Metamorphic evolution of the Archean Pony Middle Mountain Metamorphic
Suite, Tobacco Root Mountains, southwestern Montana
Department of Geology, Whitman College, Walla Walla, WA
The Pony Middle Mountain Metamorphic Suite (PMMMS) is the northernmost
of three recognized Archean terranes in the uplifted core of the Tobacco
Root Mountains (figure 1). The other terranes are the Spuhler Peak
Metamorphic Suite (SPMS) and Indian Creek Metamorphic Suite (ICMS)
(Burger et al., 1994). Previous Keck research has concentrated upon
the SPMS and ICMS with small amounts on the PMMMS. The goal of this
project is to characterize and determine the metamorphic history of
the PMMMS, and to draw correlations between the PMMMS and the other
suites present in the Tobacco Root Mountains.
The PMMMS is characterized by thick sequences of quartzofeldspatihic
gneisses (approximately 55% of outcrop) along with lesser amounts
of thick concordant units of mafic gneiss (approximately 35% of outcrop).
The latter is interpreted by Vitaliano et al., (1979) as originally
lava flows and/or thin pyroclastic layers. Many of these mafic units
can be traced along strike for several kilometers. The remaining 10%
consists of thin layers and pods of pyroxene-rich gneiss, aluminous
schists, possible iron formation rocks and banded garnet gneiss, ultramafic
pods, marble, and quarzite. Later metamorphosed mafic dikes/sills
(MMDS) are locally abundant as thin, fine-grained sheets with distinct
contacts that cross-cut the gneissic foliation of the host rocks.
The mafic gneisses can be distinguished from the MMDS based on their
coarser grain size, greater continuity, and concordant nature with
regards to the foliation of the surrounding rock types, especially
with that of the quartzofeldspathic gneisses.
Quartzofeldspathic Gneisses: The common mineral assemblage in the
quartzofeldspathic gneisses is plagioclase + quartz ± biotite
± hornblende ± garnet. Potassium Feldspar was not found
in any of the quartzofeldspathic gneisses, although it has been reported
by others (Vitaliano et al., 1979). The mafic mineral content varies
from 0 to approximately 30%. Most of the samples collected contain
garnets (0.25 to 5 mm in diameter), several of which have slight sigmoidal
tails or pressure shadows of felsic material. Foliation is often bent
around the garnet porphyroblasts, indicating pre-to-syn tectonic growth.
All of the quartzofeldspathic gneisses are strongly foliated in one
direction, with several showing evidence for cataclastic shear deformation.
The quartzofeldspathic gneisses contain mineral assemblages consistent
with metamorphism in the upper amphibolite facies (Immega and Klein,
1976). Sericite alteration of plagioclase grains occurs in most samples
and is locally abundant, with two sections showing further alteration
to phengite. Such alteration of plagioclase is consistent with a minor
late stage (Mesozoic) thermal event, possibly related to Laramide
tectonic effects and emplacement of Cretaceous intrusions (Burger
et al., 1994), and/or to minor shear deformation of the rocks.
Mafic Gneisses: These are the second most abundant units in the PMMMS.
These units are found as both thick layers that are extensive along
strike, and as centimeter- and meter-sized boudinaged layers and mafic
pods within the quartzofeldspathic gneisses. They contain medium to
coarse grains of hornblende and plagioclase ± quartz ± biotite
± garnet ± clinopyroxene ± orthopyroxene ± potassium
feldspar. The amphibole is predominantly hornblende. Most samples
(~80%) contain a green hornblende in thin section while the other
~15% contains brown hornblende. Only one sample showed both green
and brown varieties together. Actinolite occurs as a later, fine-grained
Cady (1994) described the amphibolites of the PMMMS similarly, but
described only those of "salt & pepper" texture, which has a salt
and pepper-like appearance from the distribution of small plagioclase
and quartz grains and darker hornblende and other mafic minerals.
Lowell (1994) described "wispy" amphibolites from the SPMS which are
characterized as being migmatized hornblende-amphibolite with abundant
felsic melt pockets, or "wisps" of plagioclase and quartz. The PMMMS
contains mostly salt & pepper amphibolites, but subordinate wispy
types also exist. Quartz and plagioclase inclusions in hornblende
exist locally. With a few exceptions, foliation is moderately developed
in both the amphibolites and hornblende gneisses. The amphibolites
and hornblende gneisses contain mineral assemblages consistent with
the upper amphibolite facies. Several samples show mineral assemblages
suggesting possible granulite facies metamorphism.
Pyroxene-rich Gneisses: Pyroxene gneisses were found predominantly
as centimeter- and meter-sized boudins and layers in the hornblende
gneiss sequences, as well as minor boudins in quartzofeldspathic gneisses
of intermediate mafic content. Clinopyroxene is the dominant pyroxene.
A few samples, however, showed both clinopyroxene and orthopyroxene.
Several of the pyroxene gneisses are heavily altered with replacement
of many of the pyroxene grains by very fine-grained lower temperature/pressure
minerals (talc and serpentine?). Several samples have large relict
clinopyroxene crystals (up to 1.2 cm in length) in a finer matrix
of clinopyroxene + plagioclase + quartz + hornblende + garnet ±
orthopyroxene. The large diallage clinopyroxene porphyroclasts appear
to be in disequalibrium with the rest of the minerals in the section,
and are rimmed by smaller (0. 25 to 0.75 mm) equilibrium clinopyroxene
grains. These porphyroclasts may be relict igneous pyroxenes or relicts
from a earlier metamorphism that has been partially overprinted. Minor
amounts of "starburst" amphibole, very fine-grained randomly oriented
needle-like cummingtonite, exists in a few samples. These starburst
amphiboles may be an indication of a late (?) low pressure thermal
Aluminous Schist: Minor amounts of aluminous schist exist in the
PMMMS. One thin section contained quartz + biotite + garnet + plagioclase
+ kyanite + sillimanite. The garnet porphyroblasts appear to be pre-to
syn-kinematic and range from 0.50 to 6.0 mm in diameter. Sillimanite
appears to be replacing kyanite, suggesting that this sample barely
crossed the sillimanite isograd during Proterozoic metamorphism.
Magnetite Gneiss and Banded Garnet Gneiss: Several samples contain
abundant amounts of magnetite (up to 25%). Mineralogy consists of
magnetite + clinopyroxene + orthopyroxene + garnet + quartz + plagioclase
+ hornblende. The mineralogy is similar to that of iron formation
rocks described by Immega and Klein (1976) from similar locations
and elsewhere in the southern Tobacco Roots. Immega and Klein (1976)
concluded that on the basis of electron microprobe studies of pyroxene-rich
assemblages the iron formation rocks of the Tobacco Roots are of granulite-facies
rank. Banded garnet gneiss outcrops in the northern portion of the
PMMMS. These samples contain garnet + plagioclase + quartz + muscovite
+ biotite + clinozoisite (?). The gneiss consists of alternating bands
of garnet and quartz + plagioclase. According to McClain and others
(1979), compositional modeling suggests that the banded garnet gneiss
is a metamorphosed impure iron formation with up to 30% added Al2O3
There is evidence for at least two Precambrian metamorphic events
in the SPMS and possibly the ICMS, the first being high pressure and
the second being a lower pressure (4-6 kb) event (J. Brady, personal
communication). The oldest age for rocks in the Tobacco Roots is 2.7
Ga which is correlated with upper amphibolite facies metamorphism
(Burger et al., 1994). There is also clear evidence from 40Ar/39Ar
dates on amphiboles from MMDS from the ICMS and amphibolites from
both the ICMS and SPMS for a regional thermal event at 1.8 Ga (Brady
et al., 1994). There is slight evidence for a third event that postdates
the lower pressure (4-6 kb) event. Evidence for this event comes from
randomly oriented starburst amphiboles (probably cummingtonite), which
must postdate the last ductile deformation event (J. Brady, personal
communication). It is unknown whether they represent a Proterozoic
event, or are associated with intrusion of the Cretaceous Tobacco
Root Batholith. These starburst amphiboles have been recognized in
all three metamorphic suites (J. Cheney, personal communication).
A minor period of retrograde greenschist facies metamorphism of approximately
100 Ma is attributed to Laramide tectonic activity and to the intrusion
of the Tobacco Root Batholith. The effects of this event are thought
to be minimal, except along major faults, shear zones, and along the
immediate contacts of intrusions (Vitaliano et al., 1979).
From previous studies it is known that the majority of mineral assemblages
from the PMMMS are consistent with upper amphibolite conditions of
650-750 ûC and 4-6 kb (Immega and Klein 1976). MMDS which intruded
after 2.7 Ga but before 1.8 Ga are good evidence for an 1.8-1.6 Ga
thermal event of amphibolite facies in the PMMMS, which correlates
with the age determined for metamorphism of MMDS from the ICMS (Vitaliano
et al., 1979; Brady et al., 1994; and Burger et al., 1994). The development
of low grade alteration minerals in the PMMMS, such as sericite, phengite,
chlorite, talc, serpentine and minor amounts of muscovite and biotite
is most likely the result of Cretaceous thermal metamorphism, but
may be from an earlier retrograde event.
Small amounts of rock in the PMMMS and other suites contain mineral
assemblages indicative of a higher granulite facies event. The PMMMS
rocks containing these higher grade assemblages are most commonly
pyroxene-rich boudins or pods and iron formation rocks containing
orthopyroxene. Granulite facies assemblages have been reported from
quartzofeldspathic gneisses of the PMMMS (Hanley, 1975 and Vitaliano
et al., 1979). There are several probable explanations for the existence
of sporadic granulites in the PMMMS. First, they could be relicts
of a regional granulite facies developed during the 2.7 Ga event and
subsequently overprinted by the younger 1.8 Ga amphibolite event.
The rheology of these rocks might be such that they have resisted,
in part, overprinting by subsequent metamorphism. Or, perhaps the
granulites were developed locally during prograde metamorphism in
H2O-deficient pockets during the 1.8 Ga event, due to local differences
in pH2O/pCO2 from the surroundings. Evidence for this may come from
Cheney et al., (1994) who reported pressures and temperatures in the
range of 8 kb and ~750 ûC from kyanites of the SPMS thought
to be 1.8 Ga, however, they could be Archean in age. According to
Burger et al., (1994) there is textural evidence from elsewhere in
the Tobacco Roots to suggest both scenarios. Further research into
the P-T histories of the granulites is needed in order to provide
constraints on their formation, which in turn might prove useful in
determining tectonic setting for the PMMMS and related suites in the
Hopefully, further petrologic and geothemobarometry work will shed
more insight on the relationship between the PMMMS and ICMS. Previous
workers (Levandowski, 1956, Burger, 1966, and Cordua, 1973, in Burger
et al., 1994) have separated the PMMMS and ICMS based on lithologic
characteristics. The ICMS, located in the west-central and southern
portions of the Tobacco Roots, consists of quartzofeldspathic gneiss,
hornblende gneiss, dolomitic marble, aluminous schists, quartzite,
and iron formation. Although no definitive contact has been located
between the PMMMS and ICMS they were separated on the basis of the
abundance of marble and iron formation in the ICMS, and relative absence
in the PMMMS. Data, presented by Owen (this volume), suggests that
both the PMMMS and ICMS are one unit that has been folded around the
outside of the SPMS. This along with the fact that there are minor
amounts of marble, quartzite and iron formation in the PMMMS is evidence
that they may in fact be lateral facies variations on the same unit.
The metamorphic evidence seems to be consistent with the structural
evidence for a one unit scenario for the PMMMS and ICMS. Both the
PMMMS and ICMS appear to have similar metamorphic histories, as evidence
suggests that they have both undergone at least two events; a higher
pressure event first, followed by a lower pressure thermal event.
Brady, J. B. et al., 1994, Geochemical and 40Ar/39Ar evidence for
terrane assembly in the Archean of southwestern Montana: Geological
Society of America Abstracts with Programs, v. 26, p. 232. Burger,
H. R., Brady, J. B., and Cheney, J. T., 1994, Archean rocks of the
Tobacco Root Mountains, Montana: Seventh Keck Research Symposium in
Geology (Trinity University), p. 54-59.
Cady, Pamela, 1994, Petrographic analysis of amphibolites from the
Spuhler Peak Formation, Indian Creek Metamorphic Suite, and Pony-Middle
Mountain Metamorphic Complex, Tobacco Root Mountains, Montana: Seventh
Keck Research Symposium in Geology (Trinity University), p. 64-66.
Cheney, J. T. et al., 1994, Metamorphic evolution of Archean rocks
in the Tobacco Root Mountains of s.w. Montana: Geological Society
of America Abstract with Programs, v. 26, p. 226.
Hanley, T. B., 1975, Structure and petrology of the northwestern
Tobacco Root Mountains, Madison County, Montana (Ph.D. thesis): Bloomington,
Indiana, Indiana University, p. 289.
Immega, I. P., and Klein, C., 1976, Mineralogy and petrology of some
metamorphic Precambrian iron formations in southwestern Montana: American
Mineralogist, v. 61, p. 1117-1144.
Lowell, Josh, 1994, Petrological and structural constraints on the
history of the Spuhler Peak Formation near Noble Lake, Tobacco Root
Mountains, Montana: Seventh Keck Research Symposium in Geology (Trinity
University), p. 78-81.
McClain, L. K., Mathews G. W., and Frank, C. O., 1979, Petrogenesis
of a metamorphic garnetite, Tobacco Root Mountains, southwestern Montana:
Geological Society of America Abstracts with Programs, v. 11, p. 279.
Owen, David, 1996, Archean Deformation of the Tobacco Root Mountains,
southwestern Montana: Ninth Keck Research Symposium in Geology (Williams
Vitaliano, J. C., Cordua, W. S., Hess, D. F., Burger, H. R., Hanley,
T. B., and Root, F. K., 1979, Explanatory text to accompany geologic
map of southern Tobacco Root Mountains, Madison County, Montana: Geological
Society of America map and Chart Series, MC-31.
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