RICHARD LEE ARMSTRONG Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520
Total Page：16
File Type：pdf, Size：1020Kb
RICHARD LEE ARMSTRONG Department of Geology and Geophysics, Yale University, 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. Geochemistry, 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 isotope 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 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 2082 R. L. ARMSTRONG—K-AR DATING, LACCOLITHIC CENTERS, COLORADO PLATEAU 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.