age domains within a single

monazite (Montel et al., 1996). Errors can often be claimed as low as 0.0025-0.0050 % of

the calculated age, even for Proterozoic rocks (Shaw et al., 2001).

Two other sources of error need to be considered besides just the EMP statistical

error, these being specifically, (1) the use of an average monazite conposition for ZAP

matrix corrections, which can add an additional 1% error to the uncertainty of the final

age, and (2) the uncertainty of the composition of the standards, which also can account

for an additional 1 % uncertainty on the final age (Montel et al., 1996). Cocherie and

Albarede (2001) argue that considering all the sources of error, of the process as a whole

(statistical error, ZAP corrections, standardizations), the best precision that can possibly

be claimed on a population of ages, or for a single spot age, from a monazite grain is ±2%

of the final age calculation, thus an error reported <2%, for any age, is suspect at best. 1

will follow the advice of Cocherie and Albarede (2001) and report the final absolute

errors no lower than ±2% for single spot analysis and final weighted mean age.

Weighted Mean Age and Weighted Error

The procedure in this research was to try and analyze at least 5-8 points from

homogenous monazite grains, or from individual zones within single heterogeneous

monazite grains. Workers at the University of Massachusetts recommend at least 6 points

be analyzed within each domain for statistical validity of the final age (M.L. Williams,

personal conversation, 2002). After single spot ages were determined from a series of

spot analyses (in some cases more than 5-8 points were analyzed, in a few cases less), a

simple mean age is calculated along with an uncertainty of the mean age at 2a. The

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Isoplot program version 2.49d (Ludwig, 2001) was used to determine individual ages that

did not fall within this 95% (2a) confidence level. Outliers were not used to calculate the

final weighted mean age of a population of monazite ages, and were rejected. The

equation from Montel et al. (1996) was used to calculate weighted mean age (Ax):

Where T, is the age of a single spot analysis and a is the associated error. These average

weighted mean ages are reported as the absolute ages of crystallization for a population

of spot ages within a single monazite (Appendix E). Average weighted error calculations

were also determined from ages represented within the 95% confidence interval. Average

weighted errors (Oax ) were also calculated using an equation from Montel et al. (1996):

On many populations of monazite ages the average weighted mean error, was

calculated to be less than 2% of the final weighted mean age. When this occurred the

final weighted mean error carries little geologic meaning because monazite dating with

the EMP cannot realistically be more precise than ±2%. If the final weighted mean error

was determined to be >2% then that error is reported in Table 2 as the final absolute error

(Oax )- If the weighted mean error was determined to be <2% of the weighted mean age,

the final absolute error will be calculated by using the equation:

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Or ={AJx (0.02)

Where (5f is equal to the final absolute error reported on a weighted mean age (Ax) in

Appendix E. The calculated Cax in this case will also be given in parenthesis in the text

next to the calculated Cf , mainly for comparison purposes to other published EMP

monazite ages. If no ciax is shown in parenthesis in the text then the single error reported

is Cf -

MSWD

The Mean Square Weighted Deviation (MSWD) is a way to statistical validify

whether a series of spot analyses (ages with associated errors), are of a single population,

and to verify how well these data points fit on a regression line (isochron). The equation

that solves for the MSWD is:

MSWD= l/(n - 2) I (Àx.Â)/(OAxi ^

Where n = the number of analyses, A% = is the plotted data point (weighted age), Â = the

place the data point intersects the isochron ± the error o ^ , and the summation (S) is from

i = 1 to n. The MSWD for a given number of analyses must fall below the value in the

following table for all of the ages to be considered from the same population: [see Wendt

and Carl (1991) for a complete explanation of the use of the MSWD]:

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rt MSWD ' n ■ M S W D n MSWD ’ ' H MSWD

1 - 6 2.414 11 1.943 16 1.756

2 - 7 2.265 12 1.894 17 1.730

3 3.828 8 2.155 13 1.853 18 1.707

4 3.000 9 2.069 14 1.816 19 1.686

5 2.633 10 2.000 15 1.784 20 1.667

It is of extreme importance to clarify that MSWD values were calculated from

weighted mean ages and not from the age of the isochron (regressioin line). Isochron ages

were not calculated. See Cocherie and Albarade (2001) for the complete procedure to

calculate isochron ages using EMP monazite dating.

Samples and Monazite Selection

Monazite EMP dating focused mainly on the same samples used in the pétrographie

and thermobarometry portions of the study (IV63, IV41, IV41D, IV66A, IV76A, IV50).

The data set includes three pelitic gneisses (IV63, IV41, IV41D), one leucosome

(IV66A), one meta-aplite (IV76A), and one augen gneiss (IV50).

The general procedure was to locate and date a minimum of 3-4 monazite crystals

that were within each individual pelitic gneiss sample, and at least one monazite within

the granitic composition samples. The goal was to vary the size of the monazites selected

for age dating as well as, the pétrographie location. This was suggested by Michael

Williams (personal communication, 2002) so the complete geochronology that the

monazite may have recorded can be determined. Monazites that were dated varied in size

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(5-225|xm), shape (roundish to elongate; anhedral to euhedral), and in pétrographie

location. Dated monazites included matrix grains, grains enclosed in quartz, grains

included in gamet cores, and include grains in textural equilibrium with biotite elongate

to foliation. Additionally, monazites included in other matrix minerals, such as K-

feldspar, plagioclase, and cordierite were also dated in several samples. Montel et al.

(1996) and Zhu et al. (1994b) recommended searching for monazites grains included in

gamet and quartz, which may possibly shield monazite from alteration or modification of

the U-Th-Pb system.

It should be noted that various textural relationships could possible be related to

different intervals of monazite growth and recrystallization, but a complete analysis

relating textural relationship, size, and morphology of individual monazite grians to the

age of monazite was not attempted. Although it will be shown that generally speaking

smaller (5-25pm), rounded, fairly homogenous grains represented the oldest population

and were located enclosed in gamet of pelitic rocks. Somewhat rectangular shaped,

subhedral, slightly larger grains (25-75pm) were located in the matrix of pelitic and gave

ages younger than monazite enclosed in gamet. And very large monazite (75-225pm)

occurred most often in grainitic rocks. The monazite in granitic rocks was extensively

zoned, gave ages comparable to matrix monazite in pelitic rocks, and usually showed

excellent crystalline stmcture.

Presentation of Data

Several methods of presenting and evaluating monazite age data can be found in the

literature. Montel et al. (1996), Montel et al. (2000) and Shaw et al. (2001) present final

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age data as weighted mean age histograms or frequency distribution diagrams. These

methods are used partly to separate out different age populations by summing up

individual probability density functions for each measurement. Cocherie and Albarede,

2001, use a new Th/Pb = f(U/Pb) theoretical isochron diagram (Figure 43) to calculate a

more precise weighted mean age and error. This method greatly improves statistical error

on U-Th-Pb calculations and provides a check for the possibility of Pb-loss. They also

use plots of the average weighted age to display individual ages from a series of spot

analyses. Cocherie et al. (1998) used the conventional Pb = f(Th*) isochron diagram that

calculates a Th-Pb age with a precision that depends on the “extent of the range in

variation” of the total amount of Th + U in each monazite grain.

Average weighted age diagrams and Th/Pb - f(U/Pb) isochron diagrams were

constructed for selected individual monazites. The Th/Pb = f(U/Pb) isochron diagrams of

Cocherie and Albarade (2001) are employed, not to calculate new precise ages as they

did (isochron ages), but to evaluate whether monazite remained closed to Pb-loss. If no

modification of the U-Th-Pb system has occurred, then error ellipses from a series of spot

ages should plot on a Th/Pb vs U/Pb regression line that is parallel to sub-parallel to the

theoretical adjacent isochrons (Cocherie and Albarade, 2001). When the slope of the

regression line defined by the data points is significantly different than that of the

theoretical isochrons, then it is likely that the U-Th-Pb system has undergone siginificant

modification (Cocherie and Albarade, 2001).

Finally, to be able to plot error ellipses on the U/Pb vs Th/Pb isochron diagrams a

standard error correlation technique was carried out to determine the error correlation

between both the ratios, U/Pb and Th/Pb. The equations that were used to determine the

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total error for the ratios U/Pb and Th/Pb are:

Ou/Pb - [((Lu) + ((^Pb)]

CTn/pb = [(On) + (

Individual errors from U, Th, and Pb, were determined using the counting statistics from

the EMP for each measurement (See Cocherie and Albarade (2001) for the complete

procedure for calculating and plotting error ellipses).

Monazite Dating Results

Figure 44 shows a frequency distribution histogram of all 225 U-Th-Pb monazite spot

ages determined from EMP U-Th-Pb chemical analysis. The histogram is interpreted to

represent at least two, and possibly three geologic events, in addition to the presence of a

inherited component of -2225-2000 Ma. During the calculation of weighted ages and

errors some homogeneous grains and zones (within single monazites), yielded mutiple

points with similar ages and means, with low associated MSWD values (analyses

summarized in Table 2). These well-behaved grains define the three large peaks on the

histogram and three of the four ages >2000 Ma. The oldest well-defined peak at -1750-

1675 Ma (mostly ages from monazite grains enclosed in gamet cores from pelitic rocks)

may constrain the age of peak metamorphism and the approximate timing of the Ivanpah

orogenic event. Furthermore, populations of monazite ages from grains enclosed in

gamet cores usually gave the lowest MSWD values. Quartz enclosed monazite grains

help define the second oldest peak on the histogram at -1550-1450 Ma. These monazites

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enclosed in quartz also gave very low MSWD values, whether they were within pelitic or

granitic rocks. Additional ages from monazite enclosed in other minerals of granitic rocks

(i.e. K-feldspar, biotite, sillimanite, and plagioclase) also help define the peak at -1550-

1450 Ma and give MSWD values also within statistical validity. The youngest well

defined peak on the histogram at -1350-1300 Ma ages are from one single monazite

located in the matrix of the pelitic sample IV63 and were not reproduceable in any other

other monazite grain.

Other individual monazite grains (and zones within single grains) gave a wide range

of ages and means with unacceptable MSWD values (analyses summarized in Table 3).

These particular not well-behaved monazite grains were located within the matrix of both

pelitic and granitic rocks, enclosed in a small gamet, and enclosed within the outer rim of

a large gamet. The grains located in the matrix of granitic rocks from Table 3 gave

MSWD values slightly higher than statistical requirements. All the grains presented in

Table 3 typically give a wide range of ages that are scattered throughout the histogram.

These ages may represent Pb-loss in monazite during high temperature metamorphism.

Given below is a discussion and presentation of age data for each individual sample from

both Table 2 and Table 3. Throughout the discussion the reader may also refer to

Appendix E for weighted mean age and error results and also for chemical analysis of

each monazite analyzed.

Pelitic Gneiss - IV63

A total of eight monazites in the pelitic gneiss sample IV63 were dated. Five of the

monazite grains were located within the large garnet IV63gtl near its core (IV63mztl, 2,

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3, 5, and 10). Three monazite grains were analyzed in the matrix of the sample, one

enclosed in cordierite (lV63mzt6), one enclosed in quartz (lV63mzt8), and one in textural

equilibrium with biotite and partly enclosed in quartz (lV63mzt9).

rV63mztlO, a 5pm diameter monazite, located in the gamet core, gave the oldest age

in the data set. Only 3 spot analyses were carried out on this grain because of it’s very

small size. The average weighted mean age was 2071 ± 230 Ma (MSWD = 0.16). Figure

45 shows the three individual error ellipses plotted on a Th/Pb = f(U/Pb) isochron

diagram. The calculation of a regression line (isochron) for the three points was not

carried out because of the lack of spread in the data.

This monazite also has very low Th concentration, which significantly increases error

on mean age calculations (Montel, 1996). It was determined that a relationship may

possibly exist between the size of the monazite and the amount of Th that it may contain.

In otherwords, at least in the rocks of eastem Ivanpah Mountains, the larger the monazite

the more Th (smaller absolute age errors) and the smaller the monazite less Th (higher

absolute age errors). This particular monazite may be an old inherited monazite preserved

within the gamet core. Miller and Wooden (1992) reported similar ages for Ivanpah, New

York, and McCullough Mountains zircons that they interpreted as inherited. This age of

>2000 Ma was not reproducible in any other monazite in this study, although one spot

age from IV41Dmzt2 gave an age of 2204 ± 45 Ma.

The second oldest monazite in this study, IV63mzt3, was also found included in the

large gamet IV63gtl. Generally speaking the oldest monazite grains, such as IV63mzt3,

were enclosed within gamet, smaller, and more rounded than monazite enclosed in other

minerals in pelitic samples or within granitic samples. Seven spot analyses were taken

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within this rounded 10pm diameter monazite grain. Average weighted mean age was

1752 ± 86 Ma (MSWD = 0.14). See Figure 46a for a plot of the average weighted mean

age and Figure 46b for the ages and associated error ellipses plotted on a Th/Pb = f(U/Pb)

isochron diagram. No isochron was fitted to these data because the plotting positions on

the isochron diagram are so tightly grouped that the Isoplot program can not define a

regression line to fit these data, although all seven ages fall within error on an age line

(green line shown on Figure 46a).

Within rV63mzt5 (located included in a rounded quartz grain which is enclosed in

gamet), five spot analyses were taken and the weighted mean age was calculated at 1 736

± 59 Ma (MSWD = 0.11). Figure 47 shows the five data points plotted on an average

weighted age plot. The average weighted mean plot shows that all five ages fall within

error of an age line (green line on Figure 47).

rV63mztl and IV63mzt2 were determined to have similar weighted mean ages at

1703 ± 43 Ma (MSWD = 0.75) and 1710 ± 34 Ma (oax = 21 Ma)(MSWD = 0.57).

Figures 48 and 49 show the Th compositional maps for both IV63mztl and IV63mzt2.

The location of the 10 spot analyses on IV63mztl, and the 12 spot analyses on IV63mzt2,

are shown on the two corresponding Th maps. The Th map for IV63mztl shows that this

grain contains two zones, a dark central core surrounded by a brighter rim. Similarly, the

Th map of IV63mzt2, shows that at least two zones exist within this grain. Spot analyses

were taken from both zones but the ages did not differ significantly from zl to z2. Figure

50 shows the age results for IV63mzt2 plotted on the Th/Pb = f(U/Pb) isochron diagram.

The calculation of an isochron (regression line) was carried out on these 12 ages and the

results are given (MSWD = 0.57). This sample was chosen for the calculation of an

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isochron because more point analyses were carried out on this grain compared to the

other monazites in this sample (a regression line can be regressed from the spread in

data).

The four monazite grains IV63mztl, mzt2, mzt3, and mzt5 (just discussed) all gave

similar weighted mean ages of -1700 Ma within statistical uncertainty. All 33 spot ages

(does not include the -1495 Ma single age from IV63mztl) from within these four grains

are given on a weighted mean plot (Figure 51) and a Th/Pb = f(U/Pb) isochron diagram

(Figure 52) to help define the age of the metamorphic event that these garnet enclosed

grains may represent. The average weighted mean age for all spot ages from these four

grains was determined to 1713 ± 34 Ma (MSWD = 0.33).

IV63mzt6 (enclosed in cordierite located in the matrix) was also mapped for Th

concentration (Figure 53a). The Th map suggest that several zones are present within the

grain, including a somewhat triangular shaped core, designated zl on the Th map. The

individual locations of spot analysis along with the associated ages are shown the Th

map. The final calculated ages of 13 spot analyses show extreme variation even within

individual zones, ranging from 1710 ± 34 to 991 ±28 Ma. The average weighted ages for

zl and z2 were determined to be 1459 ± 29 (Oax = 16 Ma) and 1379 ± 30, repectively.

The MSWD values of 63 and 88 fell well below acceptable numbers for zl and z2,

respectively. Figure 53b shows the data points plotted on a Th/Pb - f(U/Pb) isochron

diagram. The 1710 ± 34 Ma spot age located on the edge of the inner domain (zl) is

consistent with the ages of the monazites dated from the garnet core of IV63gtl

(IV63mztl, mzt2, mzt3, and mzt5). Two other individual ages within zl were determined

to be 1642 ± 33 Ma and 1655 ± 33 Ma. All other ages, some within zl and some within

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z2 (outer domain) are significantly younger. Possibly this particular monazite located in

the matrix may have been modified by substantial Pb-loss during a subsequent high

temperature geologic event.

rV63mzt8 (enclosed in quartz) was also mapped for Th concentration (Figure 54) and

the results show that at least three conpositional domains are present in this grain. Four

spot analyses from each zone (zl, z2, and z3) were performed on the BMP. The average

weighted mean ages for zl, z2, and z3, were 1522 ±31 Ma (MSWD = 2.0), 1540 ±31

Ma (MSWD = 2.2), and 1593 ± 32 Ma (MSWD = 1.3), respectively. Locations of

individual spot analysis within each zone and the associated ages are shown on the Th

map. Considering the consistency of the ages for all three zones in this monazite, perhaps

the grain was either completely reset or recrystallized in a later geologic event. No ages

in this grain are consistent with the >1700 Ma ages from the monazites included in

garnet, yet it is elongate to the preferential foliation direction in the sample, suggesting

that this particular monazite originally crystallized during the development of the

metamorphic fabric. The question is whether this determined age represents the age of

metamorphism or the age of a subsequent re-crystallization event that completely reset

the U-Th-Pb system? The latter interpretation is favored here because the tightly

constrained ages of -1752-1689 Ma from monazite enclosed in garnet stongly suggest the

age of metamorphism. The one metamorphic fabric that is present is intepretated to be the

result of this deformational event.

rV63mzt9, in textural equilibrium with an adjacent biotite grain that defines the NE

foliation direction, gives even younger ages than IV63mzt8. The Th map for this grain is

shown in Figure 55. The Th map reveals that two compositional domains are present in

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this grain, including a Th-rich inner inner core surrounded by a Th-poor outer rim. The

location of the 12 point analyses, as well as, the individual associated spot ages (six from

zl and six from z2) are shown on the Th map. Two ages from the inner domain were

determined to be 1576 ± 31 Ma and 1520 ± 30 Ma, consistent with some of the ages from

rV63mzt8, but four ages from zl and all six ages from z2 were determined to be within

the range of 1311 ± 26 Ma (Oax = 21 Ma) and 1353 ± 27 Ma (Oax = 15 Ma). Average

weighted mean age for zl and z2 (excluding the two >1500 Ma ages from zl) was

determined to be 1326 ± 27 Ma (OAx-21 Ma)(MSWD = 0.14). This >1300 Ma age of

crystallization was the youngest reproducible age of any monazite analyzed. The

consistency of the -1300 Ma ages, along with low associated errors and low MSWD

value, may suggest that this grain was almost completely reset at -1350-1300 Ma. The

U/Pb = f(U/Pb) isochron diagram (Figure 56) shows that 10 of the 12 spot analysis (also

excluding the two >1500 Ma ages) define an isochron (regression line) that is concordant

in the U/Pb-Th/Pb system.

Pelitic Gneiss - IV41

Five monazite grains were analyzed in IV41 (pelitic gneiss), three enclosed in garnet

(rV41mztl, mzt2, and mzt3) and two in the matrix (IV41mzt4, and mzt5). Both

IV41mztl and IV41mzt2 are enclosed in an elongate 2000 x 1000pm garnet. The garnets

long dimension is parallel with the preferential fabric in this sample, indicating it is a syn-

kinematic garnet that grew during the development of the foliation. Both IV41mztl and

rV41mzt2 were mapped for Th concentration and results shown on Figures 57 and 58,

repectively. The Th map for FV41mztl reveals two distinct zones are present within the

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grain, designated zl and z2. Nine spot analyses were carried out on zl, and 3 spot

analyses on z2. The average weighted mean age was for this monazite grain was

determined to be 1689 ± 34 Ma (Oax = 25 Ma). The average weighted mean age was

calculated using ages from both zl and z2, excluding the -1607 Ma, -1657 Ma, and 1656

Ma ages because of their close proximity (-1/2 pm) to the edge of the grain. Cocherie et

al. (1998) warn against individual spot analysis being taken near the edge of a monazite

because of the increased possibility of diffusion and Pb-loss occurring near the edge of

the grains, which would give younger ages. Considering the very small dimensions of

rV41mztl (-10pm x 10pm) this could not be avoided on at least half of the points. Notice

though that the oldest spot ages of 1749 ± 35 Ma and 1701 ± 34 Ma are located near the

center of the grain, and the youngest ages of 1607 ± 32 Ma and 1624 ± 32 Ma are located

near the edge of the grain. The younger ages may indicate that Pb-loss has occurred near

the edge of the grain, as suggested by other monazite workers. But the Pb diffusion

process may have only effected the monazite a few microns from it’s edge, with the true

age of crystallization of this grain may be indicated by spot ages near the core.

The Th compositional map of IV41mzt2 also shows two prominent zones present

within in this monazite. The brighter central core is designated as zl and the lighter rim

area that surrounds this brighter core as z2. Specific locations of all spot analyses on this

grain are shown on the Th map (Figure 58). Twleve spot analyses from zl and z2 give an

average weighted mean age of 1694 ± 34 Ma (oax = ± 20 Ma). Figure 59 shows the ages

and associated error ellipses, for the twelve spot ages from zl and z2, plotted on a Th/Pb

- f(U/Pb) isochron diagram defining an isochron (MSWD = 0.66).

IV41mzt3 is located within a small 1000pm x 800pm garnet. The Th compositional

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map shows that possibly three conpositional zones are present within this grain. The

designated zones zl, z2, and z3, are shown on the Th map along with individual spot ages

(Figure 60a). No spot analyses were carried out on z2 or z3. Nine analyses were

performed on zl and the determined individual ages were again problematic. Ages for the

nine points were determined to be between 1303 ± 28 Ma and 1645 ± 33 Ma (Oax = ± 22

Ma). The average weighted mean age was calculated to be 1386 ± 33 Ma (Oax =±20

Ma), but the MSWD of 5.6 was not within statistical requirements. Possibly this monazite

has also suffered Pb-loss due to high tenperature metamoiphism even though it’s located

enclosed within a small garnet. The isochron diagram (Figure 60b) shows the scatter of

the ages and their discordant nature. Possibly the smaller dimensions of the garnet

(compared to the other garnets used in this study) prevented effective shielding and Pb-

loss from occurring within the monazite during high-T etamorphism.

rV41mzt4 (enclosed in biotite and sillimanite) is -80 x 40pm grain that displays very

complex zoning patterns on the Th compositional map (Figure 61a). Two zones were

analyzed on the BMP (zl and z3). Zone 1 was designated as the central dark core and z3

was specified as the brighter half moon shaped outer region. No analyses were carried out

on z2. The nine spot analyses from zl and the six spot analyses from z3 showed little

consistency in ages and are discordant in the Th/Pb vs U/Pb system (Figure 61b). The

ages and associated error ellipses are shown on the isochron diagram for both zl and z3.

The blue error ellipse denotes the oldest determined age in the eentral core zl, of 1722 ±

34 Ma (oax = 26 Ma) and the green error ellipse is representative of the youngest age

with it’s associated error determined also from z l, of 1453 ± 29 Ma (Oax = 25 Ma). All

ages determined from z3 fall between these two ages. The average weighted mean age for

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all nine spot ages was determined to be 1583 ± 32 Ma (oax = 21 Ma). Similar to

IV41mzt3, the MSWD value of 6.4 was not within statistical requirements, which makes

the average invalid for interpretation purposes. All ages are shown for each spot analysis

on the Th map. Possibly this grain records both metamorphism and subsequent Pb-loss in

monazite, as high metamorphic temperatures caused the modification of the U-Th-Pb

system after -1722 ± 34 Ma. Although it is possible that even the -1722 Ma age

represents Pb-loss and partial resetting.

The monazite IV41mzt5 is located in the matrix enclosed in quartz. The Th

compositional map (Figure 62) shows that this monazite lacks any extensive zoning

patterns recognized in the other monazites of this sample, therefore no assigned zones

were given for this grain. The Th map shows the nine spot analyses taken on this grain

and the associated spot ages. Ages ranged from 1503 ± 30 Ma (oak =29 Ma) to 1606 ±

39 Ma, with an average weighted mean age determined to be 1547 ± 31 Ma (Oax = 24

Ma)(MSWD = 1.2). This average weighted mean age is consistent with the mean ages

from monazites from the matrix of IV63. Possibly the average age of 1547 ±31 Ma

represents the timing of a re-crystallization event, given the low MSWD value of 1.2

indicating that all nine ages are from the same age population.

Pelitic Gneiss + Leucosome - IV41D

Two monazites were analyzed in sample IV41D (leucosome + pelitic gneiss). As

mentioned earlier, IV41D is from the same location and the same outcrop as IV41.

IV41D was of additional interest in this study because -80% of the thin section was

granitic leucosome material, while -20% of the thin section was composed of similar

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schistose pelitic gneiss as IV41. The leucosome monazite (IV41Dmztl) analyzed from

this sample was an -100pm long euhedral grain that was determined to contain several

well-defined zones based on Th composition (Figure 63). Analyzed domains are

designated on the Th map as zl, z2, and z4 (no analysis were performed on z3). Six spot

analyses from the dark central core (zl) were determined to have an average weighted

age of 1559 ±31 Ma (CAx - 25 Ma)(MSWD = 2.1). Zone 2, one of the brightest areas on

the Th map, which concentrically surrounds zl, and another smaller zone (z3), was

determined to have an average weighted mean age of crystallization of 1526 ±31 Ma

(Oax = 23 Ma)(MSWD = 0.35). Zone 4, which had the highest Th concentration, is

located near the outer portion of the grain. Five spot ages from z4 give an average

weighted mean age of 1476 ± 30 Ma (Oax = 26 Ma)(MSWD = 0.38). The average

weighted mean ages from zl, z2, and z4 may constrain the timing of partial melting

during the crystallization of the grantic leucosome material between -1559 and 1476 Ma.

The youngest age o f-1476 Ma may give the minumum age of neocrystallization and

overgrowth. This particular monazite grain contained the highest concentration of Th of

all monazite grains analyzed, with each spot analysis exceeding 300,000 ppm (>30%) by

weight.

rV41Dmzt2 was located within the outer rim of the large garnet FV41Dgtl. This

garnet is 2000 x 1000pm and situated on the interface between the leucosome portion and

the schistose portion of the sample. The Th map (Figure 64) shows that at least three

compositional zones may be present within this grain, designated zl, z2, and z3, on the

Th map. Spot ages were determined from all three zones and varied greatly from spot to

spot ranging from 2204 ± 45 Ma to 1583 ± 36 Ma. The single age of 2204 ± 45 Ma

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coincides nicely with the average weighted mean age of the three spot analyses from

rV63mztlO of 2071 ± 230 Ma. This monazite grain may represent material shed of the

margin of Laurentia in the Early Proterozoic, such as IV63mztlO. The ages younger than

-2204 Ma may be the result of Pb-loss in monazite due to high-grade metamorphic

temperatures exceeding 700°C (Figure 65). Additional evidence that at least the central

core zl has undergone substantial modification of the U-Th-Pb system is the very high

MSWD value of 44 on the average weighted mean age of 1571 ± 98 Ma. Another

alternative intepretation is that only the brighter inner core represents inherited material

and that the concentric Th-poor zones (z2 and z3) that surround the core (zl) are due to

overgrowth and recrystallization caused by one or more latter high temperature

metamorphic events. If z2 and z3 do represent overgrowth and recrystallization then the

age of -1894 Ma located in z3 (most outer zone) is problematic. If this -1894 Ma age is

correct (no modification of the U-Th-Pb system) then this would give the approximate

age of the overgrowth of z3, but this age was not reproduceable in any monazite grains

analyzed. Furthermore, the range of spot ages within z2 ans z3 (other than the -1894 Ma

spot age) was between -1745 Ma and -1642 Ma (the -1583 Ma spot age is being

disregarded for interpretation purposes because it was located -1/2 pm from the edge of

the grain). Possibly the true age of recystallization that resulted in the two outer

concentric zones is between -1745 Ma and -1642 Ma. Another possibility is that the

-1894 Ma spot age represents Pb-loss after -2204 Ma. In any case, it is apparent that

there may be an inherited component present within this grain and at least zl has suffered

Pb-loss and modification of the U-Th-Pb system.

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Meta-Aplite Sill - IV76A

One monazite grain, IV76Amztl (-150 pm x 100pm), was analyzed in the

matrix of IV76A (meta-aplite), partially enclosed in K-feldspar and partially enclosed in

quartz. No monazite included in garnet was located in this sample. The Th compositional

map (Figure 66) shows that the grain has complex zoning patterns. The two zones that

were analyzed, zl and z2, are shown on the Th map and ages of crystallization

determined. Seven ages from zl (inner darker portion of the crystal) and seven ages from

z2 (brighter region that surrounds zl) were determined from individual spot analysis. The

seven spot ages give an average weighted mean age of crystallization for the central core

of 1520 ± 30 Ma (Oax = 23 Ma)(MSWD = 0.74). The seven spot ages from z2 give an

average weighted mean age of 1492 ± 30 Ma (Oax = 23 Ma)(MSWD = 1.2). Figure 67

shows all representative error ellipses plotted on a Th/Pb = f(U/Pb) isochron diagram. No

isochron was calculated for these set of data because the differing mean ages of the two

zones give a regression line that is not parallel to the two theoretical isochrons at 1500

Ma and 2000 Ma. The ages suggest that this granitic meta-aplite, which intruded parallel

to pelitic gneiss foliation throughout the eastern Ivanpah Mountains, crystallized at

-1520-1490 Ma. This may represent the timing of magmatic igneous activity during the

Early Proterozoic in the Eastern Mojave Desert.

Leucocratic Sill - IV66A

One monazite (IV66Amztl) was analyzed and ages of crystallization determined for

the sample IV66A. As with the meta-aplite IV76A, no monazite included in garnet was

located in this leucocratic sill. This -225pm long dipyrimidal monazite, enclosed in a

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matrix plagioclase, is extensively zoned as shown by the Th compositional map (Figure

68). The zones within the monazite mimic the crystal form as a whole. Even the inner

dark core has a pyrimidal shape. Four zones (zl, z2, z3, and z4) along with locations of

spot analyses are specified on the Th map. Average weighted ages of crystallization for

each zone are as follows: zl = 1475 ± 29 Ma (Oax - 16 Ma)(MSWD = 0.99), z2 = 1503 ±

30 Ma (MSWD = 0.14), z3 = 1416 ± 28 Ma (Oax = 21 Ma)(MSWD = 2.2), z4 = 1493 ±

30 Ma (OAx= 23 Ma)(MSWD = 3.6). The weighted ages of crystallization from zl, z2,

and z4 are within statistical error and agree well with the -1520-1490 Ma crystallization

ages from IV76A, and the three weighted mean ages o f-1559 Ma, -1526 Ma, and -1476

Ma from the leucosome monazite IV41D. The weighted mean age of -1416 Ma from z3

is problematic since it does not agree well with the ages from the other three zones

analyzed. It is also difficult to explain the -1416 Ma age from z3 since it is the innermost

zone, yet it is the youngest of the four zones. One explanation for the youngest mean age

from the innermost core (z3) and the high-Th concentration is that it represents the

possible effects of radiation damage causing U-Th-Pb open-system behavior (i.e. younger

age due to loss of daughter product).

This granitic material is also mineralogical similar to both IV76A and the leucosome

portion of IV41D, and may also represent granitic magmatism in the Eastern Mojave

Desert, with age constraints on crystallization between -1503 ± 30 Ma and -1416 ± 28

Ma.

Augen Gneiss - IV50

Finally, one large -140 x 75pm monazite was located in the matrix of IV50 (granitic

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 95

augen gneiss) enclosed in K-feldspar and quartz. The Th compositional map (Figure 69)

shows two well-defined segments of a darker inner core surrounded by a brighter outer

rim, with the brighter region also extending, in a linear fashion, through the central part

of the grain separating the two darker regions. The three zones labeled zl, z2, and z3 on

the Th map were analyzed with the BMP and are shown with the associated spot ages.

Five spot analyses were carried out on zl with the final absolute ages of individual spot

analyses ranging from 1692 ± 34 Ma (Oax =13 Ma) to 1534 ± 31 Ma (oax = 13 Ma). The

average weighted mean age for these five spot ages was 1618 ± 32 Ma, but the

population of ages do not define a statistically valid group (MSWD of 3.8). Possibly Pb-

loss and occurred within z l. Only 2 spot analyses were taken on z2, giving ages of 1515

± 34 Ma and 1493 ± 30 Ma (Oax = 24 Ma). The average weighted mean age of these two

spot analyses is 1502 ± 30 Ma (Oax = 24 Ma)(MSWD = 0.24).

Five spot analyses were carried out on z3 with the average weighted mean age

determined to be 1431 ± 29 Ma (oax = 23 Ma)(MSWD = 1.2). Notice that the oldest ages

are located within the central core regions, zl and z2, and the youngest spot ages in z3.

Figure 70 shows the five spot ages from z3 on a weighted average plot. Figure 71 shows

the data from all three zones plotted on a Th/Pb = f(U/Pb) isochron diagram. The oldest

age from zl, and the associated error of 1692 ± 34 Ma (G ax =13 Ma) is shown on the

diagram as the blue ellipse. This age is consistant with crystallization ages of

metamorphism from monazite from other samples in this study. This older age o f-1692

Ma may record the original age of crystallization during metamorphism and orogenesis,

while the younger ages within zl may record further evidence that Pb-loss occurred at or

near 1700 Ma. Zone 3, on the otherhand, which has an MSWD on the weighted mean age

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within requirements, may record a high temperature re-crytallization event causing

overgrowth on the monazite grain at -1431 Ma. Evidence for this overgrowth is shown

on the Th map. Zone 3 (Th-rich) extends around the Th-poor zl and z2 in a continuous

fashion around the outer perimeter of the grain.

Additionally, this possible re-crystallization age is also somewhat consistant

with the ages of monazite from other granitic rocks in this study, such as meta-aplite

IV76A, the leucocratic dike IV66A, and the leucosome portion of IV41D. Although this

augen gneiss probably does not represent an intrusive granite body or injected anatectic

melt, such is probably the case with IV76A, IV66A, and IV41D. Possibly these two

augen gneisses represent granitic rocks originating from an intusive event that was

associated with the orogenesis between -1752-1689 Ma (i.e. one single spot in zl gave

an age o f -1692 Ma).

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DISCUSSION AND CONCLUSIONS

Pétrographie evidence and metamorphic fabric measurements indicate that only one

deformational event is preserved in the rocks of the eastern Ivanpah Mountains. This

metamorphic event is recorded in all lithologie units and is preferentially orientated

N10°E and 69°NW. This is similar to the preferred orientation of metamorphic strike in

the Yavapai metamorphic terrane of ENE (Wooden and Miller, 1990). Dubendorfer et al.

(2001) presented evidence that two deformational fabrics are present in the nearby

Proterozoic Cerbat Mountains in northwest Arizona. Perhaps evidence of the first

metamorphic event, such as Dubendorfer et al. (2001) found, has been completely

overprinted in the eastern Ivanpah Mountains. Possibly the younger Yavapai orogeny

overprinted and completely destroyed evidence of the original fabric from the Ivanpah

orogeny, but this seems unlikely since metamorphism of Yavapai orogeny reached only

amphibolite facies conditions. Or perhaps the eastern Ivanpah Mountains simply were not

geographically situated such that they recorded a second metamorphic event.

The KFMASH grid constrains first approximation peak metamorphic P-T conditions

of this deformational event at -2-5 kb and - 625-800“C, repectively, based on the

presence of sillimanite, cordierite, and migmatites (partial melting), and the absence of

kyanite, andalusite, muscovite and opx. Gamet-biotite thermometry and GASP barometry

calculations on both the pelitic and granitic rocks agree extremely well with KFMASH P-

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T grid estimates. Peak metamorphic temperatures were determined using IV63gtl

(gamet-biotite thermometry), a t-714 °C. Garnet IV41Dgtl, the second largest garnet

analyzed among the pelitic rocks, records peak T’s as high as -720 °C. Peak P’s during

metamorphism during this time period were determined to be -4.4 kb as recorded by

GASP barometry using IV41Dgtl.

Thermobarometry from the leucocratic rocks agrees well with the peak metamorphic

conditions determined from the pelitic rocks. IV66A gave temperature as high as -712°C

and pressure as high as 3.8 kb. IV76A gave peak temperature and pressure as high as

-747 °C and 4.0 kb, respectively. The peak temperature of -747 °C is somewhat suspect

though because the biotite in this sample was highly altered, thus Kd values may have

been superficially increased due to Fe-oxidation. The peak-T from IV66A is probably a

more accurate determination of the highest temperature attained during the

metamorphism of the leucocratic rocks.

BMP monazite ages suggest that these metamorphic P-T conditions were attained

during the deformational event between 1752 ± 86 Ma and 1689 ± 34 Ma. These ages are

constrained from six monazite grains included within three large garnet porphyroblast

from samples IV63, IV41, and IV41D. Three of the four oldest mean ages of 1752 ± 86

Ma, 1736 ± 35 Ma, and 1703 ± 34 Ma, were all recorded by monazite grains within the

largest garnet studied (FV63gtl), and are concordant in the Th/Pb-U/Pb system.

Preservation of a inherited monazite population in the eastern Ivanpah Mountains (one

monazite also included in the large garnet porphyroblast IV63gtl), record ages as old as

2071 ± 230 Ma. The origin of this inherited material is most likely sediments shed off the

margin of Laurentia during the Paleoproterozoic. Wooden and Miller (1990) found zircon

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fractions from the Early Proterozoic Turtle Mountains (Eastern Mojave Desert) to have a

similar age at -2300 Ma, and interpreted these zircons to be remnants of a supracrustal

protolith.

Monazite grains located near the core of the garnets may be effectively shielded from

Pb-loss and re-crytallization. This apparent shielding effect and the preservation of older

monazite populations enclosed within garnet is consistent with the findings of other

workers. Montel et al. (2000) reported the same phenomenon in high-grade metamorphic

rocks from the Beni Bousera Hercynian kinzigites (sillimanite-cordierite-gamet-biotite

gneiss) in Morocco. In their study four monazites were dated by U-Th-Pb EMP analyses,

two completely enclosed in garnet and two located in cracks within garnet that were

connected to the matrix of the kinzigites. Hercynian ages (-310-282 Ma) were obtained

from the two monazites included in garnet, while ages from the monazites lying in the

cracks of the garnet gave ages at -124 Ma and 119 Ma, with no detectable Pb (i.e

extensive Pb-loss). They concluded that the cracks played an important role in the

resetting of the two grains located in the cracks, possibly due to the circulation of fluids.

But their most important conclusion was that the behavior of the U-Th-Pb system in

monazite is not only a function of temperature but also a function of pétrographie

position. Thermometry gave temperatures >850°C in the kinzigites, yet the monazites

completely enclosed in the garnet withstood temperatures higher than what is usually

considered the closure temperature for Pb in monazite (-700°C). They also considered

that quartz may also be an efficient shield for Pb-loss in monazite since it lacks cleavage

(similar to garnet). The conclusions of this research backs their findings on both counts

and that “shielding” by both garnet and quartz was observed in the granulite grade

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Proterozoic rocks of the Ivanpah Mountains.

Further evidence that suggest large garnet effectively shield monazite from Pb-loss is

from pelitic matrix monazites, which give ages that are usually problematic (exceptions

usually being grains included in quartz). Some single spot ages (1710 ± 34 Ma; 1722 ±

34 Ma; 1673 ± 33 Ma) within pelitic matrix monazite grains are consistant with the

-1752-1689 Ma weighted mean ages from monazite included in garnet, but these ages

were not reproduceable within any single matrix monazite analyzed in this research. The

scatter among ages within single matrix monazite (enclosed in minerals other than quartz)

are usually discordant in the U-Th-Pb system shown on the isochron diagrams. This

likely indicates that diffusion of Pb (Pb-loss) occurred in the monazite crystalline

structure as closure-T (-700 °C) was exceeded for radiogenic Pb in monazite.

Well-behaved matrix monazite, such as IV63mzt-9 (1326 ± 27 Ma) possibly indicates

a total resetting of the U-Th-Pb system in this grain, or new monazite growth, rather than

the age of crystallization due to deformation during orogenesis, even though the dated

grain in the matrix is aligned preferentially to the dominant NNE metamorphic fabric in

the rock.

Among the leucocratic rocks IV66A and IV76A, mean ages of crystallization from

matrix monazite grains were found to constrain granitic magmatism between -1520 ± 30

Ma and 1416 ± 28 Ma. Isochron diagrams for these samples show that the ages and

associated errors are concordant in the U-Th-Pb system, thus these ages most likely

constrain the crystallization age of these granitic rocks. These ages are also consistant

with the proposed widespread emplacement of plutonic rocks in the SW US during the

Early Proterozoic. No single spot ages from either of these leucocratic samples were

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found to record an age within the -1752-1689 Ma proposed timing of metamorphism and

deformation, even though they carry the same NNE preferred fabric oprientation as the

pelitic gneiss foliation that they abundantly intrude throughout the field area, although the

fabric within the leucocratic rocks is very weak in most cases.

On the otherhand, one monazite that was dated from the leucosome portion of

IV41D (matrix grain) records three ages within three different zones at -1559 ± 31 Ma,

1526 ±31 Ma, and -1476 ± 29 Ma. The two younger ages are consistant with the ages

determined from monazite in IV66A and IV76A, although the oldest age suggest that

partial melting and crystallization of leucosomes in the area may have commenced as

early as -1559 Ma. Possibly heat from large-scale magmatism (i.e. IV66A + IV76A +

mafic sills in the area) caused local partial melting in the Ivanpah Mountains.

Duebendorfer et al. (2001) was able to determine that two metamorphic events (Di =

-1737-1721 Ma - Ivanaph orogeny, and Dz = -1721-1683 Ma - Yavapai orogeny)

occurred in the nearby Cerbat Mountains, using conventional U-Pb zircon techniques,

and correlating ages to two distinct deformational fabrics that are present there. This

research was limited by the resolution of the EMP, which can not realistically separate-

out two metamorphic events that are separted in time by less than 50-100 Ma (Cocherie

and Albarade, 2001), such as Duebendorfer et al. (2001) accomplished using

conventional U-Pb methods. Additionally, evidence from Wooden and Miller (1989) and

Miller and Wooden (1992)(Figure 4) also suggest the Eastern Mojave Desert and the

Yavapai terrane are the result of separate orogenic events.

One possibility in the discrepancy between the evidence in this study and the other

research in the Eastern Mojave Desert may be the Proterozoic rocks of the eastern

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Ivanpah Mounting do indeed record the second orogenic episode that has been proposed

(from monazite included in garnet), but the two events can not be chronologically

separated given the limitations on precision of the EMP for monazite dating.

Other workers in the SW US have attempted to discern between separate

metamorphic events in Proterozoic rocks of New Mexico that are separated in time by

<50 million years by correlating monazite and the structural elements within monazite, to

the determined ages using the EMP, but this is highly unlikely, given the resolution of

±2% error on the EMP. Structural elements along with the dating of individual zones

within monazite can be used to help achieve additional precision, but still two distinct

geologic events separated in time by <50 million years will have errors on mean ages that

overlap, such as the proposed Ivanpah orogeny and Yavapai orogeny using the ±2% error

restriction on mean ages that Cocherie and Albarade (2001) have recommended. Also,

given the fact that only one fabric is present in the eastern Ivanpah Mountains, correlating

monazite ages with more than one distinct metamorphic fabric cannot be attempted.

Thus, the conclusion here is that between -1752 Ma and -1689 Ma the Eastern

Mojave Desert experienced only one continental collision episode evidenced by the one

metamorphic fabric preserved in the rocks of the eastern Ivanpah Mountains. During this

accretionary episode the Eastern Mojave Desert became sutured to what was then the

supercontinent of Laurentia. Since these determined ages overlap with the timing of the

well-documented Yavapai orogeny at -1700-1690 Ma, I conclude that the Eastern

Mojave Desert became part of the SW US during the Yavapai orogeny and not the

proposed Ivanpah orogeny. During this deformation event peak pressure and temperature

of metamorphism were -4.0 kb and >700°C.

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Granitic plutonism may have started in the Eastern Mojave Desert at -1559 Ma, and

continued to -1416 Ma causing Pb-loss in monazite in some cases, and overgrowth and

neocrystallization in monazite in other cases. Extreme heat influx into the upper crust

may have continued in the Early Proterozoic until -1326 ± 27 Ma, causing additional

overgrowth of monazite.

Suggestions for Future Workers

Based on the findings of this research, I suggest that other workers that use either

EMP monazite dating, gamet-biotite thermometry, or GASP barometry in their research,

should consider the following suggestions: (1) Use >5000pm radius garnet for gamet-

biotite thermometry, especially if cooling rates are believed to be very slow in the area.

This will maximize the possibility of extracting the peak metamorphic temperature. (2)

Large gamets (>2500pm radius) may also preserve older populations of monazite within

their core due to a shielding effect from extreme temperatures (i.e. high amphibolite

grade to granulite grade) and resulting Pb-loss. This research has shown, with a high

degree of confidence, that the two oldest age populations of monazite in the rocks of the

eastem Ivanpah Mountains can be found petrographically included in large gamet

porphyroblasts. I caution any future workers here at UNLV, or elsewhere, if they

undertake any research involving EMP dating on monazite, that they should date several

monazites from the cores of large gamets, if possible from several samples, because of

the potential shielding effect and preservation of older ages that these monazites may

record. (3) Vary the pétrographie location of analyzed monazite to determine the

complete geochronology that the mineral may record within a sample (i.e. again monazite

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included in large gamet may reveal more than one population of monazite of differering

ages). (4) Report no final absolute errors < ±2% on a single spot analysis or a weighted

mean age on a series of spot analyses. The EMP usually can be no more precise than ±2%

even under ideal conditions. My advice to future workers would be to consider setting

errors at ±3% or ±4% (if they fall below ±2%) of the weighted mean age. This admittedly

is a conservative approach, but will possibly limit any incorrect geochronologic

interpretations on ages of crystallization, metamorphism, and deformation. (5) To directly

test the hypothesis of the Ivanpah orogeny, future workers should attempt to separate out

populations of ages using more precise techniques for monazite dating (i.e. ion probe,

ICPMS). Also, a different geographic location should be studied within the Ivanaph

Mountains for sample collection and to search for evidence of a second metamorphic

fabric. The discovery of the presence of a second metamorphic fabric would be key if

populations of ages at -1700 are to be correlated to two separate and distinct orogenic

episodes during the Early Proterozoic.

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■D CD

C/) C/) 118" W 106“W

SW United States Geologic Map of 8 The Ivanpah Mountains -4 4 ° N (O'

Clarke Mountain Fault 3. 3" CD

CD Research ■D Area O /Yavapai Q. [_J Mesozoic rocks » Orogenic C Paleozoic rocks a / Terrane » 3o Proterozoic (-1700 Ma)' "O metamorphic o rocks / Mazatzal Fault Orogenic ' Terrane CD ■ Concealed fault Q. ' (1660-1600 Ma) X -32° N

Eastem Mojave Desert ■D (Ivanpah Orogenic Terrane ?) CD Modified from (>1700 Ma ?) 100 km Hewett, 1956 C/) C/)

Figure 1, Map showing the two well documented Early Proterozoic orogenic provinces in the SW US (Yavapai and Mazatzal) and the proposed Ivanpah orogenic terrane. The dashed lines represent boundaries of the Yavapai and Mazatzal tectonic provinces. The Eastern Mojave Desert is located within the purple dashed rectangular box. Rock samples were collected in the Summer, 2001 from the research area (enclosed solid rectangle on geologic map) within the Ivanpah Mountains. Modified from Wooden and De Witt (1991). o 1 0 6

114°W 106“W

-4 0 ‘ Nd Nd Province Province

Province, Nd Province CA-Pb Province

SEA-Pb Province 32° N

100 km

Figure. 2 Isotopic map of the SW US based on Pb and Nd ratios. The light blue areas represent divisional boundaries based on differences in ratios of ^“ Pb/^“ Pb, “ ^Pb/^“ Pb, and 208pb/204pb (emd= Eastern Mojave Desert; CA= central Arizona; SEA= southeastern Arizona). These three individual Nd provinces are defined from the data of Bennett and DePaolo, 1987. The boundary zone is the location where Pb isotopic ratios are similar between adjacent provinces. The three Nd isotopic provinces in the SW US are separated on the map by the heavy light grey lines. Modified from Bennett and DePaolo, 1987; Wooden and De Witt, 1991.

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114“ W

Nevada Arizona

• Las Vegas -36°N Ivanpah Valley Grand Canyon McCullough. ^ \ Mts^ V Ivanpah ^ Mountains

New /York \ Mts Providence Mts I

EASTERN \ MOJAVE \ 0 d e s e r t '^ \ REGION

-34° N

CALIFORNIA

50 km

Figure. 3 Map showing the abundance of Proterozoic metamorphic and igneous rocks in the eastem Mojave Desert and the surrounding area. The red dashed rectangular region encloses the Eastem Mojave Desert. The location of the Ivanpah Mountains and Ivanpah Valley are shown with the arrows. The locations of the Proterozoic McCullough, New York, Providence, and Cerbet Mountains are also given on the map. Modified from Wooden and Miller (1990).

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■D CD

C/) C/)

Eastern Mojave Desert Yavapai Province 8 Isotopic signatures - Nd ages =-2.2-2.0 Ga - Nd ages = -1.85 -1.60 Ga ci' - initial Pb isotope ratios - initial Pb Isotope ratios

(at ~ 1.7 Ga) — > 206Pb/204Pb = 16.10 206Pb/204Pb = 15.72 207Pb/204Pb = 15.38 207Pb/204Pb = 15.27 208Pb/204Pb = 35.65 208Pb/204Pb = 35.34 3 3" CD Metamorphic grade - Granulite facies -Amphibolite to greenschlst ■DCD facies O Q. C a Age of supracrustal rocks -2.30-1.77 Ga -1.85-1.60 Ga 3O "O O Characteristics of - Migmatitic - No migmatites recognized

CD intrusive granites - Continuous from -No -1.69 -1.67 Ga Q. -1.76 -1.64 Ga plutons recognized

Orientation of ■D metamorphic fabric -NNE - ENE-WSW CD

C/) C/) Figure 4. Table showing the differences between the Early Proterozoic rocks located in the Eastern Mojave Desert and the Yavapai orogenic terranes. This evidence may suggest that the two regions represent separate orogenic events that occurred in the Early Proterozoic. Data from Wooden and Miller (1990),

o 00 109

115“25’ W

L_J Metaaplite L J Metagranite □ Metatonaiite Meta leucocratic rocks 35" 27 30" N □ Augen gneiss Undivided gneissic rocks Amphibolite d Metabiotite-rich granodiorite d Metagabbro d Pelitic gneiss d Gneissic rocks Concealed Fault Field station location and sample number

IV66A

IV64B1

-35"23'00 N Scale 1:24,D00

Figure 5. Geologic map of a the eastemmost portion of the Ivanpah Mountains showing the abundance of Proterozic metamorphic rocks. The dominant lithologies found in the field area are pelitic gneiss, amphibolite, and a variety of metagranitic/metadioritic rocks. The field stations where the 12 samples were collected (June, 2001) that were used for thermometry, barometry, and monazite dating (plus IV5) are also shown on the map. A meta-prefix has been added to the original rock names given by Wooden and Miller (1992). Additionally, the name mafic tonalité has been changed to metatonaiite and the name trondjhemite has been given the name meta- aplite. Note: samples IV87 and IV49, are pelitic gneiss that were located within larger units of another lithology (indicated on the map). IV = Ivanpah; # = field station number. Modified from Wooden and Miller (1992).

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Figure 6. Stereographic projection showing the results of the ~200 strike and dip measurements that were carried out on the metamorphic fabric in the eastern Ivanapah Mountains. Each individual red square represents the pole to the plane of one measurement. The preferred orientation of the fabric was determined to be N10°E (strike) and 69°NW (dip).

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■D CD

C/) C/)

(O' Foliation striking NNE

3. 3" CD ■DCD O Q. C a 3o "O o

Q.CD

■D CD

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Figure 7. Two photos showing the well-developed foliation in politic gneiss of the eastern Ivanpah Mountains, Eastern Mojave Desert. The red arrow in (a) indicates the foliation strike. In (b) the blue arrow shows what appears to be a “false stratigraphy” which is the result of the well-developed foliation planes in politic gneiss in the area. Orientation of the this metamorphic fabric is generally NNE. Pg = politic gneiss. 11 2

. - f^«i'Ad^%'4L ,a A a K c A * Figure 8. Highly crystalline politic gneiss in the eastern Ivanpah Mountains with leucosome injections between foliation. The hammer indicates the foliation direction. The red arrows point out the felsic leucosome. Pg = politic gneiss.

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t .

Figure 9. Large lenticular shaped leucosome pod (indicated by the red arrow) injected between foliation of a highly weathered outcrop of pelitic gneiss in the eastern Ivanpah Mountains. The hammer indicates the NNE foliation direction. Pg = pelitic gneiss.

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(a)

Figure 10. Small leucocratic veins/dikes in the eastern Ivanpah Mountains that both cross cut foliation in pelitic gneiss, as well as, intrude the foliation plane. The red arrows in both (a) and (b) indicate where the leucocratic vein intrudes foliation and the blue arrows in both (a) and (b) show where it cross-cuts foliation. The chisel in (a) and the hammer in (b) indicate the NNE direction of foliation. Pg = pelitic gneiss.

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■D CD

C/) C/)

CD 8

3. 3" CD ■DCD O Q. C a O 3 "O O

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Figure 11. Small leucosome folds within pelitic gneiss in the eastern Ivanpah Mountains. The red arrows in both (a) and (b) indicate the locations of the leucosome folds. The chisel in (a) and the ink marker in (b) indicate the NNE foliation direction. Pg = pelitic gneiss CD ■ D O Q. C 8 Q.

■D CD

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Figure 12. Four photographs showing numerous occurrences of leucosome pods, hooks, and layers in pelitic gneiss of the eastern Ivanpah Mountains. The red arrows in (a and c) indicate the leucosome hook structures. The blue arrows in (b) indicate leucosome pods and layers. The yellow arrow in (d) shows garnet-filled lenticular leucosome pods. The green pencil in (a) and the chisel in (d) indicate the NNE foliation direction in the pelitic gneiss. Pg = pelitic gneiss. o\ 117

moar— k I #

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s ..

" V,'-

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Figure 14. Pelitic gneiss, amptiibolite gneiss, and meta-aplite occurring in botti (a) and (b) in a group of three in outcrop. Red arrow in (a) indicates both the strike (NNE) and dip (NW) direction of foliation that all three lithologies contain throughout the eastern Ivanpah Mountains. The hammer and chisel in (b) indicate the NNE strike of foliation. Pg = pelitic gneiss. Am = amphibolite gneiss, Mp = meta-aplite. Notice in (a) that the meta-aplite not only intrudes pelitic gneiss foliation but is also intruded by a 3 inch wide felsic dike (shown by the blue arrow).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 1 9

sv * a # * ' l ; Ê

Figure 15. Amphibolite gneiss between a weak plane of foliation in pelitic gneiss of the eastern Ivanpah Mountains. The hammer and chisel indicate the NNE foliation direction in both the amphibolite gneiss and the pelitic gneiss. The red arrows show the location of small mm-sized felsic layers in the amphibolite gniess. Pg = pelitic gneiss; Am = amphibolite gneiss.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CD ■ D O Q. C 8 Û.

■D CD

C/) C/)

8 ci'

a 3" CD ■DCD O Q. C a O 3 ■D O

CD Q.

■D CD

C/) C/) Figure 16. Leucocratic dike intruding through foliation in a small augen gneiss fold, then cross-cutting foliation at ~90° to foliation. The red arrows indicate the path of intmsion of the leucocratic dike. The blue arrow indicates the foliation direction within the augen gneiss. Ag = augen gneiss.

to o 121

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Figure 17. Small 2-3 m high folds within augen gneiss in the eastern Ivanpah Mountains [(a) and (b)]. These are the largest folds that were located within the field area. Ag = augen gneiss.

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■D CD

C/) C/)

8 ci'

3 yarnef 3" CD ■DCD O Q. C a O 3 ■D O

CD Q.

■D Figure 18. Garnet filled meta-aplite dike (IV76A) intruding pelitic gneiss in the eastern Ivanpah Mountains. The green CD pencil and chisel indicate the preferential NNE foliation direction in both lithologies. The red arrows show both clusters C/) and strands of small garnets within the meta-aplite. Mp = meta-aplite; Pg = pelitic gneiss. C/)

to to CD ■ D O Q. C 8 Q.

■D CD

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CD 8

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C/) C/)

Figure 19. Pétrographie image of biotite and sillimanite continuous NNE foliation wrapping around two garnet porphyroblast in the pelitic gneiss IV63. Si I = sillimanite; Bt = biotite; Gt = garnet. K) W 124

Figure 20. Pétrographie image of gamet elongate to the foliation direction in the pelitic gneiss IV63. The NNE orientation of metamorphic fabric is indicated by the red a mow. Gt = garnet; Bt = biotite.

Figure 21. Pétrographie image of quartz grains elongate to the NNE fabric orientation in the pelitic gneiss 1V63. The red arrow indicates the general orientation of the fabric on the image. Qtz = quartz

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$ Figure 22a. Pétrographie image of the large gamet IV63gt1 in the pelitic gneiss IV63 enclosing an abundance of oval shaped quartz grains. The quartz grains also enclose an abundance of minerals such as plagioclase (See Figure 22(b) below), and clean blocky biotite. Qtz = quartz; Bt = biotite; Gt = garnet.

Figure 22b. Pétrographie image of 20-30um oval plagioclase grains which are enclosed within oval shaped quartz grains. These oval shaped quartz grains are enclosed within the gamet IV63gt1 [also shown above in Figure 22(a)]. PI = plagioclase; Qtz = quartz; Gt = garnet.

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Figure 23. Pétrographie image of a cluster of 12-15 monazite grains located within the central region of the gamet IV63gt1 in the pelitic gneiss IV63 (also shown in Figure 22a). Several of these ~5-20um in diameter monazite grains were used for U-Th-Pb monazite dating and the results given in Chapter 6. Mzt = monazite; Gt = gamet.

Figure 24. Pétrographie image showing cordierite being replaced by sillimanite in the pelitic gneiss IV63. Both prismatic and needle-like sillimanite can be located within the cordierite grain. Crd = cordierite; P-Sil = prismatic sillimanite; N-Sil = needle-like sillimanite.

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Figure 25. Pétrographie image of the extremely well developed continuous foliation in the pelitic gneiss IV41. The foliation is defined by an overwhelming amount (-35% modal) of sillimanite and biotite. Felsic layers composed of quartz, K-spar, and plagioclase alternate with the sillimanite and biotite foliation. Sil = sillimanite: Bt = biotite; Qtz = quartz; Ks = K-spar (microcline).

Figure 26. Pétrographie image of sillimanite lineation within garnet of the pelitic gneiss IV41. The lineation within the gamet is parrellel to the fabric in the matrix, thus the garnet is likely to be post-kinematic to deformation. Sil = sillimanite; Gt = gamet.

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Figure 27. Pétrographie image of a pressure shadow extending off the edge of a gamet porphyroblast The pressure shadow is shaped like a tail and composed mainly of quartz with minor amounts of other felsic minerals. The pressure shadow has been outlined in red for clarity. Gt = garnet; Qtz = quartz.

Figure 28. Pétrographie image of cordierite twinning within the pelitic gneiss IV41. Crd cordierite.

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E Figure 29. Pétrographie image of the well-defined biotite foliation wrapping around a large K-spar augen in the in IV11. Finely crystalline felsic minerals are also present with biotite and also define foliation. Bt = biotite; Ff = small felsic minerals; = large K-spar augen.

Figure 30. Pétrographie image of an elongate lenticular collection of felsic minerals (mostly quartz and plagioclase) between biotite foliation in the leucocratic dike IV66A. This texture is abundant in IV66A. Note: the entire lenticular structure is not visible in the image. Bt = biotite.

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Figure 31(a). Pétrographie image of a large ~225um long euhedral dypyrimidal monazite crystal located within the matrix of the leucocratic dike IV66A. A central dark core is apparent within the monazite grain. Both the rim and the core were dated using the U-Th-Pb EMP technique and the results given in Chapter 6. Note: The long dimension of the monazite is parallel to the NE orientation of foliation (indicated by the red amow), indicating the monazite likely crystallized during the deformational event. Mzt = monazite.

Figure 31(b). Pétrographie image of a large ~100um long euhedral monazite grain located within the matrix of the leucosome portion of IV41D. Several zones (indicated by the blue interference colors) within the grain were dated using the EMP dating technique and the results given in Chapter 6. The NNE orientation of foliation is indicated by the red arrow. Mzt = monazite.

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Figure 32(a). Pétrographie image of prismatic sillimanite and minor amounts of biotite that define foliation in the meta-aplite dike IV76A. The foliation is shown wrapping around lenticular shaped collections of felsic minerals mainly quartz and plagioclase. Sil = sillimanite; Bt = biotite; Qtz = quartz.

Figure 32(b). Pétrographie image of needle-like sillimanite wrapping around a large plagioclase augen in the meta-aplite dike IV76A. The sillimanite defines a NE trending lineation in the rock. Sil = sillimanite; PI = plagioclase augen.

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C/) C/)

8 ci'

3 3" CD ■DCD O Q. C a O 3 ■D O

CD Q.

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Figure 33. Pétrographie image of fibrolite sillimanite enclosed within a plagioclase grain in the meta-aplite dike IV76A. This particuiar type of sillimanite is not associated with the sillimanite that defines the fabric seen in Figure 31. Possibly the fibrolite is the product of a post-kinematic reaction that is consuming plagioclase. F-Sil = fibrolite sillimanite; PI = plagioclase.

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8 ci'

3 3" CD ■DCD O Q. C a O 3 ■D O

CD Q.

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Figure 34. Pétrographie image of anhedral orthopyroxene (opx) in the amphiboiite gneiss IV5. The opx has irregular crystal faces and encloses several mineral phases such as clinopyroxene (cpx), plagioclase, and hornblende. Opx = ortho pyroxene; Cpx = clinopyroxene; PI = plagioclase; Hb = hornblende. 1 3 4

9 0 0 Assumes Fe-Mg exchange reactions only

CD 8 0 0

^ CD I s CO O Î &E ^ ë 6 0 0

5 0 0 100 0 log (dT/dt) In °C/Ma

Figure 35. Diagram showing how the radius of the gamet, along with cooling rates, can restrict the maximum temperature recorded by thermometry. This relationship assumes Fe-Mg exchange reactions only. The dashed red line is an example that shows how a lOOOum radius gamet at a cooling rate of ~2°C/Ma can only record a maximum temperature of ~650°C. The blue numbers represent °C/Ma (modified after Spear, 1993).

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IV63gt1

% % 6 .0 - 38.0

(D O FeO

(a) 0.0

core IV41Dgt2 rim 8.0 40.0

% S 6 0 38.0

^ 0 4.0 36.0 (D FeO O 2.0 CaO 34.0 MhO (b) 0 .0 32.0 0 250 500 pm

Figure 36. Diagram showing traverses from core to rim to adjacent biotite grains in direct contact with the garnet rims of IV63gt1 and IV41Dgt2. The diagram illustrates how FeO increases and MgO decreases from core to rim, while concentrations of CaO and MnO stay relatively constant. All garnets analyzed in this study show similar trends from core to rim to biotite close grains, although a few garnets did show a slight increase in MnO from core to rim (see Appendix A). The solid green circles on the two corresponding MgO concentration lines indicate a single analytical point (location) along the gamet traverse.

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#0.296 *0.334 0.167 0.326

0.345

0.338#

0 348

Figure 37. Pétrographie image showing decreasing Mg/Fe ratios from core to rim towards two adjacent biotite grains ofthe large gamet IV63gt1. Primarily, this trend is due to Mg-Fe exchange reactions tretween garnet and tnotite during retrogressive cooling (i.e Mg decreases and Fe increases). The yellow circles indicate the position of each individual EMP point analysis. Gt = garnet; Bt = biotite.

0.*I75 0.224 # 0.262 0.274 0272 "0.286 * 0.284 r' #0.286 # .0-271 0 266 / : '

Figure 38. Pétrographie image of three point traverses and corresponding Mg/Fe ratios within the gamet IV41gt1. It is shown that within the gamet from core to rim towards biotite, the Mg/Fe decreases, but away from biotite towards quartz, Mg/Fe stays relatively constant. The yellow circles indicate the position of each individual EMP point analysis. Gt = gamet; Bt = biotite; Qtz = quartz.

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KFMASH P-T Grid

AFM diagram for (B) Sill

AFM diagram for (A) Sill

Infinity

Kyanite

5 - Sillimanite

Andalusite rc 400 500 600 700 800 Figure 39. KFMASH petrogenetic P-T grid along witfi two corresponding Ttiompson AFM projections. The red and green lines stiowtwo univariant reactions ttiat likely account forttie mineral assemblages shown in the two AFM Thompson projections (A) and (B). Assemblage (A) contains sillimanite (Sill) + biotite (Bt) + gamet (Grt). As metamotphic paragenesis proceeded in the Ivanpah Mountains tie-lines connecting Sill + Bt were likely broken during the univariant reaction Bt + Sill = Grt + Cord (green line). AFM diagram (B) shows all four of these minerals co-existing in another rock mineral assemblage. The P-T stability fields for both assemblages are shown on the grid. The approximate location ofthe wet granite melting curve is shown as a dashed blue line. The AFM diagrams are projected from quartz, K-feldspar, and H,0.

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Garnet Radius vs Peak-T Recorded (metapelltic rocks) 8 ci' 5500 5000 5000 um 4500

E 4000 3 3500 3" CD « 3000 CD ■D ? O 2500 2250 um Q. ► • • C a 2000 O I • # 3 S 1500 "O * * O 1000 500 ## CD Q. 0 ------1 I I I 1 I I I I I I I [ I 600 610 620 630 640 650 660 670 680 690 700 710 720 730 740 ■D CD Temperature (°C)

C/) C/) Figure 40. Diagram showing how the size ofthe garnet used in garnet-biotite thermometry may restrict the peak-T recorded. The two largest garnets (-2250um radius and -SOOOum radius) used in thermometry recorded the highest peak-T’s among the metapelitic rocks used in this research.

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Gamet Radius vs Peak-T Recorded (metagranitic rocks) 8 5500 5000 4500 CD 4000 ______4000. um ^ ^ _ 3. I - 3" 3500 CD g 3000 ■DCD O 1 2500 Q. C a E 2000 O CD 3 O 1500 - ...... ■D O 1000 # e mm

CD 500 Q. 0 1 1 1 1 1 1 1 I 1 670 680 690 700 710 720 730 740 750 760 ■O CD Temperature (°C)

C/) C/) Figure 41. Diagram showing gamet radius plotted against the peak-T recorded by garnet-biotite thennometry for the metagranitic rocks used in this research. Similar to Figure 40, the largest garnet used for thermometry calcxilations yielded the highest Peak-T’s, thus to determine the maximum temperature of metamorphism using thennometry, large garnets (>2250 um) should be used. CD ■ D O Q. C g Q.

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C/) C/) Tvs P Metapelitic and Metagranitic rocks 7 CD 8 6

Kyanite Muscovite- i 5 out field Sillimanite 4 3. 3" CD 3 CD ■D Cordierite-in O Andalusite Q. 2 . ' field...... C a 3O "O 1 O 0 CD Q. 400 425 450 475 500 525 550 575 600 625 650 675 700 725 750 775 T(C)

■D CD Figure 42. Diagram showing calculated peak-P’s and T’s, as well as, retrograde T’s. The dark blue diamonds represent plotting positions for calculated peak metamorphic conditions. The red circles on the temperature axis represent individual retrograde T's C/) that were determined. Using garnet-biotite geothermometry and GASP geobarometry, peak metamorphic conditions in the Ivanpah C/) Mountains during the Proterozoic have been determined to be ~750°C and -4.0 kb, while the lowest retrograde temperature was calculated to be ~502°C. Note: No retrograde P’s could be determined by this research. The plotted retrograde T’s do not correspond to Okb pressure, but rather are included on the diagram for illustration purposes only. CD ■ D O Q. C g Q.

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(/)C/)

250

20- 200 -

CD 150- XJ CL 3. 3" !£ CD t— 10- 100 - ■DCD O Q. C a 3O 50- "O O

CD Q. 10 0 20 40 60 80 U/Pb Figure 43. Diagram showing the new Th/Pb = f(U/Pb) isochrons proposed by Cocherie and Albarade (2001). This thesis research is "D CD utilizing the method to determine if the U-Th-Pb system within monazite grains has undergone Pb-loss.

C/) C/)

LA CD "D O Q. C 8 Q.

■D CD

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35 f Magmatîsm CD and n = 225 8 neocrystallization ? 30 +

25 + CD Metamorphism due to 3*3. 20 + CD & orogenesis ? ■DCD I O 15 + Q. C g. o 3 Magmatism 10 + ■D and O neocystallizatiion ?

CD Q. inherited monazite ? Pb-loss ? r r u n - t n -r u -a J] -fl- "O CD 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300

C/) Age (Ma) C/)

Figure 44. Frequency distribution histogram of the 225 monazite spot ages determined in this research. Possibly three geologic events are recorded by monazite. The erogenic event in question may be recorded by the 63 ages between 1750-1650 Ma (most spot ages from monazite enclosed in garnet). Granitic magmatism causing overgrowth, neocystallization, and Pb-loss may be represented on the histogram from the 101 ages between 1575-1400 Ma (most ages from granitic rocks). A second recrystallization event may be recorded by the 13 ages betweeni 350-1300 Ma. Ages younger ages than 1300 Ma are probably due to modification ofthe U-Th-Pb system. The four ages between 2225 -2000 Ma may represent a inherited component (all four ages from monazite enclosed in garnet). CD ■ D O Q. C 8 Q.

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C/) C/) Table 2. Monazite Within MSWD Requirements Weighted Weighted Sample Analytical mean age mean error Pétrographie Lithology number points A* 5ax(25) MSWD location 8 politic gneiss IV63 mztl (2-10) 1703 43 0.21 garnet-core politic gneiss IV63 mzt2(1-12) 1710 21 0.57 garnet-core politic gneiss IV63 mzt3 (1-7) 1752 86 0.14 garnet-core | politic gneiss IV63 mzt5 (1-9) 1736 59 0.11 garnet-core politic gneiss IV63 mztSzl (1-4) 1522 31 2 m-encl. in qtz CD politic gneiss IV63 mzt8z2 (1-4) 1540 31 2.2 m-encl. in qtz politic gneiss IV63 mzt8z3 (1-4) 1593 32 1.3 m-encl. in qtz > 3. pelitic gneiss IV63 *mzt9z1 (3-6) 1326 21 0.14 m-encl. in qtz 4- bt I; 3" CD politic gneiss IV63 mzt9z2 (1-6) 1326 21 0.14 m-encl. in qtz + bt s pelitic gneiss IV63 mztIO (1-3) 2071 230 0.16 garnet-core CD i ■D granitic leucosome IV41D m ztlzl (1-6) 1559 25 2.1 m-encl. in qtz O Q. granitic leucosome IV41D mzt1z2 (1-7) 1526 23 0.35 m-encl. in qtz C a granitic leucosome IV41D mzt1z4 (1-5) 1476 26 0.38 m-encl. in qtz O pelitic gneiss IV41D mzt2z2 (1-5) 1703 47 0.75 garnet-rim 3 ■D pelitic gneiss IV41D mzt2z3 (1 and 3) 1707 94 3 garnet-rim O pelitic gneiss IV41 "m z tl (1-12) 1689 25 1.06 garnet-core pelitic gneiss IV41 mzt2z1 (1-12) 1694 20 0.66 garnet-core CD Q. pelitic gneiss IV41 mzt4z3 (1-6) 1578 29 2.2 encl. in sill + bt pelitic gneiss IV41 mzt5 (1-9) 1547 24 1.2 m-encl. in qtz granitic leucosome IV66A m ztlzl (1-14) 1475 16 0.99 m-encl. in plag granitic leucosome IV66A mzt1z2 (1-4) 1503 30 0.14 m-encl. in plag ■D IV66A CD granitic leucosome mzt1z3 (1-7) 1416 21 2.2 m-encl. in plag metaaplite IV76A m ztlzl (1-7) 1520 23 0.74 m-encl. in K-spar + qtz C/) C/) metaaplite IV76A mzt1z2 (1-7) 1492 23 1.2 m-encl. in K-spar + qtz augen gneiss IV50 mzt1z2 (1-2) 1502 24 0.24 m-encl. in K-spar + qtz augen gneiss IV50 mzt1z3 (1-5) 1431 23 1.19 m-encl. in K-spar + qtz * Data for mzt9 calculated using both z1 and z2 Spot ages of -1607 Ma, -1656 Ma, and -1657 Ma not used for calculating data.

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C/) C/) Table 3. Monazite Not Within MSWD Requirements Weighted Weighted Sample Analytical mean age mean error Pétrographie Lithology number points A, 5Ax(25) MSWD Range of ag es (Ma) location pelitic gneiss IV63 mztSzl (1-9) 1458 12 63 1710-1000 m-encl. in cordierite 8 pelitic gneiss IV63 mzt6z2 (1-4) 1379 30 88 1581-991 m-encl. in cordierite pelitic gneiss IV41D mztZzI (1-6) 1571 98 44 2204-1571 garnet-rim ci' pelitic gneiss IV41 mzt3 (1-9) 1386 20 5.6 1645-1303 encl. in small garnet; pelitic gneiss IV41 mz14z1 (1-9) 1583 21 6.4 1733-1453 encl. in sill + bt J granitic leucosome IV66A mzt1z4 (1-7) 1493 23 3.6 1555-1427 m-encl. in plag ! augen gneiss IV50 m ztlzl (1-5) 1618 29 3.8 1692-1538 m-encl. in K-spar + qtz

3 3" CD ■DCD O Q. C a 3O "O O

CD Q.

■D CD (/) C/)

4^ 00 CD ■ D O Q. C g Q.

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C/) C/)

CD 8

15.0 14.0 13.0 Average weighted mean age 12.0 / 2071 ± 230 Ma 3. MSWD = 0.16 3" CD _ 10.0

CD I 9,0 ■D O a 8.0 inherited monazite ? Q. C a £ 7.0 3O g 6.0 "O O 5.0 4.0 CD 3.0 1000 Ma Q. 2.0 j 500 Ma

■D 0.0 CD 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 C/) C/) U/Pb (ppm)

Figure 45. Th/Pb = f(U/Pb) isochron diagram for the gamet enclosed grain IV63mzt10. The average weighted mean age for the three spot analyses on this -5-1 Oum grain is 2071 ± 230 Ma. This grain may possibly represent inherited material shed off the margin of Laurentia during the Early Proterozoic. 1 5 0

2000 IV63mzt3

1900 --

1800 --

1700 --

1600 Mean = 1752 ± 86 Ma [2.4%] 95% conf. Wtd by data-pt arts only, Oof 7 rej. (a) MSWD = 0.14, probability = 0.99 1500 1

15.0 14.0 13.0 A verse weighted mean age 12.0 = 1752 ±86 Ma MSWD = 0.14 11.0 10.0 E 9.0 Metamorphism at -1752 Ma ? §[ 8.0 7.0 6.0 5.0 4.0 2000 Ma 3.0 1500 Ma 2.0

0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 U/Pb (ppm)

Figure 46. (a) Weighted mean age diagram and (b), Th/Pb = f(U/Pb) isochron diagram for the gamet enclosed grain IV63mzt3. Seven spot analyses within this ~10um grain enclosed in garnet gave an average weighted mean age of 1752 ± 86 Ma. The weighted mean age may represent the age of metamorphism in the eastern Ivanpah Mountains.

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r Mean = 1736 ±59 [1.7%] 95% conf. IV63mzt5 8 1860 - Wtd by data-pt errs only, 0 of 5 rej. MSWD= 0.11, probability = 0.77 n = 5 1820 î

CD 1780 -- 3. 3" CD 1740 -- ■DCD O Q. C 1700 » a 3O "O O 1660 -

CD Q. 1620 -•

1580 -- ■D CD

C/) 1540 C/)

Figure 47. Average weighted mean plot ofthe five spot ages determined from the garnet enclosed grain IV63mzt5. The weighted mean age for this monazite was determined to tre 1736 ± 59 Ma and may possibly represent the age of metamorphism in the eastern Ivanpah Mountains 152

Figure 48. Th compositional map of gamet enclosed grain IV63mzt1 showing the locations (red dots) and associated ages ofthe 10 EMP spot analyses carried out on this monazite grain. Ages are in Ma. Errors for each individual spot age are given in Appendix E.

1697 <^'^.1888 z2

'1713

1745 .

Figure 49. Th compositional map ofthe gamet enclosed grain IV63mzt2 showing the locations (red dots) and associated ages ofthe 12 EMP spot analyses carried out on this monazite grain. Ages are in Ma. Errors for each individual spot age are given in Appendix E.

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■D CD

C/) 3o" O

8 15.0 14.0 IV63mzt2 ci' 13.0 Average weighted mean age n= 12 12.0 1710 ±34 Ma

MSWD = 0.57 3 10.0 3" CD E 9.0 Metamorphism at -1710 Ma? ■DCD CL &0 O Q. C g 7.0 a O g 6.0 3 ■D 5.0 O 4.0 1500 Ma CD Q. 3.0 2.0 2000 Ma ■D CD 0.0

C/) 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 C/) U/Pb (ppm)

Figure 50. Th/Pb = f(U/Pb) isochron diagram for the garnet enclosed grain IV63mzt2. Twelve spot analyses from z1 and z2 within the grain give ages that define a regression line that is parallel to the theoretical isochrons. The weighted mean age of -1710 Ma may represent the age of metamorphism in the eastem Ivanaph Mountains.

U) CD ■ D O Q. C 8 Q.

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C/) C/)

2000

8 IV63mzt1, 2, 3, and 5 Mean = 1713 ± 34 [1%] 95% conf.) Wtd by data-pt errs only, 0 of 34 rej. ci' MSWD = 0,33, probability = 1.000 (error bars are 2a) 1900 --

3 3" CD 1800 ■DCD O Q. C a O 3 ■D 1700 O

CD Q.

1600 --

■D CD

C/) C/) 1500

Figure 51. Weighted mean plot for all ages and errors from the four monazites IV63mzt1, mzt2, mzt3, and mztS included in garnet. The average weighted mean age is determined to be 1713 ± 34 Ma. LA -pL CD ■ D O a . c g Q.

■D CD

WC/) 3o" O 3 CD 8 15.0 14.0 IV63mzt1, 2. 3, and 5 ci' O 13.0 n= 34 12.0 11.0 MSWD = 0.33 3.C 3" 10.0 CD 9.0 ■DCD O Q. i 8.0 Weighted mean age C a 1713 ±34 Ma O 7.0 3 ■D 6.0 O 5.0 CD Q. 4.0 3.0

■D 2.0 1500 Ma CD 2000 Ma C/) C/) 0.0 0.0 1.0 1.5 2.0 2.53.0 3.5 4.00.5 4.5 U/Pb

Figure 52. Th/Pb = f(U/Pb) isochron diagram for ail spot ages determined from the four monazite grains IV63mzt1, mzt2, mzt3, and mztS. LA The 34 spot ages and associated errors define an isochron that is sub-parallel to the two theoretical isochrons of 1500 Ma and 2000 Ma. LA Note: The -1713 Ma age is the weighted mean age and not an isochron age. 156

16 -- 15 ivesmzte 14 13 c n= 13 12 11 -- 10 o Metamorphism at I 8 / a. -1710 Ma then 7 6 /S. Pb-loss? 5 1710 ±34 Me 4 3 : error ellipse \ l 5 0 0 M a 2 2000 M ^ (b)o ' ■ ■ ' 1 ■ ' ' ■ 1 ■ ' " 1 ' ■ ■ ‘ 1 ■ ■ ' ' 1 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 U/Pb (ppm)

Figure 53. (a) Th-compositional map with corresponding spot ages from z1 and z2 ofthe cordierite enclosed matrix grain IV63mzt6. (b) Th/Pb = f(U/Pb) isochron diagram for the 13 spot ages from (a). The 1710 ± 34 Ma age from z1 corresponds with the mean ages determined from the garnet enclosed grains, but all other spot ages are significantly younger. Possibly this grain records both the age of metamorphism and subsequent Pb-loss.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 157

- - - ; - :

1 8 0 9

1584 1512

1476 1519

1 5 8 3

1 5 7 8 Z 2

1 4 9 3 1 5 1 3

10 um

Figure 54. Th-compositional map of the quartz enclosed matrix grain IV63mzt8. Four EMP spot analyses on each zone (z1, z2, and z3) were performed. The locations ofthe spot analyses are shown on the map with red dots with corresponding ages in Ma. The average weighted mean ages for z1, z2, and z3, were determined to be 1522 ±31 Ma, 1540 ± 31 Ma, and 1593 ± 32 Ma, respectively. Possibly this monazite records total re-crystallization between -1594 and 1523 Ma. Errors for individual spot ages are given in Appendix E.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 158

1324 1324 1353 1316

134& 1338 1520

10 um

Figure 55. Th-compositional map of the matrix grain IV63mzt9 that is partly enclosed in both quartz and biotite. Six EMP spot analyses from two zones (z1 and z2) were performed on this grain. The locations ofthe spot analyses are shown with the red dots with corresponding ages in Ma. The average weighted mean age of 10 of the spot ages was determined to be 1326 ± 27 Ma (the two oldest ages o f-1576 Ma and -1520 Ma were not used in the weighted mean age calculation). The consistency of the 10 ages used for the mean age calculation may suggest that this monazite was neocrystallized at -1350-1300 Ma.

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C/) 3o" O

8 15 14 ci' V\feighted mean age 13 1326 ±27 Ma n = 1 0 12 MSWD = 0.14 11

3"3 10 Neocrystallization between (D 9 1350-1300 Ma ? (D a. T3 Q. O 8 Q. C 7 a 3o 6 T3 O 5 4 (D Q. 3 1500 Ma 2 2000 Ma T3 1 (D 0 (/) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 U/Pb (ppm)

Figure 56. Th/Pb = f(U/Pb) isochron diagram for IV63mzt9. The average weighted mean age of 10 of the 12 spot analyses performed on this grain was determined to be 1326 ± 27 Ma. The 10 spot ages and associated errors define a regression line (bold line) that is parallel to the theoretical isochrons.

\o 1 6 0

1693 -1741 z2 1701 1700 1677

Figure 57. Th-compositional map of IV41mzt1 located within a gamet core. Twelve EMP spot analyses from two zones (z1 and z2) were carried out on this grain. The locations of the spot analyses are shown on the image with the red dots with corresponding spot ages in Ma. The average weighted mean age for 9 of 12 spot ages was determined to be 1689 ± 34 Ma. The spot ages of ~1657 Ma, -1656 Ma, and -1607 Ma were not used to calculate the mean age, mean error, or MSWD due to their close proximity (<1/2um) from the edge of the grain. Errors for individual spot ages are given in Appendix E.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 161

1702 -1718 'iT/ift 1743

Figure 58. Th-œmpositional map of IV41mzt2 located enclosed wittiin a gamet core. Twelve EMP spot analyses from two zones (z1 and z2) were canted out on this grain. The locations ofthe spot analyses are shown on the image with the red dots with comesponding ages in Ma. The average weighted mean age ofthe 12 spot ages from z1 and z2 was determined to be 1694 ± 34 Ma. Errors for individual spot ages are given in Appendix E.

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15.0 8 IV41mzt2 14.0 ci' Average weighted mean age 13.0 1694 ± 34 Ma n = 1 2 12.0 11.0 3 10.0 3" Metamorphism a t-1694 Ma ? CD 9.0 CD ■D I 8.0 O n Q. C % 7.0 a O 3 " 6.0 ■D O 5.0 4.0 CD Q. 3.0 2000 Ma 1500 Ma 2.0

■D CD 0.0 C/) C/) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 U/Pb (ppm)

Figure 59. Th/Pb = f(U/Pb) isochron diagram for IV41 mzt2. The twelve spot ages and associated errors have a weighted mean age of ~1694 Ma. The defined regression line is parallel to the theoretical isochrons of 2000 Ma and 1500 Ma. Possibly this monazite records a metamorphic event in the eastern Ivanpah Mountains at ~1694 Ma.

o\ N) 163

> 1330 1470 1303 1429 1304 1645. 1422 . 4 1448 / 1398

(a)

IV41mzt3

n = 1 0

Pb-ioss ? XII Û- 1645 ± 33 Ma ellipse 1500 Ma

2000 Ma

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 ______U/Pb (ppm)

Figure 60(a). Th-compositional map ofthe IV41 mzt3 enclosed within a small gamet. Nine EMP spot analyses from z1 were carried out on this monazite grain. The locations of the spot analyses are shown with the red dots with corresponding ages in Ma. (b) Th/Pb = f(U/Pb) isochron diagram for the monazite in (a). The isochron diagram demonstrates the possibility that extensive Pb-loss may be recorded within the grain.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 6 4

1569 0^4 j 1722

z " 7 . , r 1627 1621

(a)

IV41mzt4

zones n= 15

Metamorphism at ~1722 Ma then Pb-loss ? I one 1 n ) zone 1 CL

1722 ±34 Ma error ellipse

1453 ± 29 Ma error ellipse 500 Ma

..j —I.,-[...I—I..L. A , 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 U/Pb (ppm) (b) Figure 61 (a). Th-compositional map of the matrix grain IV41mzt4 which is situated in textural equilibrium with sillimanite and biotite. Three zones based on Th concentration are designated on the map. EMP spot analyses were carried out on z1 and z3. The locations of the spot analyses are shown with the red dots with corresponding ages in Ma. Errors for Individual spot ages are given in Appendix E. (b) Th/Pb = f(U/Pb) isochron diagram for the spot ages of the monazite grain shown in (a). Possibly this monazite records metamorphism in the eastern Ivanpah Mountains at ~1722 Ma, and subsequent Pb-loss.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 165

10 uin

Figure 62. Th-compositional map of the quartz enclosed matrix grain IV41mzt5. The location of the nine EMP spot analyses (red dots) with associated ages in Ma are shown on the map. The average weighted mean age determined for the nine spot ages is 1548 ± 31 Ma. Possibly this monazite grain records crystallization at -1548 Ma. Errors for individual spot ages are given in Appendix E.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 166

-’“M -fM eisV ''

20 um

Figure 63. Th-compositional map of the quartz enclosed matrix grain IV41Dmzt1. The location of the eighteen EMP spot analyses within three different zones (z1, z2, and z4) are shown on the Th map with the red dots along with the associated spot ages in Ma. The average weighted mean age of the three zones were determined to be 1559 ± 32 Ma (z1), 1526 ± 31 Ma (z2), and 1476 ± 30 Ma (z4). Perhaps this grain records the timing of crystallization of the grain during partial melting in the eastern Ivanpah Mountains. No analyses were carried out on z3. The pétrographie image of this monazite crystal is shown in Figure 31(b). Errors for individual spot ages are given in Appendix E.

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1659 1745 z2 31707

1844

2É04 1642 1743 1894

1674 1672

Figure 64. Th-compositional map of the grain IV41 Dmzt2. This grain is located within the outer rim of the large garnet IV41 D gtl. The location of the fourteen EMP spot analyses that were carried out on the three concentric zones (z1, z2, and z3) are shown on the map (red dots) with corresponding ages in Ma. Given the great variability of determined spot ages within this grain from -2204 Ma to -1583 Ma, perhaps this grain represents modification of the U-Th-Pb system in an inherited monazite, or possibly it may represent an inherited core and subsequent overgrowth of new monazite. Errors for individual spot ages are given in Appendix E.

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8 15

(O' 14 13 n = 14 12 1571 ±31 Ma MSWD = 44 11 3"3. CD 10

CD 9 T3 Inherited monazite + Pb-loss? O A Q. a 8 C 1894 a 7 ± 126 Ma 3o T3 6 O 5 2204 ± 45

CD error ellipse Q. 4 3 2 T3 CD 1500 Ma 2000 Ma (/) (/) 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 U/Pb

Figure 65. Th/Pb = f(U/Pb) isochron diagram for the 14 spot ages from the monazite grain in Figure 64 (IV41 Dmzt2). Possibly this monazite On represents an ~2204 Ma inherited component followed by Pb-loss during high-T metamorphism. 00 169

1503

1S57

50 um

Figure 6 6 . Th-compositional map of the matrix grain IV76Amzt1 partly enclosed in both K-spar and quartz. The location of the 14 EMP spot analyses (red dots), within the two designated zones (z1 and z2), and the associated ages in Ma are shown on the image. The average weighted mean age of the two zones were determined to be 1520 ± 30 Ma (z1 ) and 1492 ± 30 Ma (z2). This monazite possibly records the crystallization age of the grain during felsic magmatism in the eastern Ivanpah Mountains during the Early Proterozoic. Errors for individual spot ages are given in Appendix E.

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C/) C/)

IV76m2l1 8 ne 2 = 1492 ± 29 Ma (MSWD = 1,2)(n = 7) ci' zone 1 = 1520 ± 30 Ma (MSWD = 0.74)(n = 7)

Intrusion and ciystaiiization of 3 3" CD E granitic rocks between Ql 3 ■DCD -1520-1492 Ma? O Q. C a I O h- 3 ■D O

CD Q. 2000 Ma 1500 Ma ■D CD » 1 » I I » lit I I >1 I I I I I I ill ill I I t>| I it I I * I I t> I I II

C/) C/) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 U/Pb (ppm)

Figure 67. Th/Pb = f(U/PB) isochron diagram for the monazite grain (IV76Amzt1) shown in Figure 65. No isochron was calculated for eitherzl or z2. The associated error ellipses for the 14 spot ages determined for this grain, as well as the average weighted mean ages of z1 and z2, are shown on the diagram. The data is tightly constrained, suggesting that no modification of the U-Th-Pb system has taken place. o 171

’ 1491 V ' 1517 1507 1391 1499 C1462 r:iJ67 1516B 1489 1449 1508 : . 1514 5 1555 1409 '-'1523

1470Cl4iO

1418 i 4 gg -1506 .-1496 <- 1501 1460

50 um

Figure 6 8 . Th-compositional map for the plagioclase enclosed matrix grain IV66Amzt1. The location of the 28 EMP spot ana^ses carried out within four zones (z1, z2, z3, and z4) are shown on the image with the corresponding spot ages in Ma. The average weighted mean ages for the four zones were determined to tie 1470 ± 29 Ma (z1), 1503 ± 30 Ma (z2), 1416 ±28 Ma (z3), and 1493 ± 30 Ma (z4). Possit)ly these average mean ages constrain the occurrence of magmatism and felsic intrusion in the eastern Ivanpah Mountains between -1503 Ma and -1416 Ma. Errors for individual spot ages are given in Appendix E.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 172

1668) 1623 1 6 3 4 - 1 6 9 2 1595^^ >,45i

1450

50 um

Figure 69. Th-compositional map for the matrix grain IVSOmztl partly enclosed in both K-spar and quartz. The location of the 13 EMP spot analyses carried out within three zones (z1, z2, and z3) are shown on the image with corresponding spot ages in Ma. Possibly z1 records metamorphism at -1692 Ma and subsequent Pt)-loss, evidenced by the four substantially younger ages. Zones 2 and 3 may record neocrystallization between 1502 ± 45 Ma (weighted mean age of z2) and 1431 ± 29 Ma (weighted mean age of z3). Errors for individual spot ages are given in Appendix E.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 173

1500 Neocrystallization at -1431 Ma ? (zone 3]

1480

MSWD = 1.2 1460 -

1440 --

1420 -- Mean = 1431 ± 29 [1.4%] 95% conf, Wtd by data-pt errs only, 0 of 5 rej. 1400 -- MSWD = 1.2, probability = 0.30

1380

Figure 70. \Afeighted mean plot for the five spot ages from z3 located within the monazite grain

shown in Figure 6 8 . The weighted mean age of 1431 ± 29 Ma from z3 may give the approximate age of neocrystallization due to magmatism in the eastern Ivanpah Mountains.

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"O CD

C/) C/)

zone 3 = average weighted mean age 8 1431 ±29Ma(n = 5) IVSOmztl ci' MSWD =1.2 zones 1, 2, and 3

n = 13

3 3" CD Metamorphism at -1692 Ma,

CD then Pb-loss, followed by ■D O neocrystallization at -1431 Ma ? Q. C a 1692 ±34 Ma O3 "O error ellipse O

CD Q. 2000 Ma 1500 Ma ■D CD

C/) C/)

U/Pb (ppm)

Figure 71. Th/Pb = f(U/Pb) isochron diagram for the grain IVSOmztl. The error ellipses for the 13 determined spot ages within the three zones are shown on the diagram. The blue ellipse is designated on the diagram to emphasize the possibility that this spot age represents the age of metamorphism at -1692 Ma.

4^ CD ■D O Q. C 8 Q.

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C/) C/) Appendix A. Garnet Chemical analyses oxides cation proportions Analytical Total Pétrographie location

points SiOg FeO AI2 O3 CaO MgO MnO wt.% Si Fe Al Ca Mg Mn Mg/Fe of data points 8 IV63gt1-1 37.79 32.87 21.93 1.07 5.86 0.55 100.07 2.979 2.167 2.037 0.09 0.689 0.037 0.318 gt-1 to gt-7 from IV63gt1-2 37.83 33.29 21.65 1.07 6.43 0.51 100.78 2.968 2.184 2 . 0 0 2 0.09 0.752 0.034 0.344 gt-c to gt-r towards

IV63gt1-3 37.87 33.3 21.94 1.08 6.46 0.57 1 0 1 . 2 2 2.958 2.175 2.019 0.09 0.752 0.038 0.346 bt-r. IV63gt1-4 38.04 33.58 21.91 1.05 6.3 0.53 101.41 2.967 2.19 2.014 0.088 0.732 0.035 0.334

IV63gt1-5 37.75 33.86 21.31 1.04 5.62 0 . 6 100.18 2.989 2.242 1.989 0.088 0.663 0.04 0.296 CD IV63gt1-6 37.36 35.1 21.73 0.99 5.02 0.59 100.79 2.958 2.324 2.028 0.084 0.592 0.04 0.255 IV63gt1-7 37.45 34.77 21.74 1 . 1 1 4.65 0.64 100.36 2.974 2.309 2.035 0.094 0.55 0.043 0.238 3. IV63gt1-8 37.66 33.2 21.98 1 . 0 2 6.49 0.59 100.94 2.95 2.174 2.029 0.086 0.758 0.039 0.348 gt - 8 to gt - 1 2 from 3" CD IV63gt1-9 37.81 32.98 21.9 0.85 6.25 0.57 100.36 2.972 2.167 2.028 0.072 0.732 0.038 0.338 gt-c to gt-r towarcfe

IV63gt1-10 37.72 33.24 1.07 6.44 0.62 1 0 0 . 8 8 2.957 CD 21.79 2.179 2.013 0.09 0.752 0.041 0.345 bt-r. ■D IV63gt1-11 37.8 33.48 1.07 O 21.9 6.13 0.59 100.97 2.962 2.194 2.023 0.09 0.716 0.039 0.326 !' Q. IV63gt1-12 37.17 36.78 21.48 1.05 3.44 0.82 100.74 2.973 2.46 2.025 0.09 0.41 0.056 0.167 C a IV41gt1-1 37.19 33.49 21.67 1.19 5.34 0.95 99.83 2.96 2.229 2.032 0 . 1 0 1 0.633 0.064 0.284 gt1-1 to gt1-5 from O 3 IV41gt1-2 37.66 33.49 21.77 1.16 5.37 1 . 0 1 100.46 2.975 2 . 2 1 2 2.026 0.098 0.632 0.068 0.286 gt-c to gt-r towards ■D IV41gt1-3 37.43 33.98 1.16 1.04 2.962 O 21.93 5 100.54 2.249 2.046 0.098 0.59 0.07 0.262 bt-r. IV41gt1-4 37.21 35.03 2 1 . 6 8 1.17 4.41 1.07 100.57 2.961 2.331 2.033 0 . 1 0.523 0.072 0.224 IV41qt1-5 36.93 35.77 21.28 1.15 3.52 1.41 100.06 2.972 2.407 2.018 0.099 0.422 0.096 0.175 CD Q. IV41gt1-6 37.68 33.77 21.73 1 . 2 2 5.16 1.19 100.75 2.973 2.228 2 . 0 2 0.103 0.607 0.08 0.272 gtl - 6 to g tl - 8 from

IV41gt1-7 37.56 33.82 21.76 1.19 5.19 1.13 100.65 2.967 2.234 2.026 0 . 1 0 1 0.611 0.076 0.274 gt-c to gt-r towards IV41gt1-8 37.08 33.44 21.52 1.09 5.17 1.14 99.44 2.966 2.236 2.028 0.093 0.616 0.077 0.276 qtz-r. ■D IV41gt1-9 37.65 33.75 21.77 1.13 5.41 1.15 1 0 0 . 8 6 2.967 2.224 2 . 0 2 2 0.095 0.636 0.077 0.286 g tl-9 to g tl-12 from CD IV41gt1-10 37.46 33.58 21.71 1.19 5.1 1.18 1 0 0 . 2 2 2.972 2.228 2.03 0 . 1 0 1 0.603 0.079 0.271 gt-c to gt-r towards

IV41gt1-11 36.95 33.85 2 1 . 6 6 1.17 0 . 1 C/) 5.05 1.15 99.83 2.95 2.26 2.038 0.601 0.078 0.266 bt-r. C/) IV41gt1-12 37.69 34.46 21.63 1.15 4.47 1.19 100.59 2.988 2.284 2 . 0 2 1 0.098 0.528 0.08 0.231

IV41Dgt2-1 37.46 33.53 21.65 1 . 2 2 5.46 0.91 100.23 2.968 2 . 2 2 1 2 . 0 2 1 0.104 0.645 0.061 0.29 gt 2 - 1 to gt 2 - 8 from

IV41Dgt2-2 37.4 33.69 21.78 1 . 1 1 5.45 0.98 100.41 2.96 2.23 2.031 0.094 0.643 0.066 0.288 gt-c to gt-r towards

IV41Dgt2-3 37.57 34.28 21.75 1.06 5.5 0.96 1 0 1 . 1 2 2.958 2.257 2.018 0.089 0.645 0.064 0.286 bt-r.

IV41Dgt2-4 37.2 34.13 21.3 1 . 1 2 5.08 0.94 99.77 2.972 2.28 2.005 0.096 0.605 0.064 0.265

IV41Dgt2-5 37.1 35.06 21.15 1 . 1 2 4.45 1 . 0 1 99.89 2.975 2.351 1.999 0.096 0.532 0.069 0.226

IV41Dgt2-6 37.25 35.49 21.61 1.18 4.2 1.06 100.79 2.964 2.361 2.026 0 . 1 0 1 0.498 0.071 0 . 2 1 1 LA 1 7 6

c o E 2 - 2 2 L. 2 Ü c CM 2 o L N o Q. I! 05 d 2 ÈX jÿ _ 2 Q. (0 O) c r co O 05 - iî05 c r fil iO 05 . c r o 2 ■D 2 o 2 E § 2 O O) *5 00 S’ ü 2 CM 9 o CO ü C35 2 Y 05 %05 (35 à05 C35 (05 D) O) CL

C35 ■M" CO t o 00 (35 to 0 0 CO 00 Tf CO o 0 0 N 0 0 CM

(35 00 CO r- CM co N. tn (35 T“ COin COh- N 0 0 CM i n CM N N N CO CO CO N co co CO tn CD(O COCO co CO CO CO CO N CO §§i 8 I q o O O o o o O O O o o o o o o o o o o o OO d d d d d d O d d d d d d d d d d d CDd d d d d d d d d d 00 N N CM .p . tn ■M- 00 00 CO CO (35 CO 00 Tf O) N Tl" CM CM CO 00 CO to Tf Tf tn LO 2 CO CM i 1q si i 2 ■M- ■M- ■M- CO CO K 5 t n m I 2 §§q 2 tn CD t n t n CO d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d

00 00 CO to to (35 N N ■M’ t n t o N N TT ■M- CO 0 0 ID CO 1^ M (O CM N .r_ 05 o (35 05 C35 o (35 00 00 (35 (35 05 05 CJ5 O (35 o o a> 05 (35(35(35 o o o o o O o O o O o d o d O o o o o T- o Ô —— o o o o O d d d d d d d d d d d d d d d d d d d d d d d d d d d to 00 Tl" CO ■M" CO CO T f t n N CO CM CM CM CO CO 2 2 s S S 8 8 s co s q o o 08 s o O o o o o 8 I O sI O o §s s c c\i CM CM CM CM CM CM CM CM CM o Csj CM CM CM csi CM CM CM CM CM CM CM c\i CM CMCMCM c\i CM c\i tr 00 O) CO CO N Tf (3 5 C35 CO t o (35 Tf Tf 00 ir» in (D CMCMCO TT (35 CMN tJ- T- 05 'T ID o o Tj- Tf Tj- CO O CM CO t o CO o <- tn O (35 O CM lO CO o Q. CM CM CM CM CO T f CM CM CO o -a- CM COCMCM(M CMCMCMCMCM CM CO CM c\i c\i CM CM CM c\i CM CM CM CM CM CM CM c\i CMCMCMCMCMCMCMc\i CM CM CM cvi c O) co ■M- CO CM T f Tf CM 00 00 CNJ CO CMCO CM .r- S (35 N O) o 8 s 00 N- N. CO r - 05 t o N M CD CD O N OO(O CO CD h- CO CD co 2g 05 i 205 (35 (35 (35 (35 (35 G) 05 o ( 3 5 (35(35(35 (3 5 C D (35 (35 (35 G) CÜ CM CM 8 CM CM cvi c\i CM CM CM CM CM CM CM CM CM c\i CMCO CMCMCMCMCM cvi CM CM CM cvi ID CD 05 CM 05 T- ■M- co ID CO CO CD 0 0 CD CM CO T t T f 2 8 tO tO M tO CO CO < i n CO CO CM (35 q CM q o O o d O O d d d § CD d d d d §§o O o o o o o o o OO o o o o o CD o T- s i i — —■— T- ■— N G) O i O T j- G) CD CM 05 N — N 05 CO CO CM 0 0 o> G)G) O C35 S O CJ5 00 G) CD ■M- (35 o N N o c S O T - G ) (35 (35 CD q G)G) q q o O O — d d O CD d d — d d ■ -

Tf (35 CM r - CM G ) Tf N O 1 - CM T l- CD t o 1- Tf N co CM Tf Tf Tf co 05 CO T l- CM N to CO CO 05 tO tO C5 CD Tt CD in o o o IDO a y CO O) 00 Tt CO o o o CM CO CO CO co cô ID IDID co cô t o lO Tf T|- Tf Tj- ID ■M- ■M- ID in in tri IDID Tf Tt in

N t o 00 ID CO CM to lO CM CD co 00 CM CO CO tO CM ID CO CM T“ T— O — 2 2 in 8 o t- CM CM CM CM T-T- T- § 1

to to N CM CD CO 0 5 N CM ID CM CO 00 CO CD o i Tf ID(DN N 'M; Tf CO N (O C35 (35 q q O CO _£S CM cvi < ^ K l CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM 2

0 0 co CO CM CM Tl- T- r*. (D CO CO CO CD CM CM CO Tf IDID (35 CM 1- o 00 O ) a> % 0 )O 8 2 05 0 > CD o c o CM CM CM CM tn tn ?— CM iO (35 CD § CO CO CO CD tO tn uo m m CO CO CO CO Tf CO Tf CO 2 2 CO CO CO CO CO 2 2 co co co co CO CO CO CO t CO CO CO CO CO

CM -1- CD CD Tf lO 05 T- C55 Tf TC N to co 00 00 Tj- CO CD O) G) ID CM 1^ ID g O CM CO CO Th CM — 00 M tO tO Tf 05 CO CO ID N in CD CD CD 0 0 T |- 0 0 N N N N N M K CC5 i< (sZ (< (ô (< [< NN N N N NN co CO CO CO CO CO CO CO CO CO co co co co co co co CO CO CO c*5 CO CO CO CO CO CO

o CM co -M- LO co G) co N 00 O — CM s CM co T|- t o (O N 00 0 5 — T— 1— T- CM CO Tf LO (O N 00 05 é ^ 94 9 4 9 ^ CM CM CM 05 05^ o CD C7> 05 05 (35 CM CM CM ^ CM ^ CM CM CM CM CM ÇÔ CO CO CO CO 2 2 2 2 î t Q Q QÛÛ Q Q Q C35 CD 05 05 C35 (35 (35 C35 (35 0 5 C35 C35 C35 0 5 (35 C35 (35 05 (35 05 05 0 5 0 5 (35 0 5 CD 0 5 05 0 5 0 5 05 0 5 (35 05 05 05 05 05 (35 05 (35 (35 < T t CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM CM §5> > > > > > > > > > > > > > > > > > > > > > > > > > >

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CD ■D O Q. C 8 Q.

■D CD

C/) C/) oxides cation proportions Analytical Total Pétrographie location points SiOg FeO AlgOg CaO MgO MnO wt.% Si Fe Al Ca Mg Mn Mg/Fe of data points

IV29gt3-10 37.84 33.34 2 2 . 1 2 1.198 5.352 0.998 1 0 0 . 8 6 2.971 2.189 2.047 0 . 1 0 1 0.626 0.066 0.286 8 IV29gt3-11 37.78 33.55 21.93 1.252 5.488 0.97 100.96 2.969 2.204 2.03 0.105 0.643 0.065 0.292 IV29gt4-1 37.73 32.51 2 1 . 8 1.169 4.939 0.922 99.067 3.007 2.166 2.048 0 . 1 0.587 0.062 0.271 gt4-1 to gt4-6 from (O' IV29gt4-2 37.83 33.66 22.04 1.158 5.387 1 . 0 1 1 101.09 2.969 2.209 2.039 0.097 0.63 0.067 0.285 gt-c to gt-r towards

IV29gt4-3 37.94 33.89 21.99 1.184 5.071 1.009 101.09 2.98 2.226 2.036 0 . 1 0.594 0.067 0.267 bt-r.

IV29gt4-4 37.48 34.99 22.04 1 . 1 0 1 4.424 1.166 1 0 1 . 2 2.959 2.31 2.051 0.093 0.521 0.078 0.225

IV29gt4-5 37.55 35.3 21.9 1.175 4.23 1 . 1 2 2 101.28 2.967 2.332 2.039 0.099 0.498 0.075 0.214

IV29gt4-6 37.57 35.41 21.67 1.259 3.786 1 . 1 2 2 100.82 2.984 2.352 2.029 0.107 0.448 0.075 0.191 3. IV29gt4-7 37.65 33.31 2 2 1.125 5.374 0.927 100.38 2.971 2.198 2.046 0.095 0.632 0.062 0.288 gt4-7 to gt4-9 from 3" CD IV29gt4-8 37.83 33.75 22.06 1.042 5.513 0.965 101.16 2.966 2.213 2.038 0.088 0.644 0.064 0.291 gt-c to gt-r towarcà

CD IV29gt4-9 37.56 33.4 22.14 1.176 5.328 0.941 100.54 2.96 2 . 2 0 2 2.057 0.099 0.626 0.063 0.284 qtz-r. \ ■D O IV29gt5-1 37.47 35 21.72 1.23 4.36 0.03 99.81 2.954 2.307 2.018 0.104 0.512 0 . 0 0 2 0 . 2 2 2 gt5-1 to gt5-10 T Q. C IV29gt5-2 37.56 35.08 21.72 1 . 2 1 4.38 1.13 101.08 2.972 2.321 2.026 0.103 0.517 0.076 0.223 gt-m enclosed in a IV29gt5-3 36.89 34.89 2 1 . 6 6 1.15 4.39 1 . 1 100.08 2.952 2.334 2.042 0.524 O 0.099 0.075 0.224 plagioclase. 3 IV29gt5-4 37.31 34.93 21.96 1.16 4.48 1.09 100.93 2.955 2.313 2.049 0.098 0.529 0.073 0.229 "O O IV29gt5-5 37.21 35.25 21.54 1 . 2 2 4.24 1 . 1 2 100.58 2.965 2.349 2.023 0.104 0.504 0.076 0.214 IV29gt5-6 37.15 35.04 21.91 1.19 4.32 1.07 1 0 0 . 6 8 2.952 2.328 2.052 0 . 1 0 1 0.512 0.072 0 . 2 2

IV29gt5-7 2 1 . 8 8 1 . 2 2 CD 37.65 34.9 4.27 1.13 101.05 2.976 2.307 2.038 0.103 0.503 0.076 0.218 Q. IV29gt5-8 37.5 35.26 21.56 1.26 4.09 1.13 1 0 0 . 8 2.979 2.343 2.019 0.107 0.484 0.076 0.207

IV29gt5-9 37.55 34.84 21.78 1.31 4.39 1.08 100.95 2.972 2.306 2.031 0 . 1 1 1 0.518 0.072 0.225

IV29gt5-10 37.53 34.85 21.87 1 . 2 2 4.27 1.09 100.83 2.973 2.308 2.042 0.104 0.504 0.073 0.218 ■D IV87gt6-1 37.82 34.38 2 1 . 6 6 0 . 8 6 5.43 0 . 8 6 1 0 1 . 0 1 2.977 2.263 2.009 0.073 0.637 0.057 0.282 gt6-1 to gt6-7 from CD IV87gt6-2 37.47 34.4 21.82 0.87 5.49 0.87 100.92 2.955 2.269 2.028 0.074 0.645 0.058 0.284 gt-c to gt-r towards

C/) IV87gt6-3 37.66 33.98 21.69 0.84 5.58 0.85 1 0 0 . 6 2.973 2.243 2.018 0.071 0.656 0.057 0.293 bt-r. C/) IV87gt6-4 37.72 34.53 21.48 0.92 5.35 0.9 100.9 2.978 2.279 1.998 0.078 0.629 0.06 0.276 IV87gt6-5 37.47 35.46 21.76 0.93 4.7 0.85 101.17 2.962 2.344 2.027 0.079 0.554 0.057 0.236 IV87gt6-6 37.51 35.83 21.71 0.9 4.31 0.87 101.13 2.971 2.373 2.026 0.076 0.509 0.058 0.214 IV87gt6-7 37.58 36.38 21.58 0.95 4.05 0.87 101.41 2.976 2.409 2.014 0.081 0.478 0.058 0.198

IV87gte-8 37.83 33.76 21.75 0.77 5.59 0.82 100.52 2.983 2.226 2 . 0 2 1 0.065 0.657 0.055 0.295 gt 6 - 8 to gt6-13 from

IV87gt6-9 37.58 34.19 21.78 0.87 5.53 0.85 1 0 0 . 8 2.963 2.254 2.024 0.073 0.65 0.057 0.288 gt-c to gt-r towards IV87gt6-10 37.4 35.4 21.79 0.9 4.97 0.91 101.37 2.951 2.336 2.026 0.076 0.585 0.061 0.25 bt-r. CD ■D O Q. C S Q.

■D CD

C/) C/) oxides cation proportions Analytical Total Pétrographie location points SiOg FeO AlgOg CaO MgO MnO wt.% Si Fe Al Ca Mg Mn Mg/Fe of data points

IV87gt6-11 37.5 35.82 2 1 . 6 6 0 . 8 8 4.5 0.93 101.29 2.966 2.369 2.019 0.075 0.53 0.062 0.224 8 IV87gt6-12 37.5 35.95 21.62 0.83 4.32 0.95 101.17 2.971 2.382 2.019 0.07 0.51 0.064 0.214 IV87gt6-13 37.03 36.43 21.51 0.91 3.62 0.97 100.47 2.966 2.44 2.03 0.078 0.432 0.066 0.177 (O' IV87gt6-14 37.72 34.01 21.47 0.89 5.63 0.82 100.54 2.979 2.246 1.998 0.075 0.663 0.055 0.295 gt6-14 to 6-17 from IV87gt6-15 37.85 33.88 21.85 0.82 5.66 0.84 100.9 2.974 2.226 2.023 0.069 0.663 0.056 0.298 gt-c to gt-r towards

IV87gt6-16 37.49 34.39 21.78 0.91 5.63 0.82 1 0 1 . 0 2 2.953 2.265 2 . 0 2 2 0.077 0.661 0.055 0.292 qtz-r.

IV87gt6-17 37.63 34.01 21.74 0.91 5.51 0.85 100.65 2.97 2.244 2 . 0 2 2 0.077 0.648 0.057 0.289 IV87gt6-18 37.58 35.6 2 1 . 8 8 0.917 4.639 0.913 101.52 2.961 2.346 2.032 0.077 0.545 0.061 0.232 gt6-18 + gt6-19 3. IV87gt6-19 37.6 34.99 21.48 0.89 4.485 0 . 8 8 6 100.33 2.989 2.327 2.013 0.076 0.532 0.06 0.228 on gt-r near bt-r.; 3" CD IV87gt6-20 37.74 34.2 21.94 0.918 5.551 0.847 101.19 2.963 2.245 2.03 0.077 0.65 0.056 0.289 gt6-20 to 6-25 from

IV87gt6-21 37.56 34.23 21.63 0.894 5.708 0.871 1 0 0 . 8 8 2.961 2.257 2 . 0 1 0.076 0.671 0.058 0.297 gt-c to gt-r towardà ■DCD O IV87gt6-22 37.38 33.84 2 1 . 8 0.881 5.567 0.814 100.27 2.959 2.24 2.034 0.075 0.657 0.055 0.293 qtz-r. T Q. C IV87gt6-23 37.44 34.43 21.76 0 . 8 6 8 5.257 0.895 100.65 2.961 2.278 2.029 0.074 0.62 0.06 0.272 a IV87gt6-24 O 37.42 34.21 21.77 0.904 5.425 0.914 100.65 2.959 2.262 2.028 0.077 0.639 0.061 0.283 3 IV87gt6-25 37.4 33.93 21.73 0.91 5.472 0.891 100.33 2.963 2.247 2.029 0.077 0.646 0.06 0.287 "O O IV49gt1-1 38.08 32.73 21.93 0 . 8 8 6.19 0.73 100.54 2.985 2.145 2.026 0.074 0.723 0.048 0.337 gt 1 - 1 to g tl - 6 from IV49gt1-2 37.88 33.08 21.96 0.87 6.38 0 . 8 6 101.03 2.962 2.163 2.024 0.073 0.744 0.057 0.344 gt-c to gt-r towards

CD IV49gt1-3 37.88 33 21.96 0.839 6.08 0.851 100.61 2.972 2.165 2.03 0.071 0.711 0.057 0.328 bt-r. Q. IV49gt1-4 37.98 34.23 2 2 . 0 2 0.881 5.381 0.883 101.38 2.974 2.241 2.032 0.074 0.628 0.059 0.28

IV49gt1-5 37.6 35.06 2 1 . 8 6 0.863 5.063 0.934 101.39 2.96 2.308 2.028 0.073 0.594 0.062 0.257

IV49gt1-6 37.58 35.25 21.94 0.858 4.564 1.023 1 0 1 . 2 2 2.965 2.326 2.041 0.073 0.537 0.068 0.231 ■D IV49gt1-7 37.93 33.27 21.99 0.91 6.24 0.83 101.17 2.965 2.174 2.026 0.076 0.727 0.055 0.334 g tl-7 to g tl-9 from CD IV49gt1-8 37.95 32.86 2 2 . 1 2 0.823 6 . 2 0 2 0.902 100.84 2.968 2.149 2.039 0.069 0.723 0.06 0.336 gt-c to gt-r towards

C/) IV49gt1-9 38.07 32.95 2 2 . 1 0.903 6.157 0.903 101.08 2.973 2.152 2.034 0.076 0.717 0.06 0.333 qtz-r. C/) IV49gt1-10 38.19 32.92 22.29 0.871 5.848 0.87 1 0 1 2.981 2.149 2.051 0.073 0 . 6 8 0.058 0.317 g tl - 1 0 to g tl-15 IV49gt1-11 37.81 33.97 22.14 0 . 8 8 5.86 0.89 101.55 2.954 2.219 2.038 0.074 0.682 0.059 0.307 gt-m enclosed in IV49gt1-12 37.84 33.74 21.96 0.87 5.89 0.95 101.25 2.963 2.209 2.026 0.073 0.687 0.063 0.311 qtz and K-spar.

IV49gt1-13 37.91 33.79 21.89 0 . 8 8 6 0.89 101.36 2.965 2 . 2 1 2.018 0.074 0.699 0.059 0.317

IV49gt1-14 37.59 33.66 22.15 0.89 5.82 0 . 8 6 100.97 2.951 2 . 2 1 2.049 0.075 0.681 0.057 0.308 IV49gt1-15 37.87 34.26 22.07 0.92 5.47 0.95 101.54 3.13 2.368 2.15 0.081 0.674 0.066 0.285 IV47gt3-1 37.44 32.68 21.98 0.92 6.34 0.91 100.27 2.95 2.153 2.041 0.078 0.745 0.061 0.346 gt3-1 to gt3-7 from OO 179

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■DCO O Q. C S Q.

■D CO C/) C/) oxides cation proportions Analytical Total Pétrographie location points SiOg FeO AlgOg CaO MgO MnO wt.% Si Fe Al Ca Mg Mn Mg/Fe of data points

IV66Agt1-6 36.73 35.91 21.55 1.161 3.597 1.042 99.996 2.955 2.416 2.044 0 . 1 0.431 0.071 0.179 8 IV66Agt1-7 36.96 34.09 21.96 1.179 5.202 0.862 100.25 2.937 2.265 2.056 0 . 1 0.616 0.058 0.272 g tl-7 to g tl-12 from IV66Agt1-8 37.29 34.44 21.97 1.13 4.855 0.9 100.59 2.955 2.282 2.052 0.096 0.573 0.06 0.251 gt-c to gt-r towards

(O' IV66Agt1-9 37.16 34.91 21.83 1.167 4.106 0.988 100.16 2.966 2.33 2.054 0 . 1 0.489 0.067 0 . 2 1 bt-r.

IV66Agt1-10 37.06 35.37 21.77 1.198 4.092 1.027 100.52 2.955 2.359 2.046 0 . 1 0 2 0.486 0.069 0.206 i IV66Agt1-11 36.9 35.51 21.76 1.233 3.843 1.009 100.25 2.955 2.378 2.053 0.106 0.459 0.068 0.193 IV66Agt1-12 36.83 35.42 21.63 1.197 3.633 1.061 99.767 2.962 2.382 2.05 0.103 0.436 0.072 0.183 IV66Agt1-13 37.29 34.08 21.76 1.157 5.293 0.883 100.46 2.954 2.258 2.032 0.098 0.625 0.059 0.277 g tl-13 to g tl-16 from 3. IV66Agt1-14 37.13 33.99 21.89 1 . 1 2 2 4.893 0.923 99.947 2.957 2.264 2.054 0.096 0.581 0.062 0.257 gt-c to gt-r towards 3" CO IV66Agt1-15 37.24 34.27 21.84 1.063 4.842 0.892 100.15 2.962 2.279 2.047 0.091 0.574 0.06 0.252 qtz-r. T IV66Agt1-16 36.91 34.46 21.79 1.143 4.975 0.916 100.19 2.941 2.296 2.046 0.098 0.591 0.062 0.257 CD i ■D 36.94 35.17 3.34 1.24 0 . 2 2 2 O IV11gt5-1 21.13 2.58 100.4 2.966 2.361 1.999 0.4 0.084 0.169 gt5-1 to gt5-7 froriH Q. IV11gt5-2 36.94 34.92 21.13 2.5 3.27 1.25 1 0 0 . 0 1 2.971 2.348 2.003 0.215 0.392 0.085 0.167 gt-c to gt-r towards C a IV11gt5-3 36.91 35.22 2 1 . 0 1 2.71 3.04 0 . 8 8 99.77 2.979 2.377 1.998 0.234 0.366 0.06 0.154 bt-r. O 3 IV11gt5-4 36.65 35.63 21.25 2 . 6 2.85 0.93 99.91 2.961 2.407 2.023 0.225 0.343 0.064 0.143

"O IV11gt5-5 36.78 35.97 2 1 . 2 1 2 . 6 6 0 . 1 2 1 O 2.45 1.25 100.32 2368 2.427 2.017 0.23 0.295 0.085 IV11gt5-6 36.3 36.61 20.97 2.57 2.27 1.23 99.95 2.953 2.491 2 . 0 1 1 0.224 0.275 0.085 0 . 1 1 1 0 IV11gt5-7 36.51 36.7 20.7 2.59 2 1.19 99.69 2.979 2.504 1.99 0.226 0.243 0.082 0.097 Q. IV11gt5-8 36.82 35.06 21.3 2.49 3.19 1 . 2 100.06 2.964 2.36 2 . 0 2 1 0.215 0.383 0.082 0.162 gt5-8 to gt 5-14 IV11gt5-9 37.23 35.21 21.3 2.5 3.17 1.26 100.67 2.978 2.355 2.008 0.214 0.378 0.085 0.16 from gt-c to gt-r

IV11gt5-10 37.12 35.62 2 1 . 2 1 2 . 6 6 2.84 1 . 2 2 100.67 2.977 2.388 2.004 0.229 0.339 0.083 0.142 towards bt-r.

IV11gt5-11 36.74 35.64 21.14 2.78 2.63 1.27 1 0 0 . 2 2.966 2.406 2 . 0 1 1 0.24 0.316 0.087 0.132 ■D0 IV11gt5-12 37.07 35.66 2 1 . 2 1 2.78 2.56 1.27 100.55 2.978 2.396 2.008 0.239 0.307 0.086 0.128

2 1 . 1 2 . 8 C/) IV11gt5-13 37.08 35.48 2.55 1.18 100.19 2.987 2.39 2.003 0.242 0.306 0.081 0.128 C/) IV11gt5-14 37.23 36.13 21.34 2 . 8 6 2.44 1.27 101.27 2.973 2.413 2.008 0.245 0.29 0.086 0 . 1 2 IV11gt5-15 37.1 34.66 21.19 2.64 3.24 1.25 100.08 2.98 2.328 2.006 0.227 0.388 0.085 0.167 gt5-15 togt-17 from

IV11gt5-16 37.26 34.82 21.35 2.61 3.34 1.24 100.62 2.977 2.326 2 . 0 1 0.223 0.398 0.084 0.171 gt-c to gt-r towards IV11gt5-17 37.25 34.26 21.26 2.79 3.33 1.23 1 0 0 . 1 2 2.985 2.296 2.008 0.24 0.398 0.083 0.173 qtz-r.

IVSOgtM 36.26 36.97 21.69 1.327 2.706 1.301 100.25 2.932 2.5 2.067 0.115 0.326 0.089 0.13 gt 1 - 1 to g tl - 6 from

IV50gt1-2 36.59 36.66 21.84 1.4 2.72 1.326 100.53 2.944 2.466 2.071 0 . 1 2 1 0.326 0.09 0.132 gt-c to gt-r towards

IV50gt1-3 36.58 36.37 21.74 1.973 2.476 1.134 100.27 2.95 2.453 2.066 0.17 0.298 0.077 0 . 1 2 1 bt-r. 0 ■D O Q. C S Q.

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C/) oxides cation C/) proportions Analytical Total Petrograptiic location

points SiOg FeO AI2 O3 CaO MgO MnO wt.% Si Fe Al Ca Mg Mn Mg/Fe of data points 0 IV50gt1-4 36.74 35.99 21.55 2.592 2.062 1.229 100.15 2.967 2.431 2.051 0.224 0.248 0.084 0 . 1 0 2 IV50gt1-5 36.6 36.68 21.65 2.678 1.887 1.292 100.79 2.949 2.472 0.227 8 2.056 0.231 0.088 0.092 IV50gt1-6 36.29 37.44 21.39 2.165 1.569 1.396 100.25 2.95 2.545 2.05 0.189 0.19 0.096 0.075

IV50gt1-7 36.46 37.13 21.58 1.257 2.42 1.268 1 0 0 . 1 2 2.952 2.515 2.06 0.109 0.292 0.087 0.116 g tl-7 to g tl-9 from IV50gt1-8 36.62 36.62 21.69 1.665 2.654 1.31 100.56 2.947 2.465 2.058 0.144 0.318 0.089 0.129 gt-c to gt-r towards

IV50qt1-9 36.8 36.08 21.41 2.732 2.353 1.219 1 0 0 . 6 2.962 2.429 2.031 0.236 0.282 0.083 0.116 qtz-r.

IV50gt1-10 36.27 36.99 21.58 1.353 2.732 1.281 1 0 0 . 2 2.936 2.504 2.059 0.117 0.33 0.088 0.132 gt1-10togt1-14from

IV50gt1-11 36.25 36.37 21.5 1.651 2.487 1.347 99.596 2.947 2.473 2.06 0.144 0.301 0.093 0 . 1 2 2 gt-c to gt-r towards

IV50gt1-12 36.55 34.81 21.47 2.312 2.981 0 . 2 0 1 3. 2.306 1.261 98.701 2.374 2.064 0.281 0.087 0.118 bt-r. ji 03" IV50gt1-13 36.38 36.26 21.46 2.575 2.08 1.267 1 0 0 . 0 2 2.95 2.459 2.051 0.224 0.251 0.087 0 . 1 0 2 ■ IV50qt1-14 36.66 36.95 21.23 2.665 1.831 1.246 100.58 2.963 2.497 2 . 0 2 2 0.231 0 . 2 2 1 0.085 0.088 0 ...... - ....L "O O Q. C a 3O "O O

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Appendix B. Biotite Ctiemical Analyses oxides cation proportions Analytical Total Petrograptiic points SiOg FeO KgO NagO AlgOg CaO MgO TiOg wt.% Si Fe K Na Al Ca Mg Ti Mg/Fe location

IV63bt1-1 35.66 15.95 10.03 0.09 17.73 0.03 11.93 3.97 95.39 2 . 6 8 1 . 0 0 0.96 0 . 0 1 1.57 0 . 0 0 1.34 0 . 2 2 1.333 gt-r

IV63bt1-2 35.59 15.65 10.07 0.14 17.93 0.03 11.90 3.84 95.15 2 . 6 8 0.99 0.97 0 . 0 2 1.59 0 . 0 0 1.34 0 . 2 2 1.355 gt-r

IV63bt1-3 35.62 15.62 10.03 0.18 17.91 0.06 11.27 3.89 94.58 2.70 0.99 0.97 0.03 1.60 0 . 0 0 1.27 0 . 2 2 1.286 gt-r

IV63bt1-4 35.40 15.82 9.91 0.16 17.62 0.07 12.23 3.34 94.55 2.69 1 . 0 0 0.96 0 . 0 2 1.58 0 . 0 1 1.38 0.19 1.378 gt-r

IV63bt1-5 35.55 15.36 9.55 0.17 17.70 0.05 12.41 3.31 94.10 2.70 0.97 0.92 0 . 0 2 1.58 0 . 0 0 1.40 0.19 1.44 gt-r

IV63bt2-7 34.94 17.24 1 0 . 2 1 0 . 1 0 17.49 0 . 0 0 10.38 4.78 95.14 2 . 6 6 1 . 1 0 0.99 0 . 0 1 1.57 0 . 0 0 1.18 0.27 1.073 m

IV63bt2-8 35.13 17.85 9.82 0.08 17.11 0 . 0 0 9.83 4.79 94.61 2.69 1.14 0.96 0 . 0 1 1.54 0 . 0 0 1 . 1 2 0.28 0.982 m

IV63bt2-9 35.21 17.77 10.11 0.07 17.28 0 . 0 0 9.87 4.40 94.71 2.70 1.14 0.99 0 . 0 1 1.56 0 . 0 0 1.13 0.25 0.99 m

IV63bt2-10 35.33 18.06 10.13 0.07 17.15 0 . 0 0 10.19 4.57 95.50 2.69 1.15 0.98 0 . 0 1 1.54 0 . 0 0 1.15 0.26 1.006 m

IV63bt2-11 35.22 17.61 9.83 0.09 17.23 0 . 0 0 9.96 4.68 94.62 2.69 1.13 0.96 0 . 0 1 1.55 0 . 0 0 1.14 0.27 1.008 m

IV63bt3-1 36.02 13.22 9.82 0.16 17.35 0 . 0 0 14.41 3.40 94.38 2.70 0.83 0.94 0 . 0 2 1.53 0 . 0 0 1.61 0.19 1.943 m ;■

IV63bt3-2 36.17 13.41 9.87 0.18 17.24 0 . 0 0 14.03 3.50 94.40 2.71 0.84 0.94 0.03 1.52 0 . 0 0 1.57 0 . 2 0 1.865 m

IV63bt3-3 35.82 13.64 9.79 0.18 17.29 0 . 0 0 14.15 3.44 94.31 2.69 0 . 8 6 0.94 0.03 1.53 0 . 0 0 1.59 0.19 1.849 m i

IV63bt3-4 36.17 13.55 9.98 0 . 2 1 17.30 0 . 0 0 14.09 3.54 94.84 2.70 0.85 0.95 0.03 1.52 0 . 0 0 1.57 0 . 2 0 1.853 m '

IV63bt4-1 36.05 16.93 8.69 0.15 15.80 0 . 0 0 12.61 4.30 94.54 2.73 1.07 0.84 0 . 0 2 1.41 0 . 0 0 1.42 0.25 1.327 gt-c-encl-qtz

IV63bt4-2 36.30 17.28 8.44 0.31 15.55 0.03 12.37 4.45 94.72 2.75 1.09 0.81 0.05 1.39 0 . 0 0 1.39 0.25 1.276 gt-c-encl-qtz

IV63bt4-3 36.51 16.65 8.83 0.31 15.76 0 . 0 0 12.70 4.36 95.11 2.75 1.05 0.85 0.04 1.40 0 . 0 0 1.42 0.25 1.36 gt-c-encl-qtz

IV63bt5-1 36.66 17.07 9.39 0 . 2 0 16.39 0 . 0 0 11.58 4.76 96.05 2.74 1.07 0.90 0.03 1.44 0 . 0 0 1.29 0.27 1.209 gt-c-encl-qtz

IV63bt5-2 36.54 16.78 9.69 0.17 16.47 0 . 0 0 11.74 4.76 96.15 2.73 1.05 0.92 0.03 1.45 0 . 0 0 1.31 0.27 1.246 gt-c-encl-qtz

IV63bt5-3 36.66 16.94 9.67 0 . 2 0 16.59 0 . 0 0 11.90 4.78 96.74 2.72 1.05 0.92 0.03 1.45 0 . 0 0 1.32 0.27 1.253 gt-c-encl-qtz

IV63bt5-4 36.58 16.87 9.50 0.16 16.50 0 . 0 0 11.54 4.84 95.99 2.73 1.05 0.91 0 . 0 2 1.45 0 . 0 0 1.29 0.27 1.219 gt-c-encl-qtz

IV63bt6-1 36.04 17.26 9.84 0 . 1 0 17.39 0 . 0 0 10.75 4.51 95.89 2.71 1.08 0.94 0 . 0 1 1.54 0 . 0 0 1 . 2 0 0.25 1.109 m

IV63bt6-2 36.02 17.45 9.77 0 . 1 1 17.04 0 . 0 0 10.36 5.01 95.76 2.71 1 . 1 0 0.94 0 . 0 2 1.51 0 . 0 0 1.16 0.28 1.058 m

IV63bt6-3 36.13 17.87 10.00 0.09 17.23 0 . 0 0 10.32 4.58 96.22 2.72 1 . 1 2 0.96 0 . 0 1 1.53 0 . 0 0 1.16 0.26 1.029 m

IV63bt6-4 36.43 17.59 9.97 0.06 17.24 0 . 0 0 10.37 4.99 96.65 2.72 1 . 1 0 0.95 0 . 0 1 1.52 0 . 0 0 1.15 0.28 1.051 m

IV63bt6-5 35.89 17.87 9.92 0.08 16.83 0 . 0 0 10.64 4.97 96.21 2.70 1 . 1 2 0.95 0 . 0 1 1.49 0 . 0 0 1.19 0.28 1.061 m

IV63bt7-1 36.31 16.72 8 . 6 8 0.19 19.12 0.29 13.96 0 . 0 2 95.28 2.71 1.04 0.83 0.03 1 . 6 8 0 . 0 2 1.55 0 . 0 0 1.488 gt-r

IV63bt7-2 34.66 17.36 7.91 0.13 18.98 0.18 14.37 0 . 0 2 93.62 2.64 1 . 1 1 0.77 0 . 0 2 1.71 0 . 0 1 1.63 0 . 0 0 1.476 gt-r

IV63bt7-3 34.07 17.12 6 . 6 8 0.14 20.50 0.14 14.83 0 . 0 1 93.50 2.58 1.08 0.65 0 . 0 2 1.83 0 . 0 1 1.67 0 . 0 0 1.544 gt-r

IV63bt7-4 32.88 18.60 5.04 0 . 1 1 20.76 0.14 15.08 0.03 92.63 2.51 1.19 0.49 0 . 0 2 1.87 0 . 0 1 1.72 0 . 0 0 1.446 gt-r

IV63bt7-5 36.18 15.92 9.45 0.24 19.49 0 . 2 2 13.72 0 . 0 0 95.22 2.71 1 . 0 0 0.90 0.04 1.72 0 . 0 2 1.53 0 . 0 0 1.536 gt-r

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C/) C/) weighted weighted 2% error on Analytical Pétrographie Size of single spot mean age mean error singie spot age 2% error points location monazite (um) age (Ma) error (+) S a . (25) 5 f of A , IV76Amzt1z1-4 m-encl. in K-spar + qtz 1506 7.0 30.1 IV76Amzt1z1-5 m-encl. in K-spar + qtz 1503 7.0 30.1 IV76Amzt1z1-6 m-encl. in K-spar + qtz 1533 8.0 30.7 8 IV76Amzt1z1-7 m-encl. in K-spar + qtz 1498 7.0 1520 23 0.74 30.0 30 IV76Amzt1z2-1 m-encl. in K-spar + qtz 1557 7.0 31.1 ci' iV76Amzt1z2-2 m-encl. in K-spar + qtz 1500 8.0 30.0 1

iV76Amzt1z2-3 m-encl. in K-spar + qtz I486 5.0 29.7 Ii [1 IV76Amzt1z2-4 m-encl. in K-spar + qtz 1501 5.0 30.0 {II iV76Amzt1 z2-5 m-encl. in K-spar + qtz 1459 5.0 29.2 IV76Amzt1z2-6 m-encl. in K-spar + qtz 1456 5.0 29.1 iV76Amzt1z2-7 m-encl. in K-spar + qtz 1492 5.0 1492 23 1.20 29.8 30 ; IV50mzt1z1-1 m-encl. in K-spar + qtz 140x75 1623 12.1 32.5 3 i i ' 3 " IV50mzt1z1-2 m-encl. in K-spar + qtz 1668 11.7 33.4 CD IV50mzt1z1-3 m-encl. in K-spar + qtz 1692 12.8 33.8 IV50mzt1z1-4 m-encl. in K-spar + qtz 1595 13.1 31.9 CD Ii ■D IV50mzt1z1-5 m-encl. in K-spar + qtz 1534 12.0 1618 29 3.80 30.7 32 ‘I O IV50mzt1z2-1 m-encl. in K-spar + qtz 1515 33.8 30.3 Q. C IV50mzt1z2-2 m-encl. in K-spar + qtz 1493 13.4 1502 24 0.24 29.9 30 a IV50mzt1z3-1 m-encl. in K-spar + qtz 1377 22.4 27.5 3O IV50mzt1z3-2 m-encl. in K-spar + qtz 1450 23.4 29.0 "O IV50mzt1z3-3 m-encl. in K-spar + qtz 1435 22.7 28.7 O IV50mzt1z3-4 m-encl. in K-spar + qtz 1419 20.5 28.4 IV50mzt1z3-5 m-encl. in K-spar + qtz 1457 23.8 29.1 IV50mzt1z3-6 m-encl. in K-spar + qtz 1455 21.8 1431 23 1.19 29.1 29 CD Q. Not used in weighted mean age calculation

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to 8 BIBLIOGRAPHY

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Block, South Coast Geologic Society: Mohave Desert; Annual Fieldtrip Guidebook # 22, p. 41 -68 Hewett, D.F., 1956, Geology and Mineral Resources of the Ivanpah Quadrangle California and Nevada, Geological Survey Professional Paper 275, p. 17-22 Hodges, K.V., Spear, F.S., 1982, Geothermometry, geobarometry and the A^SiOs triple point at Mount Moosilauke, New Hampshire, American Mineralogist, v. 67, 1118- 1134 Hoffman, P.F., 1988, United Plates of America: birth of a craton. Annual review of Earth and Planetary Science Letters, v. 16, p. 543-603 Hoffman, P.F., 1989, Speculations on Laurentia’s first gigayear (2.0 to 1.0 Ga), Geology, V. 17, p. 135-138 Holdaway, M.J., Mukhopadhyay, B., Dyar, M.D., Guidotti, C.V., Dutrow, B.L., 1997, Gamet-biotite geothermometry revised: New Margules parameters and a natural specimen data set from Maine, American Mineralogist, v. 82, p. 582-595 Holdaway, M.J., 2000, Application of new experimental and Garnet Margules data to the gamet-biotite geothermometer, American Mineralogist, v. 85, p. 881-892 Holdaway, M.J., 2001, Recalibration of the GASP geobarometer in light of recent gamet and plagioclase activity models and versions of the gamet-biotite geothermometer, American Mineralogist, v. 86, p. 1117-1129 Howard, K.A., Stone, P., Pemokas, M.A., Marvin, R.F., 1982, Geologic and geochronologic reconnaissance of the Turtle Mountains area, Califomia: West Border of the Wipple Mountains detachment terrane in Mesozoic-Cenozoic Tectonic Evolution of the Colorado River Region, Califomia, Arizona, and Nevada, edited by E.G. Frost and D.L. Martin, Cordilleran, San Diego, Califomia, p. 341-354 Howard, K.A., Nielson, J.E., Simpson, R.W., Hazlett, R.W., Aluminas, H.V., Nakata, K., 1988, Mineral resources of the Turtle Mountains Wildemess Study Area, San Bemardino County, Califomia, United States Geological Survey Bulletin, 1713-B, p. 28 Indares, A., Martignole, J., 1985, Biotite-gamet geothermometry in the granulite facies: the influence of Ti and A1 in biotite, American Mineralogist, v. 70, p. 272-278 Jaffey, A.H., Flynn, K.F., Glendenin, L.E., Bentley, W.C., Essling, A.M., 1971, Precision measurements of the half-lives and specific activities of ^^^U and ^^*U, Physical Reviews, v. C4, p. 1889-1907 John, B.E., 1982, Geologic framework of the Chemehuevi Mountains, southeastem Califomia, in Mesozoic-Cenozoic Tectonic Evolution of the Colorado River Region, Califomia, Arizona, and Nevada, edited by E.G. Frost and D.L. Martin, Cordilleran, San Diego, Califomia, p. 317-325 Karlstrom, K.E., Bowring, S.A., Conway, C.M., 1987, Tectonic significance of an Early Proterozoic two-province boundary in central Arizona: Geological Society of America Bulletin, v. 99, p. 529-545 Karlstrom, K.E., Bowring, S.A., 1988, Early Proterozoic assembly of tectonostratigraphic terranes in southwestem North America, Joumal of Geology, v. 96, p. 561-576 Karlstrom, K.E., Bowring, S.A., 1991, Styles and timing of Early Proterozoic deformation in Arizona, Arizona Geological Society Digest 19, p. 1-10

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Kingsbury, J. A., Miller, C F., Wooden, J.L., Harrison, T.M., 1992. Monazite Paragenesis and U-Pb systematics in fd^Srijfffie eastern Mojave Desert, Califomia, U.S.A.: implications for thermochronometry. Chemical Geology, v. 110, p. 147-167 Koziol, A.M., Newton, R.C., 1988, Redetermination of the anorthite breakdown reaction and improvement of the plagioclase-gamet-AlzSiOg-quartz geothermobarometer, American Mineralogist, v. 73, p. 216-223 Labrenz, M.E., Karlstrom, K.E., 1991. Timing of the Mazatzal orogeny: Constraints from the Young Granite, Pleasant Valley, Arizona, in Karlstrom, K.E. and DeWitt, E., eds., Proterozoic geology and ore deposits of Arizona: Arizona Geological Society Digest 19, p. 225-236. Labrenz, M.E., Karlstrom, K.E., 1991, Timing of the Mazatzal orogeny: Constraints from the Young Granite, Pleasant Valley, Arizona, Arizona Geological Society Digest 19, p. 225-235 Lerch, M.F., Patched, P.J., Reynolds, S.J., 1991, Sr and Nd isotopic studies of Proterozoic rocks in west-central Arizona: Implications for Proterozoic Tectonics, Arizona Geological Society Digest 19, p. 51-56 Ludwig, K.R., 2001, ISOPLOT, A Geochronologic Toolkit for Microsoft Excel, version 2.49, Berkeley Geochronology Center, Special Publication no.la Martin, M.W., Walker, J.D., 1992, Extending the westem North American Proterozoic and Paleozoic craton through the Mohave Desert, Geological Society of America, Abstracts with Programs, v. 20, p. 753-756 Metcalf, R., 2002, conversation. University of Nevada, Las Vegas Miller, D M., Frisken, J.G., Jachens, R.C., Gese, D.D., 1986, Mineral resources of the Castle Peaks Wildemess Study Area, San Bemardino County, Califomia, United States Geological Survey Bulletin, 1713-A, p. 17 Miller, D M., Wooden, J.L., 1992, Proterozoic geology of the New York, Ivanpah, and Providence Mountains, Califomia: Field guide, U.S. Geological Survey Open-File Report 92-000, p. 1-10 Miyashiro, A., 1972, Metamorphism and Metamorphic Belts, London, George Allen & Unwin, p. 258 Montel, J.M., Foret, S., Veschambre, M., Nicollet, C, Provost, A., 1996, Electron micrprobe dating of monazite. Chemical Geology, v. 131, p. 37-53 Montel, J.M., Komprobst, J., Vielzeuf, D., 2000, Preservation of old U-Th-Pb ages in shielded monazite: example from the Beni Bousera Hercynian kinzigites (Morocco), Joumal of Metamorphic Petrology, v. 18, no. 3, 335-342 Mukhopadhyay, B., Holdaway, M.J., Koziol, A.M., 1997, A statistical model of thermodynamic mixing properties of Ca-Mg-Fe'*'^ gamets, American Mineralogist, v. 82, p. 165-181 Nielson, J.E., Frisken, J.G., Jackens, R.D., McDonnell, J.R., Jr., 1987, Mineral resources of the Fort Piute Wildemess Study Area, San Bemardino County, Califomia, United States Geological Survey Bulletin, 1713C, p. 12 Overstreet, W.C., 1967, The geologic occurrence of monazite, U.S. Geol. Surv., Prof. Pap., 530, p. 327 Parrish, R.R., 1990, U-Pb dating of monazite and its application to geological problems, Canadian Joumal of Earth Science, v. 27, p. 1431-1450 Parrish, R.R., Tirml, R., 1989, U-Pb age of the Baltoro granite, northwest Himalaya, and

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implications for monazite U-Pb systematics, Geology, v. 17, p. 1076-1079 Perchuk, L.L., Laven't'eva, I98^f^^ÇMmental investigation of exchange equilibria in the system cordierite-gamet-biotite, in Saxena, S.K., Ed., Kinetics and Equilibrium in Mineral Reactions, Advances in Physical Geochemistry, Springer, New York, New York, V. 3, p. 199-239 Patino Douce, A.E., Rice, J.M., 1993, Octehedral excess mixing properties in biotite: a model with applications to geothermometry and geobarometry, American Mineralogist, v. 78, p. 113-131 Rao, A.T., Fonorev, V.I., Konilov, A.N., Romanenko, M., 1998, Electron microprobe dating from spinel Granulite in the eastern Ghats Belt, India, Joumal Geological Society of India, v. 52, p. 345-350 Reynolds, S.J., DeWitt, E., 1991, Proterozoic Geology of the Pheonix region, central Arizona, Arizona Geological Society Digest 19, p. 237-250 Scherrer, N.C., Engi, M., Gnos, E., Jakob, V., Liechti, A., 2000, Monazite analysis: from sample preparation to microprobe age dating and REE quantification, Scweiz. Mineral. Petrogr. Mitt, v. 80, p. 93-105 Shaw, C.A., Karlstrom, K.E., Williams, M.L., Jercinovic, M.J., McCoy, A.N., 2001, Electron-microprobe monazite dating of ca. 1.71-1.63 Ga and ca. 1.45-1.38 Ga deformation in the Homestake zone, Colorado: Origin and early evolution of a persistent intracontinental tectonic zone. Geology, v. 29, no. 8, p. 739-742 Smith, H.A., Giletti, B.J., 1996, Lead diffusion in monazite, Geochimica et Cosmochimica Acta, v. 61, no. 5, p. 1047-1055 Spear, F.S., 1993, Metamorphic phase equilibria and pressure-temperature-time paths, Mineralogical Society of America Monograph Series, no. 1, p. 799 Spear, F.S., Cheney, J.T., 1989, A petrogenetic grid for pelitic schists in the system SiOz- Al203-Fe0 -Mg 0 -K20-H20, Contributions to Mineralogy and Petrology, v. 101, p. 149-164 Stewart, J.H., Poole, E.G., 1975, Extension of the Cordilleran miogeosynclinal belt to the San Andreas fault, southem Califomia, Geol. Soc. Am. Bull., v. 86, p. 205-212 Wagner, G.A., Reimer, G.M., Jaeger, E., 1977, Cooling age derived by apatite fission track, mica Rb-Sr, and K-Ar dating: the uplift and cooling history of the central Alps, Mem. 1st, Geol. Mineral., Univ. Padova, 30, p. 27 Wendt, I., Carl, C., 1991, The statistical distribution of the mean square weighted deviation. Chemical Geology (Isotope Geoscience Section), v. 86, p. 275-285 Williams, M.L., 2002, e-mail conversation on techniques of EMP U-Th-Pb monazite geochronology Williams, M.L., 1991, Overview of Proterozoic Metamorphism in Arizona, Arizona Geological Society Digest 19, p. 11-26 Williams, M.L., Grambling, J.A., 1990, Manganese ferric iron, and the equilibrium between gamet and biotite, American Mineralogist, v. 75, p. 886-908 Williams, M.L., Jercinovic, M.J., Terry, M.P., 1999, Age mapping and dating of monazite on the electron microprobe: Deconvoluting multistage tectonic events. Geology, v. 27, no. 11, p. 1023-1026 Winkler, H.G.F., 1967, Petrogenesis of Metamorphic Rocks, 2"^ Ed., Springer-Verlag, New York, New York, Inc. Wooden, J.L., Dewitt, E., 1991, Pb isotopic evidence for boundary between the Early

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Proterozoic Mohave and central Arizona crustal provinces in westem Arizona,

Wooden, J.L., Stacey, J.S., Doe, B., Howard, K.A., Miller, D M., 1988, Pb isotopic evidence for the formation of Proterozoic cmst in the southwestem United States, in Emst, W.G., ed.. Metamorphism and cmstal evolution of the Westem United States, Rubey Volume VII, Englewood Cliffs, New Jersey, Prentice Hall, p. 68-86 Wooden, J.L., Miller, D.M., 1990, Chronologic and isotopic framework for Early Proterozoic cmstal evolution in the eastem Mojave Desert Region, SE California, Joumal of Geophysical Research, v. 95, no. B12, p. 20,133-20,146 Yardley, B.W.D., MacKenzie, W.S., Guilford, C., 1990, Atlas of Metamorphic rocks and their textures, John Wiley & Sons, Inc., New York, New York, p. 9-115 Young, E.D., Anderson, J.L., Clarke, H.T., 1989, Petrology of Biotite-Cordierite-Gamet Gneiss of the McCullough Range, Nevada I. Evidence for Proterozoic Low-Pressure Fluid-Absent Granulite-Grade Metamorphism in the Southem Cordillera, Joumal of Petrology, v. 30, Part 1, p. 39-60 Zhu, X.K., O’Nions, R.K., Belshaw, N.S., Gibb, A.J., 1997, Significance of in situ SIMS chronometry of zoned monazite from the Lewisian granulites, northwest Scotland, Chemical Geology, v. 135, p. 35-53 Zhu, X.K., O’Nions, R.K., Belshaw, N.S., 1994b, Dating granulite facies metamorphism and depletion: SIMS chronometry of micron-scale monazite inclusions in gamet, Mineralogical Magazine, v. 58A, p. 1004-1005

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Graduate College University of Nevada, Las Vegas

David A. Shields

Local Address: 4281 South Pecos Rd. #105 Las Vegas, NV. 89121

Degrees: Bachelor of Science, Earth Systems Science 1999 George Mason University

Special Honors and Awards: Bemada E. French, Dept, of Geoscience, UNLV (Fall, 2001) Bemada E. French, Dept, of Geoscience, UNLV (Fall, 2002) Geological Society of America Graduate Student Research Grant (Fall, 2002)

Thesis Title: Gamet-Biotite Thermometry, Gamet-AliSiO^-Silica-Plagioclase Barometry, and EMP Monazite Geochronology of the Early Proterozoic Ivanpah Mountains, Eastern Mojave Desert, SE California

Thesis Examination Committee: Chairperson, Rodney Metcalf, Ph.D. Committee Member, Clay Crow, Ph.D. Committee Member, Terry Spell, Ph.D. Graduate Faculty Representative, John Swetnam, Ph.D.

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