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Geology of the Williams diatremes Textures of garnet peridotite inclusions Mineral compositions Garnet Clinopyroxene Orthopyroxene 9 01 i vine Spi nel 9 Phi ogopite Thermometry and barometry of inclusions

Introduction 1 Techniques of color classification 2 Summary of samples 2 Geology of the Williams diatremes 8 Inclusions from deep levels 9 Garnets from garnet peridotites 9 Megacryst garnets 9 Garnets from granulites, mafic granulites, and mafic amphibolites 19 Garnets from other inclusions in the Williams diatremes 19 Garnet-bearing inclusions in other diatremes 19 Garnets from Precambrian rocks in the Little Rocky Mountains 20 Colluvial garnets from the Williams diatremes 21 Comparison of Williams colluvial garnets with garnets from known sources 23 Panned concentrates from other diatremes 31 Big Slide diatreme 31 Bullwhacker Coulee diatreme 31 Conclusions 31 References 32 FIGURES 1. Geologic map of the Williams diatremes and vicinity, north-central Montana 10 2. Diagrams showing Ca-Mg-Fe in colluvial garnets, megacrysts, and garnets from inclusions in the Williams 1 and 4 diatremes 14 3. Histograms showing Cr203 content of color groups of peridotitic and colluvial garnets in the Williams diatremes 17 4-8. Diagrams showing: 4. Weight percent CaO versus weight percent Cr203 for garnets in the Williams 1 and 4 diatremes 18 5. Chromium cations per 12 oxygens versus Mg/(Mg +Fe) for garnets in the Williams 1 and 4 diatremes 20 6. Chromium versus aluminum cations per 12 oxygens for garnets in the Williams 1 and 4 diatremes 22 7. Ca-Mg-Fe in garnets from localities other than the Williams diatremes 29 8. Chromium cations per 12 oxygens versus Mg/(Mg +Fe) in garnets from localities other than the Williams diatremes 30 Contents Ill IV Contents TABLES 1-6. Analyses of: 1. Colluvial garnets in the Williams 1 and 4 diatremes 3 2. Garnets from fresh garnet peridotites in the Williams 1 and 4 diatremes 12 3. Garnets from altered garnet peridotites in the Williams 4 diatreme 13 4. Garnet megacrysts in the Williams 4 diatreme 24 5. Garnets from other inclusions in the Williams 1 and 4 diatremes 25 6A. Garnets from the Big Slide 1 diatreme 26 6B. Garnets from the Bullwhacker Coulee diatreme 27 6C. Garnets from other inclusions, Precambrian bedrock, terrace deposits, and Paleocene sedimentary rock 28

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