Evolution of the Precambrian Rocks of Yellowstone National ...

Evolution of the Precambrian Rocks of Yellowstone National ...

Evolution of the Precambrian Rocks of Yellowstone National Park (YNP): Low-Pressure
Metamorphism of the Jardine Metasedimentary Sequence
Carly Osborne1, Julia Baldwin1, Darrell Henry 2, David Mogk3, Paul Mueller4, and David Foster4
University of Montana, 2Louisiana State University, 3Montana State University, 4University of Florida

1

Introduction
Archean metasedimentary rocks in the Jardine
Metasedimentary Sequence (JMS) within northern YNP (Fig. 1)
are unique in their metamorphic grade and preservation of
sedimentary structures.
The JMS contains lower grade rocks compared to rocks further
to the east in the Garnet Hill or Junction Butte areas (Fig. 2)
The primary goal of this study was to use quantitative
thermobarometry to determine the metamorphic conditions of
pelitic rocks in the JMS and to determine whether metamorphic
P-T gradients are preserved in the JMS from west to east
where higher grade rocks are exposed at Garnet Hill (Parks et
al., 2011).
The JMS contains unique garnet-andalusite-staurolite
assemblages that provide important new data for interpreting
the Archean crustal evolution of the South Snowy Block region

Field Relations

Petrography & Mineral Chemistry
Metapelites contain diagnostic metamorphic assemblages including
chlorite-biotite in the western study area near Bear Creek (Figs. 2 &
3) (DM-7-7-06), increasing in grade to andalusite-staurolite-biotite
(AL72) and garnet-staurolite-biotite (JH715) further east (Fig. 3)
Eastern assemblages (e.g. JH713) are dominated by garnet-biotitechlorite with pseudomorphs of white mica after andalusite (Fig. 3)
Notable metamorphic assemblage transitions in the metapelites
include the formation of garnet followed by staurolite and
andalusite, and the breakdown of chlorite with increasing
metamorphic grade
Muscovite is rare and typically occurs as a retrograde phase
intergrown with andalusite

Fig. 2. Geologic map of
northern YNP. Boxes
outline the Jardine
Metasedimentary
Sequence (this poster)
and Bear Creek and
Garnet Hill study areas.
Sample locations for P-T
determination are labeled
on the map. Modified from
Casella et al. (1982).

DM7-7-06

Study area is located east of Gardiner, Montana (Fig. 1) and
extends east to Coyote Creek (Fig. 2) and has been termed the
Jardine Metasedimentary Sequence
Outcropping rocks consist primarily of fine-grained, sandy
biotite schists and minor intercalated meta-ironstones
These rocks typically preserve sedimentary structures such as
graded bedding, crossbeds, and sole marks and are interpreted
as low velocity turbidite deposits (Goldstein et al., 2011).
Porphyroblasts include garnet, andalusite, and staurolite.

Chl

Pressure-Temperature results
AL7-2: 586 67C, 4.1 0.9 kbar
BW7-12: 572 21C, 3.4 0.9 kbar
JH7-15: 576 15C, 5.6 1.3 kbar
KS7-16: 595 15C, 5.9 1.3 kbar
JH7-13: 609 20C, 4.7 1.5 kbar

Conclusions

Qtz

KS7-16

BW7-12

Thermocalc (Powell et al., 1998) was used to calculate average
P-T for five biotite schist samples.
Slight variations in P-T were observed, ranging from 572-609C
and 3.4-5.9 kbar across the study area.
Ti-in biotite thermometry yields consistent temperatures of 566586C.

JH7-15

AL7-2

B
t

Thermobarometry

JH7-13

The JMS metasedimentary rocks along with adjacent rocks in
the Garnet Hill area to the E are of anomalously low
metamorphic grade compared to other rocks of the northern
Wyoming Province
The JMS may represent a separate, distinctive unit that is
allochthonous within this northern part of YNP
References

Garnet-bearing mica schist from the JMS
Exposed outcrop of sandy
biotite schist near Bear
Creek, western JMS

Garnet bearing rock from Garnet Hill to the
east (higher grade)

Fig. 3. Photomicrographs (PPL) of the biotite schist from Bear Creek to the west (DM-7-7-06) to
east (JH7-13) across the JMS study area. Staurolite is most abundant in samples to the west,
whereas garnet increases in size and abundance to the east .Sample JH7-13 contains
pseudomorphs of white mica after andalusite.
JH7-15
A

B

C

Fig. 1. General geologic map of the
Beartooth-Snowy Block region. From
Mueller & Frost (2006).

D

Typical outcrop of biotite schist

Thin section scan of sample AL-7-2-01 showing andalusite and staurolite
porphyroblasts. Small staurolite grains also occur in the top left corner of the slide.

BW7-12

KS7-16

JH7-13

Garnet occurs in a range of textures,
including inclusions in And, partially
resorbed grts surrounded by foliation, large,
subhedral syn-kinematic porphyroblasts
with rotated inclusion trails, and matrix
grtovergrowing foliation.
Grt is Fe-rich with compositions ranging
from Alm73-82Prp8-15Grs3-5Sps1-9.
Slight prograde zoning in Sps is observed in
some samples, but Fe/(Fe+Mg) is
homogenous.

X-ray elemental maps of garnet. A: Ca, B: Fe, C: Mg, D: Mn

Casella, C.J., Levay, J., Eble, E., and others, 1982, Precambrian geology of the southwestern Beartooth
Mountains, Yellowstone National Park, Montana and Wyoming, in Precambrian Geology of the Beartooth
Mountains, Montana and Wyoming, MBMG Sp. Publ., 84, 1-24.
Mueller, P.A., and Frost, C.D., 2006, The Wyoming Province: a distinctive Archean craton in Laurentian North
America, Canadian Journal of Earth Sciences, 43, 1391- 1397.
Powell R., Holland, T., and Worley, B., 1998, Calculating phase diagrams involving solid solutions via non-linear
equations, with examples using THERMOCALC, Journal of Metamorphic Geology, 16,577-588.
Thurston, P., 1986, Geochemistry and Provenance of Archean Metasedimentary Rocks in the Southwestern
Beartooth Mountains [MS thesis]: Bozeman, MSU.

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.
Thanks to Donna Whitney and Ellery Frahm, Univ. of Minnesota for
assistance with the microprobe analyses.

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