RESEARCH Characteristics and Implications of Ca. 1.4 Ga

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Characteristics and implications of ca. 1.4 Ga deformation across a Proterozoic mid-crustal section, Wet Mountains, Colorado, USA

James V. Jones III1, Christine S. Siddoway2, and James N. Connelly3 1DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF ARKANSAS LITTLE ROCK, LITTLE ROCK, ARKANSAS 72204, USA 2DEPARTMENT OF GEOLOGY, COLORADO COLLEGE, COLORADO SPRINGS, COLORADO 80903, USA 3DEPARTMENT OF GEOLOGICAL SCIENCES, JACKSON SCHOOL OF GEOSCIENCES, UNIVERSITY OF TEXAS AT AUSTIN, AUSTIN, TEXAS 78712, USA

ABSTRACT

In the Wet Mountains, Colorado, Proterozoic rocks exposed along an oblique north-south tilted section preserve evidence of regional deformation and high temperature metamorphism in the middle and lower crust at ca. 1435–1365 Ma. Deformation of gneisses in the northern Wet Mountains is partitioned within discrete zones of subvertical foliation and northeast-trending folds, a product of northwest-southeast contraction or constriction associated with transcurrent deformation. Gneisses in the north are generally not migmatitic, and granitic intrusions form discrete bodies with distinct contacts. Shear zone foliation is cut by a late syntectonic dike with a U-Pb zircon age of 1430+5/–3 Ma, constraining the age of shear zone deformation in the upper crust. In the central to southern Wet Mountains, gneisses exhibit migmatitic foliation that dips moderately northeast, with dip- to oblique-slip mineral lineation throughout. Granite forms pervasive sills and interconnected sheets with gradational or indistinct contacts. Gneissic granite that yields a U-Pb zircon age of 1435 ± 4 Ma was emplaced into amphibolite gneiss containing 1436 ± 2 Ma metamorphic zircon. Younger, foliated granite sills were emplaced at 1390 ± 10 Ma. Our new results indicate contemporaneous deformation and metamorphism throughout the middle and lower crust at ca. 1.4 Ga. We interpret the zone of migmatitic crust pervaded by granite to represent a weak, low-viscosity, ﬂ owing lower crust that controlled the pattern of distributed deformation in the comparatively strong, brittle crust above. Thus, the Wet Mountains may be viewed as a deeply exhumed analog for the mid-crustal, low-viscosity layers that are inferred to exist in modern intracontinental orogenic settings and continental rift provinces.

LITHOSPHERE; v. 2; no. 2; p. 119–135. doi: 10.1130/L78.1

INTRODUCTION Throughout the subsurface of the mid-continent region, the voluminous ferroan granites emplaced at shallow levels at ca. 1.4 Ga are largely The regional context and tectonic setting for widespread Mesopro- undeformed (Bickford and Anderson, 1993). However, in exposures of the terozoic (ca. 1.4 Ga) granitic magmatism across the southwestern United Rocky Mountains and the southwestern United States (Fig. 1), ca. 1.4 Ga States has long been a subject of debate. Granites of this age were formed granites commonly exhibit foliation (e.g., Aleinikoff et al., 1993) and, in during an interval of voluminous igneous activity between 1.6 and 1.3 Ga many cases, are spatially associated with lithosphere-scale shear zones that occurred throughout Paleoproterozoic crustal provinces in Laurentia (e.g., Selverstone et al., 2000; Shaw et al., 2001; Jessup et al., 2006). and Baltica, and granites of similar age have been documented on nearly Regional shear zones and other coeval deformational features generally every other continent (Anderson and Morrison, 2005). Petrogenetic mod- record northwest-southeast shortening that has been attributed to intra- els constrained by geochemical and isotopic data (e.g., Anderson and continental tectonism at ca. 1.4 Ga driven by active convergence along the Cullers, 1999; Frost et al., 2001b; Goodge and Vervoort, 2006) typically distal southern margin of Laurentia (Nyman et al., 1994). The ﬁ nding of involve widespread partial melting of preexisting Paleoproterozoic lower contractional deformation that is contemporaneous with ferroan, alkalic crust of varying compositions (Anderson and Morrison, 2005). In North magmatism presents a serious problem from a petrological standpoint, America the alkaline, ferroan geochemistry of the granites (A-type, sensu however, because such granites are nearly universally derived from mag- lato; Frost et al., 2001a) and a perceived lack of dynamic fabric through- matic differentiation of mantle-derived tholeiite and are associated with out the plutons and batholiths (Bickford and Anderson, 1993) are viewed extensional or hotspot settings (Frost et al., 2001a). as evidence for regional plutonism within a setting of crustal extension The Wet Mountains are a critical locality in which to examine the or mantle upwelling (Anderson, 1983; Hoffman, 1989; Frost and Frost, discrepancy between petrological models and structural observations for 1997; Ferguson et al., 2004). Consistent with this model, the evidence events at ca. 1.4 Ga. Proterozoic gneisses exposed throughout the Wet for high-temperature, low-pressure metamorphism at ca. 1.4 Ga is wide- Mountains host voluminous alkalic, ferroan granites (Bickford and Ander- spread (Pedrick et al., 1998; Williams et al., 1999). The thermal perturba- son, 1993; Cullers et al., 1992; Cullers et al., 1993) and exhibit structures tion caused resetting of 40Ar/39Ar ages in Paleoproterozoic rocks exposed formed during northwest-southeast shortening (Siddoway et al., 2000) or throughout the southwestern United States, with temperatures >550 °C east-northeast–west-southwest dextral transcurrent deformation (Andro- attained in the vicinity of the Wet Mountains of southern Colorado (Fig. 1; nicos et al., 2002). Non-migmatitic mid-crustal gneisses in the north pass Shaw et al., 1999, 2005). into migmatitic gneisses that represent deeper levels in the south (Figs. 1 and 2), a transition that is accompanied by a change in structural style *Corresponding author e-mail: [email protected]. and a mode of plutonism from discrete plutons to pervasive dikes and

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110°W 105°W 110°W 105°W A B Archean Craton WYOMING lt <300°C e T PROVINCE B L E B T 300-350°C S U Cheyenne R H T Mazatzal R E deformationfront V 40°N Temperature O ne ca. 1.4 Ga Zo n >500°C io sit 350-500°C an Tr 300-350°C Approx. area <300°C of Fig. 2

MOJAVE PROVINCE 350-500°C YAVAPAI >500°C PROVINCE SOUTHERN GRANITE RHYOLITE 35°N PROVINCE MAZATZAL PROVINCE 0 100

km Grenville Front 0 100 GRENVILLE Ca. 1.1 Ga rocks Precambrian uplifts km PROVINCE Ca. 1.4 Ga plutons

Figure 1. (A) Regional index map of the southwestern United States. Precambrian exposures (gray) and ca. 1.4 Ga granites (black) are emphasized. Proterozoic crustal provinces, inferred boundaries or transition zones, and approximate age ranges are shown (Condie, 1986; Bennett and DePaolo, 1987; Karlstrom and Bowring, 1988; Wooden et al., 1988; Wooden and DeWitt, 1991). Regional qualitative strain ellipse inferred from structural data from the southern Rocky Mountains and Arizona (Graubard and Mattinson, 1990; Kirby et al., 1995; Nyman and Karlstrom, 1997; Shaw et al., 2001; Selverstone et al., 2000). (B) Map showing estimate of regional ca. 1.4 Ga temperature inferred from thermochronologic data (from plate 2 of Shaw et al., 2005, and references therein). sills (Bickford et al., 1989). In this paper we present new high-precision metasedimentary rocks, and mafi c and granitoid plutons that were formed U-Pb geochronology for zircon and titanite in order to refi ne the ages of and accreted to the southern margin of the Archean Wyoming Province magmatism, deformation, and metamorphism during the ca. 1.4 Ga event between 1.8 and 1.6 Ga (Condie, 1982; Karlstrom and Bowring, 1988; in the northern and southern Wet Mountains. We sampled granites, granite Reed et al., 1993) as part of a protracted period of Laurentian crustal gneisses, and amphibolite gneisses from exposures across the contrasting growth (Whitmeyer and Karlstrom, 2007). These exposures have been structural levels to determine the age of shear zone development in the divided into several orogenic provinces on the basis of rock ages and north versus penetrative deformation in the south. The new data help to isotopic characteristics (Fig. 1). The Yavapai Province is interpreted to refi ne our understanding of the contradictory viewpoints on the question represent a complex collage of predominantly juvenile arc terranes char- of Mesoproterozoic granite emplacement in a contractional (Nyman et al., acterized by rocks with Nd model ages between 2.0 and 1.8 Ga (Bennett 1994) versus an extensional setting (e.g., Anderson, 1983; Anderson and and DePaolo, 1987). Rocks of the Yavapai Province were accreted to the Cullers, 1999; Frost et al., 2001b; Dean et al., 2002). Our aim is to provide Laurentia margin 1.78–1.70 Ga along a belt stretching from Colorado to a sound geochronological framework for the ongoing effort to develop a Arizona and New Mexico (Fig. 1). The fi nal collisional phase of this long- tectonic model that unifi es petrological, geochemical, and structural geol- lived, progressive orogenic event occurred between ca. 1.71 and 1.70 Ga ogy data for the 1.4 Ga event in the Wet Mountains, with consequences for (Karlstrom and Bowring, 1988) and was followed by ~40 m.y. of volumi- the broader region of southwestern Laurentia. nous post-orogenic granitoid magmatism (Anderson and Cullers, 1999), erosional unroofi ng, and widespread sedimentation (Jones et al., 2009). GEOLOGIC SETTING The Mazatzal Province lies to the south of the Yavapai Province and extends across central and southern New Mexico and Arizona (Fig. 1). Exposures of Precambrian crustal rocks across the southwestern Mazatzal Province rocks are characterized by Nd model ages between 1.8 United States are made up of a diverse assemblage of metavolcanic rocks, and 1.7 Ga (Bennett and DePaolo, 1987) and were accreted to southern

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Il s efa

u lt z one

1706(5)** Crampton Mt. Twin Mt. pluton pluton Blue Ridge Five Pts. Quartzite def. zone 1706(5)*

~ 1705(8)* ~

Garell ~

~ 69

~ ~~

~ A Peak rk ~ 81 a Cañon City ns 1663(4)* ~ as pluton ~~~ Ri ~~~ v ~ e

~ 54 r A ~~~ rk 1666(22)* 84

a ~~ Five Points Gulch shear zone

~~ 44

a ~~~~

78~ s ~

R ~ Pegmatite dike 1430+5/-3 Ma ~ iv ~ e ~ r

1474(7)* Oak Creek pluton 1460(21)* 1439(8)* 1442(7)* 62 . 78 k 71 Newlin Creek W. McCoy C e p 43 35 ra shear zone Gulch pluton G 52 Rattlesnake Gulch*** Amphibolite ca. 1436 Ma G3 granite ca. 1390 Ma

k. Zones of high strain e C bbl cra rds Ha Mesoproterozoic granitic rocks N. San Isabel Quartzite granite

Paleoproterozoic granitic rocks 1362(7)* ? 1371(14)* ? Mafic gneisses 1486(36)* ? 38°N Basement gneiss and amphibolite 71 62 55 27 77 N 0 10 20 km

Geology after Tweto (1979) Ilse fault zone Siddoway et al. (2000) COLORADO U-Pb geochronology of Williams Ck./ Bear Ck. complex Bickford et al. (1989)* Amphibolite 1436±2 Ma 1706(5) Jones et al. (2009)** G2 granite 1435±4 Ma Jones (2005) *** with 2σ uncertainty G3 granite 1390±10 Ma

Figure 2. Generalized Precambrian geology of the Wet Mountains, Colorado. Areas sampled for new U-Pb geochronology and summary of new U-Pb zircon ages are indicated, along with a summary of published U-Pb ages.

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Laurentia during the Mazatzal orogeny ca. 1.66–1.60 Ga (Silver, 1965; along the approximately east-west Arkansas River canyon in the northern Karlstrom and Bowring, 1988; Amato et al., 2008). Deformation related to part of the range (Arkansas River Gorge; Fig. 3). Basement exposures the Mazatzal orogeny propagated northward into the southern part of the throughout the central and southern Wet Mountains have a narrower com- Yavapai Province (Transition Zone, Fig. 1), and the Mazatzal deformation positional range and are dominated by interlayered quartzose and quartz- front represents the approximate northern limit of these effects (Shaw and ofeldspathic gneiss, amphibolite gneiss, and metagabbro (Fig. 4). Rare Karlstrom, 1999). Various workers have challenged the juvenile arc accre- exposures of schist, marble, and calc-silicate gneiss exist. In general, met- tion model for the Yavapai and Mazatzal orogenies on the basis of zircon amorphic grade increases from north to south across the range. Whereas ages, lithological associations, and limited Hf isotopic data (Bickford and exposures in the Arkansas River Gorge record peak metamorphic condi- Hill, 2007a; Bickford et al., 2008), but alternative models are still being tions of greenschist to amphibolite facies (Siddoway et al., 2000), gneisses evaluated and debated (Duebendorfer, 2007; Karlstrom et al., 2007; Bick- of the central and southern Wet Mountains are migmatitic throughout (e.g., ford and Hill, 2007b). Boyer, 1962; Siddoway et al., 2000) and underwent upper-amphibolite- to After an ~150 m.y. tectonic lull, renewed southward growth of Lau- granulite-facies metamorphism (Brock and Singewald, 1968; Lanzirotti, rentia is inferred to have occurred during the Mesoproterozoic. This 1988). Amphibole is stable throughout the central and southern Wet Moun- interpretation is based on a large crustal province with Nd model ages tains, and 40Ar/39Ar hornblende thermochronologic data indicate tempera- of 1.5–1.3 Ga extending from northern Mexico to Labrador, Canada tures exceeding 500 °C over a wide region in the southern part of the range (Bennett and DePaolo, 1987; Patchett and Ruiz, 1989; Karlstrom et al., at ca. 1.4 Ga, compared with temperatures of 350–500 °C in the north 2001). An episode of widespread granitic magmatism, local emplace- during the same time (Fig. 1; Shaw et al., 2005; Siddoway et al., 2000). ment of mafi c dikes, and regional high-temperature, low-pressure meta- Numerous granitoid intrusions cut across the basement rock assem- morphism occurred throughout the southwestern United States between blages in the Wet Mountains, and these igneous rocks are divided into 1.47 and 1.36 Ga (Reed et al., 1993; Williams, 1991; Williams et al., 1999), two general age groups. Paleoproterozoic intrusive rocks are correlated and rocks of this age currently account for nearly 20% of all Precambrian with the Routt plutonic suite of Tweto (1987) and include foliated tonal- exposures across the region (Fig. 1). Circa 1.4 Ga granites, previously ite and granodiorite of the ca. 1705 Ma Twin Mountain and Crampton described as being A-type because of their alkalinity, anhydrous charac- Mountain plutons and weakly foliated to undeformed granodiorite of ter, and presumed anorogenic tectonic setting (Loiselle and Wones, 1979; the ca. 1663 Ma Garell Peak pluton (Fig. 3; Bickford et al., 1989). Other Anderson, 1983; Anderson and Cullers, 1999), are ferroan in nature (Frost Paleoproterozoic intrusive rocks, broadly referred to as G1 granitoids et al., 2001a), a geochemical characteristic that is indicative of mantle by Siddoway et al. (2000), are exposed as networks of coarse-grained infl uence (Frost and Frost, 1997) and is generally associated with exten- to K-feldspar-megacrystic dikes and sills that are isoclinally folded and sional tectonic environments such as continental rifting (Emslie, 1978; share the host rock foliation but have margins that are obliquely discor- Whalen et al., 1987; Eby, 1990). However, regional evidence exists for dant to compositional layering in wall rocks. Mesoproterozoic intrusive contractional to strike-slip deformation within the thermal aureoles of plu- rocks are correlated with the Berthoud suite of Tweto (1987) and contain tons and contemporaneous reactivation of northeast-striking crustal shear a variably deformed suite of Mesoproterozoic granites emplaced between zones in the Rocky Mountains and southwestern United States (Graubard 1474 and 1361 Ma (Bickford et al., 1989). In the northern Wet Moun- and Mattinson, 1990; Shaw et al., 2001; McCoy et al., 2005; Jessup et al., tains, Mesoproterozoic intrusions include the relatively undeformed West 2006). Nyman et al. (1994) suggested that ca. 1.4 Ga magmatism coin- McCoy Gulch and Hindman Gulch plutons and strongly foliated Oak cided with regional contraction arising from a convergent plate boundary Creek pluton (Figs. 2 and 3; Bickford et al., 1989; Cullers et al., 1993). In on a distal southern margin of Laurentia. the central and southern Wet Mountains, Siddoway et al. (2000) described two separate suites of presumed Mesoproterozoic granitoids that are pri- PROTEROZOIC GEOLOGY OF THE WET MOUNTAINS marily exposed as networks of sills and dikes. Coarse-grained to K-feldspar megacrystic granites referred to as G2 by Siddoway et al. (2000) are The Wet Mountains, Colorado, lie within the Yavapai Province south commonly concordant with respect to wall-rock gneiss foliation and are of the Mazatzal deformation front (Fig. 1; Shaw and Karlstrom, 1999) locally strongly deformed. A younger suite of granitoids referred to as G3 and constitute a large (100 km × 30 km) block of nearly continuous Pro- (Siddoway et al., 2000) contains fi ne-grained granitic sills that are con- terozoic exposure that is transected by Phanerozoic brittle faults (Fig. 2). cordant to discordant with respect to wall-rock gneiss foliation but com- Over the Wet Mountains, aeromagnetic anomaly patterns (Oshetski and monly contain a well-developed dynamic foliation that is parallel with the Kucks, 2000) defi ne strong northeast-trending lineaments that may corre- surrounding wall-rock and granite fabric. The youngest Mesoproterozoic spond with structures that were developed during Paleoproterozoic accre- intrusion in the entire range is the San Isabel pluton (Fig. 2), an extensive tion (Karlstrom and Bowring, 1988; Karlstrom and Humphreys, 1998). body ~10 km by 30 km as exposed in the southern Wet Mountains. The Circa 1.4 Ga plutons in the Wet Mountains are spatially associated with San Isabel pluton consists of largely undeformed monzogranite to syeno- the lineaments, suggesting that they may have been reactivated during the granite that crystallized 1371–1362 Ma (Bickford et al., 1989) at depths Mesoproterozoic (Finn and Sims, 2005). Together with the magnetic data, estimated at 17–23 km (500–700 MPa) on the basis of Al-in-hornblende a pronounced positive gravity anomaly for the Wet Mountains (Snelson geobarometry and the presence of primary, euhedral magmatic epidote et al., 2005; Pardo et al., 2008) suggests the presence of abundant Fe- (Cullers et al., 1992). Abundant sapphirine is stable in large roof pendants enriched plutonic rocks beyond the limits of current exposures. within the San Isabel batholith (Heimann et al., 2005), an indication that Metavolcanic and metasedimentary successions dominated by quartz- gneisses of the southern Wet Mountains attained temperatures ≥700 °C ose and quartzofeldspathic gneisses make up most of the basement expo- (Raymond et al., 1980) during its emplacement. sures in the northern Wet Mountains. The basement rock assemblage also In the northern Wet Mountains, granitoids such as the West McCoy includes abundant schist, calc-silicate gneiss, mafi c gneiss, and amphibo- Gulch pluton (Fig. 2) are commonly exposed as discrete, map-scale bod- lite (Fig. 3). Map-scale lithologic units range in thickness from tens to ies with sharp contacts. Although similar map-scale intrusions with dis- hundreds of meters, and contacts are commonly sharp but are locally gra- tinct contacts such as the Oak Creek and San Isabel plutons also occur dational across distances of a few meters. Superb exposures are accessed in exposures farther to the south (Fig. 2), the prevalent form of granite

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ILSE FAULT D U graben Parkdale ) 81 sz 86 Lineation Lineation (L n = 58 Mean direction: 62°/045 n = 113 Mean direction: 45°/015 B. D. 77 77 J03-TM1 Quartz diorite 1420-04 Ma (ttn) 78 89 ) sz 73 Foliation Five Points Gulch shear zone (FPSZ) - Gneiss Foliation (S 73 Sheep Basin domain - Intrusive rocks and gneiss n = 116 Mean plane: 235, 87° N n = 143 Mean plane: 331, 56° NE A. C. 1705 ± 8 Ma Crampton Mt. batholith Bickford et al., 1989 68 79 87 38 49 51

area of Approx. SHEEP BASIN DOMAIN BASIN SHEEP 82 enlargement 81 51 70

~ ~

~ ~ ~ ~

~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 64 38 35 60 43 80 56 1km 12° 2000ft MN Sheep Basin 35 N 43 25 61 19 66 Geological contact (dashed where uncertain) Fault (dashed where uncertain) Arkansas River U.S. Highway 50 80 54 1 23 J01-FP1 Deformed peg. dike 1430+5/-3 Ma

71 FPSZ 68 66 58 62 45 36 Index to geologic mapping: 1. Mapping and structural data from this study; et al. (1975a & b) Taylor geologic contacts from 2. Mapping by Siddoway et al. (2000); structural data from Siddoway et al. (2000) and this study 3. Siddoway et al. (2002); this study 59 76 29 Xgn 84 76 39 64 2 3 Asymmetric fold axial trace (dashed where uncertain) Metamorphic foliation and lineation in gneiss Metamorphic foliation and lineation in plutonic rock Symmetrical fold axial trace (dashed where uncertain) U-Pb sample locality 69 69 38°25´N 50 50 ECHO PARK 47 38°27´30˝N GRABEN 105°32´30˝W 47 Xs 58 58 52 MAP SYMBOLS MAP 56 34 3 65 72 84 55 57 50 52 70 68 88 44 46 TEXAS CREEK DOMAIN Granodiorite Pegmatite/aplite Felsic gneiss Amphibolite/mafic and calc-silicate gneiss Phanerozoic undifferentiated Quartz diorite Granite/G3 Quartz gneiss +/- biotite Schist

LT FAU K MAP UNITS MAP Paleoproterozoic intrusive rocks Paleoproterozoic gneiss and supracrustal rocks Mesoproterozoic intrusive rocks CREE 42 XAS TE Figure 3. Generalized geologic map of the eastern Arkansas River Gorge, northern Wet Mountains, Colorado. See index for sources of geologic mapping and structural data. New U-Pb ages New U-Pb ages data. mapping and structural of geologic sources See index for Colorado. Mountains, Wet northern Gorge, River Arkansas map of the eastern geologic Generalized 3. Figure (FPSZ) and shear zone Gulch Points the Five data and synthesis from Structural indicated. are 1989) et al., Mountain batholith (Bickford of the Crampton and published ages (this study) type from with the rock separately, plotted data are and lineation as poles, plotted Foliation diagrams. stereonet equal-area on lower-hemisphere, represented Sheep Basin domain are 2003). 9.1 (Holcombe, using GEOrient calculated were orientations Average the diagrams. above indicated taken were measurements which

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JONES ET AL.

~ ~

37º57´30˝N Approx. area of enlargement

0 1 2 mi

37º55´N

“The Wall”

105º12´30˝W Sample locality J01-WC1 J01-WC2 J01-WC3 Southern Wet Mountains Structural Data A. Gneiss foliation B. Gneiss lineation

37º50´N

n = 804 n=118 Fault Mean plane: 268, 66° N Mean direction: 55°/355

Foliation trajectory 105º02´30˝W C. Granite foliation D. Granite lineation San Isabel granite

Fine- to medium- grained, foliated granite (G3)

Gneissic granite (G2) 37º47´30˝N

Granitic gneiss 104º57´30˝W Migmatitic and lit-par-lit gneiss

Mafic schist, gneiss, and amphibolite n = 386 n=29 Mean plane: 237, 48° NW Mean direction: 41°/352

Figure 4. Generalized geologic map of Precambrian exposures in the southern Wet Mountains (redrafted, simpliﬁ ed, and reinterpreted from Boyer, 1962). Structural data for basement gneiss and granites (G2 and G3) are represented on lower-hemisphere, equal-area stereonet diagrams. Foliation (plotted as poles) and lineation data are plotted separately, with the rock type from which measurements were taken indicated above the diagrams. Average orientations were calculated using GEOrient 9.1 (Holcombe, 2003).

124 www.gsapubs.org | Volume 2 | Number 2 | LITHOSPHERE Downloaded from lithosphere.gsapubs.org on September 21, 2010 DEFORMATION ACROSS A PROTEROZOIC MID-CRUSTAL SECTION | RESEARCH occurs as a network of G2 and G3 granitoid sills and dikes that pervade the gneissic host rock. Igneous bodies, meters to centimeters in dimen- A B sion, form a distributed magmatic framework that makes up 40% to 100% of outcrops. Owing to the pervasive nature and indistinct contacts of the diverse granites, many of which exhibit penetrative dynamic fabrics, most of the Wet Mountains have been represented on geological maps as undifferentiated Proterozoic gneisses (e.g., Scott et al., 1978) with relatively few map-scale intrusive bodies. Notable exceptions are the detailed map of Brock and Singewald (1968) and the moderately detailed map of Boyer (1962), both of which attempted to distinguish the diverse granites and granitic gneisses that constitute the Mesoproterozoic bedrock. The location and nature of the transitional boundary between the discrete plutons within nonmigmatitic gneisses in the north to the region of diffuse “framework” magmatism within migmatites in the south coincide with a kilometer-wide zone of voluminous granite and pegmatite south of the Arkansas River Gorge (Siddoway et al., 2000) of presumed ca. 1.4 Ga age.

Proterozoic Structural Elements view to S view to S The change in the style of magmatism from north to south across the range coincides with a change in structural style and fabric orientation. C The northern part of the range exhibits upright, open folds and subvertical foliation formed at moderate metamorphic conditions, whereas the central and southern part of the range contains shallowly to moderately dipping, penetrative gneissic fabrics formed at high temperatures. In our xenolith research we selected two areas in the north and south to map in detail, with attention to deformation structures, fabric and fold orientations, and kinematics. The mapping guided the selection of samples for new U-Pb geochronology described below. Figure 3 is a compilation map for Proterozoic exposures along the Arkansas River Gorge in the northern Wet Mountains. The central element is the 2–5-km-wide Five Points Gulch shear zone (Fig. 3; Siddoway et al., 2000), a north-northwest–striking, subvertical high strain zone that exhibits an oblique lineation defi ned by aligned sillimanite. Exposures within the shear zone are dominated by K-feldspar-biotite-quartz-plagio- SE clase-muscovite gneisses with localized zones of quartz-muscovite ± sil- Figure 5. Field photographs from the eastern Arkansas River Gorge. (A) limanite “pod” rock (e.g., Pedrick et al., 1998). High-strain layers sepa- Cross-sectional view looking to south of deformed pegmatite dike (sam- rate lower strain domains with biotite foliation that is tightly folded into ple J01-FP1; Fig. 3 for location) from within the Five Points Gulch shear upright, north-northwest–trending folds. The dominant shear zone fabric zone. Overall shear sense across the zone is east side up. (B) Close-up of shear bands that offset the pegmatite dike shown in (A). (C) View down (Ssz) strikes north-northwest and dips steeply east-northeast, with an average orientation of 335/63°E (Fig. 3A). The sillimanite mineral lineation on deformed quartz diorite of the Crampton Mountain batholith (sample plunges moderately north-northeast with an average orientation of 45°/015 J03-TM1; Fig. 3 for location) with shear band and elongate mafi c xenolith. (Fig. 3B) but is locally steep to subvertical. The presence of strongly aligned prismatic sillimanite and garnet with symmetrical tails within high strain zones indicates peak metamorphic conditions of >700 °C and 500 domain (Fig. 3), and these rocks are interfolded and interfoliated with a MPa during shear zone deformation (Givot and Siddoway, 1998). Kine- mafi c and felsic association of gneisses (Siddoway et al., 2000) that origi- matic indicators across the shear zone include ductile shear bands, asym- nated as bimodal volcanic rocks (Stiles, 1997; Wearn and Wobus, 1998). metric tails on garnet, and en echelon tension-gash arrays. Many of these Subordinate schists contain mineral assemblages indicative of metamor- indicators appear in zones with minor amounts of leucosome, suggesting phism at 650 °C and <400 MPa (Siddoway et al., 2000; Goodge and Sid- onset of melting (cf. Sawyer, 2008) during one episode of movement along doway, 1997). Large cordierite poikiloblasts within one schist unit pre- the shear zone. Kinematic indicators and asymmetric folds in parts of the serve a penetrative crenulation cleavage (S1) that disrupts compositional shear zone with steeply plunging lineations show sinistral reverse-oblique, layering defi ned by quartz and opaque inclusions, inferred to be relict east-side-up displacement (Fig. 5), consistent with the juxtaposition of bedding (Siddoway et al., 2000). Microstructural relationships indicate higher temperature shear zone rocks against the lower temperature Texas S1 development during an early stage of a progressive deformation event Creek association to the west (Siddoway et al., 2000). (D1) accompanied by metamorphism (M1) involving growth of 10–20 cm We identify two domains with contrasting fabric geometries, deforma- cordierite and plagioclase poikiloblasts (Siddoway et al., 2000). Outside tion styles, and/or fabric intensity on either side of the Five Points Gulch the metamorphic megacrysts, the dominant foliation, S2, is defi ned by shear zone; these are the Texas Creek and Sheep Basin domains to the aligned micas and quartz ribbons, and S2 wraps around the cordierite poi- west and east, respectively (Fig. 3). Quartzose and quartzofeldspathic kiloblasts. S2 forms the dominant fabric and compositional layering in the gneisses of sedimentary origin dominate exposures in the Texas Creek Texas Creek association as a whole. The S2 fabric is deformed by upright,

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kilometer-scale, east-trending folds (F2) with a cumulative π−axis orientation in both gneisses and G2 + G3 granites is essentially parallel, trending tion of 45°/081 (Siddoway et al., 2000). These folds are interpreted to have toward azimuth 350–355 (Fig. 4B and 4D). Kinematic shear sense indica- formed during a second deformation event (D2) involving subhorizontal, tors, including asymmetric folds, indicate reverse (top to the south) shear north-northwest–directed shortening (Siddoway et al., 2000), and they are sense in both wall-rock gneisses and G2 and G3 granite sills. sharply truncated to the east by the Five Points Gulch shear zone. In the central Wet Mountains, Lanzirotti (1988) found evidence for East of the Five Points Gulch shear zone, along the easternmost 10 km three episodes of penetrative deformation, all attributed to northwest- of the Arkansas River Gorge, exposures in the Sheep Basin domain consist southeast shortening, in exposures in the Tyndall Mountain quadrangle of tonalite, quartz diorite, and granodiorite of the 1705 ± 8 Ma Crampton (Brock and Singewald, 1968). The fi rst two deformation events (D1 and Mountain pluton (Bickford et al., 1989) along with localized exposures of D2) are broadly correlated with ca. 1.7 Ga plutonism, constrained locally gray quartz biotite and amphibolite gneiss wall rock. On the western side by a 1692 ± 5 Ma U-Pb zircon age (Bickford et al., 1989) from granulites of the domain the north-northwest–striking fabric of the Five Points Gulch in the central part of the range (Lanzirotti, 1988; Brock and Singewald, shear zone passes into asymmetric northeast-plunging folds in the gneisses 1968). Lanzirotti (1988) suggested that the third deformation (D3) might over a distance of a few hundred meters. Northeast-striking, subverti- have coincided with the emplacement of a suite of younger deformed plu- cal foliation in gray gneisses has an average orientation of 235/87° NW tons at 1650–1615 Ma (Bickford et al., 1989), but D3 might have been con- (Fig. 3C) and is defi ned by biotite and amphibole and locally enhanced by temporaneous also with ca. 1.4 Ga metamorphism and magmatism. U-Pb dynamically recrystallized plagioclase, K-feldspar, and quartz. The folia- zircon and titanite data from 10 km to the east along Rattlesnake Gulch tion orientation is generally parallel to the western margin of the 150 km2 (Fig. 2) suggest that 1691–1680 Ma amphibolite and felsic gneiss under- Crampton Mountain pluton (Fig. 3), and abundant fl attened mafi c enclaves went at least one episode of deformation involving foliation development with aspect ratios up to 30:1 are parallel with the surrounding fabric and isoclinal folding prior to emplacement of a fi ne-grained granite sill (Fig. 5C). Well-developed biotite and/or amphibole mineral lineation in 1679 ± 2 Ma (Jones, 2005). There is also evidence of metamorphic zircon the metaplutonic rocks plunges moderately to steeply northeast with an and titanite growth or recrystallization in amphibolite between 1436 and average orientation of 62°/045 (Fig. 3D). Locally developed asymmetric 1390 Ma accompanied by at least one episode of deformation involving fabrics include C-S foliation and ductile shear bands (Fig. 5C). Kinematic fabric overprinting or reactivation (Jones, 2005). However, direct evidence indicators observed along the east-northeast–trending southern margin of of ca. 1.4 Ga magmatism is lacking from the central Wet Mountains. the Crampton Mountain pluton (Fig. 3 map) show dominantly reverse, northwest-side-up displacement with a component of sinistral offset. The U-Pb GEOCHRONOLOGY eastern boundary of the Sheep Basin domain is the Ilse fault zone, a structure of probable Neoproterozoic ancestry (cf. Timmons et al., 2002) that We undertook new U-Pb geochronology to constrain precisely the most recently was reactivated during formation of the Tertiary Parkdale age of deformation and metamorphism in the northern and southern Wet graben (Fig. 3; Fryer, 1996; Kelley and Chapin, 2004). Mountains, respectively. Samples were collected from the well-charac- In exposures ~15 km south of the Arkansas River Gorge, focused work terized structural sites described in the previous section to help assess on the Mesoproterozoic Oak Creek pluton (Dean et al., 2002) established whether the contrasting structural styles developed in the northern ver- that the intrusion has a layered character indicative of emplacement in sus the central and southern part of the range were contemporaneous or sheets and exhibits a northwest-striking solid-state foliation concordant had developed at different times. Because the age of deformation in the with that in host gneisses. An east-southeast–trending mineral-stretching Texas Creek domain of the Arkansas River Gorge was reasonably well lineation is well developed, particularly on the pluton margins, and is established (see below; Siddoway et al., 2002; Siddoway et al., 2000), associated with kinematic criteria that indicate normal-oblique displace- sampling in the northern Wet Mountains was concentrated within the Five ment with pluton side down. Points Gulch shear zone and Sheep Basin domain to address the timing of Exposures in the central and southern Wet Mountains exhibit remark- deformation in the shear zone and in metaplutonic rocks exposed in the ably consistent geometries of foliation and folds that defi ne a dominant eastern Arkansas River Gorge (Fig. 3). In the southern Wet Mountains, we east-west pattern over 100 km2 of exposure (Fig. 4). Deformation affected targeted multiple generations of deformed tabular granite intrusions that both wall-rock gneisses and granitic phases G1 through G3 that pervade occur as a network of sills and sheets in outcrop and their metamorphosed the range, as is evident where foliation trajectories cross multiple litho- Paleoproterozoic country rocks. The samples were collected from a single logic boundaries in Figure 4. The dominant compositional layering and large exposure of gneiss and granite along the Wet Mountains’ southwest- migmatitic foliation in the central Wet Mountains strikes east-west with ern escarpment near the intersection of Bear Creek and Williams Creek moderate to shallow north-northwest dips with an average orientation of (Boyer, 1962; Callahan, 2002; Perkins, 2002). This exposure, informally 270/50°N (Siddoway et al., 2000; Jones, 2005). North-plunging biotite named “The Wall” because of its subvertical aspect, nearly 100 m in lineation with an average orientation of 52°/003 is well developed in bio- height (Fig. 6), is 0.5 km east of the Pole Creek Trailhead along San Isabel tite gneiss units and at granitic gneiss–amphibolite gneiss contacts (Sid- National Forest Road 630 (Fig. 4). doway et al., 2000; Jones, 2005). Tight-to-isoclinal folds with amplitudes Sample processing and analytical techniques followed those described ranging from tens of centimeters to more than a kilometer deform the by Jones and Connelly (2006). We present isotopic data and sample loca- gneissic layering and granitic sills, providing evidence that the foliation is tion coordinates in Table 1 and associated concordia diagrams in Figures 7 a composite fabric that resulted from multiple episodes of broadly coaxial and 8. Results described in the following section are grouped according to deformation. In the southern Wet Mountains there is some discrepancy in general locations in the Wet Mountains. the foliation orientations of G2 and G3 granitic intrusions versus those of the gneissic rocks. Whereas the average foliation in gneisses strikes east- Northern Wet Mountains–Arkansas River Gorge west and dips moderately north (average orientation, 283/56°N; Fig. 4), consistent with the central domain, the well-developed, locally gneissic Deformed Pegmatite Dike (J01-FP1) foliation in G2 and G3 sills strikes east-northeast and dips moderately Within the Five Points Gulch shear zone there is an array of subver- north-northwest (average orientation, 243/43°N; Fig. 4C). Mineral linea- tical dikes oriented parallel to the north-northwest–striking shear zone

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Figure 6. Field photograph of “The Wall,” a large, vertical exposure in the southern Wet Mountains that is characterized by penetrative, moderately north-northwest–dipping fabrics. Sample localities for U-Pb geochronol- J01-WC2 ogy (see Fig. 9) are indicated by stars. G2 granite

J01-WC3 J01-WC1 G3 granite Amphibolite

View to NNE

TABLE 1. WET MOUNTAINS U-Pb ISOTOPIC DATA noitcarF thgieW noitartnecnoC derusaeM detcerroC cimota *soitar segA )aM( (mg) U PbR Common PbT 206Pb 208Pb 206Pb 207Pb 207Pb 206Pb 207Pb 207Pb (ppm) (ppm) (pg) 204Pb 206Pb 238U 235U 206Pb 238U 235U 206Pb

Deformed pegmatite dike (J01-FP1; N 38° 27.32′, W 105° 30.83′) Z1 lg prsm euh clr abr 0.002 184 46 3 2000 0.0630 0.25184 76 3.1665 110 0.09119 18 1448 1449 1451 Z2 euh tips clr lt brn abr 0.001 1318 306.8 2 13238 0.0072 0.24679 72 3.0683 84 0.09017 14 1422 1425 1429 Z3 euh tip dk brn abr 0.002 801 178.3 2 8826 0.0054 0.23654 90 2.9262 100 0.08972 20 1369 1389 1420 rba pit nrb4Z pit rba 7312.671417100.0 9441254155418321190.02614081.34651352.09240.0101 Z5 sm brn tip abr 0.001 2264 499.9 4 4540 0.0058 0.23444 90 2.9091 102 0.09000 20 1358 1384 1425

Foliated quartz diorite, Crampton Mountain pluton (J03-TM1; N 38° 28.79′, W 105° 25.36′) T1 md-lg pale brn ang frags abr 0.081 46 11.7 381 168 0.1327 0.24413 60 2.9953 108 0.08899 24 1408 1406 1404 T2 md-lg pale brn ang frags abr 0.093 40 10.6 504 133 0.1528 0.24636 60 3.0519 126 0.08985 28 1420 1421 1422 T3 md-lg pale brn ang frags abr 0.171 42 11 919 139 0.1482 0.24785 58 3.1032 122 0.09081 26 1427 1433 1443

Amphibolite (J01-WC1; N 37° 55.22′, W 105° 9.97′) Z1 md sbhd-sbrnd tan abr 0.001 354 87 2 3853 0.0615 0.24793 70 3.0911 90 0.09042 14 1428 1430 1434 Z2 lg clr sbhd abr 0.002 1108 269.2 5 6483 0.0559 0.24647 64 3.0787 82 0.09059 10 1420 1427 1438 Z3 sm clr sbhd prsm abr 0.001 833 203.7 1 11913 0.0572 0.24782 56 3.0927 72 0.09051 10 1427 1431 1436

Coarse-grained, foliated granite sill (J01-WC2; N 37° 55.22′, W 105° 9.97′) Z1 sm rnd sbhd bge abr 0.001 193 54 2 1880 0.2147 0.24875 68 3.0972 92 0.09030 16 1432 1432 1432 Z2 sm-md euh prsm clr abr 0.002 101 26.6 2 1591 0.1350 0.24908 76 3.1082 122 0.09050 26 1434 1435 1436 Z3 sm clr sbhd prsm abr 0.001 319 85.6 2 1777 0.1644 0.24865 70 3.0984 120 0.09037 26 1432 1432 1433 T1 md-lg pale brn ang-sbrnd abr 0.128 74 22.8 335 429 0.4864 0.22671 60 2.7218 78 0.08708 16 1317 1334 1362 T2 md-lg pale brn ang-sbrnd abr 0.138 84 24.6 369 452 0.4806 0.21671 52 2.6154 70 0.08753 14 1264 1305 1372 T3 md-lg pale brn ang-sbrnd abr 0.085 75 22.9 197 505 0.4249 0.23518 58 2.8202 86 0.08697 18 1362 1361 1360 T4 md-lg pale brn ang-sbrnd abr 0.063 73 25 139 511 0.6093 0.23488 68 2.8401 86 0.08770 14 1360 1366 1376 T5 md-lg pale brn ang-sbrnd abr 0.108 70 22.5 230 517 0.4847 0.23604 62 2.8505 80 0.08758 14 1366 1369 1373 T6 md-lg pale brn ang-sbrnd abr 0.101 75 24.5 257 474 0.4847 0.24064 58 2.9165 118 0.08790 28 1390 1386 1380

Fine-grained, foliated granite sill (J01-WC3; N 37° 55.22′, W 105° 9.97′) Z1 md-lg euh brn-orng abr 0.003 464 111 3 8586 0.0337 0.24742 56 3.0929 70 0.09066 10 1425 1431 1440 Z2 md sbhd tan abr 0.001 362 86 6 1165 0.0187 0.24891 60 3.1277 92 0.09113 16 1433 1440 1449 Z3 sm sbhd tan-clr abr 0.001 255 73.1 2 3380 0.0714 0.28416 76 3.9539 110 0.10092 20 1612 1625 1641 Z4 md sbhd tan-clr abr 0.001 576 139.5 2 4360 0.0250 0.25226 60 3.1977 80 0.09194 14 1450 1457 1466 Abbreviations: abr—abraded; ang—angular; bge—beige; brn—brown; clr—clear; dk—dark; euh—euhedral; frags—fragments; lg—large; lt—light; md—medium; orng— orange; prsm—prisms; sm—small; sbhd—subhedral; sbrnd—subround. *Ratios corrected for fractionation; 1 pg and 0.25 pg laboratory Pb and U blanks, respectively, and initial common Pb calculated using Pb isotopic compositions of Stacey and Kramers (1975). All fractions of zircon and titanite are extensively abraded (Krogh, 1982) unless otherwise noted. Two-sigma (2σ) uncertainties on isotopic ratios are reported after the ratios and refer to the ﬁ nal digits. PbR refers to radiogenic Pb; Common PbT refers to total common Pb.

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A Back-rotated pegmatite dike 1530 B Foliated quartz diorite (J01-FP1) (J03-TM1) 1500 .2641 1500 .2606 206 Pb 206 Pb 238 U 238 U 1450 1470 T2 1443 Ma T3 Zircon cores 1400 1440 Z5 T1 Z1 Ca. 1450 Ma .2491 .2382 1410 Z2 1350

1380 1300 Zircon tips core Z3 1430+5/-3 Ma .2341 .2158 Z6 (43%) 207 Pb 207 Pb 252 ± 300 Ma 0 Ma reference line 235 U 235 U 2.92 3.20 3.48 2.62 3.02 3.42 Figure 7. U-Pb concordia diagrams for samples from the Arkansas River Gorge, northern Wet Mountains. Ages are determined by linear regression through the data except where indicated, and probability of ﬁ t (%) is indicated in parentheses. See text for details.

ABAmphibolite (J01-WC1) G2 gneissic granite (J01-WC2) 1475 .2562 .2527 1450 206 Pb 206 Pb Z2 238 U 1450 238 U Z1 Z3 1400 Zircon 1425 Z1 207 Z3 1362 Ma T3 Avg. Pb/206Pb age .2450 Z2 .2341 T5 1350 T4 1435±4 Ma 1400 Avg. 207Pb/206Pb age Titanite T1 T2,T4, and T5 1436±2 Ma 1300 1375 1375±2 Ma (25.8%) .2338 .2155 T2 207 Pb 207 Pb 0 Ma reference line 0 Ma 235 U 43±65 Ma 235 U 2.88 3.08 3.28 2.58 2.90 3.22

C G3 fine-grained, foliated granite (J01-WC3) 1700 .2996 206 Pb 1749±28 Ma 238 U 1600 Z3

1500 Figure 8. U-Pb concordia diagrams for samples from the southern Wet Mountains. Ages are determined by linear regression through the data .2542 Z4 except where indicated, and probability of ﬁ t (%) is indicated in parenthe- Z1 1400 Z2 ses. See text for details. 1390±10 Ma 1300 (23%)

.2088 207 Pb 235 U 2.70 3.58 4.46

128 www.gsapubs.org | Volume 2 | Number 2 | LITHOSPHERE Downloaded from lithosphere.gsapubs.org on September 21, 2010 DEFORMATION ACROSS A PROTEROZOIC MID-CRUSTAL SECTION | RESEARCH foliation (Fig. 3). These intrusions both follow and sharply cut across the shear zone fabric, Ssz, and they generally lack internal foliation; thus they are interpreted to be coeval with at least one major phase of shear zone deformation. To determine the age of syntectonic dike emplacement, a A 20–30-cm-thick, subvertical pegmatite dike was sampled near the east margin of the Five Points Gulch shear zone (Fig. 3; Table 1). The pegmatite dike is concordant but truncates the shear zone foliation along parts of its margin. It is cut in turn by a series of en echelon, east-dipping shear bands with reverse-oblique kinematics (Figs. 5A and 5B), consistent with east-side-up displacement across the Five Points Gulch shear zone (Sid- doway et al., 2000). The pegmatite yielded a single population of tan to brown, euhedral to subhedral zircon with prismatic faces indicative of an igneous origin. Most of the grains contain dark brown xenocrystic cores surrounded by light tan to clear euhedral tips (Fig. 7A inset). Tips were mechanically separated from core material using tweezers, and the constituent parts of the grains were then abraded, dissolved, and analyzed separately. The 207Pb/206Pb ages of two core fractions of dark brown zircon material overlap concordia at 1451 and 1449 Ma (fractions Z1 and Z4, Table 1; Fig. 7A). Three fractions of zircon tip material (Z2, Z3, and Z5) deﬁ ne a line with intercepts of 1430+5/–3 Ma and 252 ± 300 Ma (Fig. 7A). B

Foliated Quartz Diorite of the Crampton Mountain Pluton (J03-TM1) Quartz diorite exhibiting C-S foliation was collected from exposures in the Sheep Basin domain for U-Pb titanite geochronology to determine the age of dynamic metamorphism and foliation development. The U-Pb zircon age of 1705 ± 8 Ma for emplacement of the Crampton Mountain G2 pluton was determined previously by Bickford et al. (1989). The sample site is in the southwestern part of the Crampton Mountain pluton ~1 km west of the dated locality of Bickford et al. (1989) (Fig. 3). Titanite observed in thin section forms submillimeter clusters that are elongate G3 parallel to the biotite foliation, indicating that titanite formed as a product of metamorphic recrystallization. Abundant titanite in the mineral separate consists of dark brown angular fragments that were handpicked for three fractions (T1–T3; Table 1). The 207Pb/206Pb ages range from 1443 to N 1404 Ma (Table 1), and fractions T2 and T1 overlap concordia at 1422 Ma and 1404 Ma (Fig. 7B). C

Southern Wet Mountains–Bear Creek–Williams Creek

Amphibolite (J01-WC1) Medium- to coarse-grained amphibolite is the prevalent host rock to the two generations of granite, G2 and G3. This sample contained sparse relict clinopyroxene, nearly wholly replaced by amphibole with pronounced shape preferred orientation. Prominent foliation strikes east-west and dips moderately north, hosting a north-northwest–plunging amphibole lineation (Fig. 4A and 4B). The amphibolite yielded a single population of pink to tan, subrounded to subhedral, equant zircon interpreted to be metamorphic in origin. Three fractions (Z1–Z3) plot close to concordia 207 206 and have an average Pb/ Pb age of 1436 ± 2 Ma (Fig. 8A). view to ENE

Coarse-Grained, Foliated Granite Sill (J01-WC2) Figure 9. Field photographs of granitoids exposed in the southern Wet Coarse-grained to K-feldspar-megacrystic G2 granite forms 1–5-m-thick Mountains. (A) View down on G2 granite fl oat block with gneissic fabric sills in exposures throughout the southern Wet Mountains. The granite defi ned by recrystallized K-feldspar and biotite. (B) View down on G3 fi ne- margins are generally concordant with host rock foliation but also cut grained granite sill cutting obliquely across gneissic foliation (highlighted by lines) in coarse-grained G2 granite. (C) G3 granite sill containing obliquely across compositional layering and isoclinal folds in the gneisses. well-developed foliation and small, asymmetric open folds. Folds have Moderate to strong foliation in the sills is defi ned by crystallographic pre- southward vergence that is consistent with reverse-sense, top-up-to-the- ferred orientation of biotite and dynamically recrystallized K-feldspar south–southeast kinematic indicators elsewhere throughout the sill. and quartz (Fig. 9A). The dynamic fabric strikes east-northeast and dips moderately north-northwest (Fig. 4C), hosting a pronounced downdip

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mineral lineation defi ned by biotite and quartz (Fig. 4D). Asymmetric of 1435 ± 4 Ma (Fig. 8B), interpreted to represent the time of emplace- folds and mineral microstructures record reverse-sense, top-up-to-the- ment and crystallization of the pervasive granite sills. The 1436 ± 2 Ma south–southeast kinematics. age of metamorphic zircon within amphibolite gneiss that forms part of The G2 granite yielded a single population of pink to clear, euhedral the host rock is identical within analytical uncertainty to the age of associ- to subhedral, prismatic zircon consistent with an igneous origin. Three ated G2 granite. The amphibolite gneiss contains evidence for extensive fractions (Z1–Z3) overlap concordia with an average 207Pb/206Pb age of retrogression of clinopyroxene to amphibole, an indication that heat and 1435 ± 4 Ma (Fig. 8B), and this age is interpreted to represent emplace- possibly fl uids introduced during the voluminous emplacement of G2 sills ment and crystallization of the coarse-grained granite. The granite also caused dynamic recrystallization and breakdown of primary pyroxene, yielded abundant dark brown, angular titanite fragments. Thin section leading to growth of metamorphic zircon. study revealed the presence of titanite oriented parallel to the dominant U-Pb results for metamorphic titanite from the G2 granite reveal gneissic fabric defi ned by dynamically recrystallized K-feldspar, quartz, that tectonothermal metamorphism continued after G2 emplacement at and biotite, an indication that titanite grew or recrystallized during meta- ca. 1435 Ma in the southern Wet Mountains. Two titanite age popula- morphism accompanying solid-state deformation of the granite sill. Three tions defi ne reference chords with upper intercepts at 1375 ± 2 Ma and titanite fractions (T2, T4, and T5) defi ne a line with intercepts of 1375 ca. 1362 Ma (Fig. 8B). The ages overlap the 1362 ± 7 Ma and 1371 ± 2 Ma and 43 ± 65 Ma, and two fractions (T1 and T3) are colinear along ± 14 Ma emplacement ages for the San Isabel granite (Bickford et al., a reference chord with intercepts of ca. 1362 Ma and 0 Ma (Fig. 8B). 1989). Therefore we attribute the growth of metamorphic titanite to thermal perturbation and fl uid fl ow associated with emplacement of the Fine-Grained, Foliated Granite Sill (J01-WC3) ca. 1365 Ma San Isabel granite batholith, the main body of which is Fine-grained G3 granite forms sills that cut across foliated G2 gran- exposed ~5–10 km from the sampled outcrops (Fig. 4). ite sills and foliation, with compositional layering and isoclinal folds in Zircon from the G3 granite sill (J01-WC3, Fig. 8C) defi nes a chord with both amphibolite gneiss and granitoid gneisses (Fig. 9B). G3 sills range a lower intercept of 1390 ± 10 Ma, interpreted as the age of crystallization in thickness from 0.5 to 5.0 m and exhibit solid-state biotite foliation that of the granitic sill. This age falls between the other two age groupings, strikes east-northeast and dips moderately north-northwest (Fig. 4C), with suggesting that plutonism continued and that elevated temperatures were moderately north-northwest–plunging biotite mineral lineation (Fig. 4D). sustained between dynamic and magmatic events. We interpret the 1749 Asymmetric mineral fabrics record reverse-sense, top-up-to-the-south– ± 28 Ma upper intercept for sample J01-WC3 to refl ect zircon inheritance, southeast kinematics, and entire sills are locally deformed by asymmetric thus providing an indirect constraint upon the protolith age for basement folds with east-trending axes and southward vergence (Fig. 9C). gneisses in the southern Wet Mountains. The G3 granite yielded a single population of brown to tan, equant, euhedral to subhedral prismatic zircon. We interpret that clear, euhedral Age and Kinematics of Deformation in the Northern Wet tips surrounding slightly darker, translucent xenocrystic cores represent Mountains igneous overgrowths. The small size of the zircon grains made mechanical separation of the cores from rims impossible; therefore air abrasion The earliest preserved record of deformation in the northern Wet Moun- of shorter duration was used so that the volumetrically minor amount of tains is in the Texas Creek domain (Fig. 3) of the Arkansas River Gorge. overgrowth material was retained. Four zircon fractions (Z1–Z4) defi ne Siddoway et al. (2000) interpreted D1 progressive development of a pen- a line with intercepts of 1749 ± 28 Ma and 1390 ± 10 Ma (Fig. 8C). etrative cleavage (S1) and growth of cordierite (M1) wrapped by prevalent S2 foliation to have occurred during the Paleoproterozoic, broadly DISCUSSION synchronous with emplacement of the 1663 ± 4 Ma Garell Peak pluton (Fig. 2; Bickford et al., 1989). The Texas Creek and Sheep Basin domains Interpretation of U-Pb Geochronology Results were folded during subsequent D2 north-south (Texas Creek) to northeast- southwest (Sheep Basin) shortening (Siddoway et al., 2000). Subsequent The late tectonic pegmatite dike (J01-FP1) intruded near the eastern research has recognized a defl ection of fabrics in folded gneisses around margin of the Five Points Gulch shear zone yielded zircon cores with ages of the 1474 ± 7 Ma West McCoy Gulch pluton (Fig. 2; Bickford et al., 1989) 1451 and 1449 Ma (Fig. 7A) that are interpreted to represent inherited grains and that ca. 1430–1420 Ma monazite inclusions are present in M1 cordi- derived from nearby Mesoproterozoic granitic plutons such as the Oak erite poikiloblasts in F2 fold limbs (Siddoway et al., 2002). Thus, regional Creek pluton (Fig. 2). We interpret the upper intercept age of 1430+5/–3 Ma heating and contraction during D2 were broadly coeval with G2 granitic for the zircon tips to represent growth during crystallization of the pegmatite magmatism during the Mesoproterozoic. dike, with the lower intercept refl ecting more recent Pb loss caused by a Because the Five Points Gulch shear zone truncates F2 folds of the disturbance arising from Phanerozoic tectonism. Growth of metamorphic adjacent Texas Creek domain (Fig. 3), major movement upon the shear titanite occurred from 1443 to 1404 Ma during development of the north- zone development is attributed to a third phase of deformation (D3) that east-striking foliation within quartz diorite of the Crampton Mountain plu- followed closely upon or was an outgrowth of the folding event. Our new ton (J03-TM1, Fig. 7B), which we attribute to the deformation that caused U-Pb age for a syntectonic pegmatite dike indicates that D3 deformation development of northeast-trending asymmetrical folds in the Sheep Basin in the Five Points Gulch shear zone occurred at ca. 1431 Ma. Work by domain. We interpret the range of ages from the titanite fractions to refl ect Dean et al. (2002) indicates that emplacement of the 1439 ± 8 Ma Oak pulses of metamorphism throughout a protracted Mesoproterozoic tectono- Creek pluton (Bickford et al., 1989; Cullers et al., 1993) was syntec- thermal event in the northern Wet Mountains, consistent with 40Ar/39Ar ages tonic, thus indicating that D3 deformation was widespread throughout from across the region (Shaw et al., 2005; Siddoway et al., 2000). the northern Wet Mountains. The northeast- to east-directed extensional Consistent U-Pb zircon ages of ca. 1435 Ma were acquired from fabrics that overprint regional foliation along the margin of the pluton samples of granite and amphibolite gneiss from the southern Wet Moun- (Dean et al., 2002; Siddoway et al., 2002) are generally parallel to folds tains. The coarse-grained K-feldspar-megacrystic G2 granite (J01-WC2) within the Sheep Basin domain and lineation in the Crampton Mountain yielded three concordant zircon fractions with an average 207Pb/206Pb age pluton (Figs. 2 and 3). Our new titanite ages indicate that the effects of

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D3 were through 1422–1404 Ma in the Crampton Mountain pluton. The mechanical anisotropies within host rocks. The margin of the Crampton presence of dynamically recrystallized plagioclase and K-feldspar, quartz, Mountain batholith (Figs. 2 and 3) is an example of one such element that and biotite refl ect strain at relatively high temperatures in the northern Wet may control the northeast-oriented structure of the Sheep Basin domain, Mountains during D3 deformation. which mirrors the mapped geometry of the southern margin of the batho- To summarize, the structures of the northern Wet Mountains, active in lith fairly closely (Fig. 3) but contrasts with the orientation of north-north- the interval 1444–1431 Ma, show (1) sinistral oblique motion upon the west–striking foliation (Ssz) in the Five Points Gulch shear zone. In either north-south–striking Five Points Gulch shear zone, with east-northeast– case, the Mesoproterozoic ages of folding, shear zone formation, and fab- directed opening indicated by the pegmatite dike array; (2) normal-sense, ric reactivation suggest that the different structural domains in the northern northeast-southwest–directed displacement on the margins of the Oak Wet Mountains refl ect varying mechanical responses to ca. 1.4 Ga subho- Creek pluton; and (3) north-south to northwest-southeast contraction in the rizontal, north-northwest–directed shortening, possibly with a signifi cant Texas Creek and Sheep Basin domains, respectively, forming kilometer- to transcurrent component, under moderate- to high-temperature conditions. meter-scale folds. The contrasting kinematics from coeval structures may be reconciled within a regional strain state having the maximum and mini- Age and Kinematics of Deformation in the Central and Southern mum fi nite strain axes in the plane of the earth, with the intermediate axis Wet Mountains vertical; in other words, within a transcurrent strain state (Fig. 10, inset). Alternatively, the formation of the Five Points Gulch shear zone, Oak In the central Wet Mountains, direct evidence of ca. 1.4 Ga magma- Creek pluton, and pegmatite dike array, and to some extent the distribu- tism is lacking. The orientation and style of deformation is similar to that tion of granite and pegmatite, were controlled by preexisting structures or recorded in exposures farther to the south, and Siddoway et al. (2000)

Homestake s.z. (Shaw et al., 2001) Contraction Oak Creek Vernal Mesa s.z. (Jessup et al., 2005) direction for stretching direction 10 km N folds and reverse FPSZ sense shears

Normal Sheep sense Basin zone folds

Brittle- ductile ~~~~~~~~~~~~~~~~~~~~~~~~~~ Reverse ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ sense shears transition

Music Pass pluton Sangre de Cristo Mts. (Jones and Connelly, 2006)

Subvertical fabrics, partitioned deformation Localized, discrete 20 km ~~~~~~~~~~ ~~~~~~~~~ granitic plutonism Five Points shear zone ca. 1430-1405 Ma ~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Mid-crustal magma layer ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ (Shaw et al., 2005) Oak Creek pluton (?)

Upper crustal plutons Shallow to subhorizontal (e.g., Vernal Mesa) lower-crustal flow Mid-crustal plutons Concordant, “framework” (e.g., San Isabel) granitic magmatism and granitoid sill ex. G2 and G3 granite sills Approximate level networks ca. 1435-1360 Ma of current exposure Figure 10. Schematic block diagram of the middle crust beneath southern Colorado ca. 1.4 Ga. Interpreted, reconstructed relationships illustrate contrasting structural and magmatic styles with relative position (i.e., depth) in the crust. Shallower levels of exposure are characterized by steeply dipping to subvertical fabrics, and deformation is commonly localized along discrete shear zones. Examples include the Five Points Gulch shear zone (FPSZ, Fig. 3) in the northern Wet Mountains, and the Homestake and Vernal Mesa shear zones (Shaw et al., 2001; Jessup et al., 2006) exposed to the north and northwest in Colorado. Deeper levels of exposure are characterized by moderately to shallowly dipping, penetrative gneissic fabrics and penetrative deformation. Examples include exposures in the southern Wet Mountains (Fig. 4) and Taos Range to the south (Pedrick et al., 1998) in northern New Mexico. Inset is a qualitative strain ellipse representing the orientation of different structural elements in the Wet Mountains, Colorado, relative to northwest-southeast shortening interpreted by Nyman et al. (1994) at ca. 1.4 Ga (modiﬁ ed from Siddoway et al., 2000).

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suggested that many of the deformed granites are likely correlative with The access to deep crustal levels in the southern Wet Mountains reveals the G2 and G3 granites that were dated as part of this study. In exposures a consistent top-to-the-south–southeast transport sense in the restored along Rattlesnake Gulch (Fig. 2), Jones (2005) documented syntectonic geometry of shallowly dipping fabrics that accords with the kinematics growth or recrystallization of metamorphic zircon and titanite in amphib- of deformation of steep foliation in mid-crustal exposures of the northern olite between 1436 and 1390 Ma associated with fabric overprinting or Wet Mountains. The presence of syntectonic granites that were strongly reactivation. Otherwise, available age information indicates that deforma- affected by the deformation provides evidence of subhorizontal crustal tion and metamorphism recorded in exposures of the central Wet Moun- fl ow. The earliest recognized episode of pervasive fl ow accompanied the tains is predominantly Paleoproterozoic in age (Brock and Singewald, emplacement of coarse-grained G2 granites at 1435 Ma, and the consis- 1968; Lanzirotti, 1988; Bickford et al., 1989). tent deformational record of fi ne-grained G3 dikes suggests that crustal In contrast, our new results from exposures in the southern Wet Moun- fl ow continued until the 1390 Ma emplacement of G3 granites. The simi- tains reveal that nearly all of the observed deformation occurred during larity of structures of older G2 and younger G3 granites indicates a stable the Mesoproterozoic. The earliest recognized phase of high-temperature, strain state for 45 million years, suggesting long-lived, subhorizontal fl ow penetrative deformation occurred during emplacement of coarse-grained in a magma-rich environment. G2 granitic sills at 1435 ± 4 Ma. Magmatism was accompanied by foliation development and extensive recrystallization or new growth of meta- IMPLICATIONS FOR Ca. 1.4 Ga TECTONIC MODELS morphic zircon in wall-rock amphibolite at 1436 ± 2 Ma. The observa- tion that granitic sills locally cut a preexisting wall-rock foliation requires Granite magmatism associated with contrasting structures of the north- at least one phase of earlier, likely Paleoproterozoic, deformation and ern and southern Wet Mountains occurred during the narrow time interval metamorphism; however, any isotopic record of these events has been of 1444–1431 Ma, with deformation at shallower crustal levels localized obliterated by ca. 1.4 Ga thermal effects (Shaw et al., 2005). Gneissic within discrete subvertical shear zones like the Five Points Gulch shear G2 granite sills were affected by reverse-sense, north-northwest to south- zone (Siddoway et al., 2000; Shaw et al., 2001; McCoy et al., 2005) or southeast–directed crustal fl ow. Fine-grained G3 granite sills emplaced at distributed throughout the Texas Creek and Sheep Basin domains as 1390 ± 10 Ma cut the gneissic fabric in G2 granites and wall rocks, and mesoscopic upright folds. An east-west–oriented zone containing abun- they exhibit the same kinematic sense; thus the duration of crustal fl ow dant pegmatite and granite appears to demarcate the transition in structural deformation was on the order of 45 m.y. The 1362 ± 7 Ma San Isabel styles (Siddoway et al., 2000; Collins et al., 2004). This zone of granite granite (Bickford et al., 1989) cuts all fabrics, and it is largely undeformed, may have been emplaced along a rheological and thermal barrier (Fig. 10; so it provides a minimum age for all Mesoproterozoic deformation in the cf. Shaw et al., 2005) at the upper boundary of the sill and dike networks southern part of the range. at deeper levels of the southern Wet Mountains. The more coherent body of the Oak Creek pluton (Fig. 2; Cullers et al., 1993), at approximately Circa 1.4 Ga Regional Penetrative Flow in the Lower Middle Crust the same crustal level, has a sheeted aspect (Dean et al., 2002) that may refl ect emplacement into a dilational structure controlled by mechani- New U-Pb zircon and titanite results for tectonized granites and host cal anisotropies in the host gneisses (cf. McFadden et al., 2010) within gneisses prove that the Wet Mountains underwent deformation between a unifi ed strain state (see above). The pegmatite and granite emplaced at ca. 1435 and 1360 Ma, coincident with regional granitic magmatism (Reed the boundary may have insulated the deeper crust and helped sustain the et al., 1993; Anderson and Cullers, 1999). Deformation was localized within higher temperatures at depth (Shaw et al., 2005) that are recorded by the subvertical shear zones in the northern part of the range, with development metamorphic mineral assemblages and the new zircon and titanite ages of cylindrical upright folds. The partitioning of strain upon discrete zones is reported here. At depth the voluminous G2 and G3 granite may have consistent with 1.4 Ga structural styles in exposures of the Sangre de Cristo advected heat and signifi cantly weakened the lower crust, thus permitting Mountains (Jones and Connelly, 2006), the Black Canyon of the Gunnison long-lived penetrative deformation and subhorizontal, north-northeast to (Jessup et al., 2006), the Idaho Springs–Ralston Creek shear zone (McCoy south-southeast–directed fl ow. et al., 2005), and the Homestake shear zone (Shaw et al., 2001; Selverstone Models for intraplate deformation predict that the strength of the lower et al., 2000). The shallowly to moderately dipping dynamic fabrics that are crust fundamentally infl uences the geometry and strain distribution within syntectonic with respect to pervasive granite sills of 1435–1390 Ma age, dis- an active orogen. Royden (1996) demonstrated that a weak lower crust covered in the southern Wet Mountains, are somewhat unique in the Meso- underlying a strong upper crust permits transmission of compressive proterozoic record of the southern Rocky Mountains. stresses >1000 km from a convergent boundary, leading to development The relatively deep exposure of granites and migmatitic host gneisses of a broad orogen with high average elevation but low relative relief. The in the southern Wet Mountains are a consequence of differential exhuma- spatial extent of the zone of fl ow in the southern Wet Mountains suggests tion and uplift that caused northward tilting of the entire range. Evidence that the pervasive sills of ca. 1435 Ma granite emplaced within high- of this comes from comparison of geobarometry described above from temperature host gneisses acted to weaken the lower crust and propagate the north versus south (Siddoway et al., 2000; Cullers et al., 1992). The deformation across a wide region of southwest Laurentia in Mesoprotero- contrast in depth of exposure is best explained by differential exhuma- zoic time. Consequently the exposures of deep crust in the Wet Mountains tion that caused 10°–15° of northward tilt. In addition, apatite-fi ssion-track merit further study as an analog for low-viscosity layers that are inferred thermochronology from north to south across the Wet Mountains indicates in modern intracontinental orogenic settings like Tibet and the Altiplano substantial Cenozoic exhumation that elevated a Late Cretaceous apatite (e.g., Nelson and Project INDEPTH, 1996; Brasse et al., 2002). partial annealing zone and the Eocene Rocky Mountain erosion surface The fl ow direction for deep levels in the southern Wet Mountains is (Kelley and Chapin, 2004) that had been established during and after the parallel with the contraction direction identifi ed for shallower levels in Laramide orogeny. Taking the two factors into account, a conservative the north, suggesting that north-northwest–south-southeast kinematic estimate for northward tilt of the Wet Mountains crystalline block is 15° transport controlled the overarching pattern of deformation in the Wet to 25°. Thus, during Proterozoic time the foliation dips in the central and Mountains. Our fi ndings are consistent with interpretations of Nyman et southern Wet Mountains would have been relatively shallow, at 35° or less. al. (1994) and Nyman and Karlstrom (1997) that ca. 1.4 Ga deformation

132 www.gsapubs.org | Volume 2 | Number 2 | LITHOSPHERE Downloaded from lithosphere.gsapubs.org on September 21, 2010 DEFORMATION ACROSS A PROTEROZOIC MID-CRUSTAL SECTION | RESEARCH was controlled by far-fi eld stresses transmitted from a distant plate bound- 1. Mid-crustal deformation in the northern part of the range involved ary, and the magmas may have profoundly weakened the lower crust and folding, shear zone formation, and development of subvertical fabrics facilitated the propagation of deformation throughout such a broad region. between 1431 and 1404 Ma. Strain was partitioned in discrete structural However, evidence for lower crustal fl ow in a compressional setting for domains, owing to varying mechanical responses to ca. 1.4 Ga subhori- ca. 1.4 Ga magmatism seemingly confl icts with the petrogenesis and tec- zontal, north-northwest–directed shortening or transcurrent strain. tonic association of ferroan granites (Eby, 1990; Frost et al., 2001a). We 2. At deeper levels, granite sills and dikes intruded migmatitic host recognize that ferroan granites are not commonly associated with conver- gneisses that underwent pervasive subhorizontal lower crustal fl ow 1435– gent tectonic environments and are not known to occur within modern 1390 Ma, with south-southeast–directed transport, resulting in wide- intracontinental orogenic settings. We also acknowledge the well-estab- spread, gently dipping fabrics. lished indications of mantle involvement in ferroan granite petrogenesis 3. The contrasting deformation styles and plutonism in the northern (Frost et al., 2001a) and the strong association of ferroan granites and and southern Wet Mountains were contemporaneous and broadly kine- extensional or hotspot settings (Emslie, 1978; Whalen et al., 1987; Eby, matically compatible, with minor differences attributable to increasing 1990; Frost and Frost, 1997). But we also believe that widespread evi- depth of exposure from north to south across the range or differential dence for ca. 1.4 Ga deformation in exposures throughout the southern motions during poly-stage exhumation. Rocky Mountains and southwestern United States, and particularly our 4. Metamorphic temperatures were suffi ciently high to induce meta- new evidence for long-lived magma-enhanced crustal fl ow at deeper struc- morphic growth of titanite and zircon, and thus were suffi cient to create a tural levels, warrants consideration as well. weak, low-viscosity lower crust that would fl ow in response to dynamic Shaw et al. (2005) speculated that topographically driven syncontrac- or gravitational stresses. tional extension at shallower crustal levels at ca. 1.4 Ga could reconcile Furthermore, the kinematics of mid- to lower crustal structures of the regional structural evidence for shortening with the geochemical charac- Wet Mountains is in accord with results over a broad region and thus lend teristics of coeval granites. Modern orogens and orogenic plateaus com- support to the identity of the Wet Mountains as a deeply exhumed analog monly display evidence for synchronous shortening and extension both for the mid-crustal, low-viscosity layers of the type that are inferred to parallel and perpendicular to the orogen (e.g., Burchfi el et al., 1992), exist in modern intracontinental orogenic settings. Our fi ndings are con- and the apparent contrast between shortening in the southwestern United sistent with an orogenic setting for widespread granitic magmatism at States and extension in the granite-rhyolite provinces of the mid-continent ca. 1.4 Ga and further suggest that the granites may have played a key role region might simply refl ect contrasting levels of exposure. Emplacement in weakening the lower crust, thus allowing deformation to propagate over of mantle-derived tholeiite required for generating ferroan granites could such a broad region. More work is needed to better understand the geo- be related to asthenospheric upwelling driven by thermal or convective chemistry and petrogenesis of granites exposed at deep crustal levels in removal of an orogenically thickened lithosphere (e.g., Houseman et al., the Wet Mountains and to assess the extent and geometry of transcurrent 1981) or by emplacement of a large mantle plume (Hoffman, 1989; Frost deformation locally and throughout the surrounding region in an effort to and Frost, 1997; Ferguson et al., 2004). The former mechanism is a pro- reconcile the contrasting interpretations of the tectonic environment for cess believed to occur quite commonly in convergent orogenic systems the remarkable extent of potassic, ferroan granites at ca. 1.4 Ga. (e.g., Collins, 1994; Cloos, 2003) and has been inferred along the eastern margin of Laurentia during approximately the same time (Rivers, 1997). ACKNOWLEDGMENTS The latter mechanism was likely responsible for producing extremely ferroan and alkalic granites at ca. 1.4 Ga in southern Wyoming (Frost and The ideas presented in this paper have benefi ted signifi cantly from dis- Frost, 1997). cussions with Chris Andronicos, Karl Karlstrom, Mike Williams, Colin Alternatively, the indications of transcurrent strain in the northern Wet Shaw, and Micah Jessup. Field assistance was provided by Owen Cal- Mountains, together with regional evidence for localized extension (e.g., lahan, George Perkins, Adam Krawiec, and Tom Collins III. Analytical Kirby et al., 1995) and oblique movement along ca. 1.4 Ga shear zones assistance was provided by Kathy Manser. Reviews by Pat Bickford and (e.g., Selverstone et al., 2000; Shaw et al., 2001; Jessup et al., 2006), sug- Ron Frost helped to improve the organization and clarity of the manu- gest that regional shortening at ca. 1.4 Ga may have involved a signifi cant script. Sources of funding for this research included the Keck Geology transpressional component. Ferroan granites may be associated with trans- Consortium, NSF-EAR 0101314 to CSS, NSF-EAR-0003528 to JNC, current settings (Sylvester, 1989; Düzgören-Aydin et al., 2001; Deering and the Geology Foundation and Department of Geological Sciences at et al., 2008), and a transpressional strain fi eld would allow for structures the University of Texas at Austin. indicating shortening and simultaneous tholeiitic diking and underplating in the lower crust. The spatial association of shear zones and ca. 1.4 Ga REFERENCES CITED granites suggests that these structures may have conveyed magma upward Aleinikoff, J.N., Reed, J.C., Jr., and DeWitt, E., 1993,The Mount Evans batholith in the Colorado from sites of underplating, differentiation, and additional melt generation Front Range: Revision of its age and reinterpretation of its structure: Geological Society in the lower crust (Fig. 10; Nyman et al., 1994). Dilation or extension of America Bulletin, v. 105, p. 791–806, doi: 10.1130/0016-7606(1993)105<0791:TMEBIT may have occurred locally in areas where preexisting crustal fabrics were >2.3.CO;2. Amato, J.M., Boullion, A.O., Serna, A.M., Sanders, A.E., Farmer, G.L., Gehrels, G.E., and oriented at an angle to the regional northeast-striking structural grain (e.g., Wooden, J.L., 2008, Evolution of the Mazatzal province and the timing of the Mazat- Kirby et al., 1995; Nyman and Karlstrom, 1997; Karlstrom and Hum- zal orogeny: Insights from U-Pb geochronology and geochemistry of igneous and metasedimentary rocks in southern New Mexico: Geological Society of America Bul- phreys, 1998; Dean et al., 2002; Jessup et al., 2005). letin, v. 120, p. 328–346, doi: 10.1130/B26200.1. Anderson, J.L., 1983, Proterozoic anorogenic granite plutonism of North America, in CONCLUSIONS Medaris, L.G., et al., eds., Proterozoic Geology: Selected Papers from an International Proterozoic Symposium: Geological Society of America Memoir 161, p. 133–154. Anderson, J.L., and Cullers, R.L., 1999, Paleo- and Mesoproterozoic granite plutonism of Col- Several important insights emerge from new geologic and geochrono- orado and Wyoming: Rocky Mountain Geology, v. 34, p. 149–164, doi: 10.2113/34.2.149. Anderson, J.L., and Morrison, J., 2005, Ilmenite, magnetite, and peraluminous Mesoprotero- logic data from an oblique Proterozoic crustal section in the Wet Moun- zoic anorogenic granites of Laurentia and Baltica: Lithos, v. 80, p. 45–60, doi: 10.1016/j. tains, Colorado: lithos.2004.05.008.

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Andronicos, C.L., Siddoway, C.S., and Carrick, T., 2002, Transtensional shear zones in Finn, C.A., and Sims, P.K., 2005, Signs from the Precambrian: The geologic framework of the southern Laurentia: Implications for 1.4 Ga tectonics: Geological Society of America Rocky Mountain region derived from aeromagnetic data, in Karlstrom, K.E., and Keller, Abstracts with Programs, v. 34, no. 3, p. 29. G.R., eds., The Rocky Mountain Region: An Evolving Lithosphere: American Geophysi- Bennett, V.C., and DePaolo, D.J., 1987, Proterozoic crustal history of the western United cal Union Geophysical Monograph 154, p. 39–54. States as determined by neodymium isotopic mapping: Geological Society of America Frost, B.R., Barnes, C.G., Collins, W.J., Arculus, R.J., Ellis, D.J., and Frost, C.D., 2001a, A Bulletin, v. 99, p. 674–685, doi: 10.1130/0016-7606(1987)99<674:PCHOTW>2.0.CO;2. geochemical classifi cation for granitic rocks: Journal of Petrology, v. 42, p. 2033–2048, Bickford, M.E., and Anderson, J.L., 1993, Middle Proterozoic magmatism, in Reed, J.C., Jr., doi: 10.1093/petrology/42.11.2033. et al., eds., Precambrian: Conterminous U.S.: Boulder, Colorado, Geological Society of Frost, C.D., and Frost, B.R., 1997, Reduced rapakivi-type granites: The tholeiite connection: America, Geology of North America, v. C-2, p. 540–544. Geology, v. 25, p. 647–650, doi: 10.1130/0091-7613(1997)025<0647:RRTGTT>2.3.CO;2. Bickford, M.E., and Hill, B.M., 2007a, Does the arc accretion model adequately explain the Frost, C.D., Bell, J.M., Frost, B.R., and Chamberlain, K.R., 2001b, Crustal growth by magmatic Paleoproterozoic evolution of southern Laurentia?: An expanded interpretation: Geol- underplating: Isotopic evidence from the Sherman batholith: Geology, v. 29, p. 515– ogy, v. 35, p. 167–170, doi: 10.1130/G23174A.1. 518, doi: 10.1130/0091-7613(2001)029<0515:CGBMUI>2.0.CO;2. Bickford, M.E., and Hill, B.M., 2007b, Does the arc accretion model adequately explain the Fryer, S.L., 1996, Laramide faulting associated with the Ilse fault system, northern Wet Moun- Paleoproterozoic evolution of southern Laurentia?: An expanded interpretation: Reply: tains, Colorado [M.S. thesis]: Fort Collins, Colorado State University, 119 p. Geology, v. 35, no. 1, p. e144, doi: 10.1130/G24042Y.1. Givot, R.M., and Siddoway, C.S., 1998, Deformation of mid-Proterozoic metamorphic rocks, Bickford, M.E., Cullers, R.L., Shuster, R.D., Premo, W.R., and Van Schmus, W.R., 1989, U-Pb Five Points Gulch shear zone, Colorado: Geological Society of America Abstracts with zircon geochronology of Proterozoic and Cambrian plutons in the Wet Mountains and Programs, v. 30, no. 6, p. 9. southern Front Range, Colorado, in Grambling, J.A., and Tewksbury, B.J., eds., Protero- Goodge, J.W., and Siddoway, C., 1997, Mineral reactions and petrogenetic implications of zoic Geology of the Southern Rocky Mountains: Geological Society of America Special Fe-Mn-andalusite, northern Wet Mountains, Colorado: Geological Society of America Paper 235, p. 49–64. Abstracts with Programs, v. 29, no. 5, p. 11. Bickford, M.E., Mueller, P.A., Kamenov, G.D., and Hill, B.M., 2008, Crustal evolution of southern Goodge, J.W., and Vervoort, J.D., 2006, Origin of Mesoproterozoic A-type granites in Lau- Laurentia during the Proterozoic: Insights from zircon Hf isotopic studies of ca. 1.75 Ga rentia: Hf isotope evidence: Earth and Planetary Science Letters, v. 243, p. 711–731, doi: rocks in central Colorado: Geology, v. 36, p. 555–558, doi: 10.1130/G24700A.1. 10.1016/j.epsl.2006.01.040. Boyer, R.E., 1962, Petrology and structure of the southern Wet Mountains, Colorado: Geo- Graubard, C.M., and Mattinson, J.M., 1990, Syntectonic emplacement of the approximately logical Society of America Bulletin, v. 73, p. 1047–1070, doi: 10.1130/0016-7606(1962) 1440 Ma Mt. Evans Pluton and history of motion along the Idaho Springs–Ralston 73[1047:PASOTS]2.0.CO;2. Creek shear zone, central Front Range, Colorado: Geological Society of America Brasse, H., Lezaeta, P., Rath, V., Schwalenberg, K., Soyer, W., and Haak, V., 2002, The Bolivian Abstracts with Programs, v. 22, no. 6, p. 12. Altiplano conductivity anomaly: Journal of Geophysical Research, v. 107, no. B5, 2096, Heimann, A., Spry, P.G., and Teale, G., 2005, Zincian spinel associated with Proterozoic base- doi: 10.1029/2001JB000391. metal sulfi de occurrences, Colorado: A re-evaluation of gahnite composition as a guide Brock, M.R., and Singewald, Q.D., 1968, Geologic Map of the Mt. Tyndall Quadrangle, in exploration: Canadian Mineralogist, v. 43, p. 601–622. Custer County, Colorado: U.S. Geological Survey Geologic Quadrangle Map GQ-596, Hoffman, P.F., 1989, Speculations on Laurentia’s fi rst gigayear (2.0–1.0 Ga): Geology, v. 17, scale 1:24,000. p. 135–138, doi: 10.1130/0091-7613(1989)017<0135:SOLSFG>2.3.CO;2. Burchfi el, B.C., Chen, Z., Hodges, K.V., Liu, Y., Royden, L.H., Deng, C., and Xu, J., 1992, The Holcombe, R.J., 2003, GEOrient v. 9.1, http://www.holcombe.net.au/software/ index.html. South Tibetan Detachment System, Himalayan Orogen: Extension Contemporaneous Houseman, G.A., McKenzie, D.P., and Molnar, P., 1981, Convective instability of a thick- with and Parallel to Shortening in a Collisional Mountain Belt: Geological Society of ened boundary layer and its relevance for the thermal evolution of continental con- America Special Paper 269, 41 p. vergent belts: Journal of Geophysical Research, v. 86, p. 6115–6132, doi: 10.1029/ Callahan, O., 2002, Preliminary evidence for syntectonic emplacement of Mesoproterozoic JB086iB07p06115. plutons in the southern Wet Mountains, Colorado, in Proceedings, Keck Research Sym- Jessup, M.J., Karlstrom, K.E., Livaccari, R., Connelly, J., Tyson, A., and Rogers, S.A., 2005, posium in Geology, 15th, Amherst, Massachusetts: Lancaster, Pennsylvania, Franklin Complex Proterozoic crustal assembly of southwestern North America in an arcu- and Marshall College, p. 137–140. ate subduction system: The Black Canyon of the Gunnison, southwestern Colorado, Cloos, M., 2003, Collisional delamination, lithospheric rupture and magmatism: Geological in Karlstrom, K.E., and Keller, G.R., eds., The Rocky Mountain Region—An Evolving Society of America Abstracts with Programs, v. 35, p. 95. Lithosphere: Tectonics, Geochemistry, and Geophysics: American Geophysical Union Collins, T.R., IV, Siddoway, C.S., Jones, J.V., III, and Tellio, C., 2004, Shear zone roots in the Geophysical Monograph 154, p. 21–38. middle crust: The transition from partitioned deformation to penetrative ductile fl ow Jessup, M.J., Jones, J.V., III, Karlstrom, K.E., Williams, M.L., Connelly, J.N., and Heizler, M.T., in the northern Wet Mountains, CO: Geological Society of America Abstracts with Pro- 2006, Three Proterozoic orogenic episodes and an intervening exhumation event in the grams, v. 36, no. 5, abs. 217-4. Black Canyon of the Gunnison region, Colorado: Journal of Geology, v. 114, p. 555–576, Collins, W.J., 1994, Upper- and middle-crustal response to delamination: An example from doi: 10.1086/506160. the Lachlan fold belt, eastern Australia: Geology, v. 22, p. 143–146, doi: 10.1130/0091 Jones, J.V., III, 2005, Proterozoic tectonic evolution of southern Laurentia: New constraints -7613(1994)022<0143:UAMCRT>2.3.CO;2. from fi eld studies and geochronology in southern Colorado and northern New Mexico, Condie, K.C., 1982, Plate tectonics model for Proterozoic continental accretion in the south- U.S.A. [Ph.D. thesis]: Austin, University of Texas at Austin, 204 p. western United States: Geology, v. 10, p. 37–42, doi: 10.1130/0091-7613(1982)10<37 Jones, J.V., III, and Connelly, J.N., 2006, Proterozoic tectonic evolution of the Sangre de Cristo :PMFPCA>2.0.CO;2. Mountains, southern Colorado, U.S.A.: Rocky Mountain Geology, v. 41, p. 79–116, doi: Condie, K.C., 1986, Geochemistry and tectonic setting of early Proterozoic supracrustal rocks in 10.2113/gsrocky.41.2.79. the southwestern United States: Journal of Geology, v. 94, p. 845–864, doi: 10.1086/629091. Jones, J.V., III, Connelly, J.N., Karlstrom, K.E., Williams, M.L., and Doe, M.F., 2009, Age, prov- Cullers, R.L., Griffi n, T., Bickford, M.E., and Anderson, J.L., 1992, Origin and chemical evolu- enance, and tectonic setting of Paleoproterozoic quartzite successions in the south- tion of the 1360 Ma San Isabel Batholith, Wet Mountains, Colorado: A mid-crustal gran- western United States: Geological Society of America Bulletin, v. 121, p. 247–264. ite of anorogenic affi nities: Geological Society of America Bulletin, v. 104, p. 316–328, Karlstrom, K.E., and Bowring, S.A., 1988, Early Proterozoic assembly of tectonostratigraphic doi: 10.1130/0016-7606(1992)104<0316:OACEOT>2.3.CO;2. terranes in southwestern North America: Journal of Geology, v. 96, p. 561–576, doi: Cullers, R.L., Stone, J., Anderson, J.L., Sassarini, N., and Bickford, M.E., 1993, Petrogenesis 10.1086/629252. of Mesoproterozoic Oak Creek and West McCoy Gulch plutons, Colorado: An example Karlstrom, K.E., and Humphreys, E.D., 1998, Persistent infl uence of Proterozoic accretionary of cumulate unmixing of a mid-crustal, two-mica granite of anorogenic affi nity: Pre- boundaries in the tectonic evolution of southwestern North America; interaction of cra- cambrian Research, v. 62, p. 139–169, doi: 10.1016/0301-9268(93)90098-M. tonic grain and mantle modifi cation events: Rocky Mountain Geology, v. 33, p. 161–179. Dean, R.L., Andronicos, C., Siddoway, C., and Ray, J., 2002, Syntectonic emplacement of the Karlstrom, K.E., Ahall, K.I., Harlan, S.S., Williams, M.L., McLelland, J., and Geissman, J.W., 1.44 Ga Oak Creek pluton, Wet Mountains, Colorado: Geological Society of America 2001, Long-lived (1.8–1.0 Ga) convergent orogen in southern Laurentia, its extensions Abstracts with Programs, v. 34, no. 6, p. 243. to Australia and Baltica, and implications for refi ning Rodinia: Precambrian Research, Deering, C.D., Cole, J.W., and Vogel, T.A., 2008, A rhyolite compositional continuum gov- v. 111, p. 5–30, doi: 10.1016/S0301-9268(01)00154-1. erned by lower crustal source conditions in the Taupo Volcanic Zone, New Zealand: Karlstrom, K.E., Whitmeyer, S.J., Williams, M.L., Bowring, S.A., and Jessup, M.J., 2007, Does Journal of Petrology, v. 49, p. 2245–2276, doi: 10.1093/petrology/egn067. the arc accretion model adequately explain the Paleoproterozoic evolution of southern Duebendorfer, E.M., 2007, Research focus: Crust formation in the western United States: Laurentia?: An expanded interpretation: Comment: Geology, v. 35, no. 1, p. e143–e144, Geology, v. 35, p. 191, doi: 10.1130/0091-7613(2007)35[191:RFCFIT]2.0.CO;2. doi: 10.1130/G23971C.1. Düzgören-Aydin, N.S., Malpas, J., Goncuoglu, M.C., and Erler, A., 2001, A review of the Kelley, S.A., and Chapin, C.E., 2004, Denudation history and internal structure of the Front nature of magmatism in central Anatolia during the Mesozoic post-collisional period: Range and Wet Mountains, Colorado, based on apatite-fi ssion-track thermochronol- International Geology Review, v. 43, p. 695–710, doi: 10.1080/00206810109465042. ogy: New Mexico Bureau of Geology and Mineral Resources Bulletin, v. 160, p. 41–77. Eby, G.N., 1990, The A-type granitoids: A review of their occurrence and chemical character- Kirby, E., Karlstrom, K.E., Andronicos, C.L., and Dallmeyer, R.D., 1995, Tectonic setting of the istics and speculations on their petrogenesis: Lithos, v. 26, p. 115–134, doi: 10.1016/0024 Sandia Pluton; an orogenic 1.4 Ga granite in New Mexico: Tectonics, v. 14, p. 185–201, -4937(90)90043-Z. doi: 10.1029/94TC02699. Emslie, R.F., 1978, Anorthosite massifs, rapakivi granites, and late Proterozoic rifting of North Krogh, T.E., 1982, Improved accuracy of U-Pb zircon ages by the creation of more concor- America: Precambrian Research, v. 7, p. 61–98, doi: 10.1016/0301-9268(78)90005-0. dant systems using an abrasion technique: Geochimica et Cosmochimica Acta, v. 46, Ferguson, C.B., Duebendorfer, E.M., and Chamberlain, K.R., 2004, Synkinematic intrusion p. 637–649, doi: 10.1016/0016-7037(82)90165-X. of the 1.4-Ga Boriana Canyon Pluton, northwestern Arizona: Implications for ca. 1.4-Ga Lanzirotti, A., 1988, Geology and geochemistry of a Proterozoic supracrustal and intrusive regional strain in the western United States: Journal of Geology, v. 112, p. 165–183, sequence in the Central Wet Mountains, Colorado [M.S. thesis]: Socorro, New Mexico doi: 10.1086/381656. Institute of Mining and Technology, 164 p.

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Loiselle, M.C., and Wones, D.R., 1979, Characteristics and origin of anorogenic granites: K.E., and Keller, G.R., eds., The Rocky Mountain Region: An Evolving Lithosphere: Geological Society of America Abstracts with Programs, v. 11, p. 468. American Geophysical Union Geophysical Monograph 154, p. 163–184. McCoy, A.M., Karlstrom, K.E., Williams, M.L., and Shaw, C.A., 2005, Proterozoic ancestry Siddoway, C.S., Givot, R.M., Bodle, C.D., and Heizler, M.T., 2000, Dynamic versus anoro- of the Colorado mineral belt: Ca. 1.4 Ga shear zone system in southern Colorado, in genic setting for Mesoproterozoic plutonism in the Wet Mountains, Colorado: Does the Karlstrom, K.E., and Keller, G.R., eds., The Rocky Mountain Region: An Evolving Litho- interpretation depend on level of exposure?: Rocky Mountain Geology, v. 35, p. 91–111, sphere: American Geophysical Union Geophysical Monograph 154, p. 71–90. doi: 10.2113/35.1.91. McFadden, R., Teyssier, C., Siddoway, C.S., Whitney, D., and Fanning, C.M., 2010, Oblique Siddoway, C.S., Jones, J., Williams, M.L., Andronicos, C.L., Connelly, J., and Karlstrom, K.E., dilation, melt transfer, and gneiss dome emplacement: Geology (in press), doi: 10.1130/ 2002, The Mesoproterozoic record in the Wet Mountains, Colorado: Geological Society G30493.1. of America Abstracts with Programs, v. 34, no. 6, p. 181. Nelson, K.D., and Project INDEPTH, 1996, Partially molten middle crust beneath southern Silver, L.T., 1965, Mazatzal orogeny and tectonic episodicity, in Abstracts for 1964: Boulder, Tibet: Synthesis of Project INDEPTH: Science, v. 274, p. 1684–1688, doi: 10.1126/science Colorado, Geological Society of America Special Paper 82, p. 185–188. .274.5293.1684. Snelson, C.M., Keller, G.R., Miller, K.C., Rumpel, H.M., and Prodehl, C., 2005, Regional crustal Nyman, M.W., and Karlstrom, K.E., 1997, Pluton emplacement processes and tectonic setting structure derived from the CD-ROM 99 seismic refraction/wide-angle refl ection profi le: of the 1.42 Ga Signal Batholith, SW USA; Important role of crustal anisotropy dur- The lower crustal and upper mantle, in Karlstrom, K.E., and Keller, G.R., eds., The Rocky ing regional shortening: Precambrian Research, v. 82, p. 237–263, doi: 10.1016/S0301- Mountain Region: An Evolving Lithosphere: American Geophysical Union Geophysical 9268(96)00049-6. Monograph 154, p. 271–292. Nyman, M.W., Karlstrom, K.E., Kirby, E., and Graubard, C.M., 1994, Mesoproterozoic con- Stacey, J.S., and Kramers, J.D., 1975, Approximation of terrestrial lead isotopic evolution tractional orogeny in western North America; evidence from ca. 1.4 Ga plutons: Geol- by a two-stage model: Earth and Planetary Science Letters, v. 26, p. 207–221, doi: ogy, v. 22, p. 901–904, doi: 10.1130/0091-7613(1994)022<0901:MCOIWN>2.3.CO;2. 10.1016/0012-821X(75)90088-6. Oshetski, K.C., and Kucks, R.P., 2000, Colorado Aeromagnetic and Gravity Maps and Data: Stiles, L., 1997, Petrology and geochemistry of granite in the northern Wet Mountains, south- A Web Site for Distribution of Data (online version): U.S. Geological Survey OF-00-42. central Colorado: Proceedings, Keck Research Symposium in Geology, 10th, April 1997, Pardo, J., Keller, G.R., and Holloway, S., 2008, New observations on the extent of Cambrian Keck Geology Consortium, p. 269–272, http://keckgeology.org/fi les/pdf/symvol/10th/ rifting in Colorado: An update: Geological Society of America Abstracts with Programs, Colorado/stiles.pdf. v. 40, no. 6, p. 545. Sylvester, P.J., 1989, Post-collisional alkaline granites: Journal of Geology, v. 97, p. 261–280, Patchett, P.J., and Ruiz, J., 1989, Nd isotopes and the origin of Grenville-age rocks in Texas: doi: 10.1086/629302. Implications for Proterozoic evolution of the United States mid-continent region: Jour- Taylor, R.B., Scott, G.R., Wobus, R.A., and Epis, R.C., 1975a, Reconnaissance Geologic Map nal of Geology, v. 97, p. 685–695, doi: 10.1086/629352. of the Cotopaxi 15-minute Quadrangle, Fremont and Custer Counties, Colorado: U.S. Pedrick, J.N., Karlstrom, K.E., and Bowring, S.E., 1998, Reconciliation of confl icting tectonic Geological Survey Miscellaneous Investigations Series Map I-900, scale 1:62,500. models for Proterozoic rocks of northern New Mexico: Journal of Metamorphic Geol- Taylor, R.B., Scott, G.R., Wobus, R.A., and Epis, R.C., 1975b, Reconnaissance Geologic Map of ogy, v. 16, p. 687–707, doi: 10.1111/j.1525-1314.1998.00165.x. the Royal Gorge Quadrangle, Fremont and Custer Counties, Colorado: U.S. Geological Perkins, G., 2002, Dynamic high-temperature metamorphism and deformation during Survey Miscellaneous Investigations Series Map I-869, scale 1:62,500. emplacement of the San Isabel Pluton, Wet Mountains, Colorado, in Proceedings, Keck Timmons, J.M., Heizler, M., and Karlstrom, K.E., 2002, Proterozoic and Paleozoic ancestry Research Symposium in Geology, 15th, Amherst, Massachusetts: Lancaster, Pennsyl- of regional Laramide fault networks: Using 40Ar/ 39Ar K-feldspar thermochronology to vania, Franklin and Marshall College, p. 153–156. evaluate early cooling/exhumation histories across discrete fault zones: Geological Raymond, W.H., Leiggi, P.A., and Sheridan, D.M., 1980, Sapphirine in Precambrian Rocks Society of America Abstracts with Programs, v. 34, no. 6, p. 181. Associated with Stratabound Sulfi de Deposits, Custer County, Colorado: U.S. Geologi- Tweto, O., 1979, Geologic Map of Colorado: Colorado Geological Survey, scale 1:500,000. cal Survey Bulletin 1513, 22 p. Tweto, O., 1987, Rock units of the Precambrian basement in Colorado: U.S. Geological Sur- Reed, J.C., Jr., Bickford, M.E., and Tweto, O., 1993, Proterozoic accretionary terranes of Colo- vey Professional Paper 1321-A, p. A1–A54. rado and southern Wyoming, in Reed, J.C., Jr., et al., eds., Precambrian: Conterminous Wearn, K.M., and Wobus, R.A., 1998, Early Proterozoic metavolcanics of the Arkansas River U.S.: Boulder, Colorado, Geological Society of America, Geology of North America, Canyon, Howard to Royal Gorge, central Colorado: Eos (Transactions, American Geo- v. C-2, p. 110–121. physical Union), v. 79, p. 355. Rivers, T., 1997, Lithotectonic elements of the Grenville Province: Review and tectonic impli- Whalen, J.B., Currie, K.L., and Chapell, B.W., 1987, A-type granites: Geochemical character- cations: Precambrian Research, v. 86, p. 117–154, doi: 10.1016/S0301-9268(97)00038-7. istics, discrimination and petrogenesis: Contributions to Mineralogy and Petrology, Royden, L., 1996, Coupling and decoupling of crust and mantle in convergent orogens; v. 95, p. 407–418, doi: 10.1007/BF00402202. implications for strain partitioning in the crust: Journal of Geophysical Research, v. 101, Whitmeyer, S.J., and Karlstrom, K.E., 2007, Tectonic model for the Proterozoic growth of p. 17,679–17,705, doi: 10.1029/96JB00951. North America: Geosphere, v. 3, p. 220–259, doi: 10.1130/GES00055.1. Sawyer, E.W., 2008, Atlas of Migmatites: Quebec, Canadian Mineralogist Special Publication Williams, M.L., 1991, Heterogeneous deformation in a ductile fold-thrust belt: The Proterozoic 9, Mineralogical Association of Canada, 373 p. structural history of the Tusas Mountains, New Mexico: Geological Society of America Scott, G.R., Taylor, R.B., Epis, R.C., and Wobus, R.A., 1978, Geologic Map of the Pueblo 1 × 2 Bulletin, v. 103, p. 171–188, doi: 10.1130/0016-7606(1991)103<0171:HDIADF>2.3.CO;2. Degree Quadrangle, South-Central Colorado: U.S. Geological Survey Map I-1022, Williams, M.L., Karlstrom, K.E., Lanzirotti, A., Read, A.S., Bishop, J.L., Lombardi, C.E., Pedrick, 2 sheets, scale 1:250,000. J., and Wingsted, M.B., 1999, New Mexico middle crustal cross sections: 1.65-Ga mac- Selverstone, J., Hodgins, M., Aleinikoff, J.N., and Fanning, C.M., 2000, Mesoproterozoic roscopic geometry, 1.4-Ga thermal structure, and continued problems in understand- reactivation of a Paleoproterozoic transcurrent boundary in the northern Colorado ing crustal evolution: Rocky Mountain Geology, v. 34, p. 53–66, doi: 10.2113/34.1.53. Front Range: Implications for ~1.7- and 1.4-Ga tectonism: Rocky Mountain Geology, Wooden, J.L., and DeWitt, E., 1991, Pb isotopic evidence for the boundary between the early v. 35, p. 139–162, doi: 10.2113/35.2.139. Proterozoic Mojave and central Arizona crustal provinces in western Arizona, in Karl- Shaw, C.A., and Karlstrom, K.E., 1999, The Yavapai-Mazatzal crustal boundary in the South- strom, K.E., ed., Proterozoic Geology and Ore Deposits of Arizona: Arizona Geological ern Rocky Mountains: Rocky Mountain Geology, v. 34, p. 37–52, doi: 10.2113/34.1.37. Society Digest, v. 19, p. 27–50. Shaw, C.A., Snee, L.W., Selverstone, J., and Reed, J.C., Jr., 1999, 40Ar/39Ar thermochronology Wooden, J.L., Stacey, J.S., Howard, K.A., Doe, B.R., and Miller, D.M., 1988, Pb isotopic evi- of Mesoproterozoic metamorphism in the Colorado Front Range: Journal of Geology, dence for the formation of Proterozoic crust in the southwestern United States, in v. 107, p. 49–67, doi: 10.1086/314335. Ernst, W.G., ed., Metamorphism and Crustal Evolution of the Western United States Shaw, C.A., Karlstrom, K.E., Williams, M.L., Jercinovic, M.J., and McCoy, A.M., 2001, Elec- [Rubey Volume]: Englewood Cliffs, New Jersey, Prentice-Hall, v. 7, p. 68–86. tron-microprobe monazite dating of ca. 1.71–1.63 Ga and ca. 1.45–1.38 Ga deformation in the Homestake shear zone, Colorado: Origin and early evolution of a persistent intracontinental tectonic zone: Geology, v. 29, p. 739–742, doi: 10.1130/0091-7613 MANUSCRIPT RECEIVED 4 SEPTEMBER 2009 (2001)029<0739:EMMDOC>2.0.CO;2. REVISED MANUSCRIPT RECEIVED 16 DECEMBER 2009 40 30 Shaw, C.A., Heizler, M.T., and Karlstrom, K.E., 2005, Ar/ Ar thermochronologic record of MANUSCRIPT ACCEPTED 12 JANUARY 2010 1.45–1.35 Ga intracontinental tectonism in the southern Rocky Mountains: Interplay of conductive and advective heating with intracontinental deformation, in Karlstrom, Printed in the USA

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