Download Mineralogy and Petrology of Tertiary

465
Mineralogy and Petrology of Tertiary-Quaternary
Volcanic Rocks in Caribou County, Idaho
Donald
W. Fiesingerl,
William
ABSTRACT
Basaltic lavas of Caribou County, Idaho, comprise
the extensive interconnected Blackfoot, Willow Creek,
and Gem Valley lava fields with small valley flows
occurring within Enoch, Upper, Wooley, and Slug
Creek Valleys. These lavas consist primarily of olivine
tholeiite, with minor tholeiite and tholeiitic trachybasalt. The olivine tholeiite is similar to that of the
Snake River Plain, as it is olivine and hypersthene
normative, does not contain calcium-poor pyroxene,
and shows an affinity with alkali olivine basalt on the
alkali-silica
diagram. The tholeiite and tholeiitic
trachybasalt are hypersthene and quartz normative
with silica contents ranging from 48 to 53 percent.
The trachybasalt is higher in total alkalies, Pros, and
Fe/(Fe + Mg) than the tholeiite and has alkali
feldspar as a groundmass phase.
Flows of plagioclase basalt porphyry
occur at
Cinder Island within the Blackfoot Reservoir and at
Nelson and King Canyons within the Bear River
Range east of Gem Valley. This porphyry consists of
approximately
30 percent plagioclase phenocrysts, 5
to 20 millimeters in length, set in an aphanitic to
opaque groundmass.
A potassium-rich
alkali trachyte occurs at the
southern end of Slug Creek Valley. It consists of
olivine and diopsidic augite phenocrysts in a groundmass of alkali feldspar, olivine, and augite, with
minor plagioclase, exsolved iron-titanium
oxides,
and traces of amphibole and biotite.
Olivine tholeiite is considered to be the parental
magma from which other basaltic lavas have originated. Fractionation
of 40 to 60 percent crystals from
olivine tholeiite will yield a liquid with the composition of tholeiite or trachybasalt.
As removal of
groundmass phases is required, filter pressing during
late-stage crystallization
is the proposed mechanism.
‘Department
of Geology,
Utah State University,
Logan,
94322.
*Andover
Oil, Tulsa, Oklahoma
74172.
‘Terra Resources,
Inc., En&wood,
Colorado
80111.
Utah
D. Perkins*, and Barbara J. Puchy3
In contrast to this, plagioclase basalt porphyry can be
derived by the removal of olivine and the accumulation of plagioclase, both common phenocryst phases.
Although olivine tholeiite may be derived with IO
percent partial melting of pyrolite, equilibrium between the derived olivine tholeiite and the residuum
from partial melting cannot be demonstrated. Using
mica peridotite to represent a more potassic mantle
composition, alkali trachyte may be derived with 30
percent partial melting. These contrasting
magmas
are considered to be indirect evidence for mantle
inhomogeneity or perhaps different mantle structures
beneath the Basin and Range province and the
Middle Rocky Mountain province in southeastern
Idaho.
INTRODUCTION
Tertiary-Quaternary
basaltic lavas in Caribou
County, Idaho, comprise the extensive interconnected
Blackfoot, Willow Creek, and Gem Valley lava fields
near Soda Springs and the small valley flows to the
east within Enoch, Upper, Wooley, and Slug Valleys.
Rhyolite also occurs in the region, forming three
conspicuous
domes at the south end of Blackfoot
Reservoir and two islands within the reservoir.
This paper represents the synthesis of three projects: (1) a study of the mineralogy and petrology of
the lavas in the vicinity of Gem Valley (Perkins,
1979), (2) a study of the mineralogy and petrology of
the lavas in the Overthrust Belt (Puchy, l981), and (3)
a study of basaltic volcanism in the Blackfoot Reservoir region as part of a geothermal evaluation made
in cooperation with the Department of Geology and
Geophysics at the University of Utah for the U. S.
Geological Survey.
The objectives of this paper are (a) to present the
chemistry and mineralogy of volcanic rocks representing the various lava fields and valley flows in the
region, (b) to compare the basaltic lavas of Caribou
County with those of the eastern Snake River Plain,
466
Cenozoic
and (c) to present a petrogenetic
of these volcanic rocks.
PREVIOUS
Geology
Idaho
model for the origin
WORK
A comprehensive study of the geology of southeastern Idaho was undertaken by Mansfield (1927).
As part of this study, he classified the lavas in this
region as olivine basalt, because they contain olivine
and feldspar phenocrysts in a groundmass of augite,
feldspar, olivine, and magnetite, with minor apatite
and hematite. In a later investigation,
Mansfield
(1929) also described the lavas in Portneuf Valley, the
northern end of the Gem Valley lava field, as olivine
basalt, with mineralogy similar to that mentioned
above.
In an investigation of late Pleistocene stratigraphy
in the Gem Valley area, the lavas of the Gem Valley
lava field have been described as porphyritic
olivine
basalt, having phenocrysts of plagioclase, olivine, and
augite in a groundmass of plagioclase, olivine, augite,
magnetite, and glass (Bright, 1963, 1967). Potassiumargon dates by Armstrong
and others (1975) give an
age of 0.1 + 0.03 million years for these basal&. Bright
(1963, 1967) estimated an age of 33,500-27,000 years
for one flow in Gem Valley on the basis of stratigraphic relationships with Lake Thatcher sediments
dated by carbon-14 methods.
A series of geophysical investigations
have dealt
with the origin, structure, and prebasalt topography
of the Blackfoot and Gem Valley lava fields (Mabey
and Armstrong,
1962; Mitchell and others, 1965;
Oriel and others, 1965; Mabey and Oriel, 1970;
Mabey, 1971). The geologic map of the Soda Springs
quadrangle (Armstrong,
1969) and the preliminary
geologic map of the Bancroft quadrangle (Oriel,
1968) delineate the extent of the basalt flows and the
locations of numerous eruptive centers.
The small valley flows to the east of the Blackfoot
and Willow Creek lava fields have been reported by
Mansfield (1927). The vesicular lava in Slug Valley
has been briefly described by Cressman (1964) as
olivine basalt no younger than Pliocene in age. The
distribution
of lavas in Caribou County is shown on
the generalized composite geologic maps of Mabey
and Oriel (1970) and Oriel and Platt (1980). These
maps incorporate the work of Mansfield (1927, 1929),
Armstrong (1969), Oriel(1968), and Cressman (1964).
GEOLOGIC
of
SETTlNG
Caribou County lies in both the northeastern part
of the Basin and Range province and the western part
of the Middle Rocky Mountain province (Figure 1).
r
/
I
IDAHO
_,_.-.-.
Figure
I. Index map of southeastern
County,
Idaho, the Snake River
Range and Middle Rocky Mountains
- -.A
Idaho,
showing
Caribou
Plain, and the Basin and
physiographic
provinces.
In general, the western portion of the county is
dominated by fault block mountains composed of
Paleozoic sedimentary rocks locally overlain by Tertiary sediments. The eastern portion of the county is
part of the Overthrust
Belt dominated by upper
Paleozoic and Mesozoic sedimentary rocks involved
in a westward-dipping
imbricate thrust zone (Armstrong and Cressman, 1963). The distribution of lavas
within Caribou County is shown in Figure 2. In parts
of the Blackfoot, Willow Creek, and Gem Valley lava
fields, the flows are well-exposed along the extensive
northwest-southeast
scarp system which runs through
the area. The scarp trends indicated by Armstrong
(1969) for the Soda Springs area have been observed
to continue more than 10 miles to the north, beyond
the Blackfoot Reservoir. Individual scarps may be
traced visibly for many miles, and where covered by
loess, their presence can be inferred from topography
and alignment of sink holes. The scarp system is best
exposed and most continuous immediately adjacent
to and south of the Blackfoot Reservoir.
A study of aerial photographs and ground checks
undertaken as part of the present study indicates only
minor deviation in the lateral extent of the basaltic
lavas from that reported by Mansfield (1927, 1929),
Oriel (1968), and Armstrong (1969). The occurrence
of basaltic cinder and cinder cones, however, is more
widespread
than revealed in earlier studies. In the
area south of Henry and southeast of the Blackfoot
Reservoir, numerous cinder pits have been dug and at
least six conspicuous topographic depressions in this
area are probably cinder cones masked by loess.
Throughout
the entire region, loess overlies most
Fiesinger
and
others-Volcanic
flows and cinder cones.
The western part of the Blackfoot lava field near
Tenmile Pass has a thick covering of loess with few
cinder cones and few scarps visible. The alignment of
sinks through the loess suggests the presence of
scarps at depth, which parallel the northwest-southeast trend observable to the east. Similarly, in the
eastern part of the Blackfoot
lava field, south of
Henry and east of the Blackfoot
River, volcanic
features are subdued. This area lies approximately
300 feet higher in elevation than the remainder of the
Blackfoot lava field and has a thick covering of loess.
Scarps are not well-exposed, show little relief, and are
discontinuous,
This area corresponds
with negative
magnetic anomalies interpreted by Mabey and Oriel
(1970) to represent reversely magnetized basalt that is
older than the flows to the southwest.
Mabey and Oriel (1970) also observed negative
magnetic anomalies in Lower, Wooley, and Upper
Valleys and at the southern end of Enoch Valley,
which were interpreted to result from reversely magnetized basalt. Positive anomalies attributed to normally magnetized basalt were reported for the northern end of Enoch Valley and the western side of the
Willow Creek lava field. Since there have been twenty
or more reversals in the Earth’s magnetic field since
Eocene time (Doell and Cox, 1962), specific ages of
these lavas cannot be determined from magnetic
polarity, but the presence of both normal and reverse
magnetization
shows that the flows are not all
equivalent in age. The reversely magnetized flows
would have to have an age greater than 0.7 million
years (Mabey and Oriel, 1970).
The dominant northwest-southeast
scarp system is
somewhat coincident with block fault patterns recognized within adjacent mountain ranges. This fault
pattern probably originated in the Tertiary, and this
style of faulting has continued to the present (Armstrong and Oriel, 1965). The occurrence of isolated
eruptive centers in the Bear River Range, Soda
Springs Hills, Reservoir Mountain, and Pelican Ridge
and the alignment of cinder cones in Gem Valley also
indicate an association between volcanism and faulting (Mansfield,
1927; Armstrong,
1969).
Field evidence indicates continued faulting throughout the period of volcanic activity in the region. Age
relationships between volcanism and scarp formation
are generally not clear and can be established only
locally. Cinder cones are commonly
located on
scarps, with no recognizable offset, indicating eruption along older, inactive scarps. Similarly, scarps
project through (under?) the rhyolite domes south of
Blackfoot Reservoir, but only the southwestern
portion of China Hat shows evidence of possible offset
by faulting. Other cinder cones have been truncated
by scarps indicating post-eruption
displacement. Ex-
Rocks
in
Caribou
County
467
amples include the cinder cones south and southwest
of China Hat, the cinder cone on the eastern side of
Reservoir Mountain, and cinder cones east of the
Blackfoot Reservoir.
Field evidence reveals that basaltic volcanism took
longer than the emplacement of the rhyolite domes.
Three small lobes of basalt occur on the lower slopes
of Middle Cone, on the northwestern
and southeastern sides. Associated with the basaltic lobe to the
southeast is flow-banded,
pumiceous rhyohte containing blebs of basalt (3 to 5 centimeters in diameter)
and larger xenoliths of basalt and white quartzite up
to 20 centimeters in length. Three basaltic xenoliths
were examined petrographically.
Mineralogies and
textures are somewhat typical of fine-grained basaltic
lavas from the region. Unusual features include the
presence of amphibole and alkali feldspar(?)
as
groundmass phases in one, and the presence of quartz
and plagioclase xenocrysts and rounded inclusions of
vesicular brown glass with trichites in the others.
Armstrong
and others (1975) have dated the
rhyolites of China Hat and Middle Cone at 0.1
million years, whereas Leeman and Gettings (1977)
obtained dates of about 0.05 million years. The
rhyolite of Sheep Island, within Blackfoot Reservoir,
has been dated at 1.4 million years (S. H. Evans,
University of Utah, personal communication).
SAMPLING
ANALYTICAL
AND
PROCEDURES
Samples were taken from basaltic flow units and
eruptive centers representing the various lava fields
and valley flows within the region. After a prehminary petrographic
study of more than sixty-five
specimens, twenty-six samples were selected for further study. Samples selected are relatively free of
oxidation, alteration, and vesiculation and represent
diversity as suggested by their textures, mineralogies,
and occurrences.
Sample locations are shown in
Figure 2; brief descriptions are presented in Table I.
Whole rock chemical analyses were obtained using
volumetric,
gravimetric,
and calorimetric
methods
described by Carmichael and others (1968). Mineral
analyses were obtained using the ARL EMX electron
probe micro-analyzer
at the University
of Utah,
following standard procedures for silicate and oxide
analysis. Phenocryst phases were analyzed systematically from core to rim, with an average of five spot
analyses obtained from each of ten randomly selected
phenocrysts or microphenocrysts.
Groundmass phases
were analyzed by taking two to three spot analyses
from each of twenty to twenty-five randomly selected
mineral grains. Correction procedures used on microprobe data follow the methods of Bence and Albee
Cenozoic
468
Geology
of Idaho
more exact identification. Most lavas tend to be nonporphyritic to microporphyritic
with typical basaltic
mineralogy and textures. The nonporphyritic
lavas
generally have less than 5 percent phenocrysts;
the
microporphyritic
lavas generally have more than IO
percent phenocrysts. Phenocryst assemblages consist
of plagioclase and olivine, commonly clustered in a
glomeroporphyritic
texture. The olivine phenocrysts
are euhedral to subhedral, average 0.6 to 1.0 millimeter in diameter, and commonly
have euhedral
crystals of magnetite attached. Skeletal olivine crystals are found in some lavas. The plagioclase phenotrysts average 1.5 millimeters in length, and are
commonly twinned according to the albite, Carlsbad,
and Carlsbad-albite twin laws. Plagioclase and olivine
commonly exhibit a continuous size gradation between phenocrysts and groundmass in most lavas.
A variety of groundmass textures have been observed in the basaltic lavas including ophitic, subophitic, intergranular, and hyaloophitic. Flow-banded
lavas show subparallel alignment of plagioclase phenotrysts or groundmass laths in a pilotaxitic texture.
The groundmass assemblage consists of plagioclase,
olivine, augite, iron-titanium
oxides, and apatite.
(1968) and Albee and Ray (1970) and were made
using the computer program of Nicholls and others
(1977). In all samples where iron was analyzed by
microprobe, total iron is reported as FeO. Modal
analysis consisted of mineral determination
at approximately
1,600 points covering 2 to 3 square
centimeters using a spot interval of 0.1 millimeter.
PETROGRAPHY
Colors and textures of lavas within the region are
variable and range from gray to black for the denser
lavas and gray to black to dark red for the more
vesicular lavas. Samples collected from cinder cones
tend to have distinctive flow banding and are commonly heterogeneous in mineral distribution,
grain
size, and grain orientation, whereas samples collected
from scarps and thick flow units tend to be homogeneous in texture.
Modal analyses of the twenty-six
analyzed lavas
are presented in Table 2. Undifferentiated
groundmass consists of birefringent material too small for
-1
\
, /
,/
/’
:
I
i
‘,
,
i
‘1
C’
8’
I\.\
I
I’ \/
Figure
2. Map of Caribou
County,
Idaho,
showing
the general
Enoch
Valley
and Wooley
Valley
lie east of Henry
Upper
distribution
Valley
lies
of volcanic
rock.
south of Wayan.
The Willow
Creek
Sample
locations
lava field lies north
of Henry;
are indicated
by open circles.
Fiesinger
and others-
Volcanic
Plagioclase commonly dominates the groundmass,
with laths being 0. I to 0.2 millimeter in length, and
albite twinning being ubiquitous. Groundmass olivine
occurs as equant anhedral grains, ranging from 0.1 to
0.5 millimeter in diameter. Augite is commonly in an
ophitic to subophitic relationship with plagioclase.
Single, optically continuous augite crystals greater
than 2 millimeters in length have been recognized.
The iron-titanium
oxides generally average less than
0.3 millimeter in maximum dimension. Magnetite
grains are characteristically
equant, commonly occurring as euhedral cubic crystals, whereas the ilmenite
grains are typically elongate or needlelike. In addition
to these groundmass constituents, two samples (6 and
12) contain diffuse laths of alkali feldspar and have
been classified as trachybasalt.
Interstitial glass, containing abundant microlites,
Table
I. Locations
of analyred
samples
from
lavas of Caribou
County,
Rocks
in Caribou
County
469
is present in many lavas. Glass is the major constituent
within the pillow lavas of Gem Valley (sample 5). The
outer margins of these pillow structures are similar to
those
described by Moore (1970) for Hawaiian
submarine eruptions. They contain an outer palebrown layer of sideromelane that becomes progressively opaque with an increase in microlites going
inward from the rim.
In contrast to these lavas, a distinctive
basalt
porphyry forms Cinder Island, within the Blackfoot
Reservoir (sample 24), and the flows in Nelson and
King Canyons within the Bear River Range, east of
Gem Valley (samples 22 and 23). This porphyry
consists of approximately
30 percent plagioclase
phenocrysts,
5 to 20 millimeters in length, in an
aphanitic to opaque groundmass. These phenocrysts
are commonly fractured or fragmented, suggesting
Idaho.
470
Cenozoic
Geology
that they were broken during emplacement. Twinning
is similar to that described above for microphenotrysts, and extinction patterns indicate normal concentric zoning. There is no evidence of resorption.
The lava at the southern end of Slug Valley
(samples 25 and 26) has been previously described as
ohvine basalt (Mansfield, 1927; Cressman, 1964) and
it does have the appearance of basalt in hand
specimen. In thin section, this lava is found to contain
olivine and augite phenocrysts, up to 2 millimeters in
Table
2. Modal
analyses
of laws
from
Caribou
County,
Idaho
(volume
of Idaho
diameter, in a groundmass of alkali feldspar, olivine,
and augite, with minor plagioclase, exsolved irontitanium oxides, and traces of amphibole and biotite.
Petrographically,
this lava is similar to a fine-grained
shonkinite
(Williams
and others, 1954) and has
tentatively been classified as alkali trachyte (Streckeisen, 1979).
Xenocrysts of sodic plagioclase and quartz occur
in many of the lavas and are commonly
found
throughout
the Blackfoot
lava field. The largest
percent)
Fiesinger
and others-
Volcanic
plagioclase xenocryst, measuring 1 by 2 by 4 centimeters, was found in the lavas of Sheep Island in the
Blackfoot Reservoir. Xenocrysts are more commonly
less than 5 millimeters
in maximum dimension.
Quartz xenocrysts tend to be rounded and have finegrained reaction rims of augite. Plagioclase xenotrysts show resorption features such as embayments,
honey-combed interiors, complex zoning, rounded to
irregular outlines, and replacement by iron-titanium
oxides. Xenocrysts of hypersthene, up to 3 millimeters
in diameter and with compositions
of Enso to En,,,,
have been found in lavas from Gem Valley and Upper
Valley. Xenoliths of silt are present in the lavas of
Gem Valley and are probably derived from the sediments of Lake Thatcher (Bright, 1963).
MINERALOGY
FELDSPARS
Plagioclase is the only feldspar within most samples of nonporphyritic
and microporphyritic
basalt,
pillow basalt, and basalt porphyry.
It coexists with
alkali feldspar as groundmass in trachybasalt, and it
is subordinate to alkali feldspar as groundmass in
alkali trachyte. In general, zoning in plagioclase
phenocrysts tends to be normal, from calcic cores to
sodic rims. Groundmass plagioclase is less calcic than
coexisting phenocrysts
and is zoned to more sodic
compositions.
Representative partial analyses of plagioclase phenocrysts and groundmass are presented
in Table 3; zoning trends are shown in Figure 3.
Compositions of plagioclase phenocrysts and groundmass in basaltic lavas lie between Anso and An,5 with
the average compositions
of both within the labradorite field, AnTo to Am,,. Pillow basalt (sample 5) has
the most calcic plagioclase phenocrysts,
with core
compositions
near Anso (Figure 3b). Plagioclase is
generally more calcic in nonporphyritic
basalt than in
microporphyritic
basalt. The most extensive zoning is
found in a basalt with diabasic texture from Gem
Valley (Figure 3a). The large phenocrysts
in the
plagioclase basalt porphyry (samples 22, 23, and 24)
show limited normal zoning from An-i,, to Anso.
Sample 8 contains numerous plagioclase xenotrysts of more sodic composition
than that of the
coexisting phenocrysts (Figure 3~). The phenocryst
zoning trend for this sample is more sodic than that
of the coexisting groundmass
suggesting that the
phenocryst trend may include some xenocrysts approaching equilibration with the magma. Sample 18
is unusual in that the zoning trends of the coexisting
phenocrysts and groundmass do not overlap, suggesting that the phenocrysts are out of equilibrium with
Rocks
in Caribou
County
471
the magma (Figure 3i).
Samples of trachybasalt (samples 6 and 12) have
average plagioclase compositions that are more sodic
than those of other basaltic lavas. The plagioclase is
more extensively zoned in sample 6 than in sample 12.
OLIVINE
Olivine is present as both phenocrysts and groundmass in most samples of basaltic lava and alkali
trachyte. It is restricted to the groundmass in trachybasalt (samples 6 and 12). The extent of zoning in
both phenocrysts
and groundmass
is illustrated in
Figure 4; average compositions
are presented in
Table 4. In general, zoning in olivine is normal, from
forsteritic cores to fayalitic rims. In the basaltic lavas,
phenocryst
zoning generally overlaps that of the
groundmass,
with groundmass
compositions
being
more iron-rich. Compositions
of phenocrysts range
from FOKJ to Fo40; coexisting olivines in the groundmass range in composition from Fo75 to Fo25. Some
lavas show limited compositional
variation, such as
the pillow lava (sample 5), reflecting rapid extrusion
and cooling. In four lavas, samples 2, 9, 17, and 19,
phenocryst
and groundmass
compositions
do not
overlap, suggesting that the phenocrysts were out of
equilibrium with the magma. Groundmass olivine in
trachybasalt
is more iron-rich
than that in other
basaltic lavas, with compositions
ranging from Fo4s
to Fo,o. The olivine phenocrysts in alkali trachyte are
more magnesium-rich
than those of most basaltic
lavas, with compositions ranging from Foss to Fo72.
AUGITE
Compositions
and zoning trends are illustrated in
Figure 5; representative
analyses are presented in
Table 5. Augite is present in the groundmass of all
basaltic lavas except the pillow basalt (sample 5).
Zoning is more extensive and compositions
more
iron-rich in the microporphyritic
lavas (Figure 5g)
than in the nonporphyritic
ones (Figure 5a-d). Compositions tend to be more calcic than the tholeiitic
Skaergaard trend (Brown and Vincent, 1963), consistent with the lack of a coexisting calcium-poor
pyroxene. Augite occurs as both phenocrysts
and
groundmass in the samples of alkali trachyte (samples
25 and 26) where it is noticeably diopsidic and zoned
toward hedenbergite (Figure 5e, f). The two samples
(6 and 12) of trachybasalt show distinct augite composition fields suggesting differentiation (Figure 5~).
Sample 3 is diabasic, has cooled at a relatively
slow rate, and shows a strong trend toward hedenbergite (Figure 5a). Sample 16 is aphanitic and
472
Cenozoic
Geology
hypocrystalline,
has cooled rapidly, and shows a
calcium-enrichment
trend (Figure 5d). This unusual
zoning of sample 16 is similar to that observed by
Leeman and Vitaliano (1976) in some augites of the
McKinney Basalt and by Smith and Lindsley (1971)
in a flow from the Picture Gorge Basalt. Smith and
Lindsley (197 I) proposed that this “quench trend” of
calcium-iron substitution reflects a metastable crystalliquid partitioning during rapid crystallization.
IRON-TITANIUM
OXIDES
Coexisting magnetite and ilmenite occur as groundmass phases in all lavas studied. Representative
analyses are presented in Tables 6 and 7. Analyses
were not made where these oxide phases were found
of
Idaho
to be exsolved, oxidized, or extremely small and
disseminated. Two generations of oxides are present
in sample 19. The magnetite microphenocrysts
(P) are
noticeably enriched in CrzO,, AbOr, and MgO and
depleted in CaO relative to the magnetite groundmass
(G). The major difference in the coexisting ilmenites
is the higher MgO content and lower CaO content of
the microphenocrysts.
These trends are consistent
with those reported by Carmichael and others (1974)
for fractionating tholeiitic magmas. Magnetite within
the basalt porphyry (sample 24) is noticeably enriched
in A1203 and MgO, containing 3.55 percent Al203
and 2.64 percent MgO compared with a range of 1.8
to 2.4 percent A1203 and a range of 0.5 to 2.4 percent
MgO in magnetites of the other basaltic lavas studied.
These values of Al203 and MgO tend to straddle the
typical values reported for titanomagnetites
of basic
Figure 3. Zoning
trends of feldspars
in
lavas of Caribou
County,
Idaho (molecular percent An, Ab, Or). Open circles indicate average phenocryst
compositions; filled circles indicate average
groundmass
compositions.
For normative feldspars,
filled circles indicate
olivine tholeiites;
open circles indicate
tholeiite,
trachybasalt,
and alkali trachyte. A-G q nonporphyntic
basalt; H
and I q microporphyritic
basalt; and
J-L = plagioclase
basalt porphyry.
Ab
Fiesinger
and others-
Volcanic
lavas with low silica activity, and tholeiitic lavas
(Carmichael and others, 1974). Compositions
of the
magnetite-ulvospinel
solid solution range from 30 to
54 molecular percent magnetite whereas compositions
of the coexisting hematite-ilmenite
solid solution
range from 88 to 96 molecular percent ilmenite.
PETROLOGY
CHEMISTRY
AND
CLASSIFICATION
Chemical analyses and CIPW norms of lavas
analyzed for this study are presented in Table 8. The
silica contents range from 45 to 53 percent for
basaltic lavas and from 53 to 56 percent for alkali
trachyte. Using the normative classification of Yoder
and Tilley (1962), the basaltic lavas are saturated to
oversaturated
with silica; they contain normative
Rocks
in Caribou
Counry
473
hypersthene with olivine or quartz and are classified
as olivine tholeiite and tholeiite respectively. It should
be noted that no calcium-poor pyroxene is present in
the groundmass of any of these lavas, nor is there any
evidence of olivine reaction to form a calcium-poor
pyroxene.
The occurrence of normative quartz in samples
4, 22, and 24, and the low amount of normative
olivine in sample 17 (less than 1.0 percent) is attributed to oxidation of iron. These four samples have
FeO/(FeO + Fe203) ratios ranging from 0.36 to 0.60
compared with a range of 0.73 to 0.87 for all other
basaltic lavas. The high silica content and normative
quartz content of sample 18 is attributed to the
assimilation of pumiceous rhyolite from Middle Cone
or crustal rock. Trachybasalt (samples 6 and 12) and
basalt of Upper Valley (samples 15 and 16) contain
normative hypersthene and quartz and are classified
as tholeiitic.
The designation of most basaltic lavas as olivine
474
Cenozoic
Geology
tholeiite, using normative rather than mineralogical
criteria, is consistent with other studies of lavas
from the Snake River Plain, such as Tilley and
Thompson (1970), Thompson (1975), Leeman and
Manton (1971), and Leeman and Vitaliano (1976).
Characteristics
of the Snake River Plain olivine
tholeiite include MgO between 6.4 and 11.2 percent
and phenocryst
of olivine * plagioclase set in a
groundmass of olivine, plagioclase, augite, and irontitanium oxides (Thompson,
1975).
The association between the various volcanic rocks,
and the change in normative constituents as a function
of ferrous-ferric
ratio, are illustrated on the normative
olivine-diopside-quartz
(Ol-Di-Qtz)
diagram (Figure
6). The indicator ratio (I.R.) of Coombs (1963) may
be calculated from normative constituents
((Hy +
2Qz)/Hy
+ 2(Qz + Di)] or determined from this
diagram by projecting a line from the 01 apex,
through the sample data point, to the Di-Qtz join.
There is an apparent clustering of samples of nonporphyritic olivine tholeiite (filled circles) with I.R.‘s
between 0.0 and 0.3, in the field of mildly alkaline and
transitional basalt. This is comparable with the I.R.‘s
reported for Snake River Plain basaltic lavas (Stout
and Nicholls, 1977).
The differentiation
index (D.I.) of Thorton and
Tuttle (1960), defined as the sum of normative quartz,
orthoclase, albite, nepheline, leucite, and kaliophilite,
ranges from 26 to 31 for samples of nonporphyritic
olivine tholeiite (Table 8). This range is also comparable with that of Snake River Plain basaltic lavas
(Stout and Nicholls, 1977). Samples designated as
tholeiitic have D.I.‘s ranging from 36 to 44, whereas
alkali trachyte samples have D.l.‘s greater than 48.
The alkali-silica variation diagram has been used
by Macdonald and Katsura (1964) in their study of
Hawaiian lavas to distinguish alkali-olivine
basalt
from tholeiitic basalt. Figure 7 shows that most lavas
have an alkali-olivine basalt affinity, with only three
samples in or near the tholeiite field. These three
samples have been discussed previously, with samples
15 and 16 being classified as tholeiitic and sample 18
containing xenocrysts of alkali feldspar and quartz.
The AFM diagram (Figure 8) indicates the relationships between the lavas of Caribou County, the
Snake River Plain olivine tholeiites, and the lavas
from Craters of the Moon. In general, the samples of
nonporphyritic
olivine tholeiite from Caribou County
(filled circles) lie within or near the field of Snake
River Plain olivine tholeiites. The three samples of
plagioclase basalt porphyry (22, 23, and 24; open
circles with horizontal bars) and two samples of
tholeiite (I5 and 16; filled circles with vertical bars)
are more alkali-rich than the other olivine tholeiites.
The alkali and iron enrichment of trachybasalt (samples 6 and 12; filled circles with horizontal bars) is
of Idaho
also apparent, as these two samples plot close to the
Craters of the Moon field.
Characteristics
of the Craters of the Moon-type
lavas include high Ti02, FeO, P205. and total alkalies
(Leeman and others. 1976). Samples 6 and 12 are
noticeably higher in PzOS, total alkalies, and Fe/(Fe +
Mg) than the other analyzed samples, but they contain alkali feldspar in the groundmass which has not
been reported previously for basaltic lavas of the
Snake River Plain, and they contain normative quartz.
In addition, the groundmass of these lavas is hypocrystalline rather than glassy to opaque as reported
Nonporphyritic
Basalt
UI
I
2
I
-m
I
3
I
I
.
1
4
5
.
I
7
I
l
1
I
8
9
,
1
-
-m
I
I=
IO
.
I
I-
II
0
.
I
I3
I
.
I
I
.I
ml
6
-
I2
I
14
.,
1
IS
1
L
I6
Mieroporphyrilic
I
Barolt
I7
-m
16
I
I
l
I
I9
.
I
-ml
L
20
.
I
21
Plagioclorr
Bosoll
t
1
-
.
I
.
Porphyry
22
-
23
24
I
Alkali
Trachyte
25
w
26
1
I
Fo
I
20
Figure
1
I
I
I
I
60
olivines in lavas of
1
I
80
I
I
FO
Caribou
County,
circles
above
horizontal bars indicate average phenocryst compositions; filled
circles
below
horizontal
bars indicate
average
groundmass
compositions.
Idaho
4. Zoning
trends
of
(molecular
percenr
I
I
I
40
Fo,
Fa).
Filled
Fiesinger
and others-
Volcanic
Rocks
for the ferrobasalt of Leeman and others (1976), and
the high-iron lavas of Stout and Nicholls (1977).
The two samples of alkali trachyte (25 and 26;
open diamonds) lie completely away from the basaltic
lavas indicating the lack of a petrogenetic relationship. The scatter of the remaining samples (18, 19,
and 8; open circles) is attributed to the occurrence of
quartz and alkali feldspar xenocrysts.
Similar relationships are also indicated on the iron
enrichment diagram (Figure 9). There is a cluster of
data points for nonporphyritic
olivine tholeiite (filled
circles), and a distinct separation of tholeiite (filled
circles with vertical bars), trachybasalt (filled circles
with horizontal bars), plagioclase basalt porphyry
(open circles with horizontal bars), and alkali trachyte
(open diamonds). The microporphyritic
lavas (open
circles) which contain xenocrysts of quartz and alkali
feldspar show considerable scatter. The uniqueness of
alkali trachyte is indicated as it lies well away from
any iron enrichment trend of the basaltic lavas.
Table
4. Representative
Sample
average
Weight
Fe0
Number
microprobe
Per+
analyses
of olivines
Oxides
in Caribou
Counfy
475
GEOTHERMOMETRY
Coexisting magnetite-ulvospinel
and ilmenite-hematite solid solutions in sixteen samples representing
tholeiitic lavas have been analyzed permitting the
application of the geothermometer
of Buddington
and Lindsley (1964). Microprobe analyses were recalculated using the method of Carmichael (1967a). and
the temperatures and fugacities were calculated using
the equations derived by Powell and Powell (1977).
Results of these calculations are presented in Table 9
and Figure 10. The temperatures of equilibration and
corresponding
oxygen fugacities plot close to the
fayalite-magnetite-quartz
buffer curve, indicating internal buffering of oxygen by the crystal-liquid
assemblage (Carmichael and Nicholls, 1967). Temperatures range from a high of 1lOl’C
to a low of
805” C with a corresponding range in oxygen fugacity
(-logfoz) of 8.6 to 15.0. Sample 19, containing both
microphenocryst
and groundmass
oxides, shows a
in lavas from
Weigbt
Percent
Fe
Caribou
County,
End ?&mt&
La
MgO
CIO
Fa
27.1
41.6
22.3
27.7
43.8
22.3
39.0
35.0
23.0
38.6
34.0
19.7
38.6
24‘7
0.39
0.49
0.42
0.55
0.69
0.73
0.87
38,d
58.9
31.6
39.3
65.0
31.9
53.3
61.1
40.2
67.4
59.4
34.3
67.4
43. I
0.6
0.8
0.1
1‘1.
1.1
1.3
100.4
52.2
31.4
44.7
14.7
26.7
20.9
0.66
0.71
0.69
65.9
53.0
63.4
33.1
46.7
36,d
1.1
1.1
1.1
36.5
27.8
36.5
20.1
ii.38
0.50
0.67
0.71
33.4
51.2
36.0
63.6
63.8
48.5
63.8
35.0
28.7
30.4
42.8
25.8
33.6
31.8
21.9
0.26
0.39
0.78
0.71
53.4
40.7
43. I
60.6
17.5
22.6
42.8
38.3
0.50
0.43
24.9
32.0
Idaho.
Motecutar
FTi
wTQtd
Percent
F0
End Members
La
Nonporphytitic Basalt
3(P)
3(G)
W)
5(G)
6(G)
W)
w3
12(G)
IS(P)
13(G)
lo%1
99.9
99.1
99.5
100.4
99.1
30.3
50.0
24.3
31,2
56.0
24.5
d6.4
69. I
49.2
75.1
68.0
42.9
1d.S
52.3
0.6
0.8
0.6
0.8
1.1
1.0
1.3
1001
100.8
100.9
66.0
43.6
54.1
33.0
33.4
44.8
1.1
I.0
I.1
0.6.
0.8
1.0
I.1
99.8
100.3
100.8
99.7
21.6
41.8
27.9
55.0
71.8
57.4
71.1
0.5
0.8
0.9
43.9
1.1
45.0
$8.6
55.3
38.2
0.4
0.6
1.2
1.1
100.8
99.9
99.8
99.9
43,8
32‘2
34.5
31.7
53.8
67.2
64.4
47.2
0.4
0.6
1.1
1.1
74.7
66.8
0.8
0.7
100.4
99.5
18.6
24.7
80.7
74.7
0.7
0.6
0‘8
-.
Microporphyritic Basalt
17(P)
17(G)
18(P)
18(G)
25.0
36.1
23.4
44.8
PlagioclaseBasalt Porphyry
2X’?
39. I
23(G)
24(p)
24~~1
Alkali Trachyte
25~)
I
2W’)
(P) = phenocryst
(G) = groundmass
I
I
476
Cenozoic
Geology
decrease in temperature and oxygen fugacity going
from microphenocrysts
to groundmass as is expected.
Reported iron-titanium
oxide equilibration temperatures for Snake River Plain olivine tholeiites and
Craters of the Moon high-iron lavas range from
1095” C to 915” C, and corresponding
oxygen fugacities (-log fo2) range from 9.6 to 12.8 (Stout and
Nicholls, 1977).
The presence of groundmass
olivine and augite
permits the application of a second geothermometer,
that of Powell and Powell (1974). Calculated temperatures, using average olivine and augite groundmass
compositions
and an assumed pressure of one bar,
range from lOl3O C to 993” C (Table 9). The apparent
poor correlation between the olivine-clinopyroxene
geothermometer
and the iron-titanium
oxide geothermometer reflects the uncertainty of both methods
(* 30”) and the use of average compositions for zoned
groundmass phases. In addition, the uncertainty of
the olivine-clinopyroxene
geothermometer
increases
with increasing pressure, 5” * 5’ per kilobar (Powell
and Powell, 1974).
of Idaho
PETROGENESIS
As previously indicated in the discussion of chemistry and classification,
the analyzed lavas from
Caribou County do not comprise a coherent population. To evaluate the presence of a liquid line of
descent, lavas to be considered should be fine grained
and nonporphyritic.
An assumption is made that these
lavas represent samples from an evolving magmatic
system. Because of the chemical similarity in the
samples of nonporphyritic
olivine tholeiite collected
from diverse parts of Caribou County, their low silica
content, and high MgO content, these lavas most
likely approximate
a parental magma (Figure 1 I).
Processes of assimilation and low-pressure fractionation can then be evaluated using whole-rock chemical
analyses from Table 8, mineral analyses from Tables
3,4, 5,6, and 7, and the interactive computer program
and procedures outlined by Stormer and Nicholls
(1978). The quality of fit for each test is expressed as
the sum of squares of residuals, where the residual of
each component oxide is obtained by subtracting the
n
.Hd
n
20
19
Y!Y-
En
Figure 5. Zoning trends of augiles in laws of Caribou
County,
zoning within individual
laws; small filled circles indicate
intrusion (Brown and Vincent, 1963). A-D q nonporphyritic
18
VFS
Idaho (molecular
percent Wo, En, Fs). Irregular
oval fields indicate extent of
average compositions.
Curve is zoning trend of augites from the Skaergaard
basalt; G = microporphyritic
basalt: and E and F = alkali trachyte.
Fiesinger
and others-
Volcanic
calculated difference between parent and daughter
from the observed difference between parent and
daughter. The smaller the value of the sum of squares
term, the better the fit; tests with values less than one
are given further consideration.
Acceptability
as a
petrologic model depends on consistency between the
calculated amounts of fractionated/assimilated
phases
and petrographic observations.
Representative fractionation schemes are presented in Table 10.
Tholeiite
and Trachybasalt
Models tested tc determine the relationship between parental olivine tholeiite and daughter products, tholeiite and trachybasalt,
consisted of the
fractionation of olivine, plagioclase, augite, and irontitanium oxides and the assimilation
of rhyolite
(unpublished analyses from W. P. Nash, University
Table
5. Representative
(P) = phenocryst
(C) = groundmass
average
microprobe
analyses
(weight
percent)
Rocks
in Caribou
County
477
of Utah). As shown in Table 10, tholeiite, represented
by sample 16, and trachybasalt, represented by sample
6, can be derived from olivine tholeiite (sample 10) by
fractionating
olivine, plagioclase, augite, and irontitanium oxides. The transition from tholeiite sample
15 to tholeiite sample 16 can also be accounted for by
fractionation
of these same phases. The transition
from trachybasalt sample 6 to trachybasalt sample 12
requires the fractionation of feldspar, olivine, augite,
and ilmenite and the addition of a small amount of
magnetite. Similar testing with rhyolite included as
an additional phase made no significant difference in
residuals or in the amounts of other phases, showing
that assimilation was not a significant factor in the
formation of these lavas.
Most fractionation
models require the removal of
all analyzed phases found within these basaltic lavas,
rather than just the early formed microphenocrysts,
of au&es
in laws
from
Caribou
County,
Idaho.
478
Cenozoic
Geology
of Idaho
Fiesinger
01
and others-
Volcanic
in Caribou
Counrj
479
011
HY
Figure 6. Normative
olivine-diopside-quart7
(01.Di-Qtz)
diagram
for laws of Caribou
County,
Idaho. Symbols
used: filled
circles-nonporphyritic
basalt; open circles-microporphyritic
basalt; open circles with horizontal
bars-plagioclase
basalt
porphyry;
filled circles with vertical
bars-tholeiite;
filled
circles with horizontal
bars-trachybasalt;
open diamondsalkali trachyte;
lines with arrows indicate changes in normative
constituents
with increase in ferrous-ferric
ratio. Indicator
ratio
of Coombs (1963) is shown on the Di-Qtz join.
46
Rocks
Figure 8. AFM diagram for laws of Caribou
County,
Idaho. A =
weight percent K20 + NaO;
F = weight percent Fe0 + FeO,;
M = weight percent MgO. Symbols
used: filled circles-nonporphyritic
basalt; open circles-microporphyritic
basalt; filled
circles with vertical bars-tholeilte;
filled circles with horizontal
bars-trachybasalt;
open circles with horizontal
bars-plagioclase basalt porphyry;
open diamonds-alkali
trachyte.
SRP
indicates distribution
of Snake River Plain olivine tholeiites
and COM indicates distribution
of Craters of the Moon laws
(Leeman
and others, 1976).
52
Figure 7. Alkali-silica
diagram
for lavas of Caribou
County,
Idaho.
Symbols
used: filled circles-nonporphyritic
basalt:
open circles-microporphyritic
basalt; filled circles with vertical
bars-tholeiite;
filled circles with horizontal
bars-trachybasalt;
open circles with horizontal
bars-plagioclase
basalt porphyry.
Diagonal
line is alkali olivine
basalt-tholeiite
boundary
of
Macdonald
and Katsura (1964).
Figure 9. Iron enrichment
diagram
for laws of Caribou
County,
Idaho.
Symbols
used: filled circles-nonporphyritic
basalt;
open circles-microporphyritic
basalt; filled circles with vertical
bars-tholeiite;
filled circles with horizontal
bars-trachybasalt;
open circles with horizontal
bars-plagioclase
basalt porphyry;
open diamonds-alkali
trachyte.
480
Cenozoic
Geology
olivine with magnetite, and plagioclase, as observed
in thin section. As mentioned previously, augite does
not occur as distinctive phenocrysts, but rather as an
ophitic to subophitic groundmass phase. This suggests
that a process such as filter pressing may have been in
operation, rather than gravity settling or flowage
differentiation.
This process would drive off interstitial liquid, leaving a crystal mush of microphenocrysts
and groundmass crystallization
products.
When deriving the tholeiite and trachybasalt,
Table
8. Whole-rock
‘Total
includes
chemical
4.95 percent
analyses
normative
(weight
hematite
percent)
and CIPW
of Idaho
estimated amounts of fractionated
crystals range
from 40 to 60 percent; that is, 40 to 60 percent of the
parent magma would have crystallized at the time of
separation of the daughter lavas. This is not unlike
the 65 to 70 percent crystallization
required to derive
the iron-rich Craters of the Moon lavas of Stout and
Nicholls (1977).
The fractionation
of olivine and plagioclase, the
microphenocrysts
commonly found in olivine tholeiite, has been proposed by Leeman and Vitaliano
norms
of lava
from
Caribou
County,
Idaho
Fiesinger
and
others-
Volcanic
(1976) to account for the variation in composition of
the McKinney Basalt, and also by Tilley and Thompson (1970) to account for the variation in their suite
of Snake River Plain lavas. Similarly, olivine and
plagioclase fractionation
can account for the variation within the samples of olivine tholeiite from
Caribou County. But there is still the possibility that
the slight scatter of data for olivine tholeiite may
result from the presence of more than one liquid line
of descent or the presence of more than one parental
magma. Age relationships have not been sufficiently
Table
8. continued.
Rocks
in Caribou
County
481
established to conclude that all samples of olivine
tholeiite in Caribou County are contemporaneous,
and magnetic data discussed previously
certainly
indicates otherwise.
Contaminated
Lavas
The presence of numerous xenocrysts of quartz
and feldspar, possibly derived from the incorporation
of rhyolite, together with differentiation, may account
for the compositional variation among lava samples
482
Cenozoic
Geology
18, 19, and 8. Sample 18, for example, can be
accounted for by adding approximately
25 percent
rhyolite to parental olivine tholeiite (sample 7).
Sample 19 can be derived by fractionating
a small
amount of olivine and magnetite from olivine tholeiite
(sample 9) and adding approximately
9 percent
rhyolite (Table IO). Sample 8, in turn, can be derived
by fractionating olivine, augite, plagioclase, and irontitanium oxides and adding less than 5 percent
rhyolite (Table 10).
Sample 18, however, is the only lava close to
rhyolite, but the assimilation of 2.5 percent rhyolite in
the near-surface
environment
leaving less than I
percent xenocrysts is probably unrealistic. Alternatively, the incorporation
of quartzofeldspathic
crustal
rocks at depth would have to be considered for all
three samples, but this process cannot be evaluated at
this time.
Plagioclase
Basalt
Porphyry
The texture of plagioclase basalt porphyry, represented by samples 22, 23, and 24, indicates the accumulation of plagioclase phenocrysts.
Using olivine
tholeiite (sample 9) as the parent, plagioclase basalt
porphyry (sample 24) can be derived by fractionating
olivine, augite, and less than I percent iron-titanium
oxides and adding I5 percent plagioclase (Table IO).
The accumulation of plagioclase in the plagioclase
basalt porphyry may result from processes such as
flowage differentiation,
bubble transport,
or the
of ldaho
buoyancy of plagioclase in a more dense magma.
Both olivine and plagioclase occur as dominant
phenocrysts in microporphyritic
lavas in the region,
but only plagioclase is selectively concentrated
in
these rocks suggesting that bubble transport
or
density contrast may be significant.
Densities of
olivine tholeiite were calculated over a range of
temperature from 1000°C to 1200°C using the procedure of Bottinga and Weill (1970). Densities of
plagioclase over this same temperature interval were
determined for compositions
Anso and AnTo using
data presented in Campbell and others (1978). This
range in composition encompasses that of the average
plagioclase in the three analyzed samples of plagioclase basalt porphyry.
Calculations
for anhydrous
conditions yield liquid densities consistently greater
than that of plagioclase. For calculations with water
contents greater than I molecular percent, the density
contrast is reversed, with the density of the magma
being less than that of plagioclase (Table 11). For
density contrast to have been effective in producing
the plagioclase basalt porphyry, a low water content
must be assumed for the differentiating magma. This
is consistent with the lack of resorption
features
within the plagioclase phenocrysts.
Table
9. Temperatures
County,
Idaho.
(P)
(G)
= phenocryst
= groundmass
9IO II
P
-
13-
800
900
1000
1100
Figure
IO. Temperatures
of iron-titanium
oxide
equilibration
for
lavas of Caribou
County,
Idaho.
Symbols
used: filled
circlesnonporphyritic
basalt;
open circles-microporphyritic
basalt;
filled
circles
with vertical
bars-tholeiite;
P-phenocryst;
Ggroundmass.
Curve
indicates
fayalite-magnetite-quartz
buffer.
of equilibration
for
lava
from
Caribou
Fiesinger
Table
IO. Representative
I
Is.
I+- . ..
‘;-‘,--m*
F.0‘3.
.
.
12II 7
fractionation
and others-
schemes
for
0I
0
00
---------0
0 et **
0
t
5
4
6
derivation
Volcanic
of lava
Rocks
from
in Caribou
Caribou
County
County,
483
Idaho.
I
,
Figure
I I. MgO variation
diagram
for lavas of Caribou
microporphyritic
basalt;
filled
circles
with vertical
horizontal
bars-plagioclase
basalt
porphyry.
County,
bars-tholeiW
Idaho.
Symbols
used: filled
circlesPnonporphyritic
filled
circles
with horizontal
barsPtrachybasalt:
basalt;
open
open circlescircles
with
Cenozoic
484
Table I I. Densities
plagioclase.
* z-I.5
Alkali
molecular
@m/cc)
percent
of olivine
tholeiite
and
Geology
coexisting
H20
Trachyte
The high MgO, silica, and alkali content, and low
total iron content of the samples (25 and 26) of alkali
trachyte from Slug Valley preclude the derivation of
this lava from olivine tholeiite. Comparing
these
samples with other potassium-rich
alkaline rocks,
they are most similar to orenditic rocks, saturated
rocks with K20 in excess over NazO (Sahama, 1974).
The lavas from Slug Valley are quartz normative with
a KzO:NazO ratio of approximately
2: I; but they are
noticeably less potassic than the “average” orendites
of Sahama (1974), and they contain magnetite which
is uncommon in rocks of orenditic affinity (Carmichael, 1967b). Orenditic rocks are rare and occur in
a few restricted areas, such as the Leucite Hills in
Wyoming, the Fitzroy Basin of Western Australia,
and southeastern Spain, where they are confined to
volcanic or subvolcanic environments
and lack plutonic counterparts
(Sahama, 1974).
Carmichael (1967b) considered the orenditic lavas
to be of mantle origin, and Prider (1960) proposed the
derivation of the Fitzroy Basin rocks to be from a
mica peridotite magma. In a recent review of hypotheses for the origin of highly potassic magmas, Gupta
and Yagi (1980) concluded that the origin is related to
partial melting of a phlogopite peridotite-bearing
upper mantle.
If a similar origin is to be assumed for the Slug
Valley lavas, the amount of partial melting can be
determined using an addition-subtraction
diagram
(Bowen, 1928). To represent the mantle composition,
mica peridotite analyses from Best and others (1968)
were used. These peridotites are part of an association
of highly potassic, silica-poor igneous rocks located
east of Salt Lake City, Utah, at the western end of the
east-west trending Uinta Arch. The related volcanic
rocks are reported to be similar to those of the
Leucite Hills (150 miles northeast of the Utah locality)
and range in age from upper Eocene to lower
Pliocene (Best and others, 1968). Partial melting on
the order of 30 to 35 percent is required to derive the
Slug Valley lavas from this mica peridotite.
of Idaho
Olivine
Tholeiite
The origin of olivine tholeiite, considered to be the
parental magma from which tholeiite, trachybasalt,
and plagioclase basalt porphyry have been derived,
must now be evaluated. Considering the studies of
Snake River Plain olivine tholeiite, Stout and Nicholls
(1977) proposed partial melting of a pyrolite mantle;
Leeman and Vitaliano (1976) and Leeman (1976)
proposed partial melting of a spinel-peridotite mantle,
or derivation from a deeper source and equilibration
with a spinel-peridotite mantle; and Thompson (1975)
proposed partial melting of a relatively iron-rich
spinel-lherzolite upper mantle. Thus, the percentages
of partial melting were calculated, using the pyrolite
of Green and Ringwood (1967) and a spine1 lherzolite
analysis from Bacon and Carmichael (1973) as hypothetical mantle compositions (Table 12). For a pyrolite mantle, approximately
10 percent partial melting
is required, whereas for a spine1 lherzolite mantle, less
than 2 percent melting is required. The amount of
partial melting required before liquid will separate
from the mantle depends on such variables as water
content in the mantle and conduit geometry. As the
percentages of partial melting for the pyrolite mantle
are greater than that for a lherzolite mantle, the
segregation of a larger volume of melt should occur
more readily. As demonstrated by Stout and Nicholls
(1977), the residual material derived from partial
melting of pyrolite has a composition similar to spine1
lherzolite. Thus, if the mantle is similar to pyrolite,
spine1 lherzolite represents depleted mantle. Regardless of the interpretation, spine1 lherzolite nodules are
not associated with tholeiitic magmas, nor have they
been found in lavas of the Snake River Plain or in
those of the present study.
To pursue the issue further, the residual material
was recast as a mineral assemblage of olivine, clinopyroxene, orthopyroxene,
and spinel, that is, a spine1
lherzolite using the method of Nicholls (1977b). The
temperature
of equilibration
was assumed to be
1200” C and the pressure of equilibration was taken to
be 12 kilobars (12 x 10’ kPa) based on a crustal
thickness of approximately 40 kilometers for this area
(Smith, 1978). The resulting calculated mineral comTable 12. Percentage
of partial melting
olivine tholeiites
from hypothetical
lherzolite
composition.
required to derive selected
mantles of pyrolite
and
Fiesinger
and orhers-
Volcanic
positions and modal amounts of these minerals are
relatively insensitive to moderate changes in pressure
and temperature. The next step is to estimate pressure
and temperature conditions of equilibration between
the parental magma and the calculated residual
assemblage. The procedure used was that of Nicholls
(1977a), based on the thermodynamics
of heterogeneous equilibria using activity-composition
relationships in silicate melts and solid phases (see also
Carmichael and others, 1977; Evans and Nash, 1979).
In this procedure, pressure-temperature
curves are
calculated representing
equilibration
between the
activities of components in the solid residuum and
activities of those same components in the magma.
Theoretically, the intersection of these curves would
then indicate the pressure and temperature of equilibration. As many as five pressure-temperature
curves
were calculated for a given magma-residuum
pair.
Equilibrium,
based on curve intersections, was not
indicated at any reasonable pressures and temperatures. It is thus concluded that pyrolite is not a
realistic source composition for the olivine tholeiite.
CONCLUSIONS
The volcanic rocks of Caribou County, Idaho,
consist primarily of olivine tholeiite, sharing many
characteristics
with the olivine tholeiite of the Snake
River Plain. Petrographic
similarities
include the
presence of olivine and plagioclase phenocrysts, the
lack of calcium-poor
pyroxene, the presence of
olivine in the groundmass,
and the occurrence of
ophitic to subophitic groundmass augite. Chemically,
the similarity is apparent when analyses are plotted
on AFM and the alkali-silica
diagram, and on a
comparison of the differentiation index and Coomb’s
indicator ratio.
Minor occurrences of tholeiite and tholeiitic trachybasalt may be genetically related to parental olivine
tholeiite by fractionation
of olivine, plagioclase, augite, and iron-titanium
oxides, leaving 40 to 60
percent residual liquid. As this requires removal of all
crystalline phases rather than just microphenocrysts,
filter pressing within a compressed
near-surface
magma chamber is proposed for the mechanism of
formation.
Plagioclase basalt porphyry, containing 30 percent
plagioclase phenocrysts,
occurs at Cinder Island in
the Blackfoot
Reservoir and at Nelson and King
Canyons in the Bear River Range east of Gem Valley.
Its derivation from parental olivine tholeiite may be
accomplished by fractionating
5 percent olivine and
Rocks
in Caribou
County
485
accumulating I5 percent plagioclase. Density calculations indicate an appropriate density contrast between
liquid and plagioclase under anhydrous conditions,
for plagioclase crystals to rise through the magma.
The lack of resorption features and limited zoning
suggest that water pressure was probably very low
during emplacement.
Alkali trachyte, which is high in MgO, silica, and
alkalies, occurs at the southern end of Slug Creek
Valley and is unrelated to other lavas in the area. It is
chemically similar to orendites and may be related to
the Leucite Hills in Wyoming (130 miles to the
southeast).
The origin of olivine tholeiite and alkali trachyte
has been evaluated with the partial melting of various
hypothetical mantle compositions.
The derivation of
olivine tholeiite would require less than 2 percent
melting of spine1 lherzolite or approximately
IO
percent melting of pyrolite. The migration of magma
would be facilitated with larger amounts of melt,
favoring a pyrolite model. Equilibration
studies between melts and residuum from pyrolite, recast as
spine1 lherzolite representing depleted mantle, show
that pyrolite, however, is not a reasonable source
composition. The origin of the alkali trachyte, with
its high KzO content, necessitates a potassic source,
such as mica peridotite. Mica peridotite is associated
with potassic lavas at the western end of the Uintas
(130 miles to the south) and was used as the
hypothetical mantle composition.
The derivation of
alkali trachyte requires 30 percent partial melting of
such mica peridotite. The occurrence of these two
different magmas, both of which are assumed to be
mantle derived, indicates contrasting source regions
or in homogeneities in the mantle. This may be
indirect evidence for differences in the mantle, either
in composition or structure, between the Basin and
Range province and the Middle Rocky Mountain
province.
ACKNOWLEDGMENTS
Financial support for this work was provided in
part by U. S. Geological Survey Grant No. l4-080001-G-545; a summer Fellowship,
Utah State University (to Perkins); Research Assistantships,
Utah
State University (to Perkins and Puchy); and the
College of Science, Utah State University (computer
costs). We thank William P. Nash and Stan Evans,
University
of Utah, for use of the electron microprobe, unpublished analyses of rhyolite, and age
dates.
Cenozoic
Geology
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