VI1 TABLE OF CONTENTS Page CHAPTER 1: INTRODUCTION 1 CHAPTER 2: GEOLOGY OF MOUNT MAZAMA 8 Regional Setting 8 Tectonic Framework 9 Evolution of the Volcano 15 The Climactic Eruption 18 Post-Caldera Activity 22 Petrography 23 Chemical Variation 33 CHAPTER 3: PRODUCTS OF THE CLIMACTIC ERUPTION 43 Petrography and Geochemistry 43 Mineralogy 50 Discussion 61 CHAPTER 4: MELTING EXPERIMENTS 65 Experimental Methods 66 Results 68 Discussion 73 CHAPTER 5: PHASE RELATIONS IN A WATER- UNDERSATURATED MAGMA 78 A-C-M-S System 79 FO-DI-Q System 90 Iron Enrichment 92 Fe-MG Distribution Coefficients 104 Discussion 106 CHAPTER 6: GENERATION OF DIVERGENT MAGMAS 109 Mass Balance Calculations 109 Trace Element Variations 116 Discussion 132 Implications for the Basalt-Rhyolite Association 135 Vlll CHAPTER 7: VOLCANOLOGICAL IMPLICATIONS 140 Physical Conditions in the Chamber 140 Crystal Settling 155 Depth and Dimensions of the Chamber 157 Sources of Water 159 Mode of Eruption 163 Implications for Caldera Formation 165 APPENDICES A. Chemical Analyses of Rocks and Glasses 166 B. Location of Samples 180 C. Chemical Analyses of Minerals 183 D. Experimental Data 19 3 BIBLIOGRAPHY 198