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This is to certify that the

thesis entitled

Petrogenesis of the Keetley Volcanics in
Summit and wasatch Counties, North-Central

 

  

 

Utah
presented by
LeeAnn Feher
has been accepted towards fulfillment
of the requirements for
M. S- degree in W
Major professor / (
4-7-97

Date

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MSU Ie An Affirmative ActiorVEqueI Opportunlty InethIon
Wane-m

 

PETROGENENIS OF THE KEETLEY VOLCANICS, IN SUMMIT AND WASATCH
COUNTIES, NORTH-CENTRAL UTAH

by

LeeAnn F eher

A THESIS
Submitted to
Michigan State University
in partial fiilfillment of the requirements

for the degree of

MASTER OF SCIENCE

Department of Geological Sciences

1997

ABSTRACT

PETROGENESIS OF THE KEETELY VOLCANICS, IN SUMMIT AND
WASATCH COUNTIES, NORTH-CENTRAL UTAH

by

LeeAnn Feller

The Oligocene Keetley volcanics (KV) are located in north-central Utah and occur
on the eastern end of the 45-kilometer long, east-west trending, Wasatch intrusive belt
(WIB). The WIB is part of the Cottonwood Arch and occurs along the extension of the
Uinta Arch.

The KV and the Wasatch intrusive rocks are a series of high-K, calc-alkaline
rocks. The WIB consists of a western group and an eastern group of stocks. The western
stocks consist of three chemically distinct stocks; the Little Cottonwood, Alta and Clayton
Peak, the eastern stocks are chemically similar to the Alta stock. The KV are similar to
the eastern stocks and the Alta stock with respect to emplacement ages and chemical
composition. The chemical composition of the KV ranges from 55 wt. % SiOz to 68 wt.
% SiOz and there is little variation in mineralogy In comparison with other well-
documented calc-alkaline rocks the KV do not follow typical, coherent calc-alkaline
chemical trends nor do they follow simple crystal fractionation and/or assimilation paths,
thus the origin of the chemical variation is most likely due to complex processes involving
multiple magmatic sources. The origin of the WIB and KV is probably related to partial
melting of the crust due to production of decompression mantle melts resulting from the

rollback of the Farallon plate which occurred in the Cordillera beginning 50 Ma.

For T and Mick.

iii

ACIGVOWLEDGEMENTS

Thanks Mom and Dad. I couldn’t have done this without your support. Lisa, my
sister, thank you for all the laughs when I needed them and all the cat care-taking that
allowed me to go on my adventures. Special thanks to Candice Murphy, my very best
fiiend in the whole world.

I thank Tom Vogel, my advisor, and Kathy Fishbum for all their support since my
arrival at MSU. Thanks Tom for being an excellent role model and for all your guidance, '
especially in my moments of foundering. Bill Carnbray and Kaz Fugita, my committee
members are thanked for their input and reviews of this thesis.

I would also like to thank my original inspirations, Tim Flood and Nelson Ham

who got me started on all this geology in the first place.

iv

TABLE OF CONTENTS

LIST OF TABLES ............................................................................ ' ..... vi i
LIST OF FIGURES ................................................................................ viii
INTRODUCTION ................................................................................... 1
Previous work ............................................................................... 4
Purpose and Regional Geology ............................................................ 6
Methods ...................................................................................... 9
WASATCH INTRUSIVE BELT WHOLE ROCK CHEMISTRES AND AGES ........ ll '
Major and trace element chemical compositions ....................................... II
Summary of the mineralogy and textures of the Wasatch intrusive belt ............ 14
Ages .......................................................................................... 14
CHEMICAL COMPOSITIONS AND MINERALOGY OF THE KEETLEY
VOLCANICS .............................................................................. 16
Whole rock chemical compositions of the Keetley volcanics ......................... l6
Mineralogy of the Keetley volcanics ..................................................... 22
Petrography .................................................................................. 22
Electron Microprobe Analysis ............................................................. 35
Pyroxene therrnometry ..................................................................... SO
Homblende geobarometry ................................................................. 55
Assimilation and fractional crystallization models ...................................... 57
DISCUSSION ....................................................................................... 61
Chemical and age relationships between the Keetley volcanics and
the Wasatch intrusive belt ...................................................... 61
Evaluation of the effects of Pm on the evolution of the Keetley volcanics ....... 6]
Reverse zoning in plagioclase ............................................................ 64

Evaluation of petrologic processes to produce the chemical variation within the
Keetley volcanics; fractional crystallization, magma mixing and crustal
assimilation ........................................................................ 67

Fractional crystallization ................................................ 69

Magma mixing ........................................................ 73

Assimilation and fractional crystallization ........................ 73
Origin of high-K calc-alkaline rocks and a possible petrogenetic
model for the evolution of the Keetley volcanics ......................... 75
CONCLUSIONS ............................................................................... 81
APPENDD( A ................................................................................... 83
APPENDIX B ................................................................................... 84
BIBLIOGRAPHY .............................................................................. 9O

Table 1

Table 2

Table 3

Table 4

Table 5

Table 6

Table 7

Table 8

Table 9

Table 10

Table I 1

LIST OF TABLES

Ages for the Wasatch Intrusive Belt, including the Keetley Volcanics.....15

Whole rock major element XRF analyses ...................................... 84
Whole rock trace element XRF analyses ....................................... 87
Modal point count analyses shown in percent ................................ 25
Representative plagioclase phenocryst microprobe analyses ................ 40
Representative amphibole phenocryst microprobe analyses ................. 51 .
Representative pyroxene phenocryst microprobe analyses .................. 53
Representative biotite phenocryst microprobe analyses ..................... 54

Temperatures estimated from two-pyroxene thermometry
based on end-member compositions ........................................... 56

Comparison of whole rock CIPW norm calculations of the
Keetley volcanics vs. the Fish Canyon Tufl‘. Keetley volcanic
samples are those used to calculate hornblende geobarometry ............ 58

Multiple linear regression models of Assimilation and Fractional
Crystallization for the Keetley volcanics ....................................... 59

vii

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

LIST OF FIGURES

Location of the Wasatch Intrusive Belt and the Keetley volcanics

in north-central Utah. Boxed area is the map area for John’s map
(1989). The Keetley volcanics are the southern most group of

Tertiary volcanics. Modified from John (1989). PC = Pine Creek,

A = Alta, WF = Wasatch Front, CN = Charleston-Nebo, DC = Deer
Creek .............................................................................. 2

Close up of the Wasatch Intrusive Belt, illustrating the nine stocks

as well as the Park Premier stock, the Indian Hollow plug is ofl‘ the

map area just to the northeast of the Park premier stock. The Park
Premier stock and the Indian Hollow plug intrude the Keetley

Volcanics. Modified from John (1989). The area in the smaller
rectangle outlines the 8 stocks numbered from 1 through 8 ................ 3

Schematic cross-section illustrating locations where estimated depths

of emplacement were made from west to east along the Wasatch
Intrusive Belt. The Little Cottonwood was emplaced at 11 km

and the eastern most stocks were emplaced at about 1 km.

From John (1989) ................................................................ 5

Some major and trace element chemical compositions for the

Western stocks. Open crosses are the Little Cottonwood,

open triangles are the Alta and filled triangles are the Clayton

Peak ............................................................................... 12

Some major and trace elements chemical compositions for

the Eastern stocks. Open circles are Flagstaff, filled squares are
Mayflower, open squares are Valeo, open diamonds are Pine Creek

and filled diamonds are for Ontario .......................................... 13

High-K calc-alkaline classification of the Keetley volcanics

based on divisions after McBirney (1993). Symbols for Keetley
volcanics are filled squares for Jordanelle Dam location, open

squares for Francis, open diamonds for Indian Hollow, filled

triangles for Peoa and open triangles for 1-80 ............................... l7

viii

Figure 7.

Figure 8.

Figure 9.

Figure l0.

Figure l la-b.

Figure 12a-b.

Figure 13a-b.

Figure 14a-b.

Figure 15a-b.

A typical AFM calc-alkaline difi‘erentiation trend (after Hess(l989))
plotted with the chemical trend of the Keetley volcanics.

CA = calc-alkaline. Symbols for the Keetley volcanics

are the same as in Figures 6 .................................................... 18

Comparison of high-K calc-alkaline volcanic rocks from the

Central Volcanic Zone (CVZ) of the Andes in northern Chile,

north-west Argentina, and south-west Bolivia (shaded area) to

the Keetley volcanics. Data on the CVZ is from Wilson (1989).

Most of the Keetley volcanics span the entire compositional range

of the volcanic systems within the high-K calc-alkaline volcanics

from the CVZ. Symbols for the Keetley volcanics are the same

as in Figure 6 ..................................................................... 19

Major element variation diagrams for the Keetley volcanics.

Note the large variation in MgO, T102, A1203 and the alkalis

with a small variation in SiOz. Symbols for the Keetley volcanics

are the same as in Figure 6 ..... ; ............................................... 20

Trace element variation diagram 5 for the Keetley Volcanics.
All trace elements show scatter at constant 8102. Symbols are the
same as in Figures 6 ............................................................. 23

Photomicrograph of sample 17—23 III, in plane polarized light a)

and with crossed polars b). Note oscillatory zoning of hornblende

as well as a near parallel alignment of the phenocrysts. Field of view

is 4.4 mm across ................................................................ 27

Photomicrograph of sample 20-3 Fran, in plane polarized light a)

and with crossed polars b). The hornblende phenocryst is rimmed

by smaller grains of clinopyroxene. Note the small grains of
clinopyroxene in the matrix. Field of view is 4.4 mm across ............. 29

Photomicrograph of sample 20-3 Fran under higher magnification

in plane polarized light a) and with crossed polars b). Note the
unaltered clinopyroxene surrounding the partially altered/reacted
hornblende phenocryst. Field of view is 1.2 mm across .................. 31

Photomicrograph of sample 19-3b JD, in plane polarized light a)

and with crossed polars b). At the top is a phenocryst of hornblende
that has reacted and is in disequilibrium along side a fresh, unaltered
clinopyroxene. Field of view is 1.2 mm across ............................ 33

Photomicrograph of sample 20-18 [-80, in plane polarized light a)
and with crossed polars b). Notice the two grains of quartz on the

ix

lefi and right-hand sides. Both of these grains exhibit resorbed and
embayed textures ............................................................... 36

Figure 16a-b. Photomicrograph of sample 17-2 III, in plane polarized light a) and

Figure 17.

Figure 18.

Figure 19.

Figure 20.

Figure 21.

Figure 22.

Figure 23.

Figure 24.

Figure 25.

Figure 26.

crossed polars b). A large grain of K-feldspar makes up most of the
field of view. This grain is probably a xenocryst ........................... 38

Feldspar triangle including rim, middle and matrix plagioclase
phenocryst compositions. The one analysis at the Or apex is a
K-feldspar xenocryst ........................................................... 42

Anorthite variations within plagioclase rim analyses. Diamonds
are the maximum %An and the squares are the minimum %An
within each sample ............................................................. 43

Anorthite variations within plagioclase middle analyses. Diamonds
are the maximum %An and the squares are the minimum %An within
a sample ......................................................................... 45

Anorthite variations within plagioclase matrix anlayses. Diamonds
are the maximum %An and the squares of the minimum %An within
each sample ..................................................................... 47

F eldspar triangles showing reverse zoned plagioclase phenocrysts
of two representative samples ................................................ 49

Simple pyroxene quadrilateral including all pyroxene phenocryst
compositions including rims, cores and middles ............................ 52

Comparison of the chemical compositions of the Western stocks
(superimposed open areas) to the Eastern stocks. Symbols of the
Eastern stocks are the same as in Figure 5 .................................. 62

Comparison of the chemical compositions of the Western stocks
(superimposed open areas) to the Keetley volcanics. Symbols for
the Keetley volcanics are the same as in Figure6 ........................... 63

Theoretical plagioclase feldspar phase diagram illustrating the effects
of varying pressure, where P2>P1>P3 ........................................ 66

Typical differentiation trends of calc-alkaline rocks erupted within
continental crust, 1 is for Lassen Peak, northern California and 2 is for
Crater Lake National Park, Oregon (Hyndman, 1985 and references
within). The Keetley Volcanics clearly do not follow the typical calc-

Figure 27a-d.

Figure 28.

alkaline differentiation trend. Symbols for the Keetley volcanics
are the same as in Figure 6 ..................................................... 68

Comparison of ,a typical calc-alkaline fractionation trend (shaded areas)
from the Mogollon-Datil volcanic field with the Keetley Volcanics.
Symbols for the Keetley volcanics are the same as in Figure

6 ................................................................................... 7O

Stages of change in the angle of subduction of the F arallon plate and
varying rates of convergence of the North American plate from 150

Ma to the present. Figure is modified from Cross and Pilger

(1978) ............................................................................ 7s

INTRODUCTION

The Oligocene Keetley volcanics are located in north-central Utah and occur on
the eastern end of the 45-kilometer long, east-west trending, Wasatch intrusive belt
(Figure 1) (John, 1989). The Wasatch intrusive belt is part of the Cottonwood Arch and
is continuous with the western extension of the Uinta Arch. The Keetley volcanics and the
Wasatch intrusive rocks are a series of high-K, calc-alkaline rocks. These are divided into
two groups of stocks and a sequence of volcanic rocks based on the relative location and
differences in texture among the stocks. The western stocks are coarse-grained, mostly
equigranular and include the Little Cottonwood, Alta, and Clayton Peak stocks. The
eastern stocks are in general porphyritic, characterized by coarse—grained phenocrysts in a
fine- to medium-grained groundmass, and include the Flagstaff, Ontario, Mayflower,
Glencoe, Valeo, Pine Creek, and Park Premier stocks and the Indian Hollow plug. The
two latter stocks occur in the very eastern end of the area and intrude the Keetley
volcanics (Figure 2) (John, 1989). The Indian Hollow plug is not shown on Figure 2 but
occurs just to the north-east of the Park Premier stock. The Keetley volcanics are a high-
K andesitic sequence of lahars, volcanic conglomerates, tufi‘s and breccias, and minor lava
flows (Leveinen, 1984 ). The clasts within the Keetley volcanics are characterized by fine-
to medium-grained phenocrysts in a mostly fine-grained matrix. The Keetley volcanics
have an estimated maximum thickness of at least 90 m near the Jordanelle Dam just east of
Park City, Utah (Woodfill, 1972) and extend over 330 km2 of area in north-central Utah.
The Keetley volcanics have been deposited subhorizontally between the west-east trending

Uinta Arch to the east and the north-south trending Wasatch Range to the west.

 

 

 

 

 

 

 

 

 

 

Figure 1. Location of the Wasatch intrusive belt and the Keetley volcanics in north-
central Utah. Boxed area is the map area from John’s map (1989). The
Keetley volcanics are the southern most group of Tertiary volcanics.
Modified from John (1989). PC = Pine Creek, A = Alta, WF = Wasatch
Front, CN = Charleston-Nebo, DC = Deer Creek.

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Previous Work

John (1989) estimated the depths of emplacement of the Wasatch intrusive belt
based on fluid inclusion data and contact metamorphic mineral assemblages as well as
stratigraphic relationships. The estimated depths of emplacement progressively shallow
from west to east with an estimated depth of the Little Cottonwood stock at 11 km in the
west and of the eastern stocks at 1 km in the east (John, 1989) (Figure 3). The Keetley

volcanics are the eastern most extrusive component of this whole system.

Previously determined ages have shown that the stocks of the Wasatch intrusive
belt all have approximately the same age ranging from 30 to 34 Ma (Bromfield, 1977) and
(Crittenden, 1973). Detailed U/Pb dating of zircons (Constenius, pers. comm, 1996), and
4oAr/39Ar dating (Flood, pers. comm, 1996) of minerals are in progress and may give

more insight into the age relationships within the Wasatch intrusive belt.

Based on research in progress (Vogel, pers. comm, 1996) three distinct chemical
trends occur in the three western stocks (Little Cottonwood, Alta, and Clayton Peak). '
The eastern stocks, in a gross sense, are chemically similar to the Alta stock. A goal of
this study is to compare the chemical compositions of the Keetley volcanics and the
Wasatch intrusive belt. Woodfill (1972) presented chemical analyses on seven samples
from the Keetley volcanic field. From his work he concluded that the Keetley volcanics

were derived from a trachyandesite parent. This model will be evaluated in this study.

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Purpose

The purpose of this study is two-fold. Relationships between the Wasatch
intrusive belt and the Keetley volcanics will be determined by comparing chemical
compositions and ages. The second part of this thesis is to determine if the origin of the
compositional diversity of the Keetley volcanics was produced by difi‘erentiation of a
single or multiple magmatic sources. In order to do this, the chemical and mineralogical
variations will be determined and fi'actional crystallization, mixing, and assimilation models

will be evaluated.
Regional Geology

The Wasatch intrusive belt is located in the central Wasatch Mountains and is
considered to be part of the Cottonwood area (Crittenden, 1977). The Wasatch Range
extends from the center of Utah, northward into Idaho. The western edge of the range
marks part of the eastern limit of the Basin and Range province and the eastern edge,
which in many places is not well defined, merges with the Wyoming Basin and the
southern Colorado Plateau. The east-west trending Uinta Range separates the Wyoming
Basin to the north and the Colorado Plateau to the south. All of these structures are part
of the Cordilleran fold and thrust belt (Crittenden, 1977). The Wasatch intrusive belt also
lies at the boundary between Archean basement to the north and Proterozoic basement to
the south (Hutchinson and Albers, 1992). The junction between Archean and Proterozoic
crust forms a weakness at the Uinta trend which extends west of Colorado through the

Wasatch Mountains to central Nevada (Bryant and Nichols, 1988).

The Cottonwood area lies at the boundary of the intersection of the Sevier
orogenic belt and the east-west trending Uinta arch. Sedimentary units ranging from
Precambrian through Mesozoic cratonal and miogeoclinal sediments are intruded by the
nine Tertiary plutons of the Wasatch intrusive belt and as a result the sedimentary rocks
now dip away fiom the west-east trend of the intrusive belt to the north, northwest,
northeast, east and southeast, most notably near the Little Cottonwood stock and less
obviously around the other stocks (Boutwell, 1912). The Keetley volcanics lie
subhorizontally between the west-east trending axis of the Uinta Arch and the north-south
trending Wasatch Mountains where they cover any structural continuities that may exist

between the two major structures (Boutwell, 1912).

By Late Cretaceous time the convergent continental margin of western U. S. had
developed into an Andean type continental margin are system due to subduction of the
Farallon plate under the North American plate (Miller et al., 1992, and references therein).
During the Late Cretaceous through early Tertiary time, the Laramide and Sevier orogenic
events caused by the convergence of the Farallon plate and the North American plate
formed the Cordilleran foreland fold and thrust belt in Utah, Idaho and Nevada. The
Laramide orogeny took place from 80 to 50 Ma (Miller, 1992) and the Sevier orogeny
took place between 145 to 75 Ma (Armstrong, 1968). The eastern extent of the Sevier
orogeny is referred to as the hingeline. The deformation caused by the Laramide style
deformation occurred mainly within the Colorado Plateau to the east of the hingeline.
Predominately thick-skinned deforrnational foreland uplifts and basins were produced in an
area with relatively thin sedimentary cover on Precambrian crystalline basement. The

Sevier orogenic event occurred to the west of the hingeline and produced thin-skinned,

decollement thrusting with folding being restricted to the thick miogeoclinal sedimentary
wedge and bordering cratonal strata that overlie the Precambrian basement (Miller, 1992).
More than half of the total shortening that occurred during the Sevier orogeny happened
simultaneously with the basement cored uplifis of the Laramide orogeny (Allmendinger,

1992). Hence, the two styles of deformation were occurring simultaneously for a time.

The Keetley volcanics are bounded by the eastward plunging Cottonwood Arch
and the westward plunging Uinta Arch. The Uinta Arch consists of Precambrian through
Mesozoic strata that forms a westward plunging anticline. The Uinta Arch was uplifted
during the Sevier orogeny and as a result is faulted on both flanks of the main anticline
(Crittenden, 1977). Thrust faults developed in the Cottonwood Arch during the Laramide
orogenic event. Prior to intrusion of the igneous rocks, large thrust faults and folds
formed due to west to east ramping of thick sequences of sedimentary rocks. These faults
have a present eastward dip but at the time of formation had a westward dip (Calkins,
1943). It is thought that the present eastward dip of these thrust faults was caused by

continued compression from the west that was resisted by the Uinta Arch (Eardley, I939).

The Cordilleran foreland fold and thrust belt has undergone a number of episodes
of extension, but the most notable extension occurred in the Cenozoic. The Cenozoic
extensional event formed what is known as the Basin and Range province. Extension
occurred mainly in those areas that underwent shortening during the Mesozoic and early
Cretaceous times. The general direction of extension was originally east-west and
eventually rotated to northwest-southeast. The extension was accommodated by high-

angle normal faulting and detachment faults (Wemicke, 1992). Constenius (1996) has

recently illustrated that the Cordilleran foreland fold and thrust belt was reactivated due to
a gravitational collapse caused by changes in the rate and style of North America-Pacific
plate convergence. A change fi'om compression to extension began with the westward
rollback of the Farallon plate beginning at ca. 49-48 Ma and ending ca. 20 Ma
(Constenius, 1996). One local efi‘ect of the change fi'om compression to extension noted
by Constenius (1996) is illustrated by the collapse of the Charleston-Nebo thrust sheet that

has been reactivated since compression and has moved westward.

The overall importance of the chemical and age relationships of the Keetley
volcanics and the Wasatch intrusive belt to the regional geology is how the timing of
emplacement of the magmas relates to the timing of the change fi'om compression to

extension in the Cordillera
Methods

Because the Keetley volcanics consist mainly of volcaniclastics, no stratigraphic
correlation within a field site or between field sites could be made. Therefore. the
sampling strategy that was undertaken was to collect as wide of a variety of volcanic clasts
as possible in order to sample the entire chemical variation within the volcanic sequence.
Approximately 200 samples were collected from five different locations within the Keetley
volcanic field. A total of 50 representative samples were chosen for analyses and added to

17 previously analyzed samples to give a total of 67 chemical analyses.

Several laboratory methods were used to obtain the necessary chemical and
mineralogical data for this study. Petrographic analysis of thin sections was done to

determine the mineralogy and textures. X-ray fluorescence spectrometry was used to

10

determine whole rock major element and select trace element concentrations on 67
samples. Electron microprobe analysis was done at the University of Indiana using a
Cameca Camabax SXSO electron microprobe to determine specific major element
chemical compositions of phenocrysts and some microphenocrysts in selected samples.
Statistical multiple linear regression to test fractional crystallization, mixing and
assimilation models were used to evaluate possible petrologic processes as explanations
for the chemical variation within the Keetley volcanics.

X-ray fluorescence techniques involved measuring 1.0 g of finely ground rock
powder, 9.0 g of Lithium Tetraborate and .160 g of Ammonium Nitrate. These materials
were then mixed and firsed into glass disks by melting in platinum crucibles for at least 20
minutes and subsequently poured into platinum molds and quenched. Each sample was
then analyzed for major and trace elements using a X-ray fluorescence spectrometer. The
detection limits and standard deviations for the trace elements as well as the trimmed mean
and standard deviations based on 16 repetitions for each major element are reported in
Appendix A.

Electron Microprobe analytical techniques involved obtaining two to three
analyses fi'om each of the selected phenocryst grains including plagioclase, hornblende,
pyroxene and biotite. Ten thin sections were chosen with 8 to 10 phenocrysts selected
from each for analysis. The thin sections were chosen to represent an array of the most
mafic, intermediate and most silicic end members out of the total sample array. In general,
rim compositions were obtained for all selected phenocrysts and in some cases, such as for
reversed zoned plagioclase, rim, middle, and core compositions were obtained. The probe

conditions varied depending on the mineral being analyzed. For mafic silicates a 15 kv

11

accelerating voltage and a 20 nA beam was used. The beam size was 2 microns. For
feldspars a 15 kv accelerating voltage and a 10 to 15 nA beam were used with a beam size

of 5 microns.

Wasatch Intrusive Belt Whole Rock Chemical Compositions, Mineralogy and Ages

Major and trace element chemical compositions

The Wasatch intrusive belt consists of nine stocks. These stocks are divided into
three western stocks, the Little Cottonwood, Alta, and Clayton peak; and six eastern
stocks, the Mayflower, Valeo, Flagstafi‘, Pine Creek, Ontario and Glencoe. All chemical
data for the western and eastern stocks are from Vogel (pers. comm, 1996) The western
stocks make up three distinct chemical groups based on major and trace element variation .
diagrams. The Little Cottonwood stock is the most silicic at about 70% $102, the Alta is
intermediate at 60 - 67% SiOz and the Clayton Peak is the most mafic at 55% - 60% SiOz.
Rb/ Sr - oxide trends as well as major and trace element variation diagrams illustrate the
three distinct chemical groups of the western stocks (Figure 4). The eastern stocks are
not as chemically diverse as the western stocks. They range in $102 from about 58% to
68% and can not be isolated from each other in terms of distinct chemical groups (Figure
5). In terms of Rb/Sr - oxide trends and major and trace element variation diagrams, it is
difficult to separate the eastern stocks from one another with the exception of the Ontario
stock which has the highest Rb/Sr content (Figure 5). In a broad comparison, the eastern

stocks are most similar to the Alta stock.

12

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14

Summary of the Mineralogy and Textures of the Wasatch Intrusive Belt
John (1989) summarizes the textures and mineralogy of the Wasatch

intrusive belt. In general the western stocks range from medium- to coarse-grained for the
Little Cottonwood to medium-grained for the Alta and fine-grained for the Clayton Peak.
Parts of the Little Cottonwood and the Alta are porphyritic. The eastern stocks range
from medium- to fine-grained with sedate porphyritic textures and groundmass ranging in
size from .01 - .25 mm overall (John, 1989). The Keetley volcanics are characteristically
porphyritic with fine to medium-grained phenocrysts.

In terms of mineralogy, the Little Cottonwood contains K-feldspar phenocrysts up
to 6 cm, quartz and biotite, magnetite and sphene with minor hornblende. The Alta stock
contains phenocrysts of K-feldspar, plagioclase, quartz and biotite, hornblende, magnetite,
sphene and minor amounts of clinopyroxene and illmenite. The Clayton Peak stock has
phenocrysts of K—feldspar from l-2 cm, quartz and rare phenocrysts of biotite and
contains clinopyroxene, biotite, magnetite and minor amounts of orthopyroxene, illmenite,
sphene and hornblende. The eastern stocks contain plagioclase, biotite, hornblende,
magnetite and quartz. The Park Premier also contains clinopyroxene, but no quartz.
Some of the eastern stocks contain sphene. The Keetley volcanics contain plagioclase,
hornblende, clinopyroxene, rare orthopyroxene, biotite and rare sphene as well as resorbed
quartz.

Ages

Ages for the Wasatch intrusive belt including the Keetley volcanics are listed in
Table 1. Biotite K/Ar ages (closing temperature 310°) of the Wasatch intrusive belt and

Keetley volcanics were determined by Bromfield and others (1977) and Crittenden and

15

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16

others (1973) are about 34 Ma for the eastern stocks, Keetley volcanics and the Clayton
Peak stock; 32 Ma for the Alta stock and 26 Ma for the Little Cottonwood stock. Biotite
K/Ar ages are significant for the eastern stocks and the Keetley volcanics because they
represent emplacement ages due to the quenched textures of these rocks. Zircon U/Pb
ages with a closing temperature of 750° (F eher et al., 1996) represent emplacement ages
as well and were determined to be 36 Ma for Clayton Peak stock, 33.5 Ma for the Alta

stock and 30.4 Ma for the Little Cottonwood stock.

Chemical Compositions and Mineralogy of the Keetley Volcanics

Whole Rock Chemical Compositions of the Keetley Volcanics

A total of 67 chemical analyses were done on the Keetley volcanics. The major
element and trace element analyses from XRF spectroscopy are given in Tables 2 and 3 of
Appendix B. )0: R techniques are discussed on p.10. The Keetley volcanics occur mostly
within the high-K calc-alkaline series of rocks (Figure 6) however, they do not follow the
typical AF M, calc-alkaline differentiation trend (Figure 7). The Keetley volcanics have a
large variation in chemical composition which is illustrated in Figure 8. Figure 8 compares
the entire Central Volcanic Zone of the Andes which is composed of several individual
volcanic systems to the single volcanic system of the Keetley volcanics. Most of the
Keetley volcanics span this large variation.

The major element trends of CaO, F e0 and MgO in general show decreasing
trends with an increase in SiOz, but they do not show particularly coherent trends (Figure
9). MgO has the most deviation from a linear trend (Figure 9). The alkalis (NaZO and

K20), A1203, and TiOz show scatter at constant SiOz (Figure 9). The trace elements; Rb,

KO
2

17

 

l l T I

 
 
 
 
 
   

L Shoshonitic A

High-K calc-alkaline

  
 
  

Cale-alkaline

 

Tholeiitic

l l l l i
so 55 60 65 70 75 so

SiO
2

 

 

 

Figure 6. High-K calc-alkaline classification of the Keetley volcanics based

on divisions after McBimey (1993). Symbols for the Keetley volcanics are

filled squares for Jordanelle Dam location, open squares for Francis, open diamonds
for Indian Hollow, filled triangles for Peoa and open triangles for 1-80.

FeO

 

 

Na O+K 0 M90
2 2

Figure 7. A typical AFM calc-alkaline differentiation trend afier Hess (1989)
plotted with the chemical trend of the Keetley volcanics. CA = calc-alkaline and
the solid black line is the path of differentiation. Symbols for the Keetley
volcanics are the same as in Figure 6.

KO
2

 

l9

 

 

 

 

50 60 7O 80
SiO
2

Figure 8. Comparison of high-K calc-alkaline volcanic rocks from the
Central Volcanic Zone (CVZ) of the Andes in northern Chile, north-west
Argentina and south-west Bolivia (shaded area) to the Keetley volcanics.
Data on the CVZ from Wilson (1989). Most of the Keetley volcanics span
the entire compostitional range of the volcanic systems within the high-K
calc-alkaline volcanics from the CVZ. Symbols for Keetley volcanics are the
same as in Figure 6.

20

Figure 9. Major element variation diagrams for the Keetley volcanics. Note the large
variation in MgO, TiOz, A1203 and the alkalis with a small variation in
SlOz. Symbols for the Keetley volcanics are the same as in Figure 6.

21

 

 

 

 

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22

Sr, Ba, Zr, Nb have little coherence at nearly constant SiOz. P205 shows a general
decrease with increase in silica but is not a tight linear trend (Figure 10).
Mineralogy of the Keetley Volcanics

Petrography

A total of 50 thin sections were prepared from the volcanic clasts of the

Keetley volcanics. Representative modal analyses for the Keetley volcanics are illustrated
in Table 4. A total of 800 point counts were done for each modal analysis. The
distinction between phenocrysts and matrix was based on grain size. Any grain that was
less than .5 mm was considered matrix and anything larger was considered a phenocryst.
The amount of phenocrysts in the volcanic clasts ranges from 35% to 63% (Table 4), with
a mean percentage of phenocrysts equal to 53.5%. Petrographic descriptions of the
Keetley volcanics will be given collectively due to their similarities. The major variation
being the presence or absence of pyroxene.

The Keetley volcanics are porphyritic andesites. All samples contain phenocrysts
of plagioclase and hornblende as the major phases with minor amounts of biotite in most
samples, with the exception of JD 19-9 and 1H 17-13 which contain about 10% biotite
each. In most samples plagioclase is the dominant phenocryst (Table 4). Plagioclase
occurs as medium- to fine-grained, euhedral to subhedral laths as well as microphenocrysts
within the matrix. Plagioclase laths have characteristic oscillatory zoning and many are
reversely zoned. Plagioclase melt inclusions are very common in nearly all of the thin
sections.

Homblende is the next most abundant phenocryst phase and occurs as fine- to

medium-grained, euhedral to subhedral phenocrysts. Homblende phenocrysts often

23

Figure 10. Trace element variation diagrams for the Keetley Volcanics. All trace
elements show scatter at constant SiOz. Symbols for the Keetley volcanics

are the same as in Figure 6.

24

 

 

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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25

Table 4. Modal point count anlayses shown in percent.

Sample #

JD 18-8

JD 19-9

JD 19-10
JD 19-17
JD 19-1

JD 19-2

JD 18-9B
JD 19-37

IH 16—4

IH 17-2

lH 17-13

IH 17-23

IH 17-25
l-80 15-11
l-80 20-14
PEOA 19—27
PEOA 19-28
PEOA 19-31
FRAN 20-3
FRAN 20-11

Plag

28.25
25.25
12.73
25.75
28.13
30.84
19.50
34.42
20.63
32.44
12.73
23.91
24.50
17.98
25.88
33.29
32.58
12.50
25.06
25.59

Amph

11.63
3.36
3.62
8.00
9.63

16.23

12.00
7.29

15.63

16.15
3.62

13.70

17.25

11.40

13.13
2.12
4.24
1.82

14.09
7.24

Biot

11.25
9.95
10.24
5.50
1.38
1.37
0.25
1.63
1.13
4.62
10.24
1.12
0.13
2.47
3.75
0.37
1.50
6.19
1.00
4.00

Cpx

5.88
20.40
17.35

0.50

1.63

0.12

0.75

0.00

1.50

0.00
17.35

1.25

1.25

1.18

0.00
11.85
14.48
19.42

6.23
16.10

Opaq

0.75
2.11
2.62
0.88
8.63
1.87
1.75
3.39
4.88
0.90
2.62
1.49
2.50
9.87
4.13
2.12
1.12
2.43
1.87
2.62

Qtz Matrix Phenocrysts

0.63
1.24
1.87
0.00
0.00
0.00
0.00
2.39
0.00
0.77
1.87
0.00
0.00
1.06
0.63
0.00
0.00
0.00
0.25
0.00

41.63
37.69
51.56
59.38
50.63
49.56
65.75
50.88
56.25
45.13
51.56
58.53
54.38
56.05
52.50
50.25
46.07
57.65
51.75
44.44

58.37
62.31
48.44
40.62
49.37
50.44
34.25
49.12
43.75
54.87
48.44
41.47
45.62
43.95
47.50
49.75
53.93
42.35
48.25
55.56

26

display oscillatory zoning (Figure Ila-b). Also note the near parallel alignment of the
phenocrysts in Figure Ila-b, this texture is most common in the samples from the Indian
Hollow location. Occasionally hornblende is cored by pyroxene. Inclusions of
plagioclase, opaques and some biotite are common. In most samples the hornblende
phenocrysts are surrounded by very fine-grained clinopyroxene (Figures 12a—b and 13a-b).
Disequilibrium of hornblende phenocrysts is common and is characterized by reaction rims
that are composed of opaques. In some cases the hornblende phenocrysts not only have
reaction rims but appear to have been nearly completed reacted to form an opaque mineral
(magnetite?) In these same samples the clinopyroxene is in equilibrium and appears fresh
and unaltered and does not appear to have reacted (Figure I4a-b).

In about 80% of the samples clinopyroxene exists as fine- to medium-grained,
euhedral to subhedral phenocrysts coexisting with hornblende as well as in the matrix as
microphenocrysts in most samples. One orthopyroxene phenocryst was detected by
electron microprobe anda few phenocrysts were found in thin section.

Biotite exists as phenocrysts and microphenocrysts within the matrix. Biotite
phenocrysts are less abundant than hornblende phenocrysts in most samples. They are
fine-grained, euhedral to subhedral, and are often nucleated on hornblende phenocrysts.

In most of the samples in which hornblende has reacted to opaques the biotite has reacted
as well.

Opaque minerals are found in all samples and the majority are magnetite, however,
rare grains of illmenite were detected by electron microprobe analysis.

Quartz occurs as anhedral, resorbed, embayed grains up to 1.5 mm in a few

samples and does not have a modal abundance of more than about 2.5%. Based on the

27

Figure l la-b. Photomicrograph of sample 17-23 TB, in plane polarized light a) and with
crossed polars b). Note oscillatory zoning of hornblende as well as a near
parallel alignment of the phenocrysts. Field of view is 4.4 mm across.

 

29

Figure lZa-b. Photomicrograph of sample 20-3 Fran, in plane polarized light a) and with
crossed polars b). The hornblende phenocryst is rimmed by smaller grains
of clinopyroxene. Note the small grains of clinopyroxene in the matrix.
Field of view is 4.4 mm across.

 

31

Figure l3a-b. Photomicrograph of sample 20-3 Fran under higher magnification in
plane polarized light a) and with crossed polars b). Note the unaltered
clinopyroxene surrounding the altered/reacted hornblende
phenocryst. Field of view is 1.2 mm across.

32

 

33

Figure l4a-b. Photomicrograph of sample 19-3b ID, in plane polarized light a) and with
crossed polars b). At the top is a phenocryst of hornblende that has
reacted and is in disequilibrium along side a fresh, unaltered clinopyroxene.
Field of view is 1.2 mm across.

34

 

35

resorbed, embayed texture and the rare occurrence it is probable that the quartz grains are
xenocrysts (Figure lSa—b). However, the texture of these may be due to the fact that the
quartz grains are disequilibrium phenocrysts. One large grain and a few smaller grains of
K-feldspar occur as irregular grains (Figure 16a-b). These are probably xenocrysts. Rare
mineral aggregates up to 1 cm are found throughout the samples and generally are
composed of hornblende, biotite and quartz phenocrysts.

Electmn Microprobe Analysis

Mineral chemical compositions were determined by a Cameca Carnabax electron
microprobe at the University of Indiana for plagioclase, amphibole, pyroxene, and biotite
(see p. 10-11 for a description of techniques). Magnetite is the dominant opaque mineral.

Representative plagioclase compositions are presented in Table 5. The total range
of plagioclase phenocrysts for all microprobe analyses are illustrated in Figure 17. The
one analysis that is orthoclase is probably a K-feldspar xenocryst. Two analyses are albitic
plagioclase(An1o), both of these analyses are rim analyses. Most plagioclase rim
compositions are within An3o to An” with a maximum anorthite variation of Ann to Ams
in sample 19-9. An anomalous variation from Anw to Ams occurs within sample 19-2
(Figure 18). The plagioclase middle compositions have a maximum anorthite variation of
An“ to Anso, which occurs within sample19-2 (Figure 19). The maximum anorthite
variation within the matrix plagioclase ranges from Ann to Arm and occurs within sample
19-17 (Figure 20). Several of the plagioclase phenocrysts are reversely zoned (Table 5)
and are not specific to any one sample or a group of samples. Examples of reverse zoning

are illustrated in Figure 21. Oscillatory zoned grains are also common.

36

Figure lSa-b. Photomicrograph of sample 20-18 [-80, in plane polarized light a) and
with crossed polars b). Notice the two grains of quartz on the left and
right-hand sides. Both of these grains exhibit resorbed and embayed

textures.

 

38

Figure l6a-b. Photomicrograph of sample 17-2 IH, in plane polarized light a) and
crossed polars b). A large grain of K-feldspar makes up most of the
field of view. This grain is probably a xenocryst.

 

 

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Figure 17. Feldspar triangle including rim, middle and matrix
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43

Figure 18. Anorthite variations within plagioclase rim analyses. Diamonds are the
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46

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50

Representative amphibole compositions are listed in Table 6. In all of the analyses
FeO is calculated as FeO = F e0 + .8998 *Fe203. The most variation within the amphibole
analyses is within MgO, which varies from 9.85% to 14.82%, CaO ranges from 10.75% to
11.95%, with two anomalous analyses at about 20%.

All pyroxene microprobe analyses are plotted in Figure 22 on the pyroxene
quadrilateral and representative pyroxene compositions are illustrated in Table 7. Most of
the analyses are MgO rich with intermediate CaO and fall within the clinopyroxene range.
Three analyses are within the orthopyroxene range. Sample 19-28 is the only sample
which allows for two-pyroxene thermometry to be estimated because it contains both
clinopyroxene and orthopyroxene.

Table 8 lists some representative compositions of biotite phenocrysts. There is
very little variation within the biotite compositions. K20 is consistent ranging from 8.19
to 9.43 wt. %.

Pyroxene Thermometry

Two-pyroxene thermometry was done using one orthopyroxene analysis with two
different clinopyroxene analyses from within sample 19-28 JD. The results of this are
illustrated in Table 9. The Ca-QUILF system developed by Lindsley and Frost (1992) was
used to determine the temperatures. This method requires that four phases of the FeO-
MgO-CaO-TiOz-Sioz system be used. However, only the clinopyroxene and
orthopyroxene analyses were available from sample 19-28 ID, no Fe-Ti oxide analyses
were obtained. Because of this, reasonable pressures had to be estimate for the
clinopyroxene and orthopyroxene compositions in order to obtain temperatures. Using

the QUILF program (Anderson et al., 1993) pressures of 1, 2, 4 and 6 Kb were arbitrarily

51

 

 

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Figure 22. Simple pyroxene quadrilateral including all pyroxene
phenocryst compostions including rims, cores and middles.

53

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55

used and the resulting temperatures were then determined for each pressure using the
calculated end members of the pyroxenes (Table 9). For Group 1 and 2 pyroxenes the
lowest average temperatures at which orthopyroxene are in equilibrium with
clinopyroxene are at 1 Kb = 1130 °C, at 2 Kb = 1132 °C, at 4 Kb = 1137 °C and at 6 Kb =
1142 °C.
Homblende Geobarometry

The total aluminum (AlT) content of hornblende is pressure sensitive and can be
used as a geobarometer if it is in equilibrium with the assemblage; biotite, plagioclase,
sanidine, quartz, sphene, ilmenite or magnetite, melt and possibly vapor (Hammerstrom
and Zen, 1986). This method recalculates AlT from hornblende formulas on the basis of
23 oxygens and all iron as FeO. The result is a linear relationship between increasing AlT
and increasing pressure. Hammerstrom and Zen (1986) achieved the linear relationship
with a correlation coefficient of r2 = 0.8 and an uncertainty of i 3 kb. This correlation
coefficient was refined by Hollister et a1. (1987) by analyzing calc-alkaline plutons with
intermediate compositions to those used by Hammerstrom and Zen (1986). The result
was a r2 = .97 with a reduction in the uncertainty to i 1 kb. Often the correct mineral
assemblage, as listed above, is not available to get accurate estimates of pressure. In a
study done by Johnson and Rutherford (1989), the correct mineral assemblage was
available for pressure estimates. If the correct mineral assemblage is not available, for
example if quartz and sanidine are missing, it is possible to do estimates on samples in
which the melt contains greater than 76% SiOz and 5% K20 (anhydrous), which are close
to quartz and sanidine saturation (Johnson and Rutherford, 1989). The Keetley volcanics

are lacking quartz and sanidine as primary phases and it is not possible to determine the

56

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57

composition of the melt because of the non-glassy texture of the groundmass. Therefore
the estimate of geobarometry should be used with caution. However, The CIPW norm
calculations of the Keetley volcanics are very close to the CIPW norm of the Fish Canyon
Tuff (Table 10). Therefore, this method at least provides some sort of estimate for
pressure estimates at which hornblende was stable. Using the pressure estimate equation
from Johnson and Rutherford (1989);

P = (4.09 +/- 0.27) (AlT) - (3.29 +/- 0.45), with .2 = .96,
the low pressure estimate using AlT = 1.62 is P = 3.35 Kb, the high pressure estimate
using AlT = 2.42 is P = 6.61 Kb. The total range of wt.% A1203 in hornblende is from

9.55 % to 14.11 %.

Assimilation and Fractional Crystallization Models

Table 11 illustrates the results fi’om assimilation and fractional crystallization
(AFC) models from the program Mixing (Carr, 1994) after Bryan et al., (1969) in which
the sum of the squares of the residuals is calculated by multiple linear regression. The
whole rock compositions of the most mafic (18-8 JD) and the most silicic (16-04 Fran, 15-
11 [H and 19-37 JD) end-members were chosen as the mixing magmas. Sample 17-31
was chosen as the hybrid because it has an intermediate compositions. Amphibole,
plagioclase and clinopyroxene microprobe analyses from 18-8 JD were chosen as the
fractionating phases. Sample 18-8 JD was mixed individually with each of the silicic end-
member samples and had fractional crystallization of the mafic phases superimposed on
the mixing process. The percents of fractionating phases and amounts of end-members

necessary for each combination to produce the hybrid magma are listed in Table 11.

58

Table 10. Comparison of whole rock CIPW norm calculations of the Keetley volcanics vs.
the Fish Canyon Tuff. Keetley volcanic samples are those used to calculate hornblende
geobarometry.

FCT9 17-2|H 19-37 JD 17-13lH

QTZ 13.12 14.48 21.60 2.59
OR 23.99 21.03 15.99 20.18
AB 34.19 31.80 34.13 34.55
AN 16.21 16.75 17.66 20.87
DI 1.50 2.26 0.27 6.51
HY 0.00 8.87 5.80 8.18
MT 0.00 2.99 2.92 3.59
ILM 1.14 1.06 1.00 1.86

AP 0.00 0.76 0.64 1.69

59

Table 11. Multiple linear regression models of Assimilation and Fractional Crystallization
for the Keetley volcanics.

Hybrid lava is 17-31 lH
Coef % Cum Mineral or Rock
-0.016 -1.616 18-8-3 hbl
-0.036 -3.736 18—8—8 plag
-0.027 -2.827 18-8-1 cpx
0.426 42.9 18-8 JD

0.646 65.1 16-04 IH
Min/Rock Si02 Ti02 A1203 F60 M00 M90 CaO N820 K20 P205
18-8-3 44.01 2.92 11.13 13.53 0.32 14.2 11.72 2.17 0 0
18-8-8 60.58 0 29.12 0 0.49 0 0 9.81 0 0
18-8-1 51.72 2.14 0.37 14.68 8.69 0.48 21.57 0.37 0 0

18-8JD 57.25 0.871843 7.02 0.14 3.04 6.37 3.7 2.65 0.53
16-04IH 65.52 06315.05 4.63 0.06 2.47 4.3 3.72 3.23 0.39

Hybrid 17-31IH

Obs-P 62.17 0.66 16.38 5.24 0.1 2.79 4.78 3.76 3.7 0.41
Calc—P 62.4 0.68 16.33 5.37 -0.16 2.65 4.72 3.58 3.21 0.48
Diff‘wt -0.09 -0.01 0.03 -0.12 0.26 0.14 0.06 0.18 0.48 -0.07

Sum of squares of residuals =.387

Hybrid lava is 17-31IH

Coef % Cum Min/Rock
-0.056 -5.656 18-8-3 hbl
-0.024 -2.424 18-8-4 plag
-0.022 -2.222 18-8-1 cpx
0.458 46.1 15-11 l-80
0.638 64.2 18-8 JD

Si02 Ti02 A|203 F60 Mn0 M90 CaO N820 K20 P205

1883 44.01 2.92 11.13 13.53 0.32 14.2 11.72 2.17 , 0 0
18-8-4 60.86 0 29.03 0 0.54 0 0 9.58 0 0
18-8-1 51.72 2.14 0.37 14.68 8.69 0.48 21.57 0.37 0 0
15—11l- 67.54 0.77 13.37 4.05 0.07 3.41 4 3.09 3.29 0.4

18-8JD 57.25 0.871843 7.02 0.14 3.04 6.37 3.7 2.65 0.53

Hybrid 17-31IH

Obs-P 62.17 0.66 16.38 5.24 0.1 2.79 4.78 3.76 3.7 0.41
Cale-P 62.39 0.7 16.55 5.26 -0.1 2.7 4.77 3.42 3.19 0.52
Diff‘vvt -0.09 -0.04 -0.08 -0.01 0.2 0.1 0.01 0.34 0.5 -0.11

Sum of squares of the residuals = .446

60

Table 11 (cont)

Hybrid lava 17-31 lH

Coef %Cum Min/Rock
-0.007 -0.707 18-8-3 hbl
-0.098 -9.998 18-8-4 plag
-0.041 -4.141 18-8-1 cpx
0.518 52.4 18-8 JD
0.617 62.4 19-37 JD

Sample Si02 Ti02 Al203 F60 Mn0 M90

18-8-3 44.01 2.92 11.13 13.53 0.32
18-8-4 60.86 0 29.03 0 0.54
1881 51.72 2.14 0.37 14.68 8.69

18-8JD 57.25 0.871843 7.02 0.14
19-37JD 66.6 05316.07 3.87 0.05

Hybrid 17-31IH

Obs-P 62.17 0.66 16.38 5.24 0.1
Calc-P 62.39 0.67 16.53 5.34 -0.31
Diff‘wt -0.09 -0.01 -0.07 -0.09 0.41

Sum of squares of the residuals = .749

14.2

0
0.48
3.04
1.85

2.79
2.6
0.19

11.72
0

21 .57
6.37
4

4.78
4.81
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2.17
9.58
0.37

3.7
4.04

3.76
3.45
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080 N620 K20

0
0
0

2.65
2.71

3.7
3.04
0.65

P205

0
0
0
0.53
0.28

0.41
0.45
-0.03

DISCUSSION

Chemical and Age Relationships Between the Keetley Volcanics and the Wasatch
Intrusive Belt

Overall there are three and possibly four main chemical groups that
characterize the Wasatch Intrusive Belt and the Keetley volcanics (V ogel, in preparation,
1997). The Little Cottonwood stock ranges in silica from 65% to 73%, the Alta stock is
intermediate ranging from 60% to 68% silica, and the Clayton Peak stock is the most
mafic ranging from 54%-62% silica and they have distinct Rb/Sr - oxide trends. The
chemical compositions of the eastern stocks are distinctly different fi'om the Little
Cottonwood and Clayton Peak stocks but do have similarities to the Alta stock (Figure
23). The Keetley volcanics are variable but have similarities to the eastern stocks and the
Alta stock (Figure 24). The eastern stocks and the Keetley volcanics have biotite K/Ar
ages of about 34 Ma. The Alta stock has a zircon U/Pb age of 33.5 Ma. The similarities
in the chemical composition and emplacement ages of the Alta stock, eastern stocks and
the Keetley volcanics are interpreted to mean that they may be genetically related.
Detailed zircon U/Pb (Constenius, pers. comm.,l996), and 40Ar/39Ar dating of minerals
(Flood, pers. comm, 1996) are in progress to evaluate this suggestion, and to firrther
document the emplacement history and petrologic relationships among the Wasatch

intrusive belt.
Evaluation of the effects of Pm on the evolution of the Keetley Volcanics

Change in pressure can account for the breakdown of hornblende in the Keetley

volcanics. The presence of hornblende in disequilibrium in the majority of the samples

61

62

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64

implies that there is a minimum H20 content and pressure that allows growth of
hornblende in the magma. The presence of very-fine grained pyroxenes nucleated around
reacted homblendes as well as microphenocrysts of pyroxene in the matrix suggests that

hornblende became unstable while pyroxene became stable.

In general as the amount of H20 in a silicate melt decreases the stability of hydrous
phases such as amphibole decreases (Eggler, 1972). An experimental study by Johnson
and Rutherford (1989) on the Fish Canyon Tufl‘ indicates that at 2 kb hornblende becomes
unstable with less than .25 Xmo. They found that for the Fish Canyon Tufi‘, in order for
amphibole to be stable at Pm. =2 kb Xmo must be between .25 and .75. P, T and Xmocan
be roughly estimated from hornblende instability. In calc-alkaline melts it appears that the
upper temperature stability limit of amphibole for any firgacity of H20 in the fluid may
represent either incongruent melting of the amphibole or a reaction to clinopyroxene
(Eggler, 1972). Because of the calc-alkaline nature of the Keetley volcanics and the fact
that many of the samples contain hornblende phenocrysts rimmed by clinopyroxene, this
may represent a reaction controlled by the magma moving out of the stability field of
hornblende and into the stability field of pyroxene by a lowering of the Pm” in the magma
as it ascended. The P, T and X mo conditions under which this process occurred may be
estimated to be between 930 °C and about 760 °C at 2 kb and below .25 an following
the experimental data of Johnson and Rutherford (1989).

Reverse Zoning in Plagioclase

The Keetley volcanics contain a significant population of reverse zoned

plagioclase phenocrysts. There are two possible explanations for this feature. One is that

65

an increase in pressure followed by a drop in pressure caused the rims of the plagioclase
crystals to be more anorthitic than the inner parts of the crystal. This is caused by the fact
that anorthite is more stable at elevated pressures than albite is. As a magma evolves
under ideal conditions, one would expect to see the zoning of plagioclase become less
anorthitic toward the rim. However, if there are cyclic increases and decreasses in
pressure as one would expect to see in a volcanic system, increasing pressure results in
more anorthitic rich composition of the plagioclase grains that are crystallizing (Figure
25). Another explanation for reverse zoning in plagioclase is magma mixing. The
invasion of a more anorthitic magma into the magma chamber could also result in reverse
zoning of plagioclase crystals. The variation from A1130 to Aug of the rims of the
plagioclase phenocrysts as well as the variation from A1142 to Arm in the matrix plagioclase ‘
means that the not all of the plagioclase was in equilibrium with the same magma. The
intrusion of a more mafic magma could have produced more anorthic rims on some of the
plagioclase. Also, a more mafic magma could introduce other plagioclase grains with
different rim compositions to the original magma. The fact that there is more than a
bimodal distribution in anorthite compositions of the plagioclase rims implies that there is
not a simple binary mixing cure. Instead there was probably more than one magmatic
source involved. The anorthite variation within the rims and the matrix plagioclase could
also be the result of changes in pressure as discussed above. It is difficult to distinguish
between these two processes to explain the cause of reverse zoned plagioclase. The
results of AFC modeling are consistent with magma mixing (see below) and therefore it is

quite possible that the reverse zoning of the plagioclase is due to magma mixing.

66

 

 

 

 

 

 

 

 

 

 

Figure 25. Theoretical plagioclase feldspar phase diagram illustrating the
effects of varying pressure, where P2>P1>P3.

67

Evaluation of Petrologic processes to Produce the Chemical Variation within the
Keetley Volcanics; Fractional Crystallization, Magma Mixing, and Crustal
Assimilation

Magma mixing, wall rock assimilation and fractional crystallization are all
processes that can affect the chemical compositions of magmas. Tests of petrologic
processes can be used to determine if the data set is consistent or inconsistent with a
particular model but do not prove that a particular process is the cause of the chemical
variation (Vogel, 1982). It is probable that more than one of these processes operates
simultaneously resulting in complicated chemical trends to explain. Each of these
processes are evaluated and are considered as possibilities for the chemical variation
within the Keetley volcanics. Major and trace elements are used to evaluate these
processes. Figure 26 shows the Keetley volcanics with two typical calc-alkaline
differentiation trends of calc-alkaline rocks. The Keetley volcanics do not follow this
typical trend and there is little coherence in the data array. Chemical compositions of the
Keetley volcanics show significant scatter and can not be separated into different
petrologic or chemical groups. Therefore it is very likely that more than one petrologic
process has affected the Keetley volcanics. Whether or not magma mixing , assimilation
or fractional crystallization or a combination of one or more of these processes have
affected the evolution of the Keetley volcanics is a difficult question to answer directly
because there is so little coherency in the data. However, these processes were evaluated

and compared to other calc-alkaline suites of rocks.

68

FeO

 

v v v v V v J v y

Na20+K20 M90

Figure 26. Typical differentiation trends of calc-alkaline rocks erupted within
continental crust, l is for Lassen Peak, northern California and 2 is for Crater
Lake National Park, Oregon (Hyndman, 1985 and references within). The
Keetley volcanics clearly do not follow the typical calc-alkaline differentiation
trend. Symbols for Keetley volcanics are the same as in Figure 6.

69
Fractional Crystallization

One mechanism for differentiation to produce andesites is by fi'actional
crystallization of a basalt parent, typically of plagioclase, orthopyroxene, amphibole and
magnetite (Gill, 1981). Statistical models of fi'actionation were tested and all were
unsuccessfirl, and cannot alone provide an explanation for the chemical variation within
the Keetley volcanics. Even smaller linear trends that occur within the entire data array
could not be modeled by fiactionation. Clearly, linear trends within the total data set
obviously are not trends produced by crystal fractionation. There is also no petrographic
evidence of fiactional crystallization, for example there are no accumulations of any
phenocrysts phases that seem to account for any variations in the chemistry such as

plagioclase accumulation, which would result in an overall increase in Sr.

The MELTS program (Ghiorso and Sack, 1995) was used to try to estimate
amounts and compositions of various fractionating phases over a temperature range of
1200°C to 900°C. The significant abundance of hornblende precludes the use of MELTS

program because it does not include hornblende as a fractionating phase in its calculations.

In a typical fi'actionated sequence of calc-alkaline andesites one would expect to
see near linear or curvilinear trends with little scatter within most major and trace element
chemical trends. Figure 27a-d illustrates the chemical composition of the Keetley
volcanics with recent analyses fi'om calc-alkaline rocks of the Mogollon-Datil Volcanic
Field, New Mexico thought to be produced by fractionation (Davis and Hawkesworth,
1994). Notice the tight linear arrays for the Mogollon-Datil volcanics compared to the

large variation within the Keetley volcanics (Figure 27a-d). There must be more than one

70

Figure 27a-d. Comparison of a typical calc-alkaline fiactionation trend (shaded areas)
from the Mogollon-Datil volcanic field with the Keetley volcanics.
Symbols for Keetley volcanics are the same as in Figure 6.

71

a)

 

 

 

 

 

Si0

b)

70

 

 

 

 

55

 

CaO 5—

SiO

72

G)

 

 

 

 

 

70

SK)

d)

 

 

 

70

 

 

 

 

3000

55

Ba 2000 -
1000

SK)

73

process afi‘ecting the petrogenesis of the Keetley volcanics, rather than simply one process

such as fractional crystallization.
Magma Mixing

There are many documented cases of mixing a basalt with rhyolite or dacite to
produce intermediate magmas such as andesites in several volcanic systems. Examples of
this are the Coso volcanic field (Bacon and Metz, 1984), and the Medicine Lake Volcano
(Grove et al., 1988). In both of these cases there is evidence of a hybrid magma including
relict phenocrysts from both the mafic and felsic components (Bacon and Metz, 1984), as
well as melted crustal xenoliths and reacted, resorbed and overgrown phenocrysts and
xenocrysts (Grove et al., 1988). The Keetley volcanics show some evidence of mixing.
Reacted and partially resorbed phenocrysts of hornblende are common but are believed to
have resulted from lowering of pressure as the magma ascended, as discussed above.
Mixing of basalt and silicic magma to produce an andesite, and eruption prior to re-
equilibration of a hybrid magma can produce some of the same features, such as reacted
phenocrysts, as does increasing Pmo in a magma (Halsor and Rose, 1991). The Keetley
volcanics also contain reverse zoned plagioclase phenocrysts which are probably the result

of magma mixing.

Assimilation and Fractional Crystallization

All attempts at modeling magma mixing, or fractional crystallization as sole
processes using multiple linear regression with the available end-members were
unsuccessful. Magma mixing or fractional crystallization alone can not explain the

chemical variation within the Keetley volcanics. However, combinations of crystal

74

fractionation and magma mixing within the available end members resulted in statistically
acceptable values. In these models acceptable results would include the sum of the
squares of the residuals being low, generally less than 1, the r2 values being close to one
and the mixing coefficients being geologically reasonable, meaning that the mixing and
fractionating values have to be consistent with geologic and petrologic data (V ogel,
1982). In order to produce the intermediate, hybrid lava 17-31 IH a combination of
42.9% mafic end-member plus 65.1% silicic end-member with fractionation of 1.6%
hornblende, 3.7% plagioclase and 2.8% clinopyroxene from the mafic end-member result
in a sum of the squares of the residuals = .387. This combination of mixing and crystal
fractionation permits the AFC model as a possible explanation for the some of the
chemical variation within the Keetley volcanics. Similar results with the sum of the
squares of the residuals < 1 are reproducible using other silicic end-members (Table 11.).

However, this model does not prove that AFC alone caused the chemical variation.

Quartz and rare K-feldspar xenocrysts as well as possible xenoliths are found in
some of the samples, these may indicate assimilation of a silicic granitoid crust. It is not
possible to determine if the quartz and k-feldspar xenocrysts are the result of small pieces
of a granitoid contaminant being included or if they represent relict parts of large amounts
of incorporation of a granitoid that has been significantly melted (Lange and Carmichael,
1996). Therefore, even if there was contamination by a granitoid body through
assimilation, it is not possible to estimate the amount of assimilation of such a
contaminant. However, the rare occurrence and the resorbed, embayed texture of the
quartz grains suggest that they are xenocrysts. Whether or not what appear to be

xenoliths are actually xenoliths is difficult to determine because they have very similar

75

mineralogy to the Keetley volcanics, therefore they may just be aggregates of minerals

from within the magma.

Origin of high-K calc-alkaline rocks and a Possible Petrogenetic Model for the
Evolution of the Keetley Volcanics

There are two main tectonic settings in which high-K calc-alkaline rocks may be
produced. One tectonic setting is a continental arc system and the other is a post
collisional setting. The chemical and isotopic characteristics of magmas in an continental
arc setting are believed to be the products of partial melting in the mantle wedge which is
induced by incompatible element enriched fluids derived from a subducting slab.
Subsequent to this the magmas rise to the continental crust and are firrther enriched in
incompatible elements through assimilation (Roberts and Clemens, 1993). The amount of ‘
enrichment in incompatible elements from the crust is attributed to the thickness of the
crust as well as the depth to the BeniotT zone. The thicker the crust that the magma must
pass through the more enriched in incompatibles it will become (Dickenson, 1975).
Crustal contamination of the mantle derived parent magma will produce characteristic
trace element and isotopic signatures, such as those examined in the classic continental are

setting of the Andes (Hildreth and Moorbath, 1988).

A second tectonic model for the origin of high-K calc-alkaline rocks is in post
collisional settings (Pitcher, 1987 in Roberts and Clemens, 1993). Melting of the source
rock occurs by interaction of mantle melts with the crust. Extension following thickening
would then allow mantle upwelling and underplating of the lower crust by mantle derived

mafic magmas (Sonder et al., 1987). The extra heat added to an already hot crust would

 

76

allow for partial melting to take place and these melts could then ascend through the crust

(Sonder et al., 1987, Dewey, 1984).

By Late Cretaceous time the western Cordillera had developed into an Andean
type continental margin arc system, which produced mantle melts by the continuous
subduction of the Farallon plate ( Miller et al., 1992 and references therein). The relative
rate of convergence between the Farallon plate and the North American plate increased
between 75 and 70 Ma, continued fi'om 65 to 55 Ma and began to decline between 55 and
40 Ma (Engebretson 1985). This slowing of convergence rate is attributed to a shallowing

in subduction of the Farallon plate (Miller, 1992, and references therein).

The timing of compressional vs. extensional regimes in the Cordillera is related to
the idea of migrating arc-magmatism (Coney and Reynolds, 1977). Migration of the
magmatic arc in the southern Rocky Mountains is related to Late Cretaceous - Early
Tertiary crustal shortening and simultaneous shallowing of the angle of subduction of the
Farallon plate beneath North America (Coney and Reynolds, 1977). Constenius (1996)
has recently reviewed the timing of the change from convergence to extension within the
Cordillera. Based on ages of magmatism and basin formation, Constenius (1996) has
suggested that during the Late Cretaceous to Early Eocene (72-54 Ma) accelerated
subduction and/or shallowing of the subducting Farallon plate were occurring
simultaneously with about 600 km of eastward progression of magmatism in the
Cordillera. In the early-middle Eocene (53-51 Ma) there was a reduced rate of plate
convergence, and steepening of the slab, resulting in westward migration and rollback.

This caused widespread magmatism. In the middle Eocene (49-48 Ma) the onset of

77

normal faulting and basin-fill sedimentation in the Cordillera thrust belt was related to the
westward movement of the Farallon plate from the Great Plains toward the Pacific coast.
The normal faults superimposed half grabens on the deformed Cordillera thrust belt

(Constenius, 1996).

The Uinta lineament was reactivated during the compressional stages of the Sevier
and Laramide orogenies, which lasted from 145 Ma to 50 Ma overall. The onset of the
change from compression to extension in the western US. began as a result of rollback of
the Farallon plate (49—48 Ma), which in turn resulted in a thermal weakening of the crust
(Constenius, 1996). The position of the subducted slab and the thermal state of the slab at
50, 35, 30, 20, 10 and 0 Ma were estimated by Severinghaus and Atwater (1990) based on
magnetic anomalies, the time since subduction, and age of the slab upon entry into the
trench. Based on these estimates Severinghaus and Atwater (1990) have concluded that
by 35 Ma the subducted slab had been shortened along its entire length. This shortening
or rollback of the slab is probably recorded by the westward sweep of magmatism in the
western US. noted by Coney and Reynolds, ( 1977) and Cross and Pilger (1978). Cross
and Pilger (1978) also suggest that the Farallon plate may have disassembled by breaking
off into pieces during the rollback. The model proposed by Cross and Pilger (1978) of the
rollback of the Farallon plate is illustrated in Figure 28. This figure illustrates the different
stages of convergence of the North American plate and the Farallon plate and diagrams

the break-up and roll-back of the subducted slab.

It is possible that the Wasatch intrusive belt and Keetley volcanics are a result of

the steepening and rollback of the Farallon plate. The Uinta lineament was reactivated

78

Figure 28. Stages of change in the angle of subduction of the Farallon plate and
varying rates of convergence of the North American plate from 150 Ma to
the present. Figure is modified from Cross and Pilger (1978).

 

79

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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80

during the Sevier and Laramide orogenies. This linear weakness facilitated the
emplacement of magmas related to the rollback of the Farallon plate to rise into the crust,
beginning furthest east at 50 Ma and reaching Utah between 40 and 30 Ma. According to
Severinghaus and Atwater (1990) at about 35 Ma the Farallon plate was steepening and
rolling back underneath the western US, including north central Utah. Given the ages of
the Wasatch intrusive belt and Keetley volcanics of around 34 Ma it is possible that the
mantle derived magmas triggered by this process subsequently became buoyant and

ascended into the Uinta lineament and formed the Wasatch intrusive belt.

Roberts and Clemens, (1993) evaluated a model for the origin and production of
high-K calc-alkaline rocks such as the Keetley volcanics. They determined that partial
melts from hydrated, meta-calc—alkaline and high-K calc-alkaline andesites and basaltic
andesites are the most suitable source for producing high-K calc~alkaline rocks. The melts
produced from these source rocks fall directly within the high-K calc-alkaline trend
(Roberts, and Clemens, 1993, Beard and Lofgren, 1991). The presence of high-K calc-
alkaline rocks therefore implies that there is a source of andesite at depth that is large
enough to supply a melt. Partial melting of mafic to intermediate source compositions
with H20 contents between 0.7%and 1.6% have predicted volumes of generated melt
between 30% and 60% at T= 900°C-950°C and P= 5-10 Kbar for fluid absent source
rocks (Clemens, 1990; in Roberts and Clemens, 1993) . The high volume of melt
produced in these conditions precludes the need for fluids derived from elsewhere such as
a subducting slab, to generate large enough volumes of melt that may move towards the

surface (Roberts and Clemens, 1993).

81

A possible model for the evolution of the Keetley volcanics and Wasatch intrusive
belt is that they were produced by partial melting of previously emplaced or underplated
calc-alkaline rocks, which were related to subduction of the Farallon plate. These melts
were subsequently emplaced along the Wasatch-Uinta lineament, which was a conduit
along which these magmas could have been emplaced. A model of Cambray et al. (1993)
may be applicable to the emplacement of the Wasatch intrusives. This model would
invlove the development of releasing steps along larger normal faults within the Uinta

lineament. These releasing steps could provide space for independent magmas to occur.
Conclusions

The purpose of this thesis was to determine the relationships between the Keetley
volcanics and the Wasatch intrusive belt as well as to determine the cause of the chemical
variation within the Keetley volcanics. The Keetley volcanics, the Alta stock and the
eastern stocks have similar emplacement ages ranging from 33.5 - 34 Ma as well as similar
chemical compositions. This suggests that these igneous bodies may have a genetic

relationship.

The main conclusions from this thesis are the following. The Keetley volcanics
have a significant variation in their chemical composition and this variation cannot be
explained by fractional crystallization or magma mixing as separate processes acting
independently. AFC models, however, permit magma mixing and crystal fiactionation to
produce an intermediate magma composition from mixing the most mafic and felsic end-
members with simultaneous fractionation of amphibole, plagioclase and clinopyroxene

from the mafic end-member. These models are reproducible using different end-members

82

from within the higher silica end with the most mafic end-member. The occurrence of
reverse zoned plagioclase and quartz and K-feldspar xenocrysts are geologic evidence that

some mixing/assimilation occurred.

The Keetley volcanics were most likely derived from partial melting of previously
emplaced or underplated calc-alkaline rocks. The Uinta lineament is a weakness in the
crust that was reactivated during the Laramide and Sevier orogenies. It is the conduit
along which magma ascended to form the Wasatch intrusive belt and the Keetley
volcanics. The rollback or break up of the Farallon plate is thought to have produced
mantle melts which ascended to the crust causing melting of previously emplaced calc-
alkaline rocks. The Uinta lineament acted as a conduit for the magmas to ascend further
into the crust where the Wasatch intrusive belt and Keetley volcanics resulted. Upon
ascent through the continental crust the magma underwent differentiation as well as
assimilation and mixing resulting in a very diverse chemical composition for the Keetley

volcanics.

APPENDICES

APPENDIX A

Appendix A. XRF detection limits for trace elements and statistics for major elelments
analyzed from glass disks.

 

 

 

 

 

 

 

 

 

 

 

 

Element Detection limit (ppmflIMajor Oxide TrMean (wt.%) Std. Dev.

Ba 100l|sro2 52.73 0.106
La 48]]Ti02 1.08 0.009
Cr 63IIA1203 15.02 0.077
Ni 25lIFeO 9.99 0.030
Zn 14IIMnO 0.18 0.004
Rb 13flM99 6.63 0.018
Sr 12|[CaO 10.91 0.014
Y 14][Na203 2.18 0.018
Nb 1511190 ' 0.64 0.003
Zr 14||r>205 0.14 0.005

 

 

 

 

 

83

APPENDIX B

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89

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