University of Vermont ScholarWorks @ UVM Graduate College Dissertations and Theses Dissertations and Theses 2018 Geochronologic Constraints On The iminT g Of Metamorphism And Exhumation Of The iT llotson Peak Complex In Northern Vermont Cheyne Aiken University of Vermont Follow this and additional works at: https://scholarworks.uvm.edu/graddis Part of the Geology Commons Recommended Citation Aiken, Cheyne, "Geochronologic Constraints On The iminT g Of Metamorphism And Exhumation Of The iT llotson Peak Complex In Northern Vermont" (2018). Graduate College Dissertations and Theses. 942. https://scholarworks.uvm.edu/graddis/942 This Thesis is brought to you for free and open access by the Dissertations and Theses at ScholarWorks @ UVM. It has been accepted for inclusion in Graduate College Dissertations and Theses by an authorized administrator of ScholarWorks @ UVM. For more information, please contact [email protected]. GEOCHRONOLOGIC CONSTRAINTS ON THE TIMING OF METAMORPHISM AND EXHUMATION OF THE TILLOTSON PEAK COMPLEX IN NORTHERN VERMONT A Thesis Presented by Cheyne L. Aiken to The Faculty of the Graduate College of The University of Vermont In Partial Fulfillment of the Requirements for the Degree of Master of Science Specializing in Geology October, 2018 Defense Date: June 19, 2018 Thesis Examination Committee: Laura E. Webb, Ph.D., Advisor Donald S. Ross, Ph.D., Chairperson Keith Klepeis, Ph.D. Cynthia J. Forehand, Ph.D, Dean of the Graduate College ABSTRACT The Tillotson Peak Complex (TPC) in northern Vermont records high-pressure (HP) subduction zone metamorphism that occurred during the Ordovician Taconic Orogeny, and subsequent retrograde metamorphism and deformation that occurred during the Silurian Salinic Orogeny. Previous studies have documented a polymetamorphic history, with peak metamorphic pressures possibly up to 2.5 GPa and temperatures of 550°C. Prior to this research, constraints on the timing of metamorphism in the TPC were limited to a single Middle Ordovician 40Ar/39Ar total fusion age for glaucophane. This study integrates 40Ar/39Ar step heating analyses of multiple mineral phases and U-Pb dating of titanite with field and microstructural observations to further constrain the subduction–exhumation history of the TPC. Microstructural and petrologic analyses in thin section on samples of felsic gneiss, pelitic schist, amphibolite, and blueschist suggest deformation during varied P-T conditions. The earliest and highest-pressure metamorphic event documented in the TPC samples is associated with inclusions in garnet and white mica in S1 quartz microlithons. Inclusions of paragonite, titanite, and omphacite in garnets, locally defining S1, suggest that some blueschist may have formed in the retrograde path in association with the S2 foliation. A greenschist-facies metamorphic overprint in most samples is also associated with S2, primarily defined by epidote, white mica, and chlorite. E-W trending F2 intrafolial folds are commonly rootless in outcrop, locally defined by blueschist–eclogite- facies fold noses. Kinematic indicators relative to S2 and L2 stretching lineations give a predominantly top-to-the-E shear sense. S3 crenulation cleavage development is related to folding about E-W trending F3 folds that define the map pattern of the Tillotson Peak Complex. Locally developed S4 crenulations are axial planar to the NW-trending Gilmore Antiform. Additionally, D4 deformation and metamorphism is recorded by microfractures in garnet and epidote, as well as chlorite pseudomorphs after garnet. 40Ar/39Ar step heating of multiple phases and U-Pb dating of titanite yielded ages corresponding with the Taconian to the Salinic orogenies. Ages of ~485–480 Ma are attributed to prograde–peak metamorphism (M1) and S1 development. Ages that span ~471–456 Ma are interpreted to document retrograde M2 metamorphism through greenschist to locally blueschist-facies metamorphic conditions during exhumation and S2 development. Correlation of D3 microstructures in these samples with map-scale folds suggest that E–W trending folds developed in the range of ~455–445 Ma, recorded by minimum apparent ages in the field area, and locally as plateau ages along the margin of the TPC. Younger ages ~435–405 Ma are observed locally in apparent age gradients, and are interpreted to reflect metamorphic overprinting that resulted in the chlorite pseudomorphs after garnet and the growth of actinolite, which may be related to the timing of folding about the Gilmore Antiform. Results presented here suggest the impact of Acadian retrograde metamorphism and deformation on rocks of the TPC may be less significant than previous work suggests. ACKNOWLEDGEMENTS The work presented in this paper would not have been possible without the guidance, support, and knowledge of many individuals that have been a critical aspect in all stages of this research. Most importantly, Laura Webb, you have been more than understanding and offered more constructive criticism than I could have ever asked for. I cannot thank you enough for your patience when deadlines were approaching, as well as the advice you always had to offer. Additionally, I would like to thank my committee members, Don Ross and Keith Klepeis, for their feedback throughout this long process. Dan Jones without your knowledge and assistance in the lab, this research would have not been possible. I would like to thank Dr. Andrew Kylander-Clark for his willingness to examine titanite from our samples as a part of a pilot study. Jody Smith your availability and wisdom when it came to utilize the SEM was a great benefit to this research. Finally, this research would not have been possible were it not for financial support from the Burlington Gem and Mineral Club and the Vermont Geological Society to fund analyses, and the Ronald Suiter Prize from the University of Vermont to fund travel for presenting at conferences. To Jon Kim, Joe Gonzalez, Susanne Baldwin, Evan Tam, Hannah Blatchford, John Mark Brigham, Beth Pidgeon, and anyone else that came out during field season: your input helped to formulate the interpretations established in this research. For that, I thank all of you. To my fellow graduate students, my friends, and coworkers: you made my time here memorable. Whether it be a quick hike, input on research, grabbing a beer together, or taking the time to listen to my complaints, you have my sincerest appreciation. ii TABLE OF CONTENTS ACKNOWLEDGEMENTS ....................................................................................... ii LIST OF FIGURES .................................................................................................. vi LIST OF TABLES .................................................................................................... vii CHAPTER 1: COMPREHENSIVE LITERATURE REVIEW ............................. 1 1.1 Introduction ......................................................................................................... 1 1.2 The Formation and Exhumation of High- to Ultrahigh-Pressure rocks ................. 3 1.2.1 Introduction: ...................................................................................................... 3 1.2.2 Formation of High-Pressure to Ultrahigh-Pressure Terranes: ........................... 4 1.2.3 Exhumation Mechanisms: ................................................................................. 5 1.2.4 Geochronology: ............................................................................................... 10 1.2.5 Understanding Deformation Mechanisms with Microstructures ..................... 12 1.3 Geologic Setting of the Tillotson Peak Complex ................................................. 14 1.3.1 Regional Geology ............................................................................................ 14 1.3.2 Tectonic Model for the Taconic Orogeny ....................................................... 15 1.3.3 Tectonic model for the Salinic Orogeny ....................................................... 17 1.3.4 Tectonic model for the Acadian Orogeny........................................................ 18 1.3.5 Geology of the TPC ......................................................................................... 19 1.3.6 Geology of the Belvidere Mountain Complex and ophiolites in Quebec ........ 22 1.3.7 Structural Geology ........................................................................................... 23 1.4 Geochronologic Constraints for Metamorphism in northern Vermont and southern Quebec ......................................................................................................... 25 1.4.1 Metamorphism During the Early Taconic Orogeny ........................................ 26 1.4.2 Metamorphism During the Late Taconic Orogeny .......................................... 26 1.4.3 Metamorphism during the Salinic Orogeny .................................................... 27 1.4.4 Acadian Overprinting on the rocks of the Northern Appalachians ................. 28 1.4.5 Recent Studies in the TPC ............................................................................... 29 CHAPTER 2: METHODS ....................................................................................... 50 2.1 Introduction........................................................................................................... 50 iii 2.2 Microstructural Analysis: ..................................................................................... 50 2.3 Compositional Analysis