Evolution of the Precambrian Rocks of Yellowstone National Park (YNP): High Grade Metamorphic Rocks from Junction Butte to Slough Creek

Angela Lexvold1, Lea Gilbertson1, David Mogk2, Darrell Henry3, Paul Mueller4, and David Foster4 1Univ. of Minnesota Morris, 2Montana State Univ., 3Louisiana State Univ., 4Univ. of FloridaIntroduction Geochemistry

Discussion and conclusion

GeochronologyAges of three grey gneiss samples were acquired by the LA-ICP-MS at the University of Florida. U/Pb zircon data indicates a crystallization age of 3244 ± 22 (2σ) Ma.

PetrographyMelanosome (Fig. 5a-b):• medium- to coarse-grained• tonalitic• quartz, plagioclase feldspar, biotite, muscovite, zircon, monazite, apatite, allanite

Field ObservationsThe grey orthogneiss (Fig. 4) has experienced high-grade metamorphism and ductile deformation, including: isoclinal, recumbent, and gently plunging folds; ptygmatic folds; boudins; and sheath folds. The grey gneiss is cut by in situ migmatites, and veins are random, on a cm scale, and either discordant or concordant.

Field ObservationsThe grey gneiss and metasedimentary rocks in the Junction Butte area contain discordant and concordant veins and dikes of the massive, garnet-bearing leucogranite pluton (Fig. 6). Psammitic and mica-rich xenoliths and schlieren occur at the margins of the pluton.

The Junction Butte to Slough Creek area (Fig. 1) of Yellowstone National Park contains a suite of rocks that are distinct from the Jardine Metasedimentary Sequence (JMS) and Garnet Hill terrane to the west. This suite consists of a grey tonalitic orthogneiss cut by migmatites (Fig. 2) and a distinct, garnet-bearing, leucocratic granitic pluton that cross-cuts the grey gneiss and metasedimentary rocks (Fig. 3).

Petrography•medium- to coarse-grained, homogeneous with little internal fabric•granitic – granodioritic•quartz, plagioclase, perthitic K-feldspar, muscovite, biotite, amphibole, garnet, ilmenite, epidote, zircon, apatite, secondary chlorite, opaques

Figure 2. Grey orthogneiss.

Figure 6. Leucogranite dike cross-cutting the grey gneiss.

GeothermobarometryA leucogranite sample was analyzed using the University of Minnesota’s EPMA. Thermobarometry (GBPQ) calculations yielded peak crystallization conditions of 821 – 863 C⁰ and 7.0 – 7.3 kbar (Fig 8).

1. The grey tonalitic orthogneiss is distinct from the JMS terrane in terms of composition, age, metamorphism, and deformation style

2. The Junction Butte migmatites formed by in situ partial melting whereas injection migmatites occur on Garnet Hill

3. The Junction Butte gneiss are consistent with the Mesoarchean gneiss of the eastern Beartooth Mtns. Both gneisses belong to the tonalite-trondjemite-granodiorite (TTG) suite that is common in Archean cratons (Mueller et al., 1996).

4. The leucogranite pluton is significant because it is a peraluminous, S-type granite and a dry, high-temperature crustal melt.

Figure 1. The Junction Butte to Slough Creek area is located on the eastern portion of the study area (Casella et al., 1982).

Grey gneiss Leucogranite

Figure 5. Photomicrographs of typical grey gniess assemblage. Biotite is light to dark brown and foliated. Quartz shows signs of deformation such as undulatory extinction and recrystallization. width of view is 3 mm. (a) Melanosome, ppl. (b) Same as 3a, xpl. (c) Leucosome, ppl. (d) Same as 3c, xpl.

5a b

c d

Figure 7. Photomicrographs of typical leucogranite assemblage and texture. width of view is 3mm. (a) Leucogranite, ppl. (b) Same as 5a, xpl. (c) Garnets are sub- to euhedral, 1 – 4 mm, and unzoned, ppl. (d) Same as 5c, xpl.

7a b

c d

The purpose of the study was to answer the following questions:1. How are the high-grade metamorphic rocks and leucogranite pluton of Tower Junction related to the JMS and Garnet Hill terrane?2. Is the grey gneiss related to the Mesoarchean gneiss ?3. Do the grey gneiss and leucogranite suite represent a tectonic boundary or unconformity?

Figure 3. Leucogranite outcrop.

Figure 4. Boudinaged gneiss/migmatite.

Leucosome (Fig. 5c-d):•medium- to coarse-grained•tonalitic – granodioritic•quartz, plagioclase feldspar, perthitic K-feldspar, biotite, muscovite, zircon

Figure 9. (a) Peak crystallization conditions indicate that the leucogranite was a dry crustal melt. Once the biotite melt curve is breached, garnet (b) and >15% melt is generated, and the liquid is capable of mobilization (Winter, 2001). The red dot represents the leucogranite.

ReferencesCasella, C.J., Levay, J., Eble, E., Hirst, B., Huffman, K., Lahti, V., and

Metzger, R., 1982, Precambrian geology of the southwestern Beartooth Mountains, Yellowstone National Park, Montana and Wyoming: Special Publication – State of Montana Bureau of Mines and Geology, v. 84, p. 1 – 24.

.Mueller, P.A., Wooden, J.L., Mogk, D.W., Nutman, A.P., and Williams,

I.S., 1996, Extended history of a 3.5 Ga trondhjemitic gneiss, Wyoming Province, USA: evidence from U – Pb systematics in zircon: Precambrian Research, v. 78, p. 41 – 52.

Winter, J.D., 2001, An Introduction to Igneous and Metamorphic

Petrology: Prentice-Hall Inc., Upper Saddle River, New Jersey, 697 p.

 

Figure 8: Geochemical variation diagrams showing a) dacitic composition of the gray gneisses and rhyolitic composition of leucosomes and leucogranite; b) peraluminous nature of these rocks (corundum-normative), and c) tectonic affinity with syn-collisional granites or volcanic arc granites.

This 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.

Gray Gneiss Leucosome Leucogranite Himalayan Granites

Acknowledgements