University of Al berta Petrogenesîs of Middle Jurassic to Miocene magmatism wîthin the Nechako plateau, central British Columbia: Insight from petrography, geochemistry, geochronology and tracer isotope studies Nancy Grainger @ A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Master of Science Department of Earth and Atmospheric Sciences Edmonton, Alberta Winter 2000 National Library Bibliothèque nationale 1*1 cf Canada du Canada Acquisitions and Acquisitions et Bibliographie Services services bibliographiques 395 Wellington Street 395. rue Wellington Ottawa ON KI A ON4 OttawaON KtAON4 Canada Canada The author has granted a non- L'auteur a accordé une licence non exclusive licence allowing the exclusive permettant à la National LiT,rary of Canada to Bibliothèque nationale du Canada de reproduce, loan, distribute or sell reproduire' prêter, distribuer ou copies of this thesis in microfom, vendre des copies de cette thèse sous paper or electronic formats. la fome de microfiche/fïlm, de reproduction sur papier ou sur format électronique. The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d' auteur qui protège cette thèse. tbesis nor substantial extracts fkom it Ni la thèse ni des extraits substantiels may be p~tedor othervcrise de celle-ci ne doivent être imprimés reproduced without the author' s ou autrement reproduits sans son permission. autorisation. ABSTRACT Igneous rocks of the Middle Jurassic, Cretaceous, Eocene and Miocene periods dominate the geology of the Nechako plateau in central British Columbia- Study of the geochemistry and tracer isotope values of these rocks provides insight into their different magmatic origins and tectonic settings. Middle Jurassic plutonism is isotopically primitive and supports emplacement above an active continental arc. Eocene volcanism was regionally continuous between 53 and 47 Ma with episodic related plutonism between 55 and 47 Ma. Eocene volcanisrn is isotopically primitive (87~r/86~ri= 0.7040-0.7045; ~Ndi= +0.9 to +4.6) and is interpreted as derived from an ennched upper mantle source. Magmas were variably modified by fractionation and contamination and erupted in a distal arc setting. Parental magmas of isotopically primitive Oligo-Miocene Chilcotin Group basalts are asthenosphere derived. lsotopic results from spinel Iherzolite xenoliths entrained in Oligo-Miocene lavas indicate enriched components within the upper rnantle. To the glory of God with whom al1 things are possible As with any such body of work there are innumerable people who have contributed to this project and without whom this work would not have been successfully cornpleted. I want to begin by thanking the two people who are primarily responsible for the inspiration, guidance and support of this work; Dr. Larry Heaman (University of Alberta) and Mike Villeneuve (GSC Ottawa). Mike got me hooked on geochronology and suggested the project to me. He arranged the financial support for much of the analytical work, let me use his ArIAr lab in Ottawa and was always on hand to offer advice if I needed it. Larry welcomed me into his lab at the University of Alberta and taught me most of what I know about U-Pb geochronology- He financially supported both this project and myself, as needed, over the past two years. His advice on every aspect of this project has proven invaluable. I greatly appreciate the time he spent carefully and critically reading every line of this thesis. I am very fortunate to have had the opportunity to work with him. I am indebted to al1 of those I worked with in the field during the summers of 1997 and 1998. Lori Synder (senior mapper), whose guidance and enthusiasm for geology was much appreciated. Jonah Resnick who always kept me on my toes and persisted in calling me the 'Ootsa Queens. I hope someday we have another opportunity to work together and argue about the geological problems you're so passionate about. Tina Pint who never complained about the bush I dragged her through and always made sure I was wide awake and ready to start the day- Steve Sellwood who always gave me a hug when I needed it. Elspeth Bames who had no end of good humour and common sense to share. Amber McCoy who was present for the second, successful, attempt on Dayeezcha Mountain and who was a good friend when things went awry. The Alpha crew; Karen Fallas, Mike Hrudey, Crysbl Huscroft, Marianne Qüat, Seleena Billesberger and Kelly Franz, some of whom kept me Company occasionally on traverse, others who made sure I was aware of interesting Ootsa locales and good sample sites- Also two assistants from the 7997 crew; Angelique Justason and Andy Blair- Angie and I found we shared a liking for testing the limits as well as our birthday. Andy who kept us al1 in stitches with his deadpan humour. Missing from the above Iist are three very important people; Bert Struik (GSC Pacific), Bob Anderson (GSC Pacific) and Don Maclntyre (BCGS). I owe a great debt to Bert and Bob, from whom I learned much of what I know about field methods and techniques. Both Bob and Bert provided superb leadership and organization in the field câmps I was part of. AI1 three of these people took time out from a hectic field season ta bring me to sites and help me sample in areas they were familiar with. Outside the field seasons, they provided support and engaged in discussion about the Eocene stratigraphy and other aspects of this study. Bob provided financial support for the Endako tracer isotope analyses frorn his research budget and supplied me with innumerable written and digital resources. I am also indebted to Rob Creaser, who let me work in his tracer isotope lab. He taught me Rb-Sr and Sm-Nd chemistry and helped enormously in problem solving when things got tricky. I am grateful that I was given the opportunity to do the tracer isotope work in this project, as it became an increasingly important component of this project. There is a small army of people who have helped me in laboratories at the GSC in Ottawa and the University of Alberta. I would Iike to thank Barbara Bohm for teaching me some of the crushing and mineral seperation techniques I needed and, more importantly, how io do them better. I greatly appreciate the help of Chris Bohm and Kim Toope who taught me a lot about U/Pb chemistry. Chris always carefully answered my questions regarding techniques and helped improve my lab practices. Thanks to Stacey Hagen for al1 her help in the mass spectrometry lab and work in analyzing a number of the tracer isotope samples. I need to thank Fred Quigg at the GSC for al1 his help in the Ar/Ar lab and his patient hard work processing al1 the data. Thanks to Peter Belanger and the analytical chemistry group who processed my geochemistry samples at the GSC. Thanks also to Anthony Bond who digitized rny detaiied rnaps at the GSC Vancouver- 1 am priviieged to have had the opportunity to study at the University of Alberta and 1 appreciate the financial support given to me by the Department of Earth and Atmospheric Sciences. There are many people in the department who have helped make my time at the U of A mernorable and I would like to thank everyone part of this department for making it the great place to study that it is. Particular thanks to those friends who encouraged and distracted me whe~I needed it and generaliy helped me to stay sane; Dave, George, Kathy, Elspeth, Karen and, particularly, Steph and Andrea. Thanks to Rachel, Sonya and Doug, friends outside of the University in Edmonton who helped me feel Iike 1 lived here, not just studied here. Thanks to al1 my friends who stayed in touch from a distance and kept encouraging me; Tash, Joscelyn, Andrew, Jennifer, Crystal, Sue, Ray, Janet, Jenn, Geoff, Rachel and Jeff. Finally, thanks to my family who have aiways encouraged and supported me, no matter how far away my pursuits take me. TABLE OF CONTENTS CHAPTER 1 .Introduction ...................................................................................................... 1 References......................................................................................................................... -2 CHAPTER 2 - Geochronology and geochemistry of Late Tertiary, Late Cretaceous and Middle Jurassic igneous rocks in the Nechako plateau, central British Columbia .......... 4 Introduction........................... 4 Regional Geology ................................................................................................................ 4 Previous Work ..................................................................................................................... 7 Oligocene - Micoene..................................................................................................... 7 Cretaceous ................................................................................................................... 8 Jurassic ......................................................................................................................... 8 Major and Trace Element Geochemistry ............................................................................ 9 Analytical Methods ....................................................................................................... 9 Results .......................................................................................................................... 9 U-Pb Geochronology ........................................................................................................