University of Alberta TECTONOMETAMORPHIC EVOLUTION OF THE KLUANE METAMORPHIC ASSEMBLAGE, SW YUKON: EVIDENCE FOR LATE CRETACÇOUS EASWARD SUBDUCTION OF OCEANIC CRUST UNDERNEATH NORTH AMERICA JOCHEN ERNST MEZGER (c) A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Geology Edmonton, Alberta Fa11 1997 National Library Bibliothèque nationale du Canada Acquisitions and Acquisitions et BiMiogiaphic SeMces setvices bibliographiques 395 WeUiingtorr Street 395, rue Wellinw WONK1AW -ON K1AW Canada Canada The author has granted a non- L'auteur a accordé une licence non exclusive licence allowing the exclusive permettant à la National Library of Canada to Bîbîiotheque nationale du Canada de reproduce, loan, distriaute or seIl reproduire, prêter, distniuer ou copies of this thesis in microfonn, vendre des copies de cette thése sous paper or elecîronic formats. la forme de microfiche/film, 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. thesis nor substantial extracts fiom it Ni la thèse ni des extraits substantiek may be printed or otherwise de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son permission. autorisation. "Ma perche, se ha percezione di sè non deve aver memoria? La rnemoria B una potenza dell'anima, e per piccola che sia Sanima che la pietra ha, avrà memoria in proporzione." "But why. if the stone perceives itself, should it not have memory? Memory is a power of the soul, and however small the sou1 of a stone, it will have a proportionate mernory." Umberto Eco, "The Island of the Day beforen Abstract The Kluane metamorphic assemblage (KMA) foms a 160 km NWSE striking belt of graphitic micaquartz schist with inteilayered ultramafic lenses. It is located east of the Denali fauit at the northwestem margin of the Coast Belt of the Western North American Cordillera. between island arc rocks of the lnsular Superterrane (INS) to the West and pericratonic assemblages to the east. The lithology of the KMA is distinct Rom any metamorphic assemblage in the Cordillera The dominant structural fabric of the KMA is a pervasiw NEdipping schistosity with a generally east-trending mineral stretching lineation. Mylonitic schists are characterized by a distinct monoclinic fabric that defines westward hangingwall up sense of shear. The KMA has experienced Late Cretaceous syndeformational high Pnow T metamorphism followed by low PBigh T thermal overprinting resulting from Early Eocene emplacement of the Ruby Range Batholith (RRB). Trace element and neodymiurn isotope signatures of the KMA distinguish its protolith from adjacent liysch of the Dezadeash Formation (DF) which has juvenile signatures and Late Proteroroic model ages. and schists of the pericratonic Aishihik Metamorphic Suite (AMS), part of the Yukon Tanana terrane, with ancient continental cnist source and Late Archean mode1 ages. The KMA has intemediate ~~(0)values of 4 and T,, of 1.3 Ga, indicative of homogeneous mking of ancient and jwenile material. A possible tectonic rnodel for the evolution of the KMA involves deposiüon of fine grained pelitic seditnents in a backarc basin proximal to a volcanic arc and distal to an ancient continent. Late Cretaceous eastward motion of the Kula plate resulted in collapse of the basin and oblique eastward subduction, with a leff-lateral strike-slip component, undemeath the North American continental margin (AMS). Sediments of the KMA and parts of the oceanic cnist experienced dudile deformation and high Pnow T metamorphism dunng subduction and were added to the upper plate. Change of plate motion in the latest Cretaceous resulted in Eariy Eocene sill-like intrusion of the RRB along the suture between the KMA and AMS. Continuous northward plate motion caused rapid late Eoœne upiift of the KMA and juxtaposition with the DF and INS. ACKNOWLEOGEMENTS Dr. Philippe Erdmer is thanked for introducing me ?O the exciang geology of the Northem Canadian Cordillera. His rdentless support and guidanœ throughout the course of my Ph.D. thesis is sincerely appreciated. The Yukon Geology Program, fonnerly the Yukon Geoscienœ Onice, in Whitehorse provided valuable logistical support and is thanked for usage of faciiities and field equipment, as well as sharing heliwpter tirrte. In the field discussions of the KMA and AMS with Dr. Steve Johnston of the Yukon Geoscience ûffice and of the Dezadeash Formation with Dr. Grant Lowey are sincerely appreciated. Important geochronological data was provided by Dr. Jim Mortensen of the University of British Columbia. Together with his thorough knowledge of the geology of the southwestem Yukon. it helped to improve this thesis. Able assistance and wmpanionship in the field was provided by Rob Brown (1993). Kevin Deck (1994) and Brys Francis (1995). The Bayshore Restaurant along the Alaska Highway at the southwestem end of Kluane Lake always welcomed Idirty" geologists and senred some of the best food north of latitude 60. The members of my supenrisor cornmittee, Dr. Tomas Chacko, Dr. Robert Creaser and Dr. Karlis Muehlenbachs are thanked for guidanœ during the my Ph.D. program. I am very thankfid to Dr. Thomas Chacko for lengthy discussions on metamorphic petroIogy and geothennobarometry. Introduction to the unstabk world of radiogenic isotopes by Dr. Creaser was fundamental in developing ideas on the origin of the Kluane metamorphic assemblage. It could not have been done without them. Dr. Robert Luth is thanked for help on petrochemical problems. I am abo grateful for discussions with other geologists a various meetings. In particular I would like to thank Dave Brew (USGS Menlo Park), Carol Evenchick (GSC Vancouver). Paul Wlliams (University of New Brunswick) and Lincoln Hollister (Princeton University). Even if these discussions were short, they provided me with better understanding and helped to improve my thesis. Geochemical analyses were provided by Washington State University. Oxygen isotope deteminations of the olivine serpentinïtes were undertaken by Dr. Frsd Longstafiie of the University of Western Ontario. One 4Ar-a~ranalysis was done at Queen's University of Kingston, Ontario, by Karen Fallas. The relentless help and prompt assistance of Paul Wagner of the electron microprobe lab at the ûepartrnent of Earth & Atmospheric Sciences at the University of Alberta is gratefirlly acknowleciged. The help and friendship of my fellow graduate students at the University of Alberta in the last four and a haff years made the highand lows of graduate student life more enjoyable and bearable. I am thanml to Rob Stevens for welcoming me to Edmonton and sheitering me for the first weeks. 1 have leamed a lot from discussions with Suman De, Karen Fallas, Steve Grant, George Morris, Dinu Pan& Rob Stevens, Steve Wetherup, and Taro Yamashita. Karen Fallas and George Moms read parts of my thesis and made valuable suggestions that helped to improve my English. Further thanks to Liane Schlickenrieder, Darlene Atkinson, Leslie Driver, Mario Houle, Keme-An n Shannon, Harald Hübscher and Ruth Noronha for their friendship. But most of al1 I am grateful for the support and friendship of Sabrina Trupia. The next beers will be on me! I am very grateful for the moral, and often financial, support of my fiiends and family in Germany, and especially my parents, Trude and Werner Mezger. Fieldwork for this project was funded by NSERC operating grants to P. Erdmer and reseanh grants from the Canadian Circumpolar Institute (CCI) and the Geological Society of America to the author. Several University of Alberta scholanhips provided further financial support that helped to complete this projed. TABLE OF CONTENTS CHAPTER 1 .INTROOUCTlON ..................................... 1 Statement of problem .......................................... 2 Previouswork ................................................ 4 Outline of present study ........................................ 5 CHAPTER 2 - ROCK TYPES AND STRATlGRAPHlC RELATIONSHIPS ..... 7 INTRODUCTlON .............................................. 8 THE KLUANE METAMORPHIC ASSEMBLAGE (KMA) ................ 8 Muscovite-chlontequarh schist (calcic schist) ................... 12 Biotitequartz schist (aluminous schist) ......................... 21 Olivine serpentinite ........................................ 42 RUBY RANGE BATHOLITH .................................... 49 POST-EOCENE IGNEOUS INTRUSIONS ......................... 54 DEZADEASH FORMATION (Jb) ................................ 56 GREENSCHIST (PMvs) ....................................... 59 CHAPTER 3 .STRUCTURAL STYLES OF THE KLUANE METAMORPHIC ASSEMBLAGE .............................................. 61 INTRODUCTION ............................................. 62 PRWIOUSWORK ........................................... 66 DUCTILE FABRICS OF THE KLUANE METAMORPHIC ASSEMBLAGE . 66 Planarfabncs ............................................. 66 Linearfabrics ............................................ 72 Shear-sense indicators ..................................... 74 Folds ................................................. 85 BRlTTLE STRUCTURES ...................................... 88 Faub ................................................... 88 Jointing ................................................. 93 STRUCTURALANALYSIS OF THE KMA .........................