Download Garnet Hill area

Evolution of the Precambrian Rocks of Yellowstone National Park (YNP): High Grade Metamorphic
Rocks at Garnet Hill
Introduction and Scientific
Rationale
Benjamin Parks1, Cameron Davidson1, Darrell Henry2, David Mogk3, Paul Mueller4, and David Foster4
1Carleton
•Archean metasedimentary rocks exposed in the Garnet Hill area in northern YNP
exhibit distinctive field relations, structural style, and metamorphic conditions
compared with the low-grade Jardine Metasedimentary Sequence (JMS) to the west
and high-grade orthogneisses of the Junction Butte area to the east.
• The Garnet Hill area has been injected by numerous generations of felsic dikes
and sills, which cut metasedimentary and lenticular dioritic units.
• In this study, we calculate the metamorphic conditions of the Garnet Hill
metasedimentary rocks, and have obtained radiometric ages of some of the felsic to
dioritic dikes and sills.
•The Garnet Hill area contains key evidence about the metamorphic, structural and
magmatic processes that have contributed to Archean crustal evolution in this area.
College,
2Louisiana
State Univ.,
3Montana
C
A
B
Metapelite sample 10-CO-713-02 displaying quartzofeldspathic layering (A)
aligned with bt foliation (B). X-ray map (C) of garnet crystal seen in (B) displays
Ca zoning pattern.
Field Relations
Meter-scale granitic dike cutting
pelitic schist. Hand lens for scale.
•Injection of the Garnet Hill
assemblages is interpreted to have
occurred over at least two separate
events, the older injection having
undergone more extensive
deformation, boudinaging, and
ptygmatic and isoclinal folding
followed by crosscutting by a second,
less deformed set of dikes.
• The development of ductile
structures and injection of igneous
sills and dikes are contemporaneous
with high grade metamorphism at
Garnet Hill.
Ironstone sample 10-AL-719-01 displaying anhedral crystal shape and
ingrown texture between hornblende and grunerite crystals.
State Univ.,
4Univ.
of Florida
Petrography
• Pelitic schist contains: garnet, plagioclase, sillimanite, biotite, and
quartz with staurolite inclusions preserved in garnet poikiloblasts.
Foliation defined by bt, mm-scale quartzofeldspathic layers present.
• Ironstones contain: garnet, plagioclase, hornblende, grunerite, and
quartz. Anhedral crystal texture. Hbl & grun display ingrown texture,
indicating disequilibrium.
Geochronology
Ages were obtained for hornblende diorite plutons from Garnet Hill to
determine its relation to other nearby igneous units. Ages were
determined by measuring U-Pb systematics in zircons using Laser
Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS). Ages
below are based on 207Pb/206Pb ratios from data that is <10%
discordant.
Summary
• P-T values recorded for metasedimentary units in the
Garnet Hill and JMS areas are similar (560-615°C, 4.0-5.5
kbar); however these are quite different compared to
orthogneisses at Junction Butte to the east (~800oC, 7-8
kbar).
•Field observations and P-T results from the migmatites at
Garnet Hill indicate that the felsic dikes and sills
were emplaced by injection and not in-situ melting of the
metasedimentary rocks. Their age and composition are
similar to the adjacent Crevice and Hellroaring plutons.
• Garnet Hill is the easternmost extent of the
metasedimentary sequence that includes the lower grade
JMS rocks.
• A major structural discontinuity is interpreted between the
Garnet Hill rocks and the higher grade and older (~3.2 Ga)
tonalitic orthogneisses to the east at Junction Butte.
• Age determinations from two hornblende quartz diorite bodies on
Garnet Hill are:
2798 ± 4 Ma (2σ), 10-DM-7-3-03
2815 ± 9 Ma (2σ),10-PM-7-14-01
These ages are similar to the Crevice and Hellroaring Plutons.
Geothermobarometry
Two dike generations seen
cutting hornblende diorite.
Map of the northern
border of Yellowstone
National Park
mapped by Casella et
al.,1982. The Garnet
Hill area is defined by
the reference box.
Tight to isoclinal disharmonic
folding in pelitic schists with
leucosome veins.
P-T plot of ironstone sample 10-EHG-705-01, using the Grt-bt
thermometer, the Mg- and Fe-model GBPQ barometer, and the
GRIPS barometer. The aluminosilicate triple point is plotted in gray.
• Pressure and temperature calculations using TWQ yield four
independent reactions in both metasedimentary units: the Grt-Bt
thermometer, and Mg-model GBPQ, GASP, and GRAIL barometers in
the metapelites, and Grt-Bt, Mg- and Fe-model GBPQ, and GRIPS in
the ironstone.
• Grt-Bt, GASP, and GBPQ reactions indicate metamorphic
conditions of 560-615°C and 4.0-5.5 kbar.
• However, GRAIL and GRIPS reactions yield pressures about 1.52.5 kbar higher than other barometers, suggesting that these
reactions are not appropriate for these rocks.
• The Ti-in-biotite thermometer applied to same data agrees with
the metapelite results in TWQ and yields 565-570°C.
• For diorite intrusions, iterative Al-in-hbl and hbl-plag igneous
thermobarometry yield 685-727°C and 5.8-6.5 kbar, interpreted
as crystallization pressure and temperature.
P-T plot comparing metamorphism undergone by JMS (beige
box), Junction Butte (JB) orthogneiss, and Garnet Hill (GH)
metapelites and ironstones.
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.