Transcript Evolution of the Precambrian Rocks of Yellowstone

Evolution of the Precambrian Rocks of Yellowstone National Park (YNP): Late Archean Felsic Plutons
Kate Philbrick1, Bryant Ware2, Darrell Henry3, David Mogk4, Paul Mueller5, and David Foster5
1Wellesley
College, 2Boise State Univ., 3Louisiana State Univ., 4Montana State Univ., 5Univ. of Florida
Introduction
The purpose of this study is to
determine the timing of the intrusion of
the felsic plutons in relation to the
sedimentation and metamorphism of the
surrounding metasedimentary sequence
and the plutons in the Beartooth
Mountains.
Figure 1: View of the Hellroaring
Pluton looking to the north
In the northern YNP the Jardine
Metasedimentary Sequence has been
intruded by two main igneous bodies:
the Crevice Pluton and the Hellroaring
Pluton. A dynamic mix of different
contact relations were observed
adjacent to the Crevice and the
Hellroaring Plutons. Locally, there are
areas of sharp cross-cutting contacts
Figure 5: Biotite-rich
between the granite plutons and the Figure 4:
Metasedimentary
xenoliths near the
metasedimentary rocks, areas where xenoliths in the SE
margin of Crevice
xenoliths are present indicating forceful margin of Crevice
Pluton.
emplacement (Figure 4), and some thin Pluton.
injection migmatites. The plutons have not developed a strong deformation fabric
near the contacts and clearly cross cut large folds of the metasedimentary rocks
(most notably in the Bear Creak area) indicating post kinematic intrusion.
• The Crevice and Hellroaring Plutons are distinct
in their emplacement style (bulbous magmatic
bodies intruded at 4 Kb, ~10-15 km depth)
compared with deeper seated (~20-25 km)
magmatic bodies emplaced in series of sill-like
sheets in the adjacent Beartooth Mountains.
Figure 6:
Slickenlines
from the Crevice
Pluton.
Peraluminous
• The plutons formed from the partial melting of
continental crust, perhaps from a pelitic,
metasedimentary source.
Metaluminou
s
Figure 7: View of
the Crevice
Pluton
Metasedimentary
relations from
Bear Creek
across the
Yellowstone River
Figure 9: Total alkali v. SiO2 diagram showing
restricted rhyolitic compositions compared to the
compositional range of the Long Lake magmatic
complex of the Beartooth Mountains. Crevice Pluton
= dots, Hellroaring Pluton = triangles
Figure 10: Relative Al2O3 concentrations
in samples from the Crevice and
Hellroaring Plutons
• The emplacement of these granitic bodies is
significant because they place a minimum age of
~2.8 Ga on sedimentation as well as the regional
metamorphism and deformation of the Jardine
Metasedimentary Sequence.
1000
Figure 2: View of the Crevice
Pluton looking to the east,
featuring Rattlesnake Butte
10-BW-7-02-01
10-PM-7-06-01
10-DM-06-30-08
10-KP-7-19-01
CL5
CL6
Petrography
100
Over 170 samples were collected
throughout the month of field work. Thin
sections were made for petrographic
analysis.
The
photomicrographs
of
samples 10-DM-06-30-09 and 10-KP-0719-03 show anhedral biotite against
subhedral muscovite. This relationship is
consistent throughout the pluton, including
samples from near the contact and in the
interior of both the Crevice and Hellroaring
Plutons. Photomicrographs of samples 10WF-07-06-01 and 10-BW-7-2-01 show a
good representation of the mineralogy of
the Crevice and Hellroaring Plutons that
include sub-equal modal abundances of
quartz-plagioclase
and
microcline
(perthitic).
A
B
C
Figure 8: Photomicrographs of
the Hellroaring and Crevice
Plutons. A) Texture and
mineralogy of sample 10-WF07-06-01: Biotite Quartz
Monzonite, Hellroaring Pluton.
B) Texture and mineralogy of
sample 10-BW-07-02-01:
Muscovite Granite, Crevice
Pluton. C) : Biotite-muscovite
intergrowth in interior of
Hellroaring Pluton (sample 10KP-07-19-03). Note that biotite
is anhedral against subhedral
muscovite. Crossed-polarized
light. D) : Biotite-muscovite
intergrowth in interior of
Crevice (10-DM-06-30-09).
Note that biotite is anhedral
against subhedral muscovite.
Crossed-polarized light.
References
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: in Precambrian
Geology of the Beartooth Mountains, Montana and
Wyoming, MBMG Sp. Publ., 84, p 1-24.
Sample/Chondrite
The Precambrian metasedimentary
rocks of northern Yellowstone National
Park have been intruded by two plutonic
granitic bodies, the Crevice and
Hellroaring Plutons. Montgomery (1982)
determined an age of 2.73 to 2.79 Ga
for one of the plutons in the northern
portion of Yellowstone National Park,
0.3 km east of Tower Junction. Casella,
et al. (1982) hypothesized that the
plutons have a similar geologic history
to the well-studied plutons in the
Beartooth Mountains, 40 miles to the
east.
Conclusions
Geochemistry
Field Relations
10
1
La
Ce
Nd
Sm
Eu
Gd
Tb
Ho
Tm
Yb
Lu
Element
Figure 11: Spider diagram showing
HFSE depletion characteristic of
formation in an arc environment.
Figure 12: REE diagram showing
pronounced negative Eu anomaly indicting
a crustal source for these magmas.
Acknowledgements
This project was supported through the NSF REU
program, Division of Earth Science grants EAR
0852025, 0851752, and 0851934.
D
Geochronology
Special thanks to YNP staff, Christie Hendrix, Stacey
Gunther, Carrie Guiles, Bridgette Guild and Hank
Heasler for their support and interest.
Figure 3: Map of the northern border of Yellowstone National Park showing the location
of the two major felsic plutons mapped by Casella et al.,1982. The Crevice Pluton is
located in the northwest of the map while the Hellroaring Pluton can be viewed near the
central area of the map. Sample locations and type are shown by the different colored
dots; biotite quartz monzonites are magenta, and muscovite-bearing quartz monzonites
are pink.
Time (in Ga)
2.7
For the Crevice Pluton, a U/Pb zircon age of 2.808 +/- 0.007 2σ Ga has
been determined. The inherited zircon in the Crevice Pluton is
consistent with zircon populations in the adjacent metasedimentary
rocks. The Hellroaring Pluton has a provisional U/Pb zircon age of
2.805 +/- 0.010 2σ Ga.
2.8
2.9
3.0
3.1
}
Age of the plutons
2.8 Ga
Age of deposition of the
metasedimentary sequence
2.95-2.8 Ga
Montgomery, C.W., 1982, Preliminary Zircon U-Pb Dating of
Biotite Granodiorite from the South Snowy Block,
Beartooth Mountains: in Precambrian Geology of the
Beartooth Mountains, Montana and Wyoming, MBMG
Sp. Publ., 84, p 41-44.
Figure 14: Tectonic variation diagrams indicating a
volcanic arc or collisional setting for this magmatism.
Figure 13: Phase diagram showing the
stability of magmatic muscovite, indicating a
minimum crystallization pressure of 0.38
GPa; from Winter (2010, Figure 18.5a)
Thanks to David Hawkins for his expertise and
guidance.
Thanks to Dr. Mark Schmitz for his ever extending
guidance, knowledge and patience.