University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 1977 Small laccoliths and feeder dikes of the northern Adel Mountain volcanics Celia Kathleen Whiting The University of Montana Follow this and additional works at: https://scholarworks.umt.edu/etd Let us know how access to this document benefits ou.y Recommended Citation Whiting, Celia Kathleen, "Small laccoliths and feeder dikes of the northern Adel Mountain volcanics" (1977). Graduate Student Theses, Dissertations, & Professional Papers. 7117. https://scholarworks.umt.edu/etd/7117 This Thesis is brought to you for free and open access by the Graduate School at ScholarWorks at University of Montana. It has been accepted for inclusion in Graduate Student Theses, Dissertations, & Professional Papers by an authorized administrator of ScholarWorks at University of Montana. For more information, please contact [email protected]. SMALL LACCOLITHS AND FEEDER DIKES OF THE NORTHERN ADEL MOUNTAIN VOLCAN ICS by C, Kathleen Whiting B.S., University of Redlands, 1974 Presented in partial fulfillm ent of the requirements fo r the degree of Master o f Science UNIVERSITY OF MONTANA 1977 Approved by: Chairman, Board o f Examiners nrad5atebchooi~ Date Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: EP37918 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. UMT Diassrtation PkAWâmg UMI EP37918 Published by ProQuest LLC (2013). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code uest* ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 -1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. BIRDTAIL BUTTE Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT Whiting, C. Kathleen, M.S., 1977 Geology Small Laccoliths and Feeder Dikes o f the Northern Adel Mountain Volcanics ( 74 pp.) . D irector: David D. A11 Potassium-rich basalts o f the upper Cretaceous Adel Mountain Volcanics form a thick sequence of flows and volcanic breccias intruded by radial dike swarms, laccoliths, and stocks. Complex dikes radiating from multiple centers approximately 4 miles to the west o f the center proposed by Lyons (1944) and Beall (1973), feed two sets of laccoliths arrayed along circular arcs north of the main volcanic pile. Lyons (1944) and Beall (1973) showed that the northernmost la cco lith s are emplaced in the lower part o f the upper Cretaceous Virgelle sandstone along an arc with a radius of about 20 miles. These bodies are more than h mile in diameter and relatively s ill­ like. Four smaller laccoliths arrayed along a concentric arc with a radius of about 13 miles are steep-sided, their thickness being greater than their diameter. These are emplaced along the unconformable Interface between the Two Medicine Formation and the overlying Adel Mountain volcanic p ile . The laccoliths and their feeder dikes contain prominent euhedral phenocrysts of diopsidic augite and less prominent phenocrysts of plagioclase and either biotite or olivine. Surrounding the pheno­ crysts is a fine-grained groundmass of anorthoclase, plagioclase, pyroxene, and magnetite, altered in some cases to a complex assem­ blage of zeolites. Blebs of magnetite and apatite, apparently re­ flecting phase immiscibility, are pervasive. Pétrographie analy­ sis of porphyritic textures indicates that pyroxene phenocrysts crystallized prior to intrusion of the dikes. These phenocrysts, however, show no flow features. The lack of flow differentiation features may in part be explained by turbulence during magma in­ je c tio n . 11 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGMENTS My deepest thanks to David A lt whose Interest and enthusiasm generated so many interesting discussions during the course of this work, I also wish to thank Dr. Don Hyndman»who provided fresh in­ sights on several occasions. Dr. Tom Margrave, who read the final manuscript, and Larry Williams for his assistance with the petrology. The ranchers of the St. Peter area of Montana are gratefully acknowledged fo r th e ir h o s p ita lity . I also thank J. R. McBride for his special interest in this project. m Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS Page ABSTRACT ............................................................................................................. ü ACKNOWLEDGMENTS i i i LIST OF ILLUSTRATIONS.................................................................................... vi LIST OF TABLES.................................................................................................v i i i CHAPTER I INTRODUCTION................................................................................... 1 Previous Work and Purpose of th is Stu d y ............................ 6 I I GENERAL GEOLOGY ................................................................................ 9 Sedimentary Rocks ....................................................................... 9 Eruptive Rocks ........................................................................... 13 Structure of Volcanic P ile ................................................... 15 Intrusive Rocks ........................................................................... 16 D ik e s ................................................................................................. 17 L a c c o lit h s .....................................................................................19 I I I PETROGRAPHY OF THE INTRUSIVE ROCKS...........................................33 General Description and D istrib u tio n ....... 33 Descriptive Mineralogy ...................................................... 38 A Possible Carbonatite in the AdelMountains? . 48 T e x tu re .............................................................................................51 Problems Related to Texture .............................................. 52 iv Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS (Continued) Page IV PETROGRAPHIC PROVINCE OF CENTRAL MONTANA..................................... 59 Relation of Adel Mountain Volcanics to the Central Montana Pétrographie Province ........................................... 62 V SUMMARY................................................................................................... 67 REFERENCES............................................................................................................. 69 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF ILLUSTRATIONS Figure Page 1. Location map of Adel Mountain Volcanics and Study Area . 1 2. Geologic Map of Adel MountainVo lc a n ic s ............................... 3 3. Geologic Map of Northern Adel M ountains ................................ 4 4. Igneous and Tectonic Features Associated with Lewis and Clark Lineaments ....................................................................... 5 5. Geologic Map of the Intrusive Rocks of the Adel Mountain Volcanics ......................................... 10 6. Major Structural Elements of North Central Montana . 11 7. Geologic Map of Area Remapped by A u t h o r .................. 20 8. Square B u t t e ............................................................................. 22 9. B e a ll's (1973) Suggested Mechanism fo r Radial Extension of Country Rocks .......................................................... 24 10. Block Diagram Showing the Relationship of Intrusive Dikes and Laccoliths to the Sedimentary Formation . 25 11. Formation of Small Laccoliths .................................................. 27 12. G.K. Gilbert's (1877, p. 28) Hypothetical Ideal L a c c o l it h ................................................................................. 28 13. Block Diagram Illustrating that the Formation of a Laccolith Causes a Cylindrical Block of Overlying Layers to Move Upward ......................................................... 31 14. Double Triangular Diagram Showing the Representative Compositions of the Alkali-rich Trachydolerite of the Northern Adel Mountain Volcanics ...................................... 34 15. Olivine-bearing Trachydolerite ............................................. 36 16. Sector and Fine Oscillatory Zoning in Salite Phenocrysts 39 17. Complex Intergrowths of Clinopyroxene Phenocrysts . 40 vi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF ILLUSTRATIONS (Continued) Figure Page 18. Clinopyroxene Phenocrysts Mantled by Olivine Crystals . 42 19. Magnetite Blebs with Embedded Apatite Crystals .................. 46 20. Groundmass Anorthoclase and Plagioclase Microlites . 53 21. Sketch Map o f Cretaceous-Eocene Igneous Rocks o f Central Montana .................................................................•.....................................60