Transcript Heterogeneous Growth and Dissolution of Sillimanite in

The Middle Crust of the Wyoming Province – Ground-truthing above 2000
Meters Elevation in the Beartooth Mountains, Montana and Wyoming
Darrell Henry, Dept. of Geology and Geophysics, Louisiana State University; David Mogk, Dept. of Earth Sciences, Montana State University and
Paul Mueller, Dept. of Geological Sciences, University of Florida
Introduction
Local Characteristics of Archean Rocks in the Beartooth Mountains
A significant challenge for EarthScope will be to develop geologic and geophysical
studies that can distinguish crustal discontinuities at a range of scales and at
resolutions high enough to discern structural and lithologic boundaries, and to relate
these discontinuities to structures in the mantle lithosphere. To gain an insight into
the lithologic and structural complexity of the middle crust that is likely to be
imaged over large areas in the Northern Rocky Mountains, we can examine Archean
crust exhumed from middle crustal levels in the cores of mountain ranges uplifted
during the Laramide Orogeny. The Archean rocks of the Beartooth Mountains of
Montana and Wyoming provide an opportunity to examine, first-hand, the lateral
and vertical distributions of formerly middle-crustal lithologies and the structures
superimposed on them.
Regional Distribution and General Character of
Archean Rocks of the Wyoming Province
Archean rocks exposed in the eastern and central Beartooth Mountains, Bighorn
Mountains, and samples from deep-drill cores in eastern Wyoming and Montana are
dominantly Late Archean granitoids, members of the tonalite-trondhjemitegranodiorite suite, with inclusions of older supracrustal rocks preserved as tectonic
slices or pendants in the younger magmatic rocks. These rocks are considered to be
part of the Beartooth-Bighorn Magmatic Zone (BBMT). The Archean rocks in the
western Beartooth Mountains and to the west are dominantly older high-grade
gneisses with varying abundances of metasedimentary rocks. This area is generally
termed the Montana Metasedimentary Province (MMP). The dominantly magmatic
terrane in the eastern and central Beartooth Mountains and the high-grade gneiss
terrane to the west are separated by a major discontinuity in the Archean basement
marked by a mobile belt in the North Snowy Block, western Beartooth Mountains.
Fig. 1: General outline map of the
Wyoming Province and its
subprovinces showing major areas of
exposure of Archean rocks (WGT,
Wyoming greenstone province;
BBMT, Beartooth-Bighorn magmatic
zone; MMT, Montana
metasedimentary province). The
subprovinces are defined by the
restricted occurrences of certain
lithological associations and common
geologic histories. Although not
geologically homogeneous, major
contrasts among subprovinces include
metamorphic grade, ages of
granitoids and granitic gneisses,
structural style and the compositions
and ages of supracrustal
associations. Modified from Mueller
et al. (1996).
On a finer scale in the Beartooth Mountains we recognize four geographically and geologically
distinct domains: the main Beartooth massif, the Stillwater Block, the North Snowy Block and the
South Snowy Block represent distinct lithologic, structural, and chronologic subdivisions (Fig. 2).
• The main Beartooth massif consists
predominantly of voluminous Late Archean
igneous granitic rocks (2.8-2.9 Ga) with inclusions
of metasupracrustal rocks, which exhibit wide
ranges in sizes (cm to km), composition,
metamorphic grade, and isotopic age (to 3.3 Ga).
The peak metamorphic conditions likely associated
with the early stages of granitic magmatism are
considered to be 5-7 kbar and 750-800ºC. Swarms
of mafic dikes (2-200 m wide) intruded the area
during several distinct time periods beginning 2.55
Ga and extending until 0.75 Ga.
Additional Major Geologic Features in the Beartooth
Mountains and Archean terranes to the west
• Paleozoic – Archean unconformity
is readily accessible (Fig. 6).
• The bounding thrust faults at the
margins of the uplift are exposed
(Fig. 5).
• Terrane distinctions to the west of
the Beartooth Mountains are
relatively distinctive (Fig. 7)
Fig. 2: Geographic and geologically-distinct blocks
of the Archean rocks of the Beartooth Mountains.
Fig. 6: Paleozoic sedimentary rocks
unconformably overlying the Archean
rocks of the main Beartooth massif.
Fig. 7: Highly interpretive
cross-section of the northern
Wyoming Province after the
juxtaposition of the terranes
at ~2.55 Ga. Modified after
Mogk and Henry (1988).
Issues potentially addressed by EarthScope
EarthScope could address several additional questions that are related to the
Beartooth and other Wyoming Province uplifts:
Fig. 3: Supracrustal xenoliths of varying size included in the 2.8-2.9 Ga, late Archean granitic rocks of the
eastern Beartooth Mountains. A representative P-T path of the migmatites associated with the granitic rocks
is given on the right (Maas, 2004).
• The North Snowy Block is
interpreted as a collage of
several allochthonous units
structurally juxtaposed against
the main Beartooth Massif with
latest Archean magmatism
suggesting this juxtaposition
occurred about 2.55 Ga.
Fig. 4: Large-scale isoclinal
fold in the George Lake
Marble that is cored by the
Barney Creek Amphibolite
with strong attenuation of the
units in the lower limb. The
Pine Creek Nappe Complex is
overlain by the Heterogeneous
Gneiss unit (Mogk, 1983).
• The South Snowy Block is dominated by lower grade metasedimentary rocks and a series of
locally important 2.7 Ga old granitic plutons.
• The Stillwater Block is
dominated by the mafic layered
Stillwater Complex (2.7 Ga) and
its contact aureole that developed
in older metasedimentary rocks
(Fig. 5).
Fig. 5. Structural section
through the complex at
Mountain View (after
Turner et al., 1985).
1. What is the character of the faulting at depth that brought these middle
crustal blocks to the surface, and what is the relation of these faults to
Archean structures?
2. What is the nature of the crust below the 3000 m of mid-crustal level
lithologies currently exposed in the Beartooth Mountains?
3. Are major subprovince boundaries (e.g., BBMZ-MMP) largely vertical or
horizontal structures?
4. Do any of the geologically recognizable province or sub-province
boundaries extend to the mantle?
References cited
Maas, A. T. (2004) Migmatization of Archean Aluminous Metasediments from the Eastern Beartooth
Mountains, Montana, U.S.A. MS Thesis, Louisiana State University, 141 p.
Mogk, D. W. (1983) The Petrology, Structure and Geochemistry of an Archean Terrane in the North Snowy
Block, Beartooth Mountains, Montana. PhD Thesis, University of Washington, 234 p.
Mogk, D. W. and Henry, D. J. (1988) Metamorphic petrology of the northern Archean Wyoming Province,
Southwestern Montana: Evidence for Archean Collisional Tectonics: In W. G. Ernst, ed., Metamorphism
and Crustal Evolution of the Western U.S., Rubey Volume VII, Prentice-Hall, p. 362-382.
Mueller, P. A., Wooden, J. L., Mogk, D. W., Nutman, A. P. and Williams, I. S. (1996) Extended history of a 3.5
Ga trondhjemitic gneiss, Wyoming Province, USA: evidence from U-Pb systematics in zircon.
Precambrian Research, 78, 41-52.
Turner, A.R., Wolfgram, D. and Barnes, S.J., 1985. Geology of the Stillwater County sector of the J-M reef,
including the Minneapolis adit. In: Czamanske, G.K. and Zientek, M.L. (Eds.), Stillwater Complex.
Montana Bureau of Mines and Geology, Spec. Pub. 92, 210-230.