Proterozoic Rocks of the Western United States

Abstract:
 Proterozoic rocks of the southwestern United States are found in Arizona, New Mexico, and Colorado.  The significance of these rocks is to determine the geologic history of the southwest using mineralogy, petrology, and structural data.  Two events, ~1.7 Ga and ~1.4 Ga, are the focus for this research in order to understand what type of large scale event occurred at ~1.7 Ga, and what type of heating event took place at ~1.4 Ga.  It is necessary to compare Proterozoic rocks, through research and fieldwork, from regional locations to understand what events took place.

Introduction:
 Proterozoic rocks are the basis of many current research projects involving rocks in Colorado, Arizona, and New Mexico. These rocks reflect two separate deformational events at approximately 1.7 Ga and 1.4 Ga (Livacarri et al, 2001).  It is important to study these rocks in order to interpret the geologic history of the southwestern United States.  The difficulty of this task is understanding what type of event occurred at 1.7 Ga to cause deformation in the rocks, why the 1.4 Ga rocks are found deformed as well as undeformed, and what type of event took place for such a large scale reheating occurred at approximately 1.4 Ga.
The primary objective is to obtain knowledge of the structural geology of the Proterozoic rocks in western Colorado, eastern Utah, Arizona, and New Mexico, and to interpret the differences in deformation between the Paleoproterozoic and Mesoproterozoic rocks, the forces required for the deformation, and their general location.  Paleoproterozoic rocks are highly deformed in the local area.  This includes migmitization and two intervals of deformation.  No fabrics are recognized in local Mesoproterozoic rocks.  This is in contrast to the surrounding outcrops of Mesoproterozoic rocks located in Arizona, New Mexico, and Colorado, which have experienced deformation.  Deformation of the Mesoproterozoic rocks in Arizona is described as an S-C fabric (Duebendorfer et al, 2001).  This implies that a foliation developed concurrently with shearing.

Methods:
My research will include three aspects.  The first is a structural and mineralogic description of the 1.7 Ga rocks in Western Colorado, and their deformation.  Second I will describe undeformed and deformed 1.4 Ga rocks in the local area, as well as Arizona.  Field research will be included in both the first and second parts of the study.  Photographs and rock descriptions will be provided of the Proterozoic rocks.  Descriptions of a newly found S-C fabric from 1.4 Ga rocks in Arizona, and the forces required to obtain this type of fabric will be given, as well as pictures of this type of fabric.  The final objective is to create a map in showing the locations of the 1.7 Ga rocks, as well as the deformed and undeformed 1.4 Ga rocks.
 The study will be conducted primarily through literary research.  Recent publications will be the focus of this effort, as well as literature containing information on S-C fabrics.  The field research will include visits to our local Proterozoic rocks in order to describe and photograph them.

Study Areas and Geologic Setting:
Proterozoic rocks can be found at many locations in the southwestern United States.  The locations focused on in this paper are found in Colorado, Arizona, and Utah (Figure 1).  Local outcrops of Proterozoic rocks in Colorado are located in the Colorado National Monument, Unaweep Canyon, Domingez Canyon, and Black Canyon (Figure 2).  Exposures extend into Utah, and can be seen along the Colorado River, as well as in Mee and Moore Canyons.  Arizona, New Mexico, and Eastern Colorado will also be included in the research.

Paleoproterozoic and Mesoproterozoic:
Proterozoic rocks are divided into two groups based upon age: Paleoproterozoic (1.7 Ga) and Mesoproterozoic (1.4 Ga).  Local Paleoproterozoic rocks include 1) sillimanite-bearing, biotite quartzofeldspathic migmatite and 2) meta-quartz monznite pluton (Figures 3 and 4).  The dates assigned to these two groups are discordant U-Pb zircon dates (Livaccari et al, 2001), which implies a discrepancy in the process used to date the rocks, and so the date is not certain.  The structure of these rocks includes an episode of migmitization, during which the rock was heated at a temperature high enough to result in a leucosome and melanosome.  During cooling, the first deformation (S1) resulted flattening of the leucosome and melanosome, and pinch and swell chocolate-block boudinage, and implies a flattening fabric and equal extension in all directions (Figure 3).  Folding then occurred creating a series of tight to isoclinal folds (F2), which dip WNW to ESE (Figure 4).  These folds can be found as Type 1 interference folds, or sheath folds (Figure 6).  Sheath folds are formed by either one or two periods of folding.  For example, the fabric may be folded with an axial surface N-S, and folded again with an axial surface E-W.  To form in a single deformational event, shearing would have to occur.  Evidence for sheath folds can be seen as small-scale structures where the leucosome is wrapped around in an oval shape.  Small-scale structures such as these reflect large-scale features, such domes, which are found in Westwater Canyon as well as Colorado National Monument.
The Mesoproterozoic rocks include 1) megacrystic, quartzmonzonite pluton, 2) medium-grained, garnetiferous, 2-mica granite pluton, 3) rare NW-SE-striking, biotite-hornblende dikes, and 4) tourmaline-bearing pegmatite dikes (Figure 5).  No fabrics have been found in 1.4 Ga plutons in the Grand Junction area.  This implies that no stresses were put on these rocks during or after intrusion.  Boudinage has been recognized in the hornblende dikes, which cut across brittle migmatitic host rocks.  The 1.4 Ga plutons are part of a regionally recognized thermal event, which caused the age dates of mica to become reset.  This is also significant because the intrusion of the plutons could reflect a large scale reheating, such as underplating, but it is debatable whether or not this is true.
It is necessary to understand the mineralogy of these rocks in order to interpret the types of conditions under which they formed.  An example of this is the mineral sillimanite, which indicates that the temperatures would have to be very high in order for it to form (Raymond, 1995).  Each mineral present is an indication of the temperatures and pressures under which the rock formed.  The knowledge that the rock is a migmatite implies that the rock has been metamorphosed at high temperatures and consists of two portions, a light colored rock, leucosome, and dark colored rock, melanosome.  The leucosome is non-foliated and appears igneous.  The melanosome is foliated and contains boudins.

Regional Relationships:
The Paleoproterozoic rocks of the Cerbat Mountains of northwestern Arizona are described as containing metavolcanic and metasedimentary rocks that have been intruded by granitoids and which experienced two periods of deformation (Duebendorfer et al, 2001).  The structure of these rocks includes a foliation, which strikes northeast, as well as folding.  This is similar to 1.7 Ga rocks near Grand Junction, but their composition differs. New evidence of deformed Mesoproterozoic rocks is currently being studied.  An S-C fabric has been discovered in Mesoproterozoic rocks from Arizona, which implies that a foliation developed and shearing occurred simultaneously (Figure 6).  This finding is significant because it allows an interpretation of the stresses present during 1.4 Ga intrusions, in which the stresses required are flattening as well as shearing.
Exposures of Paleoproterozoic and Mesoproterozoic rocks are found in the Wet Mountains of Colorado.  Paleoproterozoic rocks are composed primarily of seven types of gneiss, one of which is a garnet-sillimanite-biotite gneiss, two types of schist, and an amphibolite (Siddoway et al, 2000).  Structure includes foliation, lineation, folding, and shearing.  Migmatization can be found in most places, but not all.  Mesoproterozoic rocks of the area consist of plutons and intrusions.  In contrast to 1.4 Ga rocks near Grand Junction, these plutons are foliated (Sidoway et al, 2000).

Discussion:
Explanations for the deformation and intrusion of the 1.7 Ga and 1.4 Ga rocks of the southwestern United States differ in detail, but express a common idea of deformation of ~1.7 Ga rocks due to an accretionary boundary and a heating event at ~1.4 Ga.  One explanation for the deformation of ~1.7 Ga rocks is the accretion of oceanic island arcs as well as marginal basins, in association with the Yabapai and Mojave province collision (Duebendorfer, 2001).   Crustal material is attained, and a means for structural compression and burial to achieve metamorphism and the structures created is achieved.  Plate convergence is supported by mineralogical protliths, and structural features, such as thrust faulting and shearing, found in Arizona and Colorado (Karlstrom, et al).   This theory is supported by mineralogical evidence of marginal basin and arc setting protiliths discovered in the Wet Mountains (Siddoway, 2000).  Research done in the Grand Junction area also supports this.  The heat and stresses required for the formation of 1.7 Ga migmatites near Grand Junction would require a large scale event such as an accretionary boundary, which would provide heat and compression from burial, as well as compression from converging plate boundaries. ).
Each source above also provides the idea of a 1.4 Ga regional thermal event.  It is believed that this reheating caused a recrystallization, the resetting of hornblende, biotite, and muscovite, as well as the establishment of tectonic trends, as seen in the gradient of metamorphism, which increases southward, across the southwestern United States (Shaw, 2001).  The 1.4 Ga event is believed to be anorogenic due to a lack of deformation and metamorphism around the plutons, and the chemical composition of the plutonic rocks (Nyman et al, 1994).  1.4 Ga plutons near Grand Junction support this finding with the lack of deformation in the pluton and at the boundaries, and the composition of these plutons.  More research is required to fully understand the extent of the 1.7 Ga and 1.4 Ga rocks, the exact ages of each, and what type of thermal event occurred during the 1.4 Ga event.

Conclusions:
 There are many similiarities between the rocks located in western Colorado and the southwestern United States.  Rocks of ~1.7 Ga are regionally deformed, though not all in the same way, but they do reflect a large-scale possibly accretionary feature.  This can be inferred from the mineralogy, protoliths, and structure of these rocks.  Emplacement of ~1.4 Ga plutons is interpreted as an anorogenic event, but could be responsible for a regional reheating event.  Evidence for this is recrystallization of minerals in the ~1.7 Ga rocks, and lack of deformation in some of the ~1.4 Ga plutons.
 
 
 
 

Figure 1: Map of the Proterozoic rocks in Colorado.
 
 
 

Figure 4: Boudin from 1.7 Ga migmatite (Nothouroughfare Canyon).
 
 
 

Figure 5: Description of sheath folds (also named Type 1 interference folds).
 

Karlstrom, Karl E., Connelly, James N., Siddoway, Christine S., Williams, Michael L. 2000. Contrasting Styles of ca. 1.4 Ga Tectonism in the Southern Rockies: Evidence for a Fossil Rheologic Transition in a Deeply Exhumed Intracontinental Orogen?, NSF Structure and Tectonics Program, Collaborative Research Funding Cycle: 1 June 2000, p A-1 to D-10.

Livacarri, R.F., Bowring, T.J., Farmer, E.T., Garhart, K.S., Hosack, A.M., Navarre, A.K., Peterman, J.S., Rollins, S.M., Williams, C.A., Department of Physical and Environmental Sciences, Mesa State College, Grand Junction, CO 81502, and Kunk, M., Scott, R.B., Unruh, D., USGS, Federal Center, Denver, CO 80225, 2001.  Proterozoic Rocks of the Uncompahgre Plateau, Western Colorado and Eastern Utah, Geolorical Society of America Abstracts, v. 33, No. 5, 1pp.

Nyman, M.W., Karlstrom, K.E., Kirby, E., Graubard, C.M. 1994. Mesoproterozoic Contractional Orogeny in Western North America: Evidence from ca. 1.4 Ga Plutons, Geology, v. 22, p. 901-904.

Raymond, Loren A. 1995. Petrology, Brown Communications, United States, 681 pp.

Shaw, Colin A. 2001. Ca. 1.4 Ga metamorphic temperatures of the southwestern U.S. estimated from regional 40Ar/39Ar data, Geological Society of America Abstract.

Siddoway, Christine S., Givot, Rima M., Bodle, Christoher D., Heizler, Matt T.  2000. Dynamic versus anorogenic setting for Mesoproterozoic plutonism in the Wet Mountains, Colorado: Does the interretation depend on level of exposure?, Rocky Mountain Geology, v. 35, no. 1, p. 91-111.