Uranium-Lead Zircon Ages and Crustal Contamination of the Northeastern Idaho Batholith

Western Michigan University ScholarWorks at WMU

Master's Theses Graduate College

12-1984

James J. Dexter

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Recommended Citation Dexter, James J., "Uranium-Lead Zircon Ages and Crustal Contamination of the Northeastern Idaho Batholith" (1984). Master's Theses. 1506. https://scholarworks.wmich.edu/masters_theses/1506

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]. URANIUM-LEAD ZIRCON AGES AND CRUSTAL CONTAMINATION OF THE NORTHEASTERN IDAHO BATHOLITH

James J. Dexter

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 December 1984

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. URANIUM-LEAD ZIRCON AGES AND CRUSTAL CONTAMINATION OF THE NORTHEASTERN IDAHO BATHOLITH

James J. Dexter, M.S.

Western Michigan University 1984

The magmas which formed the northeastern Idaho batholith were

contaminated with, or derived from, old crnstal material as

evidenced, from previous studies, by high whole—rock *7Sr/,*Sr

ratios, relatively high a07Pb/*°‘Pb ages, and Archean to

Proterozoic upper intercept ages for batholithic zircons.

The old zircon component has been identified in this study by

separating zircons into fractions based on size and morphology.

Pb/U isotopic ratios are distributed on a chord with a lower

concordia intercept intrusive age of 73.6 + 6 m.y. Zircons from a

batholithic sample located one kilometer inward from the contact

were separated according to mineral association. This method was

not successful in segregating old inherited zircon from euhedral

zircon. Upper-intercept ages from this project, when combined with

those from other studies, range from 1700 m.y. to 2340 m.y. and’

indicate the complexity and variety of the 'older continental

crust* source terrain.

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

This project was conducted under the supervision of Ronald B.

Chase, Western Michigan University, and M. E. Bickford, University

of Kansas. The Michigan Highway Department and the Geology

Departments of Western Michigan University and the University of

Kansas provided the necessary hardware and facilities. The

research was partially funded by National Science Foundation Grant

#33790. to R. B. Chase and two grants from the Western Michigan

University Graduate Student Research Fund. The support given

these contributors is greatly appreciated.

James J. Dexter

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DEXTER, JAMES JOSEPH URANIUM-LEAD ZIRCON AGES AND CRUSTAL CONTAMINATION OF THE NORTHEASTERN IDAHO BATHOLITH

WESTERN MICHIGAN UNIVERSITY M.S. 1984

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Page

ACKNOWLEDGEMENTS...... ii

LIST OF T A B L E S ...... iv

LIST OF FIGURES...... v

INTRODUCTION ... ,...... 1

GEOLOGY...... 3

GEOCHRONOLOGY...... 9

Separation Procedures...... 9

Analytical Methods ...... 9

Results...... 10

DISCUSSION OF THERMAL AND STRUCTURAL EVENTS ...... 19

Chronology of Events ...... 19

Implications Regarding Origin of Batholithic Magma .... 22

APPENDICES

A. Location of Samples...... 25

B. Petrographic Description of Samples...... 26

C. Separation Procedures and Analytical Techniques...... 28

D. Analytical Data...... 33

E. Theoretical Basis of U-Pb Geochronology...... 35

BIBLIOGRAPHY...... 41

iii

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

1. Analytical Data ...... 34

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

Page

1. Regional setting of the Idaho batholith ...... 4

2. Geologic and sample location map...... 5

3. Concordia plot of qnartz diorite orthogneiss...... 11

4. Back-scattered electron image of (-200) mesh-size zircons...... 13

5. Concordia plot of medium-grained granitic rock from the batholith interior...... 15

6. Concordia plot of medium-grained granitic rock showing upper-intercepts...... 16

7. Foliation and structure in quartz diorite orthogneiss . . . 20

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

This research follows petrological and structural studies of the

northeastern border zone of the Idaho batholith and is a continuation

of U/Pb isotopic studies by Chase and others (1978) . The results may

be compared with other past or ongoing chemical studies of the

northern part of the Idaho batholith (Bitterroot lobe) and thus will

help in obtaining an evolutionary model for the entire batholith.

Previous workers who have attempted to determine crystallization

ages for parts of the Idaho batholith include McDowell and Kulp

(1969), Armstrong (1974), Ferguson (1975), Fleck (1980), Chase and

others (1978, 1983), Bickford and others (1981), and Garmezy and

Sutter (1983). Methods used included fission-track, potassium-argon,

rubidium-strontium, and uranium-lead. Because of either an uplift

event (Ferguson, 1975), a magmatic event (Armstrong and others, 1977),

a hydrothermal event (Criss and Taylor, 1978), or a combination of

these (Chase and others, 1978) around 38-49 m.y. ago, the radiometric

clocks pertaining to most of the methods used were reset. Although

Pb-loss in zircons resulting from such events was possible, Chase and

others (1978) suggested that zircon ages allowed an interpretation

more consistent with the geologic setting of the Idaho batholith. In

addition, it was determined that zircons were contaminated with an

older, inherited fraction and that the linear array obtained by

plotting the isotopic data probably represented a mixing line between

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

older, inherited zircon and zircon crystallized directly from the

magma. Previous evidence for such contamination of Idaho batholith

magmas was presented by Granert and Hofman (1973) on the basis of U/Pb

isotopic data from zircon (although they did not see the xenocrystic

zircon), and by Armstrong and others (1977) on the basis of high

initial *7Sr/8‘Sr ratios. The necessity of separating zircons into

representative groups that would better define linear relationships on

concordia diagrams became apparent.

This project was designed to obtain ages for thermal/ structural

events related to intrusion and crystallization of the northeastern

border of the Idaho batholith. In addition, two different methods

(outlined in Separation Procedures, p. 10 and Appendix C) were

utilized in an effort to separate representative zircons into groups

(including a group of older, inherited zircons) whose U/Pb isotopic

data would be most meaningful in determining these ages. Samples were

collected by R. B. Chase in the summer of 1976, and sample preparation

and mineral separations were performed at Western Michigan University

and the University of Kansas during winter and spring of 1978. Mass

spectrometry was conducted at the University of Kansas Isotope

Geochemistry Laboratory during the summer of 1978.

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

The Idaho batholith is a composite group of plutons produced

during subduction of oceanic crust along an easterly-dipping Benioff

zone. Subduction complexes west of the batholith have been described

by Vallier (1973), Talbot and Hyndman (1975), Hamilton (1976), and

Talbot (1976). Underflow of oceanic crust probably occurred from late

Cretaceous to middle Tertiary time (Hamilton, 1969) . The regional

setting of the Idaho batholith, which is southwest of the Montana

overthrust belt and northwest of the Sevier orogenic belt, is shown in

Figure 1. The northern half of the batholith (Bitterroot lobe) is

surrounded by metamorphosed rocks of the Middle-to Late-Proterozoic

Belt Supergroup, pre-Belt rocks of the Salmon River arch, and by

Cenozoic volcanic rocks of the Columbia Plateau.

Evolution of the northeastern part of the Idaho batholith

involved multiple intrusion, metamorphism up to sillimanite-orthoclase

grade, multiphase penetrative deformation, and gneiss doming. The

geology of the northeastern border zone of the batholith is shown in

Figure 2. The main part of the batholith is bounded on the north by

three main types of metasedimentary rocks: calc-silicate gneiss,

quartzofeldspathic gneiss, and pelitic schist. Most of these are

probably metamorphosed rocks of the Belt Supergroup. Concordant

bodies of metamorphosed anorthosite are present within the pelitic

schist unit; the schist is structurally adjacent to and beneath the

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

Snuswjap

Infrastructure

Canada

j'M o b iJ e

Bitterroot Lobe jlumbia

ateau y Boulder IBatholith

Idaho y/ V Atlanta Batholith ^ Lobe

Regional setting of the Idaho batholith (Modified from Chase and others, 1978*, Talbot and Hyndman, I973;and King (tectonic map of North America).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5

10 2 0 km _J ______l

Tertiary and Quaternary sediment*

R hyollte

Lolo Hot Springs batholith (epizonol) Outlying plutons related to main Idaho batholith (mesozonal to katazonal)

Quartz monzonlto

Main Idaho batholith (C ^gradational contact zone)

Orthogneiss

^ffir'ih.Koo1enol_~_ Meta-anorthoslte / ♦ + + + * T N J .. r5r;Lake "-r~r-” : »♦♦♦♦♦ + + ♦ v* Calc-silicate gneiss; Known W allac e Fm. In vicinity of thrusts Quartzlte and guartzo- feldspathlc gneiss; Known Koot.e" Ravalli Group near thrusts

Pelitic schist

Thrust fault

High-angle fault

Blastomylonite zone \ C - - - - - ^ -T-" $ ? *\ - - Covered contact

• % y . 'I„R2_2Wm£i\R2IW Approximate contact

' ■ ■ W Sample location

Figure 2. Geologic and sample location map; northeastern border zone of the Idaho batholith.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 6

qnartzofeldspathic gneiss. The age and history of the anorthosites

are poorly known. The metasediments are cut by epizonal, mesozonal,

and katazonal plutonic rocks, some of which are foliated. To the

east, the batholith is bounded by a north-south trending

blastomylonite zone, a normal fault zone, and Cenozoic sediments and

volcanics of the Bitterroot Valley.

The metasedimentary rocks of the northeastern border zone have

been studied in detail by Chase (1973). The qnartzofeldspathic gneiss

contains layers from 0.6 to 9 cm thick composed of varying proportions

of quartz, feldspar, and biotite; quartz-rich layers are predominant.

The pelitic schist contains layers from 0.2 to 1 cm thick of medium-

grained quartz and feldspar alternating with muscovite-sillimanite-

biotite layers of similar thickness. The qnartzofeldspathic and

pelitic units contain local boudins of calc-silicate gneiss and

amphibolite that are in sharp contact with host-rocks. Lithologic

layering in calc-silicate boudins generally is parallel to layering

and schistosity in adjacent schist and gneiss, except where

schistosity wraps around the boudin necks. All of the metasedimentary

units have been subjected to at least three phases of penetrative

deformation during which they were deformed into mesoscopic

concentric-, convolute-, and similar-style folds. The rocks were

subjected to conditions of upper amphibolite-grade metamorphism and

then were remetamorphosed under lower-pressure (cordierite-grade)

conditions (Cheney, 1975) .

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7

The earliest intrusive phase of the northeastern Idaho batholith

was quartz diorite (tonalite) represented by five separate bodies of

orthogneiss directly north of the main part of the batholith (Fig. 2).

These orthogneiss bodies are characterized by relatively uniform

distribution of individual mineral phases, lithologic discordance of

the contact of quartz diorite with qnartzofeldspathic gneiss, and

continuity of biotite schistosity across the contact, all of which

demonstrate the orthogneissic character of the quartz diorite.

Dimensionally aligned andesine crystals impart a strong lineation to

the rock (Chase, 1973). Deformed and foliated pegmatites related to

the orthogneiss are present in the orthogneiss and in the host rocks.

The orthogneiss is medium-grained and allotriomorphic granular in

texture. A petrographic description of an orthogneiss is given in

Appendix B and in Bickford and others (1981) .

Although the main Idaho batholith consists of many plutons with

poorly known contact locations, there are two main types of granitic

rocks: an earlier medium-grained granite to granodiorite and a later

porphyritic granite with large potassium feldspar megacrysts (Bickford

and others, 1981). The contact with metasediments is gradational and

consists of two different zones: a tonalite - calc-silicate gneiss

xenolith association and a granodiorite-quartzofeldspathic gneiss

xenolith association (Chase, 1973). The xenoliths become less

abundant toward the interior of the batholith over a contact zone up

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 8

to 2 bn wide. A petrographic description of a medium-grained granitic

rock is given in Appendix B and in Chase and others (1978) .

The infrastructure of the Idaho batholith has been subjected

to two phases of deformation before the injection of magma; batho

lithic rocks truncate Fx and Fa structures (Chase, 1973) . A third

period of deformation probably began shortly after intrusion of

initial phases of the Idaho batholith (the quartz diorite orthogneiss)

and may have accompanied intrusion of later stages. The northeastern

sector of the batholith is part of a gneiss dome, a large elliptical

uplift bounded by thrust faults on the. north, high-level Tertiary

plutons on the west and south, and a blastomyIonite zone on the east

(Chase, 1977). The blastomylonite zone was produced during voluminous

intrusion of the latest phases of the Idaho batholith, around 48 m.y.

ago (Bickford and others, 1981; Chase and others, 1983). Adjacent

metasediments were deformed in the zone, first by rigid-body rotation,

and then by down-dip shear parallel to reoriented axial planes (Chase,

1977) . Blastomylonites are present in basement-rocks of the

Bitterroot Valley and are dipping up to 30 degrees to the east;

mylonitic texture is weak to absent in basement-rocks east of the zone

(Abramiuk, 1981). Movement along the mylonite zone involved a multi

stage series of structural and thermal events (Chase and others,

1983). Isolated parts of the gneiss dome were faulted.

Mylonitization, retrograde metamorphism, and intrusion of felsic dikes

occurred in the fault zones.

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

Separation Procedures

Specific objectives of this research were to: 1) determine the

minimum crystallization age of an early phase of the Idaho batholith

(quartz diorite orthogneiss) and separate its zircons into groups

based on size and morphology, and 2) determine the minimum

crystallization age of a medium-grained granodiorite (part of the main f mass of Idaho batholith) and separate its zircons into groups

associated with specific minerals. The separation procedures were

designed in an effort to distinguish an old-zircon component from

younger zircon component in terms of their isotopic ratios plotted

along a chord on the concordia diagram. Samples of foliated quartz

diorite (A-33, Fig. 2) and granodiorite (A-18, Fig. 2) were broken and

pulverized. A Wilfley-Table, heavy liquids, and a Frantz Isodynamic

magnetic separator were used to obtain individual groups (fractions)

of the total zircon population. Only zircons with low magnetic-

susceptibility were analyzed. A detailed description of separation

procedures is given in Appendix C.

Analytical Methods

Concentrations and isotopic abundances of Pb and U from zircons

were determined by standard methods of mass spectrometry as described

by Bickford and Mose (1975) . Zircons were dissolved in sealed and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10

heated teflon vessels; U and PI) were separated according to methods

described by Krogh (1973) . The two samples were analyzed by isotope-

dilntion nsing a six inch radius, single filament, solid source mass

spectrometer. One of the samples (A-33) was re-run (Bickford and

others, 1981) on an automated 9-inch radius instrument with on-line 2 3 8 data reduction. The natural decay constants used were: X U =

1.55125 x 10-1°/yr., and X3*5U = 9.8485 x 10_1°/yr., published by

Jaffey and others (1971). Reduction of isotopic data was accomplished

using the isotopic composition of Pb according to the two stage growth

model of Stacey and Kramers (1975). A detailed description of

analytical techniques is given in Appendix C.

Results

Quartz diorite orthogneiss

The data from four fractions of zircon from quartz diorite

orthogneiss (A-33) are plotted on the concordia diagram (Fig. 3). The

plot of isotope ratios defines a single chord on the diagram with a

lower intercept of 73.6+ 6 m.y. and an upper intercept of 1724 + 40

m.y. The lower intercept represents the minimum age of

crystallization of zircon overgrowths in the orthogneiss whereas the

upper intercept yields the average age of rounded zircon cores. The

analytical data were interpreted by utilizing a least-squares-cubic

method as described by Tork (1966). Uncertainties in the slope and

intercepts of the best-fit line at one standard deviation were used to

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 11 Figure 3. Concordia plot of quartz diorite orthogneiss ( A - 3 3 ) .

CM O + S o +

O (0 CM 00 O CD

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obtain maximum and minimum values. These valnes are the basis for the

error in the determined ages.

The array of data supplies further evidence that zircon popu

lations in this part of the batholith consist partly of older,

inherited fractions as discussed by Grauert and Hofmann (1973), and

Chase and others (1978). The latter were the first to document visual

evidence for this older fraction by identifying round cores with clear

euhedral overgrowths. The zircons with recognizable round cores

appeared to prevail in the larger size-fractions as well as within

biotite (R. B. Chase, personal communication, 1978). Thus, zircons

from samples of quartz diorite orthogneiss were separated into

different size-fractions and a fraction of round, detrital-looking

zircons were hand-picked, mostly from the (+100) mesh-size. Four

separate groups of zircons from the sample are shown in Figure 3. The

least discordant ratios are represented by the (-200) mesh-size

fraction because it consists primarily of euhedral zircon (without

cores) that crystallized directly from the quartz-diorite magma (Fig.

4). The most discordant ratios along the chord are represented by the

hand-picked ('round') fraction because this fraction contains the

greatest percentage of rounded, older cores. The two medium-sized

fractions plot close together on the chord, between the (-200) and

('round') fractions. Thus, the chord in Figure 3 represents a mixing

line between younger zircon overgrowths plus zircons without cores,

and older zircon cores.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4. Back-scattered electron image of (-200) mesh-size zircons; z400. Length of zircon in center of photo is 175 microns.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14

Granodiorite

The data from two fractions of zircon from granodiorite (A-18)

are plotted in Figures 5 and 6, together with data obtained during the

earlier study by Chase and others (1978).

Zircons from this sample were separated into groups according to

mineral association: 1) zircons as inclusions in biotite, 2) zircons

as inclusions in apatite, and 3) zircons interstitial to quartz and

feldspar. Only 0.001 mg of zircon from apatite were obtained; this

proved to be an insufficient amount to adequately measure isotopic

ratios on the six-inch mass spectrometer. The 'biotite' and

'interstitial' fractions (labeled BIO and INT in Figure 5) are, by

themselves, inadequate for plotting a statistically meaningful chord.

When the data from these fractions are combined with arrays of data

obtained earlier (Chase and others, 1978) from the same sample plus a

sample of medium-grained quartz monzonite, a regression line yields a

lower-concordia intercept of 45 + 10 m.y. This age, within the given

error range, agrees with an age of 55 + 3 m.y. obtained by Bickford

and others (1981) on a medium-grained granite of the interior Idaho

batholith. Because the 55 m.y. age was obtained from zircons

separated on the basis of size (which effectively separated different

ratios of old/new zircon along the chord), the 55 m.y. age-date has

the least amount of error. A concordia plot of upper intercepts is

shown in Figure 6. The chord projected upward through the combined

data points intersects concordia at 1830 + 175 m.y. This agrees with

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. .50 .45 INT (4-18)

.40

BIO ( A - 18) .30 35 A -2 0 A - 20, .25 Pb/235U 2 0 7 200 .20 .15 150 interior (0 = granodiorite from this study; A =granodiorite and quartz monzonite from Chase and others, 1978). .10 100 Figure 5. Concordia plot of medium-grained granitic rock from the batholith .05 50 01 .03 .02 r .04 23B,j 206pb

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 16 Figure Figure 6. Concordia plot of medium-grained granitic rock showing upper-intercepts.

o <0

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

other data (Bickford and others, 1981; Chase and others, 1978) which

suggests an upper intercept range from 1700 m.y. to 2349 m.y. These

are the apparent mean ages of incorporated older zircons.

The method of separating the zircons into groups based on mineral

association proved to be more time-consuming and not as effective in

segregating old and new zircon components as the method of using size

and morphology described above. However, this does not preclude the

usefulness of such separation procedures. Obtaining sufficient

quantities of zircon (on the basis of mineral association) to separate

into various size-fractions would be extremely time-consuming but may

prove useful in obtaining more statistically reliable ages. These

ages could be more reliable for two reasons: 1) a greater quantity of

zircon fractions from the same sample may decrease amounts of error,

and 2) if there has been lead-loss caused by later thermal events, the

rate of lead-loss from zircon within biotite may be different from the

rate of lead-loss from zircon within apatite. Such separation

procedures, therefore, may allow more reasonable interpretations of

absolute crystallization ages. In addition, the dating of zircons

separated on the basis of mineral association may provide insights

into the crystallization histories of individual granites. Because of

high activation energy and anion affinity of the Zr4+ ion, Murthy

(1958) suggested that zircon formation is rapid and early during the

crystallization of silicate melts. Because zircon is commonly

associated with late-stage magmatic products such as pegmatites and

certain ore minerals, Moorhouse (1956) suggested that accessory

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minerals like apatite and zircon form late during the crystallization

of granites. Thus, zircons included within biotite may have formed

significantly earlier than zircons included within apatite.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. DISCUSSION OF THERMAL AND STRUCTURAL EVENTS

Chronology of Events

The history of metamorphic and deformational events affecting the

northeast border zone of the Idaho Batholith has been described by

Chase (1973), Nold.(1974), and Chase (1977), and more recently

modified by Bickford and others (1981), and Chase and others (1983).

The main contribntion of this study is to provide a minimum age for

the quartz diorite orthogneiss, an early magmatic phase of the Idaho

batholith.

The Late Cretaceous igneous-metamorphic complex of the

northeastern Idaho batholith evolved through a variety of processes:

1) multiple metamorphism and small-scale folding, 2) east-directed

thrusting and recumbent folding, and 3) synkinematic to postkinematic

intrusion of quartz-diorite (A-33 of this study is an example) and

granitic plutons (Chase and others, 1983) . Sometime prior to

synkinematic intrusion of the plutons, the rocks were subjected to two

deformational events (Chase, 1973); the timing of these events is not

known. The quartz diorite to granite plutons were intruded between 82

+ 10 m.y. (Chase and others, 1978) to 73.6 + 6 m.y. (this study).

Evidence obtained in the field to support the idea of synkinematic

intrusion is presented in Figure 7. Local development of axial plane

schistosity in quartzofeldspathic gneiss is continuous with and

parallel to foliation in quartz diorite (A-33). The incipient

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biotite-rich layer of s quartzofeldspathic \V>'! '/.VO 7 / gneiss

./* / 7 " /{ ' ' y in: ''■it '/ quartz-rich layer of ' / '/ / 7 ' / quartzofeldspathic /t » / , / / ' / ' > i I i gneiss

>- / / ' tf'' /' / // // , b.)

A-33 / ///'

0.5 m

0.2 m

Figure 7. Sketches of foliation and structure in quartz diorite orthogneiss; a.) incipient foliation in quartzofeldspathic gneiss is continuous with foliation in orthogneiss (A-33) b.)small shear of quartzofeldspathic gneiss inclusion in orthogneiss.

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

schistosity (Sz) is developed along the axial planes of flexural-slip

folds (Fj) of a third deformational event (Chase 1973). This parallel

foliation, together with the lithologic difference between

quartzofeldspathic gneiss and quartz diorite and more uniform mineral

distributions in the quartz diorite, demonstrate the orthogneissic

character of the foliated quartz diorite.

A fourth folding event was described by Chase (1973) as a close

conformity of macroscopic structural patterns to the curving border of

the Idaho batholith that either: 1) pre-dated intrusion of the main

batholith, 2) was synchronous with intrusion, 3) was a post-intrusion

event, or 4) was associated with two or all three of the above. A

small shear plane which strikes to the northeast in orthogneiss (Fig.

7) indicates that local deformation of quartz diorite occurred during

this event, or that late-stage shearing occurred during magmatic flow.

A fifth event was described by Chase (1973) as mylonitic

deformation of the Idaho batholith and metasedimentary units and

Gleitbrett-style folding of biotite schist layers and pegmatite along

the eastern front of the Bitterroot Range. According to Chase and

others (1983), development of the mylonite zone involves a long,

multi-stage series of thermal and structural events during diapiric

uprising of the Bitterroot dome in Early Tertiary time. The close

proximity of epizonal plutons (such as the feldspar megacryst-bearing

granite dated by Bickford and others at 46 m.y.) and mesozonal plutons

(such as the 55 m.y. old medium-grained granite) indicates that large-

scale vertical movement occurred during Eocene time. Mylonitization

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. probably occurred progressively at or near the upper surfaces of

several nearly crystallized plutons and extended northward along axial

surfaces of pre-existing folds in metasedimentary rocks on the east

flank of the rising dome (Chase and others, 1983) . High angle

faulting (accompanied by local retrograde metamorphism) formed the

present Bitterroot front shortly after mylonitization during a final ft stage of vertical uplift.

Implications Regarding Origin of Batholithic Magma

Data collected during this project are significant to the origin

of magma that crystallized and formed the extensive Idaho batholith.

As mentioned previously, Grauert and Hofmann (1973) were the first to

infer a palingenic origin for the batholith on the basis of U-Pb

isotopic data from zircons. Armstrong and others (1977), and Chase

and others (1978), described strontium isotope data from the batholith

that further supported this idea: #7Sr/**Sr ratios from the

batholith are mostly greater than 0.708. Hurley and others (1965)

have shown that *7Sr/#eSr ratios are greater than 0.705 for most

crystalline continental rocks, and less than that for most mantle-

derived igneous rocks. Chase and others (1978) were the first to

identify young, euhedral zircon overgrowths on older rounded cores.

An important aspect of this project was the demonstration that, by

hand-picking a rounded fraction of zircons, a larger percentage of

older, inherited crystals could be isolated. It is important to note

that the higher position of this fraction on the chord (Fig. 3) is due

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. to a greater percentage of xenocrystic zircon as opposed to an

addition of radiogenic lead or a loss of uraninm. Lead is most common

in the +2 oxidation state and has a much larger ionic radius (1.32&.

than Zr+4 (ionic radios of 0.87&) and therefore, is not easily

incorporated into the zircon lattice. Since the content of uranium in

the 'round' fraction does not significantly differ from that of the

more concordant euhedral fractions, a loss of uranium is not con

sidered likely. Another indication of the presence of an older zircon

component is the 207Pb/206Pb ages. The hand-picked fraction has a

207/206 age of approximately 1600 m.y. whereas the fraction with the

greatest percentage of euhedral zircon has a 207/206 age of about 1000

m.y. These a07Pb/20*Pb data are in agreement with those reported by

Grauert and Hofmann (1973). In addition, the hand-picked fraction

differed physically from the other fractions of zircon, as mentioned

above. The large amounts of magma that produced the Idaho batholith

were either wholly derived by partial melting of continental crust or

extensively contaminated with continental crust.

According to Watson (1979), zirconium solubility is low in

peraluminous melts and much higher in subalkaline to peralkaline

melts. The peraluminous nature of Idaho batholith magmas may explain

the presence of xenocrystic zircon. The presence of xenocrystic

zircon allows speculation as to the nature of the crustal source-rock,

as well as allowing ages (in the form of upper-intercepts on

concordia) to be obtained. As more efficient means are developed for

isolating fractions of xenocrystic zircon, the error on upper-

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. intercept ages should be reduced. The upper-iutercept ages obtained

in this project, together with those reported by Bickford and others

(1981), indicate a variety of ages ranging from 1700 m.y. to 2340 m.y

Such a wide variety of upper intercepts along fairly linear chords

probably indicates the complexity and variety of the 'older

continental crust' source terrain.

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

Location of Samples

A-33

Quartz diorite orthogneiss from talus approximately 0.4 km

north of the northern tip of South Kootenai Lake along the south

east side of a ridge. The sample was taken approximately 0.5 miles

north of the Idaho batholith contact zone (NE 1/4 NE 1/4 Section

14, T9N, R22W; Lat. 46°32'41"N, Long. 114°19'12''W; Ravalli

County, Mont.).

A-18

Granodiorite from a major phase of Idaho batholith, approx

imately 1.4 km south of its contact with quartzofeldspathic gneiss.

The sample was collected about 1.2 km southwest of the southwestern

end of South Kootenai Lake (NE 1/4 NW 1/4 Section 23, T9N, R22 V;

Lat. 46°31'44"N, Long. 114°19'52"W; Ravalli County, Mont.).

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

Petrographic Description of Samples

A-33 Foliated onartz diorite

The texture of this rock ranges from granoblastic to

lepidoblastic; the foliation, defined by alignment of biotite, is

most visible on the outcrop scale. Although melanocratic layers

technically do not exist in the rock, darker ’layers' millimeters

thick alternate with more leucocratic layers centimeters thick and

give the rock a banded appearance in outcrop. Locally, the rock

displays an isotropic texture. In thin section, polygonal grain

boundaries are common. Secondary alteration of biotite and plagio-

clase is minimal. A modal analysis (Bickford and others, 1981)

yielded 37% quartz, 52% plagioclase (andesine), and 11% biotite.

The accessories are muscovite, chlorite, zircon, apatite, opaques,

sphene, and monazite.

A-18 Granodiorite

This rock is hypidiomorphic-granular and slightly seriate. In

general, the grain boundaries are polygonal. Quartz is anhedral

with undulose extinction and commonly embays plagioclase. Anhedral

to subhedral plagioclase (andesine) and anhedral E-feldspar are

weakly zoned. Evenly scattered clusters of subhedral to anhedral

biotite are present. Secondary alteration is slightly developed

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. throughout the rock. Zircon occurs as inclusions in biotite,

apatite, and interstitial to quartz and feldspar. A modal analysi

(Chase and others, 1978) yielded 24% quartz, 57% plagiolase (An,,)

6% K-feldspar, and 13% biotite. Accessories include muscovite,

apatite, chlorite, zircon, magnetite, sphene, and epidote.

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

Separation Procedures and Analytical Techniques

Approximately 500 lbs. of rock per sample locality were broken up

by sledge hammer, run through a Bico rock crusher down to medium-

gravel and coarse-sand size, and ground down to medium- to fine

grained sand size in a disc mill. Iron filings were then removed by

utilizing various magnets. The samples were sieved using a standard

rotap and all material below 60-mesh size was processed on a Wilfley

Table. The resulting 'heavies' were then processed by means of heavy

1iquids.

The Wilfley Table heavies from sample A-18 (Idaho batholith

granodiorite) were first put through bromoform (specific gravity =

2.85). The ''light'' fraction (quartz and feldspar) was re-ground and

put through bromoform and methylene-iodide; 2.9 mg of interstitial

zircons were obtained. The ''heavy'' fraction was put through

methylene iodide (specific gravity = 3.32) and the 'lights' from this

process (apatite and biotite) were then separated. By dissolving the

apatite in warm nitric acid, 0.001 mg of zircons included in apatite

were obtained. By re-grinding the biotite and running the material

through methylene iodide, 9.8 mg of zircon included in biotite were

obtained.

The Wilfley-Table heavies from sample A-33 (quartz-diorite

orthogneiss) were processed through heavy liquids in a standard

manner. The resulting group of zircons were then sieved and a group

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 29

of 'rounded' zircons were hand picked (by means of a camel-hair brush

and binocular microscope) mostly from the largest size fraction (+100

mesh).

The procedures for preparing zircons for separation of uranium

and lead are as follows:

1. All zircons were cleaned in reagent grade HN03 on low heat,

then washed and decanted with deionized water and acetone.

2. Each zircon fraction was weighed (to the nearest 0.0005 g) in

foil boats. This weight was then cross-checked with weights

of each fraction in previously cleaned and weighed teflon

bombs.

3. Approximately 2 ml of distilled HF and 1 or 2 drops of HN03

were added to each teflon bomb. The bombs were then placed

in high-pressure steel casings; these were 'cooked' at

approximately 160°C for five days.

4. The casings were removed from the oven and cooled; the teflon

bombs were then removed, opened, and placed in a filtered-air

'clean box' where they evaporated until dry.

5. Approximately 2 ml of 3N distilled HC1 were added to each

teflon bomb. The bombs were then returned to the steel-

casings and again 'cooked for 24 hours.

6. The casing wore removed and cooled and the contents of each

bomb were emptied into previously cleaned, covered (with

parafilm), labeled, and weighed teflon beakers that were

marked I.C. (isotope composition). Each teflon bomb was

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 30

rinsed with deionized water and emptied into its respective

beaker until each beaker was approximately 3/4 fnll. The

contents were thoroughly stirred with a clean qnartz rod and

each beaker re-covered with the parafilm.

7. Each I.C. solution was weighed and recorded. About half of

each I.C. beaker was then poured into beakers marked I.D.

(isotope dilution) and quickly re-covered with parafilm to

minimize evaporation losses. The weight of each I.D. beaker

with solution was recorded. The I.C. beakers with solution

were re-weighed and recorded.

8. About 1 gram of spike solution containing U 305 and Pb5*08

tracer was added to each I.D. beaker. Weights were obtained

using a system of cross-checks and an aliquot ratio was

recorded for each sample.

9. The contents of all I.C. and I.D. beakers were then evapo

rated until dry in the filtered air 'clean box'.

The dried I.C. and I.D. samples were processed by means of an

ion exchange procedure. This procedure utilizes an organic

resin in teflon columns that will preferentially hold or

release elements (including U and Pb) upon introduction of

acids with different normalities. The steps of this

procedure are as follows:

1. The ion exchange columns were fully cleaned in acid, rinsed,

and mounted.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2. A mixture of 3N HC1 and Dow-Xl-Anion resin was added to a

depth of 2 cm in the tnbes. Care was taken to eliminate air

pockets.

3. Approximately 1 ml of 3N distilled SCI was added to the I.C.

and I.D. beakers to take np the material in solution.

4. Dissolved sample was put in the tnbes. The resin held the

zircon components.

5. One ml of 3N HC1 was added to each column. This step

releases zirconium and rare-earth elements which are

discarded.

6. One ml of 6N HC1 was added to each column. This step

releases the Pb which is saved in a cleaned vessel.

7. One ml of deionized water was added to each column. This

step releases the D which is saved,in a cleaned vessel.

8. All samples were then evaporated till dry in the filtered air

'clean box', then placed in small plastic containers. The

samples were then ready for mass-spectrometric analysis.

The samples were analyzed using a single-focusing mass

spectrometer with a 6-inch radius of curvature and a thermionic

emission ion source. Each Pb sample was loaded onto a single rhenium

filament on a bed of silica gel in the presence of HaP0j. Each

uranium sample was loaded onto a single rhenium filament with a TaO

''sponge*'. Every loaded filament was outgassed in a vacuum system

prior to loading the sample in the mass spectrometer. The sample of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 32

quartz diorite orthogneiss (A-33) was re-run on an automated 9-inch

radius mass spectrometer with on-line data reduction at a later date

by Bickford and others (1981).

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

Analytical Data

The tracer isotope contents in the spike solution that was mixed

in the I.D. beakers were: KU #1-D tracer, Pb20* = 0.620 ug/g and U 235

= 2.801 ug/g; KO #1-C tracer, Pb203 = 1.129 ug/g and U 23* = 4.744

ng/g. The blank levels were: Pb20* = 4 . 0 ng and U233 = 5.0 ug

(Bickford and Mose, 1975).

The following table gives the analytical data obtained during

this project:

4 00 t H =& vo *n *n co m o tj o vo PS o cs cs o CO vo o co CO o r** o CS o 0 d o ON vo Sb CO CO vo "S’ o o o o o o CO oo r- in O CO in CO ON cs ON o t- B r-i I • • CO CO CO vo 5is ^4* o co CS o cs o o o CS O o r- cs CO o CS in B co CS co I • n

~~-200 .02465 KU #1-C 360.7 9.3 296.30 0.0208 0.2158 34 APPENDIX E~~

~~Theoretical Basis of U-Pb Geochronology~~

~~(Much of the following discussion has been summarized from Fanre~~

~~(1977), York and Farquhar (1972), and Hamilton (1965).~~

~~Radioactive Decay~~

~~Radioactivity is a manifestation of breakdown or decay of~~

~~unstable atoms. Isotopes of unstable elements become nuclei of~~

~~different elements as a result of radioactive decay. In 1900, E.~~

~~Rutherford showed that radioactive decay follows an exponential law~~

~~and may be expressed as follows:~~

~~where P = the number of parent atoms present at time t, and 1 is a~~

~~constant of disintegration that varies in value depending on the~~

~~radioactive element of interest. Simplifying equation (1) yields:~~

~~dP — = -Xdt~~

~~which when integrated yields:~~

~~-In P = Xt + C (2 )~~

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

~~where C = the constant of integration. Since P (present number of~~

~~parent atoms) = P0 (initial number of parent atoms) when t = o, C =~~

~~-In P0 . Substituting this value in equation (2) yields:~~

~~-In P = At - In P0~~

~~In P0 - InP = Xt~~

~~In Jjl = t-t P~~

~~P0 = P eXt or (3) P = P0e-Xt~~

~~Equation (3) gives the number of radioactive parent atoms (P)~~

~~that remain (at any time t) of the initial number of parent atoms~~

~~(Po) that were present when the radioactive ''clock'* was set (when~~

~~t=o). This is the basic equation that describes all processes of~~

~~radioactive decay.~~

~~Assuming that decay of a radioactive parent produces a stable~~

~~daughter-product and that the number of daughter atoms is zero at~~

~~t=o, then the number of daughter atoms (D*) produced by the decay~~

~~of its parent is given by:~~

~~D* = P0 - P (4)~~

~~By substituting equation (3) into equation (4), the following is~~

~~obtained:~~

~~D * = Pe^t _ p~~

~~D* = P ( e ^ - 1) (5)~~

~~In general, the total number of daughter atoms (D) present in a~~

~~system where decay is occurring is:~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 37~~

~~D = D0 + D* (6)~~

~~Where D0 is the number of daughter atoms present initially (when~~

~~t=o) . Substituting equation (5) into equation (6), the following~~

~~is obtained:~~

~~D = D0 + P (eU - 1) (7)~~

~~This is the fundamental equation used to make age determinations of~~

~~rocks based on decay of a radioactive parent to a stable daughter.~~

~~Both the total number of daughter atoms (D) and the number of~~

~~radioactive parent atoms (P) are measureable quantities, and D0 is~~

~~a constant whose value can be either calculated or assumed. When~~

~~these values are obtained, equation (7) can be solved for t, which~~

~~is the age of the radioactive-decay system.~~

~~The P-Pb Method of Dating~~

~~This method of dating the crystallization ages of rocks is~~

~~based on the decay of *}*U through intermediate radioactive~~

~~daughter products ending in stable ao*Pb, and the decay of 2J5U~~

~~through intermediate radioactive daughter products and ending in~~

~~stable I07Pb. Another isotope of lead, ao4Pb, is not radiogenic and~~

~~is treated as a stable reference isotope. The isotopic composition~~

~~of lead in minerals (such as zircon) containing uranium can be~~

~~expressed as follows [by substitution in equation(7)]:~~

~~(8)~~

~~( e V - 1) (9)~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 38~~

~~Where -----306Pb anA“uu 307Pb are the isotope ratios measured at the *o«Pb 304Pb~~

~~/20«pb\ / 30 7Pb\ time of analysis;! 1 and {----- ) are the original isotope \304Pb /o \ 304Pb/0~~

~~ratios incorporated into the mineral at the time of its formation;~~

~~of analysis; and Xj are the decay constants of 33 *U and 335U,~~

~~respectively, and t = the time elapsed since the mineral was~~

~~crystallized or closed to uranium and lead. After the isotopic~~

~~composition of lead and uranium have been determined on a mass~~

~~spectrometer by isotope dilution analysis, the equations can be~~

~~solved for t using assumed values for original lead isotope ratios:~~

~~tao 7 _ 1 l n / 30 7Pb/2<)4Pb - ( 30 7Pb/304Pb)0 + l\ (10) ^"2 V 23JU/204PJ, '~~

~~The t 3oc can be calculated in a similar manner. In addition, a~~

~~'lead - lead' age can be determined by the present ratio of the~~

~~radiogenic lead isotopes:~~

~~30 7Pb 33 5P (e Xat - 1) 3<>7Pb (11) S • A t 1 \ 2°6Pb 1 ' 30~~

~~These three dates (t30,, tao?, and the 'lead-lead') will be the~~

~~same (concordant) and represent the age of the mineral only if the~~

~~mineral has remained closed to uranium, lead, and all intermediate~~

~~daughter products throughout its history. Often it is found that~~

~~minerals do not remain closed systems, and thus yield discordant~~

~~ages.~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A.O. Nier (1939) showed that discordant age patterns may~~

~~reflect the pre- or post-crystallization history of a mineral. L.~~

~~H. Ahrens (1955) was the first to observe linear relationships when~~

~~the a07Pb/a35U ratio was plotted against the ao*Pb/a,,U ratio for~~

~~discordant age sequences. G. W. Wetherill (1956) introduced a~~

~~curve called ''concordia'', which represents the locus of all~~

~~points having equal ao7Pb/a33U and ao~~

~~uranium isotope ratios fall on concordia, the ages will be concor~~

~~dant (i.e., the ao~~

~~does not fall on the curve represents a discordant age. In many~~

~~cases, such discordant points will define a linear relationship~~

~~that intersects the concordia curve in two places. Discordant~~

~~ages may be caused by a variety of reasons: 1) the gain or loss of~~

~~uranium or lead either by continuous or episodic diffusion, 2) the~~

~~loss of intermediate daughter products, or 3) the incorporation of~~

~~old U and Pb into a developing magma. Wetherill (1956) reasoned~~

~~that the upper intercept of a chord formed by episodic loss of lead~~

~~would represent the true age of the mineral while the lower inter~~

~~cept would represent the time of Pb loss from the system. Tilton~~

~~(1960) demonstrated that continuous loss of Pb from the mineral~~

~~results in a chord that curves down and passes through the origin~~

~~and thus only an estimated age of the whole system can be obtained.~~

~~The incorporation into a developing magma of older zircons in~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. herited from its source region or intruded host rocks also yields~~

~~discordant U-Pb ages as described by Grauert and Hofmann (1973),~~

~~Chase and others (1978), and Bickford and others (1981).~~

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

~~Abramiuk, N. I., 1981, A summary geologic report on the Missoula/ Bitterroot drilling project, Missoula/Bitterroot basins, Montana: U.S. Department of Energy Open File Report, GJBX-7(81/, 146 pp.~~

~~Ahrens, L. H., 1955, Implications of the Rhodesia age pattern, Geochimica et Cosmochimica Acta, no. 8, p. 1-15.~~

~~Armstrong, R. L., 1974, Geochronometry of the Eocene volcanic- plntonic episode in Idaho: Northwest Geology, v. 3, p. 1-15.~~

~~______, 1975, Precambrian (1500 m.y. old) rocks of central Idaho-The Salmon River arch and its role in Cordilleran sedimentation and tectonics: American Journal of Science, v. 275-A, p. 437-467.~~

Armstrong, R. L., Taubeneck, W. H., and Hales, P. 0., 1977, Rb-Sr and K-Ar geochronometry of Mesozoic granitic rocks and their Sr isotopic composition, Oregon, Washington, and Idaho: Geological Society of America Bulletin, v. 88, p. 387-411.

~~Bickford, M. E., and Mose, D. G., 1975, Geochronology of Precambrian rocks in the St. Francois Mountains, southeast Missouri: Geological Society of America Special Paper 165, 48 p.~~

Bickford, M. E., Chase, R. B., Nelson, B. K., Shuster, R. D., and Arruda, E. C., 1981, U-Pb studies of zircon cores and overgrowths, and monazite: Implications for age and petrogenesis of the northeastern Idaho batholith: Journal of Geology, v. 89, p. 433-457.

~~Chase , R. B., 1973, Petrology of the northeastern border zone of the Idaho batholith, Bitterroot Range, Montana: Montana Bureau of Mines and Geology Memoir 43, 28 p.~~

~~______, 1977, Structural evolution of the Bitterroot dome and zone of cataclasis: rn Geol. Soc. America Field Guide No. 1, Rocky Mountain Sec. meeting. University of Montana, Missoula, Mt., p. 1-24.~~

Chase, R. B., Bickford, M. E., and Tripp, S. E., 1978, Rb-Sr and U-Pb isotopic studies of the northeastern Idaho batholith and border zone: Geological Society of America Bulletin, v. 89, p. 1325- 1334.

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 42~~

Chase, R. B., Bickford, M. E., and Arruda, E. C., 1983, Tectonic implications of Tertiary intrusion and shearing within the Bitterroot dome, northeastern Idaho batholith: Journal of Geology, v. 91, no. 4, p. 462-470.

~~Cheney, J. T., 1975, Kyanite, sillimanite, phlogopite, cordierite layers in the Bass Creek anorthosites, Bitterroot Range, Montana: Northwest Geology, v. 4, p. 77-82.~~

Criss, R. E., and Taylor, H. P., Jr., 1978, Regional 1*0/x<0 and D/H variations in granitic rocks of the southern half of the Idaho batholith and the dimensions of the giant hydrothermal systems associated with the emplacement of the Eocene Sawtooth and Rocky Bar plntons: Geological Society of America Abstracts with Programs, v. 10, p. 384.

~~Faure, G., 1977, Principles of isotope geology: New York, John Wiley and Sons, 464 p.~~

~~Ferguson, J. A., 1975, Tectonic implications of some geochronometric data from the northeastern border zone of the Idaho batholith: Northwest Geology, v. 4, p. 53-58.~~

~~Fleck, R. J., 1980, Latest Cretaceous and early Tertiary emplacement of the Bitterroot lobe of the Idaho batholith: Geological Society of America Abstracts with Programs, v. 12, no. 6, p. 273.~~

Garmezy, L., and Sutter, J. F., 1983, Mylonitization coincident with uplift in an extensional setting, Bitterroot Range, Montana- Idaho: Geological Society of America Abstracts with Programs, v. 15, no. 6, p. 578.

~~Granert, B., and Hofmann, A., 1973, Old radiogenic lead components in zircons from the Idaho batholith and its metasedimentary aureole: Carnegie Institute Washington, Year book no. 72, p. 297-299.~~

~~Hamilton, E. I., 1965, Applied geochronology: London, Academic Press, Inc., Ltd., 267 p.~~

~~Hamilton, W. B., 1969, Mesozoic California and the underflow of the Pacific mantle: Geological Society of America Bulletin, v. 80, p. 2409-2429.~~

~~______, 1976, Tectonic history of west-central Idaho: Geological Society of America Abstracts with Programs, v. 8, p. 378-379.~~

~~Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 43~~

Hurley, P. M., Bateman, P. C., Fairbairn, H. W., and Pinson, W. H., Jr., 1965, Investigations of initial Sr^’/Sr** ratios in the Sierra Nevada plutonic province: Geological Society of America Bulletin, v. 76, p. 165-174.

Jaffey, A. H., Flynn, K. F., Glendenin, L. E., Bentley, W. C., and Essling, A. M., 1971, Precision measurements of half-lives and specific activities of a3SU and ***D: Physics Review, C, v. 4, p. 1889-1906.

~~Krogh, T. E., 1973, A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determinations: Geochimica et Cosmochimica Acta, no. 37, p. 485-494.~~

~~McDowell, F. W., and Kulp, J. L., 1969, Potassium-Argon dating-of the Idaho batholith: Geological Society of America Bulletin, v. 80, p. 2379-2382.~~

~~Moorhouse, V. V., 1956, The paragenesis of accessory minerals: Economic Geology, v. 51, no. 3, p. 248-262.~~

~~Murthy, M. V. N., 1958, On the crystallization of accessory zircon in granitic rocks of magmatic origin: Canadian Mineralogist, v. 6, part 2, p. 260-263.~~

~~Nier, A. 0., 1939, The isotopic constitution of radiogenic leads and the measurement of geologic time. Physics Review, no. 55, p 153- 163.~~

~~Nold, J. L., 1974, Geology of the northeastern border zone of the Idaho batholith, Montana and Idaho: Northwest Geology, v. 3, p. 47-52.~~

~~Stacey, J. S., and Kramers, J. D., 1975, Approximation of terrestrial lead isotope evolution by a two-stage model: Earth and Planetary Science Letters, v. 26, p. 207-221.~~

~~Talbot, James L., 1976, The structural environment of the northern Idaho batholith: Geological Society of America Abstracts with Programs, v. 8, no. 3, p. 414.~~

Talbot, J. L., and Hyndman, D. V., 1975, Consequence of subduction along the Mesozoic continental margin west of the Idaho batholith: Geological Society of America Abstracts with Programs, v. 7, p. 1290.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ______and ______, 1973, Relationship of the Idaho batholith strnctnres to Montana lineament: Northwest Geology, v. 2, p. 48-52.

~~Tilton, G. R., 1960, Volume diffusion as a mechanism for discordant lead ages: Journal of Geophysical Research, v. 65, no. 9, p. 2933-2945.~~

~~Vallier, T. L., 1973, Pre-Tertiary geology of the Snake River Canyon northeastern Oregon and western Idaho: Geological Society of America Abstracts with programs, v. 5, no. 7, p. 846.~~

~~Watson, E. B., 1979* Zircon saturation in felsic liquids: experimental results and applications to trace element geochemistry: Contributions to Mineralogy and Petrology, v. 70 p. 407-419.~~

~~Wetherill, G. W., 1956, Discordant uranium-lead ages (I): Transactions of the American Geophysical Union, v. 37, p. 320- 326.~~

~~York, D., 1966, Least-squares fitting of a straight line: Canadian Journal of Physics, no. 44, p. 1079-1086.~~

~~York, D., and Farquhar, R. M., 1972, The earths' age and geochronology: New York, Pergamon Press Inc., 178 p.~~

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