Iowa State University Capstones, Theses and Graduate Theses and Dissertations Dissertations 2020 A multiple-pulse emplacement model for the Shonkin Sag laccolith, MT Claire Elizabeth Ruggles Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/etd Recommended Citation Ruggles, Claire Elizabeth, "A multiple-pulse emplacement model for the Shonkin Sag laccolith, MT" (2020). Graduate Theses and Dissertations. 18212. https://lib.dr.iastate.edu/etd/18212 This Thesis is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Graduate Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. A multiple-pulse emplacement model for the Shonkin Sag laccolith, MT by Claire Ruggles A thesis submitted to the graduate faculty in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Major: Geology Program of Study Committee: Sven Morgan, Co-major Professor Jacqueline Reber, Co-major Professor Yuyu Zhou The student author, whose presentation of the scholarship herein was approved by the program of study committee, is solely responsible for the content of this thesis. The Graduate College will ensure this thesis is globally accessible and will not permit alterations after a degree is conferred. Iowa State University Ames, Iowa 2020 Copyright © Claire Ruggles, 2020. All rights reserved. ii TABLE OF CONTENTS Page LIST OF FIGURES .................................................................................................................. .iv LIST OF TABLES .....................................................................................................................v ACKNOWLEDGMENTS .......................................................................................................... vi ABSTRACT .............................................................................................................................vii CHAPTER 1. INTRODUCTION ................................................................................................ 1 CHAPTER 2. GEOLOGIC SETTING ........................................................................................ 7 Regional Geology ..................................................................................................................7 Petrology ...............................................................................................................................8 Emplacement History ........................................................................................................... 12 CHAPTER 3. FIELD OBSERVATIONS .................................................................................. 14 Preliminary Field Work ........................................................................................................ 14 Field Work ........................................................................................................................... 14 Field Observations ............................................................................................................... 15 Internal Contacts and Magma Mingling .......................................................................... 15 Faults and Folds .............................................................................................................. 19 Deformation Levels ........................................................................................................ 21 Bulk Magnetic Susceptibility ............................................................................................... 22 Displacement Calculations ................................................................................................... 25 CHAPTER 4. ROCK FABRIC, MAGNETIC, AND PETROGRAPHIC ANALYSES .............. 27 Thin Sections ....................................................................................................................... 27 Rock Composition, Fabrics and Textures ........................................................................ 27 Magnetic Mineralogy ...................................................................................................... 28 Features of Interest .......................................................................................................... 29 Curie Point Estimates ........................................................................................................... 33 Anisotropy of Magnetic Susceptibility ................................................................................. 34 Image Analysis .................................................................................................................... 37 CHAPTER 5. THERMAL MODELLING ................................................................................. 39 Testing Parameters ............................................................................................................... 39 Determining the Maximum and Minimum Emplacement Times ........................................... 42 CHAPTER 6. DISCUSSION .................................................................................................... 49 Evidence for Multiple Pulses ................................................................................................ 49 Emplacement Model for the SSL.......................................................................................... 54 Rock Fabric and Magnetic Analyses .................................................................................... 57 Thermal Modeling ............................................................................................................... 59 iii CHAPTER 7. IMPLICATIONS ................................................................................................ 61 CHAPTER 8. CONCLUSION .................................................................................................. 62 REFERENCES ......................................................................................................................... 64 APPENDIX SUPPLEMENTARY TABLES ......................................................................... 67 iv LIST OF FIGURES Figure 1: Regional map of the area northeast of the Highwoods Volcanic Center ........................5 Figure 2: Geologic map and cross-section of the Shonkin Sag laccolith. ......................................6 Figure 3: Image and sketch of the south-eastern margin of the Shonkin Sag laccolith looking north ...........................................................................................................9 Figure 4: Drone image perpendicular to the fringing sill wall and the kinematic cross section of the fringing sills ..................................................................................... 10 Figure 5: A generalized section of the SSL ................................................................................ 12 Figure 6: Field photos of observed features at the SSL .............................................................. 17 Figure 7: Field photos of observed features at the SSL .............................................................. 18 Figure 8: Bulk magnetic susceptibility measurements................................................................ 24 Figure 9: Microphotographs of thin sections displaying deformation levels ............................... 30 Figure 10: Microphotographs of oxides and sulfides in thin section. .......................................... 31 Figure 11: Microphotographs of features of interest in thin section ............................................ 32 Figure 12: Heating curves for Curie Point thermal analyses ....................................................... 34 Figure 13: Anisotropy of magnetic susceptibilty data. ............................................................... 36 Figure 14: Jelinek plot. .............................................................................................................. 37 Figure 15: Microphotographs of thin sections and rose diagram for Image Analysis .................. 38 Figure 16: Stills of the initial and end results of the single-pulse model ..................................... 43 Figure 17: Stills of the initial and end results of the double-pulse model .................................... 44 Figure 18: Stills of the initial and end results of the minimum-time triple-pulse model .............. 46 Figure 19: Stills of the initial and end results of the maximum-time triple-pulse model ............. 47 Figure 20: Emplacement diagram of the Shonkin Sag laccolith ................................................. 53 Figure 21: Location of the bulk magnetic susceptibility measurements ...................................... 74 v LIST OF TABLES Page Table 1: Displacement calculations for offset accommodated by the major, subvertical tangential fault ....................................................................................................... 26 Table 2: Parameters and cooling time for the parameter testing models. .................................... 41 Table 3: Parameters used for the emplacement time models. ..................................................... 42 Table 4: Cooling times for each pulse in the emplacement models. ........................................... 48 Table 5: Block sample data.