AN INTEGRATED STUDY OF THE BLUE HILLS PORPHYRY AND RELATED UNITS QUINCY AND MILTON, MASSACHUSETTS by SUZANNE SAYER B. S., Tufts University SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY August, 1974 Signature of Author Planetary .--.. ., gu s. 1974. Department of Earth and Planetar y Scienc es, August, 1974 Certified by ... ... .... .... .w o rr y~r . .. 0 -.' ,,Thesis Su i Accepted by . .... ... .. ......... ................ Chairman, Departmental Committee on Graduate Students Undgre-n - ~ N V5 1974 MIT.19 AN INTEGRATED STUDY OF THE BLUE HILLS PORPHYRY AND RELATED UNITS QUINCY AND MILTON, MASSACHUSETTS by SUZANNE SAYER Submitted to the Department of Earth and Planetary Sciences in partial fulfillment of the requirements for the degree of Master of Science in August, 1974 ABSTRACT A field and petrologic study, including two new chemical analyses and trace element determinations on three samples, was undertaken to define possible subvarieties of the Blue Hills porphyry, a member of the Blue Hills Igneous Complex. It is concluded that, the Blue Hills porphyry is geologically and mineralogically a single unit, dominantly granite porphyry, which grades into a porphyritic granite on one side. The Blue Hills porphyry becomes more aphanitic with fewer phenocrysts near the contact with the country. rocks. Textural variations correlate well with the topographic features: the higher the elevationthe more aphanitic the Blue. Hills porphyry becomes. The outcrop at the Route 128-28 intersection has traditionally been interpreted as a "fossil soil zone", but on the basis of detailed field and petrographical studies, it is reinterpreted as an extrusive facies of the Blue Hills porphyry. The controversial "spheroidal" unit with eutaxitic features, the extremely variable textures of the Blue Hills porphyry, and high-temperature pseudomorphs of quartz, all indicate that the Blue Hills porphyry was emplaced at relatively shallow depths. Results of trace element analyses of the Quincy granite and Blue Hills porphyry are nearly identical and confirm the long-held belief that the two units are comagmatic. Thus, the Blue Hills porphyry intrusion is nearly contemporaneous with the Quincy granite. The Blue Hills porphyry and Quincy granite are both younger than the aporhyolite member of the Blue Hills Igneous Complex. The aporhyolite is tentatively correlated with the Mattapan, Lynn and Newbury volcanics, with its interbedded Siluro-Devonian fossils. The 207 206 Quincy granite yields an Pb /Pb age of 437+32 MY. Hence, volcanic activity occurred in the Boston area starting with the aporhy- olite, at least 437+ 32MY ago and continued for at least 25 MY until the late Silurian Newbury volcanics. Thesis Advisor: Richard S. Naylor Title: Associate Professor of Geology T A B L E O F C 0 N T E N T S Page List of Tables 10 List of Figures 11 Inde'x Map 13 CHAPTER I - Introduction 14 Acknowledgements 18 CHAPTER II - Geologic and Tectonic Position of the Blue Hills Porphyry 20 A. Braintree Argillite 20 B. Rhombporphyry 22 C. Aporhyolite 24 C.1 General Description 24 C.2 Outcrop Descriptions: 24 Great Blue Hill and Hemenway Hill 25 Wampatuck Hill 26 Fox Hill 26 Eastern Part of the Blue Hills 26 D. Blue Hills Porphyry 27 D.1 Type Locality 28 D.2 Textural Variations 29 D.3 Contact Features of the Blue Hills Porphyry 34 E. Fine-Grained Riebeckite Granite 35 Page F. Quincy Granite .36 F.1 Contacts of the Quincy Granite 37 Rattlesnake Hill 37 Chickatawbut Hill 39 Other Contacts with the Blue Hills Porphyry 40 Contact Features of the Quincy Granite and Aporhyolite 41 Intrusive Breccia 41 F.2 General Statement on Quincy Granite Contacts 41 G. Pondville Conglomerate 42 CHAPTER III - Major and Trace Element Chemistry 43 Major Element Chemistry 43 Trace Element Chemistry 45 Analytical Procedure 45 Sample Descriptions 48 Petrographic Descriptions 48 73-4-1 48 73-4-2 52 73-4-56 52 Sample Preparation 53 Estimate of Analytical Accuracy 53 Trends of REE Abundance 54 Page 6 Results 57 Discussion 59 Conclusion 62 CHAPTER IV - Review of Isotopic Studies of the Blue Hills Complex(BHIC) 63 Summary of Isotopic Work 63 Discussion 66 Evaluation of Rb-Sr Dating 71 Quincy Granite Results 71 Blue Hills Porphyry Results 71 Aporhyolite Results 75 Discussion of Rb-Sr Systematics 79 Potassium-Argon Dating 82 Zircon Dating 85 Quincy Granite 85 Blue Hills Porphyry 88 Evaluation of All Isotopic Work 89 Quincy Granite Results 89 Blue Hills Porphyry Results 90 Aporhyolite Results 91 Conclusion 92 Page CHAPTER V -Discussion 93 Petrology and Geology of the Blue Hills Porphyry 93 Geochemical Affinity of the Blue Hills Porphyry and Quincy Granite 95 Radiogenic Ages 96 APPENDIX A - Summary of Previous Work On the BHIC 99 Nomenclature 100 Previous Workers 101 W. 0. Crosby 101 G. F. Loughlin 106 C. H. Warren 106 Aporhyolite 107 Rhombporphyry 107 Quincy Granite 107 Blue Hills Porphyry 107 Granite Porphyry 107 Quartz-Feldspar Porphyry 109 Blue Hills Porphyry Contact Features 109 Summary of Warren's Work 110 B. K. Emerson 111 M. P. Billings 113 L. LaForge 113 K. G. Bell 114 Page N.E. Chute 114 Blue Hills Porphyry-Quincy Granite Relations-1940 115 Blue Hills Porphyry-Quincy Granite Relations-1969 115 Conclusion 116 APPENDIX B - Blue Hills Porphyry and Pondville Conglomerate Contact 117 A. Blue Hills Porphyry 117 A.1 Type Locality 117 A.2 Route 128-28 Roadcut 119 B. Pondville Conglomerate 122 B.1 Matrix of the Pondville Conglomerate 123 B.2 Contact with the Wamsutta Formation 124 B.3 Contact with the "Spheroidal" Zone 124 C. "Spheroidal" Zone 124 C.1 Characteristics of Pyroclastic Units 125 C.2 Characteristics of Fossil Soil Zones 126 C.3 General Description of the "Spheroidal" Zone 126 Page C.4 Matrix of the "Spheroidal" Zone 129 Types One and Two 129 Type Three 129 Type Four 129 C.5 General Characteristics 130 C.6 "Spheroids" 131 C.7 General Features 132 D. Discussion 134 E. Conclusion 134 APPENDIX C - Peaks Used in INAA Determinations of REE Abundances 137 APPENDIX D - Abbreviations Used in the Text 138 References Cited 139 L-IST OF TABLES Page Table 1 - Modal Abundances of Minerals Present in the Blue Hills Porphyry 32 Table 2 - Chemical Analyses of the Quincy - Granite and Blue Hills Porphyry 44 Table 3 - Trace Element Abundances (ppm) 46 Table 4 - Observed Eu Abundance Divided by that Interpolated from Figures 6,7, and 9. 60 Table 5 - Isotope Ages of the Rock Units in the Blue Hills Igneous Complex 64 Table 6 - Summary of Rb-Sr Ages for Figures in this Work 81 Table 7 - Potassium-Argon Ages of the Quincy Granite and Blue Hills Porphyry 84 Table 8 - Comparison of Geological Interpretations of the Blue Hills Igneous Complex by Various Geologists Since Crosby 100 LIST OF FIGURES Page Figure 1 - Geographical names referred to in Text Figure 2 - Bedrock Map of Rock Units Figure 3 - Blue Hills Porphyry from the Type Locality 30 Figure 4 - Localities of Analyzed Samples 47 Figure - Rare Earth Abundances of Five New England Granites Normalized to Chondrites 49 Figure - Rare Earth Abundances of Three Quincy Granite Samples Normalized to Chondrites 50 Figure - Rare Earth Abundances of Three Peabody Granite and Three Cape Ann Granite Samples Normalized to Chondrites 51 Figure - Rare Earth Abundances of Three Blue Hills Porphyry Samples Normalized to Quincy Granite Rare Earth Abundances 52-53 Figure 9 - Rare Earth Abundances of Three Blue Hills Porphyry Samples Normalized to Chondrites 58 Figure 10- Concordia Diagram for Zircon from the Cape Ann, Quincy, and Peabody Granites 68 Figure 11- Sample Localities for Rubidium-Strontium Isotopic Analyses 70 Figure 12-Whole-Rock Rb-Sr Isochron Diagram for the Quincy Granite 72 Figure 13- Whole-Rock Rb-Sr Isochron Diagram for the Cleaned Quincy Granite Page Figure 14 - Whole-Rock Rb-Sr Isochron Diagram for the Blue Hills Porphyry 74 Figure 15 - Whole-Rock Rb-Sr Isochron Diagram for the Cleaned Blue Hills Porphyry 76 Figure 16 - Whole-Rock Rb-Sr Isochron Diagram for he Blue Hills Porphyry along the Southern Contact with the Pondville Conglomerate 77 Figure 17 - Whole-Rock Rb-Sr Isochron Diagram for the Aporhyolite 78 Figure 18 - Whole-Rock Rb-Sr Isochron Diagram for the Cleaned Aporhyolite 80 Figure 19-- Schematic of the Contact of the Blue Hills Porphyry with the Pondville Conglomerate at Route 128-28 Intersection 116 INDEX MAP I. INTRODUCTION The purpose of this thesis is to restudy the Blue Hills porphyry member 'of the Blue Hills Igneous Complex. The previous isotopic age determinations, new trace element data, and major element chemi- cal analyses are combined with new field studies of the Blue Hills porphyry, aporhyolite, and Quincy granite. The area of this study is limited to the Blue Hills, south of Boston, in Milton and Quincy, Massachusetts (see Index Map). The whole Boston area, including the Blue Hills, has been affected by large scale tectonic disturbances (Quinn, 1971; Woodward, 1957; Rodgers, 1952, 1967; and Lyons and Faul, 1968), and by repeated glacia- tions which have obscured many important relationships. Thus, identi- fication of units, and of their stratigraphic position, has proved contro- versial (Quinn and Moore, 1968). The Blue Hills Igneous Complex is composed of the fine-grained riebeckite granite, the Blue Hills porphyry, the aporhyolite, and the Quincy granite. Crosby (1880) noted that the Blue Hills Igneous Complex is younger than the middle-Cambrian Braintree argillite, and all subsequent geologists have agreed with this conclusion. Similarily, it is widely agreed that the Complex antedates the Pennsylvanian aged rocks of the Norfolk and Narragansett Basins (Knox, 1944; Woodworth, 1894; Shaler, Woodworth and Foerste, 1899). From fossil evidence alone, no more precise limits can be placed on the Blue Hills Igneous Complex.