Paleomagnetic and 40Ar/39Ar Geochronologic Data

from Late Proterozoic Mafic Dikes and Sills,

Montana and Wyoming

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1580
M A R C H 3, 18 4 9

U.S. DEPARTMENT OF THE 

Paleomagnetic and 40Ar/39Ar Geochronologic Da

from Late Proterozoic Mafic Dikes and Sills, 

Montana and Wyoming

By Stephen S. Harlan, John Wm. Geissman, and Lawrence W. Snee

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1580

Paleomagnetic and 40Ar/39Ar results from mafic dikes and sills in

northwestern Wyoming and western Montana indicate that they were

emplaced during a regional magmatic event at 780 to 770 Ma

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1997
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

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

Harlan, Stephen S.

Paleomagnetic and 40Ar/39Ar geochronologic data from Late Proterozoic mafic dikes 

and sills, Montana and Wyoming / by Stephen S. Harlan, John Wm. Geissman, and 

Lawrence W. Snee.

p. cm.—(U.S. Geological Survey professional paper ; 1580)

Includes bibliographical references.

Supt. of Docs. no. : I 19.16 : 1580

1. Geology, Stratigraphic—Proterozoic. 2. Geochronometry—Montana. 

3. Geochronometry—Wyoming. 4. Sills (Geology)—Montana. 5. Dikes 

(Geology)—Wyoming. I. Geissman, John William. II. Snee, Lawrence W. 

III. Title. IV. Series.

QE653.5.H37 1998

551.7′15′09786—dc21 97–17085

CIP
CONTENTS

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

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

Geologic Setting and Sampling ........................................................................................... 2

Teton Range ................................................................................................................. 2

Beartooth Mountains ................................................................................................... 3

Belt Supergroup ........................................................................................................... 4

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

Results and Interpretation .................................................................................................... 6

Paleomagnetism............................................................................................................ 6

40Ar/39Ar Geochronology ............................................................................................. 8

Discussion ............................................................................................................................ 10

Conclusions ......................................................................................................................... 13

Acknowledgments ................................................................................................................ 14

References Cited .................................................................................................................. 14

FIGURES

1. Simplified geologic map of the northern Rocky Mountains.....................................................................................

2–4. Generalized geologic map of:

2. Part of the Teton Range, Wyoming....................................................................................................................

3. Southeastern corner of the Beartooth uplift, Montana and Wyoming ...............................................................

4. Eastern margin of fold and thrust belt near Wolf Creek, Mont..........................................................................

5. In situ orthogonal vector diagrams and equal-area projections showing behavior of mafic dike samples

from dike at Mount Moran during alternating-field and thermal demagnetization ..................................................

6. Equal-area projections showing tilt-corrected paleomagnetic results from mafic dike at Mount Moran

and Christmas Lake mafic dike.................................................................................................................................

7. In situ orthogonal vector diagrams and equal-area projections showing demagnetization behavior

of samples from Christmas Lake dike and basement gneiss.....................................................................................

8. 40Ar/39Ar apparent age spectra for dike at Mount Moran, Teton Range; Christmas Lake dike,

Beartooth Mountains, Wyoming; and gabbro sill that intrudes lower Belt Supergroup sedimentary strata

near Wolf Creek, Mont..............................................................................................................................................

9. Orthographic global projection centered on lat 20°N., long 135°E., showing location of virtual

geomagnetic poles and 95 percent confidence ellipses from Mount Moran and Christmas Lake dikes ..................

TABLES

1. Analytical data for 40Ar/39Ar age determinations from mafic dikes and sills, northwestern Wyoming

and western Montana ................................................................................................................................................

2. Virtual geomagnetic poles from 780-Ma mafic intrusive rocks of western North America .....................................
Paleomagnetic and 40Ar/39Ar Geochronologic Data 

from Late Proterozoic Mafic Dikes and Sills, 

Montana and Wyoming

By Stephen S. Harlan,1 John Wm. Geissman,2 and Lawrence W. Snee 1

ABSTRACT

We report paleomagnetic and geochronologic results

from two Late Proterozoic mafic dikes exposed in Archeancored uplifts from northwestern Wyoming and 40Ar/39Ar results from a gabbro sill that intrudes lower sedimentary strata

of the Belt Supergroup in western Montana. The gabbro sill

yields a 40Ar/39Ar plateau date of 776 Ma that records the

age of sill emplacement during Late Proterozoic time. Paleomagnetic results from the mafic dike at Mount Moran, Teton

Range, and the Christmas Lake dike, Beartooth Mountains,

indicate magnetizations with similar directions and virtual

geomagnetic poles (VGP’s), which we interpret to be primary thermoremanent magnetizations acquired during dike emplacement and cooling. A hornblende from the Christmas

Lake dike yields a 40Ar/39Ar plateau date of 774 Ma, which

is statistically identical to that of the gabbro sill from western

Montana. The similarity of the dike site-mean directions and

VGP’s suggest that their magnetizations are of similar age.

40Ar/39Ar results from a hornblende-pyroxene concentrate

from the Mount Moran dike are discordant and contain excess 40Ar; a precise estimate of the age of this sample is not

possible given our data, but the paleomagnetic and geochronologic results suggest that it is of Late Proterozoic age, and

not Middle Proterozoic as previously argued. The VGP’s

from the Mount Moran and Christmas Lake dikes are similar

to those of identical age from southwestern Montana and

northern and western Canada. These mafic dikes and sills are

part of a regional magmatic event that affected the western

part of the Laurentian craton at about 780 Ma.

1U.S. Geological Survey, P.O. Box 25046, Mail Stop 963, Denver

Federal Center, Denver, CO 80225.

2Department of Earth and Planetary Sciences, University of New

Mexico, Albuquerque, NM 87131.

INTRODUCTION

Swarms of Proterozoic mafic sheets are a commo

ture of many uplifts of Wyoming and western Montana

basement-cored uplifts of the Archean Wyoming pro

they are represented by dikes of diverse trend, compo

and apparent age. Reported dike ages from the baseme

lifts range from about 2.6 Ga to 0.75 Ga, with appare

maxima at 2.56 Ga, 2.2–2.0 Ga, 1.45 Ga, 1.3(?) Ga, 1

and 0.75 Ga (Snyder and others, 1989; Baadsgaard and 

ler, 1973; Wooden and others, 1978). In western Mo

mafic intrusions are generally found as sills that intrud

imentary strata of the Middle Proterozoic Belt Supe

and have apparent ages of 1.45–1.38 Ga and 0.75 Ga. D

their wide occurrence and the recognition that mafic di

sill swarms may be sensitive recorders of fundamenta

logic and geodynamic processes (Halls, 1982), correla

dike and sills of similar age and geochemical compo

between uplifts of western Montana and Wyoming ha

limited in scope and is problematic. Although dikes in 

individual uplifts have been extensively studied, at p

there is little detailed information regarding the region

tribution of sheets of particular age groups or their geo

ical characteristics. Hence, the tectonic and geody

significance of many swarms is poorly understood.

It has long been recognized that the natural rem

magnetization (NRM) acquired by mafic dikes during 

ing can be a reliable recorder of the geomagnetic field a

be geologically stable throughout billions of years. 

properties make paleomagnetism an extremely powerf

for correlating dikes of individual swarms over wide 

(Buchan and Halls, 1990). Recent advances in geoc

logic dating methods, including the recognition that 

dikes can sometimes be dated with great precision 

U-Pb technique and, in some cases, by the 40Ar/39Ar m

(Hanes, 1988; Krogh and others, 1988; Heama
PALEOMAGNETIC AND 40Ar/ 2 39Ar GEOCHRONOLOGIC DATA FROM LATE PROTEROZOIC MAFIC DIKES AN

LeCheminant, 1993), have improved understanding of the

distribution and mechanisms of mafic dike emplacement

(Buchan and others, 1993, 1994). As part of an ongoing study

of the paleomagnetism and geochronology of several dike

swarms in basement-cored uplifts of Wyoming and Montana,

we report paleomagnetic and 40Ar/39Ar results from mafic

dikes exposed in the Teton Range of northwestern Wyoming

and the Beartooth uplift of Montana and Wyoming, and a

40Ar/39Ar date from a mafic sill in northwestern Montana

(fig. 1). We conclude that these dikes and sills were probably

emplaced during a single intrusive episode at about 780–770

Ma. Similarity of our results with paleomagnetic and geochronologic data from other well-dated mafic rocks from the

northwestern United States and Canada indicate that 780-Ma

mafic magmatism was regional in extent.

GEOLOGIC SETTING AND SAM

TETON RANGE

The Teton Range of northwestern Wyoming 

fragment of a large, northwest-trending Laramide

Cretaceous–early Tertiary) structural block that 

modified by Pliocene-Recent movement along a z

jor north-striking normal faults along its eastern m

2). Approximately 170 km2 of Archean crystallin

is exposed, including complexly deformed gneiss

phibolite, and discordant plutons. The basement ro

by a swarm of northwest-trending tholeiitic dik

probably the most spectacular of which is the 

Figure 1. Simplified geologic map of the northern Rocky Mountains showing location of Archean-cored rocks in basement up

bution of rocks of the Middle Proterozoic Belt Supergroup, and locations of areas described in this report. Tr, Teton Range; BTm

Mountains; TRm, Tobacco Root Mountains. Modified from Link and others (1993).
GEOLOGIC SETTING AND SAMPLING 

dike at Mount Moran, which has a vertical exposure of 

than 1,500 m (Reed and Zartman, 1973; Love and o

1992). The dikes are medium- to dark-gray or gray-

and consist of fine- to medium-grained diabase. The 

are undeformed and have well-defined chill margins

dikes are composed primarily of plagioclase and pyro

with minor hornblende and biotite. Tabular phenocry

plagioclase are common and show evidence of partial 

ation to sericite. Pyroxene is commonly altered to mix

of blue-green amphibole and chlorite.

K-Ar dates from the Mount Moran dike were rep

by Reed and Zartman (1973) and gave apparent ages o

Ma for a whole-rock sample from a chilled margin an

Ma and 396 Ma for plagioclase from near the dike c

Due to the discordance between the dates and the recogn

that K-Ar dates may be spurious due to the loss or ga

40Ar, Reed and Zartman (1973) dated hornblende and b

from several basement rocks collected in a traverse pe

dicular to the dike contact. Their objective was to date 

emplacement by partial to complete resetting of the 

rocks adjacent to the dike contact. They found that 

blendes, regardless of proximity to the dike contact, 

dates of 2,800 to 2,650 Ma, which closely approximat

inferred age of regional metamorphism of the base

rocks. Biotite dates from three samples gave apparent ag

1,450 to 1,350 Ma regardless of position with respect 

dike. Reed and Zartman (1973) argued that the close

otite, located 1.5 m from the 30-m-thick dike, should 

been elevated to temperatures sufficient to cause part

complete resetting of its K-Ar age. They concluded th

minimum age for the Moran dike was 1,450 Ma and 

younger dates were unreliable.

Paleomagnetic samples for this study were coll

from a shoulder east of the main face of Mount Moran 

elevation of about 2,750 m (fig. 2).

BEARTOOTH MOUNTAINS

The Archean crystalline basement of the Bear

Mountains of south-central Montana and northwestern 

oming (fig. 1) hosts an extensive array of mafic dike

trend generally northwest; some north- and northeast-t

ing dikes are also present (Prinz, 1964) (fig. 3). U

geochemical criteria, Mueller and Rogers (1973) iden

at least four geochemical groups, based primarily on v

tions in TiO2 content, that have apparent emplacement 

of 2.5 Ga, 1.3 Ga, and 0.75 Ga, based on whole-rock R

and K-Ar dating methods (Mueller and Rogers, 

Baadsgaard and Mueller, 1973). The youngest grou

dikes (group IIIA) is fine- to medium-grained diabas

consists of clinopyroxene, orthopyroxene, and plagio

with minor biotite, hornblende, and interstitial granop

One of the dikes, the Christmas Lake dike, can be trace

Figure 2. Generalized geologic map of part of the Teton Range,

Wyoming, showing location of northwest-trending dikes and paleomagnetic site sampled in this study. Modified from Love and

others (1992).
PALEOMAGNETIC AND 40Ar/ 4 39Ar GEOCHRONOLOGIC DATA FROM LATE PROTEROZOIC MAFIC DIKES AN

16 km and has a vertical exposure of 800 m (Prinz, 1964).

These dikes gave K-Ar dates that range from 770 to 720 Ma

(N = 7 samples), with a composite K-Ar isochron of 740±32

Ma (2σ). A similar date of 780 Ma was obtained by Hanson

and Gast (1967) for a dike from this same group.

Paleomagnetic samples for this study were collected

where the mafic dike exposed at Christmas Lake crosses U.S.

Highway 212 (fig. 3). The sample used for 40Ar/39Ar dating

was obtained along strike of the dike from an exposure along

the shore of Christmas Lake.

BELT SUPERGROUP

Mafic sills are common in lower sedimentary strata of

the Middle Proterozoic Belt Supergroup, a thick (> 15,000

m) sedimentary prism composed of clastic and carbonate

rocks deposited along the western margin of North America.

They have been called the Purcell sills in the United States

and Moyie sills in Canada, with these terms applied to a wide

range of rocks including diabase, diorite, quartz diorite, gabbro, metadiorite and metagabbro (Snyder and others, 1989).

Sills range in thickness from 1 to 500 m, and in some areas,

sills have inflated the Belt stratigraphic section by as much as

30 to 35 percent (Höy, 1989). Typically, sills that intrude the

Belt Supergroup are coarse grained and consist primarily of

plagioclase and clinopyroxene, although some contain significant hornblende, biotite, and granophyre. Most are extensively altered due to hydrothermal conditions following

emplacement (i.e., deuteric alteration) or possibly due to subsequent low-grade regional metamorphism and deformation.

Use of the terms “Purcell” or “Moyie” when describing the sills of the Belt Basin is confusing because no single

identifying physical or geochemical characteristic distinguishes a “Purcell” or “Moyie” sill from any other sill and

because these terms have been applied to sills that may

have been emplaced during more than one intrusive event.

Also, the term is sometimes confused with the Middle Proterozoic Purcell lavas (Höy, 1989), which may not be extrusive equivalents of any mafic sills. At least two episodes of

sill emplacement (about 1,450–1,420 Ma and 800–780 Ma)

in the Belt Basin have been recognized (Zartman and others, 1982; Höy, 1989; Ross and others, 1992), and there

may be others (Ross and others, 1992). For these reasons,

we suggest that use of “Purcell” should be avoided when referring to any of the numerous mafic sills that intrude sedimentary rocks of the Belt Supergroup.

The gabbro sample dated in this study was 

from a poorly exposed sill along Wolf Creek, app

6 km northwest of Wolf Creek, Mont. (fig. 4). Th

proximately 40 m thick and intrudes undivided se

strata of the Empire and Spokane Formations of 

part of the Belt Supergroup (Schmidt, 1978). The s

primarily of fine- to medium-grained plagioclas

nopyroxene, with minor biotite, hornblende, and g

Locally, the gabbro is coarse and consists of a hig

entiated granophyre that contains abundant, random

ed, dark-brown to black hornblende needles as muc

long from which a hornblende concentrate was 

Samples for paleomagnetic analysis were not coll

this sill because it is located near the frontal ma

fold and thrust belt, and paleomagnetic analysis 

Cretaceous volcanic and sedimentary strata have d

ed that these rocks have been subjected to local ve

rotations during Late Cretaceous–early Tertiaryco

deformation (Jolly and Sheriff, 1991).

Figure 3. Generalized geologic map of southeastern c

Beartooth uplift, Montana and Wyoming, showing loca

ic dikes that cut Archean crystalline basement and loca

dated in this study. Modified from Prinz (1964) and M

Rogers (1973).
ANALYTICAL METHODS 

Figure 4. Generalized geologic map of the eastern margin of fold and thrust belt near Wolf Creek,

Mont., showing location of the gabbro sill dated in this study. Modified from Schmidt (1978).

ANALYTICAL METHODS

At Mount Moran, we collected seven blocks of the dike,

which were oriented using a Brunton compass. In the laboratory, individual samples were obtained from these blocks using a water-cooled diamond drill bit fitted to a drill press. Ten

samples from the Christmas Lake dike were obtained with a

portable drill; individual samples were oriented using magnetic and solar compasses and a clinometer. An additional

five samples were collected at or near the contact of the

Christmas Lake dike in order to conduct a baked contact test

(Everitt and Clegg, 1962) to determine whether the observed

dike remanence was primary, or acquired at some time after

dike emplacement.

The natural remanent magnetization (NRM) of 

sample was measured using either cryogenic or spinner 

netometers. Detailed alternating field (AF) demagnetiz

was conducted on all specimens and proved effective i

lating the sample characteristic remanent magnetiz

(ChRM). Duplicate specimens from a few samples wer

thermally demagnetized using a low-magnetic inductio

nace. Demagnetization results were examined using v

diagrams, stereographic projections, and normalized in

ty decay diagrams. Sample ChRM directions, define

three or more colinear points on vector diagrams, wer

culated using principal components analysis (PCA) (K

vink, 1980); all ChRM directions used in this study 

maximum angular deviation values less than 7°. Site-

directions were calculated using statistics from Fisher (1
PALEOMAGNETIC AND 40Ar/ 6 39Ar GEOCHRONOLOGIC DATA FROM LATE PROTEROZOIC MAFIC DIKES AN

Samples for 40Ar/39Ar analysis were crushed, ground,

and sieved, and mineral concentrates were produced using

standard heavy-liquid and magnetic techniques. Mineral

concentrates for the Christmas Lake dike and the Wolf Creek

gabbro sill were hand-picked to a purity greater than 99.9

percent. The hornblende concentrate from the Teton dike

consists of a mixture of hornblende and pyroxene that could

not be purified greater than about 70 percent due to fine grain

size and intergrowths.

The 40Ar/39Ar samples were irradiated in the U.S. Geological Survey TRIGA reactor for 60 hours. Variations in neutron flux were monitored using hornblende MMhb-1, which

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

1987). Corrections for interfering isotopes were made using

production ratios derived from potassium and calcium salts irradiated with the samples. Each sample was heated in a double vacuum, low-blank resistance furnace for 20 minutes in a

series of 11 to 14 steps to a maximum temperature of 1,300°C.

After each heating step, the gas was collected and purified using Zr-Al-Ti getters, and all five naturally occurring isotopes

of argon were measured using a mass spectrometer operated

in the static mode. Apparent ages for each temperature step

and the total gas date were calculated using the decay constants recommended by Steiger and Jäger (1977). A detailed

description of analytical procedures similar to those used in

this study are given by Tysdal and others (1990).

RESULTS AND INTERPRETATION

PALEOMAGNETISM

NRM intensities from the Mount Moran dike samples

ranged from 0.43 A/m to 1.24 A/m, with a geometric mean

of 0.84 A/m. Demagnetization results showed that virtually

all samples contained two magnetization components (fig. 5).

The first component is relatively minor and is characterized

by low coercivities and laboratory unblocking temperatures

during demagnetization. The resultant direction obtained

from this magnetization is commonly of north declination

and moderate positive inclination. This component was typically removed by AF demagnetization in fields of 5 to 10

mT, or by temperatures of 100°C to 250°C in thermal demagnetization. The direction and demagnetization characteristics

suggest that this is a secondary viscous remanent magnetization that was probably acquired in the present-day field.

At greater demagnetization levels, a well-defined magnetization component of northwest declination and shallow

positive to negative inclination that trends toward the origin

of demagnetization diagrams was identified (fig. 5). This

component was usually isolated by AF fields of 15 to 130

mT and by laboratory unblocking temperatures from 300°C

to 590°C. This direction is far from the present-day field

and far from expected directions from virtually any time

period during the Phanerozoic; hence, we consi

be the ChRM of the dike and suggest that it is a 

manent magnetization (TRM) acquired during c

lowing dike emplacement. Isolation of the ChR

demagnetization and complete unblocking by tem

less than 590°C indicates that the primary carrier 

tization is low-Ti titanomagnetite.

A site-mean direction calculated from the s

vidual samples has an in situ declination of 285.6° 

nation of 11.5° (k = 36, α95 = 10.2°) and 

geomagnetic pole (VGP) at lat 15.2°N., long 152.

5.3°, δm = 10.4°). Application of tilt correctio

Mount Moran dike are problematic. Although 

Flathead Sandstone nonconformably overlies the 

summit of Mount Moran (Love and others, 1992)

attitudes for these rocks are not available. Overall

tural geometry of the Teton Range and paleoma

dence suggest that the range has been tilted west a

since Pliocene time along north-trending normal 

have uplifted the present-day Teton block (Byrd a

1994). A tilt correction based on an average strike 

E. and a 10° west tilt gives a site-mean declination 

and inclination of 1.5° (fig. 6); the resultant VGP b

11.6°N., long 148.9°E. Because cooling of a dike 

dike at Mount Moran occurs in less than a few hun

which is short relative to the time required to av

magnetic secular variation, this result is a spot re

the geomagnetic field at the time of dike emplace

Paleomagnetic results from the Christmas La

similar to those obtained from the dike at Mou

NRM intensities from the dike samples were fairl

with a range of 0.18 A/m to 0.51 A/m and geometr

0.44 A/m. Demagnetization experiments revealed 

defined components of magnetization (figs. 7A an

first component was of north declination and mod

tive inclination and was removed by AF demagne

10 to 20 mT or by thermal demagnetization of 

300°C. As with the Christmas Lake dike, the dem

tion characteristics and directions suggest that thi

bly a viscous remanent magnetization acquir

present-day field.

AF demagnetization in fields from 20 to 15

temperatures from 300°C to 590°C reveals a seco

tization of northwest declination and shallow neg

nation, which we consider the ChRM of the dik

and 7B). AF and thermal demagnetization charact

dicate that magnetization in these dikes is probab

by low-Ti titanomagnetite. A site-mean direction 

from nine of the dike samples gives a declination 

and inclination of –21.8° (k = 44, α95 = 7.9°) and 

lat 4.6°N., long 139.7°E. (δp = 4.4°, δm = 8.3°

served ChRM is similar to that reported from two 

sibly in the same dike, by Larson and others (1973

these results were based on limited AF demagneti
RESULTS AND INTERPRETATION 

Figure 5. In situ orthogonal vector diagrams and equal-area projections showing behavior of mafic dike samples from dike at Mou

ran during alternating-field and thermal demagnetization. For vector diagrams, open and solid symbols show in situ magnetization c

nents projected onto vertical and horizontal planes, respectively, at each demagnetization step. For equal-area projections, solid circ

projections on lower hemisphere. Demagnetization is in millitesla (mT) and degrees Celsius. For vector diagrams, 1 division = 0.1 A
PALEOMAGNETIC AND 40Ar/ 8 39Ar GEOCHRONOLOGIC DATA FROM LATE PROTEROZOIC MAFIC DIKES AN

Because lower Paleozoic sedimentary strata at

Beartooth Butte were tilted during Late Cretaceous–early

Tertiary uplift of the Beartooth Mountains, we have applied

a tilt correction to our data based on a N. 72° W. strike and

a southwest dip of 10°. This change has little effect on either the declination or inclination of the magnetization (i.e.,

tilt corrected declination = 292.3°, tilt corrected inclination

= –21.4°; fig. 6) or location of the VGP (lat 7.4°N., long

137.0°E.) because the axis of tilt is similar to the paleomagnetic direction.

Paleomagnetic results from two samples of basement

gneiss collected at 1 and 20 cm from the contact with the

Christmas Lake dike gave ChRM directions that are essentially identical to those from the dike samples (fig. 7C). The

remaining three samples collected at distances greater than

about 0.8 to 1.2 m from the dike gave erratic directional behavior during AF demagnetization. The similarity of directions from the two gneiss samples from the dike contact zone

with ChRM’s from the dike (fig. 6) suggest that the contact

rocks were thermally remagnetized during dike intrusion.

This suggests that the baked contact test is positive and that

the dike remanence is a primary TRM. Overall, however, the

results of this baked contact test must be considered inconclusive as we cannot demonstrate that this magnetization was

superimposed on a preexisting stable remanence. As with the

Mount Moran dike, this magnetization is unlike most expected directions of Phanerozoic age. We suggest that the magnetization observed in the dike is probably a TRM acquired

during dike emplacement and that the VGP is a spot-reading

of the geomagnetic field.

40Ar/39Ar GEOCHRONOLOGY

40Ar/39Ar results from the hornblende concentrate

from the Mount Moran dike yield a total gas date of 881±4

Ma (table 1) but a strongly discordant age spectrum (fig. 8).

Initial (700°C to 950°C) and high-temperature (1,075°C to

1,300°C) steps give anomalously old apparent ages that

range from 1,340 Ma to 830 Ma, but intermediate-temperature steps record a trough-like pattern with apparent ages of

800 to 765 Ma. Such “U-shaped” age spectra are characteristic of samples that contain excess 40Ar (Lanphere and

Dalrymple, 1976). Because excess 40Ar biases the age of

individual temperature steps toward older ages, most workers consider the trough of the “U” a maximum estimate for

the true age of the sample (Hanes, 1988). The presence of

excess 40Ar is a common phenomenon in many plutonic

hornblendes, but, in this case, the presence of pyroxene

contributes to this problem. In this sample, the 1,025°C and

1,050°C steps overlap in apparent age with a mean of

769±5 Ma (2σ). This date is probably a reasonable estimate

of the age of the sample given its similarity with apparent

ages from the Christmas Lake dike and the gabbro sill from

Wolf Creek, described below. On the basis of these data,

we suggest that the dike may be Late Proterozoic in age,

and not Middle Proterozoic as proposed by Reed and Zartman (1973). Indeed, paleomagnetic evidence from 

discussed below, confirm that the dike was emp

ing Late Proterozoic time.

In contrast to the hornblende-pyroxene c

from the Mount Moran dike, 40Ar/39Ar results from 

blende separate from the Christmas Lake dike are 

straightforward. The sample yields a total gas dat

Ma (2σ) (table 1) and a relatively flat age spectru

Low temperature fractions (700°C to 925°C) giv

ages greater than about 800 Ma, and these correla

atively high and variable 39Ar/37Ar ratios (i.e., 0.

table 1), which probably reflect the degassing 

Figure 6. Equal-area projections showing tilt-corre

magnetic results from mafic dike at Mount Moran (Te

and Christmas Lake mafic dike (Beartooth Mountains, 

Inverted triangles are individual sample ChRM dire

Mount Moran dike; circles are ChRM directions from 

Lake dike; and squares are paleomagnetic results from 

ment gneiss samples collected adjacent to Christmas 

Stars are site-mean directions and their α95 cones of 

Solid symbols are projections on lower hemisphere; op

are projections on upper hemisphere.

Figure 7 (following page). In situ orthogonal vecto

and equal-area projections showing demagnetization b

samples from Christmas Lake dike (A, B) and basement 

lected 1 cm from the dike contact (C). The gneiss samp

collected 20 cm from the dike contact give magnetizati

essentially identical to those from the dike, suggesting t

quired a thermoremanent magnetization due to thermal e

ciated with dike emplacement. Basement samples c

greater distances gave erratic and unstable demagnetiza

ior. For dike vector diagrams, 1 division = 0. 1 A/m; for 

ple, 1 division = 0.0001 A/m. Conventions are the same 
RESULTS AND INTERPRETATION 
PALEOMAGNETIC AND 40Ar/ 10 39Ar GEOCHRONOLOGIC DATA FROM LATE PROTEROZOIC MAFIC DIKES AN

loosely held excess 40Ar or contamination by relatively high

potassium phases. At greater temperatures (950°C to

1,075°C), the age spectrum is essentially flat over greater

than 87 percent of the 39ArK released with individual apparent ages varying by less than 1.02 percent. 39Ar/37Ar ratios

are relatively uniform, indicating that a single homogeneous

mineral phase was degassed over this temperature range.

These steps yield a mean plateau age of 774±4 Ma (2σ),

which is essentially identical to the total gas date; we consider the plateau date to be a reasonable estimate for the age of

emplacement of the Christmas Lake dike. Baddeleyite from

this dike gives a U-Pb date of 779 Ma (A.N. LeCheminant,

oral commun., 1994), which is statistically indistinguishable

from our 40Ar/39Ar results.

The hornblende from the gabbro sill near Wolf Creek

gives a total gas date of 788±7 Ma (table 1). The low-

(600°C to 850°C) and high-temperature (1,100°C to

1,250°C) steps record variable and discordant dates, and the

spectrum shows evidence of some excess 40Ar (fig. 8). Intermediate-temperature (900°C to 1,050°C) steps give essentially constant 39Ar/37Ar ratios and concordant dates that

define a plateau date of 776±5 Ma over about 90 percent of

the 39ArK released (fig. 8). We consider this to be the best

age of emplacement of the gabbro sill. We also note that it

is statistically identical to the date obtained from the Christmas Lake dike, exposed some 330 km to the southeast, and

that it is similar to K-Ar dates from gabbro sills t

lower Belt strata elsewhere in western Montana 

1978; Marvin and Dobson, 1979; J. D. Obrado

commun., 1993).

DISCUSSION

Both the Mount Moran dike from the Teton 

the Christmas Lake dike from Beartooth Moun

similar paleomagnetic directions and VGP’s that 

latitudes in the western Pacific Ocean (fig. 9). T

dance of 40Ar/39Ar and U-Pb dates from the Chris

dike indicates that it is Late Proterozoic in age a

geochronologic systems have not been disturbed 

quent thermal events. The baked contact test from 

mas Lake dike, although not conclusive, strongl

that the observed magnetization is a TRM acqui

cooling following dike emplacement. Because cl

peratures to 40Ar diffusion for hornblende (i.e., 

540°C) overlap unblocking temperatures for mag

suggest that the 776-Ma age from the hornblend

Christmas Lake dike probably closely approxima

of remanence acquisition in the dike. Assumin

ChRM in the Mount Moran dike is also a TRM acq

ing dike emplacement and cooling, the similarity 

magnetic directions and VGP’s from the Mount M

Beartooth dikes strongly suggest that both were e

about the same time.

Figure 8. 40Ar/39Ar apparent age spectra for dike at Mount Moran, Teton Range; Christmas Lake

dike, Beartooth Mountains, Wyoming; and a gabbro sill that intrudes lower Belt Supergroup sedimentary strata near Wolf Creek, Mont. The height of each rectangle represents analytical error in the

apparent age of the individual temperature step at ± 1σ.
DISCUSSION 
PALEOMAGNETIC AND 40Ar/ 12 39Ar GEOCHRONOLOGIC DATA FROM LATE PROTEROZOIC MAFIC DIKES AN

The VGP’s from the Mount Moran and Beartooth

dikes are distinctly different from most poles from Middle

Proterozoic rocks of about 1,400 Ma and 1,200 to 1,100 Ma

(Harlan and others, 1994) but are similar to VGP’s from

northwest-trending mafic dikes exposed in the southern Tobacco Root Mountains of southwest Montana and Late Proterozoic dikes and sills from northern and northwestern

Canada (table 2, fig. 9) (Park and others, 1995b). In the

southern Tobacco Root Mountains, Harlan (1993) found paleomagnetic and geochronologic evidence for two distinct

episodes of dike emplacement in Middle and Late Proterozoic time. Paleomagnetic results from geochemical group-B

dikes (Wooden and others, 1978) gave magnetizations of

northwest declination and shallow negative inclination,

similar to those observed in the Mount Moran and Christmas Lake dikes. 40Ar/39Ar dates from hornblende from

these dikes give apparent ages of 769±7 Ma (2σ), and biotite from host gneiss immediately adjacent to the contact

of a group B dike gave an apparent age of 780 Ma (S.S.

Harlan, unpub. data). Preliminary U-Pb data from zircon

and baddeleyite from these dikes give similar 207Pb/206Pb

dates of 782±8 Ma and 785± 8 Ma (S.S. Harlan and W. Premo, unpub. data). VGP’s from the Mount Moran, Christmas

Lake, and southern Tobacco Root Mountains dikes are

similar to those reported from (1) mafic dikes an

intrude Late Proterozoic strata in the Mackenzie 

of the northern Cordillera in Canada and (2) m

(Hottah sheets) from the northwestern Canadian 

Great Bear Lake (table 2; fig. 9). Both the dike

from the Mackenzie Mountains and the Hottah sh

northwestern Canadian shield give relatively pre

baddeleyite dates of 779 Ma (LeCheminant and 

1994) that are essentially identical to the 780- 

dates from the Christmas Lake dike, Tobacco R

tains dikes, and the gabbro sill exposed near W

Mont. The similarity of the Mount Moran VGP 

from 780- to 770-Ma dikes from elsewhere in Mo

western North America, and the 40Ar/39Ar results 

here, suggests that the age of the Mount Moran d

Proterozoic in age, and not Middle Proterozoic as 

Reed and Zartman (1973). Further isotopic deter

possibly using U-Pb technique, are needed to mo

ly identify the age of dike emplacement.

On a regional scale, the consistency of VGP

topic dates from dikes and sills from northwes

ming to northern Canada demonstrates the cohere

western margin of the North American craton 

Proterozoic time and suggests that all the 

Figure 9. Orthographic global projection centered on lat 20°N., long 135°E., showing location of

virtual geomagnetic poles (VGP’s) and their 95 percent confidence ellipses from Mount Moran and

Christmas Lake dikes (after tilt corrections as described in the text) and location of VGP’s of similar

age from elsewhere in western North America. Symbols for individual VGP’s are given in table 2.
CONCLUSIONS 

magnetizations are primary in origin (Park and others,

1995b). Mafic magmatism at 780–770 Ma appears approximately synchronous over a broad expanse of western North

America that extends from the Canadian Arctic to northwestern Wyoming, a distance greater than 2,400 km. Using

high-precision U-Pb dates on baddeleyite from mafic dikes

and sills in northern Canada and the Christmas Lake dike in

the Beartooth Mountains, Wyoming, A.N. LeCheminant

(oral commun., 1994) established a composite age of

781+6/–3 Ma for this event and has tentatively termed it

the “gunbarrel igneous event.” Emplacement of a quartz diorite plug dated as 778 Ma (U-Pb, zircon) (Jefferson and

Parrish, 1989) and eruption of the coeval Little Dal basalts

in the Mackenzie Mountains (Dud ́as and Lustwerk, 1997)

and mafic volcanic rocks of the Windermere Group of

northeastern Washington (762±44 Ma, Sm-Nd mineral isochron) (Devlin and others, 1988) may be manifestations of

this same event. Identification of 780- to 760-Ma 40Ar/39Ar

and K-Ar dates on sills from the lower parts of the Belt Supergroup, including the Wolf Creek sill from this study,

suggests that they may be more common than previously

thought, but detailed geochemical and high-precision geochronologic data are necessary to better understand their

distribution. Numerous studies (Link and others, 1993;

Harper and Link, 1986; Elston and McKee, 1982; ChristieBlick and Levy, 1989; Stewart, 1972) report the presence

of Late Proterozoic mafic intrusive and volcanic rocks to

the south, but whether they are part of this event is unclear

given the paucity of reliable isotopic dates from this region.

The cause of widespread mafic magmatism at 780 to

770 Ma remains enigmatic. LeCheminant and Heaman

(1994) suggest that mafic magmatism was a response to major extension accompanying development of a rift margin

along the western margin of the North American craton during Late Proterozoic time. Park and others (1995b) argue that

trends from dike subswarms in northwestern Canada and

Wyoming define part of a crudely radial pattern that was

associated with an ancient mantle plume that produced 

ant radiating dike swarm, similar to those of the 1.2

Mackenzie (LeCheminant and Heaman, 1989) and 0.7

Franklin (Heaman and others, 1992) magmatic events

two hypotheses may not be exclusive, as thermal effec

sociated with the mantle plume may have been a prec

to, or caused, Late Proterozoic rifting. The proposed loc

of the plume center is off the present coast of western N

America, and Park and others (1995b) argue that the p

center and other parts of the radial swarm were severed 

the craton by Late Proterozoic and early Paleozoic r

and development of a passive continental margin. The 

continental mass(es) that contain the remnants of the 

dike swarm are unknown but may include parts of east 

arctica and Australia, if parts of these continents lay adj

to the western margin of the Laurentian craton at abou

Ma to form part of the supercontinent Rodinia (Da

1991; Moores, 1991; Hoffman, 1991; Brookfield, 

Borg and De Paolo, 1994; Powell and others, 1993). Re

ly, Park and others (1995b) suggested that the approxim

800-Ma Gairdner dike swarm of western Australia m

part of the radial dike swarm formed at 780–770 Ma. I

rect, this interpretation and the hypothesis that western 

tralia lay adjacent to the North America continent c

tested by comparing high-quality paleomagnetic and 

chronologic data from this and other studies of Late Pro

zoic dikes and sheets from western North America 

those from the Gairdner dike swarm.

CONCLUSIONS

40Ar/39Ar isotopic dates from two mafic dikes 

gabbro sill from northwestern Wyoming and western 

tana yield similar Late Proterozoic dates of about 770 

The Mount Moran dike from the Teton Range and the C

mas Lake dike from the Beartooth Mountains yield si
PALEOMAGNETIC AND 40Ar/ 14 39Ar GEOCHRONOLOGIC DATA FROM LATE PROTEROZOIC MAFIC DIKES AN

VGP’s that are essentially identical to those from rocks of

similar age elsewhere in Montana and Canada. Together, the

paleomagnetic and geochronologic data from this and other

studies provide evidence for a regional mafic magmatic

event at 780–770 Ma that affected a large area of western

North America. Mafic magmatism may have been a response

to a mafic plume that preceded rifting and breakup of the

North American continent during Late Proterozoic and early

Paleozoic time. The paleomagnetic and geochronologic data

from this and other studies may be useful in testing the validity of Late Proterozoic continental reconstructions.

ACKNOWLEDGMENTS

Paleomagnetic analyses reported in this study represent

work completed during a Ph.D. dissertation at the University

of New Mexico, Albuquerque, but not previously published.

Paleomagnetic work was supported by a University of New

Mexico Kelley-Silver Fellowship and NSF grant EAR88-

16814. B. Marshall and S. Mahan provided the hornblende

concentrate from the Christmas Lake dike. R. Yeoman is

thanked for assistance in the 40Ar/39Ar dating. P. Batuello

helped collect paleomagnetic samples. K. Buchan, J. Park,

and T. LeCheminant are thanked for useful discussions regarding the paleomagnetism, geochronology, and nature and

distribution of Late Proterozoic dikes in the northwestern

United States and northern and western Canada. We thank

J.C. Reed, Jr. and R.L. Reynolds for careful and constructive

reviews of the manuscript.

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Published in the Central Region, Denver, Colorado

Manuscript approved for publication April 11, 1997

Edited by Richard W. Scott, Jr.

Graphics prepared by Stephen S. Harlan

Photocomposition by Carol Quesenberry

Online version Joan G. Nadeau