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Graduate Student Theses, Dissertations, & Professional Papers Graduate School
1981
Petrology and Origin of Archean Lithologies in the Southern Tobacco Root and Northern Ruby Ranges of Southwestern Montana
Michael L. Wilson The University of Montana
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Mansfield Library University of Montana Date: (" 1 9 8 1
PETROLOGY AND ORIGIN OF ARCHEAN LITHOLOGIES
IN THE SOUTHERN TOBACCO ROOT AND NORTHERN RUBY RANGES
OF SOUTHWESTERN MONTANA
by
Michael L. Wilson
B.A. , University of Montana, 1979
Presented in partial fu lfillm e n t of the requirements for the degree of
Master of Science
UNIVERSITY OF MONTANA
1981
Approved by:
t r U u / 'airman , Bo a ref o f c
D &m , Sraduate Scho UMI Number: EP72604
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ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 ABSTRACT
Wilson, Michael L. , M.S., Spring, 1981 Geology
Petrology and Origin of Archean Lithologies in the Southern Tobacco Root and Northern Ruby Ranges of Southwestern Montana Director: Donald W. Hyndman Jo rJ This study focuses on the parentage of an Archean assemblage exposed within tig h t synforms in the Copper Mountain area of the southern Tobacco Root Mountains and Kelly area of the northern Ruby Range. This assemblage is typical of much of the poorly understood Montana basement terrane, and is composed of: migmatic quartzofeldspathic gneiss, dolomitic marble, massive to banded amphibolite, micaceous quartzite, pelitic schist, pure quartzite or chert, massive and banded iron-formation, and small conformable fragments of peridotite and pyroxenite. Foliation parallels composition layering within, and the litho lo gic contacts between units, and may reflect an original layering. At least two periods of folding around north-northeast axes affected both areas. Tight isoclinal F] folds are common through out the quartz-feldspar gneiss, amphibolite, and iron-formation. The overall synformal structure of both areas represent F£ folding. Textural and mineralogical criteria indicate that prograde metamorphism straddled the upper-amphibolite lower-granulite grade boundary with temperatures in the 700-800°C range and pressures of 4-8 k-bars. Granulite mineralogies in mafic and ultramafic lithologies represent rocks which were locally water undersaturated. Greenschist-facies assemblages are developed across both areas and may represent a greensehist-grade thermal event which reset K-Ar clocks at 1600 Ma. Field,petrographic, and geochemical crite ria suggest the dominant quartzofeldspathic gneisses originated as a thick sequence of feldspathic sands interstratified with silicic and mafic volcanic rocks. Dolomitic marble was deposited as a siliceous calcareous ooze. Massive and banded amphibolite show nearly identical compositions which are consistent with metamorphosed flows of basalt to basaltic andesite composition. Micaceous quartzites and p e litic schists represent terrigineous influx of feldspathic sands and aluminous muds, whereas pure quartzite lenses most probably represent cherts. Iron-formations f i t into the volcanogenic Algoma-type and may have been deposited as ferriginous-si1iceous brines associated with submarine volcanism. Ultramafic masses were conformably emplaced as tectonic fragments prior to or during prograde metamorphism and deformation. A Franciscan-type fore-arc environment provides an excellent depositional and deformational model for the assemblage exposed in the Copper Mountain and Kelly areas, and explains lith o lo g ic and structural variation across the Montana basement terrane. ACKNOWLEDGMENTS
My thanks are extended to,
Dr. Don Hyndman for his guidance and friendship
Dr. Dave Fountain for his constructive criticism and insight in the Archean
Dr. Tim LaTour for discussions and observations in the field
Dr. Hal James for his excellent mapping
Dr. Peter Hooper for the use of XRF fa c ilitie s at Washington
State University
Sigma Xi, The National Scientific Society, for a grant-in-aid of research
My father for his able assistance in the field
Steve Luthy and several other graduate students, for many hours of discussion
Jon Achuff and his pi diagram computer program
Ray Larsen for preserving my sanity
My mother for her patience
And Gina Schmidt for her constant optimism, energy, and beauty. TABLE OF CONTENTS
Page
ABSTRACT ...... ii
ACKNOWLEDGMENTS ...... i i i
LIST OF TABLES...... vi
LIST OF FIGURES...... v ii
LIST OF PLATES...... v iii
CHAPTER
I. INTRODUCTION ...... 1
II. FIELD AND PETROGRAPHIC DESCRIPTION OF LITHOLOGIES. 6
QuartzofeldspathicGneiss ...... 7
Dolomitic Marble ...... 13
Amphibolite ...... 15
Pelitic Schist ...... 19
Quartzite ...... 21
Iron-Formation ...... 26
Ultramafic Rocks ...... 31
P e g m a tite ...... 34
Meta-diabase ...... 35
Paleozoic Rocks ...... 36
Cretaceous-TertiaryIntrusions ...... 37
I I I . METAMORPHISM...... 39
IV. STRUCTURE ...... 47
Copper Mountain A re a ...... 49
Kelly A re a ...... 50
i v CHAPTER Page
V. PROTOLITHS AND INTERPRETATIONS ...... 57
Quartzofeldspathic Gneiss ...... 58
Dolomitic Marble ...... 61
Amphibolite ...... 61
Pelitic Schist ...... 63
Impure and Pure Q u a r t z it...... e 66
Iron-formation ...... 69
Ultramafic Rocks ...... 73
Tectonic Interpretations ...... 73
VI. CONCLUSIONS...... 80
REFERENCES...... 83
APPENDIX 1: Location of X-R-F Samples ...... 91
v LIST OF TABLES
Table Page
1. Visually estimated modes for study-area lithologies . .11
2. Visually estimated modes for study-area lithologies . . 33
3. Comparison of folding events postulated for the Tobacco Root and Ruby R a n g e s ...... 48
4. Major-element analyses of quartzofeldspathic gneiss . . 59
5. Major-element analyses of amphibolite ...... 64
6. Major-element analyses of micaceous and pure quartzite or c h e rt...... 67
7. Major-element analysis and modally estimated chemical compositions of iron-formations ...... 70
vi LIST OF FIGURES
Figure Page
1. Index map of southwestern Montana ...... 4
2. Photograph of biotite-quartz-feldspar gneiss .... 8
3. Photograph of hornblende-quartz feldspargneiss . . . 8
4. Photograph of banded amphibolite ...... 17
5. Photograph of massive folded amphibolite ...... 17
6. Photograph of chert ridge ...... 23
7. Photograph of bedded chert outcrop ...... 23
8. Photograph of iron-formation outcrop ...... 27
9. Photograph of F 2 folds in iron-formation ...... 27
10. Cartoon of lithologic section surrounding iron-formations ...... 28
11. Pressure-temperature diagram showing prograde c o n d itio n s ...... 45
12. Map of the southern Tobacco Root M o u n ta in s ...... 51
13. Pi diagram of poles to fo lia tio n in the Copper Mountain a r e a ...... 52
14. Map of the Northern Ruby Range showing major structural domains ...... 54
15. Pi diagrams of poles to folia tion in the Kelly a r e a ...... 56
16. Niggli plots of analyzed am phibolites ...... 65
17. Schematic diagram showing possible tectonic reconstruction of the Montana basement ...... 76
v i i LIST OF PLATES ate Page
1. Geologic map of The Copper Mountain area, southern Tobacco Root Mountains ...... in pocket
2. Geologic map of The Kelly area, northern Ruby Range ...... in pocket CHAPTER I
INTRODUCTION
The geology of southwestern Montana is characterized by a series of uplifted ranges which are cored with a complexly deformed Archean supracrustal assemblage. Pre-metamorphic parentage and depositional environment are poorly understood for these sequences which consist of varying proportions of quartzofeldspathic gneiss, amphibolite, dolomitic marble, pelitic schist, quartzite, and iron-formation.
Rb-Sr analyses of gneisses (Mueller and Cordua, 1976; James and
Hedge, 1980) show that in itia l metamorphism of these strata occurred about 2750 Ma. A second thermal event reset mica and amphibole
K-Ar ages at 1660 Ma (G ile tti, 1966, 1971). Metamorphic grade ranges from greenschist to lower-granulite facies, with upper-amphibolite assemblages dominating.
Vague and inconsistent stratigraphic designations carrying genetic connotations, such as Pre-Cherry Creek Group and Dillon
Granite Gneiss, have been imposed on these metamorphic rocks (Peale,
1896; Tansley and others, 1933; Reid, 1957, 1965; Heinrich, 1960,
1963; Okuma, 1971; Garihan, 1973; and many others). These regional units have several shortcomings:
1) Separation of units is based upon presence (Cherry Creek
Group) or absence (Pre-Cherry Creek Group) of marble, or general absence of marble and/or quartzite and garnet (Pony Group) in the
1 2
section. These are extremely tenuous criteria considering litho-
facies variations and complications created during multiple folding.
2) Units are locally defined, making the necessary and long
overdue correlation of sequence across range boundaries confusing i f
not impossible. For example, the Pony Group of the Tobacco Root Mountains
is, in the lite ra tu re , roughly equivalent to the Pre-Cherry Creek
Group of the Ruby Range.
3) Implied genetic connotations may prove to be wholly unfounded.
For example, the Cherry Creek Group contains marble and has been
considered, therefore, exclusively a metasedimentary package; the
Dillon Granite Gneiss is considered a metamorphosed granitic intrusion.
Recent workers (e.g., James and Hedge, 1980) are abandoning this
nomenclature and suggest that the Montana basement terrane, the northern most extension of the Wyoming Province (Condie, 1976), is physically
and in general structurally and 1itho logica lly continuous, from the
Ruby Range in the southwest through the Tobacco Root, Madison, Gravelly,
and Gallatin Ranges in the east.
Although the basic geologic mapping is nearly complete, l i t t l e attention has been given to the Montana basement as a tectonic element.
Tectonic interpretations are either ignored, or center around a stable continental shelf model (e.g., Vitaliano and Cordua, 1979). Both
Hanley (1976) and Fountain and Desmarais (1980) however, have recognized the sim ila rity between the assemblage exposed across these basement blocks and sequences which have formed in an island-arc
setting. An integrated approach emphasising correlation of lithologic 3 packages within and across ranges is lacking, but is c ritic a l to any tectonic interpretation.
This study, is concerned with comparison of the petrology and parentage of two Archean sequences enclosing distinctive lithologies such as garnet iron-formation (Immega and Klein, 1976; Dahl, 1977), in the Copper Mountain area of the southern Tobacco Root Mountains, and Kelly area of the northern Ruby Range (Fig. 1). The Kelly area lies along the northeast flank of the Ruby Range, within the Laurin
Canyon Quadrangle. Access is by secondary road west from Alder,
Montana, and along the nearly impassable Beatch Canyon jeep tr a il.
The Copper Mountain area occupies the southern tip of the Tobacco Root
Mountains, about 15 kilometers to the northeast of the Kelly area.
Ramshorn, Bivens, and California creek roads provide dry weather access as secondary roads leaving nighway 287 between Laurin and
Alder, Montana.
Field work for this study was done during the summer of 1980.
Particular attention was given to field relationships and description of lithology and sequence in hopes of showing a consistent assemblage reflecting an original depositional environment. The Copper Mountain area was mapped at a scale of 1" = 1000' using enlarged U.S.G.S. topographic maps as a base (pi ate 1). A 1" = 500' outcrop map was prepared to better show the poorly exposed fie ld relationships in the
Kelly area (Plate 2). These maps incorporate, with many revisions and additions, some preliminary mapping done by James and Weir (1962, Figure i. Index map of part of southwestern Montana showing exposure of Archean crystalline rocks (hatchured) and the location of the Copper Mountain (A ) and Kelly (B ) study areas.
I12“00'
Porr
4 5° 30’ CD
Uj Ennis CD
Alder
Dillon co *7o
Uj
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1972), Levandowski (1956), Cordua (1973), and Smith (1980).
Foliation, lineation, and the attitude of small-scale fold axes were measured in the fie ld . A computer program was used to plot and contour pi diagrams constructed from poles to fo lia tio n in both areas. These plots are helpful in interpreting the overall structure of both areas.
Ninety thin-sections representing all lithologies in both areas were examined and compared petrographically. Modal analyses were visually estimated for all lithologies. Several polished sections of iron-formation were prepared to better observe the presence or absence, and general nature of its banding.
Major-element analysis was performed on 16 samples of amphibolite, quartzofeldspathic gneiss, and quartzite from both study areas. Samples were prepared and analyzed using X-ray fluorescence fa c ilitie s at
Washington State University, Pullman, Washington. 3.5 gram splits of sample were mixed with 7 grams of lithium tetraborate and fused into beads at 1000°C. These beads were analyzed for major elements using a Phillips PW 1410-00 XRF spectrometer. Matrix corrections were applied. CHAPTER I I
FIELD AND PETROGRAPHIC DESCRIPTION OF LITHOLOGIES
Mapping in the Copper Mountain area of Lhe southern Tobacco
Root Mountains and Kelly area of the northern Ruby Range has revealed an equivalent Archean lithologic section within tight synforms in both areas (see plates 1 and 2). Consistent alternation of major lithologies across both ranges (Burger, 1969; Okuma, 1971) suggests that this section represents an original depositional package compli cated by folding. Strict strati graphic correlations are impossible, however, since original thicknesses have been modified, and strati - graphic top rendered indeterminate during metamorphism. Since foliation parallels compositional banding and lithologic contacts, structural top is tentatively assumed to be stratigraphically up section.
The lith o lo g ic package occupying both synforms and working upwards is: quartzofeldspathic gneiss, dolomitic marble, amphibolite, micaceous quartzite and pelitic schist, pure quartzite (chert), and iron-form ation. Small masses of Precambrian meta-peridotite and pyroxenite, and intrusive bodies of Cretaceous-Tertiary quartz d io rite and granodiorite intrude both areas. Thin Proterozoic diabase dikes cut across the Copper Mountain area, and Paleozoic sediments overlie and bound the Kelly study area. Field and petro- graphic description of these units follows. The Precambrian
6 7 assemblage is described stru ctura lly upwards, and is followed by the younger Paleozoic and Cretaceous-Tertiary rocks. Visually estimated modes fo r these lithologies are given in Tables 1 and 2.
Quartzofeldspathic Gneiss
Quartzofeldspathic gneiss is the stru ctura lly lowest and lith o - lo g ica lly most abundant constituent in both the Ruby and Tobacco
Root Mountains. Cordua (1973) suggests that the unit makes up about
75 percent of the entire lith o lo g ic section in the Tobacco Roots with a total thickness of about 5 km. Quartzofeldspathic gneiss covers o about 325 km along the backbone of the Ruby Range, with an estimated structural thickness of at least 9 km (Garihan, 1973).
The Copper Mountain synform floats in a sea of quartz-feldspar gneiss which extends for several kilometers in a ll directions. Out crop of gneiss in the Kelly area is restricted to a small lobe in the southeast corner, which is sharply overlain by dolomite. This lobe forms the northern tip of a thick swath of the "Dillon Granite Gneiss” of Heinrich (1960) which extends to the south of the study area.
In the fie ld , the gneiss exhibits prominently jointed outcrops and low lying ledges of lig h t brown-gray to pink, massive to conspicu ously banded gneiss (Fig. 2). Texture and mineralogy vary considerably across the unit which consists of biotite-quartz-feldspar gneiss, quartz-feldspar gneiss, hornblende-quartz-feldspar gneiss and migmatite in order of decreasing abundance (Cordua, 1973; Smith, 1980). All 8
Figure 2. Banded biotite-quartz-feldspar gneiss from the Copper Mountain area.
Figure 3. Hornblende-quartz-feldspar gneiss from the Copper Mountain area. Brunton for scale. 9 gradations between leucocratic and mafic gneisses are present, making the unit heterogeneous in outcrop but homogeneous on a mapping scale.
Alternation of felsic and rnore-mafic layers from 0.5 cm to 15 m thick are characteristic, and garnet porphyroblasts are locally abundant enough to call some of the unit a garnet gneiss. These mineralogical and textural variations are probably due both to original differences in bulk composition and texture, as well as the effects of plastic flowage, shearing, and differentiation during metamorphism.
Migmatization has sweat out m illim eter to several centimeter- thick stringers and pods of pegmatite, and produced tabular masses of pegmatitic gneiss which grade into the other textural and mineralogical varieties. Plastic deformation within the unit has le ft small isoclinal flow folds and mylonitic textures throughout. Metamorphic diffe re n tia tio n has enhanced fo lia tio n and produced various pinch and well structures which may resemble sedimentary crossbeds.
Streaks, pods, and concordant layers of hornblende gneiss are scattered throughout the quartzofeldspathic gneiss in both lo ca litie s
(Fig. 3). Hornblende content within the gneiss commonly increases towards these bodies.
Along Currant Creek, in the extreme northeast corner of the
Copper Mountain study area, a swarm of metabasite s ills and cross cutting dikes outcrop within the gneiss. These rocks d iffe r s lig h tly from typical concordant amphibolites since some exhibit cross-cutting relationships, xenoliths, cumulate textures and chilled margins 10
(Gi 1 Irneister, 1971; Cordua, 1973). Most metabasites are fine-grained, massive, and are more pyroxene- and garnet-rich than many amphibolites.
Hornblende-plagioclase-pyroxene-garnet assemblages are common (Cordua,
1973). Because of th e ir small volume and overall s im ila rity in mineralogy and texture, metabasites were not considered separately from amphibol ites in the Copper Mountain area.
In thin-section,(Table 1) three typical mineral assemblages can be observed in the gneisses of both areas, with the most abundant mineral liste d f ir s t:
1) quartz-microcline-plagioclase +_ garnet + biotite + hornblende
(typical quartzofeldspathic gneiss)
2) quartz-plagioclase-microcline-biotite-garnet +_ sillimanite
(biotite quartz-feldspar gneiss)
3) plagioclase-hornblende-quartz + b io tite (hornblende gneiss)
The f ir s t two assemblages are identical in thin-section except for the presence or absence of accessory hornblende, or sillim anite .
Strongly undulose, strained quartz commonly makes up one th ird to nearly one half of the gneiss. Granulated seriate quartz grains up to 1 mm across are commonly wrapped by elongate, curved ribbons of sutured quartz up to 7 mm in length. Elongate quartz and feldspar, and to a lesser extent biotite and flattened garnets, commonly create a strong foliation.
Equigranular to elongate plagioclase is also undulose with poly gonal to sutured grain boundaries. Patchy zoning is locally present as is myrmekite in samples with abundant microcline. Both twinned Ta ble 1
Visually Estimated Modes For Study-Area Lithologies
Micaceous Garnet Mineral («)______Q-F Gneiss Dolomite Amphibolite Quartzite Quartzite "Pure” Quartzite (Chert) Schist Iron-formation
Quartz 35-40 Tr-3 1-10 25-70 10-45 95-9 B 30-55 30-55 Plagioclase 15-40 Tr 10-25 5-30 5-15 2-7 5-35 A n 20-30 ^ 3 0 - 6 0 “ 15-20 An 30-4 5 A n 30 ^ 1 5 - 2 5 K-Feldspar 5-25 5-35 2-20 1-7 5-30 Diopside-Salite 0-5 2-5 0-15 Tr-20 (Ferro)-Augite 10-30 (Ferro)-Hypersthene Tr Tr-30 Tr-3 5 Hornblende 0-9 35-65 1-5 Tremolite-Actinolite 0-3 0-2 Anthopayllite Tr Tr-1 G r u n e n t e 1-5 Olivine 3-7 Calcite 30-40 Tr-1 Tr Tr Dolomite 40-45 Muscov ite Tr-2 Tr Biot i te 1-25 Tr-1 1-25 1-25 3-5 3-25 Pnlogopite Tr-1 S i l 1 imanite 0-2 0-2 Tr-3 Tr-10 Kyan i te 0-2 Garnet 0-20 0-2 Tr-20 3-10 15-45 1-2 0 0-55 Z ircon T r - 1 Tr Tr-1 Tr-1 Tr A patite Tr Tr Tr Tr Tr-1 Tr Sphcne Tr Tr Tr-1 Serpentine 5-15 Sericite Tr-5 Tr-1 Chlorite Tr-1 Tr Tr-1 Tr-7 Tr 1-5 0-1 Epidote Tr Tr Tr 0-1 Hematite Tr Tr Tr-1 Tr-1 Magnetite Tr-5 1-10 0-7 0-3 Tr 10-30 Graphite 0-3 Tr -10 Ilmenite Tr Tr Tr Tr n u t i 1 e Tr Tr Tr
No. of samples 12 5 17 9 4 12 6 7 12 and untwinned plagioclase are present, and bent and offset twin lamellae are common.
Microcline forms about one-fourth of the gneiss. Both massive and "patchwork" varieties are present in augen up to 5 cm across.
Ribbon and patchy microperthite are common.
Biotite is characteristically altered to chlorite and oriented within a foliation, but locally shows no distinct orientation. It makes up 5 to 10 percent of the quartzofeldspathic gneiss and up to
25 percent of the biotite-quartz-feldspar variety.
Garnet is clustered within biotite-rich layers as amoeboidal porphyroblasts which comprise up to one-fourth of the rock.
Shapes vary from rounded to ellipsoidal or flattened indicating syndeformational growth and ro llin g .
Anhedral magnetite rimmed by chlorite forms 1-5 percent and is elongate parallel to foliation. Rounded to subrounded zircons form trace amounts in the gneiss and have been used as a crite rion to suggest a sedimentary protolith (Garihan and Okuma, 1971; Garihan and
Williams, 1976). Accessory hornblende, chlorite after b io tite , sillim anite nucleating on b io tite , muscovite enclosed in microcline, and diopside after hornblende are common.
The concordant layers of hornblende-plagioclase gneiss are clearly gradational with surrounding quartz-feldspar gneisses. They show essentially the same textures and contain more hornblende, less plagioclase, quartz, and garnet, and no microcline. Dark pleochroic 13
yellow to green hornblende makes up 10 to 25 percent of the rock
and imparts a slig h t fo lia tio n in most samples. Hornblende p o ik ilitic a lly
encloses plagioclase and quartz.
Twinned, slightly sericitized plagioclase and strongly undulose
elongate quartz compose 60 to 70 percent of the rock. Granulation has
produced small 0.2 to 1.0 mm and larger 1.0 to 1.3 mm grains of both
quartz and plagioclase. Elongate grains and ribbon structure are much
less abundant than in typical quartzofeldspathic gneiss. Microcline was not found in this assemblage but has been reported in the hornblende-
plagioclase gneiss (Cordua, 1973). B io tite , garnet, zircon, and magnetite are minor or accessory, making up 5 to 7 percent of most
samples.
Dolomitic Marble
Tan- to buff-weath.ering layers of metamorphosed carbonate form a
concordant layer between quartzofeldspathic gneiss and overlying amphibolite throughout both ranges. These dolomitic marbles serve b oth as excellent structural markers, and unambiguous representatives of metasedimentary rocks. Dolomite lies against Paleozoic sediments on the north, west, and eastern boundary of the Kelly area, and defines the limbs of both the Copper Mountain and Kelly synforms. Thick greaswood and orange lichen-covered surfaces make the presence of dolomite conspicuous, p a rticula rly on south facing slopes. Original
thickness of the unit is indeterminate due to plastic flowage during deformation. 14
In the Kelly area, a few small outcrops of hornblende-diopside- epidote gneiss define a thin band lying against the marble. These rocks represent "classic examples of metamorphosed impure carbonate" according to Harold James (written communication, 1980) who noted all gradations between these gneisses and dolomitic marble in the Carter
Creek area of the Ruby Range.
Most commonly, the marble is a massive, medium- to coarse-grained mosaic of white to tan dolomite and calcite. The presence of s ilic a te minerals such as forsterite and diopside provide the marble with a velvety sheen and suggest an impure, siliceous carbonate as a pro- to lith . Layering ranges from 5 cm to over a meter, and is loca lly defined by broken pods and stringers of quartz boudins up to a meter thick.
Burger (1966) suggests that these lenses of quartz may represent de formed layers of metamorphosed chert nodules.
In thin-section (Table 1, p. 11) calcite and dolomite form an interlocking matrix studded with varying amounts of forsterite, diopside, garnet, quartz, serpentine, phlogopite, and graphite.
Calcite can be distinguished from dolomite by its altered, dusty appearance in thin-section (Dahl, 1977). Forsterite forms small rounded grains or clusters of grains which are rimmed or completely replaced by fibrous serpentine. Diopside occurs as equant grains, which in many cases, are partially rimmed with serpentine or tremolite.
Garnets, where present, occur as small anhedral porphyroblasts. Light- brown grains of randomly oriented phlogopite are commonly s lig h tly 15 bent and make up less than one percent of the rock. Disseminated graphite, observable in hand sample, forms thin irregular rods and small anhedral grains which are associated with serpentinized forsterite.
Amphi boli te
Stratiform, melanocratic amphibolite consisting dominantly of hornblende and plagioclase, comprises much of both sections. Large lenses of amphibolite 60 to 500 meters thick are interbedded with schists, quartzites, and chert, whereas thin segregations and inter layers 2 to 3 cm to 40 meters thick, are present within quartzofelds- pathic gneiss and iron-format ion. These amphibolites are everywhere parallel to the foliation of surrounding units with the exception of metabasites, which are locally discordant in the northwestern corner of the Copper Mountain area. This discordant relationship has been used as evidence for an igneous origin for the amphibolites
(Levandowski, 1957; Heinrich, I960; Okuma, 1971; Garihan, 1973;
Cordua, 1973). The total thickness of amphibolite in the study areas is d if f ic u lt to determine because of structural complexity and its occurrence as thin interlayers within several other lithologies.
Cordua (1973) gives a rough estimate of 2.7 km as a structural thickness of amphibolite across the southern portion of the Tobacco
Roots.
Amphibolite is a general lith o lo g ic category which includes several distinct textural and mineralogical varieties in both areas: 16
1) "Salt and pepper" amphibolite with unoriented, fine- to medium-grained hornblende, pyroxene, quartz, and plagioclase in various percentages.
2) Banded or streaked hornblende-plagioclase gneiss with a well-developed foliation defined by alternating quartz-feldspar and hornblende-pyroxene-garnet-rich bands. Veined migmatites are common.
3) Massive, medium- to coarse-grained amphibolite or hornblendite containing 70 to 90 percent hornblende, and minor amounts of quartz and plagioclase.
The best exposures are on ridges and in gullies cut along major drainages where one or a ll three of these varieties may be present in a given outcrop. All three varieties commonly grade into one another, suggesting that much of the variation in this unit may be due to differentiation and partial melting during metamorphism, rather than re flecting original differences in p ro to lith . Beds of meta-conglomerate have been described within hornblende gneiss in the Ruby Range (Smith,
1980), but nothing resembling conglomerate was found in either study area.
The typical amphibolite in both areas is a dark-green to black, medium-grained, wel1-foliated rock with thin discontinuous layers of quartz and feldspar, enveloped in a hornblende rich matrix (Fig. 4).
Garnet, where present, occurs as scattered porphyroblasts or as thin segregations on the border of the quartz-feldspar layers. 17
Figure 4. Strongly banded amphibolite from the Kelly area.
Figure 5. Metamorphically enhanced massive amphibolite from the Copper Mountain area. 18
Mineral assemblages diagnostic of the granulite facies characterize
much of the amphibolite and metabasite. The typical assemblage is :
Hornblende-plagioclase-diopside-hypersthene-quartz + garnet.
Hornblende forms 25 to 60 percent of the hornblende gneiss and s a lt-
and-pepper varieties, and up to 90 percent of the massive amphibolite
(Table 1, p. 11). Euhedral to anhedral blades define the fo lia tio n
and, in some cases a lineation. Pleochroism varies from a lig h t brown
to olive high temperature color, to a blue-green lower temperature
color. Diopside, a ctin o lite and epidote commonly rim amphibole as an
alteration.
Plagioclase comprises from 5 to 25 percent of most amphibolites.
Normally and reversely zoned calcic andesine and labradorite are common.
Undulose extinction and bent twin lamellae are the result of defor mation after growth.
Quartz forms 5 to io percent of the amphibolite. Grains are equant and anhedral, commonly elongate parallel to compositional
layering, and mosaically intergrown with plagioclase.
Both clinopyroxene and orthopyroxene grains are stubby and locally undulose. Diopside is pale-green and hypersthene shows its characteristic pink-brown to pale-green pleochroism. Diopside is less abundant in Copper Mountain (3 to 5 percent) than Kelly (5 to 15 percent) amphibolites. Hypersthene commonly makes up 10 to 35 percent of the amphibolites. The genetic relationship between hypersthene and hornblende is ambiguous in these rocks (Cordua, 1973). Hornblende 19 clearly replaces hypersthene in some thin sections, whereas hornblende is replaced by hypersthene in other thin sections.
Broken sigmoidal garnets are amoeboidal with inclusions of quartz, plagioclase, and loca lly hornblende. Foliation commonly wraps around the porphyroblasts which are preferentially concentrated at the margins of quartz-feldspar segregations. Trains of anhedral magnetite enclosed in hornblende, make up 5 percent of several samples. Other accessory minerals include apatite, biotite, chlorite, epidote, and calcite.
Pelitic Schist
Aluminous schist and schistose gneiss composed of b io tite , garnet, sillimanite, quartz, plagioclase, and graphite, occur as thin discontinuous map units. They are sharply interlayered with quartzites and chert, and also occur as thin, 5 to 15 m-thick layers within iron-formation.
The more genissic, poorly foliated varieties grade into quartzites in both areas. The close association of these schists with quartzite and marble, and the presence of graphite, suggest a carbonaceous siliceous shale as a p ro to lith .
Although the schists can be followed along trend, exposures are very poor and weather readily to a micaceous s o il. Since exposure is rare, the presence of schist was in many cases inferred from s o il, flo a t, and small swales found within quartzites during mapping. Poor exposure makes adequate estimates of thickness d iffic u lt.
Hand samples are commonly schistose and medium-grained, lig h t- brown to gray-weathering rocks (Fig. 5). Abundant sillimanite, garnet, 20 and biotite are concentrated in millimeter-thick bands which alternate with coarse quartz-fel dspar segregations. Subparallel bundles of sillim a n ite are arranged in the fo lia tio n or as rosettes lying on the fo lia tio n . Anhedral garnets, 2 to 10 cm across are commonly rimmed by thin quartz-feldspar coronas. Soft, disseminated graphite is observable in hand sample. Wei 1-developed isoclinal flow folds deform b io tite , sillimanite, and graphite, as well as pegmatitic stringers which locally transgress the schistosity.
The most common schist assemblage in both fie ld areas is quartz- biotite-garnet-si1 1imanite-plagioclase +_ K-feldspar. These sillimanite- orthoclase assemblages suggest the reaction:
muscovite + quartz z—7 sillimanite + K-feldspar + H2O (Evans, 1965), was important.
Quartz makes up 30 percent of most samples as folded wisps of seriate grains, and sutured aggregates of smaller anhedral grains
(Table 1, p. 11).
Light-yellow to dark-red biotite imparts a foliation and makes up a quarter of the schist. Zircons surrounded by halos are generally included. Slender prismatic bundles of sillim a n ite nucleate on the biotite and are locally altered to a greenish pyrophyl1ite . Both biotite and sillim a n ite are crenulated around microscipic folds but are not recrystal 1ized into polygonal arcs.
Large round to spiral garnets are wrapped by the fo lia tio n as well as coronas of coarse quartz and plagioclase. These anhedral porphyro blasts are commonly poikilitic containing irregular quartz, plagioclase, and biotite. 21
Twinned and untwinned, andesine is s lig h tly undulose and
sericitize d, whereas untwinned subhedral K-feldspar is unaltered.
Palgioclase and microcline rarely make up more than 10 percent of the
schi St.
Crenulated grains and slender rods of graphite make up to 5 percent
of some sections. Accessory minerals include traces of zircon, apatite,
muscovite, epidote, and magnetite.
The more gneissic schists commonly contain the assemblage quartz-
plagioclase-biotite-K-feldspar +_ sillim a n ite . Coarse-grained elongate
quartz commonly forms ha lf of the rock, with twinned polygonal plagio
clase forming another third. Twinned perthitic microcline rarely forms
more than a trace in Copper Mountain schists, but may make up 30 per
cent in Kelly pelites. Clusters of olive-brown biotite are partially
altered to chlorite. A weak crenulated foliation can be discerned in
some of these gneisses, and locally a cross-mica fabric is well
developed. Accessory minerals include apatite, muscovite, sillim a n ite ,
and magnetite.
Quartzi te
Foliated, fine- to coarse-grained metamorphic quartzite is a con
spicuous constituent of the lithologic section exposed within both field
areas. Mappable layers of quartzite are present within, and surrounding,
schists, amphibolite and iron formation. Several lenses are commonly oriented along a given structural horizon (Cordua, 1973), which suggests 22
an original depositional geometry which has been tectonically pinched and dismembered. Thicknesses range from 5 to 150 meters in the purer chert lenses, to 10 to 200 meters in the micaceous variety. These quartzites can be distinguished on the basis of quartz content.
Prominent ridges in both areas are held up by folded lenses of
pure meta-quartzite or chert, which is composed of 95 to 98 percent medium to coarse granular quartz (Fig. 6). These vitreous quartzites are commonly white to light-brown but lo ca lly take on a light-green color from chrome mica (fuchsite). From a distance these rocks appear structureless , but upon closer examination, prove to be banded with thin layering defined by the alternation of elongate rods and mull ions of quartz, and thin mi 11imeter-selvages of mica (Fig. 7). This bedding is similar to structures reported in graded volcanic ash-chert beds in the Archean of northern Minnesota (Lavery, 1972).
The lower surrounding hills within the study areas are capped with w e ll-fo lia te d , fine- to medium-grained, micaceous quartzite. These impure gray to brown rocks commonly contain 45 to 60 percent quartz and up to a third feldspar, which alternates with 2 mm- to 1 cm-thick bands containing biotite, muscovite, sillimanite, and rarely kyanite, magnetite, and graphite. Brown to ruby-red garnets commonly scatter throughout the rock, locally making up 45 percent of a garnet quartzite.
Although garnet quartzite is not well exposed at Copper Mountain, a mappable band about 60 meters thick crops out in the Kelly area. This garnet quartzite contains less biotite and microcline, and slightly more plagioclase than surrounding micaceous quartzites (Dahl, 1977). 23
Figure 6. Thin ridge of massive, vitrous chert in the Copper Mountain area.
Figure 7. Outcrop showing bedding in chert at the Copper Mountain area. 24
Thin sections (Table 1, p. 11) show the pure meta-quartzite to be composed of quartz-microcline-plagioclase-muscovite-biotite- sillimanite. Ninety-five to 98 percent of the rock is composed of moderately to extremely undulose quartz showing a puzzlework extinction, in an annealed mosaic of 4 to 10 mm-long grains. Small round to oblong p e rth itic microcline and undulose plagioclase is commonly enclosed with quartz. Feldspar makes up about 5 percent of the purer quartzites.
Subhedral to anhedral muscovite has overgrown some plagioclase grains. Oriented b io tite makes up 5 percent of most sections. Fiborous radiating sillimanite nucleates on the biotite as well as growing as prismatic grains within small fractures in quartz grains. Small rounded to oblong grains of apatite and zircon are present in trace amounts. Near iron-formation these cherts also contain small amounts of garnet, pyroxene, and magnetite.
The banded micaceous quartzites are characterized by the assemblage quartz-microcline-garnet-biotite-plagioclase + cordierite + sillimanite + muscovite + kyanite.
Varying degrees of porphyroclastic to mylonitic textures are present within these quartzites. Quartz commonly forms 25 to 60 percent, both as larger (1 to 10 mm) anhedral undulose porphyroclasts, and fin e r
(0.1 to 0.8 mm) anhedral grains which form pressure shadows around garnet and feldspar (Table 1, p. 11). A d istin ct patchwork deformation fabric
(Boehm lamellae) is well developed in many quartz grains. Many of the 25
quartzites show thick, continuous, extremely undulose quartz ribbons and streaks indicative of intense flowage during deformation. These
3 to 7 mm-thick ribbons appear to have enveloped microcline, plagioclase, and fine-grained quartz during flowaqe. B io tite , concentrated along the boundaries of these undulose streaks, forms about 10 percent of the rock.
Subhedral to anhedral microcline is commonly s lig h tly elongate and perthitic, exhibiting plaid twinning which often has a somewhat pinched, undulose character. Some of the larger grains have been broken as evidenced by offset and rotated twinning. Twinned, zoned and unzoned plagioclase is subhedral and shows varying degrees of sericitization.
Pink garnet porphyroblasts, 0.5 to 1.0 mm across, are typically weakly helicitic, preserving a relict swirled foliation as oriented
inclusions of quartz, biotite, and sillimanite. These anhedral garnets are commonly in cip ie n tly altered to ch lo rite . Percentages vary from
3 to 10 percent in micaceous quartzites, up to 45 percent in garnet quartzi te.
Cordierite can be found in a few of the Kelly samples as small, undulose subhedra which form coronas around garnet, and contain b io tite and sillim a n ite . S illim anite can be found in some samples of quartzite from both field areas as small, prismatic, oriented grains associated with biotite and microcline. One or two rounded grains of relict kyanite are found near, as well as enclosed w ithin, sillim anite grains. 26
The rounded habit and rarity of this kyanite suggests that it is metastable with respect to sillimanite (Dahl, 1977). Traces of rounded apatite and zircon, thin rods of magnetite, r u tile , and carbonate can also be observed in most samples.
Iron-formation
Massive and banded iron-formation crops out as small pods and folded le n ticu la r bodies throughout much of the high-grade terrain in
Montana. The Copper Mountain iron-formation is the most extensive of those within the Tobacco Root Mountains with a strike length of about 10 km, and an average thickness of 40 m. The Kelly iron-formation forms several discontinuous bands some 10-50 meters thick which snake across 5 km of the Ruby Range.
Outcrops are commonly fragmented, deeply weathered, and poorly exposed except where they have been exhumed by exploration trenches or roads (Fig. 8). In some cases a weathered rubble of dense red-brown flo a t is present, whereas other outcrops form small faces with magnificently etched small scale isoclinal to open folds (Fig. 9).
A d istin ctive bright red-brown soil commonly marks outcrops as do large reddish ant h ills which provide excellent mineral separates of the unit.
Observations of iron-formation and immediately surrounding lithologies, both where trenched and naturally exposed, allowed com posite horizontal lithologic sections (Fig. 10) to be drawn for both fie ld areas. Sections in both areas are essentially identical. The 27
Figure 8. Trench showing folded iron-formation (top) grading into pelitic schist, and underlying micaceous quartzite at Copper Mountain.
Figure 9. Well developed F2 folds in the Kelly iron-formation. 28
COPPER MOUNTAIN IRON-FORMATION
KEY
Amphibolite
Massive to banded chert KELLY IRON-FORMATION Iron-formation
Micaceous quartzite
Pelitic schist
FEET 50 - 1
Figure io Composite cartoon showing lithologic sequence surrounding banded iron-formations at the Copper Mountain and Kelly areas. The thickness of units and degree of exposure vary considerably. 29
iron-formation is conformable within a white to brown, medium-grained
quartzite and locally grades into 1 to 5 meter-thick interbeds of
garnet-biotite-si11imanite schist. The schist is extremely friable
with zones made up almost e n tire ly of red garnets up to a cm across
within a reddish-brown micaceous powder. Massive to bedded, vitreous
white-to-brown cherts are also intimately interbedded with both iron-
formation and schist as well as overlying amphibolites.
These iron-formations consist primarily of interlayered quartz
and magnetite and commonly contain abundant red garnet, green to black
hypersthene, clinopyroxene, and hornblende. D ifferential weathering
of quartz and iron silica te s commonly creates a pronounced banding which is undoubtedly an original texture, possibly accentuated by metamorphic d iffe re n tia tio n . In general, banding is only moderately
developed although very delicate banding is preserved in several
places, p a rticu la rly within the Kelly area. Internal contortions in banding are observable in polished sections. Although such contortions
in banding are often attributed to soft-sediment deformation or pulsing volcanic imput of s ilic a and iron (Stanton, 1972), in this area, they seem to be a simple result of folding and shearing during deformation.
The more massive iron-formations in both study areas commonly contain less magnetite, and more amphibole, pyroxene, and garnet as porphyroblasts up to 5 cm across. These massive varieties often take on an almost skarn-like appearance in hand sample. Dahl (1977) 30
f ir s t noted the presence of both quartz iron-formation and garnet
iron-formation in the Kelly area, with a "generally lower modal quartz
and higher modal magnetite" in the latter. The quartz iron-formation
assemblage consists of quartz-magnetite-ferroaugite-hypersthene +
hornblende +_ grunerite + garnet. The garnet iron-formation assemblage
is typified by quartz-magnetite-garnet-ferroaugite-hypersthene +
hornblende + grunerite.
Quartz occurs as annealed elongate, grains up to 0.5 mm across which are segregated in bands up to 3 mm thick, or scattered throughout
the more ferromagnesian layers (Table 1, p. 11). Quartz is coarser- grained and more undulose in the massive iron-formations, perhaps as a
result of recrystal 1ization which has also destroyed much of the fine
1ayering.
Magnetite forms large (1 to 3 mm) elongate grains typically connected in discontinuous bands which embay or are infolded within quartz, garnet, and pyroxene. Smaller (0.3 to 0.8 mm) anhedral grains commonly form oriented trains or random intergrowths within garnet, pyroxene, and amphibole. Skeletal, needle-like grains can be observed in some samples.
Strongly fractured, salmon to pink garnets are commonly poikilitic containing quartz and magnetite. They are rounded to oblong in shape, and are completely embayed by pyroxene in some samples. Garnet forms
35 to 50 percent of most iron-formations, although several quartz iron- formations in the Kelly area contain no garnet. 31
Subhedral to anhedral pyroxene grains compose half of the rock.
Clinopyroxene (ferroaugite or salite) is slightly pleochroic pale-
green whereas orthopyroxene (ferrohypersthene) is pleochroic light green to lig h t pink with an inclined extinction of a few degrees. Both
pyroxenes are fractured, and rimmed or replaced by secondary dark-green hornblende and asbestiform grunerite.
Ultramafic Rocks
Subdued dark-green to grey, to reddish-brown-weathering outcrops of serpentinized peridotite and minor pyroxenite are present in both study areas. These knobby ultramafic outcrops are often elongate to elliptical in outline, with sharp contacts showing narrow shear
rhinds and slickensided serpentine where well exposed. These bodies parallel the regional foliation of both areas, and characteristically impart their dark olive color to the surrounding soil.
Several small peridotite pods crop out in the northern portion of the Kelly area along the axis of a small upfaulted block. Py roxenite outcrops on the order of 10 meters across are also scattered throughout the area. Three small ultramafic bodies were mapped within the Copper Mountain area, the largest of which forms an e llip tic a l mass within quartzite on Copper Mountain itse lf (Plate 1).
Cross-fiber serpentine, o livin e , orthopyroxene, hornblende and garnet can be distinguished in hand samples of peridotite. These hazburgites are massive, dark and medium-grained, except where amphibole is abundant and creates a faint streaky foliation. 32
In thin section, (Table 2) original textures are masked by ex
tensive serpentinization. Closely spaced serpentine-filled fractures are striking in thin section. Cross-fiber crysotile and radiating
liz a rd ite comprise 35 percent of most samples. Rounded grains of
fo rs te rite up to 0.3 mm in diameter make up about one third of the
rock, and are generally replaced by serpentine, or rimmed by brown
iddingsi te.
Hypersthene and enstatite are subrounded, colorless to pleochroic
light pink, and partially altered to serpentine. Actinolite and olivine are commonly enclosed in orthopyroxene. Pale pleochroic green hornblende forms less than 25 percent of the rock and shows sharp grain boundaries with light-green subhedral diopside. A few grains of plagioclase are present within amphibole in Copper Mountain peridoti tes.
Pleonaste, an iron-magnesiurn spinel, forms dark-green subrounded grains which are intergrown with unoriented phlogopite. Garnet is in small broken porphyroblasts which are p a rtia lly chlo ritized. Magnetite forms ragged grains and ladders intergrown with olivine . Carbonate, anthophyllite, chlorite, diopside, and phlogopite are common replace ment assemblages in most peridotites.
The meta-pyroxenites are black, dense, fine- to coarse-grained rocks composed mainly of orthopyroxene, hornblende, and diopside, a typical granulite facies assemblage. They are difficult to distinguish from massive amphibolite in hand sample. Outcrops are subdued and rounded and are commonly 10 to 20 meters across. Table 2 Visually Estimated Modes For Study Area Lithologies
Mineral (%) Peridotite Pyroxenite Peqmat ite Diabase Quartz Diorite Granodiorite Feldspar Porphyry
Quartz 10-15 1-5 10 10-20 30 Plagioclase 0-3 3-7 40-50 30-50 50-55 10-20 ^ 7 0 - 7 5 ^30-60 ^30-40 ^ 4 0 - 4 5 K-Feldspar 70-80 0-5 7-10 5-7 Diopside-Salite Tr-15 1-3 (Ferro)-Augi te 3-10 30-40 (Ferro)-Hypersthene 0-10 25-40 Enstatite 10-25 0-15 Hornblende 10-30 5-60 2-5 0-20 10-15 Trerolite-Actinolite 1-5 Tr-1 Anthophvllite Tr-1 0 1 ivine 5-35 Calcite 0-5 Tr T r Muscovite 1 Tr Biotite Tr-1 1-3 1-5 3-5 5-7 Phlogopite Tr-5 Garnet Tr-1 Tr-3 Tr-1 Magnetite Tr-15 3-10 Tr 1-3 2-5 Tr-3 3 IImenite T r Zircon Tr Tr Apatite T r Tr Tr Sphene 1 T r Tr Serpentine 5-40 3-10 Sericite Tr-1 3-5 20 Chlorite Tr-3 Tr-5 Tr- 1 T r 10 Epidote Tr-2 Tr Hematite Tr Pleonaste Tr-1 Tr-5
No. of samplas 4 3 2 2 3 2 2 ■
CJ 00 34
Colorless to pale pink orthopyroxene comprises as much as
40 percent of the rock (Table 2, p. 33). Lineated hypersthene and
enstatite are poikilitic containing one another as well as magnetite,
and diopside. They rarely form more than 5 percent of the rock.
Pale-green diopside is minor in most thin-sections although Dahl (1977)
has described a pyroxenite from the Kelly area as a mix of 50 percent
hornblende, and 50 percent diopside. Hornblende (or in some cases
edenite, a Na-rich, Fe-free hornblende) makes up 60 percent of some
samples. Pale-yellow cross-cutting veinlets of serpentine containing
secondary magnetite, form as much as 5 percent of pyroxenites, much
less than in peridotites because of the lack of olivine . Minor
phlogopite, garnet, ch lo rite , and carbonate are also present as accessories in most pyroxenites.
Workers in both the Tobacco Roots (Tendall, 1978) and Ruby Range
(Desmarais, 1978) argue’ that sharp contacts, narrow shear rhinds, and a general lack of fo lia tio n are strong evidence in favor of cold tectonic emplacement for these ultramafic bodies.
Pegmati te
Pegmatite veins and stringers a few centimeters thick, to small lenses and pods up to 20 meters across were observed in both areas.
They are ty p ic a lly pink medium- to coarse-grained rocks showing cataclastic, graphic, and gneissic texture. About 95 percent of the rock is composed of pink microcline (75 percent) and graphically intergrown quartz and plagioclase (25 percent) with minor amounts of 35 muscovite, b io tite , and anhedral garnet (Table 2, p. 33). Both foliated and massive varieties are present.
Most pegmatites are confined to the quartzo-feldspathic gneiss as pods, stringers, and tabular masses up to several meters across.
In the Kelly area, pegmatites can also be found within quartzite and as thin folded streaks within amphibolite. All occurrences in both areas are concordant with regional structure and surrounding lithology and may have been derived by local anatectic melting.
Discordant, unfoliated pegmatites have been reported from other portions of both ranges. Many o f these may be post-metamorphism, intruding during late Cretaceous plutonic activity related to emplace ment of the Tobacco Root batholith.
Meta-diabase
Thin, greenish-grey, Proterozoic diabase dikes from 10-40 meters thick, cross-cut the regional fo lia tio n in the Copper Mountain area and to the south of the Kelly area. At Copper Mountain, diabase is best exposed in sections 2 and 11 where thin linear ribs cut sharply across the quartozofeldspathic gneiss. These 5 to 10 meter-thick dikes weather brown to rust-orange and commonly display chilled margins up to 0.5 m thick.
In thin section (Table 2, p. 33) twinned and zoned subophitic plagioclase (1abradorite) and augite compose about 40 percent of the rock. Most of the plagioclase is overgrown with sericite and epidote, 36 and much of the augite is rimmed by deep blue-green hornblende, c h lo rite , and b io tite . Anhedral to subhedral quartz and microcline are in te rs titia l and make up about 10 percent of the rock. Magnetite dust or small disseminated grains make up as much as 5 percent of the rock. Carbonate and epidote comprise a small percentage as pervasive alteration products. A thorough petrographic description of Tobacco
Root diabase dikes is given by Koehler (1972).
Three sets of dikes were emplaced in the Tobacco Root-Ruby Range area during the late Precambrian (Koehler, 1972; Wooden and others,
1978). Dikes in the southern Tobacco Roots were emplaced around
1455 Ma. and are compositionally equivalent to low-K th o le iite . The other two sets are high-K quartz normative types which intruded about
1300 Ma.
Most workers (e.g., Burger, 1969; Koehler, 1972) suggest that structural weakness controlled the emplacement of the diabase as many dikes trend roughly N 75° W. Dikes with, as well as without, this general trend can be found in the Copper Mountain area.
Paleozoic Rocks
Paleozoic sedimentary rocks are not present in the Copper Mountain study area but do overlie, and form a faulted boundary on three sides of the Kelly area (Plate 2). These sedimentary rocks are not differentiated on the geologic map but include a relatively thick sequence of sandstones, dolomites, and shales which range in age from middle-Cambrian to middle-Pennsylvanian. An excellent discussion of 37 the petrology and structure within this sequence is provided by
Tysdal (1970).
Cretaceous-Tertiary Intrusions
Small isolated and irregular intrusive bodies genetically related to the Tobacco Root batholith occur within both study areas. Three small pods of quartz d io rite within section 25, comprise the only intrusive phases within the Kelly area. Two of these bodies are less than 30 meters in diameter, whereas the th ird measures 90 meters long by 150 meters wide. A small differentiated stock, approximately
450 meters wide by 900 meters long, intrudes the central portion of the Copper Mountain study area. Quartz d io rite and minor granodiorite are present within th is stock which is bordered by a few small patches of highly altered feldspar porphyry. The meager copper mineralization from which Copper Mountain derives its name, is commonly attributed to fluids associated with this stock (Levandowski, 1956; Vitaliano and
Cordua, 1979).
The quartz diorite forms resistant and jointed, light-colored outcrops in both areas. Hand samples are massive, fine- to medium- trained rocks composed of interlocking quartz and plagioclase, and unoriented b io tite . In thin-section (Table 2, p. 33) plagioclase is undulose and commonly exhibits bent twin lamellae. Subhedral quartz and a n tip e rth itic oligoclase make up about 90 percent of the rock.
Quartz and plagioclase exh ibit bimodal grain sizes and textures suggestive of cataclastic deformation. Subhedral hornblende, 38 unoriented light-brown b io tite , anhedral magnetite and accessory zircon make up the other 10 percent.
Granodiorite and feldspar porphyry could not be found in the
Copper Mountain area. The feldspar porphyry is a light-green where fresh, to light-brown where weathered. Abundant feldspar phenocrysts up to 5 mm across are set in an aphanitic groundmass. In thin-section
(Table 2, p. 33) the porphyry contains large zoned plagioclase phenocrysts. Pleochroic light-green hornblende makes up about 10 percent of the rock and is commonly rimmed or overgrown with chlo rite. Quartz is undulose in small anhedral grains. A few percent of light-brown b io tite is randomly oriented throughout the rock. Half of the rock is composed of a very fine-grained altered groundmass containing quartz, plagioclase, biotite, sericite, and chlorite.
A small amount of granodiorite is present within the Copper
Mountain stock. I t contains more K-feldspar and less hornblende than the quartz diorite. Anhedral twinned microcline makes up 7 to 10 per cent of the rock, with pleochroic straw to green-colored porphyroblasts of hornblende making up another quarter of the rock. Half of the rock is composed of euhedral , twinned and zoned plagioclase. Quartz is undulose and commonly makes up about one th ird of the rock. Light- brown b io tite is randomly oriented and commonly chlo ritized. Accessory subhedral magnetite, muscovite and zircon form up to 5 percent of the rock. CHAPTER I I I
METAMORPHISM
Equilibrium mineral assemblages determined from examination of some 90 thin-sections, suggest that Archean litho lo gies within the southern Tobacco Root and northern Ruby Ranges underwent metamor phism which straddled the upper-amphibolite lower-granulite-grade boundary. Much of the Montana basement endured sim ilar conditions including the Highland (Gordon, 1979), Madison (Thompson, 1969), and Beartooth Ranges (Van DeKamp, 1969). The following assemblages occur in the Copper Mountain and Kelly study areas:
Quartzofeldspathic Gneiss quartz-piagioclase-K-feldspar-augite-bioti te quartz-K-feldspar-plagioclase +_ garnet +_ b io tite + hornblende +_ s ill imanite quartz-garnet-piagioclase-biotite-augi te-microcli ne plagioclase-hornblende-augite-quartz +_ b io tite + hypersthene
(hornblende gneiss)
Dolomitic Marble calcite-dolomite-forsterite-diopside-serpentine graphite + quartz plagioclase
39 40
Amphi bolite hornblende-plagioclase-hypersthene-quartz +_ augite + magnetite + garnet hornblende-hypersthene-magnetite-quartz + biotite hornblende-plagioclase-quartz-garnet
Pure Quartzite (Chert) quartz-microcline-biotite-si11imanite + plagioclase + magnetite + muscovi te quartz-sil 1 imanite +_ microcline
Micaceous Quartzite quartz-m icrocline-plagioclase-biotite-garnet + muscovite + s i 11i man i te + kyan i te quartz-plagioclase-microcl ine +_ garnet + graphite quartz-piagioclase-microcline-garnet-cordieri te garnet-quartz-biotite-plagioclase-microcl ine + sillirnanite +_ magnetite
Pelitic Schist quartz-plagioclase-biotite + garnet quartz-biotite-garnet-sillimanite-plagioclase + graphite quartz-microcline-plagioclase-biotite-garnet-si11imanite + magnetite
Iron-Formation quartz-garnet-magnetite +_ hypersthene + diopside quartz-magnetite-augite-nypersthene-hornblende + garnet + grunerite 41
quartz-garnet-hypersthene-magnetite +_ anthophyllite
hypersthene-quartz-magnetite-garnet-hornblende
Peri dotite
hornblende-augite-hypersthene-olivine-serpentine-magneti te +_
plagioclase
olivine-hypersthene-serpentine-magnetite +_ garnet + spinel
Pyroxenite
augite-hypersthene-magnetite-serpentine + hornblende
Diabase
plagioclase-augi te-quartz-hornblende
Various mineralogical and textural characteristics of these rocks
can also be used to quantify grade. These include:
1) The presence of perthitic K-feldspar in gneiss, indicating con
ditions which lo ca lly reached the granulite grade.
2) Migmatitic quartzite and gneiss suggesting plastic deformation
and conditions at or above the minimum melting curve of granite.
3) Relict kyanite which formed metastably with respect to sillimanite
in quartzite.
4) Hypersthene grains with r e lic t hornblende cores in amphibolite.
5) Dark olive-green hornblende and deep-red pleochroism in b io tite ,
6) Two-pyroxene iron-formations suggesting metamorphism lo ca lly reached
the orthopyroxene zone of the granulite facies. 42
Prograde mineral assemblages and textural c rite ria suggest that
the following metamorphic reactions may have been important:
Quartzofeldspathic Gneiss
Ti biotite + si I Iimanite + 2 quartz *— 7 pyralspite + K-feldspar +
rutile + H^O
Ti b io tite + muscovite + quartz ^— 7 K-feldspar + garnet + rutile + H2O
Marble
tremolite + 3 calcite ^— 7 4 diopside + dolomite + CO 2 + H2O (Turner,
1968)
tremolite + 11 dolomite ^—7 8 forsterite + 13 calcite + 9 CO2 +
H20 (Metz, 1967)
Amphibol i te
hornblende + anorthite + hypersthene ^— ralmandine + augite + a lb ite +
h2o
hornblende + almandine + 5 quartz ^— 7 7 hypersthene + 3 plagioclase +
H20 (DeWard, 1965)
orthopyroxene + plagioclase + H 2o ^— 7 hornblende + almandine
3 orthopyroxene + 2 plagioclase^ 7 augite + 2 almandine + quartz
(Miyashiro, 1961)
Quartzite muscovite + quartz^— 7si 11 imanite + K-feldspar + (Evans, 1965) muscovite + 6 quartz + 2K ^-—^3 microcline + 2H
garnet + sill imanite (kyanite) + quartz-^—^cordierite 43 biotite + quartz^ /sillimanite + garnet + K-feldspar + H 2O
Pelitic Schist biotite + quartz^ 7 sillimanite + garnet + K-feldspar + H2O muscovite + quartz^ ^sillim anite + K-feldspar + H2O (Evans, 1965)
3 kvanite^ 73 s ill imanite
Meta-ultramafic rocks forsterite +tremolite^— 7diopside + enstatite + H2O tremolite + 2 spinel^ 73 forsterite + enstatite + 2 anorthite +
2 H20
Iron-formation anthophyl 1ite z— 7 enstatite + quartz + H2O
Hydrous retrograde greenschist facies minerals were developed after emplacement of Proterozoic diabase dikes and before deposition of overlying Paleozoic rocks. The following retrograde relationships are common: chlo rite (after garnet, b io tite , hornblende) epidote (a fter augite, plagioclase, hornblende) sericite (after plagioclase, K-feldspar) sphene (after rutile and ilmenite) cordierite (after garnet) serpentine (after olivine, orthopyroxene, diopside) tremolite-actinolite (after hypersthene, hornblende) 44
diopside (a fte r hornblende)
grunerite (a fte r hornblende)
carbonate (a fte r plagioclase)
spinel (after magnetite)
muscovite (a fte r hornblende)
hematite (after magnetite)
A rather open ended Pressure-Temperature fie ld can be drawn on the
basis of mineral breakdown, and thefir s tappearance of several meta- morphic minerals in these assemblages. When th isfie ld isreconciled with conditions suggested by electron microprobe analyses of iron-
formation by Dahl (1977) and Immega and Klein (1976), two small triangles
represent the best estimate fo r prograde metamorphism within the Copper
Mountain and Kelly areas (Fig. 11). Copper Mountain conditions appear
to have been s lig h tly lower (700-750°C, 4-6 kbar) than the Kelly area
(750-800°C, 6-8 kbar). A common boundary exists at about 750°C and
6 kbar which would correspond to a depth of metamorphism between 20 and 25 kilometers (Turner, 1981).
The sporadic occurrences of granulite mineralogy common within amphibolite, meta-pyroxenite, and iron-formation have been taken as evidence of an early granulite facies metamorphism which has been a ll but obliterated by a late r upper-amphibolite-grade event (e.g., Reid,
1963; Cordua, 1973). These two metamorphic events are commonly correlated with the 2750 Ma. and 1660 Ma. radiometric dates obtained from these ranges. Other workers, including this w rite r, suggest a 45
p h 2 o (kb)
500 600 700 800 900
TEMPERATURE (°C)
Figure 11. Pressure-temperature diagram showing meta- morphic conditions achieved in the study areas based upon mineral breakdown (A), and fields determined for the Copper Mountain (B) (Immega and Klein, 1976) , and Kelly (C) (Dahl, 1978) iron-formations. Reactions shown are from Hyndman (1972) :
Ky r And * — r Sill Minimum melting curve of granite Muse + Qtz ^ Ksp + Sill + H20 Formation of hypersthene Ksp r Perthite Anth ^ Enst + Qtz + H O Breakdown of hornblende + quartz 46 single event history, with granulite facies assemblages representing lo c a lly d rie r rocks where PH 2O < Pload during upper-amphibolite metamorphism (e.g., Garihan, 1973). The 1600 Ma. dates may represent a second greenschist facies metamorphism, which has overprinted much of the region. CHAPTER IV
STRUCTURE
At least two deformational events have been recognized across
the Tobacco Root (Burger, 1967; G illm eister, 1971: Vitaliano and
Cordua, 1979) and Ruby (Garihan, 1973; Smith, 1980) Ranges. F-j
deformation created tight, isoclinal to recumbant folds with similar-
style. These isoclines are from a few centimeters to 10 meters across, and are beautifully displayed throughout the quartz-feldspar gneiss, amphibolite, and iron-formation in the Copper Mountain and Kelly areas.
An f£ event refolded these isoclines into the broad, cylindrical flexures, which form the series of anti forms and synforms that bound and define both study areas (Table 3).
Okuma (1971) found evidence of a s lig h t warping of f-j and fg axes, and Karasevich (1980) subdivided f] and f^ into four fold generations in the Ruby Range. No evidence of an f 3 or fq. event were encountered in the study areas.
Major high-angle northwest-trending faults cut across the regional fo lia tio n of both ranges, and in some cases have controlled the em placement of Proterozoic diabase dikes (Tysdal, 1971; Garihan, 1973;
Vitaliano and Cordua, 1979). Several northeast-trending faults truncate against these northwest structures, and smaller faults of variable trend are also scattered throughout both ranges. Exposure of faults is generally poor in the study areas, with locations based
47 48
Table 3.
Comparison of Precambrian Folding Events Postulated For the Tobacco Root and Ruby Ranges
Tobacco Root Range Ruby Range (Burger, 19 6 9, Gillmeister, (Okuma, 1971, Garihan, 1973, 1971, Vitaliano and Cordua, Smith, 1980) 1979)
a. isoclinal folds trending a. isoclinal folds trending and plunging N and plunging NE b. similar-style b. similar-style c. development of foliation c. development of foliation and axial plane schisto- and axial plane schisto- sity s ity d. mechanisms: passive and d. mechanisms: passive and flexural flow flexural flow
a. open folds trending and a. isoclinal to open folds plunging N-NE trending and plunging N-NE b. refold F-, folds b. refold F^ folds c. coaxial with c. coaxial as well as non coaxial with F d. locally create fracture d. locally create fracture cleavage cleavage e. mechanisms: flexural e. mechanisms: flexural flow and passive flow? and slip, and passive flow
a. do not occur in the a. warping of around N- Tobacco Roots or are trending axes completely coaxial with b. non-coaxial with F^ c. mechanism: flexural flow? d. do not occur in the central Ruby Range or in the Kelly study area, or may be completely coaxial with F£ 49
upon juxtaposition of rock units, and the presence of gossany breccia
or siickensided float.
A wel1-developed regional fo lia tio n is also conspicuous across
the Copper Mountain and Kelly areas. Foliation parallels compositional
layering within, and the lithologic contacts between units. Layering
has been locally transposed along foliation in the noses of isoclinal
folds. The general concordance of fo lia tio n and compositional layering
suggests that much of the metamorphic fabric reflects an original
layering. Mylonitic textures, such as rodded quartz, and elongate feldspar
and garnet porphyroblast in quartz-feldspar gneiss, suggest that
shearing was important in developing some fo lia tio n (Cordua, 1973).
Lineations are prim arily defined by quartz boudinage, elongate feldspar
and garnet, and aligned hornblende in amphibolite. One orientation of
lineation is present in any outcrop (see Plates 1 and 2).
Copper Mountain Area
The Copper Mountain area lies in the center of a tight, isoclinal and overturned synform which is about 5 km across, and extends for approximately 14 km along the southwest flank of the Tobacco Roots.
Cordua (1973) divided the southern Tobacco Root Mountains into several structural domains. Domains 1 and 2 lie within the boundaries of the study area (Fig. 12).
Both limbs of the Copper Mountain synform are defined by thin bands of dolomitic marble which are surrounded by quartzofeldspathic gneiss, and dip west at about 60°- The central portion of the area 50
TOBACCO ROOT BATHOLITH
VIRGINIA CITY
PRECAMBRI AN TERTIARY CONTACT
Figure 12 Generalized map of the southern Tobacco Root Mountains showing the Copper Mountain area and major structural domains defined by Cordua (1973). Marble outcrop is stippled to show the general structural style. 51
is occupied by lenses of amphibolite, iron-formation and chert which show a double closure. Although closure to the north appears to be a relict of folding, the southern closure may be the result of juxta position of two separate folds during faulting.
Approximately 190 poles to regional foliation were plotted on the lower hemisphere of a Schmidt equal-area net to determine the trend and plunge o f the synform. This plot (Fig. 13) shows a sm all-circle geometry, suggesting a refolded fold with an average axis trending
N 6° W and plunging 20° northwest. This agrees well with the estimate of N 10° W, 25°W for the Copper Mountain synform (Cordua, 1973).
Although the overall structure is synformal , the northern portion of the fold is antiform al. Downward-facing folds (Hobbs and others,
197G) such as this are often formed during refolding of the strati- graphically inverted limb of a recumbant fold. A few post-metamorphic undulations and cross folds are also developed in amphibolite and chert outcrops in the southwest corner of the area.
Surface expression of faults at Copper Mountain is confined to a small swarm which offsets units in the southern portion of the area, and several unmappable faults within quartzite in sections 1 and 12.
Broken fragments of iron and copper-stained chert commonly mark these narrow zones, and slickensides are observed lo ca lly as w ell. Poor outcrop and th ick, 1ith o lo g ic a lly heterogeneous units may obscure exposure of other faults within the map area.
Kelly Area
The Kelly area forms the northern tip of a Precambrian wedge 52
3.S
Copper o Min
Figure 13. Contoured equal area plot of 190 poles to foliation in the Copper Mountain study area. Contour densities as shown. 53
5 km across, which extends for about 7 km along the northeast margin of the Ruby Range. The Kelly synform covers the entire map area as a relatively tight, east-southeast-plunging f£ fold bounded, on the north and west by dolomitic marble. The northern tip of an adjacent antiform trending N 40° W, occupies the extreme southeastern corner of the map area. A thick section of Paleozoic sedimentary rocks is juxtaposed against Archean lithologies to the north, east, and along the Kephart fa u lt to the west.
Folding is complex and exposure poor across much of the area.
However outcrop of several thin, and one relatively thick, bands of iron-formation help define the overall structure. Exposure within exploration trenches, and a ground magnetometer survey o f iron- formation in the northern h a lf of the area (James and Weir, 1972), lend some credence to structural interpretations.
Smith (1980) divided the Hinch Creek area to the south, into 13 structurally coherent domains. Domain 1 represents most of the Kelly area (Fig. 14). Folds and regional fo lia tio n generally trend east- west as opposed to the dominant northeast-southwest grain across the southern portion of the range. This discordance may be the result of disharmonic folding controlled by contrasts in lithologic competence.
Discordant folding has also been explained by large-scale recumbant nappes in the Ruby (Okuma, 1971; Karasevich, 1980), Tobacco Root
(Burqer, 1967; Duncan, 1978), and Beartooth (Reid and others, 1975)
Ranges. KELLY MAP AREA
L
’ . r , r y Hollow Creek
H i n c h
2miles
Figure 14. Map of the northern Ruby Range showing the location of the Kelly area and the major structural domains defined by Smith (1980). Outcrop of folded marble is indicated by the stipple pattern. 55
Lower hemisphere poles to 150 fo lia tio n attitudes were plotted for the Kelly synform (Fig. 15). This plot shows a strong concen tra tio n in the northern hemisphere, with two d is tin c t maxima representing the limbs of a fold whose average axis trends N 54° W, and plunges
60° to the southeast.
Northeast-trending faults cut across the area, creating the drainages w ithin Beatch and Wilcox canyons (Plate 2). A small eye-shaped horst is present just north of Beatch Canyon, with faults exposed in trenches cut in deeply weathered iron-formation and slickensided outcrops of quartzite along the Beatch Canyon jeep tra il. Although outcrop is sparse, Harold James (w ritten communication, 1980) believes th is block is fault bounded based on stratigraphic evidence and extensions of iron- formation using a ground magnetometer survey. James suggests that
"the concept of a squeezed up 'pumpkin seed' on a structural axis marked by a diapric ultram afic body, remains the most lik e ly interpre tation" for this locale. 56
y o S y n
Figure 15. Contoured equal area plot of 150 poles to foliation in the Kelly study area. Contour densities as shown. CHAPTER V
PROTOLITHS AND INTERPRETATIONS
The importance of considering the total Archean section exposed as a partial stratigraphic package representing a unique depositiona! environment has been underestimated. Multiple deformation and amphibolite grade metamorphism make determination of pre-metamorphic parents challenging. The metasedimentary origin of dolomitic marble, pelitic schist and micaceous quartzite is unquestionable, whereas quartzofeldspathic gneiss, amphibolite, pure quartzite, and iron- formation have an ambiguous history.
This chapter is an attempt to integrate field, petrographic and geochemical c rite ria which shed lig h t on possible pre-metamorphic parents. Major-element geochemistry is helpful in recognizing the precursors of the more elusive rock types; particularly the igneous, sedimentary, or mixed heritage of quartzofeldspathic gneiss and amphibolite. Major- element analyses of gneiss, amphibolite, and quartzites are presented here, as are analyses of iron-formation from both study areas by Bayley and James (1973) and Dahl (1977).
The f ir s t h a lf of th is chapter is devoted to the orig in of lithologies exposed within the study areas as well as across most of the Montana basement. The remainder is an attempt to compare this
Archean terrane with a fore-arc or accretionary prism environment.
57 58
Quartzofeldspathic Gneiss
Quartzofeldspathic gneiss is both the most abundant and enigmatic lithology within the basement terrane of southwestern Montana.
S tru ctu ra lly, these migmatitic gneisses form the lowermost unit in both study areas. Foliation and large-scale compositional banding w ithin the gneiss are everywhere parallel with the contacts of surrounding lith o lo g ie s . Contacts between the gneiss and overlying marble are sharp, but are gradational over a few meters with surrounding amphi- b o lite s. Concordant lenses and re la tiv e ly thick layers of hornblende gneiss are enclosed w ithin, and grade into , the dominant quartz- feldspar gneisses. On a mapping scale of 1" = 1000* these gneisses are extremely monotonous. Possible sedimentary or igneous textures and structures have been homogenized during re crysta lliza tio n and mi gmi ti zati on.
Compositionally and m ineralogically these rocks are granitic or rhyolitic in composition. Major-element analyses (Table 4) of four gneisses from the Copper Mountain and Kelly areas show calc-alkaline affinities with 69 to 75 percent Si0£ and 4 to 8 percent total alkalis.
Both Heinrich (1960) and Smith (1980) have suggested that gneisses in the Ruby Range represent a metamorphosed synkinematic pluton.
Tnis conclusion is somewhat at odds with th e ir regionally concordant contact relationships, millimeter to decimeter-scale compositional layering, and enclosed layers of hornblende gneiss and metabasite which most probably represent basic flows and minor pyroclastic rocks. Table 4. XRF chemical analyses of quartzofeldspathic gneiss from the study areas.
Quartzofeldspathic Avg. Granite Franciscan Greywacke Gneiss (Hyndman, 1972) (Bailey and others, 1969) KQF1 KQF2 KQF 3 CMQF 1 SiQ2 69 .88 74.86 72.31 73.77 73.86 71.7 Ti02 0.34 0.20 0.29 0.37 0.20 0.3 18.17 12.96 16.97 13.01 13.75 13.2 Al2°3 Fe^O ^ 0.74 0.68 0.95 2.14 0.78 0.3 Z j FeO 0.85 0 .78 1 .09 2.45 1.13 3.6 MnO 0.01 0.02 0.01 0.05 0.05 0.0 CaO 3.63 0.28 3.08 1 .42 0.72 1 .8 MgO 0.89 0.69 0.78 0 .56 0 .26 1 .8 Na20 3.73 1 .55 3.49 1 .32 3.51 2.7 k 20 1.4 1 6 .68 1 .53 4.91 5.13 1 .3 0.10 0 .04 0.03 0.07 0.14 0 . 1 P2°5 Total 99 .74 98.74 100.52 100 .22 99 . 53 96.7
* D.F. 2.61 -0.38 1.49 -1.64 2.32 -2.11
* D.F. - Discrimination Function = 10.44 - 0.21 x SiO- - 0.32 x Fe?(K (as total iron) - 0.98 x MgO + 0.55 x CaO + 1.46 x Na0(3 + 0.54 x Ko0 (Shaw,. 1972) Z Z cn <£> 60
Garihan and Williams (1976) noted that the composition of these gneisses is also sim ilar to metamorphosed arkose (feldspathic arenite) and/or siliceous shale. Rounded and elongate zircons within gneisses in the Tobacco Roots (Cordua, 1973) have also been used to suggest a mature meta sedimentary parent. A metasedimentary origin is consis tent with the conformable and compositionally variable nature of the gneiss and is plausible for at least the strongly banded micaceous- garnetiferous varieties.
Shaw (1972) developed a discrim ination function fo r quartzo feldspathic gneisses based upon the weight percent values of Si 02,
^e2®3 (as total iro n ), MgO, CaO, Na20, and J^O (Table 4). Positive values of the function indicate a probable igneous parentage, whereas negative values characterize gneisses of sedimentary origin. Two of the gneisses analyzed in this study show positive, and two show negative function values (Table 4), suggesting that both igneous and sedimentary parents are represented and interlayered. Gneisses with positive values show high potassium contents typical of igneous rocks, whereas gneisses with negative function values have lower potassium contents consistent with feldspathic sands (see Table 2).
Based upon fie ld and geochemical c rite ria , the quartzofeldspathic gneisses most probably represent a thick package o f metamorphosed fledspathic sands or “greywacke" at least 5 km thick, interstratified with rh y o lite , basalt flows and pyroclastic rocks. Hornblende gneisses and orthogneisses may represent a bimodal trondjehmitic calc-alkaline suite (Fountain and Wilson, in preparation). 61
Dolomitic Marble
As metamorphosed accumulations of carbonate sediment, these marbles represent unambiguous evidence of shallow to deep marine sedimentation. The ubiquitous presence of tremolite, diopside, and fo rs te rite suggests that these carbonates were o rig in a lly impure siliceous dolomites. Discontinuous layers of quartz boudinage up to a meter thick may represent original thin stratiform layers of chert which replaced the carbonate.
I t is d if f ic u lt to determine i f these carbonates originated as chemical percipitates or biogenic oozes. Their close association with amphibolites of possible volcanogenic origin, a common association both in Archean and recent (Karig, 1973) te rra in s, suggests deposition may have been accomplished during quiescent periods between submarine volcanic emanations.
Amphibolite
The chemical and physical characteristics of amphibolites derived from isochemical metamorphism of basalt flows or tuffs, volcaniclastic sediments, or calcareous shales (marls) are essentially identical
(Engel and Engel, 1962). This makes conclusive determination of amphibolite proto!iths d if f ic u lt and, perhaps, impossible i f a ll three rock types are present in the original section.
The amphibolites in this study occur as relatively thick and continuous lenses and layers, which are everywhere conformable to the foliation and compositional banding in surrounding lithologies. 62
Extensive recrystal 1ization and metamorphic differentiation have obliterated much of the original textural features of the protolith.
Crosscutting relationships, and igneous textures (chill margins, relict phenocrysts, xenoliths, and pillows) are not displayed in these amphibolites, but have been documented within metabasites in the southern
Tobacco Roots (Cordua, 1973; Hanley, 1976). Sedimentary textures such as r e lic t quartz pebbles (Bielak, 1978) or c a lc -s i1icate layers are also conspicuously absent, although quartzite pods up to 20 meters thick can be observed in some outcrops.
In the field, gradual variation in grain size (fine to coarse), structure (well or poorly developed banding), and mineralogical com position (percentage of quartz, plagioclase, and hornblende) can be observed across a single exposure. Bielak (1978) concluded that banded hornblende gneisses in the Winnipeg creek area of the Ruby Range were originally sedimentary rocks, whereas massive amphibolites were basalt flows. If these textural and mineralogical variations were distinct rather than la te ra lly and v e rtic a lly gradational, a heterogenous rock sequence would make better sense. These variations may simply be a relic of minor compositional and textural inhomegenities enhanced during metamorphism however.
Geochemical c rite ria (Burger, 1969; Cordua, 1973) applied to amphibolites in the Tobacco Roots suggest that they are igneous in origin on the basis of the high Cr and Ni, low K^O and SiC^and A^Og contents typical of known Archean basalts. 63
Major-element analyses and Niggli values were determined for 8 amphibolites from the Copper Mountain and Kelly areas (Table 5).
Massive, banded, and salt-and-pepper varieties show nearly identical bulk compositions, and compare well with amphibolites from the Madison
(Foster, 1962), Spanish Peaks (Spencer and Kozak, 1975), and
Beartooth (Van de Kamp, 1969) ranges. Compositions strongly resemble those of basic igneous rocks, p a rtic u la rly th o le iitic basalts, except that iron in the latter is less oxidized. Niggli C/Mg and A1-A1K/C ratios (Leake, 1964) are compatible with igneous trends (Karro dolerites) and are oblique to trends expected fo r calcareous shales
(Fig. 16).
On the basis of field and geochemical criteria, amphibolites in the Copper Mountain and Kelly areas appear to represent metamorphosed flows of tholeiitic basalt to basaltic andesite composition. There are no real differences in the composition of banded or massive am phibolites suggesting that i f sediments are present they are volum etrically minor or were derived from basic igneous rocks.
Pelitic Schist
Thin, poorly exposed layers of pelitic schist are commonly inter layered, and locally grade into micaceous quartzite, pure quartzite, and iron-form ation. The mineralogy of these schists suggests a siliceous and alumina-rich protolith. Modally estimated chemical compositions (Cordua, 1973) are consistent with re crystallized siliceous shale. The presence o f graphite in some schists suggests these terrigenous sediments were also somewhat carbonaceous. Table 5. XRF chemical analyses of amphibolite from the study areas. Amphibolite Avg. Oceanic Tholeiite Banded Salt and Pepper Massive (Hyndman, 1972) CM10 CM40 CM81A K7 K81A CM36 CM6 K16
5 3 ,.76 54 ..25 42 . Sl02 .09 48.,37 41 ..67 52., 18 56 .,05 49.,29 49 ..50
0 ,.47 0..54 1 ., 12 1 .,60 1 ..98 0. 0 .,47 0 ..77 .80 Tl°2 ,37 1 . a i 2o 3 15.,67 14 .,99 17 .,44 15.,87 21 .,43 13..16 15.,50 10,,51 15..20 4 ..33 5.. 10 7 ..21 ,56 7..21 5. 5..98 2,.40 Fe2°3 6. .13 4 ..19 FeO 4 .,96 5..84 8.,26 .7..51 8..26 5..88 4 ..80 6 ..85 8 ..00
MnO 0 ,.15 0 ,.19 0 ,.22 0..25 0..10 0..21 0 ..16 0,.23 0,.17 CaO 9..98 10 ,.06 9 ..23 9..29 10 ,.73 8,.62 10 ..94 8..70 10 ,.90
MgO 8 ..39 8,. 15 11 ,.07 6,.82 5..92 12..85 7..17 18,.08 7 ,.90
o c * Na20 1 .86 1 ,.85 2 ,.97 1 ,. 2 8 1 ,.28 1 ,.69 1 ,.63 2 .70 ,
K,0 0 ..36 0 ,.72 0 ,.26 0 ,.18 0 ,.39 0 ,.39 0 ,.71 0 ,.19 0,.26
^2^5 0 ,.07 0 ,.05 0 ,. 10 0 ,.14 0 ,.04 0 ,.04 0 ,.05 0 ,.08 0 .21 Total 100 ..69 101 ,. 84 93 ,.84 99 ,.56 100 ,.10 100 ,.10 101 .74 97,.31 99 .04
Si 125 ,.90 118,.40 86 ,.80 110,.20 87,.22 111,.60 136 ,.80 90 ,.09
Ti .83 .89 1 ,.73 2 ,.73 3,. 1 1 .60 .86 1 .05 P .07 .05 .09 .14 .05 .04 .05 .06
C 25,.04 23,.52 20 ,.41 22,.67 24,. 0 6 19 ,.78 28,.60 17 .04
A1 21 ,.59 19 ,.25 21 ,. 18 21 ,.26 26,.38 16,.58 22 .24 11 .29 o o Fm 47 .09 52 .27 54 . 36 49 .24 44 .50 60 .45 44 68 .55 o Aik 6 .26 4 .93 5 .43 6 .. 80 5 .05 3 .17 5 . 11 •J .10 K .08 .20 .08 .04 .10 .16 .21 .07 Mg .62 .63 .53 .51 .42 VO oo .59 .72 65
50 CARBONATES
30
SHALES
O .20 .40 .60 .80 1.00 mg
50
SHALES
30
al-alk
IGNEOUS FIELD DOLOMITE LIMESTONE
20 40 60 80 100
-10
Figure is. Niggli C/Mg and Al Alk/C (Leake, 1964) plots for 8 analyses of amphibolites from the Copper Mountain and Kelly areas. 66
Impure and Pure Quartzite
Three distinct varieties of quartzite are present in the study area: micaceous quartzite, garnet quartzite, and pure quartzite.
The impure micaceous quartzites are essentially quartz-microcline- plagioclase rocks with accessory garnet, sillimanite, muscovite, and biotite. They typically contain 50 to 60 percent quartz and 20 to 30 percent microcline, giving them an almost igneous appearance in thin section. Mylonitization and recrystallization destroyed original textures, with the exception of a planar layering defined by quartz and oriented micas.
Major-element analysis of quartzite lenses from both study areas reflects a silica- alumina- and relatively potassium-rich rock as expected from the mineralogy (Table 6). The composition, texture, and association of these quartzites with marbles and pelites in the fie ld , suggests sandstone as the p ro to lith . Their chemical composition
(Table 6) is sim ilar to feldspathic arenites, suggesting these sands may well have had a plutonic provenance (e .g., Pettijohn and others,
1972).
The presence of garnetiferous quartzites (40 to 45 percent garnet) which contain less biotite and microcline and slightly more plagio- clase than typical micaceous quartzites, may be of genetic significance.
Similar garnet quartzites associated with iron-formation at Broken
H ill, A ustralia (Stanton, 1976) are considered chemical sediments of volcanic o rig in (Kramm, 1972). They may however simply represent sands locally rich in clay. Table 6♦ XRF chemical analyses of quartzites and chert from the study areas.
Avg. Ortho- Avg. Arkose quartzite Franciscan Chert Micaceous (Pettijohn, (Pettijohn, (Bailey and others, Quartzites 1972) Chert 1972) 1969) K2 6 . CM 3 7 CM1 K4 2- S102 83.33 81.73 81 .48 94.77 93 .54 97 .20 95.90 Ti02 0.30 0.20 0.25 0.05 0.01 0.00 0.06 8.19 9.14 7 . 82 1 .54 1 .23 1.19 1.10 A12°3 1 .25 0.39 1 .45 0.04 0.06 1 .70 Fe2°3 0.03 FeO 1.43 0.44 0.76 0.05 0.05 0.11 0.34 MnO 0.23 0.01 0.01 0.00 0.00 0.00 0.05 CaO 0.13 0.13 0.37 0.07 0.08 0.05 0 .50 MgO 1 .35 0.77 0 .55 0.11 0.04 0.06 0.10 Na2o 0.49 0.73 2.30 0.24 0.14 0.06 0.02
k 2o 3 .47 4.97 4.43 0.24 0.05 0.09 0.26 0.02 0.03 0.01 0.02 0.03 P2°5 - 0.00 0.00 Total 100.18 98 .54 99 .43 97.12 95.05 9 8. 82 100.06 68
Lenses of nearly pure (95 to 98 percent quartz) massive to
layered quartzite are sharply interlayered with micaceous quartzite
and schist in both study areas. Major-element analyses (see Table 6)
show that these rocks have a very different composition with less
alumina, iron, and potassium, and more silica than surrounding
micaceous quartzite. This chemistry is consistent with isochemically
recrystallized orthoquartzite, quartz arenite, and chert. The pure
siliceous composition, sharp contact relationships with surrounding
schists and impure quartzites, and close association o f these rocks
with amphibolite and iron-formation, strongly supports chert rather
than pure sandstone as a p ro to lith .
The o rig in of Precambrian cherts is uncertain. Biogenic (Laberge,
1972) and inorganic (Koehler, 1972) precipita tion have been suggested
on textural and petrologic grounds. Inorganic precipitation of silica
requires concentrations of about 100 ppm of silica (10 times greater
than modern oceanic concentrations) conditions which may have been
possible under Archean weathering (Cloud, 1970) or siliceous volcanic emanations (Ingram , 1972; Siever, 1977). Such exhalations have been documented in the modern record (Kanmera, 1974) and would mesh well with amphibolite and iron-formation of a volcanogenic nature. Strikingly sim ila r massive to bedded cherts are interbedded with thin aluminum and iron rich shales and associated with massive and pillow basalts in the Mesozoic Franciscan Formation of C alifornia (Bailey and others,
1964). These rhythmically bedded Mesozoic Franciscan cherts contain 69
small amounts of pyroclastic material, but contain abundant radiaolaria
and are considered ra io laria n oozes (Pessagano, 1973).
Iron-formati on
Massive and banded garnet-bearing iron-formation occurs as thin
lenses and folded layers in both areas associated with amphibolite.
Contact relationships exposed in exploration trenches and in outcrop,
show that iron-formation is intimately interlayered with quartzite and
chert, and locally grades into thin beds of garnet-biotite-si11imanite
schi st.
Major-element analysis (Table 7) reveals a relatively siliceous
and aluminous lith o lo g y, strongly enriched in both ferrous and fe r ric
iron. The quartz iron-formation reflects its different mineralogy with higher silica, manganese, and calcium, and lower total iron,
than the garnet iron-formation.
Archean iron-formations are commonly regarded as metamorphosed,
chemically precipitated, water-deposited sediments. Some workers
favor a biogenic orig in (Cloud, 1973; Laberge, 1973; Trendall, 1977), whereas many others suggest an exhalative origin (Hutchinson and others, 1971; Goodwin and Ridler, 1973) for iron-formation based on
its presence within thick sections of mafic to felsic flows, tuffs,
and volcaniclastic sediments. The source of iron and s ilic a is the
greatest unknown in dealing with these rocks. Extensive erosion of
lo w -re lie f land masses in a C02-rich Archean atmosphere (Cloud, 1973), and submarine exhalations of iron and silica into volcano-tectonic Table 7. Chemical analyses of iron-formation from the study areas.
Iron-Formation
Kelly Area Copper Mountain Area (Bayley and James, 1973) (Dahl, 19 77) (Cordua, 19 73)* K1 K2 GIF QIF1 QIF2 CMl CM2
Si°2 40.00 39.58 38.40 50.00 71.90 39 47
Ti02 0.08 0.10 0.03 0.05 0.44 - - a i 2o 3 1.95 2.47 6.21 6.08 1.16 5 4 32.91 32.07 29.40 23.50 0.88 Fe2°3 22 17 FeO 18.87 19.22 20.60 15.00 10.80 29 27
MnO 0.73 0.75 0.75 2.07 2.83 - -
CaO 1.87 1.90 1.87 0.99 7.40 - - MgO 2.46 2.41 2.31 0.62 3.03 3 4 Na20 0.12 0.09 0.03 0.00 0.07 n.d. n.d. k 2° 0.56 0.85 0.15 0.07 0.03 n.d. n.d. 0.10 P2°5 0.09 0.09 0.07 0.01 n.d. n.d. h 2°+ 0.31 0.33 0.23 0.38 0.26 n.d. n.d. h 2o - 0.15 0.16 0.07 0.21 0.10 n.d. n.d. co2 0.15 0.16 0 .00 0.07 0.41 n.d. n.d. Total 100.24 100.13 100.14 99.11 99.32 98 99
* Partial chemical compositions based on modal analyses corrected for density and assuming the garnet is pure almandine and the orthopyroxene is ferro- hypersthene (En^) . ** n.d. = not determined 71 basins (Goodwin, 1973; Erikkson, 1980) have been suggested. Both mechanisms, as well as a combination of the two, may have been
important.
Gross (1965) divided iron-formations into two major types:
1) A1goma-Type iron-formation
a) Small, discontinuous lenses of oxide-facies iron-formation
which are m illim eters to several hundred meters th ick,
and rarely more than a few kilometers long.
b) Intimately associated with calc-alkaline to tholeiitic
volcanic flows, pyroclastics, and graywacke.
c) Archean in age, and interpreted as representing colloidal
deposition of iron and silica supplied by volcanic
emanation in a te cto n ica lly active basin.
2) Superior-Type iron-formation
a) Very continous and evenly bedded lenses tends to
hundreds of meters th ic k , and up to 100‘ s of kilometers
long, with several different facies representing
conditions of basinal depth.
b) Volcanic rocks are not everywhere d ire c tly associated
but are invariably present.
c) Proterozoic in age, representing relatively quiet shallow-
water basins supplied with iron and silica from subarial
weathering with some input possible from volcanic sources. 72
Archean iron-formations in the Copper Mountain and Kelly areas correspond nicely to the volcanogenic Algoma-type (Cordua, 1973;
Hanley, 1976). Several discontinuous layers of s ilic a te facies iron-
formation, indicating deposition at intermediate water depths and corresponding eh and pH (James, 1954), occupy both areas. Thicknesses vary from 10 to 60 meters with s trik e lengths o f 5 and 10 kilometers
in the Kelly and Copper Mountain areas respectively. Amphibolites and cherts of possible volcanic origin are intimately associated, as are micaceous quartzites and schists representing terrigenous sedimentation. A similar situation may exist within the Michipicoten basin in Ontario (Goodwin, 1973) where iron-formations considered volcanic exhalites, are enclosed exclusively within volcanic rocks on one basin edge, c la s tic sediments on the other, and both rock types in the central portion of the basin.
The tops of thin chert layers in the Copper Mountain and Kelly iron-formations show continuous millimeter-thick stringers of felds par. S im ilar features observed in Canadian iron-formations have been interpreted as original thin layers of ash (T. LaTour, verbal communi cation, 1980; B. Barnett, verbal communication, 1981).
The Copper Mountain and Kelly iron-formations may well have accumulated during the waning stages of submarine volcanism, sim ilar to ferruginous m etalliferous sediments now p e rcip ita tin g at sites of fumarolic brine emission on the floor of the Red Sea (Fryer and
Hutchinson, 1977). 73
Ultramafic Rocks
Small periodotite and pyroxenite bodies scattered about both areas, were originally emplaced as relatively cold Alpine-type ultramafic
fragments* Spinifex textures are absent, suggesting that these bodies
did not make it to the surface as ultramafic flows or komatiites, or
that original textures have been destroyed during metamorphism.
Petrographic studies (Desmarais, 1978; Tendall, 1978) indicate that these ultramafic rocks were originally partially serpentinized, and emplaced as a solid or crystal much p rio r to or during prograde metamorphism. Progressive metamorphism de-serpentinized the original metamorphic rock (Desmarais, 1978), which was la te r serpentinized as a re su lt of greenschist facies metamorphism. Desmarais and Fountain
(1980) suggest that some of the ultramafic bodies within the Ruby
Range, form boudins within hornblende gneiss, and may be tectonites rather than intrusions.
Tectonic Interpretations
The Archean sequence exposed in the Tobacco Root and Ruby Ranges is characteristic of the entire Dillon block which includes the
Highland (Duncan, 1976; Gordon, 1979), Greenhorn (Berg, 1976),
Gravelly (M illholland, 1972), Madison (McThenia, 1960; Spencer and
Kozak, 1975; Erslev, 1980), and northwestern Beartooth (Van DeKamp,
1969; Reid and others, 1975) Ranges. Very l i t t l e has been done to match this depositional sequence with a modern analog. This is in part due to the poor understanding of Archean tectonic processes as well as an 74 absence of lithologic correlation across range boundaries. Recent advances in plate tectonics (Engel and Kolm, 1972; Burke and others,
1976) strongly suggest that horizontal tectonics involving micro- continental collisions and island-arc formation is the most plausible mechanism to explain many Archean terranes.
Hanley (1976) proposed a back-arc or marginal basin (Karig,
1971; Packham and Falvey, 1971) as a depositional environment fo r the sequence w ithin the Tobacco Root Mountains. Sim ilar sequences w ithin the Cretaceous Rocas Verdes Complex of southern Chile (Tarney and others, 1976), the Barberton Mountain Land of South Africa (Eriksson,
1980) and the Sinai Supergroup of the eastern Mediterranean have also been interpreted as back-arc basins.
Although this model seems to work well fo r greenstone belt assemblages, there are at least three major problems in applying a back-arc model to the high-grade terrane of southwestern Montana:
1) Sedimentation and deformation are commonly quite diverse across small distances in back-arc environments (Karig and Moore,
1975). The Montana basement however, is characterized by its lith o lo g ic and structural continuity over great distances.
2) Back-arc settings commonly contain complete or s ig n ific a n t portions of o p h io lite sequences. The thick layered gabbros and sheeted dike swarms typical of those sequences are absent in southwest
Montana.
3) Metamorphism across back-arc basins is typically low-grade greenschist facies (Karig and Moore, 1975; Tarney and others, 1976). 75
Although a small patch of low-grade rocks is present in the Gravelly
Range, nearly the entire basement has endured upper-amphibolite facies
conditions.
Fountain and Desmarais (1980) compared the Montana basement to the Wabowden Subprovince of Canada, and by analogy, an accretionary plate margin. More specifically, I propose that the general setting of a fore-arc basin (Figure 17) may provide a modern tectonic de- positional analog for this Archean assemblage. A fore-arc basin or accretionary prism occupies the entire region lying on the trench side of the andesitic volcanic chain o f island-arc systems. These marine basins are commonly on the order of 50 to 500 km across and are fille d with 5 to 12 km of tu rb id ite and pelagic sediments imbricated with mafic volcanic rocks, chert, and slivers of ultramafic rock (Moore,
1972; Karig, 1973; Dickinson and Seely, 1979).
The Mesozoic Franciscan Formation of the C alifornia Coast ranges
(Bailey and others, 1964), the vestige o f a large marine accretionary wedge, including thick terrestrial elastics, may provide a deposi- tional and deformational analog o f the Archean assemblage exposed across the Montana basement. This rock package is s trik in g ly sim ilar and includes, in order of decreasing abundance: quartzofeldspathic
"greywacke", black and ferruginuous shale, massive and pillow basalt, serpentinite slices, massive and bedded chert, and cherty limestone.
These lith o lo g ie s occur as coherent mappable units (Fox, 1976) and chaotic shear-bounded melanges (Bailey and others, 1964) and are in 76
UPPER-SLOPE DISCONTINUITY
Madison Mylonite Zone ?
FORE-ARC BASIN OF MA1N-ARC BACK-ARC ACCRETIONARY PRISM Beartooth Range * BASIN . Dillon Block ?
NW SE
Figure 17 Schematic diagram showing the tectonic setting and possible reconstruction of the Montana basement as a partial cross- section through an Archean island arc. 77
general, physically, compositionally, and volumetrically comparable with corresponding rock types in the Montana basement.
"Greywackes" compose 70 to 80 percent of the Franciscan Formation, attaining thicknesses of 4 to 6 km (Bailey and others, 1964). Like the quartzofeldspathic gneiss, these sands are g ra n itic and quartz dioritic in composition, show large-scale compositional banding, and are interbedded with mafic volcanic rocks. Much of the Franciscan grey- wacke however, contains less K-feldspar than the Montana quartz- feldspar gneisses. Sands from the coeval Great Valley sequence to the east, do contain appreciable K-feldspar derived from the adjacent arc (D. Hyndman, personal communication, 1981). Perhaps the gneisses in the Copper Mountain and Kelly areas represent an environment similar to the transition zone between the Franciscan and Great Valley
Sequence.
Massive and pillow basalts form about 10 percent of the Franciscan package and are comparable with amphibolite and metabasite. R elict pillows have been described in Amphibolite exposed in the Ruby Creek area of the Gravelly Range (Bayley and James, 1973).
Lenses of cherty limestone, from several meters to 1200 meters thick and up to a kilometer long, are commonly in contact with these
Franciscan basalts (Bailey and others, 1964; Jones and others, 1978).
These limestones contain abundant ra diolaria and represent both shallow and deep-water carbonate deposition (Wachs and Hein, 1975).
They appear to be less abundant in the Franciscan section than are marbles in the basement terrane. 78
Fine-grained black shales interstratified with sandstones, and red ferruginous shales intimately associated with massive and bedded cherts make up another 10 percent of the Franciscan section. These ferruginous shale-chert sequences may be analogous to pelitic schist and interbedded massive to banded cherts in my study areas. Differing conditions in the Archean may be responsible for the absence of iron- formation in the Franciscan Formation. Conformable masses of serpen tin iz e d p e rid o tite and dunite analogous to those in the basement terrane, are also distributed throughout the Franciscan complex.
Franciscan structure is complex and s till poorly understood
(Jones and others, 1978). The en tire sequence is is o c lin a lly folded and locally overturned. Gently to steeply dipping faults and shear zones (Cowan, 1975; Fox, 1976) are common but are d if f ic u lt to detect as most parallel the structural grain; this may be the case in south western Montana as well. Although metamorphism of the Franciscan is dominantly blueschist facies (high pressure/low temperature) type, the upper-amphibolite to lower-granulite conditions across the Montana basement could easily be the re sult of higher Archean geothermal gradients, or a full equilibration following blueschist metamorphism.
If this model holds, the Dillon block would represent a remnant fore-arc environment. The Beartooth Range to the southeast, dominantly composed of quartz-feldspar gneiss and Archean granitic and tonalitic plutons (Butler, 1969; Weeks, 1979) could represent the eroded bowels o f the main-arc (see Fig. 19). 79
A 3 km-thick mylonite zone, documented by Erslev (1980) in the neighboring Madison Range, could represent the faulted upper-slope discontinuity between the fore-arc and main-arc of modern island-arc systems. Determining the orig in o f this shear is c r itic a l however since it may also represent a major strike-slip fault, or a crustal suture between two genetically unrelated terrains. The abundance of mylonitic textures throughout the Montana basement suggests that other unrecognized shears, analogous to those in the Franciscan terrane, may well exist.
An understanding of the sharp grade boundary (M illholland, 1976) between greenschist and amphibolite facies in the northern and southern
Gravelly Range is also important. Fountain and Desmarais (1980) have suggested that this boundary may represent the juxtaposition of two slices of d iffe rin g metamorphic grade during underthrusting in an accretionary prism.
These basement blocks represent a tectonic element which has under gone a distinctive and decipherable history. An island-arc environment provides an excellent framework fo r this Archean domain and provides a plausible point of focus for future reconstruct ions. CHAPTER VI
CONCLUSIONS
Petrology, major-element geochemistry, sequence and thickness o f Archean lith o lo g ie s exposed in the Copper Mountain and Kelly synforms, suggests an equivalent assemblage occupies both areas.
This assemblage is typical o f much o f the Montana basement and is composed of migmatitic quartzofeldspathic gneiss, dolomitic marble, massive to banded amphibolite, micaceous and pure quartzite (chert), p e litic schist, banded and massive iron-form ation, and small con formable ultramafic (peridotite and pyroxenite) masses. Several meta-diabase dikes cut across the Copper Mountain area and to the south of the Kelly area. Foliation parallels the compositional layering within, and the contacts between units, suggesting that much of the metamorphic texture re fle cts an original layering.
At least two periods of folding around north-northeast axes affected both areas. Tight isoclinal f] folds occur within the quartz- feldspar gneiss, amphibolite, and iron-formation. The overall syn- formal structure of both areas represent the f 2 folds. Pi diagrams drawn from poles to regional foliation suggest that the Copper
Mountain and Kelly synforms trend N 6° W, 20°NW, and N 54° W,
60° SE, respectively. This divergence in trend may be controlled by lithology or be the result of large-scale nappes.
80 81
Texture and mineralogy suggest that prograde metamorphic con ditions straddled the upper-amphibolite lower-granulite-grade boundary with temperatures in the 700-800°C range and pressures of 4-8 kbar.
A common boundary can be drawn fo r both areas at 750°C and 6 kbars, on the basis of electron microprobe analysis of iron-formation minerals from both areas (Immega and Klein, 1976; Dahl, 1977). These conditions correspond to a mid-crustal depth of metamorphism o f 20 to 25 km. The sporadic occurrences of granulite mineralogy in mafic and ultramafic lithologies represent locally drier rocks where P H 2O < P load during metamorphism. Greenschist-facies assemblages are developed across both areas, re fle c tin g a greenschist-grade metamorphic event which reset
K-Ar clocks at 1600 Ma.
Field, petrographic, and geochemical c rite ria suggest the dominant quartzofeldspathic gneisses originated as a thick sequence of feldspathic sands with interbedded rh yo lite and basalt flows. Massive and banded amphibolite are compatible with metamorphosed tho leiitic basalt flows.
Micaceous quartzites and pelitic schists represent terrigenous influx of feldspathic sands and aluminous (perhaps cla y-rich ) sediments, whereas pure quartzite lenses were probably cherts. Dolomitic marble formed as siliceous, calcareous oozes during quiescent non-volcanic episodes. Massive and banded iron-formations formed as ferruginous- siliceous brines associated with submarine volcanism. Ultramafic masses were conformably emplaced as cold tectonic slices before or during prograde metamorphism and deformation. Diabase dikes which 32 cut across both ranges represent a late-stage (1400-1600 Ma.) ex tension and intrusion of tholeiitic basalt.
A fore-arc or accretionary prism environment provides a cogent tectonic model fo r the assemblage exposed in the Copper Mountain and
Kelly areas and explains lith o lo g ic and structural variation across much of southwestern Montana. The Franciscan Complex-Great Valley
Sequence of California may provide a direct sedimentological and de- formational analog fo r th is Archean assemblage, with is o c lin a lly folded and sheared feldspathic greywackes , limestones, ferriginuous shales, massive and bedded cherts, massive and pillow basalt, and conformable masses of serpentinized ultramafic rocks.
In applying th is hypothesis the Dillon Block would represent the fore-arc with the Beartooth Range representing the eroded bowels of the main-arc environment. A 3 km-thick mylonite zone in the neighboring
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XRF Sample Locations From
The Copper Mountain and Kelly Study Areas 92
Appendix 1 XRF Sample Locations From The Copper Mountain and Kelly Study Areas
Copper Mountain Quadrangle, Southwestern Montana
C M —1 Pure quartzite (chert) S E h S e c . 1 T.5S. R.4W.
CM-6 Massive amphibolite N W h S e c . 1 T.5S. R.4W. C M - 10 Banded amphibolite swh Sec. 11 T.5S. R.4W. C M - 3 6 "Salt and pepper" Sec. 36 T.4S. R.4W. amphibolite s w h C M - 3 7 Quartz ite swh Sec . 6 T.5S. R.3W.
CM - 4 0 Massive amphibolite N W h S e c . 31 T.4S. R.3W.
CM-81A Massive amphibolite N E h Sec. 12 T.5S. R.4W.
C M —Q F — 1 Biotite Q-F gniess N W h Sec. 35 T.4S. R.4W.
Southwestern Montana
K-7 Banded amphibolite SE^ S e c . 25 T . 6 S . R. 5W.
K - 16 Massive amphibolite N W h S e c . 30 T . 6S . R.4W.
K-26 Quartzite N W h S e c . 30 T.6S. R . 4 W.
K — 4 2 Pure quartzite (chert) S E h S e c . 25 T . 6S . R.5W. K-81A "Salt and pepper" S E h 24 T . 6 S . R . 5W . amphibolite Sec .
K —Q F — 1 Biotite Q-F gneiss S E h Sec . 30 T . 6 S . R. 4W.
K - Q F —2 Augen Q-F gneiss S E h Sec . 30 T . 6 S . R. 4W.
K —Q F - 3 Augen Q-F gneiss S E h S e c. 30 T.6S. R.4W. GEOLOGIC MAP OF THE COPPER MOUNTAIN AREA, SOUTHERN TOBACCO ROOT MOUNTAINS, MADISON COUNTY, MONTANA
Geology by Michael L. Wilson 1981
EXPLANATION
Feldspar porphyry
Ultramafic rocks: pendotite and pyroxenite
Pegm atite
Massive and banded iron-formation
Micaceous quartzite, thin layers of pelitic schists, and pure quartzite or chert
Am phibolite
D olom itic m arble
Q uartzofeldspathic gneiss w ith layers of amphib olite and metabasite
Axis o f overturned antiform
Axis of overturned synform
Strike and dip of foliation
Trend and plunge o f lineation
Exploratio n trench
ZOO 400 Meters
No Vertical Exaggeration
PLATE I. U e s f j
Mektniu W ils **, GEOLOGIC MAP OF THE KELLY AREA, NORTHERN M L ( f t
Geology by Michoel L Wilson 1981
EXPLANATION
on Ullioira fic rocks: pendotite and pyroxenite uy\Or
Mosstve and banded iron-formation
I j o Gam etiferous q ua rtz ite
quartzite, thin layers of pelibc schist, and
S Quartzofeldspathic gneiss with layers of omphibolite
M o d , dashed where inferred
J . ' ' F a it
^Bend in secto r _ 4 _ , Axis of anhform
Aits of synform
M e : E ite is w s of ir o n - fm a ta io the eorthere Strike anil dip of bedding half ef tte otea ore based en a ground mag- K t o * survey a id r q y n g by James end Strike and dp of foliation Wer (1 972).
Strike and dip o f nerticd Motion
Trend and plirtge o f lineation
PLATE 2.