Structural and Petrologic Comparison of the Southern Sapphire Range, Montana with the Northeast Border Zone of the Idaho Batholith
Western Michigan University ScholarWorks at WMU
Master's Theses Graduate College
12-1979
Structural and Petrologic Comparison of the Southern Sapphire Range, Montana with the Northeast Border Zone of the Idaho Batholith
Stacy Lon Clark
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Recommended Citation Clark, Stacy Lon, "Structural and Petrologic Comparison of the Southern Sapphire Range, Montana with the Northeast Border Zone of the Idaho Batholith" (1979). Master's Theses. 1939. https://scholarworks.wmich.edu/masters_theses/1939
This Masters Thesis-Open Access is brought to you for free and open access by the Graduate College at ScholarWorks at WMU. It has been accepted for inclusion in Master's Theses by an authorized administrator of ScholarWorks at WMU. For more information, please contact [email protected]. STRUCTURAL AND PETROLOGIC COMPARISON OF THE SOUTHERN SAPPHIRE RANGE, MONTANA WITH THE NORTHEAST BORDER ZONE OF THE IDAHO BATHOLITH
by
Stacy Lon Clark
A Thesis Submitted to the Faculty 01 The Graduate College in partial fulfillment of the Degree of Master of Science
Western Michigan University Kalamazoo, Michigan December, 1979
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS
Dr. Ronald B. Chase, Department of Geology, Western Michigan
University, guided the field work and manuscript preparation. Dr.
J. Ronald Sides, Department of Geology, Western Michigan University,
and Dr. Donald W. Hyndman, Department of Geology, University of Montana
provided valuable assistance. The field investigation was financed by
the Graduate Research Fund of Western Michigan University, a Montana
Bureau of Mines Grant, and research funds from Burlington Northern,
Incorporated. Aerial photographs, topographic maps and access to remote
logging roads were provided by the U.S. Forest Service. Finally, I wish
to acknowledge the ranchers of the Sleeping Child area for their coopera
tion.
Stacy Lon Clark
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CLARK, STACY LON STRUCTURAL AND RETROLOGIC COMPARISDN OF THE SOUTHERN SAPPHIRE RANGE, MONTANA WITH THE NORTHEAST BORDER ZONE OF THE IDAHO &ATHOL1TH.
WESTERN MICHIGAN UNIVERSITY, M . S . , 1 9 7 9
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS
CHAPTER
I. PAGE
INTRODUCTION ...... 1
General Statement and Purpose ...... 1
Previous Investigations ...... 3
Location and Nature of this S t u d y ...... 5
Regional Tectonic Setting and General Features of the Sapphire Block ...... 6
Stratigraphy ...... 7
Structure ...... 7
Metamorphism...... 10
Proposed Regional Tectonic Models ...... 11
II. PETROGRAPHIC DESCRIPTIONS ...... 16
Bitterroot Range and Zone of C a t a c l a s i s ...... 16
Noncataclastic interior ...... 16
Zone of weak cataclasis ...... 17
Zone of strong cataclasis ...... 17
Local mylonite l e n s e s ...... 21
Sapphire Range ...... 21
Quartzofeldspathic gneiss ...... 22
Pelitic schist ...... 24
Calc-silicate gneiss ...... 26
Amphibolite ...... 31
Quartzofeldspathic orthogneiss ...... 31
Quartz diorite ...... 32
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER PAGE
Isolated granitic intrusions ...... 33
Volcanic rocks and associated dikes ...... 33
III. METAMORPHISM...... 35
Introduction ...... 35
Metamorphic conditions ...... 35
Regional metamorphism ...... 40
Thermal overprint ...... 41
Retrograde metamorphism ...... 42
IV. STRUCTURAL GEOLOGY ...... 43
Penetrative and nonpenetrative Structural Features ...... 43
Planar structures ...... 43
Lineations...... 43
Mesoscopic folds ...... 44
Structural History ...... 46
F^ f o l d i n g ...... 46
T?2 f o l d i n g ...... 46
F^ fo] di n g ...... 49
Zone of cataclasis...... 54
Faul _i n g ...... 54
V. INTERPRETATION...... 57
Timing of the Sapphire Tectonic Block Movement ...... 57
Synopsis of Deformational History of the Sapphire Tectonic Block ...... 58
Synopsis of Thermal-Structural History of the southern Sapphire Range ...... 61
iy
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Origin of the Quartzofeldspathic Orthogneiss...... 63
Choice of Tectonic Models ...... 66
VI. CONCLUSIONS...... 68
APPENDIX I ...... 70
REFERENCES CITED ...... 83
v
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ILLUSTRATIONS
FIGURE PAGE
1 Tectonic map of southwest Montana showing major faults, distribution of granitic plutons and important geological regions ...... 2
2 Index map showing information sources for the Bitterroot and Sapphire Ranges, southwest Montana ...... 4
3 Recommended terminology for the Belt Supergroup in southwest Montana ...... 8
4 Map of Sapphire tectonic block showing structural zones . . . 9
5 Block diagrams illustrating model 1 ...... 12
6 Block diagrams illustrating model 2 ...... 13
7 Block diagrams illustrating model 3 ...... 14
8 Sketch map of major rivers, streams and peaks within the study a r e a ...... 19
9 AMF diagrams for A^ (sillimanite-muscovite grade) pelitic assemblages...... 36
10 ACFK diagrams for A^ (sillimanite-muscovite grade) calc- silicate assemblages ...... 37
11 ACFK diagrams for A^ (sillimanite-muscovite grade) amphibolite and quartzofeldspathic gneiss assemblages .... 38
12 Univarient stability range of metamorphic assemblages described in the southern Sapphire Range ...... 39
13 Examples of concentric and similar-style folds from the quartzofeldspathic gneiss and pelitic schist ...... 45
14 Structural data from selected outcrops showing the relationship between the development of crenulation lineation and F£ f o l d s ...... 47
15 Structural orientation of axes of mesoscopic concentric- style F folds in the metasediments along Sleeping Child C r e e k ...... 48
vi
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16 Synoptic diagram of subarea analysis of foliation patterns in the metasediments along Sleeping Child Creek .. . 50
17 Example from subarea analysis of small circle distribution of foliations near junction of Sleeping Child and Blacktail C r e e k s ...... 51
18 Metamorphic isograds superimposed on geologic map of Skalkaho-tallow Creek area ...... 53
19 Attitudes of mineral streaks from the zone of cataclasis - contours are 32%, 24%, 16%, 8% and 1% per 1% a r e a ...... 55
20 Plot of average modes of quartzofeldspathic orthogneiss, granodiorite-granite suite from the noncataclastic interior of the Idaho batholith and rocks from the zone of cataclasis along the Bitterroot Range front ...... 65
PLATE
1 Geologic map of eastern border of Idaho batholith, Bitter root Range, and southern Sapphire Range, Montana .... in pocket
2 Structure sections of eastern border of Idaho batholith and southern Sapphire Range, Montana ...... in pocket
3 Poikilitic potassium feldspar megacryst and myrmekitic plagioclase grain boundary association from the non cataclastic interior of the Idaho batholith ...... 18
4 Crushed layers of fine grained quartz and feldspar wrap around lenticular augen of plagioclase ...... 18
5 Recrystallization in quartzofeldspathic gneiss resulted in development of elongate quartz and feldspar which parallels schistosity defined by biotite and needles of sillimanite . . 23
6 Partial recrystallization of quartz and feldspar in quartzofeldspathic gneiss resulting in "embayed" grain boundaries...... 23
7 Localized chaotic appearance of biotite cleavage flakes in pelitic s c h i s t ...... 25
8 Dark areas in fibrolite mat retain pleochroic characteristics of biotite ...... 25
vii
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9 Unoriented porphyroblast of muscovite which cuts schistosity defined by biotite and needles of sillimanite in leucocratic layer of pelitic schist ...... 27
10 Quartz and "clean" muscovite partially replacing prismatic staurolite porphyroblast ...... 27
11 Biotite and "ragged" muscovite, which define schistosity in the pelitic rocks, wraps around slightly poikilioblastic g a r n e t ...... 28
12 Biotite cleavage flakes, which define schistosity in pelitic rocks, are cut by a garnet porphyroblast...... 28
13 Partial development of polygonal texture in calc-silicate g n e i s s ...... 30
APPENDIX
1 Modal analyses of individual thin sections with geologic sketch m a p ...... 70
viii
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INTRODUCTION
General Statement and Purpose
Ravalli County, Montana can be separated into three distinct geolog
ical regions, (shown in the western sector of Fig. 1), the Bitterroot
Range, the Sapphire Range and the Bitterroot Valley. The southern Bitter
root Range is dominated by granite to granodioritic rocks of the Idaho
batholith. Cataclastic textures have been superimposed on batholithic
rock along the eastern front of the range. In the southern Sapphire
Range, regionally metamorphosed Belt Supergroup quartzofeldspathic and
calc-silicate gneisses, amphibolite and pelitic schist have suffered re
peated deformation and later igneous intrusion. Tertiary volcanics and
valley fill in the Bitterroot Valley are overlain by glacial deposits
along the east-draining tributaries of the Bitterroot Range and Lake Mis
soula sediments along the western flank of the Sapphire Mountains. A
fourth region of importance, the Dillon Block, is an exposed basement
high located southeast of the Sapphire tectonic block in southwest Mon
tana.
Past reconnaissance investigations have not revealed the complex
geology of the igneous-metamorphic terrane in the area of the present
study. The purpose of this study is to (1) fill a gap in regional map
ping of the southern Bitterroot and Sapphire Ranges, (2) compare defor
mation styles in the detached Sapphire block to those in the Bitterroot
dome, (3) suggest models which explain the development of the observed
1
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ro 50 km 1975b) Helena I 12 I and Chase, d u ( t'e Talbot, BOULDER BATHOUTH ONS I I • 0 • I I £RE LI NT LI Elkhorn Mts. Volcanlcs; Early Tertiary Volcanlcs BLOCK ssouI a ssouI regions. regions. (Adapted from Hyndman, distribution of and granitic important plutoii9 geological SAPPHIRE Mi Mi s *» V) \ ~ \ 00 * Figure 1. Tectonic map of southwest Montana showing major faults, • (Mostly (Mostly Cretaceous) .Granitic Rocks BATHOLITH «V- 00
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regional structures, and (it) compare structural styles in the study area
to those in mapped areas to the north and east.
Previous Investigations
Regional studies of the Bitterroot Range, shown in Figure 2, include
reconnaissance investigations of the entire Range (Lindgren, 190*+; Chase,
1977) the northeastern part of the Idaho batholith and adjacent regions
in the Sapphire Range (Langton, 1935), and the Hamilton 30-minute quad
rangle (Ross, 1952).
More recent published studies (Anderson, 1959; Berg, 1968; Chase,
1973, 1977; Cheney, 1972; Greenwood and Morrison, 1973; Groff, 195*+; Hall,
1968; Larsen and Schmidt, 1958; Jens, 197*+; Wold, 197*+; Ross, 1952; Weh-
renberg, 1972; Winegar, 1973, U.S.F.S. Open-File Report) and unpublished
thesis investigations (Leischner, 1959; White, 1969; Williams, 1975) de-
aescribe in detail the geology of the northeast border zone of the Idaho
batholith.
Unpublished studies (Csejtey, 1963; Desormier, 1975; Jerome:., 1968 ;
Maxwell, 1965; Montgomery, 1958; Presley, 1970) and published investiga
tions (Emmons and Calkins, 1913; Flood, 197*+; Hawley, 1975; Hughes, 1975;
Kauffman and Earll, 1963; LaTour, 197*+; McGill, 1959; Mutch, I960; Nelson
and Dobell, 1961; Noel, 1956; Poulter, 1957; Wallace, 1976, U.S.G.S. Open
File Report, Wiswall, 1976) of sectors adjacent to the study area describe
the geology of the northern and southern Sapphire Range. Chase and Wine
gar conducted separate reconnaissance investigations within the study area
in 197*+ and 1975, respectively. In the Bitterroot Valley, McMurtrey and
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114 °
li
BC 2 5km
0
Figure 2. Index map showing information sources for the Bitterroot and Sapphire Ranges, southwest Montana. Study area is shaded. 1 = Anderson (1959), 2 = Chase (1973), 3 = Chase (1977), 4 = Desormier (1975), 5 = Emmons and Calkins (1913), 6 = Flood (1974), 7 = Greenwood and Morrison (1973), 8 = Hall (1968), 9 = Langton (1935), 10 = LaTour (1974), 11 = Montgomery (1958), 12 = Nelson and Dobell (1961), 13 = Nold (1974), 14 = Poulter (1957), 15 = Presley (1970), 16 = Ross (1952), 17 = Wallace and Klepper (1976), 18 = Wehrenberg (1972), 19 = Wisx^all (1976), 20 = Williams (1975). Locations of specific studies are B = Berg (1968), C = Cheney (1974), J = Jens (1974), W = White (1969).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5
Konizeski (1956) described the geology and ground-water resources, and
Leber (1972) conducted a study of Pleistocene glaciation and deposition
of Lake Missoula sediments along the eastern margin of the Bitterroot
Range and Valley.
Location and Nature of This Study
The area studied is in central Ravalli County, Montana, within the
Hamilton South, Darby, Mountain House, Deer Mountain, Lard Mountain and
Como Peaks 7 1/2 minute quadrangles. It is approximately 200 square
kilometers in area.
The rugged Bitterroot Range in the west is cut by numerous east-
trending, steep-walled, glaciated canyons resulting in excellent rock
exposure. Maximum elevations are 3300 to 3700 meters. The unglaciated
southern Sapphire Range to the east obtains maximum elevations of 1800 to
2600 meters. Rock exposure is poor and primarily confined to cuts along
U.S. Forest Service and logging roads.
Field work was conducted during the summer of 1977. Field data were
recorded on U.S. Forest Service aerial photographs and U.S. Geological
Survey 7 1/2 minute topographic maps. Measurements of foliation, mineral
lineations, fold axes and slickenside striae were recorded for later
structural analysis. Sixty-five specimens were collected from which
sixty thin sections were prepared; thirty-eight of these sections were
point-counted (Appendix 1).
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Regional Tectonic Setting and General Features of the Sapphire Block
The Bitterroot gneiss dome (Chase, 1977), which includes the
northeast border zone of the Idaho batholith, is an elliptical-shaped
uplift of approximately 4000 square kilometers in area. The dome is
bounded by thrust faults on the north, high level Tertiary plutons on
the west and south and a 100 km-long shear zone on the east which marks
the western boundary of the Sapphire tectonic block.
Recently it has been suggested that an allochthonous 75 by 100 km
block of Belt, Paleozoic and Mesozoic sediments, the Sapphire Tectonic
Block, detached from a rising infrastructure and slid more than 25 km
to the east (Hyndman, Talbot, and Chase, 1975b). The movement of the
block as a structural unit imprinted characteristic deformation styles
on the region which define the present boundaries of the block. High-
angle, south-dipping, reverse faults with a minor left-lateral, strike-
slip component (Mutch, 1960; Desormier, 1975; l\allace, written communi
cation, 1978) define the northern boundary (Fig. 1). These faults swing
south through the Flint Creek Range, becoming low-angle, west-dipping
overthrust faults (Calkins and Emmons, 1915; Poulter, 1957; Mutch, 1960;
McGill, 1959) and form the eastern boundary of the block. The eastern
boundary of the block has recenty been re-defined by Hyndman (1979) as
the Georgetown thrust in the central Flint Creek Range while folds and
thrusts in the eastern Flint Creek Range are rocks bulldozed east by the
block. The southern boundary has been obscured by granitic plutons and
is currently taken to be the sillimanite isograd along the western part
of the boundary (Hyndman, Talbot, and Chase, 1975b).
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Stratigraphy
Precambrian Belt Supergroup sediments (Fig. 3) are probably the
oldest metasediments exposed in the Sapphire block with younger Paleo
zoic and Mesozoic sediments confined to the eastern portion of the tec
tonic unit. The combined thickness of Belt, Paleozoic and Mesozoic
sedimentary units is approximately 7400 to 12000 meters (Calkins and
Emmons, 1915; Poulter, 1957; Mutch, 1960).
In the study area and vicinity, only the Ravalli Group and lallace
Formation have been tentatively identified. In western Montana, the
Ravalli Group has been described as an olive-gray, massive, fine
grained, quartzite, with bedding planes defined by very thin (one to six
millimeters) micaceous sheets (Emmons and Calkins, 1913; Poulter, 1957).
The transition zone between the upper Ravalli Group and Lower Uallace
Formation is defined by a marked increase in argillaceous beds which
grade into shales and limestones of the tallace Formation. The Wallace
Formation consists of a thick sequence of tan to light green, thinly
bedded (2-6 centimeters), impure limestone with interbedded argillite,
siltite, and quartzite (Flood, 1974, LaTour, 1974; Presley, 1970; H s -
wall, 1976).
Structure
Deformation style within the block can be divied into three struc
tural zones as originally suggested by Calkins and Emmons (1915) (Fig.
4). The eastern zone, located east of the toe of the block, consists of
asymmetrical folds with east-dipping thrusts cut by normal faults. East
of the Philipsburg and Georetown thrusts and in a line west of the Royal
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Washington, Vicinity of Idaho and Missoula, adjacent parts Alberton, and of Montana St. Regis, MT
Cambri an Flathead quartzite or its equivalent
------unconTormiiy Monk Fm Wi ndermere ------Unconformityr~ system of Canada Huckleberry Fm ' Unconformity Pilcher Qtzite AAA Garnet Range Fm MISSOULA Li bby Fm Q. McNamara Fm Z=> §
a. Bonner Qtzite GROUP Striped Peak Fm.
Miller Peak Fm
(MIDDLE BELT Wallace Fm Wallace Fm CARBONATE)
St. Regis Fm St. Regis Fiii\Spokane R"
b UJ RAVALLI GROUP Revett Fm Revett Fm
Burke Fm Burke Fm
(PRE - RAVALLI Prichard Fm Prichard Fm OR LOWER BELT) - B a s e ------Not- - - — - - Exposed ------
PRE-BELT CRYSTALLIKE ROCKS
Figure 3. Recommended terminology for the Belt Supergroup in southwest Montana. Names in parentheses are used informally. (Adapted from Harrison, 1972, Fig. 5)
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BOULDER
r i - i - BATHOLI BATHOLI
IvlvXyX;! '_ » V \- Western Structural Zone Granite rocks X v X v X v "J/'C//
a Ph iIi psburg Thrust Middle Structural Zone b Georgetown Thrust
c Flint Creek Plutons Eastern Structural Zone d Goat Mountain Thrust
e Philipsburg batholith
f Bitterroot Dome
Figure 4. Map of Sapphire tectonic block showing structural zones. (Adapted from Hyndman, Talbot, and Chase, 1975b)
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stock and Goat Mountain fault, the middle zone is characterized by
tightly appressed, asymmetric, west-dipping, low angle thrusts which are
cross-cut by a later set of normal faults (Poulter, 1957; Mutch, 1960;
Flood, 1974; Wiswall, 1976). In the western zone, west of the Philips-
burg and Georgetown thrusts, broad, open concentric-style folds dominate
(Presley, 1970; LaTour, 1974). Three types of faults have been identi
fied in the western zone; (1) north-south trending, west-dipping, low
angle thrust faults, (2) north-south trending normal faults and (3)
east-west trending fractures and faults (Hughes, 1975).
Epizonal dioritic to granitic plutons, stocks and associated dikes
within the Sapphire tectonic block show petrologic and compositional
similarities, and equivalent radiometric ages to the Idaho and Boulder
batholiths (Hawley, 1975; Hughes, 1975; Hyndman and others, 1975a;
Hyndman, Talbot and Chase, 1975b). The presence or absence of folia
tion within individual plutons implies that magmatic emplacement
occurred during or soon after the deformation events of the Sapphire
tectonic block (Csejtey, 1963; Flood, 1974; Hawley, 1975; Hughes, 1975;
Hyndman and others, 1975a). The position of emplacement throughout the
block is structurally controlled by faults (Hyndman and others, 1975a),
tension fractures (Hughes, 1975) and folds (Presley, 1970).
Metamorphism
In general, metamorphic grade increases toward the Idaho batholith.
Metamorphism ranges from sillimanite-muscovite-grade with contact meta
morphic modifications in the southwest portion of the block to essenti
ally unmetamorphosed rocks modified only by contact metamorphism along
the eastern margin of the block.
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Proposed Regional Tectonic Models
Any tectonic model designed to explain the evolution of the Sapphire
block must account for the development of the cataclastic zone along the
eastern front of the Bitterroot Range, regional isograd patterns, deforma
tion style, and plutonic activity in and around the Sapphire tectonic
block. With the above criteria as restrictions, three possible models
can be proposed.
The first model (Fig. 5), similar to Lindgren’s (1904) original pro
posal, assumes an allochthonous block of Belt sediments and possible sli
vers of pre-Belt basement were thrust eastward, pre- or syn-tectonic to
the intrusion of the Idaho batholith. Subsequent doming of the rising
infrastructure deformed the basal shear zone of the thrust sheet which
is now the zone of cataclasis.
A second model designed to explain only the zone of cataclasis has
been suggested by Chase (verbal communication, 1978) (Fig. 6). The Belt-
Paleozoic (?) sedimentary pile remained autochthonous in relation to the
rising infrastructure. As doming continued, local shearing occurred at
the infrastructure-suprastructure boundary along the margins of the dome.
In response to infrastructural doming combined with the buttressing effect
of the Dillion block, thrust faults could have developed east of the main
intrusion by gravitational spreading and eastward movement of the sedimen
tary pile upslope on the pre-Belt basement.
The third model (Fig. 7) proposed by Hyndman, Talbot and Chase (1975b)
suggests that a rising infrastructure provided the gravity potential for a
block of Belt Supergroup and younger sediments to detach and slide down the
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preBeIt Basement BI ock
Li i I Ion preBeIt Basement BI ock
1 1 daho
3Batho I i t hi ^\\ \ i * / -’I ■' IC1-1— Superg roup
preBe i t Basement Bl ock
Figure 5. Block diagrams illustrating model 1; 1) development of thrusting across the top of the Idaho batholith and along the eastern margin of an allochthonous block, 2) intrusion of Idaho batholith during thrusting and warping of thrust sheet, and 3) normal faulting and erosion.
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Supergroup
preBeIt Basement BI ock
N\
preBeIt Basement
Supergroup
preBeIt Basement
Figure 6. Block diagrams illustrating model 2; 1) early intrusive phase domes autochthonous Sapphire block, 2) development of thrusting along eastern margin of the block caused by gravitational spreading as doming continues, and 3) normal faulting and erosion.
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daho
Supergroup
preBeIt Basement BI ock
/" ‘ C-'C. .... BathoI i t h v 7 ' Supergroup C«/o'-vy7if/4— preBeIt Basement BI ock
i'/s 'I daho ’i w ' w ■/ ^' v/ -BatheI ithM'M - i ^ •"‘’•vl —* i, W > jV , 1 — • 'i I a / Supergroup
preBeIt Basement BI ock
Figure 7. Block diagrams illustrating model 3; 1) doming and development of gravitational instability of overlying suprastructure, 2) eastward movement of block and development of zone of cataclasis and marginal thrust ing, and 3) normal faulting and erosion.
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eastern flank of the dome forming the highly complex structures of the
Sapphire Range.
All three proposed tectonic models will be discussed further in an
interpretation section which follows a description of the metamorphic
and structural geology in the study area.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PETROGRAPHIC DESCRIPTIONS
Bitterroot Range and Zone of Cataclasis
The largest part of the study area in the Bitterroot Range contains
igneous rocks of the Idaho batholith. The batholith exhibits textural
variation as one moves west from the Bitterroot V alley into the core of
the igneous complex. Three dominant zones were studied, expressed on
the geologic map (Plate 1) and described as follows:
1. Noncataclastic interior - Medium-to coarse-grained, phaneritic, hypidiomorphic-granular, holocrystalline granite-granodiorite with slightly gneissic appearance.
2. Zone of weak cataclasis - Similar compositionally to noncata clastic interior but more gneissic in appearance where feldspar and quartz wrap around feldspar augen and biotite and muscovite form a weak schistosity.
3. Zone of strong cataclasis - Similar compositionally to the above zones but highly gneissic; schistosity defined by parallelismm of muscovite and biotite, annealed quartz lenses, and elongate feldspar augen.
Noncataclastic interior
Rock compositions within the Idaho batholith vary from quartz monzo-
nite to tonalite (Chase, 1973). Appendix 1 summarizes thin section analy
sis. Although a hypidiomorphic-granular texture and isotropic fabric dom
inates, local zones of granulated feldspar and quartz, and subparallel
biotite and muscovite give the rock a foliated appearance.
Large perthitic potassium-feldspar megacrysts are poikilitic and con
tain inclusions of quartz and plagioclase. Locally, poikilitic megacrysts
16
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give the rock a "porphyritic" texture. Oligoclase-andesine shows weak to
strong normal and reverse zoning, albite, Carlsbad, alb ite-Carlsbad, and
albite-pericline twins with minor sericite and local epidote alteration.
Small peripheral grains of myrmekitic plagioclase border the potassium
feldspar megacrysts (Plate 3, Fig. 8). Biotite, partially altered to
chlorite, and muscovite are interstitial. Biotite contains apatite and
zircon. Quartz occurs as fractured, slightly undulose, interstitial
grains. Minor amounts of interstitial sphene, magnetite, and epidote
are present.
Zone of weak cataclasis
Rocks of the noncataclastic interior and the zone of weak cataclasis
are nearly identical with respect to composition and mineralogy (Appendix
1). The distinction between the units is based on differences in fabric
observed on the mesoscopic and microscopic scales.
Granulated layers of quartz and feldspar wrap around coarse feldspar
augen or rock fragments giving the rock a gneissic appearance. Ihese
augen of plagioclase and poikilitic potassium feldspar (containing inclu
sions of quartz and plagioclase) are elongated parallel to a weak folia
tion formed by the subparallel orientation of biotite and muscovite. Clus
ters of annealed grains of quartz are commonly parallel to the foliation.
Zone of strong cataclasis
The zone of strong cataclasis has been described by Ross (1952,
p. 153) as the border zone gneiss. This zone is compositionally similar
to the noncataclastic interior (Appendix 1). Many of the textural
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 8
Plate 3. Poikilitic potassium feldspar megacryst (pf) and myrme- kitic plagioclase (mp) grain boundary association from the non-cataclastic interior of the Idaho batholith. Analyzer in. (LHC 51)
Plate 4. Crushed layers of fined grained quartz and feldspar wrap around lenticular augen of plagioclase. Subparallel granulated biotite (b) and trace muscovite flakes define foliation. Analyzer in. (LHC 11)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. N .1 3 6 3 .1 122 3 122 k
H tn > tn H • • 94 7 94 r c c Cold S p rI p S Cold 9 2 6' 2 9 528 aJl. Hart Hart (A) (A) and sites of specifically described structural data (O), area. area. Numbers correspond to locations of plate samples Figure 8. Sketch map of major stream and peaks within the study
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20
details of the noncataclastic interior are preserved in the strongly
cataclastic rocks.
Potassium-feldspar megacrysts and myrmekitic plagioclase associa
tions, albite rims on plagioclase adjacent to potassium feldspar, and
clusters of uncrushed rock fragments which preserve the texture of the
noncataclastic interior suggest that cataclastic deformation has not
destroyed all textures characteristic of the noncataclastic interior.
Crushed layers of partially recrystallized fine-grained undulose
quartz, feldspar and biotite with trace amounts of muscovite form subpar
allel elongate lenses which commonly wrap around lenticular fractured
augen of plagioclase and potassium feldspar (Plate 4). Quartz occurs in
three habits, (1) isolated undulose grains, (.2) elongate sutured lenses
which wrap around lenticular augen of plagioclase and potassium feldspar,
and (3) inclusions within the augen. Crushed layers, and subparallel
granulated flakes of biotite and muscovite give the unit a uniform, east-
dipping foliated appearance. A weak lineation is defined by the align
ment of augen, quartz lenses and mica clusters within foliation planes.
Locally within the zone of strong cataclasis, black, highly crushed
mylonitic layers, ranging from a few millimeters up to a meter in thick
ness, suggest intense local shearing. Crushed layers of very fine-grained
feldspar, quartz and biotite form subparallel surfaces which wrap around
widely scattered augen of plagioclase and potassium feldspar. The align
ment of elongate feldspar and composite quartz lenses impart a weak linea
tion. Both fabric elements, foliation and lineation, observed within the
mylonite layers parallel those observed within the zone of strong catacla
sis.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 21
Local mylonite lenses
A highly crushed, red to yellow-brown, very fine-grained mylonite
occurs in lensoid masses along the Bitterroot Range front adjacent to
the Bitterroot Valley. Because of the highly granulated nature of the
rock, modal averages were not obtained.
Crushed layers composed of biotite, quartz, feldspar and an uniden
tifiable groundmass form a weak foliation which wraps around scattered
feldspar augen. Stringers of granulated biotite and muscovite give the
rock a streaked-out appearance and accentuate the weak foliation. The
foliation is offset by microfaults.
Sapphire Range
In the Sapphire Range, multiple deformed metasedimentary rocks which
contain amphibolite bodies, intrusive grantitic rocks and intrusive and
extrusive volcanic rocks are intruded by a heterogeneous quartz diorite
pluton (Plate 1). These metasedimentary rocks appear to resemble those
of the middle to lower Precambrian Belt Supergroup. Recognition of indi
vidual formations is difficult because of amphibolite-grade metamorphism,
therefore no unit names have been assigned on Plate 1. The dominant meta
sedimentary lithologies are quartzofeldspatic gneiss and pelitic schist
located in the northeastern portion of the map area. Both units exhibit
mica schistosity and contain concordant amphibolite and concordant-
discordant pegmatite bodies. Calc-silicate gneiss, interbedded with
quartzite and pelitic schist, is in concordant contact with the quartz-
diorite pluton located on the south.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 22
Quartzofeldspathic orthogneiss appears to be structurally concord
ant to metasediments along Sleeping Child Creek. heak foliation is de
fined by the preferred orientation of biotite clusters. Linear quartz
rods parallel to the foliation give the unit a gneissic appearance.
Volcanic rocks are located along the eastern and western edges of
the Bitterroot Valley in the central portion of the study area. North
east trending rhyolite to quartz-latite porphyry dikes intrude the meta-
sediments .
The average modes for the metasediments, orthogneiss and associated
igneous bodies are given in Appendix 1.
Quartzofeldspathic gneiss
Alternating blue-gray, quartz-rich layers and light-gray quartz,
feldspar, and biotite layers range in thickness from 0.5 to 40cm. Schist
osity is prominent in the biotite-rich layers. Localized zones of re
crystallized quartz and feldspar appear oriented with their long dimen
sion parallel to schistosity (Plate 5).
Fine-grained, interstitial, anhedral oligoclae-andesine shows albite
or albite-Carlsbad twins, normal or oscillatory zoning, and minor serici-
tic alteration. Orthoclase is medium-grained, anhedral and locally poi-
kiloblastic containing inclusions of quartz and feldspar. Medium-grained,
anhedral quartz, showing undulose extinction, commonly forms sutured
grain boundaries with the feldspars in the more quartz-rich layers (Plate
6). Biotite is interspersed and contains inclusions of apatite and zir
con. Chlorite alteration of biotite yields minor inclusions of sphene
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Plate 5. Recrystallization in quartzofeldspathic gneiss resulted in development of elongate quartz and feldspar which parallels weak schistosity defined by biotite and needles of silli- manite. Analyzer in. (541)
Plate 6. Partial recrystallization of quartz and feldspar in quartzo- feldspathic gneiss resulting in "embayed" grain boundaries. Analyzer in. (926)
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and opaque minerals. Trace amounts of ragged muscovite observed in one
thin section are rimmed by clusters of prismatic sillimanite. The exact
relationship and timing of replacement involving muscovite and sillimanite
is uncertain.
Pelitic schist
Fine- to medium-grained quartz- and feldspar-rich layers ranging
from 1 to 3 mm in thickness alternate with reddish brown biotite-silli-
manite-muscovite layers of similar thickness. The preferred orientation
of biotite and sillimanite results in a pronounced schistosity. Schisto
sity is absent in several thin sections (Plate 7). Locally, schistosity
is observed at slight angles to lithologic layering suggesting the devel
opment of schistosity parallel to axial-planar surfaces of large-scale
folds.
Medium-grained, slightly fractured oligoclase-andesine is confined
to the quartz-feldspar-rich layers and shows albite and albite-Carlsbad
twins. Minor normal, reverse, and weak oscillatory zoning and slight
sericitic alteration are present. Locally, poikiloblastic plagioclase
contains quartz inclusions. Myrmekitic plagioclase is present adjacent
to fractured anhedral orthoclase grains in the leucocratic layers.
Fine-grained, anhedral quartz is distributed evenly throughout the
rock. Sutured boundaries between fractured quartz and feldspar grains is
the dominant grain-to-grain relationship in the leucocratic layers. A
few elongate quartz and feldspar grains show polygonal textures along
isolated linear zones in two thin sections.
Several of the six sillimanite habits described by Cheney (1972) have
been observed; generally two or three in the same thin section. Those
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Plate 7. Localized chaotic appearance of biotite cleavage flakes (b) in pelitic schist. Trend of schistosity shown by arrows in upper left hand corner. Analyzer out. (528)
Plate 8. Dark areas in fibrolite mat (si) retain pleochroic characteristics of biotite. "Ragged" muscovite cleavage flake (m) borders fibrolite mat. Analyzer out. (1223)
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observed are, Cl) porphyroblasts, or fibrolite, intergrown with, biotite,
(2) mats of "deformed" fibrolite retaining various characteristics of
biotite (Plate 8), (3) acicular fibrolite needles which splay out in
rosette aggregates, (4) individual "clumps" of prismatic to acicular fi
brolite with little or no trace of biotite, and (5) prismatic crystals
forming discrete layers which wrap around quartz and feldspar porphyro-
blasts.
Biotite is dominately altered to chlorite and contains inclusions of
zircon, apatite and opaque minerals. Three distinct habits have been
identified. They are (1) discrete elongate crystals associated with mus
covite which defines schistosity, (2) small aggregates of biotite which
splay out in a rosette pattern and (3) unoriented prismatic crystals which
cut across schistosity. Muscovite occurs as either (1) unoriented por-
phyroblasts (Plate 9), (2) felty-appearing mica replaced by sillimanite
which is parallel to schistosity, or (3) muscovite replacing sillimanite.
Staurolite, partially replaced by muscovite and quartz, occurs along mus-
covite-sillimanite-rich layers in elongate prisms (Plate 10). The remnant
staurolite has a granular appearance. Xenoblastic, slightly poikiloblastic
garnet with quartz and feldspar inclusions displays helicitic textures in
a few grains. Schistosity locally wraps around, or is cut by, the garnet
(Plates 11 and 12).
Calc-silicate gneiss
Light-green to grey-green, hornfelsic, diopside-plagioclase-rich
calc-silicate rocks are best exposed in xenoliths within the quartz dio-
rite pluton located in the southeastern sector of the study area. TWo
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Plate 9. Unoriented porphyroblast of muscovite (m) cuts schistosity (trend of schistosity shown by arrows in upper right hand corner) defined by biotite (b) and needles of sillimanite (si) in leucocratic layer of pelitic schist. Analyzer in. (1223)
Plate 10. Quartz and "clean" muscovite partially replace prismatic staurolite (st) porphyroblast. "Ragged" muscovite (m) is largely replaced by fibrolitic sillimanite (si). Analyzer out. (557)
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Plate 11. Biotite (b) and "ragged" muscovite (m), which define schistosity in the pelitic rocks, wraps around slightly poikiloblastic garnet (g). Analyzer out. (528)
S
Plate 12. Biotite (b) cleavage flakes, which define schistosity in pelitic rocks, are cut by a garnet prophyroblast (g). Analyzer out. (528)
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distinct textures were identified, gneissic and spotted hornfelsic.
Gneisses commonly possess regular layers, 1 mm to 5 cm thick, of diopside-
rich or plagioclase-rich layers. Latour (1974) indicates that spotted
hornfelses in the Skalkaho Creek area to the north are dominated by clus
ters of white scapolite. Light-green spotted hornfels containing clusters
of scapolite were also recognized in the large xenolithic blocks within
the quartz diorite pluton in the southeast portion of the study area
(Plate 1).
Fine- to medium-grained, subhedral andesine-labradorite in the calc-
silicate gneiss shows albite, albite-percline, and albite-Carlsbaa twins,
normal, reverse, and oscillatory zoning, and minor sericitic alternation.
Andesine-labradorite exhibits two habits of occurrence; (1) large embayed
"ragged" poikiloblasts commonly with bent twin lamellae and inclusions of
diopside or (2) interstitial subhedral-euhedral grains showing well deve
loped polygonal textures (Plate 13). Colorless subhedral to anhedral
diopside also occurs in two habits; (1) as evenly distributed fine- to
medium-sized anhedral grains or (2) as large poikiloblasts, locally so
full of inclusions that they appear to be a collection of optically
oriented, medium-sized grains (described also by LaTour, 1974). llinor
amounts of actinolite occur as radiating aggregates of bladed prisms or
as "ragged” , undulose, fibrous prisms which defined a weak lineation and
are in contact with diopside. Poikiloblasts of hornblende containing
inclusions of plagioclase are present. Colorless, anhedral scapolite is
locally in contact with diopside. Trace amounts of interstitial, anhe
dral quartz, and euhedral to subhedral sphene and apatite, are present.
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Plate 13. Partial development of polygonal texture in calc-silicate gneiss composed of plagioclase (p), diopside (d), and interstitial quartz. Analyzer in. (947)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 1
Amphibolite
Gray-black, medium-grained, elongate amphibolite bodies generally
parallel schistosity and layering in the pelitic schist and quartzofeld-
spathic gneiss. Because of the poor exposure, the exact relationship
between schistosity in the gneiss and schist and hornblende lineation
in amphibolite could not be determined.
The medium-grained hornblende is green, pleochroic, euhedral to sub
hedral, and slightly chloritized. A few grains are poikilitic and contain
inclusions of plagioclase, quartz, and scapolite. Labradorite-bytownite
is twinned according to the albite and albite-Carlsbad laws. Strong to
weak reverse, normal, and oscillary zoning is present. A few grains show
slight sericitization. Colorless scapolite is present in contact with
plagioclase and hornblende. Biotite is dispersed evenly and gives the
rock a weak schistosity. Anhedral quartz, euhedral to subhedral sphene,
apatite, and epidote occur interstitially or as inclusions in the major
minerals.
Quartzofeldspathic orthogneiss
Reddish-brown quartzofeldspathic orthogneiss, located along Sleeping
Child Creek drainage, is composed chiefly of plagioclase, potassium feld
spar, quartz, and biotite. The preferred orientation of dispersed bio
tite clusters defines a weak schistosity and elongate quartz pods parallel
to schistosity give the rock a gneissic appearance. Locally, the texture
appears somewhat polygonal.
Fine-grained, subhedral to anhedral oligoclase-andesine shows weak
normal, reverse, and oscillatory zoning, albite, albite-Carlsbad twinning,
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highly irregular sutured grain boundaries, and moderate sericitic altera
tion. Albite rims in plagioclase are present adjacent to potassium feld
spar. Trace amounts of interstitial myrmekitic plagioclase are present.
Medium-grained, anhedral orthoclase, microcline, and perthite contain in
clusions of quartz and plagioclase. Slightly chloritized, brown biotite,
containing inclusions of apatite and zircon, occurs in clusters. Medium-
to coarse-grained, elongate, anhedral quartz parallels schistosity.
Trace amounts of muscovite appear in grain to grain contact with biotite
or as large "ragged" irregular porphyroblasts.
Quartz diorite
Gray, medium- to coarse-grained, phaneritic, hypidiomorphic-granular
quartz diorite to granite dominates the southern third of the study area
in the Sapphire Range. Large blocks of interbedded calc-silicate gneiss,
quartzite, and quartzofeldspathic gneiss are dispersed throughout the
pluton.
Coarse grains of euhedral to subhedral andesine display weak to
strong reverse and normal zoning, twinning according to the albite,
albite- Carlsbad, and albite-pericline laws, and slight sericitic altera
tion. Orthoclase is medium-grained, irregular and, in a few grains, con
tains microperthitic exsolution lamellae. Interspersed brown biotite
contains inclusions of apatite and zircon. Local chloritic alteration of
biotite is common and yields inclusions of opaque minerals. Euhedral to
subhedral sphene and anhedral epidote occur in clusters associated with
biotite. Trace amounts of fine-grained, "clean" muscovite are in grain-
to-grain contact with biotite.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 33 Isolated granitic intrusions
Throughout the northeastern sector of the study area are exposed
small stocks ranging in composition between granite, granodiorite and
tonalite. The granitic rocks are cut by numerous pegmatite and aplite
dikes. Gray to whitish gray, fine-to medium-grained, hypidiomorphic-to
allotromorphic-granular texture, and isotropic fabric dominate. A weak
parallelism of biotite is present along local contacts.
Euhedral to subhedral, medium- to fine-grained oligoclase is twinned
according to the albite and albite-Carlsbad laws, shows strong normal
zoning and slight sericitic alteration in the grain cores. Trace amounts
of myrmekitic plagioclase are present. Anhedral, medium-grained, micro-
perthitic microcline distinguished by grid twinning, and orthoclase are
the dominant potassium feldspars. Potassium feldspars are locally poi
kilitic and contain inclusions of plagioclase and quartz. Anhedral quartz
occurs as large irregular or fine interstitial grains. Flakes of slightly
chloritized biotite contain inclusions of apatite and zircon and are spa
tially associated with opaque minerals. Local fine-grained muscovite is
in grain-to-grain contact with biotite, although large "clean" grains are
present along fractures in rocks which exhibit foliation.
Volcanic rocks and associated dikes
A northeast-trending porphyritic dike swarm crosses the east-central
portion of the study area. Reddish, aphanitic, hypocrystalline, iron-
stained, slightly porphyritic rhyodacite and quartz latite contains sani-
dine, zoned oligoclase, and pyramidal quartz phenocrysts with, trace amo
unts of altered biotite and opaque minerals. Devitrified groundmass shows
irregular, highly altered mineral clusters.
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Extrusive volcanic rocks, striking N55°E and dipping 30°S, cover the
granitic-metasediment terrane on the east edge of the Bitterroot Valley
between Hamilton and Darby. The layered deposits are dominated by gray
to yellow gray porphyritic latite tuff, red to pink rhyodacite vitro-
phyre, and sligfrtly porphyritic welded tuff which contains sanidine,
zoned plagioclase and partially altered biotite phenocrysts. Micro
sheared volcanic breccia is less abundant than tuff. A variety of rock
fragments, including porphyritic basalt with oriented feldspar phenocrysts,
rhyodacite, obsidian, and quartz latite, are present in the breccia.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. METAMORPHISM
Introduction
Mineral assemblages and textural relationships indicate that the Belt
rocks in the southern Sapphire Range were regionally metamorphosed to con
ditions up to sillimanite-muscovite grade. Metamorphic textures developed
during this regional event are modified by a later thermal event. Features
of retrograde metamorphism are superimposed on those of earlier metamorphic
events.
Metamorphic conditions
The mineral assemblages which occur in the quartzofeldspathic gneiss,
calc-silicate gneiss, pelitic schist and amphibolite and ACFK and AMF dia
grams expressing each assemblage are given in Figures 9, 10, and 11. Trace
quantities of apatite, zircon, opaque minerals, and myrmekitic plagioclase
are present. Chloritized biotite, hornblende, garnet, and sericitized plag
ioclase result from retrograde metamorphism.
Temperature and pressure conditions within the study area can be broad
ly interpreted on the basis of experimentally established univariant sta
bility curves. Critical univariant curves, given in Figure 12 are;
1. muscovite + quartz = sillimanite + orthoclase + H~0 (Weill, 19 66; Evans, 1965).
2. staurolite + muscovite + quartz = biotite + Al-Og -}- ^ 0 (Hoschek, 1969; similar to reaction suggested by Guidotti, 1970). This reaction places an upper temperature limit on the stability of staurolite.
35
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Al 2°3 36 Si 11imanite
Pelitic assemblage; plagioclase - biotite - muscovite - quartz - sillimanite + (orthoclase) + (staurolite). 0 = Bulk composition
a i 2o 3
iman i te
Pelitic assemblages; plagioclase - biotite - muscovite - quartz - sillimanite + (orthoclase) + (almandine). 0 = Bulk composition
Almandi ne FeO / MgO
B io tite
Figure 9. AMF diagrams for (sillimanite-muscovite grade) pelitic assemblages. All assemblages contain quartz except local calc-silicate assemblages.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 37
A
Calc-silicate assemblages; diopside - plagioclase - sphene + (quartz) + (hornblende). o = Bulk composition Plagioclase
Diopside HornbIende F
A
Calc-silicate assemblages; diopside - plagioclase - Plagioclase scapolite - sphene (Scapolite), + (actinolite) + (quartz). © = Bulk composition
Diopside Actinolite
Figure 10. ACFK diagrams for A (sillimanite-muscovite grade) calc-silicate assemblages. All assemblages contain quartz except local calc-silicate assemblages.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Amphibolite assemblages; hornblende - plagioclase - biotite - scapolite - quartz 0 = Bulk composition
c
X Biotite Kornblende F
Quartzofeldspathic gneiss assemblages; Muscovite quartz - potassium feldspar - plagioclase - biotite - muscovite + (sillimanite). a Plagioclase 0 = Bulk composition
K Potassium feldspar Biotite
Figure 11. ACFK diagrams for A^ (sillimanite-muscovite grade) amphibolite and quartzofeldspathic gneiss assemblages All assemblages contain quartz except local calc- silicate assemblages.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 39
10
9
8 7
6 kb 5
3
2 1
400 500 600 700 800 900
T °C
Figure 12. Univariant stability range of metamorphic assemblages described in the southern Sapphire Range. Shaded area outlines probable stability field for meta-Belt units in the study area.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 40
3. kyanite = sillimanite (Newton, 1966).
4. chlorite + muscovite = staurolite + biotite + quartz + H2O (Hoschek, 1969). This reaction places a lower temper ature limit on staurolite.
5. andalusite = sillimanite (Newton, 1966).
Given the above reaction boundaries, metamorphic temperatures of 600-
700° C and a PT, ranging between 3.5 to 6.5 Kb can be predicted. If the Lly\J
phase boundary for the kyanite-sillimanite reaction is taken from experi
mental results by Richardson, Gilbert, and Bell (1969) or Holdaway (1971)
rather than Newton (1966), the upper limit on temperature and pressure
would remain essentially the same. However, the andalusite-sillimanite
reaction boundary produced by Richardson, Gilbert and Bell (1966) (R in
Figure 12) or Holdaway (1971) (H in Figure 12) either decreases or expands,
respectively, the lower limit of predicted metamorphic pressure for rocks
of the southern Sapphire Range.
Regional metamorphism
The first major recognizable metamorphic episode includes the develop
ment of a penetrative regional schistosity parallel to the lithologic
layers in Belt metasediments. Similar examples of a schistosity/layering
parallelism developed by syn-metamorphic deformation have been reported
in such metamorphic terranes as the Adirondack Mountains (Engel, 1949),
northern Vermont (koodland, 1965), and the Shuswap Terrane, British Col
umbia (Jones, 1959; Hyndman, 1968). During this episode, the growth and
parallelism of muscovite and biotite in the quartzofeldspathic and pel-
itic units and tremolite/actinolite in the calc-silicates defines schisto
sity. Granulation of quartz and feldspar may have occurred during this
period. Syn-to post-kinematic garnet poikiloblasts (with weak poikilitic
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 41
textures) truncate schistosity (Plate 12). A later schistosity defined
by muscovite, biotite, and sillimanite wraps around the garnet prophyro-
blasts (Plate 11). This schistosity is observed to wrap around stauro
lite poikilblasts which have been partially replaced by quartz and mus
covite (Plate 10). Since staurolite and garnet are not present in the
same thin section, schistosity developed in rocks containing staurolite
poikiloblasts cannot be related to schistosity in rocks containing garnet
poikiloblasts. The presence of quartz, "clean" muscovite, and silliman
ite in grain contact with staurolite suggests that these minerals formed
by the reaction (Guidotti, 1970):
staurolite + Na muscovite + quartz = sillimanite + K-richer muscovite + albite + ^ 0 (+ garnet).
The coexistance of tremolite/actinolite and diopside in the calc-
silicate gneiss may be caused by the lack of sufficient quartz for the
reaction:
tremolite + 3 calcite + 2 quartz = 5 diopside + 3002 + I^O.
Therefore, tremolite/actinolite is stable under amphibolite-grade con
ditions. Scapolite replaces plagioclase in the calc-silicate gneiss.
Thermal overprint
Annealing textures suggest that the thermal pulse of regional meta
morphism post-dates, or outlasted, major regional deformation in the
study area. Nearly all thin sections analyzed show some characteristics
of annealing, that is, slightly curved or straight boundaries suggesting
equilibrium (see Spry, 1969, p. 115), mixed with irregular, curved, den
tate, or sutured grain boundaries. The degree of annealing varies from
fine-grained, polygonal, elongate patches (Plate 5) to large "ragged"
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 2
embayed grains of quartz and feldspar (Plate 6). The "raggedness" of
some grain boundaries and the lack of well-defined polygonal grains sug
gest that the thermal pulse ceased before the completion of the annealing
process, or that some later deformation post-dates the thermal pulse of
the first synkinematic metamorphism. Polygonal textures which are well-
developed in the calc-silicate gneiss (Plate 13) and less so in quartz-
feldspathic gneiss (Plate 6) suggest that local factors such as grain
size, strain conditions, availibility of fluids, and local diffusion
gradients controlled the sites of annealing on the microscopic scale.
Unfractured porphyroblasts of "clean" muscovite and rosetta sprays of
sillimanite show decussate texture. Cleavage flakes of biotite (Plate
7), which originally defined a prominent schistosity, also appear dis
oriented locally in schistose rocks.
Retrograde metamorphism
The waning stages of regional metamorphism are recognized by the
replacement of amphibolite-grade mineral assemblages by those mineral
assemblages more stable under lower pressure-temperature conditions.
Prominent features of retrograde metamorphism include; 1) replacement
of sillimanite by "sericitic" muscovite, (2) alteration of plagioclase
to saussurite or sericite, and (3) the partial replacement of biotite,
hornblende, and garnet by chlorite.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. STRUCTURAL GEOLOGY
Penetrative and Nonpenetrative Structural Features
Planar structures
Layering (Sq ) is recognized on the basis of compositional variation
which may represent original sedimentary beds modified by later metamor
phic differentiation. No other sedimentary structures were observed.
Alternating concentrations of quartz-feldspar and biotite-muscovite-
(+ sillimanite) define layering in the quartzofeldspathic and pelitic
units. In the calc-silicate rocks, alternating layers with concentrations
of diopside-hornblende, tremolite/actinolite and plagioclase represent
original bedding.
Schistosity (S^) is defined by the preferred orientation of biotite
and muscovite in the quartzofeldspathic and pelitic units. The weak paral
lelism of biotite imparts a weak nonpenetrative schistosity in the calc-
silicate and amphibolite. Other nonpenetrative structural surfaces in
clude axial surfaces of mesoscopic folds, igneous contacts, joints, and
fault surfaces.
Lineations
Mineral parallelism is represented by alignment of sillimanite in the
quartzofeldspathic and pelitic units, hornblende in amphibolite and feld
spar augen, quartz rods, and mica streaks in foliation planes within the
zone of cataclasis. leak alignment of tremolite/actinolite in the
43
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calc-silicate gneiss imparts a weak lineation.
Crenulation lineation is defined by small-scale (4 to 8 mm) ridges
or crenulations on micaceous layering surfaces in quartzofeldspathic
and pelitic units (parallel to hinges of folds).
A weak lineation is represented by the intersection of layering and
schistosity where axial—plane schistosity is developed in mesoscopic folds
within pelitic units.
Other lineations include mullions, which are the weathered hinges of
large mesoscopic folds in quartzofeldspathic-pelitic gneiss, and hinges of
mesoscopic folds.
Mesoscopic folds
Concentric-style folds (Fig. 13a and b) are common in the more compe
tent quartzite-quartzofeldspathic units and rarely observed in the pelitic
units. The folded layers maintain constant thickness around poorly defined
hinge lines. Schistosity wraps around the hinges of the folds suggesting
a flexural mechanism of deformation. Crenulation lineations parallel axes
of concentric-style folds and plot as point concentrations around the
axis on stereographic projections (Fig. 14). Axes of mesoscopic concen
tric-style folds plot in clusters trending east-west (Fig. 15).
Similar-style folds were represented in pelitic schist and commonly
display axial-plane schistosity (Fig. 13c). When pelitic metasediments
dominated the local lithology, similar-style folds (on the order of a few
meters) with large amplitude/wave length ratios were observed. The best
example of these folds was observed in outcrop approximately 100 meters
southeast of Sleeping Child Hot Springs.
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a)
S2
20 cm 10 cm
c)
► ■i
Figure 13. a and b) Concentric-style folds from the quartzo- feldspathic gneiss. c) Similar-style fold in pelitic unit showing thicking in the hinge and the develop ment of axial-plane schistosity. d) Thin-section sketch of schistosity truncating layering at contact between quartzofeldspathic gneiss and pelitic schist.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 46
Structural History
Although the distribution of outcrops in the study area is not ade
quate to conduct a reliable domain analysis, some structural data are
available for reconstruction of the structural history of the region.
Three phases of folding are recognized in the southern Sapphire Range.
F^ folding
The concordance between schistosity and layering observed in the peli
tic and quartzofeldspathic units on the mesoscopic scale and examples of
schistosity truncating lithologic layers at small angles on the microscopic
scale suggests the first folding event was isoclinal (F^) with the develop
ment of penetrative schistosity (Fig. 13d). No isoclinal fold closures
were observed on the mesoscopic scale. It would be difficult to find the
closures of such folds if the amplitude/wave length ratios are large in a
region where exposures are poor.
F£ folding
The quartzofeldspathic gneiss provides the best exposures of F? struc
tures. Concentric-style folds developed by a flexural-slip mechanism with
concomitant development of coaxial crenulation lineation as shown in Figure
14 CL parallel to B^). Mica schistosity parallels lithologic layering
around the hinges of folds. Axial-plane schistosity developed locally in
the pelitic units. Mesoscopic data suggests F2 folding about an axis which
is now east-west and nearly horizontal (Fig. 15).
Within the study area, excellent exposures of F2 folds are confined
along Sleeping Child Road near Sleeping Child Hot Springs.
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N
126 136 o +
+ lineations
© - fold hinges
Figure 14. Structural data from selected outcrops showing the relationship between the development of crenulation lineation (crosses) and F^ fold hinges (circles). Refer to Figure 8 for location of numbered outcrop.
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Figure 15. Structural orientation of hinges of mesoscopic concentric- style F folds in the metasediments along Sleeping Child Creek.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 49
folding
Subarea analysis of foliation patterns, possible stratigraphic rela
tionships within the study area, and reconnaissance mapping the north
CLaTour, 1974) and east (Winegar, U.S.F.S. Open-File Report, 1975) reveal
a later folding event which produced a northeast-trending antiform (Plates
1 and 2).
Subarea analysis of foliation patterns in the study area reveals a
somewhat hetergeneous macroscopic fold pattern about axes trending north
east (Fig. 16). Small-circle distribution of foliations observed in a
few subareas (Fig. 17) imply conical-shaped folds which may have resulted
from macroscopic sagging and/or doming, refolding of concentric-style
folds, or F£ folds which were originally conical in these regions. These
subareas are near granitic stocks.
Ross (1963) describes the unmetamorphosed Ravalli Group (1500-2700
meters thick) in western Montana varying from nearly pure quartzites to
pale green to gray siliceous shales in the lower section, grading into
thick-bedded white to tan quartzite interbedded with very thin sericitic
beds toward the middle of the section, and consisting of thin-bedded argi
llaceous and quartzitic beds interbeddd with limy quartzite in the upper
part of the section. These upper 50 to 150 meters of thinly laminated
siliceous argillite are a transition zone between the Ravalli and the
thin-bedded, highly laminated, calcareous shales and impure limestones of
the Wallace Formation (1300-1800 meters thick). The metamorphic equiva
lent of the above-described middle and upper Belt sequence may be present
within the study area. Massive quartzites and quartzofeldspathic gneiss
abundant in the lower Sleeping Child Creek drainage could be correlative
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N
a~
Figure 16. Synoptic diagram of subarea analysis of poles to foliation in the metasediments along Sleeping Child Creek. Distorted great circle patterns of suggests local heterogeneity in folding about an axis trending northeast.
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N
Figure 17. Example from subarea analysis of small circle distri bution of poles to foliation near junction of Sleeping Child and Blacktail Creeks. Refer to Plate 1 for exact location.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 52
to the middle or upper Ravalli Group. Calc-silicates and pelitic rocks,
which may be correlative to the I-allace Formation, are increasingly abun
dant to the north and south of Sleeping Child Creek. Thick sequences of
metasediments are similarly described along the Lochsa and Selway Rivers,
Idaho and have been correlated with middle Belt lithologies (Greenwood and
Morrison, 1973; Reid, Morrison, and Greenwood, 1973).
To define metamorphic conditions, LaTour (1974) uses the meionite
content of scapolite and the presence or absence of calcite, talc, tremo-
lite, and diopside in the calc-silicate rocks of the Wallace Formation
plus the presence or absence of muscovite, sillimanite, and orthoclase in
the pelitic units found in the lallace Formation. Metamorphic facies
boundaries and the contact between the tallace Formation and Ravalli Group
metasediments are roughly concentric. LaTour suggests that this isograd
pattern could be used to work out the complex geology of the region even
though the region lacks outcrop continuity. Assuming that the Belt sec
tion was horizontal prior to regional metamorphism of the Sapphire Range,
an unrealistic assumption in most metamorphic terranes, the isograd pat
tern should roughly reflect the structure of the region (Fig. 18). LaTour
(1974) found this to be true within his area and that of Presley (1970)
to the north in the I'illow Creek stock area. Based on lithologic layer
ing, stratigraphic relationships (Fig. 18) and isograd patterns in the
south-west portion of his study area, LaTour speculates that an anticlinal
structure exists in the Sleeping Child Creek area with Ravalli Group meta
sediments in its core.
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+ *+•: ••• O O +++++++++ >o 4 + ♦ ♦ + + + + + + + + + + ♦ + o o 00 o o 7?+ + + + + + ** + + S A . *• •
. ■^++' 'f + t + + +4 ?v + + I© o
□ID Ravalli Gp.
E 3 K-rich quartzo feldspathic gneiss
Figure 18. Metamorphic isograds and general structural trends superimposed on geologic map of Skalkaho-Willow Creek area (after LaTour, 1974). Refer to Figure 2 for geographical location in relation to present study.
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Zone of cataclasis
The eastern margin of the Bitterroot dome is marked by a pronounced
north-south trending, 100-km-long, zone of cataclasis. The present approx
imate thickness of this zone is calculated between 900-1000 meters. On
the mesoscopic scale, crushed layers of quartz and feldspar, the preferred
orientation of biotite and muscovite define an east-dipping foliation, and
closely spaced slickenside striae define a lineation which plunges 15° to
25° east to southeast (Fig. 19). This foliation grades westward into the
unsheared to weakly-sheared granite-granodiorite of the Idaho batholith.
Mesoscopic, non-penetrative, east-dipping, thin black mylonitic layers
parallel foliation. On the microscopic scale, elongate augen of feldspar
and aligned undulose quartz impart a weak lineation which parallels folia
tion. Crushed layers of partially recrystallized quartz and feldspar
wrap around feldspar megacrysts which commonly have granulated borders
and display pressure shadows.
Faulting
Along the eastern margin of the Bitterroot Range, east to southeast
dipping, high-angle faulting has brought Wallace Formation metasediments
into contact with the zone of cataclasis. Narrow bands of mylonite asso
ciated with these faults are characteristically reddish-brown, highly-
crushed, and slightly porphyroclastic.
Faults in the southern Sapphire Range are difficult to recognize
because of poor exposure, uncertain age relationships of granitic rocks,
and lack of key marker beds. The area is characterized by north-south
trending normal faults and east-west tension fractures.
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Figure 19. Attitudes of mineral streak lineation from the zone of cataclasis - contours are 32%, 24%, 16%, 8%, and 1% per 1% area.
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According to Hughes (1975), north-south trending normal faults ob
served throughout the Sapphire block result from differential rates of
movement within the block or post-intrusion block faulting. Along Sleep
ing Child Creek at least two north-south trending faults are inferred
from tiends of volcanic intrusions. The first can be traced to the south
along Sawdust Gulch across Sleeping Child Creek to Blacktail Creek de
fining the eastern extent of the orthogneiss (Plate 1). The other fault
parallels the north-east border of the study area and is associated with
the hydro-thermal activity near the junction of Two Bear Creek and Sleep
ing Child Creek. Aphanitic dikes injected along both fault planes ob
scures fault contacts.
A southeast dip of 30° in Eocene volcanics layers (Armstrong, 1974)
located along the eastern margin of the Bitterroot Valley is a result of
deposition over an irregular dipping surface and later southeast tilting.
Southeast tilting and local fracturing of Eocene volcanic layers suggest
a final structural event developed during post-Eocene time.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INTERPRETATIONS
The degree of metamorphism, origin of the plutons and volcanic units,
regional faulting, and broad open folding mapped by others along the east
ern and northern margin of the Sapphire block provide constraints which
must be considered in constructing the deformation history and choosing
among the three proposed models: (1) syntectonic regional thrusting, (2 )
gravitational spreading and (3) detachment and gravitational sliding.
This section discusses the kinematic and dynamic interpretation of the
Sleeping Child Creek-Deer Mountain region as it fits into the regional
structural setting.
Timing of the Sapphire Tectonic Block Movement
The deposition of the mid-Cretaceous Beaverhead Conglomerate marks
the beginning of emplacement of the Idaho batholith (Axelrod, 1968; Schol-
ten and Onasch, 1977). As doming and spreading of the infrastructure con
tinued into the overlying sedimentary pile, eastward movement of the Sap
phire block occurred by one or more of the proposed mechanisms listed
above and reviewed in the introduction (p. 11-15).
Evidence from the northern and eastern margin of the Sapphire tec
tonic block suggests movement occurred in latest Cretaceous time (Hyndman,
1977). Along the northeast corner of the block, the Golden Spike Forma
tion is deformed by movement of the block. This late Cretaceous thick
wedge of conglomerate was derived from uplifts to the west and volcanic
material from the lower Elkhorn Mountains Volcanics to the southeast
57
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(Gwinn and Mutch, 1965). The lower unit of the Elkhorn Mountains V ol-
canics, an early eruptive phase of the Boulder batholith, has been dated
at 78 m.y. (Tilling and others, 1968).
Along the eastern margin of the block, late Cretaceous units are
affected by thrust faults and associated folds whereas Oligocene and Mio
cene intermontane valley sediments are not. In addition, granitic intru
sions were emplaced just syn- or post-tectonic to the development of fold
ing and thrusting into the eastern toe of the block (Csejtey, 1963; McGill,
1965; Hawley, 1975; Hyndman and others, 1975a). K-Ar dates on plutons
which intruded near the end of deformation in the block are 76 to 72 m.y.
(Hyndman and others, 1972). K-Ar dates for the Boulder batholith are 68
to 78 m.y. (Tilling and others, 1968).
Because of the close association in time between the emplacement and
consolidation of granitic magmas and deformation within the block, the
Sapphire tectonic block must have moved during the late-Cretaceous, just
prior to or in the early stages of the emplacement of the Boulder batho
lith (Hyndman, Talbot, and Chase, 1975b).
Synopsis of Deformational History of the Sapphire Tectonic Block
The northern and eastern margins of the Sapphire block are intensely
faulted and folded. Northwest trending, high-angle reverse faults dipping
to the southwest, and associated subparallel, gently-plunging drag folds
dominate structures in the northern boundary of the block (Desormier,
1975; Wallace, U.S.G.S. Open-File Report, 1976). Structures in the Lewis
and Clark line (Montana lineament) are overridden by the Sapphire block
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east of Missoula, Montana. Later reactivation of the Lewis and Clark
line during the Oligocene or Miocene time (Desormier, 1975) cut and off
set structures of the Sapphire block. Regional metamorphism is weak
along the northern boundary of the block.
Lithin the eastern structural zone, overturned, similar-style folds
with east-dipping axial planes are accompanied by west-directed thrusts
(Calkins and Emmons, 1915; McGill, 1965; Mutch, 1960). These "west-dir
ected" thrusts suggest that deformation within the eastern structural zone
may be associated with the intrusion of the Boulder batholith (Mutch, 1960)
or represent east-dipping erosional remnants or klippe of east-directed
thrusts. Intrusion of the Flint Creek plutons thermally metamorphosed
Belt, Paleozoic and Mesozoic rocks of the region and deformed pre-existing
folds ana thrust faults (Mutch, 1960; Hyndman and others, 1975a). Early-
Tertiary (Hughes, 1975) or mid-Tertiary (Mutch, 1960) north-south trending
normal faults cut all previous structures.
Folds involving Belt and Paleozoic sediments in the northern part of
the middle structural zone (Figure 4) are symmetrical-upright with west
dipping axial planes (Emmons and Calkins, 1913; Poulter, 1957). In the
south, plastic deformation resulted in tightly appressed recumbent struct
ures with west-dipping axial planes (Flood, 1974; t-.iswall, 1976). Tighter
structures to the south represent the deeper levels of tectonic develop
ment (Flood, 1974; Liswall, 1976).
kiswall (1976) recognizes three deformational events in the southern
Sapphire block; similar-to-isoclinal folding with the development of pene
trative axial-plane schistosity (Fj), concentric-style folding of schisto
sity on the mesoscopic scale (F£), and the development of a macroscopic
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structure (Fg) coaxial with the second deformational event. Wiswall sug
gests the observed deformational sequence is related to the movement of
the Sapphire block. As F^ structures in the block were driven to shallower
tectonic levels by the frontal buttressing in the Precairibrian basement
(Dillon block), folding became largely concentric in style (F^-F^). Re
gional metamorphism up to amphibolite-grade affected the extreme southern
sector of the block (Flood, 1974; Wiswall, 1976).
Plutons along the southern and eastern boundary of the block are of
two types; (1) synorogenic plutons of diorite to granitic composition
which show foliation and evidence of shearing caused by movement along
thrust planes (Flood, 1974; Hawley, 1975), and (2) post-orogenic porphy-
ritic granites injected along zones of structural weakness such as fract
ures and faults (Flood, 1974; Hughes, 1975).
The western margin of the Sapphire block consists of pelitic, quartzo
feldspathic and calc-silicate metasediments which are metamorphosed locally
up to amphibolite grade (LaTour, 1974), intruded by acidic plutons and
dikes, and later overlapped by acidic tuffs, flows, and breccias. Struct
urally the region is characterized by broad, open concentric-style folds,
which acted locally as structural loci for intrusion (Presley, 1970).
Recent mapping suggests that this region is more faulted than pre
viously thought. Apparently some large-scale thrusts are present (Wallace,
U.S.G.S. Open-File Report, 1976).
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Synopsis of Thermal-Structural History of the southern Sapphire Range
A synopsis of the regional events is presented below.
1) Sediments of the Late Precambrian Belt Supergroup and possibly
the pre-Belt basement were subjected to isoclinal folding (Fj)
and development of axial-plane schistosity. On the microscopic
scale, the growth of garnet prophyroblasts which cut across
schistosity suggests that metamorphism continued after the
development of foliation (S^).
2) Acidic plutons intruded overlying suprastructure prior to or
during F£ folding. A weak metamorphic foliation is imposed
on the plutons.
3) The isoclines were refolded about east-west trending axes (F2 ).
Metamorphic grade reached sillimanite-muscovite grade. On the
microscopic scale, schistosity wraps around garnet and staurolite
porphyroblasts. The S-^ foliation acted as slip planes for F2
flexural-slip folding. Axial-plane schistosity developed locally
in the pelitic units. The development of these east-west F2 folds
may have accompanied domal uplift and eastward movement of the
Sapphire block. As forceful emplacement of the rising mobile
infrastructure proceeded, the zone of cataclasis developed as a
result of the eastward movement of the suprastructure. As dif
ferential rates of movement in the block continued, the F2 folds
were possibly stretched and rotated into roughly east-west, nearly
horizontal orientations. Similar east-west folds are reported by
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Chase (1977) in the pelitic and quartzofeldspathic rocks near
the base of the zone of cataclasis. It seems likely, however,
that these east-west folds observed in the study area formed at
different structural levels and possibly different times from
those reported by Chase (1977).
4) During or after the eastward movement of the Sapphire block, late
Cretaceous-early Tertiary acidic plutons were injected along
the zone of cataclasis, thrusts faults, foliation planes, and
bedding-plane faults (Hughes, 1975) which acted as conduits for
intrusion. It has been postulated ( H y n d m a n , Talbot and Chase,
1975b; Hyndman, 1977) that these granitic magmas were either
detached segments of Idaho batholith magmas transported along
the shear zone, late granitic intrusives petrologically related
to the Idaho batholith, or a combination of these possibilities.
The plutons caused local doming or sagging of structures ( P r e s
ley, 1970). This intrusive phase resulted in, or was accompanied
by, broad open anticlinal folding (F^) i-n the study area. The
axis of the major fold, which plunges to the northeast, appears
curved as a result of heterogeneous folding or multiple defor
mation. The age of these events is estimated to by 60-80 m.y.
based on dating of acidic plutons in the Sapphire Range (Hughes,
1971).
5) High-angle block faults, features of retrograde metamorphism and
minor intrusions are present along the eastern margin of the
Bitterroot dome and Sapphire Range in the study area. Chase
(1977) suggests that these features represent an isostatic
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adjustment to tectonic denundation of the Bitterroot dome. Along
the eastern margin of the dome, high-angle faulting has brought
Precambrian Belt Supergroup (Wallace Fm.) against the cataclastic
shear zone resulting in a narrow north-south trending band of
mylonite. High-angle faulting has brought a foliated quartzo
feldspathic orthogneiss against Belt Supergroup metasediments in
the Sapphire Range. The age and genesis of the orthogneiss is
discussed below. Fluids migrating along these high-angle faults
resulted in retrograde alteration and secondary mineralization.
Aphanitic dikes, acidic tuffs, flows, and breccias of possible
Challis age (Eocene to Oligocene according to Armstrong, 1974)
are cut by high-angle faults suggesting ages ranging from 30-60
m.y.
6) Late Cenozoic tilting (regional ?) is evidenced by strike and
dip of locally fractured volcanic rocks.
Origin of the Quartzofeldspathic Orthogneiss
Three origins can be postulated for the foliated quartzofeldspathic
orthogneiss located along Sleeping Child Creek and other isolated locali
ties, The unit may represent a block of pre-Belt basement. Orthogneisses,
presumed to be pre-Belt and in fault contact with Belt metasediments, have
been described by Reid, Morrison and Greenwood (1973) and Armstrong (1975)
in the Clearwater orogenic zone of central Idaho. The linear nature of
the contact, sharp breaks in topography between the contact of ortho
gneiss and metasediments suggesting a fault scarp, and spatial distribu
tion of the orthogneiss implies faulting as the mechanism of emplacement.
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It is possible that the orthogneiss is an early intrusive phase of
the Idaho batholith which intruded the suprastructure and became foliated
along with the overlying suprastructure. Several similarily foliated
plutons have been described by Chase (1973), Nold (1974), and Wehernberg
(1972) in the northeast border zone of the Idaho batholith. Mineralog-
ical comparison of granitic and cataclastic rocks from the Idaho batho
lith and the orthogneiss located in the Sapphire Range is shown in Fig
ure 20 using the Streckeisen classification (1967). All three rock types
lie within the granodiorite field. A similar magma source is possible.
The orthogneiss could be a detached segment of similar body located
to the west which became foliated as it was carried eastward along the
zone of cataclasis. In such a model it would represent a weakly-sheared
upper sector of the zone of cataclasis as projected under the Sapphire
Range. Chase (1977) has described a structurally similar foliated quartz
diorite orthogneiss in the zone of cataclasis in the northeast border zone
of the Idaho batholith. Armstrong (1975) has described pre-Belt ortho
gneisses from the northwest part of the Salmon River Arch which lies 70-
80 km due west, not much farther than calculated 60 km movement of the
Sapphire tectonic block (Hyndman, 1977).
Textural, mineralogical, and structural evidence seems to support
the idea that the orthogneiss is petrologically related to the granitic
rocks of the Idaho batholith and was faulted into its present position.
However, dating of the unit will be necessary to determine the age rela
tionships of the orthogneiss.
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NoncatacIast i c I nter ior (I sample)
Zone of Cataclasis. (4 samples)
Q Quartzofeldspath ic Orthogneiss (3 samples)
T o n a I i t e 09
Dior i t e A
Figure 20. Plot of avt^age modes of quartzofeldspathic orthogneiss, granodiorite-granite suite from the non-cataclastic interior of the Idaho batholith and rocks from the zone of cataclasis along the Bitterroot Range front.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 66
Choice of Tectonic Models
The three tectonic models postulated in the introductory statements
outline the development of features observed in the Sapphire tectonic
block. Significant amongst these features are decreasing metamorphic
grade away from the Idaho batholith, intrusion of acidic plutons into the
block, folding within the block, and high-angle faults on the north and
east boundaries of the block. It was hoped that comparison of the struc
tural and thermal histories of the northeast border zone of the Idaho batho
lith and the present study area in the southern Sapphire Range would permit
descrimination between the three models.
At least one of the three models fails to explain the observed re
gional structures. If gravitional spreading had been the primary mechanism
for the development of the Sapphire block, spreading away from the Bit
terroot dome and the development of structures associated with spreading,
such as the zone of cataclasis, should form a radial pattern around the
dome. However, the zone of cataclasis does not swing west along the north
ern boundary of the Idaho batholith, but dies out along the eastern margin
of the northern Bitterroot Range (lehrenberg, 1972). Furthermore, the
great thickness of the zone argues for a large magnitude movement of the
Sapphire block.
Results of this study do not allow a distinction between syn-tectonic
regional thrusting or detachment and gravitational sliding from the mobile
infrastructure.
The isoclinal folding (F-) indicates that the units were deformed in
a passive field under plastic deformation (Donath and Parker, 1964) and
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depth of burial sufficient to reduce the viscosity of the Belt sediments.
The peripheral or interior infrastructure seems a likely environment for
plastic deformation and passive flow folding.
Concentric folding (F2-F2) indicates the suprastructure (Sapphire
tectonic block) was later deformed at a higher tectonic level or at lower
temperature and was subjected to more-brittle deformation and flexual-
flow folding (Flood, 1974; Wiswall, 1976). Movement of the suprastruc
ture to higher tectonic levels could have been the result of a rising
mobile infrastructure. As the suprastructure became gravitionally un
stable, the block began sliding to the east off the infrastructure and
buttressing resulted against the Dillon block in the southeastern part
of the Sapphire block (Wiswall, 1976). A sliding model best explains
the complex regional fold and fault patterns in the southern Sapphire
Range. However, if thrusting was syn-tectonic to the intrusion of the
Idaho batholith, the regional thrust model could equally explain the
presently observed structures of the Sapphire block.
A study of several unsolved problems within the Sapphire block could
provide meaningful data for a resolution of this problem. These include;
Cl) a structural analysis to reveal the magnitudes and directions of stress
which produced the structures observed in the toe and lateral margins of
the block, (2) determination of relative movements along thrust faults
which define the northern boundary of the block to determine if lateral
movements have occurred, and C3) comparative geochemistry and dating of
foliated and nonfoliated plutons on opposite sides of the zone of cata
clasis.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CONCLUSIONS
Pelitic, quartzofeldspathic and calc-silicate metasediments of the
Precambrian Belt Supergroup in the Sapphire Range have been subjected to
one regional metamorphic event and at least three periods of deformation.
The rock units display mineral assemblaes characteristic of sillimanite-
muscovite-grade metamorphism.
deformation was a penetrative event which produced a mica schisto-
sity parallel to lithologic layering. The development of isoclinal folds
(F-^) with high amplitude/wavelength ratios is suggested by the occasional
angular relationships between schistosity and layering observed in some
samples on the microscopic scale. Flexural-slip folding of layering and
mica schistosity (F?) resulted in the development of mica crenulation
lineation which parallels F£ fold axes. Locally, axial-plane schistosity
developed in the pelitic units. A third deformation (F^) resulted in
broad, open folds which trend to the northeast. Heterogeneous folding
during the Fg event, or a later deformational event possibly related to
the tilting of volcanics within the study area, resulted in curving of
the F2 fold axes.
Three models can be suggested to explain the observed thermal-
structure history of the Sapphire tectonic block, (1) eastward thrusting
just prior to, or during, the intrusion of the Idaho batholith, (2) local
shearing at the infrastructure-suprastructure boundary accompanied by
gravitational spreading at the margins of the block, and (3) detachment
and eastward movement of the suprastructure (Sapphire block) off a rising
68
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 69
infrastructure (Idaho batholith). Of the three models, detachment of
the suprastructure and movement down the east flank of the rising in
frastructure best fits the regional thermal-structural data now avail
able. However, syn-intrusion regional thrusting cannot be ruled out.
In addition to uncertainties in the structural evolution, several
problems remain unsolved in the southeastern Sapphire Range. These in
clude; (1) the age and structural relationship of the quartzofeldspathic
orthogneiss, (2 ) the similarity of structural styles reported in the
northeast border zone of the Idaho batholith to those observed in the
southwestern Sapphire Range, (3) stratigraphic correlation of metasedi
ments within the study area to their unmetamorphosed equivalents, and (4)
the timing and parental origin of the numerous granitic plutons located
throughout the study area.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 70
APPENDIX 1
Noncataclastic interior from the Idaho batholith (1000 counts per slide)
Slide No. LHC 51
Quartz 244
Plagioclase 448
Composition of Plagioclase ^25
Myrmekitic plagioclase 20
Potassium feldspar 240
Biotite 42
Muscovite 5
Chlorite Tr
Opaque Minerals 1
Apatite Tr
Zircon Tr
Sericite Tr
Sphene Tr
Rutite Tr
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 Tr Tr Tr
25 1 6 1 Tr Modal Analyses of zone of strong cataclasis along Bitterroot
2 1 1 Tr 1 7 7 4 Range front (1000 counts per slide) 13 13 Tr 7 Tr Tr Tr Tr Tr Tr 44 44 Tr Tr Tr Tr 61 61 74 67 260 260 213 238 201 201 232 217 An25 An25 An26 An24 411 411 471 441 LHC 1 LHC 1 LHC 11 Average mode
Lpidote Sericite Rutile Sphene feldspar Biotite Slide No. Chlorite Zircon Opaque plagioclase Potassium Apatite Quartz Plagioclase Composition of plagioclase Minerals Muscovite Myrmekitic
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ro
1 2 9 5 52 Tr Tr Tr Tr Tr Tr 249 240 441 An25
Average mode
3 7 Z 5 43 Tr Tr Tr Tr Tr 244 241 452 An ^ 2 ^ An LHC 31
4 13 61 Tr Modal Analyses of zone of Tr Tr Tr Tr Tr 253 239 Range front (1000 counts per slide) 430 weak cataclasis along Bitterroot A n 2 9 LHC 13
plagioclase Sphene Lpidote Sericite Slide No. Quartz plagioclase Potassium feldspar Biotite Rutile Chlorite Plagioclase Composition of Opaque minerals Zircon Myrmekitic Apatite Muscovite
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. u> Tr Tr 136 116 138 606 325
17 70 Tr Tr 63 Tr 117 433 541 An39 An32 8 4 71 20 Tr Tr Tr 4 Tr Tr 154 237 191 375 An27 slide) 1 1 2 12 Tr 152 368 An31 376 211 588 counts per in in the Sapphire Range 1 1 1 (1000 11 1 g 2 1 53 2 Tr Tr 190 371 1263 An An
Modal analyses of quartzofeldspathic gneiss Slide No. Biotite Composition of Epidote Quartz 153 feldspar 226 243 Sphene plagioclase Sericite Potassium Sillimanite 63 Plagioclase Chlorite Apatite Muscovite Opaque minerals Zircon
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 2 20 Tr Tr
1 1 1 1 1 2 5 2 2 1 8 Tr Tr Tr 54 54 117 190 190 251 158 158 266 926 926 584 Average Mode 330 An34 An32 1 4 CO 2 CM 161 239 249 counts per slide) C < (1000 1 4 1 3 18 11 178 344 An31 833 980 Modal analyses of quartzofeldspathic gneiss 1 1 14 Tr Tr 87 Tr 192 226 600 An34
Slide No. Sphene Biotite feldspar 400 184 Epidote Sericite Sillimanite Composition of plagioclase Potassium Zircon Chlorite 4 Quartz Opaque minerals Plagioclase 267 Apatite Muscovite
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 75
Modal analyses of pelitic schist in the Sapphire Range (1000 counts per slide)
Slide No. 1223 1082 528 1340
Quartz 63 98 78 43
Plagioclase ~r/, o o 138 270 310
Composition of plagioclase An25 An2 ^ ^ 3 0 An£g Potassium feldspar 43 79 196 Tr
Biotite 148 137 128 163
Muscovite 66 170 102 112
Sillimanite 215 227 140 236
Garnet 16 58
Sphene 13
Opaque minerals Tr 2 Tr 12
Chlorite Tr 146 70 64
Zircon Tr Tr
Apatite Tr Tr Tr
Clinozoisite Tr
Sericite 30 1 Tr 2
Pmtile Tr Tr
Staurolite
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Modal analyses of pelitic schist in the Sapphire Range (1000 counts per slide)
Slide No. 82 1220 557 Average Mode
Quartz 166 243 70 109
Plagioclase 318 248 303 287
Composition of
plagioclase An 27 An3 l £ CO An28 Potassium feldspar 167 117 117 103
Biotite 15 149 210 136
Muscovite 91 111 156 115
Sillimanite 242 118 120 185
Garnet 11
Sphene Tr Tr 2
Opaque minerals 1 5 1 3
Chlorite Tr 2 2 41
Zircon Tr
Apatite Tr Tr Tr Tr
Clinizoisite 2 6 1
Sericite Tr 3 2 5
Rutile Tr
Staurolite 13 2
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
6 3 6 4 3 10 Tr Tr 30 533 406 An53 Average Mode 3 2 2 34 Tr 55 1281 483 474 An 12 Tr Tr 143 539 304 1268 An48 3 1 6 5 13 26 Tr Tr 554 392 1279 An51 in in the Sapphire Range (1000 counts per slide) Modal analysis of calc-silicate 57 Tr 544 943 446 An 52
feldspar Sphene Sericite 25 Scapolite Slide No. 947 Hornblende Composition of Diopside Clinozoisite 543 10 plagioclase An Actinolite Apatite 10 Potassium Quartz Plagioclase 412
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 00 Tr Tr 6
An6g 4 4 6 8 8 14 14 7 70 Tr Tr 2 Tr Tr Tr Tr Tr 2 102 102 63 342 342 456 405 405 520 Average Mode 456 An in in the Sapphire Range (1000 counts per slide) Modal analyses of Amphibolite ^n66
Slide No. 482 Epidote Scapolite Tr Tr Sphene 5 Biotite 23 Plagioclase 392 Quartz 5 Sericite Composition of plagioclase Clinozoisite Chlorite Opaque Zircon Apatite Tr 5 Hornblende 570 minerals Tr
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. VO 2 1 7 2 Tr 89 Tr 85 Tr 130 674 An28 Average 6 10 10 Tr 66 Tr Tr 120 743 An30 in in the Sapphire Range (1000 counts per slide) Modal Analyses of Quartz Diorite 4 4 1 4 13 64 114 Tr Tr 139 104 664 684 952 An26
Sericite plagiolase Sphene feldspar Apatite Chlorite Zircon Slide No. Biotite Quartz Composition of plagiolase Plagioclase Potassium Opaque minerals Muscovite Myrmekitic
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 3 4 3 Tr Tr Tr Tr Tr 518 An34 Average Mode 6 5 27 17 Tr Tr 34 28
An36 SC34
2 2 1 3 Tr 375 328 280 165 152 142 934 4 6 Tr Tr Tr 58 5 269 An32 An37 counts per slide) Sapphire Range granodiorite in the 8 (1000 8 8 6 Tr 22 Tr 82 124 172 Modal analyses of Quartz Monzonite- 13 14 29 Tr 36 256 189 463 702 531 447 448 SC40 1205 SC7 An30 An33
Sericite Clinozoisite Sphene Chlorite Rutile Apotite Zircon feldspar Biotite plagioclase plagioclase Opaque minerals Potassium Composition of Slide No. Muscovite Quartz Plagioclase Myrmekitic
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. oo 2 5 2 3 41 Tr 240 217 486 An Average Mi
7 7 Tr Tr 37 445 272
8 10 36 7 524 150 An An SK7 counts per slide) Modal analyses of in in the Sapphire Range (1000 Quartzofeldspathic orthogneiss Tr 5 Tr Tr Tr Tr Tr SK25
Biotite 80 ZirconSericite Tr Tr Opaque minerals Epidote feldspar 191Chlorite 272 257 Composition of plagioclase An plagioclase Tr Slide No. Potassium Apatite Muscovite Tr Quartz 230 Plagioclae 489 Myrmekitic
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•• j-iin 0C W
f( o k , -yN f - ^ 4 ? « ♦ 4- % r described described thin sections^ Appendix Appendix I. Geologic sketch map showing the location of
sSSs.*?^ - '
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. REFERENCES CITED
Anderson, R. E., 1959, Geology of lower Bass Creek Canyon, Bitterroot Range, Montana (M. S. thesis): Univ. Montana, Missoula, Mont. 70 p.
Armstrong, R. L., 1974, Geochronometry of the Eocene volcanic plutonic episode in Idaho: Northwest Geology, v. 3, p. 1-15.
Armstrong, R. L. , 1975, Precambrian (1500 m.y. old) rocks of central Idaho - The Salmon River Arch and its role in cordilleren sedimentation and tectonics: Am. Jour. Sci., v. 275-A, p. 437-467.
Axelrod, D. I., 1968, Tertiary floras and topographic history of the Snake River basin, Idaho: Geol. Soc. America Bull., v. 79, p. 713- 734.
Berg. R. B., 1968, Petrology of anorthosites of the Bitterroot Range, Montana: in Origin of anorthosites and related rocks, a symposium: New York State Mus. and Sci. Serv. Mem. 18, p. 387-398.
Calkins, F. C., and Emmons, k. H . , 1915, Description of the Philipsburg quadrangle, Montana: U.S. Geol. Survey Geol. Atlas, Folio 196, 25 p.
Chase, R. B . , 1973, Petrology of the northeastern border zone of the Idaho batholith, Bitterroot Range, Montana: Montana Bur. Mines and Geology Mem. 43, 28 p.
Chase, R. B., 1977, Structural evolution of the Bitterroot dome and zone of cataclasis: in Geol. Soc. of America Guidebook No. 1, Rocky Mountain Sec., 1977, p. 1-24.
Chase, R. B., and Johnson, B. R., 1977, Border-zone relationships of the northern Idaho batholith: Northwest Geology, v. 6-1, p. 38-50.
Cheney, J. T., 1972, Petrologic relationships of layered meta-anorthosites and associated rocks, Bass Creek, western Montana (M. S. thesis): Uni.. Montana, Missoula, Mont., 112 p.
Csejtey, B. , 1963, Geology of the southeastern flank of the Flint Creek Range, western Montana (Ph. D. thesis): Princeton Univ. Princeton, N. J. 175 p.
Desormier, I-., , 1975, A section of the northern boundary of the Sapphire tectonic block, Missoula and Granite Counties, Montana (M.S. thesis): Univ. Montana, Missoula, Mont., 65 p.
Donath, F. A., and Parker, R. B. , 1964, Folds and folding: Geol. Soc. of America Bull., v. 75, p. 45-62.
Emmons, k. H., and Calkins, F. C., 1913, Geology and ore deposits of the Philipsburg quadrangle, Montana: U.S. Geol. Survey Prof. Paper 78, 271 p.
83
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Engel, A. E. J . , 1949, Studies of cleavage in the metasedimentary rocks of the northwest Adirondack Mountains, New York: Trans. Am. Geophys. U n . , v. 30, p. 767-784.
Evans, B. k’. , 1965, Application of a reaction-rate method to the break down equilibria of muscovite and muscovite plus quartz: Am. Jour. Sci., v. 263, p. 647-667.
Flood, R. E. , 1974, Structural geology of the Upper Fishtrap Creek area, central Anaconda Range (M. S. thesis): Univ. Montana, Missouls, Mont., 71 p.
Greenwood, k. R., and Morrison, D. A., 1973, Reconnaissance geology of the Selway-Bitterroot kilderness Area: Idaho Bur. Mines and Geology Pamphlet 54, 30 p.
Groff, S. L., 1954, Petrography of the Kootenai Creek area, Bitterroot Range, Montana (M. A. thesis): Univ. Montana, Missoula, Mont. 80 p.
Guidotti, C. V., 1970, The mineralogy and petrology of the transition from the lower to upper sillimanite zone in the Qquossoc area, Maine: Jour. Petrology, v. 11, p. 277-336.
Gwinn, V. E . , and Mutch, T. A., 1965, Intertongued Upper Cretaceous vol canic and nonvolcanic rocks, central-western Montana: Geol. Soc. America bull., v. 76, p. 1125-1144.
Hall, F. k. , 1968, Bedrock geology, north half of Missoula 30-minute quadrangle (Ph. D. thesis): Univ. Montana, Missoula, Mont. 253 p.
Harrison, J. E., 1972, Precambrian Belt basin of northwestern United States: Its geometry, sedimentation, and copper occurrences: Geol. Soc. America Bull., v. 83, p. 1215-1240.
Hawley, K . , 1975, The Racetrack pluton - a newly defined Flint Creek pluton: Northwest Geology, v. 4, p. 1-8.
Holdaway, M. J. , 1971, Stability of andalusite and the aluminum sili cate phase diagram: Am. Jour. Sci., v. 271, p. 97-131.
Hoschek, G., 1969, The stability of staurolite and chloritoid and their significance in metamorphism of pelitic rocks : Contrib. Mineralogy and Petrology, v. 22, no. 3, p. 208-232.
Hughes, G. J . , 1971, Petrology and tectonic setting of igneous rocks in the Henderson-tallow Creek igneous belt, Granite County, Montana (Ph. D. thesis): Mich. Tech. Univ., Houghton, Mich., 236 p.
Hughes, G. J . , 1975, relationships of igneous rocks to structure in the Henderson-killow Creek igneous belt, Montana: Northwest Geology, v. 4, p. 15-25.
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Hyndman, D. lv. , 1968, Mid-Mesozoic multiphase folding along the border of the Shuswap metamorphic complex: Geol. Soc. America Bull., v. 79, p. 575-588.
Hyndman, D. 1.'., and others, 1975, Petrogenesis of the Philipsburg batho- lith-Bimetallic stock, Flint Creek Range, western Montana: Montana Bur. Mines and Geology Open-File Rept., 95p.
Hyndman, D. V. , 1977, Mylonitic detachment zone and the Sapphire Tectonic Block: Geol. Soc. America Guidebook No. 1, Rocky Mountain Sec., 1977, p. 25-31.
Hyndman, D. lv. , 1979, the Bitterroot dome - Sapphire tectonic block, and example of a plutonic-core gneiss dome complex with its detached suprastructure, _in Cordilleran Metamorphic Core Complexes: P. J. Coney, M. Crittenden, Jr., and G. H. Davis, editors, GSA Mem., in press.
Hyndman, D. lv. , Obradovich, J. D., and Ehinger, R. , 1972, Potassium argon age determinations of the Philipsburg batholith: Geol. Soc. America Bull., v. 83, p. 473-474.
Hyndman, D. lv. , Talbot, J. L. , and Chase, R. B. , 1975, Boulder batholith: A result of emplacement of a block detached from the Idaho batholith infrastructure?: Geology, v. 3, p. 401-404.
Jens, J. C . , 1974, A layered ultramafic intrusion near Lolo Pass, Idaho: Northwest Geology, v. 3, p. 38-46.
Jerome, N. H. , 1968, Geology between Miller and Eightmile Creeks, northern Sapphire Range, western Montana (M. S. thesis): Univ. Montana, Missoula, Mont., 49 p.
Jones, A. G . , 1959, Vernon map area, British Columbia: Canada Geol. Survey Mem. 296, 186 p.
Kauffman, M. E . , and Earll, R. N., 1963, Geology of the Garnet-Bearmouth area, western Montana: Montana Bur. Mines and Geology Mem. 39, 40 p.
Langton, C. M. , 1935, Geology of the northeast part of the Idaho batholith and adjacent region in Montana: Jour. Geology, v. 43, no. 1, p. 27-60.
Larsen, E. S., Jr., and Schmidt, R. G., 1958, A reconnaissance of the Idaho batholith and comparison with the southern California batho lith: U. S., Geol. Survey Bull. 1070-A, 33 p.
LaTour, T. E., 1974, An examination of metamorphism and scapolite in the Skalkaho region, southern Sapphire Range, Montana (M. S. thesis) : Univ. Montana, Missoula, Mont., 95 p.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 86
Leischner, L. M., 1959, Border-zone petrology of the Idaho batholith in the vicinity of Lolo Hot Springs, Montana (M. S. thesis): Univ. Montana, Missoula, Mont., 76 p.
Lindgren, I.., 1904, A geological reconnaissance across the Bitterroot Range and Clearwater Mountains in Montana and Idaho: U.S. Geol. Survey Prof. Paper 27, 123 p.
Maxwell, J. C., 1965, Geologic map and cross sections, southwest Drummond area: _in Billings Geol. Soc. Guidebook, 16th Ann. Field Cong., .1965, in pocket.
McGill, G. E., 1959, Geology of the northwest flank of the Flint Creek Range, western Montana: Montana Bur. Mines and Geology Spec. Pub. 18, Map 3.
McMurtrey, R. G. , and Konizeski, R. L . , 1956, Progress report on the geo logy and ground-water resources of the eastern part of the Bitter root Valley, Montana: Montana Bur. Mines and Geology Inf. Circ. 16, p. 1-8.
Montgomery, J. K., 1958, Geology of the Nimrod area, Granite County, Montana (M. S. thesis): Univ. Montana, Missoula, Mont., 61 p.
Mutch, T. A., 1960, Geology of the northeastern flank of the Flint Creek Range, Montana (Ph. D. thesis): Princeton Univ., Princeton, N. J., 150 p.
Nelson, W. H . , and Dobell, J. P., 1961, Geology of the Bonner quadrangle, Montana: U.S. Geol. Survey Bull. 1111-F, p. 189-235.
Newton, R. C., 1966, Kyanite-andalusite equilibrium from 700° to 800° C: Science, v. 153, p. 170-172.
Noel, J . , 1956, Geology of the east end of the Anaconda Range and adjacent areas, Montana (Ph. D. thesis): Indiana Univ., Bloomington, Indiana, 74 p.
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Presley, M. lv. , 1970, Igneous and metamorphic geology of the Willow Creek drainage basin, southern Sapphire Mountains, Montana (M. S. thesis): Univ. Montana, Missoula, Montana, 64 p.
Reid, R. R. , Morrison, D. A., and Greenwood, W. R . , 1973, The Clearwater orogenic zone : A relict of Proterozoic orogeny in central and north ern Idaho: Belt symposium, Idaho Bur. Mines and Geology, v. 1, p. 1-56.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 87
Richardson, S. V., Gilbert, M. C., and Bell, P. M . , 1969, Experimental determination of kyanite-andalusite and andalusite-sillimanite equilibria; the aluminum silicate triple point: Am. Jour, sci., v. 267, p. 259-272.
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Scholten, R., and Onasch, C. M . , 1977, Genetic relations between the Idaho batholith and its deformed eastern and western margins: North west Geology, v. 6-1, p. 25-37.
Spry, A., 1969, Metamorphic textures: New York, Pergamon Press Ltd., 350 p.
Streckeisen, A., 1967 Classification and nomenclature of igneous rocks: Neues Jahrb. Mineral. Abhandl., v. 107, p. 144-240.
Talbot, J. L., and Hyndman, D. W. , 1973, Relationship of the Idaho batho lith structures to the Montana lineament: Northwest Geology, v. 2, p. 48-52.
Tilling, R. I., Klepper, M. R., and Obradovich, J. D., 1968, K-Ar ages and time span of emplacement of the Boulder batholith, Montana: Am. Jour. Sci., v. 266, p. 671-689.
Lallace, C. A., and Klepper, M. R . , 1978, Preliminary reconnaissance geo logic map of the Cleveland Mountain and north half of the Ravenna quadrangles, western Montana: U.S. Geol. Survey Open-File Rept. 76-527.
Leber, L. M., 1972, Correlation of Pleistocene glaciation in the Bitter root Range, Montana, with fluctuations of Glacial lake Missoula: Montana Bur. Mines and Geology Mem. 42, 42 p.
Lehrenberg, J. P., 1972, Geology of the Lolo Peak area, northern Bitter root Range, Montana: Northwest Geology, v. 1, p. 25-32.
Weill, D. F. , 1966, Stability relations in the A^Og-SiC^ system calculated from solubilities in the AlgOg-SiOgNagAIFg system: Geochem. et Cosmo- chim. Acta, v. 30, p. 223-237.
White, B. G., 1969, Structural analysis of a small area in the northeast border zone of the Idaho batholith, Idaho (M. S. thesis) : Univ. Montana, Missoula, Mont., 51 p.
Williams, R. D., 1975, Structural and metamorphic geology of the Bass Lake area, northern Bitterroot Range (M. S. thesis) : Univ. Montana, Mis soula, Mont., 53 p.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 88
tdswall, C. G. , 1976, Structural styles of the southern boundary of the Sapphire Tectonic Block, Anaconda-Pintlar lilderness Area, Montana (M. A. thesis); Univ. Montana, Missoula, Mont., 62 p.
hoodland, B. G., 1965, The geology of the Burke quadrangle, Vermont: Vermont Geol. Survey Bull. 28, 151 p.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PLATE 1
17 30
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TERTIARY?
UPPER
CRETACEOUS
PRECAMBF I AN •u» GEOLOGIC MAP OF EASTERN BORDER ZONI OF THE IDAHO BATHOLITH, BITTERROOT RANGE; AND SOUTHERN SAPPHIRE RANGE, MONTANA I
LITHOLOGIC DESCRIPTIONS
KB Quaternary alluviian: coarse-grained, poorly sorted strea gravels □ and sands.
Quaternary stream terraces: crudely stratified sand with cut-and- flll structures and cross-bedded remnant of the Hamilton, Dutch Hill, and Riverside terraces.
7*30 Glaclofluvlal deposits: lateral and end moraines, till, outwash deposits and sparse remnants of glaclolacus- i trlne detritus. TEST I ARY? Extrusive and Intrusive rocks: porphyritlc latite Cuff, red to pink rhyodacite, and welded tuff; slightly porphtritlc, iron-stained rhyodacllte $ and quart2 latite.
I ::: Small bodies of intrusive rock: quartz monzonlte, granodiorite and tonallte with dominant hypidiooorphic- to allotromorphlc-granular texture, isc tropic fabric; and minor crosscutting apllte and pegmatite dikes*
Quartz dlorite: Homblende-blotite-bearlng, medlum-to-coarse grained* hypidlomorphlc-granular rock with isotropic fabric. m Mylonlte: highly sheared and retrograde metamorphosed granitic rocks cu UPPER by mnerous microfaults and fractures.
Zone of strong cataclasls: highly sheared augen gneiss sector of tte CRETACEOUS Idaho bethollth.
Zone of weak cataclasls: weakly sheared fabric transitional between xoc of strong cataclasls and massive batholith.
Idaho batholith: blotlte-bearlng quartz monzonlte and granodlotlte with dominant hypidlomorphlc-granular texture; locally foliated.
Amphibolies: gray-black, concordant, elongate bodies containing distinct hornblende llneatlon and schlstoslty.
Meeasedlments: quartxlte, quartzofeldspathlc gneiss and politic schist PfiECAMBFIAN which dominate the northern map area: calc-sillcate gneiss interbedded with quartzlte and politic schist tc the south.
Quartzofeldspathlc orthogrteiss: biotite-quartz-feldspar rock with weak blotlte schlstoslty and elongate quart: clusters parallel to schlstoslty. *** % % *
LITHOLOGIC CONTACTS
Exact
Approximate
Inferred
Gradational textile boundary
AREAL SYMBOLS Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced withPermission 0897^354 l
m a n
67 ai 12*30 7*30
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UPPER
CRETACEOUS
d
PREC AMBRI AN
1
** .*;•» &
T .4 H.
T .3 K.
>.'.. >*>V: •' rv *vr*,-» 2 30
R .20 I./ IR.I3 ■ 43*00 2*30 114*00
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Quartz diorite: Hornblende-blotlte-bearlng, tacdlwco-coarse grained, hypidlomorphlc-granular rock with isotropic fabric. UPPER Mylonlte: highly sheared and retrograde metamorphosed granitic rocks cut by numerous microfaults and fractures.
Zone of strong cataclasls: highly sheared augrn gneiss sector of tfta CRETACEOUS Idaho batholith.
Zone of weak cataclasls: wakly sheared fabric transitional between z o o s of strong cataclasls and massive batholith.
Idaho batholith: biotite-bearing quartz monzonlte and granodlotlte with dominant hypidlomorphlc-granular texture; locally foliated.
Amphlbollte: gray-black, concordant, elongate bodies containing distinct 0 hornblende llneaclon and schlstoslty.
Metasediments: quartzlte, quartzofeldspathlc gneiss and politic schist PRECAMBR I AN which dominate the northern map area: calc-silicata ^ gneiss Interbedded with quartzlte and pelltlc schist to Che south.
Quartzofeldspathlc orthogrtelss: blotlte-quartz-feldspar rock with weak blotlte schlstoslty and elongate quartz clusters parallel to schlstoslty.
LITHOLOGIC CONTACTS
------Exact
_ . — - Approximate
Inferred
Gradational texture boundary
a r e a l s y m b o l s
. Mountain peaks
STBllCTtSE SYMBOLS 2 30
X Strike end dip of mica schlsCoaiCy in distinctly foliated onite. Vertical dip Trend and plunge of mica streak llncztism.
_ T Fault: bar and ball on down thrown side.
r 0 Fold hinges
i I
s c jl c l i i w w
43*00 »*CT U CLMB, 1 114*00
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PLATE 2
A West
UJ
2000
1000
0
STRUCTURE SECTIONS CORRESPONDS
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D N S CORRESPONDING TO AA' AND BET FROM PLATE 1
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South
I PLATE 1 I • .5 • 1 (ILMIT1I I 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.