U.S. Department of the Interior 

U.S. Geological Survey

Geology, Age, and Tectonic Setting of the 

Cretaceous Sliderock Mountain Volcano, Montana

U.S. Geological Survey Professional Paper 1602

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Geology, Age, and Tectonic Setting of the 

Cretaceous Sliderock Mountain Volcano, Montana

By Edward A. du Bray and Stephen S. Harlan

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1602

Physical and compositional development of an Upper 

Cretaceous stratovolcano in south-central Montana

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1998
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Thomas J. Casadevall, Acting Director

For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

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

Library of Congress Cataloging-in-Publication Data

Du Bray, E.A.

Geology and tectonic setting of the Cretaceous Sliderock Mountain volcano, Montana 

/ by Edward A. du Bray and Stephen S. Harlan.

p. cm. — (U.S. Geological Survey professional paper ; 1602) 

Includes bibliographical references.

1. Sliderock Mountain (Sweet Grass County, Mont.). 2. Geology, Stratigraphic— 

Cretaceous. 3. Geology—Montana—Sliderock Mountain (Sweet Grass 

County). 4. Volcanoes—Montana—Sweet Grass County. I. Harlan, Stephen 

S. II. Title. III. Series. 

QE523.S63D8 1999

551.2'1'0978664—dc21 98-18106

CIP
CONTENTS

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

Introduction and Geologic Setting.................................................................................. 1

Physical Volcanology of the Sliderock Mountain Volcano............................................ 3

Geochronology................................................................................................................ 5

Petrography..................................................................................................................... 11

Geochemistry.................................................................................................................. 11

Regional Tectono-magmatic Speculation....................................................................... 16

Acknowledgments........................................................................................................... 17

References....................................................................................................................... 18

FIGURES

1. Tectonic map of the Crazy Mountains Basin.............................................................................................................. 2

2. Simplified geologic map of Sliderock Mountain stratovolcano.................................................................................. 4

3. Photographs of Sliderock Mountain stratovolcano lahars........................................................................................... 6

4. 40Ar/39Ar age spectra and 39Ar/37Ar ratios................................................................................................................. 7

5. 39Ar/40Ar versus 36Ar/40Ar correlation diagrams....................................................................................................... 10

6. Total alkali-silica variation diagram............................................................................................................................ 11

7. Variation diagrams showing abundances of major oxides.......................................................................................... 13

8. Ternary variation diagram showing the relative proportions of Rb, K, and Sr........................................................... 14

9. Trace-element tectonic-setting-discrimination variation diagrams............................................................................. 15

10. Ternary AFM variation diagram ................................................................................................................................. 15

11. Variation diagram showing rubidium and strontium abundances............................................................................... 16

12. Variation diagram showing rubidium and potassium abundances.............................................................................. 17

TABLES

1. 40Ar/39Ar data for samples of the Sliderock Mountain stratovolcano........................................................................ 8

2. Chemical compositions of samples from the Sliderock Mountain stratovolcano, Montana....................................... 12

III

Geology, Age, and Tectonic Setting of the Cretaceous 

Sliderock Mountain Volcano, Montana

By Edward A. du Bray and Stephen S. Harlan

ABSTRACT

Sliderock Mountain volcano is an Upper Cretaceous 

stratovolcano located along the southern margin of the Crazy 

Mountains Basin in south-central Montana. The volcano is 

part of an extensive northwest-trending volcanic field that 

lies astride the folds and faults of the Nye-Bowler lineament. 

The volcano consists mostly of volcaniclastic strata, although 

basaltic andesite lava flows are also present. An extensively 

eroded, weakly mineralized hypabyssal intrusive complex, 

inferred to represent the volcano's solidified magma chamber, intrudes lava flows and associated lahars. 40Ar/39Ar data 

indicate that the oldest lavas associated with the volcano were 

erupted by about 78±1 Ma, whereas dikes representing the 

final phase of magmatism were emplaced about 74.9 Ma.

The magmatic system ranges in composition from 

basaltic andesite to dacite. Major oxide and trace element 

data are consistent with a subduction-related continental 

magmatic arc setting. However, this type of magmatic activity was not widespread in this region during Late Cretaceous 

time. The Sliderock Mountain volcano represents one of the 

easternmost manifestations of Late Cretaceous magmatic 

activity and, except for rocks of the Boulder batholith and 

cogenetic Elkhorn Mountains Volcanics 160 km to the 

northwest, is rare in south-central Montana. Compositional 

diversity within components of the volcano appears to 

reflect evolution via about 50 percent fractional crystallization involving clinopyroxene and plagioclase. During the 

compositional evolution of the magma system, the reservoir 

was periodically tapped to feed lava flows, dikes, and a 

series of coalesced hypabyssal stocks. The location of 

Sliderock Mountain volcano, as well as other igneous rocks 

of this field, may have been controlled to some extent by the 

deep-seated fault system of the Nye-Bowler lineament.

INTRODUCTION AND 

GEOLOGIC SETTING

An extensive field of Upper Cretaceous volcanic rocks 

is exposed along the southern margin of the Crazy Mountains Basin, north of the Beartooth Mountains, in south-central Montana. These volcanic rocks have long been

considered part of the Upper Cretaceous Livingston Group, 

exposed elsewhere in the Crazy Mountains, but clearly represent a proximal, physically distinct volcanic source area. 

These rocks are important in understanding the geologic and 

tectonic setting of the northern Rocky Mountains because 

they constitute one of the easternmost Late Cretaceous volcanic fields in the North American Cordillera. The volcanic, 

volcaniclastic, and intrusive rocks that comprise this field 

have been described by a number of workers including Weed 

(1893), Vhay (1934), Rouse and others (1937), Parsons 

(1942), and Brozdowski (1983), but in general, modern petrologic, geochemical, and geochronologic data are lacking 

for these rocks. In this report, we present new geologic, petrologic, geochemical, and 40Ar/39Ar geochronologic data 

from the informally named Sliderock Mountain volcano, one 

of the main volcanic centers in this field. The rocks that comprise the Sliderock Mountain volcano were studied as part of 

the U.S. Geological Survey's mapping efforts in U.S. Forest 

Service lands of the Custer-Gallatin National Forest (du Bray 

and others, 1994). The data obtained in this study are used to 

present a model for the genesis of the Sliderock Mountain 

volcano and to help define the regional tectono-magmatic 

regime in which it developed. The data and interpretations 

presented here, although largely limited to one volcanic center, provide a foundation for future, more detailed studies of 

the volcanic rocks that constitute this important Late Cretaceous volcanic field.

Sliderock Mountain volcano is located along the southern margin of the Crazy Mountains Basin (fig. 1), a complex, 

structural and physiographic basin in south-central Montana. 

The Crazy Mountains Basin originally developed as part of 

the Cordilleran foredeep that formed east of the Cordilleran 

orogenic belt during Mesozoic contraction associated with 

subduction of the Farallon plate along the western margin of 

North America (Dickinson and others, 1988). During the 

Late Cretaceous, the Crazy Mountains Basin received about 

3500 m of volcaniclastic and synorogenic sediments derived 

primarily from volcanic and tectonic activity within the adjacent thrust belt to the west, as well as locally derived volcanic, volcaniclastic, and sedimentary rocks from vents located 

along the present-day southern margin of the basin. The original configuration of the Crazy Mountains Basin was

1
CRETACEOUS SLIDEROCK MOUNTAIN VOLCANO, MONTANA

110°W

46°N

Eastern limit of 

Disturbed Belt

Geology not shown

EXPLANATION

Tertiary Fort Union Formation 

Tertiary/Cretaceous intrusive rocks 

Cretaceous rocks of the Sliderock Mountain volcano 

Cretaceous volcaniclastic rocks (Livingston Group) 

Phanerozoic sedimentary rocks 

Proterozoic Belt Supergroup 

Archean crystalline basement rocks 

Fault-Sense of displacement unspecified 

Normal fault-Ball and bar on downthrown block 

Thrust fault-Barb on upper plate

Anticline-Arrow at end of line shows direction of fold plunge 

Overturned anticline 

Syncline

Figure 1. Map of the Crazy Mountains Basin in south-central Montana showing principal structural and tectonic features (modified from 

Roberts, 1972 and Harlan and others, 1988). Gray dashed line shows the approximate margins of the Crazy Mountains Basin.
PHYSICAL VOLCANOLOGY OF THE SLIDEROCK MOUNTAIN VOLCANO

substantially modified by the progression of thrust-belt folds 

and faults of the Helena Salient of the Disturbed Belt into the 

western part of the Crazy Mountains Basin during Late Cretaceous/early Tertiary time, followed by or coeval with 

development of basement-cored foreland uplifts of the 

Bridger Range and Beartooth uplifts, and by left-lateral 

movement along the Lake Basin and Nye-Bowler fault systems (Wilson, 1936; Roberts, 1972; Dickinson and others, 

1988; Harlan and others, 1988).

The rocks of the Sliderock Mountain volcano are part of 

a northwest-trending belt of Upper Cretaceous volcanic and 

volcaniclastic rocks and their vents, which lie astride the 

northwest-trending axis of the Nye-Bowler lineament just 

north of the Beartooth uplift. These volcanic rocks have 

been considered part of the Livingston Group, a thick succession of dominantly fine grained volcaniclastic sandstone 

exposed primarily in the western part of the Crazy Mountains Basin (fig. 1). Although correlated with the Livingston 

Group, the exact relationship between the volcanic rocks 

exposed along the southern margin of the basin and those of 

the Livingston Group rocks is uncertain (Roberts, 1972). In 

general, Livingston Group sedimentary rocks are distal sediments derived from volcanic rocks associated with the 

Upper Cretaceous Elkhorn Mountains Volcanics exposed far 

to the northwest. The volcanic and very coarse grained volcaniclastic rocks exposed along the southern margin of the 

basin contrast significantly with the generally fine grained 

nature of the Livingston Group sedimentary rocks at their 

type section near Livingston, Mont. The presence of numerous vents, small stocks, dikes, and lava flows as well as the 

coarse-grained nature of the volcaniclastic rocks suggest that 

these represent a separate volcanic source area local to the 

southern Crazy Mountains Basin. Proximal volcaniclastic 

sedimentary rocks from these vents were shed northward and 

eastward and intermingled with more distal volcanic Livingston Group deposits derived from the Elkhorn Mountains 

volcanic centers far to the north and west.

Sliderock Mountain volcano (fig. 2) is an intermediate 

composition stratovolcano that consists primarily of volcaniclastic strata including lahars, mudflow deposits, volcaniclastic sandstone, and volumetrically minor pyroclastic 

flows. Intercalated with the volcaniclastic strata are basaltic 

andesite lava flows. The lava flows and volcaniclastic strata 

are cut by an intermediate composition hypabyssal stock, the 

diorite to granodiorite of Sliderock Mountain, which is 

inferred to be the solidified core of the volcano. Other minor 

stocks and plugs in the study area, including the Lodgepole 

intrusion (Brozdowski, 1983), which is well exposed in 

Clover Basin (fig. 2), are probably related to this intrusion. 

The stock and volcanic and volcaniclastic strata are cut by a 

swarm of dikes that form a crudely radial pattern along the 

north, east, and west flanks of the volcano. The dikes, whose 

orientation probably indicates a radially oriented stress 

regime related to shallow emplacement of the magma

reservoir parental to the Sliderock Mountain volcano, was 

the final episode of magmatism associated with the volcano.

The volcano developed within a terrane underlain by 

Paleozoic and-Mesozoic sedimentary rocks, the oldest of 

which are Paleozoic strata that form isolated roof pendants, 

large xenoliths, or are host rocks to the intrusive core of the 

volcano. Lower and Upper Cretaceous rocks, including the 

Kootenai Formation, Thermopolis and Mowry Shales, Frontier Formation, Cody Shale, Telegraph Creek Formation, and 

Eagle Sandstone, are intruded by the diorite to granodiorite 

of Sliderock Mountain and related intrusions. The intrusions 

have locally metamorphosed host sedimentary strata to an 

assemblage of calc-silicate minerals, including garnet and 

epidote. Where these rocks are most intensely metamorphosed, small bodies of weakly mineralized magnetitegarnet-epidote skarn are present. These skarns contain some 

sulfide minerals, especially chalcopyrite, and anomalous 

abundances of metals such as Au, Mo, Pb, Ag, Cu, and Sb 

(Hammarstrom and others, 1993). Numerous gold-bearing 

quartz veins are present in the area around Gold Hill (Crow, 

Gold Hill, Silver King, and Sulphide mines) (fig. 2). The 

diorite to granodiorite of Sliderock Mountain is locally 

weakly sericitized and contains disseminated pyrite near the 

veins. Gold placer deposits of limited size are present in the 

drainages near the gold-bearing veins.

PHYSICAL VOLCANOLOGY OF THE 

SLIDEROCK MOUNTAIN VOLCANO

The earliest volcanic activity associated with the stratovolcano was the eruption of basaltic andesite lava flows of 

Derby Ridge (du Bray and others, 1994). These rocks form 

a 400-m-thick section of lava flows, with volumetrically 

minor pyroclastic flows, block and ash flows, welded tuff, 

and interbedded lahars, erupted and unconformably deposited on moderately tilted Mesozoic sedimentary rocks.

The basaltic andesite lava flows are overlain by texturally and compositionally immature volcaniclastic strata. The 

basal approximately 50 m of these strata consist of interbedded sandstone, siltstone, and shale that probably forms the 

base of the Livingston Group in the area. These sedimentary 

rocks are overlain by at least 300 m of clast-supported lahar 

deposits (fig. 3) composed of poorly sorted, bouldery mudflows correlative with the Livingston Group (Parsons, 1942; 

p. 1177). The lahars, which form prominent rubbly cliffs, 

were probably derived by disaggregation of lavas erupted 

from the Sliderock Mountain volcano. Individual mudflows 

are massive and tens of meters thick; bedding is absent or 

poorly developed in most places. Clasts, dominantly composed of basaltic andesite, are 0.05-3 m in diameter and 

subangular to angular; some clasts in the basal part of the 

unit may be derived from the Lodgepole intrusion of 

Brozdowski (1983). The matrix of the deposits is finely
CRETACEOUS SLIDEROCK MOUNTAIN VOLCANO, MONTANA

45°40'

110°00' 109°55'

45°35'

Limit of Geologic 

Mapping

Clover ^ , ... Basin Geology not shown

Figure 2 (above and facing page). Simplified geologic map showing the Sliderock Mountain stratovolcano (modified 

from du Bray and others, 1994). All three-digit sample locality numbers are prefixed by 202.

comminuted ash and altered andesite. This unit contains thin 

interbeds of very well sorted epiclastic volcanic sandstone, 

siltstone, and shale that contain carbonized plant remains; 

minor pyroclastic flow deposits may also be present. A few 

thin (several meters) andesite lava flows are enclosed within 

the lahar sequence. A second, 300-m-thick set of porphyritic, light- to dark-gray plagioclase-clinopyroxene basaltic 

andesite lava flows overlie the lahar deposits; these rocks 

probably represent the culmination of eruptive activity at the 

Sliderock Mountain center.

The intrusive phase of the Sliderock Mountain volcano 

is a very light gray to medium-gray plagioclase-hornblende 

diorite to granodiorite that intrudes all lava flow and lahar 

deposits. The diorite to granodiorite may represent multiple 

intrusive phases that coalesced to form an intrusive complex. 

The intrusive complex probably is the solidified core of the 

stratovolcano.

Porphyritic, light-gray plagioclase-clinopyroxene-hornblende basaltic trachyandesite to trachyte dikes, 1.5 to 5 km 

long and 1 to 10 m wide, form prominent, linear ridges,
GEOCHRONOLOGY

EXPLANATION

Quaternary deposits

Cretaceous andesite-diorite intrusions

Cretaceous trachyandesite dikes

Cretcaceous diorite of Sliderock Mountain 

Cretaceous basaltic andesite lava flows 

Cretaceous lahar deposits

Cretaceous volcaniclastic rocks

Cretaceous basaltic andesite lava flows of Derby Ridge 

Cretaceous sedimentary rocks

Paleozoic sedimentary rocks

______ Fault-Sense of displacement unspecified. Dashed

where approximate

f Normal fault-Ball and bar on downthrown block. 

Dashed where approximate

• 116 Location of sample for geochemical analysis 

n 91 SR2b Location of sample for isotopic age determination 

A Geographic locality

especially north of Sliderock Mountain. The dikes form a 

crudely radial pattern and cut all intrusive and extrusive 

phases of the volcano (du Bray and others, 1994). The dikes 

are compositionally and petrographically similar to other 

igneous rocks of the Sliderock Mountain area. They probably represent the final phase of igneous activity associated 

with the volcano. Other intrusive rocks, possibly contemporaneous with and genetically related to the dikes, include 

sills of light-gray porphyritic andesite, a plug of light-gray 

porphyritic diorite, and a diorite sill, south, east, and west of 

the volcano, respectively (fig. 2).

GEOCHRONOLOGY

Six samples from Sliderock Mountain volcano were 

dated by the 40Ar/39Ar step-heating technique. Four hornblende separates were prepared from intrusive rock samples 

and two sericite separates were prepared from samples of 

altered rock that hosts precious metal deposits at Gold Hill. 

Although many of the basaltic andesite lava flows contain 

amphibole, separates of sufficient quantity and purity could 

not be prepared. Samples selected for dating were crushed 

and sieved and mineral concentrates were prepared using 

standard gravimetric (heavy liquid) and magnetic 

techniques. Mineral separates were further purified by

hand-picking. Separates were sealed in silica vials and irradiated in the central thimble of the U.S. Geological Survey 

TRIGA reactor for 30 hours. Neutron flux during irradiation was monitored using hornblende standard MMhb-1, 

which has a K-Ar age of 520.4±1.7 Ma (Samson and Alexander, 1987).

After irradiation, the samples were progressively 

degassed in a double-vacuum resistance furnace in a series 

of nine to sixteen 20-minute-long steps to a maximum temperature of 1600°C. Apparent ages were calculated using 

decay constants recommended by Steiger and Jager (1977). 

The determination of whether the individual apparent ages 

yielded a "plateau" was made using the critical value test of 

Dalrymple and Lanphere (1969) following the plateau definition of Fleck and others (1977). Plateau dates were calculated using a weighted mean, where weighting is by the 

inverse of the analytical variance (Taylor, 1982). A detailed 

description of analytical procedures similar to those used in 

this study is given by Tysdal and others (1990, App. 1). 

Results of 40Ar/39Ar are presented as age spectra and apparent 39Ar/37Ar ratios (fig. 4); analytical data are shown in 

table 1.

Two samples of the diorite and granodiorite stock of 

Sliderock Mountain stock were dated. Sample nos. 202109 

and 91SR2b gave total gas dates of 78.5±0.5 and 81.3±0.6 

Ma (2o), respectively. With the exception of low temperaOQ O''?

ture steps, both samples have relatively constant Ar/ Ar 

ratios for more than about 90 percent of the potassiumderived 39Ar (39ArK) released (fig. 4A), which indicates that 

a single homogeneous phase was degassed at higher temperatures during step-heating experiments. Amphibole sample 

no. 91SR2b has a strongly U- or saddle-shaped age spectrum 

(fig. 4A) characterized by anomalously old apparent ages in 

the low- and high-temperature steps and younger apparent 

ages for intermediate temperature steps (fig. 4A). Such a 

spectrum is characteristic of the incorporation of excess 

40Ar, a phenomenon common in plutonic amphibole (Lanphere and Dalrymple, 1976). Because the sample may have 

incorporated excess 40Ar, a maximum age is given by the 

minimum age defined by the intermediate temperature steps; 

in this case, the age of the hornblende must be less than about 

77.4 Ma. Because the shape of the age spectrum is suggestive of the presence of excess 40Ar, we analyzed the data 

using a 39Ar/40Ar versus 36Ar/40Ar inverse correlation diagram (McDougall and Harrison, 1988). In contrast to the 

apparent ages calculated for individual temperature steps in 

age spectrum diagrams, correlation analysis makes no 

assumption regarding the initial 40Ar/36Ar ratio of the sample. The 39Ar/40Ar versus 36Ar/40Ar data for sample no. 

91SR2b (fig. 5) scatter about a line (mean square of 

weighted deviates, MSWD, is 6.9) that defines an apparent 

age of 77.4 (N = 9/11 steps). The analytical error for the 

regression (±0.19 Ma, lo) has been adjusted to account for 

an external error component in the regression by multiplying
CRETACEOUS SLIDEROCK MOUNTAIN VOLCANO, MONTANA

Figure 3. Photographs of 

Sliderock Mountain stratovolcano lahars. A, Individual 

mudflows are massive and tens 

of meters thick; bedding is absent or poorly developed in 

most places. B, Close-up view 

of clast-supported lahar; clasts 

are composed of disaggregated 

basaltic andesite lava, whereas 

matrix is composed of ash and 

finely comminuted lava.

the observed error by the square root of the MSWD to 

account for the excess scatter in the regression (Ludwig, 

1988) and yields a more appropriate estimate of the analytical error of ±1.0 Ma (2o). The initial 40Ar/36Ar ratio determined from the regression is 497±13 (2o), which is 

significantly greater than the atmospheric value of 295.5. 

The high initial 40Ar/36Ar ratio clearly indicates that this 

sample contains excess 40Ar. Therefore, we consider the age 

determined by the correlation diagram to be the most reliable 

estimate for the age of sample no. 91SR2b.

Sample no. 202109 has a similarly discordant age 

spectrum (fig. 4A), suggestive of excess 40Ar incorporation,

but the sample may have also experienced minor 40Ar loss. 

The age spectrum for this sample has two steps that overlap 

and yield an apparent plateau with an age of 77.70±0.24 

Ma, but this age may be misleading because greater than 60 

percent of the 39ArK was released in a single temperature 

step at 1150°C. Degassing most of the 39ArK in a single 

step precludes resolution of the fine detail of the age spectrum. Analysis of the 40Ar/39Ar data using a correlation 

diagram gives an apparent age of 77.97±0.12 Ma (N=8/ll 

steps, MSWD=2.19), with an initial 40Ar/36Ar ratio of 

313+5, which is somewhat greater than the present-day 

atmospheric value.
GEOCHRONOLOGY

1.0

-10

s.

*£

" 0.5

100

<D

E 90

=

W

* 80

W

OP

8: 70

0

4.U«- 1 U*7

—— -92SR2B

n , ______________

1 i . . • • i

| 2021 09 Hornblende

91SR2B Hornblende

__ , , f88^

[I

50 1(

1.0

<C 1.0

1<

s 

0.5

100

<D

2 90

z

til 

CD

< 80 lw

or

8: 70

B en

)0 C

91SR3 Hornblende

————— 74.94 ± 0.13 Ma ——————

»%,, INI IIIIM ml n°TM^HS

50 1C

PERCENT 39ArK RELEASED PERCENT 'WK RELEASED

1.3

lo,

100

<D

| 90

m"

J 80

z

w 

(_* fin

202 128 Hornblende

1 ——————— 76.39 ± 0.24 Ma —————————

100

90

m"

80

|70

91JH38Sericite 

91JH36Sericite

50 

PERCENT 39ArK RELEASED

100

Figure 4.

50 100 

PERCENT 39ArK RELEASED

Ar age spectra and 39Ar/37Ar ratios for samples from Sliderock Mountain volcano. A, hornblende from the Sliderock

Mountain stock; B, hornblende from hornblende dacite of the Lodgepole intrusion; C, hornblende from an andesite dike; and D, sericite from 

altered rock associated with gold-bearing veins in the Sliderock Mountain stock. The height of rectangles for individual temperature steps in 

age spectra represent analytical error at ±lo; mean ages are given at ±2o.

Combining the two correlation ages from sample nos. 

91SR2b and 202109 yields an arithmetic mean of 

77.78±0.57 Ma (2a; error based on the standard error of the 

mean) (Taylor, 1982), which may be a reasonable estimate 

for the closure age for Ar diffusion from hornblende in the 

stock of Sliderock Mountain. However, this type of simple 

average makes no provision for the quality or precision of 

the analyses and is strictly appropriate only in cases where 

the analytical errors for the age determinations are equal in

magnitude or are unknown. In this case, however, the analytical errors between the two correlation ages are significantly different. Where analytical errors are unequal, ages 

may be calculated using a weighted mean, where weighting 

is by the inverse of the analytical variance (Taylor, 1982). 

By utilizing this method, a measure of the "reliability" of 

each age measurement is incorporated in determining the 

best estimate of the age of the sample (Fleck and others, 

1996). Calculating a weighted mean for these samples gives
8 CRETACEOUS SLIDEROCK MOUNTAIN VOLCANO, MONTANA 

Table 1. 40Ar/39Ar incremental heating data for samples from Sliderock Mountain stratovolcano, Montana.

[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 is the 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 percent of radiogenic 40Ar and percent of total 39Ar released in each 

temperature step. Temperature steps in boldface are those used in the calculation of plateau age. Conversion of volts signal to moles Ar can be 

made using a conversion factor of 1.25 2x10" 12 moles argon per volt of signal]

Temp

(°C)

900

1000

1100

1200

1300

1350

1375

1400

1425

1500

1600

Total Gas

750

850

950

1000

1033

1066

1100

1150

1200

1300

Total Gas

700

800

850

900

950

1000

1025

1050

1075

1100

1125

1150

1200

1250

1350

1450

Total Gas

40 AArR

0.01536

0.20430

0.06617

0.13035

2.9085

6.6463

0.35900

0.20929

0.12769

0.22140

0.13081

0.23788

0.09930

0.69813

2.3118

2.8069

2.6447

0.90993

0.96318

0.33431

0.21151

0.03128

0.10994

0.03248

0.03393

0.03779

0.05113

0.37415

1.5309

1.9253

1.4448

0.63996

1.7220

1.7792

2.0763

4.8678

0.31048

39 AArK

Sample no. 202109:

0.00371

0.04428

0.01479

0.02843

0.60173

1.4226

0.07625

0.04301

0.02713

0.04463

'0.02662

Sample no. 91SR2B:

0.01529

0.01155

0.07999

0.26869

0.33317

0.29739

0.10214

0.10429

0.03652

0.02329

Sample no. 202128:

0.00414

0.01504

0.00644

0.00542

0.00587

0.00859

0.06157

0.25624

0.32128

0.24564

0.10970

0.29311

0.30443

0.35655

0.83383

0.31048

40 . 39 . ArR/ Arj

Hornblende;

4.138

4.614

4.474

4.585

4.834

4.672

4.708

4.866

4.707

4.960

4.913

4.723

Hornblende;

15.556

8.598

8.728

8.604

8.425

8.893

8.909

9.236

9.154

9.082

8.817

Hornblende;

7.564

7.308

5.041

6.261

6.436

5.950

6.077

5.974

5.992

5.882

5.834

5.875

5.844

5.823

5.838

5.855

5.887

39 A ,37. ^ Ail Ar

1 93. 1 mg; J-value =

0.51

1.26

0.67

0.25

0.20

0.20

0.20

0.17

0.15

0.17

0.15

148.5 mg; J-value =

0.05

0.30

0.21

0.22

0.22

0.22

0.22

0.21

0.21

0.21

244.8 mg; J-value =

0.43

0.39

0.83

0.82

0.59

0.28

0.20

0.19

0.19

0.18

0.18

0.19

0.19

0.19

0.19

0.18

%

0.009420

14.2

38.8

42.7

56.5

91.8

96.4

82.1

76.5

70.1

74.7

52.0

0.005230

24.9

60.0

86.4

95.3

96.4

96.8

89.4

95.1

90.4

8.1.6

0.007388

2.7

13.9

8.8

11.4

6.2

5.4

18.8

36.1

39.1

48.9

92.9

94.6

93.6

93.6

95.8

95.7

40 A 07 39 . i Arji % Ar

±0.1% (Iff)

0.2

1.9

0.6

1.2

25.8

61.0

3.3

1.8

1.2

1.9

1.1

±0.2% (Iff)

1.2

0.9

6.3

21.1

26.2

23.4

8.0

8.2

2.9

1.8

±0.2% (Iff)

0.1

0.5

0.2

0.2

0.2

0.3

2.0

8.2

10.2

7.8

3.5

9.3

9.7

11.4

26.6

9.9

Apparent Age

(Ma at la)

69.0 ±2.8

76.75 ±1.30

74.46 ± 0.50

76.28 + 1.76

80.32 + 0.16

77.70 ±0.12

78.29 ±1.05

80.85 ±1.13

78.27 + 0.12

82.38 + 1.53

8 1.62 ±0.95

78.53 ± 0.25

141.1 ± 1.6

79.4 ± 3.4

80.52 + 0.35

79.40 ±0.1 8

77.79 ±0.18

82.01 ±0.19

82.16 ±0.38

85.10 ±0.39

84. 36 ±1.27

83.72 ±0.81

81.32 ±0.28

98.1 + 0.5

94.9 ± 0.5

66.0 ± 1.0

81.6 ± 4.9

83.8 ± 3.0

77.6 + 5.2

79.23 ±0.21

77.92 ±0.21

78.15 ±0.18

76.74 ±0.1 8

76. 12 ±0.44

76.65 ±0.18

76.26 ± 0.29

75.99 + 0.23

76.1 8 ±0.22

76.39 ± 0.27

76.8 ± 0.3

an apparent age of 77.96±0.24 Ma (2o). Although analytically very precise, the geologic significance of this age must 

be viewed with caution because the correlation ages may also 

represent maximum apparent ages. In addition, the high analytical precision of the age is probably geologically unrealistic because the contribution of the sample no. 91SR2b age to 

the calculated mean is minor owing to its large analytical 

error (Taylor, 1982). Given the problematic nature of the

analyzed samples due to the presence of such things as 

excess 40Ar and scatter in the isochrons, the quoted uncertainties of either mean age probably should be viewed as 

minimum values. Because of the magnitude of the adjusted 

analytical error from the regression of sample no. 91SR2b 

correlation data, we consider a reasonable and conservative 

estimate for the mean hornblende age from the stock of 

Sliderock Mountain to be about 78±1 Ma (based on the
GEOCHRONOLOGY

Table 1. 40Ar/39Ar incremental heating data for samples from Sliderock Mountain stratovolcano, Montana—Continued.

Temp 40 .ArR

39 A 40 . 39 . ATK ATR/ Ai

Sample no. 91SR3: Hornblende,

600

700

800

850

900

950

1000

1025

1050

1075

1100

1125

1150

1200

1250

1450

Total Gas

600

700

750

800

850

900

950

1000

1050

1100

1150

1350

Total gas

700

800

900

950

1000

1050

1100

1150

1350

Total gas

0.1885

0.01554

0.03290

0.01058

0.03146

0.03465

0.89419

0.74029

1.6597

4.1423

5.3529

1.8898

0.90664

0.79238

0.32979

0.84325

1.8195

3.6868

2.8953

2.3084

1.7335

1.7307

3.2692

3.3670

1.4341

0.36108

0.21684

0.23697

3.4148

3.4022

2.4473

2.6193

4.0817

3.0832

0.66482

0.32443

0.37944

0.00436

0.00342

0.00544

0.00491

0.00550

0.00616

0.15642

0.13013

0.29333

0.72681

0.94074

0.33198

0.15784

0.13950

0.05843

0.14961

Sample no. 91JH38:

0.30732

0.62046

0.49511

0.39982

0.30300

0.30192

0.57114

0.58675

0.24980

0.06160

0.03459

0.03054

Sample no. 91JH36:

0.58333

0.58297

0.42229

0.46031

0.71448

0.54029

0.11507

0.05811

0.06473

4.325

4.544

6.045

2.155

5.715

5.621

5.717

5.869

5.658

5.699

5.690

5.692

5.744

5.680

5.644

5.636

5.681

Sericite;

5.920

5.942

5.848

5.774

5.721

5.732

5.724

5.738

5.741

5.862

6.269

7.760

5.820

39. .37. /w 40 A CK Ar/ Ar % ATR

; 215.6 mg; J-value •

0.28

0.19

0.18

0.20

0.21

0.23

0.23

0.22

0.22

0.22

0.22

0.22

0.21

0.21

0.22

0.24

68.5 mg; J-value =

188.2

129.9

185.3

181.5

191.2

128.3

291.5

246.9

140.1

71.64

17.76

7.04

Sericite; 35.6 mg; J-value =

5.854

5.836

5.795

5.690

5.713

5.707

5.778

5.583

5.862

5.765

171.2

164.2

132.0

150.1

272.4

181.6

65.27

23.19

11.32

%39Ar Apparent Age 

(Maatlo)

= 0.007450 ±0.2% (la)

15.5

17.2

10.6

13.8

49.6

51.7

81.5

85.0

90.9

93.5

92.8

88.8

89.9

92.0

88.6

92.4

0.007365 ±0.1% (la)

91.0

96.5

96.5

97.4

94.6

95.6

95.9

95.6

93.7

87.6

83.3

70.2

0.00740 ±0.1% (la)

92.8

95.8

96.9

95.6

97.0

96.6

95.6

90.8

91.4

0.1

0.1

0.2

0.2

0.2

0.2

5.0

4.2

9.4

23.3

30.2

10.7

5.1

4.5

1.9

4.8

7.8

15.7

12.5

10.1

7.6

7.6

14.4

14.8

6.3

1.6

0.9

0.8

16.5

16.5

11.9

13.0

20.2

15.3

3.2

1.6

1.8

57.2 ±12.4

60.1 ±8.5

79.5 ±11.5

29 ±18

75.2 ±8.2

74.0 ±10.8

75.24 ± 0.48

74.88 ± 0.47

74.49 ± 0.24

75.02 ±0.13

74.90 ±0.11

74.93 ±0.11

75.59 ±0.34

74.77 ± 0.43

74.30 ±1.10

74.21 ±0.80

74.78 ± 0.39

76.82 ± 0.32

77.10 ±0.12

75.94 ±0.12

74.96 ±0.1 8

74.29 ±0.18

74.43 ±0.17

74.33+0.11

74.50 ±0.11

74.54 ±0.18

76.08 ± 0.69

81.2 ±1.7

100.0 ±2.3

75.55 ±0.18

76.33 ± 0.26

76.10 ±0.21

75.58 ± 0.47

74.24 ± 0.26

74.53 ±0.11

74.45 ± 0.34

75.36 ±0.91

72.87 ±1.89

76.44 ± 0.70

75.20 ±0.31

magnitude of the error from sample no. 91SR2b). Given the 

high closure temperature for Ar diffusion in hornblende 

(500°±50°C) (McDougall and Harrison, 1988), coupled with 

the fact that this hypabyssal stock intrudes overlying, 

cogenetic volcanic rocks and therefore probably cooled quite 

rapidly, the dates probably closely approximate the emplacement age of the stock.

Sample no. 202128 is a porphyritic biotite-hornblende 

dacite from the Lodgepole intrusion (Brozdowski, 1983), a 

laccolith about 5 km south of Sliderock Mountain volcano in 

Clover Basin (figs. 1, 2). The Lodgepole intrusion is

compositionally similar to intrusive rocks in the core of the 

Sliderock Mountain volcano and lithologically similar clasts 

are present in some lahars of Sliderock Mountain volcano. 

Hornblende from the Lodgepole intrusion gave a total gas 

age of 76.80±0.50 Ma. Although the spectrum for sample 

no. 202128 indicates an excess Ar component in low temperature steps, it is simply interpreted (fig. 4C), giving a plateau age of 76.39±0.24 Ma for 78.2 percent of the 39ArK 

released. The 39Ar/37Ar ratio for this interval is relatively 

constant, which indicates that a single phase was degassed 

during this heating interval.
10 CRETACEOUS SLIDEROCK MOUNTAIN VOLCANO, MONTANA

Hornblende from one of the radial andesite dikes (sample no. 91SR3) that is representative of the dike swarm gave 

a total gas date of 74.8±0.8 Ma. Overall, the age spectrum is 

simple in that individual temperature steps give essentially 

identical apparent ages and have similar 39Ar/37Ar ratios 

over virtually the entire range of 39ArK released (fig. 4Q. A 

weighted-mean plateau date for the 1075° to 1125°C steps 

(64.2 percent of the Ar^ released) gives an age of 

74.94±0.13Ma.

Finally, two sericite samples (sample nos. 91JH36 and 

91JH38) associated with potassically altered rock that surrounds a gold vein in the Gold Hill mine area were dated. 

Both sericite spectra show initial apparent ages of about 77 

Ma; apparent ages decrease in higher temperature steps, have 

relatively constant apparent ages of about 74.5 Ma in intermediate temperature steps, and increase in the highest temperature steps (fig. 4D). Overall, these spectra are indicative 

of excess 40Ar, although the high radiogenic yield of the 

samples combined with limited spread in 36Ar/40Ar ratios 

precluded precise determinations of initial 40Ar/36Ar values 

using correlation diagrams. The two samples yield similar 

plateau dates of 74.42±0.12 Ma and 74.49±0.19 Ma. 

Because the samples are from the same vein and give essentially identical dates, we calculated a weighted-mean age of 

74.44±0.20 Ma and consider this to be the best estimate for 

the age of potassic alteration.

The 40Ar/39Ar dating described here indicates that the 

Sliderock Mountain stock is probably less than about 78 to 

77 Ma, although an absolute age is difficult to quantify given 

the effect of excess Ar on the samples. If the stock is 

approximately coeval with the nearby Lodgepole intrusion, it 

could be as young as about 76.5 Ma. Alternatively, the compositionally similar intrusion may represent a slightly 

younger igneous event. Because the stock cuts the volcaniclastic and volcanic strata, their age is somewhat greater. 

Although it is not possible to precisely establish the age, an 

approximate age of about 78±1 Ma is what we consider to be 

a reasonable maximum age estimate for the initiation of 

volcanism and intrusive activity at the Sliderock Mountain 

volcano. Final magmatic activity is represented by the 

emplacement of radial dikes, which intrude all other phases 

of the volcano, at about 74.9 Ma. The 40Ar/39Ar dates of 

about 74.4 Ma from hydrothermally altered phases of the 

stock indicate that weak precious mineralization was probably genetically related to the stock and occurred during the 

final phases of igneous activity at Sliderock Mountain 

volcano.

The 78 to 75 Ma dates for igneous activity at Sliderock 

Mountain volcano are somewhat younger than the 82.4±4.1 

and 81.8±3.1 Ma (both at lo) K-Ar dates previously reported 

for rocks in the Sliderock Mountain area (Marvin and Dobson, 1979). The apparent ages of these conventional K-Ar 

dates may have been influenced by the excess 40Ar we

0.004

Air—* 

0.003

S.. 0.002

0.001

0.004

Air—s 

0.003

<

^: 0.002

91SR2b Hornblende 

Age = 77.4 ±1.0(2o) 

«Ar/»Ar, = 497±13(2o)

0.04 0.08 0.12

0.001

B

202109 Hornblende 

Age = 77.97 ±0.12 (2a)

0.04 0.08 0.12 

39Ar/40Ar

0.16 0.20

Figure 5. 39Ar/40Ar versus 36Ar/40Ar correlation diagrams 

for two hornblende separates from the stock of Sliderock 

Mountain. Circles represent 39Ar/40Ar versus 36Ar/40Ar ratios 

determined for each temperature step; lines about the points 

represent the error in the ratios at one standard deviation. For 

most steps, the errors on the ratios are smaller than the size of 

the symbols. Closed circles are those points used in the regression; open circles are those rejected. Individual points 

were rejected based on significantly different ~ Arr Ar versus 

36Ar/40Ar ratios or because of significantly different 39Ar/ 

37Ar ratios (table 1 ) which suggest that the gas evolved from 

these steps during the step-heating experiment may be due to a

4036 4036 different mineral phase. Ar/Ari is the initial Ar/Ar 

value as determined from the inverse of the 36Ar/40Ar intercept. A, sample no. 91SR2b; one of the points is not shown 

because it plots off the scale of the correlation diagram. B, 

sample no. 202109.

observed during the course of our study. Our 40Ar/39Ar dates 

are similar to conventional K-Ar dates for the Elkhorn Mountains Volcanics and volcanic rocks in the Maudlow basin 

north and west of the Crazy Mountains Basin (Robinson and 

Marvin, 1967; Robinson and others, 1968; Tilling and others, 

1968; Hanna, 1973; Marvin and others, 1989). Additional 

isotopic dating is necessary elsewhere throughout the volcanic field in order to more accurately constrain the ages of 

Late Cretaceous magmatism along the southern margin of 

the Crazy Mountains Basin and to relate magmatism to the 

regional tectono-magmatic evolution of the northern Rocky 

Mountains.
GEOCHEMISTRY 11

PETROGRAPHY GEOCHEMISTRY

Basaltic andesite lava flows of Derby Ridge are composed of light-gray to light-olive gray intersertal rocks that 

are weakly trachytically layered in some places. Crystal 

abundances in these rocks are highly variable; phenocrysts 

are subhedral to euhedral laths of zoned and albite-twinned 

plagioclase (0.6 mm) altered to clay, subhedral grains of 

pale-green clinopyroxene (0.5 mm), and anhedral grains of 

opaque oxide minerals (0.2 mm). Accessory minerals 

include apatite and hornblende. The groundmass, partly 

altered to calcite in some places, is composed of a very fine 

grained, turbid intergrowth of plagioclase and opaque oxide 

minerals; some rocks contain incipiently devitrified glass.

Light- to dark-gray basaltic andesite lava flows that 

overlay the lahar deposits have highly variable crystal contents; phenocrysts are subhedral to euhedral laths of albitetwinned and zoned plagioclase (0.6 mm), subhedral clinopyroxene (0.5 mm), and anhedral opaque oxide minerals (0.2 

mm). The groundmass is a devitrified intergrowth of finegrained plagioclase and opaque oxide minerals. The lava 

flows are characterized by an intersertal texture; trachytic 

layering is present locally.

The core of the volcano is composed of very light gray 

to medium-gray, hypidiomorphic granular diorite to granodiorite. This rock is principally composed of subhedral to 

euhedral laths of zoned and albite-twinned plagioclase (0.8 

mm) that is moderately to strongly sericitized. Hornblende 

(0.8 mm) is the principal mafic mineral and forms strawyellow to olive-green subhedral laths. About 5 percent of the 

crystals in the stock are anhedral grains of opaque oxide minerals (0.2 mm). Quartz (anhedral, very fine grained, and 

interstitial), biotite, or clinopyroxene are present in some 

samples; apatite is a ubiquitous trace phase. The stock is texturally heterogeneous and is characterized by pronounced 

grain size variation; it is fine grained in most places but is 

locally medium grained. The groundmass in some samples 

is a turbid intergrowth of very fine grained plagioclase, chlorite, calcite, and opaque oxide minerals.

The basaltic trachyandesite to trachyte dikes contain 

15-30 percent phenocrysts, including euhedral to subhedral 

laths of strongly zoned, albite-twinned, clay-altered plagioclase (1 mm) and subequal amounts of anhedral to subhedral 

pale-green clinopyroxene (0.5 mm) and subhedral to euhedral olive-green hornblende (1 mm); apatite is a trace mineral. The groundmass is a turbid, devitrified intergrowth of 

plagioclase and opaque oxide minerals; secondary zeolite 

minerals are common. Three small intrusions (du Bray and 

others, 1994), probably related to the dikes, are porphyritic 

and contain 25-40 percent phenocrysts of plagioclase, hornblende, and biotite; calcite and zeolite minerals are secondary minerals in the fine-grained groundmass and replace 

plagioclase.

Geochemical analyses were obtained for 26 samples of 

the Sliderock Mountain stratovolcano. All analyses were 

performed in the laboratories of the U.S. Geological Survey 

in Denver, Colorado. Major oxide analyses were performed 

(analysts, D.F. Siems, J.E. Taggart, and J.S. Mee) using Xray fluorescence techniques (Taggart and others, 1987). The 

ratios Fe2+: total iron as Fe2+ were adjusted to 0.85 and 

major oxide abundances recalculated to 100 percent, on an 

anhydrous basis. Trace element abundances were determined (by E.A. du Bray) by energy-dispersive X-ray fluorescence spectroscopy (Elsass and du Bray, 1982) using 109Cd 

and 241 Am radio-isotope excitation sources; the accuracy of 

this data is quantified by Yager and Quick (1992).

Rocks of the Sliderock Mountain stratovolcano are subalkaline to transitionally alkaline (table 2, fig. 6). Compositions of the lava flows stratigraphically above and below the 

lahar deposits, and most of the dikes, are indistinguishable 

(fig. 7) and cluster in the basaltic andesite and basaltic trachyandesite fields (Le Bas and others, 1986); some lava flow 

compositions extend to andesite or trachyandesite and one 

dike is composed of trachyte. Compositions of intrusive 

rocks from Sliderock Mountain stratovolcano are bimodally 

distributed. About half of these have compositions of

LJ 

O

LJ 

Q_

10

0 5

CM

a

CM 

D

BASALTIC 

TRACHYANDESITE

BASALTIC 

ANDES I 

I I I I I I I I I/I

ANDESITE 

I I I I I I I I I

43 55 67 

S i0 2 , IN WE IGHT PERCENT

Figure 6. Total alkali-silica variation diagram with IUGS classification grid (Le Bas and others, 1986) showing analyses of samples 

from the Sliderock Mountain stratovolcano (table 2). Alkalinesubalkaline divider is that of Irvine and Baragar (1971). Data are 

plotted as follows: intrusive phase, plus signs; dikes, circles; basaltic 

andesite lava flows of Derby Ridge, triangles; and basaltic andesite 

lava flows deposited atop the lahar deposits, squares.
12 CRETACEOUS SLIDEROCK MOUNTAIN VOLCANO, MONTANA

Table 2. Chemical compositions of samples from the Sliderock Mountain stratovolcano, Montana.

[Major oxide analyses (weight percent) normalized to 100 percent, volatile free. FeO/FeO* recalculated to 0.80. All trace element data are in 

parts per million. Sum;, original, pre-normalization total with total iron as Fe2O3- LOI, loss on ignition; bdl, below detection limit]

Basaltic andesite lava flows of Derby Ridee

Sam.

Si02

A1203

Fe203

FeO

MgO

CaO

Na2O

K2O

Ti02

P205

MnO

LOI

Sum,

Rb

Sr

Y

Zr

Mb

Pb

Th

Ba

La

Ce

Nd

No.- 202112

54.76

19.81

1.14

5.80

3.11

8.88

3.22

2.18

0.47

0.48

0.16

1.18

100.25

51

841

23

80

5

bdl

bdl

827

30

52

22

202114 202115

56.47

19.25

1.08

5.52

2.79

7.15

3.16

3.44

0.50

0.51

0.14

4.16

99.31

88

652

12

92

9

16

12

1170

36

61

27

58.75

17.94

1.01

5.15

2.79

5.80

4.45

2.99

0.59

0.40

0.12

2.11

99.61

73

653

17

146

12

bdl

bdl

1356

28

52

15

202116

52.85

19.48

1.06

5.42

2.52

10.72

4.59

2.16

0.45

0.51

0.24

5.97

99.43

64

860

7

68

8

17

bdl

1078

16

72

25

202124

54.99

19.04

1.31

6.67

3.51

8.20

2.49

2.52

0.62

0.50

0.15

4.96

99.67

56

858

20

100

8

bdl

bdl

924

20

57

15

202125

55.27

19.07

1.19

6.07

2.97

9.28

2.92

2.10

0.52

0.48

0.14

6.96

99.86

50

795

12

71

4

17

13

836

21

46

6

202091

55.58

19.38

1.12

5.73

3.17

8.61

3.12

2.19

0.50

0.46

0.14

1.26

99.95

63

834

22

84

10

10

13

806

24

86

24

Basaltic andesite lava flows Stock

202092

53.30

17.66

1.42

7.26

3.79

9.13

3.12

2.69

0.90

0.56

0.15

1.84

100.00

45

971

32

169

9

bdl

bdl

1013

18

67

32

202093

55.66

18.90

1.20

6.11

3.14

6.49

3.71

3.44

0.72

0.52

0.12

2.86

99.81

66

974

29

158

9

24

bdl

1723

32

67

42

Diorite to eranodiorite stock of Sliderock Mountain

Sam. No.-- 202095

SiO2

A1203

Fe203

FeO

MgO

CaO

Na2O

K2O

TiO2

P205

MnO

LOI

Sum,

Rb

Sr

Y

Zr

Nb

Pb

Th

Ba

La

Ce

Nd

55.46

18.35

1.23

6.26

2.97

7.25

3.65

3.43

0.76

0.52

0.12

1.95

99.63

69

1021

22

130

15

13

bdl

1315

40

62

46

202101

62.26

17.52

0.84

4.29

2.23

5.21

3.81

2.89

0.51

0.33

0.12

2.40

99.88

75

940

21

148

12

bdl

bdl

1560

46

51

29

202102

53.98

19.18

1.14

5.83

4.41

8.50

3.49

2.05

0.50

0.40

0.14

2.93

98.95

44

855

11

54

9

bdl

bdl

861

15

53

16

202105

69.93

16.70

0.44

2.23

0.20

1.95

4.47

3.48

0.39

0.18

0.04

3.20

99.25

66

650

8

108

13

19

bdl

2507

20

32

38

202109

62.91

17.56

0.79

4.05

2.05

5.13

3.81

2.77

0.50

0.32

0.11

1.86

99.10

67

946

21

149

13

16

bdl

1681

31

57

33

202110

67.31

16.85

0.49

2.52

0.99

3.52

4.56

3.13

0.35

0.20

0.07

2.34

99.33

60

1263

20

166

13

19

bdl

2111

28

70

22

202113

54.64

19.62

1.21

6.18

2.62

8.41

3.70

2.39

0.57

0.50

0.16

4.40

99.85

59

984

15

71

9

bdl

bdl

1114

25

57

27

202121

63.94

17.01

0.69

3.50

2.32

4.47

4.70

2.50

0.49

0.29

0.09

1.15

99.12

42

1042

20

131

9

17

bdl

1759

29

52

27

202129

54.06

18.41

1.27

6.49

4.43

8.57

3.16

2.33

0.68

0.46

0.14

3.68

99.45

59

845

25

83

5

bdl

bdl

849

36

70

22

202100

54.35

19.24

1.24

6.30

3.16

8.94

3.22

2.34

0.59

0.49

0.15

1.14

100.08

55

941

15

93

9

27

14

909

40

72

19

202119

53.39

18.93

1.31

6.70

3.95

8.97

3.29

2.16

0.68

0.48

0.14

1.61

99.55

45

861

16

57

7

6

bdl

892

32

54

26

202120

56.21

18.26

1.26

6.40

2.90

7.73

3.20

2.70

0.71

0.50

0.12

1.45

99.63

62

803

28

143

8

bdl

bdl

969

40

65

15

202094

54.65

18.79

1.23

6.28

3.90

9.24

2.73

2.02

0.61

0.43

0.13

5.53

99.87

51

851

15

72

7

5

20

859

33

66

20

Basaltic trachvandesite and trachvte dikes

202088

61.76

17.87

0.76

3.86

1.31

4.40

3.58

5.59

0.48

0.26

0.14

2.46

99.71

108

1056

23

131

13

23

15

1367

54

70

30

202089

53.91

19.69

1.22

6.23

2.74

8.59

3.74

2.57

0.62

0.53

0.16

2.34

100.02

58

1183

20

88

9

6

bdl

883

31

58

35

202111

53.16

19.80

1.28

6.51

2.87

8.89

4.03

2.17

0.58

0.54

0.17

3.37

100.01

55

832

22

62

7

13

bdl

966

21

57

32

202126

52.89

20.21

1.15

5.86

3.66

9.75

3.18

2.21

0.44

0.50

0.15

3.72

99.82

56

932

5

58

10

18

bdl

810

15

49

23

basaltic andesite or basaltic trachyandesite, and the remainder have compositions of dacite to rhyolite.Compositions of the analyzed samples are metaluminous; the average alumina saturation index (molar A^Cty
GEOCHEMISTRY 13

LJ 20 

z o 

— ct:

LJ 

- Q_

O hCNlX

— O

LJ

LJ 

Z O

LJ 

- CL

CM O h- 

•- X

\- o

LJ

Z

LJ

Z O — ct

LJ 

- CL

18

16 

.9

.7

O

(L>

O

LJ

Z

LJ 

Z O

— ct: 

LJ

- CL

o

CD h- 

^ X

o

LJ

Z

LJ

Z O

— ct: 

LJ

- CL 

O

D hO X

o

LJ

0

10

8

6

4

2

O

+ +D

D A

- D

A

A

A

0

+

O +

z

LJ 

O

LJ

CL

O

LJ

LJ. 5 

I O

- Ct 

LJ

- CL

in -z

O i-- ° 

CMX 

CL O

Z 

LJ

*- U „

- ct 4

LJ 

- CL 

O

CMI— -z 

O X "•> 

Z O

LJ

Z 

LJ 5 o ° 

ct

— LJ 

CL 4

O hCMX -7 

^ O °

LJ

55 

Si0 2

60 65 

N WEIGHT PERCENT

70

A

A

+

- ++

A

o

_A 

-O

A

SHOSHONITIC

HIGH-K CALC-ALKAL

55

Si0 2

60 65 

IN WEIGHT PERCENT

Figure 7. Variation diagrams showing abundances of major oxides in volcanic rock samples from the Sliderock Mountain stratovolcano 

(table 2). Discriminant lines on K2O versus SiC>2 diagram are from Ewart (1982). See figure 6 for explanation of symbols.

[CaO+K2O+Na2O]) for the 26 samples is 0.844. This value 

is consistent with derivation of the volcanic and plutonic 

rocks from an igneous (mantle) source and precludes significant contamination of primary mantle-derived melts by 

assimilation of crustally derived sedimentary rocks (Chappell and White, 1992).

Major oxide abundance variations in the Sliderock 

Mountain volcano rocks are similar to those of most

continental volcanic arc calc-alkaline igneous rocks. Total 

iron, A12O3, MgO, CaO, P2O5 , MnO, and TiO2 abundances 

decrease with increasing SiO2, whereas K2O and Na2O 

abundances increase with increasing SiO2 (fig. 7). The K2O 

abundances in most of these samples plot in the high-K calcalkaline series of Ewart (1982) and Na2O:K2O is approximately 1.3. The composition of the Sliderock Mountain 

rocks are similar to those of other Late Cretaceous igneous
14 CRETACEOUS SLIDEROCK MOUNTAIN VOLCANO, MONTANA

centers, including those of the Elkhorn Mountains Volcanics 

(Klepper and others, 1971) and chemically primitive 

(approximately 55 weight percent SiO2 ) parts of the Boulder 

(Smedes, 1966; Tilling, 1973) and Idaho (Norman and others, 1992) batholiths, in western Montana and eastern Idaho. 

Particularly diagnostic features of the major oxide compositions of the Sliderock Mountain rocks are high A^C^ and 

low MgO contents at about 55 weight percent SiOo (table 2, 

fig. 7).

Trace element abundances for samples of the stratovolcano are similar to those for other unevolved, mantle-derived 

magmas, including those of the more primitive parts of the 

Late Cretaceous Boulder (Tilling, 1973) and Idaho batholiths 

(Norman and others, 1992). Rubidium and Zr abundances 

for the Sliderock Mountain rocks are significantly lower and 

Sr abundances slightly higher than those for the Idaho 

batholith. Rubidium and Sr abundances are similar to and 

significantly higher, respectively, than their abundances in 

the primitive parts of the Boulder batholith. Rubidium abundances in samples of the stratovolcano increase slightly with 

decreasing Sr abundances, which is in accord with antithetic 

Rb-Sr abundance relations displayed by most igneous rocks 

(Macdonald and others, 1992). The average Rb/Sr ratio for 

these rocks, about 0.07, is very low. Yttrium and Ba abundances are similar to those for the Idaho batholith.

The degree of magmatic evolution displayed by rocks of 

the Sliderock Mountain stratovolcano is depicted by their relative Rb, K, and Sr abundances (fig. 8). Abundances of these 

elements are principally controlled by feldspar (Hanson, 

1978); Sr and K abundances are depleted and Rb abundances 

are enriched, respectively, by feldspar fractionation. Thus, 

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 represent the least 

evolved magmas. Rocks of the Sliderock Mountain stratovolcano are significantly less evolved than those of other 

subduction-related magmatic systems, including those represented by middle Tertiary, ash-flow tuffs of southern Nevada 

(du Bray, 1995) and Tertiary volcanic rocks of the Andean 

arc (du Bray and others, 1995).

Compositions of Sliderock Mountain volcano rocks plot 

near the Sr apex of the Rb-K/100-Sr ternary diagram and thus 

imply very limited evolution by feldspar fractionation. The 

array of points defined by the intrusive rocks overlaps the 

array formed by all other components of the volcano. A sample of the intrusive phase of the volcano represents the most 

evolved part of the magmatic system; two lava samples and 

a dike sample have similar compositions, however, and indicate that magma was tapped from the Sliderock Mountain 

system, to form dikes and supply lava flow eruptions, during 

the entire cooling and compositional evolution of the system.

Pearce and others (1984) recognized that granitoid 

rocks generated in various tectonic settings have distinctive 

geochemical signatures and they developed trace-element

K/100

Rb Sr

Figure 8. Ternary variation diagram showing the relative proportions of Rb, K, and Sr in samples from the Sliderock Mountain 

stratovolcano (table 2). See figure 6 for explanation of symbols.

discriminant diagrams from which tectonic setting can be 

inferred. Trace-element abundance variations in coeval volcanic and plutonic rocks generated in a given terrane should 

be similar. Consequently, compositions of the Sliderock 

Mountain rocks can be compared to the tectonic setting-trace 

element grids developed by Pearce and others (1984). Traceelement data for all of these samples plot in the volcanic arc 

field on these diagrams (fig. 9). Gill (1981) indicated that 

Ba/Nb ratios of modern arc rocks are greater than 26 and negative Nb anomalies on normalized extended trace element 

diagrams are a diagnostic geochemical characteristic of continental volcanic arc rocks. The average Ba/Nb ratio for the 

Sliderock Mountain rocks is about 130 and negative Nb 

anomalies are a ubiquitous feature of these rocks. In addition, major oxide compositions of the Sliderock Mountain 

rocks follow a calc-alkaline trend parallel to but displaced 

slightly above the Cascade trend on an AFM diagram (fig. 

10). These compositional features suggest that rocks of the 

stratovolcano represent a subduction-related, continental 

volcanic arc magma system.

Compositional data for samples of the stratovolcano are 

characterized by several groups of oxides and elements 

whose abundances show mutual covariation. These covariant relations suggest that compositional variation was controlled by a distinct mineral assemblage, which is in accord 

with the fact that the compositions of samples of the stratovolcano form linear arrays on many variation diagrams. The 

minerals inferred to have influenced these compositional 

variations, which could have controlled the variations either 

by their occurrence in the partial melting residuum and (or)
GEOCHEMISTRY 15

100

Q_ 

Q_

£ 10

WITHIN PLATE

VOLCANIC 

ARC

O

A

10 

Y N

100 

PPM

1000

Q_ 

Q_

100

1 0

Wl TH I N 

PLATE

- VOLCAN 

ARC

10 

Y

100 

Nb , IN

1000 

PPM

Figure 9. Trace-element tectonic-setting-discrimination variation 

diagrams showing compositions of volcanic rock samples from the 

Sliderock Mountain stratovolcano (table 2). Tectonic setting-composition boundaries from Pearce and others (1984). See figure 6 for 

explanation of symbols. A, niobium versus yttrium. B, rubidium 

versus yttrium plus niobium.

in crystal fractionation assemblages, occur as phenocrysts in 

the rocks.

The oxides whose abundances are most highly correlated are FeO, CaO, MgO, and A12O3 . Correlation coefficients (r2) between pairs of these elements are all greater 

than 0.70; most are greater than 0.8 and some exceed 0.9. 

Mutual abundance covariations involving these oxides 

indicate that clinopyroxene had a major influence on

CASCADE 

+ CALC-ALKALINE

A M

Figure 10. Ternary AFM diagram showing compositions of volcanic rock samples from the Sliderock Mountain stratovolcano. 

Cascade calc-alkaline trend line is from Irvine and Baragar (1971). 

See figure 6 for explanation of symbols.

compositional variation in the Sliderock Mountain stratovolcano rocks. The lack of a significant correlation between 

K2O abundances and those of this group of oxides indicates 

that neither hornblende nor biotite were important in the 

geochemical evolution of these rocks.

Abundance variations of FeO and TiO2 have an r2 value 

of 0.73, and those of CaO and P2O5 have an r2 value of 0.86. 

The lack of other oxide or element abundance covariations 

with these two pairs indicates that ilmenite and apatite, 

respectively, are additional phases that influenced compositional variation within the stratovolcano magmatic system. 

An additional consequence of fractional crystallization or 

varying degrees of partial melting is variable incompatible 

oxide or element enrichment of the resulting silicate liquid; 

a compositional covariation pair involving K2O and Rb 

(r2=0.84) may reflect either of these processes.

Hanson (1989) describes the use of trace element scatter plots in deciphering the petrogenesis of igneous rocks. 

Using plots of this type to model petrogenesis involves the 

use of the equations that define elemental abundances resulting from varying degrees of partial melting and (or) fractional crystallization. Following Shaw (1970), F is the 

fraction of liquid or melt in the system, D, is the bulk distribution coefficient, Cti is the weight concentration of element 

I in the liquid or melt, and Coi is the weight concentration of 

element I in the system. For melting, Q/C0j = !/[(!- 

F)Dj+F]. If DJ is 0 for a given element (that is, if the element 

is completely incompatible), Cij/Coi = 1/F and the amount of 

melting can be constrained using an inferred C0j and knowing the composition of samples that represent the system.
16 CRETACEOUS SLIDEROCK MOUNTAIN VOLCANO, MONTANA

The equation for fractional crystallization (Neumann and 

others, 1954), Cu/Coi = F(DM), also resolves to CH/Coi = 1/F 

if DJ is 0, and the extent of fractional crystallization can be 

calculated.

An orthogonal plot in which the abundances of a highly 

incompatible element are plotted against those of a compatible element can be used to determine whether partial melting 

or fractional crystallization is principally responsible for the 

compositional evolution displayed by a suite of associated 

samples (Hanson, 1989). Rubidium, for instance, acts 

incompatibly in most igneous systems (Arth, 1976), especially in intermediate to mafic systems such as the one 

described here, whereas Sr behaves compatibly. The distribution of data for samples of the stratovolcano on a Rb-Sr 

plot (fig. 11) suggests that the liquid parental to'these rocks 

contained about 40 ppm Rb and 1200 ppm Sr. Using that 

composition, assuming perfect Rb incompatibility (DpO), a 

bulk distribution coefficient of 2 (Arth, 1976) for Sr (in plagioclase and clinopyroxene), and the equations for melting 

and fractional crystallization, Rb and Sr abundances in liquids generated by varying amounts of melting or fractional 

crystallization can be calculated. Having done this, it is clear 

(fig. 11) that varying Rb and Sr abundances depicted by samples of the stratovolcano most probably result from fractional 

crystallization; compositions calculated for the relatively 

small amounts of melting necessary to produce the most 

evolved samples of the stratovolcano are characterized by Rb 

abundances that are far too large at any given Sr abundance. 

The curve calculated for varying degrees of fractional crystallization, however, fits the observed data fairly well. Deviations from the calculated fractional crystallization curve 

probably reflect analytical uncertainty and the weakly altered 

nature of Sliderock Mountain stratovolcano rocks.

Presuming that fractional crystallization is the dominant 

process responsible for compositional variation within samples of the stratovolcano, the fractional crystallization equation and abundance ranges of incompatible components, Rb 

for instance, can be used to evaluate the extent of crystallization required to produce the entire compositional abundance 

range. Because Rb abundances more than double, its DJ must 

be less than one (Hanson, 1989; fig. 1). For extents of fractional crystallization less than 0.8, D; must be less than 0.3, 

which further indicates Rb incompatibility during evolution 

of the stratovolcano magmatic system. Finally, the incompatibility of Rb, as well as K2O, is confirmed by a scatter plot 

for these components (fig. 12). Their incompatibility is indicated by colinearity of abundance variations and by the 

nearly zero y-intercept for the regression line through data 

for the stratovolcano. Using the regression line (instead of 

analytic data for individual samples, which may reflect weak 

rock alteration) the highest and lowest Rb contents in 

samples of the stratovolcano are about 105 and 40 ppm, 

respectively. So, 40/105=1/F and F=0.38; the extent of fractional crystallization (1-F) is 0.62. The most evolved sample 

of the Sliderock Mountain stratovolcano can be produced by

800

Sr

1 000 

I N PPM

1200

Figure 11. Variation diagram showing rubidium and strontium 

abundances in samples from the Sliderock Mountain stratovolcano 

(table 2). See figure 6 for explanation of symbols. Diamonds labeled 35% PM, 45% PM, 55% PM, 65% PM, 75% PM, and 100% 

PM are liquid compositions calculated for the indicated percentages 

of partial melting, as described in text; triangles labeled 50% FC, 

40% FC, 30% FC, 20% FC, 10% FC, and 0% FC are liquid compositions calculated for the indicated percentages of fractional crystallization, as described in text.

62 percent crystallization of a liquid represented by the most 

primitive sample.

REGIONAL TECTONO-MAGMATIC 

SPECULATION

Geochemical data presented here suggest that igneous 

products of the Sliderock Mountain stratovolcano are the 

products of arc-related magmatism. However, the Late Cretaceous age and position of the volcanic rocks of this field 

well east of the conventionally accepted locus (centered on 

the Idaho batholith) of Late Cretaceous arc magmatism is 

problematic. Dispersed magmatism, not coalesced to a narrow magmatic arc, and extending considerably inboard from 

inferred subduction zones has been presented (Lipman, 

1980) as indicators of relatively low-angle subduction. The 

age and position of the Sliderock Mountain stratovolcano 

may indicate accelerated convergence and low-angle subduction along the Late Cretaceous western margin of North 

America.

Following development of the stratovolcano, Cenozoic 

magmatic activity, as represented by the Absaroka (Smedes 

and Prostka, 1972; Harlan and others, 1996) and Challis volcanic fields (Norman and others, 1992; Janecke and Snee,
ACKNOWLEDGMENTS 17

100

80 S

Q_ 

Q_

2 60

.a" 40

20

X

ION LINC: 

SLOPE 21.6 

Y-INTERCEPT 1.17 

CORRELATION COEFFICIENT

K 2 0, IN

3 

WE IGHT PERCENT

Figure 12. Variation diagram showing rubidium and potassium 

abundances in samples from the Sliderock Mountain stratovolcano 

(table 2).

1993), by renewed plutonism in south-central Idaho (Toth, 

1987; Lewis and Kiilsgaard, 1991), and by alkalic magmatism in central Montana (Dudas, 1991) migrated westward 

across western Montana and into Idaho. Migration of the 

magmatic locus may represent decreased convergence and 

steeper subduction (Lipman, 1980) along the early Cenozoic 

western margin of North America as the subducted slab of 

oceanic lithosphere began to founder and the convecting 

asthenospheric wedge thickened (Lipman, 1980) in progressively westward positions beneath the western edge of the 

North American plate.

Dickinson and Hatherton (1967) demonstrated a relationship between the K2O content (at a given SiO2 content) 

of magma in subduction-related volcanic arc systems and 

depth to the center of the subducted slab (Benioff zone) 

beneath the arc. Lipman and others (1972) used this relationship and contoured the results to construct a map that 

portrays inferred depths to the Benioff zone beneath middle 

Cenozoic volcanic centers of the western United States. 

Their map (Lipman and others, 1972; fig. 9) indicates a 

depth of >310 km to the Benioff zone in the Sliderock Mountain area during middle Cenozoic time. The K^OgQ value of 

about 2.7 weight percent for the Late Cretaceous Sliderock 

Mountain stratovolcano (fig. 7) indicates a depth to the 

Benioff zone of about 225 km at 78-75 Ma. These observations imply that the convergence rate declined along the west 

edge of western North America and the angle of subduction 

steepened between Late Cretaceous and middle Cenozoic 

time in southern Montana. This suggestion is in accord with 

the westward retreat of arc magmatism represented by the 

Eocene Absaroka and Challis volcanic fields and the Eocene 

part of the Idaho batholith.

Finally, major crustal fault zones such as the NyeBowler lineament may have played a role in localizing volcanism along the southern margin of the Crazy Mountains 

Basin. The Nye-Bowler fault zone is a major northwesttrending zone of northeast- and northwest-trending wrench 

faults and folds caused by left-lateral fault movement during 

Late Cretaceous-early Tertiary time (Wilson, 1936; Roberts, 

1972). The trend of the Nye-Bowler lineament is subparallel 

to that of the Lake Basin fault zone exposed to the north in 

the Crazy Mountains Basin and to a regionally extensive set 

of northwest-trending faults exposed throughout southwestern Montana (Schmidt and Garihan, 1986). Many of these 

faults, including those of the Nye-Bowler lineament, are 

thought to have originally formed during Precambrian extension, perhaps as normal faults associated with development 

of the Middle Proterozoic Belt Basin to the north and west. 

Many of the Proterozoic faults of southwestern and central 

Montana were reactivated as reverse and thrust faults during 

Late Cretaceous-early Tertiary contractional deformation to 

form major uplifts during the Laramide orogeny (Schmidt 

and Garihan, 1986). Some of these faults apparently localized intrusive and volcanic centers elsewhere in southwestern Montana. For example, Schmidt and others (1990) 

suggested that emplacement of the Late Cretaceous Tobacco 

Root batholith was controlled, at least in part, by northwesttrending left-oblique slip reverse faults that were active during Laramide contraction. Similarly, Chad wick (1970) suggested that two deep-seated northwest-trending faults, 

believed to be reactivated Precambrian fault zones, in the 

Beartooth Mountains, south of the study area, are responsible for the alignment of two chemically distinct, northwesttrending belts of Tertiary intrusions and volcanic centers of 

the Absaroka-Gallatin volcanic field. The northwest alignment of the Upper Cretaceous volcanic rocks and intrusions 

in the southern Crazy Mountains Basin, essentially astride 

the axis of the Nye-Bowler lineament, strongly suggests that 

this fault zone controlled the emplacement of volcanic centers of the field. The presence of deep-seated fault zones, as 

well as low-angle subduction along the western margin of 

North America at this time, may help explain the location of 

the Sliderock Mountain volcano field well east of the 

generally accepted locus of Late Cretaceous Cordilleran 

magmatism.

ACKNOWLEDGMENTS

Thanks to I.E. Elliott, B.S. Van Gosen, EJ. LaRock, 

and A.W. West who helped map the geology of the Sliderock 

Mountain area. Jane Hammarstrom provided the two 

sericite samples dated in this study and Ross Yeoman helped 

with the Ar/39Ar analyses. We thank local landowners for 

providing access across their land to public lands administered by the U.S. Forest Service. Technical reviews by 

Karen Lund and lan Ridley helped improve this study.
18 CRETACEOUS SLIDEROCK MOUNTAIN VOLCANO, MONTANA

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Lodgepole intrusive area—Implications for the northern extent 

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Chadwick, R.A., 1970, Belts of eruptive centers in the AbsarokaGallatin volcanic province, Wyoming and Montana: Geological Society of America Bulletin, v. 81, p. 267-274.

Chappell, B.W., and White, A.J.R., 1992,1- and S-type granites in 

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related rocks: Transactions of the Royal Society of Edinburgh, 

v. 83, p. 1-26.

Dalrymple, G.B., and Lanphere, M.A., 1969, Potassium-argon 

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seismicity around the Pacific: Science, v. 157, p. 801-803.

Dickinson, W.R., Klute, M.A., Hayes, M.J., Janecke, S.U., Lundin, 

E.R., McKittrick, M.A., and Olivares, M.D., 1988, Paleogeographic and paleotectonic setting of Laramide sedimentary 

basins in the central Rocky Mountain region: Geological Society of America Bulletin, v. 100, p. 1023-1039.

du Bray, E.A., 1995, Geochemistry and petrology of Oligocene and 

Miocene ash-flow tuffs of the southeastern Great Basin: U.S. 

Geological Survey Professional Paper 1559, 38 p.

du Bray, E.A., Elliott, I.E., Van Gosen, B.S., LaRock, E.J., and 

West, A.W., 1994, Reconnaissance geologic map of the 

Sliderock Mountain area, Sweet Grass County, Montana: U.S. 

Geological Survey Miscellaneous Field Studies Map 

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Manuscript approved for publication March 12, 1998 

Published in the Central Region, Denver, Colorado 

Graphics by the authors 

Photocomposition by L.M. Carter and Joan G. Nadeau

* U.S. GOVERNMENT PRINTING OFFICE: 1998—673-048 / 20114 Region No. 8

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