The batholith in the : Revision of its age and reinterpretation of its structure

JOHN N. ALEINIKOFF JOHN C. REED, JR. U.S. Geological Survey, P.O. Box 25046, , Colorado 80225 ED DEWITT

ABSTRACT dicate that the batholith was emplaced at 1,442 dant by about 0.8%^2.1%, with a considerable ± 2 Ma and belongs to the Berthoud Plutonic spread in 207Pb/206Pb ages. Many of the zircons The Mount Evans batholith, in the central Suite. Most of the batholith has igneous tex- from these samples contain apatite, K-feldspar, Front Range of Colorado, is composed of a tures and structures, except in the vicinity of and quartz inclusions that appear to replace main phase of massive to conspicuously foliated the Idaho Springs-Ralston shear zone where zircon along cracks and imperfections from monzogranite and granodiorite and unde- those features are tectonically recrystallized rim to core. We suggest that these inclusions formed aplite and pegmatite. The Mount and foliated. Foliation elsewhere in the batho- formed during a Laramide ore-forming event Evans batholith was previously considered to lith is a flow structure. and incorporated Early and Middle Protero- be part of the 1.7 Ga Routt Plutonic Suite. New Zircons in two granodiorite samples, col- zoic radiogenic lead scavenged from the coun- U-Pb zircon ages on four samples (granodio- lected near the shear zone (just south of the try rock. The excess radiogenic lead caused the rite, monzogranite, and granite), however, in- ), are reversely discor- scatter and reverse discordance in the data.

106° 105° 40° o o Intrusive rocks of the N O Colorado mineral belt ® y«c

Sedimentary rocks

+ + + + + i- -i- + + + o o 1.0 Ga plutonic rocks N O ai O al Other~l.4 Ga ® plutonic rocks •o •o 2

Mt. Evans batholith

» * il • u 5 39° 30' 1.7 Ga plutonic rocks N O fe > wo CL > Metamorphic rocks (showing generalized <5 LU trends of foliation)

10 10 20 Miles s s I L. J I L ss Shear zone with 10 10 20 30 Kilometers Proterozoic movement _L_

Figure 1. Geologic map of the central Front Range of Colorado showing location of the Mount Evans batholith and its relations to other plutons and structural features (after Tweto, 1979; Bryant and others, 1981). Box outlines area of Figure 2.

Geological Society of America Bulletin, v. 105, p. 791-806, 16 figs., 2 tables, June 1993.

791

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Fault Idaho Springs- Foliation Mineral Location of Location of Ralston shear zone (inclined; vertical) lineation dated sample chemically analyzed sample

Figure 2. Geologic map of the Mount Evans batholith showing generalized structural trends. Compiled from Bryant and others (1981), Harrison and Wells (1959), Gobel (1972), Sheridan and Marsh (1976), and unpublished field sheets lent by Bruce Bryant. Box outlines area of Figure 5. Numbered triangles correspond to chemical analyses in Table 1; numbered dots correspond to dated samples in Table 2.

792 Geological Society of America Bulletin, June 1993

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Figure 3. Photomicrographs (crossed and uncrossed polarizers) of monzogranite from the main phase of the Mount Evans batholith showing mafic clots and tabular feldspar that define foliation. Note random orientation of biotite within mafic clots and lack of subsolidus deformation. Field of view is 8 mm across. Abbreviations: b (biotite), m (muscovite), q (quartz), o (oligoclase), s (sphene), a (allanite). Locality M9, Figure 2, near the summit of Mount Evans, Mount Evans 714-minute quadrangle.

Hie Mount Evans batholith is anomalous in INTRODUCTION tons, assigned to the Routt Plutonic Suite by composition and structure compared to most Tweto (1987), are characteristically calc-al- other 1.4 Ga plutons of the southwestern Basement rocks in the Front Range and kalic, tectonically foliated, and structurally United States. The differences probably reflect other Laramide uplifts in northern and central concordant and are generally considered to different sources of partial melting; the specific Colorado consist of Early Proterozoic meta- be syntectonic with respect to the principal tectonic setting where rocks of such disparate morphic rocks invaded by Early and Middle deformation of their wall rocks (Reed and oth- origin are temporally and spatially juxtaposed Proterozoic plutons of three general ages: ers, 1987). The -1.4 Ga plutons, the is not understood. -1.7, -1.4, and -1.0 Ga. The -1.7 Ga plu- Berthoud Plutonic Suite of Tweto (1987), are

Figure 4. Photomicrographs (crossed and uncrossed polarizers) of granodiorite from the main phase of the Mount Evans batholith near the projection of the Idaho Springs-Ralston shear zone. Note bent biotite and shear bands containing biotite altered to chlorite, new muscovite, and minor calcite. Same scale and abbreviations as Figure 3. Dated sample 3, Figure 2, along Colorado Highway 103 at elevation 9,000 ft, 0.2 mi S35°E of junction of Chicago Creek and West Chicago Creek, Georgetown 7'/2-minute quadrangle.

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Figure 5. Detailed geologic map along part of the western contact of the Mount Evans batho- lith showing relations of thejrocks of the batho- lith to the country rocks and to satellitic bodies of Silver Plume Granite. For location see Figure 2. Note rotation of structures in isolated block of wall rocks in east-central part of map with respect to structures in the main body of wall rocks. Trend of contact on detailed map differs somewhat from that shown in Figure 2 because of differences in scale of mapping.

characteristically alkali-calcic, nonfoliated or weakly flow foliated, and discordant. The —1.0 Ga plutonic rocks are chiefly nonfoli- ated alkali-calcic granite and syenite that compose the sharply discordant batholith and Lone Rock pluton (Fig. 1). Both groups of Middle Proterozoic plutons are commonly considered to be anorogenic (Anderson, 1983; Anderson and Thomas, 1985). The emplacement ages of the major groups of plutonic rocks were established by Rb-Sr whole-rock dating (Hedge and others, 500 1000 Meters 1967; Peterman and others, 1968) and U-Pb I J I I L I zircon dating (Stern and others, 1971). During 1000 2000 Feet these early studies, workers recognized that J I I I I I I I I I some of the Early Proterozoic plutonic rocks are only weakly foliated and some of the Mid- EXPLANATION dle Proterozoic rocks are conspicuously flow foliated, so that it is not always possible to Surficial Flow foliation in distinguish between —1.7 and —1.4 Ga rocks deposits /w granite without geochronology (Tweto, 1987; C. E. Hedge, 1990, oral commun.). Silver Plume Foliation In • N v The Mount Evans batholith is exposed Granite granodiorite over an area of about 225 km2 in the Front Range west-southwest of Denver (Fig. 1). Granodiorite of 'J'/'/M Mineral lineation The batholith is composed primarily of weak- the Mount Evans X in granodiorite ly to conspicuously foliated coarse-grained batholith 40 monzogranite and granodiorite (IUGS classi- Foliation in fication of Streckeisen, 1976) but also con- Gneissic pegmatite wall rocks X6 0 tains several small plutons of weakly foliated Mineral lineation in monzogranite (Fig. 2) called the granite of Biotite schist, \ wall rocks Rosalie Peak (Biyant and Hedge, 1978). Be- gneiss and 30 cause of its foliation, and its petrographic, migmatite / geochemical, and isotopic similarity (Hedge, Layering in 1969) to rocks of the -1.7 Ga Boulder Creek Fold axis 4. wall rocks batholith (dated by Peterman and others,

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1968; Stern and others, 1971; Premo and Van trending shear zones with complex Protero- components are segregated into small-scale Schmus, 1989), the Mount Evans batholith zoic and younger movement that apparently compositional layers parallel to the foliation. has been thought to be of Early Proterozoic controlled emplacement of the Laramide and A faint lineation expressed by elongation of age (Tweto, 1987). A recalculation (using the Tertiary plutons of the Colorado Mineral Belt the mafic clots is widespread but commonly program of Ludwig, 1991) of the Mount (Tweto and Sims, 1963). The shear zone dies difficult to measure. Biotite, hornblende, and Evans Rb-Sr data, however, yields an age of out to the southwest within the batholith minor muscovite in the mafic clots and layers 1,560 ± 520 Ma. The large uncertainty is due (Tweto and Sims, 1963; Tweto, 1979; Biyant are generally randomly oriented and unde- to a combination of factors, including limited and others, 1981; Graubard and Mattinson, formed (Fig. 3). Effects of subsolidus defor- enrichment of radiogenic Sr from only four 1990). mation are evident only in areas along the samples and scatter about the isochron. One The bulk of the Mount Evans batholith southwestern projection of the Idaho whole-rock analysis of the granite of Rosalie consists of two phases: (1) a main phase Springs-Ralston shear zone (Fig. 4) and in Peak indicated that it was about the same age of chiefly massive to conspicuously foliated local ductile deformation zones elsewhere. as the Mount Evans batholith (Bryant and coarse-grained porphyritic biotite-hornblende Foliation in the main phase of the batholith Hedge, 1978). granodiorite, locally ranging from monzo- has previously been interpreted as a tectonic The purposes of this paper are to (1) present granite to tonalite, and (2) an aplite-pegmatite fabric (Harrison and Wells, 1959; Hedge, new geochronologic results (using the U-Pb phase, which consists of massive to weakly 1969; Gobel, 1972). In most of the pluton, zircon method) that indicate that rocks of the foliated aplite and magnetite-bearing, two- strikes are roughly east-west and dips are Mount Evans batholith, including the granite feldspar pegmatite that form dikes, sills, and moderately to steeply north, generally paral- of Rosalie Peak, are part of the Middle Prot- irregular bodies that compose as much as 5% lel to foliation in the enclosing metamorphic erozoic Berthoud Plutonic Suite; (2) present of some outcrops. Some rocks in these bodies rocks (Fig. 2). Contacts of the batholith with structural data that suggest that the foliation resemble the granite of Rosalie Peak (Biyant the metamorphic rocks to the north and south in the Mount Evans batholith is a flow struc- and Hedge, 1978), which forms several small are conformable with the foliation. The ap- ture, not a tectonic foliation; (3) present mappable plutons in the southern part of the parent 90° discordance between the trace of chemical data that show that the Mount batholith (Fig. 2). the contact and the foliation trends along the Evans batholith is anomalous among the 1.4 The foliation that characterizes much of the northern part of the western contact (Fig. 2) Ga plutons in Colorado; and (4) discuss pos- main phase of the batholith is defined by align- is due in part to the intersection of the north- sible causes for complex U-Pb isotopic sys- ment of tabular K-feldspar phenocrysts and dipping contact with steep north-sloping to- tematics in zircons from granodiorite near the clots of biotite, hornblende, sphene, and pography and is partly an artifact of poor ex- Idaho Springs-Ralston shear zone and the opaque minerals. Locally mafic and felsic posure and sparse data. Detailed mapping Colorado Mineral Belt.

FIELD RELATIONS AND INTERNAL STRUCTURES

The Mount Evans batholith intrudes high- grade biotite gneiss, felsic gneiss, amphibo- lite, and calc-silicate gneiss. It is cut by dikes and irregular discordant bodies of biotite- muscovite monzogranite of the Silver Plume Figure 6. Xenolith of migmatitic bio- batholith (Harrison and Wells, 1959), dated at tite gneiss in foliated biotite granodiorite about 1,420 Ma (Hedge, 1969; Graubard and of the Mount Evans batholith. Inclusion Mattinson, 1990). Its relations to the Yankee contains pre-existing structures at high Creek pluton (also dated by Hedge, 1969, at angle to near vertical foliation in the en- about 1,420 Ma) are less certain. The contact closing granodiorite. The granodiorite is poorly exposed and may be gradational in also contains smaller mafic inclusions part, but locally the Yankee Creek cuts the aligned with the foliation. Scale is 15 cm Mount Evans (B. H. Biyant, 1989, personal long. Same location as Figure 4. commun.). Both the Mount Evans and the Yankee Creek are intruded by the —1.1 Ga Pikes Peak Granite (Hedge, 1969; D. Unruh, 1991, personal commun.) in the Lone Rock pluton (Fig. 2). The Mount Evans batholith lies at the southwestern end of the Idaho Springs-Ral- ston shear zone, one of a series of northeast-

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Figure 7. Photographs and line drawings of typ- ical bodies of aplite-pegmatite phase, subparallel to foliation in the main phase of the Mount Evans batholith. Medium stipple, main phase; white pat- tern, aplite-pegmatite phase; dark stipple, cover and debris; heavy short lines, foliation in main phase; light short lines, foliation in aplite- pegmatite phase. (A) Boulder with 32-cm-long rock hammer for scale; (B) close-up of part of A. See text for discussion. Located along Colorado Highway 5, 0.3 mi southeast of summit of Mount Evans, elevation 13,800 ft, Mount Evans IVi- minute quadrangle; near locality M8, Figure 2.

along part of the western contact (Fig. 5), however, shows that some of the discordance is real. Rocks of the main phase locally contain inclusions of wall rocks a few centimeters to several meters long containing metamorphic layering and foliation at high angles to folia- tion in the enclosing intrusive rocks (Fig. 6; Harrison and Wells, 1959; Hedge, 1969). These inclusions show no evidence of a sec- ond foliation parallel to that in the batholith. The detailed map (Fig. 5) shows an angular inclusion of wall rocks nearly 300 m long. Comparison of the orientations of structures in the nearby wall rocks with those in the included block suggest that the block has been rotated about 30° counterclockwise around an axis plunging 25°N55°W with respect to the wall rocks. Foliation in the rocks of the batho- lith is discordant with structures in this block. Foliation in aplite of the aplite-pegmatite phase of the batholith is generally defined by alignment of tabular K-feldspar phenocrysts and locally by leucocratic streaks and layers. Pegmatite associated with the aplite con- We conclude that foliation in the main Some bodies of aplite have massive to weakly tains large aggregates of magnetite, a mineral phase of the Mount Evans batholith is prin- foliated margins and strongly foliated cores that is abundant in rocks of the Mount Evans cipally a flow structure, not a tectonic feature, (Fig. 7); others have nonfoliated cores and batholith but lacking in pegmatite bodies as- because of foliated margins. In some places, foliation sociated with the Silver Plume batholith. Peg- (1) the undeformed character of the min- near a dike wall is parallel to the wall but matite is common along the margins of aplite erals in the mafic clots and the subhedral to sweeps smoothly through a continuous arc to bodies (Figs. 8B, 8C, and 8D), where it is euhedral shapes of the feldspar crystals that become perpendicular to the wall in the dike typically comb-textured with respect to the are aligned to form the foliation; interior (Figs. 8A and 8C). The character and adjacent main-phase rocks. Apparently initial (2) local discordance between foliation in geometry of the foliation in the aplite dem- pegmatite injection along open fractures was the batholith and structures in the metamor- onstrate that it is a flow structure. followed, and locally terminated by, emplace- phic wall rocks; In some places, aplite was emplaced after ment of aplite. Some pegmatite borders have (3) occurrence of inclusions of wall rocks formation of the foliation in the enclosing undergone minor buckling before emplace- that contain metamorphic structures discor- main phase rocks, as evidenced by aplite bod- ment of the aplite because foliation in the dant with foliation in the enclosing rocks of ies that cut across the fabric (Fig. 8B) and by aplite is at a high angle to comb layering in the the batholith and which lack any trace of sec- rotated inclusions of foliated main-phase pegmatite (Fig. 8D). Younger, discordant ond tectonic foliation parallel to the foliation rocks in the aplite (Fig. 8C). In other places, bodies of pegmatite are minor but show that in the batholith rocks; bodies of massive to strongly foliated aplite the latest pegmatite related to the Mount (4) relations between pegmatite, flow-foli- parallel foliation in the enclosing rocks Evans batholith is younger than some of the ated aplite, and foliated main-phase batho- (Fig- 7). aplite. lithic rocks, which show that no major pene-

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and quartz (2x4 mm) and phenocrysts of microcline and perthite (8 x 15 mm). Biotite, dark reddish-brown sphene, magnetite, par- tially metamict allanite, and minor horn- blende form the mafic clots that define the foliation discussed above. Sphene is variably resorbed and overgrown by additional sphene and minor epidote. Allanite is commonly overgrown by epidote. Magnetic susceptibilities of the batholith range from 2,000 to 8,000 x 10"5 SI units and average 5,000 x 10 5 SI units; this average corresponds to ~5 volume % magnetite, which causes a 900 gamma aeromagnetic high relative to surrounding rocks (Zeitz and Kirby, 1972). The most mafic parts of the batholith have biotite:hornblende ratios of 2:1 and contain as much as 1% sphene. Much of the batholith contains no hornblende but averages >0.25% sphene. Moderately per- aluminous rocks contain fine-grained primary muscovite within mafic clots. Although chiefly granodioritic, rock of the main phase of the Mount Evans batholith ranges from alkali-calcic diorite to granite in the classification scheme of De la Roche and others (1981; Fig. 9). Most of the batholith ranges in composition from tonalite to gran- odiorite and is much more mafic than all other 1.4 Ga plutons in the Front Range. Those samples that plot partially or completely in the granite field (samples M5, M9, and Mil) contain more K-feldspar than most rocks of the batholith and may represent feldspar ac- cumulation, not fractional crystallization of tonalitic to granodioritic magma. The granitic rocks could also be related to the granite of Rosalie Peak, whose age is indistinguishable Figure 7. {Continued). from that of the Mount Evans. The batholith has average Fe numbers (for example, [FeO + 0.89Fe2O3]/[FeO + 0.89Fe203 + MgO]; trative deformation could have intervened a limited zone along the southwestern pro- Table 1) as do all 1.4 Ga plutons in the Front between emplacement of the main phase of jection of the Idaho Springs-Ralston shear Range except the Fe-rich Sherman Granite the batholith and emplacement of the cross- zone does the fabric in the batholith appear to (Fig. 10). All rocks of the main phase except cuttingflow-foliated aplite and comb-textured have formed predominantly under subsolidus the most evolved granite and granodiorite are pegmatite; and conditions, as evidenced by mortar structure, metaluminous, in marked contrast to moder- (5) occurrence in weakly foliated granite of shear bands of biotite altered to chlorite and ately to strongly peraluminous 1.4 Ga plu- Mount Rosalie, shown below to be essentially new muscovite, and substantially strained tons, such as the Silver Plume and St. Vrain the same age as the main phase of the batho- quartz and feldspar. batholiths (Fig. 11). Other plutons of similar lith, of rotated inclusions of foliated main- mafic chemical composition in Colorado are phase Mount Evans rocks (Bryant and the Oak Creek pluton (1.44 Ga) in the Wet Hedge, 1978), also implying that no major MINERALOGY AND GEOCHEMISTRY Mountains (Noblett and others, 1987; Bick- episode of penetrative deformation separated ford and others, 1989) and the Vernal Mesa emplacement of the main phase of the batho- The bulk of the main phase of the batholith Quartz Monzonite (1.48 Ga) in the Black Can- lith from that of the granite of Mount Rosalie. is composed of medium- to coarse-grained, yon of the Gunnison (Hansen and Peterman, We ascribe the general parallelism between dark gray, equigranular to slightly porphyritic 1968; Bickford and Cudzilo, 1975). In Cali- foliation in the main phase of the batholith and (K-feldspar phenocrysts), foliated to massive fornia, granodiorite (—1.4 Ga) at Bowmans structures in the metamorphic wall rocks to biotite- and sphene-rich diorite to granodio- Wash in the Whipple Mountains (Anderson, the influence of wall rock anisotropy and(or) rite (chemical classification of De la Roche 1989) resembles the Mount Evans batholith in a nonisotropic stress field on flow directions and others, 1980; Fig. 9). Most rocks have a major-element chemistry (Fig. 12). during emplacement of the batholith. Only in groundmass of medium-grained oligoclase A characteristic feature of the Mount

Geological Society of America Bulletin, June 1993 797

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/6/791/3381767/i0016-7606-105-6-791.pdf by guest on 27 September 2021 Figure 8. Photographs and line drawings of typical massive to well flow-foliated main phase and irregularly shaped bodies of aplite-pegmatite phase in the Mount Evans batho- lith. Patterns as in Figure 5. (A) 6.6-m-high exposure of main phase cut by aplite-pegmatite; 32-cm-long rock hammer in lower center of photograph for scale. (B) Foliated to massive main phase cut discordantly by aplite dikes with pegmatite borders. Note 90° divergence of flow foliation in aplite dikes. (C) Flow-foliated main-phase aplite body having comb- layered pegmatite borders. Note rotation of fabric in small blocks of main phase that are cut by aplite dikes. Aplite contains layers of extremely leucocratic material that are de- flected 90° as the main sill splays into thin, vertical dikes. Flow foliation in aplite locally diverges 90° around small blocks of main phase. (D) Flow-foliated main phase cut by dike having pegmatite borders. Pegmatite has been buckled slightly along axis that parallels foliation in main phase. Aplite in core of dike has flow foliation at a high angle to axis of buckled pegmatite. See text for discussion. Located along Colorado Highway 5, 0.25 mi north of Lincoln Lake, elevation 12,500 ft, Harris Park 7V£-minute quadrangle.

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Flow- foliated

I to massive- ^J^^)

I pegmatite

Figure 8. (Continued).

Geological Society of America Bulletin, June 1993

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A Silver Plume O St. Vrain + Sherman • Mt. Evans 1500 x Mt. Rosalie O Vernal Mesa

Q\ 1.7-Ga Boulder Creek 3 Ü 1000 + ? Figure 9. R1-R2 plot (De la Roche and others, 1981) of 1.4 Ga plutons in the Front ©5 N Range, Colorado. Abbreviations: Di, di- + orite; Gd, granodiorite; Gr, granite; Mz, U« monzonite; Mz Di, monzodiorite, Q M, 500 quartz monzonite; Ton, tonalite. Bands de- o vs fining alkalinity from DeWitt (1989). Data II from this study, Bryant and Hedge (1978), bT Zielinski and others (1981), Anderson and Thomas (1985), and W. R. Hansen (1991, written commun.).

1000 1500 2000 2500 3000

Rj= 4000SÌ - ll,000(Na + K) - 2000(Fe + Ti)

Evans batholith is its high Sr concentration clase (the major Sr reservoir) was not a stable U-Pb GEOCHRONOLOGY (Fig. 13), a trait much more similar to 1.7 Ga phase. In terms of major elements, the one plutons in the Front Range, such as the Boul- sample of the granite of Rosalie Peak more Four samples (—35 kg each) from the main der Creek batholith, than 1.4 Ga plutons. The strongly resembles the Sherman and Silver phase of the Mount Evans batholith were col- high Sr contents may be due to partial melting Plume Granites than rocks of the Boulder lected for U-Pb analysis of zircon and sphene of a high-Sr parent rock or may be indicative Creek batholith (Fig. 9), but its Sr-rich nature (Fig. 2; Table 2). Dated rocks are massive to of derivation of the magma under pressure (Fig. 13) suggests that it belongs to the Mount slightly flow-foliated monzogranite from the and temperature conditions at which plagio- Evans suite of rocks. center of the batholith, strongly flow- and tec-

80 / I / Very Mg-rich Mg-rich I Average Fe-rich/ Very Fe-rich A

A A Silver Plume c 70 £ O St. Vrain u Q. +++ + Sherman — / / • Mt. Evans 7 J c x Mt. Rosalie O 60 - /O •b 53 O Vernal Mesa

Figure 10. Fe number versus Si02 plot of 1.4 Ga plutons in the Front Range, Colorado. Field 50 -• - I boundaries modified from those in DeWitt 0.5 0.6 0.7 0.8 0.9 1.0 (1989). (FeO + 0.89Fc CL)/(FcO + 0.89Fc O, + MgO) t J t *

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TABLE 1. MAJOR- AND MINOR-ELEMENT CONCENTRATIONS OF ROCKS IN THE MOUNT EVANS BATHOLITH, FRONT RANGE, COLORADO

Map no. MI M2 M3 M4 M5 M6 M7 M8 M9 MIO Mil M12 M13 M14* M15f Lab no.s 54 55 56 57 58 59 61 62 63 65 66 67 68

SÌ02 63.20 62.60 58.90 67.80 65.50 60.40 66.20 65.60 67.20 66.40 64.50 63.90 63.60 61.10 67.70 TiOj 1.03 1.09 1.40 0.79 0.67 1.24 0.72 0.79 0.69 0.76 0.95 1.07 1.09 1.34 0.63 A1203 14.90 15.20 15.10 14.60 15.80 15.00 15.30 15.10 14.60 15.00 14.70 14.30 14.90 14.50 14.20 Fe,Oj 6.52 6.65 8.88 4.67 4.05 8.02 4.53 4.76 4.55 4.94 6.29 7.06 6.89 8.82 4.57 MnO 0.08 0.10 0.12 0.06 0.03 0.10 0.07 0.06 0.07 0.05 0.09 0.10 0.08 0.12 0.06 MgO 2.00 2.12 2.60 1.20 1.33 2.53 1.55 1.09 1.50 1.14 1.89 2.09 2.03 2.05 1.21 CaO 3.87 4.32 5.15 2.97 2.43 4.61 3.13 2.46 2.61 2.15 3.13 3.47 3.80 4.51 2.45 Na20 3.01 3.15 3.10 3.13 3.18 3.03 3.37 3.05 3.21 3.10 2.98 3.02 3.21 3.07 2.84 K20 3.79 3.54 2.96 3.62 5.13 3.44 4.26 5.83 4.51 5.30 4.45 3.66 3.30 2.92 4.75 P205 0.41 0.41 0.56 0.26 0.24 0.49 0.24 0.28 0.23 0.31 0.31 0.44 0.45 0.49 0.25 LOI 0.35 0.39 0.36 0.42 0.59 0.51 0.27 0.34 0.41 0.39 0.31 0.24 0.25 0.60 0.43 Total 99.16 99.57 99.13 99.52 98.95 99.37 99.64 99.36 99.58 99.54 99.60 99.35 99.60 99.52 99.09

R1 2065 2028 1872 2420 1915 1910 2084 1784 2149 1939 2015 2123 2124 2041 2259 R2 806 866 976 664 636 913 712 613 640 581 717 755 800 869 601 A/CNK 0.93 0.90 0.86 1.01 1.04 0.88 0.97 0.96 0.98 1.02 0.96 0.94 0.95 0.89 1.00 Fe no. 0.75 0.74 0.75 0.78 0.73 0.74 0.72 0.80 0.73 0.80 0.75 0.75 0.75 0.79 0.77

Ba 1816 1919 1959 1610 3107 1681 1050 1723 1316 1509 1404 1323 1492 Rb 119 115 106 178 177 131 172 217 177 183 170 164 115 119 172 Sr 581 563 648 377 1022 649 400 501 396 469 467 434 529 577 417 Y 26 41 27 27 52 31 53 32 31 66 47 30 44 42 Zr 284 226 376 213 352 305 303 536 192 341 351 355 266 310 242 Nb 15 8 8 12 15 9 27 22 18 11 24 23 11 15 22 La 84 98 65 89 290 52 116 231 67 86 96 62 59 Ce 162 186 129 172 525 110 231 405 140 235 220 155 140

Note: map no. is number shown in Figure 2; Fe,03, all iron as Fe203; LOI, loss on ignition; R1 = 4,OOOSi - ll,000(Na + K) - 2,000(Fe + Ti); R2 = 6,OOOCa + 2,000Mg + 1.000A1; A/CNK, molar ratio of Al203/(Na20 + K20 + CaO); Fe no. = (FeO + 0.89Fe2O3)/(FeO + 0.89Fe203 + MgO); major-element analyses by X-ray fluorescence by J. E. Taggart, Jr., A. J. Bartel, and D. Seims; minor-element analyses by energy-dispersive X-ray fluorescence by Ross Yeoman. 'Dated sample A5606-3. * Dated sample A50918-2. sUSGS laboratory numbers preceded by D3218.

tonically foliated granodiorite (two samples) heavy liquids, and magnetic separator. Hand- with Daly multiplier for small signals. Two- from the northern margin (near the Idaho picked fractions of about 1 mg or less, com- sigma uncertainties in Pb/U (Ludwig, 1980, Springs-Ralston shear zone), and massive to posed of the more prismatic and elongate, 1991) of about 0.4%-0.6% reflect use of a slightly flow-foliated granite of Rosalie Peak. lighter colored, least-deformed grains, were 235U tracer in the larger fractions, whereas Granodiorite and monzogranite were ob- analyzed. Where possible, doubly terminated smaller splits were spiked with a 233U-236U tained from fresh roadcuts; the granite of Ro- crystals were selected. Five larger fractions, double spike, which lowered uncertainties to salie Peak, collected at an outcrop at about prepared by removing obviously broken or 0.13%-0.25%. Lead concentrations were de- 13,450 ft (near the summit of Epaulet Moun- damaged grains, were also analyzed. Urani- termined using a 208Pb tracer. tain), is somewhat weathered. um and lead were extracted using a modified Seven fractions of zircon, three of which All four samples yielded abundant zircon version of the procedure of Krogh (1973), and were abraded, from the monzogranite (sam- that was extracted using standard mineral the isotopic ratios were measured on a single- ple 1, A50918-2, Fig. 2, Table 2) contain light separation techniques including Wilfley table, collector VG Isomass 54E mass spectrometer brown prismatic zircon with length-to-width ratios (1/w) of 4-6. Nearly all are composed of euhedrally zoned cores and rims (Fig. 14A). The three abraded splits are about 2.8%- 6.3% discordant and form a linear array with Strongly Peraluminous o concordia intercept ages of 1,443 ± 2 and 216 1.2 ± 60 Ma (Fig. 15A). Data from the unabraded - fractions are more discordant and more scat- A A Silver Plume tered, have somewhat higher uranium con- . Mildly Peraluminous x a O St. Vrain centrations, and little variation in the Pb/U + Sherman U 1 •At ages (Table 2). We also analyzed a fraction • Mt. Evans composed of material abraded from the outer " x Mt. Rosalie 0.9 + parts of doubly terminated crystals (Aleini- 2k • + O Vernal Mesa Metaluminous m • o koff and others, 1990) in order to determine the age of the overgrowths (Fig. 15A). This fraction has a 207Pb/206Pb age of 1,421 Ma; it has more uranium and is more discordant (—10%) than the other fractions. A best-fit line calculated through the three data points Si0 (wt. percent) 2 representing abraded splits, plus the fraction

Figure 11. Si02 versus A/CNK plot of 1.4 Ga plutons in the Front Range, Colorado. A/CNK of overgrowths, has intercept ages of 1,442 ± is the molar ratio of A^OyYCaO + Na20 + K20). 1 and 185 ± 18 Ma. Because of the good

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TABLE 2. U-Pb ISOTOPIC DATA FROM ZIRCON AND SPHENE FROM ROCKS OF THE MOUNT EVANS BATHOLITH, COLORADO

Fraction* Wt. Concentration Pb composition* Ratios (percent error)5 Ages (Ma)** (mesh size) (rog) (ppm)

206 206 206 207 206 pb Pb »Pb pb 207pb pb Pb 207pb 207pb i l I U Pb ^Pb ™Pb ^ÏÏ »*Pb

1. A50918-2 (monzogranite; lat. 39°36'29", Ion g. 105°37'30") (-100+150JDEA 0.60 349.0 89.90 30036 10.982 6.9982 ,2425(.14) 3.029(.15) ,09059(.05) 1400 1415 1438 ( -100+150)DANM 5.45 447.8 111.0 11604 10.908 8.2588 ,2372(.43) 2.959(.44) .09048(.04) 1372 1397 1436 (-150+200JD 5.17 457.1 110.2 5883.3 10.783 8.4557 ,2307(.46) 2.875(.46) ,09037(.07) 1338 1375 1434 (- 150+200)DEA 0.21 421.8 103.2 5933.6 10.795 7.7841 ,2323(.19) 2.892(.21) ,09029(.09) 1347 1380 1432 (~200+ 250)D 5.54 464.9 111.9 20035 10.967 8.7022 .2318(.56) 2.892(.57) •09049(.05) 1344 1380 1436 (—325)D 6.17 527.6 126.1 8932.8 10.867 8.0800 ,2282(.49) 2.846(.49) •09046(.06) 1325 1368 1435 (-l00+150)DEDust 0.11 612.0 137.7 1090.5 9.7488 8.7923 ,2117(.09) 2.621(.12) ,08979(.08) 1238 1306 1421 sphene 0.60 65.17 18.06 710.18 9.2881 4.7535 ,2385(.29) 2.893(.41) .08799(.25) 1379 1380 1382 2. A50918-1 (granite of Rosalie Peak; lat. 39°34'21", long. 105°37'49") (+100)DEA 1.20 657.2 131.4 631.34 8.9840 4.4680 .1693(.14) 2.083(.16) ,08920(.08) 1008 1143 1409 ( -100+ 150JNMA 1.18 414.9 96.86 1984.5 10.279 5.7024 ,2113(.24) 2.630(.25) •09026(.06) 1236 1309 1431 (-150+200JDE 0.28 345.3 96.30 640.5 8.8991 3.6001 ,2268(.12) 2.834(.16) .09062(.09) 1318 1365 1439 (-150+200 EA 0.28 290.6 70.29 1897.5 10.220 4.9347 ,2142(.16) 2.673(.19) .09050(.09) 1251 1321 1436 3. R81201-2 (granodiorite; lat. 39°41'33\ long. 105°36'55") (-100+150)DE 0.70 222.0 61.22 3140.2 10.471 6.1181 ,2532(.26) 3.179(.27) .09107(.07) 1455 1452 1448 (—100+150)D 0.28 224.9 58.58 7089.1 10.790 6.8672 ,2439(.16) 3.051(.17) •09072(.06) 1407 1421 1441 (—100+ 150)DEA 0.60 225.2 63.31 7375.9 10.723 5.7050 ,2569(.13) 3.237(.15) •09137(.08) 1474 1466 1454 (-100+ 150)DEA 0.44 211.4 58.35 60445 10.958 5.8392 ,2540(.14) 3.188(.18) -09103(.10) 1459 1454 1447 (— 150+ 200)DE 0.89 204.4 56.06 9067.6 10.832 6.2320 ,2540(.13) 3.179(.15) ,09078(.06) 1459 1452 1442 sphene 0.91 43.56 14.88 329.36 7.584« 2.4851 ,2452(.24) 3.026(.41) ,08948(.30) 1414 1414 1415 4. A5606-3 (granodiorite; lat. 39°42'4 8", long. :105°36'24" ) (+100)DA 0.28 202.4 57.21 3093.4 10.459 5.5322 ,2559(.23) 3.215(.28) •09111(.15) 1469 1460 1449 (+100JDA 0.65 172.0 48.77 2156.7 10.262 5.1298 ,2530(.25) 3.174(.28) .09099(.ll) 1454 1451 1446 (~100+150)DE 0.94 202.3 55.02 13051 10.888 6.3218 ,2525(.13) 3.160(.15) .09077(.06) 1452 1448 1442 (—150+ 200)D 5.86 212.7 56.65 3201.9 10.497 6.4535 ,2462(.46) 3.087(.47) •09091(.06) 1419 1429 1445 sphene 3.20 88.91 35.84 470.20 8.3412 1.5185 ,2534(.23) 3.153(.26) .09024(.ll) 1456 1446 1431

235 235 Constants: X = 9.8485 E-10/yr; ^X = 1.55125 E-10/yr; ^u/ U = 137 gg (Steiger and Jägeri 'Abbreviations: D (diamagnetic), E (elongate), NM (nonmagnetic), A (abraded), Dust (rim material removed by abrasion). TBlank (<.1 ng) and fractionation (0.1%/a.m.u.) corrected. Blank lead composition is 1:18.7:15.6:37.2. §2

linearity of data (MSWD = 1.2) from hand- age of the rims (1,421 Ma) corresponds to the trending toward the same lower intercept of picked grains in abraded fractions, we con- timing of a geologic event (that is, emplace- —200 Ma. A split of handpicked, inclusion- clude that both the cores and rims of zircons ment of the Silver Plume Granite, as dated by free sphene (of probable igneous origin based in the monzogranite crystallized at about Graubard and Mattinson, 1990), because then on euhedral, wedge-shape morphology) is 1,442 Ma. It is unlikely that the 207Pb/2(16Pb the data should plot on a different chord nearly concordant at about 1,382 Ma. This

Mt. Evans

1500 Bowman's Wash Oak Creek Vernal Mesa

• All other 1.4-Ga

1000 ^

Figure 12. R1-R2 plot (De la Roche and others, 1981) of 1.4 Ga plutons in the south- western United States. Data from same sources as Figure 9, plus J. Stone and R. L. Cullers (1990, unpub. data), and DeWitt 500 (1991, unpub. data). Note mafic nature and low alkalinity of Mount Evans samples compared to all other 1.4 Ga samples. Same abbreviations as in Figure 9.

1000 1500 2000 2500 3000

Rj= 4000SÌ - ll,000(Na + K) - 2000(Fc + Ti)

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rim relationship displayed by zircons from the other three samples (Fig. 14B). Zircons in sample 4 (A5606-3, Fig. 2, Table 2), collected A Silver Plume near Ute Creek within the Idaho Springs- O St. Vrain Ralston shear zone, are dark brown, and + Sherman many have fractured cores. • Mt. Evans About 25% of the zircons from both gran- o Vernal Mesa odiorite samples have abundant dark and light elongate inclusions. Most of the inclusions ^ 17"Ga are solid, either partially or completely filling Boulder Creek void spaces; a small percentage are composed of fluids, some of which contain solid daugh- ter inclusions. The inclusions are at the edges

0 200 400 600 800 and in the middle of the grains (Fig. 16). This microtexture is not present in zircons from Sr (ppm) the granite or monzogranite samples collected Figure 13. Sr versus Rb plot of 1.4 Ga plutons in the Front Range, Colorado. to the south in the interior of the batholith. As discussed later, these tubes and inclusions may be caused by Laramide (Cretaceous- Tertiary) ore fluids because the granodiorite age is probably the time at which the monzo- through the four fractions has intercept ages samples are from a highly mineralized area granite passed through the closure tempera- of 1,448 ± 9 and 133 ± 53 Ma (Fig. 15B). This along the southeastern edge of the Colorado ture for sphene, estimated to be about 550 ± age is indistinguishable from our U-Pb results Mineral Belt. 50 °C (Hanson and others, 1971). for the massive monzogranite from the center Seven of nine fractions from the two gran- Zircons from the granite of Rosalie Peak of the Mount Evans batholith. odiorite samples are reversely discordant (sample 2, A50918-1, Fig. 2, Table 2) are light Zircons from both granodiorite samples (that is, plotting above the concordia curve, to medium brown, prismatic (1/w = 4-6), with collected near the northern margin of the Fig. 15C). Zircons in these seven fractions some pitting of faces. Most crystals contain batholith are prismatic (1/w = 2-4) and have were individually handpicked. In contrast, cores and overgrowths that are euhedrally euhedrally zoned cores and overgrowths, the other two splits, from which less than zoned. Isotopic data from four fractions are similar to zircons from the granite and ideal grains were removed, are about 1.5% more discordant (8.3%—30.6%) than results monzogranite. Zircons in sample 3 (R81202-2, and 2.4% normally discordant. 207Pb/206Pb from zircons in the monzogranite (Table 2). Fig. 2, Table 2), collected near Chicago ages of the seven reversely discordant frac- The two (-150+200) fractions contain signif- Creek, are clear to light brown, with slightly tions range from about 1,441 to 1,454 Ma, and icantly less U, and are less discordant, than rounded tips. Although overgrowths are both abraded and unabraded splits are re- the coarser fractions. Abest-fit line calculated present, these zircons lack the obvious core- versely discordant (by between 0.8%-2.1%).

Figure 14. Photomicrographs of zircons from dated samples. (A) Zircon from sample 1 (A50918-2, monzogranite). Note obvious overgrowths and acicular inclusions. Grains are about 150 jjim long. (B) Zircon from sample 3 (R81201-2, granodiorite). Note acicular and equant inclusions. Grains are about 150 |xm long.

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Figure IS. Concordia plots of U-Pb isotopic data from zircon in the Mount Evans batholith. (A) Monzogranite (sample 1) from interior of 0.260 batholith. Open symbols (larger than analytical error) are abraded fractions; filled symbols are unabraded. Regression through abraded splits only. Diamond is fraction of overgrowths. (B) Granite of Rosalie 0 256 Peak (sample 2). (C) Granodiorite from northern margin of batholith a? near Idaho Springs-Ralston shear zone (samples 3 and 4). Data plotted Q 252 as error ellipses. £ <0 oC\J

All zircons from these samples probably con- tain excess radiogenic Pb, which would ulti- 0.244 mately move the data up the discordia. Thus, grains that originally were more normally dis- 0.240 cordant will end up only slightly reversely 2.96

discordant (that is, the two fractions that were 207Pb/ 235 U not handpicked), whereas well-preserved (handpicked) grains with minimal discor- c dance will be driven to reverse discordance by the inclusion of whole-rock radiogenic Pb. We are unable to precisely determine the age ically, petrologically, and structurally from (Graubard and Mattinson, 1990; Graubard, of the granodiorites; we conclude that the zir- rocks of the nearby and only slightly younger 1991), formation of folds in some of the wall cons are about the same age as the granite of Silver Plume batholith, which is one of the rocks (Harrison and Wells, 1959; Moench and Rosalie Peak and monzogranite, based partly best known of the —1.4 Ga plutons in Colo- others, 1962), widespread retrograde meta- on similar 207Pb/,206Pb ages of the normally rado. The Silver Plume was derived by partial morphism (Sims and Gable, 1964; Gable and discordant fractions. A split of sphene from melting of anhydrous lower crust at depths Sims, 1969), and disturbance of isotopic sys- sample R81201-2 has a concordant age of greater than 36 km (Anderson and Thomas, tems (Peterman and others, 1968; Reed and 1,415 Ma, about 33 m.y. older than igneous 1985). Pb isotopic ratios suggest a mid-crustal Snee, 1991). Thus, not all of the -1.4 Ga sphene from the quartz monzonite to the source for the Mount Evans (Aleinikoff and anorogenic plutons of Silver and others (1977) south. Sphene from sample A5606 (within the others, in press). Emplacement of the two and Anderson (1983,1987) may be as "anor- shear zone) is reversely discordant, with a distinctly different plutons in the same local ogenic" as is widely supposed. 207pb/206pb age of 1430 Ma area requires tectonic processes that sampled The unusual reverse discordance in zircon two distinct sources at different depths during and sphene from the granodiorite samples lo- DISCUSSION AND CONCLUSIONS a time span of only —20 m.y. cated near the Idaho Springs-Ralston shear Although not syntectonic with respect to zone may have been caused by the presence The new geochronologic data indicate that the main regional deformation at —1.7 Ga as of inclusions, some of which are needle-like the Mount Evans batholith is part of the ~ 1.4 previously thought (Hedge, 1969; Bryant and apatite crystals (of presumed primaiy origin) Ga Berthoud Plutonic Suite, not the —1.7 Ga Hedge, 1978), emplacement of the Mount in zircon. Most inclusions, however, are Routt Plutonic Suite to which it was previ- Evans batholith seems to have been contem- equant and ovoid to irregular; some inclu- ously assigned (Tweto, 1986). Rocks of the poraneous with an important tectonic episode sions penetrate crystals from edge to interior batholith, however, are very different chem- that involved movement along shear zones along microfractures, and some embay exte-

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40t-'M

40'M

C D Figure 16. Photomicrographs of zircons from samples 3 and 4 showing inclusions. Abbreviations: A (apatite), K (K-feldspar), Q (quartz).

rior faces (Fig. 16). Some mineral inclusions New Brunswick, although they suggest that K-feldspar and apatite contain significant partly fill irregular tubes. These textures sug- the inclusions formed by rapid crystallization concentrations of common Pb. Thus, it is gest that the nonacicular inclusions are sec- followed by magmatic resorption. likely that these replacement mineral inclu- ondary replacements, not primary (that is, We suggest that the observed replacement sions in the zircons contain radiogenic Pb, igneous minerals), particularly because zir- textures are the result of corrosive fluids that derived from nearby 1.4 and 1.7 Ga rocks. con nucleates very early in the melt, forming dissolved parts of some zircons along pre- In the most reversely discordant fraction euhedral crystals, occasionally around other existing defects, such as cracks or metamict (206Pb/2MU age of 1,474 Ma), we calculate a euhedral grains. Three types of mineral in- zones. Although the rock is not pervasively minimum excess (beyond concordance) of clusions have been identified (by scanning altered, biotite is recrystallized to smaller about 1.5 ppm Pb. The amount of excess ra- electron microscopy-energy dispersive grain size and is slightly chloritized, and pla- diogenic Pb would be greater if the grains X-ray fluorescence): apatite (70%), K-feld- gioclase is slightly sericitized. The proximity were somewhat normally discordant before spar (25%), and quartz (5%). Major-element of the Idaho Springs-Ralston shear zone, Ter- formation of the inclusions. Scatter in 207Pb/ compositions were verified by electron mi- tiary intrusive rocks, and numerous ore-bear- 206Pb ages (1,441-1,454 Ma; Table 2; croprobe quantitative analysis; in addition, ing veins of Laramide age (probably about 60 Fig. 15C) is probably related to mixing of Pb we discovered one inclusion of albite. Similar Ma, Rice and others, 1982) to the granodiorite from sources of different ages. Zircons from textures were observed by Sullivan and van samples is additional evidence for hydrother- the interior of the batholith, many kilometers Staal (1990) in zircons from metarhyolite in mal activity in the area. In most crustal rocks, south of the shear zone, do not contain these

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replacement inclusions and are normally dis- Bowring, Karl Karlstrom, and Cinda Grau- Kessler, E. J., 1976, Rb-Sr geochronology and trace element geochemistry of Precambrian rocks in the northern Hualapai cordant in linear arrays with little scatter in bard are gratefully acknowledged. Mountains, Mojave County, Arizona [M.S. thesis}: Tucson, 207 206 Arizona, University of Arizona, 73 p. Pb/ Pb ages. We prefer the interpretation Krogh, T. E., 1973, A low-contamination method for hydrothermal of excess radiogenic Pb over the possibility of decomposition of zircon and extraction of U and Pb for iso- topic age determination: Geochimica et Cosmochimica Acta, loss of U for the cause of the reverse discor- REFERENCES CITED v. 37, p. 637-649. 207 206 Ludwig, IC. R., 1980, Calculation of uncertainties of U-Pb isotopic dance because of the scatter in Pb/ Pb Aleinikoff, J. N., Winegarden, D. L., and Walter, M„ 1990, U-Pb data: Earth and Planetary Science Letters, v. 46, p. 212-220. ages and because of the presence of Pb-bear- ages of zircon rims: A new analytical method using the air- Ludwig, K. R., 1991,1SOPLOT—A plotting and regression program abrasion technique: Chemical Geology (Isotope Geoscience), for radiogenic-isotope data: U.S. Geological Survey ing secondary inclusions. v. 80, p. 351-363. Open-File Report 91-445, 41 p. Aleinikoff, J. N., Reed, J. C., Jr., and Wooden, J. L., in press, Lead Moench, R. H., Harrison, J. E., and Sims, P. K., 1962, Precambrian From our integrated field, geochemical, isotopic evidence for the origin of Early and Middle Proter- folding in the Idaho Springs-Central City area, Front Range, ozoic rocks of the Colorado Province: Precambrian Research. Colorado: Geological Society of America Bulletin, v. 73, and geochronologic studies, we conclude the Anderson, J. L., 1983, Proterozoic anorogenic granite plutonism of p. 35-58. following. North America, in Medaris, L. G., Jr., Byers, C. W., Mick- Noblett, J. B., Cullers, R. L., and Bickford, M. E., 1987, Proterozoic elson, D. M., and Shanks, W. C., eds., Proterozoic geology; crystalline rocks in the and vicinity, central 1. Monzogranite of the main phase of the Selected papers from an International Proterozoic Sympo- Colorado: New Mexico Geological Survey Guidebook, 38th sium: Geological Society of America Memoir 161, p. 133-154. Field Conference, p. 73-82. Mount Evans batholith and the granite of Ro- Anderson, J. L., 1987, The origin of A-type Proterozoic magmatism: Peterman, Z. E., Hedge, C. E., and Braddock, W. A., 1968, Age of salie Peak were emplaced at about 1,442 Ma; A model of mantle and crustal overturn: Geological Society of Precambrian events in the northern Front Range, Colorado: America Abstracts with Programs, v. 19, p. 571. Journal of Geophysical Research, v. 73, p. 2277-2296. granodiorite of the main phase is probably the Anderson, J. L., 1989, Proterozoic anorogenic granites of the south- Premo, W. R., and Van Schmus, W. R., 1989, Zircon geochronology western United States, in Jenney, J. P., and Reynolds, S. J., of Precambrian rocks in southeastern Wyoming and northern same age. eds., Geologic evolution of Arizona: Arizona Geological So- Colorado, in Grambling, J. A., and Tewksbury, B, J., eds., 2. Field relations and textural evidence ciety Digest, v. 17, p. 211-238. Proterozic geology of the southern : Boul- Anderson, J. L., and Thomas, W. M., 1985, Proterozoic anorogenic der, Colorado, Geological Society of America Special Paper suggest that foliation in most of the batholith two-mica granites: Silver Plume and St. Vrain batholiths of 235, p. 13-32. Colorado: Geology, v. 13, p. 177-180. Reed, J. C., Jr., and Snee, L. W., 1991, 1.4-Ga deformational and is a flow foliation. The only evidence for a Bickford, M. E., and Cudzilo, T. F., 1975, U-Pb age of zircon from thermal events in the central Front Range, Colorado: Geo- Vernal Mesa-type quartz monzonite, Unaweep Canyon, logical Society of America Abstracts with Programs, v. 23, subsolidus tectonic foliation is in and near the west-central Colorado: Geological Society of America Bulle- no. 4, p. 58. Idaho Springs-Ralston shear zone. tin, v. 86, p. 1432-1434. Reed, J. C., Jr., Bickford, M. E., Premo, W. R., Aleinikoff, J. N., Bickford, M. E., Cullers, R. L., Shuster, R. D., Premo, W. R., and and Pallister, J. S., 1987, Evolution of the Early Proterozoic 3. Although similar in age, the Mount Van Schmus, W. R., 1989, U-Pb geochronology of Protero- Colorado province: Constraints from U-Pb geochronology: zoic and Cambrian plutons in the Wet Mountains and south- Geology, v. 15, p. 861-865. Evans batholith differs in chemistiy and ern Front Range, Colorado: Geological Society of America Rice, C. M., Lux, D. R., and Macintyre, R. M., 1982, Timing of structural habit from most 1.4 Ga plutons in Special Paper 235, p. 49-64. mineralization and related intrusive activity near Central City, Bryant, B., and Hedge, C. E., 1978, Granite of Rosalie Peak, a phase Colorado: Economic Geology, v. 77, p. 1655-1666. the western United States. Other rocks of of the 1700-miJ)ion-year-old Mount Evans Pluton, Front Sheridan, D. M., and Marsh, S. P., 1976, Geologic mapof the Squaw Range, Colorado: U.S. Geological Survey Journal of Re- Pass quadrangle, Clear Creek, Jefferson, and Gilpin Counties, possibly similar origin are the Oak Creek plu- search, v. 6, p. 447-451. Colorado: U.S. Geological Survey Geologic Map GQ-1337, ton and Vernal Mesa Quartz Monzonite. The Bryant, B., McGrew, L. W., and Wobus, R. A., 1981, Geologic map scale 1:24,000. of the Denver 1° x 2° quadrangle, north-central Colorado: Silver, L. T., Bickford, M. E., Van Schmus, W. R., Anderson, J. L., chemically diverse 1.4 Ga plutons preclude a U.S. Geological Survey Miscellaneous Investigations Series Anderson, T. H., and Medaris, L. G., Jr., 1977, The 1.4-1.5 Map 1-1163, scale 1:250,000. b.y. transcontinental anorogenic plutonic perforation of North common comagmatic origin, and thus, prob- De la Roche, H., Letemer, J., Grandclaude, P., and Marchal, M., America: Geological Society of America Abstracts with Pro- ably should not be considered a petrologic 1980, A classification of volcanic and plutonic rocks using grams, v. 9, p. 1176-1177. RjRj-diagram and major-element analyses—Its relationships Sims, P. K., and Gable, D. J., 1964, Geology of Precambrian rocks, suite. with current nomenclature: Chemical Geology, v. 29, Central City district, Colorado: U.S. Geological Survey Pro- p. 183-210. fessional Paper 474-C, 52 p. 4. Reverse discordancy in some zircons DeWitt, E., 1989, Geochemistry and tectonic polarity of Early Prot- Stacey, J. S., and Kramers, J. D., 1975, Approximation of terrestrial erozoic (1700-1750 Ma) plutonic rocks, north-central Ari- lead isotope evolution by a two-stage model: Earth and Plan- from the northern part of the Mount Evans zona, in Jenney, J. P., and Reynolds, S. J., eds., Geologic etary Science Letters, v. 26, p. 207-226. batholith may be caused by excess radiogenic evolution of Arizona: Arizona Geological Society Digest, Steiger, R. H., and Jäger, E., 1977, Subcommission on geochronol- v. 17, p. 149-164. ogy, convention on the use of decay constants in geo- and Pb in inclusions of apatite, K-feldspar, and Gable, D. J., and Sims, P. K., 1969, Geology and regional meta- cosmochronology: Earth and Planetary Science Letters, quartz formed during a Laramide mineraliza- morphism of some high-grade cordierite gneisses, Front v. 36, p. 359-362. Range, Colorado: Geological Society of America Special Pa- Stem, T. W., Phair, G., and Newell, M. F., 1971, Boulder Creek tion event. These inclusions may have scav- per 128, 87 p. batholith, Colorado, Part II—Isotopic age of emplacement Gobel, V., 1972, Geology and petrology of the Mount Evans area, and morphology of zircon: Geological Society of America enged radiogenic Pb from the surrounding Clear Creek County, Colorado [Ph.D. thesis]: Golden, Col- Bulletin, v. 82, p. 1615-1634. orado, Colorado School of Mines, 220 p. Streckeisen, A., 1976, To each plutonic rock its proper name: Earth Proterozoic country rocks; however, we can- Graubard, C. M., 1991, Extension in a transpressional setting: Em- Science Review, v. 12, p. 1-33. not confirm this and future studies will deter- placement of the mid-Proterozoic Mt. Evans batholith, central Sullivan, R. W., and van Staal, C. R., 1990, Age of a metarhyolite Front Range, Colorado: Geological Society of America Ab- from the Tetagouche Group, Bathurst, New Brunswick, from mine whether only zircon was altered by a stracts with Programs, v. 23, no. 4, p. 27. U-Pb isochron analyses of zircons enriched in common Pb, in Graubard, C. M., and Mattinson, J. M., 1990, Syntectonic emplace- Radiogenic age and isotopic studies: Report 3: Geological Pb-bearing fluid or whether the entire rock ment of the ~ 1440 Ma Mt. Evans pluton and history of motion Society of Canada Paper 89-2, p. 109-117. was modified. along the Idaho Springs-Ralston Creek shear zone, central Tweto, 0., 1979, Geologic map of Colorado: U.S. Geological Sur- Front Range, Colorado: Geological Society of America Ab- vey, scale 1:500,000. stracts with Programs, v. 22, p. 12. Tweto, O., 1987, Rock units of the Precambrian basement in Col- Hansen, W. R., and Peterman, Z. E., 1968, Basement-rock geochro- orado: U.S. Geological Survey Professional Paper 1321-A, nology of the Black Canyon of the Gunnison, Colorado: U.S. 54 p. ACKNOWLEDGMENTS Geological Survey Professional Paper 600-C, p. C80-C90. Tweto, O., and Sims, P. K., 1963, Precambrian ancestry of the Hanson, G. N., Catanzaro, E. J., and Anderson, D. H., 1971, U-Pb Colorado mineral belt: Geological Society of America Bulle- ages for sphene in a contact metamorphic zone: Earth and tin, v. 74, p. 991-1014. We thank Bruce Bryant for the loan of field Planetary Science Letters, v. 12, p. 231-237. Zielinski, R. A., Peterman, Z. E., Stuckless, J. S., Rosholt, J. N., Harrison, J. E., and Wells, J. D., 1959, Geology and ore deposits of and Nkomo, I. T., 1981, The chemical and isotopic record of sheets used in the compilation of Figure 2; the Chicago Creek area, Clear Creek County, Colorado: U.S. rock-water interaction in the Sherman Granite, Wyoming and Geological Survey Professional Paper 319, 92 p. Colorado: Contributions to Mineralogy and Petrology, v. 78, Carl Hedge, Zell Peterman, Bruce Bryant, Hedge, C. E., 1969, A petrogenetic and geochronologic study of p. 209-219. migmatites and pegmatites in the central Front Range [Ph.D. Zietz, I., and Kirby, J. R., Jr., 1972, Aeromagnetic map of Colorado: and Cinda Graubard for lively conversation thesis]: Golden, Colorado, Colorado School of Mines, 158 p. U.S. Geological Survey Geophysical Investigations Map and insight in the field; Marianne Walter for Hedge, C. E., Peterman, Z. E., and Braddock, W. A., 1967, Age of GP-880, scale 1:100,000. the major Precambrian regional metamorphism in the north- mineral separations and chemical extrac- em Front Range, Colorado: Geological Society of America Bulletin, v. 78, p. 551-558. tions; and Greg Meeker for technical assist- Hedge, C. E., Peterman, Z. E., Case, J. E., and Obradovich, J. D., ance on the electron microprobe. Critical 1968, Precambrian geochronology of the northwestern Un- MANUSCRIPT RECEIVED BY THE SOCIETY FEBRUARY 14,1992 compahgre plateau, Utah and Colorado: U.S. Geological Sur- REVISED MANUSCRIPT RECEIVED OCTOBER 7,1992 reviews by Zell Peterman, Paul Sims, Sam vey Professional Paper 600-C, p. C91-C96. MANUSCRIPT ACCEPTED OCTOBER 14,1992

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