'Jhi tDYapkttmt <ds suypofttd, 11tti J\an4y ,See fiward Bivzn, i:tt }tis riteY(LOY1J, to attolher- ouht arldt1t9 ,g:rt!drtafa shtd.erJ · il- t1tt biofo,,9H 1)e;yaA-rca:itt, 1btivet$1t,,y J fdhttte.$ota, nu1111:t1.. To my parents and all my other teachers .. . PETROLOGY OF .THE MIGMATITE COMPLEX ALONG THE SOUTH FORK OF THE CLEARWATER RIVER , IDAHO
A THESIS SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA
BY DIANE HELEN CARLSON
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
JUNE , 1981 ABSTRACT A nonuraniferous migmatite ,complex is exposed along the South Fork of the Clearwater River in north central Idaho. The complex is situated along the northeastern border of the Atlanta lobe of the Idaho batholith in a high-grade metamorphic terrane consisting of aluminous gneiss, ton- alitic migmatite, calc-silicate granofelses, quartzofeldspathic gneiss, quartzite, augen gneiss, and amphibolite. As the margin of the Atlanta lobe is approached, on the western border of the complex, the Droogs Creek granite intrudes the high-grade metasedimentary rocks in a lit- par-lit manner. Toward the east, the percentage of granitoid rocks de- creases, and in situ migmatite is exposed near Dutch Oven Creek. Struc- tures indicative of more advanced stages of migmatization increase west- ward through the complex. The high-grade metasedimentary rocks in the complex are steeply dipping and trend north northwest. They generally have gradational contacts, and contain abundant well-rounded zircon. Aluminous gneiss near the Crooked River contains sillimanite and garnet in addition to biotite, muscovite, feldspar, and quartz. The aluminous gneiss is on the eastern limb of an antiform cored by migmatite. The migmatite, hers named the Dutch Oven Creek (DOC) migmatite, contains discontinuous peg- matite-1 ike leucosomes which are enveloped by very thin biotite-rich melanosomes. Overall, the migmatite is tonalitic in composition but is granitic locally where layers contain up to 30 percent microcline. The migmatite is brecciated by quartz monzonite along its western con- tact and grades into calc-silicate granofelses. The calc-silicate granofelses are typically layered and contain epidote-, diopside-, and scapolite-bearing assemblages. Hornblende-rich selvages along the edges of calc-silicate xenoliths attest to local metasomatism. The granofelses grade into quartzofeldspathic gneiss and quartzite near Newsome Creek. Micaceous quartzite at Golden is folded into large- scale open folds, the limbs of which contain hinges of isoclines. Augen gneiss occurs as xenoliths in the DOC migmatite, sheets in the aluminous gneiss, and as a complexly folded composite unit in sharp contact with the Golden quartzite. Biotite amphibolites are present in every unit either as blocks or sill-like lenses, and are garnet-bearing in the aluminous gneiss. Interference structures and isoclinal foid hinges on the limbs of large-scale folds suggest that the area was affected by two and possibly three progressive(?) deformational events. Tight-to-isoclinal folds in the DOC migmatite are coaxial with the larger north-south trending anti- form. In the aluminous gneiss, sillimanite needles are coaxial with tight folds, whereas fibrolite is folded. Faults trend northwesterly in the eastern part of the complex and are randomly oriented in the west. Joints define at least two maxima at N83W;72S and N69W;54S. The entire complex is within the sillimanite zone of the upper amphibo- 1 ite facies. The orientation of sil1imanite prisms and fibrolite indicate that metamorphism and deformation were coeval. Mineral relations in the aluminous gneiss suggest that at the peak of metamorphism, temperatures between 700°-730°C and pressures exceeding 3.5 kb were attained. These ii conditions are within the range for partial melting to occur. Compositions of leucosomes in the DOC migmatite generally plot close to the isobaric cotectic surfaces in regions of low temperature in the Qz-Ab-Or-An-H 0 system and are compatible with an origin by partial melting. Leucocratic2 layers that do not plot close to cotectic surfaces in areas of low tempera- ture, may have formed by metamorphic segregation induced by(?) part ia 1 melting. iii
ACKNOWLEDGEMENTS
I would like to acknowledge Dr. James A. Grant for serving as chairman of my thesis committee. Ors. John C. Green, Timothy B. Holst, and Donald K. Harriss also served on the thesis committee and are acknowledged for their suggestions and helpful criticisms. I am grateful to Dr. Paul E. Myers of the University of Wisconsin at Eau Claire and Ms. Penny Morton, Dr. John C. Green, and Dr. Timothy B. Holst of the University of Minnesota-Duluth for acting as advisors while Dr. Grant was in England. I am especially grateful to Dr. Myers for his continued interest and encouragement in my geological endeavors and for his inspiration which led me into the field of geology . . His assistance throughout the duration of this study and in the preparation of Figure 32 are much appreciated. I would like to thank David Brekke, Scott Robinson, and Dr. Frank Karner for their assistance and cooperation in using the microprobe at the University of North Dakota-Grand Forks. I am indebted to J. Thomas Nash and the Uranium and Thorium Branch of the U. S. Geological Survey for funding this entire study. Dr. Richard Ojakangas aided in the interpretation of accessory minerals and acted as a liaison for the U. S. Geological Survey; his expertise and encouragement are gratefully acknowledged. Special thanf
TABLE OF CONTENTS
ABSTRACT .... i iii TABLE OF CONTENTS v ILLUSTRATIONS vii TABLES x
PLATES x INTRODUCTION Problem 1 Location and Access l Previous Work 1 Present Study 6 REGIONAL GEOLOGY . 8 DESCRIPTION OF ROCK UNITS IN THESIS AREA 12 General Statement ... 12 Quartzite ...... 13 Cale-silicate Granofels 16 Pyroxenes 18 Amphiboles ... 19 Minor Minerals .. 20 Aluminous Gneiss Unit . 22 Aluminous Gneiss . 24 Leucocratic Stringers, Pods, and Dikes 29 Amphibolites ... . 32 Migmatite ...... 35 Migmatite Structures 35 Melanosome . 38 Leucosomes . 41 Dikes 44 Amphibolites 47 Augen Gneiss 48 Granitoid Rocks . 51 Droogs Creek Granite 53 Santiam Creek and Fall Creek Intrusion Breccias 55 Legget Creek Granite . . . . . 57 Intrusions in the Cale-silicate Unit 58 Di abase . . . 59 STRUCTURAL GEOLOGY . 60 General Statement 60 vi
TABLE OF CONTENTS, CONT.
Compositional Layering--Foliation 60 Fo 1ds ...... 61 Tight-to-Isoclinal Folds 61 Ptygmatic Folds 66 Open Folds .... 66 Faults and Shear Zones 69 Joints ...... 72 Lineations ...... 74 Stereonet Analysis 76 Dama in I =· Surveyor Creek to Tenmi le Creek 76 Domain II = Golden to Buckhorn Creek ... 79 Domain III =· Fall Creek to Allison Creek . 79 Domain IV •· Dutch Oven Creek Migmatite .. 82 Domain V = Crooked River Aluminous Gneiss Unit 84 Stereonet Analysis of Entire Area 84 Conciusions ... 90 METAMORPHIC PETROLOGY 92 General Statement 92 Protoliths 92 Mineral Relations 94 Conditions of Metamorphism 98 Migmatization ...... 100
FAVORABILITY OF THE MIGMATITE COMPLEX 111
SUMMARY AND CONCLUSIONS 115 APPENDICES I Glossary of Terms Al II Modal Compositions of Metasedimentary Rocks A3 III Microprobe Analyses A7
REFERENCES CITED ...... 118 vii
ILLUSTRATIONS
Figure Page 1. LOCATION OF STUDY AREA 2 2. VIEW OF SOUTH FORK VALLEY 3 3. ACCESS MAP . . . . . 4 4. REGIONAL GEOLOGY MAP 9 5. TECTONIC MAP OF IDAHO 11 6. CONTACT BETWEEN GOLDEN QUARTZITE AND BUCKHORN GNEISS 14 1. COMPOSITIONAL LAYERING IN GOLDEN QUARTZITE . . 14 8. PHOTOMICROGRAPH OF DEFORMATION BANDS IN GOLDEN QUARTZITE ...... 15 9. BRECCIATED CAlC-SILICATE GRANOFELS . 17 10. HORNBLENDE-RICH SELVAGE ON CALC-SILICATE BLOCK . 17 11. LAYERED CALC-SILICATE GRANOFELS ...... 18 12. PHOTOMICROGRAPH OF SCAPOLITE IN GRANOBLASTIC TEXTURE 21 13. SILLIMANITE-MICROCLINE STRINGERS IN ALUMINOUS GNEISS 24 14. MODAL COMPOSITIONS OF ALUMINOUS GNEISS 26 15. PHOTOMICROGRAPH OF SILLIMANITE NEEDLES INCLUDED IN KINKED MUSCOVITE ...... 28 16. PHOTOMICROGRAPH OF FIBROLITE MAT RIMMED BY SILLIMANITE PRISMS ...... 28 17. MODAL COMPOSITIONS OF LEUCOCRATIC PODS, DIKES, AND STRINGERS IN ALUMINOUS GNEISS ...... 30 18. PHOTOMICROGRAPH OF ZONED PLAGIOCLASE WITH QUARTZ INCLUSIONS ...... 31 19. PHOTOMICROGRAPH OF EPITAXIAL OVERGROWTH ON PLAGIOCLASE 31 20. PHOTOMICROGRAPH OF PRIMARY MUSCOVITE EMBAYED BY MI CROCLINE ...... 33 viii
ILLUSTRATIONS, CONT.
Figure Page 21. PHOTOMICROGRAPH OF KINKED BIOTITE RIMMED BY CHLORITE, MUSCOVITE, AND MAGNETITE 33 22. LAYERING IN MELANOSOME 36 23. AMPHIBOLITE AGMATITE . 37 24. MODAL COMPOSITION OF MELANQSOME FROM DUTCH OVEN CREEK MIGMATITE ...... 39 25. MODAL COMPOSITIONS OF LEUCOSOMES IN DUTCH OVEN CREEK MIGMATITE ...... 42 26. LEUCOSOME THAT CUTS A RECUMBENT ISOCLINAL FOLD . . 43 27. MODAL COMPOSITIONS OF DIKES FROM DUTCH OVEN CREEK MIGMATITE ...... 45 28. DIKE AND LEUCOSOMES CUT BY LATE DIKE 46 29. BUCKHORN AUGEN GNEISS ...... 49 30. MODAL CLASSIFICATION OF GRANITOID ROCKS 52 31. SHEET OF LIT-PAR-LIT GNEISS IN DROOGS CREEK GRANITE 54 32. SANTIAM CREEK INTRUSION BRECCIA . . 56 33. ISOCLINAL FOLD IN SCHLIERIC TONALITE 57 34. RECUMBENT TIGHT-TO-ISOCLINAL FOLDS WITH AXIAL PLANAR FOLIATION ...... · · · · 62 35. UPRIGHT TIGHT-TO-ISOCLINAL FOLDS WITH AXIAL PLANAR FOLIATION CUT BY COPLANAR LEUCOCRATIC VEIN . . . . 62 36. SHEARED TIGHT-TO-ISOCLINAL FOLD WITH FOLDED FOLIATION ...... 63 37. FOLD SHOWING FIRST AND SECOND ORDER MEDIAN SURFACES 64 38. TIGHT FOLDS WITH AXIAL PLANAR FOLIATION, INTRUDED BY GRANITE . . 65 39. PTYGMATIC FOLDS 67 40. PTYGMATIC FOLDS 67 ix
ILLUSTRATIONS, CONT.
Figure Page 41. OPEN FOLD IN DUTCH OVEN CREEK MIGMATITE 68 42. LARGE-SCALE OPEN FOLD AT TRAIL CREEK 68 43. POLES TO FAULTS AND SHEAR ZONES . . 70 44. DISPLACED GRANITOID DIKE IN DUTCH OVEN CREEK MIGMATITE ...... 71 45. CONTOUR OF POLES TO SHEAR JOINTS 73 46. MULLIONS AT BUCKHORN CREEK . . . 75 47. POLES TO STRUCTURAL ELEMENTS FROM DOMAIN I 77
4.8a. BIOTITE GNEISS INCLUSION CONTAINING PTYGMATIC FOLDS 78 48b. STEREONET PLOT OF FOLD AXES FROM INCLUSIONS IN DROOGS CREEK GRANITE ...... 78 49. POLES TO STRUCTURAL ELEMENTS FROM DOMAIN II . . . . . 80 50a. STEREONET PLOT OF STRUCTURAL FEATURES FROM DOMAIN III 81 50b. CONTOUR OF POLES TO FOLIATION FROM DOMAIN I II 81 51a. CONTOUR OF POLES TO COMPOSITIONAL LAYERING FROM DOMAIN IV ...... 83 51b. STEREONET PLOT OF FOLD AXES FROM DOMAIN IV . . . 83 52. CONTOUR OF POLES TO GRANITOID DIKES IN DUTCH OVEN CREEK MIGMATITE ...... 85 53. POLES TO FOLIATION FROM DOMAIN V ...... 86 54a. w DIAGRAM OF POLES TO COMPOSITIONAL LAYERING AND FOLIATION FROM ENTIRE AREA ...... 88 54b. S DIAGRAM OF MEASUREMENTS IN FIGURE 54a 88 55. CONTOUR OF FOLD AXES FROM ENTIRE AREA 89 56. P- T DIAGRAM ...... 99 x
ILLUSTRATIONS, CONT.
Figure Page
57 PROJECTION OF Qz-Ab-Or-An-H20 SYSTEM 102 58 LOCATION OF MICROPROBE SAMPLES ... 104
59 Qz-Ab-Or-An-H20 PLOT FROM LOCATION 8041 . 105 60 Qz-Ab-Or-An-H20 PLOT FROM LOCATION 8042 106 61 Qz-Ab-Or-An-H20 PLOT FROM LOCATIONS 8067 AND 80163 107
TABLES
Table Page l MINERAL CHARACTERISTICS FROM DUTCH OVEN CREEK MIGMATITE, DROOGS CREEK GRANITE, AND SANTIAM CREEK TONALITE ...... 40 2 MINERAL ASSEMBLAGES FROM METASEDIMENTARY ROCKS 95
3 RADIOACTIVITY OF ROCK UNITS ...... 112
PLATES
Plate Page I GEOLOGIC MAP ALONG THE SOUTH FORK OF THE CLEARWATER RIVER, IDAHO ...... in pocket II STATION LOCATION MAP ALONG THE SOUTH FORK OF THE CLEARWATER RIVER, IDAHO ...... in pocket INTRODUCTION
Problem This thesis deals with the petrology and structure of a high- grade metamorphic terrane between the Atlanta and Bitterroot lobes of the Idaho batholith west of· Elk City, Idaho (Fig. 1). The meta- morphic terrane has a predominant north-south structural trend and consists of aluminous and augen gneisses, calc-silicate rocks, quartzites, amphibolites, migmatites, and granitoid rocks. Recent road construction along the upper South Fork of the Clearwater River (hereafter referred to as South Fork) provides spectacular exposures of contact and structural relations across the complex. The objectives of this study are to: 1) describe mappable lithologies and contact rela- tions, 2) determine the relationship of granitic rocks to other units, 3) examine the structural and metamorphic histories, and 4) investigate. the distribution of radioactive minerals in the rocks.
Location and Access Field study was confined to road cuts on Highway 14 between Surveyor Creek and Crooked River along the South Fork in the Golden and Center Star minute quadrangles (Plate I). The steep-walled South Fork valley (Fig. 2) has an average relief of 1500 feet (457 meters) and access to the adjacent area is limited to logging roads along tributaries which follow the structural grain. Location and major access routes are shown in Figure 3.
Previous Work In a preliminary geologic reconnaissance of the region drained by 2
LOCATION OF STUDY AREA IN IDAHO
t15'3s'
0 5 Miles
ar'
J8 •• I I I I -- ' " ' '.. .. . · ...
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0 200 Miles
c;ource: ATLAS 76 : GSA 8ULL. -;;- NEZPERCE N.F.
Figure 1. Location of study area. 3
Figure 2. View of South Fork valley at west- ern edge of study area. 4
-·--·-·r ·-· ·-,--·-·-·-·-·-·-·-·-·-·-·-·--·-·-·---·- I i ! ( I i I MONTANA I I ! i \ i \ i \ \ .)
10
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WASH. ( \ rm
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I ! i I I OAHO -·-·- ·-·-·-·-·-·-·-·-·-·- ·-·-·-·-·- ·- ·---· --·-·-·.J
· Figure 3. Major access routes to study area. 5 the Clearwater River, Lindgren (1904) recognized metamorphic rocks of Precambrian age in the South Fork area. Lindgren believed the granite- gneiss complex was derived in part from the Idaho batholith and in part from highly metamorphosed Precambrian granite. Thomson and Ballard (1924), in a report on gold resources in north-central Idaho, presented a generalized geologic map and structure section across the bathol ith. They postulated a "gneissoidal shell" dipping away from and enclosing the partially unroofed batholith. Outward from the granitic batholith, the shell contains lenticular and banded gneiss injected by numerous cross-cutting dikes and sills, highly deformed mica schists, and impure quartzites. Reed (1934) studied gold-bearing gravel within the district and noted that the granite intruded folded gneisses and schists. The first thorough reconnaissance mapping in the Elk City, Orogrande, Buffalo Hump, and Tenmile districts was undertaken by Shenon and Reed (1934a; 1934b). Their reports consist mainly of comprehensive descriptions of the ore deposits but they also discuss the composition and field relations of rock units. Capps (1941) mentions faulting along upper Newsome Creek and cites evidence for folding and metamorphism before and after batholith intrusion. In a discussion of the bedrock geology and placer deposits in the Elk City area, Reid (1959; 1960) suggests the pre-batholith rocks correlate lithologically to the Beltian strata found to the north and postulates four deformational events. According to Reid (1959), the first deformation occurred during the late Precambrian and produced tight recumbent isoclinal folds with axial planar foliation. The Fi 6 axial planar foliation was folded into open folds during a second deformation in the Permian and Triassic periods and was followed by broad open folding during the Nevadan orogeny. A fourth, low-tempera- ture, deformation post-dates intrusion of the Idaho batholith. The ages of the deformations have not been dated by radiometric means. Using Reid's map and structural data as a base, Hall (1961) completed a statistical analysis·of the structure along the South Fork from Peasley Creek to Elk City. Hall attempted to verify the existence of three fold generations by the use of TI and 8 stereonet plots but offered no explanation for the geologic events that produced the observed structures.
Present Study This study is based on 5 weeks of field investigation during June and July of 1980 and is supplemented by laboratory study of 350 hand samples. Field work concentrated on detailed lithologic descriptions of metasedimentary units, mapping contact and structural relations of granitic rocks in the metasedimentary rocks, and a scintillometer survey across the field area to determine any occurrences of radio- active minerals. Station locations and mappable units were plotted on the Golden and Center Star 7t minute quadrangles.· Of the 350 hand samples collected, 180 were chosen for thin section study and approximately 50 were slabbed and stained for study with the binocular microscope. Slabs and thin section heels were stained for potassium feldspar and plagioclase feldspar using sodium cobaltinitrite and amarand stains, respectively. Modal analyses of 7 stained slabs and selected thin sections are tabulated in Appendix II. Appendix I contains a glossary of terms used in this study. The anorthite content of plagioclase was determined by X-ray diffraction and by microprobe analyses. Tables of plagioclase analyses are located in Appendix III. Granitoid rocks are classified by the IUGS classification proposed by Streckeisen (1973), and migmatite terminology is after Mehnert (1968). Structural data were plotted on Schmidt equal area stereonets and contoured with a Kalsbeck counting net. In addition to this study, Zimmerman (in progress) is conducting a detail€d structural analysis within the Golden 71 minute quadrangle under the auspices of the U. S. Geological Survey Elk City 2° mapping project. 8
REGIONAL GEOLOGY
The high-grade metamorphic rocks exposed along the South Fork are part of the Salmon River arch (Armstrong, 1975)--a northwesterly trending screen of metasedimentary rocks and gneisses that separates the Atlanta from the Bitterroot lobe of the Idaho batholith (Fig. 4). Armstrong (1975) believes the Salmon River arch structure is pre- Beltian (>1500 m.y.) in age based on a 1500 m.y. whole rock Rb-Sr date for granitic and gneissic rocks that cut the metasediments and older gneisses near Shoup, Idaho (Fig. 3). Others working to the north along the western border of the Bitterroot lobe (Morrison, 1968; Reid, Greenwood, and Morrison, 1970; Greenwood and Morrison, 1973) and along the northern tip of the Atlanta lobe (Heitanen, 1961a; 196lb ; 1963a; 1963b) suggest the metasediments are lithologically similar to and may be correlative with the Prichard, Wallace, and Ravalli Groups of the Belt Series (1450 to 850 m.y.). Direct correlation of metasedi- ments within the Salmon River arch to the Belt terrane (Fig. 4) is hampered by a cover of Columbia River basalt on the west and by the Bitterroot lobe on the east. Lack of geologic mapping and radiometric age dating to the south prevents conclusive evidence for correlating the metasediments near South Fork to pre-Beltian rocks in southwestern Montana (Harrison, 1972). According to Armstrong (1975), the Salmon River arch acted as a buttress and affected Mesozoic tectonics and emplacement of the Idaho batholith. The Salmon River arch subdivides the batholith into two lobes--the Bitterroot lobe in the north and the Atlanta lobe in the south. The Bitterroot lobe was emplaced approximately 38 - 80 m.y. e \ D CENOZOIC VOLCANIC ANO SEDIMENTARY ROCKS 0 \ 0 100 zoo KILOMETERS EARLY CENOZOIC GRANITIC PLUTONS BELTIAH '·, TERRAHE \ ST fl. B LE p\..fl.TfORM r+i MESOZOIC GRANITIC PLUTONS I \.
...... BELTIAH, PALEOZOIC, AND MESOZOIC STRATIFIEO \ ROCKS OF THE PLATFORM AHO MIOGEOSYNCLINE \ D ...... ' ' ', PALEOZOIC AHO MESOZOIC SEDIMENTARY AND :oLUMBIA IGHE,OUS ROCKS OF THE EUGEOSYNCLINE PLATEAU . GRANITIC ROCKS OF THE SALMON RIVER ARCH IE. __..--·110Nl· . I ..,vo· I
PRE- BELT QUARTZITE AHO ARGILLITE OF SOUTH-CENTRAL IDAHO
PRE-BELT GNEISS AND METASEDIMEHTARY ROCKS OF THE SALMON RIVER ARCH, BASEMENT UNDlRLYING BELT SERIES IN NORTHERN IDAHO, ANO VARIOUS GNEISSES THAT MAY BE Of MESOZOIC AGE
) PRE-BELT CRYSTALLINE BASEMENT Of THE ::> ·- •mISJ PLATFORM > / I \ __.. i SAMPLE LOCALITY FOR PRECAMURIAN 1400m.y.J i A RAOI 0 METRIC DATE \. .\,, I' / / \ / I \. ' \ ,._ ..__ __ _ ...,,.. I i '" / \ I I ,,,"'' WESTERN LIMIT OF OLDER CRATON f R 0 M I ARMSTRONG ANO OTHERS, I 9 7
]Ure 4. Regional geology and tectonic map of Idaho and adjacent states (Armstrong, 1975).
\.0 10 ago (McDowell and Kulp, 1969; Armstrong, 1974), ranges in composition from foliated granodiorite on the western edge to massive granite in the core, and is cut by several smaller Tertiary batholiths (Reid and others, 1979). The Atlanta -lobe was intruded 70 - 100 m.y. ago (McDowell and Kulp, 1969; Armstrong, 1974; Armstrong and others, 1977) and is slightly older and more mafic than the Bitterroot lobe. The Granite Creek pluton (Myers, to the west of the study area and the Droogs Creek pluton(?) within it (Plate I), represent the northern- most apices of the Atlanta lobe (Fig. 4). To the west of the study area, Seven Devils Volcanics and volcanogenic sediments of the Riggins Group have been mapped near Riggins (Hamiltor., 1963) and Harpster (Myers, 1968; in press) and are thought to represent part of an ophiolite sequence associated with the Triassic subduction complex (Hyndman and Talbot, 1975). The Riggins Group contains five generations of folding (Onasch, 1977; 1978) and is separated from the Seven Devils Volcanics by the eastward dipping Rapid River thrust fault (Hamilton, 1960; 1963; 1969). sr87;sr86 ratios (Armstrong, Taubeneck, and Hales, 1977) change abruptly from 0.704 to 0.706 across the narrow zone (Fig. 5) and mark the boundary between Pr.ecambrian crust on the east and the Phanerozoic eugeosyncline on the west. Late diabase dikes and sills within the study area may be feeders for the Columbia River basalts (Miocene-Pliocene) which crop out to the north and west of South Fork (Fig. 4). ·-·-\\·- ·- ·-·-·-·- ·- ·-·1·- ·-r·- ·- ·-·-·- ·- ·- ·- ·- ·- ·-·-·--' \\\ Washington j ! Montana \ " . I \ ' I . \ ' i \ \ \ . \ I I. .) \ I -"-I \ \ I -- ---1 --..:...: '-11\/ \ I i 7 L \ . o:,....-.:=--= LOBE ., \. .10 \ I I ...... \ '--_:"../·- ·- ·- ·-·-\ 1,....,,, . ." /Ir.. \ ._,,..,-- ·---...... _,/'._./ ). Ii1/
/3} I ... j . So V. \()I · I. Oregon s/1 Isr/./j & Idaho ' ·- ·-·-·- ·- ·- ·- ·- .- .___..104'-·- ·-i ·rI ·- ·- ·-·-·- ·-·-·-·- ·- ·- ·- ·_] _.,,,... I . 0 100 zoo mil es / 0 100 zoo :soo kilometers
86 Figure 5. Tectonic map of Idaho and adjacent states which shows the relationship between sr87 ;sr ratios and the subduction complex, the Idaho batholith, and the trend of the Salmon River Arch (After Myers, in press). 12
DESCRIPTION OF ROCK UNITS IN THE THESIS AREA
General Statement The area studied is a composite of high-grade metasedimentary rocks, gneisses, and granitoid rocks which are situated along the northeastern border· of the Atlanta lobe of the Idaho batholith (Fig. 1). The principal rock units (see Plate I) are aluminous gneiss, migmatite, calc-silicate granofels, quartzite, augen gneiss, and granitoid rocks. Quartzofeldspathic gneiss is intermediate between aluminous gneiss, calc-silicate granofels, and quartzite. The grani- toid component in this composite assemblage increases westward toward Reed Creek where a pluton(?) contains steeply-dipping sheets of meta- sedimentary rocks and gneisses. Aluminous gneiss is exposed near Newsome Creek where it grades eastward into quartzite and at Crooked River where it forms the eastern limb of an antiform cored by migmatite. The calc-silicate granofels, which is most abundant between Allison Creek and Trail Creek, is cut by numerous quartz monzonite dikes and sills. An interlayered sequence of metasedirnentary and plutonic rocks between Trail Creek and Golden consists of quartzofeldspathic gneiss, quartzite (Newsome Creek), aluminous gneiss, augen gneiss, and granite. Quartzite is exposed along Newsome Creek road, near Golden, and as inclusions in granite to the west. Augen gneiss occurs as pods in the aluminous gneiss and migmatite and as a thick sheet in sharp contact with quartzite near Buckhorn Creek. Granitoid rocks as dikes, sills, and pods have compositions ranging from trondhjemite through quartz monzonite to granite. The granitoid rocks are peraluminous and contain 13 muscovite in addition to biotite. Near Fall Creek, granitoid rocks are anomalously rich in potassium feldspar.
Quartzite Quartzites are exposed along Newsome Creek road and near Golden from Tenmile Creek to one-fourth mile west of Buckhorn Creek. Con- tacts are generally gradational except at the east edge of the Golden quartzite where the contact with augen gneiss is sharp (Fig. 6). More commonly, the impure quartzites grade into quartzofeldspathic gneiss where the total feldspar content exceeds 20 percent. The quartzites are compositionally layered (Fig. 7). Trondhjemite dikes and pods with diffuse contacts cut the layering in the quartzites at Golden, and numerous joints give the outcrop a blocky appearance. Outcrops are light brown due to preferential staining by iron oxide along joints and weathered surfaces. The quartzites are medium-grained and light gray on fresh sur- faces. Layering is defined by compositional variations, particularly, the percentage of biotite. Quartzite varies in composition from nearly pure white quartz to feldspathic gneiss, but in general contains: quartz (85-95%), plagioclase (2-10%), orthoclase (0.5-5%), biotite (1-4%), muscovite (1-2%), and subrounded to well-rounded zircon in accessory amounts. Samples are slightly strained to intensely sheared as evidenced by kinked micas, bent and broken plagioclase, and polygonized quartz grains. Quartz is highly polygonized and contains deformation bands (Fig. 8). Features of recrystallization, where present, are localized along Figure 6. Contact between Golden quartzite (left) and Buckhorn augen gneiss (Location 80132). Figure 7. Compositional layering in Golden quartzite (Location 80136) 0
-+:> 15 sutured grain boundaries or in discrete zones within the grain. Quartz is poikiloblastic and contains subhedral to euhedral plagioclase inclusions.
Eigure 8. Deformation bands in quartz grain from Golden quartzite (80161). Width of field is 2 mm.
Euhedral to subhedral plagioclase (An 8) is included in and interstitial to quartz. Grains are disseminated throughout the thin section and are moderately altered to clay and saussurite. Orthoclase is less abundant than plagioclase and occurs as subhedral to euhedral inclusions in quartz and as discrete grains. Orthoclase is fractured and slightly altered to clay. Subhedral flakes of biotite define and are parallel to tional layering in the quartzite. Biotite is kinked and ranges in color from olive green to reddish brown. A few grains are 16
pseudomorphed by pennine while others are only rimmed. Muscovite also rims biotite and contains fetrograde magnetite. Muscovite is less common than biotite and occurs as inclusions in plagioclase, rims on biotite, and anhedral flakes subparallel to biotite. It is kinked and slightly recrystallized to sericite. Accessory zircon occurs as sma 11 we 11-rounded grains and as 1arger subrounded grains; no overgrowths were observed.
Cale-silicate Granofels Ca 1c-si1 i ca te granofe ls is expos.ed between Moose and Tra i 1 Creeks and at Fall and Santiam Creeks where they occur as xenoliths in granite. Near Moose Creek, horizontal dikes and vertical sills of quartz monzodiorite cut layered calc-silicate granofelses and, where the ratio of granitoid to calc-silicate rocks approaches 1:1, blocks are brecciated and rotated (Fig. 9). Hornblende-rich selvages are common along the edges of the blocks and within granite-rich layers (Fig. 10). The calc-silicate granofelses are compositionally and texturally layered (Fig. 11) and locally grade into biotite gneiss and quartzite. Because the compositional layering is believed, in part, to be due to variations in bulk composition of the protolith, and because minerals defining the layering do not have preferred orientation, the term granofels is used instead of gneiss. Fresh surfaces are light green and weather to olive green or olive brown. The granofelses have an equigranular granoblastic texture and are commonly layered. Layers vary in width from one centimeter to several meters and contain distinctive mineral assemblages. Minerals include Figure 9. Steeply dipping calc-silicate granofels is cut by horizontal dikes and vertical sills of quartz mon- zonite. Granofels is brecciated locally where ra- tio of quartz monzonite to granofels approaches one (Location 8013). Figure 10. Hornblende-rich zone on border of large calc- silicate block engulfed by quartz monzonite (Location 80173).
...... '-J 18
(Appendix IIA) plagioclase (An 28_36 ) (0-37.3%), orthoclase (0-21.1%), quartz (11.6-62.1%), hornblende (0-46 . 1%), actinolite (0-8.0%), scapolite (0-11.1%), biotite (0-1.4%) and muscovite (0-5.9%) associated with hornblende selvages, diopside (0-41.7%), ferroan diopside- hedenbergite (0-9.0%), epidote (0-30.1%), carbonate (0-9.0%), sphene (.4-5.9%), and accessory garnet, zircon, apatite, and magnetite.
Figure 11. Block of calc-silicate granofels that shows layering and discordant contact with quartz monzonite dike (Loe. 8033H).
Pyroxenes In several of the samples studied, diopside is a major con- stituent (32-42%) at several locations and occurs to the exclusion of epidote. It is pale green and is present as anhedral granular aggregates or stubby euhedral prisms with exsolution lamellae paral l el to 001. Together with quartz and plagi oclase, it forms a mosaic- granoblastic texture. 19
Ferroan diopside-hedenbergite ts found in iron-rich assemblages containing epidote and andradite(?) garnet. The iron-rich end members are distinguished from diopside on the basis of a large 2V (=80°) and weak pleochroism. According to Heinrich (p. 215, 1965), an increase in 2V, weak to moderate pleochroism, and an association with andradite are characteristic of ferroan diopside and hedenbergite. Ferroan diopside-hedenbergite occurs:as anhedral to subhedral prisms with rounded cross sections. Amphiboles Hornblende is most conspicuous as 5 - 15 mm black porphyroblasts along selvages of calc-silicate blocks intruded by quartz monzonite (Fig. 10), but is also present as finely disseminated grains in layered granofels. The fine-grained hornblende defines darker layers in layered calc-silicate granofels (Fig. 11). In thin section, horn- blende is poikiloblastic with quartz and plagioclase blebs and is commonly associated with large euhedra of sphene. Subhedral prisms to anhedral blebs are characteristic as are cuneiform-like relics inter- grown and poikilitic with quartz. Hornblende is strongly pleochroic with = pale to medium brownish green, S = medium to deep olive green, and Y= medium green; YAc varies from 17 to 20 degrees. Hornblende is slightly altered to: chlorite and magnetite, carbonate and sphene, epidote, actinolite, and diopside. Relics of optically continuous subhedral hornblende contain grains of or are rimmed by diopside sug- gesting that diopside may have formed at the expense of hornblende. Actinolite occurs as fibers and long prisms and is either con- fined to discrete layers or is finely disseminated throughout the 20
rock. It is pale green and slightly pleochroic. In Sample 8026A, actinolite is confined to a layer where it cuts across diopside and also across other actinolite fibers. Actinolite is also intergrown with diopside or forms rims on hornblende in a 2 mm wide zone sep- arating quartz-epidote-carbonate granofels from a hornblende-rich selvage. Minor Minerals Scapolite is present in iron-rich assemblages and as a minor constituent in hornblende-rich selvages. In the granofels, scapolite occurs with epidote and andradite in a plagioclase-free assemblage which, according to Deer and others (p. 388, 1977), may signify a high Pea environment or may be indicative of Ca metasomatism. The 2 scapolite (meionite end member) is colorless and subhedral to anhedral with a refractive index= 1.58 (Fig. 12). Epidote occurs as anhedral grains in diopside-free assemblages and as a secondary product of retrograde reactions. Primary epidote is light green and has embayed and curved grain boundaries. Secondary epidote is also light green but occurs as small rounded blebs associated with hornblende, plagioclase, and andradite. Epidote is commonly rimmed by carbonate and occurs as inclusions in biotite. Large (lmm-lcm) anhedral grains of carbonate with bent rhombo- hedral cleavage and embayed grain boundaries are disseminated throughout quartz-epidote granofels (Sample 801718) and also occurs as small granules or rims along the edges of hornblende, andradite, and epidote. 21
Figure 12. Subhedral to anhedral scapolite in an equigranular granoblastic texture with quartz and ferroan diop- side (Sample 801718). Upper photo is with crossed nicols and lower photo is in plane light. Width of field is 2 mm. 22
Quartz is ubiquitous throughout the calc-silicate unit and is generally unstrained with abundant inclusions of rounded zircon and quartz. Plagioclase is anhedral to subhedral, relatively unaltered, and ranges in composition from An 28 -An 36 . The more cal.cic plagio- clase occurs in the hornblende selvages. Orthoclase, where present, is anhedral, contains inclusions of hornblende, zircon, and quartz and is slightly altered to clay. Light brown sphene is abundant in all of the calc-silicate rocks, particularly in those containing hornblende. The largest sphenes are wedge-shaped and have an affinity for hornblende, whereas smaller, subrounded(?) sphenes are found in diopside and epidote-bearing granofels.
Aluminous Gneiss Unit The aluminous gneiss unit consists of sillimanite and garnet gneisses which contain pods and stringers of amphibolite and granitoid pods, dikes, and stringers. Sillimanite- and garnet-bearing aluminous gneisses are exposed between Crooked River road and South Fork Mine on the eastern border of the study area and east of Newsome Creek (Plate I). The gneisses near Crooked River are locally folded and sheared and are in fault contact with the migmatite to the west. Aluminous gneiss near Newsome Creek is gradational into quartzofeld- spathic gneiss and biotite quartzite. In general, aluminous gneiss can be distinguished from the migmatite on the basis of the following criteria: lack of migmatitic structures, percentage of leucocratic stringers and pods, and mineralogy. The aluminous gneiss unit contains less than 15 percent leucocratic 23
stringers and pods as opposed to the migmatite wfrtch. contains abundant (30 .... 60%) well ..... developed leucocratic layers, pods, and diR.es that exhibit typical migmatite structures (see mi ·gmatite description). The a luminous gneisses typically vary in composition within a few- meters and are distinctive in that trr2y contain sillimanite and or garnet in ad ... dition to varying proportions of potassium feldspar, quartz, and plagioclase (Appendix IIB). Since the gneisses contain abundant quartz and feldspar in addition to aluminous minerals (i.e. sillimanite, garnet, biotite, and muscovite), they are classified as aluminous rather than pelitic. Quartzofeldspathic gneiss does not contain aluminous minerals other than biotite and muscovite. Sillimani.te-bearing aluminous gneisses are migmatitic west of Crooked River where they contain leucocratic pods and stringers. Sillimanite-feldspar-quartz stringers, which cut compositional layering and foliation (Fig. 13), appear to be axial-planar with tight folds. Garnet-rich leucocratic pods (Appendix IIC) are also developed on a local scale. Leucocratic pods and dikes cut the aluminous gneiss and range in composition from trondhjemite to granite (Appendix IIC). The pods and dikes are fine- to coarse-grained and do not have chilled margins. Trondhjemite is pre- or synkinematic as shown by a four-meter-wide trondhjemite dike (Loe. 803) that is sheared and enclosed by a quartz- plagioclase mylonite gneiss. A 12 meter wide "pod" of two-mica granite is syn- or postkinematic and discordantly cuts sillimanite gneiss at Location 8070. The granite is medium-grained, unsheared, and contains large clots of biotite-rich granite. 24
Figure 13. Discordant sillimanite-bearing seams in aluminous gneiss from Loe. 80163. Penny for scale.
Biotite and garnet amphibolites in the aluminous gneiss (Appendix IID) occur as pods up to 5 meters in diameter and as folded stringers less than one meter 11ide. The amphibolites are foliated and commonly lineated. Aluminous Gneiss The aluminous gneiss is medium to light gray on the fresh surface and weathers to reddish brown. It is fine- to medium-grained, foliated, and compositionally and texturally layered . Representa- tive samples are listed in Appendix IIB and contain: plagioclase (0-60.9%), orthoclase (0-6.1%), microcline (0-33.6%), quartz (33.5- 71.8%), muscovite (tr-25.1%), biotite garnet (0-10.7%), 25
sillimanite (0-19.0%), and accessory amounts of sphene, retrograde magnetite and epidote, rutile, and well-rounded zircon. The ratio of plagioclase to microcline and orthoclase and the percentage of quartz (Fig. 14) vary from gneisses containing only plagioclase and quartz to those containing orthoclase in excess of plagioclase.
Plagioclase ranges in composition from An 8 to An 26 and occurs as xenomorphic equant grains pinned .by biotite and muscovite. Grain boundaries are smooth and or embayed, and quartz inclusions, where present, are round. Plagioclase is generally twinned and is not zoned within the gneisses. It is fresh to highly saussuritized and where deformed, albite twin lamellae are bent and fractured. Microcline and subordinate orthoclase occur as embayed grains in the interstices of granoblastic quartz and plagioclase and as inclu- sions and rims on muscovite. Microcline is perthitic and commonly rimmed by plagioclase or quartz. Grains are slightly to highly altered to clay and are fractured in mylonite gneisses · Quartz is the most abundant constituent in the gneiss and varies from rounded and embayed grains with slight undulose extinction to highly polygonized ribbons with incipient recrystallization e.vident alo.ng subgrain boundaries. Olive green and red brown (titanium-rich(?)) biotite define a foliation in the aluminous gneiss . Both varieties are rarely present together but where they are, the red brown variety appears to have altered to olive green biotite plus sphene and magnetite. Pennine and muscovite plus magnetite, ! sphene replace biotite first along the basal cleavage with some grains pseudomorphed by pennine. Biotite is 26
Quartz
Plagioclase Microcline Orthoclase
Figure 14. Modal compositions of the aluminous gneiss near Crooked River. 27 kinked and intergrown with muscovite in felty stringers which define a compositional layering in addition to foliation. Red brown biotite contains abundant inclusions of rounded zircon and randomly oriented sillimanite needles. Muscovite occurs as secondary rims on biotite and as primary grains commonly intergrown with biotite. In sillimanite-bearing as- semblages, muscovite contains inclusions of small sillimanite needles (Fig. 15). Primary muscovite is commonly kinked. Sillimanite is locally abundant in the aluminous gneiss and is either fibrous or forms in clusters of fine needles and pri srns. Fibrolite is commonly folded and occurs in mats rimmed by long slender sillimanite prisms (Fig. 16) or in quartz-muscovite pods up to 4 cm in length. The close association of matted fibrolite rimmed by sillimanite prisms may suggest (Tozer in Spry, r>· 57, 1979) that the fibrolite coalesced to form prisms. Needles of randomly oriented sillimanite are common in quartz and muscovite and in sagenitic biotite. In a few grains, remnant cores of biotite are present within the fibrolite mats. Pale pink to colorless almandine garnet (Fig. 16) is concentrated along biotite-rich layers and occurs with or without sillimanite. Almandine varies in habit from rounded to embayed grains and is rarely poikiloblastic. Where poililoblastic, biotite, muscovite, and quartz inclusions are disseminated through the entire grain. Fractured almandine contains muscovite, biotite, and chlorite along fractures. Almandine cuts across the biotite foliation and is also rimmed by biotite. Accessory minerals include sphene, secondary magnetite and 28
Figure 15. Kinked muscovite containing inclusions of silli- manite (Sample 7932). Width of field is 2 mm.
Figure 16. Fibrolite mat rimmed by sillimanite prisms. Rounded garnet is pale pink and not poikiloblastic (Sample 809B). Width of field is 2 mm. 29 epidote, rutile, apatite granules, and well-rounded zircon. Zircon is colorless and occurs as very well-rounded prisms and spherical grains. Leucocratic Stringers, Pods, and Dikes Leucocratic stringers, pods, and dikes range in composition from trondhjemite to alkali granite (Fig. 17) and contain (Appendix IIC) less than 14 percent biotite in addition to plagioclase (1.2-50.7%), orthoclase (0-13.2%), microcline (0-50.9%), myrmekite (0-1.1%), quartz (17.4-60.2%), muscovite (.4-2. 3%), garnet (0-13.5%), and accessory sillimanite, apatite, and rounded zircon. Plagioclase in the leucocratic pods and stringers is zoned and typically contains abundant inclusions of quartz (Fig. 18) . Zoned plagioclase ranges in composition from An 6 to An 16 outward from the core (Appendix IIIA). In the leucocratic stringers (80163), rounded grains of highly altered plagioclase (An 14 ) contain fresh epitaxial overgrowths of albite (An 6) and are common as inclusions in microcline (Fig. 19). The rounded core has an An content similar to unzoned plagioclase in the adjacent aluminous gneiss (Sample 80163 in Appendix
IIC). Slight to moderately altered microcline and fine-grained ortho- clase occur as equant grains with or interstitial to plagioclase. Microcline is commonly perthitic and contains inclusions of rounded quartz and plagioclase. Quartz is present as interstitial polygonized grains which are rarely recrystallized. Both primary and retrograde is present in the leucocratic rocks and can be distinguished on the basis of textural 30
Quartz
Plagioclase M icroc line Orthoclase
Figure 17. Modal compositions of leucocratic stringers, pods, and dikes in the Crooked River aluminous gneiss . 31
F.igure 18. Zoned plagioclase with abundant inclusions of rounded and embayed quartz. From leucocratic pod at Location 8067. Width of field is 1 rrm.
Figure 19. Epitaxial overtrowth of Albite (Ann) on altered plagioclase core (Sample 80163). Width of field is 2 mm. 32
relationships. Primary muscovite is either interwoven with biotite or is embayed by poikiloblastic microcline (Fig. 20). Secondary muscovite rims biotite and is an alteration product of plagioclase and retrograded sillimanite. Olive green and red brown biotites occur as anhedral to subhedral plates and contain abundant inclusions of rounded zircon. The bio- tites are generally kinked and are altered to chlorite, muscovite, and magnetite (Fig. 21). Sillimanite is present as fine fibrolite interstitial to quartz and as short. needles that rim biotite. It is also present along the selvage separating gneiss from leucocratic seams. Pale pink almandine garnet (lmm-2mm) is locally abundant (80678) in leucocratic pods and, based on limited microprobe analyses (Appendix IIIA), is compositionally similar to garnet in the adjacent gneiss. Almandine is fractured and filled with biotite. A few grains have poikiloblastic cores. Accessory minerals include small subrounded zircon, large well rounded zircon, and euhedral apatite needles with length to width ratios of 5:1.
Amphibolites
Fine- to medium-grained amphibolites (Appendix !ID) are foliated and contain hornblende (26.2-73.6%), biotite (0-15.9%), garnet (0-6.4%), plagioclase (6.6-35.5%), quartz (5.0-18.9%), sphene (0-3.3%), 33
figure 20. Primary muscovite embayed by poikiloblastic microcline. The microcline contains a remnant of quartz. From a biotite-rich restitic(?) clot in discordant granodiorite pod from Location 80708. Width of field is 2 mm.
Figure 21. Bent red brown biotite rimmed by chlorite, muscovite, and small granules of magnetite (Sample 80628). Width of field is 1 mm. 34
retrograde magnetite (tr-5.0%) and epidote (0-1.7%), and accessory zircon, apatite, and carbonate. Greenish brown hornblende (o<. = medium to deep olive green, , pale olive green to very pale green, and 0 =medium green) with ato c = 14-17° is the main constituent in all but one amphibolite sampled from the aluminous gneiss unit. It is subhedral to euhedral and com- monly poikiloblastic with quartz.and plagioclase inclusions. Biotite occurs as olive green and orange brown subhedral plates and as rims on hornblende. Orange brown biotite cuts across horn- blende and is altered to pennine, sphene, and magnetite. Olive green biotite rims hornblende and is altered to pennine and magnetite.
Garnet ( Zircon and apatite occur in trace amounts. Apatite is granular and zircon is very fine-grained and well-rounded. Migmatite Migmatites in the study area are best developed between South Fork Mine and Moose Creek where they contain finely-layered melanosomes and a variety of leucosomes. The contact on the eastern border near South Fork Mine is faulted against aluminous gneiss whereas that on the west is intruded and brecciated by quartz monzonite and grades into calc-silicate granofels. The contact was drawn where blocks of migmatite became more abundant than calc-silicate blocks. This migmatite, which will hereafter be referred to as the Dutch Oven Creek migmatite, is dominantly trondhjemitic and defines an antiformal structure (see structure chapter). The Dutch Oven Creek migmatite is cut by a series of composite dikes which trend N55W;64NE and range in composition from trondhjemite to alkali granite. Migmatite Structures The following structures were observed in the Dutch Oven Creek migmatite and are classified according to the nomenclature proposed by Mehnert (1968, p. 10-11): Stromatic (Layered) Structure Layered structure is the most common of all structures present in the migmatites. It is defined by the relative amount of biotite in the melanosome and by alternating stringers of leucosome (Fig. 22). Agmatic (Breccia) Structure Amphibolite blocks act as resisters to migmatization and are 36 Figure 22. Finely-layered migmatite from just east of Dutch Oven Creek (797). commonly brecciated and form agmatic structures (Fig. 23). The blocks show varying degrees of brecciation but can be fitted back together again. Schollen (Raft) Structure Rafts of migmatite are brecciated and partially assimilated by granite to the extent that they cannot be fitted back together. Schollen structure differs from schlieren structure in that rafts are of only one lithology; schlieren structure commonly contains rafts of several lithologies that are attenuated and rotated (see Santiam Creek intrusion breccia, p. 56). Folded structure is discussed in the Structure Chapter whereas Amphibolite Agmatite 2 meters Figure 23. Blocks of amphibolite that show agmatite near 8052), Sketch is from a vertical face. · Lighter stipples represerit less mafic layer. w "'.:l 38 structures indicative of advanced migmatization are described with the granitoid rocks Melanosome The melanosome is a layered gneiss consisting of alternating light and dark layers (Fig. 22) which are commonly folded or broadly warped. The ratio of feldspar to quartz remains constant through the melanosome, and layering is entirely a function of differences in the percentage of biotite. Thin (<0.5 mm) seams of biotite commonly separate the light and dark layers. Biotite foliation is parallel to the compositional layering except at the hinges of tight folds (Fig. 34). Leucosomes are g_enerally parallel to the layering except where they truncate recum- bent isoclinal folds. Toward the leucosome, the melanosome becomes coarser-grained and is slightly enriched in biotite. The light colored layers in the melanosome are generally fine-grained, laterally continuous, and have a granoblastic texture in contrast with the leucosomes which are discontinuous, medium- to coarse-grained, and have a granitic texture. Despite the presence of thin microcline-quartz layers, the melanosome is dominantly trondhjemitic in composition on an outcrop scale (Fig. 24). The melanosome is equigranular and has a biotite foliation. Feldspars are bent and fractured, and quartz is polygonized with sutured grain boundaries. Micas are kinked and have recrystallized in the hinges of tight folds. Mineral descriptions are tabulated in Table 1. 39 Quartz Plagioclase Microcline Orthoclase Figure 24. Modal composition of the melano5ame -from - the Dutch Oven Creek migmatite. lADLE l. SUMMARY or MINERAL CHARAClERISflCS FROM DUTCH OVEN CREEK MIGMATITE, DROOGS CREEK GRANITE, AND SA NflAM CREEK TONAl .ITE Du tell Oven Creek Mi g111a t. i te 11ineral Suntia!n Creek C1·eek (;1 ani te Tona Ii te Mel anoso111e Leucoso111e Dikes ------· An15-An25 Anzo-An 22 f,1122 . Equ ant to pris111atlc and Coarse-grained; eq u11 11 Fine- to med. grained ; rarely Medium- to coarse grained; rarely Zoned (Anze An15}; larger subhe - conunon ly e111bayed; contains to pri ; ma tic; Plagioclase twinned; anhedral & equant with . twinned; moderately altered cores dral grains contain qlz inclusions qtz in c lus ions arid is lamellae are (plag) embayed boundaries; inc lusions of with epitaxial overg1·m·1ths of albile; and are rinnned by micro; appear to 111yn11ekite rims vihere in bent; rare zoning; quartz and 111icrocline; 111yr111ekitic poikiloblastic with round qtz have arnund 111icro and qtz; contact with qtz; few where altered, co n- adjacent to quartz; diffuse nonnal inclusions; diffuse nor111al zoning; n1ynnek itic in contact with grains are nonna I ly zoned tains cleavarie zon i nq; fresh- to 111od. altered. fresh- to mod . altered. perthite ; smaller grains ar·e and have preferentially oriented 11111. equant, non11a 11 y zoned, and have altered cores; fre<;h to 111od. . a 1 tered. -. - -·n ner-grai neinii"anp 1ag TOcTase; ----- ...... Anhe .p. 0 41 Leucosomes Leucosomes are well-developed in the migmatite near Dutch Oven Creek, and are generally trondhjemitic in composition (Fig. 25). The leucosomes are pegmatite-like (Mehnert, 1968, p. 56) and exhibit various forms. Laterally discontinuous lenses less than .5 meter in length are corrunon as are pinch-and-swell layers and thin (2-8 cm) stringers. The lenses and stringers are convex against the adjacent melanosome and, according to Mehnert (1968, p. 61), may represent a front-like advance of the leucosome toward the melanosome. The leucosomes are undulatory and parallel to foliation except locally, where thin ptygmatic layers are oblique to layering. The lens-like leucosomes cut gently inclined isoclinal folds in the enclosing melanosome and are in turn cut by trondhjemite dikes (Fig. 26). The leucosomes are commonly enveloped by very-- thin biotite;...rich selvages (Fig. 26). The selvage is typically less than 1 mm thick irrespective of leucosome width. The leucosomes are medium- to coarse-grained, commonly zoned, and consist mainly of quartz and feldspar (Table 1). Where zoned, the cores are pegmatitic and contain large (4.5 cm) perthitic microcline in addition to minor plagioclase and quartz. In thin section, the feldspars are equant and range from anhedral to euhedral. Myrmekite occurs along the edges of plagioclases that are in contact with micro- cline and quartz. The leucosomes have been strained as evidenced by kinked micas, undulose quartz, and bent twin lamellae in plagioclase. Of the leucosomes sampled, all but two are trondhjemite or tonalite in composition (Fig. 25), and they contain less than five percent biotite. Plagioclase and quartz occur in subequal amounts with 42 Quartz Plagioclase M icrocline Orthoclase Figure 25. Modal compositions of the leucosomes in the Dutch Oven Creek mi gmat ite. 43 Figure 26. Lens-like leucosome that cuts a gently inclined isoclinal fold. Leucosomes are enclosed by thin biotite selvages (Lee. 8046). subordinate microcline. Microcline is abundant in two of the leuco- sames sampled and is confined to the pegmatitic core zone. Pods, which contain coarse-grained pseudo-hexagonal biotite tablets up to 5 cm in diameter, are also trondhjemitic in composition. In these pods, biotite is rimmed by muscovite, pennine, epidote, and magnetite and contains rounded zircon. Only secondary muscovite is present and occurs as rims on altered biotite and as inclusions in plagioclase. Quartz is polygonized and fractured and is present as inclusions in feldspar and as discrete grains. Microcline is perthitic and contains embayed inclusions cf myrmekite. Plagioclase is normally zoned (An 16:An 5) and poikiloblastic with microcline and 44 quartz. Where perthite is in contact with plagioclase, plagioclase has myrmekitic rims. Dikes Granitoid dikes vary in width from less than one meter to several meters and trend approximately NSOW; 60NE (see Fig. 52). The dikes are commonly zoned and range in composition from tonalite to alkali quartz syenite (IUGS) (Fig. 27) .. The zoning is produced by alter- nating bands of coarse-grained to pegmatitic granite and alkali syenite and medium-grained quartz monzonite and trondhjemite. The pegmatite bands or stringers are not confined to a particular zone within the dike. The dikes have a unique form in that thin stringers of dike material coalesce in the melanosome leaving xenoliths of non-rotated gneiss within the composite dikes. Other dikes coalesce and contain sheet-like apophyses which are injected conformably between layers in the adjacent melanosome. Lens-like leucosomes which cannot be traced laterally, are cut by foliated dikes. Dikes are commonly foliated and the fractures they filled have been the locus of movement within the migmatite. Offsets of several centimeters to a meter can be demon- strated by offset biotite amphibolite layers on either side of the dike. The dikes have also been offset parallel to the foliation in the enclosing paleosome by slip along biotite-rich seams (Fig. 44). Slickenside planes also cut the dikes and attest to later movement. Thin (<8 cm) muscovite pegmatite dikes cut across the northwesterly trending dikes (Fig. 28) and post-date shearing and offset. The dikes are medium- to coarse-grained and foliated where biotite is abundant. The grains are polygonized and fractured and ribbons of .45 Quartz Core ·- . • • • ®-- • ------ Plagioclase Mic roe line Orthoclase Figure 27. _Modal compositions of dikes from Dutch Oven migmatite. Circled dots represent the cores of zoned dikes; lines con- nect the cores with the composition of the rest of the dike. 46 • ® ® • • Core An 28 Rim An 1 6 An25 I l 5 CM Figure 28. Field sketch showing relationship between late dike and other structures at Location 8042 in Dutch Oven Creek migmatite. Boxes show location of microprobe samples refered to in Figure 60. 47 quartz are aligned parallel to the walls of the dikes. Strain in the foliated dikes appears to have been localized along discrete zones of ribbon quartz and sheared muscovite which wrap around the feldspars. Similar textural re1ations have been reported (Yardley, 1978) for the Skagit gneiss of northwestern Washington. The pegmatitic zones con- tain an unusua1 radia1 growth of poikilob1astic plagioclase which is enveloped by anhedral microcline. Except for the radial intergrowth and the development of quartz ribbons, the dikes are similar to the leucosomes (Table 1). Of the dikes sampled, half fall in the trond- hjemite range while the others are predominantly granite (Fig. 27). Amphibol ites Amphibolites within the migmatite gneiss occur as thin (<1 meter thick) sill-like lenses and as brecciated blocks (Fig. 23). The amphibolites have sharp contacts with the gneiss and are commonly rimmed by a thin (1-2 cm) biotite selvage. Biotite is also present as layers within the amphibolite and, together with hornblende, produce a foliation and compositional layering. The foliation within the sill- like lenses is not always conformable with that in the melanosome; .in places, a difference of 10 to 15 degrees is apparent. Texturally, the amphibolite ranges from well foliated and layered to granoblastic. It contains (Appendix II-D) hornblende (45.0-65.7%), biotite (0.3-22.2%), plagioclase (8.3-15.0%), quartz (9.3-17.4%), and accessory epidote, ilmenite, sphene, apatite, and rounded zircon. The amphibolites in the migmatite, unlike those in the aluminous gneiss, are not garnet-bearing. Fresh hornblende is the major constituent in the amphibolites and ranges in habit from poikiloblastic embayed grains to subhedral prisms. 48 Hornblende is pleochroic with o(= pale greenish brown, s = deep brown I green, 0 = medium green and = 17 to 23 degrees. Biotite commonly defines compositional layering in the foliated amphibolites and also occurs as randomly oriented clusters in non- foliated varieties. Biotite is deep brown, subhedral, and kinked. Plagioclase is interstitial to hornblende and biotite and is included in hornblende. It is anhedral and embayed and contains round quartz inclusions. Grains are slightly to moderately saus- suritized. Embayed quartz is fine-grained and interstitial with plagio- clase. Long thin plates and embayed grains of ilmenite are interstitial and commonly rimmed by sphene. Sphene is also abundant along the basal cleavage of biotite and as strings of rounded grains parallel to hornblende. Epidote is both prismatic and granular and is associated with hornblende. Fine-grained needles of apatite have a length to width ratio of 4:1. Augen Gneiss Augen gneiss is exposed near Buckhorn Creek and in the Dutch Oven migmatite and Crooked River aluminous gneiss as sheets and pods. The augen gneiss at Buckhorn Creek is distinctive in that large (<5 cm) pink microcline augen and microcline-quartz 11 pseudo-augen 11 produce a foliation in a green chloritic matrix (Fig. 29). The Buck- horn Creek augen gneiss is gradational (?) into the Fall Creek granite on the east and is in sharp contact with the Golden quartzite on the west (Fig. 6). Within the unit, the augen grade into thin 49 Figure 29. Buck.horn augen gneiss containing microcline megacrysts and discontinuous stringers of microcline-quartz. Augen gneiss is cut by epidote-coated slickensides (Loe. 7940). Nickle for scale. microcline-quartz layers (Fig. 29) and finally into well-foliated granite. Development of augen or opthalmitic structure appears to be, in part, a function of the biotite to quartz + feldspar ratio. Where biotite or chlorite comprises approximately 25 percent of the rock, opthalmitic or 11 pseudo-augen 11 structure is present. The formation of single crystal augen may also be a function of time as crystal aggre- gate 11 pseudo-augen 11 are thought to represent the incipient stage of augen development (Ohta, 1969). Discordant pods ( Granitoid Rocks Granitoid rocks within the study area range in composition from tonalite to granite (Fig. 30) and are peraluminous in that they contain muscovite in addition to biotite. Peraluminous granites form an inner Cordilleran belt of muscovite-bearing plutons that extends from Mexico to British Columbia (Miller and Bradfish, 1980). Granitoid rocks within the study area fall within this belt and, based on sr87;sr86 ratios (>0.706) and high alumina minerals, are classified as S-type granitoids (Chappel and White, 1974). The granitoid rocks become more abundant westward through the study area toward Droogs Creek where a three mile (4.7 km) wide zone of muscovite granite is exposed. The Droogs Creek granite is litho- similar to and may be cognetic with the Granite Creek pluton (Myers, 1968) five miles (8. l km) to the west of the study area. The Granite Creek pluton is continuous southward (Bond, 1978) toward the main part of the Atlanta lobe, and reconnaissance mapping south of the Droogs Creek unit suggests that it may also connect with the Atlanta lobe. Eastward through the map area, granitoid rocks vary in Q Q TrondhJc::mi+e. • Droogs Creek granite ® Granite Creek pluton {Myers, 1968) l!J Legget Creek A Intrusion breccias between Fall and Santiam Creeks 5 • Buckhorn Creek -+- Intrusive rocks in the calc-silicate unit IUGS CLASSIFICATION • • &. A •• • fl• .... l3• • •@ • • 111 • + • + p A Figure 30. Modal classification (Streckeisen, 1973) of granitoid rocks in the study area, Q =quartz, P = plagioclase, and A= alkali feldspar. U1 N 53 composition (Fig. 30) and in intrusive style. At Fall and Santiam Creeks, intrusion breccias contain rotated xenoliths of quartzite, calc-silicate granofels, amphibolite, and biotite gneiss in a matrix of grantte and schlieric trondhjemite, respectively. The unit grades eastward into granite at Legget Creek that, except for the presence of hornblende, is similar to the Droogs Creek ·granite. East of Legget Creek, the percentage of granitoid rocks decreases and thin dikes and sills of quartz monzonite, quartz monzodiorite, and diorite intrude and locally brecciate the calc-silicate unit. Small pods of two-mica granite and granodiorite cut the Dutch Oven migmatite and Crooked River gneiss. Droogs Creek Granite A three mile wide (4.7 km) zone of two-mica granite is exposed between Surveyor and Reed Creeks and is here named the Droogs Creek granite. Its eastern border is exposed near Reed Creek and consists of steeply dipping screens of metasedimentary rock separated by thin sheets of granite. The sheets thicken westward toward the more mas- sive interior where shallow dipping screens of metasedimentary(?) rocks are injected by granite (Fig. 31). The lit-par-lit gneisses show varying degrees of assimilation from amphibolite xenoliths with sharp contacts to nebulitic granite with faint compositional layering. The granite also contains detached fold hinges up to 1.5 meters in amplitude and xenoliths of biotite gneiss that contain ptygmatic folds (Fig. 48a). The Droogs Creek granite varies from medium- to coarse-grained hypidiomorphic to fine-grained allotriomorphic. It is generally 54 Figure 31. Shallow-dipping screen of lit-par-lit gneiss from the interior of the Droogs Creek granite (Loe. 8090). equigranular but locally contains microcline megacrysts. The mega- crysts have highly embayed grain boundaries and appear to have grown around and engulfed adjacent ground mass. The granite is not foliated but contains a weak biotite lineation. Fractures, which are most con- spicuous in quartz pods, generally trend N8-l0°W and dip at steep angles. Compositionally, the Droogs Creek granite varies only in the percentage of quartz (25-50%) with the plagioclase-to-microcline ratio and percentage of biotite remaining fairly constant across the unit. In general, feldspars are neither strained nor altered, except in the granite across from Tenmile Creek where grains are bent and highly altered along a shear zone. Muscovite and biotite are present in 55 minor amounts (0.5-4.0%). Mineral descriptions are summarized in Table 1. Santiam Creek and Fall Creek Intrusion Breccias Schlieric tonalite containing rotated xenoliths of calc-silicate granofels, quartzite, biotite amphibolite, and biotite gneiss (Fig. 32) are exposed across from Santiam Creek bridge and east of Fall Creek. The inclusions are folded and commonly contain detached fold hinges. The western contact of the Santiam Creek unit is gradational with muscovite granite similar to that described. at Droogs Creek and the eastern contact toward Legget Creek is not well exposed. The Fall Creek intrusion breccia is in gradational contact with coarse-grained granite on the west and medium-grained granite on the east. Contact relations between the tonalite and xenoliths are generally sharp except with biotite gneiss inclusions which have been partially assimilated. Quartzite inclusions are compositionally and texturally similar to the Golden and Newsome Creek quartzites and are the likely source of xenocrystic quartz in the enclosing tonalite. Inclusions of calc- silicate granofels are brecciated and commonly zoned. The tonalite is medium- to coarse-grained and has a peculiar texture. Equant quartz and euhedral grains produce an equigranular texture that, according to Mehnert (1968, p. 44), is indicative of feldspar blastesis. Locally, tonalite is gneissic and contains rootless isoclinal folds (Fig. 33). These folds may be due to flow within the mass or represent relict folds from partially as- similated biotite gneiss inclusions. Only trace amounts of microcline are found within the tonalite whereas biotite comprises nearly 25 se5•w1 50• I N2.!l·w- N40"W- N6o·w- o 10 21> 30 40 feel Figure 32. Cross section of Santiam Creek intrusion breccia showing orientation of rotated xenoliths contained in schlieric tonalite (Location 8040). Arrows indicate plunge of fold axes and ticks below cross section show 100 feet increments. Amphibolite is shown in black, quartzite is stippled, and calc- silicate blocks are ruled. See Plate I (enlargement A) for location. U1 o:i 57 Figure 33. Isoclinal fold in schlieric tonalite (Loe. 8040). percent of the rock. Minerals are strained, and are described in Table 1. Legget Creek Granite Granite at Legget Creek intrudes steeply-dipping sheets of metasedimentary rock in a manner similar to the Droogs Creek granite. The presence of isoclinal folds as pendants (Fig. 38) in the granite may indicate an intrusive style similar to the intrusives reported to the west by Mye-rs (in press) . The granite is non-foliated and nebulitic locally where faint compositional layering is present. The Legget Creek granite, unlike the Droogs Creek unit, contains minor hornblende (<2%) in addition to 58 biotite and abundant accessory sphene and allanite. The hornblende is generally poikilitic and interstitial to feldspars. It is olive green with a green, B = pale green, and y = greenish brown. Epidote, biotite, and carbonate are present where hornblende is altered. Euhedral prisms of accessory sphene and allanite are also associated with the hornblende. Allanite is medium brown and occurs as elongate prisms containing darker brown cracks. Light minerals and micas have habits similar to those described for the Droogs Creek granite. Intrusions in the Calc- si l icate Unit Dikes and sills ranging in composition from quartz monzonite to trondhjemite (Fig. 30) intrude the calc-silicate unit west of Allison Creek. Where the percentage of granitoid to calc-silicate rock ap- proaches 50 percent, blocks are brecciated and rotated (Fig. 9). These intrusions are notably deficient in quartz (<15%) compared with other granitoid rocks in the study area (Fig. 30). The percentage of quartz remains fairly constant throughout the unit whereas the ratio of plagioclase to microcline varies considerably. Biotite comprises less than five percent of the rock and, at several locations, defines a strong lineation. Quartz monzonite is medium-grained hypidiomorphic to coarse- grained panidiomorphic and is rarely foliated. Plagioclase contains cloudy cores rimmed by fresh overgrowths(?) of albite or is myrmekitic in the presence of microcline. Perthitic microcline is embayed and poikilitic with inclusions of rounded quartz 59 and zoned plagioclase. Minor biotite and accessory apatite and epidote are texturally similar to that described for the Droogs Creek granite. Diabase Dark gray diabase dikes and sills, which are probably feeder.s for the Columbia River basalts, cut the granite south of Santiam Creek and the calc-silicate unit. A one-meter wide sill in the calc- silicate unit is offset with the east side down and gives evidence for late Tertiary normal faulting in the area. The diabase is vesicular and or porphyritic with zoned plagio- clase phenocrysts up to one centimeter in length. Plagioclase laths in the groundmass are also zoned and, together with augite, form a subophitic texture. Zoned plagioclases have preferentially altered cores, and saussuritic alteration is common throughout the entire rock. 60. STRUCTURAL GEOLOGY General Statement Detailed descriptions of the structural features in complexly deformed rocks along the South Fork provide a framework for the petrologic study. Since the study area was confined to the South Fork valley, the continuation of folds, faults, and shear zones north and south of the river was not investigated. Descriptions and definitions of features observed in outcrop are followed by stereonet plots of structural elements for the entire area and for five domains within the study area in an attempt to delineate large-scale and superposed folds as suggested by Reid (1959) and Hall (1961). Compositional Layering - Foliation Planar minerals are generally parallel to layering, except where axial planar foliation is weakly developed in the noses of tight folds. The term foliation is, therefore, used to define both com- positional layering and minera.l parallelism except where a distinction can be made. Within the aluminous gneiss, biotite-muscovite foliation is undulatory and steeply dipping. Thin, laterally continuous bands of leucosome and biotite-rich melanosome in the migmatite are locally compressed into tight-to-isoclinal and ptygmatic folds. Within the calc-silicate rocks, compositional layering is defined by epidote-, diopside-, and quartz-plagioclase-rich bands. Although biotite-rich zones define compositional layering in the quartzite, the presence of fold hinges suggests that the layering is a transposed foliation rather 61 than primary bedding. Granitic rocks which generally have a weak biotite lineation and compositional layering show a strong gneissosity where sheared. The compositional layering is produced by differences in· the ratio of biotite to feldspar and quartz. Folds Three fold geometries are present in the study area and include: 1) tight-to-isoclinal folds with and without axial planar foliation, 2) ptygmatic folds, and 3) large-scale open folds. Tight-to-Isoclinal Folds Compositional layering is folded in the isoclinal folds and weak axial planar foliation has developed where the predominant trend of layering is parallel to the axial planes or where the folds become very tight (Figs. 34 and 35). However, folded biotite foliation pre- dominates over axial planar foliation (Fig. 36). There is evidence of shearing along the limbs of the more strongly compressed folds (Fig. 36). Two fold orders (Ramsay, 1967, p. 355) can be recognized in the folds (Fig. 37) and are defined on the basis of drawing median sur- faces (i.e. lines connecting inflection points on the fold) through the inflection points of the parasitic and major folds. The trace of the first order median surface is a line drawn through the inflection points of the folded median surface defined by the par·asitic folds. The parasitic folds are interpreted to have formed contemporaneously with the major or host fold and thus represent one folding event. Tight-to-isoclinal folds occur in all map units wi thin the study Figure 34. (Above) Recumbent tight-to-isoclinal folds with axial planar foliation defined by biotite. From Dutch Oven Creek migmatite. (Location 8046). Figure 35. (Right) Upright tight-to-isoclinal folds also from Dutch Oven Creek migmatite with axial pla- nar foliation cut by coplanar leucocratic vein. O'\ ·N 63 Figure 36. Sheared tight to isoclinal folds with folded foliation from Dutch Oven Creek migmatite. Leucocratic layers have parallel syrmletry while biotite layers are non-parallel or subsimilar. 64 F/rsf ()rder- Second Order 1 / I I 0 Figure 37. The trace of a fold that shows the first and second order median surfaces (Loe. 8023). area. They are best developed in the migmatite unit (Figs. 34 through 36) where compositional layering-foliation is folded into non-parallel folds and, as mentioned, axial planar foliation may be present. Within the aluminous gneiss, compositional-layering-foliation is locally folded into isoclinal folds that vary from less than a meter to several meters in amplitude. Folds within the calc- silicate unit, where present, fold compositional layering and are confined to blocks engulfed by quartz monzonite. Tight folds with axial planar foliation (Fig. 38) occur as pendants in the Legget Creek granite (Loe. 80130). Detached fold hinges, up 6-5 2 cm Figure 38. A piece of float that shows tight folds with ax i a 1 p1 a na r fo 1 i at i o n , i ntr ud ed by gr a ni t e . 66 to several meters in amplitude, are rotated and occur as xenoliths within the granitic units (Fig. 32). Ptygmatic Folds Ptygmatic folds are typically polyclinal (Fig. 39) and lobate and are most common in the migmatite. Locally, the ptygma exhibit rolled or convolute fold hinges. Biotite selvages are well-developed where the axial planes of the ptygma are parallel to gneissosity in the enclosing rock (Fig. 40). The inverse ·relationship between layer thickness and wavelength observed in the isoclfnal folds also exists in the ptygmatic folds. Sense of movement in the migmatite could not be determined from a study of the relationships of S- and Z-type folds. The folded stringers are generally trondhjemitic in composition and in several instances are cut by granite. In other instances (Fig. 40), no clear cross-cutting relationship nor compositional difference can be detected between ptygmatically folded stringers and nearby leucocratic pods. One might speculate that first the trondhjemite stringers formed in close association with leucocratic pods, were then folded, and later engulfed by granite. Open Folds Open folds occur in the migmatite unit and in the feldspathic and aluminous gneisses near Trail and Newsome Creeks and range in size from less than a meter (Fig. 41) to tens of meters in amplitude (Fig. 42). Fold amplitudes and wavelengths are approximately equal and granitic dikes which cut the hinge zone of the Trail Creek antiform are co- planar with the axial plane (Fig. 42). The limbs of the large-scale Figure 39. (Right) Ptygmatically folded stringers and vein- lets that appear to be emanating from pod on upper right. Also shows relationship between thickness of the stringer and fold wavelengtl. Figure 40. (Above) Ptygmatic fold with axial planes parallel to foliation in enclosing biotite gneiss. Note the concentration of biotite on the crests of the tight folds. CJ) " Figure 41. (Right) Open fold several feet in amplitude that folds compositional layering in Dutch Oven Creek migmatite. Figure 42. (Above) Large-scale open fold near Trail Creek. Granitic dike cuts nose and is co- planar with the axial plane. CJ) co 69 ' folds near Newsome Creek contain dome and basin interference struc- tures which suggests superposed folding. Faults and Shear Zones According to Hobbs, Means, and Williams (1976, p. 300) a fault is "a planar discontinuity between blocks of rock that have been displaced past one another in a direction parallel to the discontinuity 11 and a shear zone is 11 a zone across which blocks of rock have been displaced in a fault-like manner but without prominent development of visible faults''. Both faults and shear zones can be recognized in the study area including zones of fault breccia, gouge, and interlensing shear zones through which no sense of displacement can be demonstrated. With the possible exception of the Crooked River shear zone, all other shear zones and faults are small-scale features with minor dis- placement. The small scale of displacement is most conspicous in layered rocks, such as the migmatites, in which offsets are generally less than one meter. Also, juxtaposed rocks are of compatible meta- morphic grade and have similar structures. Faults in the aluminous gneiss and migmatites between Moose Creek and Crooked River strike northwesterly and dip 20 to 85 degrees northeast (Fig. 43). Faults west of Moose Creek have more random orientations. In the migmatites, compositional layering and granitoid dikes are offset by small anastomosing faults (Loe. 8050) with left-lateral displacement. The dikes are also offset by faults which are confined to biotite-rich layers (Fig. 44). Shear zones are localized within steeply-dipping foliated granitic that juxtapose 70 • •• • • •• • • 0 • 0 0 0 0 0 0 0 43 . Poles to 25 faults and shear zones measured in the study area. Open circles represent faults and shear zones east of Moose Creek, whereas black dots represent measurements taken to the west of Moose Creek. 71 ·Figure 44. Granitic dike in the migmatite that has been left-laterally displaced along a nearly horizontal fault localized in a biotite-rich seam. (Loe. 8046) biotite-amphibolite sills. Shears also detach limbs of isoclinal folds and are roughly axial planar (Fig. 3). A one-half-mile-wide shear zone near the Crooked River (Locs. 801-8056) trends N45W;62N and contains mylonitized aluminous gneiss and lenses of granitic rock. The shear zone is followed by the Crooked River (Reid, 1959, p. 8) at least 20 miles to the south near Orogrande and may extend a considerable distance northward (Bond, 1978). The occurrence of numerous gold and base metal deposits along the shear zone (Shenon and Reed, 1934) suggests that it may have considerable economic significance. 72 Crush debris and interlensing shear surfaces are found along faults which cut granitic rocks, migmatites, gneisses, and calc- silicate rocks. These faults are generally steeply dipping and randomly oriented (Fig. 43). A fault containing lenses of gneiss and fault breccia separates the migmatites from the aluminous gneiss; however, compatible mineral assemblages suggest that offset was not great. The Crooked River shear zone is cut by several faults which show lensing and gouge and it is suggested that the more brittle type of faulting postdates shearing. Diabase dikes, which may be feeders for the Columbia River basalts, are also offset by faults and it ap- pears that the more brittle type of faulting is related to the Miocene- Pl iocene block faulting episode that affected much of northern Idaho (Capps, 1941). Joints There are two types of joints in the study area: 1) extension joints and 2) shear joints (Hobbs, Means, and Williams, 1976). The extension joints involve a simple separation whereas shear joints involve small displacements along the joint surface as evidenced by slickenside striae. Extension joints are most common in the quartzite units and impart a blocky appearance to the outcrops. An insufficient number of extension joint orientations were measured in the field to establish a trend. Shear joints throughout the area have two orientations (Fig. 45): a steeply dipping east-west set and a west-northwest-trending set that N Figure 45. Contour of 23 poles to shear joints from entire study area except 3uckhorn Creek. Contours represent 4, 8, 12, 16, 20, and 36 percent per one percent area; poles to maxima are at and 74 dips southwest at approximately 50 degrees. Shear joints are most numerous near Buckhorn Creek and between Moose and Center Star Creeks. Joint surfaces are coated with quartz and feldspar, epidote, or pyrite and shear joints have well-developed slickensides which indicate right- and left-lateral movement. Li neati ons According to Weiss (1972, p. 11), lineations can be divided into two types: 11 1) a mineral lineation or 1 streaking 1 defined by the pre- ferred orientation of inequant grains or other bodies and 2) a crenula- tion lineation defined by the hinges of microscopic folds or crenula- tions.11 Crenulation lineations are more common in the study area than mineral lineations. Mineral lineations are defined by sillimanite fibers and pods, feldspar augen, feldspar and quartz pencils, and slickenside striae. Sillimanite fibers and pods are well-developed tn the Crooked River aluminous gneiss (Location 80153) and are coaxial with nearby folds which plunge at a shallow angle southward. Lineation formed by pre- ferred orientation of augen in the augen gneisses near Buckhorn Creek and Crooked River is masked by a stronger foliation and therefore was not measured. Elongate feldspar and quartz grains in the quartz mon- zonite pencil gneiss at Location 80152 plunge south at 15° and are also coaxial with the fold axis of a nearby fold. Slickensides on shear joints are defined by epidote and quartz-muscovite striae. Crenulation lineations are produced by mullions and hinges of tight, isoclinal, ptygmatic, and open folds. According to Weiss (1972, p. 11),,mullion structure is a coarse crenulation lineation that forms in the hinge regions of very large folds and except for its size, there is no difference between this structure and finer linea- tions. Mullion structure is well-developed in the augen gneiss near Buckhorn Creek (Fig. 46) and is developed to a lesser degree near the Crooked River. On the hinges of folds, particu- larly the isoclines, lineations are defined by crenulated micaceous minerals and quartz- muscovite streaks. Orientations of fold axis lineations are discussed on page 87. Figure 46. Large-scale linear struc- tures (mullions) near Buckhorn Creek (Loe. 80120). 76 Stereonet Analysis In order to evaluate superposed and large-scale folds as described by Reid (1959) and Hall (1961), lineations and poles to foliation, faults, and joints were plotted on equal-area stereonets and con- toured. Five domains were chosen from within the map area on the basis of structural similarities and temporal and spatial relation- ships of rock units and compared -with a synthesis of trends in the whole area. From west to east, the five domains include: (I) Sur- veyor Creek (west border of study area) to Tenmile Creek, (II) Golden to Buckhorn Creek, (III) Fall Creek to Allison Creek, (IV) Dutch Oven Creek migmatite, and (V) Creeked River aluminous gneiss unit. Domain I: Surveyor Creek to Tenmile Creek Domain I includes the Droogs Creek pluton and the· westward dipping metasedimentary sheets along its eastern margin. Faint compositional layering and xenoliths in the pluton dip steeply westward except at Locations 8089-91 where the layering is nearly east-west and dips northward at a shallow angle (Fig. 47). Xenoliths contain tight fold hinges and ptygmatic folds. Ptygmatic folds in biotite gneiss xenoliths are spectacularly exposed at Hanging Rock (Fig. 48a). The ptygma plunge northward and are somewhat scattered in the northeast quadrant of the stereonet (Fig. 48b); a weak biotite lineation in the enclosing granite plunges N50E at 18° and also plots within the north- east quadrant. Closely-spaced fractures in the granite trend nearly north-south and dip steeply eastward and are best developed in quartz pods. These fractures are unique to the western half of the study area and may represent cooling phenomena. Slickensides on 77 N 0 0 0 0 0 0 • 0 • •• • •• 0 0 • • oO • 0 • Figure 47. Poles to structural elements from Domain I: Surveyor Creek to Tenmile Creek. Filled black boxes represent compositional layering in granite; open boxes represent faults; black dots show the orientation of xenoliths and sheets of gneiss and metasedimentary rock; and open circles represent fractures . N ..• .. 0 Figure 48a. Ptygmatic folds contained in biotite gniess Figure 48b. Fold axes (black dots) and bio- inclusions from Hanging Rock (Location 8092). tite lineations in granite (open circles) at Hanging Rock (Loca- tion 8092). "'-.J (X) 79 shear joints that cut the quartz monzonite trend west-northwest and indicate both left- and right-lateral movement. Domain II: Golden to Buckhorn Creek Domain II represents a steeply-dipping screen(?) of gneiss and quartzite that separates the Droogs Creek granite from the granitic rocks near Fall Creek. The screen consists of the Golden quartzite and the Buckhorn Creek gneiss. Characteristic structural elements include isoclinal fold hinges in the quartzite and undulating folia- tion, mullions, and broad fold hinges in the Buckhorn Creek gneiss. Poles to foliation (Fig. 49) plot along an east-west trend whereas joints are steeply dipping and define a north-south girdle. Domain II may represent the hinge zone of a larger fold since abundant joints and mullions are thought to characterize the hinge regions of large folds (Weiss, 1972) . Additional mapping would be needed to verify such a structure. Domain III: Fall Creek to Allison Creek Domain III contains the granitic rocks which are exposed near Fall, Santi am, and Legget Creeks and the aluminous, quartzofeldspathic, and calc-silicate gneisses to the east. Unlike Domain I (Droogs Creek pluton), the granitic rocks near Fall, Santiam, and Legget Creeks were grouped with the gneisses to the east to provide enough data points to define girdles on the stereonet. Fold axes and poles to foliation are separated into lithologic units and plotted on the stereonet (Fig. 50a) in order to define structures which have been rotated by later intrusive episodes . Poles N 0 0 0 0 0 0 0 0 0 0 0 0 • 49. Plot of poles to structural elements from Domain II: Golden to Buckhorn Creek. Open circles represent foliation; black dots show joint orientations; and boxes represent faults. N N o D 0 •· • • 0 , 00 • • 1:1 ' 0 " (\) 0 0 0 D F} gure 50a. Plot of structural features from each Hgure 50b. Contour of poles to foliation shown lithologic unit in Domain III. Dots rep- in adjacent plot. Sixty-three to ta 1 resent poles to foliation; circles represent measurements. fold axes (those rotated contain an X); boxes show poles to joints; diamonds represent poles to faults. Legget Cr. granite is in black; metasedimentary sheets are in green; Santiam intrusion breccia is in blue; and the Fall Cr. granite is in red. co_, 82 to foliation trend east-west across the stereonet (Fig. 50a) and define three(?) girdles (Fig. 50b). The 8 point to the first girdle agrees with fold axes measured in an unrotated fold near Santiam Creek and to the large-scale open folds near Trail Creek (Fig. 41). Axes of folds measured near Fall and Allison Creeks correlate with the S point of the poorly defined third girdle. No fold axes were measured that correspond to the 8 point of the second girdle. Since both north and south plunging fold axes occur in Domain III, as well as in the entire area; there may be two generations of large-scale folding or, more likely, one nearly upright large fold that plunges both north and south as a result of slight axial flexuring. Foliations measured between Legget and Trail Creeks (Plate I) may define the limbs of such a fold although no hinges were ob- served near the road west of Newsome Creek. Domain IV: Migmatite Unit Domain IV includes the entire migmatite unit from Moose Creek to Center Star Mine. The migmatite unit was chosen to represent a single domain in order to bring out the relationships between the prominent tight-to-isoclinal folds, abundant granitic dikes, faults, and any large-scale structures present. Excellent exposure of structures in the migmatites is provided by unusually sharp meanders of the South Fork. Foliations are shown on Plate I and suggest a large antiformal structure as do poles to folia- tion (Fig. 51a) wh1ch define an east-west trending girdle with a S point at N18W at 6°. The s point agrees with the fold axes of tight folds having axial planar foliation, while tight folds with folded foliation plot 10 to 15 degrees west of S (Fig. 51b). Granitic dikes N N • 0 0 D I Figure 5la. Contour of 40 poles to compositional FigGre 5lb. Plot of 16 fold axes in migmatite unit. layering in migmatite unit. Contours Open circles represent isoclines with equal 4, 6, 8, 12, and 14 percent per axial planar foliation while dots show one percent area; ft is Nl8W at 6°, isoclines with folded foliation. 00 w · form a maximum at N55W;64NE (Fig. 52) and are roughly coplanar with shear zones and faults in the eastern half of the study area (Fig. 43). No relationship between granitic dikes and axial planes could be de- termined. The migmatite unit is apparently the core of a broad antiform which plunges gently northward . Tight folds with axial planar folia- tion are coaxial to the antiform. Granitic dikes are roughly coplanar with northwesterly-trending shear zones and faults. Domain V: Crooked River Aluminous Gneiss Unit Domain V consists of an eastward dipping sheet of aluminous gneisses which are exposed to the east of the migmatite unit. Poles to foliation cluster at N90W on the stereonet (Fig. 53) along the west side of the girdle (Fig. Sla) that defines the antiforma1 structure in the migmatites. This suggests that the aluminous gneiss is on the eastern flank of the antiform. Both sillimanite fibers and elongate quartz and feldspar grains in the pencil gneiss form lineations in the aluminous gneiss and are coaxial with the axes of nearby tight folds. The lineations and fold axes have approximately the same trend as the antiformal structure but plunge at a shallow angle to the south rather than north. Stereonet Analysis of Entire Area Foliation and lineation measurements from each domain were plotted on composite stereonets in order to define large-scale and superposed folds. Foliation planes and poles to foliation were plotted on beta and pi diagrams, respectively and compared with a contour diagram of lineations measured in the field. 85 N Figure 52. Contour of poles to granitic dikes in migmatite unit. Maximum is N55t·l;54NE and contours represent 4, 1:0, 14, and 20 percent per one percent area. 80 N 0 Figure 53. Poles to foliation in Crooked River aluminous gneiss unit. Contours represent 6, 8, and 20 percent per one percent area. Thirty-one total measurements . 87 The pi diagram (Fig. 54a) contains poles to 160 foliation planes and forms an east-west trending girdle. The poles do not lie along a single great circle and an attempt to draw two girdles gives only a moderately good fit. The east-west trend of the poles is similar to stereonet plots from Domains III and IV and seems to suggest large- scal e folding in the South Fork area. The S axis to the pi diagram plunges north-northwest at a shallow angle. The 160 foliation measurements were also plotted on a beta diagram (Fig. 54b) in order to determine the S maxima. Assuming homogeneous strain, a comparison of S axes and maxima should define a statistical fold axis or axes and thus give an indication of the number of deformations which affected the area (Ragan, 1973, p. 117). Figure 54b indicates that there are two S maxima which are symmetrical across an east-west line bisecting the stereonet. The S axis cor- responds with the north plunging S maximum and suggest a nearly up- right north-trending fold. A contour diagram of lineations from the entire area (Fig. 55) shows maxima in the northwest quadrant and southern poles of the stere- onet. The southern maximum represents sillimanite lineations from Domain V, rotated fold axes from Domain III, and the large-scale folds near Trail Creek (Fig. 41). Maxima in the northwest quadrant are produced by tight folds in the migmatites (Domain IV) and ptygmatic folds at Hanging Rock (Domain I). A contour diagram of fold axes and S maxima resembling Figures 54b and 55 but including data from a larger area along the South Fork, was presented by Hall (1961). He concluded that the maxima represented three distinct fold geometries and were formed by three deformational events. N • .... ,8 • Figure 54a. 'fl'diagram showing poles to 160 measure, ments of compositional layering from the Figure 54b. ·,8 diagram of same 160 measurements entire study area. Contours represent 1 l. 2, 4, and 6 percent per one percent area . represents poles to girdles. OJ ().'!) 89 N 0 Figure 55. Contour of 50 fold axes from entire area. Contours represent 4, 6, and 8 percent per one percent area. 90 The data presented here do not necessarily corroborate Hall's work (1961) since the presence of shallow plunging north and south maxima may represent large folds and deviations from the north-south maxima may be due to rotation by later intrusive episodes. Also, no correlation between maxima and distinct fold geometries could· be made except in Domain IV where the 8 axis corresponds to tight folds having axial planar foliation. Cone 1us ions The following conclusions can be made based on field observations and stereonet analysis of structural features in the study area: 1) Tight-to-isoclinal folds with axial planar foliation are coaxial with the antiformal structure in the migmatite unit. 2) Sillimanite fibers are coaxial with nearby tight folds in the aluminous gneiss; the aluminous gneiss is on the eastern flank of the antiform. 3) Tight-to-isoclinal folds are sheared where tightly compressed and are also cut by leucocratic sills and dikes in the migmatites. This would suggest folding prior to granitic dike emplacement. 4) Ptygmatic folds are pre-granite and formed with or after the trondhjemite. 5) Fold hinges which occur as xenoliths in the granitic units, lack of strong foliation in granitic rocks, and folds in calc-silicate blocks cut by quartz monzonite suggest the granitoid rocks are post- kinematic or late synkinematic. 6) Granitic dikes are coplanar with faults in the eastern half of the study area and some have been sheared. Granitic dikes may have been emplaced along northwest trending faults during migmatization and the faults were then reactivated. 7) Shear joints cut trondhjemite dikes and are post pyritization and epidotization. 8) Brittle faults, which cut shear zones near Crooked River, may be related to the Pliocene block faulting episode which affected much of north central Idaho (Capps, 1941). 9) The only field evidence which would suggest superposed folding in the study area includes dome and basin interference structures on the limbs of the Trail Creek anticline. Traces of axial planes con- necting isoclinal folds in migmatites are slightly curved and are probably due to remobilization during migmatization. 10) Trends of tight-to-isoclinal, ptygmatic, and large-scale folds do not form separate well-defined maxima which would be expected if each belonged to a separate deformational event. If tight-to- isoclinal and ptygmatic folds were refolded by a larger structure, their fold axes should plot on a girdle rather than scattered around north and south poles of the stereonet. 11) The presence of east-west trending girdles on pi plots for Domain IV and the entire area suggest that large north-south trending folds are present. Beta maxima and lineations measured in the field plot around north and south poles of the stereonet and can best be explained by slight axial planar flexuring of a large upright fold. 92 METAMORPHIC PETROLOGY General Statement The entire study area is within the sillimanite zone of the upper amphibolite facies. Bulk compositions, the abundance of rounded zircons, and the gradational nature of units suggest that the high- grade gneisses are of sedimentary origin. Mineral relations in the aluminous gneiss give the best indica- tion of P-T conditions attained during metamorphism. The P-T conditions estimated are within the range for partial melting of metasedimentary rocks assuming PH = Ptotal· To test the possibility of partial 20 melting in the study area, the compositions of leucosomes and dikes within the Dutch Oven Creek migmatite are plotted on the Qz-Ab-Or-An- H20 system (Winkler, 1979) to determine their proximity to the cotectic surfaces, and thus the likelihood of melting at temperatures esti- mated by petrographic relations. Protol iths Restricted mineralogy, gradational contacts, and the occurrence of rounded zircons indicate a sedimentary origin for calc-silicate granofels, aluminous gneiss, and quartzite. Although the amphibolites are of mafic composition, the presence of rounded zircons suggests an iron-magnesium-rich sedimentary protolith. Rounded zircon and locally high concentrations of aluminous minerals such as sillimanite and garnet indicate that the melanosomes of the migmatites .were derived from an aluminous protolith such as a clay-rich graywacke. The occurrence of mica and feldspar in the quartzites at Newsome 93 Creek and Golden indicate derivation from an impure quartz sandstone. Coarser euhedral plagioclase in quartzites near trondhjemite dikes is probably of metasomatic origin. No primary sedimentary features were observed in the quartzites. The calc-silicate granofelses are typically quartz-rich and intergradational with quartzites. Alternating layers of calcium-iron and calcium-magnesium silicate minerals probably represent fluctua- tions in the percentage of calcite, dolomite, and siderite(?). Hornblende-rich selvages were apparently formed by local metasomatism adjacent to quartz monzonite dikes and sills. The abundance of rounded zircon and aluminous minerals in addition to crude compositional layering suggest that the aluminous gneiss is also of sedimentary origin. The protolith for these rocks, as indicated by large Si02 + Al203 to K20 ratios, should be a graywacke (Pettijohn, 1975). The tonalitic migmatite at Dutch Oven Creek could have developed from a tonalite intrusive or from a graywacke. The presence of rounded zircons and lithologic heterogeneity throughout the unit favors the latter interpretation. The rounded zircon in the amphibolite layers could be interpreted as xenocrysts in sills or as detrital grains. Thin compositional layering produced by variations in the ratio of biotite to hornblende would support a sedimentary origin. Reconnaissance mapping indicates that rocks clearly resembling those along South Fork extend a considerable distance to the north (Heitanen, 1972; Morrison, 1968) and south (Otto, 1978). However, lack of detailed mapping prevents a definite correlation with rocks 94 of established age. The age of the paragneisses along South Fork is unknown but, is at least as old as the late Cretaceous batholith rocks which intrude them. Mineral Relations Properties and textural relations of essential minerals provide ' criteria for determining paragenesis and equilibrium assemblages. Mineral textures involve the shapes, sizes, distributions, and orienta- tions of grains and their boundary relations. Contiguous minerals showing stable grain boundary relations, such as smooth non-sutured boundaries, are assumed to represent equilibrium assemblages. Table 2 lists equilibrium assemblages for each unit based on the above criteria for textural equilibrium. Two distinct types of biotite are present within the study area-- a red brown (titanium-rich(?)) biotite containing rounded zircon which are enclosed by prominent pleochroic haloes and a green brown variety that also contains rounded zircon but does not have well-developed pleochroic haloes. This suggests that the zircon in the red brown biotite was more radioactive and that the red brown biotite may be older than the green brown variety or that the red brown biotite is more susceptible to the pleochroic effect because of its trace or minor element composition. Lack of crosscutting relationships prevents conclusive evidence for paragenesis. However, Myers (1968) observed the same relations in biotite and provided textural criteria which sug- gested that the green biotite formed later than the red brown biotite. Biotite is commonly replaced by muscovite plus magnetite and or sphene and is also retrograded to pennine plus magnetite. With respect to -·---- TABLE 2. METAMORPHIC MINERAL* ASSEMBLAGES** FROM METASEDIMENTARY ROCKS. Cale-silicate Hb-Rxn Quartzite granofels Zone Amphibol ites Aluminous Gneiss QPM OAP HPQ HPQ QPB DHP HPBQ HPBQ Sil GQ}GB KBQ QKB HBP HBSp Sil PB (Sil GBMPQ) KPQ (GBMPKQ) QKM SHQ GBQ Sil QB KBG QBM SOQ HBG KPM HKD Sil BQ Sil MQ PDQ (HBPQSp) Sil GQ Sil MP J(Sil BMPQ) (QPKBM) PKO (HBPQGSp) Si 1 BK (Sil GMPQ) BMQ HKPQ BGQ BMP BPQ (DPAKQSp) (ECQGSp) MQ} (Sil MBQ) (DPAHQSp) Sil QB (DPBHQCG) GKQ 1 Sil MKQ (Sil MPKQB) Sil MB (Si 1 GBMPKQ) Sil QK (Si 1 QK) BPM BGP *P = plagioclase; Q = quartz; K = K feldspar; M = muscovite; B = biotite; 0 = diopside; E = epidote; H = hornblende; S = scapolite; C = carbonate; A= actinolite; Sp= sphene; G = garnet; and Sil = silli- manite. **Assemblages in textural equilibrium are not enclosed by parentheses; parentheses enclose all minerals in a single thin section. l.O (J1 folding, biotite occurs in two distinct habits--as an s1 folded foliation and as a second (S 2) foliation parallel to axial planes of tight folds. Biotite producing the s1 foliation is kinked and is locally recrystallized. s2 biotite is also slightly kinked suggesting that it formed during the folding event. Sillimanite occurs as folded fibrolite mats and as prisms which are coaxial with tight folds. As .previously mentioned, fibrolite is rimmed by and apparently to form sillimanite prisms. In microcline-bearing assemblages, sillimanite is in textural disequil- ibrium with muscovite and quartz. Relict sillimanite needles are mantled by muscovite and muscovite is embayed by microcline. This suggests that conditions were at least past the second sillimanite isograd: Si02 + KAl3Si3D1aCOH)2 KAlSi3Ds + Al2Si05 + H20 and that sillimanite needles are preserved in certain retrograde assem- blages. Almandine is in textural equilibrium with sillimanite and biotite and, in a few samples, appears to have overgrown the s1 biotite foliation. Garnet in the microcline-rich leucocratic stringers is similar in composition to that in the gneisses (Appendix IIIA) which suggests that it is either restitic or that both formed during the same metamorphic conditions. Although textural evidence is lacking, the microcline and almandine may have formed by a reaction of the following form: KFe3AlSi3D1a(OH)2 + Al2Si05 + 2Si02 ---- KA1Si308 + Fe3Al2Si3D12+ H20 Hornblende defines a lineation and foliation in the amphibolites but does not have a preferred orientation in the calc-silicate granofelses. Orientation of the hornblende foliation in amphibolite 97 lenses deviates slightly from the s1 foliation in the enclosing gneiss. The hornblende foliation probably formed simultaneously with the s1 biotite foliation, whereas hornblende selvages in the calc- silicate granofels are a feature of metasomatism associated with intrusion of quartz monzonite. The close association of sphene with hornblende may be due to the breakdown of titanium-rich biotite. Muscovite occurs in several distinct habits: l) interwoven with biotite in sheaths parallel to s1, 2) as rims on biotite, 3) as subhedral flakes subparallel to and perpendicular to s1, 4) as randomly oriented subhedral flakes in granite, and 5) as an alteration product of plagioclase. These textural relations suggest that a late muscovite formed after s1 and may be associated with the replacement of biotite and sillimanite. Muscovite in the granite is clearly primary since it is subhedral and is not associated with any other minerals. Traces of muscovite in trondhjemite and tonalite appear to be secondary after biotite and plagioclase. The presence of perthite and myrmekite attest to subsolidus reequilibration. However, albite rims on perthitic microcline may indicate and origin by replacement rather than by subsolvus unmixing. If the perthite is due to unmixing, plagioclase lamellae should con- tain more K20 than the albite rim, whereas lamellae in replacement perthite should contain less K20 and should have the same composition as the albitic rims. Poikiloblastic microcline with embayed rims choked with inclusions of adjacent matrix may suggest a metasomatic origin for the augen gneiss. Epitaxial overgrowths on altered cores of plagioclase may repre- seQt melt that nucleated on restitic cores derived from the adjacent 98 gneiss. Identical An content of the cores and unzoned plagioclase in the gneiss support this interpretation. Length to width ratios in apatite may be useful geothermometers (Wyllie and others, 1962) with the larger ratios or more prismatic apatite indicative of temperatures within the magmatic range. Generally speaking, the granitoid rocks and leucosomes contain prismatic apatite with L:W ratios approximating 4:1 ·to 10:1, whereas gneisses commonly contain granular apatite . . Conditions of Metamorphism Mineral assemblages (Table 2) within the metasedimentary rocks are characteristic of upper amphibolite grade metamorphism (Turner, 1968). The best evidence for metamorphic conditions imposed on the area is found in mineral assemblages within the aluminous gneiss. Since sillimanite and quartz are in equilibrium in certain assemblages within the gneiss, the lower bounds of metamorphism is somewhere past the first sillimanite isograd in divariant space (Fig. 56). Assuming Holdaway 1 s (1971) triple point is more thermodynamically valid (Day and Kumin, 1980) than Richardson 1 s (1969), the lower bound of temperature is approximately 500.0 at 3.5 kb. The occurrence of quartz, biotite, and sillimanite in textural equilibrium and lack of cordierite indicates that during the metamorphic maximum, conditions were confined to the low temperature side of the QAB --+ KCGL univari- ant line (Fig. 56). Assuming sillimanite and potassium feldspar repre- sented and equilibrium assemblage before retrograde reactions, confines P-T conditions to at least the second sillimanite isograd (QM KAV). Pods containing coarse sillimanite and perthitic microcline with small 7 6 5 "'C 0 .0 .:t:. -a. 4 ' ' ' ' 3 ' 2 450 500 600 T(°C) 700 800 Figure 56. Pressure-temperature grid (After Greenwood, 1976), 1 = Holdai'>fay, 1971; 2 = Richardson, Gilbert, and Eell, 1969; 3 = Luth and others in Greenwood, 1975; 4 = Storre and Karot! quartz inclusions suggests that the metamorphic maximum straddled the second sillimanite isograd. The absence of cordierite and the presence of garnet confines pressure to above the breakdown of Fe cordierite (Fe GQA). Thus, conditions were above the QKPV-+ Land QMPV melting curves for a water saturated system and are by the KAV, QAB KCGL, and Fe GQA reaction lines as shown in Figure 56. Based on these petrographic criteria, temperatures were on the order of 700° to 730°c and pressures exceeded 3.5 kb. Assuming PH 0 - Pto ta 1 ' 2 temperatures are within the range required for partial melting to occur. Migmatization Of the mechanisms proposed for the formation of migmatites, the following are probably the most tenable: 1) partial melting of metasedi- mentary rocks with segregation of initial melt, 2) metasomatism with external control of fluid composition and chemical potential gradients, 3) closed system metasomatism with subsolidus chemical and or mechanical segregation, and 4) lit-par-lit injection of granitic magma. Criteria for determining the mechanism(s) of migmatization are summarized in Yardley (1978) and Olsen (1977) and include: 1) closeness of leucosome composition to experimentally determined melt composition, 2) existence and nature of chemical potential gradients, 3) mass balance studies (leucosome + melanosome = paleosome?) to determine if system was closed, and 4) mineralogical and textural features such as plagioclase composition, replacement textures, and relict mineral orientations. The migmatite complex along the South Fork contains both lit-par-lit and in situ migmatite. Westward toward Droogs Creek, sheets of granite are injected between layers of metasedimentary rock, whereas at Dutch 101 Oven Creek, the discontinuous nature of the leucosomes suggests that the migmatites formed in place. In order to determine whether a melt phase was present in the Dutch Oven Creek migmatite, compositions of the leuco- somes were compared with melts formed experimentally. The most appropriate experimental system is that proposed by Winkler and others (1975; 1977; 1978) which in addition to albite, orthoclase, quartz, and H20, also includes the anorthite component. Consideration of the anorthite component permits a more accurate appraisal of melt relations. Phase relations in the Qz-Ab-Or-An-H 2o system are represented in a tetrahedron bounded by four triangles (Fig . 57). Each triangle represents a four component system An-Ab-Or, An-Ab-Qz, An-Or-Qz, and Ab-Or-Qz (H 2o is not shown). Space within the Qz-Ab-Or-An tetrahedron is divided by three isobaric (Ptotal = PH = 5 kb) cotectic surfaces . 20 The surfaces include (Fig. 57a) the plag + quartz + L + V cotectic surface (E 1-E 2-E 5-P) which separates plagioclase space from quartz space, the smaller quartz + alk. feldspar + L + V cotectic surface (E 5-E 3-P) which separates quartz space from alkali feldspar space, and the plag + alk. feldspar+ L + V cotectic surface (E4-E6-P-E5) which separates alk. feldspar space from plagioclase space. Along these surfaces, plagioclase and quartz, quartz and alkali feldspar, and plagioclase and alkali feldspar coexist with melt in addition to a vapor phase. The three cotectic surfaces intersect to form a cotectic line P-E . The cotectic line gives the composition of melts coexisting 5 with quartz, plagioclase, alkali feldspar, and vapor. Because the cotectic line is situated in the An poor part of the tetrahedron, small amounts of An have a decided effect of the composition of cotectic melts. An Qz Isotherms on cotectic surfaces Plag + Qz + l + V and Plag + Alkfsp + l + V Projection of cotectic line at 670°,685°and 700°C and of isotherms on An "/. PH o= 5 Kb cotectic surfaces. 2 L+V+ plag t quartz p . H 0 = Skb 2 ... 110° ,. ,i.?-720° tz'l'3 ... 700° g/' .. /c,,,,.-::"" ·,i------12 700' I/ . ,...... ' @ 690°E,ror i.-::;;"__.,....\1.---- ..l.ES An An : plag + quar Iz + L + V 1 Figure 57. Winkler s (1979) Qz-Ab-Or-An-H o system ... 0' \ 700" at 5 kb pressure. Figure 57a shows2 co- . ,, ·- - a tectic surfaces in three dimension and . 100°--;:35- )7\1Es Figures 57b and 57c·show projections C /,-•\,_.' °!!>-----;..-----7} qua,l<+ / from An and Qz. respectively. 40 ,,,- _.... ,•>,• Ej 38 / ,. ,,./.. ,. _.....-- _,-, / +L+V/ / v 740° ® V ;...... -- 33 3'2. · Ell':3J /'?> ___ o oO Or NC: ' In order to represent a composition that lies within the tetrahedron, the cotectic surfaces are projected onto a plane. The plag + quartz + L + V and plag + alk. feldspar+ L + V surfaces are best represented by a radial projection from the An apex down onto the Ab-Qz-Or base (Fig. 57b). Similarly, the quartz+ feldspar+ L + V and the plag +quartz + L + V surfaces are projected from the quartz apex outward onto the An-Ab-Or face (Fig. 57c). Numbers on the projected surfaces represent the weight percent of An and Qz in the melt on the An and Qz projections, respectively. Isotherms connect points representing melt compositions that exist at the same temperature and indicate regions of low tempera- ture on the surfaces. The dotted line on the An and Qz projections separates the one feldspar field (left) from the two feldspar field on the right. Leucosomes and dikes from the Dutch Oven Creek migmatite at Locations 8041 (Fig. 58) and 8042 (Fig . 28) are plotted on the Qz-Ab-Or-An-H 20 system at PH = Ptotal = 5 kb (Figs. 59 and 60). Leucocratic stringers 20 from the aluminous gneiss are similarly plotted in Figure 61. Since whole rock chemical analyses were not available, weight percentages of Qz:Ab:Or:An were calculated from modal analyses assuming equal mineral densities. Plagioclase was the only feldspar analyzed (Appendix III) and since the orthoclase is perthitic, samples would plot slightly closer to the Ab apex than shown. Chemical analyses from bulk samples of melanosomes and leucosomes would have minimized the problem inherent in sampling coarse-grained heterogeneous rocks and in dealing with feldspar composition. Because minerals such as cordierite, garnet, sillimanite, or hornblende are not present in the melanosome, conditions -----® ·: ·. . - j/.···;'_ • 5 METER__J 804 l [) An ® 17 7 @ 8011C © 80418 ® B041A . Core Anl6 Rim An 6 A '-l·." . .:.. :,.. n • •I I 22 " • Anz1 An22 ,... .. ": An22 ·: An22 ,; .. An22 ® © .·-@ Figure 58. Field sketch showing location of microprobe samples and relationship between leucosomes and sheared dike at Location 8041 in the Dutch Oven Creek migmatite. Vertical face. __. 0 +:> Figure 59. Plot of leucosomes, melanosomes, and dike from Location 8041 onto Qz-Ab-Or-An-H 20 system. Qz SAMPLE An Plag Kf Qz ·qz : Ab : Or : An ® 80410 Projection of cotectic line An20 49.2 - 50.8 50.8 : 39.4 : - : 9.8 and of isotherms on An '/, • cotectic surfaces. • @ Anl7 65.3 - 34 .7 34 .7 : 54.2 : - : 11. l 0 An22 " " " 34 . 7 : 50.9 : - : 14.4 ® 8041C An22 47. l 8 . 5 44.4 44.5 : 36.7 : 8 .5 : 10 .3 p H 0 --5 kb • 2 ... 130° An21 " " " 44·.4· : 37 .2 : 8.5 : 10 . 3 ---100°- <> Anl8 " " " 44 . 4 : 38. 6 : 8.5 : 8.9 rl'/·-7rl'/ . \1. ..----" -- ___ ,,___ 100• 33 .4 2.3 64 . 4 64.4 : 26 . 7 : 2 .3 : 5 .7 .:,..-1 ,,-®E' An20 5 © 80416 + An22 51.2 19.5 29.3 29.3 : 39.9 : 19.5 : 11.3 s9o•E, o-:_,_...-· .... '!> ._\Ii J" ro'O? ro100 :i,..'O/ C!>./ @ 8041A An11* 40.5 49.5 10.0 10.0 : 36 . 0 : 49.5 : 4.5 ••" P ,_..•'_,1Y / , o 1...... _,...... '<> / • 0 0 '® An22 " " " 10.0 : 31.6 : 49.5 : 8.9 • plag + L +' 0 An22 55.4 1.1 43.5 43. 5 : 43 .2 : l :l : 12.2 I o,..... '1. I / 100 •@> / - PROXIHITV TO COTECTIC SURFACES AND LINE P - E's Ab 80410 ® 1 • -3% An below E1 and E 2 on Ab ·· Qz join. ® 5% An above E and E1 on Ab - Qz join. An An 1 2 o 7% An above E and E1 on Ab - Qz join. 1 2 ® 8041C .: . plag + quart:z + L + V + - 3% An below plag +quartz + L + V cotectic surface . <> - 5X An below pl ag + quartz + L + V surface. :: 1:3v"o . 120°- • -15% An below plag +quartz+ L + V cotect!c surface. z_---: 700° - -- - \ 7000 . 4;/ i..'_. © 80416 + An above P - E's cotectic line, / ."/" :3S ) I 38 /. quartz+ @ 8041A ")"> alk. feldsp. / 1 • itlP-6% An below plag + alk. feld + L + V and 20% Qz in front . ... I\") E\' A -:;,_, tf.V +L+ v / E" of quartz + alk. feld + L + V cotectic surfaces. 33 ";,"'5' ...... 740° 3 v v @ o Situated on plag + quartz + L + V cotectic surface. 0 0 00 0 pu 1() Q <.Tl A b cg> cff- 61.o o 6 r Average of core and rim 60. Plot of dikes and country rock from Location 8042C (see Fig. 28) onto Qz-Ab-Or-An-H 20 system. Qz SAMPLE An Plag Kf Qz Qz : Ab . Or : An Projection of cotectic line and of isotherms on An '/, 48.0 30.5 21. 5 21 . 5 : 9.8 : 30.5 : 8.2 cotectic surfaces. ® 8042A • An 17 II II L+V+ a An22 * II 21 . 5 ·: 37 . 4 : 30. 5 : 10. 6 plag +quartz @ An25 91.5 - 8.5 8.5 : 68.6 : - : 22.9 p H 0 -5- kb 2 .... 1000-- 7QQO 44.7 45.2 10 .1 10.l : 40.2 : 45.2 : 4.5 1 -- 1 '0E' ® 8042C • AnlO 12"'"/v z'" .: " '' -- z...,.,,. __ s J. : flll"""" I J 81.3 - 18.7 18.7 : 69.l : - : 12. 2 ror '·.-:.,....1 _/ i._,_\/ / An15 / * ro'b" fo10 oo if / Eli. o 1"lag alk. leldspar L V 1 0 o_...... + + 1 E> orf/ .,/ PROXIMITY TO COTECTIC SURFACES AND LINE P - E o/·· 5 I 100 I / v v 8042A v v @v/0 y v y ® 2% above plag + alk. feldspar + + V cotectic 695° •a 1t Ab Es surface and 11 % Qz above P - E 5 cotectic 1 i ne. An An @ "'23% An above Ab - Qz join; not close .· to any cotectic surface. plag + quartz + L + V 8042C ... 1300 ® e20% Qz in front quartz + alk. feldspar + L + V 120°- cotectic surface and 6-10% below plag + alk. --- , 1000 feldspar + L + V cotectic surface. 1.0 _/ "''.: 38/ .: i. ---- 36 JQuorlz+ / 12% An above Ab - Qz join; not close to any cotectic E"f/ ._,..,..,3 _./>!:'> "i alk. Mdtp. / 1· 133 .---Jl-3t • ; J> f.'J/ +l+V / E• surface. WI. ,, r / 11.0° 3 :::St v (.iii\ 0 O O t,.O "O v ' s1.0° .., C> Ab Or ()) * Average of core and rim Fiigure 61. Plot of leucocratic stringers and adjacent gneiss in a luminous gneiss on Qz-Ab-Or-An-H 20 system. Qz SAMPLE An Plag Kf Qz Qz . Ab . Or : An 1 0 E 2@01s 80678 Projection of cotectic line 59.6 14. 2 26.2 26.2 : 50.7 : 14.2.: 8.9 and of isotherms on An '/, Anl5 cotectic surfaces. • a Anl8 II II II 26.2 : 48.9 : 14.2 : 10.7 l+Vt plog ).iuortz 80163C PHO2 --Skb * .... • Ani4 18.7 56.7 24.6 24.6 : 16.1 : 56.7 : 2.6 1000-- 1000 i.?- 0 An ** II II II 24.6 : 17 .. 6 : 56.7 : 1.1 , 112'/v ?'.. :'.,. --,, -- I2, ,,, __ .'0 E'5 6 J ll Anl3 18.2 18.9 62.9 62.9 : 15.8 : 18.9 : 2.4 E I/ / / 690° ror ,."J 0 E>..:i;...... -'O...... rd-_. ... II II II 0 Anl6 co'O" 1c 0 "-> ...... co/ / 62. 9 : 15. 3 : 18. 9 : 2.9 0 + olk. feldspar+ l co cot.(:,100,...... rf...-- / v G&c;, 0 I o,.,... 'I. I 10° PROXIMITY TO COTECTIC SURFACES AND LINE P - E1 I v vov@I / / v v v y y v 5 695° Ab Es 80678 • 8% An above Pon 'Ab - Qz side of tetrahedron. An An a 10% An above and to the 1eft of P on Ab - Qz 0 Oz /, side of tetrahedron. ,.· plog + quartz + L + V .... 130° 80163 110°- , i."J 100°- -- - \ 7000 •0"-'13% Qz in front of quartz+ alk. feld + L + V sur- y a.\, face and 15% An below plag + alk. feld + L + V. 3&--70-'3S-) 1"e"/ 384y ,,,,.; quartz+ A.&?% An below plag +quartz+ L + V cotectic surface :' 31...... - ../:,\ / 1 near Qz apex. Ei/Y. I>) / +l+V / E" 33 "J'/ ..e;e 11,00 3 v v @ o o o (: 000 0 Ab i co"r:l 1 Or '-.I * Rounded core ** Epitaxial overgrowth 108 I a-re believed to be below those required for incongruent melting of biotite. As such, only feldspar and quartz are considered in the Qz-Ab-Or-An-H 2o system. Sample 80410 is from a fine-grained leucocratic layer in the melano- some (Fig. 58). Its composition and that of the adjacent gneiss plot near the Ab-Qz join (Fig. 59) in quartz space and plagioclase space, respectively. The leucocratic layer would not be completely melted until a temperature exceeding 73o 0 c were reached. Thus, it is possible that the layer was partly liquid at the P-T range estimated for the area. Sample 8041C (Fig. 58) is from a discontinuous trondhjemite pod with a pegmatite-like texture. It plots 3% An below the plag + quartz + L + V surface in quartz space. At 720°c only a small percentage of restitic quartz would remain and at lower temperatures, a slightly greater amount would be unmelted. Sample 8041A is similar in form to 8041C but differs in composition (Fig. 59). It plots within alk. feldspar space closer to the plag + alk. feldspar+ L + V surface than the quartz+ alk. feldspar + L + V surface (Fig. 59). At approximately quartz would be melted and plagioclase plus alk. feldspar would remain suspended in the melt. Slightly higher temperatures would be necessary in order for plagioclase and then alk. feldspar to melt. It appears that, at the temperatures attainable in the area, leucosomes 8041A and 8041C were not entirely melted. Adjacent melanosomes contain more quartz than the leucosomes and are situated in higher temperature regions within the tetrahedron. The foliated dike (80418) plots slightly above the P-E5 cotectic line and was entirely liquid at low temperatures ( 650°c). The granite dikes at Location 8042 (Fig. 28) plot close to the 109 plag + alk. feldspar+ L + V and quartz + alk. feldspar+ L + V cotectic surfaces and are situated in alk. feldspar space (Fig. 60). The cross- cut dike (8042A) plots 2% above the plag + alk. feldspar + L + V surface suggesting that at 685°c, plagioclase and alk. feldspar coexisted with a cotectic melt. The late dike (8042C] is situated in alk. feldspar space closer to the plag + alk. feldspar+ L + V surface than the quartz + alk. feldspar+ L + V cotectic· surface (Fig. 60). Temperatures near 7oo0 c would have been required in order for the dike to have been totally liquid. The country rock adjacent to the dikes plots on the albite side of the albite - quartz eutectic (E 1) away from any cotectic surfaces. The sillimanite- and garnet-bearing leucocratic stringers (80678 and 80163C) in the Crooked River aluminous gneiss (Plate II) plot near the plag + alk. feldspar+ L + V surface (Fig. 61). Stringer 80678 is situated in a low temperature region near the quaternary minimum (P). Thus at temperatures around 650°c all mineral constituents in the stringers would have been melted. The stringer containing plagioclase with epitaxial overgrowths (80163C) plots in alk. feldspar space below the 7oo 0 c isotherm. Since the composition of the stringer plots closer to the plag + alk. feldspar+ L + V surface than the quartz + alk. feldspar + L + V surface, plagioclase and alk. feldspar probably co- existed with the If the epitaxial overgrowths represent melt nucleated on unmelted plagioclase, then the composition probably remained near the plag + alk. feldspar+ L + V surface. The melano- some adjacent to stringer 80163C was unaffected by melting since it is situated near the quartz apex in an area of high temperature. 110 Based on limited microprobe analyses, it appears that the formation of certain leucosomes can be attributed to partial melting and that the dikes were almost entirely liquid at the time of emplacement. Adjacent rocks plot away from the isobaric cotectic surfaces in areas of high temperatures and, thus, were unaffected by migmatization. Leucosomes with compositions plotting near cotectic surfaces in regions of low temperatures do not necessarily preclude the possibility of metasomatism. In fact, certain leucocratic layers (i.e. 80410) may have formed by metamorphic segregation induced by(?) partial melting. To determine the viability of metasomatism as an additional(?) mechanism of migma- tization, whole rock chemical analyses and more detailed sampling are needed to determine whether chemical potential gradients exist between layers. 1:11 URANIUM FAVORABILITY OF THE MIGMATITE COMPLEX Uranium favorability in the migmatite complex exposed along the South Fork of the Clearwater River is generally low. A scintillometer survey across the complex yielded readings only slightly above back- ground in both metasedimentary and granitoid rocks (Table 3). The highest reading was recorded (370 cps; bkrnd = 70 cps) in a sheared granite at Location 8038 (Plate II) that contains allanite. Within the metasedimentary rocks, uranium is apparently not structurally or 1ithologically controlled. Shear zones and the hinges of antiforms show only a slight concentration of radioactivity. In mica schists and gneisses, uranium is often associated with biotite as inclusions of radioactive accessory minerals and as grain-boundary deposits (Coney and others, 1980). Biotite in the aluminous gneiss did not contain radioactive elements other than rounded zircons and the gneiss produced the same number of counts per second (cps) as the biotite-poor calc-silicate rocks. Quartzites are nonconglomeratic and are not radioactive (Table 3). It has been shown (Coney and others, 1980) that uranium and thorium contents decrease with increasing metamorphic grade. Since the complex has undergone high-grade metamorphism (second sillimanite isograd), any uranium and thorium present were probably mobilized during prograde dehydration reacti ans. Partial melting can be an effective mechanism for concentrating radioactive elements into a melt by incongruent melting of biotite containing uranium-rich inclusions (Coney and others, 1980). Although TABLE 3. RADIOACTIVITY OF ROCK UNITS IN THE MIGMATITE COMPLEX. - Rock Unit CPS* Background Location** Comments Aluminous 80-110 70 80144,148,163 gneiss 100-110 70 80164 leucocratic stringers Calc-sll 1cate 80-90 70 80141 granofels 75 75 80170 85 70 80171 Augen gneiss 150 80 80120 near nose of anticline Quartzite 70 70 80160,161 Golden and Newsome quartzites Amphibol ite 120 70 80199 M1gmat1te Paleosome 110 90 8041 120 90 . 8045 nose of small anticline 140 80 80140 Leucosome 110 90 80140 Dikes 95 70 8075 110 90 80140 Grani to1 d 70 /U 80172 quartz monzon1te rocks 85 60 80131 Legget Creek 85 80 8037 porphyritic granite 90 70 80174 110 80 80140 Moose Creek 120 80 80120 in augen gneiss 370 70 8038 sheared granite *Counts per second measured with Geometrics scintillometer. **See Plate II. N 113 partial melting may have occurred in the complex, nonuraniferous paleosomes did not allow for concentration in the leucosomes. In general, the two-mica peraluminous granites are also barren of radioactive elements. Geochemical studies (Keith ..i!!. Coney and others, 1980) suggest that regardless of alumina content, suite alkalinity, or degree of differentiation, uranium values greater than 6 ppm will not be present in rocks unless the K2o content exceeds 4 percent. Although whole rock chemical analyses are lacking, one might speculate that the peraluminous granites in the study area contain less than 4 percent K2o and thus conditions were not favorable for uranium enrichment. ln many uraniferous migmatites (Kettle complex, N.E. Washington; Rossing Deposit, Namibia; and Bancroft, Ontario area), uranium is associated with late pegmatite dikes. Lack of pegmatite in the South · Fork complex suggests that late stage exsolved fluids were not as - sociated with the peraluminous granites and that magmatic uranium deposits are unlikely. Lack of uranium associated with the migmatite complex may be attributed to several factors: 1) lack of uranium and thorium in the sedimentary protoliths, 2) liberation of radioactive elements, if ever present, by prograde metamorphism, 3) nonuraniferous paleosomes such that radioactive elements could not be concentrated through partial melting, 4) possible low K20 (<4%) values in the peraluminous granite, and 5) lack of late stage exsolved fluids associated with the granite. It should be noted, however, that occurrences of uranium have been found in the metasedimentary rocks 1t1hich separate the two lobes 114 of the batholith. These occurrences (Samples 1600-608 .i!!_ Coney and others, 1980) are apparently of the structurally controlled meta- morphic-type. 115 SUMMARY AND CONCLUSIONS The migmatite complex along the South Fork is situated on the eastern border of the Atlanta lobe of the Idaho batholith in a screen of high-grade metamorphic rocks that has been named the Salmon River arch (Armstrong, 1975). The complex consists of metasedimentary rocks, including aluminous gneiss, tonalitic migmatite, calc-silicate granofelses, quartzite, and augen gneiss, that are intruded by two-mica granite. The granitoid rocks increase in abundance westward toward the Droogs Creek granite where a 3 mile-wide two-mica granite is exposed . Structures indicative of more advanced stages of migmatization also increase westward. On the eastern side of the complex, the Crooked River aluminous gneiss contains thin stringers of garnet- or sillimanite-bearing pods which give evidence for high-grade metamorphic conditions near the second sillimanite isograd. To the west and in fault contact with the aluminous gneiss, tonalitic migmatite at Dutch Oven Creek is layered and contains leucosomes which formed in situ. The migmatite becomes schlieric and agmatitic locally where the granitoid component increases. The grani- toid rocks become increasingly more abundant west of the migmatite and calc-silicate rocks near Allison Creek are brecciated and somewhat rotated. At Legget Creek, a massive biotite-hornblende granite intrudes thin sheets of metasedimentary rocks in a lit-par-lit manner. West of Legget Creek, intrusion breccias at Santiam and Fall Creeks contain rotated xenoliths of metasedimentary rocks. Augen gneiss near Buckhorn Creek is in sharp contact with layered quartzite at Golden. The structurally complex Buckhorn augen gneiss and Golden quartzite are cut by granite dikes and pods. West of the Golden quartzite, metasedimentary sheets give way to two-mica granite. 116 TQward the interior of the Droogs Creek granite, the granite is massive and contains flat-lying sheets of lit-par-lit biotite gneiss. The metasedimentary rocks are structurally complex as shown by trans- posed foliation in the quartzite, interference structures on the limbs of large-scale folds, and three distinct fold styles. The aluminous gneiss is situated on the eastern limb of an antiform cored by the Dutch Oven Creek migmatite. Tight-to-isoclinal folds in the migmatite are roughly coaxial with the axis of the antiform. Large-scale antiforms and synforms near Golden (Zimmerman, in progress) and Trail Creek attest to broad flexuring in the area. The presence of refolded isoclinal folds north and south of South Fork (Reid, 1959; Otto, 1978; Zimmerman, in progress) indi- cate two and possibly three folding events. Lack of foliation in the two- mica granites and the abundance of rotated fold hinges suggest that the granite is late syn- to postkinematic. Deformation was contemporaneous with metamorphism in the migmatite complex since sillimanite needles are coaxial with the tight-to-isoclinal folds. The complex is within the sillimanite-orthoclase zone of the upper amphibolite facies and, based on mineral relations in the aluminous gneiss, reached temperatures between 700°-730°C and pressures in excess of 3.5 kb during the peak of metamorphism. These conditions are within the range for partial melting to occur assuming PH = Ptotal. Proximity of 20 leucosome compositions from the Dutch Oven Creek migmatite to isobaric (5 kb) cotectic surfaces in the Qz-Ab-Or-An-H 2o system (Winkler, 1979) suggest that certain leucosomes have compositions similar to low temperature melts and are compatible with an origin by partial melting. Certain leuco- cratic layers do not plot close to cotectic surfaces and or are situated in high temperature regions in the Qz-Ab-Or-An-H 2o tetrahedron. Metamorphic 117 segregation induced by partial melting may be a tenable explanation for their origin. Uranium favorability in the migmatite complex is low owing to: l) probable lack of uranium and thorium in the sedimentary protoliths, 2) mobilization of radioactive elements, if ever present, by prograde dehydration reactions, 3) nonuraniferous paleosomes such that uranium and thorium could not be concentrated by partial melting, 4) low K20 values(?) in the two-mica granites, and 5) lack of late stage fluids associated with the granite. The age of the metasedimentary gneisses is unknoWlsince lack of continuous mapping north and south of South Fork prevents correlation with rocks of known age. The gneisses are, however, older than the Cretaceous (70-100 m.y.) granites which intrude them. The gradation from in situ leucosomes in the Dutch Oven Creek migmatite on the east to schlieric and schollen structures, and finally nebulitic lit-par-lit granite on the west near Droogs Creek, suggest that the migmatite formed in association with the granite and that the granite may have ultimately formed by partial melting at a deeper level and intruded the gneisses. Lack of chilled margins and concordant contacts further suggest that the intrusion of granite and migmatization were progressive and not separate events. APPENDICES Al APPENDIX I GLOSSARY OF TERMS Agmatite * : Migmatite with a breccia-like structure; pieces can be fit back together again. Epitaxial overgrowth: An overgrowth that has the same crystallographic orientation as the host grain (i.e. albite rims on altered plagio- clase cores have albite twins in the same crystal orientation). Equilibrium assemblage: Minerals that are in contact with each other and have stable grain boundaries. Gneiss: A foliated rock which may be layered also; differs from a layered granofels in that minerals defining layers or foliation do have a preferred orientation (i.e. biotite, muscovite, etc.). Granofels: A granoblastic metamorphic rock that may be layered but is neither foliated or lineated; minerals that define compositional layering do not have a preferred orientation. Intrusion breccia: A breccia formed by intrusion of magma into country rock. Xenoliths are of mixed lithologies, rotated, and in the thesis area, are enclosed by schlieric tonalite. Differs from schollen structure in that xenoliths are of different lithologies. I-type granite: Granite derived from differentiation of a more mafic magma. It does not contain aluminous minerals other than hornblende, has a lower Sr ratio, and is rarely associated with regional metamorphism or migmatite (Chappel and White, 1974). Leucocratic: A light colored igneous looking rock containing less than 5 percent biotite. Leucosome * : Leucocratic part of a migmatite, generally rich in quartz and feldspar; the newly formed portion or neosome. Melanosome * : Mafic part of a migmatite, rich in dark minerals such as biotite and hornblende; also part of the neosome. Migmatite * : 11 Megascopically composite rock consisting of two or more petrographically different parts, one is the country rock in a more or less metamorphic stage, the other is of pegmatitc, aplitic, or granitic, or generally plutonic appearance. 11 Neosome * : The newly formed portion of a migmatite--the leucosome and the melanosome. *Terminology from Mehnert (1968). A2 Orthoamphibolite: An amphibolite formed by the metamorphism of a basaltic igneous rock. Orthogneiss: Gneiss formed from metamorphism of granitic rocks, such as granites, syenites, quartz monzonites, etc. Paleosome * : Parent rock from which a migmatite was formed. Paraamphibolite: An amphibolite derived from the metamorphism of a sedimen- tary rock, such as a marl. Paragneiss: A gneiss derived from the metamorphism of sedimentary rocks such as graywackes, shales, etc. Sagenitic: Containing or enclosing acicular minerals, such as sillimanite needles in biotite (AGI Glossary, 1972). Schlieren * : "Irregular streaks or masses with blended outlines in migmatites or hybrid magmatites. Screen: A sheet-like body of rock. S-type granite: Granite formed from partial melting of sedimentary rocks. High Sr ratios and the presence of aluminous minerals are charac- teristic. APPENDIX IIA. MODAL ANALYSES OF REPRESENTATIVE CALC-SILICATE ROCKS. Granofelses Hb-rich Selvages 8026A 801718 7927 7922C 8033H 80173 80173 8014 80171 Plagioclase 17. 0 - 13.4 . 2. €i 37.3 - 28.9 14.8 33.0 * * ( '* ( Anl 8) (An31) An 36 i (An2s)* Perthite - - - . 6 Orthoclase 6.1 - .9 - 21.1 Microcline - - - 12.7 Quartz 32.9 49.3 36.9 62.0 11. 9 58.3 20.2 32.4 11.6 Hornblende - 2.4 11. 9 20.2 1.4 45.6 46.1 38.0 Actinolite 8.0 - 2.2 Scapolite - 11.1 - - - - . 9 Biotite - - tr 1.4 - - - .6 Muscovite - - - - - .4 1. 7 . 7 5.9 Diopside 32.4 - 41. 7 Ferro-Diopside - 2.5 - 9.0 Epidote - 29.3 tr - 1.1 30.1 .9 1. 5 1. 7 Carbonate - 5.4 - tr - 9.0 - tr Garnet - tr - - . 3 Sphene 3.3 2.4 2.4 .9 5.9 .4 1. 9 3.3 3.0 Zircon .3 - .1 .3 .7 Apatite tr tr tr tr tr tr .3 tr Magnetite - - - - tr - .3 - 1.6 Chlorite - - - - - tr . 2 .3 4.6 * An content determined by Michel-Levy Method. ·;i::. w APPENDIX , IFl3 .. MODAL ANALYSES OF ALUMINOUS GNEISS. 8057A 7936 8056C 80608 8071 8083 8082 80901 8098 809B 807A 80638 80163 Plagioclase 16.9 9.7 32.7 - 8.3 5,9 16.2 16.8 3.2 2.3 60.9 11.5 15.4 Orthoclase -- - - 4.7 - - 6.0 - - - 6.1 10.6 Microcline 17.1 3.3 - - 33.6 49. l - 26.8 - - - 5.4 Perthite tr - - - - tr ------tr Myrmekite tr - - - - tr Quartz 34.4 66.3 47.4 61. 2 40.3 28.5 71.8 42.4 46.1 52.1 33.5 41. 9 53.2 Muscovite 3.2 7.0 1.6 25.1 1.0 4.5 . 9 tr tr 16.7 1. 6 Biotite 23.0 13.7 13.9 13.1 10.8 l 0. 5 11.1 8.0 21.0 31.0 5.2 23.8 10.0 Garnet - - - .2 .6 l. 5 - - 10. 7 4.2 . tr - .6 Sillimanite 5.2 tr - - .8 - - - 19.0 9.7 - - 3.0 Zircon tr . 1 .2 - - tr - tr - tr tr - . 2 Sphene Magnetite .2 - 4.0 . 2 - tr - - - .4 tr - tr Epi dote - - .2 .2 - - - - - tr Rutile ------tr 8057A 7936 8056C 80608 8071 8083 8082 80901 809C 8098 807A 80638 80163C Plagioclase 24.7 12.2 40.8 - 9.6 6. l 18.4 18.3 6.5 4.2 64.5 19.3 18.2 K-Fel dspa r 25.0 4.2 - - 38. 7 58. 6 - 35.7 - - - 10.3 18.9 Quartz 50.3 83.6 59.2 100.0 51.7 35.3 81.6 50.1 93.5 95.8 35.5 70.4 62.9 )::> APPEND! X II-C. MODAL ANALYSES OF LEUCOCRATIC STRINGERS, PODS, AND LENS ES IN ALUMINOUS GNEISS. Stringers Pods and Lenses 80163C 801638 801630 8067B 8072A 8072B 8070B 8060C 80590 803 8055B Plagioclase 16.0 1.4 9.9 50.7 46.3 47 .8 17.8 38.9 81. 2 50.3 1. 2 Orthoclase - - - - 12.4 13.2 -- tr tr Microcline 48.5 - 50.9 12.0 - - 38.9 Perthite tr tr Myrmekite tr tr - - - - 1.1 - - - 74.5 Quartz 21.0 24.7 18.3 22.3 37.8 35.5 26.8 60.2 17.4 45.2 22.5 Biotite 12.4 . 7 2.0 5.4 1.8 2. l 13. 5 - .2 2.2 . 3 Muscovite 1.4 .9 .4 1.0 1.6 1.4 1. 5 . 9 1. 2 2.3 1. 5 Garnet .7 13.5 8.16 Si 11 imani te ------.4 Zircon -- tr tr - - - tr - tr Plagioclase:: 18.7 1.4 12.5 59.6 48.0 49.5 21.0 39.3 82.4 52.7 1. 2 K-feldspar 56.7 73.5 64.3 14.2 12.8 13.7 46.0 tr tr 75.9 Quartz 24.6 25.1 23.2 26.2 39.2 36.8 33.0 60.7 17.6 47.3 22.9 ):> U'1 APPENDIX II-D. MODAL ANALYSES OF AMPHIBOLITES. From Aluminous Gneiss From Migmatite · 8086 8085 8070C 8073A 8056B 802 8046B 8044C 80162B 8077A Plagioclase 8.6 6.6 8.0 6.7 35.5 14.9 10. 7 15.0 12.0 8.3 Quartz 13.3 13.5 5.0 14.3 16.3 18.9 22.0 15.7 4.3 . 3 Hornblende 65.1 73.6 66.1 68.3 26.2 57.6 17.4 9.3 13. 7 11.0 Bi otite 2.7 2.0 15.9 - 13.0 5.6 45.0 56.7 65.7 65.3 Garnet 5.3 2.6 1.0 6.4 Sphene tr - tr - 3.3 1. 7 1. 3 tr 2.3 1. 3 Zircon - tr tr - tr - tr tr tr tr Apatite tr tr tr tr tr Magnetite 5.0 1. 7 2.3 4.3 4.0 tr Epidote - - 1. 7 - 1. 7 1. 3 .6 1. 3 .7 12.8 Carbonate - - - - tr tr - . 3 . 3 Ilmenite ------4.0 1. 7 1.0 ):,. Q) A7 APPENDIX III MICROPROBE ANALYSES Microprobe analyses of plagioclase and garnet from the Crooked River aluminous gneiss and the Dutch Oven Creek migmatite are tabu- lated in Appendices IIIA and IIIB, respectively. The analyses were obtained from a TRACOR NORTHERN TN-2000 (250 piko amps and 15 kilovolts) using a 200 second count time and an effective beam width of 0.1 - 2.5 microns. Bence-Albee Matrix and XML fitting programs were used. Stiochiometric numbers are calculated on the basis of 32 oxygen. Based on duplicate analyses (8041D2A and B), plagioclase determinations are accurate to+ 1% An. \ APPENDIX IDA. MICROPROBE ANALYSES FROM CROOKED RIVER ALUMINOUS GNEISS. ------8067 8067 8067 8067 8067 80163 80163 80163 80163 80163 131G* 84G* 82 ll3 85 Cl C2 C3 C4 C4 GNEISS RIM CORE ------·---·----- Si02 37.57 38.69 64.90 63.24 66 . 91 65.42 66.17 68 . 75 69.16 67.06 Al203 21 . 62 22.53 21. 91 22.28 22. 70 22. 72 22.78 23.19 21. 67 22.91 FeO 37 . 73 37.49 - - - - 0.13 - 0. 15 0. 14 tlgO 2. 15 2.74 0.15 - - - - 0. 19 Cao 1.08 0. 94 2. 98 3.50 3. 12 3. 16 3.10 2.80 1. 29 2.99 Na2D - - 9.26 8.64 9.61 9. 21 9.26 10.32 11. 02 10. 15 K20 - - 0.31 0.29 0.31 0.23 0.32 0.24 0. 16 0.28 MnO 3. 43 2.45 - - 0.13 Ti02 ------0.08 P205 ------0.17 llaO - Total 103. 58 104.84 99.50 97.95 102. 7fl 100 . 74 101. 84 105.49 103.45 103.70 Number of ions on the basis of 32 oxygen Si 7.90 7.95 11.35 10.89 11. 46 11.41 11.42 11. 46 11. 72 11. 40 Al 5.36 5.46 1 4.52 4. 52 4.58 4.67 4.64 4.56 4.32 4.59 Fe 2 6.63 6.44 - - . - - 0.02 - 0.02 0.02 Mg 0. 67 0. 84 0.04 - - - - 0.05 Cu 0.24 0.21 0.56 0.65 0.57 0.59 0.57 0.50 0.23 0.54 Na - - 3. 14 2.88 3. 19 3. 12 3.10 3. 34 3.62 3.34 K - - 0.07 0.06 0.07 0.05 0.07 0.05 0.03 0.06 Mn 0.61 0.43 - - 0.02 p ------0.02 Ila Ti ------0.01 Mol % Ab 83.25 80.22 83.29 82.98 82.45 85 . 86 92 .82 84.34 An 14.85 18.11 14.88 15 . 70 15 . 69 12.85 6.41 14 .14 Or 1.86 1. 67 0.53 1. 32 1.86 1. 29 0. 77 0.52 ------·- -·------·------·------ne111arks : *8067BIG is a ga r net from adjacent gneiss. 8067ll4G is a garnet from leucocratic pod . );;. co APPENDIX IDB. MICROPROBE ANALYSES OF PLAGIOCLASE FROM DUTCH OVEN CREEK MIGMATITE. - . - - ·····------·------·--· --·-- - -- ·- ---·----·-----·--·----· ------·------·--·-- -·-· - ·-·------···------·----. -·· -- --· --·--······ ··- - ·· ------···- - --· - -· P. 042 B042 8042 B042 0042 0042 80·12 8042 8042 0041 8041 8041 B041 8041 8041 804 I B041 804 I 8041 804 I fl04 L 0041 8041 8041 BO•! I Ill 112 113 113 113 M CJ C2 C3 81 ll2 B3 Cl C2 C3 C4 C5 Ill Ill 112 113 OJ 0211 02B 03 RIM INT CORE BORDER LEUCO LEUCO LEUCO RIM CORE ------···-----··------·-·-· -··------····-----·------·----·----.. ------··-··------······------· ·- -· --·-- Si02 64 . 73 64 . 27 66 . 23 65.20 58.29 66.59 68.37 68.01 68.30 64.72 65.27 64. 09 62 . 35 61.41 61 . 14 61.92 63 .84 68.31 65.(17 65.00 63 .85 64 . 39 63.06 63 . 54 65 . 9G II 1203 25.03 24 . 90 23 . 25 22 . 60 22 . 70 24 . 21 22 . 79 21. 99 23 . 55 24 . 18 25.20 23 . 64 22.75 21 . 63 22 . 99 22 . 8G 23.70 20.57 22. 78 23.88 23.59 23.24 23 .40 23.56 23 .35 FcO 0. 11 - - - 0.15 0.13 - 0.H - 0. 10 0. 24 - 0.12 0. 11 - - 0. 13 0. 12 0.21 MqO 0. 17 - - 0.15 0. 21 0.17 0.19 - o. 19 -- - 0. 15 - 0.08 - - 0. 17 0. 17 CaO 5.20 5.40 3.56 3.43 5.22 3.07 2.09 2.00 3.13 4. 94 4.85 4. 15 3.93 3.31 4. 35 4. 10 4. 23 1. 20 3.29 4.52 4. 40 4.07 4.41 4. 54 3.fiB Nil20 B. BO 8.94 9.07 9. 18 7.21 9. 77 10.65 10 . 33 10 . 19 8.90 9. 31 8.51 9.05 8. 67 B. 50 8. 51 8. 51 10.62 9.45 8.02 B. 72 9.01 B.46 B.85 9.43 1:20 0. 12 0. 20 0. 22 0.25 0.22 0.21 0.10 - 0.05 . 25 0.20 0. 43 0.1 8 0. 53 0.14 0.42 0. 35 0. 19 0.20 0.28 0.22 0.47 0. 39 0. 37 0 . f.O 11n0 ------0.10 --- -- 0.23 - -- o. JO 0. II Ti02 ------0.08 - -- - 0. 21 0. 15 P205 - -- 0. 12 ------0. 25 0.?.8 0. 1') __ Q_,_U ·---: .. __ ---- -=------=------__ . _ ---=-- --=--- .. ___- ___ _Q_,_!§. ---=-- _:___ _..: ... __- ___ _!U .l __ ...:.. __Q. J I __ _:: ______. ____-______. _ lula 1 JIM. l!i 103. H2 103 . 13 100.82 93 . fl5 104. 81 104.55 102.49 105.70 102.99 105 . 13 100 .82 98 . 26 95.94 97 . 19 98 . 38 100.75 101.06 101 .58 101 .66 101 .05 101.77 100 . 24 101.22 103. 36 .Number of ions on tt.e bas 1s of 32 oxyg en. Si 1). 01 10 . 9'/ 11 . 33 ll . 3B 9.67 11.22 11.50 11.63 ll. 37 ll .12 11 . 02 11. 21 10 . 83 10 . 41 10 . 49 10.79 11.10 11 .Bl 1I.41 I I. 16 I 1. 15 11. 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