Origin and Emplacement of the Andesite of Burroughs Mountain, a Zoned, Large-Volume Lava £Ow at Mount Rainier, Washington, USA

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Journal of Volcanology and Geothermal Research 119 (2002) 275^296 www.elsevier.com/locate/jvolgeores

Origin and emplacement of the andesite of Burroughs 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, USA b 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 to eroded 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 more evolved upward; the lower unit is heterogeneous and unzoned. The lower unit is also more phenocryst-rich and locally 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 Rainier magmatic system. Chemically heterogeneous block-and-ash-flow deposits that conformably underlie the lava flow were the earliest products of the eruptive episode. The felsic^mafic^felsic progression in lava composition resulted from partial evacuation of a vertically-zoned magma reservoir, in which either (1) average depth of withdrawal increased, then decreased, during eruption, perhaps due to variations in effusion rate, or (2) magmatic recharge stimulated ascent of a plume that brought less evolved magma to shallow levels at an intermediate stage of the eruption. Pre-eruptive zonation resulted from combined crystallization^differentiation and intrusion(s) of less evolved magma into the partly crystallized resident magma body. The zoned lava flow at Burroughs Mountain shows that, at times, Mount Rainier’s magmatic system has developed relatively large, shallow reservoirs that, despite complex recharge events, were capable of developing a felsic-upward compositional zonation similar to that inferred from large ash-flow sheets and other zoned lava flows. ß 2002 Elsevier Science B.V. All rights reserved.

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

1. Introduction

1 Present address: Department of Geological Sciences, Systematic compositional zoning in a lava £ow University of Tennessee, Knoxville, TN 37996-1410, USA. * Corresponding author. Tel.: +1-650-329-5247; or £ow group provides information about the Fax: +1-650-329-5203. evolution and evacuation of the pre-eruptive mag- E-mail address: [email protected] (T.W. Sisson). ma body because it preserves a nearly instantane-

VOLGEO 2518 9-11-02 276 K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296

Fig. 1. Map of the lava £ow and block-and-ash-£ow deposits at Burroughs Mountain, Mount Rainier, WA, showing sample locations (circles), vertical sampling traverses (brackets), and upper^lower lava boundaries (dashed lines). ous partial sampling of the magma reservoir sys- upward con¢guration of the e¡usive £ow. Unlike tem. This perspective is similar to that obtained large ash-£ow sheets, where silicic-¢rst, ma¢c-lat- from the study of chemically zoned or layered er eruption sequences are the rule, diverse zoning ash-£ow sheets (Smith, 1979; Hildreth, 1981). Un- styles in lava £ows suggest to us that competing like ash-£ow sheets, which commonly have a processes in£uence the succession of lava compo- ma¢c-over-silicic zonation (Smith, 1979), lava sitions. These processes are not well understood, £ows can be ma¢c at their base and become pro- in part due to a scarcity of well-studied zoned gressively more silicic upward (Carrigan and Ei- lava £ows. chelberger, 1990; Vogel et al., 1989), or can erupt The Burroughs Mountain lava £ow of Mount in a sequence from early silicic to progressively Rainier, WA (Fig. 1), was studied in detail be- more ma¢c compositions, preserved in successive cause it is an accessible and well-exposed repre- £ows or £ow lobes (Donnelly-Nolan et al., 1991; sentative of the large-volume andesitic lava £ows Kinzler et al., 2000; Coombs et al., 2000). that surround Mount Rainier. As such, its ¢eld, The ma¢c-upward zonation in ash-£ow sheets geochemical, and mineralogical features can help and the silicic-to-ma¢c progression of some lava to reveal the processes leading to sizeable erup- £ows have been attributed to eruption from tions of andesitic lava. The lava £ow is a pheno- chemically zoned and layered magma bodies. cryst-rich andesite that is chemically layered, with The shallow silicic portion erupts ¢rst, followed its upper layer being more ma¢c than its basal by evacuation of more ma¢c magma from pro- zone (Stockstill, 1999), similar to zoned ash-£ow gressively deeper in the chamber (Smith, 1979; sheets. However, the upper layer is itself zoned, Spera et al., 1986; Mills et al., 1997). Eichelberger becoming systematically more felsic upward. This et al. (2000) propose an alternate interpretation zoning re£ects temporal changes in erupted com- that deep silicic magma intrudes shallow ma¢c positions that provide insights into the magmatic magma, rises through it, and erupts ¢rst. The re- zoning of a crustal reservoir and the extraction of verse, silicic-upward zonation in some lava £ows magma during a relatively large andesite eruption. has been attributed to dynamic processes where two magmas of contrasting composition erupt to- gether. Lower viscosity ma¢c magma overtakes, 2. Geologic setting and description encapsulates, and outruns higher viscosity silicic magma during £ow in the conduit (Carrigan et The Burroughs Mountain lava £ow lies at the al., 1992; Carrigan, 1994) resulting in the silicic- northeast foot of Mount Rainier between 2350

VOLGEO 2518 9-11-02 K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 277 and 1300 m elevation (Fig. 1). It is a large-volume nocrysts and as groundmass grains, but are oxi- (3.4 km3), up-to-350-m-thick and 11-km extend- dized with blebby and lamellar unmixing. Apatite ing andesitic lava £ow, which terminates where it forms inclusions in phenocrysts and less obvious abutted against glacier ice. The £ow’s thickness, needles in the groundmass. Olivine has been its situation high above the adjacent valley, and found in a few thin sections as rare strongly em- ice contact features show that the lava was im- bayed phenocrysts armored by granular orthopy- pounded by and entrenched into the margin of a roxene, and its former presence is inferred in oth- thick Pleistocene glacier (Lescinsky and Sisson, er samples from rare Fe^oxide^orthopyroxene 1998) that ¢lled the present-day White River Val- symplectites. Zircon, Fe^sul¢de, and baddelyite ley. Glacial erosion has cut cirques and cli¡s into are in very low abundance (0^3 grains per thin the north and south sides of the £ow that expose section) as 0.5^5 micron-diameter, rounded grains. nearly continuous sections from the £ow base to Baddelyite forms minute blebs on or within its eroded top. The gentle upper surface of the coarsely exsolved ilmenite, and may have precipi- £ow contrasts with the steep, glacially eroded tated from ilmenite during slow cooling and oxi- £anks, and the lava’s upper surface may be close dation. to the original £ow top, now stripped of its rubbly Quenched magmatic inclusions (Bacon, 1986) carapace. The lava erupted 496 ( þ 7) kyr ago, at are locally abundant in the lower portion of the the beginning of a period of vigorous volcanic lava £ow and in the block-and-ash-£ow deposits, activity spanning 500^420 kyr ago, the onset of but have not been found in the upper part of the which marked the beginning of modern Mount lava £ow. The quenched inclusions are ¢ne- Rainier (Sisson and Lanphere, 2000; Sisson et grained, have ellipsoidal shapes and open vesicles, al., 2001). Similar large-volume lava £ows at and lack phenocrysts, with the exception of traces Mount Rainier have been traced to radial-dike- of olivine and rare resorbed plagioclase grains fed £ank vents (Sisson, unpublished mapping), that were probably entrained from the host mag- and although no direct connection is preserved, ma during mingling. we consider it likely that the Burroughs Mountain Coarse-grained equigranular inclusions of lava £ow erupted through a radial dike system vuggy gabbronorite-to-diorite are widespread in exposed immediately to the southwest (Fig. 1). the lava £ow and reach sizes of 15 cm. Tertiary The lava £ow is porphyritic andesite-to-dacite rocks in the Mount Rainier region di¡er from with abundant medium-to-coarse-grained pheno- these coarse-grained inclusions in many respects, crysts of plagioclase (to 3 mm), and lesser pyrox- and we interpret the inclusions as products of the ene and hornblende (to 1.5 mm). Plagioclase phe- Quaternary Mount Rainier magmatic system. Pla- nocrysts are sharply de¢ned laths with relatively gioclase in nearby Tertiary rocks is characteristi- simple normal and oscillatory zoning parallel to cally clouded with alteration minerals, including phenocryst faces. Embayed zones interrupt oscil- sericite and carbonate, whereas plagioclase in the latory and normal zoning in many phenocrysts, coarse-grained inclusions is glass-clear and free of but the degree of embayment is small, has been alteration. Open vugs are rare in nearby Tertiary in¢lled and overgrown by subsequent plagioclase, rocks, having been ¢lled with quartz, carbonate, and is absent from phenocryst margins. Strongly chlorite, and other low-temperature alteration resorbed spongy grains are nearly absent, but pla- minerals. Vugs in the coarse-grained inclusions gioclase phenocrysts with patchy-zoned cores and are open, with the exception of narrow tridymite mineral or crystallized melt inclusions can be laths that protrude into or cross some vugs, and found in any thin section. Some hornblende phe- vug margins are de¢ned by the idiomorphic ter- nocrysts are fresh, most are partly-to-completely minations of bounding plagioclase and pyroxene converted to opaque oxide^silicate (opacite) pseu- grains. Pyroxene grains are unexsolved, and min- domorphs, and some near the base of the £ow are erals in the coarse-grained inclusions lack melting replaced by epitaxial orthopyroxene. Titanomag- textures. Some coarse-grained inclusions have netite and ilmenite are ubiquitous as microphe- oxy^hornblende pseudomorphs after amphibole,

VOLGEO 2518 9-11-02 278 K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 and rare inclusions contain olivine rimmed by 3. Methods pyroxene, Fe^Ti oxides, and oxy^hornblende. Mi- nor apatite, and traces of zircon, baddeleyite, and Samples were collected along the length of the Th-, U-, light-rare-earth-element-rich zirconolite lava £ow, and along north and south vertical tra- (ideally CaZrTi2O7) were found in vugs in three verses from the £ow base to its eroded top, in coarse-grained inclusions that were examined order to reveal lateral and vertical variations in closely. A few micron-sized blebs of vapor-phase magma compositions (Fig. 1). Whole-rock major argentite (AgS2) and molybdenite were also found and trace-element concentrations were determined adhering to tridymite laths. Henceforth we refer by X-ray £uorescence spectrometry (XRF) for to these rocks as cognate plutonic inclusions, im- 135 samples of lava, juvenile block and ash-£ow plying that they are products of the Quaternary clasts, and assorted inclusions. Additional trace- Mount Rainier magmatic system. Although they element (including rare-earth element) concentra- need not have crystallized from the magma tions were determined for 51 samples, using La- batches that erupted as the Burroughs Mountain ser-Ablation Inductively Coupled Plasma Mass deposits, the absence of alteration in these porous Spectrometer (LA ICP^MS) (Patino et al., rocks is suggestive of solidi¢cation only a short 1999). Whole-rock compositions for the north time prior to incorporation into the Burroughs and south vertical traverses through the lava Mountain magmas. Angular xenoliths (2) of £ow, representative of the £ow as a whole, are ¢ne-grained silicic metavolcanic rock have also presented in Tables 1 and 2. Compositions of been found in the lava £ow but are very rare. quenched magmatic inclusions from the lower Block-and-ash-£ow deposits consist of angular, part of the lava £ow are presented in Table 3; slightly vesiculated andesitic-to-dacitic blocks, cognate plutonic and metavolcanic inclusion com- some with radial prismatic jointing or bread- positions are given in Table 4. Representative crusted surfaces, in a non-welded brown to red- compositions of juvenile blocks from the block- dish^brown ash matrix that has been thermally and-ash deposit and of its inclusions are given in oxidized. Matrix ash grains are chie£y angular; Table 5. pumice and glassy bubble walls are rare. Strati¢- Phenocryst compositions were determined by cation is de¢ned by concentrations of coarse electron microprobe with a four-spectrometer Ca- blocks and is more pronounced to the south of meca SX-50 (University of Tennessee) and an Burroughs Mountain toward Mount Rainier. ARL^SEMQ (Central Michigan University), us- block-and-ash-£ow deposits accumulated as mul- ing a 15 RA sample current and a focused tiple £ow units that reach aggregate thicknesses to beam. Representative electron microprobe analy- 200 m. Juvenile clasts share the overlying lava’s ses of plagioclase, pyroxene, and hornblende are phenocryst assemblage and habit, including some given in Tables 6 and 7. Glass compositions from blocks with distinctive orthopyroxene-replaced sub-pumiceous block-and-ash clasts (Table 8) hornblende. Glass in most blocks is charged were measured with a ¢ve-spectrometer JEOL with plagioclase microlites, but sub-pumiceous 8900 electron microprobe (USGS). Sodium migra- blocks concentrated in thin zones within 20 m tion in glass was minimized with a 2 RA sample of the base of the lava £ow contain abundant current, a 15 Wm spot size, and by counting Na microlite-poor vesicular glass. The conformable ¢rst for 10 s. contact between the lava and the unconsolidated Sr, Ba, and Ca intensities in plagioclase pheno- block-and-ash-£ow deposits, lacking any exposed crysts and matrix grains were measured by LA evidence of incision, as well as textural and com- ICP^MS, using a laser spot size and penetration positional similarities between lava and blocks, depth of 25 Wm. The Ba/Sr and Sr/Ca variation, discussed in following sections, suggest that lava based on count rates, were evaluated in ¢ve sam- e¡usion closely followed block-and-ash deposi- ples, which represented the total bulk composition, probably as part of the same eruptive epi- tional variation within the £ow. Five to eight pla- sode. 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) 275^296 279 four plagioclase matrix grains were analyzed in biases for these elements can be estimated from replicate in each sample. Relative count rates analyzing a known standard. Preliminary esti- were not reduced to concentration ratios because mates of these corrections were made and the rel- of a lack of appropriate standards and uncer- ative variations of these ratios based on count tainty in the Ca concentrations at the analysis rate were identical to the variation of these ratios locations. However, the corrections for mass based on corrected concentration.

Table 1 Whole-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 91 134 137 152 174 174 210 226 238 244 Unita LLLLLUUUUUUUUU SiO2 (%) 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 59.7 TiO2 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 0.92 Al2O3 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 16.9 Fe2O3 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 6.21 MnO 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 0.10 MgO 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 3.61 CaO 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 5.93 Na2O 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 4.25 K2O 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 1.61 P2O5 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 0.26 Total 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 21 Cu 28 67 43 20 25 28 33 29 47 35 40 32 42 24 Zn 67 86 66 66 63 77 77 74 76 72 69 69 75 72 Rb 42 32 38 41 32 27 34 36 33 39 38 32 31 34 Sr 494 489 489 481 523 558 586 587 602 556 563 616 579 581 Zr 173 160 168 182 173 177 173 176 176 180 179 175 172 168 Y 1717171615161717171716171918 Nb 11 11 11 11 11 11 12 11 11 12 11 11 Ba 445 413 409 443 361 412 408 435 430 408 405 417 391 415 La 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 22.2 Ce 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 47.7 Pr 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 6.19 Nd 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 24.8 Sm 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 4.95 Eu 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 1.39 Gd 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 4.10 Tb 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 0.59 Dy 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 3.23 Ho 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 0.58 Er 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 1.60 Yb 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 1.54 Lu 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 0.21 Hf 4.40 4.15 4.16 4.85 4.37 4.48 4.79 4.53 4.33 4.49 4.35 4.22 Ta 0.72 0.70 0.70 0.80 0.72 0.72 0.79 0.72 0.70 0.75 0.72 0.70 Pb 9.5 8.9 7.9 9.6 8.8 8.8 9.6 8.0 8.7 9.3 9.2 7.8 Th 5.1 4.8 4.8 5.9 5.3 5.3 5.9 5.4 5.6 5.5 5.5 5.3 U 2.2 1.9 2.0 2.5 1.8 1.9 2.1 1.9 2.0 2.0 1.9 1.9 V 116 125 122 113 132 126 127 123 126 126 122 123 Cr 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 9-11-02 280 K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296

About 65 samples were examined in thin sec- 4. Whole-rock and glass chemistry tion and seven were point counted, using a step size of 0.3 mm (1,725 points per thin section). 4.1. Major element variations Crystals larger than 0.1 mm were counted as phenocrysts (plagioclase, orthopyroxene, clinopyrox- The Burroughs Mountain lava £ow consists of ene, hornblende and magnetite/ilmenite), and medium-Kcalc^alkaline andesite and dacite ( Gill, smaller grains as matrix. 1981; Miyashiro, 1974; Irvine and Baragar, Table 2 Whole-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 49 143 177 207 241 268 299 Unita L L UUUUUU SiO2 (%) 61.9 62.9 59.5 60.9 59.1 61.0 60.9 61.0 TiO2 0.83 0.81 0.98 0.92 0.94 0.92 0.93 0.91 Al2O3 17.0 17.0 16.3 16.7 16.8 16.5 16.6 16.6 Fe2O3 5.58 5.26 6.46 6.33 6.51 6.17 6.19 6.07 MnO 0.09 0.08 0.10 0.10 0.10 0.09 0.09 0.09 MgO 2.99 2.53 3.61 3.46 3.53 3.50 3.51 3.40 CaO 5.71 5.15 6.13 6.09 5.76 5.92 5.96 5.83 Na2O 4.24 4.14 3.95 3.95 4.09 3.99 4.01 4.03 K2O 1.60 1.79 1.67 1.66 1.60 1.65 1.66 1.68 P2O5 0.21 0.20 0.24 0.22 0.23 0.23 0.22 0.24 Total 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 36 Cu 25 26 42 35 32 31 48 29 Zn 61 57 65 69 66 66 86 66 Rb 31 40 34 35 40 34 36 38 Sr 628 528 616 592 574 588 599 599 Zr 178 192 192 186 204 197 193 199 Y 1516191818171717 Nb 12 13 13 12 13 13 13 12 Ba 408 438 482 433 436 440 438 416 La 19.2 19.9 26.2 23.1 24.1 24.6 23.8 23.9 Ce 42.9 44.4 52.3 47.9 50.9 49.9 49.8 49.6 Pr 5.18 5.33 6.76 5.97 6.34 6.38 6.30 6.33 Nd 19.8 20.6 26.7 23.6 25.2 24.9 24.7 24.7 Sm 4.00 4.28 5.30 4.78 5.15 5.01 4.98 5.02 Eu 1.21 1.23 1.47 1.34 1.44 1.39 1.39 1.38 Gd 3.29 3.55 4.40 4.12 4.19 4.11 4.09 4.13 Tb 0.49 0.53 0.64 0.61 0.62 0.60 0.59 0.61 Dy 2.65 2.86 3.47 3.28 3.42 3.32 3.31 3.33 Ho 0.50 0.54 0.65 0.62 0.63 0.62 0.61 0.61 Er 1.40 1.47 1.79 1.69 1.70 1.68 1.64 1.68 Yb 1.32 1.41 1.62 1.52 1.48 1.41 1.4 1.45 Lu 0.19 0.2 0.24 0.22 0.22 0.2 0.21 0.21 Hf 3.23 3.6 4.06 3.72 3.91 3.48 3.81 3.69 Ta 0.64 0.68 0.73 0.68 0.74 0.68 0.69 0.66 Pb 9.6 8.6 6.4 8.2 5.9 7.2 9.5 5.9 Th 4.4 5.0 6.1 5.3 5.2 4.9 5.2 5.2 U 2.4 2.5 2.3 2.1 2.1 2.2 2.2 2.1 V 124 113 135 131 124 120 120 118 Cr 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) 275^296 281

1971). Whole-rock compositions range continu- SiO2 lava samples can have higher P2O5, but the ously from 57.7 to 63.6 wt% SiO2, from 5.0 to range of P2O5 values also increases, thus de¢ning 2.5 wt% MgO, and from 1.2 to 1.9 wt% K2O a fan-shaped ¢eld on the SiO2 variation diagram (Fig. 2). Whole-rock Fe2O3ðtotalÞ, CaO, TiO2, (Fig. 2). K2O variations are also not simple: K2O and MgO concentrations decrease linearly with values are highest in samples with s 61 wt% SiO2, increasing SiO2 ;Na2O concentrations are rela- are clustered near 1.5 wt% in samples with 60^61 tively constant near 4 wt% over the entire range wt% SiO2, and are scattered between 1.2 and 1.8 of SiO2 values. Whole-rock P2O5 values are low- wt% in the few samples with SiO2 6 60 wt%. est, on average, in the highest-SiO2 rocks. Lower- Block-and-ash-£ow clasts cover a nearly identi-

Table 3 Quenched 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 56.1 TiO2 1.06 1.18 1.06 0.99 1.06 1.09 0.96 Al2O3 17.4 18.2 17.8 18.1 18.2 17.6 18.1 Fe2O3 7.65 7.49 7.53 7.07 7.46 7.64 7.41 MnO 0.12 0.12 0.11 0.11 0.11 0.12 0.12 MgO 5.61 3.90 5.43 4.47 5.22 5.21 4.55 CaO 7.51 7.29 7.37 6.47 7.05 7.67 7.05 Na2O 3.41 3.43 3.71 3.37 3.35 3.50 3.66 K2O 0.98 0.89 0.93 1.20 1.08 0.97 1.11 P2O5 0.19 0.23 0.20 0.17 0.19 0.19 0.17 Total 98.0 98.4 97.6 98.3 98.0 98.4 99.2 Ni (ppm) 40 21 40 32 38 41 31 Cu 34 106 41 24 35 574 74 Zn 86 73 77 84 81 96 75 Rb 21 20 21 25 19 19 25 Sr 589 612 608 550 621 623 512 Zr 118 145 123 129 119 119 108 Y 16171613161615 Nb 10 14 10 10 10 10 6 10 Ba 281 284 278 361 273 268 287 La 14.5 16.4 14.6 13.3 13.4 13.7 Ce 29.5 30.9 30.4 25.6 28.1 28.0 Pr 3.88 4.24 4.04 3.18 3.81 3.77 Nd 16.3 17.7 16.3 12.5 16.1 15.7 Sm 3.63 3.94 3.56 2.84 3.75 3.62 Eu 1.2 1.22 1.20 1.12 1.19 1.21 Gd 3.37 3.55 3.27 2.62 3.47 3.33 Tb 0.51 0.52 0.50 0.42 0.53 0.52 Dy 2.89 3.06 2.81 2.51 2.98 2.94 Ho 0.57 0.59 0.54 0.47 0.59 0.56 Er 1.52 1.57 1.49 1.27 1.55 1.49 Yb 1.45 1.51 1.42 1.29 1.51 1.43 Lu 0.21 0.22 0.20 0.19 0.22 0.21 Hf 2.83 3.1 2.73 2.88 2.67 2.78 Ta 0.51 0.76 0.53 0.57 0.51 0.54 Pb 9.92 8.52 8.31 16.94 7.34 44.3 Th 2.5 2.4 2.6 3.2 2.5 2.5 U 0.8 1.0 0.8 1.1 0.9 0.8 V 167 135 195 122 157 172 Cr 154 22 134 105 133 110 Major oxides, Ni^Y, and Ba by XRF, rest by LA^ICPMS.

VOLGEO 2518 9-11-02 282 K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 cal compositional range as the lava £ow, although Quenched magmatic inclusions in the lava £ow with a few outliers. Block-and-ash-£ow clast com- are basaltic andesites (Table 3), comparable to the positions range from 56.7 to 63.5 wt% SiO2, from ‘basic andesite’ ( 6 57 wt% SiO2)ofGill (1981). 4.5 to 2.4 wt% MgO, and from 1.3 to 1.8 wt% Normalized to 100 wt%, to factor out hydration K2O. An element (oxide) by element (oxide) stu- of glassy groundmass, quenched inclusion compo- dent-T test con¢rms that the lava £ow and pyro- sitions range from 54.8 to 57.3 wt% SiO2, from clastic deposits cannot be distinguished on the 5.7 to 4.0 wt% MgO, and from 0.9 to 1.2 wt% basis of chemical composition (Stockstill, 1999). K2O(Fig. 2). Quenched inclusion compositions Microlite-poor matrix glasses from sub-pumi- lie on-trend with lava and pyroclast analyses on ceous blocks are rhyolitic (Table 8) and lie on SiO2 variation diagrams, except for P2O5 and the high-SiO2 extension of trends de¢ned by Na2O(Fig. 2), for which the inclusions are dis- lava and clast whole-rock compositions (Fig. 2), tinctly below low-SiO2 extrapolations of the lava including Na2O near 4 wt%. and pyroclast compositional arrays.

Table 4 Representative 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 66.4 TiO2 1.31 0.93 0.82 0.77 0.93 0.86 0.90 0.69 0.66 Al2O3 16.8 17.9 14.7 15.2 16.4 16.8 16.4 17.8 17.5 Fe2O3 9.41 8.24 9.18 9.12 8.43 5.8 6.02 6.04 2.77 MnO 0.14 0.13 0.13 0.14 0.14 0.09 0.09 0.06 0.03 MgO 5.40 5.01 6.89 7.51 5.54 3.15 2.99 2.20 2.96 CaO 8.95 7.35 7.42 7.91 7.61 5.55 4.98 4.29 4.59 Na2O 3.72 3.81 2.88 2.89 3.34 4.07 4.09 4.77 3.83 K2O 0.42 0.46 1.17 0.88 0.92 1.75 1.74 1.61 1.15 P2O5 0.23 0.18 0.18 0.15 0.21 0.18 0.20 0.11 0.20 Total 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 15 Cu 257 61 306 234 84 32 17 214 77 Zn 87 109 104 88 91 76 68 48 44 Rb 7 7 23 16 20 40 37 35 42 Sr 607 589 434 497 571 520 517 492 420 Zr 71 93 130 103 105 170 178 199 139 Y 161715141719171317 Nb 6 10 6 10 6 10 6 10 6 10 6 10 6 10 6 10 6 10 Ba 253 247 304 208 285 417 386 413 441 La 12.6 14.1 15.0 13.5 13.1 21.3 19.0 Ce 28.6 36.5 32.3 29.7 31.2 50.5 39.7 Pr 3.6 4.56 3.86 3.67 3.85 6 4.77 Nd 17.8 22.1 17.0 15.4 16.8 22.8 20.8 Sm 3.84 4.89 3.72 3.3 3.79 4.45 3.8 Eu 1.32 1.44 1 0.92 1.24 1.33 1.12 Gd 3.58 4.24 3.46 2.95 3.44 3.96 3.05 Tb 0.54 0.72 0.58 0.48 0.54 0.63 0.48 Dy 3.1 3.68 2.95 2.43 2.86 3.37 2.57 Ho 0.54 0.63 0.56 0.46 0.55 0.6 0.45 Er 1.63 1.94 1.63 1.43 1.69 1.9 1.44 Yb 1.66 1.63 1.61 1.44 1.55 1.76 1.36 Lu 0.24 0.26 0.25 0.23 0.25 0.29 0.21 Cr 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) 275^296 283

Cognate plutonic inclusion compositions scatter TiO2,P2O5,Na2O, and Al2O3. The remaining widely on SiO2 variation diagrams (Fig. 2). cognate plutonic inclusion samples (9 of 20) plot Slightly more than half (11 of 20) of the cognate among the lava samples and may have formed by plutonic inclusion samples have lower SiO2 and bulk solidi¢cation of magma like that of the lava K2O, and higher MgO, CaO, and Fe2O3ðtotalÞ £ow. The two metavolcanic xenoliths plot o¡ ma- than the lava. An origin as cumulates (£oor or jor oxide compositional arrays de¢ned by the lava sidewall) is suggested by their relatively coarse and pyroclast samples. grain sizes, textures, absence of late-magmatic minerals (quartz, K^feldspar, biotite), and their 4.2. Trace-element variations widely variable but ma¢c compositions. These relatively ma¢c cognate plutonic inclusions do not Trace-element variations in the lava and the plot on-trend with the lava and pyroclast compo- block-and-ash-£ow deposit are complex and gen- sitional arrays on SiO2 variation diagrams for erally do not de¢ne narrow trends with whole-

Table 5 Representative 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 58.3 TiO2 0.92 0.85 0.8 0.88 0.83 0.72 0.73 0.92 0.76 0.79 Al2O3 17.9 17.6 18.3 17.5 17.5 17.3 17.0 17.6 17.4 14.9 Fe2O3 6.73 5.80 5.56 5.74 5.78 4.91 4.79 6.65 5.56 6.78 MnO 0.11 0.09 0.09 0.11 0.09 0.08 0.08 0.12 0.09 0.10 MgO 4.25 3.13 3.06 3.31 3.03 2.43 2.67 4.35 3.16 6.83 CaO 6.63 5.79 5.48 6.15 5.67 5.35 5.33 6.58 5.65 7.57 Na2O 4.00 4.62 4.25 3.65 4.20 4.09 4.28 3.97 4.25 3.00 K2O 1.28 1.45 1.51 1.52 1.55 1.81 1.61 1.38 1.43 1.32 P2O5 0.17 0.21 0.22 0.22 0.21 0.19 0.18 0.21 0.17 0.07 Total 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 74 Cu 36 23 22 20 35 25 12 39 30 66 Zn 75 72 72 74 67 61 71 75 67 66 Rb 29 24 27 34 37 46 36 28 27 34 Sr 597 633 610 622 605 621 606 612 537 355 Zr 128 149 147 151 152 145 140 135 136 117 Y 1313141814161815 1418 Nb 6 10 6 10 6 10 6 10 6 10 6 10 6 10 6 10 6 10 6 10 Ba 288 418 504 415 448 433 412 382 395 280 La 13.2 16.4 20.4 18.4 19.3 18.3 15.8 14.6 Ce 31.5 41.9 43.4 40.6 40.1 42.0 37.0 35.0 Pr 3.73 4.59 5.01 4.74 4.35 4.42 4.06 3.89 Nd 15.8 18.0 18.9 21.1 18.6 18.3 16.5 15.8 Sm 3.25 3.33 3.78 3.97 3.39 3.34 3.34 3.17 Eu 1.14 1.12 1.16 1.25 1.17 1.05 1.21 1.06 Gd 2.87 2.71 3.35 3.33 2.86 2.75 2.75 2.62 Tb 0.45 0.43 0.48 0.52 0.43 0.42 0.45 0.43 Dy 2.37 2.17 2.64 2.76 2.34 2.2 2.38 2.12 Ho 0.43 0.39 0.47 0.49 0.39 0.39 0.42 0.41 Er 1.32 1.21 1.63 1.54 1.3 1.26 1.26 1.21 Yb 1.32 1.28 1.34 1.52 1.31 1.38 1.25 1.21 Lu 0.21 0.19 0.2 0.22 0.17 0.18 0.19 0.18 Cr 101 39 34 32 54 20 29 57 69 486 Major oxides, Ni^Ba, and Cr by XRF, rest by LA^ICPMS.

VOLGEO 2518 9-11-02 284 K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296

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

rock SiO2 (Fig. 3) or other indices of di¡erentia- values in rocks with 6 60 wt% SiO2. Like Rb, Zr tion. Concentrations of Sr, Cr, and Zn that are is incompatible in the phenocryst minerals, but Zr compatible in the major phenocryst minerals de- concentrations do not increase uniformly in high- crease with increasing SiO2 and decreasing MgO er SiO2 rocks. (not illustrated) but scatter in excess of analytical Quenched magmatic inclusions have distinctly precision. Variations of Rb with SiO2 resemble lower Rb, Zr, Ba, and La/Yb than the lava and those of K2O, with the highest Rb, on average, pyroclast samples (Fig. 3). Quenched inclusion Sr in rocks with s 61 wt% SiO2, and scattered Rb concentrations are similar to those of lavas and

VOLGEO 2518 9-11-02 K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 285

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

pyroclasts, but fall far o¡ the trend of increasing cognate plutonic inclusions plot o¡ the low-SiO2 Sr with decreasing SiO2 de¢ned by the lava and projection of the lava’s Sr^SiO2 array. pyroclast suites. Cognate plutonic inclusions with lava-like major element compositions also have 4.3. Spatial variations lava-like trace-element abundances. Ma¢c, poten- tially cumulate cognate plutonic inclusions di¡er Closely spaced samples were collected up the from lava samples in their lower Rb, Zr, Ba, and steep north and south sides of the lava £ow La/Yb. As with quenched inclusions, the ma¢c (Fig. 1) to explore for vertical variations in lava

VOLGEO 2518 9-11-02 286 K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 composition that might give insights into eruption ment ratios (Th/U, La/Yb) the upper^lower dynamics and chemical zonation of the pre-erup- boundary is distinct. The evolved-upward gra- tive magma reservoir. Both traverses show similar dient in the upper layer is manifest in decreasing major and trace-element variations with height in V concentrations, slightly decreasing Th/U, and the £ow and reveal that the £ow is vertically increasing La/Yb with height in the layer. No zoned (Figs. 4 and 5). Rocks from the upper por- gradient is apparent for Sr, Zr, or Ba. Rb concen- tion of the lava £ow (150^200 m thick) are slightly trations may increase slightly with height in the more ma¢c than rocks from the lower portion layer, although any Rb gradient is near the limit (100 m thick). Average Fe2O3ðtotalÞ of upper and of what can be distinguished by our analyses. lower portion samples are 6.4 and 5.8 wt%, re- The di¡erence in composition of the upper and spectively. The upper^lower transition is abrupt, lower layers is matched by small di¡erences in and the £ow consists of chemically distinct upper phenocryst abundance. Modes measured on four and lower sections. On the north traverse, the lower layer and three upper layer samples show transition is marked by a zone of rubble and that evolved lower layer rocks contain 36^45 vol% lava spines, overlain by massive lava of the base phenocrysts, whereas ma¢c upper layer rocks con- of the upper portion. The zone of rubble and tain 23^38 vol% phenocrysts. Upper and lower spines resembles a £ow top, but the overlying layers have equal quantities of ferromagnesian massive lava lacks cooling features, such as nar- minerals (8^10 vol%), and the di¡erence in pheno- row glassy columns, that would indicate a pause cryst content is due to more abundant plagioclase between emplacement of the portions of the £ow. in the evolved lower layer. Talus conceals the lower^upper boundary along the south sample traverse. Except along the north 4.4. Phenocryst abundances and compositions traverse, the upper^lower boundary is not obvious in the ¢eld and was not traced throughout Evolved, lower layer lava is illustrated by sam- the £ow during mapping. ple 97-17, which was collected from near the base In addition to being more ma¢c, the upper of the north sample traverse (Table 1). The layer has a weak vertical chemical gradient, be- groundmass (56 vol%) is dominated by very coming more evolved upward. This gradient is ¢ne-grained ( 6 0.05 mm) plagioclase laths. Pla- conspicuous for Fe2O3, MgO and TiO2 (Fig. 4) gioclase phenocrysts (34%) are chie£y euhedral and modest for CaO (not illustrated). For other laths to 2.5 mm with length to width ratios rang- major oxides, the gradient is absent or is su⁄- ing from 2:1 to 6:1 and averaging around 3:1. ciently weak to be di⁄cult to resolve above ana- Phenocrysts of hornblende (3.5%), clinopyroxene lytical precision. There may be a slight increase in (2.9%), and orthopyroxene (1.8%) are euhedral- SiO2 and decrease in P2O5 with height in the to-subhedral grains ranging from 0.1 to 1.5 mm. upper layer, whereas K2O appears to lack a ver- Ilmenite and magnetite grains (1.3%) range from tical trend. No chemical gradient was found in the 6 0.1 to 0.4 mm. Tiny needles of apatite are in- lower portion of the £ow. Samples from elsewhere cluded in all of the phenocryst minerals but are along the £ow are consistent with the lower-felsic, di⁄cult to distinguish in the matrix. upper-ma¢c division (Stockstill, 1999) but are in- Upper layer lava is illustrated by sample 97-34, su⁄cient to reveal the upper portion’s weak com- collected from the top of the north sample tra- positional gradient. verse (Table 1). The groundmass (64%) is domi- The lower-felsic, upper-ma¢c division is also nated by very ¢ne-grained plagioclase laths. Pla- apparent in compatible trace-element concentra- gioclase is the most common phenocryst (27%), tions. As a group, lower layer samples have lower with the same size range and habit as for the low- concentrations of Sr and V (Fig. 5), and Cr and er layer. Clinopyroxene (3.8%) is a marginally Zn (not illustrated) than do upper layer samples. more abundant phenocryst than orthopyroxene Incompatible trace-element di¡erences are less ob- (3.0%) and both form euhedral-to-subhedral vious, although for some elements (Rb) and ele- grains 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) 275^296 287

Fig. 4. Selected major-oxide variations with height above £ow base along north (circles) and south (squares) vertical sample sections. Filled and open symbols distinguish upper and lower compositional divisions. Analytical uncertainties are þ 2 standard errors. phenocrysts of hornblende (1.4%) range from 0.1 still, 1999). Plagioclase phenocrysts in the evolved to 0.5 mm. Ilmenite and magnetite grains (1.3%) lower layer span the full range of compositions, range from 6 0.1 to 0.4 mm, and apatite is whereas plagioclase in the more ma¢c upper layer present as tiny needles included in the phenocrysts is slightly less diverse, limited to between An41 and in the groundmass. and An53. Zoning is generally limited within sin- Plagioclase phenocryst compositions in the en- gle phenocrysts to 6 10 mol% An, and pheno- tire £ow range from An37 to An58 (representative cryst rims are consistently more sodic than cores analyses in Table 6, extensive analyses in Stock- (Table 6).

VOLGEO 2518 9-11-02 288 K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296

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

Plagioclase phenocrysts from upper and lower cryst core compositions from the upper and lower layer samples along the north traverse were exam- layers overlap, but matrix grains di¡er ^ matrix ined with LA ICP^MS to determine zoning of Ba grains from the evolved lower layer have higher and Sr. Intensities of Ba, Sr and Ca were mea- Ba/Sr and lower Sr/Ca than do upper layer matrix sured on cores and rims of plagioclase pheno- grains. Core^rim zoning is variable, but generally crysts, and on matrix plagioclase, and the Ba/Sr the phenocryst rim compositions approach or versus Sr/Ca ratios are plotted in Fig. 6. Pheno- match those of the matrix grains. This re£ects

VOLGEO 2518 9-11-02 K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 289 two types of zoning. In lower layer samples Ba/Sr but are insu⁄cient to correlate with zoning in the generally increases from phenocryst cores to rims lava. and matrix grains, whereas in many upper layer Trace quantities of zircon ( 6 5 microns) were phenocrysts Ba/Sr decreases from cores to rims found in a few lava and block-and-ash-£ow sam- and matrix grains. Rare phenocrysts in the lower ples that were examined closely by backscatter layer also have this ‘reversed’ zoning trend. Some electron imaging, regardless of whether the sam- anomalous phenocrysts in the upper layer have ples were well crystallized or glassy. Zircon is not distinctly high Ba/Sr values (Fig. 6), which may normally an early crystallizing mineral in Zr-poor signal crystallization from a high Ba/Sr melt. subduction-zone andesites, so zircon may derive Orthopyroxene and clinopyroxene phenocrysts by assimilation of zircon-bearing rock, possibly are MgO-rich with relatively small ranges in com- the cognate plutonic inclusions, or by mixing position (Table 7). Over the entire £ow, orthopy- with low-temperature zircon-saturated magma. roxene phenocrysts range from Wo2:9En61:3Fs35:8 A single xenocrystic grain of resorbed quartz to Wo3:0En67:1Fs29:9. Orthopyroxene phenocrysts and very rare resorbed biotite xenocrysts have from the lower layer have a slightly broader range also been found in the lava, but these minerals in compositions than the orthopyroxene pheno- are not present in the cognate plutonic inclusions crysts from the more ma¢c upper layer (Stockstill, and must have originated from a minor additional 1999). Clinopyroxene phenocrysts have less di- assimilant or by mixing with an evolved, quartz- verse compositions than orthopyroxene, ranging and biotite-bearing magma. Biotite is also present from Wo42:3En41:6Fs16:1 to Wo44:2En40:6Fs15:2 (Ta- as a trace groundmass constituent in well-crystal- ble 7). Hornblende analyses are shown in Table 7, lized samples.

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

Sample n SiO2 Al2O3 Fe2O3 CaO SrO BaO Na2OK2O Total Ab Or An 97-17 Phenocryst 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 44.3 5 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 41.9 5 56.0 26.8 0.39 9.16 0.14 0.03 6.15 0.30 99.0 53.9 1.7 44.4 5 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 47.6 Phenocryst 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 48.8 mid 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 50.1 core 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 97-34 Phenocryst 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 44.6 mid 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 44.8 core 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 47.3 4 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 48.4 5 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 9-11-02 290 K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296

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 north sample traverse. Lines connect phenocryst core^rim pairs. E summarizes late-crystallized compositional groups.

5. Discussion tion^di¡erentiation along liquid lines of descent leading to such evolved melts, would account 5.1. Crystallization^di¡erentiation and magma for the dominant major element trends, including mixing nearly constant Na2O over a wide range of SiO2, and decreasing P2O5 and compatible major oxides Major and trace-element variations in the Bur- with increasing SiO2. Complex variations of K2O, roughs Mountain lava and pyroclastic deposits Rb, Ba, Sr, and Zr (Figs. 2 and 3) rule out simple signal important roles for both crystallization^dif- crystal-fractionation of a single parent magma. ferentiation and magma mixing. Glasses from Instead, the Burroughs Mountain magma reser- block-and-ash-£ow clasts (Table 8) lie on the voir may have been assembled from multiple high-SiO2 extensions of lava and clast major ele- batches of similar, but not identical andesitic ment arrays (Fig. 2). Variable segregation of such magma that crystallized the same phenocryst as- evolved melts, or variable extents of crystalliza- semblages and therefore followed similar liquid

VOLGEO 2518 9-11-02 K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 291 lines of descent. Mixing between these magma blende also produce acceptable results but yield batches may have produced the scatter in trace- slightly negative coe⁄cients for clinopyroxene, element abundances. perhaps signifying a hornblende^clinopyroxene Multiple regression calculations (Bryan et al., reaction relation. Average rhyolitic matrix glass 1969), using average phenocryst compositions can be produced from the representative andesite (Tables 6 and 7), whole rocks (Tables 1 and 2, parent by crystallization of 66% the same mineral and Stockstill, 1999), and glasses (Table 8), sup- assemblage in nearly identical proportions: pla- port the inference that crystallization^di¡erentia- gioclase (69.5%), orthopyroxene (20.1%), clino- tion was a dominant process in producing the pyroxene (5.3%), Fe^Ti oxides (4.3%), and apatite Burroughs Mountain andesite^dacite suite. Rep- (0.7%) (4residuals2 = 0.007). Crystallization^dif- resentative dacite (an average of seven lava anal- ferentiation of similar parent magmas along a yses with SiO2 s 63 wt%, normalized to 100%, common liquid line of descent can account for with all Fe as FeO) can be produced from a rep- close agreement of mineral assemblages and pro- resentative andesite parent (an average of 22 lava portions estimated from whole rocks, and semi- analyses with SiO2 60^61 wt%) by 26% crystalli- independently from whole rocks and glasses. Mix- zation of an assemblage of plagioclase (69.1%), ing of unrelated magmas produced from indepen- orthopyroxene (22.9%), clinopyroxene (3.5%), dent sources, or large extents of assimilation gen- Fe^Ti oxides (3.7%), and apatite (1%) (4re- erally would not. Small variations in some trace- siduals2 = 0.07). Calculations including horn- element ratios (eg. Th/U; Fig. 5) cannot be due to

Table 7 Representative electron microprobe analyses of pyroxene and hornblende phenocrysts

Sample n SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2OK2O F Cl Total Mg# 97-17 Opx 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 63.0 core 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 58.5 mid 2 52.0 0.23 0.90 0.04 22.2 0.56 22.5 1.33 0.03 99.8 58.5 core 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 62.7 mid 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 60.7 core 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 97-34 Opx 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 60.0 core 2 52.6 0.23 1.04 6 0.03 21.5 0.54 22.4 1.22 0.03 99.6 59.2 97-12 Opx 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 54.5 Cpx 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 60.8 Cpx 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 97-14 Opx 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 64.7 Cpx 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 9-11-02 292 K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 crystallization^di¡erentiation of the lava’s pheno- low SiO2 (Figs. 2 and 3). Traces of zircon in lava crysts, and either result from assimilation or from and pyroclastic £ow samples, unusual for an- the introduction of evolved, accessory mineral-sa- desites and dacites, and the presence of zircon in turated liquids from chamber-margin cumulates. the cognate plutonic inclusions are consistent with Quenched magmatic inclusions in the lower the notion that inclusions have disaggregated and part of the lava £ow and in the block-and-ash- dispersed into the andesite^dacite magma. If the £ow deposits are indisputable evidence that the lower-SiO2 cognate plutonic inclusions are cumu- Burroughs Mountain andesite^dacite magmatic lates, they were not formed by crystallization^dif- system was also injected with new basaltic ande- ferentiation of Burroughs Mountain andesite to site liquids. Those liquids’ compositions do not lie produce dacite. The cumulates may have formed on the arrays de¢ned by lava and clast samples by crystallization^di¡erentiation of basaltic an- for some major and trace elements (Figs. 2 and 3). desite to produce andesite, but this possibility re- Mixing with such basaltic andesite liquids cannot, quires further work to be veri¢ed. therefore, have produced the dominant compositional variations in the andesite^dacite lava and 5.2. Spatial^temporal variations pyroclastic £ows. Mixing with basaltic andesite liquids might account for the deviation of some The Burroughs Mountain eruptive episode be- lava and pyroclastic £ow samples o¡ the domi- gan with compositionally diverse andesite^dacite nant compositional arrays, such as the scattering block-and-ash £ows. Compositional diversity of samples to low-P2O5 at low-SiO2. Quenched could result from tapping magma from wide- magmatic inclusions with compositions appropri- spread locations within a heterogeneous or zoned ate to plot at the ma¢c end of the andesite^dacite magma reservoir, perhaps during recharge events series are present in other Mount Rainier lava recorded by quenched inclusions, or it could re- £ows (Sisson, unpublished analyses), but are ab- sult from eruption of discrete magma batches dur- sent at Burroughs Mountain, and if such andesitic ing assembly of the shallow magma reservoir that liquids recharged the Burroughs Mountain mag- fed the subsequent lava £ow. The eruptive episode ma reservoir, they failed to quench to inclusions. switched abruptly to e¡usion of lava, with insuf- The straight-sided and relatively inclusion-free ¢cient time for incision and erosion of unconsoli- habits of plagioclase phenocrysts, lacking pro- dated block-and-ash deposits. The transition to nounced resorbtion, suggest that in£uxes of basal- e¡usive eruption might result from establishment tic andesite were too small to destabilize resident of a radial-dike-fed £ank vent, as for some other phenocrysts and took place su⁄ciently long be- large lava £ows at Mount Rainier, but we have no fore eruption that conditions stabilized in the conclusive evidence to establish this. The ¢rst lava magma reservoir and weakly resorbed grains to erupt was relatively evolved and phenocryst- were overgrown. rich and formed the lower portion of the Bur- Disaggregation of cognate plutonic inclusions roughs Mountain lava £ow. Some of this early would have a similar e¡ect on lava and pyroclast lava carried vesicular quenched basaltic andesite compositions as mixing with basaltic andesite, inclusions, suggesting further recharge events. producing scatter to low P2O5,Na2O, and Sr at Erupting lava abruptly became more ma¢c and

Table 8 Electron microprobe analyses of glass in sub-pumiceous block-and-ash-£ow clasts a Sample n SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2OK2OP2O5 Cl SO3 Total 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 95.9 SR755 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 94.6 SR755-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 96.5 a 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) 275^296 293 slightly poorer in plagioclase phenocrysts. This With time, greater extents of mixing with the slightly more ma¢c lava overrode the earliest evolved resident magma might have created the evolved lava and formed the upper portion of felsic-upward zonation of the upper lava layer. the £ow. As the eruption waned, e¡using magma The quenched basaltic andesite inclusions are became progressively more evolved, creating the the clearest candidates for a newly introduced weak felsic-upward zonation in the upper part of magma, but compositional variations in the lava the lava £ow. Progressive emplacement of andes- £ow as a whole (Fig. 2) and in the vertical sample itic lava as a vertical succession, as opposed to a sections (Fig. 5) do not result from mixing with series of distributed £ow lobes (Blake and Bruno, magmas like the basaltic andesite inclusions. Mix- 2000), may result from its con¢nement against ing with basaltic andesite inclusion magmas glacial ice (Lescinsky and Sisson, 1998; Lescinsky would produce compositions with relatively high and Fink, 2000). MgO and low P2O5, unlike the more ma¢c lava The ma¢c over felsic layering of the Burroughs samples. If zonation in the lava was due to mixing Mountain lava £ow, without a separating cooling shortly before eruption, the recharge magma was break, is similar to many chemically zoned ash- not preserved as macroscopically distinguishable £ow sheets. Two possible explanations are: (1) inclusions or streaks, and it left little evidence in layering developed from eruption of a previously the form of phenocryst textures or gross changes zoned magma body; or (2) layering developed by in phenocryst compositions. For these reasons, we the immediately pre- or syn-eruptive intrusion of infer that any sizeable syn- or immediately pre- more ma¢c magma into a crystallizing less ma¢c eruptive recharge magmas must have been very magma body. In the ¢rst interpretation, pheno- similar to the resident magmas, or more simply, cryst-rich evolved magmas could have developed that no sizeable recharges took place. Small vol- by crystallization^di¡erentiation (and perhaps as- ume Holocene tephras at Mount Rainier preserve similation) and accumulated due to buoyancy in obvious evidence of pervasive syn-eruptive mixing the upper part of a magma body. Lower portions and mingling (Mullineaux, 1974; Swanson, 1993; of the reservoir may have been occupied by less Venezky and Rutherford, 1997), as do some lava di¡erentiated and probably hotter magmas, and £ows (McKenna, 1994; Sisson, unpublished map- periodic replenishments of less evolved magma ping), but the large-volume lava £ow at Bur- into the roots of a magma body are likely to roughs Mountain is not one of these. have contributed to vertical zonation. This zona- Plagioclase trace-element ratios further illumi- tion would then have been preserved in inverted nate the magmatic and eruptive history. Di¡eren- form during eruption of the lava. The weak felsic- ces between lower and upper portions of the £ow upward zonation in the upper layer might have in matrix grain Ba/Sr and Sr/Ca (Fig. 6) shows resulted from tapping of magma from progres- that liquid compositions varied during the course sively shallower portions of the zoned reservoir of the eruption. Early erupted magma was more system as the eruption waned. evolved, richer in phenocrysts, and contained a In the second interpretation, the spatial zona- more-evolved (higher Ba/Sr) melt. Later erupted tion of the lava £ow resulted from mixing pro- magmas were more ma¢c, slightly poorer in phe- cesses at or immediately before the time of erup- nocrysts, and contained less-evolved (lower Ba/Sr) tion and does not re£ect pre-eruptive zonation of melt. Increasing Ba/Sr from plagioclase pheno- a magma reservoir. In this interpretation, the pre- cryst cores to rims and matrix grains is the sense eruptive magma system consisted of a crystalliz- of zoning expected for crystallization^di¡erentia- ing, more evolved, and phenocryst-rich magma tion of a plagioclase-rich assemblage, where Sr is body that was intruded by more ma¢c, pheno- more compatible than Ba (Blundy and Wood, cryst-poor magma(s). The resident evolved mag- 1991). This normal Ba/Sr zoning in the lower ma began to erupt ¢rst, creating the lower portion £ow unit is consistent with magma that was cool- of the lava £ow, followed by eruption of mixed ing and crystallizing in a relatively simple fashion. magma that created the upper portion of the £ow. Reversed trace-element zoning of plagioclase in

VOLGEO 2518 9-11-02 294 K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296

Fig. 7. Interpretation of the Burroughs Mountain eruptive episode and zone magma reservoir (not to scale). (1) Stage 1, high effusion rates feed ash £ow eruptions and tap magmas over a wide depth range from a vertically zoned, recharged magma reservoir. (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 with basaltic andesite continues. (3) Stage 3, sharply increased e¡usion taps magmas from deeper in the zoned reservoir, producing the more ma¢c upper portion of the lava. As the eruption waned, progressively more felsic magmas were tapped from shallower in the reservoir, producing the felsic-upward zonation in the lava’s upper layer. the upper £ow unit, beginning with plagioclase mixing of strongly diverse magmas immediately like that in earlier-erupted rocks, may signify before or during eruption, support the zoned res- that phenocrysts from the evolved portion of the ervoir interpretation for the magmatic system that magmatic system became engulfed in a less fed the Burroughs Mountain eruptions. evolved, lower Ba/Sr liquid. Mixing between resident magma and a newly injected less fractionated 5.3. A possible eruption scenario magma or simple settling of shallow plagioclase phenocrysts into deeper, less fractionated parts of Temporal variations in erupted compositions a zoned magma reservoir could account for the can result from the dynamics of magma with- reversed Ba/Sr zoning. In either instance, compo- drawal from a zoned reservoir. High withdrawal sitional and thermal contrasts were too small to rates tap magma over a wide depth range in a strongly destabilize the plagioclase phenocrysts or magma chamber, whereas low withdrawal rates to produce phenocryst-rim plagioclase distinctly tap magmas dominantly from the chamber’s more anorthitic than that in the earlier-erupted upper portions (Spera et al., 1986). High magma portion of the £ow (Table 6). These subtle zoning withdrawal rates and consequent sampling over a features are most consistent with small di¡erences wide depth range could account for the composi- in melt composition and temperature, and suggest tionally diverse Burroughs Mountain pyroclastic mixing or crystal settling within the andesite^da- £ows that began the eruptive episode (Fig. 7). cite compositional suite. The relatively simple Quenched magmatic inclusions in those deposits compositional variations in the lava £ow, and suggest that recharge events preceded the erup- the absence of features attributable to pervasive tion, but evidence is inadequate to determine if

VOLGEO 2518 9-11-02 K.R. Stockstill et al. / Journal of Volcanology and Geothermal Research 119 (2002) 275^296 295 recharge caused eruption. Disruption of the suggesting that conduit dynamics, not magma chamber margins distributed cognate plutonic in- composition, determined explosive versus e¡usive clusions throughout the magma. Lava e¡usion eruption styles. E¡usive eruption ¢rst tapped then followed, due to opening of a radial dike magma from the shallow evolved portion of the (Fig. 7) or other changes in conduit con¢gura- magma reservoir. More ma¢c lava followed, per- tions. Early lava was restricted to evolved compo- haps due to tapping of less fractionated magma sitions, perhaps signifying low withdrawal rates from deeper in a zoned reservoir. As the eruption that tapped magma only from shallow parts of waned, evolved magma from shallow in the sys- the reservoir. Quenched basaltic andesite inclu- tem increasingly dominated the erupting compo- sions record further recharge events, although sition, leading to a weak felsic-upward zonation in these were inadequate to destabilize phenocrysts the upper portion of the lava £ow. The zoned lava or to strongly modify bulk magma compositions. £ow at Burroughs Mountain shows that, at times, Erupting lava then abruptly became more ma¢c Mount Rainier’s magmatic system has developed and overrode the earlier-erupted lava, producing relatively large, shallow reservoirs that, despite the more ma¢c upper portion of the £ow, but the complex recharge events, were capable of devel- change to more ma¢c compositions was not due oping a felsic-upward compositional zonation to mixing with basaltic andesite recharge magmas. similar to that inferred from large ash-£ow sheets Instead, the shift to ma¢c compositions might and other zoned lava £ows. have resulted from greatly increased e¡usive £ux that sampled less di¡erentiated andesite from deeper in the reservoir. Alternatively, recharge Acknowledgements may have initiated a plume that brought less fractionated resident andesite magmas to shallow lev- We thank the US Department of the Interior, els where they became available for eruption National Park Service for permission to conduct (Clynne, 1999). As the eruption waned and e¡u- research in Mount Rainier National Park. Con- sive £ux declined, magma was tapped from pro- structive and insightful reviews by Julie Donnelly- gressively shallower portions of the reservoir, pro- Nolan, Mike Clynne, Jim Gardner, and Anita ducing the felsic-upward zonation of the upper Grunder led to a much-improved manuscript. part of the £ow. Lina Patino developed and guided the LA-ICP- MS analytical techniques, and R. Thomas, J.P, Brandenburg, and J. Weaver assisted with sample 6. Conclusion preparation.

We interpret the layered Burroughs Mountain lava £ow to result from the partial evacuation of References a zoned, partly crystallized andesite^dacite magma body, wherein depth of magma withdrawal Bacon, C.R., 1986. Magmatic inclusions in silicic and inter- was coupled to e¡usion rate. The reservoir was mediate volcanic rocks. J. Geophys. Res. 91, 6091^6112. assembled from similar, but not identical andesitic Blake, S., Bruno, B.C., 2000. Modelling the emplacement of magma batches that crystallized a common pla- compound lava £ows. Earth Planet. Sci. Lett. 184, 181^197. Blundy, J.D., Wood, B.J., 1991. Crystal-chemical controls on gioclase^pyroxene^amphibole^Fe-Ti oxide^apa- the partitioning of Sr and Ba between plagioclase feldspar, tite assemblage. More fractionated magmas occu- silicate melts, and hydrothermal solutions. Geochim. Cos- pied shallower portions of the reservoir. Deeper mochim. Acta 55, 193^209. portions were more ma¢c due to the combined Bryan, W.B., Finger, L.W., Chayes, F., 1969. A least-squares e¡ects of recharge events and less advanced crys- approximation for estimating the composition of a mixture. Carnegie Inst. Washington Year B. 67, 243^244. tallization^di¡erentiation. Initial eruptions pro- Carrigan, C.R., Eichelberger, J.C., 1990. Zoning of magmas by duced block-and-ash £ows that were composition- viscosity in volcanic conduits. Nature 343, 248^251. ally indistinguishable from the subsequent lava, Carrigan, C.R., 1994. Two-component magma transport and

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the origin of composite intrusions and lava £ows. In: Ryan, McKenna, J.M., 1994. Geochemistry and Petrology of Mount M.P. (Ed.), Magmatic Systems. International Geophys. Se- Rainier magmas: Petrogenesis at an Arc-Related Stratovol- ries, vol. 57, pp. 319^354. cano with Multiple Vents. M.S. Thesis. Univ. Washington, Carrigan, C.R., Schubert, G., Eichelberger, J.C., 1992. Ther- 122 pp. mal and Dynamical Regimes of Single- and Two-Phase Mills, J.G.Jr., Saltoun, B.W., Vogel, T.A., 1997. Magma Magmatic Flow in Dikes. J. Geophys. Res. 97, 17377^17392. batches in the Timber Mountain magmatic system, South- Clynne, M.A., 1999. A complex magma mixing origin for western Nevada Volcanic Field, Nevada, USA. J. Volcan. rocks erupted in 1915, Lassen Peak, California. J. Petrol. Geotherm. Res. 78, 185^208. 40, 105^132. Miyashiro, A., 1974. Volcanic rock series in island arcs and Coombs, M.L., Eichelberger, J.C., Rutherford, M.J., 2000. active continental margins. Am. J. Sci. 274, 321^355. Magma storage and mixing conditions for the 1953^1974 Mullineaux, D.R., 1974. Pumice and other pyroclastic deposits eruptions of Southwest Trident volcano, Katmai National in Mount Rainier National Park, Washington. U.S. Geol. Park, Alaska. Contrib. Mineral. Petrol. 140, 99^118. Surv. Bull. 1326, 83 pp. Donnelly-Nolan, J.M., Champion, D.E., Grove, T.L., Baker, Patino, L.C., Szymanski, D.W., Vogel, T.A., Hannah, R.S., M.B., Taggart, J.E., Bruggman, P.E., 1991. The Giant Cra- 1999. Rare earth element analysis of rocks by laser ablation ter lava ¢eld: geology and geochemistry of a composition- ICP-HEX-MS. Geol. Soc. Am. Abstr. Programs 31, 345. ally zoned, high-alumina basalt to basaltic andesite eruption Sisson, T.W., Lanphere, M.A., 2000. The geologic history of at Medicine Lake Volcano, California. J. Geophys. Res. 96, Mount Rainier volcano, Washington. Wash. Geol. 28, 28. 21843^21863. Sisson, T.W., Vallance, J.W., Pringle, P.T., 2001. Progress Eichelberger, J.C., Cherko¡, D.G., Dreher, S.T., Nye, C.J., made in understanding Mount Rainier’s hazards. EOS 2000. Magmas in Collision: Rethinking chemical zonation Trans. Am. Geophys. Union 82, 113^119. in silicic magmas. Geology 28, 603^607. Smith, R.L., 1979. Ash-£ow magmatism. Geol. Soc. Am. Spec. Gill, J.B., 1981. Orogenic Andesites and Plate Tectonics. Pap. 180, 5^27. Springer, Berlin. Spera, F.J., Yuen, D.A., Greer, J.C., Sewell, G., 1986. Dynam- Hildreth, W., 1981. Gradients in silicic magma chambers: im- ics of magma withdrawal from strati¢ed magma chambers. plications for Lithospheric magmatism. J. Geophys. Res. 86, Geology 14, 723^726. 10153^10192. Stockstill, K.R., 1999. The Origin and Evolution of the Bur- Irvine, T.N., Baragar, W.R.A., 1971. A guide to the chemical roughs Mountain Lava Flow, Mount Rainier, Washington. classi¢cation of the common volcanic rocks. Can. J. Earth M.S. Thesis. Michigan State University, 88 pp. Sci. 8, 523^548. Swanson, D., 1993. Variation in the Grain Size Distribution Kinzler, R.J., Donnelly-Nolan, J.M., Grove, T.L., 2000. Late and the Chemical Composition of the Mount Rainier Holocene hydrous ma¢c magmatism at Paint Pot Crater and C-ash Unit. B.S. Senior Thesis. University of Puget Sound, Callahan £ows, Medicine Lake Volcano, N. California and 77 pp. the in£uence of H2O in the generation of silicic magmas. Venezky, D., Rutherford, M.J., 1997. Pre-eruption conditions Contrib. Mineral. Petrol. 138, 1^16. and timing of dacite-andesite mixing in the 2.2 ka C-layer at Lescinsky, D.T., Fink, J.H., 2000. Lava and ice interaction at Mount Rainier. J. Geophys. Res. 102, 20069^20086. stratovolcanoes; use of characteristic features to determine Vogel, T.A., Eichelberger, J.C., Younker, L.W., Schuraytz, past glacial extents and future volcanic hazards. J. Geophys. B.C., Horkowitz, J.P., Stockman, H.W., Westrich, H.R., Res. 105, 23711^23726. 1989. Petrology and emplacement dynamics of intrusive Lescinsky, D.T., Sisson, T.W., 1998. Ridge-forming, ice- and extrusive rhyolites of Obsidian Dome, Inyo Craters vol- bounded lava £ows at Mount Rainier, Washington. Geology canic chain, eastern California. J. Geophys. Res. 94, 17937^ 26, 351^354. 17956.

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