The University of Maine DigitalCommons@UMaine Electronic Theses and Dissertations Fogler Library Summer 8-23-2019 Fractures, Fluids, and Metamorphism: Shear Zone Initiation in the Marcy Anorthosite Massif, Adirondacks, New York, USA James Hodge University of Maine, [email protected] Follow this and additional works at: https://digitalcommons.library.umaine.edu/etd Part of the Geochemistry Commons, Geology Commons, and the Tectonics and Structure Commons Recommended Citation Hodge, James, "Fractures, Fluids, and Metamorphism: Shear Zone Initiation in the Marcy Anorthosite Massif, Adirondacks, New York, USA" (2019). Electronic Theses and Dissertations. 3050. https://digitalcommons.library.umaine.edu/etd/3050 This Open-Access Thesis is brought to you for free and open access by DigitalCommons@UMaine. It has been accepted for inclusion in Electronic Theses and Dissertations by an authorized administrator of DigitalCommons@UMaine. For more information, please contact [email protected]. FRACTURES, FLUIDS, AND METAMORPHISM: SHEAR ZONE INITIATION IN THE MARCY ANORTHOSITE MASSIF, ADIRONDACKS, NEW YORK, USA By James Hodge B.S. College of Saint Rose, 2017 A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science (in Earth and Climate Sciences) The Graduate School The University of Maine August 2019 Advisory Committee: Scott Johnson, Professor and Director School of Earth and Climate Sciences, Advisor Chris Gerbi, Professor School of Earth and Climate Sciences, Advisor Alicia Cruz-Uribe, Professor School of Earth and Climate Sciences Martin Yates, Professor School of Earth and Climate Sciences FRACTURES, FLUIDS, AND METAMORPHISM: SHEAR ZONE INITIATION IN THE MARCY ANORTHOSITE MASSIF, ADIRONDACKS, NEW YORK, USA By James Hodge Thesis Advisors: Dr. Scott Johnson, Dr. Chris Gerbi An Abstract of the Thesis Presented in Partial Fulfillment of the Requirements for the Degree of Master of Science (in Earth and Climate Sciences) August 2019 Localized shear zones are important rheological features that influence deformation behavior throughout the Earth’s middle-to-lower crust. Therefore, the processes through which shear zones initiate and localize remains an important geologic question. The study of strain localization and shear zone initiation is made difficult due to continued deformation overprinting the microstructures which lead to initiation and obfuscating the context in which localization occurred. The Marcy anorthosite in the Adirondack Highlands, New York, is a nominally granulite-facies, plagioclase-rich massif cut by centimeter-to-meter scale shear zones which provides a natural example of shear zone localization within the middle-to-lower crust. My work focuses on the microstructural examination of shear zones at Bennies Brook within the Marcy massif to construct the sequence of geologic events which lead to shear zone initiation. I used field observations combined with optical and electron microscope observations and electron probe geochemistry to investigate how microstructural conditions changed over time as the sequence progressed, as well as explore the tectonic implications of shear zone development within the massif. My results suggest that the initiation of viscous shearing was facilitated by a combination of physical and chemical weakening during exhumation. The country rock anorthosite was pervasively fractured and/or crushed which increased permeability sufficiently to allow for the infiltration of chlorine-rich hydrothermal fluids, primarily along centimeter-wide brittle fault zones where permeability was greatest. These fluids triggered the retrograde replacement of plagioclase feldspar to scapolite and pyroxene to amphibole and quartz. This weakened the rock through the introduction of the relatively weak minerals as well as an associated reduction in grain size. In the planar zones of greatest metasomatism, this weakening was sufficient for viscous shearing to initiate. The relative timing and orientation of the shear zones supports previous work suggesting that the Adirondack Highlands underwent exhumation associated with orogenic collapse during the Ottawan orogeny (ca. 1080–1000 Ma). TABLE OF CONTENTS LIST OF TABLES ......................................................................................................................... iv LIST OF FIGURES .........................................................................................................................v Chapter 1. INTRODUCTION ...............................................................................................................1 1.1. Purpose of Study ...........................................................................................................1 1.2. Shear Zone Formation and Evolution ...........................................................................4 1.3. Retrograde Metamorphic Weakening ...........................................................................8 1.4. Geologic Setting............................................................................................................9 1.4.1. Tectonic and Polymetamorphic History ......................................................10 1.4.2. Study Site .....................................................................................................13 2. METHODS ........................................................................................................................15 2.1. Sample Collection .......................................................................................................15 2.2. Sample Preparation .....................................................................................................15 2.3. Sample Analysis..........................................................................................................15 2.3.1. Optical Microscopy ......................................................................................15 2.3.2. Scanning Electron Microscopy ................................................................... 16 3. STRUCTURAL OBSERVATIONS ..................................................................................17 3.1. Field Observations ......................................................................................................17 3.2. Microscale Observations .............................................................................................20 3.2.1. Zone I (Host Anorthosite) ............................................................................24 3.2.2. Zone II (Outer Gradient) ..............................................................................27 ii 3.2.3. Zone III (Inner Gradient) .............................................................................27 3.2.4. Zone IV (Shear Zone Core) .........................................................................29 4. GEOCHEMICAL DATA ..................................................................................................30 4.1. Plagioclase ..................................................................................................................30 4.2. Scapolite ......................................................................................................................30 4.3. Pyroxene .....................................................................................................................31 4.4. Amphibole...................................................................................................................33 4.5. Other Minerals ........................................................................................................... 33 5. DISCUSSION ....................................................................................................................35 5.1. Bimodal Plagioclase....................................................................................................35 5.2. Deformation and Metamorphism Timeline ................................................................39 5.3. Granulite-Amphibolite Transition ..............................................................................45 5.4. Implications for Adirondack History ..........................................................................47 5.4.1. Tectonic Effects ...........................................................................................48 5.4.2. Massif Properties ........................................................................................ 48 6. CONCLUSIONS................................................................................................................52 REFERENCES ..............................................................................................................................54 APPENDIX ....................................................................................................................................63 BIOGRAPHY OF THE AUTHOR ................................................................................................97 iii LIST OF TABLES Table 3.1. Summary of microstructural zones associated with shear features at Bennies Brook. .......................................................................................................23 Table 3.2. Mineral abbreviations after Whitney & Evans (2010). ..........................................23 Table 4.1. Table of EPMA data for plagioclase representative of various thin sections ........31 Table 4.2. Table of EPMA data for scapolite representative of various