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Transcript
Proterozoic Evolution of the Western Margin of the Wyoming Craton:
Implications for the Tectonic and Magmatic Evolution of the Northern Rockies
David Foster, Paul Mueller & Jim Vogl - University of Florida
David Mogk - Montana State University, Joe Wooden - U.S. Geological Survey
The Great Falls
tectonic zone is
dominantly
~1.85 Ga crust
deformed at
~1.77 Ga.
ABSTRACT
Defining the extent and nature of basement provinces that lie beneath
Mesoproterozoic to Phanerozoic sedimentary rocks in the region between
the exposed western margin of the Archean Wyoming craton and the
Neoproterozoic margin of Laurentia has been elusive because of thick
cover and voluminous Cretaceous-Tertiary magmatism. Known
Paleoproterozoic basement is exposed southwest of the Wyoming
province in the Mojave block, but the region between this area and
exposed Archean and Paleoproterozoic rocks in the Priest River complex
is poorly known. U-Pb geochronological data from small exposures of preBelt Supergroup basement along the western side of the Wyoming craton
in southwestern Montana reveal crystallization ages ranging from about
2.4 to 1.8 Ga. Rock forming events as young as 1.6 Ga are indicated by
isotopic (Nd, Pb, & Sr) signatures and xenocrystic zircon populations in
Cretaceous-Eocene granitoids, west of the exposed western margin of the
Wyoming Province. A major component of this lithosphere was primitive,
gives ages about 1.7-1.86 Ga, and occurs in a zone that appears to
extend west to the Neoproterozoic rifted margin of Laurentia along the
trend of the Great Falls tectonic zone (GFTZ). These data suggest that
the basement in this area contains some accreted juvenile
Paleoproterozoic arc-like terranes, possibly combined with a mafic
underplate of similar age. This area largely underlies the Belt Basin and
northern Idaho batholith and we refer to it as the Selway terrane. The
Selway terrane comprises these Paleoproterozoic elements along with
subordinate Archean blocks isolated from the Wyoming craton. The
Selway terrane crust has been more easily reactivated structurally and
much more fertile for magma production and mineralization than the thick
crust and lithosphere of the Wyoming craton and is of prime importance
for evaluating Neoproterozoic continental reconstructions. Major
structural features like the Lewis and Clark line, Helena salient, Eocene
metamorphic core complexes appear to have some basement or
lithospheric control. The largely continental-sourced Idaho batholith
formed mainly over Paleoproterozoic arc terranes, supporting the
hypothesis that the Selway lithosphere was chemically enriched and more
fertile for partial melting compared to areas underlain by Archean
lithosphere. Plutons of the Cretaceous SW Montana granitic province
(e.g., Boulder and Pioneer batholiths), as well as the Eocene Challis
volcanics, Absaroka volcanics, and Montana alkaline province are mainly
restricted to the GFTZ or parts of the Wyoming province known to contain
Paleoproterozoic lower crust. Spatial (e.g., orogen-orthogonal), temporal
(Cretaceous and Tertiary), and compositional (ore-bearing and non orebearing) variations in magmatism, therefore, were related to the
compositions of mantle-derived melts, depth of melting, and the relative
fertility of the lithosphere. The variations reflect characteristics established
during distinct Archean and Proterozoic episodes of crustal growth that
were and modified during the Mesozoic and Tertiary. Improved images of
the lithosphere anticipated from EarthScope data may help define the
major Precambrian provinces, their boundaries and roll in crustal
evolution, although higher resolution geophysical data of the crustal
structure may also be required.
The Selway terrane
contains crust
formed and
accreted between
2.4 and 1.6 Ga, along
with small Archean
blocks.
Aeromagnetic anomaly map of northwestern United States
with interpreted Precambrian terrane boundaries and Proterozoic
structures from Sims et al. (2004).
CB, Cheyenne belt suture; DSZ, Dillon shear zone; GFSZ, Great Falls shear zone; LC Lewis
and Clark fault zone; SF Snake River fault zone; SR Snake River Plain.
The Lewis & Clark
fault zone may have
developed along the
southern margin of
the Medicine Hat
block.
Basement uplifts in blue
Evidence for the age of buried
Precambrian crust from
inherited zircons in Cretaceous
and Tertiary plutons.
Southwest Laurentia Basement Provinces
.
The Cordilleran
fold-trust belt
is controlled by the
basement architecture.
Cretaceous - Tertiary
magmatism is
concentrated in areas
of Paleoproterozoic
Eocene
metamorphic core
complexes
are
concentrated in
areas underlain
by Proterozoic
accreted terranes
and
crust.
island arcs.
Geochronologic evidence for
Proterozoic orogeny in the
NW Wyoming Province,
and for Proterozoic rocks
in the Great Falls tectonic zone
and Selway terrane.