Download Evolution of the Precambrian Rocks of Yellowstone

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
Introduction
1Univ.
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, garnetbearing, leucocratic granitic pluton that cross-cuts the grey gneiss and
metasedimentary rocks (Fig. 3).
Grey gneiss
Gray Gneiss
Leucosome
Leucogranite
Himalayan Granites
Leucogranite
Field Observations
The 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.
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?
Field Observations
The 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.
Petrography
Melanosome (Fig. 5a-b):
• medium- to coarse-grained
• tonalitic
• quartz, plagioclase feldspar,
biotite, muscovite, zircon, monazite,
apatite, allanite
Figure 2. Grey orthogneiss.
Figure 3. Leucogranite outcrop.
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
c
Leucosome (Fig. 5c-d):
• medium- to coarse-grained
• tonalitic – granodioritic
• quartz, plagioclase feldspar, perthitic
K-feldspar, biotite, muscovite, zircon
b
d
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.
Discussion and conclusion
Figure 4. Boudinaged gneiss/migmatite.
Figure 1. The Junction Butte to Slough Creek area is located on the
eastern portion of the study area (Casella et al., 1982).
Geochemistry
of Minnesota Morris, 2Montana State Univ., 3Louisiana State Univ., 4Univ. of Florida
Figure 6. Leucogranite dike cross-cutting the grey gneiss.
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 7.
7a
Photomicrographs of
typical leucogranite
assemblage and
texture. width of view is
3mm. (a) Leucogranite,
ppl. (b) Same as 5a,
xpl. (c) Garnets are sub- c
to euhedral, 1 – 4 mm,
and unzoned, ppl. (d)
Same as 5c, xpl.
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 tonalitetrondjemite-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.
References
b
d
Geochronology
Geothermobarometry
Ages 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.
A leucogranite sample was analyzed using the University of Minnesota’s
EPMA. Thermobarometry (GBPQ) calculations yielded peak crystallization
conditions of 821 – 863 ⁰C and 7.0 – 7.3 kbar (Fig 8).
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.
Casella, 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.
Acknowledgements
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.