Precambrian geology of Lake Plateau, Beartooth Mountains, Montana
by Douglas P Richmond
A thesis submitted in partial fulfillment of the requirements of the degree of Master of Science in Earth
Sciences
Montana State University
© Copyright by Douglas P Richmond (1987)
Abstract:
The Lake Plateau area in the central Beartooth Mountains of southern Montana is comprised of
voluminous late Archean intrusive rocks ranging from quartz diorite to granite in composition, with a
variety of supracrustal inclusions. The inclusions range in size from centimeter to kilometer scale and
include biotite hornblende schists (bio-qtz-hbld-epi-plag) and pelitic schists (bio-qtz-cord-plag-gar sill).
These inclusions have experienced upper amphibolite grade metamorphism at 6-8 kbar and 580-650°C,
with penetrative deformation creating a north striking foliation. The intrusive rocks vary in modal
mineralogy and texture on a meter scale. In some places they have an hypidiomorphic-granular texture,
and in others they have weak foliation or foliated augen texture. Assimilation of inclusions is common
with foliated granites occurring at gradational contacts with inclusions. Pegmatite and aplite veins
associated with the intrusive rocks cut across nearly all Archean rocks and comprise 15-20% of the
total rock volume. Structural trends include north-south foliation with associated isoclinal folds, broad
open kilometer scale folds, and unfolded shear zones with mylonitic textures and retrograde
metamorphism to chlorite and epidote. Younger rocks include amphibolite dikes and a few Tertiary
felsic dikes.
Lake Plateau and the surrounding Beartooth Mountains evolved by: 1) burial of supracrustal rocks to
depths of 20-25 km; 2) penetrative deformation and upper amphibolite grade metamorphism; 3)
generation of high-Na intrusives such as the Long Lake granites; and 4) generation of the K-rich
granites of Lake Plateau. The Lake Plateau granitoids are interpreted as mid-crustal melts emplaced at
approximately 20 km and generated from a slightly deeper crustal source. These large volumes of
K-rich granites are different from the Na-rich rocks reported in the eastern Beartooths (Mueller and
others, 1985). Large volumes of granite with subordinate quartz diorite and a variety of supracrustal
inclusions are consistent with characteristics in younger examples of post-collisional tectonic settings.
The Beartooth Mountains represent the remaining mid-crustal evidence of development of thickened
continental crust in the late Archean by collisional tectonics and post-collisional uplift, similar in many
ways to modern day tectonic processes. PRECAMBRIAN GEOLOGY OF LAKE PLATEAU,
BEARTOOTH MOUNTAINS, MONTANA
by
Douglas P. Richmond
A thesis submitted in partial fulfillment
of the requirements of the degree ■
of
Master of Science
in
•Earth Sciences
MONTANA STATE UNIVERSITY
Bozeman, Montana
June, 1987
MMN Li
/1/3 7 ?
« f/4 J
(Lof-
APPROVAL
of a thesis submitted by
Douglas P. Richmond
This thesis has been read by each member of the thesis committee
and has been found to be satisfactory regarding content, English usage,
format, citations, bibliographic style, and consistency, and is ready
for submission to the College of Graduate Studies.
ISlo
cI, IcItf
Chairperson, Graduate Committee
Approved for the Major Department
Head,
Date
or Department
Approved for the College of Graduate Studies
, ^ 3 -S ^
Date
Graduate Dean
iii
STATEMENT OF PERMISSION TQ USE
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presenting
this
thesis
in. partial
fulfillment
of
the
requirements for a master's degree at Montana State University, I agree
that
the
Library shall make it available to borrowers under rules
the Library.
Brief quotations from this thesis are allowable
special permission,
of
without
provided that accurate acknowledgment of source is
made.
Permission
for
extensive quotation from or reproduction of
thesis may be granted by my major professor,
the Director of Libraries when,
or in his/her absence, by
in the opinion of either, the proposed
use
of the material is for scholarly purposes.
the
material
Signature
Any copying or use of
in this thesis for financial gain shall not
without my permission.
this
be
allowed
iv
ACKNOWLEDGEMENTS
.. I wish to thank the late Dr.
Mogk (Committee Chairman),
for their suggestions,
Dr.
Robert A.
David R.
guidance,
Chadwick,
Dr.
David
W.
Lageson, and Dr. Ken Emerson
and criticism during the preparation
of this thesis.
This thesis was partially funded by the NASA Early Crustal Genesis
Project through a grant secured by Dr..Mogk.
Further thanks is extended to Dr.
.Other
field
Safford,
wife
assistance was given by:
and Jim Barnaby.
Linda
and
our
Mogk for help in field mapping.
David
Hazen,
Ken
Salt,
Hugh
Extensive help in the field was given by my
dog Shade,
who,
along
with
the
other
field
assistants, helped by carrying supplies and rock samples on the arduous
trail
to Lake Plateau.
Mike Trombetta provided invaluable
assistance
and advice in the final preparation of this thesis.
Finally
Elizabeth, for
I
would
their
like to thank my
moral
and
loving
financial
help this study would not have been possible.
parents,
support.
Howard
Without
and
their
V
TABLE OF CONTENTS
Page
LIST OF TABLES.......................
LIST OF FIGURES...............
vi
vii
LIST OF PLATES.......... ..................."....................
viii
ABSTRACT............... ....................... .............,_...
ix
INTRODUCTION...................
I
REGIONAL SETTING................................................
3
ROCK UNITS......................................................
7
General Statement...........................
Inclusions............................
Biotite Garnet Schist...............................
Hornblende Biotite Schist...........................
Intrusive Units..............................
Hornblende Quartz Diorite...........................
Granite - Granodiorite - Pegmatite..................
Dikes.............
7
12
14
14
16
22
STRUCTURE.......................................................
25
TECTONIC CONDITIONS............................
31
CONCLUSIONS.............. ......................................
REFERENCES CITED
8
8
.37
39
Vi
LIST OF TABLES
Table
I.
Page
Granite characteristics of tectonic settings...............
35
vii
LIST OF FIGURES
Figure
Page
1.
Precambrian outcrops of the Beartooth Mountains............
2
2.
Archean outcrops of the Wyoming Province...................
4
3.
Photomicrograph of biotite garnet schist....... ............
8
4.
AFM composition diagram for biotite garnet schist..........
10
5.
Garnet-Cordierite phase relations in
biotite garnet schist....................
11
6 . Modal percent quartz— K-spar— -plagioclase for the
Lake Plateau intrusive units...........................
15
Photomicrograph of rounded zircon with dark reaction rim
in Lake Plateau granite....................... -........
18
8 . Photomicrograph of muscovite in Lake Plateau granite.......
19
7.
9.
Iron-magnesium-titanium compositional range for muscovite
in Lake Plateau granite................................
20
Pressure-temperature diagram showing water-saturated
granite solidus and muscovite+quartz reaction curve.... =
21
Photomicrograph of subhedral epidote of probable magmatic
origin in Lake Plateau granite.........................
21
Photomicrograph of relict ophitic texture in
continuous dikes................
23
13.
Generalized structure map of Lake Plateau..................
26
14.
Photomicrograph of quartz in granite from a shear zone.....
28
15.
Photomicrograph of mylonitized granite from a shear zone....
16.
Photomicrograph of bent and fractured plagioclase
.................
in sheared granite................... '
10.
11.
12.
29
29
viii
LIST OF PLATES
Plate
I.
Page
Geologic Map of Lake Plateau, Beartooth Mountains,
Montana......................................... . back pocket
ABSTRACT
The Lake Plateau area in the central Beartooth Mountains of
southern Montana is comprised of voluminous late Archean intrusive
rocks ranging from quartz diorite to granite in composition, with a
variety of supracrustal inclusions. The inclusions range in size from
centimeter to kilometer scale and include biotite hornblende schists
(bio-qtz-hbld-epi-plag)
and
pelitic
schists
(bio-qtz-cord-plaggar sill). These inclusions have experienced upper amphibolite grade
metamorphism at 6-8 kbar and 580-650°C, with penetrative deformation
creating a north striking foliation. The intrusive rocks vary in modal
mineralogy and texture on a meter scale. In some places they have an
hypidiomorphic-granular texture, and in others they have weak foliation
or foliated augen texture. Assimilation of inclusions is common with
foliated granites occurring at gradational contacts with inclusions.
Pegmatite and aplite veins associated with the intrusive rocks cut
across nearly all Archean rocks and comprise 15-20% of the total rock
volume. Structural trends include north-south foliation with associated
isoclinal folds, broad open kilometer scale folds, and unfolded shear
zones with mylonitic textures and retrograde metamorphism to chlorite
and epidote. Younger rocks include amphibolite dikes and a few Tertiary
felsic dikes.
Lake Plateau and the surrounding Beartooth Mountains evolved by:
I) burial of supracrustal rocks to depths of 20-25 km; 2) penetrative
deformation and upper amphibolite grade metamorphism; 3) generation of
high-Na intrusives such as the Long Lake granites; and 4) generation of
the K-rich granites of Lake Plateau. The Lake Plateau granitoids are
interpreted as mid-crustal melts emplaced at approximately 20 km and
generated from a slightly deeper crustal source. These large volumes of
K-rich granites are different from the Na-rich rocks reported in the
eastern Beartooths (Mueller and others, 1985). Large volumes of granite
with
subordinate quartz diorite and a variety of
supracrustal
inclusions are consistent with characteristics in younger examples of
post-collisional tectonic settings. The Beartooth Mountains represent
the
remaining mid-crustal evidence of development of
thickened
continental crust in the late Archean by collisional tectonics and
post-collisional uplift, similar in many ways to modern day tectonic
processes.
I
INTRODUCTION
The Lake Plateau study area encompasses sixty square kilometers in
the
central
provides
Beartooth
Mountains of southern
Montana
(Fig.
excellent exposures of Precambrian metamorphic
and
I).
It
granitic
rocks in a glacial topography of polished knobs and deep cirques. Lake
Plateau
and
is
a small area in a region of extensive
has difficult access (five hours on foot).
important
because
blocks within the Beartooth Mountains.
link
other recent studies around the area,
important
to
exposures,
But it is geologically
it is located near the boundaries
tectonic
to
Archean
of
three
major
It provides a central
and
it
is
therefore
the understanding of the genesis and evolution
of
this
Archean region.
The purpose of this study is to characterize the Lake Plateau rock
units and to establish the geologic history of this Archean terrane. By
doing soi the following questions are addressed:
1) What Archean crustal levels are now exposed at Lake Plateau?
2) What was the source for these rock units?
3) What tectonic conditions created this terrane?
4) What
constraints do these data place on theories about Archean
crustal development in the Beartooth Mountains and
Southwest Montana?
surrounding
2
LIVINGSTON
MONTANA
/NORTH
'SNOWY
■ LOCK
,STILLWATER
-^COMPLEX
LAKE
PLATEAU
SOUTH
> SNOWY
V block
RED LODGE
CENTRAL
■EARTOOTH
BLOCK
COOKE CITY
MONT
WYO
MAMMOTH
Figure
I.
Precambrian outcrops of the Beartooth Mountains. Lake
Plateau is shown in relation to the various blocks and to
the Mill Creek - Stillwater Fault Zone.
3
REGIONAL SETTING
The
Beartooth
Mountains are near the north end
outcrops
of the Wyoming Province (Fig.
numerous
mountain ■ ranges cored by Archean granites
rocks.
accretion
Karlstrom
from the south has been
and Houston,
1984),
the
Archean
This province consists of
and
In the southern part of the Wyoming Province,
Proterozoic
1982;
2).
of
supracrustal
late Archean
demonstrated
to
(Condie,
but tectonic conditions in
the
north are less clear.
In southwest Montana, Archean exposures are truncated by a west to
northwest
system of faults that have been active from the late Archean
to the present (Geissman and Mogk,
1986; ' Schmidt and Garihan,
1986).
North of these faults are extensive exposures of Proterozoic Belt rocks
and Phanerozoic sediments and volcanics. To the south are Archean-cored
Laramide uplifts (Foose and others,
that
show
these
2)
which
are
dominated
quartzofeldspathic gneisses,
1960;
Garihan and Okuma,
western
BeartoOths,
types
there
the east,
dominated
Ruby,
by
and Tobacco
metasediments
the
Root
that
west,
Ranges
include
1974; Clark, 1987). In the Madison Range and
is a complex set of te'rranes with
of
varying
intrusive
1975; Erslev, 1983; Mogk, 1984; Salt, 1987).
the Beartooth Mountains,
by major
In
1983)
schists and marbles (Heinrich and Rabbit,
that include metasediments and a wide range
units (Spencer and Kozak,
To
Schmidt and Garihan,
changing characteristics from west to east.
uplifts include the Blacktail,
(Fig.
rock
1961;
granitic intrusions
including
Lake
Plateau,
(Mueller and others,
are
1985).
4
LITTLE
BELT MOUNTAINS
%
W Y O M IN G
PROVINCE
TOBACCO BOOT
/ M OUNTAINS
D
NOBTHEBN MADISON
BEABTOOTH
M O U N TAIN S
MONTANA
W y o m in g '
SOUTHEBN M AD ISO N
BANGS
B IG H O B N
M O U N TAIN S
I ~ X
N
I B LA C K T A IL |
M O UNTAINS : TETON
/
I BANGS
BLACK
HILLS
O W L CBEEK
MOUNTAINS
GRANITE
M O U N TAIN S
W IN D
RIVER
RANGE
• A LB IO N
RANGE
---- 1
RAFT
RIVER
WASATCH
RANGE
NORTHEASTERN * '* ■ ■ * .
U IN TA
MADRE f
M OUNTAINS
M E D IC IN E BOW
M O U N TAIN S
______ W T O M .IN G
-^TJSCL
Figure 2.
ToioiABT--
Archean outcrops of the Wyoming Province (Clark, 1987, after
Condie, 1976).
This eastward progression of Archean sediments, accreted terranes,
and
major
tectonic
intrusions in southwest Montana is the product
conditions.
By
answering
the
questions
of
about
Archean
granite
compositions, source rocks, and crustal thickness at Lake Plateau, this
study will add to the understanding of these tectonic conditions and of
plate tectonics in general during the late Archean.
The Beartooth Mountains were subdivided into blocks by J.T. Wilson
(1936)
as shown in Figure I.
metasediments, including
The South Snowy block consists mainly of
metagraywackes
and
ironstones
with
minor
5
granitic
intrusions
Thurston,
1986).
volcanics
separate
Beartooths,
(Hallager,
Paleozoic
this
1980;
and
Casella
and
others,
Mesozoic . sediments
block from other Archean
and
1982;
Tertiary
exposures
in
the
making structural relationships unclear. Part of the North
Snowy
block has been mapped as a Late Archean mobile belt with
scale
eastward
thrusting
of
supracrustal
schists,
large-
marbles,
and
amphibolites over northeast-trending gneisses, schists and amphibolites
(Mogk,
1984). Along the north edge of the Beartooths is the Stillwater
Complex,
a
platinum-bearing,
layered mafic and ultramafic
with an associated contact aureole.
to
be
in
intrusion
The contact aureole, has been shown
fault contact with the adjoining North
Snowy
and
Central
Beartooth blocks (Geissman and Mogk, 1986).
Lake Plateau is part of the fourth and largest block,
Beartooth
volumes
block.
Early
the Central
studies of the central block with
its
large
of granite and granodiorite were done by Arie Poldervaart
and
his students (Eckelmann and Poldervaart, 1957, Poldervaart and Bentley,
1958,
Larsen and others,
1966,
Butler,
1966, and Butler, 1969). The
earliest of these studies suggested that the granites formed by
metasomatism of folded sedimentary rocks.
folding
have
and
been
processes
Later studies suggested that
metamorphism were contemporaneous and that
produced
(Casella,
by
flow
1969).
of rocks
The
static
rather
than
layering
by
current study indicates
sedimentary
a
magmatic
origin for the large volumes of granitic rock at Lake Plateau.
This is
consistent with recent magmatic interpretations of similar rocks
kilometers
east
others, 1985).
of
Lake Plateau in the central
block
may
(Mueller
fifty
and
6
The
intrusions
intrusions
(Mueller
to the east have been age dated at
and
others,
1985),
with 3.4 Ga
2.75
old
Ga
old
granulite
inclusions (Henry and others, 1982). Other available age dates from the
region
include 2.7 Ga for the Stillwater Complex (Lambert and
others,
1985). The published age dates nearest to Lake Plateau are from a drill
core study near Hawley Mountain, seven kilometers north of Lake Plateau
(Lafrenz and others,
for
1986).
This study yielded an age date of 2.75 Ga
a foliated biotite granite and 2.1 Ga for a poorly
granite.
Plateau
The
foliated
pink
timing and physical conditions of the magmatism at
Lake
is important to understanding the relationship of the
Central
Beartooth block to the Stillwater Complex and the North Snowy block.
7
ROCK UNITS
General Statement
The
tabular
outcrops
and
exposed at Lake Plateau are dominated
irregular
bodies
of
granodiorite
to
by
numerous
granite .and
by
pegmatite and aplite veins associated with these late to post-kinematic
intrusions.
Mineralogical
and
textural
gradational within the intrusive units.
are
variations
generally
The pegmatite and aplite veins
ubiquitous and crosscut nearly all other rock
intrusions
are
units.
Within
the
are numerous aligned xenoliths of biotite-gamet schist and
homblende-biotite schist. These inclusions range from centimeter scale
up
to
a few hundred meters by a few kilometers (Plate I).
metamorphosed
grained
in amphibolite facies with textures ranging
schist to coarse compositionally banded gneiss.
granitic
units
are
sharp
in
some
places
but
are
They
from
were
fine­
Contacts with
more
commonly
gradational, showing partial assimilation by the granite.
There
are mafic intrusive rocks including a continuous 500
meter .
wide body of hornblende quartz diorite which predates the granites, and
numerous
amphibolite
dikes
which cut the granites.
The
only
rocks
younger than Precambrian at Lake Plateau are rare Tertiary felsic dikes
associated
study
area,
with the Eocene Absaroka Volcanics which occur south of the
covering
dacite (Chadwick, 1985).
the older rocks with a layer
of
andesite
and
8
Inclusions
Biotite garnet schist
Biotite
the
from
garnet schist is the dominant rock type in inclusions
western third of the study area (Plate I).
meter
scale
in
single
outcrops,
to
Inclusion size
a
north-south
of
ranges
trending
inclusion just west of Mirror Lake that measures over two kilometers in
length.
This
irregular
sharp
inclusion
granitic
with
cut by numerous pegmatite
intrusions.
pegmatite
units, showing
is
veins
Contacts with surrounding
veins and generally gradational
units
with
varying degrees of partial assimilation.
and
by
are
granitic
Foliation
of
inclusions is typically concordant with foliation of intrusives.
The biotite garnet schist has the assemblage of
cordierite,
(Fig.
3).
titanium
Figure 3.
quartz,
garnet,
and plagioclase ( A n ^ ) , plus or minus sillimanite
Accessory minerals include allanite,
oxide,
biotite,
zircon, apatite, iron
and secondary chlorite after biotite.
Photomicrograph of biotite garnet schist.
G: garnet; C; cordierite; Q; quartz.
B:
biotite;
9
Foliation
defined
by
and
weak
compositional
centimeter
alignment
layering.
scale isoclinal
of
biotite
or broken,
both
post- and syn-kinematic crystalization.
with
grains
and many grains have random
is
open
and
of
are
locally
by
quartz
and
biotite.
It is not
orientations
poikiloblastic ranging in size up to
inclusions
folds
The biotite is brown to red-brown.
bent
Garnet
and
No
indicating
two
centimeters
internal
pattern
of
inclusions was recognized.
Quartz
of
grains
contain
strain
occurs as scattered anhedral grains and also as aggregates
in
irregular blebs to 15
strained
free,
grains showing slip bands and
recrystallized
Cordierite
centimeters
occurs
as
long.
mosaics
alteration.
blebs
of
smaller,
over
biotite
grains.
irregular
grains
growing
foliation with characteristic dusting of opaques,
pinite
These
pleocroic halos, and
Cordierite locally comprises up to 20% of the rock
in the large inclusion at Mirror Lake.
Metamorphic
temperature and pressure estimates were made for
biotite garnet schist based on microprobe analyses
done by
D.W.
the
Mogk
on biotite,
garnet, and cordierite. Analyses were done inside the rims
of garnets,
away from retrograde effects near the rim,
and on biotite
grains near, but not in contact with, the garnets (Mogk and Mueller, in
review).
Figure
minerals.
Peak
calculated
4
shows
metamorphic
the measured compositions
temperature estimates of
of
these
580-650°C
using the biotite-garnet geothermometer of Ferry and
three
were
Spear
(1978). Pressure estimates of 7-8 kilobars were calculated based on the
10
garnet-cordierite barometer calibrated by Lonker (1981). Figure 5 shows
a graphical solution to the pressure calculation.
A
CORD
GAR
J
i
l
0.2
Figure 4.
l
---- 1
I
0.5
'
0.8
AFM composition diagram for biotite garnet schist. Projected
from muscovite (after Best, 1982). Excess silicate and water
assumed. Values are calculated from microprobe data.
A: Al 2O3; F : FeO; M: MgO
Pelitic
schists
similar
to the biotite garnet
schist
of
Lake
Plateau have been described elsewhere in the region. A large pendant of
pelitic
schist
in
granite
has been reported
on
the
West
Boulder
Plateau, 15-20 kilometers northwest of Lake Plateau (Geissman and Mogk,
1986).
been
Staurolite-bearing
schists with greater than 25% biotite
reported on the West Fork of the Stillwater River,
8
have
kilometers
11
600
Figure 5.
_I_________i
I________ I—
650
700
750
800
T e m p e ra tu re
OC
Gamet-Cordierite phase relations in biotite garnet schist.
Based on the calibration of Lonker (1981).
12
NNE of Mount Douglas (Page and Nokleberg,
area,
1972). In the Cathedral Peak
12 kilometers ENE of Mount Douglas,
Butler (1966) has described
biotite garnet schist with minor amounts of staurolite, cordierite, and
anthophyllite,
and
Precambrian slates.
to
middle
with
chemical .signatures
similar ' to
those
of
He interpreted these schists as representing.lower
amphibolite facies metamorphism.
Weeks (1980)
has
mapped
extensive cordierite and staurolite-bearing schist just across the Mill
Creek
- Stillwater Fault Zone to the northwest of Lake Plateau.
widespread
Beartooth
occurrences suggest that at least the western part
These
of
the
Mountains experienced deposition of Archean sediments, which
were subsequently buried and metamorphosed prior to granite generation.
The estimates of 7-8 kilobars indicate burial of these sediments to 2025 kilometers.
Hornblende Biotite Schist
Hornblende
biotite schist comprises most of the inclusions in the
eastern two thirds of the Lake Plateau study area (Plate I),
highest
concentrations of inclusions north and east of Rainbow
The inclusions range from meter scale to tens of meters
biotite
with
garnet schist,
the
Lakes.
and, like
have both sharp and gradational contacts
the
with
the intrusive units. In contrast to the biotite garnet schist, however,
the
hornblende biotite schist spans a broad range of mineralogies
textures,
more
than
suggesting that there may be hornblende-bearing
one
protolith
or inclusions of similar
rocks
and
rocks
from
that
have
undergone different amounts of interaction with the intrusive rocks and
their associated fluids.
13
The
typical mineral assemblage of the hornblende
includes
epidote
hornblende,
with
chlorite.
contain
biotite,
plagioclase
accessory sphene,
Color index
5-10% K-spar.
biotite
(An^^y),
iron titanium
oxide,
ranges from 20-60%, and the
schist
quartz,
and
and
secondary
lighter
varieties
There is also a wide variation in modal amounts
of hornblende and biotite. Hornblende ranges from 0-50%. Biotite ranges
0-35%
from
and
is olive colored in some
samples
and
red-brown
in
others.
Rock
textures range from fine-grained,
dark schistose rock
strong alignment of hornblende and biotite imparting cleavage,
gneissic
There
rock
is
also
to more
with gradational light and dark compositional
a lineated variety with scattered
with
banding.
elongate
clots
of
hornblende and biotite in a lighter matrix.
Homblende-plagioclase
schist
(unpublished
metamorphism
empirical
in
the
phase relations in the hornblende
microprobe
mid- to
calibration
of
data
upper
Spear
from . D .
amphibolite
(1980;
1981).
Henry)
facies
biotite
indicate
using
Application
of
the
the
homblende-plagioclase geothermobarometer of Plyusnina (1982) indicates
temperatures and pressures on the order of 580°C and 7
kilobars, which
are near the temperature and pressure estimates from the biotite garnet
schist inclusions.
Rocks with similar modal mineralogy to the Lake Plateau hornblende
rocks
are
described
found
throughout the
amphibolites
the Cathedral Peak area.
central
Beartooths.
Butler
(1966)
with similar biotite-hornblende variations
in
He later suggested three possible protoliths:
I)tuffs or flows of intermediate igneous composition,
2) graywacke, or
14
3) metasomatically altered mafic rocks (Butler,
1969).
In the eastern
Beartoqths, Mueller and others (1983) have described very similar rocks
which are an abundant type of inclusion in that area, and which yield a
geochemical
origin.
signature consistent with andesitic magmas with , a
Thus, the
throughout
the
widespread occurrence of
Central
Beartooth
Block
mantle
hornblende-biotite
may
be
due
to
rocks
extensive
andesitic magmatism prior to metamorphism and granite generation.
Intrusive Units
Hornblende Quartz Diorite
Hornblende
outcrop
that
exposures
outcrops
quartz diorite occurs as a continuous,
trends north-south across Lake Plateau,
near Mirror Lake (Plate I).
500 meter wide
with
the
There are also scattered
in the eastern third of the plateau.
It appears from
best
minor
cross­
cutting relationships to be the oldest intrusion in the area. At Mirror
Lake,
the
west
large body is in contact with biotite garnet schist on
and granitic rocks on the east.
sharp
The contact with the
schist
the
is
and irregular with minor intrusions of hornblende quartz diorite
into the schist.
The contact with the granites is also sharp with very
few intrusions of granitic rock or pegmatite into the dense
rock.
Most
veins,
of these intrusions are thin (less than I cm)
although
hornblende
leucocratic
in a few places there are large outcrops of
granitic
rock with numerous inclusions of hornblende quartz diorite.
The
mineral assemblage of the hornblende quartz diorite
plagioclase (An^Q_^),
epidote,
iron-titanium
includes
hornblende, biotite, and quartz, with accessory
oxide,
sphene,
and retrograde
chlorite
and
15
sericite.
It
falls
plagioclase— K-spar
The
hornblende
in
the
ternary
quartz
diagram
diorite
(Fig.
field
on
a
quartz—
6).
and biotite comprise 25-50% of the
total
volume.
Grain
size varies from fine-grained varieties (0.25 mm) to course
to
cm),
3
with 15-20% magnetite occurring locally
in
the
(up
coarsest
varieties.
LAKE PLATEAU
MODAL %
GRANODIORITE
GRANITE
QUARTZ DIORITE
K-SPAR
Figure
6.
FLAG
(STRECKEISEN, 1976)
Modal percent quartz— K-spar— plagioclase for the Lake
Plateau intrusive units (after Streckeisen, 1976). Based on
point counts and visual estimates.
16
The
rock
granular
is
texture,
typically unfoliated
and
it
with
relict .hypidiomorphic-
is locally recrystallized
to
equilibrium
textures with triple junctions and straight grain boundaries. Foliation
is
present in some finer grained outcrops and is defined by
of hornblende and biotite grains.
two
clots
alignment
Most outcrops show scattered one
centimeter round clots of randomly oriented biotite grains.
weather
appearance.
more
easily,
Microscopically,
giving
the
weathered
surfaces
These
a
pitted
biotite grains in these clots
fine-grained epidote rims and cleavage fillings.
to
show
Hornblende is blocky,
anhedral, and locally poikilitic with rounded quartz inclusions.
Similar
rocks are described by Butler (1966) under the heading of
"hornblende-bearing
such
as
outcrops
unit
rocks",
which
also includes schistose
the hornblende biotite schist
with
that
is
described
above.
granoblastic textures may be the same
exposed at Mirror Lake.
varieties
The
early
Casella (1969)
larger
intrusive
noted
similar
amphibolites throughout the Central Beartooth Block, and he reported an
increase in these rocks from the south and east toward the core of
the
Beartooths.
Granite - Granodiorite - Pegmatite
This
texture,
granite
suite
comprises a wide
variation
in
mineralogy,
and grain size. They fall within the granite and granodiorite
fields (Fig.
6 ), and they comprise 80-90% of the Lake Plateau outcrops
(Plate I). 15-20% of this volume is pegmatite and aplite veins.
Texture and mineralogy vary from outcrop to outcrop,
are sharp to gradational,
for
and contacts
with gradational changes more common. Except
pegmatite, and aplite veins,
large distinct bodies
with
uniform
17
characteristics
interaction
contacts
are
rare.
between
Instead,
intrusive
there is. evidence
units and
inclusions.
with inclusions commonly show relict
of
extensive
Textures
compositional
near
layering
with granitic layers apparently injected into existing foliation of the
inclusions.
Further
from
contacts,
granitic foliation decreases
to
wispy biotite layers a few biotite grains thick, and within some larger
granitic
bodies little foliation is apparent.
Increased mafic content
and stronger foliation of granites near inclusions suggests
assimilation
of
extensive
inclusions or invasion of alkali-rich solutions
into
the country rock.
Pegmatite
homogeneous
and
veins,
aplite
to
veins
range
from
millimeter
zoned pegmatite veins a few meters thick
feldspar and quartz borders grading inward to mostly quartz
There
are
also
meter
scale,
scale homogeneous
aplite
veins,
with
interiors.
and
a
few
ptygmatic aplite bodies. Veins are typically planar with sharp contacts
and little deformation. They commonly parallel joint patterns, and many
large granitic boulders weather out with planar, pegmatitic faces.
The
granite
suite main assemblage
plagioclase
generally
although
near
less
than
is
microcline, 'and
in the range of ratios shown in Figure
6, with
5% each
epidote,
of
biotite,
larger amounts of these minerals
inclusions.
quartz,
Accessory
minerals
muscovite,
occur
include
and
locally,
zircon,
especially
apatite,
hornblende, garnet and iron titanium oxide.
Microscopic
textures
are typically hypidiomorphic-granular
with
various reaction textures including: myrmekite, patchy K-spar replacing
plagioclase,
albite rims on plagioclase,
and recrystallized quartz in
18
mortar texture between larger grains.
muscovite,
epidote,
In some thin sections,
and minor hornblende occur together in
biotite,
irregular
veinlets a few grains wide. Biotite is olive to olive-brown, similar to
that in hornblende biotite schist, and it occurs as scattered grains as
well
as
in
veinlets.
Garnets up to 5 mm occur in
both
aplite
and
pegmatite veins, and muscovite up to 5 cm occurs locally in pegmatites.
Zircons
Some
have
are rare,
dark
overgrowths.
but those present are conspicuously
reaction rims (Fig.
Similar
zircons
in
7),
and
granitic
others
rocks
of
rounded.
show
euhedral
the
eastern
Beartooths have been described as predominantly detrital (Eckelmann and
Poldervaart,
1957) and have been dated at more than 3.1 Ga. (Catanzaro
and Kulp, 1964).
Figure 7.
Photomicrograph of rounded zircon with dark reaction rim in
Lake Plateau granite.
19
Figure 8 .
Photomicrograph of muscovite in Lake Plateau granite. Large
size and subhedral shape indicate probable magmatic origin.
M: muscovite; Q; quartz.
Blocky
provide
time
subhedral
is also present
8) and
(Fig.
another line of evidence for high pressure conditions
of
granite
granites
meets
minerals;
emplacement.
the
muscovite (Speer,
granite
muscovite
The
muscovite in
following criteria
used
to
the
at
Lake
recognize
may
the
Plateau
magmatic
1984); I) grain size is comparable to other magmatic
2) the muscovite has subhedral to euhedral shape; and 3) the
is relatively unaltered.
Speer also reports that examples
of
magmatic muscovite have greater titanium content than examples of postmagmatic
or hydrothermal muscovite.
Figure 9 is a plot of TiOg-Fe^Dg-
MgO compositions from microprobe analyses of muscovite in Lake
granites.
The
elevated
titanium values are comparable to
magmatic muscovite shown by Speer (1984).
muscovite
and
those
If this is primary
then the pressure and temperature
must be above the two curves shown in Figure 10
of
for
magmatic
that crystallized in equilibrium with the associated
sodic plagioclase,
emplacement
Plateau
quartz
granite
(Hyndman,
20
1981).
This
places
the pressure minimum for granite
emplacement
at
about A kilobars.
Fe. O
Figure 9.
Iron-magnesium-titanium compositional range for muscovite in
Lake Plateau granite. Shown in relation to line representing
average measured compositions of magmatic muscovite (after
Speer, 1984). Based on microprobe analyses.
Microscopic
magmatic
mineral in the Lake Plateau granites (Fig.
subhedral,
where
textures suggest that epidote may also be
they
ranging up to I mm in size,
contact
biotite or quartz,
boundaries with plagioclase.
11).
a
primary
Grains
are
with straight grain boundaries
and irregular
to
myrmekitic
Some epidote grains have allanite
cores.
These textures match those described as magmatic by Zen and Hammarstrom
(1984), who interpret such magmatic epidote to be an indication of high
pressures (above 7 kilobars) during crystallization.
21
GRANITE SOLIDUS
T0C
Figure 10. Pressure-temperature diagram showing water-saturated granite
solidus and muscovite+quartz reaction curve.
Patterned
region
represents
probable pressure and
temperature
conditions of Lake Plateau granite emplacement indicated by
the presence of primary muscovite (modified from Hyndman,
1981).
Figure
11.
Photomicrograph of subhedral epidote of probable magmatic
origin in Lake Plateau granite. E: epidote; B: biotite; Q;
quartz; P: plagioclase; F; iron-titanium oxide.
22
The
with
existence
of these epidote and muscovite
other evidence,
gradational
strong
such as abundant pegmatites and assimilation
contacts
evidence
textures. combined
between the granites
and
inclusions,
for high water pressures and therefore
at
provides
deep
crustal
are four Precambrian dikes exposed within the Lake
Plateau
levels during granite crystallization.
Dikes
There
study area (Plate I).
All four are near-vertical tabular units.
Three
of the dikes are continuous, planar, and uniform in thickness (25-35 m)
with sharp contacts.
There is no apparent alteration of the older rock
units that are cut by the dikes.
irregular
contacts,
and
it
The fourth dike is discontinuous with
is
cut
extensively
by
pink
granite,
pegmatite, and aplite.
The
texture.
three
The
pigeonite (2V:
and
iron
and
assemblage
20-25),
Relict
dikes
is
all have the
same
plagioclase
(An
mineralogy
and
45-50)1 augite,
hornblende, and quartz with accessory biotite,
oxide.
ophitic
Secondary epidote and white
texture
is recognizable
mica
in
replace
most
thin
Plagioclase is subhedral to euhedral ranging up to 4 mm long
showing
replaced.
main
titanium
plagioclase.
sections.
continuous
various
stages of alteration from
clear
to
completely
Ophitic pyroxenes are partially or wholly replaced by patchy
mats of fine amphibole laths and rims of anhedral green and
blue-green
hornblende. Augite is the more common remaining pyroxene with pigeonite
recognizable
only as patchy cores (Fig.
12).
Quartz occurs as
anhedral grains and in micrographic texture with plagioclase.
minor
23
Figure
12.
Photomicrograph of relict ophitic texture in continuous
dikes. P: dark, patchy pigeonite; H: hornblende; PL:
plagioclase.
The discontinuous dike has the mineral assemblage of
hornblende,
titanium
epidote,
oxide,
clinozoisite,
and
retrograde
sphene
and
quartz,
and
plagioclase,
with accessory iron
chlorite.
phenocrysts up to 3 cm comprise 10- 20% of the rock,
Plagioclase
and they have been
almost completely replaced by epidote and clinozoisite. The matrix is a
dense, faintly foliated fabric of equigranular hornblende, epidote, and
quartz.
There
are
no recognizable pyroxenes.
Oxides are
rimmed
by
sphene.
Prinz
Beartooth
(1964)
produced
Mountains
a
comprehensive study of
to the east and south of
Lake
dikes
Plateau,
in
the
and
he
described dikes similar to those described here. The discontinuous dike
closely resembles his " Archean metadolerites" in mineralogy,
and
relationship
with
probably
intruded
granite
generation.
Precambrian
the granites.
Prinz states that
rock that was still ductile during late
texture,
these
dikes
stages
of
match
his
"Late
dolerites" which are "abundant in all parts of
the
range
The
three
continuous
dikes
24
and
are remarkably uniform in composition,
of intrusion" (Prinz,
happened
after
1964,
p.
1222).
indicating a single period
He states that this intrusion
uplift had fractured and faulted older
units
cut
dikes appear
directly
by
these dikes.
All
four
to
have
recrystallized
amphibolite facies in the late stages of granite generation or
after
when
deformation
pegmatites
the country rock was still hot.
indicates
The lack
post-kinematic timing,
and
the
of
into
shortly
penetrative
cross-cutting
and lack of granite alteration at contacts indicate a close
association between the dikes and the granites.
25
STRUCTURE
The structural geology of Lake Plateau is characterized by
broad-
scale open folding of a regional north-south striking foliation.
These
folds are cut by undeformed dikes and shear zones.
The
following
structural features were observed and measured
in
the field: foliation, including schistosity and compositional layering;
lineations
defined by preferred orientation of biotite and
aggregates, or by hinges of isoclinal to open,
contacts
hornblende
centimeter-scale folds;
of major inclusions with granitic units;
and shear zones.
A
generalized overview of this data is shown in Figure 13.
The 'foliation
imposed
fabric
generally strikes north-south and
associated
with
the
main-stage,
represents
regional,
the
upper-
amphibolite metamorphic event. Foliation is defined by strong alignment
of biotite and hornblende in schists,
gneissic inclusions.
and by compositional layering in
Individual grains are rarely bent or broken,
but
rather have recrystallized into the imposed fabric during metamorphism.
The
foliation is generally planar within inclusions,
millimeter
to
show
during
shows
some
and
some
centimeter scale intrafolial isoclinal folds
crenulation cleavage in biotite-rich varieties.
not
but
The isoclinal folds do
bent or broken grains and are interpreted to have
At contacts with intrusive
units,
the foliation is locally contorted into discontinuous ptygmatic
folds.
In
the main metamorphic event.
developed
other
creating
places,
granitic material has been injected into
compositional
layering,
and at gradational
foliation
contacts
where
LAKE
PLATEAU
/
SHEAR ZONE
FOLIATION
ATTITUDE
LAKE
PINCHOT
MIRROR
HORSESHOE
RAINBOW
WOUNDED
I MAN
LAKES
Figure
13.
Genera l i z e d structure map of Lake P l a t e a u . R e p resentative
foliation
attitudes are from granites and inclusions.
Stereonet A: Mirror Lake
synform with five degree plunge to the south-southwest.
Countours at
5-10-15%, 22% point m a x i m u m . Based on 41 points. Stereonet B : Rainbow
Lakes
antiform
with
twenty degree plunge to
the
south-southwest.
Countours at 5-10-15%, 17% m a x i m u m . Based on 53 points.
27
assimilation
granite
as
has
occurred,
relict foliation is recognizable
in
the
and
as
also show a general north-south trend with shallow
to
preferred orientation of scattered biotite grains
wispy mafic layers.
Lineations
horizontal
plunges.
aggregates
of
outcrops.
open
hornblende
Other
folds
at
Most
mineral lineations
observed
crystals within hornblende
are
elongate
biotite
schist
lineations measured are fold hinges of isoclinal
outcrop scale.
These lineations all
appear
to
and
have
occurred during development of the regional imposed fabric.
Figure 13 shows a second folding event characterized by kilometerscale open folds that plunge gently south.
foliation attitudes,
These folds are defined
changes
in
and their axes fit into
pattern
mapped over 200 square kilometers including Lake
a
by
regional
Plateau,
by
Butler (1966). He described them as open, cylindrical folds with gentle
north or south plunges.
Open folds similar to these have been reported
in the eastern Beartooths as a post-metamorphic event (Rowan, 1969; and
Mueller, 1979).
The shear zones are near vertical and do not appear to be deformed
by the open folding event.
wide
showing
characteristics.
chlorite
of
the
These are linear zones ten to thirty meters
retrograde
Biotite
metamorphism
schist
and
inclusions
ductile
have
been
result of ductile shearing.
larger
altered
schist with irregular crenulated chlorite layers and
fine grained quartz that has recrystallized from larger
also
deformation
mosaics
grains
as
Granitic units within the shear zones
show quartz recrystallization ranging from mortar texture
strained grains (Fig.
to
14),
around
to fine grained mylonitic textures
28
characterized
(Fig.
locally
15).
by smaller,
Plagioclase
recrystallized,
strain-free quartz
grains are commonly fractured (Fig.
have been replaced by mats of fine chlorite and
grains
16)
and
epidote.
The
shear zones are recognizable in outcrop as layers of fine-grained green
schist
and
red
and black
banded,
chert-like
siliceous
layers
of
mylonitized granite. They can be traced along strike as straight linear
features that commonly produce swales in the topography.
Figure 14.
Photomicrograph of quartz in granite from a shear zone.
Fine-grained
recrystallized quartz in mortar
texture
surrounds large, strained Quartz grains.
29
Figure 15.
Photomicrograph of mylonitized granite from a shear zone.
Quartz is completely recrystallized to smaller, strain-free
grains.
Figure 16.
Photomicrograph
sheared granite.
of
bent
and
fractured
plagioclase
in
30
Based
history
event
on the above structural features,
of
is
fabric,
destroyed
Lake Plateau can be constructed.
a partial
The
the upper amphibolite metamorphism with
isoclinal
evidence
deformational
folding,
of
events.
first
sedimentary
recognizable
associated
and late granite generation.
original
deformational
structures
It was followed by broad open
imposed
This
event
or
earlier
folding
without
accompanying penetrative deformation, and finally by shearing and local
retrograde metamorphism.
31
TECTONIC CONDITIONS
can
A tectonic model for the Archean rocks of the Beartooth
Mountains
be
comparing
these
developed by summarizing the available data and
by
data to theories of Archean crustal development and to models of
tectonic settings developed from younger rocks. From
the Lake
Plateau
data presented in this study, we can answer some of the questions posed
in the introduction.
First,
Plateau?
what
The
Archean
The
geobarometry
magmatic
crystallization
granites
levels
are now
exposed
at
metamorphic pressure estimate of 7-8 kilobars
cordierite-garnet
depth.
crustal
of
on
the order of 7-8 kilobars.
magmatic
based
corresponds to about 20-25 km
epidote in the granites
muscovite
with quartz
suggests
sodic
on
crustal
pressures
The existence
and
Lake
in
of
the
pIagioclase
indicates greater than 4 kilobars of water pressure (Hyndman, 1981) and
places a minimum granite emplacement depth at about 15 km.
contacts
of
xenoliths
and
granites
with
Gradational
textures
suggesting
assimilation, indicate emplacement into hot country rock, and support a
conclusion
lower
that the rocks now exposed at Lake Plateau formed at mid to
crustal levels.
supported
with
Therefore a minimum depth of 15 km is
probable
depths
of 20—25
km
during
metamorphic event and subsequent emplacement of magma.
coupled
with
biotite-gamet
30°C/km
the
600-650°C
geothermometry,
metamorphic
yields
temperature
strongly
the
mainstage
This
estimate,
estimate
a metamorphic gradient of
which is lower than most reported Archean
values
from
25-
•(Grambling.
32
1981)
and
is
similar
to
present
day
values
for . gradients
in
collisional tectonic environments (e.g. Spear and others, 1984).
The second question is: what was the source of these rock units? A
supracrustal
probable
origin
has
been determined
for
the
schists,
with
a
sedimentary origin for the biotite garnet schist based on its
pelitic assemblage,
and a possible andesitic origin for the hornblende
biotite schist based on similarities to metamorphosed andesites in
eastern Beartooths (Mueller and others,
1983).
the
The granitic units are
interpreted
to have a magmatic origin based on hypidiomorphic-granular
textures,
idiomorphic'
relationships.
within
the
crustal
zoned
plagioclase,
and
The source of these .magmas was probably partial melting
lower crust,
source
is
near the crustal levels
mantle
now
indicated by the granodioritic to
types, as opposed to tonalitic or more mafic magmas
direct
cross-cutting
source
derivatives.
exposed.
granitic
This
magma
characteristic
Other indications
of
a
of
crustal
melting source include: rounded zircons, possible anatectic textures at
contacts
with
inclusions,
pegmatite.
This
high
dehydration
reactions
in
and high water content producing
water
content
was
probably
the supracrustal rocks
abundant
generated
during
by
melting
at
pressures and temperatures near the granite solidus.
.
The
terrane?
third
The
question
answer
is:
what tectonic conditions
to this question must allow for:
created
I)
this
burial
of
supracrustal rocks to about 20 km; 2) penetrative deformation producing
north-south
foliation and isoclinal folds;
similar
to
present
partial
melting
values in thickened
3)
metamorphic
gradients
continental .crust;
within the crust to produce
granitic
magmas.
and
4)
These
33
conditions
magmatic
would
be met in a collisional tectonic
arcs and interarc basins.
basins,
and
setting
involving
The scale of the involved arcs and
their orientation is difficult to determine based on
the
limited evidence exposed in the Beartooth Mountains.
Such
the
a setting is supported by some of the regional
North
Snowy
trondhjemitic
shortening.
high-grade
surface
the
gneisses
The
metasediments
(Mogk,
are
1984),
thrust
eastward
demonstrating
In
over
east-west
wide-spread occurrence in the Beartooth Mountains
metasupracrustal rocks indicates extensive deep
rocks.
signature
Block,
geology.
In the eastern Beartooths1
the andesitic
of inclusions (Mueller and others,
existence of magmatic arcs at 2.8 Ga.
burial
crustal melting,
of
geochemical
1983 and 1985)
supports
Continued accretion of such
magmatic arcs and associated sediments may have eventually created
thickened crust,
of
the
and granitic intrusions observed at
Lake Plateau.
These
plate
ideas
tectonics.
faster-moving
allow
for
without
arc
are consistent with current theories
about
Archean
Dewey
that
thinner,
and
Windley (1981)
plates existed in the Archean.
theorize
The faster motion
dissipation of higher Archean amounts
calling on higher geothermal gradients.
systems
radiogenic
heat
They also state
that
amalgamated into cores of late Archean rigid
plates, . giving
modern-style
of
that
over
and
to
Lake Plateau probably represents mid to
deep crustal levels of such an early proto-continent.
argues
continental
rise to stable continental portions .of plates
plate tectonics.
would
two thirds of the
present
Dickinson (1981)
continental
crust
had
emerged from the mantle before 2.5 Ga as a product of accelerated plate
34
tectonic
activity.
He states that early continents formed during this
time as collages of oceanic island arcs,
lower
crust
in
thickened
Archean
and he calls on remelting
plates
to
achieve
of
internal
fractionation of the crust. This process seems to have occurred at Lake
Plateau where andesitic and quartz dioritic chemistries are subordinate
to large volumes of granite and granodiorite.
are
Studies of younger magmatism where modern plate tectonic
settings
discernible
tectonic
have
shown a strong
relationship
between
environment and granite characteristics (e.g. Pitcher, 1983). If modern
style
plate tectonics operated by the late Archean as suggested
Dewey and Windley,
granites
may
be
(e.g.
1981), then the characteristics of the Lake Plateau
clues
to the tectonic
environment
in
which
they
developed.
Pitcher
tectonic
(1983)
has summarized granite characteristics
environments.
describes Lake Plateau.
be seen in Table I.
uplift
calls
Of
these the !-Caledonian type
for
most
closely
In fact the similarities are striking,
The !-Caledonian setting is one of
five
as can
post-collision
with associated granites generated by crustal melting.
Pitcher
on a post-kinematic tensional environment to produce these
.potassium granites that contrast with the higher
sodium,
high
syn-tectonic
granites of the I-Cordilleran environment.
The
evidence
from
the Beartooth Mountains suggests
granite classifications may be valid for the late Archean.
faulting
may
have
sodium,
that
these
The
thrust
in the North Snowy Block indicates a collisional setting that
coincided
with crustal thickening and
I-Cordilleran
granites
production
in the eastern and central
of
high
Beartooth
A - TYPE
S - TYPE
M - TYPE
I - TYPE
CORD I L L E R A N
I - TYPE
CALEDONIAN
L A K E PLATEAU
Oceanic
island-arc
Andinotype marginal
c o n t i n e n t a l arc
Caledonian-type
post-closure uplift
B e s t fit:
Caledonian-type
B i o t i t e g r anite
A l kalic g r anite
Syenite
Plagiogranite
s u b o r d i n a t e to
gabbro
Tonalite dominant
D i o r i t e to g r a n i t e
Assoc, w i t h g a b b r o
Granodiorite-granite
Minor h o m b l e n d e diorite
G r a n o d iorite-granite
Minor hornblendequartz diorite
P e r thites
Interstitial
micrographic
K-spar
Pink K - s p a r
i n t e r s t i t i a l and
xenomorphic
Pink K - s p a r
i n t e r s t i t i a l and
invas i v e
Pink-spar
i n t e r s t i t i a l and
megacrysts
C o g n a t e xen o l i t h s
Bas i c m a gma blebs
Bas i c igneous
xenoliths
Dioritic xenoliths
m a y be r e s titic
Mixed xenolith
populations
Mixed xenolith
population
Multiple batholiths
p l u t o n s and sheets
Multiple cauldron
com p l e x e s
Small v o l u m e
Small plutons
Quartz dioritegabbro
Gre a t m ultiple,
linear batholiths
Com p l e x e s of
m u l t i p l e plutons
and sheets
M u l t i p l e sheets
and i r regular
bodies
Characteristically
Caldera-centered
l a c k i n g in
v o l u m i n o u s vx.
a l kalic lavas
Associated
islan d - a r c
volcanism
G r eat v o l u m e s
of a n d e s i t e and
dacite
Some have
basalt-andesite
lava "plateau"
No s u r f a c e data
P o s s i b l e andesitic
inclus i o n s
Short,
sus t a i n e d
plutonism
Very long-duration
episodic plutonism
Short, s u s t a i n e d
plutonism
Post-kinematic
L a t e - to postkinematic
plutonism
Hetcynotype
conti n e n t a l
oblique collision
Po s t - o r o g e n i c
or a n o r o g e n i c
L e u c o . monzogranite
some h i g h in
biotite
White K-spar
megacrysts
setting
Metasedimentary
xen o l i t h s
S u stained
S h o rt-lived
s y n — and p o s t —
kinematic plutonism
plutonism
Much shortening
L o w pres s u r e
metamorphism
D o m i n g and
rifting
O p e n fol d i n g
Burial-type
metamorphism
Vertical movements
Burial-type
metamorphism
D i p - s l i p and
strike-slip faulting
R e t r o g r a d e metam o r p h .
H i g h gra d e metamorph.
D u c t i l e shear i n g
m i n o r retro, m e t a m o r p h
Sn and W - g r e i s e n
and v e i n - t y p e
mineralization
C o lumbite
Cassi t e r i t e
Fluo r i t e
Porphyry copper
and gold
mineralization
P o r p h y r y copper,
molybdenum
mineralization
Rarely strongly
mineralized
No m i n e r a l i z a t i o n
recognized
Table
I.
Granite characteristics of tectonic settings (after Pitcher , 1983). Lake
Plateau granite characteristics closely match those of the I-type Caledonian
granites.
W
Ln
36
Mountains (Mueller and others, 1985). This compressional event may then
have been followed by uplift and a tensional environment that
produced
the !-Caledonian granites and later dikes observed at Lake Plateau.
37
I
CONCLUSIONS
Lake
crustal
with
Plateau represents a view of late Archean exposures of
levels on the order of 20-25 kilometers.
granite
crustal
to
granodiorite compositions
It shows
that
were
deep
intrusions
generated
by
melting and injected into upper amphibolite grade supracrustal
rocks
of sedimentary and possibly andesitic origin.
The existence
these
rocks demonstrates that by late Archean time (2.8-2.5
Ga),
of
the
Earth had begun to develop thick, differentiated continental crust.
Theories
allow
and
on ■ late Archean tectonics in southern Montana
for this thick crust with granite to
low
metamorphic
gradient
granodiorite
(25-30 °C/km
based
on
temperature estimates of 7-8 kilobars and 600-650 °C).
modern
need
to
compositions
pressure
and
The theory that
style plate tectonics were operating by the late Archean
(i.e.
Dewey and Windley, 1981) is supported by the data reported here.
The
the
characteristics of the Lake Plateau rocks match very
characteristics
regions
(Pitcher,
attributed
1983).
The
to
younger
post-collisional
following sequence of events
closely
uplift
is
thus
proposed for the tectonic history of the Beartooth Mountains:
1) Generation of magmatic arcs and interarc basins
2) Collisional tectonics resulting in:
a) Crustal shortening and thickening
b) Deep burial of supracrustal rocks
c) Penetrative
metamorphism
deformation
and
upper
amphibolite
grade
38
d) Generation of high sodium granites such as the Long
Lake
rocks 50 km east of Lake Plateau (Mueller and others, 1985)
3) Post-collisional tension and uplift resulting in:
a) Generation
of Lake Plateau granite
to
granodiorite
by
crustal melting
b) injection of amphibolite dikes
This
sequence can only represent a partial picture of the history
of this ancient terrane based on the evidence remaining in its
exposed
roots and based on theories about tectonic processes operating over two
billion years ago.
39
REFERENCES CITED
Best, M.G., 1982, Igneous and metamorphic petrology:
Freeman and Company, 630 p.
.
New York,
W.H.
Butler, J.R., 1966, Geologic ' evolution of the Beartooth Mountains,
Montana and Wyoming. Part 6. Cathedral Peak area, Montana:
Geologic Society of America Bulletin, v. 77, p. 45-64.
Butler, J.R., 1969, Origin of Precambrian granitic gneiss in the
Beartooth Mountains, Montana and Wyoming, in Larsen, L.H., Prinz,
M., and Manson, V., eds., Igneous and metamorphic geology:
Geologic Society of America Memoir 115, p. 73-101.
Casella, C.J., 1969, A review of the Precambrian geology of the
eastern Beartooth Mountains, Montana and Wyoming, in Larsen, L.H.,
Prinz, M., and Manson, V., eds., Igneous and metamorphic geology:
Geologic Society of America Memoir 115, p. 53-71.
Casella, C.J., Levay, J., Eble, E., Hirst, B., Huffman, K., Lahti, V.,
and Metzger, R., 1982, Precambrian geology of the southwestern
Beartooth Mountains,
Yellowstone National Park, Montana and
Wyoming, in Mueller, P.A., and Wooden, J.L., eds., Precambrian
geology of the Beartooth Mountains, Montana and Wyoming: Montana
Bureau of Mines and Geology Special Publication 84, p. 1-24.
Catanzaro, E.J., and Kulp, J.L., 1964, Discordant zircons from the
Little Belt (Montana), Beartooth (Montana) and Santa Catalina
(Arizona) Mountains: Geochimica et Cosmochimica Acta, v. 28, p.
87-124.
Chadwick, R.A., 1985, Overview of Cenozoic volcanism in the westcentral United States, in Flores, R.M., and Kaplan, S.S., eds.^
Cenozoic paleogeography of the west-central United States: Rocky
Mountain
Section,
Society of Economic . Paleontologists
and
Mineralogists, p. 359-382.
Clark, M.L., 1987, Protolith and tectonic setting of an Archean
quartzofeldspathic gneiss sequence in the Blacktail Mountains,
Beaverhead County,
Montana: Bozeman, Montana, Montana State
University, unpublished M.S. thesis, in preparation.
Condie, K.C., 1976, The Wyoming Archean Province in the western United
States, in Windley, B.F., ed., The early history of the Earth: New
York, Pergamon Press, 310 p.
40
Condie, K.C., 1982, Plate-tectonics model for Proterozoic continental
accretion in the southwestern United States: Geology, v. 10, p.
37-42.
Dewey, J.F., and Windley, B.F., 1981, Growth and differentiation
of the continental crust: Royal Society of London, Philosophical
Transactions, series A, v. 301, p. 189-206.
Dickinson, W.R., 1981, Plate tectonics through geologic time: Royal
Society of London, Philosophical Transactions, Series A, v. 301,
p. 207-215.
Eckelmann,
F.D.,
and Poldervaart, A., 1957, Geologic evolution
of the Beartooth Mountains, Montana and Wyoming. Part I. Archean
history of the Quad Creek area: Geologic Society of America
Bulletin, v. 68, p. 1225-1262.
Erslev, E.A., 1983, Pre-Beltian
Range, Southwestern Montana:
Memoir 55, 26 p.
geology of the Southern Madison
Montana Bureau of Mines and Geology
Ferry, J.M., and Spear, F.S., 1978, Experimental calibration of
the partitioning of Fe and Mg between biotite and garnet:
Contributions to Mineralogy and Petrology, v. 66. p.113-117.
Foose, R.M., Wise, D.U., and Garbarihi, G.S., 1961, Structural geology
of the Beartooth Mountains, Montana and Wyoming:" Geologic Society
of America Bulletin, v. 72, p. 1143-1172.
Garihan, J.M., and Okuma A.F., 1974, Field evidence suggesting a
non-igneous origin for the Dillon quartzofeldspathic gneiss.
Ruby Range,
southwestern Montana (abs.): Geological Society
of America Abstracts with Programs, v.6, p. 510.
Geissman, J.W., and Mogk, D.W., 1986, Late Archean tectonic emplacement
of the Stillwater Complex along reactivated basement structures,
northern
Beartooth
Mountains,
southern
Montana,
U.S.A.:
Proceedings of the. 6th international conference on basement
tectonics, p.25-44.
Grambling, J.A., 1981,
metamorphic rocks:
p.63-68.
Pressures and temperatures in Precambrian
Earth and Planetary Science Letters, v. 53,
Hallager, W.S., 1980, Geology of Archean gold-bearing metasediments
near Jardine,
Montana:
Berkeley, University of California,
Unpublished PhD Dissertation, 136 p.
Heinrich, E.W., and Rabbit, . J.C., 1960, Pre-Beltian geology of the
Cherry Creek and Ruby Mountains areas, southwestern Montana:
Montana Bureau of Mines and Geology Memoir 38, 40 p.
41Henry, D.J., Mueller, P.A., Wooden, J.L., Warner, J.L., and LeeBerman,
R.,
1982, Granulite grade supracrustal assemblages
of the Quad Creek area, eastern Beartooth Mountains, Montana:
Montana Bureau of Mines and Geology Special Publication 84, p.
147-156.
Hyndman, D.W., 1981, Controls on source and depth
of granitic magma: Geology, v. 9, p. 244-249.
Karlstrom,
K.E., and Houston,
R.S.,
1984,
Analysis
of a
Proterozoic suture . in
Precambrian Research, v. 25, p. 415-446.
of
emplacement
The Cheyenne Belt:
southern
Wyoming:
Lafrenz, W.B., Shuster, R.D., and Mueller, P.A., 1986, Archean and
Proterozoic granitoids of the Hawley Mountain area, Beartooth
Mountains, Montana: MGS-YBRA Field Conference, p. 79-89.
Lambert,
D.D.,
Unruh, D.M., and Simmons, E.C., 1985, Isotopic
investigations of the Stillwater Complex: A review, in Czamanske,
G.K., and Zientek, M.L., eds., The Stillwater Complex, Montana:
Geology and guide: Montana Bureau of Mines and Geology Special
Publication 92, p. 46-54.
Larsen, L.H., Poldervaart, A., and Kirchmayer, M.,. 1966, Geologic
evolution of the ■Beartooth Mountains, Montana and Wyoming.
Part 7. Structural homogeneity of gneisses in the Lonesome
Mountain area: Geologic Society of America Bulletin, v. 77,
p.' 1277-1292.
Lonker, S.W., 1981, The P-T-X relations of the cordierite-gametsillimanite-quartz equilibrium: American Journal of Science, v.
281, p. 1056-1090.
Mogk, D.W., 1984, The petrology, structure and geochemistry of an
Archean terrane in the North Snowy Block, Beartooth Mountains,
Montana: Ph.D. Dissertation, University of Washington, Seattle,
440 p.
Mogk, D.W., and Mueller, P.A., 1987, Magmatic epidote in Archean
granitoids of the Lake Plateau area, Beartooth Mountains, Montana:
Occurrences and tectonic implications: Geological Society of
America Bulletin, in review.
Mueller, P.A., 1979, Ages of deformation in the Helloroaring Plateau
area, eastern Beartooth Mountains, Montana: Canadian Journal of
Earth Sciences, v. 16, p. 1124-1129.
Mueller, P.A., Wooden, J.L., Henry, D.J., and Bowes, D.R., 1985,
Archean crustal evolution of the eastern Beartboth Mountains,
Montana and Wyoming, in Czamanske, G.K., and Zientek, M.L., eds.,
The Stillwater Complex, Montana: Geology and guide: Montana Bureau
of Mines and Geology Special Publication 92, p. 9-20.
42
Mueller, P.A., Wooden, J.L., Schulz, K., and Bowes, D.R., 1983,
Incompatible-element-rich andesitic amphibolites from the Archean
of Montana and Wyoming: Evidence for mantle metasomatism: Geology,
v. 11, p. .203-206.
Page, N.J.,
and Nokleberg,
W.J.,
1972,
Genesis of
mesozonal
granitic rocks below the base of the Stillwater Complex in
the Beartooth Mountains,
Montana:
United States Geological
Survey Professional Paper 80OD1 p. D I27-DI41.
Pitcher,
W.S., 1983, Granite type and tectonic environment, in
Hsu, K .J ., ed., Mountain building processes: London, Academic
Press, Inc., p. 19-40.
Plyusnina, L.P., 1982, Geothefmometry and geobarometry of plagioclasehornblende bearing assemblages: Contributions to Mineralogy and
Petrology, v. 80, p. 140-146.
Poldervaart, A., and Bentley, R.D., 1958, Precambrian and later
evolution of the Beartooth Mountains,
Montana and Wyoming:
Billings
Geological
Society Guidebook,
9th
Annual
Field
Conference, p. 7-15.
Prinz, M., 1964, Geologic evolution of the Beartooth Mountains,
Montana and Wyoming. Part 5. Mafic dike swarms of the southern
Beartooth Mountains: Geologic Society of America Bulletin, v. 75,
p. 1217-1248.
Rowan,
L.C.,
1969, Structural geology of the Quad-Wyoming-Line
Creek area, Beartooth Mountains, Montana: Geologic Society of
America Memoir 115, p. 1-18.
Salt, K.J.,
1987,
Archean geology of the Spanish Peaks
area,
southwestern Montana: Bozeman, Montana, Montana State University,
unpublished M.S. thesis, in preparation.
Schmidt, C.J., and Garihan, J.M., 1983, Laramide tectonic development
of the Rocky Mountain Foreland of southwestern Montana, iu Lowell,
J.D., and Gries, R., eds., Rocky Mountain Foreland basins and
uplifts: Denver, Rocky Mountain Association of Geologists, p. 271294.
Schmidt, C.J., and Garihan, J.M., 1986, Middle Proterozoic and Laramide
tectonic activity along the southern margin of the Belt Basin, in
Roberts,
M., ed., Belt Supergroup; a guide to Proterozoic rocks
in western Montana and adjacent areas: Montana Bureau of Mines and
Geology Special Publication 94, p. 217-236.
Spear,
F.S.,
1980,
NaSi ==
plagioclase and amphibole:
Petrology, v. 72, p. 33-41.
CaAl exchange
Contributions
equilibrium between
to Mineralogy
and
43
Spear, F.S., 1981, Amphibole-plagioclase equilibria: An empirical model
for the relation albite + tremolite = edenite + 4 quartz:
Contributions to Mineralogy and Petrology, v. 77, p. 355-364.
Spear, F.S., Selvefstone, J., Hickmott, D., Crowleyi P., and Hodges,
K.V., 1984, P-T paths from garnet zoning: A new technique for
deciphering tectonic processes in crystalline terranes: Geology,
v. 12, p. 87-90.
Speer, J.A., 1984, Micas in igneous rocks, in Bailey, ed., Micas:
Mineralogical Society of America Reviews in Mineralogy,
v.
13, p. 299-349.
Spencer, E.W., and Kozak, S.J., 1975, Precambrian evolution of the
Spanish Peaks area, Montana: Geologic Society of America Bulletin,
v. 86, p. 785-792.
Streckheisen, A., 1976, To each plutonic rocks its proper name: Earth
Science Reviews, v. 12, p. 1-33.
Thruston,
P.B.,
1986,
Geochemistry
and provenance of Archean
metasedimentary
rocks in southwestern
Beartooth
Mountains:
Bozeman,Montana, Montana
State University, unpublished M.S.
thesis, 74 p .
Vitaliano,
C.J., Burger, H.R. Ill, Cordua, W.S., Hanley,
T.B.,
Hess, D.F., and Root, F.K., 1979, Geologic map of southern
Tobacco
Root Mountains,
Madison County, Montana:
Geological
Society of America Map and Chart Series MG-31, map, scale
1:62,500, and text.
Weeks, G.,
1980, Precambrian geology of the
Boulder River
area,
Beartooth Mountains,
Montana:
Missoula, Montana, University .
of Montana, unpublished M.S. thesis, 58 p.
Wikstrom, A., 1984, A possible relationship between augen gneisses and
postorogenic granites in S.E. Sweden: Journal of Structural
Geology, v. 6, p. 409-415.
Wilson,
J.T., 1936,. Geology of the Mill Creek-Stillwater area,
Montana: Princeton, New Jersey, Princeton University, unpublished
PhD dissertation, 202 p.
Wooden, J.L., Mueller, P.A., Hunt, D.K., and Bowes, D.R., 1982,
Geochemistry and Rb-Sr geochronology of Archean rocks from the
interior of the southeastern Beartooth Mountains, Montana and
Wyoming, in Mueller, P.A., and Wooden, J.L., eds., Precambrian
geology of the Beartooth Mountains, Montana and Wyoming: Montana
Bureau of Mines and Geology Special Publication 84, p. 45-56.
Zen,
E., and Hammerstrom, J.M., 1984, Magmatic epidote
petrologic significance: Geology, v. 12, p. 515-518.
and
its
GEOLOGIC MAP OF
LAKE PLATEAU
PLATE I
LEGEND
UNI T B O U N D A R I E S
LITHOLOGIES
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