Origin and emplacement of the andesite ofBurroughs Mountain, a zoned, large-volume lava £ow at

Mount Rainier, Washington, USA

Karen R. Stockstill a;1, Thomas A. Vogel a, Thomas W. Sisson b;�

a Department of Geological Sciences, Michigan State University, East Lansing, MI 48824-1115, USAb Volcano Hazards Program, US Geological Survey, Menlo Park, CA 94025, USA

Received 2 February 2002; received in revised form 3 May 2002; accepted 3 May 2002

Abstract

Burroughs Mountain, situated at the northeast foot of Mount Rainier, WA, exposes a large-volume (3.4 km3)andesitic lava flow, up to 350 m thick and extending 11 km in length. Two sampling traverses from flow base toeroded top, over vertical sections of 245 and 300 m, show that the flow consists of a felsic lower unit (100 m thick)overlain sharply by a more mafic upper unit. The mafic upper unit is chemically zoned, becoming slightly moreevolved upward; the lower unit is heterogeneous and unzoned. The lower unit is also more phenocryst-rich andlocally contains inclusions of quenched basaltic andesite magma that are absent from the upper unit. Widespread,vuggy, gabbronorite-to-diorite inclusions may be fragments of shallow cumulates, exhumed from the Mount Rainiermagmatic system. Chemically heterogeneous block-and-ash-flow deposits that conformably underlie the lava flowwere the earliest products of the eruptive episode. The felsic^mafic^felsic progression in lava composition resultedfrom partial evacuation of a vertically-zoned magma reservoir, in which either (1) average depth of withdrawalincreased, then decreased, during eruption, perhaps due to variations in effusion rate, or (2) magmatic rechargestimulated ascent of a plume that brought less evolved magma to shallow levels at an intermediate stage of theeruption. Pre-eruptive zonation resulted from combined crystallization^differentiation and intrusion(s) of less evolvedmagma into the partly crystallized resident magma body. The zoned lava flow at Burroughs Mountain shows that, attimes, Mount Rainier’s magmatic system has developed relatively large, shallow reservoirs that, despite complexrecharge events, were capable of developing a felsic-upward compositional zonation similar to that inferred from largeash-flow sheets and other zoned lava flows.7 2002 Elsevier Science B.V. All rights reserved.

Keywords: Mount Rainier; Cascade Range; andesites; dacites; magmatic di¡erentiation; lava £ows

1. Introduction

Systematic compositional zoning in a lava £owor £ow group provides information about theevolution and evacuation of the pre-eruptive mag-ma body because it preserves a nearly instantane-

0377-0273 / 02 / $ ^ see front matter 7 2002 Elsevier Science B.V. All rights reserved.PII: S 0 3 7 7 - 0 2 7 3 ( 0 2 ) 0 0 3 5 8 - X

1 Present address: Department of Geological Sciences,University of Tennessee, Knoxville, TN 37996-1410, USA.* Corresponding author. Tel. : +1-650-329-5247;

Fax: +1-650-329-5203.E-mail address: tsisson@usgs.gov (T.W. Sisson).

VOLGEO 2518 9-11-02

Journal of Volcanology and Geothermal Research 119 (2002) 275^296

www.elsevier.com/locate/jvolgeores

ous partial sampling of the magma reservoir sys-tem. This perspective is similar to that obtainedfrom the study of chemically zoned or layeredash-£ow sheets (Smith, 1979; Hildreth, 1981). Un-like ash-£ow sheets, which commonly have ama¢c-over-silicic zonation (Smith, 1979), lava£ows can be ma¢c at their base and become pro-gressively more silicic upward (Carrigan and Ei-chelberger, 1990; Vogel et al., 1989), or can eruptin a sequence from early silicic to progressivelymore ma¢c compositions, preserved in successive£ows or £ow lobes (Donnelly-Nolan et al., 1991;Kinzler et al., 2000; Coombs et al., 2000).The ma¢c-upward zonation in ash-£ow sheets

and the silicic-to-ma¢c progression of some lava£ows have been attributed to eruption fromchemically zoned and layered magma bodies.The shallow silicic portion erupts ¢rst, followedby evacuation of more ma¢c magma from pro-gressively deeper in the chamber (Smith, 1979;Spera et al., 1986; Mills et al., 1997). Eichelbergeret al. (2000) propose an alternate interpretationthat deep silicic magma intrudes shallow ma¢cmagma, rises through it, and erupts ¢rst. The re-verse, silicic-upward zonation in some lava £owshas been attributed to dynamic processes wheretwo magmas of contrasting composition erupt to-gether. Lower viscosity ma¢c magma overtakes,encapsulates, and outruns higher viscosity silicicmagma during £ow in the conduit (Carrigan etal., 1992; Carrigan, 1994) resulting in the silicic-

upward con¢guration of the e¡usive £ow. Unlikelarge ash-£ow sheets, where silicic-¢rst, ma¢c-lat-er eruption sequences are the rule, diverse zoningstyles in lava £ows suggest to us that competingprocesses in£uence the succession of lava compo-sitions. These processes are not well understood,in part due to a scarcity of well-studied zonedlava £ows.The Burroughs Mountain lava £ow of Mount

Rainier, WA (Fig. 1), was studied in detail be-cause it is an accessible and well-exposed repre-sentative of the large-volume andesitic lava £owsthat surround Mount Rainier. As such, its ¢eld,geochemical, and mineralogical features can helpto reveal the processes leading to sizeable erup-tions of andesitic lava. The lava £ow is a pheno-cryst-rich andesite that is chemically layered, withits upper layer being more ma¢c than its basalzone (Stockstill, 1999), similar to zoned ash-£owsheets. However, the upper layer is itself zoned,becoming systematically more felsic upward. Thiszoning re£ects temporal changes in erupted com-positions that provide insights into the magmaticzoning of a crustal reservoir and the extraction ofmagma during a relatively large andesite eruption.

2. Geologic setting and description

The Burroughs Mountain lava £ow lies at thenortheast foot of Mount Rainier between 2350

Fig. 1. Map of the lava £ow and block-and-ash-£ow deposits at Burroughs Mountain, Mount Rainier, WA, showing sample lo-cations (circles), vertical sampling traverses (brackets), and upper^lower lava boundaries (dashed lines).

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296276

and 1300 m elevation (Fig. 1). It is a large-volume(3.4 km3), up-to-350-m-thick and 11-km extend-ing andesitic lava £ow, which terminates where itabutted against glacier ice. The £ow’s thickness,its situation high above the adjacent valley, andice contact features show that the lava was im-pounded by and entrenched into the margin of athick Pleistocene glacier (Lescinsky and Sisson,1998) that ¢lled the present-day White River Val-ley. Glacial erosion has cut cirques and cli¡s intothe north and south sides of the £ow that exposenearly continuous sections from the £ow base toits eroded top. The gentle upper surface of the£ow contrasts with the steep, glacially eroded£anks, and the lava’s upper surface may be closeto the original £ow top, now stripped of its rubblycarapace. The lava erupted 496 ( N 7) kyr ago, atthe beginning of a period of vigorous volcanicactivity spanning 500^420 kyr ago, the onset ofwhich marked the beginning of modern MountRainier (Sisson and Lanphere, 2000; Sisson etal., 2001). Similar large-volume lava £ows atMount Rainier have been traced to radial-dike-fed £ank vents (Sisson, unpublished mapping),and although no direct connection is preserved,we consider it likely that the Burroughs Mountainlava £ow erupted through a radial dike systemexposed immediately to the southwest (Fig. 1).The lava £ow is porphyritic andesite-to-dacite

with abundant medium-to-coarse-grained pheno-crysts of plagioclase (to 3 mm), and lesser pyrox-ene and hornblende (to 1.5 mm). Plagioclase phe-nocrysts are sharply de¢ned laths with relativelysimple normal and oscillatory zoning parallel tophenocryst faces. Embayed zones interrupt oscil-latory and normal zoning in many phenocrysts,but the degree of embayment is small, has beenin¢lled and overgrown by subsequent plagioclase,and is absent from phenocryst margins. Stronglyresorbed spongy grains are nearly absent, but pla-gioclase phenocrysts with patchy-zoned cores andmineral or crystallized melt inclusions can befound in any thin section. Some hornblende phe-nocrysts are fresh, most are partly-to-completelyconverted to opaque oxide^silicate (opacite) pseu-domorphs, and some near the base of the £ow arereplaced by epitaxial orthopyroxene. Titanomag-netite and ilmenite are ubiquitous as microphe-

nocrysts and as groundmass grains, but are oxi-dized with blebby and lamellar unmixing. Apatiteforms inclusions in phenocrysts and less obviousneedles in the groundmass. Olivine has beenfound in a few thin sections as rare strongly em-bayed phenocrysts armored by granular orthopy-roxene, and its former presence is inferred in oth-er samples from rare Fe^oxide^orthopyroxenesymplectites. Zircon, Fe^sul¢de, and baddelyiteare in very low abundance (0^3 grains per thinsection) as 0.5^5 micron-diameter, rounded grains.Baddelyite forms minute blebs on or withincoarsely exsolved ilmenite, and may have precipi-tated from ilmenite during slow cooling and oxi-dation.Quenched magmatic inclusions (Bacon, 1986)

are locally abundant in the lower portion of thelava £ow and in the block-and-ash-£ow deposits,but have not been found in the upper part of thelava £ow. The quenched inclusions are ¢ne-grained, have ellipsoidal shapes and open vesicles,and lack phenocrysts, with the exception of tracesof olivine and rare resorbed plagioclase grainsthat were probably entrained from the host mag-ma during mingling.Coarse-grained equigranular inclusions of

vuggy gabbronorite-to-diorite are widespread inthe lava £ow and reach sizes of 15 cm. Tertiaryrocks in the Mount Rainier region di¡er fromthese coarse-grained inclusions in many respects,and we interpret the inclusions as products of theQuaternary Mount Rainier magmatic system. Pla-gioclase in nearby Tertiary rocks is characteristi-cally clouded with alteration minerals, includingsericite and carbonate, whereas plagioclase in thecoarse-grained inclusions is glass-clear and free ofalteration. Open vugs are rare in nearby Tertiaryrocks, having been ¢lled with quartz, carbonate,chlorite, and other low-temperature alterationminerals. Vugs in the coarse-grained inclusionsare open, with the exception of narrow tridymitelaths that protrude into or cross some vugs, andvug margins are de¢ned by the idiomorphic ter-minations of bounding plagioclase and pyroxenegrains. Pyroxene grains are unexsolved, and min-erals in the coarse-grained inclusions lack meltingtextures. Some coarse-grained inclusions haveoxy^hornblende pseudomorphs after amphibole,

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 277

and rare inclusions contain olivine rimmed bypyroxene, Fe^Ti oxides, and oxy^hornblende. Mi-nor apatite, and traces of zircon, baddeleyite, andTh-, U-, light-rare-earth-element-rich zirconolite(ideally CaZrTi2O7) were found in vugs in threecoarse-grained inclusions that were examinedclosely. A few micron-sized blebs of vapor-phaseargentite (AgS2) and molybdenite were also foundadhering to tridymite laths. Henceforth we referto these rocks as cognate plutonic inclusions, im-plying that they are products of the QuaternaryMount Rainier magmatic system. Although theyneed not have crystallized from the magmabatches that erupted as the Burroughs Mountaindeposits, the absence of alteration in these porousrocks is suggestive of solidi¢cation only a shorttime prior to incorporation into the BurroughsMountain magmas. Angular xenoliths (2) of¢ne-grained silicic metavolcanic rock have alsobeen found in the lava £ow but are very rare.Block-and-ash-£ow deposits consist of angular,

slightly vesiculated andesitic-to-dacitic blocks,some with radial prismatic jointing or bread-crusted surfaces, in a non-welded brown to red-dish^brown ash matrix that has been thermallyoxidized. Matrix ash grains are chie£y angular;pumice and glassy bubble walls are rare. Strati¢-cation is de¢ned by concentrations of coarseblocks and is more pronounced to the south ofBurroughs Mountain toward Mount Rainier.block-and-ash-£ow deposits accumulated as mul-tiple £ow units that reach aggregate thicknesses to200 m. Juvenile clasts share the overlying lava’sphenocryst assemblage and habit, including someblocks with distinctive orthopyroxene-replacedhornblende. Glass in most blocks is chargedwith plagioclase microlites, but sub-pumiceousblocks concentrated in thin zones within 20 mof the base of the lava £ow contain abundantmicrolite-poor vesicular glass. The conformablecontact between the lava and the unconsolidatedblock-and-ash-£ow deposits, lacking any exposedevidence of incision, as well as textural and com-positional similarities between lava and blocks,discussed in following sections, suggest that lavae¡usion closely followed block-and-ash deposi-tion, probably as part of the same eruptive epi-sode.

3. Methods

Samples were collected along the length of thelava £ow, and along north and south vertical tra-verses from the £ow base to its eroded top, inorder to reveal lateral and vertical variations inmagma compositions (Fig. 1). Whole-rock majorand trace-element concentrations were determinedby X-ray £uorescence spectrometry (XRF) for135 samples of lava, juvenile block and ash-£owclasts, and assorted inclusions. Additional trace-element (including rare-earth element) concentra-tions were determined for 51 samples, using La-ser-Ablation Inductively Coupled Plasma MassSpectrometer (LA ICP^MS) (Patino et al.,1999). Whole-rock compositions for the northand south vertical traverses through the lava£ow, representative of the £ow as a whole, arepresented in Tables 1 and 2. Compositions ofquenched magmatic inclusions from the lowerpart of the lava £ow are presented in Table 3;cognate plutonic and metavolcanic inclusion com-positions are given in Table 4. Representativecompositions of juvenile blocks from the block-and-ash deposit and of its inclusions are given inTable 5.Phenocryst compositions were determined by

electron microprobe with a four-spectrometer Ca-meca SX-50 (University of Tennessee) and anARL^SEMQ (Central Michigan University), us-ing a 15 RA sample current and a focusedbeam. Representative electron microprobe analy-ses of plagioclase, pyroxene, and hornblende aregiven in Tables 6 and 7. Glass compositions fromsub-pumiceous block-and-ash clasts (Table 8)were measured with a ¢ve-spectrometer JEOL8900 electron microprobe (USGS). Sodium migra-tion in glass was minimized with a 2 RA samplecurrent, a 15 Wm spot size, and by counting Na¢rst for 10 s.Sr, Ba, and Ca intensities in plagioclase pheno-

crysts and matrix grains were measured by LAICP^MS, using a laser spot size and penetrationdepth of 25 Wm. The Ba/Sr and Sr/Ca variation,based on count rates, were evaluated in ¢ve sam-ples, which represented the total bulk composi-tional variation within the £ow. Five to eight pla-gioclase phenocryst core^rim pairs, and three or

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296278

four plagioclase matrix grains were analyzed inreplicate in each sample. Relative count rateswere not reduced to concentration ratios becauseof a lack of appropriate standards and uncer-tainty in the Ca concentrations at the analysislocations. However, the corrections for mass

biases for these elements can be estimated fromanalyzing a known standard. Preliminary esti-mates of these corrections were made and the rel-ative variations of these ratios based on countrate were identical to the variation of these ratiosbased on corrected concentration.

Table 1Whole-rock compositions along north vertical section through the Burroughs Mountain lava £ow

Sample 97-16 97-17 98-37 98-36 98-35 98-31 98-30 98-29 98-28 98-27 98-26 98-25 98-24 97-34

M abv base 12 18 76 91 91 134 137 152 174 174 210 226 238 244Unita L L L L L U U U U U U U U USiO2 (%) 61.9 60.7 59.6 64.3 63.4 60.8 59.9 61.7 60.3 61.3 61.5 60.8 61.6 59.7TiO2 0.86 0.93 0.91 0.87 0.82 0.97 0.97 0.96 0.92 0.94 0.91 0.91 0.93 0.92Al2O3 17.0 17.2 16.7 16.1 16.5 16.7 17.0 16.4 16.5 16.4 16.5 17.0 16.5 16.9Fe2O3 5.66 6.05 6.65 5.89 5.46 6.78 6.53 6.47 6.59 6.44 6.27 6.17 6.30 6.21MnO 0.09 0.10 0.10 0.09 0.08 0.11 0.10 0.10 0.10 0.10 0.10 0.09 0.10 0.10MgO 3.05 3.33 3.26 2.86 2.66 3.93 3.85 3.58 3.97 3.79 3.60 3.50 3.63 3.61CaO 5.47 5.59 5.38 4.98 5.18 6.04 6.11 5.91 6.10 5.91 5.85 6.02 5.95 5.93Na2O 4.26 4.29 4.21 4.07 3.79 3.89 4.00 3.98 4.00 3.99 4.04 4.08 3.76 4.25K2O 1.67 1.58 1.55 1.84 1.59 1.50 1.51 1.65 1.46 1.63 1.65 1.57 1.48 1.61P2O5 0.19 0.20 0.20 0.19 0.21 0.25 0.26 0.26 0.26 0.27 0.25 0.25 0.25 0.26Total 100.2 100.0 98.6 101.2 99.7 101.0 100.2 101.0 100.2 100.8 100.7 100.4 100.5 99.5

Ni (ppm) 16 18 165 13 10 23 22 21 23 25 20 17 25 21Cu 28 67 43 20 25 28 33 29 47 35 40 32 42 24Zn 67 86 66 66 63 77 77 74 76 72 69 69 75 72Rb 42 32 38 41 32 27 34 36 33 39 38 32 31 34Sr 494 489 489 481 523 558 586 587 602 556 563 616 579 581Zr 173 160 168 182 173 177 173 176 176 180 179 175 172 168Y 17 17 17 16 15 16 17 17 17 17 16 17 19 18Nb 11 11 11 11 11 11 12 11 11 12 11 11Ba 445 413 409 443 361 412 408 435 430 408 405 417 391 415La 19.3 17.8 20.1 18.3 18.8 17.7 20.8 21.4 20.8 18.9 20.1 21.6 23.0 22.2Ce 41.7 37.7 41.3 40.7 41.0 39.2 44.9 46.0 45.5 39.3 43.8 46.7 47.1 47.7Pr 5.30 4.88 5.46 5.21 4.62 4.99 5.95 6.08 5.27 5.28 5.77 5.99 6.36 6.19Nd 20.6 19.5 21.2 20.5 20.0 20.0 23.5 24.2 23.5 21.4 23.0 23.7 25.0 24.8Sm 4.25 4.00 4.39 4.22 3.71 4.24 4.75 4.77 4.30 4.40 4.55 4.73 5.03 4.95Eu 1.22 1.19 1.23 1.24 1.18 1.36 1.40 1.41 1.31 1.31 1.37 1.39 1.36 1.39Gd 3.61 3.59 3.77 3.56 3.11 3.44 4.03 4.05 3.51 3.81 3.85 3.86 4.31 4.10Tb 0.53 0.52 0.55 0.53 0.47 0.52 0.60 0.61 0.54 0.57 0.56 0.58 0.62 0.59Dy 2.94 2.97 3.05 3.05 2.66 2.95 3.25 3.27 2.78 3.15 2.88 3.16 3.39 3.23Ho 0.54 0.55 0.55 0.55 0.43 0.53 0.58 0.63 0.47 0.57 0.54 0.59 0.63 0.58Er 1.46 1.51 1.52 1.56 1.39 1.49 1.53 1.66 1.51 1.51 1.49 1.57 1.69 1.60Yb 1.49 1.59 1.52 1.55 1.34 1.48 1.62 1.61 1.45 1.53 1.50 1.52 1.66 1.54Lu 0.20 0.22 0.21 0.22 0.21 0.20 0.22 0.23 0.22 0.21 0.21 0.22 0.22 0.21Hf 4.40 4.15 4.16 4.85 4.37 4.48 4.79 4.53 4.33 4.49 4.35 4.22Ta 0.72 0.70 0.70 0.80 0.72 0.72 0.79 0.72 0.70 0.75 0.72 0.70Pb 9.5 8.9 7.9 9.6 8.8 8.8 9.6 8.0 8.7 9.3 9.2 7.8Th 5.1 4.8 4.8 5.9 5.3 5.3 5.9 5.4 5.6 5.5 5.5 5.3U 2.2 1.9 2.0 2.5 1.8 1.9 2.1 1.9 2.0 2.0 1.9 1.9V 116 125 122 113 132 126 127 123 126 126 122 123Cr 61 70 64 60 42 86 83 82 87 85 86 87 82

Major oxides, Ni^Y, and Ba by XRF, rest by LA-ICPMS.a L, lower unit; U, upper unit.

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 279

About 65 samples were examined in thin sec-tion and seven were point counted, using a stepsize of 0.3 mm (1,725 points per thin section).Crystals larger than 0.1 mm were counted as phe-nocrysts (plagioclase, orthopyroxene, clinopyrox-ene, hornblende and magnetite/ilmenite), andsmaller grains as matrix.

4. Whole-rock and glass chemistry

4.1. Major element variations

The Burroughs Mountain lava £ow consists ofmedium-K calc^alkaline andesite and dacite (Gill,1981; Miyashiro, 1974; Irvine and Baragar,

Table 2Whole-rock compositions along south vertical section through the Burroughs Mountain lava £ow

Sample SR857 SR858 SR859 SR860 SR861 SR862 SR863 SR864

M abv base 0.2 49 143 177 207 241 268 299Unita L L U U U U U USiO2 (%) 61.9 62.9 59.5 60.9 59.1 61.0 60.9 61.0TiO2 0.83 0.81 0.98 0.92 0.94 0.92 0.93 0.91Al2O3 17.0 17.0 16.3 16.7 16.8 16.5 16.6 16.6Fe2O3 5.58 5.26 6.46 6.33 6.51 6.17 6.19 6.07MnO 0.09 0.08 0.10 0.10 0.10 0.09 0.09 0.09MgO 2.99 2.53 3.61 3.46 3.53 3.50 3.51 3.40CaO 5.71 5.15 6.13 6.09 5.76 5.92 5.96 5.83Na2O 4.24 4.14 3.95 3.95 4.09 3.99 4.01 4.03K2O 1.60 1.79 1.67 1.66 1.60 1.65 1.66 1.68P2O5 0.21 0.20 0.24 0.22 0.23 0.23 0.22 0.24Total 100.2 99.9 98.9 100.3 98.7 100.0 100.1 99.9

Ni (ppm) 30 27 42 37 142 35 38 36Cu 25 26 42 35 32 31 48 29Zn 61 57 65 69 66 66 86 66Rb 31 40 34 35 40 34 36 38Sr 628 528 616 592 574 588 599 599Zr 178 192 192 186 204 197 193 199Y 15 16 19 18 18 17 17 17Nb 12 13 13 12 13 13 13 12Ba 408 438 482 433 436 440 438 416La 19.2 19.9 26.2 23.1 24.1 24.6 23.8 23.9Ce 42.9 44.4 52.3 47.9 50.9 49.9 49.8 49.6Pr 5.18 5.33 6.76 5.97 6.34 6.38 6.30 6.33Nd 19.8 20.6 26.7 23.6 25.2 24.9 24.7 24.7Sm 4.00 4.28 5.30 4.78 5.15 5.01 4.98 5.02Eu 1.21 1.23 1.47 1.34 1.44 1.39 1.39 1.38Gd 3.29 3.55 4.40 4.12 4.19 4.11 4.09 4.13Tb 0.49 0.53 0.64 0.61 0.62 0.60 0.59 0.61Dy 2.65 2.86 3.47 3.28 3.42 3.32 3.31 3.33Ho 0.50 0.54 0.65 0.62 0.63 0.62 0.61 0.61Er 1.40 1.47 1.79 1.69 1.70 1.68 1.64 1.68Yb 1.32 1.41 1.62 1.52 1.48 1.41 1.4 1.45Lu 0.19 0.2 0.24 0.22 0.22 0.2 0.21 0.21Hf 3.23 3.6 4.06 3.72 3.91 3.48 3.81 3.69Ta 0.64 0.68 0.73 0.68 0.74 0.68 0.69 0.66Pb 9.6 8.6 6.4 8.2 5.9 7.2 9.5 5.9Th 4.4 5.0 6.1 5.3 5.2 4.9 5.2 5.2U 2.4 2.5 2.3 2.1 2.1 2.2 2.2 2.1V 124 113 135 131 124 120 120 118Cr 87 75 91 95 106 101 92 95

Major oxides, Ni^Y, and Ba by XRF, rest by LA^ICPMS.a L, lower unit; U, upper unit.

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296280

1971). Whole-rock compositions range continu-ously from 57.7 to 63.6 wt% SiO2, from 5.0 to2.5 wt% MgO, and from 1.2 to 1.9 wt% K2O(Fig. 2). Whole-rock Fe2O3ðtotalÞ, CaO, TiO2,and MgO concentrations decrease linearly withincreasing SiO2 ; Na2O concentrations are rela-tively constant near 4 wt% over the entire rangeof SiO2 values. Whole-rock P2O5 values are low-est, on average, in the highest-SiO2 rocks. Lower-

SiO2 lava samples can have higher P2O5, but therange of P2O5 values also increases, thus de¢ninga fan-shaped ¢eld on the SiO2 variation diagram(Fig. 2). K2O variations are also not simple: K2Ovalues are highest in samples with s 61 wt% SiO2,are clustered near 1.5 wt% in samples with 60^61wt% SiO2, and are scattered between 1.2 and 1.8wt% in the few samples with SiO26 60 wt%.Block-and-ash-£ow clasts cover a nearly identi-

Table 3Quenched magmatic inclusion compositions

Sample SR865 SR866 SR867 SR868 SR869 SR870 97-29i

SiO2 (%) 54.1 55.7 53.5 56.3 54.3 54.4 56.1TiO2 1.06 1.18 1.06 0.99 1.06 1.09 0.96Al2O3 17.4 18.2 17.8 18.1 18.2 17.6 18.1Fe2O3 7.65 7.49 7.53 7.07 7.46 7.64 7.41MnO 0.12 0.12 0.11 0.11 0.11 0.12 0.12MgO 5.61 3.90 5.43 4.47 5.22 5.21 4.55CaO 7.51 7.29 7.37 6.47 7.05 7.67 7.05Na2O 3.41 3.43 3.71 3.37 3.35 3.50 3.66K2O 0.98 0.89 0.93 1.20 1.08 0.97 1.11P2O5 0.19 0.23 0.20 0.17 0.19 0.19 0.17Total 98.0 98.4 97.6 98.3 98.0 98.4 99.2

Ni (ppm) 40 21 40 32 38 41 31Cu 34 106 41 24 35 574 74Zn 86 73 77 84 81 96 75Rb 21 20 21 25 19 19 25Sr 589 612 608 550 621 623 512Zr 118 145 123 129 119 119 108Y 16 17 16 13 16 16 15Nb 10 14 10 10 10 10 6 10Ba 281 284 278 361 273 268 287La 14.5 16.4 14.6 13.3 13.4 13.7Ce 29.5 30.9 30.4 25.6 28.1 28.0Pr 3.88 4.24 4.04 3.18 3.81 3.77Nd 16.3 17.7 16.3 12.5 16.1 15.7Sm 3.63 3.94 3.56 2.84 3.75 3.62Eu 1.2 1.22 1.20 1.12 1.19 1.21Gd 3.37 3.55 3.27 2.62 3.47 3.33Tb 0.51 0.52 0.50 0.42 0.53 0.52Dy 2.89 3.06 2.81 2.51 2.98 2.94Ho 0.57 0.59 0.54 0.47 0.59 0.56Er 1.52 1.57 1.49 1.27 1.55 1.49Yb 1.45 1.51 1.42 1.29 1.51 1.43Lu 0.21 0.22 0.20 0.19 0.22 0.21Hf 2.83 3.1 2.73 2.88 2.67 2.78Ta 0.51 0.76 0.53 0.57 0.51 0.54Pb 9.92 8.52 8.31 16.94 7.34 44.3Th 2.5 2.4 2.6 3.2 2.5 2.5U 0.8 1.0 0.8 1.1 0.9 0.8V 167 135 195 122 157 172Cr 154 22 134 105 133 110

Major oxides, Ni^Y, and Ba by XRF, rest by LA^ICPMS.

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 281

cal compositional range as the lava £ow, althoughwith a few outliers. Block-and-ash-£ow clast com-positions range from 56.7 to 63.5 wt% SiO2, from4.5 to 2.4 wt% MgO, and from 1.3 to 1.8 wt%K2O. An element (oxide) by element (oxide) stu-dent-T test con¢rms that the lava £ow and pyro-clastic deposits cannot be distinguished on thebasis of chemical composition (Stockstill, 1999).Microlite-poor matrix glasses from sub-pumi-ceous blocks are rhyolitic (Table 8) and lie onthe high-SiO2 extension of trends de¢ned bylava and clast whole-rock compositions (Fig. 2),including Na2O near 4 wt%.

Quenched magmatic inclusions in the lava £oware basaltic andesites (Table 3), comparable to the‘basic andesite’ (6 57 wt% SiO2) of Gill (1981).Normalized to 100 wt%, to factor out hydrationof glassy groundmass, quenched inclusion compo-sitions range from 54.8 to 57.3 wt% SiO2, from5.7 to 4.0 wt% MgO, and from 0.9 to 1.2 wt%K2O (Fig. 2). Quenched inclusion compositionslie on-trend with lava and pyroclast analyses onSiO2 variation diagrams, except for P2O5 andNa2O (Fig. 2), for which the inclusions are dis-tinctly below low-SiO2 extrapolations of the lavaand pyroclast compositional arrays.

Table 4Representative plutonic textured and metavolcanic inclusion compositions

Sample cognate plutonic metavolcanic

98-02c 98-49c 97-37c 97-41c 97-46c 97-21c 98-32c 97-40x 98-28x

SiO2 (%) 52.6 55.3 55.7 55.1 56.4 61.2 63.2 59.4 66.4TiO2 1.31 0.93 0.82 0.77 0.93 0.86 0.90 0.69 0.66Al2O3 16.8 17.9 14.7 15.2 16.4 16.8 16.4 17.8 17.5Fe2O3 9.41 8.24 9.18 9.12 8.43 5.8 6.02 6.04 2.77MnO 0.14 0.13 0.13 0.14 0.14 0.09 0.09 0.06 0.03MgO 5.40 5.01 6.89 7.51 5.54 3.15 2.99 2.20 2.96CaO 8.95 7.35 7.42 7.91 7.61 5.55 4.98 4.29 4.59Na2O 3.72 3.81 2.88 2.89 3.34 4.07 4.09 4.77 3.83K2O 0.42 0.46 1.17 0.88 0.92 1.75 1.74 1.61 1.15P2O5 0.23 0.18 0.18 0.15 0.21 0.18 0.20 0.11 0.20Total 99.0 99.3 99.1 99.7 100.0 99.4 100.6 97.0 100.1

Ni (ppm) 68 26 120 103 51 18 14 1 15Cu 257 61 306 234 84 32 17 214 77Zn 87 109 104 88 91 76 68 48 44Rb 7 7 23 16 20 40 37 35 42Sr 607 589 434 497 571 520 517 492 420Zr 71 93 130 103 105 170 178 199 139Y 16 17 15 14 17 19 17 13 17Nb 6 10 6 10 6 10 6 10 6 10 6 10 6 10 6 10 6 10Ba 253 247 304 208 285 417 386 413 441La 12.6 14.1 15.0 13.5 13.1 21.3 19.0Ce 28.6 36.5 32.3 29.7 31.2 50.5 39.7Pr 3.6 4.56 3.86 3.67 3.85 6 4.77Nd 17.8 22.1 17.0 15.4 16.8 22.8 20.8Sm 3.84 4.89 3.72 3.3 3.79 4.45 3.8Eu 1.32 1.44 1 0.92 1.24 1.33 1.12Gd 3.58 4.24 3.46 2.95 3.44 3.96 3.05Tb 0.54 0.72 0.58 0.48 0.54 0.63 0.48Dy 3.1 3.68 2.95 2.43 2.86 3.37 2.57Ho 0.54 0.63 0.56 0.46 0.55 0.6 0.45Er 1.63 1.94 1.63 1.43 1.69 1.9 1.44Yb 1.66 1.63 1.61 1.44 1.55 1.76 1.36Lu 0.24 0.26 0.25 0.23 0.25 0.29 0.21Cr 84 118 331 380 197 41 53 12 50

Major oxides, Ni^BA, and Cr by XRF, rest by LA^ICPMS

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296282

Cognate plutonic inclusion compositions scatterwidely on SiO2 variation diagrams (Fig. 2).Slightly more than half (11 of 20) of the cognateplutonic inclusion samples have lower SiO2 andK2O, and higher MgO, CaO, and Fe2O3ðtotalÞthan the lava. An origin as cumulates (£oor orsidewall) is suggested by their relatively coarsegrain sizes, textures, absence of late-magmaticminerals (quartz, K^feldspar, biotite), and theirwidely variable but ma¢c compositions. These rel-atively ma¢c cognate plutonic inclusions do notplot on-trend with the lava and pyroclast compo-sitional arrays on SiO2 variation diagrams for

TiO2, P2O5, Na2O, and Al2O3. The remainingcognate plutonic inclusion samples (9 of 20) plotamong the lava samples and may have formed bybulk solidi¢cation of magma like that of the lava£ow. The two metavolcanic xenoliths plot o¡ ma-jor oxide compositional arrays de¢ned by the lavaand pyroclast samples.

4.2. Trace-element variations

Trace-element variations in the lava and theblock-and-ash-£ow deposit are complex and gen-erally do not de¢ne narrow trends with whole-

Table 5Representative block-and-ash-£ow clast and inclusion compositions

Sample juvenile clasts quenched incl.’s plutonic incl.’s

97-6 97-10 97-9 98-17 98-39 98-13 98-11 97-54g 97-6g 98-18g

SiO2 (%) 57.3 58.8 60.6 60.7 61.6 63.3 63.8 58.2 61.4 58.3TiO2 0.92 0.85 0.8 0.88 0.83 0.72 0.73 0.92 0.76 0.79Al2O3 17.9 17.6 18.3 17.5 17.5 17.3 17.0 17.6 17.4 14.9Fe2O3 6.73 5.80 5.56 5.74 5.78 4.91 4.79 6.65 5.56 6.78MnO 0.11 0.09 0.09 0.11 0.09 0.08 0.08 0.12 0.09 0.10MgO 4.25 3.13 3.06 3.31 3.03 2.43 2.67 4.35 3.16 6.83CaO 6.63 5.79 5.48 6.15 5.67 5.35 5.33 6.58 5.65 7.57Na2O 4.00 4.62 4.25 3.65 4.20 4.09 4.28 3.97 4.25 3.00K2O 1.28 1.45 1.51 1.52 1.55 1.81 1.61 1.38 1.43 1.32P2O5 0.17 0.21 0.22 0.22 0.21 0.19 0.18 0.21 0.17 0.07Total 99.3 98.3 99.9 99.8 100.4 100.2 100.5 99.9 99.9 99.6

Ni (ppm) 52 25 23 6 10 21 6 10 6 10 29 33 74Cu 36 23 22 20 35 25 12 39 30 66Zn 75 72 72 74 67 61 71 75 67 66Rb 29 24 27 34 37 46 36 28 27 34Sr 597 633 610 622 605 621 606 612 537 355Zr 128 149 147 151 152 145 140 135 136 117Y 13 13 14 18 14 16 18 15 14 18Nb 6 10 6 10 6 10 6 10 6 10 6 10 6 10 6 10 6 10 6 10Ba 288 418 504 415 448 433 412 382 395 280La 13.2 16.4 20.4 18.4 19.3 18.3 15.8 14.6Ce 31.5 41.9 43.4 40.6 40.1 42.0 37.0 35.0Pr 3.73 4.59 5.01 4.74 4.35 4.42 4.06 3.89Nd 15.8 18.0 18.9 21.1 18.6 18.3 16.5 15.8Sm 3.25 3.33 3.78 3.97 3.39 3.34 3.34 3.17Eu 1.14 1.12 1.16 1.25 1.17 1.05 1.21 1.06Gd 2.87 2.71 3.35 3.33 2.86 2.75 2.75 2.62Tb 0.45 0.43 0.48 0.52 0.43 0.42 0.45 0.43Dy 2.37 2.17 2.64 2.76 2.34 2.2 2.38 2.12Ho 0.43 0.39 0.47 0.49 0.39 0.39 0.42 0.41Er 1.32 1.21 1.63 1.54 1.3 1.26 1.26 1.21Yb 1.32 1.28 1.34 1.52 1.31 1.38 1.25 1.21Lu 0.21 0.19 0.2 0.22 0.17 0.18 0.19 0.18Cr 101 39 34 32 54 20 29 57 69 486

Major oxides, Ni^Ba, and Cr by XRF, rest by LA^ICPMS.

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 283

rock SiO2 (Fig. 3) or other indices of di¡erentia-tion. Concentrations of Sr, Cr, and Zn that arecompatible in the major phenocryst minerals de-crease with increasing SiO2 and decreasing MgO(not illustrated) but scatter in excess of analyticalprecision. Variations of Rb with SiO2 resemblethose of K2O, with the highest Rb, on average,in rocks with s 61 wt% SiO2, and scattered Rb

values in rocks with 6 60 wt% SiO2. Like Rb, Zris incompatible in the phenocryst minerals, but Zrconcentrations do not increase uniformly in high-er SiO2 rocks.Quenched magmatic inclusions have distinctly

lower Rb, Zr, Ba, and La/Yb than the lava andpyroclast samples (Fig. 3). Quenched inclusion Srconcentrations are similar to those of lavas and

Fig. 2. Major-oxide^SiO2 variation diagrams for Burroughs Mountain lava and pyroclastic-£ow samples. Arrows show trajectoryto matrix glass in sub-pumiceous block-and-ash-£ow clasts.

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296284

pyroclasts, but fall far o¡ the trend of increasingSr with decreasing SiO2 de¢ned by the lava andpyroclast suites. Cognate plutonic inclusions withlava-like major element compositions also havelava-like trace-element abundances. Ma¢c, poten-tially cumulate cognate plutonic inclusions di¡erfrom lava samples in their lower Rb, Zr, Ba, andLa/Yb. As with quenched inclusions, the ma¢c

cognate plutonic inclusions plot o¡ the low-SiO2projection of the lava’s Sr^SiO2 array.

4.3. Spatial variations

Closely spaced samples were collected up thesteep north and south sides of the lava £ow(Fig. 1) to explore for vertical variations in lava

Fig. 3. Trace-element^SiO2 variation diagrams for Burroughs Mountain lava and pyroclastic-£ow samples. Representative analyti-cal uncertainties are N 2 standard errors.

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 285

composition that might give insights into eruptiondynamics and chemical zonation of the pre-erup-tive magma reservoir. Both traverses show similarmajor and trace-element variations with height inthe £ow and reveal that the £ow is verticallyzoned (Figs. 4 and 5). Rocks from the upper por-tion of the lava £ow (150^200 m thick) are slightlymore ma¢c than rocks from the lower portion(100 m thick). Average Fe2O3ðtotalÞ of upper andlower portion samples are 6.4 and 5.8 wt%, re-spectively. The upper^lower transition is abrupt,and the £ow consists of chemically distinct upperand lower sections. On the north traverse, thetransition is marked by a zone of rubble andlava spines, overlain by massive lava of the baseof the upper portion. The zone of rubble andspines resembles a £ow top, but the overlyingmassive lava lacks cooling features, such as nar-row glassy columns, that would indicate a pausebetween emplacement of the portions of the £ow.Talus conceals the lower^upper boundary alongthe south sample traverse. Except along the northtraverse, the upper^lower boundary is not ob-vious in the ¢eld and was not traced throughoutthe £ow during mapping.In addition to being more ma¢c, the upper

layer has a weak vertical chemical gradient, be-coming more evolved upward. This gradient isconspicuous for Fe2O3, MgO and TiO2 (Fig. 4)and modest for CaO (not illustrated). For othermajor oxides, the gradient is absent or is su⁄-ciently weak to be di⁄cult to resolve above ana-lytical precision. There may be a slight increase inSiO2 and decrease in P2O5 with height in theupper layer, whereas K2O appears to lack a ver-tical trend. No chemical gradient was found in thelower portion of the £ow. Samples from elsewherealong the £ow are consistent with the lower-felsic,upper-ma¢c division (Stockstill, 1999) but are in-su⁄cient to reveal the upper portion’s weak com-positional gradient.The lower-felsic, upper-ma¢c division is also

apparent in compatible trace-element concentra-tions. As a group, lower layer samples have lowerconcentrations of Sr and V (Fig. 5), and Cr andZn (not illustrated) than do upper layer samples.Incompatible trace-element di¡erences are less ob-vious, although for some elements (Rb) and ele-

ment ratios (Th/U, La/Yb) the upper^lowerboundary is distinct. The evolved-upward gra-dient in the upper layer is manifest in decreasingV concentrations, slightly decreasing Th/U, andincreasing La/Yb with height in the layer. Nogradient is apparent for Sr, Zr, or Ba. Rb concen-trations may increase slightly with height in thelayer, although any Rb gradient is near the limitof what can be distinguished by our analyses.The di¡erence in composition of the upper and

lower layers is matched by small di¡erences inphenocryst abundance. Modes measured on fourlower layer and three upper layer samples showthat evolved lower layer rocks contain 36^45 vol%phenocrysts, whereas ma¢c upper layer rocks con-tain 23^38 vol% phenocrysts. Upper and lowerlayers have equal quantities of ferromagnesianminerals (8^10 vol%), and the di¡erence in pheno-cryst content is due to more abundant plagioclasein the evolved lower layer.

4.4. Phenocryst abundances and compositions

Evolved, lower layer lava is illustrated by sam-ple 97-17, which was collected from near the baseof the north sample traverse (Table 1). Thegroundmass (56 vol%) is dominated by very¢ne-grained (6 0.05 mm) plagioclase laths. Pla-gioclase phenocrysts (34%) are chie£y euhedrallaths to 2.5 mm with length to width ratios rang-ing from 2:1 to 6:1 and averaging around 3:1.Phenocrysts of hornblende (3.5%), clinopyroxene(2.9%), and orthopyroxene (1.8%) are euhedral-to-subhedral grains ranging from 0.1 to 1.5 mm.Ilmenite and magnetite grains (1.3%) range from6 0.1 to 0.4 mm. Tiny needles of apatite are in-cluded in all of the phenocryst minerals but aredi⁄cult to distinguish in the matrix.Upper layer lava is illustrated by sample 97-34,

collected from the top of the north sample tra-verse (Table 1). The groundmass (64%) is domi-nated by very ¢ne-grained plagioclase laths. Pla-gioclase is the most common phenocryst (27%),with the same size range and habit as for the low-er layer. Clinopyroxene (3.8%) is a marginallymore abundant phenocryst than orthopyroxene(3.0%) and both form euhedral-to-subhedralgrains from 0.1 to 0.7 mm. Euhedral-to-subhedral

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296286

phenocrysts of hornblende (1.4%) range from 0.1to 0.5 mm. Ilmenite and magnetite grains (1.3%)range from 6 0.1 to 0.4 mm, and apatite ispresent as tiny needles included in the phenocrystsand in the groundmass.Plagioclase phenocryst compositions in the en-

tire £ow range from An37 to An58 (representativeanalyses in Table 6, extensive analyses in Stock-

still, 1999). Plagioclase phenocrysts in the evolvedlower layer span the full range of compositions,whereas plagioclase in the more ma¢c upper layeris slightly less diverse, limited to between An41and An53. Zoning is generally limited within sin-gle phenocrysts to 6 10 mol% An, and pheno-cryst rims are consistently more sodic than cores(Table 6).

Fig. 4. Selected major-oxide variations with height above £ow base along north (circles) and south (squares) vertical sample sec-tions. Filled and open symbols distinguish upper and lower compositional divisions. Analytical uncertainties are N 2 standard er-rors.

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 287

Plagioclase phenocrysts from upper and lowerlayer samples along the north traverse were exam-ined with LA ICP^MS to determine zoning of Baand Sr. Intensities of Ba, Sr and Ca were mea-sured on cores and rims of plagioclase pheno-crysts, and on matrix plagioclase, and the Ba/Srversus Sr/Ca ratios are plotted in Fig. 6. Pheno-

cryst core compositions from the upper and lowerlayers overlap, but matrix grains di¡er ^ matrixgrains from the evolved lower layer have higherBa/Sr and lower Sr/Ca than do upper layer matrixgrains. Core^rim zoning is variable, but generallythe phenocryst rim compositions approach ormatch those of the matrix grains. This re£ects

Fig. 5. Selected trace-element variations with height above £ow base along north (circles) and south (squares) vertical sample sec-tions, and MgO^P2O5 variation diagram for vertical section samples. Filled and open symbols distinguish upper and lower com-positional divisions. Analytical uncertainties are N 2 standard errors.

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296288

two types of zoning. In lower layer samples Ba/Srgenerally increases from phenocryst cores to rimsand matrix grains, whereas in many upper layerphenocrysts Ba/Sr decreases from cores to rimsand matrix grains. Rare phenocrysts in the lowerlayer also have this ‘reversed’ zoning trend. Someanomalous phenocrysts in the upper layer havedistinctly high Ba/Sr values (Fig. 6), which maysignal crystallization from a high Ba/Sr melt.Orthopyroxene and clinopyroxene phenocrysts

are MgO-rich with relatively small ranges in com-position (Table 7). Over the entire £ow, orthopy-roxene phenocrysts range from Wo2:9En61:3Fs35:8to Wo3:0En67:1Fs29:9. Orthopyroxene phenocrystsfrom the lower layer have a slightly broader rangein compositions than the orthopyroxene pheno-crysts from the more ma¢c upper layer (Stockstill,1999). Clinopyroxene phenocrysts have less di-verse compositions than orthopyroxene, rangingfrom Wo42:3En41:6Fs16:1 to Wo44:2En40:6Fs15:2 (Ta-ble 7). Hornblende analyses are shown in Table 7,

but are insu⁄cient to correlate with zoning in thelava.Trace quantities of zircon (6 5 microns) were

found in a few lava and block-and-ash-£ow sam-ples that were examined closely by backscatterelectron imaging, regardless of whether the sam-ples were well crystallized or glassy. Zircon is notnormally an early crystallizing mineral in Zr-poorsubduction-zone andesites, so zircon may deriveby assimilation of zircon-bearing rock, possiblythe cognate plutonic inclusions, or by mixingwith low-temperature zircon-saturated magma.A single xenocrystic grain of resorbed quartzand very rare resorbed biotite xenocrysts havealso been found in the lava, but these mineralsare not present in the cognate plutonic inclusionsand must have originated from a minor additionalassimilant or by mixing with an evolved, quartz-and biotite-bearing magma. Biotite is also presentas a trace groundmass constituent in well-crystal-lized samples.

Table 6Representative electron microprobe analyses of plagioclase along rim^core traverses

Sample n SiO2 Al2O3 Fe2O3 CaO SrO BaO Na2O K2O Total Ab Or An

97-17Phenocryst 1 rim 6 57.4 26.1 0.39 8.46 0.12 0.03 6.38 0.55 99.4 55.9 3.2 40.9

6 56.2 26.6 0.42 9.13 0.14 0.03 6.07 0.43 99.0 53.3 2.5 44.35 56.8 26.4 0.42 8.76 0.14 6 0.03 6.27 0.38 99.2 55.2 2.2 42.6

mid 6 57.0 26.2 0.39 8.69 0.13 0.05 6.42 0.35 99.2 56.1 2.0 41.95 56.0 26.8 0.39 9.16 0.14 0.03 6.15 0.30 99.0 53.9 1.7 44.45 55.4 27.3 0.42 9.83 0.14 6 0.03 5.76 0.26 99.1 50.7 1.5 47.8

core 1 55.4 27.3 0.43 9.79 0.13 0.05 5.78 0.26 99.1 50.9 1.5 47.6Phenocryst 2 rim 5 55.7 27.1 0.48 9.8 0.13 0.04 5.73 0.39 99.3 50.4 2.3 47.4

6 55.3 27.3 0.43 10.0 0.13 0.05 5.56 0.35 99.1 49.1 2.0 48.8mid 5 55.2 27.4 0.42 10.2 0.13 6 0.03 5.62 0.27 99.2 49.2 1.6 49.3

6 55.0 27.5 0.46 10.3 0.13 0.04 5.51 0.24 99.2 48.5 1.4 50.1core 5 54.9 27.5 0.46 10.4 0.13 0.06 5.49 0.25 99.2 48.2 1.4 50.4

97-34Phenocryst 1 rim 5 56.7 26.2 0.43 8.88 0.15 0.04 6.22 0.41 99.0 54.6 2.4 43.1

4 56.3 26.5 0.42 9.25 0.15 0.03 6.08 0.40 99.1 53.1 2.3 44.6mid 5 56.5 26.3 0.43 8.98 0.14 0.06 6.12 0.40 98.9 53.9 2.3 43.7

4 56.3 26.5 0.41 9.16 0.12 0.05 5.99 0.38 98.9 53.0 2.2 44.8core 4 54.8 27.5 0.39 10.3 0.14 0.03 5.48 0.31 99.0 48.2 1.8 50.0

Phenocryst 2 rim 5 55.6 27.2 0.42 9.71 0.15 0.03 5.75 0.34 99.2 50.7 2.0 47.34 54.5 27.8 0.44 10.4 0.14 6 0.03 5.33 0.29 98.9 47.3 1.7 51.0

mid 5 55.2 27.3 0.43 9.86 0.15 0.06 5.61 0.31 98.9 49.8 1.8 48.45 55.7 26.9 0.45 9.39 0.14 0.04 5.83 0.34 98.8 51.9 2.0 46.2

core 5 55.1 27.3 0.48 9.99 0.16 0.04 5.55 0.3 98.9 49.3 1.8 49.0

n gives number of analyses averaged.

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 289

5. Discussion

5.1. Crystallization^di¡erentiation and magmamixing

Major and trace-element variations in the Bur-roughs Mountain lava and pyroclastic depositssignal important roles for both crystallization^dif-ferentiation and magma mixing. Glasses fromblock-and-ash-£ow clasts (Table 8) lie on thehigh-SiO2 extensions of lava and clast major ele-ment arrays (Fig. 2). Variable segregation of suchevolved melts, or variable extents of crystalliza-

tion^di¡erentiation along liquid lines of descentleading to such evolved melts, would accountfor the dominant major element trends, includingnearly constant Na2O over a wide range of SiO2,and decreasing P2O5 and compatible major oxideswith increasing SiO2. Complex variations of K2O,Rb, Ba, Sr, and Zr (Figs. 2 and 3) rule out simplecrystal-fractionation of a single parent magma.Instead, the Burroughs Mountain magma reser-voir may have been assembled from multiplebatches of similar, but not identical andesiticmagma that crystallized the same phenocryst as-semblages and therefore followed similar liquid

Fig. 6. Uncorrected LA ICP^MS count rates ratios (see text for explanation) for plagioclase in representative lower, felsic (A,samples 97-2 and 97-3; B, sample 97-16) and upper, ma¢c (C, sample 97-34; D, sample 97-46) layer samples along the northsample traverse. Lines connect phenocryst core^rim pairs. E summarizes late-crystallized compositional groups.

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296290

lines of descent. Mixing between these magmabatches may have produced the scatter in trace-element abundances.Multiple regression calculations (Bryan et al.,

1969), using average phenocryst compositions(Tables 6 and 7), whole rocks (Tables 1 and 2,and Stockstill, 1999), and glasses (Table 8), sup-port the inference that crystallization^di¡erentia-tion was a dominant process in producing theBurroughs Mountain andesite^dacite suite. Rep-resentative dacite (an average of seven lava anal-yses with SiO2s 63 wt%, normalized to 100%,with all Fe as FeO) can be produced from a rep-resentative andesite parent (an average of 22 lavaanalyses with SiO2 60^61 wt%) by 26% crystalli-zation of an assemblage of plagioclase (69.1%),orthopyroxene (22.9%), clinopyroxene (3.5%),Fe^Ti oxides (3.7%), and apatite (1%) (4re-siduals2 = 0.07). Calculations including horn-

blende also produce acceptable results but yieldslightly negative coe⁄cients for clinopyroxene,perhaps signifying a hornblende^clinopyroxenereaction relation. Average rhyolitic matrix glasscan be produced from the representative andesiteparent by crystallization of 66% the same mineralassemblage in nearly identical proportions: pla-gioclase (69.5%), orthopyroxene (20.1%), clino-pyroxene (5.3%), Fe^Ti oxides (4.3%), and apatite(0.7%) (4residuals2 = 0.007). Crystallization^dif-ferentiation of similar parent magmas along acommon liquid line of descent can account forclose agreement of mineral assemblages and pro-portions estimated from whole rocks, and semi-independently from whole rocks and glasses. Mix-ing of unrelated magmas produced from indepen-dent sources, or large extents of assimilation gen-erally would not. Small variations in some trace-element ratios (eg. Th/U; Fig. 5) cannot be due to

Table 7Representative electron microprobe analyses of pyroxene and hornblende phenocrysts

Sample n SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O F Cl Total Mg#

97-17Opx 1 rim 4 52.4 0.21 1.12 0.07 20.6 0.50 23.3 1.24 0.04 99.5 61.1

mid 5 52.5 0.32 1.19 6 0.03 19.5 0.44 23.9 1.51 0.03 99.4 63.0core 4 52.9 0.19 0.91 0.03 20.3 0.51 23.6 1.29 0.04 99.8 61.8

Opx 2 rim 4 51.6 0.22 0.89 0.03 22.0 0.56 22.3 1.53 0.03 99.2 58.5mid 2 52.0 0.23 0.90 0.04 22.2 0.56 22.5 1.33 0.03 99.8 58.5core 1 51.0 0.20 0.89 0.03 21.7 0.57 22.2 2.28 0.03 98.9 58.7

Hbl rim 1 41.6 3.56 11.9 6 0.03 12.1 0.08 14.6 10.8 2.70 0.34 0.34 6 0.03 98.0 62.7mid 1 41.2 3.55 11.6 6 0.03 13.4 0.17 14.9 10.7 2.63 0.32 0.79 6 0.03 99.3 60.7core 1 41.0 3.59 11.7 0.07 12.2 0.10 14.3 10.8 2.93 0.32 2.13 6 0.03 99.1 62.0

97-34Opx rim 4 52.3 0.18 0.82 6 0.03 21.3 0.54 23.0 1.17 0.03 99.3 60.0

mid 4 52.7 0.18 0.73 6 0.03 21.4 0.55 23.1 1.14 6 0.03 99.8 60.0core 2 52.6 0.23 1.04 6 0.03 21.5 0.54 22.4 1.22 0.03 99.6 59.2

97-12Opx rim 1 51.2 0.1 0.70 6 0.03 24.8 0.66 21.4 1.08 6 0.03 99.9 54.6

mid 1 51.7 0.08 0.49 6 0.03 24.7 0.58 21.3 0.98 0.09 99.9 54.5Cpx 1 mid 1 48.3 0.78 4.84 6 0.03 11.1 0.35 12.2 22.0 0.51 100.1 60.5

core 1 49.9 0.83 4.66 0.08 11.5 0.35 12.8 19.4 0.52 100.0 60.8Cpx 2 rim 1 50.9 0.55 2.90 6 0.03 7.4 0.27 16.4 21.3 0.27 100.0 75.5

core 1 50.9 0.37 1.97 0.04 10.8 0.39 13.8 21.4 0.42 100.1 64.0

97-14Opx rim 1 52.6 0.43 2.02 6 0.03 18.6 0.47 24.0 1.78 0.14 100.0 64.2

core 1 52.9 0.46 1.96 0.09 18.1 0.51 23.8 2.06 0.04 99.9 64.7Cpx rim 1 52.4 0.30 1.40 6 0.03 9.91 0.24 14.2 21.2 0.33 100.0 66.6

core 1 51.1 0.56 2.54 6 0.03 10.2 0.27 13.6 21.3 0.41 100.0 65.0

Abbreviations and symbols: n, number of analyses averaged; opx, orthopyroxene; cpx, clinopyroxene; hbl, hornblende;Mg#=100 Mg/(Mg+Fe).

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 291

crystallization^di¡erentiation of the lava’s pheno-crysts, and either result from assimilation or fromthe introduction of evolved, accessory mineral-sa-turated liquids from chamber-margin cumulates.Quenched magmatic inclusions in the lower

part of the lava £ow and in the block-and-ash-£ow deposits are indisputable evidence that theBurroughs Mountain andesite^dacite magmaticsystem was also injected with new basaltic ande-site liquids. Those liquids’ compositions do not lieon the arrays de¢ned by lava and clast samplesfor some major and trace elements (Figs. 2 and 3).Mixing with such basaltic andesite liquids cannot,therefore, have produced the dominant composi-tional variations in the andesite^dacite lava andpyroclastic £ows. Mixing with basaltic andesiteliquids might account for the deviation of somelava and pyroclastic £ow samples o¡ the domi-nant compositional arrays, such as the scatteringof samples to low-P2O5 at low-SiO2. Quenchedmagmatic inclusions with compositions appropri-ate to plot at the ma¢c end of the andesite^daciteseries are present in other Mount Rainier lava£ows (Sisson, unpublished analyses), but are ab-sent at Burroughs Mountain, and if such andesiticliquids recharged the Burroughs Mountain mag-ma reservoir, they failed to quench to inclusions.The straight-sided and relatively inclusion-freehabits of plagioclase phenocrysts, lacking pro-nounced resorbtion, suggest that in£uxes of basal-tic andesite were too small to destabilize residentphenocrysts and took place su⁄ciently long be-fore eruption that conditions stabilized in themagma reservoir and weakly resorbed grainswere overgrown.Disaggregation of cognate plutonic inclusions

would have a similar e¡ect on lava and pyroclastcompositions as mixing with basaltic andesite,producing scatter to low P2O5, Na2O, and Sr at

low SiO2 (Figs. 2 and 3). Traces of zircon in lavaand pyroclastic £ow samples, unusual for an-desites and dacites, and the presence of zircon inthe cognate plutonic inclusions are consistent withthe notion that inclusions have disaggregated anddispersed into the andesite^dacite magma. If thelower-SiO2 cognate plutonic inclusions are cumu-lates, they were not formed by crystallization^dif-ferentiation of Burroughs Mountain andesite toproduce dacite. The cumulates may have formedby crystallization^di¡erentiation of basaltic an-desite to produce andesite, but this possibility re-quires further work to be veri¢ed.

5.2. Spatial^temporal variations

The Burroughs Mountain eruptive episode be-gan with compositionally diverse andesite^daciteblock-and-ash £ows. Compositional diversitycould result from tapping magma from wide-spread locations within a heterogeneous or zonedmagma reservoir, perhaps during recharge eventsrecorded by quenched inclusions, or it could re-sult from eruption of discrete magma batches dur-ing assembly of the shallow magma reservoir thatfed the subsequent lava £ow. The eruptive episodeswitched abruptly to e¡usion of lava, with insuf-¢cient time for incision and erosion of unconsoli-dated block-and-ash deposits. The transition toe¡usive eruption might result from establishmentof a radial-dike-fed £ank vent, as for some otherlarge lava £ows at Mount Rainier, but we have noconclusive evidence to establish this. The ¢rst lavato erupt was relatively evolved and phenocryst-rich and formed the lower portion of the Bur-roughs Mountain lava £ow. Some of this earlylava carried vesicular quenched basaltic andesiteinclusions, suggesting further recharge events.Erupting lava abruptly became more ma¢c and

Table 8Electron microprobe analyses of glass in sub-pumiceous block-and-ash-£ow clasts

Sample n SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Cl SO3 Totala

SR754 15 74.5 0.49 13.3 1.74 0.02 0.30 1.57 4.14 3.72 0.10 0.10 0.03 95.9SR755 15 75.9 0.42 12.6 1.62 0.03 0.16 1.11 3.69 4.27 0.12 0.09 0.01 94.6SR755-melt incl. 5 74.4 0.56 14.0 1.34 0.08 0.32 1.53 4.60 2.87 0.10 0.14 0.02 96.5a Analyses normalized to sum to 100%. Total gives original sum, n gives number of analyses averaged.

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296292

slightly poorer in plagioclase phenocrysts. Thisslightly more ma¢c lava overrode the earliestevolved lava and formed the upper portion ofthe £ow. As the eruption waned, e¡using magmabecame progressively more evolved, creating theweak felsic-upward zonation in the upper part ofthe lava £ow. Progressive emplacement of andes-itic lava as a vertical succession, as opposed to aseries of distributed £ow lobes (Blake and Bruno,2000), may result from its con¢nement againstglacial ice (Lescinsky and Sisson, 1998; Lescinskyand Fink, 2000).The ma¢c over felsic layering of the Burroughs

Mountain lava £ow, without a separating coolingbreak, is similar to many chemically zoned ash-£ow sheets. Two possible explanations are: (1)layering developed from eruption of a previouslyzoned magma body; or (2) layering developed bythe immediately pre- or syn-eruptive intrusion ofmore ma¢c magma into a crystallizing less ma¢cmagma body. In the ¢rst interpretation, pheno-cryst-rich evolved magmas could have developedby crystallization^di¡erentiation (and perhaps as-similation) and accumulated due to buoyancy inthe upper part of a magma body. Lower portionsof the reservoir may have been occupied by lessdi¡erentiated and probably hotter magmas, andperiodic replenishments of less evolved magmainto the roots of a magma body are likely tohave contributed to vertical zonation. This zona-tion would then have been preserved in invertedform during eruption of the lava. The weak felsic-upward zonation in the upper layer might haveresulted from tapping of magma from progres-sively shallower portions of the zoned reservoirsystem as the eruption waned.In the second interpretation, the spatial zona-

tion of the lava £ow resulted from mixing pro-cesses at or immediately before the time of erup-tion and does not re£ect pre-eruptive zonation ofa magma reservoir. In this interpretation, the pre-eruptive magma system consisted of a crystalliz-ing, more evolved, and phenocryst-rich magmabody that was intruded by more ma¢c, pheno-cryst-poor magma(s). The resident evolved mag-ma began to erupt ¢rst, creating the lower portionof the lava £ow, followed by eruption of mixedmagma that created the upper portion of the £ow.

With time, greater extents of mixing with theevolved resident magma might have created thefelsic-upward zonation of the upper lava layer.The quenched basaltic andesite inclusions are

the clearest candidates for a newly introducedmagma, but compositional variations in the lava£ow as a whole (Fig. 2) and in the vertical samplesections (Fig. 5) do not result from mixing withmagmas like the basaltic andesite inclusions. Mix-ing with basaltic andesite inclusion magmaswould produce compositions with relatively highMgO and low P2O5, unlike the more ma¢c lavasamples. If zonation in the lava was due to mixingshortly before eruption, the recharge magma wasnot preserved as macroscopically distinguishableinclusions or streaks, and it left little evidence inthe form of phenocryst textures or gross changesin phenocryst compositions. For these reasons, weinfer that any sizeable syn- or immediately pre-eruptive recharge magmas must have been verysimilar to the resident magmas, or more simply,that no sizeable recharges took place. Small vol-ume Holocene tephras at Mount Rainier preserveobvious evidence of pervasive syn-eruptive mixingand mingling (Mullineaux, 1974; Swanson, 1993;Venezky and Rutherford, 1997), as do some lava£ows (McKenna, 1994; Sisson, unpublished map-ping), but the large-volume lava £ow at Bur-roughs Mountain is not one of these.Plagioclase trace-element ratios further illumi-

nate the magmatic and eruptive history. Di¡eren-ces between lower and upper portions of the £owin matrix grain Ba/Sr and Sr/Ca (Fig. 6) showsthat liquid compositions varied during the courseof the eruption. Early erupted magma was moreevolved, richer in phenocrysts, and contained amore-evolved (higher Ba/Sr) melt. Later eruptedmagmas were more ma¢c, slightly poorer in phe-nocrysts, and contained less-evolved (lower Ba/Sr)melt. Increasing Ba/Sr from plagioclase pheno-cryst cores to rims and matrix grains is the senseof zoning expected for crystallization^di¡erentia-tion of a plagioclase-rich assemblage, where Sr ismore compatible than Ba (Blundy and Wood,1991). This normal Ba/Sr zoning in the lower£ow unit is consistent with magma that was cool-ing and crystallizing in a relatively simple fashion.Reversed trace-element zoning of plagioclase in

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 293

the upper £ow unit, beginning with plagioclaselike that in earlier-erupted rocks, may signifythat phenocrysts from the evolved portion of themagmatic system became engulfed in a lessevolved, lower Ba/Sr liquid. Mixing between resi-dent magma and a newly injected less fractionatedmagma or simple settling of shallow plagioclasephenocrysts into deeper, less fractionated parts ofa zoned magma reservoir could account for thereversed Ba/Sr zoning. In either instance, compo-sitional and thermal contrasts were too small tostrongly destabilize the plagioclase phenocrysts orto produce phenocryst-rim plagioclase distinctlymore anorthitic than that in the earlier-eruptedportion of the £ow (Table 6). These subtle zoningfeatures are most consistent with small di¡erencesin melt composition and temperature, and suggestmixing or crystal settling within the andesite^da-cite compositional suite. The relatively simplecompositional variations in the lava £ow, andthe absence of features attributable to pervasive

mixing of strongly diverse magmas immediatelybefore or during eruption, support the zoned res-ervoir interpretation for the magmatic system thatfed the Burroughs Mountain eruptions.

5.3. A possible eruption scenario

Temporal variations in erupted compositionscan result from the dynamics of magma with-drawal from a zoned reservoir. High withdrawalrates tap magma over a wide depth range in amagma chamber, whereas low withdrawal ratestap magmas dominantly from the chamber’supper portions (Spera et al., 1986). High magmawithdrawal rates and consequent sampling over awide depth range could account for the composi-tionally diverse Burroughs Mountain pyroclastic£ows that began the eruptive episode (Fig. 7).Quenched magmatic inclusions in those depositssuggest that recharge events preceded the erup-tion, but evidence is inadequate to determine if

Fig. 7. Interpretation of the Burroughs Mountain eruptive episode and zone magma reservoir (not to scale). (1) Stage 1, high ef-fusion rates feed ash £ow eruptions and tap magmas over a wide depth range from a vertically zoned, recharged magma reser-voir. (2) Stage 2, possible opening of a radial dike (or other change in conduit con¢guration) allows e⁄cient degassing and e¡u-sive eruption. Low e¡usion rates tap magma chie£y from the evolved, shallow portion of the reservoir. Modest recharge withbasaltic andesite continues. (3) Stage 3, sharply increased e¡usion taps magmas from deeper in the zoned reservoir, producingthe more ma¢c upper portion of the lava. As the eruption waned, progressively more felsic magmas were tapped from shallowerin the reservoir, producing the felsic-upward zonation in the lava’s upper layer.

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296294

recharge caused eruption. Disruption of thechamber margins distributed cognate plutonic in-clusions throughout the magma. Lava e¡usionthen followed, due to opening of a radial dike(Fig. 7) or other changes in conduit con¢gura-tions. Early lava was restricted to evolved compo-sitions, perhaps signifying low withdrawal ratesthat tapped magma only from shallow parts ofthe reservoir. Quenched basaltic andesite inclu-sions record further recharge events, althoughthese were inadequate to destabilize phenocrystsor to strongly modify bulk magma compositions.Erupting lava then abruptly became more ma¢cand overrode the earlier-erupted lava, producingthe more ma¢c upper portion of the £ow, but thechange to more ma¢c compositions was not dueto mixing with basaltic andesite recharge magmas.Instead, the shift to ma¢c compositions mighthave resulted from greatly increased e¡usive £uxthat sampled less di¡erentiated andesite fromdeeper in the reservoir. Alternatively, rechargemay have initiated a plume that brought less frac-tionated resident andesite magmas to shallow lev-els where they became available for eruption(Clynne, 1999). As the eruption waned and e¡u-sive £ux declined, magma was tapped from pro-gressively shallower portions of the reservoir, pro-ducing the felsic-upward zonation of the upperpart of the £ow.

6. Conclusion

We interpret the layered Burroughs Mountainlava £ow to result from the partial evacuation ofa zoned, partly crystallized andesite^dacite mag-ma body, wherein depth of magma withdrawalwas coupled to e¡usion rate. The reservoir wasassembled from similar, but not identical andesiticmagma batches that crystallized a common pla-gioclase^pyroxene^amphibole^Fe-Ti oxide^apa-tite assemblage. More fractionated magmas occu-pied shallower portions of the reservoir. Deeperportions were more ma¢c due to the combinede¡ects of recharge events and less advanced crys-tallization^di¡erentiation. Initial eruptions pro-duced block-and-ash £ows that were composition-ally indistinguishable from the subsequent lava,

suggesting that conduit dynamics, not magmacomposition, determined explosive versus e¡usiveeruption styles. E¡usive eruption ¢rst tappedmagma from the shallow evolved portion of themagma reservoir. More ma¢c lava followed, per-haps due to tapping of less fractionated magmafrom deeper in a zoned reservoir. As the eruptionwaned, evolved magma from shallow in the sys-tem increasingly dominated the erupting compo-sition, leading to a weak felsic-upward zonation inthe upper portion of the lava £ow. The zoned lava£ow at Burroughs Mountain shows that, at times,Mount Rainier’s magmatic system has developedrelatively large, shallow reservoirs that, despitecomplex recharge events, were capable of devel-oping a felsic-upward compositional zonationsimilar to that inferred from large ash-£ow sheetsand other zoned lava £ows.

Acknowledgements

We thank the US Department of the Interior,National Park Service for permission to conductresearch in Mount Rainier National Park. Con-structive and insightful reviews by Julie Donnelly-Nolan, Mike Clynne, Jim Gardner, and AnitaGrunder led to a much-improved manuscript.Lina Patino developed and guided the LA-ICP-MS analytical techniques, and R. Thomas, J.P,Brandenburg, and J. Weaver assisted with samplepreparation.

References

Bacon, C.R., 1986. Magmatic inclusions in silicic and inter-mediate volcanic rocks. J. Geophys. Res. 91, 6091^6112.

Blake, S., Bruno, B.C., 2000. Modelling the emplacement ofcompound lava £ows. Earth Planet. Sci. Lett. 184, 181^197.

Blundy, J.D., Wood, B.J., 1991. Crystal-chemical controls onthe partitioning of Sr and Ba between plagioclase feldspar,silicate melts, and hydrothermal solutions. Geochim. Cos-mochim. Acta 55, 193^209.

Bryan, W.B., Finger, L.W., Chayes, F., 1969. A least-squaresapproximation for estimating the composition of a mixture.Carnegie Inst. Washington Year B. 67, 243^244.

Carrigan, C.R., Eichelberger, J.C., 1990. Zoning of magmas byviscosity in volcanic conduits. Nature 343, 248^251.

Carrigan, C.R., 1994. Two-component magma transport and

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 295

the origin of composite intrusions and lava £ows. In: Ryan,M.P. (Ed.), Magmatic Systems. International Geophys. Se-ries, vol. 57, pp. 319^354.

Carrigan, C.R., Schubert, G., Eichelberger, J.C., 1992. Ther-mal and Dynamical Regimes of Single- and Two-PhaseMagmatic Flow in Dikes. J. Geophys. Res. 97, 17377^17392.

Clynne, M.A., 1999. A complex magma mixing origin forrocks erupted in 1915, Lassen Peak, California. J. Petrol.40, 105^132.

Coombs, M.L., Eichelberger, J.C., Rutherford, M.J., 2000.Magma storage and mixing conditions for the 1953^1974eruptions of Southwest Trident volcano, Katmai NationalPark, Alaska. Contrib. Mineral. Petrol. 140, 99^118.

Donnelly-Nolan, J.M., Champion, D.E., Grove, T.L., Baker,M.B., Taggart, J.E., Bruggman, P.E., 1991. The Giant Cra-ter lava ¢eld: geology and geochemistry of a composition-ally zoned, high-alumina basalt to basaltic andesite eruptionat Medicine Lake Volcano, California. J. Geophys. Res. 96,21843^21863.

Eichelberger, J.C., Cherko¡, D.G., Dreher, S.T., Nye, C.J.,2000. Magmas in Collision: Rethinking chemical zonationin silicic magmas. Geology 28, 603^607.

Gill, J.B., 1981. Orogenic Andesites and Plate Tectonics.Springer, Berlin.

Hildreth, W., 1981. Gradients in silicic magma chambers: im-plications for Lithospheric magmatism. J. Geophys. Res. 86,10153^10192.

Irvine, T.N., Baragar, W.R.A., 1971. A guide to the chemicalclassi¢cation of the common volcanic rocks. Can. J. EarthSci. 8, 523^548.

Kinzler, R.J., Donnelly-Nolan, J.M., Grove, T.L., 2000. LateHolocene hydrous ma¢c magmatism at Paint Pot Crater andCallahan £ows, Medicine Lake Volcano, N. California andthe in£uence of H2O in the generation of silicic magmas.Contrib. Mineral. Petrol. 138, 1^16.

Lescinsky, D.T., Fink, J.H., 2000. Lava and ice interaction atstratovolcanoes; use of characteristic features to determinepast glacial extents and future volcanic hazards. J. Geophys.Res. 105, 23711^23726.

Lescinsky, D.T., Sisson, T.W., 1998. Ridge-forming, ice-bounded lava £ows at Mount Rainier, Washington. Geology26, 351^354.

McKenna, J.M., 1994. Geochemistry and Petrology of MountRainier magmas: Petrogenesis at an Arc-Related Stratovol-cano with Multiple Vents. M.S. Thesis. Univ. Washington,122 pp.

Mills, J.G.Jr., Saltoun, B.W., Vogel, T.A., 1997. Magmabatches in the Timber Mountain magmatic system, South-western Nevada Volcanic Field, Nevada, USA. J. Volcan.Geotherm. Res. 78, 185^208.

Miyashiro, A., 1974. Volcanic rock series in island arcs andactive continental margins. Am. J. Sci. 274, 321^355.

Mullineaux, D.R., 1974. Pumice and other pyroclastic depositsin Mount Rainier National Park, Washington. U.S. Geol.Surv. Bull. 1326, 83 pp.

Patino, L.C., Szymanski, D.W., Vogel, T.A., Hannah, R.S.,1999. Rare earth element analysis of rocks by laser ablationICP-HEX-MS. Geol. Soc. Am. Abstr. Programs 31, 345.

Sisson, T.W., Lanphere, M.A., 2000. The geologic history ofMount Rainier volcano, Washington. Wash. Geol. 28, 28.

Sisson, T.W., Vallance, J.W., Pringle, P.T., 2001. Progressmade in understanding Mount Rainier’s hazards. EOSTrans. Am. Geophys. Union 82, 113^119.

Smith, R.L., 1979. Ash-£ow magmatism. Geol. Soc. Am. Spec.Pap. 180, 5^27.

Spera, F.J., Yuen, D.A., Greer, J.C., Sewell, G., 1986. Dynam-ics of magma withdrawal from strati¢ed magma chambers.Geology 14, 723^726.

Stockstill, K.R., 1999. The Origin and Evolution of the Bur-roughs Mountain Lava Flow, Mount Rainier, Washington.M.S. Thesis. Michigan State University, 88 pp.

Swanson, D., 1993. Variation in the Grain Size Distributionand the Chemical Composition of the Mount RainierC-ash Unit. B.S. Senior Thesis. University of Puget Sound,77 pp.

Venezky, D., Rutherford, M.J., 1997. Pre-eruption conditionsand timing of dacite-andesite mixing in the 2.2 ka C-layer atMount Rainier. J. Geophys. Res. 102, 20069^20086.

Vogel, T.A., Eichelberger, J.C., Younker, L.W., Schuraytz,B.C., Horkowitz, J.P., Stockman, H.W., Westrich, H.R.,1989. Petrology and emplacement dynamics of intrusiveand extrusive rhyolites of Obsidian Dome, Inyo Craters vol-canic chain, eastern California. J. Geophys. Res. 94, 17937^17956.

VOLGEO 2518 9-11-02

K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296296