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1982
Mylonite zones in the crystalline basement rocks of Sixmile Creek and Yankee Jim Canyon Park County Montana
Robert Robert Burnham The University of Montana
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Mansfield Library U niversity of Montana Date: 19 8 2
MYLONITE ZONES IN THE CRYSTALLINE
BASEMENT ROCKS OF SIXMILE CREEK AND
YANKEE JIM CANYON, PARK COUNTY, MONTANA
by
Robert Burnham
B.A., Dartmouth, 1980
Presented in partial fulfillment of the requirements for the degree of
Master of Science
UNIVERSITY OF MONTANA
1982
Approved by: UMI Number: EP40141
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ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 ABSTRACT
Burnham, Robert, M.S., Fall, 1982 Geology
Mylonite Zones in the Crystalline Basement Rocks of Sixmile Creek and Yankee Jim Canyon, Park County, Montana
Director: Donald Hyndman
Yankee Jim Canyon and Sixmile Creek, approximately 50 kilometers south of Livingston, Montana, expose strongly deformed and meta morphosed Precambrian crystalline basement rocks. Mapping reveals two distinct packages of rocks: a medium-grained gneissic package, and a fine-grained schistose package. The gneissic package in Yankee Jim Canyon is subdivided into a mixed gneiss unit and a mylonitic potassium feldspar gneiss unit. In addition to the gneissic package, Sixmile Creek contains amphibole schist and micaceous schist of the schistose package.
A northeast structural trend characterizes both map areas. The study area lies along a large northeast structural trend which extends from the southern Madison Range to the north Snowy block (Reid et a l., 1975; Erslev, 1981; Mogk, 1982). In Sixmile Creek, two ductile shear zones follow a similar northeast structural trend. Yankee Jim Canyon exposes only one ductile shear zone. In these ductile shears, the width of mylonitization varies, but probably does not exceed one-half kilometer.
Adjacent to the mylonite zones, greenschist-facies retrograde metamorphism overprints amphibolite-facies prograde metamorphism only in the immediate vicinity of the mylonite zones. The localization of retrograde metamorphism may result from two processes:
1) Shear heating associated with mylonites may cause temperature increases in excess of one hundred degrees Celsius (Brun and Cobbold, 1980; Fleitout and Froidevaux, 1980). I f conduction or convection does not significantly dissipate the heat away from the mylonite zones, a retrograde aureole may form in areas of increased temperature.
2) Retrograde mineral assemblages develop more readily where an influx of water is sufficient to induce alteration. This water enters the system prior to or during mylonitization. Increased water content enhances mylonitization by increasing d u c tility . Water circulating along fractures may hydrate the rocks su fficie n tly to induce mylonitization along these zones of weakness.
A combination of shear heating and increased water content may link mylonitization and retrograde metamorphism. This paper is dedicated to my mother and father for twenty five years of
love and care. ACKNOWLEDGMENTS
I would like to especially thank the three members of my committee, Prof. Donald Hyndman, Prof. David Fountain, and
Prof. Keith Osterheld for their scie n tific insight and their c ritic a l review of this paper. Prof. John Wehrenberg assisted in x-ray diffraction for mineral identification. Shirley Pettersen typed several versions of this paper, and I might never have finished without her support. Sara Foland analyzed several samples for rare earth elements at Los Alamos Laboratories, New Mexico, and Dr. Peter Hooper analyzed further samples for whole-rock geochemistry at Washington
State University. Dr. David Mogk of the University of Washington, offered well appreciated insight of the regional geology during a crucial point of the thesis. Paul Kuhn, Jeff Mauk, Rick Moore, and
Sue Bloomfield provided assistance and encouragement during the past year. Finally, I would like to thank Lisa Conte for her moral support from afar. TABLE OF CONTENTS
Page
ABSTRACT...... ii
ACKNOWLEDGMENTS...... i i i
LIST OF TABLES ...... vi
LIST OF FIGURES...... v ii
CHAPTER
I. INTRODUCTION ...... 1
Background ...... 3
II. ROCK DESCRIPTIONS...... 6
Amphibole Schist ...... 6
Micaceous Schist ...... 10
Mixed Gneiss...... 10
Mylonitic Potassium Feldspar Gneiss ...... 15
Metamorphic Precursors -Schistose Package . . . 17
Metamorphic Precursors - GneissicPackage . . . 21
I I I . METAMORPHIC HISTORY ...... 29
Prograde Metamorphism ...... 29
Retrograde Metamorphism ...... 34
Spatial Association of Retrograde Metamorphism and Ductile Deformation ...... 45
IV. STRUCTURE...... 49
V. COMPARISON OF YANKEE JIM CANYON AND SIXMILE CREEK TO THE SOUTHERN MADISON RANGE AND THE NORTH SNOWY BLOCK ...... 59
Southern Madison Range ...... 59
North Snowy B lo c k ...... 62 i v CHAPTER Page
VI. CONCLUSION...... 67
REFERENCES...... 70
APPENDIX I ...... 76
APPENDIX I I ...... 93
V LIST OF TABLES
Table Page
1. Visually estimated modes of minerals in rocks of the schistose package ...... 11
2. Visually estimated modes of minerals in rocks of the gneissic package ...... 19
3. Neutron activation analyses for major and trace elements in amphibole s c h is t ...... 20
4. X-ray flourescence analyses for major elements . . . 26
5. Direction of elongation of porphyroblasts i n myl oni tes ...... 55
6. Comparison of some interpretations by Reid et a l. (1975) versus those by Mogk (1982) in the north Snowy b l o c k ...... 64
7. Comparison of deformational events versus metamorphic history for the study a re a ...... 67
vi LIST OF FIGURES
Figure Page
1. Location map ...... 2
2. Geologic map of Yankee Jim C anyon ...... 7
3. Geologic map of Sixmile Creek ...... 8
4. Photomicograph of dislocated hornblende annealed by actinolite ...... 9
5. Quartz-plagioclase-potassium feldspar diagram of samples from the micaceous s c h is t ...... 12
6. Photograph of outcrop from the mixed gneiss unit . . 9
7. Photograph of outcrop from the mylonitic potassium feldspar gneiss ...... 16
8. Photograph of outcrop from the mylonitic potassium feldspar gneiss ...... 16
9. Plot of Niggli c versus Niggli m g ...... 19
10. Plot of La/Yb versus £ REE in amphibole schist . . . 22
11. Quartz-plagioclase-potassium feldspar diagram of the gneissic u n its...... 23
12. Si02 versus diagram ...... 25
13. A-F-M diagram of mixed gneiss versus mylonitic potassium feldspar gneiss ...... 27
14. Photomicrograph of sillim anite at feldspar grain boundary ...... 30
15. ACFmK for prograde mixed gneiss ...... 31
16. Pressure-temperature diagram of prograde metamorphism in the mixed gneiss u n i t...... 32
17. ACFmK for prograde units of the schistose package . 35
18. Geologic cross-section of Sixmile Creek compared to ductile deformation and retrograde metamorphism ...... 36 vi i Figure Page
19. ACFmK diagram of retrograde minerals ...... 38
20. Photomicrograph of garnet altering to sericite, chlorite, and secondary b i o t i t e ...... 30
21. S ta b ility fields of retrograde m in e ra ls ...... 40
22. Peristerite solvus diagram for retrograde versus prograde plagioclase ...... 42
23. Photomicrographs of crenulation cleavage ...... 44
24. Photograph of isoclinal fold a x i s ...... 50
25. Poles to fo lia tio n diagram from Sixmile Creek . . .51
26. Poles to fo lia tio n diagram from Yankee Jim Canyon .51
27. Poles to foliation diagram from schistose units in Sixmile Creek ...... 53
28. Plot of tight to isoclinal folds in the mixed gneiss u n i t...... 53
29. Plot of lineations in the schistose units of Sixmile Creek...... 56
30. Photograph of a slickenside s u r fa c e ...... 50
31. Plot of slickenside orientations...... 56
32. Photograph of b io tite schist lacking mylonitic textures...... 61
33. Photomicrograph of a mylonite ...... 61
34. Simplified geologic map of the north Snowy block, after Reid et. al. (1975) ...... 63
v i i i CHAPTER I
INTRODUCTION
A northeast-trending, three kilometer-wide mylonite zone was
proposed by Erslev (1981) in the crystalline basement of the
southern Madison Range of southwestern Montana. Several smaller
northeast-trending shear zones were described by Reid et al. (1975)
in the crystalline basement of the north Snowy block (see Fig. 1).
Fountain (pers. comm.) examined mylonitic rocks in Emigrant Gulch
15 kilometers southwest of the north Snowy block. A large area of Pre-Belt exposures outcrop south of Emigrant Gulch along the eastern margin of Paradise Valley in Yankee Jim Canyon and Sixmile
Creek, f if t y kilometers south of Livingston, Montana (see Fig. 1).
During preliminary investigation I discovered mylonitic rocks in
Sixmile Creek and chose Yankee Jim Canyon and Sixmile Creek as the study area.
The study focuses on the metamorphic and depositional history of the area, with an emphasis on the conditions of mylonitization.
A discussion of the lithologies includes possible protolith in te r pretations, as well as petrographic and rock descriptions. The section on metamorphic history involves the relationship between mylonitization and metamorphism determined from petrography and mapping. The section on structure involves field relations and measurements of foliations, lineations, and fold axes. Finally, a
1 Livingston Bozeman approx. strike of units (Reid c X et of., 75) N'. 'Snowy Madison Block
C Sixmile Creek
Yankee Jim Canyon approx strike of units and mylonite Erslev, mi) Ip Miles
Fig.I Precambrian Basement Outlined with Study Area Shaded. modified from Ross et al (1955) 3 brief discussion of the north Snowy block and the southern Madison
Range will help in visualizing the overall northeast structural trend, and its relationship to Sixmile Creek and Yankee Jim Canyon.
Background
The nature of the boundary between the highly deformed, highly metamorphosed terrain of crystalline basement typified by the Tobacco
Root Mountains to the west, and the less deformed, less metamorphosed western Beartooth massif to the east is not well understood. Between these terrains lies a northeast structural trend that extends from the north Snowy block to the southern Madison Range.
East of the study area, unaltered granitic gneiss, granite^ tonalite, quartzofeldspathic gneiss, migmatite, and the Stillwater
Complex dominate the Beartooth massif (Casella, 1969; Butler, 1969;
Skinner et a l., 1969;and Weeks, 1980). Rb-Sr and K-Ar clocks set by a single amphibolite-grade metamorphic event yield dates of 2.6 to
2.7 b illio n years (Wooden and others, 1979; Peterman, 1981).
West of the study area crystalline basement of the northern Madison
Range (Spencer and Kozak, 1975), the Ruby Mountains (Okuma , 1971;
Garihan, 1979; Smith, 1980; Wilson, 1981), the Gravelly Range (Hadley,
1960; and Mill hoi 1 and , 1976), and the Tobacco Root Mountains (Cordua,
1973; Vitaliano and Cordua, 1979; Wilson, 1981) underwent several periods of deformation and metamorphism. In these areas, one or more prograde events reached uppermost amphibolite to granulite facies prior to a retrograde event of the greenschist facies. Quartzite 4 p e litic schist, dolomitic marble, amphibolite, iron-formation and quartzo-feldspathic gneiss comprise the bulk of the crystalline basement rocks in these ranges (Wilson, 1981).
L ittle basement outcrops between these two terrains, except in the southern Madison Range, the north Snowy block, and the two areas presented here. Foliations, map units, and ductile shear zones strike northeast in both map areas. Similar northeast-trending structures lie to the northeast in the north Snowy block and to the southwest in the southern Madison Range (Reid et a l. , 1975; Erslev, 1981; Mogk, 1982).
Alignment of the fabric of these three areas constitutes a major north east structural trend in the crystalline basement of southwestern
Montana.
Exposure of crystalline basement results from u p lift along north- northeast-trending normal faults (Hoiberg, 1940) at the western edge of both map areas. Only one mylonite zone outcrops in Yankee Jim
Canyon, where Laramide to recent faulting down-dropped the other mylonite zone and the schistose units into Paradise Valley. Five kilometers south of Yankee Jim Canyon, the high-angle Gardiner reverse fault uplifts units to the north more than 3,000 meters (Fraser et a l.,
1969). Wilson (1934) proposed that the Gardiner Fault follows an east-west line from Gardiner to the northern Madison Range. Near
Gardiner, this fa u lt represents the southernmost lim it of the Beartooth up!i f t . 5
Yankee Jim Canyon exposes two gneissic units mapped as mixed gneiss and mylonitic potassium feldspar gneiss. In addition to the gneissic units, Sixmile Creek exposes two schistose units mapped as amphibole schist. I propose two mylonite zones in Sixmile Creek and one in Yankee Jim Canyon. One mylonite zone exists at the contact between schist and gneiss. Mylonitic potassium feldspar gneiss com prises a second mylonite zone. CHAPTER II
ROCK DESCRIPTIONS
Yankee Jim Canyon and Sixmile Creek study areas were mapped
on a scale of 1:15,840 (1 in. = 4 mi.) (See Figs. 2 and 3). Field
relations, handsample descriptions, and petrography were used to
identify individual rock units. The following descriptions are
generalizations based on 275 rock samples and 72 thin sections.
Detailed petrographic descriptions are listed in Appendix I.
Schistose Package
Amphibole Schist
Amphibole schist is a fine-grained, dark green, foliated or
lineated, plagioclase-hornblende schist with minor biotite, quartz,
and sphene. Albite, epidote, and actinolite exist where the
amphibole schist underwent retrograde metamorphism (see Table 1 for
mineral percentages). Retrograde amphibole schist exhibits defor-
mational textures, such as undulose extinction and dislocated horn
blende crystals (see Fig. 4), not seen in prograde amphibole schist.
Also, epidote forms fine-grained aggregates in albite crystals, and
actinolite rims hornblende. In handsample, amphiboles form a lineation
and a crude fo lia tio n , but their extreme fine-grained size make i t
d iffic u lt to see. In thin section, the amphiboles have a bladed
appearance due to their orientation. This unit commonly interfingers
6 7
GEOLOGY OF YANKEE JIM CANYON
no^'so" Miles
Dp me
Mo bn
6 8 0 ° f 'A t
Gradational J Contact *
Q uaternary Alluvium
Precambrian Lithologies
Mixed Gneiss Mylonitic K Feldspar Gneiss Figure 2. 8
GEOLOGY OF SIXMILE CREEK
II0°45
Miles
V V 45°I5‘
n6800
Gradational j Contact <
Qu aterna ry Alluvium
Precambrian Lithologies
Amphibolite Schist aS Mixed Gneiss mg
Micaceous Schist [ms My Ion it i c kg K-Feldspar Gneiss Figure 3. 9
Fig. 4. Photomicrograph magnified 16 times in plain lig h t of actinolite annealing a fractured hornblende crystal.
Fig 6. Photograph shows veined gneiss, amphibolite pod, and fe lsic dike rock types common to the mixed gneiss unit. 10 with micaceous schist. Contacts between amphibole schist and micaceous schist are gradational over several meters.
Micaceous Schist
Micaceous schist is fine-grained, dark gray, homogeneous, and highly schistose. It weathers brown and contains thin to blocky laminations. Two rock types are present, garnet-biotite-quartz- plagioclase schist and muscovite-plagioclase-quartz schist. In both rock types, retrograde mineral assemblages include chlorite, sericite, and epidote (see Table 1). Adjacent to the ductile shear zones, bent b io tite cleavage, secondary b io tite , mortar texture, stain-free
120-degree grain boundaries in quartz, sutured quartz, and retro grade minerals demonstrate recrystallization, and deformation approaching mylonitization.
Muscovite schist characteristically contains more quartz than does biotite schist (see Fig. 5). The protolith for muscovite schist was probably a fine-grained quartzite; whereas, the protolith for biotite schist was closer to a fine-grained sandy mudstone.
Gneissic Package
Mixed Gneiss
The mixed gneiss unit contains at least eight rock types:
1. Medium-grained, biotite-plagioclase-quartz schistose veined gneiss contains local sillim anite, muscovite, chlorite, epidote, ch lo ritoid, garnet, myrmekite, and zircon. The veined gneiss comprises 11
Table 1
Visually estimated modal percentages
Schistose Package
Unaltered Altered * Muscovi te Amphibole Biotite Schist Biotite Schist Schist Schist
Quartz (35-37) (30-35) (55-65) (0-3) Plagioclase (37-40) (0-35) (0-15) (0-40)
Al bi te - (0-55) (0-12) (0-35)
Potassium Feldspar - - (0-2) -
Biotite (20-25) (3-27) - (0-1)
Muscovite - - (10-30) -
Almandine Garnet (tr-1) (0-tr) - -
Hornblende - -- (5-60) Chlorite (0-tr) (1-12) (0-tr) (0-3) Cl inozoisite-Epidote (0-tr) (0-2) (0-tr) (0-20)
Seri cite (0-tr) (tr-3) - -
Acti noli te -- - (0-55)
Tremolite --- (0-85)
Calcite - (0-tr) - (0-1)
Zi rcon - (0-tr) - -
Apati te (0-tr) -- -
Hemati te - - (2-12) - Opaque Fe Oxide (tr-2) (tr-3) (0-tr) (tr)
Sphene --- (0-2)
Number of Samples 2 3 4 5
^Altered samples taken adjacent to mylonite zones. MUSCOVITE SCHIST ■ BIOTITE SCHIST .
K-FELDSPAR PLAGIOCLASE Fig.5 MODAL ESTIMATES BASED ON 100 INTERFERENCE FIGURES. 13
approximately seventy-five percent of the mixed gneiss unit.
2. Discordant and concordant, medium-grained, black amphibolites
form elongate pods, commonly adjacent to fe lsic dikes.
3. Granular, medium-grained, weakly schistose rocks contain
greater than f if t y percent quartz, fifteen to twenty-five percent
biotite, with lesser amounts of plagioclase, sericite, and zircon.
These rocks have mineralogical and textural characteristics similar to
metaquartzites.
4. Medium-grained biotite-plagioclase-quartz migmatitic gneiss
lies adjacent to veined gneiss.
5. Medium- to coarse-grained plagioclase-hornblende-hypersthene
ultramafic pods were probably emplaced in a solid state. They are
surrounded by sheared c h lo ritic selvages.
6. Medium-grained garnetiferous pods contain approximately eighty
percent almandine garnet with minor biotite, chlorite, chloritoid,
plagioclase, and zircon.
7. Pegmatites, felsic and diabase dikes cross-cut many layers.
Because they cross-cut each other their relative ages cannot be
determined.
8. Small patches of granite or granitic gneiss show slight to moderate micaceous foliations indicative of post-emplacement deformation
(see Table 2 for mineral percentages).
Because of extreme complexities, any description of the mixed
gneiss unit is a simplification (see Fig. 6). Typically, no layer Table 2 Visually estimated modal percentages Gneissic Package Unaltered Altered Mylonitic Garnet- Granitic Amphibolite Meta- U-maf. Mi neral Veined Gneiss Veined Gneiss K-Feld. Gneiss Rich PodGneiss Quartzite Pod Quartz (30-50) (20-60) (20-40) (tr) (30-40) (0-1) (45-70) - PIagioclase (30-60) (0-40) (0-15) (5) (25-40) (15-45) (20-30) (3) AT bite - (0-45) (10-35) - - - - - Potassium Feldspar (0-4) (0-3) (15-35) - (10-30) - - - Biotite (5-25) (4-30) (0-20) (14) (10-25) (0-5) (5-15) - Muscovi te (0-15) (0-15) (0-7) - (0-2) - (3-7) - Sillimanite (0-tr) - - -- - (0-tr) - Almandine Garnet (0-4) (0-tr) - (80) - - - - Hornblende - - - -- (50-80) - (10) Hypersthene ------(87) Chlorite (0-tr) (0-5) (0-7) (tr) (0-tr) (0-1) (0-tr) - Clinazoisite-Epidote (0-tr) (0-10) . (0-60) - - - -- Seri ci te (0-tr) (tr-15) (tr-7) - (0-1) -- - Chloritoid - (0-tr) - (tr) - - - - Actinolite ------Calcite (0-tr) (0-2) (0-tr) - - -- - Fuchsite - (0-2) (0-15) - - -- - Zircon (0-tr) (0-tr) (0-tr) (tr) (tr) (0-tr) . (0-tr) - Graphite (0-1) (0-tr) - - - - (0-tr) - Apatite (0-tr) (0-tr) (0-tr) - (0-tr) - - - Rutile (0-2) (0-1) - - (0-tr) - - - Hematite (0-tr) (0-tr) (0-tr) - - - - - Opaque Fe Oxide (tr-2) (tr-3) (tr-2) (tr) (tr-1) (tr-1) (tr-1) (tr) Myrmeki te (0-1) - - - (0-tr) - - - Number of Samples 14 16 9 1 4 3 3 1 15
continues more than one hundred meters. Layers pinch out or end in
fault contacts. Numerous felsic or mafic, pods and dikes, as well as
migmatization disrupt the unit. Retrograde metamorphism and defor
mation associated with the shear zones creates additional complications.
Common deformational textures include undulose extinction, bent
b io tite cleavage, mortar texture, sutured grain boundaries, re-
crystallization and retrograde alteration. These textures are more
prominent closer to gradational contacts with the mylonitic potassium
feldspar gneiss.
Mylonitic Potassium Feldspar Gneiss
Fine- and medium-grained sericite-chlorite-fuchsite-epidote-
biotite-plagioclase-microcline gneiss exhibits mylonitic fabrics which
include reduction in grain-size, anastomosing foliations, pressure
shadows, mortar texture, sutured grain boundaries, flaser texture, as well as recrystallized quartz and biotite (see Table 2). Figures 7 and
8 are examples of mylonitic potassium feldspar gneiss in the fie ld .
Microcline porphyroblasts commonly exsolve to perthitic albite. Retro grade metamorphism and deformation, present in a ll samples, destroyed most evidence of prograde mineral assemblages. Medium-grained bio tite and muscovite indicate temperatures higher than greenschist facies.
Contacts between this unit and the mixed gneiss unit grade over at least one hundred meters. Evidence of mylonitization diminishes over a short distance beyond the contacts with the mixed gneiss unit.
Metamorphic Precursors
Intense metamorphism and polyphase deformation complicate protolith 16
Fig. 7. Mylonitic potassium feldspar gneiss located in Sixmile Creek.
Fig. 8. Mylonitic potassium feldspar gneiss located in Yankee Jim Canyon. 17
interpretations. Mineralogical and geochemical arguments used to
determine protoliths assume a relatively isochemical system, but hydrous
retrograde mineral assemblages suggest an influx of water occurred,
and elements mobilized during metasomatism alter rock chemistry.
Hydrous mass transfer may lead to erroneous protolith interpretations.
Schistose Package
The following mineralogical and textural features indicate a meta-
sedimentary origin for fine-grained schist in Sixmile Creek. Compo
sitional layering, consistent fine-grain size, interstitial micas, and
fis s ilit y of these rocks are similar to many other metasedimentary
schists. Amphibole schist is more homogeneous than compositionally
layered. When visible, measurements of compositional layering range
from 0.3 to 20 centimeters. Amphibolite-grade metamorphism and de
formation destroyed most sedimentary structures, however, re lic t clasts
appear to be preserved locally.
A plot of modal percentages on a quartz-plagioclase-potassium
feldspar diagram separates quartz-rich muscovite schist from feldspar-
rich b io tite schist (Fig. 5). Muscovite schist averages approximately
sixty-five percent quartz and fifteen percent muscovite. Biotite
schist averages th irty -fiv e percent quartz and twenty-five percent
b io tite . These compositions suggest an average sandstone protolith for the muscovite-schist, and a sandy shale for the biotite schist
(Blatt et a !., 1972, p. 270). The distinction between metaigneous and metasedimentary origin is
less certain for the fine-grained green amphibole schist. Massive,
homogeneous appearance over several meters suggests a metabasaltic
origin (ortho-amphibolite) for amphibole schist in Sixmile Creek.
It does not interlayer with other sedimentary rocks on a scale of
several to tens of centimeters as expected for metamorphosed impure
dolomite (para-amphibolite) (Walker, 1960). In the fie ld , micaceous
schist interfingers with amphibole schist; however,they interfinger
over tens of meters. A thin layer of trem olite-rich rock near the
contact with the gneissic package contains more than eighty precent
tremolite. Such a composition is not plausible for metabasalt.
Various geochemical arguments used to resolve the origin of amphi-
bolite yield equivocal results. Leake (1963, 1964) used plots of
Niggli c versus mg for basic igneous rocks and various sediments
(see Fig. 9). An empirical trend for basic igneous rocks lies
approximately perpendicular to expected trends for calcareous sedi mentary rocks.
REE and major element weight percentages of four amphibole schists were determined by neutron activation analysis at Los Alamos Laboratories
(see Table 3). Niggli c versus mg plots of the four samples are in
the v ic in ity of Leake's plot for basic igneous rocks. However, many more analyses are necessary to demonstrate a sta tistica l correlation, and the argument only holds true for isochemical systems. Deep circulating waters or metasomatism may have caused significant mass ines represent variou s sedimentary trends
Niggli c
basic / amphibole gneous schists ( of Sixmile Creek) v trend
I .7 1.0 Niggli mg = ___ MgO + FeO + MnO+ 2Fe203 Fig. 9 Comparison of trends from possible igneous and sedimentary parents,(after Leake, 1964, p. 241). 20
Table 3
Major and Trace Element Geochemistry of Amphibole Schist Samples
Sample No. BB 5 BB 7 BB 94 BB 95
AI2O3 (%) 11.7 15.0 14.6 14.0 MqO (%) 6.7 6.7 4.5 7.7 Total Fe (%)16.2 11.9 8.5 11.0 CaO (%) 9.9 8.7 9.4 8.8 Na20 (%) 2.8 3.4 3.2 4.0 Ti20 (ppm) 9700 8000 3000 4900 MnO (ppm) 2500 2300 1500 2100 K20 (ppm) 6000 7000 5000 6000 Cl (ppm) 270 200 140 200 Sc (ppm) 46 37 25 37 V (ppm) 351 261 250 223 Cr (ppm) 273 308 175 661 Co (ppm) 57 48 38 55 Cu (ppm) 300 400 300 400 Zn (ppm) 10 9 10 10 Ga (ppm) 90 100 90 100 Se (ppm) 3 3 18 3 Rb (ppm) 20 20 20 20 Sr (ppm) 400 400 300 400 La (ppm) 12 3 14 8 Ce (ppm) 21 6 29 17 Sm (ppm) 2.6 1.7 2.1 1.9 Dy (ppm) 4 3 2 3 Yb (ppm) 4 2 1 2 Hf (ppm) 2 1 2 2 W (ppm) 12 16 22 11 Th (ppm) 1 1 3 2 Cs (ppb) 1000 900 800 900 Eu (ppb) 1640 660 780 670 Tb (ppb) 900 460 900 400 Lu (ppb) 600 300 300 400 Ta (ppb) 600 600 900 600 Au (ppb) 18 30 10 10 U (ppb) 250 100 860 550
Sara Foland performed the neutron activation analyses . for these samples at Los Alamos Laboratories, New Mexico. 21
transfer of major elements which may cause deviation from the correct
field on Niggli plots.
Rare-earth patterns alleviate this problem, because REE are less
mobile than major elements (Hermann et a l., 1974). Schubert (1979)
and Balashov et al (1973) correlated lanthanide REE patterns for am-
phibolites and several possible modern equivalents. A plot showing
La to Yb ratios versus the sum of REE contains different fields of
unmetamorphosed basalts and sediments which might serve as protoliths
(see Fig. 10). The limited data available for the amphibolite schists
of Sixmile Creek suggest a basaltic origin.
Gneissic Package
The metamorphic precursor of the veined gneiss is not known.
Other rock types, such as felsic dikes, amphibolite and ultramafic
lenses, are metaigneous. A plot of gneisses on a quartz-plagioclase-
potassium feldspar diagram reveals an overall to n a litic composition
(see Fig. 11). However, several samples have quartz percentages in excess of f if t y percent and mica percentages in excess of fifteen
percent. High quartz percentage, granular texture, and in te rs titia l mica suggest sandstone protoliths, but these samples represent a smal 1 portion of the mixed gneiss unit.
Layering in the veined gneiss formed by metamorphic differen tiation or injection. Dark, biotite-plagioclase-quartz laminae alternate with biotite-poor layers resembling metaigneous rocks.
Laminae thicknesses range from 0.2 to 15 centimeters (see Fig. 6 ). • amphibole schist ug/g - - sedimentary fields IOOt — basaltic fields 5 0 -
La/Yb
1 0 -
0 .5 ug/g 5 10 5 0 100 5 0 0 1 0 0 0
SREE = La + C e+ S m + E u + Tb + Yb + Lu Fig. 10 Plot of REE for basalt and sedimenary rocks, (after Schubert, 1976, p. 303 and 304). K-FELDSPAR MYLONITIC GNEISS
MIXED "GNEISS
■■
K- FELDSPAR PLAGIOCLASE Fig.II VISUALLY ESTIMATED MODAL PERCENTAGES. 24
Alumina-rich minerals, such as sillimanite and garnet, comprise a small
percentage of some mixed-gneiss samples. Such minerals commonly
indicate some type of metasedimentary origin. Dietrich (1960) de
termined that a contaminated igneous parent may crystallize a small
percent of these alumina-rich minerals. Furthermore, many samples
contain no sillimanite or garnet, thus mineralogical and textural
arguments w ill not unequivocally resolve the protolith question for
the mixed gneiss unit.
Attempts to determine protoliths using geochemical techniques
proved equally frustrating. Tarney (1976) plotted SiO 2 versus TiC^
for calk-alkaline and metasedimentary rocks (Fig. 12). Separation of
these rock types enabled him to distinguish between modern equivalents
for Archean gneiss. He determined that Archean metasedimentary rocks separate from Archean metaigneous rocks along the same line. In this
study, samples from the mixed gneiss and mylonitic potassium feldspar gneiss units show no clear separation along the line (see Table 4 for
XRF data). Equivocal results may be due to :
1. Too few samples;
2. Insufficient sample volume to proportion the gneissic
layers accurately;
3. Fracture controlled or metasomatic contamination;
4. A combination of metasedimentary and metaigneous samples.
Lambert et a l. (1976) were able to distinguish between various cratonic provinces on a A-F-M diagram. Figure 13 shows geochemical 25
7.
40 50 60 70 8 0 90 7 S i 0o • - Mixed qneiss C- A - B iotite schist
+ - Mylonitic Potassium Feldspar Gneiss g - Amphibolite
Fig. 12 Tarney (1976, p.408) plotted this line according to an empirical study to separate Archean gneiss and calk-alkaline igneous rocks (below) from post-Archean sedimentary and meta sedimentary rocks (above). Archean metasedimentary samples did not concentrate either above or below the line in his survey. 26
Table 4
XRF Whole Rock Geochemistry for Major Elements
Mylonitic K-Feld. Mixed Gneiss Samples Amphibolite Biot. Gneiss Schist
BB-32 YJ-26 YJ-36 BB-36 YJ-6 YJ-84 YJ-98a YJ-98b BB-56 BB-98 YJ-93 BB-49
Si 02 69.39 70.59 59.29 63.90 69.39 74.87 62.72 61.02 49.15 55.15 54.23 70.70 a i 2o3 15.86 14.33 18.56 17.60 15.26 11.54 15.71 16.78 16.85 6.57 7.65 13.55 Ti02 0.50 0.59 0.78 0.54 0.53 0.44 0.78 0.84 0.52 0.25 0.81 0.66 1.75 1.83 2.55 2.34 2.64 2.58 4.22 4.28 5.05 3.99 5.64 3.06 Fe2°3 FeO 2.01 2.09 2.92 2.69 3.03 2.95 4.83 4.90 5.87 4.58 6.46 3.50 MnO 0.03 0.04 0.06 0.07 0.08 0.07 0.10 0.12 0.23 0.22 0.23 0.07 CaO 2.13 2.36 4.48 4.67 1.76 1.75 2.24 . 2.26 10.53 10.37 11.53 1.49 MgO 1.44 1.13 2.07 2.45 2.56 1.99 3.57 3.54 8.20 17.85 10.83 2.60 k 2o 3.85 4.10 5.05 2.11 2.12 1.66 3.23 3.41 1.35 0.00 0.34 1.89 Na20 2.81 2.78 3.75 3.51 2.58 2.13 2.53 2.78 2.25 1.00 2.15 2.50 0.21 0.16 0.48 0.13 0.05 0.02 0.05 0.08 0.09 0.04 0.11 0.09 P2°5
X-ray flourescence analyses performed at Washington State University under the guidance of Dr. Peter Hooper. My lonitic K Feldspar Gneiss Mixed Gneiss ■
Can. Shield Gneisses
Younger Kaapaal Lewisian a Intrusions E. Greenland m Gneisses Kaapvaal T ona lites
Aik Fig.13 Wt. Percent after Lambert et al (p. 381) 28
data of rocks from several provinces. Mixed gneiss samples d iffe r substantially from mylonitic potassium feldspar gneiss samples in the study area and this demonstrates their geochemical individuality.
Also, mixed gneiss samples plot closer to other granodioritic to tonalitic gneissic provinces; whereas, mylonitic potassium feldspar gneiss samples plot closer to the granitic rocks of Younger Kaapvaal
Intrusions. CHAPTER I I I
METAMORPHIC HISTORY
Prograde Metamorphism
Petrographic mineral identification was used to determine metamorphic grade. Prograde samples are distinguished from retrograde samples by the presence of relatively unaltered sillim anite, almandine garnet, plagioclase, hornblende, coarse-grained muscovite, or coarse grained b io tite , and the absence of chlorite, epidote, a ctin o lite , albite, chloritoid, sericite, or secondary biotite.
Metamorphic grade does not vary significantly between Sixmile
Creek and Yankee Jim Canyon. However, metamorphic grade may vary be tween prograde events of the schistose and gneissic packages. Intense retrograde metamorphism has destroyed all sign of prograde mineral assemblages adjacent to the ductile shear zones. Mixed gneiss samples taken several kilometers away from the mylonites contain prograde mineral assemblages. These assemblages include sillim anite, almandine garnet, medium-grained muscovite, b io tite , plagioclase, and quartz.
In eight samples, fib r o litic sillim anite appears at grain boundaries between feldspar crystals (Fig. 14). Figure 15 plots prograde mineral assemblages on an ACFmK diagram. Garnet and sillim anite do not contact one another, so no tie-line joins them.
Reactions listed on Figure 16 constrain prograde pressure-tem- perature conditions to the shaded area. Unaltered muscovite and the 29 30
Fig. 14. Photomicrograph magnified 40 times in plain lig h t shows oriented fib ro litic sillimanite at the grain boundary between feldspar crystals.
Fig. 20. Photomicrograph magnified 10 times in plain lig h t reveals garnet altering to se ricite , secondary b io tite and chlorite. rm
Fig. 15 ACFmK PLOT FOR PROGRADE MINERALS IN MIXED GNEISS UNIT. Mi nimum me! t \ of wet \q r a n i te
400 500 600 700 800 900 T C Fig.16 PROGRADE MIXED GNEISS MINERALS PLOTTED ON A P-T DIAGRAM. - Fe Chi. + Magnetite + Qtz. = Aim. + H2O (Hsu, 1968) S ill. - And. - Ky. (Holdaway, 1971) Muse. + Qtz. = S ill. + Orth. + H2O (Evans, 1965) Min. melt wet granite (Tuttle + Bowen, 1958) 33 lack of orthoclase lim it the prograde event to the si 11imanite- muscovite zone of the amphibolite facies.
Migmatite and myrmekite may suggest the existence of a meta morphic event in excess of the si 11imanite-muscovite zone. Bryden
(1950) mapped the lower slopes of Yankee Jim Canyon as migmatite.
Some migmatite is present, however i t comprises a small portion of the mixed gneiss unit. Minimum melting pressure-temperature fields for granitic rocks encompass a small portion of the shaded area in Figure 16.
Conditions of migmatization depend on water content and rock compo sitio n , as well as temperature and pressure. Also, myrmekitic in te r growths, common in the mixed gneiss unit, form either during granulite- facies metamorphism or from a magma (Spry, 1969, p. 104). Therefore, granulite-facies metamorphism may have preceded the amphibolite-facies event. I f so, amphibolite-grade metamorphism destroyed any diagnostic granulite facies minerals and l i t t l e remains to substantiate a higher grade event.
Viewed in thin section, sillim anite is moderately oriented with respect to micaceous schistosity. Also, medium-grained muscovite interfingers with biotite along the schistosity. This schistosity probably formed during isoclinal folding (see Page 52). Hence, si 11imanite-grade metamorphism may accompany isoclinal folding.
Titanium-poor brown b io tite commonly contains ru tile crystals along its cleavage traces which suggest alteration from a higher-grade, titanium-rich biotite. Brown biotite plus rutile may crystallize during declining temperatures following prograde metamorphism, or during a subsequent lower-grade event. 34
Prograde mineral assemblages for the schistose package do not re
s tric t pressure-temperature conditions to a specific zone in the am
phibolite facies. Micaceous schists contain garnet, muscovite, b io tite ,
quartz, oligoclase, and minor potassium feldspar. Amphibole schists
contain hornblende andesine, minor quartz, and b io tite (Fig. 17).
These minerals lim it the prograde event to the amphibolite facies, but
their s ta b ility fields include the entire facies except the si 11imanite- orthoclase zone.
Retrograde Metamorphism
Adjacent to the shear zones retrograde metamorphism overprinted the prograde rocks. Greenschist-facies mineral assemblages include epidote, clinozoisite, chlorite, sericite, secondary biotite, albite, microcline, chloritoid, actinolite, and tremolite. These retrograde minerals replace prograde minerals such as calcic plagioclase, potassium feldspar, almandine garnet, hornblende, and coarser-grained b io tite .
Pertinent retrograde alteration equations follow later in this chapter.
The intensity of retrograde metamorphism varies with distance from the ductile shear zones, and does not vary according to rock type.
Figure 18 compares a geologic cross section of Sixmile Creek with cross sections of ductile deformation and retrograde metamorphism.
Dots represent petrographic descriptions projected along strike to the cross section of Sixmile Creek. The v a ria b ility and abundance of retrograde mineral assemblages determine the degree of retrograde metamorphism. For example, samples containing greater than ten percent + Q t z + Fe-Oxide ± Sph
K
Fig. 17 PROGRADE ACFmK PLOT FOR MICACEOUS SCHIST AND AMPHIBOLITE SCHIST. GOU1 36
HIGH MOD. LOW NONE A B C DEGREE OF RETROGRADE METAMORPHISM
MYLONITE
DEFORMED a/or RECRYSTALLIZED REGIONAL B DEFORMATION DEGREE OF DUCTILE DEFORMATION
ft. m. 7 0 0 0 2120
6 0 0 0
5 0 0 0 1670 A B C Fig.18 GENERALIZED GEOLOGIC X-SECTION OF SIXMILE CREEK. Km. AMPHIBOLE SCHIST AS MIXED GNEISS MG MICACEOUS SCHIST MS MYLONITIC KG K-FELD. GNEISS
'/ GRADATIONAL CONTACT 37
retrograde minerals (except a lb ite ), and containing three or more retrograde minerals (including albite) are considered to be a high degree of retrograde metamorphism. Samples containing no retrograde mineral or a trace of one are designated none for the degree of retro grade metamorphism. A trace amount might form during the waning stages of prograde metamorphism. This diagram shows the degree to which each sample displays retrograde metamorphism and does not show increasing temperature-pressure conditions of retrograde metamorphism.
Mylonitic and/or deformational petrofabrics determine the degree of ductile shearing. The plotting scheme involves the following textures, in order of importance; reduction in grain-size, pressure shadows, anastomosing foliations, ribbon texture, mortar texture, flaser texture, grain dislocations, sutured grain boundaries, bent cleavage traces, bent twin lamellae and undulose extinction. Definitions of mylonite vary considerably. I have borrowed from descriptions by
Spry (1969, pp. 234-236), Bell and Ethridge (1973), and Hobbs et a l.
(1976, pp. 419-427) as an outline of mylonitic textures.
Agreement between the two curves is excellent. Samples taken several kilometers away from the ductile shear zones show l i t t l e or no sign of retrograde metamorphism, whereas, samples with pronounced mylonitic textures taken from the ductile shear zones contain a far greater abundance of retrograde minerals.
Retrograde minerals plotted on an ACFmK diagram (Fig. 19) include chlorite, epidote, sericite, secondary biotite, actinolite, microcline, fuchsite, chloritoid, and albite. The formation of secondary biotite -P Q tz ± Micr -p Fe-Oxide Ser ± Plag
Ep or Clinoz
Ch Bt—Phg Act —Tr Fm
Fig.19 ACFmK PLOT OF RETROGRADE EVENT.
CO CO 39 indicates that retrograde metamorphism, at least locally, reached the biotite zone of the greenschist facies. The alteration of almandine garnet suggests that retrograde pressure-temperature conditions were below the garnet-zone of the greenschist facies (Fig. 20). Figure 21 shows stability fields for all retrograde minerals. Individual stability fields cover a wide range of metamorphic grades; however, they all contain a portion of the greenschist facies.
Several reactions reported by various authors which may occur during the retrograde event include:
chlorite + biotite^-jj + quartz ^ almandine-rich garnet +
b io t it e ^ + H^O (Chakraborty and Sen, 1967)
chlorite + muscovite + epidote £ almandine-rich garnet + b io tite +
F^O (Brown, 1969)
chloritoid + chlorite + quartz ^ almandine-rich garnet + F^O
(Thompson and Norton, 1968)
Fe-chlorite + quartz + magnetite ^Almandine + H^O
(Hsu, 1968)
Compositional and textural evidence suggest several other reactions:
A1 + actinolite # hornblende
SiO^ + clinozoisite + albite ^ andesine (An^g) + H2O + A1
chloritebiotite + Fe oxide + 6 H2O
sericite + Fe oxide ^ b io tite
brown biotite + rutile ^ orange biotite (with increased titanium)
sericite after potassium feldspar
microcline after potassium feldspar
sericite after plagioclase AMPHIBOLITE Prehnite GREENSCHIST gm gm Pump Chi Bt Aim Staur Ky Musc 0rth
Mu ^ ------_ Chi
Bt ...... - ...... - J
Act
Ab low Ca Chd
Ep
Fig. 21 INTERPRETATION OF METAMORPHIC FACIES FROM MINERALS OBSERVED. After Hyndman (Manuscript, 1982). 41
Although cordierite and staurolite are not present in any samples,
they may act as important intermediate phases. Possible reactions are:
chlorite + muscovite ^ staurolite + biotite + quartz + b^O
(Hoschek, 1969)
chlorite + muscovite + quartz ^cordierite + biotite +
(Schreyer and Yoder, 1964)
In each case, the reverse reaction describes the retrograde event.
Most of these reactions require the addition of water. Progressive metamorphism drives o ff water with increasing grade which leaves relatively anhydrous rocks. In contrast, retrograde metamorphism crystallizes hydrated minerals which require additional water.
Some samples contain up to seventy percent epidote and sericite or forty percent se ricite , chlorite, and epidote. These samples could not form in an isochemical system. A large influx of water must infiltrate the area of mylonitization, perhaps along pre-existing fractures prior to or during the retrograde event.
Crystallization of retrograde epidote requires a source of calcium. The alteration of andesine or oligoclase to albite liberates calcium and aluminum necessary for the formation of many retrograde minerals. Calcic plagioclase plus water alters to albite plus epidote
(or calcite) according to a relationship illustrated by the persisterite solvus (Fig. 22) (Crawford, 1966; Cooper, 1972). PROGRADE AMPHIBOLE
STAUROLITE GNEISS 8. SCHIST >. SCHIST ISOGRAD
GARNET IS 0 GRAD
Ab + Ep or Cole BIOTITE I SOGRAD 10 20 30 40 50 60 RETROGRADE ROCKS An CONTENT
Fig. 22 PERI STER ITE SOLVUS after C r aw ford, ( I 966). 43
Optical determinations yield values of An^Qj to A n ^ j f ° r plagioclase from samples several kilometers from the mylonites.
Adjacent to the mylonites, plagioclase ranges from An,nA to An, x. vUj (5) X-ray diffraction patterns of four texturally-similar amphibole schists were compared to calculated powder patterns from Borg and
Smith (1968). The results reaffirm optical estimations. Plagioclase from amphibole schist away from the mylonites has diffraction patterns more calcic than oligoclase, whereas plagioclase from amphibole schist adjacent to the mylonites resembles low-albite patterns. Plotting the results on the persisterite solvus diagram indicates greenschist- facies retrogression.
Textural evidence supports synkinematic deformation and retrograde metamorphism. Post-deformation recrystallization of quartz forms 120 degree, strain-free, grain boundaries. Figure 23 shows the nose of a crenulation cleavage which contains recrystal 1ized quartz. The same sample also contains biotite altering to chlorite and recrystal 1ized polygonal arcs. The following sequence of events may explain these textures .
1) formation of original schistosity;
2) formation of crenulation cleavage;
3) recrystal 1ization of quartz and alteration of biotite to
chlorite, and the formation of polygonal arcs;
4) minor post-recrystallization deformation evidenced by
undulose extinction and ragged grain boundaries of quartz. 44
Fig. 23a. Photomicrograph magnified 10 times in plain lig h t of a crenulation cleavage. Recrystallized biotite and chlorite form polygonal arcs.
Fig. 23b. Photomicrograph magnified 16 times with crossed nicols taken just below figure 23a. Recrystal 1ized quartz indicated that metamorphism outlasted deformation, however the quartz shows signs of minor sutured grain boundaries and undulose extinction. 45
Oriented retrograde minerals, such as sericite, secondary b io tite ,
and chlorite further support syntectonic crystallization. Commonly,
these minerals form anastomosing foliations indicative of myloniti-
zation. Epidote crystallizes as unoriented fine-grained aggregates,
as well as coarse-grained crystals. Epidote's granular crystal habit
(Troger, 1979, p. 56) may not orient despite dynamic metamorphism.
Minor, extremely fine-grained phengite and/or clay minerals alter
from plagioclase and form perpendicular intergrowths along the 010 and
001 cleavage traces. Mineralogy and texture suggest post-tectonic
hydrothermal alteration. Hydrothermal alteration is described for
several shallow-level porphyritic intrusions in the area. Clay and/or
phengite alteration may be indirectly related to this younger hydro-
thermal a ctivity.
Spatial Association of Retrograde Metamorphism and Ductile Deformation
The localization of greenschist facies retrograde metamorphism to ductile shear zones, shown in Figure 18, is described above. Mitra and Frost (1981) proposed that thin, late Precambrian (900 to 600 m.y.) ductile deformation zones reactivated along older, larger deformation zones in the Wind River Range of Wyoming. Retrograde chlorite- actinolite-epidote greenschist-facies assemblages are spatially associa ted with the younger shear zones. The Wind River shear zones are smaller, on a scale of microscopic to tens of meters, than those described here.
Beach (1980) proposed retrograde metamorphism associated with 46 shear zones of the Lewisian complex. Careful petrological and geochemical research determined reaction equilibria, all of which require the introduction of water. He determined that calcic plagio clase alters to albite, releasing Ca and A1 during retrograde meta morphism. Aluminum reacted to form micaceous minerals, however, calcium-poor retrograde assemblages suggest that Ca le ft the system.
Furthermore, reaction equilibria support the introduction of K+ and + hydrous H ions. Fluid migration through an open system described by
Beach (1980) matches well with the model outlined here.
Metamorphic petrofabrics (see page 43) suggest that mylonites formed contemporaneously with retrograde metamorphism. Two possible explanations account for the spatial association of retrograde meta morphism and shearing in Yankee Jim Canyon and Sixmile Creek.
1) Shear heating produced elevated temperature gradients adjacent
to the mylonites. This temperature gradient induced green-
schist-facies retrogression in older amphibolite-grade rocks.
Limited conduction and convection restricted elevated
temperatures , and the resulting retrograde metamorphism, to
areas surrounding the shear zones.
2) An influx of water established a hydrous environment favorable
for retrograde mineralization. Abundant retrograde mineral
assemblages crystallize where water circulated along deep
fractures prior to or during mylonitization. 47
Various authors report that shear heating or thermal softening
play an important role during deformation (Weathers and Bird, 1979;
Brun and Cobbold, 1980; Fleitout and Froidevaux, 1980). Theoretical
calculations by Brun and Cobbald (1980) yield temperature increases
in excess of 100° C for major shear zones. Weathers and Brun (1979)
determined that temperature decreased a total of 75° C within 100
meters of the Moine Thrust. In each study, calculations of shear
heating depend on measurements of shear stress and shear strain. I
calculated neither shear stress nor shear strain in my area; therefore,
the role of shear heating remains undetermined.
I f the calculations for shear heating are correct, then increased
temperatures adjacent to the shear zone may be a prerequisite to
retrograde metamorphism in the study area. However, shear heating would not affect retrograde metamorphism which occurred during the waning
stages of prograde metamorphism. Furthermore, shear heating w ill not
liberate enough water to account for the abundant hydrous retrograde minerals found in the shear zones. Progressive dehydration occurs with
increasing metamorphic grade. Therefore, dehydrated amphibolite- facies rocks w ill not liberate substantial water during greenschist- facies metamorphism. Another mechanism must exist to introduce water
into the system. An influx of water would increase d u c tility (Beach,
1976; Beach and Tarney, 1978). Hydrated zones in Yankee Jim Canyon and Sixmile Creek exhibit the greatest degree of mylonitization and retrograde metamorphism. 48
In summary, an influx of water apparently played an important role in the spatial association of mylonitization and ductile shearing
Water entering the system via pre-existing fractures may lim it retro grade mineralization to these hydrated areas. However, water entering the system along shear zones created during mylonitization might also produce a hydrated zone where retrograde minerals crystallized. 49
CHAPTER IV
STRUCTURE
Structural complexities make it difficult to unravel the de-
formational history of the study area. Bryden (1950) mapped the
lower slopes of Yankee Jim Canyon as migmatite. Some migmatite
exists, however i t comprises a small portion of the mixed gneiss
unit. Structures observed in the fie ld suggest at least three periods
of deformation for the mixed gneiss unit and at least three periods
of deformation for the schistose units. These structures include
tight to isoclinal folds, broad open folds, crenulation cleavage, mylonites, and siickensides. Structural analysis of these defor- mational events follows.
Tight to isoclinal folds in the mixed gneiss unit represent the earliest period of deformation (see Fig. 24). The absence of tight
to isoclinal folds in the schistose units may reflect a different deformational history, a lack of clearly visible folds in a fine grained homogeneous schist, or intense transposition and obliteration of folds. Foliation outside the mylonite zones, is parallel to the axial plane of these isoclinal folds. In general, foliations in both areas strike northeast and dip northwest. Poles to foliations on a
Schmidt equal-area-net for Sixmile Creek and Yankee Jim Canyon show similar orientations (Figs. 25 and 26). The northeast-strike of foliations parallels the strike of lithologic contacts and ductile shear zones (see Fig. 2 and Fig. 3). Measurements of poles to 50
Fig. 24. Photograph taken in Yankee Jim Canyon shows an isoclinal fold in the mixed gneiss unit.
Fig. 30. Horizontal quartz slickensides exposed in a road cut in biotite schist from Sixmile Creek. 5%, 7.5/o, and 10% contours
Figure 25. Poles to foliation diagram with 40 points on a Schmidt equal-area- net of Yankee Jim Canyon.
5.3%, 10.7%, and 16 .II contours
Figure 26. Poles to fo lia tio n diagram with 56 points on a Schmidt equal - area-net of Sixmile Creek. 52
foliations from schistose units plot close to cumulative plots of a ll
fo lia tio n measurements which suggest the same structural control for
the schist as the gneiss (Fig. 27).
Measurements of tig h t to isoclinal folds were taken from widely
separated samples of fold axes. Microscopic fold axes were not
measured. Instead, measurements were taken only from well-exposed
fold axes. Axes plunge 15 degrees, approximately N 25 E.
Figure 28 shows a plot of tig h t to isoclinal fold axes from the
mixed gneiss unit. Northwest-dipping foliations in the study area are
axial planar to the isoclinal folds. Late-stage deformation,
especially ductile deformation, may account for the scatter of fold-
axis measurements. Measurable fold axes are absent within the
mylonite zones. Rotation of axes during simple shear described by
Skjernaa (1980) and Ramsay (1980), in addition to subsequent defor- mational events, may account for the scatter of fold-axis measurements.
Broad open folds observed in gneiss and schist represent the second
period of deformation. Open folds are found several kilometers away
from the ductile shear zones. These broad open folds warp foliations which are axial planar to the isoclinal folds. Therefore, the broad open folds formed after the isoclinal folds. Measurements of the broad open folds were d iffic u lt to make, their axes plunge approximately
30 degrees to N 40° W.
Formation of mylonite may represent another period of deformation.
Mylonitization deforms and destroys micaceous minerals alligned during isoclinal folding. Therefore, i t is a later event. In hand sample, 53 N
Figure 27 Poles to fo lia tio n diagram with 30 points on a Schmidt equal-area net of the schistose units in Sixmile Creek. N
Figure 28 Plot of 10 megascopic isoclinal fold axis from the mixed gneiss unit. 54
mylonite is recognized by fine-grained dark, p h y llitic laminae that
wrap around feldspar porphyroblasts. Also, elongate pressure shadows
may be present. Petrographic textures include the above, plus re
duction in grain-size, ribbon quartz, mortar texture, sutured grain
boundaries, and recrystallization. Samples are described as mylonite where these textures dominate over textures formed during regional
metamorphism. I estimate maximum widths of one-half kilometer for each
zone on the basis of these petrographic textures. However, these are
estimations and differences may result from the following:
1. On a large scale, shear strain, as evidenced by mylonitic
textures, decreases gradationally away from mylonite zones
(Fig. 18);
2. On a fine scale, highly sheared rocks lie adjacent to
less-deformed rocks;
3. Mylonite zones may thin and thicken.
The existence of two mylonite zones complicates interpretations.
They may have formed contemporaneously, although no evidence points directly to this conclusion. One mylonite juxtaposes gneissic rocks against schistose rocks. This could happen in two ways: 1) ductile shearing takes place at a pre-existing contact, where the contact acts as a zone of weakness*, or 2) tectonic transport along the mylonite zone brings together different rock units.
Mylonitic potassium feldspar gneiss has an elongate map pattern.
Mineralogically it plots slightly above the ternary minimum towards 55
quartz on a plagioclase-quartz-potassium feldspar diagram (see Fig. 11).
Therefore, this unit may have been a large granitic s ill. If so, late-
stage movement along the s ill while i t remained a crystal mush could
produce mylonites.
I was unable to determine the direction of tectonic transport along the zones. The few mineral lineations present at the contact between the gneissic and schistose package do not exhibit a consistent orientation (see Fig. 29). Furthermore, one cannot determine the direction of tectonic transport from mineral lineations alone (Hobbs et a l.,
1976). Mineral lineations in combination with fold axes may determine the direction of tectonic transport, but fold axes were not found in the mylonite zones. Therefore, lineations may point in the direction of tectonic transport or perpendicular to the direction of transport.
Measurements of elongate pressure shadows adjacent to feldspar mega- crysts hint at the transport direction. Limited data taken indicates several possible transport directions relative to the plane of schistosity (Table 5).
Table 5
Elongate Pressure Shadows
Number of Samples Direction of Elongation
3 Roughly horizontal in the plane of schistosity
Parallel to the dip of schistosity
10 Indeterminate or flattened within the plane of schistosity, they could be either horizontal or dip-slip. 56 N
Figure 29 18 mineral lineations taken from amphibole schist samples near the contact between the gneissic package and the schistose packaqe. N
Figure 31 A plot of 14 slickenside measurements on a Schmidt equal-area-net. 57
Possible mechanisms to produce these elongate porphyroblasts
include strike-slip movement, combined dip-slip and strike-slip
movement, or simple flattening. However, the sample population is
not large enough to give s ta tis tic a lly valid conclusions.
Laramide-to-Recent faulting represents the fourth period of de
formation. Foliation measurements vary considerably for samples taken
along the margin of Paradise Valley, where Laramide-to-Recent faulting disrupts the schistosity (see Fig. 2 and 3). The presence of a small scarp on an alluvial fan along the eastern margin of Paradise Valley attests to recent faulting. Fan symmetry and the lack of dissection make a Pre-Wisconsin age unlikely. Similar scarps are visible near the basement contact for fifteen kilometers to the north-northeast.
Laramide-to-Recent u p lift along the eastern margin of Paradise
Valley may have disturbed portions of the Precambrian block. No nearby
Paleozoic or Mesozoic sections rest undisturbed on basement which leaves no way to evaluate whether the Precambrian block acted as a single unit. Slickensides have predominantly near-horizontal orien tations in rocks from Yankee Jim Canyon and Sixmile Creek (Fig. 30).
Figure 31 shows a plot of slickenside orientations. Slickensides characteristically lie in moderate to steeply dipping planes of fo lia tio n . Minor late-stage, strike -slip movement within the basement block could form near-horizontal slickensides. I f late-stage strike -slip movement did not form the slickensides, they may indicate strike -slip 58 movement during the waning stages of ductile deformation. Slicken sides probably formed during Laramide deformation.
Contacts between units on the northeast side of Sixmile Creek do not continue directly across to the southwest side (see Fig. 3).
Normally, such offsets suggest the existence of a dip-slip fault along
Sixmile Creek. However, poor exposure in the valley and gradational contacts across the ductile shear zone may account for the apparent offset.
In conclusion, four periods of deformation were recognized:
D.j Tight to isoclinal folding accompanied formation of
schistosity parallel to axial planes;
D£ Broad open folds with kink bands in the nose, possibly
synkinematic with crenulation cleavage;
Ductile shearing and formation of northeast-trending
mylonite;
D^ Laramide-to-Recent b rittle faulting and formation of subhorizontal slickensides. CHAPTER V
COMPARISON OF YANKEE JIM CANYON AND SIXMILE CREEK TO THE SOUTHERN MADISON RANGE AND THE NORTH SNOWY BLOCK
Metamorphic and structural sim ilarities between crystalline base ment in Yankee Jim Canyon and Sixmile Creek, the southern Madison Range, and the north Snowy block warrant a closer examination. In a ll three areas, greenschist facies metamorphism followed a much higher prograde event, but not all map units exhibit this retrograde vent. Structural similarities between the three areas include northeast-trending map patterns, foliations, and shear zones. Direct lithologic correlations may prove impossible, but a first-o rd e r examination of sim ila ritie s and differences may shed lig h t on what is necessary for future work.
Southern Madison Range
Erslev (1981) mapped a large area of crystalline basement, near
Earthquake Lake in the southern Madison Range, seventy kilometers south west of this study area. Northeast-striking foliations with northwest dips predominate in the area. Micaceous schist from Erslev's southern domain exhibit fine-grained, homogeneous textures similar to those in
Sixmile Creek. However, the southern domain contains widespread dolomitic marble not found in Sixmile Creek.
Erslev (1981) proposed that a three-kilometer-wide mylonite zone occurred at the contact between gneiss and schist. Fine-grained meta- sedimentary schists outside the mylonite zone have finer-grained 59 60
equivalents within the mylonite zone. Mylonitization reduced grain-
size and produced ribbon quartz.
Erslev (personal comm., 1982) examined the fine-grained schistose
units in Sixmile Creek. Because of their sim ila rity in hand sample to
fine-grained mylonitic schists of the southern Madison Range, he in te r
preted that the schist in Sixmile Creek is recrystallized mylonite.
However, no coarser-grained equivalent exists in Sixmile Creek to
demonstrate grain-size reduction from a coarser unit. Also, detailed
petrography reveals no sign of mylonitic textures (see Fig. 32). In
contrast, thin sections from samples near the gneissic contact exhibit
numerous mylonitic textures (see Fig. 33). I t is d iffic u lt to determine
the extent of mylonitization in a highly recrystallized fine-grained schist. The simplest origin for the fine-grained schist involves metamorphism of a fine-grained sedimentary rock. The possibility remains that recrystallization obliterated the textures of a wider mylonite zone.
Erslev (1982) determined that retrograde greenschist-facies meta morphism was spatially associated with the three-kilometer-wide mylonite zone in the southern Madison Range. Also, he proposed mass transfer to produce the retrograde mineral assemblages. This model is similar to that proposed here for the Yankee Jim Canyon and Sixmile Creek areas.
Structural complexities make direct lithologic correlations across large distances in the crystalline basement extremely d iffic u lt i f not impossible. Also, i f attempts to correlate mylonites between the southern 61
Fig. 32. Photomicrograph magnified 10 times in plain lig h t of a b io tite schist exhibits none of the textures suggestive of myloni ti zation.
Fig. 33. In contrast to figure 32, this photomicrograph magnified 10 times with crossed nicols exhibits textures such as reduction in grain size, pressure shadows, anastimosing micaceous fo lia tio n s, mortar texture and sutured quartz which are characteristic of mylonite. Madison Range and Sixmile Creek may be problematic. In Sixmile Creek,
schist lies north of gneiss, whereas, in the southern Madison Range,
the situation is reversed. I f the terrains do correlate, i t seems
more lik e ly that two gneissic packages sandwich a schistose belt with
mylonite zones along opposite edges of the belt. However, evaluation
of this hypothesis is hindered by very limited basement exposure
between the two areas.
North Snowy Block
Tertiary volcanic and intrusive rocks cover most of the 25 kilometers
which separate Sixmile Creek, from the north Snowy block to the north
east. Reid et a l. (1975) mapped the crystalline basement of the north
Snowy block (see Fig. 34). Mogk (personal comm., 1982) re-examined the
north Snowy block, and noted several discrepancies of Reid's work.
Table 6 lis ts some of these discrepancies.
Mogk (1982) proposed the division of the Mount Delano Gneiss into three 1ithologically distinct units. Furthermore, he differentiated between metasedimentary rocks on either side of the Pine Creek nappe which Reid et a l. (1975) mapped as Davis Creek Schist. The details of Mogk's (1982) lithologic descriptions and structural interpretations are beyond the scope of this paper. However, the division of the
Mount Delano Gneiss into three d istin ct map units greatly enhances the understanding of sim ilarities and differences between geology to the north Snowy block and Sixmile Creek. 63
Km.
Fig. 34 Geology of the north Snowy block (after Reid et a l, 1975) (see page 64 for description). 64
Table 6
Symbol on Unit name by Description and interpretation Sketch map Reid et a l. (1975) ______by Moqk (1982)
A Mt. Delano Gneiss Migmatitic unit which grades into meta sediments to the west (unit D).
Mt. Delano Gneiss Mylonitic trondjemite lies structurally below the folded nappe complex, lith o - logically distinct from unit A.
Mt. Delano Gneiss "Paragneiss," veined gneiss highly "mixed" with several other rock types.
Davis Creek Schist Metasedimentary rocks, 1ithologically d istin ct, and higher metamorphic grade than unit E.
Davis Creek Schist Agreement, micaceous schist with c h lo ritic alteration, highly mylonitic along con tact with Mt. Cowen Gneiss.
Pine Creek Nappe Agreement, folded isoclinal nappe complex which consists of amphibolite, marble, and quartzite.
Mount Cowen Gneiss Agreement, potassium feldspar augen gneiss, sheared along thin band to the west.
Despite the proximity, widespread exposures, and sim ilar northeast structural trend to the north Snowy block, lithologic correlations do not match well enough to ju s tify a direct comparison. Field relations, petrography, and geochemistry of both areas must be examined to sub stantiate any direct correlation. I consider, below, two possible lithologic correlations. 65
First, the thin band of Mount Cowen Gneiss, and the units to
either side labeled C and E exhibit many similarities with rocks studied
in Sixmile Creek. In outcrop, unit C of the Mount Delano Gneiss con
tains a contorted arrangement of various rock types similar to those
observed in the mixed gneiss (Mogk, pers. comm., 1982). To the west,
the thin band of Mount Cowen Gneiss has many of the characteristics
of the mylonitic potassium feldspar gneiss of Sixmile Creek. Local
mylonitization and alteration of biotite-quartz- plagioclase-perthitic
microcline gneiss of the Mount Cowen Gneiss to epidote, sericite,
chlorite, and clay is similar to descriptions of the mylonitic potassium
feldspar gneiss of Sixmile Creek. Farther west, the correlation is
less clear, where mixed gneiss in Sixmile Creek has no equivalent in
the north Snowy block. A possible explanation involves the intrusion
of a granitic s ill, the Mount Cowen Gneiss, into different parts of the
section. S till farther west, the Davis Creek Schist (unit E) contains
more chlorite and muscovite than the schistose package in Sixmile Creek.
Compositional variations within sedimentary parents may account for some
of these differences.
A second possible correlation involves a section of fine-grained
schist, similar to that in Sixmile Creek, found in an outlier of the
north Snowy block along the northeast margin of Paradise Valley (Erslev,
personal comm., 1982). The projected strike from Sixmile Creek transects
the north Snowy block fifteen kilometers east of these outliers. Left-
lateral movement along the M ill Creek - Stillw ater Fault zone may 66 account for this offset. Horizontal displacement along the fa u lt is not well documented at M ill Creek. Near the Boulder River several authors describe le ft-la te ra l movement of three kilometers or more where the fa u lt displaces Paleozoic units (Wilson, 1936; Foose et al.,
1961; Butler, 1966). Whatever the movement, the M ill Creek-StilIwater
Fault confuses attempts to correlate units across it . Despite lith o logic variations, dominant northeast-striking foliations and map patterns suggest structural sim ilarities between the two areas. CHAPTER VI
CONCLUSION
Two northeast-trending mylonite zones discovered in the study
area range in width up to one-half kilometer. Prior to mylonitiza-
tion, prograde metamorphism reached amphibolite-grade and the rocks were intensely deformed. Greenschist-facies retrograde metamorphism
is spatially associated with the mylonite zones and probably formed
syntectonically with mylonitization.
Table 7 is a proposed sequence of structural events and their
relationship to metamorphism.
Table 7
Deformation Metamorphi sm Textural Evidence
Isoclinal folding Amphibolite facies, Well oriented micas, sillim ani te-muscovi te- moderately oriented zone si 11imani te
Broad open folding Broad open folds warp the with crenulation cleavage which is axial cleavage planar to isoclinal folds; broad open folds not found in mylonite zones
Mylonitization Greenschist facies, Polygonal arcs with chlorite biotite-zone after biotite; undulose epidote; sericite, chlorite, and secondary b io tite orien ted parallel to schistosity
Larami de-to-Recent Minor hydrothermal Alteration forms 90-degree faulting alteration intergrowths along plagio- clase cleavage traces
67 68
Hydrated retrograde mineral assemblages reflect mass transfer of
water, K+, and possibly other elements prior to, or during retrograde
metamorphism. This hydrated environment not only allowed retrograde
alteration, but it may have induced mylonitization by increasing
d u c tility . Shear heating may have further localized retrograde meta morphism, as well as the introduction of the water.
The mylonite zones formed along a large northeast structural trend.
This structural trend may extend 100 kilometers in length, from the
southern Madison Range, through Yankee Jim Canyon and Sixmile Creek, to
the north Snowy block. Northeast-striking units characterize at least
20 kilometers of the southern Madison Range, and 15 kilometers of the north Snowy block. The mylonite zones mapped here may have formed along
this northeast structure. Reactivation of pre-existing structural weak nesses such as the contact between units, could have generated the shear zones. Contacts between units might have served as the weakness.
The timing of mylonitization remains unclear. Commonly, retrograde greenschist events in the basement of southwestern Montana reset K-Ar dates at approximately 1.7 b illio n years before present (James and
Hedge, 1980). Reset dates in Yankee Jim Canyon and Sixmile Creek may yield similar results. However, if shearing controlled retrograde metamorphism, as inferred above, then neither mylonitization nor this retrograde metamorphism may be related to the regional retrograde event.
Fountain and Desmarais (1980) noticed changes in lithologies, 69 metamorphism, and structural styles, on a gross scale, between the
Beartooth Massif and the area surrounding the Tobacco Root Range. They inferred a boundary between these two pre-Belt terranes along a north east-trending line which lies 30 kilometers northwest of the structural trend described above. I f the two terranes are d is tin c t, then the boundary may be situated somewhat closer to the structural trend described here. The existence of a suture zone at such a boundry would help explain the major zone proposed. Hence, Erslev's (1981) report of a three-kilometer-wide mylonite zone brought forth speculation of a major suture zone. The lack of evidence for major tectonic transport led
Erslev (1981) to believe that such a zone did not exist. The much thinner mylonite zones exposed in Yankee Jim Canyon and Sixmile Creek probably do not by themselves constitute a major continent-continent suture zone. Typically, mylonite zones on the order of several kilometers wide, and high-grade metamorphic rocks accompany major continent-continent-col1ision zones such as the Nelson front in Manitoba
(Bell, 1971; Gibb and Walcott, 1971; Cransonte and Turek, 1976; Weber and Scoates , 1978).
In summary, the shear zones proposed in Yankee Jim Canyon and Sixmile
Creek may differ in size, structural style, metamorphic history, and position in lithologic sequence from those found in the southern Madison
Range and the north Snowy block. A tectonic framework involving the northeast structural trend awaits further study. Such a study must examine a ll three areas to resolve discrepancies between mapping, tectonic transport, lithologic interpretations, and the extent of mylonitization. REFERENCES
Balashov, Y.A., A.A. Kremenetskiy, V.M. Shvetts, 1973. Geochemical c rite ria of the nature of Precambrian amphibolites, Geochem. Intern., 9, pp. 918-931.
Beach, A., 1976. The interrelations of fluid transport, deformation, geochemistry and heat flow in early Proterozoic shear zones in the Lewisian complex, Phil. Trans. Roy. Soc., A 280, pp. 569-604.
and J. Tarney, 1978. Major and trace element patterns established during retrogressive metamorphism of granulte facies gneisses, northwest Scotland, Precamb. Res., 7, pp. 325-348.
, 1980. Retrogressive metamorphic processes in shear zones with special reference to the Lewisian complex, jjiShear Zonesin Rocks, J. Struct. Geol., 2, n. 1/2, pp. 257-264.
Bell, C.K., 1971. Boundary geology, upper Nelson River area, Manitoba and northwestern Ontario, in Geoscience Studies in Manitoba, Geol. Assoc. Can. Sp. Pap. 9, pp. 11-29.
Bell, T.H., and M.A. Etheridge, 1973. Microstructure of mylonites and their descriptive terminology, Lithos, 6, pp. 337-348.
B latt, H., G. Middleton, R. Murray, 1972. Origin of Sedimentary Rocks, Prentice-Hall Inc., New Jersey, 634 pp.
Bryden, E.L., 1950. Geology of an area north of Gardiner, Montana, M.S. Thesis, Wayne Univ., Detroit, Mich., 47 pp.
Brown, E.H., 1969. Some zoned garnets from the greenschist facies, Am. Mineral., 54, pp. 1662-1677.
Borg, L.Y., D.K. Smith, 1968. Calculated powder patterns: I. Five plagioclases, Am. Min. 53, pp. 1709-1724.
Brun, J.P., and P.R. Cobbold, 1980. Strain heating and thermal softening in continental shear zones: A review, j_n Shear Zones in Rocks, J. Struc. Geol., 2, no. 1/2, pp. 149-158.
Butler, J.R., 1966. Geological evolution of the Beartooth Mountains, Montana and Wyoming: Part 6, Cathedral Peak Area, Montana, Geol. Soc. Am. B u ll., 77, pp. 45-64.
, 1969. Origin of Precambrian granitic gneiss in the Beartooth Mountains, Montana and Wyoming, Geol. Soc. Am. Mem., 115, pp. 73-101.
70 71
Casella, C.J., 1969. A review of the Precambrian geology of the eastern Beartooth Mountains, Montana and Wyoming, Geol. Soc. Am. Mem., 115, pp. 53-72.
Chakraborty, K.R., and S.K. Sen (1967. Regional metamorphism of p e litic rocks around Kandra, Singhbhum, Bihar, Contr. Min. Petrol., 16, pp. 210-232.
Cooper, A.F., 1972. Progressive metamorphism of metabasic rocks from the Haast schist group of southern New Zealand, J. Petrol., 13, pp. 457-492.
Cordua, W.S., 1973. Precambrian geology of the southern Tobacco Root Mountains, Madison County, Montana, Ph.D. Thesis, Indiana Univ., Bloomington, 247 pp.
Cranstone, D.A., and A. Turek, 1976. Geological and geochronological relationships of the Thompson nickel belt, Manitoba, Can. J. Earth Sci., 13, pp. 1058-1069.
Crawford, M.L., 1966. Composition of plagioclase and associated minerals in some schists from Vermont, USA, and South Westland, New Zealand, with inferences about the peristerite solvus, Contr. Min. Pet., 13, pp. 269-294.
Dietrich, R.V., 1960. Banded gneisses, J. Petrol., 1, pp. 99-120.
Erslev, E.A., 1981. Petrology and structure of the Precambrian metamorphic rocks of the southern Madison Range, southwestern Montana, Ph.D. Thesis, Harvard Univ., Cambridge, Mass., 133 pp.
Evans, B.W., 1965. Application of a reaction rate method to the breakdown equilibria of muscovite and muscovite plus quartz, Am. J. Sci., 263, pp. 647-667.
Foose, R.M., D.U.Wise, and G.S. Garbarini, 1961. Structural geology of the Beartooth Mountains, Montana and Wyoming, Geol. Soc. Am. B ull., 72, pp. 1143-1172. Fleitout, L., and C. Froidevaux, 1980.. Thermal and mechanical evolution of shear zones, ^n Shear Zones in Rocks, J. Struct. Geol., 2, n. 1/2, pp. 159-165.
Fountain, D.M., and N.R. Desmarais, 1980. The Wabowden Terrane of Manitoba and the Pre-Belt basement of southwestern Montana: A comparison, Mont. Bur. Mines Geol. Sp. Publ., 83, pp. 35-46.
Fraser, G.D., H.A. Waldrop, and H.J. Hayden, 1969. Geology in the Gardiner area, Park County, Montana, U.S.G.S. B u ll., 1277, 118 pp. 72
Garihan, J.M., 1979. Geology and structure of the central Ruby Range, Madison County, Montana, Geol. Soc. Am.-Bull., 90, pp. 695-788.
Gibb, R.A., and R.I. Walcott, 1971. A Precambrian suture in the Canadian shield, Earth and Planet. Sci. Let., 10, pp. 417-422.
Grapes, R.H., and C.M. Graham, 1978. The actinolite-hornblende series in metabasite and the so-called m is cib ility gap: A review, Lithos, 11, n. 2, pp. 85-97.
Hadley, J.B ., 1960. Geology of the north part of the Gravelly Range, Madison County, Montana, in West Yellowstone Earthquake Area, B illings Geol. Soc. 11th Ann. Field Conf., Guidebook, pp. 149-153.
Hermann, A.G., M.J. Potts, and D. Knake, 1974. Geochemistry of the rare earth elements in spilites from the oceanic and continental crust, Contr. Min. Petrol., 44, pp. 1-16.
Hobbs, B.E., W.D. Means, P.F. Williams, 1976. An Outline of Structural Geology, John Wiley and Sons, New York, 571 pp.
Holberg, L., 1940. Geomorphic problems and glacial geology of the Yellowstone Valley, Park County, Montana, J. Geol., 48, pp. 275-303.
Holdaway, M.J., 1971. S ta b ility of andalusite and the aluminum silicate phase diagram, Am. J. Sci., 271, pp. 97-131.
Hoschek, G., 1967. Unterschungen zum Stabilitatsbereich von Chloritoid und staurolith, Contr. Mineral. Petrol., 14, pp. 123-162.
Hseu, L.C., 1968. Selected phase relationships in the system Al-Mn-Fe- Si-0-H: A model for garnet equilibria, J. Petrol., 9, pp. 40-82.
Hyndman, D.W., 1972. Petrology of Igneous and Metamorphic Rocks, McGraw-Hill Book Co., New York, 533 pp.
, 1982. Petrology of Igneous and Metamorphic Rocks, second edition, unpubl. manuscript, 567 pp.
James, H .I., and C.E. Hedge, 1980. Age of basement rocks in southwestern Montana, Geol. Soc. Am. Bull. Part 1, 91, pp. 11-15.
Lambert, R. St. J., V.E. Chamberlain, and J.G. Holland, 1976. The geochemistry of Archean rocks, i_n The Early History of the Earth, B.F. Windley editor, pp. 377-389.
Leake, B.E., 1963. Origin of amphibolites from northwest Adirondacks, New York, Geol. Soc. Am. B ull., 74, pp. 1193-1202. 73
Leake, B.E., 1964. The chemical distinction between ortho- and para- amphibolites, J. Petrol., 5, pp. 238-254.
Millholand, M.A., 1976. Mineralogy and petrology of Precambrian metamorphic rocks of the Gravelly Range, southwestern Montana, M.S. Thesis, Indiana Univ., Bloomington, 134 pp.
Mitra, G., and Frost, B.R., 1981. Mechanisms of deformation within Laramide and Precambrian deformation zones in basement rocks of the Wind River Mountains, Contrib. Geol., Univ. Wyo., 19, n. 2, pp. 161- 173.
Mogk, D.W., 1981. Archean rocks of the north Snowy block, Absaraka Range, Montana, Geol. Soc. Am. Abst. Prog., p. 97.
, 1981. Tectonic thickening of Archean continental crust, north Snowy block, Beartooth Mountains, Montana, Geol. Soc. Am. Abstr. Prog., 13, n. 7, p. 513.
, 1982. Petrology, structure, and geochemistry of an Archean Terrain in the north Snowy block, Montana, Ph.D. dissertation, (in progress).
Okuma, A.F., 1971. Structure of the southwestern Ruby Range, near Dillon, Montana, Ph.D. Thesis, Penn. State Univ., Univ. Park, 122 pp.
Peterman, Z.E., 1981. Dating of Archean basement in northeastern Wyoming and southern Montana, Geol. Soc. Am. B u ll., 92, pp. 137-146.
Ramsay, J.G., 1980. Shear zone geometry: A review, jji Shear Zones in Rocks, J. Struct. Geol., 2, n. 1/2, pp. 83-101.
Reid, R.R., W.J. McMannis, and J.C. Palmquist, 1975. Precambrian geology of north Snowy block, Beartooth Mountains, Montana, Geol. Soc. Am. Sp. Pap., 157, 135 pp.
Schreyer, W., and H.S. Yoder, J r., 1964. The system Mg-cordierite-H«0 and related rocks, Neues Jahrb. Min. Abh., 101, pp. 271-342.
Schubert, K., 1979. Amphibolites from the central Alps: REE - Patterns and th e ir relation to the possible parent rocks, Schweiz. Min. Petr. M itt., 59, pp. 299-308.
Skjernaa, L., 1980. Rotation and deformation of randomly oriented planar and linear structures in progressive simple shear, in_ Shear Zones in Rocks, J. Struc. Geol., 2, n. 1/2, pp. 101-109.
Skinner, W.R., D.R. Bowes, and S.G. Khomy, 1969. Polyphase deformation in the Archean basement complex, Beartooth Mountains, Montana and Wyoming, Geol. Soc. Am. B ull., 80, pp. 1053-1060. 74
Smith, K.J., 1980. Petrology and origin of Precambrian rocks in the eastern Ruby Mountains, southwestern Montana, M.S. Thesis, Univ. Montana, Missoula, 84 pp.
Spencer, E.W., and S.J. Kozak, 1975. Precambrian evolution of the Spanish Peaks area, Montana, Geol. Soc. Am. B u ll., 86, pp. 785-792.
Spry, A., 1969. Metamorphic Textures, Pergamon Press, Oxford, 350 pp.
Tarney, J ., 1976. Geochemistry of Archean high-grade gneisses, with implications as to the origin and evolution of the Precambrian Crust, jji The Early History of the Earth, B.F. Windley, editor, pp. 405-417.
Thompson, J.B., and S.A. Norton, 1968. Paleozoic regional metamorphism in New England and adjacent areas, jji Studies of Appalachian Geology, E.A. Zen, editor, pp. 319-327.
Troger, W.E., 1979. Optical Determination of Rock-forming Minerals, E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, Germany, 187 pp.
Tuttle, O.F., and N.L. Bowen, 1958. Origin of granite in light of ex perimental studies in the system NaAlSio0o-KAlSio0o-Ho0, Geol. Soc. Am. Man., 74. 153 pp. 3 8 3 8 2
Vitaliano, C.J., and W.S. Cordua, 1979. Geologic map of the southern Tobacco Root Mountains, Madison County, Montana, Geol. Soc. Am., M.C.-31.
Weathers, M.S., and J.M. Bird, 1979. D ifferential stress determined from deformation induced microstructures of the Marine Thrust, Journ. Geoph. Res., 84, n. B13, pp. 7495-7509.
Weber, W., and R.F. Scoates, 1978. Archean and Proterozoic metamorphism in the northwestern Superior Province and along the Churchill- Superior boundary, Manitoba, jjn Metamorphism in the Canadian Shield, Geol. Surv. of Can. Paper, 78-10, pp. 5-16.
Weeks, G., 1980. Precambrian geology of the Boulder River area, Beartooth Mountains, Montana, M.S. Thesis, Univ. Montana, Missoula, 58 pp.
Wilson, C.W., 1934. Geology of the thrust fa u lt near Gardiner, Montana, J. Geol., 42, pp. 649-663.
Wilson, J.T., 1936. The geology of the Mill Creek-StilIwater area, Montana, Ph.D. Thesis, Princeton Univ., New Jersey, 118 pp.
Wilson, M.L., 1981. Petrology and origin of Archean lithologies in the southern Tobacco Root and north Ruby Ranges of southwestern Montana, M.S. Thesis, Univ. Montana, Missoula, 92 pp. 75
Wooden, J.L., C.W. Montgomery, and C.J. Casella, 1979. Rb-Sr and zircon U-Pb ages of the granitic rocks of the southwest Beartooth Mountains within Yellowstone Park, Montana and Wyoming, Geol. Soc. Abst. Prog., 1, n. 6, p. 306. APPENDIX I APPENDIX I PETROGRAPHIC DESCRIPTIONS Sixmile Creek Slide No. and Rock Type Modes (*) ...... Grain Size Grain Shape Deformation Textures BB-7 Hornblende (50) fine subhedral, Highly deformed to Foliated hornblende, Amphibole bladed mylonitic, plag twin hornblende dislocated Schist Clinozoisite (5-10) aphanitic clumps lamallae dislocated and subsequently and bent, undulose annealed by a ctin o lite , A!bite - An^ (35) fine anhedral hornblende, ribbon a ctin o lite rims Quartz (2-3) fi ne anhedral quartz hornblende A ctinolite (5) fine rims hornblende
BB-95 Hornblende (50) f i ne subhedral, Low to moderate Foliated hornblende, bladed deformation, undulose no evidence of plagioclase retrograde meta- Amphibole Plagioclase fi ne subhedral An40 morphi sm Schi st Biotite 0 ) fine subhedral Sphene (2) aphanitic anhedral
BB-98 Actinolite- Tremoli te (95) medi urn anhedral, Low to moderate Unoriented, composition equidimensional deformation, textures may indicate a may have been sedimentary origin Amphi bole Quartz (3) fine anhedral, obliterated by mottled for this sample hydrous retrograde Schist* PIagioclase (trace) f i ne altered metamorphism Chlori te (2) f i ne adjacent to amphibole
* Not a common amphibole schist, this sample is probably an exoti APPENDIX I (Continued)
Slide No. and Rock Type Modes______(%) Grain Size Grain Shape Deformation Textures
RB-n Hornblende (50) fine anhedral, ragged Highly deformed, Foliated albite and hornblende amphiboles, epidote Amphibole Actinol ite (5) fine adjacent to have bent cleavage restricted to Schi st hornblende traces, hornblende albite grains Albite An^ (35) aphanitic clumps undulose Calcite (1) f i ne subhedral, veins Quartz (trace) fine anhedral
BB-10 Biotite (25) fine subhedral, ragged Mylonite, grain- Foliated biotite, size reduction, broken rolled Biotite PIagioclase - broken b io tite garnet surrounded Schist (40) fine subhedral An30 crystals, re- by chlorite. Chlorite (trace) aphanitic crystallized quartz, S-shaped b io tite , Quartz (35) fi ne anhedral bent cleavages, Garnet (trace) fine anhedral undulose plag and b io tite Apatite (trace) aphani tii subhedral
BB-49 Quartz (35) fine-med anhedral Highly deformed, Foliated re limited grain-size crystallized Biotite Biotite (25) fine subhedral reduction, broken biotite, rolled Schist PIagioclase _ b io tite grains, garnet An5 (35-40) f i ne subhedral recrystallized ribbon Cli nozoisite f i ne anhedral quartz, S shaped ! (1) biotite '- j Garnet (trace) fine anhedral 00 Seri cite (trace) aphanitic Calcite (trace) fi ne APPENDIX I (Continued)
Slide No. and Rock Type Modes U) Grain ! Grain Shape Deformation Textures
BB-89 Quartz (60) fine anhedral Highly deformed to Foliated muscovite, mylonitic, minor recrystallized Muscovite A1bite grain size re oriented quartz, Schist An-5 (20) fine anhedral duction, small weathered Muscovite (15) aphani- subhedral blocks of schist fine transported parallel Hematite (5) fine along to schistosity laminae Potassium (2) fine anhedral Feldspar
BB-106 Quartz (65) fine anhedral Highly deformed, Foliated muscovite, broken grains with lith ic fragments Muscovite Plagioclase (15) fine subhedral recrystal 1ization of quartz and Schist An '30 or annealing plagioclase are Muscovite (10) fine subhedral especially of some of the rare quartz sedimentary textures Hematite (10) fine alone observed, weathered laminae
BB-30 Quartz (40) fine anhedral Mylonitic, grain- Rolled microcline embayed size reduction, porphyroblasts, some Mylonitic Microcl'ine (25) fine to anhedral, pressure shadows, quartz recrystallized, med i urn ragged anastamosing mylonitic foliation foliations of micas almost p h y llitic , in Potassium Albite An^ (25) fine embayed and quartz, ribbon tense greenschist Feldspar Seri cite (4) aphanitic quartz, micas and alteration epidote fragmented, Gneiss Biotite (3) aphanitic S shaped micas, Clinozoisite (2) aphanitic clumps sutured quartz APPENDIX I (Continued)
Slide No. and Rock Type Modes (%) Grain Size Grain Shape Deformation Textures
Cal cite (1) fine subhedral Garnet (trace) aphanitic Chlorite or Chloritoid? (trace) aphanitic
BB-32 Quartz (35) fi ne anhedral M ylonitic, intense Mylonitic folia Myloni tic Microcline (20) fine to anhedral, grain-size re tion, recrystallized coarse embayed duction, anastomo quartz, plagio sing foliations of clase altered to Potassium Plagioclase (20) fine anhedral sericite and epidote, potassium Anc and An00„ feldspar altered ° 28? b io tite , quartz pressure shadows, to sericite. Feldspar Bioti te (15) fi ne subhedral suture quartz, Gneiss Sericite (7) aphanitic broken b io tite crystals, ribbon quartz. Epidote (3) aphanitic
RB-lOc Quartz (25) fine anhedral M ylonitic, intense Mylonitic foliation grain-size re almost p h y llitic , Mylonitic Microcline (20) medium anhedral, duction, mortar perthite, secondary embayed texture, anasto biotite, sericite Potassium Albite - An^ (20) f i ne anhedral mosing foliations chlorite and epidote around microcl ine, form as alteration Feldspar Biotite (15) f i ne subhedral pressure shadows products. Gneiss Epidote (14) fine anhedral of recrystallized quartz, ribbon quartz Serici te (5) aphanitic undulose microcline 00 Chiorite (1) aphanitic APPENDIX I (Continued)
Slide No. and Rock Type Modes (*) Grain Size Grain Shape Deformation Textures BB-41 Quartz (30) fine anhedral Highly deformed Foliated biotite, to mylonitic, b io tite wraps around Mylonitic B iotite (20) fine to subhedral minor anasti- garnet, garnet a l Potassium medium mosing fo lia tio n , tered to albite + Feldspar Albite Anr (27) fine subhedral limited grain- sericite + chi ori- Gneiss b size reduction, toid?, limited Microcline mats-medium b io tite cleavage quartz recrystalli- altering to sericite- and albite twins zation mat sericite (20) aphanitic altered bent, sutured and Garnet (2) fine anhedral embayed quartz, undulose b io tite Fuchsite 0 ) aphani tic
Mixed Gneiss Samples from Sixmile Creek
BB-6 Albite An^ (55) fine to anhedral, Highly deformed to Foliation of chlorite medi urn ragged mylonitic, ribbon and Fe oxides in quartz, sutured chlorite, chlorite chlorite - Quartz (30) fine to anhedral grain boundaries and Fe oxide altered quartz - medium of quartz, from biotite, albite Chlorite (10) fine to from b io tite medium limited grain- epidote and sericite schistose size reduction. lie within albite gneiss B iotite (1) fine subhedral crystal s Epidote (2) f i ne anhedral Fe-oxide (2) aphanitic in chlorite CO
Serici te (trace) aphanitic - APPENDIX I (Continued)
Slide No. and Rock Type Modes (%) Grain Size Grain Shape Deformation Textures
BB-17 Plagioclase (45) medi urn subhedral Slight deformation, Foliated biotite, An27 regional meta quartz embayed by morphism created plagioclase b io tite - Biotite (20) medi urn euhedral the original quartz- Quartz (35) medium subhedral schistosity, slightly plagioclase bent plagioclase twins, schistose minor undulose ex gneiss tinction in plagioclase
BB-23 Plagioclase (40) medium subhedral slig h tly deformed, Gneissic layering An b io tite - undulose bio tite into biotite-poor 25 and quartz and biotite-rich microcli ne- Quartz (30) medi urn anhedral quartz- zones, b io tite plagioclase Microcline (20) medi um subhedral forms the schistosity schistose Biotite (9) medium subhedral gneiss Muscovite (1) fine subhedral Zi rcon (trace) aphani tic euhedral
BB-25 Plagioclase (45) medium anhedral Highly deformed, Foliated micas, re An35 bent bio tite crystallized quartz cleavage, bent rounded and si 11imanite - Quartz (30) medium anhedral plagioclase twins embayed plagio b io tite - Biotite (20) fine subhedral crushed or broken clase quartz biotite grains plagioclase Orthoclase 0 ) medium anhedral 00 ro schistose Muscovite (2) fine subhedral gneiss Epidote 0 ) aphanitic -
Sillimanite (trace) aphanitic * APPENDIX I (Continued)
SIide No. and Rock Type Modes______(%) Grain Size Grain Shape Deformation Textures
BB-36 Quartz (40) fine anhedral Highly deformed to Micaceous fo lia tio n , mylonitic, limited altered patches of chlorite- Albite- (35) medi urn anhedral epidote- Anc grain-size re epidote + chlorite + b io tite J duction, sutured biotite, chlorite al bi te- B iotite (15) fine subhedral quartz, crushed alters from biotite, grains, bent limited quartz re- quartz Epidote (5) aphaniti clumps schist plagioclase and crystallization Chlorite (2) aphaniti subhedral b io tite cleavages, undulose b io tite Calcite (1) fine anhedral Microcline (1) fine anhedral Sericite (trace) aphanitic
BB-39 Quartz (45) medium anhedral Highly deformed, Foliated biotite, garnet alters to altered Plagioclase (30) medium anhedral sutured and re- crystallized quartz, se ricite , secondary schistose Mat Sericite (10) aphanitic gnei ss rounded feldspars, biotite and Biotite (15) fine-med. subhedral garnet appears chloritoid, mat dislocated along textured sericite from Chlorite aphanitic (1) the plane of potassium feldspar? Epidote (trace) aphanitic schistosity Chloritoid (trace) aphanitic Garnet (1) fine anhedral
oo CO APPENDIX I (Continued)
Slide No. and Rock Type Modes (%) Grain Size Grain Shape Deformation Textures
BB-47 Quartz (45) fine-med. anhedral Highly deformed to Foliated micas, muscovite- mylonitic, re b io tite alters to Albite An3 (35) fine-coar: anhedral b io tite crystall ized chlorite, layers of al bite- Biotite (10) fine subhedral ribbon quartz, quartz - b io tite + quartz Muscovite (5) f i ne subhedral limited grain- epidote + muscovite schistose size reduction, albite + quartz, gneiss Epidote (3) fi ne anhedral bent b io tite albite appears Calcite (1) f i ne subhedral cleavage, bent rolled plagioclase twins, Chlorite (1) aphanitic undulose biotite
BB-70 Plagioclase (45) medium subhedral Slightly deformed, Foliated biotite, muscovite- An25 undulose quartz, unaltered b io tite - slightly bent Quartz (37) fi ne anhedral quartz- plagioclase twins plagioclase Biotite (15) fine-med. subhedral schistose euhedral gneiss Muscovi te (2) fi ne subhedral Fe-oxi de (2) fine Zircon (trace) aphanitic APPENDIX I (Continued)
Slide No. and Rock Type Modes______(%) Grain Size Grain Shape ______Deformation ______Textures
Actinolite- Tremolite (50) Fine-med. anhedral Mylonite, severely Unoriented broken amphibole amphiboles, horn Albite -An5 (25) f i ne anhedral crystals, un blende altered to Epidote (15) fine anhedral dulose amphiboles, actinolite, albite grain-size re alters to epidote, Hornblende (6) fine ghosts duction original rock was Chlorite (3) aphanitic - a normal amphi bol ite not Quartz (1) fine anhedral amphibole schist
Sphene (trace) aphanitic -
Yankee Jim Canyon
YJ-5 Quartz (45) medium anhedral Moderately de Foliated biotite, si 11imanite- Plagioclase- formed, bent b io tite limited recrystal garnet- (30) medi urn subhedral cleavage, bent lized quartz, b io tite - An38 plagioclase twins, granular, fibrolitic plagioclase- Biotite (20) medi urn subhedral undulose b io tite . si 11imanite, al- quartz mandine garnet schi stose Garnet (5) medium subhedral gneiss Orthoclase (1) medium subhedral Si 11imanite (trace) aphanitic - co <_n APPENDIX (Continued)
Slide No. and Rock Type Modes (%) Grain Size Grain Shape Deformation Textures
YJ-14 Quartz (55) medi urn anhedral Highly deformed, Foliated biotite, sutured quartz, muscovite typically Plagi oclase- sillimanite- ' (30) medi urn subhedral severely bent lies along b io tite epidote- b io tite cleavages, cleavages, one b io tite - An25 Biotite (10) fine subhedral bent plagioclase muscovite grain is plagioclase- twins, undulose offset along a dis quartz Epidote (3) f i ne anhedral plagioclase and located b io tite schi stose b io tite grain, fibrolitic gnei ss Muscovi te (trace) fine subhedral sillimanite lie Chlori te (trace) aphanitic along plagioclase Serici te (trace) aphanitic boundaries, myr- meki te. Si 11imanite (trace) aphanitic
YJ-20 Garnet (90) medi urn subhedral- Slightly deformed, Unoriented, fresh garnetiferous euhedral bent b io tite almandine garnet Plagioclase (5) fine pod subhedral cleavages. porphyroblasts, B iotite (4) fine euhedral surrounded by Chlorite aphanitic amphibolite in (1) outcrop. Zircon (trace) aphanitic
CO cn APPENDIX I (Conti nued)
Slide No. and Rock Type Modes______(%) Grain Size Grain Shape Deformation Textures
Y J-26 Quartz (40) fine anhedral Mylonite, grain- Mylonitic foliation, Mylonitic size reduction, anastomosing b io tite , Albite -An^ (15) f i ne anhedral potassium sutured and re recrystal 1ized quartz feldspar Microcline (30) medium anhedral crystal 1 ized forms small pressure gneiss quartz, broken shadows, rolled Biotite (20) fine subhedral biotite crystals, microcline crystals. Epidote (trace) aphanitic dislocations of albite twins. Serici te (trace) aphanitic Apatite (trace) aphanitic
YJ-33 A1tered Quartz (35) fine anhedral Highly deformed Feldspars altered to microcline- to mylonitic, sericite, large quartz Microcline (30) fine anhedral limited grain- d istin ct vein of schist A1bi te-An^ (30) fine subhedral size reduction, epidote, biotite broken albite completely altered Sericite (2) aphanitic and microcline to chlorite. crystals un Epidote (2) medium euhedral dulose quartz. Chlorite (1) aphanitic
"J00 APPENDIX I (Continued)
Slide No. and Rock Type Modes {%) Grain Size Grain Shape Deformation Textures
YJ-36 Quartz (30) fine-med. anhedral Highly deformed Anastomosi ng altered to mylonitic, muscovite microcline- Microcline (30) medi urn anhedral limited grain- foliation, albite quartz Albi te-Ang (15) fine anhedral size reduction, altered to epidote schist recrystal 1ized and muscovite, Epidote (10) fine subhedral quartz pressure epidote unoriented. shadows, bent Fuchsite (5-10) fine-med. subhedral and dislocated Muscovite (5-10) fine subhedral albite twins.
Zircon (trace ) aphanitic -
YJ-38 Epidote (40) fine-med. subhedral Highly deformed?, Hydrous greenschist intense post alteration minerals altered Quartz (25) fine anhedral deformational dominate, weak chlorite- Chiori te (10) f i ne anhedral alteration schistosity, fuchsite- quartz- Albi te (5) fine-med. subhedral obi iterated feldspars intensely many textures. altered. epidote f i ne-med. subhedral rock Fuchsite (15) Biotite (4) f i ne anhedral
Sphene (1) aphanitic -
00 00 APPENDIX I (Continued)
Slide No. and Rock Type Modes {%) Grain Size Grain Shape Deformation Textures
Y J-41 Epidote (60) fine-coarse subhedral Highly deformed, Unoriented, late altered Quartz (10) fine anhedral dislocated albite stage shearing may chlorite- twins, un- exhibit a preferred fuchsi te- Fuchsite (10) f i ne subhedral dulose plagio- breakage direction, quartz- clase, patches brown mica length plagioclase- Plagioclase An ? (15) fine subhedral of epidote fast with high 2V, epidote Unknown brown (2) fine anhedral disrupted? mica rolled rock mica si ightly Chiorite (2) aphanitic Muscovi te (1) fine subhedral
YJ-53- Quartz (45) medium anhedral moderately de Unoriented, appears formed, bent almost granitic, Microcline (25) medi urn subhedral, grani tic ragged b io tite cleavages, mymekite, plagio s ill bent plagioclase clase altered to Plagioclase (25) medium subhedral twins, dislocated sericite An25 mi crocline twins, undulose plagio* Sericite (4) aphanitic cl ase B iotite (i) aphanitic
co APPENDIX I (Continued)
Slide No. and Rock Type Modes (%) Grain Size Grain Shape Deformation Textures
YJ-65 Plagioclase- (45) medium subhedral Moderately deformed, Foliated biotite, An30 bent albite twins, recrystal 1 ized sillimanite- bent biotite quartz crystals, b io tite - Quartz (35) medium anhedral cleavages, un layering, primary quartz- dulose b io tite and muscovite and plagioclase B iotite (19) med i urn euhedral plagioclase secondary sericite schistose Muscovite (1) fine subhedral gneiss Si 11imanite (trace) aphanitic - Zircon (trace) aphanitic -
Sericite (trace) aphanitic -
YJ-81 Quartz (45) medi urn anhedral, Low deformation, Not highly fresh limited un oriented almost b io tite - PIagioclase- (30) medium subhedral dulose quartz. granular, in te r plagioclase- Anrtr. stitial biotite, quartz 25 recrystall ized quartz, plagio rock B iotite (24) fi ne euhedral clase altered to
Sericite (1) aphanitic - sericite.
Zircon (trace) aphanitic - Ilmenite (trace) f i ne spicular KO O APPENDIX I (Continued)
Slide No. and Rock Type Modes (%) Grain Size Grain Shape Deformation Textures
YJ-96 Quartz (45) med.-coarse anhedral Highly deformed, Thick layers 4-8 sillimanite- bent bio tite centimeters, Plagioclase muscovite- cleavage, warped b io tite (45) b io tite - An30 med.-coarse subhedral biotite un fo lia tio n , plagioclase- dulose, bent muscovite in te rstia l quartz Biotite (5) medi urn subhedral and dislocated to plagioclase, schistose Muscovite (5) medium subhedral a 1 bite twins, quartz-rich and gneiss quartz plagioclase-rich Si 11imanite (trace) aphanitic fibroli tic sutured layers
YJ-99 Quartz (45-50) med.-coarse anhedral Moderately de Weakly foliated to sillimanite- formed, b io tite massive, unknown micaceous- Plagioclase (45) coarse subhedral undulose with altered colorless An plagioclase- 28 slightly bent patches, chlorite quartz cleavage, alters from biotite gneiss Biotite (5) medium subhedral slightly bent Muscovite (2) fine-med. cuts bioti a1 bite twins, sutured quartz Si 11imanite (trace) aphani tic fib r o litic
Chlorite (trace) aphanitic -
RB-9 Hornblende (55) medium euhedral Low deformation, Massive undulose horn Plagioclase (45) medi urn anhedral blende and
Amphibolite Sericite (trace) aphanitic - plagioclase
Epidote (trace) aphanitic -
Chlorite (trace) aphanitic - APPENDIX I (Continued)
Slide No. and Rock Type Modes______(%) Grain Size Grain Shape Deformation Textures
RB-43 Hypersthene (90) med.-coarse euhedral Low deformation, Massive, cumulate mildly undulose texture with horn Orthopyrox- Hornblende (8) medium euhedral hornblende blende in te rstia l enite to hypersthene, Plagioclase (2) fine-med. subhedral forms a large pod-like body in outcrop surrounded by a friable c h lo ritic selvage.
RB-49 Hornblende (80) medium euhedral Low deformation, Massive undulose plagio Amphiboli te Plagioclase (10) medium subhedral clase and b io tite Bioti te (10) fine euhedral Quartz (trace) fine anhedral
ro APPENDIX II APPENDIX 2 93 GEOLOGY OF SIXMILE CREEK
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Quaternary Alluvium
PreCambrian Lithologies ______i______Amphibolite Schist |as| Mixed Gneiss mg
Micaceous Schist [n is i Mylonitic kg K-Feldspar Gneiss Petrographic and geochemical sample locations.