Fission-Track Geochronology and Geothermometry of Late Cretaceous-Early Tertiary Epizonal Plutons, in and Adjacent to the Lombard Thrust Fault, Southwestern Montana

Western Michigan University ScholarWorks at WMU

Master's Theses Graduate College

6-1988

Jean Talanda

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Recommended Citation Talanda, Jean, "Fission-Track Geochronology and Geothermometry of Late Cretaceous-Early Tertiary Epizonal Plutons, in and Adjacent to the Lombard Thrust Fault, Southwestern Montana" (1988). Master's Theses. 1202. https://scholarworks.wmich.edu/masters_theses/1202

This Masters Thesis-Open Access is brought to you for free and open access by the Graduate College at ScholarWorks at WMU. It has been accepted for inclusion in Master's Theses by an authorized administrator of ScholarWorks at WMU. For more information, please contact [email protected]. FISSION-TRACK GEOCHRONOLOGY AND GEOTHERMOMETRY OF LATE CRETACEOUS-EARLY TERTIARY EPIZONAL PLUTONS, IN AND ADJACENT TO THE LOMBARD THRUST FAULT, SOUTHWESTERN MONTANA

by Jean Talanda

A Thesis Submitted to the Faculty of the Graduate College in partial fulfillment of the requirements for the Degree of Master of Science Department of Geology

Western Michigan University Kalamazoo, Michigan June 1988

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. FISSION-TRACK GEOCHRONOLOGY AND GEOTHERMOMETRY OF LATE CRETACEOUS-EARLY TERTIARY EPIZONAL PLUTONS , IN AND ADJACENT TO THE LOMBARD THRUST FAULT, SOUTHWESTERN MONTANA

Jean Talanda, M.S. Western Michigan University, 1988

Thrusting occurred in the central portion of the cen tral Montana sa lie n t where the timing of movements is

somewhat obscured by the multiple deformations from Archean time to the present. The relationship of this

thrusting to the associated plutons has not been established in every case. Fission-track geochronology was employed in dating several plutons associated with the Lombard thrust fault, consequently, defining a time range in which movement along the thrust occurred. Seven plutons with known structural relationships to the Lombard thrust have ages which suggest that movement along the fault occurred 73.2 +/-6.7 Ma. Fission-track length distributions suggest that all the plutons studied share a common thermal history. The distributions indicate that the plutons were brought to a shallow depth and cooled rapidly similar to undisturbed volcanic-type

rocks. Therefore, the fission-track ages obtained in

this study are very likely related to the actual crystallization of rocks.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. AC KNOWLEDGEMENT S

Matthew Fisher, my husband: Thanks for your support.

Dorothy and Edmund Talanda, my parents: I hope this paper can appropriately reflect all that I have learned. Donald S. Miller, Chairman of the Geology Department

at Rensselaer Polytechnic Institute (RPI); Ian R. Duddy, from the University of Melbourne, Australia; and, Raymond A. Donelick, RPI: They have given me their time and energy above and beyond th a t expected from any outside

university. I truly appreciate their assistance.

The su p p o rtin g s t a f f a t RPI; Mary Roden, C harles Kavanaugh and Clark Isachsen. I thank them for th e ir help. John D. Grace, W. Thomas Straw, Christopher J.

Schmidt and Ronald B. Chase; Professors in the Geology Department at Western Michigan University (WMU) : This thesis has been a challenge. Thanks for helping me with the concept, the field work and lab equipment. My sincere gratitude to my other supporters: the Graduate Research Fund at WMU; the Awards and Fellowship Committee for a financial grant; the Department of Geology at WMU, for a financial award; the Carlson's, in Montana;

Jerry Jones, for the Phoenix Memorial Reactor at U of M; Richard Berg at the Montana Bureau of Mines; and, the Michigan Department of Transportation, for their lab.

Jean Talanda ii

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University M icrdrilm s International 300 N. Zeeb Road Ann Arbor, Ml 48106

Fission-track geochronology and geothermometry of Late Cretaceous-Early Tertiary epizonal plutons, in and adjacent to the Lombard thrust fault, southwestern Montana

Talanda, Jean, M.S.

Western Michigan University, 1988

UMI 300 N. Zeeb Rd. Ann Arbor, MI 48106

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

ACKNOWLEDGEMENT ...... i i LIST OF TABLES ...... vi LIST OF FIGURES ...... v ii CHAPTER

I. INTRODUCTION ...... 1 The Problem ...... 1 II. GEOLOGICAL ...... 5 Lombard Thrust Fault ...... 6

Boulder Batholith Region ...... 7

I II . FISSION TRACK ANALYSIS ...... 10 Problems Involved in Fission Tracking ...... 10

Effects of Other Fission Track Producing Isotopes ...... 12 Physical and Age Limitations of Apatite Crystals ...... 13 Changes After Crystal Formation ...... 14 Laboratory Determinations of the Parent- Isotope/Daughter Ratio-Track Ratio ...... 15 Age Equation and Zeta Calibration Factor... 15 Geothermometry Effects of Heat and Time on Resultant Ages ...... 19 Fission-Track Length Distributions ...... 22 IV. ANALYTICAL METHODS AND RESULTS ...... 26 Sampling Techniques ...... 26 Methods of Fission-Track Dating ...... 27

i i i

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

CHAPTER IV.

Results: Relative Thermal-Neutron Fluence Measurements ...... 31 Results: Individual and Weighted Mean Zeta Calibration Factors ...... 34 R esults: Measurements of U ncertainty .. 35 Methods of Fission-Track Length Distributions ...... 35

Results: Fission-Track Length Distributions ...... 35

V. DISCUSSION OF RESULTS ...... 38 Fission-Track Length Distributions ...... 38 Fission-Track Ages ...... 43 Individual Pluton Ages ...... 44 Determination of the Timing of Movement Along the Lombard Thrust ...... 51 VI. SUMMARY ...... 53 BIBLIOGRAPHY ...... 55

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

1. Montana Sample Information and Locations ...... 4 2. L ist of Samples Not Dated ...... 27 3. Data for Fission-Track Age Analysis ...... 32

4. Fission Track-Length Distributions forSamples and Induced-Track Standards ...... 37 5. Fission-Track Ages and K-Ar References ...... 42

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

1. Fission-Track Length Distributions; Five Cases Observed by Gleadow and Others (in Press) ...... 24 2. Sample-Mica Pairs; Before and After Radiation ... 29 3. Petrographic View of Sample-Mica Pairs ...... 30 4. Determination of Gradient on Irradiation Cylinder ...... 34 5. Fission-Track Length Distributions. A. Five Cases Observed by Gleadow et a l. (in Press) B. Montana Samples ...... 39

6. Track Length Distributions for Annealed Standards and Samples ...... 40

7. Montana Fission-Track Ages ...... 45

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

INTRODUCTION

The Problem

Plutonism, volcanism and thrusting occured between 68 and 78 Ma within the fold and thrust belt of south western Montana (Hanna, 1973; Robinson, Klepper, & Obradovich, 19 68) . During the relatively short time span in which deformation occurred, the Boulder batholith and

its satellite plutons were emplaced in and near to the Lombard thrust sheet. Overprinting of thrust belt

features in this area by Cenozoic extensional faulting makes uplift history difficult to interpret. This study sought to define a time limit for the

Lombard thrust by dating selected plutons in and adjacent to the Lombard thrust sheet using fission-track geochronology and geothermometry. The plutons selected for dating were chosen for their presumed structural relationship to the Lombard th ru st. Thorough mapping of the Lombard th ru st sheet by Robinson (1963), Garihan, Schmidt, Karasevich (1982),

Schmidt & O 'N eill (1982) and Ludman (1965) has allowed the geologic framework of this area to be comparatively

well understood. Some of these selected plutons appear 1

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 to have moved with the Lombard thrust sheet where a number of them are inferred to penetrate the sheet. Thus, information regarding the age of these igneous bodies would permit the dating of the Lombard th ru st more p recisely. As an example, plutons thought to cut folds or

thrust faults are considered post-thrusting; and those cut or folded by a thrust fault have generally been considered to be pre-thrusting. Thus, if the ages of the plutons are known, the time range in which thrusting occurred can be narrowed.

In addition to this dating process, fission track

analysis has the unique ability to provide information

regarding the thermal history of the samples. By taking

measurements of track length distributions, it is possible to date the plutons and to draw inferences about their thermal histories. The study area is within the Cordilleran fold and thrust belt of southwestern Montana; specifically the southeastern margin of the Boulder batholith and the C entral Montana sa lie n t (Plate 1). Samples taken from

the southwest Montana transverse zone which bounds the

southern part of the Lombard sheet include the Butte Quartz Monzonite, the Hell Canyon pluton, the Rader Creek pluton, the 10-N pluton, the Brownback s i l l (Diamondhead sill) , the Gold Hill sill (Bone Basin sill) , the Burton

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 Park pluton, the Pipestone Pass pluton and the Cottonwood Canyon s i l l . Other samples from the Montana fold and thrust belt are the Lone Mountain pluton, the Rattlesnake Butte pluton, the Townsend pluton, the North Boulder complex and the Mt. Doherty complex (Freeman, Ruppel, & Klepper 1985; Schmidt, Smedes, Suttner, & Vitaliano, 1979; Robinson, 1963; Ludman, 1965). Exact sample locations are given in Table 1.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sanple Location 1/1^ SM 1/1, Sec.28, T.3N, R.1E 1/1, NE 1/1, Soc.1, T.1N, R.6U 1/1, SE 1/1, Sec.28, T.2S, R.6U 1/1, NE 1/1, Sec.7, T.1N,1/1, R.1E NE 1/1, Sec.26, T.6N, R.1U 1/1, SE 1/1, Sec.9, T.1N,1/1, R.8U SU 1/1, Sec.15, T.IN, R.8U 1/1, NE 1/1, Sec.8, T.1S, R.1H 15*; 111 11* 52*30"; 112 21* 59** 111 37* 1/1, SE 1/1. Sec.8. T.2N, R-2W 17*30"; 1/1, NU 1/1, 111 38*10" Sec.8, T.8N, R.1E 38*; 112 20*10*' 1/1. Sec.16, T.1N, 1/1. R.7U Sec-29, T.2N. R.2H 1/1, NE 1/1, Sec.7, T.1N, R.1E 1/1, SE 1/1, Sec.3,1/1, T.1N, SU 1/1, R.7H Sec.32, T.2N, R.2H 50*10"; 112 22* 50*20"; 112 21* 07*; 111 38*309" 16*; I12 06*30" 56*; 111 50* 07*; 111 38*30" 51*; 120 28*15"53*07"; 111 53*10" 50*; 112 28* 5 1 *3 0 " ; 111 52* 15 13*; 111 59* 15 SE 15 HE 15 SU Central uest, Sec.28, T.1S, R.3U 18 15 SE 15 NU 15 HE NE 18 HE 18 16 NU 1S 15 SU 15 NE 15 KU 15 SE HU NU

la t o phaso a Lortbard Thrust intrudes Canbrian Rocks fro n th e Main phaso o f Magna fau lt; cuts Rader Crook Pluton; KagnaCVitaliano,personal coteO thrust;shape intrudedof N.M. + fault controlled zone; by fron lato phaso of nagna intrudes Precanbrian Belt series synclino of Paleozoic rocks; folded Late Cretaceous rocks by the possibly nain piutonic ascended in body; faultby the zone nain plutonic body; folded Late Cretaceous rocks across large synclino of Paoozoic Magna ticMagna equivalent of volcanics cuts and shears volcanics; cuts possibly ascended in fault zone feeder for volcanics rocks CaMbrian and Devonian rocks cuts and shears volcanics; cuts Pro-thrusting; pro-folding; Contonporanoous; intrudes thrust Post-toctonic if fron P o s t-th ru s tin g ; cu ts Cartp Crook Pro-folding; cuts across large Pro-thrusting;soporatod and shearedPro-thrusting;separated and sheared Contonporanoous; faulted; cuts Pro-thrusting; folded * sheared; Pre-folding; s ills folded in Pro-thrusting; butress for thrusts; Hid to late thrusting;post-folding; Intrudes Precanbrian Post-thrusting; latest nagna phase Older than the nain phase nagna Table 1 Rock Type Suspoctod relationship to Quartz Honzonito Quartz Honzonito Quartz Honzonito Quartz Honzonito Quartz Honzonito Quartz Honzonito Granodiorito Quartz Honzonito Hid to late-thrusting;post-folding; Granodiorito / Granodiorito / R hyodacito Granodiorito Hornblende Rndosito Quartz Honzonito Calc-Rlkaline Suito Granodiorito

■1600 Foot 5350 5820 8200 5500 5200 •1880 ■1580 5260 5300 ■1580 8500 fUaskite1810 8800 Quartz Honzonito 5500 5200 Montana Montana Sample Information and Location E lo v a tio n

Sanple Nan* 10-N Pluton Butto Quartz Honzonito Brounback SOianondhoad ill S or ill Hell Cannon Pluton Rattlesnake Butto Pluton Rader Crook Pluton Rader Crook Pluton Tounsend Pluton Lono Mountain Stock Gold H illBono B asin S S ill i l l or North Bouldor Conplox Lono Mountain Stock Pipestone Pass Pluton Cottonuood Canyon S ill Burton Park Pluton Ht. Oohtery Conplex

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. C H A PTER I I

GEOLOGICAL SETTING

The fold and thrust belt (Sevier) structures in the

Montana area are characterized by folds and thrusts superimposed on broadly arched and thrusted crystalline basement, (Scholten, 1968). During the Middle Proterozoic time, when tectonic

development occurred along the southern margin of the Belt Basin in southwestern Montana, two fault sets were

formed. The two Proterozoic fault zones were: (1) the

northwest-trending extensional faults, which may have

been related to the opening of the Belt Basin; and, (2) the east-trending Willow Creek-Perry Line, probably a right-normal oblique fault zone. These old, northwest- trending Proterozoic shear zones that extend to the basement appear to have been reactivated from Late Cretaceous to early Paleocene during a second orogenic episode (Schmidt & Garihan, 1983, 1986). Associated folding occurred producing anticlines and synclines with large amplitudes. Allochthonous sheets were translated eastward along

detachm ent surfaces in B eltian rocks and ramped up and over the craton. As earlier thrusts were folded new

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6 thrusts formed further eastward. Plutonism, associated with thrusting, also developed eastward as evidenced by the intrusive complex called the Boulder batholith. The thrust belt structures can be generally described as an anastomating and imbricated system of east-trending, north-dipping oblique slip faults, that probably flatten with depth merging with a detachment surface in Precambrian Belt rock. It has been speculated th a t the fa u lts merge northeastward into north-trending, west-dipping thrust faults (Schmidt & O'Neill, 1982; Woodward, 1981). Schmidt and Garihan (1986) suggest that

the east-trending, north-dipping faults of the Jefferson Canyon region (southwestern Montana traverse zone)

comprise a large lateral ramp of the Lombard sheet which

merges northeastward into the north-trending, west- dipping Lombard thrust. The age of emplacement of this thrust sheet is one of the principal problems dealt with in this report.

Lombard Thrust Fault

The east-trending Jefferson Canyon fault zone (Plate 1) is inferred to connect with the north-trending Lombard thrust just north of Three Forks, Montana, to form a

major fault zone within the fold and thrust belt. This

appears to connect with the Mayflower and Cave fa u lts as seen in Plate 1. To the north, the Lombard is inferred

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 to connect with the Eldorado thrust northeast of Helena, Montana (Woodward, 1981). The entire proposed *ault zone, the Eldorado-Lombard-Jefferson Canyon zone, marks the pronounced eastward deflection of the Montana fold and thrust belt called the Helena structural salient, and defines the eastern margin of the Lombard thrust sheet.

This fault zone has carried a 11ochthonous Middle Proterozoic sedimentary rocks (Belt Supergroup) to the east over Paleozoic and Mesozoic sedimentary layers. Three other thrust faults between the Lombard and

the eastern edge of the thrust belt (Verrall, 1955) also indicate that ramping has occurred. An eastward propagating sequence of thrusts from a single detachment has been inferred by Schmidt and O'Neill (1982). Between the southernmost trace of the Lombard thrust and the Boulder batholith, larger folds may represent

areas where the Lombard sheet has stepped up to higher stratigraphic layers within the Belt Supergroup (Schmidt & O'Neill, 1982).

Boulder Batholith Region

The Proterozoic shear zones formed in the Belt Basin

of western Montana became a major influence on the style

of deformation during the Late Cretaceous-Early Tertiary

time, when the Boulder batholith was emplaced in the Helena structural salient.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The Helena S alien t is bounded to the north and south by the Lewis-Clark-Osborn and Willow Creek lineaments respectively (Woodward, 1981). During the Late Cretaceous, thrust faulting was deflected to the north of

the Boulder batholith by the Lewis-Clark-Osborn lineament, as reflected by a left-lateral tear zone. Faulting was deflected to the south of the batholith by the portions of the Willow Creek lineament, a right- lateral tear zone.

Eruption of the Elkhorn Mountain Volcanics was

followed by north-northeast trending folding. Igneous

activity began during the formation of the fold and thrust belt. The northeast-trending Boulder batholith is an epizonal pluton intruded over a 10 million year span between 68-78 Ma (Klepper, Ruppel, Freeman, & Weeks, 1971). The Boulder batholith and surrounding plutons are composed of a succession of calc-alkaline magmas differing in composition and age. Two dissimilar magma series have been proposed as sources: the main series called the Butte Quartz Monzonite, which includes the bulk of the Boulder batholith; and, the sodic series which includes the southern Boulder batholith plutons, Radar Creek and Hell Canyon (Tilling, 1973). Table 1 lists the petrology and structure of the plutons dated. The western and southwestern portion of the Boulder b ath o lith was la te r intruded and mantled by the Lowland

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 Creek volcanics during the Early Eocene, 48-50 Ma (Smedes, 1962b; Smedes & Thomas, 1965). This second phase of volcanism has obscured many of the structural relationships of the Late Cretaceous - Early Paleocene

orogeny, and may even be related to the sodic series (Tilling, 1973).

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

FISSION TRACK ANALYSIS

Problems Involved in Fission Tracking

In geology fission track analysis has many

ap p licatio n s. The method of fissio n track analysis can be used in geochronology to date a variety of rocks as

well as geothermometry to provide valuable information about their thermal history. In the following

discussions dating methods based on the formation of fission tracks will be treated separately from that of

the thermal history which deals with the healing of fission tracks.

Apatite, a common accessory mineral in many rocks,

is most often used as a low-temperature geothermometer in the fission track analysis. Apatite crystals, found in igneous rocks, have a sufficient uranium content to allow them to be used in this process. The area chosen for this study was selected because it had an abundance of

igneous rock (plutons and sills) associated with the

Lombard thrust (See Plate 1 for sample locations). Because apatite crystals are relatively stable under

weathering conditions, they have been studied as paleotemperature indicators in hydrocarbon resource 10

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 11 evaluation (Green/ Duddy, Gleadow, & Lovering [in press]); and the post-depositional, thermal evolution of sedimentary basins and orogenic belts (Lakatos & Miller, 1983) .

Fission tracking is based on the theory that an atomic parent isotope, 238 uranium ( 238 U), present in a

solid material such as an apatite crystal, is unstable and will decay spontaneously into a pair of fission fragments. Unlike other geochronologic studies which use the daughter products, fission-track dating is based on the crystal lattice distortions created during the

fission process. In a fission event, two positively charged nuclei are produced by the decay of 238 U and pass

through the solid (e.g. the apatite crystal) in which

they are encased. The nuclei recoil from each other in opposing d irectio n s forming a damaged zone in the so lid , while stripping electrons from the crystal lattice (Fleischer, Price, & Walker, 1965a). A damaged zone is left with positively charged ions that repel each other creating a linear trace within the crystal? this is called a fission track. This new track has a consistent

and characteristic cigar shape measuring only 10-20 micrometers long. Assuming that the amount of parent isotope in a

crystal remains constant from the time of its formation, the parent isotope will spontaneously decay at a constant

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 12 rate with time. It is the proportion or ratio of

o rig in a l p a re n t 23 U8 to the number of daughter tracks that allows a mineral such as apatite to be dated. This is a simple concept but there are many questions which must be answered in order to establish the reliability of fission-track geochronology. For example: (a) Is it correct to assume that a mineral with 23 8 U is formed during a geologic event and that at the closing of such an event the age of the mineral is zero? (b) Are there other isotopes present that undergo spontaneous fission creating tracks indistinguishable from those of 238 U? (c) What are the age limitations of a mineral to be dated by fission tracking? (d) After a

crystal has formed, has it been altered in any way physically or chemically to give a false parent-daughter r a tio ? (e) Has the re la tio n s h ip of p a re n t-iso to p e to daughter-track remained the same throughout time? (f) Do isotopes decay at a constant rate even under varied geologic conditions? (g) How accurately can the present laboratory techniques determine the parent-daughter ratio?

An attempt is made throughout the rest of this

chapter to address these questions.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 13 Effects of Other Fission Track Producing Isotopes

During the formation of a mineral such as apatite it 23 0 may incorporate trace quantities of within its

crystal lattice? at this time no fission tracks have yet been produced. In this report it is assumed that crystallization occurs at the end of a geologic event; when time is set to zero for those crystals formed during that event. Minerals generally contain other isotopes such as 235U and 232thorium (232Th). Each of these also has the capability to produce fission tracks identical to those of 233 U. The present known U/233U ratio of - 3 7.2527 x 10 for the vast majority of natural systems,

has helped to establish that the number of tracks produced by 23 5 U is negligible when compared to those

2 3 g 232 from U. In the same manner, fissio n tracks from

Th can also be considered as negligible the effects of 23^U and 232Th w ill be ignored (Henderson, 1982).

In addition, 238 U is also known to decay by another process called alpha emission. The proportion of u 238 decay by alpha emission has been calculated and sub tracted from the total 233U; the remaining 233U is

assumed to decay by fissio n .

Physical and Age Limitations of Apatite Crystals

Fission tracks are distinguished visually under the petrographic microscope. It is very important that

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14 crystals have a sufficient number of tracks to determine the parent-daughter ratio. The track density, the number

of tracks per unit surface area, is a function of the age of the material and its uranium content. If too few

tracks are present as in a rock with uranium-poor apatite or in a very young rock, fission tracking may be impossible. The same is true if the tracks are too dense and can not be visually distinguished from each other. This might be the case of a very old Precambrian rock or a rock with uranium-enriched apatites. Fission-track

dating has been employed for rocks as old as 20 x 109 years old.

The chosen apatite crystals must also have very few

fractures, dislocations or inclusions which may obstruct or distort tracks and make it difficult to achieve an accurate parent-daughter ratio. The physical and age limitations of the apatites in this fission track analysis did not allow several of the samples collected to be dated.

Changes After C rystal Formation

Information regarding the post-crystallization

history of a rock is generally wanting. It has been assumed, therefore, that after a given geologic event the crystals formed have not undergone any chemical or physical alteration resulting in a gain or loss of 23fl

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 15 which would change the parent-daughter relatio n sh ip . The crystals are assumed to have remained in a closed system after formation; the quantity of uranium is considered to be unchanged except upon fissio n and alpha emission. It is also assumed that the fission tracks have not been altered or destroyed.

Laboratory Determinations of the Parent-Isotope/Daughter- Track Ratio

In order to achieve an accurate parent-daughter ra tio , many steps must be properly performed to elim inate erroneous re s u lts (Gleadow, 1981). Some common problems stem from c areless track counting; the presence of track like defects in the crystals; inhomogeneous uranium

distributions; incomplete etching of tracks; mismatched mica and sample images; disconcordant ages among grains; varied neutron fluence (flux) among grains; and, different degrees of contact between the mica-sample pairs (Green, 1981). Many laboratory checks were incorporated into this study in order to reduce these kinds of errors. These checks include the dating of a sample previously dated by another technique; including

standard samples whose ages are well known; including standards of known uranium co n cen tratio n ; and, having

split samples analyzed separately by two analysts.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 16 Age Equation and Zeta C alibration Factor

The a p a t it e c ry s ta ls are mounted and the c ry sta ls are exposed by polishing. The mounts are then etched so the tracks can be counted and the samples and standards are then sent to be irradiated. After that, the tracks are counted under the microscope and eventually, the results are substituted into an age equation. In this study, age dating was determined using the

Zeta Age Equation (Equation 1) described in Hurford and Green (1981a; 1981b):

T unknown = ____ 1 (1+ lambda-d . Zwm . g . Pd . Ps) (1) lamda-d Pi where:

g Geometry facto r (0.5 for external detector method) 235U/ 238U I = Natural isotopic ratio, / (7,2527 x 10~3) 238 lambda-d = Total decay constant for U (1.55125 x 10"10 1/a)

Pd Measured induced track density for a glass dosimeter of known uranium

concentration and isotopic ratio

Pi Measured induced track density for a sample

Ps Measured spontaneous track density for a sample

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 17

Q = Measured thermal-neutron fluence T std = Age of an age-standard T unknown = Age of an unknown sample Z = Individual Zeta Calibration Factor

Zwm = Weighted Mean Zeta Calibration Factor R = Correlation coefficient

This equation was chosen over the traditional age equation because it eliminates the necessity of using 238 two, poorly determined parameters; the U fission decay constant and the thermal neutron fluence (Hurford & Green, 1981a; 1981b). This type of age determ ination is most dependant on

Z, the Zeta Calibration Factor. Each analyst determines his own Zeta Factor to correct for his own ability to reproduce ages of known standards (Equation 2). Z = (exp (lambda-d . Tstd) - 1) (2) Lambda-d . [Ps]STD . g . Pd [Pi]STD (Elaboration of these equations can be found in Hurford & Green, 1983; Fleischer & Price, 1964; Roberts, Gold, & Armani, 1968; Wagner, Reimer, Carpenter, Paul, Van der Linden, & G ijbels 1975; Gentner, 1972). The Zeta Calibration Factor is dependant on the use

of a specific method of fission track analysis called the External Detector Method. This involves the exposure of the sample crystals to thermal neutrons in a nuclear

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 18 lie reactor. As a result of neutron bombardment, u po c fissions to produce a new set of U tracks from the sample. Mica placed adjacent to the sample acts as a recorder or external detector of these induced tracks.

After irradiation, the mica is etched and the induced tracks are used to determine the present concentration of 338U (the parent) by utilizing the 235y^238u ratio, I,

assumed to be constant for most natural systems. The advantage of this method is that the fossil tracks are counted on the sample grains representing the daughter fraction and the induced tracks are counted on

the mica detector for determining the amount of parent

isotope. The result is the present parent-daughter 238 ratio, (concentration of U/concentration spontaneous tra c k s) . Another advantage of th is method is th a t i t enables ages to be determined individually on each grain within the mount. (For further discussion of the External Detector Method see Friedlander, Kennedy, Macias, & Miller, 1981; Green, 1981, 1984; Gleadow, 1981).

When "known age sta n d ard s" go through the same

irradiation process as the samples, their ratio of spontaneous to induced tracks, (Ps/Pi) STD, can be used in the Zeta Factor Equation. A glass standard (or dosimeter) also irradiated along with the samples serves to supply a material of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 19 "known uranium concentration". This is used to measure the induced track density, Pd, necessary to solve the Zeta Factor Equation (Hurford & Green, 1981, 1983). Substitution of the parameters, (Ps/Pi) STD and Pd,

into Equation 2 yields a single Zeta Calibration Factor. Repeating the whole process with many different 'known standard' allows the analyst to produce a personal Weighted Mean Zeta Factor, Zwm, to correct for his ability to reproduce the known age of a standard. After evaluation of the Weighted Mean Zeta, the unknown ages of samples may be determined by substitution of the measured

track densities Ps, Pi, Pd, in Equation 1.

Geothermometry Effects of Heat and Time on Resultant Ages

Fission track analysis is also a relatively new method of geothermometry, which takes advantage of the fact that fission tracks respond to prevailing temperatures. Once the fission-track age has been ascertained for a given rock, it can be evaluated further by geothermetric techniques. Information about the thermal history of a vertical suite of samples is unique to

fission-track dating. It is possible to date samples on

a mountain from its lowest to highest elevations; or through the depths of a deep core and learn about the thermal or burial history.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20 Fission tracks are metastable, meaning they are gradually annealed under certain combinations of heat and

time (Fleischer e t a l . , 1965a? Gleadown & Duddy, 1980). It has been discovered that fission tracks in apatite

will remain stable for extended periods of time, if once cooled (solidified below 125°C and never subjected to tem p eratu res g reater than 70°C (Dodson, 1973). If the crystals are heated above 70°C, tracks will begin to disappear. Apparently, the ions that were displaced during track formation gradually diffuse back into the

track and seal it. In fission-track geothermometry the

term annealing is used for this partial to complete erasure of tracks. How fast and how much they fade depends not only on temperature but the amount of exposure time at a given temperature. In such cases, where loss of tracks has occurred,

the fission-track clock will not date the original formation of the crystal but some later stage of its thermal history. As temperature increases the tracks begin to fade and eventually disappear, effectively resetting the age.

As determined experimentally, the resultant age from the Zeta Age Equation will be accepted as the time before

present when the given sample is cooled through the 110°C

isotherm for the last time in its history. If, however, the movement of isotherms is large such as in a period of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 21

rapid uplift and erosion, the apatite ages from depths above the annealing zone may actually record the age of

their formation while those below the isotherm record a later thermal event. It is, therefore, desirable to sample vertically as well as horizontally. Above 125°C fission tracks will be completely annealed in apatites

such that no tracks remain and the age of a sample is reset to zero. Prom that time on any tracks formed will result in the age of the event that annealed the previous tracks but will not indicate anything about the time of

rock formation (Naeser & Paul, 1969; Fleischer, Price,

Walker, 1975; Faure, 1977; Gleadow & Duddy, 1981; Harrison, 1984; Green, 1985; Gleadow, Duddy, & Lovering, 1983) .

Apatite is the most sensitive fission tracking mineral to temperatures that normally occur in the upper several kilometers of the earth's crust. Other minerals, however, can be used for fission tracking and can aid the understanding of the thermal history when used in addition to apatites. Minerals such as sphene and zircon are also valuable because of their ability to resist

annealing at temperatures greater than apatite. Dating several minerals by sampling over a large horizontal distance can reveal important thermal information about the site. If apatite and sphene were present in a granite that had quickly cooled but was

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 22 later intruded by a basaltic sill, their fission-track ages might reveal the following. Close to the granite- basal t contact the ages of both minerals were reset by heat from the intrusion to the age of the intrusion. A distance away from the intrusion, both minerals revealed the older age of the cooled granite showing they were not

affected by the intrusion. In between these areas, the ages of the two minerals would be d iffe re n t. A patite, due to i ts lower annealing tolerance, would have i ts age reset to that of the intrusion, but the age of the sphene would remain unchanged by the heat, giving the age of the

granite. As in this example, it is possible to estimate the effect of temperatures in a thermal event. (For further information see Fleischer & Price, 1964b; Green, 1985). Unfortunately, this project could not take into consideration the vertical sampling technique or the use of additional minerals so that the age and thermal history of the samples could be further evaluated. There is another aspect to track fading, however, that has given some insight into the thermal history of the rocks studied in this project. This is the degree of track fading which occurs between 70 and 125°C and

results in unique fission-track length distributions (Green, 1985) .

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 23 Fission-Track Length Distributions

Important information about thermal histories can be obtained from different track lengths within a crystal. Generally, all tracks are created with the same length even though they may have been formed at different times. Track length distributions are controlled by and repre

sentative of the cooling or thermal history of a sample; this includes the effects of time as well as temperature. Between the tem peratures of 70 and 125°C, fissio n tracks in apatites are metastable and subject to partial annealing. Therefore, if a given proportion of tracks in a crystal are distinctly shorter than the rest of the tracks (having been partially annealed), that proportion

has been influenced by a thermal event before the other tracks were formed. The degree of shortening reflects the range of thermal conditions that the shorter tracks experienced before the longer tracks were created. Regardless of the amount of shortening, the faster a crystal is cooled through its metastable zone, the more uniform the track lengths will be. The slower a crystal is cooled, the more divergent its length distribution becomes. Five types of track length distributions have been observed in c ry sta ls which are highly d istin c tiv e of their thermal histories (see Figure 1):

1. All the tracks are freshly induced by irradiation and are of maximum lengths.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 24 2. All the tracks naturally have a uniform length exhibiting <20% shortening; e.g., undisturbed volcanics

and related rocks which have cooled very rapidly and have never been significantly heated above 50°C.

3. All the tracks have been considerately shortened; e.g., undisturbed basement rocks which have

not been heated above the 125°C isotherm since emplacement. 4. The tracks have a bimodal length distribution; e.g., a two stage history with alternate high and low

temperature phases. 5. The tracks are of many mixed length distri

butions; e.g., the effects of continuous production of tracks through time.

W Case Cl Case 12 Case 13 Case M C a se <5 « 5 0 -i INDUCED UNDISTURBED ‘UNDISTURBED BIMODAL MIXED £ h 40- VOLCANIC BASEMENT l 30- fl D 20- u 10- rn I ! i z im ln n lm tl ILLU n n li 1111 iTm! til mi 1111nil 11IIMI imtmtlmilhnl D 0 5 10 12 20 0 5 10 15 20 0 5 19 15 20 0 5 10 12 29 0 5 10 152: Z

TRACK LENGTH (microns)

Figure 1. Fission-Track Length Distributions: Five Cases Observed. Source: Gleadow, A. J. W., Duddy, I. R., Green, P. F ., Laslett, G. M. & Lovering J. F. (In Press). Confined fission-track lengths in apatite: A diagnostic tool for thermal history analysis, s u b m itte d T o Earth and Planetary Science L e tte rs.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 25 Because of the sensitivity of apatite to moderately elevated temperatures, its fission-track ages rarely reflect the actual age of host rock formation. An exception to this, as determined by the track length distributions, is the case of 2, rapidly cooled volcanics or epizonal intrusives which have not been buried deeply

(< 1 km) . In this case only, the length distributions seem to be independent of ages; characterizing only the thermal history (Gleadow et al., [in press]). In order to determine whether the fission-track ages reflect the age of host rock formation, it is recommended that any

age determination be complimented by a track length distribution study.

There is still much to be learned about fission-

track length distributions because the concept is relatively new. There are problems which have not been

solved; such as unavoidable counting bias in measuring the lengths and anisotropic annealing of tracks (Laslett, Gleadow & Duddy 1983; Green, 1981). Until th is topic is more thoroughly studied, its implications will not be completely understood.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. C H A PTER I V

ANALYTICAL METHODS AND RESULTS

Sampling Techniques

For a study such as this to be of maximum utility, it is important that the study area be sampled properly and that excellent laboratory techniques be maintained. These factors are extremely important in order to achieve representative samples for petrologic analysis.

In southwestern Montana, surrounding the eastern and

southern margins of the Boulder batholith, horizontal

sampling was employed to more fully understand the time

frame of the igneous and structural evolution of the area. Vertical sampling at each locality, however, was not undertaken and therefore the regional thermal evolution could not be fully addressed. This study did, however,

successfully produce fission-track ages as well as geothermal histories for several of the igneous bodies sampled and each has been analyzed in its respective structural context.

Samples taken across the study area came from fourteen distinct igneous bodies: one from the main

igneous complex called the Butte Quartz Monzonite; and,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 27 fourteen from satellite plutons, sills and dikes (Plate 1) . Some samples could not be utilized for this study. Some of them had too few ap atites to date, whereas others had an abundance of apatites but too many dislocations or

inclusions to make them useful for dating (Refer to Table 2 for explanations).

Table 2 L ist of the Samples Not Dated

Sample Name Reasons sample not dated

Gold Hill s i l l Too many d is lo c a tio n s ; too few tracks Townsend pluton Too many d is lo c a tio n s ; too few tracks Lone Mountain stock Poor contact between mica and slide due to pinholes

North Boulder complex Too many d is lo c a tio n s ; too few tracks; highly zoned uranium Pipestone Pass pluton Poor mounts; sparse a p atite Cottonwood Canyon s i l l Too few tracks to align mica with slide Buton Park pluton Sparse apatite

Mt. Dohtery complex Sparse apatite

Methods of Fission-Track Dating

The first procedure in the fission-track dating is to obtain apatite separates, 90-300 urn in size, from

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 28 crushed, pulverized, sieved and washed sample rock. Two

heavy organic liquids, diodomethane and tetrabromoethane, in conjunction with a Franz Electromagnetic Separator

were employed to separate a p atite s from other minerals by using density differences and magnetic properties. The a p atite grains were then mounted on 27 x 46 mm slid es and

polished to a smooth and flat finish with a 0.3 micron Aluminum Oxide slurry to expose the internal portions of the grains. These surfaces were then etched in 5M Nitric Acid for five to ten seconds at room temperature to

reveal the fission tracks within the apatites.

The crystal mounts were trimmed to 10 x 15 mm,

covered with a low-uranium mica detector of the same size, and held together tightly in a small bag made of heat-shrink plastic. Individual slides were strategically stacked and packaged for irradiation. Uranium-containing glass dosimeters (SRM612) from the U.S. National Bureau of Standards were placed at the top and bottom of the stack to test for a non-uniform radiation dose. Two other tectonically unrelated but well dated age standards from Durango, Mexico and Renfrew, Canada were placed among the Montana slid e s.

The stack was wrapped in aluminum foil and

irradiated at the Phoenix Memorial Reactor at the University of Michigan in Ann Arbor, Michigan. The wrapped slid es were placed in pneumatic tubes and placed

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. within a few inches from the core to obtain the maximum

bombardment of thermal neutrons (2xl013

neutrons/cm^/second) . The samples were then sent to Rensselaer Polytechnic Institute (RPI) for acid etching and microscopic analysis.

After irradiation, all slides and their compli mentary mica detectors were carefully separated (see Figure 2) .

MICA DETECTOR

EPOXY

APATITE CRYSTAL Before Radiation

After Radiation

Figure 2. Sample-Mica Pairs; Before and After Radiation.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The micas were then etched in 39 percent aqueous hydro fluoric acid for about 18-20.5 minutes at room temperature. The mica, acting as a recorder (detector),

revealed the fission tracks induced by irradiation of the uranium in the apatite. In this manner a comparison can be made between the spontaneous fission tracks revealed on the sample mounts and the induced fission tracks recorded on the complimentary mica detector. The slide and mica pairs were then remounted on a standard petrographic slide, 27 x 46 mm, as mirror images and analyzed petrographically (see Figure 3).

Eyepiece Grid

Apatite on sample mount Tracks on mica detector

Figure 3. Petrographic View of Sample-Mica Pairs.

For complete details on the methods used in this

fissio n -tra c k study re fe r to Gleadow, 1984.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 31

Results; Relative Thermal-Neutron Fluence Measurements

Induced track density for the glass dosimeter standards (Pd) were counted at lOOOx under dry conditions on a Leitz Ortholux TM microscope. A 5 x 5 ocular grind was produced for counting tracks by calibrating the eyepiece to a 600 lines/mm diffraction grating. Fission tracks were counted on the glass standards using a combination of transmitted and reflected light. The re su lts are summarized on Table 3.

It was apparent from the results that the irradiated

package was not uniformly irradiated. The density of

tracks (Pd) in tracks/cm^ is given for the glass standards at both the top and bottom of the irradiated package. A 4.2% flux per cm of package height had been induced andthis thermal-neutron fluence (Q) was cor rected for in the analysis of the samples (see Figure 4). Q values were adjusted accordingly for fission-track age determinations. Table 3 lists the corrected values assigned to various ample positions in the irradiated package.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. N i CO 8353 8013 8013 8013 8353 8013 8013 8353 8013 8353 8013 8013 8353 8353 8353 8013 8353 10E6(Pd) 4008 10E6(Pd) 4005 10E6 10E6 10E6 10E6 10E6 8353 10E6 10E6 10E6 10E6 10E6 10E6 10E6 10E6 10E6 10E6 10E6 10E6 10E6 X X X X X X X X X X X X X X X X X X X X 1 .3 9 8 1 .3 5 0 1 .3 4 0 1 .3 9 7 1 .3 6 9 1 .3 8 0 1 .3 7 3 1 .3 8 3 4 .3 3 0 1 .3 6 3 4 .4 3 0 4 .4 6 0 4 .4 6 0 63 4 .2 9 0 962520 1 .3 5 3 4 .4 1 0 472 4 .3 8 0 263 4 .3 1 0 379 379 310 2671 10E6 10E6 10E6 10E6 10E6 10E6 665 4.310 10E6 10E6 761 10E6 460 10E5 118 10E5 665 10E6 1280 10E6 10E6 504 1.347 10E6 10E6 739 10E5 92 10E6 X X X X X X X X X X X X X X X X X X 2 .0 8 1 .1 5 1 .5 6 2 .1 7 3 .2 8 3 .2 8 6 .5 3 29 6 .1 4 115 2 .8 1 127 286 110 1 .2 1 421 3 .0 1 133 1 .2 3 10E5 10E5 Ps 10E5 Pi 25 2 .0 4 Q 10E5 10E6 864 4.56 10E5 10E6 152 6.48 10E6 102 5.95 10E5 146 10E5 75 10E5 10E6 621 3.86 10E5 313 7.56 10E5 105 10E5 10E5 10E5 58 10E5 161 X X X X X X X X X X X X X X X X X X track density track density fluenmce 1 .4 8 1 .4 8 1 .6 0 6 .2 2 1 .9 3 3 .1 5 9 .3 6 8 .8 3 4 .2 8 3 .5 6 3 .2 1 8 .1 1 Spontaneous-fission Induced-fission Thermal neutron T a b le 3 Sample 3 .6 6 Slide Tracks/cm2 Tracks- Tracks/cm2 Tracks” Neutrons/cm2 Tracks- Data for Fission-Track Age Analysis Neutron Dosimeter Neutron Dosimeter Apatite Sample Apatite Sample 6.19 Apatite Standard Apatite Sample Apatite Sample 9.90 A p atit e Apatite Sample Apatite Sample 3.56 Apatite SampleApatite Standard 8.36 2.22 Sample Code SRM612-I SRM612-II M01-15 / R M01-6 / 10-1 M01-9 / 62 M01-14 / H-l M01-5 / K M01-4 / 10-B M01-16 / L-2 M01-1 / B2 Sample Name 10-N Pluton Rader Creek Pluton Rattlesnake Butte Pluton M01-11 / T-l Brownback Diamondhead S ill S or ill Butte Quartz MonzoniteRenfrew, Canada Hell M01-8 Canyon/ Pluton 4 Rader Creek Pluton Durango, Mexico Lone Mountain Stock

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8 9 .6 50 50 21 15 30 4 0 .9 51 4 7 .4 2 6 .5 50 15 38 5 7 .5 78 Ppm Uranium — — 0 .7 0 3 8 0 .7 8 8 0 0 .9 8 8 0 0 .4 1 3 00 .7 3 8 3 1 8 .5 9 .8 0 .9 7 7 5 0 .9 1 4 5 0 .8 4 4 1 0 .9 0 8 30 .9 4 76 3 0 0 .8 2 0 80 .8 1 6 0 1 4 .8 0 .9 4 6 5 0 .5 5 4 9 — — Passed 0.9168 P assed F a ile d P assed P assed P assed 73 Passed 0.7969 85 P assed 96 90 Failed50 0.3531 31 P assed 93 # G a lb r a i th 's ti o n or tes£ results cient ...— 306 225 Passed 0.8874 428 P a sse d 182125 P assed Fields (Green 1981) R Grains Chi Squared Coeffi- ———106 — Myr 7 7 .9 9 6 .4 92.9 39 Passed 8 4 .1 8 7 .6 8 6 .69 2 .7 214 T T + 2s 150.2 15 Passed 1 2 5 .4 1 1 6 .6 — — 5 3 .8 6 6 .9 56.0 84.8 70 Passed 9 4 .6 4 6 .6 5 9 .1 3 8 .8 4 9 .7 7 2 .1 66.95 5 84.5 .2 2875 4 .1 5 9 .8 7 7 .4 1 5 0 .6 6 1 .9 7 3 .0 1 1 5 .8 — 1 5 .4 21.6 173.4 216.6 1 1 .2 1 9 .4 1 5 .4 1 3 .4 Age* Age Myr Myr Myr T rack — Fission Correla- —— 7 0 .4 * 1 4 .4 31.2 6.8 24.4 38.0 177 75.7 Mean 8.8 8 1 .6 * 3 5 .0 6 7 .6 Mean 2 8 .8 6 0 .9 * 7 7 .3 * 9 4 .4 * 2 1 .4 7 1 .4 * 1 6 .2 Pooled Age +/- 2s T - 2s 1 0 2 .0 * 4 8 .2 T T unknown 1 9 5 .0 Sample 88.6 17.4 71.2 Sample 75.5 8.6 S ta n d a rd Sample 110.0 SampleSample 68.5 9.4 Sample 114.0 36.6 Sample 73.5* Sample 7 3 .2 S lid e Neutron Dosimeter A p a tit e Apatite Standard A p a tite A p a tite A p a tite A p a tite A p a tite A p a tit e A p a tite A p a tite SRM612-1 N eu tro n D o sim eter SRM12—II M01-14 / H-l M01-6 / 10-1 M01-15 / R M01-4 / 10-B M01-16 / L-2 Sample Name Sample Code Number of tracks counted to determine the reported track density. 10-N Pluton Brownback Sill or M01-1 / B-2 Butte Quartz Monzonite M01-8 / 4 Durango, Mexico M01-5 /K Diamondhead S ill Renfrew, Canada M01-9 / 62 Hell Canyon Pluton Table 3—Continued Rader Creek Pluton Rader Creek Pluton Rattlesnake Butte Pluton M01-11 / T-l Lone Mountain Stock * R.P.I. analysisby R.A. Donelick; CN1 dosimeter; Zeta = 120. A A R.P.I. analysisby D.S. M iller;dosimeter; CN1 Zeta = 120.6+/- 1.3. Ppm Parts per million. w Myr Million years. w

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0 1 2 Pd - Bottom Dosimeter (106 tracks/cm2)

Figure 4. Determination of Gradient on Irradiation Cylinder

R esults; Individual and Weighted Mean Zeta C alibration Factor

All individual zetas and track counts were made at 1600X under dry conditions on a Leitz Ortholux TM microscope. Tracks were counted using both transmitted and reflected light.

From the individually calculated zetas a Weighted

Mean Zeta Factor, Zwm, of 342.26 +/- 9.98 was obtained.

This corresponds to a 2.92% error, from the theoretical

ideal of 350. The personal Zwm of the investigator was used in calculating the apatite fission-track ages in Table 3 (Hurford & Green, 1981, 1983).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 35 Results: Measurements of Uncertainty

All zeta and age determinations were calculated using the statistical techniques as developed by Green

(1981). See Hurford and Gleadow, 1977; Hurford and Green 1981a, 1981b, 1982, 1983 for additional information on other fission-track parameters.

Methods of Fission-Track Length D istributions

The length of individual fission tracks was measured on the same sample slides used in the dating process. A

Zeiss, MOP-3 TM, digitizing pad and a tracing tube

affixed to a Leitz 12 TM microscope allowed the simultaneous observation and measurement of confined

tracks (completely enclosed) at 1250X under dry conditions.

Results: Fission-Track Length Distributions

Annealed standards contain apatite crystals with completely annealed (healed) fission tracks. After being annealed, spontaneous fission will occur from the apatite's uranium and produce new tracks. Fresh tracks

are a maximum length to which other samples with

partially annealed tracks (shorter in length) can be compared.

Two annealed standards as well as four other commonly studied standards were compared with the Montana

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 36

fission-track length distributions (FTLD's). Because of th e d iffe re n t methods employed in these length measure ments as opposed to fission-track age determinations, two Montana samples were used in the track length study which

were not dated. These two exceptions are samples from the Gold H ill s i l l and the Townsend pluton.

The o r ie n ta tio n of the mounted c ry sta ls as viewed under the microscope is an important factor in track- length measurements. According to Gleadown et al, (in

press) and Donelick (1986), distributions measured parallel to the c-axis exhibit the longest and most

consistent average track lengths within a crystal. Because of the anisotropy associated with track lengths at various crystal1ographic orientations, only those crystals viewed with tracks roughly parallel to the c- axis were studied. The resultant data for fission-track length

distributions is shown on Table 4.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. u> 40 70 79 101 100 112 tracks counted Number Number of UM 2.798 101 2.239 13 2.770 3.052 3.539 Kurtosis of tracks UM . UM -0.387 3.636 +0.127+0.415 3.080 3.610 100 +0.194 +0.279 3.587 104 Skewness -0.075 2.179 -0.347 -0.014 -0.261 of tracks -0.535 4.343 102 -0.105 1.01 1.17 0.86 0.70 0.91 0.75 0.85 1.11 0.91 Average +/- Is UM Track Length Table 4 UM 15.52 14.63 14.82 15.02 14.65 16.49 14.56 0.92 +0.20316.47 3.127 101 14.45 14.62 Average Samples and Induced-Track Standards Track Length Fission-Track Length Distributions for (induced-tracks) (induced-tracks) 10-N 10-N Pluton Sample Sample Name Brownback S ill Diamondhead or S ill Butte Quartz Monzonite Gold H ill S ill Townsend Pluton or 14.35 1.15 Hell Canyon Pluton Rader Creek Pluton (R) 14.47 0.81 Rader Creek Pluton (10-B) Rattlesnake Butte Pluton Annealed Renfrew (8462-1) Long Long Mountain Stock Annealed Durango (8122-3)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CH A PTER V

DISCUSSION OF RESULTS

Fission-Track Length Distributions

As discussed earlier, the Montana samples were compared to freshly induced fission tracks of natural apatites. Results from Table 4 show that tracks that are induced have a narrow and symmetrical track length distribution; mean length = 16.5 urn and standard

deviation = +/- 0.9 urn. This is in good agreement with

th a t measured by Gleadow e t. a l ., (in press) where the

mean length = 16.3 urn and standard deviation = +/- 0.9 urn.

The Montana fission-track length distributions are a ll very sim ilar to those described by Gleadow e t. a l ., (in press) as volcanic and related rocks which have cooled very rapidly, never having been significantly reheated above 50°C (Figure 5). S t a t i s t i c a l a n a ly sis of the Montana FTLD's show a

mean length of 14.4 urn and a standard deviation, +/- 0.9 urn. The FTLD's are narrow, unimodel and symmetrical but

the mean length is up to 21% shorter than that for induced tracks from annealed standards (Figure 6,

statistical data on FTLD's). Twenty percent reduction in 38

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. OJ VO II §5 t t 15 20 MIXED 3 IlMllllllllllll 3 10 13 20 GOLD HILLGOLD lltH tL rfltlkltl Case TOYiNSEND-2 0 5 10 13 20 I (in press) u i III BIMODAL Case #4 11111111 HELL HELL CANYON' RATTLESNAKE im lnnltntU -m l 0 3 10 13 20 0 S 10 13 20 0 0 3 10 13 eo ) l l l l l l l u l l BASEMENT UNDISTURBED 1 0-N PLUTON i«ithnt4fMillitil mtlnniih u n i 0 5 10 13 20 0 5 10 15 20 0 3 10 13 20 . Case £3 RADAR RADAR CREEK 1 0 -B TRACK LENGTH LENGTH (microns)TRACK

et is 111 h 11 111 II hrfmnl i §2 10 LONE LONE UT. 3 VOLCANICS Case UNDISTURBED RADAR RADAR CREEK 1 111111 m 111111 I unlmihfiilhttl 0 3 10 13 29 • 5 10 15 2ft I Li B.- Montana Samples. A.- Five Cases Observed by Gleadow et al J i l ! l I u i i I Case £1 BROWNBACK SUTTE Q. M. m milim 1111 milim linllJIilrriiliV iil

INDUCED TRACKS INDUCED

0 5 11 IS 29

a S 13 15 20

0 3 13 13 23 - -

- - 10 50 n 50 40 30- 2 0 40- 50-j 10 40- 30- 2 0 30- Figure 5. Fission-Track Length Distributions. m u 20 ^ io H U. □ PQ O' u n 50 X 3 A. B.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. maximum amount determined before there I f. any measurable reduction in fission-track ages (Gleadow et. al., 1983). This, and the fact that the standard deviation is less

than one, lends confidence to the ages of the Montana samples.

AVERAGE TRACK LENGTH PER SAMPLE

1------1------1------1------1------1------r Anntaltd Standard 0122-0

ArtntaUd Standard 0462-1

LONE MOUNTAIN 1------10—N PLUTON

RATTLESNAKE BUTTE

TOWNSEND PLUTON

BUTTE QUARTZ MONZON1TE

RADAR CREEK PLUTON

HELL CANYON PLUTON

COLD HILL SILL

RADAR CREEK PLUTON 10-N

BRDVH BACK SILL

Figure 6. Truck Length Distributions for Annealed Standards) and Samples

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 41

The uniform ity among the Montana samples suggests a similar thermal relationship between them. It is, therefore, probable that all the samples experienced one and the same tectonic event in which the magma rose quickly to a very shallow depth and cooled rapidly. This is a significant finding in this study because it is only among the undisturbed volcanic-type FTLD's that the fission-track ages can be assumed to actually represent the age of rock formation and not some secondary thermal event (Gleadow e t. a l., in p ress). This idea gives credibility to the fission-track

ages obtained. It also implies that the ages will be concordant with those of other isotopic studies (Table 5).

One last point of interest, the FTLD study clearly shows the samples from the Boulder batholith, namely Rader Creek and Butte Quartz Monzonite, and the nearby Hell Canyon pluton have distributions positively skewed. Those fu rth er away from the b a th o lith , the samples; 10-N, Lone Mountain, Rattlesnake, Townsend, Brownback and Gold Hill; are negatively skewed. Although the implication is unclear, the mean length of the fission tracks appear to

be slightly less in samples farthest away from the Boulder batholith.

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

____

6 0 7 3

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Hyr homblendo=74.7 ♦ /- 3.0 C*23 Oatos Nearby K-ft- hornblende=73.9 ♦ /- 3.0 CX43 biotite=73.5 2.9 CX23 Sampled area: biotite=72.8 4 /- 3.6 C*23 East of sampled area: biotite=73.9 4 /- 3.7 C*23 Near sampled area: biot. / hnbd.=?6.9 4/-2.3 C*23 Sampled aroa:biotite=72.9,Host: biot-=72.7General pluton: ♦/- hom61ondos71.2; Sampled 73.2 area:3.0, ♦/- biotite=70.0 */-3 hbld.=76.5 General 1.0 .6 pluton: ♦ C*33 4 /- 72.0 3.54 2.4 /- 2.7 Cx23 CX23 C*2J Tobacco Root Batholith: biotite=7S 0*3 6eneral pluton: 75.0 4/- 3.3 C*23 Elkhorn HI. Volcanics north of pluton: Tobacco Root Batholith;Sand Crook S zircon=88 ill: CPb-alpha) biotito=106 CX51 hyr 115.8 125.4 116.6 150.2 9 6 .4 8 4 .5 7 7 .9 9 2 .7 8 7 .6 8 4 .8 8 6 .6 r ♦ 2s

Hyr 3 8 .8 5 9 .1 6 1 .9 5 9 .8 7 3 .0 4 9 .7 7 2 .1 9 4 .6 4 6 .6 7 1 .25 3 .8 106 f - 2s

8 .8 6 6 .9 8.6 66.99 .4 84.1 Hyr 1 5.4 19-4 54.1 92.9 1 3 .4 l i : 2 1 5 .4 1 7.4 2 8 .8 2 1 .4 36.6 77.4 150.6 3 5 .0 4 8 .2 Table 5 82 8 5 .0 6 6 .5 8 3 .2 77.6 14.4 S6.0 9 9 .1 7 9 .9 10S .1 1 1 7 .7 T 4 Is »✓- 2s

7 1 .3 8 0 .1 6 3 .8 7 3 .2 6 9 .6 5 3 .2 7 1 .2 7 9 .8 7 7 .9 1 2 6 .1 66.S 8 3 .7 9 5 .7 1 3 2 .3 5 5 .3 6 3 .8 6 4 .1 7 9 .9 9 7 -3 102.3 Hyr f - Is

4 .7 7 .7 0.1 63.3 79.5 16.2 55.2 9 .7 7 .2 6 3 .2 6 .7 5 .6 Hyr 7 .7 8 .7 14.4 1 0 .7 17 .5 ♦ / - Is 2 4 .1

Fission-Track Ages and K-Ar References X X X X Hean Rgo 4.4 K ** Hyr X unknown T Pooled Ago 6 8 .5 6 7 .6 Hear) Bge 7 5 .7 7 1 .4 7 7 .3 7 3 .5 7 0 .4 6 0 .9 9 4 .4 Fission Track 8 1 .6 Sample Kano C*43, NcHannisC*53, <19643JaffaC«63, GilottiE*73,and others Daugherty <19593<19663 and Vitaliano <1969) C1968) ; Zeta=120.2 4/- 2.1 Zota=120.6 ♦/- 1.3 C*23, ReportedC«33, Roportodby fillin by Knopf g <19643; , Klopper HcDouoll and <19663 Obradovich 10-N Pluton 110.0 Rader Crook Pluton Hell Canyon Pluton Rader Crook Pluton X, R .P .I. Anlysis btj R.R. nolick; Do CHI dosimeter Hyr, Million years , Dianondhead SillButt# Quartz Honzonite 102.0 75-5 4.3 Lone Mountain StockRattlesnake Butto Pluton 73.2 114.0 18.3 ", R .P.I. analysis by 0.S.H iller; CHI dosimeter; Brownback Sill or 68.6

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 43 Fission-Track Ages

Because of the volcanic-like FTLD's observed in the samples, the re su lta n t fissio n -trac k ages are assumed to

reflect the formation age of the plutons. The tight grouping of ages and the fact that all but one passed

Galbrith's chi squared test (Green, 1981) indicates the ages are reliable (Table 3) . Resultant ages of the selected plutons are given in

Table 5. They range in age from 68.5 to 114.0 Ma. These plutons are either part of the Boulder batholith or s a t e l li t e s of the b a th o lith , which were emplaced 69-78 Ma (Robinson, Klepper, Obradovich, 1968; Tilling, Klepper, Obradovich, 1968), 68-78 Ma (Hanna, 1973), 70-78 Ma.

(Knopf, 1964) with the majority being crystallized between 72-78 Ma. Measured ages are only significant within the range of two standard deviations. This means that an exact date of formation can not be produced with this method, but, the confidence in te rv a l in which the date lie s , can be.

Four controlling factors were interwoven in this

study, and, as discussed below, they each lend

credibility to the ages obtained. First, the analyst's personalized Weighted Mean Zeta Factor used to correct

for analytical errors, was 342. This was achieved after 23 analyses of age standards and is considered very good

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 44 when compared to the theoretical ideal of 350. Essent

ially it lends confidence to the petrographic results. Secondly, both age standards, Renfrew and Durango, irradiated with the Montana samples and analyzed with the Weighted Mean Zeta of 342 are in excellent agreement with

those dated at RPI. This indicates the Montana samples have not been processed in any significantly different manner than those of RPI's. Thirdly, RPI took seperates from the Montana samples

and packaged, irradiated and analyzed them independently. Their results are in agreement with this study except for

the age of 10-N pluton. The only noticeable difference is that the RPI ages have a much larger standard deviation because they are based on fewer grain counts. L astly, knowing th at Butte Quartz Monzonite has been previously dated several times by Potassium-Argon (K-Ar) Methods (Table 5) , i t was in te n tio n a lly placed among the samples and used as a standard or reference with which the samples could be compared. In conclusion, all of the Montana age analyses seem reliable except that of Rattlesnake Butte which is discussed in the following section.

Individual Pluton Ages

For sample rock types and their inferred relation ship to thrusting refer to Table 1. See Figure 7 for a fission-track age comparison among the Montana samples.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. poue wt pr sin fte oyih onr Frhrrpouto poiie ihu pr ission. perm without prohibited reproduction Further owner. copyright the of ission perm with eproduced R

iue . otn FsinTak Ages Fission-Track Montana 7. Figure Minion rears 40 50 130 140 150 60 70 100 110 120 160 80 90 ______£ LU =) in < O' LU o o LiJ o: O ______■ ISO TAK AGES: TRACK FISSION i m m n sadr deviation standard One w sadr deviation standard Two Proposed time of emplacement emplacement of time Proposed f obr shoot Lombard of I •Q O II CD* Si O 3 45 46

Brownback S ill

The Brownback sill is a uniform, silicic quartz monzonite that intruded into Cambrian rocks. It appears to be tectonically folded with the surrounding rock, but this is unproven (Schmidt, 1976; Smith, 1970). If it has

been folded, then it was emplaced before folding and thrusting, and therefore, before the Lombard thrust sheet was emplaced. This means the age of the Brownback sill probably provides a pre-thrusting age limitation. The fission-track age is 88.6 +/- 17.4 Ma (all dates

are given with 2 standard deviations) suggesting that

this sill was emplaced before or during Late Cretaceous deformation (68-78 Ma). The samples analyzed at RPI also confirm this.

10-N Pluton

The qu artz m onzonite sample taken from the 10-N

pluton (a composite pluton) extends beneath 10-N highway near Three Forks (Schmidt & O'Neill, 1982; Robinson, 1963) . Its close proximity to the leading edge of the

Lombard thrust sheet is interesting but inconclusive. The fission-track age for this rock is 110 +/- 15.4 Ma,

which implies that it was emplaced prior to deformation.

The age from RPI (81.6 +/- 35.0 Ma) , however, expands from pre-deformation' through the deformation period. The

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 47 fission-track ages of the 10-N pluton are therefore considered to be ambiguous. Butte Quartz Monzonite

The Butte Quartz Monzonite sample is part of the main magma series making up the majority of the Boulder batholith. Its emplacement has been dated 72-75 Ma by K- Ar studies (T illing et a l ., 1968; McDowell, 1966, 1971). Its relationship to the Lombard thrust is uncertain but

according to Knopf (1968), it is at least post-volcanic. This sample was chosen as a standard to which the Montana sample results could be compared. The fission-track age, 75.5 +/- 8.6 Ma, closely corresponds to the K-Ar age and is also supported by that of RPI. If this age is

reasonable then it means this sample was emplaced while deformation was taking place.

Hell Canyon Pluton

The Hell Canyon pluton is a homogeneous, leucocratic, quartz monzonite, derived from a late phase magma (Lambe, 1981). It was emplaced in to the Camp Creek fault which is the westward continuation of the Jefferson

Canyon fault zone and is the southwestern trace of the

Lombard thrust sheet (Schmidt & O'Neill, 1982). The Hell Canyon pluton, therefore, provides a post-Lombard age

limitation needed for this study.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 48

The fission-track age is 68.5 +/- 9.4 Ma. This date i s com parable w ith 70-74 Ma from McMannis (1963) and Tilling et al., (1968). It agrees with the magmatic information from Lambe (1981) as well as the fission- tra c k ages from RPI, 67.6 +/- 28.8 Ma, This pluton is apparently post-deformation.

Rader Creek Pluton

The Rader Creek pluton is made of granodiorite but

grades to quartz monzonite. It is compositionally

related to the pre-batholith volcanics and actually

contains mafic pipes within it. It is probably one of

the earliest, less felsic plutons to be emplaced (Lambe, 1981; Tilling et al., 1968). Because it is also intruded by the Butte Quartz Monzonite it is considered pre or syn Lombard in age. The Rader Creek pluton was dated once in this study and tw ice by RPI using rocks from d ifferen t locatio n s. All analyses were agreeable and confirmed the fission- track age of 75.7 +/- 8.8 Ma. Other isotopic studies gave ages th at lie between 74 and 75 Ma (Lambe, 1981) . This means the Rader Creek pluton was emplaced early in

the deform ation sequence and was most lik ely emplaced prior to or during thrusting.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 49 Lone Mountain Stock

The Lone Mountain stock, between Radarsburg and Three Forks, is a quartz monzonite with a fine grained equivalent around its margin, indicative of rapid cooling. It intrudes the pre-batholithic volcanics and is believed to intrude Paleozoic rocks which were previously subjected to the main phase of folding. This indicates they were probably emplaced sometime during or a fte r movement of the Lombard th ru st. The fission-track age is 73.2 + /- 13.4 Ma which is

backed by the analysis at RPI. There are no other studies with which to compare this date, but the data

suggests the stock was emplaced at some time during or

after deformation, perhaps while movement occurred along the Lombard thrust plane.

Rattlesnake Butte Pluton

The Elkhorn Mountain volcanics are intruded by

Rattlesnake Butte pluton, a compositional equivalent of the volcanics (Tilling, 1974). Its fine grained texture is a result of its emplacement at shallow depths followed by rapid cooling. It could be a feeder for the volcanics

but the evidence to confirm this is lacking. It is post- volcanic and most likely pre-deformation and pre

thrusting in age. There is no evidence that it has been folded or faulted but it was intruded into the post-

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50 v o lcan ic Radarsburg Syncline (Chadwick, 1971; Klepper, Weeks, Ruppel, 1957).

The fission-track date for this pluton is 114 + /- 18.3 Ma, (77.4-150.6 Ma) suggesting a pre-thrusting age, but certainly too general to base any conclusions. According to Chardwick (1971) the volcanics erupted from 84 to 74 Ma with the majority extruded at 78 Ma (Robinson et a l. , 1968). There are no other radiometric studies available to which this fission-track age can be compared. The fission-track age was difficult to achieve on this particular sample and will not be used to address

the age of the Lombard thrust.

Gold H ill S ill

The Gold Hill sill, a uniformly porphyritic grano- d io rite appears to be magmatically related to the Tobacco Root batholith dated at 75 Ma. The sill was emplaced concordantly along a plane of weakness within the Middle Proterozoic LaHood Formation. It was later folded along with the surrounding rocks into an anticline (Capozza, 1967; Burfield, 1967) . There is textural evidence that

it cooled slowly at great depth and then was quenched rapidly near the surface. It is pre-thrusting in age.

This pluton was too difficult to evaluate for age by fission-tracking methods but the track length distri butions confirm its cooling history and relationship to the plutons sampled.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 51 Townsend Pluton

Like R a ttle sn a k e B u tte, the Townsend pluton is a compositional equivalent of the volcanics (Tilling,

1974) . It was emplaced as a sill in Paleozoic rocks before the culmination of Late Cretaceous - Early Paleocene folding and faulting. It has been faulted and is assumed to be pre-thrusting in age. Ja ffe, G ottfried, Waring, Worthing (1959) have dated i t to be 72-76 Ma. As with Gold Hill, sill, the Townsend pluton was

much too difficult to date by fission tracking. The

track length distributions do, however, infer that this sample is thermally related to the other plutons and was brought to a shallow depth and cooled quickly.

Other Plutons

Fission-track age dating was attempted on several

other plutons. Unfortunately, for the various reasons given in Table 3, they were eliminated throughout the process.

Determination of the Timing of Movement Along the Lombard Thrust

As discussed earlier, by obtaining the age of

several plutons with known relationships to the Lombard thrust, it is possible to narrow the time range in which movement occurred along its fault plane. Constraints on

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 52 the timing of movement were made by plutons known to be pre or post thrusting (deformation). Timing for movement

along the Lombard thrust plane was bracketed between the pre-thrusting and post-thrusting plutons, and, by averaging the ages of the plutons thought to be intruded while thrusting took place. The timing of movement was

determined to be 73.2 +/- 6.7 Ma or 66.5 - 79.9 Ma. This indicates that thrusting along the Lombard thrust plane occurred approximately the same time the Boulder batholith was being intruded (68-78 Ma).

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

SUMMARY

The objectives of this study have been met. Several plutons in and adjacent to the Lombard thrust have been

dated by fission track analysis and the timing of movement along the Lombard thrust plane has been narrowed using the resultant ages. The resultant FTLD's of the Montana samples indicate

that all the samples are thermally related and fit the "undisturbed volcanic-type" rocks as discussed in Gleadow et al., (in press). This specific type of FTLD implies

these igneous bodies ascended quickly to a shallow depth of the earth's crust and cooled rapidly. Statistical treatment of the data indicates that the results are reliable. The fact that the Montana samples were determined to be the "undisturbed volcanic-type" means their fission-track ages probably represent the age of rock formation and not some later thermal event. One

point of interest in the FTLD study shows the samples

from the Bloulder batholith have positively skewed distribtuions while those further away from the batholith are negatively skewed. The mean length of the fission

tracks also appears to be slightly less in samples farthest away from the Boulder batholith. 53

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Results of the fission-track dating analyses correlate well with those of other earlier radiometric s tu d ie s . The ages of the Montana p lu to n s range from Early Cretaceous to middle Paleocene time; fiv e of which

extend from Late Cretaceous to Early Paleocene and two of which are dated from Early to Late Cretaceous (Figure 6).

Clearly, the relationships of the ages to the structural features of the plutons delineate a time frame in which movement along the Lombard thrust sheet took place. The data indicate the Lombard thrust was emplaced 73.2 + /- 6.7 Ma (66.5-79.9 Ma).

The similar timing of thrusting and the

crystallization of the Butte .Quartz Monzonite (68-78 Ma) suggests a close association between the two. Apparently some plutonic activity preceded thrusting but also con tinued after thrusting as proposed by Schmidt and O'Neill (1982). The r e s u l ts of th is study exemplify the manner in

which tectonic features, like the Lombard thrust fault c a n be dated indirectly using fission-track geochronology. In addition to this the fission-track method has the a b ility to give some additional

information about the thermal history of the samples which other dating processes do not.

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

Burfield, W. J. (1967). A gravity investigation of the Tobacco Root Mountains/ Jefferson Basin/ Boulder batholith/ and adjacent areas of southwestern Montana. Unpublished master's thesis, Indiana University, Bloomington, IN. Capozza, F. C. (1967). The geology of the Bone Basin and Perry Canyon in tru sio n s, Madison County, Montana. Unpublished master's thesis, Indiana University, Bloomington, IN. Chadwick, R. A. (1971). Volcanism in Montana. Northwest Geology, 3^, 1-20. Daniel, B, Daniel, F., & Richard, B. (1981). Radiometric dates of rocks in Montana. Montana Bureau of Mines and Geology, a Department of Montana College of Mineral Science and Technology. Bulletin 114, pp. 1-136. Dodson, H. M. (1973). Closure temperature in cooling geochronologic and petrologic systems. Contributions to Mineralogy and Petrology, 40, 259-274. Dokka, R. K. & Mahaffie, M. J. (1985). Significance of concordant fission track ages from continental basement terrains. Baton Rouge: Department of Geology at Louisiana State University, pp. 1-36. Donelick, R. A. (1986). Mesozoic-Cenozoic thermal evolution of the Atlin Terrane, Whitehorse Trough, and costal plutonic complex from A tlin, B ritish Columbia to Haines, Alaska as revealed by fission track geothermometry techniques. Unpublished master's thesis, Rensselaer Polytechnic Institute, Troy, NY. Faure, G. (1977). Principles of isotope geology. New York: John Wiley. Fleischer R. L. & Price, P. B.23^1964b). Decay constant for spontaneous fission of U. Physic Review, 133 (63 & 64) . Fleischer R. L., Price, P. B. & Walker, R. M. (1965a). Neutron flux measurement by fission tracks in solids. Nuclear Science Engineering, 22, 153-156. 55

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Freeman, V. L., Ruppel, E. T. & Klepper, M. R. (1985). Geology of p a rt of the Townsend Valley, Broadwater and Jefferson Counties. Contributions to Economic Geology, Geological Survey Bulletin, 1042-N, U.S. Government P rintin g Office, Washington. Friedlander, G., Kennedy, J. W., Macias, E. S. & M iller, J. M. (1981). Nuclear and radiochemistry. New York: John Wiley. Galraith, R. F. (1981). On statistical models for fission track counts. Mathematical Geology, 13,(6), 471-488. Garihan, J. M., Schmidt, C. J. & Karasevich, L. P. (1982). Roadlog for the Ruby Range, part of the Highland Range, and adjacent intermontane basins, southwest Montana, with emphasis on recurrent tectonic history. Tobacco Root Geologist Society, 7th Ann. Field Conference Guidebook, pp. 45-68. Garihan, J. M., Schmidt, C. J., Young, S. W. & Williams, M. A. (1983) . Geology and recurrent movement history of the Bismark-Spanish Peaks-Gardiner Fault System, southwestern Montana. Rocky Mountain Association of Geologists, 295-314. Garwin, L. (1984) . Fission track dating comes of Age. New S c ie n tis t, 23, 21. Gentner, W., Storzer, D., Gijbels, R. & Van der Linden-„ R. (1972). Calibration of the decay constant of U spontaneous fissio n . Transactions of the American Nuclear Society, 15, 125-126. Gleadow, A.J.W. (1981). Fission track dating methods: What are the real alternatives? Nuclear Tracks, (Printed in Britain), 5_(1 & 2), 3-14. Gleadow, A. J. W. (1984). Fission track dating methods - II, A manual of principles and techniques. Geology Track International. Townsville: James Cook University. Gleadow, A. J. W. & Duddy, I. R. (1980) . Early Cretaceous volcanism and the early breakup history of southwestern Australia. In M. M Cresswell & P. Vella (Ed.), Evidence from fissio n track dating of volcaniclastic sediments, uonawana v.

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Hanna, W. F. (1973). Paleomagnetism of the late cretaceous Boulder b ath o lith , Montana: American Journal of Science, 273, 778-802. Harrison, T. M. (1984). A reassessment of fission track annealing behavior in apatite. Abstracts, 4th Internatio nal Fission Track Workshop, Troy, NY.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 58 Henderson, P. (1982). Inorganic geochemistry. New York: Pergamon Press. Hurford, A. J. & Gleadow, A. J. W. (1977) . Calibration of fission track dating parameters. Nuclear Track Detection, 1_, 99-106. Hurford, A. J. & Gleadow, A. J. W. (1981). C alibration of fission track dating parameters. Nuclear Track Detection (Printed in Great Britain), 1(1), 41-48. Hurford, A. J. & Green, P. F. (1981a). A reappraisal of neutron dosimetry and uranium-238 decay constant: Values in fission track dating. Nuclear Tracks, 5, 53-61. Hurford, A. J. & Green, P. F. (1981b). Standards, dosimetry and the uranium-238 decay constant: A discussion. Nuclear Tracks, 5J1 & 2), 73-75.

Hurford, A. J. & Green, P. F. (1982). A user's guide to fission track dating calibration. Earth and Planetary Science Letters (Printed in the Netherlands), 59, 343-

Hurford, A. J. & Green, P. F. (1983). The zeta age calibration of fission track dating. Isotope Geoscience, 1_, 285-317. Jaffe, H. W. , Gottfried, D. Waring, C. L. & Worthing, H. W. (1959). Lead-alpha age determinations of accessory minerals of igneous rocks 1953-1957 (Survey Bulletin 1097-B). In studies in Geochronolgy: U. S. Geology.

Klepper, M. R., Ruppel, E. T., Freeman, V. L. & Weeks, R. A. (1971). Geology and mineral deposits, east flank of the Elkhorn Mountains, Broadwater County, Montana: Geological Survey. (Professional Paper 665) . Washington, DC: U. S. Government Printing Office. Klepper, M. R., Weeks, R. A., & Ruppel, E. T. (1957). Geology of the southern Elkhorn Mountains, Jefferson and Broadwater Counties, Montana: Geological survey. (Professional Paper 292). Washington DC: U. S. Department of Interior, U. S. Government Printing O ffice. Knopf, A. (1964). Time required to emplace the Boulder b ath o lith : Montana - A f i r s t approximation. American Journal of Science, 262, 1207-1211.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 59 Knopf, A. (1968). Geology of the northern part of the Boulder Batholith and adjacent area: Montana. (Miscellaneous Geologic Investigation, Map 1-381). Washington DC: U. S. Geological Survey Printing O ffice.

Lakatos, S. & Miller, D. S. (1983). Fission track analysis of apatite and zircon defines a buria] depth of 4 to 7 km for lowermost Upper Devonian, Catskill Mountains, New York. Geology, 5(11), 103-104. Lambe, R. N. (1981). Crystallization and petrogenesis of the southern portion of the Boulder Batholith: Montana (Doctoral d isse rta tio n , U niversity of California, 1981). Dissertation - Dr. of Philosophy in Geology. 1-171. L asle tt, G. M., Gleadown, A. J. W. & Duddy, I. R. (1983 in press). The interpretation of the isotopic ratio OOC O *3 Q U/ U in fission track dating. Math Geology.

Ludman, A. (1965). Geology of the Mt. Doherty igneous complex: Jefferson County, Montana. Unpublished master's thesis, Indiana University, Bloomington, IN. Matthews, G. W., McClain, L. K. & Johanns, W. M. (1977). Petrogenetic aspects of the Hell Canyon pluton and its relation to the Boulder Batholith: Southwestern Montana. Northwest Geology, 6_, 77-84. McDowell, F. W. (1966). Potassium Argon dating of cordilleran intrusives. Unpublished doctoral d isse rta tio n , Columbia U niversity. McMannis, W. J. (1963). LaHood formation: A coarse facies of the Belt series in southwest Montana. Geological Society of America Bulletin, 74, 407-436. Michigan Memorial Phoenix Project. (1983). Phoenix Memorial Laboratory, Ford Nuclear Reactor, Radiation, Analytical, Test, and Training Services. Ann Arbor: University of Michigan. Miller, D. S., Green, P. F., Hurford, A. J. & Naeser, C. W. (1985). Results of interlaboratory comparison of fission track age standards: Fission track workshop. Nuclear Tracks (Printed in Great Britain), 0(0), 000- 000.

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Naeser, C. W. & Paul, H. (1969). Fission track annealing in apatite and sphene. Journal of Geologists, 74, 705-710.

Naeser, C. W., Zimmermann, R. A. & Cebula, G. T. (1981). Fission track dating of apatite and zircon: An interlaboratory comparison. Nuclear Tracks (Printed in Great Britain), 5J1 & 2), 65-72. O'Neill, J. M., Ferris, D. C., Schmidt, C. J. & Hanneman, D. L. (1986). Movement along northwest trending faults at the southern margin of the Belt Basin, Highland Mountains, southwestern Montana. S tate of Montana Bureau of Mines and Geology, in Belt Supergroup: A Guide to Proterozoic Rocks of Western Montana and Adjacent Areas, 94, 209-216. Roberts, J. H., Gold, R. & Armani, R. J. -41968). Spontaneous fission decay constant of U. Physic Review, 174, 1482-1484.

Robinson, G. D. (1963) . Geology of the Three Forks Quadrangle, Montana. (U. S. Geological Survey Professional Paper 370) . Washington, DC: U. S. Government Printing Office. Robinson, G. D., Klepper, M. R. & Obradovich, J. D. (1968). Overlapping plutonism, volcanism, and tectonism in the Boulder batholith region, western Montana. In R. R. Coats, R. L. Hay, & C. A. Anderson (Ed) Studies in Volcanology (pp. 557-576). Montana: Geology Society American Members. Ross, C. P. (1963a). Evolution of ideas relative to the Idaho Batholith. U.S. Geological Survey, Denver, Colorado. Northwest Science, 37(1), 45-60.

Ross, C. P. (1963b). Some features of the Idaho Batholith. U. S. Geological Survey, Washington. International Geological Congress Report of the XVI Session, U. S. of America. Rutland, C., Vogel, T. A. & Greenwood, W. R. (1984). Major element chemical evolution of the cretaceous Elkhorn Mountain volcanics: Southwestern Montana. Abstracts with Programs - 97th Annual Meeting, Geological Society of America. Geology Society of America, 16(6), 641.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 61 Schmidt, C. J. (1976). Structural development of the Lewis and Clark Cavern State Park area, southwest Montana. In M. M. Lankston (Ed), Guidebook; Montana Bureau of Mines and Geology Special Publication, 73, 141-150.

Schmidt, C. J. & Garihan, J. M. (1983). Laramide tectonic development of the Rocky Mountain foreland of southwestern Montana. Rocky Mountain Association of Geologist, 271-294. Schmidt, C. J. & Garihan, J. M. (1986). Middle Proterozoic and Laramide tectonic activity along the southern margin of the Belt Basin. In S. M. Roberts (Ed.), Belt Supergroup (pp. 217-235). Montana Bureau of Mines & Geology, Special Publication. Schmidt, C. J. & O'Neill, J. M. (1982). Structural evolution of the southwestern Montana transverse zone. Rocky Mountain Association of Geologists, 193-218.

Schmidt, C. J., O'Neill, J. M. & Brandon, W. C. (1984). Influences of foreland structures on geometry and kinematics of the frontal fold and thrust belt, southwestern Montana. Abstracts with Programs - G.S.A., 97th Annual Meeting. Geologist Society of America, 16(6), 647. Schmidt, C. J ., Smedes, H. W., Suttner, L. J. & Vitaliano, C. J. (1979, June). Near-surface batholiths, related volcanism, tectonism, sedimentation and mineral deposition. Gregson, Montana: Guidebook for Penrose Conference - Granite I_, 1-111. Scholten, R. (1968). Model for evolution of Rocky Mountain east of Idaho batholith. Tectonophysics (Printed in the Netherlands), £(2), 109-126. Smedes, H. W. (1962). Preliminary geologic map of the northern Elkhorn Mountains, Jefferson and Broadwater Counties, Montana. (U. S. Geology Survey Mineral Investigation Field Studies, Map MF-243). Washington, DC; U. S. Government P rinting Office. Smedes, H. W. (1962b). Lowland Creek volcanics, an upper Oligocene formation near Butte, Montana. Journal Geology, 70, 255-266.

Smedes, H. W. & Thomas, H. H. (1965). Reassignment of the Lowland Creek volcanics to Eocene age. Journal Geology, 73, 508-510.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 62 Smith, J. L. (1970). Petrology, mineralogy and chemistry of the Tobacco Root b ath o lith , Madison County, Montana. Unpublished master's thesis, Indiana University, Bloomington, IN. Storzer, D ieter, Wagner, & Gunther, A. (1982). The application of fission track dating in stratigraphy; A critical review: Numerical dating in stratigraphy7 New York: John Wiley. Suzuki, M., Fukuoka, H., Shiraco, M., Kasuya, M. & Tomura, K. (1984). Basic data for d irect determination of fission track zeta constants for N.B.S. SRM 961, 962. St. Pauls Review of Science, 4(5), 141-156. Tilling, R. I. (1973, December). Boulder batholith, Montana: A product of two contemporaneous but chemically distinct magma series. Geological Society of America Bulletin, 84, 3879-3900.

Tilling, R. I. (1974, December). Composition and time relations of plutonic and associated volcanic rocks, Boulder batholith region, Montana. Geological Society of America Bulletin, 85, 1925-1930. Tilling, R. I., Klepper, M. R. & Obradovich, J. D. (1968). Potassium-argon ages and time span of emplacement of the Boulder b a th o lith , Montana. American Journal of Science, 266, 671-689. Verrall, P. (1955). Geology of the Horseshoe Hills area, Montana. Unpublished doctoral dissertation, Princeton University, Princeton, NJ. Vitalinano, C. J. (1987, February). Personal correspondence. Department of Geology, Indiana University.

Wagner, G. A., Reimer, G. M., Carpenter, B. S., Faul, H., Van der Linden, R. & Gijbels, R. (1975). The 23 8 spontaneous fission rate of U and fission track dating. Geochim, Cosmochim. Acta, 39, 1279-1286.

Woodward, L. A. (19 81). Tectonic framework of Disturbed belt of west-central Montana. American Association Petroleum Geologists Bulletin, 65(2), 291-302.

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Feltis, R. 1985, top ofytlie fladisoil degreti quadrangl e j Geology, Geologic

Freeman, V. I ., Ruppe Geology of Part 01 and Jefferson Courl Economic Geologyv U.S. Gov. Printincj

Klepper, fl. R., Rupp^ 1971, Geology and1 Elkhorn Mountains, Survey,; Prof. Pape Washington, 1-65

Klepper, M. R., Weeks of the. Southern Ell water Counties, He 292, U.1 S. Departn Washington, 1-82

Lambe, R. It., 1981, Cl southern portion o| Dissertation - Dr. division of the Un

Matthews, G. W., HeCl Petrogenetic aspec relation to the Bo Northwest Geology,

Robinson, G. D .,’1963 Montana: U. S. Geo Gov. Printing Offi

Schmidt, C. J ., O'Neil Southwest Montana 1 of geologists, 193-

' ’.REFERENCES ■ ; 'A " ■' : ■ A -A .. ... ozza, F.C., 1967, The geology of the Bone Basin and Perry Canyon Intrusions, Madison County, Montana: unpublished master's thesis Indiana University, 1-48 p. tis , R. D.,.., 1985, flap showing configuration of the top of the Madison Group, Sulfer Springs, 1 x 2 degree quadrangle, Montana: Montana Bureau of Mining Geology, Geologic Map Series, no. 39. f » eman, V, L., Ruppel, E. T., Klepper, M. R., T958, Geology of Part of the Townsend Valley, Broadwater and Jefferson Counties, Montana: Contributions to Economic Geology, Geological Survey Bulletin 1042-N, J.S. Gov. Printing Office, Washington, 481-556 p.

Dper, M. R., Ruppel, E. T., Freeman, W. L., Weeks, R. A., 1971, Geology and mineral deposits, East Flank of the ilkhorn Mountains, Broadwater County, Montana: Geological Survey, Prof. Paper 665, U.S. Gov. Printing Office, iashington, 1-65 p.

)per, M. R., Weeks, R. A., Ruppel, E. T., 1957, Geology t f the Southern Elkhorn Mountains, Jefferson and Broad water Counties, Montana: Geological Survey, Prof. Paper !92, U. S. Department of Interior, U.S. Printing Office, Jashington, 1-82 p.

>e, R. N., 1981, Crystallization and petrogenesis of the ;outhern portion of the Boulder Batholith, Montana: lissertation - Dr. of Philosophy in Geology, Graduate ivision of the Univ. of California, Berkley, 1-171 p. T hews, G. W., McClain, L. K., Johanns, W. M., 1977, 'etrogenetic aspects of the Hell Canyon Pluton and its elation to the Boulder Batholith, Southwestern Montana: orthwest Geology, Vol. 6, 77-84 p.

nson, G. D.,'1963, Geology of the Three Forks Quadrangle, ontana: U. S. Geological Survey, Prof. Paper 370, U. S. ov. Printing Office, Washington, 143 p.

idt, C. J., O'Neill, J. M., Structural Evolution of the outhwest Montana Transverse Zone: Rocky Mountain Assoc, f geologists, 193-218 p.

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