Constraints on the Forrnation of the 

Bitterroot Lobe of the Idaho Batholith, 

Idaho and Montana, fr,om U-Pb Zircon 

Geochronology and Feldspar Pb Isotopic Data 

U.S. GEOLOGICAL SURVEY BULLETIN 2008 I 
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Ave. 
Constraints on the Formation of the 

Bitterroot Lobe of the Idaho Batholith, 

Idaho and Montana, from U-Pb Zircon 

Geochronology and Feldspar Pb Isotopic Data 

By MARGO I. TOTH and JOHN S. STACEY 

U.S. GEOLOGICAL SURVEY BULLETIN 2008 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1992 

For sale by the 

Books and Open-File Reports Section 

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Library of Congress Cataloging-in-Publication Data 

T oth, Margo I. 

Constraints on the formation of the Bitterroot lobe of the Idaho Batholith, Idaho 

and Montana, from U-Pb zircon geochronology and feldspar Pb isotopic data I by 

Margo I. Toth and JohnS. Stacey. 

p. em. - (U.S. Geological Survey bulletin; 2008) 

Includes bibliographical references. 

1. Idaho Batholith (Idaho and Mont.) 2. Uranium-lead 

dating. 3. Zircon~ldaho Batholith (Idaho and Mont.) 4. Feldspar-Idaho 

Batholith (Idaho and Mont.) 5. Geology, Stratigraphic-Proterozoic. I. Stacey, 

J. S. II. Title. Ill. Series. 

QE75.B9 no. 2008 

[QE611.5.U6J 

557.3 s-dc20 91-37569 

[552'.3'09796] CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Geologic setting 2 

Geochronology 4 

Sample selection 4 

Analytical methods 4 

U-Pb zircon data 6 

Proterozoic samples 6 

Jurassic samples 7 

Cretaceous samples 7 

Tertiary samples 7 

Feldspar Pb data 8 

Discussion 10 

Acknowledgments 12 

References cited 12 

FIGURES 

1. Geologic map of the Bitterroot lobe of the Idaho batholith showing sample 

locations and sample numbers 3 

2. Concordia plots for zircon data that have Cretaceous lower intercepts 8 

3. Concordia plots for zircon data that have Tertiary lower intercepts 9 

4. Pb isotope data for feldspars from most of the samples in this study 11 

TABLES 

1. U-Pb analytical data for zircons and monazite from the Bitterroot lobe 5 

2. Summary of U-Pb zircon ages, U-Pb model ages, and sample descriptions from 

the Bitterroot lobe 6 

3. Feldspar Pb isotope data, arranged by descending 206PbP04Pb values 10 

Contents Ill 

Constraints on the Formation of the 

Bitterroot Lobe of the Idaho Batholith, 

Idaho and Montana, from U-Pb Zircon 

Geochronology and Feldspar Pb Isotopic Data 

By Margo I. Toth and John S. Stacey 

Abstract 

Zircons from Cretaceous and Tertiary plutonic rocks of 

the west-central portion of the Bitterroot lobe of the Idaho 

batholith contain abundant inherited cores that make precise 

U-Pb age determination difficult. Zircons from a tonalite 

emplaced along the western periphery of the batholith yield an 

almost concordant age of 94±1.4 Ma. Monzogranite and 

granodiorite plutons were intruded along the northern edge of 

the lobe from 75 to 71 Ma. Subsequently, the volumetrically 

more dominant plutons in the central and western parts of the 

lobe were emplaced between 59 and 54 Ma; most of the 

intrusive activity in this area probably occurred at about 

55 Ma. 

The most accurately determined upper-intercept age for 

the Cretaceous plutons is 1,779±20 Ma. This is similar to the 

1,751 ±63 Ma age that was obtained for a gneissic muscovitebiotite monzogranite of probable Proterozoic age. For Tertiary 

rocks, upper-intercept ages are more variable, ranging from 

1,812±21 Ma to 1,625±46 Ma. These results, combined with 

Pb isotopic data from feldspars, confirm that the magmas of the 

Bitterroot lobe were derived mainly from an Early Proterozoic 

lower continental crust. Our findings are consistent with 

studies in the northern and northeastern parts of the Bitterroot 

lobe by other workers. 

INTRODUCTION 

The Bitterroot lobe of the Idaho batholith has a 

complex g·eologic history that has made interpretation of 

geochronologic and isotopic data quite difficult. Important 

aspects of this geologic history include: diverse ages and 

Manuscript approved for publication September 6, 1991. 

composttwns of source rocks, variable assimilation of 

country rock, presence of restite minerals (specifically 

xenocrystic zircon), intrusive activity over a period of at 

least 40 m.y., widespread hydrothermal aureoles generated 

by Eocene plutons, rapid uplift, and mylonitization of the 

central and eastern parts of the lobe. 

Many workers have attempted different methods to 

resolve the geochronology of the Bitterroot lobe. Fleck 

(1990), Criss and Fleck (1987), Fleck and Criss (1985a, 

1985b ), Chase and others (1978), Armstrong and others 

(1977), Tripp (1976), and McDowell and Kulp (1969) have 

attempted to solve the problems through the use of Rb-Sr, 

K-Ar, and oxygen isotopic studies; Ferguson (1975) used 

fission-track dating. These studies indicated that the plutons 

of the Bitterroot lobe are mostly Paleocene in age. The 

studies also document a younger thermal event, about 

45-40 Ma, that disturbed the isotopic systems in many of 

the older rocks concurrent with the formation of a mylonitic 

zone at about 48-42 Ma along the eastern border of the 

Bitterroot lobe. 

To understand the generation of the mylonitic rocks, 

Garmezy (1983) and Garmezy and Sutter (1983) relied on 

Ar-Ar dating and detailed petrographic study. Further work 

on the mylonitic rocks was done by Chase and others (1983) 

and Arruda (1981) using U-Pb methods. 

U-Pb studies of the Bitterroot lobe include those by 

Shuster and Bickford (1985), Chase and others (1983, 

1978), Bickford and others (1981), Nelson (1981), Tripp 

(1976), and Grauert and Hoffman (1973). These studies 

have dealt mostly with the more accessible northeastern 

border of the Bitterroot lobe and the eastern zone of 

mylonites. Although Grauert and Hoffman (1973) were the 

first workers to recognize the presence of a component of 

old zircon in the granitic rocks, the work of Bickford and 

others (1981) is the most detailed study of the Proterozoic 

zircon components. 

Introduction 1 
The objective of our work is to study the evolution of 

the plutonic rocks of the Bitterroot lobe by using U-Pb data 

from zircon and monazite; we also present new data from 

interior, less accessible parts of the lobe. Common-Ph data 

from feldspars in the same samples provide additional 

information on source-rock characteristics. We compare our 

results with those of other workers and summarize the 

constraints on the ages and origins of the plutons of the 

Bitterroot lobe. 

Granitic rocks are classified in this paper according to 

Streckeisen (1973 ). Structure, petrology, and petrogenesis 

of the Bitterroot lobe is described in detail by Hyndman 

(1984), Hyndman and Foster (1987), Foster and Hyndman 

(1990), Garmezy (1983), Reid (1987), and Toth (1987, 

1983a, 1983b). 

GEOLOGIC SETTING 

The Idaho batholith is one in a series of subductionrelated, Mesozoic, granitoid batholiths extending from 

Alaska to Mexico along the Cordillera of western North 

America. The Idaho batholith is similar to the other batholiths in age and petrology, but it was emplaced into mostly 

Proterozoic miogeoclinal metasedimentary rocks instead of 

Phanerozoic eugeoclinal rocks. The batholith is 390 km 

long and 80 to 160 km wide and extends from north-central 

ldah~. and western Montana to southern Idaho, where it is 

covered by basalts of the Snake River Plain. An apparent 

arch of Proterozoic metasedimentary rocks divides the 

batholith into two parts: the Atlanta lobe in the south and the 

smaller and younger Bitterroot lobe in the north. 

The Bitterroot lobe comprises a plutonic complex of 

diverse ages and compositions that crops out over about 

14,000 km2 (fig. 1). The lobe forms a northwest-trending 

tabular body that intrudes Proterozoic metasedimentary 

rocks to the north, south, and east and that intrudes 

allochtonous metavolcanic and metasedimentary rocks to 

the west. Small, isolated Proterozoic bodies of augen gneiss 

occur along and to the north of the Lochsa River (not shown 

in fig. 1 ); these bodies have not been mapped in any detail. 

Three compositionally different plutonic phases are 

contained within the Bitterroot lobe. From oldest to 

youngest these are: hornblende-biotite tonalite and quartz 

diorite plutons (unit Kt, fig. 1); muscovite-biotite granodiorite and monzogranite plutons (unit TKg, fig. 1); and 

hornblende-biotite syenogranite, quartz syenite, and monzogranite plutons (unit Tg, fig. 1). 

Cretaceous mesozonal hornblende-biotite tonalite and 

quartz diorite plutons form the western border of the 

Bitterroot lobe (fig. 1); similar rocks also occur along the 

northeastern border as small, isolated plutons (one of these 

is shown in fig. 1). In hand specimen, the rocks are gneissic 

to well foliated, equigranular, medium grained, antl contain 

accessory sphene, epidote, apatite, zircon, allanite, 

magnetite, and thorite. Major- and trace-element geochemistry (Toth, 1987) suggests that the plutons have 

assimilated some metasedimentary material; this conclusion 

is supported by field evidence. 

Paleocene mesozonal plutons of muscovite-biotite 

granodiorite and monzogranite form the major part of the 

Bitterroot lobe. These plutons intrude tonalite and quartz 

diorite on the western, southwestern, and northern borders 

of the lobe (fig. 1). Complex migmatitic terranes sometimes 

separate different plutons and involve septa of Middle(?) 

Proterozoic, and possibly older, metasedimentary rocks. 

Inclusions of metasedimentary rock are also common along 

margins of plutons and consist of calc-silicates, quartzite, 

schist, and augen gneiss. The appearance, chemistry, and 

mineralogy of the granitic rocks are fairly homogeneous, 

but the work of Williams (1977), Toth (1983b, 1987), and 

Hyndman (1984) have outlined at least ten large plutons 

with different textural or chemical variations; more bodies 

may be present but are, as yet, unidentified. 

The plutonic rocks are typically white to light gray 

and slightly porphyritic with subhedral to euhedral feldspar 

phenocrysts 6-8 mm long in a groundmass of mediumgrained feldspar, quartz, biotite, and muscovite; mediumgrained equigranular rocks are less abundant. Accessory 

minerals consist of magnetite, ilmenite, zircon (some with 

anhedral cores and euhedral overgrowths), monazite, apatite, and rare epidote. Major- and trace-element geochemistry suggests that the plutons may belong to a single 

comagmatic suite (Toth, 1987). The plutons were most 

likely derived from a homogeneous source rock (probably 

Proterozoic metasedimentary rocks) and underwent very 

little differentiation or assimilation before emplacement 

(Toth, 1983b; Hyndman, 1984). 

Large, Eocene, epizonal bodies of subalkalic syenogranite, quartz syenite, and monzogranite intrude both 

country rock and all earlier igneous phases of the Bitterroot 

lobe along the northern, southwestern, and southern sides of 

the lobe (fig. 1). Rhyolitic to andesitic tuff and lava flows 

are preserved in two places in down-dropped fault blocks 

and locally overlie parts of the epizonal plutons on the south 

side of the Bitterroot lobe (Lund and others, 1983). The 

epizonal plutons are unfoliated, medium to coarse grained, 

and contain miarolitic cavities that commonly contain 

crystals of fluorite, smoky quartz, and microcline. Major 

minerals of the epizonal plutons are perthite, plagioclase, 

quartz, biotite, and hornblende; allanite, apatite, baddeleyite, cassiterite, fluorite, monazite, sphene, thorite, 

zircon, and iron-titanium oxide minerals are accessories. 

Major- and trace-element geochemistry (Toth, 1987) 

suggests that the plutons were probably derived from partial 

melting of a depleted upper-mantle source and assimilated 

some crustal material during emplacement. 

A 200- to 500-m-thick mylonite zone, which dips 

shallowly to the east, borders the eastern and southeastern 

sides of the Bitterroot lobe, adjacent to the Bitterroot 

2 Formation of Bitterroot Lobe of Idaho Batholith, Idaho and Montana 
0 20 40 KILOMETERS 

EXPLANATION 

I au I 

w 

Alluvial and glacial deposits, 

undivided (Quaternary) 

Epizonal granitic rocks (Eocene) 

Volcanic rocks (Tertiary) 

Mesozonal granitic rocks of the 

Bitterroot lobe (Paleocene and 

Cretaceous) 

Mylonite in TKg 

Kg 

Kt 

Ym 

Ymi 

Mesozonal granitic rocks of the 

Atlanta lobe (Cretaceous) 

Mesozonal quartz diorite and 

tonalite (Cretateous) 

Metamorphic rocks (Middle(?) 

Proterozoic) 

Migmatite (Middle(?) Proterozoic) 

-----Contact 

-----Fault 

Figure 1. Geologic map of the Bitterroot lobe of the Idaho batholith showing sample locations and 

BRM sample numbers (from table 1 ). Geology from Bond (1978), Toth (1983a), Lund and others 

(1983), and Nold (1968). 

Valley, Montana (fig. 1). The mylonite zone is younger than 

all other rock units except the Eocene epizonal plutons, 

although its formation may have been contemporaneous 

with the intrusion of the younger of the Eocene plutons. The 

formation of the mylonite zone has been attributed to 

gravity sliding of an overlying block (Hyndman, 1977) and 

to emplacement-related phenomena combined with 

extensional tectonism or crustal thinning (Garmezy, 1983; 

Geologic Setting 3 
Garmezy and Sutter, 1983; Chase and others, 1983). 

Mylonitic textures in the granodiorite and monzogranite 

plutons extend into the center of the lobe. 

GEOCHRONOLOGY 

Sample Selection 

State Highway 12, which roughly parallels the 

Lochsa River (fig. 1 ), transects the northern end of the 

Bitterroot lobe of the Idaho batholith (the highway is not 

shown on fig. 1 ), and a cross section of the lobe is 

conveniently exposed along the highway. Relationships 

between the rocks exposed along Highway 12 are described 

in Reid and others (1979). Eight samples were collected 

from this area and included a tonalitic gneiss from the 

western periphery, six granodiorites or monzogranites from 

the mesozonal plutons, and one sample of Proterozoic 

augen gneiss that caps many of the plutons exposed along 

the highway. Farther south, in the central and western parts 

of the Bitterroot lobe, three samples of monzogranite or 

granodiorite were also collected. One of these samples was 

collected close to a younger epizonal pluton, but unaltered, 

unsheared rocks were otherwise chosen. Sample locations 

are shown in figure 1; analytical data are shown in table 1; 

and brief sample descriptions are given in table 2. Samples 

of Eocene epizonal plutons were not collected. 

Analytical Methods 

Methods for chemical separation and mass 

spectrometry of Pb and U in zircons were slightly modified 

from Krogh (1973). Pb blank levels were 0.3 to 1.0 

nanograms. 

Feldspar concentrates were washed with hot 7N 

HN03 and 6N HCl and then leached for 20 minutes in warm 

5 percent HF to minimize any radiogenic Pb derived from in 

situ U and Th decay (Ludwig and Silver, 1977). Leaching 

was followed by microwave dissolution in a closed Teflon 

capsule using a solution of 48 percent HF, 15N HN03, and 

50 percent HC104 (Fischer, 1986). Pb was extracted using a 

chloride-form anion-exchange column. 

Isotope ratios were measured with a 30-cm radius, 

90° sector, computer-controlled mass spectrometer. 

Uranium analyses utilized a triple filament assembly, and 

Pb was run with silica gel on a single rhenium filament. 

Corrections for Pb isotopic fractionation were determined 

from replicate analyses of a standard sample (NBS 

SRM-981) to be 0.12±0.025 percent per mass unit. For 

feldspar Pb, reproducibility of isotope ratios with respect to 

204Pb is within 0.10 percent as determined from duplicate 

filament loadings of feldspar Pb separates. All errors are 

quoted at the 95 percent level of confidence. 

Table 1 shows that most of the 206PbP04Pb ratios in 

the zircons were greater than 1 ,000; the effects of commonPh corrections on our concordia-intercept ages are, 

therefore, generally small. Common-Ph corrections were 

made by using the composition of the appropriate feldspar 

Pb or by using model ratios of Stacey and Kramers (1975) 

corresponding to the intrusion age of the sample. Analytical 

precision is within approximately 1.0 percent for 206PbP38U 

and 207Pb;235U (at the 95 percent confidence level) as 

determined from replicate analyses of a standard concordant 

zircon sample. 

Regression lines for the data on concordia plots are 

calculated using the procedure of York (1969). Concordiaintercept errors are computed from error envelopes 

generated around the regression lines at the 95 percent 

confidence level. For almost all samples in this study, 

scatter in the data around the regression lines exceeds that 

expected from analytical errors alone. Unlike analytical 

error, this extra "geologic" scatter may vary between 

samples, and, therefore, a Student's T factor must be used to 

estimate errors for the slopes of the regression lines and 

their intercepts on concordia (Ludwig, 1980). This 

statistical limitation was not applied in some earlier zircon 

studies of the Idaho batholith (e.g., Bickford and others, 

1981; Chase and others, 1983). For example, a Student's T 

factor for four fractions is 4.3, and such a factor would 

significantly increase the errors claimed for a number of the 

intercept-model ages in those studies. Nevertheless, 

although different methods have been adopted for error 

estimation, ages obtained here are in good agreement with 

previous studies. 

Krogh ( 1982) demonstrated that abrasion could be 

used to remove the more metamict outer portions from 

Archean zircon crystals, thereby yielding more concordant 

U-Pb data. Most samples in our study contain zircons with 

Early Proterozoic cores and younger Mesozoic-Cenozoic 

overgrowths; these zircons are similar to those from farther 

east in the Idaho batholith that were described in detail by 

Bickford and others (1981). For a number of cases in our 

study, abrasion of such composite zircons has removed a 

portion of the younger material surrounding older cores and 

has resulted in data with older U-Pb model ages. 

Consequently, when data from all zircon fractions of a 

sample are combined, longer chords are obtained that 

usually yield more precisely determined concordia-intercept 

ages. For our work, abrasion was carried out in an air-driven 

cell developed by J.N. Aleinikoff and reported by Goldich 

and Fischer (1986). 

4 Formation of Bitterroot Lobe of Idaho Batholith, Idaho and Montana 
Table 1. U-Pb analytical data for zircons and monazite from the Bitterroot lobe of the Idaho batholith 

[Atomic ratios are corrected for common Pb as described in the text. Abbreviations following mesh-size designations are: M, magnetic; NM, 

nonmagnetic; A, abraded; no abbreviation indicates that the sample was not split magnetically and was not abraded] 

Sample no. Sample u Pb Atomic ratios ~12arent ages (Ma) Measured 

(mesh size) weight (ppm) (ppm) 206Pb/ 207Pb/ 207Pb/ 206Pb/ 207Pb/ 207Pb/ 206Pb/ 

(milligrams) 238u 23su 206pb 238u 235u 206pb 204pb 

BRM-3 

( + 100) ............................ 12.01 724 17.6 .02370 .2878 .08807 151 257 1384 2467 

( -400) .............................. 15.15 834 8.68 .01021 .0783 .05558 66 77 4372 183 

BRM-4 

( + 150)NM ..................... 11.04 853 63.6 .07224 .9902 .09941 450 699 161 12476 

(-150+250)M ................... 7.78 994 65.4 .06259 .8130 .09420 391 604 1512 4334 

( -325 + 400)M ................. 1 0.97 971 60.2 .05982 .7715 .09354 375 581 1499 16042 

( -400) .............................. 12.36 931 57.9 .06019 .7824 .09428 377 587 1513 6050 

BRM-5 

( + 100) .............................. 7.43 1386 59.5 .04224 .4956 .08509 267 409 1318 3458 

( -400)NM ......................... 8.66 1554 68.8 .04421 .5215 .08556 279 426 1328 5444 

BRM-6 

( +200)NM ..................... 14.3 997 30.9 .02940 .3650 .09002 187 316 1426 2035 

( + 200)NM,A ................. 17.06 922 32.3 .03314 .4186 .09159 210 355 1459 3500 

( -400)M ........................... 12.06 1391 28.3 .01958 .2190 .08109 125 201 1224 4380 

BRM-7 

( + 100) ............................ 32.59 485 52.4 .10019 1.4431 .10447 616 907 1705 1118 

( + 100)NM,A ................... 1.29 557 87.9 .15291 2.2420 .10634 917 194 1737 4683 

( -150 + 200) ....................... 9.36 602 53.5 .08468 1.1956 .10239 524 799 1668 7126 

( -400) .............................. 36.28 687 35.1 .04816 .6440 .09698 303 505 1567 12520 

(-400)NM ......................... 4.23 633 41.8 .06202 .8497 .09937 388 624 1612 5540 

(-400)M ............................. 8.10 684 39.3 .05386 .7250 .09764 338 554 1580 4668 

BRM-8 

( + 100)NM ..................... 11.25 1116 133.6 .13921 2.0530 .10696 840 1133 1748 23970 

( + 100)A ......................... 10.57 636 159.0 .24210 3.5900 .10750 1398 1547 1758 17140 

(-250)NM ....................... 19.96 972 190.0 .19010 2.7872 .10630 1122 1352 1737 54640 

(-325)M ............................. 5.96 1062 183.6 .16641 2.4170 .10534 992 1248 1720 10280 

( -400)NM ......................... 6.57 956 137.6 .16720 2.4515 .10634 997 1258 1737 6650 

BRM-9 

( + 100)NM ................... 31.74 219 3.6 .01467 .0972 .04806 94 94 102 531 

(-100+ 150)NM .............. 33.25 246 3.6 .01403 .0958 .04955 90 93 174 1284 

(-150+200) ..................... 31.54 269 4.4 .01495 .1017 .04934 95 98 164 630 

( -325 + 400) ..................... 32.59 339 5.4 .01481 .0995 .04871 95 96 134 876 

BRM-10 

( -100 + 150) ....................... 8.53 840 29.1 .03311 .4307 .09433 210 364 1515 1806 

( -150 + 200) ..................... 14.58 882 24.3 .02653 .3256 .08900 169 286 1404 2574 

( -325 + 400) ..................... 32.59 849 14.0 .01394 .1427 .07428 89 135 1049 334 

(-400) .............................. 31.27 1062 11.0 .00980 .0862 .08900 70 84 734 2994 

BRM-11 

( + 150)NM ..................... 22.17 646 16.2 .02477 .2918 .08543 158 260 1325 4340 

(-400)NM ....................... 11.62 734 10.2 .01326 .1201 .06569 85 115 796 855 

BRM-12 

( + 150) .............................. 7.98 1282 28.7 .02226 .2474 .08060 142 224 1212 4149 

( -250 + 325)M ................... 8.05 1258 36.2 .02773 .3244 .08485 176 285 1312 4077 

(-400) .............................. 18.27 1443 39.4 .02689 .3117 .08408 171 276 1295 8528 

BRM-13 

( +200)NM ..................... 17.94 762 39.0 .04964 .6646 .09710 312 517 569 8260 

(-250)M ............................. 5.99 1005 33.7 .03182 .3882 .08849 202 333 1353 7973 

Monazite .......................... 5.11 670 102.4 .00951 .0605 .04613 61 61 60 222 

Geochronology 5 
Table 2. Summary of U-Pb zircon ages, U-Pb model ages, and sample descriptions from the Bitterroot lobe 

[Errors are at the 95 percent level of confidence. MSWD, mean standard weighted deviates of Brooks and others (1972)] 

Coordinates Zircon lnterce52t-model ages 

Sample no. Oat) fractions Lower Upper MSWD Sample description 

Oong) regressed (Ma) (Ma) 

Proterozoic 

BRM-5 ....... 46~'30•N. 2 • • • Proterozoic augen gneiss at Badger Creek, 

................. 114°54'40·w. approx 90% Pb loss at approx 55 Ma. 

BRM-8 ....... 46°13'42•N. 3 24±140 1,751±63 53.2 Gneissic muscovite-biotite monzogranite, 

................. 115~'49•W. Pb loss effectively at approx 115 Ma. 

Jurassic 

BRM-4 ....... 46~'06•N. 4 170±52 2,000±200 2.5 Medium-grained, muscovite-biotite 

................. 114°52'15·w. granodiorite from near roof of pluton. 

Cretareous 

BRM-7 ....... 46°17'06•N. 6 71±9 1,779±20 6.2 Medium-grained, muscovite-biotite 

................. 115on'06·w. monzogranite. 

BRM-9 ....... 46°08'24•N. 3 94±1.4 Concordant 0.003 Medium-grained, gneissic biotite tonalite; 

................. 1150J6'06•W. some (primary?) muscovite. 

BRM-13 ..... 46~7'06•N. 2 15±5 1,801±23 0 Medium-grained biotite granodiorite; slightly 

................. 115°05'3o·w. mylonitized. 

Tertiary 

BRM-3 ....... 45°56'53•N. 2 57±1.3 1,812±21 

................. 114°46'23•W. 

BRM-6 ....... 46~1'30•N. 3 54±2.5 1,722±20 

................. l15°17'12•W. 

BRM-10 ..... 46°09'42•N. 4 47±11 1,720±130 

................. l14°47'12•W. 

BRM-11 ..... 46°13'12•N. 2 59±1.7 1,738±25 

................. 114~7'06•W. 

BRM-U ..... 46~'30•N. 3 55±5.4 1,625±46 

................. 114°56'06•W. 

• Almost coincident data; no regression possible 

U-Pb Zircon Data 

Sample locations are shown on figure 1 and are 

described in table 2. Analytical data are presented in table 1, 

and the results of age computations are shown in table 2. 

The samples are discussed below in order from Proterozoic 

to Tertiary. 

Proterozoic Samples 

BRM-5: Some of the Cretaceous and l?aleocene 

plutons clearly intrude a Proterozoic augen gneiss unit that 

0 Medium-grained biotite monzogranite. 

1.6 Medium-grained, foliated, muscovite-biotite 

monzogranite. 

89.4 Medium-grained, weakly foliated, biotite 

granodiorite. 

0 Foliated, muscovite-biotite granodiorite 

porphyritic gneiss. 

0.67 Slightly foliated, medium-grained, muscovitebiotite granodiorite; capped by Proterozoic 

augen gneiss. 

is well exposed at several points along Highway 12 (Reid 

and others, 1979). Xenoliths of petrologically similar augen 

gneiss occur along pluton margins in the center of the lobe. 

The augen gneiss was sampled near the northeastern edge of 

the lobe at Badger Creek (BRM-5, fig. 1 ). Unfortunately, 

the two data points are very discordant and almost 

coincident because the zircons in both fractions have lost 

almost 90 percent of their Pb. An attempt was made to 

abrade the remainder of these highly metamict zircons, but 

they completely disintegrated in the process. The proximity 

of this sample to the younger intrusive rocks of the batholith 

probably accounts for this discordant data. A concordia plot 

6 formation of Bitterroot Lobe of Idaho Batholith, Idaho and Montana 
is not presented for BRM-5, and the data are useless for 

geochronology. However, similar augen gneisses have been 

dated elsewhere in central and northern Idaho at 1,370±10 

Ma and 1,576±13 Ma by Evans and Fischer (1986) and 

Evans and Zartman (1990). 

BRM-8: This sample is a muscovite-biotite monzogranite from a pluton along the northwestern edge of the 

batholith and was collected between Proterozoic metamorphic country rock and a Proterozoic migmatite unit. 

Although the rock has some similarity to some other phases 

of the Bitterroot lobe in hand specimen, it is strongly 

gneissic. In thin section, the rock shows several features that 

contrast to the main plutons of the Bitterroot lobe. The 

sample is strongly foliated; contains large, euhedral grains 

of muscovite; and has a higher concentration of euhedral, 

accessory zircon and monazite, most of which form large 

radioactive haloes in the biotite and muscovite. Our data 

show that Early Proterozoic zircons in this sample are the 

least discordant of any we have analyzed and have an 

upper-intercept age of 1,751±63 Ma (fig. 2A, table 2). The 

MSWD (mean standard weighted deviates of Brooks and 

others (1972)), equal to 53, indicates that the zircons of this 

pluton must have had a complex Phanerozoic history (the 

lower-intercept model age of 24±140 Ma is meaningless). 

BRM-8 is most likely a Proterozoic remnant of either 

wallrock or source rock for the Bitterroot lobe and was 

severely affected during emplacement of the batholith. 

Jurassic Samples 

BRM-4: This sample is from a muscovite-biotite 

.. granodiorite pluton from the northeastern edge of the 

batholith. Four zircon fractions yield a significant scatter in 

the concordia diagram and a very poorly defined "age" of 

170±52 Ma (fig. 2B, table 2). This sample is within several 

miles of Proterozoic metamorphic wallrocks. The sample 

apparently assimilated material of different ages from the 

source region or wallrocks, resulting in the scatter in the 

zircon data. The U-Pb systems in the zircons may also have 

been disturbed by later events in the batholith. These data 

demonstrate the difficulty of zircon dating for many of the 

rocks of the batholith. 

Cretaceous Samples 

BRM-7: This muscovite-biotite monzogranite was 

collected near the northwestern edge of the batholith at 

Macaroni Creek where it enters the Lochsa River. Five data 

points from BRM-7 form a chord with intercept-model ages 

of 72±16 and 1,782±39 Ma. Data from abrasion of the 

plus-1 00-mesh fraction extends the chord and improves the 

precision of the intercept ages to 71±9 and 1 ,779±20 Ma 

(fig. 2C, table 2). 

BRM-9: This sample is from one of the older biotite 

tonalite gneisses at the western edge of the Bitterroot lobe 

(Toth, 1987). Three zircon fractions, each having small 

inherited components, yield a lower-intercept age of 94±1.4 

Ma (fig. 2D, table 2). 

BRM-13: This biotite granodiorite is from the 

northern part of the Bitterroot lobe. A chord for two 

fractions gives concordia intercepts of 75 Ma and 1,801 Ma 

with analytical errors of 5 and 23 Ma, respectively-these 

errors are surely underestimated (fig. 2C, table 2). These 

data plot close to the chord for BRM-7 and are nearer to the 

concordia lower intercept. It seems likely that BRM-13 was 

intruded during the same event as BRM-7, approximately 

72-71 Ma. 

Tertiary Samples 

Four widely spaced Tertiary samples have lowerintercept ages that are almost identical, in the range 59 to 54 

Ma; a fifth sample may be somewhat younger: 47±11 Ma. 

BRM-3: This biotite monzogranite is in the southern 

part of the Bitterroot lobe, close to several Eocene plutons. 

Only two zircon fractions have been analyzed from this 

sample; these fractions yielded concordia-intercept ages of 

57±1.3 and 1,812±21 Ma (fig. 3A, table 2). For two 

fractions, the computed uncertainties are for analytical 

errors only and are probably underestimates. Data for the 

minus-400-mesh fraction lie very close to concordia, and 

any geologic scatter in the data from analysis of additional 

fractions could have only a minimal effect on the precision 

of the lower-intercept age. Therefore, in the absence of 

further data, we estimate the emplacement age of this pluton 

to be about 57 Ma. 

BRM-6: This sample is from one of the monzogranite 

plutons in the north-central part of the Bitterroot lobe. The 

data for three zircon fractions that were analyzed from this 

monzogranite plot farther from the concordia than the other 

samples and give intercept ages of 54±2.5 and 1,722±20 Ma 

with an MSWD of 1.6 (fig. 3B, table 2). 

BRM-10: This is a sample of a biotite granodiorite 

from the central part of the lobe, a few km from the epizonal 

Whistling Pig pluton of Lund (1980). Four zircon fractions 

from this weakly foliated granodiorite exhibit scattered data 

about a chord on the concordia with an MSWD of 89.4 (fig. 

3C, table 2). The intercept ages are 47±11 and 1,720±130 

Ma. The Whistling Pig pluton was dated by K-Ar methods 

on hornblende at 51.3±1.0 Ma (Toth, 1983b) and clearly 

intrudes the pluton from which BRM-1 0 was collected. 

Significant hydrothermal alteration effects extend into the 

country rocks for several km around the Whistling Pig 

pluton, and it is quite likely that Pb loss occurred in sample 

BRM-1 0; this yielded the scatter in the data and the 

anomalously young age. 

BRM-11: This muscovite-biotite granodiorite is in the 

eastern part of the Bitterroot lobe, close to the mylonite 

front, and shows mylonitic textures. Two zircon fractions 

give concordia-intercept-model ages with unrealistic errors 

Geochronology 7 
0.24 

::::> 

~ 

:0 t 0.16 

A 

4 

INTERCEPTS AT 

2000 ± 200 and 170 ±52 Ma 

(MSWD = 2.54) 

5 

0.24 

0.20 

0.16 ::::> 

~ I 0.12 

0.08 

0.04 

0.00 

0.0 

c 

0.0160 

0.4 0.8 1.2 1.6 2.0 

207Pb/235U 

INTERCEPTS AT 

1900± 1000AND94± 1.4 MA 

(MSWD • .00265) 

2.4 

0.2 0.4 0.6 0.8 1.0 0.0120.__--'--~--'-~~--'---'-----L-...1---1--'----J'---J 

1.2 0.080 0.092 0.100 0.104 

207Pb/235U D 207Pb/235U 

Figure 2. Concordia plots for zircon data that have Cretaceous lower intercepts. Data points labeled "A" indicate abraded zircon 

fractions. A, Concordia plot for sample BRM-8; B, Concordia plot for sample BRM-4; C, Concordia plots for samples BRM-7 and 

BRM-13 (open squares indicate data points for sample BRM-7; open circles indicate data points for sample BRM-13); 0, 

Concordia plot for sample BRM-9. 

of 59±1.7 and I,738±25 Ma (fig. 3A, table 2). The point for 

the smallest sized fraction also plots fairly close to concordia, and, in the absence of further data, we conclude that the 

age is approximately 59 Ma. 

BRM-12: This sample is from one of the muscovitebiotite plutons in the northeastern part of the Bitterroot lobe 

and is capped by Proterozoic augen gneiss. Data from three 

zircon fractions form a linear array (MSWD=0.67) with 

intercept ages of 55±4 Ma and 1,625±46 Ma (fig. 3D, 

table 2). 

BRM-I2 is from one of the plutons that intrudes the 

Proterozoic augen gneiss of BRM-5, and the sample was 

taken about I 0 m below the roof of the pluton. The 

upper-intercept age of I ,625±46 Ma is the youngest that we 

have found in this part of the Bitterroot lobe; most of the 

other upper-intercept ages are in the range from I,780 to 

I,720 Ma (table 2). It is possible that zircons that were 

assimilated from the approximately I ,500-Ma augen gneiss 

are responsible for the younger upper-intercept age of this 

sample. 

Feldspar Pb Data 

Pb isotopic data for feldspars are presented in table 3 

and are plotted in figure 4. 

In the previous section, we concluded that monzogranite BRM-8 is probably a remnant of one of the Early 

Proterozoic source rocks that was significantly affected by 

emplacement of the batholith. The Pb isotopes are 

considerably more radiogenic in this sample than in any of 

8 Formation of Bitterroot Lobe of Idaho Batholith, Idaho and Montana 
:::> co 

C') 

C\1 

::0 

a.. (I) 

0 

C\1 

:::> 

~ C\1 

0.03 

0.02 

0.01 

::0 0.02 

~c._ 

0.01 

0.08 

A 

B 

INTERCEPTS AT 

BRM-3: 1812±21 and 57± 1.3 Ma 

(MSWD=O) 

BRM-11: 1738 ± 25 and 59± 1.7 Ma 

(MSWD=O) 

0.16 0.24 0.32 0.40 

201Pb/23su 

INTERCEPTS AT 

1722 ± 20 and 54± 2.5 Ma 

(MSWD= 1.6) 

0.16 0.24 0.32 0.40 

201Pb/23su 

0.48 

0.48 

0.04 

:::> 0.03 ~ C\1 

::0 

~c._ 0.02 

0.01 

0.04 

co::> 0.03 C') 

C\1 

::0 

c._ 

~ 0.02 

0.01 

0.08 0.16 0.24 0.32 0.40 0.48 

c 201Pb/23su 

0.08 0.16 0.24 0.32 

D 201Pb/23su 

Figure 3. Concordia plots for zircon data that have Tertiary lower intercepts. Data points labeled "A" indicate abraded zircon 

fractions. A, Concordia plots for samples BRM-3 and BRM-11 (open squares indicate data points for sample BRM-3; open circles 

indicate data points for sample BRM-11); 8, Concordia plot for sample BRM-6; C, Concordia plot for sample BRM-10; 0, 

Concordia plot for sample BRM-12. 

the other samples. The data are consistent with this being a 

Proterozoic crustal rock in which the feldspar Pb was 

equilibrated with the whole rock at the time of batholith 

emplacement during the Cretaceous. The isotopic resetting 

of Pb in feldspars is a common phenomenon and occurs at 

relatively low temperatures, as demonstrated in the study of 

the Eldora ·stock in Colorado by Doe and Hart (1963). For 

Proterozoic augen gneiss BRM-5, the feldspar Pb was also 

probably reset at the time of batholith intrusion. Although 

BRM-5 may well have been derived from the same crust 

(approximately 1,780 Ma) as the younger plutons, the 

feldspar Pb isotopic data indicate a different history: 

perhaps there was an additional U-Pb fractionation at about 

1,500 Ma when the augen gneiss was metamorphosed. From 

the zircon data, we have postulated some contamination of 

BRM-12 by BRM-5; the similar Pb isotopic compositions 

in both graphs of figure 4 may be evidence for this. 

If closed system conditions are assumed for Pb 

evolution in BRM-8 between 1,780 and approximately 100 

Ma, then the 238UP04Pb (Jl) value was 15.8 (average crustal 

J-!=:9.7). During the same period, for evolution of Pb in the 

source rock of the least radiogenic sample (BRM-1 0), Jl is 

computed to be 8.4--this is consistent with Pb evolution at a 

deep level in the Proterozoic crust (Zartman, 1965). 

Intermediate Jl values were computed for the other samples, 

and all are shown in table 3. If the linear trend evident in the 

207PbP04Pb versus 206PbP04Pb plot of figure 4 is given an 

age interpretation and if closed system evolution from a 

common isotopic source is assumed, then for a common 

crystallization age of 60 Ma, the average age of the source 

Geochronology 9 
Table 3. Feldspar Pb isotope data, arranged by descending 206Pbf04Pb values 

e38UP04Pb (Jl) values are values in source rocks that are calculated assuming a closed system between 1,760 Ma 

and time of crystallization] 

Sample 206Pb/ 207Pb/ 

no. 204pb 204pb 

BRM-8 ............... 20.552 15.808 

BRM-9 ............... 19330 15.677 

BRM-5 ............... 19307 15.637 

BRM-12 ............. 19.247 15.644 

BRM-7 .............. 19.063 15.651 

BRM-6 ............... 18.805 15.629 

BRM-11 ............. 18.478 15.597 

BRM-13 ............. 18.294 15.574 

BRM-10 ............. 18.270 15.562 

is computed to be 1,650±120 Ma. This encompasses the 

entire age spectrum of basement rocks that we have found in 

this part of the batholith. The trend in figure 4 is consistent 

with derivation of Phanerozoic rocks by remobilization of 

the Proterozoic basement rocks that they intrude. 

Because of their relatively low values of 207PbP04Pb 

with respect to 206PbP04Pb, the least radiogenic data plot 

just below the average growth curve of Stacey and Kramers 

(1975) for continental crust (fig. 4). Thus, the Early Proterozoic crust from which the batholithic rocks were derived 

was formed from comparatively unevolved material, 

probably in an island arc setting. Because Archean upper 

crustal rocks of the Wyoming craton have very high 

207PbP04Pb ratios (Wooden and Mueller, 1988; Stacey and 

Hedlund, 1983), little contribution to this Early Proterozoic 

crust could have been made from evolved continental crust 

of significantly older age--otherwise the data would plot 

above the growth curve in figure 4B. Our data, therefore, 

indicate that, during formation of the Proterozoic source 

rocks of this part of the lobe, there was little recycling of 

material from the Archean craton that is presently located 

no more than 100 km to the southeast. Shuster and Bickford 

(1985) presented some Pb isotope data from batholithic and 

Proterozoic host rocks in the vicinity of the northeastern 

boundary of the Bitterroot lobe. Feldspar data from granites 

show significant variation in 207PbP04Pb (Shuster and 

Bickford, 1985), and several points form a parallel trend 

that plots above our data. This situation indicates that the 

continental crust along the eastern edge of the lobe 

incorporated a component of more radiogenic, and perhaps 

older, crustal material when it was formed 1,700-1,800 Ma. 

Among the regions bordering the Wyoming Archean 

province for which Pb isotope data are available, data for 

the Bitterroot lobe are most similar to those from the rocks 

and ores of southwestern Colorado, southern New Mexico, 

and Arizona (Doe and others, 1979; Stacey and I;Iedlund, 

1983; Wooden and others, 1988). Galena data from Pre208Pb/ Zircon age 238U/ 

204pb (Ma) 204pb 

41.100 ~1,750 15.8 

39.042 94 11.9 

39.051 a1,500 11.8 

38.976 55 11.6 

38.984 71 11 

38.667 54 10.2 

38343 59 9.1 

38.144 75 8.5 

38.335 47 8.4 

cambrian and Mesozoic-Cenozoic ore deposits in the Belt 

Supergroup, to the north of the Idaho batholith, show 

similarly unevolved Pb isotope characteristics for 

underlying basement rocks (Zartman and Stacey, 1971). 

These observations are evidence that, during the period 

1 ,800-1 ,600 Ma, considerable additions of new crust were 

made by accretion of extensive island arc systems to the 

North American continent (Wooden and others, 1988; 

Nelson and DePaolo, 1985; Stacey and Hedlund, 1983). 

DISCUSSION 

The oldest rocks of the Bitterroot lobe are tonalite and 

quartz diorite plutons that were intruded along the western 

and northeastern margins of the lobe. A tonalite on the 

northwestern periphery of the batholith is dated at 94 Ma in 

this study, and four samples from the northeastern border 

zone of the batholith have been dated by other workers at: 

73 Ma, using U-Pb methods (Bickford and others, 1981 ); 73 

Ma, using U-Pb methods (Chase and others, 1978); 81 Ma, 

using fission-track methods (Ferguson, 1975); and 87 Ma, 

using U-Pb methods (Tripp, 1976). Ages of the tonalite and 

quartz diorite plutons thus span the period from 94 to 73 

Ma; younger phases were emplaced near the eastern edge of 

the lobe. 

Very few data exist concerning sources for the 

tonalitic and quartz dioritic rocks in this region. However, 

zircon U-Pb and feldspar Pb isotopic data indicate that the 

tonalite and quartz diorite magmas ingested some Proterozoic crustal material during emplacement (Toth, 1987). 

Two granodiorite and monzogranite plutons from this 

study were intruded during the period from 75 to 71 Ma 

along the northern edge of the lobe, and we have dated four 

plutons in the central and western parts of the lobe between 

59 and 54 Ma. In the eastern part of the lobe, Shuster and 

1 0 Formation of Bitterroot Lobe of Idaho Batholith, Idaho and Montana 
.0 

a_ 

~ N 

-.0 

41.0 

40.0 

a_ 39.0 0) 

0 

N 

38.0 

A 

37.0~----------~--------------------~--------------------~------------------~ 

15.8 

.0 15.7 

a_ 

~ 

0 

N 

-.0 

a_ 

"' 

~ 15.6 

15.5 

18.0 20.0 21.0 

Figure 4. Pb isotope data for feldspars from most of the samples in this study. Data are labeled with BRM sample numbers (from 

table 1 ). Model growth curves (labeled "S-K") are for average crust (Stacey and Kramers, 1975). MORB trend lines are from 

Tatsumoto (1978). A, Plot of 208PbP04Pb versus 206Pb/204Pb; B, Plot of 207PbP04Pb versus 206PbP04Pb. 

Bickford (1985) give a crystallization age for one sample of 

60 Ma; Bickford and others (1981) cite ages of 55 to 51 Ma 

for crystallization of the nonfoliated granites; Chase and 

others (1978) cite 66±10 Ma as the age of emplacement of 

the granites in their study; Nelson (1981) gives 

crystallization ages of 56 to 46 Ma; and Tripp (1976) reports 

a crystallization age of 67 Ma. In summary, U-Pb dating by 

many workers indicates that the granitic rocks of the 

Discussion 11 
Bitterroot lobe were first emplaced along the northern edge 

of the lobe at 72 Ma; the volumetrically more important 

granitic rocks in the central and western parts of the lobe 

were emplaced between 60 and 51 Ma. In addition, Chase 

and others (1983), Arruda (1981), and Garmezy and Sutter 

(1983) indicate that, during the period from 48 to 42 Ma, 

mylonitization that was approximately contemporaneous 

with emplacement of epizonal plutons in the Bitterroot lobe 

occurred along the eastern border of the lobe. 

The presence of older, xenocrystic zircons in all of the 

studied plutonic units indicates that the magmas were 

derived, at least in part, by partial melting and remobilization of older continental crust. Fleck (1990), Criss and Fleck 

(1987), Fleck and Criss (1985b ), and Shuster and Bickford 

(1985) demonstrate from their Rb-Sr, Pb-Pb, and rareearth-element (REE) data that the main source for granitic 

rocks in the lobe is not the exposed country rock, but is 

instead depleted lower crust. However, in the western and 

central parts of the lobe that we have studied, the data 

indicate that Pb is more radiogenic than in areas along the 

eastern border of the lobe investigated by Shuster and 

Bickford (1985). It is also possible that we have identified 

one of the Early Proterozoic (approximately 1,750 Ma) 

source rocks on the western edge of the batholith. 

Upper-intercept ages for the Cretaceous rocks in table 

2 are consistent with a common age of 1, 780±20 Ma for the 

Proterozoic basement in this northern part of the Bitterroot 

lobe. Upper-intercept ages for the Tertiary plutons are more 

variable, but most lie in the range from 1,812 to 1,720 Ma. 

Assimilation of small components of country rock during 

emplacement may have further reduced some upperintercept model ages (as discussed for BRM-12). Our 

results are similar to those of Bickford and others (1981), 

who found a range of 1,780 to 1,720 Main upper-intercept 

model ages for most of their zircon samples. However, the 

presence of older material along the eastern edge of the lobe 

is indicated by a few upper-intercept-model ages in the 

range of 2,340 to 2,000 Ma. The presence of older material 

is also evident in some of the Pb isotope data from the same 

region (Shuster and Bickford, 1985). 

ACKNOWLEDGMENTS 

Preparation of this manuscript was helped by 

discussions with M.E. Bickford, R.J. Fleck, and J.L. 

Wooden. The analytical work was carried out by L.B. 

Fischer, and we are grateful for the efforts of R. Ogg and D. 

Plume who made most of the mineral separations. 

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12 Formation of Bitterroot Lobe of Idaho Batholith, Idaho and Montana 
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Grauert, B., and Hofmann, A., 1973, Old radiogenic lead 

components in zircons from the Idaho batholith and its 

metasedimentary aureole: Carnegie Institute of Washington, 

Year Book 72, p. 297-299. 

Hyndman, D.W., 1977, Mylonitic detachment zone and the 

Sapphire tectonic block, in Geological Society America 

Field Guide No. 1, Rocky Mountain Section meeting, 

Missoula, Montana, p. 25-31. 

Hyndman, D.W., 1984, A petrographic and chemical section 

through the northern Idaho batholith: Journal of Geology, v. 

92, p. 83-102. 

Hyndman, D.W., and Foster, D.A., 1987, The role of tonalites and 

mafic dikes in the generation of the Idaho batholith: Journal 

of Geology, v. 96, p. 31-46. 

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, v. 37, p. 485-491. 

Krogh, T.E., 1982, Improved accuracy of uranium-lead zircon ages 

by the creation of a more concordant system using an air 

abrasion technique: Geochimica et Cosmochimica Acta, v. 

46, p. 637-649. 

Ludwig, K.R., 1980, Calculation of uncertainties of U-Pb data: 

Earth and Planetary Science Letters, v. 46, p. 212-220. 

Ludwig, K.R., and Silver, L. T., 1977, Lead isotope inhomogeneity 

in Precambrian igneous K-feldspars: Geochimica et Cosmochimica Acta, v. 41, p. 1,457-1,471. 

Lund, K.I., 1980, Geology of the Whistling Pig pluton, SelwayBitteiroot Wilderness, Idaho: Boulder, University of 

Colorado, unpub. M.S. thesis, 115 p. 

Lund, K.I., Rehn, W.M., and Holloway, C.D., 1983, Geologic map 

of the Blue Joint Wilderness Study Area, Ravalli County, 

Montana, and the Blue Joint Roadless Area, Lemhi County, 

Idaho: U.S. Geological Survey Miscellaneous Field Studies 

Map MF 1557-B, scale 1:50,000. 

McDowell, F.W., and Kulp, J.L., 1969, Potassium-argon dating of 

the Idaho batholith: Geological Society of America Bulletin, 

v. 80, p. 2,379-2,382. 

Nelson, B.K., 1981, U-Pb isotopic systematics in the northeast 

border zone of the Idaho batholith, Bitterroot Range, 

Montana: Lawrence, University of Kansas, unpub. M.S. 

thesis, 82 p. 

Nelson, B.K., and DePaolo, D.J., 1985, Rapid production of 

continental crust 1.7 to 1.9 b.y. ago. Nd isotopic evidence 

from the basement of the North American mid-continent: 

Geological Society of America Bulletin, v. 96, p.746-754. 

Nold, J.L., 1968, Geology of the northeastern border zone of the 

Idaho batholith, Montana and Idaho: Missoula, University 

of Montana, unpub. Ph.D. dissertation, 159 p. 

Reid, R. R., 1987, Structural geology and petrology of a part of the 

Bitterroot lobe of the Idaho batholith: U.S. Geological 

Survey Professional Paper 1436, p. 37-58. 

Reid, R.R., Bittner, E., Greenwood, W.R., Ludington, S., Lund, K., 

Motzer, W. E., and Toth, M., 1979, Geologic section and 

road log across the Idaho batholith: Moscow, Idaho, Idaho 

Bureau of Mines and Geology, Information Circular 34, 

20 p. 

Shuster, R.D., and Bickford, M.E., 1985, Chemical and isotopic 

evidence for the petrogenesis of the northeastern Idaho 

batholith: Journal of Geology, v. 93, p. 727-742. 

Stacey, J.S., and Hedlund, D.C., 1983, Lead isotopic compositions 

of diverse igneous rocks and ore deposits from southwestern 

New Mexico and their implications for Early Proterozoic 

crustal evolution in the western United States: Geological 

Society of America Bulletin, v. 94, p. 43-57. 

Stacey, J.S., and Kramers, J.D., 1975, Approximation of terrestrial 

lead evolution by a two-stage model: Earth and Planetary 

Science Letters, v. 26, p. 207-221. 

Streckeisen, A.L., chairman, 1973, Plutonic rocks-classification 

and nomenclature recommended by the lUGS 

subcommission on the systematics of igneous rocks: Geotimes, v. 18, p. 26-30. 

Tatsumoto, Mitsunobu, 1978, Isotopic composition of lead in 

oceanic basalt and its implication to mantle evolution: Earth 

and Planetary Science Letters, v. 38, p. 63-87. 

Toth, M.I., 1983a, Reconnaissance geologic map of the SelwayBitterroot Wilderness, Idaho County, Idaho, and Missoula 

and Ravalli Counties, Montana: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-1495-B, scale 

1:125,000. 

Toth, M.I., 1983b, Structure, petrochemistry, and origin of the 

Bear Creek and Paradise plutons, Bitterroot lobe of the 

Idaho batholith: Boulder, University of Colorado unpub. 

Ph.D. dissertation, 337 p. 

Toth, M.I., 1987, Petrology and origin of the Bitterroot lobe of the 

Idaho batholith: U.S. Geological Survey Professional Paper 

1436, p. 9-33. 

Tripp, S.E., 1976, Rubidium-strontium and uranium-lead geochronology of the northeastern border zone of the Idaho 

batholith, Bitterroot Range, Montana: Kalamazoo, Western 

Michigan University, unpub. M.S. thesis, 73 p. 

Williams, L.D., 1977, Petrology of a section across the Bitterroot 

lobe of the Idaho batholith: Moscow, Idaho, University of 

Idaho, unpub. Ph.D. dissertation, 212 p. 

References Cited 13 
Wooden, J.L., and Mueller, P.A., 1988, Pb, Sr, and Nd isotopic 

compositions of a suite of Late Archean igneous rocks, 

eastern Beartooth Mountains: implications for crust-mantle 

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p. 59-72. 

Wooden, J.L., Stacey, J.S., Doe, B.R., Howard, K.A., and Miller, 

D.M., 1988, Pb isotopic evidence for the formation of 

Proterozoic crust in the southwestern U.S., in, Ernst, W.G., 

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U.S. [Rubey Volume VII]: New Jersey, Prentice-Hall, 

p. 68-86. 

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correlated errors: Earth and Planetary Science Letters, v. 5, 

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Zartman, R.E., 1965, The isotopic composition of lead in microclines from the Llano uplift, Texas: Journal of Petrology, v. 

5, p. 359-408. 

14 Formation of Bitterroot Lobe of Idaho Batholith, Idaho and Montana 

'trU.S. GOVERNMENT PRINTING OFFICE: 1992-673-049146032 


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