Professional Paper 1715

U.S. Department of the Interior

U.S. Geological Survey

Petrology of the Crazy Mountains Dike Swarm

and Geochronology of Associated Sills,

South-Central Montana
Cover.  Prominent ridge northwest of Cottonwood Lake, in the Crazy Mountains, south-central

Montana, composed of the diorite and gabbro phase of the Big Timber stock and intruded by dikes of

the Crazy Mountains dike swarm.
Petrology of the Crazy Mountains Dike 

Swarm and Geochronology of Associated 

Sills, South-Central Montana

By Edward A. du Bray, Stephen S. Harlan, and Anna B. Wilson

Professional Paper 1715

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2006

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Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

du Bray, E.A., Harlan, S.S., and Wilson, A.B., 2006, Petrology of the Crazy Mountains dike swarm and geochronology 

of associated sills, south-central Montana: U.S. Geological Survey Professional Paper 1715, 21 p.

ISBN 1-411-30658-9
Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Acknowledgments.........................................................................................................................................5

Geology............................................................................................................................................................5

Analytical Methods........................................................................................................................................5

Petrography ....................................................................................................................................................8

Geochronology.............................................................................................................................................11

Whole-Rock Chemistry ...............................................................................................................................15

Major-Oxide Data................................................................................................................................15

Trace-Element Data............................................................................................................................17

Discussion.....................................................................................................................................................17

Conclusions...................................................................................................................................................19

References Cited..........................................................................................................................................19

Figures

1.  Maps showing regional geologic setting for the Crazy Mountains dike swarm ...............2

2.  Generalized geologic map of the Big Timber stock ................................................................4

3.  Total alkali-silica variation diagram showing compositions of samples

from the Crazy Mountains dike swarm .....................................................................................9

4.  Graphs showing 40Ar/39Ar age spectra for hornblende and biotite separates

from alkaline intrusive rocks in the Crazy Mountains ..........................................................12

5.  Variation diagrams showing abundances of major oxides and zirconium

in samples from the Crazy Mountains dike swarm...............................................................16

6.  Trace-element tectonic setting–discrimination variation diagram for

samples from the Crazy Mountains dike swarm ...................................................................17

7.  Ternary AFM diagram showing compositions of samples from the

Crazy Mountains dike swarm ...................................................................................................18

8.  Ternary variation diagram showing the relative proportions of Rb, K, and Sr

in samples from the Crazy Mountains dike swarm...............................................................18

Tables

1.  Composition of selected samples of the Crazy Mountains dike swarm..............................6

2.  Data for 40Ar/39Ar step heating of samples of alkaline sills in the

Crazy Mountains .........................................................................................................................13

3.  Summary of isotopic dating results for alkaline sills in the

northern Crazy Mountains.........................................................................................................15

iii
Abstract

The Crazy Mountains dike swarm is a radial array associated with the Eocene (approximately 49 Ma) Big Timber stock 

in the Crazy Mountains of south-central Montana. Dikes are 

hosted by the stock and by Paleocene Fort Union Formation 

rocks that host the stock. The spatial coincidence, field relations, and similarity of petrographic and geochemical features 

suggest that the dikes and the stock are cogenetic and coeval. 

The dikes depict a broad range of compositions, from rhyolite 

to basalt; dike composition is only weakly related to position 

relative to the Big Timber stock. The dikes range from 51 to 

71 weight percent SiO2, a slightly more restricted range than 

that exhibited by the stock. Compositional variation among 

the dikes is less systematic than is characteristic of the stock; 

dike compositions form more broadly diffuse arrays and tend 

to be slightly more alkaline. Both of these factors could result 

from late-stage magmatic-hydrothermal processes that seem 

to have weakly altered many of the dikes. Major-oxide and 

trace-element characteristics of the dikes and the associated 

stock are consistent with a mantle-dominated genesis in a subduction-related volcanic arc setting. In addition to petrologic 

and geochemical data, new 40Ar/39Ar geochronologic data for 

strongly alkaline sills in the northern Crazy Mountains are 

also presented. Dates for the sills, which are peripheral to the 

Crazy Mountains dike swarm, indicate that alkaline magmatism was temporally associated with subalkaline magmatism 

represented by the Big Timber stock and related dikes. Characteristics of the subalkaline magmatism are consistent with 

renewed volcanic arc magmatism during the Eocene westward 

hingeline retreat and subsequent southwestward migration of 

the magmatic front in response to steepening subduction along 

the western edge of North America.

Introduction

Numerous alkaline to subalkaline Eocene intrusions are 

present near the approximate center of the Late Cretaceous–

early Tertiary Crazy Mountains Basin of south-central 

Montana (fig. 1). Most of these intrusions have associated 

well-developed, cogenetic dike swarms. In this area, the 

Crazy Mountains dike swarm is associated with the elliptical 

(8 by 13 km) Big Timber stock, a composite intrusion that 

forms the core of the southern Crazy Mountains. The stock 

is hosted by sedimentary rocks of the Paleocene Fort Union 

Formation (fig. 2). Thousands of dikes, which constitute 

the Crazy Mountains dike swarm, intrude the Big Timber 

stock and its host rock. The dikes are approximately radially 

oriented relative to the stock. The compositional characteristics and variation of the Crazy Mountains dike swarm are the 

principal focus of this study. 

The Crazy Mountains and the dike swarm have been 

the subject of numerous geologic investigations. Even in the 

earliest geologic investigations of the Crazy Mountains, Wolff 

(1892, 1938) and Larsen (1940) noted the Crazy Mountains 

dike swarm. Beginning in the early 1960s, University of 

Cincinnati students initiated a new phase of geologic investigations in this area; in particular, Simms (1966) studied the 

alkaline rocks in the northern Crazy Mountains, Tappe (1966) 

mapped the Big Timber stock, and Starmer (1972) studied the 

Crazy Mountains dike swarm. Subsequently, Dudas (1990, 

1991) completed a reconnaissance geochemical study of the 

Big Timber stock. Then, du Bray and Harlan (1996) completed 

a detailed study concerning the petrogenesis and geochronology of the Big Timber stock. 

The present study is a companion to that of du Bray 

and Harlan (1996) and pertains to the petrology of the Crazy 

Mountains dike swarm and the relationship between the dike 

swarm and the stock. In addition, we present new 40Ar/39Ar 

geochronologic data for alkaline intrusions, primarily in the 

northern Crazy Mountains, which complement 40Ar/39Ar 

geochronologic data for the Big Timber stock previously published by du Bray and Harlan (1996). Because Dudas (1990, 

1991) presented and interpreted significant compositional data 

for the alkaline dikes and sills, the petrology and geochemistry 

of these rocks are not further addressed in this paper.

The Crazy Mountains dike swarm and associated Big 

Timber stock are components of the Eocene central Montana alkalic province of Larsen (1940) and Chadwick (1972) 

and are temporally related to the Eocene Challis igneous 

episode of Armstrong (1978). Like other igneous centers in 

the central Montana alkalic province, igneous rocks in the 

Petrology of the Crazy Mountains Dike Swarm and 

Geochronology of Associated Sills, South-Central 

Montana

By Edward A. du Bray, Stephen S. Harlan, and Anna B. Wilson
CRAZY

M

O

U

NTAIN

N

SYNCLINE

SHIELDS RIVER ANT CI L NI E

ROBINSO

N

ANTICLINE Ti

Ti

Ti

Ti

Ti

Gordon

Butte

Ti

Ibex Mtn

TKu

TKu

TKu

TKu

TKu

TKu TKu

TKu

2

3 4

5

6

COMB CREEK

STOCK

BIG TIMBER

STOCK

LOCO MTN

STOCK

1

EXPLANATION

TKu

Ti Tertiary intrusions

Tertiary and Cretaceous

basin sediments

Fault—Bar and ball on

downthrown side

Anticline—Dashed where

inferred

Syncline—Dashed where

inferred

40Ar/39Ar sample locality—

Locality number shown in

table 3

2

A

0 5 MILES

N

0 5 KILOMETERS

Figure 1. Regional geologic setting for the Crazy Mountains dike swarm, south-central Montana. A, Generalized 

geologic map of the Crazy Mountains showing sites of geochronologically analyzed intrusions. B, Index map for 

figure 1A, showing tectonic elements of the Crazy Mountains Basin. Both modified from Roberts (1972).

 Petrology, Crazy Mountains Dike Swarm, Montana
15 KILOMETERS

0 15 MILES

0

Tertiary and Cretaceous intrusive rocks

Tertiary Fort Union Formation

Cretaceous volcanic and volcaniclastic strata (Sliderock Mountain volcano)

Cretaceous volcaniclastic rocks (Livingston Group)

Phanerozoic sedimentary rocks

Proterozoic Belt Supergroup

Archean crystalline basement rocks

Fault—Bar and ball on downthrown side

Thrust fault

Anticline—Dashed where inferred

Overturned anticline

Syncline—Dashed where inferred

EXPLANATION

MONTANA

Area of

this map

111°W 110°W 109°W

46°N LAKE BASIN FAULT ZONE

BIG BELT UPLIFT

BRIDGER

RANGE

BEARTOOTH

MOUNTAINS

HELENA SALIENT

ROCKY MOUNTAINS

FORELAND PROVINCE

NYE-BOWLER

LINEAMENT

Eastern limit of

Disturbed Belt

Outline of

Crazy Mountains Basin

Area of

Figure 1A

B

Introduction 
MONTANA

Map area

Big Timber stock (Tertiary)

Campfire Lake apophysis

Quartz monzodiorite

Diorite and gabbro

Fort Union Formation (Tertiary)

Contact—Dashed where approximately

located

Dike

Sample Locality

Rhyolite

Trachyte/dacite

Trachyandesite/andesite

Basaltic andesite

Basalt

EXPLANATION

Tcl

Tfu

Tqm

Tdg

Big Timber Creek

Tcl

Tfu

Tfu

Limit of

geologic mapping

Tfu

Tqm

Tqm

Tqm

Crazy

Peak

Tdg

Tdg

Tdg

Tdg

Tdg

739-740

734-735

738

619

332

625

618

622

621

617

620

737

736

401

517

741

606

308

718

712

705

707

501-502

725-726

306

310

510

503 714

704

611 607

724

717

337

508

601-604

742-743

744

402 513

516

514-515

0 1 2 3 MILES

0 1 2 3 KILOMETERS

46°05'

46°00'

110°25' 110°20' 110°15'

 Petrology, Crazy Mountains Dike Swarm, Montana
Figure 2 (facing page). Generalized geologic map of the Big 

Timber stock, south-central Montana (modified from du Bray 

and others, 1993). Numbered symbols show compositions and 

sample localities for chemically analyzed dikes. Numbers on 

map correspond to last three digits (for example, 513) of sample 

numbers (for example, 202513) in table 1.

Crazy Mountains occur in an isolated range east of the Rocky 

Mountain front. Other Eocene igneous rocks in the region 

include subduction-related volcanic arc rocks in the Highwood Mountains (O’Brien and others, 1991, 1995), north 

of the Crazy Mountains, the voluminous Challis Volcanic 

Group in eastern Idaho (Hardyman, 1989; Janecke and Snee, 

1993), and rocks of the Absaroka-Gallatin volcanic province 

(Smedes and Prostka, 1972; Feeley, 2003). The Big Timber 

stock is located near the southeast margin of the southwardextending magmatic belt delineated by Armstrong and Ward 

(1991) for the interval 55-40 Ma. The petrogenesis and 

tectonic implications of the stock and its dike swarm, representing a discrete igneous center located well east of most late 

Mesozoic–early Cenozoic, subduction-related magmatism 

in western North America, are noteworthy and are briefly 

discussed in this study.

Acknowledgments

Geologic mapping and sample collection for this study 

were by E.A. du Bray, J.E. Elliott, A.B. Wilson, B.S. Van 

Gosen, and L.A. Rosenberg during 1992 as part of a mineral resource assessment of the Custer and Gallatin National 

Forests. Constructive reviews by Karen Lund and R.G. Tysdal 

considerably helped the accuracy and clarity of this study.

Geology

The oldest rocks in the central Crazy Mountains are 

those of the Paleocene Fort Union Formation, a succession 

of nonmarine sedimentary rocks at least 2,000 m thick that 

is principally composed of interbedded siltstone, mudstone, 

sandstone, and, locally, pebble conglomerate (Roberts, 1972). 

The Fort Union Formation, which hosts the dike swarm and 

the Big Timber stock, was thermally metamorphosed to hornfels as much as several kilometers from its contact with the 

stock. Metamorphism associated with stock emplacement is 

restricted to development of a narrow hornfels aureole; chlorite, formed by recrystallization of clay-rich matrix material, 

is present in some places. Metamorphism associated with dike 

emplacement is not apparent.

Du Bray and others (1993) defined two phases of the Big 

Timber stock (fig. 2). The volumetrically predominant phase 

is medium- to coarse-grained diorite and gabbro (Streckeisen, 1973). The core, or inner part, of the composite stock 

is fine- to medium-grained quartz monzodiorite that forms an 

irregularly shaped intrusion, including several large dike- or 

sill-like extensions; the two phases of the stock are marked 

by a sharp contact. Cross-cutting relations indicate that the 

quartz monzodiorite phase was emplaced after solidification 

of the diorite and gabbro phase. Du Bray and Harlan (1996) 

presented detailed petrologic and geochronologic data for the 

stock and its components.

Magmatism associated with the Big Timber stock also 

involved emplacement of a well-developed dike swarm. The 

dikes are several centimeters to several tens of meters wide 

and as much as several kilometers long. The number of dikes 

that intrude the quartz monzodiorite phase is less than that in 

the diorite and gabbro phase; most dikes are contemporaneous 

with, or older than, the quartz monzodiorite phase. The broad 

range of dike compositions, including rhyolite, trachydacite/

dacite, trachyandesite/andesite, basaltic trachyandesite/basaltic 

andesite, and trachybasalt, display only weakly systematic spatial distribution (fig. 2). Rhyolite dikes are present only near 

the stock’s core. Most of the trachydacite/dacite dikes crop 

out in the diorite and gabbro phase of the Big Timber stock, 

within 1.5 km of its contact with the inner, quartz monzodiorite phase. Basaltic trachyandesite/basaltic andesite dikes crop 

out exclusively in the Fort Union Formation, in most cases 

more than 3 km from the outer edge of the Big Timber stock. 

Trachybasalt and trachyandesite/andesite dikes do not seem 

to be systematically distributed. In addition, dikes of multiple 

compositions may be present at any sample site. Diagnostic 

cross-cutting relations are relatively rare, so relative age relations between dikes of various compositions are indeterminate. 

As suggested by Roberts (1972), dikes of the swarm (fig. 2) 

form a crudely radial pattern with respect to the Big Timber 

stock (du Bray and others, 1993). Starmer (1972) provided 

additional data concerning the outcrop character, distribution, 

and structural and tectonic features of the dike swarm.

Analytical Methods

In order to develop representative petrographic and 

compositional data for the Crazy Mountains dike swarm, 52 

dike samples were collected (fig. 2). All geochemical analyses (table 1) were performed in analytical laboratories of 

the U.S. Geological Survey in Denver, Colo. Major-oxides 

were analyzed (analysts, D.F. Siems and J.S. Mee) by X-ray 

fluorescence techniques (Taggart and others, 1987). Ratios of 

Fe2+:total iron as Fe2+ were adjusted to 0.90 and major-oxide 

abundances recalculated to 100 percent, volatile free. E.A. du 

Bray determined abundances of Rb, Sr, Y, Zr, Nb, and Ba by 

energy-dispersive X-ray fluorescence spectroscopy (Elsass and 

du Bray, 1982; Yager and Quick, 1992) using 109Cd and 241Am 

radio-isotope excitation sources. The major-oxide compositions of these samples were classified (fig. 3) as volcanic rocks 

according to the nomenclature of Le Bas and others (1986).

Analytical Methods 
1

Table 1. Composition of selected samples of the Crazy Mountains dike swarm, south-central Montana. 

[Note: Major-oxide data in weight percent (normalized to 100 percent, anhydrous); trace-element data in parts per million. Ferrous iron/total iron as FeO 

adjusted to 0.9. LOI, loss on ignition; bdl, below detection limit] 

Sample 202306 202308 202310 202332 202337 202401 202402 202501 202502 202503 202508

SiO2 63.44 59.16 62.51 56.57 57.87 55.31 58.35 58.05 59.31 67.02 59.26

Al2O3 16.12 16.10 18.05 18.72 14.77 17.19 17.29 16.34 18.31 16.17 17.69

Fe2

O3 0.54 0.74 0.47 0.73 0.93 0.94 0.81 0.74 0.60 0.37 0.51

FeO 4.40 5.99 3.79 5.95 7.53 7.63 6.57 6.02 4.84 3.01 4.15

MgO 2.58 2.91 1.43 3.17 5.26 4.08 2.68 4.01 2.12 1.12 2.38

CaO 3.86 4.94 3.83 7.92 5.42 7.48 5.74 5.80 5.35 2.41 6.67

Na2

O 4.55 4.93 4.83 3.37 3.71 3.29 3.68 4.15 4.46 4.36 4.14

K2O 3.14 3.24 3.73 1.90 2.87 2.12 2.96 2.96 3.39 4.61 3.57

TiO2 0.99 1.41 0.96 1.11 1.10 1.26 1.34 1.37 1.09 0.64 1.13

P2

O5 0.30 0.48 0.33 0.43 0.38 0.53 0.45 0.44 0.43 0.21 0.43

MnO 0.08 0.09 0.07 0.12 0.15 0.16 0.13 0.12 0.11 0.07 0.08

LOI 1.95 0.76 1.16 2.16 0.45 1.77 2.41 0.05 bdl 0.33 0.16

Rb 58 57 77 27 83 29 65 68 76 125 53

Sr 804 1,003 788 1,090 726 994 791 1,053 1,008 493 885

Y 16 22 29 28 24 34 35 23 27 14 30

Zr 185 198 275 149 237 176 202 179 173 328 208

Nb 16 20 29 21 14 25 22 25 22 16 22

Ba 1,567 1,897 1,806 1,338 1,418 1,643 1,526 1,698 1,632 1,341 1,611

La 46 68 60 47 72 63 55 53 62 49 42

Ce 70 99 110 90 86 103 85 96 92 86 78

Nd 32 22 62 24 55 66 30 19 32 28 59

Sample 202510 202513 202514 202515 202516 202517 202601 202602 202603 202604 202606

SiO2 56.28 55.50 58.08 59.77 53.39 57.62 58.44 65.23 58.33 57.44 53.95

Al2O3 16.23 16.62 17.49 17.86 16.16 16.32 17.24 17.00 16.77 17.71 16.46

Fe2O3 0.85 0.91 0.74 0.70 0.98 0.80 0.79 0.40 0.78 0.77 1.04

FeO 6.90 7.38 5.98 5.70 7.92 6.51 6.39 3.21 6.35 6.25 8.39

MgO 4.38 4.86 3.17 2.08 5.35 4.45 2.84 1.15 3.56 2.27 4.68

CaO 6.20 7.21 5.25 5.12 7.84 6.63 5.39 2.47 5.49 5.41 5.96

Na2

O 4.00 3.24 4.17 4.19 3.86 3.22 4.31 5.05 4.25 4.81 4.85

K2O 3.29 2.31 3.28 3.06 2.41 2.82 2.78 4.35 2.94 3.27 2.15

TiO2 1.32 1.39 1.27 0.89 1.12 1.09 1.23 0.86 1.00 1.17 1.80

P2

O5 0.45 0.45 0.45 0.50 0.81 0.40 0.46 0.23 0.41 0.77 0.57

MnO 0.09 0.13 0.12 0.13 0.16 0.12 0.12 0.05 0.11 0.13 0.16

LOI 3.38 2.74 2.20 1.79 1.89 1.89 0.44 1.43 0.68 2.56 1.44

Rb 74 40 69 69 28 43 71 87 47 59 42

Sr 762 817 895 925 2,059 664 860 575 987 1,312 897

Y 22 20 30 31 36 13 16 18 24 21 25

Zr 195 157 189 182 147 114 210 306 161 264 173

Nb 23 14 29 17 28 15 23 28 16 30 30

Ba 1,450 1,205 1,577 1,674 2,658 1,448 1,470 2,063 2,255 1,924 1,301

La 28 48 55 53 125 45 44 59 48 67 50

Ce 75 82 111 103 230 77 74 118 83 121 93

Nd 33 29 51 60 77 45 22 46 51 66 49

 Petrology, Crazy Mountains Dike Swarm, Montana
2

Table 1. Composition of selected samples of the Crazy Mountains dike swarm, south-central Montana.—Continued 

Sample 202607 202611 202617 202618 202619 202620 202621 202622 202625 202704 202705

SiO2 69.62 56.58 55.31 59.66 54.14 51.27 61.17 62.61 57.24 56.98 69.30

Al2O3 15.90 18.28 17.22 18.91 14.09 16.06 17.18 17.18 16.68 17.00 15.39

Fe2

O3 0.22 0.72 0.98 0.54 0.89 1.20 0.66 0.56 0.87 0.86 0.28

FeO 1.77 5.81 7.92 4.36 7.21 9.73 5.38 4.50 7.04 6.99 2.27

MgO 1.01 2.35 3.62 1.84 8.23 5.48 2.35 1.70 3.54 3.02 1.40

CaO 2.73 5.34 5.14 4.62 8.42 8.07 4.56 3.66 5.31 5.71 3.63

Na2

O 4.53 5.30 3.72 4.93 2.85 3.04 4.46 5.15 3.94 4.42 3.19

K2O 3.67 3.38 4.00 3.54 2.51 2.63 2.92 3.50 3.66 2.78 3.91

TiO2 0.38 1.55 1.41 1.10 1.07 1.93 0.76 0.59 1.00 1.62 0.45

P2

O5 0.15 0.59 0.54 0.43 0.44 0.44 0.45 0.42 0.59 0.48 0.15

MnO 0.03 0.08 0.15 0.08 0.14 0.16 0.11 0.13 0.13 0.14 0.05

LOI 4.30 0.04 2.48 2.74 3.19 4.04 1.55 1.37 1.21 2.44 5.21

Rb 58 69 81 68 54 66 56 60 68 45 55

Sr 772 1,192 700 925 764 842 1,018 1,418 1,242 904 665

Y 6 28 27 22 14 29 26 39 28 21 8

Zr 105 206 157 225 147 131 194 230 198 147 113

Nb 8 25 19 21 17 20 18 39 14 29 12

Ba 2,680 1,826 1,812 1,960 1,270 1,097 2,263 3,104 2,292 1,297 2,658

La 40 86 74 79 42 31 64 143 85 63 47

Ce 45 108 88 114 117 67 112 189 105 111 51

Nd 26 42 36 40 36 34 39 77 66 16 37

Sample 202707 202712 202714 202717 202718 202724 202725 202726 202734 202735 202736

SiO2 60.75 70.81 64.53 60.11 61.66 58.68 51.06 68.33 63.34 55.20 55.89

Al2O3 16.09 15.18 16.49 17.94 15.73 16.48 14.56 15.59 17.04 17.29 16.58

Fe2O3 0.62 0.24 0.47 0.61 0.59 0.78 1.37 0.35 0.54 0.88 0.85

FeO 5.03 1.96 3.83 4.94 4.75 6.34 11.12 2.83 4.35 7.11 6.91

MgO 3.30 0.59 1.69 2.23 3.96 3.51 6.61 1.83 1.57 4.45 5.17

CaO 4.57 1.84 3.53 4.83 4.74 5.37 6.83 2.43 4.01 7.40 6.93

Na2

O 4.76 4.55 4.85 4.27 4.24 4.12 3.30 3.80 4.66 3.39 3.19

K2O 3.20 4.25 3.42 3.43 2.89 2.81 2.15 4.15 3.38 2.48 2.67

TiO2 1.22 0.40 0.76 1.11 0.99 1.45 2.08 0.49 0.56 1.24 1.25

P2O5 0.38 0.13 0.34 0.42 0.36 0.38 0.74 0.16 0.42 0.42 0.43

MnO 0.08 0.05 0.08 0.10 0.09 0.08 0.19 0.04 0.12 0.13 0.13

LOI 1.59 0.08 0.06 1.59 2.79 0.50 1.56 2.53 0.72 1.84 2.55

Rb 67 97 70 81 50 56 70 66 72 51 58

Sr 687 660 949 834 823 867 798 876 1,249 1,011 868

Y 21 13 18 28 16 17 30 12 31 23 21

Zr 180 204 216 189 185 161 152 113 210 167 160

Nb 22 24 30 23 23 23 20 13 24 17 18

Ba 1,754 2,177 2,077 1,693 1,988 1,687 868 2,723 2,619 1,536 1,449

La 66 58 84 70 55 46 56 47 73 51 52

Ce 85 85 112 78 84 63 91 60 129 86 86

Nd 55 48 52 72 28 37 50 34 29 35 43

Analytical Methods 
3

Table 1. Composition of selected samples of the Crazy Mountains dike swarm, south-central 

Montana.—Continued 

Sample 202737 202738 202739 202740 202741 202742 202743 202744

SiO2 53.03 55.74 54.33 54.57 58.51 57.01 57.20 56.68

Al2O3 14.37 16.03 18.63 16.04 16.97 17.51 16.67 17.44

Fe2

O3 1.09 0.92 0.87 0.98 0.76 0.83 0.86 0.89

FeO 8.81 7.45 7.08 7.94 6.17 6.76 7.00 7.24

MgO 8.13 4.51 3.43 5.22 3.51 3.04 3.59 3.66

CaO 8.22 5.67 7.43 6.76 5.24 5.18 5.45 6.46

Na2

O 2.50 5.05 3.77 4.45 4.23 4.77 4.13 4.56

K2O 1.77 2.76 2.50 1.84 3.03 3.06 3.47 0.80

TiO2 1.54 1.12 1.30 1.14 1.02 1.25 0.95 1.63

P2O5 0.38 0.62 0.52 0.89 0.45 0.44 0.52 0.51

MnO 0.15 0.15 0.14 0.16 0.11 0.14 0.16 0.13

LOI 3.79 2.14 1.80 5.98 1.56 1.94 2.43 6.07

Rb 34 50 42 29 53 61 84 12

Sr 695 991 1,117 1,175 903 1,085 1,009 1,739

Y 18 22 26 33 22 19 23 31

Zr 131 164 176 294 205 172 161 154

Nb 19 19 20 62 14 21 16 27

Ba 1,024 2,019 1,638 2,685 1,838 1,810 1,816 810

La 48 59 46 95 67 30 36 32

Ce 63 120 102 172 79 93 105 97

Nd 27 47 49 87 49 10 51 29

Samples from six intrusions (mostly sills) peripheral to 

the Crazy Mountains dike swarm were dated by the 40Ar/39Ar 

step-heating method. Mineral separates were prepared by standard crushing and sieving techniques, and concentrates were 

prepared by standard gravimetric (heavy liquid) and magnetic 

separation techniques. Mineral concentrates were further purified by hand picking to an estimated visual purity of greater 

than 99.9 percent. Mineral separates were loaded in aluminum 

foil capsules, sealed in silica vials, and irradiated for 30 hours 

in the central thimble of the U.S. Geological Survey (USGS) 

TRIGA reactor in Denver, Colo. Vertical and horizontal 

gradients of neutron fluence in the irradiated package were 

monitored by 8–10 standards distributed along the length of 

each vial; the geometry of the irradiated package was such that 

each unknown sample was adjacent to at least one standard. 

We monitored neutron fluence using hornblende standard 

MMhb–1 (Samson and Alexander, 1987), which has a K-Ar 

age of 520.4±1.7 Ma (Samson and Alexander, 1987). Production ratios measured on pure K2

SO4 and CaF2 salts irradiated 

with the samples were used to correct for irradiation-produced 

40Ar (from K) and 39Ar (from Ca).

After irradiation, samples were progressively degassed in 

a double-vacuum resistance furnace in a series of eight to ten 

20-minute-long steps to a maximum temperature of 1,450°C. 

After each heating step, we collected the gas and purified it 

using Zr-Al-Ti getters, and all five argon isotopes were measured with a mass spectrometer operated in the static mode. We 

calculated apparent ages using decay constants recommended 

by Steiger and Jäger (1977). We evaluated argon data using age 

spectra, apparent 39Ar/37Ar ratios (hornblende separate), and 39Ar/40Ar versus 36Ar/40Ar correlation diagrams. The determination of whether the individual apparent ages in an age spectrum 

yielded a “plateau” was made by applying the criteria of Fleck 

and others (1977). These criteria define a plateau as comprising two or more contiguous gas fractions that yield apparent 

ages statistically indistinguishable at the 95-percent confidence 

level (using the critical-value test as applied by Dalrymple and 

Lanphere, 1969) and that together constitute greater than 50 

percent of the total potassium-derived 39Ar (39ArK) released in 

the step-heating experiment. The plateau age was calculated 

from a weighted mean, where weighting is by the inverse of the 

analytical variance (Taylor, 1982). Total gas, plateau, and correlation ages are given at ±2 σ and include the analytical uncertainty in the determination of the fluence parameter J. Complete details for procedures used in the USGS Denver Argon 

Geochronology Laboratory were presented by Snee (2002).

Petrography

Compositionally and texturally diverse rhyolite-to-basalt 

dikes associated with the Big Timber stock are light gray to 

dark greenish gray, pinkish gray, and dark brown. Most dikes 

are medium to dark gray and porphyritic, containing combinations of feldspar, quartz, clinopyroxene, amphibole, biotite, and 

olivine phenocrysts in an aphanitic to fine-grained groundmass. 

 Petrology, Crazy Mountains Dike Swarm, Montana
Phenocryst content ranges from 5 to about 50 percent and 

averages 20–30 percent. The primary groundmass for most of 

the dikes was probably glassy; however, no primary glass is 

present. The groundmass is now a devitrified intergrowth of 

feldspar, mafic silicate minerals, quartz, opaque oxide minerals, and secondary or alteration minerals. The more detailed 

petrographic descriptions that follow are based on geochemical 

groupings. Geochemical data for the 52 analyzed samples form 

a nearly continuous array but were divided into four groups 

(rhyolite, trachydacite/dacite, trachyandesite/andesite and 

basaltic trachyandesite/basaltic andesite, and trachybasalt) of 

compositionally similar rocks to facilitate synthesis of petrographic data. The descriptions below are synthesized from a 

number of samples for each of the four composition types and 

are presented to characterize mineralogic and petrographic 

variation among dikes of various compositions.

Only two of the sampled dikes (samples 202607 and 

202712) are composed of rhyolite (fig. 3). Rhyolite dikes 

account for about 5 percent of the Crazy Mountains dike 

swarm; consequently, they are not a major component of the 

swarm. Petrographic characteristics define two principal types 

of rhyolite dikes: (1) rapidly cooled or quenched dikes and 

(2) more slowly cooled, phaneritic dikes, which are petrographically equivalent to the fine- to medium-grained quartz 

monzodiorite that forms the irregularly shaped intrusion at the 

core of the Big Timber stock. In addition to the two rhyolite samples, a third sample (202705), whose composition 

is dacite, is petrographically identical to one of the rhyolite 

samples (202607). Given the petrographic characteristics of 

sample 202705, its nearly rhyolite composition, its elevated 

volatile content, and its highly altered nature, we concluded 

that the primary composition of this sample may well have 

been rhyolite; its petrographic features are synthesized with 

those of sample 202607 in the following description. Similarly, a fourth sample (202503), whose composition is also 

dacite, is petrographically most similar to the other rhyolite 

sample (202712). Consequently, petrographic features of 

sample 202503 are synthesized with those of sample 202712 

in the following description.

Trachybasalt

Basalt

Basaltic

trachyandesite

Trachyandesite

Trachydacite

Rhyolite

Dacite

Andesite

Basaltic

andesite

ALKALINE

SUBALKALINE

Principal phenocrysts

 Plagioclase 

 Plagioclase + biotite + green hornblende

 Plagioclase + brown magnesio-hastingsite

 Plagioclase + clinopyroxene + brown magnesio-hastingsite

 Plagioclase + clinopyroxene

EXPLANATION

15

10

5

0

35 40 45 50 55 60 65 70 75

SiO2, WEIGHT PERCENT

Na2O + K2O, IN WEIGHT PERENT

Figure 3. Total alkali-silica variation diagram showing compositions of 

samples from the Crazy Mountains dike swarm, south-central Montana. 

International Union of Geological Sciences classification grid (Le Bas 

and others, 1986) is also shown. Gray-shaded field defined by samples 

of the Big Timber stock (du Bray and Harlan, 1996).

Petrography 
The rapidly cooled or quenched rhyolite dikes (samples 

202607 and 202705) contain 15–25 percent phenocrysts in an 

aphanitic, intersertal groundmass composed of a turbid intergrowth of calcite, sericite, quartz, and feldspar. The principal 

phenocrysts in these rocks are subhedral to euhedral feldspar laths 1–1.5 mm long, most of which are almost entirely 

replaced by calcite and sericite. These dikes also contain trace 

amounts of anhedral quartz, opaque oxide minerals, and zircon 

but do not include mafic silicate phenocrysts. Secondary, 

subhedral flakes of sericite as much as 1 mm long constitute 

about 1 percent of each sample, and secondary opaque oxide 

minerals form pseudomorphs after titanite.

The phaneritic rhyolite dikes (samples 202712 and 

202503) are composed of hypidiomorphic granular, seriate 

rock that contains varying proportions of plagioclase, quartz, 

potassium feldspar, biotite, green hornblende, and magnetite; 

the more mafic of these two samples (202503) also contains 

clinopyroxene. These dike rocks contain accessory titanite, 

apatite, and zircon and glomerocrysts of hornblende, biotite, 

plagioclase, and opaque oxide minerals. Subhedral to euhedral, moderately compositionally zoned plagioclase laths are 

0.5–1 mm long. Anhedral, interstitial quartz grains are 0.5 mm 

in diameter. Weakly exsolved, anhedral potassium feldspar 

grains are interstitial and 0.5–1 mm in diameter. Straw-yellow 

to tan, anhedral biotite is interstitial and forms cleavage flakes 

less than 0.5 mm in diameter. Pale-green, anhedral to subhedral amphibole laths are 0.5 mm long. Anhedral grains of 

pale-green clinopyroxene are 0.1–0.5 mm in diameter.

Seven dike samples (in addition to samples 202705 and 

202712, which were included in the petrographic syntheses of 

the two types of rhyolite dikes previously described) are composed of trachydacite/dacite. As such, about 15 percent of the 

Crazy Mountains dike swarm is composed of dikes that have 

trachydacite/dacite compositions. Among these seven samples, 

we identified two groups, each having distinctive petrographic 

features; these two sample groups also define unique compositional fields (fig. 3).

The first group (three solid squares plotted wholly 

within the trachydacite field, fig. 3) of dacitic dikes (samples 

202602, 202714, and 202726) is characterized by biotite 

and (or) green magnesio-hornblende (du Bray and Harlan, 

1996) as the predominant mafic silicate mineral. These dikes 

contain 10–30 percent phenocrysts in a turbid, aphanitic, 

intersertal groundmass composed of feldspar, quartz, opaque 

oxide minerals, hornblende, and secondary and (or) alteration 

minerals, including chlorite and clay. Subhedral to euhedral 

plagioclase is the predominant phenocryst phase in these dikes 

and forms albite-twinned and moderately zoned laths 1–4 

mm long. In some samples, plagioclase was partly replaced 

by calcite, sericite, and clay minerals. Yellowish-green to 

pale-green magnesio-hornblende phenocrysts, some of which 

were partly replaced by chlorite or biotite, form anhedral to 

subhedral grains that are 0.5–1 mm long. Subhedral grains 

of tan to red-brown biotite are 1–3 mm in diameter. Opaque 

oxide phenocrysts form anhedral to subhedral grains 0.1–0.5 

mm in diameter. These dike samples contain accessory apatite 

and zircon, and some contain glomerocrysts of plagioclase, 

hornblende, biotite, and opaque oxide minerals. Dike sample 

202714 has the lowest SiO2 and total alkali contents of these 

three trachydacites and, in terms of its petrography, is transitional between the two groups of trachydacite dikes: it has 

biotite as its principal mafic silicate mineral but also contains 

distinctive brown amphibole. The major-oxide composition of 

this sample is similarly transitional between the two compositionally distinct types of trachydacite, shown by solid squares 

and the symbol × in figure 3.

The second group (four solid × symbols plotted in the 

trachydacite field, fig. 3) of dacitic dikes (samples 202306, 

202310, 202622, and 202734) contains brown or brownishgreen magnesio-hastingsite (du Bray and Harlan, 1996) as 

the predominant mafic silicate mineral. These dikes contain 

5–30 percent phenocrysts in a turbid, aphanitic, intersertal 

groundmass composed of feldspar (in part replaced by calcite), 

opaque oxide minerals, and lesser amounts of green hornblende. The groundmass was replaced, in part, by combinations of secondary chlorite, calcite, and epidote. Subhedral to 

euhedral plagioclase is the predominant phenocryst phase in 

these dikes and forms albite-twinned and moderately zoned 

laths 1–3 mm long. In some samples, plagioclase was completely replaced by clay minerals. Magnesio-hastingsite phenocrysts form euhedral laths and equant grains that range in 

length or diameter from 0.5 to 1.5 mm. Some of these amphibole grains were partly to completely replaced by chlorite. 

Opaque oxide phenocrysts form anhedral to subhedral grains 

0.2–0.5 mm in diameter. These dikes contain accessory quartz, 

apatite, titanite, and zircon and glomerocrysts of feldspar, 

magnesio-hastingsite, and opaque oxide minerals.

Thirty-nine of the samples (about 75 percent of the 

dikes) are composed of trachyandesite/andesite and basaltic 

trachyandesite/basaltic andesite. These trachyandesitic dikes 

form two petrographically distinct groups; one group (24 

samples) is characterized by brown magnesio-hastingsite 

phenocrysts (du Bray and Harlan, 1996) and the other group 

(15 samples) contains clinopyroxene as the sole mafic silicate 

phenocryst. The two groups, respectively termed “magnesiohastingsite trachyandesite” dikes and “clinopyroxene trachyandesite” dikes, are described in the following paragraphs.

The magnesio-hastingsite (du Bray and Harlan, 1996) trachyandesite dikes contain 5–30 percent phenocrysts (although 

several dikes are phaneritic and several are aphanitic) in a 

turbid, aphanitic, intersertal groundmass composed of feldspar (in part replaced by calcite), opaque oxide minerals, and 

lesser amounts of green hornblende. The groundmass has been 

replaced, in part, by combinations of secondary chlorite, calcite, and epidote. Subhedral to euhedral plagioclase is the predominant phenocryst phase in most of these dikes and forms 

albite-twinned and moderately zoned laths 1–3 mm long. 

In some samples, plagioclase has been completely replaced 

by clay minerals. Magnesio-hastingsite phenocrysts are the 

predominant phenocrysts in some dikes, forming euhedral 

laths and more equant grains that range in length or diameter 

from 0.5 to 1.5 mm. Some of these amphibole grains are partly 

10 Petrology, Crazy Mountains Dike Swarm, Montana
to completely replaced by chlorite. Most of these dikes (solid 

circles, fig. 3) also contain colorless to pale-tan, lath-shaped, 

anhedral to subhedral clinopyroxene phenocrysts that are 

0.5–2 mm long; seven magnesio-hastingsite trachyandesite 

dikes that lack clinopyroxene are plotted in the trachyandesite 

field in figure 3 (symbol ×). Opaque oxide phenocrysts form 

anhedral to subhedral grains 0.2–0.5 mm in diameter. These 

dikes contain accessory quartz, apatite, titanite, and zircon and 

glomerocrysts of feldspar, magnesio-hastingsite, and opaque 

oxide minerals. 

The clinopyroxene trachyandesite dikes include both 

crystal-rich and crystal-poor varieties. The crystal-poor dikes 

(n=8) average 8 percent phenocrysts, whereas the crystalrich dikes (n=7) contain about 35 percent phenocrysts. The 

two varieties are otherwise similar and include an aphanitic, 

intersertal groundmass composed of feldspar (in part replaced 

by calcite), opaque oxide minerals, and chlorite and lesser 

amounts of clinopyroxene and devitrified glass. Plagioclase 

is the predominant phenocryst phase in these dikes and forms 

subhedral to euhedral albite-twinned and moderately zoned 

laths 1–5 mm long. Subhedral clinopyroxene phenocrysts 

form very pale tan, lath-shaped to granular grains that range in 

length or diameter from 0.5 to 3 mm. Opaque oxide phenocrysts form anhedral to subhedral grains 0.2–0.5 mm in diameter. These dikes contain accessory apatite and glomerocrysts 

of feldspar, clinopyroxene, and opaque oxide minerals. Some 

dikes contain chlorite that appears to pseudomorph completely 

replaced olivine crystals. Secondary calcite and chlorite are 

abundant in some dikes, whereas mixtures of chlorite and clay 

seem to have replaced primary interstitial glass in others.

Similar to the rhyolite dikes, trachybasalt dikes are rare 

and account for less than 5 percent of all dikes in the swarm. 

Only two samples (202620 and 202725) are composed of 

basalt (fig. 3). The two basalt dike samples have substantially different petrographic characteristics even though their 

chemical compositions (table 1; fig. 3) are similar. Sample 

202725 contains about 25 percent phenocrysts in an aphanitic, 

intersertal groundmass composed of plagioclase, hornblende, 

chlorite, and biotite. Green, subhedral grains of calcic augite 

(du Bray and Harlan, 1996) 2.5 mm in diameter are the predominant phenocryst type in this sample. Other phenocryst 

phases include plagioclase and magnesio-hastingstite. Plagioclase forms euhedral, albite-twinned and weakly zoned laths 

about 3 mm long. Magnesio-hastingstite forms euhedral brown 

laths that are about 1.5 mm long. This type of dike contains 

accessory apatite and relatively abundant secondary chlorite. 

Minor opaque oxide minerals form anhedral grains that are 

0.5 mm in diameter. The other type of basalt dike (sample 

202712) contains only 10 percent phenocrysts, also in an 

aphanitic, intersertal matrix that is composed of plagioclase, 

magnesio-hastingsite, and opaque oxide minerals. Subequal 

amounts of olivine and magnesio-hastingsite form most of the 

phenocrysts in this type of dike. Olivine is colorless and forms 

subhedral grains as much as 2 mm in diameter. Magnesiohastingsite forms brown, subhedral to euhedral laths as much 

as 3 mm long. Minor plagioclase, in part replaced by calcite, 

forms zoned laths that are as much as 3 mm long. Some 

olivine appears to have been replaced by chlorite. This type of 

dike also contains minor amounts of opaque oxide minerals 

that form anhedral phenocrysts less than 0.2 mm in diameter.

Geochronology

Five of the six samples dated by the 40Ar/39Ar step-heating method are splits of mineral separates that were previously 

dated by the K-Ar method (Harlan, 1986; Harlan and others, 

1988). The dates reported here are for samples of alkaline 

sills, a dike, and a stock in the northern part of the Crazy 

Mountains. In addition, we report a new 40Ar/39Ar date for the 

strongly alkaline Ibex Mountain sill, west of the Big Timber 

stock. A K-Ar date for this sill was also reported by Harlan 

and others (1988), but the sample dated in the present study 

was newly collected subsequent to the completion of that 

study. Sample descriptions were presented by Harlan (1986) 

and Harlan and others (1988), and sample localities are shown 

in figure 1A of this report. Results of the 40Ar/39Ar dating are 

presented as age spectra in figure 4, and complete analytical 

data are presented in table 2.

Sample 82VP–2 was collected from a trachyte sill 

exposed along the eastern flank of the Robinson anticline 

intrusive complex (fig. 1A, locality 1). Hornblende from the 

sill previously yielded a K-Ar date of 48.8±4.6 Ma (1 σ) 

(Harlan and others, 1988). An 40Ar/39Ar analysis of the same 

mineral concentrate gave a somewhat discordant age spectrum 

with a total gas age of 50.84±0.28 Ma (2 σ) (table 2). The 

spectrum is somewhat U-shaped, older ages being indicated 

by data for the lower temperature steps, but yields a fairly 

flat profile over about 90 percent of the 39ArK released (fig. 

4). The sample did not yield a plateau according to the strict 

criteria of Fleck and others (1977), but we consider the best 

age estimate for the sample to be defined by the intermediate 

temperature steps (1,100–1,350oC), which give a reasonably 

flat release pattern and a weighted mean age of 50.61±0.14 

Ma (2 σ) (table 3). We interpret this date to best represent the 

emplacement age of the trachyte sill. The age is analytically 

indistinguishable from the previously determined K-Ar date 

but is analytically much more precise.

Five biotite separates from strongly alkaline intrusions, 

which range from mafic nepheline syenite to nepheline 

syenite, were dated by the 40Ar/39Ar method. These include 

the sill(?) at Ibex Mountain, just west of the subalkaline 

Big Timber stock (fig. 1A, locality 2), sills of the Robinson 

anticline intrusive complex (fig. 1A, localities 3 and 4), and 

the Comb Creek stock and an associated radial dike (fig. 1A, 

localities 5 and 6). Previous K-Ar dating of mineral concentrates from these intrusions gave ages ranging from 48.0±1.9 

(1σ) to 52.6±2.0 (1 σ) (table 3) (Harlan, 1986; Harlan and 

others, 1988). Interpretation of 40Ar/39Ar results for these 

samples is reasonably straightforward. Although individual 

age spectra show minor complications in low- or hightemperature steps (fig. 4), all samples yielded well-defined, 

Geochronology 11
83VP65 biotite

Tp = 50.10±0.19 (2σ)

83VP75 biotite

Tp = 50.03±0.11 (2σ)

Ibex Mountain biotite

Tp = 50.10±0.12 (2σ)

82VP2 hornblende

Tw = 50.61±0.14 (2σ)

84CC12 biotite

Tp = 50.22±0.15 (2σ)

83VPCC1 biotite

Tp = 50.06±0.17 (2σ)

60

55

50

45

40 20 40 60 80 100

60

55

50

45

40 20 40 60 80 100

APPAREN

T

A

GE (Ma)

PERCENT 39ArK RELEASED PERCENT 39ArK RELEASED

60

55

50

45

40 20 40 60 80 100

APPAREN

T

A

GE (Ma) 60

55

50

45

40 20 40 60 80 100

APPAREN

T

A

GE (Ma)

60

55

50

45

40 20 40 60 80 100

60

55

50

45

40 20 40 60 80 100

Figure 4. Graphs showing 40Ar/39Ar age spectra for hornblende and biotite separates from alkaline intrusive rocks in the Crazy 

Mountains, south-central Montana. Analytical error in the apparent age for each temperature step in the age spectra is represented 

by the height of the shaded rectangle; the proportion of 39ArK released in step-heating experiments is represented by the length of the 

shaded rectangle. Arrows show steps in the age spectra used to define the plateau (Tp) or weighted mean (Tw) dates, as described in 

the text. Analytical error in the mean age is given at ±2 σ.

12  Petrology, Crazy Mountains Dike Swarm, Montana
4

Table 2. Data for 40Ar/ 39Ar step heating of samples of alkaline sills in the Crazy Mountains, south-central Montana. 

[Note: 40Ara is measured atmospheric argon used for mass-discrimination at time of analysis; 40ArR is radiogenic 40Ar in volts signal; 39ArK is potassium-derived 39Ar in volts signal; 40ArR /

39ArK is the ratio of 40ArR to 39ArK after correction for mass-discrimination and 

interfering isotopes; 39Ar/ 37Ar = ratio of 39ArK to 37ArCa (this value can be converted to the approximate K/Ca by multiplying by 0.52); 

% 40ArK and % 39Ar are the percentage of radiogenic 40Ar and percentage of total 39Ar released in each temperature step. Conversion 

of volts signal to moles Ar can be made using a conversion factor of 1.252×10-13 moles argon per volt of signal. Steps denoted by bold 

font are those used in calculation of the plateau or weighted mean dates as described in the text. Analytical details are available in Snee 

(2002)]

Temp

(°C)

40ArR 39ArK 40ArR / 39ArK 39Ar/ 37Ar % 40ArR % 39Ar Apparent age 

(Ma ± 1V)

Sample 82VP2: Hornblende, trachyte, sill; 70.9 mg; 40Ar/ 36Ara = 298.9; J-value = 0.00769r0.1% (1V)

700 0.12645 0.03153 4.011 2.62 21.6 0.6 54.87±1.37 

800 0.11665 0.02694 4.329 1.56 20.2 0.5 59.15±1.29 

900 0.22156 0.05643 3.926 1.07 62.3 1.0 53.73±1.05 

975 0.27472 0.06863 4.003 0.84 74.7 1.2 54.76±0.35 

1,050 0.78274 0.20420 3.833 0.43 75.7 3.6 52.47±0.29 

1,075 1.1540 0.30934 3.730 0.37 86.1 5.4 51.09±0.15 

1,100 1.73822 0.47043 3.695 0.36 91.1 8.3 50.61±0.15 

1,125 4.9527 1.3374 3.703 0.35 93.7 23.5 50.72±0.08 

1,150 6.4046 1.7318 3.698 0.35 94.6 30.4 50.67±0.08 

1,200 2.1640 0.58862 3.676 0.34 92.5 10.3 50.36±0.13 

1,250 1.9220 0.52223 3.680 0.29 93.8 9.2 50.41±0.14 

1,350 1.1035 0.29962 3.683 0.27 92.3 5.3 50.45±0.21 

1,450 0.20734 0.05506 3.766 0.28 82.5 1.0 51.56±0.47 

Total gas 3.712 50.84±0.14 

Sample from Ibex Mountain: Biotite, mafic nepheline syenite, sill, 50.9 mg; 40Ar/ 36Ara

 = 298.9; J-value = 0.007781r0.1% (1V)

600 0.05104 0.04121 1.238 20.70 0.7 0.7 17.30±0.99 

700 0.42849 0.16230 2.640 53.49 56.6 2.6 36.68±0.17 

750 1.14790 0.32306 3.564 102.33 70.8 5.2 49.35±0.13 

800 1.65660 0.45697 3.265 137.22 86.0 7.4 50.18±0.08 

850 1.93705 0.53609 3.613 183.78 87.9 8.7 50.02±0.09 

900 2.05721 0.56998 3.609 250.85 89.1 9.2 49.96±0.14 

950 2.07748 0.57350 3.622 174.92 90.3 9.3 50.14±0.10 

1,000 1.74716 0.48279 3.619 137.74 88.7 7.8 50.10±0.08 

1,050 1.66418 0.46037 3.615 104.29 90.3 7.5 50.04±0.24 

1,100 2.90917 0.80330 3.622 71.88 86.7 13.0 50.13±0.08 

1,150 2.38796 0.66073 3.614 53.21 82.7 10.7 50.03±0.08 

1,350 3.98206 1.09858 3.625 15.99 79.7 17.8 50.18±0.08 

Total gas 3.574 49.49±0.12 

Sample 83VP65: Biotite; mafic nepheline syenite, sill; 59.7 mg; measured 40Ar/ 36Ara

 = 298.9; J-value = 0.00623r0.1% (1V)

600 1.05374 0.23180 4.546 99.29 58.1 4.5 50.38±0.14 

700 1.91901 0.42471 4.518 161.74 78.0 8.3 50.08±0.16 

800 6.54108 1.44775 4.518 352.04 93.4 28.3 50.08±0.19 

850 3.51108 0.77686 4.520 745.20 93.5 15.2 50.09±0.33 

900 3.32483 0.73430 4.528 340.31 93.2 14.4 50.18±0.26 

950 2.78227 0.61316 4.538 308.36 92.2 12.0 50.29±0.42 

1,000 1.72834 0.38260 4.517 160.59 80.4 7.5 50.07±0.14 

1,050 1.09643 0.24175 4.535 78.10 65.3 4.7 50.27±0.65 

1,100 0.70177 0.15125 4.640 37.65 51.0 3.0 51.41±1.00 

1,300 0.48709 0.10349 4.707 5.06 41.5 2.0 52.14±1.07 

Total gas 4.532 50.22r1.07

Geochronology 13
5

Table 2. Data for 40Ar/ 39Ar step heating of samples of alkaline sills in the Crazy Mountains, south-central Montana. 

—Continued

Temp

(°C)

40ArR 39ArK 40ArR / 39ArK 39Ar/ 37Ar % 40ArR % 39Ar Apparent age 

(Ma ± 1V)

Sample 83VP75: Biotite, mafic nepheline syenite, sill; 70.9 mg; 40Ar/ 36Ara = 298.9; J-value = 0.007825r0.1% (1V)

600 0.35848 0.10247 3.498 51.92 48.4 1.1 48.72±0.37 

700 0.97747 0.26818 3.645 93.60 87.9 2.8 50.73±0.15 

750 1.84141 0.51303 3.589 128.80 84.9 5.3 49.97±0.08 

800 2.83212 0.78754 3.596 190.88 92.0 14.5 50.06±0.11 

850 3.64399 1.01482 3.594 333.53 94.8 10.5 49.99±0.09 

900 4.53069 1.23591 3.589 293.40 94.7 13.0 49.97±0.08 

950 4.44170 1.23591 3.594 187.92 93.9 12.8 50.03±0.08 

1,000 3.54210 0.98650 3.591 122.61 91.8 10.2 49.99±0.09 

1,050 4.39253 1.22284 3.592 115.85 86.3 12.6 50.01±0.08 

1,100 3.49982 0.97170 3.602 73.45 77.4 10.0 50.14±0.08 

1,150 2.09815 0.58376 3.594 32.27 69.0 6.0 50.04±0.11 

1200 1.38955 0.38493 3.610 14.50 52.9 4.0 50.25±0.14 

1350 1.22904 0.34308 3.591 3.85 57.8 3.5 49.87±0.49 

Total gas 3.594 50.03±0.10 

Sample 83VPCC1: Biotite, mafic nepheline syenite, dike, 61.3 mg; 40Ar/ 36Ara

 = 298.9; J-value = 0.006138r0.1% (1V)

600 0.08828 0.02069 4.266 15.21 7.7 0.4 46.63±1.22 

700 1.00596 0.21530 4.672 35.74 65.4 4.0 51.01±0.30 

800 5.94678 1.29290 4.600 78.82 88.4 24.0 50.23±0.14 

900 6.14365 1.34352 4.573 116.17 92.0 25.0 49.94±0.14 

950 3.31741 0.72587 4.570 50.47 88.8 13.5 49.91±0.29 

1,000 2.68422 0.58519 4.587 34.54 86.2 10.9 50.09±0.14 

1,025 1.21498 0.26520 4.581 23.58 80.1 4.9 50.03±0.20 

1,050 0.83467 0.18252 4.573 14.05 77.5 3.4 49.94±0.43 

1,100 1.17611 0.25662 4.583 10.11 77.6 4.8 50.05±0.22 

1,250 2.26306 0.48965 4.622 2.55 70.5 9.1 50.46±0.16 

Total gas 4.589 50.11±0.18 

Sample 84CC12: Biotite, mafic nepheline syenite, stock, 53.2 mg; 40Ar/ 36Ara

 = 298.9; J-value = 0.006151r0.1% (1V)

600 0.18041 0.04671 3.862 37.26 35.6 0.9 42.35±0.78 

700 0.74269 0.16672 4.455 113.97 52.8 3.2 48.76±0.25 

800 5.20549 1.13237 4.597 322.98 94.3 22.0 50.30±0.20 

850 3.35254 0.73051 4.589 385.09 96.3 14.2 50.22±0.14 

900 1.41193 0.30669 4.604 268.53 94.5 5.9 50.38±0.19 

950 1.36432 0.29800 4.578 131.95 89.4 5.8 50.10±0.19 

1,000 2.44198 0.53064 4.602 143.76 93.6 10.3 50.36±0.15 

1,050 3.73626 0.81358 4.592 138.51 95.7 15.8 50.25±0.14 

1,100 3.45184 0.75335 4.582 100.21 96.1 14.6 50.14±0.14 

1,250 1.72665 0.37730 4.576 4.75 94.7 7.3 50.08±0.16 

Total gas 4.580 50.12±0.16 

14  Petrology, Crazy Mountains Dike Swarm, Montana
plateau-like segments over about 90 percent of the 39ArK

released in the step-heating experiments. Plateau dates are 

tightly clustered and range from 50.03 to 50.22 Ma, while 2 σ

errors range from ± 0.11 to 0.19 Ma (fig. 4; table 3). Because 

most of the samples are of fairly small hypabyssal intrusions 

that probably cooled very quickly, we interpret these biotite 

dates to represent the best estimate of the emplacement age of 

the strongly alkaline sills, dikes, and the Comb Creek stock. 

These data indicate that the strongly alkaline intrusive rocks 

were emplaced during a fairly restricted interval of time at 

about 50.10 Ma.

Results of the 40Ar/39Ar step-heating experiments are 

summarized in table 3 along with preferred ages based 

on interpretation of isotopic analyses. The 40Ar/39Ar dates 

presented here yield ages that are consistent with previously determined K-Ar dates but are analytically much more 

precise. The 40Ar/39Ar dates for alkaline rocks of the northern 

Crazy Mountains are somewhat older than biotite 40Ar/39Ar 

dates for the subalkaline Big Timber stock, which give apparent ages of about 49.2–49.3 Ma (du Bray and Harlan, 1996). 

This could be interpreted to indicate that the alkaline rocks 

of the northern Crazy Mountains predate emplacement of 

the subalkaline rocks exposed largely in the southern Crazy 

Mountains. This interpretation is problematic, however, 

because biotite dates from the Big Timber stock probably 

represent cooling ages through closure temperatures of about 

350°C and thus represent minimum ages for emplacement of 

the stock. Intrusive relationships bearing on the relative ages 

of the two distinct geochemical groups are rare and typically 

ambiguous (Dudas, 1991), and they cannot be used to distinguish the relative ages of the two groups. The limited span of 

40Ar/39Ar ages between the subalkaline and alkaline rocks (ca. 

50.6–49.2 Ma) indicates that magmas represented by these 

two distinctly different geochemical groups were very closely 

associated in both time and space and may indicate that all 

of these rocks were emplaced in as little as 1.5 m.y. On a 

regional scale, the age span of 51–49 Ma is similar to that of 

most igneous centers of the central Montana alkalic province 

and is coeval with the peak of widespread volcanism in the 

Absaroka-Gallatin volcanic field directly to the south of the 

Crazy Mountains Basin.

Whole-Rock Chemistry

Major-Oxide Data

Extensive compositional variation, as exemplified by 

SiO2 abundances that range from 51 to 71 weight percent 

(table 1), is the most obvious feature of the Crazy Mountains 

dike swarm. Compositional variation among the dikes (fig. 5) 

is not quite as extensive as that of the presumably cogenetic 

Big Timber stock and less systematic than that displayed by 

the stock. In particular, the abundance array of Na2O + K2O 

versus SiO2 for the dikes (fig. 3) is more diffuse than that 

for the stock (du Bray and Harlan, 1996; fig. 5). The diffuse 

nature of the alkali compositional array and petrographic 

features, indicating that many of the dikes were affected by 

late-stage alteration, suggest that the enhanced alkalinity 

of the dikes may be a secondary, nonmagmatic feature that 

resulted from magmatic-hydrothermal processes. Most dikes 

have alkaline to subalkaline compositions (as indicated by 

total alkali versus silica in the range between the alkalinesubalkaline dividing lines of Irvine and Baragar, 1971, and Le 

Bas and others, 1986), and the dikes, as a group, appear to be 

more alkaline than the stock. Many of the dikes have compositions that are truly alkaline, as indicated by total alkali versus 

silica that plots (fig. 3) above the alkaline-subalkaline dividing 

line of Le Bas and others (1986). The tendency toward alkaline compositions decreases in dikes having the highest SiO2

content. Most sample compositions plot in the high-potassium 6

Table 3. Summary of isotopic dating results for alkaline sills in the northern Crazy Mountains, south-central Montana. 

[Tp, plateau date; Tw, weighted mean date, for which the strict plateau criteria have not been met, as described in text. Analytical error associated with the 

K-Ar dates is ±1 V; the error in the 40Ar/ 39Ar dates is given at ±2 V. All 40Ar/ 39Ar dates are from mineral separates used in the original K-Ar dating, with 

the exception of the sample from the Ibex Mountain sill. For this intrusion, a new sample was collected, processed, and dated. K-Ar dates were previously 

reported by Harlan (1986) and Harlan and others (1988)]

Sample

locality 

(fig. 1) 

Sample No. Locality Latitude

(°N.)

Longitude

(°W.) Rock type Materialdated

K-Ar date

(Ma)

40Ar/ 39Ar date 

(Ma)

1 82VP2 Robinson anticline 46.2023 110.4592 Trachyte sill Hornblende 48.8±4.6 Tw = 50.61±0.14 

2 Ibex Mountain Ibex Mountain 46.9943 110.4479 Mafic nepheline 

syenite sill(?) 

Biotite 50.5±0.6 Tp = 50.10±0.12 

3 83VP65 Robinson anticline 46.1873 110.4918 Mafic nepheline 

syenite sill 

Biotite 50.6±1.9 Tp = 50.10±0.19 

4 83VP75 Robinson anticline 46.1875 110.4637 Mafic nepheline 

syenite sill 

Biotite 49.3±1.9 Tp = 50.03±0.11 

5 83VPCC1 Comb Creek stock 46.2984 110.4453 Mafic nepheline 

syenite dike 

Biotite 52.6±2.0 Tp = 50.06±0.17 

6 84CC12 Comb Creek stock 46.307 110.499 Nepheline syenite 

stock

Biotite 48.0±1.9 Tp = 50.22±0.15 

Whole-Rock Chemistry 15
High-K

(calc-alkaline)

series

Calc-alkaline

series

Shoshonitic

series

MgO,

IN

WEIGHT PERCEN

T

FeO*,

IN

WEIGHT PERCEN

T

Al2O3,

IN

WEIGHT PERCEN

T

CaO,

IN

WEIGHT PERCEN

T

Na2O,

IN

WEIGHT PERCEN

T

SiO2,

IN WEIGHT PERCENT

MnO,

IN

WEIGHT PERCEN

T

TiO2,

IN

WEIGHT PERCEN

T

K2O,

IN

WEIGHT PERCEN

T

P2O5,

IN

WEIGHT PERCEN

T

ZIRC

O

NIU

M,

IN PARTS PER MILLIO

N

300

0.0

0.5

0.0

0.1

0

1

2

0

2

4

1

3

2

50 60 70 50 60 70

4

3

5

0

4

8

2

6

0

4

8

2

6

0

4

10

2

6

12

8

14

16

18

15

17

250

200

150

100

Figure 5. Variation diagrams showing abundances of major oxides and zirconium in samples from the Crazy Mountains dike swarm, 

south-central Montana. Discriminant line on K2O versus SiO2 diagram from Ewart (1982).

16  Petrology, Crazy Mountains Dike Swarm, Montana
calc-alkaline field (Ewart, 1982) on a K2

O versus SiO2 diagram (fig. 5), and some plot in the shoshonitic series field. 

According to the classification scheme of Le Bas and others 

(1986), and because (Na2O-2) < K2O in almost all of the dikes, 

the trachybasalt dikes are properly termed “potassic trachybasalt,” the basaltic trachyandesite dikes are “shoshonite,” and 

the trachyandesite dikes are “latite.”

The phaneritic rocks of the Big Timber stock display 

a strong relationship between chemical composition and 

modal mineralogy. Similarly, rocks of the Crazy Mountains 

dike swarm display a strong relationship between composition and phenocryst suites (fig. 3). As previously indicated, 

samples of the dike swarm were assigned to groups on the 

basis of the types of contained phenocrysts; each group has 

a distinctive composition. A particularly noteworthy group 

of samples contains clinopyroxene and plagioclase as its 

principal phenocrysts. Samples containing these phenocrysts 

are distinguished by compositions that are at the low-silica, 

low-total-alkalis end of the compositional array formed by the 

dike swarm. A large subset of dike samples contains plagioclase, clinopyroxene, and magnesio-hastingsite as phenocrysts. 

These dikes contain low to intermediate abundances of silica 

and a distinctly higher abundance of total Na2O + K2O than the 

group that contains clinopyroxene and plagioclase as its principal phenocrysts. Another geochemical subset of the dikes 

contains intermediate abundances of silica that are greater 

than those of the two previously described groups and also 

contain intermediate abundances of total Na2O + K2O; this 

group is petrographically noteworthy given its lack of clinopyroxene phenocrysts and the presence of only plagioclase and 

magnesio-hastingsite phenocrysts. The more evolved, higher 

silica dacite and rhyolite dikes are petrographically characterized by plagioclase, potassium feldspar, biotite, quartz, and 

varying combinations of clinopyroxene, magnesio-hastingsite, 

and magnesio-hornblende.

As is the case for most subalkaline rocks, abundances 

of FeO* (total iron expressed as ferrous oxide), MgO, CaO, 

TiO2, MnO, and P2O5 in samples of the Crazy Mountains dike 

swarm decrease with increasing SiO2, whereas abundances 

of K2O and Na2O increase (fig. 5). The abundance of Al2O3

increases from about 16 to 18 weight percent at SiO2 abundances of about 50 and 60 weight percent, respectively; Al2

O3

abundances decrease gradually to about 15 weight percent at 

SiO2 abundances between 60 and 71 weight percent. None of 

the major-oxide variation arrays is strictly linear, and most 

display considerable dispersion. Variations in FeO*, MgO, 

CaO, TiO2, MnO, and P2O5 abundances form concave-upward 

curvilinear patterns, whereas those of K2O and Na2O form 

concave-downward curvilinear patterns. Variations of CaO and 

FeO* are the most systematic and nearly linear.

Trace-Element Data

Variations in trace-element abundance among samples of 

the Crazy Mountains dike swarm are considerably less pronounced than associated variations in major-oxide abundances. 

Approximate minimum and maximum values, in parts per million, for selected trace elements in the dike samples, excluding 

rare compositional outliers, are as follows: Rb, 20 and 80; Sr, 

400 and 1,300; Y, 10 and 35; Zr, 125 and 225; Nb, 15 and 30; 

Ba, 800 and 1,500; La, 30 and 90; Ce, 60 and 120; and Nd, 10 

and 70. As is true of the variations in major-oxide abundances, 

trace-element distributions are characterized by considerable 

dispersion and abundances are poorly correlated with SiO2

abundances. Abundances of Sr, Y, and Nb do not consistently 

covary with SiO2 abundances; they exhibit a weak tendency 

to decrease in samples having the highest SiO2 abundances. 

Abundances of La, Ce, and Nd show no correlation with varying SiO2

 abundance. Abundances of Ba, Rb, and Zr are consistently greater in samples having higher SiO2

 contents. The 

Rb-Sr ratios for samples of the Crazy Mountains dike swarm 

are consistently low and range from 0.01 to 0.14, averaging 

0.07±0.04.

Discussion

Granitoid magmas generated in different tectonic settings 

have distinct geochemical signatures (Pearce and others, 1984); 

these signatures help define the tectonic setting in which the 

Big Timber stock and its associated dike swarm developed. 

Trace-element abundances in the Crazy Mountains dike swarm, 

in particular those of Rb, Y, and Nb, are similar to those of 

subduction-related volcanic arc rocks (fig. 6). Gill (1981) has 

shown that La-Nb ratios of 2–7 are also characteristic of magmas generated in a subduction-related volcanic arc setting. The 

VOLCANIC ARC OROGENIC

WITHIN PLATE

10,000

1,000

100

10

1

1 10 100 1,000 10,000

Rb, IN PARTS PER MILLION

Y + Nb, IN PARTS PER MILLION

Figure 6. Trace-element tectonic setting–discrimination 

variation diagram for samples from the Crazy Mountains dike 

swarm, south-central Montana. Tectonic setting−composition 

boundaries from Pearce and others (1984).

Discussion 17
La-Nb ratios for samples of the Crazy Mountains dike swarm 

(table 1) range from 1.2 to 5.9 and average 2.8±1. Major-oxide 

compositions of the Crazy Mountains dike swarm approximate 

a calc-alkaline AFM (Na2O + K2O–FeO*–MgO) trend similar 

to that of modern Cascade volcanic arc rocks (fig. 7). These 

compositional characteristics are all similar to those exhibited 

by samples of the cogenetic Big Timber stock and are consistent with a mantle-dominated genesis in a subduction-related 

volcanic arc setting (du Bray and Harlan, 1996).

In igneous systems, abundances of Sr, K, and Rb are 

principally controlled by feldspar (Hanson, 1980); abundances of Sr and Rb are most and least, respectively, depleted 

by feldspar fractionation. As feldspar fractionation and 

differentiation proceed, K and, to an even greater extent, Rb 

are concentrated in the residual liquid relative to Sr. Compositions plotting nearest the Sr apex of the Rb–K/100–Sr 

ternary diagram (fig. 8) represent the least evolved magmas. 

Tight clustering of Sr abundances for samples of the Crazy 

Mountains dike swarm indicates that plagioclase (principal 

residence of Sr) fractionation did not strongly affect Sr abundances in compositional evolution of the Crazy Mountains 

dike swarm. Samples of the Crazy Mountains dike swarm plot 

along a short array with an origin near the Sr apex; samples of 

the most and least evolved dikes plot farthest from and nearest to the Sr apex, respectively. The shortness and position of 

this array, near the Sr apex, also suggest that Sr abundances in 

the magma reservoir represented by the dike swarm were not 

strongly affected by plagioclase fractionation. The shortness 

of the compositional evolution array on the Rb–K/100–Sr 

ternary diagram (fig. 8) also results from the significantly 

elevated Sr abundances characteristic of dike swarm samples 

(table 1); fractionation of plagioclase from a very Sr-rich 

magma causes the abundance of Sr, relative to those of K and 

Rb, in residual magma to be little affected. However, the role 

of plagioclase fractionation in the overall evolution of the 

reservoir represented by the dikes is indicated by covariations 

in Al2O3 and SiO2 abundances (fig. 5). Abundances of Al2O3

increase steadily in samples containing between about 50 

and 60 percent SiO2. This can occur only if relatively Al2O3-

rich plagioclase composes less than half of the fractionating 

assemblage. The downturn in Al2O3 abundances in samples 

containing greater than 60 percent SiO2 denotes the point at 

which plagioclase fractionation becomes a controlling factor 

in compositional evolution of the residual liquid.

The spatial coincidence, contemporaneity, field relations, and similarity of petrographic and geochemical features 

combine to indicate that dikes of the Crazy Mountains dike 

swarm and the Big Timber stock are cogenetic and are simply 

different physical manifestations of magma derived from a single reservoir. The fact that essentially all of the compositional 

range depicted by the Big Timber stock is mimicked by dikes 

of the Crazy Mountains dike swarm indicates that the magma 

reservoir, represented by the solidified stock itself and from 

which magma represented by the dikes was extracted, was 

tapped continuously during all stages of its compositional evolution. Magma represented by various intrusions in the central 

Crazy Mountains was emplaced into a shallow crustal magma 

reservoir that formed a roughly diapiric body. Emplacement 

was accompanied by development of a quasi-radial fracture 

HAWAIIAN ALKALINE

CASCADE CALC-ALKALINE

F

A M

Figure 7. Ternary AFM (Na2O + K2O–FeO*–MgO) diagram 

showing compositions of samples from the Crazy Mountains 

dike swarm, south-central Montana. Cascade calc-alkaline and 

Hawaiian alkaline trend lines from Irvine and Baragar (1971).

K/100

Rb Sr

Figure 8. Ternary variation diagram showing the relative 

proportions of Rb, K, and Sr in samples from the Crazy Mountains 

dike swarm, south-central Montana.

18  Petrology, Crazy Mountains Dike Swarm, Montana
system in the overlying and enclosing roof and wall rocks. 

Brittle fractures that cut the rocks of the Fort Union Formation were affected by periodic dilation in response to a local 

tectonic stress field that varied as emplacement of the reservoir 

represented by the stock proceeded. During these dilation episodes, which must have occurred throughout the time required 

for the magma reservoir represented by the stock to achieve 

its full compositional diversity, magma was extracted from the 

underlying reservoir and emplaced upward into a radial array 

of tension fractures.

Given the conclusion that the dike swarm and the stock 

are cogenetic, the process of reservoir compositional evolution 

identified by du Bray and Harlan (1996) for the stock is likely 

equally applicable to the compositional diversity displayed by 

samples of the dike swarm. The identified process involves 

sidewall crystallization and fractionation of varying amounts 

of plagioclase, orthopyroxene, clinopyroxene, magnesiohastingsite, hornblende, biotite, titanite, apatite, zircon, and 

magnetite. Glomerocrysts found throughout samples of the 

dikes and stock indicate that mineral assemblages contained 

therein must have played an essential role relative to chemical 

evolution of the parent magma. Crystals nucleated along the 

walls and roof of the cooling magma reservoir while residual 

silicate liquid continued mixing; some previously formed 

crystals were plucked from the solidification front and remixed 

with residual liquid. Similarly, the tectonic regime in which the 

Big Timber stock evolved (du Bray and Harlan, 1996) must 

have prevailed during development of the dike swarm. The 

Eocene Big Timber stock and its associated dike swarm are 

manifestations of renewed volcanic arc magmatism following 

shallow subduction; these igneous rocks document the easternmost extent of arc magmatism at the onset of Eocene igneous 

activity and its subsequent westward migration in response to 

steepening subduction and westward hingeline retreat.

Conclusions

The Crazy Mountains dike swarm forms a radial array 

associated with the Eocene Big Timber stock in the Crazy 

Mountains of south-central Montana. The spatial coincidence, 

contemporaneity, field relations, and similarity of petrographic 

and geochemical features indicate that the dike swarm and 

the stock are cogenetic. Furthermore, 40Ar/39Ar data presented 

here and by du Bray and Harlan(1996) indicate that alkaline 

and subalkaline magmatism were approximately coeval. Dikes 

are hosted by the stock and by the same Fort Union Formation 

rocks that host the stock.

The dikes depict a broad array of compositional types 

from rhyolite to basalt; no well-developed preferential spatial 

distribution of dikes of any particular composition was identified. Dike samples contain from 51 to 71 weight percent SiO2

, 

a range that, although similar to that exhibited by the stock, 

is not quite as extensive. Compositional variation among the 

dikes is less systematic than is characteristic of the stock; the 

dikes form more broadly diffuse compositional arrays and 

tend to be slightly more alkaline. Both of these factors could 

result from late-stage magmatic-hydrothermal processes that 

seem to have weakly altered many of the dikes. Major-oxide 

and trace-element characteristics of the dikes and the associated stock are consistent with a mantle-dominated genesis 

in a subduction-related volcanic arc setting. As discussed at 

length by du Bray and Harlan (1996), characteristics of this 

magmatic system are consistent with renewed volcanic arc 

magmatism during the Eocene westward hingeline retreat and 

with subsequent westward migration of the magmatic front in 

response to steepening subduction along the western edge of 

North America.

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Published in the Central Region, Denver, Colo.

Manuscript approved for publication August 24, 2005

Graphics by the author and

by Springfield and Springfield (Contractor)

Photocomposition by Mari L. Kauffmann

(Contractor, ATA Services)

Edited by Diane E. Lane

References Cited  21