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Uranium-Lead Zircon Ages and Crustal Contamination of the Northeastern Idaho Batholith

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Uranium-Lead Zircon Ages and Crustal Contamination of the Northeastern Idaho Batholith Western Michigan University ScholarWorks at WMU Master's Theses Graduate College 12-1984 Uranium-Lead Zircon Ages and Crustal Contamination of the Northeastern Idaho Batholith James J. Dexter Follow this and additional works at: https://scholarworks.wmich.edu/masters_theses Part of the Earth Sciences Commons Recommended Citation Dexter, James J., "Uranium-Lead Zircon Ages and Crustal Contamination of the Northeastern Idaho Batholith" (1984). Master's Theses. 1506. https://scholarworks.wmich.edu/masters_theses/1506 This Masters Thesis-Open Access is brought to you for free and open access by the Graduate College at ScholarWorks at WMU. It has been accepted for inclusion in Master's Theses by an authorized administrator of ScholarWorks at WMU. For more information, please contact [email protected]. URANIUM-LEAD ZIRCON AGES AND CRUSTAL CONTAMINATION OF THE NORTHEASTERN IDAHO BATHOLITH by James J. Dexter A Thesis Submitted to the Faculty of the Graduate College in partial fulfillment of the requirements for the Degree of Master of Science Department of Geology Western Michigan University Kalamazoo, Michigan December 1984 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. URANIUM-LEAD ZIRCON AGES AND CRUSTAL CONTAMINATION OF THE NORTHEASTERN IDAHO BATHOLITH James J. Dexter, M.S. Western Michigan University 1984 The magmas which formed the northeastern Idaho batholith were contaminated with, or derived from, old crnstal material as evidenced, from previous studies, by high whole—rock *7Sr/,*Sr ratios, relatively high a07Pb/*°‘Pb ages, and Archean to Proterozoic upper intercept ages for batholithic zircons. The old zircon component has been identified in this study by separating zircons into fractions based on size and morphology. Pb/U isotopic ratios are distributed on a chord with a lower concordia intercept intrusive age of 73.6 + 6 m.y. Zircons from a batholithic sample located one kilometer inward from the contact were separated according to mineral association. This method was not successful in segregating old inherited zircon from euhedral zircon. Upper-intercept ages from this project, when combined with those from other studies, range from 1700 m.y. to 2340 m.y. and’ indicate the complexity and variety of the 'older continental crust* source terrain. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS This project was conducted under the supervision of Ronald B. Chase, Western Michigan University, and M. E. Bickford, University of Kansas. The Michigan Highway Department and the Geology Departments of Western Michigan University and the University of Kansas provided the necessary hardware and facilities. The research was partially funded by National Science Foundation Grant #33790. to R. B. Chase and two grants from the Western Michigan University Graduate Student Research Fund. The support given these contributors is greatly appreciated. James J. Dexter ii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INFORMATION TO USERS This reproduction was made from a copy of a document sent to us for microfilming. While the most advanced technology has been used to photograph and reproduce this document, the quality of the reproduction is heavily dependent upon the quality of the material submitted. The following explanation of techniques is provided to help clarify markings or notations which may appear on this reproduction. 1. The sign or “target” for pages apparently lacking from the document photographed is “Missing Page(s)”. 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If necessary, sectioning is continued again—beginning below the first row and continuing on until complete. 4. For illustrations that cannot be satisfactorily reproduced by xerographic means, photographic prints can be purchased at additional cost and inserted into your xerographic copy. These prints are available upon request from the Dissertations Customer Services Department. 5. Some pages in any document may have indistinct print. In all cases the best available copy has been filmed. University Micrdrilms International 300 N. Zeeb Road Ann Arbor, Ml 48106 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 3 2 4 8 1 6 DEXTER, JAMES JOSEPH URANIUM-LEAD ZIRCON AGES AND CRUSTAL CONTAMINATION OF THE NORTHEASTERN IDAHO BATHOLITH WESTERN MICHIGAN UNIVERSITY M.S. 1984 University Microfilms International 300 N. Zeeb Road, Ann Arbor. MI 48106 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PLEASE NOTE: In all cases this material has been filmed in the best possible way from the available copy. Problems encountered with this document have been identified here with a check mark V . 1. Glossy photographs or pages t / 2. Colored illustrations, paper or print_____ 3. Photographs with dark background > / 4. Illustrations are poor copy______ 5. Pages with black marks, not original copy______ 6. Print shows through as there is text on both sides of page______ 7. Indistinct, broken or small print on several pages X 8. Print exceeds margin requirements _____ 9. Tightly bound copy with print lost in spine______ 10. Computer printout pages with indistinct print______ 11. Page(s)___________ lacking when material received, and not available from school or author. 12. Page(s)___________ seem to be missing in numbering only as text follows. 13. Two pages numbered ___________ . Text follows. 14. Curling and wrinkled pages ______ 15. Other ________________________________________________ _ _________________ University Microfilms International Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS Page ACKNOWLEDGEMENTS............................................... ii LIST OF T A B L E S ................................................. iv LIST OF FIGURES................ v INTRODUCTION ... ,................................................ 1 GEOLOGY........................................................ 3 GEOCHRONOLOGY................................................ 9 Separation Procedures.......................................9 Analytical Methods ...................................... 9 Results.................................................... 10 DISCUSSION OF THERMAL AND STRUCTURAL EVENTS ............ 19 Chronology of Events ..................................... 19 Implications Regarding Origin of Batholithic Magma .... 22 APPENDICES A. Location of Samples..................................... 25 B. Petrographic Description of Samples..................... 26 C. Separation Procedures and Analytical Techniques........... 28 D. Analytical Data......................................... 33 E. Theoretical Basis of U-Pb Geochronology.................. 35 BIBLIOGRAPHY....................................................41 iii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES t Page 1. Analytical Data ......................................... 34 iv Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES Page 1. Regional setting of the Idaho batholith .................. 4 2. Geologic and sample location map............................ 5 3. Concordia plot of qnartz diorite orthogneiss................ 11 4. Back-scattered electron image of (-200) mesh-size zircons....................................13 5. Concordia plot of medium-grained granitic rock from the batholith interior................................15 6. Concordia plot of medium-grained granitic rock showing upper-intercepts....................................16 7. Foliation and structure in quartz diorite orthogneiss . 20 v Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INTRODUCTION This research follows petrological and structural studies of the northeastern border zone of the Idaho batholith and is a continuation of U/Pb isotopic studies by Chase and others (1978) . The results may be compared with other past or ongoing chemical studies of the northern part of the Idaho batholith (Bitterroot lobe) and thus
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