The Alkalic intrusions of Garrison Montana: A possible extension of the Central Montana Alkalic

183 Pages·2017·8.74 MB·English

UUnniivveerrssiittyy ooff MMoonnttaannaa
SScchhoollaarrWWoorrkkss aatt UUnniivveerrssiittyy ooff MMoonnttaannaa
Graduate Student Theses, Dissertations, &
Graduate School
Professional Papers
2003
TThhee AAllkkaalliicc iinnttrruussiioonnss ooff GGaarrrriissoonn MMoonnttaannaa:: AA ppoossssiibbllee eexxtteennssiioonn
ooff tthhee CCeennttrraall MMoonnttaannaa AAllkkaalliicc PPrroovviinnccee
Rebecca Sylvan Kunz
The University of Montana
Follow this and additional works at: https://scholarworks.umt.edu/etd
Let us know how access to this document benefits you.
RReeccoommmmeennddeedd CCiittaattiioonn
Kunz, Rebecca Sylvan, "The Alkalic intrusions of Garrison Montana: A possible extension of the Central
Montana Alkalic Province" (2003). Graduate Student Theses, Dissertations, & Professional Papers. 4675.
https://scholarworks.umt.edu/etd/4675
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].
Maureen and Mike
MANSFIELD LIBRARY
The University of
Montana
Permission is granted by the author to reproduce this material in its entirety,
provided that this material is used for scholarly purposes and is properly
cited in published works and reports.
**Please check "Yes" or "No" and provide signature**
Yes, I grant permission
No, I do not grant permission
Author's Signature:.
Date:_
Any copying for commercial purposes or financial gain may be undertaken
only with the author's explicit consent.
8/98
THE ALKALIC INTRUSIONS OF GARRISON, MONTANA:
A POSSIBLE EXTENSION OF THE CENTRAL MONTANA ALKALIC
PROVINCE
By
Rebecca Sylvan Kunz
B.S. Ft. Lewis College, Durango, Colorado, 1996
Presented in partial fulfillment of the requirements
for the degree of
Master of Science
The University of Montana
2003
Approved by
3
aid W. Wyi
Dean, Graduate School
l&/c>3
Date
UMI Number: EP40139
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.
UMI EP40139
Published by ProQuest LLC (2014). 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 LLC.
789 East Eisenhower Parkway
P.O. Box 1346
Ann Arbor, Ml 48106- 1346
ABSTRACT
Kunz, Rebecca Sylvan, M.S., December 2003 Geology
The Alkalic Intrusions of Garrison, Montana: A Possible Extension of the Central Montana Alkalic
Province
Director: Donald W. Hyndman
Dikes and sills intruding late Cretaceous sedimentary rock surrounding Garrison, Montana are basalts
and diorites, which differ in composition from the rocks found to the east and southeast in the Boulder
batholith. The Boulder batholith is a calc-alkaline center associated with Cretaceous subduction. The
Garrison rocks are alkalic in composition, more closely resembling those found in the Central Montana
Alkalic Province, located 65 km to the northeast. The Adel Mountains Volcanic Held is the closest igneous
center associated with the province to the Garrison area. The Garrison rocks are late Cretaceous and were
intruded at approximately the same time as both the Adels and the Boulder batholith.
Most of the Garrison rocks are too altered for comprehensive chemical analysis, but contain intact
minerals that reveal some associations. Microprobe data on augite and plagioclase from the Garrison dikes
and sills, the Adels, and several mafic units associated with the Boulder batholith reveal that the Garrison
intrusions are closer in composition to the Boulder batholith rocks, but are still not closely related to either
igneous center. The basalts and diorites in the Garrison basin appear to be closely related to each other.
Whole-rock chemical analysis of three diorites reveal close chemical tics to the Kokoruda Ranch
complex, a shonkinitic pluton near the Boulder batholith. The Kokoruda complex and the Garrison rocks
appear to be from a source separate from the Boulder batholith and the Adel Mountains Volcanic center.
The presence of alkalic magmas on the west side of the Boulder batholith, outside the recognized boundary
of the Central Montana Alkalic Province, indicates that the processes that formed the province may be
active in the Rocky Mountain thrust-fold belt.
The placement of the Garrison dikes and sills also constrains the age of deformation associated with the
thrust-fold belt. A late Cretaceous age determined for one of the diorites and evidence of deformation of the
dikes and sills reveals that the intrusive episode occurred during deformation and may be an important key
in unveiling the local tectonic history.
ACKNOWLEDGEMENTS
I want to thank my committee members Don Hyndman, Jim Sears and Garon Smith for their time
and effort in helping me complete this project. Don provided much needed funds and time for the chemical
analyses and microprobe and Jim for extra advice when I needed it the most. I would also like to thank
Diane Johnson and Scot Cornelius of Washington State University for all their help and advice. Last, but
not least, I am grateful to Erik Katvala, Erik Burtis, Chris Hawkins, Nate Harrison, Ryan Portner and
Rachel McCool and the rest of the geology department for their help and support. They were always
willing to help with suggestions, from valuable to highly amusing, and timely distractions.
TABLE OF CONTENTS
ABSTRACT............................................................................................................................................................................ii
ACKNOWLEDGEMENTS..................................................................................................................................................iii
TABLE OF CONTENTS....................................................................................................................................................iv
LIST OF FIGURES AND TABLES..................................................................................................................................v
INTRODUCTION.................................................................................................................................................................1
Setting..........................................................................................................................................................................2
The Boulder Batholith and the Elkhorn Mountains Volcanics.............................................. 4
Flint Creek Range....................................................................................................................................................7
Adel Mountains Volcanics......................................................................................................................................7
Central Montana Alkalic Province......................................................................................................................8
Other Alkalic Intrusions........................................................................................................................................10
PREVIOUS WORK............................................................................................................................................................12
METHODS..........................................................................................................................................................................13
RESULTS.............................................................................................................................................................................18
MINERALOGY................................................................................................................................................................19
Mafic Intrusions......................................................................................................................................................19
Intermediate to Felsic Intrusions.....................................................................................................................25
Elkhorn Mountains Volcanics basalt...............................................................................................................27
Dunkleberg Ridge dike.........................................................................................................................................28
CHEMISTRY..................................................................................................................................................................33
Mineral.......................................................................................................................................................................33
Whole-rock...............................................................................................................................................................67
DISCUSSION...................................................................................................................................................................109
Models......................................................................................................................................................................109
The dikes and sills.............................................................................................................................................110
Relationship to the Boulder batholith and Adel Mountains Volcanic center...............................113
Timing.......................................................................................................................................................................120
Central Montana Alkalic Province and mantle..........................................................................................124
Magma mixing or single source model.........................................................................................................126
CONCLUSIONS..............................................................................................................................................................127
REFERENCES..................................................................................................................................................................130
Appendix A: Microprobe Results.............................................................................................................................136
Appendix B:....................................................................................................................................................................158
Appendix C: XRF Results............................................................................................................................................165
Appendix D:....................................................................................................................................................................167
iv
LIST OF FIGURES AND TABLES
Figure 1: Tectonic map..................................................................................................................................... 3
Figure 2: Regional map..................................................................................................................................... 6
Figure 3: Central Montana Alkalic province.............................................................................................. 9
Figures 4a-b: Local geological maps........................................................................................................... 15
Table 1: Minerals................................................................................................................................................. 21
Figure 5: Augite ternary diagram....................................................................................................................35
Figures 6a-i: Augite microprobe graphs.......................................................................................................36
Figures 7a-i: Augite cores vs. rim................................................................................................................. 46
Figure 8: Plagioclase ternary diagram........................................................................................................ 52
Figures 9a-f: Plagioclase microprobe graphs..............................................................................................53
Figures lOa-i: Amphibole microprobe graphs.......................................................................................... 58
Figure 11: Orthopyroxene ternary diagram.............................................................................................. 62
Figures 12a-i: Orthopyroxene microprobe graphs....................................................................................63
Figure 13: High-K graph.................................................................................................................................. 69
Figure 14: K-series graph...................................................................................................................................70
Figures 15a-i: Whole-rock Si02 graphs.........................................................................................................71
Figures 16a-i: Whole-rock Mg number graphs........................................................................................ 76
Table 2: Normative minerals.......................................................................................................................... 82
Figures 17a-q: Trace-element Mg number graphs...................................................................................84
Figures 18a-q: Trace-element Si02graphs................................................................................................ 93
Figures 19a-e: Trace-element ratio graphs................................................................................................105
Figure 20: Chondrite-normalized element graph.....................................................................................108
V
INTRODUCTION
This thesis investigates the petrology of a little-studied group of dikes and sills located along the Lewis
and Clark Line near Garrison, Montana. The area is bordered by the Purcell anticlinorium to the north and
the Lombard thrust plate to the south. Preliminary work on the Garrison intrusions by Sears et al. (1999,
2000a) indicated that two magma sources influenced the dikes and sills. They noted that a series of
andesitic intrusions became less alkaline and more calc-alkaline to the south. The Boulder batholith,
southeast of the Garrison intrusions, is calc-alkaline in composition and is considered to be subduction
related. About 60 km to the northeast is the edge of the petrographic province described by Larsen (1940)
as the Central Montana Alkalic Province. Mingling and mixing of magmas from the two regions has been
found in satellite plutons along the eastern margins of the Boulder batholith.
The purpose of this study is to determine the magmatic association of the intrusions and see if they are
the result of magma mingling and mixing, or if they are derived from just one source. If there is a relation
to the Central Montana Alkalic Province, then the traditionally recognized boundaries of the province will
need to be reconsidered, possibly including the northern part of the Boulder batholith. The batholith has a
higher K20 content in the northeast, which is closer in composition to the Central Montana Alkalic
Province. Intrusions associated with the batholith to the south and west have a higher Na20 content and are
more chemically similar to other calc-alkaline batholiths in the region. The emplacement of the Boulder
batholith coincided with late Cretaceous to Eocene thrusting in the area and the location of the intrusions
along the Lewis and Clark Line is important in constraining the timing of the deformation. As little work
has been done on the dikes and sills, this study will provide more information for deciphering the timing of
tectonic events in the area.
1
Setting
The majority of the Garrison dikes and sills lie within 5 km of the Little Blackfoot and Clark Fork
rivers, Powell County, Montana. Sills to the northwest were not included due to time and monetary
constraints. The Garrison intrusions cut late Cretaceous sedimentary rocks in the vicinity of Garrison. The
oldest exposed unit cut by the intrusions is the Cretaceous Kootenai Formation. Most of the intrusions are
found in the upper Cretaceous Colorado Group. Most of the Colorado Group was deposited in a near-shore
to terrestrial fluvial environment. Sediments were derived from predominantly western sources, where
uplift was taking place as a result of tectonic forces related to the Farallon-North America plate collision
(Wadell, 1997). The youngest unit cut by the intrusions is the Campanian to Maastrictian Golden Spike
Formation. This unit consists of fluvial sandstone and conglomerate derived from Precambrian to
Cretaceous rocks to the west, and volcanic rocks to the southeast. The Golden Spike Formation was
deposited concurrently with the eruptions of the upper Cretaceous Elkhorn Mountains Volcanics and
contains a number of lava flows and lahar deposits. The volcanic and volcaniclastic rocks are intertongued
with non-volcanically derived units and thicken to the southeast in the direction of the Boulder batholith.
The lava flows preserved in the Garrison depression are andesites that correlate with the lower Elkhorn
Mountains Volcanics (Gwinn and Mutch, 1965; Wadell, 1997).
The Garrison intrusions straddle the Lewis and Clark Line, a Precambrian lineament that extends
from Wallace, ID, to east of Helena, MT (Figure 1). It was reactivated during late Cretaceous to Eocene
deformation and became a shear zone accommodating differential motion between the Lewis-Eldorado-
Hoadley thrust slab to the north and the Lombard tectonic plate to the south. Its expression on the surface is
a series of en echelon southeast-trending folds. The width of the Lewis and Clark Line ranges from about
30-50 km (Waddell, 1997; Sears, 1996). In this varied tectonic environment, the Garrison dikes and sills
could be related to one or both of the igneous centers in the region: the Boulder batholith and associated
Elkhorn Mountains Volcanics, and the Adel Mountains volcanic field, which is part of the Central Montana
Alkalic Province.
2