Transcript Evolution of the Precambrian Rocks of Yellowstone National

Evolution of the Precambrian Rocks of Yellowstone National Park (YNP): Regional Overview
David Mogk1, Darrell Henry2,Paul Mueller3, and David Foster3
Scientific Rationale
The Precambrian rocks of northern Yellowstone National
Park provide important insights into the
petrogenesis, architecture, and evolution of the northern
Wyoming Province (WP). These rocks are distinguished by:
1Montana
Setting in the Wyoming Craton
Key Research Questions
Some of the key research questions addressed in this project
include:
• A suite of metasedimentary rocks. The Jardine
Metasedimentary Sequence(JMS), composed primarily of
biotite schists, quartzites and minor iron formation have
preserved primary sedimentary structures (graded
bedding, cross-beds, channels). Original sediments are
relatively immature and this suite is interpreted as a
turbidite sequence. In contrast, metasedimentary rocks
throughout the northern WP are more typically stable
platform sediments (marble, quartzite, pelite).
Relationship of the South Snowy
Block to the Beartooth Mountains
3) What is the igneous, metamorphic, and structural history of this
area, and what does this tell us about crustal genesis and evolution?
4) Is this suite of rocks allochthonous in their current setting (Mogk,
1984), or do they represent unusual preservation of upper crustal
rocks in the midst of mesozonal plutons and higher grade
metamorphic sequence exposed across the northern Wyoming
Province?
• The metasedimentary rocks (JMS) are interpreted as
a turbidite sequence deposited in a mid- to distal-fan
setting based on the type and distribution of preserved
primary sedimentary structures.
• These rocks were most likely deposited along an
active continental margin. The source area(s)
represent a bi-modal association of mafic and tonalitic
rocks. The age of deposition is constrained between
2.8-2.9 Ga based on detrital and magmatic zircon
ages. These rocks could not have been derived from
the contemporaneous, adjacent Long Lake Magmatic
Complex (interpreted as a continental arc).
2) What are the ages, provenance(s) and depositional environments
of the metasedimentary rocks?
Hellroaring Pluton, YNP
Setting of the South Snowy Block in the Wyoming Province
• Magmatism in this area is dominated by two epizonal
quartz monzonitic plutons (emplacement depth ~12 Km),
whereas magmatism in the Long Lake magmatic complex
in the adjacent Beartooth Mountains is at much deeper
levels (~20 Km), and with a range of compositions from
gabbroic to granitic.
Because of these differences
in lithology, metamorphism,
structural style, and
magmatism, earlier work
(Mogk, 1984) interpreted these
rocks and adjacent rocks in the
South Snowy Block as an
allochthonous unit, tectonically
emplaced in the latest
Archean. The purpose of this
study is to test that hypothesis.
Preliminary Results
1) What is the relationship of this suite of rocks to the voluminous 2.8
Ga magmatic rocks of the main Beartooth massif to the east and the
high-grade, migmatitic and mylonitic gneisses of Yankee Jim Canyon
to the west?
• Metamorphism is at an anomalously low-grade ranging
from chlorite zone to andalusite-staurolite zone in most of
the metasedimentary rocks, compared with upper
amphibolite to granulite facies rocks in most of the
northern WP.
• The structural style is dominated by kink-folding and late
mylonitic shear zones, whereas adjacent areas (Yankee
Jim Canyon) have developed pervasive ductile shear
zones.
State Univ., 2Louisiana State Univ., 3Univ. of Florida
Detrital zircon age spectra from quartzites; J-2
(JMS) could not have been derived from adjacent
Beartooths.
• Two quartz monzonitic plutons have been emplaced at
high crystal levels (~12 km), at 2.8 Ga, are
peraluminous, and have distinct compositions
compared with the LLMC. Although their emplacement
is contemporaneous with the LLMC, they appear to
have had a different source. Mafic to intermediate
plutons intruding the JMS are also 2.8 Ga, but are
geochemically identical to rocks of the LLMC.
• Metamorphism is relatively low-grade ranging from
chlorite zone in the west to staurolite-andalusite zone
in the east; the low pressure, high temperature
metamorphism is of a “Buchan" style. An injection
migmatite complex is located at Garnet Hill, and a
migmatitic orthogneiss (3.2 Ga) and garnet
leucogranite occur in the Vantage Butte areas.
• Polyphase deformation includes rare isoclinal folds
and two generations of kink folds that pre-date the
plutons. Mylonitic shear zones cut both the plutons
and the metasedimentary rocks.
Details of these results can be seen in the adjacent
posters.
Metasedimentary packages at
Bear Creek
Sketch map showing the locations of the Jardine (J) area
in the South Snowy Block, Yankee Jim Canyon (YJC),
and locations in the main Beartooth massif.
This project was supported through the NSF REU program,
Division of Earth Science grants EAR 0852025, 0851752,
and 0851934.
Geologic map of the crystalline rocks of northern YNP; Casella et al., (1982)
Long Lake Magmatic Complex, Beartooth
Mtns.
Special thanks to YNP staff, Christie Hendrix, Stacey
Gunther, Carrie Guiles, Bridgette Guild and Hank Heasler for
their support and interest.