Potassium-Argon Geochronology of a Basalt-Andesite-Dacite Arc System: the Mount Adams Volcanic Field, Cascade Range of Southern Washington

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Potassium-argon geochronology of a basalt-andesite-dacite arc system: The Mount Adams volcanic field, Cascade Range of southern Washington

MAR^n'^A^^ANPHERE } • Geological Survey, Menlo Park, California 94025

ABSTRACT widely spaced episodes of peak productivity. erupted basaltic scoria and lavas that range Large stratocone systems may remain active widely in composition. In conjunction with High-precision K-Ar dating and detailed for half a million years. Subdivision of com- preparing a detailed geologic map (Hildreth mapping have established an eruptive chro- plex stratovolcanoes into eruptive or con- and Fierstein, 1995), we worked to establish nology for a Cascade stratovolcano and its structional "stages" without detailed map- an eruptive chronology for the whole vol- surrounding array of coeval basaltic cen- ping, geochronology, and compositional canic field, cone and periphery, through an ters. Mount Adams is a 200 km3 andesitic data should be treated skeptically. Discus- ambitious program of high-precision potas- cone that stands at the center of a coeval sion of volumetric eruptive rates can be mis- sium-argon (K-Ar) dating, the results of 1250 km2 Quaternary volcanic field that leading without an adequate time scale. which we report here. The present investi- contains >60 discrete vents. K-Ar ages were Stratovolcanoes need never develop large gation, using the multiple-collector mass measured for 74 samples from 63 of the 136 upper-crustal magma chambers. Basalt spectrometer (Stacey and others, 1981), may volcanic units defined in the course of the erupts peripherally, but its ascent is sup- be the most detailed K-Ar study yet under- mapping. Within analytical error, there are pressed beneath stratovolcanoes by deep- taken of an arc volcanic field where the strat- no discrepancies between K-Ar ages and crustal domains of magma storage and hy- igraphic record is well established. Previous stratigraphic sequence. Major activity be- bridization that form where concentrated attempts to date Quaternary stratovolca- gan in the area ca. 940 ka, and inception of injection of basalt amplifies crustal melting. noes and Quaternary basalt-andesite-dacite the central stratovolcano took place at ca. arc assemblages have met with mixed suc- 520 ka. A plot of cumulative volume erupted INTRODUCTION cess, although efforts comparable to the versus time shows that between 940 and 520 present study are underway in our labora- 3 ka the eruptive rate was <0.04 km /k.y. and Lofty composite cones loom large in per- tory for the Mazama (Bacon and Lanphere, —80% of the products were basaltic. Ande- ceptions of arc magmatism, but remarkably 1990) and Tatara-San Pedro (Dungan and sites are volumetrically dominant and were few of these conspicuous and abundant vol- others, 1993) arc systems. emplaced in three main cone-building epi- canoes have ever been investigated thor- An unnecessary pessimism is widespread sodes centered at 500, 450, and 30 ka—at oughly. Not uncommonly, more is known in the volcanological community concerning eruptive rates of 1.6-5 km3/k.y. At a lower 3 about the isotopic, phenocrystic, or trace- the feasibility of K-Ar age calibration of rate of 0.05-0.1 km /k.y., the magmatic sys- element composition of a stratovolcano Quaternary arc volcanoes, centered perhaps tem remained almost continuously active than about its stratigraphy, longevity, and around the observation that time intervals between the main pulses—although breaks eruptive history. For many active centers, between eruptions are commonly much in activity as long as 30 k.y. are permitted by the historical record is well established, and, shorter than the precision limits typical of the K-Ar data. Andesitic-dacitic activity in for a very few, detailed eruptive histories K-Ar age determinations for mafic and in- the focal region and dominantly basaltic ac- have been reconstructed as far back as the termediate rocks. The present results (Ta- tivity on the periphery have coexisted for limit of routine radiocarbon dating (ca. 40 ble 1) show that such pessimism is unwar- 520 k.y., and their products are interstrati- ka). For hardly any long-lived stratovolca- ranted, at least for a long-lived system. Our fied. The last main episode of cone construc- noes, however, do we have more than a relative success reflects the following fac- tion occurred ca. 40-10 ka, the oldest an- vague impression of what "long-lived" really tors: (1) Performance of the multiple-col- desite identifiably derived from within the means, much less any real measure of epi- lector mass spectrometer, as described in present-day edifice having an age of 33 ± 14 sodicity, fluctuation in eruption rates, or the Appendix. (2) Most products of the ka. Andesites forming the south-summit time-volume-composition relationships. Mount Adams volcanic field are fairly rich in rim and the true summit have ages of 13 ± K20. Although low-K lavas are also present, 8 ka and 15 ± 8 ka, respectively. Mount Adams, in southern Washington (Figs. 1 and 2), is one of the largest Qua- the dominant suite is the most potassic in The time-volume-composition data bear ternary stratovolcanoes (composite cones) the Cascades (Fig. 3); at equivalent Si02 upon several fundamental questions con- in the Cascade Range. The 200 km3 ande- contents, the Mount Adams suite has twice cerning the long-term behavior of arc vol- site-dacite edifice stands at the center of a the K20 content of rocks from Mount St. canoes. Stratovolcanoes commonly grow in coeval 1250 km2 volcanic field that contains Helens (Hildreth and Fierstein, 1985). (3) spurts but can stay active between the >60 discrete vents, many of which have Samples selected for dating were nonvesicu-

Geological Society of America Bulletin, v. 106, p. 1413-1429, 7 figs., 1 table, November 1994.

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structional stages, usually,: on the basis of structural, morphologic, or compositional evidence. Without adequate geochronology, the reality or integrity of Such stages, how- 47°N ever useful as a first measure, has to remain suspect: shifting vents and deposition over rugged terrain can make 103 and 105 yr un- conformities indistinguishable. (4) Strato- volcanoes are built up largely from the products of central-vent eruptions, but many also have flank vents, and some (like Mount Adams) are surrounded by extensive arrays of peripheral vents. Without good age con- While Swan trol one can seldom do better than speculate about possible relationships among the magmas erupted at central, flank, and peripheral locations. Assignment of undated flank-vent units to main-cone stages can be very tenuous. Neighboring cinder cones are sometimes called "parasitic," but rarely are the age-and-origin assumptions implicit in that word well founded. (5) Speculation about "cycles" of eruptive behavior, pro- gressive evolution in size or composition of a magmatic system, or stages in development of a volcano cannot be tested or investigated adequately without detailed age ' Port/and control. (6) Delineations within complexvol-

A MOUNT HOOD canic fields of petrologic suites, of lines of magmatic descent, or of unrelated but co- 122°W 121 °W existing magma types gain plausibility when scrutinized for age relationships as well as by Figure 1. Regional location map shows distribution of Mount Adams, Indian Heaven compositional criteria. (IHVF), and Simcoe Mountains (SMVF) volcanic fields and emphasizes four river systems that drain Mount Adams. Symbols: # = selected towns; A = large stratovolcanoes; -k = Even the best geochronology won't re- selected lesser volcanic centers, principally those that help define Quaternary volcanic solve all these problems, but extensive ap- front. In southern Washington, the Quaternary volcanic zone has an east-west width of 150 plication of precise dating methods—tied km. to detailed stratigraphic relationships—re- mains the most promising (and still under- lar, nondiktytaxitic, and (to the extent pos- vals in a volcano's past, and to estimate char- utilized) approach to understanding how sible) holocrystalline—taken generally from acteristic rates of magmatic differentiation. volcanic systems work. glass-poor massive interior zones of lava But good age control also enables us to ad- flows. Selection was based typically on thin- dress recurrent uncertainties about funda- VOLCANIC GEOLOGY section examination of several samples of mental processes and properties of mag- any particular unit, scrutinized to exclude matic systems. For example: (1) In thinking Geologic Setting porosity, glass, secondary minerals, and about subvolcanic plumbing systems, age in- posteruptive alteration. In some cases, re- formation is as important as composition for Mount Adams lies 50 km north of the Co- collection in successive field seasons was interpreting spatially separated eruptive lumbia River and 50 km east of Mount St. necessary to obtain appropriate material. units as comagmatic products of a common reservoir, the duration and configuration of Importance of Geochronology in Volcanology which are seldom definable. (2) If evolved • magma (say, dacite) has erupted repeatedly Comprehensive geochronology enor- at a particular volcano, age and composi- Figure 2. Location map for Mount Adams mously enhances the value and interpret- tional data are equally essential for assessing volcanic field. Abbreviations: PP = Pikers ability of stratigraphic, geochemical, and whether such behavior represents small Peak (south summit); SC = Sunrise Camp; petrologic data sets for any complex volcan- batches recurrently injected from the deep TS = The Spearhead; Ck = Creek; FK = ic field. Good age control obviously im- crust or intermittent tapping of a shallow Fork. • = summit of Mount Adams. A = proves our ability to recognize past episodes fractionating chamber of considerable size approximate center of deeply eroded Hell- of elevated (or diminished) eruptive fre- and longevity. (3) Eruptive products of long- roaring stratovolcano (ca. 520-490 ka). quency or volumetric output, to evaluate the lived stratovolcanoes are sometimes sub- Small • = lesser vents. IHVF = Indian duration and significance of quiescent inter- divided and assigned to a number of con- Heaven volcanic field.

1414 Geological Society of America Bulletin. November 1994

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Helens (Fig. 1). Towering 3 km above neigh- TABLE 1. K-Ar AGES AND ANALYTICAL DATA FOR ROCKS FROM THE MOUNT ADAMS VOLCANIC FIELD boring valleys, its volume is surpassed only m Focal area Map K20 (wt%) Ar' r.iij, Calculated by that of Mount Shasta among the Quater- label nary Cascade stratovolcanoes. The central Sample Eruptive unit (10-»3 mol/g) (%) no. cone, predominantly andesitic, covers about 2 600 km , and the peripheral volcanic field, MA-63 andesite of South Butte asb 1.338 ± 0.002 0.2292 0.3 12 + 17 MA-44 andesite of Suksdorf Ridge ask 1.877 ± 0.013 0.2466 0.3 10 ± 16 largely basaltic, covers an additional 650 MA-51 andesite of Mount Adams summit aas 2.464 ± 0.009 0.5230 1.1 15 + 8 2 km . The area is near the center of an ex- MA-56 andesite of Pikers Peak (south summit) app 1.873 ± 0.007 0.3430 0.9 13 ±8 MA-246 andesite of Pikers Peak (Ridge of Wonders) app 1.588 ± 0.010 0.7548 1.7 33 ± 14 traordinarily extensive region of fundamen- MA-15 andesite of Big Spring Creek abs 2.349 ± 0.007 0.9647 3.4 28 ± 6 MA-484 andesite of Devils Gardens adg 1.981 ± 0.017 1.046 3.6 37 ± 8 tally basaltic Quaternary volcanism in south- MA-504 andesite of West Fork awf 2.520 ± 0.008 2.007 6.0 55 ±7 ern Washington (Luedke and others, 1983; MA-401 andesite of Morrison Creek amc 2.100 ± 0.005 1.706 8.3 56 ± 6 MA-83 basalt of Riley Creek brc 1.521 + 0.005 1.383 2.2 63 ± 14 Walsh and others, 1987). The Mount Adams MA-204 dacite of Cascade Creek dcc 3.333 ± 0.030 4.659 1.7 97 ±22 (vent agglutinate) 4.287 1.6 89 ± 22 volcanic field (>60 vents; 0.9-0 Ma) over- MA-336 dacite of Cascade Creek (intracanyon flow) dcc 3.428 ± 0.009 5.744 15.2 117 ±6 laps eastward with the somewhat older Sim- MA-965 dacite of Bird Creek Meadows dbc 2.541 ± 0.004 4.221 16.5 115 ± 5 MA-413 andesite of Parrott Crossing ape 2.619 ± 0.002 4.528 13.2 120 ±5 coe Mountains volcanic field (>80 vents; MA-354 andesite of Killen Creek (Horseshoe Lake) akc 2.339 ± 0.003 4.040 13.2 120 + 7 4.5-0.6 Ma; Sheppard, 1967a; Uto and oth- MA-124 A andesite of Twin Falls Creek atf 2.537 ± 0.005 5.022 16.1 137 ±6 MA-1011 andesite of Foggy Flat aff 2.103 ± 0.014 4.588 11.4 152 ±8 ers, 1991), and it merges southwestward into MA-290 andesite of Crofton Ridge acr 1.804 ± 0.008 4.150 5.9 160 ± 12 MA-1008 dacite of High Camp dhc 2.904 ± 0.006 8.892 26.9 213 ± 5 the largely coeval Indian Heaven volcanic MA-122 dacite of Adams Creek dac 3.592 ± 0.017 12.05 17.3 233 ± 6 field (>40 vents; ca. 0.8-0 Ma; Hammond MA-355h dacite of Sheep Creek dsc 3.282 ± 0.015 11.64 39.9 246 ±4 MA-356 andesite of Lewis River alw 2.837 ± 0.010 10.27 29.9 251 ± 6 and others, 1976; Korosec, 1989). The re- 10.23 26.6 250 ± 7 . MA-838 dacite of Lewis River dir 3.178 + 0.013 11.55 13.6 252 ±8 gion is strikingly anomalous in that low-K MA-991 dacite of Hellroaring Ditch dhd 3.468 ± 0.013 15.03 37.5 301 ± 6 tholeiite, calcalkaline basalt, and alkalic ba- 15.43 24.2 309 + 8 MA-1035 andesite west of Muddy Fork awm 2.401 + 0.005 11.35 6.2 328 ± 21 salt coexist across virtually the entire 150 km MA-144 andesite of Lookingglass Lake all 1.988 ± 0.007 9.431 13.6 329 ± 11 MA-174 andesite of White Salmon River aws 2.717 ± 0.005 12.53 37.0 320 + 7 width of the Quaternary volcanic zone (Lee- (top flow distally) man and others, 1990). MA-331 andesite of White Salmon River aws 2.805 ± 0.035 18.41 46.0 456 ± 8 (Cascade Creek base) MA-408 andesite of White Salmon River aws 2.118 ± 0.004 13.94 27.2 457 ± 11 The continental crust under the Mount (base near Buck Creek) MA-477 dacite of The Spearhead dts 2.025 + 0.006 13.81 16.1 474 ± 12 Adams region is 40-45 km thick (Mooney MA-523 andesite of Hellroaring Volcano ahv 1.831 ± 0.013 13.23 35.0 502 ± 9 and Weaver, 1989), but its composition is (Big Muddy Creek) 12.24 37.4 464 ± 14 MA-967 andesite of Hellroaring Volcano ahv 2.241 ± 0.020 16.50 33.2 511 ± 9 poorly known. Beneath the Quaternary vol- (Shadow Lake Quarry) MA-365 andesite of Hellroaring Volcano ahv 2.875 + 0.014 21.35 24.0 516 ± 10 canic field, exposed rocks consist almost en- (Hellroaring Meadow) tirely of Miocene lava of the Columbia 10 1 -10 _1 4 River Basalt Group and Oligocene to Mio- *Xe = 0.581 X 10~ yr ; \3 = 4.962 X IO yr ; ""K/K = 1.167 x 10~ mol/mol. cene Cascade-arc assemblages (Walsh and others, 1987). The inclined seismic zone that dips beneath coastal Washington (Weaver Lake Valley, (2) still older centers near that supports it lies above the central and Baker, 1988) is last detectable -100 km the Klickitat River, or (3) vents in the In- "focus" of a broader swarm of distributed west of the Mount Adams region, which is dian Heaven volcanic field. intrusions of mantle-derived basalt (Hil- virtually aseismic. Much of what has been Except on the stratocone itself, nearly ev- dreth and Moorbath, 1988; Hildreth, 1981, established about the complex tectonic set- ery unit that erupted inside the corridor, ir- Fig. 15, p. 10183). It is important to ap- ting is summarized by Swanson and others respective of age, consists of basalt or olivine preciate that andesite-dacite activity in the (1989), Smith (1993), and Hildreth and andesite. Within 5 km of the summit, on the focal region and coeval basaltic activity on Fierstein (1995). other hand, only a single basalt is known to the periphery have coexisted since ca. 0.5 have penetrated the andesitic focus during Ma. Their products are interstratified, and Volcanological Overview the half-million-year existence of the strato- even substantial basaltic shields like King volcano. Although basaltic vents are also Mountain and Goat Butte are sandwiched The stratovolcano is the largest center scattered elsewhere, the vent corridor re- between andesitic lavas from Mount along a north-trending zone of vents flects a zone of especially concentrated in- Adams. (Hammond, 1980) that has produced jection of the crust by mantle-derived basalt. The original focus of the stratovolcano lay >90% of the volume of the Mount Adams For the past 500 k.y., the volumetrically larg- —5 km southeast of the modern summit, volcanic field (Hildreth and Fierstein, est throughput of magma in Washington has where an andesite-dacite edifice at least as 1995). This 6-km-wide vent corridor ex- taken place along this corridor. The greatest large as the present-day cone was centered tends >50 km north-south and contains eruption rate of all has been near its mid- in middle Pleistocene time. The gutted rem- —50 recognized sites of eruption (Fig. 2 section, at Mount Adams, beneath which nants of this Hellroaring volcano are well and Fig. 5, below), including vents for vir- fractionation, assimilation, and mixing in exposed in Hellroaring and Big Muddy tually all of the Holocene (<10 ka) and the deep crust are maximized—yielding vo- Creeks, where lava flows and fragmental de- late Pleistocene (132-10 ka) volcanic units luminous derivative magmas and effectively posits dip radially away from a hydrother- mapped—and for most of the older ones suppressing ascent of basalt. The stratocone mally altered, dike-ridden core (Fig. 2 and as well. Nearly all units that originated is thus said to be "focal" to a more diffuse Fig. 4B, below). outside the corridor erupted from (1) mid- array of basaltic vents, just as the lower- The modern summit cone (3742 m; dle Pleistocene vents around the Trout crustal zone of melting and hybridization 12276 ft) is a much younger edifice. Be-

1416 Geological Society of America Bulletin. November 1994

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TABLE 1. (Continued.) from Mount St. Helens —3500 yr ago (layer Ye; Mullineaux, 1986). All eight of the Hol- 40 A r Peripheral vents Map K20 (wt%) Arrad Calculated* age (ka) label ocene units consist predominantly of lava Sample Eruptive unit C10 13 mol/g) (%) flows, and all issued from flank vents at no. 2100-2600 m (6800- 8500 ft) around the

MA-311 basalt of Smith Butte bsb 1.502 ± 0.011 0.2990 0.6 14 ± 13 main cone. Seven are andesitic (54.5%-62% MA-437R andesite of Little Mount Adams ala 1.743 ±0.012 1.576 5.1 63 ± 7 MA-550 basalt of Glaciate Butte (vent) bgb 1.043 ±0.005 1.045 69 ± 15 Si02) and vented effusively; the lone basalt basalt of Glaciate Butte 1.184 68 ± 10 MA-603 bgb 1.201 ± 0.003 (49% Si02) constructed the only Holocene (intracanyon) MA-502 basalt west of Clearwater Creek bew 0.755 ± 0.006 1.059 97 ±21 cinder cone in the Mount Adams volcanic MA-996 andesite east of Shadow Lake aes 2.606 ± 0.003 4.180 13.0 111 ± 6 MA-695 basalt of Potato Hill bph 1.464 ± 0.005 2.332 14.1 111 ± 10 field. No attempt was made to date Holo- MA-24 basalt of King Mountain bkm 0.9819 89 ± 40 0.766 ± 0.001 1 106 cene units by the K-Ar method. 1.277 116 ± 31 i 106 ± 25 MA-380 basalt of Meadow Butte bmb 0.649 ± 0.002 1.292 138 ± 58 MA-729 basalt of Quigley Butte bqb 0.817 ± 0.004 1.353 115 ±28 MA-381 basalt west of Draper Springs bdw 1.221 ± 0.002 2.155 122 ± 21 Styles of Eruption (east side of Red Butte) MA-9S4R basalt west of Stagman Ridge bsr 0.827 ± 0.006 1.727 145 ± 20 MA-226 basalt of Goat Butte bgt 1.141 ± 0.003 2.622 159 ± 31 In contrast with the numerous explosive MA-319 basalt west of Draper Springs bdw 0.974 ± 0.012 2.570 184 ± 29 (Southwest toe of King Mountain) dacitic eruptions of nearby Mount St. MA-302 basalt of Hcrions Bridge bhb 0.851 ± 0.003 2.564 8.2 209 ± 21 Helens, activity at Mount Adams has been MA-394 basalt of Cakey Butte beb 1.038 ±0.005 3.680 8.0 246 ± 19 MA-799 basalt east of White Salmon River bew 1.088 ± 0.008 5.147 2.6 329 ± 50 dominantly effusive. The broad apron sur- MA-40 andesite of Cakey Butte acb 0.884 ± 0.003 3.298 2.9 259 ± 35 MA-424 basalt of Bear Creek (East lobe) bbc 1.176 ± 0.005 5.470 12.8 323 ± 13 rounding the cone consists largely of lava MA-422 basalt of Bear Creek (West lobe) bbc 1.148 ± 0.002 5.781 15.9 350 ± 14 flows, the proportion of fragmental flow de- MA-535A basalt of Bacon Creek bbn 0.710 ± 0.003 3.868 11.3 378 ± 22 MA-11 dacite of Olallie Lake (stony) dol 3.575 ± 0.006 19.62 38.0 381 ±5 posits preserved being no greater than 10%. MA-350 dacite of Olallie Lake (glassy) dol 3.698 ± 0.010 16.00 5,1 301 ±231 17.51 6.1 329 ± 21 J ± 16 Summit eruptions encountered the spe- MA-93Ö andesite of The Island ati 2.082 ± 0.002 11.55 33.5 385 ±7 MA-421 basalt of Flattop Mountain bfm 1.132 ± 0.004 6.422 20.0 394 ± 12 cial conditions imposed by a glacial ice cap MA-449 andesite of Dairy Creek ade 2.046 ± 0.004 12.54 38.5 425 ±7 and by 25°-50° slopes on all sides. Spatter MA-572 basalt of West Fork bwf 0.375 ± 0.002 2.702 9.8 500 ± 34 MA-585 basalt of Clearwater Creek bel 1.341 ± 0.008 9.745 13.7 505 ± 16 and scoria layers high on the cone indicate MA-811 basalt of Swampy Meadows bsm 0.938 ± 0.002 6.992 21.8 518 ± 14 MA-398 basalt of White Salmon River bws 0.923 ± 0.003 7.402 27.7 557 ± 18 lava fountaining and strombolian activity, MA-719 andesite of Mud Spring ams 1.111 ± 0.006 9.945 28.2 622 ± 15 but more abundant ash-rich fragmental de- MA-992 andesite of Laurel air 2.276 ± 0.004 20.69 11.8 631 ± 21 MA-386 dacite of Dry Creek ddc 1.908 ± 0.004 19.09 57.2 695 ± 8 posits containing angular glassy blocks of MA-414 basalt of Outlet Falls bof 0.182 ± 0.011 2.029 3.3 775 ± 114 MA-347 andesite of Bacon Creek abc 1.414 + 0.004 18.39 31.0 903 ± 13 poorly vesiculated andesite indicate a prev- MA-428 andesite of Mount Adams Highway aah 1.880 ± 0.004 25.19 28.6 931 ± 14 alence of phreatomagmatic and steam-blast (Garbage dump) MA-705 andesite of Mount Adams Highway aah 1.894 ± 0.006 25.70 46.3 942 ± 11 eruptions promoted by interaction of rising (Skunk Creek) MA-390 basalt of Blue Jay Campground bbj 0.940 ± 0.004 12.72 10.1 940 ± 36 magma with meltwater from the ice cap. Ef- MA-846 basalt of Glenwood bgo 0.182 ± 0.002 6.176 1.3 2.36 ± 0.72 Ma fusive and fountain-fed lavas that over- MA-554 rhyolite of Clearwater Creek Tre 3.845 ± 0.044 173.9 87.2 3.14 ±0.04 Ma MA-994 rhyolite of Mann Butte Tint 3.972 ± 0.005 1722 94.5 29.9 ± 0.2 Ma flowed the summit rim built steeply dipping radial stacks of thin flows, each dominated 10 1 10 4 •\e = 0.581 X 10~ yr" ; ^ = 4.962 X 10" yr" ';«K/K = 1.167 X 10" mol/mol. by rubbly oxidized zones of scoriaceous flow

cause andesitic debris greatly predominates in moraines deposited during the last main Pleistocene glacial advance (ca. 25-14 ka) but is less abundant in older (basalt-rich) tills around Mount Adams (Hopkins, 1976; Hildreth and Fierstein, 1990, 1995), it was clear from field evidence alone that the present-day edifice higher than ~2500 m (—8000 ft) is younger than the penultimate glaciation. Lacking the K-Ar results pre- sented below, however, there might well have been a tendency to lump the andesite- dacite central activity into two main stages— without having any idea of when the shift of focus took place or any appreciation of the complexity and virtual continuity of central activity for the past 0.5 m.y. There have been no historical eruptions Si02

in the Mount Adams volcanic field, and Figure 3. K20-Si02 variation (in wt%) of Quaternary volcanic rocks from Mount Adams most of the eight Holocene eruptive units volcanic field. All data by wavelength-dispersive X-ray fluorescence spectroscopy, normal-

mapped (Hildreth and Fierstein, 1995) are ized H20-free (Hildreth and Fierstein, 1995). Field boundaries extended from those of Gill older than a distinctive ash layer erupted (1981).

Geological Society of America Bulletin, November 1994 1417

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breccia; some flows lost coherence on slopes Mount Adams was long assumed to be a be of middle Pleistocene age on the basis of steeper than —35° and transformed into monotonously andesitic volcano, but the morphology and association. lava-debris avalanches or block-and-ash current study shows that eruptive products Following Richmond and Fullerton flows. range continuously in composition from (1985), the Pleistocene age boundaries are Pyroclastic-flow deposits are rare at 52% to 68.5% Si02 on the main cone and taken to be 1.65 Ma and 10 ka, and the Mount Adams. A few thin andesitic scoria from 47% to 61% Si02 on the periphery boundaries between early, middle, and late flows are exposed on the east face of the (Fig. 3). Especially noteworthy is the wide Pleistocene are 788 ka and 132 ka. summit cone and low on the western apron. compositional range of basalt—extending A 10-m-thick dacitic ash-flow sheet interca- from alkalic to low-potassium varieties. Early Pleistocene (and Older) Activity lated with lavas of the Hellroaring volcano is During the late Pleistocene alone, represen- the only pumiceous pyroclastic-flow deposit tatives of the entire compositional spectrum Volcanic rocks of early Pleistocene age at Mount Adams. Extensive ash-fall layers of basalt erupted in proximity to each other are exposed only at the southeast fringe of of local derivation have not been recog- and to the andesitic stratovolcano. The po- the Mount Adams volcanic field (Fig. 4A), nized, although several layers of dacitic fall- tentially mixed parentage implicit in such where they are spatially and compositionally out from Mount St. Helens (Mullineaux, proximity is apparent in the varied ande- transitional with coeval parts of the Simcoe 1986) are widely preserved in the Mount sites of the field, which range—for example Mountains volcanic field (Fig. 1), the young- Adams area. If widespread fallout accom- at 55% Si02—from 0.8% to 2.3% KzO est eruptive period of which has been K-Ar panied eruption of any of the dacite lavas at (Fig. 3). dated at 1.0-0.6 Ma (Uto and others, 1991). Mount Adams, it was eroded away or ob- Dacitic magma erupted at least 18 times Two Pliocene units west of the Klickitat scured by younger deposits. in the history of the volcanic field, mostly River (Fig. 4A) apparently relate to still ear- Eruptions of basalt and olivine andesite from flank vents close to the base of the lier episodes of Simcoe magmatism (4.5-2 peripheral to the main cone produced —35 main cone. Dacitic units include 5 large cou- Ma): (1) Tholeiitic olivine basalt that forms cinder cones and constructed several shields lees, at least 10 lesser lava flows, a pumi- the Wellenbrock Spring shield (bwb) south- consisting of countless thin shingled flows ceous pyroclastic flow, a spatter-fed agglu- east of Glenwood was dated by K. Uto of fluid lava—the 70 km2 King Mountain tinate pile, block-and-ash flows, and breccia (1993, personal commun.) at 3.4 Ma and (2) shield being the largest. Rapid canyon-cut- sheets associated with thick lava flows. All low-potassium olivine tholeiite (LKOT) that ting by glaciers and glacier-fed streams fa- are stratigraphically old, antedating con- floors the northwest sector of Glenwood vored funnelling of voluminous lava flows struction of the modern summit cone. Valley gave us a rather imprecise age of into and along river valleys; resulting Quaternary rhyolite is not present in the 2.36 ± 0.72 Ma (bgo; Table 1). Overlying tongues of andesite and dacite are 10-20 km Mount Adams volcanic field. We dated two this LKOT is an apron of plagioclase-olivine long, and some intracanyon flows of basalt rhyolitic lavas of Tertiary age (Table 1), a basalt (basalt of Blue Jay campground, bbj, advanced >25 km downstream. small kipuka in upper Clearwater Creek 940 ± 36 ka) exposed in several windows (3.14 ± 0.04 Ma) in the northeast part of the northwest of Glenwood; erupted from a field and Mann Butte (29.9 ± 0.2 Ma) at the now-concealed source close to Mount Compositions of Eruptive Products southwest margin (Fig. 4A). Adams and possibly part of a shield largely covered by younger lavas (Fig. 4A), this ba- Throughout construction of Mount salt marks the onset of major activity in the Mount Adams volcanic field. Sandwiched Adams, products of central-vent eruptions K-Ar RESULTS: ERUPTIVE between these basalts (bgo and bbj) near of the stratovolcano consisted almost en- CHRONOLOGY tirely of phenocryst-rich pyroxene andesite Draper Springs (Figs. 2 and 4A) are the oldest andesites (ade, ads) in the volcanic field. (56%-62% Si02). Most of these rocks con- Of 132 Quaternary eruptive units distin- Exposed only distally, these undated tain 15%-35% plagioclase phenocrysts, a guished on the geologic map (Hildreth and andesitic lavas flowed southeastward from few percent each of clinopyroxene and ortho- Fierstein, 1995), 56 are from central or flank now-concealed vents near the subsequent pyroxene, and <1% Fe-Ti oxides, with or vents of the focal stratovolcano, 68 are from site of Mount Adams. without subordinate olivine. Such andesite peripheral vents, and 8 (not further dealt also erupted on the lower slopes of the cone with here) flowed into the Mount Adams Basaltic andesite of shield 2971 (-7 km at numerous flank vents, where olivine area from vents in the Indian Heaven vol- north of Glenwood) yields an age of 903 ± andesite (53%-57% Si02) and pyroxene canic field (Fig. 1). Table 1 gives K-Ar ages, 13 ka near the vent. A few kilometers east- dacite (63%-68.5% Si02) are likewise com- analytical data, and labels (keyed to Figs. 4 ward, the shield lavas unconformably overlie mon—both typically being relatively pheno- and 5) for 74 dated samples. Of the 33 sam- slightly more evolved lavas (andesite of cryst poor. No Quaternary product of the ples from the focal area, 23 are andesitic, 9 Mount Adams Highway, aah) that may be Mount Adams volcanic field has pheno- dacitic, and 1 basaltic; of the 41 peripheral early products of the same vent system; the crysts of quartz, sanidine, or biotite, and am- samples, 10 are andesitic, 3 dacitic, 26 ba- latter have ages of 942 ± 11 ka and 931 ± 14 phibole is present only in two peripheral saltic, and 2 are Tertiary rhyolites adjacent ka and form 50-80 m bluifs on the west rim lava flows, both of which predate inception to the volcanic field. Coupled with field of the Klickitat River gorge. Banked against of the stratovolcano (Hildreth and Fierstein, relationships, the 74 dates satisfactorily these bluffs is a thick stack of LKOT lavas 1995). All basaltic units (47%-52% Si02), bracket the ages of nearly all units erupted (basalt of Outlet Falls, bof, 775 ±114 ka) inside or outside the vent corridor, contain in the volcanic field. The principal excep- that flooded the Klickitat Canyon for at least olivine phenocrysts, and many have plagio- tions are a few isolated cones along the 30 km (Fig. 4A). As thick as 160 m, this clase and clinopyroxene as well. southwest fringe of the field, all thought to intracanyon stack still extends from rim to

1418 Geological Society of America Bulletin, November 1994

river in many places, indicating that this the base of the deeply incised pile exposed. clearly represents the flank of a large reach of the Klickitat canyon (cut into Mio- The lowermost lava flows exposed in the andesitic cone, the principal vent of which cene basalt) is no deeper today than it was in canyons of Hellroaring and Big Muddy appears to have been somewhere beneath early Pleistocene time. Compositionally dis- Creeks, respectively, give ages of 516 ± 10 the present-day summit cone rather than at tinct from the older LKOT lavas (bgo) west ka and 491 ± 8 ka; one of the uppermost the Hellroaring center 5 km southeast of it. of Glenwood, these flows probably erupted flows (near the south rim of Hellroaring The uppermost Hellroaring lavas and brec- from a vent system now concealed beneath Creek) gives 511 ± 9 ka, suggesting that cias dip westward under the modern cone the Camas Prairie-Glenwood valley. East of much of the pile was constructed very rap- and are no younger than 474 ± 12 ka; on the the Klickitat River, the Jungle Butte shield idly. Dacitic breccias of The Spearhead (dts) east flank, one of the uppermost east-dip- (bjb; Fig. 4A), which consists of calc-alka- dip northwest, away from the core of the old ping flows is dated at 511 ± 9 ka. Thus the

line olivine basalt (52% SiOz), is also of center; the K-Ar age of 474 ± 12 ka for this simplest explanation is that the younger early Pleistocene age (K. Uto, 1993, per- unit could indicate the real longevity of the southwestern andesites (aws) erupted some- sonal commun.). cone, but it might just reflect minor loss of where west of the Hellroaring cone and radiogenic Ar from this partly glassy, hy- were prevented by it from spreading east- Middle Pleistocene Eruptions Older drated dacite. ward (Fig. 4B). than the Stratovolcano Roughly contemporaneous with inception of the stratovolcano were numerous pe- Persistent Activity 450-120 ka During the 200 k.y. interval just preceding ripheral eruptions of compositionally varied inception of the stratocone (at ca. 520 ka), basalt, all of which were emplaced directly Recurrent stratocone activity gradually the main locus of activity shifted somewhat upon Tertiary rocks. Previously mentioned constructed a surrounding apron of ande- westward, providing the first clear evidence was the olivine basalt along the White site-dacite lavas that extends —15 km in of the north-south vent corridor that has Salmon River (bws, 557 ± 18 ka) southwest most directions from the present summit, dominated the volcanic field ever since. A of Mount Adams. At the western periphery, overrunning all quadrants except the east- sheet of plagioclase-rich basalt (basalt of another extensive olivine basalt flooring the erly (70°-130°) sector already occupied and McCumber Spring, bins, Fig. 4A) and an Swampy Meadows area yields an age of blocked by the Hellroaring edifice (Fig. 4B). overlying coulee of nearly aphyric pyroxene 518 ± 14 ka (bsm, Fig. 4B). Its own vent is Although the vent system and the proximal dacite (dacite of Dry Creek, ddc, 695 ± 8 ka) covered by younger deposits, but this basalt parts of these middle Pleistocene lavas are descended to the north edge of the Glen- lies at the end of a north-trending chain wholly covered by the imposing late Pleis- wood valley from sources that are now cov- (bnf, Fig. 4B) of at least seven olivine-basalt tocene summit cone, medial-to-distal expo- ered but likely to lie within the corridor. vents that may all be of similar age. On the sures are widespread. West of Laurel (Fig. 4A), a cluster of vents lower north slope of the stratovolcano, an For the extensive southwest apron helps define the south end of the corridor. extensive northeast-dipping fan of olivine- (Fig. 4B), in which as many as 40 lava flows One vent produced a coulee of hornblende plagioclase basalt (basalt of Clearwater (aws) are exposed, two basal flows yield ages andesite (andesite of Laurel, air, 631 ± 21 Creek, bcl) dated at 505 ± 16 ka crops out of ca. 457 ka, and the topmost distal flow ka), two produced scoria cones and lava in several windows. At the toe of this slope, preserved yields 320 ± 7 ka; medially, where flows of plagioclase-olivine basalt (bhc, blr), a thick intracanyon stack of LKOT lavas the stack is —300 m thick in Cascade Creek and one yielded a lobate sheet of pheno- (basalt of West Fork, bwf) dated at 500 ± 34 and disappears beneath the younger edifice, cryst-poor olivine andesite (andesite of Mud ka floors the West Fork of the Klickitat it is overlain (near Lookingglass Lake) by a Spring, ams, 622 ± 15 ka) that banks against River. Considering that exposures of this distinctive olivine-andesite unit (all) dated the others (Fig. 4A). An isolated LKOT cen- age are limited to the periphery, the wide- at 329 ± 11 ka. No attempt was made to ter at nearby Stoller Canyon (bst) appears spread emplacement of many different ba- subdivide the —140 k.y. eruptive chronology comparably old; another at Little Mountain saltic units approximately contemporaneous (ca. 457-ca. 320 ka) of this major pile of (blm) might also be this old but morpholog- with the onset of voluminous andesitic cen- shingled but broadly comformable lavas— ically seems somewhat younger. Northwest tral activity is noteworthy—and probably all of which are plagioclase-rich pyroxene an- of Trout Lake (Fig. 4A), olivine basalt (ba- not fortuitous. desite (58%-63% SiOz). It appears that salt of White Salmon River, bws, 557 ± 18 most of the pile accumulated rapidly (ca. ka) exposed only in windows on the floors of Shift of the Andesitic Focus 450 ka) and was overrun at ca. 330-320 ka the White Salmon River and Trout Lake by only a few thinner lava flows; a veneer of Creek is the oldest Quaternary lava in the Pyroxene-andesite lava flows (58%-63% glacial deposits, however, leaves this field in- sector, resting on Tertiary rocks. The vent SiO ) comparable to those of the Hellroar- terpretation uncertain. for this basalt is concealed, possibly beneath z ing volcano are also exposed 6-15 km south- Comparable but less well-exposed fans Mount Adams. west of it, where they form a medial-to- (Fig. 4B) of stratocone-derived pyroxene- distal apron as thick as 300 m. Two basal andesite flows have been dated as follows: Inception of the Stratovolcano flows (andesite of White Salmon River, aws, 328 ± 21 ka to the north, 251 ± 6 ka to the Fig. 4B), compositionally different but both northwest, 160 ± 12 ka to the south, 152 ±

Andesite-dacite lavas (57%-64% Si02) resting on Tertiary rocks at the floors of the 8 ka to the north, 137 ± 6 ka to the west, and of the Hellroaring volcano (Fig. 4B), the Cascade Creek and White Salmon River 120 ± 7 ka to the north-northwest. Repre- oldest and probably largest component of gorges, respectively, yield ages of 456 ± 8 ka senting the same time interval, dacite lava the compound Adams edifice, bank east- and 457 ±11 ka. The thick pile consists of flows emplaced on the medial-to-distal parts ward against Miocene rocks, but nowhere is numerous flows, dips 2c-7° southwest, and of the stratovolcano yield ages of 381 ± 5 ka,

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Figure 4. Four simplified maps of eruptive units emplaced during time interval indicated on each panel. Andesite production was greatest during spurts of stratocone growth at 520-490 ka, 460-425 (?) ka, and 40-10 ka; another upsurge at 125- 100 ka was basalt dominated. Contours above 1500 m for the present-day stratovolcano are shown for reference on all panels. Symbols: Large • = site of modern summit of Mount Adams. V = approximate center of deeply eroded Hellroaring stratovolcano (ca. 520-490 ka); strike-and-dip symbols represent remnants of the edifice. Small • = lesser vents, shown only if exposed. Ar- rows indicate flow directions; open-ended units are eroded distally or covered proxi- mally. Paired letters o/y signify older and younger units across a contact (where not otherwise clear from the map pattern, al = alluvium. Dated samples are located by an x, accompanied by K-Ar age (in ka) as detailed in Table 1. Three-letter symbols for dated eruptive units are identified in Table 1; undated units shown on these panels are, alphabetically: abb, andesite of Bunnell Butte; abu, andesite of Butter Cave; ade, andesite east of Draper Springs; ads, andesite of Draper Springs Camp; aeb, andesite east of Bunnell Butte; age, andesite of lower Gotchen Creek; aha, aphyric andesite west of Hole- in-the-Ground Creek; ahg, andesite of Hole-in-the-Ground-Creek; aln, andesite north of Little Mount Adams; anp, andesite north of Peterson Ridge; arw, andesite of Ridge of Wonders; asc, andesite of Sled Camp; asm, andesite of Snipes Mountain; bah, basalt of Mount Adams Highway; bcc, basalt of Cougar Creek; bee, basalt of East Canyon Creek; bgu, basalt of Guler Moun- tain; bhc, basalt of Holmes Creek; bjb, basalt of Jungle Butte; blm, basalt of Little Mountain; blr, basalt of Laurel; bms, basalt of McCumber Spring; bnf, basalts west of Ninefoot Creek; brb, basalt of Red Butte;

bsc, basalt of Spring Creek; bst, basalt of Stoller Canyon; bwb, basalt of Wellenbrock Spring; dss, dacite southeast of Snowplow Mountain; Trc, rhyolite of Clearwater Creek; Trm, rhyolite of Mann Butte; and TVs, Tertiary volcanic and sedimentary rocks, undivided. All units are described in detail by Hildreth and Fierstein (1995).

301 ± 6 ka, 252 ± 8 ka, 246 ± 4 ka, 233 ± four separate vents 6-11 km east-southeast lavas) erupted on the lower end of what is 6 ka, and 213 ± 5 ka; these and several more of the modern summit (Fig. 4B) on the east- now the Ridge of Wonders, the youngest of dacites bracketed stratigraphically within ern flank of the already extinct and de- the three (ati) yielding an age of 385 ± 7 ka. this interval are discussed in a later section graded Hellroaring edifice, which was still Numerous basaltic eruptions on the pe- (Fig. 5). Middle Pleistocene growth of the high enough to deny centrally derived lavas riphery were likewise contemporaneous stratovolcano was long, slow, intrinsically access to the easterly sector. An extensive with the protracted central (or proximal- pulsatory, but broadly continuous. bilobate flow complex (adc) near Dairy and flank) activity that incrementally built the Early in this protracted period of strato- Cougar Creeks erupted at 425 ± 7 ka, and middle Pleistocene stratovolcano. On the cone growth, eruptions of olivine-pyroxene three compositionally dissimilar scoria southwest periphery (Fig. 4B), seven com-

andesite (55%-60% Si02) took place at cones (arw, aln, ati; each with associated positionally varied scoria cones (and associ-

1420 Geological Society of America Bulletin. November 1994

Wolupt Lake Butte basaltic shield (bgt, 159 ± 31 ka) is also sandwiched between Adams-derived andesite lavas, and west of Mount Adams, the poorly exposed basalt west of Stagman Ridge (bsr, 145 ± 20 ka) appears to be, as well.

Eruptive Upsurge (120-100 ka) and Eruptive Lull (100-70 ka)

A surge in volumetric eruption rate took place in the early part of the late Pleistocene (ca. 120-100 ka). Some 30%-35% of the basalt erupted in the whole Mount Adams volcanic field was emplaced at this time, most or all of it from vents along the north-south corridor (Fig. 4C). To the south, three basaltic shields together represent 7-10 km3 of eruptive products. Olivine-basalt flows high on the surfaces of the Quigley Butte and King Mountain shields yield ages of 115 ± 28 ka (bqb) and 106 ± 25 ka (bkm), respectively. The Meadow Butte shield, crowded between them and intermediate in age (on the basis of field relations among lava flows on the shield surfaces), has proven difficult to date, the best determination yielding an age of 138 ± 58 ka (bmb). However, an olivine-plagioclase basalt (basalt west of

Draper Springs, bdw, 52% Si02) that un-

derlies the olivine basalts (50%-51% SiOz) of both Meadow Butte and King Mountain gave an age of 122 ± 21 ka. Construction of all three shields may well have overlapped in time. Gitnvtood North of Mount Adams, the Potato Hill center (bph; 111 ± 10 ka) produced a scoria

cone and a lava field (50%-54% Si02) that yWhite spread >15 km westward into the Cispus Salmon River canyon. Another thick pile of olivine- basalt lavas (basalt west of Clearwater

Creek, bew, 49% SiOz) 5-6 km southeast of Potato Hill gives an age of 97 ± 21 ka. Fi- nally, on the east flank near Cougar Creek,

undated basaltic lavas (bcc, 51%-52% Si02) from a now-concealed vent probably within the corridor are unlikely to be much younger Figurt 4. (Continued). than the directly subjacent andesite (andesite of Parrott Crossing, ape) dated at 120 ± ated lavas) are all likely of this age, including 259 ± 35 ka and 246 ± 19 ka. Low on the 5 ka. LKOT centers at Little and Guler Moun- South apron of Mount Adams, two extensive During the same interval that produced tains, but only the basalts of Flattop Moun- fans of basaltic lavas (bew, bhb) are dated at so much basalt, several voluminous units of tain (bfm, 394 ± 12 ka) and Bear Creek 329 ± 50 ka and 209 ± 21 ka; both overlie andesite and dacite erupted from central or (bbc, 350 ± 14 ka, 323 ± 13 ka) have been andesite, but their vents and proximal areas flank vents on the stratovolcano (Fig. 4C). dated. On the southeast apron of Mount are covered to the north by still younger The Killen Creek fan of pyroxene-andesite Adams, a basaltic lava (bbn) along upper Adams-derived andesite. Nearby windows lavas (akc), extending north-northwest be- Bacon Creek gives an age of 378 ± 22 ka. On of limited extent also reveal three separate yond Horseshoe Lake, gives an age of 120 ± the north wall of Trout Lake Creek, at least units of basaltic andesite and two of basalt, 7 ka, and southeast of Mount Adams an un- four vents produced a suite of basalt and one of which gives an age of 184 ± 30 ka. usually voluminous (3-4 km3) flow of simi- olivine andesite (bbc, acb) that yields ages of East of Mount Adams, the 25 km2 Goat lar andesite (andesite of Parrott Crossing,

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ape) erupted near Bench Lake at 120 ± 5 ka, filling the Klickitat River canyon for —20 km (Sheppard, 1967b). Close to nearby Shadow Lake, comparable andesite (aes) again Wolupt Lake erupted at 111 ± 6 ka, flowing —7 km southeast into upper Bacon Creek. Also on the 120-100 ka southeast flank of the stratovolcano, the Bird Creek Meadows area is floored by the most silicic unit in the volcanic field, an extensive

lava flow of pyroxene dacite (68% SiOz) emplaced at 115 ± 5 ka. At about the same time (117 ± 6 ka), another pyroxene-dacite lava (dcc) flowed >22 km down Cascade Creek and beyond from a spatter-fed source high on the southwest flank of Mount Adams. Down- stream, intracanyon emplacement of this 60- m-thick dacite defeated the former course of the White Salmon River, causing it to cut a new canyon —1 km to the east; between the mouths of Ninefoot and Green Canyon creeks, this 6-km-long diversion is now the most deeply incised reach of the White Salmon. Following this period of elevated output, an apparent lull in eruptive activity may have lasted —30 k.y. In view of the uncertainty in age determinations and the burial of the central area by the modern summit cone, it cannot be asserted with confidence that this quiescent interval was marked by no eruptive events at all; for rocks of this age, however, exposure is good, and no undated units are even remote candidates for emplacement between ca. 100 ka and the eruption of Glaciate Butte at ca. 68 ka.

Magmatic Revival and Growth # of the Modern Summit Cone Glenwood Traut Late The apparent lull was broken by peripheral and focal eruptions of basalt and basaltic andesite (Fig. 4D). Not until 10-15 k.y. later did eruptions resume of the pyroxene White andesite more typical of the focal region. Salmon First was the basalt of Glaciate Butte (bgb, R.

51%-52.5% SiOz), low on the north-northeast apron, yielding ages of 69 ± 15 ka near the vent and 68 ± 10 ka near the snout of an Figure A. (Continued). intracanyon lava flow 25 km downstream on the Klickitat River. At 63 ± 7 ka, the phenocryst-poor basaltic-andesite spatter-and- are stratigraphically constrained to be of unique penetration took place toward the scoria cone called Little Mount Adams (ala) about this age, as well (abb, asm; Fig. 4D). end of one of the longest recognized in- erupted on the east-southeast flank, accom- Also at about this time (63 ± 14 ka), the tervals of quiescence. panied by a fountain-fed stack of intracan- basalt of Riley Creek (brc, 51%-52.5% The earliest postlull andesitic units are a yon lavas that flowed >6 km down Hellroar- Si02) erupted from a now-concealed vent pair of extensive lava-flow fans (amc, awf; ing Creek. Although undated, comparably that was either central or high on the west Fig. 4D) that erupted focally, spread south phenocryst-poor basaltic andesites (ande- flank of Mount Adams (Fig. 4D). This is and northeast from the site of the main sites of Bunnell Butte and Snipes Moun- the only basalt known to have erupted cone, and funnelled distally into intracanyon tain), which form four scoria cones and as- through the andesitic focus during the 520 tongues that give ages of 56 ± 6 ka on the sociated lavas northwest of King Mountain, k.y. history of the stratovolcano; the White Salmon River and 55 ± 7 ka on the

1422 Geological Society of America Bulletin. November 1994

capping the true summit (aas) yield ages of 13 ± 8 ka and 15 ± 8 ka, respectively. These ages support the inference that the ice- Wo I apt Lake ravaged summit region has produced no Hol- ocene eruptions, as suggested by the meager 70-10 ka record of Holocene tephra and the extensive acid-sulfate hydrothermal alteration of the high cone (Hildreth and others, 1983). Most of the great edifice above timberline, and virtually all of it higher than -2500 m, was constructed between 40 and 10 ka—perhaps largely in the interval 35-15 ka.

Youngest Peripheral and Flank Eruptions

During the last main episode of central cone construction (ca. 40-10 ka), compositionally contrasting basaltic units erupted nearby (Fig. 4D). At the south toe of the andesitic apron, the Smith Butte group of cinder cones marks the source of an extensive lava-flow fan of alkalic olivine basalt (bsb), for which we obtained a stratigraphically reasonable K-Ar age of 14 ± 13 ka. At the north toe, voluminous LKOT lavas (basalt of Spring Creek, bsc) erupted from shield 5162 (Figs. 2 and 4D), on the Cispus- Klickitat drainage divide, and spread —18 km eastward and >35 km westward. Con- flicting 14C ages of 21.5 ± 0.5 ka and 41.1 ± 1.3 ka were measured on organic matter at the base of this unit (Hildreth and Fierstein, 1995; D. A. Swanson, 1993, personal commun.); no attempt was made to date it by the K-Ar method. Low-K tholeiitic basalt also Indian Me oven volcanic field erupted during this interval from the Lemei Rock shield southwest of Mount Adams (Fig. 2), flooding the Trout Lake lowland with tube-fed lavas that are 14C dated at 31-22 ka (Hildreth and Fierstein, 1995). Effusive flank vents (at knobs 9402 and 9090) on the southern slope of Mount Adams produced a large fan of olivine-pyroxene andesite lavas that now form the D prominent buttress called Suksdorf Ridge. These in turn are overlain by sparsely por- West Fork of the Klickitat River, locations km-thick accumulation of breccias, scoria, phyritic olivine andesite erupted at the separated by 30 km. At -2500 m on the and thin lavas) is a lava flow that caps the South Butte cinder-spatter cone. During the north-northeast flank of Mount Adams, Ridge of Wonders near Sunrise Camp, high last main Pleistocene ice advance, both many of the pyroxene-andesite lava flows of on the east flank. The western apron of stacks of flows were glacially eroded mar- the Devils Gardens shield (adg) appear to Mount Adams is also covered by centrally ginally and distally but not on their high predate and issue from beneath the steep derived lavas that built a major flow fan proximal surfaces; both overlie andesitic la- ice-clad cone making up the modern sum- (asc) of pyroxene andesite (with or without vas derived from the Pikers Peak vent (13 ± mit; one such flow gives an age of 37 ± 8 ka. olivine); the fan overlies the basalt of Riley 8 ka) of the main cone, thus restricting their A younger flow from a subsidiary vent on the Creek (brc, 63 ± 14 ka), and one of its ages fairly narrowly. K-Ar age determina- shield surface did not yield sufficient radio- youngest flows, an intracanyon tongue on tions gave 10 ± 16 ka for a Suksdorf Ridge genic argon to give a meaningful age. Big Spring Creek (abs), gives an age of 28 ± lava (ask) and 12 ± 17 ka for one from The oldest successfully dated andesite 6 ka. On the main cone itself, andesite form- South Butte (asb). Even though the two 40 (app, 33 ± 14 ka) unequivocally derived ing the south-summit rim (andesite of Pikers rocks contain only 0.3% radiogenic Ar from within the main summit cone (a 1.3- Peak, app) and the small fragmental cone (Table 1) and have large analytical uncer-

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Figure 5. Dacite eruptive units (63%-

68.5% Si02) mapped in Mount Adams volcanic field (Hildreth and Fierstein, 1995). As all are older than 100 ka, vents are covered for all except units dcc and perhaps dol. Unit dth is a pumiceous pyroclastic- flow deposit; the rest are lavas. Dated samples are located by an x, accompanied by K-Ar age (in ka) as detailed in Table 1. For other dacites, the limiting ages shown are based on stratigraphic relationships. Age of isolated unit dnc is poorly known and could be early Pleistocene or older. Three-letter symbols for dated units are explained in Ta- ble 1; undated units in this diagram are: dhg, dacite of Hole-in-the-Ground Creek; dim, dacite of Little Muddy Creek; dnc, dacite north of Cress Camp; dss, dacite southeast of Snowplow Mountain; dsw, dacite of Swampy Creek; dth, dacite tuff of Hellroar- ing Creek. Designations (ahv) and (aws) indicate dacitic lava flows within two stacks of predominantly andesitic lavas; the former yields an age of 516 ± 10 ka near the base of the andesite of Hellroaring volcano, and the latter closely overlies the basal flow of the andesite of White Salmon River (457 ± 11; Table 1). All units are described in detail by Hildreth and Fierstein (1995). Diagram also indicates all vents exposed (stars), ir- respective of composition; the 6-km-wide north-trending corridor encloses most known or inferred vents—other than the northwest-trending array near Trout Lake.

The two smallest Holocene lava flows, on Battlement and Suksdorf Ridges, may be the youngest eruptive units in the volcanic field, appearing to be of middle to late Neo- glacial age (Hildreth and Fierstein, 1995).

DISCUSSION OF ERUPTIVE HISTORY

Areas and ranges in thickness were estimated for each of 124 map units distinguished by Hildreth and Fierstein (1995), tainties, the ages are stratigraphically con- Fig. 2) and lavas that flowed as far as 13 km and volumes were then calculated on the ba- sistent and credible. northeast. Diverse andesitic lava flows sis of conservative and liberal reconstruc-

Holocene activity has been limited to (54.5%-62% Si02) erupted effusively from tions of the original (pre-erosional) extent flank vents fairly high (2100-2600 m) on the seven independent vents widely distributed of each. Although erosion has rendered 3742 m edifice, yielding a modest volume on the north, east, south, and northwest many individual estimates rather uncertain, (0.9-1.2 km3) of diverse products. Early in flanks. The three most voluminous (—0.4, the resulting "minimum" and "maximum" Holocene time, eruption of high-K olivine- 0.3, and 0.12 km3, respectively) postdate cumulative-volume curves are very similar in

plagioclase basalt (49% Si02) on the north fallout of Mazama ash (ca. 7 ka) but predate form (Fig. 6). Most of the 124 increments flank produced a cinder cone (Red Butte; Mount St. Helens ash layer Ye (ca. 3.5 ka). represent single flows or sets of related flows

1424 Geological Society of America Bulletin. November 1994

30%-40% of the products were basaltic. 4 00 Since inception of the stratovolcano (ca. 520 ka), even though the four main pulses pro- 350 duced 80%-85% of the total volume of the volcanic field, the magmatic system has never completely shut down. Breaks in ac- 300 tivity as long as 30 k.y. are permitted by the K-Ar data (Figs. 6 and 7), but, considering the numerous undated units and the ex- 250 tended eruptive periods represented by some of the complex units (shields and lava- 200 flow fans), few if any repose intervals are likely to have lasted that long. The persistent background activity (for example, 400-125 150 ka) was not just basaltic but, by volume, included 10%-13% dacite and 50%-57% andesite—most of these evolved products dis- 100 charging focally. As the dominant focus for ascent and eruption of nonbasaltic magma, 50 the Mount Adams stratocone system has remained effectively active throughout its entire half-million-year history (Fig. 7). 0 BASALT Corollary to this conclusion is the obser- ANDESITE vation that peripheral basaltic activity and DACITE focal andesitic-dacitic activity have been persistently contemporaneous, coexisting throughout the lifetime of the stratovolcano. Figure 6. Cumulative eruptive volume vs. time for Mount Adams volcanic field. Maxi- During that 520 k.y. lifetime, basalt is known mum (MAX) and minimum (MIN) cumulative curves are based on liberal and conservative to have penetrated the focus only once—at estimates of thicknesses and original areas of each of 124 map units (Hildreth and Fi- 63 ± 14 ka (unit brc; Fig. 4D), apparently erstein, 1995). Composition of each unit, whether basalt, andesite (including basaltic an- following one of the longest lulls in focal desite), or dacite, is indicated at bottom of diagram. Shaded boxes represent the three main activity (Fig. 7). andesitic cone-building episodes (ahv = andesite of Hellroaring volcano; aws = andesite The total volume erupted, as indicated by of White Salmon River; app = andesite of Pikers Peak). Durations of ahv and app are summing the ranges estimated for each of reasonably well known; for aws, the queried arrows schematically suggest an alternative for 124 map units (Fig. 6), lies between 231 km3 the poorly known time-volume distribution. and 399 km3 (that is, -315 ± 84 km3). By a wholly different method, Sherrod and Smith (1990) obtained a present-day volume of that probably took intervals ranging from (unit aws) that began at ca. 457 ka, compa- —225 km3 for Mount Adams and its basaltic days to decades to emplace. A few of the rably peaked time-volume behavior is as- periphery. Volume erupted is, of course, mafic shields consist of countless flows of sumed, but a prolonged alternative is also much greater, owing to repeated glacial ep- similar composition that could have illustrated in Figure 6. isodes and to reduction of the edifice to a erupted intermittently over periods of Distributing the 75-125 km3 of the Hell- dike-ridden hulk only 2100-2800 m high centuries or more, and the three main roaring cone (ahv; Fig. 4A) and the 39-52 prior to late Pleistocene construction of the cone-building units (ahv, aws, app) each km3 of the late Pleistocene cone (app + adg modern (3742 m) cone. Because erosion and contain hundreds of lavas and fragmental + asc + abs; Fig. 4D) over emplacement concealment render volume estimates for all layers emplaced over intervals lasting intervals of 25 k.y. yields cone-building but the youngest eruptive units uncertain thousands of years. eruptive rates of 3-5 km3/k.y. and 1.6-2.1 within factors of 1.5 to 5, we prefer to cite The outstanding feature of the cumulative km3/k.y., respectively. For the basalt-domi- the ranges calculated by the minimizing and curves (Fig. 6) is the volumetric dominance nated upsurge at 125-100 ka, eruption of maximizing approximations. 3 of focal andesite emplaced in three main 12-24 km in —25 k.y. similarly yields an Of the 231-399 km3 total, 83%-87% is- 3 cone-building episodes, centered at 500 ka, average rate of 0.5-1 km /k.y. sued focally from central or proximal-flank ca. 450 ka, and 30 ka. An important lesser In contrast, background rates between the vents, and only 13%—17% was produced by episode at 125-100 ka was dominated by main eruptive pulses were more than an or- vents peripheral to the stratocone. Of the eruption of basalt from vents peripheral to der of magnitude smaller. Between 940 and 124 eruptive units distinguished (Hildreth the stratocone but distributed along the 520 ka, prior to inception of the stratovol- and Fierstein, 1995), 46 basaltic units add up north-south corridor (Figs. 2 and 4C). In cano, the volumetric eruption rate was only to 20-58 km3 or 9%-15% of the total vol- three of the four episodes, most of the 0.015-0.04 km3/k.y., and -80% of the prod- ume erupted; 60 andesitic units give 208- eruptive volume was emplaced within —25 ucts were basaltic. Between 400 and 125 ka, 335 km3 (84%-89%); and 18 dacites yield k.y. For the less well-constrained episode the rate was 0.05-0.1 km3/k.y., but only 3-6 km3 (1.4%-1.6%).

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3 900 700 500 300 100 0 k0 125 ka averaged 0.05-0.1 km /k.y., only 1 1 1 1 1 1 1 1 1 • bsb-+ l%-5% of peak eruptive rates, but it prob- bgb -o- ably never really shut down. During this long K-Ar ages —•— -o • -o interval, representing half the lifetime of the -* o stratovolcano, eruptive activity was intrinsi- hmh . 0 cally sporadic but in long-term perspective m o effectively continuous (Fig. 7). Over this 275 —•— -o- k.y. timespan, numerous eruptive pulses —•— -o- —•— o produced —50 distinguishable eruptive V- • bew o units, with a wide array of andesite and da- >/ —•— o o cite focally and basalt peripherally that to- , bbc -o -o- gether amount to 6%-8% of the volume of . 9/ • o the volcanic field. "Dormancy" is an anthro-

Dacite (63%-68.5% Si02) is the most- term behavior of arc volcanoes—an impor- particular cone-building episodes can be evolved product of the Mount Adams vol- tant class of magmatic systems for which age very tenuous. -anic field. Although it constitutes only control comparable to the present data set is 4. Large stratocone systems can remain ac- -1.5% of the volume erupted, the 18 units generally lacking. tive for half a million years. Documented ex- recognized are widely distributed in most 1. Stratovolcanoes commonly grow in amples of greater longevity are rare. In the sectors around the cone (Fig. 5) and across spurts. Construction of an imposing cone Cascade arc, the Baker, Rainier, Adams, much of the time span of the volcanic field need take only l%-5% of the active lifetime Hood, Jefferson, Mazama, and Shasta an- (Fig. 6). Two dacites in the southeast quad- of the volcano. Numerous circum-Pacific desite-dacite systems are all likely to have rant (Fig. 4A) predate inception of the stra- stratocones have added 10 km3 or more dur- lasted >300 k.y. Among them, there is no tovolcano; 16 dacites (516-115 ka) were ap- ing the Holocene alone (>1 km3/k.y.), and, obvious relationship between longevity and parently derived from central or flank vents on a similar time scale, the productivity of a eruptive volume, though adequate geochro- related to it; and none is younger than 115 ± few has been much greater; for example, nology is lacking for most. There is no clear 5 ka. All are pyroxene dacite except the Fuji (Honshu) added —40 km3 between 13 relationship between longevity and evolu- sparsely porphyritic hornblende-dacite unit and 8 ka (Togashi and others, 1991), and the tion toward a "climactic" caldera-forming dnc, which is old but undated. No two dacite 250 km3 Klyuchevskoi cone (Kamchatka) is stage. A stratovolcano lifetime of 1 m.y. is units are identical; the 18 differ significantly thought to have grown entirely in the last 7 not impossible, but claims thereof should be in chemical composition (Fig. 3) and range k.y. (Khrenov and others, 1991), yielding treated skeptically until detailed investiga- widely in orthopyroxene/clinopyroxene/oli- eruption rates of 8 and 35 km3/k.y., respec- tion has linked geochronology with strati- vine proportions and in plagioclase pheno- tively. For Mount Adams, the main cone- graphic verification of the integrity of the cryst content (<1% to >25%; Hildreth and building episodes give comparable rates of system. 3 Fierstein, 1995). Despite a hint of clustering 2-5 km /k.y. (Fig. 6) —or greater if the 5. Eruptive patterns of historically active by age (Fig. 6), each of the 18 dacites rep- spurts were in reality shorter than the 25 k.y. stratovolcanoes display a remarkably wide resents an independently evolved magma intervals fixed loosely by the K-Ar data. spectrum of recurrence time scales (Simkin batch. 2. Stratovolcano systems may stay active and others, 1981). At one extreme, Sakura- between the widely spaced episodes of peak jima (Kyushu) has erupted persistently since IMPLICATIONS FOR ARC productivity. Although cone-building spurts 1955, ejecting andesitic tephra from a sum- VOLCANIC SYSTEMS can supply 80%-90% of the magma ulti- mit crater on a virtually daily basis. More mately erupted, lingering intermittent leak- common are stratocones (for example, Fue- The time-volume-composition results age (Fig. 6) shows that source regions can go, Izalco, Klyuchevskoi, Manam, Na- summarized in Figure 6 bear upon several remain secularly productive. For Mount kadake, Pavlof, Shishaldin, Villarrica) that fundamental questions concerning the long- Adams, the background trickle from 400 to erupt every 0.1-10 yr or so, mostly minor

1426 Geological Society of America Bulletin. November 1994

outbursts that typically cluster into intervals The time scale and spatial scale consid- the arrays of peripheral eruptions (Hildreth, of greater and lesser frequency. A third dis- ered should fit the process being investi- 1981, Fig. 15c). tinctive style is displayed by stratocones gated. Appropriate scales are evidently 8. Stratovolcano systems need not develop where short eruptive episodes recur only on quite different for assessing the geochemical large upper-crustal magma chambers and a time scale of a hundred to several hundred budgets of arc-magma generation, deep- need never evolve toward a caldera-forming years (for example, Fuji, Osorno, Ruiz, San crustal heating and melting, formation and stage. Arc calderas that result from collapse José, Turrialba, and, during the last 4 k.y., replenishment of upper-crustal reservoirs, of shallow reservoirs beneath stratovolca- Mount St. Helens). Finally, there is the mil- or the eruption hazards at stratovolcanoes. noes (for example, Krakatau, Mazama, San- lenary eruptive mode represented by active For mafic-to-intermediate stratovolcano torini, Villarrica) are usually associated with andesite-dacite cones that have undergone systems, most of the basalt and much of the large eruptions of ejecta zoned to rhyodacite

only a few eruptive episodes in Holocene andesite ever reaching the upper crust prob- (69%-72% Si02) or even rhyolite. At time; included here are Mounts Baker, ably erupts. For the few such systems that Mount Adams, dacite magma erupted at Hood, Rainier, and Adams. The observa- occasionally develop mid- to upper-crustal least 18 separate times (Figs. 5 and 6), but tional record is not long enough to know silicic reservoirs, a single 0.1-1 km3 leak of neither rhyolite nor rhyodacite ever ap- whether such recurrence styles really repre- rhyodacite may represent development of a peared. Construction (40-10 ka) of the sent favored modes of eruptive behavior or 10 km3 pluton, otherwise undetected. Volu- main present-day stratocone entailed accu- are merely conceptions based on partial minous throughput of andesitic and basaltic mulation of 40-50 km3 of heterogeneous

glimpses of a behavioral continuum. The magmas shows that there is little or no con- (56%-62% SiOz) andesite in hundreds of great variety of products making up Mount temporaneous storage of low-density silicic batches, without the appearance of any si- Adams—including near-vent cinders, brec- magma in the mid- to upper crust. For geo- licic diiferentiates at all—not even dacitic cias, and agglutinates, proximal-to-medial thermal resources, therefore, high volumet- ones. Of the 18 dacite units—all older than

stacks of ejecta and thin rubbly lavas, and ric eruption rates are irrelevant. Large cones ca. 115 ka—only four have Si02 >65%, and thin-to-thick lava flows on the apron—none- can be constructed rapidly without creation only one (unit dbc) evolved to 68%; their theless suggests that, at various times in its of upper-crustal reservoirs. What counts eruptive volumes are in the range 0.02-1.2 long history, this one andesite-dacite strato- for accessible geothermal energy is shallow km3 each, but only four or five of them are volcano could have displayed all of the magma storage, not throughput, and there is likely to have been as large as 0.5 km3. Mod- eruptive frequency patterns just described. little evidence that the two are closely est batches like these could fractionate from 6. Discussion of volumetric eruption rates linked. andesite in magma bodies as small as 1-5 km3, at any crustal depth, or they could ac- can be misleading without conceptual clarity 7. Andesite-dacite production in the fo- cumulate independently in dikes and pods in about time scales. For Mount Adams, the cal region and coeval basaltic activity on the hot deep crust. Recurrent eruption of small eight Holocene (10-0 ka) eruptive units add periphery have coexisted at Mount Adams 3 batches of dacite at irregular intervals and up to only 0.9-1.2 km , giving a production for 520 k.y. (Fig. 7). Scarcity or abundance 3 its secular alternation with varied andesite rate (—0.1 km /k.y.) similar to the long-term of surrounding mafic cinder cones and lavas (and even basalt) shows that there is no background rate for the period 400-125 ka has nothing to do with maturity of the stra- 3 standard sequence, no unidirectional pro- (0.05-0.1 km /k.y.) but 20-50 times smaller tovolcano system. Flank eruptions high on gression, and certainly nothing predeter- than that for the main (25 k.y. long) cone- the edifice that produce silicic andesite like 3 mined in the evolution of stratovolcanoes. building episodes (2-5 km /k.y.). Average that erupted at the summit are reasonably eruption rate for the whole volcanic field interpreted as lateral breakouts from a cen- during the past 940 k.y. is 0.25-0.4 km3/k.y. tral conduit system. Each of the diverse ba- ACKNOWLEDGMENTS and since inception of the stratovolcano at salts and olivine-andesites that erupt from ca. 520 ka is 0.4-0.7 km3/k.y. These figures vents on the lower apron or wider periphery, Judy Fierstein contributed extensively to obscure much of the information of interest however, has its own conduit from mantle or the mapping and petrological investiga- (Fig. 6) in understanding the magmatic sys- deep-crustal depths. tions on which this geochronological study tem. For comparison, Mount St. Helens, The term parasitic should be abandoned is based. She also prepared the illustrations. which originated only 36-40 k.y. ago (50 km as it implies dependence upon the main stra- Potassium measurements were made by M. west of Mount Adams, in the same arc seg- tocone and promotes the view that periph- Dyslin, L. Espos, S. T. Pribble, and D. Vivit. ment), has had an eruptive rate (Sherrod eral mafic eruptions are leaks from a central Ar measurements and age calculations were 3 3 and Smith, 1990; 79 km /40 k.y.) of -2 km / chamber or conduit. Virtually the opposite made by J. Y. Saburomaru and J. C. Von k.y.; at least half of this, however, probably is true: Andesitic stratovolcanoes are the Essen. Helpful reviews and editorial im- erupted in the last 4000 yr (Mullineaux, derivative features, and peripheral basalts provements were provided by J. Aronson, 1986), yielding a late Holocene rate of —10 the more fundamental. Stratovolcanoes are R. L. Christiansen, F. W. McDowell, D. A. 3 km /k.y. Lumping the long-term or short- built up of fractionated hybrid magmas that Swanson, A. G. Sylvester, and B. D. Turrin. term productivities of Mount Adams and rise from deep-crustal zones of melting, as- Mount St. Helens—volcanoes of contrasting similation, storage, and homogenization APPENDIX age, episodicity, and composition—into a (Hildreth, 1981; Hildreth and Moorbath, 3 Methods grand average of so many km /k.y. per kilo- 1988). Beneath such zones, centrally con- meter of arc length for the southern Wash- centrated fluxes of mantle-derived basalt A 30 x 50 km area centered on Mount Adams ington segment is more likely to confuse stall within the focal parts of broader do- was mapped geologically at a scale of 1:24 000, for publication at 1:50000 (Hildreth and Fierstein, than to benefit inquiry into sub-arc mag- mains that are injected with radially dimin- matic processes. 1995). Some 132 Quaternary volcanic units were ishing intensity by the basalts that also feed defined in the course of the mapping, almost all

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for the first time, and nearly half of them were The successful measurement of ages on the lie Lake (dol) yielded an age ¡of 381 ± 5 ka (with dated by the K-Ar method (Table 1). Each is an Mount Adams rocks resulted primarily from the 38% ""Ar,^), but massive vitrophyre near the eruptive unit in the sense of derivation from a performance of the multiple-collector mass spec- base gave ages of only 301 ± 123 ka and 329 ± 21 40 single vent or fissure. Most are simple or multi- trometer. In a typical mass analysis on this instru- ka (with only 5%-6% Arrad). Similarly, the lobate lava flows, but many are shields, fans, or ment, 120 measurements of each Ar isotope are stony interior of the intracanyon dacite of Cas- stacks of lava flows that have a common vent and made during a period of ~2 min. Linear regres- cade Creek (dcc) gave 117 ± 6 ka (with 15.2% 40 mineralogical and chemical coherence. Each unit sions of the isotope peaks generally yield values Arrad), whereas massive but incompletely devit- was delineated in the field and its integrity con- for the peaks at time of admission of the gas sam- rified near-vent agglutinate gave apparent ages of firmed or refined by microscopic and chemical ples into the mass spectrometer having coeffi- only 97 ± 22 ka and 89 ± 22 ka (with merely 40 40 work in the laboratory. Most units have narrow cients of variation of <0.02% for the Ar and 1.6%-1.7% Arrad). Isotopic exchange with at- compositional ranges, but a few are heterogene- 38Ar peaks and <0.5% for the 36Ar peak. Such mospheric Ar during secular hydration of glassy ous or zoned across ranges of l%-4% SiOz. The precision in measuring peak heights generally domains in the groundmass may be more impor- relative ages of contiguous units have been de- cannot be achieved with a single-collector mass tant than simple diffusive loss of radiogenic Ar. termined on the basis of field relationships for the spectrometer. The high precision of the multiple- Entrapment of atmospheric Ar by bubble collapse great majority. collector measurements permits calculation of and resorption during emplacement may be a For basalt, andesite, and dacite younger than 1 good-quality ages when the proportion of radio- widespread additional problem for dating agglu- Ma, whole-rock samples generally are the choice genic Ar is as low as 1%. tinates and fountain-fed lavas across the whole for conventional K-Ar dating rather than sepa- It is important to keep the variation in mass compositional spectrum from basalt to rhyolite. rated phenocrysts. The primary reason is the spectrometer discrimination as low as possible to (3) Discrepancies between the K-Ar data and lower KzO content of the phenocrysts. K20 is achieve high precision in measured ages. Because stratigraphic order are remarkably few. The larg- concentrated in the groundmass of a volcanic of error propagation, this is particularly true for est is between ages for the andesite of Cakey 40 rock, and the KaO content of the whole rock com- samples with low radiogenic Ar contents. Ar Butte (acb) (259 ± 35 ka) and the overlying basalt monly is four to five times higher than that of analyses on the multiple-collector mass spectrom- east of White Salmon River (bew) (329 ± 50 ka), 40 plagioclase. The K20 content of pyroxene gener- eter are made on batches of 25 samples. One or both of which yielded <3% Arrad. Because their ally is an order of magnitude lower than that of two splits of atmospheric Ar are analyzed at the error envelopes overlap, however, these ages are plagioclase. beginning of each batch, and the composition of not formally inconsistent with the stratigraphy. All of the ages (Table 1) are conventional K-Ar atmospheric Ar is used to calculate mass spec- Our attempt to date the very young south-slope ages measured on whole-rock samples selected trometer discrimination. Discrimination meas- sequence emplaced in the order (1) andesite of after thin-section examination. Most of the sam- ured on 34 batches of samples during the past 2 yr Pikers Peak (app) (13 ± 8 ka), (2) andesite of ples meet the usual criteria for whole-rock K-Ar has a coefficient of variation of 0.24%. Suksdorf Ridge (ask) (10 ± 16 ka), and (3) an- dating (Mankinen and Dalrymple, 1972), but a A total of 86 argon extractions were made, and desite of South Butte (asb) (12 ± 17 ka) produced few contain minor amounts of glass. The samples only 6 failed to produce useful information. Two a reasonable set of ages considering that each 40 selected for dating were crushed to 0.5-1 mm andesite samples having —2% KzO did not yield yielded <1% Arrad. (-18 + 35 mesh). Aliquants weighing—25 g were useable data, although analyzed in duplicate; both In contrast to these minor discrepancies, we call used for the Ar measurement. A10 g aliquant was rocks contained large quantities of nonradiogenic attention to the following examples of age-strat- ground to -200 mesh, and splits of the powder Ar. Two other samples yielded acceptable ages on igraphic agreement (Table 1): (1) accordant pairs were used for the KzO measurements. a second Ar extraction after an initial failed of samples from different parts of each of the fol-

K20 measurements were made in duplicate on experiment. lowing units—andesite of Mount Adams Highway each of two separate splits of sample powder by The precision of K20 measurements was al- (aah), andesite of White Salmon River (aws), ba- flame photometry after lithium metaborate fusion most always better than ±0.5%. Thus the preci- salt of Bear Creek (bbc), and basalt of Glaciate and dissolution (ingamells, 1970); each plus-or- sion of calculated ages is governed primarily by Butte (bgb); (2) ages of the andesite of Lewis minus value given in Table 1 is the standard de- the precision of the Ar measurements. Naturally, River (alw) (251 ± 6 ka) and the directly subja- viation of four measurements. Ar analyses were the measurement precision is directly related to cent dacite of Lewis River (dlr) (252 ± 8 ka); and by isotope-dilution mass spectrometry using a the proportion of radiogenic 40Ar in the rock— (3) the age of the andesite of Mud Spring (ams) 38 high-purity (>99.9%) Ar tracer and techniques which increases with age and K20. In the focal (622 ± 15 ka), which banks against (Fig. 4A) the and equipment described previously (Dalrymple area where most units are andesite or dacite, 7 out andesite of Laurel (air) (631 ± 21 ka). and Lanphere, 1969). All samples for Ar extrac- of 12 samples younger than 100 ka yielded ages tion were baked overnight at 280 "C. Mass anal- with lo- uncertainties <30%, all 7 samples 100- yses were done on a 22.68 cm radius, multiple- 200 ka have uncertainties <10%, and 15 of 16 REFERENCES CITED collector mass spectrometer with a nominal 90° samples 200-520 ka have uncertainties <5%. Bacon, C. R., and Lanphere, M. A., 1990, The geologic setting of sector magnet, using automated data collection Eruptions from peripheral vents are mainly ba- Crater Lake, Oregon, in Drake, E. T., Larson, G. L., Dy- mond, J., and Collier, R., eds., Crater Lake—An ecosystem (Stacey and others, 1981; Sherrill and Dalrymple, salt, with subordinate andesite and dacite. The study: San Francisco, Pacific Division of the American As- 1980). basalt samples are less radiogenic in general, and sociation for the Advancement of Science, p. 19-27. Cox, A., and Dalrymple, G. B., 1967, Statistical analysis of geo- Errors given for the calculated ages of individ- consequently the basalt ages have larger uncer- magnetic reversal data and the precision of potassium- ual measurements are estimates of the standard tainties. Four out of 5 samples younger than 100 argon dating: Journal of Geophysical Research, v. 72, p. 2603-2614. deviation of analytical precision. These errors ka have uncertainties <30%; 6 of 7 samples 100- 200 ka have uncertainties <20%; 10 of 12 samples Dalrymple, G. B., and Lanphere, M. A., 1969, Potassium-argon were calculated using formulas derived by Cox dating: San Francisco, W. H. Freeman and Co., 258 p. and Dalrymple (1967) and Dalrymple and Lan- 200-400 ka have uncertainties <10%; and 11 of Dungan, M. A., Singer, B., and Thompson, R. A., 1993, The tem- 13 samples 400-940 ka have uncertainties <5%. poral evolution of Volcan Tatara, 36°S, Chilean Andes: In- phere (1969). Duplicate Ar measurements were sights from K-Ar dating and photogrammetric projections: made on six samples, and ages were calculated for Eos (Transactions, American Geophysical Union), v. 74, each individual Ar measurement (Table 1). For Problems in Dating the Mount Adams Suite p. 651. Gill, J. B., 1981, Orogentc andesites and plate tectonics: Berlin, five samples, the calculated ages agreed within the Springer-Verlag, 390 p. uncertainty assigned to each age. For the sixth Summarized here are the principal difficulties Hammond, P. E., 1980, Reconnaissance geologic map and cross sections of southern Washington Cascade Range: Portland, (MA-523), the error envelopes around each age encountered in the course of the K-Ar investiga- Oregon, Portland State University, Department of Earth do not overlap; this may reflect heterogeneity of tion. (1) LKOT (0.1-0.4 wt% K20) generally Sciences, 31 p., 2 sheets, scale 1:125 000. the sample itself. For these six samples, weighted Hammond, P. E., Pedersen, S. A., Hopkins, K. D., Aiken, D., proved difficult to date with acceptable precision. Harle, D. S., Danes, Z. F., Konicek, D. L., and Stricklin, mean ages were also calculated—where weighting This intractability may be as attributable to per- C. R., 1976, Geology and gravimetry of the Quaternary ba- is by the estimated variance of individual age meas- vasively porous (diktytaxitic) texture as to intrin- saltic volcanic field, southern Cascade Range, Washington, in Pezzotti, C., ed., Proceedings, Second United Nations urements (Taylor, 1982) rather than simple arith- sically low K contents. Such porosity is often ac- Symposium on the Development and Use of Geothermal metic means. The calculation of weighted mean companied by high yields of nonradiogenic Ar. Resources, San Francisco, May 1975, Volume 1: Washing- ton, D.C., U.S. Government Printing Office, p. 397-405. ages allows data of different quality to be com- (2) Glassy samples gave ages as much as 20% Hildreth, W., 1981, Gradients in silicic magma chambers: Impli- bined without the poorer data having a dispro- too young for some of these Pleistocene lavas. cations for lithospheric magmatism: Journal of Geophysical portionate effect on the result. The platy crystalline interior of the dacite of Olal- Research, v. 86, p. 10153-10192.

1428 Geological Society of America Bulletin. November 1994

Hildreth, W., and Fierstein, J., 1985, Mount Adams: Eruptive his- Luedke, R. G., Smith, R. L., and Russell-Robinson, S. L., 1983, Stacey, J. S., Sherrill, N. D., Dalrymple, G. B., Lanphere, M. A,, tory of an andesitc-dacite stratovolcano at the focus of a Map showing distribution, composition, and age of late Ce- and Carpenter, N. V., 1981, A five-collector system for the fundamentally basaltic volcanic field: U.S. Geological Sur- nozoic volcanoes and volcanic rocks of the Cascade Range simultaneous measurement of argon isotope ratios in a vey Open-File Report 85-521, p. 44-50. and vicinity, northwestern United States: U.S. Geological static mass spectrometer: International Journal of Mass Hildreth, W., and Fierstein, J., 1990, Geologic map and geother- Survev Miscellaneous Investigations Series Map 1-1507, Spectrometry & Ion Physics, v. 39, p. 167-180. mal assessment of the Mount Adams volcanic field, Cascade scale 1:500 000. Swanson, D. A., Cameron, K. A., Evarts, R. C., Pringle, P. 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N., 1990, Compositional diversity of late Cenozoic basalts in Smith, J. G., 1993, Geologic map of upper Eocene to Holocene a transect across the southern Washington Cascades: Im- volcanic and related rocks in the Cascade Range, Washing- MANUSCRIPT RECEIVED BY THE SOCIETY NOVEMBER 11, 1993 plications for subduction zone magmatism: Journal of Geo- ton: U.S. Geological Survey Map 1-2005, scale 1:500 000, REVISED MANUSCRIPT RECEIVED MARCH 30,1994 physical Research, v. 95, p. 19561-19582. pamphlet 19 p. MANUSCRIPT ACCEPTED APRIL 14,1994

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