'Jhi tDYapkttmt &ltds 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<s go to Nan Pickett from the Universtty of Wtsconsin at Eau Clatre for her two days of field assistance during tfie summer of 1979, to John Nelson for drafting the locatton map, to Paula Berger who came to m.r aid in the preparation of four th.in sections for iv microprobe study, and to Yvonne Plornedahl and Bonnie Milligan who helped type the manuscript. I am also grateful to Julie Smith and my brother, Frank Carlson, who were a tremendous help during the final stages of thesis preparation. v 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.