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PHASE RELATIONS IN THE CONTACT AUREOLE OF THE ONAWA PLUTON, MAINE by JOHN MARSHALL MOORE, JR. B. Sc. University of Manitoba SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY August, 1960 Signature of Author .-. ... ... Departm t of Geology and Geo isics, August, 1960 Certified by . 'rt' Supervisor Acceptedby................................................. Chairman, Departmental Committee on Graduate Students V PHASE RELATIONS IN THE CONTACT AUREOLE OF THE ONAWA PLUTON, MAINE John Marshall Moore, Jr. Massachusetts Institute of Technology Cambridge, Massachusetts Submitted to the Department of Geology and Geophysics in August, 1960, in partial fulfillment of the require- ments for the degree of Doctor of Philosophy ABSTRACT The metamorphic aureole developed in low calcium slates at Onawa, Maine by an igneous body of granitic to gabbroic composi- tion has been studied petrographically and chemically in the light of chemical thermodynamic considerations. The sequence of stable mineral assemblages from slate of the low greenschist facies to quartz-sillimanite-cordierite-biotite-pota s sic feldspar hornfels is deduced from petrographic and X-ray powder diffraction data. Chemical analyses of coexisting biotite and cordierite by instrumental methods show that chemical equilibrium is closely approached in the high grade assemblages. Shifts of the stability fields of the minerals can be explained in terms of the physical variables of metamorphism. Thesis Supervisor: Professor H. W. Fairbairn Title: Professor of Geology TABLE OF CONTENTS Page List of Figures vi List of Tables viii Acknowledgements ix Part One (Condensed for publication from Part Two) I. Introduction 1-1 II. Location and Topography 1-1 III. Areal Geology 1-3 IV. Petrology and Petrography 1-3 A. Slate 1-4 B. Aureole 1-5 C. Pluton 1-10 V. X-Ray Study 1-11 A. Opaque Minerals 1-11 B. Cordierite 1-12 VI. Theoretical Considerations 1-12 VII. Choice and Preparation of Samples 1-15 VIII. Chemical Analyses 1-18 IX. Graphical Analysis 1-21 X. Summary of Conclusions 1-29 Part Two I. INTRODUCTION 1 II. A. Location and Access 3 B. Topography 5 TABLE OF CONTENTS (Cont.) Page III. FIELD OBSERVATIONS 6 A. Previous Investigations 6 B. Areal Geology 8 C. Geology in the Vicinity of the Onawa Pluton 10 1) Slate 11 2) Aureole 11 3) Intrusive 16 IV. PETROGRAPHY 17 A. Onawa Aureole 17 1) Slate 17 2) Aureole 21 3) Intrusive 33 4) Inclusions 36 B. Other Aureoles in the Vicinity 36 V. MINERALOGICAL STUDIES BY X-RAY DIFFRACTION 39 A. Mineralogy of the Slates 39 B. Mineralogy of the Hornfelses 43 1) Non-opaque minerals 43 2) Opaque minerals 45 C. Structural State of Cordierite 50 VI. THEORY 53 A. The Phase Rule 53 B. Graphical Representation 61 C. Graphical Treatment of Pelitic Rocks 67 D. Distribution of a Minor Component Between Two Phases 73 E. External Variables in Contact Metamorphism 74 TABLE OF CONTENTS (Cont.) VII. INTERPRETATION OF PETROGRAPHIC AND X-RAY DATA IN THE LIGHT OF THE PHASE RULE 79 VIII. CHEMICAL ANALYSES 83 A. Introduction 83 B. Choice of Samples 84 C. Preparation of Samples 86 D. Purity of Samples 90 E. Analytical Methods 94 F. Discussion of Analytical Procedures 97 G. Analytical Results 102 IX. DISCUSSION OF RESULTS 125 A. Precision and Accuracy of Chemical Analyses 1 25 1) Precision 125 2) Accuracy 129 B. Graphical Presentation of Data 136 1) A-F-M Plots 136 2) Distribution of manganese 147 C. Metamorphic Reactions in the Onawa Aureole 147 1) Prograde reactions 147 2) Retrograde reactions 163 3) Comparison with other localities 165 D. Correlation With Experimental Data 176 E. Suggestions for Further Research 180 BIOGRAPHY 182 BIBLIOGRAPHY 183 vi LIST OF FIGURES Part One Page 1-1. Index map of Maine 1-2 Part Two 1. Geological map of the Moosehead Lake area 4 2(a). Geological map of the Onawa pluton and its aureole 12 2(b). Structural map of the western end of the Onawa pluton (from Philbrick (1933)) 13 3. Geologic and sample location map of the vicinity of Lake Onawa 18, 19 4. X-ray powder diffraction patterns of slate 0-3, Big Wilson Cliffs, illustrating the effects of heat treatment 41 5. Hypothetical one-component pressure-temperature diagram 59 6(a) Isograd maps based on hypothetical one-component (b) system 59 7. Hypothetical three-component composition diagram 62 8. Projection through a mobile component 62 9. Projection through a pure substance 62 10. The Al 0 3-FeO-MgO-K 20 tetrahedron 71 11(a) Projections of the A-F--M-K tetrahedron on to the (b) plane Al 0 3-FeO-MgO (a) Projection through mus- covite (b) Projection through K feldspar 71 12. Mineral separation flow chart 88 13. X-ray powder diffraction intensity data for the deter- mination of quartz impurity in cordierite 93 14. Biotite-cordierite tie lines in the A-F-M triangle, projected through quartz, K-feldspar and ilmenite (a) All iron plotted as FeO 137 (b) Analyzed FeO only plotted 138 ------------------- vii LIST OF FIGURES (Cont.) Page 15. Oxidation state of iron in coexisting biotite and cordierite 142 16. Compositions of biotite, cordierite, chlorite and slate in the A-F-M triangle, projected through quartz, muscovite, and ilmenite, all iron plotted as FeO 145 17. Distribution of manganese between coexisting biotite and cordierite 148 18. Suggested prograde assemblages in pelitic hornfels 150 19. Mica-feldspar assemblages across the K feldspar isograd 157 20. Reactions involving almandite garnet (=alm) in hornfels 157 21. Comparison of Onawa assemblages with those of other localities 172 V111 LIST OF TABLES Part 2 Table Page 1. Table of Formations in the Moosehead Lake Area 9 2. X-ray Powder Data for Opaque Minerals 47 3. Distortion Index of Cordierite from the Onawa Aureole 51 4. Scheme of Replicate Analyses 98 5. Analyses of Standard Rocks W-1 and G-1 104 6. Chemical Analyses 105 7. Corrections to Chemical Analyses 113 8. Sample Calculation of Mean Corrected Chemical Analysis 121 9. Correction of Analysis of Chlorite Ch-65 122 10. Calculation of Mode of Slate 0-65 124 11. Precision of Analytical Results 126 12. Structural Formulas of Biotite and Cordierite 131 ACKNOWLEDGEMENTS I am indebted to Professor H. W. Fairbairn, my thesis super- visor, for suggesting the problem and for providing wise counsel throughout the investigation. To Professor J. B. Thompson Jr. I owe my interest in metamorphic petrology; he suggested the Onawa pluton as a thesis locality and made many valuable suggestions about the metamorphism of pelitic sediments. Dr. S. S. Philbrick was most generous with his thin sections, field data, and personal interest, which enabled me to greatly expand my limited concept of the area. Dr. G. H. Espenshade of the U. S. G. S. also kindly loaned thin sections of Maine hornfels, and went out of his way to provide location data. I appreciate the interest of Mr. G. A. Chinner and Dr. W. Schreyer of the Geophysical Laboratory in my problem; discussions with them provided many new ideas about contact metamorphism. Fellow graduate students at M. I. T. who particularly contributed to the investigation are Dr. W. C. Phinney and Dr. A. H. Brownlow, who took time from their own thesis work to instruct me in instrumental analysis, and Mr. S. R. Hart, who was always willing to discuss matters petrologic. Profs. W. H. Pinson and J. W. Winchester, Mr. J. H. Crocket and Mr. C. C. Schnetzler all gave useful advice on analytical problems. My greatest debt is to my wife, who deserves joint authorship for her part in the thesis. She contributed time, effort, and love at every stage of the project from climbing mountains through sieving rock and calculating results to typing seemingly endless tables of figures. In the preparation of the final draft, I owe thanks to Mrs. Edith x Nicholson, the typist, for her skill and patience, and again to Charles Schnetzler for consenting to proofread and collate the final portion in my absence. The support of this research by a grant from the National Science Foundation and by scholarship aid from the M. I. T. Canadian Trust Fund is gratefully acknowledged. Part One (Condensed for publication from Part Two) 1-1 I. INTRODUCTION This study is one of several completed in the last few years at M. I. T., all of which were aimed at quantitatively evaluating the extent to which chemical equilibrium is attained in various meta- morphic processes. The methods adopted were primarily petro- graphic study and chemical analysis of coexisting ferromagnesian minerals by means of instrumental procedures developed by Shapiro and Brannock (1956). The contact aureole of the Onawa pluton in Maine proved suitable for such a study because previous field and petrographic work (Philbrick, 1933, 1936) showed it to be well ex- posed, intruded into low calcium pelites, relatively free of polymeta- morphic effects and not so fine grained as to prohibit mineral sepa- rations. It thus appeared that the contact metamorphic mineral assemblages would be amenable to graphical treatment according to the methods of Thompson (1957) as a means of interpreting them in the light of chemical thermodynamic theory. II. LOCATION AND TOPOGRAPHY The Onawa pluton is located in central Maine (index map, Fig- ure 1), in Piscataquis county, in the Sebec Lake and Sebec topographic quadrangles. A map of the intrusive and its aureole, taken from Philbrick (1933), is reproduced in Figure 2(a) (Part Two, p. 12). The resistant hornfelses crop out on the peaks and slopes of the mountains which surround a swampy basin occupied by the pluton. The aureole is accessible by road at the western end.

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