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Bedrock Geology and Geochemical Analysis of the Bowdoinham 7.5' Quadrangle, Southwestern Maine

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Bedrock Geology and Geochemical Analysis of the Bowdoinham 7.5' Quadrangle, Southwestern Maine Bedrock Geology and Geochemical Analysis of the Bowdoinham 7.5’ Quadrangle, southwestern Maine Joel Frank Cubley Submitted in Partial Fulfillment of the Requirements for the Degree of Bachelor of Arts Department of Geology Middlebury College Middlebury, Vermont April 2005 Bedrock Geology and Geochemical Analysis of the Bowdoinham 7.5’ Quadrangle, southwestern Maine Joel Cubley, Department of Geology Middlebury College, Middlebury, Vermont 05753 The Bowdoinham 7.5’ quadrangle, located in southwestern Maine, is situated along the boundary between the regionally extensive Liberty-Orrington and central Maine lithotectonic belts. Detailed 1:24,000 scale bedrock mapping in the northern half of the quadrangle has resulted in the delineation of the following lithologic units (described from west to east): (1) interlayered biotite granofels and calc-silicate gneisses of the Silurian-Devonian Vassalboro Formation (central Maine sequence), (2) a previously unrecognized, deformed and recrystallized Devonian metagranitoid pluton hereby named the Hornbeam Hill Intrusive Suite, (3) migmatitic biotite gneisses and other subordinate lithologies (e.g. amphibolites, rusty schists) associated with the Ordovician Falmouth-Brunswick sequence (Liberty-Orrington belt), and (4) several small but mappable granitic pegmatite bodies of both Devonian and Permian age. The Hornbeam Hill Intrusive Suite is significant because it stitches the contact between the Vassalboro Formation and Falmouth-Brunswick sequence rocks. Thin dikes and sills of Mesozoic diabase are found in the study area, but cannot be mapped at this scale. Stratified rocks in the quadrangle have been penetratively deformed, folded, and metamorphosed to upper amphibolite conditions during the Acadian orogeny. A pervasive east-dipping foliation (generally <45º) can be found in rocks of both lithotectonic belts, as well as the Hornbeam Hill Intrusive Suite, implying that the dominant episode of deformation in this region occurred after the emplacement of that pluton. This foliation is axial planar to inclined isoclinal folding in the field area. A reconnaissance geochemical study of amphibolites from the Falmouth- Brunswick sequence shows subalkaline, tholeiitic basalt compositions with slight LREE enrichment. Tectonic diagrams show a mixture of MORB and IAT signatures suggestive of a back-arc basin environment. The geochemical characteristics of the Falmouth- Brunswick sequence amphibolites are similar to previously published geochemical studies from rocks of the Spring Point Formation of the Casco Bay Group (exposed in other parts of the Liberty-Orrington belt) and rocks of the Bathurst Supergroup in the Miramichi Highlands of New Brunswick. The Hornbeam Hill pluton is characterized by significant petrographic and geochemical variability, ranging from coarse-grained granitic gneisses to syeno-dioritic gneisses (SiO2 ranges from 54 to 72%). New U-Pb zircon (SHRIMP) dates from the intrusion yield an average metamorphic recrystallization age of 390 ± 3 Ma, and igneous crystallization appears not to be significantly older, with core ages falling within analytical uncertainty of rim ages. This age is important for two reasons: (1) it implies that no significant movement occurred along the boundary between the central Maine and Liberty-Orrington lithotectonic belts in this region subsequent to ~390 Ma, and (2) the deformational event responsible for the pervasive foliation and associated folding in the field area must be younger than ~390 Ma. i Acknowledgements This project was funded by support given to Dave West by the Maine Geological Survey (through the StateMap program). Additional funds were provided by the National Science Foundation (Grant# EAR-0207263). First and foremost, I would like to thank Dave West for giving me the opportunity to battle the mosquitoes and humidity of southwestern Maine and from it derive this entire project. Without his continued guidance and support this thesis would have never come to fruition. I would also like to express my deepest gratitude to Ray Coish for patiently answering my unending geochemical questions day in and day out, and helping me sift through the piles of diagrams to find the strongest argument possible. The hospitality of Art Hussey in welcoming Dave and me into his home during the field season was extremely generous, and his continued input on the local geology was a valuable asset. The advice and constructive criticism of Spike Berry at the Maine Geological Survey was also critical in the development of this project. The rest of the geology department faculty at Middlebury, including Jeff Munroe, Pete Ryan, Patricia Manley, and Tom Manley were extremely supportive throughout the year, and all lent valuable insights at different points during the evolution of this thesis project. I would also like to thank Bill Hegman and Lee Perlow in the geography department for their continued assistance with the GIS analyses and displays presented within. Many thanks must be extended to Gianina Farrugia, Levi Doria, Katharine North, and Trevor Cloak for their camaraderie during those late nights on the geology wing, and their humorous fatalism each and every morning. It has been a great pleasure to work with each and every one of them this year. I would like to also acknowledge the efforts of Colin Kikuchi, Brendan Condit, Nick Benjamin, John Hanley, Chris Farina, Charlie Bettigole, Mike Hennessy, Dan Stone, and Max Jones in forcing me to disregard the joys of glacial striations every once in a while, and teaching me what it really means to be a Prankster 4 Life. I must express my deepest gratitude to my mother, Cindy Cubley, for her love and support of all my endeavors, past, present, and future. Finally, I would like to thank Lauren Armstrong for being the fabric of this entire year. Without her constant encouragement, unending patience, and selfless love this thesis, and my entire senior year, would have been a much less gratifying experience. ii To Dad So was I once myself a swinger of birches; And so I dream of going back to be. It's when I'm weary of considerations, And life is too much like a pathless wood Where your face burns and tickles with the cobwebs Broken across it, and one eye is weeping From a twig's having lashed across it open. I'd like to get away from earth awhile And then come back to it and begin over. May no fate willfully misunderstand me And half grant what I wish and snatch me away Not to return. Earth's the right place for love: I don't know where it's likely to go better. I'd like to go by climbing a birch tree, And climb black branches up a snow-white trunk Toward heaven, till the tree could bear no more, But dipped its top and set me down again. That would be good both going and coming back. One could do worse than be a swinger of birches. -Robert Frost iii Table of Contents Abstract i Acknowledgements ii Dedication iii Table of Contents iv List of Figures vi Chapter I—Introduction 1 Chapter II—Regional Geology 4 Stratigraphy 4 Structure 19 Metamorphism 23 Geochemistry 25 Previous Work in the Bowdoinham 7.5’ Quadrangle 30 Chapter III—Methods 32 Fieldwork 32 Geochemical Analyses 34 Petrographic Analyses 36 Chapter IV—Results 37 Metamorphic Stratigraphy 37 Reconnaissance Petrography 47 Structure 59 Geochronology 62 Geochemistry 64 Chapter V—Discussion 81 Mapping 81 Metamorphism 87 Structure 87 Geochemistry 89 Chapter VI—Conclusions 98 Works Cited 101 iv Appendix I—Field Stations 107 Appendix II—Geochemical Sample Data Table 139 Appendix III—ICP Standards Data 142 Appendix IV—Falmouth-Brunswick Amphibolites ICP Data 143 Appendix V—Hornbeam Hill Intrusive Suite ICP Data 144 Appendix VI—Mineralogy of Thin Sections 145 Appendix VII—Fold Axis and Mineral Lineation Measurements 148 v List of Figures 1. Generalized Bedrock Geology of southwestern Maine 3 2. Regional Geology and Lithotectonic Terranes of New England 5 3. Southwestern Maine Lithotectonic Terranes 7 4. Liberty-Orrington Belt Structural Interpretation 9 5. Stratigraphic Cross-Section of the Bath 1:100,000 Map Sheet 10 6. Summary of Stratigraphic Nomenclature, southwestern Maine 12 7. Maine Pegmatite Bodies Map 15 8. Brunswick Pegmatite Field Map 15 9. Topsham Granite REE pattern 19 10. Metamorphic zones of Maine 24 11. Spring Point Formation REE and Spider Diagrams 26 12. Ordovician Tectonic Model for the Iapetus Ocean Basin 27 13. Bathurst Supergroup-Spring Point Formation Geochemical Comparison 29 14. Newberg (1984) map of northern Bowdoinham 7.5’ Quadrangle 30 15. Geochemisty sampling sites 33 16. Geochemistry sites within northern half of Bowdoinham 7.5’ Quadrangle 34 17. Field photograph of the Vassalboro Formation 39 18. Field photograph of the Unnamed Gneiss Member 41 19. Field photograph of Mixed Rocks Member 42 20. Field photograph of Hornbeam Hill pluton, garnet gneiss variant 44 21. Field photograph of Hornbeam Hill pluton, porphyritic feldspar variant 44 22. Field photograph of Devonian tourmaline-bearing pegmatite 45 23. Field photograph of Permian pegmatite 46 24. Field photograph of Mesozoic dike 47 25. Photomicrograph of Vassalboro Formation 49 26. Photomicrographs of Rusty-Schist and Coticule Member, Vassalboro Fm. 50 A. Sillimanite-rich gneiss 50 B. Coticule-rich gneiss 50 27. Photomicrograph of mafic hornblende gneiss, Hornbeam Hill pluton 52 28. Photomicrograph of poikilitic garnet gneiss, Hornbeam Hill pluton 53 29. Photomicrograph of alkalic granitic gneiss, Hornbeam Hill pluton 53 30. Photomicrograph of rusty schist, Unnamed Gneiss Member 54 31. Photomicrograph of homogenous Falmouth-Brunswick amphibolite 56 32. Photomicrograph of banded Falmouth-Brunswick amphibolite 57 33. Photomicrograph of titanite-rich Falmouth-Brunswick amphibolite 58 34. Equal Area Stereonet Projection—Fold Axes 59 35. Equal Area Stereonet Projection—Lineations 60 36. Equal Area Stereonet Projection—Foliations of stratified rocks 61 37. Equal Area Stereonet Projection—Foliations of Hornbeam Hill pluton 61 vi 38. Cathodoluminesence image of Hornbeam Hill zircon crystals 63 39. 206Pb/238U plot of Hornbeam Hill U/Th zircon analyses 63 40. Falmouth-Brunswick amphibolites Zr/TiO2-SiO2 diagram 64 41.
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