The Mount Evans batholith in the Colorado Front Range: Revision of its age and reinterpretation of its structure JOHN N. ALEINIKOFF JOHN C. REED, JR. U.S. Geological Survey, P.O. Box 25046, Denver, 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- Colorado Mineral Belt), 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 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 10 15 J I Kilometers TERTIARY OR CRETACEOUS MIDDLE PROTEROZOIC EARLY PROTEROZOIC A _/v_ I A • ++++++ ++++++ ++++++ 1 2 1 2 3 Granodiorite '1.0 Ga plutonio Other ~1.4 Ga Mount Evans -1.7 Ga Metamorphic rocks rocks plutonio rocks batholith Plutonic 1. amphibolite and 1. granodiorite and rocks calc-silicate gneiss monzogranite 2. felsic gneiss 2. granite of Rosalie 3. biotite gneiss and migmatite Peak 60 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 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 MOUNT EVANS BATHOLITH, COLORADO 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. Geological Society of America Bulletin, June 1993 793 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 ALEINIKOFF AND OTHERS 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 Pikes Peak 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, 794 Geological Society of America Bulletin, June 1993 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 MOUNT EVANS BATHOLITH, COLORADO 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.