University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 1979 The Skalkaho pyroxenite-synite complex east of Hamilton Montana and the role of magma immiscibility in its formation Jeffrey J. Lelek 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 Lelek, Jeffrey J., "The Skalkaho pyroxenite-synite complex east of Hamilton Montana and the role of magma immiscibility in its formation" (1979). Graduate Student Theses, Dissertations, & Professional Papers. 7130. https://scholarworks.umt.edu/etd/7130 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]. THE SKALKAHO PYROXENITE-SYENITE COMPLEX EAST OF HAMILTON, MONTANA, AND THE ROLE OF MAGMA IMMISCIBILITY IN ITS FORMATION by Jeffrey J. Lelek A.B., Dartmouth College, 1977 Presented in partial fulfillment of the requirements for the degree of Master of Science UNIVERSITY OF MONTANA 1979 Approved by /c/CU V jJy\Lc / î ''Chairman, Board of Examiners Dean, Graduate Scho< Date 7f UMI Number: EP37931 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 UMI EP37931 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 ProQuest ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 - 1346 ABSTRACT Lelek, Jeffrey J., M.S., Spring, 1979 Geology The Skalkaho Pyroxenite-Syenite Complex East of Hamilton, Montana, and the Role of Magma Immiscibi1ity in its Formation (130 pp) Director: Dr. Donald W. Hyndman Various lines of evidence suggest that the bimodal rocks of the Skalkaho igneous complex east of Hamilton, Montana formed from co­ existing immiscible magmas. Pyroxenites vary from nearly pure (>98%) sali te, to biotite (vermiculite) pyroxenite, to amphibole pyroxenite rich in sphene, apatite, and magnetite. These pyroxenites are juxta­ posed against syenites. Miarolitic cavities in an unusual pegma­ tite of anhydrous mineralogy attest to the shallow emplacement of the complex. Based on field relations, mineralogical similarities, and chemical considerations, the regionally uncommon pyroxenites and syenites appear both comagmatic and contemporaneously emplaced. Fractional crystallization as a differentiation mechanism may be ruled out on the basis of textural evidence such as the absence of cumulate-intercumulate phases, structural evidence such as the lack of layering, and elemental behavior. Differentiation of the two systems through the process of liquid immiscibility seems indicated. The bimodal, comagmatic, and contemporaneous nature of the rocks is compatible with such an origin. The complex is alkaline, as are many rock examples cited to illustra te im m iscibility. Parameters seen to enhance experimental immiscibility are evidenced at Skalkaho. These include high Ti, P, and K, high K^O/total alkalies, low pressure of emplacement, and probably moderate to high volatile con­ tents. Partitioning of Ti, P, and REE into the mafic fraction is consistent with the operation of liquid immiscibi 1ity as opposed to fractional crystallization. Finally, the rocks plot near opposite ends of the determined immiscibility field on the join fayalite- 1eucite-si1ica (Roedder, 1951) with potential tielines paralleling those seen to exist between known immiscible factions. 11 ACKNOWLEDGMENTS Special thanks go to Don Hyndman and Dave A lt, two believers whose excellent guidance was crucial to the evolution of this project. Discussions with Keith Osterheld, B ill Beyer, Gil Wiswall and Jim Harrington provided enlightening insights. I would also like to thank the Western Vermiculite Company for access rights and valuable information, and Lesa Anderson, whose undying patience and everpresent encouragement were invaluable to the completion of this effort. 111 TABLE OF CONTENTS Page ABSTRACT....................................................................................................... ii ACKNOWLEDGMENTS..................................... i i i LIST OF FIGURES............................................................................................. vii LIST OF PLATES............................................................................................ ix LIST OF TABLES............................................................................................ ix CHAPTER I. INTRODUCTION ................................................................................ 1 General Statement .............................................................. 1 Location of Skalkaho Igneous Complex ......................... 3 Previous Work, Mining History, and This Study . 5 II. GENERAL GEOLOGY ....................................................................... 7 Regional Setting .................................................................. 7 General Geology of the Skalkaho Complex ................. 10 Field R e la tio n s .................................................................. 13 I II. PETROGRAPHY................................................................................ 20 Pyroxenite ............................................................................... 20 Anhydrous pyroxenite ...................................................... 20 Mica pyroxenite .............................................................. 26 Amphibole pyroxenite ...................................................... 28 Paragenesis ....................................................................... 34 i V CHAPTER Page Syenitic Rocks ...................................................................... 38 Alkali feldspar syenite .............................................. 38 Syen i t e ............................................................................... 39 Paragenesis ...................................................................... 41 Hybrid Rocks .......................................................................... 42 P e g m a tite .............................................................................. 43 Apatite phase .................................................................. 47 Carbonatite ( ? ) .................................................................. 50 Trachyte ................................................................................... 53 F e n ite ....................................................................................... 54 IV. CHEMISTRY................................................................................... 57 V. PETROGENESIS OF THE COMPLEX............................................. 62 Comagatism and Contemporaneity ..................................... 62 Magmatic Differentiation .................................................. 64 Fractional crystallization ......................................... 64 Movement of volatiles .................................................. 67 Current Status of Liquid Immiscibility ..................... 68 Hon-silicate-si 1icate systems ................................. 68 S ilicate-silicate experimental work ......................... 71 The system K 20-Fe 0-Al203-SiO2 ................................. 71 Effects of added constituents ................................. 77 Elemental partitioning .................................................. 82 Mechanics of immiscible separation ......................... 88 V CHAPTER Page Lunar Examples ...................................................................... 89 Terrestrial Examples .......................................................... 91 Basaltic late-stage immiscibility ......................... 91 Basaltic early immiscibility ..................................... 93 Alkaline systems .............................................................. 97 Liquid Immiscibil ity at Skalkaho Mountain .... 102 Chemical su ita b ility ...................................................... 102 Intermediate rocks .......................................................... 108 Shallow nature .................................................................. 112 Similar plutons .............................................................. 114 VI. SUMMARY AND CONCLUSIONS...........................................................120 REFERENCES CITED ....................................................................................... 122 VI LIST OF FIGURES Figure Page 1 Location map .............................................................................................4 2 Geological map of the Skalkaho complex .....................................11 3 Sketch of trachytoid texture ....................................................... 15 4 Sketch showing distribution of mica-rich zones in pyroxenite ..............................................................................................15