RICHARD LEE ARMSTRONG Department of and Geophysics, , New Haven, Connecticut 06520

K-Ar Dating of Laccolithic Centers

of the Colorado Plateau and Vicinity

ABSTRACT Two episodes of igneous activity occurred on the Plateau Province, and in some areas mafic volcan- Colorado Plateau during Phanerozoic time. Lacco- ism continues today. lithic centers whose ages cluster about the Cre- Both pyroxene and hornblende separates con- taceous-Tertiary boundary occur along the projec- sistently gave anomalously old K-Ar dates. It was tion of the Laramide mineral belt of Colorado as thus necessary to analyze whole-rock samples from far southwest as the Carrizo Mountains. Laccolithic which the mafic minerals were removed to obtain and mafic volcanic centers of mid-Tertiary age age determinations that did not conflict with occur scattered about, and on the edges of, the stratigraphic relations.

INTRODUCTION probably older than the mid-Tertiary volcanic episode (Luedke and Burbank, 1962). Recently One of the distinctive geologic features of Godwin and Gaskill (1964) have proven that the Colorado Plateau region is the occurrence the West Elk laccolithic cluster is Eocene or of widely scattered igneous centers, the roots of younger in age. K-Ar dating has made it possi- extinct volcanoes (Gilbert, 1880; Hunt, 1956). ble to demonstrate that other centers origi- Stocks and laccoliths are all that remain in nated during both of the stratigraphically many centers and they frequently intrude defined volcanic episodes, yielding an element Upper Cretaceous marine and nonmarine sedi- of victory to both Laramide and mid-Tertiary ments, but stratigraphic relations defining schools of thought. their exact age are lacking. The problem con- cerning their age has been reviewed by Witkind ACKNOWLEDGMENTS (1964). Along the eastern margin of the Pla- The dating studies were financed by National teau, volcanism is evident at two stratigraphic Science Foundation Grant GP 5383; the mass levels. Volcanic debris in the McDermott- spectrometer was provided by grants from the Animas and correlative units that straddle the Research Corporation and the Sheffield Scien- Cretaceous-Tertiary boundary marks the first tific School of Yale University. P. A. Scholle episode of igneous activity in the region. The assisted during sample collecting, and Paul second episode of volcanism occurred during Taylor assisted in the Ar laboratory. This paper the middle of the Tertiary, beginning about was written while the author was an Honorary the end of the Eocene and continuing vigor- Fellow in the Department of Geophysics and ously during Oligocene and Miocene time. , Australian National University. Mafic volcanism has continued sporadically until the present (Hunt, 1956; Luedke and ANALYTICAL METHODS Burbank, 1966). As a rule, a direct connection between laccolithic center and stratigraphically Atomic absorption spectrophotometric tech- dated volcanic unit has been severed by erosion, niques were used for K analyses and conven- and various authors have correlated the centers tional dilution techniques for Ar with one or the other or both igneous episodes, analysis (Armstrong, 1970). The precision of there being a tendency to consider all centers the dates is 2 percent (tr) or 100/( 100 -percent to be of the same age. Most geologists who have air correction) percent, whichever is greater. worked in Colorado have tended to favor a All dates are calculated using the constants Cretaceous-Tertiary boundary (Laramide) age KX0 = 4.72 X 10-10 yr-1, KX, = 0.584 X 40 (Witkind, 1964); laccoliths near Ouray are 10-io yr-i, K /K = 0.0119 atom percent.

Geological Society of America Bulletin, v. 80, p. 2081-2086, 1 fig., October 1969 2081

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Work began on purified mafic mineral sepa- Two Groups of Dates—Two Episodes of rates, but, when pyroxene and hornblende Igneous Activity dates were obtained that defied the strati- Disregarding the three anomalous mafic graphic relations observed in the field, the mineral dates discussed above, the dates for the crushed whole rock from which the mafic laccolithic centers fall into two groups, con- minerals had been removed by bromoform sistent with the regional geology: an older was analyzed. By X-ray these whole-rock sam- group (84 to 61 m.y.) spanning the Cretaceous- ples are shown to be essentially quartz, feld- Tertiary boundary and a mid-Tertiary group spar, chlorite, and sericite. The dates obtained (46 to 24 m.y.). with such material are probably a few percent low even in the absence of later alteration San Juan Region (Armstrong, 1970). In a few cases where no mafic minerals were present, crushed whole Three volcanic samples from the eastern rock was analyzed without further treatment. San Juan Mountains associated with the Another source of trouble with altered py- Creede caldera (Ratte and Steven, 1967) roxene and hornblende separates was difficulty give dates for the Fisher Quartz Latite of in gas purification using heated titanium 26.5 m.y. and for the Mammoth Mountain sponge. The cause of this is unproven, but Rhyolite 28.2 and 26.7 m.y., averaging 27.5 CC>2 is suspected to be partially responsible, m.y., in perfect agreement with dates of 26.4 as traces of calcite were present, and bromo- m.y. for the Fisher Quartz Latite and 27.2 form absorbed on expandable clays may also m.y. and 27.8 for units that overlie and under- contribute. lie the Mammoth Mountain Rhyolite reported by Steven and others (1967). Four dates from the western San Juan Mountains (Luedke and DISCUSSION Burbank, 1966) are unacceptable as all the samples are altered and the dates are discordant Anomalous Dates with regional stratigraphy. The pre-mid-Ter- Sample descriptions, analytical data, and tiary intrusive rock near Ouray gave discordant K-Ar dates are given in Table 1. The first mat- dates, neither date in agreement with the pub- ter of note is the discordance between mafic and lished date of 66 m.y. for volcanic rocks, whole-rock samples for several of the Colorado presumably derived from the Ouray area localities: Ute Mountain, Rico, and the La (Dickinson and others, 1968). These new dates Plata Mountains. A similar observation was indicate only that alteration and mineralization first made by Stern and others (1965), who ob- occurred during the Miocene in the Ouray and tained discordant dates for mafic mineral Silverton districts. McDowell (1966) has re- separates from the La Sal Mountains laccoliths. ported K-Ar dates of 24, 25, and 27 m.y. for In that case, the older dates were attributed to other localities in the San Juan Mountains. All xenolithic contamination, and a similar ex- work to date indicates a concentration of planation appears reasonable here. volcanic activity in the San Juan region 25 to Hornblende diorite inclusions were observed 30 m.y. ago. in the Ute, Rico, and La Plata localities, and, although they were deliberately excluded from Laramide Igneous Belt the material processed for dating, there exists Damon (1968) documents the importance of the possibility that contamination with such a distinct episode of Laramide igneous activity material, perhaps derived from Precambrian (80 to 50 m.y.) in the Arizona-New Mexico- basement, has occurred. In fact, the process of Colorado region. In the Colorado Plateau region mineral separation and concentration would the laccolithic igneous rocks at Ouray (geolog- be a bias in favor of xenolithic material that is ically dated), Rico, La Plata, Ute Mountain, usually less altered than groundmass mafic and the Carrizo Mountains are members of the minerals. Whole-rock samples for these three Laramide group. The most significant observa- localities, with mafic fraction removed, gave tion that can be made for these centers is that dates in more reasonable agreement with strati- they form a geographically coherent group, a graphic relations. A date of 120 m.y. for horn- lineament of sorts, that is an exact continuation blende from La Plata reported by McDowell of the Laramide mineral belt of central Colora- (1966) may likewise be anomalous due to do (see Fig. 1). The length and distinctive char- xenolithic contamination. acter of this belt of igneous activity is remarka-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/80/10/2081/3432747/i0016-7606-80-10-2081.pdf by guest on 25 September 2021 TABLE 1. SAMPLE DESCRIPTIONS, ANALYTICAL DATA, AND K-AR DATES FOR IGNEOUS ROCKS OF THE COLORADO PLATEAU-ROCKY MOUNTAINS REGION

Radiogenic Ar STP cc X 106 (percent air N. W. correction in Date Sample Locality Lithology* Latitude^ Longitudet Material Dated* o//o KK parentheses) (m.y.) Reference Colorado-San Juan region 735 Mammoth Mountain Pyroxene-biotite- 37°41' 0" 106°39'30" Biotite 6.13, 6.27 7.02 (22) 28.2 Ratte and Steven, 1967 Rhyolite, 2 mi andesite crystal, WNW of South vitric welded tuff Fork 737 Mammoth Mountain Biotite-andesite 37°46' 5" 106°47'12" Biotite 6.63, 6.60 7.09 (32) 26.7 Ratte and Steven, 1967 Rhyolite, 2 mi E. crystal, vitric of Wagonwheel Gap welded tuff 736 Fisher Quartz Hornblende-biotite- 37°46'15" 106°49'28" Biotite (6% Chi) 4.12, 4.12 4.40 (36) 26.5 Ratte and Steven, 1967 Lathe, Wagon- andesite porphyry wheel Gap 738 Older pluton SW. of Altered diorite 37°59' 5" 107°42'30" Altered mafic (Chi 0.91, 0.95, 1.87 (69) 49.3 Luedke and Burbank, 738 R Ouray, Canyon porphyry +clay) 0.95 1962 Creek Valley Whole rock less 2.17, 2.17 2.02 (34) 23.1 heavy fraction 739 San Juan Tuff, Altered andesite 37°58'40" 107°43' 0" Whole rock 1.73, 1.69 1.75 (61) 25.5 Luedke and Burbank, Canyon Creek tuff 1962 741 Silverton Series, Altered felsite 37°52'38" 107°43'27" Whole rock 4.71, 4.69 2.80 (52) 14.8 Burbank and Luedke, 1.5 mi S. of Red porphyry of Burns 1964 Mountain Pass Formation Colorado 744 Dike, 4 mi S. of Altered hornblende- 37°38'20" 108° 3'35" Pyroxene (25% Chi, 0.558, 0.545 4.09 (36), 179 Ransome, 1901 744R Rico diorite porphyry 10% Rk) 4.19 (36) Whole rock less 2.45, 2.42 6.06 (44) 61.3 heavy fraction 745 Ute Mountain, 3 mi Hornblende-diorite 37°14'22" 108°46'32" Hornblende (5% 0.800, 0.790 17.78 (10), 481 Ekren and Houser, 1965 745R S. of Ute Peak in porphyry Chi, 3% Rk) 16.99 (9) Sun Dance Cluster Whole rock less 1.56, 1.55 5.35 (51), 83.7 of Laccoliths heavy fraction 5.24 (53) 743P La Plata Mountains, Augite monzonite 37°23'20" 108° 4'40" Augite (5% Hbl, 0.492, 0.481, 1.75 (54), 85.5 Eckel, 1949 743 Madden Creek 6% Rk) 0.462 1.59 (54) Biotite (4% Chi, 6.86, 6.89 18.04 (21), 65.0 6% Hbl) 18.22 (19)

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Radiogenic Ar STP cc + 106 (percent air N. W. correction in Date Sample Locality Lithology* Latitude^ Longitudet Material Dated* %c/ v^ parentheses) (m.y.) Reference 730 Spanish Peaks-East Adamellite porphyry 37°25'20" 104°58'29" Whole rock 2.39, 2.41 3.83 (87) 39.5 Knopf, 1936 Spanish Peak Northern Arizona-New Mexico 746 Dike crossing New Vogesite 36°38'35" 108°49'40" Pyroxene (1% 1.27, 1.28 1.57 (51) 30.6 O'Sullivan and Mex. Hwy. 13 S. biotite) Beikman, 1963 of Shiprock 748 Carrizo Mountains- Hornblende-diorite 36°51'30" 109° 7'13" Hornblende (4% 1.53, 1.53 4.20 (31) 67.9 Strobell, 1956 748R Teec Nos Pas porphyry Rk) Canyon Whole Rock less 2.77, 2.78 7.67 (44) 67.9 heavy fraction 733 Ute Park-Cimarron Biotite-diorite 36°32'15" 105° 9'30" Biotite (25% Chi, 2.81, 2.79 3.81 (40) 33.8 Smith and Ray, 1943 Canyon Palisades porphyry 20% Rk) Utah 749A La Sal Mountains- Hornblende-diorite 38°34'17" 109°17'25" Pyroxene-hornblende 1.15, 1.17, 1.11 (57) 23.5 Hunt, 1958 Grand View porphyry (5% Rk) 1.21 Mountain 750 Abajo Mountains- Altered hornblende- 37°52'15" 109°27' 0" Altered mafic (Chi) 0.520, 0.531 0.576 (74) 27.4 Witkind, 1964 750R Horsehead diorite porphyry Whole rock less 1.89, 1.86 2.13 (63) 28.2 Laccolith heavy fraction 751 Bull Mountain, Hornblende- 38° 8'35" 110°43'30" Hornblende (17% 1.30, 1.32 2.31 (23) 43.7 Hunt, 1953 751R Henry Mountains monzodiorite Rk) porphyry Whole rock less 1.66, 1.70 3.40 (46), 48.0 heavy fraction 3.15 (42), 3.24 (42)

* Mineral modifiers used in rock names are given in order of increasing abundance. t Latitude and longitude were determined using the 1:250,000 AMS 1° X 2° topographic maps that provide complete coverage of the region. * Impurities in material dated are listed only where they exceed 2 percent. Abbreviations used: Chi for chlorite, Hbl for hornblende, Rk for rock.

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ble; it appears to have no counterpart in the world today. Three of the four dates arc closely grouped between 69 and 61 m.y., and this corresponds almost exactly with the geologic time scale position of the uppermost Creta- ceous-Paleocene volcanics ot southwestern Colorado (63-m.y. era boundary established by Folinsbce and others, 1961). The date for Ute Mountains of 84 m.y. may be correct; it seems to belong to the Laramide date population, but it may be slightly anomalous as it was from the same sample ior which an Ordovician date was obtained on a hornblende separate. It would be worthwhile to date additional samples from that area to test the reality of the result. Tertiary Igneous Activity Figure 1. Outline map of Colorado Plateau and vicinity showing the location of dated igneous centers. The laccolithic centers in Utah as well as two Triangular symbols identify the Laramide centers dis- centers within the Rocky Mountains and the cussed by Damon (1968) or specifically cited in the Shiprock volcanic neck belong to the mid- text of this paper. Circles indicate all localities for w hich I eniary volcanic episode. A date of 23.5 m.y. new dates are presented. The extension of the Laramide lor pyroxene from the La Sal Mountains agrees lineament of Colorado onto the Colorado Plateau is with the dates 23 and 26 m.y. reported by boldly evident. Stern and others (1965) as their best values, selected from anomalous results. The rock and rock vogesite dates 30.6 m.y., mid-Oligocene. hornblende dates of 48 and 44 m.y. for one of Pohlmann (1967) reported a date of 31 m.y. the Henry Mountains laccoliths indicate an for a vogesite sill in the Navajo Reservation, Loccnc age and make it one of the oldest Ter- and Watson (1967) reported a 27-m.y. date for tiary volcanic centers yet dated in the state minette in Monument Valley. (Armstrong, 1970). Altered mafic and whole- rock dates for the Abajo Mountains agree on an tipper Oligocene age (Tertiary time scale of SUMMARY Kvernden and others, 1964). The development of laccoliths is not a leal tire The East Spanish Peak laccolith (39.5 m.y.) characteristic of age but of tectonic setting and Cimarron Canyon laccolith (34 m.y.) of (Mudge, 1968). This is now clear for the Colo- the Rocky Mountains are late Locenc and rado Plateau region where dioritic magmas of early Oligocene, respectively, in age. The two different igneous episodes have produced Spanish Peaks are known to postdate mid- isolated clusters of laccoliths and stocks. Geo- Eocene sediments (Knopf, 1936). chronometric data have ended the controversy Several mafic volcanic dikes of the Colorado concerning the general age of these isolated Plateau region appear to be oi the same age as centers of igneous activity, but much detailed the diontic magmas that form laccoliths. Ship- work remains to be done.

REFERENCES CITED Armstrong, R. L., 1970, of Tertiary Dickinson, R. G., Leopold, E. B., and Marvin, igneous rocks, eastern Basin and Range Prov- R. F., 1968, Late Cretaceous uplift and volcan- ince, Nevada, Utah and vicinity: Geochim. et ism on the north flank of the San Juan Moun- Cosmochim. Ada. tains, Colorado (abs.): Geol. Soc. America Burbank, W. S., and Luedke, R. G., 1964, Geol- Spec. Paper 115, p. 415. ogy of the Iron ton quadrangle, Colorado: Eckel, E. B., 1949, Geology and ore deposits of U.S. Gcol. Survey Gcol. Quad. Map GQ 291. the La Plata district, Colorado: U.S. Geol. Damon, P. E., 1968, Potassium-argon dating of Survey Prof. Paper 219, 179 p. igneous and metamorphic rocks with applica- Ekren, E. B., and Houser, F. N., 1965, Geology tions to the Basin Ranges of Arizona and and petrology of the Ute Mountains, Colorado: Sonora, p. 1-71 in Radiometric dating for U.S. Geol. Survey Prof. Paper 481, 74 p. geologists: New York, Interscience. Evernden, J. F., Savage, D. E., Curtis, G. H., and

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James, G. T., 1964, Potassium-argon dates and Geology structure and deposits of the Cenozoic mammalian chronology of North the Ship Rock quadrangle, New Mexico and America: Am. Jour. Sci., v. 262, p. 145-198. Arizona: U.S. Geol. Survey Misc. Geol. Inv. Folinsbee, R. E., Baadsgaard, H., and Lipson, J., Map 1345. 1961, Potassium-argon dates of Upper Cre- Pohlmann, H. F., 1967, The Navajo Indian Nation taceous ash falls, Alberta, Canada: New York and Dineh hi Kayah: New Mexico Geol. Soc. Acad. Sci. Annals, v. 91, p. 352-359. Guidebook, 18th Field Conf., 1967, p. 63-69. Gilbert, G. K., 1880, Report on the geology of the Ransome, F. L., 1901, The ore deposits of the Rico Henry Mountains (2d ed.): U.S. Geog. and Mountains, Colorado: U.S. Geol. Survey 22nd Geol. Survey of the Rocky Mountains Region, Ann. Rept., 1900-1901, p. 229-398. 179 p. Ratte, J. C., and Steven, T. A., 1967, Ash flows Godwin, L. H., Gaskill, D. L., 1964, Post-Paleo- and related volcanic rocks associated with the cene West Elk laccolithic cluster, west-central Creede caldera, San Juan Mountains, Colo- Colorado: U.S. Geol. Survey Prof. Paper 501 rado: U.S. Geol. Survey Prof. Paper 524H, C, p. 66-68. 58 p. Hunt, C. B., 1953, Geology and geography of the Smith, J. F., and Ray, L. L., 1943, Geology of the Henry Mountains region, Utah: U.S. Geol. Cimarron Range, New Mexico: Geol. Soc. Survey Prof. Paper 228, 234 p. America Bull., v. 54, p. 891-924. 1956, Cenozoic geology of the Colorado Pla- teau: U.S. Geol. Survey Prof. Paper 279, 99 p. Stern, T. W., Newell, M. F., Kistler, R. W., and 1958, Structural and igneous geology of the Shawe, D. R., 1965, Zircon uranium-lead La Sal Mountains, Utah: U.S. Geol. Survey ages and mineral potassium-argon ages of La Prof. Paper 294-1, p. 305-364. Sal Mountains rocks: Jour. Geophys. Research, Knopf, Adolph, 1936, Igneous geology of the v. 70, p. 1503-1507. Spanish Peaks region, Colorado: Geol. Soc. Steven, T. A., Mehnert, H. H., and Obradovich, America Bull., v. 47, p. 1727-1784. J. D., 1967, Age of volcanic activity in the Luedke, R. G., and Burbank, W. S., 1962, Geology San Juan Mountains, Colorado: U.S. Geol. of the Ouray quadrangle, Colorado: U.S. Geol. Survey Prof. Paper 575D, p. 47-55. Survey Geol. Quad. Map GQ 152. Strobell, J. D., 1956, Geology of the Carrizo Moun- 1966, Volcanism in the western San Juan tains area in northeastern Arizona and north- Mountains, Colorado: Bull. Volcanol., v. 29, western New Mexico: U.S. Geol. Survey Chi p. 345-346. and Gas Inv. Map OM 160. McDowell, F. W., 1966, Potassium-argon dating Watson, K. D., 1967, Kimberlite pipes of north- of Cordilleran intrusives: Columbia Univ., eastern Arizona, p. 261-269 in Wyllie, P. J., Ph.D. dissert., 242 p. Editor, Ultramafic and related rocks: New Mudge, M. R., 1968, Depth control of some York, John Wiley & Sons, 464 p. concordant intrusions: Geol. Soc. America Witkind, I. J., 1964, Geology of the Abajo Moun- Bull., v. 79, p. 315-332. tains area, San Juan County, Utah: U.S. Geol. O'Sullivan, R. B., and Beikman, H. M., 1963, Survey Prof. Paper 453, 110 p.

MANUSCRIPT RECEIVED BY THE SOCIETY MARCH 13, 1969

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