Transcript Metasedimentary Rocks

Evolution of the Precambrian Rocks of Yellowstone National Park (YNP): Metasedimentary Rocks
Emily Hernandez Goldstein1, Kirsten Sauer2, Jeremy Harwood3, David Mogk3, Darrell Henry4, Paul Mueller5, and David Foster5
1Columbia
Metasedimentary rocks of the Wyoming Province are typically high grade
metamorphic rocks with substantial amounts of deformation. The nature of
the protolithic sedimentary rocks is typically uncertain due to this
deformation. In contrast to this stereotype, the relatively low-grade clastic
metasediments of the Jardine Metasedimentary Sequence (JMS) of YNP
provide an opportunity to:
• investigate sedimentary features as they relate to sedimentary
environments
• consider the possible sources of the detrital material and its relation to
tectonic reconstruction
• constrain the age of deposition
Field Relations
Outcrops of metasediments contain preserved sedimentary structures which
help delineate the paleo-depositional environment.
• Preserved sedimentary structures include: graded bedding, crossbeds, sole
marks, and lenticular, coarse-grained bodies interpreted as channels (Fig. 1).
• Measured stratigraphic sections reveal an overall coarsening of the clastic
sequences from west to east (Fig. 2).
• Metaconglomerates, rip-up clasts, and well-defined erosional surfaces are
absent.
• Based on the observed lithologies and sedimentary structures, the original
sediments are interpreted as low velocity turbidite deposits.
b
Conclusions
Geochemistry
Introduction
a
Univ., 2Cornell Univ., 3Montana State Univ., 4Louisiana State Univ., 5Univ. of Florida
Whole-rock geochemical analysis provides insight into the nature of the
provenance and tectonic setting.
• The JMS is characterized by both felsic and mafic tendencies
• Felsic: Elevated Zr/Y (4-14), enriched in incompatible elements
• Mafic: High Cr, Ni, and FeOT
• Range of REE patterns- HREE depletion and LREE enrichment, Eu/Eu*
from 0.4 to 0.9.
Provenance
• The geochemistry indicates that the source was bimodal.
• Sources likely include felsic and mafic volcanics along with tonalitictrodjhemitic-granitoids.
Tectonic Setting
• Tectonic discrimination diagrams after Bhatia (1983, 1986) indicate that
the JMS has a geochemical signature most similar to a Continental Island
Arc setting. This is consistent with the bimodal nature of the provenance.
The preserved sedimentary structures,
geochemistry and geochronology indicate
these clastic metasedimentary rocks:
• resemble clastic turbidite deposits of
greenstone belts.
• were deposited on a mid-to-distal
submarine fan on an active continental
margin.
Figure 3.Tectonic setting discrimination ternary plots with JMS samples and representative Archean
compositions plotted (KOM = komatiite, BAS= basalt, TTG, FVO = felsic volcanics, GR = granite). OIA =
Ocean Island Arc, CIA = Continental Island Arc, ACM = Active Continental Margin, PM = Passive Margin
(after Bhatia 1983, 1986).
Geochronology
Isotopic analytical results and field relationships
constrain the possible depositional age range.
• Detrital zircons were analyzed by LA-ICP-MS
(207Pb/206Pb of <10% discordant) yield the ages
of source detritus from 2.9 Ga to 3.66 Ga.
• Cross-cutting relationship of 2.8 Ga granite
plutons constrain the age of deposition of the
JMS between 2.9 Ga and 2.8 Ga.
c
Figure 1. Relict sedimentary features
from the JMS (bottom photos
highlight interpreted structure).
a
c
References
Bhatia, M.R. (1983) Plate-Tectonics and
Geochemical Composition of Sandstones.
Journal of Geology, 91(6), 611-627.
Acknowledgements
b) Interbedded metamorphosed
sandstone and shale.
Metasandstone layers
highlighted by gray lamination
and metashale layers highlighted
by garnet growth from clay
minerals during metamorphism.
c) Inferred relict cross bedding in
metamorphosed sandstone
indicating flow during deposition.
• are not consistent with derivation from
the Archean rocks of the Beartooth
Mountains based on the detrital zircon
age spectrum.
Bhatia, M.R. (1985) Plate-Tectonics and
Geochemical Composition of Sandstones - a
Reply. Journal of Geology, 93(1), 85-87.
a) Bottom of sedimentary layer with
a preserved sole mark.
b
• were deposited between 2.9 and 2.8
Ga, based on the youngest detrital
zircon (2.9 Ga) and cross-cutting
granitic plutons (2.8 Ga).
This project was supported through the
NSF REU program, Division of Earth
Science grants EAR 0852025, 0851752,
and 0851934.
Figure 2. Stratigraphic sections of metasediments along an
west to east (left to right) transect.
Figure 4. Histogram of detrital zircon grain populations.
Field areas west to east: Bear Creek, Crevice Lake,
Blacktail Deer Creek East, Cottonwood Creek.
Special thanks to YNP staff, Christie
Hendrix, Stacey Gunther, Carrie Guiles,
Bridgette Guild and Hank Heasler for their
support and interest.