CaOAl2O3

K2O Na2O

MgO

TiO2

Fe2O3

MnO

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

Conclusions

References

Introduction

C D

A B

AcknowledgementsThis project was supported through the NSF REU program, Division of Earth Science grants EAR 0852025, 0851752, and 0851934.

Special thanks to YNP staff, Christie Hendrix, Stacey Gunther, Carrie Guiles, Bridgette Guild and Hank Heasler for their support and interest.

Field Relations Geochemistry

Petrography

Geochronology

The Cooke City area of Southern Montana hosts 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.

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.

▪ 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.

Figure 2 (Left):Gradation boundary between units. Figure 3 (Right): Metabasite enclave.

Undulatory extinction patterns in quartz grains along with a preferred 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.

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.

Figure 9 (Left): Modal petrography of the tonalitic and granodioritic suites.

U-Pb zircon ages were determined using Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS) on zircons mounted in epoxy ground to ~1/2 thickness and previously imaged by cathodoluminescense (CL). Ages below are based on 207Pb/206Pb ratios 2 s.e.m. errors; only data less than 10 % discordant were used to determine the ages below:o 2802 +/- 2 Ma , 11TG-7-16-01, Biotite Tonaliteo 2806 +/- 13 Ma, 11MN-7-15-01, Quartz Hornblende Dioriteo 2832 +/- 7 Ma, 11SPM-7-18-02, Biotite Tonalitic Gneiss

Compositional domains evident in CL images in some zircons were also measured. Samples 11MN7-15-01 and SPM7-18-02 showed an indication of inheritance of 3.0- 3.2 Ga components. The higher errors for these samples probably effect inadverent inclusion of this material during analysis.

Plutonic rocks in the Long Lake area to the east have 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.

Both in LLMC and Cooke City Area temporal field relationships, as a rock that appears to be an early forming enclave in one area may appear as a late-stage intrusion in another area. Variable metamorphism in the amphibolite facies is recorded in mafic xenoliths in the area. Local foliation may be the result of flow patterns during synkinematic emplacement, or later deformation.

Structure

Figure 11 (Middle right):REE diagram 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.

Figure 12 (Left): (Y+Nb)-Rb and Figure 13 (Right): Y-Nb tectonic variation diagrams after Pierce et al. (1984) indicating a volcanic arc chemical signature for samples from the Cooke City area.

Plutonic rocks are cut by shear zones with a strong mylonitic fabric, particularly in the Colter Campground area. Ductile deformation, as evidenced by discrete meter-scale shear zones, grain size reduction, and development of a strong lineation, is pervasive in this area. Two areas were selected for collecting foliation measurements.

Nb, Ta, and Ti depletion in a spider diagram along with low TiO2 across the range of SiO2 saturation indicate the presence of residual rutile and probable volcanic arc source.REE diagrams show three separate fractionation patterns:

o Patterns with moderate fractionation, positive Eu anomalies, and slightly enriched Tm, Yb and Lu compared to Ho and Er indicate that the melt was in 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 plagioclase. These samples could not have been consanguineous with other REE patterned samples, due to incompatible behavior of REE with increasing SiO2.

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.

Mueller, P. A., Wooden, J.L., Mogk, D.W., Henry, D.J., and Bowes, D.R., 2010, Rapid growth of an Archean continent by arc magmatism:

Precambrian Research, v. 183, no. 1, p. 70-88, doi: 10.1016/j.precamres.2010.07.013.

Jablinski, J.D., and Holst, T.A., 1992, Deformational history of Archean metasedimentary rocks, Beartooth Mountains, Montana, USA:

Journal of Geology, v 100P.D1-D10.Zen E., and Hammarstrom J.M., (1984) Magmatic epidote and its petrologic

significance. Geology 12:515 -518

Figure 14 and 15 (Above): Stereograph projections of foliation measurements.

Ti Yb Lu

Figure 8 ( Above): TAS diagram demonstrates the range of SiO2 in the TTG suite. (after Le Bas et al. (1986))

Tonalite: KM7-15-03; TG7-16-01

Granodiorite: KM7-17-01; MH7-14-01

Trondhjemite: SPM7-18-01

Granites

TTG suite

Diorites

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.