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University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 1971 Igneous and metamorphic geology of the Willow Creek drainage basin southern Sapphire Mountains Montana Mark Whitehead Presley The University of Montana Follow this and additional works at: https://scholarworks.umt.edu/etd Let us know how access to this document benefits ou.y Recommended Citation Presley, Mark Whitehead, "Igneous and metamorphic geology of the Willow Creek drainage basin southern Sapphire Mountains Montana" (1971). Graduate Student Theses, Dissertations, & Professional Papers. 7139. https://scholarworks.umt.edu/etd/7139 This Thesis is brought to you for free and open access by the Graduate School at ScholarWorks at University of Montana. It has been accepted for inclusion in Graduate Student Theses, Dissertations, & Professional Papers by an authorized administrator of ScholarWorks at University of Montana. For more information, please contact [email protected]. IGNEOUS AND METAMORPHIC GEOLOGY OF THE WILLOW CREEK DRAINAGE BASIN, SOUTHERN SAPPHIRE MOUNTAINS, MONTANA by Mark W. Presley A.B., Franklin and Marshall College, 1963 Present in partial fulfillment of the requirements for the degree of Master of Science UNIVERSITY OF MONTANA 1971 Approved by ÔÔ, ___________ Chairman, Board'^f Examiners De Gra ^ LU/.. Date Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: EP37940 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. UMT OisMTtatiwi PvbHahing UMI EP37940 Published by ProQuest LLC (2013). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code P r o O u e s f ProQuest LLC, 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 - 1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS I am indebted to Drs. Hyndman and Alt of the Geology Faculty of the University of Montana and Dr. Keith Osterheld of the Chemistry Department, for their helpful criticism during the research. I would also like to thank Dr. J. P. Wehrenberg of the University of Montana, who first suggested the idea of a mapping project in the southern Sapphire Mountains. The many residents of Corvallis, Montana and the Willow Creek area also deserve credit for their help during field work. The Society of Sigma Xi provided partial funds to cover field expenses. 11 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS Page ACKNOWLEDGEMENTS .......................................... Ü LIST OF TABLES ............................................. V LIST OF ILLUSTRATIONS ...................................... vi 1. Introduction ......................................... 1 Scope and Purpose ............................... 1 Previous W o r k ................................... 3 II. General Relationships ............................. 4 III. Structure ........................................... 7 IV. Willow Creek Stock .................................. 12 General Relationships .......................... 12 Petrography .........................................17 Compositional Variations ........................ 19 Discussion of CompositionDa t a .....................25 Pressure Conditions ............................ 26 V. Additional Igneous Rock Types ................... 29 Diabase Dikes ................................... 29 Quartz-Latite Dikes ............................ 30 Syenite and Vermiculite Deposits .............. 31 111 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. IV Page VI. Metamorphism .............................. 34 Rock Descriptions and Stratigraphy ............ 34 Calc-Silicate U n i t ............................... 35 Biotite-Quartz U n i t ..................... 36 Blue-Gray Quartzite Unit ..................... 36 Regional Metamorphism .......................... 37 Biotite-Chlorite Z o n e .......................... 37 Diopside Z o n e ....................................42 Contact Metamorphism ............................ 48 S c a p o l i t e ................ 56 Environment of Metamorphism ................... 57 BIBLIOGRAPHY ............................................... 61 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES Table Page 1 Determinations on granite samples ................. 21 2 Mineralogical data - biotite-chlorite zone . 41 3 Mineralogical data - diopside z o n e ..................47 4 Mineralogical data - contact z o n e ....................54 V Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF ILLUSTRATIONS Figure Page 1 Index map showing the Bitterroot Valley in western Montana ................................... 2 2 Generalized dips of compositional layers within metamorphic units of the Willow Creek a r e a ........................................... 9 3 Diagrammitic cross section across the Willow Creek drainage from the area of Dominic Buttes to the area of Skalkaho M o u n t a i n ................ 10 4 Lower hemisphere, equal area plot of poles to compositional layering .......................... 11 5 Geology of the Willow Creek stock ................. 14 6 Contour plot of plagioclase, potassium feld­ spar, and quartz from the Willow Creek stock . 15 The stability of muscovite and muscovite plus quartz ........................................ 27 8 Stratigraphie units ............................ 34 9 Metamorphic zones in the Willow Creek stock . 38 10 Biotite-chlorite zone metamorphic assemblages 40 11 Calicum-silicate metamorphic assemblages . 44 12 Biotite-quartz metamorphic assemblages . 45 13 Contact zone metamorphic assemblages ............ 50 14 Equilibrium curves for the stated reaction at 500 and 1000 bars p r e s s u r e ....................... 51 VI Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. v i l Plate Page I Geologic map of the Willow Creek a r e a ........... Pocket II Compositional variations within the Willow Creek stock: specific gravity, mafic modes, plagioclase modes ................................ Pocket Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I. Introduction Scope and Purpose This report is a study of the igneous and metamorphic petrology of the Willow Creek drainage basin in the Southern Sapphire Mountains, Montana, (Fig. 1). It includes: (1) the mapping of the lithologie units, (2) the determination of compositional variations within the Willow Creek stock, a granite body in the center of the Willow Creek area, and (3) the description of metamorphic assemblages along with the mapping of isograds. A major problem in working with batholithic granites is separating the purely regional metamorphic events from deep seated contact metamorphic events. If one assumes that the granite melts form in the highest grade portions of regional metamorphic terrains, the magma will intrude into lower grade rocks and cut across the regional isograds; contact metamorphism surrounding the granitic intrusions will thus be superimposed on the regional metamorphism. This can occur in a wide range of pressure conditions, shallow intrusions forming typical low pressure, hornfels- like contact metamorphism, whereas deeper intrusions will develop coarser-grained, higher-pressure contact aureoles. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. MONTANA Scale(ml.) o 20 L. J£. IDAHO -114 « 115' Hall (1968)' M issoula Jerome (1968) Willow Creek , Area Skalkaho Rass Fâlis Œ / Hami ton Ross (1950) 46 Lost TraU Fbss 114» Figure 1, Index map showing Bitterroot Valley in western Montana, Areas of previous work are shown and labeled by author. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. This research shows that the Willow Creek stock has been intruded into amphibolite grade regional metamorphic rocks under moderate to high pressures, and has superimposed a coarse-grained, high pressure aureole on the country rocks. Previous Work (Refer to Fig. 1) The first mapping to include part of the Willow Creek area was done by Langton (1935), who mapped the Sapphire Mountains north of Willow Creek. His work excluded the southern half of the Willow Creek drainage. Ross (1950) mapped the far western edge of the Willow Creek area as part of the Hamilton 30' Quadrangle. Stewart Ness (1968) outlined the general nature of the Willow Creek stock and the surrounding metamorphics in a senior paper at the University of Montana. This paper is the first detailed work on the metamorphic and igneous environment in the Willow Creek area. Nold (1968) mapped zones of the amphibolite and greenschist facies in the Lolo Pass area of the northern Bitterroot Mountains, approximately 30-40 miles northeast of Willow Creek. Hall (1968) continued the biotite zone of the green­ schist

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