Precambrian geologic history of the Gravelly Range, southwest Montana Aaron Fricke Senior ...

38 Pages·2009·41.42 MB·English

Precambrian geologic history of the Gravelly Range, southwest Montana
Aaron Fricke
Senior Integrative Exercise
March 11, 2009

Submitted in partial fulfillment of the requirements for a
Bachelor of Arts degree from Carleton College, Northfield, Minnesota.
Table of Contents
Abstract
Introduction........................................................................................................................1
Geologic Background.........................................................................................................2
Geologic Setting of the Gravelly Range...........................................................................7
Field Descriptions and Petrography of Major Units....................................................10
Structural Relationships..................................................................................................14
Geochemistry of Mafic Rocks.........................................................................................16
Geochemical Interpretations...........................................................................................21
Discussion.........................................................................................................................22
Conclusions.......................................................................................................................28
Acknowledgements..........................................................................................................29
References Cited...............................................................................................................31
Appendix 1........................................................................................................................34
Appendix 2........................................................................................................................35
Precambrian geologic history of the Gravelly Range, southwest Montana
Aaron Fricke
Carleton College
Senior Integrative Exercise
March 11, 2009
Advisor
Cameron Davidson, Carleton College Department of Geology
The metasedimentary rocks of the Gravelly Range, in southwest Montana, USA, are
of anomalously low metamorphic grade compared to nearby Archean rocks in the
Tobacco Root Mountains. This has led some previous workers to assign a Proterozoic
age to the rocks of the Gravelly Range. This study examines the petrology, structure,
and geochemistry of the metasupracrustal rocks of the Gravelly Range in order to
better compare these rocks to those found in the Tobacco Root Mountains to the north.
Metamorphic monazite from a phyllite in the Gravelly Range was dated to ca. 2.5
Ga, all but precluding a Proterozoic depositional age for the protolith (Gerwin 2006).
Likewise, petrologic, geochemical, and structural comparisons between the Precambrian
rocks of the Gravelly and Tobacco Root Mountains show no major incompatibilities.
Based on these comparisons, the simplest geologic history of the Gravelly Range is
very similar to that defined for the Tobacco Root Mountains by Brady et al. (2004a).
However, the structural mechanism by which the Gravelly Range escaped the high-grade
metamorphism experienced in surrounding areas remains unknown.
Keywords: Gravelly Range, Big Sky Orogeny, Tobacco Root Mountains, Geochemistry,
Montana, Precambrian.
1
INTRODUCTION
The Gravelly Range is a Laramide-age uplift in southwestern Montana located
near the northwest margin of the Wyoming province, an Archean age cratonic province
that forms a portion of the cratonic core of North America (Burger, 2004; Hoffman, 1988)
(Fig. 1). The rocks of the Gravelly Range consist of thrusted and faulted low-grade rocks
including phyllite, quartzite, marble, banded iron-formation, and amphibolite. The only
age control in the Gravelly Range is provided by a 2569 ± 45 Ma monazite age from a
sillimanite-bearing phyllite in the south-central Gravelly Range (Gerwin, 2006).
Rocks in the central Gravelly Range are of predominantly greenschist and lower
amphibolite facies (this study, Vargo, 1990). By comparison, rocks in the Tobacco
Little Belt
Mountains
Helena
e
atholith eat Foalnlisc Zon
Butte der b GrTect
ul
o MONTANA
B Tobacco Root
Highland
Mountains
Mountains
Beartooth
Bozeman Mountains
Pioneer
batholith
Dillon Tobacco
Root
Ruby
batholith
Range
Madison
Range
Gravelly
Range
WYOMING
Gallatin
Archean Range
N
exposures
0 30
IDAHO
Km
Figure 1: Location map showing the Gravelly Range in relation to other Archean
exposures in southwest Montana. Modified from Mueller et al. (2005).
2
Root Mountains, less than 30 kilometers to the northwest, record upper amphibolite
to granulite facies metamorphism (Cheney et al., 2004a). According to Harms et al.
(2004a), the metamorphism in the Tobacco Root Mountains is associated with the Big
Sky Orogeny, a ca. 1.78 Ga to ca. 1.71 Ga orogenic event along the northwest margin of
the Wyoming Province (Burger, 2004; Harms et al., 2004a).
The Precambrian geologic history of the Tobacco Root Mountains has been well
documented, most recently and completely in Special Publication 377 of the Geological
Society of America (Brady et al., 2004a). The goal of this study is to discover the extent
to which the Precambrian geologic histories of the Tobacco Root and Gravelly Ranges
are similar. This comparison, in addition to the date provided by Gerwin (2006), may
help constrain the geologic history of this enigmatic area.
GEOLOGIC BACKGROUND
The cratonic core of North America is composed of seven Archean provinces
(Hoffman, 1988). Of primary interest to this study are the Wyoming and Medicine Hat
provinces, which form the southwest portion of Laurentia (Fig. 2). Specifically, this
study focuses on Archean rocks exposed in Laramide-age uplifts in southwest Montana.
Key to understanding the complex Precambrian geologic history of this area are three
major geochronologic or structural boundaries: Giletti’s Line, the Great Falls tectonic
zone (GFTZ), and the Madison mylonite zone (MMZ).
Giletti (1966) defined a NW trending line which crosses the Gravelly Range,
based on distinct K-Ar and Rb-Sr ages recorded in the rocks (Fig. 3). Rocks north of this
line record Proterozoic ages (1.6-1.8 Ga), whereas rocks to the south of this line record
earliest Proterozoic or Archean ages (2.1-3.2 Ga). Harms et al. (2004a) suggest Giletti’s
Line represents the southern limit of thermotectonism associated with the Big Sky
Orogeny.
The Great Falls tectonic zone is interpreted to be a suture between the Wyoming
3
and Medicine Hat provinces, but the age of this suture is a matter of debate. Hoffman
(1988) proposes an Archean age for the GFTZ, while O’Neill (1998) suggests the GFTZ
represents a Mesoproterozoic suture between the two provinces.
The Madison mylonite zone lies to the southeast of the Gravelly Range, and
bisects the Madison Range. The MMZ is interpreted to be a NE-striking, NW-dipping
shear zone with a reverse sense of shear based on its placement of higher-grade rocks
over lower-grade rocks (Erslev and Sutter, 1990). The Madison mylonite zone lies
far enough to the southeast of Giletti’s Line as to be well outside the region with reset
120° N 110° N 100° N
Hearne
Province n
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o Superior
s 50° N
d
Medicine u Province
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Hat Block nic z o n e sn-a
G re at Falls te cto rT 45° N
Black
Wyoming
Hills
Province
Cheyenne
40° N
Belt
500 km
Figure 2: Schematic diagram of the major Archean cratonic
provinces of southwest Laurentia (dark gray). Arrows indicate direc-
tions of marginal convergence. Adapted from Mueller et al. 2005.
4
C A N A D A
115°
110°
Area of Precambrian rock
exposure
NE
O
NIC Z
O
IDAMONTANA G REAT FALLS TECTLMITOTULNET BAEINLST
H
O
accreted arc terrane
metamorphic core
foreland and ductile
thrust belt
TOBACCO ROOT
MOUNTAINS
MHOIGUHNRLTUAANBINYDS G ARLALNAGTIEN MBOEUANRTTAOINOSTH
RANGE
GILETTI'GS RLIANVEEmMLLazyYoldoninseoMitneADISOSN. YOMING SB.L SONCOKWY 0 100 Kilometers
RANGE RANGE W
Figure 3: Map showing the Great Falls Tectonic Zone, Giletti’s Line, and
the Madison mylonite in relation to Archean exposures in southwest
Montana. Modified from Harms et al. 2004.
Proterozoic Ar ages. However, rocks within the MMZ show evidence of a thermal event
at about 1.8 Ga, indicating the MMZ was active during the Big Sky Orogeny (Erslev and
Sutter, 1990; Harms et al., 2004a).
The Tobacco Root Mountains lie entirely to the north of Giletti’s Line, and
closer to the Big Sky collisional margin. The Tobacco Root Mountains, much like the
Gravelly Range, consist of quartzofeldspathic gneisses and amphibolites overlain by and
intercalated with metasedimentary rocks, which are divided into three metamorphic suites
(Burger, 2004). This study will focus on the two largest suites, the lithologies of which
most closely resemble those found in the Gravelly Range: the Indian Creek Metamorphic
5
Suite (ICMS), and the Pony-Middle Mountain Metamorphic Suite (PMMMS).
Harms et al. (2004a) propose a model for the Archean and Proterozoic evolution
of the Tobacco Root area, explaining the depositional and metamorphic history of the
Indian Creek and Pony-Middle Mountain Metamorphic Suites (Fig. 4). The model calls
on active rifting between ca. 3.35 and ca. 3.2 Ga, creating the volcanic protoliths of
the quartzofeldspathic gneisses and amphibolites of the ICMS and PMMMS. Ages for
this rifting are supplied by magmatic zircons from the quartzofeldspathic gneisses of
the Tobacco Root Mountains (Mueller et al., 2004). Based on detrital zircon ages from
ICMS quartzites (Mueller et al., 2004), rifting became less active between ca. 3.13 Ga
and ca. 2.85 Ga, and the supracrustal rocks of the ICMS were deposited into a supra-arc
basin during this period.
The basin was closed ca. 2.45 Ga, when collision with an unspecified body
produced a tectonothermal event along the northwest margin of the Wyoming province
(Cheney et al., 2004a; Harms et al., 2004a; Mueller et al., 2004). This thermal event is
recorded by metamorphic zircon (Krogh et al., 1997), and monazite (Mueller et al., 2004)
growth in the ICMS and PMMMS. The collision produced some of the gneissic banding
and folding described in the Tobacco Root Mountains by Harms et al. (2004b). This
deformation is thought to predate later deformation associated with the Big Sky Orogeny,
since mafic dikes and sills associated with incipient rifting ca. 2.06 Ga are seen to cross-
cut these previous fabrics (Brady et al., 2004c).
Between ca. 1.78 and ca. 1.71 Ga, the rocks of the Tobacco Root Mountains
were involved in the Big Sky Orogeny, a collision between the northwest margin of
the Wyoming province and an accreted arc terrain (Harms et al., 2004a). This orogeny
is characterized in great detail in Special Publication 377 of the Geological Society of
America (Brady et al., 2004a). The age of the Big Sky Orogeny is constrained by U-Pb
monazite ages (Cheney et al., 2004b), 40Ar/39Ar hornblende geochronology (Brady et al.,
2004b), and U-Pb zircon ages (Mueller et al., 2005).
6
In the Tobacco Root Mountains, the ICMS and PMMMS record upper
amphibolite facies metamorphism associated with the Big Sky Orogeny (Harms et al.,
2004a). The Big Sky Orogeny reoriented preexisting fabrics and structures; the resulting
extensive folding and penetrative fabrics indicate that the trend of the Big Sky Orogen
was roughly E-W in present coordinates (Harms et al., 2004b). The extent to which
petrologic, geochemical, and structural data from the Gravelly Range is consistent with
this model is the focus of this study.
1.78-1.72 Ga
NW BIG SKY OROGEN SE
aarcc ctererrtaende MMT -Big Sky Orogeny
PMMMS ICMS -ICMS-PMMMS strongly deformed and metamorphosed to upper
amphibolite facies
-Gravelly Range rocks deformed and metamorphosed to greenschist/
subcrustal lithosphere
lower amphibolite facies
NWgenerally decreasing deformation/metamorphismSE 2.45 Ga
-colission causes tectonothermal event along continental margin.
unknown
collider
-gneissic banding and folding develops in ICMS-PMMMS
-less intense deformation in Gravelly Range, monazite growth records
subcrustal lithosphere subcrustal lithosphere metamorphism at 2569 ± 45 Ma (Gerwin 2006).
NW Gravelly Range sediment deposition SE 3.13-2.85 Ga
counntiknneonwtanl -ICMS supracrustal rocks deposited in back-arc basin (arc activity low)
basement
ICMS sediment deposition -similar Gravelly Range metasupracrustal rocks deposited in back-arc basin
further to the SE, minor volcanism continues
subcrustal lithosphere
NW ICMS-PMMMS ARC-BACK ARC SYSTEM SE 3.35-3.2 Ga
unknown
continental -arc volcanism creates ICMS/PMMMS protoliths
basement
-similar arc volcanism produces protoliths of quartzofeldspathic gneiss
sliuthbocsrupshtearle ocean crust mantlelistuhbocsrpuhsetrael and amphibolite in Gravelly Range, sediments begin to accumulate in basin
Figure 4: Archean/Proterozoic geologic chronology of the Tobacco Root and Gravelly
Ranges. Rocks exposed in the Tobacco Root Mountains include Indian Creek
Metamorphic Suite (ICMS) and Pony-Middle Mountain Metamorphic Suite (PMMMS).
Summaries in italics describe the geologic history of the Gravelly Range proposed in this
study. Schematic cross sections and summaries in regular text adapted from Harms et al.
(2004a).
7
GEOLOGIC SETTING OF THE GRAVELLY RANGE
The Gravelly Range is roughly 70 kilometers in length, running SSE from
Virginia City, Montana to near the Idaho state line. The Madison River valley separates
the Gravelly Range from the Madison Range to the east. Although the highest peaks
reach elevations over nine thousand feet, slopes are relatively gentle, and tree line extends
to summits in most places.
The study area is located entirely within the Wall Creek State Wildlife
Management Area and the Beaverhead National Forest. The northern portion of
the study area is accessed via the McAtee Bridge which crosses the Madison River
roughly 30 kilometers south of Ennis, Montana. The southern portion of the study area
is accessed via NFS Route 209, which crosses the Madison River approximately 60
kilometers south of Ennis.
The study area is bounded to the north by a large talc mine operated by Rio
Tinto, and to the south by Standard Creek (see Appendicies 1 and 2). This area consists
of a package of metasupracrustal rocks including phyllite, quartzite, marble, banded
iron-formation, and pelitic schist (cf. Vargo, 1990). This metasedimentary package is
underlain by and intercalated with quartzofeldspathic gneisses and amphibolites.
To the north of the talc mine, the Cherry Creek Metamorphic Suite is exposed.
This suite consists of a metasuprasrustal sequence similar to that found south of the talc
mine, although metamorphism in this area reaches upper amphibolite grade. Here high-
grade kyanite schists are found adjacent to low-grade phyllites (Vargo, 1990). Heinrich
and Rabbitt (1960) reported a decrease in metamorphic grade throughout this area toward
the south.
The lithologies exposed south of Standard Creek resemble the metasupracrustal
rocks to the north. However, there is little evidence of amphibolite facies metamorphism
in this area (Vargo, 1990). A lack of ultramafic rocks, and the prevalance of low-pressure
mineral assemblages define this area.