Download Evolution of the Precambrian Rocks of Yellowstone National Park

Precambrian Rocks of Yellowstone National Park (YNP) and Surrounding Areas:
Plutonic Rocks of the Cooke City Area
Skyler Mavor1, Melanie Newton2, Darrell Henry3, David Mogk4, Paul Mueller5, David Foster5, Peter Stelling1, and Andrew Wulff2
1Western
Washington Univ., 2Western Kentucky Univ., 3Louisiana State Univ., 4Montana State University, 5Univ. of Florida
Introduction
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 The Cooke City area of Southern Montana hosts
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expansive exposures of Precambrian basement rocks
of the Wyoming Craton, typified by intermediate to felsic
plutonic outcrops. Modal mineralogy indicates that most
outcrops are of the tonalite-trondhjemite-granodiorite
(TTG) suite, typical of Archean cratons worldwide.
Additional rock types in the area include: granite, diorite,
quartz diorite, quartz syenite, and mafic amphibolites.
Rocks are metaluminous to weakly peraluminous and
show an I-type granitoid signature, demonstrated by the
alumina saturation (A/(C+N+K) <1.1) in 97 % of
samples, sodic nature, and generally low K2O/Na2O
values.
Archean plutonic rocks exposed in the Cooke City area
exhibit distinctive field relations, structural style, and
metamorphic conditions similar to Long Lake Magmatic
Complex (LLMC).
The Cooke City area has been injected by numerous
generations of TTG and granites and include
metasedimentary and mafic xenoliths.
In this study, U-Pb zircon ages were calculated for
some of the plutonic rocks in this area.
The Cooke City area contains key evidence about the
metamorphic, structural and magmatic processes that
have contributed to Archean crustal evolution in this
area.
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Figure 2 (Left):Gradation boundary between units.
Figure 3 (Right): Metabasite enclave.
Outcrops can be characterized and mapped as two broad compositional
units, boundaries between units are commonly gradational, with locally
sharper distinctions (Figures 2 and 3):
o Grey, slightly finer grained TTG includes plagioclase-rich tonalite,
trondhjemite and granodiorite.
o Granitic suite encompass rock types with 68-72% SiO2 values which
include granite, granitic gneiss and granodiorite which are pink at
outcrop scale.
Field relationships indicate a sequence of events:
o TTG suite intruded into metasedimentary country rock, incorporating
xenoliths of biotite schist.
o TTG suite intruded by the granite-granodioritic suite, evidenced by
brittle deformation of tonalites with cross cutting granite- granodioritic
intrusions.
o Metabasite enclaves appear to be coeval with the granite- granodioritic
bodies, as a mafic unit may appear as either an intrusion or an enclave.
Granodiorite: KM7-1701; MH7-14-01
Tonalite: KM7-15-03;
TG7-16-01
Trondhjemite:
SPM7-18-01
Ti
Yb Lu
Petrography
Figure 4 (Left): Undulatory extinction patterns in
quartz grains. Field of view: 1mm.
Figure 5 (Right): Center of picture shoes sericitic
alteration and myrmekite texture in plagioclase. Field
of view: 1mm.
Figure 6(Left): Subhedral epidote with allanite
core in Tonalite. Field of view: 1mm.
Figure 7 (Right): Euhedral epidote interpreted as
a primary magmatic phase in tonalite. Field of
view: 3mm.
orientation of biotite grains indicate a deformation episode of the Cooke City
TTG rocks, and suggests pre- or synkinematic emplacement (Figure 4).
 Plagioclase commonly displays sericitic alteration patterns and chlorite is
found replacing original biotite in samples suggesting late stage
hydrothermal alteration (Figure 5).
 Igneous accessory minerals include: zircon, apatite, allanite, and Fe- oxides.
 Epidote occurs both as anhedral crystals replacing biotite or hornblende
and as subhedral to euhedral some with allanite core (Figure 6 and 7).
Epidote can form as a primary magmatic phase in granitic or granodioritic
melts that attain pressures of at least 8 kbar, which implies emplacement of
the plutons in a mid to lower level crustal setting.
Granites
TTG suite
Diorites
Figure 8 ( Above): TAS diagram demonstrates the
range of SiO2 in the TTG suite. (after Le Bas et al. (1986))
Figure 9 (Left): Modal petrography of the tonalitic
and granodioritic suites.
Structure
Nb, Ta, and Ti depletion in a spider diagram along with low
 Plutonic rocks are cut by shear zones with a strong
TiO2 across the range of SiO2 saturation indicate the
mylonitic fabric, particularly in the Colter Campground
presence of residual rutile and probable volcanic arc
area. Ductile deformation, as evidenced by discrete
source.
meter-scale shear zones, grain size reduction, and
REE diagrams show three separate fractionation patterns:
development of a strong lineation, is pervasive in this
o Patterns with moderate fractionation, positive Eu
area.
anomalies, and slightly enriched Tm, Yb and Lu
 Two areas were selected for collecting foliation
compared to Ho and Er indicate that the melt was in
measurements.
equilibrium with amphibole, and that relative plagioclase
accumulation may have occurred.
o Patterns with high degrees of fractionation indicate
garnet in the residuum, and little interaction with
plagioclase.
o Moderately fractionated patterns with negative Eu
anomalies in low SiO2 samples indicate residual
Figure 14 and 15 (Above): Stereograph projections of
plagioclase. These samples could not have been
foliation measurements.
consanguineous with other REE patterned samples, due
to incompatible behavior of REE with increasing SiO2.
 Plutonic rocks in the Long Lake area to the east have
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been described as a continuous series spanning a
dioritic to granitic compositional range, whose
relationships may be explained by repeated coeval
injections of magmas with varying compositions from
different sources ,with emplacement ages of 2.79-2.83
Ga. Cooke City Area data demonstrate similar field
relationships, geochemistry, and ages to those of
LLMC.
Figure 12 (Left): (Y+Nb)-Rb and Figure 13 (Right): Y-Nb tectonic  Both in LLMC and Cooke City Area temporal field
relationships, as a rock that appears to be an early
variation diagrams after Pierce et al. (1984) indicating a volcanic
forming enclave in one area may appear as a late-stage
arc chemical signature for samples from the Cooke City area.
intrusion in another area. Variable metamorphism in the
amphibolite facies is recorded in mafic xenoliths in the
 U-Pb zircon ages were determined using Laser Ablation
area. Local foliation may be the result of flow patterns
Inductively Coupled Mass Spectrometry (LA-ICP-MS) on
during synkinematic emplacement, or later deformation.
zircons mounted in epoxy ground to ~1/2 thickness and
previously imaged by cathodoluminescense (CL). Ages
Mueller, P. A., Wooden, J.L., Mogk, D.W., Henry, D.J., and Bowes, D.R.,
below are based on 207Pb/206Pb ratios 2 s.e.m. errors; only
2010, Rapid growth of an Archean continent by arc magmatism:
Precambrian Research, v. 183, no. 1, p. 70-88, doi:
data less than 10 % discordant were used to determine the
10.1016/j.precamres.2010.07.013.
ages below:
Jablinski, J.D., and Holst, T.A., 1992, Deformational history of Archean
metasedimentary rocks, Beartooth Mountains, Montana, USA: Journal of
o 2802 +/- 2 Ma , 11TG-7-16-01, Biotite Tonalite
v 100P.D1-D10.
o 2806 +/- 13 Ma, 11MN-7-15-01, Quartz Hornblende Diorite Zen E.,Geology,
and Hammarstrom J.M., (1984) Magmatic epidote and its petrologic
o 2832 +/- 7 Ma, 11SPM-7-18-02, Biotite Tonalitic Gneiss
significance. Geology 12:515 -518
 Compositional domains evident in CL images in some zircons
were also measured. Samples 11MN7-15-01 and SPM7-18This project was supported through the NSF REU program, Division
02 showed an indication of inheritance of 3.0- 3.2 Ga
of Earth Science grants EAR 0852025, 0851752, and 0851934.
components. The higher errors for these samples probably
Special thanks to YNP staff, Christie Hendrix, Stacey Gunther, Carrie
effect inadverent inclusion of this material during analysis.
Guiles, Bridgette Guild and Hank Heasler for their support and
Conclusions
 Undulatory extinction patterns in quartz grains along with a preferred
A
Figure 1 (Above): Location of the Beartooth Plateau and Cooke City.
This study focuses on rocks sampled from the area to the
northeast of Cooke City. Areas of interest include the Broadwater
River (BR) area of this study and the Long Lake (LL) area of previous
studies.
Geochemistry
Field Relations
Al2O3
CaO
K2O
Na2O
B
Figure 10 (Top right): Multielement spider
diagram. The bottom red square and blue
diamonds show a slightly positive Eu
anomaly, most likely due to excess
Plagioclase. Red squares are KM7-17-01;
MH7-14-01. Blue diamonds are KM7-15-03;
TG7-16-01. Purple Circle is SPM7-18-01.
Figure
C 11 (Middle right):REE diagram
D
normalized to chondritic meteorites.
Normalization values from Sun and
McDonough (1989). Same legend as Figure
10.
Figure 12 (Bottom Right): Harker type
diagrams showing trends across silica
saturation. Same legend as Figure 8.
Geochronology
References
MgO
TiO2
Fe2O3
MnO
Acknowledgements
interest.