Three Proterozoic Orogenic Episodes and an Intervening Exhumation Event in the Black Canyon of the Gunnison Region, Colorado

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1 Three Proterozoic Orogenic Episodes and an Intervening Exhumation Event in the Black Canyon of the Gunnison Region, Colorado Micah J. Jessup, James V. Jones III, 1 Karl E. Karlstrom, 2 Michael L. Williams, 3 James N. Connelly, 4 and Matthew T. Heizler 5 Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, U.S.A. ( mjessup@vt.edu) A B S T R A C T U/Pb zircon/titanite geochronology, in situ monazite geochronology, and 40 Ar/ 39 Ar thermochronology provide an unusually complete data set for reconstructing the tectonic history of Proterozoic rocks exposed in the Black Canyon, Gunnison, Colorado. These new geochronologic data record three protracted orogenic episodes and an exhumation event between orogenic pulses: (1) Yavapai orogeny ( Ma), (2) exhumation marked by an angular unconformity beneath post-yavapai, pre-mazatzal quartzites, (3) Mazatzal orogeny (postquartzite deposition), and (4) Ma intracratonic tectonism. Supracrustal rocks of the Black Canyon succession were deposited or crystallized at or prior to Ma and were intruded by the Ma Pitts Meadow granodiorite. Paleoproterozoic hightemperature metamorphism (1650 C) associated with the Yavapai orogeny occurred between 1741 and 1690 Ma. Deposition of interorogenic quartzites took place after 1700 Ma. The Vernal Mesa monzogranite was emplaced at Ma followed by metamorphism ( C and 3 1 kbar) at Ma. 40 Ar/ 39 Ar thermochronology records Mesoproterozoic middle crustal temperatures of C, with the highest temperatures occurring near the Vernal Mesa monzogranite and the NE-striking Black Canyon shear zone. The area cooled through 350 C by 1385 Ma but variably cooled through 300 C from 1370 to 1100 Ma, suggesting long-term residence of rocks above the 250 C isotherm at 10 km crustal depths. When these results are combined with geologic data to construct generalized pressure/temperature/time/deformation paths (PTtD), a new template for the evolution of Proterozoic rocks of southwestern Colorado and the southwestern United states emerges. Online enhancements: appendixes, tables. Introduction Proterozoic rocks exposed in the southwestern United States record a tectonic evolution that resulted in the southward growth of Laurentia between 1.8 and 1.0 Ga (DePaolo 1981; Reed et al. Manuscript received May 4, 2005; accepted May 8, Geology Discipline, University of Minnesota at Morris, 600 East 4th Street, Morris, Minnesota 56267, U.S.A.; jonesjv@morris.umn.edu. 2 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.; kek1@unm.edu. 3 Department of Geosciences, University of Massachusetts, 611 North Pleasant Street, Amherst, Massachusetts 01003, U.S.A.; mlw@geo.umass.edu. 4 Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712, U.S.A.; connelly@mail.utexas.edu. 5 New Mexico Bureau of Geology and Mineral Resources, Socorro, New Mexico 87801, U.S.A.; matt@nmt.edu. 1987; Hoffman 1988; Bowring and Karlstrom 1990; CD-ROM Working Group 2002). Stages of the 800 m.yr. tectonic history include the accretion of juvenile crust, progressive assembly of tectonostratigraphic terranes, and reactivation of parts of the assembled orogen (Ilg et al. 1996). Many midcrustal rocks exposed in the Southwest record multiple metamorphic and deformational events. Williams and Karlstrom (1996) proposed looping PTt paths that involve burial by thrusting, one or more tectonothermal pulses, long-term residence at some depth, and unroofing after the final Proterozoic orogenic cycle. This study attempts to document the record preserved by metamorphic and intrusive rocks exposed in the Black Canyon region, Colorado, in an effort to construct a tectonic history that builds on [The Journal of Geology, 2006, volume 114, p ] 2006 by The University of Chicago. All rights reserved /2006/ $

2 556 M. J. J E S S U P E T A L. existing models for the evolution of the southwestern United States during the Proterozoic (fig. 1). The Black Canyon region provides excellent exposure of both Paleo- and Mesoproterozoic rocks and offers an important spatial connection between well-studied areas in the Grand Canyon, Arizona, and the Rocky Mountains (Karlstrom and Bowring 1988; Ilg et al. 1996; Shaw et al. 2001, 2005). New geochronology presented here includes: U-Pb zircon and titanite geochronology, in situ monazite geochronology, and 40 Ar/ 39 Ar thermochronology. These data are an important addition to the field relationships, preliminary geochronology, and regional tectonic model presented by Jessup et al. (2005) and better constrain Proterozoic timing and tectonic processes. A major contribution to tectonic models for the southwestern United States emerges that helps clarify the nature of three orogenic episodes. Proterozoic Rocks in the Black Canyon Region The Black Canyon region of southwestern Colorado provides a nearly 100-km-long transect of exposed Proterozoic rocks (fig. 1; Hunter 1925; Hansen and Peterman 1968; Hansen 1971, 1972; Bickford et al. 1989; Jessup et al. 2005). Metasedimentary rocks dominate the western part of the region (Black Canyon area; fig. 1B, location 1), whereas metavolcanic rocks dominate the eastern part of the region (Gunnison area; fig. 1B, locations 2, 3). The contact between the metasedimentary rocks of the Black Canyon area and the metavolcanic rocks of the Dubois and Cochetopa successions is pervasively sheared and interpreted as a zone of interlayered volcanic and immature sedimentary rocks (Olsen and Hedlund 1973; Jessup et al. 2005). The Dubois ( Ma) and Cochetopa ( Ma) successions, two different age successions of metavolcanic and metasedimentary rocks that are intruded by gabbroic sheets and plutons, represent the oldest rocks exposed in the Gunnison area (fig. 1B, locations 2, 3; table 1; Bickford and Boardman 1984; Bickford et al. 1989). The metavolcanic rocks have bimodal chemistry with end member compositions that range from tholeiitic basalts to rhyolites. Primary structures in the metavolcanogenic rocks include pyroclastic sheets, basalt flows with amygdaloidal tops, pillows lavas, and breccias (Bickford and Boardman 1984; Bickford et al. 1989). The Black Canyon succession consists of quartz-rich metasedimentary rocks, amphibolite (dated and discussed below), and schists that are distinct from the metavolcanic Dubois and Cochetopa successions (Hansen and Peterman 1968; Hansen 1971, 1981; Bickford and Boardman 1984; Jessup et al. 2005). Here, quartzofeldspathic paragneisses locally preserve primary structures such as bedding, ripple marks, and crossbedding. Paragneisses commonly grade into subordinate amounts of pelitic schist of varying composition. Based on primary structures and relationships between the metasedimetary and metavolcanic rocks, Condie and Nuter (1981) suggested deposition in an arc or immature back arc extensional basin. Still older unexposed basement may be present in the subsurface as fragments of older continental crust, as shown by inherited zircon with ages between 1870 and 1840 Ma (table 1; Hill and Bickford 2001). The youngest Proterozoic metasedimentary rocks in the Gunnison region are two quartzite bodies that unconformably overlie the Dubois succession (Hill and Bickford 2001; Jessup et al. 2005; fig. 1B). Intrusive rocks, including many pegmatite dikes (dated and discussed below), are common throughout the Black Canyon area (table 1). Proterozoic rocks of the Black Canyon region record at least four phases of pre-phanerozoic ductile deformation (Hansen 1981; Jessup et al. 2005). The D1 phase is characterized by isoclinal F1 folds of primary features such as bedding and the formation of a pervasive S1 foliation. During D2, the S1 foliation was folded by shallowly plunging north- Figure 1. Map and simplified cross section of Proterozoic rocks exposed in the Black Canyon of the Gunnison area, Colorado, after Tweto and Sims (1963) and Hansen (1971, 1972). Proterozoic rocks in the Black Canyon area consist of supracrustal rocks that are migmatized and intruded by Pitts Meadow granodiorite, Vernal Mesa and Curecanti monzogranites, and several generations of pegmatite dikes. Sample locations are indicated by black stars ( 40 Ar/ 39 Ar) or circles (monazite, zircon, titanite). Results of geochronology are shown in boxes. Inset A shows field area location (gray box) in North America. Inset B shows the distribution of Proterozoic rocks in southern Colorado (gray), including quartzites (black), the Black Canyon area (1), the Dubois (2), and Cochetopa (3) successions. Inset C is a stereonet that displays the average orientation of pegmatite dikes that crosscut the Vernal Mesa monzogranite, the average shear plane of the Black Canyon shear zone, and L4 stretching lineations (black circles). Braces on 40 Ar/ 39 Ar ages denote two mineral separates from one sample.

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4 Table 1. U-Pb Zircon Geochronology for the Black Canyon and Gunnison Region Dated material Interpretation Age (Ma) Reference Inherited zircons in Dubois Fragments of older crust Hill and Bickford 2001 Dubois succession rhyolite Rhyolite deposition to Bickford et al Granite of Tolvar Peak, Dubois Intrusion of calc-alkalic plutons into Dubois succession Bickford et al Powderhorn granite, Dubois Intrusion of calc-alkalic plutons into Dubois succession Bickford et al Cochetopa succession rhyolite Rhyolite deposition to Bickford et al Uncompahgre area Minimum protolith age Livaccari et al Amphibolite in Black Canyon area a Minimum protolith age This study Amphibolite in Black Canyon area a Oldest zircon growth in Black Canyon area This study Quartz diorite in the GAC Emplaced as a sill predeformation Bickford et al. 1989; Lafrance and John 2001 Fine-grained granite, Dubois Intrudes older metarhyolite 1700 Hill and Bickford 2001 Cochetopa succession Rhyolite deposition Bickford et al Quartzmonzonite, Uncompahgre Emplaced syn northwest-trending F Livaccari et al Metamorphic zircon in amphibolite a Metamorphic event pre-pitts Meadow granodiorite 1723 This study Tonalite of Gold Basin, Gunnison Bickford et al Pitts Meadow granodiorite a Migmatization in the Black Canyon This study Pegmatite dike, Black Canyon a Migmatization in the Black Canyon This study Granite of Wood Gulch, Gunnison Bickford et al Granite, Gunnison area Bickford et al Outer ring of GAC Syn-northeast-directed shortening 1700 Lafrance and John 2001 Quartzite deposition Exhumation Hill and Bickford 2001 Cebolla Creek quartzite a Exhumation 1700 This study Granite of Cochetopa Canyon Posttectonic Bickford et al Vernal Mesa monzogranite a Emplaced parallel to Black Canyon shear zone This study Vernal Mesa monzogranite Postkinematic in Uncompahgre area Livaccari et al Curecanti Pluton (Rb/Sr) Horizontal dike crosscuts migmatitic gneiss Hansen and Peterman 1968 Late pegmatite dike, Black Canyon a Reactivation of the Black Canyon shear zone This study Note. GAC p Gunnison Annular Complex. a Rocks dated during this study.

5 Journal of Geology O R O G E N I C E P I S O D E S I N T H E B L A C K C A N Y O N 559 west-trending F2 folds that are the dominant structure from the Cochetopa area (Afifi 1981; Wortman et al. 1990) to the Morrow Point area (fig. 1). The Black Canyon shear zone (D3) is a 4-km-wide zone of subvertical NE-striking S3 foliation characterized by upright, highly transposed isoclinal folds (Tweto and Sims 1963; Jessup et al. 2005). Fold axes become parallel with the stretching lineation in the center of the shear zone. On both sides of the Black Canyon shear zone, F2 fold axes trend north-south (e.g., Crystal Creek synform and Morrow Point antiform; fig. 1) and are interpreted to have been partially rotated in drag folds during dextral shear. The fourth phase of deformation (D4) is recorded by pegmatite dikes (292, 51 NE) that were emplaced nearly perpendicular to the stretching lineation (10 30 r243 ) during reactivation of the Black Canyon shear zone at 1.4 Ga (fig. 1C). These pegmatite dikes (dated and discussed below) are interpreted to have filled tension gashes that record dextral, with a small component of northwest-side-up, reactivation of the D3 Black Canyon shear zone (fig. 1, cross section). The NW-striking Red Rocks fault displaced Proterozoic structures by 5 km of leftlateral strike-slip movement at some time after emplacement of the Vernal Mesa monzogranite (fig. 1; Hansen 1981; Timmons et al. 2001; Jessup et al. 2005). The Black Canyon region, similar to many other areas of exposed Proterozoic rocks in the Southwest, is characterized by significant variations in metamorphic grade over tens of kilometers that probably result from polyphase metamorphism (Williams and Karlstrom 1996). Overall, there is a decrease in metamorphic grade from northwest to southeast (fig. 1; Afifi 1981). To the northwest, migmatitic schist and paragneisses form the host rock to the Pitts Meadow granodiorite (fig. 1B, location 1). These rocks reached amphibolite facies during peak metamorphism coincident with plutonism and D1-D2 deformation (Hansen and Peterman 1968; Hansen 1972; Jessup et al. 2005). To the southeast (fig. 1B, location 2), the Dubois succession reached epidote-amphibolite to lowermost amphibolite facies, possibly during D1 deformation (Shonk 1984). Still farther to the southeast (fig. 1B, location 3), the Cochetopa succession reached only upper greenschist to lower amphibolite facies (Afifi 1981). U/Pb Zircon and Titanite Geochronology U-Pb zircon and titanite geochronology were employed to determine the crystallization ages of several generations of intrusive rocks and a minimum protolith age for the Proterozoic rocks exposed in the Black Canyon area. Isotopic data are presented in table 2. Analytical methods follow those of Connelly and Mengel (2000). A list of sample locations is found in table A1, available in the online edition or from the Journal of Geology office. Sample J01-BC5: Amphibolite, Pitts Meadow Wall Rock. The oldest newly dated sample is a salt and pepper amphibolite layer from the Ute trail area of the Black Ridge quadrangle (figs. 1, 2A; tables 1, 2; Hansen 1971). The amphibolite layer contains a pervasive north-south-trending subhorizontal L- tectonite fabric and is one of many amphibolite layers within the predominantly migmatized quartzofeldspathic gneiss (Hansen 1971). Throughout the Black Canyon region, amphibolites are commonly dark gray to black, lineated, medium- to coarse-grained schist and consist of hornblende and plagioclase with subordinate biotite, quartz, titanite, epidote, apatite, zircon, diopside, and microcline (Hansen 1971). The sample yielded a diverse population of clear, colorless to beige, anhedral, elongate zircon with thin overgrowths. Three fractions (Z1, Z3, and Z4) define a line, with two analyses overlapping concordia (fig. 2A). A fourth fraction, Z2, is optically similar to the other three fractions but overlaps concordia at a younger age of 1723 Ma. Regressing the two older concordant fractions with the discordant fraction Z1 defines a line with an upper intercept age of Ma (fig. 2A). If zircon is metamorphic in origin, this date would reflect the first metamorphic event of this region and a minimum protolith age for the supracrustal rocks of the Black Canyon area of Ga (table 1). We prefer this interpretation on the basis of the observation that primary igneous zircon does not commonly crystallize in mafic volcanic rocks. If zircon is primary instead, than the age of this sample is interpreted to represent crystallization of the mafic protolith coeval with the emplacement and deposition of metavolcanic and metasedimentary rocks of the Cochetopa succession ( Ma; Bickford and Boardman 1984; Bickford et al. 1989). Sample J01-BC7: Pitts Meadow Granodiorite. Pitts Meadow granodiorite is exposed over several square kilometers at the northwestern limit of the Black Canyon area and interpreted to reach batholithic dimensions in the subsurface (fig. 1; Hansen and Peterman 1968). Compositions range from granodiorite to diorite with some very mafic enclaves that consist predominantly of hornblende. Using whole-rock Rb-Sr geochronology, Hansen and Peterman (1968) originally interpreted a crystallization age of Ma. High-grade metamor-

6 560 Table 2. Fraction U-Pb Isotopic Data Concentration Measured Corrected atomic ratios a Ages (Ma) Weight (mg) U (ppm) Pb R Common Pb T (pg) 206 Pb/ 204 Pb 208 Pb/ 206 Pb 206 Pb/ 238 Ub 2j 207 Pb/ 235 Ub 2j 207 Pb/ 206 Pb 2j 206 Pb/ 238 Ub 207 Pb/ 235 Ub 207 Pb/ 206 Pb J01-BC5 amphibolite: Z1 4 sm frag ab Z2 2 med clr edges ab Z3 5 sm clr edges ab Z4 4 sm clr clrls frag J01-BC7 Pitts Meadow granodiorite: Z1 2 sm-med euh-sub cl Z2 2 sm bge clr sub pr Z3 2 sm bge clr sub pr T1 clr paleyel ang ab T2 brn abr ang T3 clr yel ang reabr T4 brn reabr J01-BC6 deformed pegmatite: Z1 single sm bge euh ab Z2 3 sm clr bge euh ab Z3 4 sm euh bge ab Z4 single sm euh bge ab K00-BC9 Vernal Mesa granite: T1 6 dk brn ang clr ab T2 10 lt yel ang block Z1 5 euh pink clr sm ab Z2 3 clr pink euh sm ab Z3 3 euh pink clr ab , Z4 1 euh clr ab Z5 single lg clr euh b J01-BC2 late pegmatite: Z1 single sm clr brn ab Z2 single dk brn med eu Z3 single sm dk brn eu Note. ab p abraded; ang p angular; b p best; bge p beige; block p blocky; brn p brown; cl p clear; clrls p colorless; dk p dark; eu p euhedral; frag p fragments; lg p larges; lt p light; med p medium; paleyel p pale yellow; pr p prisms; reabr p reabraded; sm p small; sub p subhedral; yel p yellow. a Ratios corrected for fractionation, 1 pg and.25 pg laboratory Pb and U blanks, respectively, and initial common Pb calculated using Pb isotopic compositions of Stacey and Kramers (1975). All fractions are zircons and are extensively abraded (Krogh 1973). 2j uncertainties on isotopic ratios are reported after the ratios and refer to the final digits.

7 Figure 2. U/Pb zircon (Z) and titanite (T) geochronology. 561

8 562 M. J. J E S S U P E T A L. phism of the host rock is interpreted to be associated with the emplacement of the Pitts Meadow granodiorite as shown by complex migmatized and gradational contacts with host rocks (Hansen and Peterman 1968; Jessup et al. 2005). The contact between the Pitts Meadow granodiorite and wall rocks is concordant and parallel to the NS-striking foliation of both wall rock and pluton (Hansen 1972). Pitts Meadow granodiorite is folded into a series of north-south-trending subhorizontal F2 folds (fig. 1; Hansen and Peterman 1968; Hansen 1971; Jessup et al. 2005). Sample J01-BC7 yielded a single population of beige, clear, slightly elongate, subhedral to euhdral zircon consistent with an igneous origin. Analytical data from three fractions (Z1 Z3) plot on and very near concordia, defining a poorly constrained line with intercepts of Ma and / 1900 Ma (fig. 2B; table 2). Given the large uncertainty of the lower intercept due to the clustering of the three analyses, we prefer to pin the lower intercept at Ma (as justified by a lack of evidence for an older resetting event; see sample J01-BC6, below) to derive a slightly older upper intercept of Ma that overlaps with the 207 Pb/ 206 Pb age of the single concordant fraction (fig. 2B; table 1). We interpret this age to reflect crystallization of the Pitts Meadow granodiorite. Four titanite fractions with 207 Pb/ 206 Pb ages ranging from Ma are not colinear and likely reflect some combination of Mesoproterozoic thermal resetting and/or recystallization of ca Ma titanite and more recent Pb loss. Sample J01-BC6: Pegmatite Dike in the Ute Trail Area. Sample J01-BC6 is from an isoclinally folded 0.5-m-wide pegmatite dike with a NS-striking, solid-state S2 foliation that is parallel to the foliation preserved within the Pitts Meadow granodiorite. This dike is interpreted to represent one of the oldest generations of pegmatite dikes and is characterized by small, irregular to podlike masses that were generally emplaced along the S1 foliation in the country rock, subsequently deformed during D2, and finally truncated by Mesoproterozoic igneous bodies (Hansen and Peterman 1968). In the Pitts Meadow area, many other 20-cm-wide pegmatite dikes are folded, locally boudinaged, and sinistrally sheared (Jessup et al. 2005). In the hinge of F2 folds, such as those exposed in Crystal Creek canyon, this early generation of pegmatite dikes is parallel to the S1 foliation and is folded by the north-south trending subhorizontal F2 folds. We relate these pegmatite dikes to an early metamorphic event in which migmatization occurred during or after the formation of S1 and possibly continued into the development of F2 folds. Sample J01-BC6 was collected 100 m east of sample J01-BC7 on the Ute trail (figs. 1, 2C). This sample yielded a simple population of equant, clear, colorless-to-beige, faceted zircon consistent with an igneous growth. Three zircon fractions (Z1, Z3, and Z4) define a line with intercepts of Ma and Ma. We interpret the upper intercept to reflect the age of crystallization (fig. 2C). A fourth fraction, Z2, does not plot on this line and is presumed to contain an inherited component. The lower intercept age reflects recent Pb loss likely related to Laramide events (table 2) and implies a lack of significant thermal events in the interval between ca and 50 Ma. The Ma crystallization age overlaps with that of the Pitts Meadow granodiorite and suggests that migmatization associated with D2 deformation was synchronous with emplacement of the Pitts Meadow granodiorite (table 1). Sample K00-BC9: Vernal Mesa Monzogranite. Sample K00-BC9 is from the Vernal Mesa monzogranite exposed in the Black Canyon National Park along the south rim loop road 75 m south of the Chasm View overlook (figs. 1, 2D). The Vernal Mesa monzogranite is a km-wide pluton with northeast-striking subvertical contacts that is interpreted to have been emplaced along the preexisting Black Canyon shear zone (Jessup et al. 2005). Hansen and Peterman (1968) originally interpreted the crystallization age of the Vernal Mesa monzogranite as Ma. They noted the presence of a strong foliation, defined by the preferred orientation of microcline phenocrysts, which is parallel to the near vertical contacts of the intrusion. We interpret the fabric as being formed primarily by magmatic flow after phenocrysts had formed with minor fabric development following matrix crystallization. The composition of the intrusion ranges from monzogranite to granodiorite, and the average modal composition is 43% plagioclase, 22% orthoclase, and 15% quartz with minor amounts of biotite, opaque iron minerals, epidote, titanite, hornblende, apatite, and calcite (Hansen 1971). This sample yielded a simple population of elongate (3:1 4:1), clear, colorless, euhedral zircon typical of igneous growth (fig. 2D). Three zircon fractions (Z2, Z4, and Z5) and one fraction of dark brown titanite (T1; not used in calculation) cluster near concordia and yield an average 207 Pb/ 206 Pb age of Ma (fig. 2D). Given the simplicity of the zircon population and their apparent igneous paragenesis, we interpret the averaged 207 Pb/ 206 Pb

9 Journal of Geology O R O G E N I C E P I S O D E S I N T H E B L A C K C A N Y O N 563 age of Ma age to represent the time of crystallization. Two slightly older fractions (Z1 and Z3) are interpreted to contain a small amount of inheritance. Light yellow titanite was also recovered from this sample and was observed to occur as rim overgrowths on a couple of grains with dark brown cores. The youngest concordant fraction of yellow titanite (T2) yielded an age of 1408 Ma (fig. 2D) and is interpreted to represent new titanite growth during a later thermal pulse, probably related to pegmatite dikes that crosscut the pluton (see below). Sample J01-BC2: Crosscutting Pegmatite Dike. Sample J01-BC2 is from a 2-m-wide pegmatite dike (268, 81 N) that crosscuts the Vernal Mesa monzogranite between the Cedar Point and the Painted Wall overlooks on the south rim drive (fig. 1). This pegmatite dike is one of several west- to northweststriking pegmatite dikes (average 292, 51 NE) that crosscut the northwestern contact between the Vernal Mesa monzogranite and migmatized quartzofeldspathic gneiss (fig. 1; Hansen 1971). These pegmatite dikes cut the magmatic foliation of the Vernal Mesa monzogranite as well as leucocratic layers within the migmatitic quartzofeldspathic gneiss host rock. Several large pegmatite dikes of this generation are exposed on the Painted Wall (fig. 3). The pegmatite dike orientation, with respect to the walls of the Vernal Mesa monzogranite and Black Canyon shear zone, suggests that the dikes were injected into tension gashes that formed nearly perpendicular to the maximum D4 extension direction (X-axis) of the strain ellipse (fig. 1C and cross section). Sample J01-BC2 yielded two concordant zircon fractions (Z2 and Z3) that define an age of Ma (fig. 4E). We interpret this as the crystallization age of this pegmatite dike and, thereby, the time of D4, predominantly dextral with a component of northwest-side-up, reactivation of the Black Canyon shear zone. Detrital Zircon Geochronology The Cebolla Creek quartzite is a northeast-trending synclinal keel with a basal conglomerate that contains predominantly white quartz pebbles with smaller fragments of red and black chert (fig. 1B; Olsen and Hedlund 1973). Whereas rocks of the underlying Dubois succession are pervasively deformed (fig. 4A), weakly deformed pebbles within the quartzite (fig. 4B) suggest that it unconformably overlies the Dubois succession (Jessup et al. 2005). The 207 Pb/ 206 Pb ages of 79 detrital zircon grains from the Cebolla Creek quartzite, analyzed by Figure 3. NW-striking pegmatite dikes exposed on the 600-m-high Painted Wall as viewed toward the northwest from the base of SOB gully. Pegmatite dikes crosscut both the migmatitic gneiss (mig) exposed on the Painted Wall and Vernal Mesa monzogranite (V.M.) in the foreground. Approximate location of contact between the migmatitic gneiss and Vernal Mesa monzogranite is highlighted by the dashed black line. These dikes are dated as Ma by this study and constrain the timing of dextral reactivation of the D3 Black Canyon shear zone during D4 intracratonic tectonism. laser-ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) techniques, show an age distribution between 2.95 Ga and 1.65 Ga with minor Archean-aged detritus (6% of grains analyzed) and a peak detrital age of 1.76 Ga (see app. B, available in the online edition or from the Journal of Geology office; Jones 2005). Hill and Bickford (2001) reported a similar range of detrital zircon U-Pb SHRIMP ages ( Ma; 1738 Ma average)

10 564 M. J. J E S S U P E T A L. Figure 4. A, Photomicrograph of the typical hornblende schist of the Dubois succession that underlies the Cebolla Creek quartzite conglomerate. Elongate hornblende (Hrn) and plagioclase define the pervasive foliation. Field of view is 3 mm wide. B, Photomicrograph of the Cebolla Creek quartzite conglomerate under plane polarized light. Relatively undeformed quartz (Qtz) pebbles are surrounded by a predominantly quartz matrix with subordinate amounts of chlorite, muscovite, and oxides. Field of view is 3 mm wide. from an unnamed quartzite exposed 8 km southeast of Cebolla Creek (fig. 1B). The youngest detrital 207 Pb/ 206 Pb ages from this study agree within error (ca Ma) with U-Pb geochronology from across the region, indicating that quartzite deposition occurred after ca Ma (Cox et al. 2002; Jones 2005). In Situ Monazite Geochronology and Metamorphism High-resolution compositional mapping, age mapping, and in situ microprobe dating of monazite, combined with textural and microstructural analysis, can help constrain the timing of metamorphic and deformational events (Williams et al. 1999; Williams and Jercinovic 2002). Complete methods are described in Williams and Jercinovic (2002) and Jercinovic and Williams (2005). The procedure involves (1) full thin-section mapping to locate and classify the setting of all monazite grains, (2) highresolution elemental mapping of selected monazite grains, (3) detailed wavelength dispersive spectrometer (WDS) scanning of each compositional domain to be dated in order to determine background and to evaluate potential interferences, (4) one or more major element analyses in each compositional domain, (5) repeated trace element analyses within each compositional domain until the required statistical precision is reached, and (6) calculation of one date and associated uncertainty for each compositional domain. The critical part of the method employed is that each monazite compositional domain is analyzed numerous times to produce a single date with an associated uncertainty (Williams et al. 2006). In essence, each domain is sampled repeatedly until the precision of the resulting date reaches an acceptable value or until all available domain area has been analyzed. Results are presented as a Gaussian distribution for each compositional domain. Then, weighted mean ages are computed for combinations of domain dates that are interpreted to represent specific monazite growth events. Age uncertainties for each monazite domain were determined as follows. Uncertainties associated with background intensities (as determined by the scan and regression method; Jercinovic and Williams 2005) were set at 1% based on repeated analysis of standards at the University of Massachusetts. Background uncertainty, peak counting uncertainty, and calibration uncertainty were propagated through the data reduction algorithm to yield an uncertainty estimate for trace element analysis. Individual trace element uncertainties were propagated to a single uncertainty associated with each weighted mean trace element analysis for each compositional domain. Finally, trace ele-

11 Journal of Geology O R O G E N I C E P I S O D E S I N T H E B L A C K C A N Y O N 565 ment uncertainties for each domain were propagated through the age equation to yield the uncertainty associated with each weighted mean domain age and are represented by Gaussian distribution plots. Weighted means of domain ages are calculated when monazite domains can be interpreted to represent a single geologic event. Laboratory monazite standards were run before and after each analytical session (Williams et al. 2006). To determine the age of metamorphic events in the host rocks to the Pitts Meadow granodiorite and Vernal Mesa monzogranite, samples MJBC-43 and MJBC-120 were selected for in situ monazite geochronology. A sample (MJBC-120) from the Crystal Creek canyon 0.5 km east of the Pitts Meadow granodiorite (fig. 1) contains sillimanite (fibrolite) K-feldspar biotite garnet in a matrix of plagioclase and quartz. Garnet porphyroblasts are subhedral and inclusion-rich. Fibrolite forms complex mats within the foliation plane. The coexistence of sillimanite, K-feldspar, and lack of muscovite suggests that a reaction such as muscovite quartz r sillimanite K-feldspar (1) Figure 5. Pressure and temperature estimates for two locations in the Black Canyon area shown as light (MJBC- 43) and dark (MJBC-120) gray polygons. Numbers refer to the following reactions: 1, muscovite quartz r sillimanite K-feldspar; 2, staurolite r garnet sillimanite biotite; 3, gedrite cummingtonite r garnet anthophyllite; 4, staurolite gedrite r cordierite garnet; 5, chlorite gedrite r cordierite anthophyllite (Spear 1995). may have been crossed, and, therefore, the rock reached peak temperatures above the second sillimanite isograd (fig. 5). Monazite grains from sample MJBC-120 typically contain core and rim domains (fig. 6A 6C; table 3). Results from three separate core domains are shown in figure 7. The weighted mean of the three core domains is Ma (fig. 7). Two separate rim domains were analyzed and yield dates of and Ma. Although these rim dates may represent a single metamorphic event, it seems likely that the two rims grew at different times, probably in response to local fluid circulation events. Importantly, there is no evidence for Mesoproterozoic thermal overprint in the western Black Canyon area. The second sample (MJBC-43) is from a garnet biotite plagioclase cordierite anthophyllite staurolite schist that is exposed 3.5 km southeast of the Vernal Mesa monzogranite on the south rim of the canyon (figs. 1, 8). The garnet, cordierite, and staurolite porphyroblasts are commonly concentrated along folded compositional layers. Cordierite garnet biotite anthophyllite occur in a single thin section, whereas staurolite was identified within another thin section 0.5 m from this sample. Anthophyllite commonly extends across the foliation (fig. 8A). Polygonal grain boundaries in monomineralic assemblages characterize the microstructure of the quartz and plagioclase matrix (fig. 8B). To estimate the maximum temperature and pressure for this sample, we used both the petrogenetic grid for the KFMASH system and for cordierite-anthophyllite rocks (fig. 5; Spear 1995). Together, these reactions define an overlapping stability field of C and 3 1 kbar (fig. 5), which we interpret to characterize the PT reached by this schist. Two monazite grains were selected for analysis from sample MJBC-43. One grain is an inclusion in garnet and the other is an inclusion in cordierite (figs. 6D, 6E; fig. 8; table 3). Both grains are essentially unzoned and, thus, were analyzed as single domains. The two inclusion grains yielded dates of and Ma for the garnet and cordierite inclusions, respectively. The weighted mean age is Ma (fig. 7). Note that uncertainties associated with these dates are larger than current microprobe monazite results (using new instruments and methods at the University of Massachusetts). However, these early results are sufficiently precise to constrain the two main ages

12 Figure 6. X-ray compositional maps of Y, Th, Ca, and U in three monazite grains from sample MJBC-120 (A C) and two from sample MJBC-43 (D, E). For monazite locations in MJBC-43, see figure 8. Uncertainties are 2j.

13 Journal of Geology O R O G E N I C E P I S O D E S I N T H E B L A C K C A N Y O N 567 Table 3. In Situ Monazite Geochronology Data Sample Analyses Y Th Pb U Age (Ma) MJBC-120: M3: Core 4 13,678 26, SD (2j) a Rim Ave 5 16,625 37, SD (2j) a M6: Core 4 12,944 29, SD (2j) a Rim 8 17,214 33, SD (2j) a M12: Core Ave 8 13,320 25, SD (2j) a MJBC-43: M1: Average , SD (2j) a M2: Average , SD (2j) a a Precision (2j), including counting statistics, background regression, and calibration uncertainty. See text for explanation. Note that uncertainties associated with these dates are larger than current microprobe monazite results using new instruments and methods at the University of Massachusetts. of metamorphism (Paleoproterozoic and Mesoproterozoic) in the Black Canyon area. 40 Ar/ 39 Ar Thermochronology New 40 Ar/ 39 Ar age determinations from hornblende (5), muscovite (6), and biotite (12) separates provide additional insight into the thermal history for south-central Colorado (figs. 9 11). Analytical methods and isotopic data are provided in appendix C in the online edition or from the Journal of Geology office and briefly described here. All minerals were analyzed by the age spectrum method and step-heated using either a double vacuum resistance furnace or a defocused CO 2 laser. Samples generally were either single crystals or bulk aggregates that typically contained!20 individual grains. Sample locations are shown as black stars in figure 1, and the data are summarized in table 4. The hornblende spectra are complex and yield highly variable apparent ages (fig. 9). Hornblende separates from the Dubois and Cochetopa successions of the eastern Black Canyon region are the oldest, with apparent ages as old as 1700 Ma (fig. 9A 9C). Samples SC97-17a and SC97-17b are from the same outcrop and display similarly complex spectra; SC97-17b is younger overall and yields a more complex K/Ca spectrum. Sample SC97-18 is from an amphibolite unit within a mafic/felsic layered package from the Cochetopa succession and yields a slightly less complex spectrum (fig. 9C). For these three hornblende samples, steps yielding younger ages generally correlate with relatively high K/Ca values, indicating that a fine-grained, relatively high-k, nonretentive phase contaminated the hornblende. Spectrum complexity stems from the variable degassing of these phases. Hornblende degasses in discrete temperature windows that are related to phase changes during in vacuo step-heating (e.g., Lee et al. 1991; Wartho et al. 1991). Thus, during discrete laboratory temperature ranges, the phase that preferentially degasses will variably dominate the age spectrum. Noncontiguous heating steps that yield the oldest ages and have similar low K/Ca values are used to assign an apparent age to some of these hornblende samples (e.g., fig. 9A). Although incomplete separation between the true closure age of the amphibole and contaminating phases may exist, based on the U/ Pb data from the region, the amphiboles were expected to yield 1700 Ma dates where Mesoproterozoic thermal effects are minor (e.g.,!450 C; Shaw et al. 2005). The apparent age spread for hornblende from the Dubois and Cochetopa successions ( Ma) represents variable degrees of contamination rather than recording variable closure times for the amphiboles. In contrast to the rocks of the Gunnison area, Paleoproterozoic amphibole-bearing rocks from the Black Canyon area record Mesoproterozoic ages (fig. 9D, 9E). Samples K01-BC-52 and K01-BC-53

14 568 M. J. J E S S U P E T A L. sample SC97-16a yields a fairly complex pattern and an assigned age of 1260 Ma (fig. 10; table 4). Sample K00-BC-5 and T01-RR3 were collected from within the Black Canyon shear zone and yield ages of and Ma, respectively (fig. 10A, 10B). In the northwestern corner of Black Canyon, samples K01-BC-50 and K01-BC-51 muscovite yield similar ages of and Ma, respectively (fig. 10C, 10D). The final two muscovite samples, SC97-16a and SC97-16c, are from the same outcrop near the Morrow Point area (fig. 1, 10E; fig. 10F). Sample SC97-16c has a fairly flat spectrum with a weighted mean age of Ma, whereas sample SC97-16a is much younger at 1260 Ma. Sample SC97-16c muscovite is a rim fragment of a coarse-grained ( 1 cm) crystal extracted from a pegmatite dike, and sample SC97- Figure 7. Electron microprobe analysis monazite summary for samples MJBC-120 and MJBC-43. The weighted mean of cores from sample MJBC-120 is Ma. The weighted mean of monazite inclusions from garnet and cordierite porphyroblasts in sample MJBC43 is Uncertainties are 2j. from the Pitts Meadow granodiorite are from the Ute trail area in the northwestern corner of the Black Canyon area (fig. 1). Spectra from these samples are complex, but overall, the K/Ca spectra are more consistent than SC97-17a, SC97-17b, and SC The lack of correlation between K/Ca values and apparent age, aside from the initial heating steps, suggests that the main cause for age spectrum complexity for samples K01-BC-52 and K01- BC-53 is not contamination by a fine-grained, K- bearing phase, but rather true internal age variations that are poorly represented by the age spectrum technique. Most of the spectra yield 1500 Ma dates and represent Paleoproterozoic hornblendes that are interpreted to have undergone partial argon loss during ca. 1.4 Ga reheating. Muscovite and biotite 40 Ar/ 39 Ar data from the Black Canyon region record apparent ages less than 1.4 Ga and bear on the cooling history following 1.4 Ga magmatism and metamorphism (figs. 10, 11). Five of the six muscovite separates from the Black Canyon area provide fairly flat spectra with plateau ages between 1356 and 1389 Ma, whereas Figure 8. Enlargement of thin section (4.75 mm # 2.5 mm) from sample MJBC-43. Locations of monazite M1 and M2 and their relationship to garnet and cordierite porphyroblasts used for pressure and temperature estimates for this sample. Inset A shows anthophyllite typical of this sample. Inset B shows polygonal quartz grains.

15 Journal of Geology O R O G E N I C E P I S O D E S I N T H E B L A C K C A N Y O N 569 Figure Ar/ 39 Ar age and K/Ca spectra for hornblende samples. 16a is a 1-mm single crystal from a much finergrained schist. The discordance of the two samples from the same outcrop is probably caused by grain size and perhaps recrystallization of the younger sample, as recorded by overall closure temperature variations of the muscovite. Twelve biotite separates yield the most variable age spectra of this sample suite (fig. 11). Most of the samples have weighted mean ages with very high MSWD values that attest to significant age scatter between individual heating steps. This scatter is attributed to substantial intracrystalline age heterogeneity rather than a reflection of alteration, and therefore, it is assigned a plateau age to compare and distinguish broad populations. Samples from within the Black Canyon shear zone have dates that range from 1073 to 1270 Ma with only one date being greater than 1200 Ma (fig. 11A, 11D 11F, 11J 11L). In contrast, samples to the northwest have biotite dates of 1291 and 1370 Ma (fig. 11B, 11C). The oldest biotite date is given by SC97-19 from the Cochetopa succession at Ma (fig. 11I). No age is assigned to SC97-18 (fig. 11H), also from the Cochetopa succession, as its complex spectra probably results from chlorite alteration (Lo and Onstott 1989) that is visible in the sample and also indicated by an apparent low K 2 O concentration (see table D1, available in the online edition or from the Journal of Geology office). Discussion To summarize the tectonic evolution of the Black Canyon region, we propose two generalized PTtD paths (figs. 12, 13). The older loop begins with inherited zircon from the Dubois succession that range in age from 1870 to 1840 Ma and provide evidence for the presence of older continental crust, of an unknown extent, under the metasedimentary and metavolcanic rock of this region (fig. 12A; Hill and Bickford 2001). An early metamorphic event is recorded by metamorphic zircon in amphibolite at Ma in the host rock to the Pitts Meadow granodiorite. Following our preferred interpretation of a metamorphic origin for zircon in this sample, the Ma age provides (1) a minimum protolith age for the metasedimentary rocks exposed in the Black Canyon area (fig. 12B) and (2) evidence that these rocks reached pressure and temperature conditions that enabled growth of metamorphic zir-

16 570 M. J. J E S S U P E T A L. Figure Ar/ 39 Ar age and K/Ca spectra for muscovite samples. con at this time (fig. 12C). The Ma monazite from sample MJBC-120 (supracrustal rock) and 1723 Ma metamorphic zircon age from amphibolite suggests that metamorphism began before or during emplacement of the Pitts Meadow granodiorite. Monazite rim dates may correspond to zircon dates from pegmatites and leucosomes in the Pitts Meadow granodiorite (i.e., during magmatism), but they may also represent postemplacement fluid circulation. We interpret these results to indicate one protracted, or several localized, Paleoproterozoic metamorphic/deformational events between 1741 and 1689 Ma, including the Ma emplacement of the Pitts Meadow granodiorite. The temperature conditions of this event exceeded the second sillimanite isograd (1650 C). We envision an orogenic episode ( Ma) that involved thermal, magmatic, and deformational pulses consistent with the regionally established Yavapai orogeny (fig. 12D). We therefore term this loop the Yavapai orogenic loop. Several aspects of this local record of the Yavapai orogeny should be highlighted. First, D2 deformation created northwest-striking folds that are interpreted to have resulted from the assembly of arcs across northwest-striking tectonic elements (Jessup et al. 2005; also see Albin and Karlstrom 1991; Duebendorfer et al. 2001). The culmination of this event is marked by tectonometamorphic pulses, emplacement of the Pitts Meadow granodiorite, pegmatite dikes, and metamorphism that reached 1650 C. Second, the northeast-striking Black Canyon shear zone overprinted the northwest fabric during D3 deformation. These data are in agreement with other areas such as the Homestake shear zone, Colorado and Grand Canyon, Arizona (Ilg et al. 1996; Shaw et al. 2001). Detrital zircon from the two quartzites that overlie the Dubois succession ranges in age from 1770 to 1712 Ma (Hill and Bickford 2001) and 2950 to 1650 Ma (this study). The youngest zircon population from our results ( Ma) provides a maximum depositional age for the Cebolla Creek quartzite of 1700 Ma. The quartzite is interpreted to rest unconformably on polydeformed rocks of the Dubois succession that were exhumed prior to quartzite deposition. Although the original extent of the sedimentary basins is unknown, monazite rim ages of 1689 Ma in the Pitts Meadow area, 70 km northwest of quartzite exposures (figs. 1,

17 Figure Ar/ 39 Ar age and K/Ca spectra for biotite samples.

18 Table 4. Sample 40 Ar/ 39 Ar Data Summary Mineral Plateau age (Ma) 1j (Ma) Area Age method n % 39 Ar MSWD TGA (Ma) 1j (Ma) Latitude Longitude L# K00-BC-9 B Black Canyon Plateau K01-BC-51 B Black Canyon Plateau K01-BC-53 B Black Canyon Plateau K02-BC-31 B Black Canyon Plateau K02-BC-32 B Black Canyon Plateau SC97-15 B Black Canyon Plateau SC97-16a B Black Canyon Plateau SC97-18 B Cochetopa TGA NA NA NA SC97-19 B Cochetopa Plateau T01-RR-5b B Red Rock Fault Plateau T01-RR-8b B Red Rock Fault Plateau T01-RR-9b B Red Rock Fault Plateau K01-BC-52 H Black Canyon Plateau K01-BC-53 H Black Canyon Plateau SC97-17a H Dubois Plateau SC97-17b H Dubois Plateau SC97-18 H Cochetopa Plateau K00-BC-5 M Black Canyon Plateau K01-BC-50 M Black Canyon Plateau K01-BC-51 M Black Canyon Plateau SC97-16a M Black Canyon Plateau SC97-16c M Black Canyon Plateau T01-RR-3m M Red Rock Fault Plateau Note. TGA p total gas age, n p number of steps for plateau, % Ar p amount of 39 Ar comprising plateau, MSWD p mean sum weighted deviates for plateau steps, L# p laboratory number, B p biotite, H p hornblende, M p muscovite, NA p not available.